dp03a

ExxonMobil ProprietaryFRACTIONATING TOWERS Section Page

DEVICE SELECTION AND BASIC CONCEPTS III-A 1 of 60

DESIGN PRACTICES December, 2001

ExxonMobil Research and Engineering Company – Fairfax, VA

CONTENTSSection Page

SCOPE ............................................................................................................................................................4

REFERENCES.................................................................................................................................................4

BACKGROUND...............................................................................................................................................4KEY STEPS INVOLVED IN TOWER DESIGN - AN OVERVIEW ...........................................................4TYPES OF CONTACTING DEVICES AVAILABLE .................................................................................5

CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE.........................................................................7

CROSS FLOW DEVICES - HARDWARE DEFINITIONS ..............................................................................11

CROSS FLOW DEVICES - PROCESS DEFINITIONS..................................................................................24VAPOR HANDLING LIMITATIONS.......................................................................................................24LIQUID HANDLING LIMITATIONS........................................................................................................27OTHER PROCESS CONSIDERATIONS ..............................................................................................28

CROSS FLOW DEVICES - BASIC DESIGN CONSIDERATIONS................................................................33

CROSS FLOW DEVICES - GENERAL CONCLUSIONS ..............................................................................34

COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS,DUALFLOW TRAYS) ....................................................................................................................................34

EQUIPMENT TYPES AND APPLICATIONS.........................................................................................34

COUNTER-CURRENT DEVICES - PROCESS DEFINITIONS......................................................................38VAPOR / LIQUID CAPACITY LIMITATIONS.........................................................................................38EFFICIENCY AND TURNDOWN ..........................................................................................................38HEAT TRANSFER.................................................................................................................................39OTHER CONSIDERATIONS.................................................................................................................39

COUNTER-CURRENT DEVICES - GENERAL CONCLUSIONS ..................................................................39

NOMENCLATURE.........................................................................................................................................40

COMPUTER PROGRAMS ............................................................................................................................40

Changes shown by ➧

ExxonMobil ProprietarySection Page FRACTIONATING TOWERS

III-A 2 of 60 DEVICE SELECTION AND BASIC CONCEPTSDecember, 2001 DESIGN PRACTICES


CONTENTS (Cont)Section Page

TABLESTable 1 Trays - A Summary Of Characteristics ..........................................................................41Table 2 Counter-Current Devices - A Summary Of Characteristics............................................42Table 3A Tower Internals Selection For New Towers ...................................................................43Table 3B Tower Internals Selection For New Towers ...................................................................44Table 3C Tower Internals Selection For New Towers ...................................................................45Table 3D Relative Fouling Resistance Of Common Fractionation Devices...................................46Table 4 Tower Internals Selection For Revamps........................................................................47Table 4A Application Guidelines For Debottlenecking Fractionation Towers................................48Table 4B Tower Internals Selection For Revamps........................................................................49Table 4C Tower Internals Selection For Revamps........................................................................50Table 4D Tower Internals Selection For Revamps........................................................................51Table 5A Tower Internals Selection For Heat Transfer Service (New Towers And Revamps) .....52Table 5B Tower Internals Selection For Heat Transfer Service (New Towers And Revamps) .....53Table 5C Tower Internals Selection For Heat Transfer Service (New Towers And Revamps) .....54Table 6A Tower Internals Selection For Entrainment Removal Service........................................55Table 6B Tower Internals Selection For Entrainment Removal Service........................................56Table 7 Tower Design Checklist (Trays).....................................................................................57Table 8 Tower Design Checklist (Packing).................................................................................58

FIGURESFigure 1 Cross Flow Vs. Counter-Current Device Operation .........................................................6Figure 2 Types Of Trays (Schematic) ............................................................................................7Figure 3 Types Of Moveable Valves..............................................................................................8Figure 4 Some High Capacity Tray Designs..................................................................................9Figure 5 Enhanced Downcomer Trays ........................................................................................10Figure 6A Typical Sieve Tray Tower ..............................................................................................12Figure 6B Typical Tray Layout .......................................................................................................13Figure 7 Pass Arrangement On Multi-Pass Trays........................................................................15Figure 8 Stepped Vs. Sloped Downcomers .................................................................................16Figure 9 Modified Arc Vs. Arc Type Downcomers .......................................................................16Figure 10 Sulzer Cartridge Trays And Their Envelope Downcomers.............................................17Figure 11 Downcomer Sealing Techniques ...................................................................................18Figure 12 Bubble Area Definitions .................................................................................................20Figure 13 Free Area Definitions.....................................................................................................21Figure 14 Sketches Of Some Hardware Devices ..........................................................................23Figure 15 Jet Flooding: Its Impact On Entrainment And Tray Efficiency........................................24Figure 16 Froth Regime Vs. Spray Regime Operation ..................................................................25Figure 17 Flow Regime Within Normal Operating Range..............................................................25Figure 18 Generating Entrainment.................................................................................................26Figure 19 Downcomer Filling Components (Static Pressure Balance) ..........................................27Figure 20 Effect Of Liquid Rate On Sieve Tray Turndown.............................................................29Figure 21 Effect Of Weeping On Efficiency ...................................................................................32Figure 22 Typical Sieve Tray Performance Diagram .....................................................................33Figure 23 Random (Dumped) Packings.........................................................................................35Figure 24 Structured Packing (By Koch-Glitsch, Inc.)....................................................................35Figure 25 Various Types Of Grids .................................................................................................36Figure 26 Different Types Of Baffles..............................................................................................37





Revision Memo12/01 This update represents a major revision effort. Highlights of this revision are:

(1) Major updates to discussions and figures throughout the entire section, to reflect themerging of ExxonMobil fractionation technologies.

(2) Updated Figure 4, “Some High Capacity Tray Designs”, to include Provalve deck, UOPMultiple Downcomer Sieve Trays, and the Sulzer HiFi Tray.

(3) Updated the definition of waste area to account for three different types of wasted towerarea.

(4) Included discussion and sketches of picket fence and swept back weirs.(5) Major modifications to Vapor Handling Limitations section include:

updated hierarchy of design considerations,included emulsion regime in discussion of flow regimes.

(6) Major modifications to Liquid Handling Limitations section include:updated hierarchy of primary and secondary design considerations,expanded and updated discussions of all design considerations.

(7) Major modifications to Other Process Considerations section include:updated hierarchy of design considerations,included discussion of fouling.

(8) Inserted Table 3D, “Relative Fouling Resistance of Common Fractionation Devices”.(9) Major modifications to Tables 1, 2, 3, 4A, 7, 8 to include newer fractionation devices and

debottlenecking strategies.(10) Inserted NOMENCLATURE section.




SCOPEThis section presents guidelines which are useful for selecting the optimum contacting device for a given service. Also includedare discussions on the various process limitations which restrict the operating range of a given device. The detailed designprocedures for the various fractionating devices are covered in subsequent sections. A detailed Tower Design Checklist isincluded as Table 7 (for trays) and Table 8 (for packing). They are intended for use after the design has been completed. Forinformation on mechanical requirements see GP 5-2-1. For fractionator process information, refer to the Separations Guide(TMEE 058).

REFERENCESDESIGN PRACTICESAll Other Sections of Section III

GLOBAL PRACTICEGP 5-2-1 Internals for Towers and Drums

OTHER REFERENCES1. Atmospheric Pipestill Guide, ExxonMobil Engineering Technical Manual, EEPE-0006.2. Fractionating Tower Troubleshooting Guide, ExxonMobil Engineering Technical Manual, TMEE 021.3. Fuels Vacuum Pipestill Guide, ExxonMobil Engineering Technical Manual, EETD 076 (EE.20E.96).4. Maloney, D.P., Fuels Vacuum Pipestill Wash Zone Reliability, New Section of the Fuels Vacuum Pipestill Guide, Technical

Manual Update, EE.53E.98.5. Mobil Tower Internals Program (MoTIP) User’s Manual, Mobil Research and Development Corporation.

➧ 6. Refinery Distillation Practices, Mobil Technology Company, May, 1996.➧ 7. Separations Guide, ExxonMobil Engineering Technical Manual, TMEE 058.➧ 8. Trayed Tower Internals, MTC Process Design Practices, Mobil Technology Company, Practice No. II, Volume No

9. Attached to the Table of Contents for Section Ill - Fractionating Towers, is a list of ExxonMobil FRACTIONATIONSPECIALISTS who can provide help on a broad range of fractionation issues.

BACKGROUNDKEY STEPS INVOLVED IN TOWER DESIGN - AN OVERVIEWThe purpose of this Subsection is to define the Basic Concepts needed to understand how the various contacting devices workand to help select the best internal for a given service. Once the contacting device has been selected, the designer shouldthen read the particular Subsection that covers that device. Before beginning, however, it is important to look at all the stepsthat must be taken in order to achieve an optimum tower design. To do this, the eight key steps in designing a distillation towerare briefly reviewed below.1. Define the key separations. In many cases, where individual components can be identified, the amount of impurity

permitted in the products is given. This is needed in order to set the yield and purity level of the products being produced.2. Obtain the appropriate vapor-liquid equilibrium (VLE) and enthalpy data method. This can be done by referring to

the ExxonMobil Data Library Manual or by contacting your FRACTIONATION SPECIALIST. The use of an inappropriatedata method could lead to off-spec products or an inoperable design.

3. Calculate the theoretical trays required at different reflux rates. By running parametric cases on a plate to platecomputer program with differing numbers of trays and reflux rates, the shape of the theoretical trays vs. reflux ratio curvecan be determined. This curve will assist in determining the most economical tower height and diameter combination.





BACKGROUND (Cont)4. Estimate the tray efficiency. This step assumes that the contacting internal will be sieve or valve trays; however, a similar

process should be followed with other internals, such as packing. For most services, towers similar to the one beingdesigned have been built in the past and operating efficiencies have been determined. These are tabulated in Section III-I, Tray Efficiency, and are suitable for preliminary scoping purposes. For truly new systems, and for checking the efficiencyduring the tower design, the efficiency should be calculated via Section III-I (or the appropriate computer program), sincethe final efficiency is also influenced by the tray hardware design. The liquid flow path length (lfp) is the most importanthardware parameter with respect to tray efficiency, provided normal hydraulics (that is, weeping and entrainment belowallowable limits, and Ao/Ab and weir heights within allowable ranges). The flow path length determines the number ofmixing pools on the tray, which in turn has a direct influence on the tray efficiency. Tray efficiency increases rapidly fromthe minimum allowable flow path length (16 in [400 mm] for conventional trays) up to about 30 in (760 mm). For flow pathlengths greater than 30 in. (760 mm), liquid flow on the tray is essentially plug flow, and efficiency is independent of flowpath length. However, for flow path lengths greater than about 70 in. (1780 mm), tray efficiency starts to decrease due tothe presence of stagnant regions on the tray, depending on tray geometry.

5. Define the maximum and minimum feed rates that the tower must handle. In general, a turndown ratio of 2/1 isadequate for most services. If greater turndown than this is needed, the turndown ratio itself becomes one of the key itemsin selecting both the tray type and how it should be designed. Another term commonly used is "flexibility”, which is theability of a device to operate efficiently over a range of vapor and liquid loadings.

6. Selecting the best tower internal. Subsequent discussions plus Tables 1-3 will facilitate internals selection. For mosttrayed columns, sieve or valve trays will be the first choice whereas for packed towers a dumped packing of the 2 in. (50mm) size will normally suffice. Of course, such items as fouling tendency, allowable pressure drop, turndownrequirements, and in the case of revamps, required capacity and/or efficiency will have an important influence on theinternal finally selected. As a design technique, zoning the tower into different sections can increase flexibility and reducetower height. However, most new tower designs should not be divided into more than two sections (with the feed zonebeing the line of demarcation the two sections), due to diminishing returns on overly complex design. Contact aFRACTIONATION SPECIALIST if a design with more than two sections is being considered for a simple tower.

➧ 7. Tower sizing and tray hydraulic calculations. The key parameters to consider in the sizing and hydraulics areas arediscussed in subsequent pages of this Subsection for the various devices available. Various computer programs areavailable in PEGASYS to facilitate calculations and permit rapid optimization via parametric cases.It is strongly recommended, however, that the engineer's first internals design be done with the close guidance of aFRACTIONATION SPECIALIST, following the steps in the detailed design procedure, in order to develop the necessaryfeel for how the many variables interrelate. Subsequent designs can then be done via computer with confidence.

8. Process Control. In order to get optimal performance from any distillation tower, it must be controlled properly. As thetower diameter and number of trays increases, the lag time in the system can become quite large. The designer shouldsee Section XIl - Instrumentation and consult with the appropriate SYSTEMS ENGINEERING SPECIALIST to ensure thatthe correct process control system has been specified.

TYPES OF CONTACTING DEVICES AVAILABLEA contacting device must have good liquid and vapor handling capacities, good contacting efficiency, reasonable pressuredrop, predictable turndown characteristics and be economical. The devices available fall into two broad categories - crossflow and counter-current. They are shown conceptually on Figure 1. Subsequent paragraphs will discuss how each deviceworks.




BACKGROUND (Cont)

➧ FIGURE 1CROSS FLOW VS. COUNTER-CURRENT

DEVICE OPERATION

Downcomer

Liquid

Vapor

Vapor

Tray

LiquidIn

Contacting Zone

Cross Flow Counter-Current

DP3Af01

Cross Flow Devices (Trays). The liquid flows horizontally across a flat, level plate (called the tray) that contains a contactingdevice selected by the designer that intimately disperses the vapor into the liquid. In addition, the dispersion process mustproduce sufficient interfacial area and maintain the phases in contact with each other long enough to promote adequate masstransfer between the phases.As the liquid flows across the tray, it is contacted by the ascending vapor. At the far side of the tray, the liquid enters adowncomer which carries it to the tray below where the contacting process is repeated. Obviously, the contacting area must belarge enough to handle the required liquid and vapor rates while promoting the desired mass transfer. Likewise, thedowncomer must be large enough to handle the liquid being processed. The various process limitations that define theoperating constraints for the contacting area and the downcomer will be discussed later under CROSS FLOW DEVICES -PROCESS DEFINITIONS.Counter-Current Devices (Packing, Grids, Baffle Sections, and Dualflow Trays). With these devices, the liquid flow iscounter-current to the vapor flow. The efficiency of contact is again dependent on the area available for mass transfer. Intrays, this is provided by bubbling vapor through the liquid, thereby producing sufficient interfacial area for mass transfer. Withsuch counter-current devices as packing and baffle trays, however, the interfacial area needed for mass transfer is provided bythe surface area of the device, or by forcing the vapor to flow through descending curtains of liquid (thereby breaking them upinto droplets).Generally speaking, as the surface area of the device goes up, the efficiency goes up. However, as the surface areaincreases, the capacity falls while its cost rises. Thus, the final choice involves optimizing the capacity, efficiency, cost, andother process considerations of the various internals available. Subsequent paragraphs will discuss how this is done.





CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLESome trays used widely within the ExxonMobil circuit are sketched in Figure 2 and discussed briefly in subsequent paragraphs.The pertinent Subsection for designing each device is also noted next to the figure in parentheses.

FIGURE 2TYPES OF TRAYS

(SCHEMATIC)

Sieve(III-B)

Valve(III-E)

Jet(III-D)

Bubble Cap(III-C)

Closed Pulsating(Floating)

Fully Open

StandardCap

VacuumCap DP3AF2

1. Sieve Trays. For most fractionation services in refining and petrochemicals, seive trays will be the first choice. Thecontacting area consists of flat plates containing perforations, usually 1/2 in. (13 mm) in diameter. They are the simplesttrays to fabricate and are therefore the cheapest. They also exhibit good capacity, efficiency, and turndown characteristics(about 2/1 to 3/1). Their flat surface facilitates maintenance; thus, they may be used in fouling services provided the holesize is increased to 3/4 - 1 in. (19-25 mm).

➧ 2. Valve Trays. The valve size, shape weight and other parameters vary from vendor to vendor (Figure 3). Valve traysshould be used in lieu of sieve trays when the turndown ratio exceeds 3/1. These trays contain proprietary devicesmanufactured by Koch-Glitsch Inc. (formerly two companies, Koch and Glitsch), Saint-Gobain NorPro Corporation(formerly Norton), Sulzer Chemtech (formerly Nutter Engineering and Sulzer), and other vendors. Valve trays should beused in lieu of sieve trays when the turndown ratio exceeds 3/1; in the upstream sector, valve trays are the preferredchoice due to this high turndown capability. For design purposes their capacity and efficiency are assumed to be aboutequal to that of a sieve tray, but their cost is roughly 10% higher. Floating (pulsating) valve trays are not recommended forfouling service; however, there are cases where fixed valve trays would be recommended for fouling service (see number6 below). All else being equal (cost / delivery time, etc.), rectangular valves are preferred due to lack of rotation. Rotationeventually erodes the holes and legs, causing the valves to pop out. This occurs even if dual rotation stops are included inthe valve specification.




CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont)

FIGURE 3TYPES OF MOVEABLE VALVES

Koch-Glitsch Type A

Koch-Glitsch Type T Sulzer (Nutter) BDH

Koch-Glitsch V-1

DP3AF3

3. Jet Trays. Jet trays, an Exxon development in the 1950's, are now used primarily in heavy hydrocarbon services thathandle high liquid rates. The directional effect of the vapor as it leaves the inclined tabs helps push the liquid across thetray, thereby increasing its capacity. Unfortunately, this directional effect also reduces liquid residence time on the traysuch that efficiency becomes low for most light ends and other towers with low to moderate liquid loads. For higher liquidrates typical of pumparounds, the efficiency in heat transfer service is comparable to other cross flow devices.

➧ 4. Bubble Cap Trays. These trays are used infrequently within ExxonMobil today because they cost about 100% more, andhave a higher pressure drop, than a sieve trays. They should be considered when flexibility needs exceed 3/1 and valvetrays cannot be used because fouling is a problem. While even bubble caps will foul over time their large physicalclearances (compared to valve trays) provide a longer run length. For applications that require low pressure drop, vacuumcaps should be considered instead of standard caps.

5. Cartridge Trays. These trays derive their name from the fact that they are really a collection of trays, held together byrods, to form bundles or “cartridges.” These cartridges are then lowered into a top-flanged vessel. For convenientinstallation each cartridge usually contains 10-15 trays. The contacting device is usually a sieve or valve tray. They areused in small diameter columns (i.e., < 3 ft or 900 mm) where inaccessibility prevents the use of standard trays. Mostcartridge trays in use by ExxonMobil were made by Sulzer Chemtech, since Sulzer’s tray has the only commercially provenperipheral sealing device.

➧ 6. Fixed Valve Trays. These devices can best be described as valve trays whose valve units are fixed in the fully openposition. Sulzer Chemtech's Nutter Small V-Grid (SVG) trays on triangular pitch are considered to be an alternative tosieve trays and generally have somewhat better turndown ratio (20% higher). Larger fixed valve units such as the NutterSVG or the NorPro Provalve may be useful for extending run lengths in some fouling services (but not where stickymaterial is entrained from below). Nutter Large V-Grid (LVG) and fixed valve trays on square pitch have lower capacityand should only be used upon consultation with a FRACTIONATION SPECIALIST.

➧ Mini Fixed Valve Trays: Sulzer Mini V-Grid (MVG), Figure 4, and Koch-Glitsch VG-0. At pressures under 50 psia (345kPa) a tray with mini valves has an estimated 10-15% jet flood capacity advantage when compared to a conventional(0.5 in. [13 mm] hole) sieve tray with approximately the same efficiency. The capacity gain is similar to what is expectedwith a small hole sieve tray in low liquid rate services. V-Grid trays have the same directional effect as the NorPro Provalvetray. Koch-Glitsch's Bi-FRAC should not be considered for application. Contact your FRACTIONATION SPECIALIST formore details should an application of fixed mini valve trays exist.





CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont)

➧ FIGURE 4SOME HIGH CAPACITY TRAY DESIGNS

Downcomers

Downcomer orientationalternates from tray to tray

DowncomersTray Deck

0.5" (13mm)or

0.67" (17mm)

SULZER CHEMTECH'S NUTTER MVG TRAY NORPRO PROVALVE DECK

Liquid Flow

1.5" (38mm)

Liquid Flow

UOP MULTIPLE DOWNCOMER SIEVE TRAYS SULZER HI-FI TRAY

DP03AF04




CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont)➧ 7. Enhanced Downcomer Trays. Enhanced downcomer trays currently available include Koch-Glitsch’s Nye Trays and

MaxFrac trays, Saint-Gobain NorPro’s Triton trays (see 8d below) and the Sulzer MVGT Tray. By utilizing the area underthe downcomer for additional vapor flow, an increase in the vapor handling capacity is achievable (Figure 5). Nye traysand recently, to a more limited extent, Triton trays have successfully been applied in tower revamps. An estimated 10-15%additional jet flood capacity can be expected with similar efficiency when compared to a conventional sieve tray. It shouldbe noted that the increase in capacity is approximately proportional to the ratio of the downcomer outlet area (downcomerbottom area) to the tower cross-sectional area. Contact your FRACTIONATION SPECIALIST should an application of anenhanced downcomer tray exist.

➧ FIGURE 5ENHANCED DOWNCOMER TRAYS

NORPRO TRITON TRAYKOCH-GLITSCH NYE TRAYCONVENTIONAL TRAY

DP3AF5

Downcomer

ProvalveDeck

OrificePlate

8. Other Trays.➧ a. Multiple Downcomer Trays: UOP MD (Multiple Downcomer) and ECMD (Enhanced Capacity Multiple Downcomer)

Trays, Sulzer Hi-fi Tray. These proprietary trays are particularly useful for fractionation services handling high liquidrates. Although they have about 20% higher capacity than a conventional sieve tray, their efficiency is less than thatof sieve trays while their cost is greater. These characteristics make them unattractive for new tower designs of lessthan 15 ft (4570 mm) diameter. For high pressure, light ends towers greater than 15 ft (4570 mm) diameter, MD,ECMD or Hi-Fi trays should considered in the scoping phase, with total tower cost (internals plus shell plusinstallation) compared to the conventional 4-pass design cost. In revamp situations, these trays can be installed atlow tray spacings, and may therefore provide more theoretical stages for a given tower height. Because of the manydowncomers used in these trays, tray to tray access is limited. Thus, the entire tray may have to be removed if fullaccess to lower trays is required. This may be a significant maintenance debit for some applications. These trays arenot recommended in fouling services because of the inability to fully inspect and clean the trays in a reasonableamount of time. The small hole size typically used, 3/16 in. (4.76 mm), is a further concern in fouling services.Capacity increases of 10% relative to the standard MD tray are achievable with the ECMD tray. The ECMD tray is thehighest capacity tray used by ExxonMobil. The first installation of ECMD trays within ExxonMobil occurred in 1996.The Sulzer Hi-fi Tray is an alternate multiple downcomer tray with similar performance. Contact yourFRACTIONATION SPECIALIST if an application of multiple downcomer trays is being considered.

➧ b. Koch-Glitsch SuperFrac Tray. Koch-Glitsch uses the brand name SuperFrac to describe a variety of traytechnologies developed to improve the maximum vapor handling capacity of trays relative to standard sieve, valve,and bubble cap trays. The technologies incorporated into a SuperFrac tray may include any of the following: (1) small(mini-) valves, either fixed or movable depending on turndown requirements, (2) a patented downcomer design tominimize its size which maximizes the space for vapor flow, (3) an inlet weir to seal the downcomer and/or distributeliquid to the tray, (4) special aerated washers, used to fasten tray panels, that distribute vapor to relatively unaeratedareas of the tray deck, (5) bubble promoters to provide rapid aeration of the liquid flowing from the inlet downcomers,(6) directional orientation of some valves to distribute liquid uniformly over the tray deck, and (7) a calming box withorifices at the bottom (also called a relief downcomer) upstream of each downcomer to unload the downcomer.There are a few applications of SuperFrac trays within ExxonMobil. EMRE has evaluated limited commercial data onthis technology, including a high pressure C3 splitter. Based upon the sparse data available to EMRE, the SuperFractray has features that can allow 15% to 25% increased jet flood capacity versus standard sieve, valve, or bubble cap





trays, when the system is non-fouling and non-foaming. Contact your FRACTIONATION SPECIALIST if a Koch-Glitsch SuperFrac application is being considered.

CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont)➧ c. NorPro Provalve Tray. This tray uses large valves that are wider on the upstream end of the valve than on the

downstream end, thereby deflecting vapor and directing liquid. This directional effect promotes uniform vapordistribution throughout the bubbling area of the tray, resulting in better performance than conventional valve trays. Inaddition, the large size and unique shape of the fixed Provalve valves make them highly resistant to fouling.ExxonMobil has used Provalve trays in such services as debutanizers, coker fractionators, amine regenerators, andC8/C9 splitters. Contact your FRACTIONATION SPECIALIST for assistance regarding potential application of thistechnology.

➧ d. NorPro Triton Tray. This tray uses proprietary Provalve valves, with truncated downcomers to increase traybubbling area. FRI has verified NorPro claims of significantly higher capacity (about 20% higher) compared toconventional valve trays, with comparable mass transfer efficiency. The results of FRI tests also suggest about 10%higher capacity compared to the Nutter MVG tray. Triton trays also exhibit lower pressure drop than conventionalfixed valve and MVG trays. Contact your FRACTIONATION SPECIALIST for assistance regarding potentialapplication of this technology.

➧ e. Other Devices. For trays not mentioned in this Subsection please contact your FRACTIONATION SPECIALIST forguidance.

CROSS FLOW DEVICES - HARDWARE DEFINITIONSIn order to understand how a tray functions, it is important to define various hardware and process parameters. The hardwareparameters will be described first. Figure 6A shows a typical sieve tray tower. The particular tower in Figure 6A has sixsingle-pass sieve trays, with the sieve holes on a triangular pitch. Inlets include a reflux inlet nozzle to the top tray, a reboilerreturn distributor below the bottom tray, and a liquid distributor to the third tray (from the top). There is a total drawoff followingthe second tray (from the top), a reboiler drawoff after the bottom tray, and a third liquid outlet at the bottom of the tower (notshown) for the bottom product. The arrows in the figure depict the direction of liquid flow. The hardware parameters in Figure6B are common to all the cross flow trays.




➧ FIGURE 6ATYPICAL SIEVE TRAY TOWER

Tray Outlet Weir

Downcomer Apron

Liquid Flows ThroughClearance UnderDowncomer

Reflux Inlet Nozzle

False Downcomer

Total Drawoff with noProvision for Overflowto Tray Below

Reboiler Return DistributorCutaway Shows Slots

Seal Pan

Reboiler Drawoff

Reboiler Drawoff Box

Inlet DistributorCutaway ShowsHolesDirecting LiquidAgainstDowncomer Apron

Typical Reflux InletArrangement forSingle-Pass Trays

No Sieve Holes inDowncomer Floor

Overflow isBottom Product

DP03Af06a





➧ FIGURE 6BTYPICAL TRAY LAYOUT

Tray Support Ring

Liquid Flow

Note (1)

Weir Height(hwo)

DiameterDT

r

DowncomerClearance (c)

DowncomerWidth (Rise)

DowncomerApron

TraySpacingH

DowncomerInlet

Area (Adi)

1/2 OutletWeir

Length(Io)Perforated, Valved, Tabbed

or Capped Area (Ab)

DP3Af06b

Note:(1) Any portion of these areas within 3 in. (75 mm) of a hole is included in the tray’s bubble area (Ab).




CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)As shown, a typical tray layout contains certain key items:1 . Tower Diameter and Tray Spacing. These are the two major parameters that set the capacity of the tower. As the

distance between trays is increased (tray spacing, H), the tower capacity will increase. The most economic spacingusually falls between 18-24 in. (450-600 mm) for most services. Spacings above 36 in. (900 mm) provide little incrementalcapacity advantage and are therefore not usually recommended. Likewise, tray spacings as low as 12 in. (300 mm) canalso be used, but this increases the tower diameter (DT) required to handle a given set of vapor and liquid loadings. Inaddition, low spacings also make maintenance much more difficult. Each tray’s Section III (B, C, D and E) contains atable of minimum acceptable tray spacings for maintenance as a function of tower diameter and service (i.e., whether it isclean or fouling).

2. Contacting [bubble] area (Ab, shown shaded). The bubble area is defined as the tray deck area where the liquid/vaporcontacting occurs. It is calculated during the detailed design of the tray.

➧ 3. Downcomer Area. This is the area (Ad) that must be devoted to handling the liquid as it flows from a given tray to the traybelow. A distinction is made between three different areas associated with a given downcomer geometry, because trayperformance depends on each of these values. The downcomer inlet area (Adi) is the horizontal area available for liquidflow into the downcomer. The downcomer outlet area (Ado), also called the downcomer bottom area, represents thecorresponding horizontal area at the bottom of the downcomer. The area under the downcomer (Aud), also called thedowncomer escape area, is the downcomer clearance (c) multiplied by the downcomer outlet chord length. The edge ofthe downcomer is usually chordal in shape and its maximum width is called the downcomer width or downcomer rise (r).The difference between the width at the top and the width at the bottom of a sloped downcomer is referred to as the"downcomer kick". Section III-K contains the necessary tables to calculate downcomer areas and other geometricparameters. Alternatively, the PEGASYS computer program’s “Segment of Circles” program located within the GeometryMenu can also be used.

4. Downcomer Clearance. This is the vertical clearance (c) between the tray floor and the bottom edge of the downcomerapron. This clearance varies with liquid rate and is discussed in each tray’s Subsection under BASIC DESIGNCONSIDERATIONS.

➧ 5. Outlet Weir Height and Outlet Weir Length. As the liquid leaves the contacting area, it flows over the outlet weir as itenters the downcomer. The height of the outlet weir (hwo) is set by the designer to provide liquid holdup on the tray topromote efficient liquid/vapor contacting and to seal the downcomer upstream of the weir. The outlet weir length (Io) is thesame as the downcomer inlet chord length.Note: The "inlet" and "outlet" terminology used for weirs and downcomers can be confusing. For weirs, "inlet" refers to theupstream weir on a given tray. For downcomers, "inlet" refers to the upstream (top) part of the downcomer. Therefore, theoutlet weir is located on the downcomer inlet chord.

➧ 6. Multi-pass Trays. As the liquid rate on a tray increases, the capacity of the tower can usually be increased if the liquidflow is split into more than one path (see arrows labeled L on Figure 7). Such split-flow trays are called multi-pass trays.While single pass trays are the most common, double pass (2 pass) trays are frequently used. Three and four pass traydesigns are also available, but their use comprises less than 5% of the total trays used in ExxonMobil towers, with fourpass designs being more common than three pass. Since higher pressure towers have a higher volumetric ratio of liquidto gas, multi-pass trays with greater total downcomer area are common in these towers.





CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)

➧ FIGURE 7PASS ARRANGEMENT ON MULTI-PASS TRAYS

L L L L

LLLL

L

L

LL

LL

Four PassThree Pass

Single Pass Double Pass (2 Pass)

L

L L L

L L

DP3Af07

Center (Inboard) Downcomer

Side (Outboard)Downcomer

Off-center Downcomer

On multi-pass trays, the downcomer(s) nearest the tower centerline are referred to as “inboard” or “center” downcomers,while those farthest away are called “outboard” or “side” downcomers.

7. Sloped and Stepped Downcomers. For a given tower diameter, a certain amount of the available cross-sectional area isneeded for liquid handling (downcomer area) with the remainder available for vapor flow (bubble area). Therefore, anysteps that can be taken to reduce the area consumed by the downcomer(s) will provide additional area for vapor flow. Thisgoal can be achieved on heavily liquid loaded trays by using stepped or sloped downcomers as shown on Figure 8.Typical design for a downcomer “step” is for the step to be located at 33% of the tray spacing. Stepped downcomers arepreferred in small diameter towers [DT < 48 in. (1220 mm)] due to easier weld-in installation. Otherwise, slopeddowncomers are preferred. When a sloped downcomer is specified, often suppliers offer a “semi-sloped” design. Thisdesign has better mechanical features, since the upper vertical portion is often used as a support beam. However, theassociated reduction in average free area results in a slight loss in capacity.





➧ FIGURE 8STEPPED VS. SLOPED DOWNCOMERS

Tray Deck

Sloped DowncomerStepped DowncomerStraight Downcomer

DP3AF8

hv,hi

hv,lo

hv,hi

hv,loDashed Linesrepresentsemi-sloped design

➧ 8. Modified Arc and Envelope Downcomers. The chord length at the bottom of a downcomer must be sufficient to provideuniform liquid flow distribution onto a tray, because uniform liquid distribution results in good tray efficiency. Theappropriate Subsection in Section III provides the minimum suggested chord length for a specific type of tray. In someservices where very low liquid rates must be handled, minimizing the downcomer area can result in a downcomer chordlength that is smaller than that suggested in the appropriate Subsection. In these cases, a modified arc (also known assegmental, or swept back) downcomer can be specified (see Figure 9). A modified arc downcomer has a smaller areathan a chordal downcomer, when the chord length of the chordal downcomer is equal to the “projected” weir length of themodified arc downcomer. The modified arc downcomer thereby enables a reduction in downcomer area without sacrificinguniform liquid flow distribution onto the tray below. Modified arc downcomers can also be used to help balance weirlengths for four-pass tray designs. Some pre-1960 towers may contain a full arc-type downcomer. This style downcomerfunctions in the same manner as a modified arc but can be more expensive to build; therefore, smooth arc-typedowncomers are not specified in new designs. However, they can be retained if their area is sufficient for revampconditions. Section III-K provides geometric data for chordal and modified arc downcomers.

FIGURE 9MODIFIED ARC VS. ARC TYPE DOWNCOMERS

StraightLine

Segments

Modified Arc Arc

SmoothArc

ProjectedWeir

Length

Rise

DP3AF9





CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)For towers under 36 in. (900 mm) in diameter, cartridge or package trays are frequently used. These trays are installed insets (or bundles) held together by rods, and can be lowered into a top-flanged vessel. Since the trays are removable thedowncomer must be an integral part of the tray. These downcomers are called envelope type downcomers (SeeFigure 10). Standardized downcomer sizes for these trays can be found in Section III-E, Table 4A and 4B. However,custom sizes can be provided if one of the standard sizes does not meet process requirements.

FIGURE 10SULZER CARTRIDGE TRAYS AND THEIR

ENVELOPE DOWNCOMERS

Nutter CartridgeTray

Envelope Downcomer(Plan View)

DP3AF10

9. Downcomer Sealing Techniques. In order to prevent some of the vapor from bypassing the contacting region by flowingupward through the liquid in the downcomer, the downcomer must be “sealed" by liquid. In most tray designs with an outletweir, the liquid holdup (or clear liquid height) will sufficiently seal the downcomer clearance without additional hardwaredevices (See discussion on Downcomer Seal under Liquid Handling Limitations). When this is not possible, however,ways to provide a seal via hardware techniques are shown in Figure 11.Both the recessed inlet box and the inlet weir seal use mechanical means to help seal the downcomer. In the case of therecessed box, the seal is obtained because the bottom of the downcomer apron and the tray below are at the sameelevation. With an inlet weir, the weir height plus the liquid head over the inlet weir must be at least equal to thedowncomer clearance. By using a shaped downcomer lip, the head loss under the downcomer is reduced, allowing asmaller downcomer clearance to be used. This reduced downcomer clearance can provide a process seal for thedowncomer, although the corresponding increase in the velocity under the downcomer may cause it to exceed thespecified limit (See Downcomer Seal and Velocity under downcomer in CROSS FLOW DEVICES-PROCESSDEFINITIONS).

➧ Recessed inlet boxes are susceptible to trapping solids, which may affect tray performance. If this is a problem in anexisting tower, the boxes should be covered or removed and the edge of the downcomer panel will need to be modified inorder to properly seal the downcomer. If the recessed inlet boxes are covered, then a side vent and bottom drain arerequired. Consult GP 5-2-1 regarding drain hole specifications for recessed inlet boxes.

➧ For shaped lip downcomers, the radius tip (shaped downcomer lip) may be in the range of 1 to 2 in. (25 or 50 mm),although 1 in. (25 mm) is typical. Remember to check the location of sieve holes, valves, or jet tabs relative to the edge ofthe shaped lip to avoid getting vapor into the downcomer. Do not use shaped downcomer lips on top reflux distributors(false downcomers), or with a recessed inlet box, an inlet weir, or a bubble cap tray. Most importantly, the presence of aradius tip in conjunction with a seal pan or inlet weir creates an undesirable restriction for the liquid flow. In addition, thepresence of these downstream devices induces turbulence and defeats the purpose of streamlining the flow with a shapeddowncomer lip.





➧ FIGURE 11DOWNCOMER SEALING TECHNIQUES

C

C

Recessed InletBox

InletWeir

ShapedDowncomer

Lip

DP3AF11

SeeDetail

A

DetailA

10. Contacting Area Definitions. During the design of a given device, such terms as bubble area, free area, hole area, andwaste area are used. They are explained below.a. Bubble area (Ab). This is the area between the downcomers where vapor/liquid contacting occurs. See Figure 12.b. Hole/valve/cap/tab area (Ao). This is the open area or hole area provided within the bubble area to permit vapor to

enter, contact, and pass through the liquid on the tray. In the case of a sieve tray, it will be equal to the total area of allthe holes on a given tray.The hole area is usually expressed as a fraction or percentage of the bubble area (Ao/Ab). This ratio is determined byvarious correlations discussed in each tray's Subsection.





CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)➧ c. Free area (Af). Test data have shown that as the vapor flows through and leaves the bubble area (Ab) it expands over

the downcomer(s) and its velocity drops. Thus, an area greater than the bubble area is available for vapor flow. Thislarger area is known as the free area (Af) and is defined in Figure 13. Data analysis has shown that a combination ofbubble area and free area help determine a tray’s capacity at jet flood.For trays with sloped or stepped downcomers the average free area is used. It is defined at the bottom of Figure 13.

➧ d. Waste area (Aw). Waste area is defined as any area of a tower that cannot be used to enhance tray capacity orefficiency, thereby representing "wasted" tower area. There are three general types of waste area:• Tray deck waste area refers to any area in the bubble area that is further than 3 in. (75 mm) from the edge of a

contacting device. For rating calculations, tray deck waste area reduces both the bubble area and the free areaof a tray. Tray blanking, recessed inlet boxes, inlet weirs, shaped downcomer lips, and rectangular bubble areablanking patterns all contribute to tray deck waste area.

• Volumetric waste area results from any obstruction that inhibits vapor flow through the free area above a traydeck. Wasted volume may be represented as an effective waste area in the same way free volume is representedas an effective free area. For rating calculations, volumetric waste area reduces the free area only. Splashbaffles, distributor pipes, beams and drawoff boxes may contribute volumetric waste area.

• Downcomer waste area exists wherever there is an obstruction in the downcomer, reducing the downcomercapacity. For rating calculations, downcomer waste area reduces the effective downcomer inlet area, outlet area,and/or mean downcomer area, depending on where in the downcomer the obstruction exists.




➧ FIGURE 12BUBBLE AREA DEFINITIONS

Adi

2

22

2Adi

AdoAdo

Ado

Adi A*di

A*do

Two-Pass TraysSingle-Pass Trays

Plan View Shows InboardTray Bubble Area

lfp lfp lfp

lolud

lo lo*Ab Ab Ab

Ab = As - Adi - Ado - Aw (if any) For Side Downcomer Tray Ab = As - Adi - A*do - Aw (if any)

For Center Downcomer Tray Ab = As - A*di - Ado - Aw (if any)

* Terms with asterisks refer to center (inboard) downcomer, those without asterisks refer to side (outboard) downcomer. DP3AF12






FIGURE 13FREE AREA DEFINITIONS

Adi

Ado

Adi

Ado

Ado

AdiAdi

Ado

2

22

2 2

2222

2

Adi Adi

Af Af

Af Af

Af Af

A*diA*di

A*doA*do

Two-Pass Trays

AfAf

Single-Pass Trays

Af = As - Adi - Aw (if any)

For Side Downcomer Tray Af = As - A*di - Aw (if any)

For Center Downcomer Tray Af = As - Adi - Aw (if any)

- Aw (if any)Af = As -Adi + Ado

2

* Terms with asterisks refer to center (inboard) downcomer, those without asterisks refer to the side (outboard) downcomer.DP3AF13

2For Side Downcomer Tray Af = As - - Aw (if any)A*di + A*do

2Adi + Ado - Aw (if any)For Center Downcomer Tray Af = As -




CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)11. Other Hardware Definitions

a. Anti-jump baffle. On multi-pass trays, a baffle is centered above the center and off-center downcomer to preventliquid collision in the center of the downcomer. The specific need for this baffle varies with liquid rate and the type oftray used and is discussed fully under DETAILED DESIGN CONSIDERATIONS in each tray’s Subsection. Thetypical height of the anti-jump baffle is 14-16 in. (350-400 mm). If this criteria cannot be satisfied due to heightconstraints, then the anti-jump baffle should extend from the top of the outlet weir (or tray deck if no outlet weir ispresent) to a minimum of 2 in. (50 mm) below the underside of either the tray deck or beam located above the baffle.An isometric sketch of an anti-jump baffle is shown in Figure 14A. Other mechanical details are contained in GP 5-2-1.

➧ b. Picket fence weirs. The use of picket fence weirs to increase the effective liquid height above the tray is the mostcommon way to reduce blowing or spray regime operation. Picket fence weirs are used for low liquid rateapplications, commonly for low liquid rate wash tray (large hole sieve tray) design in fouling service. Picket fenceweirs may also be used to balance weir loading on 4-pass trays. The effective outlet weir length (that is, thecumulative length of weir over which liquid flows into the downcomer) should be no less than 30% of the actual weirlength. If less than 30% is desired, a FRACTIONATION SPECIALIST should be consulted. See Figure 14B for atypical detail of a picket fence weir.

➧ c. Swept back weirs. Use of swept back weirs with a given tray design has the effect of increasing liquid handlingcapacity slightly, while lowering the pressure drop. Swept back weirs have little effect on efficiency, except for lowliquid load systems where there is a slight decrease in efficiency caused by the reduction in bubble area. Swept backweirs can be used for sloped downcomers, where a modified arc is not feasible. For design or rating calculations, theprojected weir length (that is, the chord that connects the two points where the weir intersects the tower wall) is used,and the tray deck area between the swept back weir and the downcomer is stictly regarded as tray deck waste area.Remember to correctly account for the waste area caused by the swept back weir. See Figure 14C for a typical detailof a swept back weir.

➧ d. Minimum width (rise) for center (inboard) downcomers. To ensure good downcomer performance on centerdowncomers, the minimum width (rise) of the center downcomer is 8 in. (200 mm) at the inlet and 6 in. (150 mm) atthe outlet. These dimensions must be maintained even if anti-jump baffles are used.

➧ e. Splash baffle. When operating at low liquid rates excessive entrainment may occur. In addition, clear liquid heightbecomes lower and could result in an unsealed downcomer or poor fractionation efficiency. The installation of avertical splash baffle above the tray deck at the entrance to the downcomer is an alternative to picket fence weirs forincreasing the clear liquid height. Splash baffles (Section III-B, Figure 15) provide additional liquid head on the trayby providing additional resistance to flow into the downcomer and by acting as an entrainment deflector whenoperating in the spray regime. Splash baffles are not to be used in fouling services.





➧ FIGURE 14SKETCHES OF SOME HARDWARE DEVICES

Baffle Height14 -16"

(350 - 400 mm)Typical

Top edge of baffle should bepositioned a minimum of2" (50 mm) below the undersideof the tray deck or beamlocated above the baffle

Bottom edge of baffle should bepositioned flush with top of outletweir, if any; otherwise, with top oftray deck. See discussion inSubsections III, B, C, D and E.

DP3AF14

Tray DeckDowncomer

Picket Fence Weir

Downcomer

Waste Area

Tray Deck

Swept Back WeirWaste Area

C. Swept Back Weir

B. Picket Fence Weir

A. Anti-Jump Baffle

Pickets should be no more than 6" (150 mm)wide, and between (1/2) H and (3/4) H tall.




CROSS FLOW DEVICES - PROCESS DEFINITIONSThe purpose of this section is to provide the designer with insight into the many parameters that influence tray design. This willbe achieved by providing both a verbal description and a drawing (wherever possible) for each definition. Unless obvious ornoted otherwise, each of the items discussed below apply to all of the cross-flow trays. They will be discussed under threebroad categories: a) those affecting vapor capacity, b) those affecting liquid capacity, and c) other limitations/considerations.

➧ VAPOR HANDLING LIMITATIONS1. Jet Flooding. For the vast majority of cases, this is the limitation that sets the vapor handling capacity of all cross-flow

trays. As its name implies, the liquid is projected or “jetted" to the tray above by the vapor as it leaves the tray’s orifice. Ifsufficient liquid is carried to the tray above (i.e., entrained), it will overload the downcomers, and the tray will flood. Whenflooding occurs, the liquid begins to back up on the tray until the inter-tray space is filled with a dense froth. SeeFigure 15. This causes the next higher tray to flood and moves progressively up the tower until the liquid is carried out thetop of the tower or is removed through a liquid drawoff nozzle. When flooded, the tower fractionates poorly and is verydifficult to control.Tray capacity at jet flood is largely a function of tray geometry. Namely, tray spacing, hole area, hole size, bubble area,free area / bubble area ratio, and downcomer inlet area / tower area ratio are important factors in determining capacity atjet flood.Since jet flooding sets the maximum capacity of the tower, it must not be exceeded. Furthermore, as the percent of the jetflood velocity moves from 85% to 100%, the entrainment rate increases exponentially and the tray efficiency falls offsharply. Thus, it is essential that the designer stay within the jet flood limitations discussed in each tray’s Subsection.

FIGURE 15JET FLOODING: ITS IMPACT ON ENTRAINMENT AND TRAY EFFICIENCY

100% Flood

Inter-Tray Spaceand Downcomer

Operating Normally

50% Flood

Inter-Tray Spaceand DowncomerFull (Flooded)

% Jet Floodvs.

Efficiency

Entra

inm

ent

Percent of Jet Flooding

Tray

Effi

cien

cy

10085500

DP03Af15

2. Ultimate Capacity. Is the highest vapor load the tower can handle. Unlike jet flooding capacity, ultimate capacity cannotbe increased with hardware modifications. It represents the velocity at which the liquid is broken into such small dropletsthat most are entrained and cannot fall back to the tray via Stokes Law. For most systems, however, the Jet Floodingvelocity will limit before ultimate capacity is reached. Normally, ultimate capacity will only be a limitation in some highpressure, light ends fractionators (such as some demethanizers, deethanizers, depropanizers, etc.) It occurs on theseunits because the surface tension is low [< 5 dynes/cm (mN/m)] and the liquid can be shattered more easily into smalldroplets. Ultimate capacity can also limit deep cut vacuum towers packed with high capacity grid or structured packing.Nevertheless, it should be checked on all designs regardless of operating pressure, to ensure that no problem exists.When ultimate capacity is reached (new designs), the only practical way to increase capacity is by increasing towerdiameter. For revamps, there may be no way to avoid ultimate capacity and a new tower may be needed. Discuss suchcases with your FRACTIONATION SPECIALIST.





CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)3. Flow regimes - spray, froth and emulsion. Different flow regimes can exist on a tray (see Figure 16). The first, and

most common, is known as the froth regime. In this regime, vapor passes through the liquid on the tray as discretebubbles of irregular shape. The bubbles are formed at the tray perforations and are swept away by the froth. The frothsurface is mobile and not level. As the vapor rate decreases, the flow regime crosses from the froth regime to theemulsion regime, where the vapor is dispersed as small bubbles in the liquid. As the vapor rate increases from the frothregime, jets and bubbles of rapidly changing shape are observed. If the vapor rate is raised still further, a gas jet issuesfrom the orifice and some of the liquid is shattered into droplets. This latter regime is called the spray regime. In the sprayregime, the vapor phase is continuous whereas in the froth and emulsion regimes, the liquid phase is continuous (seeFigure 16). Spray regime operation occurs primarily at high vapor velocities and low liquid rates, whereas emulsionregime operation occurs primarily at low vapor velocities and high liquid rates (see Figure 17).

FIGURE 16FROTH REGIME VS. SPRAY REGIME OPERATION

Froth Spray

DP03Af16

FIGURE 17FLOW REGIME WITHIN NORMAL OPERATING RANGE

Operating LimitSprayRegime Froth (or Mixed)

Regime

EmulsionRegime

Lower Operating limit

Vapo

r Rat

e

Liquid Rate

Upper

DP03Af17




CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)Operation in the spray regime can be very detrimental to good tower performance, with tray efficiency dropping sharply.This occurs primarily because the liquid and vapor residence times as well as the interfacial area generated on the tray arereduced. (See later discussion on Tray Efficiency.) While spray regime operation has been observed on all the widelyused trays discussed earlier, it has been investigated primarily with sieve trays.Under spray regime conditions, the vapor rate is sufficient to “blow through" the liquid, thereby making the vapor phasecontinuous. This is where the term "blowing” originated which is another term often used to describe the spray regime.Since the liquid rate is usually set by the process itself and can not be increased, the most effective way to suppress thespray regime is to dissipate the jet leaving the orifice as quickly as possible. The common method for avoiding sprayregime operation is through use of a picket fence weir. Another way to avoid spray regime operation is to increase theopen area on the tray, thereby reducing jet velocity. A third way is to use smaller sized orifices [say 1/8 in. (3 mm)] holesvs. the standard 1/2 in. (12 mm) holes used on sieve trays. Since the distance to dissipate a jet is a function of the orificediameter, the smaller the orifice the faster the jet will dissipate.A fourth way is through the use of valve trays. Since the vapor leaves the valve element almost horizontally, its verticalvelocity component is greatly reduced and its jet more quickly dissipated. Other, less frequently used techniques arementioned in Section III-B (Sieve Trays). Their use, however, will require contacting your FRACTIONATION SPECIALISTfor guidance.

4. Entrainment. Is defined as liquid that is carried by the vapor from a given tray to the tray above. As the vapor rate in thecontacting area is increased, the amount of energy being dissipated also increases. This energy creates the interfacialarea needed to provide good contacting between the liquid and the vapor. It also expands the froth or spray height on thetray, thereby decreasing the distance between the top of the spray and the tray above. As this distance decreases further,some of the liquid is carried (entrained) to the tray above as droplets. As Figure 18 indicates, the smallest drops will beentrained to the tray above while the largest drops will fall back to the entrainment generation tray. As the quantity ofentrainment increases, the tray above becomes overloaded and “floods" and the tray’s efficiency drops sharply (see aboveunder Jet Flooding).

FIGURE 18GENERATING ENTRAINMENT

Drops

Vapor BubblesIn Liquid

Uniform VaporVelocity Profile

Initial ProjectionVelocity Of The Drops

DP03Af18

The quantity of entrainment generated is dependent on vapor rate, liquid rate, and certain hardware parameters.Correlations for predicting entrainment have only been developed for sieve trays. One correlation predicts entrainmentunder low liquid loading conditions while the other is used for the balance of the liquid rate range (see Section III-B SieveTrays). In general, tray designs shall not allow entrainment to exceed 10% of the tray’s liquid flow rate.





CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

➧ LIQUID HANDLING LIMITATIONS1. Downcomer flood. Percent of downcomer flood is the criterion that determines how close a tower is to flooding as a

result of excessive liquid height in the downcomer. Downcomer filling (hd) is defined as the clear liquid height in thedowncomer. As Figure 19 shows, it is composed of the tray pressure drop (ht) across the tray immediately upstream ofthe downcomer being considered, the head loss under the downcomer (hud), the inlet head (hi) on the tray, and the headloss due to two-phase flow through the downcomer (hdc). If there is no inlet weir on the tray, the inlet head will be thesame as the clear liquid height (hc) on the tray. If an inlet weir is present, downcomer filling will increase due to the weirheight, the crest over the weir, and added pressure drop of the liquid flowing between the downcomer apron and the inletweir. The tray pressure drop (ht) is composed of the dry tray pressure drop (hed) and the clear liquid height (hc). Each ofthe pressure drops (or heads) is expressed in inches (mm) of hot clear liquid.

➧ FIGURE 19DOWNCOMER FILLING COMPONENTS

(STATIC PRESSURE BALANCE)

hed

InletWeir

ht

hc

hc

ht

hd

ht = hed + hc

hi

hi = hc where there is no inlet weir

DP3AF19

hud

hdc

The above calculation procedure expresses the downcomer filling in inches (mm) of clear liquid. Since the liquid enters thedowncomer as a froth, the actual fluid level in the downcomer will be higher than the calculated clear liquid filling. Theexact height is dependent on the average froth density in the downcomer. As the liquid travels downward in thedowncomer, the vapor disengages and escapes out the top of the downcomer. If the downcomer is sized properly, theliquid leaving should be essentially clear liquid. Thus, there is a froth density gradient down the downcomer that rangesfrom the froth density on the tray (at the top) to partially to totally clarified clear liquid at the bottom.




CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)100% of downcomer flood and 100% downcomer froth backup both occur when the froth height in the downcomer is equalto the distance from the bottom of the downcomer to the top of the outlet weir. At this point, an incremental increase indowncomer filling would result in downcomer froth backing up to the tray above. Percent downcomer froth backup is thefroth height in the downcomer divided by the distance from the bottom of the downcomer to the top of the outlet weir.Percent downcomer flood represents the ratio of the actual vapor and liquid feed rates to the feed rates that wouldresult in 100% downcomer froth backup. Therefore, percent downcomer froth backup is simply a measure ofdowncomer froth filling, whereas percent of downcomer flood implicitly takes into account the effect of both liquid rate andvapor rate as well. If percent downcomer froth backup or percent of downcomer flood exceeds the design limit, then thedesigner must take steps to reduce ht, hud, hi, or hdc. While many variables determine the values of these head losses,downcomer clearance, hole area, downcomer inlet and outlet chord lengths, and outlet weir height are the most significanthardware parameters that can be modified to meet downcomer flood and downcomer backup requirements. If this cannotbe done, then either multi-pass trays should be considered or the tray spacing should be increased. If neither of thesesteps corrects the problem, consider using packing and consult your FRACTIONATION SPECIALIST.

2. Secondary limitations. While downcomer flood and downcomer backup limits should always be met to ensure successfultower operation, there are additional criteria that should be met whenever possible. Consult your FRACTIONATIONSPECIALIST if any of the following secondary criteria can not be met.a. Liquid rate per inch of outlet weir. The accuracy of the jet flood and downcomer flood correlations can only be

ensured within the range of operating conditions used to develop the correlations. For sieve and bubble cap trayswhen jet flood is the limiting flood mechanism, the liquid rate should be in the range 1.5 - 15 gpm/inch of weir (3.7 - 37dm3/s/m of weir). When downcomer flood is the limiting flood mechanism, the liquid rate should be in the range 2.3 -17.5 gpm/inch of weir (5.7 - 43.5 dm3/s/m of weir).

b. Downcomer choking. If the downcomer inlet area is too small and the froth on the tray cannot readily enter thedowncomer, the froth height will increase in the contacting area. This height will continue to increase until there issufficient head to “force” the froth into the downcomer or until the froth reaches the tray above, causing flooding.When the downcomer is bridged over at the entrance with froth, it is called downcomer choking. Since downcomerchoking can cause flooding, it must be avoided. Data analysis has also shown that downcomer backup may work incombination with downcomer choking to cause flooding. For this reason, a limit is also placed on the geometric mean(square root of the product) of percent downcomer backup and percent downcomer choking.

c. Velocity under downcomer. A high downcomer outlet velocity produces a channeling effect, in which liquid flowfrom the downcomer is unevenly distributed on the tray deck. Instead of generating a uniform liquid velocity profileover the entire bubble area, the sides of the tray deck hold stagnant liquid due to the overwhelming velocitycomponent in the center of the liquid flow path. Rising vapor from the tray below flows through the sides of the traydeck, which represent the path of least resistance to vapor flow. This vapor and liquid maldistribution results in poorvapor-liquid contact on the tray and poor separation efficiency. This may also lead to premature weeping in the middleof the tray, which further decreases efficiency. Additionally, high velocity under the downcomer with high vaporloading has been shown to produce a rooster tail effect (froth height is much greater at the outlet side of the tray) thatpromotes outlet side flooding. If the velocity under the downcomer exceeds the specified limit, increase downcomerclearance or increase the downcomer bottom area.

d. Downcomer seal. If a downcomer is not sealed, some vapor will bypass the tray and flow upward through thedowncomer, resulting in a drop in tray efficiency. To avoid this problem, the designer should make certain that thedowncomer is sealed at minimum rates (turndown conditions at minimum loaded tray) for new designs and revamps.For multi-pass trays, the downcomer seal must be checked for every tray pass. While a downcomer may be sealed bymechanical means (see Figure 11), it can also be sealed by process means. That is, if the sum of the clear liquidheight (hc) and the head loss under the downcomer (hud), plus 1/4 of an inch (6 mm) for allowable unseal, equals orexceeds the downcomer clearance (c) the downcomer is said to be sealed [hc + hud + 1/4 in. (6 mm) ≥ c]. For mostnormal designs, with 50% turndown, if the downcomer is sealed at the minimum rates it is usually sealed at designrates. However, if the minimum rates have a different liquid to vapor ratio (L/V) than the design rates, it is advisable tocheck for both cases since hc (for sieve trays) is dependent on both liquid and gas rates. For liquid rates < 1.5 gpm/in.of weir/pass (3.7 dm3/s/m of weir/pass) an inlet weir should be considered. If downcomer sealing criteria can’t be met,consult your FRACTIONATION SPECIALIST to determine the impact on your particular design\

➧ OTHER PROCESS CONSIDERATIONS1. Tray Efficiency. The purpose of the brief discussion below is to acquaint the designer with the key variables that affect

tray efficiency. A more fundamental discussion is provided in Section III-I, Tray Efficiency.Tray capacity and tray efficiency are the two most important criteria in tray design. While the diameter of a tower is mainlydetermined by capacity considerations, tray efficiency determines the number of actual trays, and therefore the height, ofthe tower.





CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)To achieve good efficiency the vapor must be dispersed in the liquid phase intimately and kept in contact long enough forsufficient mass transfer to occur. This means that we must be able to calculate the liquid and vapor residence times on thetray as well as the amount of interfacial area generated.The vapor residence time is the time required for the vapor to flow through the volume of froth on the tray. Likewise, theliquid residence time is the time required by the liquid to flow through the volume of froth on the tray. Both of thesevariables are dependent on liquid and gas rates as well as the weir height and bubble area on the tray. Efficiency is alsoaffected by physical properties, such as the vapor and liquid diffusivities, but such parameters cannot be changed throughtray hardware changes.To achieve good efficiency, the designer must optimize the weir height, open area, bubble area, liquid flow path length,number of liquid passes, and other variables. Likewise, excessive weeping, entrainment and operation in the spray regimemust be avoided. The only practical way to perform this optimization is by using a computer program, such as thePEGASYS - Sieve Tray Design and Rating Program, which is accessible from the PEGASYS Fractionating TowerMenu.

2. Tray turndown. Turndown (or flexibility) is the term used to define the range of loadings over which acceptable trayperformance is achieved. This usually means the range over which the tray efficiency stays at or above the design value(see Figure 20). Turndown ratio is defined as the ratio of the capacity factor at design conditions to the capacity factor atminimum turndown conditions.As the efficiency curve in Figure 20 shows, there is a relatively flat portion of the efficiency curve where design (or better)efficiency is obtained. At low vapor rates, however, excessive weeping decreases efficiency whereas at high vapor rates(above 85% of flood) excessive entrainment decreases efficiency. These falloff points generally correspond to 20%fractional weeping and 10% entrainment.An alternative to changing trays to obtain turndown capability is to operate the distillation column at higher reflux or boiluprates at the turndown conditions to compensate for the loss in efficiency. This can be particularly advantageous ifoperation at turndown conditions is anticipated to occur fairly infrequently (which is typically the case).

FIGURE 20EFFECT OF WEEPING ON EFFICIENCY

GoodOperation

20% Weepage

Vapor Rate

Frac

tiona

l Wee

page

(%)

Tray

Effi

cien

cy

DP3Af20




CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)The left hand sketch in Figure 21 shows the fractional weepage and entrainment curves for a typical sieve tray with amoderate to high liquid rate. The range of vapor rates over which the weeping is below 20% and entrainment is below10% provide a second way to look at tray turndown. This is also referred to as the tray's operating window. Sieve trayscan usually be designed to provide a turndown ratio of 2/1 to 3/1.If the liquid rate on a tray is low [say below 1.5 gpm/in. of weir/pass (3.7 dm3/s/m of weir/pass)], the operating window onthe tray is extremely small or non-existent. This is shown on the right hand sketch in Figure 21. Designing trays tofractionate under these conditions is extremely difficult and your FRACTIONATION SPECIALIST should be consulted forhelp. Also see Section III-B, BASIC DESIGN CONSIDERATIONS - Froth to Spray Regime Transition for furtherbackground.If a sieve tray cannot be designed to meet the required turndown, then valve trays with their higher turndown capabilitiesshould be evaluated (greater than 3/1). Since these trays are only marginally more expensive than sieve trays (by 0-10%)their use can frequently be justified for high turndown cases.

3. Foaming. Foaming can occur in distillation towers via several different mechanisms including:a. The presence of surface active materialsb. The presence of solidsc. Entrainment of hydrocarbon liquids into aqueous systemsd. Condensation of hydrocarbon vapors into aqueous systemse. When a second liquid phase is about to form.Foam is different than a froth primarily because it takes far longer for the foam to collapse. Foam also typically has ahigher amount of vapor compared to a froth. For example, in a laboratory foaming test, the collapse time of a froth is low,usually less than 5 seconds. On the other hand, a foam can persist much longer, sometimes for minutes or even hours.Thus, a foaming system in a tower will begin to entrain at lower loadings and will not readily disengage in the downcomers.To design for foaming, the tray is usually oversized by using a lower percent of jet flooding and downcomer flood, a low drytray pressure drop, a low downcomer entrance velocity, and a reduced allowable downcomer filling. A radius tip and largedowncomer clearance are often specified. “Standard" foaming guidelines are provided for various tray types in theirrespective Subsections in this manual. Since the degree of foaminess varies and is generally unpredictable, experience insimilar towers may be used instead to set some or all of these hydraulic criteria.In some severely foaming situations, flooding can still occur even when the above guidelines are followed. Operatingengineers should then consider process changes to eliminate the foaming or determine the cause and remove the sourceof impurity causing the foam. If these changes do not eliminate the problem, the use of anti-foam agents may provide theonly solution. Although usually effective, especially for short-term relief, users should be aware that anti-foam agents areoften costly and may adversely affect product purity.

4. Fouling. Fouling is the buildup of any type of solid deposit on a tower internals device. Fouling eventually plugs orreduces the effective size of an orifice or opening, resulting in diminished performance (efficiency, capacity, etc.) or evencomplete inoperability. Tower internals for fouling services must be specified to minimize fouling and to provide easyaccessibility for periodic cleaning of the device. Refer to Table 3D for relative fouling resistance of fractionation devices.Since fouling is strongly system dependent, the designer should contact the FRACTIONATION SPECIALIST for pastexperience and guidance.

5. Additional hydraulic considerations:a. Weeping. At low vapor velocities, the dry tray pressure drop of the tray is insufficient to support the liquid head on the

tray (hc) and some liquid begins to flow intermittently through the vapor openings. The point at which this liquidbypassing begins is called the "weep point." As the vapor rate is decreased further, more liquid pours through theholes and continuous weeping occurs.While the total quantity of liquid that weeps is constant at a given vapor rate, the weep rate per hole is sporadic. Thatis, some holes are in the weeping mode while others are in the vapor bubbling mode. Thus, at any instant in time agiven hole may be bubbling, weeping, or doing neither. This occurs on a random basis across the contacting area ofthe tray.While weeping can occur on all tray types, the designer is concerned about its effect primarily on sieve trayperformance, since this is the most widely used tray in ExxonMobil plants. Since weeping occurs only at reducedrates, it is the major factor in determining tray turndown (i.e., the range of vapor loadings over which acceptablefractionation is achieved. See Tray Turndown discussion). For sieve trays, this usually means a ratio between 2/1 to3/1.As Figure 20 shows, as vapor rate decreases, weeping increases very rapidly. Data have shown that above 20%fractional weepage, tray efficiency begins to decrease sharply.





CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)Therefore, for most designs, the lower operating limit is reached when fractional weepage exceeds 20%. By using thecorrelations presented in Section III-I (Tray Efficiency) the falloff in efficiency can be predicted for any value offractional weepage. However, the designer should reduce weeping as much as possible by reducing hole area (newdesigns) or by blanking holes (on revamps).

b. Dumping. When all the liquid flows through the holes on a tray (i.e., no liquid flows over the weir) dumping is said tooccur.Because the efficiency is extremely poor and the products produced will be off-spec, trays should not be operated inthe dumping region.

c. Dry tray pressure drop (hed). This represents the energy expended by the vapor as it flows through the contactingdevice (sieve hole, valve element, etc.) as if no liquid were present, i.e., the tray is “dry.” To increase this pressuredrop, the area available for vapor flow (hole area) must be decreased. To lower the pressure drop, the hole area mustbe increased. Since the dry tray pressure drop is proportional to the square of the vapor velocity, small changes inopen area can produce large changes in pressure drop.The dry tray pressure drop must be high enough to provide good contacting between the vapor and the liquid at bothdesign and turndown rates. However, it must be low enough at design rates to prevent excessive entrainment,operation in the spray regime, or excessive downcomer filling.On the other hand, if the designer chooses a hole area which is too large, there may be insufficient pressure drop atturndown conditions to prevent liquid from weeping through the holes. Thus, the designer must select a hole areawhich satisfies both turndown and design rate operations.The usual approach is to design the tray for maximum rates and then check it for turndown conditions. However, ifchanges in the dry tray pressure drop cannot be made that will satisfy all operating conditions, a more flexible device(such as valve trays) may be required.

d. Tray pressure drop (ht). As the vapor flows through the contacting zone on a given tray, it must overcome tworesistances in series. The first is the dry tray pressure drop (hed) and the second is the clear liquid height (hc). Thetray pressure drop (ht) is the sum of hed + hc. The tray pressure drop is critical in tray design since it is one of themajor components of downcomer filling and therefore downcomer flood. It is especially critical, however, for vacuumsystems where minimum pressure drop is required.

e. Clear liquid height (hc). This is the height of liquid on a tray, expressed in inches (mm) of hot liquid. It is the secondresistance the vapor must overcome as it passes through the tray. The clear liquid height is a function of the liquidrate and outlet weir height.The clear liquid height must be high enough to provide sufficient contact time between the liquid and the vapor formass transfer to occur. Excessive clear liquid heights should be avoided, however, since they increase the tray’spressure drop, increase downcomer filling, and may cause premature weeping. If the tower is heavily liquid loadedand hc is too high, consider increasing the number of liquid passes to reduce the liquid rate per pass.




CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

FIGURE 21EFFECT OF LIQUID RATE ON SIEVE TRAY TURNDOWN

Broad Operating Window(Mod. to High Liquid Rate)

Good Operating Range(Good Turndown)

Vapor Rate

Frac

tiona

l Wee

page

(%)

Frac

tiona

l Ent

rain

men

t (%

)

60

50

40

30

20

10

60

50

40

30

20

10

Frac

tiona

l Wee

page

(%)

Frac

tiona

l Ent

rain

men

t (%

)

Vapor Rate

No Operating Range(Poor Turndown)

No Operating Window(Low Liquid Rate)

Note: 20% fractional weepage and 10% entrainment is the maximum allowed to achieve good efficiency.DP3Af21

6. Heat Transfer. The most frequent application for trays in heat transfer service is in heavy hydrocarbon fractionators suchas atmospheric pipestilIs, cat fractionators, steam cracker primary fractionators, etc. Since these applications usuallyentail handling high liquid rates, jet trays are the tray of preference for these pumparound circuits.While single pass jet trays would be adequate for most applications, two pass jet trays may be required to avoid costlytransitions to accommodate the trays above and below the pumparound.If the liquid rate is low [say < 4 gpm/in. of diameter/pass (10 dm3/s/m of diameter/pass)] sieve trays or valve trays shouldbe considered.After using Table 5A to select the tray type, the appropriate Subsection should be used to design the tray. Then, SectionIII-F, Heat Transfer should be used to determine how many actual trays are required.For services where pressure drop is critical, packing is usually used because of its much lower pressure drop. This isespecially true in vacuum pipestills and other vacuum distillation applications. This subject is discussed further underCOUNTER-CURRENT DEVICES - PACKING.

7. Overall Flood. In reality, flooding mechanisms do not act independently. Instead, flooding is usually caused by acombination of effects, resulting in a continuous flooding curve (Figure 17 and Figure 22). For this reason, an "overallflood" correlation has been developed for cross flow fractionation devices. Overall flood is a combination of jet flood,downcomer flood, and ultimate capacity, and depends primarily on the limiting flood mechanism (that is, the floodmechanism with the highest percent of flood). If overall flood exceeds the design limit, actions must be taken to reduce thelimiting flood mechanism. It is important to note that overall flood does not take into account every possible floodingmechanism, so all other design criteria must also be met to ensure successful design.





CROSS FLOW DEVICES - BASIC DESIGN CONSIDERATIONSAs discussed earlier a fractionating tray must be operated within a certain range of vapor and liquid rates to give optimumperformance and an economical design. Outside this range, efficiency drops off and/or the tower becomes inoperable. Theeffects of vapor and liquid rates on tray performance are depicted schematically on Figure 22. These performance limitationsare summarized below.

➧ FIGURE 22TYPICAL SIEVE TRAY PERFORMANCE DIAGRAM

Liquid Rate

Vapo

r Rat

e

DP03Af22

Jet Flood (Entrainment Flood)

Downcomer Flood

Excessive Entrainment

Weep Point

Moderate Weeping

Heavy WeepingDumping

Area of Normal Operation

0

0

Maximum Vapor Rate Considerations. A very high vapor rate may cause:1. Jet flooding, excessive entrainment, spray regime operation, or exceeding the ultimate capacity of the tray.2. High pressure drop across the tray, resulting in excessive downcomer filling and subsequent downcomer flooding.Minimum Vapor Rate Considerations. A very low vapor rate may cause:1. Weeping or dumping.2. Poor contacting and tray efficiency because of inadequate vapor/liquid mixing.

These conditions can result from insufficient vapor loading or from excessive open area on the tray, both of which producea low vapor velocity through the tray openings.

Maximum Liquid Rate Considerations. High liquid rates may cause:1 . Tray flooding (due to insufficient disengaging in the downcomers), excessive tray pressure drop, and excessive

downcomer filling.2. Tray flooding due to excessive downcomer entrance or exit velocity and downcomer bridging.Minimum Liquid Rate Considerations. Low liquid rates may cause:1. Spray regime operation at high vapor rates.2. Vapor bypassing up the downcomers, if the downcomer is not sealed.




CROSS FLOW DEVICES - BASIC DESIGN CONSIDERATIONS (Cont)3. Poor contacting and low tray efficiency, because of inadequate liquid residence time on the tray due to operation in the

spray regime.To avoid these problems, the correct combination of tray type, tower diameter, tray spacing, free area, downcomer size,hole area and number of liquid passes must be utilized. This optimum can be achieved with correct use of the PEGASYSdesign program.

CROSS FLOW DEVICES - GENERAL CONCLUSIONSFor most applications, sieve trays will be the first choice. If high turndown (>3:1) is needed, valve trays should beconsidered. For most heat transfer services (pumparounds) in atmospheric pressure columns, jet trays are preferred.A summary of the key parameters for each of the major tray types used by ExxonMobil is provided in Table 1. Table 2provides a similar summary for the counter-current devices to be discussed later in this Subsection.In addition, to aid the designer in selecting the best internal for a given application, a series of "Decision Trees” (in the formof Tables) have been prepared for the design objectives outlined below.

Table 3 - Tower Internals Selection for New TowersTable 4 - Tower Internals Selection for RevampsTable 5 - Tower Internals Selection for Heat Transfer ServiceTable 6 - Tower Internals Selection for Entrainment Removal Devices

Once the design has been completed, the engineer should review Table 7, Tower Design Checklist (Trays) to be sure thatno major point has been overlooked.

COUNTER-CURRENT DEVICESTYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS)

EQUIPMENT TYPES AND APPLICATIONS1. PACKING (See Section III-G for more details.)

Although a packed tower design may result in a smaller diameter tower, the total cost of the installation with packing,packing supports, and distributors/re-distributors is more expensive than a trayed tower. In addition, packed towersare more sensitive to liquid and vapor maldistribution, and packing and liquid distributors are less tolerant to foulingthan trays. New high capacity trays should be considered first when revamping an existing tower as the revamp costsand time are much lower than converting a trayed vessel into a packed tower. Therefore, packing is usually justifiedonly for:

• Applications where pressure drop across the internals is critical, such as in vacuum distillation or in some gas cleanupunits handling recycled gas that requires compression.

• Corrosive but non-fouling services where ceramic packings are more economical than alloy trays.• Towers less than 3 ft (900 mm) in diameter where packing and tray costs are nearly equal.• Sidestream strippers where increased efficiency can reduce steam consumption and provide ENCON credits.• Revamps where an acceptable tray design cannot be achieved.• Foaming systems, such as demethanizers, TEG Contactors, and some amine units.• Applications for which a reduction in height, weight, or footprint is desired (such as offshore towers, or towers

susceptible to swaying conditions)a. Random packings (also called dumped packings) are the most frequently used counter-current devices. Their name

derives from the fact that they are dumped into the column and orient randomly.Random packings come in a number of different shapes, sizes and materials of construction (Figure 23). Basically,as the packing size increases, the capacity increases while the pressure drop, cost, and efficiency decrease. Thus,for a given design, there is an optimum economic combination of packing size, tower diameter and tower height. ForPall rings, past studies have shown that the 1.5 to 2 in. (38 - 50 mm) ring size usually provides the optimum design.

➧ Several other packings provide improved performance characteristics. These include Koch-Glitsch’s Cascade Mini Ring(CMR), Saint-Gobain NorPro's Intalox Metal Tower Packing (IMTP) also known as Metal lntalox, Sulzer Chemtech'sNutter Ring, and UOP Raschig Super-Rings. Some of these are depicted in Figure 23.There are a number of other packing types available but they are not widely used in ExxonMobil towers and thereforehave not been included here.





COUNTER-CURRENT DEVICESTYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) (Cont)

FIGURE 23RANDOM (DUMPED) PACKINGS

PALL RING KOCH-GLITSCH METAL CMR NOR PRO METAL INTALOX (IMTP) NUTTER (SULZER) RING

DP3AF23

➧ b. Structured packings. These devices are fabricated in bundles from crimped sheet metal and installed in the tower inlayers having a fixed orientation. Structured packing provides more effective surface area per unit volume and is moreefficient than random packing. Due to competitive pressures, costs are now similar between random and structuredpacking. Since the crimp height can be changed, the capacity, efficiency, pressure drop and cost can also be varied.Thus, the optimum choice must be determined by an economic study.Of all the contacting devices available, structured packing provides the lowest pressure drop per theoretical stage ofcontacting as well as the best capacity / efficiency combination. This feature makes it especially attractive in vacuumtowers and as a high capacity revamp option in other low pressure towers. Structured packing is not recommendedfor use in high pressure distillation applications or for liquid rates above 20 gpm/ft2 (13.6 dm3/s/m2), unless theapplication is a high pressure aqueous system. See Section III-G for details.There are several suppliers including: Flexipac and Gempak by Koch-Glitsch, Intalox Structured by Saint-GobainNorPro, Montz-Pak by Montz, and Mellapak and MellapakPlus by Sulzer Chemtech. The Sulzer MellapakPlus seriesstructured packings have comparable efficiency and higher capacity than the Mellapak series. ExxonMobil experiencewith MellapakPlus is limited, and a FRACTIONATION SPECIALIST should be consulted for specific applications. Anexample of structured packing, by Koch-Glitsch, Inc., is shown in Figure 24.

FIGURE 24STRUCTURED PACKING (BY KOCH-GLITSCH, INC.)

DP3Af24





2. GRIDS (also see Section III-G)Grids are similar to structured packing in that they are fabricated in panels and installed in an ordered manner. However,their efficiency characteristics are much poorer due to their high open area and low surface area per unit of volume. Thefirst grid to appear on the market (circa 1961) was Glitsch grid. It was intended for use in services where entrainmentremoval was critical but where fouling was too severe to use crinkled wire mesh screens. This made it ideal for use inwash zones of vacuum pipestills, cat fractionators, etc.Because of the grid's large physical openings, it has demonstrated good run lengths in these fouling services. However.these large physical openings, plus a relatively low specific surface area, give it the performance characteristics of a verylarge size packing. Therefore, it has a very high capacity and low pressure drop. Its efficiency, however, is very low(about 50% of that provided by the 2 in. [50 mm] Pall ring).

➧ In recent years, several new grids have come on the market. They are Flexigrid #2 and #3 (and newer styles) by Koch-Glitsch, SNAPGRID #3 and Mellagrid by Sulzer Chemtech, and Intalox grid by NorPro. Intalox grid is consideredfunctionally equivalent to Glitsch Grid EF-25A. ExxonMobil experience with Mellagrid is limited, and a FRACTIONATIONSPECIALIST should be consulted for specific applications. Pictures of these major grids are shown in Figure 25.

➧ FIGURE 25VARIOUS TYPES OF GRIDS

DP3AF25

KOCH-GLITSCH EF-25A KOCH-GLITSCHFLEXIGRID STYLE 2

SULZER SNAPGRID

SingleSheet

AsInstalled

SULZER MELLAGRID






Because of their high capacity and low pressure drop grids have also been used in heat transfer sections (pumparounds)of vacuum pipestills and other heavy hydrocarbon fractionators. The liquid is introduced on the top layer of grid via spraynozzles. In revamping pumparounds, a split bed may be required. That is, the grid is used in the bottom portion of the bedfor capacity reasons while a dumped or structured packing is used on the top where the loadings are lower, to maximizeefficiency.

3. BAFFLE SECTIONS (also see Section III-J)There are two basic types of baffle sections - sheds, and disc and donuts. These devices operate differently than grids orpacking. In baffle sections, the liquid cascades from baffle to baffle in the form of liquid curtains. As the vapor flowsthrough these curtains, the liquid is broken up into droplets and mass transfer occurs. However, this is a very inefficientliquid/vapor contacting mechanism and produces very low efficiency. These devices are sketched in Figure 26.

FIGURE 26DIFFERENT TYPES OF BAFFLES

Disc and DonutSheds

Disc

DP3AF26

Donut

Because of their high open area and large physical dimensions they are ideally suited for:• Slurry sections of cat fractionators and coker scrubbers where high temperatures and solids are present, a severely

fouling service.• Condensable blowdown drums and water quench towers where large volumes of liquids must be handled and solids

are sometimes present.For severe fouling services, baffle sections are about the only internal available if long run lengths are required. Becauseof their high open area, they have high capacity but very poor efficiency. Thus, baffle sections require a disproportionateamount of tower straightside for the functions they perform.

➧ 4. DUALFLOW TRAYS (also see Section III-L)Dualflow trays are basically sieve trays without downcomers; thus, the entire cross-sectional area of the tower is availablefor vapor and liquid flow. Unlike sieve trays, however, the vapor and liquid flow through the same holes on a periodicbasis. It is very important to ensure that dualflow trays are installed level to the ground to prevent vapor / liquid bypass.Dualflow trays are useful primarily for revamping heavily liquid loaded towers. Although they have high capacity, their poorturndown and low efficiency (vs. sieve trays) make them unattractive for new designs. In addition, they can exhibit poorstability in large diameter towers.





There are several types of dualflow trays on the market. The most common consists of a flat tray deck containing roundsieve holes. The Ripple tray, developed by Stone and Webster is a flat, perforated tray that has been crimped to form asine wave in the tray panel. The third device is the proprietary Shell Oil Turbogrid Tray which is a flat-surfaced tray whoseopenings are long, rectangular slots. These devices were all developed to maximize tower throughput.Since the industry only has reliable data for the FRI-type dualflow tray, that is the one currently being recommended forExxonMobil revamps although Ripple trays are applied in some ExxonMobil towers. ExxonMobil has not used the ShellTurbogrid Tray. If a tower to be revamped contains any type of dualflow tray, please contact your FRACTIONATIONSPECIALIST for advice.

COUNTER-CURRENT DEVICES - PROCESS DEFINITIONS

VAPOR / LIQUID CAPACITY LIMITATIONS1. Flooding. Dumped and structured packings as well as grids have similar capacity characteristics. Their vapor handling

capacity is determined by the specific packing or grid type, size, liquid loading, and system physical properties (surfacetension, density, and viscosity).a. Packed towers. Unlike trays, flooding is harder to define in a packed tower. There is no tray spacing or downcomer to

fill with liquid. What does occur is that liquid begins to accumulate in the packing and the pressure drop begins to risemore sharply. This is known as the load point. With further increases in vapor rate, the pressure drop rises almostvertically and liquid begins to “pile up” on the top of the packing. This has been observed visually at FractionationResearch Incorporated (FRI). Like trays, however, as liquid begins to accumulate in the packing and backmixingoccurs (analogous to entrainment on trays), the efficiency (HETP) becomes poorer (see efficiency discussion below).

b. Baffle sections. As the vapor rate is increased, the curtain of descending liquid is steadily raised until it is nearlyhorizontal. Simultaneously, liquid begins to pile up on the top surface of the shed or disc and donut element. Furtherincreases in rate cause entrainment to occur from a given element to the element above and flooding begins.The capacity of these devices is likewise dependent on tray spacing, percent open area, curtain area, liquid rate, andsystem physical properties. These variables are discussed more fully in Section III-J.

c. Dualflow trays. These trays flood in a manner similar to sieve trays; that is, the intertray space becomes completelyfilled with a dense froth which is entrained from tray to tray. For a given diameter, their capacity is a strong function oftray spacing and percent hole area and a weak function of hole size and liquid physical properties.

2. Ultimate capacity (see discussion under CROSS-FLOW DEVICES - PROCESS DEFINITIONS). All of the counter-current devices can also flood due to ultimate capacity. This is especially true for baffle sections and dualflow trays, evenat low operating pressure. This occurs because these devices inherently have a very high capacity and thus operatecloser to an ultimate capacity limitation to start with. Each device should be checked for this limitation by using theequations given in the appropriate Subsection of Design Practice III.

EFFICIENCY AND TURNDOWN1. Dumped and structured packings. These devices provide the highest efficiency per unit of pressure drop (i.e., lowest

pressure drop per theoretical stage). Small packing sizes have the highest efficiency, pressure drop and cost, but have thelowest capacity. Thus the designer must choose the optimum combination for a given design case.To ensure optimum efficiency at all rates, a high quality liquid distributor must be used. In fact, the distributor’s turndownusually limits before that of the packing. For most applications, a turndown of 2/1 is specified. Higher turndown, say 4/1,will result in a more expensive distributor. The selection of a liquid distributor is critical in the design of a packed bed. Thistopic is discussed extensively in Section III-G.For most applications, dumped packings should be considered first because they are lower cost. If a low pressure dropper theoretical stage and/or a short column height is required, then structured packings should be evaluated as an option.

2. Grids. Because of their high open area and relatively small amount of surface area per unit volume, their efficiency isquite low [about 50% of 2 in. (50 mm) Pall rings]. Since they are normally only used in fouling services (wash zones) or inpumparounds, their low efficiency can usually be accepted. For most grid applications, long run length and/or low pressuredrop are the two key features required. Turndown is usually limited by the spray nozzle distributors to about 2/1.

3. Baffle sections. The efficiency of these devices is usually quite poor. They provide little interfacial area for vapor/liquidcontacting. Their only redeeming feature is resistance to fouling and thus good run length. Their turndown characteristicsare poor (about 1.25/1). However, acceptable operation can usually be maintained in pumparound service when the liquidrate can be kept at or near design flow.





COUNTER-CURRENT DEVICES - PROCESS DEFINITIONS (Cont)4. Dualflow trays. This device has a peak efficiency only about 80% as high as that provided by a sieve tray. In addition,

they have a low turndown ratio of about 4/3, or 1.33/1. Their primary use is in debottlenecking relatively small diametercolumns where turndown is not a major concern. However, they are very infrequently used.

HEAT TRANSFERThe heat transfer characteristics of each of the counter-current devices parallels that of their mass transfer capabilities (seediscussion under EFFICIENCY AND TURNDOWN immediately above). For a more thorough discussion on heat transfer, referto Section III-F. Section III-F also provides the necessary correlations for each device to design a heat transfer section.

OTHER CONSIDERATIONS1. Liquid, vapor, and mixed phase inlets. When working with any of the packings or grids, great care must be taken to

ensure adequate vapor and liquid distribution. This is true because there is very little pressure drop to help correct anyvapor maldistribution problem.Likewise, the liquid will not be redistributed since most of the newer packings have poor spreading characteristics. Fortoday's packings, a high quality liquid distributor is essential. Furthermore, if there is a mixed phase inlet present, the twophases must be separated before the liquid is distributed to the packing. Otherwise, adverse kinetic effects anddisengaging problems will almost guarantee that the packed bed will perform poorly.Baffle sections usually employ a perforated pipe to distribute the liquid to the various shed elements. In the past this hasnot posed a problem, but it should be remembered that the fractionation requirement (and thus the need for good liquiddistribution) is not critical for services where these devices are normally used.Dualflow trays require good liquid distribution on the top tray deck; otherwise, vapor/liquid maldistribution will occur.Dualflow trays also require good leveling and the decks must be flat.

➧ 2. Fouling. The relative fouling resistance of common fractionation devices is shown in Table 3D. Since fouling is stronglysystem dependent, the designer should always contact the FRACTIONATION SPECIALIST for past experience andguidance.

3. Corrosive services. For many years, dumped ceramic packings have been used in highly corrosive services because oftheir low cost. Today, plastic packings are finding increasing use in these services. Since the amount of corrosion isstrongly system dependent, the designer should always contact the FRACTIONATION SPECIALIST for guidance oninternals selection first. Then, the appropriate MATERIALS SPECIALIST should be consulted to optimize the materials ofconstruction as needed.

COUNTER-CURRENT DEVICES - GENERAL CONCLUSIONSFor most fractionation applications, sieve or valve trays remain the best overall internals choice. However, if packing is neededfor process reasons, dumped packings of the 2 in. (50 mm) size will usually prove most economical.When low pressure drop per theoretical stage and/or column height restrictions apply, structured packing should be considered.Structured packing should also be considered for high capacity revamps of hydrocabon distillation towers with operatingpressures of less than 100 psia (690 kPa).For highly fouling services, baffle sections or one of the grids should be considered.For highly corrosive services, a packing made of ceramic or plastic material should be considered.To aid the designer in selecting the best internal for a given application, a series of "Decision Trees" (in the form of Tables)have been prepared for the objective outlined below.

Table 1 - Trays - A Summary of CharacteristicsTable 2 - Counter-Current Devices - A Summary of CharacteristicsTable 3 - Tower Internals Selection for New TowersTable 4 - Tower Internals Selection for RevampsTable 5 - Tower Internals Selection for Heat Transfer ServiceTable 6 - Tower Internals Selection for Entrainment Removal Service

Once the design has been completed, the engineer should review Table 8, Tower Design Checklist (Packing) to be sure thatno major point has been overlooked.




➧ NOMENCLATUREAb = Bubble area, ft2 (m2) (see Figure 12)Ad = Downcomer area, ft2 (m2)Adi = Total downcomer inlet area, ft2 (m2)

*Adi = Total downcomer inlet area on inboard tray, ft2 (m2)

Ado = Total downcomer outlet area (downcomer bottom area), ft2 (m2)*Ado = Total downcomer outlet area (downcomer bottom area) on inboard tray, ft2 (m2)

Af = Free area, ft2 (m2) (superficial area minus arithmetic average of inlet and outlet area of downcomer(s) above the tray minus the waste area) (see Figure 13)

Ao = Hole/valve/cap/tab area, ft2(m2)As = Superficial (total) tower area, ft2 (m2)Aud = Area under the downcomer (downcomer escape area), ft2 (m2)Aw = Waste area, ft2 (m2)c = Downcomer clearance between tray and downcomer apron at tray inlet, in. (mm) (see Figure 3)DT = Tower diameter, ft (mm)H = Tray spacing, in. (mm)HETP = Height equivalent to a theoretical plate, in. (mm)hc = Clear liquid height on tray, in. (mm) of hot liquidhd = Downcomer filling, in. (mm) of hot liquidhdc = Head loss due to two-phase flow through the downcomer, in. (mm) of hot liquidhed = Effective dry tray pressure drop, in. (mm) of hot liquidhi = Tray inlet head, in. (mm) of hot liquidht = Total tray pressure drop, in. (mm) of hot liquidhud = Head loss under downcomer or splash baffle, in. (mm) of hot liquidhv, hi = Distance between downcomer step (for stepped design), or straight / sloped transition (for semi-sloped

design), and the tray above, in. (mm) (see Figure 8)hv, lo = Distance between downcomer step (for stepped design), or straight / sloped transition (for semi-sloped

design), and the bottom of the downcomer apron, in. (mm) (see Figure 8)hwo = Outlet weir height, in. (mm) (see Figure 3)lfp = Flow path length (distance between inlet and outlet downcomers), ft (mm) (see Figure 12)lo = Outlet weir length, in. (mm) (see Figure 12)P = Pressure, psia (kPa abs)QL = Liquid rate, gpm (dm3/s) at conditionsr = Downcomer inlet rise (width) for chordal downcomers or downcomer inlet width for inboard downcomers,

in. (mm)

COMPUTER PROGRAMSOnce the contacting device has been selected, the designer should read the corresponding Section of Design Practice III.Each Subsection lists the various computer program(s) available under the heading COMPUTER PROGRAMS. Supportingdocumentation is also provided. For the various contacting devices, the PEGASYS computer program can be used forequipment design and rating.





➧ TABLE 1TRAYS - A SUMMARY OF CHARACTERISTICS

TRAY TYPE CAPACITY EFFICIENCY COST PERUNIT AREA FLEXIBILITY * REMARKS

Sieve Medium to High High. Equal toor better thanother traytypes.

Lowest of alltrays withdowncomers.

Medium. 2-3/1 canusually beachieved.

First choice for mostapplications; extensivedesign data available.

Moveable Valve Medium to high; as good assieve trays or better.

High. As goodas sieve trays.

Medium.About 10%greater thansieve trays.

High.Possibly up to5/1.

Not recommended forfouling services.

Fixed ValveSulzer SVG, LVGKoch-Glitsch V-0NorPro ProvalveMVGOthers

As good as sieve trays orslightly better for SVG ontriangular pitch; LVG and fixedvalve trays on square pitchhave lower capacity. SeeTable 4A for capacity of otherdevices mentioned.


About thesame assieve trays.

Medium. Do not use in foulingservices when foulantmaterial is “sticky” andentrained in vapor phase.See Table 3D.

Sulzer Mini V-Grid(MVG)

Medium to high; approximately10-15% higher than sieve traysat pressures below 50 psia(345 kPa).


At least 5%higher thansieve trays.

Medium.Slightly higherthan sievetrays.

Good alternative to sievetrays at low liquid ratewhere higher capacity isneeded.

Koch-Glitsch NyeNorPro TritonMVGT

High; approximately 10-15%higher than a sieve tray.

High. Slightlylower thansieve trays.

Medium;About 25%higher thansieve trays.

Medium.Almost asgood as sievetrays.

Good alternative to sievetrays in tower revampswhere higher capacity isneeded.

Jet Highest at low pressures andhigh liquid rates.

Low tomedium.

Low tomedium.About 5%higher thansieve trays.

Low. 1.5 or2/1.

Consider only when liquidrate exceeds 4.0 gpm/in.of diameter per pass(10.0 dm3/s/m ofdiameter per pass).

Bubble Cap Medium to high, except low tomedium at high liquid rate.

Medium tohigh.

High. At leasttwice the costof sieve trays.

3/1 to 4/1 Use for high flexibility orlow liquid rate applicationwhere fouling of valvetrays may be a problem.

UOP MultipleDowncomer (MD)UOP EnhancedCapacity MultipleDowncomer (ECMD)Sulzer Hi-Fi

Very high. Estimated to be30-40% higher than aconventional sieve tray for highliquid rate services.

Low tomedium.

Higher thansieve or valvetrays.

Low. (< 2/1) Can be installed on verylow tray spacings.Consider for revampswhere no other device isacceptable. Notrecommended for foulingservices. Limitedinspection access.

* Ratio of maximum to minimum vapor loads at which tray efficiency remains above about 90% of its design value.




➧ TABLE 2COUNTER-CURRENT DEVICES - A SUMMARY OF CHARACTERISTICS

DEVICE CAPACITY EFFICIENCY COST PER UNITAREA FLEXIBILITY * REMARKS

Random Packing(Pall Rings, MetalIntalox, Nutter Rings,etc.)

Medium. Medium. Medium to low,depending on materialof construction.

> 3/1. Good efficiency perunit of pressure drop.Mainly used in highliquid rate absorbers.

Structured Packing(Flexipac, MontzGempak, MellapakIntalox, etc.)

High to very high(large crimp).

High - depends onsurface area.

Medium; depends onmaterial.

> 3/1. Best efficiency per unitof pressure drop.Selection of packingsize allows trade-off ofcapacity and efficiency.

Glitsch GridFlexigridSnapgridIntalox Grid

Very high. Poor as fractionationdevice. Good forentrainment removaland heat transfer.

Medium to high. Low; less than2/1.

Good for high vapor-low liquid service tominimize effect ofentrainment. Used inwash zones of heavyhydrocarbonfractionators wheremoderate cokingoccurs.

Sheds and Disc-and-Donuts

Very high. Poor as fractionationdevice.

Medium. Low. less than1.5/1.

Used in severe foulingservice; e.g., slurrypumparound in catfractionator.

Downcomerless(Dualflow or Ripple)Trays

Highest in someinstances.

Medium to good, atdesign liquid andvapor rates.

Lowest to medium;royalty on RippleTray, none on FRIDualflow types.

Low. Of interest for revampsif poor flexibility istolerable.

* Ratio of maximum to minimum vapor loads at which efficiency remains above approximately 90% of its maximum value.Packing flexibility is typically limited by distributor turndown, and can be up to 10/1 with appropriately designeddistributor.





➧ TABLE 3ATOWER INTERNALS SELECTION FOR NEW TOWERS

SEE TABLE 3B

SEVERE

≤ 3/1 > 3/1

TURNDOWNRATIO

REQUIRED?

NO

ALL CASES

TURNDOWNRATIO

REQUIRED?

YES

∆P CRITICAL?

YES OR NO

EFFICIENCYCRITICAL?

≤1.5 gpm/inch*

SIEVE

≤ 3/1

VALVE

> 3/1

TURNDOWNRATIO

REQUIRED?

NO

PACKING **

ALL CASES

TURNDOWNRATIO

REQUIRED?

YES

∆P CRITICAL?

YES OR NO

EFFICIENCYCRITICAL?

>1.5 gpm/inch*

LIQUID RATE?

NONE

SEE TABLE 3B

MODERATE

FOULINGTENDENCY?

VALVE - IF< 0.25 gpm/in.(0.62 dm3/s/m)

CONSULTFRACTIONATION

SPECIALIST

SIEVE-POSSIBLEHIGH

ENTRAINMENT- USE PICKETFENCE WEIRS

STRUCTUREDPACKING -

SPECIAL LIQUIDDISTRIBUTOR

REQUIRED

DP3AT3A

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass** Apply structured packing if P < 100 psia (690 kPa) and QL / As < 20 gpm/ft2 (13.6 dm3/s/m2), or for high pressure aqueous systems;

otherwise use random packing.




➧ TABLE 3BTOWER INTERNALS SELECTION FOR NEW TOWERS

SHEDS

ALL LIQUID RATES

SEVERE

GRIDS PLUSCONSULT

FRACTIONATIONSPECIALIST

≤ 2/1


SPECIALIST

> 2/1

TURNDOWNRATIO

REQUIRED?

NO

GRIDS

≤ 2/1


SPECIALIST

> 2/1

TURNDOWNRATIO

REQUIRED?

YES

∆P CRITICAL?

NO

EFFICIENCYCRITICAL?

≤ 1.5 gpm/inch *

SEETABLE 3C

> 1.5 gpm/inch *

LIQUID RATE?

MODERATE

SEE TABLE 3A

NONE

FOULING TENDENCY?

DP3AT3B

YES

LARGE HOLE SIEVEWITH PICKET FENCE

WEIR, OR FIXED VALVE

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass





TABLE 3CTOWER INTERNALS SELECTION FOR NEW TOWERS

LARGE HOLEDIAMETER SIEVE

OR JET TRAYOR SVG

OR PROVALVE

≤ 2/1

V-GRIDOR

PROVALVETRAY

> 2/1

TURNDOWNRATIO

REQUIRED?

NO

LARGE HOLEDIAMETER SIEVE

OR SVG ORPROVALVEOR GRIDS

≤ 2/1

LARGE SIZESTRUCTUREDPACKING ORDEEP GRID

BED

> 2/1

TURNDOWNRATIO

REQUIRED?

YES

∆P CRITICAL?

YES OR NO

EFFICIENCYCRITICAL?

> 1.5 gpm/inch *

SEETABLE 3B

≤ 1.5 gpm/inch *

LIQUID RATE?

MODERATE

FOULING TENDENCY?

DP3AT3C





➧ TABLE 3DRELATIVE FOULING RESISTANCE OF COMMON FRACTIONATION DEVICES

For a given fractionation device, a larger limiting hole / lift / path size corresponds to higher fouling resistance. Therefore, thelist below is not universally valid, but instead represents a general guideline for the relative fouling resistance of devices basedon the "standard" size of the specified device. In addition to the fractionation devices listed in Table 3D, the fouling resistanceof other tower internals such as liquid distribtors must be considered.

RELATIVE FOULINGRESISTANCE FRACTIONATION DEVICE *

Caged valve tray with heavy foulant flowing on deckFloating valve traysFixed valve trays (when sticky material entrained from below)Structured packings - low crimp height [0.5 in. (13 mm) or less]Dumped packings - small diameter [1.5 in. (38 mm) or less]UOP MD and ECMD trays, Sulzer Hi-fi trayMini-valve trays or fixed valves with 0.25 in. (6.4 mm) lift or lessBubble cap trays - small [0.375 in. (9.5 mm) or less] skirt clearanceCaged valve traysSieve trays [0.5 in. (13 mm) hole diameter]Bubble cap trays - large [greater than 0.375 in. (9.5 mm)] skirt clearanceLarge fixed valve trays, such as Sulzer SVG (no sticky material entrained from below)NorPro ProValve tray deckLarge hole sieve trays [≥ 0.75 in. (19 mm)]Jet traysDumped packings - large diameter [~3.5 in. (90 mm)]Structured packings - large crimp height [~2 in. (50 mm)]Koch-Glitsch Flexigrid Style 3 or EF 25A or equivalentKoch-Glitsch Flexigrid Style 2 or equivalent

Lowest

Moderate

Baffle sections (sheds; disc and donuts)Highest Open spray chamber

* Nye tray inserts can be designed to have the same fouling resistance as the associated tray deck.





TABLE 4TOWER INTERNALS SELECTION FOR REVAMPS

DP3AT4

SEE TABLE 4B

MAXIMIZE CAPACITY -LOSS IN EFFICIENCY

TOLERABLE

STRUCTUREDPACKING

≤ 1.5 gpm/inch *

SEETABLE 4A

≤ 3/1

MULTIPASSVALVETRAYS

> 3/1

TURNDOWNRATIO

REQUIRED?

NO

PACKING **

YES

∆P CRITICAL?

> 1.5 gpm/inch *

LIQUID RATE?

INCREASE CAPACITY -SAME OR BETTER

EFFICIENCY

SEE TABLE 4C

SAME CAPACITY -IMPROVE SEPARATION

REVAMP OBJECTIVE?






➧➧➧➧ TABLE 4AAPPLICATION GUIDELINES FOR

DEBOTTLENECKING FRACTIONATION TOWERS(1) (2)

RELATIVE CAPACITY INCREASE @ CONSTANT SEPARATION EFFICIENCY (3)

PRESSURE 0 - 10% 10 - 20% 20 - 30% 30% +

LowUnder50 psia(345 kPa)

• MVG• ProValve• SuperFrac (6)

•2-Pass Nye (4)

•2-Pass Trays• Random Packing• Structured Packing

• Triton• 2-Pass MVG• 2-Pass SuperFrac (6)

• 2 Pass Nye (4)

• Random Packing• Structured Packing

• Structured Packing • Structured Packing

Moderate

50 psia(345 kPa)

to

165 psia(1140 kPa)

• Nye (4)

• ProValve• 2-Pass Trays• MD Trays• Random Packing• Structured Packing (5)

• 2-Pass Nye &Superfrac Trays (6)

• Triton• 4-Pass Trays• MD Trays• Random Packing• Structured Packing (5)

• MD Trays• Hi-fi Trays• ECMD Trays• Structured Packing (5)

• Hi-fi Trays• ECMD Trays

HighAbove165 psia(1140 kPa)

• Nye (4)

• ProValve• 2-Pass Trays• MD Trays• Random Packing (5)

• 2-Pass Nye &SuperFrac Trays (6)

• 4-Pass Trays• MD Trays• Random Packing (5)

• 4-Pass Nye &Superfrac Trays (6)

• MD Trays• ECMD Trays• Hi-fi Trays

• Hi-fi Trays• ECMD Trays

Notes:(1) Stainless steel is assumed for all internals. The cost of MD trays and random packing are generally very close. However, if stainless

steel MD trays are required, random packing will generally be less expensive.(2) If the application is in fouling or corrosive service, consult your FRACTIONATION SPECIALIST.(3) Approximate capacity increase relative to single-pass sieve tray.(4) Installing a Nye tray above the feed tray has been shown to successfully debottleneck feed trays. Also consider installing a Nye tray to

debottleneck a tray above a reboiler return, where excessive waste area due to distributor pipes may reduce effective bubble area.Consult your FRACTIONATION SPECIALIST for details of this application.

(5) Structured packing is not recommended for pressures greater than 100 psia (690 kPa) or liquid loading greater than 20 gpm/ft2(13.6 dm3/s/m2), unless the application is a high pressure aqueous system.

(6) SuperFrac trays require FRACTIONATION SPECIALIST involvement for all applications.





TABLE 4BTOWER INTERNALS SELECTION FOR REVAMPS

DP3AT4B


SPECIALIST

≤ 1.5 gpm/inch *

NO

LARGERSIZE

PACKING ORGRID

YES

∆P CRITICAL?

≤ 2/1

MULTIPASSSIEVE OR

PACKING ORGRID

NO

LARGERSIZE

PACKING ** ORGRID

YES

∆P CRITICAL?

> 2/1

TURNDOWNRATIO

REQUIRED?

> 1.5 gpm/inch *

LIQUID RATE?

MAXIMIZE CAPACITY -LOSS IN EFFICIENCY

TOLERABLE

REVAMP OBJECTIVE?

DUAL FLOW, UOP MDOR ECMD, SULZERHI-FI OR JET TRAYS

FOR HIGH LIQUIDRATES






TABLE 4CTOWER INTERNALS SELECTION FOR REVAMPS

DP3AT4C

YES

STRUCTUREDPACKING

NO

SPARE CAPACITYIN TOWER?

≤ 1.5 gpm/inch *

NO

PACKING **

YES

PACKING **

YES

INCREASE REFLUXOR DECREASE

OPERATINGPRESSURE OR

PACKING

NO

∆P CRITICAL?

> 2/1

TURNDOWNRATIO

REQUIRED?

YES

SEETABLE 4D

NO


> 1.5 gpm/inch *

LIQUID RATE?

SAME CAPACITY -IMPROVE

SEPARATION

REVAMPOBJECTIVE?

INCREASE REFLUXAND/OR DECREASE

OPERATINGPRESSURE PLUS SEE

FRACTIONATIONSPECIALIST

≤ 2/1

∆P CRITICAL?

INCREASE REFLUXAND/OR DECREASE

PRESSURE, REDUCENUMBER OF PASSES,

OR LOWER TRAYSPACING







TABLE 4DTOWER INTERNALS SELECTION FOR REVAMPS

DP3AT4D

UOP MD OR ECMDTRAYS, OR SULZER

HI-FI AT LOWERTRAY SPACING OR

PACKING

NO

PACKING **

YES

∆P CRITICAL?

≤ 2/1

PACKING **

YES

PACKING **

NO

∆P CRITICAL?

> 2/1

TURNDOWNRATIO

REQUIRED?

NO

SEETABLE 4C

YES


> 1.5 gpm/inch *

LIQUID RATE?

SAME CAPACITY -IMPROVE SEPARATION

REVAMP OBJECTIVE?






TABLE 5ATOWER INTERNALS SELECTION FOR HEAT TRANSFER SERVICE

(NEW TOWERS AND REVAMPS)

DP3AT5A

GRID

YES

GRID

< 2/1 2-3/1

SEEFRACTIONATION

SPECIALIST

> 3/1

TURNDOWNRATIO

REQUIRED?

NO

∆P CRITICAL?

MODERATE

PACKINGOR GRID

INDEPENDENT

TURNDOWNRATIO

REQUIRED?

YES OR NO

∆P CRITICAL?

NONE

SHEDS

SEVERE

FOULING?

YES

SEETABLE 5B

NO

VACUUMSERVICE?

LARGE HOLEDIAMETER

SIEVE TRAYS





TABLE 5BTOWER INTERNALS SELECTION FOR HEAT TRANSFER SERVICE


DP3AT5B

SEETABLE 5C

NONE

SEEFRACTIONATION

SPECIALIST

≤ 1.5 gpm/inch*

LARGE HOLEDIAMETERSIEVE OR

JET TRAYS

SEEFRACTIONATION

SPECIALIST

> 3/1

TURNDOWNRATIO

REQUIRED?

> 1.5, ≤ 4 gpm/inch*

JET ORLARGE HOLE

DIAMETERSIEVE TRAYS

≤ 2/1 > 2/1

TURNDOWNRATIO

REQUIRED?

> 4 gpm/inch*

LIQUID RATE?

MODERATE

SHEDS

SEVERE

FOULING?

NO

VACUUMSERVICE?

≤ 3/1

MULTIPASSLARGE HOLE

DIAMETER SIEVEOR BUBBLE CAP

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass** 4 gpm/in. of weir/pass = 10 dm3/s/m of weir/pass




TABLE 5CTOWER INTERNALS SELECTION FOR HEAT TRANSFER SERVICE


DP3AT5C

STRUCTUREDPACKING

≤ 1.5 gpm/inch *

SIEVE TRAYOR

STRUCTUREDPACKING

≤ 3/1

VALVE TRAYOR PACKING

> 3/1

TURNDOWNRATIO

REQUIRED?

>1.5, ≤ 4 gpm/inch*

JET TRAYS

≤ 2/1 > 2/1

TURNDOWNRATIO

REQUIRED?

> 4 gpm/inch *

LIQUID RATE?

NONE

FOULING?

NO

VACUUM SERVICE?

MULTIPASSSIEVE OR

VALVE TRAY ORRANDOMPACKING

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass** 4 gpm/in. of weir/pass = 10 dm3/s/m of weir/pass





TABLE 6ATOWER INTERNALS SELECTION FOR ENTRAINMENT REMOVAL SERVICE

DP3AT6A

SHEDS

SEVERE


SPECIALIST

≤ 1.5 gpm/inch *

LARGE HOLEDIAMETER

SIEVE TRAY ORSVG OR

PROVALVE

≤ 3/1


SPECIALIST

> 3/1

TURNDOWNRATIO

REQUIRED?

NO

GRID ORLARGE CRIMPSTRUCTURED

PACKING

≤ 2/1

DEEPER GRIDBED OR LARGE

CRIMPSTRUCTURED

PACKING

> 2/1

TURNDOWNRATIO

REQUIRED?

YES

∆P CRITICAL?

YES OR NO

EFFICIENCYCRITICAL?

> 1.5 gpm/inch *

LIQUID RATE?

MODERATE

SEETABLE 6B

NONE

FOULING?





TABLE 6BTOWER INTERNALS SELECTION FOR ENTRAINMENT REMOVAL SERVICE(1)


SPECIALIST

≤ 1.5 gpm/inch *

EFFICIENCYCRITICAL?

> 1.5 gpm/inch *

LIQUID RATE?

NONE

FOULING?

∆P CRITICAL?

NO

DP3AT6B

∆P CRITICAL?

YES

GRID ORLARGE CRIMPSTRUCTURED

PACKING

STRUCTUREDPACKING

TURNDOWNRATIO

REQUIRED?

YES

SIEVE TRAYOR

STRUCTUREDPACKING

VALVE TRAYOF

STRUCTUREDPACKING

TURNDOWNRATIO

REQUIRED?

NO

STRUCTUREDPACKING

STRUCTUREDPACKING

TURNDOWNRATIO

REQUIRED?

YES

SIEVE TRAYOR

STRUCTUREDPACKING

VALVE TRAYOR

STRUCTUREDPACKING

TURNDOWNRATIO

REQUIRED?

NO

≤ 2/1 > 2/1≤ 3/1 > 3/1 ≤ 2/1 > 2/1 ≤ 3/1 > 3/1

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass(1) Crinkled wire mesh screens (CWMS) or vane-type mist eliminators can also be used in many situations. See Sections III-H and V-A.





➧ TABLE 7TOWER DESIGN CHECKLIST (TRAYS)

The purpose of this checklist is to ensure the designer has addressed all major items involved in the design, revamp, or ratingof tower internals. After reviewing this list, the designer should check the pertinent Subsection for specific guidelines. Ifunresolved questions remain, your FRACTIONATION SPECIALIST should be consulted.

HYDRAULICS• Are the percents of jet flood, downcomer flood, and ultimate capacity within the allowable values?• Is the entrainment rate greater than 10%? If so, consult your FRACTIONATION SPECIALIST to determine its impact

on efficiency.• Is the weeping rate greater than 20%? If so, did you check the impact on tray efficiency?• Has the correct valve type and valve count been specified for valve trays using conventional valves? If valves are

being considered which do not have pressure drop coefficients, has the correct dry tray pressure drop been specified?• Is pressure drop critical for process reasons (vacuum operation, etc.)? If so, would structured packing have been a

better choice?• Have anti-jump baffles and picket fence weirs been specified if required?• Is downcomer backup within acceptable limits?• Is downcomer entrance velocity at or below the allowable value?• Is the downcomer sealed? Does it meet the choking criteria and other secondary criteria?• If a jet tray design, is the liquid rate between the limits of 4 and 24 gpm/in. of diameter/pass (10 and 60 dm3/s/meter of

diameter/pass)?• Have multi-pass trays been considered if the liquid rate limits tower capacity? (Sieve and valve trays only)

• Have 4-pass trays been properly balanced? (Balancing 4-pass trays involves adjusting effective weir lengths, panelhole areas and downcomer clearances so that the liquid to vapor ratio on each panel is approximately equal. See theassociated tray section for more details and guidance on balancing 4-pass trays.)

TRAY EFFICIENCY• If the chemical system is new (not used elsewhere in ExxonMobil), did you review the final value with your

FRACTIONATION SPECIALIST?• Does the calculated efficiency look reasonable when compared with values listed in Section III-I, Table 2 (Overall

Efficiencies Recommended in Past Designs)?• Was the Fluidity graph (Section III-I, Figure 8) used to set the efficiency for heavy hydrocarbon systems?

BLANKING• For revamps - does the amount and distribution of blanking meet the criteria given in the pertinent tray Subsection and

GP 5-2-1?• For new designs - has the vendor been given enough input to provide the correct amount and distribution of blanking

for the tray in question? (See pertinent tray Subsection and GP 5-2-1)• Would valve trays have been a better choice?• For cases requiring excessive blanking, has a rectangular bubble area / vertical baffle design (Section III-I) been

considered?

SPECIAL CONSIDERATIONS• Is the final design as "balanced” as possible? (See Balanced Design discussion in the Subsection for each device).• If a drawoff box is present, was the free area and bubble area calculated correctly? In many cases, these reduced

areas may require higher tray spacing to keep within flooding limits. See discussion in Section III-H.• Is there sufficient clearance between the drawoff box and the tray below to avoid tray flooding and/or downcomer

entrance problems?• For flashing feeds or reflux, have the correct internals been specified? Is there sufficient room to install a perforated

pipe distributor? Has adequate open area been provided to avoid entrainment generation? Is there sufficient tray




spacing to compensate for the waste area of the distributor as well as the total vapor rate (vapor to tray from traybelow plus vapor in feed)?

TABLE 7TOWER DESIGN CHECKLIST (TRAYS) (Cont)

• Does the reboiler drawoff box design meet the criteria given in Section III-H?• Make sure anti-vortex baffles are provided at all liquid drawoffs.• For all internals that the vendor must supply, does the Design Specification contain a table that lists the necessary

vapor / liquid rates and physical properties?• Are clear, unambiguous drawings (preferably to scale) provided for all ExxonMobil designed internals? Are all critical

process dimensions clearly shown on these drawings?• Is tray spacing sufficient for maintenance purposes or for cleaning if fouling is expected? Does the affiliate have local

specifications that may be more restrictive than those in the ExxonMobil Design Practices Manual?• Has the correct material of construction been chosen for the tower internals? The default material should be stainless

steel.• If a fouling service, are the internals acceptable? (NH4Cl deposition, coking, polymerization, “imported” solids in the

feed, etc.)• If a foaming service, have the guidelines for foaming systems been followed?• For steam stripping sections, have individual tray loadings been calculated via guidelines in Section III-I?• If minimum overhead entrainment is critical, has a deentrainment device been provided?• Are all process nozzles correctly sized?• Are vessel manholes appropriately specified?• Are all tower internals designed to be accessible for inspection and cleaning?

TURNDOWN CONSIDERATIONS• If greater than 3/1, have valve trays been considered?• If system has a liquid rate lower than 1.5 gpm/in. of weir/pass (3.7 dm3/s/meter of weir/pass), does the tray meet the

weeping, entrainment, spray / froth (primarily sieve trays), and turndown criteria? Have picket fence weirs beenconsidered?

➧ TABLE 8TOWER DESIGN CHECKLIST (PACKING)

The purpose of this checklist is to ensure the designer has addressed all major items involved in the design, revamp, or ratingof tower internals. After reviewing this list, the designer should check the pertinent Subsection for detailed guidelines. Ifquestions arise, your FRACTIONATION SPECIALIST should be consulted.

• For debottlenecking a trayed column, were tray alternatives considered?

PACKING TYPEIf a vacuum system or a distillation system less than 100 psia (690 kPa) and 20 gpm/ft2 (13.6 dm3/s/m2), was structuredpacking considered because of its low pressure drop characteristics and high capacity and efficiency versus randompacking?• Does the packing size specified fall within the range given in Table 2, Section III-G?• Is the packing choice consistent with the system's fouling tendency or would grid have been a better choice?• Is the material of construction appropriate for the fluids being handled? Packing would not be a good choice where

ammonia chloride deposition could occur. (If ceramic packing was chosen, remember some breakage will occur andchips may cause plugging of the reboiler or packed bed/support plate. Metal packings must have virtually nocorrosion rate due to the thin material used in their fabrication.)

HYDRAULICS• Is the percent of flood within the range provided in Table 3 of Section III-G? Also, is ultimate capacity at or below

85%?





• If tower pressure drop is critical, has allowance been made for the pressure drop of support plates, chimney trays, etc.as well as for the packing itself? Have design vapor pressures (and therefore volumetric vapor rates) been adjusted toreflect the actual pressure drops?

• Do the support plate and liquid distributor have enough open area to avoid flooding before the packing floods? This isespecially critical for revamps.

• If fouling is expected, has its adverse effect on capacity and run length been considered?

TABLE 8 (Cont)TOWER DESIGN CHECKLIST (PACKING)

EFFICIENCY CONSIDERATIONS• Has the type and size of packing been considered from an efficiency, pressure drop and capacity standpoint, or would

the optimum choice consist of a smaller size or different type?• Is the type of liquid distributor chosen adequate for the design liquid rate and turndown needed?• Does the distributor chosen have the turndown needed for your service?• For reviewing vendor drawings or evaluating existing distributors, does the pour point distribution meet the criteria in

Section III-G, APPENDIX A? Does the distributor design meet the other requirements of this APPENDIX? Are allmetering orifices shown with size? Is a table of liquid heads at minimum, design, and overflow rates included?

• Are clear, unambiguous drawings (preferably to scale) provided for all ExxonMobil designed internals? Are all criticalprocess dimensions clearly shown on these drawings?

• For revamps, if the existing liquid distributor was built prior to 1983, it is probably of poor design. Considerreplacement with a newer model if improved separation is one of the revamp objectives.

• Ensure that the Design Specification requires that all liquid distributors, except spray nozzle distributors, for towerswith a diameter of 3 ft (900 mm) or greater be water tested in the presence of a representative of the owner.

• Attach Flow Test Requirements (Section III-G, Table 7) and Liquid Distributor Guidelines (Section III-G, Table 6), aswell as minimum and design liquid rates for the flow test.

• Ensure that the design specification requires each spray distributor to be water tested in place prior to startup toensure optimum distribution. Consult Section III-G, SPECIAL DESIGN CONSIDERATION FOR GRIDS, GRIDSPRAY NOZZLE CONSIDERATIONS for details concerning this procedure.

AUXILIARIES• Are external strainers with correct mesh size specified? Is piping material downstream of the strainer upgraded as

required to prevent pipe scale from fouling the distributor?• For flashing feeds, ensure that the mixed phase will disengage completely before it enters the liquid distributor.

(Provide a chimney tray (preferred) or a flashbox pre-distributor to achieve this goal.)• Do reboiler returns and vapor-containing feeds have a perforated pipe distributor with adequate open area?• If a low overhead entrainment rate must be met, has a deentrainment device been provided?• For all internals that the vendor must supply, does the Design Specification contain a table that lists the necessary

vapor/liquid rates and physical properties?• Are all process nozzles correctly sized?• Are vessel manholes appropriately specified?• Is vapor distribution taken into account (as in Section III-G) in the design of collector trays and feed pipes?

• Is selection and installation of internals consistent with the guidelines provided in Section III-G, under SPECIALDESIGN CONSIDERATIONS FOR PACKING?




SPECIAL CONSIDERATIONS• Has the potential for pyrophoric fires been assessed? Have proper cleaning procedures been used to remove

pyrophoric materials (such as pyrophoric iron sulfide deposits) before the column is opened up?• Have venting and draining of internals been taken into account?• Are all tower internals (most importantly, distributors) designed to be accessible for inspection and cleaning?

dp03a

Documents

n t e r n

f o r e n t r ai n

ns f e r s e r vic e

n d r e va

u n t e r cu r

o n f r

n t t yp e s

t tra n s f e r