Tray Column Design

LKMT

Industry Academia Workshop on “Design of Refinery Distillation Columns“

October 23rd – 24th 2013

Di till ti E i t D i M th d l d P tiDistillation Equipment Design : Methodology and Practices

S K Saxena AGMS.K. Saxena, AGMEIL, New Delhi

Selection of contacting device for Columns

The selection process for tower internals can beThe selection process for tower internals can bestraightforward for some designs, but it could bedifficult for others as It depends on variousfactors like whether it’s a Grass root design or arevamp activity.

21 October 2013 2

Choices Available (Contacting Devices)

There are two broad categories of contacting devices:

• Trays (Vales/Sieves/Bubble Caps etc.)• Packed Beds (Structured or Random Packing)

21 October 2013 3

Contacting Devices:

PackingPacking TraysTrays

21 October 2013 4

Selection of contacting device depends upon

• Operating pressure

• Turndown Ratio• Turndown Ratio

• Foaming Tendency

• Solids present, dirty or polymerized solution handling

• Heat removal requirementsq

21 October 2013 5

A Typical Tray column :

21 October 2013 6

Types of Trays

ValveSieve

Trays with Down comer

SieveBubble Cap

Shed- DeckTrays without Down comer

Dual FlowBaffleDisc & DonutRi l

Shed Deck

Ripple

Multi down comer traysHigh capacity Trays (Proprietary Design)

Non Fractionation TraysCollector/ Chimney Trays

Pro valve, Triton, Nye, SuperFrac, MVG

21 October 2013 7

Non- Fractionation Trays

Valve Trays:

A li ti

Vapor Vapor and Liquid Contacting and Liquid Contacting Mechanism in Valve TraysMechanism in Valve Trays

Applications:

• Offer high capacity & good efficiency• Good turndown characteristics• Good turndown characteristics• Maximum application-widely used in almost all applications.• Cheaper and simple in fabrication.

21 October 2013 8

Sieve Trays:

VaporVapor and Liquid Contactingand Liquid Contacting Mechanism in SievesMechanism in SievesVapor Vapor and Liquid Contacting and Liquid Contacting Mechanism in SievesMechanism in Sieves

Applications:

• System where high capacity near design are to be maintained incontinuous service.

• Poor turndown characteristics as compared to valve trays• Suitable for Liquid-liquid extraction applications

21 October 2013 9

q q pp

Bubble Caps:

Vapor Vapor and Liquid Contacting and Liquid Contacting Mechanism in Bubble Mechanism in Bubble CapsCaps

A li tiApplications:

• Suitable for all services, except extremely coking, polymer formationor other high fouling conditionsor other high fouling conditions.

• Used for extremely low flow conditions i.e. good turndown characteristics& offer maximum flexibility in flow range.

21 October 2013 10

Advantages of Trays

• Tray columns exhibit wider operating range than packed columns i.e. having Potential to handle vapor & liquid loadingsliquid loadings

• Very low liquid rates leads to incomplete wetting ofVery low liquid rates leads to incomplete wetting of packing as compared to trays thus having higher contacting efficiency

• High residence time provided for liquid on trays

• If solids are present in the fluid, tray columns can be designed to permit easier cleaning.

21 October 2013 11

g p g

Comparison of Commonly Used Trays

Parameters Bubble* Sieve ValveCapacity % 100 120-150 120-150

Efficiency % 100 105 115 105 115Efficiency % 100 105-115 105-115

Flexibility or turndown

10:120:1 (possible)

2:13:1 ( possible)

4:16:1 (possible)

Entrainment 300 100 Least

Cost % 100 70-85 70

Fouling service suitability

Fair Good (for large holes)

Good

Tray spacing 24”-36” 20”-30” 18”-30”

*With bubble cap trays as base case*With bubble cap trays as base case

21 October 2013 12

OTHER TYPE OF OTHER TYPE OF TRAYS:TRAYS:

1. Baffle Trays / Shed deck

2. Ripple Tray

3 Disc and Donut tray3. Disc and Donut tray

4. Dual Flow tray

21 October 2013 13

Applications Other Types of Trays

Type Applications

Dual flow /Ripple •Same as for sieve traysDual flow /Ripple trays

Same as for sieve trays.•Higher capacity but low efficiency•Poor turndown characteristics

Baffle trays / shed deck/ Disc & Donut

•Good for heat transfer applications•Can handle coke & solids.

21 October 2013 14

CLASSIFICATION OF TRAYS

Based on Liquid Flow Paths:

Single PassSingle Pass Two PassTwo Pass

21 October 2013 15

CLASSIFICATION OF TRAYS

Three PassThree Pass Four PassFour Pass

21 October 2013 16

High Capacity Trays (HCT)

HCT

21 October 2013 17

Difference in Configuration:

Major difference in Conventional & High capacity trays are due to modification in the type/ construction of the internals :

•Modification in active area

•Modification in down comer area

R d ti i T S i•Reduction in Tray Spacing

21 October 2013 18

Types of HCT :

By modifying Active Area By Modifying Down Comer Area

Bifrac & Superfrac (M/s Koch)

Provalve (M/s Norton)

Maxfrac (M/s Koch)

Triton (M/s Norton)( )

MVG (M/s Nutter)

( )

MD, ECMD & VGMD (M/s UOP)

Hi Fi (M/ S l )Hi-Fi (M/s Sulzer)

Nye (M/s Glitch)

21 October 2013 19

Claims by HCT Vendors:

Vendor Tray Capacity increase (Upto)

Koch-GlitschNorton

NyeBifracMaxfrac

15-20%25%25%

SuperfracTritonProvalve

30%25%20%Provalve 20%

UOP ECMDVGMD

20%30%VGMD 30%

Sulzer MVG 20%

21 October 2013 20

Various types of High Capacity Valves

MVG (MINI V GRID) PROVALVEMVG (MINI V-GRID) PROVALVE

21 October 2013 21

Continuation….

O Hi Fi T b S lMD Trays by UOP Hi-Fi Trays by Sulzer

21 October 2013 22

Continuation….

ECMD Trays by UOP

21 October 2013 23

Comparison between various types of Trays

Tray Type Capacity Efficiency Cost / unit area

Flexibility Remarks

Bubble Cap Med. to High except Med. To High High ( almost 3/1 to 4/1 Use for high flexibilityplow to med. @ high liquid rates

twice the cost of sieve trays)

Sieves Med. to High High, Equal to or better than others

Lowest of all trays 2/1 to 3/1 First choice for mostapplications;

Valves Med. to High High, as good as sieves

Medium (About 110% of sieve trays)

High.Possibly up to5/1.

Not recommended forfouling services.

UOP-MD, Very High, estimated Low to Medium Higher than valve Low. (< 2/1) Consider for revampsUOP MD,UOP-ECMD,Sulzer Hi-Fi

y g ,to be 30-40% than a conventional sievetrays for high liq. rates

gtrays

( ) pwhere no other device is acceptable. Low Tray Spacing, Non-fouling services

SuperFrac Very High estimated Medium More than 105% Medium Good alternative toSuperFrac,MVG, ProValve,Triton

Very High, estimated to be 10-15 % than a conventional sievetrays for high liq. rates

Medium More than 105% of sieve trays

Medium.Slightly higherthan sievetrays.

Good alternative to sievetrays at low liquid ratewhere higher capacity is needed.

21 October 2013 24

Packed Column

Liquid DistributorLiquid Distributor

Packing

Liquid Collector/ Re-distributor

Support Plate

Bed Limiter

S t Pl tSupport Plate

21 October 2013 25

Applicability of Packed Columns in a Refinery:

• For column < 900 mm I.D: Packing are usually cheaper than trays.

• Acids & many other corrosive materials can be readily handled in packed column. Because materials like ceramic, carbon or other acid resistant material can be used in packed beds.

• Hold-up of liquid can be quite low in packed column, an p q q p ,advantage when liquid is thermally sensitive.

• Liquid tending to foam may be handled more readily inLiquid tending to foam may be handled more readily in packed column because of relatively low degree of liquid agitation by gas.

21 October 2013 26

Contd….

• Potential to handle high throughput at high efficiency.

• Packing itself acts as a foam breaker.Packing itself acts as a foam breaker.

• Extensive use in revamp application. However. acceptable in grass-root design alsograss-root design also

• Offer low P/ unit theoretical stage.

21 October 2013 27

Historical Generation of Packing in Industries

Packing

1st Generation 2nd Generation 3rd Generation

g

Raschig Rings

Lessing RingsPall Rings

HyPak GemPakFlexiPak

Cross Partition RingsBerl Saddles

IMTPCMRN tt Ri

MellaPakParlPak

Nutter Rings

21 October 2013 28

Types of Random Packing

INTALOX SADDLEIMTP INTALOX SADDLE

PALL RING RASCHIG RING

21 October 2013 29

PALL RING RASCHIG RING

Structured Packings

EIL’s Parlpak

21 October 2013 30

Comparison between various types of Packings

Packing Type

Capacity Efficiency Cost / unit area

Flexibility Remarks

Random Packing(Pall ings, Metal Intalox, etc.)

Medium Medium Medium to Low > 3/1 - Good efficiency perunit of pressure drop.

- Mainly used in Absorbers where liquid loads are highloads are high

Structured Packing(FlexiPac

High to Very High High Medium(Varies with Metallurgy)

>3/1 Best efficiency per unit of pressure drop.

(FlexiPac,GemPak, MellaPak etc.)

eta u gy)

Grid Very High Good in Medium to High < 2/1 -Good for high vapor &Grid(FLexiGrid, Intalox Grid, etc.)

Very High Good in Entrainment

removal & Heat Transfer

applications only.

Medium to High 2/1 Good for high vapor & low liquid service .

- Used in wash zones of HC fractionators where Coking formation

21 October 2013 31

predominates

Criteria for Internal selection:

Following criterions help the designer in selecting the best internal for a given application depends upon the variousinternal for a given application, depends upon the various factors but primarily on whether it’s a :

- A Grass Root Design, or - A Revamp Activityp y

21 October 2013 32

Internal Selection for New tower (Table-1):

21 October 2013 33

Internal Selection for New tower (Table-2) :

21 October 2013 34

Internal Selection for a Revamp service (Table -3):

21 October 2013 35

Internal Selection for a Revamp service (Table-4):

21 October 2013 36

Comparison between Trays & PackingParametersParameters TraysTrays Packed columnPacked columnParametersParameters TraysTrays Packed columnPacked column

RandomRandom StructuredStructuredService -Process with exothermic

reactions- HP AbsorbersP A d (HT)

-Vacuum serviceRreactions

- Fouled service- High no. of liquid withdrawals- High turndown & low liquid rates

- Pump Around (HT)- Vacuum services

-Revamps-High no. Of stages.-H.P absorbers

Capacity

Pressure Drop

rates.

1.0 1.0-1.15

1/3-1/5

1.2-1.3

1/5-1/10Pressure Drop (P)

Column size

1.0

1.0

1/3-1/5

0.9

1/5-1/10

0.8

Cost Internals Columns

1.01.0 1.2

0.91.5*0.9

21 October 2013 37

Focus on Design of Trays

Before proceeding for the design of a tray tower, the Design Specialist needs the following data from his process counterpart:

a. Vapor Loads entering the trays and its properties viz. temperature, pressure, mol. wt. & compressibility factor.

b. Liquid Loads leaving the tray and its properties viz. density, q g y p p y,viscosity, surface tension etc.

c. Allowable pressure drop per tray.d. Turn down/ turn up requiremente. Foaming tendencyf. Material of constructiong. Fouling Characteristicsg gh. Any special requirements like heat transfer etc.

21 October 2013 38

Tray Geometry: Major Components

Following are the major components of the tray :

• No of PassesNo. of Passes• Active Area • Down comers• Number of valves• Number of valves• Pitch/Layout• Calming zone• Inlet & Outlet Weir• Inlet & Outlet Weir• Tray Spacing• Under Down comer Flow Clearance (UDFC)• Column Diameter• Column Diameter• Column Height• Other additional features like chimney tray, distributors etc.

21 October 2013 39

Typical Tray Layout

Plan View

Elevation View

21 October 2013 40

Tray design :

Following are the 3 broad categories that influence the design of the tray:design of the tray:

a.a. Parameters affecting vapor capacityParameters affecting vapor capacityg p p yg p p yb.b. Parameters affecting liquid capacityParameters affecting liquid capacityc.c. Other Important parametersOther Important parameters

The same can be shown clearly from this graph…..

21 October 2013 41

Typical Performance Diagram

Area of Normal Operationapor

Rat

e

Area of Normal Operation

Moderate Weeping

Va

Heavy Weeping

21 October 2013 42

Liquid Rate

Tray design : Effect of Vapor Loadings

There are certain limitations on the vapor handling capacity of the column about which the designer shall be fully aware of before finalizing the configuration ofbe fully aware of before finalizing the configuration of the Column:

/ fThe main controlling/limiting factors are :

•Jet Flood•Flow Regimes•Entrainment

21 October 2013 43

Parameters affecting Vapor Loadings: Jet flood

Jet Flood is the state of entrainment when the down comers can't handle adequate liquid flow and liquid backs up and fills the trays & the vapor-liquid contacting mass of a tray reaches the one tray p q g y yabove, resulting in massive entrainment and liquid recycling which eventually affects the distillation and overloads the down comers.

Effect of Jet Flood on Column performance:

• This can result in loss of bottoms level low tray efficiency high tower pressure• This can result in loss of bottoms level, low tray efficiency, high tower pressure drop, control problems, etc.

• If the down comer is too small or tray pressure drop is too high flooding can beIf the down comer is too small or tray pressure drop is too high, flooding can be caused by down comer backup even if entrainment is small.

21 October 2013 44

Jet Flood Vs Column Efficiency

21 October 2013 45

Parameters affecting Vapor Loadings: Flow Regimes

Different flow regimes can occur on a tray in a column.

Following are the prominent flow regimes that can be encountered in aFollowing are the prominent flow regimes that can be encountered in a column depending on Vapor-liquid flow rates:

E l i i ti i il t l l iti d• Emulsion regime operation occurs primarily at low vapor velocities and high liquid rates i.e. the liquid phase is continuous

• Froth regimes occurs primarily when vapor passes through the liquidFroth regimes occurs primarily when vapor passes through the liquid on the tray as discrete bubbles of irregular shape, formed at the tray perforations and are swept away by the froth

• Spray regime operation occurs primarily at high vapor velocities and low liquid rates i.e. In the spray regime, the vapor phase is continuous.

21 October 2013 46

Flow Regime within Normal Operating Range

21 October 2013 47

Effect on Efficiency:

From the above graph it is clear that:

•Froth regime is the most desirable operating regime for an efficient performance of a column.

•Operation in the Spray regime can be very detrimental to good tower performance as tray efficiency dropping sharplyperformance as tray efficiency dropping sharply.

21 October 2013 48

Parameters affecting Vapor Loadings: Entrainment

Entrainment occurs because of inadequate vapor space above thevapor-liquid contacting zone on a tray. Liquid doesn't fully disengagefrom the vapor, and is carried to the tray above.

Eff t f E t i t C l P fEffect of Entrainment on a Column Performance:

• Entrainment becomes more prevalent when tower vapor loads increase. p p• Massive entrainment leads to jet flooding.

21 October 2013 49

Entrainment in a Column:

21 October 2013 50

Tray design : Effect of Liquid Loadings

Similar to vapor loadings, there are certain limitations on the liquid handling capacity of the column as well:

Liquid loading effect the following parameters :

•Down comer Flood•Down comer Flood•Under Down Comer Velocity•Down Comer Back Up limits•Down Comer Back-Up limits

21 October 2013 51

Liquid Handling Limitations: Down Comer Flood

Down Comer Flood is the criterion that determines how close a tower is to flooding as a result of excessive liquid height in the down comer.

or,

Percent down comer flood represents the ratio of the actual vapor and liquid feed rates to the feed rates that would result in p q100% down comer froth backup.

21 October 2013 52

Down Comer Filling in a Column

Down comer filling (Hd) is defined as the clear liquid height in the down comer.

Hd = Ht + Hud + Hi + Hdc

And, Dry Tray Pressure drop (Ht) is given by :

Ht = Hed + Hc

where;Hd - Down Comer Filling

Ht = Hed + Hc

Ht - Tray pressure drop,Hud - Head loss under the down comer,Hi - Inlet head on the tray,Hdc - Head loss due to flow through the down ComerHdc - Head loss due to flow through the down ComerHed - Dry tray pressure drop Hc - Clear liquid height

21 October 2013 53

Each head is expressed in inches (mm) of hot clear liquid.

Typical in a down comer

21 October 2013 54

DC Back Up:

This is the measure of the aerated liquid in the down comer, calculated in terms of clear liquid height.

• Backup is a function of dry and wet tray pressure drop and head loss under the down comer.

• If the restrictions to flow are too large and/or a foamy system is involved, the aerated liquid in the down comer can backup to the level of the outlet weir and cause tower flooding.

• Calculated backup should generally not exceed about half of the tray spacing

L b k h ld b d f f t• Lower backups should be used for foamy systems

21 October 2013 55

Other Important Parameters:

There are many other important parameters which shall be satisfied or shall be taken care of in the design viz.

• Tray Area & Efficiency• Tray Spacing• Turn-down Ratio / weeping• Tray pressure Drop• F li• Fouling• Foaming• OthersOthers

21 October 2013 56

Tray Area:

Mainly 2 Components ascertain Tray Area:

1) Calming Area : This is an area preserved for vapor1) Calming Area : This is an area preserved for vapor disengagement prior to liquid entering a down comer. The un-perforated narrow strip of active area next to the outlet weir is considered to be calming area.

2) Acti e Area : This is the tray deck area enclosed by the tower2) Active Area : This is the tray deck area enclosed by the tower wall, the outlet weirs, and the edges of the inlet areas. Valves or sieve holes are located in the active area

21 October 2013 57

Tray capacity & Tray efficiency:

Tray capacity and tray efficiency are the two most important criteria in tray design:

• Diameter of a tower is mainly determined by capacityconsiderations,• Tray efficiency determines the number of actual trays and• Tray efficiency determines the number of actual trays, andtherefore the height, of the tower.

• To achieve good efficiency, the designer must optimizeg y, g pthe weir height, open area, bubble area, liquid flow pathlength, number of liquid passes, and other variables.

21 October 2013 58

Tray Spacing:

Tray spacing along with Column Diameter is an important parameter that set the capacity of the tower.

•As the distance between trays is increased, the tower capacity will increase and the column diameter could be reduced.• Low tray spacing can also be used but this increases the column• Low tray spacing can also be used, but this increases the column diameter as to handle a given set of vapor and liquid loadings. Also, such low spacing may lead to difficult maintenance.

T i ll t i f 24 i h i f d• Typically , a tray spacing of 24 inches is preferred.

Therefore, Selection of optimum tray spacing is a function ofTherefore, Selection of optimum tray spacing is a function of column diameter and service (clean or fouled) and most importantly depends upon the experience of the equipment designer/databank

21 October 2013 59

designer/databank.

Turndown Ratio :

Turndown Ratio is the term used to define the range ofloadings over which acceptable tray performance is achieved.This usually means the range over which the tray efficiencystays at or above the design value.y g

Note : While deciding the turndown ratio, a specialist must checkthe weeping efficiency curve that how much design efficiency isobtained at turn down.

21 October 2013 60

Effect of Weeping:

This is the passage of liquid through the tray deck seams, and valve or sieve holes.

• It occurs more when vapor rates are low, and is the primary reason sieve trays lose efficiency at turndown operation.

• When 25% of the liquid load weeps through the tray, one should expect significant loss of tray efficiencyshould expect significant loss of tray efficiency

21 October 2013 61

Effect of weeping on Efficiency:

21 October 2013 62

Foaming Factor:

Foaming can occur in distillation towers via several different mechanisms including:

• Presence of surface active materials.• Presence of solids.Presence of solids.• Entrainment of HCs liquids into aqueous systems.• Condensation of HCs vapors into aqueous systems.

To design for foaming, the tray is usually oversized by using a lower percent of jet flooding and down comer flood, a low dry tray pressure drop a low down comer entrance velocity and a reduced allowabledrop, a low down comer entrance velocity, and a reduced allowable down comer filling.

21 October 2013 63

Packed Column Design :

Following are the major parameters that influence the design of the packed columns :design of the packed columns :

a.a. Vapor & Liquid Capacity limitationsVapor & Liquid Capacity limitationsp q p yp q p yb.b. Efficiency & Turn downEfficiency & Turn downc.c. Heat TransferHeat Transferd.d. Other considerationsOther considerations

21 October 2013 64

Packed Column DesignPacked Column Design: Effect of Vapor/: Effect of Vapor/LiqLiq Capacity Capacity

Phenomena of Flooding in Packed Beds

(In both Random Packing or Str ct red Packing)(In both Random Packing or Structured Packing)

Flooding occurs when liquid begins to accumulate in theFlooding occurs when liquid begins to accumulate in thepacking and the pressure drop begins to rise more sharply.With further increases in vapor rate, the pressure drop risesp p palmost vertically and liquid begins to “pile up” on the top ofthe packing.

21 October 2013 65

Packed Column Design: Packed Column Design: Efficiency & Turn DownEfficiency & Turn Down

• For most applications, random packing should beconsidered first because they are lower costconsidered first because they are lower cost.

• If a low pressure drop per theoretical stage and/or a shortcolumn height is required, then structured packing shouldg q , p gbe evaluated as an option.

• To ensure optimum efficiency at all rates, a high qualityli id di t ib t t b dliquid distributor must be used.

• The selection of a liquid distributor is critical in the design ofa packed beda packed bed

21 October 2013 66

Packed Column Design: Packed Column Design: Other FactorsOther Factors

There are various parameters that also influence the Design of the packed columns, for instance:Design of the packed columns, for instance:

a.a. Heat transfer RequirementHeat transfer Requirementqqb.b. Vapor & Liquid distributionVapor & Liquid distributionc.c. Fouling , etc.Fouling , etc.

21 October 2013 67

Future Challenge & Trends

Challenges lying ahead for this industry:

•Lower capital cost & optimized design•Energy efficient hardware•Higher continuous run length•Flexibility to handle wide variations in loads.

21 October 2013 68

21 October 2013 69

21 October 2013 70