Distillation Part2

DISTILLATION COLUMNValve trays In valve trays, perforations are covered by liftable caps. Vapor flows lifts the caps, thus self creating a flow area for the passage of vapor. The lifting cap directs the vapor to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays. Sieve trays Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters.

DISTILLATION COLUMNBecause of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications.The next few figures show the direction of vapor and liquid flow across a tray, and across a column.

DISTILLATION COLUMNEach tray has 2 conduits (may have more), one on each side, called downcomers. Liquid falls through the downcomers by gravity from one tray to the one below it. A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid.Being lighter, vapor flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapor on each tray is called the active tray area.

DISTILLATION COLUMNProprietary High Efficiency Trays Manufacturers have introduced numerous improvements to the basic tray design. Here are some samples.Koch Glitsch Bi-FRAC TraysNorPro Provavle TraysSulzer VortexTray

DISTILLATION COLUMNTrays and Plates The terms "trays" and "plates" are used interchangeably. There are many types of tray designs, but the most common ones are :

Bubble cap trays A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapor. Vapor rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.

DISTILLATION COLUMNPacking Packing are devices that are designed to increase the interfacial area for vapor-liquid contact without causing excessive pressure-drop across a packed section. Packing are generally divided into three classes:Random or dumped packingStructured PackingGrids

DISTILLATION COLUMNRandom Packing Random packing are discrete pieces of packing of a specific geometrical shape which are dumped or randomly packed into the column shell.

Random packing such as the Pall rings and saddles above can be made from many kinds of material including plastics, ceramics and stainless steels.

DISTILLATION COLUMNStructured Packing These are crimped layers of wire mesh or corrugated sheets.

Sections are normally stacked in the column.

DISTILLATION COLUMNGrid Packing These are also systematically arranged packings, but instead of wire mesh or corrugated sheets, these use an open-lattice structure.

Sections are normally stacked in the column.

DISTILLATION COLUMNPacked Column Internals The performance of packing depends greatly on the quality of the distribution of liquid and vapor.This in turn is a function of the column internals.

DISTILLATION COLUMNPackings versus Trays 1. Smaller pressure drops per unit length: PACKED (e.g. 0.04 psi/stage for packed; 0.15 psi/stage for tray; important issue for vacuum distillat'n) 2. Smaller liquid holdup (susceptibility to flooding): PACKED (e.g., important issue for batch distillation) 3. Higher L/G ratio (slope of operating line) PACKED; or MULTIPASS TRAYS 4. Lower L/G ratio: TRAY 5. Easier cooling of liquid: TRAY 6. Easier side stream withdrawal (plus easier incorporation of other tower internals such as inter-reboilers and cooling coils): TRAY 7. Better adaptation to L/G systems that foam: PACKED 8. Better for L/G systems that corrode: PACKED (random) 9. Easiest for cleaning: TRAY 10.Operation for small column diameter: PACKED (maintenance issues)

DISTILLATION COLUMNPackings versus Trays 11. Operation for large column diameter: TRAY (packings have maldistribution problems when diameter is too large) 12. Presence of solids TRAY (solids can clog void space of packed beds) 13. Feed composition variation: TRAY (feed tray position can be varied) 14. Ability to predict column performance/more accommodating of design errors TRAY 15. Chemical reactions TRAY (Larger holdup, or residence time, of liquid) 16. Lower weight: TRAY 17. Effect of temperature gradient between ambient and internal: less significant for TRAYS (expansion/contraction of shell can crush packings.) 18. Lower capital costs (often): TRAY

DISTILLATION COLUMNRandom Versus Structured Packing1. At low liquid rates ( 10 gpm/ft2, > 100-200 psia. 5. Structured packings perform less well with aqueous liquid systems because of poorer wetting on the surface of metal structured packings. .

DISTILLATION COLUMNRandom Versus Structured Packing. 6. Liquid holdup: similar values for structured and random packings. 7. Structured packings are very susceptible to corrosion. 8. Structured packings are much less sensitive to surges and plant upsets. A packed tower operating in the loading region can be easily induced into the flooding region of operation with plant upsets. 9. Inspection and maintenance of structured packings are much more difficult than random packings. 10. Cost: A tradeoff exists. The cost per unit mass is 3-10 times more expensive for structured compared to random packings; but, the former is more efficient (lower HETP) due to lower pressure drop. Also, pumping costs are less for structured packings because of the lower pressure drop and the shorter columns.

DISTILLATION COLUMNTray and Packing Manufacturers in the Net

www.sulzerchemtech.comwww.koch-glitsch.comwww.nortoncppc.comwww.jaeger.comwww.vff.de

DISTILLATION COLUMNCOLUMN REBOILERS

There are a number of designs of reboilers. They can be regarded as heat-exchangers that are required to transfer enough energy to bring the liquid at the bottom of the column to boiling point. The following are examples of typical reboiler types.

DISTILLATION COLUMN

DISTILLATION OPERATIONFACTORS AFFECTING DISTILLATION COLUMN OPERATION The performance of a distillation column is determined by many factors, for example: feed conditions state of feed composition of feed trace elements that can severely affect the VLE of liquid mixtures internal liquid and fluid flow conditions state of trays (packings)

DISTILLATION OPERATIONFeed Conditions The state of the feed mixture and feed composition affects the operating lines and hence the number of stages required for separation. It also affects the location of feed tray. During operation, if the deviations from design specifications are excessive, then the column may no longer be able handle the separation task. To overcome the problems associated with the feed, some column are designed to have multiple feed points when the feed is expected to containing varying amounts of components.

DISTILLATION OPERATIONReflux Conditions As the reflux ratio is increased, the gradient of operating line for the rectification section moves towards a maximum value of 1. Physically, what this means is that more and more liquid that is rich in the more volatile components are being recycled back into the column. Separation then becomes better and thus less trays are needed to achieve the same degree of separation. Minimum trays are required under total reflux conditions, i.e. there is no withdrawal of distillate.

DISTILLATION OPERATIONOn the other hand, as reflux is decreased, the operating line for the rectification section moves towards the equilibrium line. The pinch between operating and equilibrium lines becomes more pronounced and more and more trays are required.The limiting condition occurs at minimum reflux ratio, when an infinite number of trays will be required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty).

DISTILLATION OPERATIONInternal Liquid and Vapor Flows Internal Liquid and Vapor flows can affect the hydraulic performance of trays.

DISTILLATION OPERATIONFlooding (Priming) Flooding is brought about by excessive accumulation of liquid inside the column.This accumulation is generally caused by one of the following mechanisms:Spray entrainment floodingFroth entrainment floodingDowncomer backup floodingDowncomer choke flooding

DISTILLATION OPERATIONSpray Entrainment FloodingAt low liquid flow rates, trays operate in the spray regime, where most of the liquid on the tray is in the form of liquid drops.As vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above.The liquid accumulates on the tray above instead of flowing to the tray below.

DISTILLATION OPERATIONFroth Entrainment FloodingAt higher liquid flow rates, the dispersion on the tray is in the form of a froth.When the vapor velocity is raised, froth height increases.As this surface approaches the tray above, entrainment rapidly increases causing liquid accumulation on the tray above

DISTILLATION OPERATIONDowncomer Backup FloodingAerated liquid is backed up into the downcomer because of tray pressure drop, liquid height on the tray and frictional losses in the downcomer apron.All of these increase when liquid flow rate is increased, while tray pressure drop also increases when vapor flow rate is raised.When the backup of aerated liquid in the downcomer exceeds tray spacing, liquid accumulates on the tray above.

DISTILLATION OPERATIONDowncomer Choke FloodingAs liquid flow rate increases, so does the velocity of aerated liquid in the downcomer.When this velocity exceeds a certain limit, friction losses in the downcomer and downcomer entrance become excessive, and the frothy mixture cannot be transported to the tray below.This causes liquid accumulation the the tray above.

DISTILLATION OPERATIONWeeping/Dumping This phenonmenon is caused by low vapor flow. The pressure exerted by the vapor is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started.

DISTILLATION OPERATIONState of Trays and Packings The actual number of trays required for a particular separation duty is determined by the efficiency of the plate, and the packings if packings are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus appropriate materials should be specified for tray construction.

EXPERIENCED BASED RULESTray TowersA. For ideal mixtures, relative volatility can be taken as the ratio of pure component vapor pressuresB. Tower operating pressure is most often determined by the cooling medium in condenser or the maximum allowable reboiler temperature to avoid degradation of the process fluidC. For sequencing columns:1. Perform the easiest separation first (least trays and lowest reflux)2. If relative volatility nor feed composition vary widely, take products off one at time as the overhead3. If the relative volatility of components do vary significantly, remove products in orderof decreasing volatility4. If the concentrations of the feed vary significantly but the relative volatility do not, remove products in order of decreasing concentration.D. The most economic reflux ratio usually is between 1.2Rmin and 1.5RminE. The most economic number of trays is usually about twice the minimum number of trays.

EXPERIENCED BASED RULESTray TowersF. Typically, 10% more trays than are calculated are specified for a tower.G. Tray spacings should be from 18 to 24 inches, with accessibility in mindH. Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures, or 6 ft/s (1.8 m/s) under vacuum conditions.I. A typical pressure drop per tray is 0.1 psi (0.007 bar)J. Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and stripping typically have efficiencies closer to 10-20%K. The three most common types of trays are valve, sieve, and bubble cap. Bubble cap trays are typically used when low-turn down is expected or a lower pressure drop than the valve or sieve trays can provide is necessary.L. The most common weir heights are 2 and 3 in and the weir length is typically 75% of the tray diameter

EXPERIENCED BASED RULESTray TowersF. Typically, 10% more trays than are calculated are specified for a tower.G. Tray spacings should be from 18 to 24 inches, with accessibility in mindH. Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures, or 6 ft/s (1.8 m/s) under vacuum conditions.I. A typical pressure drop per tray is 0.1 psi (0.007 bar)J. Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and stripping typically have efficiencies closer to 10-20%K. The three most common types of trays are valve, sieve, and bubble cap. Bubble cap trays are typically used when low-turn down is expected or a lower pressure drop than the valve or sieve trays can provide is necessary.L. The most common weir heights are 2 and 3 in and the weir length is typically 75% of the tray diameter

EXPERIENCED BASED RULESTray TowersM. Reflux pumps should be at least 10% overdesignedN. The optimum Kremser absorption factor is usually in the range of 1.25 to 2.00O. Reflux drums are almost always horizontally mounted and designed for a 5 min holdup at half of thedrum's capacity.P. For towers that are at least 3 ft (0.9 m) is diameter, 4 ft (1.2 m) should be added to the top for vapor release and 6 ft (1.8 m) should be added to the bottom to account for the liquid level and reboiler returnQ. Limit tower heights to 175 ft (53 m) due to wind load and foundation considerations.R. The Length/Diameter ratio of a tower should be no more than 30 and perferrably below 20S. A rough estimate of reboiler duty as a function of tower diameter is given by:Q = 0.5 D2 for pressure distillationQ = 0.3 D2 for atmospheric distillationQ = 0.15 D2 for vacuum distillationwhere Q is in Million Btu/hr and D is tower diameter in feet

EXPERIENCED BASED RULESPacked TowersA. Packed towers almost always have lower pressure drop than comparable tray towers.B. Packing is often retrofitted into existing tray towers to increase capacity or separation.C. For gas flowrates of 500 ft3/min (14.2 m3/min) use 1 in (2.5 cm) packing, for gas flows of 2000 ft3/min (56.6 m3/min) or more, use 2 in (5 cm) packingD. Ratio of tower diameter to packing diameter should usually be less than 15E. Due to the possibility of deformation, plastic packing should be limited to an unsupported depth of 10-15 ft (3-4 m) while metallatic packing can withstand 20-25 ft (6-7.6 m)F. Liquid distributor should be placed every 5-10 tower diameters (along the length) for pall rings and every 20 ft (6.5 m) for other types of random packings