Trays and Packed Towers

7/27/2019 Trays and Packed Towers

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Course Manual Crude Distillation (Tray and Packed Tower)

Chapter 3

Tray and Packed Towers Description

MIDOR (Rev.0) Jan. 2006 Chapter # 3 Page # 1 of 72


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Chapter 3 Contents

3.1 Choice of Trays or Packing

3.2 Tray Towers3.2.1 Tower Functions3.2.2 Arrangement of Processing Steps3.2.3 Tower Design Considerations3.2.4 Tower Trays

3.2.4.1 Types of Trays3.2.4.2 Selection of Tray Type3.2.4.3 Tray Layout

3.2.4.4 Tray Stability3.2.4.5 Operating Range3.2.4.6 Satisfactory Operating Region3.2.4.7 Tray Construction3.2.4.8 Material of Construction3.2.4.9 Tray Design3.2.4.10 Tower Internals

3.3 Packed Towers

3.3.1 Packed Tower Function3.3.2 Types of Packing3.3.3 Tower Internals

3.3.3.1 Packing Support3.3.3.2 Liquid Distributors3.3.3.3 Liquid Redistributors3.3.3.4 Hold-down Plates

3.3.4 Tower Auxiliaries



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Chapter 3

Tray and Packed Towers Description

3.1 CHOICE OF TRAYS OR PACKING

The choice between a tray and backed tower for a particular application can only bemade with complete assurance by costing each design. However, this will notalways be worthwhile, or necessary, and the choice can usually be made, on the basisof experience by considering main advantages and disadvantages of each type; whichare listed below:

1. Plate towers can be designed to handle a wider range of liquid and gas flow-

rates than packed towers.2. Packed towers are not suitable for very low liquid rates.

3. The efficiency of a tray can be predicted with more certainty than theequivalent term for packing (HETP or HTU).

4. Plate towers can be designed with more assurance than packed towers. Thereis always some doubt that good liquid distribution can be maintainedthroughout a packed tower under all operating conditions, particularly in largetowers.

5. It is easier to make provision for the withdrawal of side-streams from traytowers; coils can be installed on the trays.

6. It is easier to make provision for the withdrawal of side streams from traytowers.

7. If the liquid causes fouling, or contains solids, it is easier to make provision forcleaning in a tray tower; man ways can be installed on the trays. With smalldiameter towers it may be cheaper to use packing and replace the packing

when it becomes fouled.

8. For corrosive liquids a packed tower will usually be cheaper than theequivalent plate tower.

9. The liquid hold-up is appreciably lower in a packed tower than a plate tower.This can be important when the inventory of toxic or flammable liquids needsto be kept as small as possible for safety reasons.



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10.Packed towers are more suitable for handling foaming systems.

11.The pressure drop per equilibrium stage (HETP) can be lower for packing thanplates; and packing should be considered for vacuum towers.

12.Packing should always be considered for small diameter towers, say less than0.6m, where trays would be difficult to install, and expensive.

3.2 TRAY TOWERS

A. Towers Functions

3.2.1 Fractionating Tower

Is used in referring to a counter-current operation in which a vapor mixture isrepeatedly brought in contact with liquid having nearly the same composition as therespective vapors.

Atmospheric Distillation "Tower"

Is the first step in any petroleum refinery, in which the separation of the crude oil intovarious fractions. These fractions may be products in their own right or may be feed

stocks for other refining or processing units.

Vacuum Distillation "Tower"

Is used to reduce the temperature for the distillation of heat-sensitive materials andwhere very high temperatures would otherwise be needed to distill relatively nonvolatile materials.

Stabilization "Tower"

It is a fractionation operation conducted for the purpose of removing high-vaporpressure components.

Splitting "Tower"

It is a simple distillation process, in which separation of naphtha into two streamsbefore further processing can take place.



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Stripping "Tower"

Is the process where the requirements, to strip a volatile component or group of

similar components from a relatively non-volatile solution or product by the action ofstripping gas or steam.

3.2.2 Arrangement of Processing Steps:

This applies only to situations where two or more towers in sequence are beingconsidered. Single tower are designed on the basis of the available feed, but in anymultiple product distillation train, there are a number of ways in which the towerscan be arranged. Consider, for example, a three component system consisting of

propane, isobutene and n-butane which must be separated into relatively pure

components. Two routes can be used. The first tower can be depropanizer yielding apropane product distillate followed by a C4 splitter (deisobutanizer). Alternately, thefirst tower can be a deisobutanizer yielding an n-butane bottoms product followed bya depropanizer.

The operating conditions and relative equipment sizes for the two are shown inFigure 3.1. By inspection, it is obvious that Method 1 is the better process design

because:

1. Equipment costs will be lower since the large tower will be designed for a

lower pressure.2. Operating costs will be lower since the deisobutanizer condenser load will be

approximately half that of Method 2. This will reflected in a lower reboilerduty for Method 1.

3.2.3 Tower Design Considerations

Briefly, here are some of the factors that dictate the design of a column.

As you might guess, two important factors are

1. The throughput, or flow rate of material, and2. The ease of separation.

One way to quantify the relative ease of separation is to compare the volatilities ofthe components to be separated.

Because the boiling points change with pressure, it is more convenient to look atcurves as shown in Figure 3.2 from these curves we can find the boiling point of acompound at any pressure, or conversely, the vapor pressure of a compound at any

pressure.



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Method 1

Method 2

Figure 3.1 Process Design Alternates for Production of Propane i.butane and n-butane


Relative no. of trays

Relative tower diam.

Tower design pressure, psig

Relative no. of traysRelative tower diam.Tower design pressure, psig

1

1

300


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Figure 3.2 Vapor Pressure Light Hydrocarbons



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For example, we see the vapor pressure of propane at 100F is 190 psia; and thevapor pressure of propylene is 226 psia at 100F. The ratio of these two vapor

pressures is an expression of their relative volatilities. 226/190 = 1.2. This ratio isdefined as (alpha). The nearer alpha is to 1, the greater the difficulty of separation.The more difficult separations require more trays in a column and more reflux. Allthis requires more energy. The separation of normal butane and i.pentane is mucheasier. Alpha at 100F equals 52/20 or 2.6. This separation would require fewer traysand less energy.

Alpha is somewhat higher at lower temperatures. Therefore, other things being equal,it is desirable to operate at lower temperatures and, thereby, lower pressures.However, "other things are not equal" and there is a limit as to how low a pressurewe can operate. That limit is usually the temperature of the cooling water for the

overhead condenser. Because we must condense the overhead product, we must havea coolant whose temperature is below the boiling range of the overhead. Usually it isnot economically practical to refrigerate the coolant, so water or air is normally used.The available coolant temperature is usually the factor that establishes the towerdesign pressure.

To see how alpha affects tower design, let's look at two separations. First, adepropanizer - a distillation column designed to separate propane from i.butane andheavier components. Alpha is 190/72 or 2.6. Although there are heavier components

in the bottom and lighter components in the overhead, we need only consider whatwe call the key components, namely the heaviest major component in the overheadand lightest major component in the bottoms. For this separation, propane from

butane, the tower typically requires 30 trays, 5 to one reflux, and is about 50 ft. inheight. The diameter is largely dependent on the throughput.

In contrast, let's look at a tower to separate ethyl benzene and xylene. Alpha isapproximately 1.08. A tower for this separation has about 350 trays and uses 100 to 1reflux. A large depropanizer tower might cost about $500,000, while an ethyl

benzene-xylene tower might cost about $5 million. Of course, there is a considerabledifference in the cost of utilities to operate these two columns.

An additional factor that determines tower size is purity of product. As we sayearlier, each equilibrium stage has less of the impurity than the adjacent stage. Itfollows then that we must specify the desired purity. That is, the mole fraction of animpurity that we will accept in the product. For our depropanizer we might accept,say, 1 to 2 % butane in the overhead propane product and we much tolerate a loss of2 % propane in the bottoms.



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Obviously, to achieve a lower concentration of butane in the overhead, and stillmaintain the low loss of propane in the bottom, would require more trays, morereflux, and consequently, a taller and larger diameter tower with its attendant increasein investment and utility costs. To summarize, the height of a tower and number oftrays is largely dependent on the relative volatility of the key components and the

permissible mole fraction of these key components in the products.

Tower design procedure

The design of a distillation tower can be divided into the following steps:

1. Specify the degree of separation required: set product specifications.

2. Select the operating conditions, batch or continuous, operating pressure.

3. Select the type of contacting device: plate or packing.

4. Determine the stage and reflux requirements. The number of equilibriumstages.

5. Size the tower: Trays, number of real stages.

6. Design the tower internals: Trays, distributors, packing supports.

7. Mechanical design: vessel and internal fittings.

The principal step will be to determine the number of stages and reflux requirement.

3.2.4 Tower Trays

3.2.4.1 Types of Trays

Fractional distillation requires mass and heat transfer between vapor and liquidflowing counter currently through a fractionating tower. A large number of devicesto ensure a more or less thorough contact between the rising vapors and down-coming liquid had been developed. Bubble-cap trays, valve trays, sieve trays, andgrid trays are examples of devices.



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A. Trays Having Separate Liquid Down comers

The great majority of commercial fractionations are carried out in columns where theliquid flows horizontally across each tray. The liquid contacts the rising vapor and isseparated from the vapor before flowing through down comers onto the tray below.In nearly all cases, the down comers are segmental parts of the column, and are

provided with a liquid-overflow weir to assure a minimum height of liquid on eachtray. Inlet weirs for the liquid entering onto a tray are used in some designs. Varioustypes of trays with separate liquid down comers which are in more common usetoday are illustrated in Figure 3.3.

Bubble-Cap Trays (Figure 3.3a) are so widely used in the petroleum and chemicalindustries that they are generally considered to be "the standard." All new types of

trays are compared with "a bubble-cap tray,"

The outstanding characteristic of a will designed bubble-cap tray is probably itsability to perform satisfactorily over wide ranges of liquid and vapor rates. In whichthe vapour passes up through short pipes, called risers, covered by a cap with aserrated edge, or slots. The bubble-cap tray is the traditional, oldest type of cross-flow tray, and many different designs have been developed. Standard cap designswould now be specified for most applications.

The most significant feature of the bubble-cap is that the use of risers to ensure that alevel of liquid is maintained on the tray at all vapours flow-rates.

Although there are many styles and dimensions of caps (Figure 3.4) in use, the roundbell shaped bubble-cap is quite practical and efficient (Figure 3.5).

Dimensions, it is available in sizes of 3,4,5,6 and 7 inches, but the most popular andmost adaptable size is about 4 inches O.D. The 3-inch and 6-inch are also in commonuse for the smaller and larger diameter towers.

Slots are the working part of the cap. Slots are usually rectangular of trapezoidal inshape. The rectangular slots give slightly greater capacity while the trapezoidal slotsgives slightly better performance at low vapor rates.

Shroud Ring, it is recommended to give structural strength to the prongs or ends ofthe cap.



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Figure 3.3 Trays Having Liquid Down comer



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Figure 3.4 Bubble Caps & Risers



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Figure 3.5 Bubble Cap



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Sieve or Perforated Trays (Figure 3.3b) have been in use longer than bubble-captrays but have not received the same wide acceptance. This is partly because ofinadequate performance data with respect to liquid and vapor capacities. Recentlymore attention has been given to sieve trays, and it appears as though they will findincreased use by industry. The vapour passes up through perforations in the tray andthe liquid is retained on the tray by the vapour flow. There is no positive vapourliquid seal, and at low flow rates liquid will "weep" through the holes, reducing thetray efficiency. The perforations are usually small holes, but larger holes and slotsare used.

Valve Trays (Floating Cap) (Figure 3.3c) Valve trays are proprietary designs. Theyare essentially sieve trays with large diameter holes covered by movable flaps, whichlift as the vapour flow increases.

As the area for vapour flow varies with the flow rate, valve trays can operateefficiently at lower flow rates than sieve trays the valves closing at low vapour rates.Is somewhere between a bubble-cap and a sieve tray in operating principle. It is a

bubble-cap tray where the vapour, makes one 90-degree turn to enter the liquidhorizontally, there are no risers, and the caps have no teeth. It can also be consideredas a modified sieve tray where the vapor emerges horizontally into the liquid insteadof vertically, and the perforations have variable area.

Float-Valve Trays (Figure 3.3d) is a recent development worthy of consideration,although there is very little published information about its operation. It is a valve-type tray with floating rectangular caps positioned by end-brackets. One edge, theheavy edge, of each cap is turned upward 90 degrees. At low vapor rates the lightedge of each cap opens first, and at higher vapor rates the heavy edge opens. Like theFlexitray, it can be considered as somewhere between a bubble-cap and a sieve trayin operating principle.

Uniflux Trays (Figure 3.3e) is a third newcomer to the field. It has had considerable

industrial use already, but there is very little published information about itsoperation. It is a bubble-cap tray, modified so as to reduce fabrication costsconsiderably and to have possible other advantages. The tray is made of a number ofS-sections, with the vapor making a 180-degree turn between the riser and the cap,and then emerging from one side (the downstream side) of the cap, after a 90-degreeturn through the cap slots. In this way the vapor emergence should help the liquidflow across the tray, reducing hydraulic gradient.



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B. Trays Having No Liquid Down comers

Traditionally, fractionating columns that have no liquid down comers have beenpacked columns, where the ascending vapor contacted the descending liquid in truecountercurrent action. Recent developments have substituted perforated trays, wherethe liquid and vapor both pass through the same openings, for the continuous

packing. There is not the same degree of differential contacting here as for thecontinuously packed columns, but there are many possible advantages for thismodified "packing," compared with either conventional packed columns or theconventional tray columns.

Turbo grid Trays (Figure 3.6 a) consists of a flat grid of parallel slots extendingover the entire cross sectional of the column. The slots can be stamped perforations

in a flat metal plate, or can consist of the spaces between horizontal bars. Liquidlevel on each tray is maintained by dynamic balance of liquid and vapor rates. TheTurbo grid tray has had considerable industrial applications already, but there is little

published information about its operation.

Ripple Trays (Figure 3.6 b) is the latest arrival in the field of liquid-vapor contactingdevices. It is made by corrugating a conventional sieve plate into sinusoidal waves.The perforations extend over the entire cross sectional area of the column. Liquidlevel is maintained on each tray by a dynamic balance of the fluid flows, being a very

recent development, there is little published information about its use and operation.

Figure 3.6 Trays having no Liquid Down comers



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3.2.4.2 Selection of Tray Type

The principal factors to consider when comparing the performance of bubble-cap,sieve and valve trays are: cost, capacity, operating range, efficiency and pressuredrop.

Cost

Bubble-cap trays are appreciably more expensive than sieve or valve trays. Therelative cost will depend on the material of construction used; for mild steel theratios, bubble-cap: valve: sieve, are approximately 3.0:1.5:1.0. However,comparative quotations over the last few years show the relative costs in dollars persquare foot of tray area to be: bubble-cap tray, 20; flexitray, 14; Uniflux, 10; sieve,

10; and turbo grid, 10. These are costs before installation.

Capacity

There is little difference in the capacity rating for the three types (the diameter of thecolumn required for a given flow-rate); the ranking is sieve, valve, and bubble-cap.

Operating range

This is the most significant factor. By operating range is meant the range of vapourand liquid rates over which the plate will operate satisfactorily (the stable operatingrange). Some flexibility will always be required in an operating plant to allow forchanges in production rate, and to cover start-up and shut-down conditions. The ratioof the highest to the lowest flow rates is often referred to as the "turn-down" ratio.Bubble-cap trays have a positive liquid seal and can therefore operate efficiently atvery low vapour rates.

Sieve trays rely on the flow of vapour through the holes to hold the liquid on the tray

and cannot operate at very low vapour rates, but, with good design, sieve trays can bedesigned to give a satisfactory operating range; typically, from 50 per cent to 120 percent of design capacity.

Valve trays are intended to give greater flexibility than sieve trays at a lower costthan bubble-caps.



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Efficiency

The Murphree efficiency of the three types of trays will be virtually the same whenoperating over their design flow range and no real distinction can be made betweenthem.

Pressure Drop

The pressure drop over the trays can be an important design consideration,particularly for vacuum columns. The trays pressure drop will depend on thedetailed design of the tray but, in general, sieve plates give the lowest pressure drop,followed by valves, with bubble-caps giving the highest.

Summary

Sieve trays are the cheapest and are satisfactory for most applications. Valve traysshould be considered if the specified turn-down cannot be met with sieve trays.Bubble-caps should only be used where very low vapour (gas) rates have to behandled and a positive liquid seal is essential at all flow-rates.

3.2.4.3 Tray Layout

Flow Paths, (Figure 3.7)

The simplest tray arrangement considering fluid flow and mechanical details is thecross-flow. It fits the majority of designs. When liquid flows become small withrespect to vapor flow the reverse flow tray is recommended; when liquid load is highwith respect to vapor, the double pass tray is suggested, as the path is cut in half andthe liquid gradient reduced; and for the extremely high liquid loads, the double-passcascade is suggested. A guide for tentative selection of the tray type for a givencapacity is given in Table 3.1.

Figure 3.8 and Table 3.2 identify the distribution of areas of a tray by the action oftray area.



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Figure 3.7 Liquid over Tray Flow Path



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Figure 3.8 Classification of Tray Area



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Cap Layout, (Figure 3.8) caps should be arranged on the tray in 60 equilaterallayout, with the liquid flowing into the apex of the triangle rather than parallel to the

base. The liquid flows normal to each raw of caps.

Inlet Weirs, these contribute to the uniform distribution of liquid as it enters the trayfrom the down comer. It is not recommended for fluids that are dirty or tend to foulsurfaces.

Outlet Weirs, these are necessary to maintain seal on the tray, thus insuringbubbling of vapors through liquid.

Down comer, (Figure 3.9) the down comer from a tray must be adequate to carry theliquid flow plus entrained foam and froth. The vertical and straight segmental down

comer is recommended although the segmental tapered design has been used quitesuccessfully, the wide mouth of the inlet as compared to the outlet is considered to

provide better foam disengagement conditions. The Dow comer seal on the tray isrecommended based on the liquid flow path.

The segmental, or chord down comer, shown in Figure 3.9 is the simplest andcheapest form of construction and is satisfactory for most purposes. The down comerchannel is formed by a flat plate, called an apron, which extends down from theoutlet weir. The apron is usually vertical, but may be sloped to increase the plate area

available for perforation. If a more positive seal is required at the down comer at theoutlet, an inlet weir can be fitted or a recessed seal pan used. Circular down comers(pipes) (Figure 3.17) are sometimes used for small liquid flow-rates.

Liquid Bypass Baffles, also known as redistribution baffles, these short stub bafflesguide the liquid flow path to prevent excessive by-passing of the bubble-cap field oractive area.

Weep Holes, holes for drainage must be adequate to drain the tower in a reasonable

time, yet not too large to interfere with tray action. Draining of the tower through thetrays is necessary before any internal maintenance can be started. The majority ofthe holes are placed adjacent to the outlet or down comer weir



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Figure 3.9 Segment (Chord) Down comer Designs

3.2.4.4 Tray Stability

A tray is stable when it can operate with acceptable efficiencies under conditionswhich fluctuate, pulse, or surge, developing unsteady conditions. This type ofoperation is difficult to anticipate in design, and most trays will not operate longwithout showing loss in efficiency.



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Flooding, occurs when the pressure drop across a tray is so high that the liquidcannot flow down the tower as fast as required. The pressure drop across the trayincreases to very high values, and the tray efficiency drops markedly. When the frothand foam in the down comer back up to the tray above and begin accumulating onthis tray. The down comer then contains a mixture of lower density than the clearliquid, its capacity becomes limited, disengagement is reduced, and the level rises inthe down comer. The level extends onto the tray above, and will progress to the pointof filling the tower, if not detected and if the liquid and vapor loads are not reduced.

Pulsing occurs when the vapor rate is low and unsteady, when the slots and holesopening are low, and when the liquid dynamic seal is low. With irregular vapor flowenters the caps or holes, the liquid pulses or surge, even to the point of dumping. The

best cure is a steady vapor rate and good slot or hole opening to allow for reasonable

upsets.

Dumping liquid occurs at high liquid rates and low vapor loads. Some of slots orhales will dump liquid instead of passing vapor, resulting in poor tray efficiency. Fortowers with conventional down comers, dumping usually occurs at the upstream rawof caps or holes, where the liquid has the largest head and kinetic energy.

Blowing, occurs when the vapor rate is extremely high, regardless of the liquid rates,causing large vapor streams or continuous bubbles to be blown through the liquid.

Blowing is usually accompanied by accessing entrainment of large droplets and slugsof liquid up to the tray above. The pressure drop across the tray can be quite high andincrease very rapidly with any increase in vapor rate. The efficiency and contact islow and entrainment is usually high.

Coning occurs at low liquid rate or seals. The vapor pushes the liquid back from theslots or holes and passes upward with poor liquid contact. This causes poor trayefficiency.

Entrainment occurs when mist and liquid particles carry up in the vapor from theliquid on one tray through the riser or holes to tray above. Sufficient tray spacingmust be available to prevent the quantity of material from significantly affecting theefficiency of the tower.

Puking usually occurs at a high liquid rate and low gas rate. At high liquid rate the



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liquid level on each tray will rise. As the level rises, the flow of gas up the tower isrestricted. The gas pressure in the bottom of the tower will begin to rise. It will reachthe point that a surge of a gas will suddenly move up the tower with enough velocityto carry the liquid with it.

Reducing the liquid flow rate will usually eliminate puking. Puking should not beconfused with "carryover". Puking occurs almost instantaneously. Furthermore, if theliquid rate is not reduced, the tower will puke again when the liquid stacks up.Carryover is usually caused by a high vapor flow rate. It happens continuously,whereas puking is an intermittent thing.

5.2.4.5 Operating Range

Satisfactory operation will only be achieved over a limited range of vapour and

liquid flow rates. A typical performance diagram for a sieve plate is shown in Figure3.10.

The upper limit to vapour flow is set by the condition of flooding. At flooding thereis a sharp drop in plate efficiency and increase in pressure drop. Flooding is caused

by either the excessive carry over of liquid to the next plate by entrainment, or byliquid backing-up in the down comers.

The lower limit of the vapour flow is set by condition of weeping. Weeping occurs

when the vapour flow is insufficient to maintain a level of liquid on the plate."Coning" occurs at low liquid rates, and is the term given to the condition where thevapour pushes the liquid back from the holes and jets upward, with poor liquidcontact.



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Figure 3.10 Sieve Plate Performance Diagram

Figure 3.11 Qualitative Region of Satisfactory Operation



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3.2.4.6 Satisfactory Operating Region

We have indirectly defined the region of satisfactory operation by defining thesurrounding objectionable phenomena. The direct definition is now easier to make.

The region of satisfactory operation is that range of liquid and vapor rates where

(1) The contacting efficiency of the tray is at or near maximum, and.

(2) The column is mechanically able to handle the liquid and vapor loads in asteady-state manner.

Usually, flooding is the only limitation for which there is a sharp line of distinction

between satisfactory and unsatisfactory operation. The other limitations areordinarily not so abruptly critical.

Satisfactory Operating Regions for Bubble-Cap Trays

Visual Observations

The functioning of a bubble-cap can be best understood by observing a tray inoperation. Typical profiles of vapor leaving a bubble-cup are shown in Figure 3.12.

Figure 3.12 illustrates the cap action at a reasonable liquid rate for different vaporrates. Spouting or jetting of the liquid upward (commonly called "foam" or "froth")occurs between the caps, and may extend up to the tray above. Each cap acts as acalming zone for de-aerating this froth. Liquid drains continuously from the frothonto the top of each cap, and runs over the sides of the cap down into the slots. It issucked into the vapor stream at the top of the slots, and finally emerges with the mainvapor stream closer to the tray floor. At the vapor rate is increased, vapor issues froma variable slot area at slightly increasing slot velocities until full slot opening is

reached. As the vapor rate is increased still further, the liquid inside the annular spacebetween the cap and riser is depressed down to the tray floor. Thereafter, the cap actsas a fixed orifice and the slot velocity increases at a more rapid rate.



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Figure 3.12 Bubble Cap Tray Schematic Dynamic Operation

Figure 3.13 shows that around normal design rates, the vapor has depressed theliquid down to the tray, and has pushed the froth about 2 inches away from the cap atthe top. The impinging of vapor streams from adjacent caps, as dictated by the capspacing, appears to be the important factor in vapor escape. At typical cap spacing atop view of the tray at normal vapor rates would show some vapor holes blownthrough the liquid.

Figure 3.13 Bubble Cap Performance



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As the vapor rate is increased above normal loadings, a point is reached where thetray is practically blown dry, and a vapor pocket surrounds each cap. Liquid is jettedto the tray above, causing large amounts of entrainment, poor liquid-vapor contact,and large pressure drops.

Satisfactory Operating Regions for Other Types of Trays

The other types of trays which have considerable promise today are comparativelynew. They are all proprietary devices, and most of them are being improved throughcontinuous research and development.

Trays Having Separate Liquid Down comers

Flexi trays, The two different weights of caps which are generally used on a tray arein alternate rows parallel to the outlet weir. The lighter caps open during the first 20-30% of vapor loading, and the heavier ones open from this range to about 50-70% ofvapor loading. At higher rates all caps are fully open and thrust against the blowdown spiders. This arrangement insures a wide range of vapor loading and also goodvapor distribution across the tray at low vapor loads, something which is not possiblewith either the conventional bubble-cap or sieve tray.

The smaller cap diameter, about 2 inches, and the large pitch of 3-6 inches, should

permit the Flexitray to have appreciably lower pressure drop and entrainment thanthe bubble-cup tray has, and about the same order of magnitude as a perforated tray.The liquid-handling capacity should be greater than that of a bubble-cap tray becauseof less liquid gradient across the tray.

We would expect a Flexitray column to handle about 20-40% higher vapor loadsthan a bubble-cap column and about the same loads as a sieve-tray column. TheFlexitray column should handle satisfactorily smaller vapor rates than a bubble-capcolumn and considerably smaller rates than a sieve-tray column.

Float Valve Trays. The float-valve tray is regarded as being similar to the Flexitrayin operating principle. At around 20% of the vapor loading, the light edges of thevalves are open, and the heavy edges open between 40 and 70% of design vaporloading. At higher rates, the valves are fully opened and held against the support

brackets. Proper spacing of the valves should permit the float-valve tray to havelower pressure drop and entrainment than the bubble-cap tray has, and in the sameorder of magnitude as the sieve tray. The liquid-handling capacity should be greaterthan that of a bubble-cap tray because of less liquid gradient across the tray. It is

believed that the liquid gradient would be slightly higher across the Float-valve traythan across the Flexitray, but this difference might not be significant.



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We would expect that a Float-valve tray could handle about 20-40% more vapor loadthan a bubble-cap tray, and about the same as a sieve tray. It should be able to handlesatisfactorily smaller vapor rates than a bubble-cap tray, and much smaller rates thana sieve tray.

Uniflux Trays is regarded as a modified bubble-cap tray. The vapor emerging onlyfrom the downstream portion of the caps (sections) could aid in reducing the liquidgradient across the tray. Such a reduction may or may not be significant in tray

performance. The absence of available data leaves us with the opinion that theUniflux tray should have about the same operating characteristics as a bubble-captray, but with the definite advantage of economy of fabrication and installation.

Trays Having No Liquid Down comers

Turbo grid Tray is considered as a modified packed column. The liquid and vaporhas counter currently through the same openings, so there is no liquid gradient acrossa tray. It would be expected that Turbo grid trays would have a capacity 20-50%greater than bubble-cap trays, but a much smaller operating range (say, from 100%maximum down to 50%). This would be expected because the liquid level on eachtray is maintained by dynamic balance. At low vapor rates the contacting efficiencyshould decrease appreciably. On the other hand, the Turbo grid tray should have avery small pressure drop and should be the most economical of all trays to fabricate.

Ripple Tray, has smaller openings than the Turbo grid tray for the phases to passthrough and probably has a higher efficiency. The Ripple tray would be expected tohandle 20-40% more vapor load than can be bubble-cap tray, but it should have amuch smaller operating range, comparable to that of the Turbo grid. At low vaporrates, the contacting efficiency should decrease appreciably. The Ripple tray shouldhave a pressure drop about the same as sieve trays and should be economical tofabricate.

3.2.4.7 Tray Construction

The mechanical design features of sieve trays and described. The same generalconstruction is also used for bubble-cap and valve plates.

Two basically different types of plate construction are used. Large-diameter traysnormally constructed in sections, supported on beams. Small trays are installed in thecolumn as a stack of pre-assembled trays.



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Sectional Construction

A typical tray is shown in Figure 3.14. The tray sections are supported on a ringwelded round the vessel wall, and on beams. The beams and ring are about 500 mmwide, with the beams set at around 0.6 m spacing. The beams are usually angle orchannel sections, constructed from .folded sheet. Special fasteners are used so thesections can be assembled from one side only. One section is designed to beremovable to act as a man way. This reduces the number of man ways needed on thevessel, which reduces the vessel cost.

Stacked Trays (Cartridge Trays)

The stacked type of construction is used where the column diameter is too small for a

man to enter to assemble the trays, say less than 1.2 m (4ft). Each tray is fabricatedcomplete with the down comer, and joined to the above and below using screwedrods (spacers); (see Figure 3.15).

The trays are installed in the column shell as an assembly (stack) of ten, or so, trays.Tall columns have to be divided into flanged sections so that tray assemblies can beeasily installed and removed. The weir, and down comer supports, are usuallyformed by turning up the edge of the tray.

The trays are not fixed to the vessel wall, as they are with sectional trays, so there isno positive liquid seal at the edge of the tray, and a small amount of leakage willoccur. In some designs the tray edges are turned up round the circumference to make

better contact at the wall. This can make it difficult to remove the trays for cleaningand maintenance, without damage.

Side-stream and Feed Points

Where a side-stream is withdrawn from the column, the plate design must be

modified to provide a liquid seal at the take-off pipe. A typical design is shown inFigure 3.19. When the feed stream is liquid it will be normally introduced into thedown comer leading to the feed plate, and the plate spacing increased at this point.



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Figure 3.14 Sectional Tray Design



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Figure 3.15 Stacked Tray Tower



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Figure 3.16 Cross flow Bubble-Cap Plate



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General Notes

- For 2-pass trays it's preferable to have the top and bottom trays with side

down comers. If the top and bottom must be different, use side down comerat the bottom and center at the top.

- Inlet weirs are sometimes used to distribute reflux.

- Slotted distributors are used for liquid and 2-phase feeds and pump rounds.

- Vapor feed are usually flush connections.

- Check the location of all internal piping to make sure it does not internalwith tray man ways.

Liquid feed, Two Phase feed or Pump rounds

In general, liquid feed should be directed to the entrance of the feed tray. In somecases liquid can be fed into the down comer above the feed tray. However, if vaporcan be present, or if the feed is at a temperature greatly different from the feed tray,then the feed should not be introduced into the down comer (Figure 3.19).

Vapor feed, (Figure 3.20) feed location at two or more trays should be either alleven numbered trays or all odd, for same feed details at all locations.

Draw Pan (Draw off) (Figure 3.21) used for draw off connections to side streamstrippers, side stream products, and no-foaming pump rounds. Equalizer pipearrangements for double cross flow towers are to be covered.

3.2.4.8 Material of Construction

Bubble-cap trays have been constructed of cast iron, sheet carbon steel, and sheetmetal of various alloys. Cast iron caps and trays used to be common. However, theyare very heavy and required heavy foundations and tower structures, furthermore,cast iron caps are thick and more consequently wasteful of tower cross-sectionalarea.

Sheet metal trays and caps overcome these disadvantages, as well as being cheaper;carbon steel trays are considerably cheaper then cast iron. As a result, cast iron traysare seldom specified anymore.



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Nearly all of the common metals and alloys have been used in the construction ofsheet metal trays and caps. Low carbon steel is the usual standard for non-corrosiveservice. The use of other metals and alloys are dictated by the conditions of corrosionexpected. In order to indicate the effect of materials of construction on cost, thefollowing relative tray costs are illustrative of some of the common metals andalloys.

Material Relative Tray Cost

Carbon steel 111-13% chrome type 400 218-8 type 304 2 1/218-8 Type 316 3 1/2

Monel

3.2.4.9 Tray Design

a. Bubble Cap Tray Design

The tray and caps operate as a unit or system; therefore they must be so considered indesign (Figures 3.16 and 3.17)

Standardization

The custom design of the trays for each application is usually unnecessary anduneconomical. Instead most designers utilize a standard reference tray layout and capsize to check each system. If the results of the tray hydraulics study indicateoperation unsatisfactory for the standard tray, then alterations of those featurescontrolling the out-of-line performance is in order, utilizing the same method as will

be outlined for the initial design of a custom tray. It is understood that such astandard tray cannot be optimum for every application but experience has

demonstrated that many applications fit. The economic advantages of utilizing alimited number of bubble cap sizes and designs are reflected in warehouse stocks.The standardization of layouts, down comer areas, weir lengths and many otherfeatures are reflected in savings in engineering mechanical design time.

At the same time, systems which do not adapt themselves to this standardizationshould be recognized and handled as special designs.



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Design Objectives

Each tray design should ultimately resolve and achieve the following:

1. Capacity: High for vapor and/or liquid as required. This yields the smallestcolumn diameter for a given throughput. Flexibility or adaptability to high andlow fluctuations in vapor and liquid rates.

2. Pressure drop: Low pressure drop is necessary to reduce temperaturegradients between top and bottom of the column. High pressure drop isusually (but not always) associated with uneconomical design. In somesystems pressure drop is not a controlling feature, within reasonablelimitations.

3. Efficiency: High efficiency is the objective of each tray performance. Thebetter the contact over a wide range of capacities, the higher will be theefficiency throughout this range.

4. Fabrication and installation costs: Details should be simple to maintain lowcosts.

5. Operating and Maintenance Costs: Mechanical details must account for the

peculiarities of the system fluids (coking, suspended particles, immisciblefluids, etc.) and accommodate the requirements for drainage, cleaning(chemical or mechanical), corrosion, etc., in order to keep the daily costs ofoperation and downtime to a minimum.

b. Sieve Tray Design

The basic requirements of a tray contacting stage are it should: provide good vapour-liquid contact.

Provide sufficient liquid hold-up for good mass transfer (high efficiency) hassufficient area and spacing to keep the entrainment and pressure drop withinacceptable limits. Have sufficient down comer area for the liquid to flow freely fromtray to tray.

Tray design, like most engineering design, is a combination theory and practice. Thedesign methods use semi-empirical correlations derived from fundamental researchwork combined with practical experience obtained from the operation of commercialtowers. Proven layouts are used, and the plate dimensions are kept within the rangeof values known to give satisfactory performance.



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Tray design procedure

A trial-and error approach is necessary in tray design: starting with a rough traylayout, checking key performance factors and revising the design, as necessary, untila satisfactory design is achieved. A typical design procedure is set out below

Procedure

1. Calculate the maximum and minimum vapour and liquid flow-rates/for theturn down ratio required.

2. Collect, or estimate the system physical properties.

3. Select trial tray spacing.

4. Estimate the tower diameter, based on flooding considerations.

5. Decide the liquid flow arrangement.

6. Make a trial tray layout: down comer area, active area, hole area, hole size,weir height.

7. Check the weeping rate, if unsatisfactory return to step 6.

8. Check the plate pressure drop, if too high return to step 6.

9. Check down comer back-up, if too high return to step 6 or 3.

10.Decide tray layout details: calming zones, imperforated areas, check holepitch, if unsatisfactory return to step 6.

11.Recalculate the percentage flooding based on chosen tower diameter.

12.Check entrainment, if too high return to step 4.

13.Optimize design: repeat steps 3 to 12 to find smallest diameter and trayspacing acceptable (lowest cost).

14.Finalize design: draw up the tray specification and sketch the layout.



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5.2.4.10 Tower Internals

The arrangements for a tower internal include the following:

1. Reflux. (Figure 3.18)

2. Liquid feed, vapor, and two phase. (Figure 3.19 & 20)

3. Pump around (Figure 3.19)

4. Draw-pan draw offs (Figure 3.21)

5. Tower bottom for Kettle reboiler (Figure 3.22), thermo siphon (Figure 3.23),

and once-through thermo siphon (Figure 3.24).



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Figure 3.18 Reflux Arrangement



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Figure 3.19 Liquid Feed, Two Phase Feed or Pump rounds



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Figure 3.20 Feed Vapor



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Figure 3.22 Tower Bottom for Kettle Reboiler



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Figure 3.23 Tower Bottom for Thermo siphon Reboiler



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Figure 3.24 Tower Bottom for Once through Thermo siphon Reboiler



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3.3 PACKED TOWERS

3.3.1 Packed Towers Function

Packed towers are used for distillation, gas absorption, and liquid-liquid extraction.Stripping (desorption) is the reverse of absorption.

The gas liquid contact in a packed bed tower is continuous, not stage wise, as in aplate tower. The liquid flows down the tower over the packing surface and the gas orvapour, counter-currently, up the tower. In some gas absorption towers co-currentflow is used. The performance of a packed tower is very dependent on themaintenance of good liquid and gas distribution throughout the packed bed, and thisis an important consideration in packed tower design.

A schematic diagram, showing the main features of a packed absorption tower, isshown in Figure 3.26. A packed distillation tower will be similar to the plate towers,with the plates replaced by packed sections.

Packed Tower Design Procedures

The design of packed tower will involve the following steps:

1. Select the type and size of packing.

2. Determine the tower height required for the specified separation.

3. Determine the tower diameter (capacity), to handle the liquid and vapour flowrates.

4. Select and design the tower internal features: packing support, liquid

distributor, redistributors.



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B.PackedAbsorptionC

olumn

A.Cross-Se

ctionTypicalPackedTow

er

Figu

re3.26PackedTowers


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3.3.2 Types of Pickings

Many types of packing materials have been used ranging from simple, readilyavailable solids such as stones or broken bottles to expensive complex geometricalshapes. In general, the packing material should have the following characteristics:

1. It should have a large wetted surface area of packed space so as to present apotentially large interfacial area for phase contacting.

2. It should have a large void volume. These will allow reasonable throughputsof phases without serious pressure drop.

3. It should be corrosive resistant.

4. It should have good wetting characteristics.

5. It should have a low bulk density. In large packed towers, the weight of thepacking can be quite large resulting in serious support problems.

6. It should be relatively inexpensive.

Many diverse types and shapes of packing have been developed to satisfy these

requirements. They can be divided into two broad classes:

1. Stacked packing, which are regular arrangements of the packing elements, and

2. Random packing.

Stacked packing, such as grids, have an open structure, and are used for high gasrates, where a low-pressure drop is essential, for example, in cooling towers.

Random packing are more commonly used in the process industries.

Design data for these packing are given in Table 5.3

The design methods and data given in this section can be used for the preliminarydesign of packed columns, but for detailed design it is advisable to consult the

packing manufacturers.



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Rasching rings. (Figure 3.27)

Are one of the oldest specially manufactured types of random packing, and are stillwidely used in the process industries because of their lower cost than the others, butare less efficient, and the total cost of the tower will usually be higher, the wallthickness of the rasching ring is an important factor because, as the thickness isdecreased, mechanical strength decreases. A greater wall thickness will result in anincreased pressure drop, lower free space, and reduced surface area. Best results areobtained when walls are relatively thin. The diameter and the height of rasching ringare equal. Rasching rings may be fabricated from porcelain, clays, carbon or metals.

Ball rings. (Figure 3.28)

Are essentially rasching rings in which wall stamped and bent inward this increasesthe free area and improves the liquid distribution characteristics. They are availablein a variety of materials (ceramic, metal, and plastics). Metal and plastics(polypropylene) rings are more efficient than ceramic rings.

Berl saddles. (Figure 3.29)

Are costly to produce but do have some advantage over other packing, they can bepacked with more randomness than rings, and they give a relatively large amount of

surface area per unit volume, they are available in a variety of materials: ceramics,metals, plastics, and carbon.



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Table 3.3

Particulars of Hy Contact Tower Packing (approximate values)



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Figure 3.27 Rasching Rings Packing



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Figure 3.28 Pall Rings Packing



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Figure 3.29 Berl Saddles Packing



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Intalox Saddles. (Figure 3.30)

Can be considered to be an improved type of berl saddle, their simple shape makesthem easier to manufacture than berl saddles.

Hypac and Super Intalox packing. (Figure 3.31)

Can be considered improved types of ball ring and Intalox saddle, respectively.

Lessing and cross partition rings. (Figure 3.32)

Are simple modifications of rasching ring to improve operating. Figure 3.33illustrate some other common packing shapes.

The choice of material will depend on the nature of the fluids and the operatingtemperature. Ceramic packing will be the first choice for corrosive liquids, butceramics are unsuitable for use with strong alkalis. Plastic packing are attacked bysome organic solvents and can be used up to moderate temperature, so are suitablefor distillation towers. Where the tower operation is likely to be unstable metal ringsshould be specified. For new towers, the choice will normally be between pall ringsand berl or intalox saddles.

Packing size

In general, the largest size of packing that is suitable for the larger size of columnshould be used; up to 50 mm. Small sizes are appreciably more expensive than thelarger sizes. Above 50 mm the lower cost per cubic meter does not normallycompensate for the lower mass transfer efficiency. Use of too large size in a smallcolumn can cause poor liquid distribution.



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Figure 3.30 Ionic Saddles Packing



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Figure 3.31 Hypac and Super Intalox Packing


Hypac


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Figure 3.32 Lessing and Cross Partition Rings Packing



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Figure 3.33 Some Other Packing



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Recommended size ranges are:

Column diameter Use packing size

< 0.3 m (1 ft) < 25 mm (1 in.)0.3 to 0.9 m (1 to 3 ft) 25 to 38 mm (1 to 1.5 in)> 0.9 m 50 to 75 mm (2 to 3 in.)

Packing application, Table 3.4 represents packing applications.

3.3.3 Tower Internals

The internal fittings in a packed tower are simpler than those in a plate tower but

must be carefully designed to ensure good performance. As a general rule, thestandard fittings developed by the packing manufacturers should be specified. Sometypical designs and their use are discussed in the following paragraphs.

3.3.3.1 Packing Support

The function of the support plate is to carry the weight of the wet packing, whilstallowing free passage of the gas and liquid. These requirements conflict, a poorlydesigned support will give a high pressure drop and can cause local flooding. Simple

grid and perforated plate supports are used, but in these designs the liquid and gashave to be check for the same openings. Wide-spaced grids are used to increase theflow area, with layers of larger size packing stacked on the grid to support the smallsize random packing (Figure 3.34).

The best design of packing support is one in which gas inlets are provided above thelevel where the liquid flows from the bed; such as the gas-injection type shown inFigure 3.35 and these designs have a low pressure drop and no tendency to flooding.They are available in a wide range of sizes and materials: metals, ceramics and

plastics.



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Table 3.4 Packing Type Application



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Figure 3.34 Types of Packing Support



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Figure 3.35 Typical Designs of Gas Injection Supports



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3.3.3.2 Liquid Distributors

The satisfactory performance of a packed tower is dependent on maintaining auniform flow of liquid throughout the tower, and good initial liquid distribution isessential. Various designs of distributors are used. For small-diameter towers acentral open feed pipe, or one fitted with a spray nozzle, may well be adequate; butfor larger towers more elaborate designs are needed to ensure good distribution at allliquid flow-rates. The two most commonly used designs are the orifice type, shownin Figure 3.36 and the weir type, shown in Figure 3.37. In the orifice type the liquidflows through holes in the plate and the gas through short stand pipes. The gas pipesshould be sized to give sufficient area for gas flow without creating a significant

pressure drop; the holes should be small enough to ensure that there is a level ofliquid on the plate at the lowest liquid rate, but large enough to prevent the distributor

overflowing at the highest rate. In the weir type the liquid flows over notched weirsin the gas stand-pipes. This type can be designed to cope with a wider range ofliquid flow rates than the simpler orifice type.

For large-diameter towers, the trough-type distributor shown in Figure 3.38 can beused, and will give good liquid distribution with a large free area for gas flow.

All distributors which rely on the gravity flow of liquid must be installed in the towerlevel, or misdistribution of liquid will occur.

A pipe manifold distributor, Figure 3.39 can be used when the liquid is fed to thetower under pressure and the flow-rate is reasonably constant. The distribution pipesand orifices should be sized to give an even flow from each element.



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Figure 3.38 Weir Trough Distributors

Figure 3.39 Pipe Distributor



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3.3.3.3 Liquid Redistributors

Redistributors are used to collect liquid that has migrated to the column walls andredistribute it evenly over the packing; they will also even out any misdistributionthat has occurred within the packing.

A full redistributor combines the functions of a liquid distributor; a typical design isshown in Figure 3.40.

The "wall-wiper" type of redistributor, in which a ring collects liquid from thecolumn wall and redirects it into the centre packing, is occasionally used in small-diameter columns, less than 0.6 m. Care should be taken when specifying this type toselect a design that is shown in Figure 3.41.

The maximum bed height that should be used without liquid redistribution dependson the type of packing and the process. Distillation is less susceptible to maldistribution than absorption and stripping. As a general guide, the maximum bedheight should not exceed 3 column diameters for rasching rings, and 8 to 10 for pallrings and saddles. In a large diameter column the bed height will also be limited bythe maximum weight of packing that can be supported by the packing support andcolumn walls; this will be around 8 m.

If the columns must be packed dry, for instance to avoid contamination of processfluids with water, the packing can be lowered into the column in buckets or othercontainers. Ceramic packing should not be dropped from a height of more than half ameter.

Liquid Hold-up

An estimate of the amount of liquid hold up in the packing under operatingconditions is needed to calculate the total load carried by the packing support. The

liquid hold-up will depend on the liquid rate and, to some extent, on the gas flow-rate. The packing manufacturers design literature should be consulted to obtainaccurate estimates. As a rough guide, a value of about 25 per cent of the packingweight can be taken for ceramic packing.



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Figure 3.40 Full Redistributors



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Figure 3.41 Wall Wiper Redistributor



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Wetting Rates

If very low, liquid rates have to be used, the packing wetting rate should be checkedto make sure it is above the minimum recommended by the packing manufacturer.

eunit volumperareasurfaceRacking

areasectionalcrossunitperrateliquidVolumetricrateWetting =

Norman recommends that the liquid rate in absorbers should be kept above 2.7Kg/m2 s.

If the design liquid rate is too low, the diameter of the column should be reduced. Forsome processes liquid can be recycled to increase the flow over the packing.

A substantial factor of safety should be applied to the calculated bed height forprocess where the wetting rate is likely

3.3.3.4 Hold-down Plates

At high gas rates, or if surging occurs through miss-operation, the top layers ofpacking can be fluidized. Under these conditions ceramic packing can break up andthe pieces filter down the column and plug the packing; metal and plastic packing can

be blown out of the column. Hold-down plates (Figure 5.42) used with ceramicpacking to weigh down the top layers and prevent fluidization; a typical design isprevent expansion of the bed when operating at a high-pressure drop. They similar tohold-down plates but are of lighter construction and are fixed to the column walls.The openings in hold-down plates and bed-limiters should be small enough to retainthe packing, but should not restrict the gas and liquid flow.

Installing Packing

Ceramic and metal packing are normally dumped into the column wet to ensure atruly random distribution and prevent damage to the packing. The column is partiallyfilled with water and the packing dumped into the water. A height of water must bekept above the packing at all times. Calculated bed height for process where thewetting rate is likely to be low.



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Documents

Trays and Packed Towers