Plate Column

Plate columns are widely used for distillation as well as for gas absorption/ desorption. These are vertical cylindrical towers in which liquid and gas or vapour are contacted counter-currently in stepwise manner on the plates or trays. The gas or vapour enters near the bottom, rises through the openings in the trays and then through the liquid pool on the trays in dispersed condition, thus coming into intimate contact with the liquid before leaving at the top. The liquid enters near the top, flows down from tray to tray through downcomers, crosses each tray, contacts the gas or vapour issuing from the openings on the trays and leaves at the bottom. The overall effect is multiple counter-current contact of liquid and gas or vapour while each tray provides cross flow of the two fluids. Depending on the requirement, configuration of cross flow trays may be of the three types, single pass, reverse flow, and multiple pass. However, the number of trays depends on the extent of mass transfer while the diameter of the tower depends on the quantity of liquid and gas or vapour to be handled, and in case of distillation, on the reflux ratio. Conditions leading to high mass transfer efficiencies cause operational difficulties. Each tray of the tower acts as a stage where the fluids are brought into intimate contact to permit diffusion of one or more component and then they are physically separated.

In order to achieve high tray efficiency, there should be thorough mixing of the two fluids, interfacial surface of contact should be as large as possible and time of contact as long as possible. The pre-requisite for long exposure time is that the liquid pool on the tray should be as deep as possible.

When the gas rises slowly through the openings on the tray, bubble size is large and interfacial area of contact is relatively small, the liquid is quiescent and some of the liquid may pass without contact with the gas or vapour. If, on the other hand, the gas velocity is high, there is thorough agitation and foaming. For high plate efficiency, deep liquid pool and high gas velocity are required. These in turn lead to longer plate spacing resulting in taller tower and higher fixed cost. The pressure drop within the tower also increases leading to higher operating cost. High gas velocity may also lead to flooding and stoppage of operation. An intelligent compromise should therefore be made for which one should be aware about the constructional and operational aspects of the following hardwares.

Tower shell: Towers are generally made of cylindrical shells of metals and alloys although glass, glass-lined metal, impervious carbon, plastic or even wood are used for the purpose depending upon the specific requirements and corrosion conditions. Smaller diameter towers are provided with hand holes while larger ones are provided with manholes for cleaning and repairing.

Tray and tray spacing: Trays are usually made of sheet metal. Special alloys are used for corrosive fluids. Plate thickness depends on corrosion rate. The trays are stiffened and are fastened to the shell with allowance for thermal expansion to prevent their movement by surges of gases.

Tray spacing is decided from the convenience of construction, maintenance and cost and then checked that adequate provision has been made against flooding and entrainment. The spacing normally depends on the column diameter and operating conditions, and varies from 0.15 m to 1 m. However, tray spacing of 0.5 m is quite common for trays of 1 to 3 m diameter, the recommended range being 0.3 to 0.6 m. In the column employed in chemical process industries, tray spacing generally varies between 450 and 900 mm which help to avoid premature flooding as well as excessive entrainment.

Lower tray spacing restricts allowable vapour velocity thereby promotes froth regime operation, however, may be used where tower height is a constraint. Standard tray spacing for large-diameter columns are generally either 0.46 or 0.61 m, but 0.3 and 0.91 m spacing are also used.

Tower diameter: Tower diameter should be sufficiently large to handle the liquid and gas or vapour at velocities within the range of satisfactory operation so as to ensure proper dispersion and mixing without excessive pressure drop, which may lead to flooding. Flooding may occur due to loading of liquid in the column caused by the high vapour or gas velocity in upward direction. During such counter-current flow of gas/vapour and liquid, the transfer of momentum takes place from gas/vapour phase to liquid phase. When the upward momentum exceeds the liquid weight, the flooding usually occurs. Detailed discussion on such flooding in the packed tower has been dealt within Section 6.3.2.

However in plate columns for a specific liquid flow rate, a maximum vapour flow rate exists beyond which incipient column flooding occurs because of backup of liquid in the downcomer. This condition when sustained, leads to carryout of liquid with the overhead vapour leaving the column.

Downcomer flooding takes place when liquid backup is caused by downcomers of inadequate cross sectional area to carry the liquid flow but rarely occurs if the cross sectional area of downcomer is at least 10% of the total cross sectional area of the column and if tray spacing is at least 60 cm. Another design limit is entrainment flooding which is caused by excessive carry over of liquid by vapour entrainment to the tray above. This entrainment of liquid is due to carry up of suspended droplets by rising vapour or to throw up of liquid particles by vapour jets formed at tray perforations, valves, or bubble-cap slots. During passage of gas/vapour through the liquid, drops of various sizes are usually formed. The smaller sized drops do not tend to settle when their terminal velocities become less than

the gas/vapour velocity, thus resulting their carry over to the tray above. The large sized drops however will tend to fall back after travelling certain distance in upward direction. When such drops will not fall back, these will also be carried over to the next upper tray. The superficial gas velocity at flooding is given by

VF = K (6.2) where,

VF = net vapour velocity at flooding condition, i.e. volumetric flow rate of gas at flooding divided by the net tower cross section

K = Souders and Brown factor at flooding condition in m/s may be obtained from Fair correlation presented in Figure 6.1 (adapted from Fair 1961) for different tray spacing

v = surface tension, dyne/cm

tL and tG = densities of the liquid and vapour streams in the column respectively.

More discussions on calculation of flooding velocity in plate type column have been made elsewhere (Coulson et al. 1985, Lockett 1986, Humphrey and Keller 1997, Seader and Henley 2006).

A high vapour velocity is needed for high plate efficiencies, and the actual velocity (Vn) will normally be between 70-90% of the flooding velocity. However for design, a value of 80 to 85% of the flooding velocity is usually used. The net column area, An is obtained from the relation

An = (6.3)

and Ac = An + Ad (6.4)

where Ac is the cross sectional area of the column and Ad is the downcomer area.

Figure 6.1 Values of K for use in Eq. (6.2).

The column diameter, D is then calculated from the relation D =

(6.5)

In actual practice the column diameter obtained using Eq. (6.5), is required to be rationalized.

Downcomers

The liquid flows from a plate to the plate next below through downcomers or downspouts. A portion of the tower cross section, separated by a vertical plate, generally serves as the downcomer although circular pipes are also used. Sufficient residence time has to be allowed in the downcomer for releasing any gas or vapour that might be entrapped in the liquid. The bottom of the downcomer should be well submerged within the liquid on the next lower plate so as to prevent any gas or vapour escaping through it.

Weir

Weirs are provided at the downcomer entrance to maintain the desired liquid pool on the plate.

Extension of the downcomer plate usually acts as weir. The weir height required to maintain the volume of liquid on the plate, is an important parameter in determining the plate efficiency. A high weir will increase the plate efficiency but at the expense of a high pressure drop across the plate. For columns operating above atmospheric pressure weir heights should normally be between 40 and 90 mm; 40 to 50 mm being recommended. For vacuum operation lower weir heights are used to reduce

the pressure drop; 6 to 12 mm is generally recommended.

The three commonly used plate columns are bubble-cap columns, sieve-plate columns and valve-tray columns shown in Figure 6.2. Design procedures of these columns have been dealt with elsewhere (Smith 1963, Van Winkle 1967, Billet 1979, Coulson et al. 1985, Walas 1988, Ludwig 1997). When trays are designed properly, a stable operation is achieved wherein (i) vapour flows only through the perforations or open regions of the tray between the downcomers, (ii) liquid flows from tray to tray only by means of the downcomers, (iii) liquid neither weeps through the perforations of the tray nor is carried by the vapour as entrainment to the tray above, and (iv) vapour is neither carried down by the liquid in the downcomer to the tray below nor allowed to bubble up through the liquid in the downcomer.

Figure 6.2 Three common types of plates used in column.

Bubble-Cap Column

A schematic diagram of a typical bubble-cap column used in distillation operation is shown in Figure 6.3(a). The plates are provided with a number of short cylindrical vapour risers, covered by caps through which the gas or vapour bubbles, hence the name bubble-cap column. A typical configuration of bubble-cap tray has been shown in Figure 6.3(b) along with different types of caps which are in use. The gas or vapour from the next lower plate rises through the

Figure 6.3(a) Schematic diagram of bubble-cap column.

Figure 6.3(b) Typical bubble-cap tray along with different types of caps.

vapour risers and then escapes into the liquid through slots cut in the rim or skirt of the caps. These slots being fully submerged in liquid on the plates, the gas is forced to bubble through the liquid in finely dispersed condition.

The liquid flows across each plate before passing on to the plate below through the downcomer. The flow pattern of vapour and liquid has been shown in Figure 6.3(c). Baffles are sometimes provided on the plates to ensure that liquid flows over all the caps. The gas or vapour rises through the bubble-caps in dispersed condition and produces a frothy mixture of liquid and gas providing large interfacial area for mass transfer.

Figure 6.3(c) Flow pattern of vapour and liquid on a bubble-cap tray.

Because of their capacity to handle wide ranges of liquid and gas or vapour flow rates, bubble-cap

columns have been exclusively used in the industry for a pretty long time. At present however, they are seldom used in new installations in view of their high cost and availability of equally or even more efficient equipment at much lesser cost.

Sieve-plate column

A section of a typical sieve-plate column is shown in Figure 6.4(a). Sieve plates are perforated plates usually made of stainless steel or other alloys and placed horizontally in the column. The gas or vapour from a plate rises through the perforations of the next upper plate and then bubbles through the liquid pool on the plate in finely subdivided state. Liquid flows across each tray when it comes in contact with the rising vapour or gas, then over an outlet weir, and finally into a downcomer which takes the liquid by gravity to the tray below as shown in Figure 6.4(b). A pool of liquid is retained on the plate by an outlet weir. The stated phenomena lead to form two-phase flow regime on the plate.

As noticed by the practicing engineers, any one of the flow regimes shown in Figure 6.4(c) may prevail during operation.

Figure 6.4(a) Schematic view of a typical sieve-plate column.

Figure 6.4(b) Downcomer operation over a plate.

Figure 6.4(c) Two-phase flow regimes on sieve-plate: (a) Spray, (b) Foam, (c) Emulsion, (d) Bubble and (e) Cellular foam.

Downcomers are conduits having circular, segmental or rectangular cross sections which allow the liquid to flow from a tray to the next one located below. Figure 6.5 shows different types of downcomer used in plate type column. Circular downcomers (pipes) are sometimes used for small liquid flow rates. The straight, segmental vertical downcomer is the simplest in construction and less expensive than other types of downcomers, and is satisfactory for most purposes. The downcomer channel is formed by a flat plate, called an apron, which extends down from the outlet weir. The apron is usually vertical but may be sloped or inclined to increase the plate area available for perforation. Sometimes arc type configuration is also used. The area of a downcomer varies from 8 to 25% of the column area. In this area liquid and vapour cannot come in contact with each other. Such configuration reduces significantly the vapour handling capacity of a tray. When the downcomer is sloped from the top and truncated above the tray deck, available area for mass transfer will be somewhat higher. The sloped one providing maximum active area on the lower tray, enhances the vapour handling capacity. Also, adequate space becomes available on the top of the downcomer for the liquid and vapour to separate from each other. The downcomer extending downward from the bottom tray of a column should be sealed by a seal pan in order to restrict the flow of vapour through it. If more positive seal is required, an inlet weir can be fitted. The height of clear liquid in the

downcomer is always greater than the height of clear liquid on the tray because the pressure difference across the froth in the downcomer is equal to the total pressure drop across the tray from which liquid enters the downcomer, plus the height of clear liquid on the tray below to which the liquid flows, and the head loss for liquid flow under the downcomer apron.

The diameter of the perforation varies from 2.5 to 12 mm, 5 mm being the preferred size and they are drilled on triangular pitch of 2.5 to 5 times the hole diameter. It is to be noted that the small diameter of perforation will help in producing gas/vapour bubble of small diameter and a bubble with diameter less than or equal to 3 mm, will behave like a rigid sphere (Bond and Newton 1928, Hughes and Gilliland 1952, Davies 1960, Sherwood et al. 1975) thus preventing required mass transfer between gas and liquid phases. About 70 to 80% of plate surface is perforated. The perforated area is somewhat lower than the active area which is approximately the column area minus twice the downcomer area, due to the

Figure 6.5 Different types of downcomers: (a) Segmental, (b) Sloped, (c) Inclined and (d) Arc.

following reasons: some spaces, called calming zones, on the plates just near the liquid inlet and near the overflow weir are left without perforations. This ensures minimum chances of gas or vapour escaping through the downcomer and also allows some degassing of the liquid before the same enters the downcomer. The area available for perforation may further be reduced by the obstructions caused by structural members, i.e. support rings, beams, etc. The perforated area can be calculated from the plate geometry. The relationship between the weir chord length, chord height and the angle subtended by the chord has been given in Figure 6.6 (Coulson et al. 1985).

Figure 6.6 Relation between angle subtended by chord, chord height and chord length.

If the gas or vapour velocity through the holes falls below a certain minimum, liquid may not flow over the entire plate. Moreover, in such cases, liquid will drain through the holes. This phenomenon is known as weeping. As a result, gas-liquid contact on the plates will be seriously hampered.

Weeping may be avoided by maintaining adequate velocity of gas or vapour through the holes.

For a tray to operate at high efficiency (i) weeping of liquid through the perforations of the tray must be small compared to flow over the outlet weir and into the downcomer, (ii) entrainment of liquid by the gas/vapour must not be excessive, and (iii) froth height in the downcomer must not approach tray spacing.

The gas-side pressure drop for flow of gas or vapour across each plate usually varies between 50-70 mm of water. This pressure drop is partly due to frictional loss for flow of gas or vapour through the holes, partly due to the liquid pool on the plate, and partly due to surface tension. In distillation, the required pressure drop is provided by the reboiler while in gas absorption, the same is provided by suitable blower.

Evaluation of height of column with Sieve tray

The conversion of the equilibrium stages to actual stages requires the use of an overall tray efficiency.

The most rigorous method begins with point efficiencies and then converts these efficiencies to an overall tray efficiency. Since this approach is not practicable for multi-component mixtures, a simple analytical expression developed by Lockett (1986) is recommended when no supporting experimental data are available:

EO = 0.492 [nL(aLK/HK)av]-0.245 (6.6)

where

EO = overall tray efficiency,

(aLK/HK)av = average relative volatility between the light and heavy keys, and nL = viscosity of the feed (liquid) mixture.

Since the plate spacing was selected in a prior design step, all the information is now available to

DH = additional height required for column operation.

Detailed design of sieve-tray column has been dealt within literature (Coulson et al. 1985, Barniki and Davies 1989).

In recent years, a number of novel designs of sieve plate columns have been developed. A few important ones have been mentioned here.

Linde trays use slotted plates which not only reduce the hydraulic gradient in large plates but also eliminate stagnant areas in the liquid and approach plug flow conditions.

Counter-flow trays do not have any downcomer. Liquid and gas or vapour flow counter currently through the same openings.

Turbo-grids are sheet metals stamped with slotted openings to form the tray and so arranged that alternate trays have openings at right angles.

Valve tray column

Valve trays are sieve trays with large, up to 40 mm diameter, variable openings for gas flow. The openings are covered with movable caps, which are lifted by the rising gas and act as variable orifices. As the gas flow increases, the valves are pushed up and provide larger space for gas flow.

The valves are usually circular with dome shaped or flat caps. However, rectangular caps and caps with downward cones are also in use but they cannot always rotate as freely as the circular ones. The lifts of the caps are restricted by constructional features like special legs or spiral webs. Some caps have double lift system where the inner light caps are lifted at low gas load. As the gas flow rate

The valves are usually circular with dome shaped or flat caps. However, rectangular caps and caps with downward cones are also in use but they cannot always rotate as freely as the circular ones. The lifts of the caps are restricted by constructional features like special legs or spiral webs. Some caps have double lift system where the inner light caps are lifted at low gas load. As the gas flow rate