Straight downcomer

Clearance under i ___down comer U-l

[*-Flow path-» 1 length and bubbling area

- Recessed seal pan

Figure 2. Tray component terminology.

- Recessed seal pan

Figure 2. Tray component terminology.

0 m Clin

Figure 3. Typical tray of integral truss design.

Figure 4. Cartridge tray assembly.

comer design also is particularly important at high operating pressures due to a reduction in the difference between vapor and liquid densities.

The lower limit of tray operation, meanwhile, is influenced by the amount of liquid weeping from one tray to the next. Unlike the upward force of entrainment, weeping liquid flows in the normal direction and considerable amounts can be tolerated before column efficiency is significantly affected. As the vapor rate decreases, however, a point eventually is reached when all the liquid is weeping and there is no liquid seal on the tray. This is known as the dump point, below which there is a severe drop in efficiency.

Sieve Tray. The sieve tray is a low-cost device that consists of a perforated plate that usually has holes of 1/16 in. to 1 in. diameter, a down-comer, and an outlet weir. Although inexpensive, a correctly designed sieve tray can be comparable to other styles in vapor and liquid capacities, pressure drop, and efficiency. For flexibility, however, it is inferior to valve and bubble-cap trays and sometimes is unacceptable for low liquid loads when weeping has to be minimized.

Depending on process conditions and allowable pressure drop, the turndown ratio of a sieve tray can vary from 1.5 to 3 and occasionally higher. Ratios of 5, as sometimes claimed, can be achieved only when the tray spacing is large, available pressure drop is very high, liquid loadings are high, and the system is nonfoaming. For many applications, a turndown of 1.5 is acceptable.

It also is possible to increase the flexibility of a sieve tray for occasional low throughput operation by maintaining a high reboil and increasing the reflux ratio. This may be economically desirable when the low throughput occurs for a small fraction of the operating time. Flexibility, likewise, can be increased by the use of blanking plates to reduce the hole area. This is particularly useful for initial operation when it is proposed to increase the plant capacity after a few years. There is no evidence to suggest that blanked-off plates have inferior performance to unblanked plates of similar hole area.

Dual-Flow Tray. The dual-flow tray is a high hole areas sieve tray without a down-comer. The liquid passes down through the same holes through which the vapor rises. Because no down-comer is used, the cost of the tray is lower than that of a conventional sieve tray.

In recent years, use of the dual-flow tray has declined somewhat because of difficulties experienced with partial liquid/vapor bypassing of the two phases, particularly in larger diameter columns. The dual-flow column also has a very restricted operating range and a reduced efficiency because there is no cross flow of liquid.

Valve Tray. Although the valve tray dates back to the rivet type first used in 1922, many design improvements and innumerable valve types have been introduced in recent years. A selection of modern valves as illustrated provide the following advantages (Fig. 5):

1. Throughputs and efficiencies can be as high as sieve or bubble-cap trays.

2. Very high flexibility can be achieved and turndown ratios of 4 to 1 are easily obtained without having to resort to large pressure drops at the high end of the operating range.

3. Special valve designs with venturi-shaped orifices are available for duties involving low pressure drops.

4. Although slightly more expensive than sieve trays, the valve tray is very economical in view of its numerous advantages.

5. Because an operating valve is continuously in movement, the valve tray can be used for light-to-moderate fouling duties. Valve trays can be success

Figure 5. (a) Special two-stage valve with lightweight orifice cover for complete closing; (b) and (c) two typical general purpose valves which may be used in all types of services.

Figure 5. (a) Special two-stage valve with lightweight orifice cover for complete closing; (b) and (c) two typical general purpose valves which may be used in all types of services.

fully used on brewery effluent containing waste beer, yeast, and other materials with fouling tendencies.

Bubble-Cap Tray. Although many bubble-cap columns still are in operation, bubble-cap trays rarely are specified today because of high cost factors and the excellent performance of the modern valve-type tray. The bubble cap, however, does have a good turndown ratio and is good for low liquid loads.

Baffle Tray. Baffle trays are arranged in a tower in such a manner that the liquid flows down the column by splashing from one baffle to the next lower baffle. The ascending gas or vapor, meanwhile, passes through this curtain of liquid spray.

Although the baffle-type tray has a low efficiency, it can be useful in applications where the liquid contains a high fraction of solids.


For many types of duties, particularly those involving small-diameter columns, packing is the most economical tower internal. One advantage is that most packing can be purchased from stock on a cubic-foot basis. In addition, the mechanical design and fabrication of a packed column is quite simple. Disadvantages of packing include its unsuit-ability for fouling duties, breakage of ceramic packing, and, according to some reports, less predictive performance, particularly at low liquid loads or high column diameters.

The most widely used packing is the random packing, usually Rashig rings, Pall rings, and ceramic saddles. These are available in various plastics, a number of different metals and, with the exception of Pall rings, in ceramic materials. Although packings in plastic have the advantage of corrosion resistance, the self-wetting ability of some plastic packing, such as fluorocarbon polymers, sometimes is poor, particularly in aqueous systems. This considerably increases the HETP when compared with equivalent ceramic rings.

High-efficiency metal mesh packing as shown in Figure 6 has found increasing favor in industry during recent years. One type uses a woven wire mesh that becomes self-wetting because of capillary forces. This helps establish good liquid distribution as the liquid flows through the packing in a zig-zag pattern.

Figure 6. Segment of high-efficiency metal mesh packing.

If properly used, high efficiency packings can provide HETP values in the range 6-12 in. This can reduce column heights, especially when a large number of trays is required. Such packings, however, are very expensive, and each application must be studied in great detail.

With random and, in particular, high-efficiency packing, considerable attention must be given to correct liquid distribution. Certain types of high-efficiency packing are extremely sensitive to liquid distribution and should not be used in columns more than 2 ft in diameter. Positioning of these devices and the design of liquid distribution and redistribution are important factors that should be determined only by experts.


One of the most important aspects of any distillation system is the ability to maintain the correct compositions from the columns by means of proper controls and instrumentation. Although manual controls can be supplied, this approach rarely is used today in the United States. Manual control involves the extensive use of rotameters and thermometers, which, in turn, involves high labor costs, possible energy wastage, and, at times, poor quality control. Far better control is obtained through the use of pneumatic or electronic control systems.

Pneumatic Control Systems. The most common form of distillation column instrumentation is the pneumatic-type analogue control system. Pneumatic instruments have the advantage of being less expensive than other types and, because there are no electrical signals required, there is no risk of an electrical spark. One disadvantage is the need to ensure that the air supply has a very low dew point (usually — 40°F) to prevent condensation in the loops.

Electronic Control Systems. Essentially, there are three types of electronic control systems:

1. Conventional electronic instruments.

2. Electronic systems with all field devices explosion proof.

3. Intrinsically safe electronic systems.

The need to have a clear understanding of the differences is important. Most distillation duties involve at least one flammable liquid that is being processed in both the vapor and liquid phases. Because there always is the possibility of a leak of liquid or vapor, particularly from pump seals, it is essential for complete safety that there be no source of ignition in the vicinity of the equipment. Although many instruments, such as controllers and alarms, can be located in a control room removed from the process, all local electronic instruments must be either explosion proof or intrinsically safe.

With explosion-proof equipment, electrical devices and wiring are protected by boxes or conduit that will contain any explosion that may occur. With intrinsically safe equipment, barriers limit the transmission of electrical energy to such a low level that it is not possible to generate a spark. As explosion-proof boxes and conduits are not required, wiring costs are reduced.

For any intrinsically safe system to be accepted for insurance purposes, FM (Factory Mutual) or CSA (Canadian Standards Association) approval usually must be obtained. This approval applies to a combination of barriers and field devices. Therefore, when a loop incorporates such instruments from different manufacturers, it is essential to ensure that approval has been obtained for the combination of instruments.

Auxiliary Equipment

In any distillation system, the design of auxiliary equipment such as the reboiler, condenser, preheaters, and product coolers is as important as the design of the column itself.

Reboiler. Although there are many types of reboilers, the shell-and-tube thermosyphon reboiler is used most frequently. Boiling within the vertical tubes of the exchanger produces liquid circulation and eliminates the need for a pump. A typical arrangement is shown in Figure 7.

For certain duties, particularly when the bottoms liquid has a tendency to foul heat transfer surfaces, it is desirable to pump the liquid around the heat exchanger. Because boiling can be suppressed by use of an orifice plate at the outlet of the unit, fouling is reduced. The liquid being pumped is heated under pressure and then is flashed into the base of the column where vapor is generated.

An alternative approach is the use of a plate heat exchanger as a forced-circulation reboiler. With this technique, the very high liquid turbulent flow that is induced within the heat exchanger through the use of multiple corrugated plates holds fouling to a minimum. Meanwhile, the superior rates of heat transfer that are achieved reduce the surface area required for the reboiler.

Condensers. Because most distillation column condensers are of shell-and-tube design, the processor has the option of condensing on either the shell or tube side. From the process point of view, condensation on the shell side is preferred because there is less subcooling of condensate and a lower pressure drop is required. These are important factors in vacuum duties. Furthermore, with cooling water on the tube side, any fouling can be removed more easily.

Tube-side condensation, on the other hand, can be more advantageous whenever process fluid characteristics dictate the use of more expensive, exotic materials. Capital cost of the unit then may be cut by using a carbon steel shell.

Preheaters/Coolers. The degree to which fluids are aggressive to metals and gasketing materials generally determines the selection of plate or shell-and-tube preheaters and product coolers. If fluids are not overly aggressive toward gasket materials, a plate heat exchanger is an extremely efficient preheater because a very close temperature approach may be achieved. Added economy is realized by using heat from the top and bottoms product for all necessary preheating.

Very aggressive duties normally are handled in a number of tubular exchangers arranged in series to generate a good mean temperature difference. The use of multiple tubular units obviously is more expensive than a single plate heat exchanger but is unavoidable for certain solutions such as aromatic compounds.

Vent Condenser. It is normal practice on distillation systems to use a vent condenser after the main condenser to minimize the amount of volatiles being driven off into the atmosphere. Usually of the shell-and-tube type, the vent condenser will have about one-tenth the area of the main unit and will use a chilled water supply to cool the noncon-densible gases to about 45-50°F.

Base of

Base of


Pumps. Because most distillation duties involve fluids that are highly flammable and have a low flash point, it is essential that explosion-proof (Class 1, Group D, Division 1) pump motors be supplied. Centrifugal pumps generally are specified as they are reliable and can provide the necessary head and volumetric capacity at moderate costs.

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