Bubble-plate towers

The objective of absorption equipment is to provide intimate contact between the liquid concerned and gas, thus facilitating the absorption process. When a large volume of gas is absorbed per unit volume of liquid, a marked rise in temperature may result and devices for the removal of the heat evolved are then an important feature of the equipment. Apart from packed towers, the types of equipment described here include agitated vessels, bubble-plate towers, cooler-absorbers and spray towers. 

An equimola mixture of benzene and toluene is to be separated in a bubble-plate tower at the rate of 100 kg mol/h at 1 atm pressure. The overhead product must contain at least 98 mole percent benzene. The feed is saturated liquid. A tower is available containing 24 plates. Feed may be introduced either on the eleventh or the seventeenth plate from the top. The maximum vaporization capacity of the reboiler is 120 kg mol/h. The plates are about 50 percent efficient. How many moles per hour of overhead product can be obtained from this tower ... 

Fig. 6-30. Installed cost of packed ani bubble plate towers (ENR = 760).

For equipment protection such as in condensers, storage tanks, heat exchangers, and bubble plate towers... 

Design of Packed Tower. Ideally, the interaction between vapor and liquid in packed towers is true countercurrent rather than the stepwise-countercurrent propess of a bubble-plate tower with theoretical plates. Instead of finite steps, the true countercurrent action should be treated differentially. Consider the schematic drawing of the packed tower in Fig. 7-29. Let 0 be the mols of overflow, V the mols of vapor, x and y the average mol fraction in the liquid and vapor, respectively, n distance above and m distance below the feed. Focusing on the differential section dn, V(dy/dn) must equal 0 dx/dn) for each component, and this transfer must be due to an exchange of components back and forth between the liquid and vapor. This transfer is due to the fact that the vapor and liquid at a given cross section are not in equilibrium with each other, and the rate of transfer will be a function of the distance from equilibrium thus,... 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

FIG. 14-79 Cost of trays in plate towers. Price includes tray deck, bubble caps, risers, downcomers, and structural-steel parts. The stainless steel designated is type 410 Peters and Timmerhaus, Plant Design and Economics for Cbemical Engineers, 4th ed., McGraw-Hill, New York, 1.9.91). 

Figure 24.1 Types of tower or column reactors for gas-liquid reactions (a) packed tower, (b) plate tower, (c) spray tower, (d) falling-film tower, (e) bubble column...

Plate tower. A plate tower (Figure 24.1(b)) contains, for example, bubble-cap or... 

Dispersions may be classified into two types, based upon size range of the droplets formed. Turbulence creators (mixing impellers, mixing valves, eductors, orifice plates) will produce fine emulsions of micron-size droplets. Nozzles, perforated plates, bubble caps, tower packings, etc., can form discrete drops of relatively large size which will quickly settle through the continuous phase. 

Bubble cap an inverted cup with a notched or slotted periphery to disperse the vapor in small bubbles beneath the surface of the liquid on the bubble plate in a distillation tower. 

The absorption can be performed in different types of unit, for example spray tower, packed bed, plate tower, bubble column, static mixer, and bubble tank. Gas-liquid interfacial area and liquid fraction are criteria for equipment selection. 

Many modifications of the three types of contactors just discussed have been developed in an effort to reduce costs, reduce pressure drop, equalize vapor flow through each contactor, increase plate efficiencies, or, in general, improve the operating performance of the tower. An example of this for modification of bubble-cap towers is the old Uniflux tray originally developed by Socony-Vacuum, which consisted of a series of interlocking S-shaped sections which were assembled in the form of tunnel caps with slot outlets on one side only. Segmental downcomers, similar to the downcomers in conventional bubble-cap columns, were provided. The vapors issued from the Uniflux caps in... 

Cost of trays in plate towers. Price includes tray deck, bubble caps, risers, downcomers, and structural-steel parts. 

Other Types of Wet Scrubbers. Plate towers, like sieve, valve, and bubble cap towers, and packed beds have been used in the past for dust collection but these are all subject to plugging. 

Filters, heat exchangers, dryers, bubble cap columns, cyclones, etc., are ordinarily designed and built by process equipment manufacturers. However, units of special design for one-of-a-kind operations such as packed or plate towers are quite often designed and built under the supervision of plant engineers. Thus, there is a large variety of this type of equipment, none of it essentially standard. 

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