Absorption in Plate and Packed Towers

6 ABSORPTION IN PLATE AND PACKED TOWERS 10.6A Equipment for Absorption and Distillation 

Introduction to absorption. As discussed briefly in Section 10.IB, absorption is a ma s-transfer process in which a vapor solute. 4 in a gas mixture is absorbed by means of a liquid in which the solute is more or less soluble. The gas mixture consists mainly of an inert gas and the solute. The liquid also is primarily immiscible in the gas phase i.e., its vaporization into the gas phase is relatively slight. A typical example is absorption of the solute ammonia from an air-ammonia mixture by water. Subsequently, the solute is recovered from the solution by distillation. In the reverse process of desorption or stripping, the same principles and equations hold. 

10 Stage and Continuous Gas-Liquid Separation Processes 

Various types of tray (plate) towers for absorption and distillation. In order to efficiently contact the vapor and liquid in absorption and distillation, tray (plate) towers are often used. A very common type of tray contacting device is the sieve tray, which is shown schematically in Fig. 10.6-la and in Section 11.4A for distillation. 

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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. 

Efficient contact is produced between the phases in agitated gas-liquid contactors and, therefore, this type of equipment can also be useful for those absorption and stripping operations for which conventional plate or packed towers may not be suited. It may also be useful where the operation involves the contact of three phases—say, gas, liquid, and suspended solids. The latter application could be represented by the low-pressure polymerization of ethylene with solid catalysts (F5). 

Before presenting cost data for plates and packings in distillation, absorption, and stripping towers, the following example is presented to estimate the purchase cost of an adiabatic, homogeneous, gas-phase reactor. 

Equipment Absorption, stripping, and distiUation operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed. The gas and liquid normally flow countercurrently, and the devices serve to provide the contacting and development of interfacial surface through which mass transfer takes place. Background material on this mass transfer process is given in Sec.. 5. 

The problems relating to mass transfer may be elucidated out by two clear-cut yet different methods one using the concept of equilibrium stages, and the other built on diffusional rate processes. The selection of a method depends on the type of device in which the operation is performed. Distillation (and sometimes also liquid extraction) are carried out in equipment such as mixer settler trains, diffusion batteries, or plate towers which contain a series of discrete processing units, and problems in these spheres are usually solved by equilibrium-stage calculation. Gas absorption and other operations which are performed in packed towers and similar devices are usually dealt with utilizing the concept of a diffusional process. All mass transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

Absorption is a commonly applied operation in chemical processing. It is used as a raw material or a product recovery technique in separation and purification of gaseous streams containing high concentrations of organics (e.g., in natural gas purification and coke by-product recovery operations). In absorption, the organics in the gas stream are dissolved in a liquid solvent. The contact between the absorbing liquid and the vent gas is accomplished in countercurrent spray towers, scrubbers, or packed or plate columns. 

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. 

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