Absorption and stripping

The most common alternative to distillation for the separation of low molar mass materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent that preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. If it is possible to use one of the materials already in the process, this should be done in preference to introducing an extraneous material. Liquid flowrate, temperature and pressure are important variables to be set. 

The vapor-liquid equilibrium for such systems can often be approximated by Henry s Law (see Chapter 4)  

K = vapor-liquid equilibrium K-value For A = 1, the equation takes the form  

If the number of theoretical stages is known, the concentrations can be calculated from  

For multicomponent systems, Equation 10.4 can be written for the limiting component, that is, the component with the highest Kt. Having determined the number of stages, the concentrations of the other components can be determined from Equation 10.6. 

The operations described above require mass transfer of a substance from the gas stream to the liquid. When mass transfer occurs in the opposite direction (i.e., from the liquid to the gas) the operation is called desorption, or stripping. For example, the benzene and toluene are removed from the absorption oil mentioned above by contacting the liquid solution with steam, whereupon the aromatic vapors enter the gas stream and are carried away, and the absorption oil can be used again. Since the principles of both absorption and desorption are the same, we can study both operations simultaneously. 

Absorption and stripping are usually conducted in packed columns or in trayed towers. Packed columns are preferred when (1) the required column diameter is less than 60 cm (2) the pressure drop must be low, as for a vacuum service (3) corrosion considerations favor the use of ceramic or polymeric materials and/or (4) low liquid holdup is desirable. Trayed towers are preferred when (1) the liquid/gas ratio is very low, and (2) frequent cleaning is required. If there is no overriding consideration, cost is the major factor to be taken into account when choosing between packed columns and trayed towers for absorption or stripping. 

Absorption and stripping are technically mature separation operations. Design procedures are well developed for both packed columns and tray towers, and commercial processes are common. In most applications, the solutes are contained in gaseous effluents from chemical reactors. Passage of strict environmental standards with respect to air pollution by emission of noxious gases from industrial sources has greatly increased the use of gas absorbers (also known as scrubbers) in the past decade. 

Program Manager, Rocfceidyne Division Rockwell International Canoga Park, California 

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Kohl, Absorption and Stripping in Rousseau, Handbook of Separation Process Technology, Wiley, 1987. 

Experimental K g< and Ki a data are available for most absorption and stripping operations of commercial interest (see Sec. 15). The solute concentrations employed in these experiments normally are very low, so that Ki a — Ki/i and K g< Pt where pf is the total pressure employed in the actual experimental-test system. Unlike the individual gas-film coefficient /cg, the overall coefficient will... 

Extrapolation of KgO data for absorption and stripping to conditions other than those for which the origin measurements were made can be extremely risky, especially in systems involving chemical reactions in the liquid phase. One therefore would be wise to restrict the use of overall volumetric mass-transfer-coefficient data to conditions not too far removed from those employed in the actual tests. The most reh-able data for this purpose would be those obtained from an operating commercial unit of similar design. 

This section also includes a treatment of distillation-type separations from a rate-based point of view that utilizes principles of mass transfer and heat transfer. Section 14 also presents details of that subject as applied to absorption and stripping. 

Example 2 Calculation of Kremser Method For the simple absorber specified in Fig. 13-44, a rigorous calculation procedure as described below gives results in Table 13-9. Values of were computed from component-product flow rates, and corresponding effective absorption and stripping factors were obtained by iterative calculations in using Eqs. (13-40) and (13-41) with N = 6. Use the Kremser method to estimate component-product rates if N is doubled to a value of 12. 

The HETP of a packed-tower section, valid for either distillation or dilute-gas absorption and stripping svstems in which constant molal overflow can be assumed and in which no chemical reactions occur, is related to the height of one overall gas-phase mass-transfer unit Hqc by the equation... 

Mass-transfer theory indicates that for trays of a given design the factors most hkely to inflnence E in absorption and stripping towers are the physical properties of the flnids and the dimensionless ratio Systems in which the mass transfer is gas-film-controlled may be expected to have plate efficiencies as high as 50 to 100 percent, whereas plate efficiencies as low as 1 percent have been reported for the absorption of gases of low sohibility (large m) into solvents of relatively high viscosity. 

Edmister Method (1957). Edmister has developed an improved procedure that features equations combining absorption and stripping functions as follows ... 

Edmister, W. C., Absorption and Stripping-factor Functions for Distillation Calculation by Manual- and Digital-computer Methods, A.I.Ch.E. Journal, June 1957. 

The area of absorption and stripping is difficult to correlate for the wide range of peculiarities of such systems. The correlation of Gautreaux and O Connell [22] allows... 

Several methods [17, 18, 29, 40, 62, 67, 223] for handling this design have been offered and each has introduced a concept to improve some feature. An approximation method combination of Kremser-Brown [40, 67] and a more complete method of Edmister [18] will be summarized. Figure 8-57 summarizes the system and terminology. The accepted nomenclature for absorption and stripping is located on page 121. 

Figure 8-58. Absorption and stripping factors, Ea or Eg vs. effective values Ag or Se (efficiency functions). Used by permission, Edmister, W. C., Petroleum Engr., Sept. (194 to Jan. (1948).

This method [18] is well suited to handling the details of a complicated problem, yet utilizing the concept of average absorption and stripping factors. It also allows for the presence of solute components in the solvent and the loss of lean oil into the off gas. Reference 18 presents more details than are included here. Reference 18 is Edmister s original publication of the basic method for absorbers and strippers. Reference 18 also generates the... 

Figure 8-83. Allowable mass velocity for fractionation, absorption, and stripping columns.

Edmister, W. C., Hydrocarbon Absorption and Fractionar tion Process Design Methods, Pet. Engr. May 1947-March 1949 and, Absorption and Stripping—Factor Functions for Distillation Calculations by Manual and Digital—Computer Methods. A.I.ChJE. foumal, V. 3, No. 2 p. 165 (1957). 

Kremser-Brown-Sherwood Method — No Heat of Absorption, 108 Absorption — Determine Component Absorption in Fixed Tray Tower, 108 Absorption — Determine Number of Trays for Specified Product Absorption, 109 Stripping — Determine Theoretical Trays and Stripping or Gas Rate for a Component Recovery, 110 Stripping — Determine Stripping-Medium Rate for Fixed Recovery, 111 Absorption — Edmlster Method, 112 Example 8-33 Absorption of Hydrocarbons with Lean Oil, 114 Inter-cooling for Absorbers, 116 Absorption and Stripping Efficiency, 118 Example 8-34 Determine Number of Trays for Specified Product Absorption, 118 Example 8-35 Determine Component Absorption in Fixed-Tray Tower, 119 Nomenclature for Part 2, 121... 

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). 

Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60-90% for gas absorption and stripping, 10-20%. 

Though packed absorption and stripping columns can also be designed as staged process, it is usually more convenient to use the integrated form of the differential equations set up by considering the rates of mass transfer at a point in the column. The derivation of these equations is given in Volume 2, Chapter 12. 

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