Packed towers, separations mass transfer coefficients

Separations in Packed Towers 398 Mass Transfer Coefficients 399 Distillation 401... 

The overall mass transfer coefficients Kg and Ky have units of (moIes)/(time-interfacial area-unit mole fraction driving force). In the case of a wetted-wall column, the interfacial area is known. However, for most types of mass transfer equipment the interfacial area cannot be determined. It is necessary therefore to define a quantity a that is the interfacial area per unit of active equipment volume. Although separate compilations of a can be found in handbooks and vendor literature, this parameter is usually combined with the mass transfer coefficients to define capacity coefficients (k a) and (K a) for the liquid phase and (K,a) or (k,a) for the vapor phase, which then have the dimensions of moles per unit time per unit driving force per unit of active equipment volume. The application of these composite coefficients to the design of packed towers is now demonstrated. 

You want to use an existing 7.4-m packed column to remove 99% of the methylene chloride from an entering stream containing 114 ppm. The methylene chloride will be absorbed by mineral oil in which it is extremely soluble (i.e., m is very small). The column has a surface area per volume of 5 cm /cm, and a mass transfer coefficient Ka of 8.1 10 cm/sec, based on a gas-side driving force. You plan to use an air fiux of 1.4 10 mol/cm sec. (a) Will this column work (b) You have a ten-stage absorption tower with which you may also achieve the same separation. If you still want 99% removal, what absorption factor A (= LjmG) should you use ... 

The effective interfacial area is used in mass transfer studies as an undivided part of individual and overall coefficients when it is difficult to separate and determine the effective area. The work of Shulman et.al.,65 presents a well organized evaluation of other work in addition to their own. One of the difficulties in correlating tower packing performance lies in obtaining the correct values for the effective interfacial areas of the packing on which the actual absorption, desorption, chemical reaction, etc. are completed. Figures 9-47 A, B, C, D, E, F, G present a correlation for Avater flow based on the ammonia-water data of Fellinger [27] and are valid for absorption work. 

In most types of separation equipment such as packed or spray towers, the interfacial area that is effective for mass transfer cannot be accurately determined. For this reason it is customary to report experimentally observed rates of transfer in terms of transfer coefficients based on a unit volume of the apparatus rather than on a unit of interfacial area. Such volumetric coefficients are designated as Kca, kLa, etc., where a represents the interfacial area per unit volume of the apparatus. Experimentally observed variations in the values of these volumetric coefficients with variations in flow rates, type of packing, etc., may be due as much to changes in the effective value of a as to changes in k. Calculation of the overall coefficients from the individual volumetric coefficients is made by means of the equations... 

---