where S, and 8V are respectively the height of liquid and vapor films (m), AS, and

ASv are the corresponding volume of liquid and vapor film (m3), respectively, and L and

V are the flow of liquid and vapor phases (mol s"1), respectively. K\'\a and K]°ka are the binary mass transfer coefficients, but their units are mol m"3 s"1.

Bulk liquid Liquid film Vapor film Bulk vapor

Bulk liquid Liquid film Vapor film Bulk vapor

Fig. 3. Schematic diagram of two-film theory; the direction of mass transfer is assumed from vapor phase to liquid phase.

The schematic diagram of two-film theory is shown in Fig. 3. The entire resistance to mass transfer in a given turbulent phase is in a thin, stagnant region of that phase at the interface, called a film. This film is similar to the laminar sublayer that forms when a fluid flows in the turbulent regime parallel to a flat plate.

So we can calculate a and on the condition that K\ka and Kvlka are known, and then [k'lkd\ and [k^ka\ are determined by Eqs. (181) and (185). Let us see one example about how to obtain K'na and Kvlka by means of the AIChE method.

Example: In a distillation column with sieve tray, the operation conditions are listed in Table 13. This table is designed for the convenience to be dealt with by MS Excel software. The mixture on the sieve tray is composed of 50 mol% benzene (1) and 50 mol% propylene (2) for the liquid phase, and 10 mol% benzene (1) and 90 mol% propylene (2) for the vapor phase

(T = 80°C, P = 700kPa). Compute K\\a and tc]ka (kmol s"1) on the sieve tray. Table 13

The operation conditions on the sieve tray


Physical quantity



The vapor volume flowrate (m3 h"1)

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