Two-Film Mass-Transfer Model for Gas-Liquid Systems

2 Two-Film Mass-Transfer Model for Gas-Liquid Systems 

The rate of mass transfer of A may also be characterized in terms of overall mass transfer coefficients KAg and KA( defined by 

Each of these last two equations represents the additive contribution of gas- and liquid-film resistances (on the right) to the overall resistance (on the left). (Each mass transfer coefficient is a conductance and its reciprocal is a resistance ) 

Special cases arise from each of equations 9.2-13 and -14, depending on the relative magnitudes of kAg and kA(. For example, from equation 9.2-13, if kAg is relatively large so that VkAg HAlkA(, then KAg (and hence NA) is determined entirely by kA(, and we have the situation of liquid-film control. An important example of this is the situation in which the gas phase is pure A, in which case there is no gas film for A to diffuse through, and kAg - . Conversely, we may have gas-film control. Similar conclusions may be reached from consideration of equation 9.2-14 for KA(. In either case, we obtain the following results  

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