Reactors with two process streams in countercurrent flow

Reactors with two process streams in countercurrent flow 

This is a situation with particular importance, e.g. for gas absorption. The equations are similar to the ones given in the preceeding section, though the sign of one of the mass balances is reversed  

This equation has to be combined with eq. (7.40) and (7.42). The mass balance over a section of the reactor is 

Note that the exit concentration of reactant A is measured at the end of the tubular reactor where B enters. Generally, only the entrance concentrations are known. For countercurrent processes, one may use a simple iterative method, assuming a certain exit concentration of A, 

Let us consider an example of great practical interest, the chemically enhanced gas absorption, where the mass transfer resistance in the gas phase can be neglected. For this situation eq. (5.44) can be written as 

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