In Section 12.9 we noted that co-current operation of absorbers was often employed when a single equilibrium stage was sufficient. Co-current operation has the advantage that flooding cannot occur. This means that high vapor and liquid flow rates can be used, which automatically leads to small-diameter columns.
A schematic of a co-current absorber is shown in Figure 16-9A. The analysis will be done for dilute systems using overall mass transfer coefficients. The system is assumed to be isothermal. The liquid and vapor are assumed to be in plug flow, and total flow rates are constant. The rate of mass transfer in segment dz is
which can be related to the changes in solute flow rates