The number of stages decreases while the solvent-to-feed ratio
increases as the reflux ratio increases. A high value of Nth leads to a
low solvent flow rate, thus reducing the column internal diameter.
A low value of Nth leads to high solvent flow rates, thus increasing
the column internal diameter. Besides the number of stages,
the solvent-to-feed ratio is the most important design variable in
countercurrent gas extraction, because of its great impact on the
column diameter and compression costs.