The isolation of a short polypeptide yields 230 kg of butanol extract containing 0.081 wt% peptide. We want to strip this extract with initially peptide-free saline solution at pH 9.0 to yield a product solution containing 0.2 wt% peptide. The equilibrium under these conditions is

y = 0.16x

Where y = mass fraction in butanol, and x = mass fraction in water. We want to recover 98% of this peptide.

How many countercurrent stages do we need for this extraction?