Calculate and plot the minimum-external-reflux ratio and the minimum number of equilibrium stages against percent product purity for the separation by distillation of an equimolar bubble-point liquid feed of isobutaneln-butane at 100 psia. The distillate is to have the same iC4 purity as the bottoms is to have nC4 purity. Consider percent purities from 90% to 99.99%. Discuss the significance of the results.