(120d) Mass Transfer - Thinking beyond Theoretical Stages

Equilibrium stage calculations have become nearly universal calculation tools for mass transfer columns contacting vapor and liquid in countercurrent flow. Whether the service is absorption, stripping, distillation, fractionation, quench, or evaporation, equilibrium stage models make mass and heat balances easy and complete.

In some columns, equilibrium or theoretical stages are a good model of what the vapor/liquid contacting devices provide. But theoretical stages are often a poor model of what trays and packing provide. In such cases, equilibrium stage calculations are still highly useful for establishing heat and mass balances, for finding stream conditions, and for estimating stream properties. But then the engineer must realize that correlating tray or packing performance to theoretical stages is deceptive, and he or she should not force-fit a design tray efficiency or packing HETP to a column design. Many times, a force-fit tray efficiency of 30% or a packing HETP three times the standard HETP is interpreted as equipment that doesn't work very well. In reality, it is the reliance on the theoretical stage model that doesn't work very well.

This paper discusses columns in which theoretical stages are a poor representation of tray or packing performance, and it provides guidelines to help an engineer find a more consistent and logical model.