Effect of Diffusion on Efficiency of Fractionating Tower Contact Devices

Abstract

A study has been made of 13 industrial fractionating towers having a variety of plate contact devices (S-valve, straight-through valve, rectangular-cap, and Glitch valve devices) and 11 industrial fractionating towers having a variety of cross-flow packing contact devices. It is demonstrated that the efficiency of contact devices of various designs is less in the stripping sections of the fractionating towers than in the concentrating sections. Analysis of the mechanisms of diffusion of fractionated components in vapor and liquid phases in contact devices shows that the diffusion coefficient in the vapor phase decreases from the top to the bottom of the tower as the concentration of high-boiling components increases. This reduces mass transfer from one phase to another in the contact device in the stripping section of the tower and, as a result, reduces the efficiency of the contact device in this section of the tower. It is shown that elevation of pressure in the tower and increased temperature gradient across the tower height cause further decline of the contact device efficiency in the stripping section of the tower.