(156b) Debottlenecking of a Semi-Batch Vacuum Distillation Column in a Chemical Plant

Purification of a dilute organic mixture (1 wt% light component) at a chemical plant was carried out by vacuum distillation in semi-batch mode, with continuous feed and bottoms flow but accumulation of the light component in the overhead receiver up to at least 35 times its feed concentration. The column is equipped with a forced-circulation reboiler, a spray condenser, and two beds of 3^rd-generation random packing (IMTP). This purification step was the bottleneck for production. A dynamic simulation model was constructed to allow the capacity rating of both the column and its reboiler. High pressure drop in the packed bed, combined with low heat-transfer coefficients in the reboiler due to laminar flow (in the single-phase zone) and film boiling, were found to be responsible for the current capacity limitation. To overcome these limitations, installation of 4^th-generation random packing, together with the use of turbulators inside the reboiler tubes and an increase in the recirculation flow rate, were recommended. With the proposed changes, film boiling was no longer flagged in the calculations, and the maximum feed rate to the column increased by 12%.