Control of distillation columns with dual steam and hot-oil reboilers

Abstract

Many distillation columns achieve process intensification by utilizing hot streams from other units in the plant to substitute for some of the utility-supplied steam used in the reboiler. This thermal coupling requires adequate differential temperature driving forces so that heat-exchanger area is not excessive. It also requires that the units are reasonably close together so that piping cost and pressure drops are not too large. There are two reboilers at the base of the column that operate in parallel with the two independent heat sources providing vapor boilup. If the external hot stream is vapor that is condensed in the one reboiler, the heat transferred is latent heat. If the external hot stream is hot liquid, the heat transferred is sensible heat, which requires much larger flowrates. Since pumping liquid is relatively inexpensive, larger flowrates and larger pressure drops are less important than in vapor streams.This paper explores the dynamic control of dual-reboiler distillation columns in which a hot liquid stream from another unit is fed to one reboiler and steam is fed as needed to the other reboiler. The key simulation implementation issue is how to dynamically model the heat transfer in the hot-oil reboiler since it is governed by log-mean differential temperature driving forces. Several alternative control structures are evaluated.