Plantwide design and control of processes with inerts. 1. Light inerts

This is the first paper of a series that explores the design and control of processes containing inert components. Three strategies for the removal of light inerts from a process are compared both from the standpoint of steady-state economics as well as from the standpoint of dynamic controllability. It is shown that the best structure for the case of light inerts (for the inert volatility and range of inert loadings considered) consists of a reactor followed by a full column with side draw where inert is removed as the overhead product, the column bottoms is the plant product, and the side draw is recycled back to the reactor. It is also demonstrated that for certain plant structures, a decrease in production rate can cause long periods of time when the purge valve saturates open. During this time a substantial amount of raw material and product are lost through the purge stream, wasting a considerable amount of money. It is demonstrated that this problem can be eliminated by changing the setpoint of the purge composition controller whenever production rate changes occur.