Investigation of distillation systems using heavy or intermediate entrainers for separating toluene–methanol: process economics and control

BACKGROUND The separation of methanol–toluene by introducing aniline or triethylamine (Et3N) is investigated, with aniline and Et3N as heavy and intermediate entrainers, respectively. First, the conventional extractive distillation process with aniline as the extractive entrainer (HE) and the direct/indirect separating sequences using Et3N as intermediate entrainer (IE‐D/IE‐I) have been designed and optimized with rigorous simulations. Subsequently, the dynamic performances of these optimized processes are investigated with simple temperature control strategies. Finally, the steady‐state process economics and dynamic performances for these processes are facilitated. RESULTS The IE‐I process has a 7.26% total annual cost (TAC) saving compared with the HE process and the total energy cost of the IE‐I process is 18.79% lower than that of the HE process. As to dynamic performances, the maximum transient deviation of the methanol product purity in the IE‐I process is much smaller than that in the HE process when ±20% feed flow rate disturbances are introduced. And when −20% step toluene composition disturbance is introduced, the new steady‐state methanol product purity in the HE process is 99.5 mol% while that in the IE‐I process is 99.8%. CONCLUSIONS The numerical and graphic comparisons indicate that the IE‐I process possesses advantages over the HE process to some extent. The small advantages possessed by the IE‐I process are mainly those of the use of lower‐grade heat source, lower energy consumption and smaller dynamic deviations under feed disturbances.