Process Synthesis And Control Strategy Of Hybrid Process For Separating Propylene Glycol Methyl Ether And Water Binary Mixture

Propylene glycol methyl ether and water mixture forms a binary azeotrope at atmospheric.The binary mixture,comprising 92.2 mol%water and 7.8 mol%propylene glycol methyl ether at a flow of 1000 kmol/h was separated to study liquid-liquid extraction combined with either heterogeneous azeotropic distillation or extractive distillation processes.Economic optimization based on the minimum total annual costs（TAC）was performed for the two hybrid processes.The dynamic characteristics of the two hybrid processes were studied and corresponding robust dynamic control structures were developed.The extraction effect of extractant and feasibility of hybrid processes were studied with the help of the equilibrium curves and the ternary phase diagrams with residue curve by the NRTL thermodynamic model.Based on a sequential iterative optimization procedure,the hybrid processes were optimized.The TAC and CO₂emissions for the liquid-liquid extraction combined with heterogeneous azeotropic distillation process were 3478014$/y and 2762.380 kg/s,which can achieve significant reductions of 40.24%in the TAC and 45.37%in CO₂ emissions compared with the traditional two-column heterogeneous azeotropic distillation process.The TAC and CO₂ emissions for the liquid-liquid extraction combined with extractive distillation process were 3112250$/y and 2168840 kg/s,which can achieve significant reductions of 34.18%in the TAC and 43.95%in CO₂ emissions compared with traditional three-column extractive distillation process.It is found that the hybrid processes with liquid-liquid extraction have a great advantage over the traditional special distillation process in terms of economy,energy consumption and CO₂ emissions.Based on the optimal steady states of the two hybrid processes,the dynamic control structures for the liquid-liquid extraction combined with either heterogeneous azeotropic distillation or extractive distillation processes were investigated.Two types of disturbances which are±20%feed flow rate and composition disturbances will be introduced to evaluate the dynamic performance of the control structure.The results show that the ratio control structure of reboiler duty to feed flow has a good effect on the improving the feed disturbance lag reaction and the ability of handling disturbance of the structure.The composition-temperature cascade control structure can handle the disturbances and maintain the product purity well for the liquid-liquid extraction combined with heterogeneous azeotropic distillation process.An improved dual temperature control structure can deal with the lack of product purity and improve the ability of handling disturbance for the liquid-liquid extraction combined with extractive distillation process.