Design And Control Of Conventional And Entrainer-assisted Pressure-swing Distillation For Separating Binary Azeotropes

Pressure-swing distillation(PSD)is a widely-used method for separating azeotropes.The researches about steady-state design and dynamic control of PSD process are very important for industrial practices.At present,however,the researches about PSD for separating azeotropes mainly focus on conventional PSD(C-PSD)for separating the minimum-boiling azeotropes.The researches about C-PSD for separating the maximum-boiling azeotropes are few,and the existing researches about dynamic control have demonstrated some difficulties.Besides,as a novel method for separating azeotropes,entrainer-assisted PSD(EA-PSD)can be effectively applied to separate pressure-insensitive azeotropes,which broadens the separated mixtures of PSD.However,the research about EA-PSD is few.This research aims to solve the existed dynamic problems in C-PSD for separating the maximum-boiling azeotropes and conduct systematic investigation in steady-state design and exploratory investigation in dynamic control of EA-PSD,in order to provide important references for the industrial application of PSD.(1)The dynamic control of C-PSD for separating pressure-sensitive binary maximum-boiling azeotrope was investigated.For C-PSD for separating the maximum-boiling azeotrope methanol/trimethoxysilane,the dynamic difficulties of the high-pressure column(HPC)-low pressure column(LPC)sequence were solved with three new effective control structures proposed,while four effective control structures were proposed for the LPC-HPC sequence.The results revealed that,manipulating the reboiler duty to control the sump level in the HPC and controlling the bottom flow rate of the HPC at the initial value was the crucial control strategy for the former two processes to achieve effective control.Then,this crucial control strategy was verified by the application in C-PSD for separating the maximum-boiling azeotrope ethylenediamine/water with two new effective control structures proposed.(2)The HPC-LPC sequence of EA-PSD for separating pressure-insensitive minimum-boiling azeotrope methanol/toluene was proposed.Several partially and fully heat-integrated PSD were established and their economics were compared with extractive distillation processes.The results showed that the most economical EA-PSD had 5.39% reduction of TAC and 8.32% reduction of operating cost than the most economical extractive distillation process.By establishing and analyzing the control structures of several EA-PSD processes,the dynamic control of EA-PSD for separating binary minimum-boiling azeotrope was investigated for the first time.The results revealed that: the enthalpic state of the fresh feed of the HPC had crucial influence on the dynamic controllability of the process;the control mode of the connecting flow had important influence on the dynamic controllability of the process;the application of a high-selector to the feedback control loop of the reflux flow rate was effective to improve the dynamic performances.(3)For the LPC-HPC sequence of EA-PSD for separating pressure-insensitive minimum-boiling azeotrope toluene/pyridine,n-propanol was screened as a new entrainer and two-column sequence and three-column were established.Based on the two-column sequence,the dynamic control of the LPC-HPC sequence of EA-PSD for separating the minimum-boiling azeotrope was investigated for the first time.The results showed that two-column sequence had 16.07% reduction of TAC and 14.39%reduction of operating cost than three-column sequence.For the two-column sequence,manipulating the recycling stream to control the pyridine product purity was the crucial control strategy to make the process effectively controlled.(4)Taking the separation of the maximum-boiling azeotrope phenol/cyclohexanone as an example,two parts were implemented to investigate how to improve the dynamic performances of EA-PSD for separating pressure-insensitive binary maximum-boiling azeotrope.The first point was the control mode of the recycling stream.The second was the application of a high-selector.The results showed that the control mode of the recycling flow had important influence on the dynamic controllability of the process;the application of a high-selector to the feedback control loop of the reflux flow rate or reboiler duty was effective to improve the dynamic performances.