Economic Optimization And Control Of Pressure-swing Distillation With Intermediate Connection For Separating Binary Azeotropic Mixture

Pressure-swing distillation is an effective method for separating pressure-sensitive azeotropic mixtures in industrial production,but energy consumption is the main obstacle to its development.In this article,an improved process with an intermediate connection was developed based on the two-column pressure-swing distillation.Taking the separation of ethyl acetate-ethanol,methanol-chloroform and ethylenediamine-water as examples,the energy-saving effect and controllability of pressure-swing distillation with intermediate connection were studied.Based on the sequential iterative optimization algorithm and taking the minimum total annual cost(TAC)as the objective function and using the Visual Basic software(VB)to invoke the automation server of Aspen Plus,A VB optimization program for pressure-swing distillation with intermediate connection process was developed.The optimal operating conditions and TAC of the pressure-swing distillation with intermediate connection for separating three case systems are obtained by this developed optimization program.Through the comparison of the optimization results,it can be seen that the pressure-swing distillation with intermediate connection has the effect of reducing the energy consumption of the distillation process.The pressure-swing distillation with intermediate connection process for separating ethyl acetate-ethanol is superior to the optimum of the PSD process,for 8.17 % in energy consumption savings and a 7.91 % reduction in the TAC.Compared with conventional partial heat-integrated pressure-swing distillation,the separation using the new partial heat-integrated two-column process leads to a significant reduction in the energy consumption,of up to 18.32%and with a TAC savings of up to 15.21%.After pressure optimization,the energy and TAC can save 22.41%/19.77%.For the methanol-chloroform system,the energy-saving effect of the side-line connection variable pressure distillation process is not obvious.For ethylenediamine-water separation process,compared with the optimal results of the conventional process,the pressure-swing distillation with the intermediate connection process is more promising,with 10.86%energy savings and a 8.64%reduction in the TAC.Compared with the conventional partial heat-integrated process,the new process can bring more energy savings,of up to17.86%,and a TAC reduction of 13.45%.The results show that the energy-saving effect of ethyl acetate-ethanol system and ethylenediamine-water system is obvious and the pressure has significant effect on the energy-saving effect of the pressure-swing distillation with intermediate connection.Through the analysis of the liquid composition profiles for the pressure-swing distillation with intermediate connection process and conventional pressure-swing distillation process,it can be seen that the increase of the side stream makes the liquid composition profiles in the column different,and the number of stages is re-optimized,but whether this is the main reason for improving the efficiency of the distillation column and realizing energy saving is worth further study.Dynamic controls of pressure-swing distillation with an intermediate connection process for ethyl acetate-ethanol separation were investigated.The double temperature/composition cascade control structure can perfectly achieve effective control when ±20% feed disturbances are introduced.To avoid the use of component controllers,a replacement scheme is proposed.The original product purities could be maintained using the basic control structure for the PSDIC process if the product purities in the steady state were properly increased.This alternative method provides a simple and effective control scheme in industrial production.