Separation Of Pyridine-water By Unconventional Distillation: Process Design, Economic Optimization And Control

Two unconventional distillation methods for pyridine-water separation werepresented by using pressure swing distillation and heterogeneous azeoteropicdistillation with n-heptane as an entrainer. Rigorous steady state simulations wereimplemented on chemical process simulation software Aspen Plus. To obtain theoptimized configurations for the two processes，sequential iterative optimizationprocedures combining partial economical optimization with global economicaloptimization were proposed. Whether a design can be applied in practice stronglydepends on weather it can be effectively controlled. Dynamic simulations wererealized using commercial software Aspen Dynamics to test the effectiveness ofcontrol schemes.Simulation calculation results showed that, when the feed composition is10wt%pyridine and90wt%water, compared with the pressure swing distillation without heatintegration, fully heat-integrated pressure swing distillation is more energy-saving andeconomical, with energy decreased by43.38%and total annual cost decreased by38.75%. Two control schemes using PI controllers were explored. Dynamicsimulation results revealed that the basic control scheme CS1cannot maintain thequality specifications as a result of pressure fluctuation. Then, differential temperaturecontrol and pressure-compensated temperature control were introduced on the basis ofCS1. An improved control scheme CS2was proposed. The corresponding dynamicresponses showed that control scheme CS2works effectively for the same feed flowrate and feed composition disturbances. The dynamic control performances areevidently improved. It does a pretty good job in maintaining the quality of the twoproducts.For the same feed composition, improved two-column process was chosen inheterogeneous azeotropic distillation. The optimal concentration of pyridine in recyclestream is54wt%. The basic control scheme can maintain the product purities welleven in face of large feed flow rate and feed composition disturbances.The economic and control performances were compared between fullyheat-integrated pressure swing distillation and heterogeneous azeotropic distillation.Results showed that, when the feed composition is10wt%pyridine and90wt%water, costs of two alternative processes are nearly the same, with the former saved0.37%total annual cost, and the dynamic performances are good and somewhat similar.Effect of feed composition on the economic performances of the two distillationprocesses was investigated. Results showed that, from the point of view of totalannual cost, heterogeneous azeotropic distillation has more extensively applicablescope, and from the point of view of energy consumption, two alternative processesboth have extensive applicable scope. As the concentration of water greater than orequal to80wt%, fully heat-integrated pressure swing distillation is more economical.As the concentration of water smaller than80wt%， heterogeneous azeotropicdistillation is more economical. As the concentration of water larger than or equal to50wt%, fully heat-integrated pressure swing distillation is more energy-saving. As theconcentration of water smaller than50wt%, heterogeneous azeotropic distillation ismore energy-saving.