Rigorous Simulation And Optimization Of Methanol Distillation Schemes Assisted By Pseudo-transient Models

Methanol distillation is a separation process with intensive energy and complex process structure.Its optimal design needs to find operating conditions,material flowrate and unit equipment size and structural parameters that can maximize economic benefits and energy efficiency.It needs to face the problem of complex system modeling and solving large-scale nonlinear equations,and needs reliable,robust and efficient solution strategies and optimization methods.So far,in spite of methanol distillation process design problem is widely studied,but due to the problem size and the limitation of the large-scale technology of solving nonlinear equation,the design of methanol distillation process are mainly based on process simulation,rather than process optimization based on rigorous models with all decision variables simultaneously optimized(operating parameters such as continuous variables and the equipment structure parameters such as discrete variables),and therefore unable to guarantee the optimal process system structure and operating parameters.In order to study the characteristics,advantages and disadvantages of the existing methanol distillation process system,and to obtain the optimal process,this work developed pseudo-transient models of complex methanol distillation system in equation-oriented(EO)environment,which expanded the convergence basin of rigorous simulation in EO environment.The steady-state optimization algorithm assisted by PTC models and the bypass efficiency method were adopted in this work,which effectively solves the convergence problem of nonlinear steady-state model using Newton-type algorithms and the problem of simultaneously optimizing integer variables and continuous variables.With the minimum total annual cost(TAC)as the objective function,robust and fast optimizations were implemented based on rigorous models for 8 methanol distillation processes,among which 4 processes were proposed for the first time.In this study,the optimal process characteristics of methanol distillation were obtained based on rigorous optimization and process analysis.It was found that multi-effect distillation could significantly reduce the energy consumption of the system,and the heat integration arrangements had obvious influence on equipment cost and operation cost.The operating pressure of each column of the system is an important parameter that affects the heat integration effect of the methanol distillation process.In addition,the presence of higher alkanes in the feedstock has a significant influence on the operation parameters and equipment parameters of each column and on the overall economy.It can be seen,based on the economical comparison of the 8 processes,that under the same feed condition,the TAC of the five-column scheme II is the lowest;however,the four-column scheme II proposed in this work also has the remarkable energy-saving benefits,and compared with five-column schemes,its structure are more simple,and the operation is relatively easy to control,which has more potential industrial applications and has significant reference values for the design of methanol distillation system in the future.Finally,the optimization for 8 complex methanol distillation processes shows that the steady-state optimization algorithm assisted by PTC models is robust and efficient,and is a powerful tool to solve the rigorous optimization problems of large-scale complex processes.