Process Integration And Intensification Of Distillation Systems Considering Pressure Decision Variable Optimization

The chemical industry is a typical energy-intensive industry,of which distillation is the most widely used thermal separation technology.It accounts for about two-thirds of the capital investment and energy consumption of the entire plant.However,it has low energy efficiency but the waste heat emission is large,which seriously hinders the economic and environmental benefits of related industries.Therefore,this thesis carries out research work on the purpose of improving the energy efficiency and recovering waste heat in distillation systems.To be specific,in the scope of chemical process system engineering,through process simulation and numerical optimization technology,we discuss the use of process integration and intensification methods to improve the energy efficiency and recovery of distillation process.The specific content includes:(1)An equation-oriented shortcut model is developed to reveal the relationship between pressure sensitivity and economics In this thesis,through the economic analysis of some typical cases under different operating pressures,the importance of optimizing the operating pressure on the pressure-sensitive distillation system is emphasized.An equation-oriented shortcut model considering the effect of operating pressure is developed to estimate the sensitivity of the economic benefits of the distillation system to pressure,and the reliability of the method has been verified by several case studies,confirming that the shortcut model can accurately distinguish pressure-sensitive/-insensitive system.Therefore,it can be used to determine the necessity of optimizing operating pressure.(2)An enumeration-based synthesis framework for binary multi-effect distillation is developed for screening and designing energy-efficient structures Based on the discussion of the necessity of pressure optimization,this thesis further applies the shortcut model to the multi-effect distillation and constructs a synthesis framework for screening and designing energy-efficient structures.The framework enumerates 43 alternative structures,and uses a shortcut model to initially screen a small number of structures that are more economical and the initial values required for subsequent optimization,and then determines the optimal results through a rigorous optimization model.Through numerical optimization,the research results provide a fair performance comparison between multi-effect distillation and conventional distillation.Compared with conventional distillation column,the multi-effect distillation structures can save about 50% of energy consumption and up to 30% of total annual costs.(3)Novel application of process electrification based on self-heat recuperation technology in various distillation systems Based on the self-heat recuperation technology,this thesis proposes several innovative electrical-driven distillation systems(azeotropic and zeotropic).Taking the zeotropic ethylbenzene/styrene separation as an example,the process electrification can bring about 8% to 28% total annual cost savings for different feeds.In addition,taking the azeotropic tetrahydrofuran/water and acetone/chloroform as examples,process electrification can bring about 24% and about 48% total annual cost savings,respectively.(4)Novel process intensification approach in zeotropic multicomponent distillation systems through liquid-only transfer stream is proposed In this thesis,several innovative multicomponent distillation configurations with liquid-only transfer stream are proposed.Taking the separation of ternary and quaternary aromatic hydrocarbons as an example,the simulation results show that the above structures can significantly reduce the remixing effect and improve the process economic performance.For the ternary distillation system,the structure can save up to 20% of the total annual cost.For quaternary distillation,compared with the conventional direct sequence,the structure can reduce energy consumption by18.9% and total annual cost by 11.9%.(5)Novel process integration method of extractive distillation with organic Rankine cycle and economizer is presented This thesis presents a new method of extractive distillation integrating organic Rankine cycle and economizer.The integrated system can recover the low-grade waste heat from the condenser and recycle solvent cooler and convert it into electrical work.Taking the pressure-sensitive n-heptane/isobutanol/Nmethylpyrrolidone system as an example,R227 EA was optimized and selected as the organic Rankine cycle working fluid.The results show that the direct waste heat recovery through the organic Rankine cycle can only reduce the annualized cost by 3.01%.However,the integrated system that combines the organic Rankine cycle and the economizer can reduce the total annual cost by up to 30.30%.The innovations in the thesis can be summarized as the following three points:(1)Pressure is introduced as a decision variable into simultaneous optimization of various distillation systems This thesis analyzes the issue of ignoring pressure optimization in the previous literature,and introduced pressure as a decision variable into simultaneous optimization of various distillation systems.Several case studies show that the introduction of pressure optimization has a decisive impact on improving the economic performance of the distillation system.(2)Several innovative energy-efficient distillation configurations based on the principles of process integration and intensification are developed This thesis develops several innovative energy-efficient distillation configurations based on the principles of process integration and intensification: Chapter 3 proposes a variety of new multi-effect distillation structures;Chapters 4 and 6 have been developed electrical-driven distillation structures for zeotropic and azeotropic systems,respectively;Chapter 5 develops a variety of new distillation configurations with liquid-only transfer streams;Chapter 7 proposes a new type of integration of organic Rankine cycle and economizer in extractive distillation system.(3)An innovative rigorous optimization framework for handling MINLP in distillation design is developed This thesis develops an innovative optimization framework based on Aspen Plus and MATLAB platform to solve the MINLP(mixed-integer nonlinear programming)problem in distillation design.Practice has shown that the framework can successfully handle distillation optimization problems with pressure decision variables.