Fundamental Research Of Low Transition Temperature Mixtures (LTTMs) Applying To Extraction,Extractive Distillation And Absorption Processes

Extraction,extractive distillation and absorption are important separation technologies in industry,for which the selection of solvent is especially crucial.Traditional organic solvents have disadvantages of being volatile,polluting the environment and high recovering energy consumption.Although ionic liquids（ILs）can overcome the shortcomings of traditional organic solvents,the literatures reported that ILs have some disadvantages such as poor biodegradation,high price and complicated synthesis,which limit their application in industry.In view of this point,the Low Transition Temperature Mixtures（LTTMs）are proposed as the new extraction agents in extraction,extractive distillation and absorption processes.LTTMs are environmentally friendly solvents which also possess advantages of simple preparation,low price,and tailor-made characters.So how to screen and experimentally study solvents for different systems,and use process simulation to evaluate the feasibility of this kind of solvent in industrial application,are of great significance for its industrial application.Therefore,simulation and experimental research on the application of LTTMs in extraction,extractive distillation and absorption processes are performed in this paper,and the content is shown as follows:Benzene-cyclohexane azeotrope is selected as the research system,simulation and experimental research of LTTMs in the extraction process are performed in order to demonstrate the feasibility of LTTMs which can be used as solvent for benzene extraction.A predictive thermodynamic model COSMO-SAC is applied to screen solvent from combinations of 25 HBDs and 13 HBAs.Sulfolane is selected as an HBD and tetramethylammonium bromide is selected as an HBA for the synthesis of LTTM which is used as an extractant for the separation of benzne-cyclohexane.Based on quantum chemistry theory,the optimization of molecular structure is performed,and the molecular interaction energy between HBD/HBA and the component（benzene/cyclohexane）are calculated.Weak intermolecular interaction are studied using independent gradient model（IGM）.Atoms in molecules（AIM）is adopted to study the interaction sites between the molecules,and Laplacian is calculated to determine the properties of the bonds.The calculation results show that there are mainly a hydrogen bond force and van der Waals force between the HBD/HBA and benzene/cyclohexane,and the interaction between the HBD/HBA and benzene is greater than that between HBD/HBA and cyclohexane.In order to prove the accuracy of the screening results,liquid-liquid equilibrium experiement of benzene-cyclohexane-LTTMs are carried out.The effects of temperature and the mole ratio of HBD to HBA on the extraction performance（distribution coefficient,selectivity,performance index）are also investigated.In addition,the binary interaction parameters of NRTL model between the components are regressed based on the experimental data.Moreover,benzene extraction process is simulated and optimized using Aspen Plus,and the design parameters are obtained.The simulation results show that the selected LTTMs is superior in both energy consumption and the amount of solvent compared with the traditional organic solvent sulfolane.Isopropanol-water azeotrope is selected as research system,simulation and experimental research of LTTMs in extractive distillation process are performed in order to demonstrate the feasibility of LTTMs which can be used as solvent for isopropanol-water extractive distillation process.A predictive thermodynamic model COSMO-SAC is applied to screen solvent from combinations of 28 HBDs and 9 HBAs.Oxalic acid is selected as an HBD and choline chloride is selected as an HBA for the synthesis of LTTMs which is used as an entrainer for the separation of this mixture.The molecular intertaction,acting sites and the properties of bonds are also perfomed based on quantum chemistry theory.The calculation results show that the interaction between HBD/HBA and isopropanol/water is mainly hydrogen bonding and van der Waals interaction.Vapor-liquid equilibrium experiment of isopropanol-water binary mixtures containing LTTMs is carried out and the results demonstate that LTTMs used as solvent can break the azeotrope of isopropanol-water system.In addition,the binary interaction parameters of NRTL model between LTTMs and isopropanol/water are regressed based on the experimental data.Moreover,the extractive distillation process of isopropanol-water with LTTMs as solvent is performed and the design parameters are optimized using multi-objective genetic algorithm（MOGA）with total annual cost（TAC）,global energy consumption（GEC）and extraction efficiency(E_ext,e_ext)as the optimization goal.The Pareto front is obtained under the constraints and CO₂ emissions under the corresponding design point are calculated to evaluate the environmental benefits of process.The results show that there is an optimal design point for different objective functions.Feed disturbance analysis of extractive distillation process with lowest TAC are studied and three control structures are established in order to observe its controllability.The research shows that the proposed temperature-concentration control structure can maintain the purity of the product near the set value,and the control structure c effectively work pretty well for disturbances of both feed rate and feed composition.Therefore,this study provides a reference for the separation of alcohol-water systems using LTTMs as entrainer in industry.Taking CO₂ capture as the research object,simulation and experimental research of LTTMs in the absorption process are performed in order to demonstrate the feasibility of LTTMs which can be used as absorbent for CO₂ absorption.Based on the empirical method,levulinic acid is selected as an HBD,and tetraethylammonium chloride is used as an HBA to synthesize LTTMs used for CO₂ absorption.The absorption mechanism of CO₂ capture by LTTMs is studied based on theory of quantum chemistry.The calculation results show that the interaction between the HBD/HBA and CO₂ is mainly the effect of van der Waals force and the interaction between the HBA and CO₂ is greater than that of the HBD and CO₂.The solubility of CO₂ in LTTMs is determined based on the absorption experiment,and the effects of molar ratio of HBD to HBA,the temperature and pressure on Henry coefficient are also investigated.In addition,the binary interaction parameters of the Redlich-Kwrong（RK）equation of state are regressed based on the experimental data.Moreover,the absorption process which using LTTMs as solvent is simulated by Aspen Plus and the design variables are optimized using MOGA with capital cost and operating cost as goal,and the Pareto front is obtained under the different conditions.The simulation results show that although LTTM is not as good as monoethanolamine in the amount of solvent,the energy consumption is lower than that of monoethanolamine when treating the same amount of CO₂.In addition,the control structure of absorption process is also studied,and the proposed control structure can effectively work pretty well for disturbances of both feed rate and feed composition,which demonstrates that the CO₂ capture process using LTTMs as the solvent can operate stably and has good controllability.In this paper,the feasibility of LTTMs used as solvent for extraction,extractive distillation and absorption processes is studied from the follow aspects:solvent screening,mechanism analysis,phase equilibrium experiment,process simulation and control.The good separation performance of LTTMs has been proved by experiment and process simulation.The research content of this work provides a new idea for the application of LTTMs which are used as solvents in chemical separation process,and it also provides a reference for its industrial application.