Experimental Study And Process Simulation On The Purification Of Trioxane By Extractive Distillation With Ionic Liquid

As an important chemical raw material,1,3,5-Trioxane has been used as the monomer for production of new diesel additives such as poly(oxymethylene)dimethyl ethers(PODEn),polyoxymethylene polymers(POMs),fine chemicals and a series of bulk chemicals.With the rapid growth of market economy,the production capacity of trioxane has been unable to meet the demand and the production equipment is being actively built in the world.The vapor phase distillate of the synthesis experiment of trioxane will contain impurities such as water and formaldehyde,and will form the lowest azeotrope of trioxane-water binary system.which makes it impossible to separate and refine the trioxane using conventional rectification methods.In order to reduce equipment investment and energy consumption of separation,a new process for separating and refining trioxane needs to be developed.To this end,this paper proposes an extractive distillation method that uses ionic liquid as an extractive rectifying agent to separate and purify trioxane,and carries out relevant experimental studies and simulation calculations,and has achieved the following results.(1)In this paper,the isobaric vapor-liquid equilibrium(VLE)data for the binary mixture of trioxane and water are measured at 101.3k Pa with a modified Othmer still.Experimental data is in good agreement with literature data,which shows that the experimental method is reliable and accurate.Two kinds of ionic liquid extraction rectifying agents which are [MMIM][A] and [EMIM][Ac] are designed and experimental research is conducted,the results show that adding 1mol/kg [MMIM][A]could break the azeotrope formed by trioxane and water and make the vapor-liquid equilibrium curve deviating from the original azeotropic curve.The higher the concentration of the ionic liquid becomes,the better the separation effect is,i.e.the greater the relative volatility becomes.However,adding 7mol/kg [EMIM]OAC does not break the azeotropy point.(2)In this paper,an extractive distillation experiment was carried out using a rectification device and an aqueous solution of trioxane.The concentrations of trioxanein the raw materials used were 40%,50% and 60%,respectively.The ionic liquid [MMIM][A] is fed on the top of the tower with the solvent ratio of 1:1.After 3hours of equilibrium,the sampling tests and analysis are carried out on the top of the tower.The results showed that the concentration of trioxane has increased to ≥99.5%(the concentration of trioxane in the azeotrope of trioxane and water is 70.3%),which further confirmed the new extractive rectifying agent successfully broke the azeotrope.(3)The parameters(fitted by the least squares method)of the NRTL model are obtained from the experimental data of the trioxane-water binary system and the trioxane-water-[MMIM][A] ternary system.The average relative deviations of the vapor composition of trioxane-water and the trioxane-water-[MMIM][A] systems are3.21% and 6.88%,respectively,Which shows that the fitting is good.(4)The NRTL model parameters between the trioxane,water and[MMIM][A]obtained are substituted into Aspen Plus software to carry out the extractive distillation process simulation.Various operating conditions are obtained after optimization,and both the temperature changes and vapor-liquid composition were analyzed.Meanwhile,the extraction-rectification(first extraction followed by rectification)process with benzene as the extractant was simulated and optimized.Both processes were simulated under the same raw material composition,feed volume and separation requirements and the heat load is compared.A comprehensive comparison shows that the extractive distillation method based on ionic liquid [MMIM][A] saves15% of energy compared to the extraction-rectification process based on benzene.