Purification Of Coal-to-Ethylene Glycol Via Melt Crystallization

Ethylene glycol(EG)plays an important role in chemical production,and more than90%of its applications are polyester.The market demand is increasing year by year.The production of EG from coal(syngas)has been developing rapidly in recent years under the energy background of"rich coal,poor oil,and less gas".However,a series of impurities different from the petroleum-ethylene process are produced in the oxalate hydrogenation reaction,such as 1,2-butanediol,1,2-propanediol,ethers,polyglycols,and residual esters.The presence of these impurities affects the quality of coal-to-EG,and thus affects the downstream application of EG in polyester industry.At present,distillation technology is used to separate the impurities in industry,but the impurity system is a heat-sensitive system,simultaneously,narrow boiling point and azeotropic problems are existence.Distillation is difficult to purify,and the product quality cannot meet the production requirements of polyester fiber.Therefore,it is a meaningful research topic to develop a feasible separation process for this problem.Melt crystallization technology has the unique advantages of low-temperature operation,suitable for narrow boiling,azeotropic,heat-sensitive and other special systems,and easy to obtain high-purity products.It is widely used in chemical separation.Based on the analysis of the basic properties of impurities,this study adopts the method of tubular static melt crystallization to carry out laboratory separation research on the existing impurities.For the two-component system of each impurity and EG,the steps are designed including in-situ seed crystal generation,initial crystallization,cooling crystallization,the separation of mother liquor in negative pressure,sweating,and melting.The pores situation of the crude crystals in the crystallization section is described by the fractal porosity theory.The experimental parameters of separation were investigated and optimized systematically,the multi-step separation operation is adopted to remove impurity successfully,and EG product was obtained with a purity higher than 99.8%,which proves the effectiveness of the melt crystallization method,provides a new idea for the separation of the impurity system and provides an experimental basis for industrial application of separation.The main conclusion is divided into the following four parts:1.The boiling point of 1,2-butanediol and 1,2-propanediol are similar with EG,and1,2-butanediol can also form azeotrope with EG.In this paper,the method of purifying EG from the two-component system EG/1,2-butanediol and EG/1,2-propanediol is studied through the static melt crystallization process.When removing 1,2-butanediol,the experimental parameters of purification step ? were studied,which divided into crystallization stage and sweating stage,and the optimal crystallization conditions were selected as the initial crystallization temperature was–15°C,the cooling rate was 0.01°C·min^–1,and the final crystallization temperature was–18°C.The sweating stage has a positive effect on purification.The optimal sweating conditions were sweating temperature–15.5°C and sweating time 120 min.It was determined that the step ? was crystallization+sweating,and the EG content was increased to 97.33%;The separation mode of step ? and ? was crystallization-crystallization-crystallization+sweating,and the EG content could be increased to 99.21%and?99.8%,respectively.Refer to the above process to remove 1,2-propanediol,and the EG content increased to 97.48%,99.0%,and?99.8%after separation step ???,respectively.2.The melting point of 2-methoxyethanol is 72°C lower than that of EG,the separation of EG/2-methoxyethanol system by melt crystallization has been studied.The separation parameters of each step were investigated,the selective optimization of the separation step was carried out through the study of ??? separation process.It was determined that the final separation step was step ? and ? only crystallization,step ? was crystallization+sweating,and step ? was crystallization-crystal-crystallization+sweating.The corresponding EG content was increased to 97.26%,98.34%,99.12%,and 99.83%,respectively,the removal of2-methoxyethanol was achieved.3.When polyglycol was removed,the basic law of separation EG/diethylene glycol was studied.The optimal crystallization conditions for purification step ? were the initial crystallization temperature of–16°C,the cooling rate of 0.01°C·min^–1,the final crystallization temperature of–20°C.The sweating stage was at–15.5°C for 120 min.The EG content increased to 97.78%after step I.The separation parameters of step ? and ? were investigated and optimized,and the EG content of step ? could reach 99.20%under optimal conditions.Step ? cannot achieve the separation target by crystallization+sweating,and the operation mode of crystallization-crystallization+sweating was used to increase the EG content to 99.86%.When the impurity was triethylene glycol,the separation law of EG/diethylene glycol was learnt from.After ??? purification step(crystallization+sweating operations),triethylene glycol impurity has also been removed successfully,and the content of EG was?99.8%.4.When removing the residual ester impurities,the impurities were divided into two types.One type is methyl glycolate with a melting point of–9.15°C.The separation parameters were investigated and optimized.EG content increased to 98.29%under the optimal crystallization and sweating conditions during the purification step I.In step ?,the sweating stage was discarded due to the yield factor,and the purity of EG increase to 99.11%by the separation in crystallization stage.In step ?,the effect of excessively high sweating temperature on the purification was weakened,the high purity requirements cannot be met through a single crystallization and sweating operation.The crystallization-crystallization+sweating operation was adopted to increase the purity of EG to99.83%.The other type is dimethyl oxalate,which is solid at room temperature.The typical melt crystallization process is not suitable for removing this impurity.We designed the glass sand core fixed at the bottom of the crystallization inner tube and the EG purification achieved through a solid-liquid separation process.In the separation of dimethyl oxalate,crystal can be generated during the pre-cooling process,the temperature was continued to be lowered and solid-liquid separation is performed at the final crystallization temperature.The influence of the final crystallization temperature was obvious in a wide temperature range,and the EG content in the liquid could be?99.75%by a crystallization treatment with the crystallization temperature of –29°C.