Study On Crystallization Process Of Dimethyl Terephthalate

With the improvement of people's living standards,the recycling of materials such as polyethylene terephthalate is becoming more and more significant.Dimethyl terephthalate(DMT)is one of the main products of chemical depolymerization of polyester materials.In this paper,the crystallization process of DMT was studied from three perspectives:thermodynamics of the solid-liquid equilibrium process,simulation of solid-liquid equilibrium and crystal habit,and optimization of crystallization process conditions.The mole solubility of dimethyl terephthalate(DMT)in 17 pure solvents(methanol,ethanol,n-propanol,i-propanol,n-butanol,i-butanol,ethyl acetate,i-propyl acetate,n-propyl acetate,n-butyl acetate,n-amyl acetate,methyl propionate,acetone,methyl ethyl ketone,cyclohexanone,acetonitrile and chloroform)in the temperature range of 283.15?318.15 K was determined by the laser dynamic method.The solubility of DMT in these solvents increases with increasing temperature and follows the order:chloroform>acetonitrile>esters and ketones>alcohols.Correlations of the solubility with temperature have been performed in terms of five thermodynamic models(modified Apelblat,?h,three-parameter Van't Hoff,Wilson and NRTL)and the maximum average and root-mean-square deviations are 2.32%and 31.63×10-5,respectively.Better fittings were obtained with the Wilson model and the NRTL model.In addition,the mixing properties(Gibbs energy,enthalpy,entropy,as well as activity coefficients and reduced excess enthalpy at infinitesimal concentration)were evaluated using Wilson model.The intermolecular ?-parameters,mixing energy and binding energy of the DMT solution system were calculated using the Materials Studio platform,and the differences in the solid-liquid equilibrium of DMT in different solvents(methanol,acetonitrile,acetone,ethyl acetate,and chloroform)were explained from these perspectives of molecular thermodynamics.Then,based on the data measured by PXRD,the structure of the DMT unit cell was solved.The refined DMT unit cell was used to estimate the crystal habit of DMT using the BFDH method.These results are in good agreement with the experimental results.For the methanol solution system of products by depolymerizing PET using supercritical methanol,it has been experimentally determined that the method of cooling and adding water to dissolve crystals can prepare 99.9%DMT.Then,the response surface method was used to design the experimental scheme.For the crystal products,the volume average particle size(D(4,3))mathematical model and the particle size variation coefficient(C.V.)mathematical model were established with the stirring speed(n),the water addition rate(v),the crystal growth time(t),and the volume ratio of water to methanol(r)as the influencing factors.Meanwhile,n,v,t and r were analyzed for the yield of DMT and the content of extremely fine particles(particle size<10 ?m)in the product.The process conditions determined to be beneficial to subsequent industrial operations were 200 r/min?n?300 r/min,0.5 mL/min?v?0.9 mL/min,t=10 min,and 2.5?r?3.0.