Continuous Preparation Process For Low-molecular-weight Poly (Ethylene Adipate)

Polyester polyols are macroglycols prepared by the condensation of a glycol and a dicarboxylic acid or acid derivative, which can further react with isocyanates to produce polyurethane. Nowadays industrial technologies for the manufacture of polyester polyols all use batch stirred tanks, which are benefit to the flexibility of products, but reduce the stability of product quality. Moreover, due to the limited heat and mass transfer, the batch industrial reaction time of adpic acid series polyester polyols need more than20hours. A continuous process for the preparation of polyester polyols has been developed in this thesis. An innovational six-stage bubbling reactive distillation tower (BRDT) reactor has been constructed to perform both esterification process and polycondensation process, and the total residence time for the preparation of poly (ethylene adipate)(PEA) with average molecular weight (Mn)2000is significantly shortened to about6hours.Many aspects including the reaction kinetics of PEA esterification process and polycondensation process, the principle for the construction of BRDT and the hydrodynamics of BRDT, hot model experiments for PEA continuous esterification and polycondensation process in BRDT, the modelling and simulation optimization for polyester polyols continuous synthetic process in BRDT have been studied, which provide the key technologies and basic data as well as strategies for the development and optimization of PEA continuous process.First, the effects of different parameters (including reaction temperature, feed ratio of alcohol and acid, reaction pressure, different kinds of catalyst, the catalyst dosage, et al.) on PEA esterification and polycondensation process were investigated. Under the conditions of temperature range160～230℃and feed ratio of1.2at atmospheric pressure, the second-order, shift-order and third-order reaction kinetic models for the direct esterification process between AA and EG without additional catalyst had been developed on the basis of different catalytic mechanisms of esterification reactions. Under the conditions of temperature range160-230℃, pressure2kPa and in the present of additional25ppm catalyst of tetraisopropyl titanate (TPT), the second-order kinetic model for PEA polycondensation process had been developed. Model identification results showed that the second-order model could well express the kinetic feature of both esterification and polycondensation process.Second, an innovational six-stage BRDT was constructed to perform PEA esterification and polycondensation process, which coupled reaction with distillation to remove by-products efficiently. In this tower reactor, the liquid/gas flow, residence time and pressure drop could be manipulated using special tray structure with liquid downflow and gas upflow pipes. The visual cold model experiments showed that the liquid flowed through each tray from the top to the bottom, and was discharged from the bottom of the column, while gas passed the trays from the bottom to the top. The flowing fluid on the column trays had little dead zone due to the intensive bubbling effect of the gas, ensuring there optimum intermixing. Each column tray acted as a CSTR, and the fluid through multi-stages tended to be plug flow. The diameter of gas pipe in column tray was the key factor for stable operation of this kind reactor.Third, a bench scale BRDT was built to carry out the continuous esterification and polycondensation process respectively to prepare low-molecular-weight PEA. The esterification oligomers with carboxyl groups less than1mol·kg-1and polycondensation products with Mn about2000as well as carboxyl groups less than0.02mol·kg-1had been obtained successfully. A comprehensive mathematical model for PEA continuous esterification and continuous polycondensation process in BRDT was developed; the calculated results were in good agreement with experimental data. The influence of the major operating conditions on reactor performance was further simulated, it was found that the optimal operating conditions as following:the temperature distribution was160,180,200,220,230,230℃, residence time for each column tray35min, feed ratio1.2, reaction pressure1atm, nitrogen flow rate100ml min-1for continuous esterification process; and the temperature distribution was220,230,230,230,230,230℃, residence time for each column tray30min, reaction pressure2kPa, nitrogen flow rate100ml min-1for continuous polycondensation process; these optimal conditions were also verified experimentally.Fourth, the whole flow sheet of PEA continuous process was simulated. The influence of the circulation of water and EG from polycondensation process was calculated, compared with no water and EG circulation and only EG circulation, after both water and EG generated in polycondensation process were recycled to the esterification process, the feed ratio could be decreased from1.2to1.095and the residence time of each column tray could be decreased from35min to30min in esterification process, therefore the whole process became more simple and economical.