Development Of High-efficiency Polycondensation Reactor With Structured Flow Field

Reaction and mass-transfer are coupled in melt polycondensation process,and the rate of polycondensation depends not only upon the chemical kinetics of main reaction but also upon the mass-transfer of volatile by-products through the bulk melts,the mass-transfer tends to become the rate-limiting step with the increase of melts viscosity,especially in later stage of polycondensaton process.Therefore the polycondensation device must intensify mass transfer of high viscousS fluid which should properly match the progress of polycondensation.However that is not easy to realize within wide viscosity range.A novel idea for developing such polycondensation reactor with structured flow field has been proposed from this work:the reactor is a vertical one with assembly of a variety of special film-forming elements to manipulate the desired flow field.In the reactor,there is no agitation and polymer melt flows through multi-stage film-forming elements from top to bottom to form falling film due to gravityï¼›huge gas-liquid interfacial area is generated while adequate film renewal is achieved and the residence time can be controlled by multi-stage holding dispensers. Compared to traditional horizontal polycondensation reactors,all polymer melts in new reactor are under the state of thin film,which achieves high filming efficiency and avoids me negaliVe efiect of hydrostatic head on the mass-transfer.The fluid flow in this type of reactor is expected to have little back mixing and dead zone,which may be close to plug flow.The hydrodynamic performance of single and multi-stage film-forming elements, including the hole elements,slit elements with and without supporting wires,has been investigated.Syrup solutions with Viscosity ranging from 1.5 t0 1600 Pa.s were selected as model experimental media.The performance of fluid now,film forming,and surface renewal of single element,which changs with the viscosityï¼›operation conditions and structural parameters,has been examined by using visual technique and particle image velocimetry(PIV)system separately.The results showed that the film forming efficiency of these elements were usually greater than 100 m2.m-3.s,which was much higher than that in Rotating-disk reactor and Cage-like reactor.The slit elements with supporting wires avoided the free-surface contraction of falling film and achieved high film-forming efficiency especially for low viscous fluid,but slowed the fluid flow in the film.Meanwhile,the hole or slit elements with no supporting wires effectively fastened fluid flow in the film to achieve quickly surface renewal which enhanced the mass-transfer for high viscous fluid. A series of correlations for fluid flow, film-forming area and surface renewal frequency have been obtained, which may be used for initial selection and design of the structure of film-forming elements.The mixing performance of the new reactor combined with different multi-stage elements has been studied by pulse testing using visual technique for color tracing. The results showed that little back-mixing existed in down flowing stream in the new falling-film reactor and the fluid flow through multi-stages behaved as plug flow. The mixing performance of single film-forming elements with free falling film was close to that of a CSTR while the supporting wires in the reactor significantly reduced the degree of axial back-mixing. The axial back-mixing was also reduced by more perpendicular connection of film-forming elements between adjacent layers, and the radial mixing homogeneity was also improved.Several reactors assembled with muti-stage film-forming elements of varying configurations for different demands have been analysed and their performances were evaluated by continuous melt polycondensation of poly(ethylene terephthalate) (PET) and polycarbonate (PC). The results showed that these new reactors with varying structured flow field successfully produced high molecular weight polymers with uniform quality and their reaction time was shortened significantly in comparison with that in conventional polycondensation process. The new reactors have been verified to have excellent performances, such as high efficiency, great flexibility and universality. The realistic models have been established for the continuous PET melt polycondensation process in new falling film reactors and verified by pilot-scale experimental data. The models may be used to predict the axial distribution of degree of polymerization along the reactors and provide other valuable information for optimizing the inside structures to match the proceeding of polycondensation process.Finally, the design principles for combinations of film-forming elements of new polycondensation reactor with structured flow field have been analyzed, and the designing framework of new reactor has been proposed.Due to its outstanding performance, the new falling-film polycondensation reactor with structured flow field may have various potential applications. It can be adapted to many melt polycondensation processes, as well as some devolatilization processes for high viscous fluid without reactions.