【作者】 奚桢浩；

【导师】 张志斌； 赵玲；

【作者基本信息】 华东理工大学 ， 化学工程， 2010， 博士

【摘要】 理想的反应器应该能够提供满足反应所需的流场结构。缩聚反应器需强化高粘流体的传递过程并使传递与反应进程正确耦合。高粘缩聚反应器的传递性能主要取决于其成膜和表面更新特性。本文在研究孔式、栅缝式等简单结构的降膜元件流动和传递特性基础上,设计了流场结构化的新型高粘缩聚反应器：反应器采用立式结构,通过降膜内构件的组合使聚合物熔体在重力作用下形成稳定降液膜流动；降膜元件结构类型、尺寸及其组合方式与物系流变性相适应,始终满足较大成膜面积和较快表面更新的结合；设置多级溢流持液器,强化过程流体分散与混合,实现过程停留时间可控,并使反应器内物料整体流动趋于平推流。所构建的流场结构化的新型高粘缩聚反应器成功应用于聚酯与聚碳酸酯的熔融缩聚过程,其能效远优于传统缩聚反应器。以粘度范围1.5 Pa·S～1600 Pa·S的糖浆溶液为模拟物料,利用可视化技术以及激光粒子成像测速系统实验研究了不同结构类型降膜元件及其组合方式下的流体流动、成膜、更新以及混合等性能。结果表明：液膜流动受过程粘性力、惯性力、重力与表面张力共同作用,成膜通道面积接近的前提下,开孔元件最易形成降膜,且形成的膜面流动速度最快,栅缝元件次之,而带有支撑件的栅缝元件最为困难,支撑件的引入使液膜流动减缓,却可有效地改善降膜自由面的收缩；降膜元件所形成的成膜面积随粘度、流量与通道尺寸的增大而增大,特别是在流体粘度较高时,几类降膜元件的成膜效率均接近100m2.m-3.s,甚至数百,高于传统卧式缩聚反应器的50-80 m2.m-3.s,其中带有支撑件的栅缝元件的成膜效率的关键在于其膜厚的控制；自由降膜元件液膜表面更新快,而支撑件的引入则明显减小了液膜表面更新,甚至带来更新频率数量级的变化。因此,低粘阶段应选择带有支撑件的栅缝元件,以有效减缓膜面收缩,获得较大的传质面积；而随着粘度的升高,需要逐渐减少支撑件数量,控制合适的膜厚以保证成膜效率；而在更高粘度区域,流体流动性差,宜选择无支撑件的栅缝元件甚至开孔元件,以强化流体流动,获得较快的膜面更新。基于实验分别建立了不同降膜元件的液膜流动模型、成膜及更新频率计算模型,为新型缩聚反应器结构设计提供计算依据。停留时间分布测试结果表明多级降膜结构的新型缩聚反应器内物料流动一般接近平推流,其中单级开孔元件和无支撑件的栅缝元件的混合效果接近于一独立的CSTR,而支撑件的引入可降低过程轴向返混程度,使整体流型趋于平推流；改变相邻元件间的连接方向增加降液膜膜面的交错,可在一定程度上减小轴向返混程度,并且改善径向混合效果。以传质性能与反应进程的正确耦合为原则,合理设计并组合多级多通道降膜元件,构建了多种型式的流场结构化的降膜缩聚反应器,适用于不同的物系粘度变化范围；并用不同分子量的聚酯以及聚碳酸酯为原料考察了所构建新型反应器的缩聚性能。10kg／hr中试实验结果表明：所设计的流场结构化新型缩聚反应器不但能代替现有卧式缩聚反应器用于常规纤维级聚酯生产,而且可直接缩聚生产瓶级以及更高分子量聚酯产品,可在较短停留时间内实现聚合物分子量的快速提升,同样工艺条件下仅半小时即可达到传统卧式缩聚反应器2小时的缩聚效果,能效大幅度提高,同时所得缩聚产品分子量分布窄；该类反应器同时具有较强的通用性,同样结构的反应器可以分别适用于聚酯以及聚碳酸酯的熔融缩聚过程。此外,建立了流场结构化降膜缩聚反应器中的聚酯熔融缩聚过程模型,模拟了过程反应与传质的耦合效果。最后,基于流场结构化新型缩聚反应器及降膜内构件的特点,总结了该类反应器内结构选型设计依据,并以过程反应与传质的合理匹配为原则,提出了降膜缩聚反应器的设计思路与计算框架。流场结构化的新型缩聚反应器由于能够通过成膜内构件结构及其组合变化在较宽的粘度范围实现形成传质界面大的薄层液膜和表面快速更新的有效结合,并使其传递性能与反应进程正确匹配,性能优势突出,有望成为高粘缩聚反应器的一次重大进步。更多还原

【Abstract】 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 gravity；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 viscosity；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.更多还原