Three-dimensional Numerical Simulation Of Reactive Extrusion For Polycondensation Of Poly-phenylene Terephthalamide

Poly-phenylene terephthalamide (PPTA) is a typical high performance fiber with excellent strength, modulus property, acid-alkaline resistance, heat resistance and low density. The continuous production of PPTA is realized via reactive extrusion (REX) with low temperature solution polycondensation. The REX process of PPTA involves complex transport phenomena in complex geometry of screw channel, as well as in company with reaction. To manufacture the PPTA product efficiently and stably, the screw geometry should be well designed according to the requirements of polycondensation stage. In the present work, the reaction process of PPTA in a co-rotating twin screw extruder is investigated by computational fluid dynamics methods. A 3D numerical model for REX of PPTA is established and the relationship between flow field and reaction is systematically studied. Based on these results, an integrated scheme has been proposed for PPTA polycondensation extrusion process. Moreover, a virtual simulation platform is developed to realize the parameterization and modularization for PPTA REX. The numerical results provide theoretical approach and guide for the flow structure optimization in REX device and the polycondensation control of PPTA. The detail results are as follows.Based on finite element numerical method,3D numerical model for PPTA REX in a co-rotating twin screw extruder was established. A series of PPTA solutions with various molecular weights were synthesized via controlling the molar ratio. It was found their rheological behavior can be described by a power-law fluid model. Further, a systematical rheokinetics model of PPTA reaction system was put forward for the first time. Coupling with the rheokinetics equation, kinetics equation and governing equations of hydrodynamics, including continuity equation, momentum equation, energy equation and laminar diffusion equation of species concentration, the mathematic descriptions of PPTA REX was accomplished. The mesh superposition technique (MST) was implemented to eliminate the possible negative volume meshes arising due to screw rotating, and the evolution scheme was employed to improve the convergence in nonlinear problems. In order to ensure the accuracy of the numerical method and reliability of the numerical results, verification and validation work was performed. The check results confirmed the established model and its results.The micromixing behavior of multi-component reaction was a critial factor for the reaction in pre-polycondensation. Thus, the effects of premixing condition, segregation and residence time distribution (RTD) were investigated with model reactions. The premixed and non-premixed feedings corresponded to the microfluid and macrofluid, respectively. Generally, for multicomponent polycondensation, the reaction extent by premixed condition was higher than that by non-premixed condition. Under non-premixed condition, the homogenization was enhanced with increasing screw rotating speed, resulting in higher reaction extent. It illustrated that for non-premixed feeding, higher rotating speed could compensate the reduction of residence time and lead to higher reaction extent. While higher rotating speed had a detrimental influence upon reaction extent under premixed condition. It could be concluded the effect of rotating speed depends on the feeding condition. The simulated results of orthogonal test also indicated that feeding condition and flow rate had greater impacts on reaction yield. Besides, specific throughput, defined as the ratio of flow rate and rotating speed, was found control the micromixing behavior. Once the specific throughput remained constant, the micromixing would not change. The relevant results provided improvement guidance for the extruder design and operation.The main polycondensation section was a critical stage in improving the molecular weight of PPTA. It was found the main energy source in REX was the reaction heat. Keeping the temperature of barrel at 80℃, the temperature rise in REX process was less than 1.2℃, which could ensure stable extrusion. It indicated the heat removal through barrel can satisfy the requirement of PPTA polycondensation. Besides, the effect of screw geometry on PPTA polycondensation was investigated systematically. Full flight element (FF) provided the best pumping capacity, and its pumping capacity could be improved with increasing screw lead. Kneading block (KB) and screw mixing element (SME) show their abilities in axial mixing. The larger stagger angle of KB, the better mixing performance. The generated longer residence time resulted in higher molecular weight of PPTA product. The study of unconventional elements was represented by farrel asymmetric modular mixing elements and polygon. The RTD results of these elements were attractive for residence time, while the radial mixing and heat transfer were restricted due to their non-intermeshing design. Comparatively, FF, KB and SME were more suitable for PPTA polycondensation.The REX of PPTA had typical multi-scale characteristics. The polycondensation occurring at the interface of species was influenced by the flow field, and the flow field would also change with the evolutions of molecular weight and material viscosity. Hence, the desired flow field should be matched to control the reaction (micro-scale), which could be achieved via adjusting the screw geometry (macro-scale). The main goal in pre-polycondensation stage was to improve the micromixing, and it was found FF with smaller lead and SME could split fluid repeatedly and enhance species homogenization with better radial mixing. As the main polycondensation stage was concerned, its responsibility was to improve the degree of polycondensation. The simulation results indicated that the employment of reverse screw element and KB prolonged the residence time and the molecular weight was improved as a result. To eliminate the aggregation of PPTA molecular chain in post-polycondensation stage, the dispersive mixing performance should be enhanced. Thus, higher strain and stress were required for this stage. The results indicated KB had higher strain and stress over that of FF. The distinction was more obvious with larger stagger angle, suggesting KB should be of prime consideration into the design of post-polycondensation extruder to break up the PPTA jelly under the premise of enough pumping capacity.A virtual platform for numerical simulation of PPTA reactive extrusion was constructed, which realized the parameterization and modularization in REX simulation. Inputting data through the user’s interface, the platform can complete the geometric construction and discretization of twin screw extruders automatically, then the operational parameters and physical properties can be introduced by Polyflow. After achieving convergence, post-processing can also be executed efficiently. With this platform, the detailed information of flow field and reaction extent in twin screw devices can be well simulated, and the analysis of operational and design optimization can be conducted.