CFD Modeling And Intensification Of Transport And Reaction Processes In Highly Viscous Polymerization Reactors

Polymerization reactor is one of the core equipments in polymer industry.It is of great significance for the development and application of polymerization reactor to deeply understand and master the transfer processes and reaction processes as well as the relationship between them.The influence mechanism of the complex physical properties of the polymerization system,the reactor structure and the operation conditions on the transfer and reaction processes has been a key and difficult issue in the field of polymerization engineering.In addition,problems such as low mixing and heat transfer efficiency,large concentration and temperature gradients of reactive species and wall sticking caused by the high-viscosity characteristic and nonNewtonian properties of the polymerization process will significantly affect the operational characteristics and the polymerization results.How to realize the polymerization process intensification is also one of the key research directions.Based on the computational fluid dynamics(CFD)method,the comprehensive CFD coupled model towards polymer quality was established,which involves the complex reactor structure,rheological properties,operation conditions and polymerization reaction process.The transfer processes and reaction process in the polymerization reactors were simulated,which realized the qualitative and quantitative prediction and interpretation of the reactor characteristics and polymer properties.The model provides guidance for the design and optimization of industrial non-ideal reactors and the process intensification.The CFD method was applied to simulate the hydrodynamics,power consumption and heat transfer performance in an industrial stirred polymerization reactor.The influence law of Bingham-pseudoplastic fluid on the flow,mixing and heat transfer was revealed.It was found that the high viscosity characteristic and non-Newtonian properties of the polymerization process significantly reduced the mixing and heat transfer efficiency.The strong shear flow zone,which is also called cavern,was formed near the impeller and the fluid stagnates near the coil in the reactor.The process intensification strategy based on the active intensification principle is proposed.The combination of a pitched-blade impeller and a large blade impeller is applied to effectively improve the flow between coils,reduce the thickness of the laminar thermal boundary layer,eliminate the impact of stagnant zones and then improve the mixing performance and heat transfer efficiency in the reactor.The hydrodynamics and heat transfer performance of the shell side of Sulzer Mixer Reactor(SMR)were analyzed by experimental and CFD methods.Compared with the straight tubular reactor(STHE)and the straight tubular reactor with segmental baffles(STHE-SG),the heat transfer performance of SMR is improved by 20%～130%and 20%-60%(Re=0.004-20),respectively.The velocity and temperature distributions are more uniform and the residence time distribution is narrower.In addition,the mechanism of radial mixing and heat transfer enhancement of SMR reactor based on the passive intensification principle is revealed by CFD simulation,that is,the special spatial structure of SMR strengthens the radial flow of fluid,so that the fluid can carry out efficient diversion and confluence processes while changing the flow direction constantly,which realizes the process intensification of mixing and heat transfer processes.Furthermore,considering the characteristics of high viscosity and non-Newtonian properties of the polymerization process,the influence of the special tube structure in SMR on the reactor operation characteristics was investigated.The effects of rheological parameters(i.e.the yield stress,the consistency coefficient and the powerlaw index)on hydrodynamics,residence time distribution,heat transfer performance and mixing efficiency were studied by experiments and CFD simulations.It was found that the residence time distribution of Bingham-pseudoplastic fluid was wider than that of the Newtonian fluid.When treating high-viscosity/ultra-high-viscosity fluids at low flow rates,the rheological parameters has little effect on heat transfer performance.Compared with the shear-based flow in STHE,the effective elongational flow and higher shear stress in SMR are helpful to improve the mixing efficiency and heat transfer performance.In general,SMR has better comprehensive performance in handling highly viscous non-Newtonian fluids.Based on the passive intensification principle,a new reactor(LPDMR)was designed for the application of laminar fluid flow with high viscosity or ultra-high viscosity.The hydrodynamics,heat transfer performance,mixing efficiency,residence time distribution characteristics were studied by CFD method,and compared with those of the SMR reactor.It was found that under the same reactor size,the radial mixing degree of the fluid in LPDMR is greater than that in SMR due to the effect of the disturbence elements.The intensity of segregation at the outlet of LPDMR is one order of magnitude lower than that of SMR,indicating that the distribution mixing efficiency of LPDMR is higher.Because there is no leakage flow near the wall in LPDMR,the residence time distribution is narrower than that of SMR.When Re<0.1,the heat transfer performance of LPDMR is equivalent to that of SMR or even better.When Re>0.1,the heat transfer performance of LPDMR is apparently better than that of SMR,but the pressure drop of LPDMR is significantly higher.Furthermore,the effects of structural parameters of the disturbence elements on the flow,mixing and heat transfer were discussed.It was found that when the length-diameter ratio is 1,the adjacent elements are staggered settled,and the number of crossbar is 2,the comprehensive performance of heat transfer and pressure drop is better.Based on the moment method and Flory distribution theory,a CFD model coupled with polymerization kinetics was established by user-defined function(UDF)and C++programming.The coupled CFD model was applied to simulate the industrial styrene thermal polymerization process,which involves the tower reactors with several tower and stirred sections.The complex fluid properties and transfer processes are considered in the coupled model.The calculated polymer product properties(i.e.conversion,molecular weight and molecular weight distribution)are more accurate than those of the ideal reactor model(ideal CSTR+PFR model)based on Aspen.It was found that as the polymerization proceeds,the mixing efficiency was greatly reduced with the decrease of rotational speed and the increase of fluid viscosity.The non-ideality of the reactors becomes significant.The coupled model was further analyzed to explore the effects of operating conditions,including temperature,rotational speed and volume flow rate,on hydrodynamics characteristics and polymer product properties.The establishment of the CFD coupled model links the polymer properties with the transfer processes,and can be applied to determine the appropriate operating conditions and optimize the design of industrial non-ideal reactors.