CFD Modeling Of Solution Polymerization Processes Towards Molecular Weight Distribution

Grasping the transport and reaction characteristics within polymerization reactors is highly valuable to polymerization reactor design and scale-up.However,it has always been challenging to understand how the polymerization mechanism,physical transport processes,reactor configuration and operating conditions affect polymer product quality in the field of polymer reaction engineering.Therefore,computational fluid dynamics(CFD)modeling of molecular weight distribution(MWD)and copolymer composition distribution(CCD)can establish a quantitative relationship between chemical process engineering and polymer product quality,thus revealing the transport and reaction behaviors of different polymerization processes,which is of great theoretical significance and potential prospect for industrial polymerization processes.In this study,a CFD approach coupled with polymerization kinetics is developed to analyze flow and mixing,mass and heat transfer,and polymerization reaction for styrene free radical polymerization,butadiene coordination polymerization and ethylene-propylene copolymerization using vanadium-based Ziegler-Natta catalyst towards molecular weight distribution and copolymer composition distribution,respectively.From laboratory scale to industrial scale and from homogeneous homopolymerization to heterogeneous copolymerization,the solution of polymer product quality indices such as molecular weight distribution by CFD is realized.Firstly,a coupled CFD model capable of solving molecular weight and polydispersity index for styrene solution polymerization process with high viscosity and variable viscosity is developed,and the effects of flow and mixing on polymer product quality for free radical polymerization are revealed.It is found that,imperfect mixing at the inlet and top of the stirred tank greatly affects styrene polymerization process.With the increase of impeller speed,the polymerization behavior in the stirred tank tends to ideal CSTR,and the monomer conversion and the polymer molecular weight are decreased.Secondly,the integrated CFD method has been applied to an industrial cis-polybutadiene process,and the cis-polybutadiene solution polymerization process by tank reactor in China is analyzed comprehensively.The results show that,the mixing behavior with double helical ribbon agitator is very close to ideal CSTR at the top and in the middle of the reactor,but there apparently exists a nonideal mixing region in the feeding zone at the bottom of the reactor.The obvious temperature and concentration gradients in the deteriorated area at the bottom of the reactor are responsible for the enormous deviations from ideal CSTR,and another turbine agitator mounted at the bottom can be used for mixing enhancement in the feeding zone.Thirdly,the above CFD model for homogeneous homopolymerization process at steady state has been extended to heterogeneous copolymerization process at transient state.And the effects of operating conditions on copolymer molecular weight and copolymer composition for ethylene-propylene copolymerization using vanadium-based Ziegler-Natta catalyst are discussed in terms of hydrodynamics,gas-liquid mass transfer and polymerization reaction.It is revealed that,the polymerization rate is mainly determined by the ethylene concentration in the liquid phase,and it reaches a maximum by the joint actions of mass transfer,polymerization and mixing.The rapidly continuous feeding of gas mixture makes the monomer saturated in the liquid phase quickly,and then the copolymer composition is almost unchanged during the polymerization process.Finally,the MWD and CCD distributions inside the polymerization reactor are calculated for ethylene-propylene copolymerization process by introducing Flory distribution and Stockmayer distribution based on CFD method.And the spatial-temporal variations of MWD and CCD distributions for ethylene-propylene copolymers are discussed.It turns out that,as the polymerization proceeds,MWD distribution is narrower and narrower,but CCD distribution is wider and wider.The MWD and CCD distributions are wider at the bottom of the reactor due to larger fluctuation of feed compositon,and narrower at the top because of sufficient flow,mixing and polymerization.