Flow Mixing And Mathematical Modeling Of The Vertical Stirred Bed Reactor For Gas Propylene Polymerization

Polypropylene is one of the five general synthetic resin materials in the world,widely used in automobiles,medical and other industries.At present,the world’s most advanced polypropylene production processes are the gas phase processes and the bulkgas phase combined processes.The Novolen process is a typical gas-phase process,which has the advantages of high space-time yield,high operating flexibility,and high single-pass conversion of monomers,and has been Considered one of the most suitable technologies for the production of multiple varieties of polypropylene.It adopts two vertical single helical ribbon stirred reactors,The stirred bed is in a dense-phase subfluidized state under agitation and the fluidizing gas.The heat of polymerization reaction is mainly removed by the vaporization of the propylene condensate sprayed onto the bed,so it can obtain high space-time yield and single-pass conversion under a high packing density.The two reactors can be operated in parallel to produce homopolymers and random copolymers,and can also be operated in series to produce homopolymers,random and impact copolymers.It can theoratically develop a full range of polypropylene grades.The operation of the industrial reactor shows that the agglomeration and wall sticking are easy to occur when the copolymer was produced,which awfully affects the long-term stable operation of the reactor.This is because the single helical ribbon agitator mainly causes convective particle flow,and the local mixing of the particles is poor.In the reactor,the uneven particle dispersion and local dead regions are prone to exist,resulting in particles agglomerates or even forming large lumps to a shut down of the device.At this time,the ethylene content in the sticky and agglomerated materials exceeds the normal range,while the rubber content is lower than the designed value.The quality of the polymer doesn’t meet market requirements.Therefore,accurately and comprehensively describing the gas-solid flow and dispersion state in the vertical stirred reactor,and the in-depth study of the matching relationship between fluid mechanics,heat transfer characteristics and product properties are the keys to achieving long-term stable operation of the Novolen device and the development of new brands.Based on these,the research carried out in this paper is as follows:(1)A cold model experimental device of a vertical single helical ribbon stirred reactor(referred to as stirred bed)was established,and the residence time distribution of particles was measured by pulse method to explore the macroscopic mixing of particles in the stirred bed.A compartment model of "PFR+double CSTRs" was proposed to describe the lag,short circut and expotional tail.An image processing method of plane reconstruction of cylindrical surface was established to characterize the laminar flow of particles on the wall.It is found that the particle flow pattern in the ascending zone along the wall and the central descending zone is plug flow,and the interface is located on the inner diameter of the helical ribbon.Based on this,a multistage(each stage is a CSTR)particle circulation model is further established,and the derived particle residence time distribution curve can accurately reproduce all the characteristics of the RTD experimental data such as time lag,short circuit,decay oscillation,and exponential tail,which indicates that the fed particles will circulate many times until they are discharged,nearly forming a CSTR mode.(2)The magnetic particle tracking technique was established to study the translational,rotational and pulsation characteristics of incohesive particles,liquidcontained particles and cohesive particles in the stirred bed.The results show that the helical ribbon caused the circulation of particles under the above three experimental systems.The magnetic particles rotate while being transported,and the long-time angle distribution of the north and south poles conforms to random distribution.The vertical velocities of cohesive particles and liquid-holding particles are significantly lower than those of incohesive particles,indicating that the relative slip between the particles and the ribbon increases as the interparticle force increases.The particle velocity ratio in the descending zone and the ascending zone gradually approaches 1 as the stirring speed increases.At this time,the residence times in the two zones are basically equal.The particles showed very stable regular fluctuations on the bed surface and bottom.After counting the number and period of fluctuations,it was found that they were basically only related to the stirring speed,and the vertical slip speed ratio was used to predict the fluctuation period.In addition to the main circulation motion,the particle motion is superimposed with tiny displacement pulsations,and the pulsation signal intensity of liquid-holding particles is the highest,followed by cohesive particles,and incohesive particles get the weakest.Spectrum analysis shows that the pulsation spectrum of liquid-holding particles is wider,followed by cohesive particles and incohesive particles.This shows that the particle pulsation is more violent and disordered under the action of the liquid bridge force,followed by the solid bridge force,and the pulsation of the incohesive particle is weak and more orderly.(3)The evolution of gas phase flow regime and mass transfer characteristics in a stirred bed were investigated by pressure pulsation and gas phase tracer methods.At first,the agitation increases the voidage and caused a large drop of the bed pressure drop.While under agitation,the voidage of the whole bed remained basically stable.Secondly,the axial dispersion coefficient in the stirred bed is proportional to the gas velocity,which is much larger than the molecular diffusion coefficient,while the effect of stirring speed on the axial dispersion coefficient is very weak.Finally,a steady point source tracer gas was introduced along the axis of the reactor and the radial tracer concentration distribution of the downstream was measured.It was found that the gas flow in the bed was divided into two regions,and the interface was located at the inner diameter of the single helical ribbon.They are also called the central region and the wall region,and there is almost no gas exchange between the two regions.Under the conditions of different stirring speed and gas velocity,the concentration distribution in the central descending region can be divided into two types:Type I distribution conforms to the dispersed plug flow model,and the radial dispersion coefficient is proportional to the gas velocity and much larger than the molecular diffusion coefficient.Type II distributions are more uniform in the radial direction and do not fit dispersed plug flow model.According to the relative size of the particle circulation flux and the gas flux,when the particle circulation flux is small and the gas flux is large,the airflow remains pluf flow to form a type I distribution;when the particle circulation flux is large and the gas flux is small,the airflow is scattered to form a Type Ⅱ distribution.Based on this,the gas flow regime was drawn to determine the flow transition boundary.(4)A liquid-sprayed stirred bed heated by a jacket was established,and volatile petroleum ether was sprayed from the top and bottom respectively,simulating the process of introducing propylene condensate into the bed in an industrial device.The results show that the influence of fluidization gas velocity on temperature distribution is very weak,and the spray amount of condensate at the top only changes the temperature values at different positions of the bed,but does not change the shape of the axial and radial temperature distribution curves.At the same time,the heat transfer inside the bed is mainly by particle convection.Combined with the gas-solid flow model in chapters 4 to 6,a multi-stage cycle heat transfer model for a stirred bed is established,and the dynamic temperature change and steady-state temperature distribution in the stirred bed are predicted,which are in good agreement with the experimental data.(5)Based on the established particle multi-stage circulation model of stirred bed,gas flow regime diagram and multi-stage heat transfer model,coupled with the kinetics of propylene homopolymerization and copolymerization,a two-reactors in series model was established.Firstly,the axial temperature distribution of the industrial reactor was simulated,and it was found that the existing temperature detection points could not monitor the maximum temperature in the reactor.Secondly,the average molecular weight of propylene homopolymerization and ethylene-propylene copolymerization products were predicted by the method of moments,and it was found that the molecular weight and polymer dispersity index(PDI)of copolymers were larger than those of homopolymers.Copolymerization molecular weight is more sensitive to hydrogen concentration variation than homopolymerization.When the hydrogen concentration increased,the average molecular weight and PDI of the copolymer decreased rapidly.When the mass ratio of ethylene in the feed gas exceeds 40%,the rubber content in the copolymer remains basically unchanged after 50%.