Continuous emulsion polymerization of styrene in a single Couette–Taylor vortex flow reactor

The effect of the geometrical and operational parameters on the mixing characteristics of a Couette–Taylor vortex flow reactor (CTVFR) were investigated and were correlated with the same parameters by using the tank‐in‐series model. Continuous emulsion polymerization of styrene was conducted at 50°C in a CTVFR to clarify the effects on kinetic behavior and reactor performance of operational parameters such as rotational speed of inner cylinder (Taylor number), reactor mean residence time, and emulsifier and initiator concentrations in the feed streams. It was found that steady‐state monomer conversion and particle number could be freely varied only by varying the Taylor number. In order to explain the observed kinetic behavior of this polymerization system, a mathematical model was developed by combining the empirical correlation of the mixing characteristics of a CTVFR and a previously proposed kinetic model for the continuous emulsion polymerization of styrene in continuous stirred tank reactors connected in series (CSTRs). On the basis of these experimental results, it was concluded that a CTVFR is suitable for the first reactor (prereactor) of a continuous emulsion polymerization reactor system. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1931–1942, 2001     

Continuous production of solid polystyrene in back‐mixed and linear‐flow reactors

A mathematical model has been developed to predict the steady state performance of a continuous bulk styrene polymerization process with catalytic initiation for solid polystyrene. The polymerization section contains one boiling CSTR, followed by multiple linear‐flow reactors. The devolatilization section consists of two polymer pre‐heaters and two high‐solids flashes. The polymer moment equations were solved simultaneously with the reactor modeling equations. The non‐linear algebraic equations were solved by a Newton‐Raphson iteration technique to give the steady‐state styrene monomer weight fraction in a CSTR. The coupled, non‐linear ordinary differential equations were numerically integrated using a single‐step, 4th‐order Runge‐Kutta technique, followed by a multi‐step Adams‐Moulton technique. The resulting computer simulation model is capable of evaluating how the production rate and product quality are affected by feed composition, temperature, initiator type, initiator concentration, and residence time. Several case studies were given for commercially important crystal‐clear and impact‐resistant resins. A binary initiation system gives a good balance of monomer conversion, polymer molecular weights, and rubber grafting compared to a single initiation system. The styrene dimer/trimer occur in low concentrations but can be substantially reduced with a low temperature initiator. The ideal mean residence time is approximately one minute or less in a shell‐and‐tube devolatilization pre‐heater. Low flash chamber vacuum is more effective than high polystyrene melt temperature to reduce the volatile content of the final product. The water injected to the low volatile melt shows promising improvement in the second‐stage polystyrene devolatilization.     

Mixing performance comparison of milliscale continuous high‐shear mixers

Mixing performance of two continuous flow millilitre‐scale reactors (volumes 9.5 mL and 2.5 mL) equipped with rotor‐stator mixers was studied. Cumulative residence time distributions (RTD) were determined experimentally using a step response method. Distributions were measured for both reactors by varying impeller speed and feed flow rate. The mixing effect was determined by measured RTDs. Computational fluid dynamics (CFD) were used to verify that the residence time distribution in the measurement outlet agreed with the outlet flow. The mixing power of both reactors was determined using a calorimetric method. The reactor inlet flow rate was found to affect mixing performance at 1–13 s residence times but the effect of impeller speed could not be noted. Both milliscale reactors are close to an ideal continuous stirred‐tank reactor (CSTR) at the studied impeller speed and flow rate ranges. The specific interfacial area was found to depend on the reactor inlet flow rate at constant impeller speed for the case of copper solvent extraction.

     

Continuous free-radical polymerization of styrene in the stirred tank reactor. Effect of inhibitors

The influence of inhibitors (p-benzoquinone) on the conversion and the molecular weight distribution (MWD) during free radical polymerization (AIBN) of styrene was examined. A continuous stirred tank reactor (CSTR) was used for the experiments. The concentration of the inhibitor and initiator, the mean residence time and the reaction temperature were modified. The measurement of the residence time distribution (macroscopic mixing) and the estimation of the segregation number showed an ideal mixing behaviour (HCSTR). The conversion and the MWD were calculated. As the results show, it is possible to calculate the reaction in a HCSTR by only using the kinetic constants measured in discontinuous runs.     

Integration of CFD and polymerization for an industrial scale cis-polybutadiene reactor

A computational fluid dynamics (CFD) approach, coupled with anionic polymerization kinetics, was used to investigate the solution polymerization in a 12?m³ industrial scale cis-polybutadiene reactor. The kinetic model with double catalytic active sites was integrated with CFD by a user-defined function. The coupled model was successfully validated by the plant data and then used to investigate the key operating variables. Also, predictions of CFD model were compared with those of continuous stirred tank reactor (CSTR) model. Although the reaction mixture is well mixed in the middle and at the top of the reactor, there exists a poor mixing feeding zone at the bottom, which leads to serious deviations from the ideal CSTR. The polymerization process with nonideal mixing is very sensitive to the inlet temperature and the feeding rate. Enhancing the mixing performance in the feeding zone could be an effective way to improve the product quality.     

Study of suspension polymerization of styrene with a circular loop reactor

A circular loop reactor was devised and used to study the suspension polymerization of styrene. The transient droplet diameter distributions and the final particle size distributions were measured by changing the impeller diameter and the impeller speed. The effects of the impeller diameter on the size distributions and mean sizes of the final polymer particles were investigated. In the case of lower mixing power, the mean polymer droplet diameters depend upon impeller diameter in the early stage of polymerization, but become almost identical irrespective of the impeller diameter after the middle stage. In the case of higher mixing power, the mean polymer droplet diameters are almost identical irrespective of the impeller diameter throughout polymerization. The final mean particle sizes are correlated only with mixing power.     

Increasing PVC suspension polymerization productivity by using temperature‐programmed reactions

Vinyl chloride polymerizations are known to be autoaccelerating. The reaction rate increases with conversion. Because of this phenomenon, substantial reactor productivity at early conversion can be lost because the heat‐removal capacity of the reactor is not fully utilized until near the end of the polymerization. For this reason it is desirable to speed up the polymerization at the beginning and slow it down near the end. This rate adjustment can be achieved by running the polymerization hotter in the beginning and then cooling. We have written a scientifically based computer model of the polymerization designed specifically to simulate such temperature‐programmed reactions. The model does a complete heat balance on the polymerization, has a molecular weight predictor, and will be described and demonstrated for a polymerization at 50°C using sec‐butyl peroxydicarbonate (SBP) as initiator. By using this single initiator and a very simple straight‐line temperature‐programmed reaction, the time to 80% conversion can be reduced from 335 minutes to 240 minutes. This is a substantial increase in productivity.     

Using Tomography to Characterize the Mixing of Non‐Newtonian Fluids with a Maxblend Impeller

The flow field inside a cylindrical mixing vessel was visualized by electrical resistance tomography (ERT), a non‐intrusive measurement technique. Six tomography planes, each containing 16 sensing electrodes, measured the mixing time in the agitation of pseudoplastic fluid exhibiting yield stress. The effects of various parameters such as impeller types, impeller speed, fluid rheology, power consumption, Reynolds number, and absence of baffles on the mixing time were investigated. The Maxblend impeller was able to improve the mixing performance of non‐Newtonian fluids in a batch reactor. The mixing quality could be further enhanced by decreasing the xanthan gum concentration and using baffles in the mixing vessel.     

Influence of thermal runaway in styrene–acrylonitrile bulk copolymerization revealed by computational fluid dynamics modeling

The computational fluid dynamics (CFD) and kinetic-based moment methods coupled approach is adopted to simulate the bulk copolymerization of styrene–acrylonitrile (SAN) in a stirred tank reactor. Numerical simulations are carried out to investigate the impacts of impeller speed, monomer ratio, initiator ratio, and initial reaction temperature on the copolymerization process and product properties. Particularly, the Chaos theory is selected as a criterion for evaluating the occurrence of the thermal runaway. The Flory's and Stockmayer's distributions are employed to calculate chain length distribution and copolymer composition distribution of copolymer. The simulation results highlight that the appearance of thermal runaway can be postponed by properly increasing the rotation speed, decreasing the initiator loadings, initial acrylonitrile contents and initial reactor temperature. Furthermore, significant differences exist in the product properties that predicted by the ideal and non-ideal models, which demonstrates that the temperature heterogeneity plays a crucial role in SAN copolymerization. This study could offer references for the safe operation and design of polymerization processes.     

Free radical solution oligomerization of styrene in a continuous-stirred tank reactor train

For free radical oligomerization of styrene, a scheme for calculating the molecular weight distribution and conversion in a continuous-stirred tank reactor (CSTR) train is developed, which also allows the calculation of molecular weight distribution (MWD) for batch reaction. Calculations show that under conventional or near dead-end condition: (1) increasing initial initiator concentration, reaction time and reaction temperature, and decreasing initial monomer concentration cause P?_n and P?_w to decrease and MWD to narrow; (2) increasing initial initiator concentration, reaction time and reaction temperature, and increasing monomer concentration cause monomer conversion to increase; (3) a single CSTR gives a lower rate of oligomer production, but a narrower MWD than does a batch reactor.     

MICROMIXING EFFECTS ON BARIUM SULFATE PRECIPITATION IN A DOUBLE-JET SEMI BATCH REACTOR

The effects of turbulent mixing on barium sulfate precipitation in an imperfectly mixed double jet semi batch reactor were investigated experimentally and theoretically. When two feed solutions in separate streams were fed into the semi batch reactor, the precipitation was significantly altered by the impeller speed and the feeding time. Generally, in the range of low impeller speed (below 400 rpm), the suspension was segregated vertically in the reactor and the average particle size increased with increasing impeller speed. However, in the range of high impeller speed (above 400 rpm) the suspension was homogeneously dispersed in the reactor, but the trend of the turbulent mixing effect on the precipitation was opposite to that in range of the low impeller speed. The precipitation in the semi batch reactor was controlled by particle mass transfer and micromixing of the feed streams, both of which were promoted by increasing the impeller speed. At low impeller speed the influence of the mass transfer was dominant so that the particle size increased with increasing impeller speed, but at high impeller speed it was surpassed by the influence of micromixing so that the trend was reversed because enhanced micromixing generates a large number of small particles in the reactor.

To model our hypothesis for the effects of imperfect mixing on the reaction precipitation in the semi batch reactor, a micromixing-limited plug flow-ideal semi batch series reactor model was developed. The model predicts that enhanced micromixing created high supersaturation levels in the premixing region (plug flow reactor) which reduces the average particle size. The model also predicts the effect of feeding time on the precipitation in the semi batch reactor. These predictions are in excellent agreement with the experimental data. An interesting prediction of our model is that micromixing in the premixing region plays an important role in the overall reaction precipitation and its effect is greatly intensified as the turbulent mixing intensity is increased, which is opposite to our common sense.     

MICROMIXING EFFECTS ON BARIUM SULFATE PRECIPITATION IN AN MSMPR REACTOR

The Effects of non-ideal and nonhomogeneous mixing on barium sulfate precipitation in an MSMPR reactor were observed experimentally and analyzed theoretically. To generate nonhomogeneous mixing the unmixed feed streams were fed to the reactor at the same location (joint feeding mode) or a plug flow reactor was connected to the MSMPR reactor. These nonhomogeneous mixing conditions resulted in significant reductions in particle size and increases in particle numbers. These non ideal mixing effects were dependent on the impeller speed, feed stream velocity and residence time in the connected plug flow reactor and are believed to result from elevated supersaturation levels in a premixing zone which are controlled by turbulent micromixing

To model the effect of nonhomogeneous mixing (premixing) in the MSM PR reactor a plug flow-stirred lank reactor series model was developed. The plug flow reactor represents the premixing region of the MSMPR reactor in which turbulent micromixing is important, and the stirred tank reactor describes the homogeneous mixing region of the MSMPR reactor where particle growth is important. The model predicts that the premixing effect is strongly dependent on micromixing of the feeds in the premixing region, and thus, as the turbulent mixing intensity in this region is increased, the particle size in the product suspension is reduced and the particle population is increased. These predictions of the model arc in good agreement with the experimental data. An interesting prediction of the model is that as the impeller speed increases, the precipitation of barium sulfate in an MSMPR reactor deviates increasingly from the precipitation in a perfectly mixed (ideal) reactor.     

Continuous emulsion polymerization of vinyl acetate. II Operation in a single Couette–Taylor vortex flow reactor using sodium lauryl sulfate as emulsifier

Continuous emulsion polymerizations of vinyl acetate were conducted at 50°C in a single continuous Couette–Taylor vortex flow reactor (CCTVFR) using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator. The polymerization can be carried out very smoothly and stably, but the steady‐state monomer conversion attained in a CCTVFR is not as high as that in a plug flow reactor (PFR), but only slightly higher than that in a continuous stirred tank reactor (CSTR), even if the Taylor number is adjusted to an optimum value. Also, the effects of operating variables, such as the emulsifier, initiator, and monomer concentrations in the feed and the mean residence time on the kinetic behaviors were almost the same as those observed in a CSTR. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2755–2762, 2002     

Simulation and optimization of the continuous tower process for styrene polymerization

The continuous tower process, a popular industrial process for the manufacture of polystyrene, was simulated and optimized. A kinetic model for the thermal polymerization of styrene, which takes into account the Trommsdorff effect and the volume change accompanying the reaction, was developed. This was used to formulate model equations for the continuous flow stirred tank reactor (CSTR) and plug flow reactor (several sections) in the tower process. The model can predict monomer conversion, number‐ and weight‐average molecular weights, polydispersity index (PDI), and temperature at various locations in the unit, under specified operating conditions. Multiobjective optimization of this process was also carried out, for which an adaptation of a genetic algorithm (GA) was used. The two objectives were maximization of the final monomer conversion and minimization of the PDI of the product. The conversion in the CSTR was constrained to lie within a desired range, and polymer having a specified value of the number‐average molecular weight was to be produced. The optimal solution was a unique point (no Pareto sets were obtained). The optimal solutions indicated that the tower process is operated under near‐optimal conditions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 775–788, 2004     

Optimization of polymer synthesis through distributed control of polymerization conditions

An optimal control methodology is applied to the goal of lowering polydispersity while increasing conversion in polymerization reactions. An illustration using initiator, heat, and monomer flux control profiles for free‐radical polymerization of styrene in a plug flow reactor is provided and compared with available experimental data. The design calculations use a kinetic model that includes the gel effect. The reactor designs show that distributed initiator, heat, and monomer fluxes along the length of the reactor lower the polydispersity of the styrene polymers and increase conversion for a given reaction time. The monomer flux maintains a nearly constant monomer concentration in the reactor. The initiator and heat fluxes are highly correlated. The temperature rises as a result the heat flux; but the initiator flux results in a lower initiator concentration relative to the initiator cofeed case. At a reaction time of 120 min, a conversion of 44% and a polydispersity of 1.73 have been achieved. The theoretical designs, although not proven to be globally optimal, are of high quality. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2922–2928, 2002     

Mixing effect on emulsion polymerization in a batch reactor

The emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab‐scale reactor, which was equipped with a pitched blade turbine, four baffles, a U shaped cooling coil, and a temperature controller. Potassium persulfate was used as the initiator and sodium dodecyl sulfate as the surfactant. The effects of impeller speed, mounting baffles, and reaction temperature on the monomer conversion, polymer nano particle size and number, and molecular weight were examined in detail. An increase in the impeller speed up to 250 rpm enhanced the polymer properties but further agitation reduced the conversion, particle size, and molecular weight. The installation of the baffles enhanced the particle size and molecular weight but reduced the conversion and particle number. The use of baffles resulted in a narrower size distribution throughout the polymerization process. While the particle size and molecular weight were reduced with an increase in the reaction temperature, the monomer conversion and particle number were improved. POLYM. ENG. SCI., 55:945–956, 2015. © 2014 Society of Plastics Engineers     

Characterisation of the mixing of non‐newtonian fluids with a scaba 6SRGT impeller through ert and CFD

An attempt has been made to study the mixing of yield‐pseudoplastic fluids with a Scaba 6SRGT impeller using electrical resistance tomography (ERT) and computational fluid dynamics (CFD). The ERT system with four sensor planes, each containing 16 equispaced stainless steel electrodes, was used to measure the mixing time. The multiple reference frames (MRF) technique and the modified Herschel–Bulkley model were applied to simulate the impeller rotation and the rheological behaviour of the non‐Newtonian fluids, respectively. To validate the model, the CFD results for the power consumption were compared to the experimental data. The validated model was then employed to obtain further information regarding the averaged impeller shear rate, impeller circulation, and pumping capacities. The CFD and ERT data were utilised to investigate the effect of the impeller power, fluid rheology, and impeller size on the mixing time. The mixing time results obtained in this study were in good agreement with those reported in the literature. © 2011 Canadian Society for Chemical Engineering     

A study of the soap-free emulsion polymerization of styrene

A study on the soap-free emulsion polymerization of styrene has been accomplished in this work. The polymerization reaction was carried out in a batch reactor under isothermal condition. Potassium persulfate was used as an initiator. In our experiments, the effects of agitation speed, monomer concentration, and initiator concentration on the number and size of polymer particles, on the conversion of monomer and molecular weight of polymers were investigated. In addition, the systems in the presence of emulsifier or CaSO₃ were investigated and discussed in comparison with a system free of them.     

Continuous emulsion polymerization of vinyl acetate. Part I: Experimental studies

An interesting feature of commercial continuous emulsion polymerization reactors is the important phenomenon of sustained oscillations. Laboratory investigations of emulsion polymerization in a continuous stirred tank reactor (CSTR) have shown that the conversion, number of polymer particles and other related properties often oscillate widely with time. These oscillations can lead to emulsifier levels too small to adequately cover polymer particles resulting in excessive agglomeration and reactor fouling. Herein is reported an extensive experimental study of vinyl acetate emulsion polymerization in a CSTR. The effects of initiator and emulsifier concentrations, mean residence time and rate of agitation on the production rate and size distribution of polyvinyl acetate latices were investigated. Domains of reactor operation which give rise to sustained oscillations and massive agglomeration of the latex are mapped which give rise to sustained oscillations and massive agglomeration of the latex are mapped out. The effect of different start-up policies on reactor transients were also investigated.     

Polystyrene latex synthesized in presence of ultrasonic initiation

The low water solubility of styrene (St) monomer increase the need for a good initiator system to speed up the emulsion polymerization and remove unreacted monomers. Polymerization of styrene monomer in water was performed at 30, 50, and 70°C under ultrasonic irradiation using sodium dodecyl sulfate as surfactant and ammonium persulfate as initiator. Ultrasonic energy was used as a tool to speed up the polymerization. Combining ultrasonic and ammonium persulfate led to a higher conversion and higher rate of polymerization. Ultrasonic energy has an effect on the particle size distribution. The particle size distribution increases with an increase in the monomer conversion of styrene for ultrasonic polymerization, whereas the particle size distribution did not change with an increase in the monomer conversion compared with the conventional thermal polymerization results. Higher molecular weights were obtained under ultrasonic irradiation. FE‐SEM and TEM pictures show different morphology with changing temperature polymerization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011