Non-Newtonian and non-isothermal analysis of flow during polymerization of methyl methacrylate in a model twin screw extruder. Part II

The flow analysis network (FAN) method was modified to analyze the flow during polymerization of methyl methacrylate (MMA) in a model counter-rotating nonintermeshing screw extruder. The shear viscosity of the reactive mixture in the twin screw extruder was considered as a mixture of polymer and monomer. Thus, the reaction viscosity of the mixture of polymer and monomer was taken to be an explicit function dependent on the shear rate, temperature, conversion, and molecular weight. A new flow path method was developed to calculate the residence time distribution, which related to the degree of conversion. The numerical prediction of the velocity, temperature, viscosity, and pressure profiles during reaction in the model twin screw extruder is described.     

Short Stopping of Runaway Methyl Methacrylate Polymerizations

Polymerization reactions are generally risky reactions for two reasons. On the one hand, they are strongly exothermic reactions; and on the other hand, the viscosity of the reaction masses can strongly increase during the course of the reaction. Especially in homogeneous systems, the increase of viscosity causes a sharp decrease of the heat transfer coefficient. Moreover, there are several polymerization systems which show strongly autocatalytic behavior at higher polymer volume fractions. In the case of loss of agitation or cooling, it is necessary to bring the reaction back to safe operating conditions. The addition of a so-called stopping agent is one of the possibilities to prevent a reaction system from runaway under such circumstances. These stoppers are radical-trapping agents which can react with free radicals by formation of a terminated product with respect to the polymerization process. With respect to the free radical, inhibition and chain growth are the two reactions which compete with each other. According to the reaction rate ratio of these two reactions, these agents are classified into retarders and true inhibitors. In the case of an inhibitor the rate of inhibition is much faster than the rate of chain growth, in the case of a retarder both reaction rates are of nearly the same order of magnitude. Surveys of the literature with respect to the inhibition of polymerization reactions are given in [1,3]. The present paper deals with the inhibition of radical solution and suspension polymerizations of methyl methacrylate using 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl in an adiabatic reaction calorimeter.     

Winsor I-like (micro)emulsion polymerization of styrene initiated by oil-soluble initiator

Batch emulsion polymerization of styrene initiated by an oil-soluble initiator and stabilized by non-ionic emulsifier (Tween 20) has been investigated. The rate of polymerization vs. conversion curve shows the two non-stationary rate intervals typical for the non-stationary-state polymerization. This behavior is a result of the continuous particle nucleation and the decrease of monomer concentration at the reaction loci with increasing conversion. The initial increase of the polymerization rate is attributed to the increase of particle number and the polymerization proceeding under the monomer-saturated condition—the Winsor I-like (micro)emulsion polymerization. The decrease of the polymerization rate is the result of the depressed transfer of monomer from the monomer reservoir to the reaction loci. Above 50 °C the monomer emulsion separates into two phases: the upper transparent monomer phase and the lower blue colored (microemulsion) phase. The polymerization mixture consists of the microdroplets (act as the reaction loci) and large degradable monomer droplets (act as the reservoir monomer and emulsifier). The continuous release of emulsifier from the monomer phase and the microdroplets induce the continuous particle nucleation up to high conversion. The initial formation of large particles results from the agglomeration of unstable growing particles and monomer droplets. The size of large (highly monomer-swollen) particles decreases with conversion and they merge with the growing particles at ca. 40-50% conversion. The coarse initial emulsion transformed during polymerization to the fine (semitransparent) polymer emulsion as a result of the continuous particle nucleation, the shrinking of highly monomer-swollen polymer particles and the depletion of monomer droplets. The low overall activation energy of polymerization is mainly ascribed to the decreased barrier for entering radicals into the latex particles with increasing temperature.     

Catalytic reaction performed in the liquid–liquid system at batch and semibatch operating mode

Hydrolysis of the propionic anhydrite catalysed with sulphuric acid at batch and semibatch operating conditions has been investigated using the reaction calorimeter RC1 Mettler Toledo. Due to a limited solubility of the anhydrite in the aqueous phase where the reaction takes place, mass transfer with simultaneous chemical reaction has to be considered. Contributions of both phases to the reaction mixture change during the reaction progress, so that a complex, strongly non-linear behaviour of the reactor has been noticed. Influence of the concentration of the catalyst, the reaction temperature as well as the initial volume fraction of the organic phase in the reaction mixture and the stirrer speed on the overall conversion rate have been determined directly from calorimetric measurements. Some indications related to the safe and efficient performance of the investigated process as well as to a simplified experimental kinetic model have been formulated.     

Automatic polymerization reactor with on-line data measurement and reactor control

An automatic polymerization reactor has been developed for on-line measurement of conversion, viscosity, and molecular weight distribution of homogeneous free radical polymerization. For on-line measurements during polymerization a fraction of the reaction mixture is pumped as bypass through a densimeter and a viscometer. In time intervals of 15 min samples are taken, diluted semi-automatically, and then injected into a high performance gel permeation chromatography for determination of the molecular weight distribution of the polymers. A microcomputer collects these data, calculates the conversion of reaction and the output data for different control units. Methyl methacrylate is polymerized batch-wise in solution with ethyl acetate as solvent and dicyclohexyl peroxydicarbonate as initiator. Semi-batch polymerizations with constant reaction rate are performed by feeding initiator which is controlled by open-loop and closed-loop control.     

Multiobjective dynamic optimization of batch free radical polymerization process catalyzed by mixed initiator systems

The optimal control policies for batch free radical polymerization of styrene catalyzed by a binary mixture of monofunctional initiators have been determined using a multiobjective dynamic optimization technique. The process objectives considered in the optimization include monomer conversion, polymer molecular weight, initiator residue level, and total reaction time. It is illustrated through model simulations and experiments that the performance of the batch polymerization process can be improved significantly through the use of optimal initiator mixture and polymerization temperature programming. This paper also illustrates how the multiobjection optimization technique can be used effectively to solve complex polymerization reactor optimization problems with detailed reaction models.     

Dynamics of a continuous stirred tank reactor for styrene polymerization initiated by a binary initiator mixture. II: Effect of viscosity dependent heat transfer coefficient

The dynamic behavior of the solution polymerization of styrene in a continuous stirred tank reactor is analyzed with a mixture of tert-butyl perbenzoate and benzoyl peroxide as an initiator system. In the modeling of the reactor, a viscosity dependent reactor wall heat transfer coefficient is used to account for the changing heat transfer efficiency as monomer conversion and polymer molecular weight increase. The steady state and bifurcation behaviors have been investigated with the reactor residence time, initiator feed composition, initiator concentration, feed solvent volume fraction, and coolant temperature as bifurcation parameters. Unlike the reactors with constant heat transfer coefficient, the present system exhibits relatively simple steady state and dynamic bifurcation behaviors. Oscillatory behavior is observed only when the solvent volume fraction in the feed exceeds 0.2. The dynamic simulation of the reactor also indicates that a feedback temperature controller may fail to maintain the reactor temperature when the heat transfer coefficient changes as a result of process disturbances.     

甲基丙烯酸甲酯本体聚合中物性参数的测定及关联

甲基丙烯酸甲酯(MMA)本体聚合过程中比热容、黏度和导热系数等物性参数是影响聚合动力学和体系传热的重要因素.研究了聚合转化率和温度对MMA本体聚合体系密度、比热容、黏度和导热系数的变化,发现随着转化率增大,体系比热容减小,密度、黏度和导热系数增大,并存在黏度发生突变的临界转化率;随着体系温度增大,密度和导热系数减小,比热容和黏度突变对应的临界转化率增大.建立了能描述各物性参数随转化率(聚合物浓度)和温度变化的数学关联式,计算值和实验值吻合较好,建立的各关联式能较好地预测MMA本体聚合中的物性的变化,可为聚合配方及传热的设计提供基础.     

Modeling and control of continuous stirred tank reactor for thermal copolymerization

A mathematical model is developed for solution copolymerization in a continuous stirred tank reactor. For the thermal copolymerization of styrene and acrylonitrile (SAN), the kinetic rate expression for thermal initiation is derived by applying the pseudo-steady-state hypothesis to the intermediates, and the kinetic parameters are estimated by experimental investigation. The moment equations of living and dead polymer concentrations are derived by applying the pseudokinetic rate constantmethod. The model is used to calculate the conversion, the copolymer composition, the weight-average molecular weight, and the polydispersity. It is demonstrated that this model can predict the industrial data very well under various operating conditions. The dynamic analysis of the reaction system enables us to determine the polymer properties against the changes in the operation parameters. It is noticed that the monomer conversion is controlled to some extent by the reaction temperature and the feed monomer fraction. The monomer conversion control of a solution copolymerization reactor is treated with different control algorithms. The fuzzy/proportional–integral–derivative controller shows satisfactory performances for both setpoint tracking and disturbance rejection and can be easily applied to continuous polymerization processes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:921–931, 1998     

Polymerization of butyl methacrylate by light scattering and the formation of regions of local inhomogeneity

The polymerization of butyl methacrylate (BMA) at ～60°C above the polymer glass transition temperature has been studied by determining the spectra of the scattered light, and the angular dependence of the total (integrated) scattering intensity. Both phonon velocity and attenuation show a relaxation behaviour during conversion, but the frequency dependent macroscopic shear viscosity does not contribute to the relaxation mechanism. The behaviour of the Rayleigh-Brillouin intensity ratio and the dissymmetry of the total scattered intensity shows that small regions of local inhomogeneities at low concentrations are formed during the polymerization reaction.     

Nonisothermal suspension polymerization of vinyl chloride for enhanced productivity

In the present study, a mathematical model is developed to numerically predict nonisothermal batch suspension polymerization of vinyl chloride. Free volume theory was used to consider diffusion‐controlled reactions. Model predictions were validated against field data obtained in a pilot scale stirred tank reactor. Variable temperature trajectory was considered during the course of the reaction to improve productivity by reducing the polymerization time for a certain conversion. Variable temperature during the course of the polymerization was successfully implemented by considering the predefined K value. By using variable temperatures during the course of the reaction, the density of the short branches per 1,000 monomer units as a criterion for structure defect remained relatively unchanged. Maximum reduction in reaction time relative to the isothermal case with the same K value and final conversion was 44% for the best temperature trajectory. J. VINYL ADDIT. TECHNOL., 22:470–478, 2016. © 2015 Society of Plastics Engineers     

Experimental study and computer simulation of a two-stage continuous polymerization process for the production of methyl methacrylate

The two major problems encountered in industrial liquid-phase addition polymerization are: the heat released by highly exothermic reactions and the great increase in viscosity with conversion. The high rate or heat generation, coupled with the low thermal diffusivity of the reacting system, often lead to thermal runaway. Even with the process kept under marginal control, large temperature variations broaden the product molecular-weight distribution. Temperature control is particularly difficult in the Trommsdorff region, where reaction rate rapidly increases as temperature rises and viscosity builds up. A two-stage process is developed in this work to attack these problems and to achieve continuous operation of poly(methyl methacrylate) bulk polymerization. This process utilizes a continuous stirred-tank reactor (CSTR) as a first-stage prepolymerizer and a spray tower as the second-stage finishing reactor. Use of a CSTR offers good temperature control and product uniformity during the early stages of reaction and eases delivery of the reacting system to the second stage at the desired conversion and molecular-weight level. Spraying the partially polymerized mixture into the tower as fine droplets prior to the onset of gel effect eliminates the problems of transporting, agitating, and mixing a reacting system with a rapidly increasing viscosity. Heat of reaction is efficiently removed by a countercurrent stream of nitrogen in the tower, in direct contact with the falling droplets. The high surface-to-volume ratio of these small droplets facilitates heat transfer, and the problem of heat buildup can be efficiently controlled. Products from the bottom of the tower can then be melt-processed by conventional methods, such as extrusion. Experiments performed in the laboratory have demonstrated the feasibility of this proposed concept. Process optimization was in no way achieved due to serious space and equipment limitations. The process was thus further examined by computer simulation and model parameter sensitivity study. A practical design was recommended based on the model predictions.     

Transformation of liquid to amorphous solid: The time to vitrify for styrene polymerization

A model is presented for the calculation of the time to vitrify vs. temperature for isothermal polymerization by the chain growth mechanism. The model is based on the glass transition temperature (T_g) rising from its initial value to the reaction temperature. The relationships between T_g and the volume fraction of polymer and monomer, the volume fraction of polymer and the extent of reaction, and the extent of reaction and time are also required. In a plot of temperature vs. time the vitrification curve is generally S-shaped; the time passes through a maximum just above the glass transition temperature. The model applies to linear polymerization in which monomer and high molecular weight polymer are the dominant species, i.e., to chain reactions. In this communication the model is applied to the bulk polymerization of styrene by the free radical mechanism.     

Modeling and optimization of a high-pressure ethylene polymerization reactor using gPROMS

A gPROMS implementation of a comprehensive steady-state model of the high-pressure polymerization of ethylene in a tubular reactor is presented. Model outputs along the reactor length include the complete molecular weight distribution and branching indexes, as well as monomer conversion, average molecular weights, reactants’ compositions, and reactor temperature and pressure. A detailed calculation of physical and transport properties, such as the reaction mixture density, heat-transfer capacity, viscosity and global heat-transfer coefficient is also included. The reactor model is included in an optimization framework that is used to determine the best operating conditions for producing a polymer with tailor-made molecular structure in terms of the complete molecular weight distribution, branching and polydispersity.     

Influence of mixing performance on polymerization of acrylamide in capillary microreactors

Nonliving free radical polymerization of acrylamide was chosen as a model reaction to investigate the effect of mixing performance on the polymerization in capillary microreactors. The polymerization rate was enhanced by increasing the volumetric flow rate and the reaction temperature at a constant residence time. However, higher temperatures led to lower Mn and larger PDI. The reaction mixture viscosity increased significantly during the polymerization. Both diffusion and dispersion coefficients were calculated to evaluate the mixing performance in microreactors. The capillary microreactor with a larger inner diameter led to higher monomer conversions, lower Mn and larger PDI compared to the capillary microreactor with a smaller inner diameter, which could be explained through a heat balance analysis for the polymerization and the dispersion effect. Moreover, it was found that the addition of a pre‐mixing stage minimized the effect of insufficient mixing between the initiators and the monomers on the polymerization. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1828–1840, 2018     

Poly(phenylene sulfide): Polymerization kinetics and characterization

The polycondensation kinetics of aromatic nucleophilic substitution on 1,4-dichlorobenzene by sodium sulfide has been investigated at 195°C in N-methyl pyrrolidone. The reaction follows second-order kinetics. The rate is bimodal with an initial slow rate till 50% conversion followed by a faster rate between 50 and 97% conversion. The specific reaction rates have been evaluated as 3.97 × 10^?3 L m^?1 s^?1 and 1.02 × 10^?2 L m^?1 s^?1 for the initial and later part (50–97%) of the reaction. The development of the degree of polymerization with reaction time was followed by end-group analysis and intrinsic viscosity measurements of polymer samples collected at different conversions. The reaction differs from conventional polycondensation reactions in two aspects. Polymer formation occurs at low conversions, and a significant amount of uncreacted monomer is present even at very high conversions. Unlike other precipitation polymerization reactions, the polymer chain continues to grow even after precipitation.     

Experimental investigation of vinyl chloride polymerization at high conversion—reactor dynamics

A series of suspension polymerizations of vinyl chloride monomer (VCM) was carried out in a 5-L pilot plant reactor over the temperature range, 40–70°C. The reactor pressure and monomer conversion were monitored simultaneously every 7–8 min. The critical conversion X_f, at which the liquid monomer phase is consumed, was considered to occur when the reactor pressure fell to 98% of the vapor pressure of VCM for suspension at the polymerization temperature. The reactor model predictions of pressure are in excellent agreement with the experimental data over the entire conversion and temperature ranges studied. The mechanism of reactor pressure development for VCM suspension polymerization is discussed herein in some detail. For isothermal batch polymerization, the reactor pressure falls in two stages due to the effect of polymer particle morphology on pressure drop. The first stage is due to the volume increase of the vapor phase as a result of volume shrinkage due to conversion of monomer to polymer. The monomer phase is not yet consumed at this stage, but it is trapped in the interstices between primary particles creating a mass transfer resistance; therefore, the reactor pressure drops slowly. The second stage is due to both the volume increase of the vapor phase and to the monomer in the vapor phase diffusing into the polymer phase because of the subsaturation condition with respect to monomer in the polymer phase. The reactor pressure drops dramatically with an increase in monomer conversion at this stage. The present model can be used to predict reactor dynamics during suspension polymerization under varying temperature and pressure conditions.     

Zur kinetik und teilchenbildung der fällungspolymerisation von acrylsäure in gegenwart von polystyrol-block-polyethylenoxid-copolymeren

A surfactant system containing polystyrene-block-poly(oxyethylene) and water was used for the free radical precipitation polymerization of acrylic acid in toluene. The use of this stabilizing system makes it possible to produce poly(acrylic acid) dispersions with a particle diameter ranging from 50 to 300 nm and solid contents up to 40 wt.-%, which remain stable for months. The rate of polymerization, which was measured on-line in a reaction calorimeter shows a strong autocatalytic behavior. The maximum of the polymerization rate and the corresponding time of appearance depend strongly on the water content. The influence of the composition of the surfactant (block length), its concentration and the concentration of initiator and monomer on the polymerization rate and the particle diameter were studied. Especially by varying the block length of the copolymer, the number of particles, respectively the particle diameter, can be controlled. To describe the course of the polymerization process, a model based on the theory of homogeneous particle nucleation is suggested. The influence of water on the swelling equilibrium of the poly(acrylic acid) particles is taken into account.     

顺丁橡胶聚合过程控制

丁二烯溶液聚合过程是将溶剂中的丁二烯在催化剂作用下引发聚合得到顺式1,4-含量在96%－98%的聚丁二烯胶液的工艺过程。聚合过程是整个橡胶生产中最关键的步骤,聚合反应的好坏直接影响到成品质量,通过对聚合进料量、单体浓度、反应温度、催化剂用量及配比、转化率、聚合物门尼粘度等项目的控制达到稳定聚合反应,优化产品质量的目的。     

Synthesis of cationic flocculant by radiation-induced copolymerization of methyl chloride salt of N,N-dimethylaminoethyl methacrylate with acrylamide in aqueous solution

The radiation-induced copolymerization of the methyl chloride salt of N,N-dimethylaminoethyl methacrylate (DMAEM·MC) with acrylamide (AAm) was used to prepare a cationic polymer flocculant. The polymerization rate increased with increasing dose rate, polymerization temperature, monomer concentration and mole fraction of AAm in the monomer mixture. The molecular weight of the copolymer was also found to increase with monomer concentration and mole fraction of AAm, but at high concentration and fraction of AAm, intermolecular crosslinking tends to occur during the polymerization to form water-insoluble copolymer. A water-soluble copolymer having various molecular weights and cationic strengths can be synthesized by selecting suitable reaction conditions; i.e., this radiation process can provide a much higher molecular weight copolymer with a wide range of cationic strength. The flocculation effect was evaluated using sludge from wastewater of sugar manufacture. It was found that the radiation-polymerized copolymer DMAEM·MC–AAm has an excellent flocculation effect.