DEVELOPMENT OF BATCH EMULSION POLYMERIZATION PROCESSES

Although emulsion polymerization has been used for a long time, relatively little attention has been paid to the technological issues of this polymerization technique. This paper describes the research on chemical engineering aspects of emulsion polymerization in (semi-)batchwise operated stirred tanks. The objective of this work was to improve the operation of current processes and to allow for improvements in the development of novel emulsion polymerization processes. For this purpose, different issues have shown to be important, for which the work described in this paper has been focused on four topics: emulsification, colloidal stability, rheology in high solids polymerization and heat transfer. These topics have been studied using the polymerization of styrene and vinyl acetate as two representative model systems

Our results reveal that sufficient emulsification is essential for proper control of the polymerization process. For the emulsifier used in this study, the colloidal stability of the polymer particles is mainly governed by the physico-chemical properties of the reaction mixture. During high solids emulsion polymerization, the particle size distribution of the polymer particles considerably influences the Theological properties of the reaction mixture and thereby the flow pattern in the reactor. Heat transfer to the reactor wall depends strongly on reactor geometry, impeller type and diameter as well as stirrer speed. Additionally, the physical properties of the reaction mixture, being related to solids content, conversion and monomer type, are important for heat transfer.     

DEVELOPMENT OF BATCH EMULSION POLYMERIZATION PROCESSES

Although emulsion polymerization has been used for a long time, relatively little attention has been paid to the technological issues of this polymerization technique. This paper describes the research on chemical engineering aspects of emulsion polymerization in (semi-)batchwise operated stirred tanks. The objective of this work was to improve the operation of current processes and to allow for improvements in the development of novel emulsion polymerization processes. For this purpose, different issues have shown to be important, for which the work described in this paper has been focused on four topics: emulsification, colloidal stability, rheology in high solids polymerization and heat transfer. These topics have been studied using the polymerization of styrene and vinyl acetate as two representative model systems

Our results reveal that sufficient emulsification is essential for proper control of the polymerization process. For the emulsifier used in this study, the colloidal stability of the polymer particles is mainly governed by the physico-chemical properties of the reaction mixture. During high solids emulsion polymerization, the particle size distribution of the polymer particles considerably influences the Theological properties of the reaction mixture and thereby the flow pattern in the reactor. Heat transfer to the reactor wall depends strongly on reactor geometry, impeller type and diameter as well as stirrer speed. Additionally, the physical properties of the reaction mixture, being related to solids content, conversion and monomer type, are important for heat transfer.     

The effect of particle size distribution on the olefin polymer reactor dynamics

Mass and energy balances in a reactor have been derived to study the effect of particle size distribution (PSD) for each reaction mechanism on the reactor dynamics. It was observed that the PSD affects both bed height and particle volume. A feasible region for reactor operation has been calculated by using physical constraints. In a nonisothermal polymerization system, the reactor temperature does not change appreciably as catalyst injection rate increases. A unique steady state solution is found in a gas-phase continuous stirred-bed propylene polymerization reactor. The eigenvalues of the system of equations indicate that the steady state is unstable. A comparison with published data allows the observation that the actual reactor dynamics may be readily explained by using only the PSD derived from a simple reaction mechanism.     

Progress of polymer reaction engineering: From process engineering to product engineering

Polymer reaction engineering studies the design, operation, and optimization of reactors for industrial scale polymerization, based on the theory of polymerization kinetics and transfer processes (e.g., flow, heat and mass transfer). Although the foundation and development of this discipline are less than 80 years, the global production of polymers has exceeded 400 million tons per annum. It demonstrates that polymer reaction engineering is of vital importance to the polymer industry. Along with the maturity of production processes and market saturation for bulk polymers, emerging industries such as information technology, modern transportation, biomedicine, and new energy have continued to develop. As a result, the research objective for polymer reaction engineering has gradually shifted from maximizing the efficiency of the polymerization process to the precise regulation of high-end product-oriented macromolecules and their aggregation structures, i.e., from polymer process engineering to polymer product engineering. In this review, the frontiers of polymer reaction engineering are introduced, including the precise regulation of polymer chain structure, the control of primary aggregation structure, and the rational design of polymer products. We narrow down the topic to the polymerization reaction engineering of vinyl monomers. Moreover, the future prospects are provided for the field of polymer reaction engineering.     

Modeling of branching density and branching distribution in low-density polyethylene polymerization

Low-density polyethylene (ldPE) is a general purpose polymer with various applications. By this reason, many publications can be found on the ldPE polymerization modeling. However, scission reaction and branching distribution are only recently considered in the modeling studies due to difficulties in measurement and computation of scission effect and branchings of polymer. Our previous papers [Kim, D.M., et al., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of CSTR. Chemical Engineering Science 59, 699-718; Kim, D.M., Iedema, P.D., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of a tubular reactor. Chemical Engineering Science, submitted for publication] are concerned with the scission reaction during ldPE polymerization and its effect on molecular weight distribution (MWD) of ldPE for various reactor types. Here we consider branching distributions as a function of chain length for CSTR and tubular reactor processes. To simultaneously deal with chain length and branching distributions, the concept of pseudo-distributions is used, meaning that branching distributions are described by their main moments. The computation results are compared with properties of ldPE samples from a CSTR and a tubular reactor. Number and weight average branchings and branching density increase as chain length increases until the longest chain length. The concentrations of long chain branching (LCB) are close to those of first branching moment in both CSTR and tubular reactor systems. The branching dispersity, a measure for the width of the branching distribution at a certain chain length, has the highest value at shorter chain length and then monotonously decreases approaching to 1.0 as chain length increases. Excellent agreements in branching dispersities between calculation with branching moments and prediction with assumption of binomial distribution for a tubular reactor and CSTR processes show that the branching distribution follows a binomial distribution for both processes.     

Inferential sensors for estimation of polymer quality parameters: Industrial application of a PLS-based soft sensor for a LDPE plant

Low-density polyethylene (LDPE) and ethylene vinyl acetate (EVA) copolymers are produced in free radical polymerization using reactors at extremely high pressure. The reactors require constant monitoring and control in order to minimize undesirable process excursions and meet stringent product specifications. In industrial settings, polymer quality is mainly specified in terms of melt flow index (MI) and density. These properties are difficult to measure and usually unavailable in real time, which leads to major difficulty in controlling product quality in polymerization processes. Researchers have attempted first principles modeling of polymerization processes to estimate end use properties. However, development of detailed first principles model for free radical polymerization is not a trivial task. The difficulties involved are the large number of complex and simultaneous reactions and the need to estimate a large number of kinetic parameters. To overcome these difficulties, some researchers considered empirical neural network models as an alternative. However, neural network models provide no physical insight about the underlying process. We consider data-based multivariate regression methods as alternative solution to the problem. In this paper, some recent developments in modeling polymer quality parameters are reviewed, with emphasis given to the free radical polymerization process. We present an application of PLS to build a soft-sensor to predict melt flow index using routinely measured process variables. Issues of data acquisition and preprocessing for real industrial data are discussed. The study was conducted using data collected form an industrial autoclave reactor, which produces LDPE and EVA copolymer using free radical polymerization. The results indicated that melt index (MI) can be successfully predicted using this relatively straightforward statistical tool.     

Particle size distribution and its effect on the olefin polymer reactor dynamics over olefin polymerization catalysts with multiple active sites

Mass and energy balances in a reactor have been derived to study the effect of particle size distribution for multiple active site catalyst systems on the reactor dynamics. It was found that multiple active sites in a Ziegler-Natta catalyst affect neither the reactor dynamics nor the particle size distribution, as opposed to a system which uses single site catalysts. It was discovered that a simple reaction model with a single type of active site dominant adequately explains the reactor dynamics and the particle size distributions for a continuous stirred-bed reactor for polymerization of propylene over a Ziegler-Natta catalyst.     

Modeling gel effect in branched polymer systems: Free‐radical solution homopolymerization of vinyl acetate

In this work a comprehensive mathematical framework is developed for modeling gel effect in branched polymer systems with application in the solution polymerization of vinyl acetate. This model is based on sound principles such as the free‐volume theory for polymer chains diffusion. The model predictions for monomer conversion and number‐ and weight‐average molecular weights were found to be in good agreement with published data in the literature. Moreover, the joint molecular‐weight distribution–long chain branching distribution is calculated by direct numerical integration of a large system of nonlinear ordinary integral‐differential equations describing the mass conservation of macromolecular species in a batch reactor. This allows studying the effect of process conditions such as initiator and solvent concentration on the product quality. It is believed that this work might contribute to a more rational design of polymerization reactors. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Polymer reaction engineering: From reaction kinetics to polymer reactor control

Polymer reaction engineering is a relatively “young”, very broad, multidisciplinary, rapidly developing field. It is the combination of polymer science, chemistry and technology with process engineering principles. The outcome of this high degree of synergism has evolved over the last fifteen or so years towards an area that includes any or all of the following: polymerization and post-polymerization (chemical modification) reaction kinetics; mathematical modelling and process simulation; polymer reactor design and scale-up; sensor development and process monitoring; and polymer reactor optimization, state estimation and computer control. This article will attempt to give an overview of the results obtained in our laboratory over the last seven years from systematic studies of polymer reaction engineering and polymer production technology problems. These problems cover all aspects of polymer reaction engineering mentioned above. Going from fundamentals to practice, the basic premise of the article is that only by adopting a holistic approach can one devise effective strategies in order to achieve the final objective of more efficient polymer reactor design and control, and hence improved production systems of polymeric materials.     

Particle size distribution in olefin continuous stirred-bed polymerization reactors

The control of polymer particle size and PSD is of industrial importance. Very fine particles pack poorly, thereby limiting reactor capacity, and present a dust explosion hazard. In olefin polymerization, a particle size distribution (PSD) in the polymerization reactor has been derived using population balances. Three reasonable reaction mechanisms for Ziegler-Natta catalysts, i.e., a simple reaction model, an active site reduction model, and a two sites model, have been used to derive the average number of active sites. It was observed that the PSD depends not only on residence time, but also on the reaction mechanism. It was also found that multiple active sites change the PSD slightly. The PSD, however, does not depend on initial catalyst volume.     

A composite catalytic polymer membrane reactor using heteropolyacid-blended polymer membrane

The vapor-phase MTBE decomposition was examined in a shell and tube-type catalytic membrane reactor (CMR). 12-Tungstophosphoric acid (PW) was used as a catalyst and poly-2,6-dimethyl-1,4-phenylene oxide (PPO) was used as a polymer material. A single-phase CMR (PW-PPO/Al₂O₃, type-1) and a composite CMR (PW-PPO/ PPO/ Al₂O₃, type-2) were successfully designed and characterized. It was revealed that the single-phase PW-PPO/ Al₂O₃ showed perm-selectivities for reaction products. The selective removal of methanol through the catalytic membrane shifted the chemical equilibrium toward the favorable direction in the MTBE decomposition. The PWPPO/ PPO/ Al₂O₃ showed the better performance than PW-PPO/ Al₂O₃. The enhanced performance of PW-PPO/ PPO/ Al₂O₃ CMR was due to the intrinsic perm-selectivity of PW-PPO and the additional separation capability of sub-layered PPO membrane.     

Measurement and control of polymerization reactors

The measurement and control of polymerization reactors is very challenging due to the complexity of the physical mechanisms and polymerization kinetics. In these reactors many important variables, which are related to end-use polymer properties, cannot be measured on-line or can only be measured at low sampling frequencies. Furthermore, end-use polymer properties are related to the entire molecular weight, copolymer composition, sequence length, and branching distributions. This paper surveys the instrumentation technologies, which are of particular interest in polymerization reactors with emphasis on, for example, measurement of viscosity, composition, molecular weight, and particle size. This paper presents a hierarchical approach to the control system design and reviews traditional regulatory techniques as well as advanced control strategies for batch, semibatch, and continuous reactors. These approaches are illustrated by focusing on the control of a commercial multiproduct continuous emulsion polymerization reactor. Finally, the paper captures some of the trends in the polymer industry, which may impact future development in measurement and reactor control.     

Emulsion copolymerization: Kinetics model and reactor performance

The Dow process for producing perfluorinated ionomeric membranes includes several emulsion copolymerizations involving gaseous tetrafluoroethylene and a second liquid phase monomer. The choice of the organic phase monomer depends on the desired product. The emulsion copolymerization reactor model was developed by extending the Smith-Ewart-Gardon theory for emulsion polymerization processes. Population balance techniques and Flory-Huggins solution theory were applied. The resulting coupled partial differential equations were solved using the method of characteristics. The reactor model, with minimal adjustable parameters, predicts most polymerization results, including molecular weight, reaction rates in the three process stages, latex particle size, polymer composition, and the composition drift as a function of reaction time. The analysis and reactor model is used in the manufacturing process to set process conditions to obtain the desired properties in the polymer product.     

Principles and limits of polymer latex tailoring

The preparation of a synthetic latex is a very complex process that is affected by the monomers selected, surfactants, initiators and the polymerization process. The semi-continuous process is the most frequent as it enables the control of the polymerization heat removal as well as the control of the composition of the copolymers comprising several types of monomer units. Some aspects of copolymerization in emulsion and particle growth in the case of the semi-continuous process are discussed. Copolymers usually comprise 4–5 comonomers, some of them with functional groups. Functional groups serve as loci for crosslinking, improve colloid stability, increase polarity, improve adhesion, cause alkali-solubility and/or alkali-swellability. High value polymer latices with special particle morphology, composition and other characteristics can be tailor-made by using special polymerization recipes that are still mostly empirical.     

Einfluß des stofftransportes bei der polymerisation von äthylen und 1-okten mit Ziegler-Katalysatoren

The polymerization of ethylene and 1-octene with supported Ziegler-catalysts was investigated with regard to the influence of mass transport of monomers on the kinetics, molecular weight and molecular weight distribution. In the case of the polymerization of ethylene, it was found that for certain conditions of reaction the mass transport of ethylene can influence the kinetics of polymerization respectively the catalyst efficiency strongly. The molecular weight and molecular weight distribution of the polyethylene formed are practically not affected by the conversion as well as particle size of catalyst and polymer. The molecular weight distribution however is affected by the concentration of the catalyst. The polymerization process of ethylene in suspension is distinguished by chemical and physical processes. A continuous chain initiation, for example, is based on the continuous reduction of the catalyst particles to small pieces during the course of polymerization. An apparent chain termination respectively catalyst deactivation can occur when catalyst particles are encapsulated within the growing polymer particles. The polymerization of 1 -octene for similar conditions of reaction gave polymers which were solved completely in the system used. The molecular weight distribution of the polymer formed nevertheless was very broad. This indicates that the mass transport of the monomers through the solid phase of polymer cannot be the main reason for the broad molecular weight distribution of the polymers which are produced by heterogeneous Ziegler-catalysts in suspension.     

OPTIMUM REACTOR DESIGN IN METHANATION PROCESSES WITH NONUNIFORM CATALYSTS

A generic methodology is developed to design a heterogeneous catalytic reactor for methanation processes. For the optimization of a heterogeneous catalytic reactor, nonuniform catalyst pellets such as a layered catalyst are considered with respect to reaction type, reactor performance, and component distribution inside the catalyst. Heterogeneous uniform and nonuniform catalyst models were developed to analyze the effect of mass and heat transfer between both bulk phase and catalyst surface and inside a catalyst pellet. Then, concentration profiles of hydrogen and carbon monoxide in the catalyst pellet and along the reactor axis were obtained by analyzing simulation results. It was shown that the application of different types of nonuniform catalyst pellets at a certain number of separate zones within a reactor could produce higher catalyst performance than a reactor with uniform catalyst. Furthermore, it proved a significant decrease of catalyst deactivation behavior such as coking and sintering. Layered catalysts were optimized to maximize an overall reactor performance over the catalyst lifetime, achieving capital cost reduction characterized by reactor size, catalyst amount, and degree of catalyst deactivation. Last, temperature control throughout the reactor operating periods was strategically planned for a reactor operation with distribution of nonuniform catalyst pellets. This methodology can also be usefully applied to the design of heterogeneous catalytic reactors for other processes such as hydro-treating process and cracking process.     

A numerical study on the effects of acidic catalyst on the polymerization of nylon-6

The effects of acetic acid on the polymerization characteristics of nylon-6 are investigated in a reactor model that consists of a continuous flow stirred tank reactor (CSTR) and a tubular reactor connected in series. Mathematical models for the CSTR and the tubular reactor have been established and solved by numerical methods. In the CSTR, the monomer conversion and the molecular weights are increased as the feed acetic acid concentration is increased. In the tubular reactor, the acid acts as both a catalyst and a modifier for the polymerization reaction. The effects of the feed acetic acid content on the zeroth, first and second moments and the polydispersity index of the polymer have been discussed.     

环流流化床反应器中乙烯气相聚合反应研究

研究开发了１种新型的用于乙烯气相聚合反应的环流流化床反应器。通过对工业Ａ催化剂和实验室自制的ＱＣＰ－０１催化剂的乙烯气相聚合反应评价及聚合物产品的颗粒形态等方面的研究,认为环流流化床反应器具有聚合反应平稳、催化剂的聚合活性高、产品粒径分布均匀等特点,是１种具有良好应用前景的新型反应器。     

Polymer property control in a continuous styrene polymerization reactor using model-on-demand predictive controller

The model-on-demand (MoD) framework was extended to the model predictive control (MPC) to design a multiple variable model-on-demand predictive controller (MoD-PC). This control algorithm was applied to the property control of polymer product in a continuous styrene polymerization reactor. For this purpose, a local auto-regressive exogenous input (ARX) model was constructed with a small portion of data located in the region of interest at every sample time. With this model an output prediction equation was formulated to calculate the optimal control input sequence. Jacket inlet temperature and conversion were chosen as the elements of regressor state vector in data searching step. Simulation studies were conducted by applying the MoD-PC to MIMO control problems associated with the continuous styrene polymerization reactor. The control performance of the MoD-PC was then compared with that of a nonlinear MPC based on the polynomial auto-regressive moving average (ARMA) model for disturbance rejection as well as for setpoint-tracking. As a result, the MoD-PC was found to be an effective strategy for the production of polymers with desired properties.