Online model-based control of an emulsion terpolymerisation process

A multivariable constrained model predictive control was formulated and implemented to control online an emulsion terpolymerisation process. A generic multilayer control architecture for real-time implementation of optimal control policies for particulate processes was also developed. The methodology was implemented within a multi-tiered software environment for real-time implementation of model-based control strategies. A full mechanistic dynamic model for emulsion terpolymerisation, developed in our previous work, is used as a real-time soft-sensor and incorporated within the MPC strategy. The simultaneous control of key polymer properties such as the particle size distribution (PSD), average molecular weight (Mn) and terpolymer composition were investigated using case studies involving manipulation of flow rates of monomers (styrene, MMA, MA), surfactant and initiator, and the reactor temperature. Although some offsets were observed with a degree of model mismatch, the experimental results agreed well with predictions.     

Population balance model versus lumped model for emulsion polymerisation: Semi-batch and continuous operation

In this article, a systematic comparison is made of a detailed population balance model of the emulsion polymerisation process that accounts for the dynamic evolution of the entire particle size distribution with a simple model of the process that accounts only for the dynamic evolution of the average particle size and the total particles. Both models account for the underlying mechanisms of the process to the level admissible within their respective frameworks. The predictions of the two models are compared under both batch and continuous operation. The aim is to elucidate the degree of disparity of model predictions for batch operation, and to elucidate the ability of the simple lumped models to predict oscillatory dynamics for continuous operation. The focus is on the comparison of the predictions of important control variables such as solids content (conversion), total particles and average particle size, but also those of important particle phenomena of nucleation, growth and coagulation. It is found that with respect to these lumped control variables, the simple model performs well in matching the detailed model, and hence will be a very valuable tool for the purpose of on-line feedback control. However, the detailed models will be important for distributed control variables such as the entire particle size distribution.     

Operation of semi-batch emulsion polymerisation reactors: Modelling, validation and effect of operating conditions

A detailed dynamic model was developed for a styrene emulsion polymerisation semi-batch reactor to predict the evolution of the product particle size distribution (PSD) and molecular weight distribution (MWD) over the entire range of monomer conversion. A system exhibiting zero-one kinetics was employed, with the model comprising a set of rigorously developed population balance equations to predict monomer conversion, PSD and MWD. The modelling equations included diffusion-controlled kinetics at high monomer conversion where the transition from the zero-one regime to a pseudo-bulk regime occurs. The model predictions were found to be in good agreement with experimental results. Both particle growth and the PSD were found to be strongly affected by the monomer feedrate. Reactor temperature had a major influence on the MWD which was, however, insensitive to changes in the monomer feedrate. These findings were confirmed experimentally. As a result, it seems reasonable to propose that the use of the monomer feedrate to control the PSD and the reactor temperature to control the MWD are appropriate in practical situations. Consequently, an optimal monomer feed trajectory was developed off-line (using the validated reactor simulation) and verified experimentally by producing a polymer with specific PSD characteristics.     

On-line multi-variable predictive control of molar mass and particle size distributions in free-radical emulsion copolymerization

A multi-variable model predictive control (MPC) was formulated to solve control problems associated with a combination of regulation and targeting desired set-points. We investigated the simultaneous control of key polymer properties: the particle size (PSD) and molecular weight distribution (MWD) by manipulating the flow rates of the monomers (styrene, MMA), surfactant, initiator and the temperature of the reactor. A multi-input-multi-output (MIMO) formulation was constructed for the constrained optimal control problem to maximize the width of the PSD (with M_n at a constant set-point), and to maximize the average molar mass. The strategy developed within a gPROMS-API-DCS environment allowed real-time implementation of model-based control of the process. The optimal control problem was implemented via an interface to a dynamic optimization code. Major improvements in process operation and polymer property control resulted on the implementation of our multi-variable MPC algorithm. The manipulation of the four flow rates and the temperature increased the degree of freedom in the system and achieved tighter PSD and MWD control. The on-line performance of MPC for MWD and PSD control was found to be satisfactory.     

On the role of an unconventional rigid rodlike cationic surfactant on the styrene emulsion polymerization. Kinetics, particle size and particle size distribution

An unconventional amphiphile (1-[ω-(4′-methoxy-4-biphenylyloxy)octyl]pyridinium bromide, PC8) was used as surfactant in the emulsion polymerization of styrene. At low surfactant concentration (6, 12 or 36 mmol l⁻¹), curves of polymerization rate versus conversion obeyed the typical behavior characterized by intervals I, II and III. However, at high concentration (48 or 72 mmol l⁻¹) the interval II was not observed. The particle size distribution curves showed two families of polymer particles, indicating the participation of at least two mechanisms of particle formation, one being the simple micellar nucleation and the other probably the coagulative nucleation of precursor particles. The latter was considered to occur during the nucleation interval.     

Measurement methods of particle size distribution in emulsion polymerization

The particle size distribution of polymer always develops in emulsion polymerization systems, and certain key phenomena/mechanisms as well as properties of the final product are significantly affected by this distribution. This review mainly focuses on the measurement methods of particle size distribution rather than average particle size during the emulsion polymerization process, including the existing off-line, on-line, and in-line measurement methods. Moreover, the principle, resolution, performance, advantages, and drawbacks of various methods for evaluating particle size distribution are contrasted and illustrated. Besides, several possible development directions or solutions of the in-line measurement technology are explored     

Inferential closed-loop control of particle size distribution for styrene emulsion polymerization

In this work, a new control strategy for controlling the particle size distribution (PSD) in emulsion polymerization has been proposed. It is shown that the desired PSD can be achieved by controlling the free surfactant concentration which in turn can be done by manipulating the surfactant feed rate. Simulation results show that the closed-loop control of free surfactant concentration results in a better control of PSD compared to open-loop control strategy, in presence of model mismatch and disturbances. Since the on-line measuring of ionic free surfactant concentration is difficult, conductivity which is related to it is measured instead and used for control purposes. The closed-loop control of conductivity also results in a better control of PSD compared to open-loop control strategy, but its performance is not as good as controlling free surfactant concentration in presence of model mismatch.     

Online inferential product attribute estimation for optimal operation of emulsion terpolymerisation: Application to styrene/MMA/MA

An advanced model for process design and control of emulsion terpolymerisation was developed. A test case of emulsion terpolymerisation of styrene (Sty), methyl methacrylate (MMA) and methyl acrylate (MA) was investigated on state of the art facilities for predicting, optimising and control end-use product properties including global and individual conversions, terpolymer composition, the average particle diameter and concentration, glass transition temperature, molecular weight distribution, the number- and weight-average molecular weights and particle size distribution.The model equations include diffusion-controlled kinetics at high monomer conversions, where transition from a ‘zero-one’ to a ‘pseudo-bulk’ regime occurs. Transport equations are used to describe the system transients for batch and semi-batch processes. The particle evolution is described by population balance equations which comprised a set of integro-partial differential and nonlinear algebraic equations. Backward finite difference approximation method is used to discretise the population equation and converts them from partial differential equations to ordinary differential equations. The model predictions were experimentally validated in the laboratory and were found to be in excellent agreement, thus paving the way for further application of the model.     

Maximizing production and polymer quality (MWD and composition) in emulsion polymerization reactors with limited capacity of heat removal

In this work, an on-line control strategy based on reaction calorimetry was used to maximize the production of styrene/n-butyl acrylate latex with desired copolymer composition and molecular weight distribution in reactors with limited capacity of heat removal. For this purpose, nonlinear model-based controllers were used to maximize the production maintaining simultaneously the ratios of each comonomer concentration and of the overall unreacted monomer concentration to chain transfer agent at the required values.     

Optimal operating policies for closed-loop recycling HPLC processes

In this paper, optimal operating policies for HPLC separation processes based on a closed-loop recycling strategy are presented. Successful application of closed-loop recycling HPLC processes had been reported, mainly in the pharmaceutical industry, but there is still limited knowledge about the dynamic behaviour of this process and little work on optimal operation of closed-loop recycling systems has been presented in the open literature. This paper investigates different operating policies for a closed-loop recycling HPLC system using an equilibrium-dispersive model coupled with a dynamic optimisation technique. The operating policies include: conventional recycling, recycling with peak shaving and recycling with peak shaving and multiple feed injection. The determination of optimal operating policies is aimed to maximise the amount of purified product(s) and thereby achieve better column performance whilst keeping the matrix cost constant. The practical implementation of these operating policies will also be considered.     

High‐Solid‐Content Emulsions of PVC: Scale‐Down of an Industrial Process for Enhanced Understanding of Particle Formation Part 3: Production of Bimodal Latexes

A scale‐down study of an industrial reactor for the production of polyvinyl chloride (PVC) via an emulsion polymerization process was carried out in order to understand the cause of batch‐to‐batch variations in product quality. The results in Part 2 of this series of papers indicated that a large excess of base is required to control the particle size distribution (PSD) of the seed process. Here, it is demonstrated that the flow rate of the initiator and the second‐stage surfactant are the most important parameters for PSD control. Altering the time point at which the initiator and surfactant are injected allows controlling the relative volume fractions of large and small particles.     

High‐Solid‐Content Emulsions of PVC: Scale‐Down of an Industrial Process for Enhanced Understanding of Particle Formation Part 1: Introduction and Scale‐Down

A scale‐down study of an industrial reactor for the production of polyvinyl chloride (PVC) via an emulsion polymerization process was carried out in order to understand the source of batch‐to‐batch variations in product quality. In Part 1, an analysis of the plant is presented and the industrial recipe scaled down to a pilot‐scale reactor. In the following Parts 2 and 3 a systematic analysis of the main process parameters revealed that particle generation and stabilization actually occurred in a manner slightly different from what was thought at the production site.     

High‐Solid‐Content Emulsions of PVC: Scale‐Down of an Industrial Process for Enhanced Understanding of Particle Formation Part 2: Preliminary Analysis of Seed Production

A scale‐down study of an industrial reactor for the production of polyvinyl chloride (PVC) via an emulsion polymerization process was performed in order to understand the source of batch‐to‐batch variations in product quality. In Part 2 of this series of three papers, it is demonstrated that a large excess of base is required to control the particle size distribution of the seed process. Although differences exist between the critical micelle concentration and the surface area occupied by a surfactant molecule for linear and branched isomers of the surfactant sodium dodecyl benzene sulfonate, the characteristics of the molecules from different suppliers were reasonably similar.     

Calorimetric estimation for a batch-loop emulsion polymerisation reactor

To facilitate the online monitoring and control of a pilot-scale polymerisation reactor, state estimation techniques are investigated. Specifically, a batch-loop reactor is employed for the emulsion polymerisation of methyl methacrylate. The reactor consists of jacketed tubular sections fitted with in-line static mixers, thus providing mixing homogeneity and improved temperature control. A direct estimation of the reaction rate is attained through measurements of process and jacket side temperatures, and thus a calorimetric method of estimation. This is compared with a Kalman filter based calorimetric approach, in which there is compensation for model uncertainties and measurement noise. For both estimation methods, no knowledge of the kinetic model for polymerisation is needed. Experimental results indicate that with an accurate model of the process energy balance, in which, for example, the recycle pump energy input is described, the Kalman filter approach is found to provide excellent prediction of conversion, for both high and low conversions, for this pilot-plant reactor system. The approach does not require any (approximate) kinetic knowledge, and is thus considerably easier in implementation than the extended Kalman filter approaches.     

Computationally efficient solution of population balance models incorporating nucleation, growth and coagulation: application to emulsion polymerization

A computationally efficient solution technique is presented for population balance models accounting for nucleation, growth and coagulation (aggregation) (with extensions for breakage). In contrast to earlier techniques, this technique is not based on approximating the population balance equation, but is based on employing individual rates of nucleation, growth and coagulation to update the PSD in a hierarchical framework. The method is comprised of two steps. The first step is the calculation of the rates of nucleation, growth and coagulation by solving an appropriate system of equations. This information is then used in the second step to update the PSD. The method effectively decomposes the fast and the slow kinetics, thereby eliminating the stiffness in the solution. In solving the coagulation kernel, a semi-analytical solution strategy is adapted, which substantially reduces the computational requirement, but also ensures the consistency of properties such as the number and mass of particles.     

乳液聚合中乳胶粒粒径大小及分布的影响因素

在乳液聚合中,乳胶粒的大小及分布对乳液的性能及其应用有很大的影响,同时也反映了乳液聚合反应进行的过程。本文综述了影响乳胶粒粒径大小及分布的各种因素,如聚合工艺、乳化剂、单体种类、聚合温度、引发剂等,并介绍了不同粒径乳液的性能及其应用。     

A novel approach to prepare large-scale and narrow-dispersed latex particles by emulsion polymerization based on particle coagulation mechanism

In order to understand the mechanism of narrow particle size distribution of the final latex during particle coagulation, a series of experiments were performed to investigate the effect of polymer nature on particle coagulation capability. In particular, thermodynamics and kinetics in aqueous phase were considered to illustrate the detail process of particle coagulation. The final particle size decreased with the increasing side chain length of alkyl methacrylate from 181.5 nm in MMA to 131.6 nm in EMA, 119.3 nm in PMA, and 115.1 nm in BMA, indicating that the particle coagulation capability was proportional to the hydrophilicity of polymer. With increasing polymer hydrophilicity, the affinity between surfactant molecules and particle surface decreased, thus enhancing the particle coagulation capability. Moreover, the critical length of oligomer radical also increased with increasing hydrophilicity and the efficiency of radical capture decreased, thus increasing the saturation of monomer concentration in the inner part of particle, promoting particle coagulation. Combining these results and the La Mer Diagram, a novel approach was developed to prepare large-scale, narrow-dispersed, and high solid content polymer latex based on particle coagulation mechanism. Three criteria, namely, rapid nucleation, fast coagulation, and a long growth period, should be met to produce latex with a narrow particle size distribution.     

测定压敏胶乳液表面张力的简易珠滴重量法

介绍了一种利用常用的酸式滴定管、移液管和电子天平测定乳液表面张力的简易方法。首先通过测定已知表面张力液体的珠滴质量 ,由σ =D×M计算出D值 ,然后测定压敏胶乳液的珠滴质量 ,并利用同样的公式计算出乳液的表面张力。我们利用此方法测定了 5种压敏胶乳液的表面张力。结果表明 ,这种测定乳液表面张力方法操作简便 ,实用性强 ,结果准确可靠。     

加氢裂化尾油非临氢降凝催化剂研究

以辽化加氢裂化尾油为原料,在常压非临氢条件下评价了HZSM-5/Al2O3和HM/Al2O3催化剂的降凝效果。考察了反应温度、催化剂使用时间和空速对白油收率、凝点和闪点的影响。通过对催化剂的比表面积、孔径分布和酸中心分布进行表征,发现催化剂的这些理化性质与催化剂的催化性能有着密切的关系。实验结果表明,在常压非临氢条件下,HZSM-5分子筛催化剂比丝光沸石分子筛有更好催化降凝效果和催化稳定性。     

Challenges in particle size distribution measurement past, present and for the 21st century

The field of particle size distribution (PSD) characterization and measurement has experienced a renaissance over the past ten years. This revitalization has been driven by advances in electronics, computer technology and sensor technology in conjunction with the market pull for PSD methods embodied in cost effective user friendly instrumentation. The renaissance can be characterized by at least four activities. (1) End user innovation exemplified by techniques such as hydrodynamic chromatography (HDC), capillary hydrodynamic fractionation (CHDF) and field flow fractionation methods (SdFFF, FlFFF, and ThFFF). (2) Revitalization of older instrumental methods such as gravitational and centrifugal sedimentation; (3) Evolution of research grade instrumentation into low cost, routine, user friendly instrumentation exemplified by dynamic light scattering (DLS). (4) The attempt to meet extremely difficult technical challenges such as: (a) providing a single hybrid instrument with high resolution over a very broad dynamic range (4+ decades in size; e.g., Fraunhofer/Mie; photozone sensing/DLS); (b) PSD measurement of concentrated dispersions (acoustophoretic, dielectric measurements, fiber optic DLS (FOQELS)); (c) in-situ process particle size sensors (in-line or at line, e.g., FOQELS); (d) routine measurement of particle shape and structure (e.g., image analysis). Instrumental methods resulting from these activities are discussed in terms of measurement principles and the strengths and weaknesses of these methods for characterizing PSDs. Business and societal driving forces will impact customer perceived instrumentation and knowledge needs for the 21st century and the ability to meet the specific difficult technical challenges in particle size distribution characterization mentioned above. Anticipated progress toward meeting these technical challenges is discussed in conjunction with the associated anticipated advances in required technologies.