Miniemulsion polymerisation in pseudo-bulk regime: Mathematical modelling, prediction and optimal strategy of operation

A dynamic mathematical model based on pseudo-bulk kinetics was developed for styrene miniemulsion polymerisation and validated using experimental data. Key findings are that the developed model match reasonably with experimental data and is able to predict the polymerisation attributes and product properties such as fractional conversion, average molecular weight, molecular weight distribution (MWD), average particle size and particle size distribution (PSD) in miniemulsion polymerisation. Optimal strategy of semibatch operation was developed using reaction temperature and monomer feed rate as process variables with specified initial conditions.     

Semi-batch emulsion copolymerization of styrene and butyl acrylate for production of high solids content latexes: Experiments and mathematical model

Polymer latexes with high solids content exhibit several advantages such as lower costs of transport and storage and shorter drying and film formation times. To keep the apparent viscosity at satisfactory levels, the particle size distributions should be either broad or multimodal. In the present work, the production of high solid content latexes by emulsion copolymerization of styrene and butyl acrylate was studied in a semi-batch process in which the nucleation of a second population of particles is promoted by the instantaneous (shot) feeding of an additional charge of emulsifiers. Latexes of about 64 wt% of solids with bimodal particle size distributions and reasonably low apparent viscosity were produced. A representative mathematical model for the copolymerization processes that accounts for the changes in the particle size distribution that occurred in the process is presented, tested and validated with the experimental data measured during the experiments.     

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.     

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.     

High solids content emulsion polymerisation without intermediate seeds. Part II. In situ generation of bimodal latices

A process for the production of high solid content latexes with low viscosity that does not require the use of intermediate seeds is developed. It is shown that the second population of a bimodal latex can be generated by controlled homogeneous nucleation through the use of a combination of an initiator that generates charged free radicals and a mixed surfactant system that is very rich in non-ionic surfactant. The process was shown to be reproducible, and was used to produce a latex with a solid content of almost 77% (v/v) and a viscosity of 1.5 Pa s at a shear rate of 20 s⁻¹.     

High solids content emulsion polymerisation without intermediate seeds. Part III. Reproducibility and influence of process conditions

The reproducibility and robustness of a unseeded process for the production of high solid content, low viscosity latex are examined. A series of runs showed that the experiments are very reproducible both in terms of the particle size distribution and the shear viscosity of the final latex. It is also shown that it is the surfactant concentration and feed profiles that are the most sensitive issues in maintaining product specifications since they control the rate of generation of small particles.     

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.     

Analysis of shear-induced coagulation in an emulsion polymerisation reactor using computational fluid dynamics

Shear-dependent coagulation is a costly problem for the latex manufacturing industry, due to product degradation and reactor downtime. In this study, a method for calculating the shear-dependent coagulation rate in emulsion polymerisation is developed. The method combines simple models for coagulation (only binary collisions being considered) with the effects of rheology on the flow field, using computational fluid dynamics (CFD) to solve the detailed flow field in the reaction vessel. By using the local shear rates (LSR), the method developed provides a more detailed and system-specific assessment compared with using an average shear rate (ASR) for calculating the coagulation rate. The difference in the predictions between the ASR and the proposed LSR method was investigated. It was found that the ASR and LSR methods predict different coagulation rates, especially for more sophisticated coagulation models where the coagulation rate is not linearly dependent on the shear rate. The LSR method was also used to study the effect of the rheology of the latex, of the impeller speed and of the reactor design on the coagulation rate. It was found that the LSR method is useful for providing both visual and numerical means to identify regions with elevated coagulation rates in the modelled reaction vessel. The treatment provides estimates of the amounts of coagulum formed on the vessel walls and on the impeller.     

Particle formation and growth in ab initio emulsifier-free emulsion polymerisation under monomer-starved conditions

Particle formation and growth in the monomer-starved emulsifier-free emulsion polymerisation of monomers with different water solubility including methyl acrylate (MA), methyl methacrylate (MMA), and vinyl acetate (VA) were studied. The rate of formation of precursor particles, via homogenous nucleation, is proportional to the monomer concentration in the water phase. One may think that the maximum number of particles will be obtained when the water phase is saturated with the monomer. The number of PMA particles showed a maximum when the water phase was starved with the monomer. The number of PVA particles did not show any sensitivity to the monomer concentration in the water phase. More unexpectedly the final number of PMMA particles showed a minimum when the water phase was just saturated with the monomer. The minimum in the final number of PMMA particles was correlated with the enhanced rate of particle growth due to the gel effect. Under monomer-starved conditions, the number of particles produced was in the order of water solubility of the monomers; MA > VA > MMA. A reverse order was produced under monomer-saturated conditions as particle coagulation became progressively more important for some of the monomers.     

Feedback controllability assessment and control of particle size distribution in emulsion polymerisation

In this article, the importance of particle size distribution (PSD) control as a means for the inferential control of the rheology of emulsion polymers is illustrated. A controllability assessment is presented to illustrate the attainability or otherwise of bimodal PSD using feedback control through a consideration of the process mechanisms—measurement limitations and process constraints that prevent the implementation of feedback corrections. The suitability of a batch-to-batch iterative feedback PSD control is demonstrated, which could act in addition to any in-batch feedback control, the latter being less feasible in certain cases, as argued in this article. A detailed population balance model is used for the batch-to-batch control, which simplifies model update and feedback correction.     

Studies of fluid bed granulation in an industrial R&D context

Following a brief overview of recent developments in laundry powder processing by comparison with other areas of industrial application, and current drivers and trends, a review is presented of collaborative research between Unilever R&D and Sheffield University on fluidised bed granulation using melt binders.These fall apart into 4 areas:

1.

Contact zone studies;

2.

Growth similarity for non-ideal systems;

3.

Growth and breakage decoupled;

4.

Scale-up rules based on similarity of growth kinetics.

The emphasis of the paper is on the identification and relative quantification of the dominant mechanisms, which are found in the balance of fluxes of solids and binder, together with heat transfer and conduction, not so much binder viscosity. These mechanisms allow for the control of the growth kinetics and rapid scale-up.Together, this lays the broad ground-work for fluidised bed granulation. Some concluding remarks provide pointers for the future of research in this area:

•

There is a strong requirement for multi-dimensional phase volume distribution based population balances.

•

Many processes require pre-granulation, and liquid-liquid contact in the spray-zone is scarcely investigated.

•

Simple combined Lagrangian-Eulerian modelling implemented in commercial code environment can be especially useful to investigate the effects of the relations between material properties and process conditions on growth kinetics.

     

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.     

State multiplicity in CSTR-separator-recycle polymerisation systems

This article continues earlier work (Comput. Chem. Eng. 24 (2000) 209) concerning the design and control of isothermal reactor-separator-recycle systems. The multiplicity behaviour of six reaction systems of increasing complexity, from one-reactant, first-order reaction to chain-growth polymerisation, is investigated. Below a critical value of the plant Damkohler number, Da<Da^cr, the only steady state involves infinite flow rates. Feasible steady states become possible if the critical value is exceeded, Da>Da^cr. For one-reaction systems, one stable steady state is born at a transcritical bifurcation. For consecutive-reaction systems, including polymerisation, a fold bifurcation can lead to two feasible steady states. Moreover, the transcritical bifurcation is destroyed when two reactants are involved. If the gel-effect is included, a maximum of four steady states are possible. When multiple steady states exist, the achievable conversion is constrained by the instability of the low-conversion branch. This has practical importance for polymerisation systems when the radicals’ quasi-steady state assumption is not valid or the gel effect is significant.     

Formation of crystalline particles from phase change emulsion: Influence of different parameters

Solidification or crystallization of phase change emulsion in the form of fine emulsion drops in a direct contact coolant at temperatures below their freezing point was studied. This work is mainly focused on the size and shape of the generated particles from phase change emulsified fats. Size of the particles is the major or key factor being considered during their formation, however, other factors that govern the particle size and shapewere also observed. The operating parameters of the process were optimized in order to obtain particles of smaller size ranges in the window of current operating conditions. The crystallization of complex emulsion matrices is very difficult to control in the bulk at desired requirement. Hence, the emulsion drop to particle formation has advantage in comparison with the bulk solidification or crystallization. The main objective of this work is to achieve spherical emulsion particles in a direct contact cooling system. Parameters like:stability, characterization, viscosity, and the effect of different energy inputs were examined. Moreover, the effects of the capillary size, interfacial tension, temperature of the emulsion on the particle size were also monitored.     

Advanced modelling and optimal operating strategy in emulsion copolymerization: Application to styrene/MMA system

A model-based framework for advanced optimal operation of copolymerization processes was developed and tested experimentally in conjunction with a distributed control system (DCS) suitable for industrial application. A test case of emulsion copolymerization of styrene (Sty) and methyl-methacrylate (MMA) was investigated for predicting and optimizing key product properties including molecular weight distribution (MWD), particle size distribution (PSD), copolymer composition and conversion. The model equations include diffusion-controlled kinetics at high monomer conversions, where transition from a ‘zero-one’ to a ‘pseudo-bulk’ regime occurs. The reactor mass and energy balances describe the system transients for batch and semi-batch operations. Population balance equations, describe the particle evolution and comprise sets of integro-partial differential and nonlinear algebraic equations. The models were solved using an efficient numerical scheme suitable for on-line monitoring and control. The model predictions were found to be in good agreement with experimental results for measurements carried out with variable monomer feed rates, variable temperature and variable composition of styrene-MMA, from 25/75 to 75/25 proportions. The manipulation of these variables was found to affect the PSD significantly. The experimental results confirmed the accuracy of our optimization scheme for the desired conversion, MWD and PSD.     

In situ mass-suspension polymerisation

In situ mass-suspension polymerisation of MMA was carried out in a single reactor. The mass polymerisation was carried out in a gently agitated monomer layer of a two stratified layers of monomer and water in the reactor. The degree of conversion at which mass polymerisation changed to suspension polymerisation, by increasing the rate of agitation, was altered systematically. The polymer content of the monomer/polymer solution, formed during the mass polymerisation stage, significantly affected the evolution of the particle size distribution. Mass-suspension polymerisation was found to be more vulnerable to drop coalescence and process failure than conventional suspension polymerisation. The results indicate the importance of the transition stage in a typical suspension polymerisation during which the rate of polymerisation is very low and the adsorption of stabiliser on the surface of drops is completed. The polymer beads from the mass-suspension polymerisation had a very broad size distribution with a large contribution from satellite particles.     

Nanoparticle formation by highly diffusion-controlled emulsion polymerisation

Emulsion polymerisations of several monomers with different water solubilities including styrene, butyl acrylate, methyl methacrylate, vinyl acetate, and methyl acrylate were carried out under highly diffusion-controlled conditions. The monomer was placed on top of an aqueous solution of an emulsifier and an initiator, while being gently stirred. Polymerisation occurred by diffusion of monomer from the monomer phase via the interface to the micellar solution. Nanoparticles as small as 25 nm were produced as a result of reduced particle growth and delayed depletion of emulsifier micelles. Nanolatexes with relatively high solids content (20%) but with a low surfactant/monomer ratio (1/50) were obtained. The monomers with the highest solubility, with the exception of methyl acrylate, produced the smallest particles. The rate of diffusion-controlled polymerisation was found to be almost proportional to the saturation monomer concentration in the water phase . The results were compared with those obtained with a high rate of agitation which allowed kinetics of polymerisation to become the rate determinant. While particles obtained by kinetics-controlled emulsion polymerisation of the monomers were large, as well as similar in size, particles obtained from diffusion-controlled runs were small, but different in size. On the other hand, particles made by kinetics-controlled emulsion polymerisation had increasing surface coverage (by emulsifier) with monomer solubility in water. Whereas, nanoparticles made by diffusion-controlled emulsion polymerisation reached almost a constant surfactant coverage independent of the monomer type (except for methyl acrylate), a surface coverage as low as 0.20 was found to be sufficient for stabilisation of nanoparticles.     

Dynamic simulation of emulsion formation in a high pressure homogenizer

A simulation model for emulsification in high pressure homogenization (HPH), based on a population balance approach, is developed assuming it to be controlled by three simultaneous processes; fragmentation, coalescence and adsorption of a macromolecular emulsifier. The aim is to investigate the implications of adding a set of models together; studying the effects of dynamics, size effects and process interactions.For fragmentation, turbulent inertial and turbulent viscous forces are included using a dynamic model based on the Weber and Capillary number. It was extended to include a deformation time scale.The rate of adsorption and coalescence is assumed to be controlled by the collision rate of macromolecular stabilizer and bare interface, modeled using convective and diffusive transport in turbulent flow.By comparing simulation results to general trends found in the literature, it can be concluded that the models can reproduce the general HPH process well. By dividing the active region of emulsification in the homogenizing valve into discrete steps, the dynamic process could also be examined, indicating the homogenization process being composed of three stages with coalescence predominantly found in the last one.     

Polymer encapsulation of surface-modified carbon blacks using surfactant-free emulsion polymerisation

Polymer encapsulations of two different grades of water-dispersible, surface-sulfonated carbon black, Sterling-4620 with an aggregate size of 260 nm and Black Pearls-800 (BP-800) with an aggregate size of 45.7 nm, were carried out using surfactant-free emulsion polymerisation of butyl acrylate, methyl methacrylate and allyl methacrylate. High levels of initiator were required because carbon black acts as an efficient radical trap. Although polymerisation in the presence of Sterling-4620 proceeded satisfactorily using ammonium persulfate (APS), the much larger specific surface area of BP-800 inhibited polymerisation with APS and the sodium salt of 4,4′-azobis(4-cyanopentanoic acid), a non-oxidising initiator, was necessary for effective polymerisation in the presence of BP-800. Several polymer-encapsulated Sterling-4620 and BP-800 products were prepared successfully using different amounts and compositions of polymer, and have been characterised using solvent extraction, pyrolysis, thermogravimetry, infrared spectroscopy, transmission electron microscopy, differential scanning calorimetry and dynamic mechanical thermal analysis.