Safety

In most processes, polymer production is limited by the thermal characteristics of the reactor. Therefore, there is a strong interest in working close to the maximum heat removal capacity of the reactor. However, in some occasions, the safety of the operation might be compromised, and hence safety considerations must be integrated into the process strategy. A research project aiming at the experimental determination of key safety characteristics including heat of reaction, total adiabatic temperature, rates of temperature and pressure rise, reaction onset temperature, system vapor pressure versus temperature, maximum temperature and pressure, two-phase flow regime and temperature of no return is in progress.  Process upsets conditions such as mischarge of reactants/catalysts, loss of stirring, loss of cooling and exposure to fire will be considered. This information will be integrated in the optimization and on-line control strategies.

Process Optimization.

A common problem in emulsion polymerization is to maximize production of a high quality polymer under safe conditions. Often, a compromise should be reached in this multivariable optimization problem because high production may lead to lower quality polymer or process conditions that yield an improvement in a given final property are deleterious for other properties. Process optimization is an on-going activity in our lab. Different approaches are being used. In some cases, classical optimization methods based on comprehensive mathematical models developed for a particular system are used. In other cases, optimization should be performed based on limited information. For example hybrid mathematical models that included rigorous material and energy balances and empirical equations for polymerization rates and molecular weights have been successfully used to maximize production rate and scrub resistance of vinyl acetate-VeoVa 10 latexes in industrial reactors. In another case, fuzzy logic was used to incorporate the operator experience in an on-line optimization. On-line process optimization for incorporation in on-line control strategies is the challenge for the next years.

Advanced Mathematical Modeling

Mathematical modeling is a way of summarizing and quantifying our knowledge about polymerization in dispersed media, and is required for process intensification. In addition, models are powerful tools for improving the understanding of complex processes, which may lead to better product quality. They may also be used to identify advantageous process conditions and risky situations that must be avoided.  Intelligent use of models in experimental design may lead to substantial savings of time (and money). Models can also facilitate on-line control of emulsion polymerization reactors and are useful for the education and training of personnel. Advanced models for polymerization kinetics, multimonomer copolymerization, polymer composition and sequence distribution, molecular weight distribution, long and short chain branching, gel content, particle morphology and particle size distribution have been developed. Modeled systems include conventional emulsion polymerization, miniemulsion polymerization, microemulsion polymerization and dispersion polymerization. Mathematical modeling will be an on-going task in the research program.

Transforming semibatch processes into continuous processesMicrog

An approach for process intensification is to transform semibatch processes into continuous polymerizations. Several systems will be considered:

Continuous Emulsion Polymerization in a Loop Reactor

The continuous loop reactor is a continuous tubular reactor with recycle, whose employment for manufacture of emulsion polymers presents several advantages. Its large heat transfer area/reactor volume ratio allows high conversion in short residence times to be achieved. This results in a substantial reduction of the reactor volume. Because of the small volume and short residence time, the loop reactor can be used with great flexibility and minimum losses in the manufacture of different emulsion polymers. The small volume and the absence of headspace make the process intrinsically safe. The effect of the process conditions, the dynamic behavior of the reactor and the start-up procedures on the latex properties were investigated for the emulsion copolymerization of vinyl acetate and VeoVa 10 carried out under industrial-like conditions, namely, using technical grade monomers to produce a high solids content (55wt%) latex. Current research includes the evaluation of reactor performance for high solids all acrylic latexes, as well as the use of the loop reactor to produce seeds. In both cases the control of the particle size distribution is critical.

 

Microemulsion Polymerization in Continuous Reactors

Microemulsions are thermodynamically stable dispersions produced by using surfactants able to reduce the interfacial energy values close to zero. Both oil-in-water (direct) and water-in-oil (inverse) microemulsions can be produced, but higher volumes of the polymeric phase can be attained using inverse microemulsions. The polymerization of inverse microemulsions of aqueous solutions of water-soluble monomers is an attractive way to produce high-molecular weight water-soluble polymers that can be employed as flocculants in wastewater treatment and enhanced oil recovery. The kinetics of inverse microemulsion polymerization initiated by both UV radiation and thermal initiators have been studied. In addition, the feasibility of producing these polymers in continuous reactors assessed. A mathematical model for the process has been developed. This process is not easy to scale-up because a large amount of heat is release in a short period of time, yielding to temperature increases that in turn results in poor quality products.

 

Continuous Polymerization in alternative reaction geometries (Couette-Taylor Reactor)

In multiphase systems it is important to achieve good mixing conditions in order to favor heat and mass transfer, avoiding phase segregation or coagulation. Conventional reactor geometries (tank and tube) do not comply these requirements. Tank reactors have a good mixing characteristics, but have important limitations on heat removal capacity and are often troublesome for scaling-up. On the other hand, tubular reactors have good heat removal capacity, but when multiple phases are present it becomes difficult to obtain good mixing profile. The Couette-Taylor reactor complies with both good heat and mass transfer exigencies. This reactor geometry has both the macroscopic mixing characteristics of a tank reactor and the heat removal capacity typical of a tubular reactor. Such reactor is composed by two concentric cylinders, the inner one is a cylinder serving as a stirrer, while the outer shell is a conically shaped and fixed double jacket. The reaction takes place in the gap between both shells. In this project the Couette-Taylor reactor will be used to study the continuous production of composite polymer/clay nanoparticles (see below). The flow pattern and the dynamic behavior of the reactor will be studied. This strategy of using the Couette-Taylor reactor relies on the belief that such geometry will enable attaining high conversions at low residence times avoiding problems of segregation and phase separation.