Emulsion polymerization reactor process types

The present review paper, therefore, refers firstly to the particle formation mechanism in emulsion polymerization, the complete understanding of which is indispensable for establishing a correct kinetic model, and then, deals with the present subject, that is, what type of reactor and operating conditions are the most suitable for a continuous emulsion polymerization process from the standpoint of increasing the volume efficiency and the stability of the reactors. 

Some possible subdivisions of the field are listed in Table I. We can characterize the type of polymerization process—i.e., mass, suspension, emulsion, etc.—and the type of reactor and how it is operated. The chemical classification of the reaction affects the mathematics as well as the product. The last distinction is between the rate of product formation and the distribution of the products obtained. 

In the following section on reactor and process types, the preponderance of work on mass/solution polymerization is pointed out with a brief review of the more limited work on suspension and emulsion polymerization. In another broad view of the field (the next section) the distinction between polymerization rate and product distribution is discussed, particularly the preoccupation with molecular weight distribution. 

Reflecting the importance of continuous emulsion polymerization processes, numerous investigations have been carried out to date, which are categorized into three groups (1) studies on the reactor configuration (stirred-tank reactors, tubular type reactors such as a simple tubular reactors, pulsed tubular reactors... 

Types of Reactor Processes Batch Reactors Semibatch Reactors Continuous Reactors Emulsion Polymerization Kinetics Other Preparation Methods... 

Summary. All of the phases and the physical and chemical mechanisms discussed in this section are important during the course of an emulsion polymerization reaction. They influence the reaction kinetics and the properties of the latex produced. Not all of the phenomena that can occur are understood in a quantitative manner. Nevertheless, considerable advances have been made in the fundamental understanding and the commercial exploitation of emulsion polymerization processes. The remainder of this chapter will focus on reactor types and reaction kinetics. 

In emulsion polymerization, monomers are polymerized in the form of emulsions and polymerization in most cases involve free-radical reactions. Like suspension polymerization, the emulsion process uses water as the medium. Polymerization is much easier to control in both these processes than in bulk systems because stirring of the reactor charge is easier due to lower viscosity and removal of the exothermic heat of polymerization is greatly facilitated with water acting as the heat sink. Emulsion polymerization, however, differs from suspension polymerization in the nature and size of particles in which polymerization occurs, in the type of substances used as initiators, and also in mechanism and reaction characteristics. Emulsion polymerization normally produces polymer particles with diameters of 0.1-3//. Polymer nanoparticles of sizes 20-30 nm are produced by microemulsion polymerization (Antonietti et al., 1999 Ytldiz et al., 2003). 

The following types of reactor processes have been described in the literature for continuous emulsion polymerization ... 

The semi-continuous emulsion polymerization processes is characterized by continued addition of reaction ingredients such as monomer, emulsifier, initiator, or water to the reaction system throughout the polymerization. In this emulsion polymerization process, two major types of feeds are used for the introduction of ingredients to the reactor neat monomer feed (M) or monomer emulsion feed (ME). In M feed method, the feed contains only monomer and all the other ingredients are initially in the reactor. Otherwise, the major components of the ME feed are a monomer, a part from the emulsifier, and water. But it... 

Semibatch processes are usually carried out using two different feed types neat monomer feed, where only monomer is flowed into reactor, or monomer emulsion feed, in which aqueous emulsifier is added with the monomer. The systems, for which a critical flow rate has been reached, such that the rate of polymerization is controlled by the rate of monomer addition, are called starved [46]. In this case, there are no more monomer droplets and the monomer concentration in the polymer particles is lower than the saturated concentration. If the feed rate is higher than the polymerization rate, the monomer accumulates in the reactor as monomer droplets. This is usually termed a flooded system. In this case, the particles are completely saturated with monomer, and reaction kinetics resembles those of a batch emulsion polymerization. 

Continuous stirred tank reactors are used commercially for solution, bulk (mass), and emulsion polymerization of vinyl monomers. In bulk homogeneous polymerization processes (e.g., polystyrene), the reactor system usually consists of a single CSTR or multiple CSTRs and an extruder-type devolatilizer to remove unreacted monomer, which is then recycled to the reactor. As monomer conversion increases, the viscosity of the polymerizing fluid increases and the overall heat removal efficiency decreases. When styrene is polymerized in bulk in a stirred tank reactor, monomer conversion is limited to about 30-40% due to an increasing viscosity of the polymerizing fluid above this conversion level. However, the overall monomer conversion can be very high because unreacted monomer is constantly recycled to the reactor. 

Particle nucleation and particle growth are important steps in emulsion polymerization because they affect the overall polymerization rate and polymer properties. Thus, initiator concentration and surfactant type/concentra-tion have significant effects on the polymerization kinetics. In a batch emulsion process, particle nucleation and growth steps can be separated to some extent by employing a multistage reaction process. In a continuous process, both particle nucleation and growth steps occur simultaneously unless a seed reactor is provided to separate these two effects. In general, latex particle size distributions obtained by batch and continuous processes are quite different. 

Industrial chemical processes are categorized as batch, semi-batch or continuous, and the manufacture of emulsion polymers is carried out in aU these process types. The processes differ not only in equipment type and economics of operation but also in the specific properties imparted to the polymer and the emulsion. In a batch emulsion polymerization, all ingredients are added to a vessel, polymerization is initiated and the reaction proceeds to completion over a period of time. Thus, conditions in the reactor gradually change from monomer -i- water —> polymer -i- water, passing through all intermediate ratios of monomer/polymer. 

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