Polymerization reactions reactor design

Process Technology. In commercial addition and condensation polymerization processes reactor design is an important factor for the quality and economics of the polymer. Combining macromolecular kinetics with reactor and process design has led to a new concept called reaction engineering. D. C. Chappelear and R. H. M. Simon review this novel concept in Chapter 1. 

In this pyrolysis, sub atmospheric partial pressures are achieved by employing a diluent such as steam. Because of the corrosive nature of the acids (HE and HCl) formed, the reactor design should include a platinum-lined tubular reactor made of nickel to allow atmospheric pressure reactions to be mn in the presence of a diluent. Because the pyrolysate contains numerous by-products that adversely affect polymerization, the TFE must be purified. Refinement of TFE is an extremely complex process, which contributes to the high cost of the monomer. Inhibitors are added to the purified monomer to avoid polymerization during storage terpenes such as t7-limonene and terpene B are effective (10). 

Reactor Design. The continuous polymerization reactions in this investigation were performed in a 50 ml pyrex glass reactor. The mixing mechanism utilized two mixing impellers and a Chemco magnet-drive mechanism. 

This monomer is ethylene when R is hydrogen, propylene when R is a methyl group, styrene when R is a benzene ring, and vinyl chloride when R is chlorine. The polymers formed from these four monomers account for the majority of all commercial plastics. The polymers come in great variety and are made by many different processes. All of the polymerizations share a characteristic that is extremely important from the viewpoint of reactor design. They are so energetic that control of the reaction exotherm is a key factor in all designs. 

There are innumerable industrially significant reactions that involve the formation of a stable intermediate product that is capable of subsequent reaction to form yet another stable product. These include condensation polymerization reactions, partial oxidation reactions, and reactions in which it is possible to effect multiple substitutions of a particular functional group on the parent species. If an intermediate is the desired product, commercial reactors should be designed to optimize the production of this species. This section is devoted to a discussion of this and related topics for reaction systems in which the reactions may be considered as sequential or consecutive in character. 

Polymerizing, Decomposing, and Rearranging Substances Most of these substances are stable under normal conditions or with an added inhibitor, but can energetically self-react with the input of thermal, mechanical, or other form of energy sufficient to overcome its activation energy barrier (see Sec. 4, Reaction Kinetics, Reactor Design, and Thermodynamics). The rate of self-reaction can vary from imperceptibly slow to violently explosive, and is likely to accelerate if the reaction is exothermic or self-catalytic. 

Photonitrosylations show an extremely high tendency for filming, and the difficulties in implementing the caprolactam synthesis on an industrial scale are also linked to reactor designs not taking into account the competitive secondary reactions leading to polymerized material. 

While it should be self-evident that a rational reactor design demands a knowledge of both the fluid dynamic environment and the detailed process kinetics, the latter are rarely available. In many instances this leads to the severe limitation of many important reactions by an inadequate fluid dynamic intensity. Some of these are known to be fast, e.g., liquid-phase nitrations, while others are (incorrectly) assumed to be slow, e.g., most polymerizations. In these circumstances the pragmatic approach is to use a high-intensity reactor for each system and then to assess the impact upon the space-time productivity. Obviously, an intrinsically slow system is resistant to further acceleration and this will rapidly become evident. One significant qualification of this contention involves the very... 

The use of precision density measurements for monitoring polymerization reactions can be done rapidly and automatically using commercially available instrumentation. The method is independent of the reactor size and design but suffers from sampling difficulties. The examples of this paper show the rapidity of data collection and three distinct sampling problems pump failure from either monomer attack or polymer scale formation, monomer phase separation in the density cell, and the lag time for rapid polymerizations. Techniques have or can be devised to avoid or reduce the influence of these problems. 

Two types of polymerization units are designed by Universal Oil Products Company. The U.O.P. Reactor-type unit contains the catalyst in tubes which are surrounded by water in a jacket for the purpose of removing the heat liberated by the exothermic polymerization reaction. The steam generated in the water jacket normally is used to preheat the feed. A feed-to-products heat exchanger furnishes the remaining heat requirements. Conventional depropanizer and debutanizer columns are used to fractionate the product. Figure 3 shows a flow diagram of a reactor type of polymerization unit. 

As important as kinetic mechanism are the phase changes that occur in polymerization. Only a small fraction of polymerizations are carried out only in one phase thus thermodynamics, heat and mass transfer, and the kinetics of the phase change itself all play a role in determining the properties of the product polymer. Table IV indicates the principal types of kinetic mechanisms and reaction media which arise in polymerization reactors. Each of these classes of systems has its own peculiar problems so that polymerization reactor design can often be much more challenging than the design of reactors for short chain molecules. 

To provide an illustration of some of the challenges of polymerization reaction engineering, we shall discuss here a few intriguing but practically important research problems which arise in the design of polymerization reactors. These examples reflect the author s own interest and are selected from research projects currently underway at the University of Wisconsin. 

During polymerization, a certain amount of polymer evaporates. One design option giving additional cooling capacity is to condense, in an external heat exchanger, the polymer from the vapor phase and to return the liquid to the reactor bulk. We assume that, under the most demanding conditions, 30% of the heat generated by the polymerization reaction can be removed in the condenser. Under these conditions, the minimum temperature difference becomes ... 

Highly viscous polymeric reactions (e.g., the hydrolytic polyamide reaction) are often carried out in a gear-pump reactor (Tadmore and Klein, 1970). This type of reactor is often difficult to operate because the clearance of the gear teeth is increased by wear caused by flow and the reaction process. For smaller viscosity of the melt, a screw reactor or a twin-screw extruder is often used. Sterbecek et al. (1987) used a twin-screw extruder (i.e., Wemer-Pfleiderer extruder ZSK 83) for studying fast ion-catalyzed polymerization (6-caprolactam) in a melt. They indicated that power input and quality of product in such a reactor depends on the slot width between reactor wall and impeller in a twin screw extruder. They provided an optimum design of a twin-screw reactor for a fast ion-catalyzed polymerization in a melt. 

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