Reactor choice polymerization reactions

Polymerization reactions. Polymers are characterized by the distribution of molecular w eight about the mean as well as by the mean itself. The breadth of this distribution depends on whether a batch or plug-flow reactor is used on the one hand or a continuous well-mixed reactor on the other. The breadth has an important influence on the mechanical and other properties of the polymer, and this is an important factor in the choice of reactor. 

Of course, it is not always necessary, or even desirable to produce a monodisperse MWD. A certain broadening may be designed into the product by the choice of reactor type. Or, the engineer (chemist) may choose to produce a broad or bimodal MWD by blending two or more narrowly distributed products. As with most aspects of polymerization reaction engineering, the possibilities are endless. 

Dimethyl sulfoxide is a favored solvent for displacement reactions in synthetic chemistry. The rates of reaction in DMSO are many times faster than in an alcohol or aqueous medium [6]. Dimethyl sulfoxide is the solvent of choice in reactions where proton (hydrogen atom) removal is the rate determining step. Reactions of this type include olefin isomerizations and reactions where an elimination process produces an olefin. Another application that uses DMSO is its use as an extraction solvent to separate olefins from saturated paraffins [7]. Several binary and ternary solvent systems containing DMSO and an amine (e.g., methylamine), sulfur trioxide, carbon disulfide/ amine, or sulfur trioxide/ammonia are used to dissolve cellulose, and act as spinning baths for the production of cellulose fibers [8,9]. Organic fungicides, insecticides, and herbicides are readily soluble in DMSO. Dimethyl sulfoxide is used to remove polymer residues from polymerization reactors. 

How fast is the runaway of the deshed reaction Generally, industrial reactors are operated at temperatures where the deshed reaction is fast. Hence, a temperature increase above the normal process temperature will cause a significant acceleration of the reaction therefore, in most cases, this period of time is short. For polymerization reactions, where decomposition of the reaction mass is not critical, this time will determine the choice of technical risk reduction measures. The concept of time to maximum rate under adiabatic conditions (TMRad) as used for decomposition reactions can be applied to the polymerization itself, starting from the process temperature. It allows estimation of the probability of entering a runaway situation, as explained below for decomposition reactions. 

Frontal polymerization carried out as described above can be turned into a continuous process. In order to do this, it is necessary to move the newly formed polymer and the reactive mixture in the direction opposite to the direction of spreading of a thermal front at a velocity equal to the velocity of the front development to feed the reactor with a fresh reactive mass.254 Control of the process, choice of process parameters and proper design of the equipment require solving the system of equations modelling the main physical and chemical processes characteristic of frontal reactions. 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

From an operational point of view, the choice of an appropriate polymerization reactor depends on six requirements temperature control mixing product accumulation and reactor foul-up follow-up separation processes the desired form of the product and safety. Heats of polymerization are typically high, so that maintaining the reactor at a desired temperature level is not always a simple task. Temperature can become spatially nonuniform and globally out of control (causing inconsistency of the reaction medium). Nonuniformity in temperature can lead to localized zones of poor mixing or even dead zones. In a polymerization reactor, temperature, mixing, viscosity,... 

In homogeneous catalysis often a reaction takes place between a gaseous reactant and a liquid reactant in the presence of a catalyst that is dissolved in the liquid phase. Examples are carbonylations, hydroformylations, hydrogenations, hydrocyanation, oxidations, and polymerizations. Typically, reactants such as oxygen, hydrogen, and/or carbon monoxide have to be transferred from the gas phase to the liquid phase, where reaction occurs. The choice of reactor mainly depends on the relative flow rates of gas and liquid, and on the rate of the reaction in comparison to the mass and heat transfer characteristics (see Fig. 8.2). 

Membranes are classified by whether the thin permselective layer is porous or dense, and by the type of material (organic, polymeric, inorganic, metal, etc.) this membrane film is made from. The choice of a porous vs. a dense film, and of the type of material used for manufacturing depends on the desired separation process, operating temperature and driving force used for the separation the choice of material depends on the desired permeance and selectivity, and on thermal and mechanical stability requirements. For membrane reactor applications, where the reaction is coupled with the separation process, the thin film has also to be stable under the reaction conditions. 

One of the most important areas for application of concepts discussed in the previous section is the selection of polymerization reactors. The properties of polymers depend on their molecular weight distribution (M WD) and so the design should ultimately use this as its basis. The subject is a vast one, and so only the basic concepts will be briefly discussed. Several excellent reviews now exist, covering various aspects of the area from a chemical reaction engineering viewpoint see Shinnar and Katz, Keane, and Gerrens, [18, 19, 20]. The latter presents a masterful survey of the effects of the choice of reactor type. 

Separation and recovery of contaminants, solvents and unreacted monomers are necessary steps in many bulk and solution polymerizations. The cost of these steps may dominate the overall economics. They should be considered as an integral part of reactor design since their economic importance may dictate the choice of a particular reaction scheme. As one example, gas-phase processes constitute an increasing segment of polyolefin production due to the simplicity of product recovery. The polymer is obtained directly from the reactor as a dry, free-flowing powder. There are no separation steps in this... 

The choice of initiator is made on the basis of the t5 e of reactor, the residence time in each zone, the desired reaction temperature, and initiator cost. A principal factor in choosing an initiator is the half-life. The longer the half-life of the initiator, the higher the degree of polymerization, but the higher the cost of the peroxide. Table 4 gives some common initiators, half-lives, and costs. 

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