Semibatch reactor polymerization reactions

Gas phase olefin polymerizations are becoming important as manufacturing processes for high density polyethylene (HOPE) and polypropylene (PP). An understanding of the kinetics of these gas-powder polymerization reactions using a highly active TiCi s catalyst is vital to the careful operation of these processes. Well-proven models for both the hexane slurry process and the bulk process have been published. This article describes an extension of these models to gas phase polymerization in semibatch and continuous backmix reactors. 

Semibatch reactors are commonly used for small-volume chemical production. This reactor type is frequently used for biological reactions and for polymerization. In the batch reactor. 

Figure 7. Degree of polymerization with reaction time in a controlled semibatch reactor. Key ---, WADP -------,...

Mitra et al. (1998) employed NSGA (Srinivas and Deb, 1994) to optimize the operation of an industrial nylon 6 semibatch reactor. The two objectives considered in this study were the minimization of the total reaction time and the concentration of the undesirable cyclic dimer in the polymer produced. The problem involves two equality constraints one to ensure a desired degree of polymerization in the product and the other, to ensure a desired value of the monomer conversion. The former was handled using a penalty function approach whereas the latter was used as a stopping criterion for the integration of the model equations. The decision variables were the vapor release rate history from the semibatch reactor and the jacket fluid temperature. It is important to note that the former variable is a function of time. Therefore, to encode it properly as a sequence of variables, the continuous rate history was discretized into several equally-spaced time points, with the first of these selected randomly between the two (original) bounds, and the rest selected randomly over smaller bounds around the previous generated value (so as... 

You are asked to design a semibatch reactor to be used in the production of specialized polymers (ethylene glycol-ethylene oxide co-polymers). The semi-batch operation is used to improve the molecular-weight distribution. Reactant B (EG) and a fixed amount of homogeneous catalyst are charged initially into the reactor (the proportion is 6.75 moles of catalyst per 1000 moles of Reactant B). Reactant A (EO) is injected at a constant rate during the operation. The polymerization reactions are represented by the following liquid-phase chemical reactions ... 

In a kinetic investigation of the catalytic liquid-phase phenol oxidation carried out in a semibatch slurry reactor [6], it has been found that homogeneous stepwise polymerization reactions are enhanced in the bulk liquid-phase due to the high liquid-to-solid volumetric ratio. The rate of phenol disappearance has been expressed on the basis of power-law kinetics as a sum of heterogeneous and homogeneous (polymerization) contributions, thus... 

Semibatch solution polymerization is a well-established method for producing acrylic resins. In the semibatch solution process, solvent is commonly charged to the reactor and heated to the desired reaction temperature, typically 100-140 °C. Monomer, initiator, and other ingredients are slowly fed to the reactor over a period of 4-6 h to achieve a desired solids content (typically up to 70% by weight solids). During polymerization, the solvent may be refluxed to help remove the heat of polymerization [40]. At the end of the polymerization, a hold time and post charge of initiator solution are common to techniques designed to reduce... 

In a semibatch emulsion homopolymer reactor, monomer is converted into polymer via a strongly exothermic reaction [2, 42]. First, water, surfactant, and initiator are loaded into the reactor. Then, monomer and heat are added to start the polymerization reaction, and after some period of time (in which the reaction sites have been formed), heat generated by the polymerization is removed in order to have an adequate reactor temperature control. Once the entire monomer load has been added, the temperature is increased to exhaust the monomer down to a small prespecified value. The monomer addition and heat exchange rates must be coordinated so that the batch is finished as soon as possible with adequate safety margin and... 

Polymerization of reactants is a common occurrence in many reactions. Although this is also a parallel scheme, it will be noticed that high concentrations of A combined with low concentrations of B will favor the desired product R. Thus, a semibatch reactor (SBR) would be the preferred candidate since the above condition is met in this reactor. We will see the design equations and principles of operation of SBRs later in this chapter. On the other hand, the common BR, PFR, and MFR would all give lower selectivities because they all allow the second reaction to proceed without hindrance. 

The batch emulsion polymerization is commonly used in the laboratory to study the reaction mechanisms, to develop new latex products and to obtain kinetic data for the process development and the reactor scale-up. Most of the commercial latex products are manufactured by semibatch or continuous reaction systems due to the very exothermic nature of the free radical polymerization and the rather limited heat transfer capacity in large-scale reactors. One major difference among the above reported polymerization processes is the residence time distribution of the growing particles within the reactor. The broadness of the residence time distribution in decreasing order is continuous>semibatch>batch. As a consequence, the broadness of the resultant particle size distribution in decreasing order is continuous>semibatch>batch, and the rate of polymerization generally follows the trend batch>semibatch>continuous. Furthermore, the versatile semibatch and continuous emulsion polymerization processes offer the operational flexibility to produce latex products with controlled polymer composition and particle morphology. This may have an important influence on the application properties of latex products [270]. 

In general, the optimization of polymerization processes [2] focuses on the determination of trade-offs between polydispersity, particle size, polymer composition, number average molar mass, and reaction time with reactor temperature and reactant flow rates as manipulated variables. Certain approaches [3] apply nonhnear model predictive control and online, nonlinear, inferential feedback control [4] to both continuous and semibatch emulsion polymerization. The objectives include the control of copolymer composition. 

As an example, let us consider suspension polymerization. In suspension polymerization, the monomers from the monomer droplets transfer into the polymer particles as long as monomer droplets exist. Thus, regarding the change of monomer concentration in the space of polymerization, that is, polymer particles, this reaction can be considered a semibatch reactor from the viewpoint of the change of monomer concentration. Figure 10 depicts the calculated results of crosslink density distribution of suspension polymerization and homogeneous phase polymerization under Flory s simplified conditions [48, 49]. In Fig. 10, the abscissa indicates the reactivity (0) at ttie time of each primary polymer formation and the ordinate shows the probability of crosslink density of each primary polymer. In the figure, for example, the line with l/ = 0.6... 

For a typical semibatch emulsion polymerization system, the initial reactor charge comprises water, surfactants, and sometimes a small proportion of monomers. When the reaction temperature (e.g., 80 °C) is reached, a persulfate initiator solution is added to the initial reactor charge to generate free radicals. This is then followed by the continuous addition of monomers (or monomer emulsion) over a period of time (normally a few hours). The appearance of the reaction mixture is transformed from transparent into translucent, opaque... 

After the mbber latex is produced, it is subjected to further polymerization in the presence of styrene (CgHg) and acrylonitrile (C H N) monomers to produce the ABS latex. This can be done in batch, semibatch, or continuous reactors. The other ingredients required for this polymerization are similar to those required for the mbber latex reaction. 

The experimental semibatch apparatus and procedure have been described in several places through the text of Wisseroth s publications ( 1, 7-9). so the details will not be repeated here. For nearly all of his work the reactor volume was one liter, temperature was 80 C, pressure was 30 atm (441 psia), and the feed was polymerization grade I assume that the reactor gas composition was 99% CsHgand 1% inerts. The range of catalyst loading was from 11 to 600 mg of TiCils per batch. The reaction time was varied from 0.5 to 6 hours. The weight ratio of alkyl-to-TiC 3 in the catalyst recipe was varied from 0.5 to 32. No data are reported from a continuous gas phase reactor. 

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