Semibatch/continuous reaction systems

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]. 

There is no doubt that the discipline of interfacial phenomena is an indispensable part of emulsion polymerization. Thus, the goal of this chapter is to offer the reader an introductory discussion on the interfacial phenomena related to the emulsion polymerization process, industrial emulsion polymerization processes (primarily the semibatch and continuous reaction systems), some important end-use properties of latex products, and some industrial apphcations. In this manner, the reader may effectively grasp the key features of emulsion polymerization mechanisms and kinetics. Some general readings in this vital interdisciphnary research area [1-6] are recommended for those who need to familiarize themselves with an introduction to the basic concepts of colloid and interface science. 

Semibatch reactors are especially important for bioreactions, where one wants to add an enzyme continuously, and for multiple-reaction systems, where one wants to maximize the selectivity to a specific product. For these processes we may want to place one reactant (say, A) in the reactor initially and add another reactant (say, B) continuously. This makes Ca large at all times but keeps Cg small. We will see the value of these concentrations on selectivity and yield in multiple-reaction systems in the next chapter. 

There are many factors that influence the outcome of enzymatic reactions in carbon dioxide. These include enzyme activity, enzyme stability, temperature, pH, pressure, diffusional limitations of a two-phase heterogeneous mixture, solubility of enzyme and/or substrates, water content of the reaction system, and flow rate of carbon dioxide (continuous and semibatch reactions). It is important to understand the aspects that control and limit biocatalysis in carbon dioxide if one wants to improve upon the process. This chapter serves as a brief introduction to enzyme chemistry in carbon dioxide. The advantages and disadvantages of running reactions in this medium, as well as the factors that influence reactions, are all presented. Many of the reactions studied in this area are summarized in a manner that is easy to read and referenced in Table 6.1. 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

Many of these difficulties can be overcome by choosing an appropriate configuration of the photoreactor system. One such a system is the mechanically agitated cylindrical reactor with parabolic reflector. In this type of reactor, the reaction system is isolated from the radiation source (which could also simplify the solution of the well-known problem of wall deposits, generally more severe at the radiation entrance wall). The reactor system uses a cylindrical reactor irradiated from the bottom by a tubular source located at the focal axis of a cylindrical reflector of parabolic cross-section (Fig. 40). Since the cylindrical reactor may be a perfectly stirred tank reactor, this device is especially required. This type of reactor is applicable for both laboratory-and commercial-scale work and can be used in batch, semibatch, or continuous operations. Problems of corrosion and sealing can be easily handled in this system. 

Semibatch Operation In semibatch operation the rates of mass flow into and out of the system are unequal (see Fig. 3-1 c). For example, benzene may be chlorinated in a stirred-tank reactor by first adding the charge of liquid benzene and catalyst and then continuously adding chlorine gas until the required ratio of chlorine to benzene has been obtained. Operation of this kind is. batch from the standpoint that the composition of the reaction mixture changes with time. However, from a process standpoint the chlorine is added continuously. The system is still an ideal stirred-tank reactor if the... 

The differences between a single CSTR and a batch reactor are similar to those between semibatch and batch reactors, except that they are usually more pronounced. The addition of more reactors to a series system tends to reduce some of the observed performance differences. A typical example of different behavior is the heat release profile. An advantage often cited for continuous reactor systems is a constant heat load with fully used reactor volume. Batch reactors are not usually operated full, and the heat load is nonuniform. In addition, portions of the batch reaction cycle are devoted to charging and emptying the reactor and sometimes for heating the reagents to polymerization temperature. Thus, the production rate per unit volume can be higher in a continuous system. 

Typical reaction systems are illustrated in Figure 14.14. A kind of batch reactor with nanopowder spread and pasted on a wall can react with molecules easily (a). A frequently seen system is a fixed bed reaetor packed with nanopowder which reacts with a flowing gas (b). The solid material is not removed until the reaction is almost complete, while the reacted gas is removed continuously. This is a kind of semibatch reaction system, which was used to produce the results shown in Figures 14.4, 14.7, and 14.8. 

Van de Vusse [16, 17] also performed experiments on the chlorination of n-decane, a reaction system of the type considered here, in a semibatch reactor. In such a reactor the chlorine gas is bubbled continuously through a batch of n-decane. In some experiments the n-decane was pure, in others it was diluted with dichlorobenzene. In some experiments the batch was stirred, in others not. The experimental results could be explained in terms of the above considerations. In all experiments y > 1 (from 150 to 500), hence the rate of the process was limited by diffusion, but the selectivity was only affected when Cgo/C i < y. This condition was only fulfilled for the experiments in which n-decane (B) was diluted. For only these experiments were the selectivities in nonstirred conditions found to differ from those with stirring. 

Much of the literature on scale-up of reaction systems has focused on continuous systems. However, scale-up methods for batch and semibatch operations have been included in several books, including Oldshue (1983), Whitaker and Cas-sano (1986) Carberry and Varma (1987) Froment and Bischoff (1990), Tatterson (1991), Hamby et al. (1992), and Baldyga and Bourne (1999). Correlations for heat transfer, mass transfer, Uquid-liquid dispersions, solids suspensions, and dissolntion are available and are discussed iu these references and in several chapters of this book. Mixing requirements for scale-up of homogeneous reactions are discussed in Chapter 13, including explanation of the limitations of the nsnal mixing scale-up parameter of equal power per unit volume. The reader is referred to the texts listed in the references, in which these correlations are well developed. These correlations are not reproduced in this chapter. 

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... 

These mixing systems offer high flexibility because they can be operated in batch, semibatch, or continuous modes. Adequate mixing is a prerequisite for the success of chemical processes in terms of rninirnizing investment and operating costs. In addition, chemical reactions with... 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

Semibatch or semiflow processes are among the most difficult to analyze from the viewpoint of reactor design because one must deal with an open system under nonsteady-state conditions. Hence the differential equations governing energy and mass conservation are more complex than they would be for the same reaction carried out batchwise or in a continuous flow reactor operating at steady state. 

The same example was solved using MINOPT (Rojnuckarin and Floudas, 1994) by treating the PFR model as a differential model. The required input files are shown in the MINOPT manual. Kokossis and Floudas (1990) applied the presented approach for large-scale systems in which the reactor network superstructure consisted of four CSTRs and four PFR units interconnected in all possible ways. Each PFR unit was approximated by a cascade of equal volume CSTRs (up to 200-300 CSTRs in testing the approximation). Complex reactions taking place in continuous and semibatch reactors were studied. It is important to emphasize that despite the complexity of the postulated superstructure, relatively simple structure solutions were obtained with the proposed algorithmic strategy. 

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

Figure 4-17 shows a special type of semibatch reactor system in which there is a continuous feed, no withdrawal of product, and a mass m i of component A initially in the reactor. The application of Eq. (4-12) will be considered for a reactor of this type when the rate equation is nOt first order and when the volume of the reaction mixture varies. Since there is no exit stream, Eq. (4-12) takes the form... 

Catalytic reactions can be run in batch mode or as a continuous process. In a batch process the reactants, catalyst, and other reaction components are loaded in an appropriate vessel, the reaction is run, and the produets are removed from the vessel after some time and separated from the catalyst. In a continuous system the reactants are passed over the eatalyst and the products removed at the same rate as the reactants are added. Another, intermediate, option is semibatch operation one of the reaetants is added, gradually or step-wise, to the reaetor, which has been loaded with catalyst and the other reaetant(s). 

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