Selectivity semibatch reactors

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

As expected, heat exchanged per unit of volume in the Shimtec reactor is better than the one in batch reactors (15-200 times higher) and operation periods are much smaller than in a semibatch reactor. These characteristics allow the implementation of exo- or endothermic reactions at extreme operating temperatures or concentrations while reducing needs in purifying and separating processes and thus in raw materials. Indeed, since supply or removal of heat is enhanced, semibatch mode or dilutions become useless and therefore, there is an increase in selectivity and yield. 

Example 5.3.1.5. Selectivity versus mode of operation in semibatch reactors... 

In order to illustrate how the mode of operation can positively modify selectivity for a large reactor of poor heat-transfer characteristics, simulations of the reactions specified in Example 5.3.1.4 carried out in a semibatch reactor were performed. The reaction data and process conditions are essentially the same as those for the batch reactor, except that the initial concentration of A was decreased to cao = 0.46 mol litre, and the remaining amount of A is dosed (1) either for the whole reaction time of 1.5 h with a rate of 0.1 mol m s", or (2) starting after 0.5 h with a rate of 0.15 mol m " s". It is assumed that the volume of the reaction mixture and its physical properties do not change during dosing. The results of these simulations are shown in Fig. 5.3-15. The results of calculation for reactors of both types are summarized in Table 5.3-3. 

It is clear from the presented data that the yield and selectivity in a large semibatch reactor can be improved compared to those in a small batch reactor that has much better heat-transfer capability. This has been achieved by decreasing the rate of heat evolution, which has been obtained by lowering the instantaneous concentration of reactant A. The results also indicate that the dosing policy can have a very significant influence on reactor performance. 

Solution From the kinetic expressions given in Example 7.3, it can be concluded that n/ri = k Clca. In order to increase the selectivity of the reactions, the Clcii should be minimized. A semibatch reactor would thus be preferred, in which BA is charged at the beginning of the batch and CL fed continuously during the reaction. In any case, charging all of the chlorine at the beginning of the reaction will be impractical. 

In pulp and paper processing, anthraquinone (AQ) accelerates the delignification of wood and improves liquor selectivity. The kinetics of the liquid-phase oxidation of anthracene (AN) to AQ with NO2 in acetic acid as solvent has been studied by Rodriguez and Tijero (1989) in a semibatch reactor (batch with respect to the liquid phase), under conditions such that the kinetics of the overall gas-liquid process is controlled by the rate of the liquid-phase reaction. This reaction proceeds through the formation of the intermediate compound anthrone (ANT) ... 

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. 

We have presented a general reaction-diffusion model for porous catalyst particles in stirred semibatch reactors applied to three-phase processes. The model was solved numerically for small and large catalyst particles to elucidate the role of internal and external mass transfer limitations. The case studies (citral and sugar hydrogenation) revealed that both internal and external resistances can considerably affect the rate and selectivity of the process. In order to obtain the best possible performance of industrial reactors, it is necessary to use this kind of simulation approach, which helps to optimize the process parameters, such as temperature, hydrogen pressure, catalyst particle size and the stirring conditions. 

The temperature-controlling features of this reaction scheme dominate selection and use of the reactor. However, the semibatch reactor does have some of the advantages of batch reactors temperature programming with time and variable reaction time control. 

The proposed model takes another approach. It was developed for multistage semibatch reactors with stationary solids and continuous co-current reactors with moving solids. It also allows for a crosscurrent stream such as gas sparged separately into any number of stages. The residence time of each stage is divided into a number of finite time intervals. Within each interval, the individual reactions are treated as successive rather than simultaneous. The model accuracy is controlled by selecting the number of intervals. 

In continuous processes the reactants are fed to the reactor and the products withdrawn continuously the reactor operates under steady-state conditions. Continuous production will normally give lower production costs than batch production, but it lacks the flexibility of batch production. Continuous reactors will usually be selected for large-scale production. Processes that do not fit the definition of batch or continuous are often referred to as semicontinuous or semibatch. In a semibatch reactor, some of the reactants may be added or some of the products withdrawn as the reaction proceeds. A semicontinuous process can be one which is interrupted periodically for some purpose, for instance, for the regeneration of catalyst. 

As seen from Table 10.2 [14], when a mixture of methanol, H2O, and N2 is used at supercritical conditions, PO selectivity is only 3.5%. When N2 is replaced by CO2, PO selectivity is increased to 41%. The high PO selectivity of aroimd 94% can be obtained in neat CO2 at supercritical conditions, however, the STY is about one order of magnitude smaller than that for semibatch reactors imder normal pressure in MeOH solvent. 

Propylene epoxidation with O2/H2 mixture in MeOH or BuOH solvent can proceed with a selectivity above 88% over Pd/TS-1 catalyst in semibatch reactors. The addition of a small amoimt of Pt is effective to maintain cationic Pd and enhances the catalytic performance. 

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... 

Semibatch reactor (Fig.. b)—A tank in which one reactant is charged initially and another reactant is added continuously during flie operation. This mode of operation is used when it is desirable to maintain one reactant (the injected reactant) at low concentration to improve the selectivity of the desirable product and to supply (or remove) heat. 

In this chapter, the analysis of chemical reactors is expanded to additional reactor configurations that are commonly used to improve the yield and selectivity of the desirable products. In Section 9.1, we analyze semibatch reactors. Section 9.2 covers the operation of plug-flow reactors with continuous injection along their length. In Section 9.3, we examine the operation of one-stage distillation reactors, and Section 9.4 covers the operation of recycle reactors. In each section, we first derive the design equations, convert them to dimensionless forms, and then derive the auxiliary relations to express the species concentrations and the energy balance equation. 

To reduce the design equation to dimensionless form, we have to select a reference state and define dimensionless extents and dimensionless time. The reference state should apply to all operations, including those with an initially empty reactor, and should enable us to compare the operation of a semibatch reactor to that of a batch reactor. Therefore, we select the molar content of the reference state, (A tot)o. as the total moles of species added to the reactor. The dimensionless extent is defined by... 

The small reactor volumes and the flexible arrangement of microstructured devices can be applied to design multipurpose plants, and traditional batch and semibatch reactors can be replaced. A considerable process intensification and enhanced product selectivity and yield have been shown [20]. Furthermore, continuous reactor operation may help in providing consistent product quality. 

One of the best reasons to use semibatch reactors is to enhance selectivity in liquid-phase reactions. For example, consider the following two simultaneous reactions. One reaction produces the desired product D... 

The influence of heat transfer on yield and selectivity in scaling up batch and semibatch reactors will be illustrated using a series reaction, taking place in an ideal jacketed stirred-tank reactor. This reaction is composed of two irreversible elementary steps, both exothermic and both with first order kinetics ... 

When a reactor is charged with liquid A and B is a gas that is added continuously, it becomes a semibatch reactor. The rates of reaction depend on the concentration of B in the liquid phase, which is a function of gas solubility, pressure, and agitation conditions. However, we are often concerned with the relative reaction rates and the selectivity, which do not depend on Cb if the reaction orders are the same for both reactions. The reactions are treated as pseudo-first-order, and equations are developed for an ideal batch reactor with irreversible first-order kinetics... 

Figure 3.4 Selectivity with parallel reactions in a semibatch reactor.

To get a high selectivity, a semibatch reactor could be used, with A fed continuously to an initial charge of B, as described in Chapter 3. Because of imperfect mixing, there will be regions near the feed pipe where is greater than the bulk concentration and Cg is somewhat depleted. These differences lower the ratio C jC and decrease the local selectivity. The extent of the decrease depends on the relative rates of mixing and reaction. 

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. 

Optimization can be done by proper reactor choice followed by a suitable temperature progression in the case of a batch or semibatch reactor, or by temperature profiling in the case of a tubular reactor. An even more effective way is to optimize reactant concentrations, pressure, and/or temperature by applying certain simple rules of kinetics and manipulation of the chemistry (wherever possible). Hence the combined efforts of chemist and chemical engineer are needed to optimize selectivity in a given complex reaction. 

We are required to conduct the reaction in a semibatch reactor and, for a few selected ... 

The kinetics of reactions is specific for different reaction systems and processes and valid for isothermal and nonisothermal reactors. The effects of the kinetics on the conversion, selectivity, or yield depend on the reaction and may be quite pronounced. Liquid or gas phase reactions with high heat capacity can be performed in specific reactors, which operate isothermally or not. We will study the most common cases such as semibatch reactors, recycle reactors, fixed-bed reactors, and reactors with membranes. 

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