Semibatch reactors examples

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. 

Example 5.4.5.1. Application of the E-model for simulation of the coupling of I-naphthol with diazotized sulphanilic acid in a semibatch reactor (after Baidyga and Bourne, 1989b). 

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. 

Remark 5 Note also that reactors of various distribution functions can be treated with this approach, on the grounds that different distribution functions are usually approximated via cascades of CSTRs. In this case, we can treat the number of CSTRs as a variable or provide a variety of alternative reactors each featuring different numbers of CSTRs. Kokossis and Floudas (1990), present examples for batch, semibatch reactors and different distribution functions. 

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. 

Figure 7.4 Example addition reaction performed in a semibatch reactor under isothermal conditions at 80°C with a feed time of 6 hours. The feed was interrupted between 3 and 4 hours. The heat release rate immediately decreases to zero and recovers its initial value after resuming the feed.

The second type of semibatch reactor is when some material is removed from the reactor during the batch. The material is typically one of the products of the reaction. A common example is in fermentors producing ethanol in which the byproduct carbon dioxide is vented off during the batch cycle. 

Although a wide variety of catalysts have been employed to crack PE, zeolites have proven particularly effective. For example, Garforth et al. reported that activation energies (La) measured when PE was catalytically cracked by HZSM-5, HY, and MCM-41 were much lower than when no catalyst was present. [66] They concluded that HZSM-5 and HY have similar activities and that both of these zeolites were more effective than MCM-41. Manos and co-workers found that catalytic cracking of PE by HZSM-5 and HY was effective in producing gasoline size hydrocarbons in a laboratory semibatch reactor [67, 68]. Mordi and co-workers reported that H-Theta-1 and H-Mordenite... 

For other worked examples of semibatch reactors, see H. S. Fogler, Elements of Chemical Reaction Engineering, 3rd ed., Prentice-Hall, 1992, pp. 190-200, and N. H. Chen, Process Reactor Design, Allyn and Bacon, Inc, 1983, Chap. 6. 

Rework Example 4-10. Plot the molar flow rates of A, B, and C as a function of reactor length (i.e., volume) for different values of between kf = 0.0 (a conventional PFR) and kj = 7.0min , What parameters would you expect to affect your results the most Vary the parameters k,k, Kc,Ffji to study how the reaction might be optimized. Ask such questions as What is the effect of the ratio of It to or of k, t to What generalizations can you make How would your an.swer change if the reactor temperature were raised significantly What if somEone claimed that membrane reactora were not as safe as. semibatch reactors What would you tell them ... 

Example 4-11 Isothermal Semibatch Reactor with Second-Order Reaction... 

Example 9-8 Multiple Reactions in a Semibatch Reactor Professional Reference Shelf... 

CDPSI Ag The production of propylene glycol discussed in Examples 8-4, 9-4, 9-5, 9-6, and 9-7 is carried out in a semibatch reactor. 

Example 5-5 Hexamethylenetetramine (HMT) is to be produced in a semibatch reactor by adding an aqueous ammonia solution (25 wt % NH3) at the rate of 2 gpm to an initial charge of 238 gal (at 25°C) of formalin solution containing 42% by weight formaldehyde. The original temperature of the formalin solution is raised to 50°C in order to start the reaction. The temperature of the NH4.OH solution is 25°C. The heat of reaction in the liquid phase may be, assumed independent of temperature and concentration and taken as —960 Btu/lbbf HMT. If the reactor can be operated at a temperature of 100°C, the rate of reaction is very fast in comparison with the rate of heat transfer with the surroundings. Temperatures higher than 100°C are not desirable because of vaporization and increase in pressure. 

Example 9,2 Valuable product V is produced in a semibatch reactor where the following simultaneous, liquid-phase chemical reactions take place. 

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... 

Example 4-6 Calculating X in a Reactor with Pressure Drop Example 4 7 Gas-Phase Reaction in Microreactor—Molar Flow Rate Example 4-8 Membrane Reaeior Example CDR4.1 Spherical Reactor Example 4.3.1 Aerosol Reactor Example 4-9 Isothermal Semibatch Reactor Profe.ssional Reference Shelf R4.1. Spherical Packed-Bed Reactor. ... 

Another classification of a semibatch reactor is one in which a gas forms or a solid precipitates during the reaction. Here, also, a volatile product may be fractionated off continuously. An example is batch esterification with continuous distillation. An example of this type of reaction, which will be dealt with later in this text, is the esterification of ethyl alcohol with acetic acid to form ethyl acetate. 

Figure 4.18 Reactants molal ratio for an example semibatch reactor.

This assessment criterion shows very clearly the fundamental difference between BR and SBR. Especially for the isothermally operated BR the example discussed has shown, that the ratio of Damkoehler to Stanton number should be significantly smaller than 1 to be sure of safe operating conditions. Exactly the opposite is required for the SBR. Semibatch reactors must be operated in an ignited state in order to be able to utilize the degree of heat production rate. Batch reactors have to be operated in the extinguished state with the consequence already outlined in Section 4.3.1.3, that they are suitable for slow and moderately exothermic processes only. 

Design of a semibatch reactor for the Haloform oxidation of diacetone alcohol (a continuation of Example 5.3). 

In this section, we present the use of MATLAB to model chemical reactors. We focus on two examples, a semibatch reactor and a fixed-bed reactor. Both represent common cases of study such as the production of polymethyl methacrylate (PMMA) in suspension and the oxidation of SO2 to SO3 that is one of the steps in the production of sulfuric acid via heterogeneous method. The models are based on explicit algebraic equations and differential equations. Thus, we use ODEXX function in MATLAB to solve the concentration, temperature, and/or pressure profiles along the operation of such equipment. 

We can envision a mechanism of one or more steps for each of these unit operations and we can write a rate equation for each step. We can then relate each of these individual rate equations to an overall rate constant. For a mechanism with two or more steps in series, one step will be slower than the other steps we say this slow step is the rate controlling step. For example, a gas—liquid reaction in a laboratory-sized reactor is either heat transfer controlled or reaction rate controlled. If we cannot supply heat fast enough to maintain the reaction or if we cannot remove heat fast enough to control the reaction, we say the reaction is heat transfer controlled. If, on the other hand, we can supply or remove heat faster than required by the reaction, then we say the reaction is reaction rate controlled. In general, laboratory-sized batch and semibatch reactors have large heat transfer surface area to reaction volume ratios therefore, transferring heat to... 

Example 8.3.A Simulation of Semibatch Reactor Operation (with L.H. Hosten )... 

Batch and semibatch reactors are most often used for low production capacities, where the cost of labor and dead time are only a small fraction of the unit cost of the product. They are generally encountered in the area of specialty chemicals and polymers and in pharmaceuticals, in particular, in plants with a wide variety of products. Large batch reactors are used in fermentations for antibiotic production, an example of which is given in Fig. 8-1. 

The balance equations for column reactors that operate in a concurrent mode as well as for semibatch reactors are mathematically described by ordinary differential equations. Basically, it is an initial value problem, which can be solved by, for example, Runge-Kutta, Adams-Moulton, or BD methods (Appendix 2). Countercurrent column reactor models result in boundary value problems, and they can be solved, for example, by orthogonal collocation [3]. The backmixed model consists of an algebraic equation system that is solved by the Newton-Raphson method (Appendix 1). 

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