Semibatch reactors, temperature

The batch process is similar to the semibatch process except that most or all of the ingredients are added at the beginning of the reaction. Heat generation during a pure batch process makes reactor temperature control difficult, especially for high soHds latices. Seed, usually at 5—10% soHds, is routinely made via a batch process to produce a uniform particle-size distribution. Most kinetic studies and models are based on batch processes (69). 

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. 

Semibatch reactors are operated in two different modes (1) Some components of the reaction mixture are loaded into the reactor. After the operating conditions have reached the required level, the other components are dosed continuously or portion-wise, whereby temperature and pressure are kept as close as possible to profiles determined as optimum ones. This mode of... 

Semibatch reactors are often used to mn highly exothermic reactions isothermally, to run gas-liquid(-solid) processes isobarically, and to prevent dangerous accumulation of some reactants in the reaction mixture. Contrary to batch of)eration, temperature and pressure in semibatch reactors can be varied independently. The liquid reaction mixture can be considered as ideally mixed, while it is assumed that the introduced gas flows up like a piston (certainly this is not entirely true). Kinetic modelling of semibatch experiments is as difficult as that of batch, non-isotherma experiments. 

The temperature rise due to this exothermic reaction then approaches the adiabatic temperature rise. The final steady state is always characterized by conditions T = T, and c = 0. A batch reactor, in which a zero order reaction is carried out, always has a unique and stable mode of operation. This is also true for any batch and semibatch reactor with any order or combination of reactions. 

A semibatch reactor is run at constant temperature by varying the rate of addition of one of the reactants, A. The irreversible, exothermic reaction is first order in reactants A and B. 

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. 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

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. 

As long as the final temperature is less than some critical onset temperature where a secondary decomposition reaction occurs, then the process can safely handle a cooling system failure. If a batch reactor temperature cannot be assured to remain less than the onset temperature after a cooling system failure, then a semibatch operation should be used. As noted in Section 3.1.3.5, it is necessary to assure that reactant concentration is not increasing above an onset concentration where a similar decomposition could occur with a cooling system failure. 

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 ... 

Transient CSTR, Batch, and Semibatch Reactors with Heat Exchanger—Ambient Temperature Not Spatially Uniform... 

Figure 18. Degree of polymerization with time in response to step change in reaction temperature in controlled semibatch reactor. Key ------, WADP -------,...

A semibatch reactor is a type of batch configuration used particularly for processes, which employ very reactive starting material. Only one reactant, plus solvent if required, is present in the reactor at the start of the reaction. The other reactant(s) is then added gradually to the first, with continued stirring and control of the temperature. Through control of the rate of addition of one reactant, the temperature of the reacting mixture may be kept uniform as the reaction proceeds. 

The stirred-iank reactor may be operated as a steady-state flow type (Fig. 3-lu), a batch type (Fig. 3- b), or as a non-steady-state, or semibatch, reactor (Fig. 3-lc). The key feature of this reactor is that the mixing is complete, so that the properties of the reaction mixture are uniform in all parts of the vessel and are the same as those in the exit (or. product) stream. This means that the volume element chosen for the balances can be taken as the volume V of the entire reactor. Also, the composition and temperature at which reaction takes place are the same as the composition and temperature of any exit stream. 

The performance of semibatch reactors under isothermal conditions was studied in Sec. 4-8. When the temperature is not constant, an energy balance must be solved simultaneously with the mass-balance equation. In general, the energy balance for a semibatch reactor (Fig. 3-1 c) will include all four items of Eq. (3-2). Following the nomenclature of Sec. 4-8, let Fq and iq be the total mass-flow rates of feed and product streams and Hq and the corresponding enthalpies above a reference state. Then, following Eq. (3-2) term by term, the energy balance over an element of time At is... 

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. 

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... 

To express the temperature changes, we write the energy balance equation. For semibatch reactors, the expansion work is usually negligible, and assuming isobaric operation, the energy balance equation is... 

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