Semi-batch operations

These two factors mean the semi-batch reactor is a commonly-used reactor type in the fine chemicals and pharmaceutical industries. It retains the advantages of flexibility and versatility of the batch reactor and compensates its weaknesses in the reaction course control by the addition of, at least, one of the reactants. 

Figu re 7.1 Semi-batch reactor compound A is initially charged and B is fed during the reaction, providing additional control of the reaction course. 

For an irreversible bimolecular second-order reaction as the example reaction, the rate equation is 

By convention in this chapter, the reactant A will be initially charged into the reactor, whereas B will be added with a constant molar feed rate FE during the 

Equations 7.4 and 7.5 form a system of differential equations for which no analytical solution is known. Thus, the description of the behavior of the semi-batch reactor with time requires the use of numerical methods for the integration of the differential equations. Usually, it is convenient to use parameters which are more process-related to describe the material balance. One is the stoichiometric ratio between the two reactants A and B  

The macroscopic material and energy balances for semi-batchwise operated perfectly mixed stirred tank reactors are given by Equations 4.24 and 4.25, respectively  

In general the initial charge contains a seed, which is used principally to avoid the lack of reproducibility of the nucleation stages when the seed is produced in situ. In addition, it should be remarked that nucleation is very scale sensitive (Meuldijk et aL, 2003). Besides the seed latex, the initial charge in a semi-batch process contains a fraction of the total amount of water to be used, surfactant and initiator. Under some circumstances certain amount of the monomer(s) can also be present. The rest of the formulation ingredients are fed to the reaction mixture at a constant flow rate. Note that in many cases time-dependent feed rates are used. These feed rate profiles can be calculated by using empirical knowledge of the process or by application of optimisation techniques based on mathematical models. 

The amount of polymer in the seed usually represents less than 5% by weight of the total polymer in the final latex product and hence the properties of the seed polymer are negligible with respect to the copolymer formed afterwards. However, the amount of polymer and the particle size of the seed are important. Thus, for a given amount of seed polymer, the lower the particle size, the higher the number of particles and as a consequence the specific particle surface area per unit volume of the water phase will be larger. As a result of this higher specific particle surface area, secondary nucleation is less probable to occur during 

When latex products with a bimodal particle size distribution are desired, secondary nucleation can be induced by mid-course pulse wise emulsifier additions to the reaction mixture. 

5 Eeeding of monomer as a monomer in water emulsion or as neat monomer 

The comparative experiments are carried out only in semi-batch operation mode the experimental conditions and the operation procedure are identical for both the SCISR and the STR, and the specific effective power inputs for the two reactors are controlled rigorously to be identical. 

The experimental run in the study on the final treatment of the precipitate is carried out in the SCISR operated continuously under the optimized conditions for about 48 h to accumulate a large enough amount of product for spray drying. The whole of the reacted precipitate separated out is fully mixed and is then used for the spray drying experiment. 

The results of the normal-designed experiments are listed in Table 13.3, where I, II, and III denote the summations of the Sauter mean diameters, d22, at Levels 1, 2, and 3, respectively and R is the extreme difference at a certain level. From Table 13.3 it follows that the order of influence of various factors on the mean size of the particles in the precipitate is D C B F H G E A. The influences of the latter three factors, G, E, and A, can be considered as very weak, while the most serious factors leading to gelation of the reaction mixture are, in order, A and G, suggesting the reaction temperature cannot be too high and the amount of dispersion agent cannot be too large. 

The ageing time, Factor H, exhibits a monotonous increasing influence on the mean size of the particles, suggesting that the particles of hydrated silica have a significant tendency to coalesce under stationary conditions. Therefore, the ageing time before further processing of the precipitate should not be too long. 

Both the Factors E, the dripping rate of H2S04, and G, the amount of dispersion agent, exhibit monotonously increasing influences on the mean diameter of particles but both their effects are insignificant. 

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Using a batch reactor, a constant concentration of sulfuric acid can be maintained by adding concentrated sulfuric acid as the reaction progresses, i.e., semi-batch operation. Good temperature control of such systems can be maintained, as we shall discuss later. 

The product crystals were agglomerates of needles or dendrites. Loose floes of dendroid strontium carbonate are compacted by agitation, which is an important factor in controlling the habit of product particles. Semi-batch operation produces larger particles compared to batch or continuous operation. 

Conventional (semi)batch-operated, mechanically stirred reactors equipped with a multinozzle system for feeding components of the reaction mixture predominate at this stage. 

Most reactor operations involve many different variables (reactant and product concentrations, temperature, rates of reactant consumption, product formation and heat production) and many vary as a function of time (batch, semi-batch operation). For these reasons the mathematical model will often consist of many differential equations. 

It becomes necessary to incorporate a total mass balance equation into the reactor model, whenever the total quantity of material in the reactor varies, as in the cases of semi-continuous or semi-batch operation or where volume changes occur, owing to density changes in flow systems. Otherwise the total mass balance equation can generally be neglected. 

Both flow terms are zero for the case of batch reactor operation, and the outflow term is zero for semi-continuous or semi-batch operation. 

For the semi-batch operation the total mass balance is... 

A batch reactor is an agitated vessel in which the reactants are precharged and which is then emptied after the reaction is completed. More frequently for exothermic reactions, only part of the reactants are charged initially, and the remaining reactants and catalysts are fed on a controlled basis this is called a semi-batch operation. For highly exothermic reactions and for two-phase (gas-liquid) reactions, loop reactors with resultant smaller volumes can be used. 

If agitation fails during a semi-batch operation, the transfer of heat will essentially stop. The resulting increase in temperature depends on the concentration of the reactants at that moment, the global kinetics, and the mass transfer rate. The effect of the temperature increase is easily simulated in a reaction calorimeter both with and without addition of reactants. 

When the charging rate (semi-batch) or the mass of reactant added in one step (batch) or the continuous feed ratios get out of phase, the heat generation may be larger than expected. This results in difficulties in the transfer of the heat generated, especially in cases where the cooling capacity has a limited safety margin Also, it can lead to accumulation of reactant in a semi-batch operation. 

Good process and equipment design practices for batch and semi-batch operating systems, particularly for those involving reactive or otherwise hazardous substances, are ... 

A technique is described [228] for solving a set of dynamic material/energy balances every few seconds in real time through the use of a minicomputer. This dynamic thermal analysis technique is particular useful in batch and semi-batch operations. The extent of the chemical reaction is monitored along with the measurement of heat transfer data versus time, which can be particularly useful in reactions such as polymerizations, where there is a significant change in viscosity of the reaction mixture with time. 

With initial conditions for the initial molar quantities of A and B (VCA, VCB), the initial temperature, T, and the initial volume of the contents, V, specified, the resulting system of equations can be solved to obtain the time-varying quantities, V(t), VCA(t), VCB(t), T(t) and hence also concentrations CA and CB as functions of time. Examples of semi-batch operations are given in the simulation examples HMT, SEMIPAR, SEMISEQ, RUN, SULFONATION and SEMIEX. 

A scrubber with recycle where part of the caustic inventory is periodically pumped from the reservoir and replaced with fresh, 20 wt% caustic. This will be referred to as a semi-batch operation. The operating periodicity may be in the order of hours or days... 

Semi-batch operations are usually conducted in a stirred vessel. One of the reactants is loaded into the vessel as a single charge and the other... 

Considering the normal mode of semi-batch operation, when A is fed steadily to the vessel and nothing is removed during reaction, the material balance becomes... 

Gas-phase reaction in continuous and semi-batch operations... 

Mathematical models derived from mass-conservation equations under unsteady-state conditions allow the calculation of the extracted mass at different bed locations, as a function of time. Semi-batch operation for the high-pressure gas is usually employed, so a fixed bed of solids is bathed with a flow of fluid. Mass-transfer models allow one to predict the effects of the following variables fluid velocity, pressure, temperature, gravity, particle size, degree of crushing, and bed-length. Therefore, they are extremely useful in simulation and design. 

Variations in Contacting Pattern — Semi-batch Operation... 

A characteristic of semi-batch operation is that the concentration CB of the reactant added slowly, B in Fig. 1.2, is low throughout the course of the reaction. This may be an advantage if more than one reaction is possible, and if the desired reaction is favoured by a low value of CB. Thus, the semi-batch method may be chosen for a further reason, that of improving the yield of the desired product, as shown in Section 1.10.4. 

In semi-batch operation, when the initial charge of A has been consumed, the flow of B is interrupted, the products discharged, and the cycle begun again with a fresh charge of A. If required, however, the advantages of semi-batch operation may be retained but the reactor system designed for continuous flow of both reactants. In... 

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