The Semi-Batch Reactor

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 

An example of a highly exothermic reaction carried out in this manner is provided by the manufacture of dodecylbenzene sulphonic acid. The reactor is charged with dodecylbenzene and 20% oleum is fed in continuously at a rate which is regulated to give a uniform temperature during the sulphonation reaction. The conversion is thus effectively controlled by the rate of oleum addition rather than the rate of the chemical reaction. 

In another type of semi-batch reactor, all the reactants are placed in a reactor and, as reaction proceeds, one of the products is continuously removed some esterification reactions are carried out in this way, water being boiled-off continuously, otherwise reaction would cease when the equilibrium position was reached. 

In principle, each of these terms may be significant and, since the volume and composition of the reactant mixture both change with time, the resulting equations may be very complex and incapable of analytical solution. 

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 

A semi-batch reactor with one feed stream and heat transfer to a cooling jacket is shown in Fig. 3.11. 

A total mass balance is necessary, owing to the feed input to the reactor, where 

The density in the reactor, p, may be a function of the concentration and temperature conditions within the reactor. Assuming constant density conditions 

All important components require a component balance. For a given reactant A 

Whenever changes in temperature are to be calculated, an energy balance is needed. With the assumption of constant Cp and constant p, as derived in Sec. 1.2.5, the balance becomes 

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For a simple A + B C reaction the semi-batch reactor balance is... 

Figure 5.137. Program SEMISEG Varying THETAMIX (0.03, 0.1, 0.3 and 0.6, curves D, C, A, B) gave these results for the semi-batch reactor. Note that when THETA > 1 the reactor becomes continuous.

Keller (1998) describes the semi-continuous reaction process of a vinyl ketone K with lithium acetylide LA to yield lithium ethinolate LE an intermediate in the vitamin production. In an undesired side reaction an oligomer byproduct BP is produced. During the process, reactant K is fed to the semi-batch reactor at a rate to maximize the selectivity for LE. 

The total balance for the semi-batch reactor assuming constant density is... 

In addition to these three principal types of reactor, there is also the semi-batch reactor in which one reactant is added gradually to the others. This is a convenient manner of operation for some highly exothermic reactions since the temperature can be controlled by adjusting the rate of addition. 

This effect is especially important in the semi-batch reactor. If the temperature difference between reactor and feed is important and/or the feed rate is high, this term may play a dominant role, the sensible heat significantly contributing to reactor cooling. In such cases, when the feed is stopped, it may result in an abrupt increase of the reactor temperature. This term is also important in calorimetric measurements, where the appropriate correction must be performed. 

In a more recent study [15], Westerterp and Molga introduced a set of dimensionless numbers (cooling, reactivity, and exothermicity numbers) characterizing the stability of heterogeneous slow liquid-liquid reactions in the semi-batch reactor. They demonstrated that the key parameter is the cooling number Co ... 

This criterion allows distinguishing two states in a batch reactor, no-runaway and runaway. For the semi-batch reactor, there are four different states, no-igni-tion, runaway, marginal ignition, and QFS. 

As with the batch reactor, the semi-batch reactor operates discontinuously. The difference with true batch operation is that for the semi-batch reactor, at least one of the reactants is added as the reaction proceeds (Figure 7.1). Consequently, the material balance as well as the heat balance will be affected by the progressive addition of one of the reactants. Also, as with the batch reactor, there is no steady state. There are essentially two advantages in using a semi-batch reactor instead of a batch reactor ... 

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. 

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 ... 

Additionally the semi-batch reactor with constant cooling medium temperature, also in cases where a stationary temperature can be achieved, shows a high sensitivity to its control parameters, that is, initial temperature and coolant temperature. This means that even for small changes in these temperatures, the behavior of the reactor may suddenly change from a stable situation into a runaway course. 

A reaction A—-—>P is to be performed in a CSTR. The reaction follows first-order kinetics and at 50 °C, the conversion reaches 99% in 60 seconds. The reaction is to be performed in a CSTR with the same productivity as the semi-batch reactor (Worked Example 7.1). The overall heat transfer coefficient of the reactor is 500 Wm"2 K"1. The maximum temperature difference with the cooling system is 50 K. 

The semi-batch reactor produces 5 m3 of product solution every 2 hours. Therefore, in a continuous reactor, the flow rate must be 2.5m3h 1 or 6.94-10-4 mV1. 

Comparative tests have been performed in the semi-batch reactor system to evaluate the Ru/Ti02 cataly versus a more conventional nickel-based catalyst. These tests show that rutlienium at only 3% metal loading has about the same activity as nickel at S0% metal loading. This comparison is only for short-term activity of the catalyst. As demonstrated in the continuous flow tests, the nickel catalyst loses activity readily in tlie first hours on stream, while the ruthenium maintains its activity. 

A solution of the semi-batch reactor model equations yields the results that would be expected from a real reactor. If the control action has been effective, p should remain constant. Any error in the computation of the exact amount of the required... 

The semi-batch process is similar to the batch process except feed addition occurs during the batch cycle. Products may also be removed during the semi-batch process. The addition/removal policy allows one to control the reaction rate or heat release during reaction. The semi-batch reactor also may provide more complete... 

Semi-batch reactor. In the semi-batch reactor, the reactor is filled during operation so Q/ is specified and positive for some time and Q = 0. The solution to Equation 4.55 then determines the change in volume of the reactor during the filling operation. 

The development of the semi-batch reactor energy balance follows directly from the CSTR energy balance derivation by setting Q = 0. The main results are summarized in Table 6.9 at the end of this chapter. Note in particular that Equations 6.81-6.83 in the semi-batch reactor Table 6.9 are identical to the corresponding Equations 6.72-6.74 in the CSTR Table 6.8. 

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