Exothermic semi-batch reactor

Figure 5.166. Exothermic semi-batch reactor with feed control.

Some batch reactions have the potential for very high energy levels. If all the reactants (and sometimes catalysts) are put into a kettle before the reaction is initiated, some exothermic reactions may result in a runaway. The use of continuous or semi-batch reactors to limit the energy present and to reduce the risk of a runaway should be considered. The term semi-batch refers to a system where one reactant and, if necessary, a catalyst is initially charged to a batch reactor. A second reactant is subsequently fed to the reactor under conditions such that an upset in reacting conditions can be detected and the flow of the reactant stopped, thus limiting the total amount of potential energy in the reactor. 

Temperature control of a large, highly exothermic semi-batch chemical or polymer reactor can be an involved problem. The reaction may be auto-acceleratlng. Heat transfer rates can vary during the process. Random disturbances can enter the process from many sources. Changes In... 

A process is described [224] in which an exothermic reaction takes place in a semi-batch reactor at elevated temperatures and under pressure. The solid and liquid raw materials are both toxic and flammable. Spontaneous ignition is possible when the reaction mass is exposed to air. Therefore, the system must be totally enclosed and confined in order to contain safely any emissions arising from the loss of reactor control, and to prevent secondary combustion reactions upon discharge of the materials to the atmosphere. Further, procedures and equipment are necessary for the safe collection and disposal of solid, liquid, and gaseous emission products. 

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. 

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 ... 

A fast exothermal reaction is to be performed in a semi-batch reactor. In order to control the temperature course of the reaction, one of the reactants is added at a constant rate, producing a constant heat flow. The reactor is cooled with water from a river (at 15 °C in winter). The cooling water should not be rejected at a temperature higher than 30 °C. 

Gibson, N., Maddison, N., and Rogers, R. L., Case Studies in the Application of DIERS Venting Methods to Fine Chemical Batch and Semi-batch reactors, Hazards from Pressure Exothermic Reactions, Unstable Substances, Pressure Relief and Accidental Discharge, IChemE, Symp, Ser. No. 102, EFCE Event No. 359, pp. 157-169, 1987. 

If our semi-batch process is exothermic and we lose flow of heat transfer fluid to the reactor jacket, the ensuing event is unlikely to become a runaway with regard to temperature if we block flow of LR into the process. Upon losing cooling to the process, the thermal balance for our semi-batch reactor becomes... 

In most exothermic processes carried out in a stirred tank reactor the bulk of the heat of reaction is carried away by external cooling, either via the wall of the reactor, or by means of an internal or extenal heat exchanger. Internal heat exchangers are usually vertical tube bundles or coils (spirals), mounted coaxially with the impeller shaft. When external heat exchangers are used, the reactor contents are pumped through it and back into the reactor. We consider here the CSTR the semi-batch reactor is treated in section 8.4.4. The steady-state heat balance for a first order reactor in a cooled CSTR can be written as follows ... 

In semi-batch or continuous operation, the feed rate allows control of the reaction course. Flence it plays an important role concerning the safety of the process. With an exothermal reaction, it is important to be able to limit the feed rate by technical means. One possibility is feed by portions, a method that is only applicable for semi-batch reactors. This mode of addition is the traditional way of limiting accumulation. In this case, the addition must be controlled by the conversion that is, the next portion is added only if the previous portion has been consumed by the reaction. Different criteria can be used to follow the reaction the temperature, the appearance of the reaction mass, chemical analysis, and so on. For a well designed process, the additions can also be performed on a time basis. 

Let us consider for instance an exothermic reaction A + B products carried out in a semi-batch reactor cooled from the wall (Tq). The tank initially containing A, B is added at a constant rate during the time tj. 

Fig. 4. Thermal runaway of a semi-batch reactor for the exothermic reaction A + B products. All characteristic times are normalized by the reaction time tR = 1/k Introduction time tj = 0.3. ...

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. 

The advantages of a semi-batch reaction, that is, a better selectivity in the case of multiple reactions or a better control of the reaction course in the case of exothermal reactions, are obtained if the reaction rate is controlled by the progressive addition of one or more reactants. Indeed, this objective can only be achieved if the added reactant is immediately converted and does not accumulate in the reactor [3]. Often a reaction is said to be feed controlled only because a reactant is fed. This is not always the case, since the feed rate must be adapted to the reaction rate, and the concentration of the added compound (B) is maintained at a low level during the reaction. 

An exothermal reaction is to be performed in the semi-batch mode at 80 °C in a 16 m3 water cooled stainless steel reactor with heat transfer coefficient U = 300 Wm"2 K . The reaction is known to be a bimolecular reaction of second order and follows the scheme A + B —> P. The industrial process intends to initially charge 15 000 kg of A into the reactor, which is heated to 80 °C. Then 3000 kg of B are fed at constant rate during 2 hours. This represents a stoichiometric excess of 10%.The reaction was performed under these conditions in a reaction calorimeter. The maximum heat release rate of 30Wkg 1 was reached after 45 minutes, then the measured power depleted to reach asymptotically zero after 8 hours. The reaction is exothermal with an energy of 250 kj kg-1 of final reaction mass. The specific heat capacity is 1.7kJ kg 1 K 1. After 1.8 hours the conversion is 62% and 65% at end of the feed time. The thermal stability of the final reaction mass imposes a maximum allowed temperature of 125 °C The boiling point of the reaction mass (MTT) is 180 °C, its freezing point is 50 °C. 

An exothermal reaction is to be performed in a 2.5 m3 stirred tank reactor as an isothermal semi-batch process at 80 °C. The specific heat of the reaction is 180kjkg 1, the specific heat capacity of the reaction mass is 1.8 kj kg 1 K 1, and the accumulation is 30%. The reaction is to be at atmospheric pressure and boiling point is 101 °C (MTT). There is a secondary reaction (decomposition) that is uncritical below 105 °C, that is, Tm4 = 105 °C. The decomposition energy is 150kjkg 1 and this decomposition releases 5 liters of a toxic, but not flammable, gas per kg reaction mass, measured at 25 °C and atmospheric pressure. 

For fast exothermic reactions, temperature control can be a problem. This is often solved by external circulation of part of Ihe reactor content through a heat exchanger, or by adding an internal heat exchange area. Alternatively, semi-batch operation can be applied, i.e., part of a reactant can be fed steadily over time or at certain intervals. Ihis also minimizes the occurrence of unwanted side reactions. 

The contacting pattern is not the only issne that determines the batch, semi-batch, or continnons mode of reactor operation. Sometimes, heat transfer and control of exothermic reactions may reqnire... 

Semi-batch operation, or the gradual addition of one or more reactants to a reactor, limit the quantity of reactants inside the vessel and increase safety when compared to batch processes in which all reactants are included in the initial batch charges (see Fig. 8.6). For an exothermic reaction, in a semi-batch process the total energy of reaction available in the reactor at any time is minimized. Gradual addition can help in controlling the rate and... 

An industrial batch reactor has neither an inflow nor an outflow of reactants or products while the reaction is being carried out. Batch reactions can be carried out in droplet microreactors, where nanoliters of fluid are individually manipulated using techniques such as electrowetting on dielectric (EWOD) and surface tension control. Semibatch reactors are used in cases where a by-product needs to be removed continuously and to carry out exothermic batch reactions where a reactant has to be added slowly. Microfluidics allows precise control of concentration and temperature, which allows batch and semi-batch reactions to be carried out in a continuous manner. Figure 1 shows the general components of a sin5)le industrial-reactor setup, compared with a laboratory-scale setup to carry out a reaction with microfluidic chips. 

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