Safe Semi-batch Reactors

In semi-batch operation, many elements determine the process safety. Among them we mention the temperature control strategy, the feed control strategy, and also the choice of reactant(s) to be initially charged and the reactant(s) to be fed. 

Concerning the temperature control strategy, semi-batch reactions are often at constant temperatures (isothermal). Another simple temperature control strategy is the isoperibolic mode, where only the jacket temperature is controlled. In rare cases, other temperature control strategies, such as adiabatic or non-isothermal, are used. 

The feed may also be controlled in different ways constant feed rate, by portions, governed by the reactor temperature, and so on. 

These different temperature and feed control strategies and their impact on reactor safety, together with general rules for assessing and improving process safety, are presented below. The choice of the reactor temperature and feed rate is also of primary importance for safety and this point will be discussed in the last section of this chapter. 

---

It should be noted that there are cases in which some selectivity will be lost in choosing a semi-batch mode over a simple batch reactor. If the desired product decomposes by a consecutive reaction, the yield will be higher in the batch reactor [177]. If, on the other hand, the reactants are producing by-products by a parallel reaction, the semi-batch process will give the higher yield. In any case, if the heat production rate per unit mass is very high, the reaction can then be run safely under control only in a semi-batch reactor. 

The rules for intrinsically safe batch and semi-batch reactor operations are extensively discussed by Steensma [175] and Steinbach [177,178]. 

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. 

Steensma, M., "Safe Operation of a Cooled Semi-Batch Reactor, Slow Liquid-Liquid Reactions," Chem. Eng. Sci., 43,2125 (1988). 

Steensma, M., and K. R. Westerterp, "Thermally Safe Operation of a Semi-Batch Reactor for Liquid-Liquid Reactions—Slow Reactions," Ind. Eng. Chem. Res., 29, 1259-1270 (1990). 

Neither the process conditions nor the technical equipment of the reactor were adapted to the nature of the reaction. Moreover the effect of the increase in batch sized was overlooked. The process had to be changed to semi-batch operation in order to ensure a safe control of the reaction. 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

In the event of an alarm, an automatic sequence of actions should make the plant safe without the intervention of an operator. The sequence is likely to be simplest for continuous or semi-batch processes typically it may involve no more than stopping the reactant feeds, provided there are no problems with accumulation of reactants. Batch reactors are more difficult, particularly if they contain large amounts of unreacted material and are more likely to require the provision of protective measures such as emergency relief venting, or the provision of dump tanks with drown out facilities. 

The industrial process [23, 212, 213] of ethoxylation and propoxylation is usually a semi-batch process. The starter alcohol and KOH are mixed and water is removed by distillation. In a second step, monomers are fed into the reactor, where the feed rate is chosen so as to be able to remove the heat of polymerization and to keep the latent heat of polymerization of unreacted monomers in a safe state. By this process, homopolymers and random copolymers are accessible. Block copolymers are produced by successive feeds of the respective monomers. Catalyst is removed by addition of acids and subsequent crystallization and filtration of precipitated salts. An optional fourth step is the removal of volatile compounds by distillation. 

Extremely fast reactions are sometimes carried out in semi-batch fashion to prevent a runaway temperature. In this case, one reagent is continuously added to a cooled reactor containing the other reagent or catalyst, and the addition rate is controlled to maintain a given batch temperature or a given heat removal rate. For safe operation, the temperature is kept high enough to ensure that a low concentration of the added reactant is required. 

---