Semi-batch reactions

The Center for Chemical Process Safety (CCPS) has identified the need for a publication dealing with process safety issues unique to batch reaction systems. This book, Guidelines for Process Safety in Batch Reaction Systems, attempts to aid in the safe design, operation and maintenance of batch and semi-batch reaction systems. In this book the terms batch and semi-batch are used interchangeably for simplicity. The objectives of the book are to ... 

Singh, J. (1993). "Assessing Semi-Batch Reaction Hazards. The Chemical Engineer, 537 (February 5), 21, 23-25. 

Aslund, B.L. and Rasmuson, A.C., 1992. Semi batch reaction crystallization of benzoic acid. American Institution of Chemical Engineers Journal, 38, 328-342. 

Continuous versus (semi)batch, reaction time, flow rates Materials of construction... 

The depolymerized Nylon used in the hydrogenation process was obtained by the ammonolysis of a mixture of Nylon-6 and Nylon-6,6 (described elsewhere, see reference 2). Hydrogenation reactions were conducted in 300 cc stirred pressure vessels. For semi-batch reactions hydrogen was constantly replenished to the reactor from a 1L reservoir to maintain a reactor pressure of 500 psig and all of the reactions were conducted with the same operating parameters and protocol. In continuous stirred tank studies hydrogen flow was controlled using... 

Dumping may be more practical for a small continuous reaction system than for a large batch or semi-batch reaction. 

Usually, isothermal calorimeters are used to measure heat flow in batch and semi-batch reactions. They can also measure the total heat generated by the reaction. With careful design, the calorimeter can simulate process variables such as addition rate, agitation, distillation and reflux. They are particularly useful for measuring the accumulation of unreacted materials in semi-batch reactions. Reaction conditions can be selected to minimize such accumulations. 

The procedure proposed in this guideline will be illustrated with a few examples of semi-batch reactions. 

A semi-batch reaction is designed to involve the following steps ... 

Heat flow calorimetry indicated that failure of the heater control to switch off and of cooling water to switch on (case (e)) was not a problem, since the steam heating is unable to exceed 100°C and, for this semi-batch reaction, the total temperature increase only reduces further any reactant accumulation. This means that cases (b), (f) and (d) are all worse than case (e). 

As with vapour pressure systems, for continuous or semi-batch reactions it may be possible to reduce the relief system size by taking into account the reduced accumulation of reacting mass See A2.4.1 and reference 10. 

A sulfonation reaction is performed as a semi-batch reaction in 96% sulfuric acid as a solvent. The total charge is 6000 kg with a final concentration of 3molT1. The reaction temperature is 110°C and Oleum 20% is added in a stoichiometric excess of 30% at constant rate over 4 hours. Under these conditions, the maximum accumulation of 50% is reached after approximately 3 hours addition. 

The heat balance of an isothermal semi-batch reaction is represented graphically in Figure 7.2. The maximum heat exchange rate (qeXiialx) calculated for a constant temperature of the heat carrier is also represented in the diagram. It increases linearly with time until the upper limit of the jacket is reached. In this example, the upper limit of the jacket is not reached during the feed time of four hours. 

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. 

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. 

Mrf represents the mass of the reaction mixture at the end of the feed, MrW the instantaneous mass of reactant present in the reactor, and Xal the fraction of accumulated reactant The ratio of both masses accounts for the correction of the specific energy, since the adiabatic temperature rise is usually calculated using the final reaction mass, that is, the complete batch. In Equation 2.5, the concentration corresponds to the final reaction mass this is also the case for the specific heat of reaction obtained from calorimetric experiments, which is also expressed for the total sample size. Since in the semi-batch reaction, the reaction mass varies as a function of the feed, the heat capacity of the reaction mass increases as a function of time and the adiabatic temperature rise must be corrected accordingly. 

Using the thermogram represented in Figure 7.7, assess the thermal safety of the substitution reaction example A + B —> P (see Section 5.3.1) performed as an isothermal semi-batch reaction at 80 °C with a feed time of 4 hours. At industrial scale, the reaction is to be in a 4 m3 stainless steel reactor with an initial charge of 2000kg of reactant A (initial concentration 3molkg 1). The reactant B (1000kg) is fed with a stoichiometric excess of 25%. 

Following these mles ensures a safe semi-batch reaction, even in cases of technical deviations or deviations from the normal operating conditions. 

Figure 7.13 Semi-batch reaction with feed rate adapted to the T -limitation at 113°C. Upper diagram heat release rate and cooling capacity on left scale, feed on right scale. Lower diagram temperature after cooling failure on left scale, feed and conversion on right scale time (h).

Ubrich, O., Srinivasan, B., Stoessel, F. and Bonvin, D. (1999) Optimization of semi-batch reaction system under safety constraint, in European Control Conference, European Union Control Association, Karlsruhe. 

The thermal time constant, defined in Equation 9.5, is useful for calculating heating and cooling times, which often take up a considerable amount of the cycle time of batch and semi-batch reactions. We start from Equation 9.4 that, after variable separation, combines with Equation 9.5, to become... 

For semi-batch reactions, it is important to check if the rating as class 2 is due to the control of the accumulation by the feed rate. Often reactions belong to class... 

A sulfonation reaction is performed as a semi-batch reaction in 96% sulfuric acid as a solvent The total charge is 6000 kg with a final concentration of 3 mol T1. The... 

For semi-batch reactions, it is wise to analyze a sample of the mixture present in the reactor before feed of the reactant is started. In fact, this mixture is often preheated to the process temperature before feeding, hence it is exposed to the process temperature and it could be useful to interrupt the process at this stage, in case of necessity. Such a thermogram assesses the thermal risks linked with such a process interruption (see case history at the beginning of this chapter). 

D. Ruppen, D. Bonvin, D.W.T. Rippin, Implementation of adaptive optimal control operation for a semi-batch reaction system, Comp. Chem. Eng. 22 (1998) 185-189. 

The model equations developed for the semi-batch reaction process can be modified for the batch process by ignoring the inflow of material and energy terms. 

Gladkii(16) at the State Scientific Research Institute of Industrial and Sanitary Gas Cleaning at Moscow did work on the three-phase calcium sulfite slurry oxidation system, finding that the liquid phase oxidation (pH 3.6-6) is first order with respect to the sulfite species. He pointed out, on the basis of pH versus time data from his semi-batch reaction, that the slurry oxidation had different periods in which either reaction kinetics or solid-liquid mass transfer controlled the oxidation rate. He also presented an omnibus empirical correlation between pH, temperature, and the liquid phase saturation concentration of calcium sulfite solution for predicting the slurry oxidation rate. The catalytic effect of manganese... 

Diadsorption of the two nitrile functions is another major point The DCA content increased with the ratio catalyst/dinitrile and, in semi-batch reaction, when the rate of addition of dinitrile was lowered (4). 

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