Isothermal Semi-batch Reactors

With single irreversible second-order reactions, the maximum of the heat release rate is reached at the beginning of the feed. At this stage, the heat exchange area may only be partially used, due to the increasing volume. This limits the effective available cooling capacity. Therefore, the knowledge of the maximum heat release 

Besides the heat release rate, the feed rate also affects the maximum reactant accumulation, a further important safety related parameter. The accumulation governs the temperature (T ) which may be reached in the case of a cooling failure. If the feed is immediately halted at the instant the failure occurs, the attainable temperature is expressed by 

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. 

Another question is important for the safety assessment At which instant is the accumulation at maximum In semi-batch operations the degree of accumulation of reactants is determined by the reactant with the lowest concentration. For single irreversible second-order reactions, it is easy to determine directly the degree of accumulation by a simple material balance of the added reactant. For bimolecular elementary reactions, the maximum of accumulation is reached at the instant when the stoichiometric amount of the reactant has been added. The amount of reactant fed into the reactor (Xp) normalized to stoichiometry minus the converted fraction (A), obtained from the experimental conversion curve delivered by a reaction calorimeter (X = Xth) or by chemical analysis, gives the degree of accumulation as a function of time (Equation 7.18). Afterwards, it is easy to determine the maximum of accumulation XaCfmax and the MTSR can be obtained by Equation 7.21 calculated for the instant where the maximum accumulation occurs [7]  

Since the accumulation is determined by a balance between feed rate and reaction rate (reactant depletion), it can be influenced by using different feed rates or different temperatures. This offers the possibility of optimizating the process conditions (discussed in Section 7.9). 

---

Figure 7.2 The different terms of the heat balance of an isothermal semi-batch reactor (in kW) as a function of time. The maximal cooling capacity of the reactor (qama,) obtained with cold water at 5°C is also represented. The difference between both curves q and qa represents the cooling effect by the feed. Its disappearance at the end of the feed at 4 hours is visible.

The use of calorimetry for on-line optimisation of isothermal semi-batch reactors. Chemical Engineering Science, 56 (17), 5147-56. 

This reaction is conducted in an isothermal semi-batch reactor. The desired product in this system is C. The objective is to convert as much as possible of reactant A by the controlled addition of reactant B, in a specified time //= 120 min. It is not appropriate to add all B initially because the second reaction will take place, increasing the concentration of the undesired by-product D. Therefore, to keep a low concentration of product D and at the same time increase the concentration of product C, the reactant B has to be fed in a stream with concentration = 0.2. A mechanistic model for this process can be found in [11]. 

Empirical grey models based on non-isothermal experiments and tendency modelling will be discussed in more detail below. Identification of gross kinetics from non-isothermal data started in the 1940-ties and was mainly applied to fast gas-phase catalytic reactions with large heat effects. Reactor models for such reactions are mathematically isomorphical with those for batch reactors commonly used in fine chemicals manufacture. Hopefully, this technique can be successfully applied for fine chemistry processes. Tendency modelling is a modern technique developed at the end of 1980-ties. It has been designed for processing the data from (semi)batch reactors, also those run under non-isothermal conditions. 

If relief sizing is for a continuous or semi-batch reactor, then it may be appropriate to use isothermal calorimetry to determine the amount of reactant accumulation under worst case conditions. The mass of the accumulation, rather than the "all-in 1 batch mass, can then be used for relief system sizing and this can reduce the required relief system size. It should sbe noted that it will still be necessary to carry out suitable adiabatic tests, as described below. Further information is given by Singh1101. 

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%. 

There are different ways of controlling a semi-batch reactor with a non-isothermal reaction course ... 

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 liquid-phase isothermal semi-batch operations with uniform injection rate, we can use the energy balance equation (Eq. 3.1.38) to determine the needed heating (or cooling) load and to estimate the isothermal HTN. Recall that the first term in Eq. 9.1.38 expresses the rate of heat transfer to the reactor... 

After completing the chemical reaction, we may need to cool the liquid contents of the batch or semi-batch reactor before discharging it. For a batch or semi-batch reactor equipped with internal coiled loops or a jacket containing finished product at temperature TReaction) the time to cool the liquid volume using an isothermal cooling fluid at... 

In this chapter the most important operation modes of reactors are considered. Models are developed by combining simple reaction kinetics for single-phase reactions with mass balances for five ideal model reactors the ideal batch reactor the semi-batch reactor the plug flow reactor the perfectly mixed continuous reactor and the cascade of perfectly mixed reactors. For isothermal conditions, conversions can be calculated on the basis of chemical kinetics only. 

The hydrogenation of toluene was performed at ambient temperature and pressure in a semi-batch isothermal stirred reactor with commercial 5% Ru-act catalyst. Hydrogen was automatically added to the system at the same rate at which it was consumed. Particle size of the catalyst used and efficiency of stirring ... 

In this chapter, we first consider uses of batch reactors, and their advantages and disadvantages compared with continuous-flow reactors. After considering what the essential features of process design are, we then develop design or performance equations for both isothermal and nonisothermal operation. The latter requires the energy balance, in addition to the material balance. We continue with an example of optimal performance of a batch reactor, and conclude with a discussion of semibatch and semi-continuous operation. We restrict attention to simple systems, deferring treatment of complex systems to Chapter 18. 

Although semi-analytical solutions are available in some cases [5], these are cumbersome and it is more usual to employ a numerical method. A simple example is presented below which illustrates the solution of the design equation for a batch reactor operated isothermally the adiabatic operation of the same system is then examined. 

Knowledge of these types of reactors is important because some industrial reactors approach the idealized types or may be simulated by a number of ideal reactors. In this chapter, we will review the above reactors and their applications in the chemical process industries. Additionally, multiphase reactors such as the fixed and fluidized beds are reviewed. In Chapter 5, the numerical method of analysis will be used to model the concentration-time profiles of various reactions in a batch reactor, and provide sizing of the batch, semi-batch, continuous flow stirred tank, and plug flow reactors for both isothermal and adiabatic conditions. 

The three types of isothermal heat flow calorimeters described above can be used to measure heat flow in semi-batch reactions, where one or more reactants are charged to the reactor and the other reactants are added at controlled rates throughout the reaction. With careful design the heat flow calorimeters can simulate process variables such as feed rate, stirring, distillation and reflux . 

---