Maximum production rate from batch reactors

On the other hand, if it is possible to use a temperature progression scheme and if one desires to obtain the maximum amount of the desired product per unit time per finit reactor volume, somewhat different considerations are applicable. If Ex > E2, one should use a high temperature throughout, but if E2 > Eu the temperature should increase with time in a batch reactor or with distance from the reactor inlet in a plug flow reactor. It is best to use a low temperature initially in order to favor conversion to the desired product. In the final stages of the reaction a higher temperature is more desirable in order to raise the reaction rate, which has fallen off because of depletion of reactants. Even though this temperature increases the production of the undesirable product, more of the desired product is formed than would otherwise be the case. Thus one obtains a maximum production capacity for the desired product. 

The maximum conversion of reactants which can be achieved in an isothermal batch reactor is determined by the position of thermodynamic equilibrium. If this conversion is regarded as unsatisfactory, the use of a simple batch reactor may be abandoned in favour of a reactor which permits removal of products from the reaction mixture. Alternatively, the reactor temperature may be changed to obtain a more favourable equilibrium however, this may result in an unacceptable reduction in the net reaction rate. Such conflicts are often resolved by the use of optimisation procedures (see Sect. 8). 

For the situation in which each of the series reactions is irreversible and obeys a first-order rate law, eqnations (5.3.4), (5.3.6), (5.3.9), and (5.3.10) describe the variations of the species concentrations with time in an isothermal well-mixed batch reactor. For consecutive reactions in which all of the reactions do not obey simple first-order or pseudo first-order kinetics, the rate expressions can seldom be solved in closed form, and it is necessary to resort to numerical methods to determine the time dependence of various species concentrations. Irrespective of the particular reaction rate expressions involved, there will be a specific time at which the concentration of a particular intermediate passes through a maximum. If interested in designing a continuous-flow process for producing this species, the chemical engineer must make appropriate allowance for the flow conditions that will prevail within the reactor. That disparities in reactor configurations can bring about wide variations in desired product yields for series reactions is evident from the examples considered in Illustrations 9.2 and 9.3. 

It is perhaps not generally realised that a switch from batch to continuous reactor design has intrinsic beneficial PI and safety implications when exothermic reactions and their associated runaway risks are involved. For batch operation, the time during which the reaction exotherm is generated is only a fraction of the batch cycle time. In order to control the reactions, it is imperative that provision is made to cope with the maximum likely heat evolution load so as to inhibit runaway. On the other hand, the heat exchanger provision for a continuous process operating at the same production rate needs to be considerably less than that for the batch equivalent, because the heat load is uniformly time distributed rather than being concentrated in a fraction of the batch residence time. Hence, continuous versions of batch processes have both safety and intensification benefits. 

After these estimates, it would be advisable to do bench scale experiments in a semi-batch reactor under well controlled conditions. One should measure the concentrations of the main product P and the byproduct X as functions of time, for various feed rates. From these data one may find the additional information require to predict more accurately the maximum feed rate that can be allowed to obtain a given selectivity. One will need a numerical solution of s. (3.25a) and (3.25b) now. One can also proceed in a purely empirical manner. Since Ais process is apparently not very sensitive to scale-dependent factors, the bench scale results may be applied with confidence on the commercial scale. 

This study demonstrates the principal possibility of hydrogen production in an outdoor photobioreactor (PhBR) incorporating a cyanobacterial mutant of Anabaena variabilis (PK84) under aerobic conditions. A computer-controlled helical tubular PhBR was operated over 4 summer months. A maximum rate of 80 mL H2 per hr per reactor volume (4.35 L) was obtained on a bright day (400 W m 2) from a batch culture. Also the culture was grown in chemostat mode at dilution rate D of 0.02 h 1. The maximum efficiency of conversion of light to chemical energy of H2 in the PhBR was 0.33% and 0.14% on a cloudy and a sunny day, respectively. 

Table 1 shows that the maximum ethanol concentration in this test was 67.7 g/1, in the outlet of the third reactor, for a flow rate of 8.1 ml/h. In this condition, the TRS and glucose concentrations were 3.3 and 0.8 g/1, respectively. The system is stable for all flow rates. The lowest flow rate, 8.1 ml/h, was more difficult to control, which was what led to some more pronounced oscillation of the variable values in this condition. It was expected in the continuous run that similar results than the ones obtained in the batch run will be achieved. As in batch operation, the cycle time is considerably longer than the reaction time, if the continuous process had reached a similar performance than the one obtained with the batch run in the continuous operation, we could save cycle time (times to clean, to fill ant to empty the reactor). Therefore, from the industrial point of view, the continuous process would lead to higher productivities. 

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