Chemical reactors batch reactor optimization

L. L. Simon, M. Introvigne, U. Fischer, K. Hungerbuhler, (2008), Batch reactor optimization under liquid swelling safety constraint, Chemical Engineering Science, 63, 770. 

Batch reactor optimization (Luyben, 1996) is a common issue in chemical engineering. One very typical problem is finding the residence time for isothermal batch reactors that maximizes/minimizes the conversion of an intermediate compound. 

The reactions are still most often carried out in batch and semi-batch reactors, which implies that time-dependent, dynamic models are required to obtain a realistic description of the process. Diffusion and reaction in porous catalyst layers play a central role. The ultimate goal of the modehng based on the principles of chemical reaction engineering is the intensification of the process by maximizing the yields and selectivities of the desired products and optimizing the conditions for mass transfer. 

Most accidents in the chemical and related industries occur in batch processing. Therefore, in Chapter 5 much attention is paid to theoretical analysis and experimental techniques for assessing hazards when scaling up a process. Reaction calorimetry, which has become a routine technique to scale up chemical reactors safely, is discussed in much detail. This technique has been proven to be very successful also in the identification of kinetic models suitable for reactor optimization and scale-up. 

The RC1 is an automated laboratory batch/semi-batch reactor for calorimetric studies which has proven precision. The calorimetric principle used and the physical design of the system are sound. The application of the RC1 extends from process safety assessments including calorimetric measurements, to chemical research, to process development, and to optimization. The ability of the RC1 to generate accurate and reproducible data under simulated plant scale operating conditions may result in considerably reduced testing time and fewer small scale pilot plant runs. 

The conversions, selectivities, and kinetics are ideally obtained in a small batch reactor, the operating conditions and catalyst formulation are determined from a bench-scale continuous reactor, the process is tested and optimized in a pilot plant, and finally the plant is constructed and operated. While this is the ideal sequence, it seldom proceeds in this way, and the chemical engineer must be prepared to consider aU aspects simultaneously. 

Just like chemical processes, biocatalytic reactions are performed most simply in batch reactors (Figure 5.5a). On a lab scale and in the case of inexpensive or rapidly deactivating biocatalysts, this is the optimal solution. If the biocatalyst is to be recycled, but the mode of repeated batches is to be maintained, a batch reactor with subsequent ultrafiltration is recommended (batch-UF reactor Figure 5.5b). The residence times of catalyst and reactants are identical in all batch reactor configurations. 

This is the most common mode of addition. For safety or selectivity critical reactions, it is important to guarantee the feed rate by a control system. Here instruments such as orifice, volumetric pumps, control valves, and more sophisticated systems based on weight (of the reactor and/or of the feed tank) are commonly used. The feed rate is an essential parameter in the design of a semi-batch reactor. It may affect the chemical selectivity, and certainly affects the temperature control, the safety, and of course the economy of the process. The effect of feed rate on heat release rate and accumulation is shown in the example of an irreversible second-order reaction in Figure 7.8. The measurements made in a reaction calorimeter show the effect of three different feed rates on the heat release rate and on the accumulation of non-converted reactant computed on the basis of the thermal conversion. For such a case, the feed rate may be adapted to both safety constraints the maximum heat release rate must be lower than the cooling capacity of the industrial reactor and the maximum accumulation should remain below the maximum allowed accumulation with respect to MTSR. Thus, reaction calorimetry is a powerful tool for optimizing the feed rate for scale-up purposes [3, 11]. 

Toulouse, C., Cezerac, J., Cabassud, M., Lann, M.V.L. and Casamatta, G. (1996) Optimization and scale-up of batch chemical reactors impact of safety constraints. Chemical Engineering Science, 51 (10), 2243-52. 

N. Aziz, M.A. Hussain, I.M. Mujtaba, Optimal control of batch reactor comparison of neural network based GMC and inverse model control approach, in Proceedings of the Sixth World Congress of Chemical Engineering, Melbourne, Australia, 23-27 September 2001. 

C. Filippi, J.L. Greffe, J. Bordet, J. Villermaux, J.L. Barnay, B. Ponte, and C. Georgakis. Tendency modeling of semi-batch reactors for optimization and control. Chemical Engineering Science, 41 913-920, 1986. 

R. Luus and O.N. Okongwu. Towards practical optimal control of batch reactors. Chemical Engineering Journal, 75 1-9, 1999. 

The main physicochemical processes in thin-film deposition are chemical reactions in the gas phase and on the film surface and heat-mass transfer processes in the reactor chamber. Laboratory deposition reactors have usually a simple geometry to reduce heat-mass transfer limitations and, hence, to simplify the study of film deposition kinetics and optimize process parameters. In this case, one can use simplified gas-dynamics reactor such as well stirred reactor (WSR), calorimetric bomb reactor (CBR, batch reactor), and plug flow reactor (PFR) models to simulate deposition kinetics and compare theoretical data with experimental results. 

Katz, S. 1960a. Best control actions in batch or pipeline reactors. Conference on Optimization Techniques in Chemical Engineering, New York University, May 18, 1960, pp. 57-78. 

This paper presents an on-line model based level control of a batch reactor with reaction rate uncertainties. The analyzed chemical batch process is catalyzed by a catalyst which decomposes in the reactor therefore it is fed several times during the batch. The chemical reaction produces a vapour phase by-product which causes level change in the system. The on-line control method is based on the shrinking horizon optimal control methodology based on the detailed model of the process. The results demonstrate that the on-line optimization based control strategy provides good control performance despite the disturbances. 

In fine and specialty chemicals production the process cost is a less relevant aspect on the other hand, the time taken to realize the industrial production is typically the critical factor. This aspect, together with the limited resources dedicated for R D, determine the preference in companies for multipurpose catalysts with respect to optimized, but more specific, catalysts. This applies also to the process itself where simpler, not optimized, batch reactors are preferred to better, but less flexible, more complex operations. This is one of the key aspects to consider in evaluating the use of multiphase operations in the synthesis of this class of chemicals. 

More detailed treatment of optimizing the operation of batch reactors can be found in R. B. Aris, An Introduction to Chemical Reactor Analysis, Prentice-Hall, Englewood Cliffs, NJ, 1960. 

Through the use of a model for a batch reactor for a particularly complex reaction, we have demonstrated the value of modeling in optimization of process conditions and in evaluation of possible hazards. For a very complex system like the present one, it is most probably easier and more cost effective to do the modeling than to run the experiments needed for proper analysis. To save laboratory data acquisition time, it is always better to plan an experimental strategy based on the anticipated need in advance. This model has been successfully used in two scale-ups. Data from these scale-ups have been used to refine the model. These refinements included a better understanding of the chemistry of the process. Plots similar to the ones presented in Figures 6-10 were used in the Reactive Chemicals Review of the present process. 

One common problem in chemical engineering is the optimization of batch reactors (Luyben, 1996). In Section 3.15.1, we showed how to find the residence time for an isothermal batch reactor that maximizes the quantity of an intermediate compound. 

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