Optimization of reactors

Optimization of Reactor Conversion for Muitiple Reactions Producing Byproducts... 

The overall inventory. In the preceding chapter, the optimization of reactor conversion was considered. As the conversion increased, the size (and cost) of the reactor increased, but that of separation, recycle, and heat exchanger network systems decreased. The same also tends to occur with the inventory of material in these systems. The inventory in the reactor increases with increasing conversion, but the inventory in the other systems decreases. Thus, in some processes, it is possible to optimize for minimum overall inventory. In the same way as reactor conversion can be varied to minimize the overall inventory, the recycle inert concentration also can be varied. 

The comparison of the results obtained from model particle systems with experience of biological systems shows a similar tendency on many points. Therefore it proved to be very advantageous for the basic investigations, especially for the comparison of different reactor types, to use suitable model particle systems with similar properties to those of biological material systems. This permitted the performance of test series under technically relevant operating conditions, similar to those prevailing in bioreactors, in a relatively short time. The results are more reproducible than in biological systems and therefore permit faster and more exact optimization of reactors. 

Once the structure of the recycle and separation has been established, some important degrees of freedom can be optimized that can have a very significant effect on the overall process economics. Start by considering the optimization of reactor conversion. 

Obviously, the use of purges is not restricted to dealing with impurities. Purges can also be used to deal with byproducts. As with the optimization of reactor conversion, changes in the recycle concentration of impurity might change the most appropriate separation sequence. 

Figure 13.23 Optimization of reactor conversion and recycle impurity concentration using a univariate search.

Optimization of reactor geometry in terms of charging and removal of materials... 

The activation energy for the side reaction is 84.1 kJ/mol, making it nearly 3 times more responsive to temperature than the main reaction. At 453 K, where the side reaction starts playing an important role, this translates to a 5 percent increase in the side reaction per kelvin compared to less than 2 percent for the main reaction. At very high temperatures the side reaction completely dominates the picture. Control and optimization of reactor temperature is essential for economic operation. 

Shelokar, P. S., Jayaraman, V. K. and Kulkami, B. D. (2003). Multi-objective optimization of reactor-regenerator system using ant algorithm. Pet. Sci. TechnoL, 21, pp. 1167-1184. 

Optimization of reactor performance based on the following criteria substrate conversion, selectivity, space-time yield, enzyme and coenzyme consumption. 

Chapter 7 briefly dealy with the catalyst deactivation problem and presents elementary information on the use of heterogeneous models for the optimization of reactors experiencing catalyst deactivation. 

Y. T. Shih and P. W. Carr, Flow-Rate Dependence of Post-Column Reaction Chromatographic Detectors and Optimization of Reactor Length for Slow Chemical Reaction. Anal. Chim. Acta, 167 (1985) 144. 

In fact, enzyme inactivation by exposure to the reaction temperature is unavoidable, since temperature exerts opposite effects o enzyme aetivity and stability so that operation temperature is always a compromise between the two. Enzyme inactivation during reaction operation occurs no matter how stable the enzyme is, since in any case operation is prolonged to the point in which a significant fraction of the initial activity is lost. Actually, the residual activity at which the biocatalyst should be disposed off is a quite relevant criterion for the optimization of reactor operation. 

A different reactor can change the regime. A good aim is to look for a reactor which operates in Regime I, i.e. high kio where the chemistry is not restrained by mass transfer. The final choice must obviously take account of requirements for heat transfer, particle suspension, foam control, materials of construction and a feasible size for the full scale equipment. Only in the simplest cases are there sufficient degrees of freedom to allow economic optimization of reactor type. 

Two important questions that arise when dealing with the optimization of reactor superstructures thus arise ... 

Optimization of reactor operation policy is of paramount importance if improvement of product quality and increase of business profits are sought. In very specific terms, optimization of the reactor operation conditions is equivalent to producing the maximum amount of polymer product, presenting the best possible set of end-use properties, with minimum cost under safe and environmentally friendly conditions. This optimum solution is almost always a compromise. Increase of polymer productivity is usually obtained with the increase of the operational costs (increase of reactor volumes, reaction temperatures and reaction times, for instance). Besides, the simultaneous improvement of different end-use properties is often not possible (the improvement of mechanical performance is usually obtained through increase of molecular-weight averages, which causes the simultaneous increase of the melt viscosity and decrease of product processibihty). Therefore, the optimization can only be performed in terms of a relative balance among the many objectives that are pursued. 

Measurement techniques for the resolution of concentration and temperature profiles in chemical reactors with heterogeneously catalyzed gas-phase reactions are a very useful tool not only for a better understanding of the reaction sequence and derivation of reaction kinetics but also for the elucidation of the coupling between catalytic reaction kinetics and mass and heat transport. The combination of numerical simulations of the reactive flow in catalytic reactors incorporating microkinetic reaction schemes and those recently developed invasive and noninvasive in situ techniques can today support the optimization of reactor design and operating conditions in industrial applications. 

By a plot of 1/ Ta versus Xa, the reaction time needed to reach a certain conversion is easily determined by graphical integration (area below the curve) as schematically depicted in Figure 4.10.14. Of course, for an optimization of reactor design, it is always helpful to know the exact rate expression, but for a brief estimation the method outlined by Figure 4.10.14 is helpful. 

As the kinetics of a chemical reaction are influenced by a multitude of different parameters such as pressure, temperature, concentrations of the reactants, mole-cularity and presence and type of a catalyst, the kinetics of any individually given reaction are to be evaluated empirically - sometimes including the development of an appropriate functional correlation. At the same time, there is a strong interest in the kinetics of a reaction, first to better understand the reaction mechanism and second to facilitate a basis for the optimization of reactor designs and process parameters. 

Undoubtedly, much can be gained from the optimization of reactor operating conditions. Often neglected, however, is the fact that much more can be gained by optimizing the reactor size at the same time. This fact will be pointed out and illustrated whenever appropriate to emphasize the importance of optimizing with respect to both reactor size and operating conditions. 

---