Optimum design practical considerations

The various mathematical methods for determining optimum conditions, as presented in this chapter, represent on a theoretical basis the conditions that best meet the requirements. However, factors that cannot easily be quantitized or practical considerations may change the final recommendation to other than the theoretically correct optimum condition. Thus, a determination of an optimum condition, as described in this chapter, serves as a base point for a cost or design analysis, and it can often be quantitized in specific mathematical form. From this point, the engineer must apply judgment to take into account other important practical factors, such as return on investment or the fact that commercial equipment is often available in discrete intervals of size. 

It is important to match mixing equipment capabilities with process requirements. While it is desirable to have an optimum design and operating conditions for every step in the process sequence, it is seldom practical to do so. For example, specialty and pharmaceutical processes require the use of multipurpose reactors. An important consideration is to understand how less-than-ideal equipment wiU function in aU stages of operation. 

Summary. It is possible to rapidly estimate the economically optimum number and size of CSTRs in series required to achieve a given disturbance attenuation. The design obtained from this analysis will always have equal-sized tanks due to the dependence of the constraints on the product of the tank residence times. The tanks will typically be between 10 and 30 m in size with residence times between 2 minutes and 1 hour. This is consistent with industrial practice and contradicts the recommendation in the literature, based on frequency response arguments, that tank sizes should be split in a ratio of about 1 4 or greater (Shinskey, 1973 Moore, 1978 McMillan, 1984). This provides a simple illustration of the ability of the integrated approach to balance operability and economic considerations in a way that qualitative argument cannot. 

In actual practice, however, there are two distinct Topliss Schemes, namely (a) For aromatic substituents and (Z>) For aliphatic side-chain substituents. It is pertinent to mention here that the said two schemes were so meticulously designed by taking into consideration both electronic and hydrophobicity features (i.e., substituents) with acommon objective to arrive at the optimum biological active substituents . 

The past decade has seen considerable advances in the development of synthetic metallopor-phyrin complexes. " We have now designed numerous practical and efficacious methods of synthesizing porphyrins with halogens at the o-aryl and also the P-pyrrole positions. The methodology provides facile access to a large number of porphyrins in optimum yields and purity (Scheme 29.25). Demetallation with trifluoroacetic acid followed by insertion of iron yielded the corresponding hemins. ... 

At the molecular level, the choice of surfactant for a given application must take into consideration the type of emulsion desired and the nature of the oil phase. As a general rule, oil-soluble surfactants will preferentially produce w/o emulsions while water-soluble surfactants yield o/w systems. Because of the role of the interfacial layer in emulsion stabilization, it is often found that a mixture of surfactants with widely differing solubility properties will produce emulsions with enhanced stability. Finally, it is usually safe to say that the more polar the oil phase, the more polar will be the surfactant required to provide optimum emulsification and stability. Such rules of thumb, while having great practical utility, are less than satisfying on a theoretical level. One would really like to have a neat, quantitative formula for the design of complete emulsion systems. A number of attempts have been made over the years to develop just such a quantitative approach to surfactant selection. Some such approaches are briefly discussed. 

The understanding of chemical equilibrium and optimum problems is extremely important from an academic and practical point of view, particularly in reactor design and control. However, the relationship between these two problems is not well understood. Historically, equilibrium-optimum considerations have been proclaimed in the famous Le Chatelier s principle. In chemistry, this principle is used to influence reversible chemical reactions. For example, the equilibrium conversion of an exothermic reaction, that is, a reaction liberating heat, is more favorable at lower temperature, so cooling of the reaction mixture shifts the equilibrium to the product side. Le Chatelier s principle is part of the curriculum of university students in chemistry and chemical engineering. Unfortunately, the relation between this principle and the analysis of equilibria and optima often is not presented clearly. In particular, there is no explicit explanation of how to apply Le Chatelier s principle, which has been formulated for closed systems at equilibrium (so at zero value of the net overall reaction rate), to continuous-flow reactors, in which the reaction rate certainly is not zero. This section is based on an article by Yablonsky and Ray (2008), which aims at bridging this gap between the concepts of equHihrium and optimum. 

Theory vs. Reality. Further questions and dilemmas arise in the practical implementation of theoretically optimum-solutions. These include considerations like environmental conditions, mass-, volume-, harnessing- and power consumption limitations, parameters of available components (e.g. radiation hardened components), costs of hardware/software development, implementation, manufacturing, functional validation and system integration. The definition of fault-containment regions [4] within a complex system (e.g. Philae lander) is also an important step in preparing the design concept. 

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