Engineering reaction

Achievements made wifhin fhe field of reaction engineering will increase fhe applicability of biocatalysts even more. For example, the use of microreactors is still in its infancy. Cascade catalysis and multi step conversions [81], a common domain of biocatalysis, will boost the application of biocatalysis for the transformation of highly reactive compounds or intermediates. Moreover, this might diminish operating time and costs as well as consumption of auxiliary chemicals and use of energy. For example, Bacher et al. published fhe six-step synthesis of labelled riboflavin using eight different enzymes in one reaction vessel [82]. 

Since before recorded history, we have been using chemical processes to prepare food, ferment grain and grapes for beverages, and refrne ores into utensils and weapons. Our ancestors used mostly batch processes because scaleup was not an issue when one just wanted to make products for personal consumption. 

The throughput for a given equipment size is far superior in continuous reactors, but problems with transients and maintaining quaHly in continuous equipment mandate serious analysis of reactors to prevent expensive malfunctions. Large equipment also creates hazards that backyard processes do not have to contend with. 

Not until the industrial era did people want to make large quantities of products to sell, and only then did the economies of scale create the need for mass production. Not until the twentieth century was continuous processing practiced on a large scale. The first practical considerations of reactor scaleup originated in England and Germany, where the first large-scale chemical plants were constmcted and operated, but these were done in a trial-and-error fashion that today would be unacceptable. 

In the United States two major textbooks helped define the subject in the early 1960s. The first was a book by Levenspiel that explained the subject pictoriaUy and included a large range of applications, and the second was two short texts by Aris that concisely described the mathematics of chemical reactors. While Levenspiel had fascinating updates 

The selection of a suitable solvent is critical, as solvents may also strip essential hydration shells from the enzyme. This is similar to short-chain alcohols. Although numerous organic solvents can be used, several aspects have to be considered when choosing a suitable solvent. These include solvent compatibility, inertness, low density, toxicity, and flammability (Adamczak and Krishna, 2004). Lipases have been tested in various organic solvents such as tcrt-butanol and n-hexane (Eltaweel et al., 2005 Herndndez-Rodriguez et al., 2009 Li et al., 2006). 

Despite their positive effects on enzyme stability, excessive use of organic solvents decreases the yield, which is mainly due to the dilution of the substrates (Ji et al., 2006). Samukawa et al. (2000) tested the use of different solvents on Lipozyme TL activity, and it was found that the yield increased along with the hydrophobic-ity of the solvent. In contrast, hydrophilic solvents are much less efficient (Doukyu and Ogino, 2010 Klibanov, 1997). For example, the use of acetone (log P = -0.23) showed a less than 20% yield compared to 80% when n-hexane was used. The negative effect of hydrophobicity is due to the solvent s interaction with the essential water layer surrounding the lipase molecule (Iso et al., 2001), which results in unfavorable conformational changes in the enzyme structure and reduces activity. 

On the other hand, from an environmental point of view, the use of organic solvents should be minimized because of their harmful environmental impacts. In addition, they are usually expensive and require separation from the product. The need for high yields with easy separation to produce products of higher purity while using environmentally friendly processes has led to a search for new technologies and new solvents. Any alternative to organic solvents should dissolve reaction substrates and reduce the excess alcohol inhibition effect and, at the same time, avoid a difficult separation of the solvent. In this regard, supercritical fluids (SCFs) have been put forward and have shown potential (Romero et al 2005). Further discussion on the use of SCFs, as a reaction medium, is covered in Section 6.5. 

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Levenspiel, O., Chemical Reaction Engineering, 2d ed., Wiley, New York, 1972. 

M. Koukohk and J. Matek, Proceedings of the 4th European Symposium on Chemical Reaction Engineering, Bmssels, 1968, pp. 347—359. 

The minimum polydispersity index from a free-radical polymerization is 1.5 if termination is by combination, or 2.0 if chains ate terminated by disproportionation and/or transfer. Changes in concentrations and temperature during the reaction can lead to much greater polydispersities, however. These concepts of polymerization reaction engineering have been introduced in more detail elsewhere (6). 

Early ia the development of chemical reaction engineering, reactants and products were treated as existing ia single homogeneous phases or several discrete phases. The technology has evolved iato viewing reactants and products as residing ia interdependent environments, a most important factor for multiphase reactors which are the most common types encountered. 

H. Simka, P. Merchant, P. Futerko, and K. F. Jensen, 13th International Symposium on Chemical Eeaction Engineering, Reaction Engineering Science and... 

M. E. Edwards, Chemical Reaction Engineering of Polymer Processing Reaction Injection Moulding Inst. Chem. Eng. Symp. Ser. 8(87), 783—796 (1984). 

Reaction Engineering. Electrochemical reaction engineering considers the performance of the overall cell design ia carrying out a reaction. The joining of electrode kinetics with the physical environment of the reaction provides a description of the reaction system. Both the electrode configuration and the reactant flow patterns are taken iato account. More ia-depth treatments of this topic are available (8,9,10,12). 

Cropley, J. B. Heuristic Approach to Complex Kinetics, pp. 292-302 in Chemical Reaction Engineering—Houston, ACS Symposium Series 65, American Chemical Society, Washington, DC (1978). 

In turbulent flow, axial mixing is usually described in terms of turbulent diffusion or dispersion coefficients, from which cumulative residence time distribution functions can be computed. Davies (Turbulence Phenomena, Academic, New York, 1972, p. 93), gives Di = l.OlvRe for the longitudinal dispersion coefficient. Levenspiel (Chemical Reaction Engineering, 2d ed., Wiley, New York, 1972, pp. 253-278) discusses the relations among various residence time distribution functions, and the relation between dispersion coefficient and residence time distribution. 

Dente and Ranzi (in Albright et al., eds.. Pyrolysis Theory and Industrial Practice, Academic Press, 1983, pp. 133-175) Mathematical modehng of hydrocarbon pyrolysis reactions Shah and Sharma (in Carberry and Varma, eds.. Chemical Reaction and Reaction Engineering Handbook, Dekker, 1987, pp. 713-721) Hydroxylamine phosphate manufacture in a slurry reactor Some aspects of a kinetic model of methanol synthesis are described in the first example, which is followed by a second example that describes coping with the multiphcity of reactants and reactions of some petroleum conversion processes. Then two somewhat simph-fied industrial examples are worked out in detail mild thermal cracking and production of styrene. Even these calculations are impractical without a computer. The basic data and mathematics and some of the results are presented. 

A few excellent books are also available on reaction engineering in the widest sense and from a fundamental point of view. These books treat the subject with mathematical rigor, yet are too inclusive to have any space left for details on experimental procedures. Here, the reader can find more insight and practical examples on the development and scale-up of... 

A surface scientist working on molecular scale of catalysis may become disappointed by seeing how little quantitative use can be made in reaction engineering of the newest and theoretically most interesting instrumental techniques. It may be of some solace to them that it is not their fault. The quantitative consequences of important insights will have to evolve from much closer cooperation between physicists, chemists and engineers. This will require people reasonably well informed in all three fields. 

Brotz, W. 1965, Fundamentals of Chemical Reaction Engineering. Addison-Wesley, Reading, MA... 

Carberry, J. J. 1976, Chemical and Catalytic Reaction Engineering. McGraw-Hill, New York. 

Fogler, H. S. 1998, Elements of Chemical Reaction Engineering 2nd Edition, Prentice Hall, Inc., Englewood Cliffs, NJ... 

In reaction engineering, laboratory catal54ic reactors are tools or instruments to study how catalysts behave in some desired reaction. Quantitatively, the investigator wants to know how much of the desired product can be made per unit weight of catalyst, how much raw material will be used, and what byproducts will be made. This is the basic information needed to estimate the costs and profitability of the process. The economic consequence of our estimates also forces us to clarify what the rate limiting steps are, and how much transfer processes influence the rates, i.e., everything that is needed for a secure scale-up. Making the... 

Other correlations exist and each is good for the particular range where the experimental measurements were made. For searching out other more appropriate correlations, the reader is referred to the copious literature and to the major books on reaction engineering. 

Many authors contributed to the field of diffusion and chemical reaction. Crank (1975) dealt with the mathematics of diffusion, as did Frank-Kamenetskii (1961), and Aris (1975). The book of Sherwood and Satterfield (1963) and later Satterfield (1970) discussed the theme in detail. Most of the published papers deal with a single reaction case, but this has limited practical significance. In the 1960s, when the subject was in vogue, hundreds of papers were presented on this subject. A fraction of the presented papers dealt with the selectivity problem as influenced by diffitsion. This field was reviewed by Carberry (1976). Mears (1971) developed criteria for important practical cases. Most books on reaction engineering give a good summary of the literature and the important aspects of the interaction of diffusion and reaction. 

For additional details of the many possibilities, the reader should refer to the basic books on Reaction Engineering mentioned in the References. 

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