CONTENTS Section 3 PRACTICAL PROBLEMS

The effective control of corrosion reactions must be based on an understanding of the mechanism of such reactions and on the application of this knowledge to practical problems. The work, regarded as a whole, represents an attempt, therefore, to present the subject of corrosion as a synthesis of corrosion science and corrosion engineering. Thus in the planning of the content an attempt has been made to strike a suitable balance between the primarily scientific and the primarily practical aspects, and so the nature of individual sections ranges from the fundamental and theoretical to the essentially practical. 

The tables and plots shown in the previous section are clear and have some inmitive content. But in the age of computers we wish to reduce all such information to equations, which can then be used by computers to interpolate and extrapolate the data and to apply it to practical problems. Furthermore, these equations will allow us to estimate VLE for systems for which we have little or no data, often with considerable success. 

A principal assumption for similarity is that there exists a viscous boundary layer in which the temperature and species composition depend on only one independent variable. The velocity distribution, however, may be two- or even three-dimensional, although in a very special way that requires some scaled velocities to have only one-dimensional content. The fact that there is only one independent variable implies an infinite domain in directions orthogonal to the remaining independent variable. Of course, no real problems have infinite extent. Therefore to be of practical value, it is important that there be real situations for which the assumptions are sufficiently valid. Essentially the assumptions are valid in situations where the viscous boundary-layer thickness is small relative to the lateral extent of the problem. There will always be regions where edge effects interrupt the similarity. The following section provides some physical evidence that supports the notion that there are situations in which the stagnation-flow assumptions are valid. 

The contents of the present contribution may be outlined as follows. Section 6.2.2 introduces the basic principles of coupled heat and mass transfer and chemical reaction. Section 6.2.3 covers the classical mathematical treatment of the problem by example of simple reactions and some of the analytical solutions which can be derived for different experimental situations. Section 6.2.4 is devoted to the point that heat and mass transfer may alter the characteristic dependence of the overall reaction rate on the operating conditions. Section 6.2.S contains a collection of useful diagnostic criteria available to estimate the influence of transport effects on the apparent kinetics of single reactions. Section 6.2.6 deals with the effects of heat and mass transfer on the selectivity of basic types of multiple reactions. Finally, Section 6.2.7 focuses on a practical example, namely the control of selectivity by utilizing mass transfer effects in zeolite catalyzed reactions. 

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Stress corrosion cracking (see also in Section 8.6 in pressurized ammonia vessels and tanks is a problem which has been discussed in many papers [1252]-[1265], [1271]-[1276], The mechanism of this phenomenon, the influence of water, and the role of oxygen are not yet completely understood, in spite of extensive research. A review is given in [1277]. As it is generally accepted that addition of water may inhibit stress corrosion [1263], [1264] it has become a widely used practice to maintain a water content of 0.2% in transport vessels [1264]. Protection may also be achieved by aluminum or zinc metal spray coating [1275], [1277], More recent research [1273], [1277], however, has shown that water may not give complete protection. 

This constitutes an MCDM problem, which is solved by selecting for all experimental conditions, i.e. solvent compositions, the response giving the worst result. At each experimental condition the lowest value of the response is shown in bold. Windows in which the separation is possible are outlined in this way and the highest value of the bold profile (indicated by the asterisk) is chosen as the optimal one. In practice the chromatographer would, for this example, be well advised to choose a slightly lower modifier content since the separation would be more robust (see Section 6.2). 

Poly( inyl chloride), poly(acrylonitrile) and the high acrylonitrile copolymers have presented the major problems with respect to reducing the residual monomer content to extremely low levels. These are in the glas state tmder the conditions where monomer removal must be carried out. In principle, the temperature should be raised above the glass temperature to facilitate monomer removal. In practice, however, the systems are usually lattices or slurries of suspension polymer and coagulation could become a problem. In any case, both the rubbery and glassy states must be considered in any discussion of the monomer removal problem. The basic principles of the transport of gases in both situations have been presented briefly and with appropriate literature references in the introductory section of this review. 

Figure This Math Challenges for Families (www.figurethis.org) is designed for middle school students and their families. Math challenges are brightly illustrated and emphasize practical application of math skills. Kids use step-by-step problem solving to answer questions like, "How fast does my heart beat " and "How many Fridays are Friday the thirteenths " Each challenge includes hints, answer explanation, extra questions, and suggestions for further reading, as well as information about how its content relates to the real world. A parents section includes an article on using children s literature to study math concepts.

The usefulness of this text will be greatly enhanced if you supplement it with information from current periodicals, basic references, statistical collections, abstracting services, indexes, and electronic data obtained from the World-Wide Web. This will allow you to update the data given in the text and—even more important—provide practice in finding the information chemical professionals need in day-to-day analysis and problem solving. The most important sources in this section are labeled with two asterisks and the next most important with one asterisk. If you are using the book for self-directed study, you should subscribe or have access to the doubled-asterisked periodicals and read the single-asterisked periodicals, become acquainted with the contents of the references with asterisks, locate a library with national and international references, and obtain a copy of the Statistical Abstract of the United States (Sections 0.1.1 and 0.1.2). 

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