PROBLEMS IN STRATEGY

Problems in Strategy Our discussion on strategy was hmited to the more straightforward aspects. This section has some challenging problems without worked answers. These are more difficult than the revision problems. 

Figure 16.10 shows another threshold problem that requires only hot utility. This problem is different in characteristic from the one in Fig. 16.9. Now the minimum temperature difference is in the middle of the problem, causing a pseudopinch. The best strategy to deal with this type of threshold problem is to treat it as a pinched problem. For the problem in Fig. 16.10, the problem is divided into two parts at the pseudopinch, and the pinch design method is followed. The only complication in applying the pinch design method for such problems is that one-half of the problem (the cold end in Fig. 16.10) will not feature the flexibility offered by matching against utility.

Strategy Problem 1 The wrong substitution pattern . Making aromatic compounds m-substituted with two o -directing groups is always a problem. What strategies can you suggest An example (TM 412) is the alkyl hahde used in the synthesis of some steroids. 

Adaptive Control. An adaptive control strategy is one in which the controller characteristics, ie, the algorithm or the control parameters within it, are automatically adjusted for changes in the dynamic characteristics of the process itself (34). The incentives for an adaptive control strategy generally arise from two factors common in many process plants (/) the process and portions thereof are really nonlinear and (2) the process state, environment, and equipment s performance all vary over time. Because of these factors, the process gain and process time constants vary with process conditions, eg, flow rates and temperatures, and over time. Often such variations do not cause an unacceptable problem. In some instances, however, these variations do cause deterioration in control performance, and the controllers need to be retuned for the different conditions. 

Definition / An expert system is a computer program that manipulates large amounts of symboHc knowledge using quaUtative techniques, to solve problems that can otherwise be solved only by expert human problem solvers. Expert systems capture the human problem solver s expertise in the form of domain-specific knowledge and domain-independent problem-solving strategies. 

Pohution prevention techniques must be evaluated through a thorough consideration of ah media, hence the term multimedia. This approach is a clear departure from previous pollution treatment or control techniques where it was acceptable to transfer a pollutant from one source to another in order to solve a waste problem. Such strategies merely provide short-term solutions to an ever increasing problem. As an example, air pollution control equipment prevents or reduces the discharge of waste into the air but at the same time can produce a solid (hazardous) waste problem. 

The Online Learning Center is a comprehensive, exclusive website that provides a wealth of electronic resources for instructors and students alike. For students, the OLC features tutorial, problem-solving strategies and assessment exercises for every chapter in the book that were developed by Ian Hunt and Rick Spinney from the University of Calgary. You can also access the Essential Student Partner from the OLC. Log on at www.mhhe.com/carey. 

Martin, D. J., and Michaelis. (1992). Research and Technology Strategy to Help Overcome the Environmental Problems in Relation to Transport Global Pollution Study. Luxembourg EEC. 

During our previous discussion of strategies for working synthesis problems in Section 8.9, we said that ids usually best to work a problem backward, or retrosyntheticnlly. Look at the target molecule and ask yourself, "What is an immediate precursor of this compound " Choose a likely answer and continue working backward, one step at a time, until you arrive at a simple starting material. Let s try some examples. 

STRATEGY The OH ions added to the buffer solution react with some of the acid of the buffer system, decreasing the amount of acid and increasing the conjugate base by the same amount. We solve this problem in two steps. First, we find the new molar concentrations of the acid and its conjugate base. Then we can rearrange the expression for Ka to obtain the pH of the solution, just as in Example 11.1. 

A central problem in complex materials systems of any kind involves testing to deteet flaws, analysis to predict their effect on remaining service life of the system, and repair strategies to overcome them. For the structural materials discussed in this chapter, these problems are uneharted territory in need of exploration by chemical engineers. 

A practical problem in solution preparation usually requires a different strategy than our standard seven-step procedure. The technician must first identify a suitable conjugate acid-base pair and decide what reagents to use. Then the concentrations must be calculated, using pH and total concentration. Finally, the technician must determine the amounts of starting materials. The technician needs a buffer at pH = 9.00. Of the buffer systems listed in Table 18-1. the combination of NH3 and NH4 has the proper pH range for the required buffer solution. 

This is an electrochemical stoichiometry problem, in which an amount of a chemical substance is consumed as electrical current flows. We use the seven-step strategy in summary form. The question asks how long the battery can continue to supply current. Current flows as long as there is lead(IV) oxide present to accept electrons, and the batteiy dies when all the lead(IV) oxide is consumed. We need to have a balanced half-reaction to provide the stoichiometric relationship between moles of electrons and moles of Pb02. 

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