Infeasible

The overlap in the shifted curves as shown in Fig. 6.15a means that heat transfer is infeasible. At some point this overlap is a maximum. This maximum overlap is added as a hot utility to correct the overlap. The shifted curves now touch at the pinch, as shown in Fig. 6.156. Since the shifted curves just touch, the actual curves are separated by AT ,in at this point (see Fig. 6.156). 

Looking at the heat flows in Fig. 6.18a, some are negative, which is infeasible. Heat cannot be transferred up the temperature scale. To... 

As the amount of temperature cross increases, however, problems are encountered, as illustrated in Fig. 7i8c. Local reversal of heat flow may be encountered, which is wasteful in heat transfer area. The design may even become infeasible. 

Figure 7.8 Designs with a temperature approach or small temperature cross can be accommodated in a single 1-2 shell, whereas designs with a large temperature cross become infeasible. (From Ahmad, Linnhoff, and Smith, Trans. ASME, J. Heat Transfer, 110 304, 1988 reproduced by permission of the American Society of Mechanical Engineers.)...

Figure 16.3 shows the situation below the pinch at the pinch. If a cold stream is matched with a hot stream with a smaller CP, as shown in Fig. 16.3a (i.e., a steeper slope), then the temperature differences become smaller (which is infeasible). If the same cold stream is matched with a hot stream with a larger CP (i.e., a less steep slope), as shown in Fig. 16.36, then temperature differences become larger (which is feasible). Thus, starting with ATmin at the pinch, for temperature differences to increase moving away from the pinch,... 

This is a case in which the tick-off" heuristic has caused problems. The match is infeasible, and its duty must be reduced to 6 MW to be feasible without either stream being ticked off (Fig. 16.226). 

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

Introduction Reactive distillation is a unit operation in which chemical reaction and distiUative separation are carried out simultaneously within a fractional distillation apparatus. Reactive distillation may be advantageous for liqiiid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reactants, when a reaction can be driven to completion by removal of one or more of the products as they are formed, or when the product recoveiy or by-product recycle scheme is complicated or macfe infeasible by azeotrope formation. 

The fuel value of most solid wastes is usually sufficient to enable self-supporting combustion, leaving only the incombustible residue and redlicing the volume of waste eventually consigned to sanitaiy landfills to only 10 to 15 percent of the original volume. The heat released by the combustion of waste can be recovered and utilized, although the cost of the recoveiy equipment or the distance to a suitable point of use for the heat may make its recoveiy economically infeasible. 

Applicability/Limitations. In-situ treatment can be used when it is uneconomical to haul or when infeasible or uneconomical to dig or pump the contaminated waste matrix for treatment in a reactor. This approach should be used whenever excavation or removal causes an increased threat to human health. It can reduce the cost of a remediation program. Because chemicals are applied to the contaminated waste matrix, specifically soil and groundwater, a potential exists for reaction with the soil. Permeability problems can occur as the result of precipitate formation. This can result in inadequate mixing of the contaminant with the treatment chemical. Gas generation may also occur. 

Immediately above the pinch, the number of rich streams is equal to the number of the MSAs, thus the feasibility criterion given by Eq. (5.8a) is satisfied. The second feasibility criterion (Eq. 5.12a) can be checked through Fig. 5.7. By comparing the values of with G, for each potential pinch match, one can readily deduce that it is feasible to match Si with either R or R2 immediately above the pinch. Nonetheless, while it is possible to match S2 with R2, it is infeasible to pair S2 with R] immediately above the pinch. Therefore, one can match Si with Ri and S2 with R2 as rich-end pinch exchangers. 

Similarly, the gaseous path diagram and MSAs are rejHesented by Fig. 7.11. Because of thermodynamic infeasibility, oil caimot be used to intercept any node. 

FMO theory was developed at a time when detailed calculations of reaction paths were infeasible. As many sophisticated computational models, and methods for actually locating the TS, have become widespread, the use of FMO arguments for predicting reactivity has declined. The primary goal of computational chemistry, however, is not tc... 

A question which chemical enumerators should not ignore is that of the extent to which their results are realistic in the physical world. Thus in [BlaCSla] it is stated that the number of alkanes (paraffins) with 40 carbon atoms is 62,491,178,805,831. Can we really be sure that all these compounds can exist or could it be that factors not catered for in the enumeration render some of them chemically infeasible In this connection we should note the paper [KleD81], in which it is shown jthat because of such factors the chemical tree enumerations by Polya and others give numbers that are consistently higher than the number of compounds that are in fact chemically possible. This does not detract from the mathematical value of these results it merely shows that care is needed in relating them to problems of real life. 

Experimental work with styrene in tubular reactors has been reported (39) where viscosities were relatively low due to conversions below 32%. However, Lynn ( ) has concluded that a laminar flow tubular reactor for styrene polymerization is probably technically infeasible due to the distortion in velocity... 

These asymptotic forms may be useful for conceptual studies, but the real design calculations must be based on the full Ergun equation. Turning to the case of compressible fluids, scaleup using geometric similarity with Sr = Sl = S is generally infeasible. Simply stated, the reactors are just too long and have too much inventory. 

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