Trading off

Increasing the chosen value of process energy consumption also increases all temperature differences available for heat recovery and hence decreases the necessary heat exchanger surface area (see Fig. 6.6). The network area can be distributed over the targeted number of units or shells to obtain a capital cost using Eq. (7.21). This capital cost can be annualized as detailed in App. A. The annualized capital cost can be traded off against the annual utility cost as shown in Fig. 6.6. The total cost shows a minimum at the optimal energy consumption. 

Reducing waste from feed impurities which undergo reaction. If feed impurities undergo reaction, this causes waste of feed material, products, or both. Avoiding such waste is most readily achieved by purifying the feed. Thus increased feed purification costs are traded off against reduced raw materials, product separation, and waste disposal costs (Fig. 10.2). 

Figure 10.7 Effluent treatment costs should be included with raw materials costs when traded off against separation costs to obtain the optimal recovery. (From Smith and Petela, Chem. Eng., 513 24, 1991 reproduced by permission of the Institution of Chemical Engineers.)...

Ahmad, S., Heat Exchanger Networks Cost Trade-ofFs in Energy and Capital, Ph.D. thesis, UMIST, U.K., 1985. 

Increasingly, maintenance engineers think in terms of the performance and maintenance of equipment over the whole life of the field. This is often at the centre of the decision on capex-opex trade-offs for example spending higher capex on a more reliable piece of equipment in anticipation of less maintenance costs later in the life of the equipment. 

The starting point of this approach is that the 3D restoration is implemented by the solution of the variational problem for the trade-off functional M , which favors in a weighted manner measured data (functional A) and a priori knowledge (functional B) ... 

There are no fixed rules that determine how much larger the neighbour list cutoff should be than the non-bonded cutoff. Clearly there will be a trade-off between the size of the cutoff and the frequency at which the neighbour hst must be updated the larger the difference, the less frequently will the neighbour list have to be updated. There may also be storage implications if the list is too large. 

Since the product srj, appears in Eq. (10.84), there is a trade-off possibility between particle size and the angle of observation. That is, the Debye scattering theory applies with the same level of accuracy to larger molecules at smaller angles and to smaller molecules at larger angles. 

A brief review of the figures of merit (1) for thermal imaging (4) and gas detection is given to show the various trades-off required to image the thermal environment and detect atmospheric contamination. 

Useful materials incorporating fire-retardant additives are not always straightforward to produce. Loadings of 10% are common, and far higher levels of flame retardants are used in some formulations. These concentrations can have a negative effect on the properties and functions for which the materials were originally intended. Product-specific trade-offs are generally necessary between functionaUty, processibiUty, fire resistance, and cost. 

Potassium fluoride [7789-23-3], KF, is the most frequently used of the alkaU metal fluorides, although reactivity of the alkaU fluorides is in the order CsF > RbF > KF > NaF > LiF (6). The preference for KF is based on cost and availabiUty traded off against relative reactivity. In its anhydrous form it can be used to convert alkyl haUdes and sulfonyl haUdes to the fluorides. The versatility makes it suitable for halogen exchange in various functional organic compounds like alcohols, acids and esters (7). For example, 2,2-difluoroethanol [359-13-7] can be made as shown in equation 9 and methyl difluoroacetate [433-53 ] as in equation 10. 

Fig. 6. Trade-off between cetane number and aromatics for NO emissions (85).

Increased heat utilization does not always mean a trade-off, and many studies have shown a reduction in energy consumption as well as capital cost which in itself is a remarkable recommendation for a systematic approach to network design. 

System Effectiveness. A system is designed to perform some intended function in a prescribed fashion. This overall capabihty is termed system effectiveness. Figure 2 illustrates the design trade-offs that constitute the components of system effectiveness. 

Availability. The system attributes of maintainabiUty and reflabiUty must both be considered. The trade-offs are rather complex and difficult to capture with any one measure. However, the term availabihty has been used to quantify these attributes simultaneously. The availabihty is sometimes related by inherent availabihty ... 

The mean time between failures MTBF is used as a measure of system reflabiUty, whereas the mean time to repair MTTR is taken as a measure for maintainabihty. Eor example, a system with an MTBF of 1200 h and a MTTR of 25 h would have an availabihty of 0.98. Furthermore, if only an MTBF of 800 h could be achieved, the same availabihty would be realized if the maintainabihty could be improved to the point where the MTTR was 16 h. Such trade-offs are illustrated in Figure 3, where each curve is at a constant availabihty. 

Fig. 3. System availability trade-off curves. MTBF = mean time between failures MTTR = mean time to repair.

Definite and known procedures for foUow-up must be provided for, with the design group assessing the value of each idea and suggestion presented by the review committee. The actions taken are known to the committee and subject to further review. With such organization, the trade-offs can be acted upon at the appropriate level. 

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