Minimum utilities

Cerda, J., Westerberg, A. W., Mason, D., and Linnhoff, B., Minimum Utility Usage in Heat Exchanger Network Synthesis—A Transportation Problem, Chem. Eng. ScL, 38 373 1983. 

The pinch design method developed earlier followed several rules and guidelines to allow design for minimum utility (or maximum energy recovery) in the minimum number of units. Occasionally, it appears not to be possible to create the appropriate matches because one or other of the design criteria cannot be satisfied. 

J. Cerda and A. W. Westerberg, Minimum Utility Usage in Heat Exchanger Nehrork Synthesis-A. Transportation Problem, DRC Report No. 06-16-80, Camegie-MeUon University, Pittsburgh, Pa., 1980. 

Gupta, A., and Manousiouthakis, V. (1993). Minimum utility cost of mass exchanger networks with variable single component supplies and targets. Ind. Eng. Chem. Res. 32(9). 1937-1950. 

As has been mentioned before, for minimum utility usage no heat should be passed through the pinch. Let us illustrate this point using the cascade diagram. Suppose that we use kW more than the minimum heating utility. As can... 

Minimum Utility Targets Using Mathematical Programming (Optimization)... 

The design procedure starts by identifying the minimum utility cost for a given heat-transfer driving force. Next, the fixed and operating costs are traded off by iterating over the driving forces until the minimum total annualized cost TAC is attained. 

Once the minimum utility cost has been identified, tradeoffs between operating and fixed costs must be established. This step is undertaken iteratively. For given values of minimum approach temperatures, the pinch diagram is used to obtain minimum cooling cost and outlet gas temperature. By ccmducting enthalpy balance around each unit, intermediate temperatures and exchanger sizing can be determined. Hence, one can evaluate the fixed cost of the system. Next, the minimum approach temperatures are altered, until the minimum TAC is identified. 

The Mctxmin Rule.—For each state, this rule selects the minimum utility and that act with the maximum value among this collection of minima is chosen, i.e., choose the act that gives max< min ufi. 

Comparing the composite curve, Figure 3.22, with Figure 3.237 shows that the heat introduced to the cascade is the minimum hot utility requirement and the heat removed at the bottom is the minimum cold utility required. The pinch occurs in Figure 3.23b where the heat flow in the cascade is zero. This is as would be expected from the rule that for minimum utility requirements no heat flows across the pinch. In Figure 3.23b the pinch temperatures are 80 and 90°C, as was found using the composite stream curves. 

Determine the pinch temperatures and the minimum utility requirements for the streams set out in the table below, for a minimum temperature difference between the streams of 20°C. Devise a heat exchanger network to achieve the maximum energy recovery. 

The construction of the problem table to find the minimum utility requirement and the pinch temperature is facilitated by using a spreadsheet. The calculations in each cell are repetitive and the formula can be copied from cell to cell using the cell copy commands. 

Determine the pinch temperature and the minimum utility requirements for the process set out below. Take the minimum approach temperature as 15°C. Devise ... 

Find the minimum utility requirements for this process, for a minimum approach temperature of 10 °C. 

Cerda J, Westerberg AW, Mason D and Linnhoff B (1983) Minimum Utility Usage in Heat Exchanger Network Synthesis - A Transportation Problem, Chem Eng Sci, 38 373. 

Figure 23.41 illustrates the overall trade-offs as a function of the number of intermediate steam mains for the case of maximized energy recovery13. Two curves are shown in Figure 23.41. The upper curve corresponds with minimum fuel requirement and the lower corresponds with minimum utility cost. Both correspond with no condensing power generation from utility steam. Once the number of intermediate mains has been chosen, then the settings for those mains are chosen, either for the upper curve to correspond with the minimum fuel for the utility steam boilers or for the lower curve for minimum utility cost (fuel and power). As the number of intermediate steam mains is increased, the fuel required and the utility cost both decrease. However, the decrease in cost diminishes as the number of steam mains increases.13...

These aldehydes can then be used to produce a variety of useful derivatives the aldehydes themselves have minimum utility. The major derivatives are alcohols, formed either by direct hydrogenation [Eq. (3)], or by an aldol condensation followed by hydrogenation [Eq. (4)]. 

Production and distribution quantities can vary from a minimum utilization to full capacity utilization... 

Resources are controlled with minimum utilization and maxi-... 

Secondly, we consider minimum utilization scenarios as control parameters. Minimum utilization has to be set if required by production processes e g. in order to ensure process stability and product quality. But minimum utilizations defined higher than required may lead to lower profits due to less optimization flexibility as shown in fig. 93. 

However, minimum utilization scenarios do not influence profits and volumes as assumed due to the same reason as in case of production variance the basis plan already led to a situation of high utilization, minimum utilization limits are not a hard constraint in the specific case. 

Fig. 6. Temperature—relative enthalpy plots showing network parameters of minimum utility for (a) the case requiring infinite area (b) where heat balance...

Fig. 7. Heat-exchange network solution space (2), where line A represents the minimum utility for feasibility, ie, infinite area required. Region B is the...

Research on the synthesis of economically optimal heat exchanger networks (HENs) has been performed for over 15 years (Nishida et al., 1981). As a result of this research, two general conclusions have emerged (1) the optimum network generally features minimum or close to minimum utility consumption, and (2) the optimum network generally has a mini-... 

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