Heat exchange networks minimum utility targets

Figure 17.9 shows a heat exchanger network designed with the minimum number of units and to satisfy the energy target at ATmi = 20°C. On the basis of the following utilities and cost data, it has a total annual cost of 14.835 x 106 ( -y 1). 

This chapter focuses on heat exchanger network synthesis approaches based on optimization methods. Sections 8.1 and 8.2 provide the motivation and problem definition of the HEN synthesis problem. Section 8.3 discusses the targets of minimum utility cost and minimum number of matches. Section 8.4 presents synthesis approaches based on decomposition, while section 8.5 discusses simultaneous approaches. 

A.E.S. Konukman, M.C. Camurdan, U. Akman, Simultaneous flexibility targeting and synthesis of minimum-utility heat exchanger networks with superstructure-based MILP formulation, Chem. Eng. Processing 41 (2002) 501-518. 

III. Heat/Process Integration Study Pinch analysis is well established for finding optimal utilities, heat transfer area, optimal fresh water consumption, minimum cooling water demand, reduced emissions targets and so on (Smith, 2005 Kemp, 2007). One application of pinch analysis to retrofitting the heat exchanger network of a crude... 

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. 

Remark 1 The above statement corresponds to the simultaneous consideration of all steps shown in Figure 8.20, including the optimization loop of the HRAT. We do not decompose based on the artificial pinch-point which provides the minimum utility loads required, but instead allow for the appropriate trade-offs between the operating cost (i.e., utility loads) and the investment cost (i.e., cost of heat exchangers) to be determined. Since the target of minimum utility cost is not used as heuristic to determine the utility loads with the LP transshipment model, but the utility loads are treated as unknown variables, then the above problem statement eliminates the last part of decomposition imposed in the simultaneous matches-network optimization presented in section 8.5.1. 

In summary, six heat exchangers is the minimum for this network when it is required that the hot and cold utilities be minimized as well. As discussed in Section 10.4, the minimum number of heat exchangers for this system is five, which can be achieved either by breaking heat loops, usually at the price of exceeding the MER targets, or by stream splitting. ... 

Be able to determine the minimum cooling and heating utilities (MER targets) for a network of heat exchangers using the temperature-interval (TI) method, the composite-curve method, or the formulation and solution of a linear program (LP). 

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