Heat cascade

Now cascade any surplus heat down the temperature scale from interval to interval. This is possible because any excess heat available from the hot streams in an interval is hot enough to supply a deficit in the cold streams in the next interval down. Figure 6.18 shows the cascade for the problem. First, assume that no heat is supplied to the first interval from a hot utility (Fig. 6.18a). The first interval has a surplus of 1.5 MW, which is cascaded to the next interval. This second interval has a deficit of 6 MW, which reduces the heat cascaded from this interval to -4.5 MW. In the third interval the process has a surplus of 1 MW, which leaves -3.5 MW to be cascaded to the next interval, and so on. 

The initial setting for the heat cascade in Fig. 6.18a corresponds to the shifted composite curve setting in Fig. 6.15a where there is an overlap. The setting of the heat cascade for zero or positive heat flows in Fig. 6.186 corresponds to the shifted composite curve setting in Fig. 6.156. 

Finally, the heat cascade is shown in Fig. 6.21. Figure 6.21a shows the cascade with zero hot utility. This leads to negative heat flows, the largest of which is -1.84 MW. Adding 1.84 MW from a hot utility as shown in Fig. 6.216 gives = 1.84 MW, Qcmm = 1-84 MW, hot stream pinch temperature =... 

Example 6.5 The stream data for a heat recovery problem are given in Table 6.7. A problem table analysis for AT , = 20°C results in the heat cascade given in Table 6.8. The process also has a requirement for 7 MW of power. Two alternative combined heat and power schemes are to be compared economically. 

Heat Pumps. Because of added capital and complexity, heat pumps are rarely economical, although they were formerly commonly used in ethylene/ethane and propylene/propane spHtters. Generally, the former spHtters are integrated into the refrigeration system the latter are driven by low level waste heat, cascading to cooling water. 

A problem table heat cascade for A Tmm = 10°C is given the Table 18.15. Hot utility is to be provided by a hot oil circuit with a supply temperature of 400°C. Cooling water is available at 20°C. 

Al) Energy balances on each hot process and utility stream in each temperature interval (TI) of the multiperiod MILP transshipment model. Each energy balance involves the residuals (heat cascaded) to and from the TI and the heat transferred in each stream match in the TI. 

Remark 1 For (i) and (ii) we assumed a values of HRAT. For (iii) and (iv) we can have H RAT = TIAT since we decompose into subnetworks based on the location of the pinch point(s). We can also have in (iii) and (i v)EM AT = TIAT < HRAT for each subnetwork which may result in less units, less total area, and less investment cost. By relaxing EM AT — TIAT, that is, being strictly less than H RAT, more opportunities to make matches are introduced in the heat cascade. Note, however, that for the network derivation we may have EM AT < HRAT of EM AT = HRAT depending on which criterion of feasible heat exchange is considered. In principle, if EM AT < H RAT, EM AT can take any small value e close to zero. Also note that EM AT is not a true optimization variable but simply a requirement for feasible heat exchange that can even be relaxed (i.e., may be e from zero). If EM AT - TIAT = e > 0, then the only specification in the above problem statement is that of HRAT based upon which (i) and (ii) are obtained. We will discuss later on how such a specification can be overcome. 

Fig. 7. Heat cascade of transshipment model proposed by Papoulias and Grossmann (1983a,b,c).

Use alternate separation techniques and/or more efficient heat usage techniques such as heat cascading (Branan, 2012)... 

Set Declarations. In Example 10.8, variables are defined with reference to whether there is a match between a hot stream and a cold stream, and if a match exists, over which interval in the heat cascade. Hence, ym,C2 is a binary variable representing whether hot stream 2 is matched to cold stream 2, and 0hi,ci,3 is a continuous variable that represents the heat transferred between hot stream 1 and cold stream 1 in interval 3. 

Another possibility for the reduction of exergy losses in the heat cascading between the gas turbine cylce and the steam turbine cycle is the "2 or 3 pressure steam cycle" as used in modem conventional GST-cycles based on natural gas, chapter 1. Therefore it is proposed here to take the gas-plus steam-turbine cycle, GST, into consideration, in particular with a "3-pressure-steam-turbine-cycle". 

FIG. XXIV-22. Process heat cascade water cracking —> Bray ton cycle — desalination. 

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