Cold composite stream

In a similar manner to constructing the hot-composite line, a cold composite stream is plotted (see Fig. 9.3 for a two-cold-stream example). 

Next, both composite streams are plotted on the same diagram (Fig. 9.4). On this diagram, thermodynamic feasibility of heat exchange is guaranteed if at any heat-exchange level (which corresponds to a horizontal line), the temperature of the hot composite stream is located to the right of the cold composite stmam. Therefore, the cold composite stream can be slid down until it touches the hot composite stream. The ptoint where the two composite streams touch is called... 

Figure 9.3 Construcdng a cold composite stream using superposition (the dashed line represents the composite line).

Solution Figures 9.6-9.8 illustrate the hot composite stream, the cold composite stream and the pinch diagram, respectively. As can be seen from Fig. 9.8, the two composite streams touch at 310 K on the hot scale (300 K on the cold scale). The minimum heating and cooling utilities are 2,620 and 50 kW, respectively, leading to an annual operating cost of... 

Figure 9.7 The cold composite stream for the pharmaceutical process.

Composite curves were developed for heat recovery targeting (Linnhoff et al., 1982). The word composite reveals the basic concept behind the composite curves method A system view of the overall heat recovery system. One hot composite stream represents all the hot process streams, while one cold composite stream represents all the cold process streams. In this manner, the problem of assessing a complex heat recovery system involving multiple hot and cold streams is simplified as a problem of two composite streams. In essence, the hot composite stream represents a single process heat source, while the cold composite stream represents a single process heat sink. 

Minimum Temperature Approach AT For a feasible heat transfer between the hot and cold composite streams, a minimum temperature approach must be specified, which corresponds to the closest temperature difference between the two composite curves on the T H axis. This minimum temperature approach is termed as the network temperature approach and defined as AT an-Maximal Process Heat Recovery The overlap between the hot and cold composite curves represents the maximal amount of heat recovery for a given AT r - In other words, the heat available from the hot streams in the hot composite curve can be heat-exchanged with the cold streams in the cold composite curve in the overlap region. 

Specifying the hot utility or cold utility or AT m fixes the relative position of the two curves. As with the simple problem in Fig. 6.2, the relative position of the two curves is a degree of freedom at our disposal. Again, the relative position of the two curves can be changed by moving them horizontally relative to each other. Clearly, to consider heat recovery from hot streams into cold, the hot composite must be in a position such that everywhere it is above the cold composite for feasible heat transfer. Thereafter, the relative position of the curves can be chosen. Figure 6.56 shows the curves set to ATn,in = 20°C. The hot and cold utility targets are now increased to 11.5 and 14 MW, respectively. 

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. 

A new design situation would start from the grand composite curves of each of the processes on the site and would combine them together to obtain a picture of the overall site utility system12. This is illustrated in Figure 23.27, where two processes have their heat sink and heat source profiles from their grand composite curves combined to obtain a site hot composite curve and a site cold composite curve, using the procedure developed for composite curves in Chapter 16. Wherever there is an overlap in temperature between streams, the heat loads... 

The basic idea in this case is to consider all kink points of both the hot and cold composite curves and draw vertical lines at these kink points. These vertical lines define the enthalpy intervals. Note that the list of kink points includes supply and targets of hot and cold streams. To illustrate such an (El) partitioning let us consider the example used in section 8.3.1.2 for which the temperature interval partitioning is depicted in Figure 8.2 of section 8.3.1.3. The (El) partition for this example is shown in Figure 8.11. Note that in this example there are six Els. 

On the cold composite curve, each stream that is to be heated must enter or leave an exchanger at the pinch point. On the hot composite curve, each stream that is to be cooled must enter or leave an exchanger at the pinch point. 

Example 4.29 Minimum utilities by composite curve method Table 4.19 shows the hot and cold process streams and their heat capacities for the process shown in Figure 4.43. 

The corresponding cold streams (sinks for heat) are shown plotted in temperature-enthalpy coordinates in Fig. 3A. Fig. 3B shows the curves combined to give a composite cold curve. Again, where streams have a common temperature range, the enthalpies and heat capacity flow rates are combined. The cold composite curve, as shown in Fig. 3B, is the single-stream equivalent of the two streams in Fig. 3A in terms of temperature and enthalpy. 

The two curves can now be matched to determine how much heat can be recovered from the hot streams in the process to the cold streams in the process. The composite curves are shown in Fig. 4 matched such that there is a minimum temperature difference (Ar in) of 10°C. The hot composite curve must be above the cold composite curve by at least at all points for feasible heat transfer. The relative position of the two curves has been adjusted such that they are separated by a specified minimum temperature difference. In this case, ATmin = 10°C. The overlap between the composite curves represents the heat recovery potential between the hot and cold streams in the process, shown in Fig. 4. The monotonic nature of the construction of the composite curves allows for maximum overlap between the curves. This in turn allows the construction to determine the maximum heat recovery potential (Qrec)- By maximizing the heat recovery, the residual demand for heating and cooling utilities is minimized. In Fig. 4, the part of the cold composite curve that projects beyond the start of the hot composite curve represents the external heating utilities required The part of the hot... 

Fig. 3 Cold composite curve. (A) Individual temperature-enthalpy profiles for cold streams. (B) Combination of profiles for two cold streams.

To understand the true significance of the pinch, first divide the composite curves at the pinch, as illustrated in Fig. 6. Above the pinch (in temperature terms), the process is overall a heat sink. Heat recovery is possible from the composite hot stream to the composite cold stream above the pinch, but there is a residual amount of heat (Gh, ) that is required to be imported from hot utility to satisfy the enthalpy imbalance. Below the pinch (in temperature terms), the process acts overall as a heat source. Heat recovery is possible from the composite hot curve to the cold composite curve below the pinch, but a residual amount of cooling (2c ,i ) is required to satisfy the enthalpy imbalance. Overall, the process above the pinch therefore acts as a heat sink, and that below the pinch acts overall as a heat source. 

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