Pinch above

The CP inequality for individual matches. Figure 16.2a shows the temperature profile for an individual exchanger at the pinch, above the pinch.Moving away from the pinch, temperature differences must increase. Figure 16.2a shows a match between a hot stream and a cold stream which has a CP smaller than the hot stream. At the pinch, the match starts with a temperature difference equal to The relative slopes of the temperature-enthalpy... 

If the number of hot streams at the pinch above the pinch is greater than the number of cold streams, the cold streams must be split to satisfy the constraint. If the number of cold streams at... 

CmOC, above pinch — above pinch F i S, above pinch i, above pinch... 

The net heat fiow across Pinch is zero. Consequently, the system can be split into two stand-alone subsystems, above and below the Pinch. Above the Pinch there is need only for hot utility, while below the Pinch only cold utility is necessary. For given AT the hot and cold utility consumption identified so far becomes Minimum Energy Requirements (MER). No design can achieve MER if there is a cross-pinch heat transfer. 

Fig. 6.7a. Above the pinch (in temperature terms), the process is in heat balance with the minimum hot utility Qnmin- Heat is received from hot utility, and no heat is rejected. The process acts as a heat sink. Below the pinch (in temperature terms), the process is in heat balance with the minimum cold utility Qcmin- No heat is received, but heat is rejected to cold utility. The process acts as a heat source.

Consider now the possibility of transferring heat between these two systems (see Fig. 6.76). Figure 6.76 shows that it is possible to transfer heat from hot streams above the pinch to cold streams below. The pinch temperature for hot streams for the problem is 150°C, and that for cold streams is 140°C. Transfer of heat from above the pinch to below as shown in Fig. 6.76 transfers heat from hot streams with a temperature of 150°C or greater into cold streams with a temperature of 140°C or less. This is clearly possible. By contrast. Fig. 6.7c shows that transfer from hot streams below the pinch to cold streams above is not possible. Such transfer requires heat being transferred from hot streams with a temperature of 150°C or less into cold streams with a temperature of 140°C or greater. This is clearly not possible (without violating the ATmin constraint). 

In choosing to transfer heat, say XP, from the system above the pinch to the system below the pinch, as shown in Fig. 6.8a, then above the pinch there is a heat deficit of XP. The only way this can... 

Analogous effects are caused by the inappropriate use of utilities. Utilities are appropriate if they are necessary to satisfy the enthalpy imbalance in that part of the process. Above the pinch in Fig. 6.7a, steam is needed to satisfy the enthalpy imbalance. Figure 6.86 illustrates what happens if inappropriate use of utilities is made and some cooling water is used to cool hot streams above the pinch, say, XP. To satisfy the enthalpy imbalance above the pinch, an import of (Q mjj,+XP) is needed from steam. Overall, (Qcmin+AP) of cooling water is used. ... 

In a situation as shown in Fig. 6.12a, with the optimal AT in at the threshold, then there is no pinch. On the other hand, in a situation as shown in Fig. 6.126, with the optimum above the threshold value, there is a demand for both utilities, and there is a pinch. 

In design, the same rules must be obeyed around a utility pinch as around a process pinch. Heat should not be transferred across it by process-to-process transfer, and there should be no inappropriate use of utilities. In Fig. 6.13a this means that the only utility to be used above the utility pinch is steam generation and only cooling water below. In Fig. 6.136 this means that the only utility to be used above... 

The point of zero heat flow in the grand composite curve in Fig. 6.24 is the pinch. The open jaws at the top and bottom represent Hmin and Qcmin, respectively. Thus the heat sink above the pinch and heat source below the pinch can be identified as shown in Fig. 

Figure 6.30 shows the grand composite curve plotted from the problem table cascade in Fig. 6.186. The starting point for the flue gas is an actual temperature of 1800 C, which corresponds to a shifl ed temperature of (1800 — 25) = mS C on the grand composite curve. The flue gas profile is not restricted above the pinch and can be cooled to pinch temperature corresponding to a shifted temperature of 145 C before venting to the atmosphere. The actual stack temperature is thus 145 + 25= 170°C. This is just above the acid dew point of 160 C. Now calculate the fuel consumption ...

Figure 6.33 shows a steam turbine integrated with the process above the pinch. Heat Qhp is taken into the process from high-pressure steam. The balance of the hot utility demand Qlp is taken... 

The process requires (Qup + Qlp) to satisfy its enthalpy imbalance above the pinch. If there were no losses from the boiler, then fuel W would be converted to shaftwork W at 100 percent efficiency. However, the boiler losses Qloss reduce this to below 100 percent conversion. In practice, in addition to the boiler losses, there also can be significant losses from the steam distribution system. Figure 6.336 shows how the grand composite curve can be used to size steam turbine cycles. ... 

Thus the appropriate placement of heat pumps is that they should be placed across the pinch. Note that the principle needs careful interpretation if there are utility pinches. In such circumstances, heat pump replacement above the process pinch or below it can be economic, providing that the heat pump is placed across a utility pinch. Such considerations are outside the scope of the present text. 

Process cooling by level 2 by this arrangement across the pinch is 0.54 — 0.14 = 0.40 MW. The balance of the cooling demand on level 2, 0.8 — 0.4 = 0.4 MW, together with the load from level 1, must be either rejected to the process at a higher temperature above the pinch or to cooling water. 

Solution Figure 7.2 shows the stream grid with the pinch in place dividing the process into two parts. Above the pinch there are five streams, including the steam. Below the pinch there are four streams, including the cooling water. Applying Eq. (7.3),... 

Rgura 7.2 To target the number of units for pinched problems, the streams above and below the pinch must be counted separately, with the appropriate utilities included. 

Figure 13.3 shows a process represented simply as a heat sink and heat source divided hy the pinch. Figure 13.3a shows the process with an exothermic reactor integrated above the pinch. The minimum hot utility can be reduced by the heat released by reaction, Qreact-... 

By comparison, Fig. 13.36 shows an exothermic reactor integrated below the pinch. Although heat is being recovered, it is being recovered into part of the process which is a heat source. The hot utility requirement cannot be reduced because the process above the pinch needs at least Q//m-,n to satisfy its enthalpy imbalance. 

There is no obvious benefit from integrating an exothermic reactor below the pinch. The appropriate placement for exothermic reactors is above the pinch. ... 

Figure 13.4a shows an endothermic reactor integrated above the pinch. The endothermic reactor removes Qreact from the process above the pinch. The process above the pinch needs at least Qn in to... 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

Fig. 14.1a. The background process (which does not include the reboiler and condenser) is represented simply as a heat sink and heat source divided by the pinch. Heat Qreb is taken into the reboiler above pinch temperature and rejected from the condenser at a lower temperature, which is in this case below pinch temperature. Because the process sink above the pinch requires at least Q min to satisfy its...

If botb reboiler and condenser are integrated with the process, this can make the column difficult to start up and control. However, when the integration is considered more closely, it becomes clear that both the reboiler and condenser do not need td be integrated. Above the pinch the reboiler can be serviced directly from hot utility with the condenser integrated above the pinch. In this case the overall utility consumption will be the same as that shown in Fig. 14.16. Below the pinch the condenser can be serviced directly by cold utility with the reboiler integrated below the pinch. Now tlje overall utility consumption will be the same as that shown in Fig. 14.1c. 

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