Composite curves pinch point

A horizontal distance between the column grand composite curve pinch point and the vertical axis represents excess heat, and therefore the scope for reduction in reflux ratio. For smaller reflux ratios, the column grand composite curve will move toward the vertical axis, and hence reduce the reboiler and condenser duties, which may be estimated by... 

Figure 10.1 Composite Curves, Pinch Point, and Minimum Energy Requirements Supertargeting...

More than 7.5 MW could be added from a hot utility to the first interval, but the objective is to find the minimum hot and cold utility. Thus from Fig. 6.186, QHmin = 7.5MW and Qcmm = 10MW. This corresponds with the values obtained from the composite curves in Fig. 6.5a. One further important piece of information can be deduced from the cascade in Fig. 6.186. The point where the heat flow goes to zero at T = 145°C corresponds to the pinch. Thus the actual hot and cold stream pinch temperatures are 150 and 140°C. Again, this agrees with the result from the composite curves in Fig. 6.5a. 

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 ...

The reason for this simple relationship is that the concept of minimum reflux implies an infinite number of stages and thus no change in composition from stage to stage for an infinite number of stages each way from the pinch point (the point where the McCabe-Thiele operating lines intersect at the vapor curve for a well-behaved system, this is the feed zone). The liquid refluxed to the feed tray from the tray above is thus the same composition as the flash liquid. 

The rectifying or stripping section of a column must operate somewhere between total reflux and minimum reflux conditions. The range of feasible operation of a column section can thus be defined for a given product composition. It can be seen in Figure 12.19 that these section profiles are bounded for a stage column by the distillation line and the pinch point curve. As noted previously, the pinch point curve provides a minimum reflux boundary for both staged and packed columns,... 

Also shown in Figure 12.19 is the residue curve projected from the same product composition. The area enclosed within the residue curve and the pinch point curve thus provides the feasible compositions that can be obtained by a packed column section from a given product composition. For any given product composition, the operation leaf of feasible operation for a column section can be defined by plotting the distillation line (or residue curve) and the pinch point curve1314. The column section must operate somewhere between the total and minimum reflux conditions. 

From the point of view of the composite curves, the location of the pinch and the A Tmin at the pinch would depend on which kind of streams were located in the region of the point of closest approach between the composite curves. If only liquid streams were present around the point of closest approach of the composite curves, then in the above example, ATmin = 10°C will apply. If there were only gas streams in the region around the point of closest approach, then in the above example, ATmin = 20°C would apply. If there was a mixture of liquid and gas streams at the point of closest approach, then A Tmin = 15°C would apply. 

Because the treatment line with a flowrate mwr is pinched with the composite curve at this point, the following relation holds ... 

By calculating the class 1 FI target, the process engineer can identify the critical uncertainty point and critical constraint (appearance of new pinches, nonnegative heating or cooling, and so on). This uncertainty point and constraint limit the resilience of a completely countercurrent (e.g., infinitely cyclic) HEN structure able to mimic the composite curves thus they seem the most likely uncertainty point and constraint to limit the resilience of a practical but well-designed (almost completely countercurrent) HEN structure. 

As shown in Figure 8.1, this takes place between the points B and H. In fact, point H is defined as the intersection of TB - 30 and the segment DEss the cold composite curve approaches the hot one. Therefore, the pinch point occurs between the temperatures ... 

Remark 1 Since we cannot bring the two composite curves closer, the pinch point represents the bottleneck for further heat recovery. In fact, it partitions the temperature range into two subnetworks, one above the pinch and one below the pinch. Heat flow cannot cross the pinch since there will be violations in the heat exchange driving forces. As a result, we need a hot utility at the subnetwork above the pinch and a cold utility at the subnetwork below the pinch. In other words, having identified the pinch point, we can now apply the first law analysis to each subnetwork separately and determine the hot and cold utility requirements. These can be read from the T - Q diagram since they correspond to the horizontal segments AG and CD, respectively. Hence, for our example we have ... 

Figure 2.14 illustrates the overall approach by pinch-point analysis. The first step is extraction of stream data from the process synthesis. This step involves the simulation of the material-balance envelope by using appropriate models for the accurate computation of enthalpy. On this basis composite curves are obtained by plotting the temperature T against the cumulative enthalpy H of streams selected for analysis, hot and cold, respectively. Two aspects should be taken into account ... 

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. 

We plot the sets of temperature versus enthalpy rate values in Figure 4.44. This is a composite diagram for the heat integration problem. It is apparent from the figure that the closest vertical approach of the two curves occurs at an enthalpy change rate of 10,000 kW. This is the pinch point for the two composite curves, and occurs where the temperature of the streams that are to be heated is 120°C and the temperature of the streams that are to be cooled is 153°C. This A7mm of 33°C is simply a consequence of the starting enthalpy rates that were initially chosen. 

Finally, from the results of these observations, we can prove that a pinch point is any point where a line emanating from the distillate composition is tangent to a residue curve. The vapor composition in equilibrium is on that same line. The reflux ratio to reach that pinch point is the ratio a/b. 

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