Pinch points

This procedure is a reasonable approximation only when no pinch points exist within the tower and when the absorption factors vary in a regular manner between the bottom and the top of the tower. 

Pinch Point—This is defined as the differenee between the exhaust gas temperature leaving the evaporator seetion and the saturation temperature of the steam. Ideally, the lower the pineh point, the more heat reeovered, but this ealls for more surfaee area and, eonsequently, inereases the baek-pressure and eost. Also, exeessively low pineh points ean mean inadequate steam produetion if the exhaust gas is low in energy (low mass flow or low exhaust gas temperature). General guidelines eall for a pineh point of 15-40°F (8-22 °C). The final ehoiee is obviously based on eeonomie eonsiderations. 

Another important parameter is the temperature difference between the evaporator outlet temperature on the steam side and on the exhaust gas side. This difference is known as the pinch point. Ideally, the lower the pinch point, the more heat recovered, but this calls for more surface area and, consequently, increases the back pressure and cost. Also, excessively low... 

Some conditions require breaking up the exchanger into multiple parts for the calculations rather than simply using corrected terminal temperatures. For such cases one should always draw the q versus temperature plot to be sure no undesirable pinch points or even intermediate crossovers occur. 

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. 

Solution Henstebeck diagrams were produced based upon using different key component pairs. These showed where pinch points were occurring and the resulting understanding allowed a proper computer simulation to be selected. (See Section 3 Fractionators Graphical... 

By constructing a pinch diagram for the problem, find the minimum cost of MSAs needed to remove phenol from Rg and R2. How do you characterize the point at which both composite streams touch Is it a true pinch point ... 

We are now in a position to incorporate material balance into the synthesis procedure with the objective of allocating the pinch point as well as evaluating excess capacity of process MS As and load to be removed by external MSAs. These aspects ate assessed through the mass-exchange cascade diagram. 

Having completed the design above the pinch, we can now move to the problem below the pinch. Figure 5.9 illustrates the streams below the pinch. It is worth noting that immediately below the pinch, only streams R, R2 and S1 exist. Stream S3 does not reach the pinch point and, hence, will not be considered when the... 

Simultaneously screen all MSAs even when there are no process MSAs Determine the MOC solution and locate the mass-exchange pinch point... 

Having identified the values of all the flowrates of lean streams as well as the pinch location, we can now minimize the number of mass exchangers for a MOC solution. As has been previously mentioned, when a pinch point exists, the synthesis problem can be decomposed into two subnetworks, one above the pinch and one below the... 

Maximum steam exit temperature Minimum pinch point temperature difference... 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

Newby et al. found that increasing the PO turbine pressure resulted in higher steam flow (for a given pinch point temperature difference in the HRSG), increased PO turbine power and overall plant efficiency. However, at the highest pressure of 100 bar attempts to increase the steam flow further resulted in incomplete combustion in the main combustor and the overall thermal efficiency did not increase substantially at this pressure level. 

Harvey et al. gave a parametric calculation of the thermal efficiency of this plant, as a function of turbine inlet temperature, the reformer pinch point temperature difference and the pressure level in the reformer (the compressor overall pressure ratio, r). 

Figure 8-25. Example of typical pinch point for critical region for high-pressure distillation. Used by permission, Wichterle, I., Kobayashi, R., and Chappeiear, P. S., Hydrocarbon Processing, Nov. (1971) p. 233, Gulf Publishing Co., all rights reserved.

Figure 15.11 Heat-recovery diagram showing the pinch point...

Unfired cycle This cycle is very similar to the mentary-fired case except there is no added fuel heat input. The approach temperature and pinch point are even more critical, and tend to reduce steam pressures somewhat. Similarly, the gas turbine exhaust temperature imposes further limits on final steam temperature. 

Dual pressure For comparison, a combined cycle scheme with dual pressure is shown in Figure 15.13. In this case, the waste heat recovery boiler also incorporates a low-pressure steam generator, with evaporator and superheater. The LP steam is fed to the turbine at an intermediate stage. As the LP steam boils at a lower temperature than the HP steam, there exists two pinch points between the exhaust gas and the saturated steam temperatures. The addition of the LP circuit gives much higher combined cycle efficiencies with typically 15 per cent more steam turbine output than the single pressure for the same gas turbine. 

The pressure balance provides an insight into the hydraulics of catalyst circulation. Performing pressure balance surveys will help the unit engineer identify pinch points. It will also balance two common constraints the air blower and the wet gas compressor. 

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