Pinch Method
The idea is to consider a plant as a thermodynamic system in which hot process streams must be cooled down and cold ones must be heated up. The trivial (and expensive) approach is to use cold and hot utilities to achieve those objectives. The alternative (and cheaper one) is to use the heat available in the hot streams to heat the cold ones. This process is called heat integration. One strategy for heat integration is the so-called "Pinch Method." The pinch point of a process is its bottleneck for heat integration. To understand this a bit better, we need some definitions.
a) Superstream: A hot superstream is defined as the equivalent to the sum of all individual hot streams within the same temperature interval. The heat content of the hot superstream in a particular temperature interval is Q = CA, where G is the heat capacity flow rate (Cpwi). The cold superstream is defined in an analogous manner.
b) Carnot Efficiency Plot: We can plot the Carnot Efficiency (1 - T./T) of both the hot superstream and the cold superstream vs. the heat exchanged. Notice that the Carnot Efficiency is nothing but a modified temperature process.
c) The cold stream is displaced horizontally until both curves are separated at a particular temperature by the Tmin. This process minimizes the generation of entropy and therefore it is the thermodynamic optimum (Forciniti et al., 1985).
d) Heat exchangers are used to allow the hot and cold streams to exchange heat with each other in the region in which they overlap.
e) Steam and cooling water are finally used to satisfy the heating and cooling requirements that cannot be satisfied by heat integration.
Questions:
1) Construct the hot and cold superstreams using the data from Table 1 (Polley and Haslego, CEP October 2002, 48-51).
2) Identify the Pinch point if the minimum delta T is 0.5, 10, 15, or 20°C. Indicate Tho and Told at the pinch point.
