Environmental Engineering Reference
In-Depth Information
On the other hand, the number of cold streams below the pinch at the pinch needs to be
smaller than or equal to the number of hot streams below the pinch at the pinch. If this is not
the case then we need to split a hot stream to achieve this guideline. At the same time, the
CP(FCp) of the hot stream below the pinch at the pinch shall be greater than or equal to the
CP(FCp) of the cold stream below the pinch at the pinch in order to be able to match them in
a heat exchanger. If this is not the case the cold stream needs to be split into two lighter loads
cold streams.
Before closing the pinch design method (PDM) general description with its rules and
heuristics let us summarize the methodology in a step-by-step fasion.
Briefly, the design procedure known as the pinch design method can be lumped as
follows:
•
Divide the problem at the pinch into two separate sub-problems
•
The design for the separate sub-problems is started at the region of minimum choice
known as the pinch point and then moving away
•
Temperature feasibility requires constraints on the CP(FCp) values to be satisfied for
matches between streams “at the pinch” for the two problems above the pinch and
below the pinch
•
The loads on individual heat exchangers are determined using the tick-off heuristic to
minimize the number of units
•
Away from the pinch there is usually more freedom in the choice of matches. In this
case the designer can choose on the basis of his/her process knowledge
Having described the pinch design methodology, let us solve the numerical example
mentioned above.
The pinch point has divided the problem into two sub-problems one above the pinch and
another below the pinch. The division lies at 310 ºC on the hot streams side and 300 ºC on the
cold streams side. The problem minimum approach temperature used is
∆T_min=10
ºC, and
the minimum heating and cooling utiltities are 2876 kW and 50 kW, respectively.
Using the minimum number of units formula mentioned before we can estimate that the
network minum number of units shall be 5 units. Now we will synthesize a feasible HEN that
realizes these two design objectives; minimum utiltities consumtion and minimum number of
units.
In such situations two pinch points will be arised and three sub-problems will be
produced. One sub-problem above the 330 ºC-320 ºC near pinch point; one sub-problem
below the 310 ºC-300 ºC pinch point and a third sub-problem in between the two pinch
points. Handling situations like that is not difficult but less systematic than one pinch point
situation [1]. The impact of such situation in the systematic design of the HEN is generating a
HEN design that exhbits more number of units than the estimated minimum number of units
HEN originally calculated without considering the near pinch point.
