Pinch analysis is a powerful tool for designing heat recovery systems. I provided a brief introduction in Chemical Processing’s July 2019 issue, “Take the Heat Off Pinch Analysis." That column focused on pinch targets. Today, I’d like to move on to pinch design.
First, a quick recap: Process streams that are cooled down, releasing heat, are called “hot streams.” Those that are heated, absorbing heat, are called “cold streams.” Note that “hot” and “cold” do not refer to the temperature of the streams. Rather, they denote the direction of heat transfer.
When we examine heating and cooling demands in most processes, there is a “pinch,” characterized by a “pinch temperature,” that divides the process into two distinct regions:
- The region above the pinch temperature has a net heat deficit. An external utility heat source (e.g., steam or a furnace) completes the energy balance.
- The region below the pinch temperature has a net heat surplus. An external utility heat sink (e.g., air or cooling water) removes the excess heat.
To minimize the utility heating and cooling requirements, and thus achieve the pinch “energy target,” we must design independent heat integration systems for these two regions. This is the basis of the pinch principle: Do not transfer heat across the pinch.
If you don’t transfer heat across the pinch, you are guaranteed to achieve the energy target for the process. However, designing heat recovery systems isn’t always easy, even for seemingly simple systems.
Pinch analysis was initially developed to improve new plants. Following the pinch principle, the design approach starts by separating the streams into two groups: those above the pinch temperature, and those below it. A systematic procedure is then followed, whereby heat transfer matches are created between the hot and cold streams in the above pinch group, until all available heat is consumed. At this point, at least one cold stream will be short of heat; the deficit must be satisfied by a utility heat source.
Following the same approach for the group of streams below the pinch will result in at least one hot stream with excess heat that will need removal by a utility heat sink. However, this systematic procedure often results in complex heat integration designs, with large numbers of heat exchangers and multiple stream splits (i.e., process streams that are divided into two or more parallel branches). These designs generally require simplification to ensure operability or to reduce costs. Consequently, most practical designs use somewhat more energy than the pinch target.
Pinch analysis also applies to revamps. In these cases, the design technique needs significant modification for several reasons. In most revamps, strong incentives exist to maximize use of any existing heat exchangers, even if they are not ideal when viewed from a pinch perspective. This often results in revamp designs that markedly differ from new plant designs. Revamps must account for existing equipment and plot space. This can limit the opportunity to add new heat exchangers. Finally, revamps typically occur during turnarounds, when time is at a premium. This makes it hard to justify complex projects that are difficult or time-consuming to execute.
Many different retrofit design procedures have been proposed. Most approaches start by identifying the existing heat exchangers in which heat crosses the pinch. The various approaches then use different methods to correct these inefficiencies. However, as in new plant designs, these methods often lead to overly complex designs, thus reducing achievable savings. The resulting revamps typically include modifications to existing heat exchangers (e.g., tube bundle replacements, to increase heat transfer capability), installation of new heat exchangers as well as piping to change the way existing heat exchangers are connected.
Pinch analysis is a powerful tool, though its use must be tempered with realism. Properly applied, it can often reduce process heating and cooling requirements by 15% or more.
B. Linnhoff, D. W. Townsend, D. Boland, G. F. Hewitt, B. E. A. Thomas, A. R. Guy, and R. H. Marsland, A User Guide on Process Integration for the Efficient Use of Energy, Rev. 1 ed., pp. 14–18, I.Chem.E., Rugby, U.K. (1994)