Introduction to Pinch Technology for Energy Integration
Pinch technology is the technology that provides a systematic methodology for energy saving in processes and total sites. The methodology is based upon thermodynamic principles. Pinch Analysis was first developed in the late 1970s as a technique for optimization of thermal heat recovery, and rapidly gained wide acceptance as yet practical approach to the design of Heat Exchanger Networks (HENs). Since then, it has evolved into a general methodology for optimization, based on the principles of process integration.
It has been applied successfully not only to energy systems (heat recovery, pressure drop recovery, power generation), but also to fresh water conservation, wastewater minimization, production capacity de-bottlenecking, and management of chemical species in complex processes.
Application of Pinch Technology
Applying Pinch technology to HEN synthesis and retrofit, the engineers can calculate the energy requirement for any process, and produce thermally efficient and practical designs. Energy savings are significant compared to previous best designs. Pinch technology also applies to optimization of the supply-side, consisting of on-site utilities, such as boilers, furnaces, steam and gas turbines, cogeneration, heat pumps, and refrigeration systems.
Consider the example of a process with only two streams, one cold (to be heated) and one hot (to be cooled), represented on a Temperature-Enthalpy (T-H) diagram. The temperature axis represents the available driving forces for heat transfer, while the enthalpy axis shows the supply and demand of heat. In this case, both hot and cold duties will be supplied by utilities (e.g., steam and cooling water).
Now consider recovering some heat from the hot stream to the cold stream, the optimum value of the Minimum Approach Temperature (ΔT_min) is first determined based on the economic tradeoff between cost savings from heat recovery and capital cost of the heat exchangers. The T-H curves are then moved horizontally until the closest vertical approach between the hot and the cold curves is equal to the ΔT_min. This point is called the process pinch. The enthalpy overlap represents the optimum amount of heat recovery between the two streams. The residual duties of the two streams must be supplied by utilities, and represent the energy targets. Notice that the duties on both hot and cold utilities are reduced by an identical amount, and equal to the amount recovered. A lower value of ΔTmin will reduce utility consumption, but will require more heat exchanger area. This concept will be discussed in details in the next section.
For processes with multiple cold streams, the individual process thermal duties can be combined into a single “cold composite curve” drawn on a T-H diagram, which represents the enthalpy demand profile of the process as shown in graph below. Similarly, all the thermal duties for hot streams can be combined into a single “hot composite curve”, which represents the enthalpy availability profile of the process. Composite curves are produced by summing enthalpy changes of individual streams in their respective temperature intervals. When both curves are plotted on the same T-H diagram, they show the opportunity for heat recovery as well as the net heating and cooling targets. The concept of composite curves reduces the multi-stream problem to a 2-stream problem.
The process pinch separates the overall process into two distinct thermal domains: (a) a net heat sink above the pinch temperature, to which hot utility must be supplied, and (b) a net heat source below the pinch temperature, to which cooling must be provided. Recent developments in the field of energy efficiency optimization now advocate the need to not develop the hot composite curve using the current state-of-art methods where only one global ΔT_min (minimum temperature approach between hot and cold composite curves) is used.
The recent US patent by the author in the reference list at the end of this chapter is introducing a new method where each hot stream will have its own ΔT_min that allow him/her to use for best waste heat recovery. In such way, the waste heat recovery problem will have several degrees of freedom for the problem optimization, versus the one parameter problem optimization application currently used in pinch technology.
This point will be discussed in more details in next chapter and for now let us continue with the rules and heuristics of the pinch technology. This Pinch technology is now well documented in several literatures and the references 1 to 4 at the end of this chapter are only few main examples presenting pinch technology and are not intended to be comprehensive or inclusive.
In this article, I will show how we can use pinch technology for energy utility targeting, selection of utility mix and heat exchanger network synthesis using pinch design method in a very simple way. Even though this book main message is the what is next the pinch technology in process industry and the era beyond Pinch technology, we admit that after almost three decades of its emanation in the late seventies and early eighties for a reason or another, the technology still has merits and it is the most widely used method for energy integration especially in oil, gas, refining and petrochemical industries.