Energy in a Pinch: How Did We Get Here?

The 1970s was a period of spiraling energy prices, driven largely by the Arab oil embargo of 1973 and the energy crisis of 1979. The price of crude oil rose more than fourfold over the decade, resulting in rampant inflation that contributed to recessions in many parts of the world, including the United States. One consequence of this was a sharpened the focus on energy efficiency, which became both an economic and a political priority.

Fortuitously for the process industries, the 1970s also marked the emergence of pinch analysis – a tool that has led to significant energy savings and continues to drive energy efficiency improvements. Table 1 is a summary of reported savings in several sectors, published by Natural Resources Canada around 2011.

Table 1: Pinch Project Energy Savings by Industrial Sector
(source: Natural Resources Canada)

Energy is one the of the largest operating costs in oil refineries, and it takes many forms, although thermal energy – heating and cooling – is dominant. For example, atmospheric crude units use thermal energy primarily to drive large fractionation columns. A moderately large (100,000 bpd) atmospheric crude unit consumes around 250 million Btu/h at a cost of roughly $9,000,000/year¹.

Energy is also a major operating cost in petrochemicals. Ethylene plants, for example, have complex energy requirements. They need very high temperatures (typically above 1,400°F) in their cracking furnaces, and very low temperatures (typically below -300°F) in their gas separation units. These extremes of temperatures, combined with the large scale of ethylene plants and the high level of complexity of their heating and cooling systems, result in very high thermal energy costs.

We use heat exchangers to transfer thermal energy, and most process units contain a significant number of them to handle their heating and cooling requirements. Some gains in energy efficiency can be achieved by improving individual heat exchangers. However, pinch analysis provides a way to look at all of the heat exchangers together as a “heat exchanger network,” and this can lead to much larger improvements.

The pinch approach is based on two key concepts that were put forward in the 1970s:

Energy targets can be quantified before designing a heat exchanger network². If you know how much heat must be added to or removed from each stream in your process, and you know the temperatures of each stream, you can calculate the minimum amount of heat that must be added to and removed from the process using external heating and cooling utilities. This can be done without knowing beforehand which streams will transfer heat between one another in each of the heat exchangers.

The Pinch Principle³. In most processes, there is a temperature (the “pinch temperature”) that divides the process into two distinct thermodynamic regions.

• Above the pinch, the process has a shortage of heat. We can minimize the use of external heat sources (“hot utilities”) by transferring heat between process streams, and only then use hot utilities to supply the remaining heating requirements.
• Below the pinch, the process has a surplus of heat. We can minimize the use of external cooling media (“cold utilities”) by transferring heat between process streams, and only then use cold utilities to satisfy the remaining cooling requirements.

The pinch concept is represented simply in Figure 1. The minimum required heat from hot utilities, Q_{h, min}, is supplied above the pinch_. This provides the net amount of heat needed to satisfy the heat balance above the pinch, after accounting for all of the heat that can be exchanged between the process streams. There is no excess heat left when we reach the pinch temperature, so heat flow = 0 at the pinch. This fact leads to the simplest statement of the Pinch Principle: “Don’t transfer heat across the pinch!”

Below the pinch, heat is exchanged between process streams to the maximum extent possible, leaving a residual amount of heat, Q_{c, min}, that must be removed in cold utilities.

Figure 1: The pinch divides a process into two distinct heat flow regions

Hot and cold composite curves (Figure 2) provide a more detailed and quantitative way to represent these concepts, and they are used extensively in pinch studies.

The hot composite curve is the sum of all the heat sources (or “hot streams”) within the process, defined by their heat load and temperature level. The cold composite curve is the corresponding sum of all the heat sinks (or “cold streams”). We plot these curves together on a single temperature-heat flow chart in Figure 2.

Within the process, heat can be recovered wherever hot streams are available at temperatures above those of cold streams. On the chart, this corresponds to portions of the hot composite curve positioned vertically above portions of the cold composite curve, with heat flow taking place vertically downwards between the two curves.

The heat flow in the process takes place in heat exchangers, whose sizes increase as the temperature difference between the hot and cold streams decreases. We therefore specify a minimum allowable temperature difference, ΔT_min, to ensure the equipment size doesn’t become too large. This corresponds to the minimum vertical separation between the hot and cold composite curves in the chart.

Most composite curves show a “pinch” where the vertical separation of the curves approaches, and eventually reaches, the ΔT_min value. The pinch divides the process into two distinct regions:

• Above the pinch, heat integration is possible where the hot composite curve sits above the cold composite curve, but there is a net heat deficit. Heat (Q_h) from an external hot utility is needed to satisfy the deficit.
• Below the pinch, heat integration is possible where the hot composite curve sits above the cold composite curve, but there is a net heat surplus. A heat sink (Q_c) must be provided by an external cold utility, to remove the excess heat.

If we allow heat to flow from the region above the pinch, where there is a shortage of heat, to the region below it, where there is excess heat, we violate the pinch principle. This increases both the hot and cold utility duties, which adds to energy costs.

Figure 2: Hot and cold composite curves. The curves represent the overall heat release and heat demand of a process as a function of temperature.

Composite curves and the pinch principle are the best-known elements of pinch analysis. They are used to establish “energy targets,” which provide a measure for the potential to save energy by heat integration. Other tools and techniques are used to convert targets into changes in the heat exchanger network, which can be incorporated in the design of the plant – either when the plant is built, or during a plant revamp. We will address this topic in our next blog.

Looking to improve energy efficiency at your facility and understand how to practically apply pinch principles into your analysis? Becht’s team of experts can help you uncover hidden savings. Contact us to get started.

References

¹Energy cost based on a 100,000 bpd atmospheric crude unit with natural gas heating at $4/mmBtu. Additional data in: Rossiter, Alan P., ‘Improve Energy Efficiency via Heat Integration,’ Chem. Eng. Prog., Vol. 106, No. 12, pp. 33-42, December 2010.

²(E.C. Hohmann, 1971, “Optimum Networks for Heat Exchange,” PhD thesis, University of S. California).

³(B. Linnhoff and J.R. Flower, 1978, “Synthesis of Heat Exchanger Networks” (2 parts), AIChE Journal, 24, 633).