Topic last reviewed: 10 April 2013
Sectors: Downstream, Upstream
Pinch analysis is a systematic technique for analysing heat flow through an industrial process, based on fundamental thermodynamics (Reference 1). The main application is in the design of heat exchanger networks or preheat trains for high energy efficiency and low emissions, both for new plant designs and retrofits (Reference 2, 3). Pinch methods are also used for debottlenecking processes and to optimize the use of utilities (especially steam at various pressure levels), and in improving the energy efficiency of distillation systems (Reference 4). Some of the more recent developments in pinch analysis focus on the management of material resources, such as water and wastewater (Reference 5, 6), and hydrogen (Reference 7).
The key concepts are illustrated in the hot and cold composite curves (Figure 1), which represent the overall heat release and heat demand of a process as a function of temperature. When these two curves are placed together on a single temperature-enthalpy plot (as in Figure 1) it is apparent that heat can be recovered within the process wherever there is a portion of the hot composite curve above a portion of the cold composite curve — i.e. heat can flow from a higher temperature part of the process to a lower temperature part. The maximum opportunity for heat recovery is Qi. There has to be a minimum allowable temperature approach, Delta-T min, to ensure that equipment sizes are reasonable.
Figure 1: Typpical hot and cold composite curves
A ‘pinch’ occurs where the vertical distance between the hot and cold composite curves equals Delta-T min. Systematic techniques for establishing energy targets and designing heat exchanger networks and other pinch applications have been developed based on an understanding of the pinch and its relevance to energy usage. Detailed procedures for establishing energy targets and designing heat exchanger networks and utility system optimization are described in more detail in References 1, 2 and 3.
|Years experience in the industry:||21+|
Project examples in the industry
Mostly in refining, especially crude units and FCCs. (Upstream applications include Claus plants, amine units and distillation systems.)
Range of application:
|Process units with significant demand for purchased heating (50 MMBtu/h or more), especially if the process has a complex preheat train|
|Guideline capital costs:||Generally consulting services and software, which are comparatively inexpensive
|Guideline operational costs:||Reduction of 10–25% of purchased fuel in oil refining applications (Reference 3, p. 21)
|GHG reduction Potential:||Up to 25% of total refinery GHG emissions (Reference 1, pp. 303–306; Reference 8, pp. 88, 341).|
|Time to perform engineering and installation:||6 months- 2 years|
|Typical scope of work description:||Due to the availability of software, engineers in a wide range of organizations can carry out pinch analysis, at least at a basic level. However, most advanced pinch analysis work is carried out by specialized consultants, often in small companies. Some of the larger engineering companies maintain a capability in the technology, and several academic institutions are engaged in both research and industrial applications. Whoever performs the pinch work itself, it is important that pinch activities should not be carried out in isolation. Rather, they should be integrated into a larger engineering organization to ensure the proper flow of information, evaluation of results, and ultimate implementation of projects (Reference 9).
The scope of a typical pinch project (Reference 2) includes acquiring data (primarily heat loads and temperatures, and economic parameters) for the process under investigation. Software is then used to analyse the data, generate targets and identify inefficiencies in the process. A new design is then developed (or improvements are identified for an existing design) using various pinch tools. This work is often carried out in conjunction with simulations of the process and a model of the utility system to refine the design and confirm the savings (Reference 3, pp. 8–17). After completion of these activities the pinch design is typically transferred to an in-house engineering group or general engineering contractor for detailed design work and implementation. Some recycling back to the pinch specialists may be necessary as the detailed design work proceeds.
Pinch analysis is particularly valuable in large, complex industrial facilities, where systematic methods are needed to identify the best opportunities to improve energy efficiency (Reference 3, p.6). It should be used in grassroots designs where purchased energy is 50 MMBtu/h or more, and in developing energy-saving revamp projects for similar facilities.
|Operational:||Optimized utility consumption
Improved energy efficiency
|Commercial:||Efficient deployment of capital
|Environmental:||Reduced greenhouse gas (GHG) emissions
Pinch analysis provides a systematic approach for identifying heat integration improvements. It is entirely possible to conduct a process design without using pinch techniques. However, the pinch method readily identifies opportunities that are very difficult to find without it, especially in complex designs. Also, because pinch analysis generates targets for economic heat recovery, it provides a convenient way of quantifying how close any given design option is to the optimum. No other techniques that are currently available commercially are as effective at optimizing heat recovery systems.
Operational issues/ risks
Pinch analysis is generally used in the early stages of process design, when opportunities for improvement are being identified and evaluated. The ‘ideal’ pinch design may include heat exchangers that are inherently hazardous because of risks associated with large pressure differences between streams or cross-contamination of products. Rigorous application of pinch design methods can also lead to complex heat exchanger networks that are difficult to control. Pinch designs should therefore be checked to determine whether these problems are present. Hazard analyses should always be carried out, and dynamic simulations can be conducted if needed to address operability concerns.
- Energy efficiency improvements of up to 25%
- Reduce greenhouse gas emissions due to reduced fuel firing
- Reduce the size and number of furnaces, boilers and cooling towers in new designs
- Achieve energy efficiencies and debottlenecking with retrofits
Industry case studies
- Reference 1, pp. 313–378, describes a number of case studies from several industry sectors.
- Reference 2 is a detailed case study of a recent crude unit retrofit pinch analysis.
- Reference 8, pp. 335–343, summarizes results from seven case studies in several industry sectors.
- Reference 10 is a detailed case study of an FCC unit retrofit pinch analysis, available online.
Pinch analysis application for refinery FCC unit
Reference 10: Pinch Analysis: For the Efficient Use of Energy, Water & Hydrogen—Oil Refining Industry: Energy Recovery at a Fluid Catalytic Cracking (FCC)
This case study is a simple application of using pinch analysis to reduce energy consumption in a refinery fluid catalytic cracking (FCC) unit. One of the FCC unit products is a slurry that is cooled in an air cooler from >300°C to 121°C before being pumped to a tank. This slurry product is an off-take from the same pump that circulates the FCC bottoms pumparound (BPA) at a temperature of 343°C. In the existing process configuration, the FCC feed is preheated by several process streams, including the BPA, before being heated to its reactor inlet temperature using a direct-fired furnace (assumed in this example to be firing refinery fuel gas). The BPA is also used to generate steam.
In this case study two different projects are developed, with combined savings of 6.55MW or 32% of the FCC’s total energy demand of 20.3 MW. One of the projects (‘Project 2’) is a simple re-piping opportunity, where the slurry off-take point is moved from the BPA pump discharge at 343°C to the steam generator discharge at 232°C.
The steps in the pinch analysis include:
Step 1: Data collection on heat loads and temperatures for all process streams, and all heating and cooling utilities.
Step 2: Generate energy targets for the process, and for each of its heating and cooling utilities, based on an appropriate minimum temperature approach for all heat exchanges (i.e. the smallest temperature difference that should be allowed between the hot and cold streams). There is a trade-off between capital cost (the lower the approach temperature, the higher the capital cost) and heating/cooling utility cost. For this case study, the minimum approach temperature was set at 30°C.
Step 3: Identify the major inefficiencies in the heat exchanger network using pinch analysis.
Step 4: Define options for reducing the largest inefficiencies.
Step 5: Evaluate competing options using economic analysis, taking into consideration the overall saving in utility costs.
Step 6: Select the best option or combination of options.
Baseline scenario: The base design for efficiency comparison is the existing system that preheats feed against process streams, including the BPA, ahead of the fired furnace. The base design also includes steam generation using excess heat from the BPA., Heat from the slurry product is rejected in an air cooler from 343 to 121°C. The energy utilization of the fired heater is 20.3 MW.
Energy efficiency Project 2: Move the slurry product off-take point from the BPA pump discharge at 343°C to the steam generator discharge at 232°C. Therefore, the air cooler heat load is reduced as the inlet temperature of the slurry is reduced from 343 to 232°C.
- Heat recovered that would otherwise be lost in air cooler = 0.91 MW
- Energy efficiency improvement of fired heater (assuming savings of 0.91 MW out of 20.3 MW total) = 4.5%
- Reduction in GHG emissions (assuming displacing 0.91 MW output from refinery fuel gas-fired furnace assuming 85% efficiency, 96% on-stream factor) = 0.2 metric tonnes CO2/hr or 1,900 tonnes CO2/yr
Estimated cost savings:
- Savings in energy cost = Canadian $143,000/yr (year 2003 costs)
- Capital cost for piping changes = Canadian $120,000 (year 2003 costs)
- <1 year simple payback
- Kemp, I.C. (2006). ‘Pinch Analysis and Process Integration: A User Guide on Process Integration for the Efficient Use of Energy (Second Edition)’. Butterworth-Heinemann (Elsevier).
- Rossiter, A.P. Improve energy efficiency via heat integration. In ‘Chem. Eng. Prog.’, Vol. 106, No. 12, pp.33–42, December 2010.
- Natural Resources Canada (2003). ‘Pinch Analysis: For the Efficient Use of Energy, Water & Hydrogen’.
- Linnhoff, B. (1994). Pinch analysis: building on a decade of progress. In ‘Chem. Eng. Prog.’, Vol. 90, No. 8, p.32.
- Wang, Y.P. and Smith, R. (1994). Wastewater minimisation. In ‘Chem. Eng. Sci.’, Vol. 49, No. 7, pp. 981–1006.
- Manan, Z.A., Tan, Y.L. and Foo, D.C.Y. (2004). Targeting the Minimum Water Flow Rate Using Water Cascade Analysis Technique. In ‘AIChE Journal’, Vol. 50, No. 12. pp. 3169-3183.
- Alves, J.J. and Towler, G.P. (2002). Analysis of Refinery Hydrogen Distribution Systems. In ‘Industrial & Engineering Chemistry and Research’, Vol. 41, No. 23, pp. 5759-5769.
- Rossiter, A.P. (ed.) (1995). ‘Waste Minimization through Process Design’. McGraw-Hill, New York.
- Rossiter, A.P. (2004). Succeed at Process Integration. In ‘Chem. Eng. Prog.’, Vol. 100, No. 1, pp. 58–62, January 2004.
- Natural Resources Canada (2012). ‘Pinch Analysis: For the Efficient Use of Energy, Water & Hydrogen—Oil Refining Industry: Energy Recovery at a Fluid Catalytic Cracking (FCC) Unit’.