Pinch analysis (2022)

Topic last reviewed: November 2022

Sectors: Downstream, Midstream, Upstream

Pinch analysis is a systematic technique for analysing heat flow through an industrial process and is based on fundamental thermodynamics [Reference 1]. The pinch method readily identifies opportunities that are very difficult to find without it, especially in complex heat exchanger networks and when there are significant heat duty requirements. Also, because pinch analysis generates targets for heat recovery, it provides a convenient way of quantifying how close any given design option is to the optimum. Specific pinch expertise is required to identify the optimum capital expenditure versus against the incremental savings breakeven point, taking into account potential physical constraints.

This technique has been widely applied in downstream sectors, particularly in refineries and petrochemical facilities, where there are complex networks of heat exchangers and available hot and cold streams. The main application is in the design of heat exchanger networks or preheat trains to reduce the external heat duty requirements and subsequently lower emissions. Application of pinch analysis in upstream operations has been more limited as upstream facilities have lower complexity processing units and varying production profiles over the lifetime of the facilities, introducing a different complexity for optimization of heating and cooling. For improved energy efficiency, detailed pinch analysis techniques can be considered but require appropriate expertise. The extent of the analysis should take into consideration the main energy drivers for a site (e.g., upstream facilities tend to need energy for driving equipment, downstream facilities tend to have high heat requirements).

Pinch analysis can be utilized both for new plant designs and retrofits [Reference 2] [Reference 3]. Pinch methods are also used for debottlenecking processes, 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] [Reference 6], hydrogen [Reference 7], and carbon pinch [Reference 13].

This topic will primarily focus on temperature pinch analysis, which aims to recover hot/cold duty and thus reduce net hot and cold utility demand.

Key concepts

The key concept underlying the pinch principle is that over the range energy (Q_i ) required by the process, which includes both heating (Q_h) and cooling (Q_c), heat can be transferred from a higher temperature level to a lower temperature level by heat recovery in the process. By doing that in an optimum fashion, the net heat input (Q_h) by burning fuel in fired heaters or steam boilers is minimized, and hence the associated cost and emissions are minimized. At the same time, the net waste heat discharged to cooling water or ambient air (Q_c ) is minimized as well.

These concepts are illustrated in the hot and cold composite curves shown in 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. Q_i is the maximum opportunity for process heat integration, i.e., the maximum opportunity for heat recovery from the hot stream to the cold stream. There has to be a minimum allowable temperature approach, ΔT_min, to ensure that equipment sizes are reasonable.

A ‘pinch’ occurs where the vertical distance between the hot and cold composite curves equals ΔT_min. The selection of the ΔT_min is based on the trade-off between the equipment and operating costs. In general, a larger ΔT_min will result in lower equipment cost and higher utility cost. 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 Reference 1, Reference 2, and Reference 3.

The pinch point divides the system into two thermodynamic regions: above pinch and below pinch. The above-pinch region requires external heat and the ‘belowpinch’ region requires external cooling. Cross-pinch heat transfer means heat is transferred from the above-pinch region to the below-pinch region for a given ΔT_min. This will result in additional external heating and cooling requirements over the thermodynamic minimum, resulting in an increase in fuel demand of fired heaters and steam reboilers (Q_h) as well as the heat rejected in cooling water (Q_c ).

These pinch techniques are currently built into software applications to allow for the curves and heat exchanger networks to be developed automatically. In some software applications, proposed heat exchanger pairings can automatically be determined. However, these should be used with caution as there could be some other key considerations, such as physical distance between the heat exchangers and changes to the process control requirements. For more details, refer to constraints discussed in the sections that follow.

Technology maturity

Commercially available	Yes
Offshore viability	Yes but opportunities are likely limited for upstream brownfield due to space and routing limitations
Brownfield retrofit	Yes
Years of experience in industry	30+
Years of experience in oil and gas industry	30+

Project examples in the industry

Pinch techniques are most beneficial, recognizing the additional design and operational complexity of heat exchanger networks, where there are heat exchangers in series recovering heat right upstream of a fired heater, for example in downstream refining, especially in preheat trains of crude units (greater than 80% of the scope) and fluid catalytic cracking (FCC) units.

In simple upstream units, pinch principles can be applied to obtain an energyefficient design. With built-in software readily available and automatic data extraction, it could be beneficial to check whether the system has been fully optimized by checking the hot and cold composite curves and required hot and cold utilities across the various production profiles.

Examples of available hot streams and cold streams that could be included for upstream facilities for heat recovery and development of composite curves are given in Table 1.

Table 1: Hot and cold streams

Hot streams (to be cooled down)	Cold streams (to be heated up)
Gas turbine exhausts	Feed into acid gas regenerator
Product rundowns to meet export or storage specifications, e.g., produced oil	Feed into distillation column (e.g., oil stabilization unit)
Stream exiting acid gas regenerator bottoms	Utility (such as boiler feedwater and thermal oil) to be preheated
Distillation condenser stream	Stream (cold gas and condensate) exiting dew point system
Produced water, which needs cooling if too high to meet disposal specifications
Gas sent to dew point systems
Gas entering the molecular sieve dehydration unit

Key metrics

Range of application	Process units with significant demand for purchased heating and/or cooling, especially if the process has a complex heat exchanger network
Energy key performance indicators	Actual reduction of hot utility (Q_h) and/or cold utility (Q_c ) compared with pinch target hot/cold utility
Guideline capital costs	Methodology (i.e., cost to perform pinch analysis): costs for consulting services and software, which are comparatively inexpensive. Alternatively, depending on the size of the organization, in-house capability can be developed and utilized, and the cost is only the software cost. Costs to implement the pinch analysis results: Capital expense for new heat exchanger (if required, as it could also be beneficial to change the order of the heat exchangers) and associated installation costs (e.g., piping for rerouting, civil, instrumentation). In general, cost– benefit analysis between the capital expenditure cost for the new exchanger and associated installation and operating expenditure (OPEX) savings, i.e., the cost of the energy savings is to be calculated prior to any design changes.
Guideline operational costs	Reduction of 8% – 25% of purchased fuel in for heating (values based on oil refining applications) [Reference 3, p. 21], [Reference 15]. Operational costs refer to maintenance costs of new heat exchangers against savings of hot and/or cold utility.
Greenhouse gas (GHG) reduction potential	Up to 25% of total refinery GHG emissions [Reference 1, pp. 303–306] [Reference 8, pp. 88, 341]. The actual GHG reduction potential will depend on the starting point, i.e. quality of the heat integration during the design stage.
Time to perform engineering and installation	Methodology: 1–3 months (depending on the availability of plant simulation and compatibility with pinch software). Implementation of pinch analysis results: 2–3 years depending on complexity of installation (refer to Heat Exchangers topic) and shutdown window (e.g. turnaround) available for implementation.
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. For 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 eventual implementation of projects [Reference 9]. The scope of a typical pinch project [Reference 2] includes: Data acquisition: (primarily heat loads and temperatures, and economic parameters) for the process under investigation. Pinch analysis: 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]. Incorporation into design: 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].

Decision drivers

Technical	Complexity of heat exchanger network Specifically for upstream: pinch analysis can be used for multiperiod cases to model/ design time-based field production profiles. This will ensure that the facilities are designed and operated in an energy-efficient way over the life cycle of the production profile.
Operational	Optimized utility consumption Improved energy efficiency Reduced risk of production constraints due to legislation (i.e., limits on fired heater size)
Commercial	Efficient deployment of capital Greenfield: Reduction of Primary Utility Equipment Size (capital expenditure) Brownfield: Reduction of modification/replacement of primary energy suppliers (e.g., Fired heaters) Reduced associated taxation
Environmental:	Reduced GHG emissions and associated cost of emission charges/credits Reduced associated pollution emissions (e.g., sulphur oxides)

Alternative technologies

No other techniques that are currently commercially available are as effective at optimizing heat recovery systems as pinch analysis. Alternative methods, such as deterministic-optimization-based methods and stochastic-optimizationbased methods, have been developed, although they are not available in commercial tools [Reference 11] [Reference 12].

Operational issues/risks

General

Pinch analysis is best performed during the early project design phase as modifications to the heat exchanger network are most cost-effective then. Pinch analysis on an existing brownfield asset is nevertheless still possible, but implementation of identified opportunities may require more CAPEX compared to a greenfield design.

Good heat exchanger design practices should be observed while implementing the results of pinch analysis. An ideal pinch design should be modified to reflect the required operability, flexibility, and control requirements. The ‘ideal’ pinch design may include heat exchangers that are inherently hazardous because of risks associated with large pressure differences between streams or crosscontamination of products. The design temperature, design pressure, and associated safety facilities should be adequately modified to reflect such a match. Ideal pinch analysis may lead to a network design with multiple parallel heat exchangers. The number of parallel heat exchangers required can be reduced by paying a small penalty in energy or they should be adequately designed to avoid low velocities/shear stress. Low velocities/shear stress could lead to high fouling rates and consequently poor energy efficiency. Pinch designs should be modified as required to address control, operability, and flexibility issues. An experienced subject-matter expert should be consulted as necessary. Hazard analyses should always be carried out, and dynamic simulations can be conducted if needed to address operability concerns.

Brownfield application

In particular, for brownfield application introduction of new exchangers as a result of pinch analysis should be reviewed for control, start-up, shutdown, and other operability cases. Before the design basis and constraints are set for the pinch study, all assumptions should be challenged to ensure a sustainable, energy-efficient design is developed. For the resulting design, adequate modifications should be made to ensure the final design is flexible, operable, and controllable. These are generally managed through a proper design review and change management work process.

Additionally, constructability, distance between streams, and space constraints are to be considered in a brownfield application. This can be mitigated by limiting the streams within the pinch scope based on distance between the streams.

Opportunities/business case

Opportunities

Energy savings of up to 25% depending on the starting point
Reduction in the size and number of furnaces, boilers, and cooling towers in new designs; for example, the heat exchanger(s) can replace the entire, or part of the duty provided by a furnace
Improve energy efficiencies and realize debottlenecking with retrofits
Reduction in associated GHG emissions to provide heating or cooling duty required from utilities

Industry case studies

Reference 1 (pp. 313–378) describes 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
Reference 15 is a detailed case study conducted in 2021 in an oil refinery
Reference 1 is a case study conducted for a stabilization unit in an upstream facility

Case study 1: Application of pinch technology in an oil refinery

Reference 15, published in 2021, investigated the opportunities for increased energy efficiency using pinch technology at the heat exchanger network for the crude preheat train of a crude distillation unit. The study involved the following key steps using simulation tools:

Development of detailed process simulation
Development of process data extraction and heat exchanger network grid representation
Pinch analysis of the heat exchanger network and retrofit analysis

The key highlights from this study are:

The energy targets are a function of the ΔT_min and the cost index is quite sensitive to this variation
The heat exchanger rearrangement combined with economic analysis (acceptable payback versus energy cost savings) result in 8% total energy savings compared with the base case

For more details, refer to Reference 1.

Case study 2: Pinch technology application at upstream seperation and stabilization plant

Reference 16, published in May 2016, investigated the use of pinch analysis to identify the opportunities and effectiveness of heat integration. This method was conducted on a generic process flow schema of an upstream separation and stabilization plant in an upstream facility as shown in Figure 2.

The ΔT_min used was 10°C and based on the pinch analysis the study concluded that there would be no cold utility required. The conventional design includes a product (water-cooled) cooler to reduce the temperature to 40°C. Based on the pinch analysis, this indicated that the product cooler could be eliminated with savings of 345,300 MJ/hour of energy.

Although the study indicated that the cooler is not required based on pinch principles, this trim cooler may still be required to ensure that the final product specifications are met and that the process can be controlled. The new sizing of the cooler can then be reduced, which is a saving in terms of CAPEX.

References

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