PINCH TECHNOLOGY

[Type the company name]PINCH TECHNOLOGYSEMINAR

[Type the author name]15-Jan-15

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PINCH TECHNOLOGY

INTRODUCTIONPinch analysis is a methodology for reducing energy consumption of processes by calculating thermodynamically feasible energy targets (or minimum energy consumption) and achieving them by optimizing heat recovery systems, energy supply methods and process operating conditions. It is also known as "process integration", "heat integration", "energy integration" or "pinch technology". The techniques were first developed in the early 1980's by teams led by Professor Bodo Linnhoff at University of Manchester (UK). Pinch technology has been successfully used in a wide range of industries, including non-chemicals: food industry, paper mill, etc to improve the energy efficiency of the process and reduce the global energy bill. The method is based on thermodynamic principles and allows to determine the heat exchangers network and utility system. Thetechniquesareapplicabletoanyprocessthatincludesheatingandcooling,e.g.,oilrefining,petrochemicals,organicandinorganicchemicals, pharmaceuticals,mineralprocessing,etc. The key components are illustrated in hot and cold composite curves, which represent the overall heat release and heat demand profiles of a process as a function of temperature. Theregionof closestapproachthepinchisakeydesignparameter .It analyzes possible heat exchanges between cold streams (requiring heat) and hot streams (releasing heat) in order to minimize irreversibilities. The process data is represented as a set of energy flows, or streams, as a function of heat load (or enthalpy) against temperature. These data are combined for all the streams in the plant to give composite curves, one for all "hot streams" (releasing heat) and one for all "cold streams" (requiring heat). The point of closest approach between the hot and cold composite curves is the pinch temperature (pinch point or just pinch), and is where design is most constrained. Hence, by finding this point and starting design there, the energy targets can be achieved using heat exchangers to recover heat between hot and cold streams. In practice, during the pinch analysis, cross-pinch exchanges of heat are often found between a stream with its temperature above the pinch and one below the pinch. Removal of those exchanges by alternative matching make the process reach its energy target. Main advantages of this approach includes: It is mainly a graphical method which allows the engineer to keep a physical approach of involved phenomenon while other optimization techniques are purely numerical The energy minimization is performed without any knowledge of the heat exchanger network which is designed afterwards A very deep knowledge of the analyzed process is not required to apply the method and retrieve substantial savings It takes into account the whole process or the whole plant, providing a systematic approach instead of focusing on a specific unit or equipment It is demonstrated that its use can reduce both capital and operating costs. Emissions are consequently also minimized.Pinch analysis has several advantages over "conventional" design approaches: A systematic procedure. It guarantees an optimum solution without relying on luck or inspired guesses by the design engineer. A common denominator methodology. Based on fundamental thermodynamics, Pinch analysis applies to all processes and technologies, continuous and batch, new and retrofit Proven energy savings. Reductions of 15% or more in energy cost are typical, even where processes have already been optimized by conventional methods Automatic pollution prevention. Reduced CO2,SOx and NOx emissions are the natural consequences of better energy efficiency. Lower cost debottlenecking. Pinch analysis shows us how to make better use of existing equipment and systems, and thus minimizes new.

The Applications include:In new plant design: Generate options to Improve energy efficiency Optimize utility system design Reduce emissions (especially CO2.SOx and NOx) Reduce capital cost Existing facilities: Analyze design and operating data to develop options to Improve energy efficiency Debottleneck process units Reduce emissions

Pinch analysis is a rigorous ,structured approach that may be used to take a wide range of improvements related to process and site utility. This includes opportunities such as reducing operating costs, debottlenecking processes, improving efficiency, and reducing and planning capital investment.Major reasons for the success of pinch analysis are the simplicity of the concepts behind the approach and the impressive results it has been obtained worldwide. It analyzes a commodity, principally energy, hydrogen or water in terms of its quality and quantity recognizing the fact that the cost of using that commodity will be a function of both.In general we are using high value utilities in our process and rejecting waste at a lower value. Pinch analysis now has an established track record in energy saving, water reduction and hydrogen system optimization. In all cases, the fundamental principle behind the approach is the ability to match individual demand for a commodity with a suitable supply. The suitability of the match depends on the quality required and quality offered. In the context of utility management, the commodity may be heat, with its quality measured as temperature; or it may be water or hydrogen, the quality of which would be purity or pressure. When considering any pinch type problem, whether it be related to energy, water or process gas, the same principles apply: Process can be defined in terms of supplies and demands (sources and sinks) of commodities(energy, water etc.). The optimal solution is achieved by appropriately matching suitable sources and sinks The defining parameter in determining the suitability of matches is quality, e.g. temperature or purity Insufficient transfer of sources means that the optimal solution cannot be achieved. In fact, the amount of inefficient transfer is equal to the wasteful use of the imported commodities

ENERGY PINCHEnergy is fundamental in industrial economics and yet is often overlooked in the drive for profitability. Pinch Technology provides a systematic methodology for energy saving in processes and total sites. The methodology is based on thermodynamic principles. Figure 1 illustrates the role of Pinch Technology in the overall process design. The process design hierarchy can be represented by the "onion diagram" as shown below. The design of a process starts with the reactors (in the "core" of the onion). Once feeds, products, recycle concentrations and flow rates are known, the separators (the second layer of the onion) can be designed. The basic process heat and material balance is now in place, and the heat exchangernetwork (the third layer) can be designed. The remaining heating and cooling duties are handled by the utility system (the fourth layer). The process utility system may be a part of a centralized site-wide utility system.

A Pinch Analysis starts with the heat and material balance for the process. Using Pinch Technology, it is possible to identify appropriate changes in the core process conditions that can have an impact on energy savings (onion layers one and two). After the heat and material balance is established, targets for energy saving can be set prior to the design of the heat exchanger network. The Pinch Design Method ensures that these targets are achieved during the network design. Targets can also be set for the utility loads at various levels (e.g. steam and refrigeration levels). The utility levels supplied to the process may be a part of a centralized site-wide utility system (e.g. site steam system). Pinch Technology extends to the site level, wherein appropriate loads on the various steam mains can be identified in order to minimize the site wide energy consumption. Pinch Technology therefore provides a consistent methodology for energy saving, from the basic heat and material balance to the total site utility system.In order to start the Pinch Analysis the necessary thermal data must be extracted from the process. This involves the identification of process heating and cooling duties. Figure 2(b) shows the flow-sheet representation of the example process which highlights the heating and cooling demands of the streams without any reference to the existing exchangers. This is called the data extraction flow-sheet representation. The reboiler and condenser duties have been excluded from the analysis for simplicity. In an actual study however, these duties should be included. The assumption in the data extraction flow-sheet is that any process cooling duty is available to match against any heating duty in the process. No existing heat exchanger is assumed unless it is excluded from Pinch Analysis for specific reasons.

THERMAL DATAStream no.Stream typeStart temperature(Ts)(C)Target temperature(Tt)(C)Heat capacityFlow rate (CP)(kW/C)

1Hot1808020

2Hot1304040

3Cold6010080

4Cold3012036

Starting from the thermal data for a process, Pinch Analysis provides a target for the minimum energy consumption. The energy targets are obtained using a tool called the "CompositeCurves".CONSTRUCTION OF COMPOSITE CURVESComposite Curves consist of temperature-enthalpy (T-H) profiles of heat availability in the process (the hot composite curve") and heat demands in the process (the "cold composite curve") together in a graphical representation. Figure 3 illustrates the construction of the "hot composite curve" for the example process, which has two hot streams (stream number 1 and 2, see Table 1). Their T-H representation is shown in Figure and their composite representation is shown in Figure 3(b).

The construction of the hot composite curve (3(b)) simply involves the addition of the enthalpy changes of the streams in the respective temperature intervals. In the temperature interval 180C to 130C only stream 1 is present. Therefore the CP of the composite curve equals the CP of stream 1 i.e. 20. In the temperature interval 130C to 80C, both streams 1 and 2 are present, therefore the CP of the hot composite equals the sum of the CPs of the two streams i.e. 20+40=60. In the temperature interval 80C to 40C only stream 2 is present, thus the CP of the composite is 40. The construction of the cold composite curve is similar to that of the hot composite curve involving the combination of the cold stream T-H curves for the process.

DETERMINING THE ENERGY TARGETS

The composite curves provide a counter-current picture of heat transfer and can be used to indicate the minimum energy target for the process. This is achieved by overlapping the hot and cold composite curves, as shown in Figure 4(a), separating them by the minimum temperature difference DTmin (10C for the example process). This overlap shows the maximum process heat recovery possible, indicating that the remaining heating and cooling needs are the minimum hot utility requirement (QHmin) and the minimum cold utility requirement (QCmin ) of the process for the chosen DTmin.

THE PINCH PRINCIPLEThe point where DTmin is observed is known as the "Pinch" and recognizing its implications allows energy targets to be realized in practice. Once the pinch has been identified, it is possible to consider the process as two separate systems: one above and one below the pinch, as shown in Figure 5(a). The system above the pinch requires a heat input and is therefore a net heat sink. Below the pinch, the system rejects heat and so is a net heat source.

In Figure 5(b), amount of heat is transferred from above the pinch to below the pinch. Thesystem above the pinch, which was before in heat balance with QHmin, now loses units of heat to the system below the pinch. To restore the heat balance, the hot utility must be increased by the same amount, that is, units. Below the pinch, units of heat are added tothe system that had an excess of heat, therefore the cold utility requirement also increases by units. In conclusion, the consequence of a cross-pinch heat transfer () is that both the hotand cold utility will increase by the cross-pinch duty ().To summarize, the understanding of the pinch gives three rules that must be obeyed in orderto achieve the minimum energy targets for a process: Heat must not be transferred across the pinch There must be no external cooling above the pinch There must be no external heating below the pinchViolating any of these rules will lead to cross-pinch heat transfer resulting in an increase inthe energy requirement beyond the target. The design procedure for heat exchanger networks ensures that there is no cross pinch heat transfer. For retrofit applications the design procedure corrects the exchangers that are passing the heat across the pinch.

REFERANCES

1. Introduction to Pinch Technology by Linnhoff March2. Pinch Analysis for the efficient use of Energy, Water and Hydrogen