PROCESS INTEGRATION FOR ENERGY CONSERVATION IN DOUBLE PIPE HEAT EXCHANGER THROUGH PINCH TECHNOLOGY

Transcription

1 PROCESS INTEGRATION FOR ENERGY CONSERVATION IN DOUBLE PIPE HEAT EXCHANGER THROUGH PINCH TECHNOLOGY Suneela Sardar, Syed Shamim Raza and S. R. Malik Department of Chemical Engineering, NFC IE&FR Faisalabad, Pakistan Correspondence Author: com Abstract: Energy conservation is the prime concern for all process industries in the current scenario of energy crisis. Questions regarding energy efficient processes, appropriate utility mix for the processes, reducing emissions, increasing plant capacities, improving product qualities and attaining the one coherent plan for the overall site can be best answered through pinch technology. Pinch analysis is used to represent the application of pinch technology to design the heat exchanger networks and study industrial processes. This paper presents the process integration carried out in double pipe heat exchanger through pinch technology. For the pinch analysis of double pipe heat exchanger, steps like identification of process streams, thermal data extraction, and selection of?tmin and construction of composite curves are carried out. Composite curve method and problem table method are used for the calculation of double pipe heat exchanger. At the end improvements in double pipe heat exchanger are mentioned. Future perspectives of pinch technology are also given. Keywords: Heat Exchanger, Pinch Technology 1. Introduction: Pinch Technology was introduced by Linnhoff and Vredeveld to represent a new set of thermodynamically based methods that guarantee minimum energy levels in design of heat exchanger networks. Over the last two decades it has emerged as an unconventional development in process design and energy conservation. Pinch technology provides a systematic methodology for energy saving in process and total sites. It is used to identify appropriate changes in the core process conditions that can have impact on energy savings. Pinch analysis starts the heat and material balance for the process then targets for energy savings can be set prior to the design of the heat exchanger networks. Pinch design methods ensure the achievements of design during the network design. Targets can also be set for the utility loads at various levels (e.g. steam and refrigeration levels). 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. industrial processes. 2. Literature Survey: Over the last two decades pinch technology provides a systematic methodology for energy savings in process and total sites. Basis of Pinch analysis is on the First and second laws of thermodynamics. The First law of thermodynamics provides the energy equation for calculating the enthalpy changes (AH) in the streams passing through a heat exchanger. The Second law determines the direction of heat flow; heat energy may only flow in the direction of hot to cold. This prohibits "temperature crossovers" of hot and cold steam profiles through the exchanger unit. In practice the hot steams can only be cooled to a temperature defined by the "temperature approach" of the heat exchanger. Temperature approach is the minimum allowable temperature difference (ATmin) in the stream temperature profile for the heat exchanger unit. The temperature level at which ATmin is observed in the process is referred as "PinchPoint" or "Pinch Condition". [1] Pinch analysis represents the application of the tools and algorithms of pinch technology for studying

2 2.1 Pinch Principle: The point where ATmin 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 fig. 1 (a). Tt had an excess of heat, therefore the cold utility requirement also increases by units. In conclusion the consequence of a cross-pinch heat transfer in Fig.l (a) is that both the hot and cold utility will increase by the cross pinch duty in fig (a). The understanding of pinch gives three rules that must be obeyed in order to achieve the minimum energy targets for aprocess:- Heat must be transferred across the pinch There must be no external cooling above the pinch There must be no external heating below the pinch.[2] Development of Pinch Technology Approach: Q c, n, The Pinch Divides the Problem into Source and Sink (a) H Q Hmin +a+p+y When the process involves single hot and cold streams it is easy to design an optimum heat recovery exchanger networks intuitively by heuristics method. In any industrial set up the number of streams is so large that the traditional design approach has been found to be limiting in the design of a good network. With the development of Pinch technology in the late 1980s, not only optimal network design was made possible, but also considerable process improvements could be discovered. Both the traditional and pinch approaches are depicted in fig.2 [3] Traditional Dcsij?" Approach.Design o7***n U^** eat Exchang System Pinch Technology Approach JJesign of Core Process eat Exchani!< System By violating the three golden rules Q HrTlin and Q Cmrr1 are each increased by a-fp+y. Fig. 1. (b) In Fig. 1 (b) amount of heat is transferred from above the pinch to below the pinch. The system 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 hat balance, the hot utility must be increased by the same amount that is units. Below the pinch, units of heat are added to the system that Fig. 2. Representation of Traditional and Pinch Design Approaches 2.2 Traditional Design Approach: First, the core of the process is designed with fixed flow rates and temperatures yielding the heat and mass balance for the process. Then the design of a heat recovery system is completed. Next, the remaining duties are satisfied by the use of the utility system. Each of these exercises is performed <ET>

3 independently of the others. [4] 2.3 Pinch Technology Approach: Process integration using the pinch technology offers a novel approach to generate targets for the minimum energy consumption before heat recovery network design. Heat recovery and utility system constraints are then considered in the design of the core process. Interactions between the heat recovery and utility systems are also considered. The pinch design can reveal opportunities to modify the core process to improve heat integration. This pinch approach is unique because it treats all processes with multiple streams as a single, integrated system. This method helps to optimize the heat transfer equipment during the design of the equipment. [4] 2.4 Areas of Application of Pinch Technology: Pinch originated in the petrochemical sector and is now being applied to solve wide range of problems in mainstream chemical engineering. Wherever heating and cooling of process materials take place there is potential opportunity. Thus initial applications of technology were found in projects relating to energy saving in industries as diverse as iron and steel, food and drinks, textiles, paper and cardboard, cement, base chemicals, oil and petrochemicals. Early emphasis on energy conservation led to the misconception that conservation is the main area of application for pinch technology. The technology when applied with imagination can affect reactor design, separator design and the overall process optimization in any plant. It has been applied to processing problems that go far beyond energy conservation. It has been employed to solve problems as diverse as improving effluent quality, reducing emissions, increasing product yield, debottlenecking, increasing throughput and improving the flexibility and safety of the processes. Since its commercial introductions, pinch technology has achieved an outstanding record of success in the design and chemical manufacturing facilities. Documented results reported in the literature show that energy costs have been reduced by 15-40%, capacity debottlenecking achieved by 5-15% for retrofits, and capital cost reduction of 5 10% for new designs. [5] 2.5 Double Pipe Heat Exchangers: Atypical double pipe heat exchanger consists of one pipe placed concentrically inside another one of the large diameter, with appropriate end fittings in each pipe to guide the fluids from one section to the next. The inner pipe may have the external longitudinal fins welded to it either internally or externally to increase the heat transfer area for the fluid with the lower heat transfer coefficient. The double pipe section can be connected in various series or parallel arrangements for either fluid to meet pressure drop imitations andlmtd requirements. [5] 2.6 Applications of the Double Pipe Heat Exchanger: The major use of double pipe heat exchanger is for sensible heating or cooling of the process fluid where small heat transfer areas (typically up to 50cm) are required. They may also be used for the small amounts of boiling or condensation on the process fluid side. The advantages of double pipe heat exchangers are largely in the flexibility of application and piping arrangement, plus the fact that they can be erected quickly from standard components by maintenance crews. [5] 3. Methodology: 3.1 Steps of Pinch Analysis: In any pinch analysis problem a well defined stepwise procedure is followed. These steps are not necessarily performed on a once through basis, independent of one another. Additional activities such as re-simulations and data modification occur as the analysis proceeds and some iteration between the various steps is always required. 1. Identification of hot, cold and utility streams in the process: 'Hot streams' are those that must be cooled or are available to be cooled. 'Cold streams' are those that mustbe heated. 'Utility streams' are used to heat or cool process

4 streams, when heat exchanger between process streams is not practical or economic. The identification of streams needs to be done with care as sometimes, despite undergoing changes in temperature, the stream is not available for heat exchanger. 2. Thermal data extraction for process and utility streams: For each hot, cold and utility stream identified, the following thermal data is extracted from the process material and heat balance flow sheet: Supply temperature (Ts C): the temperature at which stream is available. Target temperature (Tt C): the temperature of the stream must be taken to. Heat capacity flow rate (Cp KW/ C): the product of flow rate (m) in kg/sec and specific heat (Cp KJ/Kg C)asin(l). CP = m*cp (1) and pinch analysis is carried out. A few ATmin values based on Linnhoff March's application experience for shell and tube heat exchanger is shown in Table II Table 2. No Industrial Sector Experience DT m in Values 1 Oil Refining 20-40C 2 Petrochemical 10-20C 3 Chemical 10-20C 4 Low Temp Processes 3-5C Composite curves consists 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. Fig.3 below illustrates the construction of hot composite curve for the process which has two hot streams. Their T-H representation is shown in Fig.3 (a) and their composite representation is shown infig.3 (b). Enthalpy change (dh) associated with a stream passing through the exchanger is given by the First Law of Thermodynamics as shown in Table I. STREAM NUMBER TABLE 1: TYPICAL STREAM DATA STREAM NAME SUPPLY TEMP. C TARGET TEMP. C HEAP CAP. FLOW. kw /C ENTH. CHANGE kw 1 FEED REAC.OUT PRODUCT RECYCLE O a O Enthalpy Change,kW (a) 3. Selection of initial ATmin value: The temperature of the hot and cold stream at any point in the exchanger must always have a minimum temperature difference (ATmin). This ATmin value represents the bottleneck in the heat recovery. In mathematical terms, at any point in the exchanger Hot stream temperature (TH) temperature (TC) > ATmin Cold stream To begin the process an initial ATmin value is chosen Fig. 3: Composite Curves 200O <b) Enthalpy Change, kw The construction of the hot composite curve simply involves the addition of the enthalpy changes of the streams in the respective temperature intervals.

5 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. Fig.4 illustrates the construction of Grand composite curve. SHIFTED observations canbe made from the fig.5 a. An increase in ATmin value results in higher energy costs and lower capital costs. b. A decrease in ATmin value results in lower energy costs and higher capital costs. c. An optimum ATmin exists where the total annual cost of energy and capital cost is minimized. \ 4 1 * Total Cost.. # # i ^^-"-""^ Energy Cost \ Fig. 4. Grand Composite Curve ENTHALPY <H> 5. Estimation of minimum energy costtargets:...-optimum DTmin DTmin Capital Cost * Once the ATmin is chosen, minimum hot and cold utility requirements can be evaluated from the composite curves. The GCC provides information regarding the utility levels selected to meet QHmin and QCmin requirements. If the unit cost of each utility is known, the total energy cost can be calculated by using (2). u Total energy cost=z Qu x Cu (2) u=l 6. Estimation of HEN capital costtargets: The capital cost of a heat exchanger network is dependent upon three factors: The numb er of exchangers The overall network area The distribution of area between the exchangers 7. Estimation of optimum ATmin value by Energy-Capital trade off: To arrive at an optimal ATmin value, the total annual cost (sum of total annual energy and capital cost) is plotted at varying ATmin values. Three key Fig. 5. Energy-Capital Cost Trade off (Opt DTmin) The optimum heat recovery level or the ATmin optimum for the process can be determined by varying the temperature approach. 8. Estimation of practical targets for HEN design: The heat exchanger network designed on the basis of the estimated optimum ATmin value is not always the most appropriate design. Recognizing the significance of pinch temperature allows energy targets to be realized by design of appropriate heat recovery networks. 9. Design of HEN: The design of HEN is best executed using the "Pinch Design Method". The systematic application of PDM allows the design of a good network that achieves the energy target within practical limit. This method incorporates the two fundamentally important features: It recognizes that the pinch region is the most constrained part of the problem(consequently it starts the design at pinch and develops by

6 moving away) b. It allows the design to choose between match options. The design of network based on certain guidelines like the "CP Inequality Rule", "Driving Force Plot", "Stream Splitting" and "Remain Problem Analysis". After the network has been designed according to the pinch rules, it can be further subjected to energy optimization. Optimizing the network involves both topological and parametric changes of the initial design in order to minimize the total cost. problem. The overlap between the composite curves represents the maximum amount of heat recovery possible within the process. The "overshoot" at the bottom of the hot composite represents the minimum amount of external cooling required and the "Overshoot" at the top of the cold composite represents the minimum amount of external heating (Hohmannl971). T A 4. Calculations: 4.1 Composite curve method: To handle multiple streams, we add together the heat loads or heat capacity flow rates of all streams existing over any given temperature range. Thus, a single composite of all hot streams and a single composite of all cold streams can be produced in the T/H diagram, and handled in just the same way as the two-stream problem. In Fig. 6 three hot streams are plotted separately, with their supply and target temperatures defining a series of "interval" temperatures T1T5. Between Tl and T2, only stream B exists, and so the heat available in this interval is given by CPB (T1-T2). T ' L Fig. 7. Values of AHfor each stream Fig. 8 shows a typical pair of composite curves in fact, for the four-stream problem given in Tables. h? H Interval (Ti-T 2 ) (B) (T2-T3 ) (A+B+C) Ts ' Fig. 6. Formation of the hot composite curve (T3-T4 ) P>T=)(A) H (A+C) However between T2 and T3 all three streams exist and so the heat available in this interval is (CPA + CPB+ CPC) (T2-T1). A series of values of AH for each interval can be obtained in this way, and the result re-plotted against the interval temperatures as shown in Fig.7 The resulting T/H plot is a single curve representing all the hot streams, known as the hot composite curve. A similar procedure gives a cold composite curve of all the cold streams in a ( O Heat flow!kwl Fig. 8. Composite curvesfor four-stream problem 1) 4.2 Problem table method: The problem table is the name given by Linnhoff and Flower to a numerical method for Determining the pinch temperatures and the minimum utility requirements; Linnhoff and Flower (1978). Once understood, it is the preferred method, avoiding the need to draw the composite curves and man oeuvre the composite cooling curve using, for example, tracing paper or cut-outs, to give the chosen minimum temperature difference on the diagram.

7 The procedure is as follows: Convert the actual stream temperatures Tact into interval temperatures Tint by subtracting half the minimum temperature difference from the hot stream temperatures, and by adding half to the cold stream temperatures: 100 C 65 C 60 C 7K~ _105 C 70 C 65 C Table 3: Actual and Shifted Temp. Stream number and type Actual temperatures Shifted temperatures TS C) TT C) SS C) ST C) Hot Hot C 7K 35 C Fig. 9. Streams and Temperature intervals Table C 3DoC Cold Cold Shifted temperatures, which are set at l/2atmin (5 C in this case) below hot stream temperatures and l/2atmin above cold stream temperatures as shown in Table 3. Carry out a heat balance for the streams falling within each temperature interval. For the nth interval: AHn = (ZCPC -ZCPH)x(ATn) Where, AHn = net heat required in the nth interval, ZCPC = sum of the heat capacities of all the cold streams in the interval ZCPH = sum of the heat capacities of all the hot streams in the interval ATn = Interval temp difference = (Tn-1 - Tn) Table 4. Stream number & type Actual temp. TS'C TT C Avg Temp. Tavt; "C cp kj/kg k m kg/s ITeat capacity flow rate CP kw/k 1 lot Hot S1 100C S2 65C S3 60C S4 50C S5 35C Interval Si-Si+1 number i C CPHOT - CPCOLD (KW/ C) Hi (kw) Surplus or deficient Surplus Surplus Surplus Deficit Cascading the heat from one interval to the next implies that the temperature difference is such that the heat can be transferred between the hot and cold streams. The presence of a negative value in the column indicates that the temperature gradient is in the wrong direction and that the exchange is not thermodynamically possible. This difficulty can be overcome if heat is introduced into the top of the cascade: Introduce just enough heat to the top of the cascade to eliminate all the negative values; seefig.9&10. Cold Cold

8 From hot utility OkW 1 AH ( 952KW 1.952kW.. 2 AH 3.0S3SKW from hot utility kw 1 AH 1.952m kw 2 AH KW temperature of waste heat in an industrial area that is available for export. Depending on the temperature of this waste heat, it can be used for district heating or power generation. 6.2 Total Site Analysis: kw_ 3 AH (MkW 5016KW 4 AH -B.2845kW To cold utility [a) nfeasible BkW KW.. 3 AH 01 kw 8.2B4KW 4 AH -e.2s45kw OkW la cold utility (b) Feasible Fig 10. Feasible and Infeasible HeatCascades 5. Improvements: The preference is to pass the hot fluid through the inner tube to reduce heat losses, while the annulus is reserved for the high viscosity stream to limitthe pressure drop. The inner pipe should be constructed of more thermally conductive material as compare to outer pipe to maximize the heat transfer. In double pipe heat exchanger the efficiency is more in counter current flow arrangement as compare to co-current flow arrangement so counter current flow is preferred. The insulation of the outer pipe should be proper to decrease the heat losses. 6. Future Outlook: 6.3 NetworkPinch: Typically, refinery and petrochemical processes operate as parts of large sites or factories. These sites have several processes serviced by a centralized utility system. There is both consumption and recovery of process steam via the steam mains. The site imports or exports power to balance the on-site power generation. The process stream heating and cooling demands, and co-generation potential, dictate the site-wide fuel demand via the utility system. In such large sites, usually the individual production processes and the central services are controlled by different departments which operate independently. The site infrastructure usually suffers from inadequate integration. To improve integration, a simultaneous approach to consider individual process issues alongside site wide utility planning is necessary. Similar to a single process, a Total Site Analysis using Pinch Technology can be used to calculate energy targets for the entire site. For example, how much low pressure, medium pressure, and high pressure steam should the site be using? How much steam can be raised and how much power it can generate? This also helps to identify key process changes that will lower the overall site utility consumption. The development of Pinch Technology started in the late 1970s and still continues. Besides applications in energy conservation, new developments in Pinch Analysis are being made in the areas of water use minimization, waste minimization, hydrogen management, plastics manufacturing, and others. A few of key areas of research are mentioned below. 6.1 Regional Energy Analysis: By examining the net energy demands of different companies combined, the potential for sharing heat between companies can be identified. These analyses can lend insight into the amount and When optimizing energy consumption in an existing industrial process, a number of practical constraints must be recognized. Traditional Pinch Technology focuses on new network designs. Network Pinch addresses the additional constraints in problems associated with existing facilities. This analysis identifies the heat exchanger forming the bottleneck to increasing heat recovery. Then provides a systematic approach to remove this bottleneck. This step-by-step method provides an approach for implementing energy savings in a series of consecutive proj ects. Top Level Analysis: (

9 Gathering the required data in industrial areas is not an easy task. With a Top Level Analysis, only efficiencies and constraints of the utility system are used to determine which utility is worth saving. Data can be gathered from those processes or units that use these utilities. A pinch analysis can then be performed on this equipment. Optimization of Combined Heat and Power: Typically, multiple steam turbines are used in complex steam systems. CHP optimization gives a way to determine the load distribution in a network of turbines with a given total load. 6.4 Water Pinch: In view of rising fresh water costs and more stringent discharge regulations, Pinch Analysis is helping companies to systematically minimize freshwater and wastewater volumes. Water Pinch is a systematic technique for analyzing water networks and reducing water costs for processes. It uses advanced algorithms to identify and optimize the best water reuse, regeneration, and effluent treatment opportunities. It has also helped to reduce losses of both feedstock and valuable products in effluent streams. 6.5 Hydrogen Pinch: The Pinch Technology approach applied to hydrogen management is called Hydrogen Pinch. Hydrogen Pinch enables a designer to set targets for the minimum hydrogen plant production and/or imports without the need for any process design. Methods have also been developed for the design of hydrogen distribution networks in order to achieve the targets. Hydrogen Pinch also lends insight into the effective use of hydrogen purification units. By comparing the two methods it is noted that the Problem table provides the simple framework for numerical analysis and it is applicable even where simplified assumptions are invalid. Energy targeting is a powerful design and process integration aid. With all of the tools that pinch analysis provides, one of the most important challenges before process engineers is to properly integrate pinch tools into the conceptual process design phase. Using pinch technology tools and understanding the process does not ensure the desired results. These tools must be applied at the right point in the process design phase. It is Pinch Technology's role to identify "what might be". However, input from other engineering disciplines ultimately determines "what can be". References [1] B. Linnhoff and G.T. Polley, "General Process Improvements through Pinch Technology", Chem. Engg. Progress, June [2] Morgan, S., "Use Process Integration to Improve Process Designs and the Design Process", Chem. Engg. Progress, 88(9), pp.62-68, Sept [3] B. Linnhoff, "Pinch Technology Has Come of Age", Chem. Engg. Progress, July 1984 [4] Su Ahmad, Stephen G. Hall, Steve W. Morgan, and Stuart J. Parker, "Practical Process Integration - An Introduction to Pinch Technology", Aspen Technology, (2002). [5] Ian C. Kemp, "Pinch analysis and process integration", 2nd edition, Butterworth-Heinemann Publishers, Jan Conclusions: From above it is concluded that: Composite curves give conceptual understanding of how energy targets can be obtained. The problem table and its graphical representation, the GCC, give the same results more easily.