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Keywords:

  • absorption heat pump;
  • process integration;
  • energy upgrading;
  • pulp and paper

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

Integration of absorption heat pumps (AHP) in industrial processes has not yet been fully exploited due to the lack of clear implementation guidelines for this technology. In this work, a systematic methodology for the integration of AHPs in a process has been developed and is presented. Guidelines are formulated for the proper selection of heat sources and sinks that will maximise the benefit derived from heat pumping while respecting process constraints and operating requirements of the AHP. The principles of AHP operation and its efficient process integration are thus described. The methodology relies on data extracted from a Pinch Analysis of the plant. The advantages and outputs of the methodology are illustrated using an AHP implementation in a Kraft pulping process. Two realistic implementation options are presented along with their detailed design and preliminary economic evaluation.

L'intégration de pompes à chaleur à absorption (PCA) dans les procédés industriels n'a pas encore été exploitée pleinement en raison du manque de lignes directrices de mise en œuvre claires pour cette technologie. Dans ce travail, une méthodologie systématique pour l'intégration de PCA dans un procédé a été créée et est présentée. Des lignes directrices sont formulées pour choisir adéquatement les sources de chaleur et puits qui maximiseront l'avantage dérivé des pompes à chaleur tout en respectant les contraintes des procédés et les exigences relatives au fonctionnement des PCA. Les principes du fonctionnement des PCA et leur intégration efficace dans les procédés sont par conséquent décrits. La méthodologie repose sur les données extraites d'une analyse Pinch de l'usine. Les avantages et résultats de la méthodologie sont illustrés à l'aide d'une mise en œuvre des PCA dans un procédé kraft. Deux options de mise en œuvre réalistes sont présentées avec leur conception détaillée et évaluation économique préliminaire.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

Heat pumps (HP) are thermal machines used to increase the temperature at which a certain amount of heat is available. Absorption heat pumps (AHP) are emerging as a potential alternative to the more common vapour recompression heat pumps (VRHP). AHPs use the effect of pressure on the absorption–desorption cycle of a solution to create a temperature increase of the available heat, that is, the temperature lift. They are thermally driven and, when judiciously positioned into an industrial process, they can be operated in such a way that heat is pumped and energy is recovered at the same time.

Early methods to identify and evaluate heat pumping opportunities generally used a unit operation-oriented approach. Large utility consumers, such as evaporators and distillation columns, were easily identified and a HP could be integrated within the chosen unit (Flores et al., 1984; Ferre et al., 1985; Supranto et al., 1986; Eisa et al., 1987). More recent work introduced a complete process approach for HP integration and showed the importance of taking into account the thermodynamics of process integration, particularly Pinch Analysis (Linnhoff, 1993). The integration procedure for some types of HPs, such as conventional VRHP, is well-defined and discussed in the literature (Ranade et al., 1986; Ranade, 1988; Chappell and Priebe, 1989; Wallin et al., 1990; Wallin and Berntsson, 1994). It is shown that for a correct assessment of heat pumping opportunities, a full understanding of the thermodynamics and economics of both processes and utility systems, together with the associated interactions is needed. However, integration of more complex configurations like AHPs and absorption heat transformers in a process has not been thoroughly investigated. Marinova et al. (2007) presented guidelines for the implementation of a trigeneration unit (cold, heat, and power production simultaneously) in a Kraft process. They clearly showed that both process and AHP constraints must be considered simultaneously to ensure proper implementation and energy gains.

In view of many failed or deceiving AHP process integrations, yielding no or very little benefit, guidelines are formulated for the most efficient retrofit integration of an AHP into a process, simultaneously considering thermal and thermodynamic constraints from both the process and the AHP systems. The integration should begin by extracting the process stream data and ideally proceed by considering the heat exchanger network (HEN) to maximise the internal heat recovery, which generally is a more attractive proposition than the implementation of HPs (Yee and Grossman, 1990; Sama, 1996). This preliminary phase, while strongly suggested, is however not mandatory for the correct implementation and application of the method described here.

WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

Numerous technical and scientific publications have been dedicated to the fundamental principles, operation, and design of HPs (Ziegler and Riesch, 1993; Herold et al., 1996; Costa et al., 2009). In order to produce the temperature lift in a HP, a refrigerant fluid is circulated between the evaporator (E) and the condenser (C) which operates at a higher temperature and pressure (Figure 1a). The available heat, which is generally not purchased, is fed into the HP through the evaporator and released at the condenser. The circulation of refrigerant between the evaporator and the condenser is obtained by means of a compression device, a compressor in the case of VRHP (Figure 1a); this type of HP is driven by high-quality energy (shaft work or electricity). The compression system of an AHP consists of a secondary loop (Figure 1b) in which a binary solution is circulated between the absorber (A) operating at the same pressure as the evaporator, and a desorber, generally called the generator (G). The refrigerant is the most volatile component of the binary solution. The vaporised refrigerant exiting the evaporator is absorbed into the solvent-rich solution in the absorber, causing the release of additional useful heat, QA. The weak solution thus formed is pumped into the generator where the refrigerant is desorbed at a higher pressure and temperature under the effect of the driving force, supplied as heat, QG. In a real machine, a fifth heat exchanger (solution heat exchanger; SHX) is inserted between the generator and the absorber to preheat the weak solution with the rich solution, thus reducing the demand for high-quality driving energy. This configuration of an AHP is also referred to as an AHP type I. When an AHP is properly integrated into an industrial process, the driving energy is generally not purchased, and represents an additional economic incentive.

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Figure 1. Principle of heat pumping: (a) VRHP, (b) AHP.

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Some of the working fluid pairs that have been proposed for industrial uses of AHPs include H2O/LiBr, H2O/H2SO4, NH3/H2O, H2O/NaOH, H2O/glycerol, H2O/nitrate salts, and H2O/zeolites. The most commonly used working pairs for industrial applications are NH3/H2O and H2O/LiBr with NH3 and H2O as the refrigerants, respectively.

If the high and low pressure (LP) zones of the machine are reversed, the driving heat is supplied at the evaporator-generator pair, at the intermediate temperature level, while the useful heat is released by the absorber at higher temperature and by the condenser at lower temperature; this configuration is the absorption heat transformer (AHT) or AHP type II (Figure 2). For the thermal analysis of an AHP, it is convenient to represent the two circulation loops in the vapour–liquid phase diagram of the binary solution (Figure 3a), which shows the thermal operating constraints imposed on the system, the pure refrigerant evaporation line on the low temperature side and the solvent crystallization line on the high-temperature side. This type of diagram led to a very simplified schematic representation of AHPs which illustrates how the machine operates (Figure 3b).

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Figure 2. Schematic of an AHT (AHP type II).

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Figure 3. AHT representations. (a) In H2O/LiBr phase diagram. (b) Schematic of AHT. (c) Double effect AHT. (d) Double-lift AHT.

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To overcome the limitations imposed by the thermodynamics and the irreversible effects associated with heat transfer, the coupling of several cycles with internal heat transfer is also possible. To achieve higher-cycle performance, a double cycle that takes advantage of the higher availability (exergy) associated with a higher-temperature input is used. The double effect cycle represents such a cycle variation (Figure 3c). Other high-performance cycles are also possible and are presented in the literature (Ziegler and Riesch, 1993; Ziegler, 1999). It is possible to reach higher-temperature lifts by coupling cycles, especially for low temperature applications; the double-lift cycle represents one such cycle variation (Figure 3d).

The coefficient of performance (COP) is used to define the heating performance of HPs. It is defined as the ratio of the heat delivered by the HP to the driving energy; for an AHP type I, COP = (QC + QA)/(QG). Typical COP values for different configurations are presented by Ziegler and Alefeld (1987).

HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

Pinch Analysis is a technique used to maximise internal heat recovery within a process. Its scientific principles and utilisation procedures have been presented in engineering manuals (Linnhoff, 1993; Linnhoff et al., 1994). A basic step of the method is the representation in the temperature versus enthalpy diagram of the aggregate of all possible heat transfers between process streams. It consists of two composite curves, one for the streams that can be used as heat sources (hot streams) and one for the streams that can be used as heat demands (cold streams). The development of the composite curves, their relation in the T versus ΔH graph and their significance have been described elsewhere (Kemp, 2007). The composite curves are used to determine the pinch temperature and energy targets as minimum heating requirement (MHR) and minimum cooling requirement (MCR) for a given process. The thermodynamics of Pinch Analysis dictates a fundamental rule: there must not be transfer of heat from above to below the pinch-point. If this happens, the process suffers a double penalty: the simultaneous increase of the cooling and heating requirements of the process. On the other hand, an HP must transfer heat in the opposite direction from below to above the pinch-point; so it should be integrated in such a way that the heat source is situated where there is an excess of heat, that is, below the pinch, and the heat sink where there is a need for heat, that is, above the pinch (Ranade and Nihalani, 1986; Bakhtiari et al., 2007).

The composite curve diagrams can be used to position HPs in the process so as to maximise the overall energy benefit. The composite curves diagram are preferred rather than the grand composite curve diagram (Kemp, 2007), because it represent all streams being involved in the process and thus, later can be used to redesign the HEN. Figure 4a illustrates the simple case of a VRHP. When, as shown, the heat is supplied to the evaporator by a hot stream below the pinch-point, the MCR is reduced by the amount QE; similarly, if the heat is released by the condenser to a cold stream above the pinch-point, the MHR is reduced by the same amount QC. It can be easily verified that when this condition is not met, the benefit of implementing the HP is practically nil. The case of an AHP is slightly more complex, because there can be three points of heat exchange between the process and the HP (Figure 4b). The condenser and absorber must release their heat above the pinch-point to reduce the MHR; the generator which is at a higher temperature can thus only be above the pinch-point and, to reduce the MCR, the evaporator must be below the pinch-point as in the case of the VRHP. The overall energy gain is effectively the same as for a VRHP of equal power, but the need for shaft work is eliminated. The correct positioning of an AHP type II (AHT) is also presented in Figure 4c.

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Figure 4. Correct positioning of HP based on Pinch Analysis; (a) VRHP, (b) AHP type I, (c) AHP type II (AHT).

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The difficulty when implementing an AHP in a process is to determine the streams that will best fit the AHP, and what is the best configuration and working pair, taking into account the working fluid phase diagrams and other design and technical constraints. The objective of the proposed method is to answer those questions and to show how the process, the working fluid pair and the types of AHP are intimately linked.

METHODOLOGY FOR AHP POSITIONING IN A PROCESS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

The method considers both the process and AHP thermodynamics and design aspects. In this section, the process aspects to be considered for the implementation of AHPs are presented followed by AHP design aspects and interrelation between both. A list of tasks is introduced for the optimal integration of an AHP in a process.

Process Analysis

The method assumes that a Pinch Analysis of the process has been performed and thermal data on all streams are available. Since heat pumping is more expensive than heat exchanging, the HEN should first be considered and the process should be heat-integrated. It should be mentioned that other energy recovery methods such as condensate return and water closure are recommended before Pinch Analysis and HP implementation. In general, they are cheaper energy recovery measures and they help reduce the pinch-point temperature, which facilitates the implementation of AHPs.

The process streams that could be used by the HPs should be selected on the basis of their location in the composite curve and their heat loads. In the first attempt for the implementation of an AHP, streams which are heated or cooled by a utility are selected, because their heat content is wasted to the environment and not used in the internal heat recovery network. HEN reconfiguration and the utilisation of aftermath streams may be considered in later attempts as will be discussed later. The available streams can be classified into four groups; heat sources (H) and sinks (C) (hot and cold streams) above (A) and below (B) the pinch-point, that is, HA, HB, CA, and CB (step 1). As shown in Figure 4, all available streams above or below the pinch-point have the potential to be used in the AHPs. Some HB streams could be used as heat sources for an AHP type I and CA streams as heat sinks for an AHP type II. Some HA streams could be used as the driving energy for AHP type I and CB streams as low temperature heat sinks for an AHP type II.

Since the objective of a HP is to reduce MHR and MCR by energy transfer from below to above the pinch-point, each potential streams combination for an AHP contains a CA and a HB stream. An opportunity stream (a CA or a HB stream) should be selected to produce a catalogue of different stream combinations between the opportunity stream and the corresponding available CA or HB streams. The opportunity stream may be chosen in order to solve a particular heating or cooling requirement identified in a process. Streams with temperature closer to the pinch-point and high-heat loads are most desirable whenever feasible, because they require a lower temperature lift while supplying a large amount of energy to be upgraded. For each stream combination, the maximum evaporator temperature is estimated taking into account the target temperature of the HB stream and the heat exchanger temperature approach ΔT. The minimum condenser/absorber temperature (for AHP type I) or absorber temperature (for AHP type II) is estimated for each of selected heat sink stream and chosen ΔT as will be discussed in the Interactions Analysis Section. In order to produce a set of thermodynamically feasible options, some of the choices should be eliminated because of process technical constrains such as physical distances between streams, and maximum or minimum allowable temperature and pressure of process streams.

AHP Analysis

In this methodology, the phase diagrams of different working pairs are used to estimate generators temperature (for AHPs type I) and condensers temperature (for AHPs type II) for the different AHP configurations. The components loads can be estimated in the first steps of the procedure by the fast estimation method proposed by Ziegler and Alefeld (1987), which is presumed to yield COP values with an accuracy of 10% in most cases, sufficient for the purpose of preliminary estimation. Different AHP designs and technical constrains such as maximum or minimum allowable operating temperatures and pressures, and the risk of crystallization should be considered to eliminate some of the potential choices. In the final step of the procedure, a reliable thermodynamic and costing model for AHPs should be used for a high-quality design and cost evaluation.

Interactions Analysis

The temperatures of the AHP components should be estimated considering the interrelation between the process and the AHP. The procedure is illustrated in Figure 5 with two examples; a single-stage AHP types I and II. As mentioned earlier, the maximum evaporator temperature is estimated [solid line in Figure 5a (TE) and b (TE/G)] as a function of the target temperature of the HB stream and heat exchanger temperature approach ΔT. Then, the minimum condenser/absorber temperature (for AHP type I) or absorber temperature (for AHP type II) is estimated [square dot line in Figure 5a (TC/A) and b (TA)] for each selected heat sink stream and chosen ΔT. The dashed lines in Figure 5a and b show the generator temperature (TG) for a single-stage AHP type I and condenser temperature (TC) for a single-stage AHP type II. This phase-diagram mapping should be completed for all selected configurations and working fluid pairs.

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Figure 5. Estimation of the maximum and minimum temperatures (a) AHP type I and (b) AHP type II.

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The controlling stream determines the total amount of useful heat to be produced by the AHP type I (QA + QC), or by the AHP type II (QA). The capacity of the components releasing heat to the process is subsequently computed by means of the equation for the COP and taking into account the basic relation QG = QA + QC − QE. It is desirable to have the opportunity stream also as the controlling stream. If it is not the case, one could decide to eliminate that option or consider a combination of streams as the heat source or heat sinks, depending on the situation. From the calculated heat loads and selected temperatures of the generator (for AHP type I) or calculated loads and selected temperatures of the condenser (for AHP type II), and using selected process stream and site utility information, the appropriate streams for the generator (case of AHP type I) or the condenser (case of AHP type II) can be identified. Later, a set of thermodynamically feasible options should be produced by eliminating some of the potential choices, considering both the process and AHP design and technical constraints as discussed above.

A thermodynamic and costing model for AHPs is used for cost evaluation of thermodynamically feasible options. As different economic criteria are used industrially, economic evaluations will be made with three different criteria or combinations there of: pay-back period (PBP), annual net profit, and maximum allowed investment cost. Finally, all the practically possible and economically feasible opportunities are identified and can be used in a decision-making procedure as to whether the AHP implementation is interesting and detailed design is required. It should be mentioned that the chosen ΔT approach may have an effect on the stream selection and the implementation economics. A sensitivity analysis of ΔT should be done in order to identify the optimal feasible implementation of an AHP in the process.

There are some situations where no practically possible and economically feasible opportunity is identified. In those cases, reconfiguration of the HEN should be considered. The HEN should be modified in such a way as to make other CA or HB streams (which are currently used in the HEN) available for heat pumping at temperatures closer to the pinch-point to avoid large temperature lifts and yield a practical AHP design. In general, the new HEN configuration causes some increase of the total heat exchanger area (capital cost), but the utility demand does not change. In this case, the additional heat exchanger area should be considered in the economic evaluation to determine whether the economic of implementation has been improved by the HEN reconfiguration or not.

Procedure

The procedure for the retrofit of AHPs into manufacturing processes as can be summarised by the following sequence of steps:

  • 1.
    Selecting the process streams potentially usable by an AHP.
  • 2.
    Producing a set of different stream combinations (CA/HB).
  • 3.
    Estimating the temperatures of the AHP components.
  • 4.
    Computing all AHP heat loads.
  • 5.
    Identifying appropriate streams for the generator (type I) and condenser (type II).
  • 6.
    Producing a set of thermodynamically feasible options.
  • 7.
    Dimensioning and costing.
  • 8.
    HEN reconfiguration (if required).

The proposed method introduces several features which reveal superior designs to what could be achieved by either of the two conventional methods for designing AHPs and integration of traditional compression-based HPs. Indeed, it considers the interactions between the AHP and the process. It determines the streams that will best fit the AHP, and the best configuration and working pair, taking into account working fluid phase diagrams and other design and technical constrains.

CASE STUDY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

Context

Pulp and paper manufacturing is among the most energy consuming industrial sectors. For example, typical Kraft mill in Canada consumes 25 GJ/adt1 of pulp produced and a recently built mill consumes 12 GJ/adt (Browne, 1999). In the Kraft pulping process, the lignin which binds the cellulosic fibres in the wood chips is solubilised by a strong alkaline solution at high temperature and pressure (Smook, 2002). This operation is conducted in reactors called digesters. The spent liquor from the digesting step, the black liquor, is concentrated in a series of evaporators and burned in recovery boilers. In the combustion process, a smelt of sodium sulfide and sodium carbonate is produced and recovered. These chemicals are then recausticised with lime and recycled back to the digester. The cellulosic fibres are separated from the black liquor before it is concentrated in a series of counter-current washers and to form the paper pulp. The pulp is bleached, dried to about 90% solids, cut in sheets and baled for shipment to customers. A simplified schematic of the complete Kraft process is given in Figure 6.

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Figure 6. Schematic representation of the Kraft process.

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The mill supporting this study produces 700 ton of high-grade bleached Kraft pulp per day. The average steam requirement of the mill is 161 MW (19.1 GJ/adt) produced by four boilers. High-pressure steam is produced at 3034 kPa (T = 371°C) by two recovery boilers (64% of total load), a biomass boiler using wood residue (27%) and a bunker oil boiler (9%). About 25% of the high-pressure steam is used directly, 25% is throttled down to medium pressure (MP) at 965 kPa (178°C) and 50% to LP at 345 kPa (144°C).

Data were taken from an existing simulation of the mill to develop the composite curves (Figure 4); 61 streams were selected as potential heat sources or sinks (Mateos-Espejel et al., 2008b). A minimum temperature approach ΔTmin of 10°C has been chosen, which yields the pinch temperature of 71°C; where hot and cold pinch temperatures are 76 and 66°C, respectively. The maximum heating and cooling requirements of the process are 178 and 65.2 MW, while the minimum heating and cooling requirements (MHR and MCR) are 122.8 and 10 MW, respectively. It was shown in a different study that by maximisation of the internal heat recovery, there is an opportunity to reduce the steam consumption of the mill by 30 MW (Mateos-Espejel et al., 2008a).

Background

Several feasibility studies of implementation of AHPs in the P&P Industry have been published (Abrahamsson et al., 1994; Gidner et al., 1996; Costa et al., 2009). None of them is based on the results of Pinch Analysis. Since the pinch-point temperature, which plays an important role for the correct positioning of AHPs, is not presented and different mills have different pinch-points, it is not easy to find if those implementations are well positioned or not. However, it can still be determined for which pinch-point ranges they would have been positioned correctly. For example, in the study by Gidner et al. (1996), an AHP type II is implemented in the evaporation section in which the evaporator, generator, absorber, and condenser temperature are 95, 69, 125, and 20–25°C, respectively. For the correct positioning of AHP, considering ΔTmin of 10°C, the pinch-point should have been between 90 and 120°C, which seems high for a Kraft process.

In a previous study by Costa et al. (2009) presented a preliminary feasibility study of the implementation of various AHP configurations in a Kraft pulping process using data from the same manufacturing mill. They have shown that the implementation of AHPs is feasible and cost-effective. Since the implementation of the pumps into the process did not observe the principles that are formulated here, in some cases the overall energy benefit is not as assumed in those studies. As an example, a double-lift AHP type II was installed to recover heat from the steam flashed at the discharge of the digester. That installation violated the pinch rule by using a stream at 96°C to produce LP steam, therefore, affecting a transfer of heat loads rather than a reduction of steam demand. The present case study shows how the proposed method can be used to avoid such errors and correctly position an appropriately designed AHP in the process.

Procedure

Step 1: Table 1 shows the selected streams that are available heat sources and sinks in the process after maximisation of the internal heat recovery. HB1 stands out as an interesting heat source; its temperature (75.9°C) is just below the hot pinch temperature and it carries a high load (12.2 MW). There are five heat sinks available, CA1–CA5. However, it should be noted that because of high-pulp concentration, CA1 and CA2 will not be considered due to the difficulties of handling such concentrated streams.

Table 1. Selected streams
SectionDescriptionState (C or H)Tstart (°C)Ttarget (°C)Flow (kg/s)Heat load (kW)
Heat sinks above the pinch-point (CA)
 BleachingPulp through vapormixer 3C68.675.058.81471
 BleachingPulp through vapormixer 4C71.985.051.72626
 Evap. #3Liquor through HXCC104.9132.214.08907
 Evap. #2Liquor through HXCC101.4127.814.57363
 DearatorCondensate and fresh waterC6610067.69653
Heat sinks below the pinch-point (CB)
 Fresh water from 4 °C (winter) or 20 °C (summer) to any temperature below the pinch-point
Heat sources above the pinch-point (HA)
 BoilersFlue gas from CR2H164.0105.863.54314
 BoilersFlue gas from CR3H199.0117.647.64625
 BoilersFlue gas from bark boilerH182.0126.547.72863
Heat sources below the pinch-point (HB)
 Evap. #2Vapor condensingH75.974.95.312 211
 BleachingEffluent, washer 1H58.133.058.86150
 BleachingEffluent, washer 2H68.633.067.09911
 BleachingEffluent, washer 3H68.233.018.82768
 BleachingEffluent, washer 5H66.833.09.51338
 BleachingEffluent washer 4H73.133.036.06027

Step 2: As a result, the combination map is HB1& CA3, HB1& CA4, and HB1& CA5.

Step 3: In the next step the temperatures are estimated. Considering the temperature of the heat source stream (75.9°C) and ΔT = 10°C, the maximum evaporator temperature should be 65.9°C. The minimum condenser/absorber temperature or absorber temperature depending upon the type of HP that may be utilised (AHP type I and type II) are estimated as 142, 138, and 110°C for CA3, CA4, and CA5, respectively. In this example, six different configurations (single-stage, double effect, and double-lift AHP type I or type II) for the two common working fluid pairs (LiBr-H2O and NH3-H2O) are considered; this gives 36 different combinations. Table 2 shows the thermodynamically feasible configurations for the resulting combinations. Those that are not presented in the table are too remote from feasible conditions; some would lead to excessive generator temperatures for an AHP type I, some to very low condenser temperatures for an AHP type II others would fall in the crystallization zone. The estimated generator temperature for the AHP type I cases and condenser temperatures for the AHP type II cases are presented for the 20 remaining feasible configurations.

Table 2. Thermodynamically feasible configurations
CaseH.Si* streamConfigurationWorking pairCOPQG (MW)QE (MW)QA (MW)QC (MW)TC (°C)Low-TH.SiTG (°C)High-TH.So
  • *

    H.Si, heat sink; H.So, heat source; SS, single-stage; DL, double-lift; HP, high-pressure steam; MP, medium pressure steam; NF, not feasible; FW, fresh water; FG, flue gas.

1CA3Type I SSH2O-LiBr1.725.173.735.173.73  250HP
2DLH2O-LiBr1.36.852.053.974.93  190HP
3NH3-H2O1.237.241.664.414.49  200HP
4Type II SSNH3-H2O0.425.127.085.127.0810NF  
5DLH2O-LiBr0.297.444.763.548.6630FW  
6NH3-H2O0.275.256.953.298.9130FW  
7CA4Type I SSH2O-LiBr1.724.33.14.33.1  220HP
8DLH2O-LiBr1.35.691.713.34.1  190HP
9NH3-H2O1.236.021.383.663.74  190HP
10Type II SSNH3-H2O0.425.127.085.127.0810NF  
11DLH2O-LiBr0.297.444.763.548.6635FW  
12NH3-H2O0.275.256.953.298.9130FW  
13CA5Type I SSH2O-LiBr1.725.614.045.614.04  160HP
14NH3-H2O1.556.233.426.233.42  170HP
15DLH2O-LiBr1.37.412.234.315.34  140HP-MP-FG
16NH3-H2O1.237.851.84.784.87  150HP-MP-FG
17Type II SSH2O-LiBr0.475.736.55.736.530FW  
18NH3-H2O0.425.127.085.127.0825FW  
19DLH2O-LiBr0.297.444.763.548.6630FW  
20NH3-H2O0.275.256.953.298.9130FW  

Step 4: Once the COP value has been estimated by the Ziegler & Alefeld method (Ziegler and Alefeld, 1987), the load for each component can be computed and their values are given in Table 2 (Q's). For all AHPs type II cases, fresh water could be used as low temperature heat sink; except in cases 4 and 10, where the condenser temperature is too low and that will not be considered. For all AHPs type I, high-pressure steam could be used as the high-temperature driving energy. For some of them, MP steam could also be used (cases 15–16) and for others, part of the required heat load could come from flue gases (cases 15–16) (step 5).

Step 5: It consists of eliminating additional combinations by considering design and operating constraints. Some of the cases are clearly impractical and are readily eliminated. Considering the corresponding working pair equilibrium diagram; cases 1, 2, 3, 7, 8, and 9 are eliminated because of the high-generator temperature (they have generator temperature at 190°C and above), cases 4 and 10 are eliminated because 10°C would be required for the condenser temperature. Cases 1 and 7 could also be eliminated because of the high risk for crystallization and Case 14 is eliminated because of the high-operating pressure of its cycle.

Figure 7a and b illustrate why cases 1 and 10 are eliminated. In Figure 7a, the condenser/absorber and evaporator temperatures are used to illustrate the thermodynamic cycle of case 1. The estimated generator is clearly too high for current AHP technology (250°C). Also, because of the high-LiBr concentration in the solution side, there is a potential risk of crystallization during operation. Figure 7b shows that case 10 is eliminated because of the low condenser temperature. Reaching a temperature of 10°C in the condenser with a ΔT of 10°C, would entail a heat sink at or below 0°C, which is not practical. It should be mentioned that it has been verified by observation of the site mill layout that all selected streams are relatively close to each other and will not require extensive piping work. Another reasonable constraint to be considered to eliminate additional cases is the fact that as much as possible of the available energy of the selected heat source (12.2 MW) should be upgraded. Cases 13, 15, and 16 are thus eliminated, because they use only 33%, 18%, and 15% of the available 12.2 MW, respectively. At the end of this elimination procedure, 8 cases (cases 5, 6, 11, 12, 17, 18, 19, 20) remain from the 36 identified originally; they are all single-stage or double-lift AHPs type II. They will now be compared on the basis of economic and thermodynamic criteria for the final choice.

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Figure 7. Eliminated cases in LiBr-H2O and NH3-H2O equilibrium diagram; (a) case 1; high-generator T and risk of crystallization, (b) case 10; low condenser T.

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Step 6: Case 17, which has the highest estimated delivered heat (5.73 MW) as a single-stage AHP type II, and case 19, which has the highest estimated delivered heat (3.54 MW) among the double-lift AHPs type II, are chosen to be dimensioned and cost estimated, as they are likely to be the most attractive options.

Step 7: For this purpose, a simulation model for the single-stage and the double-lift AHP type II using the LiBr/H2O working pair was used (Bakhtiari, 2009). It calculates the internal mass and energy balances of each component as well as the heat transfer between external and internal streams in a steady-state operation. The parameters of a cycle such as output temperatures, pressure of each components, heat load of heat each exchanger and COP are calculated from input values such as input temperatures and mass flow rates. The results from this simulation were validated with data from the literature (Herold et al., 1996). Table 3 presents the basic design parameters and simulation results. The useful delivered energy in the absorber is calculated at 5.83 and 3.89 MW for cases 17 and 19. The power to be supplied to the generator and evaporator were then calculated at 5.76 and 6.55 MW for case 17 and 7.45 and 4.76 MW for case 19. The COP is also calculated as 0.48 and 0.32 for cases 17 and 19. Figure 8 shows the feasible cycles in the LiBr/H2O equilibrium diagram.

Table 3. Basic design parameters for selected cases
VariableUnitEGASHXCE1-A1
Case 17 (SS AHP type II)
 LMTD°C9.310.317.310.912.8 
 PressurekPa26.33.926.326.33.9 
 U.A.kW/°C693.5559.2337.0100.0498.4 
 QMW6.55.85.81.16.4 
Case 19 (DL AHP type II)
 LMTD°C11.211.622.914.58.84.2
 PressurekPa24.24.333.324.24.324.2–33.3
 U.A.kW/°C425.0642.2169.993.0945.5904.8
 QMW4.87.53.91.38.33.8
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Figure 8. The selected cycles in LiBr/H2O equilibrium diagram; (a) case 17; single-stage AHP type II, (b) case 19; double-lift AHP type II.

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The economic feasibility of those two AHP implementations was assessed. The installed cost of each device was estimated on the basis of output heat in $/kW. The following cost compiled by the US DOE (RCG/Hagler-Bailly, 1990) were used,2 581 and 656 $/kW for single-stage and double-lift, respectively (those costs are almost identical to those presented by Berntsson and Frank, 1997). The annual rate of operations, steam and cooling water costs were taken as 8640 h, 62.5 $/MWh, and 1 $/MWh, respectively as suggested by mill personnel. Maintenance and other operating costs were neglected. The estimated installed cost of the AHPs and the annual saving in steam and cooling water are presented in Table 4. The simple payback times (SPB) given by Equation (1) are 1 and 1.2 years for the single-stage and double-lift AHP type II.

  • equation image(1)
Table 4. Economic evaluation of selected cycles
CaseSS AHP type II (case 17)DL AHP type II (case 19)
Installed cost [M$]3.42.6
Steam saving [M$/a]3.22.1
Cooling saving [M$/a]0.110.11
SPB11.2

Life-cycle costs of the two case studies were compared using the net present value (NPV; Costa et al., 2009). The initial investment cost and the yearly energy savings were estimated over the life cycle, which was set at 15 years. A discount rate of 7%, which is usual for energy projects and a yearly escalation rate of 4% for fuel price have been used for the time span of the investment. Figure 9 shows the evolution of the NPV over 15 years; the abscissa gives the time elapsed from the date of investment and the intersection with the nil value line indicates the time required for the projects to become cost-effective. It is about 1 year in the two cases for the current steam price. The figure shows that the two options are interesting not only in the short term (quite short PBT), but also in the long term. The initial investment for case 19 is more attractive although both cases produce almost the same SPB. However, case 17 which produces high-yearly revenue perform best in the long term, even if the initial investment is higher. It should be noted that neither operating costs nor potential GHG emission credits were taken into account. The latter could be a significant factor of payback time reduction.

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Figure 9. Net present value for the two case studies.

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Step 8: Since two practically possible and economically feasible configurations have been already identified, the HEN reconfiguration is not considered in this case study (step 8). The authors presented another study in which the HEN reconfiguration and the effect of water closure on the pinch-point are also presented (Bakhtiari et al., 2010).

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

The implementation of an AHP in a process should be supported by a complete process analysis respecting both process and machine constraints. The opportunities for correct positioning of AHPs in a process were identified and guidelines were formulated for its appropriate positioning in the process. Results from a case study based on a real plant has validated the method and illustrated its usefulness.

In the proposed method, cases with different configurations and different working fluid pairs are compared economically with each other. At the present time, there is no reliable cost estimation model for AHPs, considering different configuration and working pairs and this work has shown that there is a need for the developement of such a tool.

One important conclusion from the case study is that even for a heat-integrated process, there can still be room for additional utility savings. For the selected potential heat source, eight different thermodynamically and technically feasible configurations were identified. It was found that a single-stage and a double-lift AHPs of type II were the most realistic implementations and their thermal design and preliminary economic evaluation is presented. Simple pay back time is about 1 year for the current steam price for the two cases. The NPV has also been computed and has shown that the two options are also interesting in the long term. The single-stage AHP type II is the best configuration for the actual case considered; because of almost the same SPB, better performance in the long term and much simpler design.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES

This work was supported by a grant from the R&D Cooperative program of the Natural Science and Engineering Research Council of Canada. The industrial partners to this project and most specially the mill that supplied the data are gratefully acknowledged.

  • 1

    adt: air dried ton.

  • 2

    All costs in this paper are given in 2008 Canadian dollars.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WORKING PRINCIPLE OF ABSORPTION HEAT PUMPS
  5. HEAT PUMPS INTEGRATION IN A PROCESS AND PINCH ANALYSIS
  6. METHODOLOGY FOR AHP POSITIONING IN A PROCESS
  7. CASE STUDY
  8. CONCLUSION
  9. Acknowledgements
  10. REFERENCES
  • Abrahamsson, K., G. Aly, A. Jernqvist and S. Stenstrom, “Applications of absorption heat cycles in the pulp and paper industry,” in “Proc. Inter. Absorption Heat Pump Conf.,” New Orleans, LA, USA (1994).
  • Bakhtiari, B., “Process Integration of Absorption Heat Pumps,” PhD thesis, Ecole Polytechnique de Montreal, Montreal, Canada (2009).
  • Bakhtiari, B., E. Mateos, R. Legros and J. Paris, “Integration of an absorption heat pump in the Kraft pulping process: Feasibility study,” in “PAPTAC—93rd Annual Meeting,” Montreal, Canada, pp. A239A244 (2007).
  • Bakhtiari, B., L. Fradette, R. Legros and J. Paris, “Opportunities for the integration of absorption heat pumps in the pulp and paper process,” Energy In Press, Corrected Proof, Available online 27 April 2010.
  • Berntsson, T. and P. A. Frank, “Learning from experiences with Industrial Heat Pumps,” in “Caddet Analysis Sries,” NL (1997).
  • Browne, T. C., “Réduction des coûts énergétiques dans l'indurstrie des pâtes et papiers,” PAPRICAN, Montreal, Canada (1999).
  • Chappell, R. N. and S. J. Priebe, “Integration of heat pumps into industrial processes,” in “Simulation of Thermal Energy Systems: Winter Annual Meeting of the ASME,” San Francisco, CA (1989).
  • Costa, A., B. Bakhtiari, S. Schuster and J. Paris, “Integration of Absorption Heat Pumps in a Kraft Pulp Process for Enhanced Energy Efficiency,” Energy 34, 254260 (2009).
  • Eisa, M. A. R., R. Best, P. J. Diggory and F. A. Holland, “Heat Pump Assisted Distillation. V: A Feasibility Study on Absorption Heat Pump Assisted Distillation Systems,” Int. J. Energy Res. 11, 179191 (1987).
  • Ferre, J. A., F. Castells and J. Flores, “Optimization of a Distillation Column With a Direct Vapor Recompression Heat Pump,” Indus. Eng. Chem. Proc. Des. Dev. 24, 128132 (1985).
  • Flores, J., F. Castells and J. A. Ferre, “Recompression Saves Energy,” Hydrocarbon. Process. 63, 5964 (1984).
  • Gidner, A., A. Jernqvist and G. Aly, “An energy Efficient Evaporation Process for Treating Bleach Plant Effluents,” Appl. Thermal Eng. 16, 3342 (1996).
  • Herold, K. E., R. Radermacher and S. A. Klein, “Absorption Chillers and Heat Pumps,” United States of America: CRC Press (1996).
  • Kemp, I. C., “Pinch Analysis and Process Integration, Second Edition: A User Guide on Process Integration for the Efficient Use of Energy,” IChemE Rugby, UK: Elsevier (2007).
  • Linnhoff, B., “Pinch Analysis—A State-of-the-Art Overview,” Chem. Eng. Res. Design 71, 503522 (1993).
  • Linnhoff, B., D. W. Townsend, D. Boland, G. F. Hewitt, B. E. A. Thomas, A. R. Guy and R. H. Marsland, “A User Guide on Process Integration for the Efficient Use of Energy,” Rugby, UK: Institution of Chemical Engineers (1994).
  • Marinova, M., E. Mateos-Espejel, B. Bakhtiari and J. Paris, “A new methodology for the implementation of Trigeneration in industry: Application to the Kraft process,” in “1st European conference on Polygeneration,” Tarragona, Spain (2007).
  • Mateos-Espejel, E., M. Marinova, S. Bararpour and J. Paris, “Energy Implications of Water Reduction Strategies in Kraft Process (Part I: Methodology),” in “PAPTAC—93rd Annual Meeting,” Montreal, Canada (2008a).
  • Mateos-Espejel, E., M. Marinova, D. Diamantis, L. Fradette and J. Paris, “Strategy for Converting a Conventional Kraft Pulp Mill into a Sustainable “Green” Mill,” in “World Renewable Energy Congress (WRECX),” Glasgow, Scotland (2008b).
  • Ranade, S. M., “New Insights on Optimal Integration of Heat Pumps in Industrial Sites,” Heat Recovery Sys CHP 8, 255263 (1988).
  • Ranade, S. M. and A. Nihalani, “Industrial heat pumps: Appropriate placement and sizing using the grand composite,” in “IETC 1986,” (1986).
  • Ranade, S. M., A. Nihalani, E. Hindmarsh and D. Boland, “Industrial heat pumps: a novel approach to their placement, sizing and selection,” in “21st Intersociety Energy Conversion Engineering Conference: Advancing Toward Technology Breakout in Energy Conversion,” San Diego, CA (1986).
  • RCG/Hagler-Bailly, I., “Opportunities for industrial chemical heat pumps in process industries,” Final Report prepared for the US DOE office of Industrial Technologies (1990).
  • Sama, D. A., “Heat exchanger network optimization strategy based on reducing the number of heat exchanger,” in “Proc. ASME,” Montpellier, Fr (1996).
  • Smook, G. A., “Handbook for Pulp & Paper Technologists,” Vancouver, Canada: Angus Wild Publication Inc. (2002).
  • Supranto, S., R. Jaganathan, S. Dodda, P. J. Diggory and F. A. Holland, “Experimental Study of the Operating Characteristics of a Heat Pump Assisted Distillation System Using R11 as the External Working Fluid,” Appl. Energy 25, 187204 (1986).
  • Wallin, F. and T. Berntsson, “Integration of Heat Pumps in Industrial Processes,” Heat Recovery Sys. CHP. 14, 287296 (1994).
  • Wallin, E., P. A. Franck and T. Berntsson, “Heat Pumps in Industrial Processes—An Optimization Methodology,” Heat Recovery Sys. CHP. 10, 437446 (1990).
  • Yee, T. F. and I. E. Grossman, “Simultaneous Optimization Models for Heat Integration. II. Heat Exchanger Network Synthesis,” Comp. Chem. Eng. 14, 11651184 (1990).
  • Ziegler, F., “Recent Developments and Future Prospects of Sorption Heat Pump Systems,” Int. J. Therm. Sci. 38, 191208 (1999).
  • Ziegler, F. and G. Alefeld, “Coefficient of Performance of Multistage Absorption Cycles,” Int. J. Refrigeration 10, 285295 (1987).
  • Ziegler, F. and P. Riesch, “Absorption Cycles. A Review With Regard to Energetic Efficiency,” Heat Recovery Sys. CHP. 13, 147159 (1993).