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Heat integration of heat pump assisted distillation into the overall process
Applied Energy 162 (2016) 1–10
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Heat integration of heat pump assisted distillation into the overall process Minbo Yang, Xiao Feng ⇑, Guilian Liu School of Chemical Engineering & Technology, Xi’an Jiaotong University, Xi’an 710049, China
h i g h l i g h t s Heat integration of heat pumps, distillation columns and background processes. An approach to identify the placement of a heat pump via pinch technology. A smaller temperature lift of heat pumping for a distillation column. The heat integration scenario reduces the energy consumption of the overall process.
a r t i c l e
i n f o
Article history: Received 5 January 2015 Received in revised form 3 October 2015 Accepted 6 October 2015 Available online 11 November 2015 Keywords: Pinch analysis Distillation Heat pump Grand Composite Curve Energy savings
a b s t r a c t Reducing the energy consumption of distillation processes can lead to significant cost savings in refineries and the chemical process industry because distillation is a widely used and energy-intensive separation technology. A distillation column can be heat integrated with heat pumps to reduce the energy supplied by the utility, and it can also be integrated into the overall process to save energy for the overall process. However, previous studies have not adequately investigated the synergistic effect of integrating heat pump assisted distillation into overall processes. In this paper, a systematic design methodology is proposed for the simultaneous heat integration of distillation, its background process and heat pump systems. Such a holistic heat integration approach can lead to considerable energy savings for the overall process. The proposed methodology also includes systematic identification for the energy-optimum placement of the heat pump and its matching with process streams. Furthermore, the impacts of distillation process modifications on the holistic heat integration strategy are examined. A case study is presented to illustrate how the proposed design method is applied and to demonstrate its effectiveness in saving energy. For the case study, the hot and cold utilities are reduced by 61.5% and 20.6% compared to energy consumptions for the base case. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The chemical industry consumes vast amounts of energy which is mainly obtained from burning fossil fuels, e.g., coal, natural gas and crude oil. This results in unsustainable life cycle impacts on the environment [1]. In process industries, separation processes are an essential step to recover or purify the desired products, or to remove undesired wastes or byproducts. Distillation is a widely used and energy-intensive separation technology [2,3], as energy consumption associated with distillation processing accounts for 40–60% of the total energy consumption of the chemical industry [4,5]. Due to the rising cost of energy and the increasingly strict environmental regulations, reducing the energy consumption of ⇑ Corresponding author. Tel.: +86 18611446202. E-mail address: [email protected] (X. Feng). http://dx.doi.org/10.1016/j.apenergy.2015.10.044 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
distillation processes is of high demand and has received considerable research attention. In a conventional distillation column, high quality heat is supplied to a bottom reboiler and waste heat is rejected from an overhead condenser. Many studies have proposed different ways to reduce the energy consumption of distillation processes. Some options for improving the energy efficiency of distillation operations include making changes to the design or operating conditions of the column, such as preheating the feed [6] and adjusting the reflux ratio [5,7]. System-wide approaches that utilize energy in an integrated manner within the distillation process are another attractive option to improve energy efficiency. For example, the concept of heat integration may be used to systematically realize heat recovery of distillation columns by making use of heat sources and heat sinks available within the distillation process [8]. Such heat integrated design options include thermally-coupled
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Nomenclature COP CP H1 H2 Hpoc QC QC,min Qcond QH QH,min Qreb Q1 Q2 Tcond TP
coefficient of performance of heat pump heat capacity flow rate, kW/°C heat flux at temperature T1 on the GCC, kW heat flux at temperature T2 on the GCC, kW heat flux at the heat pocket, kW total required cold utility of overall process, kW minimum cold utility of background process, kW heat load of overhead condenser, kW total required hot utility of overall process, kW minimum hot utility of background process, kW heat load of bottom reboiler, kW rejected heat of heat pump, kW heat taken in by heat pump, kW temperature of overhead condenser, °C temperature of the pinch point, °C
distillation [9], multiple-effect distillation [10], introduction of side reboilers and/or side condensers [11] and heat pump assisted distillation [12]. Among the aforementioned options, considerable attention has been paid by the academic and industrial communities [13–15] to the benefits of heat pumps for distillation processes as heat pumps can upgrade heat from a lower temperature level to a higher one. There are great potentials to reduce the energy consumption of distillation processes when they are appropriately integrated with heat pumps. Different schemes have been proposed for upgrading energy within a distillation column through heat pumps. One scheme is to use an external heat pump circuit in which a refrigerant fluid, different from the mixture in the column, is employed. This circuit has an evaporator and a condenser, which is coupled with the condenser and the reboiler of a distillation column, respectively [5]. Another scheme, known as mechanical vapor recompression heat pump, compresses the column overhead vapor stream in the compressor directly and uses it as a heating medium [16]. The bottom product also can be taken as a heat-exchanging medium of the heat pump with a bottom flashing arrangement [14,17]. A thermal vapor recompression heat pump is similar to the mechanical vapor recompression heat pump, in which the compressor is replaced by a stream ejector [13]. Also, an absorption heat pump transfers the heat using absorption pairs and can also be integrated in distillation processes [18,19]. Although the heat pump schemes described above are operated with different operating principles and integration concepts, all of them are applied to recover heat from the overhead condenser for use by the bottom reboiler. To allow heat exchange between a heat pump and a distillation column, the temperature lift via the heat pump should be greater than the temperature difference between the reboiler and the condenser. However, as an increase in the temperature lift of a heat pump decreases the coefficient of performance (COP), which is the ratio of the heat rejected to the external energy consumed by the heat pump, care must be taken to avoid excessive temperature lift [20]. In addition, with the aid of Column Grand Composite Curve [21], it is thermodynamically feasible to use heat pumps to exchange heat with side streams from intermediate trays of a distillation column, which requires a small temperature lift [22] but is rarely applied in practice. The basic idea of the aforementioned methods is to improve the internal energy utilization within distillation columns so that the demand for external energy, namely utility [8], can be reduced. Besides, a distillation column can also be designed in the context of the overall process with the aim of utilizing energy from other
Treb Tsup Ttar T1 T2 DTmin W
gC
temperature of bottom reboiler, °C supply temperature, °C target temperature, °C temperature of heat pump condenser, °C temperature of heat pump evaporator, °C minimum temperature difference for heat exchange, °C external work, kW Carnot efficiency
Superscripts 1 heat pump 1 2 heat pump 2 real corresponding real value
processing units to reduce the energy consumption of the overall process. For the sake of simplicity, the overall process without considering the distillation process is referred to as the background process in this paper. In the early 1980s, Linnhoff et al. applied the concept of pinch analysis and proposed a tool based on Heat Flow Cascade to optimize the allocation of available heat between a distillation process and its background process [23]. Their research indicates that if the operating temperatures of both the condenser and the reboiler are above or below the pinch temperature of the background process, distillation columns may be heat-integrated with the background process. If this is not the case (i.e. the distillation process is located across the pinch point), heat integration between the distillation column and its background process is not favored. In such situation, the column operating conditions can be modified so that the reboiling temperature is below the pinch or the condensing temperature is above the pinch. Subsequently, Bandyopadhyay [24] proposed two modifications: through side exchangers and through feed preheating, for the heat integration of a distillation column with the background process. Kravanja et al. [25] investigated the heat integration of a biochemical plant with the process modifications of heat loads and temperature levels. However, making process modifications to distillation columns requires careful consideration when the products are very sensitive to any changes in the operating conditions. Moreover, for the purpose of saving energy, heat pumps can also be heat integrated with the background process. Townsend and Linnhoff [26] proposed that a heat pump should absorb heat below the pinch temperature and reject it above, namely the across-pinch rule. In further research, Wallin et al. [27] adopted Composite Curves to guide the choice of heat pump types for various industrial processes and revealed the heat load limits of heat pump installations. Benstead and Sharman [28] matched heat pump with the Grand Composite Curve (GCC) of a process using the mirror image technique, which is relatively simple and clear. Besides, the shape of the GCC can also suggest which type of heat pump is most suitable [29]. Yang et al. [30] analyzed the dynamic changes of pinch temperature and GCC of industrial processes with heat pumps integrated. However, since both Composite Curve and GCC are composed of two sets of process streams (i.e. hot and cold streams), these pinch-based methods did not consider the matches between process streams and heat pumps, and such work was mainly done by computer programs [28,31,32]. In summary, three energy-efficient approaches have been explored so far: heat pump assisted distillation, heat integration
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between distillation columns and the background process, and heat integration between heat pumps and the background process. However, to the best of our knowledge, no studies have systematically addressed the simultaneous heat integration of heat pumps, distillation column and its background process. Obviously, the heat in the background process can be upgraded by heat pumps to reboil the liquid in the bottom reboiler or the rejected heat from the condenser can be lifted to heat cold streams in the background process. Therefore, the purpose of this work is to present a systematic methodology for the simultaneous heat integration of distillation, its background process and heat pumps in a holistic manner. Following previous heat integration approaches, the methodology of this work is developed based on the pinch conception. Firstly, this paper presents three scenarios of the heat integration concept relying on different GCCs. Secondly, the energy-optimum placement of a heat pump is identified with the minimum temperature lift. Thirdly, GCC is used as a guide to determine the streams to be matched with the heat pump. On this basis, the effects of distillation process modifications on heat integration and their technoeconomic impacts are further investigated. Compared with the conventional heat pump assisted distillation scheme, the proposed heat integration concept provides a smaller temperature lift for heat pumping and reduces considerably utility requirements of the overall process. Note that the minimum temperature difference for all the heat exchangers is set as DTmin in this work. 2. Distillation integrated into the overall process with heat pumps As presented by Townsend and Linnhoff [23], for a distillation column that is not located across the pinch point of its background process, either the reboiler or the condenser of the column can be integrated with streams in the background process. To be specific, the reboiler can take in heat from hot streams, reducing the demanded heat supplied by hot utility; or the condenser can reject heat to the cold streams, decreasing the demanded hot utility of the background process. In this case, the possibility of integrating a heat pump can be examined by the method of Townsend and Linnhoff [26]. When a distillation process is located across the pinch point, changing the location of the pinch temperature can be considered so that the operating temperatures of distillation columns are now located above or below the new pinch point. Yang et al. [30] analyzed the dynamic changes of GCC when heat pumps are integrated with a heat exchanger network. They found that the pinch temperature of a heat exchanger network can be changed by the choice for the placement of heat pumps. Based on this observation, heat pumps can be strategically applied to change the pinch temperature of the background process, which allows better heat integration.
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cannot be integrated with the background process directly. Thus, the total hot utility demand of the overall process is QH = QH,min + Qreb and the total cold utility demand is QC = QC, min + Qcond. Referring to the work of Yang et al. [30], a heat pump can be placed to change the pinch point. As shown in Fig. 1b, the heat pump absorbs heat Q2 at temperature T2 and rejects heat Q1 at temperature T1. When the amount of heat Q1 is larger than the deficient heat H1 at temperature T1 on the BPGCC, the pinch point is changed from point P to P1 with the introduction of a heat pump to the existing process and a new BPGCC is established. Consequently, the required hot utility for the background process is QH,min H1 and the required cold utility is QC,min + (Q1 Q2) H1. Furthermore, the distillation column is now located below the new BPGCC, as shown in Fig. 1c. For the overall process, the total hot utility is QH = QH,min H1 and the total cold utility is reduced to QC = QC,min + (Q1 Q2) H1 + Qcond Qreb. It should be noted that, in order to fully meet the heat demands of the distillation process, T1 should not be lower than Treb and the amount of Q1 should not be smaller than H1 + Qreb. A heat pocket may exist in the GCC when surplus of local heat sources lies above the pinch point or deficit of local heat sinks lies below the pinch point. Since the local heat source can satisfy the local heat sink when they are in the same heat pocket, the heat pocket can be excluded when the heat integration of heat pumps with the background process is considered [30]. However, in this work, these pockets existing in the GCC are strategically used to facilitate the heat recovery between a distillation process and its background process with the assistance of heat pumps. Fig. 2a shows a GCC of a background process with a heat pocket above the pinch point, and a distillation column is located across the pinch point. When a heat pump is placed across the pinch point (Fig. 2b), the amount of absorbed heat (Q2) is smaller than the amount of surplus process heat (H2) at temperature T2, while the amount of rejected heat (Q1) is equal to the amount of deficient heat (H1) at temperature T1, but is larger than the amount of deficient heat at the pocket (Hpoc). As shown in Fig. 2b, the pinch point is now changed from point P to P1. The new BPGCC shows that the temperature of the new pinch point is higher than T1 and Treb. Now, the heat integration between the distillation column and the background process can be achieved because the distillation column is located below the new pinch point of the updated BPGCC, as shown in Fig. 2c. For the overall process, the total hot utility is QH = QH,min Hpoc and the total cold utility is QC = QC,min + (Q1 Q2) Hpoc + Qcond Qreb. By comparing the two cases in Figs. 1 and 2, the reboiler of the distillation column in Fig. 1 absorbs heat Qreb from the heat pump, while the reboiler in Fig. 2 absorbs heat Qreb from the local heat source in the heat pocket. With the existence of the local heat source above the original pinch point, the temperature lift for heat pumping is reduced. In this way, the heat in the heat pocket is used by grade and better energy efficiency can be achieved.
2.1. Scenario 1: Increase in the pinch temperature For a conventional distillation column, the bottom reboiler absorbs heat Qreb at temperature Treb and the overhead condenser rejects heat Qcond at temperature Tcond. By assuming no subcooling or superheating for the condensation and evaporation, heat surplus and deficit of a distillation column can be represented as a quadrilateral box in the T H diagram, as shown in Fig. 1a [23]. The Grand Composite Curve of a background process (BPGCC) is also depicted in Fig. 1a, which shows the cascade heat flow of the background process, requiring the minimum hot utility QH,min and the minimum cold utility QC,min. Fig. 1a illustrates the case in which the distillation column is located across the pinch point and so it
2.2. Scenario 2: Decrease in the pinch temperature Fig. 3 illustrates the case of BPGCC to be integrated with a distillation column when the position of the pinch point is shifted downward. In the base case, a distillation column is located across the pinch point of BPGCC (Fig. 3a). Then, a heat pump is placed across the pinch point, as shown in Fig. 3b. It is noted that the amount of absorbed heat Q2 is larger than the surplus process heat H2 at temperature T2, and the rejected heat Q1 is equal to the deficient heat H1 at temperature T1. With the integration of a heat pump, the pinch point is changed from point P to P1, and the
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Fig. 1. Heat integration based on the increased pinch temperature 1.5-column.
Fig. 2. Heat integration based on the increased pinch temperature (a heat pocket above the original pinch point) 1.5-column.
Fig. 3. Heat integration based on the decreased pinch temperature 1.5-column.
M. Yang et al. / Applied Energy 162 (2016) 1–10
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Fig. 4. Heat integration based on the decreased pinch temperature (a heat pocket below the original pinch point) 1.5-column.
distillation column can be placed above the new pinch point of the updated BPGCC, as shown in Fig. 3c. For the overall process, the overall requirement of cold utility is QC = QC,min H2 and that of hot utility is QH = QH,min (Q1 Q2) H2 + Qreb Qcond. It should be noted that the temperature T2 cannot be higher than Tcond and the amount of heat Q2 cannot be smaller than H2 + Qcond. As similarly considered in the first scenario, here we will look at the case of having a heat pocket below the pinch point as well as having a distillation column being located across the pinch point (Fig. 4a). A heat pump is placed across the pinch point, as shown in Fig. 4b. When the amount of absorbed heat Q2 is larger than heat Hpoc, the pinch point is shifted downward from P to P1, which now allows the heat integration between the distillation column and the background process, as shown in Fig. 4c. For this case, T2 is higher than Tcond but the temperature of the new pinch point is lower than both T2 and Tcond. For the overall process, the total hot utility is QH = QH,min (Q1 Q2) Hpoc + Qreb Qcond and the total cold utility is QC = QC,min Hpoc. The condensation for distillation is realized by releasing heat to the heat pump (Fig. 3) while the heat from the column condenser is rejected to the local heat sink existing in the heat pocket (Fig. 4),
which has been made more energy-efficient by having a smaller temperature difference for heat pumping. 2.3. Scenario 3: No change in the pinch temperature Sections 2.1 and 2.2 describe how heat integration between a distillation column and its background process can be made possible by changing the temperature of the pinch point. However, there is potential for achieving better heat recovery without changing the pinch temperature when heat pumps are integrated. As shown in Fig. 5, heat pumps can be placed at two different locations. Heat pump 1 absorbs the heat from the background process and rejects the heat to the reboiler of a distillation column, while Heat pump 2 absorbs the heat from the condenser of a distillation column and rejects the heat to the background process. Besides, the amount of absorbed heat Q 12 of heat pump 1 is smaller than the surplus process heat at temperature T 12 , and the amount of rejected heat Q 21 of heat pump 2 is smaller than the deficient heat at temperature T 21 . Thus, the pinch temperature is not changed, and accordingly, the total hot utility required for the overall process is decreased from QH,min + Qreb to QH,min Q 21 and the total cold utility is decreased from QC,min + Qcond to QC,min Q 12 . Compared with scenarios 1 and 2, the heat integration in scenario 3 is achieved without changes in the pinch temperature at the expense of an increase in the number of heat pumps. For all heat pumps in scenarios 1–3, their temperature lift can be smaller than the temperature difference between the reboiler and the condenser of the distillation column. Also, heat pumps are employed to supply heat required for the distillation column as well as for some of the heat demand in the background process. Such synergetic benefits gained from the proposed heat integration options provide more opportunities in heat recovery, leading to further energy savings. However, the specific characteristics of a distillation column and its background process will determine whether heat integration can be achieved and which is the better scenario. 3. Investigation of heat pumps placement
Fig. 5. Heat integration with no change in the pinch temperature single-column.
Section 2 describes in considerable detail the heat integration of heat pumps, a distillation column and its background process. When designing or retrofitting a heat exchanger network, the
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is the COP which is given in Eqs. (3) and (4), with which the amount of heat Q2 at temperature T2 can be calculated as shown in Eq. (5). When T1 and Q1 are fixed, the amount of Q2 is linearly related to temperature T2.
COP ¼
Q1 Q1 ¼ W Q1 Q2
COP ¼ gC
T 1 þ 0:5DT min þ 273:2 ðT 1 þ 0:5DT min Þ ðT 2 0:5DT min Þ
Q1 T 1 þ 0:5DT min þ 273:2 ¼ gC ðT 1 þ 0:5DT min Þ ðT 2 0:5DT min Þ Q1 Q2
Fig. 6. Example for identifying the operating parameters of heat pumps singlecolumn.
operating conditions of heat pumps and the streams to be matched with heat pumps still need to be determined. These topics are discussed here.
ð3Þ
ð4Þ
ð5Þ
where gC is the Carnot efficiency. Step 3: The BPGCC in Fig. 6 shows that the amount of surplus process heat increases with decreasing temperature below the pinch point. Considering Eq. (5) together with the BPGCC, the amount of heat Q2 to be rejected by the heat pump as well as temperature T2 can be determined. This allows the heat integration between heat sources and heat sinks in the distillation column and its background process, and the minimum temperature lift for the heat pump is identified, as shown in Fig. 6b.
3.1. Operating conditions of heat pumps
3.2. Process streams integrated with heat pumps
A large temperature lift through a heat pump should be avoided because it can reduce the COP of the heat pump [27]. Here, an approach is presented to systematically determine the energyoptimum operating conditions of a heat pump, leading to the smallest possible temperature lift, while the desired heat integration can still be fully realized. For convenience, this design concept is illustrated with an example depicted in Fig. 6.
After the operating conditions of the heat pump have been determined based on the BPGCC, the next step is to select process streams to be integrated with the heat pump. So far, the stream releasing its heat to the heat pump is treated as a single stream, and so is the stream absorbing heat from the heat pump. However, more than one process stream can be present in the GCC, and consequently stream splitting and combination may be necessary to integrate process streams with the heat pump when the most appropriate conditions identified from the previous sections are desired [28]. In general, there is more than one feasible option for the selection of process streams. Due to the high capital cost of heat pumps [27], and to reduce the number of heat pumps used, an assumption is made that a heat pump is allowed to simultaneously absorb heat from different process hot streams and reject heat to different process cold streams. Consequently, multiple heat exchangers may need to be applied in the heat pump system. The total number of process streams integrated with the heat pump determines the number of heat exchangers to be used in the heat pump system, which indicates the configurational complexity of the heat pump system. For a vapor compression heat pump, at least two heat exchangers are required, one of which is an evaporator and the other is a condenser. From this observation, the minimum number of process streams is preferred when streams for heat integration are to be selected.
Step 1: The BPGCC shows the overall heat deficit or surplus and its temperature level for a background process. Based on the BPGCC and the operating conditions of the distillation column, either the heat Q1 available at temperature T1, or the heat Q2 available at temperature T2 can be selected for coupling with a heat pump. As shown in Fig. 6a, when the amount of Q1 is smaller than the sum of Qreb + Hpoc, the distillation column cannot be fully integrated into the overall process; on the other hand, when the amount of Q1 is larger than the sum of Qreb + Hpoc, this surplus heat Q1 (Qreb + Hpoc) brings no savings for hot or cold utilities. Hence, the optimal amount of heat Q1 is Hpoc + Qreb. With this amount of heat available for heat integration, the lowest temperature T1 for the heat pump can be determined such that the deficient heat H1 equals Q1 from the BPGCC, as shown in Fig. 6b. To be clear, it is noteworthy that the temperature of cold streams presented on the GCC is higher than its real value by 0.5DTmin and the temperature of hot stream presented on the GCC is lower than its real value by 0.5DTmin. Therefore, Eqs. (1) and (2) can be derived:
T real ¼ T 1 þ 0:5DT min 1
ð1Þ
T real ¼ T 2 0:5DT min 2
ð2Þ
where the superscript real represents the actual stream conditions. Step 2: The amount of heat Q2 to be rejected at temperature T2, which corresponds to the amount of heat Q1 to be absorbed at T1, can be estimated by simple empirical formulas or a rigorous mathematical model of heat pumps. One practical measure for evaluating the performance of a vapor compression heat pump
Fig. 7. Determining the streams to be integrated with heat pump single-column.
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Fig. 7 shows the enlarged part of Fig. 6 in which the heat pump is integrated with BPGCC. The shaded parts in Fig. 7 represent the feasible regions for selecting hot and cold streams to be integrated with the heat pump. All the hot and cold streams are expressed as a series of segments in the T H diagram. In Fig. 7, a hot stream is expressed as a segment with a negative slope and a cold stream is expressed as a segment with a positive slope. If the use of a single hot or cold stream is desired to satisfy the corresponding heat demand of the heat pump, it is necessary to consider both the temperature and heat load of the stream. The procedure of selecting a single stream is illustrated in Fig. 7. First, shift all the segments of streams to the left until the right end vertexes, indicated by A and B, of the two shaded regions are located on the segments of cold and hot streams, respectively, as shown in Fig. 7. Any streams that cannot meet this condition should not be considered for selection, for example, cold stream 1. If the segment of a process stream does not intersect with the vertical axis in Fig. 7, for example, cold stream 2, the heat load of this kind of stream is not big enough and this stream should not be considered as well. Next, if a part of the segment appears outside of the shaded region, such as hot stream 1, this stream should also be excluded due to the infeasibility in heat exchange of the background process. Finally, hot stream 2 and cold stream 3 are the feasible streams for integrating with the heat pump. In this situation, if cold stream 3 and hot stream 2 are selected to be integrated with the heat pump, only two heat exchangers (an evaporator and a condenser) are needed. If a single hot or cold stream cannot be selected for the integration with the heat pump, the combination of several process streams is necessary and the way to select streams and combine them should reflect the heat transfer feasibility constrained by the shaded regions. As a result, the number of heat exchangers in the heat pump system is increased. 4. Process modifications for distillation process Process modifications can be considered as a way to alter operating conditions or change the process flowsheet, leading to modification of process data which may give more opportunities for heat recovery, reduce the energy target or allow a simpler or cheaper network [33]. For a distillation process, process modifications generally include changing the operating pressure, adding an intermediate reboiler or condenser, splitting the heat loads, changing the reflux ratio and preheating the feed, etc. [23]. These modifications change the compositions of product streams and the heat loads of the condenser and reboiler. Process modifications for distillation should not compromise the quality of products. Also, any potential benefit gained from process modifications for the distillation should be judged together with energy consumption because improving product quality may lead to potential increase in heat duties of the condenser and reboiler. Any physical limitations or practical constraints associated with process modifications should be considered for determining the degree of implementation in practice. As shown in Fig. 8a, a distillation column is located across the pinch point of the original BPGCC. It is not feasible to fully integrate this distillation column into the overall process by process modifications only because considerable change in column pressure is needed to ensure feasibility for heat integration. Referring to the design concepts proposed in this work, Scenario 1 is appropriate for further consideration. However, some part of condensation of the distillation column is not feasible for heat integration with the background process, as may be deduced from the new BPGCC (Fig. 8a). Hence, process modifications of the distillation column may now be used to achieve heat recovery of the distillation column with the background process.
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4.1. Changing the operating pressure Changing the operating pressure of a distillation column influences the operating temperatures of the condenser and reboiler. Raising the operating pressure increases the temperatures of the condenser and reboiler while lowering the operating pressure decreases the corresponding temperatures. Permissible maximum and minimum bounds for changing column pressure exist as illustrated by the shaded region in Fig. 8b. It should be noted that the new adjusted column pressure for improving heat integration should be within the acceptable operation range and without compromising product quality. For the example given in Fig. 8b, operating pressure should be reduced to move the quadrilateral box downward to make the heat integration of the distillation column with the new BPGCC feasible, which is depicted by the dotted quadrilateral box. 4.2. Feed preheating and addition of intermediate side reboiler Feed preheating provides an increase of energy input to the feed tray as well as a reduction in the heat duty of the reboiler. Addition of an intermediate side reboiler can supply heat at a lower temperature than that of the reboiler, which facilitates evaporation of liquid in an intermediate tray of the column and can reduce the heat duty of the reboiler as well. Both feed preheating and addition of an intermediate side reboiler do not reduce the overall duty of hot utility required for the distillation column but they are able to accept lower quality of hot utility, leading to savings in energy cost. The operating conditions of the distillation column are updated which can accommodate lower grade hot utility, as illustrated in Fig. 8c. The updated conditions for the distillation column are now located within the feasible design region given by the new BPGCC, which makes the heat integration feasible. In summary, the heat integration between a distillation process and its background process can be systematically achieved through integration of heat pumps with process modifications in an integrated manner. It should be noted that process modifications may make it easy for integrating heat pumps with the background process, but any potential negative impacts, for example, degradation of distillation product quality or increase of heat duties, should be collectively evaluated together with the benefits gained from the heat integration. 5. Case study This case study is used to illustrate how the proposed methodology can be effectively applied for achieving the heat integration between a distillation process and its background process. Fig. 9 shows a simplified flowsheet of a chemical process, and the detailed stream data are listed in Table 1. The background process consumes hot utility 40 kW and cold utility 148 kW. When the distillation column is included, the total consumptions of hot and cold utilities are 78 kW and 188 kW, respectively. The energy flow for the background process is analyzed by pinch analysis. When DTmin is assumed to be 10 °C, the average pinch temperature is found to be 75 °C (i.e. 80 °C for hot streams and 70 °C for cold streams), and the minimum hot and cold utilities are 10 kW and 118 kW, respectively. The GCC of the background process is given in Fig. 10. As shown in Fig. 11, the distillation process is located across the pinch point. Here, a vapor compression heat pump is used to facilitate the heat integration. With the across-pinch rule, R142b with a critical temperature of 136.45 °C is chosen as the working fluid of the heat pump. Following the design procedure described in Sec-
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Fig. 8. Illustration of process changes of a distillation process: (a) base case; (b) pressure change; (c) feed preheating and addition of an intermediate reboiler 1.5-column.
Fig. 9. Flowsheet of the case study (modified from Ref. [34]) single-column.
Although the operating conditions of the heat pump are determined by taking into account the minimum temperature lift for heat pumping, none of the hot streams can solely meet the heat duty required for the integration with the heat pump, as shown in Fig. 11. It means that at least three heat exchangers (one condenser and two evaporators) are required in the heat pump system. In Fig. 11, hot stream 3 is located close to the feasible region and thus it is selected as the heat source to supply heat to the heat pump. Now, new operating conditions for the heat pump are determined as Q2 equals 38.8 kW and T2 (based on the actual temperature) equals 54.6 °C, as shown in Fig. 12. The external energy to be supplied for the heat pump is 9.2 kW and the COP of the heat pump is 5.2. The heat pump absorbs 38.8 kW of heat from hot stream 3 and its temperature is reduced from 72.4 °C to 64.6 °C. The heat pump rejects 48 kW of heat to cold stream 2 and its temperature is increased from 75.2 °C to 87.2 °C. In this way, only two heat exchangers are required in the heat pump system, and a further increase in the temperature lift of heat pumping is only 0.2 °C. Fig. 13 shows the improved flowsheet after implementing the solutions described above. The total hot utility is 0 kW and the total cold utility is 119.2 kW. The temperature lift of the heat pump is 42.6 °C which is smaller than the temperature difference
Table 1 Stream data of the process. Stream
CP/(kW/°C)
Tsup/°C
Ttar/°C
Heat load/kW
Hot 1 Hot 2 Hot 3 Cold 1 Cold 2
1 2 5 1.8 4
180 130 80 30 60
80 40 50 120 100
100 180 150 162 160
Column
Reboiler Condenser
130 (liquid) 60 (vapor)
130 (vapor) 60 (liquid)
38 40
tion 4, Q1 is found to be 48 kW, and T1 is determined as 92.2 °C (i.e. real temperature is 97.2 °C) on the BPGCC. Assuming that the Carnot efficiency gC is 60%, when the heat pump absorbs heat from the heat source available in the background process, Q2 is found to be 38.9 kW and T2 is 59.8 °C (i.e. real temperature is 54.8 °C), based on Eq. (5) and the BPGCC. On the other hand, when the heat pump absorbs the heat from the condenser of distillation column, T2 (based on the actual temperature) should not be higher than 50 °C because DTmin is 10 °C. Therefore, the former option is better than the latter one.
Fig. 10. The GCC of the background process in the case study single-column.
M. Yang et al. / Applied Energy 162 (2016) 1–10
9
Fig. 11. Identification of heat pump placement 1.5-column.
Fig. 12. Improvement of heat pump placement single-column.
Fig. 14. Simplified flowsheet for the case study single-column.
Fig. 13. Resulting flowsheet after implementing the heat integration singlecolumn.
between the reboiler and the condenser. However, it can be seen that the heat exchanger network is more complex than the original one. It is possible to simplify the flowsheet given in Fig. 13 through process modifications for the heat pump. By maintaining the determined operating conditions, namely, the four parameters T1, Q1, T2 and Q2 of the heat pump, driving force for the heat exchange related to hot stream 3 and cold stream 2 in the heat pump can be slightly changed, as designed in Fig. 14. As a result, the minimum temperature difference in the updated flowsheet of Fig. 14 is 9.6 °C in the evaporator and 7.9 °C in the condenser of the heat pump, compared to the original value of 10 °C. Although the reduction in driving force in the exchanger leads to larger heat exchanger areas for the reboiler and condenser, two exchangers, heat exchangers 3 and 8, can be removed in the simplified flowsheet. The details of the simplified flowsheet are shown in Fig. 14. In this case study, a heat pump is placed to absorb heat from a hot stream below the original pinch point and to reject heat to a cold stream above the original pinch point. This increases the pinch temperature from 75 °C to 175 °C. With the introduction of the heat pump, the distillation column can be fully integrated into the overall process, i.e. the reboiler of the distillation col-
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umn absorbs heat from a hot stream in the background process and the condenser of the distillation column rejects heat to the cold utility. Consequently, the application of the heat pump reduces hot utility and cold utility by 48 kW and 38.8 kW, respectively. 6. Conclusions In this work, a novel design methodology based on pinch analysis is proposed to systematically analyze the simultaneous heat integration of heat pumps, a distillation process, and its background process. For the case in which a distillation process is located across the pinch point, heat pumps can be used to facilitate the heat integration between the distillation process and its background process. Three possible scenarios for such heat integration are presented. (1) A heat pump is introduced, causing the pinch temperature to increase. The process heat below the new pinch point can satisfy the heat duty of the reboiler. (2) A heat pump is introduced, causing the pinch temperature to decrease. The condenser of the distillation column can provide heat for the cold streams above the new pinch point. (3) Two heat pumps are introduced to integrate the distillation column and the background process. In the case study considered in this paper, the proposed heat integration scheme provides 61.5% and 20.6% reduction in hot and cold utilities, respectively. Also, the temperature lift of the heat pump is determined to be 42.6 °C, which is much smaller than the temperature difference between the reboiler and the condenser (70 °C). This illustrates that the proposed heat integration method allows a smaller temperature lift for heat pump, compared with the traditional method for heat pump assisted distillation, and provides more opportunities for energy savings. In the proposed work, conceptual understanding for the simultaneous integration of heat pumps and distillation columns with the background process is obtained. As the design procedure proposed in this paper is based on graphical methods using pinch analysis, it is relatively easy to apply and can give clear design insights. However, this work only discusses the aspect of improving the energy efficiency of distillation columns by implementing heat-integrated heat pumps without considering the investment cost. Our future work will investigate the impacts of capital investment on the overall process. Acknowledgements Financial support from the National Basic Research Program of China (973 Program: 2012CB720500) and the National Natural Science Foundation of China under Grant No. 21276204 is gratefully acknowledged. References [1] Nigam PS, Singh A. Production of liquid biofuels from renewable resources. Prog Energy Combust Sci 2011;37:52–68. [2] Sarbatly R, Chiam C-K. Evaluation of geothermal energy in desalination by vacuum membrane distillation. Appl Energy 2013;112:737–46.
[3] Gorak A, Sorensen E. Distillation: fundamentals and principles. Academic Press; 2014. [4] Humphrey J. Separation process technology. New York: McGraw-Hill; 1997. [5] Waheed MA, Oni AO, Adejuyigbe SB, Adewumi BA, Fadare DA. Performance enhancement of vapor recompression heat pump. Appl Energy 2014;114: 69–79. [6] Soave G, Feliu JA. Saving energy in distillation towers by feed splitting. Appl Therm Eng 2002;22:889–96. [7] Abolpour B, Abolpour R, Shamseddini A, Kamyabi S, Hamzehee F. Optimization of the reflux ratio for methanol-water stage distillation column. Res Chem Intermed 2013;39:681–92. [8] Jana AK. Heat integrated distillation operation. Appl Energy 2010;87:1477–94. [9] Caballero JA, Grossmann IE. Optimal synthesis of thermally coupled distillation sequences using a novel MILP approach. Comput Chem Eng 2014;61:118–35. [10] Sayyaadi H, Saffari A. Thermoeconomic optimization of multi effect distillation desalination systems. Appl Energy 2010;87:1122–33. [11] An W, Yu F, Dong F, Hu Y. Simulated annealing approach to the optimal synthesis of distillation column with intermediate heat exchangers. Chin J Chem Eng 2008;16:30–5. [12] Fonyo Z, Mizsey P. Economic application of heat pumps in integrated distillation systems. Heat Recovery Syst CHP 1994;14:249–63. [13] Kiss AA, Flores Landaeta SJ, Infante Ferreira CA. Towards energy efficient distillation technologies – making the right choice. Energy 2012;47:531–42. [14] Jana AK. Advances in heat pump assisted distillation column: a review. Energy Convers Manage 2014;77:287–97. [15] Chua KJ, Chou SK, Yang WM. Advances in heat pump systems: a review. Appl Energy 2010;87:3611–24. [16] Bruinsma OSL, Spoelstra S. Heat pumps in distillation. Distillation & Absorption Conference. Eindhoven, Netherlands; 2010. [17] Maiti D, Jana AK, Samanta AN. Intensified thermal integration in batch reactive distillation. Appl Energy 2013;103:290–7. [18] Palenzuela P, Roca L, Zaragoza G, Alarcon-Padilla DC, Garcia-Rodriguez L, de la Calle A. Operational improvements to increase the efficiency of an absorption heat pump connected to a multi-effect distillation unit. Appl Therm Eng 2014;63:84–96. [19] Fonyo Z, Benkö N. Comparison of various heat pump assisted distillation configurations. Chem Eng Res Des 1998;76:348–60. [20] Feng X, Berntsson T. Critical COP for an economically feasible industrial heatpump application. Appl Therm Eng 1997;17:93–101. [21] Dhole V, Linnhoff B. Distillation column targets. Comput Chem Eng 1993;17:549–60. [22] Rivera Ortega P, Picón Núñez M, Torres Reyes E, Gallegos Muñoz A. Thermal integration of heat pumping systems in distillation columns. Appl Therm Eng 1999;19:819–29. [23] Linnhoff B, Dunford H, Smith R. Heat integration of distillation columns into overall processes. Chem Eng Sci 1983;38:1175–88. [24] Bandyopadhyay S. Thermal integration of a distillation column through sideexchangers. Chem Eng Res Des 2007;85:155–66. [25] Kravanja P, Modarresi A, Friedl A. Heat integration of biochemical ethanol production from straw – a case study. Appl Energy 2013;102:32–43. [26] Townsend D, Linnhoff B. Heat and power networks in process design part I: criteria for placement of heat engines and heat pumps in process networks. AIChE J 1983;29:742–8. [27] Wallin E, Franck P-Å, Berntsson T. Heat pumps in industrial processes—an optimization methodology. Heat Recovery Syst CHP 1990;10:437–46. [28] Benstead R, Sharman F. Heat pumps and pinch technology. Heat Recovery Syst CHP 1990;10:387–98. [29] Kemp IC. Pinch analysis and process integration: a user guide on process integration for the efficient use of energy. 2nd ed. Butterworth-Heinemann; 2007. [30] Yang M, Feng X, Chu KH. Graphical analysis of the integration of heat pumps in chemical process systems. Ind Eng Chem Res 2013;52:8305–10. [31] Wallin E, Berntsson T. Integration of heat pumps in industrial processes. Heat Recovery Syst CHP 1994;14:287–96. [32] Swaney RE. Thermal integration of processes with heat engines and heat pumps. AIChE J 1989;35:1003–16. [33] Kemp IC. Pinch analysis and process integration: a user guide on process integration for the efficient use of energy. IChemE 2007. [34] Klemes J, Friedler F, Bulatov I, Varbanov P. Sustainability in the process industry: integration and optimization. McGraw-Hill; 2010.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Heat integration of heat pump assisted distillation into the overall process Minbo Yang, Xiao Feng ⇑, Guilian Liu School of Chemical Engineering & Technology, Xi’an Jiaotong University, Xi’an 710049, China
h i g h l i g h t s Heat integration of heat pumps, distillation columns and background processes. An approach to identify the placement of a heat pump via pinch technology. A smaller temperature lift of heat pumping for a distillation column. The heat integration scenario reduces the energy consumption of the overall process.
a r t i c l e
i n f o
Article history: Received 5 January 2015 Received in revised form 3 October 2015 Accepted 6 October 2015 Available online 11 November 2015 Keywords: Pinch analysis Distillation Heat pump Grand Composite Curve Energy savings
a b s t r a c t Reducing the energy consumption of distillation processes can lead to significant cost savings in refineries and the chemical process industry because distillation is a widely used and energy-intensive separation technology. A distillation column can be heat integrated with heat pumps to reduce the energy supplied by the utility, and it can also be integrated into the overall process to save energy for the overall process. However, previous studies have not adequately investigated the synergistic effect of integrating heat pump assisted distillation into overall processes. In this paper, a systematic design methodology is proposed for the simultaneous heat integration of distillation, its background process and heat pump systems. Such a holistic heat integration approach can lead to considerable energy savings for the overall process. The proposed methodology also includes systematic identification for the energy-optimum placement of the heat pump and its matching with process streams. Furthermore, the impacts of distillation process modifications on the holistic heat integration strategy are examined. A case study is presented to illustrate how the proposed design method is applied and to demonstrate its effectiveness in saving energy. For the case study, the hot and cold utilities are reduced by 61.5% and 20.6% compared to energy consumptions for the base case. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The chemical industry consumes vast amounts of energy which is mainly obtained from burning fossil fuels, e.g., coal, natural gas and crude oil. This results in unsustainable life cycle impacts on the environment [1]. In process industries, separation processes are an essential step to recover or purify the desired products, or to remove undesired wastes or byproducts. Distillation is a widely used and energy-intensive separation technology [2,3], as energy consumption associated with distillation processing accounts for 40–60% of the total energy consumption of the chemical industry [4,5]. Due to the rising cost of energy and the increasingly strict environmental regulations, reducing the energy consumption of ⇑ Corresponding author. Tel.: +86 18611446202. E-mail address: [email protected] (X. Feng). http://dx.doi.org/10.1016/j.apenergy.2015.10.044 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
distillation processes is of high demand and has received considerable research attention. In a conventional distillation column, high quality heat is supplied to a bottom reboiler and waste heat is rejected from an overhead condenser. Many studies have proposed different ways to reduce the energy consumption of distillation processes. Some options for improving the energy efficiency of distillation operations include making changes to the design or operating conditions of the column, such as preheating the feed [6] and adjusting the reflux ratio [5,7]. System-wide approaches that utilize energy in an integrated manner within the distillation process are another attractive option to improve energy efficiency. For example, the concept of heat integration may be used to systematically realize heat recovery of distillation columns by making use of heat sources and heat sinks available within the distillation process [8]. Such heat integrated design options include thermally-coupled
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Nomenclature COP CP H1 H2 Hpoc QC QC,min Qcond QH QH,min Qreb Q1 Q2 Tcond TP
coefficient of performance of heat pump heat capacity flow rate, kW/°C heat flux at temperature T1 on the GCC, kW heat flux at temperature T2 on the GCC, kW heat flux at the heat pocket, kW total required cold utility of overall process, kW minimum cold utility of background process, kW heat load of overhead condenser, kW total required hot utility of overall process, kW minimum hot utility of background process, kW heat load of bottom reboiler, kW rejected heat of heat pump, kW heat taken in by heat pump, kW temperature of overhead condenser, °C temperature of the pinch point, °C
distillation [9], multiple-effect distillation [10], introduction of side reboilers and/or side condensers [11] and heat pump assisted distillation [12]. Among the aforementioned options, considerable attention has been paid by the academic and industrial communities [13–15] to the benefits of heat pumps for distillation processes as heat pumps can upgrade heat from a lower temperature level to a higher one. There are great potentials to reduce the energy consumption of distillation processes when they are appropriately integrated with heat pumps. Different schemes have been proposed for upgrading energy within a distillation column through heat pumps. One scheme is to use an external heat pump circuit in which a refrigerant fluid, different from the mixture in the column, is employed. This circuit has an evaporator and a condenser, which is coupled with the condenser and the reboiler of a distillation column, respectively [5]. Another scheme, known as mechanical vapor recompression heat pump, compresses the column overhead vapor stream in the compressor directly and uses it as a heating medium [16]. The bottom product also can be taken as a heat-exchanging medium of the heat pump with a bottom flashing arrangement [14,17]. A thermal vapor recompression heat pump is similar to the mechanical vapor recompression heat pump, in which the compressor is replaced by a stream ejector [13]. Also, an absorption heat pump transfers the heat using absorption pairs and can also be integrated in distillation processes [18,19]. Although the heat pump schemes described above are operated with different operating principles and integration concepts, all of them are applied to recover heat from the overhead condenser for use by the bottom reboiler. To allow heat exchange between a heat pump and a distillation column, the temperature lift via the heat pump should be greater than the temperature difference between the reboiler and the condenser. However, as an increase in the temperature lift of a heat pump decreases the coefficient of performance (COP), which is the ratio of the heat rejected to the external energy consumed by the heat pump, care must be taken to avoid excessive temperature lift [20]. In addition, with the aid of Column Grand Composite Curve [21], it is thermodynamically feasible to use heat pumps to exchange heat with side streams from intermediate trays of a distillation column, which requires a small temperature lift [22] but is rarely applied in practice. The basic idea of the aforementioned methods is to improve the internal energy utilization within distillation columns so that the demand for external energy, namely utility [8], can be reduced. Besides, a distillation column can also be designed in the context of the overall process with the aim of utilizing energy from other
Treb Tsup Ttar T1 T2 DTmin W
gC
temperature of bottom reboiler, °C supply temperature, °C target temperature, °C temperature of heat pump condenser, °C temperature of heat pump evaporator, °C minimum temperature difference for heat exchange, °C external work, kW Carnot efficiency
Superscripts 1 heat pump 1 2 heat pump 2 real corresponding real value
processing units to reduce the energy consumption of the overall process. For the sake of simplicity, the overall process without considering the distillation process is referred to as the background process in this paper. In the early 1980s, Linnhoff et al. applied the concept of pinch analysis and proposed a tool based on Heat Flow Cascade to optimize the allocation of available heat between a distillation process and its background process [23]. Their research indicates that if the operating temperatures of both the condenser and the reboiler are above or below the pinch temperature of the background process, distillation columns may be heat-integrated with the background process. If this is not the case (i.e. the distillation process is located across the pinch point), heat integration between the distillation column and its background process is not favored. In such situation, the column operating conditions can be modified so that the reboiling temperature is below the pinch or the condensing temperature is above the pinch. Subsequently, Bandyopadhyay [24] proposed two modifications: through side exchangers and through feed preheating, for the heat integration of a distillation column with the background process. Kravanja et al. [25] investigated the heat integration of a biochemical plant with the process modifications of heat loads and temperature levels. However, making process modifications to distillation columns requires careful consideration when the products are very sensitive to any changes in the operating conditions. Moreover, for the purpose of saving energy, heat pumps can also be heat integrated with the background process. Townsend and Linnhoff [26] proposed that a heat pump should absorb heat below the pinch temperature and reject it above, namely the across-pinch rule. In further research, Wallin et al. [27] adopted Composite Curves to guide the choice of heat pump types for various industrial processes and revealed the heat load limits of heat pump installations. Benstead and Sharman [28] matched heat pump with the Grand Composite Curve (GCC) of a process using the mirror image technique, which is relatively simple and clear. Besides, the shape of the GCC can also suggest which type of heat pump is most suitable [29]. Yang et al. [30] analyzed the dynamic changes of pinch temperature and GCC of industrial processes with heat pumps integrated. However, since both Composite Curve and GCC are composed of two sets of process streams (i.e. hot and cold streams), these pinch-based methods did not consider the matches between process streams and heat pumps, and such work was mainly done by computer programs [28,31,32]. In summary, three energy-efficient approaches have been explored so far: heat pump assisted distillation, heat integration
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between distillation columns and the background process, and heat integration between heat pumps and the background process. However, to the best of our knowledge, no studies have systematically addressed the simultaneous heat integration of heat pumps, distillation column and its background process. Obviously, the heat in the background process can be upgraded by heat pumps to reboil the liquid in the bottom reboiler or the rejected heat from the condenser can be lifted to heat cold streams in the background process. Therefore, the purpose of this work is to present a systematic methodology for the simultaneous heat integration of distillation, its background process and heat pumps in a holistic manner. Following previous heat integration approaches, the methodology of this work is developed based on the pinch conception. Firstly, this paper presents three scenarios of the heat integration concept relying on different GCCs. Secondly, the energy-optimum placement of a heat pump is identified with the minimum temperature lift. Thirdly, GCC is used as a guide to determine the streams to be matched with the heat pump. On this basis, the effects of distillation process modifications on heat integration and their technoeconomic impacts are further investigated. Compared with the conventional heat pump assisted distillation scheme, the proposed heat integration concept provides a smaller temperature lift for heat pumping and reduces considerably utility requirements of the overall process. Note that the minimum temperature difference for all the heat exchangers is set as DTmin in this work. 2. Distillation integrated into the overall process with heat pumps As presented by Townsend and Linnhoff [23], for a distillation column that is not located across the pinch point of its background process, either the reboiler or the condenser of the column can be integrated with streams in the background process. To be specific, the reboiler can take in heat from hot streams, reducing the demanded heat supplied by hot utility; or the condenser can reject heat to the cold streams, decreasing the demanded hot utility of the background process. In this case, the possibility of integrating a heat pump can be examined by the method of Townsend and Linnhoff [26]. When a distillation process is located across the pinch point, changing the location of the pinch temperature can be considered so that the operating temperatures of distillation columns are now located above or below the new pinch point. Yang et al. [30] analyzed the dynamic changes of GCC when heat pumps are integrated with a heat exchanger network. They found that the pinch temperature of a heat exchanger network can be changed by the choice for the placement of heat pumps. Based on this observation, heat pumps can be strategically applied to change the pinch temperature of the background process, which allows better heat integration.
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cannot be integrated with the background process directly. Thus, the total hot utility demand of the overall process is QH = QH,min + Qreb and the total cold utility demand is QC = QC, min + Qcond. Referring to the work of Yang et al. [30], a heat pump can be placed to change the pinch point. As shown in Fig. 1b, the heat pump absorbs heat Q2 at temperature T2 and rejects heat Q1 at temperature T1. When the amount of heat Q1 is larger than the deficient heat H1 at temperature T1 on the BPGCC, the pinch point is changed from point P to P1 with the introduction of a heat pump to the existing process and a new BPGCC is established. Consequently, the required hot utility for the background process is QH,min H1 and the required cold utility is QC,min + (Q1 Q2) H1. Furthermore, the distillation column is now located below the new BPGCC, as shown in Fig. 1c. For the overall process, the total hot utility is QH = QH,min H1 and the total cold utility is reduced to QC = QC,min + (Q1 Q2) H1 + Qcond Qreb. It should be noted that, in order to fully meet the heat demands of the distillation process, T1 should not be lower than Treb and the amount of Q1 should not be smaller than H1 + Qreb. A heat pocket may exist in the GCC when surplus of local heat sources lies above the pinch point or deficit of local heat sinks lies below the pinch point. Since the local heat source can satisfy the local heat sink when they are in the same heat pocket, the heat pocket can be excluded when the heat integration of heat pumps with the background process is considered [30]. However, in this work, these pockets existing in the GCC are strategically used to facilitate the heat recovery between a distillation process and its background process with the assistance of heat pumps. Fig. 2a shows a GCC of a background process with a heat pocket above the pinch point, and a distillation column is located across the pinch point. When a heat pump is placed across the pinch point (Fig. 2b), the amount of absorbed heat (Q2) is smaller than the amount of surplus process heat (H2) at temperature T2, while the amount of rejected heat (Q1) is equal to the amount of deficient heat (H1) at temperature T1, but is larger than the amount of deficient heat at the pocket (Hpoc). As shown in Fig. 2b, the pinch point is now changed from point P to P1. The new BPGCC shows that the temperature of the new pinch point is higher than T1 and Treb. Now, the heat integration between the distillation column and the background process can be achieved because the distillation column is located below the new pinch point of the updated BPGCC, as shown in Fig. 2c. For the overall process, the total hot utility is QH = QH,min Hpoc and the total cold utility is QC = QC,min + (Q1 Q2) Hpoc + Qcond Qreb. By comparing the two cases in Figs. 1 and 2, the reboiler of the distillation column in Fig. 1 absorbs heat Qreb from the heat pump, while the reboiler in Fig. 2 absorbs heat Qreb from the local heat source in the heat pocket. With the existence of the local heat source above the original pinch point, the temperature lift for heat pumping is reduced. In this way, the heat in the heat pocket is used by grade and better energy efficiency can be achieved.
2.1. Scenario 1: Increase in the pinch temperature For a conventional distillation column, the bottom reboiler absorbs heat Qreb at temperature Treb and the overhead condenser rejects heat Qcond at temperature Tcond. By assuming no subcooling or superheating for the condensation and evaporation, heat surplus and deficit of a distillation column can be represented as a quadrilateral box in the T H diagram, as shown in Fig. 1a [23]. The Grand Composite Curve of a background process (BPGCC) is also depicted in Fig. 1a, which shows the cascade heat flow of the background process, requiring the minimum hot utility QH,min and the minimum cold utility QC,min. Fig. 1a illustrates the case in which the distillation column is located across the pinch point and so it
2.2. Scenario 2: Decrease in the pinch temperature Fig. 3 illustrates the case of BPGCC to be integrated with a distillation column when the position of the pinch point is shifted downward. In the base case, a distillation column is located across the pinch point of BPGCC (Fig. 3a). Then, a heat pump is placed across the pinch point, as shown in Fig. 3b. It is noted that the amount of absorbed heat Q2 is larger than the surplus process heat H2 at temperature T2, and the rejected heat Q1 is equal to the deficient heat H1 at temperature T1. With the integration of a heat pump, the pinch point is changed from point P to P1, and the
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Fig. 1. Heat integration based on the increased pinch temperature 1.5-column.
Fig. 2. Heat integration based on the increased pinch temperature (a heat pocket above the original pinch point) 1.5-column.
Fig. 3. Heat integration based on the decreased pinch temperature 1.5-column.
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Fig. 4. Heat integration based on the decreased pinch temperature (a heat pocket below the original pinch point) 1.5-column.
distillation column can be placed above the new pinch point of the updated BPGCC, as shown in Fig. 3c. For the overall process, the overall requirement of cold utility is QC = QC,min H2 and that of hot utility is QH = QH,min (Q1 Q2) H2 + Qreb Qcond. It should be noted that the temperature T2 cannot be higher than Tcond and the amount of heat Q2 cannot be smaller than H2 + Qcond. As similarly considered in the first scenario, here we will look at the case of having a heat pocket below the pinch point as well as having a distillation column being located across the pinch point (Fig. 4a). A heat pump is placed across the pinch point, as shown in Fig. 4b. When the amount of absorbed heat Q2 is larger than heat Hpoc, the pinch point is shifted downward from P to P1, which now allows the heat integration between the distillation column and the background process, as shown in Fig. 4c. For this case, T2 is higher than Tcond but the temperature of the new pinch point is lower than both T2 and Tcond. For the overall process, the total hot utility is QH = QH,min (Q1 Q2) Hpoc + Qreb Qcond and the total cold utility is QC = QC,min Hpoc. The condensation for distillation is realized by releasing heat to the heat pump (Fig. 3) while the heat from the column condenser is rejected to the local heat sink existing in the heat pocket (Fig. 4),
which has been made more energy-efficient by having a smaller temperature difference for heat pumping. 2.3. Scenario 3: No change in the pinch temperature Sections 2.1 and 2.2 describe how heat integration between a distillation column and its background process can be made possible by changing the temperature of the pinch point. However, there is potential for achieving better heat recovery without changing the pinch temperature when heat pumps are integrated. As shown in Fig. 5, heat pumps can be placed at two different locations. Heat pump 1 absorbs the heat from the background process and rejects the heat to the reboiler of a distillation column, while Heat pump 2 absorbs the heat from the condenser of a distillation column and rejects the heat to the background process. Besides, the amount of absorbed heat Q 12 of heat pump 1 is smaller than the surplus process heat at temperature T 12 , and the amount of rejected heat Q 21 of heat pump 2 is smaller than the deficient heat at temperature T 21 . Thus, the pinch temperature is not changed, and accordingly, the total hot utility required for the overall process is decreased from QH,min + Qreb to QH,min Q 21 and the total cold utility is decreased from QC,min + Qcond to QC,min Q 12 . Compared with scenarios 1 and 2, the heat integration in scenario 3 is achieved without changes in the pinch temperature at the expense of an increase in the number of heat pumps. For all heat pumps in scenarios 1–3, their temperature lift can be smaller than the temperature difference between the reboiler and the condenser of the distillation column. Also, heat pumps are employed to supply heat required for the distillation column as well as for some of the heat demand in the background process. Such synergetic benefits gained from the proposed heat integration options provide more opportunities in heat recovery, leading to further energy savings. However, the specific characteristics of a distillation column and its background process will determine whether heat integration can be achieved and which is the better scenario. 3. Investigation of heat pumps placement
Fig. 5. Heat integration with no change in the pinch temperature single-column.
Section 2 describes in considerable detail the heat integration of heat pumps, a distillation column and its background process. When designing or retrofitting a heat exchanger network, the
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is the COP which is given in Eqs. (3) and (4), with which the amount of heat Q2 at temperature T2 can be calculated as shown in Eq. (5). When T1 and Q1 are fixed, the amount of Q2 is linearly related to temperature T2.
COP ¼
Q1 Q1 ¼ W Q1 Q2
COP ¼ gC
T 1 þ 0:5DT min þ 273:2 ðT 1 þ 0:5DT min Þ ðT 2 0:5DT min Þ
Q1 T 1 þ 0:5DT min þ 273:2 ¼ gC ðT 1 þ 0:5DT min Þ ðT 2 0:5DT min Þ Q1 Q2
Fig. 6. Example for identifying the operating parameters of heat pumps singlecolumn.
operating conditions of heat pumps and the streams to be matched with heat pumps still need to be determined. These topics are discussed here.
ð3Þ
ð4Þ
ð5Þ
where gC is the Carnot efficiency. Step 3: The BPGCC in Fig. 6 shows that the amount of surplus process heat increases with decreasing temperature below the pinch point. Considering Eq. (5) together with the BPGCC, the amount of heat Q2 to be rejected by the heat pump as well as temperature T2 can be determined. This allows the heat integration between heat sources and heat sinks in the distillation column and its background process, and the minimum temperature lift for the heat pump is identified, as shown in Fig. 6b.
3.1. Operating conditions of heat pumps
3.2. Process streams integrated with heat pumps
A large temperature lift through a heat pump should be avoided because it can reduce the COP of the heat pump [27]. Here, an approach is presented to systematically determine the energyoptimum operating conditions of a heat pump, leading to the smallest possible temperature lift, while the desired heat integration can still be fully realized. For convenience, this design concept is illustrated with an example depicted in Fig. 6.
After the operating conditions of the heat pump have been determined based on the BPGCC, the next step is to select process streams to be integrated with the heat pump. So far, the stream releasing its heat to the heat pump is treated as a single stream, and so is the stream absorbing heat from the heat pump. However, more than one process stream can be present in the GCC, and consequently stream splitting and combination may be necessary to integrate process streams with the heat pump when the most appropriate conditions identified from the previous sections are desired [28]. In general, there is more than one feasible option for the selection of process streams. Due to the high capital cost of heat pumps [27], and to reduce the number of heat pumps used, an assumption is made that a heat pump is allowed to simultaneously absorb heat from different process hot streams and reject heat to different process cold streams. Consequently, multiple heat exchangers may need to be applied in the heat pump system. The total number of process streams integrated with the heat pump determines the number of heat exchangers to be used in the heat pump system, which indicates the configurational complexity of the heat pump system. For a vapor compression heat pump, at least two heat exchangers are required, one of which is an evaporator and the other is a condenser. From this observation, the minimum number of process streams is preferred when streams for heat integration are to be selected.
Step 1: The BPGCC shows the overall heat deficit or surplus and its temperature level for a background process. Based on the BPGCC and the operating conditions of the distillation column, either the heat Q1 available at temperature T1, or the heat Q2 available at temperature T2 can be selected for coupling with a heat pump. As shown in Fig. 6a, when the amount of Q1 is smaller than the sum of Qreb + Hpoc, the distillation column cannot be fully integrated into the overall process; on the other hand, when the amount of Q1 is larger than the sum of Qreb + Hpoc, this surplus heat Q1 (Qreb + Hpoc) brings no savings for hot or cold utilities. Hence, the optimal amount of heat Q1 is Hpoc + Qreb. With this amount of heat available for heat integration, the lowest temperature T1 for the heat pump can be determined such that the deficient heat H1 equals Q1 from the BPGCC, as shown in Fig. 6b. To be clear, it is noteworthy that the temperature of cold streams presented on the GCC is higher than its real value by 0.5DTmin and the temperature of hot stream presented on the GCC is lower than its real value by 0.5DTmin. Therefore, Eqs. (1) and (2) can be derived:
T real ¼ T 1 þ 0:5DT min 1
ð1Þ
T real ¼ T 2 0:5DT min 2
ð2Þ
where the superscript real represents the actual stream conditions. Step 2: The amount of heat Q2 to be rejected at temperature T2, which corresponds to the amount of heat Q1 to be absorbed at T1, can be estimated by simple empirical formulas or a rigorous mathematical model of heat pumps. One practical measure for evaluating the performance of a vapor compression heat pump
Fig. 7. Determining the streams to be integrated with heat pump single-column.
M. Yang et al. / Applied Energy 162 (2016) 1–10
Fig. 7 shows the enlarged part of Fig. 6 in which the heat pump is integrated with BPGCC. The shaded parts in Fig. 7 represent the feasible regions for selecting hot and cold streams to be integrated with the heat pump. All the hot and cold streams are expressed as a series of segments in the T H diagram. In Fig. 7, a hot stream is expressed as a segment with a negative slope and a cold stream is expressed as a segment with a positive slope. If the use of a single hot or cold stream is desired to satisfy the corresponding heat demand of the heat pump, it is necessary to consider both the temperature and heat load of the stream. The procedure of selecting a single stream is illustrated in Fig. 7. First, shift all the segments of streams to the left until the right end vertexes, indicated by A and B, of the two shaded regions are located on the segments of cold and hot streams, respectively, as shown in Fig. 7. Any streams that cannot meet this condition should not be considered for selection, for example, cold stream 1. If the segment of a process stream does not intersect with the vertical axis in Fig. 7, for example, cold stream 2, the heat load of this kind of stream is not big enough and this stream should not be considered as well. Next, if a part of the segment appears outside of the shaded region, such as hot stream 1, this stream should also be excluded due to the infeasibility in heat exchange of the background process. Finally, hot stream 2 and cold stream 3 are the feasible streams for integrating with the heat pump. In this situation, if cold stream 3 and hot stream 2 are selected to be integrated with the heat pump, only two heat exchangers (an evaporator and a condenser) are needed. If a single hot or cold stream cannot be selected for the integration with the heat pump, the combination of several process streams is necessary and the way to select streams and combine them should reflect the heat transfer feasibility constrained by the shaded regions. As a result, the number of heat exchangers in the heat pump system is increased. 4. Process modifications for distillation process Process modifications can be considered as a way to alter operating conditions or change the process flowsheet, leading to modification of process data which may give more opportunities for heat recovery, reduce the energy target or allow a simpler or cheaper network [33]. For a distillation process, process modifications generally include changing the operating pressure, adding an intermediate reboiler or condenser, splitting the heat loads, changing the reflux ratio and preheating the feed, etc. [23]. These modifications change the compositions of product streams and the heat loads of the condenser and reboiler. Process modifications for distillation should not compromise the quality of products. Also, any potential benefit gained from process modifications for the distillation should be judged together with energy consumption because improving product quality may lead to potential increase in heat duties of the condenser and reboiler. Any physical limitations or practical constraints associated with process modifications should be considered for determining the degree of implementation in practice. As shown in Fig. 8a, a distillation column is located across the pinch point of the original BPGCC. It is not feasible to fully integrate this distillation column into the overall process by process modifications only because considerable change in column pressure is needed to ensure feasibility for heat integration. Referring to the design concepts proposed in this work, Scenario 1 is appropriate for further consideration. However, some part of condensation of the distillation column is not feasible for heat integration with the background process, as may be deduced from the new BPGCC (Fig. 8a). Hence, process modifications of the distillation column may now be used to achieve heat recovery of the distillation column with the background process.
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4.1. Changing the operating pressure Changing the operating pressure of a distillation column influences the operating temperatures of the condenser and reboiler. Raising the operating pressure increases the temperatures of the condenser and reboiler while lowering the operating pressure decreases the corresponding temperatures. Permissible maximum and minimum bounds for changing column pressure exist as illustrated by the shaded region in Fig. 8b. It should be noted that the new adjusted column pressure for improving heat integration should be within the acceptable operation range and without compromising product quality. For the example given in Fig. 8b, operating pressure should be reduced to move the quadrilateral box downward to make the heat integration of the distillation column with the new BPGCC feasible, which is depicted by the dotted quadrilateral box. 4.2. Feed preheating and addition of intermediate side reboiler Feed preheating provides an increase of energy input to the feed tray as well as a reduction in the heat duty of the reboiler. Addition of an intermediate side reboiler can supply heat at a lower temperature than that of the reboiler, which facilitates evaporation of liquid in an intermediate tray of the column and can reduce the heat duty of the reboiler as well. Both feed preheating and addition of an intermediate side reboiler do not reduce the overall duty of hot utility required for the distillation column but they are able to accept lower quality of hot utility, leading to savings in energy cost. The operating conditions of the distillation column are updated which can accommodate lower grade hot utility, as illustrated in Fig. 8c. The updated conditions for the distillation column are now located within the feasible design region given by the new BPGCC, which makes the heat integration feasible. In summary, the heat integration between a distillation process and its background process can be systematically achieved through integration of heat pumps with process modifications in an integrated manner. It should be noted that process modifications may make it easy for integrating heat pumps with the background process, but any potential negative impacts, for example, degradation of distillation product quality or increase of heat duties, should be collectively evaluated together with the benefits gained from the heat integration. 5. Case study This case study is used to illustrate how the proposed methodology can be effectively applied for achieving the heat integration between a distillation process and its background process. Fig. 9 shows a simplified flowsheet of a chemical process, and the detailed stream data are listed in Table 1. The background process consumes hot utility 40 kW and cold utility 148 kW. When the distillation column is included, the total consumptions of hot and cold utilities are 78 kW and 188 kW, respectively. The energy flow for the background process is analyzed by pinch analysis. When DTmin is assumed to be 10 °C, the average pinch temperature is found to be 75 °C (i.e. 80 °C for hot streams and 70 °C for cold streams), and the minimum hot and cold utilities are 10 kW and 118 kW, respectively. The GCC of the background process is given in Fig. 10. As shown in Fig. 11, the distillation process is located across the pinch point. Here, a vapor compression heat pump is used to facilitate the heat integration. With the across-pinch rule, R142b with a critical temperature of 136.45 °C is chosen as the working fluid of the heat pump. Following the design procedure described in Sec-
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Fig. 8. Illustration of process changes of a distillation process: (a) base case; (b) pressure change; (c) feed preheating and addition of an intermediate reboiler 1.5-column.
Fig. 9. Flowsheet of the case study (modified from Ref. [34]) single-column.
Although the operating conditions of the heat pump are determined by taking into account the minimum temperature lift for heat pumping, none of the hot streams can solely meet the heat duty required for the integration with the heat pump, as shown in Fig. 11. It means that at least three heat exchangers (one condenser and two evaporators) are required in the heat pump system. In Fig. 11, hot stream 3 is located close to the feasible region and thus it is selected as the heat source to supply heat to the heat pump. Now, new operating conditions for the heat pump are determined as Q2 equals 38.8 kW and T2 (based on the actual temperature) equals 54.6 °C, as shown in Fig. 12. The external energy to be supplied for the heat pump is 9.2 kW and the COP of the heat pump is 5.2. The heat pump absorbs 38.8 kW of heat from hot stream 3 and its temperature is reduced from 72.4 °C to 64.6 °C. The heat pump rejects 48 kW of heat to cold stream 2 and its temperature is increased from 75.2 °C to 87.2 °C. In this way, only two heat exchangers are required in the heat pump system, and a further increase in the temperature lift of heat pumping is only 0.2 °C. Fig. 13 shows the improved flowsheet after implementing the solutions described above. The total hot utility is 0 kW and the total cold utility is 119.2 kW. The temperature lift of the heat pump is 42.6 °C which is smaller than the temperature difference
Table 1 Stream data of the process. Stream
CP/(kW/°C)
Tsup/°C
Ttar/°C
Heat load/kW
Hot 1 Hot 2 Hot 3 Cold 1 Cold 2
1 2 5 1.8 4
180 130 80 30 60
80 40 50 120 100
100 180 150 162 160
Column
Reboiler Condenser
130 (liquid) 60 (vapor)
130 (vapor) 60 (liquid)
38 40
tion 4, Q1 is found to be 48 kW, and T1 is determined as 92.2 °C (i.e. real temperature is 97.2 °C) on the BPGCC. Assuming that the Carnot efficiency gC is 60%, when the heat pump absorbs heat from the heat source available in the background process, Q2 is found to be 38.9 kW and T2 is 59.8 °C (i.e. real temperature is 54.8 °C), based on Eq. (5) and the BPGCC. On the other hand, when the heat pump absorbs the heat from the condenser of distillation column, T2 (based on the actual temperature) should not be higher than 50 °C because DTmin is 10 °C. Therefore, the former option is better than the latter one.
Fig. 10. The GCC of the background process in the case study single-column.
M. Yang et al. / Applied Energy 162 (2016) 1–10
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Fig. 11. Identification of heat pump placement 1.5-column.
Fig. 12. Improvement of heat pump placement single-column.
Fig. 14. Simplified flowsheet for the case study single-column.
Fig. 13. Resulting flowsheet after implementing the heat integration singlecolumn.
between the reboiler and the condenser. However, it can be seen that the heat exchanger network is more complex than the original one. It is possible to simplify the flowsheet given in Fig. 13 through process modifications for the heat pump. By maintaining the determined operating conditions, namely, the four parameters T1, Q1, T2 and Q2 of the heat pump, driving force for the heat exchange related to hot stream 3 and cold stream 2 in the heat pump can be slightly changed, as designed in Fig. 14. As a result, the minimum temperature difference in the updated flowsheet of Fig. 14 is 9.6 °C in the evaporator and 7.9 °C in the condenser of the heat pump, compared to the original value of 10 °C. Although the reduction in driving force in the exchanger leads to larger heat exchanger areas for the reboiler and condenser, two exchangers, heat exchangers 3 and 8, can be removed in the simplified flowsheet. The details of the simplified flowsheet are shown in Fig. 14. In this case study, a heat pump is placed to absorb heat from a hot stream below the original pinch point and to reject heat to a cold stream above the original pinch point. This increases the pinch temperature from 75 °C to 175 °C. With the introduction of the heat pump, the distillation column can be fully integrated into the overall process, i.e. the reboiler of the distillation col-
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umn absorbs heat from a hot stream in the background process and the condenser of the distillation column rejects heat to the cold utility. Consequently, the application of the heat pump reduces hot utility and cold utility by 48 kW and 38.8 kW, respectively. 6. Conclusions In this work, a novel design methodology based on pinch analysis is proposed to systematically analyze the simultaneous heat integration of heat pumps, a distillation process, and its background process. For the case in which a distillation process is located across the pinch point, heat pumps can be used to facilitate the heat integration between the distillation process and its background process. Three possible scenarios for such heat integration are presented. (1) A heat pump is introduced, causing the pinch temperature to increase. The process heat below the new pinch point can satisfy the heat duty of the reboiler. (2) A heat pump is introduced, causing the pinch temperature to decrease. The condenser of the distillation column can provide heat for the cold streams above the new pinch point. (3) Two heat pumps are introduced to integrate the distillation column and the background process. In the case study considered in this paper, the proposed heat integration scheme provides 61.5% and 20.6% reduction in hot and cold utilities, respectively. Also, the temperature lift of the heat pump is determined to be 42.6 °C, which is much smaller than the temperature difference between the reboiler and the condenser (70 °C). This illustrates that the proposed heat integration method allows a smaller temperature lift for heat pump, compared with the traditional method for heat pump assisted distillation, and provides more opportunities for energy savings. In the proposed work, conceptual understanding for the simultaneous integration of heat pumps and distillation columns with the background process is obtained. As the design procedure proposed in this paper is based on graphical methods using pinch analysis, it is relatively easy to apply and can give clear design insights. However, this work only discusses the aspect of improving the energy efficiency of distillation columns by implementing heat-integrated heat pumps without considering the investment cost. Our future work will investigate the impacts of capital investment on the overall process. Acknowledgements Financial support from the National Basic Research Program of China (973 Program: 2012CB720500) and the National Natural Science Foundation of China under Grant No. 21276204 is gratefully acknowledged. References [1] Nigam PS, Singh A. Production of liquid biofuels from renewable resources. Prog Energy Combust Sci 2011;37:52–68. [2] Sarbatly R, Chiam C-K. Evaluation of geothermal energy in desalination by vacuum membrane distillation. Appl Energy 2013;112:737–46.
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