Self-Heat Recuperation Technology for Energy Saving in Chemical

Jul 22, 2009 - An innovative self-heat recuperation technology has been developed for heating and cooling thermal processes, in which not only latent ...
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Ind. Eng. Chem. Res. 2009, 48, 7682–7686

Self-Heat Recuperation Technology for Energy Saving in Chemical Processes Yasuki Kansha, Naoki Tsuru, Kazuyoshi Sato, Chihiro Fushimi, and Atsushi Tsutsumi* CollaboratiVe Research Center for Energy Engineering, Institute of Industrial Science, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan

An innovative self-heat recuperation technology has been developed for heating and cooling thermal processes, in which not only latent heat but also sensible heat are circulated in a feed-effluent heat exchanger of the thermal process by compressing the effluent stream without any heat addition. Applying this technology to the thermal processes, the amount of energy required was determined using a commercial process simulation tool, PROII. The proposed self-heat recuperation technology, in which the heat of an effluent stream is recuperated and reused for feed stream heating by gas and/or vapor recompression, was found to drastically reduce the energy consumption. 1. Introduction The reduction of carbon dioxide (CO2) emission has become a major target in efforts to suppress global warming. The combustion of fossil fuel for heating produces a large amount of CO2. Thus, the reduction in energy consumption is one of the most concerned issues. So far, to reduce the energy consumption, heat recovery technology, which exchanges heat between the hot and cold streams in a process, has been applied to thermal processes. A simple example of this technology is the application of a feed-effluent heat exchanger. By using a feed-effluent heat exchanger in a thermal process, the heat is exchanged between feed (cold) and effluent (hot) streams to recirculate the self-heat of the stream. An additional heat source may be required depending on the minimum temperature difference between two streams for heat exchange. Recently, a heat pump has been applied to thermal processes in which the ambient heat or the process waste heat is generally pumped up for heating the process stream by using working fluid compression.1-8 Although the heat pump is well-known to reduce the energy consumption and exergy loss of the process, the heat load and capacity of the process stream are often different from those of the pumped heat. Thus, the normal heat pump is still possible to provide large exergy loss in the heat exchanger. In heat recovery technologies, vapor recompression has been applied to evaporation,9,10 distillation,11-13 and drying14 in which the vapor evaporated from the process is compressed to a higher pressure and condensed providing a heating effect. The condensation heat of the stream is recirculated as the vaporization heat in the process by using vapor recompression. In this paper, an innovative self-heat recuperation technology, in which not only the latent heat but also the sensible heat of the process stream can be circulated without any heat addition, is developed and applied to a thermal process in the gas stream and the phase change stream between vapor and liquid. By means of the compression of the effluent stream, the effluent stream is heated to provide the minimum temperature difference for heat exchange with the feed stream, and all of the self-heat of the process stream is recirculated based on exergy recuperation to reduce the energy consumption. * To whom correspondence should be addressed. Tel: +81-3-54526727. Fax: +81-3-5452-6728. E-mail: [email protected].

2. Thermal Process Using Self-Heat Recuperation Technology 2.1. Gas Stream. Generally, chemical process units consist of several functions such as heating, cooling, reaction, and separation. Figure 1 shows a simple thermal process [I] in which a gas stream is heated from the standard temperature T0 to a certain operating temperature T1 of the following process (X) (e.g., reactor) by a heater (FH) (1 f 2) and cooled to the standard temperature by cooling water (CW) (3 f 4). The temperature-heat diagram of the thermal process [I] is shown in Figure 1b. The total heating duty, Qtotal, is provided by FH. If the temperatures of the streams 2 and 3 are the same, the I , is always equal to the external external heating load, QFH I cooling load QCW, expressed by I Qtotal ) QFH ) QICW

(1)

In the case of a thermal process [II] using a feed-effluent heat exchanger (HX) with a minimum temperature difference ∆Tmin (self-heat exchange thermal process), the heat of the effluent stream can be reused to preheat the feed stream up to T2, reducing overall energy consumption, as shown in Figure 2. This process is a typical energy saving process by heat recovery. In this process, the process stream is preheated from T0 to T2 with a heat exchanger (HX) (1 f 2) and heated with a heater (FH) from T2 to T1 (2 f 3). The effluent stream from the following process (X) is cooled with the HX for self-heat exchange (4 f 5) and finally cooled to T0 by CW (5 f 6). In

Figure 1. Simple thermal process [I] (for the gas stream): (a) flow diagram, (b) temperature-heat diagram.

10.1021/ie9007419 CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009

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Figure 4. Thermal process [IV] (for the vapor/liquid phase change stream): (a) flow diagram, (b) temperature-heat diagram.

Figure 2. Thermal process with a feed-effluent heat exchanger [II] (for the gas stream): (a) flow diagram, (b) temperature-heat diagram.

Figure 5. Thermal process [V] with a feed-effluent heat exchanger (for the vapor/liquid phase change stream): (a) flow diagram, (b) temperatureheat diagram.

Figure 3. Thermal process with a self-heat recuperation technology [III] (for the gas stream): (a) flow diagram, (b) temperature-heat diagram.

Figure 2b, the overlapping interval corresponds to the heat transfer duty from the hot effluent stream to the cold feed stream II ). Thus, the heat recovery by (the self-heat exchange load: QHX means of self-heat exchange can reduce the external heating II ) as follows: load (QFH II II QFH ) Qtotal - QHX ) QIICW

(2)

II where QCW represents the external cooling load (5 f 6 in Figure 2). Even with a self-heat exchange process, the heater is still required because the effluent stream should be heated to provide ∆Tmin for heat exchange. Moreover, as expressed by eq 2, the heat generated by FH should be dispersed in CW. The proposed thermal process of the gas streams for heat circulation by using the self-heat recuperation technology is shown in Figure 3a. In this process, the feed stream is heated up with a heat exchanger (HX) (1 f 2) from a standard temperature T0 to a set temperature T1. The effluent stream from the following process (X) is compressed with a compressor to recuperate the heat of the effluent stream (C) (3 f 4) and the temperature at the exit of the compressor rises up to T1′ because of the adiabatic compression. The stream 4 is cooled with the HX for self-heat exchange (4 f 5). The effluent stream is then decompressed with an expander (E) to recover part of the work of the compressor (C). The effluent stream is finally cooled to

T0 with cooling water (CW) (6 f 7). Thus, self-heat recuperation leads to perfect internal heat circulation. Note that the total heating duty, Qtotal, is equal to the internal self-heat exchange load, QIII HX, without any external heating load as shown in Figure 3b. 2.2. Vapor/Liquid Streams. In the case of a simple thermal process [IV] for vapor/liquid streams (Figure 4a), the phase change occurs from liquid to vapor. The temperature-heat diagram of the process of vapor/liquid streams without any heat recovery is shown in Figure 4b. In Figure 4b, Tb represents the boiling temperature of the stream at an operating pressure. Figure 5 shows a vapor/liquid stream thermal process [V] with heat recovery by using a feed-effluent heat exchanger as a self-heat exchange process. By using the feed-effluent heat exchanger with a minimum temperature difference ∆Tmin, the heat of the effluent stream from the following process (X) can be reused (4 f 5) to preheat the feed stream up to Tb (1 f 2). However, in this case the pinch point appears at the location with the minimum temperature difference between feed and effluent streams. Therefore, most of the latent heat of the phase change cannot be recovered by self-heat exchange. As can be V , is much larger seen in Figure 5, the external heating load, QFH V because the latent heat of than the self-heat exchange load QHX the stream is usually much larger than the sensible heat. Figure 6a shows the proposed thermal process of the vapor/ liquid streams for heat circulation by using the self-heat recuperation technology. In this process, the feed stream is heated using a heat exchanger (HX) (1 f 2) from the standard temperature T0 to a set temperature T1. The effluent stream from the following process (X) is adiabatically compressed with a

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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 Table 1. Energy Requirement and the Self-Heat Exchange Load for the Proposed and Benchmark Thermal Process of Butane (at ∆Tmin ) 10 K) proposed system

benchmark system

T1 [K]

WC [kW]

WE [kW]

Wneta [kW]

QHX [kW]

rb [-]

QFH [kW]

QHX [kW]

350 400 450

30.0 34.0 37.5

25.6 25.4 25.0

4.4 8.6 12.5

147.7 313.7 497.0

1.45 1.44 1.43

31.0 34.6 38.0

116.7 279.1 459.0

a

Wnet ) WC - WE. b r: pressure ratio in the compressor.

Table 2. Energy Requirement and the Self-Heat Exchange Load for the Proposed and Benchmark Thermal Process of Benzene (at ∆Tmin ) 10 K) proposed system

Figure 6. Thermal process [VI] with a self-heat recuperation technology (for the vapor/liquid phase change stream): (a) flow diagram, (b) temperature-heat diagram.

Wneta [kW]

QHX [kW]

rb [-]

QFH [kW]

QHX [kW]

400 450 500

34.93 34.92 37.79

1195.4 1364.3 1550.5

1.55 1.48 1.42

897.3 897.3 897.3

298.1 467.0 653.2

a

compressor (C) (3 f 4) to provide ∆Tmin. The latent heat can then be exchanged between the feed and effluent streams because the boiling temperature of the effluent stream rises to Tb′ by compression. Thus, the effluent stream is cooled with the HX for self-heat exchange (4 f 5) while recuperating its heat. The effluent stream is depressurized by a valve (VL, 5 f 6) and finally cooled to T0 with the cooling water (CW) (6 f 7). With self-heat recuperation, this leads to perfect internal heat circulation including not only the sensible heat but also the latent heat (vaporization heat and condensation heat) in the process. It can be seen in Figure 6b that the total heating duty, Qtotal, is VI , and that the equal to the internal self-heat exchange load, QHX total heating duty of the feed stream is equal to the total cooling duty of the effluent stream. This indicates that self-heat is recirculated in the process without any additional heat source. The external cooling load stems from the work of the compressor. Therefore, a large amount of energy required in the thermal process can be reduced using self-heat recuperation technology. 3. Process Simulation for the Proposed Thermal Process The process simulation was conducted to clarify the energy saving effect of the proposed self-heat recuperation technology for the proposed thermal processes. The schematic flow diagrams for gas streams and for vapor/liquid streams are shown in Figures 3a and 6a. In the self-heat recuperative thermal process of the gas streams, the energy requirement of this thermal process is equal to the net work (Wnet) as follows: Wnet ) WC - WE

(3)

where WC and WE represent the shaft work of the compressor and expander, respectively. In the self-heat recuperative thermal process of the vapor/ liquid streams, the net work is equal to the work for compressor as follows Wnet ) WC

(4)

The external heating load in the conventional self-heat exchange thermal process is calculated from the following equation: QFH ) |H3,b - H2,b |

(5)

benchmark system

T1 [K]

Wnet ) WC. b r: pressure ratio in the compressor.

where Hi,b is the enthalpy of the stream i in the self-heat exchange thermal process. The energy requirement (Wnet, QFH) and the self-heat exchange load (QHX) were calculated for a real fluid stream by using the commercial simulator PRO/II version 8 (Invensys). As a real fluid, butane was used for the gas stream while benzene (boiling point is 353.2 K) was used for the vapor/liquid stream. In the calculations for both cases, the streams were heated from 300 K to a set temperature T1, and the flow rate of the stream, F, was 100 kmol/h. The Soave-Redlich-Kwong equation of state (SRK) was used. The minimum temperature difference for heat exchange was assumed to be 10 K. The pressure ratio in the compressor was set to keep a constant temperature increase of 10 K due to compression. The efficiency of the heat exchanger was 100% (i.e., no heat loss). 4. Simulation Results The simulation results are summarized in Tables 1 and 2. It can be seen that the energy requirements of both the self-heat recuperative thermal processes of gas streams and vapor/liquid streams were drastically reduced compared to the conventional self-heat exchange thermal process when the adiabatic efficiencies of the compressor and expander are 100%. In the conventional self-heat exchange thermal process, the external heating load QFH is discarded as the external cooling load QCW. In contrast, in the self-heat recuperative thermal process of gas streams, the work of the expander WE is recovered according to eq 3, leading to a substantial reduction of the net work Wnet. In the self-heat recuperative thermal process of the vapor/liquid streams, the total cooling duty is recuperated and reused as the total heating duty by using self-heat recuperation by which not only the sensible heat but also the latent heat can be exchanged between the feed and the effluent streams. This results in a significant reduction in energy requirement for the self-heat recuperative thermal process of vapor/liquid streams. The effects of the adiabatic efficiencies of the compressor and expander on the energy requirement for gas streams and vapor/liquid stream are examined and plotted in Figure 7a,b, respectively. Although the energy requirement of the self-heat recuperative thermal process for a gas stream is slightly increased with the decrease of the adiabatic efficiencies of the compressors and expanders, a considerable energy saving is still achieved compared to the conventional self-heat exchange

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QFH ) |H3,b - H3,b | )

∫

T3

T2

FCP dT

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(6)

where Ti is the temperature of the stream i in the self-heat exchange thermal process (Figure 2), F is the flow rate, and CP is the heat capacity at constant pressure. Since ∆Tmin ) T3 T2 for a constant CP, eq 6 can be rewritten as QFH ) FCP∆Tmin

(7)

The first law of thermodynamics can be expressed as: dU ) dQ + dW

(8)

where U is the internal energy, Q is the heat, and W is the work. In the isentropic process (dQ ) 0), the work for an ideal gas is given by the following equation: dW ) dU ) Cv dT

(9)

where Cv is the heat capacity at constant volume. Thus, the works of adiabatic compression and expansion in the self-heat recuperative thermal process for an ideal gas stream (Figure 3) is calculated from the following equations: Figure 7. Energy requirement of the proposed thermal process with selfheat recuperation technology: (a) for the gas stream, (b) for the vapor/ liquid phase change stream.

thermal process. The loss of work for compression is transformed into heat as additional heating for the process stream. Even if the adiabatic efficiency is 0%, the energy requirement of the self-heat recuperative thermal process is the same as that of the conventional self-heat exchange thermal process. In the case of the self-heat recuperative thermal process for a vapor/liquid stream, a remarkable reduction in energy consumption (about 95%) was obtained as shown in Figure 7b. The energy requirement is slightly increased with the decrease of adiabatic efficiencies of the compressors and expanders. In this case, the latent heat is predominant in self-heat exchange. Thus, the self-heat recuperation technology can achieve a significant energy savings because of the perfect heat circulation of the latent heat as well as the sensible heat. 5. Discussion In the conventional self-heat exchange thermal process (Figures 2 and 5), the addition of high temperature heat from an external heat source is required for heating the process stream to provide the minimum temperature difference for feed-effluent heat exchange. The self-heat is reused to preheat the feed stream in the feed-effluent heat exchanger. The low temperature heat of the effluent stream is exhausted, for which the heat load is equal to the additional high temperature heat load. Therefore, in the conventional self-heat exchange thermal process, the additional high temperature heat is transformed to the low temperature heat, causing exergy loss. Instead of the additional heat source, ambient heat or process waste heat can be used by means of the heat pump. This leads to a reduction in exergy loss of the conventional self-heat exchange thermal process. In the self-heat recuperative thermal process, all of self-heat of the process stream is recirculated based on exergy recuperation, in which exergy of the process stream is recuperated and this stream heat is reused in the process, without any external heat source, by compressing the process stream. In the case of the conventional self-heat exchange thermal process for an ideal gas stream, the additional heat load (QFH) is represented from eq 5 by the following:

WC ) |FCv(T4 - T3)|

(10)

WE ) |FCv(T6 - T5)|

(11)

where Ti represents the temperature of the stream i. For the ideal gas, the temperature difference between streams 3 and 4 (∆T3f4) in the self-recuperative thermal process is equal to ∆Tmin. The shaft work for the adiabatic compression (WC) of the self-heat recuperative thermal process is smaller than the additional heat load (QFH) of the conventional self-heat exchange thermal process because of the relation between the constant pressure and constant volume heat capacities (CP - CV > 0). Therefore, the net work (Wnet ) WC - WE) of the selfrecuperative thermal process is much smaller than the additional heat load (QFH) of the conventional self-heat exchange thermal process. In the case of an ideal vapor/liquid system with the conventional self-heat exchange thermal process, QFH is given by the following equation: QFH )

{

FCP,V∆T2f3 + QPC, at CP,V e CP,L FCP,V∆Tmin + QPC, at CP,V > CP,L

(12)

where the temperature differences (∆T2f3) between streams 2 and 3 is larger than ∆Tmin and CP,V and CP,L are the heat capacities of the vapor and liquid, respectively. In the self-recuperative thermal process, the net work (Wnet ) WC) is examined from eq 10. Therefore, Wnet of the selfrecuperative thermal process is much smaller than QFH of the conventional self-heat exchange thermal process because of the relation between the constant pressure and constant volume heat capacities (CP - CV > 0). Thus, for both the ideal gas and vapor/ liquid systems, self-heat recuperation can drastically reduce the energy consumption. From the process simulation results, it can be seen that the energy consumption of the self-heat recuperative thermal process was 1/3-1/22 of that of the conventional selfheat exchange thermal process. Figure 8 shows conceptual temperature-heat diagrams of heat integration systems by (a) conventional pinch technology and (b) self-heat recuperation technology in feed-effluent heat exchange. The lines which represent the cold and hot streams are plotted in the temperature-heat diagram. The cold stream

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recuperative thermal process was drastically reduced (1/3-1/ 22) compared with that of the conventional self-heat exchange thermal process. Therefore, this technology is a very promising technology for energy saving. Acknowledgment This research was financially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 8. Conceptual temperature-heat diagrams of heat integration systems by (a) the conventional pinch technology and (b) the proposed self-heat recuperation technology.

means the process stream in which the temperature is increasing, and the hot stream means the process stream in which the temperature is decreasing. If ∆Tmin is negligible, the hot and cold stream lines are perfectly overlapped. In the conventional pinch technology,15-20 these lines can be moved horizontally within the temperature limits until the nearest points (pinch points) are separated by the minimum temperature difference. From these plots, the lines have identified the region in which the heat is exchanged between the hot and the cold streams for heat recovery. Beyond the area of overlap, the curves identify the need for an additional heat source and sink (cf. Figure 8a). The pinch technology is based on a concept of energy cascading utilization, in which the heat energy is used from high grade heat to low grade heat and low grade waste heat is discarded from the process. On the other hand, in the self-heat recuperation technology, the hot stream line is vertically shifted by using the adiabatic compression for the hot stream. The shaft work of the compression is required to circulate the internal heat in the process and exhausted with the process stream. If the heat capacity is independent from the pressure, hot and cold stream lines are in parallel and always separated by the minimum temperature difference. Therefore, exergy loss of the heat exchange becomes the minimum and all of the self-heat can be recirculated without any additional heat source in the self-heat recuperative thermal process (cf. Figure 8b). The self-heat recuperation technology is based on exergy recuperation in which degraded heat can be recuperated and reused. 6. Conclusion In this paper, the innovative self-heat recuperation technology for heating and cooling thermal processes using pressure change was proposed, in which not only sensible heat but also latent heat of the process stream are circulated in the thermal process. By compressing the effluent stream, the stream temperature is increased to provide the minimum temperature difference for heat exchange, and the stream heat is circulated in the process based exergy recuperation, leading to a reduction in the energy requirement of the process. The energy requirement of the proposed self-heat recuperative thermal process and the conventional self-heat exchange thermal process using an additional heat source were calculated by using a process simulator. The results show that the energy requirement of the self-heat

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ReceiVed for reView May 7, 2009 ReVised manuscript receiVed July 1, 2009 Accepted July 7, 2009 IE9007419