**Self-Heat Recuperation: Theory and Applications**

Yasuki Kansha1, Akira Kishimoto1, Muhammad Aziz2 and Atsushi Tsutsumi1 *1Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo 2Advanced Energy Systems for Sustainability, Solution Research Laboratory Tokyo Institute of Technology Japan* 

## **1. Introduction**

78 Heat Exchangers – Basics Design Applications

This work was performed as a part of the research supported by Provincial Secretariat for

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Subscripts:

1 fluid 1 2 fluid 2 w wall

**9. References** 

Hill

**8. Acknowledgment** 

Since the 1970s, energy saving has contributed to various elements of societies around the world for economic reasons. Recently, energy saving technology has attracted increased interest in many countries as a means to suppress global warming and to reduce the use of fossil fuels. The combustion of fossil fuels for heating produces a large amount of carbon dioxide (CO2), which is the main contributor to global greenhouse gas effects (Eastop & Croft 1990, Kemp 2007). Thus, the reduction of energy consumption for heating is a very important issue. To date, to reduce energy consumption, heat recovery technology such as pinch technology, which exchanges heat between the hot and cold streams in a process, has been applied to thermal processes (Linnhoff et al. 1979, Cerda et al. 1983, Linnhoff et al. 1983, Linnhoff 1993, Linnhoff & Eastwood 1997, Ebrahim & Kawari 2000). A simple example of this technology is the application of a feed-effluent heat exchanger in thermal processes, wherein heat is exchanged between feed (cold) and effluent (hot) streams to recirculate the self-heat of the stream (Seider et al. 2004). To exchange the heat, an additional heat source may be required, depending on the available temperature difference between two streams for heat exchange. The additional heat may be provided by the combustion of fossil fuels, leading to exergy destruction during heat production (Som & Datta 2008). In addition, many energy saving technologies recently developed are only considered on the basis of the first law of thermodynamics, i.e. energy conservation. Hence, process design methods based on these technologies are distinguished by cascading heat utilization.

Simultaneously, many researchers have paid attention to the analysis of process exergy and irreversibility through consideration of the second law of thermodynamics. However, many of these investigations show only the calculation results of exergy analysis and the possibility of the energy savings of some processes, and few clearly describe methods for reducing the energy consumption of processes (Lampinen & Heillinen 1995, Chengqin et al 2002, Grubbström 2007). To reduce exergy reduction, a heat pump has been applied to thermal processes, in which the ambient heat or the process waste heat is generally pumped to heat the process stream by using working fluid compression. Although it is well-known that a heat pump can reduce energy consumption and exergy destruction in a process, the

Self-Heat Recuperation: Theory and Applications 81

Figure 1 (a) shows a thermal process for gas streams with heat circulation using self-heat recuperation technology. In this process, the feed stream is heated with a heat exchanger (1→2) from a standard temperature, T1, to a set temperature, T2. The effluent stream from the following process is pressurized with a compressor to recuperate the heat of the effluent stream (3→4) and the temperature of the stream exiting the compressor is raised to T2 through adiabatic compression. Stream 4 is cooled with a heat exchanger for self-heat exchange (4→5). The effluent stream is then decompressed with an expander to recover part of the work of the compressor. This leads to perfect internal heat circulation through selfheat recuperation. The effluent stream is finally cooled to T1 with a cooler (6→7). Note that the total heating duty is equal to the internal self-heat exchange load without any external heating load, as shown in Fig. 1 (b). Thus, the net energy required of this process is equal to the cooling duty in the cooler (6→7). To be exact, the heat capacity of the feed stream is not equal to that of the effluent stream. However, the effect of pressure to the heat capacity is small. Thus, two composite curves in Fig. 1 (b) seem to be in parallel. In addition, the exergy destruction occurs only during the heat transfer in the heat exchanger. The amount of this

In the case of ideal adiabatic compression and expansion, the input work provided to the compressor performs a heat pumping role in which the effluent temperature can achieve perfect internal heat circulation without exergy destruction. Therefore, self-heat recuperation can dramatically reduce energy consumption. Figure 1 (c) shows a thermal process for vapor/liquid streams with heat circulation using the self-heat recuperation technology. In this process, the feed stream is heated with a heat exchanger (1→2) from a standard temperature, T1, to a set temperature, T2. The effluent stream from the subsequent process is pressurized by a compressor (3→4). The latent heat can then be exchanged between feed and effluent streams because the boiling temperature of the effluent stream is raised to Tb' by compression. Thus, the effluent stream is cooled through the heat exchanger for self-heat exchange (4→5) while recuperating its heat. The effluent stream is then depressurized by a valve (5→6) and finally cooled to T1 with a cooler (6→7). This leads to perfect internal heat circulation by self-heat recuperation, similar to the above gas stream case. Note that the total heating duty is equal to the internal self-heat exchange load without an external heating load, as shown in Fig. 1 (d). It is clear that the vapor and liquid sensible heat of the feed stream can be exchanged with the sensible heat of the corresponding effluent stream and the vaporization heat of the feed stream is exchanged with the condensation heat of the effluent stream. Similar to the thermal process for gas streams with heat circulation using self-heat recuperation technology mentioned above, the net energy required of this process is equal to the cooling duty in the cooler (6→7) and the exergy destruction occurs only during heat transfer in the heat exchanger and the amount of this exergy destruction is indicated by the gray area in Fig. 1 (d). As well as the gas stream, the effect of pressure to the heat capacity is small. Thus, two composite curves in Fig. 1 (b) are closed to be in parallel. As a result, the energy required by the heat circulation module is reduced to 1/22–1/2 of the original by the self-heat exchange system in gas streams and/or

To use the proposed self-heat recuperative thermal process in the reaction section of hydrodesulfurization in the petrochemical industry as an industrial application, Matsuda et al. (2010) reported that the advanced process requires 1/5 of the energy required of the

exergy destruction is illustrated by the gray area in Fig. 1 (b).

vapor/liquid streams.

heat load and capacity of the process stream are often different from those of the pumped heat. Thus, a normal heat pump still possibly causes large exergy destruction during heating. In heat recovery technologies, vapor recompression has been applied to evaporation, distillation, and drying, in which the vapor evaporated from the process is compressed to a higher pressure and then condensed, providing a heating effect. The condensation heat of the stream is recirculated as the vaporization heat in the process by using vapor recompression. However, many investigators have only focused on latent heat and few have paid attention to sensible heat. As a result, the total process heat cannot be recovered, indicating the potential for further energy savings in many cases. Recently, an energy recuperative integrated gasification power generation system has been proposed and a design method for the system developed (Kuchonthara & Tsutsumi 2003, Kuchonthara et al. 2005, Kuchonthara & Tsutsumi 2006). Kansha et al. have developed self-heat recuperation technology based on exergy recuperation (2009). The most important characteristics of this technology are that the entire process stream heat can be recirculated into a process designed by this technology based on exergy recuperation, leading to marked energy savings for the process.

In this chapter, 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 heat addition, and the theoretical analysis of this technology are introduced. Then, several industrial application case studies of this technology are presented and compared with their conventional counterparts.
