Self-heat recuperation (SHR) theory: principle and applications

2.1 Energy analysis

The traditional thermal process entails providing the required heat through fuel combustion and then transferring the heat to the stream through heat exchange. However, during combustion, chemical energy of higher quality is converted into thermal energy of lower quality, leading to significant exergy losses. To reduce the energy consumption of the process through heat recovery, heat is generally exchanged between the incoming cold stream and the outgoing hot stream. However, the outgoing hot stream still needs to burn additional fuel to provide some of the heat needed to meet the heat transfer load, leading to exergy loss and energy consumption. To resolve this issue, in the SHR heat utilization process, it is proposed to convert high-quality electrical energy into heat energy using a compressor to improve the quality of the stream. This can make the output stream to match the heat transfer load required by the feed stream. In the process of SHR, the heat recovered through all process streams are recycled (Kotani et al. 2012). Hence, the energy consumption of the heat process based on SHR technology could be much lower than that of the traditional heat process using the input-discharge heat transfer method (Kansha et al. 2010a). Therefore, this process achieves great energy savings without needing additional heat from burning fuel, as indicated in Figure 1.

Figure 1:

Thermal process of self-heat recuperation. Reproduced with permission from Kansha et al. (2009), copyright 2009 American Chemical Society.

Figure 1(a) shows the thermal process in which the gas stream does not undergo a phase change, based on the theory of SHR. In this process, the feed stream passes through the heat exchanger (1–2) and its temperature rises from T₁ to T₂. After the heat-exchanged stream passes through the compressor with an adiabatic compression for upgrading (3–4), the temperature further rises from T2 to T3. Thereafter, the stream passes through a heat exchanger to exchange heat with the feed stream (4–5), passes through an expander (5–6) to decompress the heat-exchanged stream, and further passes through a cooler (6–7) to reduce the temperature to T₁. As such, a self-heating cycle process is realized. It can be seen in Figure 1(b) that using electric energy with the highest energy quality coefficient (or exergy rate) to upgrade the thermal energy of the stream can meet the heat load for heating the feed stream without requiring additional heat input. It should be noted that the energy required for this process is equal to the cooling load of the cooler, and the exergy loss occurs only in the heat exchanger for the hot and cold streams [as indicated in the shaded area of Figure 1(b)].

Figure 1(c) shows the thermal process of a gas/liquid stream thermally cycled based on the theory of SHR. In this process, the feed stream passes through the heat exchanger (1–2) and its temperature rises from T₁ to T₂. After the heat-exchanged stream passes through the compressor with the adiabatic compression and is upgraded (3–4), the temperature further rises from T₂ to T₃. Herein, the boiling point of the stream rises from T _a to T _b , which enables the latent heat to be exchanged between the feed and the discharge. Subsequently, the stream passes through a heat exchanger to exchange heat with the feed stream (4–5), then passes through a valve (5–6) to decompress the heat-exchanged stream, and further passes through a cooler (6–7) to reduce the temperature of the stream to T₁. Similar to the above process, a self-heating cycle process is also achieved. It can be seen in Figure 1(d) that using electric energy with the highest energy quality coefficient (exergy rate) to upgrade the thermal energy of the stream can exchange the sensible heat of the gas and liquid in the feed stream with the outgoing stream. Simultaneously, the heat required for the evaporation of the feed stream is exchanged with the heat released during the condensation of the effluent stream without requiring additional heat input. Herein, the SHR theory integrates both the sensible and latent heats of the flow, to achieve optimum energy saving in the process (Kansha et al. 2009). It should be noted that the energy required for the process is also equal to the cooling load of the cooler, and the exergy loss of the process occurs only in the heat exchanger for the hot and cold streams, as indicated in the shaded area of Figure 1(d).

2.2 Exergy analysis

Kansha et al. (2013) conducted exergy analysis using temperature entropy diagram while Tsutsumi and Kansha (2017) and Chen et al. (2021) performed exergy analysis using energy conversion diagram and temperature entropy diagram. They carried out the thermodynamic analysis of thermal cycle process based on the theory of SHR, which further clarified the energy conversion process in the thermal cycle process. Figure 2(a)–(d) illustrates the process module, the temperature T-entropy S diagram, the energy conversion diagram, and the simplified energy conversion diagram, respectively, for the heat circulation system based on self-heat recuperation theory. Here, the heat Q₁(Ex, An) represents the area of the region 1-2-7-5 in Figure 2(b), in which the feed stream temperature increases from T₀ to T. The stream with heat Q₁ is upgraded by the compressor input electric W_in (Ex₁ + Ex₆, 0) with a further temperature increase from T to T + ΔT. It should be noted that W_in(Ex₁ + Ex₆, 0) represents the electric input to the compressor, which includes the electric input from the outside and output from the system. As such, the net electric of the system is defined as W_min(Ex₁, 0), as shown in Figure 2(c).

Figure 2:

Self-heat recuperation thermal circulation system. Reproduced with permission from Tsutsumi and Kansha (2017), copyright © 2017 AIDIC Servizi S.r.l.

Meanwhile, the heat flow formed after compression (the energy is represented by Q₃, as shown in Figure 2(a)–(c)) is heat exchanged with the cold flow, and the heat flow is cooled to T₀ + ΔT. According to the conservation of energy, since the heat absorbed in the heat transfer process is equal to the heat released, the area of 3-4-6-7 is equal to the area of 1-2-7-5 while the area of 2-3-4-11 is equal to the area of 1-5-6-11. One can see that the exergy of the heat flow formed after compression is actually equal to the area of 3-4-9-10, whereas the anergy is equal to the area of 6-7-10-9. Thus, the exergy of Q₃ can be expressed as the total area of 2-11-9-10 and 1-5-6-11, that is, Ex + An₂, whereas the anergy of Q₃ is An − An₂. Similarly, the exergy loss in the heat transfer process is the difference between the area of 3-4-9-10 and the area of 1-2-10-8, which is equal to the difference between the area of 2-3-4-11 and the area of 1-11-9-8, that is, the area of 5-6-9-8. Thus, Ex_loss = T₀S_gen = An₂.

However, the flow after heat transfer still contains some heat values. So the temperature of the flow (T₀ + ΔT) is reduced to T₁ through the compressor turbine, and then cooled to T₀, so that the flow returns to the initial state (that is the state of energy Q₁). In this process, the compressor turbine emits electrical energy W_out, which is the difference between T₀ + ΔT and T₁ multiplied by the difference between S₁ and S₀, and can be expressed by the area of Ex₆ in Figure 2(b). In addition, the exergy of the heat released during the cooling from T₁ to T₀ is Ex₂ = Ex₁ − An₂, and the anergy is An₂, which also explains the reason why An₂ was used in the derivation of Q₃ expression in the previous literature (Tsutsumi and Kansha 2017).

As shown in Figure 2(c), the cold flow (heat Q₁) is compressed by the compressor (power consumption, W_min) to form the hot flow, and the heat in the hot flow Q₃ is used to achieve the heat transfer of the cold flow. Due to the limitation of the minimum heat exchange temperature difference, the other part of the heat Q_out is not enough to further heat exchange of the cold flow; thus, the hot flow is expanded and cooled into the cold flow with heat of Q₁. Here, the exergy loss during heat transfer is An₂. It can be seen from Figure 2(d) that in the SHR thermal cycle, the net input work is used to make up for the exergy loss that occurs in the heat exchange process. At the same time, it is cooled and converted into heat Q_out.

2.3 Entransy analysis

Generally, the exergy rate is only applicable to the heat-to-power conversion process, not to the pure heating/cooling heat exchange process (Chen et al. 2013). As such, another energy analysis named as entransy analysis is always used. Here, the entransy represents the total ability of an object for the heat transfer. In the reversible process, the object has no entransy dissipation. On the contrary, in the irreversible process, the object will have entransy dissipation (Chen et al. 2011), which will reduce the ability of the object to transfer heat. The pure heating/cooling heat exchange process is irreversible; thus, the irreversibility of the heat exchange process can be measured by the entransy dissipation rate (Guo et al. 2007). Wu and Guo (2014) carried out entransy analysis on the thermal process of gas and steam/liquid based on the theory of SHR and revealed the reason for the energy saving based on the theory of SHR from the perspective of heat transfer.

Figure 3(a) shows the comparison of the entransy dissipation rates of processes I–III. Here, process I represents the simple thermal process of the gas, process II stands for the heat exchange process of the gas inlet and outlet, and process III is the gas SHR process. Figure 3(b) shows the comparison of the transmissive dissipation rates for processes IV–VI. Here, process IV represents the steam/liquid simple thermal process, process V represents the steam/liquid inlet and outlet heat exchange process, and process VI represents the steam/liquid SHR process. The shaded part in the figure represents the entransy dissipation rate of each thermal power process. The entransy analysis of the SHR heat process shows that its entransy dissipation rate is lower than those of the simple heat process and the heat exchange process of the incoming and outgoing materials. Since the compressor can compress and upgrade the stream adiabatically, the enzymatic dissipation rate of the heat exchange process is reduced. Therefore, the fundamental reason for energy saving based on the theory of SHR is the reduction of the irreversibility of the heat exchange process by inputting part of the electrical work.

Figure 3:

Comparison of the entransy-dissipation rates. Reproduced with permission from Wu et al. (2014), copyright 2014 American Chemical Society.

3.1 Distillation

Although distillation is widely used in chemical industry separation processes, the reboiler always consumes a lot of heat, which is generally supplied by fuel combustion to generate steam. This process does not only result in significant exergy loss but also leads to the emission of a large amount of carbon dioxide. Various energy-saving processes, such as vapor recompression distillation, thermally coupled distillation column, and dividing wall distillation column, have been proposed. However, in these processes, only the heat loads of the reboiler and the condenser can be matched, whereas the utilization of the sensible heat of the compressed steam is not considered. Based on the theory of SHR, the distillation process can be divided into two modules: preheating and rectification modules. In the preheating module, the effluent stream from the rectification module is heat-matched with the feed stream. In the rectification module, the heats of the reboiler and condenser are matched. Therefore, the gas–liquid sensible heat of the feed stream can be exchanged with the sensible heat of the effluent stream, and the heat of evaporation can be exchanged with the heat of condensation of each module.

Based on the theory of SHR, Kansha et al. (2010b, 2012a) proposed a new integrated energy-saving process module for the distillation, as shown in Figure 4(a). In the preheating module, the distillation overhead vapor compression stream and the bottom product stream are heat-matched to the feed stream. In the distillation module, the compressor is used to compress and upgrade the vapor at the top of the column, which is matched with the reboiler at the bottom of the column for heat exchange. Therefore, the gas–liquid sensible heat of the feed stream can be exchanged with the sensible heat of the effluent stream, and the heat of evaporation can be exchanged with the heat of condensation of each module. Figure 4(b) shows the temperature calorific diagram of the SHR distillation process, in which each number corresponds to the flow number in Figure 4(a), and T_std and T_b are the standard temperature and boiling temperature of the feed stream, respectively. The latent and sensible heats of the feed stream are exchanged with the effluent stream, and the heat of condensation overhead of the rectification column is heat exchanged with the heat of evaporation of the reboiler. As such, the heats of streams 4 and 12 can meet the heat load of the process after being upgraded by the compressor with the full utilization of the heat, thereby reducing energy loss. Matsuda et al. (2011) conducted a preliminary feasibility study on the application of SHR in the heavy chemical industry. They used the actual operation data of the benzene distillation process to examine the benefits of SHR from an industrial perspective. They developed an advanced SHR industrial distillation method. The study considered economic factors, applying a compressor. The comparison revealed that the energy consumption of the advanced process was 51 % of conventional industrial distillation, at the same time, the exergy input and energy input of the system were 24.4 and 60.1 % of those for the conventional industrial distillation, respectively. By utilization of the electrical energy, the energy introduced to the process as well as the energy consumption were significantly reduced.

Figure 4:

Self-heat recuperation distillation process. Reproduced with permission from Kansha et al. (2012a), copyright 2012 Elsevier.

Azeotropic distillation can be used to separate azeotropic mixtures, but it requires two distillation columns and consumes a large amount of energy. In recent years, the use of membrane separation or pressure swing adsorption to replace azeotropic distillation, vapor recompression distillation column, and thermally coupled distillation column has been proposed. However, none of these alternatives have fully utilized the thermal energy of the process stream, and they still require additional heat input. Here, the emergence of SHR theory provides the possibility to solve this problem. Researchers have verified the feasibility of its application in azeotropic rectification through theoretical simulation (Kansha et al. 2010c). It can maximize energy utilization rate within systems by only considering energy consumption. Li et al. (2022) used a multiobjective genetic algorithm method to simulate and optimize SHR azeotropic distillation using ethanol-water azeotropic distillation as an example. By identifying the appropriate design point on the Pareto front of the process, calculations show that the SHR distillation (SHRAD) system obtains great results compared to the traditional azeotropic distillation method. Specifically, the total annual cost (TAC) and CO₂ emissions were reduced significantly and the thermodynamic efficiency increased by 74.31 %. Also, two dynamic control structures are simultaneously proposed that differ from the conventional azeotropic distillation. They can maintain the product purity near the set point with a small overshoot. However, since the azeotropic distillation based on the theory of SHR requires multiple compressors, system energy saving and economic analysis need to be considered. Fan et al. (2019a) and Chen et al. (2018) carried out heat exchanger network optimization for multiple processes. Their work included detailed economic analysis and environmental studies (Yang et al. 2020). Wu et al. (2024) designed two novel heterogeneous azeotropic distillation processes by adding pure ethyl acetate as a self-solvent (HAD-PE) and refluxing part of the organic phase (HAD-OPR). They introduced SHR into the process (HAD-PE-SHR and HAD-OPR-SHR). The introduction of SHR sufficiently improves the heat utilization of the conventional process system, significantly reduces the total energy consumption, improves the exergy efficiency, and significantly reduces the TAC and CO₂ emissions of the process.

The industry obtains pure oxygen and nitrogen provided by an air separation unit. However, the commonly used cryogenic distillation needs to consume a lot of energy. To reduce the energy consumption, Kansha et al. (2011a) verified the feasibility of SHR theory in low-temperature air separation process. In recent years, the double-tower structure has been optimized, whereas the single-tower type low-temperature air separation process has been proposed. In fact, many factors could affect the process energy consumption, such as oxygen and nitrogen purity, compressor efficiency, a minimum temperature difference of the main condensers, and others. To explore the role of the SHR theory in the single-column low-temperature air separation process, air separation in oxy-combustion units (Fu et al. 2014, 2016a) and IGCC (Fu et al. 2016b) systems have been analyzed in detail. Although systems that fully utilize the latent heat and sensible heat of the process steam can greatly reduce energy consumption, research on the economic and environmental aspects of such processes is still insufficient.

To address the high energy consumption issue in distillation columns, various improved distillation column structures, such as heat-integrated distillation column (HIDiC) and dividing wall distillation column (DWC), have been proposed. However, energy matching, energy upgrading, and reuse in new energy-saving distillation towers are seldom considered. By using the theory of SHR, HIDiC (Kansha et al. 2011b) and DWC (Xia et al. 2018a) could be optimized. Kansha et al. (2010a) matched the condensing heat recovery and evaporative heat in the column to achieve the optimal heat transfer. This was done by combining the SHR technology to change the process pressure based on the conventional thermally coupled distillation column. It is found that with the use of SHR, the energy consumption of HIDiC can be greatly reduced compared to the conventional HIDiC. In addition, the theory of SHR can consider not only the upgrading of energy using electrical energy with a high exergy rate but also the utilization of chemical energy with a higher exergy rate. For example, the coupling of reactive distillation with a dividing wall distillation column essentially utilizes the chemical energy generated by the reaction (Novita et al. 2015). Similarly, various explorations have been made on this process, where not only the total energy consumption, total cost, and environmental impact are analyzed, but also various schemes using heat exchanger networks for comparison are designed (Fan et al. 2019b; Li et al. 2019a,b). Wen et al. (2023) coupled the conventional double-tower distillation air separation process with dividing wall column and proposed the cryogenic air separation process with dividing wall column process based on the SHR theory. As shown in Figure 5, the process couples single distillation column with a crude argon column (CC) and the dividing wall column (DC). After the rectification process of DC, a large amount of heat is released from the top of the column, which is compressed and improved by SHR through the compressor to match the heat transfer with the material at the bottom of the column. By establishing a systematic calculation sequence, the process parameters satisfying the purity of the product are optimized with the aim of the lowest TAC. As a result, the process energy consumption of the cryogenic air separation process with dividing wall column process based on SHR is reduced by 26.19 %, with a reduction of the TAC by 31.93 % and the CO₂ emission reduction by 25.18 %.

Figure 5:

Process flowsheet of the cryogenic air separation process with dividing wall column process. Reproduced with permission from Wen et al. (2023), copyright 2023 CIESC Journal.

Pressure swing distillation (PSD) is an effective method for separating pressure-sensitive azeotropes and has been widely used in industries. However, it consumes high energy. Various optimization processes, such as double-effect rectification, mechanical heat pump technology, and HIDiC, have been proposed. However, the full use of system energy reserves remains a problem. To further realize energy savings in the PSD process, the theory of SHR has been also applied, revealing that more energy savings can be realized compared to the traditional PSD and thermally integrated PSD processes (Xia et al. 2017). In the PSD process applying the theory of SHR, the energy of various processes has been optimized using the heat exchanger network. A detailed analysis of the total annual cost of the process (although it is concluded that the addition of a compressor will increase the process cost) (Xia et al. 2018b) shows that it not only improves energy efficiency but also reduces carbon dioxide emissions (Cui et al. 2020). Also, the economic advantages of the process can be reflected with increased operating time (Li et al. 2019a,b). Xia et al. (2017) coupled SHR with PSD (SHR-PSD) process and optimized the thermal integration of the process by heat exchanger network. They concluded that this SHR-PSD process can reduce the energy consumption by 72.39 % and TAC by 36.65 % compared to the conventional PSD process, while simultaneously reducing CO₂ emissions. Later, Luyben (2018) proposed a simpler heat exchanger system based on the study of Xia et al. (2017). They studied the dynamics and plant-wide controllability, which showed lower energy cost. Thereafter, Cui et al. (2020) proposed a novel electrical-driven self-heat recuperative pressure-swing azeotropic distillation technology, comparing it with the conventional thermal-driven PSD and PSDs integrated with heat integration and heat pump technologies. In the subsequent work (Cui et al. 2022; Yang et al. 2023), it is found that the total investment cost of PSD based on SHR can be reduced by 20 % and carbon emissions by 25 %. In addition, the study of the dynamic control structure and the process of highly integrated and interacted process shows that SHR theory has no conflict between the steady-state advantage and dynamic controllability, and thus it should be a broader and deeper study way for the SHR-PSD.

The theory of SHR has also been applied to other distillation processes, such as methanol-water distillation (Zhen et al. 2015a, 2015b), crude oil distillation (Chen 2015; Kansha et al. 2012b), natural gas recovery (Kim et al. 2012; Long and Lee 2013, 2014), propane-propylene separation (Christopher et al. 2017) processes, etc. And, all of them show good energy saving effect. As a unit operation with high energy consumption in chemical industry, the distillation has attracted the attention of many researchers. As a new energy-saving theory, SHR can be widely used in various distillation processes. Combined with other energy-saving technologies, the distillation unit has been optimized by SHR theory in terms of energy consumption, economy, and environment. Thus, the advantages of applying the SHR theory for distillation is reflected not only in the recompression and upgrading of the top steam but also in the reuse of reaction heat in the reactive distillation column. As stated above, in terms of energy conversion, the theory of SHR uses energy with a higher exergy rate to increase the energy performance of the thermal energy of the stream. It does not use additional heat to increase the stream’s exergy rate, which could effectively reduce the exergy loss in the process.

3.2 Drying

In chemical unit operations, drying reduces transportation costs by reducing product weight and size. This enables stable long-term storage of products and improves thermal efficiency. However, the drying process is very energy-intensive due to the high latent heat of the evaporation of water. There are two ways to save energy in the drying process: one is to strengthen the heat and mass transfer processes, and the other is heat recovery to achieve efficient energy utilization. For the latter, energy-saving methods, such as exhaust gas recirculation heat recovery, pinch technology, and mechanical recompression heat pump, have been proposed. However, these methods still cannot fully recover the heat of the drying medium, drying product, and evaporating water.

To improve the energy utilization rate in the drying system, Fushimi et al. (2010) proposed a drying system based on the theory of SHR, which was verified by simulation to show an energy-saving effect. Aziz et al. (2011a) also proposed a lignite drying system based on the theory of SHR, in which the hot stream is adiabatically compressed and upgraded by the compressor. Consequently, the exergy rate of the thermal energy of the hot stream is improved by using electric energy with a higher exergy rate. Since this system matches the heat load with the cold stream in the system, the sensible heat recovered from the drying medium during the drying process, the latent and sensible heat of evaporating water, and the sensible heat of the dried product can be all recycled. Figure 6 shows a structural diagram of a fluidized bed drying system based on the theory of SHR. Here, the wet lignite is heated to the specified temperature by the preheater (1a), and then the main drying process is carried out in the fluidized bed dryer (2). The moisture in the wet lignite is heated and evaporated, and then the fluidized medium is heated through the heat exchanger (1b). The heat exchanger in the dryer provides the heat required for drying by mixing steam and air, and the heat-exchanged steam and air mixture is circulated into the fluidized bed through the preheater (1a). The steam and air mixture coming out of the fluidized bed is superheated by the heat exchanger (3) and sent to the compressor for adiabatic compression and upgrading to form a closed cycle of drying medium, leading to the realization of the optimal energy savings. Subsequently, Aziz et al. (2012) proposed an improved SHR drying process for low-rank coal drying. Compared to the previously proposed SHR drying process, although the energy consumption is slightly increased, the heat exchange performance in the fluidized bed dryer is improved due to the use of pure steam condensation heat exchange. It reduces the size of the heat exchanger, and the heat load matching of the heat is optimized, reducing the exergy loss of the heat exchange process.

Figure 6:

Self-heat recuperation brown coal drying. Reproduced with permission from Aziz et al. (2012), copyright 2012 Elsevier.

Biomass has great potential to mitigate global warming. However, its high moisture content leads to higher transportation costs and lower thermal efficiency. Drying is one solution to the problem, but the biomass drying process requires a lot of heat. To solve this problem, Aziz et al. (2011b) applied the theory of SHR for biomass drying and verified its feasibility. However, since many factors affect the energy consumption of biomass drying systems, detailed simulation and experimental analysis (Liu et al. 2012a) by considering factors such as fluidization velocity, fluidizer type (Liu et al. 2014), and fluidization medium (Liu et al. 2015) have been carried out. It is found that the fluidized bed dryer can reduce drying time and drying surface area, and simultaneously carbon dioxide can replace air as the drying medium. To make full use of the energy in the drying system and improve the energy utilization rate of the system, researchers improved the process and used exergy analysis to optimize the system (Liu et al. 2012b, 2013). Fushimi and Fukui (2014), Fushimi and Dewi (2015) simplified the SHR drying system with a complicated heat exchange process and found that simplifying the feed preheating can reduce energy consumption. This also shows that the simple drying heat exchange system design can also achieve a certain energy saving effect. Sun et al. (2022) introduced SHR into the biomass drying system, achieving an energy-saving ratio of up to 93.4 %. The theory of SHR has also been applied to other drying processes, such as paddy fluidized bed drying system (Yao et al. 2017), wood fixed bed drying system (Bai et al. 2017), freeze-drying (Bando et al. 2017), etc. Simultaneously, thermodynamic and economic analyses have also been carried out (Chen et al. 2022).

The drying process applying the theory of SHR increases the thermal energy performance of the hot stream by adiabatic compression of the hot stream of steam and air. It makes the thermal energy of the hot stream to be more fully utilized and greatly reduces the energy loss of the system. Also, through the circulation of the drying medium, the heat in the system can be fully recovered and utilized. Most research on the drying process focuses on the reduction of energy consumption, which could be achieved through continuous improvement of the drying system based on the theory of SHR. In addition, the influencing factors such as fluidizing medium, dryer type, compressor adiabatic efficiency, compression ratio, drying medium flow rate, and fluidization speed can be analyzed by it. But there are few studies on system economy and environment.

3.3 Absorption

With global warming, the disposal of the greenhouse gas carbon dioxide has attracted widespread attention. Carbon capture and storage (CCS) technology has been developed as one of the technologies for carbon dioxide processing, which entails the collection of carbon dioxide produced in industry and storing it in a specific location. It includes three stages: capture, transportation, and storage (Bassano et al. 2014). The capture stage has three ways: precombustion capture, combustion capture, and postcombustion capture. The application of SHR theory in the capture stage has been mainly studied.

The most commonly used carbon dioxide capture way is the chemical absorption via ethanolamine (MEA). Figure 7(a) shows the schematic diagram of conventional carbon dioxide capture, which includes an absorption column, a heat exchanger, a desorption column with a condenser, and a reboiler. After combustion, flue gas and ethanolamine solution containing a small amount of carbon dioxide (lean liquid) enter the absorption tower. The lean liquid absorbs carbon dioxide in the flue gas and becomes a carbon dioxide-rich ethanolamine solution (rich liquid) and enters the desorption tower after heat exchange with the lean liquid from the desorption tower. The heat required for the desorption column is provided by the bottom reboiler. The absorption process in the absorption tower will release heat, and part of the heat will enter the rich liquid for heat exchange with the poor liquid. The desorption tower requires additional heat input from the bottom reboiler due to the mismatch between the overhead condenser and the bottom reboiler. This results in high energy consumption in the carbon dioxide absorption process.

Figure 7:

CO₂ absorption process. Reproduced with permission from Kishimoto et al. (2011), copyright 2011 American Chemical Society.

To achieve higher energy saving, Kishimoto et al. (2011) proposed a carbon dioxide absorption process based on the theory of SHR. By upgrading and recycling chemical energy and thermal energy, a small amount of electrical energy is used to form a self-heating cycle, which can reduce energy input. Figure 7(b) shows the schematic diagram of the carbon dioxide absorption process of SHR, in which the mixture of carbon dioxide and steam discharged from the top of the desorption tower is subjected to adiabatic compression and upgraded by the compressor. Then, the heat exchange occurs with the reboiler at the bottom of the tower. This process can effectively reduce the energy consumption of the desorption tower. Some of the heat-exchanged mixtures is refluxed, while the remaining part is sent to the subsequent section. In addition, the reaction heat released by the absorption tower is upgraded by the heat pump cycle and then used in the desorption tower, which reduces the heat loss in the process. Also, Kishimoto et al. (2012) applied the theory of SHR to the carbon monoxide shift unit and precombustion carbon dioxide capture unit in IGCC to realize energy recycling and reuse of the whole process. Compared with the traditional process, the energy consumption of this new process is reduced by 2/3, indicating a good application prospect. Subsequently, Kansha et al. (2017) investigated the CO₂ absorption process based on SHR through simulation and experimental methods, in which the reaction heat that accompanies absorption is successfully supplied through a heat pump to achieve thermal decomposition in a regenerator, thereby achieving recirculation of all process heat, indicating great energy saving potential.

SHR can also be applied in the process of removing acid gas. For example, Qian et al. (2023) established a model of acid gas removal (AGR) coupled with a power generation unit based on SHR theory. Here, steam compression and mechanical steam recompression are introduced respectively into AGR unit. The calculated energy and exergy balance of each unit shows that AGR based on SHR theory can enhance the resolution of acidic gas in the analyzer, and the introduction of steam compression SHR greatly improves the economy of AGR with a reduction of the operating cost by 3.75 %. In addition, the improved process uses low-pressure steam phase change cooling to replace part of the water cooling process, resulting in the reduced exergy loss in the distillation cooling process by 17.67 %. Thus, in the AGR process, energy saving and consumption reduction can be realized through SHR. Long and Lee (2017) proposed several distillation columns based on SHR with improved waste heat utilization modules to increase the AGR energy efficiency. They can be applied for both close-boiling and wide-boiling mixtures. Such proposed cases address the problem of high compressor cost resulting from the large temperature difference between the top and bottom of the tower through SHR technology with side reboiler or side SHR. In addition, several other proposals can be considered by using the generated steam from the top steam heat of the tower to provide steam for other consumers. After the SHR is further coupled into an integrated generator, the simulated results indicated that the load and operating cost of the reboiler of the SHR-based AGR can be reduced by 62.5 % and 45.9 % compared to the conventional AGR, respectively, with lower TAC and CO₂ emissions.

The research on the carbon dioxide absorption process is not only limited in terms of energy consumption; the economic and environmental aspects have been also discussed in detail (Andika et al. 2017). Chen et al. (2019) took carbon dioxide from combustion flue gas in a high-density triple bed circulating fluidized bed (TBCFB) system as the capture object and used the theory of SHR to couple the waste heat recovery of flue gas and carbon dioxide capture (as the detail description in Section 3.4). It recycles the reaction heat and condensation heat in the system after compression and upgrading, thereby reducing the exergy loss in the process and improving the energy utilization rate. Also, from the energy consumption and economic analysis, it is found that the system is superior to the traditional carbon dioxide capture process. Song et al. (2014, 2015) also used the SHR theory for the PSA carbon dioxide capture process. They successfully recycled the heat generated from the adsorption reaction to reduce energy consumption.

3.4 TBCFB polygeneration system

Based on the theory of SHR, Hou et al. (2022) proposed a coal-based polygeneration carbon cycle system with a TBCFB unit for efficient, clean, and low-carbon utilization of coal. As shown in Figure 8, this system mainly includes three parts: gasification island, chemical island, and power island. In this system, the pyrolysis gas generated by TBCFB is circulated as the pyrolysis atmosphere, and the generated flue gas is used as the power generation working medium of the gas turbine to achieve the purpose of carbon cycle.

Figure 8:

Process diagram of coal-based polygeneration carbon cycle (CBPCC) system. Reproduced with permission from Hou (2022), copyright 2022 Taiyuan University of Technology.

Before the proposal of Hou et al. (2022), Wang et al. (2018) started from the gasification island and used Aspen plus to simulate the TBCFB auto thermal cycle system as shown in Figure 9. Taking Huolinhe lignite in Inner Mongolia as an example, the operation parameters, such as suitable gasification reaction temperature, gasification agent dosage, and solid heat carrier circulation amount, were explored, and the material balance and energy balance in the system were analyzed. Using semi-coke as heat-carrying particles, on one hand, on the premise of satisfying heat transfer, the circulation amount of heat-carrying particles can be greatly reduced, which can reduce the difficulty and cost of system operation. On the other hand, the catalytic activity of semi-coke can promote pyrolysis and gasification reactions, improving its efficiency. With this, TBCFB can realize self-heating cycle by using circulating heat-carrying particles. This state represents the minimization of energy within the system, harnessing the advantages of SHR theory fully.

Figure 9:

Diagram simulation of TBCFB system by Aspen Plus. Reproduced with permission from Wang et al. (2018), copyright 2018 CIESC Journal.

Thereafter, Chen et al. (2019) used the theory of SHR to optimize the chemical absorption CO₂ capture process. As shown in Figure 10, this process utilizes the heat pump 1 to recover the reaction heat released by the absorption tower for the desorption tower to realize the recycling of the reaction heat between the exothermic reaction and the endothermic reaction in the system. At the same time, the compressor 2 is used to upgrade the vapor at the top of the absorption tower, which is used for the reboiler at the bottom of the absorption tower. The heat pump 3 is used to upgrade and recycle the waste heat of the system. Compared with the traditional process, the optimized process reduces the energy consumption of the capture process by 41.36 %, improves the exergy efficiency by 25.36 %, saves the standard coal consumption by 37 % per year, and reduces the total capture cost by 12 %, despite the increase of the investment cost by 27.76 %. Thus, by using the theory of SHR, the utilization of low-level thermal energy in the system can be greatly enhanced, indicating its great application potential for energy saving.

Figure 10:

Process simulation of CO₂ capture by chemical absorption method based on self-heat recuperation. Reproduced with permission from Chen et al. (2019), copyright 2019 CIESC Journal.

Furthermore, as shown in Figure 11, Liu et al. (2021) modeled the process of the low-temperature methanol washing unit and methanol rectification unit in the methanol synthesis process of the chemical island. The energy-saving optimization of the two units is also carried out using the theory of SHR, in which the heat exchanger network design is carried out for the optimized process. As a result, compared to the traditional methanol synthesis process, the total energy consumption of the Rectisol process is saved by 25.8 %, whereas the total energy consumption of the methanol distillation unit is saved by 32.3 %. Hence, this process can improve the exergy rate of the material through adiabatic compression of the compressor and redesign the heat exchange process of the system through the design of the heat exchanger network, achieving energy savings within the system.

Figure 11:

Flowchart of synthesis gas to methanol in TBCFB system. Reproduced with permission from Liu et al. (2021), copyright 2021 CIESC Journal.

Based on the above research, Hou et al. (2022) used Aspen plus to simulate the carbon cycle system of coal-based polygeneration by carrying out material, energy, and exergy balance calculations for the system, in which the optimal operating conditions of the system with energy utilization efficiency as the optimization goal are determined. The results show that the energy saving of the coal-based polygeneration carbon cycle (CBPCC) system based on SHR is 13 %, which is equivalent to 149,000 tonnes/year of CO₂ emission reduction compared with the conventional coal-based polygeneration (CBP). CBPCC has higher energy utilization efficiency and exergy efficiency, especially, the weak link of the TBCFB gasification unit system can be known through exergy analysis. Here, the coal-based polygeneration carbon cycle system fully integrates the various units based on SHR theory. Although the energy utilization efficiency of the system has been improved by optimizing the SHR energy of each unit, the close connection between the various parts of the system is not reflected. Thus, it is necessary to upgrade the local SHR theory optimization to the overall system SHR theory optimization, which could fully reflect the advantages of the SHR theory in the system energy optimization level. In response to this, Hou (2022) compared the coal-based polygeneration carbon cogeneration (CBPCC), the coal-based polygeneration carbon cogeneration with partial SHR (PSHR-CBPCC), and the coal-based polygeneration carbon cogeneration with integral SHR (ISHR-CBPCC) in subsequent study. Compared with the conventional CBPCC, the energy utilization efficiencies of the PSHR-CBPCC and ISHR-CBPCC systems are improved by 2.7 and 4.2 %, respectively, and the exergy efficiencies are improved by 2 and 4 %. The energy amounts saved by both are converted into reductions of 28 and 90.1 % of carbon dioxide emissions, respectively, which can effectively reduce carbon dioxide emissions in CBPCC. The comparison of the three process systems is shown in Tables 1 and 2.

3.5 Other processes

The theory of SHR has also been applied in other processes, such as evaporation systems, magnetocaloric cyclers, thermoelectric devices, and chemical production systems. As well known, solution evaporation crystallization is widely used in industry. However, due to the large amount of heat required for the phase transition of the solvent evaporation, the evaporation process always consumes a lot of energy. To solve this problem, the theory of SHR has been applied to those evaporation systems, such as ammonium sulfate and vitamin production (Wu et al. 2014). It is found that SHR of single-stage and two-stage evaporation systems differ in terms of energy savings. The two-stage evaporation system further recovers the energy in the outlet stream of the single-stage evaporation system, improving the heat recycling within the system and the energy utilization efficiency of the system. The effects of different concentration solutions and different boiling point solutions on the evaporation system have been also investigated. Han et al. (2014a) found that the energy-saving effect of the double-stage mechanical vapor recompression (MVR) evaporation system based on SHR for ammonium sulfate solution processing rises with increased evaporation concentration of the ammonium sulfate solution. They found an increase in energy savings from 30 to 55 % at the saturated concentration compared to the single-stage MVR evaporation system. Subsequently, Han et al. (2014b) also found that the overall energy efficiency of the MVR evaporation concentration system could be improved through SHR with the boiling point elevation in the effective range. The double-stage MVR evaporation concentration system based on SHRT is limited in its energy-saving characteristics when dealing with highly concentrated solutions with a high boiling point elevation.

In the theory of SHR, it is recommended not only to use adiabatic compression for upgrading but also to consider stream upgrading by applying magnetocaloric effect and thermoelectric devices, which can be useful. For example, the magnetocaloric cycler based on the theory of SHR has no heat input, in which only the magnetization and demagnetization of ferromagnetic materials are utilized for thermal cycling (Kotani et al. 2013a). The experimental and simulation results on the magnetocaloric cycler have verified its feasibility, but the fluid flow, bed geometry, and so on need to be further optimized (Kotani et al. 2013b, 2014). Here, the use of adiabatic magnetization and demagnetization instead of compression and expansion is an extension of the theory of SHR. Although the temperature of ferromagnetic materials is affected by the Curie temperature, they have application prospects in energy cascade utilization within systems. Thermoelectric devices use the Peltier effect to improve the quality of logistics. They can be applied for either gaseous streams or nongaseous streams. Here, the influencing factors such as surface area and heat transfer coefficient can be simulated and analyzed (Rasfuldi et al. 2015) to provide a theoretical basis for the application of thermoelectric devices in nongaseous streams.

The theory of SHR can also be applied in various chemical production processes, by which the energy-saving optimization can be carried out in various stages using the exergy analysis and energy analysis. For example, for the naphtha hydrodesulfurization reaction system, Matsuda et al. (2010) carried out exergy analysis and energy analysis by using the theory of SHR, comparing it with the traditional methods. The process of applying SHR can lead to a great energy-saving effect. It is pointed out that the reason for energy saving is to replace the direct input heat by input work. Also, the load of the cooler as well as the exergy loss can be greatly reduced. Other processes, such as the indirect production process of dimethyl ether (Kansha et al. 2015), methanol synthesis process (Kansha et al. 2014, 2019; Liu et al. 2021), carbon dioxide conversion methanol process (Chaniago et al. 2019), biodiesel production process (Fu et al. 2015), seawater desalination process (Mizuno et al. 2013) process, etc., have also been studied in detail. All these studies indicated that the energy-saving effect can be achieved by using the theory of SHR in each stage of the process.