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An Advanced Cryogenic Air Separation Process Based on Self-heat Recuperation for CO2 Separation
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 1673 – 1676
The 6th International Conference on Applied Energy – ICAE2014
An advanced cryogenic air separation process based on selfheat recuperation for CO2 separation Qian Fu, Yasuki kansha, Chunfeng Song, Yuping Liu, Masanori Ishizuka, 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
Abstract An advanced cryogenic air separation process for oxy-combustion is proposed based on self-heat recuperation technology. Compared with the conventional double-column cryogenic air separation process, only one distillation column is used in the proposed process. The heat of N2 product gas from the top of the distillation column is recirculated by exchanging latent heat with the liquid O2 in the bottom and feed streams, largely reducing the energy consumption. The simulation results showed that the energy consumption of the proposed cryogenic air separation process was decreased by 30% comparing with the conventional process, when producing O2 with low purity (95 mol%) and low pressure (120 kPa). © Published by Elsevier Ltd. This © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: Oxy-combustion; CO2 capture; cryogenic air separation; self-heat recuperation.
1. Introduction Carbon dioxide capture and storage (CCS) technology is a promising technology to reduce the CO2 emission [1]. Currently, CCS is still not commercialized due to its intensive energy cost, especially for the CO2 capture processes [2]. Usually, there are three kinds of technologies for CO2 capture: postcombustion, pre-combustion, and oxy-combustion [3]. Among them, oxy-combustion technology is a competitive technology, in which nearly pure oxygen (around 95%) is used for combustion. Thus, the main products are CO2 and H2O, which can be easily separated in post processes. However, the energy input for the oxy-combustion is also intensive due to the O2 production process. Cryogenic air separation process is commonly used for large amount of O2 production in industry. The conventional process usually contains one high pressure column and one low pressure column. To reduce * Corresponding author. Tel.: +81 3 5452 6727; fax: +81 3 5452 6728. E-mail address: [email protected].
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.189
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Qian Fu et al. / Energy Procedia 61 (2014) 1673 – 1676
the energy consumption, the condenser in high pressure column is combined with the reboiler in low pressure column [4]. Although the energy consumption can be reduced by exchanging the latent heat of N2 condensation in high pressure column and O2 vaporization in low pressure column, a large amount of energy for making pressure difference between the high and low pressure columns is required. Recently, to reduce the energy consumption in the distillation process, self-heat recuperation technology has been developed, which significantly reduced the energy consumption [5-6]. In this study, an advanced cryogenic air separation process for oxy-combustion is proposed based on self-heat recuperation. Only one distillation column is used in the proposed process. N2 gas from the top of the distillation column is compressed and exchanged latent heat with the liquid in the bottom and feed streams. Both the conventional and proposed processes are simulated for comparison. 2. Cryogenic air separation process 2.1. Conventional cryogenic air separation process Fig. 1 shows the flow sheet of a typical conventional double-column cryogenic air separation process. In this process, ambient air is firstly compressed, cooled to room temperature, and then subcooled to near dew point in the main heat exchanger before feeding into the high pressure (HP) column. In HP column, liquid N2 and O2-rich air liquid are separated. Then the liquid N2 is subcooled, depressured and fed into the top of LP column as liquid reflux, while the O2-rich air liquid is subcooled, depressured and fed into the middle of LP column. After distillation in LP column, high purity O2 is obtained at the bottom, while high purity N2 and low purity N2 are obtained at the top of LP column. These products are then depressured and change heat with subcooler and main heat exchanger, respectively.
Fig. 1 (A) Process configuration of conventional cryogenic air separation process; (B) distillation module of proposed process
2.2. Proposed cryogenic air separation process Fig. 2 shows a cryogenic air separation process for oxy-combustion based on self-heat recuperation technology. This process is similar to the process proposed for steel-works furnace reported by Kansha et al [6]. However, the product O2 purity is different, which is 95% for oxy-combustion, and detailed stream flow are different. The main principle of the proposed process is to exchange the heat of compressed N2 from the top of the distillation column (DC) with that of the depressured O2 from the bottom in the heat exchanger HX1. Then the N2 and O2 are adjusted to near boiling point and dew point before flow back to the DC by exchanging heat with other streams, which are shown in Fig. 2. Thus, neither condenser nor reboiler is required in this process, which significantly reduced the energy consumption.
Qian Fu et al. / Energy Procedia 61 (2014) 1673 – 1676
2.3. Process simulation Both the flow sheets were simulated with the process simulator PRO II 9.1, using Peng-Robinsons equation as the state. The simulation assumptions are referred to the previous work [6], in which the composition of feed air was 80 mol% N2 and 20 mol% O2 at standard temperature (25°C) and pressure (1 atm). The stage number for HP, LP and DC column were 7, 20 and 100, while the pressure for HP, LP and DC column were 446 KPaG, 50 KPaG and 200 KPaG, respectively. The minimum temperature differences in the heat exchangers were assumed to be 1.5 K in HXM, 6 K in HXsc, 1.2 K in HX1, 6 K in HX2, 10 K in HX3, and 24 K in HX4. The minimum temperature between the condenser in HP column and the reboiler in LP column is assumed to be 1 K. The adiabatic efficiency in all compressors were assumed to be 100%. We assumed that the flow rate of feed air was 167, 000 m3/h at standard temperature and pressure, and the purity of the product O 2 was around 95%. 3. Results and discussion As no energy input is required for the distillation module, the energy input of the cryogenic air separation process is only due to the work of compressors. The simulation results showed that the total energy consumption of the conventional cryogenic air separation process was around 9049 kW, while it was only 5707 kW for the proposed process based on self-heat recuperation technology. The specific energy consumption for the proposed process was 21.3 KJ/mol-O2, while it was 14.71 KJ/mol-O2 for the conventional process. Thus, the energy consumption of the proposed process was reduced by around 31%. The main reason for the energy saving is the good internal heat circulation in the proposed process. Fig. 2 showed the temperature-heat duty diagram of heat exchangers (Fig. 2A) after distillation column and the main heat exchanger (Fig. 2B) in the proposed process. It can be seen that the heat paring in each heat exchanger matched very well, which could largely reduce the exergy losses. Due to the good internal heat circulation, high pressure is not required in the distillation column of our proposed process. A
B
Fig. 2. Temperature-heat duty diagram of heat exchangers after the distillation module (A) and main heat exchanger (B) for the proposed cryogenic air separation process
Fig. 3 shows the energy and material flow diagram of both processes. The boxes represent units, lines represent the flow of materials and energy, and the red lines represent the energy input or output during the systems. We assumed that a heat exchanger can be divided into a heat transmitter and a heat receiver. It can be seen that the main heat exchanger had the largest duty for both processes, which were 11.5 MW (conventional process) and 13.8 MW (proposed process), respectively. In addition, the heat (4.86 MW) of N2 condensation was recirculated for the vaporization of O2 in the distillation column of proposed process, largely reducing the energy input. Thus, a high pressure was not required for the distillation column in the proposed process in contrast to the high column (446 kPaG) in the conventional process.
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Fig. 3 Energy and material flow diagram of the total cryogenic process (A), distillation module of conventional (B) and proposed (C) process
4. Conclusion An advance cryogenic air separation process for oxy-combustion based on self-heat recuperation technology was proposed in this study. Compared with the conventional double column cryogenic air separation process, only one distillation column was used in the proposed process. The simulation results showed that the energy consumption of the proposed cryogenic air separation process was decreased by 31% in contrast to the conventional process, when producing O2 with low purity (95 mol%) and low pressure (120 kPa). References [1] Haszeldine RS. Carbon capture and storage: how green can black be?. Science 2009;325:1647–52. [2] Metz B, Davidson O, de Coninck H, Loos M, Meyer L. IPCC special report on carbon dioxide capture and storage. Intergovernmental Panel on Climate Change, Geneva (Switzerland). Working Group III, 2005. [3] Yang H, Xu Z, Fan M, Gupta R, Slimane RB, Bland AE, Wright I. Progress in carbon dioxide separation and capture: A review. Journal of Environmental Sciences 2008;20:14-27. [4] Fu C, Gundersen T. Using exergy analysis to reduce power consumption in air separation units for oxy-combustion processes. Energy 2012;44:60-68. [5] Kansha Y, Tsuru N, Fushimi C, Tsutsumi A. Integrated process module for distillation processes based on self-heat recuperation technology. Journal of chemical engineering of Japan 2010;43:502-7. [6] Kansha Y, Kishimoto A, Nakagawa T, Tsutsumi A. A novel cryogenic air separation process based on self-heat recuperation. Separation and Purification Technology 2011;77:389-96.
Biography Qian Fu holds a position as projected researcher at the Institute of Industrial Science, the University of Tokyo, Japan. He previously earned his Ph. D degree from the Department of Systems Innovation, the University of Tokyo in 2013. His main research interest is energy saving based on self-heat recuperation technology.
ScienceDirect Energy Procedia 61 (2014) 1673 – 1676
The 6th International Conference on Applied Energy – ICAE2014
An advanced cryogenic air separation process based on selfheat recuperation for CO2 separation Qian Fu, Yasuki kansha, Chunfeng Song, Yuping Liu, Masanori Ishizuka, 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
Abstract An advanced cryogenic air separation process for oxy-combustion is proposed based on self-heat recuperation technology. Compared with the conventional double-column cryogenic air separation process, only one distillation column is used in the proposed process. The heat of N2 product gas from the top of the distillation column is recirculated by exchanging latent heat with the liquid O2 in the bottom and feed streams, largely reducing the energy consumption. The simulation results showed that the energy consumption of the proposed cryogenic air separation process was decreased by 30% comparing with the conventional process, when producing O2 with low purity (95 mol%) and low pressure (120 kPa). © Published by Elsevier Ltd. This © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: Oxy-combustion; CO2 capture; cryogenic air separation; self-heat recuperation.
1. Introduction Carbon dioxide capture and storage (CCS) technology is a promising technology to reduce the CO2 emission [1]. Currently, CCS is still not commercialized due to its intensive energy cost, especially for the CO2 capture processes [2]. Usually, there are three kinds of technologies for CO2 capture: postcombustion, pre-combustion, and oxy-combustion [3]. Among them, oxy-combustion technology is a competitive technology, in which nearly pure oxygen (around 95%) is used for combustion. Thus, the main products are CO2 and H2O, which can be easily separated in post processes. However, the energy input for the oxy-combustion is also intensive due to the O2 production process. Cryogenic air separation process is commonly used for large amount of O2 production in industry. The conventional process usually contains one high pressure column and one low pressure column. To reduce * Corresponding author. Tel.: +81 3 5452 6727; fax: +81 3 5452 6728. E-mail address: [email protected].
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.189
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Qian Fu et al. / Energy Procedia 61 (2014) 1673 – 1676
the energy consumption, the condenser in high pressure column is combined with the reboiler in low pressure column [4]. Although the energy consumption can be reduced by exchanging the latent heat of N2 condensation in high pressure column and O2 vaporization in low pressure column, a large amount of energy for making pressure difference between the high and low pressure columns is required. Recently, to reduce the energy consumption in the distillation process, self-heat recuperation technology has been developed, which significantly reduced the energy consumption [5-6]. In this study, an advanced cryogenic air separation process for oxy-combustion is proposed based on self-heat recuperation. Only one distillation column is used in the proposed process. N2 gas from the top of the distillation column is compressed and exchanged latent heat with the liquid in the bottom and feed streams. Both the conventional and proposed processes are simulated for comparison. 2. Cryogenic air separation process 2.1. Conventional cryogenic air separation process Fig. 1 shows the flow sheet of a typical conventional double-column cryogenic air separation process. In this process, ambient air is firstly compressed, cooled to room temperature, and then subcooled to near dew point in the main heat exchanger before feeding into the high pressure (HP) column. In HP column, liquid N2 and O2-rich air liquid are separated. Then the liquid N2 is subcooled, depressured and fed into the top of LP column as liquid reflux, while the O2-rich air liquid is subcooled, depressured and fed into the middle of LP column. After distillation in LP column, high purity O2 is obtained at the bottom, while high purity N2 and low purity N2 are obtained at the top of LP column. These products are then depressured and change heat with subcooler and main heat exchanger, respectively.
Fig. 1 (A) Process configuration of conventional cryogenic air separation process; (B) distillation module of proposed process
2.2. Proposed cryogenic air separation process Fig. 2 shows a cryogenic air separation process for oxy-combustion based on self-heat recuperation technology. This process is similar to the process proposed for steel-works furnace reported by Kansha et al [6]. However, the product O2 purity is different, which is 95% for oxy-combustion, and detailed stream flow are different. The main principle of the proposed process is to exchange the heat of compressed N2 from the top of the distillation column (DC) with that of the depressured O2 from the bottom in the heat exchanger HX1. Then the N2 and O2 are adjusted to near boiling point and dew point before flow back to the DC by exchanging heat with other streams, which are shown in Fig. 2. Thus, neither condenser nor reboiler is required in this process, which significantly reduced the energy consumption.
Qian Fu et al. / Energy Procedia 61 (2014) 1673 – 1676
2.3. Process simulation Both the flow sheets were simulated with the process simulator PRO II 9.1, using Peng-Robinsons equation as the state. The simulation assumptions are referred to the previous work [6], in which the composition of feed air was 80 mol% N2 and 20 mol% O2 at standard temperature (25°C) and pressure (1 atm). The stage number for HP, LP and DC column were 7, 20 and 100, while the pressure for HP, LP and DC column were 446 KPaG, 50 KPaG and 200 KPaG, respectively. The minimum temperature differences in the heat exchangers were assumed to be 1.5 K in HXM, 6 K in HXsc, 1.2 K in HX1, 6 K in HX2, 10 K in HX3, and 24 K in HX4. The minimum temperature between the condenser in HP column and the reboiler in LP column is assumed to be 1 K. The adiabatic efficiency in all compressors were assumed to be 100%. We assumed that the flow rate of feed air was 167, 000 m3/h at standard temperature and pressure, and the purity of the product O 2 was around 95%. 3. Results and discussion As no energy input is required for the distillation module, the energy input of the cryogenic air separation process is only due to the work of compressors. The simulation results showed that the total energy consumption of the conventional cryogenic air separation process was around 9049 kW, while it was only 5707 kW for the proposed process based on self-heat recuperation technology. The specific energy consumption for the proposed process was 21.3 KJ/mol-O2, while it was 14.71 KJ/mol-O2 for the conventional process. Thus, the energy consumption of the proposed process was reduced by around 31%. The main reason for the energy saving is the good internal heat circulation in the proposed process. Fig. 2 showed the temperature-heat duty diagram of heat exchangers (Fig. 2A) after distillation column and the main heat exchanger (Fig. 2B) in the proposed process. It can be seen that the heat paring in each heat exchanger matched very well, which could largely reduce the exergy losses. Due to the good internal heat circulation, high pressure is not required in the distillation column of our proposed process. A
B
Fig. 2. Temperature-heat duty diagram of heat exchangers after the distillation module (A) and main heat exchanger (B) for the proposed cryogenic air separation process
Fig. 3 shows the energy and material flow diagram of both processes. The boxes represent units, lines represent the flow of materials and energy, and the red lines represent the energy input or output during the systems. We assumed that a heat exchanger can be divided into a heat transmitter and a heat receiver. It can be seen that the main heat exchanger had the largest duty for both processes, which were 11.5 MW (conventional process) and 13.8 MW (proposed process), respectively. In addition, the heat (4.86 MW) of N2 condensation was recirculated for the vaporization of O2 in the distillation column of proposed process, largely reducing the energy input. Thus, a high pressure was not required for the distillation column in the proposed process in contrast to the high column (446 kPaG) in the conventional process.
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Fig. 3 Energy and material flow diagram of the total cryogenic process (A), distillation module of conventional (B) and proposed (C) process
4. Conclusion An advance cryogenic air separation process for oxy-combustion based on self-heat recuperation technology was proposed in this study. Compared with the conventional double column cryogenic air separation process, only one distillation column was used in the proposed process. The simulation results showed that the energy consumption of the proposed cryogenic air separation process was decreased by 31% in contrast to the conventional process, when producing O2 with low purity (95 mol%) and low pressure (120 kPa). References [1] Haszeldine RS. Carbon capture and storage: how green can black be?. Science 2009;325:1647–52. [2] Metz B, Davidson O, de Coninck H, Loos M, Meyer L. IPCC special report on carbon dioxide capture and storage. Intergovernmental Panel on Climate Change, Geneva (Switzerland). Working Group III, 2005. [3] Yang H, Xu Z, Fan M, Gupta R, Slimane RB, Bland AE, Wright I. Progress in carbon dioxide separation and capture: A review. Journal of Environmental Sciences 2008;20:14-27. [4] Fu C, Gundersen T. Using exergy analysis to reduce power consumption in air separation units for oxy-combustion processes. Energy 2012;44:60-68. [5] Kansha Y, Tsuru N, Fushimi C, Tsutsumi A. Integrated process module for distillation processes based on self-heat recuperation technology. Journal of chemical engineering of Japan 2010;43:502-7. [6] Kansha Y, Kishimoto A, Nakagawa T, Tsutsumi A. A novel cryogenic air separation process based on self-heat recuperation. Separation and Purification Technology 2011;77:389-96.
Biography Qian Fu holds a position as projected researcher at the Institute of Industrial Science, the University of Tokyo, Japan. He previously earned his Ph. D degree from the Department of Systems Innovation, the University of Tokyo in 2013. His main research interest is energy saving based on self-heat recuperation technology.