Format And Type Fonts 661 CHEMICAL ENGINEERING TRANSACTIONS Volume 21, 2010 Editor J. J. Klemeš, H. L. Lam, P. S. Varbanov Copyright © 2010, AIDIC Servizi S.r.l., ISBN 978-88-95608-05-1 ISSN 1974-9791 DOI: 10.3303/CET1021111 Please cite this article as: Kishimoto A., Kansha Y., Fushimi C. and Tsutsumi A., (2010), Design methodology of absorption process (use of MEA absorbent) based on self-heat recuperation technology, Chemical Engineering Transactions, 21, 661-666, DOI: 10.3303/CET1021111. Design Methodology of Absorption Process (use of MEA absorbent) based on Self-heat Recuperation Technology Akira Kishimoto, Yasuki Kansha, Chihiro Fushimi, 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 a-tsu2mi@iis.u-tokyo.ac.jp A chemical absorption process is the most commercially used process for CO2 separation in flue gases. However, the process needs a huge amount of energy to strip CO2 in a stripper. To overcome this problem, a new gas separation process using chemical absorption method based on self-heat recuperation technology is proposed in this study for energy saving. In this process, the regeneration energy of absorbent is provided from the exhausted heat of absorber and stripper by using a compressor and heat exchangers. The process is divided into two modules (absorber and stripper), in which the internal heat circulation is maximized. Then, the process is reconstructed by combining these modules. We evaluated the amount of energy consumption of the process as compared with the conventional gas separation process for CO2 by using a commercial process simulator (PRO/II). From the simulation results, energy consumption of the proposed process decreased to one-third at that of conventional heat recovery process. Thus, the proposed process based on self-heat recuperation technology is a very promising process for energy saving of gas separation. 1. Introduction The concern over the CO2 separation technology has risen because of Global warming. Absorption, adsorption, cryogenic separation and membrane separation are generally used to capture CO2. These separation methods are commercially available to capture not only CO2 but also natural gas in refinery plants and waste gases in other industries. The critical issue of gas separation process using the chemical absorption is that the process needs a huge amount of energy to strip CO2 in a stripper. Nowadays, it has been often discussed that amine-based chemical absorption processes to improve the effectiveness and efficiency of CO2 capture (Goto et al., 2009). One of the most famous amine for CO2 capture is a monoethanolamine (MEA). It is well known that MEA required a large amount of energy (4.0 GJ/t-CO2) for CO2 stripping. Therefore, Goto et al. (2009) developed a new absorbent to reduce the regeneration energy in the stripper. In addition, Leites (2003) analyzed some industrial CO2 chemical absorption processes and proposed the optimization methods for process energy based on examination of driving force and quasi-static analysis, but the study still has not proposed optimizing the system configuration. They have paid less attention to process configuration. In 662 contrast, Barchas and Davis (1992) proposed an energy saving process. In their study, they installed a flash column to CO2 rich stream between an absorber and a stripper. However, the process lost the effect of reduction energy while CO2 partial pressure and the absorption process pressures were low. Figure 1 shows a schematic diagram of the conventional CO2 separation process (The heat recovery process) which uses amine absorption method. The process equips a heat exchanger (HX1) for reboiler heat recovery. We analyzed the heat input and output of the processes, and found that the required energy is consumed in the following three places; i) process after cooler of the S3, ii) the lean solvent cooler between absorber and stripper, and iii) the reflux condenser at the top of stripper. Absorber (COL1) Stripper (COL2)Flue Gas Condenser S2 Reboiler Exhaust Gas HX1 S4 S5 S6 S7 S11 S12 S13 S14 S3 P1 P2 S21 S15 S1 Absorbent (Rich Amine) C1 H2 C2 S17 H2O Lean Amine Amine S19 S18 S20 M1 M2 S10 S9 S8 CO2 ProductSP1 S16 B1 Figure 1: Base MEA stripping process (The heat recovery process) Next, we analyzed the heat recovery system of the conventional absorption process from the energy and material balance. Recovered heat is only a part of the exothermic reaction heat in the absorber and the vaporization heat in the stripper. This means that the conventional process could afford to recover more exhausted heat in the process. Kansha et al. (2009, 2010) proposed the design technology for process energy saving, in which not only the latent heat but also the sensible heat of the process stream can be circulated without any heat addition. Based on the study, we propose CO2 separation process of lower energy consumption as an industrial application in this study. 2. Proposed process based on self-heat recuperation CO2 separation process using chemical adsorption method based on self-heat recuperation has been proposed. The proposed process is designed by the following important steps. 2.1 Process Division The CO2 separation process consists of an absorber and a stripper. The process shown in Figure 1 is divided into two simple processes, and then the heat balance of each simple process is analyzed. Therefore, energy input and output of the CO2 separation process can be distinguished. In addition, heat-balanced simple processes are named absorption 663 and stripping modules. A feed and effluent in each module are set at 25 O C and 1 atm as the standard condition. 2.2 Design of self-heat recuperative module In the aforementioned analysis, pairs of feed and effluent streams are selected to recover the heat. To recover the heat among selected pairs, the self-heat recuperation technology is adopted. In the self-heat recuperation technology, the stream condition is changed by means of the compression of the process stream and the stream can provide self heat to stream of the pair in the heat exchanges, leading to that the self heat of the process stream is recirculated based on exergy recuperation to reduce the process energy consumption. Following series of steps, self-heat recuperative module is constructed. 3. Simulation In this paper, process simulation with PRO/II (Invensys plc.) was conducted to calculate the process energy consumption of absorption process. We used a standard amine package model of PRO/II (Invensys plc.) in the software for this simulation. In thermodynamics data, liquid enthalpy is calculated by adding a correction for a heat of reaction. The liquid enthalpy and liquid phase density is calculated by ideal method. Table 1. Concentration of each gas in flue gas Flow rate of flue gas 100 kmol/h Concentration of CO2 in flue gas N2 77 mol% CO2 10 mol% H2 O 10 mol% O2 3 mol% Degree of CO2 removal >99 % Purity of recovery CO2 90 % Vapor phase enthalpy, entropy and density are calculated by SRKM. We assumed that the flue gas was a natural gas, and that the amount of flue gas and CO2 were 100 and 10 kmol/h, respectively. (Table 1). 4. Results and Discussion 4.1 The conventional heat recovery process The conventional heat recovery process and its energy consumption are shown in Figure 2. The inlet flue gas (S0) was set to the standard condition. S0 was heated to 42.2 O C, 101.3 kPa. The heated flue gas (S1; 42.2 O C, 101.3 kPa) was compressed, and supplied to an absorber (S1→B1→S2, 45.0 O C, 101.3 kPa). In the absorber, aqueous MEA absorbs CO2 from the flue gas (COL1), and the remaining gases are discharged from the top of the absorber (S3). The influent rich amine into the absorber is pumped, accordingly the influx is kept liquid phase (S4→S5→S6). The conditions of S4 and S6 were 55.5 O C and 111.4 kPa, and 107.3 O C and 501.3 kPa respectively. The heat of S4 includes a part of exothermic reaction heat, which is effectively used as a part of CO2 stripping energy. The stripper column (COL2) needs a huge amount of energy. The heat 664 of the rich amine sorbent from the bottom of stripper is exchanged with the heat of lean amine solvent (S15→S16), and the rich amine sorbent is heated by recovery (S5→S6). The absorbed CO2 is stripped by a part of heat of exothermic reaction and the reboiling heat in the reboiler. S6 is fed into the stripper, and then CO2 gas and steam are discharged from the top of the stripper (S7). CO2 gas was cooled, and the steam was condensed by a condenser (C1; 45 O C, 186.3 kPa), then CO2 and the condensed water (S8) were separated by a separator (SP1). The condensed water is returned to the stripper as the reflux flow (S10). CO2 gas is released through the separator (S8→SP1→ S9). Aqueous MEA is boiled, and water changed to the steam in a bottom reboiler (H2) of the column, and the vapor stream is returned to the stripper (S11→S12→S13→ COL2, 121.4 O C, 186.3 kPa). Lean amine absorbent was pumped to the same pressure as the absorber (S11→S14→P2→S15→S16). In mixers (M1, M2), MEA and H2O are mixed with the lean amine absorbent (S16→S17→M1→S18→S19→M2→S20). The aqueous MEA was cooled by a cooler (C2) (S20→C2→S21; 45 O C, 186.3 kPa). The total energy consumption of the process was 507.1 kW. The heat recovery process aimed to recover the exhausted heat of the C2. However, the C2 still wastes a large amount of heat of reboiler duty. In addition, C1 is wasted heat in the condenser. The sum of C1 and C2 duties was 389.6 kW. The wasted heat is larger than recovered heat in HX1. It can be understood that we can achieve more energy saving if these consuming energies are recirculated into the process. In this simulation, the flue and effluent gases were set at 25 O C and 1 atm as the standard condition in order to compare with the energy consumption of self-heat recuperative process under the same condition. Absorber (COL1) Stripper (COL2) Condenser S2 Reboiler Exhaust Gas HX1 S4 S5 S6 S7 S11 S12 S13 S14 S16 S3 P1 P2 S21 S15 S1 Rich Amine C1 R1 C2 S17 H2O Lean Amine Amine S19 S18 S20 118.7kW 65.6 o C 333.4 kW Energy consumption: =B1+P1+R1+P2=7.1+0.9+499.1+0.0=507.1 kW 55.5 o C, 111.3 kPa 55.5 o C, 501.3kPa 107.3 o C, 501.3 kPa 121.4 o C, 186.3 kPa M1M2 S10 S9 S8 CO2 ProductSP1 B1 0.9 kW 59.9 o C, 111.3 kPa 121.5 o C,186.3 kPa 94.9 kmol/h 9.98 kmol/h 45.0 o C,186.3 kPa 270.9 k W 499.1 kW Flue Gas 25.0 o C 101.3 kPa 100 kmol/h 42.2 o C 101.3 kPa S0 46.3 o C,111.3 kPa 77.8kW 45.0 o C 111.3 kPa HX0 0.0 kW Figure 2. Energy consumption of heat recovery process 4.2 The proposed process At first, the process shown in Figure 1 was divided into two processes for absorption and stripping. We considered the enthalpy balance of each process in which the conditions of the feed and effluent were set at 25 O C and 1 atm. In these processes, influent enthalpy is same as effluent enthalpy without reaction heat, that is, the absolute 665 value of heat of exothermic reaction is equal to the absolute value of heat of endothermic reaction. Under the condition, the total energy consumption is expressed by the following equation; stabtotal QQQ  (1) where Qab is energy consumption in the absorption process, Qst is energy consumption in the stripping process, respectively. We designed self-heat recuperative modules from these simple processes. The proposed process, which is reconstructed from the absorption and stripper self-heat recuperative modules, is shown in Figure 3. In these modules, streams recovered waste heat to maximize the recovered energy. Flue Gas S2 CO2 Product Exhaust Gas S14 S28 S7 P2 S37 S1 Rich Amine COMP1 HX4 HX2 S33 H2O Amine S35 S34 S36 B1 S3 S4 S5 S6 S9 S10 S8 S11 S12 S13 S15 S19 S21 S27 S23 S29S30 S22 HX1 HX3 EX1 C1 S17 C2 C5 C4 P1 S25 HX5 S18 S20 S16 C3 EX2 SP1 M1 M2 absorber (COL1) Stripper (COL2) S24S26 SP2 Lean Amine S32 P3 S31 V1 V2 Input energy ①Absorption process: B1-EX1=7.1-5.8=1.3 kW ②Desorption process: P1+COMP1-EX2+P2=0.2+95.5-21.3+0=74.4 kW ③Heat pump: 82.0 kW (COP=2) ①+②=75.7 kW ①+②+③ =157.7 kW Figure 3. Self-heat recuperative process (proposed process) The inlet flue gas (S1 25 O C, 1 atm) was compressed by a blower (B1) and heated by a heat exchanger (HX1) to supply a process heat to the absorption column (COL1) (S1→ B1→S2→HX1→S3). The remaining gases are discharged from the top of COL1 (S7: 59.9 O C, 111.3 kPa), expanded by an expander (EX1), and then the vapor temperature decreased by adiabatic expansion (S7→EX1→S8 56.7 O C, 111.3 kPa). S2 was heated, and S8 was cooled by HX1 (S2; 28.4 O C→HX1→S3; 45 O C, S8; 56.7 O C→HX1→S9; 46.2 O C). Afterwards, the gas stream (S9) was discharged from the process and cooled (S9→C1→S10; 25 O C, 111.3 kPa). The absorbent (S4; 55.5 O C) was discharged from the bottom of COL1, and then S4 was cooled by the lean amine in a heat exchanger (HX2) and a cooler (C2) (S4→HX2→S5 35.8 O C→C2→S6 25 O C, 101.3 kPa). S6 is connected to a pump (P1), and is heated in a heat exchanger (S11→HX3→S12; 107.3 O C, 501.3 kPa). CO2 and other gases (S13) were compressed by a compressor (COMP1, 95 kW) (S13; 108.5 O C, 186.3 kPa→COMP1→S14; 363.4 O C, 1300.0 kPa). The absorbent (S23, lean amine) were discharged from the bottom of the stripper, and the absorbent were heated by a heat exchanger (HX4; 121.4 O C). To minimize the exergy 666 loss, only latent heat should be recovered in HX4. However, in the absorption process, gas from COL2 which consists of vapor and CO2 (S13) provides not only latent heat but also sensible heat in HX4, results in exergy loss. The absorbent (S27) was heated by a heat exchanger (HX5) by exothermic reaction heat transported from absorber (S26→ HX4→S27→HX5→S28). HX4 and HX5 have a role of a reboiler. Thus, all of the exhausted heat from cooler is expressed (C1+C2+C3+C4+C5). In contrast, in the proposed process, almost of all exhausted heat is used as the heat of endothermic reaction in the stripper. If we assume that the heat of exothermic reaction is transported to heat of endothermic reaction. The heat required on the reboiler is provided from the heat of exothermic reaction by using heat pump. A performance of the heat pump is expressed the coefficient of performance (COP). Here, the COP is expressed as received heat from exothermic reaction heat divided by supplied compressor work. The total energy consumption of this process was 157.7 kW (Figure 3) while the COP of heat pump was 2. It can be seen that the proposed process decreased the energy consumption by using the compression of the process stream as compared with the conventional process and that waste heat reflects to decrease the energy consumption. Table 2. The comparison of energy consumption in the total process Absorber Stripper Overall Heat recovery process 7.1 500.0 507.1 Proposed process 1.3 74.4 75.7 (157.7) 5. Conclusion In this process, we proposed a new process using a chemical absorption method based on self-heat recuperation technology to reduce amount of energy consumption and we evaluated the amount of energy consumption of the process as compared with the conventional gas separation process for CO2 by using a commercial process simulator. From the simulation results, energy consumption of the proposed process decreased to one-third at that of conventional heat recovery process. 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