- Email: [email protected]
Exergy analysis of indirect dimethyl ether production process
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Exergy analysis of indirect dimethyl ether production process Fatemeh Bahadori , Mehran Nalband Oshnuie ⁎
T
Chemical Engineering Department, Urmia University of Technology, Urmia, Iran
ARTICLE INFO
ABSTRACT
Keywords: Exergy Lost work Dimethyl ether
The decreases in fossil fuel reservoirs, along with several recent environmental crises of pollutants emission, have caused us to find more efficient methods for reducing energy consumption. Exergy analysis is the procedure of finding and improving the less exergetically efficient devices in a process. In this paper, exergy analysis of dimethyl ether production from methanol is investigated. It is observed that main lost work in the process takes place in the shell and tube heat exchangers. Thus, it is proposed to replace the two heat exchangers with an LNG heat exchanger. It is shown that preheating the streams in the LNG associated with a shell and tube heat exchanger reduces 97% and 54% of the lost work for those two heat exchangers.
Introduction The crude oil crisis, as well as the need for long-term sustainable development, requires us to find new green and renewable resources for providing energy to the world. One of the best alternatives to fossil fuels is dimethyl ether (DME), which can be easily liquefied and transported [1,2]. The main significance of dimethyl ether is its considerable potential for use as an alternative for propane in liquefied petroleum gas (LPG), gasoline, and diesel in the automotive industry as a green and renewable energy source. Another application of dimethyl ether is its use as a fuel in the electric power generation, and the feedstock for production of dimethyl sulfate, methylation process, acetic acid, and other common useful materials [3]. The lack of sulfur and nitrogen components in DME, as well as its high oxygen content, decreases the emission of pollutants such as SOx, NOx, and CO during combustion [2]. Also, the cetane number of dimethyl ether is higher than diesel which makes it suitable for use in diesel engines [4,5]. Dimethyl ether is derived from different sources including renewable materials such as biomass, agricultural products and waste; and fossil fuels such as natural gas and coal [6,7]. DME is produced via two main techniques: (1) the one stage direct method, and (2) the two-stage indirect method [8]. During the onestage method, dimethyl ether is directly produced from syngas; whereas in the two-stage process, the syngas is first transformed to the methanol and then dimethyl ether is produced from methanol [9]. The increase in the energy consumption of industrial processes enhances the cost of processes. Thus it is critical to enhancing energy use of processes. Exergy analysis is one of the most suitable techniques to improve the energy consumption of systems. In thermodynamics, the
⁎
exergy of a system is the maximum useful obtainable work during a thermal or chemical process from a specific amount of energy [10–13]. A key goal of energy and exergy analysis is the computation of input and output energy for several possible designs before construction of a factory [14–16]. The energy balance of a system is a form of the first law of thermodynamics or the energy conservation equation; however, in real-world processes, a part of exergy is wasted during the process. Thus, the input and output of exergy are not balanced according to the second law of thermodynamics. Therefore, it is important to choose the most efficient process. Exergy analysis is aimed at identifying the patterns of energy waste in the system [17–19]. Although the production of dimethyl ether has been well-studied, analyzing and improving the exergetic efficiency of indirect dimethyl ether process has not been completely performed. As one of the key studies on the energy consumption of the DME production process, Kotas et al. analyzed the separation unit of dimethyl ether process using energy integration method. Their energy integration on distillation columns significantly reduced the energy consumption of the production unit [10]. Zhang et al. have performed exergy analysis for dimethyl ether production by biomass steam gasification. They investigated the use of the process and proposed a new configuration for dimethyl ether production using biomass which improved the exergetic efficiency of utilization [20]. Because of the environmental crisis created by fossil fuels, improving the energy consumption of industrial process is on critical demand [20]. In this study, exergy analysis of indirect dimethyl ether production on the different process equipment is investigated, and a novel configuration for the process is proposed to reduce the energy
Corresponding author. E-mail address: [email protected] (F. Bahadori).
https://doi.org/10.1016/j.seta.2018.12.025 Received 4 March 2017; Received in revised form 13 December 2018; Accepted 14 December 2018 2213-1388/ © 2018 Elsevier Ltd. All rights reserved.
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
F. Bahadori, M. Nalband Oshnuie
Fig. 1. Process flow diagram for conventional dimethyl ether production process. Table 1 Relation for the calculation of exergy in convective flow [23]. Exergy Kinetic Exergy Potential Exergy Physical Exergy Chemical exergy
Table 3 Exergetic efficiency of process equipment.
Relation
EXK =
1 m. 2
V2
EXP = mg (Z Z0) EXPH = (H H0) T0 (S
EXCH = µ
µ0 = nRTln
S0) f f0
0 = reference point, f = fugacity, µ = chemical potential.
consumption. Process description Fig. 1 shows a schematic of the indirect dimethyl ether production process. As shown, the incoming feed of methanol at 15.5 bars pressure is preheated in E-100 and E-101, and then is mixed with the unreacted recycled stream of methanol after the separation process in the distillation column. The flow of methanol is heated to 250 °C and then
Equipment
Output exergy/Input exergy (–)
Lost work (KW)
Exergetic efficiency (%)
pump Compressor Valve1 Valve2 mixer E-100 E-101 E-102 E-103 Reactor DME Tower Methanol Tower
0.93 1.23 0.93 0.99 0.94 0.12 0.71 0.61 0.64 0.08 0.05 0.06
8.93 361.50 1570.46 35.07 974.24 1104.82 5198.51 52371.73 730.01 3251.61 4599.50 2125.01
0.93 0.86 0.93 0.99 0.94 0.23 0.47 0.26 0.52 91.84 95.38 93.8
injected into the fixed bed reactor to be converted to DME. The product is transported to an E-101 heat exchanger for cooling and after decreasing its pressure to 10.4 bars injected into the T-100 distillation column. The overhead stream of distillation column includes 99.5
Table 2 Exergy relations for styrene unit [17–19,22–24]. Device
Lost Work
Efficiency
Expansion Valve
EX = EXi
EXe =
(m . e )i + W
(m. e )e
Compressor/Expander/Pump
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
Heat Exchenger
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
Cooler/Heater
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
EXe T
EXi T EXi P EXe P (m . e )i (m . e)e W Irreversibility 1 Cold Duty
(m . e )i
T T0 (m . e)e W
Q 1
= Air Cooler
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
Separator/Mixer/TEE
W (m . e)i
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
(m . e )e (m . e)i
Reactor
EX =
Qj 1
=
T0 Tj
Colmn
EX = EXi
Cycle/Process
Summation of irreversibility of all device
EXe =
(m . e )i + W
(m. e )e
i = internal, e = external, c = cold side, h = hot side, j = each component, 0 = reference point. 143
Irreversibility EXi
Wmin Wmin + LW Total irreversibility of cycle or process =1η Total consumed power in cycle or process
=
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
F. Bahadori, M. Nalband Oshnuie
Fig. 2. Suggested process for dimethyl ether production process.
The incoming methanol as a feedstock is converted into DME and water over the γ-Al2O3 catalyst in the fixed bed reactor. The operating pressure of the reactor is in the range of 10–20 bars and the reaction is performed at approximately 300 °C [20]. The proposed reaction for synthesizing DME is as follows:
2CH3 OH
CH3 OCH3 (DME ) + H2 O
(1)
Exergy analysis The concept of exergy Exergy is a measure of the maximum beneficial work possible during a mechanical, chemical, or thermal process in an equilibrium state of the system. In fact, exergy is related to the irreversibility of a process and measures the performance loss during the process. Thus, the exergy analysis specifies the source, the magnitude, and the location of thermodynamic inefficiencies in the processes; especially in thermal systems [22].
Fig. 3. Exergy efficiency of each equipment in the current and modified plant.
Exergy analysis for an open system (control volume) An open system exchanges energy with its surroundings in the following three ways: 1. The energy exchange due to heat transfer 2. The energy exchange by work 3. The energy transfer by convective flow Exergy of heat transfer Consider a thermal engine between two thermal sources with temperatures of T0 and T, respectively. The exergy of a thermal system is the total work performed reversibly by the engine. Therefore, by considering T > T0, the exergy of heat transfer EXQ for a thermal process is written as follows [23]: Fig. 4. The lost work of each equipment in the current and modified plant.
EXQ = Q 1
percent of DME. The conversion of the methanol to DME is about 70–85%. After a reduction in pressure and temperature, the unreacted methanol downstream is separated from water in the T-101 methanolwater separation column. The distilled methanol is sent back to the mixer as the recycled stream [21].
T0 T
(2)
Where Q is the transferred heat during the process. Exergy of work As exergy is an assessment of the maximum work from a specific form of energy, therefore the exergy of work EXw is defined as following [23]: 144
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
F. Bahadori, M. Nalband Oshnuie
EXw = W
(3)
of the heat exchangers. For this purpose, an LNG heat exchanger associated with a shell and tube heat exchanger is appended for increasing the efficiency of the thermal process. It is shown that the exergetic efficiency and lost work of the process are significantly improved.
Where W is the work done on the system. Exergy of convective flow Exergy of a convective flow EXS of a system consists of four components: Kinetic Exergy of the stream EXK, Potential Exergy EXP, Physical Exergy EXPh, and Chemical exergy (EXCh). Thus we have:
EXS = EXK + EXP + EXPh + EXCh
References
(4)
[1] Sumalatha B, Venkata Narayana A, Kiran Kumar K, John Babu D, Venkateswarulu TC. Design and simulation of a plant producing dimethyl ether (DME) from methanol by using simulation software ASPEN PLUS. J Chem Pharm Res 2015:897–901. [2] Semelsberger TA, Borup RL, Greene HL. Dimethyl ether (DME) as an alternative fuel. J Power Sources 2006;156:497–511. [3] Boehman AL. International DME Association IDA, DME: developments, opportunities and challenges. In: Conference on the development and promotion of environmentally friendly heavy duty vehicles such as DME trucks, Washington DC, March 17, 2006. [4] Olah GA, Goeppert A, Prakash S. Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J Organic Chem 2009;74:487–98. [5] Fleisch TH, Basu A, Sills RA. Introduction and advancement of a new global fuel; the statues of DME development in China and beyond. J Natural Gas Sci Eng 2012. [6] Arcoumanis C, Bae C, Crookes R, Kinoshita E. The potential of di-methyl ether (DME) as an alternative fuel for compression-ignition engines: a review. Fuel 2008;87:1014–30. [7] Zhao X, Ren M, Liu Z. Critical solubility of dimethyl ether (DME)+diesel fuel and dimethyl carbonate (DMC)+diesel fuel. Fuel 2005;84:2380–3. [8] Papari S, Kazemeini M, Fattahi M. Modeling-based optimization of the direct synthesis of dimethyl ether from syngas in a commercial slurry reactor. Chin J Chem Eng 2013;21:611–21. [9] Kansha Y, Ishizuka M, Song C, Tsutsumi A. An innovative dimethyl ether (DME) production using self-heat recuperation. Chem Eng Trans 2014;39:109–14. [10] Kotas TJ. The exergy method of thermal plant analysis. Malabar, USA: Krieger Publishing; 1995. [11] Khodami R, Abbas Nejad A, Ali Khabbaz MR. Experimental investigation of energy and exergy efficiency of a pulsating heat pipe for chimney heat recovery. Sustain Energy Techn Assess 2016;16:11–7. [12] Dadak A, Aghbashlo M, Tabatabaei M, Younesi H, Najafpour Gh. Using exergy to analyse the sustainability of fermentative ethanol and acetate production from syngas via anaerobic bacteria (Clostridium ljungdahlii). Sustain Energy Tech Assess 2016;15:11–9. [13] Xydis G, Nanaki E, Koroneos C. Exergy analysis of biogas production from a municipal solid waste landfill. Sustain Energy Tech Assess 2013;4:20–8. [14] Tippawan P, Arpornwichanop A. Energy and exergy analysis of an ethanol reforming process for solid oxide fuel cell applications. Bioresour Technol 2014;157:231–9. [15] Ghannadzadeh A, Sadeqzadeh M. Diagnosis of an alternative ammonia process technology to reduce exergy losses. Energy Conv Manag 2016;109:63–70. [16] Jorgensen L, Haglind F, Clausenm LR. Exergy analysis of a combined heat and power plant with integrated lignocellulosic ethanol production. Energy Conv Manage 2014;85:817–27. [17] Dimopoulos GG, Stefanatos IC, Kakalis NMP. Exergy analysis and optimization of a steam methane pre-reforming system. Energy 2013;58:17–27. [18] Gómez MR, Garcia RF, Gómez JR, Carril JC. Review of thermal cycles exploiting the exergy of liquefied natural gas in the regasification process. Renewable Sust Energy Rev 2014;38:781–95. [19] Hajjaji N, Pons MN, Houas A, Renaudin V. Exergy analysis: an efficient tool for understanding and improving hydrogen production via the steam methane reforming process. Energy Policy 2012;42:392–9. [20] Zhang X, Solli C, Edgar GH, Tian X, Zhang S. Exergy analysis of the process for dimethyl ether production through biomass steam gasification. Ind Eng Chem Res 2009;48:10976–85. [21] Azizi Z, Rezaeimanesh M, Tohidian T, Rahimpour MR. Dimethyl ether: a review of technologies and production challenges. Chem Eng Process 2014;84:150–72. [22] Aljundi IH. Energy and exergy analysis of a steam power plant in Jordan. App Therm Eng 2009;29:324–8. [23] Simpson P, Lutz AE. Exergy analysis of hydrogen production via steam methane reforming. Int J Hydrogen Energy 2007;32:4811–20. [24] Taghizadeh-Damanabi A, Bahadori F. Exergy analysis of ethylbenzene dehydrogenation to styrene monomer. Energy Environ 2018:1–18.
Table 1 presents the relation of different forms of exergy for convective flows. The potential and kinetic exergy of streams are negligible compared to the total exergy of a system. Thus, the Eq. (12) is rewritten as follows [23]:
EXS = EXPh + EXCh
(5)
The exergy relation for process equipment is calculated to study irreversibility of a dimethyl ether production unit. The unit consists of a reactor, two towers, two compressors, four heat exchangers, and a pump. Table 2 illustrates the exergy relations for process equipment. Results The aim of this study is improving the exergetic efficiency of the indirect dimethyl ether production process and reducing the energy consumption in the process. For evaluation of energy quality in the dimethyl ether production process, the exergy rate associated with the process equipment is calculated and presented in Table 3. The table shows the input exergy, the output exergy, and the exergetic efficiency of the compressor, the two valves, the mixer, the four heat exchangers, the reactor, the DME Tower, and the methanol tower. As shown in Table 3, the highest exergetic efficiency is obtained for valve 2, the DME tower, mixer, the methanol tower, valve 1, the pump, and the reactor, in the decreasing order. However, the lowest exergetic efficiency is obtained in the four heat exchangers. Thus modifying the equipment with small exergetic efficiency is recommended. In the proposed process for dimethyl ether production, the configuration of heat exchangers must be changed. Fig. 2 shows the proposed flow diagram for the modified plant. In the modified plant, two shell and tube heat exchangers E-100 and E-101 with exergetic efficiency of 23% and 47%, respectively substituted by LNG-101 and E-101 with exergetic efficiency of 82% and 96%, respectively. The temperature of the inlet stream of the reactor increases to 230 °C and 250 °C in LNG associated with E-101. The lost work of valve 1, the mixer, and the heat exchangers reduces by the implementation of the proposed scheme. Fig. 3 shows exergy efficiency of the process equipment in the current and modified plant. It is illustrated that the efficiency of heat exchangers are significantly improved. Fig. 4 illustrates the lost works for the process equipment in the current and modified plants in the dimethyl ether production process. It is seen that the lost work of E-102 is significantly decreased in the modified plant 52371.7KW to 1538.8KW. Conclusion In this paper, exergy analysis of the dimethyl ether production from methanol is investigated to increase the exergetic efficiency of the process. It is observed that the lowest exergetic efficiency is related to the two heat exchangers. Thus, it is proposed to improve the efficiency
145
Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Exergy analysis of indirect dimethyl ether production process Fatemeh Bahadori , Mehran Nalband Oshnuie ⁎
T
Chemical Engineering Department, Urmia University of Technology, Urmia, Iran
ARTICLE INFO
ABSTRACT
Keywords: Exergy Lost work Dimethyl ether
The decreases in fossil fuel reservoirs, along with several recent environmental crises of pollutants emission, have caused us to find more efficient methods for reducing energy consumption. Exergy analysis is the procedure of finding and improving the less exergetically efficient devices in a process. In this paper, exergy analysis of dimethyl ether production from methanol is investigated. It is observed that main lost work in the process takes place in the shell and tube heat exchangers. Thus, it is proposed to replace the two heat exchangers with an LNG heat exchanger. It is shown that preheating the streams in the LNG associated with a shell and tube heat exchanger reduces 97% and 54% of the lost work for those two heat exchangers.
Introduction The crude oil crisis, as well as the need for long-term sustainable development, requires us to find new green and renewable resources for providing energy to the world. One of the best alternatives to fossil fuels is dimethyl ether (DME), which can be easily liquefied and transported [1,2]. The main significance of dimethyl ether is its considerable potential for use as an alternative for propane in liquefied petroleum gas (LPG), gasoline, and diesel in the automotive industry as a green and renewable energy source. Another application of dimethyl ether is its use as a fuel in the electric power generation, and the feedstock for production of dimethyl sulfate, methylation process, acetic acid, and other common useful materials [3]. The lack of sulfur and nitrogen components in DME, as well as its high oxygen content, decreases the emission of pollutants such as SOx, NOx, and CO during combustion [2]. Also, the cetane number of dimethyl ether is higher than diesel which makes it suitable for use in diesel engines [4,5]. Dimethyl ether is derived from different sources including renewable materials such as biomass, agricultural products and waste; and fossil fuels such as natural gas and coal [6,7]. DME is produced via two main techniques: (1) the one stage direct method, and (2) the two-stage indirect method [8]. During the onestage method, dimethyl ether is directly produced from syngas; whereas in the two-stage process, the syngas is first transformed to the methanol and then dimethyl ether is produced from methanol [9]. The increase in the energy consumption of industrial processes enhances the cost of processes. Thus it is critical to enhancing energy use of processes. Exergy analysis is one of the most suitable techniques to improve the energy consumption of systems. In thermodynamics, the
⁎
exergy of a system is the maximum useful obtainable work during a thermal or chemical process from a specific amount of energy [10–13]. A key goal of energy and exergy analysis is the computation of input and output energy for several possible designs before construction of a factory [14–16]. The energy balance of a system is a form of the first law of thermodynamics or the energy conservation equation; however, in real-world processes, a part of exergy is wasted during the process. Thus, the input and output of exergy are not balanced according to the second law of thermodynamics. Therefore, it is important to choose the most efficient process. Exergy analysis is aimed at identifying the patterns of energy waste in the system [17–19]. Although the production of dimethyl ether has been well-studied, analyzing and improving the exergetic efficiency of indirect dimethyl ether process has not been completely performed. As one of the key studies on the energy consumption of the DME production process, Kotas et al. analyzed the separation unit of dimethyl ether process using energy integration method. Their energy integration on distillation columns significantly reduced the energy consumption of the production unit [10]. Zhang et al. have performed exergy analysis for dimethyl ether production by biomass steam gasification. They investigated the use of the process and proposed a new configuration for dimethyl ether production using biomass which improved the exergetic efficiency of utilization [20]. Because of the environmental crisis created by fossil fuels, improving the energy consumption of industrial process is on critical demand [20]. In this study, exergy analysis of indirect dimethyl ether production on the different process equipment is investigated, and a novel configuration for the process is proposed to reduce the energy
Corresponding author. E-mail address: [email protected] (F. Bahadori).
https://doi.org/10.1016/j.seta.2018.12.025 Received 4 March 2017; Received in revised form 13 December 2018; Accepted 14 December 2018 2213-1388/ © 2018 Elsevier Ltd. All rights reserved.
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
F. Bahadori, M. Nalband Oshnuie
Fig. 1. Process flow diagram for conventional dimethyl ether production process. Table 1 Relation for the calculation of exergy in convective flow [23]. Exergy Kinetic Exergy Potential Exergy Physical Exergy Chemical exergy
Table 3 Exergetic efficiency of process equipment.
Relation
EXK =
1 m. 2
V2
EXP = mg (Z Z0) EXPH = (H H0) T0 (S
EXCH = µ
µ0 = nRTln
S0) f f0
0 = reference point, f = fugacity, µ = chemical potential.
consumption. Process description Fig. 1 shows a schematic of the indirect dimethyl ether production process. As shown, the incoming feed of methanol at 15.5 bars pressure is preheated in E-100 and E-101, and then is mixed with the unreacted recycled stream of methanol after the separation process in the distillation column. The flow of methanol is heated to 250 °C and then
Equipment
Output exergy/Input exergy (–)
Lost work (KW)
Exergetic efficiency (%)
pump Compressor Valve1 Valve2 mixer E-100 E-101 E-102 E-103 Reactor DME Tower Methanol Tower
0.93 1.23 0.93 0.99 0.94 0.12 0.71 0.61 0.64 0.08 0.05 0.06
8.93 361.50 1570.46 35.07 974.24 1104.82 5198.51 52371.73 730.01 3251.61 4599.50 2125.01
0.93 0.86 0.93 0.99 0.94 0.23 0.47 0.26 0.52 91.84 95.38 93.8
injected into the fixed bed reactor to be converted to DME. The product is transported to an E-101 heat exchanger for cooling and after decreasing its pressure to 10.4 bars injected into the T-100 distillation column. The overhead stream of distillation column includes 99.5
Table 2 Exergy relations for styrene unit [17–19,22–24]. Device
Lost Work
Efficiency
Expansion Valve
EX = EXi
EXe =
(m . e )i + W
(m. e )e
Compressor/Expander/Pump
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
Heat Exchenger
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
Cooler/Heater
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
EXe T
EXi T EXi P EXe P (m . e )i (m . e)e W Irreversibility 1 Cold Duty
(m . e )i
T T0 (m . e)e W
Q 1
= Air Cooler
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
Separator/Mixer/TEE
W (m . e)i
EX = EXi
EXe =
(m . e )i + W
(m. e )e
=
(m . e )e (m . e)i
Reactor
EX =
Qj 1
=
T0 Tj
Colmn
EX = EXi
Cycle/Process
Summation of irreversibility of all device
EXe =
(m . e )i + W
(m. e )e
i = internal, e = external, c = cold side, h = hot side, j = each component, 0 = reference point. 143
Irreversibility EXi
Wmin Wmin + LW Total irreversibility of cycle or process =1η Total consumed power in cycle or process
=
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
F. Bahadori, M. Nalband Oshnuie
Fig. 2. Suggested process for dimethyl ether production process.
The incoming methanol as a feedstock is converted into DME and water over the γ-Al2O3 catalyst in the fixed bed reactor. The operating pressure of the reactor is in the range of 10–20 bars and the reaction is performed at approximately 300 °C [20]. The proposed reaction for synthesizing DME is as follows:
2CH3 OH
CH3 OCH3 (DME ) + H2 O
(1)
Exergy analysis The concept of exergy Exergy is a measure of the maximum beneficial work possible during a mechanical, chemical, or thermal process in an equilibrium state of the system. In fact, exergy is related to the irreversibility of a process and measures the performance loss during the process. Thus, the exergy analysis specifies the source, the magnitude, and the location of thermodynamic inefficiencies in the processes; especially in thermal systems [22].
Fig. 3. Exergy efficiency of each equipment in the current and modified plant.
Exergy analysis for an open system (control volume) An open system exchanges energy with its surroundings in the following three ways: 1. The energy exchange due to heat transfer 2. The energy exchange by work 3. The energy transfer by convective flow Exergy of heat transfer Consider a thermal engine between two thermal sources with temperatures of T0 and T, respectively. The exergy of a thermal system is the total work performed reversibly by the engine. Therefore, by considering T > T0, the exergy of heat transfer EXQ for a thermal process is written as follows [23]: Fig. 4. The lost work of each equipment in the current and modified plant.
EXQ = Q 1
percent of DME. The conversion of the methanol to DME is about 70–85%. After a reduction in pressure and temperature, the unreacted methanol downstream is separated from water in the T-101 methanolwater separation column. The distilled methanol is sent back to the mixer as the recycled stream [21].
T0 T
(2)
Where Q is the transferred heat during the process. Exergy of work As exergy is an assessment of the maximum work from a specific form of energy, therefore the exergy of work EXw is defined as following [23]: 144
Sustainable Energy Technologies and Assessments 31 (2019) 142–145
F. Bahadori, M. Nalband Oshnuie
EXw = W
(3)
of the heat exchangers. For this purpose, an LNG heat exchanger associated with a shell and tube heat exchanger is appended for increasing the efficiency of the thermal process. It is shown that the exergetic efficiency and lost work of the process are significantly improved.
Where W is the work done on the system. Exergy of convective flow Exergy of a convective flow EXS of a system consists of four components: Kinetic Exergy of the stream EXK, Potential Exergy EXP, Physical Exergy EXPh, and Chemical exergy (EXCh). Thus we have:
EXS = EXK + EXP + EXPh + EXCh
References
(4)
[1] Sumalatha B, Venkata Narayana A, Kiran Kumar K, John Babu D, Venkateswarulu TC. Design and simulation of a plant producing dimethyl ether (DME) from methanol by using simulation software ASPEN PLUS. J Chem Pharm Res 2015:897–901. [2] Semelsberger TA, Borup RL, Greene HL. Dimethyl ether (DME) as an alternative fuel. J Power Sources 2006;156:497–511. [3] Boehman AL. International DME Association IDA, DME: developments, opportunities and challenges. In: Conference on the development and promotion of environmentally friendly heavy duty vehicles such as DME trucks, Washington DC, March 17, 2006. [4] Olah GA, Goeppert A, Prakash S. Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J Organic Chem 2009;74:487–98. [5] Fleisch TH, Basu A, Sills RA. Introduction and advancement of a new global fuel; the statues of DME development in China and beyond. J Natural Gas Sci Eng 2012. [6] Arcoumanis C, Bae C, Crookes R, Kinoshita E. The potential of di-methyl ether (DME) as an alternative fuel for compression-ignition engines: a review. Fuel 2008;87:1014–30. [7] Zhao X, Ren M, Liu Z. Critical solubility of dimethyl ether (DME)+diesel fuel and dimethyl carbonate (DMC)+diesel fuel. Fuel 2005;84:2380–3. [8] Papari S, Kazemeini M, Fattahi M. Modeling-based optimization of the direct synthesis of dimethyl ether from syngas in a commercial slurry reactor. Chin J Chem Eng 2013;21:611–21. [9] Kansha Y, Ishizuka M, Song C, Tsutsumi A. An innovative dimethyl ether (DME) production using self-heat recuperation. Chem Eng Trans 2014;39:109–14. [10] Kotas TJ. The exergy method of thermal plant analysis. Malabar, USA: Krieger Publishing; 1995. [11] Khodami R, Abbas Nejad A, Ali Khabbaz MR. Experimental investigation of energy and exergy efficiency of a pulsating heat pipe for chimney heat recovery. Sustain Energy Techn Assess 2016;16:11–7. [12] Dadak A, Aghbashlo M, Tabatabaei M, Younesi H, Najafpour Gh. Using exergy to analyse the sustainability of fermentative ethanol and acetate production from syngas via anaerobic bacteria (Clostridium ljungdahlii). Sustain Energy Tech Assess 2016;15:11–9. [13] Xydis G, Nanaki E, Koroneos C. Exergy analysis of biogas production from a municipal solid waste landfill. Sustain Energy Tech Assess 2013;4:20–8. [14] Tippawan P, Arpornwichanop A. Energy and exergy analysis of an ethanol reforming process for solid oxide fuel cell applications. Bioresour Technol 2014;157:231–9. [15] Ghannadzadeh A, Sadeqzadeh M. Diagnosis of an alternative ammonia process technology to reduce exergy losses. Energy Conv Manag 2016;109:63–70. [16] Jorgensen L, Haglind F, Clausenm LR. Exergy analysis of a combined heat and power plant with integrated lignocellulosic ethanol production. Energy Conv Manage 2014;85:817–27. [17] Dimopoulos GG, Stefanatos IC, Kakalis NMP. Exergy analysis and optimization of a steam methane pre-reforming system. Energy 2013;58:17–27. [18] Gómez MR, Garcia RF, Gómez JR, Carril JC. Review of thermal cycles exploiting the exergy of liquefied natural gas in the regasification process. Renewable Sust Energy Rev 2014;38:781–95. [19] Hajjaji N, Pons MN, Houas A, Renaudin V. Exergy analysis: an efficient tool for understanding and improving hydrogen production via the steam methane reforming process. Energy Policy 2012;42:392–9. [20] Zhang X, Solli C, Edgar GH, Tian X, Zhang S. Exergy analysis of the process for dimethyl ether production through biomass steam gasification. Ind Eng Chem Res 2009;48:10976–85. [21] Azizi Z, Rezaeimanesh M, Tohidian T, Rahimpour MR. Dimethyl ether: a review of technologies and production challenges. Chem Eng Process 2014;84:150–72. [22] Aljundi IH. Energy and exergy analysis of a steam power plant in Jordan. App Therm Eng 2009;29:324–8. [23] Simpson P, Lutz AE. Exergy analysis of hydrogen production via steam methane reforming. Int J Hydrogen Energy 2007;32:4811–20. [24] Taghizadeh-Damanabi A, Bahadori F. Exergy analysis of ethylbenzene dehydrogenation to styrene monomer. Energy Environ 2018:1–18.
Table 1 presents the relation of different forms of exergy for convective flows. The potential and kinetic exergy of streams are negligible compared to the total exergy of a system. Thus, the Eq. (12) is rewritten as follows [23]:
EXS = EXPh + EXCh
(5)
The exergy relation for process equipment is calculated to study irreversibility of a dimethyl ether production unit. The unit consists of a reactor, two towers, two compressors, four heat exchangers, and a pump. Table 2 illustrates the exergy relations for process equipment. Results The aim of this study is improving the exergetic efficiency of the indirect dimethyl ether production process and reducing the energy consumption in the process. For evaluation of energy quality in the dimethyl ether production process, the exergy rate associated with the process equipment is calculated and presented in Table 3. The table shows the input exergy, the output exergy, and the exergetic efficiency of the compressor, the two valves, the mixer, the four heat exchangers, the reactor, the DME Tower, and the methanol tower. As shown in Table 3, the highest exergetic efficiency is obtained for valve 2, the DME tower, mixer, the methanol tower, valve 1, the pump, and the reactor, in the decreasing order. However, the lowest exergetic efficiency is obtained in the four heat exchangers. Thus modifying the equipment with small exergetic efficiency is recommended. In the proposed process for dimethyl ether production, the configuration of heat exchangers must be changed. Fig. 2 shows the proposed flow diagram for the modified plant. In the modified plant, two shell and tube heat exchangers E-100 and E-101 with exergetic efficiency of 23% and 47%, respectively substituted by LNG-101 and E-101 with exergetic efficiency of 82% and 96%, respectively. The temperature of the inlet stream of the reactor increases to 230 °C and 250 °C in LNG associated with E-101. The lost work of valve 1, the mixer, and the heat exchangers reduces by the implementation of the proposed scheme. Fig. 3 shows exergy efficiency of the process equipment in the current and modified plant. It is illustrated that the efficiency of heat exchangers are significantly improved. Fig. 4 illustrates the lost works for the process equipment in the current and modified plants in the dimethyl ether production process. It is seen that the lost work of E-102 is significantly decreased in the modified plant 52371.7KW to 1538.8KW. Conclusion In this paper, exergy analysis of the dimethyl ether production from methanol is investigated to increase the exergetic efficiency of the process. It is observed that the lowest exergetic efficiency is related to the two heat exchangers. Thus, it is proposed to improve the efficiency
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