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Improved processes of light hydrocarbon separation from LNG with its cryogenic energy utilized
Energy Conversion and Management 52 (2011) 2401–2404
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Improved processes of light hydrocarbon separation from LNG with its cryogenic energy utilized Ting Gao, Wensheng Lin ⇑, Anzhong Gu Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Available online 5 March 2011 Keywords: Liquefied natural gas (LNG) Cryogenic energy utilization Light hydrocarbon separation Process Economic analysis
a b s t r a c t Liquefied natural gas (LNG) often consists of some kinds of light hydrocarbons other than methane, such as ethane, propane and butane, which are of high additional value. By efficiently utilization of LNG cryogenic energy, these light hydrocarbons (Cþ 2 ) can be separated from LNG with low power consumption and LNG is gasified meanwhile. Two novel light hydrocarbon separation processes are proposed in this paper. The first process uses a demethanizer working at higher pressure (about 4.5 MPa). The methane-riched natural gas from the demethanizer can be compressed to pipeline pressure with low power consumption. The other one uses a demethanizer working at lower pressure (about 2.4 MPa). By cascade utilization of LNG cryogenic energy, the methane-riched natural gas from the demethanizer is entirely re-liquefied. Then the liquid product is pressurized to pipeline pressure by pumps instead of compressors, reducing the power consumption greatly. By both of the two processes, liquefied ethane and LPG (liquefied petroleum gas, i.e. Cþ 3 ) at atmosphere pressure can be obtained directly, and high ethane recovery rate can be gained. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane price to the economics of the light hydrocarbon separation plants are studied, and the economics are compared for these two processes. The results show that recovering light hydrocarbons from LNG can gain great profits by both of the two processes, and from the view of economics, the low pressure process is better than the high pressure process. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Natural gas is often liquefied for efficient transportation, and liquefaction is a high energy consumption process. On the other side, liquefied natural gas (LNG) should be gasified for normal use at the receiving site, and great cryogenic energy is released during the gasification process (about 840 kJ/kg), which can be utilized to recover energy and enhance economic performance [1,2]. The cryogenic energy of LNG can be utilized in several ways, such as air separation [3], cryogenic power generation [4], seawater desalination [5], and so on. Recently, a lot of LNG sources among the international trades are rich gas, which contain more than 10% of light hydrocarbons other than methane (such as ethane, propane and butane). Ethane is a kind of quality and clean raw materials for the production of ethylene, thus it has high additional values. By utilizing the cryogenic energy of LNG during its gasification, ethane and LPG (liquefied petroleum gas, i.e. Cþ 3 ) can be produced with low power consumption [6–9]. There have been some patents about separating light hydrocarbons (Cþ 2 ) from LNG as early as 1960 in America, and several new
⇑ Corresponding author. Tel.: +86 13764193350. E-mail addresses: [email protected] (T. Gao), [email protected] (W. Lin). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2010.12.040
patents have been registered in these years. However, these techniques are usual as a means of heat value control, and the Cþ 2 separated from LNG are always stay at high pressure, which is inconvenient for transportation and marketing [10–12]. In recent years, researches for the production of Cþ 2 form LNG by utilizing its cryogenic energy have developed in China. Hua et al. [13,14] proposed several improved processes. Ref. [13] suggested a new process which integrated the two parts of the heat exchanger networks and the light hydrocarbon separation process, and by heat integration and optimization, the power consumption for separation was reduced greatly. However, the pressure of the separated Cþ 2 is still high. Ref. [14] gave a further optimization for the heat exchanger networks, and designed a process which got rid of the compressor. Besides, this process utilized the cold energy of LNG þ to sub-cool the separated Cþ 2 , and thus the C2 remained liquid state at normal pressure. However, ethane was not further separated from Cþ 2 in this process, thus the product was not available to be directly used. Based on the existing researches, two novel light hydrocarbon separation processes are proposed in this paper. By both of these two processes, liquefied ethane and LPG at atmosphere pressure can be obtained directly with acceptable power level, and high ethane recovery rate can be gained. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane
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T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404
price on the economics of the light hydrocarbon separation plants are studied, and the economics are compared for these two processes. 2. Process structure For the two processes proposed in this paper, the first one, which was called ‘‘high pressure process’’, uses a demethanizer working at higher pressure (about 4.5 MPa). The methane-riched natural gas from the demethanizer can be compressed to pipeline pressure with low power consumption. The second one, which was called ‘‘low pressure process’’, uses a demethanizer working at lower pressure (about 2.4 MPa). By cascade utilization of LNG cryogenic energy, the methane-riched natural gas from the demethanizer is entirely re-liquefied. Then the liquid product is pressurized to pipeline pressure by pumps instead of compressors, reducing the power consumption greatly. HYSYS software (AspenTech) is used for the process simulations and calculations.
and released from the top of the demethanizer. The methaneriched natural gas is then compressed to the pipeline pressure by a compressor. The Cþ 2 released from the bottom of the demethanizer enters into the deethanizer after being depressurized to 0.2 MPa by a throttle. The deethanizer works at 0.11 MPa, and liquefied ethane (LC2) at atmosphere pressure is obtained at the top of the deethanizer, while LPG (LCþ 3 ) at atmosphere pressure is obtained at the bottom of the deethanizer. 2.2. 2 Low pressure process The low pressure process is shown in Fig. 2. The LNG at atmosphere pressure is firstly pressurized to 1.5 MPa by pump 1, and then it is heated twice and becomes gas–liquid two-phase fluid. For a two-phase fluid, the sensible cooling capacity of the gas phase
Table 1 LNG receiving station conditions.
2.1. 1 High pressure process The high pressure process is shown in Fig. 1. As shown in Fig. 1, the LNG at atmosphere pressure is firstly pressurized to 4.5 MPa with a pump, and then it enters into the demethanizer after being pre-heated by using the heat energy from the condenser of the deethanizer. The demethanizer works at 4.3 MPa, by which more than 99.99% of methane is recovered
Parameters
Value
Composition (mol%) CH4 C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 N2 LNG storage pressure (MPa) LNG temperature (°C) LNG heat value (MJ/Nm3) Pipeline pressure (MPa) LNG imported quantity (t/a)
90.16 5.22 3.10 0.45 0.82 0.04 0.03 0.18 0.125 158.3 40.2 7.65 1.10 106
Table 2 Process settings.
Fig. 1. High pressure process.
Fig. 2. Low pressure process.
Parameters
Value
Compressor adiabatic efficiency Pump adiabatic efficiency Pressure drop of heat exchanger Number of stages in the demethanizer Number of stages in the deethanizer Pressure drop of the demethanizer Pressure drop of the deethanizer
85% 75% 10 kPa 25 20 20 kPa 15 kPa
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T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404 Table 3 Process performance. Parameters
High pressure process
Low pressure process
Natural gas production Natural gas purity (methane content) Natural gas heat value Ethane production
98.5 t/h 98.8%
97.83 t/h 99.3%
36.19 MJ/Nm3 9.03 t/h
36.01 MJ/Nm3 9.68 t/h
LPG production Cþ 2 recovery rate Ethane recovery rate Power consumption
13.93 t/h 90.38%
14.38 t/h 95.20%
85.67% 1534 kW
91.78% 1016 kW
Heat load of the reboiler in the demethanizer
14.38 MW (natural gas consumption: 1.04 t/ h, about 1%)
8.86 MW (natural gas consumption: 0.64 t/h, about 0.65%)
and the latent cooling capacity of the liquid phase can be utilized separately. Afterwards the gas–liquid two-phase fluid enters into a liquid–vapor separator, and the separated methane-riched gas is liquefied by using the sensible cooling capacity of the LNG in heater 1 and is then pressurized to 2.4 MPa with pump 3; meanwhile, the Cþ 2 -riched liquid is pressurized to 2.5 MPa with pump 2 and then enters into the demethanizer. The demethanizer works at 2.4 MPa. The methane-riched natural gas released from the top of the demethanizer is liquefied by using the latent cooling capacity of the LNG in heater 2, and then it mixes with the methaneriched gas out of pump 3. The mixture is firstly pressurized to 5 MPa by pump 4, and then its cold energy is further utilized for the condenser of the deethanizer. Afterwards it is further pressurized to the pipeline pressure by pump 5. The Cþ 2 released from the bottom of the demethanizer enters into the deethanizer after being depressurized to 0.2 MPa by a throttle. The deethanizer works at
Table 4 Economic comparison.
Investment (million CNY) Equipment Others Total
High pressure process
Low pressure process
45 7.5 52.5
60 7.5 67.5
Operation cost (million CNY/a) Electricitya 11.87 LNG cold energyb 10.54 Payout for the workersc 0.57 Others 6.4 Total 29.38
7.86 10.54 0.57 6.4 26.37
Income (million CNY/a) Natural gasd Ethanee LPGf Total Net profitg (million CNY/a) Payback periodh (year)
1092.2 609.05 627.37 144.22 74.64 1.90
1062.1 568.2 607.76 113.86 54.91 1.96
Notes
Purity: 99.99%; heat value: 47.51 MJ/Nm3 Heat value: 46.12 MJ/Nm3
Output pressure of natural gas: 8 MPa
0.11 MPa, and liquefied ethane (LC2) at atmosphere pressure is obtained at the top of the deethanizer, while LPG (LCþ 3 ) is obtained at the bottom of the deethanizer. For these two processes, the temperature of the reboiler in the demethanizer is about 20–70 °C, and the heat energy required for this reboiler can be provided by combusting a fraction of the methane-riched natural gas; the temperature of the reboiler in the deethanizer is about 20 to 35 °C, and this reboiler can be directly heated by air or water.
3. Process performance Take one of the LNG receiving stations in China as the example, whose gas source parameters and operating conditions are shown in Table 1, these two processes are simulated by the software HYSYS (some of the settings or parameters for the components in the processes are shown in Table 2), and the performance of these two processes are calculated, as shown in Table 3. It can be seen from Table 3 that the performance of the low pressure process is better than the high pressure process. However, the high pressure process is more simple and compact, thus it is more suitable for the cases where the space is limited. Furthermore, the low pressure process requires accurate temperature matching, and thus its adaptability is worse than the high pressure process. Therefore, the high pressure process is also preferable for the cases where the conditions change frequently.
a
Electricity price: 0.9 CNY/kW h. LNG cold energy price: 10 CNY/t(LNG). c Ten workers, salary: 50,000 CNY/a for each worker; welfare factor: 14%. d The amount and the heat value of the natural gas decrease after light hydrocarbon separation, causing a loss of incomes for natural gas. Heat value is used to measure the value of the natural gas, and the price of it is 0.11 CNY/MJ. e Ethane is sold as the raw material of ethylene, the price of it should be higher than the price measured with its heat value. In order to estimate ethane price by using its heat value, the heat value price of ethane is assumed as 1.4 times than the heat value price of natural gas: 0.154 CNY/MJ. f LPG is sold as fuel, the price of it is measured with the heat value price of natural gas: 0.11 CNY/MJ. g 35% of taxes is eliminated. h One year of construction period is considered. b
Fig. 3. Changes of profit with ethane content and ethane price.
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4. Economic analysis 4.1. Economic comparison Although the low pressure process has better efficiency and more production than the high pressure process, the high pressure process is simpler, and therefore requires less equipment investment and smaller occupied area. An economic analysis and comparison for these two processes is studied, and the results are shown in Table 4 (the operation time is assumed as 8600 h per year). From Table 4 we can see, recovering light hydrocarbons from LNG can gain great profits by both of these two processes, and from the view of economics, the low pressure process is better than the high pressure process. 4.2. Effects of the ethane content and the ethane price Considering ethane is the main source of increasing income, the economics of light hydrocarbon separation plants is very sensitive to the ethane content and the ethane price. As a result, the effects of the ethane content and the ethane price on the economics of these two processes are further investigated. On the basis of the composition listed in Table 1, the effect of the ethane content is studied when changing the molar fraction of methane and ethane while the molar fraction of the other components is fixed. Meanwhile, the effect of the ethane price is studied by altering the times of the heat value price of ethane over that of the methane (a). The results are shown in Fig. 3. Fig. 3 indicates that both of these two processes have great potential for economic benefits. As long as the ethane price is higher than 1.2 times of its heat value price, the light hydrocarbon separation plants can make profits by using the high pressure process when the ethane content of LNG is higher than 5%, while it is 4% for the low pressure process. 5. Conclusion Two novel light hydrocarbon separation processes are proposed in this paper. By both of the processes, liquefied ethane and LPG at atmosphere pressure can be produced directly with acceptable
power level, and the cryogenic energy of LNG is utilized reasonable. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane price to the economics of the light hydrocarbon separation plants are studied, and the economics are compared for these two processes. The results show that, recovering light hydrocarbons from LNG can gain great profits by both of the processes, and from the view of economics, the low pressure process is better than the high pressure process. However, the high pressure process is more preferable for the cases where the space is limited as well as the cases where the conditions change frequently.
References [1] Liu HT, You L. Characteristics and applications of the cold heat exergy of liquefied natural gas. Energy Conversion Manage 1999;40:1515–25. [2] Lin WS, Zhang N, Gu AZ. LNG (liquefied natural gas): a necessary part in China’s future energy infrastructure. Energy 2010;35:4383–91. [3] Yang CC, Kaplan AL, Huang ZP. Cost-effective design reduces C2 and C3 at LNG receiving terminal. Oil Gas J 2003;101(21):50–3. [4] Michael HD, Soo CJ, Paul DD. Air separation process utilizing refrigeration extracted from LNG for production of liquid oxygen, 2008216512A1[P], 200809-11, USA. [5] Lin WS, Huang MB, He HM, Gu AZ. A transcritical CO2 Rankine Cycle with LNG cold energy utilization and liquefaction of CO2 in gas turbine exhaust. J Energy Res Technol-Trans ASME 2009;131 [Paper 042201]. [6] Lin WS, Huang MB, Shen QQ, Gu AZ. Comparison of seawater desalination methods with LNG cold energy utilization. In: Proceedings of 9th international conference on sustainable energy technologies, Shanghai, China; 2010. [7] Yang CC, Huang ZP, Barclay M, Wheeler F. Processes help turn rich LNG into lean gas. LNG J 2006:11–2 (June). [8] Coyle D, Vega F, Durr C. Natural gas specification challenges in the LNG industry. In: 15th international conference and exhibition on liquefied natural gas, Barcelona, Spain; 2007. [9] Yang CC, Bothamley G. Maximizing the value of surplus ethane and costeffective design to handle rich LNG. In: 15th international conference and exhibition on liquefied natural gas, Barcelona, Spain; 2007. [10] Prim E. System and method for recovery of Cþ 2 hydrocarbons contained in liquefied natural gas, 0158458A1[P], 2003-08-21, USA. [11] Reddick K, Belhateche N. Liquid natural gas processing, 0188996A1[P], 200310-09, USA. [12] Winningham HG, Anderson TX. Process for extracting ethane and heavier hydrocarbons from LNG, 7165423B2[P], 2007-01-23, USA. [13] Hua B, Xiong YQ, Li YJ, Yang XM. Simulation and optimization of the process of light hydrocarbon recovery from LNG. Nat Gas Ind 2006;26(5):127–9. [14] Xiong YQ, Li YJ, Hua B. Optimized design of recovery process of light hydrocarbons from LNG. J South China Univ Technol 2007;35(7):62–6.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Improved processes of light hydrocarbon separation from LNG with its cryogenic energy utilized Ting Gao, Wensheng Lin ⇑, Anzhong Gu Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Available online 5 March 2011 Keywords: Liquefied natural gas (LNG) Cryogenic energy utilization Light hydrocarbon separation Process Economic analysis
a b s t r a c t Liquefied natural gas (LNG) often consists of some kinds of light hydrocarbons other than methane, such as ethane, propane and butane, which are of high additional value. By efficiently utilization of LNG cryogenic energy, these light hydrocarbons (Cþ 2 ) can be separated from LNG with low power consumption and LNG is gasified meanwhile. Two novel light hydrocarbon separation processes are proposed in this paper. The first process uses a demethanizer working at higher pressure (about 4.5 MPa). The methane-riched natural gas from the demethanizer can be compressed to pipeline pressure with low power consumption. The other one uses a demethanizer working at lower pressure (about 2.4 MPa). By cascade utilization of LNG cryogenic energy, the methane-riched natural gas from the demethanizer is entirely re-liquefied. Then the liquid product is pressurized to pipeline pressure by pumps instead of compressors, reducing the power consumption greatly. By both of the two processes, liquefied ethane and LPG (liquefied petroleum gas, i.e. Cþ 3 ) at atmosphere pressure can be obtained directly, and high ethane recovery rate can be gained. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane price to the economics of the light hydrocarbon separation plants are studied, and the economics are compared for these two processes. The results show that recovering light hydrocarbons from LNG can gain great profits by both of the two processes, and from the view of economics, the low pressure process is better than the high pressure process. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Natural gas is often liquefied for efficient transportation, and liquefaction is a high energy consumption process. On the other side, liquefied natural gas (LNG) should be gasified for normal use at the receiving site, and great cryogenic energy is released during the gasification process (about 840 kJ/kg), which can be utilized to recover energy and enhance economic performance [1,2]. The cryogenic energy of LNG can be utilized in several ways, such as air separation [3], cryogenic power generation [4], seawater desalination [5], and so on. Recently, a lot of LNG sources among the international trades are rich gas, which contain more than 10% of light hydrocarbons other than methane (such as ethane, propane and butane). Ethane is a kind of quality and clean raw materials for the production of ethylene, thus it has high additional values. By utilizing the cryogenic energy of LNG during its gasification, ethane and LPG (liquefied petroleum gas, i.e. Cþ 3 ) can be produced with low power consumption [6–9]. There have been some patents about separating light hydrocarbons (Cþ 2 ) from LNG as early as 1960 in America, and several new
⇑ Corresponding author. Tel.: +86 13764193350. E-mail addresses: [email protected] (T. Gao), [email protected] (W. Lin). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2010.12.040
patents have been registered in these years. However, these techniques are usual as a means of heat value control, and the Cþ 2 separated from LNG are always stay at high pressure, which is inconvenient for transportation and marketing [10–12]. In recent years, researches for the production of Cþ 2 form LNG by utilizing its cryogenic energy have developed in China. Hua et al. [13,14] proposed several improved processes. Ref. [13] suggested a new process which integrated the two parts of the heat exchanger networks and the light hydrocarbon separation process, and by heat integration and optimization, the power consumption for separation was reduced greatly. However, the pressure of the separated Cþ 2 is still high. Ref. [14] gave a further optimization for the heat exchanger networks, and designed a process which got rid of the compressor. Besides, this process utilized the cold energy of LNG þ to sub-cool the separated Cþ 2 , and thus the C2 remained liquid state at normal pressure. However, ethane was not further separated from Cþ 2 in this process, thus the product was not available to be directly used. Based on the existing researches, two novel light hydrocarbon separation processes are proposed in this paper. By both of these two processes, liquefied ethane and LPG at atmosphere pressure can be obtained directly with acceptable power level, and high ethane recovery rate can be gained. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane
2402
T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404
price on the economics of the light hydrocarbon separation plants are studied, and the economics are compared for these two processes. 2. Process structure For the two processes proposed in this paper, the first one, which was called ‘‘high pressure process’’, uses a demethanizer working at higher pressure (about 4.5 MPa). The methane-riched natural gas from the demethanizer can be compressed to pipeline pressure with low power consumption. The second one, which was called ‘‘low pressure process’’, uses a demethanizer working at lower pressure (about 2.4 MPa). By cascade utilization of LNG cryogenic energy, the methane-riched natural gas from the demethanizer is entirely re-liquefied. Then the liquid product is pressurized to pipeline pressure by pumps instead of compressors, reducing the power consumption greatly. HYSYS software (AspenTech) is used for the process simulations and calculations.
and released from the top of the demethanizer. The methaneriched natural gas is then compressed to the pipeline pressure by a compressor. The Cþ 2 released from the bottom of the demethanizer enters into the deethanizer after being depressurized to 0.2 MPa by a throttle. The deethanizer works at 0.11 MPa, and liquefied ethane (LC2) at atmosphere pressure is obtained at the top of the deethanizer, while LPG (LCþ 3 ) at atmosphere pressure is obtained at the bottom of the deethanizer. 2.2. 2 Low pressure process The low pressure process is shown in Fig. 2. The LNG at atmosphere pressure is firstly pressurized to 1.5 MPa by pump 1, and then it is heated twice and becomes gas–liquid two-phase fluid. For a two-phase fluid, the sensible cooling capacity of the gas phase
Table 1 LNG receiving station conditions.
2.1. 1 High pressure process The high pressure process is shown in Fig. 1. As shown in Fig. 1, the LNG at atmosphere pressure is firstly pressurized to 4.5 MPa with a pump, and then it enters into the demethanizer after being pre-heated by using the heat energy from the condenser of the deethanizer. The demethanizer works at 4.3 MPa, by which more than 99.99% of methane is recovered
Parameters
Value
Composition (mol%) CH4 C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 N2 LNG storage pressure (MPa) LNG temperature (°C) LNG heat value (MJ/Nm3) Pipeline pressure (MPa) LNG imported quantity (t/a)
90.16 5.22 3.10 0.45 0.82 0.04 0.03 0.18 0.125 158.3 40.2 7.65 1.10 106
Table 2 Process settings.
Fig. 1. High pressure process.
Fig. 2. Low pressure process.
Parameters
Value
Compressor adiabatic efficiency Pump adiabatic efficiency Pressure drop of heat exchanger Number of stages in the demethanizer Number of stages in the deethanizer Pressure drop of the demethanizer Pressure drop of the deethanizer
85% 75% 10 kPa 25 20 20 kPa 15 kPa
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T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404 Table 3 Process performance. Parameters
High pressure process
Low pressure process
Natural gas production Natural gas purity (methane content) Natural gas heat value Ethane production
98.5 t/h 98.8%
97.83 t/h 99.3%
36.19 MJ/Nm3 9.03 t/h
36.01 MJ/Nm3 9.68 t/h
LPG production Cþ 2 recovery rate Ethane recovery rate Power consumption
13.93 t/h 90.38%
14.38 t/h 95.20%
85.67% 1534 kW
91.78% 1016 kW
Heat load of the reboiler in the demethanizer
14.38 MW (natural gas consumption: 1.04 t/ h, about 1%)
8.86 MW (natural gas consumption: 0.64 t/h, about 0.65%)
and the latent cooling capacity of the liquid phase can be utilized separately. Afterwards the gas–liquid two-phase fluid enters into a liquid–vapor separator, and the separated methane-riched gas is liquefied by using the sensible cooling capacity of the LNG in heater 1 and is then pressurized to 2.4 MPa with pump 3; meanwhile, the Cþ 2 -riched liquid is pressurized to 2.5 MPa with pump 2 and then enters into the demethanizer. The demethanizer works at 2.4 MPa. The methane-riched natural gas released from the top of the demethanizer is liquefied by using the latent cooling capacity of the LNG in heater 2, and then it mixes with the methaneriched gas out of pump 3. The mixture is firstly pressurized to 5 MPa by pump 4, and then its cold energy is further utilized for the condenser of the deethanizer. Afterwards it is further pressurized to the pipeline pressure by pump 5. The Cþ 2 released from the bottom of the demethanizer enters into the deethanizer after being depressurized to 0.2 MPa by a throttle. The deethanizer works at
Table 4 Economic comparison.
Investment (million CNY) Equipment Others Total
High pressure process
Low pressure process
45 7.5 52.5
60 7.5 67.5
Operation cost (million CNY/a) Electricitya 11.87 LNG cold energyb 10.54 Payout for the workersc 0.57 Others 6.4 Total 29.38
7.86 10.54 0.57 6.4 26.37
Income (million CNY/a) Natural gasd Ethanee LPGf Total Net profitg (million CNY/a) Payback periodh (year)
1092.2 609.05 627.37 144.22 74.64 1.90
1062.1 568.2 607.76 113.86 54.91 1.96
Notes
Purity: 99.99%; heat value: 47.51 MJ/Nm3 Heat value: 46.12 MJ/Nm3
Output pressure of natural gas: 8 MPa
0.11 MPa, and liquefied ethane (LC2) at atmosphere pressure is obtained at the top of the deethanizer, while LPG (LCþ 3 ) is obtained at the bottom of the deethanizer. For these two processes, the temperature of the reboiler in the demethanizer is about 20–70 °C, and the heat energy required for this reboiler can be provided by combusting a fraction of the methane-riched natural gas; the temperature of the reboiler in the deethanizer is about 20 to 35 °C, and this reboiler can be directly heated by air or water.
3. Process performance Take one of the LNG receiving stations in China as the example, whose gas source parameters and operating conditions are shown in Table 1, these two processes are simulated by the software HYSYS (some of the settings or parameters for the components in the processes are shown in Table 2), and the performance of these two processes are calculated, as shown in Table 3. It can be seen from Table 3 that the performance of the low pressure process is better than the high pressure process. However, the high pressure process is more simple and compact, thus it is more suitable for the cases where the space is limited. Furthermore, the low pressure process requires accurate temperature matching, and thus its adaptability is worse than the high pressure process. Therefore, the high pressure process is also preferable for the cases where the conditions change frequently.
a
Electricity price: 0.9 CNY/kW h. LNG cold energy price: 10 CNY/t(LNG). c Ten workers, salary: 50,000 CNY/a for each worker; welfare factor: 14%. d The amount and the heat value of the natural gas decrease after light hydrocarbon separation, causing a loss of incomes for natural gas. Heat value is used to measure the value of the natural gas, and the price of it is 0.11 CNY/MJ. e Ethane is sold as the raw material of ethylene, the price of it should be higher than the price measured with its heat value. In order to estimate ethane price by using its heat value, the heat value price of ethane is assumed as 1.4 times than the heat value price of natural gas: 0.154 CNY/MJ. f LPG is sold as fuel, the price of it is measured with the heat value price of natural gas: 0.11 CNY/MJ. g 35% of taxes is eliminated. h One year of construction period is considered. b
Fig. 3. Changes of profit with ethane content and ethane price.
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T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404
4. Economic analysis 4.1. Economic comparison Although the low pressure process has better efficiency and more production than the high pressure process, the high pressure process is simpler, and therefore requires less equipment investment and smaller occupied area. An economic analysis and comparison for these two processes is studied, and the results are shown in Table 4 (the operation time is assumed as 8600 h per year). From Table 4 we can see, recovering light hydrocarbons from LNG can gain great profits by both of these two processes, and from the view of economics, the low pressure process is better than the high pressure process. 4.2. Effects of the ethane content and the ethane price Considering ethane is the main source of increasing income, the economics of light hydrocarbon separation plants is very sensitive to the ethane content and the ethane price. As a result, the effects of the ethane content and the ethane price on the economics of these two processes are further investigated. On the basis of the composition listed in Table 1, the effect of the ethane content is studied when changing the molar fraction of methane and ethane while the molar fraction of the other components is fixed. Meanwhile, the effect of the ethane price is studied by altering the times of the heat value price of ethane over that of the methane (a). The results are shown in Fig. 3. Fig. 3 indicates that both of these two processes have great potential for economic benefits. As long as the ethane price is higher than 1.2 times of its heat value price, the light hydrocarbon separation plants can make profits by using the high pressure process when the ethane content of LNG is higher than 5%, while it is 4% for the low pressure process. 5. Conclusion Two novel light hydrocarbon separation processes are proposed in this paper. By both of the processes, liquefied ethane and LPG at atmosphere pressure can be produced directly with acceptable
power level, and the cryogenic energy of LNG is utilized reasonable. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane price to the economics of the light hydrocarbon separation plants are studied, and the economics are compared for these two processes. The results show that, recovering light hydrocarbons from LNG can gain great profits by both of the processes, and from the view of economics, the low pressure process is better than the high pressure process. However, the high pressure process is more preferable for the cases where the space is limited as well as the cases where the conditions change frequently.
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