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Oct 16, 2015 - In this paper, a conceptual process design for a novel natural gas liquefaction plant developed especially for offshore application is ...
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Process Design for the Offshore Production of Liquefied Natural Gas with Nonflammable Refrigerants Chul-Jin Lee, Kiwook Song, Seolin Shin, Youngsub Lim, and Chonghun Han Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01620 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Process Design for the Offshore Production of Liquefied

Natural

Gas

with

Nonflammable

Refrigerants Chul-Jin Lee†, a, Kiwook Song†, b, Seolin Shinb, Youngsub Limc, Chonghun Hanb,* a

Engineering Development Research Center, Seoul National University, Gwanak 1, Gwanak-ro,

Gwanak-gu, Seoul, 151-742, Korea b

School of Chemical and Biological Engineering, Seoul National University, Gwanak 1,

Gwanak-ro, Gwanak-gu, Seoul, 151-742, Korea c

School of Naval Architecture and Ocean Engineering, Seoul National University, Gwanak 1,

Gwanak-ro, Gwanak-gu, Seoul, 151-742, Korea



These authors equally contributed to the presented work

To whom correspondence should be addressed: Chonghun Han Tel: +82-2-880-1887. E-mail: [email protected]

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Abstract In this paper, a conceptual process design for a novel natural gas liquefaction plant developed especially for offshore application is presented. Onshore liquefied natural gas (LNG) production usually utilizes mixed refrigerant (MR)-based cycles for high efficiency in terms of operating cost. This paper proposes a cascade process using nitrous oxide and nitrogen refrigerants. The liquefaction process can be subdivided into a pre-cooling section in the vapor phase, a condensation section, and a sub-cooling section in liquid phase. The vapor compression refrigeration cycle of nitrous oxide is applied to the pre-cooling and the condensation sections, whereas the gas compression refrigeration cycle of nitrogen is applied to the sub-cooling section of LNG. The proposed process shows enhanced efficiency compared to the conventional turbinebased processes with the specific power comparable to the MR processes.

Keywords: Process Design; FLNG; Liquefaction; Offshore; Refrigerant

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1. Introduction Global natural gas demand is expected to surge, motivated by a growing preference for lowcarbon, environment-friendly energies and uncertainty in policies related to nuclear power generation. Although conventional scale natural gas plants with reserves of 5 to 100 trillion cubic feet (TCF) are being fully explored and developed, the impetus to monetize mid-scale (0.5 to 5 TCF) and small-scale gas reserves is growing. In particular, floating LNG (FLNG) production is especially gaining interest because it provides the opportunity to develop smaller or remote fields. LNG-FPSO (Floating production, storage and offloading) employed in offshore natural gas reserves can be repeatedly used for stranded gas resources. The natural gas liquefaction processes for existing base-load plants are mostly based on mixed refrigerant (MR) processes or cascade processes, both of which are based on hydrocarbon refrigerants. For FLNG, the minimization of flammable inventory is important for safety, driving interest in refrigeration cycles that contain no hydrocarbons. The nitrogen recycle expander plant, well known and extensively used in the air separation industry, is a good alternative for offshore applications. Turbine-based processes offer the advantages of safety, an easy start-up and a small layout. However, they have been rejected in onshore applications owing to low efficiency in terms of compression power requirements [1-4]. Therefore, the continous investigation is required for nonflammable refrigerant-based natural gas liquefaction cyle in FLNG due to its inherant safety and rising demand for small scale reservoir despite of relatively low efficiency compared to hydrocarbon refregeration cylcle [5]. To increase the capacity and efficiency of turbine-based processes, modifications to these processes have been proposed. Dubar suggested double and triple expander processes by dividing the refrigerant stream into two or three portions [6]. Dual expander processes proposed

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by Statoil is a similar concept [7]. Another alternative is to add a pre-cooling unit. The CO2 precooled dual expander process was suggested by Statoil [8]. Air Products and Chemicals proposed the AP-HN processes that use HFC as the pre-cooling refrigerant [9]. Yoon et al. suggested the new cascade process using N2O and other hydrocarbons as a refrigerant and compared its efficiency with the conventional hydrocarbon-based cascade liquefaction process [10]. In this study, we have developed a novel process design of turbine-based natural gas liquefaction plant for offshore application. To meet the safety criteria for LNG-FPSO, the process utilizes only nonflammable refrigerants, especially nitrous oxide (N2O). The remainder of this paper is organized as follows. In Section 2, the results of the thermodynamic analysis of carbon dioxide (CO2, R744) and nitrous oxide (N2O, R744a) are discussed. Then, in Section 3, the developed novel cascade process of N2O-N2O-N2 for offshore LNG production is presented. Results are shown and discussed in Section 4, and finally, the conclusions are presented in Section 5.

2. Thermodynamic Analysis of Carbon Dioxide and Nitrous Oxide CO2 and N2O are nonflammable refrigerants that are good candidates for offshore applications. Both candidates show similar thermodynamic properties as a refrigerant such as molecular weight, boiling point, critical point and triple point as shown in Table 1. The critical point of CO2 is 73.8 bar and 31.3 °C while the critical point of N2O is 72.5 bar and 36.4 °C. If we neglect the supercritical phase and restrict the operating range to the sub-critical region, then the critical pressure imposes an upper limit on the compressor discharge pressure in the vapor-compression refrigeration system. Further, the critical temperature determines the upper limit of the cooling

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water or air temperature in the condenser. The critical temperature of N2O is slightly higher than that of CO2, which makes N2O favorable for the flexible selection of condenser working fluid. The triple point of the refrigerant imposes a lower limit in the operating range of the vapor compression refrigeration system; the triple point of CO2 is 5.18 bar, -56.56 °C and the triple point of N2O is 0.88 bar, -90.82 °C as presented in Table 1. The temperature of the triple point of N2O is much lower than that of CO2, which makes N2O favorable for use in LNG plants. Table 1. Thermodynamic properties of refrigerants Refrigerant

MW kg/kmol

Boiling Point ºC

Critical Point P, bar T, ºC

Triple Point P, bar T, ºC

N2O

44.01

-88.5

72.5

36.4

0.88

-90.82

CO2

44.01

-78.4

73.8

31.3

5.18

-56.56

The typical liquefaction process of natural gas at approximately 50 bar can be divided into the following three sections: [11] 1. Pre-cooling section in the vapor phase (-33.2 ~ 26.9 °C), 2. Condensation section (-73.2 ~ -33.2 °C), 3. Sub-cooling section in the liquid phase (-153.1 ~ -73.2 °C). Given that the triple point of CO2 is 5.18 bar and -56.56 °C, CO2 can only be applied for the pre-cooling section. Meanwhile, N2O can be used for both the pre-cooling and condensation sections. N2O is a nonflammable, nontoxic gas, but it needs to be handled with caution as it is classified as an oxidant. Also careful consideration is needed when selecting the material.

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3. Design of N2O-N2O-N2 Cascade Process For the design of the liquefaction cycles, the following assumptions are considered: [12-26]

① The composition of the pre-treated clean natural gas feed is as given in Table 2, which is referred from the normal rich natural gas of North Africa [12].

② The mass flow rate of feed gas is 228 ton/h, which corresponds to approximately 2 MTPA (million tons per annum) of LNG production. The LNG rundown flow rate is 206.9 ton/h.

③ The pressure drop in all heat exchangers (condensers, after-coolers and LNG exchangers) is ignored.

④ The adiabatic efficiency of turbo machinery (compressors and turbines) is 80%.

⑤ The discharge temperature of all condensers and after-coolers is 30°C based on the assumption that 10 °C cooling water and 10 °C temperature approach is applied.

Table 2. Composition of natural gas feed. Component Nitrogen Methane Ethane Propane i-Butane

Mole Fraction 0.04 0.875 0.055 0.021 0.003

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n-Butane i-Pentane Total

0.005 0.001 1

A cascade process of N2O-N2O-N2 refrigerants for natural gas liquefaction is proposed in this paper as shown in Figure 1. A cycle using N2O is applied for the pre-cooling section and another cycle of N2O is applied for the condensation section. N2 refrigerant is applied for the sub-cooling section of LNG. Cycles with N2O refrigerants are vapor compression refrigeration systems and utilize the latent heat of the refrigerant. The N2 cycle is operated in the gaseous phase utilizing only sensible heat. When it compares to the conventional hydrocarbon-based cascade liquefaction process, the coldest N2 refrigerant only recycles to its former refrigeration cycle of Condensation Unit whereas the methane refrigerant goes back to the Pre-Cooling Unit in the conventional process. The produces LNG also works as a make-up refrigerant in the conventional process but the LNG is only produced and stored as an end product in the proposed scheme as depicted in Figure 2. Aspen HYSYS v7.3 is used for steady-state simulation and Peng-Robinson thermodynamic model was used as a property package [10].

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Figure 1. The proposed N2O-N2O-N2 cycle for the natural gas liquefaction.

Figure 2. The conventional hydrocarbon-based cascade process for the natural gas liquefaction.

3.1. The Pre-cooling Section The N2O refrigerant is utilized in the pre-cooling section to cool down both the refrigerant itself and the natural gas to -33°C. The refrigeration cycle is configured as a multi-stage vapor compression refrigeration system with three different expansion stages. Figure 3 shows the process flow diagram of the pre-cooling unit. Streams Pre-1 to Pre-21 are N2O refrigerant streams, whereas NG-1 to NG-4 are natural gas streams. The refrigerant is compressed in the 8 Environment ACS Paragon Plus

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compressor modules Pre-C1, Pre-C2, and Pre-C3. The discharge pressure of Pre-C3 is 65 bar and the refrigerant is cooled and condensed in E-101 to an ambient temperature of 30°C. The first expansion pressure at Pre-5 is 35.57 bar, the second expansion pressure at Pre-10 is 19.46 bar, and the third expansion pressure at Pre-14 is 10.65 bar. The Pre-5 stream cools the NG-1 and Pre-2 streams while the refrigerant evaporates. Similarly, Pre-10 cools NG-2 and Pre-8. Pre-14 exchanges heat with NG-3, the Pre-12 stream and the Con-3 stream of the condensation unit. LNG-109 is a regenerative heat exchanger to cool down Con-2 from 30°C to -14°C at Con-3.

Figure 3. The pre-cooling section of the proposed design.

3.2. The Condensation Section Natural gas is liquefied in the condensation section; see the process diagram depicted in Figure 4. The objective of this section is to cool down the natural gas to -78.6°C. The refrigeration cycle using N2O as the working fluid is also configured as a multi-stage vapor compression

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refrigeration system with three different expansion stages. Streams Con-1 through Con-21 are N2O refrigerant streams, whereas NG-4 through NG-7 are natural gas streams. The refrigerant is compressed in the compressor modules Con-C1, Con-C2, and Con-C3. The discharge pressure of Con-C3 is 12.5 bar and the refrigerant is cooled in E-102, LNG-109, and LNG-102, in series. The boiling temperature of N2O at 12.5 bar is -31.24°C, and Con-4 is in the liquid phase at -33°C. A portion of the refrigerant is expanded in Con-JT1 to 6.17 bar, and it cools the NG-4 and Con-4 streams using latent heat. Second-stage expansion pressure is 3.04 bar while the final stage expansion pressure is 1.5 bar in Con-JT3. The Con-16 stream at -81.61°C is used for the liquefaction of natural gas and cooling of Con-14; it also cools down the Sub-4 stream of the sub-cooling unit.

Figure 4. The condensation section of the proposed design.

3.3. The Sub-cooling Section

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The sub-cooling section in Figure 5 is a gas compression refrigeration system; no phase change of the refrigerant occurs in this section. The N2 gas is compressed in Sub-C1, Sub-C2, and SubC3 to 70 bar, and cooled in E-105 to 30°C. E-103 and E-104 are after-coolers with a discharge temperature of 30°C. LNG-110 is a regenerative heat exchanger to cool down Sub-2 to -64.51°C at Sub-3. A portion of Sub-3 goes through expansion in Sub-X2 and is utilized as the cold stream in Sub-10. Other portions are cooled in LNG-105 in the condensation unit, further cooled in LNG-106 by N2 itself to -124°C and then expanded in Sub-X1 to 18.1 bar and -158.1°C. This stream is used for sub-cooling LNG to -155°C.

Figure 5. The sub-cooling section of the proposed design.

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4. Results and Discussion The simulation results are summarized in Tables 3 and 4. The specific power of the developed N2O-N2O-N2 cascade cycle is 1138 kJ/kg. The efficiencies of other conventional liquefaction processes are shown in Table 5. The performance of the proposed process is further compared with other hydrocarbon based refrigeration cycle as well as nitrogen based refrigeration cycle as depicted in Table 6. The proposed cycle gives enhanced efficiency compared to the existing turbine-based processes with the specific power comparable to the MR processes [27]. It is difficult to compare the liquefaction processes precisely based only on the specific power, since the conditions are not standardized. The specific power of the liquefaction process is considerably case-sensitive, and it varies owing to many conditions, such as feed gas composition, feed gas pressure, the temperature of cooling water in heat exchangers, pressure drop in heat exchangers, and adiabatic efficiency of turbo machinery. The N2O-N2O-N2 cascade cycle proposed in this paper has potential advantages in practice owing to the simplicity and the reliability of the single-component refrigerant system. To help to estimate economic feasibility of the N2O-N2O-N2 cascade cycle proposed in this paper, total capital investment and total operating cost were analyzed by using Aspen Process Economic Analyzer (APEA) v8.8. The system cost was based on 1st quarter in 2014. Each unit process component was mapped with an equipment model for economic evaluation. Because the size of multi-stream plate fin heat exchanger is required to estimate its cost, Aspen Exchanger Design & Rating (EDR) v8.8 was used for the sizing. The estimated costs of major equipment are displayed in Table 7 and Table 8 shows total capital investment. Total operating cost is shown in Table 9.

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Table 3. The results of the N2O-N2O-N2 process. Variable Natural gas feed flow rate LNG production Pre-cooling section N2O flow rate N2O compressor duty Condensation section N2O flow rate N2O compressor duty Sub-cooling section N2 flow rate N2 compressor duty N2 expander duty N2 net compressor duty

Value 228 206.9

Dimension [ton/h] [ton/h]

885.5 17.99

[ton/h] [MW]

271.4 6.38

[ton/h] [MW]

1159 53.6 12.55 41.05

[ton/h] [MW] [MW] [MW]

Table 4. The specific power of the proposed N2O-N2O-N2 process. Variable LNG production Total compressor duty Specific power

Value 206.9 65.42 1138

Dimension [ton/h] [MW] [kJ/kg]

Table 5. The specific power of conventional and proposed liquefaction processes. [27] Process

C3MR

Cascade

DMR

SMR

Single N2

Double N2

Proposed

Specific power [kJ/kg]

1054

1218

1080

1253

3266

1426

1138

Table 6. The performance comparison of the proposed.

Feed gas mass flow, kg/s LNG production, kg/s Liquefaction ratio, %

C3H8-C2H4-CH4 Yoon et al. [10] 158.5 146.2 92.2

C3H8-N2O-N2 Yoon et al. [10] 158.5 147.7 93.2

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N2O-N2O-N2 Proposed 63.3 57.5 90.7

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Total cooling capacity, MW Specific energy, kJ/kg Specific power, kW/kg/h Total compressor power, MW Cycle COP

329.1 1530 0.43 223.7 1.47

325.81 1316.2 0.37 194.4 1.68

113.7 1138 0.38 78.0 1.46

Table 7. Major equipment cost Equipment

Compressor

Cooler

Cold box

Heat exchanger Turbine Separator

ID Con-C1 Con-C2 Con-C3 Pre-C1 Pre-C2 Pre-C3 Sub-C1 Sub-C2 E-100 E-101 E-102 E-103 E-104 LNG-100 LNG-101 LNG-102 LNG-103 LNG-104 LNG-105 LNG-106 LNG-107 LNG-108 LNG-109 LNG-110 Sub-X1 Sub-X2 V-100 Total

Mapped Model

Centrifugal Compressor

TEMA shell and tube heat exchanger

plate-fin heat exchanger

plate-fin heat exchanger Gas turbine Vertical Pressure Vessel

Cost

Group Total

(Million USD)

(Million USD)

1.7 1.8 2.2 1.9 1.9 2.0 7.0 6.4 0.2 1.9 0.1 1.3 2.0 0.4 0.2 1.1 0.3 0.4 0.4 1.7 0.7 0.8 0.1 3.7 1.9 5.5

Ratio (%)

24.8

52.1%

5.5

11.5%

6.0

12.6%

3.9

8.1%

7.4

15.5%

0.1

0.1

0.1%

47.7

47.7

100%

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Table 8. Total capital investment Cost (Million USD) Direct costs Equipment & Material Piping Instrumentation Electrical Insulation and paint Direct Totals

88.6

Indirect costs Construction Expenses Taxes and Permits Engineering Overheads Contract Fee Contingencies Indirect Totals

12.0 5.1 5.0 3.6 3.2 21.3 50.3

Total

51.9 25.5 3.0 7.7 0.5

138.9

Table 9. Total operating cost Cost (Million USD/yr) Utilities Electricity Cooling water Utility Cost

48.8 2.2 51.0

Operating Labor Cost Maintenance Cost Operating Charges Plant Overhead General & Administrative Cost Total Operating Cost

1.2 1.2 0.3 1.2 4.4 59.3

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5. Conclusion For offshore natural gas production, it is known that the use of hydrocarbons as refrigerants poses a safety issue. In this paper, a novel process of natural gas liquefaction for FLNG that was developed using nonflammable refrigerants was illustrated. The conventional processes include nitrogen expander process and its derivatives. Pre-cooling with CO2 or HFC has also been proposed. N2O has some advantages over CO2, because the critical temperature is higher and the triple point temperature is lower. In particular, N2O can be utilized for the liquefaction of natural gas at high pressure. In this research, a cascade process of N2O-N2O-N2 (pre-cooling, condensation, sub-cooling) was designed for LNG production. The proposed cycle was found to be suitable for LNG-FPSO with low specific power comparable to conventional processes.

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■ Appendix. Heat and Material Balance Pre-Cooling Section Stream

NG-1

NG-2

NG-3

NG-4

Pre-1

Pre-2

Pre-3

Pre-4

Pre-5

Pre-6

Temperature, °C

30.0

8.3

-14.0

-33.0

74.6

30.0

8.3

8.3

5.3

8.3

Pressure, bar

65.0

65.0

65.0

65.0

65.0

65.0

65.0

65.0

35.6

35.6

Mass Flow, ton/h

228

228

228

228

885

885

885

364

364

364

Stream

Pre-7

Pre-8

Pre-9

Pre-10 Pre-11 Pre-12 Pre-13 Pre-14 Pre-15

Pre16

Temperature, °C

8.3

-14.0

-14.0

-17.0

-14.0

-14.0

-33.0

-36.0

-33.0

-0.5

Pressure, bar

65.0

65.0

65.0

19.5

19.5

65.0

65.0

10.7

10.7

10.7

Mass Flow, ton/h

521

521

146

146

146

375

375

375

375

375

Stream

Pre17

Con-2

Con-3

Con-4

Temperature, °C

48.5

30.7

84.2

30.0

20.6

30.0

-14.0

-33.0

Pressure, bar

19.5

19.5

35.6

35.6

35.6

12.5

12.5

12.5

Mass Flow, ton/h

375

521

521

521

885

271

271

271

Pre-18 Pre-19 Pre-20 Pre-21

Condensation Section Stream

NG-4

NG-5

NG-6

NG-7

Con-1

Con-2

Con-3

Con-4

Con-5

Con-6

Temperature, °C

-33.0

-47.9

-64.5

-78.6

42.8

30.0

-14.0

-33.0

-47.9

-47.9

Pressure, bar

65.0

65.0

65.0

65.0

12.5

12.5

12.5

12.5

12.5

12.5

Mass Flow, ton/h

228

228

228

228

271

271

271

271

271

71

Stream

Con-7

Con-8

Con-9

Con10

Con11

Con12

Con13

Con14

Con15

Con16

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Temperature, °C

-50.9

-47.9

-47.9

-64.5

-64.5

-67.5

-64.5

-64.5

-78.6

-81.6

Pressure, bar

6.2

6.2

12.5

12.5

12.5

3.0

3.0

12.5

12.5

1.5

Mass Flow, ton/h

71

71

200

200

107

107

107

93

93

93

Stream

Con17

Con18

Con19

Con20

Con21

Sub-4

Sub-5

Temperature, °C

-78.6

-33.6

-50.0

-0.3

-12.6

-64.5

-78.6

Pressure, bar

1.5

3.0

3.0

6.2

6.2

70.0

70.0

Mass Flow, ton/h

93

93

200

200

271

495

495

Sub-Cooling Section Stream

NG-7

NG-8

NG-9

NG10

NG11

NG12

NG13

Sub-1

Sub-2

Sub-3

Temperature, °C

-78.6

-104.0

-124.0

-155.0

-163.1

-163.1

-163.1

110.3

30.0

-64.5

Pressure, bar

65.0

65.0

65.0

65.0

1.2

1.2

1.2

70.0

70.0

70.0

Mass Flow, ton/h

228

228

228

228

228

21

207

1159

1159

1159

Stream

Sub-4

Sub-5

Sub-6

Sub-7

Sub-8

Sub-9

Sub10

Sub11

Sub12

Sub13

Temperature, °C

-64.5

-78.6

-104.0

-158.1

-127.0

-64.5

-127.0

-127.0

-115.7

-81.6

Pressure, bar

70.0

70.0

70.0

18.1

18.1

70.0

18.1

18.1

18.1

18.1

Mass Flow, ton/h

495

495

495

495

495

664

664

1159

1159

1159

Stream

Sub14

Sub15

Sub16

Temperature, °C

25.6

104.8

30.0

Pressure, bar

18.1

35.6

35.6

Mass Flow,

1159

1159

1159

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Acknowledgment This research was supported by the Brain Korea 21 Plus Program in 2015, by Institute of Chemical Processes in Seoul National University, by MKE and grant from the LNG Plant R&D Center funded by the Ministry of Land, Transportation and Maritime Affairs (MLTM) of the Korean government, by the IT R&D program of MOTIE/KEIT (10049155, Development of equipment control algorithm based on plasma monitoring for efficiency improvement of 10 nm etch process), by Engineering Development Research Center (EDRC) funded by the Ministry of Trade, Industry & Energy (MOTIE). (No. N0000990), by a grant (14IFIP-B085984-02) from Smart Civil Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport(MOLIT) of Korea government and Korea Agency for Infrastructure Technology Advancement(KAIA), by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20132010201760), (No. 20132010500050), and (No. 20152010201850).

Abbreviations and Acronyms FLNG: Floating liquefied natural gas FPSO: Floating production, storage and offloading HFC: Hydrofluorocarbon LNG: Liquefied natural gas MR: Mixed refrigerant TCF: Trillion cubic feet

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