BOG Handling Method for Energy Saving in LNG Receiving Terminal

21st Europeean Symposium m on Computer Aided Process Engineering – ESCAPE 21 E.N. Pistikoopoulos, M.C. Georgiadis G and A.C. Kokossis (Editors) © 2011 Elsevier B.V. All rights r reserved..

BOG Handlin H ng Methood for En nergy Sa aving in LNG Receivving Terrminal Chansaem m Park, a You ungsub Lim,a Sangho Leee,a Chonghun n Han a a

School off Chemical and d Biological E Engineering, Seoul S Nationa al University, SSan 56-1, Shillim-doong, Kwanak-g gu, Seoul, 1511-742, Korea

Abstractt Generatioon of Boil-offf gas (BOG) in liquefied natural n gas (L LNG) receivinng terminal affects connsiderably operating energyy costs and saafety issue. For that reasonn, the BOG handling method m is deeterminant forr design of LNG L receivin ng terminal. This study proposes the concept of new desiggn for BOG handling and calculates the design u sensitiv vity analysis ffor minimum send-out casse. This desiggn provides variables using 21.9% eneergy saving an nd 0.197y paybback period. Keywordss: LNG Recceiving Term minal, BOG, Re-condenser, Cryogeniic Energy, Sensitivityy Analysis.

1. Background The LNG G receiving terrminal receivves LNG from m carriers to store s in storagge tanks as liquid conndition. In order to supplyy to customerr as gas, LNG is vaporizeed through vaporizatioon process an nd then delivvered as the natural n gas in nto the downnstream gas pipeline neetwork as sho own in Figuree 1(Park et al, 2010). Becau use the LNG aabsorbs the heat in thee tank, unload ding arms andd cryogenic piipe lines, the natural n gas coontinuously evaporatess from the LN NG due to heaat leakage. Th his vapor is called boil-off ggas (BOG). It can causse safety problems that too much BOG generate g insidee a storage tannk, whereas overtreatinng of BOG can c consume excess energ gy. Thus prop per handling oof BOG is important design factor in LNG receiiving terminal.

gure 1. Processs Flow Diagrram of Base Case C Fig The existting BOG han ndle methodss are re-cond densation and direct comprression (E. Querol et al, 2010). Thee former is thaat BOG comp pressed to arou und 10bar thrrough BOG

C C. Park et al

1830

w enough ssend-out LNG G in re-condeenser to obtaain a liquid compressoor is mixed with mixture, thhen the pressu ure is increaseed through hiigh pressure (H HP) pump annd then it is G is compresssed to pipelinne pressure vaporized by sea waterr. The later iss that the BOG m c hen go to pip peline with nnatural gas. than 2 compression stages and th through more Generally direct comprression methood has higher operating cost than re-conndensation. G receiving terminal t m use combined method of re-ccondensation and direct Most LNG n Figure 1, BO OG from storage tanks is compressed c compression. As shown too 10bar and NG goes to re-condenser annd then the then a porrtion of BOG possible to l iquefy by LN h pressure and vaporized. The rest iss compressed to pipeline mixture iss pumped to high p rectly.. This pressure thhrough high pressure (HP) compressor and transport to pipeline dire b o compare new w design casse. Most of case is deetermined as base case in this study to operating energy in LNG G receiving iss used by 5 eq quipment in Fiigure 1. Tablee 1 presents c of base case. c operating costs perating Cost of o Base Case Table 1. Op U Unit

BOG Coompressor mpressor HP Com mp HP Pum mp LP Pump Sea Watter Pump Sum

Operating O Cost(kW) C 1333.52 433.69 612.37 62.09 41.93 2483.60

gy Saving 2. Analyssis for Energ mperature of ree-condensation n is decreased d, additional B BOG can be In base caase, if the tem condensedd by re-condeenser and opeerating pressurre of re-condenser can be decreased. This studyy proposes neew design thaat utilize cold stream after HP pump to cool down i Figure 2. Thhis method prrovides lower operating preessure in reBOG streaam as shown in B nse. It can reduce r condenser and more BOG flow raate to conden energyy of BOG mpressor. compressoor and HP com

Figure 2. Process Fllow Diagram of Proposed Method

BOG Handdling Method for Energy saaving in LNG Receiving Terrminal

1831

3. Case Study S In order too verify propo osed design annd obtain optim mal design vaariables, the caase study is carried ouut for minimu um send-out ccase. The reasson for choice of minimum m send-out case is thaat the design variables v shouuld be specifieed considering g the worst casse. In LNG receiving terminal, t the worst w case to ccondense BOG G is minimum m send-out casse. 3.1. Assum mption Total opeerating costs involve BOG compressor, HP compresssor, HP pumpp, LP pump and sea water w pump. In n design of heeat exchangerr for exchangee between higgh pressure LNG and BOG, B the min nimum approaach temperaturre is assumed 10Ȕ. 3.2. Sensittivity Analysiss In this sttudy, 2 desig gn variables is used as manipulated m variables. v Areea of heat exchangerr and re-condeensing pressurre are specified d using sensitivity analysis..

4. Resultts Figure 3 and a Table 2 present p operati ting cost when n the re-condeensing pressurre is 10bar. In this figgure and tablle show effeccts of area of o heat exchan nger on operrating cost. Proposed designs decreease operatingg cost of HP compressor c co omparing withh base case by condennsing more BO OG. Also the m more area of heat exchanger is increasedd, the more operating costs is decreased. But, beccause the min nimum approach temperaturre violation t large areaa of heat exchaanger, increasse of area cannot reduce thee operating occurs in too costs.

Figurre 3. Operatin ng Energy Prroportion at 10 1 bar Table 2. Op perating Energ gy at 10 bar 2

7m

2

9m

2

11 1m

2

13m m

22

15m

Base

BOG Compp(kW)

133 33.516

13333.516

1333.5 516

1333.51 16

1333.5166

1333.516

HP Comp(kkW)

120 0.2301

55.733304

0

0

0

433.6914

HP pump(kW W)

626 6.3512

629.22263

631.71

629.988 87

628.53077

612.3708

LP pump(kW W)

62.09

662.09

62 2.09

62.0 09

62.099

62.09

C . Park et al

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SW Pump(kkW)

41.82164

41.799765

204 41.772

27 41.7812

41.784933

41.9344

Sum(kW)

218 84.009

21222.362

2069.0 088

2067.37 76

2065.9211

2483.602

a Table 3 prresent operatinng cost when the area of heeat exchanger is 23m2. In Figure 4 and ow effects of rre-condensing g pressure on operating cost st. Decrease this figuree and table sho ure reduces opperating of BOG compresssor and LP puump. But it of re-conddensing pressu ncreases sharp ply operating cost of HP reduces BOG quantity possible to coondense. It in compressoor in less than a certain presssure.

Figu ure 4. Operatiing Energy Proportion P at 23m2 Operating Enerrgy at 23m2 Table 3. O 10bar

9..5bar

9bar

8.5baar

8barr

Base

133 33.516

12933.538

1251.9 935

1208.54 45

1163.1788

1333.516

HP Comp(kkW)

0

0

0

40.7190 06

151.91811

433.6914

W) HP pump(kW

4.2873 624

628.99881

811 633.68

37 635.473

632.45977

612.3708

W) LP pump(kW

62.09

58.433764

529 54.785

94 51.1329

47.480588

62.09

SW Pump(kkW)

41.79552

41.899345

955 41.99

27 42.1142

42.250977

41.9344

Sum(kW)

61.688 206

20222.857

397 1982.3

85 1977.98

2037.2877

2483.602

BOG Compp(kW)

Sensitiivity analysis result is preseented in Tablee 4. The optimal pressure is calculated e at each area of heat exchanger. Thhe area of heatt exchanger afffects capital ccost on on et al, 2006. The electricitty price is assu umed as 0.04882$/kWh equatioon in S.G. Yoo Korea. The opttimal variablees are at 27m2 and 8.5 consideering electricitty market in K r % of operatingg cost compariing with base case. c bar. It reduces 21.9% Table 4. Ressults of Sensitivity Analysis HX Areaa (m2) 5

mal Optim Pressure((bar) 11.5

Consuumption (kkW) 21991.1

Saving S (kW) 292.5

X Cost($) HX 36419.7 3

Payback PPeriod(y) 0.295

BOG Handdling Method for Energy saaving in LNG Receiving Terrminal 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35

11 10.5 10 9.5 9.5 9 9 9 8.5 8.5 8.5 8.5 8.5 8.5 8.5

2147.1 2108.5 2069.1 2053.0 2026.9 2014.4 1984.2 1983.2 1978.0 1955.6 1939.0 1938.4 1937.9 1937.6 1937.6

336.4 375.0 414.5 430.6 456.7 469.2 499.4 500.4 505.6 528.0 544.6 545.2 545.7 546.0 546.0

37358.9 38247.7 39099.4 39922.1 40720.9 41499.7 42261.2 43007.7 43740.7 44461.7 45171.8 45871.9 46563.0 47245.6 47920.4

1833 0.263 0.242 0.223 0.220 0.211 0.210 0.200 0.204 0.205 0.200 0.197 0.199 0.202 0.205 0.208

5. Conclusion This study proposes cooling energy of high pressure LNG is utilized to cool down BOG stream. Proposed method provides reduction of operating energy. When area of the heat exchanger is 27m2 and re-condenser is operated in 8.5bar, result of the case study is the most desirable. This method provide 21.9% energy saving and PBP is 0.197y.

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