Thermodynamic analysis of extraction processes for the utilization of LNG cold energy

Thermodynamic analysis of extraction processes for the utilization of LNG cold energy G.S. Lee, Y.S. Chang*, Department Kyungnam, *Department Korea Received

of Mechanical Korea of Mechanical II April

1995;

M.S. Kim* Engineering,

University

Engineering,

revised

and S.T. Ro*

12 June

Seoul

of Ulsan, National

Ulsan

University,

680-749, Seoul

151-742,

7995

Thermodynamic analysis of the extraction processes from a constant pressure LNG (liquefied natural gas) vessel was performed in this study. LNG was assumed to be a binary mixture of 90% methane and 10% ethane by mole fraction (83:17 by mass fraction). The changes in the thermodynamic properties and the amount of utilizable cold energy were predicted during the extraction processes. Both vapour and liquid extraction processes were investigated using a computer model. During vapour extraction, the temperature of the LNG in the vessel increases dramatically, and the extracted vapour composition of methane decreases rapidly near the end of the extraction process. Utilizable cold energy has a maximum at a residual mass ratio of about 0.2. It was found that the temperature gradient due to the vapour composition change had a major effect on the behaviour of the cold energy during the vapour extraction process at a constant pressure. During the liquid extraction process, the changes in the thermodynamic properties and utilizable cold energy are negligible. When the pressure of the vessel increases, the total cold energy which can be utilized from LNG decreases. Keywords: LNG; extraction

processes; cold energy

Nomenclature Enthalpy ( kJ ) Specitic enthalpy I kJ hg ’ I Mass (kg) Residual mass ratio Rate of mass change in the vr~el Extracted mass How rate t kg , ’ ) Pressure ( kPa) External cold energy per unit time Absorbed cold energy to maintain pressure per unit time (kW) Total cold energy per unit time t = tkW) Quality Absorbed cold energy per unit ttmr extracted mas\ fou r;rtc (= 0 ,/~ir,.)

(kg

Total cold energy per unit time per extracted mass flow rate (= (i,,,,lti,) Temperature (“C, K) Time (s) Internal energy (kJ ) Volume of the vessel (m’) Specific volume ( m3 kg- ’ ) Liquid mass fraction Vapour mass fraction Overall mass fraction

7‘ I l I’ 1 \ \

s ’ )

(k-W) constant OX + 6,. )

per

unit ’ I

The use of foh\il fuels as cncrgy resource\ causes ;I serious air pollution problem. and the cm~s~~on of CO, gas as ;I combustion product is huou II to ha\ c ;I major effect OII global warming. The LIW of energy resources wjith le\s 01.

kg-‘)

Subscripts 1

( kJ kg

unit (kJ

g c\ I 7

Liquid phase Vapour phase Control volume More volatile component Le\a volatile component

(methane) (ethane)

no

tntluencc on air pollution is required, and the rational of available energy is recommended. Liquefied natural gas (LNG) is known as a clean energy \(lurcc H’hich i\ commonly used as domestic and industrial LIW

Cryogenics

1996 Volume 36, Number 1 35

Extraction

processes

for utilization

of LNG co/d energy:

fuel for combustion. LNG is composed of X5-W% methane by mole fl-action. a few percent ethane, and propane depending on its production site. Since moisture and sulf‘tu are contained in crude natural gases. they should hc removed during the liquefying process’. LNG is transported and stored at atmospheric pressure and at a temperature of about -160°C. In a real application. LNG should be vapor-i/led in order to be supplied as a natural gas, which is the final form of the fuel as supplied. During the vaporiration process. latent heat of vaporization and any sensible heat required to superheat the \apour should be supplied to the LNG. These are frequently termed ‘cold energies’, and generally are supplied by sea water. which would otherwise be ;I source‘ of useful energy. It LNG is used as a fuel in cogeneratlon systems. the waste heat of exhaust gases and the cold energy of LNG can he utilized at the same time. We intend to focu\ on the cold energy utili7ation. During extraction of LNG from a storage vessel. the ternperature and extracted gas composition may change. and the amount of utilizable cold energ) may vary since LNG is a multi-component mixture. Therefore. there is a strong need for the prediction of temperature variations. ccmposition changes. and utilizable cold energy variations during extraction of LNG. In previous studies, an evaluation of the boil-off ratio 111 a large storage vessel was performed’ and safe designsof the storage system were discussed’. The isothermal and adiabatic leak processesof zeotropic refrigerant mixtures were simulated by Kim and Didion” to show the composition changesduring leak processes.As for the utilization of LNG cold energy, Acker t’t (11.~has suggesteda dual usageof LNG, i.e. LNG ser\.esas a tnarine engine fuel and its cold energy is utilized to chill shrimp. In this paper, thermodynamic proper-t) changessuch ;I\ temperatureand composition. and cold cnergk I ariatlons 111 a storage vessel during vapour or liquid extraction processes.are simulated. In both cases.I,NG i\ assutnedto hc under a constant pressureand heat should be ~uppl~c~lto the storage vessel to maintain this pre\surc; this heat I\ defined as ‘absorbed cold energy’. Extracted lapour or liquid absorbsheat and then becomessuperheatedIO ;I certain temperature. which is selectedas 0°C for a refercncc: this heat is defined as ‘external cold energy’. The total cold energy which can be utilized during this processis the \um of these two heats.

Modelling of the extraction storage vessel

process

from

a

Schematic diagramsof the modrlled systemsfor the extrastion processesare shown in F‘i,ylr~ Irr for vapour extraction, and Figure Ih for liquid extraction. The system is composedof a constant-volume vessel with a small hole through which LNG is extracted either as a vapour or ;I liquid. The extraction processis assumedto be isobaric d the LNG in the vesselis assumedto be at a vapour-liquid equilibrium state. Pressure was taken at 300, 600. and 2000 kPa to show the effect of pressureon the extraction process.Thermodynamic propertie? of LNG are estimated using the modified Carnahan-Starling-DeSantis equationof-state and the appropriate mixing rules“. It is assumedthat the extracted vapour hasthe samecon1position as the vapour inside the vessel during ~apout-

36

Cryogenics 1996 Volume 36, Number 1

G.S. Lee et al. Externalheatexchanger

Externalhe&kxchanger (b)

(a)

Figure1 Schematicdiagramof isobaricextraction processes; (a) vapour extraction, (b) liquid extraction extraction. Similarly, for the liquid extraction process,the composition of the extracted liquid is assumedto have the samecomposition as that of the liquid inside the vessel. In analyzing the extraction process,it is assumedthat the vessel is initially tilled with a mixture of methaneand ethane (overall composition of 90/10 by mole fraction) and that the volume of the saturatedliquid occupies90% of the total volume of the vessel, which is shown in the following equation:

Simulation of the extraction processeswas conducted with a quasi-steady-stateassumptionwith finite time increments. The simulation was done by extracting a minute massfrom the vesseland then readjusting the state of the LNG in the vesselto a new thermodynamic equilibrium, accounting for the massloss. Becausethe total massof the ith component in the vessel is the sum of the massesof the component i in both the liquid and vapour phases,the overall massfraction of the ith component in the vessel is expressed in the follmving: ;, = I,( 1-q)

(2)

+ ?‘,y

where .x, and y, are the massfractions of the ith component in the liquid and vapour phases,respectively, and zi representsthe overall massfraction of the ith component. q is the quality defined by the massof the vapour phasedivided by the total mass in the system. The specific volume of I-NC; in the vesselbecomes ,’ z

1’

(3)

I)1

u here V is the volume of the vessel. First. a vapour extraction processis considered where a small amount of vapour. Am, is extracted from the system. Since only vapour is extracted, the extracted massof the ith component is hm, = Am,, = (am)~,. and by defining a parameter E= Atnltn as the fraction of the extracted mass with respect to the total, a new overall massfraction at a new quasi-steady-stateof LNG in the vessel is expressed as follows: ._’ , =

\,1t71 + v,m,--hnr, _

tt7-Am

.I-,( 1-y )+\‘,q--E??, l--E

(4)

The specific volume after the extraction is representedas

Extraction

processes

for utilization

Results After a small amount of LNG is extracted, the new composition and specific volume of LNG are determined by Equations (4) and (5). respectively. After updating the state of LNG in the vessel, the above procedure is repeated until q = 1. For the case of a liquid extraction. the extracted mass ot the ith component Am, is equal to Am,,. which, in turn, is equal to (Am ).u,by the definition of the liquid mass fraction. Overall composition after the liquid extraction i\ expressed as x W11’?y7+Vl, .,’ = 2 +,

-z-m-

n1-Al?7

.U,( I-y

)+J,y-t\, l-t

The same procedure as described above 14 u\rd for the v ap our extraction. The absorbed cold energy r detined previously I ia represented as

Q, = $ + tj1,h,

(71

The first term represent\ the rate of cnthalpy change with a constant volume and pressure within the vessel, and the second term represents the release of enthalpy via Inas\ flow. The absorbed cold energy per unit time per unit ma\\ flow rate is rearranged as follow\: (hl

where h, is the specitic enthalpy of the extracted LNG. i.c for the vupour extraction, 17, is equal to h,. and. for the liquid extraction, h, is equal to 17,. Therefore. it is \traighrforward to assume that the cold energy per unit mass flo\r is closely related to the residual mass ratio. The reaidurl mass ratio. m,. is defined as the residual mass divided h! the initial mas\ in the LNG vessel. The thermodynamic propertie\ of LNG t ov~erull Compaqsition, temperature, quality, 5pecitic volume, internal energy, enthalpy, etc. ) change during the extraction proct’\\ because LNG is a non-azeotropic mixture. The ub\orhcd cold energy 14 expre\$ed by using the thermodynsnrr~ properties a4

of LNG cold energy:

G.S. Lee et al.

and discussion

The vapour-liquid equilibrium diagram, at three different pressures, is shown in Figure 2 as a function of overall mass fraction of methane in the vessel. The end points of the extraction paths represent the state where the vessel is only tilled nith the saturated vapour (quality of 1). In FigUI’~ 2. the relationships between the temperature, overall composition and liquid and vapour compositions at specitied pressures, are shown. The overall composition of methane and the specific volume of the mixture in the vessel are calculated from the extraction model in this study. In the extraction process at a lower pressure, the temperature difference between the upper dew line and the lower bubble line becomes greater, and the property changes during the extraction process are rather significant. The changes dur~ng vapour extraction are greater than those for liquid extraction.

Vapour

extraction

In I.‘i,q~r.c~.J. the changes in the thermodynamic properties and the utili/.able cold energy during the vapour extraction process are presented as a function of residual mass ratio IU, for a pre\sure of 600 kPa. Negligible changes are found until the residual mass ratio becomes about 0.25, while dramatic changes occur when the residual mass ratio is less than 0.25. In Fig74re 3~. for the composition changes, the more volatile component, methane, tends to evaporate easily even at a lower temperature. and occupies most of the \ apour phase until the residual mass ratio becomes 0.25. .\ftcr reachtng a mass ratio of 0.25. however, a higher tem~~CI';I~LII-C is required in order to extract a unit mass of the \apour at a constant pressure. Consequently. the vapour c~ompositions of methane decrease dramatically due to a preferential evaporation of the more volatile component a11d an extraction of the vapour phase. in which the main ~~on~ponenti\ methane. The specific volumes for the vapour
Liquid extraction

I 9 J

where l.c. i\ the specifc \olurnc ot the extracted LNG. Lor convenience. each term in Equatton t c)) is dehned a\ y,,,,,<. respectively. It i\ known that the absorbed 9f.hdill. und ~~~~~~~~~~~ cold energy is dependent not only on thc\e properties hut also on the rate of change of these properties with respccr to the mass in the vessel. When extracting a pure \ubstan~e. the second and third terms in Equation I 9 ) arc /era because of constant specilic volumes and cnthalpie\. i.e. dX/dtll = 0 (X can be I’(. I'?. 17,. or 17, ). md absorbed cold energ: i\ equal to c/,,~~,~.. a non-rero con4tant

~160 0.0

I 0.2 Mass

Figure2

vapour librium u res

I 0.6

1 0.4

fraction

of

I 0.8

1.0

methane

Mass fraction changes (thick solid lines) during the and liquid extraction processes, and vapour-liquid equidiagram of methaneiethane mixture at different press-

Cryogenics

1996 Volume

36, Number

1 37

Extraction

processes

for utilization

of LNG

cold

G.S. Lee et al.

energy:

cl.005 -60

0.004

G e -80

-100

0.8

0.6

04

Residual

0.2

mass

0.003

2 5

L

E c”

-120

1 .o

“E

2 2 ;;; tp

1.o

0.0

J 0.001 0.8

ratio

0.6

0.4

Residual

0.2

0.0

ratio

mass

(b)

I

600

d

t

0' 0.8

1 .o

0.4

0.6 Residual

mass

00

0.2

I 0.8

1 .o

I 0.6

Residual

ratio

CC) Figure3 Changes methaneiethane

sitions

and

in (90/10

temperature,

the thermodynamic by mole fraction)

(b)

specific

I 0.2

I 0.4 mass

0.0

ratio

Cd) properties mixture at 600

volume

and kPa

of vapour

and

the utilizable (the starting

liquid,

cold point

energy of extraction

(c) contribution

during the is a residual

of each

term

vapour mass

in Equation

extraction ratio

(9).

process

of I); (a)

(d)

of

compo-

utilizable

cold

energy

per unit extracted mashRou rate at constant presaurc.4,. are presentedin Figlr~ 3~,.In order to investigate the variation, it is convenient to analysethe three terms in Equation (9). The first term. qpulc.i:, the latent heat contribution. which gives a constant value throughout the extraction process. The secondterm. y d,c ,,,,. give\ little contribution to 4 ,. And the last term, 4dhd,,,.maintainsit \,alue of 30-30 k.l kg ’ until the residual massratio becomes0.3. and then has a maximum value near171, = 0.2. The term 4,,,,<,,,, hasthe Freatest influence on the existence of the maximum value of y ,. Neglecting the secondterm. onI4 two term\. 4,,,,,(, and 4c,,,,,,,, remain. and since Y,,~,,~doe\ not contain derivati\c\ of. properties. it is easily supposedthat y,,L,,cgive\ the only constant contribution to y,,. Thus. the term y~,,,J,,,.which includes derivative5 of enthalpieh. contributes to the maximum value of y:,. It is \uppo5ed that the temperature gradient due to the vapour composition change mahes a major contribution to 4 Xbecausethe enthalpy of the 1apoutis a strong function of temperature. Thih supportsthe fact that the variation of the absorbedheat at constant pre\\ure is similar to that of the temperature gradient of the \ap our phase. The total cold energ!, L/,,),.ab\orbed cold energ?, 4 ,. and external

cold

energy.

L/, . art‘

presented

;14 ti function

of

residual mash ratio in Figlrr.cz .qtl. The hehaviour c~f the external cold energy. which i\ related to the tcmpt~raturc difference between the xy\tcnl and the c‘n\ironmt~nt, 14

38

Cryogenics

1996 Volume 36, Number 1

expected to depend strongly on the temperaturein the vessel, and thus can be observed to decreasesharply in the vicinity of the residual mass ratio of 0.2. Since the total cold energy is the sum of both the absorbedcold energy and the external cold energy, y,,), hasa nearly constantvalue until the residual massratio is equal to 0.3, when it reaches It\ maximum value, and finally approachesthe asymptotic value correspondingto the total cold energy of pure ethane. Liquid

extraction

In F‘iglrr-e1 the changesin composition, temperature, and utilizable cold energy are shown during the liquid extraction as a function of the residual massratio m, at 600 kPa. In contrast to the vapour extraction process, temperature, \‘apour and liquid compositions did not change significantly. and the utilizable cold energy was uniform throughout the processbecausethere was no intense evaporation when the liquid was extracted. In this process,only a small amount will evaporate to till the space left by the extracted liquid. In F~~~uw4~. the overall composition increasessteeply when the residual massratio. 1~1,. is less than 0.2. which indicates that methane is the major component in the LNG \w\el at the end of the extraction process. The temperature change is negligible. as shown in Fig~r-e 4u, and the specItic

volumes

of

liquid

and

\-apour

are

almost

constant

Extraction

processes

for utilization

of LNG

10 Residual

mass

cold

0.8

ratio

energy:

0.6 Residual

0.4 mass

G.S. Lee et al.

0.2 ratio

Figure 5 Composition change in the extracted mixture ferent pressures; (a) during the vapour extraction, (b) the liquid extraction

ti ! 0 1.o 0.8

+ 0.6 Residual

04 mass

02

00

ratio

(h)

Changes in the thermodynamic propertles and the utilizable cold energy during the liquid extraction process of methane/ethane (90110 by mole fraction) mixture at 600 kPa (the starting point of extraction is a residual mass ratio of 1); (a) compositions and temperature, (b) utilizable cold energy

Figure4

throughout the liquid extraction: thl\ can he explained bb the fact that the composition and temperature changesarc negligible. The total cold energy. L/,,>,.absorbedcold energy. (1,. and external cold energy. ~1,. during the liquid extraction process,are presentedas a function of the residual massratio as shown in Figlr~ lh. The ma.ioritl of the total cold energy is external cold energy, so the abhorbedcold energq to maintain a constant pressurein the ~e\sel was negligible in the liquid extraction process.

0.0

at difduring

ure. even the lessvolatile ethane evaporatesintensively, so the fraction of ethane in the extracted mixture increases; the extracted composition has a reduced methane fraction before ~1,= 0.2, and therefore methane is depleted less at the sameresidual massratio. Beyond this point, the relative amount of methane in the extracted mixture is more for higher pressures.Contrary to the vapour extraction case, during liquid extraction at higher pressure, the extracted liquid contains more ethane becausethe evaporation rate of ethane is increased at higher pressures.However, the composition variation with pressureis negligible. The temperature changeswhich correspond to the composition changes during the vapour and liquid extraction processesare shown in Figure 6. The temperature gradient about ~1,= 0.2 is larger at lower pressures,and, asdiscussed previously. hasthe main effect on the variation of utilizable cold energy. The changes in the absorbed. external, and total cold energy are shown in Figure 7 for the vapour extraction process. AI a lower pressure, the difference between the maximum value and the average value of the absorbedcold energy is greater than that at higher pressuresbecausethe temperature gradient is greater at a lower pressure.As the external cold energy is greatly influenced by the vesseltemperature, its value becomes smaller at a higher pressure when the saturation temperature is high, which lowers the driving temperature difference for the heat transfer from the environment. In other words, the smaller temperature dit‘ferencebetweenthe saturationtemperatureand the ambi-20 I

Pressure

effect

on vapour

and

liquid

~~

extraction

As shown in Figure 2. when the pre\\ure I\ IOU the path of the overall composition change approachesthe bubble line. Since methane ij more volatile. it claporates much faster than ethane. the les\ volatile component. at a IOU temperature when the pressurei\ low. Therefore. at bubble points for the sameoverall composition. the vupour composition of methaneat a lower pre\xure is much greater than that at a higher pressure.When the ~apour i\ extracted. the vapour composition of methanein the Besselreduce\ more remarkably at lower pressure\ compared to that at highrt pressures.As a result. the overall composition of methane decreasesand its extraction path move\ clo\er to the bubble line. Thi\ phenomenacan he explained in F’i,yri/-r,5 which showsthe extracted composition change. At ;I higher pre\\-

vapor extractlon liquid extraction

-40 u L

-60

-1601

1.o

I 0.8

I 0.6 Residual

Figure6

residual cesses;

mass

I

I

0.4

0.2

o'.O

ratio

Temperature changes in the vessel with respect to mass ratio during the vapour and liquid extraction pro(a) P= 200 kPa, (b) P= 600 kPa, (cl P= 2000 kPa

Cryogenics

1996 Volume 36, Number 1 39

Extraction Table

1

processes

Saturated

P (kPa)

for utilization

properties

of LNG cold energy:

of methane

and ethane

and absorbed,

Component

TI’C)

“1 (m3hl

200

Methane Ethane

-152.67 -74.85

0.002421 0.001898

0.2922 0.2581

600

Methane Ethane

-134.33 -47.46

0.002613 0.002049

2000

Methane Ethane

-107.12 -6.96

0.003071 0.002411

‘)

“g (m3k!3

G.S. Lee et al. external

‘) h IkJ kg ‘I

and total

cold

energies

in the vapour

extraction

h, (kJ kg ‘)

qA (kJ kg-‘)

qE (kJ kg-')

qtot (kJ

-814.90 -590.47

-331.47 -123.16

487.47 470.77

328.42 117.83

815.89 588.60

0.1035 0.09075

-784.49 -519.56

-311.08 -98.47

448.75 430.81

301.92 81.90

750.67 512.71

0.02989 0.02600

-650.45 -402.78

-311.85 -82.67

377.37 352.59

281.24 17.15

658.61 369.74

process kg-')

J

200

Conclusions

000

Thermodynamic analysis of the vapour and liquid extraction processeswas carried out for the LNG, which is assumedto be a binary mixture of 90% methaneand 10% ethane by mole fraction. Initially, the liquid volume in the cesselis assumed to be 90% of the total volume of the vessel. During vapour extraction, large changesin temperature, composition, and specific volume were found, owing to the preferential evaporation of the more volatile component. The extracted composition, until the residual mass ratio becomes0.2, is mainly methane,and beyond this point ethane is a major component in the extracted vapour. The property changes during the liquid extraction processwere negligible throughout the process. The extracted mixture contains less methane for higher pressuresfor both the liquid and vapour extraction processes. The utilizable cold energy of LNG was predicted during the extraction processes.In the vapour extraction process, the absorbedcold energy is greater than the external cold energy, and the total cold energy. which is the sum of the absorbedand the external cold energies. has a maximum for a residual massratio of about 0.2. On the other hand, the external cold energy was the major sourceof utilizable cold energy for the liquid extraction processes.The total utilizable cold energy decreasesat higher pressuresfor both the vapour and liquid extraction processes. In practice, the utilizable cold energy and thermodynamic properties can be predicted if the temperature of the mixture in the vessel and the cumulative mass of the extracted mixture are measured.

800 600 400 200

1.0

Figure 7 Absorbed, tion of residual mass

I 0.8

I 0.6 Residual

I 0.4 mass

I 0.2

I 00

ratio

total and external cold ratio during the vapour

energies extraction

as a funcprocess

ent temperaturetends to reduce the utilizable external cold energy. The total cold energy is representedby the \um of the absorbedand the external cold energy. In Figure 8, the behaviours of the absorbed. external. and total cold energy are shownduring the liquid extraction process.Since most of the total cold energy is external cold energy, and the external cold energy is reduced by an increasein the vesseltemperature,the total and the external cold energy are decreasedat higher pressures. For verification of the results, we comparedthem to the saturatedproperties and cold energiesof pure methaneand ethane as shown in Table 1. When comparing the re\ulth with the values of the table, it can be verified that at the initial stage the properties and cold energies of LNG are similar to those of pure methane. and in the final stageare similar to those of ethane.

1000,

1

Acknowledgement This work has been sponsoredby the Turbo and Power Machinery Research Center (TPMRC) at Seoul National University in Korea, and the authors appreciatethe helpful support from this centre.

References I 2

7 i w ,I~(b) ,A --(a)

1

/- - ~.

Ot---‘---b;/--l---_~---i

00

5

Figure 8 Absorbed, total and external cold energies tion of residual mass ratio during the liquid extraction (a) P= 200 kPa, (b) P= 600 kPa, (c) P= 2000 kPa

as a funcprocess,

6

40

1

1.0

Cryogenics

0.8

1996

0.6 Residual

Volume

0.4 mass

0.2 ratio

36, Number

Lee. J.H. Natural gas industrk of Korea Proc Thermal and Fluid hi’,wrrrng Con/ Korran sot Mech Engr ( 1994) 1 - 13 Chun. B.I., Kim, Y.M. and Kim, K.K. A study on the computation of hull temperature distribution and boil-off ratio of MRV type LNG carrwr 7‘rans Korecm Sot Mech Engr ( 1994) 18 986-996 Fulford, N.J. and Slatter, M.D. Developments in the safe design of I.NG tanks Cn;ogenics (1988) 28 810-817 Kim. MS. and Didion, D.A. Simulation of isothermal and adiabatic leah procewes of zeotropic refrigerant mixtures WAC & R Research ( I995 1 1 3-20 Acker. Jr. G., Brett, C.E., Bell, S., Midkiff, K.C. and Song, Y.K. txpcncncc using LNG as a marine engine fuel Marine Tech& Sot I (1989) 23 33-39 Kim. M.S., Kim, T.S. and Ro, S.T. Estimation of thermodynamic proprrtles of refrigerant mixtures usmg modified Camahan-Starling quatlon of state Trtrns Korean Sock Mech Engr (1991) 15 2189-2205