——... .. —. ._ ___ ..

THERMODYNAMIC

+4s EH’J

CONSIDERATIONS ON A BOIL-OFF GAS”””““”” ‘“““’ +RECOVERY SYSTEM

CONSIDERATIONS THERMO-DYNAMIQUES SUR LE SYSTEME DERECUPERATION DU GAZ D’EVAPORATION (BOIL-OFF)

D. Lanzi and G. Vareschi

Q$“

Tk

SNAM S.p.A., Italy

$($%

$ ‘~~’O. Sguera

\ \k5 SNAMPROGETTI S.p.A., Italy‘-:;,, ,$ff;a “

ABSTRACT

The Panigaglia LNG regasification terminal is equipped with a system recovering the boil-off gasproduced during both the unloading and the regasification phases. The system consists of acompression station and an absorption column that condenses the boil-off by means of two packingbodies, made of INTALOX METAL ring-type, DN 1“ or 2, depending on the quantity of boil-offavailable. The above configuration, which was built during the 1991 terminal revamping, was designedto optimise the previous plant facilities and compression works by compressing the boil-off up to 30bar. At present, to reduce the operating costs deriving from the electric power consumption due tocompression of the gas to high pressures, the possibility of running the column at a pressure less than25 bar has been evaluated.A new processing solution has been studied. This configuration permits using the LNG coming fromthe low-pressure pumps and therefore an LNG temperature of 15°C less than the one used in theoriginal system. The maximum boil-off rate that can be absorbed has been calculated at differentoperating pressures. The thermodynamic (material and energy balance) behaviour of the column hasbeen checked and the thermodynamic studies and statistical analysis have given satisfactory andconsistent results. In parallel, the operating parameters referred to a period of about one year, duringboth the unloading and the operating phases, have been identified and analysed.

RESUME

Le terminal m6thanier de Panigaglia pour I’exploitation de LNG est equipe d’un systeme derecuperation du gaz d’evaporation produit pendant Ies phases de dechargement et gazeification. Lesysteme comprend une station de compression et une colonne de recondensation qui condense Ie gazdevaporation avec deux “packing bodies” en “INTALOX METAL ring-type”, DN 1” et 2“ en fonction dela quantite de gaz d’evaporation disponible. Cette structure a ete implantee en 1991 avec la renovationdu terminal de gazeification, elle a ete realisee pour optimiser le precedent systeme et aussi pourcomprimer Ie gaz d’evaporation a une pression superieure a 30 Bar. Actuellement, pour reduire Iescofits derivants des consummations d’electricity pour la compression du gaz a une pression plushaute, on a envisage de pouvoir utiliser la colonne de recondensation a une pression inferieure a 25Bar. Une nouvelle solution de traitement du gaz d’evaporation a done ete etudiee. Cette solutionpermet I’exploitation du LNG refoule par Ies pompes a basse pression avec une temperature du LNGde 15°C inferieure a celle qui est utilisee clans Ie systeme actuel. La quantite de gaz devaporation quipeut ~tre absorbee a ete calculee aux differences pressions d’exploitation. Une verification thermo-dynamique a ete faite et prise en consideration pour la colonne de condensation (material and energybalance), Ies etudes et Ies analyses statistiques ont donne des resultats satisfaisants et convergent.En parallele Iesdechargement etanalyses.

parametres d’exploitationgazeification, se referant

du gaz d’evaporation produit pendant Ies phases dea une periode denviron un an, ont ete enregistres et

Portions

DISCLAIMER

of this document may be illegible

in electronic image products. Images are

produced from the best available original

document.

THERMODYNAMIC CONSIDERATIONS ON A BOIL-OFF GASRECOVERY SYSTEM

1. INTRODUCTION

The Panigaglia LNG terminal is currently the only Liquefied Natural Gas regasification plant inItaly and, with its capacity of 3.5*109 standard cubic meters per year, is strategically important andmeets the energetic diversification needs.The plant was renewed in the early 90’s, according to the new environmental and safety requirements,to make it conform to the evolution of the world gas market and meet the needs of the nationalnetwork.Some improvements have been necessary to maintain the facilities at the highest level of technical andeconomic efficiency and have brought about an evolution in the initial process philosophy of the firstretrofitting.Currently the plant configuration can be divided into 4 main sections: Reception and Storage, LNGRegasification, Boil-off gas Recovery System and Gas Correction System.

2. BOIL-OFF GAS COMPRESSORS: THE FIRST ARRANGEMENTThe Panigaglia plant was built between 1967 and 1970 and was designed to receive and to

process LNG coming from Marsa El Brega in Lybia.As the Libyan liquefied gases required the cracking of the heavy hydrocarbons they contained, theterminal process consisted mainly in fractionating, separating and removing heavy compounds whichwere then subjected to catalytic steam reforming after desulfurization.The effluent from reforming was cooled, caustic washed for CO* removal, dried, compressed totransmission network pressure, mixed with the light hydrocarbons from the fractionation section, andlastly sent to the national network.The boil-off gas (BOG) produced during ship unloading was sent back to the tankers and the portion inexcess was discharged to vent.At the end of the 70’s, in order to follow the changes in the LNG world market, as both natural gasprices and NG consumption were rising, BOG recovery became interesting.BOG was compressed (maximum pressure 30 ata [3.040”1 Oe Pa]), mixed with the effluent comingfrom the dryers (pressure about 26 ata [2.634*1 OGPa]) and then compressed to the pressure networkby two centrifugal compressors.

3. BOIL-OFF RECOVERY SYSTEM: CURRENT SITUATIONIn the early 90’s the terminal was completely renewed to obtain a less complex process and

thus a plant that was easier to run, more flexible and reliable and above all suitable for receiving thelight LNG produced by almost all of the existing liquefaction plants.At present (see Attachment 1), BOG is recovered by 2 three-stage reciprocating compressors withopposed horizontal, non lubricated type pistons which guarantee a total capacity of 11000 Kg/h andmay be used in different operating situations.The smaller compressor (item K-1050) has a capacity of 1000 Kg/h and is always in operation, whilethe other compressor (item K-1 051 ) has a capacity of 10000 Kg/h and is used only during shipunloading. BOG is taken from the storage tanks vent collector (DN=24° [0.61 O m]), compressed to apressure of about 29.5 bara [29.5*105 Pa] and finally sent to the recovery system. Compressor K-1051works with an inlet temperature of -150”C and has an automatic trip system to shut down the machinewhen the suction temperature is higher than -50”C. This system is by-passed for 8 minutes duringstart-up when the inlet temperature can reach -15°C. If the temperature is still high after 8 minutes,

.

then the unit is stopped by the system. In this period of transition the unit works at full load to reach in ashort time a temperature of less than -50 ‘C. Before the suction line of the BOG compressors a branchis installed to send a part of the gas to the ship by means of a blower having a capacity of about 11000Sm3/h and a discharge pressure of 1.6 ata [1.621 *105 Pa]. In this way the tankers may receive vapourswhen requested. In the current arrangement BOG can be used as fuel gas for the submergedcombustion vaporisers, mixed with the NG coming from the network or directly from the vaporisers’outlet. Control of the fuel gas system is based on two regulation valves: the first from 70 bara [70’105Pa] to 3.3+4.2 bara [3.3’105+4.2’105 Pa] to control the NG pressure coming from the vaporisers andthe second for the BOG pressure control. Downstream of the mixing point the fuel, heated up to 5 ‘Cby an electric exchanger, goes to a slug catcher and then feeds the four combustors. Before eachcombustor another regulation valve keeps the fuel pressure at a value between 1.3 bara [1.3*105 Pa]and 2 bara [2*105 Pa]. It is generally preferred to use NG from the network or from the vaporisers asfuel gas instead of BOG, because the BOG quality is very changeable during different plant chargeconditions and moreover pressure instability was registered inside the feed collector of the combustorsdue to regulation problems. Most of the boil-off gas recovery is ensured by the absorption column, avertical mixing counter-flow condenser with two different sections at the top and bottom. The towercontains inside two layers of packing (Intalox 1” & 2- Raschig rings type) where the compressed gasis absorbed by part of the LNG coming from the secondary pumps in thermodynamic equilibriumconditions. The operating ratio, considering a light type of LNG, is about 4.8 Kg/h of fresh LNG every 1Kg/h of BOG absorbed with an ideal ratio of about 1:4 to optimise the absorption capacity between theLNG sent to the higher packing and the LNG sent to the lower packing. The tower operating pressure,which is between 24 [24*105 Pa] and 28 bara [28*105 Pa], is controlled by a differential pressurecontroller set at a slightly higher pressure value than the booster pumps pressure. The DifferentialPressure Transmitter controls in split range two valves respectively to discharge the excess BOG tovent if the differential pressure increases too much and to pressurise the absorption tower with gastaken from the outlet of the vaporisers in case of a too low differential pressure. The LNG coming outfrom the tower is mixed with the LNG from the booster pumps and is pumped by the secondary pumpsup to about 82bara[82’105 Pa].

4. THE RETROFITTING OF THE BOIL-OFF RECOVERY SYSTEMUpdating the boil-off system (see Attachment 2) will assure a higher rated flow in the

absorption column, a higher level of reliability and flexibility to run the plant in every operating conditionand a global cost reduction in terms of maintenance and running.The pressure of the tankers arriving at the Panigaglia is normally higher than the operating pressure ofthe onshore LNG storage tanks. This configuration, which is typical of old tank designs, represents themain reason for the production of a large amount of BOG.Several factors contribute to produce BOG, but in our plant configuration the main reason is thedifferential pressure during ship unloading that can reach values as high as 1000 mm H20 gauge.Moreover it depends on the LNG flow rate during ship unloading, on the heat leakage through thetransfer line and through the tank walls, as well as on the thermal contributions of the unloading pumpsand of the vapours stored in the tanks. But it also depends on the management of some processconditions like circulation, recycles and setting the values of the pressure controllers that regulate theopening of a butterfly valve on the tanks discharge vent collector. Normally all these sources generatea large boil-off amount which may easily reach peaks of about 32000 kg/h.The two BOG compressors, currently in use, were installed at the beginning of the 80’s when the Iant

?$still had its old arrangement. In fact in the old process the machines worked at 29.5 bara [29.5’1 O Pa].When the plant was renewed the new process was studied so as to save the compression system thatwas not old enough to be substituted. This is why the absorption system and the booster pumps wereset at a slightly lower pressure than the compressors discharge pressure. The boil-off recovery systemcurrently in use is obsolete. It needs many expensive maintenance operations, has an high electricpower consumption and requires, for start-up, that the automatic trip system be by-passed, thuscausing an incorrect working of the machine during the transition period and a greater wear of wearingparts.For all these reasons it has been decided to substitute the machines in order to reach a higher level offlexibility and reliability by increasing the absorption capacity of the recovery system and at the sametime minimizing the investment costs, while maintaining most of the installed equipment.

5. CONSIDERATIONS ABOUT NEW ABSORPTION COLUMN5.1 Operating procedures

It was decided to define the best column operating pressure in order to minimise theproduction costs and to increase the maximum absorption capacity. The column pressure isdetermined by the operating pressure of booster pumps (item P-101 A/B/C/D) and by the set point ofthe PDT located at the bottom of the column.The discharge pressure of the BOG compressors can be calculated by adding the column pressure tothe pressure drop along the connecting pipe.Considering a typical LNG composition with a density in flowing conditions of about 453 Kg/m3, theoperating point of the column and BOG compressors is defined by the factors listed below:●

●

●

●

Iiuuid seal in the storaae tank: the operating range of the LNG level in the tank ma vary fromz

25.955 m to 0.850 m. This variation produces a pressure fluctuation of about 1 bar [1O Pa] on thesubmerged pumps suction.submerued pumrx: their rated capacity is about 500 m3/h with a head of 50 m. In an operatingrange of 300+600 m3/h, the head varies from 58 m to and 42 m, which produces a pressurefluctuation of about 0.7 bar [0.7*105 Pa].booster pumps: their rated capacity is about 250 m3/h with a head of 490 m in the currentconfiguration (7 impellers). In the operating range of 200+300 m3/h, the head varies from 520 to450, which produces a pressure fluctuation of about 3.1 bar [3.1*105 Pa].PDT set their operating range may be vary from 0.4to2bar[0.4’105 Pa;2’105 Pa].

All these factors, along with fluctuations due to density variations calculated in the range off 5Y0, mayproduce a remarkable pressure change in the absorption column.Particularly if the pumps operate in nominal conditions, the column pressure is about 28 bara [28*105Pa], while it reaches 29.5 bara [29.5*1 05 Pa] in the compressors discharge.Three alternatives were examined to reduce the operating pressure of the BOG compressors,considering a typical LNG composition, a PDT set at 1 bar [105 Pa] and the booster pumps running atrated capacity.Each alternative differs from the other two in the mechanical arrangement of booster pumps:

Booster pumps Column pressure at Maximum* compressorsarrangement pumps rated capacity pressure at discharge

(bara) [Pa] (bara) [Pa]

Alternative 1 5 impellers 20.3 [20.3’105] 23.1 [23.1*10~

Alternative 2 6 impellers 23.5 [23.5*1 05] 26.4 [26.4’10~

Alternative 3 6 impellers (modified) 22 [22*1 05] 25.2 [25.2”10~

* This value is calculated considering only one pump running at its rated capacity.

In the first case the gain in terms of pressure decrease is very high, but under the threshold of 22 bara[22’105 Pa] the absorption capacity of the column looses 300 Kg/h of BOG every bar below this value.In the second alternative the pressure is still high and not so different from the current situation.Therefore the best solution is the third, where the plant reaches maximum efficiency in terms of BOGrecovered and running costs.In order to modify the process conditions and to obtain the best performances, the design of the boil-offrecovery system in the new configuration has been planned as follows:●

●

●

●

●

exis~n~ column at a new operating pressure of 22 bara [22*105 Pa] (current pressure is 28 bara[28’10 Pa])two LNG supply pumpsa new set of reciprocating compressors: hvo machines of about 8000 kg/h sized for ship unloadingand another machine of 2000 kg/h for normal operating conditionsa cooling system of return vapours at the blower dischargeutility development.

6. ABSORPTION COLUMN6.1 Current arrangement:

At present, the absorption column works at about 28.70 bara [28.70’105 Pa], the LNG comingfrom the secondary pumps discharge has a temperature value close to -134°C, and the boil-offtemperature is about 38”C.

In order to avoid fluid dynamic troubles and to run the column far from flooding conditions, the LNGflow rate sent to the column must be close to 125 m3/h. In this condition the boil-off amount that can beabsorbed in the current plant arrangement has been evaluated.

10 (LNG to the 1°

I

packing from thesecondary pumps

~ discharge)

(’LfiGto the 2° packingfrom the secondarypumps discharge)

The LNG composition considered to evaluate the amount of BOG that the plant can currently absorb inthe above-mentioned conditions is listed below:

Flow 10 and 12 (LNG) composition

CHq=91 .359

C2HG=7.150

C3H8=0.631

iso-CgH10=0.037

n-CdH10=0.040

iso-C5H1z=0.003

n-C5H1z=0.002

NZ=0.778

Flow 15 (Boil-off) composition

CH4=86.353

C2HG=0.017

C3H*=0

iso-C4H10=0

n-C4H1F0

iso-C5H12=0

n-C5H12=0

N2=13.630

,.

Flow 10 and 12 (LNG)

p=23.593 Kmol/m3

MW=l 7.35 Kg/Kmol

T=-134°C

P=2870 Kpa

Total amount =125 m3

Flow 15 (Boil-off)

p=l.146 Kmol/m3

MW=I 7.68 Kg/Kmol

T=37.94 ‘C

P=2870 Kpa

According to the material balance:15+10+12=18

As we did not know the total amount of BOG gas, or the total amount and composition of the LNGcoming out from the column bottom, we started with a first trial starting value of BOG gas flow to solvethe material balance and then used the energy balance to verify this value. We also supposed that theLNG coming out is in thermodynamic equilibrium at its bubble point.The energy balance is:

HOUT- HIN= AH = H15+ Hlo + H12- H18=O

The results were recorded in the following diagram:

1 1 1 1 I

lCOMYJ Im -

$Z : ““-.Zz;s~ -

0--mxca -

-4xwo , , , , ,lm 123X 13X0 13533 14030

Soil-offflow rate (l@)

The amount of boil-off that the plant can absorb according to the specified conditions is 13505 Kg/h,The LNG coming out from the column bottom has a bubble temperature of -98.93 ‘C.

6.2 New arrangement

In the new arrangement the column will receive LNG for boil-off absorption from the boosterpumps rather than from the secondary pumps. in this way the decrease in the rated capacity of thecolumn due to the pressure reduction can be compensated. In fact the LNG is colder by about 15 ‘C(the current temperature is about -134 ‘C) and will be sent to the column by two supply pumps locatednear-by. The plant absorption capacity will increase and there will be a gain in terms of energyconsumption because of the pressure reduction at the pumps discharge and at the compressorsdischarge and because of the elimination of high energy lamination from the outlet of the secondarypumps.

,.

Control of inlet LNG flow rate will be ensured by two regulation valves driven by the measurementsignal coming from the BOG flow meter. In this way it will be possible to keep the right ratio betweenLNG and BOG and between the upper and bottom packing, without having hydraulic problems.In the new configuration the ideal ratio between the LNG sent to the higher packing and the amountsent to the lower one is 1:6.The same calculation, made for the new column arrangement considering a lower pressure (22 bara)and a lower LNG temperature (-150 “C) without changing the LNG and BOG compositions, gave thefollowing results.

1

10 (LNG to the 1°packing from thebooster pumps

-~ discharge)

12(LNGto the 2° packingfrom the boosterpumps discharge)

I I

15

11

18(BOG) (LNGout)

Flow 10 and 12 physical properties Flow 15 physical properties

MW=l 7.35 Kg/Kmol the MW=l 7.68 Kg/Kmol

T=-150 “C T=37.94 ‘C

P=22*105 Pa P=22*105 Pa

P=25.04 Kmol/m3 p=O.8720 Kmol/m3

Total amount=l 25 m3

, I ,m -

- -

mico -

5°: “’-’-’’’......,.:s=-20xcn

‘5=4cmYJ -

40XQI -

-wmo -, , I

140YI 14W0 Imx 15SM IecalBoil-off flowrata (Kgh)

In the future arrangement the plant will absorb 14800 Kg/hat least, because we considered for the inletboil-off gas temperature, the same value we had in the previous arrangement. Actually it is expected tobe lower because the boil-off gas will be compressed at lower pressure.The LNG coming out from the column bottom has a bubble temperature of -107.46 ‘C.

7. LNG SUPPLY PUMPSTwo centrifugal pumps will be installed at the base of the column (one in operation and the

other stand-by) to feed the column from the booster pumps outlet.Each pump, which assures a head of 115 m at the rated capacity of 140 m3/h, has been designed toassure maximum reliability in cryogenic service. In fact, its seal system is based on the conventionaldouble back-to-back mechanical seals. The seal housing is filled with light oil, while the area betweenthe seals is pressurised by a piston system which is designed to be self-contained, and self-compensating in order to provide a constant differential pressure higher than the pressure inside theseal housing.Between the pump system and the seal housing, the insulation chamber is relieved to suctionpressure. In this section the heat transfers from room temperature and from the heated seal chamberare sufficient to keep the natural gas in a gas phase avoiding liquid contact at the back of the lowerseal face and eliminating the possibility of icing up to the seal. The vapour inside the insulationchamber represents also a reference pressure, independent of variations in suction pressure, in orderto pressurise the seal housing at a slightly higher level, precluding the possibility of process productleakage outside.

8. BOG COMPRESSORSTwo different process configurations were studied:

El) a mixer before the first stage suction to cool the boil-off gas with LNG, followed by a K.O. drum,common to the three compression units, and a water intercooler between the second and the thirdstage.

b) no precooler before the first stage suction, but two intercoolers between the first and the secondstage and between the second and the third stage.

In both configurations there will be an aftercooler for each compressor.The second solution was preferred for the following reasons:. the mixer and the K.O. drum installation, which cause a very low compressors suction pressure

because the storage tanks pressure is already low, is avoided;. there are no pumps to empty out the K.O. drum;● in the second solution the boil-off flow rate sucked up from the storage tanks is the same as the

one sent to the column, while in the first case it is less because of the LNG vapours coming fromthe LNG injection in the mixer, when the boil-off temperature is higher than -120 “C, as is shown inthe following tables:

&

KO Recycle~R1TM

BOGfrom

T COMPRESSION AFTER crhmn

LNG 1INrr f_’fUN.F.R

SPRAYER

,.

COOLDOWN BEFORE THE COMPRESSORS SUCTION

Stream name I BOG fromtanks

Phase vapour

Rate (kg/h) 14360.024I

Temperature (“C) -100

~Molecular weiaht

1

16571.262 1397.454

4--w--0.039 -0.203

[-0.1 63*1 0~ [-O.850*1 06]o I 1

To the column To recy~

=4-+=[2400000] [101 400]

17.743 17.743

I&l

tRecycle

BOOfrom taoks v column

COMPRESSIONAFTER

UIWT COOLER

NO COOLDOWN BEFORE THE COMPRESSORS SUCTION

Stream name BOG from Compresso LNG To the column To recycletanks r suction Sprayer

Phase vapour Vapour liquid Vapour vapour

Rate (kg/h) 14360.024 14594.678 nla 14360.024 234.654

Temperature PC) -100 -97.794 nla 42 33.428

Pressure (bara) [Pa] 1.014 1.014 nla 24 1.014

[101 400] [101400] [2400000] [101400]

Molecular weight (Kg/Kmol) 17.779 17,779 nla 17.779 17.779

Enthalpy (M Kcal/h) [KJ/h] -0.185 -0.173 nla 0.682 0.011

[-0.774’1 o~ [-O.724*1 06] [2.855”1 06] [0.046’106]

Mole fraction liquid o 0 nla o 0

The right size of each machine has been defined to satisfy the specified operating conditionsand to control the LNG tank pressure during ship unloading and holding operations. Besides thecapacity of each compressor will be manually or automatically controlled by operating the suctionvalves unloaders and clearance pockets provided on each cylinder to enable 5 operational steps(0,25,50,75 and 100% for the discontinuous operation compressor, and 0,20,50,70 and 100% for thesmaller machine).This will give the plant greater flexibility.Moreover in future the plant operators will be able to put each machine into any load operationimmediately after start-up, like a room temperature compressor. The first intercooler, which isautomatically controlled, shall permit start-up over a wide range of suction gas temperatures (fromroom temperature to -160 “C) without cooling-down procedures and blowing-off natural gas to the flareline.From an economic point of view, unscheduled downtime, frequent planned maintenance shutdowns,high maintenance expenditures and process gas losses to surroundings or to the flare, which have anegative impact on plant economy, will be minimised.The new ultra-low temperature BOG compressors will be able to achieve long-time operation withoutmaintenance: from 9000 hours to 30000 operating hours, depending on the parts of the machine.To assure the maximum level of availability it is planned to carry out a global maintenance project withthe supplier. We will start implementing the planned maintenance and later (one year after theinstallation), once we have established a breakdown/maintenance trend, we will pass on to the nextstage which is the predictive maintenance in order to optimise this operation and to reduce costs.For these reasons we expect to divide the maintenance program me into two steps: preventive andpredictive maintenance, respectively. In the first phase we will carry out the standard maintenanceprogramme at specific time intervals, then we will try to extend the time intervals exceeding the timelimit set by the equipment manufacturer. This is to reduce the equipment downtime and the spare partscosts, cutting-down unneeded maintenance operations. To switch over to this type of maintenance wehave planned to equip the new compressors with a modern monitoring system for piston drop andcompressor frame vibrations. This type of remote monitoring together with a periodic oil analysis,periodic vibration tests and periodic evaluation of the thermodynamic parameters shall allow reaching alife extension for each part of the machine, as well as increasing the operating hours between twomaintenance intervals.

9. RETURN VAPOUR COOLING SYSTEMThe vapour return system will be updated to control the maximum temperature of the vapour

going back to the ship so to turn off toward the ship a major mass flow rate.As in the new arrangement the plant will be able to recover up to 15000/16000 Kg/h of BOG bycolumn, the ship unloading process has been improved considering that the ship may receive a limitedportion of the total BOG production.To control boil-off production during ship unloading the boil-off return will have to be regulated in termsof flow; flow modulation shall consist in reducing the return vapours rate during the first ship unloadingstep, when the maximum LNG rate is 3200 m3/h and in increasing the vapour rate when the LNG ratedflow is 4000 m3/h. In this way the total gas volume sent to the ship remains the same but themodulation in terms of flow avoids exceeding the maximum absorption limit of the column.The cooling system at the blower outlet will keep the maximum temperature at about -110 ‘C, so thatthe tankers will be able to receive such vapours.The cooling system consists of a mixing system located at the blower outlet used to cool the naturalgas stream by spraying LNG coming from a bleeding of the booster pumps discharge. Downstream ofthe mixer a K.O. drum with a drain accumulator assures gas/liquid separation. This arrangementallows cooling also the recycling vapours that are always present because of the high capacity of theblower.The cooling system has been preferred on the discharge side rather than on the suction side becausein the latter case a major LNG flow rate would have been necessary, with a consequent decrease inthe maximum amount of vapours coming from the storage tanks. Furthermore the above configurationallows reducing the overall dimensions and simplifying the layout.In order to define the best process solution, the two configurations have been simulated consideringseveral thermodynamic conditions and referring to two typical LNG compositions (light LNG1 [442Kg/m3] and. heavy LNG2 [471 Kg/m3]). A summary of these calculations referred to the samethermodynamic conditions with two possible configurations is listed below:

*

BOGfromtanks ‘

T

r“)BLOWER

Recycle

➤To Ship

LNGSPRAYER

DRUM

COOLDOWN ON DISCHARGE SIDE

Stream name BOG Blower Blower LNG K.O. drum To ship To recyclefrom suction discharge Sprayer draintanks

Phase vapour mixed Vapour liquid liquid vapour Vapour

Rate (kg/h) 6105.7 9214.4 9214.4 550.6 46.9 6608 3110.1

Temperature (“C) -145 -135 -96.4 -155 -155 -115 -116.3

Pressure (Pa) 1.01 4*105 1.014* 105 2.062”105 2.5’105 2.062’105 2.062’105 1.014’105

Molecular weight 18.670 18.651 18.651 18.935 47.541 18.612 18.612(Kg/Kmol)

Mole fraction liquid 0.0000 0.0002 0.0000 1.0000 1.0000 0.0000 0.0000

Vapour

Rate (kg/h) 6105.7 9210.986 9214.457 nla nla 6608 3110.116

Molecular weight 18.670 18.647 18.651 nfa nla 18.612 18.612(Kg/Kmol)

Density (Kg/m3) 1.811 1.675 2.660 nla nfa 2.986 1.464

Liquid

Rate (kg/h) nla 3470 nla 550.608 46.948 nla nia

Molecular weight nla 42.364 nla 18.935 47.541 nla nla\Kg/Kmol)

Density (Kg/m3) rda 684.462 nla 684.462 684.137 nla nla

.=

&

BLOWER r tecycle

!’KODRUM

Drain

BOGfrom tanks

LNGSPRAYER

I COOLDOWN ON SUCTION SIDE I

3To

rec cle

vapour

3662.131

Blowerdischarge

vapour

LNGSprayer

Liquid

I Stream name

I Phase

lRate (kg/h) 6059.54 10270.13 10270.13 765.145 206.429 6608

w -145 I-149.99 -111.39

2.220*1 05

-155

=4-112.82

1.418

18.451

1

1.014* 105 1.014”105 1.752*1 05

I Molecular weiaht 18.670 18.451 18.451 18.935 31.950 18.451

0 1

Vapour

Rate (kg/h) 6059.54 10270.13 10270.13 N/a nla 6608 3662.131

Molecular weight 18.670 18.451 18.451 N/a nla 18.451 18.451

Density (Kg/m3) 1.811 1.867 3.116 N/a nla 3.116

Liquid

Rate (kg/h) nla nla nla 765.145 206.429 nla nla

Molecular weight n/a nla nla 18.935 31.950 nla nla(Kg/Kmol)

Density (Kg/m3) nla nla nla 471.039 637.648 nla nla

.-

The only disadvantage is represented by a light condensation at the blower suction when the coldBOG (-145 / -130 “C) is mixed with the recycle vapours. The mist formation is not dangerous forblower operation and will be drained into a specific vessel.

10. UTILITIES DEVELOPMENTThe plant retrofitting required also an adaptation of the plant cooling system. Currently this

system consists of two sections: a closed section with fresh water that feeds the plant users and anopen section with sea water to cool down the fresh water.The sea water cooling system comprises two parallel pumps of 350 rr?/h, one running and the otherused as a spare pump, feeding MO parallel heat exchangers. They are both driven by an electricalmotor. One of them can be powered by the diesel generator that gives electric power to the mostimportant electric utilities in case of electric power failure. The sea water is treated with an additivebefore going inside the heat exchanger to avoid the growth of marine organisms inside the exchangerpipes.The fresh water system is closed and restoring is assured by the water formed inside the vaporisersduring combustion. The water is added with a corrosion inhibitor before the distribution pumps suction.The following table describes the thermal duty in the new arrangement:

ITEM AT (“C) Flow rate (m3/h) Duty (KJlh)

Air correction system compressors 20-K-I 1 13 64 3.483* I OeAIB “

Air correction system compressors 20-K-I O 13 48 2.612*106AIB

20-K-I 1 AfB and 20-K-IO A/B increase 13 60 3.266* I OG

Blower K-1 001 3.2 5.6 7.503”104

Instrumentation air compressor K-1 120 15 8.2 5.1 50*1 05

Emergency diesel engine 20-E-4 8 8 2.679’105

Air conditioning 9 23.5 8.855* I 05

Boil-off compressor 25-K-201 10 80 3.349* I 06

Lube oil cooler 25-K-201 10 12 5.024’105

Boil-off compressor 25-K-202 10 15 6.280’105

Lube oil cooler 25-K-202 10 2 8.373* I 04

Boil-off recovery system aftercooler E-1057 10 40 1.674’1Oe

Total 11.23 (AT 366.3 17.343* 106medium)

This table shows that the sea water consumption is:Sea water density = 1020 Kg/m3

Sea water AT= 6.7°C

Sea water massive flow rate =cooling water total duty 4142420

= 6,7= 620000 Kg/h

sea water AT

sea water massive jlow rate 620000Sea water volumetric flow rate =

= 1020= 610m3/h

sea water density

According to the calculation the sea water system must be developed as follows to ensure the rightheat exchange in the whole plant

.

. elimination of one existing 350 m3/h pump; the other pump (which can be fed by the dieselgenerator) will be kept to assure cooling of the critical equipment in case of electric power failure.

● installation of two new pumps with a 700 m3/hcapacity and a 25 m head.

The fresh water system is over-designed, and therefore it does not need any improvement.In case of electric power failure the cooling water for the emergency diesel engine will be guaranteedby a diesel pump that will startup automatically 30 seconds after the engine. The valve on the coolingwater line toward the” plant will be shut down and the cooling water will go only to the preferentialusers.

11. CONCLUSIONSThe new configuration has been planned to reach a higher level of reliability and performance thatpermits operating the terminal with much more flexibility and reliability.The project may be summarised in a lines:. reduction of running and maintenance costs. minimisation of project costs● increase of BOG capacity absorption● increase of flexibility in BOG compressor management.

The terminal was designed to meet the clauses of the purchase contracts drawn-up in those days.With the extensions of the new final arrangement it will now be possible to run the plant at full loadwith easier operating management in compliance with today’s contracts and according to the gasmarket situation.An update of the BOG recovery system will allow reinforcing safety and reducing the environmentalimpact. Moreover it will contribute to consolidating the corporate policy adopted by SNAM during thelast years both in Italy and in an international context too.

A~AcH~EIvT ‘i PANIGAGI,JA 1,.NGTERMINALCURRENT FLOW PROCESS DIAGRAM

BOIL-OFF

BOIL-OFFMPRESSORS

BOIL.OFF ~.

I4TUFIAL GAs rl 1 7,+s

.. . . . .

3 tLUNUflK T .- r ,.. .

PUMPS “!.. -” COMPRESSORS

ATTACHMENT 2 PANIGACiLIA LNG TERMINALTHE UPDATE OF BOIL-OFF RECOVERY SYSTEM

UNLOAOING ARMSVAPOR RETURN ARM

A i PUMPS I

BOOSTERPUMPS

uSECONDARY

PUMPS

NANRIU (MS Ii 1

NEYV BOIL OFFCOMPRESSORS

—

TO PIPELINENETWORK

●BOIL-OFF

VENT r

>

N,MEMBRANES

AIR

RISERCOMPRESSORS