WO2011062505A1

WO2011062505A1 - A plant for recovering bog from lng stored in tanks

Info

Publication number: WO2011062505A1
Application number: PCT/NO2010/000419
Authority: WO
Inventors: Per Helge S. Madsen
Original assignee: Hamworthy Gas Systems As
Priority date: 2009-11-17
Filing date: 2010-11-17
Publication date: 2011-05-26
Also published as: NO20093356A1; NO332123B1

Abstract

A plant for recovering BOG from LNG stored in tanks includes a recondenser (SD1) used to feed LNG into pumps (A1, A2, A3) boosting LNG to be passed through vaporizers units (VU1, VU2, VU3) as to produce NG by regasification of LNG, BOG being compressed using a compressor (C1). According to the present invention BOG is, after compression in the compressor (C1), passed through heat exchangers (B1, B2, B3) situated downstream the pumps (A1, A2, A3), in which BOG in heat exchange with LNG is condensed mainly into liquid and thereafter fed into the recondenser (SD1).

Description

A plant for recovering BOG from LNG stored in tanks

The present invention relates to a plant for recovering boil-off gas (BOG) originating from liquefied natural gas (LNG), for instance, stored in tanks, particularly but not exclusively on seagoing vessels.

A common technique for conveying natural gas from extraction sites is to liquefy gas at or near these sites and transport LNG to the market in specially designed storage tanks, often arranged on a sea-going vessel.

The process of liquefying natural gas involves compression and cooling to a cryogenic temperature, e.g. about -160°C. Then, LNG carriers may bring ashore significant amounts of LNG being unloaded into special tanks onshore, before either transported by road or rail on LNG carrying vehicles or revaporized and conveyed by pipelines.

However, it is often more advantageous to revaporize LNG before unloading gas from the seagoing carrier into pipelines, for instance. US-Patent No. 6,089,022 discloses re- gasifying LNG from carrier vessels before revaporized gas is transferred onshore. LNG is flowed through one or more vaporizers positioned on the vessel. Seawater taken from the surrounding water is flowed through a vaporizer to heat and vaporize LNG before vaporized gas is unloaded into suitable facilities onshore.

US-Patent No. 6,945,049 describes regasification of LNG transported with a floating carrier vessel before the cargo is to be unloaded comprising boosting and flowing LNG into an LNG/coolant heat exchanger in which LNG is evaporated, and flowing evaporated natural gas (NG) into a NG/steam heat exchanger, in which NG is heated before being transferred onshore as superheated vapour. LNG in the LNG/coolant heat exchanger is evaporated by thermal exchange against a coolant entering the heat exchanger as a gas and leaving the same in a liquefied state. Moreover, coolant is flowed in a closed circuit and through at least one coolant/seawater heat exchanger in which liquefied coolant is evaporated before entering the LNG/coolant heat exchanger, and the pressure in evaporated coolant is controlled.

At an atmospheric pressure LNG boils at slightly above -163 °C, and is usually loaded, transported and unloaded at this temperature. To deal with both such low temperature and BOG, special materials, insulation and handling equipment are required. Due to heat leakage, the surface is constantly boiling, generating vaporized natural gas (BOG) from LNG, e.g. methane.

Although many LNG transportation systems are unable to utilize or recover BOG and, thereby, involving a particular problem during unloading of LNG, one solution to handle and condense BOG is presented in Figure 1. The plant includes a recondenser in the form of a suction drum. BOG from the cargo tanks is compressed by means of a BOG compressor and is passed along with LNG into the suction drum. LNG from the cargo tanks is pumped at a temperature of -160 to -150 °C into the suction drum having typically a pressure of 500 kPa. Then, LNG including recondensed BOG is passed into at least one vaporizer as to unload LNG in the form of NG. The at least one vaporizer is fed by means one common or more LNG high pressure pumps. The suction drum constitutes a buffer tank for the other components of the plant being typically arranged on at least one skid. Additionally, the suction drum acts as a separator tank to separate gas being produced at different operating modes of the plant.

Mainly two different types of LNG carriers having regasification facilities exists, i.e. shuttle and regasification vessels (SRV) and floating storage and regasification units (FSRU). A SRV brings ashore cargo from a LNG terminal and unloads high pressure gas at either a receive jetty or subsea buoy. When emptied, the SRV returns to the LNG terminal to get a new cargo. A FSRU stands stationary and supplies high pressure gas onshore at a receive jetty or subsea buoy. When needed, a LNG carrier refills the FSRU.

Regasification facilities for such carriers have normally a regasification capacity of minimum 50 tons/hour and maximum 500-1000 tons/hour. As the capacity of a typical regasification skid amounts to about 250 tons/hour, the number of skids needed is from 1 to 4 for the respective carriers.

For typical LNG carriers having a loading capacity of 160000 m³, the BOG rate is 3-5 tons/hour. During loading LNG carriers, displacement of gases due to LNG filling causes an increased amount of gas to be removed from the tanks. Thus, during loading displaced gases amount to 10 tons/hour or even more. The BOG rate for a SRV is typically 3-5 tons/hour, whereas this amounts to near 10 tons/hour for a FSRU during LNG loading.

Normally, LNG from the cargo tanks is transferred into the suction drum subcooled at 500 kPa, and having a temperature with boiling point just above atmospheric. LNG boosted to 500 kPa is at a temperature near the ambient, e.g. 0-20 °C. When mixing BOG and LNG passed into the suction drum, LNG is heated due to cooling and condensation of BOG. If BOG mass corresponds to 7-10 % of the LNG mass, the result is that LNG gets at the boiling point at 500 kPa. Hereinafter the liquid of BOG and LNG passed from the suction drum is denoted regasification fed.

LNG need to be boosted before regasification as to subcool the regasification fed. As already mentioned above, such boosting is performed by means high pressure pumps situated in front of the vaporizers producing NG. The pumps are multistage centrifugal pumps, for instance. Such pumps and their engines are often but not necessarily of a submerged pot type. In cases of poorly subcooled LNG, gas is generated, mainly due to pump cooling. Some regasification facilities is configured to handle unwanted gas, when starting the pump, and also when forming a minor continuous amount of gas in connection with the pump housing. However, if unexpected amounts of gas are generated, it is impossible to handle the unwanted gas, resulting in a lowered liquid level, and finally damaged pumps and engines. In addition, gas being produced within the pumps has to be returned to the cargo tanks, resulting in increased BOG flow.

If having LNG at the boiling point fed to the regasification plant, the production of gas is too large. Thus, it is impossible to recondense 7-10 % by mass of BOG to LNG, e.g. if now denoted 8 %, involving that approximately 4 % BOG to LNG is applicable.

For a SRV having a BOG rate of 3-5 tons/hour, it has to be available 75-125 tons/hour, which is more than a normal minimum regasification rate. A FSRU with a maximum BOG rate of 10 tons/hour is in need of 200 tons/hour, which is far beyond a typical minimum regasification rate.

The main objective of the present invention is to solve the problems associated with state of art solutions mentioned above. This is achieved by a plant for recovering BOG from LNG stored in tanks, including a recondenser used to feed LNG into at least one pump boosting LNG to be passed through at least one vaporizer as to produce NG by regasification of LNG, BOG being compressed using a compressor, wherein BOG is passed through at least one heat exchanger situated downstream the at least one pump, in which BOG in heat exchange with LNG is condensed mainly into liquid and is returned into the recondenser. Preferentially, the respective heat exchanger is preferentially in the form of a printed circuit heat exchanger, and BOG is leaving the heat exchanger at a temperature of -145 to -135 °C. Further the recondenser is in the form combined with a suction drum, and the pump is in the form of a multistage centrifugal pump.

The present invention is now discussed in further detail with reference to the accompanying drawings, in which:

Fig. 1 is a simplified schematic flow diagram of a prior art BOG recovery plant, shown in combination with a number of vaporizers used when regasification of NG from LNG;

Fig. 2 is a simplified flow diagram of one embodiment of the present invention, in which BOG is cooled in heat exchange with LNG within heat exchangers and is passed into a recondenser mainly in the form of liquid; and

Fig. 3 is a simplified flow diagram of one embodiment of the present invention.

At first, the following LNG and BOG compositions may be considered as representative examples:

Similar to the prior art plant, the present invention includes a recondenser in the form of a combined suction drum SD1, see Fig. 2. LNG from cargo tanks, not illustrated is fed into the recondenser. LNG being passed on from the recondenser is boosted by means of at least one high pressure pump Al, A2, A3, e.g. a multistage centrifugal pump, and in the form of a submerged pot type pump. Boosted LNG is then subject to regasifica- tion within at least one vaporizer unit VU1, VU2, VU3 as to produce NG for further transport onshore. The vaporizer units may be of any suitable type, e.g. a sea- water/propane loop, direct seawater, steam heated having an intermediate water/glycol loop, direct steam, etc.

Contrary to the prior art plant presented in Fig. 1, BOG from the cargo tanks is fed into the recondenser SD1 along with LNG first after BOG has been passed through at least one heat exchanger Bl, B2, B3. BOG compressed using a compressor CI is cooled within the respective heat exchanger in heat exchange with LNG and is routed into the recondenser mainly in the form of a liquid. Downstream the pumps Al, A2, A3 high pressure LNG has typically a temperature of -150 to -140 °C. Thus, BOG passing the heat exchangers is cooled in an interval of -145 to -135 °C. At 500 kPa and such temperatures BOG is having a liquid portion of 70-100 % by mass dependent on the BOG composition. When at least 70 % BOG is condensed and cooled substantially, the flow of LNG needed for the recondenser is considerably reduced. BOG rates of 3-5 and 10 tons/hour may be handled by LNG in an amount of 20-30 and approximately 50 tons/hour, respectively. Even at the highest BOG rates, the recondenser is able to operate at the lowest regasification rates in a plant with a capacity of 50-1000 tons/hour, for instance. Each heat exchanger is preferentially a compact printed circuit heat exchanger.

As shown in Fig. 3, LNG is fed into the suction drum at -155,0 °C and 550,0 kPa, whereas the regasification fed, i.e. LNG including liquefied BOG from the heat exchangers, leaves the suction drum at -151,3 °C and 550,0 kPa. BOG enters the respective heat exchanger at 0,0 °C and 600,0 kPa and is passed into the suction drum at - 140,0 °C and 550 kPa

The discussion above as regards the present invention are to be construed merely illustrative for principles according to the invention, the true spirit and scope of present invention being defined by the patent claims. Although LNG and NG is especially mentioned when discussion the present invention and also for sake of simplicity in the patent claims, this fact is actually not excluding that any appropriate type of liquefied gases such as ethane, propane, N₂, C0₂ is applicable. As an alternative, it is understood that the present plant also may be installed onshore.

Claims

P a t e n t c l a i m s 1.

A plant for recovering BOG from LNG stored in tanks, including a recondenser (SDl) used to feed LNG into at least one pump (Al, A2, A3) boosting LNG to be passed through at least one vaporizer unit (VU1, VU2, VU3) as to produce NG by regasifica- tion of LNG, BOG being compressed using a compressor (CI), c h a r a c t e r i z e d i n that BOG is passed through at least one heat exchanger (Bl, B2, B3) situated downstream the at least one pump (Al, A2, A3), in which BOG in heat exchange with LNG is condensed mainly into liquid and is returned into the recondenser (SDl).

2.

A plant according to the claim 1, c h a r a c t e r i z e d i n that the at least one heat exchanger (Bl, B2, B3) is in the form of a printed circuit heat exchanger.

3.

A plant according to the claim 2, c h a r a c t e r i z e d i n that BOG is leaving the at least one heat exchanger (Bl, B2, B3) at a temperature of -145 to -135 °C.

4.

A plant according to any of preceding the claim, c h a r a c t e r i z e d i n that the recondenser is in the form of a suction drum (SDl).

5.

A plant according to any of preceding the claim, c h a r a c t e r i z e d i n that the at least one pump (Al, A2, A3) is in the form of a multistage centrifugal pump.