Plant for regasification of LNG

Abstract

A plant for regasification of LNG includes a pump boosting LNG pressure, an LNG/coolant heat exchanger producing NG from LNG from the boosting pumps, and a closed coolant loop extending through the LNG/coolant heat exchanger and including a heat exchanger. The coolant from the heat exchanger is passed through the LNG heat exchanger as a gas and leaving in a condensed state to produce NG by thermal exchange. A heating medium is used within the heat exchanger to provide coolant in a gaseous state. An NG/coolant heat exchanger is arranged in connection with the LNG/coolant heat exchanger and is connected to the closed coolant loop. LNG is preheated within the LNG/coolant heat exchanger and NG is trim heated within the NG/coolant heat exchanger using liquid coolant from at least one heat exchanger.

Claims

1. A plant for regasification of liquefied natural gas (LNG), comprising: at least one pump boosting LNG pressure; an LNG/coolant heat exchanger producing natural gas (NG) from LNG being flowed from the at least one boosting pump; a closed coolant loop extending through the LNG/coolant heat exchanger and including at least one first heat exchanger, a coolant from the at least one first heat exchanger being passed through the LNG/coolant heat exchanger as a gas and leaving in a condensed state so as to produce NG by thermal exchange; a heating medium being used within the at least one first heat exchanger configured to provide coolant in a gaseous state, and a second heat exchanger configured to provide heated liquid coolant and being part of the closed coolant loop, wherein an NG/coolant heat exchanger is arranged in connection with the LNG/coolant heat exchanger and is connected to the closed coolant loop, whereby LNG is preheated within the LNG/coolant heat exchanger and NG is trim heated within the NG/coolant heat exchanger using liquid coolant from the second heat exchanger; and a coolant supply that supplies the coolant to the closed coolant loop, wherein the first heat exchanger and the second heat exchanger are connected in series in the closed coolant loop, and the NG/coolant heat exchanger is connected between the first heat exchanger and the second heat exchanger in series in the closed coolant loop so that all coolant from the coolant supply is supplied to the second heat exchanger first, then to the NG/coolant heat exchanger and finally to the first heat exchanger.

2. A plant according to claim 1, wherein the closed loop includes a pressure controlling mechanism by which the pressure through the second heat exchanger and NG/coolant heat exchanger is maintained above the boiling pressure by seawater at a temperature range of 5-35 C.

3. A plant according to claim 2, wherein the pressure controlling mechanism comprises a pump and a valve, the valve controlling the pressure in coolant from the pump through the heat exchanger and NG/coolant heat exchanger above the boiling pressure.

4. A plant according to claim 1, wherein the LNG/coolant heat exchanger and NG/coolant heat exchanger are printed circuit heat exchangers.

5. A plant according to claim 1, wherein the heat exchangers included in the closed coolant loop are semi welded plate heat exchangers.

6. A plant according to claim 1, wherein the at least one booting pump is multistage centrifugal pumps.

7. A plant according to claim 3, wherein the coolant pump is a centrifugal pump.

8. A plant according to claim 1, wherein the coolant is propane.

9. A plant according to claim 1, wherein the heating medium is seawater.

10. A plant according to claim 9, wherein an external heater is arranged to preheat seawater fed into the heat exchanger in connection with the NG/coolant heat exchanger.

11. A plant according to claim 9, wherein an external heater is arranged to preheat seawater fed into all of the heat exchangers.

Description

(1) Embodiments according to the present invention are now to be described in further detail, in order to exemplify its principles, operation and advantages. The description refers to the following drawings, not necessarily to scale, where like parts have been given like reference numerals:

(2) FIGS. 1 to 4 are simplified schematic flow diagrams of the regasification plant according to various embodiments of the present invention; and

(3) FIG. 5 is a simplified flow diagram of one embodiment of the present invention.

(4) The present regasification plant comprises basically two circuits: a coolant circuit and a NG circuit. Propane is often preferred as a coolant due to thermodynamic properties and freezing point but any suitable fluid having an evaporation temperature of about 0 C. in the pressure ranges 200-2500 kPa may be suitable.

(5) As illustrated in FIG. 1, for instance, LNG is fed from onboard tanks (not shown) and into at least one high pressure pump A1, A2 which boosts LNG pressure, and from which boosted LNG is flowed into a LNG/coolant heat exchanger B. Each pump is a multistage centrifugal pump, for instance, being submerged pot mounted. LNG temperature upon entering the LNG/coolant heat exchanger is typically 160 C., and it is preheated to 20 C. and higher before exit. Preheating is effected by means of phase transition for liquefied coolant similar to U.S. Pat. No. 6,945,049. The LNG/coolant heat exchanger may be a compact printed circuit heat exchanger PCHE made from stainless steel or any suitable material.

(6) NG leaves the LNG/coolant heat exchanger B in an evaporated state and enters a NG/coolant heat exchanger C in which NG is trim heated before conveyed onshore as superheated vapour. The trim heating is performed by temperature glide for liquefied coolant. The vapour temperature is typically 5-10 C. below seawater inlet temperature.

(7) The coolant circuit is fed from a coolant supply H, e.g. a tank, and driven by a pump E into a semi welded plate heat exchanger D. Although illustrated as being mounted outside the coolant supply, the pump, e.g. a centrifugal pump, may also be of the submerged pot mounted type like the pumps A1, A2 mentioned above. Coolant is heated by means of seawater passing through the plate heat exchanger opposite of coolant, typically up to 2-5 C. below ingoing seawater temperature. Then, heated coolant is fed into the NG/coolant heat exchanger C to provide for trim heating of NG.

(8) Cooled coolant leaving the NG/coolant heat exchanger C is pressure relieved by means of a control valve F before it enters at least one semi welded plate heat exchanger G1, G2. The control valve may be replaced by any suitable means, e.g. a fixed restriction. An objective of the control valve is to maintain pressure from the pump E through the two heat exchangers D, C above boiling pressure of coolant at seawater temperature. Within each plate heat exchanger G1, G2 coolant is evaporated using seawater, each being passed on opposite sides through the heat exchangers.

(9) Then, evaporated coolant is passed on to the LNG/coolant heat exchanger B to be condensed while LNG is evaporated on each side within the heat exchanger when preheating LNG. Condensed coolant from the heat exchanger is at last returned into the tank H. Many optional variations are possible, and these are illustrated in a not-exhaustive manner in the drawings. As shown in FIGS. 2 and 4, the preheating and trim heating heat exchangers B, C may be combined to one common heat exchanger. Such common heat exchanger is having one LNG/NG path and at least one separate path for coolant in preheating and trim heating portions, respectively. Seawater being passed into the heat exchanger D may be preheated using an external heater K of appropriate type, see FIGS. 3 and 4. The same could also be done for seawater into skid being preheated using an external heater of appropriate type, see FIGS. 3 and 4. Any suitable coolant than seawater is applicable. Although, many are presented in the drawings as being a single heat exchanger, it is understood that each may be supplemented with additional heat exchanger dependent on capacity and available equipment.

(10) The regasification plant may be installed on a Shuttle Regasification Vessel (SRV) or Floating Storage Regasification Units (FSRU). The regasification plant and its heat exchangers are specially designed for marine installations and for cryogenic working conditions. The plant is based upon proven equipment with extensive references. Compared with the prior art, semi-welded plate heat exchangers are used between the propane and seawater and at least one smaller propane circulating pump may be used.

(11) Without considered mandatory, heat exchangers suitable for the present plant are designed for handling LNG with the following typical composition:

(12) TABLE-US-00001 Standard Composition (Mole %) liquefied Nitrogen 0.34% Methane (C1) 89.50% Ethane (C2) 6.33% Propane (C3) 2.49% Butane (C4) 1.26% Pentane (C5) 0.08% Hexane (C6) 0.0%

(13) Moreover, basic data input data may be: LNG-Flow: 50-300 tons/hour each skid LNG inlet temperature: 160 C. Gas outlet temperature: typically 5-10 C. below seawater temperature LNG inlet pressure: 4000-20000 kPa LNG outlet pressure: 200-600 kPa below inlet pressure Inlet seawater temperature: 5-35 C.

(14) According to FIG. 5 showing a simplified flow diagram of one embodiment of the present invention, LNG at a pressure of 500 kPa and temperature of 160 C. enters the LNG/Propane PCHE heat exchanger. It leaves with a temperature of 20 C. having a pressure of 1,120e+004 kPa and enters the NG/coolant heat exchanger from which superheated vapour leaves with a temperature of 2 C. and a pressure of 1,105e+004 kPa.

(15) In the LNG/coolant PCHE and NG/coolant PCHE heat are exchanged against propane circulating in a closed loop. Propane enters the LNG/coolant PCHE at approximately 5.4 C. and 400 kPa as gas in which the propane is condensed and leaves the PCHE as liquefied at 19 C. and approximately 253.0 kPa. In the NG/coolant PCHE propane enters at 7 C. and 800 kPa as gas and leaves after condensation as liquefied at approximately 11.9 C. and 650 kPa. Propane in the closed loop is first pumped by the pump E and heated against seawater in the plate heat exchanger D in which seawater enters at a temperature of 11 C. and having a pressure of 250 kPa and leaves at 3 C. and 100 kPa. Propane enters at a temperature of approximately 18.4 C. and 900 kPa and leaves for entering the NG/coolant PCHE in the condition specified above. Seawater enters the plate heat exchangers G1, G2 at a temperature of 11 C. and 250 kPa before exiting at 3 C. and 100 kPa. Propane enters at approximately 11.9 C. and 500 kPa and leaves for entering the LNG/coolant PCHE in the condition specified above

(16) 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, N.sub.2, CO.sub.2 is applicable. As an alternative, it is understood that the present plant also may be installed onshore.