Method for turndown of a liquefied natural gas (LNG) plant

Abstract

A method for turndown of a liquefied natural gas (LNG) plant, the plant including a liquefaction unit arranged in a flow path of the plant, includes removing LNG from a first location in the flow path downstream of the liquefaction unit; vaporizing the removed LNG, or heating the removed LNG so that the removed LNG is transformed to gas phase; and re-admitting the vaporized or transformed LNG to the flow path at a second location upstream of the liquefaction unit. A corresponding LNG plant is also provided.

Claims

1. A method for operation of a liquefied natural gas (LNG) plant, wherein the plant comprises: an inlet for receiving natural gas; a CO.sub.2 removal unit; a drying and mercury-removal unit; a pre-cooling or refrigeration unit; a liquefaction unit; an end flash or N.sub.2 stripping unit; and an LNG storage tank; wherein natural gas enters at the inlet, flows along a flow path through the CO.sub.2 removal unit, the drying and mercury-removal unit, the pre-cooling or refrigeration unit, the liquefaction unit and end flash or N.sub.2 stripping unit in turn and is stored as liquefied natural gas in the LNG storage tank and is stored as liquefied natural gas in the storage tank; the plant further comprising an LNG pump connected to the end flash or N.sub.2 stripping unit; and an LNG vaporizer connected to the LNG pump; the method comprising the steps of: removing LNG from the end flash or N.sub.2 stripping unit; passing the removed LNG through the LNG pump, to pump the removed LNG to a pressure of about 5-10 MPa; passing the pressurized LNG to the LNG vaporizer to vaporize the pressurized LNG so that the pressurized LNG is transformed to gas phase; and re-admitting the vaporized LNG to the flow path at a point downstream of the inlet and upstream of the liquefaction unit; and wherein the method is carried out during turndown of the LNG plant, when the LNG storage tank is full or when there is an interruption in supply of natural gas through the inlet, and the method is carried on until the LNG can be loaded from the LNG storage tank, or the supply of natural gas at the inlet is recommenced.

2. The method according to claim 1, wherein the vaporized LNG is re-admitted at a rate less than the plant's full production rate.

3. The method according to claim 2, wherein the vaporized LNG is re-admitted at a rate that corresponds to about 30% of the plant's full production rate or the turndown rate of the plant.

Description

(1) These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.

(2) FIG. 1 is a block diagram of an LNG plant according to prior art.

(3) FIG. 2 is a block diagram of an LNG plant according to an embodiment of the present invention.

(4) FIG. 3 is a block diagram of an LNG plant according to another embodiment of the present invention.

(5) FIG. 1 is block diagram of an LNG plant 10 according to prior art. The plant 10 comprises, in sequence: an inlet 12 for receiving natural gas, a CO.sub.2-removal unit 14, a drying and mercury-removal unit 16, a pre-cooling or refrigeration unit 18, a liquefaction unit 20, and an LNG storage tank 22. A main flow line 24 runs from the inlet 12 to the LNG storage tank 22. The general operation of such an LNG plant is known to the person skilled in the art, and will not be explained in further detail here.

(6) In a prior art start-up procedure, natural gas is flared downstream of the CO.sub.2-removal unit 14, as illustrated in FIG. 1 by reference F. Flaring of natural gas, however, causes losses of natural gas and unwanted emissions.

(7) FIG. 2 is a block diagram of an LNG plant 10 according to an embodiment of the present invention. The LNG plant 10 in FIG. 2 comprises, in sequence: an inlet 12 for receiving natural gas, a CO.sub.2-removal unit 14, a drying and mercury-removal unit 16, a pre-cooling or refrigeration unit 18, a liquefaction unit 20, an end flash or N.sub.2 stripping unit 21, and an LNG storage tank 22. A main flow line or path 24 runs from the inlet 12, through the various units 14-21, and to the LNG storage tank 22. A rundown line to the LNG storage tank 22 is designated 25.

(8) In addition, the plant 10 comprises an LNG pump 26 and an LNG vaporizer 28. The LNG pump 26 is in fluid communication with the LNG storage tank 22 via line 30, and with the LNG vaporizer 28 via line 32. Further, the LNG vaporizer 28 is in fluid communication with the main flow line 24 at a location 34 between the last of the gas pre-treatment unit 14-16, namely the drying and mercury-removal unit 16, and the pre-cooling unit 18 via line 36. The LNG pump 26 is adapted to pump LNG removed from the LNG tank 22 via line 30 to a pressure of about 5-10 MPa. The vaporizer 28 is adapted to vaporize the removed (and pressurized) LNG, by heating below the critical pressure of LNG. Said lines may for example be pipes, piping, or the like.

(9) During start-up of the plant 10, i.e. when the temperature of heat exchangers in the liquefaction unit 18 is above a production temperature (they may for instance be at ambient temperature) following e.g. a production stop, the ordinary gas flow at the inlet 12 is shut off, and LNG may be removed or extracted from the LNG storage tank 22 and provided to the LNG pump 26 by means of line 30. The removed LNG is then pumped to a pressure of about 5-10 MPa by means of the LNG pump 26. The pressurized LNG is then supplied via line 32 to the LNG vaporizer 28 where it is vaporized and hence is transformed to gas phase. Thereafter, the vaporized LNG is fed or readmitted or otherwise returned into the main flow path 24 via line 36.

(10) The re-admitted vaporized LNG is then transported or re-circulated in the main flow path 24 through the liquefaction unit 20 for cooling heat exchangers (not shown) in the liquefaction unit 20. The re-circulating natural gas acts as a heat sink for a refrigerant of the heat exchangers, and is hence not directly used as a refrigerant in the heat exchangers.

(11) The method according to this embodiment is carried on until the heat exchangers reach a production temperature, typically from about 35 C. in the pre-cooling unit 18 down to below 100 C. in the liquefaction unit 20, and then the regular production process follows.

(12) The LNG pump 26, the LNG vaporizer 28, and the lines 30, 32, 36 in FIG. 2 are dimensioned and/or controlled such that the vaporized LNG is re-admitted at a rate that corresponds to about 1-10%, or specifically 1-5%, of the full or regular production rate of the plant 10. Such control may be performed by a control means (not shown) of the plant 10.

(13) FIG. 3 is a block diagram of an LNG plant 10 according to another embodiment of the present invention. The LNG plant 10 in FIG. 3 comprises, in sequence: an inlet 12 for receiving natural gas, a CO.sub.2-removal unit 14, a drying and mercury-removal unit 16, a pre-cooling or refrigeration unit 18, a liquefaction unit 20, an end flash or N.sub.2 stripping unit 21, and an LNG storage tank 22. A main flow line or path 24 runs from the inlet 12, through the various units 14-21, and to the LNG storage tank 22. The line between the liquefaction unit 20 and the end flash or N.sub.2 stripping unit 21 is designated 23, and a rundown line to the LNG storage tank 22 is designated 25.

(14) In addition, the plant 10 comprises an LNG pump 26 and an LNG vaporizer 28. The LNG pump 26 is in fluid communication with the end flash or N.sub.2 stripping unit 21 via line 30, and with the LNG vaporizer 28 via line 32. Further, the LNG vaporizer 28 is in fluid communication with the main flow line 24 at a location 38 between the inlet 12 and the first gas pre-treatment unit, namely the CO.sub.2-removal unit 14, via line 40. The LNG pump 26 is adapted to pump LNG removed from the LNG tank 22 via line 30 to a pressure of about 5-10 MPa. The vaporizer 28 is adapted to vaporize the removed (and pressurized) LNG, below the critical pressure of LNG. Said lines may for example be pipes, piping, or the like.

(15) During turndown of the plant 10, e.g. when the LNG tank 22 is full or when there is an interruption or significant decrease in supply of natural gas through the inlet 12, the ordinary gas flow at the inlet 12 is purposely or unintentionally shut off, and LNG is removed or extracted from the end flash or N.sub.2 stripping unit 21 and supplied to the LNG pump 26 by means of line 30. The removed LNG is then pumped to a pressure of about 5-10 MPa by means of the LNG pump 26. The pressurized LNG is then supplied via line 32 to the LNG vaporizer 28 where it is vaporized and hence is transformed to gas phase. Thereafter, the vaporized LNG is fed or readmitted or otherwise returned into the main flow path 24 via line 40.

(16) The re-admitted vaporized LNG is then transported or re-circulated in the main flow path 24 to keep the plant 10 operating at a reduced rate. The LNG pump 26, the LNG vaporizer 28, and the lines 30, 32, 40 in FIG. 3 are dimensioned and/or controlled such that the vaporized LNG is re-admitted at a rate that corresponds to about 30% of the full or normal production rate of the plant 10, or at a rate equal to the turndown rate of the plant 10. Such control may be performed by the above-mentioned control means.

(17) The method according to this embodiment is carried on until the LNG can be loaded from the storage tank 22 as usual, or the supply of natural gas at the inlet 12 is recommenced, for instance, and full production in the plant 10 can resume.

(18) Optionally, lines 42 and 44 may be provided to supply vaporized LNG also at other locations. Vaporized LNG may for instance be supplied via line 42 in case the CO.sub.2-removal unit 14 is malfunctioning, or via line 44 in case the drying and mercury-removal unit 16 is out of order. Further, the LNG may alternatively be taken from line 23 between the liquefaction unit 20 and the end flash or N.sub.2 stripping unit 21 via line 46, or from the LNG storage tank 22 via line 48. The optional and alternative lines are illustrated with dashed lines in FIG. 3, and said lines may for example be appropriate pipes, piping, or the like.

(19) The LNG plant 10 according to the present invention typically has a minimum capacity of 1 MTPA (million metric tonnes per annum). However, the present invention could also be applied to plants having a capacity down to 0.1 MPTA, for example.

(20) The person skilled in the art will realize that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

(21) For instance, instead of vaporizing the removed LNG, the removed LNG can be heated, typically above its critical pressure, such that the LNG changes or transitions to gas phase. In such a case, the vaporizer 28 may be replaced by a heater adapted to heat the removed LNG so that the removed LNG is transformed to gas phase.