GB2494627A

GB2494627A - Method and apparatus for cooling boil off gas on a ship, barge or floating platform

Info

Publication number: GB2494627A
Application number: GB201115461A
Authority: GB
Inventors: Alan Roderick Duckett
Original assignee: Liquid Gas Equipment Ltd
Current assignee: Liquid Gas Equipment Ltd
Priority date: 2011-09-07
Filing date: 2011-09-07
Publication date: 2013-03-20
Also published as: GB201115461D0

Abstract

The present invention provides a method of and apparatus for cooling, preferably liquefying, a first boil off gas (BOG) stream from a liquefied first cargo in a floating transportation vessel, said liquefied first cargo having a boiling point of greater than -110Â°C at 1 atmosphere, said method comprising at least the steps of: passing refrigerant in a cooled, optionally expanded, refrigerant stream (310a, 310b) and a warmed intermediate heat transfer fluid (210a, 210b, 270) to a refrigerant heat exchanger (300); heat exchanging the cooled, optionally expanded, refrigerant stream (310a, 310b) with the warmed intermediate heat transfer fluid (210, 270) in the refrigerant heat exchanger (300) to provide a warmed refrigerant stream (320) and a cooled intermediate heat transfer fluid (220, 280); passing a first boil off gas stream (10a, 10b) from a liquefied first cargo and the cooled intermediate heat transfer fluid (220, 280) to a first cargo heat exchanger (200); heat exchanging the first boil off gas stream (10) with the cooled intermediate heat transfer fluid (220, 280) in the first cargo heat exchanger (200) to provide a cooled first cargo stream(20) and the warmed intermediate heat transfer fluid (210a, 210b, 270).

Description

METHOD OF COOLING BOIL OFF GAS AND AN APPARATUS

THEREFOR

The present invention relates to a method of cooling, particularly reliquefying, a boil off gas from a liquefied cargo, such as liquefied petroleum gas (LPG), on a floating transportation vessel, and an apparatus therefor.

Floating transportation vessels, such as liquefied gas carriers and barges are capable of transporting a variety of cargoes in the liquefied state. In the present context, these liquefied cargoes have boiling points of greater than -110°C when measured at 1 atmosphere and include liquefied petroleum gas, liquefied petrochemical gasses such as propylene and ethylene and liquid ammonia. For instance, liquefied petroleum gas is a useful fuel source, such as for heating appliances and vehicles, as well as being a source of hydrocarbon compounds. LPG comprises one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylenes and ethane.

Petroleum gases can be extracted from natural gas or produced in the refining of crude oil. It is often desirable to liquefy petroleum gases in a liquefaction facility at or near the source of extraction or manufacture. As an example, petroleum gases can be stored and transported over long distances more readily as a liquid than in gaseous form because they occupy a smaller volume and may not need to be stored at high pressures. For instance, LPG can be stored at atmospheric pressure if maintained at or below its boiling temperature, such as at -42 t or below, being the boiling point of the propane component. Alternatively, LPG may be stored at higher temperatures if it is pressurized above atmospheric pressure.

The long distance transportation of LPG or other liquefied cargo having boiling pointofgreaterthan -110 °Cwhen measured at 1 atmosphere may be carried out in a suitable LFG carrier, such as an ocean-going vessel having one or more storage tanks to hold the liquefied cargo. These storage tanks may be insulated and/or pressurized tanks. During the loading of the storage tanks and the storage of liquefied cargo such as LPG in the storage tanks, gas, such as petroleum gas, may be produced due to the evaporation of the cargo. This evaporated cargo gas such as petroleum gas is known as boil off gas (BOG). In order to prevent the build up of BOG in the storage tank, a system may be provided on the carrier to re-liquefy the BOG so that it can be returned to the storage tank in a condensed state. This can be achieved by the compression and cooling of the BOG. In many systems, the compressed BOG is cooled and condensed against seawater. Alternatively, the BOG can be re-liquefied against a refrigerant in a refrigerant circuit.

International patent application no. PCT/NO2O1 0/000166 (WO 2011/002299) discloses a method for the storage and transport of [PG on [PG carriers having two cargo types. Vaporized gas from a first cargo type is condensed in a reliquefaction unit to provide a first condensed vapor which is passed to a heat exchanger. Vapor from a second cargo type is then passed to the heat exchanger where it is condensed by means of heat exchange with the first condensed vapor. The two condensed vapors leaving the heat exchanger are then returned to their respective cargo types.

The method of International patent application no. PCT/NO2O1 0/0001 66 provides limited control of the condensation of the vapor of the second cargo type. In particular, there is little freedom for varying the cooling provided by the first condensed vapor to the vapor from the second cargo type, particularly to prevent sub-cooling when the vapor of the second cargo type is condensed. A need exists to provide an alternative method of cooling, particularly liquefying, boil off gas from a liquefied cargo having a boiling point of greater than -110 °C when measured at 1 atmosphere in a floating transportation vessel. In particular, a method which provides improved control of the cooling process, particularly to reduce or prevent sub-cooling of condensed boil off gas is desirable.

The present invention addresses this problem by providing a method of and apparatus for cooling a first boil off gas from a liquefied first cargo in which an intermediate heat transfer fluid is provided to transfer the cooling duty from a refrigerant, such as a second cargo, to the first boil off gas.

Comparing the terminology used herein with that of International patent application no. PCT/N020101000166, the first boil off gas from a liquefied first cargo of the present invention is comparable to the vapor from the second cargo type of International patent application no. The use of an intermediate heat transfer fluid in the present invention allows the separation of the heat exchange between the first boil off gas and the refrigerant into two heat exchange operations which are carried out in different heat exchangers. The physical properties of an intermediate heat transfer fluid, such as flow rate, temperature and/or pressure can be manipulated to adjust the rate of heat exchange between the refrigerant and the first boil off gas in order to better control the cooling process and avoid sub-cooling.

In a first aspect, the present invention provides a method of cooling a first boil off gas from a liquefied first cargo in a floating transportation vessel, said liquefied first cargo having a boiling point of greater than -110 °C at 1 atmosphere, said method comprising at least the steps of: -passing refrigerant in a cooled, optionally expanded, refrigerant stream and a warmed intermediate heat transfer fluid to a refrigerant heat exchanger; -heat exchanging the cooled, optionally expanded, refrigerant stream with the warmed intermediate heat transfer fluid in the refrigerant heat exchanger to provide a warmed refrigerant stream and a cooled intermediate heat transfer fluid; -passing a first boil off gas stream from a liquefied first cargo and the cooled intermediate heat transfer fluid to a first cargo heat exchanger; -heat exchanging the first boil off gas stream with the cooled intermediate heat transfer fluid in the first cargo heat exchanger to provide a cooled first cargo stream and the warmed intermediate heat transfer fluid.

The first cargo heat exchanger and the refrigerant heat exchanger are not the same heat exchanger. Thus, the first boil off gas stream does not exchange heat with the cooled, optionally expanded, refrigerant stream in the same heat exchanger. The heat is transferred between the first boil off gas stream and cooled, optionally expanded, refrigerant stream in the different heat exchangers by the intermediate heat transfer fluid.

The refrigerant can be more volatile than the first cargo, such that the first cargo has a lower vapor pressure than that of the refrigerant, when measured under identical pressure and temperature.

In one embodiment, the first boil off gas stream may be provided from a liquefied first cargo storage tank.

In another embodiment, the method may further comprise the steps of: -expanding the cooled first cargo stream to provide an expanded first cargo stream; -passing the expanded first cargo stream to a liquefied first cargo storage tank.

In a further embodiment, the cooled intermediate heat transfer fluid and warmed intermediate heat transfer fluid can be passed between the refrigerant heat exchanger and the first cargo heat exchanger in one or more intermediate heat transfer fluid streams. For instance, the warmed intermediate heat transfer fluid may be present in a warmed intermediate heat transfer fluid stream, and the cooled intermediate heat transfer fluid may be present in a cooled intermediate heat transfer fluid streams. If necessary, the intermediate heat transfer fluid streams may be passed between the first cargo heat exchanger and the refrigerant heat exchanger by mechanical fluid transfer means, such as pumping, compression or blowing, depending upon the phase of such streams. Adjustment of the mechanical fluid transfer means, to manipulate the mass flow of the intermediate heat transfer fluid, can provide improved control of the heat exchange, particularly cooling, of the first boil off gas stream.

In another embodiment, the cooled intermediate heat transfer fluid may be at least partially, preferably fully, condensed and/or the warmed ntermediate heat transfer fluid may be at east partially, preferably fully, gaseous.

In yet another embodiment, one or both of the temperature and pressure of the intermediate heat transfer fluid may be adjusted to provide improved control of the heat exchange, particularly cooling of the first boil off gas stream.

In a yet further embodiment, the refrigerant heat exchanger may be located at a point gravitationally higher than the first cargo heat exchanger. Providing at least one intermediate heat transfer fluid connection between the heat exchangers, for instance in the form of a pipe, enables condensed cooled intermediate transfer fluid to fall from the refrigerant heat exchanger to the first cargo heat exchanger and gaseous warmed intermediate heat transfer fluid to rise from the first cargo heat exchanger to the refrigerant heat exchanger. Thus, the first cargo heat exchanger can be gravitationally fed with condensed cooled intermediate transfer fluid by the refrigerant heat exchanger. The gaseous warmed intermediate heat transfer fluid, being less dense that the condensed fluid, can rise into the refrigerant heat exchanger. Typically at least one intermediate heat transfer fluid connection can be provided between a first port at or near the bottom of the refrigerant heat exchanger and a first port at or near the top of the first cargo heat exchanger.

In yet another embodiment, the cooled, optionally expanded, refrigerant stream can be at least partially, preferably fully, condensed.

In a still further embodiment, the method may further comprise the steps of: -passing the warmed refrigerant stream to a cooflng, preferably liquefying, unit to provide the cooled refrigerant stream; -optionally expanding the cooled refrigerant stream to provide a cooled, expanded refrigerant stream.

In a further embodiment, the refrigerant may be selected from one or more of the group comprising halohydrocarbons, particularly haloalkanes, such asASHRAE Nos. R-11, R-12, R-13, R-14, R-20, R-21, R-22, R-23, R-30, R-31, R-32, R-40, R-41 and R-50; C2 to C4 hydrocarbons, such as ethane, propane, n-butane, i-butane, propylene, n-butylene and i-butylene; and ammonia.

In an alternative embodiment, the refrigerant may comprise a second cargo of the floating transportation vessel. For instance, the refrigerant may be derived from the boil off gas from a liquefied second cargo. In such embodiments, the method may further comprise the steps of: -cooling a second boil off gas stream from a liquefied second cargo in a cooling, preferably liquefying, unit to provide a cooled second cargo stream as the cooled refrigerant stream; -expanding the cooled second cargo stream to provide a cooled, expanded second cargo stream as the cooled, optionally expanded refrigerant stream.

More than one cooling, preferably liquefying, unit may be provided to meet the requirements of the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (the IGC Code) and other owner and/or insurer requirements. Typically at least one standby cooling, preferably liquefying, unit is provided for redundancy, should a running unit fail or be required to be shut down for repair or maintenance.

In a further embodiment, the step of cooflng the second boil off gas stream may comprise the steps of: -compressing the second boil off gas stream to provide a compressed second boil off gas stream; -cooling the compressed second boil off gas stream to provide the cooled second cargo stream.

In a still further embodiment, the step of heat exchanging the cooled, expanded, refrigerant stream with the warmed intermediate heat transfer fluid may provide a warmed second cargo stream as the warmed refrigerant stream, such that the method further comprising the steps of: -separating the warmed second cargo stream to provide a second cargo feed stream and a second cargo return stream.

In another embodiment, the method further comprises the steps of: -expanding the second cargo return stream to provide an expanded second cargo return stream; -passing the expanded second cargo return stream to a liquefied second cargo storage tank.

In yet another embodiment, the method may further comprise the steps of: -compressing the second cargo feed stream to provide the compressed second boil off gas stream.

The second boil off gas stream and the second cargo feed stream may be compressed in different compressors of a compressor system operating at different suction pressures, or they may be compressed in different stages of a multi-stage compressor operating at different suction pressures.

Thus, the second boil off gas stream may be a low pressure stream, the second cargo feed stream may be an intermediate pressure stream and the compressed second boil off gas stream produced at the discharge of the compressor system or multi-stage compressor may be a high pressure stream. Compressing the second cargo feed stream improves the coefficient of performance of the method of cooling because warming which would be incurred if this stream were to be returned to the liquefied second cargo storage tank and then passed to the low pressure suction of the compressor is avoided.

In a further embodiment, the second cargo may be selected from one or more the group comprising C2 to C4 hydrocarbons, such as ethane, propane, n-butane, i-butane, propylene, n-butylene and i-butylene; and ammonia.

In another embodiment, the liquefied first cargo may be selected from one or more of the group comprising C2 to C4 hydrocarbons, such as ethane, propane, n-butane, i-butane, propylene, n-butylene and i-butylene; and ammonia.

In a further embodiment, the intermediate heat transfer fluid is selected from the group comprising a refrigerant as defined herein, particularly ammonia; oil, such as silicone or mineral oil; and polyalkylene glycol, such as ethylene glycol.

In preferred embodiments, the refrigerant such as the second cargo, intermediate heat transfer fluid and first cargo are each different i.e. different compounds and/or compositions.

In a further embodiment, the first cargo is butane such as n-butane and/or i-butane, the intermediate heat transfer fluid is ammonia and the second cargo is propane.

In a second aspect, the present invention provides an apparatus to cool a first boil off gas stream from a liquefied first cargo in a floating transportation vessel, said liquefied first cargo having a boiling point of greater than -110 °C at 1 atmosphere, said apparatus comprising at least: -a first cargo heat exchanger having a first inlet for a first boil off gas stream, a first outlet for a cooled first cargo stream and at least one port for an intermediate heat transfer fluid in fluid connection with at least one port of a refrigerant heat exchanger; -a refrigerant heat exchanger having a first inlet for a cooled refrigerant stream, said first inlet in fluid connection with a first outlet of a cooling, preferably liquefying, unit, a first outlet for a warmed refrigerant stream, said first outlet in fluid connection with a first inlet of the cooling, preferably liquefying, unit, and at least one port for an intermediate heat transfer fluid in fluid connection with at least one port of the first cargo heat exchanger; -a cooling, preferably liquefying, unit for cooling, preferably liquefying, the warmed refrigerant stream at a first inlet to provide the cooled refrigerant stream at a first outlet.

In one embodiment of the second aspect, the refrigerant heat exchanger may be located gravitationally higher than the first cargo heat exchanger.

The refrigerant heat exchanger need not be located directly above the first cargo heat exchanger. However, the first cargo heat exchanger is preferably located with respect to the refrigerant heat exchanger so that it can be gravitationally fed with cooled, preferably condensed, intermediate heat transfer fluid from the refrigerant heat exchanger.

The present invention is applicable to any floating transportation vessel for a liquefied cargo having a boiling point of greater than -110 °C at 1 atmosphere, such as a LPG carrier. The present invention may be utilized in floating transportation vessels where the liquefied cargo storage tanks are fully refrigerated to maintain the cargo in liquid phase at approximately atmospheric pressure by lowering the temperature, as well as in those vessels in which the cargo in the storage tanks is maintained in the liquid phase by a combination of reduced temperature and increased pressure.

The method and apparatus disclosed herein utilize different heat exchangers for the first boil off gas heat exchanger and the refrigerant heat exchanger. Two or more fluids may transfer heat in the same heat exchanger. However, the two or more fluids may not comprise both the first boil off gas stream and the refrigerant stream. For instance, the heat exchangers utilized herein may be shell and tube heat exchangers comprising a shell side and a tube side. The intermediate heat transfer fluid may be present in the shell side of the heat exchanger, while the cooled refrigerant or the first boil off gas may be present in the tube side.

The shell defines the envelope or external surface of the heat exchanger.

The refrigerant, such as the second cargo, may have the same or different volatility to that of the first cargo. In a preferred embodiment, the refrigerant is more volatile i.e. has a higher vapor pressure, compared to the first cargo. In such an embodiment, the first cargo would be less volatile i.e. have a lower vapor pressure than the refrigerant.

The vapor pressures of common liquefied cargos and refrigerants are known to the skilled person. If necessary, vapor pressure can be measured experimentally by methods known in the art. In order to be able to compare the vapor pressure of the first cargo and refrigerant, vapor pressure measurements should be carried out using the same experimental procedure, under identical temperature and pressure conditions e.g. standard temperature and pressure.

The heat exchange steps occurring in the first cargo and refrigerant heat exchangers are indirect, such that the intermediate heat transfer fluid and (i) the first boil off gas stream or (ii) the cooled refrigerant stream involved in the heat exchange in a particular heat exchanger are not in direct contact.

Embodiments of the invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which: Figure 1 shows a schematic diagram of a method of and apparatus for cooling, particularly reliquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to one embodiment of the present invention; Figure 2 shows a schematic diagram of a method of and apparatus for cooling, particularly reliquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to a further embodiment of the present invention; Figure 3 shows a schematic diagram of an arrangement of refrigerant and first cargo heat exchangers which can be used in embodiments of the method of and apparatus for cooling, particularly reliquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to the present invention.

Typically, floating transportation vessels store liquefied petroleum gas (LPG) comprising one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylenes and ethane in one or more storage tanks under one or both of reduced temperature (versus ambient) and increased pressure (versus atmospheric) in order to maintain the petroleum gas in a liquefied state.

The storage tank may be risulated, refrigerated and/or pressurized.

Vaporization of the [PG in the storage tank, for instance due to imperfect thermal insulation, will result in the formation of petroleum gas in the overhead space of the storage tank. In order to prevent the build-up of this gas, it is removed from the storage tank as a boil off gas stream. The removed boil off gas is normally reliquified before it is returned to the storage tank.

Figure 1 provides a schematic diagram of an apparatus 1 of a first embodiment of the present invention. A first cargo circuit 50, intermediate heat transfer fluid circuit 250 and a refrigerant circuit 450 are provided.

The first cargo circuit 50 and the refrigerant circuit 450 are linked via the intermediate heat transfer fluid circuit 250. There should be no heat transfer between the first cargo circuit 50 and the refrigerant circuit 450 other than through the intermediate heat transfer fluid circuit 250.

Turning to the first cargo circuit, this comprises at least one liquefied first cargo storage tank 100, a first cargo heat exchanger 200, optionally one or more mechanical fluid transfer devices 5 and optionally one or more first cargo pressure reduction means 25. A first boil off gas stream ba can be drawn from liquefied first cargo storage tank 100 and passed to the first cargo heat exchanger 200. In certain embodiments, the first boil off gas may flow around first cargo circuit 50 by natural circulation. In other embodiments, a mechanical fluid transfer means such as a blower or compressor is provided. A blower 5 is shown in the apparatus 1 of Figure 1, providing first boil off gas stream lOb at its discharge.

The first boil off gas stream 1 Ob is passed to the first inlet of first cargo heat exchanger 200, where it is heat exchanged, more particularly cooled, against cooled intermediate transfer fluid to provide a cooled first cargo stream 20 at a first outlet. The cooled first cargo stream may be a partly or fully condensed stream. The first cargo heat exchanger 200 may be a shell and tube heat exchanger, a plate heat exchanger or a plate fin heat exchanger. Typically, the first boil off gas stream 1 Ob may be passed to one or more tube bundles in a shell and tube heat exchanger, while the intermediate heat transfer fluid is present in the shell-side.

The cooled first cargo stream 20 may be optionally passed through a first cargo pressure reduction device 25, such as a Joule-Thomson valve, to provide an expanded first cargo stream 30 which can be returned to the first cargo storage tank 100. The expanded first cargo stream 30 may be a partially or fully condensed stream.

The intermediate heat transfer fluid circuit 250 comprises first cargo heat exchanger 200, refrigerant heat exchanger 300 and one or more optional mechanical fluid transfer means 215, such as a pump or compressor. A cooled intermediate heat transfer fluid stream 220 is passed to a first port of first cargo heat exchanger 200, where it is heat exchanged, more particularly warmed, against the first boil off gas stream 1 Ob to provide a warmed intermediate heat transfer fluid stream 210a ata second port. A mechanical fluid transfer means 215 takes the warmed intermediate heat transfer fluid stream ba at its suction and provides warmed intermediate heat transfer fluid stream lOb at its discharge. Warmed intermediate heat transfer fluid stream 1 Ob is passed to a first inlet of the refrigerant heat exchanger 300, where it is heat exchanged, more particularly cooled, with a cooled, expanded refrigerant stream 310b to provide the cooled intermediate heat transfer stream 220.

The intermediate heat transfer fluid may be a vapor, a liquid or a mixture of vapor and liquid. The phase or the proportion of vapor and liquid phases of the intermediate heat transfer fluid may remain constant or may vary at different points in the intermediate heat transfer fluid circuit 250. In the embodiment of Figure 1 the intermediate heat transfer fluid remains in the liquid phase throughout the circuit. Consequently, mechanical fluid transfer means 215 is shown as a pump. The mechanical fluid transfer means 215 can be used to adjust the mass flow of the cooled intermediate heat transfer stream 220 which is passed to the first cargo heat exchanger 200, thereby controlling the cooling of the first boil off gas stream lOb.

The intermediate heat transfer fluid may be a polyalkylene glycol, such as ethylene glycol.

The embodiment of Figure 1 shows a generalized refrigerant circuit 450, such as a closed refrigerant circuit e.g. in a refrigerant pack. In other embodiments, such as that of Figure 2, the refrigerant circuit may be a circuit for cooling, preferably liquefying, the boil off gas from a liquefied second cargo. This is discussed in more detail below. In Figure 1, the refrigerant circuit 450 comprises the refrigerant heat exchanger 300, a cooling, preferably liquefying, unit 400, and optionally one or more refrigerant pressure reduction devices 315. The refrigerant may comprise a single component or a plurality of components.

A warmed refrigerant stream 320 is passed to a first inlet of a cooling, preferably liquefying, unit 400. The warmed refrigerant stream may be partly, typically fully vaporized. The cooling unit 400 typically compresses and then cools the refrigerant against one or more ambient sources, such as air or water, particularly sea water, to provide a cooled refrigerant stream 31 Oa at a first outlet. The compression may be carried out by a single stage compressor, a system of compressors in series or a multi-stage compressor. Such refrigerant units are known ri the art. The cooled refrigerant stream 31 Oa may be partly, typically fully, condensed.

In this way, the latent heat of vaporization of the refrigerant can be maximized for use in the refrigerant heat exchanger 300.

The cooled refrigerant stream 310a may be optionally passed to a refrigerant pressure reduction device 315, such as an expander or Joule-Thomson valve to provide cooled, expanded refrigerant stream 310b.

Cooled, expanded refrigerant stream 31 Ob can then be passed to the first inlet of the refrigerant heat exchanger 300. The cooled, expanded refrigerant stream 310b is heat exchanged, more particularly warmed, against the warmed intermediate heat transfer stream 210b to provide the warmed refrigerant stream 320 at a first outlet. The refrigerant heat exchanger 300 may be similar to those discussed above for the first cargo heat exchanger 200. When the refrigerant heat exchanger 300 is a shell and tube heat exchanger, the cooled, expanded refrigerant stream 310b may be present in one or more tube bundles, with the intermediate heat transfer fluid present in the shell-side.

Figure 2 shows a schematic diagram of an apparatus 1 of a further embodiment of the present invention. The reference numerals which are identical to those of Figure 1 relate to corresponding streams and devices.

The first cargo circuit 50 is equivalent to that of the embodiment of Figure 1.

The refrigerant in this embodiment comprises a second cargo of the floating transportation vessel, and in particular a more volatile cargo.

Refrigerant circuit 450 therefore corresponds to a cycle for the cooling, preferably liquefaction, of boil off gas from the liquefied second cargo.

Boil off gas produced in liquefied second cargo storage tank 500 is cooled and optionally liquefied in a cooling, preferably liquefying, unit 400. Unit 400 comprises a compressor 600, a compressor driver 605 and one or more cooling devices 375. Second boil off gas stream 360 is drawn from liquefied second cargo storage tank 500 to the suction of a compressor 600. Compressor 600 may be a system of compressors or a multi-stage compressor. The latter is shown in Figure 2. The compressor 600 is driven by a compressor driver 605, which is typically a turbine, such as a gas turbine or an electric motor. The compressor 600 compresses the second boil off gas stream 360 to provide a compressed second boil off gas stream 370.

The second compressed boil off gas stream 370 can be cooled in one or more cooling devices 375. Usually the compressed second boil off gas stream 370 is cooled against an ambient source, such as a seawater stream. Typically the seawater stream can have a temperature of +36 C or below, more typically +32 C or below. The one or more cooling devices 375 provide a cooled second cargo stream 31 Oa as the cooled refrigerant stream. The cooled second cargo stream 310a may be a partially or fully condensed stream.

The cooled second cargo stream 310a may optionally be passed to a second cargo pressure reduction device 315, such as an expander or a Joule-Thomson valve, to provide a cooled, expanded second cargo stream 310b. The cooled, expanded second cargo stream 310b can then be passed to a first inlet of a second cargo heat exchanger 300 as the refrigerant heat exchanger.

The second cargo heat exchanger 300 and the first cargo heat exchanger 200 can be connected by at east one intermediate heat transfer fluid conduit via at least one port in each exchanger. Two conduits 260a, 260b are shown in the embodiment of Figure 2. In this embodiment, the first and second cargo heat exchangers 200, 300 are preferably shell and tube heat exchangers. The first and second cargos can be present within one or more tube bundles in their respective heat exchangers. The intermediate heat transfer fluid can be present in the shell-side of each heat exchanger.

When the second cargo heat exchanger 300 is located gravitationally above the first cargo heat exchanger 200, the warmed intermediate heat transfer fluid is a vapor and the cooled intermediate heat transfer fluid is a condensed liquid, then the intermediate heat transfer fluid can circulate between the two heat exchangers naturally, without the need of mechanical intervention. In order for efficient circulation, the at least one port of the second cargo heat exchanger 300 should be at or near the bottom of the exchanger while the at least one port of the first cargo heat exchanger 200 should be at or near the top of the heat exchanger.

Ammonia functions well as an intermediate heat transfer fluid in this embodiment.

Thus, when the tube bundles of the first cargo heat exchanger 200 containing the first boil off gas come into contact with cooled intermediate heat transfer fluid as a condensed liquid, the heat exchange between the two fluids can provide cooled first cargo stream 20 and evaporate the condensed intermediate heat transfer fluid to provide warmed intermediate heat transfer fluid as a vapor. Being less dense than the liquid, the warmed intermediate heat transfer fluid vapor will rise into the second cargo heat exchanger 300 through the at least one intermediate heat transfer fluid conduit 260a, 260b.

The warmed intermediate heat transfer fluid vapor can heat exchange and condense with the cooled expanded second cargo in the one or more tube bundles of the second cargo heat exchanger 300 and condense. The condensed fluid will fall under gravity to the bottom of the second cargo heat exchanger, and flow through the port down the at least one intermediate heat transfer fluid conduit 260a, 260b and return to the pool of condensed liquid intermediate heat transfer fluid in the first cargo heat exchanger.

When the cooled intermediate heat transfer fluid is at condensed, increasing the pressure of the intermediate heat transfer fluid in the first cargo heat exchanger 200 can lead to an increase in temperature of the condensate used to cool the first boil off gas stream, and vice versa. This can be used to adjust the temperature difference between the first boil off gas stream and the cooled intermediate heat transfer fluid, thereby controlling the cooling of the first boil off gas stream.

Unlike the warmed and cooled intermediate fluid transfer streams 220, 210 of Figure 1, intermediate heat transfer fluid conduits 260a, 260b may carry both warmed and cooled intermediate heat transfer fluid. Thus, the intermediate heat transfer fluid circuit in this embodiment comprises the first and second cargo heat exchangers 200, 300 and the at least one intermediate heat transfer fluid conduit 260a, 260b.

The heat exchange in the second cargo heat exchanger 300 provides a warmed second cargo stream 320 as the warmed refrigerant stream at a first outlet. The warmed second cargo stream 320 may be a multi-phase stream comprising gaseous and condensed phases. The warmed second cargo stream can be passed to a gas/liquid separation device 325, such as an open flash vessel, to provide a second cargo feed stream 330 and a second cargo return stream 340. The second cargo feed stream 330 can be a vapor stream, typically with an intermediate pressure compared to second boil off gas stream 360, which is typically a low pressure stream.

The second cargo feed stream 330 can therefore be passed to an intermediate stage of multi-stage compressor 600 (or an intermediate pressure suction of a multiple compressor system), for compression to provide part of compressed second boil off gas stream 370 at the discharge.

Second cargo return stream 340, can be passed to second return stream pressure reduction device 345, such as an expander or a Joule-Thomson valve, where it is expanded to provide expanded second cargo stream 350. Expanded second cargo stream 350 can be passed back to liquefied second cargo storage tank 500.

Figure 3 shows an alternative arrangement of first cargo heat exchanger and refrigerant heat exchanger 300 that can be used in the present invention. The remaining portions of the refrigerant circuit and the first cargo circuit may be as described above for Figure 1 and/or Figure 2. The reference numerals which are identical to those of Figure 1 relate to corresponding streams and devices.

In a similar manner to the arrangement of Figure 2, the first cargo heat exchanger 200 may be located gravitationally below refrigerant heat exchanger 300, but in this embodiment is displaced horizontally so that it is not directly below the refrigerant heat exchanger 300. Two intermediate heat transfer conduits 270, 280 may be provided. A first intermediate heat transfer conduit 270 can extend upwards from a port on the top of the first cargo heat exchanger 200 and enter the refrigerant heat exchanger 300 at a port in its side near its bottom. This conduit 270 can carry warmed intermediate heat transfer fluid as a vapor from the first cargo heat exchanger to the refrigerant heat exchanger e.g. as a warmed intermediate heat transfer fluid stream. A second intermediate heat transfer conduit 280a, 280b, 280c can extend downwards from a port at the bottom of the refrigerant heat exchanger 300 and enter the first cargo heat exchanger 200 at a port in its side near its bottom. This conduit can carry cooled, condensed, intermediate heat transfer fluid from the refrigerant heat exchanger 300 to the first cargo heat exchanger 200 e.g. as a cooled intermediate heat transfer fluid stream.

Furthermore, an intermediate heat transfer fluid storage tank 700 can be optionally provided in the intermediate heat transfer fluid circuit 250. For instance, the second intermediate heat transfer fluid conduit 280a may pass to an intermediate heat transfer fluid storage tank 700, which can act as a reservoir for cooled, condensed intermediate heat transfer fluid.

Further portions 280b, 280c of the second intermediate heat transfer fluid conduit can pass the cooled intermediate heat transfer fluid to the first cargo compressor 200. A heat transfer fluid control valve 705 may be located in the second conduit between the intermediate heat transfer fluid storage tank 700 and the first cargo heat exchanger 200 to regulate the flow of the cooled, condensed intermediate heat transfer fluid to the first cargo heat exchanger in order to control the cooling of the first boil off gas stream. Portion 280b of the second intermediate heat transfer fluid conduit is upstream of the heat transfer fluid control valve 705, while portions 280c is downstream of the control valve. The heat transfer fluid control valve 705 can control the level of the cooled, condensed intermediate heat transfer fluid in the bottom of the first cargo heat exchanger 200, thereby determining the surface area of the tube bundle or bundles through which the first boil off gas stream is passed, which is contacted by the cooled intermediate thermal transfer fluid, thus controlling the cooling of the first boil off gas stream.

The person skilled in the art will understand that the invention can be carried out in many various ways without departing from the scope of the appended claims. Thus, the invention encompasses the combination of one or more of the optional or preferred features disclosed herein.

Claims

<claim-text>CLAIMS1. A method of cooling a first boil off gas from a liquefied first cargo in a floating transportation vessel, said liquefied first cargo having a boiling point of greater than -110 °C at I atmosphere, said method comprising at least the steps of: -passing refrigerant in a cooled, optionally expanded, refrigerant stream (310a, 310b) and a warmed intermediate heat transfer fluid (210a, 210b, 270) to a refrigerant heat exchanger (300); -heat exchanging the cooled, optionally expanded, refrigerant stream (310a, 310b) with the warmed intermediate heat transfer fluid (210, 270) in the refrigerant heat exchanger (300) to provide a warmed refrigerant stream (320) and a cooled intermediate heat transfer fluid (220, 280); -passing a first boil off gas stream (ba, lOb) from a liquefied first cargo and the cooled intermediate heat transfer fluid (220, 280) to a first cargo heat exchanger (200); -heat exchanging the first boil off gas stream (10) with the cooled intermediate heat transfer fluid (220, 280) in the first cargo heat exchanger (200) to provide a cooled first cargo stream (20) and the warmed intermediate heat transfer fluid (210a, 210b, 270).</claim-text> <claim-text>2. The method of claim I wherein the cooled intermediate heat transfer fluid and warmed intermediate heat transfer fluid are passed between the refrigerant heat exchanger (300) and the first cargo heat exchanger (200) in one or more intermediate heat transfer fluid streams (210a, 210b, 220, 260a, 260b, 270, 280a, 280b, 280c).</claim-text> <claim-text>3. The method of claim 1 or claim 2 wherein the cooled intermediate heat transfer fluid (220, 280a, 280b, 280c) is at least partially, preferably fully, condensed and the warmed intermediate heat transfer fluid (210a, 210b, 270) is at least partially, preferably fully, gaseous.</claim-text> <claim-text>4. The method of claim 3 wherein the refrigerant heat exchanger (300) is located at a point gravitationally higher than the first cargo heat exchanger (200) such that condensed cooled intermediate transfer fluid falls from the refrigerant heat exchanger (300) to the first cargo heat exchanger (200) and the gaseous warmed intermediate heat transfer fluid rises from the first cargo heat exchanger (200) to the refrigerant heat exchanger (300).</claim-text> <claim-text>5. The method of any of the preceding claims wherein the cooled, optionally expanded, refrigerant stream (310a, 310b) is at least partially, preferably fully, condensed.</claim-text> <claim-text>6. The method of any of the preceding claims further comprising the steps of: -passing the warmed refrigerant stream (320) to a cooling, preferably liquefying, unit (400) to provide the cooled refrigerant stream (310a); -optionally expanding the cooled refrigerant stream (310a) to provide a cooled, expanded refrigerant stream (310b).</claim-text> <claim-text>7. The method of any of the preceding claims wherein the refrigerant is selected from one or more of the group comprising halohydrocarbons, C2 to C4 hydrocarbons, and ammonia.</claim-text> <claim-text>8. The method of any of claims 1 to 5 wherein the refrigerant comprises a second cargo of the floating transportation vessel, said method comprising the further steps of: -cooling a second boil off gas stream (360) from a liquefied second cargo in a cooling, preferably liquefying, unit (400) to provide a cooled second cargo stream (310a) as the cooled refrigerant stream; -expanding the cooled second cargo stream (310a) to provide a cooled, expanded second cargo stream (310b) as the cooled, optionally expanded refrigerant stream.</claim-text> <claim-text>9. The method of claim 8 wherein the step of cooling the second boil off gas stream (360) comprises the steps of: -compressing the second boil off gas stream (360) to provide a compressed second boil off gas stream (370); -cooling the compressed second boil off gas stream (370) to provide the cooled second cargo stream (31 Oa).</claim-text> <claim-text>10. The method of claim 9 wherein heat exchanging the cooled, expanded, refrigerant stream (310b) with the warmed intermediate heat transfer fluid (210) provides a warmed second cargo stream as the warmed refrigerant stream (320), said method further comprising the steps of: -separating the warmed second cargo stream (320) to provide a second cargo feed stream (330) and a second cargo return stream (340).liThe method of claim 10, further comprising the steps of: -expanding the second cargo return stream (340) to provide an expanded second cargo return stream (350); -passing the expanded second cargo return stream (350) to a liquefied second cargo storage tank (500).12. The method of claim 10 or claim 11 further comprising the step of: -compressing the second cargo feed stream (330) to provide the compressed second boil off gas stream (370).13. The method of any of claims 8 to 12 wherein the liquefied first and second cargo are different and are selected from one or more of the group comprising C2 to C4 hydrocarbons and ammonia.14.An apparatus (1) to cool a first boil off gas stream (10) from a liquefied first cargo in a floating transportation vessel, said liquefied first cargo having a boiling point of greater than -110 °C at I atmosphere, said apparatus comprising at least: -a first cargo heat exchanger (200) having a first inlet for a first boil off gas stream (lOb), a first outlet for a cooled first cargo stream(20) and at least one port for an intermediate heat transfer fluid in fluid connection with at least one port of a refrigerant heat exchanger (300); -a refrigerant heat exchanger (300) having a first inlet for a cooled refrigerant stream (310b), said first inlet in fluid connection with a first outlet of a cooling, preferably liquefying, unit (400), a first outlet for a warmed refrigerant stream (320), said first outlet in fluid connection with a first inlet of the cooling, preferably liquefying, unit (400), and at least one port for an intermediate heat transfer fluid in fluid connection with at least one port of the first cargo heat exchanger (200); -a cooling, preferably liquefying, unit (400) for cooling, preferably liquefying, the warmed refrigerant stream (320) at a first inlet to provide the cooled refrigerant stream (aba, 310b) at a first outlet.15. The apparatus (1) of claim 14 wherein the refrigerant heat exchanger (300) is located gravitationally higher than the first cargo heat exchanger (200).</claim-text>