Gas Liquefaction System with Multiple Refrigerant Cycles

The present disclosure relates to feed gas liquefaction systems and methods. Embodiments disclosed herein specifically refer to liquefaction of natural gas, such as methane or a mixture of light-weight hydrocarbons.

In several industrial applications, a feed gas needs to be chilled and liquefied, for instance for transportation purposes.

Specifically, natural gas requires to be liquefied and obtain liquefied natural gas (LNG) to reduce the volume thereof and ensure safe transportation thereof, in so-called LNG vessels. Other gaseous products, such as oxygen or nitrogen, may also require liquefaction for the purpose of ease of transportation,

Several liquefaction cycles have been developed in the art, in an attempt to improve the liquefaction process and make it more efficient also from the point of view of energy consumption.

Liquefaction of natural gas proved to be particularly challenging. Natural gas is extracted from gas fields, treated in scrubber units to remove impurities, such as water, mercury (Hg), heavier hydrocarbons and the like, and subsequently compressed, chilled and liquefied. Natural gas consists mainly of methane (CH), but may include percentages of heavier hydrocarbons. The chemical composition of the natural gas may fluctuate over time in an unpredictable manner. The pressure at which natural gas is delivered from the gas field may also fluctuate over time in a way which cannot be predicted.

The liquefaction system shall quickly react to pressure or compositional fluctuations of the natural gas, to prevent negative effects on the operation of the liquefaction system, such as fouling of the heat exchangers due to precipitation of solidified heavier hydrocarbons contained in the incoming flow of natural gas.

The liquefaction systems of the current art are not satisfactory from this point of view. The large thermal inertia of the components of these systems make adaptation of the operating conditions to changing pressure and/or composition of the feedgas particularly slow.

A gas liquefaction system and a liquefaction method overcoming or alleviating the drawbacks of the current systems and methods mentioned above would be welcomed in the art.

According to one aspect, disclosed herein is a system for liquefying a pressurized feed gas, the system including a high-temperature refrigerant circuit and a low-temperature refrigerant circuit. The first high-temperature refrigerant circuit includes a first compression arrangement, a first heat rejection device and a first pressure reduction device. The low-temperature refrigerant circuit includes a second compression arrangement, a second heat rejection device and a second pressure reduction device.

The high-temperature refrigerant circuit further includes a first high-temperature heat exchanger, adapted to circulate the feed gas in heat exchange with a first stream of vaporizing first refrigerant and cool the feed gas by heat exchange with the first stream of vaporizing first refrigerant. Furthermore, the high-temperature refrigerant circuit includes a second high-temperature heat exchanger, adapted to circulate compressed first refrigerant of the first refrigerant circuit and compressed second refrigerant of the second refrigerant circuit in heat exchange with a second stream of vaporizing first refrigerant and cool the compressed first refrigerant and second refrigerant by heat exchange with the second stream of vaporizing first refrigerant.

The high-temperature refrigerant circuit further comprises a main refrigerant line, which extends through a hot side of the second high-temperature heat exchanger to the first pressure reduction device. The high-temperature refrigerant circuit further comprises a first secondary refrigerant line and a second secondary refrigerant line. The first secondary refrigerant line extends from the main refrigerant line, downstream of the second high-temperature heat exchanger, through the first pressure reduction device and to the first high-temperature heat exchanger. The second secondary refrigerant line extends from the main refrigerant line, downstream of the second high-temperature heat exchanger, through the first pressure reduction device and to the second high-temperature heat exchanger.

The cooled first refrigerant, which exits the first heat rejection device, is therefore further cooled in the second high-temperature heat exchanger and split into two streams after having passed through the hot side of the second high-temperature heat exchanger. At this stage the first refrigerant is still in a liquid state or almost in a liquid state and is split into first and second secondary streams which are then delivered to the first high-temperature heat exchanger and to the second high-temperature heat exchanger. In some embodiments, the full stream of compressed and chilled first refrigerant is split into separate first and second secondary streams before entering the first pressure reduction device.

In embodiments, the first pressure reduction device comprises a first pressure reduction unit, arranged in the first secondary refrigerant line, and a second pressure reduction unit, arranged in the second secondary refrigerant line. The cooled first refrigerant stream exiting the hot side of the second high-temperature heat exchanger is thus expanded after splitting into two secondary streams, which are then directed through the cold side of the first high-temperature heat exchanger and second high-temperature heat exchanger.

The low-temperature refrigerant circuit further comprises a low-temperature heat exchanger, wherein cooled feed gas from the first high-temperature heat exchanger and cooled second refrigerant from the second high-temperature heat exchanger are further cooled and liquefied in heat exchange with a flow of vaporizing second refrigerant.

The first high-temperature heat exchanger, wherethrough the feed gas flows, is small compared to the heat exchangers of the prior art and has therefore a smaller thermal inertia. A faster reaction of the system to pressure and composition fluctuations of the feed gas is thus achieved.

According to a further aspect, disclosed herein is a method for liquefying a pressurized feed gas. The method includes the following steps:

Further features and embodiments of the method and of the system of the present disclosure are outlined below and set forth in the appended claims.

illustrates a simplified diagram of a feed gas liquefaction systemaccording to the present disclosure in one embodiment. The liquefaction systemcomprises a high-temperature refrigerant circuitand a low-temperature refrigerant circuit.

In the following description, reference will be made specifically to natural gas as feed gas, but some of the advantages of the system and method according to the present disclosure can be achieved also with feed gas of different nature.

The refrigerants of the high-temperature refrigerant circuitand low-temperature refrigerant circuitcan each include a pure gas, or a gas mixture. In preferred embodiments both refrigerants are mixed refrigerants, usually with a different gas composition. Therefore, in the following description reference will usually be made to “mixed refrigerants”.

The high-temperature mixed refrigerant circuitand the low-temperature mixed refrigerant circuitare interlaced, as will be described in more detail below, in that the refrigerants flowing in both circuits is chilled in a high-temperature heat exchanger common to both low-temperature mixed refrigerant circuitand high-temperature mixed refrigerant circuit.

The high-temperature mixed refrigerant circuitincludes a first compression arrangement, a first heat rejection device, and a first pressure reduction device. Similarly, the low-temperature mixed refrigerant circuitincludes a second compression arrangement, a second heat rejection deviceand a second pressure reduction device. Reference numberM indicates a driver, such as an electric motor, a gas turbine, a steam turbine, or the like, to drive the first compression arrangement. Reference numberM indicates a driver, such as an electric motor, a gas turbine, a steam turbine or the like, to drive the second compression arrangement.

Each heat rejection devicesandcan include one or more heat exchangers, wherein the respective first mixed refrigerant and second mixed refrigerant is cooled in heat exchange with a coolant fluid, such as air or water.

The pressure reduction devices may include one or more throttling or lamination valves, one or more expanders, or combinations thereof. As will be explained in detail below, the first pressure reduction deviceincludes two separate pressure reduction unitsA,B, through which separate streams of the first mixed refrigerant are expanded. Each pressure reduction unit can include one or more pressure reduction members, such as valves and/or expanders.

The natural gas liquefaction systemfurther includes a feed gas (natural gas) duct, which extends through a pre-treatment section, a high-temperature heat exchanger arrangementand a low-temperature heat exchangerand leads to a liquefied natural gas collection reservoir or the like, schematically shown at. The liquefied natural gas (LNG) stored in the reservoircan be loaded in an LNG vessel or delivered through a pipeline. The high-temperature heat exchanger arrangementforms part of the high-temperature refrigerant circuitand the low-temperature heat exchangerforms part of the low-temperature refrigerant circuit.

In short, compressed feed gas, in the exemplary embodiment natural gas, is pre-treated in the pre-treatment sectionto remove impurities, such as moisture, mercury and other contaminants, and is subsequently cooled and liquefied by removing heat therefrom in the high-temperature heat exchanger arrangementand low-temperature heat exchanger.

The high-temperature mixed refrigerant circuitand the low-temperature mixed refrigerant circuitare interlaced, in that the high-temperature heat exchanger arrangementis used to chill the feed gas, as well as mixed refrigerant circulating both in the high-temperature mixed refrigerant circuitand in the low-temperature mixed refrigerant circuit.

More in detail, the high-temperature heat exchanger arrangementcomprises a first high-temperature heat exchangerA and a second high-temperature heat exchangerB. The first mixed refrigerant circuit comprises a primary refrigerant line, which delivers compressed and cooled first mixed refrigerant from the first heat rejection devicethrough the high-temperature heat exchanger arrangement. The primary refrigerant lineextends through a hot side of the second high-temperature heat exchangerB, in which the compressed and cooled first mixed refrigerant flowing from the first heat rejection deviceis further cooled by heat exchange with a flow of vaporizing first mixed refrigerant counterflowing in the cold side of the second high-temperature heat exchangerB.

Downstream of the second high-temperature heat exchangerB the primary refrigerant lineis bifurcated atinto a first secondary refrigerant lineA and a second secondary refrigerant lineB. The first pressure reduction unitA of the pressure reduction deviceis located in the first secondary refrigerant lineA and the second pressure reduction unitB of the pressure reduction deviceis located in the second secondary refrigerant lineB. In the schematic diagram ofthe first pressure reduction unitA and the second pressure reduction unitB are represented as expansion or throttling valves, for instance Joule-Thomson valves.

The compressed and cooled stream of first mixed refrigerant from the first heat rejection deviceis thus split into a first stream which is depressurized in the first pressure reduction unitA and flows through the first high-temperature heat exchangerA, and a second stream which is depressurized in the second pressure reduction unitB and flows through the second high-temperature heat exchangerB.

The feed gas ductextends through the hot side of the first high-temperature heat exchangerA such that the feed gas is chilled by heat exchange against the vaporizing first stream of first mixed refrigerant.

The vaporizing second stream of first mixed refrigerant, which expands in the second pressure reduction unitB, flows through the second secondary refrigerant lineB and through the cold side of the second high-temperature heat exchangerB and removes heat from the stream of first mixed refrigerant flowing along the primary refrigerant linethrough the hot side of the second high-temperature heat exchangerB. The first mixed refrigerant exiting the first high-temperature heat exchangerA and the second high-temperature heat exchangerB is collected at the suction side of the first compression arrangementfor compression and delivery to the first heat rejection device.

The low-temperature mixed refrigerant circuitincludes a linewhich extends from the second heat rejection devicethrough a hot side of the second high-temperature heat exchangerB. Therefore, the vaporizing second stream of first mixed refrigerant flowing through the second secondary refrigerant lineB and through the cold side of the second high-temperature heat exchangerB removes heat also from the pressurized second mixed refrigerant delivered by the second heat rejection deviceand circulating in the low-temperature mixed refrigerant circuit.

The low-temperature mixed refrigerant circuit further extends through a hot side of the low-temperature heat exchanger. The pressurized and chilled second mixed refrigerant exiting the hot side of the low-temperature heat exchangeris expanded in the pressure reduction deviceand the expanded and vaporizing second mixed refrigerant flows through the cold side of the low-temperature heat exchangerin heat exchange with the feed gas flowing through lineand in further heat exchange with the pressurized second mixed refrigerant along line.

The second mixed refrigerant exiting the cold side of the low-temperature heat exchangeris delivered to the suction side of the second compression arrangementfor compression and delivery to the second heat rejection device.

Thus, chilling capacity of the vaporizing expanded first mixed refrigerant stream flowing through the first high-temperature heat exchangerA is entirely used to cool the incoming compressed feed gas (natural gas), while the chilling capacity of the vaporizing expanded first mixed refrigerant stream flowing through the second high-temperature heat exchangerB is used to chill the compressed first mixed refrigerant circulating in the high-temperature mixed refrigerant circuit, and the compressed second mixed refrigerant circulating in the low-temperature mixed refrigerant circuit.

The dimension, and therefore the thermal inertia, of the first high-temperature heat exchangerA can therefore be small, to provide a fast adaptation of the liquefaction systemto variable conditions of the feed gas entering line, for instance in response to fluctuations of the pressure and/or chemical composition of the feed gas.

illustrates a more detailed diagram of a natural gas liquefaction system according to the invention in an embodiment. The same reference numbers used inare used into designate the same or corresponding components, elements or parts.

The liquefaction systemofcomprises a high-temperature mixed refrigerant circuitand a low-temperature mixed refrigerant circuit.

The high-temperature mixed refrigerant circuitincludes a first compression arrangement, a first heat rejection device, and a first pressure reduction device. In the embodiment of, the first compression arrangementincludes a compressor train driven into rotation by a driverM, such as an electric motor, a gas turbine or a steam turbine. The compressor train of the compression arrangementincludes a first compressorA and a second compressorB. The delivery side of the first compressorA is fluidly coupled to a suction side of the second compressorB through an intercoolerA, which forms part of the first heat rejection device. The first heat rejection devicefurther includes a heat exchangerB fluidly coupled to the delivery side of the compression train, i.e. to the delivery side of the second compressorB.

Between the intercoolerA and the suction side of the second compressorB a liquid/gas separatoris provided, which separates liquefied first mixed refrigerant, collecting at the bottom of the liquid/gas separator, from gaseous first mixed refrigerant, collecting at the top of the liquid/gas separator. A further liquid/gas separatorcan be arranged upstream of the suction side of the first compressorA, to remove liquid first mixed refrigerant, if any. Liquid collecting at the bottom of the liquid/gas separatorcan be recovered or disposed of in conventional manner.

Similarly, the low-temperature mixed refrigerant circuitincludes a second compression arrangement, a second heat rejection deviceand a second pressure reduction device. Reference numberM indicates a driver, such as an electric motor, a gas turbine, a steam turbine or the like, to drive the second compression arrangement. A liquid/gas separatorcan be arranged upstream of the suction side of the compression arrangement. Liquid collecting at the bottom of the liquid/gas separatorcan be recovered or disposed of in conventional manner.

The low-temperature mixed refrigerant circuitcan further include a liquid/gas separatoradapted to receive compressed and cooled second mixed refrigerant from the second heat rejection deviceand separate liquid second mixed refrigerant at the bottom and gaseous second mixed refrigerant at the top thereof.

The pressure reduction devices may include one or more throttling or lamination valves, one or more expanders or combinations thereof. As in the embodiment of, the first pressure reduction deviceincludes two pressure reduction unitsA,B, such as two Joule-Thomson or other expansion valves, through which separate streams of first mixed refrigerant are expanded.

The natural gas liquefaction systemfurther includes a feed gas duct, which extends through a pre-treatment section, a high-temperature heat exchanger arrangementand a low-temperature heat exchangerand leads to a liquefied natural gas collection reservoir or the like, schematically shown at. The liquefied natural gas (LNG) stored in the reservoircan be loaded in an LNG vessel or delivered through a pipeline.

As shown in the schematic diagram of, similarly to, in the embodiment ofthe high-temperature heat exchanger arrangementcomprises a first high-temperature heat exchangerA and a second high-temperature heat exchangerB. The first mixed refrigerant circuit comprises a primary refrigerant line, which delivers compressed and cooled first mixed refrigerant from the heat exchangerB of the first heat rejection devicethrough the high-temperature heat exchanger arrangement. More specifically, the primary refrigerant lineextends through a hot side of the second high-temperature heat exchangerB, in which the compressed and cooled first mixed refrigerant flowing from the first heat rejection deviceis further cooled by heat exchange with a flow of vaporizing first mixed refrigerant counterflowing in the cold side of the second high-temperature heat exchangerB.

Downstream of the second high-temperature heat exchangerB the primary refrigerant lineis bifurcated atinto a first secondary refrigerant lineA and a second secondary refrigerant lineB. The first pressure reduction unitA of the pressure reduction deviceis located in the first secondary refrigerant lineA and the second pressure reduction unitB of the pressure reduction deviceis located in the second secondary refrigerant lineB.

The compressed and cooled stream of first mixed refrigerant from the heat exchangerB is therefore split into a first stream which is depressurized in the first pressure reduction unitA and flows through the first high-temperature heat exchangerA, and a second stream which is depressurized in the second pressure reduction unitB and flows through the second high-temperature heat exchangerB.

The feed gas ductextends through the hot side of the first high-temperature heat exchangerA such that the feed gas is chilled by heat exchange against the vaporizing first stream of first mixed refrigerant flowing through the first secondary refrigerant lineA.

The vaporizing second stream of first mixed refrigerant, which expands in the second pressure reduction unitB, flows through the second secondary refrigerant lineB and through the cold side of the second high-temperature heat exchangerB and removes heat from the stream of first mixed refrigerant flowing along the primary refrigerant linethrough the hot side of the second high-temperature heat exchangerB. The first mixed refrigerant exiting the first high-temperature heat exchangerA and the second high-temperature heat exchangerB is processed through the liquid/gas separator, if present, and collected at the suction side of compressorA for compression and delivery to the intercoolerA of the first heat rejection device.