GB2563021A

GB2563021A - Refrigeration circuit system and method of maintaining a gas seal of a compressor system

Info

Publication number: GB2563021A
Application number: GB1708550.7A
Authority: GB
Inventors: Bub Andreas; Garthe Daniel; Kamann Martin
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2017-05-30
Filing date: 2017-05-30
Publication date: 2018-12-05
Also published as: WO2018219964A1; GB201708550D0

Abstract

A mixed refrigeration circuit system 102 for a gas liquefaction process comprises a heat exchange arrangement 104, 166, 168, a compressor system 140, a drum (separator for liquid refrigerant) 118 and a fluid conduit network. The compressor system is in fluid communication with the heat exchange arrangement, and comprises a gas seal system housing a gas seal panel 152. The drum is in fluid communication with the heat exchange arrangement and has a vapour outlet 120 coupled to the gas seal system. The fluid conduit network is arranged to connect the heat exchange arrangement, the compressor system and the drum in a circuit. The refrigerant may comprise any of ethane, propane, butane, or pentane and the system does not require an external seal gas. The seal gas prevents the loss of valuable refrigerant. A hydrocarbon-rich gas liquefaction system and method of maintaining a gas seal of a compressor system is also claimed. The compressor system may comprise a first (144) and second (146) compressor in fluid communication with second & third drums/separators 174, 192. The vapour for the gas seal may be tapped off the vapour fractions leaving the third separator.

Description

REFRIGERATION CIRCUIT SYSTEM AND METHOD OF MAINTAINING A GAS SEAL OF A COMPRESSOR SYSTEM [0001] The present invention relates to a mixed refrigeration circuit system of the type that, for example, circulates a mixed refrigerant through a heat exchanger arrangement. The present invention also relates to a method of maintaining a gas seal of a compressor system, the method being of the type that uses a gas to prevent leakage of a mixed refrigerant from the compressor system.

[0002] In the field of hydrocarbon recovery from natural gas, and other gases from air, closed refrigerant cycles are used to cool or condense process fluids. As refrigerants, pure coolants or mixed refrigerants are used. For example, for the liquefaction of natural gas the refrigerant typically comprises one or more of the following components: nitrogen, methane, ethane or ethylene, propane, butane and pentane. In the closed cycle, the refrigerant is typically compressed, liquefied, depressurised and vaporised.

[0003] Refrigerants such as nitrogen and methane are less scarce as compared with ethane or ethylene, propane, butane and pentane. For example, nitrogen can often be sourced from an existing air separation plant and stored in a Liquid Nitrogen (LIN) storage vessels. Methane can be supplied from a methane rich fraction of a feed gas in the process for liquefaction of natural gas. In contrast, providing a reliable supply of ethane or ethylene, propane, butane and pentane is more problematic than nitrogen and methane. In many cases, these refrigerant components have to be transported over long distances if they are to be made available on site.

[0004] Regardless of the choice of refrigerant, a reliable supply of the refrigerant component or components and a minimisation of the cycle losses are important factors that influence the availability, i.e. up-time, and operating costs of a processing plant. The minimisation of the loss of one or more refrigerants becomes increasingly important, where the provision of one or more refrigerant components is difficult and/or expensive.

-2[0005] During operation of the plant, refrigerant losses can be attributed to losses caused by inefficiencies in a gas seal system of a refrigerant compressor, for example a so-called turbo compressor. In this regard, refrigerant from a discharge of the compressor is typically used as seal gas in the dry gas seal system of the compressor. A portion of the seal gas used is recycled to a suction side of the compressor while the remainder of the seal gas is sent to “flare” as a permanent refrigerant loss.

[0006] For many applications, in order to preserve valuable refrigerants, it is important to provide compressors with proper seal gases with a sufficiently high pressure, even during plant shutdown. After a refrigerant cycle in a plant is shut down, it is desirable for the refrigerant “inventory” to remain inside the closed refrigerant cycle and not lost to flare. One way to achieve this is to install non-return valves and shut down valves in line with the compressor in order to lock the entire inventory of cycle refrigerant inside the compressor system and to avoid necessarily depressurising the compressor after a shutdown of the refrigerant cycle. However, this solution often requires a reliable seal gas supply with a higher pressure than the so-called “settle-out” or equalised pressure of the compressor. In this respect, an external seal gas, for example a pre-treated natural gas, can be employed. However, use of such external seal gases contaminates the refrigerant in the refrigerant cycle and this is a considerable disadvantage. Alternatively, a so-called “seal gas booster” or compressor can be used to compress low pressure refrigerant to the required seal gas pressure level. Whilst such an approach avoids contamination of the refrigerant in the refrigerant cycle, the use of the seal gas booster represents an additional plant cost. Furthermore, the problem of loss of significant amounts of valuable refrigerant components to flare during normal operation remains, because process gas exiting from the discharge side of the compressor arrangement is used as seal gas, which results in a loss of refrigerant components, for example ethane, ethylene, propane, butane and pentane.

[0007] According to a first aspect of the present invention, there is provided a mixed refrigeration circuit system for a gas liquefaction process, the system comprising: a heat exchange arrangement; a compressor system in fluid

-3communication with the heat exchange arrangement, the compressor system comprising a gas seal system; a drum in fluid communication with the heat exchange arrangement, the drum having a vapour outlet; and a fluid conduit network arranged to connect the heat exchange arrangement, the compressor system and the drum in a circuit; wherein the vapour outlet of the drum is operably coupled to the gas seal system.

[0008] The system may comprise a refrigerant. The refrigerant may be a mixed refrigerant.

[0009] The drum may be a buffer for liquid phase refrigerant. The drum may be operably coupled to the heat exchange arrangement. The drum may be operably coupled to the heat exchange arrangement to act as a buffer for liquid refrigerant.

[0010] The drum may be a separator. The separator may be a receiver. The separator may be coupled at a mid-point in a liquid refrigerant flow path of the heat exchange arrangement. The separator may be coupled to a high pressure end of the compressor system.

[0011] Two separators may be provided; a first separator may be coupled to the heat exchanger arrangement and a second separator may be coupled to a high pressure end of the compressor system.

[0012] The system may further comprise: a vapour stream path in fluid communication with the vapour outlet; wherein the vapour stream path may comprise a tapping point in fluid communication with the gas seal system.

[0013] The gas seal system may comprise a gas seal panel; and the vapour outlet may be in fluid communication with the gas seal panel.

[0014] The drum may be operably coupled to the compressor system.

[0015] The compressor system may comprise an initial stage and a final stage; and the drum may be operably coupled to the final stage of the compressor system.

-4[0016] The inlet port of the drum may be in fluid communication with the final stage of the compressor system. The inlet port of the drum may be in fluid communication with outlet of the final stage of the compressor system, which may be the high pressure end of the compressor system.

[0017] The system may further comprise: a valve arrangement arranged to isolate selectively a fluid outlet of the drum and fluid communication between an inlet of the drum and the compressor system.

[0018] According to a second aspect of the present invention, there is provided a hydrocarbon-rich gas liquefaction system comprising the mixed refrigerant circuit system as set forth above in relation to the first aspect of the invention.

[0019] According to a third aspect of the present invention, there is provided a method of maintaining a gas seal of a compressor system in a mixed refrigeration circuit, the method comprising: circulating a mixed refrigerant in the refrigeration circuit, the refrigeration circuit comprising the compressor system, a drum and a heat exchange arrangement through which the mixed refrigerant flows; tapping off a portion of the refrigerant from a vapour stream of the drum and using the portion of the refrigerant as a seal gas for the compressor system.

[0020] The portion of the refrigerant tapped off may be substantially free of hydrocarbon components molecularly heavier than methane. The portion of the refrigerant tapped of may be substantially free of heavier components of the mixed refrigerant. The proportion of the refrigerant tapped off may comprise substantially only lighter components of the mixed refrigerant.

[0021] The portion of the refrigerant tapped off has an ethane concentration less than about 60% of the ethane concentration of the refrigerant in the refrigeration circuit.

[0022] The portion of the refrigerant tapped off may have a propane concentration that is less than about 20% of the propane concentration of the refrigerant in the refrigeration circuit.

-5[0023] The portion of the refrigerant tapped off may have a butane concentration that is less than about 50% of the butane concentration of the refrigerant in the refrigeration circuit.

[0024] The portion of the refrigerant tapped off may have a pentane concentration that is less than about 50% of the pentane concentration of the refrigerant in the refrigeration circuit.

[0025] The method may further comprise tapping off a refrigerant feed off the portion of the refrigerant and feeding the tapped off refrigerant feed to a gas seal system of the compressor system.

[0026] The drum may have an operational pressure associated therewith; the compressor system may have a settle-out pressure associated therewith. The settle-out pressure may be an equalising pressure of a blocked-in compressor system; and the operational pressure of the drum may be greater than the settle-out pressure of the compressor system.

[0027] It is thus possible to provide a system and method that reduces the loss of valuable refrigerant components of a mixed refrigerant circuit system. In this regard, the system and method maintains the gas seal of the compressor system whilst minimising leakage of valuable, for example difficult to procure and/or more expensive, refrigerant components, for example heavier refrigerant components. The system and method permits the seal to be maintained both during operation of the refrigeration circuit and after shutdown of the refrigeration circuit system. Since the vapour phase fluid leaving the drum comprises a significantly smaller portion of valuable, for example heavier, refrigerant components, i.e. a higher proportion of lighter, less valuable refrigerant components than heavier components, than contained in a stream of refrigerant circulating in the system, the use of the gas from the separator as a seal gas serves to reduce significantly loss of, for example ethane or ethylene losses, by approximately 50% and substantially mitigate loss of high boiling point refrigerant components, for example propane, butane or pentane. Furthermore, the need to provide seal gas from a source external to the refrigeration circuit in order to prevent loss of refrigerant components during periods of extended

-6shutdown is obviated. This also overcomes the problem of contamination of the refrigerant in the refrigeration circuit. Additionally, the need to employ a seal gas booster is obviated due to the continued availability of seal gas. The operating and capital costs associated with a plant comprising the system and employing the method are therefore reduced.

[0028] At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0029] Figure 1 is a schematic diagram of a refrigeration circuit system constituting an embodiment of the invention; and [0030] Figure 2 is a schematic diagram of a refrigeration circuit system constituting another embodiment of the invention.

[0031] Referring to Figure 1, a hydrocarbon-rich gas liquefaction system 100 comprises a refrigeration circuit system 102. The refrigeration circuit system 102 comprises a fluid conduit network coupling in fluid communication various functional elements of the refrigeration circuit system 102 describe herein. In this example, the refrigeration circuit system 102 comprises a heat exchange arrangement comprising a first heat exchanger 104 having, for example, a plurality of coils inside a common shell, which extends vertically when in use. In use, fluids travelling in an upward flow direction through the coils are cooled and refrigerant travelling in a downward flow direction inside the shell, but external to the coils, is vaporised and superheated.

[0032] The first heat exchanger 104 has a gas inlet port 106 and a liquefied stream outlet port 108, the liquefied stream outlet port 108 being in fluid communication with an outlet Joule-Thomson device 107, for example an outlet valve.

[0033] The first heat exchanger 104 also comprises a first refrigerant inlet port 110 and a second refrigerant inlet port 112. The first refrigerant inlet port 110 is in fluid communication with a first refrigerant outlet port 114 via a first fluid path within the first heat exchanger 104. The first refrigerant outlet port 114 is in fluid communication with an inlet port 116 of a first drum or receiver, serving in this example as a first separator 118, which also serves as a buffer for liquid phase

-7refrigerant. A vapour outlet port 120 of the first separator 118 is in fluid communication with a third refrigerant inlet port 122 of the first heat exchanger 104, the third refrigerant inlet port 122 being in fluid communication with a second refrigerant outlet port 124 of the first heat exchanger 104 via a second fluid path within the first heat exchanger 104.

[0034] The second refrigerant outlet port 124 is in fluid communication with a first refrigerant return inlet port 126 via a first Joule-Thomson device 128, for example a first valve.

[0035] A liquid fractions outlet port 130 of the first separator 118 is in fluid communication with a fourth refrigerant inlet port 132, which is also in fluid communication with a third refrigerant outlet port 134 via a third fluid path within the first heat exchanger 104. The third refrigerant outlet port 134 is in fluid communication with a second refrigerant return inlet port 136 via a second JouleThomson device 138, for example a second valve.

[0036] A driving element 142, for example a motor, is operably coupled to compressor system 140, the compressor system 140 comprising a first compressor 144 and a second compressor 146. The driving element 142 is operably coupled to the first compressor 144 and the second compressor 146 via a common drive shaft 148. The first and second compressors 144, 146 constitute stages of the compressor system 140. The compressor system 140 comprises, in this example, a gas seal system 150 comprising a gas seal panel 152 having a seal gas inlet port 154.

[0037] A fourth refrigerant outlet port 156 of the first heat exchanger 104 is in fluid communication with an inlet port 158 of a second drum, serving in this example as a suction vessel 160, a vapour fractions outlet port 162 of the suction vessel 160 being in fluid communication with an inlet port 164 of the first compressor 144.

[0038] The heat exchange arrangement also comprises a second heat exchange 166 and a third heat exchanger 168. An outlet port 170 of the first compressor 144 is in fluid communication with an inlet port 172 of a third drum, serving in the example

-8as a second, intermediate, separator 174, via the second heat exchanger 166. A liquid fractions outlet port 176 of the second separator 174 is in fluid communication with the second refrigerant inlet port 112 of the first heat exchanger 104. The second refrigerant inlet port 112 is in fluid communication with a fifth refrigerant outlet port 178 via a fourth fluid path within the first heat exchanger 104, the fifth refrigerant outlet port 178 being in fluid communication with a third refrigerant return inlet port 180 via a third Joule-Thomson device 182, for example a third valve.

[0039] A vapour fractions outlet port 184 of the second separator 174 is in fluid communication with an inlet port 186 of the second compressor 146, an outlet port 188 of the second compressor 146 being in fluid communication with an inlet port 190 of a fourth drum, serving in this example as a third, high pressure, separator 192, via the third heat exchanger 168. A vapour fractions outlet port 194 of the third separator 192 is in fluid communication with the first refrigerant inlet port 110 of the first heat exchanger 104 via a first non-return valve 196. A liquid fractions outlet port 198 of the third separator 192 is also in fluid communication with the inlet port 172 of the second separator 174 via a fourth Joule-Thomson device 200, for example a fourth valve.

[0040] It should be understood that the compressor system 140 also comprises the suction vessel 160, the second heat exchanger 166, the third heat exchanger 168, the separator 174 and the third separator 192.

[0041] In this example, the seal gas inlet port 154 of the gas seal panel 152 is in fluid communication with the vapour outlet port 120 of the first separator 118. This can be achieved in any appropriate manner, for example by providing a tapping point in a fluid stream path 204 from the vapour outlet port 120 of the first separator 118 that extend to and provides the fluid communication with the third refrigerant inlet port 122 of the first heat exchanger 104.

[0042] In general, the refrigerant comprises several components, for example nitrogen, methane, ethane or ethylene, propane and/or butane, and is vaporised and superheated in the first heat exchanger 104 at low pressure.

-9[0043] In operation, a hydrocarbon-rich stream, for example natural gas, to be liquefied is fed into the first heat exchanger 104 via the gas inlet port 106 where the hydrocarbon-rich stream is pre-cooled, liquefied and then sub-cooled. The liquefied and sub-cooled hydrocarbon-rich stream is then fed to a further processing stage, for example a nitrogen-separation stage, and/or is expanded via the outlet JouleThomson device 107 into a storage vessel operating at, for example, approximately atmospheric pressure. The first heat exchanger 104 provides cooling by virtue of the refrigerant flowing through it in the refrigeration circuit system 102.

[0044] In this regard, superheated refrigerant flows from the fourth refrigerant outlet port 156 to the inlet port 158, where the refrigerant enters the suction vessel 160, which protects the first compressor 144 against potential liquid entrainment from the first heat exchanger 104. The refrigerant mixture stream to be compressed flows into the first compressor 144, which compresses the refrigerant mixture to an intermediate pressure, whereafter the pressurised refrigerant is cooled and partially condensed in the second heat exchanger 166 that serves as a compressor intercooler. The cooled and partially condensed refrigerant then enters the second separator 174 via the inlet port 172 of the second separator 174. The liquid fractions from the second separator 174 comprise high boiling point refrigerant components and are used as a so-called Heavy Mixed Refrigerant (HMR) to pre-cool the natural gas in the first heat exchanger 104.

[0045] The vapour fractions from the second separator 174 are fed to a suction side of the second compressor 146 where the vapour fractions are further compressed in the second compressor 146 and partially condensed in the third heat exchanger 168 that serves as an aftercooler. The partially condensed stream leaving the third heat exchanger 168 enters the third separator 192 via the inlet port 190.

[0046] The high-pressure liquid fractions exiting the third separator 192 via the liquid fractions outlet port 198 are expanded in the fourth Joule-Thomson device 200 before returning to the second separator 174, thereby recirculating the refrigerant mixture upstream of the second compressor 146. The vapour fractions

-10exiting the third separator 192 comprise significantly less high boiling point components and hence less valuable refrigerant components and are cooled and partially condensed in the first heat exchanger 104 before flowing to the first separator 118, which is a cold mixed refrigerant separator. The liquid fractions exiting the first separator 118 are rich in medium boiling point components and are used as a Medium Mixed Refrigerant (MMR) for the liquefaction of the natural gas in the first heat exchanger 104. The liquid fractions from the first separator 118 are sub-cooled in the first heat exchanger 104 and expanded in the second JouleThomson device 138. The refrigerant then re-enters the first heat exchanger 104 via the second refrigerant return inlet port 136 and acts as coolant and is vaporised as the refrigerant travels downwards through the first heat exchanger 104 after the refrigerant has mixed with refrigerant also flowing downwards through the first heat exchanger and originating from above the second refrigerant return inlet port 136. The vapour fractions exiting the first separator 118 are rich in light boiling point components and show a very low concentration of valuable refrigerant components, for example ethane, ethylene, propane, butane and pentane. These vapour fractions are substantially free, in this example, of hydrocarbon components molecularly heavier than methane, and are used as a Light Mixed Refrigerant (LMR) for subcooling of the liquefied natural gas (LNG) in the first heat exchanger 104. In this regard, the vapour fractions exiting the first separator 118 flow through the first heat exchanger 104 where they are liquefied and sub-cooled and then expanded in the first Joule-Thomson device 128. The refrigerant then enters via the inlet port 126 and acts as coolant while the refrigerant is vaporised at low pressure as the refrigerant travels downwards through the first heat exchanger 104, mixing with the refrigerant re-entering the first heat exchanger 104 via the second refrigerant return unlet port 136.

[0047] The operational pressure in the first separator 118 is greater than a settleout pressure of the compressor system 140 (between inlet and outlet ports thereof), the settle-out pressure being an equalising pressure of the compressor system 140 when the inlet and outlet ports of the compressor system 140 are closed to contain the refrigerant in the compressor system 140, i.e. when the compressor system 140

- 11 is “blocked-in”. A portion of the vapour fractions exiting the first separator 118 is tapped from the fluid path 204 and flows, in this example, to the gas seal panel 152 where the vapour fractions are used as a seal gas in the gas seal system 150 of the compressor system 140.

[0048] In another example (Figure 2), instead of vapour fractions being tapped off from the fluid path 204 coupling the vapour fractions outlet port 120 of the first separator 118 to the second refrigerant inlet port 122, the vapour exiting the third separator 192 is used to serve as the seal gas for the compressor system 140. In order to enable the use of the vapour fractions from the vapour fractions outlet port

194 of the third separator 192 as the seal gas, the structure of the refrigeration circuit system 102 differs, as follows, from the structure described in the previous example.

[0049] As mentioned above, the vapour fractions outlet port 120 of the first separator 118 no longer serve as a source of the seal gas for the compressor system 140 and is simply in fluid communication with the second refrigerant inlet port 122 of the first heat exchanger 104.

[0050] Instead, the vapour phase outlet port 184 of the second separator 174 comprises a first tapping point 210, which is in fluid communication with the inlet port 158 of the suction vessel 160 via a fifth Joule-Thomson device 212, for example a fifth valve.

[0051] The heat exchange arrangement comprises a fourth heat exchanger 214 disposed in-line between the third head exchanger 168 and the inlet port 190 of the third separator 192. The first non-return valve 196 is no longer employed and a second non-return valve 216 is disposed in-line between the third and fourth heat exchangers 168, 214. A second tapping point 218 is provided between the third heat exchanger 168 and the second non-return valve 216 and is in fluid communication with the inlet port 172 of the second separator 174 via a sixth Joule-Thomson device 220, for example a sixth valve.

[0052] In operation, refrigerant flows into the third separator 192, but when the compressor system 140 ceases operation, the anti-surge valves 212 and 220 of the

- 12 compressor system 140 open in response thereto, and the refrigerant circuit system 102 “settles-out”. The liquid fractions outlet port 198 ofthe third separator 192, which is in fluid communication with the inlet port 172 of the second separator 174 during normal operation of the refrigerant circuit system 102, is closed by closing the fourth valve 200 in response to a compressor stop signal. Indeed, in this example, the valves 128, 138, 182, 200 are closed or only those required to isolate the third separator 192 from the compressor system 140 are closed. In some examples, the provision of non-return valves, in-line, serves the same purpose.

[0053] The second non-return valve 216, which is downstream of a tie-off of the anti-surge line with the anti-surge valve 220 of the compressor system 140 and upstream of the inlet port 190 of the third separator 192, ensures that pressure in the third separator 192 stays at a normal compressor discharge pressure. This means that the liquid and vapour of the third separator 192 and fluid conduits, for example piping, downstream ofthe third separator 192 remain under pressure at a pressure that is higher than the settle-out pressure mentioned above. The vapour fractions leaving the vapour fractions outlet port 194 of the third separator 192 therefore flow to the compressor system 140 for use as seal gas for as long as the pressure in the third separator 192 is higher than the settle-out pressure.

[0054] It should be understood that the vapour fractions from the vapour fractions outlet port 194 of the third separator 192 can be used as seal gas during operation of the compressor system 140. Again, using these vapour fractions serves to reduce the loss of valuable refrigerant components, since these fractions possess a low concentration of ethane, ethylene, propane, butane and pentane. In addition, the benefits of using the vapour from the third separator 192 are optimal during shut down of the compressor system 140, since no external seal gas or seal gas booster is required.

[0055] As can be seen from the above examples, through partial condensation in several stages a light mixed refrigerant can be produced within the refrigerant cycle, which can be used to reduce significantly the loss of valuable refrigerant components, for example one or more of ethane, propane, butane and pentane.

- 13Furthermore, the above examples show how a light mixed refrigerant can be produced and used as a seal gas, thereby helping to reduce the loss of valuable refrigerant components.

[0056] The skilled person should appreciate that the above-described implementations are merely examples of the various implementations that are conceivable within the scope of the appended claims. Indeed, although in the above examples the refrigerant used comprises a mixture of refrigerant components, the vapour from a high-pressure receiver of a refrigerant cycle with a pure coolant, for example nitrogen or propane, can be used to supply the compressor system 140 with seal gas during normal operation or during a plant shutdown. In this case, depressurisation of the compressor system 140 is not required if sufficient highpressure seal gas is available during an extended shutdown period. In addition, a seal gas booster is not required.

Claims

1. A mixed refrigeration circuit system (100) for a gas liquefaction process, the system comprising:

a heat exchange arrangement (104);

a compressor system (140) in fluid communication with the heat exchange arrangement (104), the compressor system (140) comprising a gas seal system (152);

a drum (118) in fluid communication with the heat exchange arrangement (104), the drum (118) having a vapour outlet (120); and a fluid conduit network arranged to connect the heat exchange arrangement (104), the compressor system (140) and the drum (118) in a circuit; wherein the vapour outlet (120) of the drum (118) is operably coupled to the gas seal system (152).

2. A system as claimed in Claim 1, wherein the drum (118) is a buffer for liquid phase refrigerant.

3. A system as claimed in Claim 1 or Claim 2, wherein the drum (118) is a separator.

4. A system as claimed in Claim 1 or Claim 2 or Claim 3, further comprising:

a vapour stream path (204) in fluid communication with the vapour outlet (120); wherein the vapour stream path (204) comprises a tapping point (202) in fluid communication with the gas seal system (152).

5. A system as claimed in any one of the preceding claims, wherein the gas seal system (152) comprises a gas seal panel; and the vapour outlet (120) is in fluid communication with the gas seal panel (152).

-156. A system as claimed in any one of the preceding claims, wherein the drum (118) is operably coupled to the compressor system (140).

7. A system as claimed in any one of the preceding claims, wherein the compressor system (140) comprises an initial stage (144) and a final stage (146); and the drum (118) is operably coupled to the final stage of the compressor system (140).

8. A system as claimed in Claim 7, further comprising:

a valve arrangement (138, 196) arranged to isolate selectively a fluid outlet (130) of the drum (118) and fluid communication between an inlet (116) of the drum (118) and the compressor system (140).

9. A hydrocarbon-rich gas liquefaction system comprising the mixed refrigerant circuit system (100) as claimed in any one of the preceding claims.

10. A method of maintaining a gas seal of a compressor system (140) in a mixed refrigeration circuit (100), the method comprising:

circulating a mixed refrigerant in the refrigeration circuit (100), the refrigeration circuit (100) comprising the compressor system (140), a drum (118) and a heat exchange arrangement (104) through which the mixed refrigerant flows; tapping off a portion of the refrigerant from a vapour stream (204) of the drum (118) and using the portion of the refrigerant as a seal gas for the compressor system (140).

11. A method as claimed in Claim 10, wherein the portion of the refrigerant tapped off has an ethane concentration less than about 60% of the ethane concentration of the refrigerant in the refrigeration circuit (100).

-1612. A method as claimed in Claim 10 or Claim 11, wherein the portion of the refrigerant tapped off has a propane concentration that is less than about 20% of the propane concentration of the refrigerant in the refrigeration circuit (100).

13. A method as claimed in Claim 10 or Claim 11 or Claim 12, wherein the portion of the refrigerant tapped off has a butane concentration that is less than about 50% of the butane concentration of the refrigerant in the refrigeration circuit (100).

14. A method as claimed in any one of Claims 10 to 13, wherein the portion of the refrigerant tapped off has a pentane concentration that is less than about 50% of the pentane concentration of the refrigerant in the refrigeration circuit (100).

15. A method as claimed in any one of Claims 10 to 14, further comprising tapping off a refrigerant feed off the portion of the refrigerant and feeding the tapped off refrigerant feed to a gas seal system (152) of the compressor system (140).

16. A method as claimed in any one of Claims 10 to 15, wherein the drum (118) has an operational pressure associated therewith;

the compressor system (140) has a settle-out pressure associated therewith, the settle-out pressure being an equalising pressure of a blocked-in compressor system; and the operational pressure of the drum (118) is greater than the settle-out pressure of the compressor system (140).