Technical field [001] The present invention relates to system for and method of producing liq uefied product gas.

Background art

[002] The energy demand in the world is increasing, and the forecast is a con- tinued growth. Gas as an energy carrier has received increased attention recent years, and it is predicted that gas will become even more important. In order to transport gas over longer distances liquefied gas is often regarded as the best option, especially overseas. Most commonly used gas is natural gas and when liquefied, it is often referred to as LNG, liquefied natural gas. As used herein, the term Natural Gas shall refer to a gaseous mixture of hydrocarbons where an es sential part is methane.

[003] Liquefying gas typically requires cooling the gas to extremely low temper ature and when LNG is concerned the liquefaction process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydro- carbons, which could cause difficulty downstream. The natural gas is then con densed into a liquid at close to atmospheric pressure by cooling it to approxi mately -162 °C typically pressure at about atmospheric pressure. An important issue for LNG production is the energy demand. High energy demand per kg produced LNG, i.e. specific energy consumption, makes it less profitable and less environmental friendly.

[004] WO 2010/071449 A2 discloses a method for optimizing the efficiency of a LNG liquefaction system of the type comprising a heat exchanger system hav ing a cooling circuit employing a gas-expansion cooling cycle comprising a gas eous cooling agent and a LNG liquefaction system. There is disclosed a system of heat exchanges for liquefying the LNG by means a cooling system comprising a gas cooling agent stream, at least one circulation compressor, at least one aftercooler and at least one gas expander.

[005] EP 2 331 897 B1 discloses a system for producing liquefied and sub cooled natural gas by means of a refrigeration assembly using a single phase refrigerant gas and a method for producing liquefied and sub-cooled natural gas by means of a refrigeration assembly using a single phase refrigerant gas. Even if the system and the method disclosed therein may be advantageous as such there is a need for improve the efficiency and technical robustness and control lability of such system and method. [006] An object of the invention is to provide a system for and method of pro ducing liquefied product gas which of straightforward structure and yet provides good efficiency.

Disclosure of the Invention [007] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[008] A system for producing liquefied product gas comprises at least two com pressors, at least two expanders and a system of heat exchangers arranged to a closed single phase refrigerant gas flow circuit, and a gas liquefaction passage arranged in connection with the system of heat exchangers so as to liquefy the product gas, wherein the least two compressors are coupled in series with each other in the refrigerant circuit, and the at least two expanders are coupled in par allel which each other in the refrigerant circuit, and wherein the compressors and the expanders are mechanically coupled with each other by a drive system.

[009] This kind of a system makes it possible to produce liquefied product gas with good efficiency and still being a mechanically simple system.

[0010] According to an embodiment of the invention the closed single phase re frigerant gas flow circuit is provided with a refrigerant gas buffer assembly con- trollably connected to an upstream side of the first compressor of the compres sors in series, and to the downstream side of the last compressor of the com pressors in series.

[0011] According to an embodiment of the invention the drive system comprises a transmission, having a first transmission ratio for at least one of the at least two compressors and a second transmission ratio for at least one of the at least two compressors.

[0012] According to an embodiment of the invention the drive system comprises an electric motor configured to operate the drive system and the compressors coupled to the system.

[0013] According to an embodiment of the invention a refrigerant gas cooler is arranged downstream to each one of the compressors in the refrigerant gas flow circuit.

[0014] According to an embodiment of the invention the refrigerant gas flow cir- cuit is provided with a first circuit section which connects the system of heat ex changers to the first one of the compressors in series and the last one of the compressor in series back to the system of heat exchangers.

[0015] According to an embodiment of the invention the refrigerant gas flow cir cuit is provided with a second circuit section which connects the system of heat exchangers to the expanders and the expanders back to the system of heat ex changers.

[0016] According to an embodiment of the invention the second circuit section comprises a first and a second branch, the first branch connects the system of heat exchangers to at least one of the at least two expanders, and the second branch connects the system of heat ex-changers to at least one of the at least two expanders .

[0017] According to an embodiment of the invention the first and a second branch are branched and connected in the system of heat exchangers. [0018] According to an embodiment of the invention the first branch is configured to be subjected to smaller extent of heat transfer in the system of heat exchang ers than the second branch.

[0019] According to an embodiment of the invention the drive system is config- ured to run at constant speed and the closed single phase refrigerant gas circuit is provided with a refrigerant gas buffer assembly connected to upstream side of the first compressor of the compressors in series, and to the downstream side of the last compressor of the compressors in series.

[0020] According to an embodiment of the invention the system comprises four compressors coupled in series with each other in the refrigerant circuit, and two expanders coupled in parallel which each other in the refrigerant circuit.

[0021] According to an embodiment of the invention the system comprises a first compressor, a second compressor, a third compressor and a fourth compressor arranged in series, wherein the first and the second compressors are coupled to the drive system with a first transmission ratio and the third and the fourth com pressors are coupled to the drive system with a second transmission ratio.

[0022] According to an embodiment of the invention the system comprises a first expander and a second expander wherein the first and the second expanders are coupled to the drive system with the second transmission ratio. [0023] Method of operating a system for producing liquefied product gas, com prising steps of circulating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrigerant gas at compressed state to the system of heat exchang ers, leading the stream of the refrigerant gas to the at least two expanders and further back to the system of heat exchangers, operating the compressors and the expanders by a common drive system mechanically coupling the compres sors and the expanders with each other.

[0024] According to an embodiment of the invention the method comprises cir culating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrig erant gas at compressed state to the system of heat exchangers, leading the stream of the refrigerant gas to the at least two expanders and further back to the system of heat exchangers, operating the compressors and the expanders by a common drive system mechanically coupling the compressors and the ex panders with each other, and controlling the cooling power of the system by ad- justing the amount of the refrigerant gas in the closed single phase refrigerant gas flow circuit by leading a portion of the refrigerant from the flow circuit to the refrigerant gas buffer assembly controllably from the upstream downstream side of the first last compressor of the compressors in series, or leading a portion of the refrigerant from the refrigerant gas buffer assembly controllably to the flow circuit at the upstream side of the first compressor of the compressors in series.

[0025] According to an embodiment of the invention the method comprises cir culating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrig erant gas at compressed state to the system of heat exchangers, leading the stream of the refrigerant gas to the at least two expanders and further back to the system of heat exchangers, and operating the compressors and the expand ers by a common drive system mechanically coupling the compressors and the expanders with each other such that at least one of the at least two compressors is rotated with a first speed and at least one of the at least two compressors is rotated with a second speed.

[0026] According to an embodiment of the invention the method comprises cir culating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrig erant gas at compressed state to the system of heat exchangers, branching a first stream of the refrigerant gas and leading the first stream of the refrigerant gas to a first expander and further from the first expander back to the system of heat exchangers, branching a second stream of the refrigerant gas and leading the second stream of the refrigerant gas to a second expander and further from the second expander back to the system of heat exchangers, connecting the first and the second streams of the refrigerant gas and leading the thus connected first and second streams of the refrigerant gas to the at least two compressors.

[0027] According to an embodiment of the invention the method comprises start ing the system using a starter device and de-coupling or deactivating the starter device after the system has reached its steady state condition, and controlling the cooling power of the system by adjusting the amount of the refrigerant gas in the closed single phase refrigerant gas flow circuit by leading a portion of the refrigerant from the flow circuit to the refrigerant gas buffer assembly controllably from the downstream side of the last compressor of the compressors in series, or leading a portion of the refrigerant from the refrigerant gas buffer assembly controllably to the flow circuit at the upstream side of the first compressor of the compressors in series.

[0028] According to an embodiment of the invention the method comprises the drive system is operated by an alternating current electric motor and after the system has reached its steady state condition the electric motor of the drive sys tem is operated with a rotational speed corresponding to a grid frequency, the motor being connected directly online the grid.

[0029] A computer readable memory device associated with the system for pro- ducing liquefied product gas comprising instructions which, when executed by a computer, cause the computer to carry out a method according to the invention.

[0030] The exemplary embodiments of the invention presented in this patent ap plication are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as charac teristic of the invention are set forth in particular in the appended claims. Brief Description of Drawings

[0031] In the following, the invention will be described with reference to the ac companying exemplary, schematic drawing, in which

Figure 1 illustrates a system for producing liquefied product gas according to an embodiment of the invention. Detailed Description of Drawings

[0032] Figure 1 depicts schematically a system 10 for producing liquefied gas (also referred to as a system in the following), which is applicable for liquefying for example natural gas LNG. The system 10 for producing liquefied gas is com- prised of a number of compressors and expanders by means of which a refriger ant gas can be brought into extremely low temperature, below minus 163 Celsius degrees, which refrigerant, when brought into heat transfer communication with a product gas, for example natural gas, causes the product gas to condense and liquefy. [0033] The system 10 comprises generally at least two compressors and at least two expanders 14.1.14.2 connected to a common mechanical drive system 16. The drive system is a mechanical force transmission system known to a skilled person in the art as such. The drive system 16 connects the compressors and the expanders with each other in fixedly manner, such that they are constrained to rotate together as a unitary assembly. This way also their relative rotational speeds in respect to each other are fixed in forcedly manner. In the advantageous embodiment of the figure 1 there are four compressors 12.1, 12.2, 12.3, 12.4 connected to the common mechanical drive system 16 with two expanders 14,1,14.2. The drive system comprises an electric motor 28 which is connected to the drive system via a first transmission 30 which has a first transmission ratio. In the figure 1 the first 14.1 and the second compressor 12.2 are in direct con nection with each other (transmission ratio 1:1). The Second compressor 12.2 is coupled to the third compressor 12.3 via a second transmission 32, which has a second transmission ratio in a range of about 1:1,8 to 1:2, such that the third compressor rotates about 2 times faster than the first and the second compres sors. In the drive system at least one of the at least two compressors is rotated with a first speed and at least one of the at least two compressors is rotated with a second speed. The third compressor 12.3, the fourth compressor 12.4 and the first expander 14.1 and the second expander 14.2 are in direct connection with each other, having the transmission ratio 1:1 between them. This way the drive system comprises a transmission, having a first transmission ratio for at least one of the at least two compressors and a second transmission ratio for at least one of the at least two compressors. [0034] As an example, when the motor is running at speed of 1500 rpm, the first and the second compressors are running at speed of 20000 rpm and the third compressor, the fourth compressor, the first expander and the second expander are running at speed of 36000 rpm. [0035] The compressors 12.1, 12.2, 12.3, 12.4 are coupled with each other in series in the refrigerant circuit 20 forming at least four stages of compressors. After, or downstream in the flow direction of the refrigerant gas, each one of the compressors 12.1, 12.2, 12.3, 12.4, there is a refrigerant gas cooler 26 arranged to the refrigerant circuit 20 so as to cool down the refrigerant gas after each com- pressor. Advantageously an overall pressure ratio produced by the compressors is 8-9. The expanders 14.1, 14.2 are arranged in parallel with each other in the refrigerant circuit 20.

[0036] The system 10 for producing liquefied product gas comprises further a system of heat exchangers 18. In a preferred embodiment, the system of heat exchangers 18 is a heat exchanger which comprises different streams receiving and giving out heat in the same unit i.e. a so-called multi-stream heat exchanger. The system of heat exchangers 18 is provided for cooling the product gas by means of refrigerant gas in a closed single phase refrigerant gas flow circuit 20, which is for sake of simplicity called as refrigerant circuit 20. The refrigerant gas is advantageously nitrogen and it is circulated in the system without phase changes, in gaseous form. The system of heat exchangers 18 has a gas lique faction passage 19 arranged in connection with the system of heat exchangers so as to liquefy the product gas. The gas liquefaction passage 19 is in flow com munication with a source of product gas 22 from which the product gas is deliv- ered in gaseous form. Respectively the gas liquefaction passage 19 is in flow communication with a gas consumer or a product gas storage 24 into which the product gas is delivered as liquefied product gas.

[0037] The closed single phase refrigerant gas flow circuit 20 of the system 10 for producing liquefied gas is provided with a first circuit section 20.1 which con nects the system of heat exchangers 18 to the first one of the compressors 12.1 in series and the last one of the compressor 12.4 in series back to the system of heat exchangers, and the refrigerant gas flow circuit is provided with a second circuit section 20.2 which connects the system of heat exchangers 18 to the ex panders 14.1, 14.2 and the expanders back to the system of heat exchangers. The refrigerant gas flow circuit 20 is also arranged to pass through the system of heat exchangers 18 such that the refrigerant circuit is in heat transfer communi- cation with the gas liquefaction passage 19.

[0038] The second circuit section 20.2 comprises a first and a second branch loops 20.2’, 20.2” in which the first and the second expanders 14.1, 14.2 are arranged. The first branch 20.2’ connects the system of heat exchangers18 to at least one of the at least two expanders 14.1, 14.2, which in the embodiment of the figure 1 means that the first branch 20.2’ connects the system of heat ex changers 18 to the first expander 14.1 and the second branch 20.2” connects the system of heat exchangers 18 to the second expander 14.2. The branching point 21 of the second circuit section 20.2 is within the system of heat exchangers 18. The branch point is arranged such that the first branch 20.2’ is leading the refrigerant gas out from the system of heat exchangers 18 sooner, and this way first branch 20.2’ is configured to be subjected to smaller extent of heat transfer in the system of heat exchangers 18 than the second branch 20.2”. Respectively, a connection point 23 connecting the refrigerant gas flows from the expanders 14.1, 14.2 is reconnecting the branch loops inside the system of heat exchangers 18. This way the refrigerant gas is lead at compressed state first to the system of heat exchangers 18, after which there is a step of branching a first stream of the refrigerant gas and leading the first stream of the refrigerant gas to a first ex pander and further from the first expander back to the system of heat exchangers 18, and the branching including a second stream of the refrigerant gas and lead- ing the second stream of the refrigerant gas to a second expander and further from the second expander 14.2 back to the system of heat exchangers 18. The first and the second streams of the refrigerant gas are connected and the con nected first and second streams of the refrigerant gas is lead back to the at least two compressors. [0039] The closed single phase refrigerant gas flow circuit 20 of the system 10 for producing liquefied gas comprises a refrigerant gas buffer assembly 34. The buffer assembly 34 comprises a buffer tank 35. The buffer tank 35 is controllably connected through a conduit 36 and a valve 38 therein to an upstream side of the first compressor 12.1 of the compressors in series. Additionally buffer tank 35 is controllably connected through a conduit 40 and a valve 42 therein to a downstream side of the last compressor 12.4 of the compressors in series. By means of the buffer assembly 34 the cooling power of the system 10 can con- trolled by adjusting the amount of the refrigerant gas in the closed single phase refrigerant gas flow circuit 20. This can be accomplished so that when the valve 42 in the conduit 40 connecting the downstream side of the last compressor 12.4 to the buffer tank 35 is opened, an amount of refrigerant gas pressurised by the compressors enters the buffer tank 35 and the amount of the gas in the closed single phase refrigerant gas flow circuit 20 is decreased. Now an amount of re frigerant gas is stored in the buffer tank 35 and is not participating to the cooling and liquefying the product gas. If the valve 38 in the conduit 36 connecting the upstream side of the first compressor 12.1 to the buffer tank 35 is opened an amount of the pressurized refrigerant gas is returned back to the closed single phase refrigerant gas flow circuit 20. Advantageously the cooling power of the system 10 is controlled by controlling the amount of the refrigerant gas in the circuit 20 while the drive system 6 is configured to run at constant speed.

[0040] As is mentioned earlier the system for producing liquefied product gas 10 is provided with the electric motor 28 which is coupled with the first transmis- sion 30 to the mechanical drive system 16. The motor is advantageously a three phase AC motor connected to a three phase electrical grid 44. The electric grid is an interconnected network for delivering electricity from producers to consum ers. The motor 28 is connected to the grid 44 by means of a starter device 46 by means for which it is possible to start the system 10 with moderate or acceptably low starting current. The starter device 46 comprises a soft start unit 50 and a selection functionality, such a switch system 48, by means of which switch sys tem the grid may be connected either directly or through the soft start unit 50 to the motor 28. The soft start unit 50 may be for example a star-delta starter, vari able frequency drive or generally a device that reduces the torque applied to the electric motor. The system 10 is provided with a computer controller 52 which is configured to control the operation of the system 10. There is a memory device 54 attached to the computer controller 52 which contains a computer program which when executed by the computer controller 52 causes the system to oper ate in a manner as described herein. The motor is started using the starter device and after reaching a predetermined speed of the engine or a state of operation of the system 10, the starter device is decoupled or deactivated. After the system has reached its steady state condition the cooling power of the system may be performed by adjusting the amount of the refrigerant gas in the closed single phase refrigerant gas flow circuit by leading a portion of the refrigerant from the flow circuit to the refrigerant gas buffer assembly controllably from the upstream side of the first compressor of the compressors in series, or leading a portion of the refrigerant from the refrigerant gas buffer assembly controllably to the flow circuit at the upstream side of the first compressor of the compressors in series. [0041] Generally, when the system 10 for producing liquefied product gas com prises at least two compressors 12, at least two expanders 14 and a system of heat exchangers 18 arranged to a closed single phase refrigerant gas flow circuit 20, and a gas liquefaction passage 18 arranged in connection with the system of heat exchangers so as to liquefy the product gas, liquefied product gas is pro- duced by circulating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors coupled in series, leading the refrigerant gas at compressed state to the system of heat exchangers 18, leading the stream of the refrigerant gas to the at least two ex panders 14 and further back to the system of heat exchangers 10, wherein the compressors and the expanders are operated by a common drive system 16 me chanically coupling the compressors 12 and the expanders 14 with each other.

[0042] Particularly, since the closed single phase refrigerant gas flow circuit 20 is provided with a refrigerant gas buffer assembly 34, the refrigerant gas is circu lated in the closed single phase refrigerant gas flow circuit, compressed in at least two compressors 12 and lead the refrigerant gas at compressed state to the system of heat exchangers 18. Further the stream of the refrigerant gas is led to the at least two expanders 14 and further back to the system of heat ex changers 18. The compressors and the expanders are operated by a common drive system 16 mechanically coupling the compressors and the expanders with each other, and the cooling power of the system is controlled by adjusting the amount of the refrigerant gas in the closed single phase refrigerant gas flow cir cuit 20 by leading a portion of the refrigerant from the flow circuit to the refrigerant gas buffer assembly 34 controllably from the downstream side of the last com pressor 12.4 of the compressors in series, or leading a portion of the refrigerant from the refrigerant gas buffer assembly 34 controllably to the flow circuit at the upstream side of the first compressor 12.1 of the compressors in series. [0043] While the product gas liquefaction power is controlled mainly by the buffer assembly 34, according to the invention the drive system 16 is configured to run at substantially constant speed during normal operation, such that the speed is preferably ruled directly by a 3-phase grid frequency to which the motor 28 is connected. Advantageously in this case the drive system 6 is operated by an alternating current electric motor 28 and after the system has reached its steady state condition the electric motor of the drive system is operated with a rotational speed corresponding to a grid frequency, the motor being connected directly online the grid 44.

[0044] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred em bodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the in vention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodi ment when such combination is technically feasible.

List of numbered embodiments

1) A system for producing liquefied product gas comprising at least two compressors, at least two expanders and a system of heat exchangers arranged to a closed single phase refrigerant gas flow circuit, and a gas liquefaction pas- sage arranged in connection with the system of heat exchangers so as to liquefy the product gas, wherein the least two compressors are coupled in series with each other in the refrigerant circuit, and the at least two expanders are coupled in parallel which each other in the refrig erant circuit, and wherein the compressors and the expanders are mechanically coupled with each other by a drive system.

2) A system for producing liquefied product gas according to the embod- iment 1, wherein the closed single phase refrigerant gas flow circuit is provided with a refrigerant gas buffer assembly controllably connected to an upstream side of the first compressor of the compressors in series, and to the downstream side of the last compressor of the compressors in series.

3) A system for producing liquefied product gas according to the embod- iment 1 or 2, wherein the drive system comprises a transmission, having a first transmission ratio for at least one of the at least two compressors and a second transmission ratio for at least one of the at least two compressors.

4) A system for producing liquefied product gas according to the embod iment 1 or 3, wherein the drive system comprises an electric motor configured to operate the drive system and the compressors coupled to the system.

5) A system for producing liquefied product gas according to the embod iment 1 , 3 or 4, wherein a refrigerant gas cooler is arranged downstream to each one of the compressors in the refrigerant gas flow circuit.

6) A system for producing liquefied product gas according to the embod- iment 1, 3, 4 or 5, wherein the refrigerant gas flow circuit is provided with a first circuit section which connects the system of heat exchangers to the first one of the compressors in series and the last one of the compressor in series back to the system of heat exchangers, and wherein the refrigerant gas flow circuit is provided with a second circuit section which connects the system of heat exchangers to the expanders and the expand- ers back to the system of heat exchangers.

7) A system for producing liquefied product gas according to the embod iment 6, wherein the second circuit section comprises a first and a second branch, the first branch connects the system of heat exchangers to at least one of the at least two expanders, and the second branch connects the system of heat exchangers to at least one of the at least two expanders.

8) A system for producing liquefied product gas according to the embod iment 7, wherein the first and a second branches are connected to each other downstream the at least two expanders and are connected to the first circuit sec tion. 9) A system for producing liquefied product gas according to the embod iment 7 or 8, wherein the first and a second branch are branched and connected in the system of heat exchangers.

10) A system for producing liquefied product gas according to the embod iment 8, wherein the first branch is configured to be subjected to smaller extent of heat transfer in the system of heat exchangers than the second branch.

11) A system for producing liquefied product gas according to the embod iment 1, wherein the drive system is configured to run at constant speed and the closed single phase refrigerant gas circuit is provided with a refrigerant gas buffer assembly connected to upstream side of the first compressor of the compressors in series, and to the downstream side of the last compressor of the compressors in series.

12) A system for producing liquefied product gas according to the embod iment 1, wherein the system comprises four compressors coupled in series with each other in the refrigerant circuit, and two expanders coupled in parallel which each other in the refrigerant circuit. 13) A system for producing liquefied product gas according to the embod iment 1 , wherein the system comprises a first compressor, a second compressor, a third compressor and a fourth compressor arranged in series, wherein the first and the second compressors are coupled to the drive system with a first trans- mission ratio and the third and the fourth compressors are coupled to the drive system with a second transmission ratio.

14) A system for producing liquefied product gas according to the embod iment 13, wherein the system comprises a first expander and a second expander wherein the first and the second expanders are coupled to the drive system with the second transmission ratio.

15) A system for producing liquefied product gas according to the embod iment 4, wherein the motor is provided with a soft start unit which is configured to connect the motor speed directly under control of electric grid frequency.

16) Method of operating a system according to embodiment 1 for produc- ing liquefied product gas, comprising steps of circulating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrigerant gas at compressed state to the system of heat exchangers, leading the stream of the refrigerant gas to the at least two expanders and further back to the system of heat exchangers, oper- ating the compressors and the expanders by a common drive system mechani cally coupling the compressors and the expanders with each other.

17) Method of operating a system according to embodiment 2 for produc ing liquefied product gas, comprising steps of circulating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrigerant gas at compressed state to the system of heat exchangers, leading the stream of the refrigerant gas to the at least two expanders and further back to the system of heat exchangers, oper ating the compressors and the expanders by a common drive system mechani cally coupling the compressors and the expanders with each other, and control- ling the cooling power of the system by adjusting the amount of the refrigerant gas in the closed single phase refrigerant gas flow circuit by leading a portion of the refrigerant from the flow circuit to the refrigerant gas buffer assembly control- lably from the downstream side of the last compressor of the compressors in series, or leading a portion of the refrigerant from the refrigerant gas buffer as sembly controllably to the flow circuit at the upstream side of the first compressor of the compressors in series.

18) Method of operating a system for producing liquefied product gas ac cording to embodiment 3, comprising steps of circulating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrigerant gas at compressed state to the system of heat exchangers, leading the stream of the refrigerant gas to the at least two expanders and further back to the system of heat exchangers, and operating the compressors and the expanders by a common drive system me chanically coupling the compressors and the expanders with each other such that at least one of the at least two compressors is rotated with a first speed and at least one of the at least two compressors is rotated with a second speed.

19) Method of operating a system for producing liquefied product gas ac cording to embodiment 7, comprising steps of circulating a refrigerant gas in the closed single phase refrigerant gas flow circuit, compressing the refrigerant gas in at least two compressors, leading the refrigerant gas at compressed state to the system of heat exchangers, branching a first stream of the refrigerant gas and leading the first stream of the refrigerant gas to a first expander and further from the first expander back to the system of heat exchangers, branching a sec ond stream of the refrigerant gas and leading the second stream of the refrigerant gas to a second expander and further from the second expander back to the system of heat exchangers, connecting the first and the second streams of the refrigerant gas and leading the thus connected first and second streams of the refrigerant gas to the at least two compressors.

20) Method of operating a system for producing liquefied product gas ac cording to anyone of the preceding embodiments 14 - 17, comprising a step of starting the system using a starter device and de-coupling or deactivating the starter device after the system has reached its steady state condition, and con trolling the cooling power of the system by adjusting the amount of the refrigerant gas in the closed single phase refrigerant gas flow circuit by leading a portion of the refrigerant from the flow circuit to the refrigerant gas buffer assembly control- lably from the downstream side of the last compressor of the compressors in series, or leading a portion of the refrigerant from the refrigerant gas buffer as sembly controllably to the flow circuit at the upstream side of the first compressor of the compressors in series.

21) Method of operating a system for producing liquefied product gas ac cording to embodiment 18, wherein the drive system is operated by an alternating current electric motor and after the system has reached its steady state condition the electric motor of the drive system is operated with a rotational speed corre- sponding to a grid frequency, the motor being connected directly online the grid.

22) A computer readable memory device comprising instructions which, when executed by a computer, cause the computer to carry out a method of any one of the embodiments 16 to 21.