System For Gathering Gas From A Gas Field Comprising A High Efficient High Pressure Compressor

Abstract

A system includes a high pressure compressor having a gas input and a gas output. The gas input is coupleable to a gas field for receiving a gathered gas with a first gas pressure, and the gas output is coupleable to a gas export system for providing the gas with a second gas pressure, which is higher than the first gas pressure, to the gas export system. The system also includes a measuring unit coupled to the gas input. The measuring unit measures an input mass flow value indicative of an input mass flow at the gas input. The system also includes a mass flow control unit that is coupled to the measuring unit for receiving the measured input mass flow value and the gas input for controlling the input mass flow such that the measured input mass flow value corresponds to a set point input mass flow value.

Description

[0001] The present application is a .sctn.371 nationalization of PCT Application No. PCT/EP2012/059386, filed May 21, 2012, and designating the United States, which, in turn, claims the benefit of EP 11168122.7, filed on May 30, 2011, the entire disclosures of which are incorporated by reference herein.

FIELD

[0002] The present embodiments relate to a system for gathering gas from a gas field and to a method for controlling a gas mass flow of a system for gathering gas from a gas field.

BACKGROUND

[0003] In the technical field of gas gathering, gas is gathered from conventional gas field types, and unconventional gas field types exist.

[0004] In conventional gas fields, the stored gas includes an initially high pressure such that further compression by external compressors is not needed in the beginning. After years of free-flow production, a high pressure central compressor (e.g., of a centrifugal type compressor) is installed for another couple of years. In the end of the gas field lifetime, low pressure wellhead compressors are implemented and connected to the respective wellheads of the gas field. The high pressure central compressor and the low pressure wellhead compressor are adapted for sucking the gas out of the gas field and generating a high pressure gas flow for subsequent gas export systems, such as pipelines or gas generators.

[0005] In unconventional gas fields, such as tight gas, shell gas or coal bed methane gas fields, the gas pressure inside the gas field decreases in a short time interval, such that central high pressure compressors and low pressure wellhead compressors are used from the beginning of the gathering of the gas from the gas field.

[0006] Up to now, the low pressure wellhead compressors and the high pressure central compressor are configured as separate and independent subsystems. The low pressure wellhead compressors and the high pressure wellhead compressor may form independent and separated units. The high pressure central compressor is optimized for a small range of inlet pressures and inlet mass flow values. The low pressure wellhead compressors are optimized for individual ranges of respective inlet pressures and mass flows. Each low pressure wellhead compressor thus produces an output pressure and an output mass flow of gas without considering the small range of inlet pressures and mass flow of gas at which the high pressure central compressor works efficiently. A variation of the gas pressure in the gas field does not affect the efficiency of the low pressure wellhead compressors dramatically, whereas the efficiency of the subsequent high pressure central compressor is affected dramatically during pressure changes. The gathering of the gas is thus inefficient if the high pressure central compressor is not working under an efficient working point for which the high pressure central compressor is configured for (e.g., the inlet gas pressures and inlet mass flow of the high pressure central compressor vary).

SUMMARY

[0007] The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary.

[0008] The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, an efficient gas gathering system for gathering gas from a gas field is provided.

[0009] A system for gathering a gas from a gas field and a method for controlling a gas mass flow of a system for gathering gas from a gas field are provided.

[0010] In one embodiment, a system for gathering a gas from a gas field is provided. The system includes a high pressure compressor having a gas input and a gas output. The gas input is coupleable to the gas field for receiving the gas gathered from the gas field with a first gas pressure. The gas output is coupleable to a gas export system for providing the gas with a second pressure to the second gas export system. The second gas pressure is higher than the first gas pressure. The system also includes a measuring unit that is coupled to the gas input. The measuring unit measures an input mass flow value that is indicative of an input mass flow at the gas input. The system also includes a mass flow control unit. The mass flow control unit is coupled to the measuring unit for receiving the measured input mass flow value. The mass flow control unit is coupled to the gas input for controlling the input mass flow such that the measured input mass flow value corresponds to a set point input mass flow value.

[0011] According to another embodiment, a method for controlling a gas mass flow of a system for gathering gas from a gas field is provided. According to the method, an input mass flow at a gas input of a high pressure compressor is measured. The gas input receives the gas gathered from the gas field with a first gas pressure. The high pressure compressor includes a gas output for providing gas with a second pressure to a gas export system. The second gas pressure is higher than the first gas pressure. An input mass flow value that is indicative of an input mass flow at the gas input is measured by a measuring unit that is coupled to the gas input. The measured input mass flow value is received by a mass flow control unit. The input mass flow is controlled by the mass flow control unit, which is coupled to the gas input. The mass flow control unit controls the input mass flow such that the input mass flow value at the gas input corresponds to a set point input mass flow value.

[0012] By the above-described system and by the above-described method for controlling a gas mass flow, gas (e.g., natural gas) may be gathered from a conventional gas field or from an unconventional gas field, in which tight gas, shell gas or coal bed methane gas is gathered.

[0013] The high pressure compressor is installed between the gas field and the gas export system. The high pressure compressor is adapted for receiving the gas from the gas field with a first pressure (e.g., approximately 10 to 14 bars) and adapted to compress the gas up to a second gas pressure of approximately 60 to 90 bars used by the gas export system.

[0014] The high pressure central compressor may, for example, be of a centrifugal type compressor and is most suitable for the gas gathering in order to generate the high gas pressure for the subsequent gas export system. The high pressure compressor includes a small efficient range regarding the inputted mass flow and input pressure in which the high pressure compressor works efficient. Due to changes in inlet conditions, such as in the mass flow and in the inlet gas pressure, a high impact on the operation efficiency is caused.

[0015] The gas export system includes, for example, a pipeline system for transporting the gas to a desired destination. The export gas system may also include a gas power plant for generating electricity or further gas treatment systems.

[0016] The measuring unit is coupled to the gas input of the high pressure compressor and is adapted for measuring a variety of parameters that are indicative of the input mass flow of the gas at the gas input of the high pressure compressor. For example, the measuring unit measures the input mass flow value (e.g., kg/s). Additionally, the measuring unit may measure the first gas pressure of the gas at the gas input and the second gas pressure at the gas output. The measuring unit is also adapted for measuring the output mass flow at the gas output of the high pressure compressor. Additionally, the measuring unit may measure the composition of the gas as well as the temperature of the gas at the gas input and the gas output of the high pressure compressor.

[0017] The measured values (e.g., the measured input mass flow value) are provided to the mass flow control unit. The mass flow control unit compares the measured values (e.g., the input mass flow value) with a given set point input mass flow value. If there is a difference between the input mass flow value and the set point input mass flow value, the mass flow control unit controls the input mass flow value such that the input mass flow value corresponds to the set point input mass flow value.

[0018] The set point input mass flow value defines the mass flow at the gas input of the high pressure compressor, by which the high pressure compressor is working at a working point of the high pressure compressor and thus is working with the highest efficiency. The working point is dependent on the pressure relation between the first gas pressure at the gas input and the second gas pressure at the gas output, the mass flow of the gas through the high pressure compressor (e.g., the mass flow at the gas input) and the rotational speed of the high pressure compressor.

[0019] Under given first pressures and second pressures, a set point input mass flow value is thus given for running the high pressure compressor at a working point of the high pressure compressor and hence at a best efficiency. A deviation of the input mass flow value causes a severe effect on the efficiency of the high pressure compressor. For example, if the measured input mass flow value differs dramatically to the set point input mass flow value (e.g., at given first and second gas pressures), compressor surge may occur.

[0020] The mass flow control unit is adapted for reducing the deviation of the measured input mass flow value from the given set point input mass flow value. The mass flow control unit may control various subsystems for influencing the input mass flow of the gas at the gas input. For example, the mass flow control unit may control upstream installed low pressure compressors, delivery valves or a mass flow in a return flow tubing and/or a mass flow of a bypass tubing, which will be explained in more detail below.

[0021] By one or more of the present embodiments, even when the gathered mass flow of gas from a gas field varies dramatically, a predetermined and desired input mass flow at the gas input of the high pressure compressor is adjustable by the mass flow control unit such that the high pressure compressor is working approximately at a working point of the high pressure compressor. Hence, the efficiency of the high pressure compressor and hence the gathering of gas from a gas field is more efficient.

[0022] According to a further exemplary embodiment, the system includes a first low pressure compressor that is connectable to a first wellhead arrangement of the gas field for receiving the gas from the first wellhead arrangement. The first low pressure compressor is connected to the gas input for providing the gas to the gas input with a first mass flow. The system also includes a second low pressure compressor that is connectable to a second wellhead arrangement of the gas field for receiving the gas from the second wellhead arrangement. The second low pressure compressor is connected to the gas input for providing the gas to the gas input with the second mass flow.

[0023] The mass flow control unit is connected to: a) the first low pressure compressor for controlling the first low pressure compressor; and b) the second low pressure compressor for controlling the second low pressure compressor such that the first mass flow and the second mass flow are controllable for controlling the input mass flow at the gas input.

[0024] Each wellhead arrangement is connected to a borehole through which gas of the gas field is gathered. A gas field may include a plurality of boreholes. A respective wellhead arrangement is attached to each borehole for gathering gas. Each wellhead arrangement is connected to a respective low pressure compressor. For example, if a wellhead arrangement delivers gas with approximately 1 to 4 bars to the low pressure compressor, the low pressure compressor increases the pressure up to approximately 5 to 20 bars. By increasing the pressure, a sucking effect is achieved, such that the gas is sucked out of the respective borehole, so that the respective mass flow through the low pressure compressor is also increased.

[0025] For the low pressure wellhead compressors, screw-type or reciprocating compressors may be used. These kinds of compressors are relatively insensitive to changes in inlet conditions. The compressors have a large operating range and may easily handle a wide range of inlet pressure variations and inlet mass flows variations under which an effective operation may still be provided.

[0026] For example, the measuring unit is adapted for measuring each mass flow and pressure at a respective gas input of a respective low pressure compressor and at a gas output of a respective low pressure compressor. This measured data is sent to the mass flow control unit. The mass flow control unit also receives the information of the input mass flow at the gas input of the high pressure compressor. If more or less mass flow is needed at the gas input of the high pressure compressor, the mass flow control unit controls the respective low pressure compressors (e.g., the first low pressure compressor and the second low pressure compressor).

[0027] For example, the mass flow control unit receives the information of the working points of the respective low pressure compressors. For adjusting the desired input mass flow of the high pressure compressor, each low pressure compressor is thus controlled under consideration of respective working points such that each low pressure compressors is operated more efficiently. The overall efficiency of the system is thus increased.

[0028] Additionally or alternatively, according to a further exemplary embodiment, the system includes a first delivery valve that is connectable to a first wellhead arrangement of the gas field for receiving the gas from the first wellhead arrangement. The first delivery valve is connected to the gas input for providing the gas to the gas input with a further first mass flow. The system further includes a second delivery valve that is connectable to a second wellhead arrangement of the gas field for receiving the gas from the second wellhead arrangement. The second delivery valve is connected to the gas input for providing the gas to the gas input with a further second mass flow.

[0029] The mass flow control unit is connected to: a) the first delivery valve for controlling the first delivery valve; and b) the second delivery valve for controlling the second delivery valve, such that the further first mass flow and the further second mass flow are controllable for controlling the input mass flow at the gas input.

[0030] The first and second delivery valves may be installed between the respective first and second wellhead arrangement and the respective first and second low pressure compressors. Alternatively, the first and second delivery valves may be installed behind (e.g., downstream) of the respectively first and second low pressure compressors.

[0031] By controlling the respective delivery valves, the further first mass flow and the further second mass flow of the gas gathered from a respective borehole is adjustable and controllable individually, such that a desired input mass flow of the gas at the gas input of the high pressure compressor is adjustable.

[0032] In gas gathering systems (e.g., in conventional gas gathering systems), gas may be gathered without using low pressure compressors. In this case, only the delivery valves are installed downstream of the wellhead arrangement in order to control mass flow from the respective wellhead arrangement to the gas input of the high pressure compressor.

[0033] According to a further exemplary embodiment, the system includes a return flow tubing that is connected to the gas output for bleeding off a part of the gas from the gas output and is connected to the gas input for injecting the part of the gas in the gas input. The mass flow control unit is coupled to the return flow tubing for controlling the bleeding off of the part of the gas from the gas output and for controlling the injecting of the part of the gas in the gas input such that the input mass flow at the gas input is controllable.

[0034] For example, if the measured input mass flow value is below the set point input mass flow value, a part of the gas may be bled off from the gas output and injected at the gas input, so that the input mass flow is increased at the gas input. In the return flow tubing, a control valve that is controllable by the mass flow control unit may be installed.

[0035] By using the return flow tubing, an anti-surge system is established. The mass flow control unit may act as a supervisory system and controls the anti-surge system. By controlling the input gas parameters (e.g., the input mass flow of the gas) and by influencing the upstream located systems, such as the low pressure compressors (e.g., wellhead compressors), the anti-surge system for the high pressure compressor is safer and more efficient.

[0036] According to a further exemplary embodiment, the system further includes a bypass tubing that is connected to the gas input for bleeding off a further part of the gas from the gas input. The mass flow control unit is coupled to the bypass tubing for controlling the bleeding off of the further part of the gas from the gas input such that the input mass flow at the gas input is controllable.

[0037] For example, if the input mass flow is higher than the set point input mass flow value, the bypass tubing may bleed off the gas from the gas input (e.g., to the environment or to a further use), for example, into a gas generator. By bleeding off the gas from the gas input, the input mass flow is thus reduced until the desired set point input mass flow value is adjusted.

[0038] By one or more of the present embodiments, the high pressure compressor of a gas gathering system is controlled and operated more efficiently. For example, predetermined set point input values (e.g., set point input mass flow values) of the gas at the gas input of the high pressure compressor are determined, so that already at the design phase, the high pressure compressor may be optimized for predetermined operating ranges in order to run more efficiently at the given set point input mass flow values. For example, the high pressure compressor may be configured with a certain number of stator rings, cooling systems, etc., so that already at the design phase of the high pressure compressor, an optimized high pressure compressor is constructable.

[0039] By one or more of the present embodiments, optimized inlet conditions for the high pressure compressor are generated, for example, by adapting and controlling the low pressure compressors. By controlling the low pressure compressors dependent on the desired input values of the high pressure compressor, an interconnected efficient and effective gathering system is achieved.

[0040] Embodiments have been described with reference to different subject matters. For example, some embodiments have been described with reference to an apparatus, whereas other embodiments have been described with reference to a method. However, a person skilled in the art will gather from the above and the following description that, unless otherwise noted, in addition to any combination of features belonging to one type of subject matter, any combination of features relating to different subject matters (e.g., between features of the apparatus and features of the method) is considered as to be disclosed with this application.

BRIEF DESCRIPTION OF THE DRAWING

[0041] FIG. 1 shows a gas gathering system according to an exemplary embodiment.

DETAILED DESCRIPTION

[0042] In FIG. 1, similar or identical elements are provided with the same reference signs.

[0043] FIG. 1 shows a system for gathering a gas from a gas field 170. The system includes a high pressure compressor 100 that includes a gas input 101 and a gas output 102.

[0044] The gas input 101 is coupleable to the gas field 170 for receiving the gas gathered from the gas field 170 with a first gas pressure p1. The gas output 102 is coupleable to a gas export system 130 for providing the gas with a second gas pressure p2 to the gas export system 130. The second gas pressure p2 is higher than the first gas pressure p1.

[0045] A measuring unit 110 is coupled to the gas input 101. The measuring unit 110 measures (e.g., by sensor elements 111) an input mass flow value (e.g., in kg/s) that is indicative of an input mass flow {dot over (m)}.sub.1 at the gas input 101.

[0046] The system also includes a mass flow control unit 120. The mass flow control unit 120 is coupled to the measuring unit 110 for receiving the measured input mass flow value and the gas input 101 for controlling the input mass flow {dot over (m)}.sub.1 such that the measured input mass flow value corresponds to a set point input mass flow value.

[0047] The measuring unit 110 may also measure further mass flow affecting parameters (e.g., the first pressure p1 and the temperature at the gas input 101 and/or the second pressure p2, the output mass flow {dot over (m)}.sub.2 and/or the temperature at the gas output 102).

[0048] All measured data is provided to the mass flow control unit 120. The mass flow control unit 120 may calculate based on the inputted data values, such as the input mass flow {dot over (m)}.sub.1, the first pressure p1, the second pressure p2, the output mass flow {dot over (m)}.sub.2, the respective gas temperature at the gas input 101 and the gas output 102, a set point input mass flow value (e.g., kg/s) at which the high pressure compressor 100 is operated approximately at a working point. The mass flow control unit 120 may be connected to the high pressure compressor 100 for controlling, for example, the speed of rotation of the high pressure compressor 100.

[0049] In FIG. 1, the tubing that transports the gas between the system devices are shown by the solid lines, whereas the data lines for transmitting measuring data and control data are shown with the dotted lines.

[0050] Besides the high pressure compressor 100, in FIG. 1, further components of the system for gathering gas are shown. From the gas field 170, gas may be gathered through a plurality of boreholes. At each borehole, a respective wellhead arrangement 151-154 is attached. Downstream of each wellhead arrangement 151-154, a respective delivery valve 161-164 is coupled. By controlling the delivery valves 161-164, a respective mass flow from a respective wellhead arrangement 151-154 to the gas input 101 may thus be controllable.

[0051] In order to increase the amount of gathered gas from the gas field, in all or in some tubes between the gas input 101 and a respective first wellhead arrangement 151-154, a respective low pressure compressor 141-144 is installed. Each low pressure compressor 141-144 may increase the pressure from approximately 1 to 4 bars to approximately 5 to 20 bars.

[0052] By controlling the low pressure compressor 141-144, the mass flow of gas that is delivered by a respective wellhead arrangement 151 to the gas input 101 is adjustable. The mass flow control unit 120 is adapted for controlling the respective low pressure compressors 141-144 individually, so that each low pressure compressor 141-144 may deliver a predetermined input mass flow {dot over (m)}.sub.1 of gas to the gas input 101. The mass flow control unit 120 is adapted, for example, for controlling the rotational speed of each low pressure compressor 141-144.

[0053] In the exemplary embodiment shown in FIG. 1, four low pressure compressors 141-144, four wellhead arrangements 151-154 and four delivery valves 161-164 are shown. In other exemplary embodiments, more or fewer low pressure compressors 141-144, wellhead arrangements 151-154 and delivery valves 161-164 may be installed.

[0054] A gathering group may be formed by one low pressure compressor 141-144, one wellhead arrangement 151-154 and one delivery valve 161-164. Each gathering group is coupled to a respective borehole. All gathering groups deliver gas to a common collecting manifold to which the high pressure compressor 100 is coupled. A gas field may include a plurality of boreholes. A respective gathering group is coupled to each borehole.

[0055] The system may also include a further high pressure compressor 100. First gathering groups are connected to the one high pressure compressor 100, and second gathering groups are connected to the further high pressure compressor.

[0056] The high pressure compressor 100 is adapted for increasing the received gas at the gas input 101 from approximately 10 to 14 bars to approximately 60 to 90 bars at the gas outlet 102. For the gas export system 130, a high gas pressure that is needed, for example, for the further processing of the gas is provided.

[0057] As shown in FIG. 1, the system includes a return flow tubing 103 to which a bleeder valve 104 is connected. The bleeder valve 104 is controlled by the mass flow control unit 120. If the measured input mass flow {dot over (m)}.sub.1 is lower than the set point input mass flow, the mass flow control unit 120 controls the bleeder valve 104 such that a part of the gas is bled off from the gas outlet 102 and injected into the gas input 101, such that the input mass flow value is increased until the input mass corresponds to the set point input mass flow value.

[0058] The term "including" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Also, elements described in association with different embodiments may be combined.

[0059] It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims can, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.

[0060] While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.

Claims

1. A system for gathering a gas from a gas field, the system comprising: a high pressure compressor comprising a gas input and a gas output, wherein the gas input is coupleable to the gas field for receiving the gas gathered from the gas field with a first gas pressure, wherein the gas output is coupleable to a gas export system for providing the gas with a second gas pressure to the gas export system, and wherein the second gas pressure is higher than the first gas pressure; a measuring unit coupled to the gas input, wherein the measuring unit is configured to measure an input mass flow value that is indicative of an input mass flow at the gas input; a first low pressure compressor that is connectable to a first wellhead arrangement of the gas field for receiving the gas from the first wellhead arrangement, wherein the first low pressure compressor is connected to the gas input for providing the gas to the gas input with a first mass flow; a second low pressure compressor that is connectable to a second wellhead arrangement of the gas field for receiving the gas from the second wellhead arrangement, wherein the second low pressure compressor is connected to the gas input for providing the gas to the gas input with a second mass flow; and a mass flow control unit, wherein the mass flow control unit is coupled to the measuring unit for receiving the measured input mass flow value, wherein the mass flow control unit is further connected to the first low pressure compressor for controlling the first low pressure compressor, and the second low pressure compressor for controlling the second low pressure compressor such that the first mass flow and the second mass flow are controllable for controlling the input mass flow at the gas input such that the measured input mass flow value corresponds to a set point input mass flow value.

2. The system according to claim 1, further comprising: a first delivery valve that is connectable to the first wellhead arrangement of the gas field for receiving the gas from the first wellhead arrangement, wherein the first delivery valve is connected to the gas input for providing the gas to the gas input with a further first mass flow; and a second delivery valve that is connectable to the second wellhead arrangement of the gas field for receiving the gas from the second wellhead arrangement, wherein the second delivery valve is connected to the gas input for providing the gas to the gas input with a further second mass flow, wherein the mass flow control unit is connected to the first delivery valve for controlling the first delivery valve and the second delivery valve for controlling the further second mass flow such that the further first mass flow and the further second mass flow are controllable for controlling the input mass flow at the gas input.

3. The system according to claim 1, further comprising: a return flow tubing that is connected to the gas output for bleeding off a part of the gas from the gas output and is connected to the gas input for injecting the part of the gas in the gas input, wherein the mass flow control unit is coupled to the return flow tubing for controlling the bleeding off of the part of the gas from the gas output and for controlling the injecting of the part of the gas in the gas input such that the input mass flow at the gas input is controllable.

4. The system according to claim 1, further comprising: a bypass tubing that is connected to the gas input for bleeding off a further part of the gas from the gas input, wherein the mass flow control unit is coupled to the bypass tubing for controlling the bleeding off of the further part of the gas from the gas input such that the input mass flow at the gas input is controllable.

5. A method for controlling a gas mass flow of a system for gathering gas from a gas field, the method comprising: measuring an input mass flow at a gas input of a high pressure compressor, wherein the gas input receives the gas gathered from the gas field with a first gas pressure, wherein the high pressure compressor comprises a gas output for providing gas with a second gas pressure to a gas export system, and wherein the second gas pressure is higher than the first gas pressure; measuring, by a measuring unit that is coupled to the gas input, an input mass flow value that is indicative of an input mass flow at the gas input; receiving, by a mass flow control unit, the measured input mass flow value, wherein a first low pressure compressor is connectable to a first wellhead arrangement of the gas field for receiving the gas from the first wellhead arrangement, wherein the first low pressure compressor is connected to the gas input for providing the gas to the gas input with a first mass flow, wherein a second low pressure compressor is connectable to a second wellhead arrangement of the gas field for receiving the gas from the second wellhead arrangement, and wherein the second low pressure compressor is connected to the gas input for providing the gas to the gas input with a second mass flow; and controlling, by the mass flow control unit coupled to the gas input, the input mass flow, wherein the mass flow control unit is coupled to the measuring unit for receiving the measured input mass flow value, and wherein the mass flow control unit is further connected to the first low pressure compressor for controlling the first low pressure compressor and the second low pressure compressor for controlling the second low pressure compressor such that the first mass flow and the second mass flow are controllable for controlling the input mass flow at the gas input such that the measured input mass flow value corresponds to a set point input mass flow value.

6. The system according to claim 1, further comprising: a return flow tubing that is connected to the gas output for bleeding off a part of the gas from the gas output and is connected to the gas input for injecting the part of the gas in the gas input, wherein the mass flow control unit is coupled to the return flow tubing for controlling the bleeding off of the part of the gas from the gas output and for controlling the injecting of the part of the gas in the gas input such that the input mass flow at the gas input is controllable.

7. The system according to claim 2, further comprising: a bypass tubing that is connected to the gas input for bleeding off a further part of the gas from the gas input, wherein the mass flow control unit is coupled to the bypass tubing for controlling the bleeding off of the further part of the gas from the gas input such that the input mass flow at the gas input is controllable.

8. The system according to claim 3, further comprising: a bypass tubing that is connected to the gas input for bleeding off a further part of the gas from the gas input, wherein the mass flow control unit is coupled to the bypass tubing for controlling the bleeding off of the further part of the gas from the gas input such that the input mass flow at the gas input is controllable.