PROCESS FOR COOLING A GAS BY MEANS OF A REFRIGERATION CYCLE

Abstract

In a process for cooling hydrogen by means of a refrigeration cycle, a cycle fluid (4), which is nitrogen, is cooled to a temperature lower than −100° C., at least one portion (8-1) of the cooled cycle fluid is expanded in a turbine (T1) in order to cool the at least one portion of the cycle fluid, which produces a two-phase fluid (6) at the outlet of the turbine, the two-phase fluid is separated in a phase separator (V1), and at least one portion of the gas (8) produced in the phase separator is sent to a first heat exchanger (E1) in order to exchange heat indirectly with the feed gas (1) to be cooled, which produces a cooled feed gas (2) and a heated cycle gas (9), which is compressed in a compressor (C1) and then cooled in a cycle.

Description

BACKGROUND

The present invention relates to a process for cooling a gas, or even for liquefying a gas, by means of a refrigeration cycle.

It is common practice to cool a gas by means of a closed refrigeration cycle, which produces cold using a turbine that expands the cycle gas, this turbine being coupled to a compressor that compresses the cycle gas (generally air or nitrogen).

The gas at the outlet of the turbine is generally in gaseous form, with 0% liquid, but sometimes the process requires liquid to be produced, for example in order to feed a thermosiphon. Thus, a liquid phase may be formed, which typically constitutes up to 10 mol % or more of the overall expanded flow. In order to produce such an increase in the liquid fraction at the outlet of the turbine, the expansion ratio of the turbine is kept constant (or at least below 11), but the inlet temperature of the turbine is reduced, typically lower than −115° C. (preferably below −130° C. for a nitrogen cycle).

SUMMARY

One aim of the present invention is to present a process that allows the required liquid fraction to be produced at the outlet of the turbine without reducing the temperature at the inlet of the turbine, but optionally by increasing the ratio between the outlet pressure of the turbine and the inlet pressure of the turbine to 11 or above 11 (preferably above 12, or even 16), the two pressures being in bar absolute.

Another aim is to simplify the design of the heat exchanger.

Another aim is to reduce the specific energy of the cycle.

Another aim is to produce one portion of the cycle fluid as a liquid product.

The invention relates to processes such as the recovery of cold from liquefied natural gas, wherein the inlet temperature of the gas passing through the turbine is set by the external conditions (for example if the gas to be expanded has been cooled by vaporization of liquefied natural gas through a heat recovery unit, the inlet temperature of the gas passing through the turbine is set depending on the temperature of the liquefied natural gas to be vaporized (typically −120° C.)).

Units for cooling and optionally for liquefying, for example units producing LNG, nitrogen, oxygen or hydrogen, are cooled by a closed cycle of an intermediate fluid (typically nitrogen with a purity of greater than 90 mol %, preferably greater than 99 mol %, or even 99.9 mol %) comprising at least one compressor, at least one booster, at least one turbine and at least one Joule-Thomson expansion valve. Turbines supply a large portion of the cold energy by extracting energy from the cycle (by extracting enthalpy from the intermediate fluid).

In certain cases, in addition to the production of cold gas, the process requires the production of liquefied gas, either for export or for supplying cold energy needed for cooling and/or liquefying, at least partially, the feed flow, for example in order to feed a thermosiphon.

The liquid is produced either using a Joule-Thomson expansion valve or by the turbine. This production of liquid is more effective if it is performed in the turbine but this requires a turbine with a high expansion ratio.

It is well known, in air separation, for up to 10% liquid to be produced at the outlet of an air turbine. This production is achieved by lowering the inlet temperature and with an expansion ratio of lower than 11.

The problem solved by the present invention is that of producing a liquid fraction at the outlet of the turbine by increasing the ratio between the outlet pressure and the inlet pressure of the turbine. This provides an additional parameter to be adjusted and allows the cycle to be highly effective.

JP2002 164389, WO2014/019698A2 and “Performance and Optimization of Hydrogen Liquefaction Cycles” by Nandi et al, International Journal of Hydrogen Energy, vol. 18, no. 2, 1993, describe the use of a refrigeration cycle that involves the gas to be cooled itself; it is therefore implicit that the feed gas has the same composition as the cycle fluid.

In CN112361713A, the cycle fluid is hydrogen, as is the feed gas. It may be observed that the cycle is used to bring the feed gas to its liquefaction temperature. In the present case, nitrogen cannot bring hydrogen to its liquefaction point since nitrogen would freeze at a higher temperature.

According to one subject of the invention, provision is made for a process for cooling a feed gas, which is hydrogen, by means of a refrigeration cycle, wherein:

• • a) a cycle fluid, which is nitrogen, is cooled to a temperature lower than −100° C., or even lower than −120° C.,
  • b) at least one portion of the cooled cycle fluid is expanded in a turbine in order to cool the at least one portion of the cycle fluid, which produces a two-phase fluid at the outlet of the turbine, and either
  • c) the two-phase fluid is separated in a phase separator, and
  • d) at least one portion of the gas produced in the phase separator is sent to a first heat exchanger in order to exchange heat indirectly with the feed gas to be cooled, which produces a cooled feed gas and a heated cycle gas, which is compressed in a compressor and then cooled in a cycle according to step a), and
  • e) at least one fraction of the liquid of the phase separator is vaporized in a second heat exchanger by way of indirect heat exchange with the cooled feed gas in order to further cool the feed gas or even to liquefy it,
  • or
  • f) the two-phase fluid is heated directly in a first exchanger by way of heat exchange with the feed gas to be cooled, which produces a cooled feed gas and a heated cycle gas, which is sent to a compressor as cycle fluid before being cooled according to step a).

According to other optional features:

• • the ratio between the inlet pressure and the outlet pressure in bar absolute of the turbine is equal to or greater than 11, preferably greater than 12, or even greater than 16,
  • the two-phase fluid contains a proportion of liquid between 5 and 20 mol % of liquid,
  • the two-phase fluid contains a proportion of liquid greater than 5 mol %, if not greater than 7 mol %, if not greater than 9 mol %, preferably up to 15 mol %, of liquid,
  • the amount of liquid produced in the turbine is adjusted by adjusting the outlet pressure of the cycle compressor and therefore the inlet pressure of the turbine, thus modifying the expansion rate of the turbine,
  • one portion of the cooled fluid is expanded in the turbine and another portion of the cooled cycle fluid is cooled in the first exchanger until it is completely liquefied, forming a liquid, this liquid is then expanded in a valve, the expanded liquid and the two-phase fluid at the outlet of the turbine are either mixed in the phase separator or mixed before being sent directly to the first heat exchanger in order to be heated,
  • all of the liquid of the two-phase fluid sent to the phase separator or directly to the heat exchanger originates from the turbine,
  • the cycle fluid contains at least 90 mol %, or even at least 99 mol %, of nitrogen,
  • during step a), the cycle fluid is cooled by an external source of cold in a third heat exchanger, for example by vaporizing liquefied natural gas at less than −100° C. or heating natural gas at less than −100° C., and is sent directly to the turbine without passing through the first heat exchanger,
  • one portion of the liquid formed in the phase separator serves as a liquid product of the process,
  • gas is added, from an external source, to the cycle downstream of the compressor, this gas having the same composition as the cycle fluid, in order to increase the inlet pressure of the compressor in order to increase the proportion of liquid produced by the turbine,
  • the cooling in step a) takes place by way of indirect heat exchange in a heat exchanger other than the first heat exchanger (or the second heat exchanger, if present),
  • the cycle fluid is cooled in step a) to a temperature greater than −192° C.,
  • the cycle fluid is not cooled
  • the cycle fluid is cooled in step a) to a temperature lower than −192° C.,
  • the cooled feed gas is then liquefied,
  • the cooled feed gas is then liquefied by a refrigeration cycle in which hydrogen or helium circulates.

According to the invention, a refrigeration cycle comprising a cycle fluid (which is nitrogen) is compressed in a cycle compressor, cooled by way of heat exchange with an external source of cold energy (such as liquefied natural gas) and introduced at less than −100° C., preferably less than −120° C., into an expansion turbine in order to extract work from the cycle fluid. The expansion rate of the turbine may be equal to or greater than 11 (preferably greater than 12, or even greater than 16) and therefore the expanded fluid comprises at least 5 mol %, up to 10 mol %, or even up to 20 mol %, of liquid. The expansion rate is the ratio between the inlet pressure and the outlet pressure, the two pressures being in bar absolute.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 shows a process according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a gas 1, which is hydrogen, is cooled in a first heat exchanger E1 by way of indirect heat exchange with a closed refrigeration cycle 8.

Then, the gas is optionally cooled in a second heat exchanger E2 in order to form a fluid 3 at −190° C. (gas or liquid) against a liquid fraction 8-1 of the cycle fluid.

The cycle compressor C1 compresses the cycle fluid, which is nitrogen, from a first pressure to a second pressure. The compressed gas 4, which is typically compressed at an ambient temperature, is cooled in a third heat exchanger E3 by way of heat exchange with liquefied natural gas 10 or another fluid to −120° C. The liquefied natural gas 10 is heated, or even vaporized, in the heat exchanger E3, forming a heated fluid 11. The cycle fluid leaves the exchanger E3 at −115° C. as gas 5.

Otherwise, the gas 4 may be cooled in the heat exchanger E1.

The gas 5 is optionally divided into two portions, one 5-1 of which is expanded in a turbine T1, the ratio between the outlet pressure and the inlet pressure exceeding 11. A liquid fraction is produced at the outlet of the turbine, which represents 5 to 20 mol % of the expanded flow 6. The fraction is preferably greater than 5 mol %, if not greater than 7 mol %, if not greater than 9 mol %.

The other portion 5-2 of the gas is cooled in the first heat exchanger E1, having been sent to the exchanger at an intermediate temperature thereof. At the outlet of the first exchanger E1, said portion is expanded as fluid 7 in a valve JT1. Then, the two expanded flows 6,7 are mixed and sent to a phase separator V1, forming a gas 8 and a liquid. The liquid is optionally divided in two, one portion 8-1 being sent to a heat exchanger E2, where it is vaporized. The remainder 8-2 serves as a by-product. The liquid vaporized by vaporizing the portion 8-1 is returned to the separator V1. The gas 8 is heated in the heat exchanger E1, forming the gas 9, which is sent to the compressor C1 at the first pressure.

All of the liquid in the phase separator V1 preferably originates from the turbine T1. The amount of liquid produced by the turbine is controlled by the pressure of the cycle: the level of liquid in the phase separator V1 (thermosiphon) located downstream of the turbine T1 is detected by the level control LC1, which will act in cascade on the pressure control PC1 downstream of the compressor C1, which, in the event of a low level of liquid in the separator V1, will open the make-up valve JT2 for making up the stock of the cycle 12 in order to deliver a gaseous make-up flow from an external source, for example an air separation unit, having the same composition as the cycle fluid. Making up additional cycle stock will allow more liquid 8 to be vaporized, and will therefore increase the pressure at the inlet of the compressor C1. The inlet guide vanes (IGV) of the compressor C1 will open and increase the pressure of the cycle until the liquid level in the phase separator V1 reaches equilibrium. It should be noted that the presence of the flow 5-2 and of the valve JT1 is not essential to the invention. However, if the increase in the pressure of the cycle described above is not sufficient for the liquid level in the separator V1 to reach equilibrium, a second setpoint of the LC1 (liquid level lower than the previous setpoint) may control this valve JT1 in order to further increase the liquid production toward the phase separator V1.

The valve JT1 therefore fulfils two functions: firstly, providing an adjustment that can be more responsive to the amount of liquid in the phase separator V1 and secondly, providing a portion of the production of liquid feeding the phase separator V1 without depending on the performance of the turbine T1 or on the pressure and temperature conditions at the suction end of the turbine (fluid 5, 5-1). Finally, this valve JT1 enables an increase in flexibility of the unit for the production of liquid, in particular for instances of reduced operation.

This passage of the flow 5-2 in the first exchanger E1 and the valve JT1 therefore each enable an increase in reliability and flexibility of the liquid production system.

It is also possible to send the two-phase fluid 6 or the mixture of fluids 6,7 directly to the heat exchanger E1 in order to be heated, without passing through the phase separator.

The invention firstly allows the design of the heat exchanger to be simplified by dispensing with a liquefaction passage dedicated to the production of liquid and secondly allows the specific energy of the cycle to be reduced to 5%, or even to 10%, depending on the arrangements.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Claims

1. A process for cooling a hydrogen feed gas utilizing a refrigeration cycle, wherein: a) a nitrogen cycle fluid is cooled to a temperature lower than −100° C.,b) at least one portion of the cooled cycle fluid is expanded in a turbine in order to cool the at least one portion of the cycle fluid, which produces a two-phase fluid at the outlet of the turbine,and eitherc) the two-phase fluid is separated in a phase separator, andd) at least one portion of the gas produced in the phase separator is sent to a first heat exchanger in order to exchange heat indirectly with the feed gas to be cooled, which produces a cooled feed gas and a heated cycle gas, which is compressed in a compressor and then cooled in a cycle according to step a), ande) at least one fraction of the liquid of the phase separator is vaporized in a second heat exchanger by way of indirect heat exchange with the cooled feed gas in order to further cool the feed gas or even to liquefy it,orf) the two-phase fluid is heated directly in a first exchanger by way of heat exchange with the feed gas to be cooled, which produces a cooled feed gas and a heated cycle gas, which is sent to a compressor as cycle fluid before being cooled according to step a).

2. The process as claimed in claim 1, wherein the ratio between the inlet pressure and the outlet pressure in bar absolute of the turbine is equal to or greater than 11.

3. The process as claimed in claim 1, wherein the two-phase fluid contains a proportion of liquid between 5 and 20 mol % of liquid.

4. The process as claimed in claim 3, wherein the amount of liquid produced in the turbine is adjusted by adjusting the outlet pressure of the cycle compressor and therefore the inlet pressure of the turbine, thus modifying the expansion rate of the turbine.

5. The process as claimed in claim 1, wherein one portion of the cooled fluid is expanded in the turbine and another portion of the cooled cycle fluid is cooled in the first exchanger until it is completely liquefied, forming a liquid, this liquid is then expanded in a valve, the expanded liquid and the two-phase fluid at the outlet of the turbine are either mixed in the phase separator or mixed before being sent directly to the first heat exchanger in order to be heated.

6. The process as claimed in claim 5, wherein the flow of the cooled portion in the first exchanger is variable in order to adjust the level of liquid in the phase separator.

7. The process as claimed in claim 1, wherein all of the liquid of the two-phase fluid sent to the phase separator or directly to the heat exchanger originates from the turbine.

8. The process as claimed in claim 1, wherein the cycle fluid contains at least 90 mol % of nitrogen.

9. The process as claimed in claim 1, wherein, during step a), the cycle fluid is cooled by an external source of cold in a third heat exchanger and is sent directly to the turbine without passing through the first heat exchanger.

10. The process as claimed in claim 1, wherein one portion of the liquid formed in the phase separator serves as a liquid product of the process.

11. The process as claimed in claim 1, wherein gas is added, from an external source, to the cycle downstream of the compressor, this gas having the same composition as the cycle fluid, in order to increase the inlet pressure of the compressor in order to increase the proportion of liquid produced by the turbine.

12. The process as claimed in claim 1, wherein the cycle fluid is cooled in step a) to a temperature greater than −192° C.

13. The process as claimed in claim 1, wherein the cooled feed gas is then liquefied.

14. The process as claimed in claim 13, wherein the cooled feed gas is then liquefied by a refrigeration cycle in which hydrogen or helium circulates.