PROCESS TO SEPARATE NITROGEN FROM METHANE BY PERMEATION AND CRYOGENIC DISTILLATION

Abstract

A process for the separation of nitrogen from a feed stream containing at least methane and nitrogen, with a methane content between 4 and 12% mol. consists of at least the following steps: separation of the feed stream by means of a rubbery-type membrane to produce a permeate enriched in methane at a pressure greater than 2 bara and a non-permeate which is a nitrogen-enriched residue gas at a pressure greater than 2 bara and processing of the high-pressure residue gas in a cryogenic separation unit to produce a methane rich liquid and a nitrogen-enriched gas wherein the pressure of the membrane permeate is controlled as a function of the nitrogen concentration in the nitrogen-enriched gas.

Claims

1. A process for the separation of nitrogen from a feed stream comprising methane and nitrogen, with a methane content between 4 and 12% mol, the process comprising the steps of: a) separating the feed stream by means of a rubbery-type membrane to produce a permeate enriched in methane at a pressure greater than 2 bara and a non-permeate that is a nitrogen-enriched residue gas at a pressure greater than 2 bara; b) processing of the high-pressure residue gas in a cryogenic separation unit to produce a methane rich liquid and a nitrogen-enriched gas; and c) measuring the nitrogen concentration of the nitrogen-enriched gas, wherein the pressure of the membrane permeate is controlled as a function of the nitrogen concentration in the nitrogen-enriched gas.

2. The process according to claim 1, wherein the pressure of the membrane permeate is increased if the nitrogen concentration in the nitrogen-enriched gas increases.

3. The process according to claim 1, wherein the pressure of the membrane permeate is decreased if the nitrogen concentration in the nitrogen-enriched gas decreases.

4. The process according to claim 1, wherein the pressure of the membrane permeate is varied using a valve on a conduit into which the membrane permeate flows.

5. The process according to claim 1, wherein the feed stream is cooled upstream of the membrane by indirect heat exchange with at least one of the permeate and the non-permeate.

6. The process according to claim 5, wherein the feed stream and the products of the separation are the only fluids indirectly exchanging heat in the heat exchanger.

7. The process according to claim 5, wherein the feed stream is cooled in a first heat exchanger by indirect heat exchange with at least one of the permeate and the non-permeate and the non-permeate is cooled in a second heat exchanger distinct from the first heat exchanger by indirect heat exchange with at least one fluid produced in the cryogenic separation unit.

8. The process according to claim 1, wherein the cryogenic separation unit comprises at least one distillation column.

9. The process according to claim 7, wherein the cryogenic separation unit comprises at least first and second distillation columns,

10. The process according to claim 7, wherein the cryogenic separation unit comprises at most three distillation columns.

11. The process according to claim 1, wherein the feed stream is cooled upstream of the rubbery type membrane by indirect heat exchange with at least one of the permeate and the non-permeate.

12. The process according to claim 1, further comprising the steps of: i) introducing a stream derived from the high pressure residue gas into the first distillation column to produce a nitrogen-rich gas stream and an impure methane liquid stream containing at least 5% mol. nitrogen; ii) at least partially condensing at least part of the nitrogen-rich gas stream in a heat exchanger to produce a nitrogen rich stream; iii) sending at least part of the liquid nitrogen rich stream to the second distillation column to be separated; iv) separating the at least part of the liquid nitrogen rich stream to form a methane enriched liquid; v) sending at least part of the methane enriched liquid to the third distillation column at a first height; vi) condensing a top gas of the second distillation column In a bottom reboiler of the third distillation column; vii) sending a liquid nitrogen rich stream from the top of the second distillation column into the third distillation column at a height above the first height; viii) recovering a methane rich bottom liquid from third distillation column and then pumping the methane rich bottom liquid to a medium pressure of at least 3 bara; and ix) combining at least part of the permeate from step a), at least part of the impure methane liquid stream and at least part of the methane rich bottom liquid from step b) viii to form a methane product.

13. The process according to claim 9, wherein the first distillation column operates at a first pressure and the second distillation column operates at a second pressure, the second pressure greater than the first pressure by at least 5 bars.

14. The process according to claim 1, wherein no external source of refrigeration is used in the process.

15. The process according to claim 1, wherein the only compressor or compressors pressurizing a gas is or are methane product compressors or feed gas compressors.

16. The process according to claim 1, wherein the feed stream contains between 2% mol. and 10% mol. nitrogen and wherein the high-pressure residue gas contains at least 1.1 times more nitrogen than the feed stream and at most 50% mol. nitrogen.

17. The process according to claim 16, wherein the feed stream contains between 10% mol. and 40% mol. nitrogen and wherein the high-pressure residue gas contains at least 1.1 times more nitrogen than the feed stream and at most 80% mol. nitrogen.

18. The process according to claim 1, wherein the membrane operates at a temperature less than 10° C.

19. The process according to claim 1, wherein the feed stream sent to the membrane is cooled to the temperature less than 10° C. by heat exchange with at least one of the permeate and non-permeate of the membrane.

20. The process according to claim 1, wherein the methane content of the permeate is less than the methane content of the methane rich product of the cryogenic columns.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The invention will now be described in greater detail, referring to the FIGURE.

[0073] FIG. 1 represents a process according to the an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0074] All described and/or depicted features on their own or in any desired combination form the subject matter of the invention, irrespective of the way in which they are combined in the claims or the way in which said claims refer back to one another

[0075] The process including a permeation step in block M and a further cryogenic distillation step involving three distillation columns K01, K02, K03, columns K02 and K03 being thermally linked to form a double column, with the top gas of column K02 being condensed in a bottom condenser of column K03 and thereby evaporating the bottom liquid of column K03.

[0076] A gaseous stream 1 containing at least nitrogen and methane preferably contains less than 13% mol, nitrogen, possibly less than 10% mol nitrogen and even less than 7% mol nitrogen. The stream may contain at least one component from the following list: water, carbon dioxide, benzene, ethane, propane, butane, pentane, heptane, toluene, carbon monoxide.

[0077] Gaseous stream 1 preferably contains at least 80% mol methane, still more preferably at least 90% mol methane.

[0078] The feed stream 1 may contain between 2% mol and 10% mol nitrogen.

[0079] Alternatively the feed stream 1 may contain between 10% mol and 40% mol nitrogen.

[0080] The gaseous stream 1 is cooled in a heat exchanger E05 and separated in a rubbery type membrane M to form a permeate 4 enriched in methane at at least 3 bars and a non-permeate enriched in nitrogen. The permeate 4 is warmed in heat exchanger E05.

[0081] Preferably the non-permeate 3 is at a pressure of at least 3 bara and contains at least 1.1 times more nitrogen than the feed stream 1. The non-permeate 3 may contain at most 80% mol nitrogen or at most 50% mol nitrogen.

[0082] The membrane operates at less than 10° C.

[0083] The non-permeate enriched in nitrogen is also warmed in heat exchanger E05 and then optionally purified in purification unit P to remove any components which would freeze at temperatures encountered during the distillation process, such as water, carbon dioxide to form gaseous stream 7. Stream 7 is cooled in heat exchanger E01 and divided in two, one part 11 being cooled in heat exchanger E01 and the rest being used to control the temperature of stream 9 downstream of E01. Stream 9 is sent to reboiler E04 in which bottom liquid 13, 15 from column K01 is vaporised to form a stream sent to the bottom of first distillation column K01, as reboil.

[0084] Stream 9 is condensed in heat exchanger E04 and sent to an intermediate level of column K01 as feed.

[0085] Stream 11 is the top feed for first distillation column K01.

[0086] The top stream 19 from column K01, enriched in nitrogen, is cooled in heat exchanger E01 (not at the warm end, the drawing is schematic) and is sent as gaseous feed to the bottom of column K02. Heat exchanger E01 is typically composed of several heat exchangers.

[0087] The bottom stream 13 from first distillation column K01 is enriched in methane and is divided in three. One part 15 is previously described, part 17 is warmed in exchanger E04 and the rest is pumped in pump P02, vaporised in exchanger E01 and forms stream 6.

[0088] The operating pressure of column K01 is at least 5 bars greater than that of column K02.

[0089] The bottom liquid of column K02 is cooled in subcooler E03, expanded and sent as feed to column K03. Top gas from column K02 is sent to the condenser E02 at the bottom of column K03 and part of the condensed liquid formed is sent as stream 23 to the top of the column K03.

[0090] Bottom liquid 25 is removed from column K03, pressurized in pump P01 and then warmed in subcooler E03. It Is then mixed with liquid 17 and the mixture is vaporized in exchanger E01 to form gaseous stream 5. Stream 5 is expanded in a valve, is mixed with the permeate 4 following valve expansion of permeate 4 and the two phase mixture is separated in phase separator S1. The gas from phase separator S1 is compressed in compressor C1, cooled in cooler R1, separated again in phase separator S2, compressed in compressor C2 and cooled in cooler R1 and then mixed with stream 6 to form treated methane rich gas 3.

[0091] In an alternative example, the phase separators S1 and/or S2 are not present and the mixture formed by mixing stream 4 and stream 5 remains entirely gaseous.

[0092] Top gas 2 from column K03 is a nitrogen-enriched gas which is warmed in subcooler E03 and then in exchanger E01.

[0093] The purity of the stream 2 is measured downstream of heat exchanger E01 using an analyzer AIC.

[0094] A pressure indicator and controller PIC measures the pressure of the permeate 4 and controls a valve to increase or reduce the flowrate of stream 4. The pressure indicator and controller is itself controlled by the analyzer AIC. The higher the pressure of the permeate 4, the lower the flowrate of the permeate 4 and the lower the nitrogen content in the permeate 4. If the permeate flowrate 4 decreases, the non-permeate flowrate 3 necessarily increases and thus the flowrate 7 sent to the cryogenic unit K01, K02, K03 increases, at the same time as the nitrogen concentration in the non-permeate 7 feeding the cryogenic unit decreases.

[0095] This should generally have the effect of reducing the overall methane recovery for the entire process (permeation and cryogenic separation) by increasing the content of methane in the nitrogen 2 rejected from the cryogenic separation. The pressure level of the permeate 4 is optimized, for example manually, to maximize that methane recovery which can be monitored with an analyzer AIC on the waste stream 2.

[0096] Alternatively an automatic control/cascade can be installed with the controller AIC on stream in cascade of the PIC on stream 4.

TABLE-US-00001 Stream 2 4 5 6 7 1 Rejected 3 Membrane MP gas from HP gas from Membrane Feed gas Nitrogen Treated gas permeate cryo cryo residue Flow rate NCMH 28000 900 27000 20000 2640 4300 7880 Pressure bara 70 1 57 12 12 57 68 Temperature ° C. 50 30 40 32 30 30 33 Composition Nitrogen mol % 5.7 99.3 2.4 2.7 2.9 1.2 13.4 Methane mol % 84.2 0.683 87.132 83.921 96.827 96.149 84.999 Ethane mol % 8.9 0.000 9.283 11.926 0.242 2.513 1.452 Propane mol % 1.0 0.000 1.037 1.367 0.007 0.133 0.075 i-Butane mol % 0.02 0.000 0.022 0.029 0.000 0.002 0.001 n-Butane mol % 0.042 0.000 0.043 0.057 0.000 0.004 0.002

[0097] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0098] The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0099] “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

[0100] “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0101] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0102] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0103] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.