Process and device for the cryogenic separation of a methane-rich stream

Abstract

In a process for the cryogenic separation of a methane-rich feed stream containing between 3 and 35% of oxygen and also nitrogen, the feed stream is cooled in order to produce a cooled stream, at least one portion of the cooled stream is sent to a distillation column, a bottom stream is withdrawn from the distillation column, the bottom stream being enriched in methane compared to the feed stream, a stream enriched in oxygen compared to the feed stream is withdrawn from the distillation column, and a nitrogen-rich stream is sent to the column.

Claims

1. A method for the cryogenic separation of a methane-rich feed stream containing oxygen and nitrogen, the method comprising the steps of: i) cooling the methane-rich feed stream to produce a cooled stream; ii) introducing at least a portion of the cooled stream to a distillation column under conditions effective for a distillation of the cooled stream, the distillation column having a distillation section; iii) withdrawing a bottom stream from the distillation column, the bottom stream being enriched with methane compared with the methane-rich feed stream; iv) withdrawing an oxygen-enriched stream from the distillation column, the oxygen-enriched stream having a higher percentage of oxygen as compared with the methane-rich feed stream; and v) introducing a nitrogen-rich gaseous stream coming from an external source to a lower part of the distillation column, such that the nitrogen-rich gaseous stream participates in the distillation of the cooled stream; and vi) wherein the nitrogen-rich gaseous stream introduced in step v) is in an amount effective to prevent a ternary mixture of methane, oxygen, and nitrogen of having a ternary concentration falling within a flammability zone for the ternary mixture while within the distillation column, wherein the methane-rich feed stream contains between 3% and 35% oxygen.

2. The method as claimed in claim 1, wherein the methane-rich feed stream contains between 65% and 97% methane.

3. The method as claimed in claim 1, wherein the methane-rich feed stream contains between 3% and 35% in total nitrogen and oxygen.

4. The method as claimed in claim 1, wherein the methane-rich feed stream contains between 3% and 35% nitrogen.

5. The method as claimed in claim 1, wherein the nitrogen-rich gaseous stream contains at least 90% nitrogen, or even at least 95% nitrogen.

6. The method as claimed in claim 1, wherein the nitrogen-rich gaseous stream is sent to a bottom of the distillation column, wherein the bottom of the distillation column is a portion of the distillation column that is located below the distillation section.

7. The method as claimed in claim 1, wherein the methane-rich feed stream is sent to a condenser/reboiler where the methane-rich feed stream is at least partially condensed while partially vaporizing a bottom liquid in order to form a vaporized gas and an at least partially liquefied feed stream, the at least partially liquefied feed stream is sent from the condenser/reboiler to the distillation column and the vaporized gas is mixed with the nitrogen-rich gaseous stream.

8. The method as claimed in claim 1, wherein a nitrogen-rich liquid stream is vaporized by exchange of heat with the methane-rich feed stream in order to produce the nitrogen-rich gaseous stream.

9. The method as claimed in claim 1, wherein the methane-rich feed stream contains between 3% and 10% oxygen.

10. A method for the cryogenic separation of a methane-rich feed stream containing oxygen and nitrogen, the method comprising the steps of: i) cooling the methane-rich feed stream to produce a cooled stream; ii) introducing at least a portion of the cooled stream to a distillation column under conditions effective for a distillation of the cooled stream to create a bottom stream and a top stream, wherein the bottom stream is enriched with methane compared with the methane-rich feed stream, wherein the top stream is enriched with oxygen as compared with the methane-rich feed stream; iii) withdrawing the bottom stream from the distillation column; iv) withdrawing the top stream from the distillation column; and v) introducing an effective amount of a nitrogen-rich gaseous stream to a lower part of the distillation column, wherein the nitrogen-rich gaseous stream comes from an external source, wherein the effective amount of the nitrogen-rich gaseous stream is based on an amount of nitrogen needed to prevent a ternary mixture of methane, oxygen, and nitrogen of having a ternary concentration falling within a flammability zone for the ternary mixture while within the distillation column, wherein the methane-rich feed stream contains between 3% and 35% oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

(2) FIG. 1 shows a ternary diagram.

(3) FIG. 2 shows diagram in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

(4) The invention will be described in more detail with reference to the figures, FIG. 2 of which shows a simplified diagram of the method according to the invention.

(5) A stream of feed gas 1, which may be biogas, comprises between 30% and 50% methane, with a CH.sub.4/CO.sub.2 ratio of between 1 and 2. It also contains air gases with a nitrogen/oxygen ratio greater than 3.7 and is saturated with water. The gas 1 is purified by drying and desulfurization and to eliminate the carbon dioxide that it contains by permeation and/or adsorption in a treatment unit 2, so that it substantially contains nothing more than methane, nitrogen and oxygen. A typical composition of the treated gas 4 could be 68% methane, 31% nitrogen and 3% oxygen. This feed gas 4 produced by the treatment unit 2 is cooled in a heat exchanger 3 of the blade or fin type at a pressure of between 6 and 15 bar. The gas 4 is sent to a condenser/reboiler 5 of a simple distillation column 6. The gas cools in the condenser/reboiler 5 and is at least partially condensed, while heating the bottom of the column 6. The fluid 7 produced by condensing the gas 4 is expanded in a valve 8 at a pressure between 1.1 and 5 bar absolute and then sent to the head of the column 6 as a liquid 9. The temperature of the liquid 9 must be greater than 90.7K in order avoid the risk of solidifying the methane.

(6) This liquid then separates in the column 6 in order to form a head gas 10 containing 84% nitrogen and 5% oxygen. This gas 10 heats in the exchanger 3 in order to form the residual gas 11. The bottom liquid 12 of the column 6 is withdrawn with a composition of less than 100 ppm of oxygen, traces of nitrogen and the rest being methane. The bottom liquid 12 is sent to the reboiler 5, where it partially vaporizes. The gas 15 formed is sent to the column bottom through the pipe 21. The remaining bottom liquid 13 vaporizes in the exchanger 3 in order to form a pure gaseous methane product 14.

(7) A liquid nitrogen storage 16 is connected to the exchanger 3 by a pipe 17 in order to vaporize the liquid nitrogen. The vaporized nitrogen 18 is sent through a pressure-reducing valve 19 and the pipe 20 to the bottom of the column 6, mixed with the vaporized methane 15 coming from the reboiler 5. The vaporized nitrogen contains at least 90% nitrogen, or even at least 95% nitrogen. Mixing the nitrogen with the vaporized methane better disperses the nitrogen in the column and avoids the formation of flammable pockets.

(8) In order to start up the column 6, the storage 16 contains liquid nitrogen for inerting the column.

(9) The nitrogen 20 may also come from an air-separation apparatus producing gaseous nitrogen or a gaseous nitrogen system. Otherwise liquid nitrogen from an air-separation apparatus may vaporize in the exchanger 3 in order to supply the gas 20.

(10) The feed gas may contain up to 10% oxygen or up to 5% oxygen.

(11) 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.

(12) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(13) 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.

(14) 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 a range is expressed, it is to be understood that another embodiment is from the one.

(15) 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.

(16) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

(17) 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.