METHOD FOR PURIFYING BIOGAS THROUGH MEMBRANES AT NEGATIVE TEMPERATURES

Abstract

The invention relates to a method for membrane permeation of a gas flow including methane and carbon dioxide, wherein said gas flow is cooled to a temperature of 0° C. to −60° C. before being fed into a membrane separation unit.

Claims

1. Process for purifying, by membrane permeation, a gas stream that comprises methane and carbon dioxide, said process comprising the following successive steps: compressing the gas stream to a pressure between 5 and 20 bar; a first step of cooling the compressed gas stream to a temperature between 0° C. and 15° C.; drying the cooled and compressed gas stream to obtain a water content≤0.1 ppm; cooling the dried, cooled, and compressed gas stream using a heat exchanger to a temperature between 0° C. and −60° C.; separating the cooled, dried, cooled, and compressed gas stream through first, second, and third membrane stages, each of said first, second and third membrane stages providing an associated CO2-depleted retentate and a CO2-enriched permeate, he first membrane stage receiving the cooled, dried, cooled, and compressed gas stream, the second membrane stage receiving the CO2-depleted retentate from the first membrane stage, the third membrane stage receiving the CO2-enriched permeate from the first membrane stage; recovering a methane-enriched gas stream by recovering the CO2-depleted retentate from the second membrane stage and reheating the recovered CO2-depleted retentate from the second membrane stage to a temperature between 0° C. and 20° C.; and after said step of separating, the CO2-enriched permeate from the second membrane stage and the CO2-depleted retentate from the third membrane stage are recovered and reheated in the heat exchanger to a temperature between 0° C. and 20° C. and then mixed with the gas stream to be purified before said step of compressing.

2. The process according to claim 1, wherein the gas stream is cooled to a temperature between −20° C. and −45° C. before being introduced into the membrane separation unit.

3. The process according to claim 1, wherein the separation step (e) involves first, second and third membrane stages that each provide a CO.sub.2-depleted retentate and a CO.sub.2-enriched permeate, with the first stage receiving the gas stream resulting from said step of cooling the dried, cooled, and compressed gas stream, the second stage receiving the retentate from the first stage and third stage receiving the permeate from the first stage.

4. The process according to claim 1, wherein the CO2-enriched permeate from the second membrane stage and the CO2-depleted retentate are reheated in the heat exchanger to different temperatures.

5. The process according to claim 1, wherein the CO2-enriched permeate from the third membrane stage is reheated to a temperature between 0° C. and 20° C. and then sent to a vent or a vent treatment system.

6. The process according to claim 1, wherein the CO2-enriched permeate from the third membrane stage is reheated and then sent to a liquefaction unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawing(s). It is to be noted, however, that the drawing(s) 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.

[0040] The FIGURE shows an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The invention will now be described in greater detail with the aid of the FIGURE which is a diagram of the plant according to invention.

[0042] The crude biogas 1, containing 43.6% CO.sub.2, 54.6% CH.sub.4, 0.8% N.sub.2 and 0.2% O.sub.2, saturated with water, at 5° C. and at a pressure of 0.1 barg, is mixed with the recycled stream 24, containing 66.6% CO.sub.2. The stream 2 is then sent to the compressor 3, where it is compressed to 9.6 barg, before being cooled to 5° C. After cooling, the water is removed in a separator, then the gas is reheated up to 15° C. The stream of gas 6 is then sent to the dryer 7. The stream 8 of dry gas, containing 51.2% CO.sub.2, then passes through the exchanger, in which it is cooled to −30° C. The stream of cooled gas enters into a first membrane state, where it is separated into two fractions. The retentate 12 is depleted in CO.sub.2 and contains no more than 30% CO.sub.2; it is sent to a second membrane stage. The permeate 16 is enriched in CO.sub.2 and contains 90% CO.sub.2; it is sent to a third membrane stage. The second membrane is stage in turn produces two fractions, the stream 14 depleted to 1.3% CO.sub.2, and the stream 15 enriched to 73% CO.sub.2. The third membrane stage also produces two fractions, the stream 18 depleted to 38% CO.sub.2, and the stream 19 enriched to 99.3% CO.sub.2. The CO.sub.2-rich stream 19 is reheated in the exchanger 9 from −30° C. to 25° C., and then sent to the vent. The stream 14, referred to as biomethane, contains 99.5% of the methane contained in the crude biogas 1, and is reheated to 13.4° C. and then sent to its final use (injection into the network, or fuel gas for vehicles). The streams 15 and 18 are heated to 13.4° C., mixed and sent to upstream of the compressor 3.

[0043] Compared to a similar process according to the prior art at ambient temperature, this process makes it possible to reduce the number of membranes and the specific electricity consumption, and if necessary the operating pressure. This is what the table below shows:

TABLE-US-00001 Specific electricity Operating pressure Number of consumption (bar) membranes (kWh/Nm3) Conventional 12 18 0.24 process at ambient T Cold membranes 10 7 0.207

[0044] Depending on the desired applications, the stream of biomethane and/or the vent stream may be produced at a temperature below ambient temperature, in order to be sent to liquefaction units, thus reducing the electricity consumption of the latter.

[0045] 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.

[0046] The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0047] “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.

[0048] “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.

[0049] 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.

[0050] 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.

[0051] 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.