CN1713949A - Membrane separation method - Google Patents

Abstract

A high purity stream of methane can be obtained from crude natural gas, especially exhaust gas from waste landfills, by a process that includes first removing moisture (4), then feeding the dried crude gas mixture to a gas-liquid contact absorber (10) to strip heavy hydrocarbon compounds in a primarily carbon dioxide by product stream. Methane enriched gas from the absorber (10) is separated in a membrane separation unit (20) which provides permeates enriched in carbon dioxide that is recycled to the absorber and a purified product stream of methane.

Description

Membrane separation method

technical field

The invention relates to a membrane separation method for refining natural gas. More particularly, the present invention relates to a process comprising the removal of heavy hydrocarbon contaminants by absorption treatment of a crude gas feed prior to operation with a membrane separation unit to separate carbon dioxide from methane.

Background technique

Refined natural gas, typically about 97 mole percent methane, about 3 mole percent carbon dioxide and trace amounts of water vapor, is an important commercial product for applications such as high calorific value fuels and feedstocks for chemical production processes. Crude natural gas, that is, methane mixed with pollutants, is available from various sources such as underground wells. Exhaust gases from solid waste landfills are also an increasingly valuable source of crude methane. Such crude gases typically contain 10-50 mole percent carbon dioxide, 50-80 mole percent methane, and small amounts of pollutants including heavy hydrocarbons. Carbon dioxide can be used in food processing and other purposes. The crude natural gas mixture can thus provide two valuable industrial materials, methane and carbon dioxide.

Membrane separation is a very efficient method of separating carbon dioxide from methane. However, the separation performance of selective gas permeable membranes is often adversely affected by the presence of contaminants, especially heavy hydrocarbons, in the crude gas mixture. Removal of heavy hydrocarbons is therefore required for viable methane membrane separations. Furthermore, natural gas containing heavy hydrocarbon contaminants is not commercially viable for transport from source to consumer. Therefore, the so-called "pipeline specifications" for refining natural gas have low concentration limits for heavy hydrocarbons. For this reason, removal of heavy hydrocarbons from carbon dioxide and methane is also desirable.

Certain processes for stripping hydrocarbons from crude natural gas to enable subsequent membrane separation of methane and carbon dioxide have employed unit operations such as dew point control (“DPC”), temperature swing adsorption (“TSA”), and pressure swing adsorption ("PSA") as an integral part of the methane concentration process. By broad definition, DPCs, TSAs, and PSAs each require substantial amounts of cooling, steam, and clean gas to operate effectively. These auxiliary facilities are expensive and thus significantly increase the product cost.

Membrane separations typically operate at maximum efficiency when the feedstock is compressed. Compressed costs can reduce the economic justification of such approaches. In addition, in order to obtain the ideal pure methane product concentration, membrane separation usually includes multi-stage membrane separation units connected in series, that is, more than one stage. Multiple stages can create a potentially wasteful by-product stream, which further reduces the attractiveness of membrane separation for refining methane. Mainly for the above reasons, membrane separation methods have so far not been found to be of great benefit for the commercial production of methane from landfill off-gas.

An attractive method for concentrating and recovering methane and carbon dioxide from landfill gas is disclosed in US 5,681,360 assigned to Acrion Technologies. The "Acrion" method involves, in one or two vessels, absorbing pollutants normally present in waste gas with a relatively small amount of carbon dioxide absorbent present in the gas. This process produces a methane-enriched stream and a carbon dioxide-enriched stream. This methane-rich stream contains a small but not negligible portion of carbon dioxide which is left to be separated to provide a refined methane product. This carbon dioxide rich stream contains wasted methane and additional methane may be required to facilitate processing by combustion.

It would be desirable to have an integrated and cost and energy efficient process that produces a highly concentrated methane component from crude natural gas with reduced methane losses in waste.

Contents of the invention

A very efficient method and system for refining methane from crude natural gas has been discovered. The new method and system feature preabsorption of heavy hydrocarbon compounds with a carbon dioxide absorbent followed by membrane separation of the methane-rich absorbed product. Notably, the permeate gas from the downstream main membrane separation unit operation is returned to provide absorbent for the upstream absorption operation. In a preferred multi-stage membrane separation embodiment, the permeate gas from the second and optionally higher membrane stages is recycled back to the feedstock to the absorption unit, thereby providing efficient feedstock recovery.

Accordingly, the present invention provides a method for separating methane from a crude gas mixture containing methane, carbon dioxide and heavy hydrocarbon compounds, the method comprising: absorbing heavy hydrocarbons from the crude gas mixture with a carbon dioxide-enriched composition; heavy hydrocarbon compounds, thereby providing an intermediate gas mixture substantially free of heavy hydrocarbon compounds; separating the intermediate gas mixture with a selective gas permeable membrane to form (a) a methane-rich product mixture, and (b) a carbon dioxide-enriched composition; and, using the obtained carbon dioxide-enriched composition for absorbing heavy hydrocarbon compounds from a crude gas mixture.

The present invention also provides a method for separating methane from a crude mixture containing methane, carbon dioxide and hydrocarbon compounds, the method comprising the steps of:

(A) compressing the crude gas mixture and removing water therefrom to produce a dehydrated feed gas comprising methane, carbon dioxide and heavier hydrocarbon compounds,

(B) contacting the feed gas in an absorber unit with a liquid absorbent condensed from a first-stage permeate gas mixture containing a majority of carbon dioxide, said heavy hydrocarbon compound being substantially completely absorbed into the absorbent, to form liquid by-products containing carbon dioxide and heavy hydrocarbon compounds,

(C) separately removing the liquid by-product and an intermediate gas mixture comprising methane and carbon dioxide substantially free of heavy hydrocarbon compounds from the absorber unit,

(D) Make the intermediate gas mixture contact the raw material side of the first membrane that preferentially permeates carbon dioxide compared to methane in the first-stage membrane separation unit, so that the intermediate gas mixture selectively permeates through the membrane, so as to pass through the membrane in the membrane separation unit. side forming the first stage permeate gas mixture, and

(E) A first stage retentate gas mixture richer in methane than the intermediate gas mixture is removed from the feed side of the first stage membrane separation unit.

The present invention further provides a system for producing refined methane from a crude mixture comprising methane, carbon dioxide and volatile organic compounds, the system comprising:

(a) a dryer operated to remove water from the crude mixture; and a compressor operated to increase the pressure of the crude mixture to a pressure suitable for absorbing heavy hydrocarbons,

(b) A counter-current gas-liquid direct contact absorber, located downstream of the dryer and compressor, suitable for substantially complete absorption of heavy hydrocarbon compounds from the crude mixture into a liquid carbon dioxide absorbent, and suitable for producing substantially intermediate gas mixtures free of heavy hydrocarbon compounds,

(c) a first-stage membrane separation unit having a first membrane that preferentially permeates carbon dioxide over methane, a feed chamber on the side of the membrane in fluid communication with the intermediate gas mixture, and a feed chamber on the side of the first membrane opposite the feed chamber a permeation chamber adapted to receive the first stage permeate gas of the intermediate gas mixture selectively permeable through the first membrane,

(d) a condenser operative to liquefy the first stage permeate gas, and

(e) a recycle transfer line in fluid communication between the absorber and the permeate chamber of the first membrane separation unit, operative to transfer the first stage permeate gas to the absorber.

Description of drawings

Figure 1 is a schematic flow diagram of one embodiment of the present invention.

Detailed ways

Referring to FIG. 1 , it can be seen that in one embodiment of the invention, a crude natural gas stream 1 is processed to produce a refined methane stream 32 . The crude natural gas includes large amounts of methane and carbon dioxide, and includes small amounts of various pollutants such as oxygen, nitrogen, hydrogen sulfide, water, and hydrocarbons other than methane. The crude gas is pretreated to remove water. This is done by compressing the gas in compressor 2 and drying it in dryer 4 . The dryer is any type of dehumidifier known in the art, such as a cooling coil coalescing filter. Typically, water is removed in condensed liquid stream 3.

The dehydrated crude gas stream 5 is then conditioned for absorption removal of heavy hydrocarbon compounds. The conditioning takes place in the compressor 6 and heat exchanger 8, which respectively increase the pressure and temperature of the absorber feed gas 9 to values suitable for hydrocarbon removal.

The conditioned absorber feed gas 9 is fed into an absorber 10 . Again, any conventional apparatus suitable for performing gas-liquid contact absorption can be used. Preferably, the absorption unit is a vertically placed tower. Such columns are typically packed with packing particles, or fitted with sieve trays or bubble-cap trays, as is used in the industry for the fractionation of fluid mixtures. The feed gas is usually introduced between the top and the bottom, preferably between near the bottom and half the height of the absorber, while a gas stream 12 already depleted in heavy hydrocarbons and containing a considerable amount of methane is withdrawn from the top. Absorbent stream 26 is passed into the column between the top and bottom, above the point of introduction of the feed gas. Preferably, the absorbent stream is introduced near the top of the absorber, as shown in FIG. 1 . Absorbent stream 26 is a carbon dioxide enriched composition. This stream can be condensed by, for example, an in-line condensing unit, an external reflux condenser of the column or an internal condensing heat exchanger in the top of the column. Carbon dioxide flows down through absorber 10, absorbs heavy hydrocarbons from the feedstock, and exits the bottom of the tower as by-product stream 14.

The overhead product 12 that has been depleted of heavy hydrocarbons enters the first-stage membrane separation unit 20 . An optional compressor (not shown) can be used to convey the stream to the separation unit 20 . This intermediate gas mixture is substantially free of heavy hydrocarbon compounds which would otherwise be detrimental to the membrane or adversely affect membrane separation performance. The terms "substantially" and "substantially completely" are used in this context and elsewhere to mean that the properties associated therewith are present, though not completely or totally, but in substantial amount. For example, "substantially free of heavy hydrocarbon compounds" means that the gas mixture has been substantially devoid of these hydrocarbons, but not necessarily completely free of such hydrocarbons.

Membrane separators known in the art can be used. The separation unit used in this invention is characterized by having a selective gas permeable membrane 21 which preferentially permeates carbon dioxide over methane. That is, carbon dioxide permeates through the membrane faster than methane. The membrane 21 has two sides and divides the separation unit into a feed compartment 25 and a permeate compartment 23 . The intermediate gas mixture 12 entering and in contact with the membrane 21 permeates into the permeation chamber. In the permeate chamber it is withdrawn and returned to the absorber as first stage permeate gas mixture 26 . This first stage permeate gas mixture is rich in carbon dioxide and thus ideally serves as the absorbent fluid in the absorber column.

The retentate gas mixture on the feed chamber side of the membrane 21 is depleted of carbon dioxide by the action of the membrane separation process and thus enriched in methane. For some product applications, the methane concentration in the first stage retentate gas mixture may be satisfactory. In this case, the first-stage retentate gas mixture can be stored or used directly in subsequent process unit operations. In general, refined methane for high calorific value fuel applications should have higher methane concentrations and fewer pollutants than can be achieved with single-stage membrane separation. For this purpose, a second-stage membrane separation is performed.

The first stage retentate gas mixture 22 can be fed into the feed chamber 35 of a second stage membrane separation unit 30 . The second stage permeation chamber 33 is located on the opposite side of the second membrane 31 which is also permeable preferentially to carbon dioxide over methane. Gas selectively permeates as a result of contact of the first stage retentate gas mixture with the second membrane to form a carbon dioxide enriched second stage permeate gas mixture 36 and provides a highly methane enriched second stage retentate gas mixture 32. This highly methane-rich gas mixture typically has a sufficiently high methane concentration to be used as a high calorific value fuel, and thus can be discharged from the second-stage membrane separation unit to storage or directly to a combustion process for conversion into heat energy.

The second stage permeate gas mixture 36 is substantially concentrated in carbon dioxide and contains a small amount of methane which permeates through the second membrane. The second stage permeate gas 36 is recycled through the membrane separation unit for methane recovery. This second stage permeate gas is usually at a pressure so low that it cannot be fed directly to the absorber along with the first stage permeate gas 26 . By the time the second stage permeate can be recycled back to crude feed gas 1, the permeate has been dried. Thus, the second stage permeate is preferably returned to the dry crude gas mixture 5 upstream of the compressor 6, as shown in FIG. 1 .

The composition of the crude gas feedstock to the refining process is variable and depends on the source of the crude gas. For example, the raw gas mixture typically contains about 30 vol% carbon dioxide, 60 vol% methane, and about 10 vol% other contaminants including hydrogen sulfide, water, oxygen, nitrogen, and hydrocarbons other than methane. The other hydrocarbons may be classified as "light hydrocarbon compounds" or "heavy hydrocarbon compounds". As used herein, the term "heavy hydrocarbon compound" refers to a compound formed only of hydrogen and carbon and containing more than 6 carbon atoms. Heavy hydrocarbons typically enter and block the pores of the selective gas permeation membrane, a phenomenon often referred to as "plasticization". Plasticization has an adverse effect on membrane separation performance, often to the point where the membrane separation performance of the module becomes practically impractical.

In an exemplary embodiment of this invention, the crude gas mixture is compressed to about 2.1 MPa (300 psi) and dried in a coalescing water filter to remove substantially all of the water. The dried crude gas mixture is compressed to about 6.0 MPa (870 psi) and heated to about 35°C in a finned tube heat exchanger before being introduced halfway up the packed absorber column. The absorber typically operates at about 5.5-7.6 MPa (800-1100 psi). This pressure range makes the novel process ideal for refining methane from raw gas from a raw natural gas source, ie, a natural subterranean geologically formed well. These sources typically provide the crude gas at elevated pressures not far below the absorber operating pressure. Thus, the efficiency of the present invention is increased by the fact that only a small energy input is required to compress the crude gas to the operating pressure. The present new absorption process can refine crude gas from abandoned landfills, however, these sources produce very low pressure crude gas. Raising landfill off-gas to absorber operating pressure typically requires significant energy input. This makes the new method less preferred for treating landfill off-gas.

The crude gas mixture is countercurrently contacted in the absorber with an absorbent rich in carbon dioxide to provide an overhead stream containing about 45% by volume methane, 50% by volume carbon dioxide and about 5% by volume including hydrogen sulfide, oxygen, Pollutants including nitrogen and light hydrocarbon compounds. The absorbent is condensed by cooling to -5°C at the top of the column where the absorbent begins to descend as a liquid through the column. Absorption of heavy hydrocarbons into the absorbent is much a one-way operation compared to other countercurrent fractionation processes. That is, the crude gas flows upward from the point of entry into the absorber, and the absorbent flows downward from the point of entry. When the two streams contact each other, the heavy hydrocarbons are stripped from the crude gas and leave the bottom with the absorbent. The bottoms product is a liquid stream containing about 97 vol% carbon dioxide and about 3 vol% heavy hydrocarbon compounds. Substantially all of the heavy hydrocarbon compounds are discharged to the absorber bottoms.

As the crude gas passes upward through the absorber, it contains less and less heavy hydrocarbons. At the top, the gas is substantially free of heavy hydrocarbon compounds and exits the absorber as overhead gas. The overhead gas from the absorption tower enters the feed end of the first hollow fiber membrane module. The permeate gas mixture has a composition of about 90% by volume carbon dioxide, about 10% by volume methane, and pollutants including light hydrocarbon compounds. This gas mixture is compressed, cooled and returned from the first membrane module to the top of the absorber where it contacts the upwardly flowing gas.

An advantageous feature of the new process derives from the high pressure, ie typically above 5.5 MPa (800 psi), at which the absorption of heavy hydrocarbon compounds in the absorber takes place. After the first stage permeate gas has been compressed to a suitable high pressure to allow it to return to the absorber, the first stage permeate gas can be condensed to a liquid state using a medium of only mild cooling temperature. For example, brine or water at a temperature of about -5°C to about 20°C can be used to liquefy carbon dioxide at high pressure. In contrast, hydrocarbon-carbon dioxide fractionation at low pressure typically requires reflux condensation at very low temperatures requiring more expensive and difficult to handle cryogenic cooling media with coolant temperatures below about -50°C.

After contacting the feed side of the first membrane, the gas is removed from the first stage membrane separation unit. This first stage retentate gas mixture has a composition of about 60% by volume methane, about 30% by volume carbon dioxide and the remainder including light hydrocarbons other than methane, water, oxygen and nitrogen.

The first stage retentate gas mixture is fed into the second stage gas separation membrane unit so that it contacts one side of the second permselective membrane. The composition of the second stage permeate gas mixture is about 62% by volume carbon dioxide, and about 35% by volume methane. Although the amount of methane in the permeate is small, it is worth intercepting. Thus, the second stage permeate gas mixture is recycled back to the dry crude gas. The retentate gas mixture from the second stage separation unit has a composition of about 98 volume percent methane, light hydrocarbon compounds, and about 2 volume percent carbon dioxide. This mixture is suitable for industrial applications, mainly for generating calorific value by combustion as fuel.

Membrane separation units that can be used in this invention are known in the art. The main component of this type of membrane separation unit is a selective gas permeable membrane. Typically, the film has a polymeric composition.

A wide variety of polymeric materials have desirable gas permeation properties and can be used as membranes in the present invention. Representative materials include polyamides, polyimides, polyesters, polycarbonates, esters of copolycarbonates, polyethers, polyetherketones, polyetherimides, polyethersulfones, polysulfones, fluoroethylene polymers Compounds and copolymers such as polyvinylidene fluoride, tetrafluoroethylene, copolymers of tetrafluoroethylene and perfluorovinyl ether or perfluorodioxole, polybenzimidazole, polybenzoxazole, Polyacrylonitrile, cellulose derivatives, polyazoaromatics, poly(2,6-dimethylphenylene oxide), polyphenylene ether, polyurea, polyurethane, polyhydrazide, polymethylimide, polycondensation Aldehyde, cellulose acetate, nitrocellulose, ethyl cellulose, styrene-acrylonitrile copolymer, brominated poly(xylene oxide), sulfonated poly(xylene oxide), tetrahalogen substituted polycarbonate, tetrahalogen Substituted polyesters, esters of tetrahalogen substituted polycarbonates, polyquinoxalines, polyamide-imides, polyesteramides, mixtures, copolymers, substituted materials and the like thereof. Other similar suitable gas separation layer membrane materials include polysiloxanes, polyacetylenes, phosphorus nitrogen chain polymers, polyethylene, poly(4-methylpentene), poly(trimethylsilylpropyne), Poly(trialkylsilylacetylene), polyurea, polyurethane, mixtures, copolymers, substituted materials and the like thereof. It is further contemplated that polymerizable materials, ie, materials that cure to form polymers such as sulfurized silicones and the like, may be suitable gas separation layers for the multicomponent gas separation membranes of the present invention. Preferred materials for the dense gas separation layer include combinations of aromatic polyamides and aromatic polyimides.

Membranes can be in many shapes such as flat sheets, pleated sheets, rolls, tubes, ribbons, and hollow fibers, just to name a few. The membranes may be housed in any conventional type of housing or vessel suitable for providing feed gas supply, permeate and retentate gas removal. The vessel also provides a high pressure side (for feed and retentate gas) and a low pressure side (for permeate gas) of the membrane. For example, flat sheet membranes can be stacked in a plate and frame assembly, or wound in a spiral assembly. Large numbers of hollow fiber membranes can be assembled into a bundle of membrane modules, typically encapsulated with a thermosetting resin within a cylindrical housing, in a co-current configuration through the bundle. Hollow fiber modules are generally preferred because they provide a large membrane surface in a small volume. The final membrane separation unit consists of one or more membrane modules housed individually in a pressure vessel, or multiple elements together in a sealed housing of appropriate diameter and length.

For improved performance, hollow fiber membranes generally include a very thin selective layer forming part of a thicker structure. This can be, for example, a monolithic asymmetric membrane comprising a dense skin region and a microporous support region forming the selective layer. Such membranes are described, for example, in US 5,015,270 to Ekiner. As a further and more preferred example, the hollow fiber membranes may be of the so-called "composite membrane" type, ie membranes with multiple layers. Composite membranes typically consist of a porous but non-selective support membrane providing mechanical strength coated with a thin and selective layer of another material responsible for the separation performance. A wide variety of polymers can be used as the matrix. Typical support membrane materials include polysulfone, polyethersulfone, polyetherimide, combinations of polyimides and polyamides, blends, copolymers, substituted materials, and the like. Typically, such composite membranes are made by solution casting (or, in the case of hollow fibers, spinning) a supported membrane, followed by solution coating of the selective layer in the separation step. Hollow fiber composite membranes can also be made by simultaneous coextrusion spinning of both the matrix material and the separation layer, as described in US 5,085,676 to Ekiner. The entire disclosures of the aforementioned patents are hereby incorporated herein. Membrane separation units useful in the present invention are available from Air Liquide, S.A., MEDAL unit of Houston, Texas.

Although specific forms of the invention have been selected in the foregoing in order to fully and adequately describe the invention to those skilled in the art, it is understood that substantially equivalent or superior results will be obtained. Various alternatives and modifications of properties and/or properties are considered to fall within the scope and spirit defined by the appended claims.

Claims (12)

1, a kind of being used for from the method for the thick admixture of gas separation of methane that contains methane, carbon dioxide and heavy hydrocarbon compounds, this method comprises: absorb heavy hydrocarbon compounds with a kind of composition that is rich in carbon dioxide from this thick admixture of gas, thereby a kind of medium-gas mixture that is substantially free of heavy hydrocarbon compounds is provided; Separate this medium-gas mixture with a selective gas permeable membrane, to form the composition that (a) is rich in the product mixture of methane and (b) is rich in carbon dioxide; With, the composition that is rich in carbon dioxide that is obtained is used for absorbing heavy hydrocarbon compounds from thick admixture of gas.

2, a kind of being used for from the method for the crude mixture separation of methane that contains methane, carbon dioxide and hydrocarbon compound, this method comprises the steps:

(A) compression this thick admixture of gas, and therefrom except that anhydrating contains the dehydration unstrpped gas of methane, carbon dioxide and heavy hydrocarbon compounds with generation,

(B) make this unstrpped gas in an absorber unit, contact the liquid-absorbant of the first order permeate gas mixture of the self-contained most of carbon dioxide of a kind of condensation, described heavy hydrocarbon compounds is absorbed fully basically and enters in the absorbent, to form the liquid by-product of carbonated and heavy hydrocarbon compounds

(C) shift out this liquid by-product and medium-gas mixture respectively from this absorber unit, this medium-gas mixture contains methane and carbon dioxide, is substantially free of heavy hydrocarbon compounds,

(D) this medium-gas mixture is contacted than the methane feed side of first film of permeable for carbon dioxide more preferably in first order film separation unit, make this medium-gas mixture selectively penetrating cross this film, with film see through side form described first order permeate gas mixture and

(E) shift out the first order that more is rich in methane than medium-gas mixture from the feed side of first order film separation unit and ooze residual air body mixture.

3, the described method of claim 2, wherein heavy hydrocarbon compounds is absorbed in the process of one way by this absorptive unit and enters absorbent.

4, the described method of claim 2, wherein the heavy hydrocarbon compounds absorption that enters absorbent occurs in greater than under about 5.5MPa (800psi).

5, the described method of claim 2 further comprises:

(F) make the first order ooze residual air body mixture and in the film separation unit of the second level, contact, make first to ooze residual air body mixture selective permeation second film than the methane feed side of second film of permeable for carbon dioxide more preferably, with form second level permeate gas mixture and

(G) from the film separation unit of the second level, shift out and ooze the second level that residual air body mixture more is rich in methane than the first order and ooze residual air body mixture.

6, the described method of claim 5 further comprises this second level permeate gas mixture is added in the dehydration unstrpped gas.

7, the described method of claim 2, wherein contact and absorption step comprise:

(B-1) unstrpped gas of will dewatering is introduced this absorption tower from the feed points of a vertical counter current contacting Gas-Liquid Absorption tower of placing,

(B-2) the most of carbon dioxide in the permeate gas mixture of the condensation first order at least, with formation liquid CO 2 absorbent,

(B-3) the liquid CO 2 absorbent is added from feed points top this absorption tower and

(B-4) discharge byproduct from this absorption tower.

8, the described method of claim 7, wherein being condensate in this absorption tower of carbon dioxide carried out.

9, the described method of claim 7, wherein feed points is positioned at this top, bottom, absorption tower and is lower than half position highly.

10, the described method of claim 2 further comprises with temperature being higher than-5 ℃ cooling medium condensation first order permeate gas mixture approximately.

11, a kind of being used for from the system of the crude mixture production refine methane that contains methane, carbon dioxide and VOC, this system comprises:

(a) drier, its running is anhydrated to remove from crude mixture; And compressor, its running increases to the pressure that is suitable for absorbing heavy hydrocarbon with the pressure with crude mixture,

(b) the adverse current gas-liquid directly contacts absorber, it is positioned at drier and compressor downstream, be suitable for from crude mixture heavy hydrocarbon compounds absorbed basically fully and enter in the liquid CO 2 absorbent, and be suitable in one way, producing the medium-gas mixture that is substantially free of heavy hydrocarbon compounds

(c) first order film separation unit, its have than methane more preferably permeable for carbon dioxide first film, be positioned at the permeate chamber that film and medium-gas mixture fluid are communicated with the feed chamber of side and are positioned at the relative side with feed chamber of this first film, this permeate chamber is suitable for receiving the first order infiltration gas of medium-gas mixture selectively penetrating by first film

(d) condenser, its running with liquefy this first order infiltration gas and

(e) recirculation transfer line, fluid is communicated with between the permeate chamber of this pipeline and the absorber and first film separation unit, turns round to send first order infiltration gas to absorber.

12, the described system of claim 11, wherein feed chamber is suitable for receiving the first order and oozes the residual air body, and this system further comprises:

(f) second level film separation unit, its have than methane more preferably permeable for carbon dioxide second film, be positioned at this second film and the first order and ooze the permeate chamber that residual air body fluid is communicated with the feed chamber of side and is positioned at the relative side with feed chamber of this second film, this permeate chamber be suitable for receiving the first order ooze residual air body mixture selectively penetrating by second film second level infiltration gas and

(g) echo-plex pipeline, fluid is communicated with between the permeate chamber of this pipeline and second level film separation unit and the crude mixture of absorbent upstream, and running is to add second level infiltration gas in the crude mixture that has compressed and dewatered.

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