CA1062176A - Gas sweetening by membrane permeation - Google Patents

Gas sweetening by membrane permeation

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Publication number
CA1062176A
CA1062176A CA214,264A CA214264A CA1062176A CA 1062176 A CA1062176 A CA 1062176A CA 214264 A CA214264 A CA 214264A CA 1062176 A CA1062176 A CA 1062176A
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CA
Canada
Prior art keywords
membrane
sour gas
methane
gas components
steps
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA214,264A
Other languages
French (fr)
Inventor
Donald L. Klass
Carl D. Landahl
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Gas Technology Institute
Original Assignee
Institute of Gas Technology
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
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Abstract

ABSTRACT

A method for sweetening sour gas containing hydrogen sulfide and/or carbon dioxide sour gas components by delivering the sour gas as a feed mixture to a semi-permeable membrane being highly selective for such sour gas components, collecting permeated sour gas components, and then collecting the rejected, sweetened methane gas. In preferred form, the feed sour mixture is delivered to an anisotropic membrane having a separation factor in excess of about 20, having a high solubility parameter of greater than about 9, and having a permeation constant sufficient to assure efficient sweetening. In another preferred form, the sour feed gas mixture is delivered at a feed pressure which is substantially greater than the permeate pressure at which the permeated sour gas components are collected. The rejected sweet methane gas has at least about 99% concentration of methane and higher fuel components with less than 1% carbon dioxide and less than about 20 ppm of hydrogen sulfide.

Description

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SPECIFICA~ION

This invention relates to a method for sweetening sour gas by delivering sour feed gas mixtures to a semi-permeable membrane which is highly selective for the sour gas components, so that said sour gas components are permeated and sweetened gas is rejected.
It is a recognized goal in the art to sweeten sour gas containing hydrogen sulfide and carbon dioxide as the sour com-p9nents. These acidic components are objectionable because they result in corrosion, and their presence reduces the concentra-tion of methane which upsets desired fuel levels. The hydrogensulfide component is further objectionable because of its offensive smell and because of its recognized toxicity. The art is concerned with this problem and several methods for sweetening gas have been disclosed such as the use of solvent absorbers as shown in U. S. 3,710,546. The use of molecular sieves has also been disclosed in U. S. 3,470,677. The use of a bundle of semi-permeable capillary filters to separate hydrogen 8ulfide from methane has been disclosed in U. S. 3,534,528. It i8 desirable to provide the art with an alternative method for sweetening sour gas which is reliable, efficient, and which may - be practiced in a variety of ways to extend the advantages of such an alternative method.
It is therefore an object of the invention to provide i an alternative method in the art for sweetening sour gas by - ~
- Qelectively permeating the acid gas components through a membrane ~`
having separation factors favorable to said acid gas components.
It is another object of the invention to provide such a method which imultaneously removes the acid gas components wh~le rejecting the methane gas to thereby increase the methane concentration to sweet levels.
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Yet another object of thè invention is to provide such a method for sweetening sour gas wherein non-homogenous membranes are used to advantage to attain improved permeation o~ the acid gas components.
It is yet still another object of this invention to ` provide a method of the type described for sweetening sour gas where feed pressures and permeate pressures are used to advantage to attain improved separation of the acid gas components by permeation.
10The term "sour gas" is used in its accepted manner in the art to indicate a gas mixture having from about 80 to ; about 90% methane or light paraffins, with the balance being acid components consisting of hydrogen sulfide and carbon dioxide.
~t is usually provided that the carbon dioxide acid component is present in substantially larger amounts than the hydrogen sulfide component. Generally, the carbon dioxide is present up to about 10% and the hydrogen sulfide is generally present in amounts less than 1%. The term "sweet gas" is used to identify a high fuel level gas containing at least about 99~ methane or light paraffins and less than about 1% of the acid components. In general, the carbon dioxide is present in amounts less than 1%
and trace amounts of hydrogen sulfide are present, say, less than about 20 ppm.
The invention disclosed herein provides that sour gas 'i5 reliably sweetened by simultaneously removing the acid gas components by permeation where the membrane which is highly selective for such acid gas components. The rejected methane is collected at high concentrations to obtain the sweet gas.
The use of the term "methane" herein is intended to likewise include the possible presence of any light paraffins, and such ' ~
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terms should be understood as conveying this meaning for the purposes of the present invention. As used in this application the concentrations of methane, carbon dioxide and hydrogen sulfide are considered relative to the total of these three components.
It is a feature of the invention that anisotropic me~ranes are used to particular advantage because they are highly selective for the acid gas components and they have accept-able permeability constants to allow efficient permeation of the lO acid gas components. It has been found that such anisotropic membranes overcome a common problem arising with homogenous membranes in that desired high solubility parameters of greater than 9 have high separation factors, usually in excess of 20, but also share the disadvantage of having a relatively low permeability constant. When using homogenous membranes to practice the invention, it is necessary to balance the factors of high separation and lower permeability in selecting a membrane. ~
It has been found that more favorable combinations of separation ~ -factors and permeability constants are attained with aniso-20 tropic membranes, as well as composite membranes, both being non-homogenous. The term "anisotropic" is recognized in the art a~ representing an integrally formed membrane having a thin skin on one side an~ the balance comprising a thicker, more 4 porous material. It is characteristic of anisbtropic membranes that their permeation properties are not uniform in all directions. In particular, permeation-rates through anisotropic me~branes compared with homogenous membranes of the same overall ~hickness~are considerably higher.~ Anisotropic membranes are preferred in a "sheet" configuration but may be used to advant-30 age in other forms including tubular or concentricO

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A variety of anisotropic membranes may be utilized such as polymers of polyvinylidene chloride ~particularly near homopolymers), polyacrylonitrile, or the cellulo~e acetate membranes. The useful membranes include the cellulose diacetate and the cellulose triacetate membranes. Also us--ful are ma~erials such as gelatin, nylon 6 and 6/6, polyvinyl alcohol membranes, polystyrene, polyurethane, PVC, vinylidene copolymers, and the like.
The multi-layer or composite membranes generally provide a thin layer membrane having a high solubility parameter polymer supported by a thicker but lower solubility parameter polymer.
The use of the term "permeated gases" or "permeate"
refers to the gases which are preferentially adsorbed by the membrane. Such permeate gases may be carried by the membrane to a location on the other side of the membrane.
Current belief holds that permeability of the gas through a membrane is characterized by two features: 1) solubility of the gas in the membrane material and 23 diffusion of the gas through a membrane. Permeation of any single gas is therefore viewed as being the product of the solubility and diffusivity of a given gas in the membrane. Each gas h'as a particular permeability constant (K) for a given membrane. The rate of permeation of a gas is fuither influenced by variables such as membrane thickness, nature of the membrane, layers of the membraneiinvolved, differential pressures, temperatures, and possibly still other factors. The permeability constant (K) ts computed in terms of flow (cu cm) at a standard conditions per time (s) at specified thickness (cm), effective surface area (sq cm), and pressure differential across the membrane lcm Hg):

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. cu.cm .(STP?..-.cm K, sq cm-s-cm HG
The ratio of the permeability constants for two gasses under the ; same conditions is known as the separation factor (2); and is computed as the ratio of the permeability of the sour gas com-. ponents with respect to that of methane through a given polymer membrane:
7 H2S/CH4; CO2/CH4 The non-homogenous membranes selected have the desired high solubility parameters that are greater than about 9, generally 10 in the range of 9.4-15Ø These membranes have the desired high . separation factors for either or both of the sour components hydrogen sulfide and carbon dioxide, and have acceptable perme-ability constants to attain efficient permeation, when used in the advantageous forms described.herein.
In order to increase the efficiency of collecting rejected methane, it is necessary to increase the membrane area so that the permeate may be more efficiently collected. It has . :
been found that the required membrane area may be substantially . reduced if higher feed pressures are used and lower permeate :i 20 pressures are provided in the sweetening gas permeation process.

It is generally provided that the feed pressure be at least greater than atmospheric. In practice, it is preferred that the feed pressure be substantially greater, say, at least 30-fold . - over the.permeate pressure. The feed pressure can be attained in various ways such as from the flow rate of the sour gas . mixture along the closed path against the face of the membrane.
The permeate pressure may likewise be provided in various ways, such as.providing.an enclosure to one side of the membrane and evacuating the chamber formed therein. Such pressure gradients may be provided by resort to the usual skills recognized by ; practitioners. .

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The following examples illustrate representative teachings of features of the invention, but such examples should not be construed as being exclusi.ve embodiments.
EXAM2LE l . _ Permeation of Acid Gas Components in a Square Permeator Cell A square permeator cell is provided which receives feed gas from a row of ports so that such gas flows across the membrane and exits at a parallel row of ports along the opposite edge of the membrane. The permeated gas flows out ts rows of ports placed at opposite sides of the membrane. The membrane is supported on filter paper caulked around the edge with a silicone rubber sealant. Feed gas pressure is regulated and fed to the cell assembly and to a manifold leading to a gas chromatograph.
The nonpermeated or rejected gas flows from the cell through a variable restrictor to a flow indicator or the chromatograph.
` Permeated gas is led to a flow indicator or the chromatograph directly for operation near atmospheric pressure. During ^~ measurements on the permeated gas side of the membrane at sub- -atmospheric pressures, a pump is placed in the line before the flow indicator and chromatograph. Adjustments of the variable restrictor in the reject stream allows variation of the ratio of permeated gas flow to reject flow. The reject flow is set approximately and time is allowed for equilibration of flow through the membrane. The reject and permeated gas flows are measured and the feed, reject and permeated gas streams are analyzed in succession. The feed gases are analyzed by mass spectrometry.
-- A sour~gas feed mixture is delivered to the square pérmeator cell, and said mixture has hydrogen sulfide in con- :
aentrations ranging from about 0.02 to 0.56%; and carbon dioxide ., ~ ~ , :

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in a concentration ranging from 1.0 to 10.3%. The feed pres-sures range from 19 to 215 psia while the permeated gas stream pressure was maintained at near vacuum (01.-0.3 psia) or at atmo-spheric pressure. A run composed of about 90~ methane, 10~
carbon dioxide, and 0.02 to 0.4~ hydrogen sulfide showed an increase in sweetening performance with increased feed pressure for a permeate pressure of 15 psia. An unexpected enrichment of methane and propane in the reject gas was observed during all runs, the ethane concentration ranging up to 170% of that in the feed. Propane showed a greater increase than ethane. The propane concentration in the reject gas ranged up to 230% of that in the feed.

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EXAMPL~ 2 Membrane Permeability Constants and Separation ~actors A polyamide membrane, nylon 6, having a solubility parameter of 14,was evaluated at different temperatures for a sour gas mixture including 1.1 mol ~ of carbon dioxide and 0.5 mol % of hydrogen sulfide. A permeation cell is used similar to that described in Example 1, except the cell was modified to allow the output side of the membrane to be swept with helium into sample loops of known volume which was switched into the chromatograph carrier stream for analysis. The input side of the membrane was held at a constant total pressure 65 psia.
A polyvinyl alcohol membrane having a solubility parameter of 13 was evaluated under the same conditions as the - nylon 6 membrane. Both procedures provide that, following selected sample loop valve actuation, permeated gas sample passes successively through one chromatographic column, one side of a dual-bead thermistor detector, a second chromatographic column, the other side of the detector, and finally to atmosphere. The gases are analyzed by mass spectrometry for methane and carbon dioxide, and by the iodine titration method for hydrogen sulfide.
The results are shown in following Table 1.

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The above table confirms the correlation between high solubility parameters and high sour-gas-component-to-methane separation factors. Both of the membranes tested had high solubility parameters, and exhibited moderate to high separation f~ctors for hydrogen sulfide ana carbon dioxide with respect to methane. A value of 50 was obtained at 70C for nylon 6 and hydrogen sulfide, while a value upwards of lO0 was obtained at 50C for the polyvinyl alcohol membrane and carbon dioxide. Th~
hydrogen sulfide-methane separation factor for polyvinyl alcohol decreased above 50C, but the nylon 6 membrane showed an - lO increased hydrogen sulfide-methane separation factor as the temperature increased from 30C to 70C. The carbon dioxide separation factors tended to decrease with increasing temperature over the range of 30~C to 70C.
Similar procedures were followed to evaluate the following membranes:
Membrane Solubility Parameter - Gelatin ll -Polyacrylonitrile 15 - -Nylon 6/6 14 The results are shown in following Table 2.

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The gelatin is plasticized with glycerin. All three membranes show high separation factors, the separation factor for polyacrylonitrile being 160 at 30C and 120 at 50C. Both methane and carbon dioxide permeability increased with tempera-ture, with methane showing the greater increase.
Heat stabilized nylon 6/6 showed high separation factors, generally showing improvement over nylon 6 as a membrane sweetening system.
The pigmented animal gelatin membrane showed an increase in the hydrogen sulfide-to-methane separation factor at 50C compared with the high value of 200 at 30C.
The claims of the invention are now presented and the terms used therein may be further understood by reference to the preceding specification~ :

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Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A method for sweetening methane by reducing hydrogen sulfide and carbon dioxide sour gas components, including the steps of delivering a feed mixture of methane and the sour gas components to a non-homogenous membrane having selective permeability for both said sour gas components, simultaneously removing the sour gas components which permeate said membrane, and collecting the sweetened and rejected methane.
2. A method which includes the steps of Claim 1 above, wherein said membrane selective for sour gas components has a high solubility parameter in excess of about 9, and wherein said membrane has a high separation factor and a permeability constant balanced to attain efficient permeation of the sour gas component.
3. A method which includes the steps of Claim 1 above, wherein said membrane selective for sour gas component permeation is a membrane selected from anistropic membranes and composite membranes.
4. A method which includes the steps of Claims 3 above, wherein said non-homogenous membrane is an anistropic membrane which has a separation factor in excess of about 20.
5. A method which includes the steps of Claim 4 above, wherein said anistropic membrane is of the class consist-ing of cellulose acetate, cellulose diacetate, and cellulose triacetate, and permeation of the sour gas components through the membrane is conducted at temperatures not substantially ?xcess of 50°C.
6. A method which includes the steps of Claim 1 above, wherein the mixture of methane and sour gas components is delivered at a feed pressure to the membrane, and the sour gas components are collected at a permeated pressure, said feed pressure being substantially in excess of said permeated pressure.
7. A method which includes the steps of Claim 6 wherein said feed pressure is substantially in excess of atmospheric pressure, and said permeated gas pressure is no greater than atmospheric pressure.
8. A method which includes the steps of Claim 7, wherein the feed pressure is greater than said permeate pressure, by at least about 30-fold.
9. A method which includes the steps of Claim 1 above, wherein a sour gas mixture of methane and said sour gas components contains from about 80 to about 90% methane, and substantially the balance being said sour gas components, the carbon dioxide sour gas component being present in amounts substantially greater than said hydrogen sulfide gas component, and said rejected, sweetened methane being collected in concentra-tions of at least about 99%.
10. A method which includes the steps of Claim 1 above, wherein said membrane is selected from the class consisting of cellulose acetate, polyacrylonitrile, nylon, gelatin, poly-vinyl alcohol, polyvinylidene chloride, vinylidene, copylymers, polystrene, and polyurethane.
CA214,264A 1974-03-21 1974-11-20 Gas sweetening by membrane permeation Expired CA1062176A (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4370150A (en) * 1980-08-21 1983-01-25 Phillips Petroleum Company Engine performance operating on field gas as engine fuel
US4493716A (en) * 1982-10-12 1985-01-15 W. L. Gore & Associates, Inc. Apparatus for the separation of hydrogen sulfide from a gas mixture
US5053058A (en) * 1989-12-29 1991-10-01 Uop Control process and apparatus for membrane separation systems

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4370150A (en) * 1980-08-21 1983-01-25 Phillips Petroleum Company Engine performance operating on field gas as engine fuel
US4493716A (en) * 1982-10-12 1985-01-15 W. L. Gore & Associates, Inc. Apparatus for the separation of hydrogen sulfide from a gas mixture
US5053058A (en) * 1989-12-29 1991-10-01 Uop Control process and apparatus for membrane separation systems

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