ADOUCISSEMENT DU GAZ PAR IMPREGNATION A MEMBRANE

GAS SWEETENING BY MEMBRANE PERMEATION

Abrégé anglais

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.

Revendications

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
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?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.
14

Description

7~i ~
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
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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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