Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates in general to the
purification o~ hydrocarbon gas streams containing as
impuriti~s H2S and C02, and more particularly to process
whereby H2S is selectively adsorbed from such hydrocarbon
gas streams using zeolitic molecular sie~es having minimal
catalytic activity with respect to reaction between H2S
and C02 to form COS.
The gas phase trea~ments of hydrocarbon feedstocks,
particularly natural gas, to remove H2S and other ~mpurities
by selective adsorption and absorption techniques is well
known. Natural gas, for example, commonly contains water,
hydrogen sulfide7 carbon dioixide, plus other sulfur com-
pounds and heavier hydrocarbons in various concentrations
depending upon its source. The end use of the natural
gas dictates which impurities must be removed and the
extent of that removal When the gas is tv be transported
by pipeline, ~here are specifications for its water and
corrosive sulfur, as hydrogen sulfide, contents. Trans-
mission and some other end uses do not require removal
of carbon dioxide except in those instances where a mlnimum
heating value needs to be met. Natural gas feed to a
liquifaction unit requires much more thorough clean-up to
protect against solids ~ormation by water and carbon dioxide
in the cryogenic equipment.
The æelective adsorption character of molecular
sieve has been quite ideal for these purifications for the
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reason that the order of adsorption selectivity is~
H20 ? H2S > C02 > CH4. Thus when crude natural gas is
passed through a molecular sieve adsorbent bed, the impuri-
~ies adsorb in zones and it is possible to adsorb only the
water, or water and H2S, or H20, H2S ~nd C0~ to any desired
extent.
It has been found, however, tha~ when both H2S
and C02 are present in the feedstock, COS is frequently
: present in the product gas, i.e. after treatment in a ~ :
molecular sieve purification unit, in higher concentrations
than in the feed. This is apparently due to the fact that
the molecular sieve serves as a catalyst for the reaction.
2 2 > 2
and also due to the fact ~hat the COS, once produced in
the adsorption bed is not retained therein as an impurity
adsorbate because of its low polarity and low boiling point
compared with the same properties of the other impurity i~
mo~ecules present.
Accordingly, it is the principal object of the : ~
present invention to provide a means to suppress the ;~:
formation of COS when sweetening hydrocarbon gas streams ~.
containing both H2S and C02 using molecular sieve adsorbents.
This object, we have found7 is accomplished in
the process wherein a hydrocarbon stream containing H2S
~ and C02 is contacted in the vapor phase at a temperature of
from 60 to 120F and a pressure of 200 to 1200 psia. with
a molecular sieve adsorbent to selectively adsorb H2S and
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C2 and thereafter recovering a hydrocarbon product having
a decreased concentration of H2S, by the improvement which
comprises utilizing as the said molecular sieve adsorbent
a crystalline zeolite having a pore diameter of at least
5 Angstroms, at least 45 per cent of the framework aluminum
atoms thereof being associated with at least one species
of alkaline earth metal cation having an atomic number
less than 56, preferably calclum, and containing in the
adsorbed state thereon rom 0.7 to 3 weight per cent water.
Although the preferred feedstock for treatment
in accordance with the present process is CO2 - containing
sour natural gas, any hydrocarbon of mixture of hydro-
; carbons containing H2S and C02 which is in the vapor s~ate
at a temperature within the range of 60F to 120F and a
pressur~ of from 200 to 1200 psia and w~ich is less strongly
adsorbed than ~I2S is suitably treated. The preferred
natural gas feeds~ock contains 9 in addition to methane,
water in an~ concentration up to saturation, ~p to 5 mole
per cent H2S, from 0.5 to 55 mole per cent CO2 and not
more than 25 mole per cen~ hydrocarbons having more than
one carbon atom. Commonly such hydrocarbon feedstock~
will also contain organic sulfur compounds such as mer-
captans.
; The drawing is a schematic flow diagram showing
a three bed process system suitably employed in the
practice of the present process~
. !
_ 4 _
,
~.
.. ... .. . .. .. . .
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The molecular ~ie~e zeolite adsorbent can be
any naturally occurring or synthetic cr~stalline zeoli~e
which contains at least 45 equivalent per cent beryllium,
magnesium, calcium, or strontium cations or mixtures of
any two or more of suoh cations and which has in this
ca~ion form a pore dlameter of at least 5 Angstroms. The
calcium cation forms of zeolite A and zeolite X as deined
in U.S.PO 2,882,243 and U.S.P. 2,883,244 respect~vely, have
been found to ha~e especially low catalytic activity with .
respect to the reaction of H2S and CO~ and are particularly
preferred in the present process. Other suitable zeolites :~
include the calcium cation forms of mordPnite, chabazite,
faujasite and zeolites Y disclosed in U.S.P. 3,130,007;
zeolite T disclosed in U.S.P. 2,950,952; zeolite L dis- ::
closed in U.S.P. 3,216~789; and zeolite disclosed in `~
Canadian Patent No. 993,432, issued July 20, 1976. .
The required water loading on the zeolite ad- -
sorbent is readily at~ained by any conventional means. In
cyclic continuous operation in which an adsorbent bed is : :
periodically desorbed by means of a hot purge gas, commonly
a portion of the purified product gas, it i~ convenient to j ::
inject water vapor into that purge gas stream in appro~
priate amounts such that after desorption and cool down
of the bed is complete the requisite water loading remalns :
on the bed.
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The following example is illu~trative of the
present process:
Example 1.
(a) With refer~nce to the drawing, a natural gas
feedstock having the following composi~ion wa~ employed
in the proce~s,
CH4 ..... .....95. mole ~/O
2 .106 "
C~2 ~ 3.
H2S ..... ~. .006 "
1~ the drawing it i5 to be under~tood that each of the
three ad~orbent beds shown are ~quivalent and:each i~ turn
would~ in conventional operation, undergo ~he step~ of
adsorption9 hot purge desorp~ion and cool-down in prepara-
: tion for the nexe cycle of the same three stepsO For
simplicity the~var~ou~ valves~ manifolds, pump~, etc.
ordinarily used in this conventional ~hree~bed type of
oparation have been omitted. The drawing show~ the sim~
taneous operation in each of the three b~ds.
.
The sforesald feedstock i9 ~ed at a pressure
1045 psia through line lO to ad~orb~r 12 which contains
: as the ad~orbent z~olite A havlng 80 aquivalen~ per cent
.. . .
calcium cations and 20 equivalent par cent aodium catlons
..
and cont~ining 2.6 weigh~-% ad~orbed H2O, Adsorber 12 is
.
operated during t~i~ adsorption st~p at 92~F. The efluent
rom the adsorber 20 i~ es3entiall~ pure me~h~ne, In due
courae a~ ad~orpticn fr~nt for each o~ tha component~ ~20g
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H2S and C02 are formed in the adsorber with H20 front being
closest to the ingress end of the bed and the C02 front
being nearest the egress end of the bed. Since in this
embodiment it is the purpose to remove only the H2S, the
C2 front is permitted tG break through the egress end o~
he adsorber and comingle with the product methane which
is in the main removed from the system through line 14.
A portion of the product methane is continuously passed
through line 16 to ~he top of adsorber 18 which at the
beginning of the adsorption stroke in adsorber 12 had just
finished being hot purge desorbed and con~ains essentially
H~S - free product methane. The adsorber is at a tempera~
ture of 500F. The purified methane entering adsorber 18
is at a temperature of 92F. and in its passage through
adsorber 18 cools that adsorber until a temperature of
125F. is reached. The thus heated gas leaving bed 18 is
passed through line 20, furnace 22 where the temperature is
raised to 550F., and line 24 into adsorber 26 which a~ the -~
beginning of the adsorption fill stroke in adsorber 12 has
just completed a downward adsorption ~11 stroke u~ing
feedstock of the same composition as i8 currentl~ be~ng
in~roduced through line-10, The water con~ent of the purge
gas fr~m furnace 22 is in~ected with water through line 28
to rai~e the water vapor content to 00185 mole per cent.
Tha desorbate stream from adsorber 26 which contains the
H20 and H2S previously adsorbed is fed through line 30 to
sulfur recovery unit 32. Stack gases are passed from the
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~tem through line 34 and sulfur collected from line 36.
The COS content o~ the product methane leaving the system
through line 14 is less than 8 ppm,
(b) Using the same procedure, feedstock and appara~us
as set orth in part (a) above, except that ~he zeolite
adsorbent in adsorber 12 contained less than 0,~7 weight-%
adsorbed H20, the COS content of ~he product methane
leavlng the system ~hrough line 14 is about 45 ppm.
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