Molecular sieve abstract
In the process for purifying hydrocarbon gas streams containing
hydrogen sulfide and carbon dioxide as impurities by contact thereof
with zeolitic molecular sieve adsorbents to selectively adsorb the
impurities, the formation of carbonyl sulfide by the zeolite catalyzed
reaction of H.sub.2 S with CO.sub.2 is greatly suppressed by employing
as the selective adsorbent certain cation forms of molecular sieve
zeolites which contain from 0.7 to 3 weight percent adsorbed water.
Molecular sieve claims
What is claimed is:
1. A cyclic process comprising the steps of:
(a) an adsorption purification stroke wherein a hydrocarbon stream
containing H.sub.2 S and CO.sub.2 is contacted in the vapor phase
at a temperature of from 60.degree. F. to 120.degree. F. with a
molecular sieve adsorbent in a fixed bed to selectively adsorb H.sub.2
S and a hydrocarbon product is recovered which is substantially
free of H.sub.2 S;
(b) a purge desorption stroke wherein a portion of the substantially
H.sub.2 S-free hydrocarbon product recovered in step (a) is heated
to above 120.degree. F. and passed countercurrently through the
adsorption bed to desorb substantially all of the adsorbate molecules
selectively adsorbed in step (a) and flush same from the bed;
(c) a cool-down stroke wherein the bed is cooled to below 120.degree.
F. by the cocurrent purge therethrough of a portion of the substantially
H.sub.2 S-free hydrocarbon recovered in step (a) at a temperature
of from 60.degree. F. to 120.degree. F.;
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 percent of the framework aluminum
atoms thereof being associated with at least one species of alkaline
earth metal cation having an atomic number of less than 56 and
injecting into said hydrocarbon stream prior to passage through
the bed in step (b) a sufficient amount of water vapor to impart
a substantially uniform adsorbed water loading of from 0.7 to 3.0
weight percent to the molecular sieve adsorbent.
2. Process according to claim 1 wherein the hydrocarbon of the
stream being treated is methane containing not greater than 5 mole
percent H.sub.2 S from 0.5 to 55 mole percent carbon dioxide, and
the molecular sieve adsorbent is the calcium cation form of zeolite
A having at least 45 equivalent percent calcium cations.
Molecular sieve description
The present invention relates in general to the purification of
hydrocarbon gas streams containing as impurities H.sub.2 S and CO.sub.2
and more particularly to process whereby H.sub.2 S is selectively
adsorbed from such hydrocarbon gas streams using zeolitic molecular
sieves having minimal catalytic activity with respect to reaction
between H.sub.2 S and CO.sub.2 to form COS.
The gas phase treatments of hydrocarbon feedstocks, particularly
natural gas, to remove H.sub.2 S and other impurities by selective
adsorption and absorption techniques is well known. Natural gas,
for example, commonly contains water, hydrogen sulfide, carbon dioxide,
plus other sulfur compounds 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 to be transported by pipeline, there
are specifications for its water and corrosive sulfur, as hydrogen
sulfide, contents. Transmission and some other end uses do not require
removal of carbon dioxide except in those instances where a minimum
heating value needs to be met. Natural gas feed to a liquefaction
unit requires much more thorough clean-up to protect against solids
formation by water and carbon dioxide in the cryogenic equipment.
The selective adsorption character of molecular sieve has been
quite ideal for these purifications for the reason that the order
of adsorption selectivity is: H.sub.2 O>H.sub.2 S>CO.sub.2
>CH.sub.4. Thus when crude natural gas is passed through a molecular
sieve adsorbent bed, the impurities adsorb in zones and it is possible
to adsorb only the water, or water and H.sub.2 S, or H.sub.2 O,
H.sub.2 S and CO.sub.2 to any desired extent.
It has been found, however, that when both H.sub.2 S and CO.sub.2
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
and also due to the fact that 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 molecules 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 H.sub.2 S and CO.sub.2 using
molecular sieve adsorbents.
This object, we have found, is accomplished in the cyclic process
comprising the steps of the cyclic process comprising the steps
of (a) an adsorption purification stroke wherein a hydrocarbon stream
containing H.sub.2 S and CO.sub.2 is contacted in the vapor phase
at a temperature of from 60.degree. F. to 120.degree. F. with a
zeolitic molecular sieve adsorbent in a fixed bed to selectively
adsorb H.sub.2 S and a hydrocarbon product is recovered which is
substantially free of H.sub.2 S; (b) a purge desorption stroke wherein
a portion of the substantially H.sub.2 S--free hydrocarbon product
recovered in step (a) is heated to above 120.degree. F. and passed
countercurrently through the adsorption bed to desorb substantially
all of the adsorbate molecules selectively adsorbed in step (a)
and flush same from the bed; (c) a cool-down stroke wherein the
bed is cooled to below 120.degree. F. by the cocurrent purge therethrough
of a portion of the substantially H.sub.2 -free hydrocarbon recovered
in step (a) at a temperature of from 60.degree. F. to 120.degree.
F.; 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 percent of the framework aluminum
atoms thereof being associated with at least one species of alkaline
earth metal cation having an atomic number of less than 56 and
injecting into said hydrocarbon stream prior to passage through
the bed in step (b) a sufficient amount of water vapor to import
a substantially uniform adsorbed water loading of from 0.7 to 3.0
weight percent to the molecular sieve adsorbent.
Although the preferred feedstock for treatment in accordance with
the present process is CO.sub.2 -containing sour natural gas, any
hydrocarbon of mixture of hydrocarbons containing H.sub.2 S and
CO.sub.2 which is in the vapor state at a temperature within the
range of 60.degree. F. to 120.degree. F. and a pressure of from
200 to 1200 psia and which is less strongly adsorbed than H.sub.2
S is suitably treated. The preferred natural gas feedstock contains,
in addition to methane, water in any concentration up to saturation,
up to 5 mole percent H.sub.2 S, from 0.5 to 55 mole percent CO.sub.2
and not more than 25 mole percent hydrocarbons having more than
one carbon atom. Commonly such hydrocarbon feedstocks will also
contain organic sulfur compounds such as mercaptans.
The drawing is a schematic flow diagram showing a three bed process
system suitably employed in the practice of the present process.
The molecular sieve zeolite adsorbent can be any naturally occurring
or synthetic crystalline zeolite which contains at least 45 equivalent
percent beryllium, magnesium, calcium, or strontium cations or mixtures
of any two or more of such cations and which has in this cation
form a pore diameter of at least 5 Angstroms. The calcium cation
forms of zeolite A and zeolite X as defined in U.S. Pat. No. 2882243
and U.S. Pat. No. 2883244 respectively, have been found to have
especially low catalytic activity with respect to the reaction of
H.sub.2 S and CO.sub.2 and are particularly preferred in the present
process. Other suitable zeolites include the calcium cation forms
of mordenite, chabazite, faujasite and zeolites Y disclosed in U.S.
Pat. No. 3130007; zeolite T disclosed in U.S. Pat. No. 2950952;
zeolite L disclosed in U.S. Pat. No. 3216789; and zeolite omega
disclosed in pending U.S. application Ser. No. 655318 filed July
24 1967.
The required water loading on the zeolite adsorbent is readily
attained 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
is convenient to inject water vapor into that purge gas stream in
appropriate amount such that after desorption and cool-down of the
bed is complete, the requisite water loading remains on the bed.
The following example is illustrative of the present process:
EXAMPLE 1
In the drawing it is to be understood that each of the three adsorbent
beds shown are equivalent and each in turn would, in conventional
operation, undergo the steps of adsorption, hot purge desorption
and cool-down in preparation for the next cycle of the same three
steps. For simplicity, the various valves, manifolds, pumps, etc.
ordinarily used in this conventional three-bed type of operation
have been omitted. The drawing shows the simultaneous operation
in each of the three beds.
The aforesaid feedstock is fed at a pressure of 1045 psia through
line 10 to adsorber 12 which contains as the adsorbent zeolite A
having 80 equivalent percent calcium cations and 20 equivalent percent
sodium cations and containing 2.6 weight-% adsorbed H.sub.2 O. Adsorber
12 is operated during this adsorption step at 92.degree. F. The
effluent from the adsorber 12 is essentially pure methane. In due
course an adsorption front for each of the components H.sub.2 O,
H.sub.2 S and CO.sub.2 are formed in the adsorber with H.sub.2 O
front being closest to the ingress end of the bed and the CO.sub.2
front being nearest the egress end of the bed. Since in this embodiment
it is the purpose to remove only the H.sub.2 S, the CO.sub.2 front
is permitted to break through the egress end of the adsorber and
commingle 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 the top of adsorber 18
which at the beginning of the adsorption stroke in adsorber 12 had
just finished being hot purge desorbed and contains essentially
H.sub.2 S-free product methane. The adsorber is at a temperature
of 500.degree. F. The purified methane entering adsorber 18 is at
a temperature of 92.degree. F. and in its passage through adsorber
18 cools that adsorber until a temperature of 125.degree. F. is
reached. The thus heated gas leaving bed 18 is passed through line
20 furnace 22 where the temperature is raised to 550.degree. F.,
and line 24 into adsorber 26 which at the beginning of the adsorption
fill stroke in adsorber 12 has just completed a downward adsorption
fill stroke using feedstock of the same composition as is currently
being introduced through line 10. The heated purge gas from furnace
22 is injected with water through line 28 to raise the water vapor
content to 0.185 mole percent. The desorbate stream from adsorber
26 which contains the H.sub.2 O and H.sub.2 S previously adsorbed
is fed through line 30 to sulfur recovery unit 32. Stack gases are
passed from the system through line 34 and sulfur collected from
line 36. The COS content of the product methane leaving the system
through line 14 is less than 8 ppm.
(b) Using the same procedure, feedstock and apparatus as set forth
in part (a) above, except that the zeolite adsorbent in adsorber
12 contained less than 0.7 weight-% adsorbed H.sub.2 O, the COS
content of the product methane leaving the system through line 14
is about 45 ppm. |