Molecular sieve abstract
In the purification of light hydrocarbons to remove carbonyl sulfide
impurity by selective adsorption thereof on molecular sieve adsorbents,
significant losses of the hydrocarbon are incurred by its coadsorption
on the adsorbent with the COS impurity. Conventional desorption
procedures produce a desorbate which is a non-commercial mixture
of COS and hydrocarbon. The purge desorption method of the present
invention utilizing a mixture of CO.sub.2 and a non-sorbable purge
gas permits recovery of the coadsorbed hydrocarbon in a pure and
commercially useful form.
Molecular sieve claims
What is claimed is:
1. Process for removing carbonyl sulfide from a hydrocarbon feedstock
which comprises:
(a) providing a feedstock of hydrocarbons having from 2 to 6 carbon
atoms and containing from 5 to 5000 ppm (v.) carbonyl sulfide;
(b) passing said feedstock in the liquid phase through a fixed
bed containing a molecular sieve adsorbent capable of adsorbing
carbonyl sulfide and molecules of said hydrocarbon, whereby carbonyl
is selectively adsorbed and a portion of said hydrocarbon is coadsorbed
and a purified hydrocarbon effluent from the bed is obtained;
(c) terminating the passage of said feedstock through said bed
at a time the adsorbent has remaining from 0.5 to 20 percent of
its capacity to adsorb the carbonyl sulfide impurity;
(d) draining the b ed of its void space held liquid;
(e) cocurrently to the direction of flow into the bed in step (a),
introducing a substantially non-sorbable purge gas containing from
about 1 to about 10 volume percent carbon dioxide into the bed to
desorb the coadsorbed hydrocarbon constituent of the feedstock while
retaining the carbonyl sulfide impurity constituent in the adsorbed
state;
(f) recovering the hydrocarbon from the effluent from the adsorption
bed in step (e); and
(g) regenerating the adsorption bed by desorbing the impurity constituent
therefrom.
2. Process according to claim 1 wherein the hydrocarbon is an olefin
or diolefin containing from 3 to 6 carbon atoms.
3. Process according to claim 1 wherein the hydrocarbon is a paraffin
containing from 2 to 4 carbon atoms.
4. Process according to claim 3 wherein the hydrocarbon is propane.
5. Process according to claim 4 wherein the feedstock contains
from 50 to 500 ppm (v.) carbonyl sulfide.
6. Process according to claim 1 wherein the hydrocarbon is propane
and the non-sorbable purge gas employed in step (e) contains from
1 to 5 volume per cent carbon dioxide.
Molecular sieve description
FIELD OF THE INVENTION
The present invention relates to improved processes for the adsorption
purification of hydrocarbon feedstocks and more particularly to
such processes which are improved by virtue of the use of novel
procedures to recover hydrocarbons co-adsorbed on the adsorbent
along with the impurity constituent to be removed.
BACKGROUND OF THE INVENTION
The purification in the liquid phase of organic feedstocks such
as hydrocarbons containing sulfur compound impurities by the selective
adsorption of the impurity compounds on molecular sieve adsorbents
is well know in the art. For example, the liquid phase purification
of petroleum-derived full range hydrocarbon feedstocks containing
carbonyl sulfide is disclosed in U.S. Pat. No. 3654144 issued
to J. J. Collins, Apr. 4 1972. Therein the process comprises passing
the sour hydrocarbon feed in the liquid phase through a fixed bed
of a bivalent metal cation form of zeolite A to selectively adsorb
the COS. The periodic regeneration of the adsorbent bed is accomplished
in the vapor phase using a non-sorbable purge gas such as nitrogen
for displacement of the void space liquid as well as desorption
of the COS impurity. This procedure is typical of the prior art
processes insofar as regeneration is concerned. The entire volume
of the gas stream used for displacement and purge desorption is
isolated from the system and is largely a waste material. Equally
disadvantageous economically, is the fact that the full bed void
space volume of vapor phase hydrocarbon remaining in the bed along
with a significant quantity of "sponged" liquid hydrocarbon
and coadsorbed hydrocarbon becomes admixed with the purge gas and/or
the desorbed COS impurity constituent and is either not recovered
as product or must be reclaimed by a further purification procedure.
In commonly assigned U.S. application Ser. No. 155125 filed Feb.
11 1988 now U.S. Pat. No. 4899016 by K. R. Clark and P. Richman
there is disclosed an improved process for the partial recovery
of "sponged" liquid hydrocarbon. In accordance with this
process the adsorbent bed regeneration procedure involves the use
of a portion of the purified feedstock in the vapor phase to countercurrently
remove the liquid held in the bed void space and a relatively non-sorbable
purge gas to sequentially displace the purified feedstock vapor
from the bed using plug-flow followed by a displacement of the non-sorbable
purge gas from the bed void space using another portion of the purified
feedstock in the vapor phase, and finally cooling and refilling
the bed in a direction cocurrent with the flow of feedstock into
the bed during the adsorption purification stage with a portion
of the purified feedstock in the liquid phase.
A particularly difficult problem arises in processes for the purification
of propane and other light hydrocarbons containing carbonyl sulfide
in which liquid phase adsorption on molecular sieve adsorbents is
employed to selectively adsorb the COS. It has been found that as
much as 26 weight per cent of the light hydrocarbon treated is coadsorbed
with the COS. Since inert gas purging of the adsorbent bed ordinarily
desorbs COS and light hydrocarbon simultaneously, the desorbed hydrocarbon
effluent from the bed is too highly contaminated with COS to be
commercially useful.
SUMMARY OF THE INVENTION
There has now been discovered a novel process for purifying light
hydrocarbon feedstocks containing from 5 to 5000 ppm (v.) COS as
an impurity in which the COS is selectively adsorbed and the loss
of hydrocarbon due to coadsorption is very significantly reduced
compared with prior known adsorption processes. The process comprises
the steps of:
(a) providing a feedstock of hydrocarbons having from 2 to 6 carbon
atoms and containing from 5 to 5000 ppm (v.), preferably 10 to 1000
ppm (v.) carbonyl sulfide;
(b) passing said feedstock in the liquid phase through a fixed
bed containing a molecular sieve adsorbent capable of adsorbing
carbonyl sulfide and molecules of said hydrocarbon, whereby carbonyl
is selectively adsorbed and a portion of said hydrocarbon is coadsorbed
and a purified hydrocarbon effluent from the bed is obtained;
(c) terminating the passage of said feedstock through said bed
at a time the adsorbent has remaining from 0.5 to 20 per cent of
its capacity to adsorb the carbonyl suflide impurity;
(d) draining the bed of its void space held liquid;
(e) cocurrently to the direction of flow into the bed in step (a),
introducing a substantially non-sorbable purge gas containing at
least about 0.5 volume per cent, and preferably from about 1 to
about 10 volume per cent, carbon dioxide into the bed, preferably
at an elevated temperature, to desorb the coadsorbed hydrocarbon
constituent of the feedstock while retaining the carbonyl sulfide
impurity constituent in the adsorbed state;
(f) recovering the hydrocarbon from the effluent from the adsorption
bed in step (e); and
(g) regenerating the adsorption bed by desorbing the impurity constituent
therefrom.
THE DRAWINGS
FIG. 1 of the drawings is a schematic flow diagram for a process
embodiment of the present invention in which carbonyl sulfide is
removed as an impurity constituent from a propane feedstock.
FIG. 2 is a plot of data showing the propane content of adsorption
bed effluent as a function of the time of CO.sub.2 - containing
purge gas flow into the bed during the process step of purge recovery
of propane.
FIG. 3 is a plot of data showing the CO.sub.2 and COS content of
adsorption bed effluent as a function of the time of CO.sub.2 and
COS containing purge gas flow into the bed during the process step
of purge recovery of propane.
DETAILED DESCRIPTION OF THE INVENTION
The enhanced recovery of purified hydrocarbon product which is
obtained by the practice of the present process is attributable
to the discovery that a non-sorbable purge gas containing a relatively
small concentration of carbon dioxide is capable of selectively
desorbing the hydrocarbon from a molecular sieve adsorbent which
contains sponged hydrocarbon and hydrocarbon coadsorbed with carbonyl
sulfide.
The molecular sieve adsorbent used in the process is not a narrowly
critical aspect of the invention and is selected with regard to
the molecular dimensions, polarity, volatility and the like of the
COS and any other impurity constituent to be removed, such as H.sub.2
S and H.sub.2 O. Zeolitic molecular sieves such as 13X, 4A and 5A
widely used for adsorption-purification processes are preferred
adsorbents, with zeolite 5A being a particularly preferred adsorbent.
Non-zeolitic adsorbents such as the AlPO.sub.4 's, SAPO's and MeAPO's
more recently discovered and made available commercially are also
suitably employed. Such materials are described in detail in the
patent literature, for instance, U.S. Pat. Nos. 4567029 4310440
and U.S. Pat. No. 4440871.
The light hydrocarbon feedstock composition is also not narrowly
critical. It is preferred, however that the major proportion of
the feedstock is an olefin or diolefin containing from 3 to 6 carbon
atoms, or a paraffin containing from 2 to 4 carbon atoms. In general
hydrocarbons containing from 2 to 6 carbon atoms are suitable for
treatment by the present process. Propane is a particularly preferred
hydrocarbon, either alone or in admixture with another light hydrocarbon
or with a molecular species not significantly adsorbed by the adsorbent
employed by reason of molecular size or degree of polarity, such
as highly branched chain hydrocarbons, aromatics and the like. In
general the COS concentration of the feedstock is from about 5 to
2000 ppm (v.) and is preferably from about 50 to 500 ppm (v.). Other
impurity constituents which are more strongly sorbed on the molecular
sieve adsorbent employed, such as mercaptans and H.sub.2 S, can
also be present in the feedstock.
The non-sorbable purge gas stream containing CO.sub.2 used to recover
the sponged and coadsorbed propane from the adsorbent can be any
of the inert gases, hydrogen, nitrogen and methane or mixtures thereof.
The CO.sub.2 content should be at least 0.5 vol.-% and is preferably
within the range of about 1 to about 5 vol.-%
The temperature and pressure conditions, flow rates, adsorption
bed sizes and configurations will vary depending upon the feedstock
being treated but their selection is well within the routine skill
of the art in view of the present specification.
ILLUSTRATIVE EMBODIMENT
The various embodiments of the present invention are illustrated
by the following specific process system which concerns the removal
of carbonyl sulfide from propane. The source of the propane is a
sour natural gas stream which has been initially treated in an adsorption
bed to remove most of the water and H.sub.2 S present, which has
been further treated in a deethanizer to remove the ethane, and
is the overhead effluent from a depropanizer. As such the feedstock
typically contains propane, ethane, butanes, hydrogen sulfide and
carbonyl sulfide:
With reference to FIG. 1 of the drawings, a sour natural gas stream
at a pressure of 5155 kg/m.sup.2 (gauge) is passed to an adsorption
unit 4 through line 2 and a sweetened product effluent comprised
of methane, ethane, propane, CO.sub.2 and COS is removed from the
bed through line 6 and then fed through expander 8 and line 10 to
(separator) 12 at a pressure of 1524 kg/m.sup.2 (gauge). The COS
and the C.sup.+.sub.2 hydrocarbons in the liquid state in separator
12 are passed to deethanizer column 16 through line 14. The residual
(overhead) gas from separator 12 consists essentially of methane,
but also contains about 3.4 per cent (vol.) carbon dioxide and 1.5
ppm (v.) COS. A portion of this gas stream is utilized to regenerate
adsorption bed 16 and the remainder is employed to regenerate the
propane purification beds as described hereinafter. Product methane
containing C.sub.2 hydrocarbon is removed from the system through
line 20. The bottoms from deethanizer 16 are partially recycled
through line 22 reboiler 24 and line 26 and the remainder is passed
to depropanizer column 28 through line 30 and valve 32. The overhead
from column 28 comprises propane and about 320 ppm (v) COS impurity.
Carbon dioxide, if present, is below the detectability level of
25 ppm (v.) using gas chromatographic analysis. This gas stream
is passed at a temperature of about 22.degree. C. through line 34
valve 36 and line 38 into the top of adsorbent bed 40 containing
about 41000 kilograms of zeolite 5A molecular sieve adsorbent agglomerates.
Bed 40 has, at the beginning of the adsorption purification step,
been regenerated, cooled to 32.degree. C. and filled with feedstock.
The flow rate of feedstock into bed 40 is from about 25436 to 30000
kilograms per hour. Purified propane is recovered through line 42
while COS is retained in bed 40 as an adsorbate. The bed capacity
is such that the adsorption-purification step proceeds for 8.0 hours,
after which time feedstock flow is diverted via valve 36 through
line 44 and into adsorbent bed 46. During the 8 hour period, bed
40 is engaged in adsorption-purification, adsorbent bed 46 is undergoing
regeneration following a similar adsorption-purification step therein.
As the first stage of bed regeneration, the void space liquid is
drained through line 48 valve 50 line 52 and valve 36 and passed
as a portion of the feedstock to bed 40 through line 38. Draining
and displacement of the liquid from bed 46 can be aided by the introduction
of residue gas flowing from separator 12 through line 20 line 54
furnace 56 line 58 valve 60 and line 62. Following completion
of the liquid drainage step the temperature of the residue gas stream
is increased to 288.degree. C. and is maintained at that temperature
for about 4.5 hours at a flow rate of 113275 standard cubic meters
per day in order to purge from the bed the void space propane vapor
and the propane coadsorbed with the COS impurity on the molecular
sieve adsorbent. The propane-containing effluent from bed 46 during
this purging step is recycled to deethanizer column 16 via line
64 valve 66 line 68 and line 26 passed to depropanizer column
28 along with the bottoms from deethanizer column 16 as described
hereinabove, and thereafter to adsorption bed 40 from which purified
propane is recovered and removed from the system. The duration of
the purge step in which sponged and coadsorbed propane is recovered
is readily determined by monitoring the concentration of propane
in the bed effluent. With reference to FIG. 2 it is observed in
the process embodiment described herein that in the first few minutes
after purging with residue gas at 288.degree. C. begins, the propane
content of the bed effluent is about 53 volume -%. At the end of
the first 1.5 hours of this purging, the propane content of the
bed effluent has decreased to about 7.5 volume -%. It is believed
that this propane effluent is primarily sponged (void space) propane.
Over the next 3 hour period of continued purging the propane content
of the bed effluent remains relatively unchanged, after an initial
slight rise from 7.5 volume -%, at about 11 volume -%. During this
period the propane is believed to be produced in the bed by displacement
of coadsorbed propane from the molecular sieve by virtue of purging
action of the non-sorbable constituents of the residue gas and the
displacement action the carbon dioxide constituent of the residue
gas. After 4.5 hours elapsed purge time, the propane concentration
of the bed effluent declines rapidly, indicating that the desorption
of coadsorbed propane is nearly complete.
These beliefs are clearly supported by the data of FIG. 3 taken
over the same time period as those in FIG. 2 by monitoring both
the influent and effluent streams from the same adsorption bed.
The CO.sub.2 concentration of the stream entering the bed is essentially
constant at about 3.4 volume -% as is the inlet COS concentration
at about 3 ppm (v.). Until after about 4.5 hours of purging the
concentration of both CO.sub.2 and COS in the bed effluent are much
smaller than the inlet concentrations, indicating adsorption of
both of those substances on the bed adsorbent. Upon near completion
of the displacement of propane from the adsorbent by the CO.sub.2
and an exhaustion of the bed capacity for COS, the concentration
of both CO.sub.2 and COS begin to increase very rapidly in the bed
effluent, signaling the time for termination of recycle purge recovery
of propane and the time for beginning the bed regeneration to remove
adsorbed COS impurity from the bed and from the system. In the case
of bed 46 in FIG. 1 COS is removed from the system through line
64.
While the invention has been illustrated and described in terms
of enhanced propane recovery in its purification with respect to
COS, it will be understood that the principles involved can be applied
to other purification processes in which the hydrocarbon to be recovered
is an olefin, diolefin or paraffin having from 2 to 6 carbon atoms,
for example ethane, butane, propylene and butylene. |