Note: Descriptions are shown in the official language in which they were submitted.
PROCESS FOR REMOVING CARBONYL-SULFIDE FROM LIQUID
HYDROCARBON FEEDSTOCKS
FIELD OF THE INVENTION
The present invention relates to a process for
removing sulfur, present in the form of carbon oxysulfide
or carbonyl sulfide, from liquid hydrocarbons More
particularly, the present invention relates to a process
5 for the removal of carbonyl sulfide from hydrocarbon
feedstocks containing propylene and to the conditioning of
the absorbent material used in the process.
BACKGROUND OF THE INVENTION
Industrial applications of liquid hydrocarbons and
10 particularly, liquified oleEinic hydrocarbons, have become
more increasingly specialized. The technology as presently
developed utiliæes highly efficient ca~alysts to convert
these liquified hydrocarbon feeds~ocks into final product
such as polymers. However, these highly efficient
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15 catalysts are very sensitive to contaminants, particularly
sulfur contaminants, found in these hydrocarbon feedstocks.
In a~dition to the well known suIfur compounds such as
hydrogen sulfide and mercaptans, the hydrocarbon feedtocks
normally contain a small quantity of carbonyl sulfide (COS).
; ~ 20 Usually COS is present to the extent of only several
hundred parts per million (ppm) by weight. However, even
this small amount is normally beyond the allowable limits
of an acceptable product. Since carbonyl sulfide is almost
always formed when carbon, oxygen, and sulfur or their
25 compounds, such as carbon monoxide, carbon disulfide and
~; the like, are brought together at high temperatures, this
compound is most frequently found in the hydrocarbon
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feedstocks resulting from thermal and/or catalytic cracking
operations, although, in some cases, it has been found in
virgin petroleum fractions.
To some extent, carbonyl sulfide is not as reactive as
5 its companion in hydrocarbons, hydrogen sulfide. According
to Kirk-Othmer's Encyclopedia of Chemical Technology, Vol.
13, pages 384-386, 1954 edition, carbonyl sulfide reac-ts
slowly with the aqueous alkalimetal hydroxides and is only
-slowly hydrolyzed to carbon dioxide and hydrogen sulfide.
10 This relatively unreactive characteristic of carbonyl
. . .
sulfide makes it e~tremely difficult to remove from
petroleum streams by conventional desulfurization
techniques.
The presence of COS, even at very low concentrations,
15 oftentimes renders olefins valueless for many purposes.
For example, high purity olefins are required for the
satisfactory production of many polymeric products,
especially those useful as plastics, including polymers of
ethylene~ propylene, and the like. As a result, there has
20 been a real need to improve techniques for removing COS
from hydrocarbons, especially those used for polymer
production.
Some of the known methods for removing carbon
oxys~lfide (COS) from hydrocarbon streams include the
25 following. In British Patent Specification No. 1,142,339,
published February 5, 1969, the inventors teach a process
~or the removal of COS from gas mixtures in which
unsaturated compounds such as propyne and propadiene are
present, comprising passing said mixtures in liquid phase
30 at atmospheric or superatmospheric pressures over a
substance which contains one or more of the oxides of
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; cadmium, zinc, nickel or cobalt supported on a carrier. It
is stated that this process reduces the COS concentration
to less than one (1) ppm.
~; 3~U.S. Patent No. 4,290,879 to Woodall et al, teaches
the removal of carbonyl sulfide from propane and other
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similar liquified petroleum gas products by mixing liquid
methanol with the untreated liquified gas and subsequently
contacting the liquid mixture with solid potassium
hydroxide. The C3S concentration i~s reduced to less than
5 one (1) ppm by volume.
U.S. Patent No. 3,315,003 to Khelghatian, teache~s that
carbonyl sulfide can be effectively removed from normally
gaseous hydrocarbons by first liquifying the hydrocarbons
and then contacting them with soda-lime. The effluent gas
10 must subsequently be dried to remove the moisture
therefrom.
U.S. Patent No. 3,284,531 to Shaw et al, teaches that
COS can be removed by passing a fluid hydrocarbon through a
- bed of a n anhydrous, weakly ~asict anion exchange resin.
U.S. Patent No. 3,282,831 to Hamm, discloses a method
for removing COS from a hydrocarbon stream by utilizing an
anionic exchange resin which is in the hydroxyl cycle and
which is not fully hydrated.
The problems in purifying propylene and the like
20 olefins are singularly complicated by the nearly identical
boiling points of propylene and COS which makes COS removal
by fractionation unsuitable. As a result, the levels of
COS impurity in propylene stocXs are often times
intolerably high.
Still other disadvantages are encountered in the
heretofore known procedures for the removal of COS from
hydrocarbons; particularly those to be used for olefin
polymerizatlon. For example, some of the established
methods introduce water or other contaminants into the
30 hydrocarbon stream, all of which must be removed by
:~ additional processing in order to place the hydrocarbon in
~ i suikable condition for use. Any such additional
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processin~, as we}l as any requirement to employ elevated
emp~raturos ad~ mat~rlally an~ u~e~lrably ~o ~he ~t
35 the o~eration.
;~ None of the aoove methods can reduce the COS content
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to less than fifty (50) parts per billion (ppb) by weight.
Accordingly, it can be seen that there is a need for a
process to reduce the COS concentration in a hydrocarbon
stream to 50 ppb by weight or lower.
SUMMARY OF THE INVENTION
; The present invention is directed to a process for the
removal of carbonyl sulfide from hydrocarbon feedstocks,
and more particularly from olefinic hydrocarbon feedstocks
containing propylene and from about 1 to 10 ppm by weight
10 of COS. In accordance with the present invention, COS is
removed by passing the hydrocarbon feed over a
conditioned absorbent material preferably comprising nickel
deposited on a support material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to the removal of
carbonyl sulfide (COS), sometimes referred to as carbon
oxysulfide, from liquid hydrocarbon streams. Of particular
interest is the treatment of liquid hydrocarbon streams
containing olefins which streams are to be subsequently
20 subjected to polymerization using polymerization catalysts.
As stated previously, hydrocarbon streams containing
propylene present a special problem for removal of COS by
fractiona-tion because of the nearly identical boiling
~- points of propylene and COS. The present invention is,
25 therefore, particularly useful Eor COS removal from
hydrocarbon streams containing propylene.
The subsequent discussion will describe the invention
in terms o~ treating liquid hydrocarbon feedstocks which
essentially contain a ma~or amount of propylene and minor
30 amounts of propane and impurities such as COS. It should,
however, be unaerstood that the present invention is
applicable to the treatment of liquid hydrocarbon streams
ln general an~ ole~nl~ l1qul~ hy~rocabon ~troam~ ln
,
parti~cular, i.e., hydrocarbon streams con-tainin~ ethylene,
- ~ 35 propylenej butenes or any combination thereof since these
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oleEins will react like propylene when contact-ed with the
absorbent material.
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I' has been found that propylene abscrbs onto the
absorbent material when contacted with the hydrocarbon
feedstocks containing propylene during the COS removal from
said feedstocks and that the propylene absorption reaction
5 is exothermic, occurring to a greater extent during .start
up. Under certain conditions, the termperature rise durinq
propylene absorption may be very important, more
particularly at the surface of the material of which the
temperature may be much higher than that measured with a
- 10 thermocouple, and it may thus damage the ahsorbent material.
In addition the high temperatures cause undesired
side-reactions, more particularly propylene dimerization
and trimerization. The dimers are hexenes which
copolymerize with propylene and break the regularîty of the
15 linear chain of isostatic propylene. As a result, the
copolymer has a lower cristallinity than polypropylene, and
thus a lower melting point; its mechanical resistance is
also lower.
The Applicants have found that an excessive increase
20 in the temperature of the absorbent material can be avoided
by conditioning the material with a minor amount of the
hydrocarbon to be treated. When the hydrocarbon feedstock
contains propylene the conditioning comprises passing over
~- the material an inert gas flow containing a minor amount of
25 propylene.
The conditioning is conducted for a time at sufficient
temperature and pressure under the inert gas flow,
containing the minor amount of propylene, to condition the
~ absorbent materlal without causing an excessive increase in
;- 30 temperature of said absorbent material.
The inert gas, used in the conditioning step is
generally nitrogen. It is important that the inert gas
does not contain oxygen, or contains the least possible
amount of oxy~en, preferably less than 10 ppm.
The propylene contained in the inert gas flow of the
conditioning step can be pure propylene, but most often a
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minor amount of propylene in gaseous Eorm is taken from the
propylene feed that is -to be treated to remove COS. This
propylene feed is polymer grade propylene that contains the
small amount of COS.
It is preferable to begin the conditioning procedure
by passing essentially pure inert gas over the absorbent
material, before introducing a minor amount of propylene in
- the inert gas flow. ~he propylene concentration in the
inert gas flow preferably ranges from about 0.1 to 5 vol %,
10 more preferably about O.S to 2 vol %, with about 1 vol %
being most preferred.
The conditioning step is preferably carried out at
about atmospheric pressure at or below ambient temperature
preferably below about 15C. The conditioning step is
15 continued until the propylene concentration at the outlet
e~uals that introduced indicating that the absorption
reaction is complete. It is also possible to monitor the
conditioning step by the passage of an exotherm, shown by
thermocouples introduced within the absorbent material.
It is Xnown that, when the absorbenk material is
prepared ex situ and stored under a non-oxidizing
atmosphere (usually stabilized under C02), the traces of
oxygen usually present therein have a negative effect on
the properties of the absorbent material. This negative
25 effect can be remedied if, before the above-mentioned
conditioning step, the stored absorbent material is
pretreated by passing a gaseous flow over said material, at
a temperature between about 150 to 250C; preferably at
about atmospheric pressure. The gaseous flow can be
30 entirely inert, however, it is preferred that the gaseous
flow at fir~t be inert followed by a mixture of an inert
.
gas and hydroyen wherein the hydrogen conentration is
gradually increased from 1 to more than 95 vol %.
As above, the inert gas used in the pretreatment is
35 generally nitrogen. In the pretreatment it is also
important that the inert gas does not contain oxygen, or
contains the least possible amount of oxygen, preferably
~ less than 10 ppm.
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In the pretreatment the inert gas flow is continued,
prior to the introduction of hydro~en, until the
concentration of the non-oxidizing gas at the outlet is
sufficiently low (e.~. lower than 0.1 vol %). When said
5 non-oxidizing gas is carbon dioxide, two small exothermal
endothermic temperature variations may be observed during
the pretreatment. Each endotherm being associated with a
rapid increase of CO2 conentration in the vent gas.
Hydrogen is then introduced, first at a concentration of
10 about 1 vol % in the inert gas flow, then at concentrations
- gradually incresing to over 95 vol % while measuring the
bed temperature which should not be allowed to rise above
300~C, preferably not above 250C.
Following hydrogen pretreatment, the absorbent
15 material is cooled under hydrogen flow to ambient
temperaturer purged free of hydrogen with an inert gas
flow, then condition accordin~ to the above conditioning
procedure.
The COS removal process of the present invention
20 reduces the COS concentration in the treated hydrocarbon
feedstocX to 50 parts per billion by weight (ppb) or lower.
The original COS concentration may be as high as 1000 parts
per million by weight (ppm) or higher depending on the
process of making and the origin of the hydrocarbon
25 feedstock. Due to the expense and specialization of the
present inventionl it is preferrd to utilize other less
costly and less complex processes to reduce the COS
concentration to 70 ppm or less prior to treatment with the
absorbent of the present invention.
While~ the subseguent discussion and examples may
describe the absorbent material as a nickel absorbent
~ material, the nickel absorbent material is only preferred
: ~ and should not limit the reasonable scope of the present
inv~n~ion. It i~ envisioned that the pretreatmen~ and
35 conditioning of the present invention would be usefu~ for
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treating any absorbent material tha-t has an excessive
increase in temperature, during COS removal, that could
cause side-reactions and/or damage to the absorbent
material.
The absorbent material of the present invention
preferably comprises nickel deposited on a sup~ort material.
Silica, silico-aluminas, alumina, kieselguhr and other
similar materials can be utilized as the support. When
nickel is used the nickel is preferably present both as
10 metallic nickel and as nickel oxide. The metallic nickel
should constitute from about 35 to about 70 wt.~ of the
total nickel. Prefereably the absorbent comprises from
about 40 to about 70 wt.% total nickel and from about 30 to
about 60 wt.~ support material.
The nickel can be deposited on the support by any of
the several methods well known to those skilled in the art.
For example r nickel can be depositd on the support by
dissolving nickel nitrate in water, mixing the solution
with the support and precipitating the nickel, for example
;;`20 in tne form of nickel carbonate, and subsequently washing,
drying and calcining the precipitate. The nickel deposited
in this manner is then partially reduced by means of
hydrogen to form metallic nickel in a quantit~ of from
about 35 to about 70 wt % of the total quantity of nickel
25 deposited, the remainder being in the form of nickel oxide.
In general, the size of the nickel crystallites after
reduction is from about 10 to about 200 A. The size of
the nickel crystallites depends on the extent of reduction
-carried out. In fact, if the degree of reduction is
30 increased, the si~e of -the crystallites is increased but
the absorbent material obtained does not have the desired
properties. On the other hand, if the degree of reduction
is too low, the crystallites still have good dimensions but
the quantity o~ nickel available in this case i.5 too small
35 to ensure successful purification of the feedstock.
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The specific surface area of the absorbent material
obtained after reduction is generally between 100 and 200
' m2/g.
The particle size o~ the absorbent material depends
5 especially on the pressure loss allowed in the reactor; it
has been noted, however, that it is advantageous to use the
absorbent material in finely divided form. Preferably, the
particle size of this material does not exceed about 3.5 mm
and is most preferably from about 1 to about 2.5 mm.
10 In utilizing the latest generation of Ziegler-type
catalysts in the production of polypropylene, it is
essential that the propylene feedstock contain less than 50
ppb and preferably less than 30 ppb of COS. It has been
unexpectedly found that by passing the propylene feedstock
15 over an absorbent material conditioned according to the
present invention and consisting essentially of from about
40 to about 70 wt.~ nickel deposited on support materials
selected from the group consisting of silica,
silico-aluminas, alumina, kieselguhr and similar materials,
20 wherein the nickel is present both as metallic nickel and
as nickel oxide and wherein the metallic nickel represents
from about 35 to about 70 wt.% of the total nickel, the
feedstock obtained has a COS content not exceedin~ 30 ppb.
This result is unexpected due to the degree of purity
25 obtained and due ~to the fact that this process can be
carried out elther in the presence or absence of water.
In polypropylene production, the liquid hydrocarbon
feedstock generally comprises more than 75 wt.% propylene,
more particularly, from about 85 to about 99 wt.%
30 propylene, and from about 1 to about 10 ppm COS. In one
embodiment of the present invention, the liquid propylene
feedstock is passed over the conditioned absorbent material
at a temperature of from about 0C to about 9oDC and under
sufficient pressure to keep the medium in the liquid phase.
35 The liquid hourly space velocity (LHSV) utilized is from
about 0.1 to about 20 and preferably from 0.2 to about 15.
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- The examples which follow are given in order to
provide a better illustration of the process of the present
invention, but without thereby restricting its scope.
Example I
5 a. absorbent Material
.
An absorbent material was prepared in situ, comprising
43.3 wt.~ of silica as support, on which nickel was
deposited, wherein the nickel is present in the forms of 34
wt.~ of NiO and of 22.7 wt.~ of metallic nickel.
Before reduction, the absorbent material contained
about 49 wt.% of nickel.
The absorbent material was finely divided so as to
obtain particles of about 1 mm average dimension.
The specific area of said material was of 145 m2/g.
15 b. Conditioni ~ Step
A nitrogen flow was passed during 4 hours over the
absorbent material, under atmospheric pressure, at a
temperature of 20C, and with a gaseous hourly space
velocity (GHSV) of 125 l/l.h. During a further 12 hours,
20 the conditioning was continued under the same conditions
with nitrogen containing 1 vol ~ propylene.
c. Purification of the Feed
A liquid hydrocarbon feedstock containing 99 vol % of
propylene, 1.5 ppm of COS and less than 5 ppm (detection
25 limit) of hexenes, was passed on the conditioned absorbent
material, at a temperature of 30C, under a pressure of 1.5
MPa (15 bars) sufficient to maintain the feed in the liquid
phase, and with a liquid hourly space velocity (LHSV) of 10
; l/l.h.
After 5 hours, the purified feed contained 19 ppb of
COS and less than 5 ppm (detection limit) of hexenes.
~- Example II
.
An absorbent material was prepared according to the
procedure ~e~crlbed in Example ~.a. It was stored under
35 carbon dioxide during one month.
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The absorbent material was pretreated by passing a
gaseous flow thereon, at a temperature of 180C and under
atmospherlc pressure, said gaseous flow being formed first
of nitrogen during 14 hours, then of a mixture of nitrogen
5 and hydrogen during a further 24 hours, the hydrogen
concentration therein being increased by about 5 vol % per
hour up to more than 95 vol %. The absorbent material was
cooled under said flow of nitrogen and hydrogen, then
purged free of hydrogen with a nitrogen flow.
The absorbent material was conditioned as described in
Example I.b., and the purification procedure of Example
I.c. was repeated with the conditioned material. Results
similar to Example I were obtained.
Example III Comparative
Example I was repeated with the omission of the
conditioning step I.b. After ~ hours, the purified feed
contained 24 ppb of COS and 200 ppm of hexenes.
Example IV Compar_tive
.
An absorbent material was prepared as described in
20 Example I.a. and stored under carbon dioxide during one
month.
A liquid hydrocarbon feedstock containing 99% of
propylene, 2.7 ppm of COS and less than 5 ppm (detection
limit) of hexenes, was passed on the absorbent material, at
25 a temperature oE 25~C, under a pressure of 1 5 MPa ~15
bars) sufficient to keep the feed in the liquid phase,
and with a LHSV of 5 l/l.h. ~fter 5 hours, the purified
,
feed contained 700 ppb of COS.
Exam~le V
A liquid hydrocarbon Eeedstock containing 99% of
propylene and having a residual COS content of 2.7 ppm was
passed over an absorbent material consisting of 43.3% by
~ weight of siIica as the support, on which nickel was
; ~ deposite~, the nickel being present in the form of NiO to
: 35 the extent of 34% by weight and in the form of metallic Ni
~ to the extent of 22.7% by weight.
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Before reduction, the absorbent material contained
about 49~ by weight nickel.
The absorbent material was finely divided to give an
average particle size of about 1 mm.
5The specific surface area of this material was 145
m2/g
The above mentioned feedstock was thus passed over the
a'osorbent material at ambient temperature, at a sufficient
pressure to keep the feedstock in the liquid phase (15
10 bars), and at an LHSV of 5 l/l.h.
The purified feedstock had a COS content of 18 ppb.
Æxample VI
A liq~id hydrocarbon feedstock containing 99 wt.~
propylene and having different residual COS content was
15 passed over the same absorbent ma-terial as in Example V.
The nickel containing absorbent material had a nickel
content of about 49% by weight. The absorbent material was
finely divided so as to give an average particle size of
about 1 mm. The specific area of this material was about
20 145 m2/g.
The feedstock was passed over said nicXel containing
material under various operating conditions, which are
indicated in Table I.
As can be seen from the result~, the purified
25 feedstock had a COS content lower than 30 ppb, even when
the feed contained water, which is known to be detrimental.
Table I
30 LHSV Temperature H20 Content COS
bed (C) (ppm) in out
__ _ _ __ _ ~ _ ppm ppb
4.95 20 13 1.8 22
-~ 355.05 25 8 4.5 20
4.8 23 8 3.1 18
9.3 16 14 1.85 15
15.05 15 14 1.3 24
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Example VII
A liq~id hydrocarbon feedstock con-taining 95.6 wt.~
propylene, 3.8 wt.~ propane and 0.6 wt.~ C4, the water
content of which being less than 10 ppm, and having
5 different residual COS content was passed over the same
absorbent as described in Examples V and VI except that the
, particles had an average diameter of 3.2 mm. This example
is given to illustrate the activity of the catalyst over a
long period of time.
10The feedstock was passed under a pressure of 14 bar
over a bed containing 2 liters of a nickel containing
' absorbent material.
The other operating conditions such as LHSV and
temperature bed are indicated in Table II.
Table II
:
DayTemperature LHSV COS
bed (C) in out
, 20 ppm ppb
1 14 9.4 2.8 25
9 9.3 1.4 23
12 6 9.7 4.2 21
, 25 19 7 9.7 2.55 20
9.7 3.0 11
3~ 7 9.75 1.9 16
"' 39 2 9.85 1.85 23
52 9 9.6 0.85 20
30 58 3 10.15 0.8 22
' 68 11 9.65 2.2 20
'' 82 6 9.75 1.95 15
88 1 9.8 0.8 15
35 This example shows that even after 88 days the activity of
',, the catalyst remained,very high.
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