Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR REMO~rING CARBO~YL-SULFID~ PROM LIOUID
HYDROCARBON EEEDSTOCKS
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~IELD OF THE INVENTION
The present invention relates to a process for
removing sulfur, present in the form of carbon oxysulfide
or carbonyl sulfide, from liguid hydrocarbons. ~,ore
particularly, ~he present invention relates to a process
5 for the removal of carbonyl sulfide from hydrocarbon
feedstocks containing propylene and to the conditionin~ of
.he absorbent material use~ in the process.
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~ACKGROUND OF THE INVE~TION
Industrial applications of liquid hydrocarbons and
O particularly, liquified olefinic hydrocarbons, have become
more increasingly specialized. The technolo~y as presen.ly
developed utilizes highly efficient ca~alysts to convert
these liquified hydrocarbon feedstocks into final product
such as polymers. ~owever, these highly efficient
5 ~atalysts are very sensitive to contaminants, particularly
4~ ur contaminants, found in these hydrocarbon feeds~ocks.
In aadi.ion to the well known sulfur compounds such 2s
hydro~en sul.~ide and mercaptans, the hydrocarbon feedtocks
normally contain a small quantity of carbonyl sul ide (COS).
O Usu.~lly 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
S compounds, such as carbon monoxide, carbon disulfide and
the like, are brough~ to~ether at high temperatures, this
compound is most freouently found in the hydrocarbon
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eedstocks resulting from thermal and/or catalytic cracking
operationsl although, in some cases, it has been found in
virgin petroleum fractions.
To some extent, carbonyl sulfide is not as reacti~e as
its companion in hydrocarbons, hydrogen sulfide. According
to Rirk-Othmer's Encyclopedia of Chemical Technoloay, Vol.
`13, pages 384-386, 1954 edition, carbonyl sulfide reacts
slowly with the aqueous alkalimetal hydroxides and is only
slowly hydrolyzed to carbon dioxide and hydrogen sulfide.
) This rel2~ively unreactive characteristic of carbonyl
sulfide makes it extr~mely~difficult to remove from
petroleum streams by conventio-nal desul~urization
techniques.
The presence of COS, even at very low concentrations,
oftentimes renders olefins valueless ~or many purposes.
For example, high purity olefins are reguired for the
satisfactor~ production of many polymeric products,
especially those useful ,~s plastics, including polymers of
ethylene, propylene, and the like. As a result, there has
O b~n a real need to improve techniques for removing COS
~rom hydrocarbons, especially those used for polymer
produc~ion.
Some of the known methods for removing carbon
oxysulfide (COS) from hydrocarbon streams include the
~5 following. In British Patent Specification No. 1,142,339,
published February 5, 1969, the inventors teach a process
for the removal of COS from gas mixtures in which
unsaturated compounds such as propyne and propadiene are
present, comprising p2ssing said mixtures in liquid phase
30 at atmospheric or superatmospheric pressures over a
subs.ance which contains one or more of the oxides of
cadmium, zinc, nickel or cobalt supported on a carrier. It
is stated that this process re~uces the CO5 concentration
to less than one (1) ppm.
U.S. Patent No. 4,290,879 to Woodall et al, teaches
~e remo~21 of carbonyl sulfide from propane and other
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similar liq~ified petrole~m gas products by mixing liguidmethanol with the untreated liquified gas and subsequently
contacting the liquid mixture with solid potassium
hydroxide. The COS concentration is reduced to less than
, one (1) ppm by volume
~ .S. Patent No. 3,315,003 ~o ~C)lelgha~1an, t~che~ ~hAt
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
must subsequently be dried to remove the moisture
therefrom.
~ .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 basic, anion exchange resin.
.S. Patent No. 3,282,831 to ~amm, discloses a method
for removing CoS from a hydrocarb~n 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 'he like
3 olefins are sin~ularly complicated by the nearly identical
~iling points o8 propylene and COS which makes COS removal
by frac'ionation unsuitable. As a result, the levels of
COS impurity in propylene stocks are often times
intolerably high.
Still other disadvantages are encountered in the
heretofore known procedures for the removal of COS ~rom
hydrocarbons, particularly those to be used for olefin
polymerization. For example, some of the established
methods introduce water or other contaminants into the
0 hydrocarbon stream, all of which must be removed by
additional processing in order to place the hydrocarbon in
suitable condition for use. Any such additional
processing, as well as any requirement to employ elevated
temperatures adds materially and undesirably to the cost of
:5 the operation.
None of the above me~hods can reduce the COS content
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to less than fifty (50) parts per billlon (ppb) by weight.
Accordingly, it can be seen that there ls a need for a process to
reduce the COS concentration in a hydrocarbon stream to 50 ppb by
weight or lower.
S~MMARY OF THE INVENTION
The present invention is dlrected to a process for the
removal of carbonyl sulfide from hydrocarbon feedstocks, and more
particularly from olefinlc hydrocarbon feedstocks containing
propylene and from about 1 to 10 ppm by weight of COS. In
accordance with the present invention, COS 1s rPmoved by passlng
the hydrocarbon feed over a condltioned absorbent material
preferably comprislng nickel depos~ed on a support materlal.
DETAILED DESCRIPTION-OF THE PREFERRED EMBODIMENTS
.
The present invention relates to the removal of carbonyl
sulfide (~OS), sometlmes referred to as carbon oxysulfide, from
liquid hydrocarbon streams. of partlcular interest is the
treatment of llquid hydrocarbon streams containing oleflns which
streams are to be subsequently sub;ected to polymerization using
polymerization catalysts. AS stated prevlously, hydrocarbon
streams containing propylene present a special problem for
rempval of COS by fractionation because of the nearly identical
boiling points of propylene and COS. The present invention ls,
therefore, particularly useful for COS removal from hydrocarbon
streams containing propylene.
The subsequent dlscusslon will describe the inventlon ln
terms of treating ll~uld hydrocarbon feedstocks which essentially
contaln a ma~or amount of propylene and minor amounts of propane
and lmpurities such as COS. It should, however, be understood
that the present invention is appllcable to the treatment of
liquld hydrocarbon streams ln general and oleflnlc liquld
hydrocabon streams ln general and oleflnlc llquid hydrocabon
streams ln partlcular, l.e., hydrocarbon streams containlng
ethylene, propylene, butenes or any combinat.ion thereof since
these olefins wlll react like propylene when contacted wlth the
absorbent material.
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It has been found that propylene absorbs onto th~
absorbent material when contacted with the hydrocarbon
feedstocks containing propylene during the COS removal from
said feedstocks and that the propylene absorption reaction
, is exothermic, occurring to a~greater extent during start
up. Under certain conditions, the termperature ri~e durinq
propylene absorption may be very important, more
particularly a~ the surface of the material of which the
temperature may be much higher than that measured with a
O thermocouple r and it may thus damage the absorbent material.
In addition the hig~ temperatures cause undesired
side-reactions, more particularly propylene dimerization
and trimerization. The dimers are hexenes which
copolymerize with propylene and break the regularity of the
linear chain of isos~atic propylene. As a result, the
copolymer has a lower cristallini.y than polypropylene, and
thus a lower melting point; its mechanical resistance is
also lower.
~ ' The Applicants have found that an excessive increase
'O ~ the temperature of the absorbent material can be avoided
by concitioning the material with a minsr amount of ~he
hyàrocarbon to be treated. ~en the hydrocarbon feedstoc~
contains propylene the conditioning comprises passing over
the materi21 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 rninor amount o propylene, to condition the
~bsorbent material without causing an excessive increase in
30 temperature of said absorbent material.
The inert g2s, 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 oxygen, preferably less than 10 ppm.
The propylene contained in the inert gas flow of the
conditlonlnq step can be pure propylene, but most often a
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mlnor amount of propylene in gaseous form ls taken from the
propylene feed that is to be treated to remove COS. This
propylene feed is polymer grade propylene that contalns the small
amount of COS.
It is preferable to begln the conditloning procedure by
passlng essentially pure lnert gas over the absorbent material,
before lntroducing a minor amount of propylene 1n the inert gas
flow. The propylene concentration in the inert gas flow
preferably ranges from about 0.1 to 5 vol ~, more preferably
absut 0.5 to 2 vol ~, with about 1 vol % belng most preferred.
The conditionlng step is preferably carrled out at about
atmospherlc pressure at or below ambient temperature preferably
below about 15C. The copdltioning step is continued until the
propylene concentration at the outlet equals that introduced
indicating that the absorption reaction ls complete. It ls also
possible to monltor the conditloning step by the passage of an
exotherm, shown by thermocouples introduced wlthln the absorbent
material.
It is known that, when the absorbent material is prepared ex
situ and stored under a non-oxidizlng atmosphere (usually
stabllized under CO2), the traces of oxygen usually present
thereln have a negatlve effect on the propertles of the absorbent
ma~erlal. This negative effect can be remedled if, before the
above-mentloned conditionlng step, the stored absorbent materlal
ls pretreated by passlng a gaseous flow over sald materlal, at a
temperature between about 150 to 250C; preferably at about
atmospherlc pressure. The gaseous flow can be entirely inert,
however, lt ls preferred that the gaseous flow at first be inert
followed by a mixture of an inert gas and hydrogen wherein the
hydrogen conentration is gradually increased from 1 to more than
95 vol %.
As above, the inert gas used in the pretreatment ls
generally nltrogen. In the pretreatment it is also important
that the lnert gas does not contain oxygen, or contains the least
posslble amount of oxygen, preferably less than 10 ppm.
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In the pretreatment the inert gas flow is contin~ed,
prior to the introd~c~ion of hydrogen, until the
concentration of the non-oxidizing gas at the o~tlet 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 d~ring
the pretreatment. Each endotherm being associated with a
rapid increase of Co2 conentration in the vent gas.
Hydrogen is then introd~ced, first at a concentration of
10 about 1 vol % in the inert gas flow, then at concentrations
gradually incresing to over 95`vol ~ while meas~ring the
bed temperature which should not be allowed to rise above
300C, preferably not above 250C.
Following hydrogen pretreatment, ~he absorbent
15 material is cooled under hydrogen flow to ambient
temperature, purged free of hydrogen ~ith an inert gas
flow, then condition according to the above conditioning
proced~re.
The COS removal process of the present invention
'0 red,uces the COS concentration in the treated hydrocarbon
~edstock to 50 parts per billion by weight (ppb) or lower.
The original COS concentration may be as hi~h 2S 1000 parts
per million by weight (ppm) or higher depending on the
process of making and the origin of the hydrocarbon
~5 feedstock. Due to the expense and specialization of the
present invention, 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 subseq~ent 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
invention. It is envisioned that the pretreatment and
35 conditioning of the present invention would be useful for
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treating any absorbent material that has an excessive
increase in temperature, during COS removal, tha~ could
cause side-reactions and~or damage to the abs~rbent
material.
The absorbe"t material of the present invention
preferably comprises nickel deposited on a support 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
;0 metallic nickel and as nickel oxide. The metallic nickel
should constitute from about~35 to about 70 wt.~ of the
total nickel. Preferèàbly the absorbent comprises from
about 40 to about 70 w~.% 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, nickel can be depositd on the support by
dissolving nickel nitrate in water, mixing the solution
wi.h the support and precipitating the nickel, for example
20 in the form of nickel carbonate, and subseauently washing,
~rying and calcining the precipitate. The nickel deposited
in this manner is then partially reduced by means of
hydrogen to form metallic nickel in a quantity of L rom
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 cf 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 ou~. In fact, if the degree of reduction is
30 increased, the size 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 of nickel available in this case is 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 be~ween 100 and 200
m2/9 .
The particle size ~f the absorbent material depends
5 especially on the pressure 105s 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.
0 In utilizing the latest ~eneration of Ziegler-type
catalysts in the produGtion of polypropylene, it is
essential that the propylene feedstoick contain less than 50
ppb and preferably less than 30 ppb of COS. It has been
unexpectedly found that by passing the propylene feedstock
5 over an absorbent material conditioned according to the
present invention and consisting essentially of from about
40 to abou, 70 wt.% nickel deposited on support materials
selected ,rom the group consisting of silica,
~ilico-alumin2s, alumina, kieselguhr and similar materials,
~o w~e~ein the nickel is present bo~h as metallic nickel and
.. as nickel oxide and wherein the metallic nickel represents
from about 35 to abou~ 70 w~.~ of the total nickel, the
feedstock ob~ained has a COS content no~ exceeding 30 ppb.
This result is unexpected due to the degree of purity
'~ obtained and due to the fa~t that this process can be
carried out either in the presence or absence o~ water.
In polypropylene production, ~he liquid hydr~carbon
feedstock generally comprises more than 75 wt.% propylene,
more particularly, from about 85 to about 99 wt.~
30 propylene, and from about 1 to abdut 10 ppm COS. In one
embodiment of the presen. inven~ion r the liquid propylene
feedstoc~ is passed over the condi.ioned absorbent material
at a temperature o from about 0C to abou~ ~O~C and under
sufficient pressure to keep the medium in the liquid phase.
35 The liquid hourly space veloclty (LBSV) utilized is from
about 0.1 to about 20 and preferably from 0.2 to abo~t 15.
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The examples which follow are given in order to
provide a better llustration of the process of the present
invention, but without there~y restricting its scope.
Exa~Dle 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 i5 present in the forms of 34
wt.% of NiO and of 22.7 wt.~ of metallic nickel.
10Before 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.
5 b. Conditlon_na Step
A nitrogen flow was passed during 4 hours over the
absorbent material, under atmospheric pressure, at a
tempera~ure of 20C, and with a g2seous hourly space
yelocity (GHSV) of 125 l/l.h. During a further 12 hours,
'0 the conditioning W2S continued under the same conditions
wlth ni.rogen containing 1 vol % propylene.
c. uri ication of .he Feed
A liquid hydrocarbon feedstock conteining 99 vol % of
propylene, 1.5 ppm of COS and less than 5 ppm (detection
'5 limit) of hexenes, W2S passed on the conditioned absorbent
material, at a temperature of 30DC, 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.
~0After 5 hours, ~he purified feed contained 19 ppb Or
C~S and less than 5 ppm ~detection limit) of hexenes.
ExamDle II
An absorbent material w~s prepared according to the
procedyre ~escribed in ~xample I.a. ~t was stored under
,5 carbon dioxide during one month.
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The absorbent material was pretreated by passing a
gaseous flow thereon, at a ~emperat~re of 180-C and under
atmospheric pressure, said g~seous flow being formed first
of nitrogen during 14 hours, ~hen of a mixture of nitrogen
and hydrogen during a further 24 hours, the hydrogen
concentration therein being increased by about 5 vol ~ per
ho~r 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.
0 The absorbent material was conditioned as described in
Example I.b., and the~rification procedure of Example
I.c. was repeated with the conditioned material. Results
similar to Example I were obtained.
Exam~le III Com~arative
!5 Example I W25 repeated with the omission of the
conditioning step I.b. After 5 hours, the purified feed
'contained 24 ppb of CoS and 20G ppm of hexenes.
Exam~le IV Com~arative
. An absorbent material was prepared as aescribed in
20 ~ample I.a. and stored under carbon dioxide during one
month.
- A liGuid hydrocarbon feedstock containing 99~ of
propS~lene, 2.7 ppm of COS and less than 5 ppm (detection
limit) of hexenes, W2S passed on the absorbent material, at
25 a tempera~ure of 25~C, under a pressure of 1.5 MPa (15
bars) sufficient to keep the feed in the li~uid phase,
and with a ~HSV of 5 l/l.h. After 5 hours, the purified
feed contained 700 ppb of COS.
Exam~le V
A liguid hydrocarbon feedstock 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 silica as the support, on which nickel was
deposited, 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 e~'ent of 22.7~ by weigh~.
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Before red~ction, the absorbent material contained
aboUt 49% by welght nic~el.
The absorbent material was finely divided to give an
avera9e particle size of abo~t 1 mm.
The specific s~rface area of this material was 145
m2/g .
The above mentioned feedstock was thus passed over the
absorbent material at ambient temperature, at a sufficient
press~re to keep the feedstock in the liquid phase (15
) bars), and at an ~HSV of 5 l/l.h.
The purified feedst~ck had a COS content of 18 ppb.
ExamPle VI
A liq~id hydrocarbon feedstock containing 99 wt.~
propylene and having different resid~al COS content was
passed over .he same absorbent material 2s in Example V.
The nickel containing absorbent material had a nickel
con-ent of abo~t 49% by weight. The absorbent material was
finely divided so as to give an average particle size of
a,bou' 1 mm. The specificiarea of this material was abou~
0 145 m2/g.
The feeds~ock w2s pzssed over said nickel containing
material under ~arious operating conditions, which zre
indicated in Table I.
~ s can be seen from the results, the purified
'5 feedstock had a COS content lower than 30 ppb, even when
the feed contained water, which is known to be detrimental.
Table I
30 LBSVTemperature B20 Content COS
bed (rC)(ppm) in out
P~m PPb
.
4.95 20 13 1.8 22
355.05 25 8 4.5 20
4.8 23 8 3.1 1B
9.3 16 14 1.85 15
15.C5 15 14 1.3 24
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Example VII
A liquid hydrocarbon feedstock containing 95.6 wt.%
propylene, 3.8 wt.~ prop~ne and 0.6 wt.~ Cq, the water
content of whiCh being less than 10 ppm, and having
different residual COS content was passed over the same
absorbent as described in ~xamples V and VI except that the
particies 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.
The feedstoc~ was passed under a pressure of 14 bar
over a bed containing ~liters of a nickel containing
a~sorbent material. -
The other operating conditions such as LRSV andempera!ure bed are indicated in Table II.
Table II
~Day Temperature LRSV COS
bed (C) in o~t
~m ~b
; . 14 9.4 2.8 25
~ 9 9.3 1.4 23
12 6 9.7 4.2 21
19 7 9.7 2.55 20
9.7 3.0 11
34 7 9.75 1.9 16
39 2 9.85 1.85 23
52 9 9.6 0.85 20
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
This example shows that even after 88 days the activity of
the catalyst remained very high.
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