Note: Descriptions are shown in the official language in which they were submitted.
3Çi
~3209 -1-
10 METHOD FOR REMOVING CARBONYL
5ULFID~ IN GAS TREATING PROCESSES
~ackground of the Invention
. The present invention relates to a vapor liquid
absorption process for the treatment of gas stxeams such
as natural gas streams, and particularly to the treatment
20 of such gas streams ~or elimination of caxbonyl sulfide by
its conversion to hydrogen sulfide and removal at least o~
the formed hydrogen sulfide.
In ~he processing of natural gas ~treams and the like
for the removal of the acid gases hydrogen sulfide and
25 carbon dioxide, alone or.in combination, a wide variety of
absorbents can be used. Typically in such processe~, the
gas stream is passed countercuxrent to the liquid flow of the
absorbent in an absorption tower such that the gas stream
rich in the impurity to be removed is initially contacted
30 with nearly or partially spent absorbent; while as the
gas progresses through the absorption tower~ it meets an
increasingly lean absorbent having a greater absorptive
potential for the impurity, As a consequence/ a high degree
of ex~rao~iGn of a gaseous constituent can be realized. -
35 The spent absorbent from the base vf ~he absoxption toweris no~mally passed to regenerakion ox stripping facilities
~ 3
13209 -2-
l for removing the absorbed acid gases to enable re~ycling
of the absorbent to the system. A~sorbents may be lost
by entrainment in the gas stream during regene~ation
practices; by virtue of irreversible reactions with
some of the constituents and by normal degradation.
A particularly difficult to remove sulfur compound
is carhonyl sulfide and is an undesirable constituent
ln most gas streams containing it, including natural
gas streams.
io In U.S. Patent 3,961,015 to Dailey, there is described
a continuous process for txea~ing natural gas containing
carbonyl sulfide, carbon dioxide and hydrogen sulfideO
The process comprises the step of contacting the gas
stream in countercurrent flow with an aqueous ethanol-
15 amine solution as an absorbent for carbon dioxide
- and hydrogen sulfide in a first absorbtion zone at
a net temperature at which the capacity o the absorbent
for hydrogen sulfide and carbon dioxide per unit
volume is high. The gas stream is passed from the
20 first absorbent zone to a second absorption-reaction
zone where it is brought into countercurrent contact
with an aqueous ethanolamine solution maintained at
a higher temperature and at a temperature at which
carbonyl sulf ide will hydrolyze in the presence of water
25 present or provided by the gas stream to hydrogen
~ulfide and carbon dioxide. Reaction occurs with
some absorption of the formed hydrogen sulfide and carbon
dioxide.
The gas stream is then passed to a third absorptiQn
30 zone where it is placed in countercurrent contact with
a cooled, lean aqueous ethanolamine solution for
absorption of the residual formed hydrogen sulfide
and carbon dioxide either present in the initlal gas
feed or formed in the absorption-reaction zoneO
3~ The solutions from the third absorption zone and the
36
.
1320g -3
1 second absorptlon~reaction zone are then combined to
become the absorptlon solution in the irst absorption
zone.
. The practice of the invention requires the ùse of a
5 single absorption medium, albeit at different temperatures,
capable of not only absorbing the undesired original
constituents but also the reaction products of one or
more of the original constituents.
Another limitation in the practice of U.S~ Patent
10 No. 3,961!015 is the limitation on the quantity and
temperature of the hot absorption reactive medium that
can be used in the second stage. If the temperature i~
too high or the volume too great, the first-stage
absorption medium is less effective in its capacity to
15remove the acid gases, increasing thereby the total
requirement of the circulating ab~orption medium.
- There is a need, therefore, ~or a process ln which
a hot intermediate reaction absorption mPdium does not
affect ~he overall operating capacity ~or the sys~em~
20increase circulation rates and tower sizesO
Summary of the Invention
The present invention pertains to improvements in
vapor-liquid absorption systems for treatment of gas streams
25containing carbonyl sulfide. ~he improvement resides in
providing a closed-loop hydrolysîs system containing an
aqueous alkanolamine hydrolysIs medium prior to a stage
of absorption or between stages o~ absorption in which the
gas stream containing carbonyl sulfide is ~ed and where
30hydrolysis of carbonyl sulfide into absorbable constituent~
occurs, and from which the gas stream sweeps the products
of the hydrolysis into an absorption stage where absorption
occurs.
More particularlyp the present invention provides a
3scon~inuous process or purifying gas streams contai~ing
.,
~g~36
13209 -4-
1 carbonyl sulfide and as acid gas constituents, hydrogen
sulfide or mixtures of hydxogen sulfide and carbon
dioxide.
As an essential step of the prscess,.the gas stream
5 to be treated is passed ~hrough a circulating closed-loop
hydrolysis zone containing an aqueous alkanolamine
hydrolysis medium capable of accepting and promoting
thermal hydrolysis of car~onyl sulfide maintained at a
temperature sufficient to hydrolyze substantially all o~
the carbonyl sulfide to form a resultant gas stream
containing the formed hydrogen sulfide and carbon dioxide.
~ he resultant gas stream is then passed ~rom the
hydrolysis 20ne to an absoprtion zone containing an
absorbent for and which is lean with respect to at least
hydrogen sulfide where the foxmed hydrogen sulfide alone
or with carbon dioxide is removed from the gas ~tream.
As a first step in the process~ the gas stream may be
passed through a first absorptio~ zone containing an
absorbent for at-least hydrogen sulfide, preferably in
zocountercurrent flow to the absorbent, to extract
substantially all of the hydrogen sulfide presen~ in the
gas stream t~ leave a gas stream substantially free of
hydrogen sulfide but still containin~ carbonyl sulfide.
Carbon dioxide~ if present, may, depending on ~he a~sorbent
25U5ed, be extracted with the hydrogen sulfide. If this
step is employed; the absorbent of the absorption zone
following hydrolysis is preferably the same as the
first and passed from the second absorption ~one to the
first absorption zone. The spent absorbent from the
30process is normally stripped of the acid gases and
recycled back to the processO
~ 9~
13209 _~_
Drawings
~ IG. 1, illustrates a simplified apparatu~ which may
be used to carry out the practice of the invention~
FIG. 2, is a modifica~ion of FIG. l, illustrating a
5 system for heating the gas ~tream prior to ~ntroduction
to the hydrolysis zone and for cooling the gas stream
containing the products of reaction before it enters the
second absorption zone.
,
. ' , . ~, ' . ' ' '
.
. , ' " ' '
s
l36
13209 -6
1 etailed Description
According to the present invention, there is provided
a continuous proces 5 for treating of gas streams wherein
one constituent, in the present instance carbonyl sulfide~
then passed through an essentially closed-loop hydrolysis
zone containing an aqueous alkanolamine hydrolysis medium
for promoting hydrolysis of carbonyl sulfide and maintained
at a temperature sufficient to cause hydrolysis of carbonyl
sulfide into the readily absor~able constituents, hydrogen
10 sulfide and carbon dioxide, which are swapt from the
hydrolysis zone by gas ~low into a following absorption
zone where the products of the hydrolysis reaction, namely
hydrogen sulfide and carbon dioxide~ may be readily
removed from the gas stream.
An essential step of the process of the invention i5
to employ in the closed-loop hydrolysis zone an "aqueous
alkanolamine hydrolysis medium" which means an aqueous
alkanolamine composition in equilibrium wi~h carbon dioxide
nd hydrogen sulfide, the products of hydrolysis, and
20 which will take up by some mechanism carbonyl sulfide and
promote its hydrolysis to hydrogen sulfide and carbon
dioxide at the ~emperlture employed during its residence
in the medium. Also essential is that the medium be
contained in a sepa~ate closed-loop reaction zone and not
25 intermixed with the absorbents used in the following
absorption zone and, if used, a preceding absorption æone.
In operation, the products of hydrolysis are cantinuously
removed from the aqueous alkanolamine hydrolysis medium
such that the capacity of the medium to promote hydrolysis
30 will not be lost, nor will the medium ~e irreversibly
consumed. Although not`required, the medium can ~e the
same absorbent as that employed in other absorp~ion zones
used in the processO
The presently contemplated aqueous alkanolamine
35 hydrolysis medium includes ethanolamine solutions such a5
~9~6
13209 ~7~
1 solutions of monoethanolamine, diethanolamine, methyldi-
ethanolamine, propanolamine, diisopropanolamine and the like~ -
The absorbents employed have a limited capacity for
carbonyl sulfide and provide the alkaline conditions
essential to promote its hydrolysis. This condition may
require pxomotion with a base or the like. Typically,
the alkanolamines can comprise from 1% to about 50% by
weight of the aqueous alkanolamine hydrolysis medium.
The ethanolamines are preferred.
An ethanolamine which may be used directly is an aqueous
solution of diethanolamine, preferably one containing from
about 10% to a~out 30% by welght diethanolamine. If
monoethanol~mine is employe~, it i5 intrc~uced with an
alkali metal hydroxide or salt such as sodium hydroxide,
15 potassium hydroxide, sodium carbonate, potassium carbonate
and the like, which revert to the carbonate or bicarbonate
state to inhibit or pr~vent irreversible reaction between
the carbonyl sulfide and the monoethanolamine. A typical
solution is an aqueous solution containing from about
~0 10% to abou~ 20% by weight monoethanol~nine and from about
2% to about 5% by weight OL an alkali metal hy~roxide
or alkali metal salt, or their mixtures, reported as the
alkali metal hydroxide, the balance being water.
It is presently preferred that the aqueous alkanolamine
25 hydrolyis medium be at a pH from about 8 to about 12
maintained at a temperature from about 150F to about
300F, preferably rrom about 150F to about 280Fo
The absorption solutions employed for extraction of the
acid gases in the absorption towers may be the same or
30 different than the aqueous aklanolam~ne hydrolysis m~dium. ,
L36
13209 -8-
1 ~he~ may have the capacity to absorb hydrogen sulfide
and carbon dioxide or be selective to the absorption of
hydrogen sulfide. Selective absorbents include Selexol,
a mixture of dimethyl ethers of propylene glycol,
diisopropano~amine; methyl diethanolamine and the like.
The absorbents are normally employed at ambient temperature
to maximize their absorpti~e capacity.
While the gas stream to undergo treatment may ~e fed
directly to a hydrolysis reactor containing the aqueous
i0 alkanolamine hydrolysis medium, more typically as
illustrated in FIGS. 1 and 2, the ~as stream is pre- .
processed to remove free hydrogen sulfide alone or with
carbon dioxide if present.
With reference now to FIG. 1 in the process of the
15 invention, the gas stream to undergo treatment may pass by
line 10 to the base of absorption tower 12~ then by line 14 j~
to the closed-loop hydrolysis system containing in
hydrolysis reactor 16 the aqueous alkanolamine absorption .
medium, and then following hydrolysis of carbonyl sulfide,
20 by line 18 to the second absorption tower 20. Lean
absorbent enters the second absorption tower by line 22,
collects at its base 24, and passes ~y liq~lid seal ~6 to
absorption tower 1?, and is removed by line 28 as spent
absorbent for regeneration. The treated gas leaves by
25 line 30.
If absorption tower 12 is eliminated the fe~d gas passes
directly to hydrolysis reactor 16 and the spent solution
from absorption tower 20 is regenerated for recycle~
If desired, separate absorbents may be employed in
30 towers 12 and 20 necessitating separate regeneration
facilities with the eliminatîon of flow communication
between the towers.
, .
3S
36
1320~ _g_
1 As an alternate embodiment, only a portion of the lean
absorbent may be fed to tower 20, with the balance ~o
tower 12.
The instant invention will be described in greater
5 detail in terms of the treatment of gas streams, such
as natural gas streams, containing readily removable acid
gas constituents such as hydrogen sulfide and carbo-n
dioxide and the more difficult to remove the sulfur species,
carbonyl sulfide.
I0 With reference to FIGS. 1 and 2, the feed gas, such as
natural gas, is fed to the base of absorption tower 12
countercurrent ~o the flow of an absorbent liquid there-
through. Absorption tower 12 as well as absorption tower
20, are vessels containing trays or packing and stagewise
15 contact is made by the countercurre~t flow of gas and
absorbent. Lean, or relatively lean absorbent enters
at the top of each ~tower, absorbs one or both of the
acid gas components from the gas stream as lt flows downward
through the trays or packing, and spent absorbent leaves
20 by line 28 to a regeneration, reactivation, flashing or
stripping section (not shown) where the acid gases are
removed and the lean absorbent retu-.ned for service by its
addition in line 2~ to upper absorption tower 2~ Where
vessels contain packing, the packing is frequen~ly divided
25 into two or more beds of packing to reduce mechanical loads
which would tend to crush the packing at the bo~tom of ~he
tower as well as provide for redistribution o th~ absorbent
10wing downward, which in tall packed sections, may have a
tendency to flow in channels rath~r than egually dLstributed
30 across the cross~section of the packed towerO
In the practice of the invention, the gas flowing upward
to absorption tower 12, is deple~ed of the bulk o~ ~he acid
gases which are absorbable ln the absorbent used. Massive
absorption of ~he absorbable acid gas cons~ituen~s origin-
35 ally prcsen~ in the gas prior to allowing the ga~ to
~ ~ ~9~ 3
132~9 10-
1 enter the hydrolysis reaction zone 16 is required to permit
more complete hydrolysis of carbonyl sulfide. Carbonyl
sulfide will hydrolyze in accordance with .~he reaction-
. ~OS t ~2 = ~25 ~ C2
The reaction is reversible but favored in the forwarddirection at the temperatures employed, as indicated by
the equilibrium constants at various temperatures:
io
;. . ~.
' lH25] [C2]
1COS j IH201
15 .
where~ `
.
at 100F K ~ 63,330
20 at 200F K ~ 12~951
at 300~ X = 3,272
Despite Eavorable equilibrium driving the r~action
25forward or to the risht, the reaction proceeds very slowly
at low temperatures. At higher temperatures and in an
aqueous alkanolamine hydrolysis medium~ the reaction
proceeds more rapidly and will go to essential completion
if the initial concentrations of hydrogen sulfide or
30carbon dioxide are not too high~ ~his may necessitate
mass removal of the bulk of at least one of these
constituents, usually hydrogen sulfide, in tha first
absorption tower 12.
L3E;
13209
1 The gas then leaves bulk absorption tower 12 and .
passes by line 14, with or without heat exchange to
hydrolysis reaction zone 16 for contact with, preferably
in countercurrent flow, the aqueous alkanolamine hydrolysis
S medium. Contact is for a time sufficient to achieve
essentially complete hydrolysis of carbonyl sulfide.
Residence times of from about 2 to about 5 seconds, depend~
ing on temperature, will suffice. '
~owever, in many commercial installations the hydrogen
10 sulfide and carbon dioxide appear, in .the gas to ~e
treated, in concentrations sufficiently low as not to
inhibit materially the''completeness of the hydrolys.is
reaction rate, with reaction rate rather than equiiibrium
being the controlling fac~or. 'In such instances the first
15 absorber 12 for the bulk of hydrogen sulfide, carbon
dioxide, or both, is eliminated. The gas with or without
heat exchange is fed directly to hydrolysis reactor 160
With re~erence 'to FIG5. 1 and 2, the aqueous alkanolamine
hydrolysis medium entsrs hydrolysis reactor 16 by line 40
20 at a temperature suitable to the hydrolysis of carbonyl
sufide, passes downward in countercurrent flow to the gas
stream and exits at the Dase by line 42. It is pumped by
pump 44 through heat exchanger 46 which adds heat as
required to maintain the aqueous alkanolamine hydrolysi's
25 medium at the desired operating temperature for use in
hydrolysis reactor 16. The agueous alkanolamine hydrolysis
medium returns to hydrolysis reactor 1~ by line 42 to close
the loop. A suitable heating medium is steamO
Because the aqueous alkanolamine hydrolysis medium is in
30 equilibrium with hydrogen sulfide and carbon dioxide; as . ..
well as the other consti~uents of ~he gas stream~ the
absorbent is not consumed and regeneration is not required.
In operation, the products of hydrolysis leave with the
flow of process gas ~hrough hydrolysis reactor 160
.
.
3136
13209 -12-
1 Stated another way, as the gases pass up through
hydrolysis reactor 16 in countercurrent to the aqueous
alkanolamine hydrolysis medium, carbonyl sulfide reacts
with water present in the gas stream and/or in the aqueous
alkanolamine hydrolysis medium and is converted to hydrogen
sulfide and carbon dioxide which are stripped by the
flowing gas from the aqueous alkaline hydrolysis medium,
except for the small equilibrium quantities which remain
dissolved in the aqueous alkanolamine hydrolysis medium at
lO the operating conditions.
The reaction products, along with the balance of the
gas stream, leave through line 18 and pass to absorption
tower 22 in countercurrent flow to the absorbent fed
thereto which serves to strip one or both of the acid
15 gases ~rom the gas stream. The absorben~ from tower 20
is, as shown, passed by liquid seal ~6 to absorption tower
12 and, in the case of FIG. 2~ by line 38 and pump 36
to absorption tower 12,
~he conditions for employing the apparatus shown in
20 FIG. 2 are to meet the requirement of a low temperature
gas entering absorption tower 20 to maximize removal of
the acid gas constituents,as the capacity of most
absorbents for the acid gases are inverse functions of
temperature. It can serve to meet the situation where
25 low temperature is required by the concentration of the
acid gas constituents, or by greater economy of size of
absorbing facilities, or greater energy conservation.
In the arrangement of FIG. 2~ the gas leaving bu1k
absorption zone 12 is passed by ~ine 14 in indirect heat
30 exchange with the gas leaving hydrolysis reactor 16 in
heat exchanger 32. The gas entering reactor 16 is
partially heated to the hydrolysis temperature while the
gas leaving the hydrolysis rea~tor 16 is partially cooled
in exchanger 32 for ~eed to absorption tower 20~ Any
35 additional hea~ to be removed from the ~as stream i9
~9~36
13209 -13-
1 removed by passage of a cooling medium such as water
through heat exchanger 34.
Since heat exchangers 32 and 34 create pressure losses
in gas flow, a long pipe seal 26, as shown in FIG. 1, may
be required. To avoid its use, there may be employed a
pump 36 to pump the partially spent absorben~ from the
base of absorption tower 20 to the upper ~eed level of
absorption tower 12.
9~36
1320~ -14-
CONTROLS A AND E~ ~ND EXAMP~E 1
There is processed a gas stream of the feed gas
composition as shown in Table I below. Employing a
conventional ambient temperature selective absorbent
for hydrogen sulfide in each instance, for other than ~he
hydrolysis zone, Control A shows 'che total sulfur removed
and C3S removed from the gas stream for a given absorbent
flow rate.
10 For Control B, the absorbent ~low rate was doubled,
almost making the gas treating facility twice as large.
As shown, this increases total ~ulfur removal to 97.5%~
but only 72.4% of the COS is removed.
Instead of increasing the absorbent flow ratet the
15 absorption is ~plit between the two absorption zones with
an intermediate closed-loop hydrolysis zone operated at
200F. The aquesus alkanolamine hydrolysis medium is a
35% by weight diethanolamine solution~ With the same
net flow of absorbent as in Control A, tokal sulfur
20 removal is increased to 99~3% and the amoun~ of COS
remaining is nil~
$~g~36
13209 -15-
1 TABLE I
Trea~ed Cas~
Component Feed Gas Control A Control B Exam~le 1
.. .
~2S123.28 1.02 0.35 1.02
.
COS11.16 6.Z9 3.~8 c 5 pp,m
~' :
C0~1,887.~81,598.4g 1,198.55 1,598.44
CO5,236.955,235.2~ 5,2~9.99 5,~35.24
~23,651.303,647.91 3,650.27 3,647.91
CH410.30 10.27 10~23 lC.27
N241.21 41.21 41~21 41.21
Ar~on17.6717.67 17.67 __17.67
Total10,~79.3510,558.0510,151.3510,551.76
% S Removed - 94.6 97.5 99.3
% COS Removed - 43.6 7204 100.0
~5 * Moles per hour~ dry basis
.
-
