Note: Descriptions are shown in the official language in which they were submitted.
OXI~ATIV~ REMOVAL OF HYDROGEN
SULFIDE FROM GAS ST~EA~S
tCase No. lVl)
~ac ound o~ the Invention
This invent$on relates to the removal of hydrogen sulfide
from gas streams by contacting the hydrogen sulfide containing
gas stream with an aqueous solution containing thiosulfate ions
to form elemental sulfur with the subsequent regeneration of
thiosul~ate ion by the oxidation of hydrogen sulfide.
Hydro~en sulfide is often present in many gas streams as a
contaminant which prevents the untreated discharge of gas streams
to the atmos~here or the use of components of certain gas streams
for other prod~ctive purposes. Over the years many
desulfurization processes have been developed in attempts to
produce gas streams substantially free of hydrogen sulfide. Many
of these processes are operated in a cyclical manner and employ
regenerable oxidizing agents to oxidize hydrogen sulfide to
elemental sulfur which is recovered and to discharge to the
atmosphere or reuse the gas stream. Regeneration of the reduced
oxidizing agent in an aqueous absorbing medium, after its
oxi~ation of the hydro~en sulfide to sulfur is typically
accomplished by contacting the aqueous ~edium with air or oxygen
gas.
Many such processes have been attempted often ~sing a rela-
tively expensive organic material in the solution. A common
detriment attributed by those skilled in the art with many of
these so-called wet desulfuri~ation processes, particularly those
involving some organic derivatives such as quinones; is the
formation of thiosulfate ions as a by-product. Much effort and
creativlty has been dirècted towards the reduction or elimination
of thiosulfates or polythionates present in the oxldizing liquor
used to absorb the hydrogen sulf i~e from the gas stream. Several
30 such at~empts are described ~or example, in U.S. Patent
~.
4.347,212. One Patent. U.S. Patent No. 3,773,662, describes a
process for removing thiosulfates from solutions by introducing
hydrogen sulfide gas into the liquid stream to decompose the
polysulfide and suppress hydrolysis of elemental sulfur during
cooling thus increasing the recovery of sulfur from a sulfur
plant. The presence of thiosulfate was treated as an undesirable
nuisance.
It has long been recognized that the polythionate or
thiosulfate salt in aqueous solution will react with hydrogen
sulfide to form sulfur. For instance, U.S. Patent No. 1,832,325
teaches a process for removing hydrogen sulfide by oxidizing
sodium thiosulfate to sodium tetrathionate and reactin~ it with
hydrogen sulfide. It also disclosesthe use of buffering salts
such as sodium bicarbonate or acid sodium phosphates to keep the
hydroxide ion concentration below a value above which sodium
tetrathionate would become unstabLe. O~idation of the
thiosulfate to tetrathionate is accomplished by the use of
certain catalyst of nickel or cobalt.
U.S. Patent 1,937,196 teaches the removal of hydrogen
sulfide from a gas stream by the contact of a sodium carbonate
solution which is later regenerated by heating and aeration.
This patent teaches the use of thiosulfate salts such as sodium
thiosulfate, ammonium thiosulfate, zinc th~os~lfate, iron
thiosulfate and the Llke for the absorption of sulfur dio~ide
wherein the thiosulfate is re~enerated by contact with hydrogen
sul~ide. Sul~ur is recovered after such contact with the
thiosulfate solution having absorbed sulfur dioxide.
In U.S. Patent No. 3,536,618 there is described a sulfur
removal step wherein ammonium thiosulfate is reacted with hydrogen
sulfide in a gas stream whlch is then decomposed to recover
ammonium, hydrogen sulfide, water and elemental sulfur. The
resulting aqueous stream is recycled to remove sulfide salts ~rom
a hydrocarbon product stream by a simple absorption. This
1~ ~
process does not deal with the clean-up of a gas stream but does
disclose the reaction of a thiosulfate ~ith hydrogen sulfide to
form elemental sulfur.
Hydrogen sulfide removal in the presence of an activated
carbon catalyst and stoichiometric amounts of oxygen is described
in U.S. Patent 4,263,271. There is no liquid phase involved in
the discussion with respect to this pate~t.
The removal of hydrogen sulfide from a gas stream by contact
with a regenerable washing solution having a pH of between about
5 and 10 and containing solubilized vanadium, thiocyanate ions, a
carboxylate complexing a~ent, one or more water soluble quinones
and one or more water soluble nonquinone aromatic compounds
capable of solubilizing tetravalent vanadium is describe~ in U.S.
Patent 4,432,962 which issued on February 21, 1984. The absorbed
hydrogen sulfide is converted to elemental sulfur and after
oxidative regeneration of the washing solution is separated from
the regenerated solution.
U.S. Patent No. 4,243,648 also describes a hydrogen suLfide
removal and conversion method wherein a hydrogen sulfide
containing gas stream is contacted with a regenerable washing
solution capable of absorbing hydrogen sulfide and converting the
hydrogen sulfide to hydrophobic elemental sulfur particles.
After regeneration of the washing solution having a pH of between
S and 10 which contains solubilized vanadium, thiocyanate ions, a
solubiliziny agent for tetravalent vanadium and a water soluble
carboxylate complexing agent, the elemental sulfur form is
separated. The patent describes this process as being
particularly useful for removal of hydrogen sulfid~ from gas
streams produced by the sweetening of sour natural gas,
processing of ores, the destructive distillation of coal and oil
shale, the gasification or liquifaction of coal, the use of
geothermal fluids to generate electricity or other processes
which produce hydrogen sulfide containing gases.
~ 2~
Notwithstanding the disclosures of the foregoing prior art,
much is still to be accomplished in the removal of hydrogen
sulf ide from contaminated gas streams. Previously hydrogen
sulfide was removable only by employing multiple unit operations
such as, for example, an amine scrubber followed by a Claus unit
which in turn was followed by a tail gas treatment and still
sulfur was left in the gas stream. To accomplish a technical
solution to the problem where removal of hydrogen sulfide and
sulfur recovery is simply and erficiently accomplished the
invention herein described was made~ Many advantages of the
present invention will become manifest in view of the following
description.
Summary of the Invention
The present invention accomplishes the removal of hydrogen
sulfide from contaminated gas streams and the recovery of
elemental sulfur by the contact of such gas stream with a
buffered aqueous solution enriched in thiosulfate ions at an
initial pH of from 4.5 to 6.5 at the liquid inlet of the first
reaction zone for a residence time sufficient to react a portion
of the hydrogen sulf ide in the gas stream to elemental sulfur
thereby reducing the thiosulfate to elemental sulfur. The
resulting wash stream, now lean in the thiosulfate ion, is passed
to a second reaction zone where it contacts the gas stream, now
lean in hydrogen sulfide but fortified with at least a
stoichiometric amount of oxygen such that the hydrogen sulfide
remaining in the contaminated gas stream is oxidized to the
thiosulfate in this second reaction zone thus enriching the
solution. The now cleaned ~as stream is then removed f}om the
second reaction zone and the liq~id aqueous solution enriched in
thiosul~ate ion is recycled to the ~irst reaction zone to contact
additional hydrogen sulfide. The initial contact of the gas
stream and liquid in the second reaction zone is made at a pH of
from 5.0 to 7.0
The advantages of this invention are many. The process of
the invention can be easily adapted to treat many gas streams
contaminated with a wide range of hydrogen sulfide
S concentrations: from vent gases containing onl~ small amounts of
hydrogen sulfide to refinery effluent gases which may be
substantially pure hydrogen sulfide. Hydrogen sulfide removal
can be accomplished without forming noxious sulfur dioxide gases.
It is particularly useful in instances wherein acid gases removed
from the sweetening of natural gas containing hydrogen sulfide
and carbon dioxide are treated in the process to recover the
sulfur so that the carbon dioxide can then be used for enhanced
oil recovery methods.
Another advantage paramount in the operation of the method
of this invention is that great variations of hydrogen sulfide
concentration in the contaminated gas stream heing treated can be
acco~modated in the practice of the invention without destroying
the operability thereof as will be seen from the following
discussion.
The novel features of this invention which are considered as
characteristic thereof are set forth in particular in the
appended claims. The invention itself, however, both as to its
construction and method of operation, together with the ob~ects
and advantages thereof, though not to be limited thereby, will be
best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
Brief DescriPtion of the Drawinq
Fig. l is a diagram showing the process of this invention.
Fig. 2 shows a specific preferred embodiment of the process
of this invention in diagrammatic form.
i~
Details of the Invention
While the prlsence o~ thiosulfates was previously considered
to be a detriment in processes of recoverying ele~ental sulfur
present as hydrogen sulfide in gas streams, it has now been
discovered that the presence of thiosulfate ions in aqueous
solutions ean be used to the advantage of recovering hydrogen
sulflde from gas streams as in the form of elemental sulfur. The
process of this inve~tion operates utilizing two reaction zones,
on~ wherein the thiosulfate ion in the buffered aqueous solutlon
contacts hydrogen sulfide in the gas stream reacting to form
elemental sulfur, and the other where hydrogen sulfide remaining
in the gas stream is oxidized to restore the thiosulfate
concentration in the aqueous solutisn.
The reaction zones provide the location for liquid gas
contact in ways known to the engineer such as, for example,
bubble plate column, a back mixing reactor or the packed bed,
which is preferred.
The reaction in the first reaction zone is preferably
con~cted at temperatures at which elemental sulfur is molten to
facilitate the separation of elemental sulfur from the aqueous
20 absorhing solution. A suitable operating pressure is maintained
to reduce the vaporization of water occurring from the solution
to ~inimize the need for water make up and chemical treatment;
typically a fa~r minimu~ for operation is 6 kg~cm ~or low
pressure ~pplication since the partial pressure of steam at
oPerating temperature is 5 kg/cm and, as later stated, hi~her
pressures of 3~ kg/cm or even greater are useful, even up over
150 kg/cm . The initial contact of the gas stream with buffered
liquid aqueous solution rich in thiosulfate ions occurs at a pH
in the range of 4.5 to 6.5 preferably from 5.5 to 6Ø In this
30 pH range the reaction between the thiosulfate ion in solution and
the hydrogen sulfide contaminate of the gas stream is relatively
fast. The contact preferably occur~ on a ~olid packing In the
,,i~ ~
reaction zone such as saddles or carbon spheres or pellets or
some other well-~nown tower packinc3 material. While not acting
as a catalyst, the surface of the packin~ does provide sites for
contact between the gas and liquid and the reaction for
conversion of hyrogen sulfide to sulfur.
At steady state operation of the process of this invention,
the concentration of the thiosulfate ion in the solution is such
that approximately half of the hydrogen sulfide in the gas stream
reacts in accordance with the followinc~ formula:
10 I. 2H S ~ 2H ~ S O ~ 4S + 3H O
2 3 ~ Z
to form sulf~r leaving a gas stream lean in hydrogen sulfide to
exit the first reaction zone and to enter the second reaction
zone in the presence of a stoich~etric amount of oxygen sufficient
to oxidize the remaining hydrogen sulfide, at steady state,
almost half of that originally present, to the correspanding
thiosulfate. This oxidation reaction causes the rest of the
hydro~en sulfide to be removed and thus when exiting the second
reaction zone the gas strea~ could be depleted or substantially
free, of hydrogen sulfide, depending upon the gas stream
requirements. For instance should the situation be a ~as stream
to be dischar~ed to the at~osphere a 50 part per million hydrogen
sulfide concentration can c~e tolerated and easily obtained. 0~
course, the gas stream would contain considerable water through
the practice of this invention because contact with the gas
2~ stream occurs at an elevated temperature and therefore preferred
operation is that a condenser and water knock-out drum be placed
in the line for removal of the condensate strea~.
The liquid solut$on now being enriched in thiosulfate ions
exits the second reaction zone at a pH less than when it entered,
usually at a pH at which it is desirable to reintroduce the
regenerated th1o~ulfate strearn into the first reaction zone to
again contact the hydrogen 6ul~ide ln tho ga~ stre~m, L.o, An
~L2~
initial pH of from 4.S to 6.5.
The PH of the liquid solution is p~imarily controlled and
adjusted by introducing an alkali metal hydroxide or ammonium
hydroxide into said li~uid solution prior to the contact with
oxygen and the hydrogen sulfide lean gas strea~ prior to the
second reaction zone. This pH i5 adjusted to a pH of from 5.0 to
7.0, preferably from 5.8 to 6.3, at the inlet of the second
reaction zone. Thus, this becomes the initial pH of the
thiosulfate lean solution prior to regeneration by contact with
the gas stream lean in hydrogen sulfide. The reaction in the
second reaction zone proceeds as follows:
II.2 H 5 + 202~ 2H ~ S O I H O
The thiosulfate ion in the buffered aqueous solution would
be present in the form of the ionized salt of either am~onium or
sodium or potassium with ~.odium thiosulfate being preferr.ed. The
sodium thiosulfate is preferred because limited soLubility of
sodium sulfate, commonly called Glauber's salt, eases the
separation in the purge stream.
The buffered liquid aqueous solution enters the first
reaction zone at an initial pH of from 4.5 to 6.5 which, in the
practice of the process of this invention, is the pH of the
regenerated liquid aqueous solution recovered frorn the second
reaction zone.
After contact and reaction with the hydrogen sulfide in the
contaminated ~as stream a thiosulfate lean liquid aqueous
solution is removed from the first reaction zone at a higher pH,
and the elemental sulfur, preferably in its molten.state is
separated. The thiosulfate lean stream is then contacted with an
alkali metal hydroxide or ammonium hydroxide, usually the alkali
hydroxide of the same salt of the thiosulfate being used,
preferably ammoniu~ hydroxide or sodium hydroxide to adjust the pH
to the desired initial pH for introduction into the ~econd
~2~
reaction zone or regeneration zone, to contact the gas stream
now including at least a stoichiometric amount of oxygen for
the oxidation of hydrogen sulfide to the thiosulfate. The
oxygen can be introduced in a gaseous mixture such as air when
dilution of the contaminated gas stream is not important or as
pure oxygen when the presence of the diluents, such as nitrogen
in the case of air, are undesirable.
The liquid aqueous solution is buffered by employing the
alkali metal carbonate, alkali metal acid phosphate, ammonium
carbonate, or ammonium phosphate, as well known to those skilled
in the art, to buffer the solution in the range of pH of 5.0
to 7Ø The acid phosphate buffering agent, such as sodium
acid phosphate is preferred. The buffering agents are
preferably added with the alkali hydroxide. When carbon dioxide
is present in the gas stream some or all of the buffering may
be accomplished by in-situ formation of carbonate buffering
agents.
Turning now to the drawings, specifically Fig. l~ the gas
stream contaminated with hydrogen sulfide entering through
line 14 contacts the buffered aqueous solution enriched in thio-
sulfate ions at the selected pH of from about 4.5 to 6.5,
prefer~bly from 5.5 to 6.0 in a first reaction zone lO. The
solution enters the reaction vessel 12 through line 16. The
hydrogen sulfide containing gas stream remains in contact with
the buffered aqueous solution enriched in thiosulfa~e ions in
the first reaction zone for residence time sufficient to react
a portion of the hydrogen sulfide to elemental sulfur. The
residence time may be expressed in terms of time, usually
expressed in seconds. In any case the gas and the solution
should be in contact for from 15 seconds to lO0 seconds. From
this information, using conventional engineering parameters, a
process engineer can calculate reactor size and bed dimensions.
Of course, in instances where there is a low loading of
hydrogen sulfide in the gas stream and a high concentration of
- 9a -
thiosulfate ion in the solution, all of the hydrogen sulfide may
be reacted with thiosulfat~ ion to form elementaL sulfur in the
first reaction zone. This emphasizes one of the characteristics
of the practice of the process of this invention. If all
5 hydrogen sulfide is reacted in the first reaction zone, no
hydrogen sulfide would be available for oxidation into
thiosulfate ion in the second reaction, or oxidation, zone. Thus
the buffered aqueous solution enriched $n thiosulfate will
continue to circulate and be reacted in the first reaction zone
10 without any regeneration until there is hydrogen sulfide in the
exit gas stream to be oxidized into thiosulfate ion. Thus the
system equalizes itself.
Conversely, if there is a considerable hydrogen sul~ide in
the gas stream being treated in excess of the amount of
L5 thiosulfate needed, all of the thiosulfate ions will be reduced
to elemental sulfur and a large amount of thiosulfate produced in
the second reaction zone from the hydrogen sulfide leaving the
first reaction zone 10 until the thiosulfate concentration is
sufficient to convert about half of the hydrogen sulfide in the
20 gas stream into elemental sulfur allowing the other half to exit
the first reaction zone to be oxidized to thiosulfate. Tt i5 in
this manner that the practice of the process of this invention
bPcomes self-levelin~ and is able to tolerate wide and sudden
shifts in hydrogen sulfide concentrations in the gas stream.
25 Thus, it can be seen that the concentration of thiosulfate ions
in solution is not critical since it balances itself against the
amount of hydrogen sulfide in the gas at the pH operational
ranges disclosed herein.
Returning to the drawing the first reaction zone 10 is
30 preferably a packed bed reaction zone filled with solld packing
material such as Berl saddles, Raschlg r~ngs, Intalox saddles or
carbon pellets and the like to allow the contact sur~ace and
reaction time to be suff1cient ~or the formation ~P el~mentaL
r
,~ 10
sulfur on the surface.
The first reaction zone 10 is opera~d preferably at a
temperature wherein elemental sulfur is liquid or molten, i.e.
between 270 degrees F. (130 C) and 320 degrees F. (160 C)
preferably 300 degrees F. (150 C) since it'is within this range
that molten sulfur has its lowest viscosity.
Since the reaction liquid is an aqueous material and the
temperature is relatively high, often above the boiling point of
water, it is preferable that the process be conducted at elevated
pressures, even though such elevated pressures are not required
for the reaction itself to proceed. In order to reduce water
loss during the reaction taking place in the first and second
reaction zones of this process elevated pressures are used in
both reaction zones. Of course, the pressure is primarily
dictated by the source of the gas stream contaminated with
hydrogen sulfide and the ultimate disposition of the product gas
depleted in hydrogen sulfide. If the product gas is to be placed
into a pipeline being operated at elevated pressures then a
c~mpressor would be placed on the lnlet side of the process of
this invention such that the gas would be compressed in the first
instance. This would materially reduce the amount of water
consumed in the process and thus make it operate more
efficiently. If the product gas is a flue gas to be vented to
-the atmosphere then some minimum pressure will be selected in
order to optimi2e the operations giving considerati~n to pressure
drop and water loss.
After reaction in the first reaction zone 10 the aqueous
solution now lean in thiosulfate ions and containing elemental
sulfur leaves the first reactor 12 through llne 1~ an~ into a
sulfur separator 20 where the sulfur separates from the aqueous
liquid lean in thiosulfate ions and is drawn from the sulfu~
separator 20 to storage through line 22. The liquid aqueous
solution lean in thiosulfate ion is drawn from the sulfur
separator ~0 through line 24, through pump 26 and thence line 28
~o ~he oxidation reactor vessel 30 containing the second reaction
zone 32. Preferably, this reaction zone comprises two contact
beds 32a and 32b which like the first reaction zone, may be
packed with a ring or cera~ic saddle packing or, preferably,
solid carbon spheres. The buffered aqueous liquid solu~ion now
lean in thios~1late ions passes through line 28 to the oxidation
reactor vessel 30 where, ~ust prior to enter$ng the second
reaction zone, the pH of such buffered liquid aqueous solution
lean in thiosulate ions is determined by sensor 34 which, in the
preferred operating embodiment of this invention, is responsively
connected ~o a valve which introduces a base such as an alkali
metal hydroxide, preferably sodium hydroxide or ammonium
hydroxide through line 36 to adjust the pH of the buffered liquid
aqueous solution lean in thiosulfate ions such that the initial
contact of the solution with the gas stream in the second
; reaction zone is at a pH of from 5.0 to 7.0 preferably from about
5.8 to 6.3.
Returning to the reduction reactor lZ for a moment, the gas
stream contaminated with hydrogen sulfide exits said reduction
reactor 12 through line 3~ as a stream lean in hydrogen sulfide.
Prior to contacting the liquid solution in the second reaction
zone 32 the gas stream lean in hydrogensulfide is mixed with at
least ~ stoichiometric amount of oxygen; either in the form of
pure oxygen or a gas mixture such as air, sufficient to convert
the hydrogen ~ulfide in the hydrogen sulfide lean gas stream to
the thiosulfate ion in accordance with Equation II. The oxygen
enters the gas stream through line 40 in response to the presence
of hydrogen sulfide in the hydrogen sulfi~e depleted product gas
stream. Should it be necessary, in order to remove the required
amount of hydrogen sulfide in the product gas strea~ a
supplemental stream of oxygen may optionally be injected into the
oxidation reactor 30 through line 42 in between the beds 32a and
~,J
12
32b which form the second reaction zone.
The temperature and pressure maintained in the oxidation
reactor 30 within the second reaction ~one 32 are selected based
upon the same considerations and criteria discussed hereinbefore
with respect to the first reaction zone except that little, i~
any, elemental sulfur is produced. Since the oxidation of
hydrogen sulfide i5 an exothermic reaction the temperature of the
liquid solution exiting the second reaction zone 32 may he
somewhat higher than the te~perature entering such reaction zone.
The liquid aqueous solution, now rich in thiosulfate ions
exits the oxidation reactor through line 44 pump 46 line 48 and
through a heat exchanger, or heater 50, where the solution,
enriched in thiosulfate ions, re-enters line 16 for return to the
reduction reactor 12 and the first reaction zone 10.
1~ The heater 50 would normally be used only slightly during
steady state operations ~ut would be necessary to preheat the
buffered aqueous liquid solution rich in thiosulfate ions during
the start up of the process. Since the oxidation of hydrogen
sulfide to the thiosulfate is an exothermic reaction it
substantially replaces the heat lost in the reduction reaction
which is an endothermic reaction and the loading on the heater 50
during the operation i5 small, if necessary at all depending upon
heat losses and operating condition.
The gas stream, after traversiny through the second reaction
zone, is now depleted in hydrogen sulfide and exits the oxidation
reactor 30 through line 5Z. If the product gas exiting through
line 52 is to be vented to the atmosphere there probably would be
little interest in attempting to recover the not insubstantial
amounts of water vapor included in the gas stream. Should it be
desirable to recyc~e such water vapor as shown on Fig. 1, the gas
stream, depleted in hydrogen sulfide, would pass through
condensor 54 and thence through line 56 to a water knock-out drum
5~ where the condensed water separates from the product gas
8'~
stream and e~its the knock-out drum 58 throu~h line 60, pump 62
and line 64 through which it i5 returned to the liquid aqueous
solution lean in thiosulfate ions in line 28. The product gas
would exit water knock-out drum 58 through line 66 where, if
natural gas it would be returned to a natural gas pipeline. If
the product gas i5 carbon dioxide for use in enhanced oil
recovery then it would either go to a pipeline or storage or
compressor for further processing. The quantity of hydrogen
sulfide remaining in the depleted gas stream of course is
dependent upon the requirements and specifications of the end use
to which the product gas is to be put. It is a great advantage
of the process of this invention that substantially all of the
hydrogen sulfide can be removed from the gas stream if desired.
During the operation of the process of this invention a
certain amount of thiosulfate is oxidized to the sulfate form.
If the sodium thiosulfate is the source of the thiosulfate ion
being used in the buffered liquid a~ueous solution, Glauber's
salt, sodium sulfate, i5 produced and should be purged :From the
system. In the flow diagra~ of Fig. 1 there is shown a solution
purge stream 70 coming off of line 28 containing the liquid
solution lean in thiosulfate ion. The purged stream proceeds
through line 70 to a crystallizer 72 wherein the sulfate is
precipitated and removed from the system through line 74. The
liquor thus separated from the salt leaves crystallizer 72 and
returns to line 28 through line 76.
While the foregoing description is believed to be sufficient
to enable one of ordinary skill in the art to practice the
instant invention, for purposes of further descrip~ion only a
preferred embodiment of the instant invention wherein the
oxidation and reduction reactor have been accorded is described
with reference to Fig. 2.
In this particular embodiment a specific instance of the
separation of carbon dio~ide and hydrogen sulfide recovered as an
acid gas from natural gas and therefore containing a small amount
of me~hane is described. The flow sheet.of such an embodiment is
shown on Fig. 2. The gas stream is contaminated with 4% hydrogen
sulfide and enters the first reaction zone 210 in the reduction
reactor 212 being the upper portion of a sin~le column 213
housing both reaction zones. The gas stream enters through line
219 where it contacts a buffered liguid aqueous soiution rich in
thiosulfate ions entering the first react:ion zone 210 through
line 216 having an inltial pH of 5.6. Contact is made in the
first reaction zone 210 at a temperature of 300 degrees F. and
pressure of 490 psi (35 Kg/cm ) where it proceeds downwardly
through a packe~ car~on ~ed as the first reac-tion
zone 210 to a draw-off tray 217 from which is drawn the liquid
aqueous solution now lean in thiosulfate ions mixed with molten
sul~ur formed in the first reaction zone 210 through line 218 to
a sulfur separator 220 wherein the sulfur and liquid solution
lean in thiosulfate are separated with the sulfur being withdrawn
fro~ the bottom separator 220 through line 22Z and the liquid
aqueous solution lean in thiosulfate ion being withdrawn throu~h
line 228 throu~h which it returns to the oxidation reactor
section 230 of the unitary reaction vessel 213 and ~s introduced
into the second reaction zone 232 which is also a packed bed
where the gas/liqui~ contact is accomplished. The liquid aqueous
stream lean in thiosulfate is at a temperature of about 295
~5 degrees F. upon introduction into the second reaction zone 232.
Prior to the introduction of the liquid solution lean in
thiosulfate into the second reaction zone 232 the pH is
determined by a sensor 234 which operates a controller for a
caustic make-up 235a and water make-up 235b which introduce a
sufficient amount of caustic and water throu~h line 236 $nto line
2Z8 to adjust the pH of the liquid aqueous solution lean ~n
thiosulfate to 6.3, It is introduced into the second reaction
zone 232 at this ~nitial pH to contact the liquld solution~
,r~r
~ 15
-
The gas stream exiting the first reaction zone Z10 through
the draw-off tray 217 is now lean in hyd~ogen sulfide though
there still may be roughly one-half of the original hydrogen
sulfide remaining in the gas stream which proceeds through the
5 draw-off tray 217 to the second reaction zone 232 wherein it
c~ntacts the liquid solution lean in thiosulfate entering through
line 228 and oxygen entering through line 240. The amount of
oxy~en introduced into the second reaction zone 232 through line
240 is at least the amount sufficient to oxidize the hydrogen
10 sulfide remaining in the gas stream lean in hydrogen sulfide to
the thiosulfate in the solution thus increasing the concentration
of the thiosulfate ion in the solution and regenerating same for
recirculation to the first reaction æone 210.
The needed amo~nt of oxy~en introduced through line 240 is
15 determined by a sensor 241, which monitors the hydrogen sulfide
content of the product gas, and a controller 241a which meters
the oxygen entering through line 240 in response to a signal from
the sensor 241. The liquid solution rich in thiosulfate ion
exits the oxidation reactor section 230 through line 244 pump 246
20 and re~urn line 248 for recycling to the reduction reactor 212
and the first reaction zone 210. The liquid solut$on enriched in
thiosulfate ions exits the oxidation reactor 230 at a pH of about
5.8.
The purified product gas stream exits the oxidation reactor
25 230 throu~h line 252 and gas outlet baffle 251. The product gas
is at a te~perature of 280 degree F. (140 C) and pressu~e of 300
p.s.i. (21 Kg/cm J and passes through a heat exchanger 253 where
some heat is removed th~ough indirect heat exchange to heat the
entering gas stream contaminated with hydrogen sulfide. The
30 prod~ct gas then proceeds further to a condenser 254 where water
in the product gas is further condensed thence through line 256
into a water knock-out drum 258 where the water i5 removed
through line 260 pump 262 and line 264 which returns the water to
'`'';~'
16
the inlet gas stream which enters heat exchanger 253 through line
265 and proceeds through line 268 to hea~er 250 which is u~ed
mainly for start-up of the unit and to add such heat as may be
necessary to increase the temperature of the sour gas stream
being treated to the inlet conditions of the reduction reactor
section 212.
During start-up the liquid solution rich in thiosulfate ion
is withdrawn from line 248 through line 27~ where it is joined by
the sour gas stream entering through line 268 and proceeds thence
throu~h line 274 into the heater 250 and from said heater through
line 276 where line 214 is joined for introduction of the mixed
streams to the reactor vessel 213. The heater 250 receives fuel
gas through line 278 and its operation is controlled by sensor
280 which measures the temperature of the stream in line 276 and
responsive to such sensor 280 a control valve 282 limits the
amount of fuel gas entering the heater 250. As in the previously
discussed embodiment of the instant invent,ion, a purge stream is
removed from the liquid solution rich in thiosulfate to remove
the sulfates formed in the reaction through line 270. ~n this
embodiment the product gas~ containing only 50 parts per million
hydrogen sulfide, exits the water knock-out drum 258 through line
~66. Having thus described the foregoing invention those of
ordinary skill in the art will readily perceive many varations
~ and modifications thereof which remain within the scope and the
spirit of such description and appended claims.
