Note: Descriptions are shown in the official language in which they were submitted.
1075~3'7~
This invention relates to the recovery of sulfur,
and more specifically, to the removal of sulfur present in
various forms including elemental sulfur vapor, liquid elemental
sulfur droplets, and a mixture of the sulfur compounds hydrogen
sulfide and sulfur dioxide, all present in dilute concentration
in entraining gases. In particular, the invention is intended
primarily to remove significant proportions of elemental and
combined sulfur from the effluent gases of Claus process plants
operating to convert hydrogen sulfide to elemental sulfur for
recovery.
Claus process plants are used in such industries
as petroleum refineries and natural gas treating plants, to
convert the poisonous hydrogen sulfide evolved from petroleum
refining operations or scrubbed from natural gas (which as
collected from some gas wells may contain as much as 25~ to 40
hydrogen sulide~. In the Claus process, hydrogen sulfide is
reacted with air to form elemental sulfur and water as the major
products. The elemental sulfur so formed is condensed from the
~apor state and largely recovered from the uncondensed gases
which are principally water vapor and nitrogen, with some un-
condensed sulfur vapor, unreacted hydrogen sulfide and sulfur
dioxide, and some carbon disulfide and carbon oxysulfide from
the ~eed to (or formed in) the Claus plant operation.
The uncondensed effluent gases from ~laus process
plants are usually incinerated so that residual sulfur is
discharged as dilute sulfur dioxide from the incinerator stack.
Total emission of sulfur dioxide is measured in the stack and
it is this emission which is subject to upper limits by
environmental agencies. Despite the considerable progress that
has been made in reducing the proportion of sulfur in various
1~'75~79
forms that passes out in the effluent gases and is finally
discharged as sulfur dioxide, ultimately chemical and physical
equilibria set upper limits on the efficiency of the plants and
further recovery must be achieved by a separate plant process
wherein the conditions of operation permit additional sulfur
recovery. Several such processes are known in the art and some
are in commercial operation.
Water-scrubbiny of effluent gas streams is widely
practised, but attempts to apply this procedure to Claus
process ef1uent gas encounter a serious difficulty, namely
the tendency toward formation of colloidal sulfur in the
aqueous medium. Colloidal sulfur can be formed by the contact
of sulfur vapor with water. It is also produced by the complex
series of chemical reactions which occur when hydrogen sulfide
and sulfur dioxide react in the presence of water. In this
sequence of reactions, thiosulfuric and polythionic acids are
formed, some of which exhibit intermediate stability, and
gradually decompose with formation of elemental sulfur in
colloidal form. In this form, the sulfur cannot be readily
separated from the water. Moreover, it causes serious problems
in operation by plating out, over time, in critical regions of
the plant.
It has now been found that the problem of conkrolling
the deposition of elemental sulfur, whether from pre-existing
25 elemental sulfur vapor or from the aqueous reaction of hydrogen
sulfide and sulfur dioxide, can be solved by the addition of
certain catalytic solids, in finely divided particulate form,
to the aqueous scrubbing medium. These catalytic solids are
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1~74~ 79
those known in the art for their ability to catalyze the vapor
phase reaction of hydrogen sulfide and sulfur dioxide in s~stems
which op~rate above the water dewpoint, namely alumina and
~ctivated carbon. It could not be predicted, however, that the
presence of these catalytic solids would, for practical purposes,
substantially completely inhibit t~le aqueous phase reaction
sequence which has been referred to above, and which leads to
formation of colloidal sulfur in the bulk aqueous phase, nor
that such catalytic solids would entirely inhibit the formation
of colloidal sulfur when elemental sulfur vapor contacts the
aqueous system. It is now found that such inhibition does
occur in the presence of such catalytic solid particles. On
the basis of ,his finding, it becomes possible to devise
processes for scrubbing of Claus process effluent gas streams
lS employing aqueous slurries of activated carbon or alumina.
It has also been found that the finely divided
catalytic solids which are effective for the present invention
and which accumulate sulfur thereon during the process of the
invention by deposition of sulfur, thereby undergoing a
decrease in effectiveness, can be regenerated to restore them
substantially completely to their original activity without
destroying or consuming a significant proportion of the
catalytic material.
The in~ention therefore consists in a process for
treating a gas stream for ~he removal of sulfur vapor, entrained
liquid sulfur droplets, and admixtures of hydrogen sulfide
and sulfur dioxide, present in dilute concentration in said
gas stream, which comprises intimately contacting the gas
. ,
-- 3 --
stream with an acidic aqueous suspension of a finely divided
catalytic material of the group al~ina and activated carbon,
the acidity of said suspension being that generated by
absorption of acidic components of the gas stream.
S The invention further consists in a method for
the regeneration of the finely divided catalytic solid material
having an accumulation of sulfur deposited thereon by the
process for treating a gas stream as aforesaid, said method
comprising
(1) separating the finely divided material from
the aqueous medium wherein it has contacted the gas stream,
(2) passing through said material a fluid of
the group consisting of a liquid solvent for sulfur and gases
non-reactiv~ with said material, at temperature sufficient to
dissolve sulfur when the fluid is a liquid solvent and to melt
and volatilize sulfur when said fluid is gaseous, until
absorbed sulfur has been separated from the catalytic material
sufficiently to restore its catalytic activity. The invention
still further consists in regenerating a finely divided
catalytic solids material by the method as aforesaid and
returning the regenerated catalyst in the form of an aqueous
dispersion as the catalyst in a process for treating a gas
stream as aforesaid.
The intimate contacting of the gas stream with the
aqueous slurry, in accordance with the process of the present
invention, may be achieved by use of any suitable type of gas-
liquid contacting equipment. Such equipment includes, for
example, scrubbing towers, such as packed towers and sieve
~075879
plate or bubble cap towers, cyclone scrubbers, jet scrubbers,
spray scrubbers, cascade scrubbers and Venturi scrubbers. ~
Preferred as the most practical device for contacting the gas
with the slurry is the Venturi scrubber.
The aqueous suspensions or slurries of alumina or
activated carbon used can be prepared readily with, for example,
commercially available finely divicled grades of alumina used in
Claus process converters or with various types of powdered or
- relatively fine granular activated carbon on the market. The
proportion of the suspended solid material in the aqueous
suspension for the process of this invention generally is in
the range from substantially 1~ to substantially 10% by weighti
proportions below the foregoing have uneconomically low capacity
for removing sulfur in its various forms from the gas, and
proportions`greater than the foregoing may tend to create
problems in handling the slurries. A preferred range of solid
in the aqueous suspenslon is from 3-% to 7~.
In the process of the invention, during the
contacting step, the finely divided catalytic material acts as
a deposition site for the sulfur removed from the gas stream
as well as acting to catalyze reaction of hydrogen sulfide and
sulfur dioxide; hence, there is a limit to thP amount of sulfur
that can be removed from a gas stream by a given quantity of
catalytic material before it must be freed of accumulated
sulfur. In general, a practicable limit for deposition of
~ulfur on the catalytic material is a weight equal to the dry
weight of the material itself, after which it is expedient to
regenerate the material for reuse by removal and separation
-- 5 --
~75~7.~
of the sulfur therefrom. Thereafter the catalytic material
can again be dispersed in water and the suspension thereof~
used to contact a gas stream for removal of sulfur according
to the invention.
Because the gas streams requiring sulfur removal
generally are at temperatures above the atmospheric boiling
point of water and are not saturated with moisture, there
will be significant amounts vf heat transferred to the aqueous
- suspension and evaporation of water therefrom. A specific
slurry operating temperature can be set at which water
evaporation from the slurry will be effectively nil. This
` temperature can be maintained, for example, by provision of
a heat exchanger in a slurry circulation loop. In an
alternative mode, heat removal can be accomplished by allowing
evaporation of water from the slurry and employing make-up
water to maintain the slurry volume. Combinations of the
features of the alternative modes also can be used. The
cooling provision preferably will maintain the temperature of
the contacting suspension in the temperature range from 60C
to 80C, most preferably from 65~C to 75C.
The efficiency of removal of sulfur in all its
various forms from a gas stream by the process of the invention
depends on several factors. These include, among others, the
; efficiency of the gas~ uid contacting apparatus used to
achieve the desired intimate contacting, the specific type and
proportion of the catalytic material in the suspension, and
the degree of sulfur loading on the material from gas previ3usly
contacted. Under conditions of good efficient operation it can
;
i~75~79
be expected that 50~ or mQre of the sulfur in all its various
- forms will be removed from the effluent gas stream of a Claus
process p ant, thus reducing, by more than half, the amount of
sulfur that must be incinerated and dissipated into the
atmosphere or passed to further sulfur recovery units.
Inasmuch as there is a limit to the amount of
sulfur that a given quantity of catalytic material can remove
from a gas before the material becomes inefficient, ~t is
necessary to regenerate the material to restore its efficiency
if the process is to operate economically~ R~generation of the
catalytic material is easily achieved, once it has been
separated from the aqueous phase, by heating the material above
- the melting point of sulur, fox example with a stream of air,
superheated steam, other inert gas, or mixture of any of the
foregoing, to drive off the sulfur in a stream of gas from which
it can be condensed. Preferably superhea~ed steam or a mixture
of hot combustion gases and superheated steam is used in
regenerating activated carbon, as such gases avoid consumption
of the carbon by partial combustion thereof. It is also
possible to remove the sulfur from the catalytic material and
ther~by regenerate it by dissolving the sulfur therefrom with
a suitable liquid solvent for sulfur, for example carbon
disulfide or xylene, in a suitable extraction process, then
separating the solid catalytic material from the liquid
solution, for example by filtration. The whole overall process
of recovering regenerated catalyst from an aqueous suspension
of solid catalytic material with the deposited sulfur thereon
is facilitated by the fact that the solid catalytic material is
-- 7 --
~075879
readily separated from the aqueous suspension b~ filtration and
can be regenerated in situ on the filter medium tafter drying
if desired), for example by passage of either hot inert gas or
gases therethrough to drive off the sulfur or by passage of
liquid solvent for sulfur therethrough to dissolve the sulfur;
by such operations the catalytic material can ~hen be recovered
from the filter medium, reslurried in water, and recycled for
intimate contacting again with a gas stream containing sulfur
and sulfur compounds.
The following examples are given to illustrate
operability and practicality of various aspects of the invention,
but are not intended to place any limitations on the invention
which is defined in the ensuing claims. The examples establish
the operability and effectiveness of the aqueous suspensions of
solid catalytic material separately and individually to remove
-~ mixtures of hydrogen sulfide and sulfur dioxide from a stream
of gas and to remove sulfur vapor from a stream of gas, in each
case without the development of colloidal sulfur in the aqueous
phase. The examples further establish the regenerability of
~he solid catalytic material after it has become loaded with
sulfur removed from a gas stream containing sulfur in a form
which deposits free sulfur on the solid. All percentages
expressed throughout the disclosure and claims are percentages
by weight unless otherwise specifically indicated.
The following example illustrates the effectiveness
of an aqueous suspension of activated carbon in removing sulfur
. ~
vapor from a gas stream.
-- 8 --
:.
)7~t37~1
EXAMPLE 1
To prepare a suspension of activated carbon, 1~0 gm
"Aqua Nuchar A" (trademark) activated carbon was dispersed in
100 ml of ~ater in a vigorously stirred vessel at room temperature.
A stream of preheated nitrogen gas flowing at a rate of 1.2
litres/min. was led over a body of molten sulfur held at 182C
(360F), and the resulting stream of dilute sulur vapor was led
via a heated tube at a temperature of 160C into the vigorously
stirred suspension~ The exit gas from the suspension was passed
through a plug o glass wool at ambient room temperature in a
2.5 cm diameter glass tube to retain sulfur which had passed
unabsorbed through the aqueous suspension. After two hours of
operation, the suspension was filtered through a filter paper
and yielded a clear filtrate, indicating the absence of any
colloidal sulfur that might have formed from the contact of
sulfur vapor with water. The filter cake of carbon and sulfur
was dried for two hours at 110C then extracted in a Soxhlet
apparatus with carbon disulride solvent for two hours. From
the extract, 0.244 gm of solid sulfur was obtained. Soxhlet
extraction of the glass wool plug with carbon disulfide then
was found to yield 0.166 gm of solid sulfur. The efficiency
of removal of sulfur from the gas stream by the aqueous
suspension of activated carbon ~hus was 59.5~.
The following example illustrates the effectiveness
of bauxite (alumina) in aqueous suspension for removing dilute
admixed hydrogen sulfide and sulfur dioxide from a gas stream.
EXAMPLE 2
Finely ground bauxite prepared by grinding a Claus
'.~
~ g _
~:)75 !37g
process commercial pelleted catalyst, in the amount of 2.5 gm,
~as dispersed in 250 ml water in a laboratory flas~ having`an
inlet line for a gas stream, an outlet for withdrawal of
aqueous slurry, and a top opening for connection to the bottom
of a contacting device comprising a commercial laboratory
vacuum jacketed dlstillation column section of two-inch (five
cm) inside diameter having five si~eve tray plates with down-
comers. A "Moyno" (trademark) positive displacement pump was
connected to the flask to withdraw slurry therefrom continuously
and pump it to the top of the distillation column, from whence
it flowed by gravity down the column and back into the flask
` while a gas stream, forced into the flask through the gas inlet,
flowed up through the column and contacted the slurry on the
sieve plates, thence to a vent pipe. A test gas stream was
prepared by continuously metexing nitrogen gas, hydrogen sulfide
gas, and sulfur dioxide gas at rates of 3 litres/min., 60 ml/min.,
and 30 ml/min. respectively into the inlet line, the proportions
giving a mixture containing substantially two volume percent
hydrogen sulfide, one volume percent sulfur dioxide, and ninety-
seven volume percent nitrogen. ~he stream of test gas waspassed into the flask and up through the distillation column to
contact the continuously recirculated slurry over a period of
50 minutes, during which time the temperature of the circulating
~ slurry was maintained at 77C by application of heat to the flask.
- 25 At the end of the measured time period, the slurry was filtered
- and the filtrate found to be clear and free of colloidal sulfur.
The recovered bauxite was dried then extracted with carbon
disulfide and found to contain 17.5% free sulfur (based on the
- 10 -
107~7~
original dry sul~ur-free bauxite weight) which had been
removed from the test gas containing no free sulfur. The
efficiency of removal of sulfur from the test gas was
calculated to be 6.6~; this relati~ely low efficiency was at
least partly attributable to the lower effectiveness of alumina
as a catalyst in the invention and also perhaps to the lack of
sufficient intimate contact of the test gas with circulating
slurry in the simple five plate contacting column.
The following example illustrates the effectiveness
of activated carbon in aqueous suspension for removing dilute
admixed hydrogen sulfide and sulfur dioxide from a gas stream,
and illustrates further the regeneration of the acti~e carbon
for reuse in said operation after it has been in use long enough
to become extensively loaded with absorbed free sulfur. By use
of appropriate convenient apparatus the example also illustrates
the ease with which successive absorptions and regenerations of
the active carbon can be carried out.
EXAMPLE 3
The relatively simple apparatus for this example
comprised essentially a glass tube (4.4 cm outside diameter)
with a sintered glass plate in the bottom section, about 25 cm
from the ~op. Connections at the top and bottom of ~he tube
permitted introduction of an aqueous suspension of carbon on
top of the sintered plate, introduction of a gas stream below
the plate to bubble up through aqueous suspension thereon and
venting of the gases at top; the connections also permitted
the introduction of steam or other regenerating fluid abo~e
the sintered plate and withdrawal of fluid medium below the
~7~79
plate for a regeneration step. The apparatus was enclosed in
a heating facility so that the temperature thereof could be
raised to a high level during reyeneration of carbon on the
plate. To start the operation, a flow of 1.3 litres/min. of
nitrogen gas into the tube through the bottom was established,
and 5 grams of "Nuchar SN" (trademark) activated carbon
suspended in lO0 ml of water was added on top of the plate
where it remained as the gas bubbled up through it. Heat was
applied by the heating facility to maintain the aqueous
suspension at a steady temperature of substantially 70C.
Flows of hydrogen sulfide and sulfur dioxide ~ere then added
to the flow of nitrogen into the tube, at rates of 40 and 20
ml/min. respectively, and the operation continued for a
measured period of two hours, during which time the
concentration of sulfur dioxide in the exit gas bei~g vented
above the suspension was continuously monitored with a Miran
(trademark) infra-red analy~er. The slight increase .in this
monitored concentration which occurred during the two-hour
interval indicated a substantially uniform and only slightly
decreasing efficiencv of conversion of the sulfur dioxide
during the conversion period. ~uring the period, occasiona7
additions of a few milliliters of water were made to the
suspension to maintain its volume substantially around lO0 ml,
compensating ~or the water lost in the gas stream by evapor-
ation. By periodic iodometric titration analysis of samplesof both the feed gas and the exit gas vented above the aqueous
suspension, the efficiency of removal of sulfur compounds from
the feed gas during passage through the carbon suspension was
- 12 -
1C~7~137~3
calculated to be 41.5%. At the end of the two-hour operation
-the feed gas streams were ha]ted and a slight pressure app~ied
above the aqueous suspension -to force the water thereo~ through
the plate and leave a filter cake of carbon with absorbed
sulfur on the plate. A water-cooled condenser was then
connected to the bottom of the tuble and a flow of superheated
steam introduced above the carbon with the temperature in the
tube being raised by means of the superheated steam to
substantially 900F (482C) as measured by a thermocouple
inserted in the carbon, thereby melting, vaporizing, and
entraining the sulfur deposited on the active carbon and
removing it to the condenser belo~ where the steam and sulfur
condensed and were recovered as a white milky dispersion of
sulfur in water. At the end of the two hours of steaming the
regeneration of the active carbon by removal of sulfur was
considered adequate, and the gas stream of mixed nitrogen,
hydrogen sulfide, and sulfur dioxide was re-established through
the tube with the same flow rates as previously, and with an
addition of 100 ml of fresh water to the loose, powdered carbon
on the plate as a suspension medium therefor. Again continuous
infra-red analyzer monitoring of the sulfur dioxide
concentration in the exit gas for the ensuing two hours snowed
that only a slight increase occurred in the concentration during
this interval, indicating an efficiency of conversion that
decreased only slightly during the interval. Periodic iodometric
analysis of samples of the feed gas and exit gas established that
the efficiency of conversion of the sulfur compounds to free
sulfur in the suspension during the interval was 50~. At the
.
- 13 -
1~)75~379
end of this interval the water of the aqueous suspension was
filtered through the plate and the carbon filter cake again
steamed for two hours with a flow of superheated steam as
previously, to remove sulfur therefrom. Following this the
flow of the mixed gas stream was again established as previously,
with water added to the carbon on the plate to form a suspension
through which the gas stream was bubbled for a further two-hour
reaction period and with continuous infra red monitoring of the
exit gas. Again the monitoring established that there was only
a slight decrease in the conversion efficiency during the two
hours of operation, and periodic iodometric analysis of the feed
and exit gas streams showed that the efficiency of conversion
during the run was 45.5%. These efficiency of conversion values
were within a reasonable range in the same order of magnitude
and established adequately that the active carbon could be
regenerated and reused without loss of efficiency. To confirm
the authenticity of the sulfur removal efficiencies for the
three successive two-hour reaction periods, the milky dispersions
of sulfur in water obtained rom the condenser during the first
two regeneration periods were separately evaporated at 110C
and the weights of sulfur in the residues measured; the carbon
with its absorbed sulfur from the third two-hour reaction
period was extracted in a Soxhlet extractor with carbon
.~.
disulfide, then the carbon disulfide was eva~orated and ~he
weight of sulfur residue determined. The weights of sulfur
thus recovered from the three successive regeneration steps
~- were in satisfactory agreement with the calculated values of
sulfur removal from the feed gas based on the analysis of the
feed gas and vent gas compositions.
- 14 -
~1:)75875~
The following example also illustrates the
effectiveness of activated carbon in aqueous suspension for
removing dilute admixed hydrogen sulfide and sulfur dioxide
from a gas stream and additionally illustrates the regeneration
of the active carbon for reuse in said operation after it has
been in use long enough to become extensively loaded with
absorbed free sulfur, like the preceding Example 3, but this
time the regeneration is achieved by extracting the sulfur from
the carbon with hot liquid xylene.
F.XAMPLE 4
- ~s in Example 3, the apparatus for removing the
dilute sulfur compound gases from the gas stream included the
glass tube with sintered glass plate in the bottom section, top
connections for a vent and introduction of aqueous medium and
` 15 regenerating medium, and bottom connections for introduction of
~ feed gas and removal of liquid extractant. Likewise as in
; Example 3, a flow of 1.3 liters/min. of nitrogen gas, 40 ml/min.
of hydrogen sulfide, and 20 ml/min. of sulfur dioxide was
: bubbled through a suspension of 5 grams of "Nuchar SN" activated
carbon in suspension in 100 ml of water on the sintered plate
for periods of two hours, with the suspension being maintained
at substantially 70C by a heating facility around the tube and
wit~ occasional additions of liquid water to the suspension to
maintain its volume around 100 ml. As previously, in each of
these two-hour reaction periods 9.4 grams of sulfur, in the
form of the gaseous sulfur compounds, was introduced into the
carbon suspension. The concentration of sulfur dioxide in-the
exit gases was monitored throughout the two-hour reaction
~L075~79
periods and found to increase only slightly during each two-
hour run. At the end of eacll of five such successive absor~tion
runs, the feed gas flow was stoppecl, and filtration commenced,
aided by application of inert gas pressure above the carbon
S suspension. At the end of this operation a wet carbon filter
cake remained on the sintered plate. At this point the bottom
of the reactor tube was connected to a tared liquid recovery
flas~ through a water trap, and a flow of xylene vapor from a
source of boiling xylene was connected to the top of the tube.
The xylene vapor forced residual water through the sintered
` plate as xylene condensed on the carbon, and the xylene in turn
filtered through the carbon filter cake and sintered plate into
the recovery flask. In filtering through the filter cake the
hot xylene condensate dissolved sulfur absorbed by the carbon,
and the flow of xylene vapor was continued until the temperature
in the filter cake reached 260F (127C), i.e. well above the
boiling point of the water-xylene azeotrope, indicating that all
the water was removed from the filter cake and the carbon was
completely contacted by th~ xylene. When the xylene flow was
stopped, the liquid xylene in the recovery flask was distilled
therefrom for reuse and the weight of the residue of sulfur
extracted from the carbon during the regeneration period and
remaining in the flask was measured. The weights of sulfur so
obtained for the five successive regeneration periods following
the five absorption runs respectively were 5.2, 3.9, 3.9, 4.2
and 4.4 grams. These weights corresponded respectively to 55.3%,
41.5%, 41.5~, 44.7% and 46.8% of the sulfur fed (as sulfur
compounds) to the active carbon suspension during the two-hour
- 16
~75~
absorption runs preceding the respective regeneration periods.
These sulfur recoveries established the effectiveness of ha~t
-liquid xylene to remove absorbed sulfur from the active carbon
and also established that, althou~h an initial high value of
sulfur removal efficiency was not fully maintained on
regeneration, the drop in efficiency was tolerable and was not
continuous, the recoveries (and corresponding removal
efficiencies) clearly having levelled off at an acceptable level
near 50%.
The foregoing examples have illustrated various
aspects of the invention. In addition it can be pointed out
that the invention may be adapted to achieve a reduction in
the overall amount of sulfur present as sulfur dioxide in
dilute concentration in, for example, a furnace stack gas or
lS a sulfide ore roasting process effluent. Such gaseous streams,
normally still containing sulfur dioxide even after primary
treatment but containing no hydrogen sulfide, may be treated
in accordance with the present invention by first adding a
stoichiometric proportion of hydrogen sulfide thereto to
achieve reaction with the sulfur dioxide to foxm elemental
sulfur, then intimately contacting the augmented stream with
an a~ueous suspension of solid catalytic material in accordance
with the process of the invention as previously described
herein.
Numerous modifications of the specific expedients
described herein can be made without departing from the scope
of the invention which is defined in the following claims.
