U.S. patent number RE32,009 [Application Number 06/368,748] was granted by the patent office on 1985-10-22 for degassing molten sulfur.
This patent grant is currently assigned to Exxon Research and Engineering Co.. Invention is credited to Thomas H. Ledford, Howard Lerner.
United States Patent |
RE32,009 |
Ledford , et al. |
October 22, 1985 |
Degassing molten sulfur
Abstract
A method for reducing the hydrogen sulfide, hydrogen
polysulfides and/or ash content of molten sulfur is disclosed. The
subject invention is directed at the addition of an effective
amount of a degassing agent selected from the class consisting of
inorganic phosphorous compounds, urea and urea derivatives without
the necessity for also adding sulfur dioxide to the molten
sulfur.
Inventors: |
Ledford; Thomas H. (Baton
Rouge, LA), Lerner; Howard (Parsippany, NJ) |
Assignee: |
Exxon Research and Engineering
Co. (Florham Park, NJ)
|
Family
ID: |
26870298 |
Appl.
No.: |
06/368,748 |
Filed: |
April 15, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
174504 |
Aug 1, 1980 |
04299811 |
Nov 10, 1981 |
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Current U.S.
Class: |
423/578.1;
423/567.1 |
Current CPC
Class: |
C01B
17/0232 (20130101) |
Current International
Class: |
C01B
17/00 (20060101); C01B 17/02 (20060101); C01B
017/027 () |
Field of
Search: |
;423/567,578,578R,578A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2254375 |
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May 1973 |
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DE |
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1393967 |
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May 1975 |
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GB |
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1402274 |
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Aug 1975 |
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GB |
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1433822 |
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Apr 1976 |
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GB |
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Other References
Rennie, "The Removal of H.sub.2 S dissolved in Liquid Sulphur"
Quarterly Bulletin, Alberta Sulphur Research Ltd., vol. IX, No. 4
Jan.-Mar. 1973, pp. 19-23..
|
Primary Examiner: Thomas; Earl C.
Attorney, Agent or Firm: Mazer; Edward H.
Claims
What is claimed is:
1. A method for reducing the hydrogen sulfide and hydrogen
polysulfides concentration in molten sulfur comprising the addition
to the molten sulfur of an effective amount of a degassing agent,
said degassing agent selected from the class consisting of
inorganic .[.phosphorous.]. .Iadd.phosphorus .Iaddend.compounds,
urea and urea derivatives without the further addition to the
molten sulfur of sulfur dioxide .Iadd.and/or a gas which oxidizes
molten sulfur to sulfur dioxide.Iaddend..
2. The method of claim 1 wherein the inorganic .[.phosphorous.].
.Iadd.phosphorus .Iaddend.compounds are selected from the class
consisting of phosphates, phosphines, phosphites, phosphine oxides,
phosphine sulfides and hypophosphates.
3. The method of claim 2 wherein the inorganic .[.phosphorous.].
.Iadd.phosphorus .Iaddend.compounds are selected from the class
consisting of triammonium orthophosphate and diammonium hydrogen
phosphate.
4. The method of claim 1 wherein the urea derivatives are selected
from the class consisting of mono-, di-, tri- and tetra ureas,
thiourea and cyclic ureas.
5. The method of claim 1 wherein the rate of addition of the
degassing agent to the molten sulfur ranges from about 1 part per
billion to about 5000 parts per million by weight.
6. The method of claim 5 wherein the rate of addition of the
degassing agent to the molten sulfur ranges from about 50 to about
200 parts per million by weight.
7. The method of claim 1 wherein the hydrogen sulfide and hydrogen
polysulfides are at least partially removed from the molten sulfur
in a sulfur installation comprising an inlet, an inspection box
communicating with the inlet, an outlet communicating with the
inspection box, and a sulfur storage facility communicating with
the outlet, the molten sulfur flowing from the inlet through the
inspection box and the outlet to the sulfur storage facility, the
method comprising the addition of the degassing agent to the
inspection box, whereby the degassing agent becomes intermixed with
the molten sulfur to thereby release at least a portion of the
hydrogen sulfide and hydrogen polysulfides during the residence
time of the molten sulfur in the sulfur storage facility. .[.8. A
method for reducing the hydrogen sulfide or hydrogen polysulfides
present in molten sulfur which comprises treating the molten sulfur
with an effective amount of a degassing agent, said degassing agent
consisting essentially of inorganic phosphorous compounds, urea and
urea derivatives..]. .[.9. The method of claim 8 wherein the
inorganic phosphorous compounds are selected from the class
consisting of phosphates, phosphines, phosphites, phosphine oxides,
phosphine sulfides and hypophosphates..]. .[.10. The method of
claim 8 wherein the urea derivatives are selected from the class
consisting of mono-, di-, tri-, and tetra-ureas, thiourea, and
cyclic urea..]. .[.11. A method for reducing the concentration of
hydrogen sulfide and hydrogen polysulfides in molten sulfur in a
sulfur installation of the type having an inlet, an inspection box
communicating with the inlet, and outlet communicating with the
inspection box, and a sulfur pit communicating with the outlet, the
molten sulfur flowing from the inlet through the inspection box and
outlet into the sulfur storage facility, the method comprising the
addition of an effective amount of a degassing agent to the
inspection box, the degassing agent intermixing with the sulfur to
release at least a portion of the hydrogen sulfide and hydrogen
polysulfides during the residence of the molten sulfur in the
sulfur storage facility..]. .[.12. The method of claim 11 wherein a
source of the degassing agent is disposed at an elevation above
that of the inspection box, the degassing agent being added by
gravity flow to the inspection box..].
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for the removal of H.sub.2 S
from molten sulfur. More specifically this invention relates to the
removal of residual H.sub.2 S from molten sulfur by the addition to
the molten sulfur of one or more compounds described hereinafter
which promote the removal of H.sub.2 S and hasten the decomposition
of hydrogen polysulfides to H.sub.2 S. Hastening this decomposition
permits convenient disposal of the released H.sub.2 S before the
sulfur is shipped by the manufacturer.
During the manufacture of elemental sulfur, particularly by the
reaction of:
small quantities of H.sub.2 S and hydrogen polysulfides, typically
150-300 parts per million by weight (wppm), may remain in the
molten sulfur. During storage the hydrogen polysulfides decompose
to hydrogen sulfide, which is slowly released by the sulfur. This
slow release of hydrogen sulfide is undesirable for several
reasons. Since the threshold odor limit of H.sub.2 S in air is
relatively low, approximately 0.1 parts per million by volume
(vppm), this slow release may create nuisance odors. Moreover,
H.sub.2 S is highly toxic, flammable and may form explosive
mixtures with air. Accordingly, the presence of H.sub.2 S in molten
sulfur is being limited in many locations by increasing stringent
regulations.
Several methods have been used to decrease the residual hydrogen
sulfide and hydrogen polysulfides concentration in sulfur. One
method has been merely to allow a "weathering off" process to take
place before the sulfur is transported. But, this is a relatively
slow process that requires large sulfur storage capacity.
A method for increasing the rate of this "weathering off" process
uses a circulatory spraying system. However, this requires the
purchase and use of additional circulatory equipment and may not
reduce the residual hydrogen sulfide and hydrogen polysulfide to
sufficiently low levels.
Other methods have involved the addition of chemical compounds and
sulfur dioxide to the molten sulfur to convert the hydrogen sulfide
and hydrogen polysulfides to elemental sulfur. British Pat. No.
1,393,967 describes a method for reducing the amount of hydrogen
polysulfides and hydrogen sulfide by in situ reaction to form
elemental sulfur. Sulfur dioxide and an additive, which is an
inorganic alkali metal compound, ammonia, an ammonium compound or a
basic organic compound with at least one amino group, is injected
into the liquid sulfur while the sulfur is flowing through a
conduit. Table I of this patent indicates that large amounts of
H.sub.2 S remain in the molten sulfur when SO.sub.2 is not added
together with the additives noted. U.S. Pat. No. 3,447,903
discloses the addition of certain classes of compounds together
with SO.sub.2 to molten sulfur for a variety of purposes, including
the removal of small quantities of hydrogen sulfide. Among the
classes of compounds disclosed are:
(A) Ammonia and its organic derivatives having a K.sub.B greater
than 10.sup.-10 ;
(B) Primary, secondary and tertiary alkyl, aryl and cyclic amines
having a K.sub.B greater than 10.sup.-10 ;
(C) Quaternary basic nitrogen compounds that decompose between
20.degree. and 160.degree. C. to yield ammonia or its inorganic
derivative having a K.sub.B value greater than 10.sup.-10 ; and
(D) Quaternary basic nitrogen compounds that on heating to
temperatures between 20.degree. and 160.degree. C., decompose to
yield an organic amine having a K.sub.B value greater than
10.sup.-10. However, this process requires the further addition of
sulfur dioxide to react with the H.sub.2 S in place and form sulfur
rather than liberating the H.sub.2 S.
Use of SO.sub.2 to remove H.sub.2 S from molten sulfur is not
desirable for several reasons. Sulfur dioxide is difficult to
store, since it is gaseous at ambient conditions. Thus, means must
be provided for injecting the SO.sub.2 directly into the molten
sulfur and for adjusting the relative quantities of SO.sub.2 and
the other additive metered into the molten sulfur. In addition,
SO.sub.2 is toxic, a strong irritant to the eyes and mucous
membranes and an air contaminant. Moreover, SO.sub.2 is corrosive
to the carbon steel surfaces of some sulfur handling facilities.
These factors may complicate or restrict its use in commercial
facilities.
U.S. Pat. No. 3,278,269 describes several classes of compounds
which may be added to molten sulfur to improve the friability of
solidified sulfur. The compounds disclosed include the
following:
A. Liquid and solid primary, secondary and tertiary alkyl, aryl,
and cyclic amines having a K.sub.B value greater than 10.sup.-10 in
which the amino nitrogen is attached to a primary, secondary or
aromatic hydrocarbon;
B. Quaternary nitrogen compounds that decompose between
20.degree.-160.degree. C. to yield ammonia; and
C. Quaternary nitrogen compounds that on heating to temperatures
between 20.degree. and 160.degree. C. decompose to yield an organic
amine having a K.sub.B value greater than 10.sup.-10 in which the
carbon atom directly attached to the amino nitrogen is primary,
secondary or aromatic.
U.S. Pat. No. 3,364,655 discloses that H.sub.2 S can be removed
from liquid sulfur by atomizing the sulfur, and that this process
can be further improved by the addition of ammonia.
In Z. Anal. Chem. 166 (1959) pages 274-283 Schmidt and Talsky
describe a method to convert hydrogen polysulfides to hydrogen
sulfide by the addition to the sulfur of sulfites or cyanides.
It is desirable to provide a process in which the residual hydrogen
sulfide and hydrogen polysulfides are reduced to relatively low
levels without the use of excessively large, or corrosion resistant
storage facilities.
It is also desirable to provide a process in which the hydrogen
sulfide and hydrogen polysulfides can be reduced to relatively low
levels in the molten sulfur by the addition of degassing agents
that do not require complex addition or metering facilities.
It is also advantageous to provide a process in which the hydrogen
sulfide and hydrogen polysulfides are reduced to lower levels by
the addition of chemical compounds that do not significantly
increase the ash content of the product sulfur and may, in certain
instances, reduce the product sulfur ash content.
SUMMARY OF THE INVENTION
The subject invention is a method for reducing the concentration of
hydrogen sulfide and hydrogen polysulfides in molten sulfur
comprising the addition to the sulfur of an effective amount of a
degassing agent selected from the class of compounds consisting of
inorganic phosphorous compounds, urea and urea derivatives without
the further addition to the molten sulfur of sulfur dioxide. Among
the preferred inorganic phosphorous compounds are the phosphates,
phosphines, phosphites, phosphine oxide and hypophosphates. Among
the preferred urea derivatives are mono-, di, tri- and tetra ureas,
thiourea, and cyclic ureas. In one preferred method an aqueous
solution of the degassing agent or a melt of the degassing agent is
metered by gravity flow into the molten sulfur. In another method,
an aqueous solution or a melt of the degassing agent is pumped into
the molten sulfur.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plot of the percentage of the hydrogen sulfide and
hydrogen polysulfides removed from samples during a 2.5 hour period
as a function of the degassing agent concentration for three
degassing agents.
FIG. 2 is a simplified drawing of a molten sulfur facility with
means for adding a degassing agent to the sulfur.
FIG. 3 is a plot of the residual H.sub.2 S concentration in sulfur
as a function of time after starting the addition of a specific
degassing agent .
DETAILED DESCRIPTION OF THE INVENTION
Molten sulfur from conventional sulfur recovery sources typically
may contain as much as 700 parts per million by weight (wppm) of
H.sub.2 S and hydrogen polysulfides. The degassing agents used to
convert hydrogen polysulfides to hydrogen sulfide preferably should
be effective at relatively low concentrations and be relatively
easily volatilized from the sulfur so that they do not adversely
affect the residual ash content of the product sulfur. The
compounds described hereinafter have been found to be effective
degassing agents at relatively low concentrations, thereby
minimizing their effect on the product sulfur ash content. Many of
the compounds described hereinafter are readily volatilizable. This
further minimizes their effect on product sulfur ash content. It
has been found that certain classes of compounds are effective
degassing agents as indicated below:
A. Sulfur compounds comprising dithionates, dithionites,
thiosulfates, bisulfides and bisulfites. Among the preferred
compounds are easily volatilizable salts, such as ammonium,
phosphonium and alkyl ammonium salts. Specific preferred compounds
are ammonium thiosulfate and ammonium bisulfite. A non-ammonium
salt which is effective is sodium dithionite.
B. Inorganic and organo phosphorous compounds including phosphates,
hypophosphates, phosphines, phosphites, phosphine oxides and
phosphine sulfides.
Preferred compounds are the easily volatilized salts, such as
ammonium phosphonium and alkyl ammonium salts. Specific preferred
compounds are triammonium orthophosphates, diammonium hydrogen
phosphate and triphenyl phosphine.
C. Urea and urea derivatives, including but not limited to urea,
thiourea, mono-, di-, tri-, and tetra-ureas and cyclic ureas.
Preferred compounds are those easily volatilized, such as urea and
thiourea.
Samples of candidate degassing agents were screened for
effectiveness according to the following procedure. Samples of
molten sulfur were taken from the combustor rundown line of a
commercial sulfur plant so that sulfur having the highest H.sub.2 S
levels would be obtained for testing purposes. The samples,
approximately 150 ml each, were poured from a sample dipper
directly into spoutless 300 ml Berzelius beakers which also were
used to contain the sulfur during analysis. The samples were kept
hot during transport to the laboratory. A three-hole rubber stopper
equipped with a thermometer and two unequal lengths of glass tubing
was placed in the beaker opening. One piece of glass tubing
extended below the surface of the sulfur while the other terminated
above the surface. Each compound screened was added to a stirred
sulfur sample maintained at 130.degree. C. Dry nitrogen was sparged
through each sammple for 2.5 hours with the released H.sub.2 S and
exiting nitrogen being directed through two gas absorption bottles
containing a 3% zinc acetate solution. After the sulfur was purged
for 2.5 hours, the two gas absorption bottles were removed and
replaced by two gas absorption bottles containing fresh 3% zinc
acetate solution positioned as previously described.
Although lead sulfide is known in the art to be highly effective in
promoting the decomposition of hydrogen polysulfides to H.sub.2 S,
this compound is not used commercially to degas molten sulfur
because of toxicity concerns from the presence of trace amounts of
lead in the sulfur and because lead sulfide would increase the ash
content of the sulfur. This known effectiveness of lead sulfide was
used as a standard to determine the degassing characteristics of
the compounds screened. One gram of lead sulfide was then added to
the sulfur samples containing the degassing agents to decompose any
hydrogen polysulfides which had not been removed by the degassing
agent added during the first 2.5 hour period. Dry nitrogen was
again sparged through the molten sulfur for a 2.5 hour period, the
exiting nitrogen and H.sub.2 S passing into the absorption train.
The gas absorption bottles from the first and second 2.5 hour
periods were poured into separate beakers for titration by a
standard iodometric method. A known quantity of standardized iodine
solution was added to each beaker and the unreacted iodine
subsequently back-titrated with a standardized thiosulfate
solution. This method is described in more detail in the Analytical
Chemistry Of Sulfur And Its Compounds, Edited by J. H. Karchmer,
Part I, John Wiley & Sons, NY Pages 63-66 (1970), the
disclosure of which is incorporated herein by reference. The
effectiveness of each compound screened was determined by comparing
the amount of H.sub.2 S removed during the first 2.5 hour period
with that removed during the second 2.5 hour period. The higher the
ratio of the H.sub.2 S removed during the first period relative to
the second period, the more effective was the additive. Analytical
results for various compounds tested are shown in Tables I, II and
III at additive concentrations of 200, 100 and 50 weight parts per
million (wppm) respectively, in the molten sulfur. In Table I, all
samples having the same test group number were drawn on the same
day and have comparable initial H.sub.2 S values.
TABLE I ______________________________________ TESTS AT 200 WPPM
ADDITIVE LEVEL First 2.5 To- hr. With Second 2.5 tal Test Additive
hr. With PbS (ppm Group Additive (ppm H.sub.2 S) (ppm H.sub.2 S)
H.sub.2 S) ______________________________________ 1 None 246 282
528 1 H.sub.2 O(<1000 ppm) 167 355 522 2 NH.sub.4 Cl 218 272 490
2 NH.sub.4 HSO.sub.4 259 266 525 3 (NH.sub.4).sub.2 C.sub.2 O.sub.4
314 310 624 3 (NH.sub.4).sub.2 S.sub.2 O.sub.3 619 3 622 3 Na.sub.2
S.sub.2 O.sub.4 583 6 589 3 (C.sub.6 H.sub.5).sub.3 P 625 32 657* 4
NH.sub.4 SCN 331 190 521 4 (NH.sub.4).sub.3 PO.sub.4 497 4 503 5
CO(NH.sub.2).sub.2 486 18 504 6 (NH.sub.4).sub.2 S.sub.2 O.sub.3
213 159 372 7 (NH.sub.2).sub.2 CS 614 0 614 8 NH.sub.4 CO.sub.2 H
335 498 833 ______________________________________ *Some phosphine
carried by the sparge gas into the solution causes an artificially
large value because the analysis cannot distinguish between
phosphines and H.sub.2 S.
TABLE II ______________________________________ TESTS AT 100 WPPM
ADDITIVE LEVEL First 2.5 Hrs Second 2.5 Hr. Total Additive With
Additive With PbS H.sub.2 S (wppm)
______________________________________ NH.sub.4 HSO.sub.3 289 153
442 (NH.sub.4).sub.2 SO.sub.3 421 25 446 (NH.sub.4).sub.2 S.sub.2
O.sub.3 428 2 430 (NH.sub.4).sub.3 PO.sub.4 374 93 467
(NH.sub.4).sub.2 SO.sub.3 401 145 546 (NH.sub.4).sub.2 S.sub.2
O.sub.3 418 162 580 (NH.sub.4).sub.2 HPO.sub.4 496 74 570
(NH.sub.4).sub.3 PO.sub.4 272 310 582
______________________________________
The relative degassing ability of the compounds may be seen more
clearly by their effectiveness at low concentrations in the sulfur.
The relative effectiveness of three selected compounds in degassing
sulfur at varying concentrations in the sulfur may be seen from
FIG. 1, in which the degassing agent concentration is plotted
against the percent of the total H.sub.2 S released in the first
2.5 hours.
The data in Table I show that the ammonium ion is not the portion
of the ammonium salt that was active in degassing the molten
sulfur. For example, ammonium chloride and ammonium bisulfate were
totally ineffective as degassing agents. Even ammonium salts of
weaker acids such as oxalic, formic and carbonic acids were much
less effective than the best degassing agents tested, ammonium
thiosulfate, ammonium phosphate, and sodium dithionite.
The ash content of the molten sulfur sample with no degassing agent
present and with one degassing agent, ammonium thiosulfate, was
determined and is presented in Table IV. In both series of tests
and the ash determination involved merely burning a 100 gram sample
of the sulfur in a pre-weighed crucible and weighing the ash
remaining in the crucible after combustion and subsequent cooling
in a desiccator.
The possibility that some interaction of ammonium thiosulfate with
either carbon steel or concrete from the molten sulfur facility
might cause increased sulfur ash content, also was examined.
Samples of sulfur were kept at 130.degree.-140.degree. C. for 24
hours while suspended within them were small blocks of concrete and
small samples of carbon steel. Two thousand wppm of ammonium
thiosulfate was added to one sample while another was left with no
degassing agent. Both the treated sulfur and the untreated control
were sampled from their top strata and from near the bottom of
their respective containers. These tests were done with samples
both low and high in ash content. The results are shown in Table
IV.
TABLE IV ______________________________________ ASH TESTS IN
PRESENCE OF CONCRETE AND CARBON STEEL Treated Sample (2.000 ppm
Untreated Sample (NH.sub.4).sub.2 S.sub.2 O.sub.3) (No Additive)
Ash Content (wppm) Ash Content (wppm)
______________________________________ Low Ash Sulfur Sample from
top 8.7 2.9 Sample from 25.6 -- bottom High Ash Sulfur Sample from
top 269 Sample from 10.280 11.780 bottom
______________________________________
The ammonium thiosulfate caused no corrosion of metal or damage to
concrete in these tests. The rate of corrosion of carbon steel
actually decreased after the addition of ammonium thiosulfate.
The increase in the ash content of the sample drawn from the top of
the high ash sulfur may be attributable in part to ash containing
compounds rising from the bottom to the top of the sample.
In laboratory tests it was found advantageous to have continuous
stirring of the sulfur during the degassing process. In two hour
tests, substantially similar to those previously described, a
significantly greater amount of H.sub.2 S was degassed when the
sample was stirred, than when no stirring was done, a shown in
Table V.
TABLE V ______________________________________ H.sub.2 S REMOVAL
WPPM FROM SAMPLES No Stirring Continuous Stirring
______________________________________ 265,296 408
______________________________________
It was believed that continuous mixing was necessary for reducing
the hydrogen sulfide and hydrogen polysulfide concentrations to
relatively low levels in commercial facilities. Surprisingly,
however, in a commercial-sized installation the sulfur could be
degassed to relatively low levels merely by gravity feeding the
degassing agent into the molten sulfur through the inspection box
or look box in the inlet line to the sulfur pit. After addition of
the degassing agent was started, the residual concentration of
hydrogen sulfide and hydrogen polysulfides decreased with time to
relatively low levels. Such a degassing agent addition system has
several advantages over complex mixing, metering and/or
recirculation systems. The simplicity of this system greatly
decreases equipment purchase and operating costs, while providing a
more reliable, trouble-free addition system.
In FIG. 2 a preferred embodiment for adding the degassing agent to
a conventional molten sulfur facility is shown. The facility
comprises an inlet, inspection box, outlet and sulfur pit. In this
figure, many lines, valves and all instrumentation has been omitted
for simplicity. Molten sulfur leaving the sulfur recovery system
passes through inlet 10 at a temperature in excess of 114.degree.
C., typically about 130.degree. to about 165.degree. C. The sulfur
then passes through seal 20 and line 30 into inspection box 40. The
molten sulfur, typically containing about 50 to about 650 wppm of
hydrogen sulfide and hydrogen polysulfides, leaves inspection box
40 through line 50 for storage and cooling in a sulfur storage
facility such as sulfur pit 60. A solution, preferably an aqueous
solution, of at least one of the degassing agents is metered into
inspection box 40 from storage tank 70 through line 80.
Alternatively, if the melting point of the specific degassing agent
utilized is below the temperature of the molten sulfur, the
degassing agent may be added as a melt. To minimize the
equipment
TABLE VI
__________________________________________________________________________
EFFECT OF ADDING AMMONIUM THIOSULFATE IN A CONVENTIONAL SULFUR PIT
ON THE H.sub.2 S CONCENTRATION OF SULFUR Time after Addition of
H.sub.2 S Concentration in Sulfur: wppm Ash Content of Sulfur: wppm
(NH.sub.4).sub.2 S.sub.2 O.sub.3 Started. Hrs. In Sulfur Pit
Exiting Sulfur Sulfur Pit From Loading Truck
__________________________________________________________________________
O(i.e. before addition 219, 220, 220, 236 -- 74.85 -- started) 20
231, 237 190, 194 -- 74, 71 43 241, 176 218, 264 -- 71, 70 66 181,
188 -- -- -- 70 199, 114 164, 164 -- 47, 55 89 201, 189, 210 -- --
-- 171 213, 224 -- -- --
__________________________________________________________________________
required, storage tank 70 may be placed above inspection box 40,
and the degassing agent gravity fed. The flow rate is controlled by
metering valve 90. The degassing agent solution or melt also may be
metered into the molten sulfur installation using a conventional
pump. A filter 100 also may be added to minimize contamination of
the molten sulfur and to prevent plugging of metering valve 90.
In many locations the tail gas from the reaction of H.sub.2 S and
SO.sub.2 for producing elemental sulfur is treated to remove small
amounts of H.sub.2 S by a series of reactions in an aqueous
solution containing inorganic salts. Occasionally these inorganic
salts are carried into the product sulfur. In one test a degassing
agent ammonium thiosulfate was added as an aqueous solution having
55 wt. % ammonium thiosulfate to molten sulfur having a temperature
ranging between about 125.degree. and 150.degree. C. The
concentration of ammonium thiosulfate in the sulfur entering the
sulfur pit without mechanical agitation was approximately 118 wppm.
The average residence time of the sulfur in the pit was 70-80 hrs.
It was noted within the first few hours that a thick crust formed
on the sulfur surface. As indicated in Table VI the ash content of
the sulfur, measured in a sample taken from the pit surface, was
74-85 wppm before the addition of ammonium thiosulfate was started.
After 70 hours from the start of the ammonium thiosulfate addition,
the ash content of the sulfur dropped to 47-55 wppm but the H.sub.2
S content of the sulfur was not significantly reduced. Later tests
with an aqueous solution containing 20-40 wt. % urea showed that
it, too, has the property of causing this separation of ash from
sulfur and formation of a layer that may be skimmed from the sulfur
surface.
To determine if the crust formed on the surface of the sulfur may
have inhibited the volatilization of hydrogen sulfide from the
sulfur pit, a test using an aqueous solution having 55 wt. %
ammonium thiosulfate as the degassing agent was done in a
four-stage sulfur pit having a total average residence time of
about 6 hours, but with mechanical sulfur recirculation.
Stages 1, 2 and 3 were equipped with sulfur recirculation pumps
having 160, 130 and 150 GPM capacities, respectively, to
recirculate the sulfur within the same stage. These results are
presented in Table VII and indicate that on a commercial scale
ammonium thiosulfate effectively catalyzes the degassing of
sulfur.
TABLE VII
__________________________________________________________________________
EFFECT OF AMMONIUM THIOSULFATE ON THE PERFORMANCE OF SULFUR
DEGASIFICATION FACILITY WITH CIRCULATION Ammonium Set of Time After
Addition Thiosulfate Residence Time Samples of (NH.sub.4).sub.2
S.sub.2 O.sub.3 in the Addition Rate, of Sulfur in H.sub.2 S
Concentration in Sulfur, wppm No. Run Started, Hrs. wppm Each
Stage, Hrs. 1st Stage 2nd Stage 3rd Stage 4th Stage
__________________________________________________________________________
(Inactive with- out recircula- tion pump) 1 -- 0 1.2 259-263 228
204 185-194 2 -- 0 1.6 339 339 328 315 3 -- 0 1.6 275 211 168 170 4
51/2 50 1.4 76 20 6 5
__________________________________________________________________________
It should be noted that the hydrogen sulfide and hydrogen
polysulfides concentration in Table VII decrease after the addition
of ammonium thiosulfate when the sulfur was recirculated. The data
of Tables VI and VII thus indicate that ammonium thiosulfate is an
effective degassing agent, and that it also may decrease the sulfur
ash content.
FIG. 3 is a plot of the residual H.sub.2 S concentration in molten
sulfur contained in a non-stirred sulfur pit where the sulfur, to
which about 100 wppm of urea was added, had an average residence
time ranging from about 3.5 to about 6 days. The residual
concentration of H.sub.2 S decreased significantly, illustrating
that continuous use of urea also decreases the residual H.sub.2 S
concentration of the sulfur even in the absence of sulfur
agitation. The ash content of the sulfur, both at the commencement
and termination of the test, remained at less than 5 wppm, thus
indicating that the urea addition did not adversely affect the ash
content of the sulfur.
The concentration of degassing agent utilized is not critical and
will be dependent on the specific agent used. Generally, the rate
of addition of the degassing agent to the molten sulfur will range
between 1 (one) part per billion and 5000 parts per million by
weight, and preferably from about 50 to about 200 parts per million
by weight.
While the invention has been described with respect to specific
compounds and a specific embodiment, it will be understood that
this disclosure is intended to cover any variations, uses, or
adaptations of the invention including such departures from the
present disclosure as come within known or customary practice in
the art to which the invention pertains as a fall within the scope
of the invention.
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