Finishing process for the removal of sulfur compounds from a gas stream

The invention relates to a finishing process for the removal of low level concentrations of mercaptans, sulfides and disulfides from a gas stream, especially a natural gas stream. The sulfur compounds are washed from the gas stream with a solution containing hydrogen peroxide, sodium carbonate or sodium hydroxide, and ammonia or an amine.

RELATED APPLICATION 
The process of this invention is similar to a process described in U.S. 
patent application Ser. No. 352,812, now U.S. Pat. No. 4,435,371. 
TECHNICAL FIELD 
A process has been developed for the removal of mercaptans and organic 
sulfides from a gas stream, more specifically, a natural gas stream. The 
present invention provides a process for the nearly total removal of 
sulfur compounds from a gas stream. 
BACKGROUND ART 
Removal of sulfur compounds from gas streams has been of considerable 
importance in the past and is even more so today due to environmental 
considerations. Gas effluent from the combustion of organic materials, 
such as coal, almost always contains sulfur compounds and sulfur removal 
processes have concentrated on removing hydrogen sulfide since it has been 
determined to be a significant health hazard. With increasing emphasis on 
the elimination of sulfur discharge to the atmosphere and the utilization 
of natural gas streams that were heretofore unusable due to their sulfur 
content, attention is turning to the removal of sulfur compounds from gas 
streams. 
The process of the present invention provides a finishing means for the 
removal of low level concentrations of mercaptans, sulfides and disulfides 
from a gas stream, specifically natural gas. 
Numerous natural gas wells produce what is called in the industry "sour 
gas." Sour gas is natural gas that contains hydrogen sulfide, mercaptans, 
sulfides and disulfides in concentrations that make its use unacceptable. 
Considerable effort has been expended to find an effective and cost 
efficient means to remove these objectionable sulfur compounds from 
natural gas. The commercially available processes for the desulfurization 
of natural gas have been primarily concerned with the removal of hydrogen 
sulfide. At present the interstate pipeline system for natural gas has a 
limit of one-quarter gram or 4 ppm of hydrogen sulfide. This can usually 
be obtained by the use of alkanol amine, iron sponge and zinc compound 
processes. However, none of these processes are suitable or effective for 
treatment of natural gas containing low levels of mercaptans, sulfides and 
disulfides. 
U.S. Pat. No. 4,283,373 by the present inventors discloses a process which 
consists of contacting a gas stream with alkali metal salts of 
sulfonamides or resins containing sulfinamide functionalities in the 
presence of an iron sponge bed. Further, U.S. Pat. No. 4,311,680 discloses 
a process wherein an iron sponge bed is enhanced through the use of 
hydrogen peroxide. These processes and others contained in U.S. Pat. Nos. 
632,400, 1,934,242, 4,027,002, 4,238,463 and 4,278,646 all describe 
processes resulting in the removal of most of the hydrogen sulfide. Some 
work has demonstrated that some of the mercaptans, as well as some of the 
sulfides and disulfides and residual sulfides are removed. However, these 
processes do not meet the more severe demands placed on special uses of 
natural gas or gas streams where lower sulfur levels are required. 
A very pure natural gas is required for foundry applications wherein metals 
susceptible to sulfur poisoning or degradation are melted or alloyed. 
Specifically, furnaces which melt aluminum alloys require extremely low 
sulfur gas since the sulfur compounds react with aluminum. Aluminum alloys 
can be directly fired by natural gas in graphite crucibles; however, the 
graphite crucible is porous to gases so that the molten aluminum alloy 
absorbs sulfur compounds which are converted to aluminum sulfide. The 
presence of this sulfide adversely affects the alloy so that poor molds 
are obtained. Thus, it has been found desirable to reduce total sulfur to 
1 to 6 ppm level in the natural gas. 
There is a present need for an efficient low cost finishing process which 
will essentially remove all of the mercaptans, sulfides and disulfides 
from a gas stream. 
All the references cited do not suggest or disclose a process for the 
removal of mercaptans, sulfides, disulfides and residual sulfides from a 
gas stream through the use of a scrubbing solution which consists of 
hydrogen peroxide alone or in combination with ammonia or an amine. 
DISCLOSURE OF THE INVENTION 
There is disclosed a process for removing mercaptans, sulfides, disulfides 
and residual sulfides from a gas stream that is essentially free of 
hydrogen sulfide which consists of contacting said gas stream with a 
solution containing ammonia or an amine and hydrogen peroxide. Further, 
there is disclosed a process for the removal of sulfur compounds from a 
gas stream which comprises contacting said gas stream with a solution 
consisting of (a) 1 to 25 percent by weight hydrogen peroxide, (b) 1 to 15 
percent by weight of sodium carbonate or sodium hydroxide; (c) 1 percent 
by weight to saturation of ammonia or an amine, and (d) 1 to 5 percent by 
weight sodium silicate. 
The process of the present invention can be carried out in single or 
multi-stage operations. Single stage operation is preferred for natural 
gas with sulfur levels of 10 to 50 ppm, consisting of sulfur compounds 
other than H.sub.2 S. For natural gas containing large amounts of H.sub.2 
S a preferred process would consist of a first unit which would remove 
most of the H.sub.2 S. For gases containing larger amounts of H.sub.2 S 
the alkanol amine scrubber is recommended and for lesser amounts an iron 
sponge bed continuously regenerated with hydrogen peroxide is preferred. 
The remaining mercaptans, sulfides and disulfides are then removed in a 
second unit which contains a scrubbing solution which consists of hydrogen 
peroxide alone or in combination with ammonia or an amine. 
The treatment unit or scrubber is fabricated from any material which is 
inert to hydrogen peroxide and ammonia or the amine, such as aluminum or a 
polyolefin coated vessel. The scrubber is designed so as to provide a fine 
dispersion of bubbles through the peroxide/amine or ammonia solution, i.e. 
the scrubber can be fitted with distributor plates or more conveniently 
filled with a packing such as berl saddles, polyethylene cylinders, 
vermiculite, or aluminum protruded packing. It would be obvious to a 
skilled chemical engineer that numerous devices would be appropriate for 
use of the present invention. The requirement is that the gas stream to be 
treated have sufficient contact with the treatment solution so as to 
effect economical, efficient, and almost total removal of mercaptans, 
sulfides, disulfides and residual sulfides from the gas stream. Further, 
the scrubber filled with the peroxide/amine or ammonia solution can have 
the gas passed upwards through the solution or it can be a trickle bed 
type system in which the hydrogen peroxide/amine or ammonia solution flows 
downward through the treatment vessel with the gas flow in the same or 
opposite direction. 
Another means for contacting the gas stream with the scrubber solution 
consists of a spray dispersion of the solution into the gas stream. This 
is accomplished by the scrubbing solution being sprayed under pressure 
into a treatment vessel through which the sour gas is passing. The liquid 
is collected at the bottom of the treatment vessel and either rejuvenated 
and recirculated to the treatment vessel or discarded. 
The treatment solution which contains hydrogen peroxide and ammonia or an 
amine can contain hydrogen peroxide at a concentration that can be varied 
over a wide range, with the preferred range being 3 to 25 percent H.sub.2 
O.sub.2 by weight. A more preferred concentration of the hydrogen peroxide 
is 10 to 25 percent by weight. It has been found that a scrubber solution 
of hydrogen peroxide/amine or ammonia is most preferred. This solution has 
been more effective than hydrogen peroxide solution alone in removing the 
more refractory sulfur compounds whose chemical structure is not known. 
The hydrogen peroxide/amine or ammonia mixture is, however, less stable. 
The half-life for H.sub.2 O.sub.2 in a 0.8 molar ammonium hydroxide 0.2 
molar peroxide solution was 40 hours at room temperature. This suggests 
that an operation where small amounts of ammonium hydroxide and H.sub.2 
O.sub.2 solution are constantly and individually added to the treatment 
unit would be the optimum method of operation. 
It has been found that the hydrogen peroxide/amine or ammonia scrubbing 
solution may be advantageously stabilized through the addition of 1.0 to 
5.0 percent by weight sodium silicate to total solution. 
Representative of other oxidizing agents that may be useful in the process 
of the present invention are tert-dibutyl peroxide, t-butyl hydroperoxide, 
cumene hydroperoxide, dicumyl peroxide and other commercially available 
organic peroxides and hydroperoxides. The most preferred oxidizing agent 
is hydrogen peroxide. 
Representative of the amines that are useful in the process of the present 
invention are methyl amine, dimethyl amine, ethyl amine, diethyl amine, 
ethanol amine, diethanol amine, propanol amine and dipropanol amine. 
Preferred amines are methyl amine, dimethyl amine, trimethyl amine and 
diethanol amine. Ammonia is more preferred than the amines. 
The amine in the scrubbing solution may be anhydrous, aqueous solutions of 
the amine or water/alcohol solutions of the amine. Alcohols of 1 to 4 
carbon atoms are useful in preparing the water/alcohol amine solutions. 
Such alcohols include methanol, ethanol, propanol, isopropanol, butanol 
and isobutanol. 
The concentration of ammonia or the amine in the scrubbing solution bed is 
preferably high. It has been found that concentrations of 0.2 Normal to 
saturated solutions are appropriate for a batch-type; however, the more 
concentrated solutions are preferred. In fact, the addition of anhydrous 
ammonia or anhydrous amines is advantageous since lower concentrations 
only result in the unnecessary addition of liquid material to the 
treatment vessel which eventually has to be removed. 
The use of the alcohol cosolvent in the preparation of the scrubbing 
solution is only required when the amines have limited water solubility. 
Only when the amine has limited water solubility should the alcohol 
cosolvent be used. 
To one skilled in chemistry it is readily apparent that anhydrous amines, 
and anhydrous ammonia, will, when placed in an aqueous media, form the 
hydrates thereof, i.e., ammonium hydroxide as well as aqueous ammonia. The 
process of the present invention contemplates these hydrates and has found 
the use of ammonia dissolved in water to be especially useful. 
The applicants have found that the use of a caustic solution will enhance 
the process of the present invention. Aqueous solutions of NaOH, KOH and 
Na.sub.2 CO.sub.3 have been found to be appropriate. 
The process of the present invention may be conducted on a continuous basis 
or batch type. In the total batch type operation the fluid-filler reactor 
is operated until the scrubbing solution is no longer effective and is 
then replaced by fresh scrubbing solution. In continuous operation a rate 
of addition of hydrogen peroxide and ammonia or amine is established to 
effect the desired removal of sulfur compounds while exhausted solution 
containing flocculent sulfur is discharged from the reactor. Further, the 
hydrogen peroxide may be continuously added to the solution with only 
intermittent addition of ammonia or the amine. This is preferred since the 
hydrogen peroxide decomposes during the reaction while the amine and 
ammonia do not. 
The amine or ammonia in either anhydrous or hydrated form may be metered in 
the gas stream prior to entry into the peroxide solution containing 
reactor. 
BEST MODE FOR CARRYING OUT THE INVENTION 
The process of the present invention was demonstrated using a cylindrical 
glass reactor measuring 4 inches (10.16 cm) by 48 inches (121.9 cm) fitted 
with metal plates clamped at either end to which were attached the inlet 
and outlet connections. The gas flow was metered at 27 psig (.apprxeq.186 
kPa) with a Brooks pressure rotometer unit which measured flow between 200 
cc per minute and 1.2 liters per minute under standard conditions. The 
glass reactor was filled with quarter-inch, half moon, ceramic saddles, 
available from the Norton Company, with a void space of approximately 4.8 
liters for the scrubber solution. 
The process of the present invention was utilized on a sour natural gas 
well from the Newburg formation of Ohio. The well-head analysis of the 
sour gas was as follows: 
______________________________________ 
Compound ppm by wt 
______________________________________ 
H.sub.2 S 134.4 
CH.sub.3 SH 2.1 
C.sub.2 H.sub.5 SH 
16.9 
C.sub.4 H.sub.9 SH 
5.9 
C.sub.5 H.sub.11 SH 
1.7 
Sulfides 12.9 
Residual sulfides 
0.2 
______________________________________ 
The natural gas prior to treatment by the process of the present invention 
was first contacted with an iron sponge treater so as to essentially 
remove all the H.sub.2 S. The analysis of the gas stream after passing 
through the iron sponge treater analyzed by a Barton Titrator measured 22 
ppm sulfur by weight. This 22 ppm sulfur by weight consisted of about 8 to 
10 ppm of sulfides and disulfides and 12 to 14 ppm of mixed mercaptans. 
Passage of this prior treated gas through 4.7 liters of 3 percent H.sub.2 
O.sub.2 (0.88 molar) at a gas flow rate of 340 cc per minute (standard) 
lowered the sulfur level to 3 ppm. When the scrubbing solution was changed 
to 0.88 molar H.sub.2 O.sub.2 (3 percent) and 2.4 molar NH.sub.4 OH and a 
gas flow of 340 cc per minute the total sulfur content was reduced to 
about 1 ppm.

EXAMPLES 1 THROUGH 6 
The treatment vessel was as described above, however, the flow rate and 
concentration of the peroxide and ammonium hydroxide were varied in the 
following examples. The temperature of the reaction was ambient 
temperature. Table I sets out the results and variables for Examples 1 
through 6. 
TABLE I 
______________________________________ 
Example 
Scrubber Gas Flow Inlet S- Outlet S- 
No. Solution Rate Content ppm 
Content ppm 
______________________________________ 
1 3% H.sub.2 O.sub.2 
360 cc/min 
22 3 
(0.88 M) 
4.9 l 
2 3% H.sub.2 O.sub.2 
360 cc/min 
22 1 
(0.88 M) 
3.4 M 
NH.sub.4 OH 
4.7 l 
3. 11.5% 890 cc/min 
26 2 
H.sub.2 O.sub.2 
(3.4 M) 
4.2 l 
4. 11.5% 935 cc/min 
32 2 
H.sub.2 O.sub.2 
(3.4 M) 
4.2 l 
5. 11.5% 1.2 l/min 27 2 
H.sub.2 O.sub.2 
1 liter 
6. 11.5% 1.2 l/min 28 2 
H.sub.2 O.sub.2 
______________________________________ 
EXAMPLES 7 THROUGH 13 
The process of the present invention was also used on a natural gas well 
which is produced from the Clinton formation of Ohio. This gas contained 
approximately 18 ppm of unknown sulfur compounds. In addition this gas had 
been previously treated to remove essentially all of the H.sub.2 S. The 
treatment vessel and procedure were as described in Examples 1 through 6, 
however, the scrubbing solution, the flow rate and the concentration of 
the scrubbing solution were varied. 
Table II sets out the variables with the results. 
TABLE II 
______________________________________ 
Scrubber 
Exam- Solution 
ple (Total Gas Flow Inlet S- Outlet S- 
No. Volume) Rate Content ppm 
Content ppm 
______________________________________ 
7 11.5% 650 cc/min 
18 11 
H.sub.2 O.sub.2 
(4.2 l) 
8 11.5% 200 cc/min 
18 11 
H.sub.2 O.sub.2 
(4.2 l) 
9 25% 330 cc/min 
18 12 
H.sub.2 O.sub.2 
(3.5 l) 
10 5% Na.sub.2 CO.sub.3 
370 cc/min 
18 9 
11.5% 
H.sub.2 O.sub.2 
(4.0 l) 
11 5% Na.sub.2 CO.sub.3 
900 cc/min 
18 12 
11.5% 
H.sub.2 O.sub.2 
(4.0 l) 
12 3.2 M 1.2 l/min 18 12 
NH.sub.4 OH 
3.4 M 
H.sub.2 O.sub.2 
(4.1 l) 
13 3.2 M 500 cc/min 
18 6 
NH.sub.4 OH 
3.4 M 
H.sub.2 O.sub.2 
(4.1 l) 
______________________________________ 
From Table II it is evident that natural gas which contains 18 ppm by 
weight of sulfur compounds is made up of 7 ppm of sulfur compounds that 
are easily removed by H.sub.2 O.sub.2. This indicates that the removed 
compounds are probably mercaptans and sulfides. The remainder about 11 ppm 
are more refractory sulfur compounds. About 2 ppm was further removed by 
using Na.sub.2 CO.sub.3 --H.sub.2 O.sub.2 solution while an additional 5 
ppm was removed with the NH.sub.4 OH--H.sub.2 O.sub.2 scrubbing solution. 
It is evident that the NH.sub.4 OH--H.sub.2 O.sub.2 scrubbing solution is 
more effective than Na.sub.2 CO.sub.3 --H.sub.2 O.sub.2 or H.sub.2 O.sub.2 
alone for the desulfurization of the gas stream. 
INDUSTRIAL APPLICABILITY 
The process of this invention which employs the use of hydrogen peroxide 
and ammonia or an amine has numerous industrial applications. The need for 
effective and economic means for finishing a gas stream which contains low 
level concentrations of sulfur compounds has been long felt. This 
invention will be useful in treating effluent from coke ovens, sewage 
plants, paper mills and in particular, sour natural gas streams, wherein 
the use of the natural gas requires extremely low levels of sulfur 
compounds. 
While certain representative embodiments and details have been shown for 
the purpose of illustrating the invention, it will be apparent to those 
skilled in this art that various changes and modifications may be made 
therein without departing from the scope of the invention.