Hydrogen sulfide removal with sulfur-containing esters

Sulfur-containing esters, such as dimethyldithiodipropionate, are excellent solvents for hydrogen sulfide and can be used for the purification of gas streams containing hydrogen sulfide. The obtained solution can be stored and used as H.sub.2 S source.

BACKGROUND OF THE INVENTION 
The removal of hydrogen sulfide from other fluids is a problem arising in a 
variety of industrial processes. Thus hydrogen sulfide is a byproduct in 
coal gasification. Hydrogen sulfide is also a common contaminant in 
natural gas streams. A variety of methods have been proposed to remove 
hydrogen sulfide from gas streams. There is a continued need for efficient 
ways to remove hydrogen sulfide from gas streams, particularly in view of 
environmental laws and the fact that hydrogen sulfide is a rather 
dangerous compound. 
THE INVENTION 
It is one object of this invention to provide a process for the removal of 
hydrogen sulfide from fluid streams, particularly from gas streams. 
Another object of this invention is to provide a new group of selective 
solvents for hydrogen sulfide. 
Still a further object of this invention is to provide a solution of 
hydrogen sulfide in a high concentration in a solvent. 
Yet another object of this invention is to provide a process of low 
pressure H.sub.2 S storage. 
These and other objects, advantages, details, features and embodiments of 
this invention will become apparent to those skilled in the art from the 
following detailed description of the invention and the appended claims. 
In accordance with this invention, it has been found that sulfur containing 
esters are good solvents for hydrogen sulfide. 
Thus, in accordance with a first embodiment of this invention, a process 
for the removal of hydrogen sulfide from a gas stream is provided. In 
accordance with this process, the hydrogen sulfide-containing gas stream 
is contacted with a sulfur-containing ester such as to produce a hydrogen 
sulfide solution in this ester as well as a gas stream having reduced 
hydrogen sulfide content as compared to the starting material. 
Broadly speaking, the solvent for hydrogen sulfide comprises one or more 
sulfur-containing esters such as esters of sulfur-containing acids. The 
esters useful in this invention can be defined as containing 
(a) 1 to 4, preferably 1 or 2 ester groups --COOR, wherein R is a 
hydrocarbyl radical, having 1 to 20 C atoms, preferably an alkyl radical, 
and 
(b) 1 to 6 sulfur groups selected from --SH and --(S).sub.x --, x being an 
integer of 1 to 6, preferably 1 to 4, most preferably 1 or 2. The sulfur 
groups and the ester groups are attached to one or more hydrocarbon 
structures each having 1 to 10 carbon atoms. The hydrocarbon structures 
are preferably alkyl structures. Generally, the ester group will be 
separated from the sulfur structure by one or more, preferably one or two 
carbon atoms. 
Examples of the so-defined esters of sulfur-containing acids which are 
useful in accordance with this invention are alkyl esters of sulfur (--SH, 
--(S).sub.x --) containing aliphatic acids. Among the alkyl esters of 
sulfur-containing aliphatic acids are the presently preferred selective 
solvents for hydrogen sulfide, namely methyl 3-mercaptopropionate: 
CH.sub.3 --O--CO--(CH.sub.2).sub.2 --SH, dimethylthiodipropionate: 
CH.sub.3 --O--CO--(CH.sub.2).sub.2 --S--(CH.sub.2).sub.2 
--CO--O--CH.sub.3, and dimethyldithiodipropionate: CH.sub.3 
--O--CO(CH.sub.2).sub.2 --S--S--(CH.sub.2).sub.2 --CO--O--CH.sub.3. These 
preferred sulfur-containing esters have two carbon atoms or a 1,2-ethylene 
group between the sulfur group and the ester group. 
The contacting of the hydrogen sulfide-containing fluid with the 
sulfur-containing ester or esters is carried out under standard conditions 
and in standard equipment. The specific process and equipment utilized 
depends to a significant degree upon the degree of hydrogen sulfide 
removal desired, the specific ester used as the selective solvent and the 
nature of the fluid from which the hydrogen sulfide is to be removed. 
Typically, the purification is carried out in a counter-current contacting 
column into which the ester of the mercapto acid is introduced at the top 
and the hydrogen sulfide-rich ester is withdrawn at the bottom. The gas 
stream containing hydrogen sulfide is introduced at the bottom of this 
column and the purified gas stream is withdrawn from the top. The 
contacting is usually carried out at a temperature above the melting point 
of the ester of the mercapto acid and normally under pressure conditions 
which correspond to the pressure of the gas stream to be purified. These 
conditions can be further characterized as follows: 
Temperature Range: any temperature above freezing point and below the 
decomposition temperature of the specific ester or mixture of esters being 
used. Lower temperatures favor greater H.sub.2 S solubility. 
Pressure Range: anywhere from a vacuum to higher pressure can be used. 
Higher pressures increase H.sub.2 S solubility. The pressure used is such 
as to keep the ester or ester mixture in the liquid phase at the operating 
temperature. 
Since the sulfur-containing ester dissolves a very high quantity of 
hydrogen sulfide, the product obtained, namely the solution of the 
hydrogen sulfide in the sulfur-containing ester can be stored and used as 
a source for hydrogen sulfide in other chemical processes, e.g., by 
heating this solution to free dissolved hydrogen sulfide as a gas. 
In accordance with a further embodiment there is thus provided an H.sub.2 S 
solution which is usable as an H.sub.2 S source. The H.sub.2 S content of 
the solution lies in the range of 5 ppm to saturation. The H.sub.2 S 
solution comprises the sulfur-containing ester (or a mixture of two or 
more of such esters) and H.sub.2 S. The quantity of hydrogen sulfide 
dissolved in the sulfur-containing ester solvent will preferably be 
between 1 vol. % (liquid volume) and saturation. Furthermore, an 
embodiment of this invention resides in a process to store H.sub.2 S. This 
process comprises contacting H.sub.2 S gas or liquid with the 
sulfur-containing ester defined to obtain a solution as described above. 
This solution is then placed into a liquid storage vessel where it remains 
under a relatively low pressure and in equilibrium with an H.sub.2 S 
containing atmosphere until it is used. The solution is readily usable as 
H.sub.2 S source by simple heating it to release H.sub.2 S gas. 
In the gas purification process described above, it is preferred to pass 
the solution of sulfur-containing ester, which is rich in hydrogen 
sulfide, to a regeneration zone wherein hydrogen sulfide is removed from 
this rich solution by heating the solution and/or by subjecting the 
solution to reduced pressure. The so-regenerated mercapto acid ester is 
then recycled to the contacting step and reutilized.

Into a packed column 1 a stream of natural gas containing H.sub.2 S is 
introduced at or near the bottom via conduit 2. A stream of liquid 
absorbent is introduced at or near the top of the column via line 3. A 
stream of natural gas being essentially free of H.sub.2 S removed via 
conduit 4 and passed to further use or storage. From the bottom of the 
column 1 H.sub.2 S containing absorbent is withdrawn via line 5. This rich 
absorbent stream is heated in heater 6, reduced in pressure at the 
pressure let-down valve 7 and passed to stripper 8. Lean adsorbent, i.e. 
essentially H.sub.2 S-free adsorbent, is withdrawn from the bottom of 
stripper 8 via line 3, H.sub.2 S gas is withdrawn from the top via line 9. 
The H.sub.2 S stream is passed to storage or further use whereas the lean 
adsorbent is recycled to extraction column 1. 
Typical conditions in the extraction column 1 are: 
Pressure: 100 to 500 psig 
Temperature: 0.degree. to 120.degree. F., preferably ambient. 
The following example is given to more fully illustrate the invention 
without undue limitation of its scope. 
EXAMPLE 
Crude dimethyldithiodipropionate (61.0 wt. % solution additionally 
containing 36.3 wt. % methyl 3-mercaptopropionate and 2.7 wt. % 
dimethylthiodipropionate) was saturated with H.sub.2 S at 80 F. and 
atmospheric pressure by dissolving liquid H.sub.2 S in the solvent under 
pressure and then reducing the pressure to atmospheric. 
The saturated solution was vacuum distilled at 5 mm Hg to separate the 
H.sub.2 S. Based on the volume of recovered solvent it was determined that 
the saturated solution contained 13% (liquid volume) hydrogen sulfide. 
Reasonable variations and modifications which will become apparent to those 
skilled in the art can be made from this invention without departing from 
the spirit and scope thereof.