Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2000-05-30
2003-04-22
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S575000, C423S576200
Reexamination Certificate
active
06551570
ABSTRACT:
FIELD OF INVENTION
The present invention relates to the removal of hydrogen sulfide from gas streams using a reaction medium comprising non-aqueous Lewis bases.
BACKGROUND TO THE INVENTION
Many reservoirs of natural gas contain hydrogen sulfide and carbon dioxide which are acid gases which can be extremely corrosive when combined with each other and water. Natural gas containing such acid or sour gases must be purified (or “sweetened”) to remove or decrease the concentration of such gases prior to the purified natural gas (“sweet gas”) being forwarded to consumer, industrial and other markets.
The most commonly-practised process technology for acid gas removal is the absorption of the acid gases from the natural gas stream by a regenerable absorbing solution in a gas processing plant. In such procedures, a regenerable absorbing solution is passed in countercurrent contact with the natural gas stream to absorb the H
2
S and CO
2
, as well as other sulfur compounds, from the natural gas stream, thereby reducing their concentration in the natural gas stream and purifying the natural gas stream.
The acid gas laden solution then is regenerated by steam stripping at elevated temperature and the regenerated solution is cooled and recirculated back to the natural gas contacting stage. Acid gases stripped from the solution in the regeneration step are vented from the gas processing plant for further processing, including, in some case, incineration to sulfur dioxide. The present invention is concerned with a manner of processing sour natural gas streams.
Chemicals that are commonly employed in such procedures include amines, esters and similar regenerable materials in which the acid gases may be absorbed. The most commonly-employed amines for this procedure include monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA).
The present invention provides novel procedures for treatment of hydrogen sulfide-containing gas streams. As described in more detail below, the process of the invention includes a step of reacting hydrogen sulfide and sulfur dioxide to form sulfur (sometimes termed the Claus reaction) in a reaction medium comprising a non-aqueous Lewis base, preferably quinoline. The processes described herein are applicable to other gas streams which contain hydrogen sulfide, including Claus process tail gas streams and industrial flue gas streams.
SUMMARY OF THE INVENTION
In accordance with the present invention, a reaction medium comprising non-aqueous Lewis bases, having a pK
b
value of about 6 to about 11, preferably about 8 to about 10, particularly quinoline, are used to remove hydrogen sulfide from gas streams, particularly in the sweetening of sour natural gas streams by effecting reaction between hydrogen sulfide and sulfur dioxide in the reaction medium. The reaction of hydrogen sulfide with sulfur dioxide, which may be in the form of a reaction product with the Lewis base, proceeds in accordance with the equation:
2H
2
S+SO
2
→3S+2H
2
O
known as the Claus reaction.
It is well known that sulfur dioxide is soluble in many amines, including quinoline, forming an equi-molar solid reaction product, itself soluble in quinoline and quinoline-water mixtures.
The inventors herein use the reaction product in an original manner to provide improved procedures for removing hydrogen sulfide from gas streams. The present invention uses a reaction comprising a Lewis base which has pK
b
values from about 6 to about 11, preferably about 8 to about 10. Although strong Lewis bases, (pK
b
less than about 6) tend to react irreversibly with sulfur dioxide, preventing the Claus reaction from occurring, weaker Lewis bases (pK
b
greater than about 11) do not appear to catalyze the Claus reaction. The Lewis bases of intermediate basicity (pK
b
from about 6 to about 11), as used herein, react reversibly with sulfur dioxide and catalyze the Claus reaction. Quinoline (pK
b
9) is the preferred amine but other amines with the required pK
b
values can be used, such as 2,4,6-trimethyl pyridine (pK
b
7).
Accordingly, in one aspect of the present invention, there is provided a process for the removal of hydrogen sulfide from a gas stream by reaction with sulfur dioxide, which comprises effecting the reaction in a reaction medium comprising a non-aqueous Lewis base with a pK
b
value in the range of about 6 to about 11 and which reaction medium:
a) absorbs sulfur dioxide and reacts chemically therewith to form a reaction product;
b) absorbs hydrogen sulfide;
c) removes the hydrogen sulfide from the gas stream through contact of the gas stream with the reaction medium in the presence of free sulfur dioxide, and/or the reaction product;
d) acts as a catalyst for the overall reaction of the hydrogen sulfide with sulfur dioxide to produce sulfur; and
(e) has the capacity to absorb sulfur dioxide in sufficient quantity to remove substantially all the hydrogen sulfide from the gas stream, notwithstanding short term variations in the stoichiometric balance between the hydrogen sulfide and the sulfur dioxide in the reaction medium.
The reaction medium may consist essentially of the non-aqueous Lewis base or may further comprise a miscible diluent of vapour pressure below about 0.39 psi at a temperature of about 120° C.
The process of hydrogen sulfide removal provided herein may be effected in a manner in which sulfur dioxide is continuously absorbed by the reaction medium to react with hydrogen sulfide in the gas stream at a temperature of about 120° to about 155° C., preferably about 120° to about 130° C., to produce liquid sulfur, and the liquid sulfur so produced is continuously removed from the process.
The latter procedure is particularly useful in a natural gas sweetening operation or for the processing of a hydrogen sulfide-containing gas stream where a continuous operation is required.
The process of hydrogen sulfide removal provided herein may be effected in a manner in which the gas stream is so contacted, intermittently or continuously, with a body of the reaction medium to react the hydrogen sulfide with sulfide dioxide in the reaction product to form sulfur until the reaction medium is depleted of its capacity to react with hydrogen sulfide.
The latter procedure is particularly useful for scavenging operations to remove lesser amounts of hydrogen sulfide on an intermittent operational basis from gas streams having a variety of sources. The procedure may be operated at a temperature above or below the melting point of sulfur and down to the solidification point of the reaction medium. The sulfur usually is permitted to accumulate in the body of the reaction medium until the reaction medium is depleted.
When the reaction medium becomes depleted of the ability to react with hydrogen sulfide, which may be detected by any conventional sensing device, the reaction medium is regenerated. Since regeneration may take a variety of forms, including replacement of the depleted reaction medium by a fresh charge of the reaction medium or a charge of reaction medium regenerated from a previous batch. Regeneration may be effected by reforming the reaction product of the sulfur dioxide and the non-aqueous Lewis base. Sulfur may be removed intermittently as desired from the reaction medium.
GENERAL DESCRIPTION OF THE INVENTION
In one specific aspect of the present invention, there is provided a continuous process for the removal of hydrogen sulfide from a gas stream, which comprises contacting a reaction medium comprising a non-aqueous Lewis base having a pK
b
value of about 6 to about 11 having the capacity to absorb sulfur dioxide in sufficient quantity to remove substantially all the hydrogen sulfide from the gas stream, notwithstanding short term variations in the stoichiometric balance between the hydrogen sulfide and the sulfur dioxide in the reaction medium with the gas stream in the presence of sulfur dioxide in the reaction medium to react with the hydrogen sulfide at a temperature above the melting point of sulfur to form liquid sulfur, in accordance with the equatio
Ellenor David T. R.
Meffe Silvano
Smith James W.
Walton Peter S.
Apollo Evironmental Systems Corp.
Sim & McBurney
Vanoy Timothy C
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