Compositions – Absorptive – or bindive – and chemically yieldive
Reexamination Certificate
1999-11-12
2003-09-30
Toomer, Cephia D. (Department: 1714)
Compositions
Absorptive, or bindive, and chemically yieldive
C252S189000
Reexamination Certificate
active
06627110
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 cases by incineration to sulfur dioxide.
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 is concerned with a manner of processing sour natural gas streams.
SUMMARY OF THE INVENTION
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. One benefit of the present invention, as described below, is that there is no necessity to precisely control the stoichiometry of the gases for a complete removal of hydrogen sulfide to be effected.
In accordance with the present invention, a reaction medium comprising a non-aqueous Lewis base, having a pK
b
value of about 6 to about 11, preferably about 8 to about 10, particularly quinoline, is used to remove hydrogen sulfide from gas streams, particularly in the sweetening of sour natural gas streams but also for other applications, such as tail gas cleanup, 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. However, such material has a measurable sulfur dioxide vapor pressure which becomes significant at temperatures above about 50° C. and hence is unsuitable as a storage medium for sulfur dioxide, at least for low pressure applications.
The inventors herein have discovered, surprisingly and unexpectedly, that non-aqueous Lewis bases, in the presence of water, hydrogen sulfide and sulfur product and over time form non-aqueous Lewis base miscible components which have a storage capacity for sulfur dioxide and which do not exhibit any significant sulfur dioxide vapor pressure. The storage capacity of the solution enables the reaction of sulfur dioxide and hydrogen sulfide to be carried out without precise control of the input stoichiometry, providing a “fly-wheel” effect. This aspect of the invention is particularly important for tail gas cleanup, such as of Claus reaction tail gas streams, where variations in gas composition commonly occur.
The exact nature of the miscible components which are formed in the non-aqueous Lewis base are unknown but are complex sulfur-oxide species, including sulfate and polythionate species, and the term “complex sulfur-oxide species” is used herein to denote such miscible components. However, their effect is to provide a medium uniquely suited to remove hydrogen sulfide from a gas stream containing hydrogen sulfide by reaction with sulfur dioxide.
The inventors herein use the complex sulfur-oxide species in an original manner to provide improved procedures for removing hydrogen sulfide from gas streams. The present invention uses a reaction medium 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 Lewis base but other amines with the required pK
b
values can be used, such as 2,4,6-trimethyl pyridine (pK
b
7). In addition, the Lewis bases may be used in diluted form with miscible liquids, such as glycols and N-methyl pyrrolidone (pK
b
8 to 10).
One aspect of the present invention is directed specifically to scavenging hydrogen sulfide from gas streams, such as sour gases, on an intermittent or continuous basis, using a reaction medium which is a non-aqueous Lewis base “pre-loaded” with sulfur dioxide, but yet exhibits no significant sulfur dioxide vapor pressure, thereby providing a reservoir of reactant for the hydrogen sulfide. By contacting a gas stream containing hydrogen sulfide with the reaction medium, the hydrogen sulfide is absorbed and reacts with the pre-loaded sulfur dioxide at ambient temperature, about 50 to about 35° C., to produce solid sulfur and water, according to the Claus reaction.
In this procedure, the pre-loaded sulfur dioxide is not stripped from the reaction medium and yet is available for reaction with the hydrogen sulfide, enabling removal of hydrogen sulfide from sour gas down to below about 4 ppm in one pass through the reactor.
The pre-loaded sulfur dioxide is present in the reaction medium in the form of complex sulfur-oxide species, including sulfate and polythionate species, and it is those species which react with the hydrogen sulfide absorbed from the gas stream.
Accordingly, in one aspect of the present invention, there is provided a reaction medium for removing hydrogen sulfide from a gas stream, which comprises a non-aqueous Lewis base having a pK
b
value of about 6 to about 11 containing complex sulfur-oxide species, including sulfate and polythionate species and exhibiting no sulfur dioxide vapor pressure and having the capacity to absorb hydrogen sulfide form a gas stream and to react with absorbed hydrogen sulfide to produce sulfur and water.
The reaction medium which is provided in accordance with this aspect of the present invention is formed by reacting hydrogen sulfide with sulfur dioxide to form sulfur and water. A stoichiometric excess of sulfur dioxide reacts with the in-situ produced sulfur and water to form the complex sulfur-oxide species. This operation may be effected in two steps or a single step. The gas stream may contain hydrogen sulfide and sulfur dioxide in a mole ratio of 1:1 to about 1.2.
Accordingly, in a further aspect of the present invention, there
Ellenor David T. R.
Meffe Silvano
Smith James W.
Walton Peter S.
Apollo Environmental Systems Corp.
Sim & McBurney
Toomer Cephia D.
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