Absorber for liquid redox processes

A method and apparatus for contacting a gas and a liquid such as may be utilized in removing sulfur from gas streams, using a liquid redox process, are provided, which prevent or avoid buildup of elemental sulfur in the process equipment by introducing the hydrogen sulfide contaminated gas through a plurality of gas inlets tangentially distributed within the inner wall of the reaction vessel containing a liquid redox scrubbing solution.

BACKGROUND OF THE INVENTION 
The present invention relates generally to contact of, by or between a gas 
and a liquid and, more particularly, to an apparatus for and method of gas 
and liquid contact such as adapted for use in liquid redox processes for 
the removal of sulfur from gases, such as natural gas, for example. 
Hydrogen sulfide (H.sub.2 S) is often encountered as a part of or in 
conjunction with various gas streams, such as in process streams formed or 
produced in various industrial processes and in natural gas extracted from 
the ground. In the presence of oxygen, H.sub.2 S can form various 
sulfates. Such sulfates can act as pollutants (which can contribute to 
acid rain), and can also themselves be corrosive or otherwise damaging to 
machinery and equipment, as well as storage and transportation devices 
such as pipelines, for example, with which such materials and associated 
process streams may come into contact. 
Currently, generally at least about 15 percent of U.S. natural gas 
production is treated for H.sub.2 S removal. In general, it is common that 
high or higher quality natural gas reserves are and have to date been 
preferentially produced. Thus, it is likely that future natural gas 
production will include a larger proportion of subquality gas, for 
example, natural gas which contains sulfur in various amounts. 
Various processes for the removal of H.sub.2 S from gas streams are know or 
have previously been proposed. One such process employs a basic agent such 
as an amine as an absorbent. In practice, such an amine absorbent can be 
regenerated for reuse as an absorbent through the application of heat, for 
example, by treatment with steam. A variety of non-regenerative processes 
using Fe-based solids, or liquid-based processes using caustic triazine, 
are also widely employed. 
A category of processes, termed in the industry as "liquid redox" 
processes, are frequently used to effect removal of H.sub.2 S from H.sub.2 
S-containing natural gas (termed "sour" gas) and other streams. In such 
processing, the removed sulfur material is commonly converted either to 
solid elemental sulfur for subsequent sale or to sulfur cake for ultimate 
disposal. The term "liquid redox" refers to reduction and oxidation 
processing which is believed to occur in the liquid phase. Generally, in a 
typical liquid redox process, a reduction-oxidation ("redox") system is 
used in which sour gas is exposed to a liquid form of sulfide 
precipitation agent, catalyst material or absorbent (for example, a metal 
oxide, in which a metal cation changes from a higher valence state to a 
lower state upon reaction with the H.sub.2 S), and the gas, now with a 
substantially reduced level of H.sub.2 S (such gas commonly being referred 
to as "sweet" gas) is piped onward to its intended use. Conventional 
liquid redox processes typically employ iron in such a reduction-oxidation 
cycle. In the cycle, the iron is alternately reduced in an absorber 
yielding elemental sulfur and oxidized in an oxidizer/regenerator yielding 
water. 
After passing through the absorber, at least a portion of the sulfur will 
have precipitated out of the absorbent solution as elemental sulfur. The 
precipitation agent/catalyst/absorbent solution is then typically sent to 
some form of regeneration apparatus, such as an oxidizer, so as to restore 
the metal cation in the solution to the desired higher valence state, so 
that the solution upon return to the absorber will again serve to absorb 
H.sub.2 S from the gas stream. 
Liquid redox processes are generally not economical for treatment 
processing of streams which contain very small amounts of sulfur, e.g., 
where the H.sub.2 S concentration is less than a few hundred ppm or the 
total amount of sulfur is less than a few hundred pounds per day. 
For economic reasons, regenerative processes employing liquid redox are 
more attractively applied to streams where the H.sub.2 S concentration is 
relatively dilute (e.g., no more than a few percent or less) and where the 
total amount of sulfur removed is above about 50-100 lbs. per day, 
preferably above about 200-300 lbs. per day, and less than 25 long tons a 
per day. When conversion to elemental sulfur is desired for environmental 
or regulatory reasons, such liquid redox processing can be particularly 
preferred when total sulfur is on the order of 10 tons or less of sulfur 
per day. 
Such liquid redox processes are favored since they operate at ambient 
temperatures and have high selectivity for H.sub.2 S. While one of the 
major attributes of liquid redox processes for removing H.sub.2 S from 
subquality natural gas is the rapid reaction rate of H.sub.2 S with the 
liquid redox solution and the subsequent precipitation of elemental 
sulfur, the process can be marred by a tendency of the sulfur to deposit 
on internal contact surfaces in the absorber, such as walls of the 
absorber itself as well as internal contact surfaces such as may be formed 
by contact devices such as static mixers, packing and the like which are 
commonly housed in and used in association with absorbers to facilitate 
and enhance contact of the gas and liquid. 
The tendency for such sulfur deposition primarily emanates from the fact 
that there is always a zone of stagnant fluid associated with a fixed 
surface. That is, a thin "boundary layer" of non-moving fluid is commonly 
present adjacent to non-moving surfaces. As a result, sulfur precipitation 
and deposition onto such non-moving surfaces adjacent to a non-moving 
fluid boundary layer can be essentially continuous and can eventually act 
to block or clog one or more of the absorber and associated processing 
equipment and piping, for example. For example, should an absorber clog, 
the absorber must typically be taken out of service and cleaned, resulting 
in plant downtime and an economic penalty to the user. 
One possible technique that could be used to keep such surfaces free from 
solid sulfur deposits would be to maintain a large flow of recirculated 
liquid through the absorber to produce turbulence to act to scrub stagnant 
zones within the absorber. However, in addition to the uncertainty in 
achieving uniform turbulence throughout the absorber, maintaining a large 
circulation of liquid can be quite costly both in terms of the additional 
amounts of absorbent which must be maintained in a circulation loop as 
well as the increased energy and equipment costs associated with 
establishing and maintaining such constant circulation and turbulence. 
It would therefore be desirable to provide a method and apparatus for 
assuring the prevention of buildup of elemental sulfur on non-moving 
surfaces within an absorber, without having to resort to continuous 
circulation of absorbent within the absorber. 
SUMMARY OF THE INVENTION 
A general object of the invention is to provide an improved apparatus and 
method for contacting a gas and a liquid. 
A more specific objective of the invention is to overcome one or more of 
the problems described above. 
The present invention comprises in part an improved absorber for liquid 
redox systems for removal of hydrogen sulfide from "sour" gas. The 
invention includes an improved absorber structure, in which the sour gas 
is introduced into the absorber through, preferably, a plurality of inlet 
apertures which are configured so that the sour gas enters the interior of 
the absorber substantially tangentially to the wall of the absorber, thus 
creating a scrubbing effect, for preventing elemental sulfur buildup. 
The general object of the invention can be attained, at least in part, 
through a specific gas and liquid contactor apparatus. The contactor 
apparatus includes an essentially hollow housing providing direct contact 
between a liquid and a gas. The liquid is axially introduced into the 
housing and the gas is tangentially introduced into the housing through a 
plurality of gas inlet openings tangentially distributed about the housing 
whereby turbulent mixing of the liquid with the gas is obtained and 
material deposition on the internal surface of the housing is avoided. 
The prior art fails to provide a gas and liquid contactor apparatus and 
method for contacting a gas and a liquid which avoids or prevents the of 
buildup, deposition or accumulation of elemental sulfur on non-moving 
surfaces within the contactor, without having to resort to continuous 
large scale circulation of contact materials, such as the liquid, within 
the contactor. In particular, the prior art fails to provide an absorber 
for a liquid redox process for the removal of sulfur from a gas stream and 
an associated method for removing sulfur from a gas stream which avoids or 
prevents such buildup, deposition or accumulation of elemental sulfur on 
non-moving surfaces within the absorber and does so without resorting to 
such continuous large scale circulation of absorbent within the absorber. 
The invention further comprehends an absorber for a liquid redox process 
for the removal of sulfur from a gas stream. The absorber includes an 
elongated housing. The housing provides a contact zone between an axially 
introduced liquid redox solution and a sulfur-containing gas introduced 
into the contact zone through a plurality of gas inlet openings 
tangentially distributed about the housing. 
The invention still further comprehends a method for removing sulfur from a 
gas stream. In accordance with such method, a sulfur-containing gas stream 
is contacted with a liquid redox solution in an essentially hollow 
housing. In the housing, the liquid redox solution is passed axially and 
the sulfur-containing gas stream is tangentially introduced through a 
plurality of gas inlet openings tangentially distributed about the 
housing. This results in turbulent mixing of the liquid with the gas with 
at least a portion of the sulfur originally contained in the gas stream 
forming a solid, with the mixing preventing deposition of solid sulfur on 
the internal surface of the housing. 
The invention still further comprehends a method for contacting a 
sulfur-containing gas with a liquid redox solution. Such a method includes 
the steps of: 
a) passing a liquid redox solution axially through a contact zone of an 
elongated housing, 
b) tangentially introducing a sulfur-containing gas into the contact zone 
through a plurality of gas inlet openings tangentially distributed about 
the housing, and 
c) contacting the axially passing solution with the tangentially introduced 
gas to form solid sulfur. 
In such a method, the tangential introduction of the gas prevents 
deposition of the solid sulfur on the internal surface of the housing. 
As used herein, references to "absorbent agent" are to be understood to 
generally refer to a precipitation agent, catalyst material, absorbent 
material and the like employed in liquid redox processing to 
regeneratively interact with hydrogen sulfide to permit and facilitate the 
removal thereof 
Other objects and advantages will be apparent to those skilled in the art 
from the following detailed description taken in conjunction with the 
appended claims and drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The present invention provides an apparatus and method for contacting a gas 
and a liquid which apparatus and method avoid or prevent solid deposition 
on internal surfaces of a contactor apparatus without having to resort to 
continuous circulation of absorbent within the contactor. 
Referring to FIG. 1, a liquid redox hydrogen sulfide removal system, 
generally designated by the reference numeral 10, is shown. The system 10 
includes a gas and liquid contactor or absorber 15, an expansion valve 16, 
a flash tank 17, an oxidizer or regenerator 20, an air or oxygen source 21 
and associated pump 22, and a solution pump 23. 
In general, sour gas containing hydrogen sulfide is introduced into the 
absorber 15 via a conduit 25 at a point 30 and, after hydrogen sulfide has 
been removed, treated or sweet gas exits from the absorber 15 at a point 
31, via a conduit 32. 
With the passage of a certain residence time in the absorber 15, hydrogen 
sulfide contained in the feed gas interacts with an absorbent agent in the 
absorber 15. Various absorbent agents can be utilized in the practice of 
the invention, including an absorbent agent selected from a number of 
known absorbent agents, for example, an absorbent agent bearing iron 
cations. As a result of interaction between the sour gas and the absorbent 
agent, elemental sulfur particles precipitate out of the solution, and the 
metal cation is converted to a metal hydride. 
Exhausted solution, bearing elemental sulfur, is processed via a conduit 35 
through the expansion valve 16 to relieve the pressure on the solution and 
subsequently passed on to the flash tank 17. Gases and vapors, composed 
largely of water vapor, are passed from the flash tank 17 via a conduit 
38. Such gases and vapor can be simply vented or flared to the atmosphere 
or utilized elsewhere within the facility, as may be desired. 
Liquid, composed of the exhausted solution, is passed from the flash tank 
17, via a conduit 40, to the oxidizer regenerator 20. An oxidizing agent, 
typically air or other oxygen-bearing gas, is introduced into the 
regenerator 20 from the source 21, such as via the pump 22. As a result of 
reaction between the oxidizing agent and the exhausted solution, the metal 
hydride typically is converted into water, which is vented as moisture 
with the spent air, shown as a process stream 42, and metal cations (e.g., 
Fe.sup.2+) which are returned to the absorber 15 via a process line 44 and 
the solution pump 23. The sulfur is removed from the oxidizer regenerator 
20 via a conduit 46 using any of a number of known techniques for removing 
elemental sulfur from a liquid, such as by processing with a settling 
vessel, as is known in the art. Typically, fresh absorbent agent must be 
periodically added to the system, since some of the redox solution is 
typically lost with the sulfur removed via the conduit 46. 
A section of the absorber 15, designated by the reference numeral 50 and in 
accordance with a preferred embodiment of the invention, is shown in FIGS. 
2 and 3. As shown, the absorber 15 has an improved configuration, in which 
the subquality ("sour") gas enters an inner absorber region, e.g., a 
contact zone 52, through a plurality of spaced tangential inlets 54. The 
tangential inlets 54 are preferably spaced and distributed about the inner 
surface 56 of the absorber 15. While the tangential inlets 54 completely 
eliminate stagnation by the surface 56, the tangential inlets 54 also 
provide intense and turbulent mixing of the gas with the liquid redox 
solution, thereby eliminating the need for packing or static mixers, which 
are generally required in various prior contactor devices in order to 
ensure adequate contact of the gas with the liquid. 
In the invention, the tangential gas flow resulting from the specified gas 
introduction generally does not permit or allow buildup of sulfur on the 
walls because of the continuous scraping action of the gas along the wall. 
As will be appreciated, one of ordinary skill in the art guided by the 
teachings of the present disclosure may vary the number and size of the 
gas inlets dependent upon the particulars of a specific design 
installation, such as required capacity, required or permissible pressure 
drop in the gas flow, etc. It will be appreciated that the distribution of 
the tangential gas inlet openings along the length of the housing will 
better ensure avoidance of the occurrence of non-moving fluid adjacent the 
internal surface of the housing along the length thereof. 
The improved absorber of the invention is configured to replace existing 
packed tower and static mixer absorber systems. Thus, in accordance with 
one preferred embodiment, the absorber contactor of the invention 
constitutes an essentially hollow housing such as wherein one or more 
specified liquids and gases may contact and interact. The term 
"essentially hollow" is used to indicate that the housing is free of 
contact or mixing enhancing devices such as packing, static mixers and the 
like such as are commonly associated with contactor and absorber 
constructions. In the practice of the invention, the inclusion of such 
contact or mixing enhancing devices are in general undesired as such 
devices may undesirably hinder or preclude the desired turbulent mixing 
and sought avoidance of zones of stagnant fluid associated with fixed 
surfaces. 
The tangential absorber 15 constitutes a gas-liquid contactor in which gas 
is introduced through tangential openings distributed through the wall of 
the absorber whereas the liquid is introduced axially either concurrently 
or countercurrently to the ultimate direction of the gas flow. In the 
illustrated embodiment of FIG. 2, the liquid is shown axially introduced 
in the direction of the arrow 60, countercurrently to the ultimate 
direction of the gas flow which, as shown in FIG. 1, is out from the top 
of the absorber 15, at the point 31. 
As identified above and in accordance with one preferred embodiment of the 
invention, the described tangential absorber substantially eliminates 
stagnation points inside the absorber, such as are generally associated 
with packed-bed absorbers or absorbers containing a static mixer. It will 
be appreciated that the incidence of sulfur attachment and deposition on 
the internal surface of such as absorber is eliminated or drastically 
reduced. 
In general, the absorber and liquid redox processing of the invention may 
advantageously be employed in conjunction with the processing of 
sulfur-containing streams containing sulfur in relative amounts of between 
about 100 ppm to no more than about 50 volume percent, measured in terms 
of hydrogen sulfide, with an upper concentration of sulfur of about 4 to 5 
volume percent measured in terms of hydrogen sulfide being common for 
streams of natural gas processed in accordance with the invention. 
It is to be understood that the discussion of theory, such as the 
discussion of the chemical interaction of hydrogen sulfide and the 
absorbent agent liquid redox solution, for example, is included to assist 
in the understanding of the subject invention and is in no way limiting to 
the invention in its broad application. 
The invention illustratively disclosed herein suitably may be practiced in 
the absence of any element, part, step, component, or ingredient which is 
not specifically disclosed herein. 
While in the foregoing detailed description this invention has been 
described in relation to certain preferred embodiments thereof, and many 
details have been set forth for purposes of illustration, it will be 
apparent to those skilled in the art that the invention is susceptible to 
additional embodiments and that certain of the details described herein 
can be varied considerably without departing from the basic principles of 
the invention.