Note: Descriptions are shown in the official language in which they were submitted.
2~~.~~~~1
1 _
T 7099
SOLUTION REMOVAL OF H2S FROM GAS STREAMS
The presence of significant quantities of H2S in various
"sour" industrial gaseous streams poses a persistent problem.
Although various procedures have been developed to remove and
recover this contaminant, most such processes are deficient, for a
variety of reasons.
In one cyclic method currently attracting attention, the sour
gas is contacted with an aqueous polyvalent metal chelate or
complex reactant solution to produce solid sulphur which is
recovered either prior to or subsequent to regeneration of the
reactant. Preferred reactants are iron (III) complexes in which the
iron (III) forms complexes with specified organic acids and
derivatives thereof.
While sour gaseous streams that contain relatively low concen-
trations of H2S may be treated sucessfully in a variety of ways if
deep removal; e.g., greater than 9S percent removal of H2S, is not
required,, removal of this level, or greatex, demands efficiencies
of operation if excessive costs of operation and materials are not
to be incurred.
One scheme for carrying out the gas treatment utilizes a
20 two°stage contacting procedure in which a venturi-shaped contacting
zone is utilized as an initial or primary contacting stage to
remove the bulk of the H2S, and a follow-up or "clean-up",stage,
such as a packed-column or sparged tower, is provided for removing
the remainder of the H2S in the gaseous stream.
These configurations have a number of drawbacks, such as
susceptibility to plugging, high gas pressure drop, and high cost.
It has been determined that the H2S removal rate by iron cheiate or
complex systems is not limited by the reaction rate of the iron
with the H2S, but by the rate of absorption of t'he H2S into the
30 reactant solution:
2 _
The present invention provides a process with an efficient
contacting technique to insure good absorption rates of the H2S
into the contacting solution, while avoiding or minimizing plugging
and high pressure drop might have great utility. U.S.
patent 4,664,902, and U.S. patent 4,758,416, describe a multizone
contact procedure in which a specified contact zone comprises a
plurality of serial flow contact sections. In one embodiment, a
first contact section of the specified contact Zone comprises a
plurality of discrete channels which provide a diverted flow path
for the gas-solution mixture in process, the channelled section
being followed by a redistribution section which is adapted to
allow radial mixing and redistribution of solution in the gas,
while inhibiting plugging. The invention is an improvement on this
technique.
To this and the process for the removal of H2S from a sour
gaseous stream according to the present invention comprises
a) feeding the sour gaseous stream to a first contacting zone,
and intimately contacting the sour gaseous stream in said
first contacting zone with an aqueous reactant solution
containing solubilized Fe(III) chelate of an organic acid, or
mixture of said acids, and solubilized Fe(II) chelate of said
acid or acids, at a temperature below the melting point of
sulphur, and at a sufficient solution to gas ratio and
conditions effective to convert H2S to sulphur and inhibit
sulphur deposition, and praducing a gas~solution mixture
comprising sour gas and aqueous reactant solution;
b) passing gas-solution mixture from step a) through a plurality
of enclosed contacting sections in serial flow communication
in a second contacting zone; under conditions to convert HZS
~o sulphur and at a temperature below the melting point of
sulphur, the first contacting section of said second
contacting zone comprising a plurality of discrete sulphur
deposition resistant channels; each discrete channel providing
a diverted flow path for gas-solution mixture through the
section, such that gas-solution mixture is directed at least
CA 02015987 2000-O1-19
- 3 -
initially at an angle acute to that of the direction of flow
of the gas-solution mixture entering the section; the second
contacting section through which gas-solution mixture is passed
comprising an enclosed mixing section operative to or adapted
to allow radial mixing of gas-solution mixture and re-
distribution of solution in gas, and to inhibit plugging due
to sulphur formation, the third contacting section through
which gas-solution mixture is passed comprising a plurality of
discrete sulphur deposition resistant channels, each discrete
channel providing a diverted flow path for gas-solution
mixture through the section, such that gas-solution mixture is
directed at least initially at an angle acute to that of the
direction of flow of the gas-solution mixture entering the
section; and producing a gas-reactant solution mixture
containing solid sulphur in said second contacting zone, the
reactant solution of said gas-reactant solution mixture having
a reduced content of solubilized Fe(III) chelate of said acid
or acids and the gas of said mixture having a reduced H2S
content;
c) passing the gas-solution mixture from the third contacting
section through an addition contact section of said second
contact zone and contacting the gas-solution mixture with
aqueous reactant solution containing solubilized Fe(III)
chelate of said acid or acids and solubilized Fe(II) chelate
of said acid or acids, and forming a gas-solution mixture
having an increased solution to gas ratio; and
d) separating the gas having reduced H2S content from gas-
reactant solution mixture produced in step c).
The gas having reduced H2S content may be separated from the
w
solution in the second contacting zone, but is preferably separated
in a separate vessel or step. If further purification is necessary
or desired, the spray contacting procedure of steps a) and b) may
be repeated, or other contacting techniques or schemes, such as use
of a sparged tower or towers, may be used. In such cases,
appropriate measures will be taken for separation of the further
- 4 -
purified gas and regeneration of the aqueous reactant solutions)
employed. For example, the solution produced by step c) and
additional solution from further purification or contacting steps
may be combined and regenerated in a single regeneration step,
sulphur removal being accomplished prior to or after the
regeneration. Preferably, however, the gas having reduced H2S
content from step d) will simply be separated from the reactant
solution, and a spent reactant solution containing sulphur will be
recovered. In this case, sulphur will be removed from the spent
reaetant solution containing sulphur, and the spent reactant
solution from which sulphur has been removed will be regenerated,
producing a reactant solution having an increased concentration of
Fe(III) chelate.
In a second embodiment, the invention relates to a process for
~ the removal of H2S from a sour gaseous stream comprising
a) feeding the sour gaseous stream to a first contacting zone,
and intimately contacting the sour gaseous stream in said .
first contacting zone with an aqueous reactant solution
containing solubilized Fe(III) chelate of an organic acid or
mixture of said acids, and solubilized Fe(Ii) chelate of said
acid or acids, at a temperature below the melting point of
sulphur, and at a sufficient solution to gas ratio and
conditions effective to convert H2S to sulphur and inhibit
sulphur deposition, and producing a gas-solution mixture
comprising roux gas and aqueous reactant solution;
b) passing gas-solution mixture from step a) through a plurality
of enclosed contacting sections in serial flow communication
in a second contacting zone; under conditions to convert H2S
to sulphur and at a temperature below the melting point of ,
sulphur, the first contacting section of said second
contacting zone comprising a plurality of discrete sulphur
deposition resistant channels, each discrete channel providing
a diverted flow path for gas-solution mixture through the
section, such that gas-solution mixture is directed at least
initially at an angle acute to that of the direction of flow
~fl~.~~~
- 5 -
of the gas-solution mixture entering the section; the second
contacting section through which gassolution mixture is passed
comprising an addition contact section in which the
gas-solution mixture is contacted intimately with additional
aqueous reactant solution containing solubilized Fe(III)
chelate of said acid or acids and solubilized Fe(II) chelate
of said acid or acids to produce a gas-solution mixture having
an increased solution to gas ratio; the third contacting
section through which gas-solution mixture is passed
comprising a plurality of discrete sulphur deposition
resistant channels, each discrete channel providing a diverted
flow path for gas-solution mixture through the section, such
that gas-solution mixture is directed at least initially at an
angle acute to that of the direction of flow of the
gas-solution mixture entering the section; and producing a
gas-reactant solution mixture containing solid sulphur in said
second contacting zone, the reactant solution of said
gas-reactant solution mixture having a reduced content of
solubilized Fe(III) chelate of said acid or acids and the gas
of said mixture having a reduced H2S content;
c) separating the gas having reduced H2S content from gas-
reactant solution mixture produced in step b).
The gas having reduced H2S content may be separated from the
solution in the second contacting zone, but is preferably separated
in a separate vessel or step. If further purification s necessary
or desired, the spray contacting procedure of steps a) and b) may
be repeated, or other contacting techniques or schemes, such as use
of a sparged tower or towers, may-b$ used. In such cafes;
appropriate measures will be taken for separation of the further
purified gas and regeneration of the aqueous reactant solutions)
employed. For example, the solution produced by step c) and
additional solution from further purification or contacting steps
may be combined and regenerated in a single regeneration step;
sulphur removal being accomplished prior to or after th~ xe-
generation. Preferably, however, the gas having reduced ki2S content
- 6 -
from step b) will simply be separated from the reactant solution,
and a spent reactant solution containing sulphur will be recovered.
In this case, sulphur will be removed from the spent reactant
solution containing sulphur, and the spent reactant solution from
which sulphur has been removed will be regenerated, producing a
reactant solution having an increased concentration of the Fe(III)
chelate of the given acid or acids. The regenerated solution will
then be passed to the first contacting zone for use as aqueous
reactant solution therein.
In another embodiment, which may be preferred in some situa-
tions, the sulphur is separated after regeneration. That is, the .
spent reactant solution containing sulphur is regenerated,
producing a regenerated reactant solution containing sulphur,
sulphur is then removed from said regenerated solution, and the
regenerated reactant solution from which sulphur has been removed
is passed to the first contacting zone for use as the aqueous
reactant solution therein. Sulphur may also be removed during
regeneration, although this is not preferred.
As used herein, the term '°direction of flow" merely refers to
the direction the bulk of the gas-solution mixture is proceeding at
the respective entrances of the sections at any given time, it
being recognized that a minor portion or portions of the mixture
may have, at least temporarily, directional movements different ,
from the movement of the bulk or mass of the gas-solution mixture.
5 The acute flow path angles of the channels of a contacting section
may vary considerably, but preferably the angles to the'direction
of flow will range from about 5° to about 60°, most preferably
from about 15° to about 4~°. Angles approaching 90° are
less
desirable, since such angles will increase the possibility of
30 sulphur deposition and plugging. A limited amount of "abrupt"
change of the flow of the gas-solution'mixture may thus be
tolerated in the invention, provided the radial mixing and
redistribution section or sections of the invention are employed,
as described more fully hereinafter. The channels may be oriented
3~ in different directions with respect to each other, while
~~~~~z~g~
_,_
maintaining acute angles to the direction of flow. If a channel has
a wide acute angle, or if the channel is positioned near the wall
or walls of the second contacting zone, the flow of the
gas-solution mixture will be directed to and contact the wall or
walls of the second contacting zone, and secondary channels, at an
obtuse angle to the direction of flow, communicating with these
channels, may provide flow of the gas-solution mixture into the
radial mixing sections. Preferably, the ratio of the length of the
first contacting section to the length of the second contacting
zone (length referring to the distance through the zone and section
in the direction of flow) is no greater than about 0.5, preferably
no greater than about 0.3. As used herein, the term "sulphur
deposition resistant" refers to the quality or character of the
walls of the discrete channels in being free or at least sub-
stantially free of sites where sulphur, present or produced in the
gas-solution mixture, may deposit. Such a suxface may be produced
by polishing, such as by electropolishi.ng, or it may be formed by
coating the surface with a suitable material, such as teflon type
materials.
The addition contact section or sections accomplish the
important function of adding and redistributing solution in the gas
and inhibiting sulphur deposition. The enclosed contact sections)
contains or contain means for allowing addition of additional
solution, such as sprayers, spargers, etc., and will be of
5 sufficient width and length in the direction of flow to allow good
mixing and prevent plugging due to sulphur formation. Those skilled
in the art may determine by experimentation the minimum effective
width and length of the addition contact section or sections
(length referring to the distance through the sections in the
direction of flow) and the appropriate ratio of the length of the
respective channeled sections to the addition or redistribution
sections. In practice, the ratio will preferably range ~rom about
0.1 to about 10, preferably from about 0.3 to about 4. Normally,
the ratio of the length of the contacting section to the widest
dimension of the section will range from about 0.2 to about 5,
_$_
preferably about 0.3 to about 2. The dimensions of the first
contacting zone are not critical, other than that it must be of a
size where good distribution of the reactant solution in the
gaseous stream is achieved. In this regard, the first contacting
zone is an important part of the invention, since good initial
intimate mixing of the gas stream and the reactant solution is
important for efficiency.
As specified, in the first embodiment, at least two contacting
sections are required in the second contacting zone, but beyond
this, the number of addition contact sections in the second
contacting zone is not critical. In the second embodiment, the bulk
of the redistribution sections may be omitted, with the addition
contact sections spacing apart at least the majority of the
contacting sections. The addition of additional ferric-ferrous
chelate solution in the addition contact zone not only increases
gas-solution contact but provides additional wetting liquid for the
channel surfaces following the addition section so that plugging is
inhibited. The total number of contacting and addition contact
sections will be determined primarily by the amount of H2S to be
removed and the desired degree of gaseous stream purity. Normally,
from 2 to 20 or 30 contacting, or channeled sections will suffice,
with from 1 to 20 or 30 addition contact sections being sufficient.
It is a requirement of the invention that the contacting sections
and the addition contact sections alternate in the sequence of
flow, so that sulphur deposition and plugging are inhibited. The
shape of the enclosing walls of the contacting sections is riot
critical, but a generally cylindrical shape is preferred. The
invention is admirably suited for use in the type of structure
commonly referred to as a pipeline contactor, and the addition
contact sections are formed from spaces between the sections
containing the structured, channeled internals.
It is a preferred aspect of the invention that, by suitable
flow rates and design of the channeled sections and the means of
addition, tb.e flow of the gas-solution mixture through the second
contacting zone will reach or approximate plug flow. Suitable
_ g _
structures for providing the channeled flow include, but are not
limited to, chevron-type mixers, such as Koch static mixers or
Glitsch Gempak mixers. The velocity of the gas treated may vary
widely. Suitable gas velocities may range from about 0.3 m/s to
about 15 m/s, with a range of from about 1.5 m/s to about 9 m/s
being preferred. As noted, the aqueous reactant solution to gas
ratio must be sufficient to provide effective removal of H2S while
inhibiting or preventing sulphur deposition in the reaction zones.
Preferably, the solution to gas ratio will range from 0.2:100 to
30:100, most preferably from 0.5:100 to 5:100, all by volume. Such
ratios will generally be sufficient to provide good wetting of the
channel surfaces so that sulphur deposition is inhibited or
prevented. The addition contact sections comprise means, such as
spray cones or nozzles, for addition of the chelate liquid.
The iron chelates employed are coordination complexes in which
irons forms chelates with an organic acid. The organic acid may
have the formula
Y Y
N-R-N
/
y Y wherein
- from two to four of the groups Y are selected from acetic and
propionic acid groups;
- from zero to two of the groups Y are selected from 2-hydroxy-
ethyl, 2-hydroxypropyl, and
X
/
-CH2CH2N
X,
wherein X is selected from acetic and propionic acid groups, and
wherein R is ethylene, propylene or isopropylene or alternatively
cyclohexane or benzene where the two hydrogen atoms replaced by
2j nitrogen are in the 1,2 position.
~~~ ~~a~
- 10 -
Exemplary chelating agents for the iron include aminoacetic
acids derived from ethylenediamine, diethylenetriamine,
1,2-propylenediamine, and 1,3-propylenediamine, such as EDTA
(ethylenediamine tetraacetic acid), HEEDTA (N-2-hydroxyethyl
ethylenediamine triacetic acid), DETPA (diethylenetriamine
pentaacetic acid); amino acetic acid derivatives of cyclic,
1,2-diamines, such as 1,2-diamino cyclohexane-N,N-tetraacetic acid,
and 1,2-phenylene-diamine-N,N-tetraacetic acid, and the amides of
polyamino acetic acids disclosed in Bersworth U.S. patent
No. 3,580,950. The Fe(III) chelates of nitrilotriacetic acid and
N-(2-hydroxyethyl) ethylenediamine triacetic acid are suitable
chelating agents.
A further suitable iron chelate is the coordination complex in
which iron forms a chelate with nitrilotriacetic acid (NTA).
' The iron chelates are supplied in solution as solubilized
species, such as the ammonium or alkali metal salts (or mixtures
thereof) of the iron chelates. As used herein, the term
"solubilized" refers to the dissolved iron chelate or chelates,
whether as a salt or salts of the aforementioned cation or cations,
or in some other form, in which the iron chelate or chelates exist
in solution. Where solubility of the chelate is difficult, and
higher concentrations of chelates are desired, the ammonium salt
may be utilized, as described in a similar process in U.S.A. patent
No. 4,871,520. However, the invention may also be employed with
more dilute solutions of the iron chelates, wherein the steps taken
to prevent iron chelate precipitation are not critical.
The regeneration of the reactant is preferably accomplished by
the utilization of oxygen, preferably as air. As used herein, the
term "oxygen" is not limited to "pure" oxygen, but includes air,
air enriched with oxygen, or other oxygen-containing gases. The
oxygen will accomplish two functions, the oxidation of Fe(II) iron
of the reactant to the Fe(III) state, and the stripping of any
,..
~~J~j)~
- 1.1 -
residual dissolved gas (if originally present) from the aqueous
admixture. The oxygen (in whatever form supplied) is supplied in a
stoichiometric equivalent or excess with respect to the amount of
solubilized iron chelate to be oxidized to the fe(III) state.
Preferably, the oxygen is supplied in an amount of from about 20
per cent to about 500 per cent excess. Electrochemical regeneration
may also be employed.
The particular type of sour gaseous stream treated is not
critical, the only practical limitation being the reactivity of
the stream itself with the solutions employed, as will be evident
to those skilled in the art. Streams particularly suited to removal
of H2S by the practice of the invention are, as indicated,
naturally-occurring gases, recycled C02 used in enhanced oil
recovery, synthesis gases, process gases, and fuel gases produced
by gasification procedures, e.g., gases produced by the gasific-
ation of coal, petroleum, shale, tar sands, etc. Particularly
preferred are coal gasification streams, natural gas streams,
produced and recycled C02 streams, and refinery feedstocks composed
of gaseous hydrocarbon streams, especially those streams of this
type having a low ratio of H2S to C02, and other gaseous hydro-
carbon streams. The term "hydrocarbon streams)°', as employed
herein, is intended to include streams containing significant
quantities of hydrocarbon (both paraffinic and aromatic), it being
recognized that such streams contain significant "impurities" nor
technically defined as a hydrocarbon. Again, streams containing
principally a single hydrocarbon, e.g., ethane, axe eminently
suited to the practice of the invention. Streams derived from the
gasification and/or partial oxidation of gaseous or liquid
hydrocarbon may be treated by the invention. The H2S content of the
type of streams contemplated will vary extensively, but, in
general, will range from about 0.005 per cent to about 10 per cent
by volume. C02 may or may not be present, but if present, may range
in content from about 0.1 per cent to about 99.0 per cent (or more)
by volume. In this context, the invention may 'be used to remove H2S
from various C02 strearas, e.g., supercritical C02 streams.
- 12 -
Obviously, the amounts of H2S and C02 present are not generally a
limiting factor in the practice of the invention. The stream
treated may also have been treated initially for H2S removal, by
this or some other technique.
The temperatures employed in the contacting zones are not
generally critical, except that the reaction is carried out below
the melting point of sulphur. In many commercial applications, such
as removal of H2S from natural gas to meet pipeline specifications,
absorption at ambient temperatures is desired. In general,
temperatures of from 10 °C to 80 °C are suitable, and
temperatures
of from 20 °C to 60 °C are preferred. Total contact times may be
varied widely, but will preferably range from about 0.5 second to
about 10 seconds, with total contact times of about 1 second to
about 5 seconds being most preferred.
Similarly, in the regeneration or stripping zone or zones,
temperatures may be varied widely. Preferably, the regeneration
zone should be maintained at somewhat lower temperatures compared
to the contacting zone. In general, temperatures of from about
10 °C to 80 °G, preferably 20 °C to 50 °C, may be
employed.
2p Pressure conditions in the contacting zones may vary widely,
depending on the pressure of the gas to be treated. For example,
pressures in the contacting zones may vary from 0.1 MPa up to
15 MPa or even 20 MPa. Pressures of from 0.1 MPa to about 10 MPa
are preferred. In the regeneration zone, pressures may be varied
considerably, and will preferably range from about 0.1 MPa to about
or 40 MPa. Residence times for given volumes of admixture and
oxygen will also vary, but preferably will range from about 1
minute to about 60 minutes, most preferably from about 1 minute to
about 40 minutes. The pressure, fluid flow, and temperature
30 relationships involved are well understood by those skilled in the
art, and need not be detailed herein. Other conditions of operation
for this type of reaction process are further described in U.S.
Patent ~Io. 3,068,065, and in the aforementioned patent
specification. Preferably, pH in the regeneration zone will range
from about 6.5 to about 8.5, and the molar ratio of the
~~:~z; 3r
- 13 -
nitrilotriacetic acid to total solubilized iron is from about 1.0
to 1.5. The process is preferably conducted continuously.
As indicated, the H2S, when contacted, is rapidly converted in
the process of the invention by the solubilized Fe(III) chelate of
the organic acid or acids to solid elemental sulphur. Since the
iron chelates per se have limited solubility in water, the iron
chelate compound or compounds are preferably supplied, as indicated
previously. The amount of solubilized Fe(III) chelate of the
organic acid or acids supplied or employed in solution is an amount
sufficient to reduce the H2S concentration or content of the stream
to the desired level. If total or substantially total removal is
desired, the total amount supplied will generally be on the order
of at least about two mots per cool of H2S. Ratios of from about 2
cools to about i5 cools of solubilized Fe(III) chelate of the organic
acid or acids per cool of H2S may be used, with ratios of from about
2 cools per cool to about 5 mots of solubilized Fe(III) chelate per
cool of H2S being preferred. The molar ratio of the Fe(III) chelate
of the acid or acids to the Fe(II) chelate of the acid or acids
present in the contacting solution will normally be less than about
6, and will preferably range from about G.2 to about 6, most
preferably about 0.5 to about 6. The chelate solution will
generally be supplied as an aqueous solution having a concentration
of from about 0.1 molar to about 3 molar; a concentration of from
about 0.5 to about 1.5 molar is preferred. The total iron
concentration, as the chelates, will range from about 0.01
per cent, preferably about 0.5 per cent, to about 7 per cent by
weight, based on the weight of the solution and the iron. As
indicated, the solubilized iron chelates of the acid or acids may
be formed in aqueous solution by the reaction of elemental iron or
of an appropriate salt, oxide, or hydroxide of iron and the
specified acid, in the presence of alkali metal or ammonium ions,
or with the ammonium or alkali metal salt.
In order to describe the invention in greater detail,
reference is made to the accompanying schematic drawing. Figure 1
illustrates an embodiment of the invention wherein at least one
~~ ~~~"1
- 14 -
redistribution section is utilized and the sour gas contacting
zones are vertically disposed, sulphur removal is accomplished in a
separate step before regeneration, and regenerated solution is
returned to the contacting zone for use as the contacting solution.
Figure 2 illustrates an embodiment wherein additional chelate
solution is supplied. All values axe calculated or merely
exemplary, and all flows, unless stated otherwise, are continuous.
As shown, sour gas, e.g., a natural gas stream containing
about 0.5 per cent H2S, in line 1 flows into generally cylindrical
column 2 wherein it is intimately contacted in zone 3 thereof with
a spray of an aqueous mixture from line 4 which comprises aqueous
0.35 M solution of ammonium Fe(III) NTA chelate, which mixture also
contains 0.15 moles per liter of ammonium Fe(II) NTA chelate and
about 0.25 mole per liter of ammonium thiosulfate, pH of the
solution being adjusted to 7.5 to 8 by the addition of ammonium
hydroxide. The solution is produced by utilization of the reducing
effect of the H2S in the gaseous stream. That is, the initial
solution employed in the contacting zone illustrated is a 0.35 M
aqueous solution of Fe(III) NTA also containing about 1.0 M
ammonium ion. After startup, and reaction with H2S in the gaseous
stream, regeneration, described hereinafter, is controlled, so that
regeneration of the ammonium Fe(III) NTA acid complex is not
complete, in the ratios mentioned.
In zone 3, the gas stream containing H2S and the aqueous
reactant mixture are intimately mixed to form a gas-reactant liquid
mixture, sulphur almost immediately forming, and the gas-reactant
liquid mixture is passed downwardly in cocurrent relationship to
the first section S of contacting zone 6.
Although a spray device is illustrated, other suitable devices
or techniques which provide intimate mixing or contacting of the
gas and aqueous reactant mixture may be employed. For example,
sparging units, cone sprayers, as well as venturis, may be utilized
to produce a well mixed gas-solution or liquid mixture.
In any event, contacting section 5 comprises a chevron type
flow directing element which provides a plurality of discrete
- 15 -
channels for the passage and direction of the gas-reactant liquid
mixture at a 30° angle to the direction of flow to the side of the
cylindrical column. In this illustration, the element used is a
Koch SMV. mixing element. To insure that the surfaces of the
channels are resistant to sulphur deposition, the mixing element
(and all those described hereinafter for zone 6) and the walls of
zone 6 are electropolished before use. At least substantial plug
flow overall through zone 6 is obtained. The superficial velocity
of the gas is 6 m/s, and the liquid to gas volumetric flow ratio is
2:100. The width of the channels of the element is about 2.5 cm,
and the diameter of the column is about 30.5 cm. The length of
element 5 in the direction of flow is about 30.5 cm. As indicated,
other types of elements may be employed. At the outlets of the
channels of element 5, gas-reactant liquid mixture from the
channels enters an open section 7 (also about 30.5 cm in length in
the direction of flow) of contacting zone 6 where gas-liquid
reactant mixture may mix radially and where redistribution of the
solution and gas occurs. The open section also inhibits plugging,
due to sulphur formation, which might occur if multiple chevron
elements were placed end to end. The gas-reactant liquid mixture,
with increasing solid sulphur content, passes through chevron
element 8, which is identical to element 5. Element 8 may or may
not be misoriented with respect to element 5.
The gas-reactant liquid mixture, upon leaving the channels of
element $, passes through addition contact section 9, where the
dimensions and operation are similar to that occurring in section
7. In section 9, the gas-liquid reactant mixture is contacted with
additional reactant solution from sprayer 10, which is supplied via
header 11 connected to line 12. As shown in the drawing, additional
sprayers are provided in zone 6, following the next two contacting
sections. The gas-liquid reactant mixture may be fresh mixture or
regenerated mixture, as described hereinafter, or it may be
supplied from some other source. The flaw of the gas-reactant
liquid mixture through the remaining corresponding sections of zone
6 and the addition of additional chelate solution is similar to
J 7J ~J
~~.'~f~%~'~
- 16 -
that described with respect to the first four sections, and need
not be described, except to note that the H2S in the gas stream is
continually being reduced, with concomitant sulphur formation and
reduction of the Fe(III) chelate concentration.
The volume of solution supplied in the addition contact
sections is not critical, sufficient solution being added simply to
increase H2S removal and insure wetting of the upper portions of
the respective channel sections. Preferably, the ratio of chelate
solution supplied in the addition contact sections to that supplied
in contact zone (3) will range from 0.1-1.0 to 2.0, but this is not
critical.
The operation of the embodiment shown in.Fig~ is similar
to that of Figure 1 the only difference being the addition of
chelate liqu or solution in the first section (section 7)
, following the first contact section.
At the lower end of column 2, the gas-reactant liquid mixture,
now containing solid sulphur, passes from vessel 2, and is sent via
line 13 to a separating unit or vessel 14 where the natural gas is
separated from the liquid and sulphur. Purified natural gas is
removed overhead via line 15, and "spent'° reactant liquid and
sulphur are removed via line 16.
As those skilled in the art will recognize, solution concen-
trations, sulphur content, and ferric-ferrous ligand concentrations
and ratios must be regulated to achieve appropriate HzS removal.
To maintain appropriate Fe(III) concentrations and provide sulphur
removal, stream 16 is sent for regeneration and sulphur removal.
More particularly, the aqueous admixture in line 16 is sent to
a depressurization and degassing unit 17, which also serves as a
sulphur concentration or thickening zone. A minor portion, e.g., 2
to 5 per cent by volume of the admixture in settler or thickener
17, and containing an increased sulphur concentration, is
continuously withdrawn from the lower portion of settler or
concentrator 17 and sent via line 18 to sulphur recovery in unit
19.
_ 17
Sulphur recovery may be accomplished in any suitable fashion,
such as by filtration. For example, sulphur may also be recovered
by that method described in U.S. patent No. 4,705,676. As those
skilled in the art will recognize, sulphur may be removed after
regeneration, if desired. In any event, solution recovered during
sulphur recovery may be returned to any suitable point in the
process, if proper adjustment is made. Preferably, however, the
solution recovered is sent to the regeneration zone, as shown, via
lines 20 and 21.
The major portion of the aqueous admixture in vessel 17 is
removed via line 21 fox regeneration of the Fe(III) chelate of
nitrilotriacetic acid. In regeneration zone or column 22, which may
be a sparged tower, the admixture is contacted cocurrently with
excess air in line 23 to convert Fe(II) chelate of NTA to Fe(III)
chelate of NTA. Air velocity in the regenerator is in the range of
0.03 to 0.9 m/s, the temperature of the liquid is about 45 °C, and
overall pressure is about 0.2 MPa. Spent air is removed via line
24, and regenerated admixture, having a ratio of Fe(III) chelate of
NTA to Fe(II) chelate of NTA of about 2.5, is returned via line 25
to line 4, which connects, as mentioned, to column 2 and line 12.
Again, as will be understood by those skilled in the art, the
solutions or mixtures employed may contain other materials or
additives for given purposes. Fox example, U.S. Patent
No. 3,933,993 discloses the use of buffering agents, such as
phosphate and carbonate buffers. Similarly, U.S. Patent
No. 4,009,251 describes various additives, such as sodium oxalate,
sodium formate, sodium thiosulfate, and sodium acetate, which are
beneficial, and other additives, such as additives to improve
sulphur separation, or antifoaming and/or wetting agents, may be
employed.
