Note: Descriptions are shown in the official language in which they were submitted.
f~7~
RE:MOVAL O~ HYDROGEN Sl~LFIDE: FRO~l GAS~S
This invention relates to an improved process and
to an improved reagent for the removal of hydrogen sulfide
from gases and the recovery of sulfur. More particularly,
the invention relates to improvements in the removal of
hydrogen sulfide from gas streams in an oxidation-reduction
system utilizing a reagent comprising an iron-chelate com-
plex wherein the iron in its ferric state oxidizes the
hydrogen sulfide to elemental sulfur and is concomitantly
reduced to the errous state and wherein the reagent is
regenerated by oxidation of the iron to the ferric state.
Numerous processes have been suggested for the
removal of hydrogen sulfide rom gas streams, including (1)
scrubbing with an alkaline or caustic solution, (2) inciner-
- ation to form sulfur dioxide and scrubbing with an alkaline
or caustic solution, (3) various dry oxidation processes
using a solid catalyst or the like (e.g. the claus process),
(4) various wet oxidation processes using a basic or alkaline
solution containing a suspended or dissolved catalyst or
oxidizing agent, and (5) selective absorption with an amine
such as monoethanolamine or diethanolamine. However, the
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foregoing types of processes are subject to various limita-
tions which in many cases detract from their commercial
feasibility. For example, in some instances the efficiency
of removal of hydrogen sulfide is low or the reagent, solu-
tion, or catalyst is expensive, unstable, or not easily
regenerated. In other cases, disposal of ~aste products
poses a serious problem. In still other cases, the operating
or equipment costs are excessive or the process is diEicult
to control.
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~'7~
It has also been suggested to effect removal o~
hydrogen sulfide in an oxidation-reduction system by con-
tacting the gas stream with a solution of a polyvalent metal
cation (such as iron) complexed with a chelating agent (such
as ethylene diamine tetra-acetic acid or a sodium salt thereof).
Iron in the ferric state oxidizes the hydrogen sulfide and
is reduced to the ferrous state, the solution b~ng regener-
ated by oxidation to convert the iron back to the fsrric
state. For example, processes using a chelated iron reagent
are disclosed in the following U~S. patents:
- Inventor Patent No. Date
Hartley et al 3,068,065 Dec. ll, 1962
Pitts et al 3,097,925 July 16, 1963
Meuly et al 3,226,320 Dec. 28, 1965
Roberts et al 3,622,273 Nov. 23, 1971
Robarts et al 3,676,356 July 11, 1972
In addition, the above-listed Roberts et al patents refer
to Czechoslovakian Patents Nos. 117,273, 117,274, and
117,277 as also disclosing the use of chelated iron solutions
for this purpose.
A serious problem in the use of a chelated iron
solution arises from the inherent instability of the sol-
ution, particularly at higher pH levels. For example, if
an aqueous solution of a complex of iron with ethylene
diamine tetra-acetic acid (EDTA) or with nitrilotriacetic
acid (NTA) is used, it is nec~ssary to exercise careful con-
trol over the pH of the solution and the relative amounts
- of iron and chelating agent. Although the solubility o~
hydrogen sulfide is greatest at the higher pH levels, at
these conditions the complex readily decompose~ and iron in
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t7 ~ 3
the ferric state is precipitated as ~erric hydro~i~e. I~
too little chelating agent is used relakive to the iron content,
the iron in the ~errous state is loosely bound and tends to
precipitate as ferrous sulfide at high pH levels. If the amount
of chelating agent is too great relative to the iron content,
the iron in the ferrous state is bound too strongly so that the
solution is difficult to regenerate by oxidation.
According to the present invention the difficulties
encountered in the prior art systems are avoided by means of
a novel and improved chelated iron solution and a process for
using the same for removlng hydrogen sulfide from gas streams.
The novel and improved solution contains two different types
of chelating agents, one of which is capable of binding iron
in the ferrous sta~e to prevent formation of ferrous sulfide,
and the other of which is capable of binding iron in the ferric
state to prevent formation of ferric hydroxide.
The invention in its broad aspect comprehends a
composition for use in removing hydrogen sulfide from a gas,
which composition comprises an aqueous solution of chelated
iron containing at least two iron chelating agents. One of
the iron chelating agents is an amine type chelating agent
which is capable of binding iron in the ferrous state to
prevent precipitation of ferrous sulfide, and at least one
of the chelating agents is a polyhydroxy type chelating agent
which is capable of binding iron in the ferric state to
prevent precipitation of ferric hydroxide.
The invention further comprehends a continuous
oxidation-reduction process for the removal of hydrogen sulfide
from a gas stream by contacting the gas stream with an aqueous
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chelating iron solutlon containing iron in the ferric state
for oxidizing hydrogen sulfide to elemental sulfur, thereby
being reduced to the ferrous state. The elemental sulfur is
separated from the solution, and chelated iron solution is
regenerated by oxidation by contacting the solution with an
oxygen-containincJ gas to convert iron in ferrous state to the
ferric state. The aqueous chelated iron solution contains
at least two iron chelating agenks, at least one of the iron
chelating agents being an amine type chelating agent capable
of binding iron in the ferrous state to prevent precipitation
of ferrous sulfide. At least one oE the chelating agents is
a polyhydroxy type chelating agent capable of binding iron
in the ferric state to prevent precipitation of ferric hydroxide,
thereby to improve the stability of the solution.
In the accompanying drawings:
Fig. 1 is a schematic process flow diagram showing
one method of practicing the invention wherein oxidation of
hydrogen sulfide and regeneration of the solution are carried
out concurrently in the same reaction zone; and
Fig. 2 is a schematic process flow diagram showing
an alternate method of practicing the invention wherein
regeneration of the solution is effected in a separate
reaction zone.
The present ;nvention utilizes a unique reagent
for removal of hydrogen sulfide which comprises an aqueous
alkaline solut~on of iron ancl two difEerent types of chelating
agents selected for different purposes, as described in
.
~ 4 -
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~ 7~3 11
greater de~ail below. The hydrogen sulfide-con-taining gas
stream is contacted or scrubbed wi~h the chelated iron
solution in which the iron is in the ferric state to effect
oxidation of the hydrogen sulfide to elemental sulfur with
concomitant reduction of the iron from ~he higher valence
or ferric state to the lower valence or ferrous state~ The
solution is regenerated, in the same reaction zone or in a
separate reaction zone, by aeration or the like to oxidize
the iron to the ferric state.
The chemistry of the oxidation-reduc~ion system is
represented by the following equations:
(1) H2S (g) ~ ~ZS (aq-)
(2) H2S (aq.) + OE ~-~ HS- + H20
_ -2 o
(4) 2Fe+3 -~ S~2--~2Fe~2 + S
(5) 2Fe~2 + 1/2 2 + FI20 ~ 2Fe~3 ~ 2 OE
However, since the iron in the system is present in two dif-
ferent valencè states there are also competing side reactions
which can occur, resulting in loss o iron and rendering the
solution ineffective for removal of hydrogen sulfide.`
(A) Fe~2 ~ S~2-~FeS~
(B) Fe~3 ~ 3(OH) ~Fe(OEI)
Although the process can be operated over a wide
range of pH, it is prefexred to maintain the pH of the sol-
ution at from about 7 to about 13, with the optimum range
being from about 8 to about 10.5. At the preferred and
optimum ranges of pH hydrogen sulfide is absorbed effectively
by the alkaline solution, but.the side reactions (A) and (B)
would predominate in an ionic iron solution so that all
- 30 ferrou~ and ferric ions would soon precipitate out.
,.
In accordance with the present invention, the iron
is complexed with two ~ifferent types of chelating agents to
prevent loss of iron from the solution at the aforementioned
pH levels. The Type A chelating agent is selected to prevent
side reaction (A) by forming a complex with ferrous ions in
solution:
Fe 2 ~ ChelA ~-~(Fe ChelA)+2
This complex binds ferrous ions sufficiently strongl~ to
maintain the concentration such that the solubility product
constan~ for ferrous sulfide is not exceeded. Therefore,
ferrous ions in solution will be preferentially bound as
chelates of Type A, and no ferrous sulfide will precipitate.
The (Fe ChelA)~2 complex is readily oxidized by atmospheric
oxygen to Fe+3 ~ ChelA, as shown in Equation (5). The Type
B chelating agent is selected to prevent side reaction ~B)
by forming a complex with ferric ions in the solution-
- Fe~3 + chelB ~-~(Fe~ChelB)+3
This complex binds ferric ions sufficiently strongly to
maintain the concentration such that the solubility pxoduct
constant for ferric hydroxide is not exceeded. Therefore,
ferric ions in solution will be bound as Type B chelates~
and no ferric hydroxide will precipitate. The (Fe Chel~)~3
complex readily reac~s with sulfide ions to produce elemental
sulfur, as shown in Equation (4).
From Equations (4) and (5) it can be seen that the
rate of oxidation of ferrous iron must be twice the rate of
hydrogen sulfide absorption. A sufficiently aerated solution
can absorb only a small amount of oxygen~ Oxygen absorption
appears to be a critical step in regenerating the solution,
and the rate of aeration may limit the capacity of the process
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or the stability of the solu-tion. Therefore i-t is impor-
tant that the scrubbing solution be aerated as completely
and efficiently as possible. As indicated in Equations (1)
through (5), two equivalents of hydroxide are removed by the
hydrogen sulfi~e, and two equivalents of hydroxide are
produced. This makes pH adjustments minimal.
Ag the Type A chelating agent, the invention uses
(either singly or as a mixture) the polyamino polycarboxylic
acids, the polyamino hydroxyethyl polycarboxylic acids, or
tha polyphosphonomethylamines, the latter being phosphorus
analogs of the polyamino polycarboxylic acids. Usually the
aforementioned types of chelating agents will be used in the
form of theix alkali metal salts, particularly the sodium
salts. The polyamino polyacetic acids and the polyamino
hydroxyethyl polyacetic acids, or their sodium salts, are
particularly desirable.
As the Type B chelating agent, the invention uses
the sugars, the reduced sugars, or the sugar acids. Examples
of suitable sugars are the disaccharides, such as sucrose,
lactose, and maltose, and the monosaccharides, such as glu-
cose and fructose. Examples of suitable sugar acids are
gluconic acid and glucoheptanoic acid, which may be used in
the form of their alkali metal salts particularly,sodium
salts. The reduced sugars, however, are preferred for the
Type B chelating agent since there is no possibility o~
hydrolysis or oxidation at a potential aldehyde group.
Examples of suitable reduced sugars are sorbitol and mannitol.
As described in more detail in the specific examples
below, excellent results have been obtained using a mixture
of the sodium salte of ethylene diamine tetra-acetic acid
.
1,
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and N-hydroxyethyl ethylene diamine triacetic acid as the
Type A chelating agent and using sorbitol as the Type B
chelating agent. Aqueous solutions of the aforementioned
Type A chelating agents are available commercially from the
Dow chemical co. under the trademarks "Versene lO0" (Na~EDTA)
and ~Versenol 120~ (~a3HEDT~). The use of this mixture of
Type A chelating agents is particularly advantageous since
it insures the desired iron complexing effect not only in
the optimum pE range of from about 8 to about 10.5 but also
at pH levels above and below this range.
The chelated iron solution of the present invention
is prepared by dissolving a suitable iron salt in water and
adding the required amounts of the Type A and Type B chelating
agents. To this solution the alkaline material is then added
to provide a concentrate which can be diluted with water as
required to obtain the operating solution having the desired
pH and iron content. The iron content of the solution may
vary over a wide range, dependent upon the gas being treated
and other factors. Solutions having an iron content of from
about 200 ppm to about 5000 ppm are preferred. In preparing
the concentrate it is desirable always to add the chelating
agents before the alkaline ayent so as to avoid precipitation
of iron. However, the presence of the two types of chelating
agents improves the stability of the solution so that no
gxeat care is required in making up the solution to prevent
precipitation of iron 'hydroxide.
For economy, the amounts of the respective chelating
agents need be no greater than required to complex the amount
of iron present in either valence state, and in general lesser
amounts can be used. In particular, it is desirable, for
.
ease of regeneration, that the molar ra-tio of ~ype A
chelating agent ~ iron be not greater than 2:1 and pre-
ferably from about 1:1 to about 1.5:1. The iron s~lt is
preferably a ferric salt such as ferric chloride, ferric
sulfate, or ferric n.itrate. However, it is also possible
to use a ~errous salt.such as ferrous sulfate, but in this
case the solution must be aerated prior to use in order to
insure oxidation of the chelated iron to the ferric state.
The alkaline material is preferably sodium carbonate or
sodium hydroxide or mixtures thereo~, although other com-
patible alkaline compounds may be used.
The process flow for the oxidation-reduction
system using the chelated iron solu~ion of the present in-
, .. ..
vention will depend upon the hydrogen sulfide content of the
gas stream being treated and the nature of the other com-
ponents of the gas stream. Fig. l illustrates a process
flow in which the oxidation of hydrogen.sulfide and the
regeneration of th~ chelated iron solution are carried out
concurrently in the same reaction zone, this arrang~m~nt
~ 20 being referred to as aerobic absorption processing or aerobic
: operation. The process flow of Fig. 1 is particularly
adapted for use in treating a waste gas stream containing a
relatively low concentration of hydrogen sulfide (e.g. 50-
100 ppm or less) and which is free of hydrocarbons or other
. materials which shuuld not b~ mixed with air.or oxygen~
Referring to Fig. 1, the reaction system compris~s
an absorption tower or scrubber 10 containing a central con-
tact zone illustrated schematically at 11. This zone may
comprise any suitable liquid-vapor contac~ing means such as
the conventional packed beds, plates or trays. The inlet
gas containing hydrogen sulfide is in-troduced into the tower
10 through a blower 12 and a conduit 13 below the contact zone
11 for passage upwardly therethrough. A flow control damper
15 is provided in the conduit 13. Typically, the inlet gas
has a low hydrogen sulfide content on the order of 50 ppm and
is free of hydrocarbons, e.g. the off-gas from a xanthate
plant producing rayo~ or cellophane, or from a sewage plant.
The chelated iron solution of the present invention is
supplied by a line 14 to sprays or distribution nozzles 16
located in an enlarged upper section 17 of the tower 10 and
passes downwardly through the contact zone 11 in counter-
current relation to the upwardly flowing gas stream. The
treated gas exits from the tower 10 through a demister zone 18
in the section 17 and an outlet 19 having a flow control
damper 21. Make-up water may be added to the system, as
required, through a line 22 communicating with sprays 23
located above the demister zone 18. Make-up chelated iron
solution may be added, as ~required, through a line 24 com-
municating with the tower 10 below the contact zone 11.
In the arrangement illustrated in Fig. 1 the
bottom portion of ~e absorption tower 10 is used as a
reservoir ~or the chelated iron solution which fills the
bottom of the tower to a level, indicated at 26, below the
point of introduction of gas through the conduit 13. The
chelated iron solution is continuously recirculated from
the bottom of the tower 10 to the nozzles 16 through a line
27, a pump 28, and a line 29 connected to the line 14. A
portion of the chelated iron solution may be bled from ~he
system through a line 31, as may be required.
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For regeneration of the chelated iron solukion,
atmospheric air is drawn -through a screened inlet 32 by
blower 33 and is supplied through a line 34 to nozzles 36
disposed in the lower portion of the tower 10 so that the
air is bubbled through the volume of solution in the bottom
of the tower, thereby thoroughly aerating the solution to
oxidize the ferrous .iron to ~erric iron. The effluent air
passes upwardly through the tower 10 along with the feed
gas and exits with the treated gas through the outlet 19.
In the contact zone 11 the hydrogen sulfide in
the inlet gas is oxidized to elémental sulfur by the chelated
iron solution, as heretofore described, and the sulfur solids
are present as a slurry in the treating solution in the bot-
tom of the tower. A portion o~f`th~s slurry, usually in the
~orm of a froth, is continuously withdrawn from the tower
10 through a line 37 to a slurry tank 38. The sulfur slurry
is withdrawn from the bottom of the slurry tank through a line
39 by a pump 41 and is supplied through a line 42 to a fi:l-
tration step, in this case a continuous drum filter ~3. A
portion of the sùlfur slurry is recirculated to the tank 38
through a line 44.
. Wet sulfur product is recovered from the drum fil-
ter 43 through a line 46 and may be washed (not shown), to
the extent that the water balance of the system permits, in
ordèr to recover absorbed chelated iron. If desired, the wet
sulfur product may be dried in an autoclave (not shown) to
obtain a dr~ high purity sulfur product. The filtrate is
withdrawn from the drum filter 43 through a line 47 to a
receiver 48. Vapor or gas is removed from the top of the
3Q receiver 48 through a line 49 by a vacuum pump 51 and is
..
introduced by a line 52 to the absorption tower 10 belo~7
the contact zone 11. Liquid filtrate is withdrawn from the
bottom of the receiver 48 through a line 53 by a pump 54
and is recirculated through line 14 to the absorption tower
lO. A portion of the iltrate may be bled from the system
through a line 56, as desired.
Fig. 2 illustrates a process flow, in accordance
with the invention, which is particularly adapted for the
treatment of gas streams containing hydrocarbons and xela-
tively high concentrations of hydrogen sulide, e.g. a sour
natural gas containing 1-5% hydrogen sulfide. In this system
the removal of hydrogen sul~id~ and the regeneration of the
chelated iron solution are carried out in separate reaction
zones, this arrangement being rearred to as anaerobic ab-
sorption processing or anaerobic operation.
Referring to Fig. 2~ a venturi scrubber 60 is
utilized for primary contact in order to accommodate the -
high hydrogen-sulfide concentration in the feed gas. ~he
gas is introduced to the scrubber through a line 61, and a
portion of the chelated iron solution is introduced to the
scrubber 60 through a line 62. The lower portion of the
scrubber 60 co~nunicates with the lower portion of an ab-
sorption tower 63 by means of an enlarged conduit 64. The
gas flows rom the scrubber 60 and passes upwardly through
a contact zone 66 in countercurrent relation with a down~
wardly flowing portion o~ the chelated iron solution sup-
plied from line 62 and a line 67 to nozzles or sprays 68
disposed above the contact zone 66. The treated gas exits
from the top o the tower 63 throuyh a line 69 a~ter passiny
through a demister zone 70.
- 12 ~
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Chelated iron solu-tion accumulates in the boffDm
portions of the scrubber 60 and the tower 63, as indicated
by the liquid level 71. A portion of the solution may be
bled from the ~ottom of ~e scrubber 60 through a line 72,
as desired. The solution accumulating in the bottom of the
tower 63 is withdrawn through a line 73 by a pump 74 an~ is
discharged through lines 76 and 77 into an oxidizer or re-
generation vessel 78. If necessary, a heat exchanger or
coolèr 79 may be interpo~e~ in the line 76. In the vessel
78 the chelated iron solution is oxidiæed or regenerated
by introduction of atmospheric air drawn through a screened
inlet 81 by a blower 82 and supplied by a line 83 to nozzles
84 located in the ~wer portion of the vessel 78 below the
liguid level indicated at 86. The air bu~s through and
aerates the solution, as previously described, and exits
from the vessel 78 through a conduit 87. ~he regeneratad
solution is continuously withdrawn from the bottom of the
vessel 78 through a line 88 by a pump 89 and is recirculated
thraugh lines 62 and 67 to the scrubber:60 and the tower 63.
The sulfur slurry i5 continuously withdrawn from
the ves~el 78 through a conduit 91 to a drum filter 92, as
previously described in connection with Fig. 1. Wet sulfur
product is removed at a line 93, and filtrate is passed to
a receiver 94 through a line 96. Vapor or gas is wi.~hdrawn
from ~e receiver 94 through a line 97 by a vacuum pump 98
and is vented through a line 99 into the air exit conduit
87 of the vessel 78. Filtrate is withdrawn from the bottom
of the receiver 94 through a line 101 and is recirculated
. by a pump 102 through line 77 to the regeneration vessel 78,
a portion of the filtrate being bled from the system through
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a line 103, if desired.
In either the Fig. 1 or the Fig. 2 process flow
arrangements the operating temperature and pressure are not
critical and may vary over a wide range. Practically
speakiny, however, the process will normally be operated at
ambient or room temperature and at atmospheric pressure or
slightly above.
The following examples will serve to illustrate
the invention but are not to be construed as limiting the
invention:
- Example 1
A chelated iron concentrate was prepared using a
concentrated aqueous solution of Na4EDTA (~RSEN~ M 100) and
a concentrated aqueous solution of Na3HEDTA (VERSENO~, 120 )
as the Type A chelating agents and using sorbitol as the
Type B chelating agent. The composition of the concentrate
was as follows on a weight per cent basis:
; Water 55.9%
FeCl3 (39 wt. % aqueous solution) 13.4
VERSENE M Powder (Na4EDTA) 6.3
VERSENOL 120 (41 wt. % aqueous
solution Na3HEDTA) 6.3
Sorbitol (70 wt~ % aqueous
solution) 6O3
~aOH (50 wt. % aqueous solution) 3.6
Na2C3 8.2
100.0%
The concentrate was diluted with sufficie~t water to
provide an operating solution having an iron content of 200
ppm. This solution was used succes~fully for about two
weeks in a continuous aerobic operation using a pilot plant
~ .
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scale countercurrent absorption column. The p~l of -the
solution was maintained within t~e range of from about 8.5
to about 9.5. The gas treated was an off-gas from a xanthate
process having a hydrogen sulfide content which varied be-
tween about 25 ppm and about 70 ppm. The outlet gas had a
hydrogen sulfide content of 5 ppm or less, and the efficiency
of hydrogen sulfide removal ranged from about 80% to about
95% dependent upon the hydrogen sulfide content of the feed
gas.
Example 2
A laboratory scrubber of the frit-ted glass disk
type was employed to treat an inlet air stream containing 1
to 2% hydrogen sulfide with a chelated iron solution. The
solution concentrate was prepared by mixing 10 ml. of 39
wt. % aqueous ferric chloride with 64 g. of SEQLENE ES-40
and 10 g. of Na4EDTA, and then adding 20 g. of 50 wt. %
aqueous sodium hydroxide. This concentrate was then diluted
with sufficient water to provide 500 g. of solution. SEQLENE
ES-40 is a 40 wt. % aqueous solution of the sodium salt of
glucoheptanoic acid as available commercially from Pfanstiehl
Labs., Inc.
Over a period of 24 hours of continuous operation
there was substantially complete removal of hydrogen sulfide,
and the sulfur was readily recovered from the used solution
by filtration. The operation was continued successfully for
more than 200 hours with the addition of small amounts of
fresh chelated iron solution from time to time as required.
Example 3
The procedure of Example 2 was followed using a
chelated iron solution prepared from a concentrate of 2.5 ml~
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:
of 39 wt. % aqueous ferric chlori~e, 3.1 g. of ~ucro~e, 4 g.
of Na4EDTA, and 5 g. of sodium carbonate, the concentrate
being diluted with water to provide 175 g. of solution.
Substantially complete removal of hydrogen sulfide was ob-
tained over a continuous operation of 12 days.
Example 4
Following the same procedure as in Example 2,
substantially complete removal of hydrogen sulfide was ob-
tained using another chelated iron solution. The concentrate
was prepared by mixing 2.5 ml. of 39 wt. % aqueous ferric
- chloride with 16 g. of SEQLENE ~I ES-40, 5.2 g. of DEQUEST 2000
4 g. of 50 wt. % aqueous sodium hydroxide, and 2 g. of sodium
carbonate. This concentrate was diluted with water to pro-
vide 175 g. of operating solution.
DEQUEST M 2006 is an aqueous solution of the sodium
salt of tris (phosphonomethyl) amine as available commer-
cially from Monsanto Co. DE~U~ST 2054, which may also be
used, is an aqueous solution of the sodium salt of N,N,~',N'
tetrakis (phosphonomethyl) hexamethyl~ne diamine as available
commercially from Monsanto Co.
Example 5
The procedure of Example 2 was followed in
effectively removing hydrogen sulfide with a chelated iron
solution prepared from a concentrate comprising 14 ml. of
39 wt. % aqueous ferric chloride, 33.6 g. of Na4EDTA, 13 g.
of 70 wt. % aqueous sorbitol, and 14 g. of sodium carbonate~
The concentrate was diluted with water to obtain 3785 ml. or
1 gallon of operating solution. It was noted that when
- ` hydrogen sulfide without air was introduced, a small amount
of ferrous sulfide was formed, but when air was reintroduced
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along with ~he hydrogen sulfide, the ferrous sulfide was
oxidized and disappeared,
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