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U’TVU’UI Le
UNITED STATES PATENT OFFICE
2,405,872
REMOVAL OF HYDROGEN SULPHIDE
, FROM GAS STREAMS
Ronald E. Reitmeier, Louisville, Ky., assignor to
The Glrdler Corporation, Louisville, Ky., a cor
poration of Delaware
No Drawing. Application March 7, 1945,
1
Serial No. 581,547
26 Claims. (Cl. 23—2)
This invention relates to the purification of
gaseous mixtures and more particularly to the
removal of hydrogen sulphide from gaseous mix
tures containing the same, and involves as an
essential feature the use of soluble dichromates.
Although suitable for removal of hydrogen
sulphide from any gaseous mixture containing
in contact with the dichromate, provided the
solution is maintained within the proper acid pH
range.
The dlchromates are recognized as better oxi
dizing agents than the chromates. However, in
order to have the dichromate present to the great
est degree. it is necessary to maintain the solution
well within the acid pH range, since the greater
the pH, the greater will be the tendency of the
the same. the process of my present invention
is particularly suited for the puri?cation of car
bon dioxide which is to be used for the manu 10 chromate to exist in the form of the monochro
facture of dry ice. It is important that carbon
mate. Maintaining low pH increases the tend
dioxide. when used for that purpose, be. substan
ency of the chromate to exist in the dichromate
form, but normally the more acid the solution
the less will be the concentration of hydrogen
It has been proposed in Hackhofer et al. Pat 15 sulphide in the solution. In other words. maxi
ent 2,044,116, to use alkali metal monochromate
mum concentration of the oxidizing dichromate
solutions for the absorption of carbonic acid
may be attained by maintaining the solution in
from gases containing it and impurities such as
the
acid pH range, but this tends to decrease the
oxygen, nitrogen and rare gases. The absorption
concentration of hydrogen sulphide in the solu
is at superatmospheric pressures and the subse 20
tion so that the dichromate has very little hy
quent recovery of carbonic acid is effected by the
drogen sulphide to work on. I maintain the
reduction of pressure, the impurities remaining
solution in the acid pH range so as to insure the
behind in the solution. In this patent it is speci
presence of a considerable quantity of dichromate,
?ed, however, that such impurities as hydrogen
so as to secure the benefit of its superior oxi
sulphide should be removed before introducing 25 dizing
action, and as an important feature of my
the gas mixture into the chromate solution (p. 1,
invention I add a metal compound capable of
col. 2, lines 28-33). Thus the process is not suit
counteracting-the tendency toward elimination
able for the separation of hydrogen sulphide and
of
hydrogen sulphide from an acid solution, and
carbon dioxide.
capable of retaining an increased concentration
80
Reich, in Patents 1,519.932, 2,122,586, and
of sulphide in the solution in contact with the
2,225,131, uses hexavalent chromium compounds
dichromate.
such as sodium chromate and sodium dichromate
While sodium, potassium and ammonium di-‘
for the oxidation of impurities in the puri?cation
chromates are referred to above as being suitable
of carbon dioxide, and states (Patent 2,225,131,
for the practice of the process of the present
p. 1, col. 1, lines 53-55) that the sodium chro 35 invention, the sodium dichromate is preferred.
mate solutions are over 50 times as effective as
tially free from contaminating materials, espe
cially hydrogen sulphide.
Any concentration of the dichromate may be em
ployed. but from an economic standpoint it is
the chromium compound should be maintained
preferable to ‘use a solution containing in the
in chromate form by the continuous addition of
neighborhood of 2% sodium dichromate.
alkali compounds in the same proportion as the 40
The metal salt which is added to the dichroé
chromate is converted to the dichromate. Thus
mate solution must be a salt of a metal the car
the presence of dichromates is indicated as dis
bonates and chromates of which are soluble under
advantageous.
.
the conditions required for carrying out my proc
I have discovered, in contrast to the prior art
ess as set forth more in detail hereinafter. Metal
dichromate solutions. - He further specified that
above referred to, that soluble dichromates, such
as sodium, potassium and ammonium dichro
mates, are more effective than chromates as oxi
dizing and purifying agents when the dichro
mates are used under proper conditions. I have
salts of zinc, copper. cadmium, mercury, arsenic.
tungsten, tin and vanadium may be employed.
Although salts of all of those metals are effective
for the purposes of the present invention, the salts
of zinc. copper, cadmium and mercury, in the
found, for example, that hydrogen sulphide is 50 order
given. are the most effective. It has been
very effectively removed from a gaseous mixture
found that the anion of the metal salt is not par
when the gaseous mixture is scrubbed with an
ticularly important, but the sulphates or the
aqueous solution of the dichromate containing a
chlorides
are preferable.
metal salt capable of retaining a relatively large
The metal salt added to the dichromate solu
coneentration of the sulphide in such solutions 65 tion may be in any concentration up to the point
3
4
of saturation, but it has been found that a dichro
“ mate solution containing 2 to
ZnSOv'lHzO is
present process is concerned. The net effect I
be represented by the following reaction:
most effective and economical in removing hy
drogen sulphide.
As above noted. the addition of such metal salts
to the dichromate solution under proper condi
It will be noted that MeSOiv is used up in
first reaction and acid is produced, and t
MeSOi is produced and acid is used in the s
greater concentration thereof may be readily
acted upon by the dichromate. but I have found 10 end reaction, but that the second reaction :
produces NaOH which will neutralize the at
that uvier some conditions the bene?t of the
and the overall result is to make the solution m
added meial salt may be counteracted at least
basic.
to some degree if the dichromate solution is too
Since reaction i is much more rapid than
acid. Consequently, in some cases it may be nec
action 2, if no buffer were added the absorpt
essary to add to the solution a further ingredient
of only a small amount of H28 into the solut
to serve as a buffer to maintain the solution with
would reduce the pH to such a value, due to
in the proper acid pH range during the entire
action i, that no further MeS would be forrr
period of the process. The particular pH range
Under such conditions the oxidation reactio:
will vary within the acid range. depending upon
the particular metal salt that is added to the di 20 would not take place to any great extent, i
there would then take place only the oxidatior
chromate solution. and with some particular metal
such small quantities of His as might be retail
salts the range may be very narrow. On the
in the solution. I have found, for example, t
other hand, when certain metal salts are added
the solubility of HgS in solutions whose pH is I
to the dichromate solution the desired pH may be
than I, is great enough that reaction 9. will
attained and maintained without the specific ad
take place with sufficient rapidity to insure cc
ditlon of a buffer. due to reactions occurring in
plete removal of H28 from a gas stream as fast
the practice of the process. Therefore it is a
the B28 is introduced into the scrubbing soluti
feature of the present invention, and within the
The solubility of CdS in solutions whose pH is I
scope thereof, that the solution be maintained
within the required pH range, with or without the 30 than 2 is great enough that reaction 2 will
take place with sufficient rapidity to insure cc
addition of a buffer, depending upon the partic
plete removal of H28 from a gas stream as [9.51
ular metal salt employed. In other words, in
the H28 is introduced into-the scrubbing soluti
the carrying out of the present invention it is par
In the case of Cu, the lower pH limit is i, am
ticularly important that the solution of dichro
the case of Zn the lower pH limit is I.
mate and added metal salt be maintained within
For the reasons above pointed out it is imp
the proper pH range, and a buffer may be added
tant that when mercury and copper salts
when} necessary to maintain such pH range.
added to the dichromate solution the pH rai
In order that the reasons for, and the manner
will be maintained between about 1 and abou
of, maintaining a particular pH range may be
more clearly understood, I will brie?y set forth 40 When using the mercury salt the desired
range is maintained without the addition of 1
an example of the reactions which I believe take
buffer. when cadmium salts are added the sc
place in the course of my present process. If the
tion should be maintained within a pH m:
added metal salt be a sulphate, the ?rst reaction
is:
from about 2 to about 7, and it is usually for
45 necessary to add a buffer to maintain that
range. When zinc salts are used, the pH rai
should be maintained between about 4 and ab
7, the preferred pH range being between 4.5 s
(where Me represents one of the metals pre
viously mentioned). Under proper conditions the
6.5, and it being necessary usually to add a bu:
metal sulphide which is produced will precipitate 50 to maintain the desired range. As will be 1
and be available, along with hydrogen sulphide,
derstood by one skilled in the art, the neces:
for oxidation by the dichromate in the solution.
for adding a buffer, and the amount of such bu:
added when necessary, will depend somewhat
However, the above reaction is a reversible one,
and if the acidity of the solution becomes too
the concentration of the dichromate and
great. the MeS will go back into solution to re 55 added metal salt in the solution.
form MeSOi and H25, so that only a relatively
Where it is found necessary to buffer the sc
tion, the buffer may be comprised of any s1
small amount of sulphide will be available for
oxidation by the dichromate. Therefore it is im
able material capable of maintaining the sc
portant that the acidity of the solution be main
tion within the desired pH range, depending u‘,
tained sufficiently low to insure that precipitation 00 the particular added salt employed as above (
of the metal sulphide does take place, thereby
cussed, but buffers which cause precipitation
in increase the concentration of sulphide made
the metal salt, or which are oxidized by the
chromate are to be avoided. The type of r
available in the solution for oxidation by the ill
chromate. As the sulphides of copper, mercury,
terial suitable as buffers is well known in
cadmium and zinc have different degrees of sol 65 art and stated in the literature. Borax has b
ubility, practical operation of my process makes
found to provide a satisfactory buffering acti
it necessary to select a pH for the solution in ac
The concentration of the buffer will of cor
cordance with the solubility of the sulphide of
depend upon how much buffering action is
the metal of the particular salt used.
quired to maintain the solution within the pro
pH range, depending upon the particular mi
The ?rst reaction above referred to is a very
rapid one, and is followed by a second reaction
salt that is added to the dichromate solution, 1
involving the dichromate and resulting in the
also upon the concentrations of the dichrom
formation of free sulphur. Whether this is a
and added metal salt in the solution. When (
t‘ons sei ves to increase the retention of sulphur in
the solution in the form of a sulphide. so that a
single reaction or a series of reactions is not de?
nitely known, but this is immaterial so far as the
ploying solutions containing 2% NazCnOr-Zl
and 2 to 3% ZnSOr'IHzO, it has been found t
a oorax concentration or about 0.5 to 1.5% is
satisfactory.
A solution which is particularly effective
removing hydrogen sulphide from gaseous
mixtures containing the same. contains 2%
NMCl'zQ': EH20. 291- to 3% ZDSOr'II-IQO, and 0.5%
to 1.5% NQQBiO‘i-IOHZO.
In the carrying out of the present invention
the gas stream from which the hydrogen sulphide
5. A process for the removal of hydrogen 2
phide from gaseous mixtures containing
same, which comprises scrubbing such gase
mixtures with an aqueous solution of a dich
mate and a zinc salt, together with a butler wh
acts to maintain the solution within an acidic
range which will permit substantial preclpltat
of zinc sulphide.
6. A process for the removal of hydrogen 5
is to be removed may be scrubbed with a solu— 10 phide from gaseous mixtures containing the sai
tion of the kind hereinbeiore described, at any
which comprises scrubbing such gaseous mixtu
desired pressure. Also. such solutions may be
used at any temperature between the freezing
with an aqeous solution of sodium diehrometi
zinc salt, and a buller, which latter acts to ma
tain said solution within an acidic pH range wh
will permit substantial precipitation of zinc s
point and the boiling point of the solution. but
it is desirable that it be used at the temperature
at which the gas to be scrubbed is available.
Any well known equipment commonly used
for effecting contact between a gas stream and
an absorption medium may be employed for car
phide.
7. A process' for the removal of hydrogen s
phide from gaseous mixtures containing the sa:
which comprises scrubbing such gaseous mixtu
rying out the present invention. For example, 20 with an aqueous solution of sodium dlchromz
the gas stream may be passed countercurrently
zinc sulphate and a butler, which latter ma
through the absorption solution in a tower packed
tains an acidic pH range which will permit 51
with quartz or Raschig rings in a well known
stantiai precipitation of zinc sulphide and will p
manner. It is not intended, however, that the
vent formation of substantial amounts of moi
inventior. be limited to any particular equipment
chromate.
for carrying out the process since. as indicated
8. A process for the removal of hydrogen s
above. any suitable equipment may be employed.
phide from gaseous mixtures containing the sex
In practice two towers may be used and the
which comprises scrubbing such gaseous mixtu
gases passed therethrough in series, and separate
with an aqueous solution of a dichromate C(
portions of the solutions circulated through each. 30 taining a zinc salt, and which said solution
so that the main absorption is in the ?rst, and
maintained within the pH range of 4 to 7 by 1
the second acts on the small residual hydrogen
action of a buffer.
sulfide. When the solution at‘ the ?rst tower be—
comes spent, it may be replaced by a fresh solu
tion, and the gas ?ow may be through the towers
9. A process for the removal of hydrogen 5
phide from gaseous mixtures containing the sax
which comprises scrubbing such gaseous mixtu
in the reverse sequence.
with an aqueous solution of dichromate and
Having thus described my invention what I
copper salt, and which said solution is maintair
of such acidity as will insure retention of sulph
in the solution.
1. A process for the removal of hydrogen sul 40
10. A process for the removal of hydrogen s
phide from gaseous mixtures containing the
phide from gaseous mixtures containing the sax
same. which comprises scrubbing such gaseous
which comprises scrubbing such gaseous mixtu
claim as new and desire to secure by Letters Pat
ent is:
mixtures with an aqueous solution of a dichro
mate and a salt of a metal selected from the
group comprising zinc. copper, cadmium, mer
cury, arsenic. tungsten. tin and vanadium, and
in which said solution is maintained of such
acidity that considerable quantitiesof sulphide
are maintained in the solution for action there
I on by the dichromate.
2. A process for the removal of hydrogen sul
phide from gaseous mixtures containing the
same, which comprises scrubbing‘ such gaseous
mixtures with an aqueous solution of an alkali
metal dichromate and a salt of a metal selected
from the group comprising zinc, copper, cad
mium. mercury, arsenic, tungsten, tin and vana
dium, and in which said solution is maintained
of such acidity as will permit substantial precipi
tation of the sulphides of said last mentioned
metals and prevent substantial torn-axon o1‘
monochromates.
3. A process for the removal of hydrogen sul
phide from gaseous mixtures containing the same,
which comprises scrubbing such gaseous mixtures
with an aqueous solution of dichromate and a zinc
salt, and which said solution is maintained of ‘
with an aqueous solution of a dichromate ant
copper salt, and which said solution is maintair
within the pH range of 1 to 7.
11. A process for the removal of hydrogen s‘
phide from gaseous mixtures containing the sar
which comprises scrubbing such gaseous m
tures with an aqueous solution of a dichrom:
and a copper salt, together with a buffer whi
acts to maintain the solution within an acidic j
- range which will permit substantial precipitati
of copper sulphide.
12. A process for the removal of hydrogen s‘
phide from gaseous mixtures containing the sat
which comprises scrubbing such gaseous rnixtu:
with an aqueous solution of sodium dichroma
a copper salt and a buffer, which latter acts
maintain said solution within an acidic pH rar
which will permit substantial precipitation
copper sulphide.
13. A process for the removal of hydrog
:luipiuue from gaseous mixtures containing t
same, which comprises scrubbing such gasec
mixtures with an aqueous solution of sodit
dichromate, copper sulphate and a butter, whi
latter maintains an acidic pH range which vi
permit substantial precipitation of copper s1
such acidity as will insure retention of sulphide in
the solution.
phide and will prevent formation of substant
4. A process for the removal of hydrogen sul Ti) amounts of monochromate.
'
14. A process for the removal of hydrog
phide from gaseous mixtures containing the same.
which comprises scrubbing such gaseous mixtures
sulphide from gaseous mixtures containing t
with an aqueous solution of a dichromate and a
same, which comprises scrubbing such gaseo
zinc salt, and which said solution is maintained
mixtures with an aqueous solution of a <
within the pH range of 4 to 7.
75 chromate containing a copper salt, and whit
2,405,672
said solution is maintained within the pH range
of l to 7 by the action of a buffer.
\
15. A process for the removal oi’ hydrogen
sulphide from gaseous mixtures containing the
same. which comprises scrubbing such gaseous
mixtures with an aqueous solution at dichromate
and a cadmium salt, and which said solution is
maintained 0! such acidity as will insure reten
8
sulphide from gaseous mixtures containim
same. which comprises ;~:-:ubbing such gal
mixtures with an aqueous solution of so
dichromate. a cadmium salt and a buffer. v
latter acts to maintain said solution withi
acidic pH range which will permit substa
precipitation of cadmium sulphide.
19. A process for the removal of mm:
tion of sulphide in the solution.
sulphide from gaseous mixtures containing
16. A process for the removal of hydrogen 10 same. which comprises scrubbing such ga:
sulphide from gaseous mixtures containing the
mixtures with an aqueous solution of so
same, which comprises scrubbing such gaseous
dichromate, cadmium sulphate and a" b‘
mixtures with an aqueous solution of a di
which latter maintains an acidic pH range in
chromate and a cadmium salt, and which said
will permit substantial precipitation of cadr
solution is maintained within the pH range of 15 sulphide and will prevent formation of sub:
2 to '7.
tial amounts of monochromate.
17. A process for the removal of hydrogen
20. A process for the removal of hydr
sulphide from gaseous mixtures containing the
sulphide from gaseous mixtures containing
same. which comprises scrubbing such gaseous
same. which comprises scrubbing such ga:
mixtures with an aqueous solution of a di 20 mixtures with an aqueous solution of a
chromate and a cadmium salt. together with a
chromate containing a cadmium salt. and u
bu?er which acts to maintain the solution with
said solution is maintained within the pH r
in an acidic pH range which will permit sub
of 2 to 7 by the action of a buffer.
'
stantial precipitation 01‘ cadmium sulphide.
18. A process tor the removal of hydrogen 25
RONALD E. REITIWEIE
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