Note: Descriptions are shown in the official language in which they were submitted.
~Z~9~28
-- 1 --
K 7463
GAS COMPOSITION MODIFICATION
A number of gasification processes in existence or being
developed, e.g., gasification of coke, residues, coal, etc.,
produce synthesis gases having various quantities of H2, CO, C02
and H2S, 2S well as minor "impurity" components of NH3 and HCN.
In the case of synth~sis gases derived from the gasification of
coal, for example, the ratio of CO to H2 may range from 0.9 to
12:1, and the gas may contain from 0.05 per cent to 10 per cent
by volume of H2S. If the "syn-gas" is to be used for fuel
purposes, the ratios mentioned are generally satisfactory, and
little need be done except elimination of contaminants such as
H2S and HCN.
On the other hand, if other uses for the synthesis gas are
contemplated, such as hydrogen production or use as a feedstock
for synthesis operationsJ the ratio of H2 to CO may become
critical, and ad~ustment of the H2/CO ratio to the right range
may require great expense. Accordingly, a process that provided a
ready method of adjustment of the H2/CO ratio from such gaæes,
even to the production of hydrogen alone, could have great
economic importance. The inven~ion relates to such a processO
Accordingly, the lnvention relates to a process comprising
a) contacting a gaseous stream containing H2, CO, and H2S with
an H2S--~elective absorbent in an absorption zone and
absorbing the bulk of the H2S in said stream, producing a
partially purified gas stream containing a minor portion of
H2S;
b) contacting at least a portion of the partially purified gas
stream with a water shift catalyst under condltions to react
CO and water in a conversion zone, and converting CO and
water to H2 and C02, and producing a modified gas stream
having an increased ratio oE H2 to CO and containing a minor
portion of ~2S;
~r
lZ19~;28
-- 2 --
c~ passing the modified gas stream to a contacting zone and
contacting the modified gas stream with an aqueous reactant
solution, t-ne solution containing an effective amount of an
oxidizing reactant comprislng oxidizing polyvalent metal
S ions or a polyvalent metal chelate of nitrilotriacetic acid
or of an acid having 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-hydroxy propyl, and
X
-CH2CH2N
wherein X is selected from acetic acid and propionic acid
groups; and
- R is ethylene, propylene or isopropylene or alternatively
cyclohexane or benzene where the two hydrogen atoms
replaced by nitrogen atoms are in the 1,2 position; and
mixtures thereof, and converting H2~ in the modified gas
stream in the contacting zone to sulphur, and recovering a
substantially sulphur-free gas stream having an increased
ratio of H2 to C0. In an additional embodiment, the
substantially sulphur-free gas stream having an increased
ratio of H2 to C0 is passed to an absorption zone which
contains an absorbent selective for C02. Carbon dioxide is
absorbed, and a gas stream having a high H2/C0 ratio or
comprising H2 having a substantially reduced C02 content i8
~Z~91Z8
-- 3 --
produced. The invention thus provides an efficient method of
producing a product stream containing a wide range of H2/CO
compositions, ranging to the point of virtually pure
hydrogen. Additionally, an optional embodiment provides for
removal of minor quantities of COS, if present, in the
streams.
The source of the gaseous stream (containing H2, CO, and
H2S) is not critical. Thus, the streams mentioned, i.e., streams
derived from the gasification of coke, residues, coal, etc., are
eminently suited to the invention. Other streams containing the
components mentioned, and in which it is desired to adjust the
ratio of H2 to CO and remove H2S, may also be treated according
to the invention, so long as other components therein do not sub-
stantially adversely affect the absorbents, catalysts, etc. em-
ployed herein. In this regard, if the absorbents chosen are sen-
sitive to HCN, removal of this contaminane before the stream is
treated according to the invention is preferred. For example, the
stream may be treated as described in U.S. Patent Serial ~o.
4~497,784 entitled ~emoYa1 of HCN from Gaseous Streams, by
20 Diaz, filed November 29, 1983. 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 e~tensively3 but, in
general, will range from about 0.05 per cent to about 10 per cent
25 by volume. CO content may vary considerably, and may range from
about 30 per cent to over 80 per cent by volume. H2 content may
also vary, but normally will range from about lO per cent to
about 50 per cent by volume. C02, of course, may be present.
Obviously, the concentrations of H2S, CO and H2 present are not
30 generally a limiting factor in the practice of the invention. In
some of the most economically attractive gasification processes,
the CO to H2 volume ratios may be quite high, as mentioned
previously.
,~r~, ',
~Z191~8
In the first step of the process, the gas stream selected is
contacted or mixed with an absorbent selective for H2S in a
manner or under conditions that will absorb the bulk of the H2S,
preferably at least 80 per cent by volume. Any of the known
H2S-selective absorbents conventionally used (or mi~tures
thereof) which do not react substantially with the other
components of the gas stream, may be employed. Those skilled in
the art will recognize that most H2S-selective absorbents tend to
absorb C02, and if any of this gas is present, it will also be
absorbed. Given these qualifications, the particular absorbent
chosen is a matter of cholce. Aqueous alkali metal carbonate and
phosphate solutions, e.g., aqueous potassium and sodium carbonate
and phosphate, carbitol (diethylene glycol monoethyl ether), and
certain aqueous alkanolamines, such as alkyl diethanolamines, may
be used. Suitable alkanolamines include methyldiethanolamine,
triethanolamine, or one or more dipropanolamines, such as di-n-
propanolamine or diisopropanolamine. Aqueous methyldiethanol-
amine, triethanolamine and dipropanolamine solutions are
preferred absorbents, particularly methyldiethanolamine and
diisopropanolamine solutions. The solutions may ontain very
minor amounts of physical solvents, such as substituted or
unsubstituted tetra-methylene sulphones.
If diisopropanolamine is used, either high purity diiso-
propanolamine may be used, or technical mixtures of dipropanol-
amine such as are obtained as the by-product of diethanolamine
production may be employed. Such technical mixtures normally
consist of more than 90% by weight of diisopropanolamine and 10%
by weight or tess of mono- and tri-propanolamines and possibly
trace amounts of diethanolamine. Concentrations of aqueous
alkanolamine solutions may vary widely, and those skilled in the
art can adjus~ solution concentrations to achieve suitable
absorption levels. In general, the concentration of alkanolamine
in aqueous solutions will be from 5 to 60% by weight, and
preferably between 25 to 50% by weight. If COS is present in the
gas, it may be removed in the absorbent, or may be hydrolyzed, as
described herein.
Suitable temperature and pressu}e relationships for
different hydrogen sulphide-selective absorbents are known, or
can be calculated by those skilled in the art. In general, the
temperatures employed in the absorption zone are not critical,
and a relatively wide range of temperatures, e.g., from 0 to
100 C may be utilized. A range of from about 0 to 85 C is
preferred.
Similarly, pressure conditions in the absorption zone may
vary widely, depending on the pressure of the gas to be treated.
For example, pressures in the absorption zone may vary from one
atmosphere up to 150 or even 200 atmospheres. Pressures of from 1
atmosphere to about 100 atmospheres are preferred. As indicated,
what is required in the absorption zone is that the bulk of the
15 H2S, preferably at least 80 or 90 per cent by volume, be
absorbed. Given the solvents and parameters mentioned, those
skilled in the art may adjust the conditions of operation to
achieve this result. It is thus an advantage of the invention
that 811 of the H2S need not be removed at this point.
The absorption step thus produces a "purified" gas stream
which has most of the H2S removed, leaving a minor portion of
H2S, e.g., less than about 10 per cent to 20 per cent by volume
N2S in the stream. The absorption liquid or solvent, being
"loaded" or "semi-loaded", is preferably "regenerated" in
suitable cyclic techniques, producing a stream rich in H2S and a
"lean" absorbent which can be recycled for use in the absorption
steps. Suitable techniques for these procedures are well known,
and form no part of the present invention. See, for example,
Canada patent 729,090, U.S. patent 3,989,811 and U.S. patent
4,085,192. Thus, in the regeneration or stripping zone,
temperatures may be varied widely, the only requirement being
that the temperatures be sufficient to reduce the H2S content in
the absorbent to a level sufficient so that, when returned to the
absorption zone, the absorbent will effectively absorb H2S from
the gas to be treated. Preferably, the temperature should be
~21912~
-- 6 --
sufficient to reduce the H2S content in the load absorbent to a
level which ~orresponds to an equilibrium loading for an H2S
content having less than 50 per cent (preferably 10 per cent) of
the H2S content of the treated gas. Equilibrium loading conditions
for H2S and C02 at varying concentrations, temperatures and
pressures for different hydrogen sulphide-selective absorbents
are known or can be calculated by known methods and hence need
not be detailed herein. In general, temperatures of from about
90 C to 1~0 C, preferably from 100 C to 170 C, may be
employed.
Similarly, in the regeneration or desorption zone, pressures
will range from about 1 atmosphere to about 3 atmospheres. As
noted J the pressure-temperature relationships involved are well
understood by those skilled in the art, and need not be detailed
herein. Contact times in the absorption zone, insofar as
meaningful, will depend, inter alia, on the velocity of the gas
stream treated, the absorbent employed, and the type of contactor
employed. In a tray column, for examples contact time might
usefully be described as the total time a given volume of gas is
present in the given absorber, recognizing the gas liquid contact
may not occur coneinuously in such a unit. Given these qualifi-
cations, "contact" times will normally range from 1 second to 30
seconds, preferably from 1 second to 20 seconds.
In sum, the conditions for the absorption and regeneration
should be so specified that the bulk of the ~2S, preferably 80 to
90 per cent and most preferably at least 95 per cent~ by volume,
of the H2S in the gas is absorbed. Such conditions, including
choice of solvents and, e.g., number of trays, if a tray
contactor is used, will provide that very little C02 is absorbed.
Any C02 or other gases ab40rbed will be released on regeneration,
and are treated wieh the ~2S, e.g., in a Claus unit.
The partially "purified" gas i4 now passed to a conversion
zone wherein it contacts water, preferably as vapour, in the
presence of a catalyst for the reaction of water and C0, and
under conditions suitable for the conversion. Since one mole of
water reacts with one mole of C0 to produce the hydrogen and C0
1219~
and since equilibrium is not easily reached, the volume of H2
produced varies directly with the water and CO supplied. Suitable
conditions, i.e., temperatures, pressures, contact times,
catalysts, etc., are known to those skilled in the art. For
example, Kirk-Othmer, Encyclopedia of Chemical Technology (~nd
Edition), Volume 4, pages 431 and 432 (1967), the Catalyst
Handbook, Chapter 6 (1970), and Catal. Rev. - Sci. Eng., Volume
21(2) pages 275-318 (19803 describe suitable conditions and
catalysts for treating the purified stream. Appropriate ~atalysts
include Fe/Cr for high temperature shift, and Cu/Zn for low
temperature shift. The high temperature Fe-based supported
catalystY have a higher sulphur tolerance than the Cu/Zn
catalyst. However, the latter system, since it operates at low
temperature~, can convert a higher proportion of CO and thus
achieve a pronounced modification of the CO/H2 ratio. This is
possible because the equillbrium of the water-shift reaction
C~+H O ~ CO +H
lies to the right at lower temperatures. As indicated, the ratio
of H~/CO is ad~usted to the extent desired by controlling the
volume of water supplied to the conversion zone. Depending on the
conditions applied and the volume of H2S remaining in the stream,
at least some COS, if present, may be converted. Optionally, a
COS conversion zone may be employed after the shift zone to
remove any COS present in the stream. The hydrolysis of COS is
shown by the following formula:
COS+H20 ~ H2S+C02
Water is added, in the COS conversion zone, in the required
amount. Any catalyst demonstrating activi~y for this reaction may
be employed. Preferred catalysts are Ni, Pd, Pt, Co, Rh or In. In
general, most of these materials will be provided as solids
deposited on a suitable support material, preferred amorphous
support materials being the aluminas, silica aluminas, and
silica. Crystalline support materials such as the alumino-
silicates, known as molecular sieves (zeolites), synthetic or
natural, may also be used. The selection of the particular
12~L9~28
catalyst (and support, if employed) are within the skill of those
working in the field. Platinum on alumina is preferred.
The temperatures employed in the optional hydrolysis zone
are not critical, except in the sense that the temperatures
employed will allow substantially complete conversion of the COS.
Temperatures will range from about 50 ~C to 150 C or even
200 C, although a range of from about 50 C to about 150 ~C is
preferred. Those skilled in the art may adjust the temperatures,
as needed, to provide efficient reaction temperatures. Contact
times will range from about 0.5 second to about 10 seconds, with
contact times of 1 second to 3 seconds being preferred. Pressures
employed ln the hydrolysis zone may be atmospheric, below
atmospheric, or greater than atmospheric. If higher temperatures
and a high temperature catalyst are employed in the shift zone,
the gas stream exiting the shift reactor or the optional COS
hydrolysis zone should be pa~sed through a heat exchange zone,
the heat from the gas preferably being utilized to heat the gas
stream entering the shift zone.
In accordance with the invention, the remainder of the H2S
in the gas stream (and any ~2S produced by hydrolysis) is removed
by contacting the stream with an aqueous reactant solution> the
solution containing an effective amount of a reactant comprising
oxidizing polyvalent metal ions, such as iron, vanadium, copper,
manganese, and nickel, or a specified polyvalent metal chelate,
and mixtures thereof. As used herein, the term "mixtures thereof"
includes mixtures of the polyvalent metal ions, mixtures of the
polyvalent metal chelates, and mixtures of polyvalent metal ions
and polyvalent metal chelates. The specified chelates are
chelates of a polyvalent metal and ~itrllotriacetic acid or an
acid having the formula:
Y
~-R-N
Y Y wherein
:12~ 28
- from two to four of the groups Y are selected from ace~ic and propionic acid groupsj
- from zero to two of the groups Y are selected from 2-hydroxy
ethyl, 2-hydroxy propyl, and
-CH2CH2N~
X, ~herein X is selected from
acetic acid and propionic acid groups; and
- R is ethylene, propylene or isopropylene or alternativelycyclohexane or benzene where the two hydrogen atoms replaced
by nitrogen atoms are in the 1,2 position; and mixtures
thereof. The oxidizing polyvalent metal should be capable of
oxidizing hydrogen sulphide, while being reduced itself from
a higher to a lower valence state, and should then be
oxidizable by oxygen from the lower valence state to the
higher valence state in a typical redox reaction. Iron,
copper and manganese and particularly iron are preferred as
polyvalent metals for the polyvalent metal chelate of
nitrilotriacetic acid. Most preference is given to the
Fé(lII) chelate of nitrilotriacetic acid. Iron(III) is
preferred as polyvalent metal for the polyvalent metal
chelate of an acid having the aforementioned formula. Most
preference is given to the Fe(III) chelate of N-(2-hydroxy-
ethyl)ethylenediamine triacetic acid. O$her polyvalent
metals (or chelates thereof) which can be used include lead,
mercury, palladium, platinum, tungsten, nickel, chromium,
cobalt, vanadium, titanium, tantalum, zirconium, molybdenum,
and tin.
In accordance with the invention, a substantially sulphur-
free gas stream having an increased H2/~0 ratio is recovered. The
conditions of operation of the oxida~ive removal of the remainder
of the H2S from the gas stream, sulphur recovery, and
regeneration of the oxidizing reactant solution are adequately
~219~2~3
- 10 -
described in U.S. patent 4,409,199 (Blytas), issued October 11,
1983, and U.S. patent 4,356,155 (Blytas and Diaz), issued October
26, 1982.
The product produced, from this stage, will depend on the
degree of conversion in the previous shift step. The gaseous
stream is treated under appropriate conditions with an absorbent
selective for CO2 in the presence of H2 or H2 and CO. If the
shift reaction has been utilized to adjus~ the H2/CO ratio to a
given point, the product will be H2 and CO, in the given ratio.
On the other hand, if the CO is reacted to extinc~ion, the gas
stream product will be comprised predominantly of hydrogen. Those
skilled in che art may select appropriate CO2-selective
absorbents, pressures, temperatures, etc., to separate the
hydrogenJCO2 or hydrogen/CO2/CO mi~tures. Suitable absorbents
include aqueous alkanolamines, sodium or potassium carbonate
solutions, tri-potassium phosphate, or solutions of sterically-
hindered amlnes in aqueous or organic solvents, or in
combinations of,amines and potassium carbonate. Conditions for
designing absorption and regeneration may be selected on the
20 basis of the specific case considered. Characteristics of the
aqueous alkanolamines, alkali metal carbonates, and potassium
metaphosphate are wel] known, as described in Gas Purification by
A.L. Kohl and F.C. Riesenfeld (1960)o Use of sterically-hindered
amines for CO2 absorption is described in U.S. patent 4 ~ 112 ~ 050
(1978)~ U~S~ patent 4~112~051 (1978)~ and U.S. patent 4~100~257
(1978)~ Preferably, temperatures will range from 10 C to 80 C,
and pressures will preferably range from 1 atmosphere to 100
atmospheres. The CO2 absorption is preferably conducted as a
cyclic process in which the CO2-"loaded" absorbent is regenerated
or stripped, the "lean" absorbent being returned for use, and the
C2 being recovered or vented.
Off-gases from the bulk H2S absorption-regeneration
procedure are preferably oxidized to produce sulphur. The
liberated H2S is preferably treated by that process known as the
"Claus" process. In the "Claus" process, elemental sulphur is
~2~9~2~3
prepared by partial oxidation of the H2S to sulphur dioxide,
using an oxygen-containing gas (including pure oxygen), followed
by the reaction of the sulphur dioxide with the remaining part of
the hydrogen sulphide, in the presence of a catalyst. This
process, which ls used frequently at refineries, and also for the
workup of hydrogen sulphide recovered from natural gas, is
carried out in a plant which typically comprises a combustion
chamber followed by one or more catalyst beds between which are
arranged one or more condensers in which the reaction products
are cooled and the separated liquid elemental sulphur is
recovered. To some extent, the amount of elemental sulphur
recovered depends on the number of catalyst beds employed in the
Claus process. In principle, 93% of the total sulphur available
can be recovered when three beds are used.
Since the yield of recovered elemental sulphur, relative to
the hydrogen sulphide introduced, is not quantitative, a certain
amount of unreacted hydrogen sulphide and sulphur dioxide remains
in the Claus off-gases. These gases may be inclnerated in a
furnace or treated in other ways known to those skilled ~n the
art.
In order to describe the invention with greater particulari-
ty, reference is made to the accompanying schematic draw~ng. All
values are merely exemplary or calculated, and should not be
taken as delimiting the invention.
As shown, a gas stream containing 2 per cent ~12S, 5 per cent
C02, 48 per cent C0 and 35 per cent H2 (all by volume), enters
absorber or contactor (1) via line (2). Absorber (1) is a tray
contactor, although any suitable contacting device (such as a
venturi) may be employed. An absorbent mixture, e.g., a mixture
comprising 45 per cent by volume of water and 55 per cent by
volume of sulfolane, enters contactor (1) via line (3). For
illustrative purposes, it will be assumed that the gaseous stream
enters at 200 mscf per hour, while the absorbent mixture enters
at 20 M gallons per hour. Pressure of the gas in line (2) ls 100
psig, and the temperature is 45 C. The countercurrent flow of
~LZ~91;~8
- 12 -
liquid and gas, 2S illustrated, provides for good contact and
absorption of the H2S in the stream. Approximately 97 per cent by
weight of the H2S in the stream is absorbed, and the partially
purified gas ls removed overhead via line (4).
The H2S-containing ("loaded") absorbent exits absorber (1)
via line (5), and passes to stripping or regeneration column (6)
wherein the H2S is stripped from the absorbent, preferably by
heat supplied as steam. "Lean" absorbent is returned via line (3)
for re-u~ilization in absorber (1) while H2S is removed via line
~7). The H2S in line (7) may be treated in any suitable fashion,
but is preferably sent to a Claus unit. If C02 has been absorbed
to any extent, provision may also be made for its removal or
recovery.
Upon exit from rontactor (1), the gas stream, which has a
substantially reduced H2S content, passes via line (4) to reactor
or contact zone (8) wherein it is contacted with water supplied
via line (9) and with a catalyst containing Fe/Cr on activated
alumina. The gas in line (4) is preferably heat exchanged with
the exit gas in line (10) before entry into reactor (8). The
temperature of the exit of reactor (8) is about 300 C, pressure
about 1000 psig, and total contact time in zone (8) is 2 seconds.
In this illustration, sufficient water~ as vapour, is supplied in
a ratio of 0.3 mols per mol of C0 in the gas stream. More or less
water may be supplied, the determining factor being the de8ree of
conversion desired. If the COS in the original stream has not
been absorbed by the absorbent in contactor (1), or if some
remains in the gas stream in line (4), it may be hydrol~zed also
in zone (8) to some de8ree. An optional COS hydrolysis zone (11)
i8 shown (dotted lines) in line (10), the outlet line from zone
3G (8). Suitable catalysts and conditions for such removal are as
described, supra; see the aforementioned U.S. patent 4,409,199.
In accordance with the invention, the gas stream, containing
the modified gas stream, and posæible COS hydrolysis products,
passes via line (10) to contactor (12) where it is contacted with
an aqueous reactant solution to produce sulphur. Contactor (12)
lZ8
is a tray contactor, although any suitable contacting device (such as
a venturi) may be employed. An aqueous oxidizing reactant solution,
e.g., a solution containing 0.4 molar of the Fe(III) chelate of
N-(2-hydroxyethyl)ethylenPdiamine triacetic acid or of nitrilo-
triacetic acid, enters contactor (12) via line (13). The gaseousstream enters at 225 mscf per hour, while the reactant solution
enters at 400 gallons per hour. Pressure of the gas in line (10)
is 800 psig, and the temperature of the gas, having exchanged
heat with line (4), is 50 C. Reactant solution is supplied at a
temperature of 40 DC. The countercurrent flow of liquid and gas,
as illus~rated, provides for good contact and reaction of the ~2S
in the stream to sulphur. As will be understood by those skilled
in the art, water and the Fe(II) complex or chelate of
N-(2-hydroxyethyl~ethylenediamine triacetic acid or of nitrilo-
triacetic acid are also produced by the reaction.
Upon exit from con~actor (12), the modified gas stream,which is now substantially free of H2S, passes through line (14) to
absorption zone (15), as more fully described hereinafter.
Concomitantly, reactant mixture, containing some Fe(II) chelate
of N-(2-hydroxyethyl)ethylenediamine triacetic acid or of nitrilo-
triacetic acid and sulphur, is forwarded via line (16) to re-
generation zone (17). As shown in dotted line boxes, the sulphur
may be removed prior to regeneration or after regeneration.
Preferably, sulphur is removed before regeneration.
In regenerator (17), oxygen is supplied, via line (18), in
molar excess. Preferably, the oxygen is supplied as air, in a
ratio of about 2.0 or greater per mole of Fe(II) chelate in the
mixture. Temperature of the mixture is preferably around 40 C,
and pressure is suitably 20 to 30 psig. Regeneration in this
manner has the added advantage of removing some water vapour,
thus aiding in prevention of water build-up in the system and
reducing bleed and make-up problems. It is not necessary that all
of the Fe(II) chelate be converted.
Regenerated absorbent mixture, i.e. 9 an absorbent mixture in
which at least the bulk of the Fe(II) chelate has been converted
9~'8
- 14 -
to the Fe(III) chelate, is removed via line (13) and returned to
contactor (12).
Any suitable absorbent for removing C02 from the H2/CO
mixture in the stream may be employed in absorber (15). For
example, aqueous diisopropanolamine/sulfolane mixtures may be
employed. Suitable C02 absorption removal procedures and
conditions are known to those skilled in the art, and form no
part of the present invention. Suitably, the C02 removal
procedure ls conducted with a regenerable absorbent, the desired
modified stream being removed via line (18), and the loaded
absorbent being removed for regeneration via line (19).
While the invention has been illustrated with particular
apparatus, those s~illed in the art will appreciate that, except
where specified, other equivalent or analogous units may be
employed. The term "zone", as employed in the specification and
claims, includes, where suitable, the use of segmented equipment
operated in series, or the division of one unit into multiple
units because of size constraints, etc. ~or example, an
absorption colu~n might comprise two separate columns in which
the solution from the lower portion of the first column would be
introduced into the upper portion of the second column, the
gaseous material from the upper portion of the first column being
fed into the lower portion of the second column. Parallel
operation of units, is of course, well within the scope of the
invention.
Again, as will be understood by those skilled in the art,
- the solutions or mixtures employed, e.g., the oxidizing reactant
solutions, may contain other materials or additives for given
purposes. For example, U.S. Pat. No. 3,933,993 discloses the use
f buffering agents, such as phosphate and carbonate buffers.
Similarly, U.S. Pat. No. 4,009,251 describes various additives,
such as sodium oxalate, sodium formate, sodium thiosulphate, and
sodium acetate, which may be beneficial.
