Note: Descriptions are shown in the official language in which they were submitted.
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SULFUR DIOXIDE SCRUBBING SYSTEM AND PROCESS FOR PRODUCING
POTASSIUM PRODUCTS
FIELD OF THE INVENTION
[0001] The present invention relates to a process for the preparation of
potassium
thiosulfate, or potassium sulfites and/or potassium bisulfites.
BACKGROUND OF THE INVENTION
[0002] The thiosulfate ion, S2032, is a structural analogue of the S042-
ion in which one
oxygen atom is replaced by one S atom. However, the two sulfur atoms in S203'
are not
equivalent. One of the S atoms is a sulfide-like sulfur atom that gives the
thiosulfate its reducing
properties and complexing abilities.
0- S-
S = S = 0 <=> 0 .. S .. 0
a
[0003] Thiosulfates are used in leather tanning, paper and textile
manufacturing, flue-gas
desulfurization, cement additives, dechlorination, ozone and hydrogen peroxide
quenching,
coating stabilizers, as an agricultural fertilizer, as a leaching agent in
mining, and so on.
[0004] Due to these complex-forming abilities with metals, thiosulfate
compounds have
also been used in commercial applications such as photography, waste treatment
and water
treatment applications.
[0005] Thiosulfates do however readily oxidize to dithionates,
trithionates,
tetrathionates, and finally to sulfates:
2S2032- + 302 ¨> 2S2062
S2062 + 02 ¨> 2S042
7S2032 + 3/202 ¨> 2S3062- +2S4062
2S3062 + 602 ¨> 6S042
54062- + 502 ¨> 4S042-
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[0006] Due to this transformation, thiosulfates are used as fertilizers in
combination with
cations such as ammonium, potassium, magnesium and calcium. The ammonium,
alkali metal
and alkaline earth thiosulfates are soluble in water. Water solubilities of
thiosulfates decrease
from ammonium to alkali metals to alkaline earth thiosulfates.
[0007] Potassium (K) is a primary plant nutrient. Potassium is associated
with
movement of water, nutrients, and carbohydrates in plant tissue. If potassium
is deficient or not
supplied in adequate amounts, growth is stunted and yields are reduced.
Potassium stimulates
early growth, increases protein production, improves the efficiency of water
use, is vital for stand
persistence in cold weather, and improves resistance to disease and insects.
[0008] Potassium thiosulfate fertilizer contains the highest percentage of
potassium in
liquid form, compared to other sources of potassium such as potassium chloride
(KC1), potassium
nitrate (KNO3), and potassium sulfate (K2SO4). In addition, it combines
potassium with sulfur
(17%) which is also an essential plant nutrient.
[0009] It is contemplated that potassium thiosulfate could be produced by
several
alternative routes such as:
I. Reaction of S and S032- in neutral or alkaline medium
Reaction of S2- and S032- (via SO2 and HS032-)
Oxidation of Potassium Hydrosulfide (KSH)
IV. Ion Exchange reaction between alkaline thiosulfates and potassium
chloride or
nitrate
V. Salt exchange between alkaline thiosulfates and Potassium Chloride or
Nitrate
VI. Oxidation of Potassium Polysulfide
[0010] However, some of these alternatives present serious difficulties or
disadvantages.
Route I and II are longer processes and require the use of sulfur dioxide SO2.
Both these routes
are described when the scrubbing of the air pollutant sulfur dioxide is an
objective. Route III
requires handling of potassium hydrosulfide as a raw material which is not
favorable due to a
hydrogen sulfide environment. Routes IV and V suffer from the drawback that
ion exchange and
salt exchange require expensive raw materials and equipment, and also require
a step of final
stripping due to the need for working with dilute solutions. The prior art has
been unsuccessful
producing high purity potassium thiosulfate via Route VI with low byproducts.
Thiosulfates, in
general, are susceptible to further conversion to sulfite and sulfate under
adverse temperature, and
pressure.
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SUMMARY OF THE INVENTION
[0011] Described herein are processes, preferably continuous, for the
preparation of
potassium thiosulfate in relatively high concentration with relatively low
amounts of soluble or
solid byproducts. The process may be used to produce a substantially clear
solution having a
relatively high percentage of potassium, and almost neutral pH, making it very
suitable as a
chlorine-free liquid fertilizer, for example as a foliar fertilizer, starter
fertilizer, in furrow fertilizer
and the like, and optionally in combination with other fertilizers.
[0012] Also described herein are processes, preferably continuous, for the
preparation of
potassium sulfite, potassium bisulfite or mixtures thereof as a substantially
clear solution having a
relatively high percentage of potassium and almost neutral pH, making it
suitable as a liquid
fertilizer, for example as a foliar fertilizer, in furrow fertilizer and the
like.
[0013] Generally, the process of the present invention provides a method
for preparing
potassium thiosulfate, comprising the following steps:
Step (1a): providing a potassium hydroxide or potassium carbonate
solution for
neutralizing acid forming components;
Step (lb): providing a SO2 contacting solution, comprising or containing
at least
some potassium sulfite or potassium bisulfite or potassium thiosulfate;
Step (2): providing sulfur dioxide gas;
Step (3): reacting these to absorb the SO2 gas and to form a reaction
mixture
comprising potassium sulfite, potassium bisulfite or mixtures thereof, which
may be sulfite rich
potassium thiosulfate;
Step (4): adding sulfur or sulfide containing compound containing sulfur
having
the oxidation state of 0, -2 or of between 0 and -2 to the reaction mixture
and optionally
potassium hydroxide or potassium carbonate, and reacting the mixture under
suitable conditions
to form potassium thiosulfate; and
Step (5): recovering the potassium thiosulfate and, optionally
concentrating the
potassium thiosulfate.
[0014] Furthermore, the present invention provides a process for preparing
potassium
sulfite, potassium bisulfite or a mixture thereof, comprising the following
steps:
Step (I a): providing a potassium hydroxide or potassium carbonate
solution for
neutralizing acid forming components;
Step (lb): providing a SO2 contacting solution, comprising at least
potassium
sulfite;
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Step (2): providing sulfur dioxide gas; and
Step (3): reacting
these to form a reaction mixture comprising potassium sulfite, or
potassium bisulfite or a mixture thereof, and recovering the potassium
sulfite, or potassium
bisulfite or a mixture thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIGURE 1 is
a schematic representation of a process according to an
embodiment of the invention.
[0016] FIGURE 2 is
a schematic representation of a process according to yet another
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] It is an
object of the present invention to provide a method for production of
potassium thiosulfate by reaction of an intermediate with a sulfite/bisulfite
composition wherein
relatively inexpensive raw materials, such as sulfur, water, sulfur dioxide
and hydrogen sulfide
gas are used, and wherein relatively high purity potassium thiosulfate can be
obtained. Potassium
hydroxide, potassium carbonate, potassium polysulfide, potassium sulfide or
potassium bisulfide
are other raw materials that may be used in this approach.
[0018] It is
another objective of the present invention to produce a concentrated
potassium thiosulfate solution.
[0019] It is still
another objective of this invention to produce potassium thiosulfate with
relatively low residual contamination from byproducts, in particular soluble
or solid byproducts.
[0020] It is still
another objective of the present invention to produce potassium
thiosulfate by a continuous operation approach.
[0021] It is still
another objective of the present invention to produce potassium
thiosulfate by using waste gas, such as hydrogen sulfide, and converting the
waste gas to sulfur or
sulfur dioxide or for direct absorption for this operation.
[0022] It is still
another objective of the present invention to use sulfur dioxide as
pristine gas by burning sulfur with oxygen or by using sulfur dioxide from
waste tail gas
operations or coke operations or flue gas containing sulfur dioxide.
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[0023] It is still another objective of the present invention to provide a
method which
allows production of a stable potassium thiosulfate product of close to
neutral pH, and with a
shelf life sufficient for commercial use.
[0024] One or more of the objectives as described above are obtained with
the processes
as described below. Also, one or more of said objectives are obtained with the
apparatus as
described below.
[0025] One or more of the objectives as described above are obtained with
the process
according to the present invention, describing (preferably continuous)
processes for the
preparation of potassium thiosulfate in relatively high concentration with
relatively low amount
of soluble contaminants such as sulfite, and sulfate.
[0026] In a preferred embodiment, controlling process parameters such as
mole ratio of
the raw materials, pH, temperature, concentration and composition of the
intermediate potassium
sulfite/bisulfite can result in a preferred clear solution with a high
percentage of potassium in
liquid form. The liquid may have an almost neutral pH, which makes this
suitable as a liquid
fertilizer, such as a foliar fertilizer. The potassium thiosulfate may be used
as such, or in
admixture with other compatible fertilizers, micronutrients, additives, and/or
the like.
[0027] One or more of the objectives as described above are obtained with
the processes
according to the present invention describing preparation of potassium
thiosulfate from an
intermediate rich in sulfite and preferably under appropriate conditions, and
using preferred mole
ratios of raw materials, producing a liquid solution of potassium thiosulfate
in relatively high
concentration with relatively low amounts of solid or soluble byproducts.
[0028] Generally, the process for preparing potassium thiosulfate of the
present
invention comprises the following steps:
Step (la): providing a potassium hydroxide or potassium carbonate
solution for
neutralizing acid forming components;
Step (lb): providing a SO2 contacting solution, comprising or containing
at least
some potassium sulfite or potassium thiosulfate;
Step (2): providing sulfur dioxide gas;
Step (3): reacting these (i.e., the potassium hydroxide or potassium
carbonate, the
SO2 contacting solution and the SO2 gas) to form a reaction mixture comprised
of potassium
sulfite, or potassium bisulfite or a mixture thereof (The reaction mixture may
be sulfite rich
potassium thiosulfate);
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Step (4): adding (i) sulfur and/or (ii) hydrogen sulfide and/or (iii)
potassium
polysulfide and/or (iv) potassium sulfide and/or (v) potassium bisulfide to
the reaction mixture,
and reacting the mixture under suitable conditions to form potassium
thiosulfate; and
Step (5): recovering the potassium thiosulfate and optionally
concentrating the
potassium thiosulfate.
[0029] Recovering the potassium thiosulfate means separating the potassium
thiosulfate
from the process to obtain a product that can be stored, transported and sold.
[0030] The process for preparing potassium thiosulfate provides a
preferably continuous
non-regenerative sulfur dioxide scrubbing system and process for the
production of potassium
products including potassium thiosulfate.
[0031] The above described process steps 1-3 can also be used to provide
potassium
sulfite, or potassium bisulfite or a mixture thereof. Recovering said
potassium sulfite, or
potassium bisulfite or a mixture thereof means separating said product from
the process to obtain
a product that can be stored, transported and sold.
[0032] The SO2 provided in the process may originate from any source, which
includes
for example: burning sulfur with oxygen; sulfur dioxide from tail gas, coke
operations, or by
converting hydrogen sulfide to sulfur dioxide, or from flue gas of other
processes containing SO2.
[0033] In addition to producing potassium products, the SO2 gas scrubbing
and
absorption system may be utilized as a Tail Gas Treating Unit (TGTU) or as a
Flue Gas
Desulfurization (FGD) system for emissions control by treating and cleaning
gas streams
containing acidic sulfur components and other acidic gas components before
being discharged to
atmosphere.
[0034] In one embodiment, the present invention utilized as a Flue Gas
Desulfurization
(FGD) and/or Tail Gas Treating Unit (TGTU) relates to a continuous process and
recovery
system for the recovery of sulfur dioxide (SO2) contained in combustion gas
streams for the
purpose of limiting SO2 gas emissions and for the purpose of producing
products containing
sulfur and potassium.
[0035] The SO2 which is reacted is not regenerated as SO2, but is recovered
as potassium
sulfite/bisulfite mixture, and/or potassium thiosulfate, solution products
containing sulfur and
potassium; these products can be used as agricultural fertilizers, or can be
used for other
purposes.
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[0036] The process may include control of SO2 gas generation and
production to control
production rate of products to meet market demand and to control 02, SO3 and
NOx
concentrations in SO2 gas for low sulfate generation and low vent stack gas
emissions.
[0037] The process may include additional process steps for removal of
excess water in
a SO2 gas Quench Tower and/or in an evaporator/concentrator, preferably at the
end of the
process, such as for example an evaporator/concentrator to concentrate the
potassium thiosulfate
if necessary.
[0038] Small amounts of sulfate as SO3 generated in combustion processes
producing
SO2 gas may be partially removed along with excess water in the SO2 gas Quench
Tower as dilute
sulfuric acid; the sulfuric acid may then be utilized in other process
equipment such as pH control
of cooling tower or may be neutralized and discharged as waste water.
[0039] Further sulfate removal from potassium sulfite, or potassium
bisulfite or a
mixture thereof and/or potassium thiosulfate products, if necessary, may be
done in the product
filtration step of the process, to remove solid crystals of potassium sulfate
which have low
solution solubility in the concentrated potassium sulfite, or potassium
bisulfite or a mixture
thereof solution and/or potassium thiosulfate product solution.
[0040] With proper design and control of the combustion system and the SO2
recovery
system, it is possible to limit oxidation of SO2 and sulfite in the SO2
recovered products resulting
in products low in sulfate. The SO2 gas stream contaminants which may be
either wholly or
partially eliminated or removed include but are not limited to: 02, SO3, NOx,
HC1, 1120 and fly
ash.
[0041] Potassium hydroxide or another base-potassium source like potassium
carbonate
is used (step la) as the alkaline feed for process reactions, pH control and
the source of potassium
for the process and potassium products produced. Potassium hydroxide is the
preferred alkaline
feed and source of potassium for the process.
[0042] The potassium base is used for neutralizing acid forming components
such as
SO2 or H2S.
[0043] The potassium base is preferably used as a solution, such as for
example 0.1
Molar to 13.5 Molar solution. Preferably, the solution has a concentration of
about 5 to 13.5
Molar. Optionally, a base-potassium source solution is formed by combining dry
or solid
potassium hydroxide or potassium carbonate with water. The dry or solid
potassium hydroxide or
potassium carbonate may be in any suitable form, such as beads, flakes or
pellets.
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[0044] Either a solution of potassium sulfite, or potassium bisulfite or a
mixture thereof
or a potassium thiosulfate solution (or a mixture comprising potassium
thiosulfate with potassium
sulfite and/or potassium bisulfite) may be utilized as the primary SO2
scrubbing solution (step lb)
in a SO2 absorber to scrub and absorb SO2 from a gas stream containing SO2 to
produce
scrubbing solution containing potassium sulfite/bisulfite.
[0045] The primary SO2 scrubbing solution is typically a concentrated
recirculating
solution of potassium sulfite, or potassium bisulfite or a mixture thereof
formed from the addition
of SO2, potassium hydroxide and dilute potassium sulfite, or potassium
bisulfite or a mixture
thereof or water from other process streams.
[0046] Alternatively, concentrated potassium thiosulfate from a downstream
reactor may
be recycled to the absorption tower if the hot SO2 gas is used to evaporate
excess water, or in the
case there is excess alkalinity in the potassium thiosulfate in the downstream
reactor which can be
carried forward to the absorption tower, or in the case where reduced
oxidation to sulfate is
desired.
[0047] A secondary SO2 scrubbing step in 2nd Stage SO2 Scrubber may be
utilized for
additional SO2 absorption and recovery. Optionally, a final gas treating step
using fiber bed gas
filters may be utilized to filter out sulfur containing particulates, for
additional control of vent
stack gas emissions.
[0048] In one embodiment, where dilute solution of potassium sulfite, or
potassium
bisulfite or a mixture thereof from other places in the process is added to
the primary SO2
scrubbing solution, a concentrated solution of potassium sulfite, or potassium
bisulfite or a
mixture thereof is obtained. This concentrated solution of potassium sulfite,
or potassium bisulfite
or a mixture thereof may be marketed as the potassium product or may be
further processed to
produce the potassium thiosulfate product.
[0049] The total potassium salt concentration of this concentrated solution
of potassium
sulfite, or potassium bisulfite or a mixture thereof is preferably about 35-41
wt% potassium
sulfite, potassium bisulfite and potassium sulfate, even more preferably about
38-40 wt% total
salt solution concentration. The pH preferably is between about 7 to about
8.5.
[0050] In another embodiment, a potassium thiosulfate solution may be
utilized as the
primary SO2 scrubbing solution to scrub and absorb SO2, thereby producing a
sulfite rich
potassium thiosulfate solution that may be further processed to produce
potassium thiosulfate
product solution that is low in sulfite and sulfate content. This sulfite rich
potassium thiosulfate
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solution contains potassium sulfite, potassium bisulfite and potassium
thiosulfate; the relative
amount of potassium sulfite and potassium bisulfite being dependent on the pH.
[0051] The process may include process steps for reacting intermediary
products of
potassium sulfite, or potassium bisulfite or a mixture thereof or sulfite rich
potassium thiosulfate
in a sulfur reactor, H2S gas contactor/reactor, or potassium polysulfide,
sulfide, or hydrosulfide
reactor to produce potassium thiosulfate that is low in sulfite.
[0052] In one embodiment (embodiment i), sulfur may be added to the process
in a
sulfur reactor for the purpose of converting the SO2 recovery product (the
potassium sulfite, or
potassium bisulfite or a mixture thereof product obtained after step 3) to
potassium thiosulfate
product.
[0053] In another embodiment (embodiment ii), sulfur may also be added as
H2S from
H2S acid gas streams and H2S from hydrocarbon gas streams typically generated
in oil refineries
and gas plants. The H2S component in these gas streams may be contacted,
absorbed and reacted
to also produce the potassium thiosulfate product. When contacting, absorbing
and reacting H2S
contained in hydrocarbon gas streams, the present invention may be utilized as
a replacement for
amine units typically used for H2S removal and recovery.
[0054] In a further embodiment, sulfur is provided as a sulfide, as in
embodiment (iii) a
potassium polysulfide; embodiment (iv) potassium sulfide; or embodiment (v)
potassium
bisulfide, wherein any of these sulfur compounds is contacted with the
solution of potassium
sulfite, or potassium bisulfite or a mixture thereof or a potassium
thiosulfate solution containing
sulfite, for reaction of the polysulfide, sulfide or bisulfide with sulfite to
form additional
thiosulfate.
[0055] Generally, the conversion of sulfite/bisulfite intermediate (i.e.,
the potassium
sulfite, or potassium bisulfite or a mixture thereof) to produce the potassium
thiosulfate product
takes place at near atmospheric pressure in a sulfur reactor or in an H2S
contactor at H2S
containing gas pressure, or in a liquid-liquid reactor, as the sulfide
generally is liquid.
[0056] Additional sulfur may also be added to a sulfur/sulfide burner, to
increase SO2
production.
[0057] The potassium thiosulfate product is particularly suitable as a
chlorine-free
potassium plant nutrient with a very high potassium concentration in liquid
form which can be
applied through all types of irrigation systems with no plugging of the drip
lines. The liquid could
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also be converted to form crystalline solid potassium thiosulfate. However,
the potassium
thiosulfate preferably is used in liquid form.
[0058] The potassium product obtained with the process as described above
preferably is
a fertilizer product, and more preferably a liquid fertilizer product.
[0059] The present invention provides a process for preparing a highly
concentrated
solution of potassium thiosulfate (K2S203), of which the amount generally is
about 40-56 wt%,
preferably about 48-56 wt% and most preferably about 50-56 wt%.
[0060] The thiosulfate can be of relatively high purity, and the amount of
products other
than potassium thiosulfate and water is about 5 wt% or less, preferably about
2 wt% or less.
Particular impurities are preferably as follows: generally, the amount of
sulfate is about 1 wt% or
less (measured as potassium sulfate), preferably, about 0.8 wt% or less, and
even more preferable
about 0.4 wt% or less. Generally, sulfite is present in an amount of about 1
wt% or less,
preferably about 0.7 wt% or less (measured as potassium sulfite). Generally,
sodium is present in
an amount of about 1 wt% or less.
[0061] Preferably, the process is performed in such a way that the
described products of
such good quality can be obtained.
[0062] The relatively low amount of other products allows the potassium
thiosulfate to
have a good storage stability. The preferred storage stability is at least
half a year or more,
preferably 1 year or more, and even more preferably 2 years or more at ambient
temperature.
Storage stable means that the liquid remains clear, i.e. no crystals or
deposition is visible with the
naked eye.
[0063] Preferably, the intermediate, sulfite/bisulfite mixture is formed at
such a pH that
the composition of intermediates favors a high amount of sulfite presence. In
one embodiment,
the intermediate potassium sulfite/bisulfite is provided at a high pH, which
maximizes SO2
absorption but is low enough to prevent the absorption of carbon dioxide, if
present in these SO2
waste gas streams.
[0064] The high amount of sulfur and potassium in both the intermediary
potassium
sulfite, or potassium bisulfite or a mixture thereof and in the potassium
thiosulfate product is
controlled by controlling the solution concentrations. The solution
concentrations are controlled
by controlling the water content of the product solutions. The water content
of the product
solutions is controlled by controlling one or more of three process control
variables. One is direct
control of water addition to the process. The second is removal of water from
the process by
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condensing water out of the SO2 gas stream. The amount of total sulfur,
calculated as potassium
sulfite, in the potassium sulfite, or potassium bisulfite or a mixture thereof
or sulfite rich
potassium thiosulfate solution, which is withdrawn from the scrubbing section
for reaction with
sulfur or sulfide, is generally between about 20 wt% and 41 wt%, preferably
between about 30
wt% and 41 wt%. The third process control variable to control water content is
concentration by
evaporation/boiling of water out of the products. Concentrating would
preferably be done on the
final potassium thiosulfate product but may also be done on the potassium
sulfite, or potassium
bisulfite or a mixture thereof product.
[0065] This non-regenerative SO2 absorption and recovery system is less
complex and
less costly to build and operate than other, regenerative SO2 recovery
processes or other TGTU's
such as SCOT units. This is because there is no need for processing to
regenerate SO2 or the SO2
absorption agent used to absorb SO2, or for conversion to H2S for amine
absorption, regeneration
and recycle as is done in SCOT TGTU's. Both SO2 and potassium absorption agent
are
incorporated into sulfur and potassium containing products that have market
value primarily as
agricultural fertilizers, preferably liquid fertilizer, such as preferably a
fertilizer for foliar spray.
DESCRIPTION OF FURTHER PREFERRED EMBODIMENTS OF THE INVENTION
[0066] FIGURE 1 is a schematic process scheme according to the process of
the present
invention.
[0067] FIGURE 2 is another schematic process scheme according to the
present
invention.
[0068] The schemes set forth in FIGURES 1 and 2 will be referred to in the
detailed
description below. The lines and unit numbers have the following meaning,
while same numbers
in FIGURES 1 and 2 have the same meaning:
[0069] Processing Units are denoted with the following numbers in the two
schemes:
T : Scrubber of SO2 containing gas
T2: Second scrubber of SO2 containing gas, coming from Ti
T3: packed absorber tower for potassium bisulfite reaction with
polysulfide,
sulfide, hydrosulfide, sulfur or H2S
Fl: fiber bed Gas Filter
R3: Sulfur reactor for reaction of liquid sulfur with potassium
bisulfite
/potassium thiosulfate
El, E2, E3, E4: Heat exchangers
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PI, P2, P3, P4: Pumps
[0070] The following lines numbers represent piping as follows in both
schemes:
01: SO2 containing gas
02A, 02B, 02C: potassium hydroxide or potassium carbonate solution
03: elemental sulfur feed or H2S feed gas or polysulfide or sulfide or
hydrosulfide stream
04: evaporated water stream
05: water feed
05C: process water from Fl
06: vent stack gas stream
07: CO2 vented from the reactor
08: concentrated potassium thiosulfate or potassium bisulfite
09: potassium thiosulfate solution
10: potassium bisulfite or potassium thiosulfate-S02 absorption solution
11: mixture of potassium sulfite and potassium bisulfite solution
12: potassium bisulfite or potassium thiosulfate-S02 absorption solution
13: recycle stream in H2S absorption
14: recycle stream for SO2 absorption
15: potassium bisulfite or potassium thiosulfate-S02 absorption solution
16: vent stack gas stream
100711 The preferred embodiment of the invention involves a multi-step
process for
converting sulfur dioxide into the final potassium thiosulfate product.
[0072] An optional preliminary step involves pretreating the sulfur dioxide
combustion
gas feed stream in a Quench Tower to cool the gas, condense out excess water
and remove some
of the other acidic gas components such as sulfuric and hydrochloric acid.
[0073] The first steps involve absorption and reaction of the sulfur
dioxide gas
component from the combustion gas stream (step 2) into a solution containing
potassium sulfite,
or potassium bisulfite or a mixture thereof or potassium thiosulfate (step
lb), while utilizing
potassium hydroxide or potassium carbonate as the alkaline absorption agent
(step la) to provide
a potassium sulfite, or potassium bisulfite or a mixture thereof product (step
3).
[0074] The next step (step 4) involves reaction of the sulfite (S032), that
may be present
in the form of bisulfite (HS03-), with either sulfur, hydrogen sulfide,
potassium polysulfide,
potassium sulfide or potassium hydrosulfide as an alternative source of
sulfur.
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[0075] The reaction product formed is potassium thiosulfate (K25203), as
described in
step 5. Potassium thiosulfate is a highly water soluble potassium salt
containing two sulfur atoms,
making potassium thiosulfate solution high in sulfur content. Optionally, the
potassium
thiosulfate solution can be concentrated by boiling or evaporating excess
water out of solution to
produce a concentrated potassium thiosulfate solution or dry potassium
thiosulfate product.
[0076] The SO2 feed gas formed upstream of the potassium thiosulfate
process may be
from several different sources including but not limited to (i) incineration
of Claus off-gas from
refinery and gas plant sulfur recovery units (SRUs), (ii) incineration of
sulfur bearing solids,
liquids or gasses including H2S/Acid Gas streams, (iii) SO2 streams from
regenerative SO2
recovery processes, (iv) pure or concentrated SO2 from other SO2 generation
processes and (v)
SO2 in flue gas from another combustion or incineration process. The total
available quantity of
SO2 in the SO2 feed gas sets the amount of potassium thiosulfate product that
may be produced.
[0077] In a preferred embodiment, additional sulfur bearing feeds may be
added to the
upstream combustion process for the purpose of controlling and increasing
production rate to
meet market demand for the potassium thiosulfate and or potassium sulfite, or
potassium bisulfite
or a mixture thereof products produced. Preferably, the combustion process is
carried out in
incineration and waste heat recovery process equipment that controls and
limits the amount of
excess oxygen in the SO2 gas stream (01). Limiting excess oxygen reduces
further oxidation of
SO2 and the absorbed SO2 as sulfite to sulfate.
[0078] Preferably, in a preliminary process step, SO2 is pretreated. In
such pretreatment,
the SO2 feed gas is cooled and excess water is condensed. Furthermore,
preferably, the SO2 is
scrubbed to remove acidic and or particulate components from the hot SO2
combustion gas.
Although this process step is preferred, it is not a process requirement. In
case such step is
applied, this is preferably done in a Quench Tower utilizing the dilute acid
solution formed from
the condensation of water vapor present in the SO2 feed gas along with acid
components that may
also be present.
H2O + SO3 H2SO4 (sulfuric acid) in quench acid solution
H20 + HCl(gas) HC1(aq) (hydrochloric acid) in quench acid solution
[0079] The Quench Tower typically utilizes a circulation pump to circulate
the dilute
acid solution through a heat exchanger and to the gas/liquid contact zone
inside the Quench
Tower in order to contact, scrub and cool the SO2 feed gas. The Quench Tower
may alternatively
be any kind of gas/liquid contacting process equipment including but not
limited to spray, packed
or tray tower, venture scrubber, inline spray or inline static mixer. As
explained, the process
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objective is to cool the hot SO2 combustion gas, condense excess water and to
scrub gas for acidic
gas components such as sulfuric and hydrochloric acids which would add
undesirable
components to the final potassium thiosulfate and or potassium sulfite, or
potassium bisulfite or a
mixture thereof product. In cases where there is little or no excess water
vapor in the SO2 feed gas
to be condensed, process water may be added. Process water may also be added
to dilute acid
concentration in Quench Tower to limit corrosion.
[0080] Preferably the SO2 gas is cooled to about 50 C (120 F) or less but
higher
temperatures may be acceptable depending on the desired distribution and
balance of water in the
process. The dilute acid formed may be utilized for its acid content in
another application or it
may be neutralized and disposed of as waste water. The SO2 in the SO2 feed gas
is slightly
soluble in the dilute acid formed and represents a very minor loss of SO2 to
the rest of the
process. If desired this minor amount of SO2 can be recovered from the dilute
acid using a
stripping process for SO2 recovery.
[0081] For SO2 feed gas formed from combustion of a concentrated sulfur
source such
as elemental sulfur there is very little or no water vapor formed and no water
would be condensed
in the Quench Tower. In this case water may be added if cooling and scrubbing
is desired. The
Quench Tower may also incorporate gas filters to further remove acid fume
particles. In any case,
the quench operation may be omitted in favor of downstream process
alternatives for cooling and
removal of excess water and contaminates.
[0082] The next process step recovers the SO2 component from the SO2 gas
stream (01)
to form a solution composed of a mixture of potassium sulfite and potassium
bisulfite solution
(11) referred to as potassium sulfite, or potassium bisulfite or a mixture
thereof The ratio of
sulfite to bisulfite depends on solution pH controlled by the addition of
potassium hydroxide or
potassium carbonate from stream (02A).
SO2 + KOH ¨> KHS03 (potassium bisulfite) pH about 5
SO2 + 2KOH K2S03 + H20 (potassium sulfite) pH about 10
2S02 + K2CO3 + H2O ¨> 2KHS03 + CO2T
[0083] The pH of the solution of potassium sulfite, or potassium bisulfite
or a mixture
thereof may be controlled over a broad range from about pH 5 to about pH 10
with potassium
hydroxide but has preferably a pH of between about 7-9, and more preferably, a
pH between
about 8.0-8.5 to enhance SO2 absorption efficiency, reject CO2 absorption and
increase solution
solubility. If no CO2 is present a pH of up to about 10 may be controlled for
maximum solution
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concentration allowing potassium thiosulfate production with sulfur reaction
in R-3 as
concentrated potassium thiosulfate without the use of evaporator, E-4.
[0084] If potassium carbonate is used the pH preferably is limited to less
than about 9,
more preferably less than about 8.5 to enable disengagement of CO2 from the
carbonate in
solution. There is however the ability to disengage some remaining CO2, if
any, in the third and
fourth process steps.
[0085] The SO2 gas is contacted by the solution of potassium sulfite, or
potassium
bisulfite or a mixture thereof or potassium thiosulfate solution in the SO2
Absorber (T-1) where
gas/liquid contact results in the absorption and reaction of SO2 from the SO2
gas stream. T-1
typically utilizes a circulation pump, P1 to circulate the potassium sulfite,
or potassium bisulfite
or a mixture thereof or potassium thiosulfate solution through a heat
exchanger E-1 and to the
gas/liquid contact zone in order to contact and absorb SO2 from the SO2 feed
gas stream (01).
[0086] The SO2 Absorber may alternatively be any kind of gas/liquid
contacting process
equipment including but not limited to spray, packed, tray or bubble tower,
venture scrubber,
inline spray or inline static mixer.
[0087] Both the absorption and reaction of SO2 with solution of potassium
sulfite, or
potassium bisulfite or a mixture thereof or potassium thiosulfate solution
produces heat which is
removed by the SO2 Absorber Cooler (E-1). The reaction is primarily with the
more alkaline
potassium sulfite component in the potassium sulfite, or potassium bisulfite
or a mixture thereof
or potassium thiosulfate solution to form additional potassium bisulfite at a
lower solution pH:
SO2 + K2S03 + H20 ¨> 2KHS03
[0088] The solution of potassium sulfite, or potassium bisulfite or a
mixture thereof or
potassium thiosulfate solution pH is controlled with the addition of potassium
hydroxide or
potassium carbonate from stream (2A) which adds alkalinity back into the
potassium sulfite, or
potassium bisulfite or a mixture thereof solution to form more potassium
sulfite:
KHS03 + KOH --> K2503 + H20
2KHS03 K2CO3 ¨> 2K2S03 + H20 + CO2t
Cooling may be controlled to operate T-1 at any process temperature from about
10-95 C (50-
200 F) depending on chosen process objectives.
[0089] Preferably temperatures are controlled to about 60 C (140 F) or
less to limit
water vapor in vented gas stream. Limiting water vapor in gas reduces the
steam plume
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(condensed water vapor) in the final vent gas that is typically discharged to
atmosphere through a
vent stack. If steam plume formation from waim water vapor saturated vent gas
mixing with
cooler atmospheric air is not an environmental issue then, higher T-1
operating temperatures may
be utilized in order to allow excess water to leave the process with the vent
gas. This can
eliminate the use of a Quench Tower for excess water removal and even
eliminate the need for E-
1 and E-2 cooling requirements. hi this case process temperature is controlled
by cooling from
natural evaporation of water as water vapor from the solution of potassium
sulfite, or potassium
bisulfite or a mixture thereof or potassium thiosulfate solution. This would
also help to produce a
more concentrated solution potassium sulfite, or potassium bisulfite or a
mixture thereof or
potassium thiosulfate with sulfite solution.
[0090] To the solution of potassium sulfite, or potassium bisulfite or a
mixture thereof
(or potassium thiosulfate with sulfite) in 1-1 process water from stream (05)
can be added to the
process which feeds forward to 1-2 and then to 1-1. T-1 may be any suitable
process equipment
for gas/liquid contacting but is preferably a contacting spray or packed bed
tower.
[0091] The addition of a second stage gas contactor, SO2 Scrubber (T-2) may
be
required for extended contacting of the SO2 gas stream for additional SO2
recovery and removal.
This allows for very low loss of SO2 with vent gas so very low SO2 emissions
are obtained in the
vent gas discharged to atmosphere. T-2 operates as a gas/liquid contactor in
the same manner as
T-1.
[0092] In one embodiment, the scrubbing solution is a dilute or weak
potassium sulfite,
or potassium bisulfite or a mixture thereof solution. The solution pH may be
controlled with the
addition of potassium hydroxide or carbonate from stream (02B) over a broad
range of pH from
about 5-10 but preferably about 7-9 and more preferably about 8.0-8.5 to
enhance SO2 absorption
efficiency and reject CO2 absorption. The dilute solution of potassium
sulfite, or potassium
bisulfite or a mixture thereof generated passes to and becomes part of T-1
potassium sulfite, or
potassium bisulfite or a mixture thereof solution. The produced T-1 potassium
sulfite, or
potassium bisulfite or a mixture thereof solution is considered an
intermediate product that
undergoes further processing in the third process step to become potassium
thiosulfate. However,
potassium sulfite, or potassium bisulfite or a mixture thereof may also be
taken off from the
process as a separate product.
[0093] In another embodiment, the scrubbing solution contains potassium
thiosulfate,
and optionally some potassium sulfite, or potassium bisulfite or a mixture
thereof. The pH
preferences as described in the former paragraph apply equally.
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[0094] In a further embodiment, a last process operation of the step to
recover SO2 and
clean the gas before discharge to atmosphere is to filter the gas through
fiber bed Gas Filter (F-1).
This Gas Filter has proven to be useful in addition to gas contacting in the
Quench Tower, T-1
and T-2 because of some residual submicron acid fume particles remaining in
the gas stream that
are not absorbed into solution. This dilute acid collected by the Gas Filter
may be fed forward
with process water as shown in the schemes 1 and 2, or may alternatively be
drawn off separately
for use as dilute acid or neutralized for waste water disposal.
[0095] The next process step (step 4) involves conversion of potassium
sulfite, or
potassium bisulfite or a mixture thereof (which may also be sulfite rich
potassium thiosulfate)
produced in the SO2 absorption process step, to potassium thiosulfate, by way
of reaction with
additional sulfur.
[0096] The additional sulfur may be elemental sulfur having a chemical
valence of 0.
The additional sulfur may also be hydrogen sulfide sulfur having a chemical
valence of -2 as a
gas or as a liquid in a potassium-based solution as potassium sulfide and or
potassium
hydrosulfide. The additional sulfur may also be a mixture of elemental and
hydrogen sulfide
sulfur with potassium hydroxide as potassium polysulfide, which may have an
average valence
between 0 and -2.
[0097] Additional potassium hydroxide or potassium carbonate may be added
as
required to maintain potassium thiosulfate reaction product pH and in the case
where potassium
thiosulfate reaction product has elevated pH with excess KOH from reaction
with potassium
sulfide the potassium thiosulfate can be recirculated to T-1 for reaction with
SO2 to lower pH and
consume excess KOH.
[0098] The chemical reactions to produce potassium thiosulfate may be
represented by
the chemical equations:
KHS03 + S + KOH ¨> K2S203 + H20 (reaction with sulfur)
KHS03 + K2S03 + H2S ¨> 1.5K2S203 + 1.5H20 (reaction with hydrogen sulfide)
2KHS03 + KHS ¨> 1.5 K2S203 + 1.5H20 (reaction with hydrosulfide)
KHS03 + K2S + SO2 ¨> 1.5 K2S203 + 0.5H20 (reaction with sulfide and SO2)
5KHS03 + K2S4 + 2KOH¨> 4.5K2S203 + 3.5H20 (reaction with polysulfide).
[0099] In one embodiment (embodiment i), in this process step (step 4)
potassium
sulfite, or potassium bisulfite or a mixture thereof or sulfite rich potassium
thiosulfate is reacted
with additional elemental sulfur. This step can be carried out in any process
equipment that
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promotes contacting of reaction mixture with elemental sulfur. The potassium
thiosulfate/sulfur
reactor (R-3) is preferably a Continuously Stirred Tank Reactor (CSTR).
[00100] The elemental sulfur feed, stream (03) may be either solid or
liquid/molten sulfur
but preferably molten so that the stirring action in the reactor breaks up the
molten sulfur feed as
it cools to form small solid sulfur particles. The small sulfur particles
advantageously have a large
total surface area available for reaction with the potassium thiosulfate
solution in the reactor.
[00101] The reaction mixture is primarily potassium thiosulfate solution
that contains
sulfite from the potassium sulfite, or potassium bisulfite or a mixture
thereof feed stream (11) into
the potassium thiosulfate/sulfur reactor (R-3). The reaction taking place is
between the sulfite
component supplied by the potassium sulfite, or potassium bisulfite or a
mixture thereof and
elemental sulfur in the potassium thiosulfate solution to produce additional
potassium thiosulfate.
The accumulating potassium thiosulfate is preferably taken off on a continuous
basis from the
continuous process but the reaction could be done on a batch basis if desired.
[00102] Preferably the sulfur is suspended in the reacting solution with
stirring action in
the CSTR. Alternatively the sulfur may be reacted in a fluidized bed reactor
where the sulfur is in
suspension in the reacting potassium thiosulfate solution by means of a
circulation pump or
updraft impeller or impellers. Also alternatively the sulfur may be in a
static bed where the
reacting potassium thiosulfate solution is circulated through the bed.
[00103] The reaction may be carried out at any temperature but the reaction
is slow and
faster reaction is favored at higher temperatures. Preferably the reaction
temperature is controlled
with heating coils to between about 70 and about 105 C, and more preferably
about 93-99 C
(200 to 210 F) for optimum reaction rates with solid sulfur particles and to
limit R-3 to low or
atmospheric reactor pressure. Higher reactor temperatures may also be employed
but at
temperatures above about 121 C (250 F), sulfur is in the molten/liquid state
and higher reactor
pressure and higher stirring agitation is required to keep molten sulfur
disbursed into small
droplets.
[00104] The reaction pH is controlled to neutral or slightly alkaline with
the addition of
potassium hydroxide or potassium carbonate, stream (02C). If potassium
carbonate is used or if
any residual carbonate is present from the potassium bisulfite produced, there
will be CO2 vented
from the reactor in stream (07). The pH generally is between about 5 and about
9, preferably
between about 6 and about 8.
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[00105] In another embodiment of this step 4 (embodiment ii), this process
step is done
with hydrogen sulfide gas, and is carried out in any process equipment that
promotes gas-liquid
contacting for the absorption and reaction of H2S gas with the potassium
thiosulfate plus
potassium sulfite, or potassium bisulfite or a mixture thereof reaction
mixture.
[00106] Preferably the absorption solution is primarily a potassium
thiosulfate solution or
sulfite rich potassium thiosulfate solution containing some sulfite from
potassium sulfite, or
potassium bisulfite or a mixture thereof addition, this solution preferably is
controlled to neutral
or slightly alkaline pH. Reaction temperature is controlled with process
cooling preferably above
about 50 C (120 F) to minimize condensation of hydrocarbons and preferably
below about 82
C (180 F) to limit moisture in unabsorbed T-3 vent gas. The pH generally is
between about 5
and about 9, preferably between about 6 and about 8.
[00107] Preferably the absorption and reaction of H2S gas is carried out in
a packed
absorber tower referred to as the Hydrogen Sulfide Absorber Tower (T-3) in
FIGURE 1. The H2S
feed gas stream (03) may consist of 100% H2S gas to gas streams containing
less than 1% H2S.
Preferably the H2S gas stream is a concentrated H2S gas stream containing 50%
or more H2S and
is referred to as acid gas in gas plants and refineries coming off amine units
used to remove H2S
from hydrocarbon streams. The absorption and reaction is selective for H2S and
rejects CO2 and
hydrocarbons that may be in the H2S, acid gas feed stream (03) to T-3.
[00108] This embodiment of the process, step 4, for selectively absorbing
and reaction of
H2S with potassium sulfite, or potassium bisulfite or a mixture thereof (or
sulfite rich potassium
thiosulfate) to produce potassium thiosulfate may be used in special
applications for H2S recovery
and removal from gas streams. Using potassium sulfite, or potassium bisulfite
or a mixture
thereof as a selective, non-regenerative absorption agent for H2S may be used
as a process
alternative for amine treating H2S containing streams. In many small and
remote locations not
having the required utilities and infrastructure to support a regenerative
amine treating process
operation, potassium sulfite, or potassium bisulfite or a mixture thereof may
be imported or
produced on site to be used as a non-regenerative H2S recovery reagent
producing potassium
thiosulfate. This process step may also be used in large gas plants and
refineries in place of amine
units to off load amine treating capacity and the acid gas produced by amine
treating going to
sulfur recovery units and thus increase overall amine treating and sulfur
recovery capacity.
[00109] In large sulfur recovery operations that are typically found in oil
refineries and
gas plants, this potassium thiosulfate process may be used to supplement or
substitute sulfur
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recovery operations resulting in increased total sulfur recovery capacity or
alternative sulfur
recovery operations for the facility.
[00110] Sulfur bearing H2S/Acid Gas may alternatively be diverted from the
main Claus
Sulfur Recovery units to an incinerator for combustion to form an SO2 gas
stream. This
effectively off loads the main Claus SRU for more total sulfur recovery
capacity for the facility.
Also directing H2S/Acid Gas to the T-3, H2S Absorber off loads additional H2S
gas going to the
Claus SRUs. Treating incinerated Claus off gas in the SO2 absorption step of
the process
eliminates the need for other more process demanding regenerative Claus tail
gas treating units
such as SCOT units which typically recycle H2S/Acid Gas back to the Claus SRU
which reduces
Claus SRU capacity.
[00111] In another embodiment (embodiment iii), this process step (step 4)
to react
potassium sulfite, or potassium bisulfite or a mixture thereof or sulfite rich
potassium thiosulfate,
is performed with potassium polysulfide.
[00112] In yet another embodiment (iv), potassium sulfide is used, while in
embodiment
(v), potassium bisulfide is used.
[00113] Any of these embodiments may be carried out in process equipment
that
promotes mixing of the reaction mixture. This is a liquid-liquid reaction
which takes place much
faster than with elemental sulfur which is a solid-liquid reaction. In this
liquid-liquid reaction
case, elevated reaction temperature and strong mixing is not required. The
reaction pH is
controlled to neutral or slightly alkaline with the addition of potassium
hydroxide or potassium
carbonate, stream (02C). If potassium carbonate is used or if any residual
carbonate is present
from the potassium sulfite, or potassium bisulfite or a mixture thereof
produced, there will be
CO2 vented from the reactor in stream (07). The pH generally is between about
5 and about 9,
preferably between about 6 and about R.
[00114] The final step is recovering the potassium thiosulfate product,
i.e. separating the
potassium thiosulfate from the process to obtain a product that can be stored,
transported and
sold. This step of collecting the potassium thiosulfate, may optionally
additionally comprise
concentrating the potassium thiosulfate solution. If the potassium thiosulfate
solution is
concentrated, this is done preferably by boiling or evaporating excess water
out of solution to
produce a concentrated solution or dry potassium thiosulfate product, if
required. The potassium
thiosulfate solution is typically concentrated to about a 50% or slightly
higher solution
concentration having a total sulfur concentration of about 17% S using an
evaporator or Solution
Concentrator (E-4).
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[00115] The degree of concentration required to produce potassium
thiosulfate at about
17% S concentration depends on the amount of excess water that is present in
the potassium
sulfite, or potassium bisulfite or a mixture thereof/sulfite rich potassium
thiosulfate feed obtained
in step 3, and in the sulfur feed streams added in step 4 of the process.
Elemental sulfur contains
no water and can eliminate the need for the concentration step to produce
concentrated potassium
thiosulfate solution if the potassium sulfite, or potassium bisulfite or a
mixture thereof feed to
step three is sufficiently concentrated. H2S gas or polysulfide solution feed
to the third step does
add water to the potassium thiosulfate produced and generally requires
concentration in the final
process step to remove excess water.
[00116] Although not typical, concentration of the potassium thiosulfate
solution may be
accomplished by limiting cooling and allowing the temperature in T-1 and T-2
to be sufficiently
high to allow water to exit in the vent stack gas as water vapor. This allows
for production of a
concentrated potassium sulfite, or potassium bisulfite or a mixture thereof
solution.
[00117] Additionally the potassium sulfite, or potassium bisulfite or a
mixture thereof-
SO2 absorption solution in T-1 and T-2 may become potassium thiosulfate-S02
absorption
solution if desired by allowing and controlling recycle of potassium
thiosulfate between T-1, T-2
and T-3 or R-3 through streams (11, 12 & 15). This potassium thiosulfate-S02
absorption mode
of operation transports potassium thiosulfate solution to the T-1 and T-2 SO2
gas absorption
system to allow evaporation of excess water into the vent stack gas stream
(06, 16). SO2
absorption by the potassium thiosulfate solution in T-1, T-2 produces a
sulfite rich potassium
thiosulfate solution which recycles back to T-3 or R-3 for reaction of sulfite
to produce additional
potassium thiosulfate low in sulfite.
[00118] An additional benefit of utilizing potassium thiosulfate solution
in T-1 and T-2 is
lower sulfate formation caused by partial oxidation of the sulfite component
in the potassium
thiosulfate absorption solution by residual oxygen in the SO2 combustion gas
stream (01). Lower
oxidation is realized from lower sulfite concentration in the potassium
thiosulfate solution and
possibly also from the potassium thiosulfate component in potassium
thiosulfate acting as an
oxidation inhibitor. Another benefit is the ability to redistribute alkalinity
between step two and
step three process solutions. Any excess alkalinity coming into step three may
be redistributed to
step two in stream (12) to where the bulk of alkalinity is required for SO2
absorption and then
recycled back to step three in stream (11).
[00119] Lower concentration potassium thiosulfate may hold some potassium
sulfate in
solution. However, concentrated potassium thiosulfate has a low tolerance for
potassium sulfate
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in solution. After concentrating, the potassium thiosulfate solution may be
cooled before it is sent
to storage in stream (08). On cooling, any significant amount of potassium
sulfate above its
solubility limit of about 0.9% will crystalize and drop out of solution. The
potassium sulfate
crystals, if any, may be filtered out of the concentrated potassium
thiosulfate solution and
processed as a separate product if desired.
[00120] As previously described, production volume of potassium bisulfite
and potassium
thiosulfate may be increased by diverting or adding more sulfur containing
feed streams to the
SO2 combustion gas equipment or incinerator to increase SO2 production. One of
the feed gas
streams to the incinerator is from the unabsorbed gas exit T-3, stream (07).
Excess H2S gas to 1-3
helps to produce potassium thiosulfate with low residual sulfite concentration
and the excess H2S
in stream (07) adds to SO2 combustion gas generation.
[00121] The present invention furthermore provides for the continuous
production of
potassium thiosulfate by (A) providing a sulfur dioxide containing gas stream
(01) and potassium
containing base (02) to at least one absorption tower (T-1, T-2), while
absorbing the gas into an
absorption solution containing sulfite and/or thiosulfate (10, 15), which is
circulated over the
absorption tower (T-1, 1-2), and (B) removing part of the absorption solution
for obtaining a
potassium sulfite containing solution (11) for (C) reacting the potassium
sulfite containing
solution with sulfur, hydrogen sulfide, potassium sulfide, potassium bisulfide
and/or potassium
polysulfide (03) with the addition of further potassium base if required (02)
to obtain potassium
thiosulfate in a vessel (1-3, R-3), recovering potassium thiosulfate (09), and
(D) optionally
concentrating the potassium thiosulfate in a heat exchanger (E-4) to obtain
concentrated
potassium thiosulfate (08).
[00122] The present invention furthermore provides for a continuous
production of
potassium sulfite, or potassium bisulfite or a mixture thereof by (A)
providing a sulfur dioxide
containing gas stream (01) and potassium containing base (02) to at least one
absorption tower
(T-1, T-2), while absorbing the gas into an absorption solution containing
sulfite (10, 15), which
absorption solution is circulated over the absorption tower (T-1, T-2), and
(B) removing part of
the absorption solution for obtaining a potassium sulfite / potassium
bisulfite containing solution
(11) and (D) optionally concentrating the potassium sulfite / potassium
bisulfite in a heat
exchanger (E-4) to obtain concentrated potassium sulfite / bisulfite (08).
[00123] The embodiments and preferences described for the process, and as
exemplified
by the description of the drawings apply to the continuous process description
provided in the
former two paragraphs.
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