Note: Descriptions are shown in the official language in which they were submitted.
2~
Where a gas mixture includes both sulfur dioxide
and lower valence nitrogen oxides, and removal of both is
desirable, these new processes for removing and recovering
sulfur dioxide and lower valence nitrogen oxides may be
combined into a single step, or may be effected sequentially,
with the latter offering certain processing advantages over
the combined process.
The conversion of lower valence nitrogen oxies to
higher valence nitrogen oxide~ to facilitate their removal
from a gas mixture is, broadly, not an entirely new concept.
Thus, for example, U.S. Patents 1,420,477; 3,733,3~3; and
3,~27,177 propose oxidizing oxides of nitrogen to remove them
more easily from gaseous stream. U.S. Patent 3,873,672
employs a related method for removing sulfur dioxide from gas
mixtures containing it. All of these primarily seek simply
removal of the pollutants and are costly to operate. None
discloses removing pollutant nitrogen oxides or sulur dioxide
or both from combustion gases and recovering them as useful
products in a practical and economic manner. Combustion gases
from the burning of carbonaceous fuels also include substantial
quantities of carbon dioxide. These quantities may be in the
range of about 5% to ahout 20~ by volume. Although emission
of carbon dioxide to the atmosphere is not presently under
severe attack as an environmental probl~m, capture and re-
covery of carbon dioxide may be highly desirable to provide
raw materials for other commercially valuable products. The
recovery techniques that form important parts of the processes
of this invention permit recovery of large volumes of carbon
dioxide in high purity at low cost, thus contributing to the
overall efficiency and economy of these processes.
This invention provides processes for treating gas
2 -
;
~296~ ~
mixtures including carbon dioxide and at least one lower
valence nitrogen oxide for the removal of these lower valence
nitrogen oxides comprising treating a gas mixture including
these gases with aqueous media including alkali metal carbo-
nate, alkali metal bicarbonate and an oxidant to convert the
lower valence nitrogen oxides to higher valence nitrogen
oxides in which alkali metal carbonate and alkali metal bi-
carbonate are present in stoichiometric excess of the amount
required to form alkali metal salts of the higher valence
nitrogen oxicles, then recovering a product aqueous media from
the treating that includes these alkali metal salts. The
recovered product aqeuous media is itself a valuable product
that may, for example, be used as a liquid fertilizer. Pre-
ferably, however, the process also comprises recovering the
alkali metal salts from the product aqueous media, and re-
cycling the product aqueous media to the treating process.
The gas treating is preferably effected at temperatures in
the range of about 50C. to about 60C. where the gas mixture
is combustion fuel gas. The oxidant effects conversion from
lower valence to higher valence nitrogen. The carbonate/
bicarbonate system immediately forms nitrite and nitrate salts
of the higher ~alence nitrogen oxides thus removing them from
the gas mixture and facilitating their recovery.
The most common oxide of ni~rogen in combustion gases
is the lower valence nitric oxide (NO), which generally
exceeds the concentration of nitrogen dioxide by a factor o F
about 10. In these combustion gas mixtures, lower valence
oxides of nitrogen may be present in concentrations in the
range of about 200 to about 20,000 parts per million. But
since these mixtures are evolved at such rapid rates, for
example, from about 1,000 cubic feet per minute to about
1,000,000 cubic feet per minute from a 500 megawatt power
plant, the quantity emitted to the atmosphere is very large
and creates a serious air pollution problem. The processes
of this invention are especially applicable in minimizing
these emissions, or at least lowering them to, say, about 50
parts per million, without seriously impeding the high flow
rate of the gas stream from the power plant to the atmosphere.
These processes employ an acceptor system that ca~tures and
holds the higher valence nitrogen oxides formed in this
process in solution in non-volatile form, and permits the
concentration of these salts ~o rise to a level where recovery
is commercially practicable. The acceptor system, including
alkali metal carbonate and alkali metal bicarbonate, permits
recycling of the product aqueous media while retaini.ng the
higher valence nitrogen oxides in solution as nitrite and
nitrate salts of alkali metal. And the carbonate/bicarbonate
acceptor system, in addition, promotes highly selective re-
covery of alkali metal nitrates and nitrites in commercial
quantities and crystallization at higher temperatures than
would be expected, thus increasing the yield and minimizing
the energy needed to evaporate and cool the product aqueous
media to effect crystallization of the nitrate and nitrite
` products.
Combustion gas mixtures amenable to the new processes
also include carbon dioxide and may include some carbon mono-
xide. The carbon dioxide concentration in combustion and
similar gases is normally in the range of about 5~ to about
20%, more commonly about 10% to about 16~ by volume. During
the treating of gas mixtures in accordance with this in-
vention, the carbon dioxide in the gas mixtures is drawn intosolution and forms alkali metal carbonates and bicarbonates
-
2~31
by reaction with an alkali metal salt such as the hydroxide.
The presence of these alkali metal carbonates and
bicarbonates in the aqueous media and their inter-conversion
plays an important part in a number of different ways in the
practice of this invention, completely aside from their role
in removing oxides of nitrogen and sulfur from the gas stream.
For example, the product aqueous media may be treated to
recover some or all of this carbon dioxide absorb~d. More-
over, the presence of the carbonates in the product aqueous
media facilitates the crystallization and recovery of such
alkali metal salts as potassium nitrate and potassium nitrite
by lowering their solubilities to a major degree. In the
produc~ aqueous media, conversion of carbonate to bicarbonate,
as by the addition o carbon dioxidle, lowers the pH of the
product aqueous media. In turn, that facilitates and
expedites the oxidation o~ nitrite to nitrate by the oxidant
present in that media. Conversely, conversion of bicarbonate
to carbonate by heating permits not only the recovery of
relatively pure carbo~ dioxide in large quantitie~, but also
facilitates the recovery of the product potassium nitrate by
crystallization in higher purity and yield.
The alkali metal carbonate and bicarbonate used in
the aqueous treating media may include any of the alkali
metals, but potassium i5 pre~erred. Potassium carbonate t in
particular, is very soluble in aqueous media, and has the
surprising effect of reducing the solubility and raising sub-
stantially the temperature at which the nitrate and the
nitrite may be crystallized. This means less energy is needed
for concentration and refrigeration to cool the product
aqueous media to crystallize the nitrate, and a higher
potassium nitrate selectivity and yield during crystallization.
9Ei~
Potassium nitrate is a valuable commercial product and is
especially valuable as a fertilizer, both for its fixed
nitrogen and its potassium.
The alkali metal is preferably introduced to the
treating step of the processes of this invention as alkali
metal hydroxide, although it can be supplied as alkali metal
carbonate. Preferably, the alkali metal hydroxide is electro-
lytically derived from alkali metal halide. Thus, for
example, electrolysis of potassium chloride produces potassium
hydroxide in aqueous media, as well as hydrogen and chlorine
gases. The hydrogen gas may be used to make an oxidant such
as hydrogen peroxide, and ~he chlorine gas to make an oxidant
such as hypochlorite. Alternatively, chlorine it~elf may be
used as the oxidant.
The alkali metal carbonate and bicarbonate used in
the treating step of the processes of this invention forms
during the treating process by reaction with carbon dioxide
from the gas mixture with the alkali metal hydroxide fed
` thereto.
Oxidation of lower valence nitrogen oxides with
preferred oxidants produces a product aqueous media including
nitrites, nitrates, carbonate and bicarbonates of alkali
metal. Because nitrates are more aasily removed from such
media than are nitrites, the nitrites are desirably oxidized
to nitrates. At the carbon dioxide content normally prevail-
ing in combustion gases, e.g., around 14%, both carbonate and
bicarbonate are present in the product aqueous media and the
pH is likely to be 9 or greater. Surprisingly, lowering the
pH to less than about 9 facilitates oxidation of nitrites
to nitrates in the product aqueous media. This lowering of
the pH may be effected by adding sufficien~ acid to nPutralize
the carbonate and convert it to bicarbonate, or, preferably,
by adding carbon dioxide to the media.
In these processes, this lowering of pH and con-
version of nitrite to nitrate is best effected as the first
step in the product recovery so as to utilize any unused
oxidant in the product aqueous media remaining from the gas
treating and to raise the nitrate concentration prior to its
recovery by crystallization. However, this conversion step
may be postponed in the recycling of the product aqueous
ld media and may even ollow the recovery of nitrate, pro~ided
sufficient oxidant is present or is added to the recycling
media to efect the oxidation.
Another important step in recovering products from
the product aqueous media is decarbonation, where the bi-
carbonate formed in the treating step of the processes is
converted to carbonate and carbon clioxide by heating the
pxoduct a~ueous media and driving off the carbon dioxide.
This heating may be effected by steam stripping or by
evaporation. Generally, ~ome evaporation of water is re-
quired to facilitate the crystallization of the product.
The carbon dioxide so made is of high purity and may be
captured for use in other processes, used for lowering the
pH of the product aqueous media to facilitate oxidation of
nitrite to nitrate, or both. Although decarbonation pre-
ferably follows conversion of nitxite to nitrate, decarbona-
tion may follow immediately after the gas treating step of
the process. Following the conversion of the bicarbonate
to carbonate, the product aqueous media is cooled to recover
alkali metal nitrates, preferably by crystallization.
Since the carbonate/bicarbonate containiny product
aqueous media employed in the gas sontacting holds alkali
.,~, , ' .
3L~l2~316~
metal nitratas and nitrites s~rongly in solution, the product
aqueous media may be recycled through gas treating st~ps many
times to raise the alkali metal nitrate concentration suffi-
ciently high to make product recovery practicable and economic.
Thus, when ~he nitra~e concentration rises into the range of
about 2~ to about 45% based on the weight of the product
aqueous media, recovery may then be effected efficiently.
Surprisingly, the presence of potassium carbonate in the
product aqueous media has a pronounced effect in reducing
the solubility of the potassium nitrate and ~hus makes its
recovery much easier, and more economical. Not only is the
temperature at which crystallization takes place significantly
higher than where the potassium carbonate is not present but
the yield per pass is also improved.
The oxidants which may be used in these processes
are among those known to oxidize lower valence nitrogen
oxides, including chlorine, chlorine dioxide, hypochlorite,
ozone, and hydrogen peroxide. Of t:hese, hydrogen peroxide
is especially preferred bPcause product recovery from the
product aqueous media is cleanest and simplest and the
hydrogen peroxide may be made at low cost. ~ut halogen-
containing oxidants such as chlorine or hypochlorite are
more active and oxidize lower valence rlitrogen oxides faster
than hydrogen peroxide, and may be preerable where this
benefit outweighs the added difficulties that chlorides
present in separating and recovering products from the
product aqueous media.
Initial oxidation in the vapor phase followed by
liquid phase oxidation proceeds faster than oxidation in the
liquid phase only. Thus, oxidation may be initiated, and
wholly or partly completed, before treatment with oxidant
'~ /~. .
~z~
aqueous media begins. Oxidation in the vapor phase is favored
by having the oxidant in gaseous form where it can be com-
mingled with the gas mixture and directly contact the nitric
oxide or nitrogen dioxide. Of the oxidants mentioned above,
ozone, chlorin~ and chlorine dioxide are gases at ambient
temperatures. Ozone and chlorine dioxide, though effective,
are high in cost. Hydrogen peroxide, though a relatively
high boiling material, can be flash vaporized into the gas
stream to also serve as a vapor phase oxidank. PrPferably,
this vapor phase oxidation is effected only for the purpose
of oxidizing nitric oxide (NO) to nitrogen dioxide (NO2),
with the remaining oxidation being carried out in the aqueous
treating media. Where the absorption rate from the gas
stream is low, the gas-liquid contacting equipment becomes
larger and more contacting stages become necessary to achieve
high removal efficiencies. Because of the huge volumes of
off-gases that must be treated at power plants and the like,
the capital investment can become sizeable. For this reason,
addition of initial gas phase oxidation to the process may
be desirable.
Where hydrogen peroxide is the oxidant, the peroxide
is preferably produced at the site of the treating process
by one of the known processes for its production. Incor-
porating the peroxide supply process directly into the
treating process effects considerable economy because most
peroxide producing processes require expensive puri~ication
steps, such as extraction and distillation, and movement of
large quantities of water. For example, where hydrogen
peroxide is made by the air oxidation of 2-alkylanthraquinone,
hydrogen peroxide is produced in an anthraquinone-containing
organic solvent phase, which is then extracted with water.
6~1~
The water layer ~ontaining hydrogen peroxide is then removed
and concentrated by distillation. The large amounts of
water that must be removed and the considerable decomposition
and loss of peroxide that takes place during this recovery
is costly. These steps may be eliminated by extracting the
hydrogen peroxide directly into the recycling aqueous media
used in this process, thus avoiding the investment in
recovery equipment and the costly recovery concentration
steps. Moreover, the hydrogen gas produced in the electro-
lytic formation of potassium hydroxide or other alkali met~l
hydroxide may be used to advantage in the anthraquinone re-
duction step of this process, thereby lowering substantially
the cost of producing this oxidant.
Where chlorine or hypochlorite is the oxidant,
chlorine produced in the electroly~;ic formation of alkali
metal hydroxide from an alkali metal chloride may be
utilized either directly, where chLorine is the oxidant,
or indirectly, where hypochlorite .is the oxidant. Thus,
chlorine may be converted to hypochlorite by the reaction
`~ 20 of alkali metal hydroxide with chlorine in water, and the
resulting aqueous hypochlorite may then be fed to the treat-
ing process of the invention. The reaction of chlorine,
hypochlorite or chlorine dioxide with gas mixtures including
lower valence nitrogen oxides and carkon dioxide resul~s in
rapid oxidation of nitric oxide to nitrogen dioxide, which
then forms nitrite and nitrate in the alkali metal carbonate/
bicarbonate aqueous treating media. Unreacted chlorine or
nitrogen dioxide carried over from the primary gas scrubbing
may be removed in a cleanup secondary treatment of the gas
mixture by scrubbing with an aqueous carbonate/bicarbonate
solution. This precludes emission of chlorine to the
-- 10 --
~2~336;~
atmosphere with the gas mixtures.
The use of chlorine, chlorine dioxide or hypo-
chlorite as the oxidant makes product separation and recovery
more difficult than where hydrogen peroxide is the oxidant.
However, alkali metal chlorides and alkali metal nitrates
in the product aqueous media may be separated from one
another in this process by a combination of steps such as
decarbonation and crystallization of potassium chloride
followed by isolation and lixiviation of the remaining
alkali metal chloride/alkali metal nitrate product. Some
o~ the alkali metal chloride so recovered may be recycled
to electrolysis for formation of additional oxidant and
alkali metal hydroxide. Alternatively, decarbonation and
evaporation of the product aqueous media may be followed
simply by lixiviation of the solids resulting from the
decarbonation/evaporation step.
The processes of this invention also permit the
recovery of sulfur dioxide from gas mixtures regardless of
wh~ther nitrogen oxides are present or not and regardless
of whether removal of those nitrogen oxides is necessary or
desira~le. Sulfur dioxide removal may be effected in a
number of ways. First, SO2 may be removed from gas mixtures
by treating them with an aqueous media including alkali
metal carbonate and bicarbonate and oxidant. The ratio of
alkali metal to sulfur dioxide (potassium: sulfur dioxide)
muæt be at least about 2. Where lower valence nitrogen
oxides are present, the process may be conducted so as to
oxidize sulfur dioxide to sulfate with the corresponding
alkali metal salts forming in the product aqueous media
together with alkali metal nitrites and nitrates. Thereafter,
product separation and recovery may proceed as described
~ 962~3
ahove. Alternatively, the gas mixtures may be treated
countercurrently with a controlled amount of oxidant and
excess alkali metal carbonate/bicarbonate in the aqueous
media so that the sulfur dioxide forms alkali metal sulfite
salt in the first stage of the gas-liquid contacting. The
oxidant oxidizes only the lower valence nitrogen oxides
passing to the later stages, and thus does not oxidize the
sulfite to sulfate. Again, the ratio of alkali metal to
sulfur dioxide must be at least about 2. Here, however,
the sulfur values may be recovered as an alkali metal sulfite
salt such as sodium sulfite, or the sulfite may be oxidized,
preferably with oxygen to make alkali metal sulfate. As
before, the higher valence nitrogen oxides are trapped in
; the solution as alkali metal nitrites and nitrates.
Rather than removing sulfur dioxide concomitantly
with lower valence nitrogen oxides, however, the processes
of this invention preferably remove sulfur dioxide separately,
and before the lower valence nitrogen oxides are removed.
To do this, the gas mixture, including carbon dioxide and
sulfur dioxide, with or without lower valence nitrogen
oxides, is treated with aqueous media comprising the carbonate
and bicarbonate of an alkali metal where the ratio of the
alkali metal to sulfur dioxide is at least about 2. A
product aqueous media is recovered that includes alkali
metal carbonate, alkali metal bicarbonate and alkali metal
sulfite formed from the sulfur dioxide. Sulfur dioxide
removal efficiency of this process is very high, about 98
or 99~, and the process is effective whether or not nitrogen
oxides are present and whether or not they are necessarily
or desirably removed.
- 12
~2~6Z~
By contrast, the widely adopted Wellman-Lord and
lime-limestone processes for removal of sulfur dioxide from
combustion gases are only able to remove about 90~ of the
sulfur dioxide present from ~hese mixtures. As with the
processes of this invention for removing lower valence
nitrogen oxides from gas mixtures, this process or the
removal of sulfur dioxide from gas mixtures is particularly
useful where the gas stream contains low concentrations of
sulfur dioxide and where the gas stream is in large volume
and must flow at a high velocity, such as where the gas
mixture is a combustion gas from a power plant or other
high fuel-consuming plant.
Although this new sulfur dioxide removal process
is superior to the Wellman-Lord process, the Wellman-Lord
process concept may alternatively be combined with the
processes of this invention for removing lower valence
nitrogen oxides from gas mixtures including both sulfur
dioxide and lower valence nitrogen oxides where there is a
direct need for sulfur dioxide as a product in preference
to alkali metal sulfate or sulfite. Moreover, ~he Wellman-
Lord proce~s and others capturing sulfur dioxide as bisulfite
may be combined with the sulfur recovery methods of this
invention. Thus, where the product aqueou~ media from gas
scrubbing includes such bisulfite, the alkali metal carbonate
may be added to the media in an amount sufficient to convert
the bisulfite to sulfite. Air oxidation of sulfite to
sulfate, and recovery of sulfate by crystallization may
then follow.
As in the process for removing and capturing lower
valence nitrogen oxides from gas mixtures, the preferred
alkali metal is potassium and the preferred source for that
13 -
~ ~ f.3~ r~ ~
potassium is electrolytically derived potassium hydroxide
made, for example, from the electrolysis of potassium
chloride. However, potassium carbonate is also acceptable
as a source of potassium.
The sulfur dioxide removal process of this invention
not only achieves very high removals of sulfur dioxide, but
also can recover large quantities of carbon dioxide at low
cost. The employment of an excess of alkali metal beyond
that needed for reaction with the sulfux dioxide to produce
; lO alkali metal sulfite not only ensures a high sulfur dioxide
removal efficiency, but, depending upon the extent of the
excess and the degree of recycling, may also absorb large
amounts of carbon dioxide from the gas mixture which can
then be recovered in the form of highly pure carbon dioxide
upon decarbonation of the alkali metal bicarbonate. The
concentration of alkali metal sulfite may be xaised to a
high level by recycling in a manner similar to that employed
with the nitrogen oxides removal. The sulfur values in the
product aqueous media are preferably recovered as alkali
matal sulfate, although alkali metal sulfite may be
crystallized and recovered as such and may be preferable
where the alkali metal is sodium. The sulfate is preferably
formed by oxidation with a low-cost oxidant such as oxygen,
or an oxygen-containing gas such as air. The by~products of
such oxidation are gaseous and consist of evaporated water
and air. Much water can be evaporated in this step, as the
oxidation is exothermic and the heat of reaction can be used
directly. Decarbonation and oxidation may also be effected
simultaneously if recovery of carbon dioxide is unnecessary
or undesirable. The alkali metal sulfate is recovered from
the aqueous media by crystallization. Crystallization of
- 14 -
2~6Z~3
potassium sulfate from the potassium carbonate-containing
product aqueous media is particularly efficient. Surprisingly,
the solubility of potassium sulfate in potassium carbonate
solutions i5 reduced to very low levels, even where the
solution is hot. This is one of the important features
of the process. For example, the solubility of potassium
sulfate in water at 70C. is 19.5 grams/100 milliliters of
water, but drops to 0.11 grams/100 milliliters of aqueous
solution containing 67 grams of potassium carbonate, a
remarkable reduction in solubility.
Alkali metal values removed from the product aqueous
media with the sulfate or sulfite salts may be replenished
with alkali metal hydroxide or carbonate and fed directly
to the recycling product aqueous media. Where the product
aqueous media includes sulfite, nitrite and nitrate salts
in solution, these may be separated from one another, as by
~irst converting the nitrites to nitrates, then oxidizing
the sulfite to sulfate, and finally by separating the sulfate
and nitrate by selective crystallization. The presence of
alkali metal carbonates in the product aqueous media reduces
the solubility of both sulfates and nitrates and aids in
the selectivity of the processj thus permitting first
sulfates, at around 70C., and then nitrates to crystallize
as the product aqueous media is cooled. The solubility of
potassium nitrate in aqueous media containing potassium
carbonate is reduced to about one tenth of that in water
alone. Thus, at 40 C., only 6 grams of potassium nitrate
dissolve in 100 milliliters of aqueous solution containing
67 grams potassium carbonate whereas 64 grams of potassium
nitrate will dissolve in 100 milliliters of water alone
at 40C.
~:~L2~
Where the new sulfur dioxide removal process of
this invention is combined with one of the new processes
for removing lower valence nitrogen oxides from gas mixtures,
even greater advantages accrue from the integration of these
two processes into a sequential overall process for removing
and preferably recovering both gases from gas mixtures such
as combustion gas mixtures. Thus, where the oxidant for
oxidation of lower valence nitrogen oxides to higher valence
nitxogen oxides is hydrogen peroxide, the electrolytic
formation of alkali metal hydroxide also produces hydrogen
which may be used to make hydrogen peroxide at the site of
the overall process, sharply decreasing the cost of making
hydrogen peroxide. Where chlorine or hypochlorite is the
oxidant, the same electrolysis process produces chlorine gas,
which may be used as the oxidant itself or converted to
hypochlorite. Alkali metal c~loride recovered from the
product aqueous media may be recycled to electrolysis again
thereby decreasing the cost of the overall process.
Where the gas mixture treated includes not only
sufficient sulfur dioxide to require its removal, but includes
some dinitrogèn trioxide, nitrogen dioxide, or ~oth, the
processes of this invention for removing sulfur dioxide
also tend to trap these oxides as alkali metal nitrites
and nitrates along with the sulfur dioxide as sulfite.
The resulting product aqueous media may then be subjected
to the product recovery steps described above to separate
the sulfur values from the nitrates and nitrites~ Where
removal and recovery of lower valence nitrogen oxides from
the same gas stream follows the recovery of sulfur dioxide,
the product aqueous media in the sulfur dioxide removal
cycle may be bled to the nitrate recovery step of the lower
- 16 ~
. , .
valence nitrogen oxide removal process after the sulfur
values in the sulfur dioxide removal product aqueous media
cycle are removed.
The processes of this invention provide a substantial
breakthrough in making the elimination of gaseous pollutants
from such gas mixtures as combustion gases both practicable
and economic. No practicable process is now known for
removing and recovering lower valence nitrogen oxides from
such gas mixtures, and none ~ermits the recovery of a saleable
product to offset the high operating cost and capital cost
of the plant and equipment needed to effect the removal.
The presence of sulur dioxide in combustion gases only
compounds the problems that industry faces today and no
process exists for achieving the near 100~ removal and
recovery of sulfur dioxide from gas mixtures whether or not
lower valence nitrogen oxides are present. The processes
o~ this invention meet all these needs. Lower valence
nitrogen oxides and particularly nitric oxide, which are
typically present in quite low concentration in combus ion
gas mixtures, cannot be removed by alkaline scrubbing aione.
Nor does oxidation of nitric oxide alone solve the problem.
Combining the use of an alkali metal carbonate/bicarbonate
scrubbing media which forms an alkaline acceptor, with an
oxidant makes possible the trapping of nitrogen dioxide and
other higher valence nitrogen oxides as they form. The
resulting product aqueous media is particularly amenable to
treatment for removal and recovery of commercially valuable
products in commercial quantities. The alkali metal nitrites
and nitrates formed in the product aqueous media are strongly
held by this acceptor system and may be recycled time and
time again through the treating process until the concentration
of each rises to a level where its recovery is commercially
practicable. Carbonates in the product aqueous me~ia
strongly reduce the solubility of the nitrates, and permit
them to be crystallized as the temperature of the media is
lowered. Nitrites in the produ~t aqueous media are readily
converted to nitrates by lowering the pH of the product
aqueous media to less than about 9, which expedites the
oxidation of nitrite to nitrate. q~he treating step of the
process of this invention may be summarized by the ollowing
reactions, where the oxidant is hydrogen peroxide and the
alkali metal is potassium:
2 KOH + C02 --~ K2C03 + H2
K2C3 + C2 + 2 2 KHCo3
NO Removal and Conversion
NO + 1/2 H22 + K2C03 ~ KN2 + KHC03
2 2 2 3 > KN02 + RN03
~ 2 XHC03
KN02 + H202 ~~ KN03 H2o
S2 Removal and Conversion
S2 + 2 KHC03 - >K2S03 +2 C02 ~ H20
K2S03 + 1/2 2 ~ 2 4
The processes of this invention are illustrated in
Figures 1, 2, 3 and 4. Figure 1 illustrates an embodiment
where the gas mixture includes carbon dioxide and lower
- 18 -
6;~
valence nitrogen oxides with or without sulfur dioxide, and
gas treatment is made to remove all three simultaneously.
The oxidant is hydrogen peroxide, the alkali metal is
potassium, and the oxides of lower valence nitrogen and
sulfur, if present, are remo~ed together.
Figure 2 illustrates the embodiment where the gas
mixture includes carbon dioxide, sulfur dioxide and lower
valence nitrogen oxides, the alkali metal is potassium, the
oxidant is hydrogen peroxide, the sulfur dioxide is converted
to sulfite by reaction with carbonate and bicarbonate, and
the oxides of lower valence nitrogen are thereafter oxidized
to higher va:Lence nitrogen oxides with hydrogen peroxide
in a single scrubber.
Figure 3 illustrates the embodiment where the gas
mixture includes sulfur dioxide, carbon dioxide, and lower
valence oxides of nitrogen, the alkali metal is potassium,
the oxidant is hydrogen peroxide~ and the sulfur dioxide is
~ormed and removed in ~he first stage as potassium sulfi~e
in product aqueous media. The lower valence nitrogen oxidès
in the sulfur dioxide-free gas mixture are removed and re-
covered in a second stage using the hydxogen peroxide oxidant
and aqueous media CGntaining potassium carbonate/bicarbonate.
Figure 4 discloses an embodiment where the gas
mixture includes carbon dioxide and lower valence nitrogen
oxides, the alkali metal is potassium, the oxidant is
chlorine, chlorine dioxide or alkali metal hypochlorite,
and the product aqueous media is processed to separate the
potassium chloride from the potassium nitrate product.
Referring now to Figure 1, a combustion gas mixture
3~ including carbon dioxide, sulfur dioxide and lower valence
nitrogen oxides enters scrubber 2 where the gas is intimately
- 19 -
~.
contacted, either cocurrently or countercurrently, with
aqueous media entering scrubber 2 via line 4. Though the
drawing shows only one scrubber, there may be several, and
each may have multiple stages. The gas mixture exits scrubber
2 via line 5, containing only small amounts of lower valence
nitrogen oxides and virtually no sulfur dioxide.
The aqueous media entering scrubber 2 via line 4
includes potassium nitrite and nitrate, carbonate and bicar-
bonate, and an oxidant~ hydrogen peroxide. Recycled aqueous
media may also enter scrubber 2 via lines 7 and 4. With
sulfur dioxicle in the gas mixture entering scrubber 2, the
recycled aqueous media also includes some alkali metal
sulfate.
The following reactions take place within scrubbar 2:
eactions of C02
2 KOH + C02 ~ K2C3 + H2o
K2C3 + C2 H20 ~ 2 KHC03
N0 Con ersion
N0 + 1/2 ~22 ~K2C03 ~ KN02 ~ KHC03
22 + 2 K2C3 ~ KN03 + KN0 +
2KHC03 ~ H20
KN02 + H202 ~ KN03 + ~2
S2 Conversion
S2 + H202 + 2 K2C03----~K2S~ + 2 KHC03
- 20 -
~3 Z9~2~3
If the gas mixture includes a large amount of carbon
dioxide, the potassium carbonate formed in scrubber 2 changes
to bicarbonate in large part. This lowers the pH in scrubber
2 into the range of about 7 to about 10. Where ~he pH is
within this range varies with the temperature, carbon dioxide
partial pressure in the gas mixture, and the quantity of
potassium carbonate and bicarbonate present. Hydrogen per-
oxide oxidiæes the oxides of lower valence nitrogen to higher
valence nitrogen oxides which then react with the potassium
bicarbonate and carbonate in the aqueous media to form
potassium nitrate and nitrite. Any sulfur dioxide present
oxidizes to potassium sulfate. The scrubbing system operates
most economically when all of the oxidant is consumed in the
scrubbing step, and that is readily accomplished with counter-
current flow of scrubbing solution to the gas mixture.
The product aqueous media from the treating opera-
tion, including potassium nitrate, potassium nitrite,
potassium sulfate, potassium carbonate and potassium bi-
carbonate, leaves scrubber 2 through line 5 and passes to
separator 6. From there, a number of process variations
may be employed for separating and recovering the products
in that media.
Potassium sulfate has a low solubility in water
compared to the other products in the product aqueous media,
particularly with potassium carbonate present. By contrast~
potassium carbonate and potassium nitrite are highl~ soluble
even when cold, compared to the others. By converting most
of the bicarbonate to carbonate, the sulfate and nitrate
potassium salts may be crystallized from the solution with
3~ no bicarbonate contamination. The carbonate and nitrite
remain in solution. The major portion of the product aqueous
- 21 -
Z~6Z~3
media is recycled through line 7 to increase the concentration
of potassium nitrate in the product aqueous media to a
sufficiently high level to make product recovery practicable
and economic without excessive evaporation.
Because of its low water solubility and the high
sulfur content of many fossil fuels, potassium sulfate tends
to crystallize from solution and is removed from separator
6 via line 9 together with the product aqueous media from
which potassium nitrate and sulfate products are to be
recoveredO Product aqueous media is removed via line 7
from separator 6 and may be recycled to scrubber 2 through
line 4.
The product aqueous media passes via line 9 to
decarbonator-evaporator 10 where a portion of the bicarbonate
i9 normally converted to carbonate with removal and recovery,
if desired, of carbon dioxide. Co~version of bicarbonate
to carbonate is desirable because potassium bicarhonate
tends to crystallize with and contaminate the potassium
nitrate, but potassium carbonate will not. The pH of the
product aqueous media which includes potassium nitrate,
potassium nitrite, potassium bicarbonate and potassium
carbonate, together with residual potassium sulfate, may be
further adjusted by adding potassium hydroxide through line
11 to prevent crys~allization of potassium bicarbonate.
Distillate water and carbon dioxide leave evaporator-decar-
bonator 10 by line 12. The decarbonation and carbon dioxide
formation are effected by thermally decomposing bicarbonate
by evaporation or steam stripping as follows:
2 KHCO3 > K2CO3 + CO~ + H2O
i,:
1~962~ `
The carbon dioxide removed via line 12 may be recovered as a
product or partially used to convert potassium carbonate to
bicarbonate to facilitate the oxida~ion of potassium nitrite
to nitrate.
Concentrated solution from evaporator-decarbonator
10 passes to crystallizer-separator 13 via line 1~ for
recovery of crystalline potassium sulfate via line 15.
Product aqueous media exiting the potassium sulfate
crystallizer-separator 13 in line 16 is then deep cooled
in crystallizer-separator 17 to recover potassium nitrate
which is removed via line 18. Potassium nitrite, which is
highly water soluble, remains in solution in the potassium
carbonate-containing aqueous media, and recycles in lines
19, 8 and 4 and is ultimately oxidized to potassium nitrate
and recovered as such. Impurities accumulated in the product
aqueous media may be purged through line 28. This purged
material may be used as a fertilizer or fertilizer supplement.
The potassium carbonate/potassium bicarbonate
system performs numerous important functions in the practice
of this invention. For example, in the trea~ing step, this
sys~em insures substantial removal of sulfur dioxide and
lower valence nitrogen oxides from the gas mixture~ In
product recovery, the system lowers the water solubility of
potassium sulfate and potassium nitrate, making recovery of
each more selective and higher in yield, thus reducing energy
needs for evaporation and refrigeration.
The following data show the unexpectedly strong
impact of potassium carbonate in reducing the solubilities
of potassium sulfate and potassium nitrate in aqueous media.
Potassium sulfate has a solubility at 70C. of 19,5 grams in
100 milliliters of water, but only 0.11 gram in 100 milliliters
- 23 -
. ~
1.~2~6'~
of aqueous solution containing 68 grams of potassium carbonate.
Potassium nitrate has a solubility at 20C. of 33 grams in
100 milliliters of water, but only 2.8 grams in 100 milli-
liters of water containing 68 grams of potassium carbonate
at 20C. Thus, potassium sulfate can be crystallized with
substantially complete removal from aqueous potassium
carbonate at temperatures above about 70C., following which
the potassium nitrate may be crystallized with increasing
recoveries at lower temperatures, ~ay from about lO~C. to
about 50C. Separation and recovery of substan~ially pure
sulfate and nitrate products is an important and unexpected
result of this process, and makes possible the recovery of
commercial quantities of these proclucts from gas mixtures
containing low concentrations of sulfur dioxide and lower
valence nitrogen oxides for the first time.
The product aqueous media in line 19, which contains
principally potassium carbonate and lesser amounts of
potassium nitrite and nitrate, is recharged with hydrogen
peroxiae oxidant. This may be fed via line 21 from an out-
side source, or may be produced by autooxidation of certainorganic compounds in on-site hydrogen peroxide plant 22.
In such a plant 22, hydrogen peroxide may be pro
duced by the cyclic reduction and oxidation of an alkyl
anthraquinone such as 2-ethylanthraquinone in an organic
solvent, as follows:
~ 1 ~ 2 ~ ~ i
- 24 -
1~29~8
OH O
~ /2HS ~ ;j,C2H5
Hydrogen peroxide may be extracted from the
anthraquinone-containing organic phase directly into the
recycling product aqueous media entering plant 22 through
line 23 and leaving through line 24, or into a separate
water strPam which is then added to the recycling product
a~ueous media. The hydrogen entering plant 22 via line 25
may be supplied from electrolytic cell 27 via line 28~
This lowers the cost of producing hydrogen peroxide signi-
~icantly because hydrogen is expensive.
Makeup potassium i5 conveniently produced as
potassium hydxoxide in electrolytic cell 27. Saturated
potassium chloride solution enters cell 27 through line 29,
and is electrolytically converted to chlorine at anode 30
and to potassium hydroxide and hydrogen at cathode 31.
Chlorine passes from the cell via line 32 and hydrogen via
line 28. A cationic-exchange membrane 33 is used to obtain
a potassium chloride-free potassium hydroxide product which
passes from cell 27 via line 11. A portion of the potassium
hydroxide solution is fed via line 34 to line 8; another
portion, to crystallizer 13 via line 11 for adjustment of
pH there to a value in the range of about 10 to about 12.
To convert nitrite remaining in the product aqueous
3~ media to nitrate rapidly, carbon dioxide produced in the
decarbonation step may be fed via line 12 to line 8.
- 25 -
112962~ ~
There, carbon dioxide converts the carbonate to bicarbonate
as follows:
C2 ~ K2C03 ~ 2 ~ 2 KCH03
Optionally, carbonation and oxidation may be expedited by
pressurizing carbon dioxide into recycling product aqueous
media in line 8. Various mineral acids may alternatively
be used to convert carbonate to bicarbonate to expedite
the nitrite oxidation but they are costlier than the carbon
dioxide made in this process.
Conversion of nitrite to nitrate, and recycle of
oxidant-fortified product aqueous media to scrubber 2 via
line 4 makes the process fully cyclic.
Figure 2 shows the removal of sulfur dioxide and
lower valence nitrogen oxides from a gas mixture containing
them and carbon dioxide, where hydrogen peroxide oxidizes
only the lower valence nitrogen oxides. Sulfur dioxide is
removed rom the gas-liquid contacting in the produc~ aqueous
media as alkali metal sulfite. These results are obtained by
providing an alkali metal to sulfur dioxide weight ratio of
at least about 2, and by limiting the am~unt o oxidant to
approximately the amount required for the oxidation of lower
valence nitrogen oxides to higher valence nitrogen oxides.
The product aqueous media thus includes (where the alkali
metal is potassium) potassium nitrate, potassium nitrite,
potassium sulfite, potassium carbonate and potassium bicar-
bonate. Countercurrent or staged countercurrent gas-liquid
contacting achieves these results. Packed, spray type or
other efficient contactvrs may be used. This process
embodiment advantageously reduces the quantity of oxidant
peroxide needed because sulfite in the product agueous media
- 2~ -
~1 '29~28
may be oxidized to sulfate with air or other oxygen-containing
gas separately rather than with the more expensive oxidant.
In Figure 2, gas mixture 101 including at least one
lower valence nitrogen oxide, carbon dioxide and sulfur
dioxide, enters near the bottom of tower 102. Alkaline
aqueous media containing alkali metal (here potassium) carbo-
nate and bicarbonate together with hydrogen peroxide enters
near the top of tower 102 via line 104 and passes downwardly
and countercurrently to the gas mixture entering in line 101.
Near the bottom of tower 102, potassium carbonate and bi-
carbonate in the alkaline aqueous media react with sulfur
dioxide to form potassium bisulfite and potassium sulfite in
the aqueous media as follows:
S2 + KHC03 3 KHS03 + C02
KHS03 + RHC03 --------~ ~2S03 + C02
Lower valence nitric oxide is unaffected and passes
upwardly. Near the top of the tower, it reacts with hydrogen
peroxide to form higher valence nitrogen oxides which pass
into solution as potassium nitrite and potassium nitrate.
The product aqueous media passing from tower 102
via line 105 thus includes potassium nitrate and potassium
nitrite, potassium sulfi~e and potassium sulfate and potassium
carbonate and potassium bicarbonate. This product aqueous
media passes via line 105 to decarbonator 106. There,
elevated temperature and stripping s~eam decompose at least
a portion of the bicarbonate, forming carbonate and carbon
dioxide which is removed and recovered if desired from line
107~ Alternatively, as shown~ the carbon dioxide may be fed
to nitxite to nitrate converter 132 in line 118 via line 107
and mixed there with recycling product aqueous media
~ 27 -
6 ". ~ ~
containing potassium nitrite and hydrogen peroxide entering
converter 132 from line 118. The carbon dioxide converts
carbonate in the product aqueous media to bicarbonate,
lowers the pH below 9, and thus expedites conversion of
nitrites to nitrates with hydrogen peroxide in the product
aqueous media. Where nitrites are to be recovered, this
step is omitted.
Decarbonated product aqueous media then passes via
line 108 to oxidizer 110 to which air or other oxygen-
containing gas is added via line 111. The oxygen oxidizes
at least a portion of the potassium sulfite to potassium
sulfate under neutral or alkaline conditions, as follows:
K2SO~ ~ 1/2 2 ~ 2 4
Any remaining bicarbonate decomposes to carbonate and carbon
dioxide and a substantial amount of water evaporates during
this oxidation. Water, air and carbon dioxide pass overhead
from oxidizer 110 via line 109.
The product aqueous media passes from oxidizsr 110
via line 112 to hot ~rystallizer~separator 113 where potas-
sium sulfate crystallizes and is recovered via line 114.
Potassium nitrate and nitrite remain in solution. Potassium
hydroxide may be added to oxidizer 110 via line 140.
Recovery of potassium nitrate, replenishment of
product aqueous media with hydrogen peroxide, conversion of
nitrite to nitrate, and furnishing of makeup potassium are
effected as in Figure lo Here, product aqueous media from
the potassium sulfate crystallizer-separator 113 passes via
line 115 to potassium nitrate crystallizer 116 which operates
in the range of about 10C. to about 40C. Potassium nitrate
is removed via line 117 and product aqueous media containing
- ~8 -
.~ .
1~2962~
principally potassium carbonate and lesser amounts of potas-
sium nitrate and nitrite pass through lines 118 and 104 to
scrubber 102, making the process cyclic~ Oxidant hydrogen
peroxide for the proc~ss is produced in plant 119, and fed
to line 118 via line 133. Alternatively, hydrogen peroxide
may come from another source to line 118 via line 1300
Air and hydrogen are fed to plant 119 via lines
120 and 121, respectively. Hydrogen may be obtained ~rom an
outside source or from electrolytic cell 122. There,
potassium hydroxide is made from potassium chloride which
enters cell :L22 via line 123. At anode 124, chlorine gas
forms and is taken overhead via line 125. Potassium ions
pass through cationic permselective membrane 126 to cathode
127, where hydrogen gas forms and passes overhead via line
128. Where hydrogen peroxide is made on site, hydrogen
peroxide passes via line 129 and 121 to hydrogen peroxide
plant 119. Potassium hydroxide for~s at cathode 127 and
passes from cell 122 via line 130 to sulfi~e oxidizer 110
and recycle line 104.
Figure 3 illustrates an embodiment of the process
of the invention where a gas mixture including sulfur dioxide,
carbon dioxide and lower valence nitrogen oxides are treated
in two separate stages, the first for ~he removal of sulfur
dioxide, and the second for the removal and recovery of
lower valence nitrogen oxides. Sulfur dioxide is removed
by employing aqueous media comprising alkali metal carbonate
and bicarbonate where ~he ratio of alkali metal to sulfur
dioxide is at least about 2. Lower valence oxides of nitrogen
are removed from the gas mixture by treating the mixtures
with aqueous media comprising alkali metal carbonate and
bicarbonate and hydrogen peroxide.
- 29 -
~, ,.
1~29~2~
Removal of sulfur dioxide from gas mixtures and
particularly combustion gas mixtures according to this
process is strikingly different from any process previously
proposed. In particular, this process uses more alkali metal
than is required to react with sulfur dioxide so that sub-
stantial carbonate and bicarbonate are present in the product
aqueous media. Sulfur dioxide is absorbed from the gas
mixture and forms sulfite rather than bisulfite as in the
Wellman-Lord process. In that process, the scrubber solution
is usually slightly acidic in order to recover the sulfur
values as sulfur dioxide. To that end, Wellman-Lord decomposes
sodium bisulfite thermally to form sulfur dioxide and sodium
sulfite which is recycled to gas contact. This process calls
for an overall ratio of alkali metal to sulfur dioxide of
less tha~ 2 and more preferably bet.ween 1 and 2, whereas
the new process of this invention e!mploys a ratio of alkali
metal to sulfur dioxide of at least 2 and preferably at
least 3. As a result, the process of this invention achieves
removal efficiencies of about 99~, whereas the Wellman-Lord
process can only reach a practical maximum of about 90%.
Importantly, the process of this invention does not destroy
nitrogen oxides present in the gas mixture with the sulfur
dioxide. By contrast, in the Wellman-Lord and lime-limestone
processes, lower valence nitrogen oxides in the gas mixture
are reduced by sulfite-bisulfite solution to n~tro~en and
nitrous oxide (N2O) and are lost as a potential source of
fixed nitrogen. In the process of this invention, the lower
valence nitrogen oxides are not destroyed but pass freely to
subse~uent steps for th ir removal and recovery.
Referring now to Figure 3, a gas mixture including
sulfur dioxide, lower valence nitrogen oxides and carbon
~ 30 -
~129628
dioxide, such as in a typical combus~ion gas, enters scrubber
202 via line 201, and is intimately contacted, cocurrently
or countercurrently, with aqueous media entering via line
- 208, and including principally alkali metal carbonate and
bicarbonate such as potassium carbonate and bicarbonate,
together with a lesser amount of potassium sulfite carried
from recycle stream 207. The sulfur dioxide is removed from
the gas mixture and transfers to the product aqueous media
as potassium sulfite. Makeup potassium hydroxide entering
via lines 237 and 208 is converted first to potassium
carbonate in stream 208 and then to potassium bicarbonate
by reaction with the carbon dioxide in the gas mixture.
In turn, the potassium bicarbonate reacts with the more
acidic sulfur dioxide to form potassium sulfite. Excess
potassium carbonate and bicarbonate effect conversion of
potassium bisulfite to sulfite. The reactions taking place
in scrubber 202 are as follows:
2 KOH ~ CO~ 3 K2CO3 ~ H2o
K2CO3 ~ CO2 + H2o > 2 KHCO3
S2 ~ 2 RHCO - -3 K SO + 2 C02 ~ H2
(NO + NO) * 2 K2CO3 > 2 KN 2 3
A substantial portion of the product aqueous media
leaving scrubber 202 in line 206 is normally recycled to
provide good contact between gas and aqueous phases. That
portion removed for product recovery is usually decomposed
by boiling or by countercurrent steam stripping in decar-
bonator 211 in which the potassium bicarbonate is convertedto carbon dioxide and potassium carbonate. The following
~ 31 ~
1129~8
reaction takes place in zone 211 at boiling temperatures;
2 KHCO3 ~ K2CO3 + CO2 + H2o
The carbon dioxide and steam so formed leave zone 211 via
line 213 from which the water may be condensed and the carbon
dioxide dried and recovered as a product, or transferred to
converter 255 via line 212. The decarbonated product aqueous
media comprising potassium carbonate, po~assium sulfite and
potassium sulfate in stream 214 passes to sulfite oxidizer/
evaporator 215 where potassium sulfite is oxidized to
potassium sulfate. Though the oxidation may be done with
hydrogen peroxide, atmospheric oxygen costs less and is
preferably used. The oxygen~containing gas is fed to oxidizer
215 via line 217.
Oxidation of sulfite to sulfate by atmospheric
oxygen proceeds rapidly under neutral or alkaline conditions
and may be expedited by operation at elevated pressures and
temperatures. Any additional base needed to raise pH may
be fed to zone 215 as potassium hydroxide through line 271,
but this is generally not necessary. The reaction taking
~o place in oxidizer 215 is as follows:
K2SO3 ~ 1/2 2 -- ~ K2S4
Potassium sulfate has a significantly lower solu-
bility in water than potassium sulfite and has a low
sol~ility in potassium carbonate solutions. Accordlngly,
potassium sulfate may readily be removed from the product
aqueous media by crystallization. Normally, the discharge
218 from sulite oxidizer 215 is a slurry because of sub-
stantial water removal and low solubility of potassium
sulfate in product aqueous madia. The slurry in line 218
- 32 -
112962~ `
may then be cooled and potassium sulfate removed in crystal-
lizer-separator 220. The recovered potassium sulfate is
normally centrifuged and removed via line 221 for drying
and packaging, and the product aqueous media is removed in
line 222.
Produc~ aqueous media in line 222, which is
principally potassium carbonate, may also contain a small
amount of potassium sulfate, unoxidized potassium sulfite
and potassium nitrite. That product aqueous media is re-
cycled to zone 202 for scrubbing via lines 222, 207 and 208.
Some of this may be bled, however, via lines 223 and 224,
respectivelyl to oxidizer 215, to line 258, or both. Addi-
tional potassium hydroxide is added to the recycling product
aqueous media via line 237 to compensate for potassium
removed in the potassium sulfate product.
The gas stream passing from scrubber 202 via line
203, substantially free of sulfur dioxide, passes to scrubber
204 where lower valence nitrogen oxides and some carbon
dioxide are xemoved. Clean gas emerges via line 205. Unlike
the embodiment disclosed in Figure 1, however, prior removal
of sulfur dioxide reduces substantially the quantity of
oxidant required, and precludes formation of more than a
small quantity of potassium sulfate during r~moval and
recovery of lower valence nitrogen oxides. Crystallization
and recovery of potassium nitrate is simpler than in Figure 1
because the product a~ueous media includes only nitrate,
nitriter carbonate and bicarbonate, but little sulfateO
Potassium makeup required for producing the potassium nitrate
product may be from potassium carbonate via line 224, from
potassium hydroxide electrolytically made in cell 231,
or both.
- 33 -
1 1 2 9 6 ~ ~ `
Product aqueous media comprising unconsumed hydrogen
peroxide, potassium nitrate and po~assium nitrite, potassium
carbonate and potassium bicarbonate~ passes ~rom zone 204 via
line 251 and is recycled to scrubber 204 via lines 252 and
253, until the concentration of nitrite, nitrate or both
reaches a predetermined minimum. Thereafter, at least some
of the product aqueous media in line 251 enters the product
recovery cycle via line 254. Preferably, the product aqueous
media to be subjecte~ to product recovery passes through
line 254 to nitrite converter 255. Carbon dioxide from
decarbonator 211 passes via line 212 to converter 255 to
convert carbonate to bicarbonate, thus lowering the pH to
less than about 9, and facilitating oxidation of nitrite to
nitra~e by unconsumed oxidant hydrogen peroxide. Product
aqueous media, now rich in potassium nitrate and potassium
bicarbonate, passes from nitrite converter 255 via line 256
to decarbonator~evaporator 257. There, a substan~ial portion
of bicarbonate is converted to carbonate and carbon dioxide
by thermal decomposition. Carbon clioxide and water pass
overhead via line 209 and the carbon dioxide may be recovered
and used elsewhere, or may be fed into line 245 to convert
potassium carbonate to hicarbonate in that line and to
facilitate oxidation of potassium nitrite there to nitrate.
Pressurizing carbon dioxide aids conversion of carbonate to
bicarbonate and facilitates formation of carbonic acid in
the product aqueous media. Oxidation of nitrite to nitrate
proceeds faster when carbonic acid is present and carbonate
is absent.
The product aqueous media leaving the decarbonation
tower 257 via line 258 contains primarily potassium nitrate
and nitrite and potassium carbonate, and is cooled in
- 34 -
1129~2~
crystallizer 260 to around 10C. Thereupon, potassium nitrate
crystallizes and is removed via line 261 following centri-
fuging. Potassium carbonate and potassium nitrite remain
in the product aqueous media which recycles via lines 262
and 253 to zone 204 following addi~ion thereto of oxidant
hydrogen peroxide via lines 245 and 253. Some of the re-
cycling aqueous media may be bled into line 258 from line
245 via line 259.
Hydrogen peroxide may be produced on site, as
described above, from line 243, or from an outside source
via line 244. Where peroxide is made on site, hydrogen is
fed to plant 241 via line 235 and air, via line 242. Pr~-
ferably, hydrogen is supplied in whole or in part fxom
electrolytic cell 231.
Makeup potassium is conveniently produced in
electrolytic cell 231 as well. To cell 231, a concentrated
solution of potassium chloride is fed via line 238 to the
anode compartment where it is elect:rolytically converted to
chlorine at anode 233 and to potassium hydroxide and hydrogen
at cathode 234. Chlorine passes from the cell via line 236
and hydrogen via line 235. Cationic membrane 232 i5 used
to produce a chloride-free potassium hydroxide product which
passes from cell 231 via line 237.
Figure 4 illustrates an embodiment of the processes
of this invention wherein a gas mixture including carbon
dioxide and lower valence nitrogen oxides are treated for
removal and recovery of those lower valence nitrogen oxides
using gaseous chlorine or alkali metal hypochlorite with an
aqueous alkali metal carbonate/bicarbonate acceptor system.
The alkali metal is potassium. Sulfur dioxide, lf present
in the gas mixture, may be removed by oxidation with the same
1 1.2~6~
oxidant, or, preferably, before ~reatment with the oxidant
as in Figure 3.
Where sulfur dioxide is not present, or if presentt
is removed in a prior step, as in Figure 3, the product
aqueous media from the treatment with oxidant includes
potassium nitrate, potassium nitrite, potassium chloride
and potassium carbonate and bicarbonate. Potassium carbonate
diminishes the solubility of potassium chloride in aqueous
media from, for example, 30% in water at 50C. to around 3%
at the same temperature in a solution containing 50%
potassium carbonate Potassium chloride may therefore be
crystallized and removed from the more soluble potassium
nitrate and nitrite which remain in the product aqueous media.
When using chlorine or hypochlorite as the oxidant,
chlorine carryover may occur at the reduced pH's that arise
in the scrubber, and a two-stage scrubbing system is pre-
ferred. Hypochlorites are strong oxidizing agents in a pH
range of about 6.5 to about 8 where a substantial amount of
hypochlor~us acid is present.
Oxidation and removal of lower valence nitrogen
oxides takes place primarily in the ini~ial scrubbing stage.
Unreacted chlorine and nitrogen oxides carried to the second
scrubbing stage are removed because of the higher alkalinity
and pH there than in the first stage. The same result may
be obtained in a single countercurrent scrubbing column but
with less flexibility and ease. The reaction of chlorine
and hypochlorites with nitric oxide is quite rapid.
Referring now to Figure 4, the gas mixture compris-
ing at least one lower valence nitrogen oxide and carbon
dioxide enters scrubber 302 via line 301. The gas mixture
is intimately contacted by aqueous media including potassium
- 36 -
1129~2~
carbonate and bicarbonate, potassium nitrate and nitrite
and the oxidant. Where ~he oxidant is potassium hypochlorite,
the oxidant is in solution and is accompanied by potassium
chloride. This solution enters scrubber 302 via line 362.
Where chlorine is the oxidant, it may be added to the gas
mixture before the mixture enters scrubber 302 or may be
fed directly to scrubber 302 through a separate inlet for
reaction with potassium carbonate and bicarbonate to form
hypochlorite. The scrubbed gas stream passes from scrubber
302 via line 303 to second stage 306 and is treated with
aqueous potassium carbonate entering via line 347 to remove
the last traces of chlorine. The scrubbed gas exits scrubber
306 via line 307 and passes to the atmosphere. The potassium
carbonate conta~ning aqueous solution passes from scrubber
306 via line 304, and is partially recycled to line 347 via
line 305. The remainder of the solution passes via line
304 to scrubber 302.
The following reactions take place in scrub~ers
302 and 306:
Zone 306
2 KOH + C02 ______ ~ K2C03 + 2
K2CO3 ~ CO2 H2o ~ 2 KHCO3
Zone 302
Cl + K CO3 ~ KOCl + KCl + CO2
C12 + 2 KHCO3 - > KOC1 + KCl + 2 CO2 ~ H2O
C 2 H20 - HOCl ~ HCl
KOCl + H2O - HOC1 + KCl
- 37 -
:1 L29628
NO Conversion
2 NO + 3 KOCl (3 KCl) + 2 KHCO3 32 KNO3 ~ 6 KCl -~
2 CO2 ~ ~2
S2 Conversion
S2 ~ KOCl ~KCl) ~ 2 KHCO3 2 4
~ C2 + H20
The oxidant, whether potassium hypochlorite,
10 hypochlorous acid, chlorine, chlorine dioxide or a mixture
of two or more of these, oxidizes the lower valence nitrogen
oxides to higher valence nitrogen oxides which react with
the potassium carbonate and bicarbonate to form non-volatile
potassium nitrate and nitrite.
Makeup potassium hydroxide! and oxidant for the
system are conveniently produced in electrolytic cell 351.
A saturated potassium chloride solution enters cell 351 via
line 319 from dissolver 318. Potassium chloride is fed to
dissolver 318 via line 359, and i~ dissolved therein with
~0 water entering via line 350 and with recycled potassium
chloride entering via line 317. The potassium chloride is
electrolytically converted to chlorine at anode 354 and to
potassium hydroxide and hydrogen at cathode 3S3. Chlorine
passes frvm the cell via line 357 and hydrogen via line 355.
A cationic-exchange membrane 352 is used ~o obtain potassium
- chloride-free potassium hydroxide which passes from cell
351 via line 360.
Where aqueous potassium hypochlorite is the oxidant,
potassium hydroxide and chlorine gas are introduced to a
30 potassium hypochlorite reactor 361 through lines 360 and
357 respectively, in the ratio produced in the electrolysis
38 -
.. . . . .. .. ... .. . .. . ... . .. . . . . . ..... . .. .. . .. ....
1~29628
namely 2:1. The potassium hypochlorite oxidant passes to
scrubber 302 via line 362.
Where chlorine is the oxidant, chlorine passes
through lines 357 and 358 to line 301, where it commingles
with the entering gas mixture before that mixture enters
scrubber 302. Potassium hydroxide is, in this embodiment,
fed to scrubber 306 via line 347.
The product aqueous media including potassium
nitrate, potassium nitrite, potassium carbonate and potassium
bicarbonate and potassium chloride leaves scrubber 302 via
line 310. Several process variations may be used to separate
and recover the products, one of which follows.
The product aqueous media from scrubber 302 passes
via line 310 to decarbonator-evaporator-separator 312, in
which a substantial portion of the potassium chloride is
crystallized. The hot slurry passes to centrifuge 315 via
line 313. Alony with the evaporation, bicarbonata in the
product aqu~ous media may be at least partially converted to
carbonate and carbon dioxide in aecarbonator-evaporator-
~0 separator 312, and the relatively pure carbon dioxide may
be taken overhead via line 311 and recovered for use in
other processes.
Additional potassium carbonate recycles ~o evapo-
rator 312 through line 348 to provide a high potassium
carbonate concentration therein. This high concentration
reduces the soluhility of potassium chloride and a major
portion may be crystallized from the aqueous media while
retaining the potassium nitrite and nitrate in solution.
Potassium chloride solids centrifuged from solution
in centrifuge 315 are discharged via line 317 and recycled
to electrolysis via dissolver 318. The aqueous media
- 39 -
1 1~9~
including principally potassium nitrate and carbonate leaves
through line 316 and passes to crystallizer 320 where the
solution is cooled, generally ~o a temperature of abou~ 30C.,
to crystallize mixed potassium nitrate-potassium chloride.
The slurry of potassium nitrate-potassium chloride passes
from crystallizer 320 via line 321 to contrifuge 322 and
the solids, principally potassium nitrate and potassium
chloride, are removed through line 3Z3 and fed ~o lixiviator
330. There, the more soluble potassium nitrate is selec-
tively dissolved, or lixiviated, from the solids mixture.
The aqueous media comprising principally potassium carbonate
passes from centrifuge 322 via line 324 and recycles via
lines 347 and 348 to decarbonator-evaporator-crystallizer
312 to promote the crystallization of potassium chloride
therein.
In lixiviator 330, the mixed potassium nitrate and
chloride solids are contacted intimately, usually in a
fluidized bed, with a warm tabout 70 to about 80C.)
highly concentrated recycling aqueous potassium carbonate
solution. The potassium nitrate tends to dissolve pre-
ferentially into the solution but leaving the potassium
chloride undissolved. The aqueous potassium carbonate-
potassium nitxate solution passes from lixivia~or 330 via
line 331 for transfer to the potassium nitrate crystallizer
340. The solution is cooled there to around 20C. and the
potassium nitrate crystallizes. The potassium nitrate
solids slurry passes via line 341 to centrifuge 342 and is
centrifuged. The potassium nitrate solids are removed via
line 343 for drying and packaging.
The aqueous potassium carbonate-containing media
passes from centrifuge 34Z via line 344 into line 345 for
- 40 -
~ 129628
return to lixiviator 330 to remove additional potassium
nitrate~
Undissolved potassium chloride solids in lixiviator
330 are passed via line 332 to centrifuge 333. The aqueous
media comprising principally potassium carbonate and
potassium nitrate transfers via line 335 to join the main
extract stream in line 331. The undissolved potassium
chloride solids from centrifuge 333 are recycled to decar-
bonator-evaporator-crystallizer 312 via line 334.
The reactions taking place in the elactrolytic cell
and the hypochlorite reactor are:
Electrolytic cell 351
2 KCl ~ H O -~ 2 XOH ~ C12~ ~ H2
Hypochlorite reactor 361
2 KOH + C12 - ~ KOCl ~ KCl
In the embodiment of Figure 4, any suifur dioxide
in the gas mixture is preferably removed inltially as by
the process described in reference to s~age 1 of Figure 3.
EXAMP~E I
A gas mixture consisting of 400 parts per million
of nitric oxide and 99.96% nitrogen was passed in a s~ream
countercurrent to an aqueous scrubbing solution consisting
of 20~ potassium carbonate and 1~ hydrogen peroxide
(stabilized) at 50C. through a 10 foot glass column having
a 2 inch diameter and packed with 1/2 inch Pall-type rings.
The gas stream was fed at a rate corresponding to about
- 41 -
1129~2~ `
0.1 foot per second superficial gas velocity, and the liquid
flow rate was jus~ below the flooding point. The concentra~
tion of nitric oxide in the gas stream was reduced to about
30 parts per million which ~orresponds to a removal effi-
ciency of about 92%. The scrubbing solution was recircu-
lated until the hydrogen peroxide concentration was reduced
to approximately 0.1% without any loss in scrubbing efficiency.
The aqueous scrubbing solution was analyzed and
the yield of potassium nitrate and po~assium nitrite formed
corresponded closely to the amount of nitric oxide removed
from the gas mixture. The molar ratio of potassium nitrite
to potassium nitrate was about l:l and the usage of hydrogen
peroxide was 1.1 times the theoretical requirement.
EXAMPLE II
The same gas mixture treated in Example I was
treated in the same equipment and under the same conditions
as under Example I, except that a scrubbing solution was
used consisting of about 18% potassium bicarbonate, about
6% potassium carbonate and about 1~ hydrogen peroxide in
water. The efficiency of nitric oxide removal from this
gas stream was 91% and the ratio of potassium nitra~e to
potassium nitrite was about 1.2:1. The yield of potassium
nitrate and potassium nitrite corresponded closely to the
quantity of nitric oxide removed from the stream and the
usage of stabilized-hydrogen peroxide was about 1.05 times
the theoretical requirement.
EXAMPLE III
The removal of nitric oxide was again tested using
the equipment described in Example I but with an aqueous
- 42 ~
~129628
scrubbing solution consisting of about 18~ po~assium bicar-
bonate, about 6~ potassium carbonate, about 10~ potassium
nitrate and about 1% hydrogen peroxide, again at 50C. The
nitric oxide removal efficiency from the gas stream remains
, at 92~. The treating solu~ion was heated and evaporated
after use to convert bicarbonate to carbonate and to adjust
the potassium nitrate concentration to approximately 12~.
The solution was cooled to 0C. and nearly 100% pure potassium
nitrate product was crystallized from this product aqueous
media wi~h little or no potassium carbona~e or potassium
nitrate contamination.
EXAMPLE IV
A gas mixture comprising 800 parts per million of
sulfur dioxide and 99.92~ nitrogen was tested in the absorp-
tion tower described in Example I using a gas velocity of
about 2.0 foot per second and an aqueous solution including
about 20% potassium carbonate~ The pH of the aqueous media
was about 11.2; the reaction temperature, about 50C.
Efficiency of sulfur dioxide removal was higher than 99%.
Removal efficiency was then tested with an aqueous media
including about 18~ potassium bicarbona~e and about 6%
potassium carbonate. Removal efficiency was again higher
than 99~. The pH of the scrubber solution was 9.2, and
the temperature was again 50C.
By contrast, removal of sulfur dioxide from the
same gas stream with an aqueous solution containing 10%
sodium sulfite at 50C., in accordance with the Wellman-
Lord process, produced a removal efficiency of only about
91~ at peak, which then declined as the concentration of
sodium bisulfite began to rise in the recycled aqueous
scrubbing media. The pH of the sodium sulfate scrubbing
~ 43 -
1~9~
solution was 7.2 at the beginning of the test.
EXAMPLE V
The solubilities in aqueous solution of potassium
nitrate, potassium sulfate and potassium chloride are sub~
stantially reduced in the presence of potassium carbonate.
As Table l below shows, an aqueous solution containing 67 grams
of potassium carbonate ln 100 milliliters o~ water xeduces
the solubility of potassium nitrate to one-tenth that in water
without potassium carbonate present and that of potassium sul-
fate by greater than one hundredth that in water alone.Potassium chloride solubilities are reduced by one-fourth to
one-eighth.
TABLE 1
SOLUBILIT~ OF KNO3, K2S04 AND KCl IN AQUEOUS K2C03
SOLUTIONS IN GRAMS PER 100 MIL~ILITERS _
IN AQUEOUS MEDIA INCLUDING
IN WATER ~LONE ~;7 g K CO /100 ml solution
_ 2 3 _ _
m p., C: _10 40 70 _ 10 40 70
XNO3 21.5 64.0 138.0 2.80 6.00 13.00
K2S4 9.1 14.5 19.5 0.07 0.09 0.11
KCl 31.0 40.0 48~5 4.00 8.00 12.00
Nitrogen oxides have valences ranging from ~l to ~5,
as represented by the following compounds:
~ 44
1~2962~
Valence ~ormula Compound
~1 N2O Nitrous oxide
+2 N0 Nitric oxide
~3 N2O3 Dinitrogen trioxide
+4 N2,N24 Nitrogen dioxide and
Dinitrogen tetroxide
~5 N205 Dinitrogen pentoxide
~3 HNO2 Nitrous acid
+5 HNO3 Nitric acid
1~ As used herein, references made to increasing or
oxidizing an oxide of lower valence nitrogen to an oxide o~
higher valence nitrogen means oxidizing from a lower vaience
state to a higher valence state, e.g., from nitric oxide (NO)
to nitrogen dioxide (NO2) and from nitrogen dioxide (N0~) to
dinitrogen pentoxide (N205) or nitric acid (HN03).
~ s used herein, alkali metals include potassium, sodium,
lithium, rubidium and cesium and alkaline earths include
calcium, magnesium, and strontium.
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