Note: Descriptions are shown in the official language in which they were submitted.
16
PRODUCTION OF MIXED ACID FEED EOR
CHLORINE DIOXIDE GENERATION
The present invention relates to the production of
chlorine dioxide, and more particularly to the
production of a mixture of hydrochloric acid and
sulphuric acid for utilization therein.
In U.S. Patent No. 3,864,456, assigned to the
applicant of this application, there is described a
process for the production of chlorine dioxide and
chlorine wherein sodium chlorate is reduced with added
chloride ions in an aqueous acid reaction medium
containing sulphuric acid at a low total acid normality
in the range of about 2 to about 4.8 norma'. The
reaction medium is maintained at its boiling point
under a subatmospheric pressure, so that chlorine
dioxide and chlorine are removed from the reaction
medium in yaseous admixture with steam. By-product
anhydrous neutral sodium sulphate is deposited from
reaction medium once the reaction medium becomes
saturated therewi~h after start up. The gaseous
mixture of chlorine dioxide, chlorine and steam removed
from the reaction zone is contacted with water, usually
after at least partial condensation of the steam, to
form a chlorine dioxide solution also containing
dissolved quantities of chlorine. -
It has previously been sugges-~:ed in U.S. Patent
~Jo. 3,347,628 to form an aqueous chlorine dioxide
solution from a gaseous mixture of chlorine dioxide,
chlorine and steam removed from a chlorine dioxide
generator to which external steam is added to dilute
the gases, by contact of the gaseous mixture with
water. In this prior process, the chlorine gas
remaining from the absorption is reacted with sulphur
dioxide and water to form sulphuric acid and
hydrochloric acid, which are fed to the chlorine
dioxide generator.
As is set forth in detail in U.S. Patent No.
4,086,329 assigned to the applicant of this
application, the latter concept is not directly
utilizable in the process of U.S. Patent No. 3,864,456,
since the chemical efficiency of ~hlorine dioxlde ~ r~
.1. 1~'7(,~
production under boiling reaction medium,
suhatmospheric pressure and low total acid normality
conditions is less than 100% in the latter process. As
described in U.S. Patent No. 4,086,329, a critical
adjustment of the hydrogen and chloride ion
concentration of the acid feed is required to take this
inefficiency into account, otherwise continuous
operation is impractical.
It has now been found that the presence of
dissolved unreacted sulphur dioxide in the hydrochloric
acid and sulphuric acid mixture resulting from the
reaction of chlorine, sulphur dioxide and water and
forwarded to the reaction medium is detrimental to the
chlorine dioxide-generating process, even in very small
concentrations. Often cell liquor, i.e. the sodium
chlorate solution resulting from diaphragmless
electrolysis of sodium chloride solution, is used as
the source of sodium chlorate feed to the reaction
medium. Such cell liquor usually contains dissolved
quantities of sodium dichro~ate, as a result of the
beneficial utilization of such chemical in the
electrolysis reaction. The presence of dissolved
quantities of sulphur dioxide in the mixed acid stream
has been found to reduce the dichromate ions to
trivalent chromium, which in turn causes the anhydrous
sodium sulphate precipitat~ to form as very fine
crystals which are very difficult to filter or
otherwise separate from the reaction medium. Further,
in the absence of dichromate ions from the reaction
medium, the presence of dissolved quantities of sulphur
dioxide in the mixed acid stream has been found to
decrease the efficiency of chlorine dioxi~e production.
A procedure in which the by-product chlorine from
the chlorine dioxide absorption is reacted with sulphur
dioxide and water to form a mixture of sulphuric acid
and hydroch]oric acid-for reuse in the chlorine dioxide
generator nevertheless is a commercially-attractive
one. Since the hydrochloric acid is used to provide at
least part of the chloride ion requirement and part of
3~ 8 ~
the acid requirement for the chlorine dioxide-producing
process, the overall amount of so~lium sulphate produced
per mole of chlorine dioxide produced is decreased when
compared to a process wherein sodium chloride provides
all the chloride ion requirement and sulphuric acid
provides all the acid requirement, as is apparent from
consideration of the following equations:
NaC103 + (l-x)NaCl + x HCl + 2-x H2 SO4 __~
C1O2 + 1/2C12 + H2O + 2-x Na2SO4 ~1)
NaC103 + 5(1-x) NaCl ~ 5x ~Cl ~ 6-5x H2S04____~
3C12 ~ 3H2O + 625X Na2 4 (2)
wherein x is the molar proportion of HCl which is used
and is a decimal value which is less than or equal to
1.00. Equation (1) represents the reaction which
produces chlorine dioxide and the extent to which the
reaction of equation (1) predominates over equation t2)
is the efficiency of chlorine dioxide production.
It will be seen that the proportion of sodium
sulphate which is produced declines as the proportion
of hydrochloric acid usod in place of sodium chloride
and sulphuric acid increases. The requirements of pulp
mills for sodium sulphate have declined while
requirements for chlorine dioxide have increased. The
ability to produce less sodium sulphate through the
use of hydrochloric acid, therefore, is beneficial.
Further, since chlorine gas from the absorption is
reacted to form reutilizable chemicals, the necessity
for separate absorption of chlorine, usually in sodium
hydroxide solution to form hypochlorite, is
substantially decreased. With the increasing tendency
to substitute chlorine dioxide for a substantial
proportion of the chlorine which has formerly been used
to effect bleaching in the first stage of a multistage
bleaching operation, the requirements for chlorine have
decreased while those for chlorine dioxide have
increased.
In has surprisingly been found that the presence
of dissolved sulphur dioxide in the mixture of
hydrochloric acid and sulphuric acid produced by the
reaction of sulphur dioxide, chlorine and water can be
avoided by utilizing a defined excess of chlorine in
the reaction.
The excess of chlorine which is required to avoid
the presence of dissolved sulphur dioxide in the mixed
acid product increases substantially linearly with
increasing strength of mixed acid produced. This
result is surprising since it has previously been
believed that, since chlorine and sulphur dioxide react
in stoichiometrically-related quantities, the acid
strength should have little or no effect on the
required chlorine partial pressure.
The effect of the excess chlorine on dissolved
sulphur dioxide concentration in the mixed acid product
is substantially Iinear in~ character. Thus, for a
given strength of mixture of hydrochloric acid and
sulphuric acid, the dissolved concentration of sulphur
dioxide decreases to zero and thereafter a dissolved
concentration of chlorine increases linearly with
increased excess gaseous chlorine concentration.
The reaction of sulphur dioxide, chlorine and
water is effected, in the present invention, in a
reaction zone, which may comprise a single reaction
vessel or two or more reaction vessels, having an inlet
for gaseous sulphur dioxide, an inlet for gaseous
chlorine and air, an inlet for water to act as reactant
and absorption medium for the acids, an outlet for a
mixture of hydrochloric acid and sulphuric acid, and an
outlet for excess chlorine and air. In accordance with
the present invention, the partial pressure of chlorine
in the outlet mixture of chlorine and air determines
the concentration of sulphur dioxide present in the
mixed acid product stream.
~
When the flow rate of water to the reaction zone
is such as to produc~ a substantially fixed total acid
normality of mixed acid product stream, an increase in
the flow rate of chlorine such as to increase the
partial pressure of chlorine at the outlet of the
reaction zone alters the concentration of dissolved
gases in the mixed acid outlet stream in a linear
manner.
~ or example, at a mixed acid strength of 6.5 N,
considered as the total acid normality of the mixture
of hydrochloric acid and sulphuric acid, a partial
pressure of chlorine at the gaseous outlet of l00 mm Hg
results in a dissolved sulphur dioxide concentration of
l gpl in the mixed acid, while a partial pressure of
chlorine of about 150 mm Hg results in a dissolved
sulphur dioxide concentration of 0 gpl. As the
chlorine partial pressure increases further, the mixed
acid has a dissolved chlorine concentration which
increases with incrèased chlorine partial pressure.
Even such a small amount of dissolved SO2 as l gpl
has an adverse effect on the chlorine dioxide-producing
reaction, in terms of decreased efficiency and, in the
case of the presence of dichromate ions, in terms of
settlability and recoverability of by-product sodium
sulphate In accordance with this invention, the
dissolved concentration of sulphur dioxide is
controlled at 0 gpl by suitable control of the partial
pressure of chlorine at the gaseous outlet from the
reaction zone.
The presence of dissolved chlorine in the mixed
acid feed does not appear to adversely affect the
chlorine dioxide-producing process, but nevertheless,
other than a very minor amount to ensure the absence of
dissolved sulphur dioxide, dissolved chlorine usually
is avoided, since such chlorine merely constitutes a
dead load on the system, in that it leaves the
generator zone with the gaseous products of reaction
and i.s recycled to the sulphur dioxide-chlorine-water
reaction zone in the chlorine feed stream.
. ,; . ~
'7~)f~6
The partial pressure of chlorine required to
achieve an a~sence of dissolved sulphur dioxide from
the mixed acid product increases in linear manner as
the total acid normality increases. In other words,
the partial pressure of chlorine at the outlet varies
linearly with the total acid normality of a mixed acid
from which diss~lved sulphur dioxide is absent.
For example, a mixed acid stream of 6.5 N total
acid normality and 0 gpl dissolved SO2 is produced at
150 mm Hg partial pressure of gaseous chlorine at the
gaseous outlet while a mixed acid stream o~ 14 N total
acid normality and 0 gpl dissolved SO2 is produced at
550 mm Hg parti.al pressure of gaseous chlorine.
In view of the linear relationships noted above,
the partial pressure of chlorine at the gaseous outlet
of the reaction zone is readily, reliably and
reproducibly used to control total acid normality and
concentration of dissolved gases in the mixed acid.
The invention is described further, by way of
illustration, wit~ reference to the accompanying
drawings, in which:
Figure 1 is a schematic flow sheet of one
embodiment of the method of the invention,
Figure 2 is a graphical representation of the
variation of dissolved gas content in mixed acid with
ch~orine partial pressure at fixed total acid
normality; and
Figure 3 is a graphical representation of the
variation of acid normality with chlorine partial
pressure at zero percent dissolved sulphur dioxide in
the mixed acid.
Referring to Figure 1, chlorine dioxide is formed
continuously in accordance with the process of U.S.
Patent No. 3,864,456 in a chlorine dioxide generator
10. Chlorine dioxide, chlorine and steam are formed in
the generator 10 as the gaseous products of reaction
are continuously removed by line 12. Anhydrous neutral
sodium sulphate is also formed in the generator 10 as
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7~ 316
the solid product of xeaction is continuously or
intermi-tently removed by line 14.
The generator 10 holds an aqueous acid reaction
medium containing chlorate ions which are continously
fed thereto in the form of a sodium chlorate solution
by line 16. The sodium chlorate solution fed by line
16 may take the form of cell liquor, in which case the
feed stream also contains sodium chloride. The
reaction medium is maintained at its boiling point
under a subatmospheric pressure and has a total acid
normality of about 2 to about 4.8 normal. The acid is
provided by a mixture of sulphuric acid and
hydrochloric acid, continuously fed to the generator by
line 18.
The gaseous mixture of chlorine dioxide, chlorine,
steam and air resulting from bleed air stream 19 is
forwarded, usually after an initial cooling to condense
at least a major proportion of the steam in the stream
in a cooler (not shown), to a chlorine dioxide absorber
20 to which water is fed by line 22 to dissolve the
chlorine dioxide therefrom and form a product solution
stream of chlorine dioxide solution in line 24. ~ome
of the chlorine contained in the gaseous mixture in
line 12 is also dissolved in the chlorine dioxide
solution.
?S The residual gas stream in line 26 is forwarded to
an air ejecter 27 whereby the vacuum is drawn for the
generator 10. The chlorine and air stream, now at
substantially atmospheric pressure, is forwarded by
line 28 to a reaction zone 30 wherein the chlorine,
supplemented, if required, by external source chlorine
in line 3~, is reacted with sulphur dioxide fed by line
34 and water fed by line 36.
The reaction zone 30, in the illustrated
embodiment, comprises a primary reactor 38 and a
tail-gas reactor 40. The chlorine feed stream 28 and
the sulphur dioxide feed stream 34 are directly fed to
the primary reactor 38 for reaction with a weak acid
solution in line 42 emanating from the tail-gas reactor
40 to form an a~ueous mixture of hydrochloric acid and
sulphuric acid of desired strength.
The primary reactor 38 may comprise a falling film
absorber while the secondary reactor 40 may comprise a
packed tower. A reaction zone 30 for the mixture of
sulphur dioxide, chlorine and water utilizing this
combination is described in copending Canadian Patent
~pplication Serial No. 388,780 filed October 27, 1981.
As is set forth in more detail therein, in a
falling film reactor, the water constitutes the falling
film and the chlorine and sulphur dioxide gases are
readily absorbed by the aqueous phase for reaction.
Integral cooling passages in the falling film absorber
enable the exothermic reaction to be controlled by the
passages of a cool heat exchange medium, usually water,
therethrough.
As the gases pass through the falling film
absorber and reaction with the water occurs, the
partial pressure of chlorine and sulphur dioxide in the
gaseous phase decreases, -thereby resulting in a
decrease of the mass transfer rate of the gases to the
liquid phase. Accordingly, for an increasing
proportion of the sulphur dioxide and chlorine to
react, an increasing reactor volume must be employed.
The un:reacted gases, along with the air present in
line 28, are forwarded from the falling film primary
reactor 38 by line 48 to the packed tower tail gas
reactor 40, wherein the remainder of sulphur dioxide is
reacted with chlorine and the water fed by line 36 to
form a weak mixture of hydrochloric acid and sulphuric
acid, which passes by line 42 to the primary reactor
38. The proportion of unreacted gases passing from the
primary reactor 38 to the secondary reactor 40 may vary
widely, depending on a balance of falling film reactor
volume and tail gas reactor volume. Since the tail gas
reactor 40 relies for cooling on the volume of water
fed thereto and reactor size, it is usual for at least
a major proportion of the reaction to be effected in
gl.~
the falling film reactor 38, usually at least about 75%
of the reaction and typically about 80%.
The aqueous mixture of hydrochloric acid and
sulphuric acid which results from the reaction zone 20
is forwarded by line 44 to the acid feed line 18.
i Additional quantities of sulphuric acid and
hydrochloric acid required to maintain the
stoichiometry of the xeactions occurring in the
generator 10 at the prevailing chlorine dioxide
efficiency, are added by line 46, as described in
detail in U.S. Patent No. 4,086,329.
The total acid normality of the aqueous mixture in
line 44 is determined by the relative flow rates of
sulphur dioxide, chlorine and water to the reaction
zone 30 and is preferably is about 7 to about 9 normal
typically about 8 normal, because of evaporative
requirement considerations of the generator 10. In the
generator 10, it is preferred to maintain a
substantially constant volume of reaction medium under
continuous operation conditIons, which necessitates
boiling off of water entering and being formed in the
generator. Assuming that other sources of water to the
generator remain the same, as the strength of the mixed
acid in line 44 increases, the volume of water required
to be evaporated decreases. However, dS the volume of
water decreases, the production rate of chlorine
dioxide declines. As the strength of mixed acid
decreases, the volume of water required to be
evaporated increases and hence the external heat
requirement increases.
,o As mentioned above, the total acid normality is
preferably about 7 to about 9. At total acid
normalities below about 7 normal, the evaporative heat
requirements increase dramatically and below about 6
normal are a signific2nt economic burden. At total
acid normalities above about 9 normal, the volume of
water required to be evaporated decreases significantly
producing a significantly decreased production rate~
When water from another source can be fed to compensate
. .
~:~7~ 6
for the decreased volume present in the mixed acid,
the acid strength may range up to about 14 normal.
The evaporative load on the reaction medium
usually is such as to produce a weight ratio of steam
to chlorine dioxide in the product gas stream of about
7:1, although, based on the strength of the mixed acid
feed and the volume of water from other sources, the
weight ratio may vary from about 4:1 to about lO:l.
In accordance with this invention, the reactions
effected in the reaction zone 30 are carried out in the
presence of excess chlorine gas, the unreacted
chlorine gas exiting the tail gas reactor 40 and hence
the reaction zone 30 along with the air by line 50.
The chlorine present in the latter stream may be
removed by contacting the same with sodium hydroxide
solution, before venting the air through an exhaust
fan.
The reaction of sulphur dioxide, chlorine and
water effected in the reaction zone 30 is controlled in
this invention so as to avoid the presence of dissolved
sulphur dioxide in the mixed acid s~ream in line 44.
This control is achieved by controlling the partial
pressure of chlorine in line 50, which, in turn, is
controlled by the chlorine feed in line 32 and/or the
air feed in line 19 and/or the sulphur dioxide feed in
line 34. As the strength of the mixed acid stream
which contains no dissolved sulphur dioxide increases
with increasing quantities of sulphur dioxide for the
same water volume, then the excess of chlorine required
increases, so that the partial pressure of chlorine in
line S0 also increases.
The procedure descrlbed above with respect to
Figure l enables the procedure of U.S. Pz,tent No.
4,086,329 to be utilized without the difficulties which
result from the presence of dissolved sulphur dioxide
in the mixed acid stream and with a mixed acid stream
of controlled total acid normality.
The inventlon is illustrated further by the
following Examples:
`,. .~
~,
1 lL'^'(~16
Example 1
This Example illustrates the detrimental effect of
the presence of dissolved sulphur dioxide on a chlorine
dioxide generating process.
(a) A 16 ton/day chlorine dioxide generator
contained a boiling reaction medium having a total acid
normality of 3.5 N, a temperature of 70C and a
subatmospheric pressure of 190 mm Hg. A gaseous
mixture of chlorine dio~ide, chlorine and steam was
removed from the generator. ~
Cell li~uor containing 490 gpl NaClO3 and 110
gpl NaCl was fed to the generator at a flow rate of 10
USGPM. A mixture of hydrochloric acid and sulphuric
acid having a total acid normality of 4 N and
containing 2 N HCl and 2N H2SO4, was also fed to the
generator at a f]ow ra e of lOUSGPM. The reaction
medium had an orange color as a result of the presence
of dichromate ion fed thereto with the cell liquor.
After start up, the reaction medium became
saturated with anhydrous s~dium sulphate which was
removed from the generator in a slurry with reaction
medium, filtered to separate it from the reaction
medium, and the reaction medium was returned to
reaction vessel by a circulatory pump.
While the generator ~as running normally in this
fashion, the mixed acid feed stream was changed to one
containing sulphur dioxide in a dissolved concentration
of 18 gpl for a period of three hours. This resulted
in the reaction medium turning brown, the density of
the reaction medium rlsing from 1.5g/cc to 1.7g/cc, the
circulating pump amperage rising from 58 to 64 amps,
the chlorine dioxide production rate rising
dramatically from 16 to 24 TPD and severe
decompositions of chlorine dioxide. After complete
shut-down of the generator necessary because of the
severe decompcsitions, the crystals were examined and
it was found that, while normally 90 to 95~ of the
crystals are retained on a 200-mesh screen, only 40 to
7~ 16
50% of the crystals were retained on the screen,
indicating a substantial diminution of crystal size.
(b) The experiment set forth in l(a) above was
repeated except that the concentration of dissolved
S2 in the mixed acid stream was 0.1 to 0.2 gpl.
S After only about 15 minutes, the orange liquor started
to turn brown, the circulating pump amperage rose and
the crystals settled much more slowly.
The results of Examples l(a) and l(b) demonstrate
the significantly adverse results which are obtained in
lQ a very short period of time when dichromate ion is
present in the reaction medium and dissolved SO2 is
present in the mixed acid feed, even, as in the case of
Example l(b), when present in very small quantities.
(c) A 10 litre laboratory scale generator was set
up containing a boiling reaction medium having a total
acid normality of 3.8 N, a temperature of 71C and a
subatmospheric pressure of 170 mm Hg. A gaseous
mixture of chlorine dioxide, chlorine and steam was
removed from the generator. ~
To the reaction medium were fed a 5.75 molar
stream of sodium chlorate at a rate of 6.7 ml/min, a
300 gpl stream of sodium chloride at a rate of 6.7
ml/min and an 18 N stream of sulphuric acid. The
chemical efficiency of chlorine dioxide production was
91~ based on conversion of sodium chlorate to chlorine
dioxide.
Thereafter, sulphur dioxide was fed to the
reaction medium at a rate of 1.8 g/min, equivalent to
an amount of 200 gpl of sulphur dioxide if dissolved in
a mixed acid feed. The efficiency of chlorine dioxide
production fell to only 73.8% based on the conversion
of sodium chlorate to chlorine dioxide.
Although the amount of equivalent dissolved
sulphur dioxide in this experiment was abnormally high,
3S it was used to demonstrate that the presence of sulphur
dioxide in the chlorine dioxide generator produces a
significant decrease in production efficiency. Lesser
amounts of dissolved sulphur dioxide produce lesser
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7~ 6
detrimental effects on efficiency, but any decrease in
efficiency presents a considerable economic penalty
since the primary source of the chlorine dioxide,
namely sodium chlorate, is an expensive chemical.
Example 2
This Example illustrates the process of the
nvention.
(a~ The reaction zone 30 illustrated in Figure 1
was set up. The reaction of sulphur dioxide, chlorine
and water was carried out to form a mixture of
hydrochloric acid and sulphuric acid. The following
~able sets forth the flow rate data for one specific
operation.
Table I
Stream Line No Feed Rate
15 Chlorine feed Line 28 1245 pph + 300 pph air
Sulphur dioxide
feed Line 34 780 pph
Water feed Line 36 ~9.8 USGPM
HCl/H2SO4 Line 44 ll.0 USGPM of 8 N acid
(free from dissolved S02)
Tail gas stream Line 48 117 pph SO2, 510 pph C12,
300 pph air
Weak acid str~am Line 42 10 USGPM of 1.5 N ccid
Chlorine vent gas
25 stream Line 50 380 pph Cl2, 300 ~ph air
(ie. 252 mm Hg Cl2 pp)
(b) The experiment set forth in Table I was
repeated with the partial pressure of chlorine in line
50 being varied to determine the effect thereof on the
dissolved concentration of gases in the mixed acid
stream in line 44 at a fixed total acid normality of
the mixed acid stream. The flow rates were adjusted to
provide a total acid normality of stream 44 of 6.5
normal.
The identity and concentration of dissolved gases
were determined for each value of chlorine partial
pressure. The results were plottcd graphically and
appear as Figure 2 of the accompanying drawings. As
~ 1'7()bl16
14
can be seen from Figure 2 the relationship between C12
partial pressure and dissolved gas concentration at
fixed acidity was determined to be a substantially
linear one. As the partial pr~ssure increased, the
dissolved concentration of sulphur dioxide decreased to
a zero value at about 150 mm Hg pp C12 and, as the
partial pressure increased further, chlorine is an
increasingly greater dissolved concentration was
present in the mixed acid.
(c) The experiment set forth in Table I again was
1~ repeated with the partial pressure of chlorine in line
50 again being varied but this time to determine the
upper limit of the total acid normality of stream 44
which still had zero dissolved sulphur dioxide
concentration in the mixed acid stream.
The partial pressure of chlorine necessary to
produce a varying strength of- mixed acid which had zero
dissolved sulphur dioxide was determined for each value
of acid strength. The results were plotted graphically
and appear as Figure 3 of the accompanying drawings.
As may be seen from Figure 3, the relationship between
C12 partial pressure and total acid normality at zero
concentration of dissolved sulphur dioxide was
determined to be a substantially linear one. As the
partial pressure increased from about 1~0 mm Hg to
about 550 mm Hg, the total acid normality o the mixed
acid stream which could still be produced with zero
dissolved sulphur dioxide was increased from 6.5 to 14
normal.
In summary of this disclosure, the present
invention provides procedures for controlling the
characteristics of a mixed hydrochloric acid and
sulphuric acid feed stream used in chlorine dioxide
production by controlling the partial pressure of
chlorine taking part in reaction with sulphur dioxide
and water to form the mixed acid. Modifications are
possible within the scope of this invention.
