Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~9~l3~
HIGH EFFICIENCY PRODUCTION OF CHLORINE
DIOXIDE
. .
The present invention relates to the production of
chlorine dioxide at high e~ficiency.
In U.S. Patent No. 4,081,520, assigned to the
applicants of this application, there is described a pro-
cess for the production of chlorine dioxide at high e~ficiency
using sodium chlorate, sulphuric acid and methanol. The
mechanism whereby chlorine dioxide is formed is that chlorine
which is coproduced with the chlorine dioxide is reacted with
the methanol to form chloride ions which then reduce the
chlorate ions to form chlorine dioxide and chlorine. The
overall reaction may be represented by the following equation:
2NaCl03 ~ 2H2S4 + CH30H - 2Cl02 + 2NaHS04 + HCHO + 2H20
1~ The reaction medium from which the chlorine dioxide
is formed and which contains sodium chlorate, methanol and
sulphuric acid is maintained at its boiling point, generally
in the range of about 50 to about 85C, under a subatmospher-
ic pressure. The evaporated water serves to dilute the
chlorine dioxide for removal from the reaction zone. The
reaction medium has a high total acid normality in excess of
about 9 normal and the by-product deposited from the reaction
medium once saturation is reached after start up is a sodium
acid sulphate, which may be sodium bisulphate ~NaHS04) or
sodium sesquisulphate (Na3H(S04)2).
The process of the prior patent is highly efficient
in terms of the conversion of chlorate ions to chlorine
dioxide. Close to 100% efficiency is attained and the chlor-
ine dioxide is removed from the reaction zone vir-tually uncon-
taminated by chlorine, which may be beneficial in manyinstances of end use of the chlorine dioxide.
The drawbac~ to this prior art process which hereto-
fore has inhibited commercial implementation thereof is the
form and nature of the solid by-product. Since the sodium
sulphate precipitates in an acid form, rernoval o this
material from the reaction zone results in a loss of the acid
Y
~9~3~
values therein. In addition, sodium acid suiphates are
difficult to handle ph~sically and exhibit deliquescence.
The present invention enables this prior art
problem to be overcome by metathesizing the solid phase sodium
acid sulphate in a unique manner to form solid phase neutral
sodium sulphate and recovering the acid values for recycle to
the reaction zone.
Although the present invention is particularly
described herein with respect to the conversion of sodium
lO acid sulphate formed in the procedure of the aforementioned
U.S. Patent No. 4,081,520 to a neutral sodium sulphate, the
present invention is broadly applicable to the conversion to
a neutral sulpha~e of a solid acid sulphate recovered from
any high acidity sulphuric acid based chlorine dioxide gener-
15 ating process in which the reaction medium is maintained atits boiling point under a subatmospheric pressure and in
which the acid sulphate precipitates from the reaction medium
in the reaction zone.
For example, the process of the invention is applicable
to the conversion of sodium acid sulphate by-product recovered
from a chlorine dioxide generator wherein added chloride ions
are used as the reducing agent for the chlorate in the
presence of hi~h acidity sulphuric acid, as described in
Canadian Patent No. 825,084, to the assignee of this
application.
Further, the chlorine dioxide generatin~ process
producing the sodium acid sulphate may be one in which
sulphur dioxide is used to reduce the coproduced chlorine
to chloride ions, in analogous manner to the methanol in the
30 process of U.S. Patent No. 4,081,520 as discussed above.
The use of sulphur dio~ide is descxibed in U.SO Patent
No. 3,233,~88, assigned to Hooker Chemicals & Plastics
Corporation.
In the process of the present invention, the solid
35 phase acid sulphate removed from the reaction zone is contac- ;
ted with water in the presence of a water-soluble alcohol or
ketone, preferably methanol, to form solid phase neutral
sul hate and to recover the sulphuric acid.
i ..1~,
~9~L3~
- There has been a prior suggestion to utilize water
and methanol or other water-soluble alcohol or ketone to
convert sodium acid sulphate to neutral sodium sulphate. Such
procedure is described in U.S. Patent No. 4,104,365 in the name5
S of Howard and T^~ley. The p~x~ss of the latter patent is directed to the
recovery of neutral ~um sulphate fram the liquld phase effluent from
high total acid normality sulphuric acid based ~hlorine di-
oxide producing processes. Su~h processes do not operate at
the boiling point of the reaction medium and do not crystallize
10 the by-product sodium sulphate from the reaction medium in
the reaction vessel. The starting material in this prior art
process is an aqueous solution of the so~ium acid sulphate,
in contxast to the solid phase starting materia} used in this
invention~
lS The prior art procedure involves an initial strip-
ping step which is said to be required to remove dissolved
gases and residual sodium chlorate which otherwise inhibit
the reaction. Such an operation is not required for the
process of this invention since the starting material is
20 solid phase sodium acid sulphate produced in the chlorine
dioxide generating reaction zone.
The process of the present invention is distinguish
ed from the prior art procedure not only on the ground that
the starting materials are in different physical forms but
25 also since the volumes of wate.r and methanol required to be
added to the aqueous effluent to form solid phase neutral
sodium sulphate in the prior art process axe very much greater
than the volumes used in the process of this inventlon. As a
consequen e of these large volumes of water and methanol, con-
30 siderable evaporation is required in the prior art process,first of all to recover the methanol used and secondly to con-
centrate the aqueous sulphuric acid to a concentration which
is suitable for reuse in the generator.
In the process of the present invention, the weight
35 ratio of water to sodium acid sulphate (calculated as
Na3H(SO4~2) is about 0.4:1 to about 1.4:1, preferably about
O.6:1 to about 0.8:1. Ratios of water to sodium acid sulphate
within the reclted range are critical to the process of the
~ 31
present invention, in that less than an 0.4:1 weight ratio
leads to only poor conversions of acid sulphat~ to neutr~l
sulphate while greater than a 1.4:1 weight ratio leads to
large quantities of sodium sulphate being dissolved in the
5 aqueous phase. By using weight ratios within this critical
range, the aqueous phase which results contains sulphuric acid
of a sufficient acid normality to permit recycle thereof to
the chlorine dioxide generating process without concentration.
In contrast, in a typical operation according to the
10 process of U.S. Patent No. 4,104,365 reLerred to above, the
weight ratio of water to sodium acid sulphate 5considered as
Na3H(SO4)2) is about 1.83:1 and the weight of water re~uired
to be removed to enable the sulphuric acid solution to be re-
used is about 3.61b. per lb. of neutralsodium sulphate re-
15 covered (or about 4.14 lb. per lb. of cio2 formed).
The weight ratio of methanol to sodium acid sulphate(calculated as Na3H(S04)2) is less critical than the water
weight ratio and may vary up to about 2:1. The weight ratio of
methanol to sodium acid sulphate is preferably about 0.3:1 to
20 abou~ 0.8:1 for the preferred weight ratio of water to sodium
and sulphate referred to above. In view of its miscibility in
water, the presence of the methanol decreases the volume o~
water in which the sodium sulphate is able to dissolve and
hence inhibits dissolution of the neutral sodium sulphate in
25 the aqueous phase.
As the weight ratio of methanol increases, the pro-
portion of neutral sodium sulphate dissolved in the aqueous
phase decreases until a weight ratio of methanol is reached
beyond which further quantities of methanol do not increase
30 the yield of solid phase neutral sodium sulphate, the yield
limit being about 80 to 85 wt.~. No benefit, therefore, is
gained by increasing the weight ratio of methanol to sodium
acid sulphate beyond about 2~
When the preferred process of the invention i5 '
35 adopted, namely, when the chlorine dioxide is produced by the
process of the above-mentioned U.S. Patent No. 4,081,520, a
proportion of the aqueous phase resulting frcm the metathesis
may be recycled directly to the chlorine dioxide producing
reaction medium to provide at least part, preferably all, of
93L31
the methanol requirement thereof. This recycle stream also
provides part of the sulphuric acid requirement of the chlorine
dioxide-formin~ process. Methanol which is recycled to the
reaction medium in this way does not require to be recovered
5 from the recycled proportion of the aqueous phase. Methanol
is removed from the remainder of the aqueous phase to provide
a sulphuric acid solution suitable for recycle to the genera-
tor.
The quantities of methanol used in this invention
10 contrast markedly with those usea in the prior art procedure
of U.S. Patent No. 4,104,365 wherein typically a weight rati~
of methanol to sodium acidsulphate (consiaered as Na3H(SO4)
of about 9.33:1 is used and must be recovered for reuse and
so that the sulphuric acid solution can be concentrated to an
15 acid normality suitable for recycIe to the chlorine dioxide
producing process.
The steam reguirement of the process of the present
invention for evaporation of the aqueous phase is limited to
that required to strip methanol and in a typical preferred
20 embodiment of the invention gives rise to a cost of about
$2.50 per ton of chlorine dioxide produced (calculated at a
cost of ~3 per 1000 lb. of steam). This steam requirement
is substantially less than for the process of U.S. Patent No.
4,104,365 wherein heat is required to strip substantial quan-
25 tities of methanol and to concentrate the sulphuric acid solu-
tion and in a typical embodiment thereof gives rise to a cost
of about $35.00 per ton of chlorine dioxide produced, i.e.
nearly fifteen times the cost of steam required for the pro-
cess of this invention.
Usually the water and methanol are added to the
solid phase sodium acid sulphate as a solution containing the
required proportions of wat~r and methanol. Since, however,
the role of the methanol is exclusively to effect the decrease
in solubility of the neutral sodium sulphate in the aqueous
35 phase, the methanol may be added after initial addition of
the water.
The above description of the process of the inven-
tion has been made with respect to the use of-methanol, in
view of the ready availability of the solvent and ~he effec-
9~
tiveness of the solvent in the process of the invention.
Other water-soluble alcohols and ketones, however, may be
used, if desired, for example, methanol, n~propanol, iso-
propanol and acetone.
The metathesis reaction used in this invention
may be effected over a wide range o~ -temperatures, us~ally
from about 10 to about 70C. The reaction proceeds effective-
ly at room temperature labout 20 to 25C) although elevated
temperatures usually are preferred as the rate of reaction
10 increases with increasing temperature. Preferably, the
temperature is in the range of about 20 to about 50C.
The metathesis reaction of the present invention
may be effected in any convenient manner. Although a batch
operation may be effected, continuous operation is preferred
~5 since the process of the invention is associated with a
continuous chlorine dioxide producing process.
The metathesis may be effected in a simple reaction
vessel or in a decantation-washing column, such as is described
in detall in Canadian Patent No. 1,117,731 to the applicant
20 herein.
Intermixing of the water-methanol solution with
the sodium acid sulphate to effect the metathesis reaction
may be assisted by stirring in a reaction vessel. Although
stirring speeds up the mass transfers involved in the metathesis,
25 high shear is unnecessary and gentle stirring only need
be used, although consuming a longer period of time.
The reaction time required for completion of the
metathesis may vary widely, and usually is from about 10 '
minutes at high stirring to about 60 minutes in a decantation
30 washer.
The invention is described further, by way of
illustration, with reference to the accompanying drawing
which is a flow sheet illustrating one embodiment of the
invention.
Referring to the drawing, a chlorine dioxide genera-
,.......
., ~
tor 10 produces a gaseous mixtuxe of chl~rine dioxide and
stPam in line 12 from which chlorine dioxide is absorbed into
.water to provide an aqueous solution thereof for utilization
in bleaching wood pulp or any other desired end use.
The generator 10 produces the chlorine dioxide in
accordance with the procedure of the aforementioned U.S. Patent
No~ 4,081,520 from sodium chlorate solution fed to the genera-
tor 10 by line 14, sulphuric acid fed to the generator 10 by
line 16 and methanol fed to the generator 10 by line 18.
The aqueous reaction medium, which has a t~tal acid
normality of greater than about 9, is maintained at its boil-
ing point below a temperature above which substantial decom-
position of chlorine dioxide occurs, usually in the range of
about 30 to about 85C, under a subatmospheric pressure
15 corresponding to the boiling point, usually in the range of
about 20 to about 400 mmHg, and sodium acid sulFhate oontinuously
precipitates from the reaction medium once the reaction medium
reaches saturation after start up~
The volume of the reaction medium in the generator
20 10 is maintained substantially constant by balancing the
volume of aqueous phase entering the generator 10 with the
volume of water evaporated from the reaction medium to form
the gaseous product stream 12 and the volume of-water removed
as slurry medium for the solid phase sodium acid sulphate.
The sodium acid sulphate precipitated ~rom the reac-
tion medium in the generator 10 is forwarded by line 20 to a
reactor 22. The sodium acid sulphate, usually sodium sesqui-
sulphate, may be removed from the generator 10 in the form of
a slurry with reaction medium and separated therefrom by fil-
30 tration prior to passage to the reactor 22.
In the reactor 22, the sodium acid sulphate is con-
tacted with water fed by line 24 and methanol fed by line 26.
The weight ratio of water to sodium acid sulphate (calculated
as Na3H(SO4)2) in the reactor is about 0.4:1 to about 1.4:1,
35~preferably about 0.6:1 to about 0.8:1. The weight ratio of
methanol to sodium acid sulphate ~calculated as Na3H(SO4)2)
is up to about 2:1, preferably about 0.3:1 to about 0.8^1.
The temperature of the media contacting the solid
phase sodium acid sulphate in the reactor 22 is preferably
. ~ ,. . . ~ . .
3~L
about 20 to about 50C and the metathesis reactlon pro-
duces solid anhydrous neutral sodium sulphate. The meta-
thesis of the sodium acid sulphate produces sulphuric acid
in addition to the neutral acid sulphate. The neutral sodium
5 sulphate and the aqueous phase are separated in any convenient
manner, such as on a filter, and the neutral sodium sulphate
is removed by line 28, for utilization as desired, typically
to make up sodium and sulphur values in a pulp mill with
which the chlorine dioxide generator 10 is associated.
The aqueous phase, containing sulphuric acid~
methanol and some dissolved sodium sulphate, is removed by
line 30 and is split into two streams, with typically approxi-
mateIy one-third of the volume of the aqueous phase being re~
cycled by line 32 to the methanol feed line 18 to the chlorine
15 dioxide generator so as to provide at least part, preferably
all, of the methanol requirement of the generator lO, any
remainder of such requirement being fed by line 33 to the
methanol feed line 18. The sulphuric acid content of the
aqueous phase in line 32 provides part of the sulphuric acid
20 requirement of the reaction medium in the generator 10.
The remaining typically approximately two-thirds of
the volume of the aqueous phase is forwaraed by line 34 to a ~,
methanol stripper 36 wherein the methanol is removed from the
aqueous phase. The methanol vapor is passed by line 38 to a
25 condensor 40 to result in liquid methanol, which is forwarded
by line 42 to the methanol feed line 26 to the metathesis reac
tor 22, the balance of the methanol requiremint of the reaator 22 being fed
by line 44.
The methanol-depleted sulphuric acid solution is re-
30 cycled by line 46 to the sulphuric acid feed stream for thechlorine dioxide generator 10 in line 16. Additional sulphur-
ic acid requirement is fed by line 48 to the sulphuric acid
feed line 16.
The process described above with respect to the
35 drawing, therefore, produces chlorine dioxide wHich is essen-
tially chlorine-free at high efficiency without the necessity
for the addition of any catalytic species to the reaction
medium. At the same timel the by-product sodium sulphate is
obtained in a neutral, preferably anhydrous neutral form,
13~
so that no sulphuric acid ls lost from the system with th~ by-
product. The system uses one of the reactants, namely,
methanol, in the conversion of the acid sulphate to the neutral
sulphate.
The invention is illust~ated fw~er by the follcwIng e~les;
Example 1
This Example illustrates the preparation of sodium
acid sulphate in accordance with the procedure of U.S. Patent
No. 4,081,520.
A chlorine dioxide generator was run to form chlor-
in~ dioxide from sodium chlorate, sulphuric acid and methanol.
The reaction medium was held at its boiling point under a sub-
atmospheric pressure and sodium sesquisulphate deposited from
the reaction medium. The operating parameters are set forth
15 in the following Table I:
TABLE I
Operating conditions:
- temperature 74C
- pressure 135 mm Hg
Reac~an_s concentration and feed
rate:
- MeOH 33%, 3.4 ml/min.
~ H2S4 9M, 3.6 ml/min.
NaClO3 6.74M, 10.5 ml/min.
Generator liquor concentrations:
2 4 9.3N
- NaClO3 l.lM
Crystal Na3H(So4)2
Chlorine dioxide production:
- rate 0O48 g/l/min.
- efficiency based on
chlorate ~ 99%
- gas analysis C1O27 99%~ C12~1%
Example II
This Example illustrates metathesis of sodium sesqui-
sulphate.
A series of experiments was conducted in which 100g
samples o solid sodium sesquisulphate were contacted with
water and/or me~ l under a variety of conditions. In each
13~
25 case, the weight of sodium sulphate recovered, the proportion
of sulphuric acid remaining in the svdium sulphate and the
normality o~ the aqueous phase at the end of the experiment
were determined. The results are reproduced in the following
Table II:
~,
. ~ . .
3~
o
. ~ . a~ r 1` ~ ~ o ,1 o ~ o o ~ ,~ a~
dP
~r~
O ~ ~ O ~ ~
Z ~-- I I C5~ 1 1 1 1 1 1 1 ~ I ~ ~ ~1 N r~l I I I
0-~ ~
~.5 ~ o o
H
.~ t` 0300 ~ ~100 ~'0000 Cl ~10000 C~
.
~ U o O O O O o O O N O ~ Lrl ~ O O 1` r` u~ U') U~
'~
O-~ -OOOOOOOC~ OO~
O ~i N ~`1 ~ ~`1 ~
',
O
.. .. o .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
~
Z; o o o o o o o o o o ~ ~ ~I o o _I o ~ ~J
~ ~ :
O -1 N ~~ U l ~ 1` CO ~ o ~ ~ O
~i . --I ~1 ~ N
.
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12
,~
_. o ~ ~ ~ ~ U~
N
~ O ~
d'
5~Id . ~ ........... .....
~ Z. ~ o O ~ 1 oo O O O O O O O O N ~ ~ N O -1
d~
.
-~
u u~
.
u~ O O O O O C~ v o
~ N N ~J N ~ ~r ~ ~p ~ ~ r-l r--1 ~ n
- ~
3~
- ~I N ~ ~ 1'-- N cr ~ (0 CO O ~ N N Nl IY N ~
O C~ o O O o o o o o C~ ~1 ~ O O ~ O O C~:
'~ ~
11 Z
~: --l o o o o o o o o ~ o a s~ o o c~ D C~
J~ ~ a~ O ~I N ~ ~r U~ 0
N ~ ~ ~ N t~
~ ~
0~ u~
~r
oP o o o o o
' 1 1 0 .,~
o~
V ~ a~
,
3~
~ ~ ~r ~ ~ ,
~: C~ o ~ o o
~ o
~' OOOOO
. ~I N tr) ~ u~
1~ ~
14
Notes:
1. A yield of 81.3 g of Na2SO~ is attained at 100%
conversion.
2. Calculated for liberated H2SO4 in aqueous phase.
S 3. SM means slow magnetic stirring just sufficient to
form a vortex~
4. - means that the quantity of sodium sulphate was not
determined.
5. FM means fast magnetic stirring sufficient to cause
splashing.
6. In this experiment NaHSO4 was convert~d to Na3H~SO4)
7. 6.4 N actually reached due to incomplete metathesis~
8. HP means hand pouring from one beaker to anQther
steadily for the "stirring time".
15 ~. 5.6 N actually reached due to incomplete metathesis.
10~ 5.8 N actually reached due to incomplete metathesis
11. 6.8 N actually reached due to incomplete metathesis.
12. 6.5 N actually reached due to incomplete metathesis.
13. 5.7 N actually reached due to incomplete metathesis.
20 14. 5.1 N actually reached due to incomplete metathasis.
15. 6.Q N actually reached due to incomplete metathesis.
16. 16.7 ppm Cr ~VI~ added to liquor.
17. 167 ppm Cr CVIl as K2CrO4 added to liquor.
18. 1670 ppm Cr ~VI~ added to liquor.
19. Cr ~III) present in Na3H~SO4)2 - 4~.8 ppm and in
Na2SO4 - 76~2 ppm.
20. ON means overniyht.
21. VS means very slow at approx. 75 rpm.
The results of the above Table II illustrate a
number of points. A comparison of experiments 1 and 2
illustrates that the addition of methanol for additional
volume is not very helpful in the metathesis~ Experiment
3 shows that me~hanol alone is unable to achieve metathesis.
A com~arison of Experiment A with Experiment 2 shows that
increasing the volume o~ water also increases the metathesis
Experiments 6~ 7 and 8 show that little or no metathesis is
attained in the absence of stirring.
Experiments 9 to 14 s~ow the ef~ect of increasing
quantities of water and were used to determine the minimum
: . :
9~3~
weight ratio o~ water to sodium sesquis~lphate required for
complete metathesis. Experiments 15 to 17 show the actual
quantities of sodium sulphate (Na2SO4) recovered using water
only. Experiments 1~ to 21 were usea to determine the
effect of minimal stirring and some or all the acid sulphate
was converted to Na2SO4.
Experiments 22 to 26 illustrate the effect o~ water
and methanol mixtures on metathesis at room temperature
while Experiments 27 to 29 illustrate the effect of such
m~xtures at 48C. Experiments 30 to 33 illustrate the
effect of increasing quantities of methanol on yield o~
Na~;04
Experiments 34 and 35 attempt to determine the
minimum water requirement while Experiments 36 to 39 attempt
to determine the effect of temperature and stirring on
marginal cases~
Experiments 41 to 44 illustrate the effect of
chromium on the pxocess while Experiment 45 shows the
e~fect of prolonged slow stirring.
20 Exam~ple III
This Example illustrates the use of solvents other
than methanol.
The procedure of Example II was repeated except that
acetone and ethanol were substituted for methanol in two runs
25 effected at 19C ~or 10 minutes with slow stirring. The
results are reproduced in the following Table III:
TABLE III
Solvent Weight Ratio Weight Ratio ~t. of 5 H2S04 No~E~ity of
2 3 ( 4~2 ven : Na2SO~ in Aqueous Phase
Na3H(.SO4)2 Recovered Na2S04
Acetone 0.75:1 0.4:1 62.4 l.S 5.1
Ethanol 0.75:1 0.4:1 75.9 0.13 5.1
The results of the above Table III illusbrate the
3s utility of acetone and ethanol in the metathesis reaction.
SUMMARY OF DISCLOSURE
. _
In summary of this disclosure, the present invention
provldes a highly efflcient chlorine dioxide generating pro-
.. ... ~ _... . .. ..
613~3~ '
cess in which chlorine dioxide uncontaminated by chlorine isformed and neutral sodium sulphate by-product is produced.
Modifications are possible within the scope of this invention. :'
,
.
.
