Note: Descriptions are shown in the official language in which they were submitted.
- 1 - 12~ 2
METHOD OF OXIDIZING SULFIDE-CONTAINING LIQUO~
This inven-tion relates generally to a method of oxidizing
sulfide-containing aqueous solution and, more specifically, to a
method of treating a sodium sulfide-containing aqueous solution,
such as a green liquor, white liquor and orange liquor in the
kraft pulping process, for oxidizing the sulfide with oxygen in
the presence of a specific catalyst.
In the kraft pulping process, it is known to be effective
to use a cooking liquor containing polysulfide for the digestion
of wood. In such a pulping system it is indispensable to treat
the spent cooking liquor and to prepare a polysulfide-containing
cooking liquor therefrom. Thus, the spent liquor, which is
called black liquor, is concentrated and combusted. The melt of
inorganic chemicals, which is called smelt, obtained as a
residue from this combustion is dissolved to form green liquor
essentially consisting of sodium carbonate, sodium thiosulfate
and sodium sulfide~ The sodium carbonate is then converted into
sodium hydroxide by causticiziny. The causticized clear liquor
is called white li~uor.
Various processes have been proposed for oxidizing sodium
sulfide solutions with -the oxidation-reduction catalysts.
Desirable catalysts for the oxidation of sodium sulfide should
have the following characteristics:
(1) To have high catalytic activity for the oxidation of sodium
sulfide;
(2) To permit a sufficient amount of oxygen to access to the
catalyst surface;
(3) To have electrical conductivity so that electron transfer
resulting in the oxidation of sodium sulfide easily occurs
(~) To have resistance to alkali;
(5) To be easily recovered from a reactor and regenerated; and
(6) To be available with low costs.
Catalysts proposed in the prior art processes are not
entirely satisfactory. For exampler United States patent No.
4,024,229 issued to Smith et al discloses a process for the
production of sodium polysulfide from sodium sulfide, in which
an aqueous sodium sulfide solution, oxygen and a ca-talyst are
brought into contact with each other to oxidize the sodium
sulfide to sodium polysulfide. The catalyst is particulate
carbon wetproofed with a hydrophobic substance such as
polytetrafluoro-ethylene (PTFE). The object of the wetproofing
treatment is to prevent the catalyst from being flooded by the
reductant (aqueous sodium sulfidel. In the oxidation-reduction
catalyst of Smith et al, the wetproofing treatment should be
conducted so that a portion thereof is encapsulated with PTFE or
the like hydrophobic substance. It is however difficult to
impart desired degree of wetproofing properties uniformly to
particulate carbon. Further, such a treatment tends to decrease
active sites of the catalyst. In addition, the surface of the
catalyst particles tends to be covered with PTFE as a result of
the wetproofing treatment, causing plugging of the catalyst
pores. This prevents the oxidant from being sufficiently fed to
the catalyst surface. Thus, the catalyst disclosed in the Smith
et a] patent fails to meet with the criteria (1) and (2)
described above.
Further, there are proposed a lot of methods in which
sodium sulfide is oxidized using activated carbon powder as a
catalyst. For example, Yoshida et al [Netsusokutei 8(1) 1981,
2-5] report that, in the reaction of an aqueous sodium sulfide
solution having dispersed carbon black powder, surface
functional groups of the carbon black play a part in
accelerating the oxidation of sodium sulfide. From the
industrial point of view, however, such a method in which the
oxidation is effected by bubbling air or oxygen through an
aqueous solution containing sulfides and activated carbon powder
with stirring is not advantageous because the separation and
recovery of the activated carbon powder for reuse encounters
considerable difficulties.
The present invention has been made in the light of the
above problems of the conventional method and contemplates the
2~X
provision of a sodium sulfide-oxidation catalyst which
satisfies all of the above criteria.
In accordance with one aspect of the present
invention there is provided a method for the oxidation
of a sulfide contained in an aqueous solution,
comprising contacting the aqueous solution with an
oxygen-containing gas in the presence of an activated
carbon catalyst which is a particular catalyst having
an average particle diameter of 0.2-4 mm, a pore volume
of at least 0.25 cc/g in pores with diameters of not
smaller than 100 A and a pore volume of at least 35% of
the total pore volume in pores with diameters of not
smaller than 100 A.
In another aspect, the present invention provides a
process of treating an aqueous solution containing
sulfide and suspended solids, comprising the steps of:
(a) flowing said aqueous liquid up through a bed
of granular solids for filtering off the su~pended
solids, whereby a filtrate which is substantially free
of the suspended solids is obtained; and
(b) contacting said filtrate and an oxygen-
containing gas concurrently with an activated carbon
catalyst in a ixed bed to oxidize the sodium sulfide,
said activated catalyst being a particulate catalyst
having an average particle diameter of 0.2-~ mm, a pore
volume of at least 0.25 cc/g in pores with diameters of
not smaller than 100 A and a pore volume of at least 35%
of the total pore volume in pores with diameters of not
smaller than 100 A.
The process according to the present invention is
suitably adopted for the treatment of a sodium sulfide-
containing solution to convert the sulfide into as much
polysulfide as possible or into sodium hydroxide. White
liquor or green liquor in the kraft pulping systems may
be advantageously treated in accordance with the process
of the present invention. The
~ ~ j
~L2~2~2
-- 4 --
resulting sodium polysulflde-containing liquor may be suitably
used as a cooking liquor for polysulfide pulping process. The
sodium hydroxide-containing liquor may be used as caustic liquor
in the bleaching process.
The present invention will be described in detail below
with reference to the accompanying drawings, in which:
Fig. 1 is a cross~sectional, elevational view
diagrammatically showing an up-flow type filtrating device used
in the process according to the present invention;
Fig. 2 is a cross section taken on line II-II in Fig. 1;
Fig. 3 shows the yield of sodium polysulfide with hours on
stream;
Fig. 4 shows the catalytic activity with hours on stream;
Fig. 5 shows the relationship between conversion of sodium
sulfide and selectivity to sodium polysulfide in the oxidation
of aqueous sodium sulfide usiny various oxidation catalysts; and
Fig. 6 shows the relationship beween the paxticle size and
the activity of various catalysts.
The catalyst in the present invention is a particulate
activated carbon catalyst. The par-ticulate activated carbon may
be prepared from a variety of raw materials such as wood chips,
petroleum pitch, coal and palmshells. The particulate catalyst
should have an average particle diameter of 0.2-4 mm, a pore
volume of at least 0.25 cc/g in pores with diameters of not
25 smaller than 100 A and a pore volume of at least 35 % of the
total pore volume in pores with diameters of not smaller than
100 A.
The present inventors have found that in order to effect
the oxidation of sodium sulfide efficiently it is important that
as much oxygen as possible should diffuse into pores of the
catalyst and arrive at active sites, and that, to achieve this
purpose, it is essential that the catalyst should have a large
amount of pores with large diameters. More particularly, the
volume of pores with a diameter of 100 A or more should be at
35 least 0.25 cc/g, preferably at least 0.35 cc/g, and should be at
least 35 % of the total pore volume of the catalyst.
972~l2
A particulate activated carbon catalyst having a
large amount of pores with a diameter of 100 A has
generally a large total pore volume and, hence, becomes
small in bulk density. Thus, it is generally preferred
that the catalyst have a hulk density of 0.5 g/cc or
less.
The average particle size of the particulate
catalyst should be 0.2-4 mm, preferably O.5~2 mm. With
a particulate catalyst having an average particle size
of greater than 4 mm, the diffusion of oxygen to the
active surfaces tends to be inhibited, causing the
lowering of the catalytic activity. While an average
particle size of less than 0.2 mm is preferable with
respect to the diffusion o oxygen, the catalyst with
such a too small average diameter causes problems in
operation of the process on an industrial scale such as
pressure loss in the oxidation reactor.
For the purpose of the present specification, the
pore volume of the particulate catalyst is determined as
follows. The volume of the pores with diameters of
loo A or more is determined from the distribution of
pores with a pore diameter of 35 A or more measured by
means of a mercury penetration porosimeter (Auto Pore
9200TM manufactured by Micrometritics Inc., U.S.A.).
The volume of pores with diameters of lass than 100 ~ is
calculated according to the Cranston-Inkly method from
the isothermal nitrogen-adsorption-desorption curve
measured by an automatic gas adsorption-desorption
device (Sorptomatic 1800TM manufactured by Carlo Erba,
Italy). The total pore volume is a sum of the above two
pores vvlume.
The oxidation of a sulfide-containing solution
according to the present invention is effected by
contacting the solution with an oxygen-containing gas
2~
5a
such as air in the presence of either the particulate
catalyst or the fiber catalyst as described above. The
follow.ing reactions occur when aqueous sodium sulfide is
subjected to such catalytic oxidation:
Na2S + 2 + 2H2 = 2Na2S2 + 4NaOH (1)
2Na2S + 202 + H20 = Na S O + 2NaOH (2)
2Na2S2 + 32 = 2Na2S23
~L~9~212
Polysulfide (Na2Sx where x is a number of 2-5)is produced in the
reaction (1), but is shown as sodium disulfide here for the
purpose of simpliclty.
Thus, in order to produce as much polysulfide as possible,
i-t is necessary to expedite the reaction (1) while suppressing
the reactions ~2) and (3)~ In view of the fact that the
reactions (2) and (3) require a higher oxygen to sulfide molar
ratio than in the reaction (1), the selectivity to polysulfide
may be improved by suitably selecting the reaction conditions.
More particularly, the yield of polysulfide may be improved by
allowing the reaction ~1) to proceed effectively in the presence
of a relatively small amount of oxygen and by preventing the
polysulfide from contacting with oxygen. This may be achieved
by the use of a catalyst with high activity and efficiently
effecting the contact between the oxygen-containing gas
(oxidant) and aqueous sodium sulfide solution (reductant).
Thus, in one preferred embodiment of the process of the
present invention, the catalyst is packed in a reactor through
which the oxidant and reductant are flown cocurrently in a
trickle flow. Counter-current contact between the oxidant and
reductant is disadvantageous because the polysulfide produced is
contacted with fresh oxidant so that the reaction (3) is
facilitated. Downward flow of the reactants is also important,
especially when the catalyst used is a fibrous catalyst, to form
thin liquid films over the surfaces of the fibrous catalyst and
to effect the diffusion of the oxidant smoothly and
homogeneously. If the oxidant and reductant are flown upwardly,
the gas (oxidant) is bubbled through the liquid so that the
oxidation cannot effectively proceed even if a fibrous catalyst
with a large outer surface area is employed.
In the above-described downward concurrent flow, it is
preferred that a portion of the oxidant is supplied from an
intermediate portion or portions of the catalyst bed. When all
the necessary oxygen is supplied from the top of the reactor,
polysulfide produced at the upper portion of the catalyst bed
can contact with a large amount of oxygen, so that the
~L297~2
polysulfide is further oxidized according to the reaction (3).
By reduciny the amount of the oxidant fed from the top of the
reactor, the reaction (3) at the upper portion of the catalyst
bed can be inhibited. The remainder of the oxidant, generally
25 % or less of the total oxidant feed, is supplied to the
reaction system in a later stage or stages.
When the perfect oxidation [(at least 80 ~ conversion of
Na2S in the reaction t2)] of the aqueous sodium sulfide solution
according to the reaction (2) is intended for the production of
as much NaOH as possible, it is important that the sulfide
solution should be contacted with a large amount of oxygen.
Thus, the oxidant and rèductant are flown through a fixed bed of
the catalyst and brought into either counter-current contact or
cocurrent contact. The oxidant is supplied from the bottom of
the reactor while the reductant is supplied from the top.
For the purpose of improvin~ the efficiency of the gas-
liquid contact on the catalyst bed, it is preferred that the
fixed bed of the catalyst be divided into a plurality of
vertically spaced apart catalyst layers and that a liquid
distributor be provided betwe0n each adjacent layers. The
liquid distributor may be a perforated plate having a total area
of the opening of 15-35 %. Generally, a liquid, during its
passage down through a packed tower, tends to flow along the
inside wall of the tower and become non-uniform in the middle
and lower portions. In the present invention, in order to
prevent such deflected liquid flows and to increase the reaction
efficiency, a relatively large number of the liquid dispersing
plates are desirably provided, for example with a distance of
1 2 m. Such dispersing plates also serve as a reinforcing
member for the fixed catalyst bed~
The oxidation is preferably performed at a temperature of
50-130 C and a pressure of 0-10 Kgtcm2G with the oxidant
(oxygen-containing gas) to reductant (aqueous sodium sulfide
solution~ feed ratio of 10-500 normal-liter/liter and a weight
hourly space velocity of 0.5-500 hour~1. Since the white liquor
and green liquor obtained in the kraft pulping process generally
~2~ L2
-- 8 --
have a temperature of 70-100 C, it is not necessary to heat or
cool the reactor if such liquors as such are to be treated.
With regard to the reaction pressure, a higher pressure is more
preferred because the diffusion of oxygen into pores of the
particulate catalyst or to the surfaces of fibrous catalyst
covered wi-th liquid films is more facilitated. However, too
high a pressure is not advantageous for the production of sodium
polysulfide because the occurrence of the side reactions (2) and
(3) are expedited. The oxidant/reductant feed ratio is based on
the feed rates at the inlet port of the fixed bed reactor. The
diffusion rate of oxygen to the catalyst surface depends on the
feed ra-tio; i.e. the higher the feed ratio, the more becomes the
amount of diffusable oxygen. However, an excess feed ratio over
the above range is not advantageous in the case of the
production of polysulfide because of the occurrence of the side
reactions (2) and (3). The weight hourly space velocity may be
selected from the above-described range in consideration of the
activity of the catalyst, the conditions under which the
oxidation is performed and the kind and amount of the desired
products.
White liquor produced in the kraft pulping system contains
a large amount (generally 50~300 ppm) of suspended solids such
as calcium carbonate and ferrous sulfide. Such suspended solids
tend to cause the plugging of the catalyst bed and adversely
affect the activity and selectivity of the catalyst. Therefore,
when the sulfide-containing solution to be oxidized contains
such suspended solids in an amount of 20 ppm or more, it is
preferred that the solution be pretreated for the removal of the
suspended solids so that the content of the suspended solids is
decreased to 5 ppm or less, preferably 3 ppm or less. The
removal of the suspended solids may be effected by filtration.
In a preferred embodiment according to the present
invention/ the filtration is performed with the use of a
pressure-type, deep bed upflow filtering device. Fig. 1 depicts
one preferred embodiment of such an upflow filtering device.
The device includes a tank or housing 11 generally cylindrical
12~7;2~.~
g
in shape. A circular supporting plate 12 is secured within the
housing 11 Eor supporting thereon a bed of granular solids 20
serving as a supporting layer for a filter bed 21 provided on
the supporting layer 20. A plurality of liquid distributers 13
5 are mounted through the supporting plate 12 to distribute an
incomming liquor supplied through an inlet pipe 1 uniformly to
the filter bed 21 via supporting layer 20.
The supporting layer 20 is constituted from a plurality
(generally 3-4) of sub-layers formed of granular solids with
10 different grain sizes in the range of 2-50 mm, the sub-layers
being arranged so that the grain size of one sub-layer is
smal ler than its adjacent lower sub-layer.
The filter bed 21 is also composed of two or more sub-
layers of granular solids with particle sizes ranging from
15 0.3-1.8 mm, the sub-layers being arranged so that the particle
size of one sub-layer is smal ler than its adjacent lower sub-
layer.
Disposed adjacent to the upper surface of the filter bed 21
is a filtrate recovering member, general ly designated as 30, for
20 collecting the filtrate which has been passed through the filter
bed 21. As shown in Fig. 2, the recovering member 30 includes a
central cylinder 34 from which a plurality (four in the
illustrated case) of horizontal collecting pipes 31 extend
radially outwardly and welded in their ends to the interior wall
25 of the housing 11. Each pipe 31 is provided with a multiplicity
of small openings 32 at its lower side. The diameter of the
openings 32 is general ly smal ler than the particle size of the
filter granules in the upper portion of the filter bed 21. The
filtrate recovering member 30 is preferably disposed at a
30 position so that the lower side of the pipes 31 is in contact
with or embeded in the upper portion of the filter bed 21. More
preferably, the recovering member 30 is so disposed that the
openings 32 are located at a level lower by about 30-300 mm than
the top surface of the filter bed 21. As shown in Fig. 1, the
35 central cylinder 34 has a narrowed lower end connected to an
outlet pipe 35 extending out through the wall of the housing 11.
~2~72~,
- 10 -
The filtrate recovering member 30 may have any other
desired structure than the above. For example, the collecting
pipe 31 may be in the form of a volute or grids. The small
openings 32 may also be formed in the side or upper surface of
the pipe 31. The outlet pipe 35 need not extend downwardly.
Since the filtration is performed under pressure, the filtrate
can be discharged from the filter device even when the pipe 35
is oriented horizontally or upwardly.
The filter device shown in Fig. 1 is further provided with
an air inlet pipe 2t an air injection nozzle 15 connected to the
pipe 2, an air discharge port 3, a liquid discharge port 4 and a
wash liquid discharge port 5. These components are provided for
regenerating the filter bed when it is loaded with suspended
solids to an extent that the filtering operation can be no
longer continued satisfactorily.
In operation, white liquor containing suspended solids is
pumped through the inlet pipe 1 and into the lower space beneath
the supporting plate 12. The incoming liquor is uniformly
distributed by the distributers 13 and flown up through the
supporting bed 20 and filter bed 21. During the passage through
the bed 21, the suspended solids in the liquor are removed. The
filtrate is collected in the collecting pipes 31 and discharged
from the filter device through the outlet pipe 35. The filtrate
thus obtained is then subjected to the oxidation treatment as
described above.
It is important that the upper space 22 above the filter
bed 21 should be filled with the filtrate throughout the
filtering operation. ~y so doing, even if the pressure of the
liquor to be treated is increased for increasing the filtering
rate, the granular solids of the filter bed 21 are not
fluidized. That is, since the upper space 22 is filled with a
liquid which is maintained in a static state because of the
provision of the recovering member 30, the pressure of the
incomming fluid applied to the bed is balanced with the back
pressure exerted from the static liquid in the space 22.
When the pressure drop in the filter bed 21 reaches a
2~
predetermined level, the fil-tering operation is stopped to
conduct the regeneration thereof. Thus, the supply of the
liquor to be treated is stopped and the liquid in the upper
space 22 is discharged from the outlet 4. Then, air is fed
through the pipe 2 to stir the bed and to release the trapped
solids from the filtering granules. The air is withdrawn
overhead through the outlet 3. Then, the air supply is stopped
and the liquor is fed through the pipe 1 to discharge the liquid
in the housing from the outlet 5 together with the solids which
have been released from the filtering granules. The feed rate
of the liquor is adjusted to fluidize the largest filtering
granules. When the feed of the fluid from the pipe 1 is
stopped, the granules are spontaneously arranged in the original
state and then the bed of the regenerated granules is set.
When the activity of the catalyst according to the present
invention is decreased, the deactivated catalyst ls then
subjected to a regeneration treatment. The regeneration is
preferably carried out while maintaining the catalyst in the
packed state as such, without taking it out of the reactor.
The regeneration is effected by con-tacting the catalyst
with an acid solution, preferably 1-5 % hydrochloric acid, in an
amount of 0.5--3 liters per liter of the packed catalyst. Nitric
acid solution may be also used for the regeneration.
The deactivated catalyst generally carries sulfides such as
sodium sulfide which, upon contact with hydrogen chloride,
produce undesirable hydrogen sulfide as follows:
Na2S + 2HCl = 2NaCl + H2S (4)
Thus, it is important that the catalyst should be pretreated for
the removal of the sulfides which deposit thereon before
conducting the treatement with acid.
The removal of the sulfides from the catalyst may be
effected by washing with water. Thus, water is fed to the
reactor and flown through the catalyst bed. Alternatively, the
pretreatment may be effected by converting the sulfides by
reaction with oxygen according to the reaction (2) described
above. In this case, an oxygen-containing gas such as air is
~2~3~2~2
- 12 -
fed to the reactor for contact with the catalyst bed.
The pretreated catalyst is then contacted with an acid
solution. Thus, the acid solution is fed to the reactor and
flown through the catalyst bed. The pretreated catalyst
sometimes contains a small amount of sulfides which remain
unremoved in the pretreatment step, causing the generation of
hydrogen sulfide in the acid treatment step. In such a case, it
is preferred that an oxygen-containing gas be fed to the reactor
simultaneously with the acid solution. By this, the hydrogen
sulfide may be converted into elemental sulfur according to the
following reaction-
H2S + 1/202 = H2O + S (5)The elemental sulfur may be easily removed from the catalyst bed
in the succeeding sodium sulfide solution oxidation stage as
polysulflde:
S ~ Na2S = Na2S2 (6)
The catalyst thus treated with the acid solution is then
washed with water to complete the regeneration. The resulting
catalyst is ready for use again ln the oxidation of sulfide
containing solution~
The following examples will further illustrate the present
invention.
Example 1
A white liquor containing, in average concentrations, 31.4
g/liter of Na2S (calculated as Na2O), 74.8 g/liter of NaOH
(calculated as Na2O) and 17.6 g/liter of Na2CO3 (calculated as
Na2O), 3.0 g/liter of Na2S2O3 (calculated as S), 0.8 g/liter of
Na2SO3 (calculated as S) and 140 ppm of suspended solids was
treated as follows. The white liquor was filtered with an up-
flow type deep bed filter device as shown in Fig. 1 at a rate of
400 liter/hour to obtain a filtrate having a suspended solids
content of 3 ppm. The filtering device has a cylindrical
housing with an inside diameter of 12 inches and a filter bed
formed from 73 liters of anthracite particles with an effective
diameter of 1.0 mm and 1.4 mm.
~L29~72~
- 13 -
The filtrate was then subjected to an oxidation treatment
in a cylindrical reactor which had an inside diameter of 8
inches and a height of 2 m and a liquid distributor having an
opening area of 30 % and which was provided with a fixed bed of
particulate activated carbon catalyst No. 1 (50 liters) having
the physical properties shown in Table 1. The filtrate and air
were flown cocurrently downward through the fixed bed reactor in
a trickle flow. The oxidation was performed at a temperature of
80 ~C under atmospheric pressure with an air to liquid feed
ratio of 50 normal-liter/liter. The relationship between the
yield of sodium polysulfide and hours on stream and between the
catalytic activity and hours on stream are shown in Figs. 3 and
4, respectively ~plotted by "o"). The catalyst was found to be
deactivated after about 3000 hours from the starting of the
oxidation. Thus, the oxidation was stopped to carry out a
regeneration treatment by washing with water, then with diluted
hydrochloric acid and finally with water again. The regenerated
catalyst was used again for the oxidation of the sodium sulfide-
containing filtrate.
Example 2
Particulate activated carbon catalysts Nos. 2-5 having the
properties shown in Table 1 were packed in a glass tube fixed
bed reactor having a diameter of 26 mm and a length of 50 cm.
An aqueous solution having a composition shown in Table 2 was
treated with respective catalysts under the conditions shown in
Table 3. The catalyst No. 1 used in Example 1 was also tested
for its activity in the same manner as above. The relationship
between the conversion of sodium sulfide and the selectivity to
sodium polysulfide in each catalyst is shown in Fig~ 5~ The
relationship between the catalyst activity and particle size is
shown in Fig. 6. In Figs. 5 and 6, the numbers in the circles
indicate the catalyst numbers. The term "catalyst activity"
used herein is defined by the following equation:
12
- 14 -
K = (1/Ci - 1/Co) x V
where K: Catalyst activity (liter/g-hour)
V: Weight hourly space velocity ~hour~1)
CO: Content of Na2S in feed (g/liter as Na20)
Cl: Content of Na2S in product (g/liter as Na20)
7~
o ~ ¦ t o ¦ t~ ¦ t N l
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. . u~ . . ~ t~
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tx~ m o o o o o o t~
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~72~2
- 16 -
Table 2
Composltlon of Aqueous Solution
Na2S (g/liter, in terms of Na2O) 31.4
NaOH (g/liter, in terms of Na2O) 74.8
Na2CO3 (g/liter, in terms of Na2O) 17.6
Table 3
Oxidation Conditions
Amount of Catalyst (cc) 100
Reaction Temperature (C) 80
10 Reac-tion Pressure Atmosphere
Air/Liquid Feed Ratio
~normal-liter/liter) 30
Reaction Time (hour) 4
Type of ContactCocurrent trlckle flow
Example 3
Using the catalyst No. 1 used in Example 1 and the same
reactor as used in Example 2, oxidation tests were carried
out under various reaction conditions as shown in Table 4. The
catalytic activities of the catalyst in respective oxidation
conditions are also summarized in Table 4.
~72~l2
- 17 -
Table 4
Catalytic Activity
Experiment No. 1 2 3 4 5 6 7
Temperature
(C) 60 80 95 80 80 80 80
. _ _
Pressure
(kg/cm2G) 0 0 0 5 10 0 0
AirjLiquid
Feed Ratio
10 (normal-lit./lit.) 40 40 40 40 40 100 450
Weight Hourly
Space Velocity
(hour~1~ 9.0 9.09.0 24.0 52049.0 9.0
Catalytic
15 Activity
(lit./g hour) 0.441.011.73 2.64 5.881.70 2.07
___ __ _
Comparative Example 1
Activated catalysts Nos. 6-8 having less than 0.25
20 cc/g of pore volume in pores having diameters of 100 A or more
were tested in the same manner as in Example 2 to obtain the
results shown in Figs. 5 and 6. The physical properties of the
catalysts No. 6-8 are summarized in Table 1.
Comparative Example 2
Activated catalyst No. 4 (I kg~ used in Example 2 was mixed
with 500 g of an emulsified liquid containing 60 g of
polytetrafluoroethylene with stirring for impregnation
therewith, followed by drying at 190 C in a hot air dryer,
thereby to obtain wetproofed carbon catalyst No. 9 having the
30 ph~sical properties shown in Table 1. The catalyst No. 9 was
tested in the same manner as in Example 2. The test results
were as shown in Figs. 5 and 6.
From the results shown in Fig. 5, it will be appreciated
that the oxidation of sodium sulfide involves simultaneous,
2~ ~
- 18 -
consecutive reactions shown by the reaction formulae ~1)-(3)
above and that the selectivity to sodium polysulfide is lowered
with the increase in the conversion of sodium sulfide~ The
catalysts used in Example 2 exhibit superior selectivity as
compared with the catalysts of Comparative Example 1. The
catalyst impregnated with PTFE (Comparative Example 2) is also
inferior to the catalysts of the present invention. The results
shown in F`ig. 6 indicate that the catalysts of this invention
have improved catalytic activity as compared with the catalysts
of Comparative Examples with similar catalyst diameters.
Comparative Example 3
The particulate activa-ted carbon catalyst No. 1 used in
Example 1 was impregnated with PTFE in the same manner as in
Comparative Example 2 to obtain catalyst No. 10 having the
properties shown in Table 1. Using catalyst No. 10, a catalyst
activity test was performed for 170 hours in the same manner as
in Example 1 except that the feed rate of the solution was
varied. The results are shown in Figs. 3 and 4 (plotted by the
mark "x").
From the results shown in Fig. 3, it will be seen that the
catalyst according to the present invention exhibits good
catalytic activity and produces sodium polysulfide with a high
selectivity in a stable manner for a long period of time in the
oxidization treatment of white liquor from an actual kraft
pulping system. The catalytic activity of wetproofed catalyst
No. 10 of Comparative Example 3 is inferior in comparison with
catalyst No. 1 of the present invention, though both catalysts
have similar pore characteristics. This is considered to be
attributed to insufficient gas/liquid/solid contact in the
catalyst bed which results from the wetproofing treatment.
Thus, it will be appreciated that a particulate activated carbon
catalyst, so far as it has a large pore volume in pores with
diameters of 100 A, can be suitably used for oxidation of sodium
sulfide without any special treatment such as a wetproofing
treatment.
