U.S. patent application number 10/892030 was filed with the patent office on 2004-12-23 for process for manufacturing and using a more stable formulation of sodium chlorite.
Invention is credited to Franco, Nicholas, Keramati, Barzin, McWhorter, Thomas E., Rosenblatt, Aaron A..
Application Number | 20040258607 10/892030 |
Document ID | / |
Family ID | 29419347 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040258607 |
Kind Code |
A1 |
Rosenblatt, Aaron A. ; et
al. |
December 23, 2004 |
Process for manufacturing and using a more stable formulation of
sodium chlorite
Abstract
Method for producing stable sodium chlorite by pelletization of
granular sodium chlorite with metal salt or salts forming hydrates
with water of hydration in the pellets being more than 5% of the
anhydrous weight of the sodium chlorite. Pelletized sodium chlorite
can be used to produce chlorine dioxide gas by passing a dilute
mixture of chlorine gas and an inert gas through a bed of the
pellets.
Inventors: |
Rosenblatt, Aaron A.; (New
York, NY) ; McWhorter, Thomas E.; (Allentown, PA)
; Franco, Nicholas; (Bethlehem, PA) ; Keramati,
Barzin; (Bethlehem, PA) |
Correspondence
Address: |
RATNERPRESTIA
P O BOX 980
VALLEY FORGE
PA
19482-0980
US
|
Family ID: |
29419347 |
Appl. No.: |
10/892030 |
Filed: |
July 15, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10892030 |
Jul 15, 2004 |
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10150835 |
May 17, 2002 |
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Current U.S.
Class: |
423/477 |
Current CPC
Class: |
C01B 11/024
20130101 |
Class at
Publication: |
423/477 |
International
Class: |
C01B 011/02 |
Claims
1. (Canceled)
2. (Canceled)
3. (Canceled)
4. (Canceled)
5. (Canceled)
6. (Canceled)
7. A method of producing chlorine dioxide gas by passing a dilute
mixture of chlorine gas with an inert gas being one of air or
nitrogen through a bed of sodium chlorite pellets containing water
of hydration greater than about 5% of the anhydrous weight of
sodium chlorite.
8. A process according to claim 7 including the step of using a
chlorine gas concentration less than 10% chlorine.
9. A process according to claim 7 including the step of maintaining
the chlorine dioxide partial pressure in the product at less than
150 mm Hg.
10. A process according to claim 7 including the step of sizing the
solid sodium chlorite pellets to pass through a 3 mesh screen, and
be retained in a 30 mesh screen.
11. A process according to claim 7 including the step of
maintaining a relative humidity less than 50% in the dilute chorine
gas.
Description
BACKGROUND OF THE INVENTION
[0001] Sodium chlorite is a strong oxidizer which is manufactured
as either a solid or as an aqueous solution. It is used in various
well-known processes, for example, to bleach textiles, and to
oxidize contaminants in drinking water and waste water. As
described in chapter 12 of the Fourth Edition of Handbook of
Chlorination and Alternative Disinfectants edited by George
Clifford White sodium chlorite is also used to produce chlorine
dioxide which is a powerful and selective oxidant and disinfectant.
Chlorine dioxide produced in these processes is used for many
purposes including, treatment of drinking water and waste water,
disinfection and preservation of food, and sterilization of medical
devices as disclosed in U.S. Pat. No. 4,681,739. In many of these
processes, sodium chlorite is supplied as an aqueous solution. In
some processes, such as the Gas:Solid process described in the
White reference, sodium chlorite is used as a granular solid.
Sodium chlorite is also supplied as a solid to some users who
dissolve it near the point of use and use it as an aqueous
solution.
[0002] U.S. Pat. No. 5,110,580 teaches that in order for the
Gas:Solid process to function efficiently two conditions must be
satisfied. First, the gas must be humidified. Otherwise, the
patentees teach, the Gas:Solid reaction may cease to function.
Secondly, pre-treatment of the sodium chlorite is necessary to
remove some or all of the sodium hydroxide. Otherwise, the chlorine
flowing through the solid bed will preferentially react with the
sodium hydroxide until the hydroxide is consumed and start-up of
the process will be delayed.
[0003] U.S. Pat. No. 4,044,103 describes a process for making
storage-stable sodium chlorite. In this process a salt which forms
a hydrate is added to sodium chlorite with water and then dried.
The final composition of the material is such that it contains a
weight of water (as hydrate) equal to at least about 5% of the
anhydrous weight of the sodium chlorite. If this criteria is met,
patentees teach that the resulting sodium chlorite formulation does
not propagate decomposition upon sudden local heating, even to a
high temperature. Patentees describe various ways to mix the sodium
chlorite and the hydrating salt including, adding dry ground
hydrated salt to a paste of sodium chlorite and drying the
resulting paste, and adding a solution of hydrating salt to dry
sodium chlorite and drying the resulting paste.
[0004] It is important to note that all of the processes taught in
the '103 patent use large amounts of excess water which is
evaporated away at elevated temperatures (typically 45.degree. C.)
after the salts are mixed. Patentees also teach the use of various
hydrating salts with high melting points and combinations of salts
with high melting points to prevent melting of the product in hot
weather. In all of the examples cited, drying of the product is
required, typically at 45.degree. C.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention is a process for manufacturing
agglomerated sodium chlorite particles suitable for, inter alia,
producing chlorine dioxide by the Gas:Solid process. A particular
useful form of agglomerated particles are pellets.
[0006] The pellets manufactured according to the present invention
do not propagate the decomposition reaction of sodium chlorite even
when locally heated to very high temperatures. These pellets resist
melting at temperatures normally found in the work environment.
Compared to other sodium chlorite formulations these pellets are
more resistant to combustion when mixed with a fuel. Also the
pellets are approximately uniform in size and are approximately
spherical in shape. The pellets produced by the present invention
permit uniform flow of gas through a bed of the pellets with less
pressure drop than a comparable bed of flakes of the same
composition. The pellets produced according to the present
invention are permeable to the mass transfer of gas into and out of
the particles during the Gas:Solid process. They dissolve rapidly
when mixed with water, do not require humidification of the gas
when used in the Gas:Solid process, and do not require pretreatment
for removal of hydroxide in order to achieve rapid start-up of
chlorine dioxide production in the Gas:Solid process.
[0007] Thus, in one aspect the present invention is a method for
producing a stable form of sodium chlorite comprising the steps of,
introducing water and granular sodium chlorite combined with one of
a metal salt or salts that form hydrates in an amount sufficient so
that when fully hydrated the water of hydration is more than about
5% of the anhydrous weight of the sodium chlorite with water the
amount of water being less than the weight of water required to
fully hydrate the salts, but more than 5% of the anhydrous weight
of the sodium chlorite, into a pelletizing apparatus and recovering
pellets being of a size to pass a three mesh screen pellet.
[0008] The present invention includes using a salt that is also
sufficiently soluble in water having a melting point above
50.degree. C. added in solution form in an amount to raise the
melting point of the final particles to more than 50.degree. C.,
while adding less water than required to fully hydrate the full
amount of hydrating salts in the mixture.
[0009] In another aspect the present invention is a method of
producing chlorine dioxide gas by passing a dilute mixture of
chlorine gas with an inert gas being one of air or nitrogen through
a bed of sodium chlorite pellets containing water of hydration
greater than about 5% of the anhydrous weight of sodium
chlorite.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a plot of self-heating rate against temperature
for a sample of technical grade sodium chlorite from an
Accelerating Rate Calorimetry Test.
[0011] FIG. 2 is a plot of self-heating rate against temperature
for a sample of sodium chlorite pellets, produced according to the
present invention, from an Accelerating Rate Calorimetry Test.
[0012] FIG. 3 is a schematic representation of a test apparatus
used to evaluate prior art compositions and compositions according
to the present invention.
[0013] FIG. 4 is a portion of a plot of chlorine dioxide against
time for a test using the apparatus at FIG. 3.
[0014] FIG. 5 is another portion of the plot of chlorine dioxide
against time for the test of FIG. 4.
[0015] FIG. 6 is a plot of percent chlorine dioxide against time
for both dry and humidified gas feed streams using the compositions
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Solid sodium chlorite is commonly manufactured, shipped and
stored as approximately 80% sodium chlorite with approximately 20%
diluent salts. Some of the diluent salts, such as sodium chloride,
are inert stabilizers. Others, such as sodium hydroxide, sodium
carbonate and other alkali salts, serve to reduce the risk of
accidental acidification of the sodium chlorite, which would
produce chlorine dioxide gas.
[0017] In the 80% solid sodium chlorite form typically supplied
commercially, sodium chlorite can react with any fuel to produce
vigorous combustion. In many cases, a source of ignition is
required in order to produce combustion of a fuel/sodium chlorite
mix. In other cases, when the typical 80% formulation of sodium
chlorite is mixed with reducing agents (for example some petroleum
products), sodium chlorite can spontaneously begin to burn with the
reducing agent. If the typical 80% formulation of sodium chlorite
is heated to approximately 150.degree. C., the material may begin
to undergo self-heating caused by the exothermic decomposition of
the sodium chlorite into oxygen, sodium chlorate and sodium
chloride. If the heating continues (either from an external heat
source or from self-heat) the material may undergo a "runaway
reaction".
[0018] FIG. 1 is a plot of self-heat rate against temperature
produced in an Accelerating Rate Calorimetry (ARC) test of a sample
of commercially available technical grade sodium chlorite. Of
particular note is that the onset temperature of self-heating is
about 151.degree. C., the maximum self-heat rate (a key measure of
the potential for a runaway reaction) is about 154.degree. C. per
minute which occurs at about 200.degree. C. In this case the very
high self-heat rate at about 200.degree. C. indicates a runaway
reaction.
[0019] It is to be noted that in the commercial sample tested, if
the sodium chlorite was heated locally to the point at which it
begins to visibly decompose, the decomposition reaction propagates
through the entire sample of sodium chlorite. If this occurs when
sodium chlorite is piled loosely, the result is a foaming reaction
that spreads slowly through the pile until all of the sodium
chlorite is consumed. Various sources in the literature indicate
that this reaction is the exothermic decomposition of sodium
chlorite into sodium chlorate, sodium chloride, and oxygen. Various
references indicate that the heat of decomposition causes the
sodium chlorate to melt and form a white foam as oxygen bubbles
through it. If external heating continues, or if heat produced in
the reaction cannot be dissipated, the sodium chlorate may break
down further, releasing still more sodium chloride, heat and
oxygen. If the sodium chlorite begins to decompose in a similar way
inside a container such as a drum, the evolving oxygen and
expanding foam may burst the container.
[0020] The potential for decomposition of sodium chlorite is
especially of concern where solid sodium chlorite is used as a
reagent, e.g. in the Gas:Solid process. Local decomposition may be
initiated in this process by the ignition of small amounts of
contaminants. Heating of the sodium chlorite may also be
exacerbated by the exothermic reaction of chlorine (one of the
reagents in the process) with the sodium hydroxide contained in the
sodium chlorite.
[0021] The Gas:Solid process involves the reaction of dilute
gaseous chlorine with solid sodium chlorite. In this process, the
dilute chlorine flows through a bed of granular solid sodium
chlorite. In order for this reaction to occur efficiently, the bed
of sodium chlorite must be permeable enough to permit the
percolation of gas through the bed. There must also be adequate
surface-to-volume ratio in the solid particles to permit mass
transfer of gas into and out of the solid particles.
[0022] Solid sodium chlorite is commercially available in the USA
in two forms:
[0023] 1. Flake material made by drying a solution of sodium
chlorite and diluent salts on a drum flaker. The particle size of
the resulting material is non-uniform. In the Gas:Solid process
this non-uniformity results in non-uniform flow of gas through the
solid bed. This causes channeling of the gas, which, in turn, can
cause premature breakthrough of unreacted chlorine.
[0024] 2. Powder material made by spray-drying a solution of sodium
chlorite and other salts. The result is a fine powder that packs
into containers so gas cannot readily permeate the powder.
[0025] In the present invention, dry sodium chlorite powder or
small dry granules of sodium chlorite, containing other salts which
can form hydrates, are mixed with an amount of water sufficient to
partially hydrate the hydrating salts. The dry mixture of salts may
also contain inert salts such as sodium chloride. The water and the
mixture of sodium chlorite and hydrating salts are mixed in a
manner so that the water is dispersed in an approximately uniform
distribution through the powder, with mechanical agglomeration to
form larger particles of solids (pellets) bound together by water.
The amount of water added is equal to at least about 5% of the
anhydrous weight of the sodium chlorite. The amount of water is
also sufficient to permit the formation of coherent pellets, with
only small amounts of unagglomerated powder remaining. The amount
of water added is less than that required to form "wet" paste
containing more water than required for complete hydration of the
hydrating salts. Therefore no drying is required.
[0026] Table 2 sets forth a typical composition of the commercially
available sodium chlorite powder used in all cases cited herein
referring to sodium chlorite powder.
1TABLE 2 Composition (% by weight) of Sodium Chlorite Powder
Constituent % by weight Sodium chlorite 79.8 Sodium sulfate 4.8
Sodium carbonate 6.1 Sodium chloride 8.5 Sodium hydroxide 0.29
Water 0.39 Sodium chlorate 0.13
[0027] Pellets produced with excess water are pasty and tend to
stick together or form larger pellets. Pellets made with excess
water may be dried as taught in the art.
[0028] When the water and dry sodium chlorite powder are mixed, the
resulting hydration is typically exothermic. Depending on the
composition of the pellets, the heat released may be sufficient to
heat the pellets to a temperature above the melting point of the
hydrated salt. Following agglomeration, the pellets are cooled
until they solidify. Depending on the formulation of the pellets,
different amounts of cooling are required.
[0029] With the process of the present invention no drying is
required. Experimentation and tests have shown that solidification
of the pellets produced according to the process of the present
invention require cooling, not drying. As part of the development
of the present invention the inventors have shown:
[0030] 1. Solidification rate is faster when the pellets are
exposed to cool air than when exposed to warm air under the same
conditions.
[0031] 2. Solidification rate appears unaffected by relative
humidity.
[0032] 3. Solidification occurs in sealed containers where there is
no convection to remove evaporated water.
[0033] 4. In sealed containers, solidification occurs faster near
the wall of the container than in the center. In the same
container, the solidified pellets near the wall are noticeably
cooler than the ones near the center.
[0034] 5. Large pellets can be immersed in cold water. A solid
paste layer will form on the outside of the pellet, slowing the
dissolution of the pellet. The cooling effect of the water will
cause the pellet to harden, even under water.
[0035] As with prior art compositions hydrating salts may be added
to the sodium chlorite used in the present invention either in dry
form or in solution in the water that is added.
[0036] According to the prior art the melting point of the product
can be increased by adding salts whose hydrates have high melting
points. The '103 patent points out the dangers of adding toxic
salts or salts that can react with sodium chlorite. Experimentation
has also shown that some salts listed as safe in the '103 patent
can produce hazardous conditions when used in the current
invention. It has been shown that salts of magnesium, when added in
solution to form pellets in the current invention, will produce
substantial volumes of chlorine dioxide gas during the curing
process and will continue to produce this gas for hours or even
days after the pellets are produced. This gas can become a
breathing hazard in the workplace. If the gas is evolved in sealed
containers, it may reach sufficient concentrations to explode
spontaneously. The mechanism of this reaction is not understood.
Tests show, however, that when solutions of magnesium sulfate or
magnesium chloride were added to powdered 80% sodium chlorite,
visible concentrations of chlorine dioxide were produced over the
period of several hours, and lesser amounts were still evolving
several days later. Addition of solutions of potassium carbonate
produced less chlorine dioxide than the magnesium salts, but still
produced enough to cause concern. Addition of water to the same
sodium chlorite powder caused no evolution of gas. Addition of
water containing sodium carbonate and sodium phosphate to the same
sodium chlorite powder caused no gas evolution.
[0037] The following example is illustrative of the present
invention.
EXAMPLE
[0038] Sodium chlorite powder of the composition shown in Table 2
is metered at a rate of 900 pounds per hour into a 3-foot pan
pelletizer (also known as a pan agglomerator) produced by many
companies such as Mars Mineral of Mars, Pennsylvania. The 3-foot
dimension refers to the approximate diameter of the rotating pan.
The pan rotates at a constant speed of 20 RPM with its axis of
rotation disposed at an angle of 60 degrees from the horizontal.
Water is sprayed at about 1 liter per minute into the pan slightly
below the point of powder entry. Pellets begin to build up in the
pan and grow in diameter through the process of "snowballing" until
pellets begin to overflow the side of the pan near the bottom.
Pellets conveyed away from the pelletizer are maintained in a
shallow layer and agitated until they cool and harden. This can be
accomplished in a number of ways including but not limited to;
packing the pellets into partially filled drums and rolling the
drums during the cooling process, drawing cool air through the
drums into which the pellets are being filled, conveying on a belt
conveyor with cool air blowing over the pellets, passing the
pellets through a vibrating screener with cooling air provided, or
using any of a variety of devices (such as fluidized beds, cooling
screw conveyors, spiral cooling systems) that provide a combination
of cooling and agitation. When the pellets have cooled and hardened
they are free flowing and can be readily packed in drums for use in
the Gas:Solid process or other processes.
[0039] The time required for cooling depends on the pellet
formulation and the cooling process. For the composition shown in
Table 2 cooling time varies from a minute or less in forced
convection to 18-24 hours if the pellets are cooled in drums.
[0040] Pellets produced using the formulation shown in Table 2
begin to soften at about 40-50.degree. C. Therefore they should
only be used in climates or applications where they will not be
exposed to temperatures above that level.
[0041] The prior art teaches that the melting point of the pellets
can be increased by adding salts whose hydrates have a high melting
point. In order to produce high melting point formulations, and to
meet the other criteria of the present invention, the salt to be
added must achieve its effect at a concentration low enough so that
it can be added at that level without adding too much water in the
process. Sodium phosphate can achieve the effect of raising the
melting point when it is incorporated at a level of about 9%. When
water containing salt is used in the pelletizing process, the ratio
of water feed to powder feed must be increased.
[0042] Pellets produced according to the invention by adding water
to sodium chlorite powder containing hydrating salts exhibited
thermal stability far superior to flaked material. FIG. 2 is a plot
of the self-heat rate against temperature measured in an ARC test
of pellets produced by adding water to the sodium chlorite
composition shown in Table 2. The resulting pellets contained water
equal to about 14% by weight. It should be noted that the maximum
self-heat rate for the pellets is about 3.degree. C. This is only
about 2% of the rate measured for commercially available flakes.
This indicates a far more stable material.
[0043] Table 3 shows the key data measured in the ARC test. The
data in Table 3 were taken using a 10 inch column of salt in each
test cartridge with a flow of a gas mixture of 4% chlorine in
commercially pure nitrogen without pre-humidification of the
contents of the column.
2TABLE 3 Minutes Calculation of to Steady Minutes NaClO2 gm
Consumed State to Cl2 At Breakthru Based % Materials Wt (gm)
Production Break on Cl2 Consumed Reacted Vulcan Flake* 588 15.0 406
199 42 -3 Mesh Pellets 496 2.5 303 148 41 -3 + 6 Mesh Pellets 478
1.8 390 191 55 -6 + 12 Mesh Pellets 424 3.0 388 190 62 -12 + 28
Mesh Pellet 442 6.0 486 238 75 *Run indicated no difference
relative to runs with pre-humidification
[0044] As the sodium chlorite in a cartridge becomes depleted,
unreacted chlorine gas begins to breakthrough in the product
stream. A fall in chlorine dioxide production provides evidence of
breakthrough. This has been confirmed experimentally by other
analytical techniques such as passing the product stream through a
flask containing ammonia. When chlorine breakthrough occurs, a
white cloud occurs in the flask. FIG. 5 shows a typical test during
the time when breakthrough occurs. Table 3 shows the time required
for different types of cartridges to breakthrough. It is clear that
smaller mesh size pellets lead to longer times to breakthrough,
though there appears to be little difference between pellets in the
-3+6 mesh range vs. pellets in the -6+12 range. Time to
breakthrough is a function of the depth (vertical length) of the
cartridge as well as the flux (pounds per hour of chlorine flow per
square foot of cartridge cross section). Thus, larger pellets can
be used for deeper cartridges with low flux, while smaller pellets
are better for shallow cartridges producing high flux.
[0045] Tests were also run in which flake material and pellets were
sealed in metal cans of about 1 liter volume. Small holes were
punched in the can tops. A small propane torch of the type sold in
home centers and hardware stores, was used to heat the sides of the
cans. After less than 10 seconds of heating, the can containing
flakes began to react, making audible sounds and the torch was
removed. Within 45 seconds after removal of the torch, the flaked
material began to foam up out of the hole in the can top and form a
mushroom of hot foam. The reaction continued for 3-4 minutes until
all of the sodium chlorite had been converted to foam. Foam
overflowing the can set fire to small pieces of combustible
material near the can. In one test, the reaction blew the lid off
the can.
[0046] The can containing pellets was heated by the same propane
torch for more than 10 minutes. The side of the can where the flame
impinged was glowing red-orange for most of this time. There was no
visible reaction except for a small amount of steam venting from
the hole in the can top. After 10 minutes, the torch was removed
and the can was opened. The pellets immediately adjacent to the can
wall where the torch impinged had been melted and apparently
decomposed. The pellets immediately surrounding the pellets in the
torch impingement area were melted, but apparently not decomposed.
The pellets in the rest of the can were soft, but apparently
undamaged.
[0047] Pellets and flakes were mixed with polyester fiberfill
(pillow stuffing) to simulate contact and mixing with finely
divided fuel. A propane torch was used to ignite the mixtures. When
the torch was applied to the flake mixture, the mixture ignited
instantly and flared up consuming the entire mixture in a period of
time of from one to two seconds. When the torch was applied to the
pellet mix, the fiber melted. If the torch was applied to pellets
sitting in molten polyester, the mix in the propane flame burned,
but self-extinguished when the torch was removed.
[0048] In another test, motor oil was poured onto piles of both
flakes and pellets. In both cases the mixture burned vigorously,
though the pellets required longer heating with a propane torch
before ignition occurred.
[0049] The heat testing provides qualitative evidence that the
pellets formed according to the invention are much more stable than
the commercially available flake material, but that both flakes and
pellets can burn when intimately mixed with fuel and ignited.
[0050] FIG. 3 is a schematic diagram of the test apparatus mixture
used to evaluate the use of pellets and flakes in the Gas:Solid
process. The apparatus 10 induces a source of gas 12 which is a
mixture of 4% chlorine in nitrogen. The gas is passed through a 2
inch diameter cartridge 20 containing a column of about 10 inches
long of sodium chlorite. Flow of the gas mixture is regulated by
flow control led by valve 16 and measured by flow indicator 14. A
pressure gauge 18 is included to measure pressure of the gas
mixture exiting source 12. The output represented by line 22 of the
cartridge 20 flows through a UV spectrophotometer 24 which measures
the concentration of chlorine dioxide. Before the gas flows through
the cartridge 20 it can be passed through a humidifier 26, or the
humidifier 26 can be by-passed by closing valves 28, 30. If the
humidifier 26 is by-passed, the gas flowing into the cartridge has
an extremely low relative humidity. Based on the specifications of
the gas supplier, the un-humidified gas mixture had a water vapor
content of <0.5 ppm.
[0051] FIG. 4 is a plot of the chlorine dioxide concentration of
the gas flowing from the cartridge 20 into the spectrophotometer 24
for the beginning of a typical test. All tests were based on a feed
gas of 4% chlorine in nitrogen flowing at 3.3 standard cubic feet
per hour (scfh). At steady state this produced about 7.7% chlorine
dioxide in nitrogen at the outlet of cartridge 20. This is the
concentration that is produced if 4% chlorine is nitrogen feed gas
reacts completely with sodium chlorite. In all tests, the system
was completely purged with nitrogen before the test began. FIG. 4
shows the absorbance of the product stream where the feed gas is
humidified and the sodium chlorite is in flake form which does not
contain hydrated salts. From FIG. 4 it is apparent about 15 minutes
is required before the concentration of the chlorine dioxide stream
reaches approximately 7.7%. FIG. 5 shows the end of the same test
noted by chlorine breakthrough at the outlet of cartridge 20. From
FIG. 5 it can be seen that chlorine breakthrough occurred
approximately 406 minutes after the test was begun.
[0052] Calculations based on the retention time of the gas in the
system indicate that about 0.5 minutes is required for blends of
nitrogen and chlorine or chlorine dioxide to displace the nitrogen
which fills the system at the start and reach the
spectrophotometer. Therefore the start of production is much
quicker when the sodium chlorite is in the form of pellets
containing hydrated salts, whereas there is a considerably longer
delay in production when the cartridge is filled with flakes. The
reason for this difference is unknown.
[0053] U.S. Pat. No. 5,234,678 teaches that humid nitrogen/chlorine
(or air/chlorine) feed gas is required in order for the Gas:Solid
process to function properly. FIG. 6 shows production of chlorine
dioxide in a column using pellets (with hydrated salts) and
humidified feed gas vs. production in an essentially identical
column using the same type pellets and very dry feed gas. It is
believed that water is available for the Gas:Solid reaction from
the hydrated salt by mechanisms that are not well understood.
[0054] Table 4 shows pressure drop over the length of a cartridge
at gas flows of 6.5 scfh and different pellet sizes. This data was
gathered using cartridges of 2 inches in diameter with a bed of
flakes or pellets 10 inches long. Most cartridges used in
production of sodium chlorite in the Gas:Solid process would be 2-5
times longer and would have proportionally higher pressure drop for
the same flux. Since, for safety reasons, most of the Gas:Solid
generators operate under vacuum or at very low pressure, there is
often very little differential pressure to drive the gas through
the system. Pressure drop of more than 1-3" of mercury in a column
is often problematic. It can be seen that in all cases except the
extremely fine -28 mesh pellets, pellets create a much lower
pressure drop than flakes for the same gas flux.
3 TABLE 4 PRESSURE DROP ACROSS COLUMNS Materials Wt (gm) cm H2O In
H2O in H.sub.g Vulcan Flake 588 21.2 8.35 0.614 +3 Mesh Pellets 454
0 0 0.000 -3 + 6 Mesh Pellets 478 2.5 0.98 0.072 -6 + 12 Mesh
Pellets 424 1.4 0.55 0.040 -12 + 28 Mesh Pelts 444 8.0 3.15 0.232
-28 Mesh Pellets 446 181.0 71.3 5.244
[0055] These data indicate that pellets larger than 3 mesh may
cause premature breakthrough in most practical applications, while
pellets smaller than 28 mesh cause excessive pressure drop in most
practical applications. Therefore pellets in the -3+28 mesh range
are preferred in most applications.
[0056] Having thus described our invention what is desired to be
secured by Letters Patent of the United States is set forth in the
appended claims which should be read without limitation.
* * * * *