Title of Invention

A PROCESS OF CONTINOUSLY PRODUCING CHLORINE DIOXIDE

Abstract The present invention relates to a process of continuously producing chlorine dioxide by the reduction of chlorate ions with hydrogen peroxide as a reducing agent in a tubular reactor, characterised by comprising the steps of: (a) feeding hydrogen peroxide and a metal chlorate or chloric acid or a mixture thereof at one end of the tubular reactor to form a reaction mixture; (b) reducing chlorate ions in the reaction mixture in said tubular reactor to form chlorine dioxide; and (c) recovering a product containing chlorine dioxide at the other end of said tubular reactor.
Full Text The present invention relates to a method of producing chlorine dioxide by the reduction of chlorate ions with hydrogen peroxide as a reducing agent in a tubular reac¬tor.
Chlorine dioxide is primarily used in pulp bleaching, but there is a growing inter¬est of using it also in other applications such as water purification, fat bleaching or re¬moval of phenol from industrial wastes. Since chlorine dioxide is not storage stable it must be produced on-site.
Production of chlorine dioxide in large scale is usually perfonned by reacting alkali metal chlorate or chloric acid with a reducing agent such as chloride ions, methanol or hydrogen peroxide at subatmospheric pressure, as described in, for example, EP pat¬ent 445493, US patent 5091166 and US patent 5091167. These production methods are highly efficient but are only suitable for production in large scale, for example at pulp mills consuming considerable amounts of chlorine dioxide for bleaching. In small scale appli¬cations, for example water purification, the chlorine dioxide is generally produced by re¬acting sodium chlorite with an acid.
EP patent 612686 disclose production of chlorine dioxide from alkali metal chlo¬rate and hydrogen peroxide at substantially atmospheric pressure.
US patent 5376350 discloses a method of producing chlorine dioxide from chlo¬rate ions and a reducing agent in a plug flow reactor which is suitable for production in small scale. Although the method works well it is still desirable to further improve the ef¬ficiency.
It is an object of the invention to provide an improved process suitable for small scale production of chlorine dioxide from metal chlorate or chloric acid and a reducing agent. Particulariy, it is an object to provide a process involving a high production rate of chlorine dioxide and low consumption of chemicals in a reactor with low space require¬ments. These objects are achieved by a process of continuous[y producing chlorine diox¬ide by the reduction of chlorate ions with hydrogen peroxide as a reducing agent in a tu¬bular reactor, preferably in the presence of a mineral acid, most preferably sulfuric acid, wherein the preferable degree of chlorate conversion to chlorine dioxide in the reactor is above about 75%, preferably from about 80 to 100%, most preferably from about 95 to 100%.
According to one aspect of the invention the process comprises the steps of:

(a) feeding hydrogen peroxide and a metal chlorate or chloric acid or a mixture thereof and optionally a mineral acid at one end of a tubular reactor to form a reaction mix¬ture;
(b) reducing chlorate ions in the reaction mixture to in said tubular reactor to form chlo¬rine dioxide, wherein the degree of chlorate conversion to chlorine dioxide in said re¬actor Is from about 75% to 100%; and
(c) recovering a product containing chlorine dioxide at the other end of said tubular reac¬tor.
According to another aspect of the invention the process comprises the steps of:
(a) feeding hydrogen peroxide and a metal chlorate or chloric acid or a mixture thereof and optionally a mineral acid at one end of a tubular reactor to form a reaction mix¬ture, wherein the molar ratio H2O2:CI03" fed to the reactor is from about 0.5:1 to about 2:1, preferably from about 0.5:1 to about 1:1;
(b) reducing chlorate ions in the reaction mixture to in said tubular reactor to form chlo¬rine dioxide; and
(c) recovering a product containing chlorine dioxide at the other end of said tubular reac¬tor.
In step (a) it is particularly preferred to feed hydrogen peroxide, a metal chlorate, pref¬erably" alkali metal chlorate such as sodium chlorate, and a mineral acid, preferably sul¬furic acid. In order to achieve a high degree of chlorate conversion it is normally advis¬able to feed hydrogen peroxide in an amount exceeding the stochiometric amount which is 0.5 mol H2O2 per mol ClOs". However, it has surprisingly been found that too much of hydrogen peroxide has a negative impact on the chlorate conversion.
According to another aspect of the invention the process comprises the steps of:
(a) feeding hydrogen peroxide, a metal chlorate, preferably alkali metal chlorate such as sodium chlorate, and sulfuric acid at one end of the tubular reactor to form a reaction mixture, wherein the sulfuric acid feed has a concentration from about 70 to about 96 wt%, preferably from about 75 to about 85 wt% and preferably a temperature from about 0 to about lOO"C, most preferably from about 20 to about SO"C;
(b) reducing chlorate ions in the reaction mixture to in said tubular reactor to form chlo¬rine dioxide; and

(c) recovering a product containing chlorine dioxide at the other end of said tubular reac¬tor.
It has been found that if the sulfuric acid feed has a concentration within the specified range, no external heating or cooling is needed as the energy from the dilution is suffi¬cient for operating the reactor adiabatically. It has also been found that the specified temperature range facilitates stable operation of the process.
According to still another aspect of the invention the process comprises the steps of:
(a) feeding hydrogen peroxide, a metal chlorate, preferably akali metal chlorate such as sodium chlorate, and sulfuric acid at one end of the tubular reactor to form a reaction mixture;
(b) reducing chlorate ions in the reaction mixture to in said tubular reactor to form chlo¬rine dioxide; and
(c) recovering a product containing chlorine dioxide at the other end of said tubular reac¬tor, wherein from about 2 to about 10 ftg H2S04, preferably from about 3 to about 5 kg H2SO4 is fed per kg CIO2 produced.
It has surprisingly been found that it is possible to operate at a chlorate conversion de¬gree above about 75% in spite of the comparatively low amount of sulfuric acid fed.
According to a particulariy preferred embodiment the invention concerns a proc¬ess of producing chlorine dioxide by the reduction of chlorate ions with hydrogen perox¬ide as a reducing agent in a tubular reactor comprising the steps of.
(a) feeding hydrogen peroxide, a metal chlorate, preferably alkali metal chlorate sjch as sodium chlorate, and sulfuric acid at one end of the tubular reactor to form a reaction mixture, wherein the molar ratio H2O2:CI03" fed to the reactor is from about 0.5:1 to about 2:1, preferably from about 0.5:1 to about 1:1, and wherein the sulfuric acid feed has a concentration from about 70 to about 96 wt%, preferably from about 75 to about 85 wt% and preferably a temperature from about 0 to about 10"C, most pref¬erably from about 20 to about 50"C;
(b) reducing chlorate ions in the reaction mixture to in said tubular reactor to form chlo¬rine dioxide; and
(c) recovering a product containing chlorine dioxide at the other end of said tubular reac¬tor, wherein from about 2 to about 10 kg H2SO4, preferably from about 3 to about 5 kg H2S04 is fed per kg ClOj produced.

It is evident that it is possible to combine the features of all the aspects and em¬bodiments described above. Other features that are particularly preferred in all the as¬pects and embodiments of the invention will now be described.
The product recovered contains chlorine dioxide, oxygen and optionally a metal salt of the mineral acid. Normally it also contains unreacted chemicals such as mineral acid and small amounts of chlorate ions. However, it has been found possible to avoid any substantial formation of chlorine.
It is preferred to operate without recirculating unreacted chemicals such as chlo¬rate or sulfuric acid from the product back to the reactor. In many applications the com¬plete product mixture can be used without separation, for example in water purification. Another option is to separate the gaseous product, i.e. chlorine dioxide and oxygen and use the chlorate containing liquid as a feed in another chlorine dioxide generator, for ex¬ample in processes as described in the earlier mentioned EP patent 445493, US patent 5091166 and US patent 5091167.
Although an ideal tubular reactor normally is operated with a plug flow without any backmixing, it has been found that the process of the invention is highly effective even if it is operated without any substantial concentration gradients in the reactor.
The reaction mixture in the bulk of the reactor preferably contains from 0 to about 2, most preferably from 0 to about 0.1 moles per litre of chlorate ions, and from about 3 to about 10, most preferably from about 4 to about 6 moles per litre of sulfuric acid. It is preferred to maintain the concentration of chlorate and sulfate below saturation 0 avoid crystallization of metal salts thereof.
All the chemical feeds, i.e. hydrogen peroxide, metal chlorate or chloric acid and he mineral acid are preferably supplied as aqueous solutions. It has been found that too nuch water in the system increases the energy consumption and decreases the chemi-cal efficiency, while to little water results in loss of stability. Therefore, the hydrogen per¬oxide feed solution preferably has concentration from about 30 to about 70 wt%, most preferably from about 40 to about 60 wt%. The chlorate feed solution, preferably alkali netal chlorate such as sodium chlorate, suitably has a concentration from about 0.5 notes per litre to saturation, preferably from about 3 to about 6 moles per litre, most preferably from about 4.5 to about 5.5 moles per litre. The mineral acid feed, preferably julfuric acid, preferably has concentration from about 50 to about 96 wt%, most prefer-ibly from about 75 to about 85 wt%. It is preferred not to add any substantial amounts of ihloride ions to the reactor except the chloride always present as an impurity in the chio-

rate feed. Preferably conventional alkali m"etal chlorate without extra added chloride is used which normally contains less than about 0.5, often less than about 0.05, preferably less than about 0.02, most preferably less than about 0.01 wt% of alkali metal chloride cal¬culated as NaCI in NaCIOs.
It has been found that the chemical efficiency Is improved by a high operating pressure, although too high a pressure result in such a high chlorine dioxide partial pres¬sure that safety problems might occur. Suitably the pressure in the reactor is from about 125 to about 900 mm Hg (about 16.7 to about 120 kPa), preferably from about 350 to about 760 mm Hg {about 46.7 to about 101 kPa), most preferably from about 500 to about 650 mm Hg (about 66,7 to about 86.7 kPa). The chlorine dioxide partial pressure is further decreased by oxygen and/or steam formed in the reactor. Although normally not necessary, it is possible also to supply extra inert gas such as air. The temperature is preferably maintained from about 30""C to the boiling point of the reaction mixture, most preferably at about the boiling point.
It has been found that the degree of mixing of the reactants affects the efficiency and it is preferred that the chlorate feed is substantially uniformly dispersed in the mineral acid at the inlet of the reactor to avoid any substantial radial concentration gradients over the cross section of the reactor. It Is also preferred that the chlorate feed is mixed with the hydrogen peroxide prior to being dispersed into the mineral acid. In order to minimize the radial concentration gradients it has been found favourable to use a tubular reactor with a inner diameter from about 25 to about 250 mm, preferably from about 70 to about 130 mm.
It has surprisingly been found possible to achieve a very high chlorine dioxide production rate, preferably from about 0.2 to about 7 kg/hr, most preferably from about 0.45 to about 4.5 kg/hr, and a high degree of chlorate conversion in a comparatively short tubular reactor preferably having a length from about 50 to about 500 mm, most preferably from about 100 to about 300 mm. It has also been found favourable to use a tubular reactor having a ratio of the length to the inner diameter from about 12:1 to about 1:1, most preferably from about 3:1 to about 1.5:1. A suitable average residence time in the reactor is from about 1 to about 100 minutes, preferably from about 4 to about 40 minutes.
A small scale production plant normally consist of only one tubular reactor, but it is possible to arrange several, for example up to about 10 tubular reactors in parallel, for example as a bundle of tubes.

The invention will now be further described in connection with the following exam¬ples which, however, not are intended to be interpreted to limit the scope of the invention.
Example 1: A process according to the invention was performed by continuously feeding a tubular reactor having an internal diameter of 100 mm and a length of 300 mm with 45 ml/min. of aqueous 5 M sodium chlorate, 46 ml/min. of 78 wt% sulfuric acid and 10 ml/min of 50 wt% hydrogen peroxide. The reactor was operated at a pressure of 630 mm Hg and a temperature of SO"C. Experiments performed at different molar ratios H2O2 to NaCIOs showed that the chlorate conversion degree was affected significantly as ap-

length of 300 mm. The inner diameters of the reactors were 100 and 150 mm, respec¬tively. The reactors were continuously fed with aqueous 5 M sodium chlorate, 78 wt% sulfuric acid and 50 wt% hydrogen peroxide. The reactors were operated at a pressure of 630 mm Hg and a temperature of 60C. It was found that the size of the reactor affected

Example 3: A process was performed by continuously feeding a tubular reactor having an internal diameter of 100 mm and a length of 300 mm with aqueous 5 M sodium chlorate, 78 wt% sulfuric acid and 50 wt% hydrogen peroxide. The reactor was operated at a pressure of 630 mm Hg and a temperature of 60°C. The amount of sulfuric acid fed was varied to achieve different compositions of the reaction mixture in the reactor. The re¬sults are shown in the table below:





WE CLAIM:
1. A process of continuously producing chlorine dioxide by the reduction of
chlorate ions with hydrogen peroxide as a reducing agent in a tubular reactor,
characterised by comprising the steps of:
(a) feeding hydrogen peroxide and a metal chlorate or chloric acid or a mixture thereof at one end of the tubular reactor to form a reaction mixture;
(b) reducing chlorate ions in the reaction mixture m said tubular reactor to form chlorine dioxide; and
(c) recovering a product containing chlorine dioxide at the other end of said tubular reactor.

2. The process as claimed in claim 1, wherein the degree of chlorate conversion to chlorine dioxide in said reactor is form above 75% to 100%.
3. The process as claimed in any one of the claims I to 2, wherein the molar ratio H203:C103" fed to the tubular reactor is from 0.5.l to 2:1.
4. The process as claimed in claim 3, wherein the molar ratio H202:C103" fed to the tubular reactor is from 0.5:1 to 1:1.
5. The process as claimed in any cne of the claims 1 to 4, wherein step (a) includes feeding a metal chlorate and a mineral acid.
6. The process as claimed in any one of the claims 1 to 5, wherein step (a) also includes feeding sulfuric acid having a concentration from 70 to 96 wt% to the tubular
reactor.
7. The process as claimed in claim 6, wherein the sulfuric acid feed has a
temperature from 20 to 50"C.

8. The process as claimed in any one of the claims 6 to 7, wherein from 2 to 10 kg H2SO4 is fed per kg C102 produced.
9. The process as claimed in claim 8, wherein from about 5 kg H2SO4 is fed per kg C102 produced.
10. The process as claimed in any one of the claims 1 to 9, wherein substantially no unreacted chlorate or mineral acid from the product in step (c) are recirculated back to the reactor.
11. The process as claimed in any one of the claims 1 to 10, wherein the inner diameter of the tubular reactor is from 25 to 250mm.
12. The process as claimed in any one of the claims 1 to 11, wherein the tubular reactor has a ratio of the length to the inner diameter from 12:1 to 1:1.
13. The process as claimed in any one of the claims 1 to I2,wherein the pressure in the tubular reactor is from 125 to 900 mm Hg {from 16.7 to 120 kPa).
14. The process as claimed in any one of the claims 1 to 13, wherein the reaction mixture in the bulk of the reactor contains from 0 to 2 moles per litre of chlorate ions and from 3 to 10 moles per litre of sulfuric acid.
15. The process as claimed in any one of the claims 1 to 14, wherem the reaction mixture in the bulk of the reactor contains from 0 to 0.1 moles per litre of chlorate ions.
16. The process as claimed in any one of the claims 1 to 15, wherein it is operated without any substantial concentration gradients in the reaction mixture in the tubular reactor.

17. The process as claimed in any one of the claims 1 to 16, wherein no substantial
amounts of chloride ions are added to the reactor.
18. A process of continuously producing chlorine dioxide substantially as herein
described and exemplified.

Documents:

476-mas-1998 abstract-duplicate.pdf

476-mas-1998 abstract.pdf

476-mas-1998 claims-duplicate.pdf

476-mas-1998 claims.pdf

476-mas-1998 correspondence-others.pdf

476-mas-1998 correspondence-po.pdf

476-mas-1998 description (completed)-duplicate.pdf

476-mas-1998 description (completed).pdf

476-mas-1998 form-19.pdf

476-mas-1998 form-2.pdf

476-mas-1998 form-26.pdf

476-mas-1998 form-4.pdf

476-mas-1998 form-6.pdf

476-mas-1998 petition.pdf


Patent Number 207546
Indian Patent Application Number 476/MAS/1998
PG Journal Number 27/2007
Publication Date 06-Jul-2007
Grant Date 14-Jun-2007
Date of Filing 09-Mar-1998
Name of Patentee M/S. AKZO NOBEL N V
Applicant Address P O BOX 9300, NL-6800 SB ARNHEM
Inventors:
# Inventor's Name Inventor's Address
1 JOEL TENNEY 1464 WOOD THRUSH WAY, MARIETTA, GA 30062.
PCT International Classification Number C 01 B11/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 08/821,164 1997-03-20 Russia