WO1998013296A9

WO1998013296A9 - Method for producing chlorine dioxide using methanol, chloride, and hydrogen peroxide as reducing agents

Info

Publication number: WO1998013296A9
Application number: PCT/US1997/018156
Authority: WO
Filing date: 1997-09-29
Publication date: 1998-06-04

Abstract

A method is described for producing chlorine dioxide by reacting alkali metal chlorate with methanol, metal chloride, and hydrogen peroxide in an aqueous acidic medium. Combining these reducing agents causes an unexpected enhancement in chlorine dioxide generation.

Description

METHOD FOR PRODUCING CHLORINE DIOXIDE USING METHANOL, CHLORIDE, AND HYDROGEN PEROXIDE AS REDUCING AGENTS

FIELD OF INVENTION

The present invention relates to a method for producing chlorine dioxide.

BACKGROUND OF THE INVENTION

Chlorine dioxide is employed in a wide variety of industrial applications, including bleaching wood pulp for paper making, bleaching textiles, treating water, and abating odors. The use of chlorine dioxide for bleaching wood pulp has increased because chlorine dioxide is more environmentally friendly than chlorine or hypochlorite, which can leave larger quantities of chlorinated organic compounds in bleaching effluent.

In a typical commercial processes for generating chlorine dioxide, sodium chlorate is reacted with a reducing agent in a strongly acidic aqueous medium. A metal chloride salt, sulfur dioxide, methanol, or hydrogen peroxide is commonly used as the reducing agent. The typical acid used is sulfuric acid or hydrochloric acid, generally to obtain an acidity of between about 3 to 10 N for the reaction mixture.

The reduction of sodium chlorate with sodium chloride can be represented by the following formula:

NaC10₃ + NaCl + H₂SO₄ > ClO₂ + '/_ Cl₂ + Na₂SO₄ + H₂O (1)

A principle disadvantage of this process is the formation of half a mole of chlorine gas for each mole of chlorine dioxide produced. At one time this chlorine gas was used for bleaching pulp. This use, however, is now disfavored because of environmental concerns.

Less chlorine gas is generated if sodium chloride is replaced by sulfur dioxide or methanol in this process. The methanol based process is termed the Solvay process, while the sulfur dioxide process is termed the Mathieson process. However, the reaction between chlorate and either sulfur dioxide or methanol is very slow, resulting in a low rate of chlorine dioxide generation. The following reactions occur initially, resulting in generation of chloride ion, which acts as a reducing agent:

ClO₃ ^" + 3SO₂ + 3H₂O - — > Cl- + 3H₂SO₄ (2) 2ClO₃- + 3CH₃OH — > 2C1- + 2HCOOH + 4H₂O + CO₂ (3)

The chloride ions reduce chlorates present in the reaction mixture according to formula (1) shown above, resulting in the production of chlorine gas. The chlorine gas reacts with the sulfur dioxide or methanol to regenerate chloride ions according to the following formula:

SO₂ + Cl₂ + 2H₂O - — > 2HC1 + H₂SO₄ (4)

CH₃OH + 3C1₂ + H₂O - — > 6HC1 + CO₂ (5)

The overall reaction using methanol as reducing agent is as follows:

9NaClO₃ + 2CH₃OH- + 6H₂SO₄ - — > 9ClO₂ + (6)

3Na₃H(SO₄)₂ + V4CO₂ + 1.5HCOOH + 7H₂O When using sulfur dioxide or methanol as reducing agent, however, at least some chlorine gas by-product is produced. Also, when using methanol as reducing agent at high chlorate concentrations and acidity, the chloride formed in reactions 2-5 can be consumed in the subsequent formation of chlorine dioxide faster than it is generated. When the chloride is exhausted, chlorine dioxide generation ceases and a phenomenon known as "whiteout" results, i.e. , the reaction medium becomes clear. To avoid this, it is often necessary to continuously add a small amount of sodium chloride, or to avoid certain high concentrations of chlorate and acid.

Hydrogen peroxide has been used as a reducing agent in chlorine dioxide generation to eliminate production of chlorine. Using hydrogen peroxide also results in a significantly faster chlorine dioxide generation rate than other processes. The reaction using hydrogen peroxide is represented by the following formula:

2NaClO₃ + H₂O₂ + H₂S0₄ > 2ClO₂ + Na₂SO₄ + 2H₂0 + O₂ (7)

An important disadvantage of this process, however, is that hydrogen peroxide is much more expensive than methanol, sodium chloride, or sulfur dioxide. For this reason, the hydrogen peroxide based process is not used as commonly as the methanol and sulfur dioxide based processes.

There is therefore a need for a method of producing chlorine dioxide that is efficient and economical, which does not generate substantial amounts of chlorine, and which reduces the possibility of a whiteout condition. SUMMARY OF THE INVENTION

The present invention relates to a method for producing chlorine dioxide by reacting alkali metal chlorate with reducing agents in an aqueous acidic medium, wherein the reducing agents are methanol, chloride, and hydrogen peroxide. We have determined that combining these reducing agents causes an unexpectedly strong enhancement in the rate of chlorine dioxide generation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a continuous process embodiment of the invention.

Figure 2 is a graph showing chlorine dioxide generation using the process of the invention at atmospheric pressure, 60° C, and 10 N H₂SO₄. The points on the graph indicated with a " + " indicate amounts of chlorine dioxide generated for combinations of sodium chloride, methanol, and hydrogen peroxide in the equivalent strength molar ratios shown. The dotted line connecting the points for 100% CH₃OH and 100% H₂O₂ or NaCl shows the expected chlorine dioxide generation rate for a combination of sodium chloride, methanol, and hydrogen peroxide reducing agents.

Figure 3 is a graph showing chlorine dioxide generation using the process of the invention at a sub-atmospheric pressure of 300 mm Hg, 60^° C, and 10 N H₂SO₄. The points on the graph shown with a " + " indicate the amount of chlorine dioxide generated for a combination of sodium chloride, methanol, and hydrogen peroxide in the ratios shown. The dotted line is an approximation of the expected amount of chlorine dioxide generation for a combination of these reducing agents.

DETAILED DESCRIPTION OF THE INVENTION

We have determined that an apparent synergism between chloride, methanol, and hydrogen peroxide reducing agents results in the generation of chlorine dioxide at a higher rate than would have been expected. This higher rate allows more chlorine dioxide production from a given generator volume, thereby reducing capital cost. It also allows, for example, a smaller generator to be used to meet a given chlorine dioxide demand. In addition, by varying the ratio of chloride to methanol to hydrogen peroxide, one can increase or decrease the chlorine dioxide generation rate. This degree of flexibility in choosing a desired chlorine dioxide generation rate is unique to the present invention. It also offers great flexibility in optimizing the cost of the process.

For example, it has been found, at atmospheric pressure, that by substituting 20% of the methanol in the methanol type process described above with 10% sodium chloride and 10% hydrogen peroxide (measured an equivalent strength basis), the generation rate of chlorine dioxide is unexpectedly high. The generation rate of chlorine dioxide obtained using hydrogen peroxide or sodium chloride is known to be higher than that obtained using methanol. But the increase obtained by substituting only 10% hydrogen peroxide and 10% sodium chloride in the methanol process represents more than 50% of the increase obtained if 100% hydrogen peroxide (or sodium chloride) is used.

As noted above, the invention results in surprising benefits at atmospheric pressure. Preferred pressures are between about 400 and 900 mm Hg. We have determined, however, that the benefits of the invention can also be enjoyed when the process is carried out at sub-atmospheric pressure, a preferred sub-atmospheric pressure being from about 100 mm Hg to 400 mm Hg. When operating at sub-atmospheric pressure, the temperature of the reaction may need to be adjusted to account for lowered boiling points resulting from the lowered pressure.

The method of the invention is preferably carried out as a continuous process. In one embodiment, depicted in a flow diagram in Figure 1 , sodium chlorate is reacted with hydrogen peroxide, methanol, and sodium chloride reducing agents in the presence of concentrated sulfuric acid. The reactants can be introduced together, but preferably are introduced separately into a conventional reaction vessel. The reaction can be carried out at atmospheric pressure, with air, or other inert gas such as nitrogen, circulating through the reaction vessel. The reaction should be maintained substantially in a steady state by continuously feeding the reactants, and by ensuring that they are evenly distributed in the reaction medium. The chlorine dioxide gas that is generated can be collected and absorbed outside of the reaction vessel. Water vapor and other gaseous byproducts should also be continuously removed from the reaction vessel, and vented to a chlorine dioxide absorber. Reaction medium containing alkali metal salt ("Na₂SO₄ Saltcake" in Figure 1), and unreacted chlorate, acid and reducing agents should also be continuously removed (e.g. , "H₂SO₄ Effluent" in Figure 1). Sodium acid sulfate deposited in the reaction at sub-atmospheric conditions can be removed and subjected to a metathesis reaction to form neutral sodium sulfate and acidic aqueous solution.

Preferably, the reaction medium that is withdrawn from a reaction vessel running at atmospheric pressure is cascaded into a second reaction vessel operating at sub-atmospheric pressure, such as a "single vessel process" (SVP™) reactor. For the preferred reactants, the withdrawn medium from a reaction vessel at atmospheric pressure contains largely sulfuric acid, with lesser amounts of chlorine dioxide, sodium chlorate, sodium sulfate, hydrogen peroxide, methanol, and chloride. The withdrawn medium, new reducing agents, sodium chlorate, and sulfuric acid are preferably separately added to the second vessel, and the second vessel kept at a sub-atmospheric pressure of between 100 and 400 mm Hg, preferably between 100 and 300 mm Hg. Chlorine dioxide gas is recovered outside of the first and second reaction vessels. In the cascade process run under these conditions, the only by-product generated in the second reactor can be a neutral metal salt, or acidic salt cake, depending on the acidity of the reaction medium.

If desired, catalysts that enhance the generation of chlorine dioxide are added to the reaction as well. Such catalysts include, e.g., silver nitrate, manganese sulfate, vanadium pentoxide, ruthenium oxide, rhodium oxide, and palladium oxide.

The process is conducted in a temperature range of between about 20° C and about 140° C, preferably between about 35° C and 80° C, and most preferably between about 50° C and 75° C.

Suitable acids for use in the reaction include, e.g. , sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chloric acid. Sulfuric acid is preferred. The acid normality is maintained in the aqueous reaction medium between about 1 N to 15 N, preferably between about 4 N and 12 N. Most preferably, the normality is maintained at between about 7 N and 10 N for atmospheric conditions, and between about 4 N and 5 N or 7 N to 10 N for subatmospheric conditions. Performance of the process at 7 N to 10 N generates acid salt cake while performance at 4 N to 5 N generates neutral saltcake.

Alkali metal chlorates that can be used in this process include, e.g., sodium chlorate and potassium chlorate. Sodium chlorate is preferred. The alkali metal chlorate concentration employed in the reaction is between about 0.01 M and saturation concentration, preferably between about 0.01 M and 4 M, most preferably between about 0.05M and 0.3 M at atmospheric pressure and between about 0.3M and 1.5 M at sub- atmospheric pressure.

Chloride agents that can be used in this process include hydrogen chloride and salts such as sodium chloride, potassium chloride, lithium chloride, barium chloride, magnesium chloride, calcium chloride and aluminum chloride. Alkaline metal salts are preferred. Sodium chloride is most preferred.

The optimum percentage of hydrogen peroxide with respect to the total amount of reducing agent used in the process, on an equivalent strength basis, depends on the chemical costs, chlorine dioxide demand, and byproduct demands. It can vary from 1 % to 99%, but for a typical paper mill is preferably less than 50%, more preferably less than 30% , and most preferably between about 5 and 10% . The amount of total reducing agent consumed in the reaction is preferably from about 100% to 120% of the stoichiometrically calculated amount.

If desired, the method of the invention can be practiced by adding hydrogen peroxide and sodium chloride to an existing chlorine dioxide generator that uses a methanol reducing agent. Alternately, the method can be practiced by adding methanol and sodium chloride to an existing generator that uses a hydrogen peroxide reducing agent. Alternately, the method can be practiced by adding methanol and hydrogen peroxide to an existing generator that used sodium chloride reducing agent.

The invention is illustrated by the following examples, which are intended to merely exemplify the invention, not to limit its scope.

EXAMPLE 1 : PARTIAL SUBSTITUTION OF METHANOL WITH SODIUM CHLORIDE AND HYDROGEN PEROXIDE REDUCING AGENTS AT ATMOSPHERIC PRESSURE

Sodium chlorate (27 grams/liter) and sulfuric acid (588 grams/liter) were added to a laboratory reaction vessel equipped with a nitrogen sparger to obtain a 600 ml reaction mixture having a chlorate concentration of 0.25M and an acid normality of 12 N. The reactor was operated at atmospheric pressure and 60° C.

Different equivalent strength molar ratios of sodium chloride, methanol and hydrogen peroxide, were added to the reactor based on the stoichiometric amount that was equivalent to 0.15 moles sodium chloride. As shown in reaction formula (1) above, only one mole of salt is added for every mole of chlorate in the process for producing chlorine dioxide.

The amount of methanol that was equivalent to 0.15 moles of salt was calculated by the following equation:

0.15 x 32/4.5 = 1.07 gram. The "32" in this equation is the molecular weight of methanol. The "4.5" is derived from the reaction stoichiometry for producing chlorine dioxide with methanol shown in equation (6) above, in which one mole of methanol produces 4.5 moles of chlorine dioxide.

The amount of hydrogen peroxide that was equivalent to 0.15 moles of salt was determined by the following equation:

0.15 x 34/2 = 2.55 grams The "34" in this equation is the molecular weight of hydrogen peroxide. The "2" is derived from the reaction stoichiometry for producing chlorine dioxide with hydrogen peroxide shown in equation (7) above, in which one mole of hydrogen peroxide produces 2 moles of chlorine dioxide.

Figure 2 shows the amount of chlorine dioxide (in grams) generated over 1.5 minutes when methanol was used alone (shown as ratio "0: 100:0"), methanol was mixed with sodium chloride and hydrogen peroxide in different equivalent strength ratios, and where sodium chloride (ratio "0:0: 100") or hydrogen peroxide (ratio " 100:0:0") was used alone. The rate of chlorine dioxide generation in this system was calculated by the following formula:

ClO₂ generation = (grams of ClO₂/67.5 grams/mole)/(0.6 liter x 1.5 min)

The amount of chorine expected to have been generated is shown by the dotted line. The generation rates calculated were as follows:

Reducing Agent Generation Rate (10² moles/liter-min.)

100% CH₃OH 6.6

10:80: 10 (NaCl:CH₃OH:H₂O₂) 9.6

20:40:40 (NaCl:CH₃OH:H₂O₂) 10.6

30:30:40 (NaCl:CH₃OH:H₂O₂) 10.8

100% NaCl 12.2

100% H₂O₂ 12.2

Figure 2 shows that the amount of chlorine generated using sodium chloride, methanol, and hydrogen peroxide reducing agents was higher at all three ratios tested than would have been expected. For example, the generation rate obtained for the 10:80: 10 ratio was 7.7 x 10 ^'2 moles/(liter-minute). This was significantly higher than the generation rate obtained using methanol alone (6.6 x 10^~2 moles/(liter-minute)) and represented about 50% of the increase that was obtained when using 100% hydrogen peroxide or 100% sodium chloride.

Thus, use of combined methanol, sodium chloride, and hydrogen peroxide reducing agents resulted in a surprising increase in the generation rate of chlorine dioxide over that obtained using methanol or hydrogen peroxide alone.

EXAMPLE 2: PARTIAL SUBSTITUTION OF METHANOL WITH SODIUM CHLORIDE AND HYDROGEN PEROXIDE REDUCING AGENTS AT SUB- ATMOSPHERIC PRESSURE

The experiment described in Example 1 was carried out as described therein, except that the process was carried out at a sub-atmospheric pressure of 300 mm Hg and 490 grams/liter of sulfuric acid was used to obtain a reaction mixture having an acid normality of 10 N. The results of this experiment are shown in Figure 3. The "expected" line shown in Figure 3 is an approximation of what is expected for the combination of methanol, sodium chloride, and hydrogen peroxide. Substitution of 20% of methanol with 10% hydrogen peroxide and 10% sodium chloride resulted in a generation rate of 9 x 10^"2 moles/(liter-minute). The chlorine dioxide generation rate increase represented about 50% of the total increase achieved by replacing methanol with 100% hydrogen peroxide. A great cost savings is achieved by using only a small percentage of hydrogen peroxide.

These results show that a combination of NaCl:CH₃OH:H₂O₂ in a 10:80: 10 equivalent molar percent ratio generated chlorine dioxide at an unexpectedly high rate.

Claims

What is claimed is:

1. In a method for producing chlorine dioxide wherein alkali metal chlorate is reacted with a reducing agent in an aqueous acidic medium, the improvement comprising reacting said alkali metal chlorate with a combination of methanol, chloride, and hydrogen peroxide reducing agents.

2. A method for producing chlorine dioxide comprising reacting alkali metal chlorate with reducing agents and an acid, wherein said reducing agents comprise methanol, chloride, and hydrogen peroxide, said hydrogen peroxide comprising less than 50% , on an equivalent strength basis, of said reducing agents.

3. The method of claim 2 wherein said chloride is sodium chloride.

4. The method of claim 3 wherein said acid is selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chloric acid.

5. The method of claim 4 wherein said acid is sulfuric acid.

6. The method of claim 3 wherein said alkali metal chlorate is sodium chlorate.

7. The method of claim 3 wherein said reaction is carried out at a temperature of between about 20° C and about 140° C.

8. The method of claim 3 wherein said reaction is carried out at a temperature of between about 35° C and about 80° C.

9. The method of claim 3 wherein said reaction is carried out at a temperature of between about 50° C and about 75° C.

10. The method of claim 3 wherein said reaction is carried out a pressure of between about 400 and 900 mm Hg.

11. The method of claim 3 wherein said reaction is carried out at about atmospheric pressure.

12. The method of claim 3 wherein said reaction is carried out a pressure of between about 100 mm Hg and 400 mm Hg.

13. The method of claim 3 wherein the acid normality of said reaction medium is between about 4 N and 12 N.

14. The method of claim 13 wherein the acid normality of said reaction medium is between about 7 N and 10 N.

15. The method of claim 3 wherein a catalytic species for enhancing generation of chlorine dioxide is added to said reaction.

16. The method of claim 15 wherein said catalytic species is selected from the group consisting of silver nitrate, manganese sulfate, vanadium pentoxide, ruthenium oxide, rhodium oxide, and palladium oxide.

17. The method of claim 3 wherein said step of reacting is carried out at a pressure between about 400 and 900 mm Hg and said alkali metal chlorate is reacted at a concentration of between about 0.05 and 0.3 M.

18. The method of claim 3 wherein said step of reacting is carried out at a pressure of between about 100 and 400 mm Hg and said alkali metal chlorate is reacted at a concentration of between about 0.3M and 1.5 M/liter.

19. The method of claim 3 wherein said hydrogen peroxide comprises less than 30% on an equivalent strength basis, of said reducing agents.

20. The method of claim 3 wherein said hydrogen peroxide comprises between 5 and 10%, on an equivalent strength basis, of said reducing agents.

21. The method of claim 3 comprising a continuous process.

22. A method for producing chlorine dioxide comprising reacting alkali metal chlorate with reducing agents in an aqueous acidic solution, said reducing agents comprising methanol, sodium chloride, and hydrogen peroxide, said hydrogen peroxide comprising less than 50%, on an equivalent strength basis, of said reducing agents, wherein said method is conducted at: a. a temperature of between about 35° C and about 80° C; b. an acid normality of between about 4 N and 12 N; c. a pressure of between about 400 and 900 mm Hg; and d. an alkali metal chlorate concentration of between about 0.01 mole/liter and 4 moles/liter.

23. The method of claim 22 wherein said acid is selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chloric acid.

24. The method of claim 23 wherein said acid is sulfuric acid.

25. The method of claim 22 wherein said alkali metal chlorate is sodium chlorate.

26. The method of claim 22 wherein said reaction is carried out at a temperature of between about 50° C and about 75° C.

27. The method of claim 22 wherein said reaction is carried out at about atmospheric pressure.

28. The method of claim 22 wherein the acid normality of said reaction medium is between about 7 N and 10 N.

29. The method of claim 22 wherein said hydrogen peroxide comprises less than 30%, on an equivalent strength basis, of said reducing agents.

30. The method of claim 22 wherein said hydrogen peroxide comprises between 5 and 10%, on an equivalent strength basis, of said reducing agents.

31. The method of claim 22 wherein a catalytic species for enhancing generation of chlorine dioxide is added to said reaction.

32. The method of claim 22 comprising a continuous process.

33. A method for producing chlorine dioxide comprising continuously feeding alkali metal chlorate, reducing agents, and acid into a reaction chamber, while removing chlorine gas, water vapor, other gaseous byproducts, alkali metal salt, unreacted chlorate and acid, methanol, sodium chloride, and hydrogen peroxide, wherein: a. said reducing agents comprise methanol, sodium chloride, and hydrogen peroxide, and said hydrogen peroxide comprises less than 50%, on an equivalent strength basis, of said reducing agents; b. said process is conducted at a temperature of between about 35° C and about 80°; c. said process is conducted at an acid normality of between 4 N and 12 N; d. said alkali metal chlorate is present in said reaction medium in a concentration of between about 0.01 mole/liter and 4 moles/liter.

34. The method of claim 33 wherein said acid is selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chloric acid.

35. The method of claim 34 wherein said acid is sulfuric acid.

36. The method of claim 33 wherein said alkali metal chlorate is sodium chlorate.

37. The method of claim 33 wherein said reaction is carried out at a temperature of between about 50° C and about 75° C.

38. The method of claim 33 wherein said reaction is carried out a pressure of between about 400 and 900 mm Hg.

39. The method of claim 33 wherein said reaction is carried out at about atmospheric pressure.

40. The method of claim 33 wherein said reaction is carried out a pressure of between about 100 mm Hg and 400 mm Hg.

41. The method of claim 33 wherein the acid normality of said reaction medium is between about 7 N and 10 N.

42. The method of claim 33 wherein said hydrogen peroxide comprises less than 30%, on an equivalent strength basis, of said reducing agents.

43. The method of claim 33 wherein said hydrogen peroxide comprises between 5 and 10%, on an equivalent strength basis, of said reducing agents.

44. The method of claim 33 wherein a catalytic species for enhancing generation of chlorine dioxide is added to said reaction.

45. The method of claim 44 wherein said catalytic species is selected from the group consisting of silver nitrate, manganese sulfate, vanadium pentoxide, ruthenium oxide, rhodium oxide, and palladium oxide.