Method for generating chlorine dioxide

Details Method for generating chlorine dioxide Method for generating chlorine dioxide

2001-03-23

2002-08-20

Warden, Sr., Robert J. (Department: 1744)

Chemical apparatus and process disinfecting, deodorizing, preser

Process disinfecting, preserving, deodorizing, or sterilizing

Using disinfecting or sterilizing substance

C422S037000, C422S028000, C210S754000, C423S477000, C426S335000, C131S300000

Reexamination Certificate

active

06436345

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for generating chlorine dioxide using carbon dioxide and sodium chlorite.
2. Background Art
Chlorine dioxide is a strong oxidizer and is widely used as a bleaching and/or disinfectant agent, with hundreds of tons being generated and used each day in the paper and water treatment industries. Chlorine dioxide is also used in considerably smaller quantities in treating agricultural produce and certain medical applications. Chlorine dioxide is well known as an algaecide, fungicide, germicide, deodorant, bleach, and general antiseptic. Currently, the equipment required for generating chlorine dioxide usually requires gaseous chlorine, sulfuric acid, or a combination of sodium hypochlorite (bleach) and acid with either sodium chlorite or sodium chlorate. Because one or more hazardous materials are typically required to generate the chlorine dioxide or produced as a byproduct, the use of chlorine dioxide has been somewhat limited. The cost of chlorine dioxide generating units and the need for trained personnel to operate and maintain the generating units has also hampered the wider utilization of chlorine dioxide. Accordingly, there exists a need for a method that is capable of readily and safely producing chlorine dioxide without requiring the use of more hazardous chemicals or generating them as reaction byproducts.
Chlorine dioxide is a hazardous material. Pure chlorine dioxide is an oily, dark amber liquid and is extremely unstable at temperatures above −40° C. Chlorine dioxide is also explosively unstable as a gas in concentrations greater than 10% by volume in air or at partial pressures above 76 mm Hg (1.46 psig). Above these levels chlorine dioxide may detonate if it contacts flammable organic solvents or other oxidizable materials. Chlorine dioxide is also pressure sensitive and may decompose violently if compressed, making it impractical to store or ship. Pure solutions of chlorine dioxide may also detonate if exposed to bright light or an ignition source such as heat, a spark, or an open flame. The upper boundary for safe chlorine dioxide concentrations in aqueous solutions is about 8 g/L at 30° C., with most generating systems being designed to operate well below that limit, typically producing solutions having 1-3 g/L chlorine dioxide.
It is well known that chlorine dioxide is formed by reaction of sodium chlorite and an acid. For instance, U.S. Pat. No. 3,591,515 to Lovely teaches the use of various acidifying agents to generate chlorine dioxide at pH levels below 6 and form a dry powder fungicide. U.S. Pat. No. 4,330,531 to Alliger discloses a twin-compartment container for generating chlorine dioxide by reacting solutions of lactic acid and sodium chlorite. U.S. Pat. No. 4,585,482 to Tice, et al. discloses a biocidal composition that uses a chlorine dioxide-liberating compound and a hydrolyzable organic acid-generating polymer (such as a methylvinylether/maleic anhydride copolymer) for lowering the pH of the composition and slowly releasing the chlorine dioxide.
Chlorine dioxide is generally formed in one of two ways, either by reducing a chlorate ion (ClO
3
−
) in an acidic medium according to reaction [1]:
ClO
3
−
+2H
+
+e
−
→ClO
2
+H
2
O  [1]
or by oxidizing a chlorite ion (ClO
2
−
) according to reaction [2].
ClO
2
−
→ClO
2
+e
−
  [2]
The choice of reducing agent for chlorine dioxide generation from chlorate has a great bearing on optimum reaction conditions, byproducts, and economics. Production from chlorite ion (ClO
2
−
) is rather uneconomical. Indeed reaction [2] is reversible and chlorite is commonly synthesized from chlorine dioxide. Reducing agents typically used for producing chlorine dioxide from chlorate are sulfur dioxide (SO
2
), methanol (CH
3
OH), chloride ion (Cl
−
), and hydrogen peroxide (H
2
O
2
). The associated half reactions are represented by reactions [3]-[6].
SO
2
+2H
2
O→SO
4
−2
+4H
+
+2e
31
  [3]
CH
3
OH+H
2
O→HCOOH+4H
+
+4e
31
  [4]
2Cl
−
→Cl
2
+2e
−
  [5]
H
2
O
2
→O
2
+2H
+
+2e
−
  [6]
Combining reaction [1] with reactions [3]-[6] produces reactions [7]-[10].
2ClO
3
−
+SO
2
→2ClO
2
+SO
4
−2
  [7]
4ClO
3
−
+CH
3
OH+4H
+
→4ClO
2
+HCOOH+3H
2
O  [8]
ClO
3
−
+Cl
−
+2H
+
→ClO
2
+Cl
2
+H
2
O  [9]
2ClO
3
−
+H
2
O
2
+2H
+
→2ClO
2
+O
2
+2H
2
O  [10]
The byproducts formed are sulfate ion (SO
4
2−
), formic acid (HCOOH), chlorine (Cl
2
), and oxygen (O
2
). The acid equivalents required per mole of chlorine dioxide produced differ and are zero for sulfur dioxide, one for methanol, two for chloride, and one for hydrogen peroxide. Acid consumption is also influenced by the process conditions used in particular commercial designs.
In all systems, a side reaction may occur, reduction of chlorate to chloride according to reaction [11].
ClO
3
−
+6H
+
+6e
−
→Cl
−
+3H
2
O  [11]
Steps must be taken to minimize this reaction by careful choice and control of reaction conditions. The basic mechanism of chlorine dioxide formation has been extensively studied and has previously been described in detail elsewhere by Haller and Northgraves in
TAPPI
(a publication of the Technical Association of the Pulp and Paper Industry) (April 1955) and Lenzi and Rapson in the
Pulp Paper Magazine of Canada
(1962). In each of the disclosed mechanisms, chloride plays a crucial role as evidenced by its presence in all chlorate-based reaction media and by the trace amounts of chlorine in the chlorine dioxide formed. No chlorine dioxide is formed if chloride ion is not present in the reaction medium. Chloride ion is introduced into the system by reduction of chlorate to chloride (reaction [11] above) or by addition of chloride in the feed. In a 1956
TAPPI
article, Rapson proposed the following mechanism where chloride ion is the reducing agent.
HClO
3
+HCl→HClO
2
+HClO  [12]
HClO
3
+HClO
2
→2ClO
2
+H
2
O  [13]
HClO+HCl→Cl
2
+H
2
O  [14]
The formation of byproducts, other than those identified in reactions [7]-[10], is governed by the chlorate salt and acid selected. In all commercial processes, the chlorate salt used is sodium chlorate (NaClO
3
) and, to date, the most commonly used acids have been sulfuric (H
2
SO
4
) and hydrochloric (HCl). Consequently, the most common byproducts are sodium sulfate (Na
2
SO
4
) and sodium chloride (NaCl). Depending upon process conditions, sulfate is recovered as neutral crystalline sodium sulfate, sodium sesquisulfate (Na
3
H(SO
4
)
2
), or is dissolved in an acidic effluent. If hydrochloric acid is used, sodium chloride is recovered in a crystalline form or in an internally recycled solution.
Commercial chlorine dioxide generation systems can be broadly divided into atmospheric and sub-atmospheric processes. Atmospheric processes include the Mathieson, Solvay, and Rapson R2 processes which use sulfur dioxide, methanol, and sodium chloride, respectively, as the primary reducing agents. Each of the processes use air to strip and dilute the chlorine dioxide and have an overflow of spent sulfuric acid. In the 1950s, the Mathieson process was dominant, followed by the Solvay process. The Mathieson process was developed in 1950 by Olin-Mathieson Chemical Corporation and generated chlorine dioxide by reducing

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