Catalytic Chlorine Dioxide - How it Works
Cloxide™ Catalytic – the next generation
Clearwater Technology plc have introduced an innovative approach to surpass conventional chlorine dioxide generation.
Cloxide™ Catalytic incorporates reliable, proven components and technologies to take advantage of chlorine dioxide’s unique properties.
Step One... No matter how you make chlorine dioxide, you must first start with chlorous acid. Chlorous acid is a simple compound that occurs when the sodium (Na+) ion in sodium chlorite (NaClO2) is replaced by a hydrogen (H+) ion. The resultant compound, chlorous acid (HClO2), is the real precursor for the formation of chlorine dioxide (ClO2). Its formation is shown by Reaction (1).
Na+ClO2- + H+ —> H+ ClO2- + Na+ (1)
In traditional technologies acid is added to sodium chlorite in high concentrations to encourage the displacement of Na+ with H+. However, since residual sodium ions remain in solution, the equilibrium of the reaction cannot be shifted entirely to chlorous acid unless a large excess of acid is added. Therefore a mixed solution exists under normal circumstances. The inability of Reaction (1) to go to completion inhibits the further formation of chlorine dioxide by limiting the amount of chlorous acid available. This is especially true in dilute solutions or when a weak acid is used.
Cloxide™ Catalytic forms a solution of chlorous acid without residual Na+. By using cation exchange resin in the H+ form, we are able to remove the Na+ from the sodium chlorite and replace it with H+ to form pure chlorous acid. When the cation resin is exhausted, i.e. all of the cationic sites are in the Na+ form, the resin must be regenerated with a hydrogen source. This may be done either onsite or off-site, depending upon the conditions of the application.
It is important to remember that the acid used during regeneration does not enter into the production of chlorous acid and never contacts the sodium chlorite. The acid is merely used as a hydrogen source for the regeneration of the cation exchange resin.
Step Two... Chlorine Dioxide
Once chlorous acid is formed, the reaction to chlorine dioxide may proceed in one of two ways: via the disproportionateness of chlorous acid to chlorine dioxide or via the oxidation of chlorous acid to chlorine dioxide.
5 HClO2 —> 4 ClO2 + HCl + 2 H2O (2)
HClO2 - e- —> ClO2 + H+ (3)
Reaction (2) is extremely time and concentration dependent. Reaction (3) often yields unwanted chlorinated byproducts (chlorine is commonly used in this reaction to donate an electron since it is an oxidising agent). Unwanted byproducts are also formed if electrochemical methods are used to oxidise the chlorous acid.
Conversion of chlorous acid to chlorine dioxide via Reaction (2) is typically less than 65% after one hour, leaving residual chlorite in the chlorine dioxide product. Conversion of chlorous acid to chlorine dioxide via Reaction (3) can be as high as 95%, but these conversion efficiencies require high concentrations of chlorous acid and chlorine. They also often require the presence of additional acid or complicated and expensive electrochemical generators. Neither of these processes has proved suitable for the commercial and small industrial market.
Cloxide™ Catalytic generates chlorine dioxide from chlorous acid without chlorine or excess acid, without chemical mixing, without electricity and without residual chlorite in the chlorine dioxide product stream .
By utilising catalytic technology in the Conversion Cartridges, we convert virtually all of the chlorous acid to chlorine dioxide via Reaction (2). This occurs instantaneously and can be achieved in dilute solutions.
This means that there is virtually no chlorite residual (as confirmed by independent analysis) in the chlorine dioxide product. Because of the high conversion rates associated with Cloxide™ Catalytic, the need for onsite storage of chlorine dioxide is removed.
Further, since Cloxide™ Catalytic can operate under high pressures, the chlorine dioxide product can be dosed directly into pressurised lines without the need for additional booster equipment. All of this occurs with chlorine dioxide concentrations of less than 700 mg/l.
For full technical and installation details please contact your local Clearwater Chlorine Dioxide product specialist