CHLORINE DIOXIDE GENERATION
There are multiple reaction chemistries used to generate chlorine dioxide on the scale required for industrial and oilfield use. In general, they either oxidize sodium chlorite or reduce sodium chlorate. These methods have specific characteristics (safety, efficiency, cost) that strongly influence their suitability for various purposes. Below are brief descriptions of the most common processes along with their pros and cons. In all cases the primary precursor (chlorite/chlorate) is in aqueous form, as are sodium hypochlorite (bleach) and acid (sulfuric/hydrochloric).
There are two basic methods of feeding the chemicals into the generation process – pumps or vacuum eduction. It is my belief that the vacuum eduction process is much to be preferred for two reasons.
- Motive water flow through an eductor is used to create a vacuum that in turn pulls the precursor chemicals into the system to form ClO2. If motive water flow is lost, chemical flow stops, and the reaction shuts down. Thus, it is inherently much safer than pump fed systems.
- Since chemicals are fed under vacuum, any leaks will draw air into the system rather than result in a pressurized leak of chemical to the outside environment.
Sodium Chlorite/Bleach/Acid (3-Part)
2NaClO2 + NaOCl + 2HCl > 2ClO2 + 3NaCl
This is the most efficient and safest reaction chemistry available for most applications. With properly designed equipment, reaction efficiency is typically 98+% and precursors are readily available from numerous sources.
Sodium Chlorite/Acid (AC)
In this process, chlorite is acidified to produce chlorous acid which disproportionates to produce chlorine dioxide. While researchers state that several reaction pathways can occur in this reaction, the general equation is as follows:
5NaClO2 + 4HCl > 4ClO2 + 5NaCl + 2H2O
While this reaction is used relatively commonly, it does have several serious drawbacks, such as
- Poor yield, thus higher cost. Note that only 80% of the sodium chlorite is converted to ClO2 (4 moles of ClO2 are produced from 5 moles of NaClO2).
- To achieve reasonable reaction efficiency and speed, acid must be overfed significantly versus stoichiometry to achieve a pH of <0.5. This typically requires an acid overfeed of 3x to 5x. This adds further cost as well as corrosion concerns.
- Precursors are reacted in concentrated form, thus resulting in ClO2 concentrations of well over 100,000 ppm in the generator. Refer to safety discussion.
This generation chemistry has been used for many years for large volume applications such as pulp and paper bleaching. Small scale generators were developed relatively recently that are suitable for industrial and oilfield use. Sodium chlorate is reacted with hydrogen peroxide and sulfuric acid. The ratio of chlorate to peroxide is fixed, so these two precursors are combined into one product. Thus, the three chemistry’s are available in the form of two precursor products. Several sources of these chemistry’s are now available.
2NaClO3 + H2O2 + H2SO4 > 2ClO2 + O2 + Na2SO4 + 2H2O
Chlorate chemistry has the advantage of being economical. Sodium chlorate is produced as an interim step in the production of sodium chlorite; thus, it is less expensive as a precursor. However, the reaction chemistry causes a couple of concerns.
- The reaction requires high purity sulfuric acid of 78% concentration. If the acid is not sufficiently pure (less than approximately 50 ppm iron content), the reaction may be inefficient or the reaction chamber may suffer damage due to micro-“puffs”. The generator must then be flushed/cleaned of the contaminated acid, repaired if necessary, and a clean acid supply provided. Thus, the purity of the acid supply and its handling is of high importance.
- The sulfate present in the solution from the sulfuric acid can be problematic in many oilfield waters. If even low levels of barium or strontium are present, they will form an insoluble precipitate with the sulfate and will scale equipment and downhole formations.
- Precursors are reacted in concentrated form, thus resulting in ClO2 concentrations of well over 100,000 ppm in the generator.