Technical Information Paper

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SCOPE Provide basic information on measures to protect personnel and equipment in the handling and storage of chlorine dioxide as a gas and as an aqueous solution. It is vitally important that any new chlorine dioxide facility design be undertaken with a supplier of chlorine dioxide technology in order to cover all of the unique design parameters these standards.

DEFINITION Chlorine dioxide is a powerful oxidizing agent, used to produce bleached pulp. Chlorine dioxide has other applications in the mill including sterilization of drinking water and general water treatment (bactericide).

Chlorine dioxide is manufactured on-site at pulp mills since it is chemically unstable and cannot be transported. Many different chemical processes are used by pulp mills to produce chlorine dioxide. The basic chemistry used is sodium chlorate based:

Sodium chlorate + strong acid + reducing agent Þ chlorine dioxide + by-products

The acid and reducing agent can vary depending on the process selected, which are determined by the mill’s operating economics and the usefulness of the by-product streams. Chlorine dioxide is made as a gas and absorbed in chilled water to produce a solution strength ranging from 8 to 12 grams of chlorine dioxide per liter of solution.

MATERIAL Properties of Chlorine Dioxide

Equilibrium Solubility in Water (3):

SAFETY PRECAUTIONS

CAS Registration: 10049-04-4

DOT Number: NA 9191

NIOSH RTECS Number: F03000000

Health Hazards (4)

Chlorine dioxide gas is a severe respiratory and eye irritant. Chlorine dioxide can affect the body if it is inhaled or if it comes in contact with the eyes or skin. It can also affect the body if it is swallowed.

Chlorine dioxide may cause irritation of the eyes, nose, throat, and lungs. It may produce coughing, wheezing, and severe breathing difficulties which may be delayed in onset.

A physician should be contacted if anyone develops any signs or symptoms and suspects that they are caused by exposure to chlorine dioxide.

Reported exposure to a worker of 19 parts per million for an unspecified time period proved fatal. Repeated acute exposure of workers to undetermined concentrations is stated to have caused eye and throat irritation, nasal discharge, cough, wheezing, bronchitis, and delayed onset of pulmonary edema. Repeated exposure may cause chronic bronchitis.

Personal Protective Equipment (4, 8)

Respiratory protective equipment is required for entry into areas where airborne concentrations are or may be above 0.1 parts per million chlorine dioxide. Use only NIOSH/MSHA approved devices and the following protective measures in the event of a spill of solution or a gas leak:

Cartridge type respirators should never be used for entry into an area with a suspected chlorine dioxide release. These respirators are intended for escape purposes only.

Wear neoprene or butyl rubber gloves to protect the skin. Wear a rubber suit, and neoprene-coated boots, together with chemical goggles or full face shield for a large leak.

Quantitative dose-effect information on chlorine dioxide is sparse. Permissible workplace exposure limits have been set based on analogy with ozone, and by extrapolation from laboratory test using various species of small animals exposed mostly to high concentrations of chlorine dioxide (5). The following recommended exposure limits should be followed:

• 8-hour time-weighted average (TWA): 0.1 parts per million. [OSHA 1988, (6)]

• Short-term exposure limit, 15 min. (STEL): 0.3 parts per million. [ACGIH 1991, (5)]

• Immediately dangerous to life or health (IDHL): 5 parts per million. [NIOSH 1994, (7)]

First Aid (8)

Inhalation: Move to fresh air. If person is not breathing, give CPR. If breathing is difficult, give oxygen. Keep person warm and resting. Obtain medical attention immediately.

Eyes: Immediately flush with lukewarm water for 15 minutes, holding eyelids apart. Obtain medical attention immediately.

Skin: Remove contaminated clothing immediately. Wash exposed skin with soap or mild detergent and water, or flush with water for 15 minutes. Obtain medical attention.

Ingestion: Have person rinse mouth with water. Give 8 to 10 ounces (240 - 300 ml) of drinking water to a conscious person. Do not induce vomiting. If vomiting occurs, rinse mouth and repeat administration of drinking water. Obtain medical attention immediately.

Chlorine dioxide gas is unstable and decomposes readily to chlorine and oxygen under upset conditions. The decomposition reaction is as follows:

ClO₂ Þ 1/2 Cl₂ + O₂ + 26.3 kcal/mole

The decomposition reaction, commonly referred to as a "puff", is exothermic and propagates through the gas at a rate proportional to the partial pressure of ClO₂ in the vessel. Chlorine dioxide gas may decompose readily when concentration in air exceeds 10 per cent. Decomposition of chlorine dioxide at low concentrations can be initiated by:

• Reactive metals, such as iron, copper, etc.
• An electric spark or static electricity occurs
• Temperatures rise above 100^oC
• Organic contaminants, especially hydrocarbon greases, oils, and rubber
• Dust and rust particles are present
• Sunlight enters the vessel or pipe
• Sudden pressure fluctuations

Introduction of catalytic or reactive impurities usually initiate decompositions. Most oxidizable substances, particularly insolubles in the feed streams or reaction liquor, may initiate decomposition. Care must be taken in: selecting corrosion-resistant materials for the raw materials-handling systems; maintaining the filters in the chemical feed systems; and avoiding oxidizable greases and other lubricants in all parts of the plant.

When the partial pressure of chlorine dioxide is in the 100 mm Hg range or less, decompositions are generally mild. However, decompositions become much more violent as the concentration increases. Once a decomposition starts, it is exothermic and the fire continues to burn as long as chlorine dioxide is present. The force of the decomposition increases with increasing partial pressure. At a chlorine dioxide partial pressure of 300 mm Hg and above, potentially damaging detonations can result.

For atmospheric reactors, air is primarily used to dilute the chlorine dioxide gas concentration to about a 12 per cent concentration (partial pressure 91 mm Hg). Safety design features and safety interlocks must be put in place to ensure proper dilution of chlorine dioxide gas and a safe shutdown.

For vacuum operated plants, water vapor and other reaction components are used primarily to maintain a safe gas concentration between the reactor and the chlorine dioxide absorber. The chlorine dioxide gas concentration between the generator vapor condenser and the absorber operates between 40 to 50 volume per cent. Safety design features and safety interlocks must also be put in place to maintain proper dilution of the chlorine dioxide gas and ensure a safe shutdown.

All reactors (and chlorine dioxide storage tanks) are equipped with explosion hatches which raise to relieve the sudden pressure resulting from a "puff." For an atmospheric reactor, the accompanying rise in temperature should shut off feed valves while simultaneously opening a purge valve which admits oil-free air to the reactor, diluting and cooling the decomposing gas. For a vacuum operated chlorine dioxide plant, chemical feeds should be shut off with an indication of a decomposition (i.e., high temperature, high pressure). Also, water and/or air dilution should be supplied to the generator gas space to help quench or dilute the chlorine dioxide decomposition. There are other important safety features and interlocks, which the mill must incorporate in a mill’s chlorine dioxide plant with the guidance of their chlorine dioxide technology supplier.

Chlorine dioxide gas and aqueous solutions are highly corrosive. Piping material selection should be based on the particular process application. The following materials are typically used for chlorine dioxide handling:

Storage and handling of chlorine dioxide (11)

The chlorine dioxide tank storage area receives 8-12 grams per liter of chlorine dioxide solution from the absorber in the chlorine dioxide plant. The storage tank area consists of chlorine dioxide storage tank(s), chlorine dioxide transfer pump(s), with the storage tanks usually in a dike.

Pressure Relief Design (12)

Chlorine dioxide decomposition is a propagating reaction, similar to a combustion front or flame. For the chlorine dioxide concentrations typical in storage, the gas phase decomposition is classified as a deflagration. Leung, et al (12), presents a method for sizing pressure relief systems for chlorine dioxide storage tanks. Important factors to determine the design relief area include: pressure rating of the tank, volume of the tank, and chlorine dioxide solution’s concentration and temperature. We design a storage tank and it’s relief system with the worse case scenario in mind. For instance, if there is a power failure of the sweep fresh air fan that draws the air across the tank, can the tank and it’s relief system handle a decomposition of the highest possible chlorine dioxide concentration? The analysis also assumes a low level in the tank, which is the worst possible decomposition.

A HAZOP type of analysis for a mill’s chlorine dioxide storage system results in a safe operating system.

Ventilation of Chlorine Dioxide Storage Tank (13)

Providing a sweep of fresh air across the vapor space of storage tank has shown to minimize the probability and violence of decompositions. Below is a graph which shows the air sweep rate (cubic feet per minute) per cross section area of the chlorine dioxide storage tank (square feet) for various chlorine dioxide solution concentrations and temperatures. This assumes a uniformly mixed gas space while maintaining a chlorine dioxide partial pressure of 40 mm Hg.

Normally, the chlorine dioxide gases swept from the storage tank are sent to a chemical or chilled water scrubber via a fan. Ideally, the chlorine dioxide from the storage tank gas is recovered with a chilled water scrubber and sent to the chlorine dioxide absorber as chilled water make-up.

Other Important Design Features (13, 14)

There are other important design features which should be considered in the design of a chlorine dioxide storage tank. These include:

• Since light catalyzes the decomposition of chlorine dioxide, pigment can be used in the manufacture of FRP tanks and pipes to block light from reaching the chlorine dioxide liquid and chlorine dioxide vapor.

• The aspect ratio (height to diameter ratio) for chlorine dioxide storage tanks should be at least 2.0 or greater. This will reduce the severity of decompositions by minimizing the available liquid surface area, which diffuses chlorine dioxide gas. The smaller tank area requires less sweeping air.

• A floating roof on the liquid surface can reduce the surface area of the gas liquid interface, reducing the chlorine dioxide diffused to the gas space in the tank. This allows the storage of strong chlorine dioxide solution, 12-14 gram per liter, with a small concentration of chlorine dioxide in the gas space. The potential benefits are: lower water input to the bleach plant, less water chiller requirements, and smaller chlorine dioxide storage and transfer systems. Ma (14) provides a good discussion on floating roofs for chlorine dioxide storage. Note, when designing a chlorine dioxide tank using a floating roof, the tank design should assume no floating roof is used. This will provide a safe design basis for the storage tank pressure rating and pressure relief design.

Tank Entry

Chlorine dioxide storage tanks should be inspected on an annual basis to determine the condition of the lining. It is important to follow safe procedures when entering a chlorine dioxide tank.

Drain the tank completely and flush several times with chilled water or with a solution containing a chlorine dioxide reducing agent (e.g., white liquor). Isolate the tank so no chlorine dioxide liquid or vapor can re-enter the tank. Force ventilate the tank with air by opening the manhole and the relief lid(s). This should be done until the odor has been diminished and the oxygen content is at an acceptable level.

After following these steps, and the mill’s confined space entry procedure, the tank can be inspected. Mill personnel should carry the appropriate personal safety equipment and wear the proper type of protective clothing.

The loss of chlorine dioxide solution from storage tanks or from other process equipment and piping should be contained and flushed with large quantities of water or neutralized with a solution containing a chlorine dioxide reducing agent (e.g., white liquor). The chlorine dioxide storage tanks should be in a dike to contain a spill, and prevent damaging the surrounding area. The volume capacity of the dike area should not be less than 110% of the greatest amount of liquid that can be released from the largest tank within the dike. The chlorine dioxide solution will release chlorine dioxide into the atmosphere and until disposed of neutralized or covered with foam will expose workers and perhaps the public to risk of gas inhalation.

The chlorine dioxide storage area should be equipped with a sump located at the lowest point in the dike floor. This sump should be capable of handling rain water, wash water, chlorine dioxide solution, and miscellaneous debris.

To prevent a hazardous situation, provisions must be taken in draining the dike area. Upon collecting fluid in the dike area, liquid should be tested to determine if it is safe to drain to the process sewer. If the dike area is located such that gravity permits, manual drain valves will be opened to drain the dike area to the process sewer, or a booster pump or jet can be used to discharge to another holding source for reuse or disposal.

As an option, the dike area can contain a submersible sump pump or an air diaphragm pump capable of pumping the fluid to a process sewer or another holding source. In either case, the valves and/or the pump(s) should be manually turned on from outside the dike area.

References

14. Ma, Raymond; "Floating Roof for Chlorine Dioxide Storage Tank," Pulp and Paper Canada 95:3 (1994) T129-132.
15. Chlorine Dioxide Environmental and Technical Information Manual, COFI Industrial Hazardous Chemical Handling Task Force June 1997
16. Jim Hopmans "Chlorine Dioxide Releases: Guidelines for Dispersion Modeling", to be published 1999 CPPA Annual Meeting