What is an ozone destruct unit?

An ozone destruct unit is used to catalytically destroy excess ozone gas produced in an ozonation process. These processes include:

1. Offgas from an ozone contacting tank
2. Excess ozone produced in a CVD process
3. Exhaust from an ozone analyzer

Although ozone gas is unstable and will thermally decompose on its own, it is often advantageous to destroy it more rapidly.

Why do we want to destroy ozone?

Ozone is a very corrosive gas, causing damage to many materials, including non-stainless metals and most plastics and rubber. In addition, personnel exposure limits set by OSHA are 0.1 ppm over an eight hour period and 0.3 pm for 15 minutes. The IDLH for ozone is 5 ppm. Please see the MSDS sheet for ozone for more information on the toxicity of ozone.

Depending on local regulations it may also be illegal to discharge ozone gas into the atmosphere.

How does it work?

The ozone gas is passed through a bed of manganese dioxide which catalytically converts the ozone back to oxygen gas. This process is exothermic and often produces enough heat to make the reaction go very quickly. This type of process is often referred to as a Thermal-catalytic destruct unit.

This process is a true catalytic process and the catalyst bed is not consumed. A catalyst bed should last 1-5 years depending on usage and the carrier gas of the ozone. If the ozone is produced in air, the life of the catalyst bed will be shorter than if the ozone is produced in oxygen gas. See the section on Bed Fouling for more information.

Why do some ozone destruct units have heaters?

Ozone gas applications can be either wet or dry. If the ozone is first contacted with water, such as in a contact tank for treating water, the ozone offgas is saturated. Even though the conversion of ozone to oxygen is exothermic, a heater is required to keep the catalyst material above the dewpoint. This is especially true on start-up of the destruct unit before it comes to temperature. This is also true if the exotherm is not enough to keep the catalyst dry. The catalyst will lose its reactivity if it becomes wet and must be either replaced or baked in an oven at 500F until it is dry.

Examples of dry applications are those in which the ozone gas does not come into contact with water, such as in a CVD process, downstream of a gas phase process ozone analyzer, or in a process where excess ozone from a generator must be eliminated.

What is the catalyst used in the ozone destruct unit?

The catalyst is known as a hopcalite-type of catalyst whose primary constituents are manganese dioxide and copper oxide. The material is not toxic or dangerous and can be disposed of in a Class 1 landfill. Please see the MSDS sheet for the catalyst material.

Can I buy replacement catalyst?
Replacement catalyst is available in 1,5 and 45 pound containers.


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