Catalyst Protection

Catalysts are often used in chemical processes to increase the speed of chemical reactions. Very often the reactants are hot gases that derive from high temperature processes.

Sometimes the catalyst is in the form of a granular bed in which the gases pass through a relatively thick bed of catalyst. Alternatively the catalyst may be in the form of a stack of layers of mesh, providing a tortuous path for the gas. In either case any solids conveyed along with the gas will be collected on the inlet side of the catalyst. The solids will accumulate, increasing the pressure drop across the catalyst until the plant has to be shut down and the catalyst cleaned. This can be a dirty, time-consuming and expensive operation.

Clearly it would be desirable to filter the process gas before passing it through the catalyst. However, the gas is often hot and is required to be hot for the catalytic reaction to work. It is much preferable to filter the gas at high temperature. The alternative would be to cool the gas, filter it and then heat it up again.

Traditionally hot gas filtration has been performed by electrostatic precipitators. However, these are not very efficient and still require the catalyst to be cleaned at quite frequent intervals.

The above photo shows a Caldo ceramic filter being used to protect a high temperature catalyst on a waste sulphuric acid recovery plant. The The filter replaced a much larger electrostatic precipitator which can be clearly seen behind the ceramic filter.

The people in the above photo included a delegation of engineers from JSC-Slavneft in Russia, who were interested in using a similar filter on the waste sulphuric acid plant at their oil works at Yaroslavl. They subsequently ordered a filter and it was installed in 2005.

The second photo shows this filter at Slavneft's site, in January 2006. The ambient temperature was -32°C, but inside the filters it was 510°C and about +5psig. The vessels, constructed in type304 stainless steel, performed perfectly producing the required capacity at a pressure drop well within specified limits. The particulate removal efficiency was virtually 100%.

Caldo ceramic filters have also been used to clean gases before oxidation catalysts. These have been used on, for instance, exhaust gases from mould burnout furnaces in the lost wax process for investment casting. In this process residual wax in the moulds cracks into lower molecular weight hydrocarbons and soot in the furnace, creating the risk of an unpleasant smell. The ceramic filter removes the soot, allowing the VOC catalyst to react the hydrocarbons with oxygen to produce CO₂ and water.