The zinc oxide process is similar to the iron sponge process. It uses a solid bed of granular zinc oxide to react with the H2S to form water and zinc sulfide:
The rate of reaction is controlled by the diffusion process, as the sulfide ion must first diffuse to the surface of the zinc oxide to react. High temperature (>250°F) increases the diffusion rate and is normally used to promote the reaction rate.
Zinc oxide is usually contained in long, thin beds to lessen the chances of channeling. Pressure drop through the beds is low. Bed life is a function of gas H2S content and can vary from 6 months to in excess of 10 years. The spent catalyst is discharged by gravity flow and contains up to 20 weight percent of sulfur.
The process has seen decreasing use due to increasing disposal problems with the spent catalyst, which is classified as a heavy metal salt.
