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Mist Eliminator

Mist Eliminator Introduction

Within an acid plant there is the necessity to remove acid mist and droplets from the gas stream exiting the drying and absorption towers.  The primary reasons for trying to capture, collect and remove the mist and droplets are:

• To prevent damage to downstream equipment

• To avoid undesirable atmospheric emissions

• To recover valuable acid from the gas stream

Acid mist and droplets are formed in one of three ways; Mechanical, Condensation or Chemical Reaction.

Mechanically formed droplets usually range in size from 10 to 100 microns.  They generally form when acid is re-entrained due to localized high velocity gas tearing droplets away from a liquid film or from the splashing or spray generated from a liquid distribution device.

Mist or fume are much smaller in size (3 microns or less) and are generated from the sudden or shock cooling of hot gas containing sulphur trioxide.   Chemical reaction between sulphur trioxide and water will also produce a mist or fume.  This generally occurs at the inlet of the absorber towers.

Fibre filters have proven to be an effective device for capture, collection and removal of mist and droplets from gases.  In sulphuric acid applications, fibre bed mist eliminators are used to remove acid mist and entrained droplets from the process gas exiting the drying and absorbing towers.  Proper and effective removal of the acid mist and droplets will extend the life of the downstream equipment and prevent undesirable atmospheric emissions.

Types of Mist Eliminators

There are generally two types of mist eliminators: Impaction and Brownian Diffusion Types so-called because of the primary collection mechanism employed in their design.  Impaction type mist eliminators employ impaction and to a lesser extend interception methods to capture, collect and remove acid mist.  As such, impaction devices are effective for the the larger particles and are less efficient for the smaller submicron particles.  To collect the submicron particles, a Brownian diffusion device must be used.