The roles of gas bubbling, wall crystallization and particulate deposition in CaSO 4 scale formation

Abstract

The objective of this work was to seek conditions leading to rapid deposition of a tenacious calcium sulfate scale layer from a falling film flowing on a heated stainless steel surface. The main results were as follows: (A) The rate of calcium sulfate scale deposition was considerably enhanced (by a factor of 5–10) when boiling occurred on the film flowing on the heat transfer surface; (B) Filtration of suspended calcium sulfate particles from the feed solution led to a very large reduction (by a factor of 20–30) in the rate of scale deposition. This result is entirely opposite to that encountered in the formation of calcium carbonate scales where particle filtration greatly enhances the rate of deposition of a calcium carbonate scale layer and increases considerably scale tenacity. These observations suggest that the dominant mechanisms in calcium sulfate formaiton is particulate deposition while the dominant mechanism in calcium carbonate scale formation is wall crystallization. (C) Pure calcium sulfate scale deposits were found to be far less adherent than deposits containing co-precipitated calcium carbonate. The co-precipitated calcium carbonate seems to act as a bonding cement, enhancing considerably the tenacity of the calcium sulfate scale layer.