Inhibition of calcium sulfate depositions by a fluidized bed

Abstract

The calcium sulfate scale inhibiting capability of a fluidized bed was investigated in four heat exchanger tubes at concentration factors of standard seawater between 1 and 3. In all experiments, deposition on the tube walls was prevented as long as the hemihydrate solubility line was not exceeded, yielding a practical procedure to assure scale-free operation of a multi-stage flash/fluidized bed evaporator. The scale layers deposited at temperatures above the hemihydrate line consisted initially of stellated conglomerates of hemihydrate crystals, which frequently seemed to be mixed with substantial amounts of gypsum. This initially formed layer was replaced by an anhydrite layer at a rate enhanced by increasing temperatures and decreasing pH values.