Gypsum scale formation on a heated copper plate under natural convection conditions and produced water remediation technologies review

Abstract

Scaling or crystallization fouling of unwanted salts is one of the most challenging and expensive problems encountered in different applications such as heat exchangers and thermal water treatment technologies. Formation of dihydrated calcium sulfate scale, also known as gypsum, on a heated copper plate is studied in lab. The copper plate, held at a given temperature, is immersed in a supersaturated solution of calcium sulfate prepared at a given concentration. The flow conditions are governed by natural convection. A parametric study, in which surface temperature and the degree of supersaturation are varied, is set up and a scale inception time curve is plotted. No scale is observed at a supersaturation index smaller or equal to 1.4. Both higher temperatures and higher concentrations result in faster scale induction; however, the effect of temperature is more significant at lower degrees of supersaturation. SEM images of scale samples show needle-like crystals, the thinnest of which formed at a supersaturation index of 2.0. The classical nucleation theory of Mullin provides an excellent fit for the results. Interfacial energies calculated out of this model are in the reported ranges.