Mechanislm of Calcium Sulfate Scale Deposition o n Heat-Tra nsf e r S urf aces David Hasson and Joseph Zahavi' Department of Chemical Ifngineering, Technion-Israel Institute of Technology, Ilaifa, Israel
Investigation of Cas04 scale deposition in an annular heat exchanger shows that surface nucleation along a heated tube occurs at a nonuniform rate, highest a t the downstream edge, and leads to the formation of a correspondinlgly nonuniform scale layer, thickest at the downstream edge. Rates of nucleate front propagation and scale-layer growth increase with surface temperature, decrease with increasing flow velocity, diminish with fine filtration, but are unaffected by the degree of recirculation. Transient nucleation is a predominant mechanism in Cas04 scale formation. A kinetic model based on this mechanism is presented.
C.kI,ciI:\I SULF.iTt; cry,qtalliziiig from an aqueous solution appears i i i three form:+-gypsuin (CaS04.2HZO), calcium sulfate heinihydrate (CaS04.11'2H,O), aiid anhydrite (C'aSOd). A s all three varieties have solubilities which decrease with iiicre:iyiiig teriiper:it~uie (Figure I ) , CaS04 scale teiids to precil~it'atefroin solut,ioii oil heat,-triiiisfer surfaces. 111saline 1v:itcr c o i i ~ ~ e r s i nsingle-purpose ~i, distillati~iiunits 1)ecoiiie more economic as the Irine top temperat,iire :md conctiitrntioii :we increased. 1l:tiiy efforts have therefore been tievotcd to t#he dctcrinin:ition of liniitiiig conditioiis under which :I distillation unit caii be operated without risking ])recil)itatioiiof the C:iS04 present i r i sea water. The geiier:iI apl)roncli has bceii to wtahlish solubility limits of the various CaS04 modificatioirs i n sea-water coiicentratrs. The coiiceiitratioii fact,or-teiiiljrr:iture diagram prepared by Standiford and Siiiek (1961) on i.he basis of sparse so1ul)ility data has heen widely used bot'h as a design tool and for correlat,ioii of the scaliiig performarice of operatiiig units. A cornprehensive plot of such d a h on a n amended solubility diagram has k~eeii recently givcw hj. Lu aiid Fabuss (1968). Scaling perforniancc~ can be only pirtly evaluated from soluldity comidrratioiis alone. Kinetic as1)ects inust, nljo lie considered, siiice iioiiscaling operation C R I I be practically achieved uiider certai II supersaturatioir conditimiq. Though anhydrite is the st'able pliase above about 40°C, Partridge and n'hitc (1929) aiid Laiigelier et a/.(1950) fouiid that the niet,:istable hcmihydratr was precipitntcd iiiucli iiiore r:Illi
