Characterization of the effectiveness of anti-scalants in suppressing scale deposition on a heated surface Xianhui Li, Hilla Shemer, David Hasson ⁎, Raphael Semiat Rabin Desalination Laboratory, Wolfson Faculty of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel HIGHLIGHTS • AS effectiveness determined by depositing CaSO 4 scale on heat exchanger at various Re numbers. • Augmented scaling with Re number indicates mass transfer controlled precipitation. • Scaling which is unaffected by Re number denotes surface reaction controlled precipitation. • AS assessed by the threshold concentration at which scaling is unaffected by transport hydrodynamics • The shift to surface reaction control is significantly affected by the temperature level. abstract article info Article history: Received 31 March 2016 Received in revised form 19 June 2016 Accepted 21 June 2016 Available online 25 June 2016 A widely used technique for controlling scale deposition in desalination practice is by dosage of anti-scalants. Fundamental techniques for assessing the relative effectiveness of different anti-scalants are of considerable in- terest. The objective of the present research was to extend a technique characterizing the effectiveness of anti- scalants by identifying the threshold concentration at which scale deposition is unaffected by transport hydrody- namics and is purely surface controlled. A lower threshold level denotes higher inhibition effectiveness. The sys- tem investigated consisted of an electrically heated annular flow heat exchanger enabling measurement of calcium sulfate scale deposition on a stainless steel heating surface at a constant surface temperature. Application of the technique is illustrated by scale deposition measurements performed at varying Reynolds numbers in the absence and presence of three anti-scalants and at different scale deposition temperatures. © 2016 Elsevier B.V. All rights reserved. Keywords: Calcium sulfate Scale inhibition Heat exchanger Mass transfer control Surface control 1. Introduction A common method for controlling scale formation in desalination practice is by the use of anti-scalants (AS). Anti-scalants are polymeric compounds, endowed with the capability of delaying precipitation of sparingly soluble salts and inducing morphological changes leading to decreased adhesion of scale deposits on flow surfaces. The conspicuous advantage of this scale control technique is that, when properly applied, an anti-scalant can suppress scale formation with very low dosage and hence, at an affordable cost. The scale suppression process involves complex phenomena related to transport of the inhibiting molecules from the solution bulk to the scaling surface. This is followed by adsorp- tion/reaction of the molecules with active growth sites of the scale ma- trix thus interfering with nucleation and crystal formation processes [1– 3]. The fundamentals of inhibition mechanisms, particularly from their quantitative aspects, are poorly understood so that the effects of operat- ing parameters on inhibition effectiveness are largely unpredictable. Application of anti-scalants is still a substantially empirical technology. Against this background of dearth of guiding theory, practical needs have prompted development of a variety of empirical techniques to as- sist selection of an optimal AS, each of which has its own advantages and limitations [4]. A technique that has the advantage of being related to intrinsic physico-chemical properties of inhibitor-crystal structure of the scaling species is based on the following considerations. Scale deposit forma- tion involves two processes in series: diffusional transport of the crystal forming ions towards the crystallizing deposit and incorporation of the ions on growth sites of the crystal lattice by surface reaction. Scale precipitation in the absence of anti-scalants is usually mass-transfer controlled [5–7]. An anti-scalant acts to retard the kinetics of scale depo- sition. The inhibition process is known to involve adsorption of anti- scalant molecules on active growth sites of the crystallization surface thereby retarding scale growth. At a sufficiently large concentration, Desalination 397 (2016) 38–42 ⁎ Corresponding author. E-mail address: hasson@tx.technion.ac.il (D. Hasson). http://dx.doi.org/10.1016/j.desal.2016.06.022 0011-9164/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal