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Early Events in the Precipitation Fouling of Calcium Sulphate Dihydrate under Sensible Heating Conditions
Author(s) -
Fahiminia Feridoun,
Watkinson A. Paul,
Epstein Norman
Publication year - 2007
Publication title -
the canadian journal of chemical engineering
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.404
H-Index - 67
eISSN - 1939-019X
pISSN - 0008-4034
DOI - 10.1002/cjce.5450850515
Subject(s) - fouling , arrhenius equation , chemistry , thermodynamics , supersaturation , analytical chemistry (journal) , precipitation , aqueous solution , nucleation , chromatography , activation energy , meteorology , membrane , biochemistry , physics , organic chemistry
Supersaturated aqueous solutions of calcium sulphate, an inverse solubility salt, were circulated through a 9 mm i.d. stainless steel tube in which they were subjected to a constant and uniform heat flux at Reynolds numbers ranging from 2100 to 36 000. Precipitation fouling of the tube surface, which was monitored and measured thermally, resulted in calcium sulphate dihydrate (gypsum) scales. Measured delay times decreased with increasing bulk solute concentration, C b , in accord with classical nucleation kinetics, decreased with increasing surface temperature T s , its reciprocal correlating with T s in the Arrhenius manner, and decreased with increasing fluid velocity V up to V ≈ 0.5 m/s before flattening out. Initial linear fouling rates, R fo , at C b = 3400 ppm, also yielded Arrhenius plots, with fouling activation energies increasing almost eight‐fold over the 0.1–1.6 m/s velocity range. This result implies a shift in the mechanism of fouling rate control with increasing velocity from mass transfer, which is weakly temperature dependent, to surface integration, which is strongly temperature dependent. The “Initial Fouling Rate Model,” which simulates this mechanistic shift with a unique dependence of the surface‐integration step on the residence time of the fluid at the tube wall, was used to represent, by least‐squares fitting, 84 fouling rate data points at different values of V and T s , with three adjustable parameters. Observed trends of R fo with V and clean wall T s were qualitatively captured by the model, but quantitative agreement was imperfect. Substantial quantitative improvement was effected by introducing into the model an additional temperature dependence of the surface‐integration term—resulting in one additional adjustable parameter—rationalized as a method of accounting for the previously neglected temperature‐dependent nucleation that accompanies crystal growth. The fouling investigation was supplemented by an experimental study of crystal growth kinetics.