Roya Sheikholeslami

Thermodynamics and kinetics for mixed calcium carbonate and calcium sulfate precipitation

Abstract The effects of CaSO4 on CaCO3 precipitation were studied in the batch tests at 60, 70 and 80°C in mixtures having calcium carbonate as the dominant salt and at a given total initial calcium concentration of 0.03 M with sulfate concentration ranging from 0 to 0.01 M . Solubility products and rate constants were determined from thermodynamic and kinetic studies and the results indicated that even minute amounts of calcium sulfate affect the thermodynamics, kinetics and the scale structure and no longer the solubility data and rate constants for the pure salt were applicable. Presence of CaSO4 from 0.002 to 0.01 M increased the calcium carbonate solubility product more than an order of magnitude. The effect of salt mixture on the solubility constant of non-dominant salt (calcium sulfate) was reverse as calcium sulfate solubility increased to its pure value with increases in its molar ratio. In addition, the rate equation suggested in the literature (J. Colloid Interface Sci. 37 (1971) 824; J. Colloid Interface Sci. 36 (1971) 166) for pure salt was not applicable to the experimental data. The general observations indicated that the presence of CaSO4 had weakened the CaCO3 scale which is usually very adherent. The experimental results did take into account the effect of solution ionic strength, however, they suggest that data for pure salt precipitation seem not to be extendable to co-precipitation.

Assessment of the effect of velocity and residence time in CaSO4 precipitating flow reaction

Abstract The effect of flow hydrodynamics on kinetics of precipitation of sparingly soluble salts that cause fouling of process equipment has not been addressed previously. Precipitation of sparingly salts from solutions is usually considered to be due to crystallization on the surface. Precipitation in the bulk or in the boundary layer and its subsequent deposition has been traditionally neglected and to our knowledge, no previous research work has focused on this important area. Computational fluid dynamics was used as a tool to examine the effect of flow on and the location within the flow domain where calcium sulphate precipitation occurs. Initially, isothermal conditions were used to isolate the effect of flow; non-isothermal conditions were examined for turbulent flows usually encountered in heat exchange systems. Precipitation kinetics was modelled using a simple second-order reaction usually used in fouling of calcium sulphate. In laminar flow, the characteristics of the velocity distribution led to the emergence of radial concentration gradients and as a result a radial diffusive flux was induced. Similar behaviour was evident under turbulent conditions but not to the same extent. In turbulent flow, particles were mainly produced in the viscous sub-layer rather than in the turbulent bulk. The reduced velocity within the laminar flow or turbulent viscous sub-layer increased the residence time within these regions, enabling precipitation to take place and more particles to form. The generated particles could deposit by particulate fouling in addition to the fact that consumption of ionic species by precipitation in the boundary layer would induce ionic concentration gradient resulting in diffusion of ions to the walls and further crystallization fouling on the walls. Deposit layer formed by crystallization on the surface has a different structure than that formed by particulate fouling. The effect of velocity and residence time on fouling, its mechanisms, and deposit structure should not be overlooked.

Mechanisms, thermodynamics and kinetics of composite fouling of calcium oxalate and amorphous silica in sugar mill evaporators—A preliminary study

Abstract Deposition of amorphous silica (SiO 2 ) and calcium oxalate monohydrate (COM) on the calandria tubes of evaporators cause serious processing problems in many cane sugar mills. Previous studies on scale formation have concentrated on fouling of individual compounds and the development of inhibiting methods for each component. Since SiO 2 and COM co-exist in sugar mill evaporators, this paper investigates the mechanisms and behavior of composite fouling of COM and SiO 2 in binary systems. Batch tests conducted at different pH (6–8) and temperatures (60–80°C) show that the presence of SiO 2 in the supersaturated solutions of COM decreased the precipitation rate of COM and as such increased COM solubility. However, the presence of COM in the supersaturated solutions of SiO 2 accelerated SiO 2 polymerization and lowered the level of initial silica supersaturation required for polymerization. The formation of COM–SiO 2 complexes is used to propose the mechanism for the co-precipitation of SiO 2 and COM.