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Initial fouling rate and delay time studies of aqueous calcium sulphate sealing under sensible heating conditions Fahiminia, Feridoun


Calcium sulphate scaling can give rise to operating problems and additional costs in industrial systems and desalination units. In order to control scale formation it is desirable to understand the mechanisms of its inception and growth. The present study focuses on understanding how process variables such as fluid velocity and temperature affect the initial fouling rate and delay time. For initial fouling rate, previous studies of calcium sulphate scaling have demonstrated conflicting behaviour with respect to fluid velocity: some investigations report an increase, others a decrease, and still others no change of initial fouling rate with increasing fluid velocity. For the delay time, most researchers have focused on bulk precipitation rather than on wall surface crystallization. For wall surface crystallization, no values have been reported for calcium sulphate delay time activation energies. In this work a theoretical model for initial crystallization fouling rate in turbulent flow, where attachment is treated as a physico-chemical rate process in series with mass transfer (Epstein, 1994), was examined. According to the model, mass transfer is directly proportional to the friction velocity, and attachment is inversely proportional to the square of this velocity. Therefore, at a given wall temperature, it follows that if the initial fouling rate is mass transfer controlled (low fluid velocity), the deposition flux increases as the fluid velocity increases. If, however, the initial fouling rate is attachment controlled (high fluid velocity), the deposition flux will decrease as the fluid velocity increases. Therefore as the fluid velocity is lowered the initial fouling rate goes through a maximum at a given wall temperature. In addition, this maximum initial fouling rate can be expected to increase and move towards higher critical velocities as the wall temperature increases. Fouling experiments were performed in a Tube Fouling Unit (TFU) (Wilson and Watkinson, 1996; Rose et al., 2000) using aqueous calcium sulphate solutions as recirculating fluid. To meet the required operating conditions, some modifications were made on the TFU. Mainly two different sets of experiments were performed, one using a concentration of 3400 ppm of calcium sulphate in solution, and the other in which a range of concentrations from 3100 to 3600 ppm was covered. The first set of experiments was performed over film Reynolds numbers of 2100 - 36000, clean inside wall temperatures of 66 - 87°C and bulk temperatures of 50°C, to observe the effect of velocity on both initial fouling rate and delay time. In the second set, performed to extract crystal surface energies, the Reynolds number and bulk temperature were kept constant at 20000 and 50°C, respectively. Also, some extra experiments were performed to investigate deposit removal occurrence and the filter pore size effect on the fouling behaviour. All the experiments were performed in a 9.02 mm i.d. electrically heated, stainless steel tube. The main features of the model were qualitatively demonstrated with calcium sulphate solutions, i.e. a maximum in experimental initial fouling rate at a given wall temperature over a range of fluid velocities, and an increase in the maximum rate and in the corresponding critical velocity as the wall temperature was increased. Calcium sulphate scaling results showed that as the velocity increased from 0.1 to 1.6 m/s, the fouling activation energy, ΔE[sub f], increased from 66 to 620 kJ/mol. This observation was consistent with the model, but the maximum fouling activation energy was significantly larger than the kinetic activation energy, ΔE, reported by other investigators. Modeling results showed an optimal solution, with an average absolute percent deviation in initial fouling rates of 67.4 % from the fit of the model. ΔE was evaluated as 262.5 kJ/mol. To reduce the deviations between the model predictions and experimental results, the model was refined by nominally taking the number of nucleation sites into account. This was done by inserting a simple function of wall temperature in the surface integration term. The results of the refined model indicated a substantial reduction in the average absolute percent deviation. From classical nucleation theory and delay time measurements the effective surface energy values were determined as 7.5 to 9.9 mJ/m² over a range of wall temperatures from 73 to 82°C. These values are close to the values of 7.9 and 14.6 mJ/m² that have been reported by Linnikov (1999) for surface nucleation on a metal surface and by Hasson et al. (2003) on a polymeric membrane surface under laminar flow conditions, respectively. Also, from delay time measurements it was possible to use Branch's (1991) approach, applied by him to black liquor fouling, for generating delay time activation energies. Calcium sulphate delay time activation energies for wall surface crystallization were determined for the first time over a range of fluid velocities. It was shown that as the velocity increased, delay time activation energies increased and approached a value around 172 kJ/mol. In order to separate the contribution of surface reaction (integration) from that of mass transfer, purely chemical activation energy values were generated through kinetic studies of calcium sulphate precipitation in a jacketed-glass reactor. The activation energies were determined as 210 and 254 kJ/mol for initial concentrations of 3400 and 3100 ppm, respectively. These values were smaller than the maximum fouling activation energy of 620 kJ/mol extracted from fouling experiments. This observation again suggested that the number of surface nucleation sites plays an important role in the wall surface crystallization process. Removal effects were studied by increasing the fluid velocity while simultaneously eliminating the concentration driving force. No continuous deposit removal was detected at a velocity of 0.7 m/s. Finally, at higher wall temperatures filter pore size had no impact on the delay time and the initial fouling rate. However, at lower wall temperatures the initial fouling rate increased with filter pore size, indicating the occurrence of bulk precipitation and particulate fouling at these temperatures.

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