Chott El Fejjej using a three-phase fluidized bed reactor — modelling aspects

Cecarbonation is considered in the context of research on the prevention of scaling. To apply it to the geothermal waters of Chott El Fejjej in El Hamma (Gabes, Tunisia), we examined water characteristics having a high permanent calcic hardness (100°F) accelerating the scaling and a high temperature at emergence (65°C) favouring the exhaust of CO2. In addition to these natural factors which predispose water to scaling, we introduced air bubbling with a distributor, permitting transport of CO2 (leading quickly to the oversaturation in calcium carbonate) and injected seeds that contribute, by adsorption, to the acceleration of calcium carbonate precipitation. This is a clean technology of decarbonation without chemical reagents. This research links our previous work in 2002 where we used a pilot reactor having a capacity of 50 L. We achieved a continuous pilot reactor having a capacity of about 1 m3 higher performance. This reactor was conceived in order to allow the study of some key parameters of importance in scaling at different radial and axial levels. It determined reactor efficiency by its output (fundamental objective) and characterization of the flow state. Decarbonation was performed to reach a target residual complete alcalimetric title (CAT) 6°F (1°F = ! 12.2 mg HCO3 /L), which stemmed from our previous experimental work. The target output was reached and was even exceeded under specific conditions where working parameters were optimised. We investigated the reactor output at its gradual filling up. This method showed an evolutionary aspect and a study of modelling kinetics according to two methods: a gait according to diffusional and Reddy models, and a gait adopting two approaches — first-order kinetics and the Haas and Karra model. The preferred model was the one of Haas and Karra that was perfectly applied (R2 . 0.99) and that is original in the domain of decarbonation under the complex, specific conditions of water characteristics such as temperature, etc.

The decarbonation using seed scale and air bubbling is a new clean technology without the addition of chemical reagents. This technology is based on transportation of CO2 by air bubbling (exhaust encouraged by a high temperature of about 65°C) and an adsorption in surface by seeds whose contribution is incontestable. The logical choice was a three-phase fluidized bed continuous reactor with water and air cocurrent flows. The unit is comprised of an air compressor, a flowmeter for air and a flow meter for water. The distribution of air is conceived according to different configurations permitting flexibility of choice. The optimization research on the pilot reactor’s working parameters obtained results about the seed nature (scale), the best scale concentration, scale size, the ratio of air flow:water flow, the contact and reaction time. Using these optimized working parameters, we investigated different variants. After a study of the reactor in continuous flow we could conclude that the output determined to different local points calculated (in percent) on the basis of the vestigial CAT is nearly the same everywhere. On the other hand, a gradual investigation on the pilot reactor, with the progression of its replenishment, showed a notable difference along the localization of points (in height): the output was appreciable from the beginning of seed injection and the target output was reached after 20 min. The modelling aspect was studied by adopting two configurations: a gait according to two diffusional and Reddy models and a gait adopting two approaches: first-order kinetics and the Haas and Karra model. Although the diffusional and Reddy models are more commonly used for the precipitation of calcium carbonate, they did not prove to be so valid: the diffusional model is not valid (0.70<R2<0.86) and the Reddy model is acceptable but is not very valid (0.88<R2<0.95). A first-order kinetics is not very valid (0.83<R2 <0.9). To research the best modeling, we adopted a more elaborate expression (model of Haas and Karra). This model gave perfect validity (R2. 0.99). Although this model is usable in the conception and management of disinfection systems, we see that it is perfectly applicable in the case of decarbonation, a completely different domain, where there is complexity of water constitutions, a temperature of 65°C and other very particular conditions.