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Contenido archivado el 2024-05-29

Modelling CaSO3/CaSO4 precipitating flow in water treatment devices

Final Activity Report Summary - FOULING SIMULATION (Modelling CaSO3/CaSO4 precipitating flow in water treatment devices)

Scaling, i.e. the deposition of sparingly soluble salts on process equipment, remains a limiting factor in water treatment. The scaling layers decrease the efficiency of separation measures and the according removal measures deteriorate the economic and ecologic balances of the process.

This project aimed to identify appropriate mathematical models for the simulation, or prediction, of calcium carbonate and calcium sulphate (CaCO3 / CaSO4) scaling phenomena occurring at membrane separation and heat transfer. These predictions contributed to better understanding and preventing scale formation via improved process design.

The first emphasis was the investigation of chemical and physical properties for the matter involved with respect to the range of operating conditions. The properties sought for were:
1. the solubility limits for the scales and the solution osmotic pressure;
2. the kinetics of scale formation;
3. the equation of state of saline water;
4. the heat capacity of saline waters;
5. the transport properties for momentum, heat and mass.

A tool was developed to determine the thermodynamic equilibrium of saline solutions given that, in theory, all properties sought for were derivable from this state. The solubilities of pure salts and the corresponding saline solution composition were indeed found to be well predictable by this approach. All other properties were modelled by alternative semi-empirical and empirical models as part mechanisms and interaction parameters for the thermodynamic approach were either missing or unreliable. Hence, the kinetic description of the scale formation was clearly the bottleneck.

The second emphasis was the description of transport mechanisms involved in scale formation. A standalone flow simulation code was developed to implement the property data and specific boundary conditions for heat and mass transfer in simplified water treatment devices. The corresponding simulations revealed the transient behaviour of the precipitating flow and the concentration and temperature polarisation effects leading to scale formation. As an analogue to the modelling efforts an empiric procedure was developed for evaluation of the simulations. The results for nanofiltration of a single salt solution demonstrated that the scale formation correlated with the wall concentration of the dissociation products. This supported the assumption that the surface crystallisation was the prior scaling mechanism.

In summary, the process of scale formation was consistent with many coupled mechanisms. In this study, these part mechanisms were analysed with respect to modelling and prediction. An overall model was developed that allowed for the estimation of scaling propensities for simplified aqueous solutions and apparatus geometries. The issues to be addressed for future 'ab-initio' prediction of real systems were identified as:
1. improved and consistent physical property models for the involved matter;
2. the exploration of matter interactions (wall-solute-solvent);
3. multi-component transport algorithms applicable to commercial Computational fluid dynamics (CFD) tools.