Effect of thermohydraulic conditions on fouling of calcium oxalate and silica

Calcium oxalate (CaOx) and amorphous silica (SiO2) have been identified as the most intractable components of scale formed in sugar mill evaporators. Effects of thermohydraulic conditions such as surface superheat (5-15 K), fluid velocity (0.8-1.8 m/s), and bulk subcooling (20-30 K) on the composite fouling process of CaOx and SiO2 and deposit characteristics were investigated in a dynamic system that contained a fouling loop under subcooled nucleate flow boiling. The rate of composite fouling was found to increase with increasing superheat as a result of enhanced bubble activities at the boiling surface and increase in deposit strength. For a constant surface superheat, the fouling rate reached a maximum value at a subcooling of 25 K, but remained constant over a range of fluid velocity ( greater than or equal to1.1 m/s). This was attributed to the counterbalancing effects of deposit consolidation and hydrodynamic lift forces at the fouled tube wall. Scanning electron microscopy (SEM) analysis of composite deposits demonstrated the presence of two layers arising from deposit consolidation by CaOx in SiO2 matrix through microlayer evaporation during nucleate boiling. Chemical microanalysis by energy dispersive X-ray spectroscopy (EDS) also indicated that the degree of deposit consolidation increased with surface superheat, fluid velocity, and bulk subcooling because of an increase in heatflux. (C) 2005 American Institute of Chemical Engineers.