Characterization of sugar juice heat exchanger tube deposit

Abstract

Fouling of heat exchanger surfaces during sugar manufacture reduces productivity and increases energy demand. This study characterizes a deposit (including its internal structure) formed in a sugar factory's evaporator unit using a variety of X-ray diffraction and scanning electron microscopy techniques (including X-ray powder diffraction, X-ray fluorescence, elemental mapping with energy dispersive spectroscopy and backscattered electron imaging). Calcium sulfate dihydrate, calcium oxalate mono- and dihydrate, hydroxyapatite, amorphous silica and organic matter are present in the deposit. The composition of the deposit (which contains three layers) varies along the height of the tube. There are noticeable differences in the composition and porosity among the layers in the deposit. A porous structure consisting of a mixture of amorphous silica, calcium oxalate dihydrate and organic matter is attached to the surface of the deposit in contact with sugar juice, while a denser morphology of amorphous silica and hydroxyapatite is attached to the tube wall. Elemental mapping identifies an association between Si, Al, Mg, Fe and O, suggesting the presence of a silicate compound as a minor component in the deposit. An attempt is made to rationalize the formation of the observed phases.