"Near zero liquid discharge membrane distillation crystallization"
, in JKU Linz, Linz, 2022
Near zero liquid discharge membrane distillation crystallization
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Membrane Distillation (MD) is considered a promising method for the desalination of saltwater. However, a general economic and environmental drawback of MD is the saline solution left behind. Similarly, important details and analyzes are still missing to enable a physiochemical understanding, including the operating time, and the inclusion of salt kinetics.
An MD Crystallization plant can improve the problem with residual brine. This process consists of two important parts, one is the MD and the other is the seeding crystallization. This thesis is about the construction of a pilot-scale MD crystallization plant at the Institute of Chemical Engineering in collaboration with Wabag and the separation of feed - solutions with high salt content in this plant. For better control of the crystallization and to prevent scaling, a seed crystal of quartz sand SiO2 was added. In the experiments with a 22.25 % NaCl solution, a permeate flux of 1.6 kg m-2 h-1 was obtained and it was found that the flux increased again during crystallization. The water recovery was 59 % and 42 % and a yield of 133 g NaCl was obtained, corresponding to a salt recovery of 33.25 %. Furthermore, an experiment was carried out with two salts with different solubilities. For fractional crystallization, the two salts CaCl2 (720 g L-1 at 20°C) and NaCl (359 g L-1 at 20°C) were used to produce a solution with a salinity of 24.3 %. The permeate flux was 0.77 kg m-2h-1 the water recovery was 45 % and 29 % of the salt was recovered.
In summary, it could be shown that a scale-up to pilot scale is feasible. A 22.25 % NaCl solution could be concentrated until crystallization. In addition to desalinated water, NaCl could also be obtained as a product. Fractionated crystallization also enabled the separation of mixed salt solutions into individual salt fractions. The salts were further examined using an optical microscope.