Modeling of alkaline scale formation in falling film horizontal-tube multiple-effect distillers

In this study, a mathematical model is developed, coupling mass transfer with chemical reaction, to calculate alkaline scale formation from simultaneous desorption–deposition of CO2–CaCO3 from evaporation of water falling film in horizontal-tube multiple-effect (ME) distillers. The saturation levels, calculated by the Oddo–Tomson saturation index (OTSI) and by the calcium carbonate precipitation potential (CCPP), are found to be positive which indicate scaling conditions. The effect of top brine temperature (TBT), concentration factor (CF), pH and salinity on CaCO3 deposition rates, scale thickness and fouling resistance (Rf) are fully described and interpreted. In addition, their effect on the CO2 desorption rates is also discussed. The amount of CO2 desorbed decreases from 36.4 g/ton feed water in the first stage to 32.5 g/ton in the last stage. The specific CaCO3 deposition decreases from 127.3 g/ton feed water in the first stage to 100.1 g/ton in the last stage. This corresponds to an average alkaline scale thickness of 165–130 μm and fouling resistance of 0.31–0.25 m2K/kW. CaCO3 deposition rates increase with increasing top brine temperature, salinity and concentration factor and decrease with increasing pH values of the seawater.