A numerical study of CaCO₃ deposition on the membrane surface of direct contact membrane distillation

This numerical study investigates the effect of CaCO₃ deposition on the performance of direct contact membrane distillation (DCMD). It is the first numerical study to directly evaluate the deposition and film thickness of CaCO₃ on the membrane surface. Therefore, a 3D computational fluid dynamics (CFD) model, which combines momentum, heat, species transport reaction, and the Eulerian wall film model, is used to analyze the CaCO₃ mass deposition rate and scaling deposition thickness on the membrane surface. HSC Chemical Software is used to evaluate the ion compositions and reaction kinetics, including the activation energy (E_a) and pre-exponential factor (A_r). The CFD model is validated with experimental results. The results reveal that the CaCO₃ deposition rate increases significantly at a high temperature, velocity, and salinity of the solution. The CaCO₃ mass deposition increased by 15 times and scaling deposition thickness increased from 0.05 nm to 1 nm when the temperature was raised from 318.15 K to 348.15 K, whereas an increase of approximately 25 % in deposition was observed with an increase in velocity from 0.17 to 1.38 m/s at a concentration of calcium (Ca⁺²) ions, 0.03 % and bicarbonate (HCO₃⁻) ions 0.004 %. The results also indicate that CaCO₃ deposition decreases the flux and efficiency of the DCMD by 2 % with a low concentration of Ca⁺² ions, 0.03 %, and HCO₃⁻ ions, 0.004 %. The findings indicate that the high feed salinity (75.23 g/l) increases CaCO₃ mass deposition and scaling deposition thickness on the membrane surface due to the high concentration of Ca⁺² ions, 0.07 %, and HCO₃⁻ ions, 0.01 %, significantly reducing the permeate flux by 14 %. The study's model development and implementation can be adjusted to offer guidance for studying the MD process in treating high saline water with fouling issues.