Numerical study of CaCO3 scaling in submerged vacuum membrane distillation and crystallization (VMDC)

The kinetics of CaCO3 scaling in a submerged vacuum membrane distillation and crystallization (VMDC) operation was evaluated using numerical methods incorporating the “mass jump” method to evaluate the temperature and concentration on the membrane surface, with experimental validation. As the concentration at the membrane surface was higher than the bulk feed solution, a scaling model based on the surface integration model was more accurate than the ion transfer model, with the energy activation (Ea) of 89.2 kJ/mol and zero order rate constant (k0) of 5.81 × 108 m4/kg·s. The CaCO3 deposition rate was significantly affected by temperature polarization and concentration polarization, indicated by 40% increase and 23% decrease on CaCO3 deposition rate when assuming no temperature polarization and concentration polarization, respectively. However, competing effects of temperature polarization and concentration polarization resulted in only an 8% increase of the CaCO3 deposition rate when both were neglected. Investigation of CaCO3 scale accumulation over 20 h in solution conducive to bulk precipitation (0.35 g/L CaCl2 and 0.73 g/L NaHCO3) and solutions below bulk precipitation threshold (0.0875 g/L CaCl2 and 0.1825 g/L NaHCO3) resulted in 9-11% higher CaCO3 accumulation when both temperature polarization and concentration polarization were neglected in the calculation. This can be a concern in prediction of CaCO3 scaling on longer operation time test. The model development method and implementation of this study can be adapted to provide guidance in study of MD process for high saline water treatment with inevitable scaling.