Gypsum scaling in membrane distillation: Impacts of temperature and vapor flux

Mineral scaling by sparingly soluble gypsum (CaSO4∙2H2O) is a persistent challenge to membrane distillation (MD). The underlying relationship between the thermodynamic state of the precipitating solution and the point of flux decline due to rapid mineral growth remains unclear. In this work, a series of experiments along with a semi-empirical model are executed to examine the thermodynamic state of the feed solution at the feed/membrane interface to evaluate and compare the critical point of scaling. The experiments were deliberately designed in a way to decouple the influence of feed temperature and vapor flux. The thermodynamic state of the precipitating solution at the membrane interface is evaluated using the saturation index and the nucleation energy barrier derived from the chemical potential difference between the dissolved ions and the gypsum mineral. The model is rooted in established heat and mass transfer relationships and reflects the testing conditions used to carry out the experiments. The model is built upon experimental results across a range of operational conditions, with the bulk feed solution temperature ranging from 50 to 80 °C (at a constant flux) and the trans-membrane water flux ranging from 10 to 40 L m−2 h−1 (at a constant feed temperature). It was observed that interfacial saturation index calculated at the induction point was not consistent across different experiments, confirming that gypsum scaling in MD is controlled by kinetics instead of thermodynamics. We also found that temperature plays a more important role than vapor flux in affecting the critical recovery. Lastly, we also provide theoretical reasoning to support the experimental observation that gypsum scaling in MD is largely dominated by heterogeneous nucleation onto the membrane surface.