Water distillation performance of carbon nanotube membrane: Non-equilibrium molecular dynamics simulation

Carbon nanotube (CNT) has a great potential as membrane material for water distillation because of its smooth and hydrophobic surface. We numerically investigated the distillation performance of CNT membrane for direct contact membrane distillation using non-equilibrium molecular dynamics (NEMD) simulation by varying diameters and lengths of the CNT and system operating conditions such as temperature, temperature difference between feed (hot) and permeate (cold) reservoirs, and sodium chloride (NaCI) concentration in the feed reservoir. It was found from the NEMD simulations that the distillation performance is enhanced by increasing system temperature, reservoir temperature difference, and CNT diameter, and decreasing CNT length, atomic attraction strength between water molecules and CNT, and NaCI concentration. The NEMD simulation overpredicts the water vapor transport by approximately an order of magnitude as compared with the results from the Knudsen diffusion model. The simulated flooding pressure is in good agreement with the theoretical prediction by the Young-Laplace equation using the MD-calculated contact angle. Most importantly, the permeability of the CNT membrane is two orders-of-magnitudes higher than a common polymer-based membrane made of Polytetrafluoroethylene (PTFE) due to almost two order-of-magnitude higher Knudsen diffusion of the CNT membrane than that of the PTFE membrane.