Production of pure water and dry salt from NaCl solutions via percrystallisation by supported carbon membranes

Abstract

Membrane percrystallisation is an alternate crystallisation process where a membrane disperses aqueous salt solution into a thin film on the permeate side. This thin film can be instantaneously vaporised to form pure water vapor and dry, crystallised salt under vacuum conditions. This work demonstrates, for the first time, that supported carbon membranes are a viable configuration for the percrystallisation of saline solutions. Within the work, the carbon materials in powder and supported membrane form were produced from polyvinylpyrrolidone (PVP) by pyrolysis at temperatures ranging from 450 and 800 °C. Porosity analysis identified that the materials do not adsorb gas. This suggests that the materials are non-porous or the pores are not accessible at the analysis conditions. All produced membranes were of supported nature with approximate thickness of 8 μm. The flux measurement revealed that the membranes, despite being similar in thickness, resulted in variable flux exhibiting a parabolic profile, with a maximum flux obtained (18 kg m−2 h−1) when tested for percrystallisation at 50 °C. The membranes maintained structural integrity when operated with 3.5 to 10 wt% NaCl solutions at 50 °C. However, membranes began to degrade when operating with 20 wt% solutions at 70 °C. The solid product analysis showed that the NaCl produced by percrystallisation is crystalline NaCl, with various morphological properties. The analysis also revealed that the particle size and shape depends on the solution flux through the membrane with smaller crystals at higher fluxes. The results of this study are used to propose a mechanism of salt formation for the percrystallisation of aqueous salt solutions.