Novel percrystallisation process by inorganic carbon membranes

Abstract

This thesis demonstrates, for the first time, the use of sucrose derived inorganic membranes for crystallisation. In addition, this thesis also shows, for the first time, a process where solvents and solutes are separated in a single step, contrary to polymeric membrane crystallisation which requires two extra steps downstream to filter and dry crystals. There are two further major aspects of this first demonstration as a contribution to the body knowledge in the area of crystallisation. The third novelty is the demonstration of the percrystalisation transport mechanism, occurring as the solution permeates through the membrane where a wet thin film is formed on the permeate face of the membrane. The wet thin film forms a solid-liquid-vapour interface. At this interface, effective evaporation of the solvent occurs under a vacuum, resulting in the crystallisation of the solute on the membrane surface. Hence, a unique single step separation process occurs as the solvent evaporates and concomitantly the crystallised solute is continuously ejected from the surface of the membranes. This novel process is called percrystallisation reflecting the single step permeation and crystallisation. Additionally, this process is able to produce essentially pure water at rates comparable to reverse osmosis. This is possible, even at a concentration that far exceeds the capabilities of reverse osmosis. Further, high production crystallised solute rates are achieved as one metre square of membrane can deliver crystallised NaCl fluxes of up to 48,000 kg m-2 per year. The fourth additional demonstration is related to morphology of the membrane and process operations in affecting crystal formation. For instance, by varying carbonisation temperature and feed temperature or vacuum pressure, it is possible to tune the crystallite size and particle size state of the treated solute. Indeed this thesis demonstrates how sodium chloride particles are produced with a much smaller and narrower particle size distribution compared to conventional methods. Likewise, changes in the carbon content of the membrane, feed temperature, and feed concentration are able to produce nickel sulphate with different hydration states. This thesis further shows examples how versatile membrane percrystallisation can be for processing several types of solutes such as paracetamol (a pharmaceutical analgesic), Vitamin C (food and pharmaceutical compound) and several nitrate and sulphate salts for hydrometallurgy applications. This thesis opens a window of research opportunities to effectively crystallise compounds of interest for a wide spectrum of industries.