Carbon molecular sieve membranes for desalination

Abstract

This work reports on the development of novel carbon molecular sieve (CMS) membranes for desalination. The membranes were prepared by vacuum impregnation of precursor solutions containing phenolic resin into porous alumina tubes. Subsequently, the membrane tubes were calcined in an inert atmosphere, leading to the carbonisation of the resin impregnated into the porous alumina substrate. A systematic and parametric study was undertaken to investigate the effect of: (i) substrate porosity, (ii) precursor solution, (iii) vacuum impregnation time and (iv) carbonisation temperature on the performance of the resultant membranes. High quality CMS membranes were successfully prepared. High water fluxes were achieved using a pervaporation setup with values of 27 kg m–2 h–1 observed at 75 °C whilst delivering salt rejection in excess of 96% for a synthetic aqueous solution of 0.3 wt% NaCl chosen to represent brackish water. Increasing the salt concentration resulted in the water flux reducing, which was most likely due to salt concentration polarisation. However, the CMS membranes continued to perform well for processing sea water (3.5 wt% NaCl) at room temperature delivering water fluxes in the region of 9.4 kg m–2 h–1 with a salt rejection close to 100%. The high performance given by the CMS membranes in terms of water fluxes are at least one order of magnitude higher than those reported for other inorganic membranes derived from silica or zeolites. One important finding of this work is that the water flux increased by a factor of 70 as the concentration of the phenolic resin precursor solution decreased from 40 to 1 wt%. This result indicates that high resin concentration led to a higher amount of carbonised resin in the porous substrate and a higher resistance to water diffusion. The precursor solutions with a low resin concentration allowed for the formation of ideal CMS structures, which polymerised into the inter-particle pore domains of the porous alumina substrate. As such a molecular sieving structure was formed as it preferentially allowed for the diffusion of the smaller molecule water with a kinetic diameter of 2.6 A, and to a higher degree rejected the passage of hydrated ions with sizes in excess of 7 A. A second important finding is that the functionality of the carbonised resin plays a role in delivering CMS membranes with high performance. CMS materials carbonised at 500 °C still retained some functional aromatic groups which were hydrophilic and impacted on membrane performance. However, increasing the carbonisation temperature to 700 °C lead to optimal microporosity, enhanced surface area and reduced hydrophilicity as the functional groups were almost completely removed. Further increases of carbonisation temperature to 800 °C resulted in the formation of larger, mesoporous structures and the unfavourable reduction of salt rejection. A third important finding is that best CMS membranes were prepared by longer vacuum times of 300 to 600 s, instead of short times of 30 or 60 s. This result was unexpected as longer exposure times should and did increase the amount of resin impregnated into the porous substrate. This in turn should have led to higher resistance to water transport. However, this work shows that short exposure times are not sufficient for resin impregnation. Instead a thin film is formed upon contact with the porous substrate. Longer times are required for the solvent to break down the thin film to allow the resin to enter the porous substrate which is aided by the diffusion of the solvent from the precursor solution towards the low pressure region (i.e. the vacuum line). To explain the effect of CMS structure formation by vacuum impregnation, a systematic investigation was carried out to study the effect vacuum pressure as a function of the resin concentration in the precursor solution, vacuum time, mass balance of resin and solvent from the precursor solution, species retained in the tube (i.e. porous substrate) and species diffused through the tube. Based on this study, a model of CMS structure formation by vacuum impregnation is proposed which include two regions of (i) film formation and (ii) CMS structural impregnation into pores of the alumina substrate.