Nanofiltration of organic solvents

Abstract

This article provides an insight into the mechanisms affecting solvent flux through dense membranes, and forms a basis for rejection studies of organic solute compounds from organic solvents. ABSTRACTTransport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms.The work aims to enhance the understanding of non-aqueous nanofiltration by focusing on the flux performance of organic solvents through a dense 2 μm polydimethylsiloxane composite membrane. The flux of alcohols, n-alkanes, i-alkanes and cyclic compounds were studied in deadend mode, at pressures of 10-900 kPa. Fluxes of 10-80 l/m 2 h were obtained for alkanes and cyclic compounds, whereas alcohol flux was around two orders of magnitude lower. The results suggest that the solvent flux through polydimethylsiloxane takes place via two distinct mechanisms -namely hydraulic and chemical transport. Hydraulic transport appears to dominate at pressures above 300 kPa, whereas chemical transport becomes more apparent at lower pressures.Comparison of the hydraulic transport data with a Hagen-Poisuelle model gives good agreement for similar solvents. Swelling effects caused by solvent-membrane interactions are identified as playing a major role in solvent flux behaviour, and compressibility effects are also thought to account for deviations from the Hagen-Poisuelle model. Viscous flow was verified by a nonseparation of mixtures of n-alkane and cyclic compounds, which suggests that the polydimethylsiloxane layer cannot sustain a dense structure when used in organic solvent nanofiltration applications.

Separation of acetic acid from dilute aqueous solution by nanofiltration membrane

“…The permeation flux at 1 bar is 84.5 L m 2 h −1 and it increases quickly with rising pressure and reaches 89.2 L m 2 h −1 at 5 bar. The phenomenon of flux increasing with pressure has been observed in other reports also: n ‐hexane flux increased with pressure as reported by Bhanushali, flux of n ‐heptane and xylene (mixture of isomers) with pressure as reported by Robinson et al, flux of benzene and toluene went up with pressure was reported in White's study . This illustrates that pressure boost is an effective method for increasing permeation flux.…”

“…In the literature, different solvent, solute and membrane parameters influencing the transport model were experimentally identified and different models were developed . However, transport models for Nanofiltration system are limited for specific experimental data.…”

Organic solvent nanofiltration with a Poly(dimethylsiloxane) membrane: Parameters affecting its sieving properties

“…Regardless of the model used, the pore sizes were found to be of the same order of magnitude as sizes reported in the literature with only a few nanometers difference if any. The differences observed for the pore radii obtained through both models from two distinct pools of data (using either the values of solvent fluxes or the PEG retention results) may result from the roughness of the models (cylindrical pores, single diameter, etc.)…”

“…The mechanisms of aqueous nanofiltration (NF) are fairly well documented but uncertainties remain about this process when organic solvents are used . This is mainly owing to additional solvent–solute–membrane interactions . Many studies have attempted to model mass transfer through such membranes.…”

Multilayer adsorption equilibrium model for gas adsorption on solids

“…Sorption of solvents in polymers is a well studied phenomenon (Astarita and Joshi 1978;Robinson et al 2004) as it plays a major role in transport processes through dense membranes. Flory-Huggins thermodynamics theory is the most used approach to model this phenomenon (Flory 1953).…”