Effect of swelling in non-aqueous nanofiltration with polydimethylsiloxane (PDMS) membranes

Transport 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. A dense polydimethylsiloxane (PDMS) composite membrane was used to assess the flux and separation performance of a range of organic solute compounds and organic solvents. Solvent flux was modelled with the Hagen- Poisuelle equation and found to fit the model well, with swelling effects being the most likely cause of some deviations. The effect of solvent type and membrane swelling on solute rejection will be discussed

The assessment of materials for crossflow nanofiltration of organic/organic liquids and the development of scale-up options

With the aqueous applications of crossflow filtration being well established, comparable developments in the field of organic/organic liquid systems remain in their infancy. Progress within the field has been hindered by the fact that there are few systems which are both robust to hydrocarbon solvents and provide good fluxes/separations under realistic operating conditions. The authors of the current paper have explored a number of materials for crossflow filtration of organic media and found that the dense organic polymer PDMS (polydimethyl siloxane) affords the best results (see Figure 1). Building on initial results, a full assessment of the membrane performance has been undertaken. Using a laboratory set-up, a range of pure and mixed hydrocarbon streams have been passed across the PDMS to assess performance with time and under variable operating conditions. Recent papers and presentations by the afore mentioned authors have considered transport mechanisms across a 2 μm PDMS membrane supported on PAN. Results from flat sheet experiments have been used to design a larger scale unit. The operation of this system has shown excellent read across in terms of flux and selectivity. It is hoped that the work detailed within this presentation will prompt other workers in the field to consider the development of novel organic polymers to build on the applicability of filtration for organic/organic separations

Nanofiltration of organic solvents

Transport 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/m2 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

Evidence for swelling-induced pore structure in dense PDMS nanofiltration membranes

A dense polydimethylsiloxane (PDMS) membrane was used to assess the flux and separation performance of a range of solutes (e.g. poly-nuclear aromatics and organometallics) and organic solvents (e.g. heptane and xylene). Solvent flux was modelled with the Hagen-Poiseuille equation and found to fit the model well, with the degree of swelling influencing the effective pore size and porosity of the membrane. The rejection mechanism for low-polarity solutes was found to be predominantly size exclusion. The rejection varied with solvent type and rejections were higher in poorer-swelling solvents. For instance, the rejection of 9,10 Diphenylanthracene was 2% in a pure heptane solvent compared with 15% in xylene. It is postulated that dense PDMS membranes exhibit the characteristics of a porous structure when swollen with solvent, and that the degree of swelling impacts on the separation performance of the membrane. A comparison between the Hildebrand solubility parameters for the PDMS membrane and the challenge solvent was found to be a good indicator of flux/rejection performance

Non-aqueous nanofiltration: solute rejection in low-polarity binary systems

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using alkyl and aromatic solvents and low-polarity, sulphur bearing, organometallic and polynuclear aromatic (PNA) solute compounds. Rejection was found to be dependent on transmembrane pressure, crossflow rate (hydrodynamic conditions), solute size and the degree of swelling induced by the solvent. Rejection increased progressively with pressure whilst a threshold condition was observed above which further increases in crossflow had a negligible influence on rejection. Measurements over the molecular weight range 84-612 g/mol showed the membrane to have a molecular weight cut-off in the region 350-400 g/mol to all but one of the tested PNA compounds (i.e. rubrene). An additional correlation using molecular dimensions instead of molecular weight showed the cut-off size of the membrane to be in the region of 1-2 nm, with all data falling on a well defined rejection/size curve. Solvent type influenced membrane swelling to an extent dependent on the relative magnitude of the solubility parameters for the solvent and PDMS; similar values led to more swelling, higher fluxes and lower rejections. Results support the concept of viscous solvent flow whilst solute transport could be either predominantly viscous or a combination of viscous and diffusive. With larger molecules a size exclusion mechanism was dominant. A new model is proposed that takes account of solute transport by a combination of viscous and diffusive mechanisms and this is shown to well represent the experimental data

Solvent flux through dense polymeric nanofiltration membranes

This work examines the flux performance of organic solvents through a polydimethylsiloxane (PDMS) composite membrane. A selection of n-alkanes, i-alkanes and cyclic compounds were studied in deadend permeation experiments at pressures up to 900 kPa to give fluxes for pure solvents and mixtures between 10 and 100 l m-2 h-1. Results for the chosen alkanes and aromatics, and subsequent modelling using the Hagen-Poiseuille equation, suggest that solvent transport through PDMS can be successfully interpreted via a predominantly hydraulic mechanism. It is suggested that the mechanism has a greater influence at higher pressures and the modus operandi is supported by the non-separation of binary solvent mixtures and a dependency on viscosity and membrane thickness. The effects of swelling that follow solvent-membrane interactions show that the relative magnitudes of the Hildebrand solubility parameter for the active membrane layer and the solvent(s) are a good indicator of permeation level. Solvents constituting a group (e.g. all n-alkanes) induced similar flux behaviours when corrections were made for viscosity and affected comparable swelling properties in the PDMS membrane layer

The influence of polarity on flux and rejection behaviour in solvent resistant nanofiltration - experimental observations

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using mixtures comprising xylene, cyclohexane or n-heptane with oxygenate components at concentrations up to 75%. The effects of polarity on flux and rejection performance were determined through a test matrix of solvent type, concentration, filtration pressure, crossflow rate and the degree of membrane crosslinking. In all cases involving alcohols, the more polar compound in the feed mixture was partially rejected by the membrane and the extent of rejection was dependent on the polarity as quantified by solubility parameter. The rejection-concentration profiles for several alcohol/solvent mixtures exhibited a maximum, with the highest rejection around 30%. Mixtures containing MTBE did not separate, i.e. no rejection was observed. Rejection increased with increasing pressure and crossflow rate but was largely unaffected by the degree of membrane crosslinking. Component flux was affected by the oxygenate concentration in the mixture, which was attributed in part to changes in the degree of membrane swelling with composition. Experimental findings suggest that the separation is primarily governed by multicomponent solvent/oxygenate/membrane swelling equilibria, and results compare favourably with swelling isotherms available in the open literature

Maria gordon buse, MD: A family affair through six decades of diabetes discovery

Maria Gordon Buse, MD, is a product of wartime Europe. She completed her professional education in four languages on three continents and continues a nearly 60-year career as an investigator, educator, and practicing endocrinologist. This brief reprisal is written collabora-tively by her biological offspring and in-tellectual progeny, an appropriate reflection of a career where family and work were joyfully intertwined in an irresolvable way