Solvent resistant nanofiltration: developing understanding of transport mechanisms

In recent years the possibility of using polymeric nanofiltration (NF) membranes for non-aqueous separations has been explored. There is, however, significant debate concerning fundamental mechanisms where concepts include solution-diffusion and ‘pore’ flow. This paper presents nanofiltration and swelling data for polyacrylonitrile (PAN)/poly-dimethylsiloxane (PDMS) composite membranes with a range of low and higher polarity solvents, some of which contained solutes in the range 84-612 MW. The influences of parameters such as crossflow rate, applied pressure, solute size and solvent polarity on filtration performance are presented and measures of flux and solute rejection are related to membrane swelling. More comprehensive descriptions of the experimental apparati and results are shown in [1-7]

Nanofiltration - a method for solute removal from fuel simulants

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using alkyl and aromatic solvents and low-polarity, sulphur bearing, organometallic (OM) 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 cut-off in the region 350-400 g/mol to all but one of the tested PNA compounds (rubrene). An additional correlation using molecular dimensions instead of molecular weight showed the cutoff size 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

The removal of solutes from organic solvents using nanofiltration

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 is discussed

Solvent induced swelling of membranes - measurements and influence in nanofiltration

This paper describes improvements to an apparatus for in-situ determinations of swelling where a linear inductive probe and electronic column gauge with an overall resolution of 0.1 μm was used for measurements of seven variants of polyacrylonitrile (PAN)/polydimethylsiloxane (PDMS) composite nanofiltration membranes in a range of alkane, aromatic and alcohol solvents. The unswollen membranes incorporated PDMS layers between 1 and 10 μm nominal thickness and were manufactured with a radiation and/or thermal crosslinking step. The tested membranes exhibited a range of swelling dependent on the degree of crosslinking, the initial PDMS layer thickness and the type of solvent. With no applied pressure the PDMS layer on some radiation crosslinked membranes swelled as much as ~170% of the initial thickness whilst other membranes were restricted to a maximum swelling of ~80%. When a pressure up to 2000 kPa was applied to a membrane then swelling could be reduced to ~20% of the value obtained at zero applied pressure. By vertically stacking up to 3 membrane samples it was possible to determine the swelling of PDMS layers as thin as 1 μm, although higher imposed pressures rendered some results unreliable as the measurement resolution of the apparatus was approached. The results of the swelling experiments are contrasted with crossflow nanofiltration performance in terms of solvent flux and solute rejection

Nanofiltration: a technology for selective solute removal from fuels and solvents

This paper describes the principal features of solvent resistant nanofiltration, and in particular its potential in fuel processing. Experimental data for both fuel simulants and a representative petrol fuel are presented to illustrate the salient features. The solute rejection mechanism for low polarity mixtures was size exclusion with a membrane cutoff in the region of 1-2 nm. The extent of solute rejection was dependent on the degree of membrane crosslinking, the membrane swelling induced by the feed and the applied (filtration) pressure. Nanofiltration experiments with the petrol fuel showed a good correlation with the data obtained for the fuel simulants, both in terms of permeate flux and solute rejection. Provided that higher polarity oxygenates were not present in the fuel, it was possible to remove undesirable polynuclear aromatic and organometallic solutes to an extent that was sufficient to reduce valve deposits (by 64%) and emissions gases (by up to 17%) in engine tests. These improvements significantly better the changes in engine performance that are brought about by the more traditional addition of fuel additives such as detergents. The technology provides a method for removing undesirable solutes from mixtures without the need for excessive energy input

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

New experimental measurements of solvent induced swelling in nanofiltration membranes

The paper describes developmental apparatus for in-situ determinations of membrane swelling and shows representative examples of the data that can be acquired. The apparatus principally comprises a linear inductive probe and electronic column gauge with an overall resolution of 0.1 μm which was used in two configurations to assess the swelling propensity of polyacrylonitrile (PAN)/polydimethylsiloxane (PDMS) nanofiltration membranes in a range of alkane, aromatic and alcohol solvents. In the absence of an applied pressure on the membrane, experiments showed a maximum expansion for the PDMS layer of 169% using an n-heptane solvent whose solubility parameter (δ) was close to that of PDMS. With more polar solvents falling in the range δ = 23.6-29.2 MPa0.5, swelling of the PDMS was much reduced (<14%) and comparable shrinkage of the PAN support layer was also observed. If a mechanical pressure was applied to the membrane then swelling was reduced. For example with a xylene solvent, over the pressure range 0-10 bar a progressive decline in membrane swelling from 118% to 50% was observed. At 20 bar swelling was further reduced to 33%. When xylene or heptane solvent was mixed with methanol, ethanol or propanol, reduced swelling of the PDMS layer occurred as the concentration of alcohol increased. The extent of swelling was closely related to the value of the mixture solubility parameter (δmixture) where a higher value of δmixture led to less swelling. The results of the swelling experiments are compared to some of the authors previously published results for crossflow nanofiltration and shown to support the salient features

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

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