14 research outputs found
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