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Review date: July 24, 2003
PNNL-SA-27883

Membrane
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Objective

Membrane separations are well developed for gases and liquids. The use of membranes has become ubiquitous in many important separations that are based on molecular weight and charge of a molecule or atom. For example, in water desalination the use of reverse osmosis membranes for the removal of cations (Na+) and anions (Cl-) for water purification is well established. There are commercial processes based on membranes for gas enrichment, i.e. N2. The knowledge base of membrane separations in liquids and gases is very extensive, but very little work has been done in the supercritical regime.

There are tremendous advantages to reap using supercritical fluid solvents for membrane separations. The solute molecule can have solubilites in the supercritical fluid approaching those seen in liquids while the mass transfer coefficient of the solute can be comparable to that seen in a gaseous solvent. Therefore, membrane fouling and mass transfer limited separations could be diminished and enhanced, respectively using a supercritical fluid solvent process.

Membrane based separations can significantly improve the overall efficiency of existing supercritical processes by eliminating the depressurization step that is used to effect the separation of the solute from the fluid solvent. This would significantly enhance energy efficiency and substantially lower the costs associated with commercial dense-fluid separation processes. Current technology requires a transition to the vapor phase to effect separation. Membrane separation has the potential for eliminating that step, thus improving efficiency for solvent recycle and product recovery.

Additionally, membrane separations can be used to rapidly and effectively separate value or waste products from the core of a reverse micelle system, while offering the advantage of recycling the surfactant(s) for recycle and reuse. Thus, non-polar solvents can be used to selectively extract both non-polar and polar species via the use of reverse micelles and all segments of the stream can separated for either collection, disposal, or recycle.

This advancement offers potential for improvement in parts cleaning via the ability to entrain particulates and polar species; natural products extraction via the ability to extract and separate more species in a single step; and in separations via the ability to separate value products from bulk streams.

Project Description

The investigation of the use of supercritical fluid membrane separations was continued and enhanced. The fundamental understanding of the separation process as a function of temperature and pressure was the goal for this advanced separation scheme. The molecules studied by membrane separation in sub- and supercritical solvents included large molecular weight fluorocarbon polymers, dextrans and a biomacromolecule. These solute molecules either had a high natural solubility in the fluid solvent, e.g. perfluoropolymer in liquid CO2, or a solvent additive, such as, reverse micelles were used to dissolve the dextran and biomacromolecule into both sub- or supercritical fluids.

Technical Accomplishments

Experiments were conducted to demonstrate membrane separations in subcritical and supercritical fluid microemulsions. The separations include cytochrome c (a protein) from a surfactant, and two dextrans of differing molecular weight. Neither the protein nor the dextrans are directly soluble in the fluids. However they are soluble in a microemulsion in the fluids.

The solvent system consisted of sub- or supercritical propane and ethane. The surfactant AOT (Dioctyl sulfosuccinate, sodium salt) was used to form the microemulsion solutions. Cytochrome c and the dextrans (having narrow nominal molecular weights) had a chromophore label specific to the molecule. This allowed permeate and retentate to be characterized via UV/VIS spectrometry.

A high pressure membrane filtration system was assembled from various commercially available and in-house fabricated components. These include syringe pumps (Isco), high pressure fittings and adapters (High Pressure Equipment), a high pressure membrane holder (Millipore), a view cell (in-house), and two small autoclaves (in-house). The membranes (Millipore) were an ultrafiltration type that allowed dissolved species having a molecular weight below a given cutoff to pass through. UV/VIS absorption spectra were acquired with a Varian/Cary 2200 UV/VIS Spectrometer.

Solutions of the microemulsions in the fluids were prepared in the view cell. Measured amounts of dextrans or protein, AOT, and water were added to the view cell. The view cell was then sealed, connected to the pressure system and pressurized with either ethane or propane. It was stirred with a magnetic stirring bar. The view cell was heated when the separation was to be carried out in the supercritical regime. Flow through the system was initiated maintaining a constant differential pressure. Once enough fluid had flowed through the apparatus, the experiment was stopped and the apparatus was depressurized. Permeate and retentate were collected and dissolved in hexane. UV/VIS absorption spectra were acquired of these solutions.

Cytochrome c was separated from the surfactant AOT in liquid propane at 20°C and 2,000 psi using a 10,000 nominal molecular weight membrane. The majority of the protein (MW about 13,000, lmax 414 nm) was retained as evidenced by the peak in the spectrum of the retentate and lack of spectral activity in this region of the permeate spectrum.

NMR spectrum of cytochrome c separated from AOT in liquid propane

A second separation of two labeled dextrans from an AOT microemulsion in propane at 22ºC, and 2,000 psi using a 5,000 nominal molecular weight membrane was undertaken. The spectra indicate that virtually all of a 40,000 nominal molecular weight dextran labeled with a chromophore having a lmax of about 590 nm is retained while most of a 3,000 nominal molecular weight dextran labeled with a chromophore having a lmax of about 400 nm passed through the membrane.

A third separation of two labeled dextrans from an AOT microemulsion in supercritical ethane at 40ºC, 7,250 psi using a 10,000 nominal molecular weight membrane is shown at right. The spectra indicate that a 3,000 nominal molecular weight dextran labeled with a chromophore having a lmax of about 590 nm preferentially passed through the membrane while virtually all of a 40,000 nominal molecular weight dextran labeled with a chromophore having a lmax of about 440 nm was retained.