Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2006 / A protocol already exists for fabrication of a capillary membrane having an internal ultrafiltration
skin supported by a finger-like pore structure in the external capillary wall (Jacobs
and Leukes, 1996; Jacobs and Sanderson, 1997). These membranes have been produced at
the Institute of Polymer Science, University of Stellenbosch, South Africa.
Two major applications emerged from the development of these internally skinned
membranes. One application was in the production of potable water by Ultra-filtration (UF)
from sources containing coloured water. A second application was in the immobilization of a
white rot fungus in a ."gradostat" membrane bioreactor. Here a nutrient gradient through the
membrane wall and fungal mat can be established and manipulated in order to stimulate
continuous production of secondary metabolites (extra-cellular enzymes). These enzymes are
useful in the degradation of polycyclic aromatic compounds, notably PCB species in
contaminated water and soils (Jacobs and Sanderson, 1997).
Two objectives emerged from experiences with the above applications. The first objective
was to improve membrane performance in UF applications. In this case a reduction was
sought in trans-membrane pressure differential required to attain a desired flux without
sacrificing rejection. The pressure required for a given desired flux across a membrane
depends on the resistance of the membrane skin layer and of its supporting sub-layer which
together comprises the capillary wall and defmes its overall structure. If any of these
resistances could be reduced, the overall resistance to transport of water would be reduced.
Then it would be possible to operate the membrane at lower trans-membrane pressure
differences. On the other hand, operation with higher pressure would also increase flux but
require a thicker capillary wall to resist this pressure. In the attempt to optimise these
properties of the capillary membrane, capillary membranes produced in the study reported
here were tested to find the relationship of flux performance with the structures that resulted
from varying key parameters affecting structure and integrity.
The objective in the case of immobilizing fungi in membrane bioreactor applications was to
attain thicker walls thus providing better support for the fungal mass. The internally skinned
capillary membrane has finger-like microvoids that start next to the UF skin layer and extend
across the capillary membrane wall and open at the external membrane periphery, giving an
ideal structure for retaining the fungal biomass. The idea of a membrane with this type of
morphology to immobilize white rot fungi was to anchor the growing fungus within these
microvoids which imitate the natural environment in which these organisms live, that is, in
the fibrous structure of decaying wood. The requirement to inoculate the microvoids with
fungal spores (reproductive cells), implies that they need to be accessible from the outside,
requiring a membrane wall that is externally unskinned.
In the formation ofthe capillary membrane the processes of formation of the porous UP skin
and the finger-like microvoids are mainly governed by diffusion of solvent out of a polymer
dope (gel phase) and of non-solvent into the dope phase. Such exchanges are of primary
importance between the bore fluid (containing non-solvent) and dope (containing solvent) or
between the external spinning bath (high in solvent content) and dope. Diffusion effects also
occur between the nascent pore voids and the precipitating polymer matrix. There are also
expected to be some convection effects due to shear between the bore fluid and the moving
dope gel phase and due to shrinkage ofthe gel phase.
The variables selected for experimentation m the study reported here were: the dope
extrusion rate (DER); dope composition (viscosity effects); bore fluid flow rate (BFF); bore
fluid composition and wall thickness and diameter effects (determined largely by spinneret
dimensions). Each of these has an expected effect on membrane structure and its resulting
performance. Most were varied over narrow ranges indicated in the literature and by
experience to be effective and critical. In addition, the effects of altering the walI thickness
were investigated by using two different spinneret sizes.
The external spinning bath composition (solvent content) was reported in the literature to be
a particularly important parameter in the formation of externally unskinned membranes.
Maintaining a high content of solvent in the external spinning bath could prevent skin
formation. Too high a solvent content could, however, prevent phase transition and lead to
later precipitation ofa dense skin on contact with the non-solvent in the later (humidification
and rinsing) steps in the fmishing of the capillary membrane product. The external bath
composition was therefore varied so as to find the bath composition that would match the
cloud point for the polymer dope employed.
As expected, the thickness of the membranes increased with DER increase. However, it was
found that there is a critical wall thickness where an external skin layer is formed as a result
of increasing the DER. A certain volumetric ratio ofDER to BFF (1,5:1 for this study) was
therefore maintained in order to produce externally unskinned membranes. This shows that
although the final membrane structure is detennined by the casting dope formulation, the
fabrication protocol plays an equally important role in controlling structural properties and
perfonnance. There was no significant change with the membrane thickness as a result of
changing BFF but the voids became longer and more in number as the BFF was increased.
Too high solvent content (99% NMP in this study) resulted in an external skin layer being
formed. According to Smolders et.al. (1992), when the solvent content in the external
spinning bath is too high, the polymer at the surface of the newly fonned membrane slowly
dissolves in the external spinning bath re-forming a dope-like solution. When the newly
formed membrane passes through the humidifier, the dope-like solution solidifies to form an
external skin. At the same instance, too low solvent (93% for this study) resulted in external
skin being fonned. Externally unskinned membranes were formed at 94 and 96% NMP bath
composition. The use of a small spinneret resulted in very thin walled externally unskinned
membranes.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:cput/oai:localhost:20.500.11838/895 |
| Date | January 2006 |
| Creators | Jack, U |
| Publisher | Cape Peninsula University of Technology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | http://creativecommons.org/licenses/by-nc-sa/3.0/za/ |
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