7 Workshop: Nanofiltration and
masstransport
Workshop lLeader: ir. Harmen Zwijnenberg, University of Twente
Most of the current membrane applications concern aqueous media. Within the
field of organic solvents limited research is carried out. This also showed during the
workshop as only four people participated in it. However, it also shows the challenges
that still lie in this field.
7.1 Food research
Within the group of food-research with poreus membranes the following items
came up:
• Most interesting is the use of the membrane in the application itself.
• Most people like to work for a solution to make the application work.
• More realistic feed solutions should be used for the experiments as often problems arise in translating ideal solutions to practical solutions.
• The development in this field as it is in others, firstly carried out to innovate and
then to improve existing models.
• A challenge lies in the correlation of membrane and solute properties to predict
membrane performance.
• As a hope or demand to industry: Tey should take serious the research carried out
on applications in order to improve the research and their processes.
7.2 Solvent resistant nanofiltration membranes
Within the group of solvent resistant membranes several problems and items could
be identified:
Current commercial nanofiltration membranes for organic solvent lack a reproducible
flux and retention. They differ largely per sample and per batch. Membrane preparation
itself is still not “state of the art”.
The retention often decreases over time suggesting that the membranes are not
stable.
A large difference exists between polar and apolar solvents. Not necessarily the
membranes that are resistant towards one of
them are also resistant to the other category.
Also (hot) water sometimes seems a problem
for the membranes, especially ceramic ones.
Dead-end setup’s are pressurized by nitrogen
which dissolve well in organic solutions. Does
the influence the measured solute retention or
measure selectivity between solvents?
Theoretically, at the permeate side the solvents
are depressurized and dissloving nitrogen might
sweep away the organics solvents with a high
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vapor pressure, thus influencing both measure retention and selectivity.
7.3 Discussion
The main problem for membranes in organic solvents is their stability. This was
also the main topic during the discussion between the participants of the workshop.
The first topic that came up and which seemed to concern everybody was the
stability of the membranes commercially available nowadays. It was the general opinion
that often membranes specifically designed for solvent applications were not suitable
for their applications, even though the information on the manufacturer sheet suggested
the opposite. Several hypotheses were expressed that might explain these disappointing
results. Firstly it was noted that there were in general two classes of solvents and two
classes of membranes accordingly; hydrophilic and hydrophobic (or polar and apolar).
Both types of membranes were mostly only stable in one of the two types of solvents.
Besides the influence of minor constituents in the feed it was obvious that mixtures of
solvents with different characters could therefore damage the membrane. Other
problems lie in using acids and hot water. It was observed at least once by all of the
participants that either acids or water had an (irreversible) negative effect on the
membrane properties. Also from ceramic membranes it is known that water and acids
can change both flux and retention dramatically due to modification of the surface or
fouling.
The second topic was related to the above one and that was that the membrane
properties seem to change in time. During the start of an experiment the membrane
performed well but after a while a decrease in performance was observed.
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Another problem that was noted by most of the researchers was that the membrane
properties differed largely per sample and per batch. Especially fluxes could vary
over 100%. However, it should also be mentioned that the small sample sizes often
used add to this effect. Nevertheless it was generally concluded that the membrane
production itself did not yet seem to be state of the art.
Concerning the transport model applicable to solvent nanofiltration, actually nobody
had a clue. Most people were of the opinion that existing models were not really able
to describe it. The work of Machado seemed to be an interesting start in this,
nevertheless it was felt that it is limited in its use by the incorporating of the transport
of solutes. The subject was not further investigated during the discussion.
For some time there was a short discussion about the use of gas (N2)-pressurized
dead-end setups as a valid method for selectivity and retention measurements. Most
researchers were not experiencing problems, although it was onetime found for a
mixture of hexane/triglycerides and a low membrane permeability. It is known that
some organic solvents absorb nitrogen well. Theoretically a problem might also rise
when a mixture of solvents is used. During the expriment, at the permeate side the
solvents are depressurized and the dissolving nitrogen might sweep away the organic
solvents with the highest vapor pressure. This effect is the largest when the heat of
vaporization is low. As a result both the measured retention and selectivity are
influenced.
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