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9 2017 | international-dairy.com · 25
Fig. 2: Permeation of ß-lg and casein as a function of time (black:
membrane 1, red: membrane 2).
rpm, active membrane surface 12.6 cm²). Even though it is known that a
stirred test cell is not able to completely reproduce the filtration behavior
of a SWM it is suitable to deliver a meaningful result for comparison of
different membranes under identical conditions. For the filtration tests,
skim milk at 20 °C was used and protein permeation was measured using
reversed phase high performance liquid chromatography (RP-HPLC). The
permeation is defined as the ratio of a certain component in permeate
and retentate and is a characteristic for filtration efficiency. The main
goal in this specific fractionation process is a high permeation of whey
proteins, i.e. β-lactoglobulin (2 – 8 nm) as the main component, while caseins
(80 – 400 nm) should be held back completely at the same time.
The filtration performance of both membranes regarding permeation of
caseins and β-lactoglobulin (β-lg) is depicted in Fig. 2.
Initially, the permeation of both casein and β-lg drops due to deposit layer
formation as it can be seen in Fig. 2. After 30 min the permeation is constant
and a steady state is reached. Permeation of β-lg was 27 and 25%, permeation
of casein 2 and 0% for membrane 1 and 2, respectively. α-lactalbumin
(α-la) showed the same trend in permeation as β-lg for both membranes with
slightly higher absolute values (data not shown). These results could not be
predicted only by nps, since it was expected that the membrane 2 with the
wider pore size would result in a higher permeation of caseins. The actual pore
size distribution by means of capillary flow porometry gave a first hint, but a
complete retention of casein by membrane 2 was unexpected. A possible explanation
for this is that the larger pores of membrane 2 (Fig. 1) were blocked
initially directly after the filtration began. This effect does not occur with
membrane 1 probably because of some pores being too large for blockage.
Filtration performance of polymeric
microfiltration membranes in SWM
To verify whether the findings obtained by the stirred test cell can be
transferred to industrial scale SWM, the filtration performance of the
same both membranes was investigated using SWM type 6338 having a
diameter of 6.3“ and a length 38”. Results are shown in Fig. 3. As already
seen in Fig. 2, a complete retention of caseins for membrane 2 and the initial
reduction of permeation of β-lg for both membranes were observed.
Permeation of β-lg was 47 and 40% for membrane 1 and 2, respectively.
Thus, despite the fact that permeation in a stirred test cell occurs differently
in terms of absolute value compared to SWM, the stirred cell is a powerful
tool to characterize membrane filtration performance. It is suitable for comparing
different membranes by means of protein permeation and retention.
Fig. 3: Filtration performance of polymeric microfiltration membranes
used in SWM (black: membrane 1, red: membrane 2).
Acknowledgements:
This project of the FEI (AiF 16836 N) is supported via AiF within the programme
for promoting the Industrial Collective Research (IGF) of the German
Ministry of Economic Affairs and Energy (BMWi), based on a resolution
of the German Parliament.
Literature:
1 Ulbricht, M.; Schuster, O.; Ansorge, W.; Ruetering, M.; Steiger, P. (2007):
Influence of the strongly anisotropic cross-section morphology of a
novel polyethersulfone microfiltration membrane on filtration performance.
Separation and Purification Tech. 57 (1): 63–73.
2 Yu Wang, K.; Chung, T.-S.; Gryta, M. (2008): Hydrophobic PVDF hollow
fiber membranes with narrow pore size distribution and ultra-thin skin
for the fresh water production through membrane distillation. Chem.
Eng. Sci. 63 (9): 2587–2594.
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