Nanofiltration is a pressure driven
separation process. The filtration process takes place on a
selective separation layer formed by an organic semipermeable
membrane. The driving force of the separation process is the
pressure difference between the feed (retentate) and the filtrate
(permeate) side at the separation layer of the membrane. However,
because of its selectivity, one or several components of a dissolved
mixture are retained by the membrane despite the driving force,
while water and substances with a molecular weight < 200 D are
able to permeate the semipermeable separation layer. Because
nanofiltration membranes also have a selectivity for the charge of
the dissolved components, monovalent ions will pass the membrane and
divalent and multivalent ions will be rejected.
Our first nanofiltration unit has been
delivered to a dairy customer in Europe with more than 3000 m2 of
effective membrane area.
Example of Application in the Dairy
Industry:
In the dairy industry the nanofiltration
process is used to concentrate and partially demineralize liquid
whey. Due to the selectivity of the membranes most of the monovalent
ions, the NPN, the organic acids, and some of the lactose will pass
the membrane.
The typical composition of the different
products achieved by nanofiltration of a liquid whey is shown in the
table below:
|
|
Raw whey
|
Concentrated whey
|
Permeate
|
Losses
|
|
Total solids
|
6.5 %
|
18.5 %
|
0.37 %
|
|
|
Total protein [% TS]
|
12.8
|
13.5
|
0
|
0
|
|
NPN [% TS]
|
0.5
|
0.36
|
4.32
|
31%
|
|
Lactose [% TS]
|
74.4
|
77.0
|
26.2
|
1.3%
|
|
Organic acids [% TS]
|
3.4
|
3.37
|
4.9
|
5%
|
|
Ashes [% TS]
|
7.7
|
5.3
|
68.8
|
28%
|
|
Fat [% TS]
|
0.7
|
0.8
|
0
|
0
|
The composition of the ashes of the raw whey
and the concentrated whey shows the preferential transport of monovalent ions
through the membranes:
| |
Raw
whey |
Concentrated
whey |
Permeate |
Losses |
| Ca
[%TS] |
0.42 |
0.41 |
0.7 |
6% |
| Mg
[%TS] |
0.12 |
0.12 |
0.21 |
6% |
| Na [%TS] |
0.61 |
0.38 |
6.5 |
40% |
| K [%TS] |
2.36 |
1.48 |
25.2 |
40% |
| S [%TS] |
0.16 |
0.15 |
0.55 |
12% |
| P
[%TS] |
0.62 |
0.57 |
2.05 |
12% |
| Cl [%TS] |
1.44 |
0.68 |
21.1 |
54% |
For the nanofiltration process, the maximum achievable
demineralization rate is about 35 % with a concentration factor of about 3.5 to
4. To reach a higher demineralization rate it is necessary to dilute the
partially demineralized whey with demineralized water and run the nanofiltration
process again. By applying this so-called diafiltration step, it is possible to
achieve a demineralisation rate of up to 45 %. On using a special pretreatment
of the whey, developed by SAFIR / EURODIA, it is possible to get a
demineralization rate of up to 75 % with nanofiltration.
The maximum concentration to be achieved
by nanofiltration is between 22 % TS and 24 % TS. Above this
concentration, problems can occur due to the precipitation of
phosphates; in addition, the viscosity of the concentrated whey is
increasing exponentially above 24 % TS.
The concentration that can be most
economical achieved with nanofiltration is 18 % TS at a
demineralization rate of 28 %. Because of the logarithmic decrease
of the permeate flow rate when the concentration increases, further
concentrating from 18 % TS up to 24 % TS would require doubling the
membrane area.
Running the nanofiltration process on
lactic acid whey will give a product that is dramatically decreased
in the lactic acid content.
In some specific cases, it is advantageous
to use nanofiltration to replace evaporation and partial
demineralization. But for most of the dairy applications, a
combination of different processes, including nanofiltration, is
required to obtain the most valuable products. Please go to the
Industries Section/ Dairy of this website for more details.
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