Nanofiltration

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 demineralization 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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