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Membrane-Based Dairy Separation A Comparison of Nanofiltration and Electrodialysis.

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Dev. Chem. Eng. Mineral Process. I3(1/2), pp. 43-54, 2005.
Membrane-Based Dairy Separation:
A Comparison of Nanofiltration and
Electrodialysis
G. Rice, S. Kentish*, V. Vivekanand, A. Barber*,
A. O'Connor and G. Stevens
Department of Chemical and Biomolecular Engineering, University of
Melbourne, Melbourne, Victoria 3010, Australia
'Burra Foods, PO Box 3 79, Korumburra, Victoria 3950, Australia
Two membrane technologies are compared for the separation of potentially valuable
lactose and calcium jkom a currently underutilised dairy stream, i.e. whey
ultrafiltration permeate. Nanofiltration, a pressure driven separation through a
porous membrane, is able to simultaneously concentrate and demineralise this
stream, giving up to 70% reduction in unwanted monovalent salts. Electrodialysis, an
electrically driven separation of ions through ion exchange or porous membranes, is
able to demineralise whey permeate to higher levels, but has lower selectivity between
mono- and divalent ions. The technologies are evaluated with regard to membrane
perm-selectivity, efficiency, cost and suitability to the given separation, based upon
research presented in the literature. It is concluded that nanofiltration oflers the most
potential f o r separation between undesirable monovalent ions and valuable divalent
ions and lactose, while electrodialysis is more suitable for higher levels of
demineralisation where a purer lactose product is required.
* Author for correspondence (sandraek@unimelb.edu.au).
43
G. Rice, S. Kentish, V. Vivekanand,A. Barber, A . O'Connor and G. Stevens
Introduction
Membrane separation processes are used within the dairy industry for many
applications. In particular, ultrafiltration is extensively used to extract valuable
proteins from cheese whey. Membranes can also be used to separate caseins from
slum milk, for volume reduction, and in the recovery of caustic fiom cleaning
wastewaters [ 11.
Increasingly, this technology is also being considered to hrther concentrate
lactose and mineral salts from the whey ultrafiltration permeate. While this stream is
rich in lactose and calcium, it is often underutilised or treated as waste. However,
more stringent environmental regulations are making disposal of this permeate stream
more expensive and subsequently alternative uses for the lactose content are often
sought.
"he lactose available within this stream can be used in food products including
baked goods, infant formulae, sauces and beverages, to name a few. It also has
application in non-food products, such as pharmaceuticals and as a fermentation
substrate [2]. Lactose derivatives, such as lactulose, lactitol and lactobionic acid, are
also potential products with uses in pharmaceuticals, as well as in sweeteners and in
animal feed [3].
The divalent ions present in ultrafiltration permeate, i.e. calcium, magnesium and
zinc, have been identified as important minerals for healthy living, promoting strong
bones and regulating biochemical pathways [4]. These minerals can be of value when
sold in conjunction with the lactose. Conversely, the monovalent ions, sodium,
potassium and chloride, are undesirable components of food products due to their
salty taste, and they decrease the value of potential lactose and calcium-based
products. Removal of these ions has also been shown to reduce costs associated with
downstream lactose crystallization, due to increased yields [51 and reduced crystal
washing [6].
Two methods have been proposed for the concentration and separation of
ultrafiltration permeate, nanofiltration (NF) and electrodialysis (ED). Both techniques
are capable of removing significant quantities of sodium, potassium and chloride ions,
which aids downstream lactose processing and utilization. In addition, minerals such
as calcium may also be separated out and used as a supplement in existing and new
products.
Whey Ultrafiltration Permeate
The composition of the ultrafiltration (UF)permeate which is to be treated by NF/ED
is obviously dependent upon the operating conditions of the upstream stages, and the
origins of the whey. Typical input parameters are summarized in Table 1, which gives
a range of values for UF permeates from sweet whey, acid whey and skim milk. The
permeate may also contain trace minerals including iron, zinc, copper, manganese,
iodine, chromium, molybdenum, nickel, silicon and tin [7]. Approximately 30% of
the total minerals present in the original milk persist in this UF permeate [7].
Conversely, the concentration of fat remaining is negligible.
44
Membrane-Based Dairy Separation: Comparison of Nanofiltration & Electrodialysis
Table 1. Composition of UF whey permeates [8-131.
Component
Concentration
(% w/w)
Lactose
3.8 - 4.9
Protein
0.01 - 0.4
Ash
0.4 - 0.7
Total Solids
4.5 - 5.8
Water
Ash components
94.2 - 95.5
Concentration
(mg/I OOg)
Ca2+
10 - 95
Na’
25 - 65
K’
110 - 150
c1-
80 - 120
Mg2’
4-8
Po:-
20 - 55
Citrate
60 - 120
The pH of the permeate stream is important as it may dictate the type of
membrane that can be used. Some studies have also shown that the pH of the solution
can have an effect on the permeability of divalent ions and lactose through such
membranes [ 141. The pH is dependent on the origins of the permeate stream, whether
from sweet or acid whey, but typically has values between 6.3 [IS] and 6.7 [lo] for
sweet whey permeate and 4.6 for acid whey permeate [ 101.
Nanofiltration
Nanofiltration is also sometimes referred to as ultraosmosis, ‘leaky’ or ‘loose’ reverse
osmosis [16], or tight ultrafiltration. This is because the nanofiltration pore size
(molecular weight range 300-1000 Da. [ 13) falls between that of ultrafiltration and
reverse osmosis. Often nanofiltration membranes possess an electric charge, due to
attachment of charged functional groups to the membrane matrix [17]. This
combination of charge and small pore size allows NF membranes to both concentrate
solutions and simultaneously separate ions on the basis of their size, charge, and ionic
interactions with both the membrane and the other ions in solution.
4s
G. Rice, S. Kentish, V. Vivekanand,A. Barber, A. O'Connor and G. Stevens
The driving force for separation in NF is the difference between the applied
transmembrane pressure (typically 6 to 40 bar [18]) and the osmotic pressure. In
general an increase in charged solute concentration leads directly to an increase in
osmotic pressure. This means that osmotic pressure limits the concentrations
achievable by nanofiltration. As the concentration of retained solids increases, so too
does the osmotic pressure. This increase therefore reduces the driving force, and
lowers the flux through the membrane, and thus the level of separation that is
achievable [ 121.
NF membranes are made from a wide range of materials including polymers such
as aromatic polyamides, polysulfones, and polyvinyl alcohols, as well as inorganic
materials such as alumina, silica and hematite, and more recently other materials such
as ceramics [19]. They can be made with varylng permeabilities, displaying water
fluxes between 10-100 l/m2h at a pressure of 10 bar [20]. NF membranes often have
either an asymmetric or thin-film composite smcture, in which a thin film at the top
(facing the feed solution) acts as the selective bamer. This is backed onto a porous
support which has no influence on the permeability or solute rejection properties of
the membrane [2 11.
Important membrane characteristics which are likely to affect the performance
include pore diameter, pore length, membrane material and membrane charge [8].
Negatively charged membranes are common in industry as they have improved
fouling resistance against organic components such as colloids and proteins [5],
however positively charged membranes are also commercially available. The fixed
charge affects the separation characteristics of the membrane by means of the Donnan
exclusion mechanism. Ions that possess the same charge as the membrane, or co-ions,
attempting to migrate through the narrow pores of the membrane are repelled by the
fixed charge of the membrane. Ions that possess opposite charge to the membrane, or
counter-ions, are subsequently enriched in the membrane phase. In order to maintain
electrical neutrality in the external solution, counter-ions will tend to remain with
their co-ions. Thus, their movement across the membrane will also be restricted.
Therefore, in a negatively charged membrane, anion repulsion is the primary
determinant of solute rejection [19]. This mechanism of Donnan exclusion also
applies to ion exchange membranes during electrodialysis.
A number of studies into NF membranes have found that rejection is based upon
both solute size and charge. It has been widely found that NF membranes reject
multivalent ions better than monovalent ions, due to the combination of steric
hindrance and ionic interactions. Rautenbach and Albrecht [22] found that in general
the following ranking often applies for reverse osmosis membranes:
Rejection of cations:
Fe" > Ni2' = Cu*' > Mg2+> Ca*+> Na' > K'
Rejection of anions:
PO,'- > SO,2- > HC0,- > Br- > C1- > NO,- = F'
Rejection of neutral organic solutes generally increases with the molecular weight or
diameter of the solute.
46
Membrane-Based Dairy Separation: Comparison of Nanofiltration & Electrodialysis
Due to the fact that NF membranes exhibit features common to both UF and RO
membranes, there is a certain amount of controversy surrounding the mass transfer
mechanisms governing solute separation. As mass transport in NF appears to fall
withn the transition region of the pressure-dnven solution diffusion of RO, and the
convective pore flow of UF, it is considered to involve both mechanisms of transport,
in addition to the electrochemical effects related to membrane charge [23].
Additionally, it has been found that due to component interactions, the behaviour of
multi-component salt solutions, such as dairy streams, are difficult to predict from
studies of binary mixtures [8]. For example, it has been observed that the rejection of
a divalent ion differs when in solution with monovalent ions and uncharged solutes.
Also, the concentration of other components can have a large influence on transport
properties. Lately, several papers have been published whch try to address multicomponent solute transfer in NF membranes, taking into account these interactions
and mass transport mechanisms [ 17,24-3 11.
Electrodialysis
Electrodialysis uses an electrical current to exploit the differences in the charge-tosize ratio of solutes to achieve separations. In general, charged solutes will migrate in
an electncal field. The migration of charged solutes in opposite directions can be
exploited to physically separate solutes using semi-pemeable membranes. Two types
of membranes can be employed, either ion exchange membranes and/or porous
membranes.
Ion exchange membranes carry a fixed electrical charge, making them permeable
only to oppositely charged solutes, i.e. a cation-exchange membrane is negatively
charged allowing the passage of cations, and an anion-exchange membrane is
positively charged. A classical electrodialysis cell thus consists of a series of
alternating anion and cation exchange membranes, separated by alternating
compartments of feed and brine solution (see Figure 1). The application of an electric
driving force over the system results in an increase in particular ions in alternating
compartments, whle the other compartments become depleted in these ions [32].
Alternate cells in an electrodialysis stack are therefore referred to as concentration
and dilution cells respectively. Feed is circulated through the dilution cells, and 5%
brine carrier solution through the concentration cells.
The method by which ion-exchange membranes permit only the passage of
oppositely charged solutes is the same as the principle behnd charged NF
membranes, namely DoMan exclusion. Mobile counter-ions enter the membrane
matrix, while mobile co-ions are mostly excluded from the membrane due to the
membrane fixed charge, making it difficult for them to traverse the membrane [32].
Porous membranes can also be used to limit the charge-induced transfer of solutes
through a membrane, and thus enhance separation selectivity, due to size exclusion
[33]. Figure 2 depicts such a typical electrophoretic membrane contactor. In h s
application, feed enters a separation chamber which is composed of two adjacent
compartments delimited by a porous membrane. llus membrane acts as a contactor
between the two streams through which mass transfer is talung place. A voltage is
47
G. Rice, S. Kentish, V. Vivekanand,A. Barber, A. O’Connor and G. Stevens
applied across the system, and two electrodes are located on either side, isolated from
the separation chamber by ion exchange membranes [33].
When charge is applied, charged components migrate towards the electrodes, from
one chamber to the next, through the porous membrane. The rate at which a
component passes through the membrane is determined by its electrophoretic
mobility, which is a fbnction of the solute size and charge [34]. The outlet
composition of the streams is therefore determined by the extent of component
transfer through the central membrane. The porous membrane can be a microfiltration
(MF), UF or NF membrane, depending on the size exclusion required.
Figure 1. Electrodialysis stack, where A is an anion-exchange membrane and C is a
cation-exchange membrane (the dotted line shows slow movement of calcium ions
through the cation-exchange membrane).
48
Membrane-Based Dairy Separation: Comparison of Nanofiltration h Electrodialysis
Figure 2. EIectrophoretic separation chamber, where C is a cation-exchange
membrane, A is an anion exchange membrane, and PM is a porous membrane (dotted
lines show slow ion movement).
With either the classical electrodialysis arrangement or the electrophoretic
separation chamber, the transport of charged solutes through the membrane is
dependent on the membrane characteristics. Separation of ions is achieved on the
basis of rate of transport through the membrane. In ion-exchange membranes,
strongly charged ions are retained more strongly by the membrane resin, and therefore
migrate more slowly through the membrane. Therefore these membranes show a
higher permeability for monovalent ions than multivalent ions [35]. Conversely, when
a porous, uncharged membrane is used, the rate of migration through the membrane is
determined by the size:charge ratio. Highly charged small molecules will travel the
fastest [34]. Multivalent ions are therefore expected to pass through a porous
membrane faster than monovalent ions.
Other desirable ion-exchange membrane properties include low electrical
resistance, good mechanical stability (low degree of swelling or shrinlung in
transition from dilute to concentrated ionic solutions), and high chemical stability (pH
range 0-14, and in the presence of oxidizing agents) [32].
49
G. Rice, S.Kentish, V. Vivekanand,A. Barber, A. O’Connor and G. Stevens
The demineralisation rate is determined by operational factors such as the flow
rate, temperature, conductivity, viscosity, pH of the process water and product,
product inlet pressure and pressure difference between stacks [36].
The electrical driving force and the current density are also important. The
limiting current density of the membrane is the maximum current that can pass
through a membrane area without creating adverse effects [32] (e.g. higher electrical
resistance or lower current utilization.) It is determined by the ion concentration in the
diluate flow stream and by concentration polarization effects. Similarly, the current
utilization is the fraction of electrical current that is actually being used for ion
transport. This is affected by factors such as the membrane selectivity, osmotic and
ion-bound water transport, and the current passing through the stack [32].
The application of an electric field can also give rise to electroosmosis. This
phenomenon is the backward.flow of a liquid medium along the walls of an
electrophoresis chamber, or through a charged membrane, and is proportional to the
applied voltage, surface charge and ionic strength of solution [37]. The driving force
for electroosmosis is the formation of an electrical double-layer at the surface-liquid
interface. Any surface charges are compensated for by charges of opposite sign
present in the electrolyte. Ions in this double layer migrate towards the electrode of
opposite sign, carrying solvent with them. The effect of electroosmosis is to oppose
the bulk flow of components to the electrode, resulting in a counteraction of
electromigration [34].
Comparison of NF and ED
Following from the above descriptions and discussion of the principles and
characteristics of NF and ED, it can be seen that both methods are capable of
achieving a separation of lactose and divalent salts from undesirable monovalent salts
such as KCl and NaCl. The choice between the two technologies must therefore take
into account a number of factors, including:
0
level of demineralisation
0
solids concentration
0
efficiency of ion separation
0
efficiency of lactose rejection
0 costs (capital and operating)
In general, studies of NF membranes have shown that solvent demineralisation of
at least 30% is achievable, with a 70% reduction in monovalent ions [38]. ED, on the
other hand, is reported to be capable of achieving higher levels of demineralisation.
Levels up to 90% are possible [6, 391; however this method becomes uneconomical
for demineralisation above 50% when compared to alternative techniques such as ion
exchange [39].
Nanofiltration is capable of concurrently concentrating the permeate solution by a
factor of 3-4 [40], with total solids of up to 25% economically achievable [6].
Conversely, very little concentration can be achieved during electrodialysis,
necessitating further downstream concentration operations.
50
Membrane-Based Dairy Separation: Comparison of Nanofiltration & Electrodialysis
NF membranes show a good selectivity between monovalent and divalent ions.
While rejections of monovalent ions can be as small as 510% (dependent on
operating conditions), rejection of divalent ions such as calcium remain above 80%
[ 18,413. This is of course dependent on membrane characteristics. A study comparing
the monovalent versus divalent separation efficiency of NF and ED (using both ionexchange and non-selective membranes) concluded that NF membranes showed
greater selectivity for ions of differing valences, and therefore was more efficient at
separating mono- and multivalent ions [35]. In the case of UF permeate, where
calcium ion rejection is desired by the membrane, NF would thus appear to be the
most suitable choice. However, this study was based on only a small selection of
commercially available membranes. Monovalent perm-selective ion-exchange
membranes are now available, albeit at a high price, which are likely to further
increase the separation efficiency between different charged salts.
The literature has revealed that loss of lactose through the membrane is a factor
for both NF and ED. Lactose loss in ED is directly related to the applied voltage, due
to the linkage with ion flux (electroosmosis), and the level of demineralisation.
Values of 1.2-6.0%loss have been quoted for ion-selective ED [39,42], and between
0.1-1.0% for new commercial NF membranes [18]. During NF, lactose losses can be
minimized by using membranes with tight pores and by reducing concentration
polarization; and for ED by controlling electroosmosis. Alternatively, further
permeate processing can be used to remove this lactose downstream.
In general ED shows higher operating and membrane costs than NF. The cost of
demineralisation in ED is directly proportional to both the feed salt concentration and
the level of separation required [32]. This is due to the combination of energy
required for the desalting process (i.e. dssociation of ions and transfer of ions through
the ion-exchange membrane), and irreversible dssipation of energy due to fixtion of
ions passing through the membrane matrix [32]. This process can also consume
significant levels of process water, necessary for the brine flow through the
concentration cells of the electrodialysis stack, with figures quoted at 0.3-1 litre/ litre
of feed [39].
The energy requirements for NF are associated with the high-pressure pumps
required to drive the separation. Energy is also irreversibly lost due to friction
associated with transport of water through the membrane (independent of feed
concentration) and of ions through the membrane (dependent on feed concentration)
[32]. A h g h concentration feed will require a hgher transmembrane pressure to
create high flux, and hence achieve separation. Process water requirements are
significantly lower than for ED, with estimates of 0.05-0.1 litre/ litre of feed [39].
Conclusions
Both nanofiltration and electrodialysis can be utilised to separate the lactose and
mineral components of ultrafiltration whey permeate, which is currently often
underutilised or treated as a waste stream. Of the two processes, nanofiltration shows
the lowest operating and capital costs [16], and is more perm-selective towards the
undesirable monovalent salts (Na, K, Cl). It is thus useful when a calcium-rich lactose
product is required, for use in tailored dairy and other food products. Furthermore,
51
G. Rice, S. Kentish, V. Vivekanand,A . Barber, A . O’Connor and G. Stevens
this process is capable of simultaneously providing much greater solids
concentrations.
Electrodialysis is more appropriate when a pure lactose product is required. This
process is less perm-selective, and all the minerals can be removed, including
calcium. Furthermore, the process provides hgher levels of demineralisation.
Acknowledgements
This research was funded by an Australian Research Council Linkage Projects award
and by Burra Foods, and this support is gratefully acknowledged. Access to
infrastructure from the Particulate Fluids Processing Centre, a Special Research
Centre of the Australian Research Council, is also gratefully acknowledged.
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