Membrane Processing
Membrane processing is a technique that permits concentration and
separation without the use of heat. Particles are separated on the basis of
their molecular size and shape with the use of pressure
and specially designed semi-permeable membranes. There are some fairly new developments in terms of
commercial reality and is gaining readily in its applications:
- proteins can be
separated in whey for the production of whey protein concentrate (WPC)
- milk
can be concentrated prior to cheesemaking at the farm level
- apple juice and wine can be clarified
- waste treatment and product recovery is possible in
edible oil, fat, potato, and fish processing
- fermentation broths can be
clarified and separated
- whole egg and egg white ultrafiltration as a preconcentration
prior to spray drying
The following topics will be covered in this section:
When a solution and water are separated by a semi-permeable membrane, the
water will move into the solution to equilibrate the system. This is known as
osmotic pressure If
a mechanical force is applied to exceed the osmotic pressure (up to 700 psi),
the water is forced to move down the concentration gradient i.e. from low to
high concentration. Permeate designates the liquid passing through the
membrane, and retentate (concentrate) designates
the fraction not passing
through the membrane.
![[image]](../deicon/membrane.gif)
Membrane Processing
Reverse osmosis (RO) designates a membrane separation
process, driven by a
pressure gradient, in which the membrane separates the
solvent (generally
water) from other components of a solution. The membrane configuration is usually cross-flow. With reverse osmosis,
the membrane
pore size is very small allowing only small amounts of very low molecular
weight solutes to pass through the membranes. It is a concentration process using a 100 MW cutoff, 700 psig, temperatures less than 40°C
with cellulose acetate membranes and 70-80°C with
composite
membranes. Please click above link for a schematic diagram of these membrane processes.
Hyperfiltration is the same as RO.
Ultrafiltration (UF) designates a membrane separation process, driven by a
pressure gradient, in which the membrane fractionates components of a liquid as
a function of their solvated size and structure. The membrane configuration is usually cross-flow. In UF, the
membrane pore size is larger allowing some components to pass through the pores
with the water. It is a separation/ fractionation process using a 10,000 MW
cutoff, 40 psig, and temperatures of 50-60°C with polysulfone membranes. In
UF milk, lactose and minerals pass in a 50% separation ratio; for example, in
the retentate would be 100% of fat, 100% of protein, 50% of lactose, and 50% of free
minerals. Diafiltration is a specialized type of ultrafiltration process in which the retentate is diluted with water and re-ultrafiltered, to reduce the concentration of soluble permeate components and increase further the concentration of retained components.
Please click above link for a schematic diagram of these membrane processes.
Microfiltration (MF) designates a membrane separation process similar to UF but
with even larger membrane pore size allowing particles in the range of 0.2 to 2
micrometers to pass through. The pressure used is generally lower than that of
UF process. The membrane configuration is usually cross-flow. MF is used in the dairy industry for making low-heat sterile milk
as proteins may pass through but bacteria do not. Please click above link for a schematic diagram of these membrane processes.
- Open Tubular:
- Tubes of membrane with a diameter of 1/2 to 1 inch and
length to 12 ft. are encased in reinforced fibreglass or enclosed inside a
rigid PVC or stainless steel shell. As the feed solution flows through the
membrane core, the permeate passes through the membrane and is collected in the
tubular housing. Imagine 12 ft long straws!
- Hollow Fibre:
- Similar to open tubular, but the cartridges contain several
hundred very small (1 mm diam) hollow membrane tubes or fibres. As the feed
solution flows through the open cores of the fibres, the permeate is collected
in the cartridge area surrounding the fibres.
- Plate and Frame:
- This system is set up like a plate heat exchanger with
the retentate on one side and the permeate on the other. The permeate is
collected through a central collection tube.
- Spiral Wound:
- This design tries to maximize surface area in a minimum
amount of space. It consists of consecutive layers of large membrane and
support material in an envelope type design rolled up around a perforated steel
tube.
Electrodialysis is used for demineralization of milk products and whey for infant formula and special dietary products. Also used for desalination of water.
Principles of operation:
Under the influence of an electric field, ions move in an aqueous solution. The ionic mobility is directly proportioned to specific conductivity and inversely proportioned to number of molecules in solution. ~3-6 x 102 mm/sec.
Charged ions can be removed from a solution by synthetic polymer membranes containing ion exchange groups. Anion exchange membranes carry cationic groups which repel cations and are permeable to anions, and cation exchange membranes contain anionic groups and are permeable only to cations.
Electrodialysis membranes are comprised of polymer chains - styrene-divinyl benzene made anionic with quaternary ammonium groups and made cationic with sulphonic groups. 1-2V is then applied across each pair of membranes.
Electrodialysis process:
Amion and cation exchange membranes are arranged alternately in parallel between an anode and a cathode (see schematic diagram). The distance between the membranes is 1mm or less. A plate and frame arrangement similar to a plate heat exchanger or a plate filter is used. The solution to be demineralized flows through gaps between the two types of membranes. Each type of membrane is permeable to only one type of ion. Thus, the anions leave the gap in the direction of the anode and cations leave in the direction of the cathode. Both are then taken up by a concentrating stream.
Problems:
Concentration polarization. Deposits on membrane surfaces, e.g. proteins - pH control is important. Prior concentration of whey, to 20% TS, is necessary before electrodialysis.
Ion exchange is not a membrane process but I have included it here anyway because it is used for product of protein isolates of higher concentration than obtainable by membrane concentration.Fractionation may also be accomplished using ion exchange processing. It
relies on inert resins (cellulose or silica based) that can adsorb charged
particles at either end of the pH scale. The design can be a batch type,
stirred tank or continuous column. The column is more suitable for selective
fractionation. Whey protein isolate (WPI), with a 95% protein content, can be
produced by this method. Following adsorption and draining of the deproteined
whey, the pH or charge properties are altered and proteins are eluted. Protein
is recovered from the dilute stream through UF and drying. Selective resins may
be used for fractionated protein products or enriched in fraction allow
tailoring of ingredients.
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