Introduction

In engineering, a membrane is a thin movie of stuff ( most normally, but non ever, a man-made polymer ) with a selective permeableness. Membrane-based separation procedures make usage of this selective permeableness. A considerable figure of different membrane procedures have found industrial applications. We ‘ll cover with microfiltration ( MF ) , ultrafiltration ( UF ) , nanofiltration ( NF ) and rearward osmosis ( RO ) . In add-on, we talk about electrodialysis and pervaporization.

We classify the membrane processes harmonizing to driving force:

Pressure

Concentration

Electrical potency

Temperature

These procedures are used in chemical, environmental and nutrient industry, nevertheless ; intent of this survey is discoursing about nutrient applications.

History

As it is known the biological membranes as selective barriers in cells and tissues are of import in industry. The belongings of selective permeableness ( perm-selectivity ) of natural membranes was discovered by Gallic physicist Abbe Nollet every bit early as 1748. Nollet discovered the relation between a semi-permeable membrane and osmotic force per unit area. There is a go oning development of new membranes in industrial applications in improved belongingss.

TYPES & A ; APPLICATIONS

Harmonizing to driving forces, membrane procedures are divided into four chief groups ; force per unit area difference, concentration difference, electrical potency difference and temperature difference. They have besides some subgroups as shown below:

I”P: contrary osmosis

nanofiltration

ultrafiltration

microfiltration

I”C: pervaporation

I”I? : electrodialysis

I”T: membrane distillment

Rearward Osmosis

Definition

The term “ osmosis ” means self-generated transportation of H2O from a more dilute solution into a more concentrated one through a membrane. Application of force per unit area stronger than the osmotic force per unit area causes H2O transportation in the opposite way from the concentrated solution to the less concentrated medium. This is the rule of “ rearward osmosis ” .

Applications

The chief applications of RO are for concentrating fluids, by remotion of H2O. For milk processing,

the concentration of milk on the farm for cut downing transit cost

for yogurt production at a concentration factor of about 1.5, to avoid the add-on of skimmed milk pulverization

for ice pick devising, besides to cut down usage of expensive skimmed milk pulverization

for cheese doing to increase the capacity of the cheese VATs

for retrieving rinse H2O

These applications are used in milk processing. However there are besides other applications for other nutrients such as fruits and vegetable juices and egg as shown below:

For concentrating tomato juice from about 4.5 Brix to 8-12 Brix

For concentrating fruit juices such as apple, pear, Prunus persica and apricot

For concentrating java and tea infusions with small loss of solids

For concentrating and drying egg white to approximately 20 % solids

Advantages

RO permits the usage of comparatively low temperatures. It reduces volatile loss caused by the stage alteration in vaporization and is really competitory from an energy position point.

RO is advantageous to minimise thermic reactions such as Browning.

RO can be used to bring forth a concluding merchandise, as in the instance of tomato paste and fruit purees or to partly concentrate prior to vaporization.

Disadvantage

From energy position point RO uses much higher force per unit areas than other membrane procedures in the scope 2-8 MPa and requires greater energy costs and particular pumps.

Nanofiltration

Definition

Nanofiltration is a comparatively recent membrane filtration procedure used most frequently with low sum dissolved solids H2O such as surface H2O and fresh groundwater, with the intent of softening ( polyvalent cation remotion ) and remotion of disinfection byproduct precursors such as natural organic affair and man-made organic affair.

Nanofiltration is besides going more widely used in nutrient processing applications such as dairy, for coincident concentration and partial ( monovalent ion ) demineralization.

Applications

Nanofiltration has been used for partly cut downing Ca and other salts in milk and whey, with keeping values of 95 % for lactose and less than 50 % for salts.

NF has been investigated for taking pesticides and constituents responsible for the colour from land H2O, every bit good as for sublimating H2O for carbonation and soft drinks.

NF is used for fractionating oligosaccharides with prebiotic potency, produced by the enzymatic dislocation of different complex saccharides.

Advantages

NF provides possible for bettering the heat stableness of the milk.

NF membranes have more unfastened pores than RO membranes.

NF removes toxic and unwanted bivalent ions ( ions with two or more charges such as lead Fe, Ni, quicksilver )

Nanofilters are close in size to RO filters but cost much less to run.

Particular belongingss of nanosized atoms can be exploited.

Ultrafiltration

Definition

Ultrafiltration uses a finely porous membrane to divide H2O and microsolutes from supermolecules and colloids. Suspended solids and solutes of high molecular weight are retained, while H2O and low molecular weight solutes pass through the membrane.

Applications

This separation procedure is used in industry and research for sublimating and concentrating macromolecular ( 103 – 106 Da ) solutions, particularly protein solutions. Ultrafiltration is applied in cross-flow or dead-end manner and separation in ultrafiltration undergoes concentration polarisation.

Milk Processing

UF has been used to concentrate cheese whey ( 6.5 % ) incorporating about 10-12 % protein, to bring forth dressed ores which could so be dried to bring forth high protein pulverizations.

The usage of UF reduces the milk required to do cheese by 6 % by capturing the soluble whey proteins in the cheese curd alternatively of leting them to go through into whey.

Oliveseed and vegetable proteins

For soybean, separation of low molecular weight peptides from soy hydrolysates, the dissociation of phytate from protein, remotion of oligosaccharides, the remotion of trypsin inhibitor and public presentation of different membrane constellations.

For cotton seed the usage of different extraction conditions has been evaluated, as have the functional belongingss of the isolates produced by UF.

Animal Merchandises

Slaughterhouse wastes contain significant sums of protein. Two of import watercourses that could be concentrated by ultrafiltration are blood and waste H2O.

Another of import stuff is gelatin. It can be concentrated from really dilute solutions by UF.

Advantages

Increased output, likely because of inclusion of some of the non-casein proteins in the curd

Lower energy ingestion

Decreased volume of whey

Many have been successful in footings of bring forthing good quality dressed ores and isolates, peculiarly with soya bean.

Disadvantages

Extra extraction processs are required because of the necessity of optimisation of extraction conditions with regard to clip, temperature, PH and antinutritional factors.

Because of that oil-rich seed and vegetable proteins are near to their solubility bounds and farther concentration will do them to come out of solution and advance farther fouling. Fouling and cleansing of membranes was found to be a series job during ultrafiltration.

Microfiltration

Definition

Microfiltration refers to filtration procedures that use porous membranes to divide suspended atoms. As a solid-liquid separation procedure, MF is progressively going a preferable option to the more expensive traditional procedures of dead-end filtration and centrifugation. In the nutrient industry, MF is extensively used for the elucidation of cloudy fluids.

Applications

In sterilisation of vino and beer, the chief aim is to take barm cells, which are rather big so the merchandise is clear and bright.

One application in the nutrient industry has been in the intervention of juices and drinks.

Clarification and biological stabilisation of vino musts and unrefined vino have been described for MF. This avoids the demand for ticketing and pasteurisation.

Other nutrient applications have been reported with meat and vegetable merchandises including the followers:

to retrieve precipitated soybean protein.

to take soil, coagulated proteins, fats and other particulatematerials from the provender.

Advantages

MF is strictly physical procedure ; it can hold advantages over traditional methods, which may affect chemical additives, in footings of the quality of merchandise every bit good as the cost of processing.

Recovery of precipitated soybean protein by MF has advantages over recovery of these proteins by utilizing UF.

MF membranes have comparatively big pore diameters. They retain suspended solid atoms such as micro-organisms and broken cells but allow protein molecules to pervade.

Removal of suspended atoms and colloidal stuff by MF is indispensable for cut downing the rate of fouling in the subsequent UF or RO stairss.

Disadvantage

Bacterial remotion from whole milk by MF is a job because the size scope of bacteriums convergences with the fat globules and, to a lesser extends with the casein micelles.

MF has non yet realized its full potency due to the terrible jobs of flux diminution due to fouling.

Comparison of These Four Membrane Processes

Typical Range of Application of Pressure-Driven Membrane Separation Processes

Procedure

Typical operating force per unit area scope ( MPa )

Limit atom size scope ( millimeter )

Medium frequency

0.1-0.3

100-10000

UF

0.2-1

1-100 ( 100-1000000 Dalton )

NF

1-4

0.5-5 ( 100-1000 Dalton )

Ro

3-10

10-100 ( Dalton )

These four types of membrane procedures are harmonizing to force per unit area difference and they are the most common procedures in nutrient applications. However, other types as it is mentioned supra such as pervaporation, electrodialysis and membrane distillment are used seldom. Therefore, they are mentioned in brief.

Pervaporation

Pervaporation is a new membrane technique which is used to divide a liquid mixture by partially zaping it through a nonporous permselective membrane.

Mass conveyance by pervaporation across permselective membranes involves three stairss

Upstream breakdown of the provender constituents between the fluxing liquid mixture and conceited surface bed of the membrane.

Diffusion of the constituents in the surface bed through the membrane

Desorption of these constituents at the downstream surface of the membrane.

Due to its high selectivity for liquid mixtures that can non be expeditiously processed one conventional distillment, pervaporation is the utile alternate to distillment and a possible rival to energy devouring separation procedures such vacuity and extractive distillment.

Electrodialysis

Electrodialysis ( ED ) is an electrochemical membrane procedure by which ions are transported through perm-selective membranes under the consequence of an electrical field. Ions are separated by ED membranes harmonizing to their chart. ED is most normally used for the transportation of ions into or from tangible fluid.

The applications of ED in nutrient processing are ;

operations of desalinating

demineralisation

de-acidification or acidification of liquid nutrients

Under the consequence of electric field, negatively charged anions tend to migrate towards the positive electrodes ( anode ) but are stopped by cationic membranes. Cations tend to travel in the opposite way and are stopped by anionic membranes. The merchandise does non touch the electrodes. Electrolysis occurs in the electrode watercourse and non in the merchandise or bearer.

De-acidification has the great advantage of avoiding add-on of chemicals.

The high mineral content of whey and whey merchandises is sometimes unwanted. ED provides a technologically sound method for demineralising whey merchandises.

Membrane Distillation

The most of import illustration of liquid-liquid membrane contactors is membrane distillment. In this procedure a warm, salt incorporating solution is maintained on one side of the membrane and a cool, pure distillation on the other. Because the solutions are at different temperatures their vapour force per unit areas are different ; as a consequence, H2O vapour flows across the membrane.

Membrane distillment offers a figure of advantages over alternate force per unit area driven procedures such as rearward osmosis, because the procedure is driven by temp. gradient, low class waste heat can be used and expensive high force per unit area pumps are non required. While the high concentrated solutions limit the flux in RO, they can be treated with membrane distillment.

The most of import application is sea H2O desalinization which is non related to nutrient applications.

GENERAL CHARACTERISTICS OF MEMBRANE MATERIALS

The bulk of commercial membranes are made of a broad assortment of organic polymers ; polyolefines, polymeric amides, hydrocarbons and besides ; asymmetric-flat-sheet membranes of cellulose ethanoate or cellulose triacetate, all right hollow fibres of aromatic polymeric amides or cellulose triacetate.

The chief features of these membrane stuffs are:

high permeate flux

good keeping capableness harmonizing to the specific application

good mechanical strength

chemical stableness and inertness

bio-inertness ( if used with enzymes and other biologically active stuffs )

thermic stableness

opposition to cleansing and sanitizing agents

opposition against microbic action

smooth, fouling opposition surface

conformity with all other nutrient safety demands.

long service life

low-cost cost

Decision

Membrane engineering will go on to be applied on big graduated table in the nutrient industry. Some industries e.g. , wet maize refinement and oil seeds, have merely late begun to use this engineering in their operations. Sing the immense sum of H2O used, these industries may see the greatest growing of membrane applications in this decennary.