BY-PRODUCTS TECHNOLOGY

Module 3. Processing and utilization of whey

Lesson 28
ELECTRODIALYSIS PROCESS FOR DEMINERALISATION

28.1 Introduction

Electrodialysis is a membrane process based on electrochemical laws governing the migration of ions under the effect of direct current at a given potential through an ion-selective semipermeable membrane. This process removes ionic (electrically charged) species from non-ionic species. In dairy industry, it is used to demineralise whey and other milk derivative solution.

28.2 Electrodialysis Membrane

The key to the electrodialysis process is the use of ion-selective membranes. Electrodialysis membranes are thin sheets of cation or anion exchange resins, usually reinforced with synthetic fibres necessary to give mechanical strength. Ion selective membranes that allow passage of positively charged cations (Na⁺, K⁺) are called cation membranes. Membranes that allow passage of negatively charged ion (Cl^-, PO₄^3-) are called anion membranes. An example of a cation membrane is styrene-divinyl benzene copolymer with sulfonic acid groups as active sites. The corresponding anion-selective membrane has the same copolymer with quaternary ammonium groups or tertiary amine compounds as anion exchanging active sites. Most commercially available membranes are having effective pore sizes of 10-20 A°, which are slightly greater than atomic dimensions and therefore, impermeable to flow of liquids and to diffusion of large molecules. General properties of electrodialysis membranes is given in Table 28.1. Desirable characteristics & properties of electrodialysis membranes are as follows:

• It should have high ion selectivity, low water transfer & low electrical resistance.

• It should have strength to withstand pressure difference across the channels, as the membranes are very thin.

• It should have dimensional stability.

• It should have chemical resistance mainly to oxidising and basic solutions used during cleaning.

Table 28.1 Properties of electrodialysis membranes

28.3 Electrodialysis Module

To achieve separation by electrodialysis, cation and anion membranes are altered with plastic spaces in a stack or module configuration with positive electrode (anode) at one end and cathode at other end. The spacers, whilst creating the flow channel, also support the membranes and include turbulence promoters. There are two main types of spacers: i) sheet flow spacers ii) tortuous flow spacers with linear flow velocities, 5-10 cm/sec and 30-50 cm/sec, respectively. It would be expected that the use of higher flow rate should alter advantages through increased turbulence and reduced boundary layer problem. Alternate cation and anion membranes are called ‘cell pairs’, which form a ‘stack’ or module. Electrodialysis module accommodates between 5 and 500 cell pairs. At the end of the stack are two electrodes that create the driving force over the whole stack. The electrodes are rinsed with their own rinse solution to avoid the possibility of scale formation. The preferred solution for rinsing of electrodes is sulphuric acid, rather than hydrochloric acid, which can lead to chlorine production. The stack may operate either in vertical or in horizontal positions. The typical batch-wise layout consists of several stacks or modules, each consisting of 200 pairs of cells in parallel connection. The line is completed with pumps, tanks, valves and instrumentation for controlling and recording flow and pressure in the stack, as well as pH, temperature, and conductivity of whey and brine. A typical layout is shown in .

28.4 Principle of Electrodialysis Process

When a DC voltage is applied across the electrodes, electrical potential created causes anions to move in the direction of anode and cations towards cathode. The ion-selective membranes form barrier to ions of opposite charge. The result is: anions attempting to migrate to anode will pass through anion membranes and are stopped by cation membranes: cations trying to migrate to cathode pass through cation membranes but are stopped by anion membranes. Hence, memberanes form alternate compartments of ion-diluting cells and ion-concentrating cells. The ion removal medium is generally a brine solution prepared from hydrochloric acid. This solution is continuously replaced to prevent high concentrations of ions being accumu-lated, leading to high osmotic pressure. By circulating whey through diluting cells and brine solution through concentrating cells, free mineral ions leave the whey and collect in brine stream. The flow is generally co-current to prevent the development of large pressure differences.

28.4.1 Electrodialysis process

The driving force to operate electrodialysis unit is generally provided by a high direct current voltage of up to 4 V per cell. For currents of up to 100 A, current density is generally in the range of 10-200 A/m². In the safe design of equipment, possible hazards through the existence of such currents may be taken into account. In general, increasing voltages result in increased rates of demineralisation.

The maximum operating voltages and currents are, however, restricted by the phenomenon of concentration polarization. Concentration polarization occurs when the current-carrying capacity of a particular solution of ions is exceeded. The limiting current density is reached when ions from the ionization of water are being transported through the membranes, and is higher at higher ion concentrations in the feedstock. The limiting current and hence limiting current density, therefore, influences the membrane area required for ion transfer under any given set of conditions. High voltages are limited by practical heat generation considerations. Thus, overall, in the easily stages of operation, a stack is voltage limited owing to the high conductivity and high allowable current density. In the later stages, the stack is current limited because of the low conductivity of solutions.

28.4.1.1 Processing modes

i) Within the module

A process option, which is available on some plants is current reversal during operation. Since ion movement is reversed, the flow between the chambers must also be switched. The effect of current reversal is to prevent and remove embedded deposits on or within the membranes, thus extending their life. This process option has been available for some time, but has not been utilized by all manufacturers. Both “Corning” and “Ionics” now include this technology as standard for their whey processing plants.

ii) Overall

Three modes of operation are commonly used:

a) Batch

A batch system, which is often used for demineralization rates above 70%, can consist of one membrane stack. An initial feed charge is recirculated through demineralization module until required level of demineralisation is achieved. High velocity of feed through the stacks is required to avoid fouling and polarization results in a low residence time and thus a low degree of demineralisation per pass. This leads to the need for recycling of the feed to achieve useful level of demineralisation. Batch processing allows for simple control of process, with operat-ing times determined by batch size, membrane area and extent of demineralisation.

b) Continuous

In continuous operation, feed is passed once through a series of modules to achieve the required demineralisation. The continuous electrodialysis method is advantageous when products with lower levels of demineralization (often limited to 60-70%) are desired. The brine cells as well as the line for electrode rinsing are connected in parallel. Variation in flow and feed quality require more complicated controls than batch operation to achieve a constant product. Voltage and current programmes for individual stacks are controlled by the various stream sensors. There is independent hydraulic control for each stack. An electrodialysis plant can easily be made automatic and supplied with a CIP system. Cleaning sequence include water rinse, alkaline solution cleaning (pH 9), water rinse, cleaning with HCl (pH 1) and final water rinse.

c) Feed and bleed

This is a combination of batch and continuous modes and involves recycling of feed stock with continuous small removal of product, compensated by equivalent feed rate. Again, greater capacity can be achieved by the use of parallel lines and multiple stages within a line. Residence times are, in general, 10-30 min.

28.5 Membrane Fouling

Membrane fouling is a major problem in electrodialysis of whey. Such fouling can involve both inorganic and organic molecules.

28.5.1 Inorganic fouling

Fouling by inorganic molecules results in general from operating too close to or above the limiting current. Under these circumstances, hydrogen ions and hydroxyl ions are transferred, causing a change in pH which will in turn result in precipitation of calcium salts such as calcium carbonate, calcium sulphate and calcium phosphate. Precipitation of these salts can occur on either the anionic or cationic membranes, but is more generally found on the brine side of the cationic membranes. These precipitates cause scaling on the surface of the membranes, which may be removed by normal acid cleaning conditions. Colloidal silicates are also well recognized inorganic foulants.

28.5.2 Organic fouling

Organic fouling of anionic membranes is more difficult to overcome. Many organic molecules, such as protein and protein fractions, amino acids, humic substances, are partially charged and are attracted to the anionic membranes during processing. Although many are deposited on the surface of membrane, some become embedded within the membrane itself. This results in increased electrical resistance within the system leading to a loss of performance. At a current density of 20-25 mA/cm², and with continuous operation, there is a danger of irreversible protein deposition. Effective cleaning of the anionic membranes can only be achieved by washing with alkaline solutions. As a consequence, however, of the alkaline effect, the lifespan of the anionic membranes is shorter than that of the cationic.

28.6 Some Developments

In order to avoid membrane fouling, a new commercial electrodialysis process, called transport depletion, has been developed. This method involves the substitution of selective anion membranes by non-ionic membranes of regenerated cellulose. In this way, no concentration polarization of the whey constituents occurs on the membrane surface. Higher current densi-ties can be applied, the lifespan of the membranes is longer and the cleaning process is simpler.

Another modification of electrodialysis is ion substitution. When applying this method, a third liquid stream, containing an ion selected for substitution, passes through the apparatus in addition to the usual whey and brine stream. A particular feature of this method is the structure of the ion membranes as compared to those used for conventional electrodialysis. The whey runs between two cation-selective membranes and the sodium is separated, into the brine, on the cathode side of the apparatus. At the same time, another cation, e.g., potassium travels through the membrane into the whey stream. The anions remain in the whey and form new salts.

A further modification of electrodialysis is electro-osmosis, which has been industrially applied in the USA, Japan and Norway since 1967.

Selected references

Delaney, R.A.M. 1976. Demineralisation of whey. Aust. J. Dairy Technol., 31: 12.
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