Electrodialysis | BiotechStudies

ELECTRODIALYSIS

It was introduced by in 1950s by Juda and McRae to demineralize brakish water.

What is electrodialysis?

Electrodialysis (ED) is a membrane process wherein ions are transported through ion-exchange membranes from one solution to another under the influence of an electrical potential. The electrical charges of ions allow them to be driven through solutions and water-swollen membranes when a voltage is applied across these media.

Use of first time electro dialysis

• Synthesizing ion exchange membrane paved the way to its industrial application. Major developments were made since then, concentrating mostly on water desalination. Electro dialysis Reversal (EDR) was commercially introduced into the market in 1967 by Ionics. It was well developed both economically and technologically and was a reliable process for twenty years. Advantages of the process included slime reduction on membrane surfaces, automatic cleaning of electrodes, polarization scale prevention and reduced cost of hydraulics. New technology was developed based on ion-exchange resin beads filling desalting compartment to later be introduced into electrode ionization (EDI) process. First commercial system we introduced in 1991. EDI resulted in higher levels of water purity, thus, stagnating the EDR technology production market.

• Nowadays, electrodialysis has a wide array of applications. Major part of the application is brackish water desalination and boiler feer and process water treatment. However, both processes present costs problems. Waste treatment also has a big commercial use as well as table salt production. Demineralization of food products shows a good potential use, although it comes with high membrane selectivity.

Procedure

• Electrodialysis is a process that uses electrical potential difference to move ions in solution through ion exchange membranes. The process includes the use of several selective membranes that can only pass through either anions or cations as a stack. The membranes are separated from each other by inert spacers which provide mechanical stability and turbulisation.

• This results in increase of mass transfer coefficient and decrease in concentration polarization at membrane surface.

• Inlet feed containing ions is fed to the electrodialysis cell which contains parallel semi permeable membranes charged with electrical potential.

• Electrochemical reaction takes place in order to create an electric field and introduce electric current into the system. As a result, oxygen is formed at the anode and hydrogen at the cathode.

Reactions at the cathode is

2e − + 2 H 2 O → H 2 (g) + 2 OH −

And at the anode,

H 2 O → 2 H + + ½ O 2 (g) + 2e − or 2 Cl − → Cl 2 (g) + 2e −

Electrical field is applied to the cell forcing the movement of ions. Cations permeate across cation membranes but cannot pass through anion membranes, while anions go through anion membranes but cannot pass through cation membranes.

This results in three types of streams:

● Dilute stream, or product stream

● Concentrate stream, or brine, which becomes concentrated in ions

● Electrode stream, which passes over the electrodes

Applications

The fermentation product exists in the broth as Na + cations and anions. Application of an electric potential to the electrodes causes the ions to move through the solution at velocities proportional to the strength of the electric field. Average ionic velocities are surprisingly slow, usually in the order of 1 mm/min but the ions do not have to travel very far to reach the membranes because the solution compartments of ED stacks are typically about 1 mm thick. The combined motion of Na + to the right and Ac- to the left carries the total electric current through the bulk solution (unless other ions are present). Electrode reactions transfer the current from the solution to the anode and cathode. 02 gas and H+ ions are generated at the anode while H2 gas and OH - ions are generated at the cathode. The anion and cation-exchange membranes, designated as A and C in figure, are selectively permeable to ions of a specific charge. The anion-exchange membrane allows Ac - ions to be carried by the electric potential out of the center compartment and into the anode compartment. Similarly, the cation-exchange membrane is permeable to Na + ions that enter the cathode compartment to form NaOH. This is not the most efficient way to recover a weak acid from a fermentor. One of the reasons is the relatively low concentration of the NaAc in the broth. This array of alternating anion- and cation-exchange membranes is the membrane arrangement most commonly found in ED. The solutions between the membranes are alternately enriched in or depleted of NaAc when the electrodes are energized. The enriched and depleted solutions are withdrawn from their respective compartments to achieve useful changes in the electrolyte content of solutions without substantially affecting the non-electrolyte composition of the solutions. It is this selectivity for electrolytes that often makes ED the process of choice for certain separations, for example desalting of protein solutions or whey and recovery of salts of organic acids from fermentation broth.

Specific application of ED and EDR requires a certain configuration of the membrane stack. The membranes are produced in the form of foils composed of fine polymer particles with ion

exchange groups anchored by polymer matrix. Impermeable to broth under pressure, membranes are reinforced with synthetic fiber which improves the mechanical properties of the membrane. The two types of ion exchange membranes used in electrodialysis are:

● Cation transfer membranes which are electrically conductive membranes that allow only positively charged ions to pass through. Commercial cation membranes generally consist of crosslinked polystyrene.

● Anion transfer membranes, which are electrically conductive membranes that allow only negatively, charged ions to pass through. Usually, the membrane matrix has fixed positive charges from quaternary ammonium groups (-NR3 +OH-) which repel positive ions.

Both types of membranes shows common properties: low electrical resistance, insoluble in aqueous solutions, semi-rigid for ease of handling during stack assembly, resistant to change in pH from 1 to 10, operate temperatures in excess of 46ºC, resistant to osmotic swelling, long life expectancies, resistant to fouling and hand washable.

It depends on the manufacturer but usually each membrane is 0.1 to 0.6 mm thick and is either homogeneous or heterogeneous, according to the connection way of charge groups to the matrix or their chemical structure.

The spaces between the membranes represent the flow paths of the demineralized and concentrated streams formed by plastic separators which are called demineralized and concentrate water flow spacers respectively. These spacers are made of polypropylene or low density polyethylene and are alternately positioned between membranes in the stack to create independent flow paths.