Bipolar Membrane Electrodialysis (EDBM)
Introduction
The basic aspect of electrodialysis with bipolar membranes (EDBM) is the combination of electrodialysis for salt separation with electrodialysis water splitting for the conversion of a salt into its corresponding acid and base. The bipolar membranes enhance the splitting of water into protons and hydroxide ions.
Bipolar membranes are a special type of layered ion exchange (IX) membrane where the two polymer layers one is only permeable for the anions and the other only for cations. Unlike membrane processes EBM isn’t applied for separation purposes but to get a reaction in the bipolar junction of the membrane where the anion and the cation permeable layers are in direct contact.
The main requirements for a bipolar membrane are:
Excellent long-term stability
A low passive drop in potential
A high rate of water splitting
High permselectivity
as well as good mechanical stability
Water splits into hydroxide ions and protons. The produced hydroxide ion and proton are separated by migration in the respective membrane layer out of the membrane.
2H2O ↔ H3O+ + OH-
Unlike a water splitting at electrodes during electrolysis, no gases are formed as a side product to this reaction, nor are gases used. Electrodialysis with bipolar membranes (EDBM) can replace electrolysis with water splitting at the electrodes and can also have a wider variety of applications.
Fig.1, Schematic process of the salt ion split by applying bipolar membranes to their corresponding acids and bases.
The potential difference needed for the electrodialytical production of one-molar solutions is 0.83 V which is equivalent to an energy consumption of 22 Wh. In contrast, the electrolytical production of a theoretical drop in potential is equivalent to an energy consumption of 55 Wh. Further advantages of bipolar membrane technology include the comparatively simple apparatus configuration, the option of a stack-like set-up as well as the low investment costs.
Especially the potential to split water without having reactive gases involved made it a promising technology already fifteen to twenty years ago.
Characteristics of EDBM
Splits water into acid (H＋) and alkali (OH－) with relatively low voltage.
Due to the absence of electrode reaction, no oxidation-reduction reaction takes place. Therefore, there will be no byproduct.
Produces acid and alkali from inorganic salt and organic acid salt in a single process.
Controls the concentration rate of acid and alkali.
Different from electrolytic processing, no electrodes are required for through every single cell and, therefore, fewer gases are generated.
Produces less waste solution.
Can withstand continuous operations for an extended time since no regeneration process, like ion exchange resin process, is required.
Application examples of EDBM
・Production of organic acid from organic acid salt
・Production of amino acid from amino acid salt
・Production of acid and alkali from waste solution with salt
・Production of acid and alkali from inorganic salt
1) Introduction
ELECTRODIALYSIS REVERSAL (EDR)
Electrodialysis is a membrane process that uses alternating Anion–selective membranes (AMs) and Cation-selective membranes (CMs), placed between an Anode (+) and a Cathode (-). Due to the applied electric field, anions will move towards the Anode and cations will move towards the Cathode. Anions are stopped by the CMs and the cations by the AMs, creating a process flow with low ion concentration (Dilutant) and a process flow with high ion concentration (Concentrate).
A pair of a CM and a AM and both areas between these membranes is a Cell Pair. A Cell Pair is the basis unit of a stack, and is repeated (n) times. The number of cell pairs in an actual stack varies depending on
the electrodialysis system, with as many as 600 cell pairs in a typical industry-scale system.
In electrodialysis suspended solids which carry positive or negative electrical charges can increase the resistance of the membrane dramatically, are deposited on the membrane surface. However, in electrodialysis the problem has been eliminated to a large extent by reversing in certain time intervals the polarity of the applied electrical potential which results in a removal of charged particles that have been precipitated on the membranes. This technique is referred to as electrodialysis reversal (EDR).
Mineral Extraction Plant
Electrodialysis Bipolar Membrane (EDBM) Technology Installed in 2017/18 and presently mothballed
Overview
Plant extracts salts from waste brine water to produce caustic soda, hydrochloric acid, magnesium hydroxide and gypsum
Electrodialysis bipolar membrane plants and evaporators are used in chemical processing plants for acid and base recovery.
Feed: Brackish water and waste brine concentrate from a desalination plant (RO concentrate)
The combined heat and power plant is a 6.5 MW Solar Taurus 70 gas turbine generator set with HRSG.
Unit Operations
1. Filtration - Cartridge Filters, Nanofiltration, and Brackish Water RO (BWRO)
a. The cartridge filters protect the NF system which protects the downstream RO.
b. System also consists of NF booster pump and NF skids configured as 3 stage Nano Filters and BWRO skid.
2. Degasification
a. The degasification tower removes CO2 from the water.
3. Ion Exchange
a. The ion exchange system consists of a Weak Acid Cation (WAC) system for hardness removal, a chelating resin bed for hardness polishing, as well as resin systems for removal of arsenic and silica.
4. Electrodialysis Reversal (EDR)
a. Operated with feed water containing a higher Total Dissolved Solids (TDS) concentration
b. System includes filters and metering pumps
5. Electrodialysis BiPolar Membrane (EDBM)
a. Supplied by GE/Suez with 7 skids per train and 8 stacks per skid
b. Complete system includes cartridge filters, caustic heat exchangers, salt water pumps, acid and caustic pumps
6. Acid Evaporation / Concentration
a. The evaporator concentrates acid from 4 to 35%
b. Acid evaporators(2) and acid tank farm and pumps and loading system
7. Caustic Evaporator
a. The evaporator concentrates caustic from 4 to 50 %.
b. Caustic evaporators (3), caustic centrifuge, pumps and caustic tanks
8. Calcium sulfate (gypsum) and Magnesium hydroxide Precipitation and Solids Removal
a. System includes precipitation reactor, clarifiers, vacuum drum filters, Sludge transfer pumps and dewatering Vacuum pumps and overflow and homogenizing tanks
9. RO and NF Membrane Systems
a. Industrial Nanofiltration unit
b. Industrial RO Unit for Brackish Water (BWRO)
10. Power Generation
a. Power – 6.5 MW, Solar Taurus 70, 60Hz, 4160V; Combustion turbine and heat recovery steam generator
b. (2) 200 KW emergency generators
11. Support Facilities
a. Tank farm
b. Cooling towers
c. Evaporation pond
d. Truck Loading
