Wednesday, December 5, 2012

Electrolysis of Brine(Nacl) -Sodium hydroxide(NaOH) production.

The ion exchange membrane cell plant is equipped with 20 electrolysers of type AZM-T-5.6-F2 consist of three blocks of monopolar filler press type electrolysers and it corresponds to three single electrolysers. The basis electrolysis element is anode chamber and a cathode chamber separated by an ion exchange membrane, Type – Flemion – 893
For electrolysis secondary purified brine is being fed to the anode chamber, purified H2O to the cathode chamber, and while applying DC current, movement and reaction of substance in the electrolyser will take place. In anode chamber electrolysis of NaCl occurs, turning Cl- into Cl2 on the anode mesh, Na+ ions moves to the cathode chamber through the ion exchange membrane. Generated Cl2 gas & anolyte flow up to the anode gas seperator by the driving force due to the gas lift effects, where chlorine gas is separated and anolyte is recycled through circulation pipe. While a part of anolyte overflow as depleated brine.
Anode side reaction
             NaCl --> Na+ + Cl-
               Cl- --> ½ Cl2 + e-   

In cathode chamber, decomposition of water occurs, produces hydrogen gas and OH- ion. OH- ions forms NaOH together with the sodium ions that has passed through the membranes. Generated H2 gas and NaOH flow up to the cathode gas seperator by gas lift effects, where H2 gas is separated and caustic is recycled through circulation pipe, while a part of caustic soda overflows as product [32%].
Cathode side reaction
      2H2O + 2e- --> H2 + 2O-   
      Na++OH- -->NaOH
Water in the anode chamber will move to the cathode chamber through the membranes by electro osmosis and osmosis. Membrane do not permit the movement of chloride ions and NaCl diffusion to the cathode chamber, transfer of hydroxide ions to the anode chamber. Transfer of those ions results in a loss of current efficiency. Hydroxide ions if any diffused form the cathode chamber to the anode chamber will produce oxygen gas, hypochlorite and chlorate by reacting with chlorine gas
       2 NaOH + Cl2 --> NaCl + NaOCl + H2O 
       6 NaOH + 3 Cl2--> NaCl3 + 5 NaCl + 3H2O 
       4 NaOH + 2Cl2 --> O2 + 4 NaCl + 2 H2O
electrolyser : AZM – T – 5- 6 F2 type electrolyser consist of the following major components
1. Anode elements
          The electrolyser contains 15 anode elements. It consists of one anode frame and a pair of anode meshes spot welded on both sides of anode frame.
Anode frame made of Titanium composed of rectangular pipe to form anode chamber, conductive rods welded with ribs to pass electrical current from equalizing burbars on to anode meshes.
 Anode mesh is titanium made expanded metal coated with RuO2 catalyst with an area of 1.71 m2 [1.5 m x 1.14m] 
2. Cathode elements
There are 18 cathode elements in one electrolyser, which comprises three types consisting of twelve C2 – Type elements with a pair of cathode mesh on both sides of cathode frame, three C-1 type with one cathode meshlocated on Fixed head side and three C-3 type with one cathode mesh located on movable head side. Cathode Frame made of stainless steel [Sus – 3ios] is composed of rectangular pipes to form cathode chamber, equalizing busbar to conduct electrical current, ribs and coil springs to support cathode meshes. Cathode mesh is copper made punched metal with nickel plating and Raney nickel plating and is attached on the cathode frame by using sealing gaskets, bolts, special jigs 4 nuts: effective area of one cathode element is same as that of anode element, such as 1.7q x 2m2 = 3.42m2.
3. Gaskets
EPDM (Ethylene Propylene diene monomel) gaskets with special structure are placed between flemion membrane & anode / cathode elements.
4. Flemion membrane
The membrane material consist of a copolymer made up of tetrafluoroethylene and carboxylated / sulphonised perfluoro vinyl ether. There are thirty sheets of membranes with effective area of 1.71m2 contained in one electrolyser & are installed between anode & cathode elements.
5. Anode gas seperator bottom header
Both are made of titanium and equipped with nozzles to connect anode element by flexible hoses one butterfly valve is installed in b/w the above for anolyte recycling
6. Cathode gas seperator & bottom header
Both are made of stainless steel and equipped with nozzles to connect cathode elements by flexible hoses. One butterfly valve is installed in between the above for caustic recycling
  7. Cell frame
Electrolyser supports are made of carbon steel & consists of base support, movable head, fixed head, sliding rail for frame, He-rods and insulators. Anode & cathode elements membranes and gaskets are placed between movable head and fixed head and are fastened together by the press unit.
8. Insulation frame
This is for insulation of each block of electrolyser and are made of rubber lines carbon steel.
9. Linking busbar
There are four linking busbars made of copper are is installed in one electrolyser.
1)    Pure sodium hydroxide
2)    Electrical energy consumption only about 77% of that of ‘Hg’ process
3)    No ‘Hg’ asbestos used
4)    Flexibility of operation
5)    Less space requirement as compared to another
1)    NaOH – 33% only produced
2)    Cl2 gas contains O2
3)    Very high purity brine required
      4) Present high cost and short life time of membrane  
Major impurities present in membrane process
Calcium and Mg
Ca2+, Mg2+ forms precipitate on the membrane layer and reduce current efficiency by alloy using hydroxyl ions to penetrate further in to the membrane from catholyte. There fore in the feed brine the content of Ca2+ combined with Mg should be kept below 20ppm
 Strontium and SiO2 
They form hydroxide and silicate on the membrane & decrease current efficiency by combined effect. Limiting value is 0.02ppm or less when SiO2 content will be in the level of 5ppm and Sr 0.06ppm or less when SiO2 content is lower than 5ppm.

Al decreases the current efficiency allowable range is <0 .1ppm=".1ppm" span="span"> 
Ba & I
Ba and I forms ppt and reduce current efficiency by combined effect
Allowable values are
Ba < 1.0 ppm when I2 is < 0.1ppm
Ba < 0.5 ppm when I2 is < 0.2 ppm

Iron form ferric oxide allows pH and can ppt at the anode phase ‘O’ the membrane and reduce current efficiency. Max allowable value in the feed brine is 1ppm or less.
‘Ni’ is in the feed brine affects membrane voltage performance and to be kept below 0.01 ppm or less. It rarely comes from the brine and the principle source is from the cathode due to the attach of hypochlorite and diffuces through the membrane during electrolyser shutdown.
Pb and Hg
Membrane will allow heavy metals in the brine and effects. Cathode coatings and voltage performance of the electrolyser. Its limiting values 0.2 ppm and 10ppm respectively.
Protection of electrolysers from electric corrosion by leak current
The circuit of electrolyser is protected from grounding for preventing current leakage to the ground and therefore avoiding electric corrosion. Current leakage occurer during fluid flow in or out from the electrolysers. For preventing corrosion of electrolysers and pipings, following methods are adopted.
1.           Leak current via the feed brine
When leak current flows out via the piping of feed brine, galvanic corrosion develops on anode gas separator and anolyte  recycle pipe. An anticorrosive electrode is provided. Inside the pipe in the feed brine inlet of each electrolysers. This anticorrosive electrode is electrically connected with the intercell busbar between the electrolyser on the anode side for keeping the potential of anticorrosive electrode nearly identical with that of the anode potential that is higher than the electrolyser potential. In this way, leak current will flow out only from the anticorrosive electrode to the feed brine side without affecting electrolyser.
2.           Leak current through the depleated brine is prevented by providing an anticorrosive electrode inside the depleated brine line of each electrolysers, electrode is electrically connected with the anode side busbar.
3.           Leak current through the catholyte is eliminated by installation of a current breaker on the catholyte outline of each electrolyser
4.           To prevent the leak current – inside the electrolyser sacrifice electrodes are installed at the inlet and outlet nozzle of first and third block
5.           In case an accidental grounding is sustained in a piping system, galvanic corrosion will take place in the piping system & equipments. To avoid such electrolytic corrosion & anticorrosive electrode is provided in common main piping of feed brine and depleated brine line.
6.           For the purpose of protecting the wiring from an excessive current in the case that the anticorrosive electrode and the connected wires short circuit to the ground the line leading to the anticorrosive electrode is provided with a trigger fuse [5A]. A alarm is displayed on a panel in the control room in the case of blow out of the trigger fuse for periodical checking the leak current, connect a DC ammeter to a plug in type terminal of the trigger fuse.
The secondary purified brine is fed to the brine head tank by the secondary brine pump through a heat exchanger or cooling or heating purpose. Purified brine is admitted to the electrolysers through a pressure control valve PICA – 3201. Then it flows in branches to the anode chambers of each electrolyser through a rotameter. Demineralized water from the head tank is admitted to the electrolysers through a pressure control valve PICA – 3202 and flows to the cathode chamber of each electrolysers. Through rotameters. During normal operation both control valve are to be put in ‘auto’ operation.

During electrolysis, at anode chamber, decomposition of 30% of NaCl in feed brine taxes place, forming depleated brine at the anode side, which overflows from the anode gas seperator and enters the depleated brine receiver. Chlorine gas enters at the anode gas seperator is separated from the anolyte & is collected into the chlorine gas main piping and sent to the treatment section, at cathode chamber, caustic soda is produced overflows from the cathode gas seperator & flows through the current breaker to the receiving tank [32% lye tank]. Caustic soda lye produced is pumped to the main storage tank through a cooler in which it is cooled by cooling tower water. The level in the caustic soda receiving tank is maintained by pumping through a level control valve, LIC – 3102 which can be put ‘auto’ operation. The H2 gas generated in the cathode chamber enters the cathode gas seperator when the gas is separated from the catholyte and is collected in the H2 gas main piping and to the H2 gas treatment section. The pressure of hydrogen gas in the main piping is that of H2 holder pressure. 
Monopolar and Bipolar systems
In the electrolysers for NaOH production membrane are clamped vertically b/w mesh like cathode and anode. The cells are filled with electrolytes and gas separating means are provided outside. Many cells unit can be stocked by a like fitter press to constitute one electrolyser with ample production capacity
Industrial electrolysers for NaCl electrolysis using membrane cell process and classified into different cell arrangement.
1.     Monopolar system
In the monopolar system cells are electrolyser are electrically connected in parallel and the electrolysers are connected in series.
Mono polar cells consisting of the following elements.
1.     Dimensionally stable titanium anode plate held in a frame with +ve busbar method
2.     The cation exchange membrane
3.     A cathode plate in a frame with negative busbar attached
A number of cells are assembled together to form one monopolar electrolyser. Since individual cells in a monopolar electrolyser are connected in the voltage across the electrolyser is same as the voltage across individual cell. For conversion of Hg cell to membrane are cell monopolar system is preferred as the existing installation can be used.
Bipolar system
These are constructed in modules. A module consist of rectangular box like construction where are face is anode and other is cathode. In the middle there is a partition place which divides the modules into anode and cathode compartments.
These modules are joined together with the membrane in b/w them. The anode chamber of one of module, the membrane and the cathode chamber of the next module constitute one cell. Several such modules are assembled together to form one bipolar electrolyser. Since the individual cells in bipolar electrolyser are connected in series, voltage across one electrolyser is the sum of individual voltage. In bipolar system electrolysers are connected in parallel.





Robert Melon said...
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