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.
MEMBRANE
CELL PLANTS
Advantages
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
Disadvantages
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
STRUCTURE 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.
Aluminium
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
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.
Nickel
‘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.
Operation
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.
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