Engineering Essays | Chemical and Process
Engineering
Process Description
Strong brine (300gll) is fed to the anolyte compartment at
a temperature of 55°C.
The anolyte compartment is separated
by the catholyte compartment which contains 35% caustic solution,
by a cationexchange membrane. The solutions in the cell are subjected
to an electrical current.
Several processes occur simultaneously in the membrane phase
in an operating cell. Sodium ions and hydroxide ions migrate
under the cumulative effect of concentration and electrical
potential gradients, with sodium ion the major potential carrier.
In general these gradients are not expected to be linear,
especially for composite, surface treated, or fabric backed
materials.
The flow of sodium ions is accompanied by a net
electrosmotic water flow in the same direction. This diffusion
of water is a function of the membrane used and it is sensitive
to external NaOH solution concentration. The effect of temperature
is less significant!
Transfer of chloride ion Cl- to the catholyte compartment
from the anode compartment is prevented as the membrane presents
an effective hydrodynamic barrier, and also the electrical
potential gradient across the membrane opposes chloride transport.
Chlorine gas saturated with wafer vapour and small amounts
of oxygen evolve from the anode, while the depleted brine
is discharged to be further treated and then re-saturated
with sodium chloride in order to be fed again to the cell.
The amount of oxygen is an indication of the cell inefficiency
as it results from the reaction of hydroxyl ions with molecular
chlorine that otherwise would have been retrieved as a product.
At the catholyte compartment caustic of strength 35% is produced,
that is removed from the cell to be further concentrated.
Hydrogen evolves from the cathode saturated with water. A
rough schematic of the process is presented in the figure
below:
Figure 1 Membrane Cell schematic
1 Modern Chlor Alkali Technology, page 158. The Materials of Construction. The Membranes.
The membrane is the key component of the membrane cell. The
energy requirement and the quality of the solution produced
in a membrane cell depend on the membrane performance. The
requirements for a membrane are as follows:
1.) Durability under the conditions of chlor-alkali electrolysis.
2.) High selectivity for sodium ion transport.
3.) Low electrical resistance.
4.) Sufficient mechanical strength for practical use.
The membrane is exposed to chlorine and strong caustic solution
at high temperature.
Only ion-exchange membranes made of perfluoropolymer can
withstand such severe conditions. The first membranes that
showed significant potential for use in the chloralkali process
were made of perfluorosulfonate polymer. These proved to be
durable in
chlor-alkali cells, but they were relatively inefficient.
In pursuit of greater efficiency, a perfluorocarboxylate polymer
membrane was developed. Because of its low water content,
the perfluorocarboxylate membrane effectively inhibits the
migration of hydroxyl ions from the catholyte, making it possible
to obtain a high current efficiency, with a 32%-35wt% caustic
soda in the catholyte.
However, the voltage drop for such
a highly efficient membrane is relatively large. Further development
resulted in a two-layered membrane, composed of a thin, high-efficient
carboxylate membrane on the cathode side and a supporting
high conductivity membrane, either carboxylate or sulfonate,
on the anode side.
The ion-exchange groups of the original polymers are in the
perfluorosulfonate form,
-S02, or the ester form, -CO2R. These polymers are processible
by melt and are formed into membranes by extrusion, heat pressing,
or lamination. In most cases, membranes
are embedded with some reinforcement, such as woven fabric
made of polytetrafluoroethylene (PTFE), so that even with
a thickness of ca. 250 µm, they exhibit sufficient mechanical
strength for practical handling. Prior to use, the ion exchange
groups are converted to -S03Na or -C02Na by caustic solution.
Because the performance of the membrane is the most important
element in the efficiency of the membrane cell, many refinements
of the technology have been made in the membrane manufacturing
technology. The improvement of hydrophilicity by covering
the surface with a porous, non-conductive inorganic material
brought about a great reduction in cell voltage. To reduce
the amount of current screening caused by fabrics, membrane
reinforcement with dispersed microfibriles or interwoven fabrics
of electrolyte-soluble fabrics and PTFE have been contrived.
The membranes are supplied commercially from Du Pont (Nafion)
and from Asahi Glass (Flemion).
Summary
The membrane in a chlor-alkali electrolyser is exposed to
chlorine on one side and strong caustic on the other. Only
perfluoropolymers have been found to withstand these conditions,
combined with appended groups having ion-exchange properties.
The first membrane ever to be constructed produced a low concentration
caustic solution and was deficient in limiting hydroxyl ions
back migration. Then the idea of an asymmetric membrane having
sulfonic acid groups on the anodic side and converted groups
on the cathodic side to overcome back migration of hydroxyl
ions was developed. These composite membranes operate efficiently
at high caustic strengths. They have got better resistance
to hydroxyl ions back migration. Nowadays membranes consist
of a film of perfluorosulfonate polymer, Teflon reinforced
fabric and a perfluorocarboxylate polymer all bonded together.
Membranes are supplied commercially from Du Pont (Nafion)
and from Asahi Glass (Flemion).
REFERENCES
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Textbook of Anaesthesia 3rd Edition, Churchill Livingstone
Al-Shaikh, B., Stacey, S. (2001) Essentials
of Anaesthetic Equipment 2nd Edition, Harcourt Publishing
Branson, R.D., Hess, D.R., Chatburn,
R.L., (1995) Respiratory Care Equipment, JB Lippincott
Company Publishing
Draeger (2003), Series on Advanced
Ventilation, [online] at URL: http://www.draeger.com/us/MT/Library/or/case_studies/advanced_vent.pdf,
last viewed 8/5/03
Goerig, M., Filos, K., Ayisi, K.W.,
(1987), George Edward Fell and the Development of Respiratory
Machines, In: Atkinson, R.S., Boulton, T.B., (Eds.), (1987),
The History of Anaesthsia, Royal Society of Medicine
Rendell-Baker, L., Pettis, J.L., (1987),
The Development of Positive Pressure Ventilators, In:
Atkinson, R.S., Boulton, T.B., (Eds.), (1987), The History
of Anaesthesia, Royal Society of Medicine
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