(227g) Modelling of Electrodialytic Removal of Multiple Ions From Synthetic Solutions

Drinking
water is slowly becoming a very precious commodity with rapid urbanization of
world. Uncontrolled pollution in every aspect of mother earth (water, soil,
environment and air) is posing challenge to flora and fauna. Days are not very
far when we will have no ground water available for drinking purposes.
Therefore, means to recycle used water through low cost technology needs to be devised.
Commercial waste water treatment reduces BOD/COD and provides water that can be
drained in channels and rivers. To make this water drinkable we need to
critically analyse its contents and pass it through adsorption bed, membrane
module and UV treatment. We are looking for application of electrodialysis to
purification of water containing metal ions from a synthetic solution of
sucrose. Electrodialysis (ED) is claimed to be more energy economic process
compared to conventional reverse osmosis and ultrafiltration.

Electrodialysis is a unit operation commonly used   to separate the ions from water through
selectively permeable membranes (cationic/anionic) and electric voltage. The
feed solution (containing electrolytes) is passed between two membranes (cation
exchange & anion exchange). The cathode and anode compartments are isolated
from the middle feed chamber and voltage is applied across these membranes.
Dissociated salts in the feed move towards their respective anion and cation
exchange membranes under the influence of electric potential. Lee et al. [2006]
discussed no of isses concerning the performance of ED operation. Factors those
govern the process are (i) Membrane, (ii) Ions, (iii) medium and (iv) potential
applied. For a given ion, the efficiency of separation depends on the membrane
type (surface charge, porocity, resistance etc.). The rate of adsorption,
diffusion, desoprtion of ions from one side to other decides the resistance.

Current
density was estimated theoretically for a known concentration and flow range
and the behaviour was noted for batch recirculation electrodialysis. The cell
was operated below limiting current density (lcd). A linear concentration profile (with increase
in current density) was assumed between the bulk to
membrane surface while the concentration at membrane surface was obtained from ?limiting
current density'. This value approaches zero while the current density
approaches limiting value.

Removal
of excess calcium ion from sugar solution using ED was conducted. Process
parameters (flow, voltage and concentration) influencing removal efficiency of
Ca²⁺ ions were estimated. Limiting current density and current
densities of solutions containing multi ions were also estimated using the
model. The method of calculating the limiting current density was verified from
literature data and found to be quite accurate. The limiting current density of
the process was estimated theoretically using Nernst-Planck equation [1&2]
and found to decrease with time which supports experimental findings.
Diffusivity and mass transfer coefficients were calculated [4&5] and found
to match well with literature [3]. The proposed model can be used for estimation
and design of an electrodialysis stack to achieve a desired output.

Key
words: electrodialysis, limiting current
density, current density, mass transfer coefficient.

References

1.     Lee
Hong-Joo, Strathmann Heiner, Moon Seung-Hyeon. ?Determination of the limiting
current density in electro-dialysis desalination as an empirical function of
linear velocity'. Desalination 190 (2006) 43-50.

2.     Geraldes
Vitor, Afonso Maria Dina. ?Limiting current density in the electrodialysis of
multi-ionic solutions'. Journal of Membrane Science 360(2010) 499-508.

3.     Jae-Hwan Choi, Hong-Joo Lee, and Seung-Hyeon Moon. ?Effects
of Electrolytes on the Transport Phenomena in
a Cation-Exchange Membrane'. Journal of Colloid and Interface Science 238, 188?195 (2001).

4.     Treybal
Robert E. ?Mass-Transfer Operations'. Third
Edition, McGraw-Hill publications.

5.     
Bruce E. Poling, John M. Prausnitz, John P.
O'Connell. ?The Properties of Gases and Liquids'. Fifth Edition, McGraw-Hill
publications.