Friday, April 1, 2011

DETERMINATION OF VOLUMETRIC MASS TRANSFER COEFFICIENT BY SODIUM SULPHITE OXIDATION METHOD

DETERMINATION OF VOLUMETRIC MASS TRANSFER COEFFICIENT BY SODIUM SULPHITE OXIDATION METHOD

AIM:
To determine the KLa through the SodiumSulphite oxidation method.

THEORY:
The sodium sulphite combines with oxygen to give sodium sulphate, with CuSO4 as catalyst in the reactor. The concentration of sodium sulphite  at various time points is inversely proportional to the oxygen transfer rate.

Na2SO3 + 0.5O2                Na2SO4

The kinetics of this reaction is independent of the sulphite concentration. The reaction consumes oxygen at a rate that is sufficiently fast so that transport of O2 from gas to liquid through liquid film is the rate-limiting step. The rate of the reaction is zeroth order in Na2SO3. If the reaction is not fast enough the reaction occurs in the liquid film around the gas bubbles. This would decrease apparent film thickness and give incorrectly high values of KLa. Concentrations of unreacted sulphite are determined by reacting the sulphite with excess iodine and then back titration of the iodine with thiosulphite. It is important  to note that the DO is zero through the reaction.

SO32- + I2                   2I-+ SO2

S2O72- +I2                   2I-  +2SO2

Rate of sulphite consumption
KLa  =
[O2]* - [O2]

[O2] = 0 and [O2]* = 8.43*10-3 g/l

Moles of O2*32
OTR =
Time * volume of reactor

Oxygen consumed = { [Na2SO3]f    -   [Na2SO3]I }/ 2
Where  Volume  Of  Standard   Na2SO3  = 0.5ml
MATERIALS REQUIRED:

·         0.5M  Na2SO3
·         1% starch solution
·         0.1M Na2S2O7
·         0.003M CuSO4, 5H2O
·         Standard Na2SO3 (0.1M to 0.5M)
·         0.1M Iodine solution (20 g/l KI + 12.7 g/l I2)
·         Measuring cylinder
·         Distilled water
·         Test tubes, eppendorfs etc.

PROCEDURE:
Sampling:
·         Fermentor is filled with 0.5M Na2SO3 and 0.003 M CuSO4. 5H2O (2.5 litre).
·         Agitation is given at 400 rpm.
·         After the aeration begins collect samples every 10 mins.
·         Estimate the concentration of Na2SO3 by titration.

Titration:
·         Burette solution: 0.1 M Na2S2O7.
·         Flask solution: 500µl of Na2SO3 solution (sample or standard) + 15 ml iodine solution + 50µl starch.
·         During titration colour changes to straw yellow. Add the starch solution at this point only and not before.
·         Estimate the concentration of Na2SO3 in sample from standard curve.

RESULT:
Thus the KLa through the SodiumSulphite oxidation method is determined

INFERENCE:
Thus KLa  is calculated by sodium sulphite oxidation method. The  KLa calculated is generally higher than that is obtained from other methods because of the following reasons.oxygen supplied to the reactor has to transfer through the gas-liquid interface. since sodium sulphite is available in the interface itself it leads to the reaction and hence higher  KLa values. KLa is greatly affected by aeration rate .Hence proper aeration must be supplied to the reactor.