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Measurement of oxygen diffusivity in non-Newtonian liquids
Chemical
Engineering
Science,
Vol.
46.
No.
5/6,
p.
1527,
1991
ooo-2509/91 %3.00 + 0.00 Pergamon Press plc
Printed in Great Bnmin.
Measurement
of oxygen
(Receioed
2 October
diffusivity
in non-Newtonian
1989; accepted
Dear Sirs, PotXek and Stejskal (1986) give values of the oxygen diffusivity in aqueous solutions of polymers obtained from absorption measurements on a wetted sphere. To evaluate the diffusivity from the oxygen absorption rate, m, they used saturated oxygen concentrations in the liquids, c*, the values of which they also measured by an independent method. They reported a profound decrease in the oxygen solubility with increasing polymer concentration. For instance, in a 0.75 wt % Stipix solution they found that the ratio of oxygen solubility in the polymer solution to that in water, e*/o:, was 0.758. That contradicts the findings of other authors (including us) who found that the oxygen solubility was independent of polymer concentration up to about 2 wt %. Concerning the method used for the measurement of c* they only say that “The values of oxygen concentration at saturation in used liquids were determined by means of Clark electrode modified by Cerkasov.” The signal of the oxygen probe, however, is proportional to the equilibrium oxygen pavtial pressure in the bulk of the measured liquid and it cannot be used for the determination of the oxygen concentration without knowing the oxygen solubility in the studied medium. Thus it is not possible to determine the ratio of the oxygen solubility in the polymer solution to that in water as a ratio of the steady-state signal of the probe immersed in a vigorously agitated polymer solution, M, to that in water, M,, as they did:
In fact, the value of M/M, represents the ratio of the oxygen diffusion resistances of the membrane and the liquid film in the two systems. In this situation the probe signal decreased owing to the increasing thickness of the liquid film adhering to the probe membrane as a result of the higher viscosity of the solution. The decrease in the probe signal appears regardless of the oxygen solubility. Correct methods for interpretation of the probe signal obtained under conditions
liquids
2 August 1990)
when liquid film resistance is significant can be found in Linek et al. (1988). The diffusivities, D, were evaluated PotBEek and Stejskal (1986) from the relation D = Km’/(c+
- c)~
where c is the oxygen concentration in the liquid leaving the sphere, and K is a function of the sphere radius, liquid flow rate and liquid viscosity and density. It is obvious from the above relation that underestimation of the c* value leads to an increase in the value of D, e.g. in 0.25 wt % Stipix solution PotSeek and Stejskal (1986) found the value of diffusivity to be 2.92 times higher compared to water. The error in the diffusivity is higher the closer c* is to c. In this way the anomalously high diffusivities in polymer solutions observed by Pot&&k and Stejskal (1986) can be explained. The error necessarily appears in a further analysis of mass transfer processes in an absorption apparatus. The analysis (PotSEek and Stejskal, 1969) carried out on the basis of the suspect values is obviously pointless. V. LINEK J. SINKULE Department of Chemical Engineeering Institute of Chemical Technology CS-166 28 Prague 6, Czechoslovakia REFERENCES Linek, V.. Vacek. V.. Sinkule. J. and Ben& P., 1988, Measurement of Oxygen by Membrane Couered Probes: Guidelines for Application in Chemical and Biochemical Engineering. Ellis Horwood, Chichester. PotPl&k, F. and Stejskal, J., 1986, Diffusivity of oxygen in non-Newtonian liquids. Chem. Engng Sci. 41, 3223-3226. Pot&&k, F. and Stejskal, J., 1989, Oxygen absorption in polymeric solutions in a bead column. Chem. Engng Sci. 44, 194-196.
1527
Engineering
Science,
Vol.
46.
No.
5/6,
p.
1527,
1991
ooo-2509/91 %3.00 + 0.00 Pergamon Press plc
Printed in Great Bnmin.
Measurement
of oxygen
(Receioed
2 October
diffusivity
in non-Newtonian
1989; accepted
Dear Sirs, PotXek and Stejskal (1986) give values of the oxygen diffusivity in aqueous solutions of polymers obtained from absorption measurements on a wetted sphere. To evaluate the diffusivity from the oxygen absorption rate, m, they used saturated oxygen concentrations in the liquids, c*, the values of which they also measured by an independent method. They reported a profound decrease in the oxygen solubility with increasing polymer concentration. For instance, in a 0.75 wt % Stipix solution they found that the ratio of oxygen solubility in the polymer solution to that in water, e*/o:, was 0.758. That contradicts the findings of other authors (including us) who found that the oxygen solubility was independent of polymer concentration up to about 2 wt %. Concerning the method used for the measurement of c* they only say that “The values of oxygen concentration at saturation in used liquids were determined by means of Clark electrode modified by Cerkasov.” The signal of the oxygen probe, however, is proportional to the equilibrium oxygen pavtial pressure in the bulk of the measured liquid and it cannot be used for the determination of the oxygen concentration without knowing the oxygen solubility in the studied medium. Thus it is not possible to determine the ratio of the oxygen solubility in the polymer solution to that in water as a ratio of the steady-state signal of the probe immersed in a vigorously agitated polymer solution, M, to that in water, M,, as they did:
In fact, the value of M/M, represents the ratio of the oxygen diffusion resistances of the membrane and the liquid film in the two systems. In this situation the probe signal decreased owing to the increasing thickness of the liquid film adhering to the probe membrane as a result of the higher viscosity of the solution. The decrease in the probe signal appears regardless of the oxygen solubility. Correct methods for interpretation of the probe signal obtained under conditions
liquids
2 August 1990)
when liquid film resistance is significant can be found in Linek et al. (1988). The diffusivities, D, were evaluated PotBEek and Stejskal (1986) from the relation D = Km’/(c+
- c)~
where c is the oxygen concentration in the liquid leaving the sphere, and K is a function of the sphere radius, liquid flow rate and liquid viscosity and density. It is obvious from the above relation that underestimation of the c* value leads to an increase in the value of D, e.g. in 0.25 wt % Stipix solution PotSeek and Stejskal (1986) found the value of diffusivity to be 2.92 times higher compared to water. The error in the diffusivity is higher the closer c* is to c. In this way the anomalously high diffusivities in polymer solutions observed by Pot&&k and Stejskal (1986) can be explained. The error necessarily appears in a further analysis of mass transfer processes in an absorption apparatus. The analysis (PotSEek and Stejskal, 1969) carried out on the basis of the suspect values is obviously pointless. V. LINEK J. SINKULE Department of Chemical Engineeering Institute of Chemical Technology CS-166 28 Prague 6, Czechoslovakia REFERENCES Linek, V.. Vacek. V.. Sinkule. J. and Ben& P., 1988, Measurement of Oxygen by Membrane Couered Probes: Guidelines for Application in Chemical and Biochemical Engineering. Ellis Horwood, Chichester. PotPl&k, F. and Stejskal, J., 1986, Diffusivity of oxygen in non-Newtonian liquids. Chem. Engng Sci. 41, 3223-3226. Pot&&k, F. and Stejskal, J., 1989, Oxygen absorption in polymeric solutions in a bead column. Chem. Engng Sci. 44, 194-196.
1527