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The combined effect of gas evolution and surface roughness on the rate of mass transfer
Electrochimica Acta. 1973,Vol. 18, PD. 279-281. Pergamon Press.Printed in Great Britain
SHORT COMMUNICATION THE COMBINED EFFECT OF GAS EVOLUTION AND SURFACE ROUGHNESS ON THE RATE OF MASS TRANSFER M. G. FOUAD, G. H. Chemical
Engineering
Department,
SEDAHMED
Alexandria
and H. A. EL-ABD University,
Alexandria,
Egypt
(Received 22 February 1972; as amended, 28 Augusf 1972)
INTRODUCTION
where N is the number of mols./cm2. are based on the projected area.
Several investigations have heen done on the effect of gas evolution on the rate of mass transfer at smooth electrodes; but no work has been published on rough gas evolving electrodes. Surface roughness enhances the rate of mass transfer in the case of ordinary natural and forced convection[l, 21. It might be interesting to combine surface roughness with gas evolution to increase the rate of production in electrolytic oxidation and reduction, EXPERIMENTAL
densities
RESULTS AND DISCUSSIONS Figure 1 shows the effect of H2 discharge rate on mass transfer at different degrees of roughness. The equivalent thickness of the diffusion layer 8 (calcuIated with D = 0.506 x 10m5 cm2 s-l) was between OGO36 and 0@08 cm. The mass transfer coefficient increases with degree of roughness to an extent which is higher than the increase in the true area of the electrode caused by surface roughness. This increase of mass transfer rate can be explained as follows. Surface roughness increases the number of active sites at which nucleation of bubbles takes place[4, 51; the increase in the number of bubbles results in an increase in the rate of mass transfer in accordance with the displacement mechanism[6]. The fact that the height of the protrusions in all three grades of roughness used is larger than the mean diffusion layer thickness, results in increasing the effective cross section for diffusion in the case. of rough electrodes; in addition, turbulent wakes may form downstream from the protrusions due to separation of the hydrodynamic boundary layer with a consequent increase in the rate of mass transfer[7]. The big jump between K-values for the electrodes with a roughness of 0.25 and O-45 mm (Fig. 1) is accounted for in part by the fact that the effective cross section area for diffusion in the case of the roughness of 0.45 mm is much higher than for the roughness of 0.25 mm (the increase k true area due to roughness of 0.25 mm is 12 and 33 per cent for the roughness of 0.45 mm). In addition, the intensity of turbulence due to boundary layer separation is larger in the case of the roughness of 0.45 mm[SJ. Figure 2 shows the effect of oxygen discharge rate on the mass transfer coefficient at different degrees of roughness. The thicknesses of the diffusion layer (calculated with D = 0.506 x lo-’ cm’s_‘) were between 0@033 and O-0048 cm. In contrast to hydrogen, surface roughness
TECHNIQUE
The cell was a rectangular plexiglass container divided into two compartments by a tight glass wool diaphragm. Electrodes of nickel having a width of 3.3 cm and a height of 10 cm, with 2 cm electrode diaphragm separation, were used; each electrode fits exactly into the cell. Three electrodes of different degrees of roughness were used in addition to a mirror like polished electrode. Roughness was in the form of horizontal parallel grooves engraved in the ekctrode; the pattern was similar to that u&d in a previous investigati&[l]. The electrolyte was O-1 M K*Fe(CNL + O-1 M LFe(CNL + 2 N NaOH. Before el&t~oly& the electrodes Gere~insulated on one side by polystyrene and degreased on the other side with trichloroethylene, then washed with alcohol and distilled water. After electrolysis, the change in Fe&N):-, Fe(CN)dconcentration was about 7 per cent; this change in concentration is corrected for in the calculation of the mass transfer coefficient. Ferricyanide was determined by iodometry[3], ferrocyanide by titrating the solution against standard KMn04 solution[3]. Temperature was kept constant at 25°C. Results are expressed by plotting the mass transfer coefficient us gas discharge rate. The gas discharge rate was calculated by using Faraday’s law, the mass transfer coefficient K according to :
N’fF’ f
Current
Co = KC,, 279
SHORT
280
COMMUNICATION
17 -
16 -
15 14 -
,” 8 .f 2 y
13tzII-
IO 9-
.i? 0
a-
:< 0.2
0.4
06
0.8
Vh2 cm3/cmf
I.2
I.0
I.4
Fig. 1. Influence of hydrogen evolution rate on the mass transfer coefficient at various degrees of roughness. 0 smooth electrode, c.d. : 30, 60, 91, 121, 152. 182 m A/cm2 ; x rough electrode, peak-to-valley height (p.v.h): O-1 mm peak to peak width (p.p.w.): 0.52 mm, increase in area (ia.): 8%: c.d.: 25, 59, 91,152,182 mA/cm*. 8 rough electrode, p.v.h.: 0,25 mm; p.p. w: 1,03 mm; i.a. 12x, c.d.: 30, 61, 91, 121, 152, 182 mA/cm*; l rough electrode, p.v.h.: O-45 mm; p.p.w.: l-03 mm; i.a.: 33%; c.d.: 30, 61, 91, 121, 151, 182 mA/cm*. results in decreasing mass transfer at oxygen evolving electrodes except at low degrees of roughness and high discharge rates. A satisfactory explanation for this behaviour could not be found. However, it may possibly be explained by the fact that part of the active area of the electrode becomes blanketed with gas bubbles clinging to the lower side of the protrusion. The release of these bubbles by buoyancy is hindered by their location under the lower side of the protrusion. Of course thii effect exists in the case of hydrogen evolution too but it is more apparent in the case of oxygen, probably because the bubble size is larger and the rate of its increase with current density is much higher than for hydrogen bubbles[4], ie the blanketed area is larger in the case of oxygen evolution. On the other hand, the enhancing factors discussed in the case of hydrogen, namely, the increase. in nucleation sites and wake formation are operating in the case of oxygen evolution like-wise, but they may have little to add to the already existing highly turbulent mass transfer which is suggested by the relatively large mass transfer coefficients observed with the smooth O2 evolving electrode. In conclusion, the use of a rough hydrogen evolving electrode can be recommended as a means for enhancing the rate of production in industrial electrolytic reduction, while for electrolytic oxidation a smooth oxygen evolving electrode is preferable. It should be added that this conclusion
is valid within
7-
min
the set of conditions
used in the
61 0
I 0.1
1 0.2
I 0.3 V.,
I 0.4
1 05
cm’/cm’.
I B6
I 0.7
min
Fig. 2. Influence of oxygen evolution rate on the mass transfer coefficient at various degree-s of roughness. 0 smooth electrode, c.d. 30, 61, 91, 121, 151, 182 p.v.h.: O-1 mm; m.4/cm2 ; x rough electrode, p.p.w.: O-52 mm; i-a.: 8%; c.d.: 30, 61, 91, 151, 182 mA/cm*. 0 rough electrode, p.v.h.: O-25 mm; p.p.w.: 1.03 mm; i.a.: 12%; c.d.: 30, 61, 89, 122, 151, 182 MA/cm’; l rough electrode, p.v.h.: O-45 mm; p.p.w.: l-03 mm; i.a.: 33%; c.d.: 30, 61, 91, 121, 151, 184 mA/cm*. present work particularly the current the mode of surface roughness.
density range and
NOMENCLATURE
CO bulk concentration
of KaFe(CN)6 or K=+Fe(CN)s. mole/cm” current density of the cathodic reduction of I KSFe@DQ6 or the anodic oxidation of K,Fe(CN)6, A/cm2 based on the projected area F the Faraday, 96500 Coulomb g. eq. or K4Fe(CN),, D diffusion coefficient of K,Fe(CN& cm*/s. s diffusion layer thickness, cm V gas discharge rate, cm”/cm* min. K mass transfer coefficient, cm/s. N mass transfer rate, mol!cm* s. REFERENCES
1. M. G. Fouad and A. A. Zatout. Electrochim. Acta 14, 909 (1969). 2. M. G. Fouad and H. A. El-Abd, accepted for publication. 3. A. I. Vogel, A Text book of Quantitative Analysis, 2nd edn. Longmans (1960).
SHORT
COMMUNICATION
4. L. J. J. Janssen and J. G. Hooglend, Elecfrochim Acta 15, IO13 (1970). 5. J. P. Glas and J. W. Westwater. ht. J. heat mass transfer. 7, 1427 (1964).
281
6. N. Ibl and J. Venczel, Metaiiober-fiche, 24. 365 (1970). Hyakoaynamics. 7. V. G. Levich, Physico-chemical Prentice-Hall ,Englewood Cliffs, New Jersey (1962).
SHORT COMMUNICATION THE COMBINED EFFECT OF GAS EVOLUTION AND SURFACE ROUGHNESS ON THE RATE OF MASS TRANSFER M. G. FOUAD, G. H. Chemical
Engineering
Department,
SEDAHMED
Alexandria
and H. A. EL-ABD University,
Alexandria,
Egypt
(Received 22 February 1972; as amended, 28 Augusf 1972)
INTRODUCTION
where N is the number of mols./cm2. are based on the projected area.
Several investigations have heen done on the effect of gas evolution on the rate of mass transfer at smooth electrodes; but no work has been published on rough gas evolving electrodes. Surface roughness enhances the rate of mass transfer in the case of ordinary natural and forced convection[l, 21. It might be interesting to combine surface roughness with gas evolution to increase the rate of production in electrolytic oxidation and reduction, EXPERIMENTAL
densities
RESULTS AND DISCUSSIONS Figure 1 shows the effect of H2 discharge rate on mass transfer at different degrees of roughness. The equivalent thickness of the diffusion layer 8 (calcuIated with D = 0.506 x 10m5 cm2 s-l) was between OGO36 and 0@08 cm. The mass transfer coefficient increases with degree of roughness to an extent which is higher than the increase in the true area of the electrode caused by surface roughness. This increase of mass transfer rate can be explained as follows. Surface roughness increases the number of active sites at which nucleation of bubbles takes place[4, 51; the increase in the number of bubbles results in an increase in the rate of mass transfer in accordance with the displacement mechanism[6]. The fact that the height of the protrusions in all three grades of roughness used is larger than the mean diffusion layer thickness, results in increasing the effective cross section for diffusion in the case. of rough electrodes; in addition, turbulent wakes may form downstream from the protrusions due to separation of the hydrodynamic boundary layer with a consequent increase in the rate of mass transfer[7]. The big jump between K-values for the electrodes with a roughness of 0.25 and O-45 mm (Fig. 1) is accounted for in part by the fact that the effective cross section area for diffusion in the case of the roughness of 0.45 mm is much higher than for the roughness of 0.25 mm (the increase k true area due to roughness of 0.25 mm is 12 and 33 per cent for the roughness of 0.45 mm). In addition, the intensity of turbulence due to boundary layer separation is larger in the case of the roughness of 0.45 mm[SJ. Figure 2 shows the effect of oxygen discharge rate on the mass transfer coefficient at different degrees of roughness. The thicknesses of the diffusion layer (calculated with D = 0.506 x lo-’ cm’s_‘) were between 0@033 and O-0048 cm. In contrast to hydrogen, surface roughness
TECHNIQUE
The cell was a rectangular plexiglass container divided into two compartments by a tight glass wool diaphragm. Electrodes of nickel having a width of 3.3 cm and a height of 10 cm, with 2 cm electrode diaphragm separation, were used; each electrode fits exactly into the cell. Three electrodes of different degrees of roughness were used in addition to a mirror like polished electrode. Roughness was in the form of horizontal parallel grooves engraved in the ekctrode; the pattern was similar to that u&d in a previous investigati&[l]. The electrolyte was O-1 M K*Fe(CNL + O-1 M LFe(CNL + 2 N NaOH. Before el&t~oly& the electrodes Gere~insulated on one side by polystyrene and degreased on the other side with trichloroethylene, then washed with alcohol and distilled water. After electrolysis, the change in Fe&N):-, Fe(CN)dconcentration was about 7 per cent; this change in concentration is corrected for in the calculation of the mass transfer coefficient. Ferricyanide was determined by iodometry[3], ferrocyanide by titrating the solution against standard KMn04 solution[3]. Temperature was kept constant at 25°C. Results are expressed by plotting the mass transfer coefficient us gas discharge rate. The gas discharge rate was calculated by using Faraday’s law, the mass transfer coefficient K according to :
N’fF’ f
Current
Co = KC,, 279
SHORT
280
COMMUNICATION
17 -
16 -
15 14 -
,” 8 .f 2 y
13tzII-
IO 9-
.i? 0
a-
:< 0.2
0.4
06
0.8
Vh2 cm3/cmf
I.2
I.0
I.4
Fig. 1. Influence of hydrogen evolution rate on the mass transfer coefficient at various degrees of roughness. 0 smooth electrode, c.d. : 30, 60, 91, 121, 152. 182 m A/cm2 ; x rough electrode, peak-to-valley height (p.v.h): O-1 mm peak to peak width (p.p.w.): 0.52 mm, increase in area (ia.): 8%: c.d.: 25, 59, 91,152,182 mA/cm*. 8 rough electrode, p.v.h.: 0,25 mm; p.p. w: 1,03 mm; i.a. 12x, c.d.: 30, 61, 91, 121, 152, 182 mA/cm*; l rough electrode, p.v.h.: O-45 mm; p.p.w.: l-03 mm; i.a.: 33%; c.d.: 30, 61, 91, 121, 151, 182 mA/cm*. results in decreasing mass transfer at oxygen evolving electrodes except at low degrees of roughness and high discharge rates. A satisfactory explanation for this behaviour could not be found. However, it may possibly be explained by the fact that part of the active area of the electrode becomes blanketed with gas bubbles clinging to the lower side of the protrusion. The release of these bubbles by buoyancy is hindered by their location under the lower side of the protrusion. Of course thii effect exists in the case of hydrogen evolution too but it is more apparent in the case of oxygen, probably because the bubble size is larger and the rate of its increase with current density is much higher than for hydrogen bubbles[4], ie the blanketed area is larger in the case of oxygen evolution. On the other hand, the enhancing factors discussed in the case of hydrogen, namely, the increase. in nucleation sites and wake formation are operating in the case of oxygen evolution like-wise, but they may have little to add to the already existing highly turbulent mass transfer which is suggested by the relatively large mass transfer coefficients observed with the smooth O2 evolving electrode. In conclusion, the use of a rough hydrogen evolving electrode can be recommended as a means for enhancing the rate of production in industrial electrolytic reduction, while for electrolytic oxidation a smooth oxygen evolving electrode is preferable. It should be added that this conclusion
is valid within
7-
min
the set of conditions
used in the
61 0
I 0.1
1 0.2
I 0.3 V.,
I 0.4
1 05
cm’/cm’.
I B6
I 0.7
min
Fig. 2. Influence of oxygen evolution rate on the mass transfer coefficient at various degree-s of roughness. 0 smooth electrode, c.d. 30, 61, 91, 121, 151, 182 p.v.h.: O-1 mm; m.4/cm2 ; x rough electrode, p.p.w.: O-52 mm; i-a.: 8%; c.d.: 30, 61, 91, 151, 182 mA/cm*. 0 rough electrode, p.v.h.: O-25 mm; p.p.w.: 1.03 mm; i.a.: 12%; c.d.: 30, 61, 89, 122, 151, 182 MA/cm’; l rough electrode, p.v.h.: O-45 mm; p.p.w.: l-03 mm; i.a.: 33%; c.d.: 30, 61, 91, 121, 151, 184 mA/cm*. present work particularly the current the mode of surface roughness.
density range and
NOMENCLATURE
CO bulk concentration
of KaFe(CN)6 or K=+Fe(CN)s. mole/cm” current density of the cathodic reduction of I KSFe@DQ6 or the anodic oxidation of K,Fe(CN)6, A/cm2 based on the projected area F the Faraday, 96500 Coulomb g. eq. or K4Fe(CN),, D diffusion coefficient of K,Fe(CN& cm*/s. s diffusion layer thickness, cm V gas discharge rate, cm”/cm* min. K mass transfer coefficient, cm/s. N mass transfer rate, mol!cm* s. REFERENCES
1. M. G. Fouad and A. A. Zatout. Electrochim. Acta 14, 909 (1969). 2. M. G. Fouad and H. A. El-Abd, accepted for publication. 3. A. I. Vogel, A Text book of Quantitative Analysis, 2nd edn. Longmans (1960).
SHORT
COMMUNICATION
4. L. J. J. Janssen and J. G. Hooglend, Elecfrochim Acta 15, IO13 (1970). 5. J. P. Glas and J. W. Westwater. ht. J. heat mass transfer. 7, 1427 (1964).
281
6. N. Ibl and J. Venczel, Metaiiober-fiche, 24. 365 (1970). Hyakoaynamics. 7. V. G. Levich, Physico-chemical Prentice-Hall ,Englewood Cliffs, New Jersey (1962).