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Photocatalysis in a slurry reactor
1158
DAVIDS. HACKERand JOHNB. Bm
veloped incorporating the Mie-Rayleigh radiation scat-tering by small particles to serve as a sensitive tracer for departure from first order attenuation coefficient,‘ p =
Pdl+[cl). An inert particulate slurry was used to obtain attenuation data. These results were then used to determine the regime in which kinetic data for the photocatalyzed reduction of methylene blue by a suspension of zinc oxide could be studied. The results of these latter measurements were compared with those for similar systems reported in the literature. Rate coefhcients obtained in this study for the reduction step of the dye were in good agreement with values reported by Cunningham for 0.1% ZnO suspensions in the photochemical reduction of PNDA. However, it was observed that the surface reaction rate constant was not independent of active site concentration as would be expected from the reaction pathways based on a single absorption site mechanism. The available data were shown to bear the following relationship to the surface available. &3= 4.11 X 10IJ[ZnO]“” cm’ set-’ molecules-’
[ 121Panifilov A. V., Mazurkevich !I..and Pakhomova E. P.. Kinet. Carol. USSR 1960 10 915. [ 131Cunningham J. and Zainal H., J. Phys. Chem. 197276 2362. [14] Yoneyama H., Toyoguchi Y. and Tamura H., J. Phys. Chem 197276 28 3460. [IS] Kittel C. Introduction to Solid State Physics, 3rd edn, p. 302. Wiley, New York 1%7. [la] Hughmark G. A., A.LCh.E. J. 197420 1 202. [17] Hayduk W. and Laudie H., A.LCh.E. 1 197420 3. APPENDIX A
*We present a short proof that mass transfer is not controlling in this process. Consider a solution of concentration, c,. The flux equations at steady state may be written the sum of two resistances, liquid film and surface reaction. Hence, N. = k,A, (c, - c,) (bulk liquid to particle surface)
(Al)
iV. = k,A,c, (surface reaction).
(A2)
The molar flux of methylene blue reactant, N., is given in terms of the catalyst surface area A,, based on a unit volume of slurry, and the mass transfer coefficient k, and the surface reaction rate coefficient, k.. Combining eqns (Al) and (A2) and solving for the liquid concentration cl, there is obtained
where [ZnO] is the weight percent of solid in solution. c,.A, Acknowkdgemenfs-Professor Hacker wishes to express his appreciation to Professor J. Dranoff and the Department of
Chemical Engineering at Northwestern University for the opportunity to develop this program while on sabbatical leave from the University of Illinois. In particular he would like to acknowledge the generosity of Professor J. Butt in providing the necessary equipment for this work and the assistance of N. Ivich in experimentation. REF&RENCgS [l] Davis H. S., Thompson G. and Crandall S. G., J. Am. Chem. Sot. 194954 2340. [2] Cassano A. and Smith J. hf., A.1Ch.E. .I. 196612 1124. [3] Schorr V., Boval V. and Smith J. hi.. Ind. Engng Chem. Pruc. Des. Dcu. 197110 509. [4] Rudd D. F., Chem. Engng Sci. 196012 51. [5] Hi F. B. and Felder R. hf., A.I.Ch.E. J. 1%5 11 873. [6] Jacob S. hi. and Dranoff J. S., A.LCh.E. J. 1%9 15 141. [7] Cerda J., Irazoqui H. A. and Cassano A. E., A.LCh.E. J. 1973 19 %3. [8] Calvert J. G. and Pitts J. N., Photochemisfry Wiley, New York (1966). [9] Dallavalle J. M., Micromeretics, 2nd edn. Pitman, New York pp. 197-200, 1948. [lo] Sattertield C. N. and Sherwood T. K., The Role ojDi#‘usion in Catalysis, pp. 43-45. Addison-Wesley, New York (1%3). [ 1I] Markham hf. C. and Laidler K. J., J. Phys. Chem. 195357 363.
N.
1
1
-k,+k,
Mass transfer can be neglected provided k,/k. * 1. The mass transfer coefficient for a suspended solid in a turbulent flow field is given as[l6] k, =3.6N Pa--2’3.0 where 0 is the mean square velocity of a particle in a turbulent field N,. = Peclet number defined as dpUll D,, = the diffusivity of specie 1 through 2. We can characterize the present experiqent for particles of HI-50~ as having an average veIoc$y of U- 10cmlsec at these mixing rates, giving the value d,U - 2.5 x lo-‘(cm’lsec). The diffusion of methylene blue in aqueous solution for relatively dilute systems can be estimated from the paper by Hayduk and Laudie ll7l to be of the order of 10-l’ cmlsec. Hence one obtains a N,,, =25O_and an estimate of the mass transfer coefficient of - 1cmlsec. This value is now compared with an experimental rate coefficient using a surface area factor proportional to the weight of zinc oxide per volume of shury. For a 2% ZnO slurry our experiments gave k, = 1.5 x lo-‘cmlsec. It is clear that the criterion L/k. > 1 is satisfied and the assumptions of minimal mass transfer is valid.
DAVIDS. HACKERand JOHNB. Bm
veloped incorporating the Mie-Rayleigh radiation scat-tering by small particles to serve as a sensitive tracer for departure from first order attenuation coefficient,‘ p =
Pdl+[cl). An inert particulate slurry was used to obtain attenuation data. These results were then used to determine the regime in which kinetic data for the photocatalyzed reduction of methylene blue by a suspension of zinc oxide could be studied. The results of these latter measurements were compared with those for similar systems reported in the literature. Rate coefhcients obtained in this study for the reduction step of the dye were in good agreement with values reported by Cunningham for 0.1% ZnO suspensions in the photochemical reduction of PNDA. However, it was observed that the surface reaction rate constant was not independent of active site concentration as would be expected from the reaction pathways based on a single absorption site mechanism. The available data were shown to bear the following relationship to the surface available. &3= 4.11 X 10IJ[ZnO]“” cm’ set-’ molecules-’
[ 121Panifilov A. V., Mazurkevich !I..and Pakhomova E. P.. Kinet. Carol. USSR 1960 10 915. [ 131Cunningham J. and Zainal H., J. Phys. Chem. 197276 2362. [14] Yoneyama H., Toyoguchi Y. and Tamura H., J. Phys. Chem 197276 28 3460. [IS] Kittel C. Introduction to Solid State Physics, 3rd edn, p. 302. Wiley, New York 1%7. [la] Hughmark G. A., A.LCh.E. J. 197420 1 202. [17] Hayduk W. and Laudie H., A.LCh.E. 1 197420 3. APPENDIX A
*We present a short proof that mass transfer is not controlling in this process. Consider a solution of concentration, c,. The flux equations at steady state may be written the sum of two resistances, liquid film and surface reaction. Hence, N. = k,A, (c, - c,) (bulk liquid to particle surface)
(Al)
iV. = k,A,c, (surface reaction).
(A2)
The molar flux of methylene blue reactant, N., is given in terms of the catalyst surface area A,, based on a unit volume of slurry, and the mass transfer coefficient k, and the surface reaction rate coefficient, k.. Combining eqns (Al) and (A2) and solving for the liquid concentration cl, there is obtained
where [ZnO] is the weight percent of solid in solution. c,.A, Acknowkdgemenfs-Professor Hacker wishes to express his appreciation to Professor J. Dranoff and the Department of
Chemical Engineering at Northwestern University for the opportunity to develop this program while on sabbatical leave from the University of Illinois. In particular he would like to acknowledge the generosity of Professor J. Butt in providing the necessary equipment for this work and the assistance of N. Ivich in experimentation. REF&RENCgS [l] Davis H. S., Thompson G. and Crandall S. G., J. Am. Chem. Sot. 194954 2340. [2] Cassano A. and Smith J. hf., A.1Ch.E. .I. 196612 1124. [3] Schorr V., Boval V. and Smith J. hi.. Ind. Engng Chem. Pruc. Des. Dcu. 197110 509. [4] Rudd D. F., Chem. Engng Sci. 196012 51. [5] Hi F. B. and Felder R. hf., A.I.Ch.E. J. 1%5 11 873. [6] Jacob S. hi. and Dranoff J. S., A.LCh.E. J. 1%9 15 141. [7] Cerda J., Irazoqui H. A. and Cassano A. E., A.LCh.E. J. 1973 19 %3. [8] Calvert J. G. and Pitts J. N., Photochemisfry Wiley, New York (1966). [9] Dallavalle J. M., Micromeretics, 2nd edn. Pitman, New York pp. 197-200, 1948. [lo] Sattertield C. N. and Sherwood T. K., The Role ojDi#‘usion in Catalysis, pp. 43-45. Addison-Wesley, New York (1%3). [ 1I] Markham hf. C. and Laidler K. J., J. Phys. Chem. 195357 363.
N.
1
1
-k,+k,
Mass transfer can be neglected provided k,/k. * 1. The mass transfer coefficient for a suspended solid in a turbulent flow field is given as[l6] k, =3.6N Pa--2’3.0 where 0 is the mean square velocity of a particle in a turbulent field N,. = Peclet number defined as dpUll D,, = the diffusivity of specie 1 through 2. We can characterize the present experiqent for particles of HI-50~ as having an average veIoc$y of U- 10cmlsec at these mixing rates, giving the value d,U - 2.5 x lo-‘(cm’lsec). The diffusion of methylene blue in aqueous solution for relatively dilute systems can be estimated from the paper by Hayduk and Laudie ll7l to be of the order of 10-l’ cmlsec. Hence one obtains a N,,, =25O_and an estimate of the mass transfer coefficient of - 1cmlsec. This value is now compared with an experimental rate coefficient using a surface area factor proportional to the weight of zinc oxide per volume of shury. For a 2% ZnO slurry our experiments gave k, = 1.5 x lo-‘cmlsec. It is clear that the criterion L/k. > 1 is satisfied and the assumptions of minimal mass transfer is valid.