Variability Of Ozone Reaction Kinetics

Variability of ozone reaction kinetics in batch and continuous flow reactors - S.W. Hermanowicz, W.D.Bellamy and L.C.Fung, Science Direct, UCB, 1999

Rajarshi Guha (07302009) ACRE Seminar, Chemical Engineering Department, IIT Bombay

Variability of Ozone kinetics • Reaction of ozone with surface water shows a variability in reaction rates • Reaction rates in continuous flow experiments are higher than batch experiments • Reaction order is described by n-th order (0.4-2) kinetics

Experimental Setup • A cylindrical bubble column reactor is used • Water and Ozone flow is maintained from the bottom until a S.S. conc has been achieved • 3 residence time has been taken for each continuous experiment • Then all flows are stopped and batch experiment starts • Water for experiment comes from a water treatment plant • Ozone conc has been measured by titration

Data Analysis •

rO3=kncO3^n n=1 is the simplest choice, but order can vary

•

C=C0exp(−k1Bt) for batch reactors

•

D−cO3−k1C cO3 Θ=0 for continuous reactors (CSTR),D is transferred ozone dose, cO3 is S.S. conc and Θ is hydraulic residence time

•

In all cases k1c > k1b

•

In CFR ozone is continuously contacted with raw water which exerts some instantaneous ozone demand

•

In batch experiment some of these demands has been partially fulfilled First order reaction rate coefficients for different steady state ozone concentrations cO3 (for bath systems “k1”=k1B; for continuous flow “k1”=k1C).

Finding the rate law parameters • 3 characteristic rate constants have been proposed • Initially first order kinetics (if logc03 v/s t is linear) • Curve significantly upward (n-th order kinetics) • Curve significantly downward (n-th order kinetics)

• The eqn has been integrated analytically to obtain c(t) • This c(t) is then fitted to the observed ozone conc using a nonlinear least square method to find kn and n

Importance of specific reaction rates • Kn and n decreases when more ozone has been transferred • This indicates a change in sp. reaction Rate r(c)/c • n<1: c decreases faster than r(c), r(c)/c will increase • n>1: r(c) will decrease faster than c, r(c)/c will decrease • n=1: r(c)/c=dc/dt and it remains constant

Reaction kinetics for different n

Conclusion • We can obtain an expression for ozone demand which attributes to the difference of reaction rates in continuous and batch experiments • Rate for continuous flow expt: • Rate of batch expt: rd=kncO3^n • Hence Instantaneous ozone demand: cd=D−cO3−kn cO3^n Θ