Influence of Waste- Water Constituents on Mass Transfer

3 Influence of (Waste-)Water Constituents on Mass Transfer 

Substituted phenols as well as phenol itself are typical constituents of (bio-)refractory waste waters and can increase a(0 3 (Gurol and Nekouinaini, 1985). They studied the influence of these compounds in oxygen transfer measurements and attributed this effect to the hindrance of bubble coalescence in bubble swarms, which increases the interfacial area a. When evaluating the effect of these phenols on the ozone mass transfer rate, it is important to note that these substances react fast with ozone (direct reaction, kD= 1.3 103 L mol 1 s 1, pH = 6-8, T = 20 °C, Hoigne and Bader, 1983 b). 

The same effect was also observed for tertiary butyl alcohol (TBA), a substance which is frequently used as a radical scavenger in kinetic experiments (see Section B 4.4). Depending on its concentration (c(TBA) = 0-0.6 mM) alpha values up to afO = 2.5 were measured (Gurol and Nekouinaini, 1985). Since TBA is hard to oxidize by molecular ozone in contrast to phenols (kD(TBA) 3 10 3 L moF1 s pH = 7, Hoigne and Bader, 1985), the effect of TBA on kLa can also be studied in mass transfer experiments with ozone. 

The importance of a correct evaluation of kLa(03) or kla 02) was confirmed in a study on the simulation of (semi-)batch ozonation of phenol (Gurol and Singer, 1983). It was shown that a close match between the measured and the calculated data was only obtained when kLa(02) was measured as a function of the residual phenol concentration. The oxygen mass transfer coefficient was observed to change from kLa(02) = 0.049 s 1 at c(M) = 50 mg IF1 phenol to kLa(02) = 0.021 s- at c(M) = 5.0 mg L 1 phenol. 

The decrease in the alpha factor to values below a = 1 can be due to a decrease in either kL or a or both. Two theories are commonly used to explain the reduction in kp. the barrier effect and the hydrodynamic effect. In the barrier theory, the presence of the surfactants at the phase interface creates an additional resistance to mass transfer due to diffusion through the surfactant layer. In the hydrodynamic theory, the layer of surfactant molecules at the gas-liquid interface depresses the hydrodynamic activity (Gurol and Nekouinaini, 1985). 

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