Oxygen interfacial mass transfer rate

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). 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

Calculate mass transfer coefficient in a 60 m3 fermenter with a gas and liquid interfacial area of a = 0.3 m2-m 3, given pbroth = 1200kg m-3. The small reactor has working volume of 0.18m3, 1 vvm aeration rate. Oxygen transfer rate (OTR) is 0.25kmol in 3 h 3. There are two sets of impellers, and flat-blade turbine types of impeller were used, HL= 1.2/),. Find the exact specifications of a large fermenter. 

Re = Reynolds number S = fluid constant p = density t = dynamic viscosity g = aerated 0 = not aerated qg gas throughput n = stirrer speed di = impeller diameter OTR = oxygen transfer rate from gas to liquid phase Fl = mass transfer coefficient a = specific interfacial surface area ... 

In Fig. 2 the key parameters are presented for recombinant E. coli batch cultivation in a 60-1 working volume airUft tower loop reactor at constant aeration rate up to 16 h, whereupon the temperature was increased from 30 to 42 °C and gene expression was induced. At the same time concentrated Luxia-Bertani (LB) medium was added to the reactor. To avoid oxygen limitation, the aeration rate was increased (Fig. 2 a). At 12 h the foaming increased and SE9 was added to the medium. The bubble velocities (Fig. 2b) and the specific gas/liquid interfacial area (Fig. 2 c) quickly increased and passed a narrow maximum, but kLa dropped and the OTR was not influenced (Fig. 2d). After the induction of the gene expression by a temperature increase and medium supplement the dissolved oxygen concentration with respect to the saturation increased due to the elevation of the aeration rate (Fig. 2 a) the mean bubble velocity (Fig. 2 b) and specific interfacial area (Fig. 2 c) decreased, OTR increased and kLa remained at low values (Fig. 2d). The mass transfer coefficient with respect to the liquid phase kL dropped from about 1.67 to 0.67 ms after the addition of SE9 to the medium [51]. 

Fig. 2 c, d c specific gas/liquid interfacial area, and d oxygen transfer rate (OTR) and volumetric mass transfer coefficient ki a of oxygen... 

Two-phase system properties can strongly influence cultivation conditions, especially if growth is oxygen transfer limited. To treat the oxygen transfer rate quantitatively it is necessary to determine the volumetric mass transfer coefficient, kj a, the dissolved oxygen concentration in t e liquid bulk. Op, and at the gas liquid interface. Op. The specific interfacial area is influenced by the Sauter bubble dieuneter, dg, and the relative gas holdup, e q, according to eq. (ToO) ... 

The specific momentum and mass transfer conditions were observed in the thin liquid layer covered by a phospholipid monolayer and subjected to forced variations of the interfacial area. Due to the Marangoni effects, the absorption rate for oxygen was increased when compared either to static condition or to area variations without the monolayer in the system. 

Another method to remove pollutants from air is to absorb the pollutants into a nonvolatile liquid such as oil. During absorption, pollutants move from the gas phase to the liquid phase. This movement is an example of mass transfer. To move between phases, the pollutants must cross the liquid-gas interface. Increasing the interfacial area increases the rate of mass transfer. An easy way to increase the interfacial area is to bubble the gas through the liquid. This concept is effective in delivering oxygen to water in fish tanks and it is effective in delivering benzene to oil. 

The liquid side volumetric physical mass transfer coefficient was determined from the desorption rate of oxygen. Detailed description of the experimental set up, procedure and analysis of data is given by Tosyali [30]. Methods of estimating the interfacial CO2 concentration, diffusivities of CO2 and OH in the liquid phase, reaction rate constant, which are all required in data analysis, can be found elsewhere [31, 32]. ... 

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