Gas Distributor Design

The effects of gas distributor design and its extent depend on the superficial gas velocity and fiow regime in which the BC operates. In the case of heterogeneous flow, the sparger has a negligible influence on the bubble size and gas-liquid mass transfer because the bubble dynamics are determined by the rate of coalescence and breakup, which are controlled by the liquid properties and the nature and frequency of bubble collisions (Chaumat et al., 2005). Hence, the sparger effect is more pronounced at lower superficial gas velocities (Uq 0.15m/s) while it is much less important at Uq 0.20 m/s and nonexistent at Uq 0.30m/s (Han and Al-Dahhan, 2007). Viscous liquids are also not affected by the gas distributor design if the column is sufficiently tall (Zahradnik et al., 1997). 

Furthermore, the mechanisms that dominate gas holdup (e.g., surface tension, particle wettability, ionic force of surfactant, viscosity, and density) require consideration. If the liquid undergoes viscosity or density changes through, for example, particle addition, the initial bubble diameter does not affect gas holdup in the heterogeneous flow regime. If, on the other hand, the other mechanisms are affected 

The distribntor effect can be quite significant such that the gas-liquid mass transfer correlation can vary by up to a factor of 2 (Lau et al., 2004). The extent to which the gas distributor affects gas holdup and bubble dynamics depends on the BC geometry and snperficial gas velocity. The taller the column is, the smaller the influence of the initial bubble diameter will be on the global gas holdup. A higher superficial gas velocity increases the probability and frequency of bubble collisions and decreases the effect of the initial bubble diameter and gas distributor design. 

Bubble formation and orifice activity are two important factors determining stability. Synchronous bubble formation, where almost all holes are active instantaneously, tends to produce a uniform bubble and gas holdup distribution. The uniform bubble distribution leads to a more stable homogeneous flow regime, less liquid recirculation, and higher gas holdup and gas-liquid mass transfer. Asynchronous orifice operation is often accompanied by alternating or oscillating orifice activity, which leads to flow instability. The instability creates more bubble-bubble interaction and leads to lower gas holdup and gas-liquid mass transfer. Hence, the gas distributor affects the critical superficial gas velocity at which the transition regime is detected. 

Perforated plates are defined by a critical flow rate above which the orifice operation is asynchronous and the liquid flow in the sparger region is relatively unstable. As the hole spacing decreases, the critical flow rate decreases as well. At the same time, perforated plates require a minimum pressure drop in order to achieve uniform orifice activity. In other words, a critical flow rate is also created at the lower end such that a lower flow rate would lead to instability as well (Kang et al., 1999 Ruzicka et al., 2003 Su and Heindel, 2005a). This effect would produce additional complications in making comparative analysis between research works using different open area ratio adjustment methods. 

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Many factors affect gas holdup in three-phase fluidized systems, including bead size and density, liquid physical properties, temperature, sparger type, and fluid superficial velocities (Bly and Worden, 1990). System parameters such as reactor and gas distributor design can have... 

In (12-17) measurements in fluidized beds of 4 cm, 15 cm and 45 cm diameter are reported. On the laboratory scale i.e. in the 4 cm dia. bed the catalyst screening was carried out using 1-butene and butadiene as a feedstock (12). Scale-up problems including the gas distributor design and the redispersion of gas in the bed by screen plates were studied in two pilot plants with bed diameters of 15 and 45 cm, respectively (12,13,14). The hydrocarbon feed varied in composition from 30 to 35 mole % n-butenes, 30 to 32 mole % butadiene, 29 to 35 mole % i-butene and about 7 mole % butane. 

In the operation of fluidized-bed reactors, the quadratic response (A/Jj qc u2) of industrial gas distributor designs must be kept in mind, because even if the fluidization velocity is lowered only slightly, an unacccpt-... 

The conditions derived above would require knowledge of K, Dx, and kL L-Both Dx and kLaL would be functions of gas velocity, fluid properties, gas distributor design, and the thickness of the basket. These functions would be obtained experimentally. 

Bauer W, Werther J, Emig G. Influence of gas distributor design on the performance of fluidized bed reactor. Ger Chem Eng 4 291, 1981. 

Gas holdup is a key parameter to characterize the macroscopic hydrodynamics of three-phase fluidization systems. The gas holdup depends on gas and liquid velocities, gas distributor design, column geometry (diameter and height), physical and rheological... 

Bauer, W. and J. Werther. Scale-up of Fluid Bed Reactors with respect to Size and Gas Distributor Design - Measurements and Model Calculations, Proc. 2nd World Congress Chem. Engng. 3 (1981) 69-72. 

Nguyen, T. V., Gas Distributor Design lor Proton-Exchange-Membrane Fuel Cells, Journal of the Electrochemical Society, Vol, 143, No. 5, 1996, pp. L103-L105. 

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