Bubble residence time

These design fundamentals result in the requirement that space velocity, effective space—time, fraction of bubble gas exchanged with the emulsion gas, bubble residence time, bed expansion relative to settled bed height, and length-to-diameter ratio be held constant. Effective space—time, the product of bubble residence time and fraction of bubble gas exchanged, accounts for the reduction in gas residence time because of the rapid ascent of bubbles, and thereby for the lower conversions compared with a fixed bed with equal gas flow rates and catalyst weights. 

These requirements are not aH mutuaHy compatible. In the case of noncatalytic reactions, bubble residence time may be the significant parameter. 

In this section, a general formulation will be given for the effect of bubble residence-time and bubble-size distributions on simultaneous and thermodynamically coupled heat- and mass-transfer in a multicomponent gas-liquid dispersion consisting of a large number of spherical bubbles. Here one can... 

Before analyzing the subject of gas holdup, it has to pointed out that while ub is the bubble rising velocity and Z/ub is the bubble residence time, the superficial gas velocity present in the following equation is equivalent to the superficial gas velocity in fixed beds ... 

Tmobiie tjme for transition of bubble surface to rigidity, s tr average bubble residence time, s... 

H/D F D scfm Bubble Residence Time Sparger Pressure... 

The phenomenon of gulf streaming arises from the formation of violent circulating currents induced by bubbles. These currents may become especially significant in large commercial beds (especially shallow beds). Davidson and Harrison (1971) have shown that circulation velocities of 200 mm/s can exist in large beds. As a result, the bubble residence time is smaller, leading to lower conversions. No simple way has yet been found to overcome this problem thus, a pilot plot of appropriate size seems almost unavoidable. 

OBRs can exhibit enhanced mass transfer between gases and liquids. The main mechanisms for the enhancement are increased hold-up and increased breakage of bubbles (reducing size, thereby increasing interfadal surface area and reducing rise velocity, which increases bubble residence time further). Figure 5.16 compares the mass transfer coefficient for an OBR with that of an STR for an air-water system on the basis of power density (Ni and Gao, 1996). 

Another example of the OBRs use with solid particles is as a photochemical reactor with solids suspension, in this case the vortical flow patterns being used to suspend catalytic titania particles to convert organics in wastewater. The tita-nia needs to be activated by ultraviolet, and the reaction requires the presence of oxygen, so air is bubbled through. The gas-liquid mass transfer is enhanced by the oscillation of the fluid, as it increases hold-up time (bubble residence time) and reduces bubble size (increasing surface area and further increasing hold-up time). The flow patterns simultaneously ensure good exposure of the titania particles to the radiation from an axially located ultraviolet lamp. 

We are now in a position to calculate tiie minimum total bubble area required from which one can calculate tiie airflow required using the bubble residence time. The calculations are somewhat circuitous but involve well-known principles ... 

Increased slurry depth increases air bubble residence time in the absorber sump/reaction tank and thereby oxygen utilization. However, the power requited to inject air increases with depth. Increased depth with constant tank diameter also increases slurry residence time, which may increase oxygen utilization and gypsum crystal size. 

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