Bubble enhanced circulation

Katz et al. [127] and Nakajima and Shiozawa [128], for example, found that coefficients on the upper finned tubes in a bundle are higher than coefficients on the lower tubes as a result of bubble-enhanced circulation. Similar results are reported by Arai et al. [129] for a bundle of Thermoexcel-E tubes. It is quite probable that with certain types of enhanced tubes it is sufficient to use the special tubes only in the lower rows since the bubble-enhanced circulation in the upper rows is so high that enhanced tubes are not effective there. 

Mass transfer in a gas-liquid or a liquid-liquid reactor is mainly determined by the size of the fluid particles and the interfacial area. The diffusivity in gas phase is high, and usually no concentration gradients are observed in a bubble, whereas large concentration gradients are observed in drops. An internal circulation enhances the mass transfer in a drop, but it is still the molecular diffusion in the drop that limits the mass transfer. An estimation, from the time constant, of the time it wiU take to empty a 5-mm drop is given by Td = d /4D = (10 ) /4 x 10 = 6000s. The diffusion timescale varies with the square of the diameter of the drop, so... 

These parts are used in fluidized beds for various purposes. For example, gas distributors and various types of baffles are installed to decrease the size of the bubbles. Moreover, draft tubes are used to enhance gas or solid circulation. Other devices such as horizontal baffles limit circulation and backmixing of solids and gas. Horizontal or vertical tubes are used for heat management. Devices used to control or improve fluidization behavior, to improve fluidization of cohesive particles or to achieve solids recovery are within the various internals met in fluidized bed reactors (Kelkar and Ng, 2002). Immersed tubes in small diameter beds may lead to slugging. Furthermore, attrition of particle breakage may change the size distribution and possibly change the fluidization behavior. 

The enhancement in the bed-wall heat transfer coefficients in bubble columns as well as the two- and three-phase contactors as compared to the single phase pipe flow is likely because of the strong circulation flow pattern in the continuous phase. 

The above analysis can be applied for the case of bubble columns. From Figure I it can be seen that, near the wall the axial component of the liquid velocity is downwards, whereas the radial component of the liquid velocity is towards the wall in the top half of the circulation cell and away from the wall in the lower half of the circulation cell. As a result, for one circulation cell the enhancement factor is given by the following equation ... 

Compared to bubble columns, airlift reactors have better liquid circulation but lower rates of mass transfer and mixing. These rates are enhanced in modified airlift reactors with perforated single or coaxial draft tubes. This enhancement is because of the breakup of gas bubbles into smaller bubbles when crossing perforated tubes. The gas-liquid interfacial area and the gas-liquid mass transfer coefficient increases. Similar effect can be achieved with the addition of packing to the riser. ... 

DuPont s new tetrahydrofuran plant, which is slated to start up in the second quarter of 1996 in Asturias, Spain, is based on a new process that is claimed to be environmentally friendly [133], The process is harnessed with a circulating fluidized-bed (CFB) reactor for the partial oxidation of n-butane. The catalyst used for this stage is vanadium phosphorus oxide (VPO). To enhance the catalytic life in the CFB reactor, attrition-resistant catalyst particles have been developed. A highly selective catalyst for hydrogenation of maleic acid to THF also has been developed. This THF conversion reaction is carried out in a bubble column reactor under relatively mild conditions. This process has several significances ... 

Echocardiographers recognizedby chance in the 1960s that the tiny air bubbles incidentally introduced into the circulation after intravenous injections produce transient echo enhancement in the right side of the heart. Such bubbles do not pass through the normal pulmonary circulation, but systemic enhancement... 

It is also known that the ellipsoidal reflector is occasionally filled with very small air bubbles which introduced by circulating the water in the ellipsoidal reflector, the focused pressure could never be enhanced to the expected level. The shock wave energy absorbed by the tiny bubbles stimulates their... 

Theoretical and experimental studies demonstrated that segmented gas/liquid flow enhances the heat transfer considerably compared to a single flow. Multiphase flow simulations revealed mainly two mechanisms explaining the increase of the Nusselt number, namely the circulation within the liquid slugs and the disturbing and renewing of the fluid layer near the wall by the gas bubbles. Detailed multiphase simulations for cylindrical chaimels by Lakehal et al. [19] lead to the... 

The heat transfer rates in bubble columns are much higher than that anticipated from single phase flow considerations. This enhancement is ascribed solely to the bubble-induced turbulence and liquid circulation. Little work has been reported on heat transfer, both at wall and to/from immersed surfaces, in bubble columns employing non-Newtonian media. Nishikawa et al. reported the first set of data on the effect of shearthinning viscosity of CMC solutions on jacket and coil heat transfer coefficients [7]. They reconciled their results for Newtonian and power law liquids by introducing the notion of an effective viscosity estimated via Equation 3, provided the gas velocity was greater than 40 mm/s. For superficial gas velocity lower than this value, the effective shear rate varies as for coil heat transfer... 

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