Relatively Very High Mass Transfer Rates

1 Relatively Very High Mass Transfer Rates 

The high power input in a relatively small volume of the ejector results in very high levels of turbulence in this section. The mixing shock phenomenon discussed later in Section 8.6 disperses the gas as very fine bubbles in the liquid. This leads to very 

In a majority of the cases, a mass transfer limitation is eliminated through increase in the driving force for gas-liquid mass transfer. The first alternative is to increase dissolved gas concentration, [A ]. Assuming that Henry s law applies, which is true for most sparingly soluble gaseous solutes, the solubility [A ] is given by 

When pure solute (e.g., hydrogen) is used and also when the liquid-phase reactant has a marginal contribution to the total pressure  

Increase in total pressure implies a higher capital cost of the reactor and is therefore not the optimum solution. As mentioned earlier, the gas-liquid mass transfer and solid-liquid mass transfer coefficients offered by a venturi loop reactor are at least an order of magnitude higher than their nearest competitor, the conventional stirred tank reactor or gas-inducing reactor. Therefore, if the gas-liquid mass transfer coefficient can be so increased by a factor of 10 through the use of a venturi loop reactor, then the sole reason for excessive reactor pressure to achieve the required higher value of mass transfer rate vanishes. In conclusion, the venturi loop reactor can allow operation at much lower pressures as compared to the stirred reactor or its variants. 

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There is considerable information available in the hterature on the design of ejectors (steam jet ejectors, water jet pumps, air injectors, etc.) supported by extensive experimental data. Most of this information deals with its use as an evacuator and the focus is on ejector optimization for maximizing the gas pumping efficiency. The major advantage of the venturi loop reactor is its relatively very high mass transfer coefficient due to the excellent gas-liquid contact achieved in the ejector section. Therefore, the ejector section needs careful consideration to achieve this aim. The major mass transfer parameter is the volumetric liquid side mass transfer coefficient, k a. The variables that decide k a are (i) the effective gas-hquid interfacial area, a, that is related to the gas holdup, e. The gas induction rate and the shear field generated in the ejector determine the vine of and, consequently, the value of a. (ii) the trae liquid side mass transfer coefficient, k. The mass ratio of the secondary to primary fluid in turn decides both k and a. For the venturi loop reactor the volumetric induction efficiency parameter is more relevant. This definition has a built in energy... 

SLMs have relatively high mass transfer rates, as the distance that the species must diffuse between the source and receiving phases is very small. SLMs are however prone to reagent loss as a result of leaching of the membrane liquid phase into the adjacent aqueous phases and as there is a relatively small amount of extractant in the membrane, any loss can seriously impact separation performance (Kentish and Stevens, 2001, Kolev, 2005). 

It is instructive to consider the relative rates of mass transfer in fixed and fluidized bed reactors. The rapid rate in the fluidized bed is due not so much to the high mass transfer coefficients involved, but to the very large... 

The value of kC02 is 20 s 1 (or 7200 h ) at room temperature [73]. The value of kia, the interphase mass transfer coefficient, for our system must be estimated based on literature reports of mass transfer rates in relatively similar systems. Although the estimate of k a obtained below is highly uncertain, it is orders of magnitude less than kC02 but very much larger (hence, less ratedetermining) than kf. 

The above analysis indicates that the rates of the mass transfer steps are significantly higher than the rate of the reaction on the catalyst surface. This is anticipated since (i) turbulence in the liquid phase is much higher than that in the fixed beds used earlier (Krishna and Sie 2000) and (ii) the small particle size used ensures relatively very high catalyst surface area. Inga and Morsi (1997) had suggested an approach similar to that outlined earlier in step 4. They defined a dimensionless parameter, f), which is a ratio of the mass transfer resistance to the total resistance ... 

There are two main periods of evaporation. When a drop is ejected from an atomiser its initial velocity relative to the surrounding gas is generally high and very high rates of transfer are achieved. The drop is rapidly decelerated to its terminal velocity, however, and the larger proportion of mass transfer takes place during the free-fall period. Little error is therefore incurred in basing the total evaporation time on this period. 

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