Mass transfer slugs

Smaller bore diameters naturally produce slugs of smaller diameter [31,97]. Typically, a smaller length can also be generated thereby. As a consequence, internal circulation in the slug and specific interface between the slugs are increased. It is assumed that the impact of the increase in internal circulation on mass transfer/ reaction processing is generally more dominant. 

Interphase mass transfer between liquid-liquid slugs... 

Fletcher, C. D., and M. A. Bolander, 1986, Analysis of Instrument Tube Ruptures in Westinghouse 4-Loop PWRs, Rep. NUREG/CR 4672, EGG-2461, Idaho Natl. Eng. Lab., Idaho Falls, ID. (4) Ford, W. D., H. K. Fauske, and S. G. Bankoff, 1971a, Slug Expulsion of Freon-113 by Rapid Depressurization of a Vertical Tube, Int. J. Heat Mass Transfer 14 33-140. (4)... 

Mass transfer to the liquid phase around a slug can be treated with the thin concentration boundary layer assumption through Eq. (1-63). Van Heuven and Beek (VI) completed these calculations for a slug with viscous and surface tension forces negligible (Eod > 100, M < 10 ). The results can be represented by... 

The resistance to mass transfer within a slug in a liquid of low viscosity has been measured by Filla et ai (F5), who found that kA) was approximately proportional to the square root of the diffusivity within the bubble, p, as predicted by the thin concentration boundary layer approximation. In addition, kA JA was independent of slug length for 1 < L/D < 2.5. 

Figure 16 Relative increase of friction and mass transfer due to gas-liquid Taylor flow, compared to developed laminar flow in small tubes. represents the dimensionless length of a liquid slug. Re the Reynolds number based on the liquid.

The influence of interphase mass transfer between liquid-liquid slugs was investigated for nitration of aromatic compounds in a capillary-flow reactor (see Figure 5.2) [22]. This was achieved by changing flow velocity via volume flow setting, while residence time was kept constant by increasing the capillary length. 

Conversion to the mononitrated benzene derivative increased linearly with increasing flow velocity because of enhanced mass transfer. The formation of phenol by-products increased in the same manner for similar reasons. In turn, consecutive by-products, dinitrated aromatics, were formed in a linear decreasing fashion. This was explained by a mass-transfer-induced removal of the mononitrated product from the reacting slug. 

Hydraulic design aims at the realization of an intensive heat and mass transfer. For two-phase gas-liquid or gas-solid systems, the choice is between different regimes, such as dispersed bubbly flow, slug flow, churn-turbulent flow, dense-phase transport, dilute-phase transport, etc. 

Other known disadvantages of packed-bed reactors are low external and internal mass transfer rates. For trickle-bed reactors, a representative value for both and k/ig is 0.06 sec [18]. As was pointed out previously, the corresponding values for monolith reactors are higher due to the enhanced radial mass transfer in liquid slugs and to shorter diffusion length in both the liquid film and the solid catalyst. 

The amount of gas transferred to the liquid slug from the hemispherical ends is equal to the mass transferred from the liquid slug to the wall i.e.. 

The liquid film has a varying thickness and is alternately exposed to the gas and to the liquid with different concentrations. However, the film damps the effect of varying concentration, and the concentration at the wall is almost constant. The time constant for diffusion in the liquid film is bj/2D = 0.1 sec. (Eq. 32), and the contact time for the gas bubble and the liquid slug is 0.02 sec. Thus the wall concentration will be almost constant, and the mass transfers directly from the gas bubble and through the liquid slug can be added using the same driving potential. 

Since the residence time in cocurrent downflow is very short, it is necessary to recirculate the liquid and the gas. Also, the best performance from a mass transfer point of view is when the gas and liquid volume flow rates are about equal, and with a bubble/slug length of about O.S-2 cm. In these cases the molar flow of gas is much less than that of liquid, and the gas component will be consumed before the liquid component reaches complete conversion. Without recirculation, new gas must be added to the liquid further down in the reactor. 

The literature on measurement of mass transfer in vertical tubular reactors is very sparse. Kasturi and Stepanek (K3, K4) have presented data for a, ki a, and kca measured under identical conditions in the case of annular flow, annular spray flow, and slug flow. For the aqueous systems used (COj, air, NaOH) they have proposed the following correlation for the interfacial area fl = 0.23[(l - a)/QJ(AP/Z)i( whereQt is incm /sec and AP/Z is in N/m . Correlations for true liquid-side and gas-side mass-transfer coefficients by the same authors are difficult to generalize, as viscosity and surface tension were not varied. 

Air Sparging Gas sparging or injection of air bubbles has been effectively used to reduce concentration polarization and enhance mass transfer. " The secondary flows around bubbles promote mixing and reduce the thickness of the concentration polarization boundary layer. When the bubble diameter exceeds that of the membrane (tubular or hollow fiber), slugs are then formed further increase in bubble diameter has no effect on flux improvement. Large slugs can displace most of the boundary layer and cause the pressure to pulsate. This results in enhancing the flux. 

Ghosh, R. Cui, Z.F. Mass transfer in gas-sparged ultrafiltration upward slug flow in tubular membranes. J. Membr. Sd. 1999, 162, 91-102. 

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