Pulsing flow regime, mass transfer

Model application in the pulsing-flow regime The mass transfer coefficient in the liquid-solid film is evaluated by means of the Dhai wadkar and Sylvester correlation (eq.3.433), and is found to be 0.45 s. Then, the several parameters of the model eq. (5.379) are shown in Table 5.18. 

The hydrodynamics control the mass transfer rate from gas to liquid and the same from liquid to the solid, often catalytic, particles. In concurrently operated columns not only the gas-continuous flow regime is used for operation as with countercurrent flow, but also the pulsing flow regime and the dispersed bubble flow regime (2). Many chemical reactors perform at the border be-... 

Mass transfer from gas to liquid in the pulsing flow regime... 

Conventional trickle bed reactors work under steady state conditions, whereby components in the. liquid, mostly via gas-to-liquid mass transfer are converted by the solid catalyst. Such a process is highly non-linear and thus it is questionable whether steady operation will provide an optimal conversion and selectivity in particular. Operation in the dynamic mode will provide an extra parameter to optimise the production. Trickle beds happen to show just naturally a dynamic flow regime in which gas/liquid discontinuities occur the pulsing flow regime. 

Ultimately at high frequencies the pulses overlap and we arrive in the dispersed bubble flow regime. Thus we consider the pulses to be zones of the bed already in the dispersed bubble flow, spaced by moving compartments that are still in the gas-continuous flow regime. This concept is very helpful in calculating mass transfer and mixing phenomena, as well as in pressure drop relations (9) where it appears that above the transition point the pressure drop can be correlated linearly with the pulse frequency. Pulses are to be considered as porous to the gas flow as is shown when we plot the pulse velocity versus the real gas flow rate, figure 5. 

The value of k a, a being the gas-liquid contact area per unit volume, k the corresponding liquid side mass transfer coefficient, is considerably higher in the pulsing than in the gas-continuous flow regime. It has been tried in the past, and partially success-full, to correlate the mass transfer data to the energy dissipation rate in the bed. We made the premise, that pulses are parts of the bed already in the dispersed bubble flow regime and therefore must accredit for an increase in the transfer rate proportional to their presence in the bed. 

Taking the active pulse height as 0.05 m and the pulse velocity as 1 m/s, we derive for the mass transfer coefficient in the gas-continuous zone, 11, a value of 10 m/s and in the pulse proper, k, a value of 6 10 m/s. These values compare very well with those given in literature (5, 6) for both gas-continuous and dispersed bubble flow regimes. An estimate of k can also be made by means of the penetration theory, taking the respective liquid in and outside the pulse as the basic for the calculation of the con-... 

Mass transfer correlations from liquid to solids are based on total interfacial area of the solids. Hence, the correlations include partial wetting effects. As expected, the mass transfer coefficient increases with Increasing liquid flow rate. It is somewhat insensitive to the gas flow rate. The sensitivity to the gas rate increases in the pulse and dispersed bubble regime. 

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