Dispersed bubble regime

Fluidization Regime. As for traditional fluidization applications, the fluidization regime—dispersed bubble, coalesced bubble, or slugging—in which a three-phase fluidized bioreactor operates depends strongly on the system parameters and operating conditions. Generally, desirable fluidization is considered to be characterized by stable operation with uniform phase holdups, typical of the dispersed bubble regime. It would be useful to be able to predict what conditions will produce such behavior. 

Overall gas holdup increases with gas velocity in the dispersed bubble regime for both low and high density particle systems (Davison, 1989 Tang and Fan, 1989 Bly and Worden, 1990 Nore et al., 1992 Pottboff and Bohnet, 1993). As gas velocity increases and the system enters the coalesced and slugging regimes, the rate of increase in the overall gas holdup decreases (Bly and Worden, 1990). 

Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dispersed bubble regime. A typical application of this reactor is the hydrogenation of residues. 

Transition velocity tjetween Ug j, dispersed bubble regime and coalesced bubble regime... 

The presence of pulp, even at very low pulp consistencies (0.1%) in the column leads to enhanced bubble coalescence and hence a narrowing of the gas velocity for the dispersed bubble regime as the pulp consistency increases (Reese et ah, 1996). Bubble coalescence inhibitors such as inorganic salts (e.g., sodium chloride and sodium phosphate dibasic) and organic compounds (e.g., ethanol, n-pentanol, iso-amyl alcohol, and benzoic acid) can be effectively applied to the liquid at concentrations up to 200 ppm to inhibit bubble coalescence behavior in three-phase fluidization (Briens et al., 1999). With the addition of the bubble coalescence inhibitor, the bed hydrodynamics at low gas velocities are significantly different from the case without the... 

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. 

Despite the Friedel model is considered a general one, independent of the flow pattern, in order to assess its possible influence, the flow pattern has been characterized for all the runs. The flow pattern map proposed by Moreno-Quiben Thome (2007) has been adopted, and it was found that, based on that classification, the flow pattern in all the runs could be associated with either the intermittent or dispersed bubble regimes. In Figure 5, the ratio between the calculated and experimental data is reported, as a function of the vapor quality, with the flow pattern as a parameter. 

Figure 5.16c indicates that as the channel size was reduced to Jh = 0.866 mm, the dispersed bubbly flow pattern vanished from the flow regime map. Figure 5.16a-c indicates that the slug-churn flow transition line shifted to the right, as the channel size was reduced. Similar trends were also found in small circular tubes by the... 

Figure 3.3 Horizontal flow regime map curves A, B, (Fr) versus Xn curve C, K versus Y curve D, T versus X . (AD, annular dispersed DB, dispersed bubble SW, stratified wavy I, intermittent SS, stratified smooth.) (From Taitel and Dukler, 1976b. Copyright 1976 by American Institution of Chemical Engineers, New York. Reprinted with permission.)...

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-... 

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-... 

It is shown, that the performance of a pulsing packed column can be split up into its two component parts, the pulses and the zones in between pulses. The pulses can be described as parts of the bed already in the dispersed bubble flow regime the zones-in between the pulses as parts of the bed still in the gas-continuous regime. The pulse frequency is linearly dependent upon the real liquid velocity. The properties of the pulse, like holdup, velocity and height are quite independent upon all the parameters except gas flow rate. 

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. 

In slurry reactors, an attempt is made to realize intensive and intimate contact between a gas-phase component, usually to be dissolved in the liquid phase, a liquid-phase component and a finely dispersed solid. In this respect, slurry reactors are related to packed-bed reactors with the various gas/liquid flow regimes that can be realized (such as trickle flow, pulsed flow and dispersed bubble flow). Also, there is much similarity with three-phase fluidized beds. 

Different authors have identified various flow regimes in large channels. In both vertical and horizontal configurations these include bubbly, dispersed bubbly, slug, pseudo-slug, churn, annular, annular mist and dispersed droplet flows. An important difference in minichannels is that the liquid flow is preferentially laminar. Surface tension effects have more and more influence as the hydraulic diameter is reduced. Gravity becomes negligible compared to surface tension so that the orientation is less influential. 

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