Bubbly flow regime

Fig. 5.2.1 Flow regimes in a trickle-bed reactor (after Sie and Krishna [2]). Typical conditions for research and industrial reactor operation are indicated. The black line indicates the boundary between the pulsed flow regime and the spray, trickle and bubble flow regimes.

Malnes, D., 1966, Slip Ratios and Friction Factors in the Bubble Flow Regime in Vertical Tubes, Norwegian Rep. KR-110, Inst, for Atomenergi, Oslo, Norway. (5)... 

Figure 1. Schematic of the bubble-flow regime in porous media. Open space corresponds to bubbles, dotted space is the aqueous surfactant solution, and cross-hatched areas are sand grains.

A - effective viscosity of foam in bubble-flow regime, mPa s... 

The Achwal-Stepanek correlation can be used in a bubble flow regime (Ramachandran and Chaudhari, 1984) ... 

The effect of moderately high viscosity (> 100 cP) is to prevent the formation of the churn-turbulent flow regime so that the bubbly flow regime persists at higher superficial gas/vapour velocitiesj(see Annex 3). . , -4... 

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. 

Of interest is a recent theoretical relation for eg in gas-liquid bubble columns based on liquid circulation and claimed to be valid both in the homogeneous bubble flow regime and in the chum-turbulent regime, also for non-Newtonian fluids. For power law fluids with... 

A bubble column is a contactor which involves the passage of a discontinuous gas phase in the form of bubbles, through a continuous phase, which can either be a liquid or a homogeneous slurry. The various flow regimes that can occur in a bubble column have been characterized by Shah et al. (1). The bubbly flow regime occurs for low superficial gas velocities (< 0.05 m/s), and is distinguished by bubbles of uniform... 

This correlation was in reasonable agreement with the low gas flow (Gg < 0.01 g cm-2 s1) data of Sato et al.74 for a benzoic acid-water system with 5.5- and 12.2-mmparticle diameter. Hirose et al.38 made extensive measurements in trickle-flow, pulsed-flowrand bubble-flow regimes and correlated the enhancement factor in Ks due to parallel gas flow with the liquid velocity. They found that this enhancement factor (ratio of Ks in the presence of gas flow to Ks in the absence of gas flow) was inversely proportional to the total liquid holdup and to a first approximation has the value 2. Their data for Ks as a function of liquid velocity... 

From Fig. 6-17, it is clear that this value of Ks is most likely to be achieved under pulsed- or bubble-flow regime with superficial liquid velocities greater than 1 cm s" . It will not be achieved under trickle-flow conditions. 

Recommendations The gas holdup in the bubble-flow regime can be estimated using either Cq. (7-13) or F.q. (7-14). For the estimation of liquid holdup in the bubble-flow regime, use of Eq. (7-9) is recommended. In the pulsed-flow regime, the data of PERC and Eq. (7-15) would be useful. More experimental work with the hydrocarbon systems is needed. 

To this author s knowledge, no data are currently available on the RTD in the gas phase for cocurrent gas-liquid upflow through a packed column For unpacked bubble-columns with large length-to-diameter ratios, the gas phase is usually assumed to be in plug flow. The same should be true for the bubble-flow regime in a packed bubble-column. 

Charpentier4 suggests that if no reliable data or correlations for a given packing are available, as a first approximation, a conservative value for kLciL in the bubble-flow regime is 0.15 s-1. In the pulsed- and spray-flow regimes, use k, at values 100 percent greater than those calculated from the relations... 

Recommendations The best available correlations for fq nL, fcL, and aL are the energy correlations shown in Figs. 7-19, 7-18, and 7-17, respectively and, at least for large packing size, their use is recommended. For the estimations of fcLaL, the suggestions of Charpentier are possible alternatives. For shaped particles and in the bubble-flow regime, the correlations ofAlexander and Shah for fcLaL are recommended. Future work in this area should consider small particles and the hydrocarbon systems. 

Snider and Perona3 measured Ksas, the volumetric liquid-solid mass-transfer coefficient, for the case of hydrogenation of a-methyl styrene on 3-mm alumina spheres coated with palladium catalyst. The results were obtained in the bubble-flow regime. The measurements of Ks, the liquid-solid mass-transfer coefficient in a nonreacting system, were first reported by Mochizuki and Matsui.20 They... 

Chen8,9 studied the gas holdup of a 7-cm i.d. 244-cm long column randomly packed with open-end screen cylinders of various sizes (1.27 cm x 1.27 cm and 1.9 cm x 1.9 cm) and screen meshes (8-14 mesh). The results with an air-water system were obtained in the bubble-flow regime. The screen cylinders were found to reduce the gas holdup. The results showed that for t/0g < 4 cm s, the gas holdup was a linear function of gas velocity, a result similar to the one obtained in an unpacked bubble-column bul not in a column packed with Raschig rings or other conventional packings. He also showed that for low gas velocity, l/0G < 3.64 cm s 1 the parameter (hG - 1ig)//ig was a unique linear function of liquid velocity (independent of gas velocity). Here, /iG is the gas holdup at zero liquid velocity. He also obtained a relationship between the gas holdup and the slip velocity between gas and liquid. All the data were graphically illustrated, however, no analytical correlation was presented. 

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