Gas and Liquid Velocities

Increasing either the gas velocity or the liquid droplet velocity in a scrubber will increase the efficiency because of the greater number of collisions per unit time. The ultimate scrubber in this respect is the venturi scrubber, which operates at extremely high gas and liquid velocities with a very high pressure drop across the venturi throat. Figure 29-8 illustrates a commercial venturi scrubber unit... 

From the visual studies on the flow patterns for circular, trapezoidal and rectangular channels it may be concluded that as the tube diameter decreases, transitions between flow regimes occur at different combinations of superficial gas and liquid velocities. 

Most of heat transfer correlations are based on data obtained in flow boiling from relatively large diameter conduits and the predictions from these correlations show considerable variability. Effects of superficial liquid and gas velocity on heat transfer in gas-liquid flow and its connection to flow characteristics were studied by Hetsroni et al. (1998a,b, 2003b), Zimmerman et al. (2006), Kim et al. (1999), and Ghajaret al. (2004). However these investigation were carried out for tubes of D = 25—42 mm. These data, as well as results presented by Bao et al. (2000) in tubes of L> = 1.95 mm and results obtained by Hetsroni et al. (2001), Mosyak and Hetsroni (1999) are discussed in the next sections to clarify how gas and liquid velocities affect heat transfer. Effects of the channel size and inclination are considered. 

Sum of the superficial gas and liquid velocities Thermal conductivity Heat flux... 

The recovery of copper powder from wastewater of electronic industries was investigated in three-phase inverse fluidized-bed electrode reactors(0.102m ID x 1.0m). Effects of gas and liquid velocities, current density, distance between the two electrodes and amount of fluidized particles on the recovery of copper powder were examined. The addition of a small amount of gas or fluidized particles into the reactor resulted in the decrease in the powder size of copper recovered as well as increase in the copper recovery. The value of copper recovery exhibited a maximum with increasing gas or liquid velocity, amount of fluidized particles or distance between the two electrodes but increased with increasing current density. 

P 28/The Hquid feed was introduced by a pump and the gas feed using a mass-flow controller [10], The reaction was carried out using liquid flows of 20.7-51.8 ml h and gas flows of 1.7-173 mlrnin . The gas and liquid velocities amounted to 0.02-1.2 and 0.03-3.0 m s , respectively. The reaction was performed in mixed flow regimes, including bubbly, slug and annular patterns. The specific interfacial areas amoimted to about 5000-15 000 m m . The reaction was conducted at room temperature. 

Conversion normalized by residence time (Figure 5.31) increased nearly linearly with the Weber number owing to an increase in specific interfacial area [10], The normalization is needed because by increasing gas and liquid velocities both interfacial area and time are affected in an antagonistic manner. 

For gas-liquid bubble flow, F and F+ are the gas and liquid velocities, respectively, and the zero-level set of 4> marks the bubble interface, which moves with time. For gas-droplets flows, on the other hand, F and V+ represent the... 

Using the specific volumes of the gas VG and liquid VL, the gas and liquid velocities can be written in the following forms ... 

In bubble columns the static head of the fluid is the dominant component of the pressure drop and consequendy it is important to determine the void fraction of the dispersion. All quanuties will be measured as posidve in the upward direction, this being the direction of flow of the dispersed phase. Assuming that the gas bubbles are of uniform size and are uniformly distributed over any cross section of the column, the gas and liquid velocities relative to the column are... 

In these models the phases are treated as if they are separate and flow in well defined but unspecified parts of the cross section. Only the simplest case, in which the phases are allowed to have different but uniform velocities, will be considered here. An overall momentum equation will be given and it will be seen that merely allowing the gas and liquid velocities to differ leads to considerable complexity. Two empirical correlations from the pioneering work of Martinelli and co-workers will then be described. These methods can be used for the churn and annular flow regimes. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

In brief, increasing the gas-and liquid velocities or mass fluxes, the liquid viscosity, or gas density, or reducing the particle diameters, increases the pressure drop. 

Dimensionless, entirely empirical correlations The problem is, that the pressure drop depends on many variables the gas-and liquid velocities, uL and ug, the gas-and liquid densities, pL and pc, the particle-diameter, dp, and eventually the diameter and shape distribution, the surface tension, at, the viscosity, pi, of the liquid and eventually of the gas, Pa, the bed porosity, e, and the column diameter and height, and the type of distributor. 

In principle, the difference between the gas-and liquid velocity should be used instead of the mean interstitial gas velocity, but in their operating conditions, the mean interstitial liquid velocity is small compared to the mean interstitial gas velocity. To a first approximation, ate was considered to be proportional to the external geometric area, ac, of the packing ... 

Foaming (and emulsion). The low gas and liquid velocities in packing suppress foam formation. The large open area of the larger random packing promotes foam dispersal. Both attributes make... 

Switching from trays to structured packing can aggravate foaming. While the low gas and liquid velocities help, the solid walls restrict lateral movement of foams and give support to the foams. 

Fig. 8 shows the time and azimuthally averaged radial liquid saturation profiles at varying superficial gas and liquid velocities at the middle axial position (2.5D). The figure shows that liquid saturation is nearly flat, which suggests a fair uniformity of liquid distribution. Moreover, with increasing liquid velocities, liquid saturation increases. Similar trends were obtained at all scan heights. 

Fig. 9 illustrates the effect of superficial gas and liquid velocities on the cross-sectionally averaged liquid saturation at the middle of the column (2.5D axial position). It is obvious that the effect of gas velocity on the liquid saturation is not significant within the range of flows studied. This could be due to the fact that solid and liquid holdups are very small, leaving enough space for the gas to flow upwards without significant interactions with the liquid phase flowing downward. 

Fig. 8. Effect of superficial gas and liquid velocities on the liquid saturation Fig. 9 Effects of gas and liquid superficial velocities on the cross-radial profile at axial position of 2.5D. sectionally averaged liquid saturation.

The pressure drop across the length of the packing was extremely small and was beyond the detectable limit of the available pressure gauges. The full scale of the available transducer (validyne differential transducer) was 200 units, which correspond to a pressure drop of 1 cm of water. In our study, it was observed that the maximum pressure drop within the gas and liquid flow rates studied was about 1 mm of water (20 calibration units). A trend was seen wherein the pressure drop increases with increasing gas and liquid velocities. 

As shown in Fig. 4, the effect of pressure is to increase the corresponding kLa. Similarly to a, kLa depends on pressure only for gas and liquid velocities above some critical velocities. The effect of pressure can be due either to an increase in kL or in a, or in both. A literature review on the pressure effect on kL in different gas-liquid contactors reveals that kL may be considered as independent of reactor pressure. Hence, kLa should vary with pressure only via the effect of the interfacial area. Following the assumption of the presence of small bubbles in the liquid films, gas-liquid mass transfer can be split into a mass transfer from the continuous gas to the liquid film, with a mass transfer coefficient equal to the one at atmospheric conditions and a mass transfer from the bubbles to the surrounding liquid, as if bubbles were suspended in a stagnant medium. Then, contribution brought about by bubbles is calculated as the product of the excess interfacial area ab and the mass transfer coefficient of a bubble in a stagnant medium (Sh = 2) ... 

The Effect of Gas and Liquid Velocities and Solid Size on Solid Suspension in a Three-Phase Bubble Column Reactor... 

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