Radial gas holdup profile

Figure 7.6 Radial gas holdup profiles for small (a and c) and large (b and d) bubble column diameters (Veera and Joshi, 1999).

Clearly, there is a need for complete mapping of liquid and gas velocities and turbulence intensities in bubble columns. Until recently only data of Hills (1974) reported liquid time averaged velocities and radial gas holdup profiles taken under identical operating conditions. Yao et al. (1990) present in addition to such data also gas velocity and turbulence intensity profiles. Some data on radial holdup distribution and axial liquid velocity in industrial size columns were presented by Kojima et al. (1980) and Koideetal. (1979). 

The upper surface of the internal is defined as the zero of the axial position. Axial positions are positive above this and negative below this. The radial profiles of the gas holdup at five axial positions were measured, as shown in Fig. 3. The radial profile of the gas holdup becomes much flatter after flowing through the internal, with an increase in the gas holdup near the wall and a decrease in the center region as compared with the gas holdup below the internal. As the distance above and away from the internal increases, the profile becomes more and more similar to that before contact with the internal. The local radial profiles of the gas holdup at axial positions 144 and 209 cm show almost no difference. This shows that the influence of the internal on the radial profile of the gas holdup becomes weaker and weaker with increasing distance away from the internal and is no longer felt beyond a certain distance. The analysis of the local gas holdup profiles shows that the influence of the internal persists to a distance of about 1.4 m. In order to show the effect of the internal on the radial profile of... 

Fig. 6 presents the experimental results on the gas holdup profiles with and without the internal at the axial position of 74 cm for different superficial gas velocities. The internal causes extra flow resistance, which in turn decreases the liquid circulation velocity and increases gas holdup [5]. In addition, the experimental results show that the radial profiles of the local gas holdup with an internal are flatter than those without the internal. Therefore, a properly designed internal can have dual function of increasing the gas holdup and improving its radial profile. 

The bubble size distribution is closely related to the hydrodynamics and mass transfer behavior. Therefore, the gas distributor should be properly designed to give a good performance of distributing gas bubbles. Lin et al. [21] studied the influence of different gas distributor, i.e., porous sinter-plate (case 1) and perforated plate (case 2) in an external-loop ALR. Figure 3 compares the bubble sizes in the two cases. The bubble sizes are much smaller in case 1 than in case 2, indicating a better distribution performance of the porous sinter-plate. Their results also show the radial profile of the gas holdup in case 1 is much flatter than that in case 2 at the superficial gas velocities in their work. 

Fig. 3. Radial profiles of the bubble size with Fig. 4. Influence of internals on the gas holdup different gas distributors (air-water system) [22]. (air-water-solid slurry system) [23].

Fig. 3. Radial profile of the gas holdup at different solid holdups...

Figure 3 shows the radial profile of the gas holdup in the riser with increasing superficial gas velocity under different solid holdups. The gas holdup increases with increasing superficial gas velocity at the different solid holdups. At a low superficial gas velocity, the liquid velocity... 

Small bubbles and flow uniformity are important for gas-liquid and gas-liquid-solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m. 

Keywords Airlift reactor Internal Radial profile Gas holdup Bubble size Bubble breakup... 

Liquid mixing time decreases sharply for an initial increase in the gas sparging rate and approach an asymptotic value that is determined by the height and diameter of the downcomer and the liquid properties [5]. A higher liquid velocity shortens the gas residence time and results in a decrease of gas holdup and interfacial area. The radial profile of the liquid is parabolic. These are disadvantageous for mass transfer. The mounting of internals in a fixed bed is often used to improve the radial profile of the liquid velocity. This motivates us to mount internals in an EL-ALRs to improve the radial profile of the gas holdup and the liquid velocity and to intensify turbulence. 

The radial profiles of the gas holdup and bubble rise velocity become more uniform after passing through the internal. With increasing distance from the internal, the radial profiles of the gas holdup and bubble rise velocity change back to be similar to that below the internal. At 144 cm above the internal, the radial profiles of the gas holdup and the bubble rise velocity are the same as those below, while 0S and[Pg.86]

The radial distribution of interfacial area for a two-phase system is shown in Figure 11. As discussed earlier, the gas holdup fraction is a strong function of radial position, but the Sauter mean bubble size is a less pronounced function of radial position. Consequently, the radial distribution of interfacial area has a shape similar to the gas holdup radial profile. Therefore, the inter facial area is well described with a third order polynominal equation. 

FIGURE 11.18 Some aspects of the heterogeneous regime (a) radial profiles of gas holdup and static pressure, (b) scehematic of liquid circulation pattern. 

The measurements of the local properties of two-phase systems during cultivation indicate that radial profiles of ds are fairly uniform. Also, their longitudinal variations are fairly moderate, except in the neighborhood of the aerator (1, 4). The same holds true for the spacial variations of the local relative gas holdups. At low superficial gas velocities the specific interfacial area, a, is fairly uniform also At high superficial gas velocities (turbulent or heterogeneous flow range) the radial profile of a has a shape of an error function, with its maximum in the column center (5). The behavior of these parameters near the aerator depends on the aerator itself and on the medium character. 

Overall gas holdup and its time-averaged radial profile or cross-sectional distribution should be the same for two reactors to be dynamically similar. ... 

The internals of the bubble column reactor may have a dramatic impact on the flow patterns of the bubbles and the liquid. Companies have not divulged details about the internals to date. Some details of the US DOE pilot plant (22.5 inch 0.57 m diameter) have been published [ 106]. In this report the dimensions of the cooling tubes, their location, and their number are provided. These cooling coils occupied about 10% of the total volume of their commercial reactors slurry volume. The gas holdup and bubble characteristics as well as their radial profiles were determined in a column that was about the size of the US DOE reactor [107-109]. Dense internals were found to increase the overall gas holdup and to alter the radial gas profile at various superficial gas velocities. The tube bundle in the column increased the liquid recirculation and eliminated the rise of bubbles in the wall region of the column. These results indicate that further studies of bubble column hydrodynamics are directed toward larger scale units equipped with heat exchange tubes. 

Carbonell and Guirardello (1997) performed simulations to establish the hydrodynamics (pressure drop, radial gas and slurry holdup distribution, effective eddy viscosity, and liquid-phase velocity profile). They also superimposed the thermal cracking reactions by accounting the radial variations in these transport properties so as to predict the heavy oil conversion. They found that the liquid recirculatory patterns (backmixing) strongly affect the product yields. However, the validation of the experimental was not carried out using these models. 

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