Bubble phase characteristics

The distribution of gas flow in the fluidized bed is important for the analysis of the fundamental characteristics of transport properties in the bed. One common method to estimate the gas flow division is based on the two-phase theory of fluidization, which divides the superficial gas flow in the bed into two subflows, i.e., bubble phase flow and emulsion phase flow, as shown in Fig. 9.14. According to the theory, the flow velocity can be generally expressed as... 

Local axial and radial gas-phase characteristic measurements were made at steady-state conditions in a 0.108-m-i.d. slurry bubble column apparatus with a two-point electrical conductivity probe. 

Radial distributions of gas-phase characteristics were measured from the wall to the center of the column in 1/4-inch increments. For gas-liquid flows, steady-state operation was achieved in 10 minutes, whereas for gas-liquid-solid flows, measurements were not performed until one hour after flow conditions were established. At the end of each run, average gas holdup was measured by quick closure of the feed stream valve. The sampling rate for the conductivity probes was 0.5 millisecond per point, and the total sample time for each local measurement was 60 seconds. These sampling conditions are comparable to those of another investigator of gas-phase characteristics in bubble columns (11). 

For each local measurement of gas-phase characteristics, approximately one hundred to six hundred bubbles were measured. The fraction of bubbles rising vertically varied from 20 to 50 percent. Since analysis of bubble length and velocity can only be made for vertically rising bubbles, fifty to three hundred bubbles1 lengths and velocities were obtained for each local position in the bubble column. 

In the preceeding sections, development of the measurement technique and analysis of gas-phase characteristics in a slurry bubble column have been made along with some comparison of the experimental data with other correlations from the literature. Up to this point, analysis of gas-phase characteristics has included only single or binary liquid components. Recently, a large effect on gas holdup and bubble size has been observed for multicomponent liquid mixtures that contain small concentrations of surface-active species (24). In their study, mixtures of alcohols and water at alcohol concentrations less than 0.1 percent caused a dramatic increase in gas holdup (up to a factor of 2) and a decrease in bubble size (up to a factor of 4) compared to those observed for the water system. The authors think the effect is the result of- interaction between molecules of different species, leading to an enrichment of one species in the interface. Therefore, in multicomponent liquid mixtures, it is necessary to have knowledge of the presence of surface-active species as well as the physical properties of the fluid. 

The conductivity probe technique has been applied successfully to gas-phase measurements in a slurry bubble column. The presence of solids does not appreciably change the gas-phase characteristics for a volume fraction of solids less than 5 percent. The radial distribution functions of gas holdup and interfacial area increase significantly from the wall to the center of the column. The average Sauter mean bubble diameter is greater than the Sauter mean bubble diameter measured near the wall. 

For multicomponent liquid mixtures in a slurry bubble column, the gas-phase characteristics appear to be greatly altered if small concentrations (less than 0.1 percent) of a surface active species are present (24). The degree to which the gas-phase characteristics are changed appears to be related to the type and concentration of the surface-active component and the turbulence in the bubble column. 

The bubble phase formed by gas in excess of that required for the onset of fluidization. The bubble is usually surrounded by a cloud of gas-soM mixture and is characterized by an indentation caused by suction due to the upward movement of the bubble. The solids that flU up this region are called the wake. The bubbles are usually large and move faster than the surrounding emulsion gas flowing at u p thus giving rise to the cloud. This behavior is usually characteristic of Geldart A and B particles (see Section 11.4.2.2). 

Gas phase properties As stated before, all the model equations involve parameters that are determined by the behavior of bubbles, either alone or in groupings, and the analysis becomes more of an exercise in bubble fluid mechanics than in reactor design. For plug-flow gas phase reactors there are a number of correlations that relate in-reactor bubble properties as a function of the inlet conditions. These are available for the bubble volume Vb, the bubble rise velocity Vb, the surface to volume ratio a, and the number of bubbles per unit volume N. In addition, if bubbles are spherical (or approximately so), information on db allows determination of a and Vb- However, these correlations are subdivided by the gross characteristics of bubble formation, namely whether there is a gas phase consisting of discrete bubbles, or whether there is interaction among bubbles with some coalescence, commonly termed a swarm bubble phase. 

One of the most characteristic phenomena of gas-solid fluidized beds is the formation of gas bubbles, which dominate the behavior of fluidized beds in relatively low-velocity regimes. In analyzing the behavior of bubbling fluidized beds, it is essential to distinguish between the bubble phase and the emulsion phase, the latter consisting of particles fluidized by interstitial gas. A bubbling bed can conveniently be defined as a bed in... 

In the following, several fundamental aspects of the dynamics of single and multibubbles in liquid-solid suspensions are discussed, which are of paramount importance to the transport processes in the three-phase fluidization system. Specifically, four subjects are covered (1) plenum bubble behavior, (2) bubble rise characteristics, (3) bubble coalescence, and (4) bubble breakup. The effects of pressure and temperature on these phenomena are illustrated. 

The most advanced and realistic description of fluidized beds is the Kunii-Levenspiel model [18]. According to this model, the bubble phase is assumed to move in the reactor following the characteristics of a plug flow, while the gas flow in the emulsion phase is assumed to be negligible. The cloud and wake phases are presumed to possess similar chemical contents. The transport of the reacting gas from the bubble phase to the cloud and wake phases and vice versa prevails. The volume element, AV, therefore consists of three parts, as in Figure 5.34 ... 

Three different flow patterns can be observed in a fluidized bed reactor (Figure 6.13). For a bubble flow, the solid particles are evenly distributed in the reactor. This flow pattern resembles fluidized beds where only a liquid phase and a solid catalyst phase exist. At high gas velocities, a flow pattern called aggregative fluidization develops. In aggregative fluidization, the solid particles are unevenly distributed, and the conditions resemble those of a fluidized bed with a gas phase and a solid catalyst phase. Between these extreme flow areas, there exists a slug flow domain, which has the characteristics typical of both extreme cases. An uneven distribution of gas bubbles is characteristic for a slug flow. 

Often the variable of interest is not the number of bubbles, but some other characteristic, e.g., the total surface area of the bubble phase in a compartment. In this case, the surface area of the bubble phase in compartment 1, S, can be related to the number of bubbles of size jAV in compartment 1,. . With s. 

The example calculation has indicated that fluctuations in the total surface area of the bubble phase in the first compartment have a standard deviation of 19.3 percent of the mean and a characteristic time constant of 0.270 seconds. These fluctuations are fairly large and relatively slowly varying compared to white noise. 

Turbulent Dispersion Coalescence. After the dispersed phase leaves the motionless mixer, it will tend to coalesce to an equilibrium drop or bubble size characteristic of the shear field in the downstream piece of pipe. This coalescence is not just a phenomenon of the downstream tailpipe but is a process happening in parallel with dispersion. It is not as well understood. We do know that just like dispersion, coalescence is affected by volume concentration and is promoted by turbulence. Coalescence is strongly affected by surface chemistry effects. The role of many chemicals added to stabilize dispersions is to slow down the coalescence rate. 

The effectiveness of a fluidized bed as a ehemical reactor depends to a large extent on the amount of convective and diffusive transfer between bubble gas and emulsion phase, since reaction usually occurs only when gas and solids are in contact. Often gas in the bubble cloud complex passes through the reactor in plug flow with little back mixing, while the solids are assumed to be well mixed. Actual reactor models depend greatly on kinetics and fluidization characteristics and become too complex to treat here. 

---