Sparged Vessels Bubble Columns

The size of gas bubbles depends upon the rate of flow through the orifices, the orifice diameter, the fluid properties, and the extent of turbulence prevailing in the liquid. What follows is for cases where turbulence in the liquid is solely that generated by the rising bubbles and when orifices are horizontal and sufficiently separated to prevent bubbles from adjacent orifices from interfering with each other (at least approximately 3fi apart). 

Very slow gas flow rate [118], [20(o C, ) /(gFor waterlike 

Large gas rates [71], Re = 10 000 to 50 000 Jets of gas which rise from the orifice break into bubbles at some distance from the orifice. The bubbles are smaller than those described above and nonuniform in size. For air-water and orifice diameters 0.4 to 1.6 mm 

Rising Velocity (Terminal Velocity) of Single Bubbles [83] 

Typically, the steady-state rising velocity of single gas bubbles, which occurs when the bouyant force equals the drag force on the bubbles, varies with the bubble diameter as shown in Fig. 6.1. 

The volume fraction of the gas-liquid mixture in the vessel which is occupied by the gas is called the gas holdup, tpG. If the superficial gas velocity in the vessel is vG, then vJ G is the true gas velocity relative to the vessel walls. If the liquid flows upward, cocurrently with the gas, at a velocity relative to the vessel walls vL/( 1 - tpG), the relative velocity of gas and liquid, or slip velocity, is 

Equation (4-21) will also give the slip velocity for countercurrent flow of liquid if vL for the downward liquid flow is assigned a negative sign. 

The holdup for sparged vessels can be correlated through the slip velocity by the following equation developed from data presented by Hughmark (1967)  

Knowing vG, and vL, equations (4-21) and (4-22) can be combined to estimate the gas holdup and slip velocity. The following restrictions apply when using equation (4-22), satisfactory for no liquid flow (vL - 0), cocurrent liquid flow up to v, = 0.1 m/s, and also for small countercurrent flow  

If a unit volume of a gas-liquid mixture contains a gas volume (pG made up of n bubbles of diameter dp, then n = 6 pG/JMfp3. If the interfacial area per unit volume is a, then n = alndp. Equating the two expressions for n provides the specific area 

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The bubble column is shown in Figure 6.2c. In this type of equipment, gas is sparged from the bottom into a liquid contained in a large cylindrical vessel. A large number of gas bubbles provide a very large surface area for gas-liquid contact. Turbulence in the liquid phase creates a large liquid-phase mass transfer coefficient, while the gas-phase coefficient is relatively small because of the very... 

So far, we have considered only mass transfer within a single phase - that is, mass transfer between fluids and solid surfaces. For gas absorption and desorption, in which mass transfer takes place between a gas and a liquid, packed columns are extensively used, while bubble columns and sparged stirred vessels are used mainly for gas-liquid reactions or aerobic fermentation. As the latter types of equipment are discussed fully in Chapter 7, we shall, at this point, describe only the performance of packed columns. 

For gas absorption, the equipment possibilities are generally packed columns plate distillation towers, possibly with mechanical agitation on every plate deep-bed contactors (bubble columns or sparged lagoons) and mechanically agitated vessels or lagoons. Packed towers and plate distillation columns are discussed elsewhere. Generally these... 

The gas may be sparged through the liquid as a stream or cloud of bubbles (bubble columns, plate columns, and sparged vessel). 

The most simple fermenter is the bubble column fermenter (or tower fermenter), which is usually a long cylindrical vessel with a sparging device at the bottom as shown in Figures 19.5a and b. The fermenter contents are mixed by rising air bubbles that also provide the oxygen needs... 

Three-phase slurry bubble columns, in which the fine catalyst particles are suspended in the liquid and a gas is sparged into the vessel. The gas rises due to the buoyancy difference between the gas and the liquid-solid slurry, driving a circulation. These reactors usually exhibit vigorous mixing pattern driven by cross-sectional variation in the gas volume fraction. Typical schematic of a three-phase slurry bubble column is shown in Figure 6.1a. Particle sizes are typically less than 500 pm. 

Sparging/bubble column + sparging stirred vessel... 

Use of novel devices -Two-blade paddles with three slits -Two-blade paddle with 168 thin needles (co = 10-120 s- ) Both sparging/ stirred vessel and sparging/ bubble column Novel devices gave reduced power consumption and speeds compared with two-blade paddle, six-pitched blade turbine, and six-blade disc turbine 0.16 0.11 0.083-0.83 Sodium dodecyl benzene sulfonate and chicken egg albumin with viscosity modified by glycerol... 

Gas sparged chemical reactors are designed and used in many different geometries. These reactors are usually continuous in gas, and batch or continuous in liquid. Some of the geometries in use are bubble columns, pipe and static mixer reactors, stirred vessels, packed columns, tray columns, spray columns, jet loop reactors, and venturi ejector reactors. Design equations for each geometry are based on correlations and simpUfying assumptions, such as uniform kLa in the stirred vessel. Other gas-Uquid reactors include spray columns and spray combustors. 

Frmdamentally, air-lift bioreactors are a modification of the bubble colunms that generate air-flow for medium circulation unidirectionally by having at least two columns—a raiser column and a downer column. They are either a draft tube or an external loop bioreactor. The bubbles sparged into a draft tube generate upward flow and medium pours into the annular space between the draft tube and bioreactor vessel and flows downwards. An essential design feature to consider is the bioreactor ratio of the height (H) to the diameter (D). Values of H/D of five or more are needed for sufficient mixing (18). Efficiency of medium circulation depends on the rate of aeration and on the ratio of the cross-sectional area of the draft tube to the total... 

Hydrodynamics of slurry reactors include the minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large variety in reactor types (sec Fig. I) and the fact that most correlations are of an empirical nature. We will therefore focus on sparged slurry columns and slurries in stirred vessels. 

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