Distributor aeration

Improperly designed, eroded, or even missing restriction orifices used for steam purge or aeration nozzles could cause catalyst attrition. Catalyst attrition is also caused by broken air and stripping steam distributors. 

The effect of downcomer aeration, of distance between the distributor plate and the draft tube inlet, and of the distributor plate design configuration on solid circulation rate is discussed below. For ease of presentation for materials of different densities, the solid particle velocity in the downcomer rather than the solid circulation rate is used. 

Aeration of the downcomer can also be provided with a conical distributor plate (No. 3 flow) with greatly increased solids circulation rate as shown in Fig. 8. At lower downcomer aeration, the solids circulation rate is essentially similar to that without downcomer aeration at a distributor plate location ofL = 21.7 cm. At higher downcomer aeration, however, a substantial increase in solids circulation rate is realized with the same total gas flow rate. Apparently, a minimum aeration in the downcomer is required in order to increase substantially the solids circulation rate. For polyethylene beads, this critical aeration rate is at a downcomer superficial... 

The same kind of phenomenon was not observed when distributor plate was located closer to the draft tube inlet atL = 14.1 cm and when only No. 7 and No. 8 or No. 7 and No. 3 flows were used. When all three flow injection locations were used, substantial improvement in solids circulation rate is possible even at L = 14.1 cm as shown in Fig. 9. The critical downcomer aeration velocities (superficial velocities based on downcomer area) for the data shown in Fig. 9 were determined through tracer gas injection experiments to be 0.29 m/s at L = 21.7 cm and 0.22 m/s at L = 14.1 cm. 

Example 35 Steady-state heat transfer in bubble columns 149 Example 36 Time course of temperature equalization in a liquid with temperature-dependent viscosity in the case of free convection 153 Example 37 Mass transfer in stirring vessels in the G/L system (bulk aeration) Effects of coalescence behavior of the material system 156 Example 38 Mass transfer in the G/L system in bubble columns with injectors as gas distributors. The effects of coalescence behavior of the material system 160... 

The volumetric gas-liquid mass transfer coefficient, khaL, largely depends on power per unit volume, gas velocity (for a gassed system), and the physical properties of the fluids. For high-viscosity fluids, kLaL is a strong function of liquid viscosity, and for low-viscosity fluids (fi < 50 mPa s), kLaL depends on the coalescence nature of the bubbles. In the aeration of low-viscosity, pure liquids such as water, methanol, or acetone, a stable bubble diameter of 3-5 mm results, irrespective of the type of the gas distributor. This state is reached immediately after the tiny primary bubbles leave the area of high shear forces. The generation of fine primary gas bubbles in pure liquids is therefore uneconomical. 

The air-life fermentor consists of two concentric columns. The outer column has a conical bottom section with a perforated plate acting as a gas distributor. The inner column is positioned over this plate. Compressed air, enriched with CO2, is used to lift a suspension of mineral and bacterial culture in medium through this inner column. The suspension then falls to the reservoir and is air-lifted up the column again. The air-lift fermentor provides a good supply of oxygen and is able to keep high pulp densities (25%) of ore fully suspended in the medium. An illustrative example is provided by the work of Helle and Onken (47). Pachucas operate in a similar fashion to the air-lift fermentor, but there is no inner column. Air enters at the conical base of the reactor, aerates the medium, and suspends and circulates the mineral particles. Pachucas are useful reactors for building up culture stocks. 

Permeability and Deaeration Various states of fluidization and pneumatic conveying exist for bulk solid. Fluidization and aeration behavior may be characterized by a fluidization test rig, as illustrated in Fig. 21-25. A loosely poured powder is supported by a porous or perforated distributor plate. The quahty and uniformity of this plate are critical to the design. Various methods of filling have been explored to include vibration and vacuum fiUing of related permeameters... 

Kaye, Powder Technology, 1,11 (1967) Juhasz, Powder Technology, 42, 123 (1985)]. Two key types of measurements may be performed. In the first, air or gas is introduced through the distributor, and the pressure drop across the bed is measured as a function of flow rate or superficial gas velocity (Fig. 21-26). In the second, the gas flow is stopped to an aerated bed, and the pressure drop or bed height is measured as a function of time, as the bed collapses and deaerates (Fig. 21-27). 

The rate of de-aeration of powder under different conditions is an important property in powder handling and processing. The rate of collapse of the powder in a fluidized bed is one way of measuring the rate of de-aeration but it is specific to the operating conditions the rate depends on, for example, whether or not the plenum chamber (space under the distributor) is vented simultaneously with the stopping of the aeration gas flow. 

Feed velocities leaving the feed pipe or feed sparger should not exceed 10 ft/s (305) and preferably be less than 4 to 5 ft/s (111). High velocities may disturb the liquid surface or cause excessive aeration in the distributor or parting box. 

Because of the low liquid velocity, the difference in Eq in the concurrent and the countercurrent tower reactors is slight. Therefore, they can be compared directly. In Table 1 the concurrent single-stage tower reactor with a porous plate gas distributor (Reactor A) and the countercurrent one with a perforated plate gas distributor (Reactor C) are compared. One recognizes that OTR, kLa as well as the productivity, Pr, is much higher in Reactor A than in Reactor C due to the more efficient aerator. 

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