Bed expansion

The shape of the bed expansion curve can be used as an indicator of the likely behaviour of a group A powder (see section 3.2) and the ratios of bed heights at wMB and wMF are uniquely related to the ratio of the velocities. The measurement of bed expansion is, therefore, a useful check on the velocity ratio. 

Abrahamsen67 found the relationship between the bed heights and fluidization velocities in the following form  

An alternative method is to use two pressure probes (see earlier) strapped together but with their measuring holes separated by a 

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Bed Expansion and Bed Density. Bed density can readily be deterrnined for an operating unit by measuring the pressure differential between two elevations within the bed. This is a highly useful measurement for control and monitoring purposes. 

A. A. Avidan, Bed Expansion and S olids Mining in Eligh-Velocity Fluidi dBeds, Ph.D. dissertation. City University of New York, 1980. 

Fig. 6. Bed expansion as affected by resin type, flow rate, and temperature, where A represents a strong base styrenic resin in the CF form at 4°C, and B...

These design fundamentals result in the requirement that space velocity, effective space—time, fraction of bubble gas exchanged with the emulsion gas, bubble residence time, bed expansion relative to settled bed height, and length-to-diameter ratio be held constant. Effective space—time, the product of bubble residence time and fraction of bubble gas exchanged, accounts for the reduction in gas residence time because of the rapid ascent of bubbles, and thereby for the lower conversions compared with a fixed bed with equal gas flow rates and catalyst weights. 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

GLS Fluidized with a Stable Level of Catalyst Only the fluid mixture leaves the vessel. Gas and liquid enter at the bottom. Liquid is continuous, gas is dispersed. Particles are larger than in bubble columns, 0.2 to 1.0 mm (0.008 to 0.04 in). Bed expansion is small. Bed temperatures are uniform within 2°C (3.6°F) in medium-size beds, and neat transfer to embedded surfaces is excellent. Catalyst may be bled off and replenished continuously, or reactivated continuously. Figure 23-40 shows such a unit. 

Backwashing - After exhaustion, the bed is backwashed to effect a 50 percent minimum bed expansion to release any trapped air from the air pockets, minimize the compacmess of the bed, reclassify the resin particles, and purge the bed of any suspended insoluble material. Backwashing is normally carried out at 5-6 gpm/ft. However, the backwash flow rates are directly proportional to the temperature of water. 

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed. 

Bed A mass of ion-exchange resin particles contained in a column. Bed depth The height of the resinous material in the column after the exchanger has been properly conditioned for effective operation. Bed expansion The effect produced during backwashing when the resin particles become separated and rise in the column. The... 

Knowing the bubble rise velocity, the bed expansion can be predicted from a material balance on the bubble phase gas. Thus, total gas flow through the bubble phase equals absolute bubble velocity times the volume fraction E of bubbles in the bed. 

The degree of bed expansion contributes to the efficiency of fluidised bed/expanded bed adsorption as a composite function of liquid distribution, liquid and particle properties (size, shape and density) and process conditions. Besides being an important design feature, the degree of bed expansion may be used as a quick and simple measure of bed stability.48... 

Findings with PDU. Work with the PDU largely paralleled the bench-scale reactor tests there was one important addition—extensive three-phase fluidization studies. As was mentioned, the PDU is equipped with a traversing gamma-ray density detector that is capable of measuring bed density to within dbO.Ol specific gravity units. Thus, we could measure and correlate fluidized bed expansion as a function of liquid and gas velocities and physical properties, and could also determine the... 

Backwash rates must be lower where anthracite is used to avoid the risk of washing the media out of the filter. Bed expansion is therefore higher, at 50 to 100%, depending on grain size and flow rate. 

To backwash carbon requires a minimum of 30 to 35% bed expansion, with a flow rate of 8 to 10 gpm/sq ft for 15 minutes. Because of the presence of fines in new carbon, the initial backwash should be double this time period. 

Size the filter for 30-inch bed depth and gravel underbed, with a service flow rate of 2 to 5 gpm/sq ft. Backwash typically every 24 hours for 10 min at 12 to 14 gpm/ sq ft to achieve 35 to 40% bed expansion. 

Bed expansion (freeboard) is 50% minimum (thus, the resin tank must be at least double the volume of the resin requirement). Resin bed expansion is a function of backwash rates and temperature. 

The space above a media bed in a closed vessel that permits bed expansion and backwashing to take place. The allowance typically is 25 to 60% of the volume occupied by the media. 

Ostergaard and Theisen (07) have proposed a qualitative explanation for the observation that the reduction of bed expansion decreases with increasing particle size. 

Rough correlations have been made of minimum fluidization velocity, minimum bubbling velocity, bed expansion, bed level fluctuation, and disengaging height. Experts recommend, however, that any real design be based on pilot plant work. 

Gravity flow in downflow carbon beds is usually controlled at hydraulic loadings less than 9.78 m3/h/m2 (4 gal/min/ft2). Upflow carbon beds with bed expansion should be considered when headloss is expected. It should be noted that TSS will break through an upflow carbon bed at about 10% bed expansion. 

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