Bed geometry

Operational Considerations. The performance of catalytic incinerators (28) is affected by catalyst inlet temperature, space velocity, superficial gas velocity (at the catalyst inlet), bed geometry, species present and concentration, mixture composition, and waste contaminants. Catalyst inlet temperatures strongly affect destmction efficiency. Mixture compositions, air-to-gas (fuel) ratio, space velocity, and inlet concentration all show marginal or statistically insignificant effects (30). 

Both catalyst space velocity and bed geometry play a role. The gas hourly space velocity (GHSV) is used to relate the volumetric flow rate to the catalyst volume. GHSV has units of inverse hour and is defined as the volume flow rate per catalyst volume. 

The size of the catalyst bed depends mainly on the degree of VOC reduction requited (14). VOC destmction efficiencies up to 95% can usually be attained using reasonable space velocities (14). However, the low GHSVs, and subsequently high catalyst volumes requited to achieve extremely high (eg, 99%) conversions, can sometimes make catalytic oxidation uneconomical. Conventional bed geometries may be found in the Hterature (14). 

Thermal treatment of a material in a gas oxidizing atmosphere is the simplest concept. This can be done in air, air diluted in N2, dry air, or in ultrahigh purity O2. In the laboratory practice, calcination is done in flowthrough beds, aided by fluidization, or in static box furnaces. Important aspects are the bed geometry, the removal of the generated gases, and temperature gradients. 

To construct a model which will give behavior similar to another bed, for example, a commercial bed, all of the dimensionless parameters listed in Eqs. (37) or (39) must have the same value for the two beds. The requirements of similar bed geometry is met by use of geometrically similar beds the ratio of all linear bed dimensions to a reference dimension such as the bed diameter must be the same for the model and the commercial bed. This includes the dimensions of the bed internals. The dimensions of elements external to the bed such as the particle return loop do not have to be matched as long as the return loop is designed to provide the proper external solids flow rate and size distribution and solid or gas flow fluctuations in the return loop do not influence the riser behavior (Rhodes and Laussman, 1992). 

Figure 11. Projections of solid circulation rate at constant total flow and changing bed geometry—results of example calculation.

Kerr (7-9) has shown the critical role of the calcination environment and bed geometry in the formation of USY zeolites ("deep bed" vs."shallow bed"calcination). Ward (10) prepared USY zeolites by calcining ammonium Y zeolites in flowing steam. The work done by Kerr and Maher et al. (11) has clearly demonstrated that USY zeolites are formed as a result of aluminum expulsion from the framework at high temperatures in the presence of steam. The nature of the non-framework aluminum species has not been completely clarified. Obviously, their composition will be strongly affected by the preparation procedure of the USY zeolite. Table II shows different oxi-aluminum species assumed to be formed during thermal dealumination of the zeolite framework. 

The operation of fluidized beds is connected to fluid mechanics within the beds (Nicastro and Glicksman, 1984). For example, heat and mass transfer are greatly influenced by the solid and fluid flow patterns, which are in turn affected by the bed geometry and internal configuration. Consequently, a thorough knowledge of fluid dynamics is a prerequisite to the successful design of a commercial unit. 

If the processes just described are assumed to characterize the transfer of mass and energy in a fixed-bed adsorber, the conservation principles may be applied to them to describe the temperature and concentration as a function of time and position. Presenting the equations for a fixed-bed geometry has the advantage of including also equations, as special cases, for transient adsorption in single particles or groups of particles in batch systems. 

The properties of the product depend on the bed geometry during treatment, and three distinct types of treatment are usually distinguished shallow bed (SB), the zeolite layer is less than 3 mm thick and is slowly heated to the activation temperature under vacuum normal bed (NB), the zeolite layer is thicker, but is also heated under vacuum deep bed (DB), a thick layer of zeolite is gradually heated under atmospheric pressure. The DB process gives the most stable product. 

Figure 2. Graphs of percent recoveries for pyrene, perylene, benzo[ghi]-perylene, and coronene using 1 8 sorbent bed geometries, one containing ca. 70% dead volume (light shading) versus one with zero dead volume (dark shading).

The variation in thermal decomposition yields, which depend on bed geometry, pressure, solvents, etc, are in agreement with the suggested role of internal aliphatic or hydroaromatic hydrogen in stabilizing free radicals in the competitive evolution of light species and tar. 

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