Freeboard

A generic multipurpose fluidized bed is illustrated in Figure 2 (1). The soHds are contained in a vessel and gas is introduced into the system via a distributor, which is typically a drilled plate at the bottom of the vessel. A plenum chamber is provided below the distributor plate. The height of the soHds level above the distributor is called the bed height, and the vertical space above the bed height is called the freeboard. A splash zone may exist as a transition between the bed and freeboard. Cyclones, located either in the freeboard or external to the vessel, are used to remove soHds from the gas stream. Diplegs can return entrained soHds directly to the bed. 

Fig. 2. Multipurpose fluidized bed where 1 represents the sheU 2, soHd particles 3, the blower 4, the gas distributor 5, the heat exchanger for fluidizing gas 6, internal heating or cooling 7, external heating or cooling 8, cyclones 9, the soHds feeder 10, soHds offtake 11, Hquid feed 12, the freeboard 13, the...

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

Flue particles ia a fluidized bed are analogous to volatile molecules ia a Foiling solution. Therefore, the concentration of particles ia the gas above a fluidized bed is a function of the saturation capacity of the gas. To calculate the entrainment rate, it is first necessary to determine what particle sizes ia the bed can be entrained. These particles are the ones which have a terminal velocity less than the superficial gas velocity, assuming that iaterparticle forces ia a dilute zone of the freeboard are negligible. An average particle size of the entrainable particles is then calculated. If all particles ia the bed are entrainable, the entrained material has the same size distribution as the bed material. 

The bed level is not weU defined in a circulating fluidized bed, and bed density usually declines with height. Axial density profiles for different CFB operating regimes show that the vessel does not necessarily contain clearly defined bed and freeboard regimes. The sohds may occupy only between 5 and 20% of the total bed volume. 

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiUary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular soflds. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the sohds become suspended in the air and move freely through the bed. 

To estimate the slumping motion of the kiln bed which periodically exposes a fresh, vapor saturated surface at the bed—freeboard interface must be considered. Based on Pick s second law in a bed of porosity, S, and for an effective diffusion coefficient, the mass-transfer coefficient on the bed side is... 

C. Oxygen and steam are also injected above the bed to increase carbon conversion and reduce yields of methane and other hydrocarbons. The freeboard 2one above the bed operates as much as 150 to 230°C above the bed temperatures (24). 

Boottop and Freeboard Areas. The boottop is that part of the hull that is immersed when the ship is loaded and exposed when the ship is empty. The freeboard is the area from the upper limit of the boottop to the main deck. The boottop suffers mechanical damage from tugs, piers, and ice. 

In the freeboard areas, commercial ships use organic 2inc-rich primers extensively and usually topcoat them with a two- or three-coat epoxy system. U.S. Navy ships use an organic 2inc-rich primer, two to three coats of an epoxy-polyamide coatings, and a siUcone-alkyd topcoat (16) the entire dry system is 150—225 )J.m thick. 

Spouted beds are used for coarse particles that do not fluidize well. A single, high velocity gas jet is introduced under the center of a static particulate bed. This jet entrains and conveys a stream of particles up through the bed into the vessel freeboard where the jet expands, loses velocity, and allows the particles to be disentrained. The particles fall back into the bed and gradually move downward with the peripheral mass until reentrained. Particle-gas mixing is less uniform than in a fluid bed. 

Freeboard and Entrainment The freeboard or disengaging height is the distance between the top of the fluid bed and the gas-exit... 

At least two ac tions can take place in the freeboard classification of solids and reaction of solids and gases. 

Dust Separation It is usuaUy necessaiy to recover the solids carried by the gas leaving the disengaging space or freeboard of the fluidized becl GeneraUy, cyclones are used to remove the major portion of these sohds (see Gas-Sohds Separation ). However, in a few cases, usuaUy on small-scale units, filters are employed without the use of cyclones to reduce the loading of solids in the gas. For high-temperature usage, either porous ceramic or sintered metal has been employed. Multiple units must be provided so that one unit can be blown back with clean gas while one or more are filtering. 

Depth tends to be determined from the retention time and the surface overflow rate. As surface overflow rates were reduced, the depth of sedimentation tanks was reduced to keep retention time from being excessive. It was recognized that depth was a valid design parameter and was more critical in some systems than retention time. As mixed-liquor suspended-solids (MESS) concentrations increase, the depth should also be increased. Minimum sedimentation-tank depths for variable operations should be 3.0 m (10 ft) with depths to 4.5 m (15 ft) if 3000 mg/L MESS concentrations are to be maintained under variable hydraulic conditions. With MESS concentrations above 4000 mg/L, the depth of the sedimentation tank should be increased to 6.0 m (20 ft). The key is to keep a definite freeboard over the settled-sludge blanket so that variable hydraulic flows do not lift the solids over the effluent weir. 

A typical NO, le cl in the combustion gas is around 107 rng/lVlJ (0,25 lb/l(F Btii), and the (X) leyel tends to be high (near 86 rng/MJ [0,20 lb/l(f Btii]), Only one design has used secondaiv air, and this lowered the NO, to 86 rng/AlJ and the (X) to about 43 rng/AlJ (0,10 lb/l(h Btii), NO, reduction by selectiye noncatalytic reduction (SNCR) has not been tested in a bubbling AFBC, but without the assistance oF secondary air, it maybe diFFicult to distribute the ammonia adequately across the Freeboard to achie e the desired effect. 

Overpressure of Store drum at proper temperature material in drum, Keep drum away from heat source due to external heat input or self reaction is complete before drumming heating. Allow adequate freeboard for material Provide adequate sprinkler protection Thermally initiated venting (e.g., melt-out bungs) CCPS G-3 CCPS G-15 CCPS G-22 CCPS G-29... 

The rotational operation of a CFB leads to a vortex motion in the freeboard which tends to inhibit particle loss by elutriation. Because of the relatively compact nature of the CFB and the operating flexibility provided by the rotational motion, the CFB has been proposed for a variety of applications including coal combustion, flue gas desulfurization, gas combustion, coal liquefaction and food drying. 

Freeboard The space provided above the resin bed in an ion-exchange column to allow for expansion of the bed during backwashing. 

A laboratory study of surface-treatment tanks by Braconnier et al." showed the effects of cross-drafts and obstructions to airflow on capture efficiency. They found that, without obstructions, capture efficiency decreased with increasing cnrss-draft velocity but the importance of this effect depended on freeboard height. In their study, cross-draft direction was always perpendicular to the hood face and directed opposite to the hood suction flow. Follow cro.ss-draft velocities (less than 0.2 m s ), efficiency remained close to 1.0 for the three freeboard heights studied. With higher cross-draft velocities, efficiency decreased as freeboard height decreased. For example, when the crossdraft velocity was 0.55 m s , efficiency decreased from 0.90 to 0.86 to 0.67 as freeboard height decreased from 0.3 m to 0.15 m to 0.1 m, respectively. 

A similar effect was observed for changes in hood flow rate. With a fixed cross-draft velocity, capture efficiency decreased with decreasing hood flow rate. This effect was much more important when freeboard height was small. Their results showed that when hood flow rate was 1.5 m s m, efficiency remained close to 1.0 as long as the cross-draft velocin. was less than 0.45 in s. The most severe conditions tested were a hood flow rate equal to 0.33 m s" nr- and crossdraft velocity equal to 1.15 m s. Under these conditions, capture efficiency was equal to 0.83 for freeboard hei t equal to 0.3 m, but decreasing to 0.4 when freeboard height was decreased to 0.1 m. 

Equation 4-371 assumes that for each operation a volume of cuttings is put into the reserve pit plus a mud volume equivalent to the circulation system. The additional volumes are attributed to mud dilution and maintenance. An additional contingency is added through 3 ft of freeboard. The reserve pit should be located in an area where capacity may be increased in an emergency or additional fluid trucked away. 

Fluid beds can be fired with gas and oil across the top of the slumped bed since sufficient freeboard exists with coal firing to prevent particle elutriation. Oil, gas or dual-fuel burners so arranged could also provide the means for bed preheating, especially if the flame is redirected down to the fluidization zone. 

Activated carbon filters are employed primarily as RW contaminant removal systems for chlorine (by chemisorption) and various organics such as trihalomethanes (THMs), petroleum products, and pesticides (by adsorption). In addition, they act as physical filters and therefore incorporate sufficient freeboard in their designs to permit periodic backwashing. 

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. 

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