Distributor entrainment

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. 

Another type of distributor, not shown in Fig. 14-64, is the spray nozzle. It is usually not recommended for hquid distribution for two reasons. First, except for small columns, it is difficult to obtain a uniform spray pattern for the packing. The fuU-cone nozzle type is usually used, with the need for a bank of nozzles in larger columns. When there is more than one nozzle, the problem of overlap or underlap arises. A second reason for not using spray nozzles is their tendency toward entrainment by the gas, especially the smaller droplets in the spray size distribution. However, some mass transfer in the spray can be expected. 

The term mist generally refers to liquid droplets from submicron size to about 10 /xm. If the diameter exceeds 10 /xm, the aerosol is usually referred to as a spray or simply as droplets. Mists tend to be spherical because of their surface tension and are usually formed by nucleation and the condensation of vapors (6). Larger droplets are formed by bursting of bubbles, by entrainment from surfaces, by spray nozzles, or by splash-type liquid distributors. The large droplets tend to be elongated relative to their direchon of mohon because of the action of drag forces on the drops. 

A value of Cj greater than 0.8 ft/sec in the distributor openings is likely to flood the distribtitor, or result in hea 7 entrainment. 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

Vapor-Liquid Gravity Separator Design Fundamentals The critical factors in the performance of a horizontal separator are the vapor residence time and the settling rate of the liquid droplets. However, two other factors enter into the design—the vapor velocity must be limited to avoid liquid entrainment, and there must be sufficient freeboard within the vessel to allow for a feed distributor. For vertical separators, the design is based on a vapor velocity that must be less than the settling velocity of the smallest droplet that is to be collected, with due allowance for turbulence and maldistribution of the feed. The vapor residence time is a function of the vapor flow rate (mass), vapor density, and volume of vapor space in the separator, based on the following ... 

To discern the stability of the clusters in the fluidized bed, Hays et al. [26] added baffles to their 6 inch (0.15 meter) diameter fluidized bed. The baffles resembled a simple grating commonly used for floor decking, and were positioned at 1.6 and 2.5 feet (0.5 and 0.76 meters) above the distributor plate. The bed height was 2.5 feet (0.76 meters). As shown in Figure 11.10, the presence of baffles resulted in an increase in the entrainment flux at higher gas velocities. One explanation for this behavior is that the clusters formed in the bed impact the baffles at the high gas velocities and are broken up, which results in smaller clusters and higher entraimnent rates. 

Losses of amine up the stack are undetectable in the CANSOLV SO2 Scrubbing System. Losses from evaporation do not occur because the amine is in salt form in solution, and losses from entrainment are avoided by using trough type distributors instead of spray nozzles to distribute the amine over the packing. In extreme cases where upsets in gas flow are anticipated, a demister may also be used. 

Shallow beds are easier to maintain in stable fluidization and of course exert a smaller load on the air blower. Pressure drop in the air distributor is approximately 1 psi and that through the bed equals the weight of the bed per unit cross section. Some pressure drop data are shown in Table 9.14. The cross section is determined by the gas velocity needed for fluidization as will be described. It is usual to allow 3-6 ft of clear height between the top of the bed and the air exhaust duct. Fines that are entrained are collected in a cyclone and blended with the main stream since they are very dry... 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

---