Solid-Gas Separations

Centrifugal Separation Centrifugal force can be utilized to enhance particle collection to several hundredfold that of gravity. The design of cyclone separators for dust removal is treated in detail in Sec. 17 under Gas-Solids Separations, and typical cyclone designs are shown in Fig. 17-43. Dimension ratios for one family of cyclones are given in Fig. 17-36. Cyclones, if carefully designed, can be more efficient on hquids than on solids since liquids coalesce on capture and are easy to drain from the unit. However, some precautions not needed for solid cyclones are necessary to prevent reentrainment. 

James B Dunsotl/ M.S./ Principal Consultant, E. 1. duPont de Nemours h- Co. Member Amencan Institute of Chemical Engineers Registered Professional Engineer. (Delaware) (Gas-Solid Separations)... 

The primary need for gas-solid separation processes is for gas cleaning the removal of dispersed finely divided solids (dust) and liquid mists from gas streams. Process gas streams must often be cleaned up to prevent contamination of catalysts or products, and to avoid damage to equipment, such as compressors. Also, effluent gas streams must be cleaned to comply with air-pollution regulations and for reasons of hygiene, to remove toxic and other hazardous materials see IChemE (1992). 

Cyclones are the principal type of gas-solids separator employing centrifugal force, and are widely used. They are basically simple constructions can be made from a wide range of materials and can be designed for high temperature and pressure operation. 

IChemE (1992) Dust and Fume Control a User Guide, 2nd edn (Institution of Chemical Engineers, London). Jacob, K. and Dhodapkar, S. (1997) Gas-Solid Separations, in Handbook ofSeparation Processes for Chemical Engineers, 3rd edn, Schweitzer, P. A. (ed.) (McGraw-Hill). 

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. 

If gas-liquid and gas-solid separations are dependent on the saturation vapor pressure of the chemical component undergoing equilibration (a) What is the expected effect when the temperature of the system is raised (b) If the system is a gas-liquid system sketch what a plot of log VT vs. 1 IT would look like including when the T is below the freezing point of the stationary phase, (c) Why might it be better to sample the vapor phase above a solution as a sample to determine trace materials in the solution ... 

Dust Separation It is usually necessary to recover the solids carried by the gas leaving the disengaging space or freeboard of the fluidized bed. Generally, cyclones are used to remove the major portion of these solids (see Gas-Solids Separation ). However, in a few cases, usually 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 filters have been employed. Multiple units must be provided so that one unit can be blown back with clean gas while one or more are filtering. 

As cyclones are less effective as the particle size decreases, secondary collection units are frequently required, i.e., filters, electrostatic precipitators, and scrubbers. When dry collection is not required, elimination of cyclones is possible if allowance is made for heavy solids loads in the scrubber (see Gas-Solids Separations see also Sec. 14). 

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