Gas-solids separators cyclones

K.T. Scmrau, Gas solid separation—cyclones, in Chemical Engineers Handbook, McGraw-Hill, New Yoik, 1984, pp. 20.77-20.121. 

M. Pell, and J.B. Dunson, Gas-solid separation-cyclones. Chemical Engineers Handbook, 7th ed., McGraw-Hill, New York, 1999, pp. 17,26-17,32. 

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. 

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. 

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. 

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). 

The cyclone separates solid particles from the outlet gas. Several cyclones may be combined to form a multistage cyclone system. More than one multistage cyclone system can be placed inside or outside the bed. Other types of gas-solid separators (e.g., filters) are also used for gas-solid separation although the cyclone is most widely adopted in fluidized beds. The dipleg/standpipe returns the particles separated by the cyclone into the dense bed. The outlet of a dipleg may be located in the freeboard or immersed in the dense bed. 

The entrance and exit geometries have significant effects on the gas and solid flow behavior in the riser. The efficiency of the gas-solid separator can affect the particle size distribution and solids circulation rate in the system. In a CFB system, particle separation is typically achieved by cyclones (see Chapter 7). The downcomer provides hold volume... 

The load characteristic of circulating fluidized bed boilers is closely related to the gas-solid flux through the combustor. Thus, choice of coal size and the proper gas-solid separator deserves attention. Generally, the fraction of particles below 0.1 mm is smaller than 10%, which could be removed only by a cyclone separator, while the major fraction of coarser particles could be easily separated by inertia or gravity (Li and Kwauk, 1981). The diagram of ash balance for a circulating fluidized bed boiler, shown in Fig. 22, can... 

Based on the experience of these 75 t/h boilers, a 220 t/h CFBC boiler has been designed and is now being fabricated. A two-stage channel separator, as shown in Fig. 39, is used on these CFBC boilers. This is followed by multi-cyclones. By using this combination of gas-solid separators, and with fly ash reburn, the combustion efficiency has reached 97.5% for a 12,000-kJ/kg low-grade coal. 

If the product in no way adheres to the dryer parts and simple cyclone collectors are sufficient for gas-solids separation, batch operation of a spray dryer may be considered. Otherwise, the time and costs for cleaning the large equipment parts make them rarely economical for other than continuous processing of a single material. 

The DWSA installation can be divided into two main parts. The first part consists of an air preheater, fluidised bed reactor, solid fuel dosing vessel with on-line mass determination system and a hot gas cleaning section, containing a cyclone and a ceramic candle filter (Schumacher type). In the fluidised bed reactor the solid fuel is gasified with air to produce a low calorific value (LCV) gas that is cleaned of fly ash and unreacted solid carbonaceous material. Air and also additional nitrogen can be preheated and is introduced into the reactor by four nozzles just above the distributor plate. The reactor is electrically heated in order to maintain a constant temperature over bed as well as freeboard section. The solid fuel is fed into the bed section in the bottom part Just above the distributor by a screw feeder from beside. The hot gas cleaning section ensures a good gas-solid separation efficiency, with filter temperatures of about 500 C. 

Entrainment Most fluidized bed reactors employ one or more cyclone, either inside the freeboard region at the top of the vessel or located externally, to capture entrained solids that are then returned continuously to the base of the fluidized bed via a standpipe and a mechanical (e.g., slide) valve or aerated nonmechanical valve (see Ref. ° for details of solid return systems). A flapper gate, acting as a check valve, is commonly employed to prevent backflow of gas up the standpipe. While cyclones are by far the most popular, other gas-solid separators like impingement separators, electrostatic precipitators, filters, and scrubbers are sometimes provided, especially as second- or third stage separators. 

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