Disengaging zone

The separation of gas-liquid (or vapor-liquid) mixtures can be enhanced by installing a mesh pad at the top of the disengagement zone to coalesce the smaller droplets to larger ones. If this is done, then the KT in Equation 8.3 is normally specified to be 0.11 m s-1, although this can take lower values down to 0.06 m-s-1 for vacuum systems8. 

Above the bed in a fluidized-bed gasifier, the cross-sectional area of the reactor is increased to produce a disengaging zone where the superficial gas velocity is below fluidization velocity.598 This allows the entrained sand particles to fall back down and maintain the bed inventory over time. This larger cross-sectional area zone, or freeboard, can be extended to obtain the total desired gas-phase residence time for complete devolatilization. Eroded bed material or fine char and ash particles that escape the reactor can be collected in a cyclone and either returned to the bed or removed from the system. 

Deposits were collected from the disengagment section and from the horizontal pass. A summary of properties for deposits removed from both locations appears in Table 3. Surface temperatures were controlled to about 500°C for probes in the disengagement zone. Specific deposition rates in the disengagment section are higher for the combustion tests than the gasification tests due to the higher fuel feed rate for gasification with the same total deposit mass. 

Disengagement Zone Cyclone Stack Cyclone Stack Post flare Disengagement Zone Disengagement Zone ... 

Compositions of combustion deposits from probes in the disengagement zone and horizontal pass show that virtually all of the potassium in the deposits can be accounted for as chloride (30%) and sulfate (70%). Although the probe locations employed are not representative of commercial gasifier designs, the collected deposits show K to be present primarily in other forms, either as bed particles or soil minerals adhering to these organic material rich deposits. 

Fig. 9 Two-pass trays 1) capped areas 2) center liquid downcomers 3) side liquid downcomers 4) disengagement zones 5) seal pan and 6) cross-sectional view.

Solids Recovery Equipment. Gas take-off from the surface of the fluid bed is accompanied by entrainment of solids, particularly of the fine and intermediate size range created in part by attrition. A hindered settling or disengaging zone above the bed is provided and the fine particles with low free-fall velocities are carried over into dust-recovery equipment consisting of one or a combination of dry or wet cyclones, multiclones, and electrostatic precipitators. Bag filters are prone to plug but must sometimes be used where expensive solids are involved. Cyclones may be placed within the reactor shell (Fig. 4-4a) or external to it (Fig. 4-4E>), depending on the freedom of the cyclone operation from mechanical troubles. 

Another important feature of a modem fluidized-bed reactor for polyolefin production is the disengaging zone the diameter of the reactor is increased at its topmost part to form the disengaging zone. This expanded cross section of the reactor slows down the upward movement of the polymer particles and prevents or minimizes them from being carried out by the fluidizing gas. Using this design procedure, the circulating gas velocity can be increased for better heat transfer. 

A typical fluidized bed reactor has a length-to-diameter ratio of ca 7 and a disengagement zone at the top. Uniform fluidization is achieved by ethylene flow through a distribution plate at the reactor bottom, and rapid circulation is needed to remove heat. Conversion is about 2% per pass. Unreacted ethylene enters the disengagement zone, separates from the entrained polymer particles, and is Altered, cooled, compressed, and recycled. A catalyst is continuously fed to the reactor without diluent, and polymer particles are continuously removed from the bed through a system of valves. Reactor temperatures of 70-100°C are common, with pressure of 1.4-3.5 MPa (200-500 psig). 

Disengagement zone. Region above the splash zone in which the upward flux and suspension concentration of fine particles decreases with increasing height. 

Dilute-phase transport zone. Region above the disengagement zone in which all particles are carried upwards particle flux and suspension concentration are constant with height. 

The height from the bed surface to the top of the disengagement zone is known as the transport disengagement height (TDH). Above TDH the entrainment flux and concentration of particles is constant. Thus, from the design point of view, in order to gain maximum benefit from the effect of gravity in the freeboard, the gas exit should be placed above the TDH. Many empirical correlations for TDH are available in the literature those of Horio et al. (1980) presented in Equation (7.37)... 

Briens CL, Bergougnou M A, Barton T. Reduction of particle entrainment from gas-fluidized beds. Prediction of the effect of disengagement zones. Powder Technology 62 135-138, 1990. 

Sulfur Removal by Selective Adsorption. The ConocoPhillips S Zorb process uses selective adsorption to remove sulfur from FCC gasoline." The feed is combined with a small amount of hydrogen, heated, and injected into an expanded fluid-bed reactor, where a proprietary sorbent removes sulfur from the feed. A disengaging zone in the reactor removes suspended sorbent from the vapor, which exits the reactor as a low-sulfur stock suitable for gasoline blending. 

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