Dense fluidized beds

Contactive (Direct) Heat Transfer Contactive heat-transfer equipment is so constructed that the particulate burden in solid phase is directly exposed to and permeated by the heating or cooling medium (Sec. 20). The carrier may either heat or cool the solids. A large amount of the industrial heat processing of sohds is effected by this mechanism. Physically, these can be classified into packed beds and various degrees of agitated beds from dilute to dense fluidized beds. 

Fluidized Beds Fluidized beds are reactors in which small particles (with average size below 0.1 mm) are fluidized by the reactant gases or liquids. When the linear velocity is above the minimum required for fluidization, a dense fluidized bed is obtained. As the superficial velocity increases, the bed expands and becomes increasingly dilute. At a high enough linear velocity, the smallest particles entrain from the bed and have to be separated from the exhaust gases and recycled. 

A schematic diagram of the process is shown in Fig. 19.25. Crude methanol is vaporized at the bottom of the reactor and passes through a dense fluidized bed at 775°F and 25 psig. The methanol is converted to hydrocarbons and water. The catalyst is separated from reaction products in a disengager section at the top of the reactor, the reactor... 

Balzer, G., Boelle, A. and Simonin, O. (1995), Eulerian gas-solid flow modeling of dense fluidized bed. Fluidization VIII, Int. Symp. Of the Engineering Foundation, Tours, p. 1125. 

Balzer G, Simonin O (1993) Extensions of Eulerian gas-solid flow modelling to dense fluidized bed prediction. In Proc 5th Int Symp on refined flow modelhng and turbulence measurements, ed Viollet PL, Paris, pp 417-424... 

Most gas-solid systems experience a range of flow regimes as the gas velocity is increased. Several important gas-solid fluidization regimes for the chemical process industry are sketched in Fig 10.1. In dense fluidized beds regions of low solid density may be created. These gas pockets or voids are frequently referred to as bubbles. 

Balzer G, Simonin O (1993) Extension of Eulerian gas-solid flow modeling to dense fluidized beds. Rapport HE-44/93.13, Laboratorie National d Hydraulique, EDE, Chatou, Erance... 

Balzer G, Boelle A, Simonin O (1995) Eulerian gas-solid flow modelling of dense fluidized bed. Eluidization Vlll, International Symposium of the Engineering Eoundation, Tours, 14-19 May, pp 1125-1134... 

Steam or gas injection is required at the bottom of the retort to start the fluidization. However, some or all of this gas may be first used in the ball stripping section. Vapor emitted by retorting adds greatly to the volumetric flow of fluidizing gas as it rises up through the retort. The vessel cross-section is increased accordingly to maintain constant conditions for the dense fluidized bed. 

Parmentier J-F, Simonin O, Delsart O A functional subgrid drift velocity model for filtered drag prediction in dense fluidized bed, AICHE J 58 1084—1098, 2012. 

In DPMs, each particle is tracked individually and all collisions are calculated, thus providing a more reliable and detailed representation of the fluidized bed. The model was introduced by Hogue and Newland (1994), Hoomans et al. (1996), and Tsuji et al. (1992), and it employs either a hard-sphere approach for dilute systems or a soft-sphere approach for dense fluidized beds. 

Dijkhuizen W, Bokkers GA, Deen NG, van Sint Annaland M, Kuipers JAM Extension of PIV for measuring granular temperature field in dense fluidized beds, AIChE J 53 108, 2007. 

The fluidized bed is usually divided into different vertical zones (Fig. 1). At the bottom there is the dense fluidized bed, above which the freeboard is located. The term freeboard denotes the space above the dense fluidized bed. The height between the dense bed surface and the gas outlet will be named here freeboard height /fpB- It could be argued whether it would be better to denote by freeboard height the total distance between the dense bed surface and the top of the vessel. However, most authors prefer the gas outlet as the upper limit of the freeboard, since this latter location determines the carryover of solids. 

The recycle cyclone, if this is used to reduce earbon losses, must be placed at a sufficient elevation above the relatively dense fluidized bed to provide a sufficiently long standleg to balance the circulation loop pressure profile. The bed elutriation will tend to result in recycled fine particles having low bulk density compared to the dense fluidized bed. Fine recycled particles must be injected into the combustion zone effectively to provide additional carbon conversion. 

Three fluidized bed processes are available for license from Mobil Badger (now ExxonMobil-Badger). These are MBR (Mobil benzene reduction), MOG (Mobil olefins to gasoline), and MOI (Mobil olefin interconversion). All three use zeolite catalyst and a dual dense fluidized bed reactor-regenerator system. The MOI process is shown in Fig. 17. The other two processes are conceptually very similar. 

Tai CY, Chien W-C, Chen C-Y. Crystal growth kinetics of calcite in a dense fluidized-bed crystallizer. AIChE J 45 1605-1614, 1999. 

The process uses a dense fluidized bed and the hydrocarbon feed containing the C4-C7 olefins is continuously passed through the bed under conversion conditions in the presence of the catalyst. The catalyst is continuously circulated between the fluidized bed and a regenerator. The fluidizable catalyst can transfer heat from the latter to the former thereby helping to supply some of the thermal needs of the conversion reaction, which is endothermic. The operation of the process is similar to that of conventional FCC. 

The dense fluidized bed conversion conditions include temperature in the range 540 "C to 650 C and pressure from 0.10 to 0.45 MPa, catalyst/oil weight ratio of 0.1 to 10, and a weight hourly space velocity (WHSV) of 1 to 10 h. Because the catalyst used in the process has lower cracking activity relative to conventional FCC catalysts, a higher temperature compared with that for conventional FCC may be used in order to achieve a higher conversion to the desired light olefins. 

The first commercial application of truly fluidized catal)4ic cracking was in 1942 at the Standard Oil Co. of New Jersey s Baton Rouge, LA. refinery. The vaporized oil was cracked in a dense, fluidized bed of catalyst ( bed cracking ). Details of that Model I FCC are given by Avidan et al. (1989). Various hardware improvements were made to bed crackers over the ensuing 30 years, but the advent of Davison s synthetic Y zeolites signalled the demise of bed crackers as state-of-art. 

---