Day31 - CE 561 Lecture Notes Fall 2009 Day 31 A bit about...

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CE 561 Lecture Notes Fall 2009 p. 1 of 17 Day 31: A bit about fluidized bed reactors Note: This set of notes is adapted, with minimal alterations, from the CD-ROM that accompanies ‘Elements of Chemical Reaction Engineering’ by H. Scott Fogler, who seems to have relied heavily on the work of Levenspiel. The fluidized bed heterogeneous catalytic reactor, in a variety of configurations, is one of the most widely used reactor types in the petrochemical industry. It is substantially more complex than the fixed bed reactors that we have just been considering. However, it has several advantages over fixed bed reactors that have allowed it to become the dominant reactor type for a large number of processes, the most important of which is fluidized catalytic cracking of gasoil to produce gasoline. Two key features of the fluidized bed reactor compared to the fixed bed reactor are rapid heat transfer by the moving solids (which helps to overcome heat removal and addition limitations found in fixed bed reactors) and the ability to use very small catalyst particles (which help to avoid internal mass transfer limitations found in the large catalyst pellets used in fixed bed reactors). Fluidization occurs when small solid particles are suspended in an upward-flowing stream of fluid, as shown in the figure below. The fluid velocity is sufficient to suspend the particles, but not large enough to carry them out of the vessel. The solid particles swirl around the bed rapidly, creating excellent mixing among them. The material fluidized is almost always a solid and the fluidizing medium is usually a gas, but may occasionally be a liquid. The characteristics and behavior of a fluidized bed are strongly dependent on both the solid and fluid properties. Nearly all of the significant commercial applications of fluidized-bed technology concern gas-solid systems, so these are treated here. The material that follows is based on what is seemingly the best model of the fluidized-bed reactor developed thus far--the bubbling-bed model of Kunii and Levenspiel. From D. Kunii and O. Levenspiel, Fluidization Engineering (Melbourne, Fla.: Robert E. Krieger Publishing Co., 1969).
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CE 561 Lecture Notes Fall 2009 p. 2 of 17 We are going to use the Kunii-Levenspiel bubbling-bed model to describe reactions in fluidized beds. In this model the reactant gas enters the bottom of the bed and flows up the reactor in the form of bubbles. As the bubbles rise, mass transfer of the reactant gases takes place as they flow (diffuse) in and out of the bubble to contact the solid particles, where the reaction product is formed. The product then flows back into a bubble and finally exits the bed when the bubble reaches the top of the bed. The rate at which the reactants and products transfer in and out of the bubble affects the conversion, as does the time it takes for the bubble to pass through the bed. Consequently, we need to describe the velocity at which the bubbles move through the column and the rate of transport of gases in and out of the bubbles. To calculate these parameters
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