Fluidization transport reactor

Whenever the difiusional limitation is broken through the use of fine catalyst powder in a bubbling fluidized bed, a new limitation arises related to the hydrodynamics of the system. In the bubbling fluidized bed, it is not possible to fully exploit the very intrinsic kinetics of the powdered catalyst. Fast fluidization (transport) reactor configuration offers excellent potential to break this limitation. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Sasol uses both fixed-bed reactors and transported fluidized-bed reactors to convert synthesis gas to hydrocarbons. The multitubular, water-cooled fixed-bed reactors were designed by Lurgi and Ruhrchemie, whereas the newer fluidized-bed reactors scaled up from a pilot unit by Kellogg are now known as Sasol Synthol reactors. The two reactor types use different iron-based catalysts and give different product distributions. 

FIG. 23-24 Reactors with moving catalysts, a) Transport fluidized type for the Sasol Fischer-Tropsch process, nonregenerating, (h) Esso type of stable fluidized bed reactor/regeuerator for cracldug petroleum oils, (c) UOP reformer with moving bed of platinum catalyst and continuous regeneration of a controlled quantity of catalyst, (d) Flow distribution in a fluidized bed the catalyst rains through the bubbles. 

Oxco [Oxidative coupling] A process for converting natural gas to transport fuels and chemicals, based on the oxidative coupling of methane to ethane in a fluidized-bed reactor. Developed in Australia by the Division of Coal and Energy Technology, CSIRO, and BHP. 

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

Figure 23.2 depicts some of the essential features of (a) fluidized-bed, (b) fast-fluidized-bed, and (c) pneumatic-transport reactors (after Yates, 1983, p. 35). 

Figure 23.2 Some features of (a) a fluidized-bed reactor (b) a fast-fluidized-bed reactor and (c) a pneumatic-transport reactor...

Co/ Fe mixed oxide, while in the second one, a fluidized-bed reactor, the catalyst that in the first step has been reduced by the olefin is reoxidized with air. The catalyst is continuously transported from the first reactor to the regenerator, by means of a C FB R. 

The second reaction vessel in a catalytic cracker is called the regenerator. The solid catalyst from the reactor is combined with a compressed air stream from an air blower, and the solid and gas phases flow upward into a bed of fluidized solid catalyst. The early designs used a bubbling bed reactor in which the velocity in the bed is slightly above the minimum fluidization velocity. More recent designs use a transport fluidized-bed reactor. A typical air-to-oil weight ratio is 0.54. 

Modification of silica gel with volatile or gaseous compounds is performed in the vapour phase. Industrial-scale reactors and laboratory scale gas adsorption apparatus have been used. In the industrial field, fluidized bed and fluid mill reactors are of main importance. In fluidized bed reactors,82 the particles undergo constant agitation due to a turbulent gas stream. Therefore, temperatures are uniform and easy to control. Reagents are introduced in the system as gases. Mass transport in the gas phase is much faster than in solution. Furthermore, gaseous phase separations require fewer procedural steps than solution phase procedures, and may also be more cost-effective, due to independence from the use and disposal of non-aqueous solvents. All these advantages make the fluidized bed reactors preferential for controlled-process industrial modifications. 

The catalyst support may either be inert or play a role in catalysis. Supports typically have a high internal surface area. Special shapes (e.g., trilobed particles) are often used to maximize the geometric surface area of the catalyst per reactor volume (and thereby increase the reaction rate per unit volume for diffusion-limited reactions) or to minimize pressure drop. Smaller particles may be used instead of shaped catalysts however, the pressure drop increases and compressor costs become an issue. For fixed beds, the catalyst size range is 1 to 5 mm (0.04 to 0.197 in). In reactors where pressure drop is not an issue, such as fluidized and transport reactors, particle diameters can average less than 0.1 mm (0.0039 in). Smaller particles improve fluidization however, they are entrained and have to be recovered. In slurry beds the diameters can be from about 1.0 mm (0.039 in) down to 10 Jim or less. 

Transport fluidized bed reactor is used for the Sasol Fischer-Tropsch process (Fig. 19-23a). 

Advantages of transport reactors include low gas and solid backmix-ing (compared to fluidized beds) and the ability to continuously remove deactivated catalyst (and add fresh catalyst), thereby maintaining catalyst activity. The fluid and catalyst are separated downstream by using settlers, cyclones, or filters. 

Courutier, M. F., Karidio, I., and Steward, F. R. A Study on the Rate of Breakage of Various Canadian Limestones in a Circulating Transport Reactor, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 788-793. Somerset, Pennsylvania (1993). 

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