Catalysts for Fluidized Beds

Attrition-resistant microspheroidal catalyst particles for fluidized beds are manufactured by means of spray drying. The catalyst particles, for instance zeolite crystals, are suspended in an aqueous sol or hydrogel of binder particles that, after processing, serve as a mechanically strong and porous matrix. Typical binders are silica-alumina and alumina gels, clays (e.g. kaolin, bentonite), and 

Diameter and size distribution of the droplets strongly depend on the design of the spraying device, the so-called atomizer. Typical devices are high-pressure nozzles, gas-liquid nozzles, and high-speed centrifugal disks. 

Pietsch, W. (2002) A omeration Processes, Wiley-VGH Verlag GmbH, Weinheim. 

Maurer, T. (2004) Investigation of Mass Transport Phenomena in the Conversion of Methanol to Olefins over Technical Alumina/ZSM-5 Catalysts. Dissertation, Shaker, Aachen, University of Karlsruhe. 

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Catalyst composition also depends on the type of reactor used. Fixed-bed iron catalysts are prepared by precipitation and have a high surface area. A silica support is commonly used with added alumina to prevent sintering. Catalysts for fluidized-bed application must be more attrition-resistant. Iron catalysts produced by fusion best satisfy this requirement. The resulting catalyst has a low specific surface area, requiring higher operating temperature. Copper, another additive used in the preparation of precipitated iron catalysts, does not affect product selectivity, but enhances the reducibility of iron. Lower reduction temperature is beneficial in that it causes less sintering. 

All catalysts, operated either in laboratory or conmiercially, are deactivated during their use. Deactivation is very important in commercial operation because it influences the choice of the operational conditions and fixes the cycle length between regenerations and the total life of the catalyst. Some catalysts remain active for a decade (catalysts for oxidation of SO2 and for ammonia synthesis) whereas others must be regenerated after a few minutes of operation (catalysts for fluidized bed hydrocarbon cracking). 

Minimum fluidization velocities for spherical particles in air are shown in Figure 9.2. Equation (9.2) applies for particles up to about 300 microns in size, which includes most fluidized catalysts. For fluidized-bed combustion or metallurgical processes, the particles are much larger, and Eq. (9.1) must be used. For very large sizes, the laminar-flow term in Eq. (9.1) becomes unimportant, and varies with the square root of dp ... 

Catalysts for fluidized-bed reactors have to be spherical as well. The appropriate particle size fraction for gas-solid systems can be estimated after Geldart [1] from the density difference between soKd and gas. Most widely used catalysts for fluidized beds and risers are Geldart-type B powders with particle diameters ranging from 40 to 500 pm or solid densities between 1.4 X 10 and 4 x 10 kg/m, respectively. When fluidization is provided by a Kquid as in ebullated-bed reactors, the particle sizes may be substantially larger because of the higher buoyancy in these systems. However, all types of fluidized-bed catalysts must exhibit high mechanical stability because they are exposed to abrasion on reactor walls and internals, collisions between particles and shear forces exerted by the surrounding fluid. 

Catalysts for fluidized bed gasification of biomass should be efficient for the reforming of hydrocarbons and have high selectivity for syngas and high resistance to attrition and carbon deposition. They should also be relatively low cost, because the formation of ash and char necessitates the continual removal, and replenishment with fresh or regenerated material, of the bed inventory. In dual fluidized bed gasifiers, the inventory is also exposed to... 

Contractor RM, Bergna HE, Chowdhry U, Sleight AW. Attrition resistant catalysts for fluidized bed-systems. In Grace JR, Shenult LW, Bergougnou MA, eds. Fluidization VI. New York Engineering Foundation, 1989, pp 589 596. 

Iron-based catalysts are used in both LTFT and HTFT process mode. Precipitated iron catalysts, used in fixed-bed or slurry reactors for the production of waxes, are prepared by precipitation and have a high surface area. A sihca support is commonly used with added alumina to prevent sintering. HTFT catalysts for fluidized bed apphcations must be more resistant to attrition. Fused iron catalysts, prepared by fusion, satisfy this requirement (Olah and Molnar, 2003). For both types of iron-based catalysts, the basicity of the surface is of vital importance. The probability of chain growth increases with alkali promotion in the order Li, Na, K, and Rb (Dry, 2002), as alkalis tend to increase the strength of CO chemisorption and enhance its decomposition to C and O atoms. Due to the high price o Rb, K is used in practice as a promoter for iron catalysts. Copper is also typically added to enhance the reduction of iron oxide to metallic iron during the catalyst pretreatment step (Adesina, 1996). Under steady state FT conditions, the Fe catalyst consists of a mixture of iron carbides and reoxidized Fe304 phase, active for the WGS reaction (Adesina, 1996 Davis, 2003). 

For fluidized bed reactors, special water-soluble, combustion catalyst and slag modifier combination products are available. These products are diluted and typically are fed at rates of from 20 to 60 lb per day for every 10 tons of bed extraction. 

A mechanistic model for propane steam reforming on a bimetallic Co-Ni catalyst in fluidized bed reactor... 

One example of this type of reactor is in the synthesis of catalyst powders and pellets by growing porous soHd oxides from supersaturated solution. Here the growth conditions control the porosity and pore diameter and tortuosity, factors that we have seen are crucial in designing optimal catalysts for packed bed, fluidized bed, or slurry reactors. 

The development of zeolite-containing catalysts has led to the development of binders. Modem catalyst technology (especially for fluidized-bed catalytic cracking and hydrocracking) selects binders which may have a variety of properties of their own (catalytic, trapping of poisons, etc.)... 

Using fixed dolomite guard beds to lower the input tar concentration can extend Ni catalyst lifetimes. Adding various promoters and support modifiers has been demonstrated to improve catalyst lifetime by reducing catalyst deactivation by coke formation, sulfur and chlorine poisoning, and sintering. Several novel, Ni-based catalyst formulations have been developed that show excellent tar reforming activity, improved mechanical properties for fluidized-bed applications, and enhanced lifetimes. 

In some situations the dynamics of the cooling system may be such that effective temperature control cannot be accomplished by manipulation of the coolant side. This could be the situation for fluidized beds using air coolers to cool the recirculating gases or for jacketed CSTRs with thick reactor walls. The solution to this problem is to balance the rate of heat generation with the net rate of removal by adjusting a reactant concentration or the catalyst flow. Such a scheme is shown in Fig. 4.24. 

IV. Instrumentation for UV Raman Characterization of Working Catalysts The Fluidized-Bed Reactor... 

Currently available data for the flow properties of the fluidized catalyst bed are fragmentary, since the local motion of the emulsion phase is diflicult to measure experimentally. Therefore, it is useful to clarify the flow properties of the bed in terms of our knowledge of bubble columns. First, the fluid-dynamic properties of the bubble columns will be explained then, the available data will be adapted to apply to fluid catalyst beds. The reader will be able to picture an emulsion phase of carefully prepared catalyst particles operating in intense turbulence for fluidized beds under conditions of practical interest. This turbulence distinguishes the flow properties of fluid catalyst beds from those of widely studied teeter beds. 

Figure 45 illustrates the plot of (f/c f mf)/cb versus (C/g Umi) according to Eq. (5-8) for fluidized beds. In the figure, Eq. (5-4) is also shown for slugging beds. The mean gas holdup for the FCC-catalyst bed is taken from Fig. 36. The plot for the FCC bed shows clearly that the bed is fluidized smoothly, without slugging. The plot also shows data for a bed of fluidized glass beads of mean diameter of 287 m. (Data are taken from W13 cf. Fig. 14). The bed behavior is seen to approach that of the slugging bed as(f/c f/mf) increases beyond 20cm/sec. Also, the averagers is approximately 0.8 for Uq < 20 cm/sec, showing the possibility of bulk recirculation of the emulsion.

Fig. 45. (Ua U i)/h as a function of (I/q I4it) for fluidized beds. FCC-catalyst beds show no indication of slugging.

Scientific approaches to improve bed fluidity are potentially important for fluidized bed technology. Also, further quantitative relations between bubble splitting and bed properties would be very helpful in planning and scaling-up fluidized catalyst beds. 

Wheeler has summarized the work on internal diffusion for catalytic cracking of gas-oil. At 500°C the rate data for fixed-bed operation, with relatively large ( -in.) catalyst particles and that for fluidized-bed reactors (very small particle size) are about the same. This suggests that the effectiveness factor for the large particles is high. Confirm this by estimating rj for the -in. catalyst if the... 

Highly acidic natural clays, montmorillonite are complex layers of S1O4 and AIO4 tetrahedra. They also contain small amounts of MgO and Fe20j. These impurities are leached with sulfuric aid, which also adds protons to increase maximum pK values from -3.0 to -8.2. These clays were the first cracking catalysts used with fixed and moving beds. However, they were quickly replaced by the superior synthetic silica-aluminas that were ideal for fluidized beds. Today, they are used as the matrix in zeolite-based cracking catalyst. 

Xu, C., Zhu, J. 2004. One-step prep>aration of highly dispersed metal-supported catalysts by fluidized-bed MOCVD for carbon nanotube synthesis. Nanotechnology 15,1671-1681. 

Vapor Phase Hydrogenations of Nitro Compounds. Catalytic hydrogenation of nitro compounds can be carried out in the vapor phase, provided the boiling point of the compound is low enough and the material is thermally stable. These two conditions effectively limit this process to aliphatic and relatively simple aromatic nitro compounds such as nitrobenzene or nitroxylene. Eady vapor-phase hydrogenation processes used fixed-bed catalysts. However, fluidized-bed catalytic vapor-phase hydrogenations, such as the one illustrated by the process for aniline, have become more common (see Amines, aromatic, aniline and derivatives). 

Partially because of the backmixing behavior and partially because of the elEciency of contact between fluid- and catalyst-phases, fluidized beds are less efficient than fixed beds, at least in terms of the amount of catalyst required to attain a given conversion. Although plug flow seems reasonable for the motion of the bubbles, particularly in the Geldhart A-A regions, bubble-emulsion interchange. 

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