Fluidized catalyst beds stability

Heat and mass transfer constitute fundamentally important transport properties for design of a fluidized catalyst bed. Intense mixing of emulsion phase with a large heat capacity results in uniform temperature at a level determined by the balance between the rates of heat generation from reaction and heat removal through wall heat transfer, and by the heat capacity of feed gas. However, thermal stability of the dilute phase depends also on the heat-diffusive power of the phase (Section IX). The mechanism by which a reactant gas is transferred from the bubble phase to the emulsion phase is part of the basic information needed to formulate the design equation for the bed (Sections VII-IX). These properties are closely related to the flow behavior of the bed (Sections II-V) and to the bubble dynamics. 

A small-scale fluidized-catalyst test unit has been described by McReynolds (25). A standard gas-oil is vaporized and passed through a fluidized catalyst bed (400 g. of powdered catalyst) at 920°F. and at a rate of 925 ml. (800 g.) of liquid per hour for thirty minutes. The product is condensed, stabilized, and fractionated to 400°F. end-point gasoline. The activity of the catalyst is reported in terms of total conversion and per cent D + L."... 

The major difficulties with these processes are controlling heat removal from the reactor the stability of the catalyst, both mechanical and chemical and catalyst loss. The latter two problems are due to the use of the fluidized bed reactor. Yields of acrylonitrile from this process are about 70%, based on propylene feed. 

Graham LJ, Jovanovic G. Dechlorination of p-chlorophenol on a Pd/Fe catalyst in a magnetically stabilized fluidized bed Implications for sludge and liquid remediation. Chem Eng Sci 1999 54 3085-3093. 

In addition to the requirements with respect to size, shape, and mechanical stability, the nature of the active phase also has to be adopted when the same catalyst is applied in different reactor concepts mainly due to differing process conditions. Vanadium phosphorous oxide composed of the vanadyl pyrophosphate phase (VO)2P207 is an excellent catalyst for selective oxidation of H-butane to maleic anhydride [44-47]. This type of catalyst has been operated in, for example, fixed-bed reactors and fluidized-bed-riser reactors [48]. In the different reactor types, different feedstock is applied, the feed being more rich in //-butane (i.e. more reducible) in the riser-reactor technology, which requires different catalyst characteristics [49]. 

The use of a fluidized-bed reactor has a number of advantages in the MTO process. The moving bed of catalyst allows the continuous movement of a portion of used catalyst to a separate regeneration vessel for removal of coke deposits by burning with air. Thus, a constant catalyst activity and product composition can be maintained in the MTO reactor. Figure 12.10 demonstrates the stability of a 90 day operation in the fluidized-bed MTO demonstration unit at the Norsk Hydro Research Center in Porsgrunn, Norway. A fluidized-bed reactor also allows for... 

Y. H. Lin, M. H. Yang, T. F. Yeh and M. D. Ger Catalytic Degradation of High Density PolyethyleneOver Mesoporous and Microporous Catalysts in a Fluidized-Bed Reactor Polym. Degrad. Stabil., 86, 121 (2004). 

Traces of water vapor in the gas stream tend to hydrolyze Cr-O-Si linkages, which tends to destabilize surface Cr(VI) [74,75]. Free Cr03 decomposes at a temperature of about 200 °C. Therefore, the purity of the fluidization gas used during the calcination of the catalyst can be important. Because the catalyst itself evolves moisture as silanol groups condense, the rate of temperature rise can also influence the stability of Cr (VI), because a rapid temperature rise results in a gas stream with a higher water concentration. Similarly, the bed depth during fluidized-bed activation can determine moisture concentrations and therefore control Cr(VI) levels. Much commercial art has developed around these principles to achieve a compromise between catalyst quality and catalyst production rate (Section 20). 

The ammoxidation reaction can, on the other hand, be performed continuously in fixed-bed and fluid-bed reactors, and by-products (particularly CO2) can be easily removed. The fluidized bed has some advantages in terms of heat transfer but demands are made on the mechanical durability of the catalyst and so catalyst choice is limited. The long-term stability of the catalysts is also important and so multicomponent systems are recommended [e.g. 1,12]. The separation of the nitrile formed can be achieved by condensation, centrifugation, filtration, or rectification. Sometimes the formation of hazardous by-products (HCN, CO) must be considered. 

Ostrowski et al. [254] analyze the catalytic partial oxidation of methane at 700°C-750°C in a Ni/a-Al203 catalyst enclosed as fixed bed or fluidized bed in a membrane reactor. The SIL-1 membranes employed exhibited over more than 100 h of operation high thermal and mechanical stability. In both cases, the separation selectivity was... 

Regarding the economical viability of the plant, the retention and stability of acylase are essential features for the process. An ultrafiltration unit retains acylase as the mobile catalyst in the reactor. Alternatively, acylase can be immobilized in a fixed or fluidized bed. A mobile catalyst system is preferred compared to the immobilized form, as the mobile catalyst system avoids mass-transfer limitations. Additionally, regeneration of the catalyst and scale-up of the reactor are much easier as compared to the process with the immobilized acylase. With respect to the deactivation of the catalyst, the thermal as well as the operational stability of acylase has been evaluated extensively [128, 129]. At a pH of 7, acylase appears to be sufficiently stable for L-amino acid manufacture. 

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. 

Fluidized bed reactors allow improved heat management for fast exothermic reactions thus increasing the performance of gas-solid reactors, but within narrow operating windows. Fluidized bed reactors impose special demands on the mechanical stability of the catalyst and are difficult to scale-up. 

A new preparation method is described to synthesize porous silicon carbide. It comprises the catalytic conversion of preformed activated carbon (extrudates or granulates) by reacting it with hydrogen and silicon tetrachloride. The influence of crucial convoaion parameters on support properties is discussed for the SiC synthesis in a ftxed bed and fluidized bed chemical vapour deposition reactor. The surface area of the obtained SiC ranges ftiom 30 to 80 m /g. The metal support interaction (MSI) and metal support stability (MSS) of Ni/SiC catalysts are compared with that of conventional catalyst supports by temperature programmed reduction. It is shown that a Ni/SiC catalyst shows a considnable Iowa- MSI than Ni/Si(>2- and Ni/Al203-catalysts. A substantially improved MSS is observed an easily reducible nickel species is retained on the SiC surface after calcination at 1273 K. 

Surface properties of the support material influence the adsorption states of Pt(acac)2, Cr(acac)3 and V(acac)3 as well as the decomposition pathways of the adsorbates and finally the dispersion of the catalytic compound. The deposited particles are more mobile on silica and hence, more capable to agglomerate. Alumina-supported Pt may be stabilized by coordinativdy unsaturated Al " " surface ions similar arguments may apply for the stabilization of amorphous Cr203 on alumina. Because the metal acetyl acetonate decomposition is accompanied by deposition of carbonaceous compounds an additional air treatment of the samples is required. Finally, the fluidized-bed technique has been proven to be applicable for preparation of catalyst particles of uniform dispersion of the catalytic compound throughout the whole bed of particles. 

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