Catalyzed fluidized beds

Special attention has to be paid to a definition of attrition rates in the case of continuous processes where fresh solid material is continuously added. This is particularly the case in heterogeneously catalyzed fluidized bed processes where fresh make-up catalyst must be added to compensate for attrition losses. The fresh catalyst may contain elutriable fines which add to the measurable elutriation rate thus leading to an apparently higher attrition rate. 

Outline a methodology for the selection of the most plausible model for the solid catalyzed fluidized-bed reactor design. 

Gas-phase reactions catalyzed by solid catalysts are normally carried out in gas-particle operation in either fixed or fluidized beds. The possibility of using gas-liquid-particle operations for such reactions is, however, of interest in certain cases, particularly if the presence of a liquid medium for the transfer of heat or mass is desirable. 

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

A fluidized-bed reactor for a solid-catalyzed gas-phase reaction (of A) operates with relatively small catalyst particles. Based on the information given below, calculate the follow-... 

Pass gas upward through a bed of fine particles. For superficial (or inlet) gas velocities much in excess of this minimum the bed takes on the appearance of a boiling liquid with large bubbles rising rapidly through the bed. In this state we have the bubbling fluidized bed, BFB. Industrial reactors particularly for solid catalyzed gas-phase reactions often operate as bubbling beds with gas velocities Wq 5 30 u. ... 

In this section, some analytical solutions of fluidized-bed models are presented. Specifically, model solutions will be given for the case of a gas-phase reactant and a single solid-catalyzed reaction of the form A —> products and bubbling fluidized bed (Type B fluidization). The same analysis holds for a reaction of the form A + B —> products, if the reaction depends only on the concentration of A. Some solutions for the cases of a single reversible reaction, for two reactions in parallel, and two reactions in series will be given as well. 

In the presence of hydrogen and carbon monoxide, the surface catalyzed reduction of "NO" controlled the overall "NO" destruction. Thus the presence of hydrogen and carbon monoxide decreased the corn-sumption of carbon to nearly zero. The rate was significantly enhanced by hydrogen over the temperature range employed for the fluidized bed combustor. 

A typical fluidized-bed reactor system consists of biomass preparation (i.e. drying and grinding), a biomass feeding system, a fluidized-bed pyrolyzer, a char removal system, and a cooling system for bio-oil collection. Char removal from the pyrolysis reactor is beneficial since the char can catalyze vapor cracking, thereby decreasing the bio-oil yield [11],... 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

Description The exothermic reaction is catalyzed by a copper chloride catalyst in a single-step, fluidized-bed reactor at temperatures of 220°C. Heat of reaction is recovered by producing 10 bar g steam or heating other heat-transfer fluids. 

Two types of bioreactors are commonly used in industry for performing enzyme-catalyzed transformations the stirred tank reactor and the packed or fluidized bed reactor [28]. Substitution of these weU-established reactors by a system based on microfluidic technology wiU happen only if there are clear and compelling advantages in so doing. It is therefore necessary to consider, in a rigorous and... 

Fluidized bed reactors have received increased interest in recent years owing to their application in coal gasification. The section on fluidized beds discusses critical areas in fluid bed reactor modeling. Computer simulation of both solid-catalyzed gas phase reactions as well as gas-solid reactions are included. 

The preparation of compounds with Si-N bonds, especially silazanes, without the intermediacy of chlorosilicon compounds has been difficult until quite recently. As part of a fundamental investigation of the reactions of silicon metal with simple small molecules, a new direct reaction of silicon was discovered (29). Silicon reacts with dimethylamine in a fluidized bed reactor to give Tris in excellent yield. The reaction must be catalyzed by copper. 

Fluidized beds, non-catalyzed solids, 595 Foam separation, 635 data, 644 equipment, 644... 

These acid-catalyzed changes are brought about by passing hot gas oil vapors either through a mechanically circulated bed of the larger pellets or upward through a fluidized bed of more finely divided catalyst particles. The upflow of vapors provides the fluidizing action of the catalyst, hence the units are referred to as fluidized cat crackers or FCC units. To avoid loss of... 

This model represents one of the first modeling approaches used to describe large industrial beds that has been fully documented and discussed in the open literature. The model was adopted by the Shell company designing a fluidized bed reactor for their solid catalyzed hydrogen chloride oxygen process. However, the model has many of the same limitations as the Davidson-Harrison model because is was developed before the importance of bubble clouds and wakes were realized. 

The solid phase could be a reactant, product, or catalyst. In general the decision on the choice of the particle size rests on an analysis of the extra-and intra-particle transport processes and chemical reaction. For solid-catalyzed reactions, an important consideration in the choice of the particle size is the desire to utilize the catalyst particle most effectively. This would require choosing a particle size such that the generalized Thiele modulus < gen, representing the ratio of characteristic intraparticle diffusion and reaction times, has a value smaller than 0.4 see Fig. 13. Such an effectiveness factor-Thiele modulus analysis may suggest particle sizes too small for use in packed bed operation. The choice is then either to consider fluidized bed operation, or to used shaped catalysts (e.g., spoked wheels, grooved cylinders, star-shaped extrudates, four-leafed clover, etc.). Another commonly used procedure for overcoming the problem of diffu-sional limitations is to have nonuniform distribution of active components (e.g., precious metals) within the catalyst particle. 

Packed and fluidized beds are commonly used in industrial mass-transfer operations, including adsorption, ion exchange, chromatography, and gaseous reactions that are catalyzed by solid surfaces because they offer a dramatical increase in the sur-... 

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