Fluidized catalyst beds formation

The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and ECC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline. 

The three-phase fluidized-bed reactor (ebulliated-bed reactor) differs from suspension reactors in the use of larger catalyst particles (0.1 to 5 mm) and the formation of a well-defined agitated catalyst bed. Whereas suspension reactors can operate in both batch and continuous mode with regard to the liquid phase (and catalyst), the ebulliated-bed reactor only operates in continuous mode, and hence is generally not the appropriate choice for tire production of fine chemicals. 

Control of oxides of nitrogen can be accomplished by catalysts or ab-sorbants, but most control systems have concentrated on changing the combustion process to reduce the formation of NOj. Improved burners, change in burner location, staged combustion, and low-temperature combustion utilizing fluidized-bed systems are all currently in use. These combustion improvement systems do not generate waste products, so no disposal problems exist. 

The highly oxygenated bio oil can be de-oxygenated, and thereby upgraded, over acidic zeolite catalysts through the formation of mainly water at low temperatures and C02 and CO at higher temperatures [1-3], Successful catalytic pyrolysis of woody biomass over Beta zeolites has been performed in a fluidized bed reactor in [4]. A drawback in the use of pure zeolitic materials has been the mechanical strength of the pelletized zeolite particles in the fluidized bed. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

In US Patent 5,569,785 an attrition-resistant zeolite catalyst is described that can be used for the production of methylamines in fluidized bed reactors. The technology claims to provide improved temperature control because of better heat transfer and more efficient solids handling in the fluidized bed. The process also offers more precise temperature control to maintain the activity of the catalyst and eliminate the formation of hot spots that lead to catalyst deactivation. 

We successfully demonstrated that hydrogen could be efficiently produced by catalytic steam reforming of carbohydrate-derived bio-oil fractions in a fluidized bed reactor using a commercial nickel-based catalyst. Greater steam excess than that used for natural gas reforming was necessary to minimize the formation of char and coke (or to gasify these carbonaceous solids) resulting from thermal decomposition of complex carbohydrate-derived compounds. 

Solids of group A have small particle diameters (% 0.1 mm) or low bulk densities this class includes catalysts used, for example, in the fluidized-bed catalytic cracker. As the gas velocity u increases beyond the minimum fluidization point, the bed of such a solid first expands uniformly until bubble formation sets in at u = //mb > mr. The bubbles grow by coalescence but break up again after passing a certain size. At a considerable height above the gas distributor grid, a dynamic equilibrium is formed between bubble growth... 

Cracking is carried out in a fluid bed process as shown in Fig. 7.9. Catalyst particles are mixed with feed and fluidized with steam up-flow in a riser reactor where the reactions occur at around 500°C. The active life of the catalyst is only a few seconds because of deactivation caused by coke formation. The deactivated catalyst particles are separated from the product in a cyclone separator and injected into a separate reactor where they are regenerated with a limited amount of injected air. The regenerated catalyst is mixed with the incoming feed which is preheated by the heat of combustion of the coke. 

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. 

The HTFT process operates with an iron based catalyst at about 350°C with the syngas passing through a fluidized bed of finely divided catalyst. Low temperatures cannot be used as the two phase system (gas and catalyst) would become defluidized by the formation of liquid waxes. At the higher temperature of the HTFT process, the catalyst is much more active than it is in the LTFT process and the hydrocarbon production rate is much higher. 

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