Fluidized catalyst beds steady reaction

First, it is of common interest to unsteady processes and their models. Chemical unsteadiness must be taken into account in many cases. For example, studies with variations in catalyst activity, calculations of fluidized catalyst bed processes (when the catalyst grain "is shaking in a flow of the reaction mixture and has no time to attain its steady state), analyses of relaxational non-stationary processes and problems of control. Unsteady state technology is currently under development [14,15], i.e. the technology involving programmed variation of the process parameters (temperature, flow rate, concentration). The development of this technology is impossible without distinct interpretation of the unsteady reaction behaviour. 

Consider the reaction A - B taking place in the dense, or particulate, phase of a bubbling bed of fluidized catalyst particles (Fig. 10). It is in steady opera-... 

Later, Furusaki et al. (F17) studied the hydrogenation of ethylene by fluidized Ni catalyst to obtain the axial reactivity distribution. Here the samples of bed gas were removed by a traveling sampler placed at the center of the bed during steady reaction, so that the sample taken in the dense phase shows an average of the concentration in the bubble and emulsion phase. Figure 74 shows an example of the axial concentration profile. 

A theory has been developed which translates observed coke-conversion selectivity, or dynamic activity, from widely-used MAT or fixed fluidized bed laboratory catalyst characterization tests to steady state risers. The analysis accounts for nonsteady state reactor operation and poor gas-phase hydrodynamics typical of small fluid bed reactors as well as the nonisothermal nature of the MAT test. Variations in catalyst type (e.g. REY versus USY) are accounted for by postulating different coke deactivation rates, activation energies and heats of reaction. For accurate translation, these parameters must be determined from independent experiments. 

Equipment. A vertical fixed-bed reactor, made of a 0.168 m I.D. and 0.5 m long 316 stainless steel tube with an axial thermowell, was used. The amounts of catalyst used for the steady state and dynamic experiments were 6.35 and 18.69 g, respectively. The reactor tube was heated by a fluidized bed sand bath. The reaction gases, O2 and CO, and the diluent, He, were metered through rotameters qnd mixed prior to their entry to the reactor. The mixing junction was designed such that either of the reaction gases or CO2 could be introduced or removed from the stream to simulate a step increase or decrease of the component in question. The effluent from the reactor was analyzed by gas chromatography in 4 minutes. 

Based on the results of Dalla Betta and co-workers, it is clear that the steady-state activity of a completely sulfur-poisoned Ni or Ru methanation catalyst is 102-104 times lower than that of the fresh catalyst. However, a typical industrial methanation process would more probably involve a catalyst only partly poisoned by sulfur. Bartholomew and co-workers (23, 113, 157) attempted to assess how sulfur poisoning of only a portion of the catalyst would affect its activity/selectivity properties in fixed-bed and fluidized-bed reactors. Data in Table XII show the effects on specific activity and product distribution of partially presulfided Co/A1203 and Ni/Al203 catalysts in a fixed bed. Catalysts were presulfided with 10 ppm H2S at 725 K, and reaction was carried out with sulfur-free feedgas. Corresponding data are listed in Table XIII for catalysts partially presulfided and then studied in a fluidized-bed reactor under the same conditions. The decrease in H2 uptake... 

The thermal simulation was verified by choosing a benzene concentration of zero (no reaction) and natural convection cooling only. An ambient temperature of 20°C was assumed and, to minimise calcur-lation time, the accumulation terms in the separation regions were neglected. For a 1.2 kW power input, the model predicted a steady-state catalyst temperature of 473°C which was reached about seven hours after heating was begun. A temperature loss of 42 0 between the pebble benzene mixer and the catalyst was predicted while the difference between the catalyst and the fluidized bed preheater was 57 C. This loss was attributed to the increased... 

The conversion variable X does not have much meaning in flow systems not at steady state, owing to the accumulation of reactant. However, here the space time is relatively short (t = 0.02 h) in comparison with the time of decay t = 0.5 h. Consequently, we can assume a quasi-steady state and consider the conversion as defined by Equation (ElO-5.10) valid. Because the catalyst decays in less than an hour, a fluidized bed would not be a good choice to carry out this reaction. 

The types of reactors used for catalytic and noncatalytic gas-solid reactions are also often similar. Moving-bed reactors are used in blast furnaces and cement kilns. Fluidized-bed reactors are used for the roasting of sulflde ores and regeneration of catalytic cracking catalyst, and fixed-bed reactors are used to remove sulfur compounds from ammonia synthesis feed gas. When regeneration of the solid reactant is desired, two or more reactors operating in parallel are required if continuous, steady-state operation is to be achieved. 

Flowing-solids fluidized bed reactors are useful when the catalyst changes activity with time on stream as well as being in the form of fine particles. By arranging for a metered constant input of fresh catalyst and a commensurate withdrawal of equilibrium catalyst from the bed, one can arrange to establish a steady state of catalyst activity at any desired level. This type of reactor is more useful for the study of catalyst decay than for kinetic studies of the reaction. Nevertheless, reactors of this type are quite useful in catalyst testing under industrial conditions. 

The main drawback of kinetic models, based only on steady-state data, is associated with the fact, that start-up and transient regimes cannot be reliably modeled. Kinetic models for nonstationary conditions should be applied also for the processes in fluidized beds, reactions in riser (reactor) - regenerator units with catalyst circulation, as well as for various environmental applications of heterogeneous catalysis, when the composition of the treated gas changes continuously. 

It needs to be pointed out that the results discussed above may not necessarily be the same as in the phot-scale experiments where the catalyst circulation between reactor and regenerator is estabhshed. The circulation of catalyst can maintain a steady average coke content but with certain distribution since the catalyst particles will have residence time distribution due to the back-mixing in fluidized bed. This wiU lead to the change of reaction results to certain extent. But the results obtained from microscale fluidized bed reactor can be used to guide the design and operation of phot-scale setup. 

Alternatively, the sub-surface or bulk 0 species may diffuse to the surface to refill the vacancy. Note that only reactors operating in a redox decoupling mode, like CFBs, would derive oxygen from the bulk because the oxidation step is carried out in a distinct fluidized-bed reactor. Obviously this diffusion process is slower than the surface re-oxidation by molecular (gaseous) O2, but depending on the cation reducibihty during catalyst equilibration , part of the bulk becomes reduced. At the steady state (rR = rred = Tox) there is a hmited amount of 0 species able to participate in the reaction at each cycle (turn-over). 

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). 

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