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Gas phase hydrodynamics

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. [Pg.149]

During the past five decades, gas-solids hydrodynamics studies principally have concentrated on solids phase measurements and characterization and have largely ignored the gas phase. Pneumatic conveying is an example solids are the commodity of interest the gas phase is only important in the sense that power requirements for blowers and compressors should be minimized. In studies of bubbling and turbulent fluidized beds, experimentalists study the spatial and temporal distribution of bubbles, but, typically, they employ solids measurement devices from which gas phase hydrodynamics are inferred. Circulating fluidized bed researchers also have devoted considerable attention to the solids phase, but since 1988 only 40 publications have appeared that deal with gas phase hydrodynamics. [Pg.256]

Gas Phase Hydrodynamics in Circulating Fluidized Bed Risers 261 Table 2... [Pg.261]

As suggested, RTD measurements should be combined with other techniques to best quantify riser gas-phase hydrodynamics. Injection and detection methods are critical to interpreting the data. Iso-kinetic injection at different radii may help deconvolute inlet boundary conditions and flow structure. Multiple detectors along the riser length also are preferred. However, combining radial gas sampling, as practiced with steady state tracers, with radioactive impulse experiments could provide sufficient data to completely characterize riser gas-phase hydrodynamics. [Pg.274]

Horio et al. describe a two-color optical fiber probe to measure gas phase hydrodynamics [102]. Ozone is injected into the riser from a 2 mm ID nozzle only 20 mm upstream of the measuring section. This section is exposed to ultraviolet... [Pg.274]

Data generated using the experimental techniques described above are used to formulate hydrodynamic models that may be used to predict reactor performance. In this section, studies that employ chemical reactions to evaluate mass transfer and contacting efficiency are described. Selected references are shown in Table 7. Dry et al. have applied hot air pulses as a reacting tracer [87]. Chemical reactions used to probe gas phase hydrodynamics include thermal decomposition of sodium bicarbonate, ozone decomposition, coal combustion, and FCC coke combustion. [Pg.276]

Some studies, such as Fujima et al., concentrate on one aspect of this multi-faceted problem [105]. For example, they set out to show the relationship between sulfur capture, particle concentration, and firing conditions. Boyd and Friedman, on the other hand, summarize the effect of many process variables on combustion performance [107]. The final DOE report concerning the NUCLA experimental program concludes that radial gas mixing is poor Gas samples withdrawn across the radius of the reactor at two heights 7 m apart had similar concentration profiles [106]. They also found that secondary air had little effect on gas phase hydrodynamics... [Pg.276]

Deducing gas phase hydrodynamics based on radial and axial concentration measurements is the ideal manner in which to scale-up. However, this approach is often prohibitive in terms of cost and time investment so we rely on cold flow experiments for hydrodynamic scaling. [Pg.278]

In summary, a single model has not been developed that can fully characterize riser gas phase hydrodynamics. The studies indicate that under dense phase conditions, typical of commercial FCC riser operation, a simple axial dispersion model may be adequate to characterize gas mixing. Under dilute conditions, a two-phase core-annular model is a good first approximation to the flow structure. However, both radial dispersion and radial gas velocity profiles must be accounted for to provide a realistic and reliable interpretation. The model suggested by Martin et al. should be further developed and applied to risers of different geometry operating with different powders [83]. However, contact efficiency may provide the simplest means from which scale-up criteria can be developed. [Pg.286]


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