Data on gas phase dispersion

Data on gas phase dispersion in three-phase sparged columns arc scarce. For two-phase systems a few correlations arc available [64, 67, 71, 72] which are shown... 

Data on gas phase dispersion are rather scarce, and in general they reveal considerable scatter. Towell and Ackerman (52) proposed the following empirical equation... 

This correlation includes also the data of other authors. Most recently, experimental data on gas phase dispersion have been reported by Mangartz and Pilhofer (79). On the basis of their findings with various liquids these authors conclude that the bubble rise velocity in the swarm (uq = uq/ q) is a characteristic varied>le which mainly Influences gas phase dispersion. Mangartz and Pilhofer (79) recommend the subsequent correlation ... 

Data on gas phase dispersion are few. Mangartz and Pilhofer (71) correlated their own results under consid-... 

Data on solids phase dispersion are similar to those of the liquid phase, at least for small particle diameters as generally prevailing in slurries [15, 76]. At zero liquid rates, if the particles are only suspended by the gas flow, a solids concentration profile will be established ... 

To this author s knowledge, no data on three-phase stirred columns are available. Preliminary observations indicate that the axial dispersion in the gas phase is considerably reduced by the presence of solid particles. Under certain conditions, even for a very low L/dc (where L is the length and dc the diameter of the stirred column) the gas phase may move essentially in plug flow. 

This equation can be recommended for design purposes. The relative velocity can be determined by applying equation (10) and (7) for the churn turbulent regime, neglecting the liquid velocity. Additionally, it is to consider, that equation (1 ) is valid only for a column diameter of 100 mm. However, there is a strong dependency of the gas phase dispersion coefficient on the column diameter. In comparing our results with literature data, we got the following dependency ... 

Collier, J. G., and D. J. Pulling, 1962, Heat Transfer to Two-Phase Gas-Liquid System, Part II, Further Data on Steam-Water Mixtures in the Liquid Dispersed Region in an Annulus, UK Rep. AERE-R-3809, Harwell, England. (4)... 

The equations describing the concentration and temperature within the catalyst particles and the reactor are usually non-linear coupled ordinary differential equations and have to be solved numerically. However, it is unusual for experimental data to be of sufficient precision and extent to justify the application of such sophisticated reactor models. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the catalyst bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on reaction rate. A useful approach to the preliminary design of a non-isothermal fixed bed catalytic reactor is to assume that all the resistance to heat transfer is in a thin layer of gas near the tube wall. This is a fair approximation because radial temperature profiles in packed beds are parabolic with most of the resistance to heat transfer near the tube wall. With this assumption, a one-dimensional model, which becomes quite accurate for small diameter tubes, is satisfactory for the preliminary design of reactors. Provided the ratio of the catlayst particle radius to tube length is small, dispersion of mass in the longitudinal direction may also be neglected. Finally, if heat transfer between solid cmd gas phases is accounted for implicitly by the catalyst effectiveness factor, the mass and heat conservation equations for the reactor reduce to [eqn. (62)]... 

Wiedmann et al. (1980) have compared the mixing of nonaerated liquids, aerated liquids, and slurries in a turbulent flow. They found that the torque required for stirred, aerated liquids is lower than that for nonaerated stirred liquids because of the decrease in the density of the gas-liquid mixture. The concentration distribution of the particles in aerated suspension becomes more uniform with increasing impeller speed, whereby the torque is higher than that for aerated liquids but lower than that for nonaerated slurries. For gas-liquid-solid systems, very limited data on dispersion of solids and gas phase are available, and further studies are necessary with different designs and for systems with different physical properties. The available literature has been reviewed by Stiegel et al. (1978), Shah et al. (1982), and Shah and Sharma (1986). 

Recommendations Under trickle-flow conditions, for the gas-phase axial dispersion, Eq. (6-53) is recommended. For the liquid-phase axial dispersion in hydrocarbon systems, use of Eq. (6-51) is recommended. More experimental data with a variety of hydrocarbon systems are, however, needed. Backmixing under pulsed-flow conditions has not been yet studied. Both experimental as well as theoretical work on this subject is needed. 

Significant literature on the axiaj dispersion in gas and liquid phases for countercurrent-flow packed-bed columns have been reported. Trickle- and bubble-flow regimes have been considered. Unlike the holdup, there is quite a discrepancy in the results of various investigators. Almost all the RTD data are correlated by a single-parameter axial dispersion model. A summary of the reported axial dispersion studies in countercurrent flow through a packed bed is given in Table 8-1. 

NMR adsorption isotherms for Ru/SiOi catalysts have been obtained using explicit calibration (89). Although the pressure over the sample could be adjusted in situ, no volumetric data were taken simultaneously, probably because of the important spillover effects in this catalytic system (see Section III.A). The NMR study was performed at pressures between 10 and 760 Torr and at temperatures between 323 and 473 K (only the 323-K results are reviewed here). The dispersion of the catalyst was determined from the irreversible H NMR signal as 0.29. The metal loading was 8 wt% so that a monolayer coverage on 1 g of catalyst corresponds to 2.8 cm of H2 under standard conditions. It is typical for an NMR sample to contain 0.5 g of material in a 1-cm sample volume, and the pores in the powder make up about half the volume. If such a sample of this catalyst is under 760 Torr of hydrogen, the gas phase corresponds to one-third of a mono-layer, and it can make a detectable contribution to the NMR signal. 

In addition to the above methods utilizing conventional ionization modes, the field ionization technique has appeared [75]. The very intense electric field (about 1 V/A), produced by an electrode, results in the ionization of molecules in the gas phase. This soft ionization technique is often used competitively with Cl, since it does not pollute the source and may yield sufficiently reproducible results. The transit time of ions in the source is on the order of 10 to 10 second. The radical molecular ions (M ) produced are characterized by a low internal energy, and thus can be detected easily. As a result of dispersion within the source, however, sensitivity is about two orders of magnitude lower than that of El. As in the case of El, the fragments produced by FI can furnish interesting structural data on carbohydrates, amino acids, peptides and cardenolides [76],... 

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