Packed beds void fraction

Adsorbent particle radius and diameter Adsorbent packed bed void fraction Effective pore diffusion coefficient Superficial gas velocity... 

Inserting the relation for T of eqn (3.19) in eqn (3.18), and making an empirical adjustment to the constant (from 72 to 60), produces an expression for AP that is in exact agreement with the Blake-Kozeny equation at the normal packed bed void fraction of 0.4 but which, beyond that point, reflects the above tortnosity considerations for expanded beds, e > 0.4 ... 

Such a comparison is given in Fig. 7 where the two heat transfer parameters X, and aw plus the external catalyst surface ap, the bed void fraction e and the pressure drop Ap are given for a selection of different random and regular catalyst packings in a tube of 50 mm internal diameter and a mass flow velocity of G. = 1 kg/(m2 s). 

The residence-time distribution in the liquid phase of a cocurrent-upflow fixed-bed column was measured at two different flow rates. The column diameter was 5.1 cm and the packing diameter was 0.38 cm. The bed void fraction was 0.354 and the mass flow rate was 50.4 g s l. The RTD data at two axial positions (which were 91 cm apart in Run 1 and 152 cm apart in Run 2) are summarized in Table 3-2. Using the method of moments, estimate the mean residence time and the Peclet number for these two runs. If one assumes that the backmixing characteristics are independent of the distance between two measuring points, what is the effect of gas flow rate on the mean residence time of liquid and the Peclet number Hovv does the measured and the predicted RTD at the downstream positions compare in both cases ... 

Ethylene and oxygen are fed in stoichiometric proportions to a packed-bed reactor operated isothermally at 260°C. Ethylene is fed at a rate of 0.30 lb mol/s at a pressure of 10 atm. It is proposed to use 10 banks of 11-in.-diameter schedule 40 tubes packed with catalyst with 100 tubes per bank. Consequently, the molar flow rate to each Qibe is to be 3 X lO " Ib inol/s. The properties of the reacting fluid are to be considered identical to those of air at this temperature and pressure. The density of the j -itL-catalyst particles is 120 Ib/ft and the bed void fraction is 0.45. The rate law is... 

Bed Characteristics Diameter Type of inert bed Pre packing depth Catalytic bed depth Post packing depht Catalyst weight Bed void fraction... 

Consider a fixed-bed column with downward cocurrent flow of liquid and gas phases. The column is packed with 0.3 cm diameter catalyst particles, with bed void fraction of 0.48, bed diameter of 5 cm, and bed length of 150 cm. Gas and liquid fluxes (both superficial) are 10 and 10 kg/m -h, respectively. Under reaction conditions the relevant gas properties are average molecular weight = 10, density 0.06g/cm, viscosity = 0.6cP, surface tension = lOdynes-cm, specific gravity = 0.9, and the molecular diffusivity of reactant = 10 ft /h. From these data estimate... 

Pressure drop calculations in the primary and secondary reformer models are based on the Ergun relationship, as presented in Reference 25. Both the laminar flow and turbulent flow terms are included. The natural parameter that arises from the Ergun equation, which can be updated with measured pressure drop information, is the TURBULENT DP COEF term in the models of both the primary and secondary reformers. This term affects only the pressure drop, whereas another term, the bed void fraction, which might also have been used as the parameter to update with measure pressure drop, also affects all the reaction rates. The bed void fraction affects the amount of catalyst in a fixed volume reformer tube, and is not an appropriate parameter to use in this case. The void fractions of typical packed beds are shown in Figure 5.70 of Reference (26). Void fractions of 0.4 to 0.6 are typical, and can be determined for specific catalysts sizes and shapes from vendor specification sheets, by measurement, or, with more difficulty, by calculation. 

The constant relating pressure drop to V po in Equation 1-4 for the dry line is actually the summation of the effects of packing shape factor, bed void fraction, and hydraulic radius of the packing as indicated in Equation 1-2. This constant can be determined from dry line pressure drop measurements for any particular type and size of packing, as long as the gas-phase flow is turbulent. 

If a one-dimensional flow through a packed bed of granular material is assumed (e.g., in moving-bed gasifiers), the pressure drop over a certain bed height Hbed complies to Equation (3.77). It uses the representative diameter based on equivalent specific surface ds, the density of the gas p, the superficial velocity in the bed u, and the dimensionless bed void fraction e for pressure drop calculation (see also Section 3.12.3.2). 

Wo gas velocity determined for the column cross-section in m/s e - bed void fraction (free packing volume, equal to its free cross-section) Pcr- gas density in kg/m ... 

Re = R nolds number, dpS UolV Sc = Schmidt number, V/D D = axial dispersion coefficient dp = Diameter of particle or empty tube = Fraction voids in packed bed Uq = Superficial velocity in the vessel. 

Relations expressing the fractional voids in a ring packed bed are useful in estimated the e values for a/e determinations [47]. The average deviation is 2.6%. 

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