Packed bed regime

Packed bed regime (fixed or moving bed operation). Here the particle hold-up is typically in the region 0.5-0.7. The particle size suitable in the packed bed regime is usually larger than 1 mm because smaller particle sizes result in unacceptably high pressure drops. 

T. R. Melli, W. B. Kolb, J. M. deSantos, and L. E. Scriven, "Cocurrent Downflow in Packed Beds Microscale Roots of Macroscale Plow Regimes,"... 

Note that the abscissa in Figure 2.4 starts at a value of 0.4, which corresponds to the voidage of a randomly packed bed. Equation 2.44 is valid for the extremes of flow regimes but strictly requires correction for the intermediate case (Khan and Richardson, 1990 Di Felice, 1994). 

In deriving expressions for the packed bed friction factor, three separate flow regimes are normally considered (see Figure 2.11) as follows. 

Very refined measurements at various positions of the packed bed were made by Jolls and Hanratty (J6), who used an active sphere (electrode) in a packed bed consisting of 1-inch inert spheres. The overall mass-transfer data for the turbulent flow regime suggest a dependence of... 

C. Packed Bed CFD Simulation Issues 1. Packed Bed Flow Regimes... 

Appendix B includes a review and a classification of conversion concepts. It also investigates the potentials to develop an all-round bed model or CFSD code simulating the conversion system. This review also contains a great deal of information on the heat and mass transport phenomena taking place inside a packed bed in the context of PBC of biomass. The phenomena include conversion regimes, pyrolysis chemistry, char combustion chemistry, and wood fuel chemistry. The main conclusions from this review are ... 

When authors illustrate the subject of thermochemical conversion of solid fuels in the literature, the conversion zone in a packed bed is divided into different process zones (drying zone, pyrolysis zone, char combustion zone, and char gasification zone), one for each thermochemical conversion process. The spatial order of this process zones is herein referred to as the bed process structure or conversion process structure. The conversion process structure is a function of conversion concept. Even more important, the bed process structure can only exist in the diffusion controlled conversion regime when the conversion zone has a significant thickness. 

To find how pore resistance influences the rate evaluate Mj or then find from the above equations or figures, and insert < into the rate equation. Desirable processing range Fine solids are free of pore diffusion resistance but are difficult to use (imagine the pressure drop of a packed bed of face powder). On the other hand a bed of large particles have a small Ap but are liable to be in the regime of strong pore diffusion where much of the pellets interior is unused. 

A packed bed reactor converts A to R by a first-order catalytic reaction, A R. With 9-mm pellets the reactor operates in the strong pore diffusion resistance regime and gives 63.2% conversion. If these pellets were replaced by 18-mm pellets (to reduce pressure drop) how would this affect the conversion ... 

Our reaction A R proceeds isothermally in a packed bed of large, slowly deactivating catalyst particles and is performing well in the strong pore diffusion regime. With fresh pellets conversion is 88% however, after 250 days conversion drops to 64%. How long can we run the reactor before conversion drops to... 

Temporal analysis of products (TAP) reactor systems enable fast transient experiments in the millisecond time regime and include mass spectrometer sampling ability. In a typical TAP experiment, sharp pulses shorter than 2 milliseconds, e.g. a Dirac Pulse, are used to study reactions of a catalyst in its working state and elucidate information on surface reactions. The TAP set-up uses quadrupole mass spectrometers without a separation capillary to provide fast quantitative analysis of the effluent. TAP experiments are considered the link between high vacuum molecular beam investigations and atmospheric pressure packed bed kinetic studies. The TAP reactor was developed by John T. Gleaves and co-workers at Monsanto in the mid 1980 s. The first version had the entire system under vacuum conditions and a schematic is shown in Fig. 3. The first review of TAP reactors systems was published in 1988. 

The operation of fluidized-bed reactors can be seen as the transition region between con-tinuous-stined tank and packed-bed reactors. In a fluidized bed, a bed of solid particles is fluidized by the upward flow of the gas or liquid stream, which may be inert or contain material relevant to the reaction. The several fluidization regimes are shown in Figure 3.51. 

For the fluidized bed process the bed expansion as a consequence of an increase in linear flow rate has to be considered. In a simplified picture diffusive transport takes place in a boundary layer around the matrix particle which is frequently renewed, this frequency being dependent on velocity and voidage, as long as convective effects, e.g. the movement of particles are neglected. Rowe [74] has included these considerations into his correlation for kf in fluidized beds, which is applicable for a wide range of Reynolds numbers, including the laminar flow regime where fluidized bed adsorption of proteins takes place (Eq. 19). The exponent m is set to 1 for a liquid fluidized bed, a represents the proportionality factor in the correlation for packed beds (Eq. 18) and is assumed as 1.45. 

A regime map of Fo versus the solid volume fraction, ap, for various gas-solid flows was presented by Hunt (1989), as shown in Fig. 4.3. Hunt (1989) suggested that except when Fo > 1 and ap > 0.1, use of the pseudocontinuum model is inappropriate. Thus, from Fig. 4.3, it can be seen that the pseudocontinuum model is applicable to packed beds, incipient fluidized beds, and granular flows, whereas it is not applicable to pneumatic transport flows, dilute suspensions, bubbling beds, and slugging fluidized beds [Glicksman and Decker, 1982 Hunt, 1989]. 

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