Axial packings

TEM). The banding periodicity varies with age, species, pathology, and state (e.g., wet vs. dry) of the specimen. Molecularly different stmctures, like heterotypic type 1 and 111 collagen fibrils, show a similar liquid-like lateral ordering and axial packing. 

Fleischmann, M. and Jansson, R.E.W. (1982) The reaction engineering of electrochemical cells. Part I. Axial packed bed electrodes. Electrochimica Acta, 27,1023. 

Pulsed Columns. The efficiency of sieve-plate or packed columns is increased by the appHcation of sinusoidal pulsation to the contents of the column. The weU-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. This leads to a substantial reduction in HETS or HTU values. 

High water-content fluids are used in some hydrauhc systems where work-stroke speeds are very low, eg, large freight elevators and large forging and extmsion presses. Pressures in these systems may be from 13.8—20.7 MPa (2000—3000 psi). Vertical in-line pumps with packed plungers and special axial—piston pumps are used with these fluids. 

Packed Red Reactors The commonest vessels are cylindrical. They will have gradients of composition and temperature in the radial and axial directions. The partial differential equations of the material and energy balances are summarized in Table 7-10. Example 4 of Modeling of Chemical Reactions in Sec. 23 is an apphcation of such equations. 

Sohd Catalysts Processes with solid catalysts are affected by diffusion of heat and mass (1) within the pores of the pellet, (2) between the fluid and the particle, and (3) axially and radially within the packed bed. Criteria in terms of various dimensionless groups have been developed to tell when these effects are appreciable. They are discussed by Mears (Ind. Eng. Chem. Proc. Des. Devel., 10, 541-547 [1971] Jnd. Eng. Chem. Fund., 15, 20-23 [1976]) and Satterfield (Heterogeneous Cataly.sls in Practice, McGraw-Hill, 1991, p. 491). 

Axial Dispersion Vermeiilen et al. [Chem. Eng. Prog., 62(9), 95 (1966)] summarized many of the data for packings. Their correlation for the continuous phase is shown in Fig. 15-36. For the dispersed phase, their correlation is given by... 

The axial dispersion coefficient [cf. Eq. (16-51)] lumps together all mechanisms leading to axial mixing in packed beds. Thus, the axial dispersion coefficient must account not only for moleciilar diffusion and convec tive mixing but also for nonuniformities in the fluid velocity across the packed bed. As such, the axial dispersion coefficient is best determined experimentally for each specific contac tor. 

Correlations for axial dispersion in beds packed with very small particles (<50 Im) that take into account the holdup of liquid in a bed are discussed by Horvath and Lin [J. Chromatogr, 126, 401 (1976)]. 

TABLE 16-10 Coefficients for Axial Dispersion Correlations in Packed Beds Based on Eq (16-79)... 

FIG. 16 11 Axial dispersion coefficient correlations for well-packed beds for e = 0.4. 

Axial Dispersion Effects In adsorption bed calculations, axial dispersion effects are typically accounted for by the axial diffusionhke term in the bed conservation equations [Eqs. (16-51) and (16-52)]. For nearly linear isotherms (0.5 < R < 1.5), the combined effects of axial dispersion and mass-transfer resistances on the adsorption behavior of packed beds can be expressed approximately in terms of an apparent rate coefficient for use with a fluid-phase driving force (column 1, Table 16-12) ... 

A comparison of the axial-dispersion coefficients obtained in oscil-lating-spiral and dense-bed crystalhzers is given in Table 22-5. The dense-bed column approaches axial-dispersion coefficients similar to those of densely packed ice-washing cohimns. 

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. 

In towers with inert packing, both radial and axial gradients occur, although conduction in the axial direction often is neglected in view of the preponderant transfer of sensible enthalpy in a flow system. 

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