Axial mixing

Consider a steady-flow chemical reactor of length L through which fluid is flowing at a constant velocity u, and in which material is mixing axially with a dispersion coefficient D. Let an nth-order reaction be occurring. 

Back-mixing (axial dispersion is not considered in dynamic similarity), for both the liquid and the gas, increases with increasing column diameter and gas superficial velocity. Consequently, if PFR exist in a small reactor it will be lost, and therefore the reactor performance will be reduced. One remedy is to extend the batch cycle-time. 

Temperature danger of hot spots with temperature gradients high solids mixing axial temperature... 

The solids are kept in suspension if the pumping capacity of the impeller causes strong enough circulation of the liquid. In most processes, complete suspension of the particles is not required. Often, so-called off-bottom suspension is sufficient, which means that all particles are moving above the bottom of the tank with some vertical velocity. Radial flow impellers are usually not very effective in suspending solid particles. Actually, about three times more power is required for a radial turbine to provide the same degree of uniformity compared to an axial turbine. This is because the radial turbines pick up particles from the bottom of the tank by the suction side of the impeller, which is only half of the total flow from the impeller. Due to the appearance of an upper and a lower circulation zone, the contents of the two zones are not sufficiently mixed. Axial impellers are therefore most frequently used for the suspension of solids in stirred tanks [65]. 

Plug or mixed Axial variation See references under Models with varying gas-phase flow concentrations ... 

Proton and F NMR spectra of the unsymmetrical chelate tt-CsHjZr (tfac)3 can be similarly interpreted. In addition, the spectra show the presence of two isomers in a molar ratio of 3 2. The isomers dilfer in the orientation of the mixed axial-equatorial ligand. 

Traffic volume is expressed in ESALs. The conversion of mixed axial loads to ESAL is conducted with the use of equivalency coefficients related to the terminal serviceability value, pt, and the structural number, SN, of the pavement. For the design of new flexible pavements, it is suggested that equivalency coefficients corresponding to p, = 2.5 and SN = 5 be used (see Table 12.3). 

Longitudinal mixing, back mixing (axial plug flow the driving force for [6.18]... 

FIGURE 4.34 yr (axial dispersion model), /md (maximum-mixed axial dispersion model), and ysD (segregated axial dispersion model) at M = 1.00. M = vaCob/(vbcoa). R = KAtCoAVB/VA-... 

At the present time, mechanisms for segregation, even in the simple tumbUng drum, remain obscure, and work on more complex and industrially common blender geometries is extremely limited. Three distinct types of de-mixing are moderately well characterized in tumblers radial de-mixing, axial de-mixing, and competitive patterned de-mixing. We describe each of these in turn. 

Back- mixing - axial backmlxing profiles - solids mixing - bed and dense phase expansion (50, 35)... 

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