Axial mixing of the liquid

Siemes and Weiss (SI4) investigated axial mixing of the liquid phase in a two-phase bubble-column with no net liquid flow. Column diameter was 42 mm and the height of the liquid layer 1400 mm at zero gas flow. Water and air were the fluid media. The experiments were carried out by the injection of a pulse of electrolyte solution at one position in the bed and measurement of the concentration as a function of time at another position. The mixing phenomenon was treated mathematically as a diffusion process. Diffusion coefficients increased markedly with increasing gas velocity, from about 2 cm2/sec at a superficial gas velocity of 1 cm/sec to from 30 to 70 cm2/sec at a velocity of 7 cm/sec. The diffusion coefficient also varied with bubble size, and thus, because of coalescence, with distance from the gas distributor. 

Axial mixing of the liquid is an important factor inthedesignof trickle bed reactors, and criteria were proposed to establish conditions that limit axial mixing. Mears [Chem. Eng. Sci. 26 1361 (1971)] developed a criterion that when satisfied, ensures that the conversion will be within 5 percent of that predicted by plug flow ... 

Vail et al. [68] used a steady state technique involving steady injection of aqueous NaCl solution at a given level of the column, to monitor the axial mixing of the liquid. The fluidized bed contained air, water and 0.87 mm spheres with a density of 2700 kg/m. The concentration profiles of tracer measured below the injection point provided information about axial dispersion coefficients and the characteristic mixing length (ratio of the axial dispersion with respect to the fluid velocity). It was observed that the presence of solids and the increase of the gas and liquid velocities were all factors promoting axial mixing of the liquid phase. 

Riquarts, R, H. (1981) A Physical Model for Axial Mixing of the Liquid Phase for Heterogeneous Row Regime in Bubble Columns , Ger. Chem. Eng. 4,18-23. 

Measurement of axial mixing in the liquid phase of a fluidized bed is performed by analysis of the residence time distribution of step or pulse signals [55], By plotting the dimensionless E-function of the output signal versus the dimensionless time, the moments of the residence time distribution may be calculated according to Eqs. (7) and (8), the first dimensionless moment /q describing the mean residence time and the second dimensionless moment U2 standing for the variance of the distribution. 

Monodisperse silica nanoparticles of a diameter of 200-500 nm were obtained [329]. This is explained by the well-defined residence time with reduced axial dispersion of the liquid segments [328,329]. In addition, by moving the liquid segments through a tube recirculation flow sets in, which is very effective in liquid mixing. 

Axial mixing in the liquid, induced by the upflow of the gas bubbles, can be substantial in commercial-scale bubble columns, especially in the chum turbulent regime. Due to typically small particle size, the axial dispersion of the solid catalyst in slurry bubble columns is expected to follow closely that of the liquid exceptions are high-density particles. The liquid axial mixing can be represented by an axial dispersion coefficient, which typically has the form... 

Since the liquid flow rates are generally rather low it may be necessary to account for axial mixing in the liquid phase. This is done in terms of axial effective diffusion. The axial effective diffusivity for the liquid phase is given by Bdxkes [40] ... 

The axial mixing in the liquid phase of random packings is investi ed by many scientists [115-126,203-213]. 

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