Mixing fluid motion

Fluid mixing is a unit operation carried out to homogenize fluids in terms of concentration of components, physical properties, and temperature, and create dispersions of mutually insoluble phases. It is frequently encountered in the process industry using various physical operations and mass-transfer/reaction systems (Table 1). These industries include petroleum (qv), chemical, food, pharmaceutical, paper (qv), and mining. The fundamental mechanism of this most common industrial operation involves physical movement of material between various parts of the whole mass (see Supplement). This is achieved by transmitting mechanical energy to force the fluid motion. 

Inline motionless mixers derive the fluid motion or energy dissipation needed for mixing from the flowing fluid itself. These mixers iaclude orifice mixing columns, mixing valves, and static mixers. 

Mixing of fluids is a discipline of fluid mechanics. Fluid motion is used to accelerate the otherwise slow processes of diffusion and conduction to bring about uniformity of concentration and temperature, blend materials, facihtate chemical reactions, bring about intimate contact of multiple phases, and so on. As the subject is too broad to cover fully, only a brier introduction and some references for further information are given here. 

Mixing is accomplished by the rotating action of an impeller in the continuous fluid. This action shears the fluid, setting up eddies w hich move through the body of the system. In general the fluid motion involves (a) the mass of the fluid over large distances and (b) the small scale eddy motion or turbulence which moves the fluid over short distances [21, 15]. 

The general equation for fluid motion in a mixing system contains no less than 13 terms. Of these terms, nine define geometric boundary conditions. If these can be fixed, and strict geometric similarity be adhered to, the equation can be simplified and written as... 

Now, let us assume that die water body is kept well mixed by turbulent fluid motion in the tank (Fig. 23.2b). The vertical motion of a particle is now given by the algebraic sum of the local vertical current velocity vz and the settling velocity vs vz tot = vs + vz. The total particle flux, E Fs, across an interface at an arbitrary depth z0 is given by the integral of Eq. 23-12 over the area Am whereby vs is replaced by the total vertical particle velocity, vz tot ... 

In the research laboratory it is important to recognize the effect of mixing on reaction rate or on other performance criteria. Energy must he supplied to produce fluid motion, thus, to compare mixing with different equipment or with different sizes of the same type impeller, it is essentia] that the comparisons be made on the basis of equal power input. 

M 10] [M 11] [P 9] [P 10] Sonic irradiation with the use of air bubbles caused considerable gross fluid motion for a dye solution [23, 24], Owing to the uniform distribution of the air pockets, mixing is induced in the complete micro chamber and not localized to one part only. 

The holes act as sources and sinks for fluid motion [23], They draw the dye from in the surrounding region and expel it in a jet-like action into other parts of the liquid. These jet-like structures are mainly responsible for mixing. Where flow symmetries delineated by the formation of internal stagnation zones exist, mixing is retarded. Best mixing results are obtained for the five-bubble arrangement. 

The transfer of heat by convection involves the transfer of energy from the surface to the fluid on a molecular scale and then the diffusion of this heat through the fluid by bulk mixing due to the fluid motion. The basic heat transport mechanism in convection is still conduction which is, of course, governed by Fourier s law. This law states that the heat transfer rate, q, in any direction, n, per unit area measured normal to n is given by ... 

For mold pellets and other suspended particles with densities close to that of the continuous phase, the agitation in a stirred mixing vessel creates the dominant force for relative fluid motion between the two phases. The intrinsic gas-liquid mass-transfer coefficient under these conditions is given by Calderbank (1967) as... 

Fluidized leaching and washing belong to counter-down, or mode III, operation for simultaneous particle-fluid motion. A basic case for uniform particle size will be established, and it will then be extended to mixed particle sizes. 

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