Mixing is defined as the reduction of inhomogeneinty in order to achieve a desired process result. The inhomgeneity can be one of the concentration, phase or temperature.Agitation is a mean whereby mixing of phases can beaccomplished and by which mass and heat transfer can beenhanced between phases and with external surfaces.
• DEPENDS ON REYNOLDS , FROUDE AND POWER NUMBERS.•THE RATIO OF DIAMETERS OF IMPELLER AND VESSEL FALLS IN THERANGE, D/D,=0.3-0.6,•WITH COMMERCIALLY AVAILABLE MOTORS AND SPEEDREDUCERS, STANDARD SPEEDS ARE37, 45, 56, 68, 84, 100, 125, 155, 190, AND 320 rpm•EXPERT OPINIONS DIFFER SOMEWHAT ON THIS FACTOR. AS AFIRST APPROXIMATION, THE IMPELLER CAN BE PLACED AT 1/6 THELIQUID LEVEL OFF THE BOTTOM.
MECHANISM OF MIXING DISTRIBUTION MECHANISM OF MIXINGDIFFUSION DISPERSION
: The process whereby materials aretransported to all regions of the vessel by bulk circulationcurrents is called distribution Dispersion facilitates rapid transfer ofmaterial throughout the vessel. The degree ofhomogeneity as a result of dispersion is limited by thesize of the smallest eddies which may be formed in aparticular fluid.This size is given approximately as theKolmogorov scale of mixing, or scale of turbulence, λ.
ASSESING MIXING TIMEThe mixing time tm is the time required to achieve agiven degree of homogeneity starting from the completelysegregated state.Usually, mixing time is defined as the time after which theconcentration of tracer differs from the final concentration Cf by lessthan 10% of the total concentration difference (Cf − Ci). tm = 4 tc
REYNOLDS NUMBER •Where Da is the impeller (agitator) diameter in m, •N is rotational speed in rev/s, •ρ is fluid density in kg/m3, • μ is viscosity in kg/m · s.
POWER NUMBER •Where Da is the impeller (agitator) diameter in m, •N is rotational speed in rev/s, •ρ is fluid density in kg/m3, •P is the power
UNGASSED NEWTONIAN FLUIDS:Mixing power for non aerated fluid depends on:•the stirrer speed•the impeller diameter and geometry•the properties of the fluid (i.e density and viscosity)The relationship of these variables is expressed in terms ofdimensionless numbers such as Reynolds number, Re andpower number, Np :POWER NUMBER is given by: Np = P/Ni3 p Di5
Once value of Np is known, the power required is calculated by: P=NppNi3Di5where:P = power; ρ = fluid density; N = stirrer speed, D = impellerdiameter
the apparent viscosity is not constant for non-Newtonian fluids but varies with the shear rates orvelocity gradients in the vessel. Several investigatorshave used an average apparent viscosity μa, which isused in the Reynolds number as follows: