2. 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 be
accomplished and by which mass and heat transfer can be
enhanced between phases and with external surfaces.
7. • DEPENDS ON REYNOLDS , FROUDE AND POWER NUMBERS.
•THE RATIO OF DIAMETERS OF IMPELLER AND VESSEL FALLS IN THE
RANGE, D/D,=0.3-0.6,
•WITH COMMERCIALLY AVAILABLE MOTORS AND SPEED
REDUCERS, STANDARD SPEEDS ARE
37, 45, 56, 68, 84, 100, 125, 155, 190, AND 320 rpm
•EXPERT OPINIONS DIFFER SOMEWHAT ON THIS FACTOR. AS A
FIRST APPROXIMATION, THE IMPELLER CAN BE PLACED AT 1/6 THE
LIQUID LEVEL OFF THE BOTTOM.
9. : The process whereby materials are
transported to all regions of the vessel by bulk circulation
currents is called distribution
Dispersion facilitates rapid transfer of
material throughout the vessel. The degree of
homogeneity as a result of dispersion is limited by the
size of the smallest eddies which may be formed in a
particular fluid.This size is given approximately as the
Kolmogorov scale of mixing, or scale of turbulence, λ.
10.
11. ASSESING MIXING TIME
The mixing time tm is the time required to achieve a
given degree of homogeneity starting from the completely
segregated state.
Usually, mixing time is defined as the time after which the
concentration of tracer differs from the final concentration Cf by less
than 10% of the total concentration difference (Cf − Ci).
tm = 4 tc
12.
13. 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.
14. 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
15. 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 of
dimensionless numbers such as Reynolds number, Re and
power number, Np :
POWER NUMBER is given by:
Np = P/Ni3 p Di5
16. Once value of Np is known, the power required is calculated
by:
P=NppNi3Di5
where:
P = power; ρ = fluid density; N = stirrer speed, D = impeller
diameter
17. the apparent viscosity is not constant for non-
Newtonian fluids but varies with the shear rates or
velocity gradients in the vessel. Several investigators
have used an average apparent viscosity μa, which is
used in the Reynolds number as follows:
