LIQUID

MIX

ING

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.

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.

DEFINATION

A BASIC STIRRED TANK DESIGN

BASIC DESIGN FACTORS

THE VESSEL

THE BAFFLES

IMPELLER TYPES•RADIAL FLOW IMPELLERS •AXIAL FLOW IMPELLERS

IMPELLERS

RADIAL FLOW IMPELLERS

AXIAL FLOW IMPELLERS

Liquid flow from the impeller is initially directed towards the wall of the reactor; i.e. along the radius of the tank.

Liquid flow from the impeller is directed downwards towards base of reactor, i.e. in the direction of the axis of the tank.

a) a radial flow, "Rushton", turbine which produces considerable turbulence near the impeller, b) a "pitched blade" impeller with flat, angled blades that generates a diverging but generally axial flow, c) a hydrofoil impeller with carefully profiled blades that develop a strong, more truly, axial flow of low turbulence. Impellers suitable for viscous fluids are: d) a helical ribbon with a blade that travels close to the wall of the tank to force good overall circulation and e) an anchor that produces strong swirl with poor vertical exchange, even when baffled with stationary breaker bars or "beaver tail" baffles.

IMPELLER SIZE: • 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,

IMPELLER SPEED:

•WITH COMMERCIALLY AVAILABLE MOTORS AND SPEED REDUCERS, STANDARD SPEEDS ARE 37, 45, 56, 68, 84, 100, 125, 155, 190, AND 320 rpm

IMPELLER LOCATION

•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.

MECHANISM OF MIXING

MECHANISM OF

MIXING

DISTRIBUTION

DISPERSION

DIFFUSION

DISTRIBUTION: The process whereby materials are transported to all regions of the vessel by bulk circulation currents is called distribution

DISPERSION: 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.

λ = (v3/pm)1/4

TRACER TECHNIQUES

A dye is introduced and the time for attainment of uniform color is noted.A concentrated salt solution is added as tracer and the measured electrical conductivity tells when the composition is uniform. The color change of an indicator when neutralization is complete when injection of an acid or base tracer is employed.The residence time distribution is measured by monitoring the outlet concentration of an inert tracer that can be analyzed for accuracy. The shape of response curve is compared with that of a thoroughly (ideally) mixed tank. 

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 = 4tc

POWER IN AGITATED VESSELS

REQUIRED TO DRIVE IMPELLER

AVERAGE POWER CONSUMPTION PER UNIT VOLUME INDUSTRIAL BIOREATORS 1-2Kw cubic metre for small vessels 10kw cubic metres for large vessels

Friction in the stirrer motor gearbox and seals reduces the energy transmitted to the fluid; therefore, the electrical power consumed by stirrer motors is always greater than the mixing power by an amount depending on the efficiency of the drive.

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.

THE FLOW IS LAMINAR IN THE TANK FOR N’RE < 10,TURBULENT FOR N’RE > 104, AND FOR A RANGE BETWEEN 10 AND 104, THE FLOW IS TRANSITIONAL, BEING TURBULENT AT THE IMPELLER AND LAMINAR IN REMOTE PARTS OF THE VESSEL

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 of dimensionless numbers such as Reynolds number, Re and power number, Np : POWER NUMBER is given by:

Np = P/Ni3 p Di

5

Once value of Np is known, the power required is calculated by:

P=NppNi3Di

5

where: P = power; ρ = fluid density; N = stirrer speed, D = impeller diameter

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:

UNGASSED NON NEWTONIAN FLUIDS:

IMPROVING MIXING IN FERMENTERS

REDUCE MIXING TIMES BY RAISING THE POWER INPUT

USE OF BAFFLES

THE IMPELLER SHOULD BE MOUNTED BELOW THE GEOMETRIC CENTER OF THE VESSEL.

USE OF MULTIPLE IMPELLERS