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Mixing Aeration and Agitation in STR

By Beatrice Sims,2014-05-07 19:09
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Mixing Aeration and Agitation in STR

    Mixing: Aeration and Agitation in a Stirred Tank Reactor Maintain uniform conditions in the vessel (solid, liquid, gas

    concentration, Temperature, pH).

     Disperse bubbles throughout the liquid, promote bubble break-up,

    increase gas-liquid interfacial transfer (bigger the interfacial area

    for diffusion, the better)

     Promote mass transfer of essential nutrients

Mixing is effected by

     Aeration and agitation in a Stirred Tank Reactor

     Aeration (and consequent fluid circulation) in an Air Lift Reactor

    Schematic of Standard tank configuration

Agitators in Bioreactors

    Rushton Turbine Impeller in Glass Bioreactor

Types of agitator

     ( (apparent viscosity) < 50 cP, high N (rotational speed) ? turbine (rushton or inclined blade) like above

    Remote clearance: D (agitator diameter) / T (tank diameter) : 0.25-0.5)

    Vessel baffled (in general, four strips of metal running parallel to the wall of the bioreactor, protruding into the liquid) to prevent vortex (similar to flow behaviour about a sink plug hole) formation at high agitation speeds

    The impact of turbine blade pitch on flow pattern

Flat blade ? Radial flow (radial means perpindicular to the shaft of

    the bioreactor. - outwards)

Sketch and measure:

    Pitched/inclined blade/propeller ? axial component (axial means that a proportion of the primary flow is parallel to the shaft

    up/downwards)

Sketch and measure:

    Marine propellers ? three blades, wide range of N, high shearing effect at high rotational speeds

Sketch and Measure:

High Viscosity Solutions

     High (? anchors.helical ribbons ( and propellers)

    Anchors, helical ribbons:D/T >0.9

    Lower speeds, vessels generally not baffled

     Intermig agitator ? axial pumping impeller requires less energy

    and lower gas through-put to produce same mass transfer

    coefficient as turbine.

Insert Intermig Picture Here:

     For adequate particle suspension and dispersal, may require

    profiled vessel base; inclined-blade agitators preferable

    Dimensionless Numbers in Agitated/Aerated Systems

    We use dimensionless numbers in agitated/aerated systems to help us characterise the design and performance of the process, however in a scale independent manner.

The first dimensionless number presented is the power number, NP

    PN P35ND

    This number in conjunction with Impeller Rotational Speed (N), Impeller Diameter (D) and Liquid Density () allow us to calculate the

    Mechanical Power (P) being transmitted to the fluid by a turbine/impeller of a given design.

    Reynolds Number is the second key number in the set of dimensionless numbers. Again similar to applications in pipes, etc., the Reynolds number indicates the degree of turbulence experienced in a stirred tank reactor.

    2ND NRe(

Where ( is the viscosity of the liquid in which the agitator is turning.

Flow Number (N) Useful measure of the pumping capacity of an Q

    impeller. Again the number is design specific and independent of

    scale.

    Q NQ3ND

Aeration Number (N) Useful measure of the gas dispersion Qg

    capabilities of the impeller.

    QgN Qg3ND

    P = agitator power (W) (N.B. Shaft power only) D = impeller diameter (m)

    -3 = fluid density (kg m)

    -1N = impeller speed (s)

    -2( = fluid viscosity (Ns m)

    3-1Q = fluid flow rate (m s)

    3-1Q = gas flow rate (m s) g

The Relationship of Power Number and Reynolds Number

Relationship has three phases each phase corresponding to the

    three phases of liquid flow, laminar, transition and turbulent A plot of Ln N vs Ln N ? straight line, slope 1 PRe

    Turbulent flow, N independent of N (also constant) pRe

    Bioreactors are, in the main, in turbulent flow. This means that the power number is constant for a given impeller design. Power numbers for a variety of impellers in turbulent flow have been well characterised, therefore if we know the impeller diameter and the rotational speed of the impeller (both easy to measure) we can subsequently estimate the mechanical power input to the bioreactor.

    It is important to note that all of the correlations presented apply to ungassed, single phase fluids only ? no allowances for aeration or

    suspensions.

In general the Gassed Power is less than the calculated ungassed

    power. A general rule of thumb for the calculation of gassed power is

    P = 0.6 P g

Example

    Calculate the specific power requirement (P/V) for a standard configuration STR, fully baffled, fitted with a Rushton turbine and

    0containing water at 25C. The vessel diameter is 0.5m. The impeller speed is 300rpm.

Solution

Standard STR? T = 0.5m

     D = T/3 = 0.167m

     H = T = 0.5m

    33 V = ? T/4 = 0.098m

    300??2;;10000.167?2ND60(N Re3(110x

    5 N139445~1.4x10Re

    ? fully turbulent flow, therefore from the Power Number Reynolds Number correlation graph, (curve 1 is a Rushton turbine remember

    not to misread the log scale!)

N=5 P

    3535P=NND = (5)(1000)(300/60)(0.167) = 81W p

    Power input per unit volume is a useful comparitive measure between bioreactors of different scales

P8133828W/m~1kW/m

    V0.098

    3Typical Specific Power Consumptions (P/V) kW/m

Mild agitation 0.1

     Suspending light solids

     Blending of low viscosity liquids Moderate Agitation 0.4

     Gas dispersion, liquid-liquid contacting

     Some heat transfer

    Intense Agitation 1.0

     Suspending heavy solids, emulsification

     Blending pastes, dough 4.0 Industrial-scale fermenters 0.5-5

    Lab-Scale fermenters 5-10

Reynolds Number ranges for Rushton turbine

    1Re < 10 laminar flow

    1410 < Re < 10 transitional flow

    4Re < 10 ? turbulent flow

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