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AgitationIn

Bioreactor

Prepared bySaurabh Jyoti Sarma

27th Feb ‘12

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Introduction

The primary purpose of aeration is to provide microorganisms in submerged culture with sufficient oxygen for metabolic requirements, while agitation should ensure that a uniform suspension of microbial cells is achieved in a homogeneous nutrient medium (Stanbury et al, 2003)

It is one of the major energy consuming/expensive tasks of the fermentation process

The structural components of fermenter involved in agitation are:

- Agitator/stirrer/impellers - Baffles

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Agitator/stirrer

Mostly, made up of stainless steel Connected to a motor either from the top or from the

bottom of the fermenter

Agitation helps in - Gas liquid mass transfer- Liquid-liquid mass transfer- Liquid-solid mass transfer- Suspension of solid- Blending miscible liquids- Heat transfer- minimizing local variations in concentration and

temperature

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Impeller: Types

They are the major component of an agitator

There are four types of impellers, namely

1. Disc turbine

2. Vaned disc

3. Open turbine and

4. Marine propeller

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Impeller: Types

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3

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1

3

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Structure and design of agitator

Source: McCabe et al. 2001

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Structure and design of agitator

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Location of impeller

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

In some cases there is provision for changing the position of the impeller on the shaft

Criteria developed by Dickey (1984) are based on the viscosity of the liquid and the ratio of the liquid depth to the tank diameter, h / Q

Whether one or two impellers are needed and their distances above the bottom of the tank are identified in following table:

Source: http://www.pacontrol.com/process-informationbook/Mixing%20and%20Agitation%2093851_10.pdf

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Source: http://www.pacontrol.com/process-informationbook/Mixing%20and%20Agitation%2093851_10.pdf

Location of impeller

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Impeller design: Dimensionless numbers

To help in characterization of the design and performance of the process

Power number: NP = P/ρN3D5

This number in conjunction with Impeller Rotational Speed (N), Impeller Diameter (D) and Liquid Density (ρ) allows to calculate the Mechanical Power (P) being transmitted to the fluid by a turbine/ impeller of a given design

Reynolds number: NRe = ρND2/µ

The Reynolds number indicates the degree of turbulence experienced in a stirred tank reactor. Where µ is the viscosity of the liquid in which the agitator is turning

Aeration Number : NQg = Qg/N D3

Useful measure of the gas dispersion capabilities of the impeller (Qg = gas flow rate (m3 s-1)

Source: http://freedownload.is/doc/mixing-aeration-and-agitation-in-str-6487241.html

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Relationship of power number and Reynolds number

Source: http://freedownload.is/doc/mixing-aeration-and-agitation-in-str-6487241.html

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Relation ship of power number and Reynolds number

Source: http://www.pacontrol.com/process-informationbook/Mixing%20and%20Agitation%2093851_10.pdf

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Relation ship of power number and Reynolds number

Conditions of viscous (laminar) flow is rare in fermentation process, the majority of fermentations exhibiting flow characteristics in either transition or turbulent zone (Stanbury et al, 2003)

As mainly turbulent flow is used for bioreactors , 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

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Role of impellers in liquid flow

Source: Shou-Hu 2007

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Role of impellers in liquid flow

The “Axial flow three blades” provides less shear and more uniform velocity in the entire discharge area than the “propeller three blades.”

For animal cells an axial-flow pattern is preferred

The “propeller three blades” is used extensively in microbial fermentation to enhance oxygen transfer

Purpose Axial flow Radial flow

Gassing Less suitable Highly suitable

Dispersing Less suitable Highly suitable

Suspending Highly suitable Less suitable

Blending Highly suitable Suitable

Source: Shou-Hu 2007

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Baffles

These are metal plate fixed to the internal surface of the fermenter

They are roughly one tenth of vessel diameter

Four baffles are normally used

For vessels over 3-dm3 diameter six to eight baffles may be used

They are used to prevent formation of a vortex Extra cooling coils may be attached to the baffles to

improve cooling capacity of the fermenter

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Role of baffles

Baffles also promote axial mixing even with radial flow impeller

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Agitation, viscosity and rheology of fermentation media

Rheology : Study of flow of matter

The shearing force F acts on the area on the top of the element. This area is given by A = dz ´dx . We can thus calculate the shear stress which is equal to force per unit area i.e.

Source: http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section1/Fluid_mechanics.htm

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The term u/y is the change in velocity with y, or the velocity gradient (or shear rate), and may be written in the differential form du/dy. The constant of proportionality is known as the dynamic viscosity, µ

For Newtonian fluid : τ = µ. du/dy

Agitation, viscosity and rheology of fermentation media

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Agitation and rheology of fermentation media

Source: http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section1/Fluid_mechanics.htm

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Rheology of non Newtonian fluid

Bingham plastic rheology: τ = τ0 + n. du/dy Where n is coefficient of rigidity, τ0 is threshold stressExample: Mycelial fermentation broth, toothpaste, clay

Pseudo plastic rheology : τ = k. (du/dy)n Where k is apparent viscosity, n is flow behavior indexExample: Mycelial fermentation broth ,polymer solutions

Dilatant rheology: Not exhibited by fermentation broth, found in liquid cement slurry

Casson body (plastic) rheology: Example: Mycelial fermentation broth

Source: Stanbury et al, 2003

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Organism Rheological type ReferencePenicillium chrysogenum Bingham plastic Deinedoerfer and

Gaden (1955)Streptomyces kanamyceticus Bingham plastic Sato (1961)Penicillium chrysogenum Pseudo plastic Deinedoerfer and

West (1961)Endomyces sp. Pseudo plastic Taguchi et al (1968)Penicillium chrysogenum Casson body Roels et al (1974)

Rheological nature of fermentation broth

Source: Stanbury et al, 2003

For determination of the nature of the fermentation broth, construction of a rheogram is necessary

A viscometer can be used for this purpose

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Thank you