Airlift and Mechanical Agitators in Fermenters

Table of Content

The given assignment thesis is on the subject “compare/contrast the comparative virtues of air-lift and mechanical fomenters in fermenters ” . In this some cardinal belongingss of air-lift and mechanical fomenters are explained. First, there is a little debut of air-lift fermenter and mechanical fomenters. Second, each belongings is explained with it ‘s similarity and differences.

Air-Lift Fermenters

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Airlift Fermenters are pneumatically agitated fermenters and holding four distinguishable zones, such as, riser, gas-liquid centrifuge, downcomer and base. These four zones divide the fermenter into upward and downward parts i.e. two stage flow parts. Furthermore, the major cause of circulation in the air-lift fermenters is the fractional gas clasp ( & A ; Delta ; & A ; epsi ; G ) which exists between the riser ( & A ; epsi ; GR ) and the downcomer ( & A ; epsi ; GD ) . The driving force for the circulation of fluid is the hydrostatic force per unit area difference. The underside of riser and the underside of the base creates this hydrostatic force per unit area difference. The force per unit area difference at the underside of the fermenter is:

& A ; Delta ; PB = & A ; rho ; L g ( & A ; Phi ; R – & A ; Phi ; D )

Here, & A ; Delta ; PB is the force per unit area difference,

& A ; rho ; L = denseness of the fluid ( denseness of gas is considered negligible )

g = gravitational invariable

& A ; Phi ; R, & A ; Phi ; D = fractional gas hold up of riser and downcomer, severally.

General correlativity to foretell gas keep up in air-lift fermenters is:

& A ; Phi ; R = a ( JG ) & A ; alpha ; ( AD/ AR ) & A ; beta ; ( µeff ) & A ; gamma ;

Here, & A ; Phi ; = gas hold up

JG = superficial gas speed

µeff = effectual viscousness of the liquid

Airlift fermenters are categorized into two classs based on their physical constructions. In some airlift fermenters baffles are placed in the fermenter to make a typical flow channel of the cringle. These are known as Internal-loop airlift reactors. In 2nd sort of airlift fermenters, i.e. External-loop reactors the downcomer and the riser are two separate tubings connected by horizontal subdivisions at the underside and the top.

Mechanical fomenter

Agitators play a cardinal function in the blending phenomena of a fermenter. They improve both O transportation capableness and commixture. Furthermore, gas distribution in the fermenter is the map of impeller and non of sparger. Therefore, an ideal fomenter should supply rapid agitation so that all the gas bubbles should scatter throughout the fermenter and increase their abode clip inside the liquid so that no air can get away before all the O2 is used up.

It should besides shear big bubbles into smaller 1s. Sometimes excessively much of stirring can be damaging, as it can consequence biological contents ( e.g. , carnal cells ) of fermenter and besides can do stratification of reactor contents with multiple impeller systems. While blending is traveling on impeller should maintain all the solid atoms suspended in the medium, maintain optimal substrate and biomass concentration and avoid hot spots or unvarying temperature in the fermenter.

A assortment of impeller designs are proposed, but the most preferable are turbine and disc type impellers. Furthermore, for cellular/biological systems which are extremely sensitive to shear forces paddle and marine impellers are of peculiar involvement. The Rushton impeller design was preponderantly in usage in the mid 1980s and is normally found on research lab and industrial fermenters. It is a disc turbine with 6 level blades and pumps fluid in radial waies.

Another type of impellers, axial hydrofoil impellers ( e.g. , A200 impeller ) are progressively popular. This is due to the fact ; they can pump liquid in either upward or downward way, holding low energy demands, cut down maximal shear rates and give first-class public presentation with syrupy agitations, such as mycelia agitations. New impeller designs are introduced 1980s onwards e.g. , , A6000 and A315 Lightnin. These are the impellers which can do high flow and holding really low shear rates.

Sometimes two fomenters are set at an angle of 900 to each other for better consequences. Normally used impellers are ; Three-bladed propellors, simple propellors, coiling thread, coiling prison guard with bill of exchange tubing, paddle, gate ground tackle, Banbury sociable, Six bladed phonograph record turbine, Anchor impeller and Z-blade sociable.

Comparisons and Contrasts

This subdivision will give all the similarities and differences in air-lift and mechanical fomenters. Some of the virtues which are taken into consideration are:

  1. Blending
  2. Aeration
  3. Practicality
  4. Energy usage
  5. Cost


Blending operations in any fermenter is measured on the footing of Mixing/Blend times and Rate of blending. Blending clip is defined as the clip required to bring forth the mixture of a coveted quality. The rate at which blending operation returns to its concluding province is known as the Rate of a commixture operation. Mixing clip for a peculiar given experiment with operating variables is shown as:

thulium = degree Fahrenheit ( & A ; rho ; , µ , N, D, g, geometrical dimensions of the system )

As per dimensional analysis, this functional relationship can be rearranged as:

Ntm = & A ; theta ; m = degree Fahrenheit ( & A ; rho ; ND2/µ , DN2/g, geometrical dimensions as ratios )

Assuming Froude figure DN2/g is non much of a importance and for geometrically similar systems,

& A ; theta ; m = degree Fahrenheit ( & A ; rho ; ND2/µ ) = degree Fahrenheit ( Re ) , here, Re = Reynolds figure

ReI = ( & A ; rho ; ND2/µ ) ,

ReI = impeller Reynolds figure

D = impeller diameter

N = rotational velocity

& A ; rho ; = the liquid denseness

µ = viscousness

Impellers sometimes may be assumed as pumping devices, due to the axial and radial flow created by them in the fermenter. In pumping figure the dimensionless entire volumetric flow rate ‘Q ‘ discharged by an impeller is shown as:

NQ = Q/ND3

Blending clip of impeller ( terbium ) is besides made dimensionless and multiplied by impellers rotational velocity, which is given as:

Nb = tbN

In to the full disruptive conditions both dimensionless pumping figure and blend clip are independent of Reynolds figure.

When Nre & gt ; 10,000 so the flow in armored combat vehicle is disruptive, on the other side when Nre & A ; lt ; 10 the flow is laminal. Therefore, between 10 and 10,000 the flow is in a passage scope and hence flow is disruptive around the fomenter and laminar in the stray parts of the fermenter.

Impellers such as, propellors, paddles and turbines are chiefly used for low syrupy mediums and in turbulent and passage governments.

Stirred armored combat vehicle reactors using mechanical fomenters can execute intense commixture. Around fomenters higher shear rate & A ; gamma ; ( s-1 ) is present which can damage delicate solid suspended atoms, such as biological cells/enzymes. Airlift fermenters possess simple mechanical constructions without any traveling portion such as impeller and hence has homogenous field of shear uniform throughout the fermenter.

Mechanical fomenters by and large improved O transportation capableness and blending public presentation of the system as compared to when mechanical agitation was non employed ; but, the O transportation efficiency was decreased by mechanical agitation.

To better the rate of commixture and to understate the whirl formation in moved armored combat vehicles baffles are employed. However, this causes addition in power ingestion by the system.

Harmonizing to the elaborate flow form visual image surveies fomenters such as, ground tackle and gate fomenters cause unstable gesture towards the walls of the fermenter, but create a dead part in the locality of the shaft. Furthermore, there is a small top to bottom turnover and therefore perpendicular concentration gradient signifiers. One solution to this is the add-on of prison guard or coiling thread to the shaft. These two impellers are used in combination as they create consequence in two different waies and consequences in diminishing the dead part in the locality of shaft. Helical thread pump the liquid medium upwards near the wall and screw pumps the liquid in downward way towards the shaft part.

Due to the gesture of the impeller a secondary circulation develops in the armored combat vehicle. As impeller moves fluid at the underside of the fermenter remains dead. Due to the motion of impeller the liquid at higher degree moves and its centrifugal force creates secondary flow in perpendicular way. This creates an imbalanced force per unit area force within the liquid and consequences in whirl. Harmonizing to the viscousness and other belongingss of the fluid this is called double-celled or one-celled.

Conventional stirred fermenters have a broader scope of applications but they perform ill in extremely syrupy non-Newtonian media, have a ill defined blending pattern comparative to airlift reactors and can non be aerated at a high rate because of impeller implosion therapy.

In moved armored combat vehicle reactors, cell mass concentrations, volumetric productivenesss, and specific power inputs are higher than in airlift reactors. In airlift efficiencies of O transportation, specific productivenesss with respect to substrate and oxygen ingestions, and output coefficients are well higher than in moved reactors.

As no mechanical agitating parts are present in it, there is no demand of shaft bearings, magnetic driven fomenters and seals. The absence of all these parts lessening danger of taint and besides brings down fermenter cost. Absence of these parts facilitates easier cleansing and sterilisation of the reactor. The injected gas used in air-lift fermenters serves double intent. It agitates the medium and secondly aerates the medium at the same clip. This helps in conveying down the cost for excess energy for agitation.

In airlift fermenters flow of the liquid is due to the difference in densenesss of the riser and the downcomer subdivisions of the reactor. Therefore, even if random motions are superimposed on it overall directivity of liquid flow is present. In contrast in automatically agitated fermenters the chief energy sourcing bring oning the flow of fluid is focal. In automatically agitated fermenters a great shear force is present near the fomenter which goes on cut downing with the addition in distance from the fomenter to the fermenter wall, whereas in airlift there is unvarying shear force throughout the fermenter.

Airlift fermenters have many chemical and biological procedures applications as either three or two stage reactors. In airlift fermenters the fluidization of solids is a direct effect of liquid circulation within the fermenter. Hence they offer simple and extremely effectual solid fluidization.

The mixing efficiency of an airlift fermenter is better than the automatically agitated system, with regard to power ingestion. Besides airlift fermenter performs better and gives more homogenous distribution when content of solid is high in the medium. Therefore airlift fermenters have higher blending efficiency than stirred tank reactors with the same power input. However, blending times are relatively longer when gas stage is involved.

For design and operation of an air-lift cognition of liquid circulation speeds, liquid-phase commixture times and axial commixture is prerequisite. Axial commixture is characterised by Bodenstein Numberss and axial scattering coefficients. In airlift fermenters blending in the top is highest among all subdivisions of the fermenter followed by the commixture in rise, which is better than the commixture in downcomer. There is besides an intensive commixture at the underside of the fermenter. This is due to the rapid flowing and turning about of the fluid. Impingement of the downflow fluid at the underside helps in intensive commixture.

BO = VL / D

Here, BO is a dimensionless commixture parametric quantity

L is the distance between two measured points

Furthermore, in airlift fermenters the undermentioned correlativities were established:

BoLG = & A ; szlig ; ( Fr1/3 ) & A ; lambda ;

Bo = K ( tm / technetium )

Here, Fr = Froude figure,

BoLG = Bodenstein figure based on superficial speed of the gas,

thulium = commixture clip

technetium = circulation clip

K, & A ; gamma ; , and & A ; szlig ; are invariables based on the geometry of the fermenter and fluid used.


Airlift fermenters are normally employed aerophilic agitations. In airlift reactors the gas is sparged in the riser. The gas-liquid scattering travels in upwards way with the co-current. This subdivision has higher gas hold up than any other subdivision of the fermenter and most of the gas-liquid mass transportation takes topographic point in this subdivision. Afterwards the liquid gas scattering enters in the gas-liquid centrifuge, a gas detachment zone. Here harmonizing to its design most of the spread gas is removed. Then the gas free liquid comes in downcomer and travels through the underside to the base of fermenter from where it once more enters into the riser.

The fluctuation in dissolved O affects the productiveness of the fermenter. At the underside of the fermenter where force per unit area and molar fraction of O2 is maximum the liquid is lacking in O. Furthermore, in the riser the sum of O2 additions as air is provided in the riser and therefore gas-liquid mass transportation occurs. Furthermore, as liquid rises force per unit area and dissolved O both lessenings in the system and hence driving force becomes smaller. When both these rate of forces becomes equal, the concentration profile becomes maximal and from this phase DO decreases. Hence in downcomer no gas scattering occurs and dissolved O profiles decreases. In this subdivision most of the O is consumed.

The overall Kla ) vitamin D O transportation coefficient was given by the equation calculated by Chisti, 1989,

Kla = 1.27 * 10-4 ( PG/ VL ) 0.925


( PG/ VL ) = & A ; rho ; LgUgr / ( 1+ ( Ad/ Ar ) )

( Kla ) R and ( Kla ) vitamin D values are selected in a mode that both two equations are satisfied:

Kla = [ ( Kla ) R Ar + ( Kla ) d Ad ) ] / [ Ar + Ad ]

( Kla ) vitamin D = & A ; Psi ; ( Kla ) R

The value of & A ; Psi ; as recommended by Chisti ( 1989 ) is fixed at 0.8.

In moved fermenters aeration does non effects the commixture clip to a larger extent. In it aeration depends on the power input and fluid flow construction inside the fermenter. Therefore coveted consequences can be achieved by counterbalancing bead in power by induced flow from aeration. Hence, there is really minimum alteration in the quality of blending. In it increase in m ( mixing clip, s ) is compensated by the liquid flow from aeration.

Power ingestion

Power ingestion is one of the most of import facets in planing a fermenter. At higher aeration rates there is a lessening in mechanical power ingestion. In moved armored combat vehicle fermenters due to the different sort of fluid blending mechanisms and unstable form we can see power ingestion in low syrupy fluids systems and high syrupy fluids systems.

In low syrupy fluids high velocity propellors are of 1/3 diameter of the fermenter and runs at 10-25 Hz.

In agitated systems the power input of low viscousness systems is expressed as:

P = degree Fahrenheit ( µ , & A ; rho ; , N, g, D, DT, others )

Here, µ = viscousness of a Newtonian liquid

& A ; rho ; = denseness

D = impeller diameter

N = rotary motion velocity

DT = armored combat vehicle diameter

By utilizing dimensional analysis, the figure of variables can be reduced and new equation is expressed as ;

NP = p/ & A ; rho ; N 3 D5 = degree Fahrenheit [ ( & A ; rho ; N D2/µ ) , ( N2D/g ) , ( DT/D ) , ( W/D ) , ( H/D ) .. ]

Here, NP = power figure

The simplest signifier of power jurisprudence can be given as ; NP = degree Fahrenheit ( Re, Fr )

Re and Fr are Reynolds figure and Froude figure, severally.

In high syrupy fluids the fluid in the locality of the impeller is influenced by the impeller and hence the flow is laminal, unlike in low syrupy fluids where flow is disruptive. Most of the non-Newtonian fluids show this behavior. Equation explicating this is:

& A ; gamma ; ang = KSN, KS is ths map of type of impeller and fermenter.

In air-lift fermenter energy economic system can be farther increased by add-on of 2nd sparger in the upper portion of downcomer.

The power demands of moved fermenters increase with the addition in sum of solid in the medium, despite at the same rotary motion velocity. Whereas in airlift fermenters for similar reaction rates and mixing clip the power input is less than a one-fourth of the moved armored combat vehicle.


The coveted consequences determine the construction of blending equipments. The power, capital cost, fomenter size and torsion influences the overall cost of the fermenter system. Hence, there is a trade off in the operating cost. As some reactions require local turbulency and some require high local turbulency hence consequently cost varies as per reaction and coveted merchandises. Table1 explains the cost of equipments harmonizing to their capacities.

Absence of any traveling portion saves a batch of production cost in air-lift fermenters. This besides prevents wear and tear inside the fermenter among traveling portion and hence reduces care and sterilization costs.


The automatically agitated fermenters are preferred in industry due to its well defined graduated table up features, general public presentation and “ off-the-rack ” convenience. Air-lift fermenters have great application in biochemical and waste intervention industry due to their low cost of operations.


In the concluding analysis, air-lift fermenters and automatically moved fermenters both have their strong points ; therefore they are suited to peculiar demands.

Airlift fermenters has unvarying shear force across the fermenter, less wear and tear due to absence of traveling parts, less care and production cost and less power ingestion. These belongingss make them suited for biochemical reactions in fermenter.

Whereas, automatically moved fomenters are of different sorts, successfully in usage in industry from past many decennaries, have good defined graduated table up belongingss and have “off-the-rack ” convenience ” . Therefore they are preferred in industrial graduated tables.


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