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Compare/contrast the relative merits of air-lift and mechanical agitators in fermenters
The given assignment thesis is on the topic "compare/contrast the relative merits of air-lift and mechanical agitators in fermenters". In this some key properties of air-lift and mechanical agitators are explained. Firstly, there is a small introduction of air-lift fermenter and mechanical agitators. Secondly, each property is explained with it's similarity and differences.
Airlift Fermenters are pneumatically agitated fermenters and having four distinct zones, such as, riser, gas-liquid separator, downcomer and base. These four zones divide the fermenter into upward and downward regions i.e. two phase flow regions. Moreover, the major cause of circulation in the air-lift fermenters is the fractional gas hold (ΔεG) which exists between the riser (εGR) and the downcomer (εGD). The driving force for the circulation of fluid is the hydrostatic pressure difference. The bottom of riser and the bottom of the base creates this hydrostatic pressure difference. The pressure difference at the bottom of the fermenter is:
ΔPB = ρL g (ΦR - ΦD)
Here, ΔPB is the pressure difference,
ρL = density of the fluid (density of gas is considered negligible)
g = gravitational constant
ΦR, ΦD = fractional gas hold up of riser and downcomer, respectively.
General correlation to predict gas hold up in air-lift fermenters is:
ΦR = a (JG)α (AD/ AR)β (µeff)γ
Here, Φ = gas hold up
JG = superficial gas velocity
µeff = effective viscosity of the liquid
Airlift fermenters are categorized into two categories based on their physical structures. In some airlift fermenters baffles are placed in the fermenter to create a distinctive flow channel of the loop. These are known as Internal-loop airlift reactors. In second kind of airlift fermenters, i.e. External-loop reactors the downcomer and the riser are two separate tubes connected by horizontal sections at the bottom and the top.
Agitators play a key role in the mixing phenomena of a fermenter. They improve both oxygen transfer capability and mixing. Moreover, gas distribution in the fermenter is the function of impeller and not of sparger. Therefore, an ideal agitator should provide rapid agitation so that all the gas bubbles should disperse throughout the fermenter and increase their residence time inside the liquid so that no air can escape before all the O2 is used up. It should also shear large bubbles into smaller ones. Sometimes too much of stirring can be detrimental, as it can effect biological contents (e.g., animal cells) of fermenter and also can cause stratification of reactor contents with multiple impeller systems. While mixing is going on impeller should keep all the solid particles suspended in the medium, maintain optimum substrate and biomass concentration and avoid hotspots or uniform temperature in the fermenter.
A variety of impeller designs are proposed, but the most preferred are turbine and disc type impellers. Moreover, for cellular/biological systems which are highly sensitive to shear forces paddle and marine impellers are of particular interest. The Rushton impeller design was predominantly in use in the mid 1980s and is usually found on laboratory and industrial fermenters. It is a disc turbine with 6 flat blades and pumps fluid in radial directions. Another type of impellers, axial hydrofoil impellers (e.g., A200 impeller) are increasingly popular. This is due to the fact; they can pump liquid in either upward or downward direction, having low energy demands, reduce maximum shear rates and give excellent performance with viscous fermentations, such as mycelia fermentations. New impeller designs are introduced 1980s onwards e.g.,, A6000 and A315 Lightnin. These are the impellers which can cause high flow and having very low shear rates.
Sometimes two agitators are set at an angle of 900 to each other for better results. Commonly used impellers are; Three-bladed propellers, simple propellers, helical ribbon, helical screw with draft tube, paddle, gate anchor, Banbury mixer, Six bladed disc turbine, Anchor impeller and Z-blade mixer.
Comparisons and Contrasts
This section will give all the similarities and differences in air-lift and mechanical agitators. Some of the merits which are taken into consideration are:
- Energy use
Mixing operations in any fermenter is measured on the basis of Mixing/Blend times and Rate of mixing. Mixing time is defined as the time required to produce the mixture of a desired quality. The rate at which mixing operation proceeds to its final state is known as the Rate of a mixing operation. Mixing time for a particular given experiment with operating variables is shown as:
tm = f (ρ, µ, N, D, g, geometrical dimensions of the system)
As per dimensional analysis, this functional relationship can be rearranged as:
Ntm = θm = f (ρND2/µ, DN2/g, geometrical dimensions as ratios)
Assuming Froude number DN2/g is not much of a importance and for geometrically similar systems,
θm = f (ρND2/µ) = f (Re), here, Re = Reynolds number
ReI = (ρND2/µ),
ReI = impeller Reynolds number
D = impeller diameter
N = rotational speed
ρ = the liquid density
µ = viscosity
Impellers sometimes may be assumed as pumping devices, due to the axial and radial flow created by them in the fermenter. In pumping number the dimensionless total volumetric flow rate 'Q' discharged by an impeller is shown as:
NQ = Q/ND3
Mixing time of impeller (tb) is also made dimensionless and multiplied by impellers rotational speed, which is given as:
Nb = tbN
In fully turbulent conditions both dimensionless pumping number and blend time are independent of Reynolds number.
When Nre > 10,000 then the flow in tank is turbulent, on the other side when Nre <10 the flow is laminar. Therefore, between 10 and 10,000 the flow is in a transition range and hence flow is turbulent around the agitator and laminar in the isolated parts of the fermenter.
Impellers such as, propellers, paddles and turbines are principally used for low viscous mediums and in turbulent and transition regimes.
Stirred tank reactors employing mechanical agitators can perform intense mixing. Around agitators higher shear rate γ(s-1) is present which can damage fragile solid suspended particles, such as biological cells/enzymes. Airlift fermenters possess simple mechanical structures without any moving part such as impeller and hence has homogeneous field of shear uniform throughout the fermenter.
Mechanical agitators generally improved oxygen transfer capability and mixing performance of the system as compared to when mechanical agitation was not employed; but, the oxygen transfer efficiency was decreased by mechanical agitation.
To improve the rate of mixing and to minimise the vortex formation in stirred tanks baffles are employed. However, this causes increase in power consumption by the system.
According to the detailed flow pattern visualisation studies agitators such as, anchor and gate agitators cause fluid motion towards the walls of the fermenter, but create a stagnant region in the vicinity of the shaft. Moreover, there is a little top to bottom turnover and hence vertical concentration gradient forms. One solution to this is the addition of screw or helical ribbon to the shaft. These two impellers are used in combination as they create effect in two different directions and results in decreasing the stagnant region in the vicinity of shaft. Helical ribbon pump the liquid medium upwards near the wall and screw pumps the liquid in downward direction towards the shaft region.
Due to the motion of the impeller a secondary circulation develops in the tank. As impeller moves fluid at the bottom of the fermenter remains stagnant. Due to the movement of impeller the liquid at higher level moves and its centrifugal force creates secondary flow in vertical direction. This creates an unbalanced pressure force within the liquid and results in vortex. According to the viscosity and other properties of the fluid this is called double-celled or single-celled.
Conventional stirred fermenters have a broader range of applications but they perform poorly in highly viscous non-Newtonian media, have a poorly defined mixing pattern relative to airlift reactors and cannot be aerated at a high rate because of impeller flooding.
In stirred tank reactors, cell mass concentrations, volumetric productivities, and specific power inputs are higher than in airlift reactors. In airlift efficiencies of oxygen transfer, specific productivities with regard to substrate and oxygen consumptions, and yield coefficients are considerably higher than in stirred reactors.
As no mechanical agitating parts are present in it, there is no requirement of shaft bearings, magnetic driven agitators and seals. The absence of all these parts decrease danger of contamination and also brings down fermenter cost. Absence of these parts facilitates easier cleaning and sterilization of the reactor. The injected gas used in air-lift fermenters serves dual purpose. It agitates the medium and secondly aerates the medium at the same time. This helps in bringing down the cost for extra energy for agitation.
In airlift fermenters flow of the liquid is due to the difference in densities of the riser and the downcomer sections of the reactor. Therefore, even if random movements are superimposed on it overall directionality of liquid flow is present. In contrast in mechanically agitated fermenters the main energy sourcing inducing the flow of fluid is focal. In mechanically agitated fermenters a great shear force is present near the agitator which goes on reducing with the increase in distance from the agitator to the fermenter wall, whereas in airlift there is uniform shear force throughout the fermenter.
Airlift fermenters have many chemical and biological processes applications as either three or two phase reactors. In airlift fermenters the fluidization of solids is a direct consequence of liquid circulation within the fermenter. Hence they offer simple and highly effective solid fluidization.
The mixing efficiency of an airlift fermenter is better than the mechanically agitated system, with respect to power consumption. Also airlift fermenter performs better and gives more homogeneous distribution when content of solid is high in the medium. Therefore airlift fermenters have higher mixing efficiency than stirred tank reactors with the same power input. However, mixing times are comparatively longer when gas phase is involved.
For design and operation of an air-lift knowledge of liquid circulation velocities, liquid-phase mixing times and axial mixing is prerequisite. Axial mixing is characterised by Bodenstein numbers and axial dispersion coefficients. In airlift fermenters mixing in the top is highest among all sections of the fermenter followed by the mixing in rise, which is better than the mixing in downcomer. There is also an intensive mixing at the bottom of the fermenter. This is due to the rapid flowing and turning around of the fluid. Impingement of the downflow fluid at the bottom helps in intensive mixing.
BO = VL / D
Here, BO is a dimensionless mixing parameter
L is the distance between two measured points
Moreover, in airlift fermenters the following correlations were established:
BoLG = ß(Fr1/3)λ
Bo = k( tm / tc)
Here, Fr = Froude number,
BoLG = Bodenstein number based on superficial velocity of the gas,
tm = mixing time
tc = circulation time
k, γ, and ß are constants based on the geometry of the fermenter and fluid used.
Airlift fermenters are commonly employed aerobic fermentations. In airlift reactors the gas is sparged in the riser. The gas-liquid dispersion travels in upwards direction with the co-current. This section has higher gas hold up than any other section of the fermenter and most of the gas-liquid mass transfer takes place in this section. Afterwards the liquid gas dispersion enters in the gas-liquid separator, a gas disengagement zone. Here according to its design most of the dispersed gas is removed. Then the gas free liquid comes in downcomer and travels through the bottom to the base of fermenter from where it again enters into the riser.
The variation in dissolved oxygen affects the productivity of the fermenter. At the bottom of the fermenter where pressure and molar fraction of O2 is maximal the liquid is deficient in oxygen. Furthermore, in the riser the amount of O2 increases as air is provided in the riser and hence gas-liquid mass transfer occurs. Moreover, as liquid rises pressure and dissolved oxygen both decreases in the system and therefore driving force becomes smaller. When both these rate of forces becomes equal, the concentration profile becomes maximum and from this stage DO decreases. Hence in downcomer no gas dispersion occurs and dissolved oxygen profiles decreases. In this section most of the oxygen is consumed.
The overall Kla)d oxygen transfer coefficient was given by the equation calculated by Chisti, 1989,
Kla = 1.27 * 10-4 (PG/ VL) 0.925
(PG/ VL) = ρLgUgr / (1+ (Ad/ Ar))
(Kla)r and (Kla)d values are selected in a manner that both two equations are satisfied:
Kla = [(Kla)r Ar + (Kla)d Ad)] / [Ar + Ad ]
(Kla)d = Ψ (Kla)r
The value of Ψ as recommended by Chisti (1989) is fixed at 0.8.
In stirred fermenters aeration does not effects the mixing time to a larger extent. In it aeration depends on the power input and fluid flow structure inside the fermenter. Therefore desired results can be achieved by compensating drop in power by induced flow from aeration. Hence, there is very minimal change in the quality of mixing. In it increase in m (mixing time, s) is compensated by the liquid flow from aeration.
Power consumption is one of the most important aspects in designing a fermenter. At higher aeration rates there is a decrease in mechanical power consumption. In stirred tank fermenters due to the different kind of fluid mixing mechanisms and fluid pattern we can consider power consumption in low viscous fluids systems and high viscous fluids systems.
In low viscous fluids high speed propellers are of 1/3 diameter of the fermenter and runs at 10-25 Hz.
In agitated systems the power input of low viscosity systems is expressed as:
P = f (µ, ρ, N, g, D, DT, others)
Here, µ = viscosity of a Newtonian liquid
ρ = density
D = impeller diameter
N = rotation speed
DT = tank diameter
By using dimensional analysis, the number of variables can be reduced and new equation is expressed as;
NP = p/ρ N 3 D5 = f [(ρN D2/µ), (N2D/g), (DT/D), (W/D),(H/D)..]
Here, NP = power number
The simplest form of power law can be given as; NP = f (Re, Fr)
Re and Fr are Reynolds number and Froude number, respectively.
In high viscous fluids the fluid in the vicinity of the impeller is influenced by the impeller and hence the flow is laminar, unlike in low viscous fluids where flow is turbulent. Most of the non-Newtonian fluids show this behaviour. Equation explaining this is:
γang = KSN, KS is ths function of type of impeller and fermenter.
In air-lift fermenter energy economy can be further increased by addition of second sparger in the upper part of downcomer.
The power requirements of stirred fermenters increase with the increase in amount of solid in the medium, despite at the same rotation speed. Whereas in airlift fermenters for similar reaction rates and mixing time the power input is less than a quarter of the stirred tank.
The desired results determine the structure of mixing equipments. The power, capital cost, agitator size and torque influences the overall cost of the fermenter system. Hence, there is a trade off in the operating cost. As some reactions require local turbulence and some require high local turbulence therefore accordingly cost varies as per reaction and desired products. Table1 explains the cost of equipments according to their capacities.
Absence of any moving part saves a lot of production cost in air-lift fermenters. This also prevents wear and tear inside the fermenter among moving part and hence reduces maintenance and sterilisation costs.
The mechanically agitated fermenters are preferred in industry due to its well defined scale up characteristics, general performance and "off-the-shelf" convenience. Air-lift fermenters have great application in biochemical and waste treatment industry due to their low cost of operations.
In the final analysis, air-lift fermenters and mechanically stirred fermenters both have their strong points; therefore they are suitable to particular needs.
Airlift fermenters has uniform shear force across the fermenter, less wear and tear due to absence of moving parts, less maintenance and production cost and less power consumption. These properties make them suitable for biochemical reactions in fermenter.
Whereas, mechanically stirred agitators are of different kinds, successfully in use in industry from past many decades, have well defined scale up properties and have "off-the-shelf" convenience". Therefore they are preferred in industrial scales.
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