Bubble and Airlift bioreactors types and applications

The ppt discusses about the design of bubble and airlift bioreactor. It also deals with their application, advantages and disadvantages. Pictorial representation of bioreactors are provided for better understanding of the bioreactors. It helps to gain knowledge on the different type of bioreactors

1. BT 8591 BPE Unit – 1 Types of Bioreactors ( 2)

2. Bubble column Bioreactor Bubble column bioreactors are tall column bioreactors where gas is introduced in the bottom section for mixing and aeration purposes . The vessel used for bubble column bioreactors is usually cylindrical with an aspect ratio of 4-6 . Usually the height-to-diameter ratio is 4-6 . Gas is sparged at the base through perforated pipes or plates or metal porous spargers . . O2 transfer, mixing and other performance factors are influenced mainly by gas flow rate and rheological properties of the fluid. Mixing and mass transfer can be improved by placing perforated plates or vertical baffles in the vessel. Does not have a draft tube.

3. Bubble column Bioreactor In the bubble column bioreactor, the air or gas is introduced at the base of the column through perforated pipes or plates, or metal micro porous spargers and causes a turbulent stream to enable gas exchange. The flow rate of the air/gas influences the performance factors —O2 transfer, mixing. The bubble column bioreactors may be fitted with perforated plates to improve performance. The reactants are compacted in the presence of finely dispersed catalyst and thus produce the products using fermentation method.

4. BCR Bubble column – Basic design Bubble column with perforated plates

5. Bubble column bioreactor In bubble-column reactors, aeration and mixing are achieved by gas sparging ; this requires less energy than mechanical stirring. Bubble columns are structurally very simple. They are generally cylindrical vessels with height greater than twice the diameter. Other than a sparger for entry of compressed air, bubble columns typically have no internal structures. A height-to-diameter ratio of about 3:1. horizontal plates are sometimes installed in tall bubble columns to break up and redistribute coalesced bubbles.

6. Bubble column bioreactor Bubble-column hydrodynamics and mass-transfer characteristics depend entirely on the behaviour of the bubbles released from the sparger . Different flow regimes occur depending on the gas flow rate, sparger design, column diameter and medium properties such as viscosity. Homogeneous flow occurs only at low gas flow rates and when bubbles leaving the sparger are evenly distributed across the column cross-section. In homogeneous flow, all bubbles rise with the same upward velocity and there is no backmixing of the gas phase . Liquid mixing in this flow regime is also limited, arising solely from entrainment in the wakes of the bubbles.

7. Bubble column bioreactor Under normal operating conditions at higher gas velocities, large chaotic circulatory flow cells develop and heterogeneous flow occurs as illustrated in Figure 13.6. In this regime, bubbles and liquid tend to rise up the centre of the column while a corresponding downflow of liquid occurs near the walls. Liquid circulation entrains bubbles so that some backmixing of gas occurs. Liquid mixing time in bubble columns depends on the flow regime. For heterogeneous flow, the following equation has been proposed for the upward liquid velocity at the centre ofthe column for 0.1 < D< 7.5 m and 0 < u G < 0.4 ms -l:

8. Bubble column bioreactor – hydrodynamics

9. Bubble column bioreactor

10. Bubble column bioreactor Values for gas-liquid masstransfer coefficients in reactors depend largely on bubble diameter and gas hold-up. In bubble columns containing non viscous liquids, these variables depend solely on the gas flow rate. Exact bubble size and liquid circulation patterns are impossible to predict in bubble columns. Accurate estimation of mass transfer coefficient is difficult. The following correlation has been proposed for non-viscous media in heterogeneous flow.

11. Bubble column bioreactor where kla is the combined volumetric mass-transfer coefficient and u G is the gas superficial velocity. Eq . (13.3) is valid for bubbles with mean diameter about 6 mm, 0.08 m < D < 11.6 m, 0.3 m < H< 21 m, and 0 < u G < 0.3 m s -1. If smaller bubbles are produced at the sparger and the medium is noncoalescing , kLa will be larger than the value calculated using Eq. (13.3), especially at low values of u G less than about 10 -2 m s - 1.

12. BCR – adv & disadv Advantages Disadvantages The main disadvantage of  bubble column  reactors is backmixing , which adversely affects product conversion.

13. BCR – Applications Bubble columns are applied industrially for production of bakers' yeast, beer and vinegar, and for treatment of waste water .

14. Air lift Bioreactor As in bubble columns, mixing in airlift reactors is accomplished without mechanical agitation. Airlift reactors are often chosen for culture of plant and animal cells and immobilised catalysts because shear levels are significantly lower than in stirred vessels. Their distinguishing feature compared with the bubble column is that patterns of liquid flow are more defined owing to the physical separation of up-flowing and down-flowing streams. As shown in Figure 13.7, gas is sparged into only part of the vessel crosssection called the riser . Gas hold-up and decreased fluid density cause liquid in the riser to move upwards. Gas disengages at the top of the vessel leaving heavier bubble-free liquid to recirculate through the downcomer . Liquid circulates in airlift reactors as a result of the density difference between riser and downcomer .

15. Air lift Bioreactor

16. Air lift Bioreactor Figure 13.7 illustrates the most common airlift configurations. In the internal-loop vessels of Figures 13.7(a) and 13.7(b), the riser and downcomer are separated by an internal bafHe or draft tube; air may be sparged into either the draft tube or the annulus. In the external-loop or outer-loop airlift of Figure 13.7(c), separate vertical tubes are connected by short horizontal sections at the top and bottom. Because the riser and downcomer are further apart in external-loop vessels, gas disengagement is more effective than in internal-loop devices. Fewer bubbles are carried into the downcomer , the density difference between fluids in the riser and downcomer is greater, and circulation of liquid in the vessel is faster. Accordingly, mixing is usually better in external-loop than internal-loop reactors.

17. Air lift Bioreactor Airlift reactors generally provide better mixing than bubble columns except at low liquid velocities when circulatory flowpatterns similar to those shown in Figure 13.6 develop. The airlift configuration confers a degree of stability to liquid flow compared with bubble columns; therefore, higher gas flow rates can be used without incurring operating problems such as slug flow or "spray formation. Several empirical correlations have been developed for liquid velocity, circulation time and mixing time in airlift reactors; however there is considerable discrepancy between the results . Equations derived from hydrodynamic models are also available ; these are usually relatively complex and, because liquid velocity and gas hold-up are not independent, require iterative numerical solution.

18. Air lift Bioreactor Several other empirical mass-transfer correlations have been developed for Newtonian and non-Newtonian fluids in airlift reactors . Performance of airlift devices is influenced significantly by the details of vessel construction . For example, in internal-loop airlifts, changing the distance between the lower edge of the draft tube and the base of the reactor alters the pressure drop in this region and affects liquid velocity and gas hold-up. The depth of draft-tube submersion from the top of the liquid also influences mixing and mass-transfer characteristics.

19. Air lift Bioreactor Airlift reactors have been applied in production of single cell protein from methanol and gas oil; they are also used for plant and animal cell culture and in municipal and industrial waste treatment. Large airlift reactors with capacities of thousands of cubic metres have been constructed. Tall internal-loop airlifts built underground are known as deepshaft reactors; very high hydrostatic pressure at the bottom of these vessels considerably improves gas-liquid mass-transfer. The height of airlift reactors is typically about 10 times the diameter; for deep-shaft systems the height-to-diameter ratio may be increased up to 100.

20. Air lift Bioreactor

21. Air lift Bioreactor

22. Air lift bioreactor Advantages Simple design with no moving parts. Less maintenance & less risk. Easier sterilisation. Low energy requirement compared to stirred tanks. Greater heat removal compared to stirred tanks . Disadvantages Greater air through put & high pressures needed. Foam breaking in efficient. Development of dead zones. Relatively high costs for compressing flow rate, difficult to adjust . Non uniform nutrient supply. Insufficient mixing .

23. ALF – other configurations

24. ALF – other conf

25. ALF – uses Air - lift fermenter  is suitable to plant and mammalian cells  fermentation  due to its low shear rate, protect cells from damage. The application of  air - lift fermentation  is the production of monoclonal antibodies. ...