Fermentation technology-chapter-viiviii-ix-x


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Fermentation technology-chapter-viiviii-ix-x

  1. 1. Chapter VIIHeat Transfer in Fermentation 1
  2. 2. IntroductionSeveral important chemical engineering concepts inBioprocess Engineering are transport phenomena (fluid flow,mixing, heat and mass transfer), unit operations, reactionengineering, and bioreactor engineering.Fluid flow, mixing, and reactor engineering are skipped in thisclass. They are available more detail in several chemicalengineering books.We start with the heat transfer in bioreactors 2
  3. 3. Two types of common heat transfer applicationin bioreactor operation • In situ batch sterilization of liquid medium. In this process, the fermenter vessel containing medium is heated using steam and held at the sterilization temperature for a period of time; cooling water is then used to bring the temperature back to normal operating conditions • Temperature control during reactor operation. Metabolic activity of cells generates heat. Some microorganisms need extreme temperature conditions (e.g. psycrophilic, thermophilic microorganisms) Heat transfer configurations for bioreactors: jacketed vessel, external coil, internal helical coil, internal baffle-type coil, and external heat exchanger. 3
  4. 4. Pro’s and cons of the heat exchanger configurations • External jacket and coil give low heat transfer area. Thus, they are rarely used for industrial scale. • Internal coils are frequently used in production vessel; the coils can be operated with liquid velocity and give relatively large heat transfer area. But the coil interfere with the mixing in the vessel and make cleaning of the reactor difficult. Another problem is film growth of cells on the heat transfer surface. • External heat exchanger unit is independent of the reactor, easy to scale up, and provide best heat transfer capability. However, conditions of sterility must be met, the cells must be able to withstand the shear forces imposed during pumping, and in aerobic fermentation, the residence time in the heat exchanger must be small enough to ensure the medium does not become depleted of oxygen. 4
  5. 5. Heat exchangers in fermentation processes • Double-pipe heat exchanger • Shell and tube heat exchanger • Plate heat exchanger • Spiral heat exchanger In bioprocess, the temperature difference is relatively small. Thus, plate heat exchanger is almost never being used The concepts and calculation for heat exchangers and their configurations are available in the text book ( Pauline Doran, Bioprocess Eng Principle, chapter 8) 5
  6. 6. Chapter VIIIMass Transfer in Fermentation 6
  7. 7. IntroductionThe Fick’s law of diffusion dC A J A = − DAB dyRole of diffusion in Bioprocess• Scale of mixingMixing on a molecular scale relies on diffusion as the final step in mixingprocess because of the smallest eddy size• Solid-phase reactionThe only mechanism for intra particle mass transfer is molecular diffusion• Mass transfer across a phase boundaryOxygen transfer to gas bubble to fermentation broth, penicillin recoveryfrom aqueous to organic liquid, glucose transfer liquid medium into mouldpellets are typical example. 7
  8. 8. Film theoryThe two film theory is a useful model for mass transferbetween phase. Mass transfer of solute from one phase toanother involves transport from bulk of one phase to theinterface, and then from the interface to the bulk of the secondphase. This theory is based on idea that a fluid film or masstransfer boundary layer forms whenever there is contactbetween two phases. According to film theory, mass transferthrough the film is solely by molecular diffusion and is themajor resistance. CA1i CA1 Bulk fluid 1 Bulk fluid 2 CA2i CA2 Film 2 Film 1 8
  9. 9. Convective mass transfer It refers to mass transfer occurring in the presence of bulk fluid motion N A = ka( C Ao − C Ai ) k: mass transfer coefficient [m/s] a: area available for mass transfer [m2/m3] CAo: concentration of A at bulk fluid CAi: concentration of A at interface For gas-liquid system, A from gas to liquid: N AL = k L a( C ALi − C AL ) N AG = kG a( C AG − C AGi ) 9
  10. 10. Overall mass transfer coefficientRefers to the book Geankoplis (2003), Transport Processes andSeparation Process Principles, 4th ed, chapter 10.4.Oxygen transport to fermentation broth can be modeled asdiffusion of A through stagnant or non-diffusing B. 1 1 m = + K G a kG a k L a ( N A = K L a C AL − C AL * )If A is poorly soluble in the liquid, e.g. oxygen in aqueoussolution, the liquid-phase mass transfer resistance dominatesand kGa is much larger than kLa. Hence, KLa ≈ kLa. 10
  11. 11. Oxygen transfer from gas bubble to cell Eight steps involved: i. Transfer from the interior of the bubble to the gas-liquid interface ii. Movement across the gas-liquid interface iii. Diffusion through the relatively stagnant liquid film surrounding the bubble iv. Transport through the bulk liquid v. Diffusion through the relatively stagnant liquid film surrounding the cells vi. Movement across the liquid-cell interface vii. If the cells are in floc, clump or solid particle, diffusion through the solid of the individual cell viii. Transport through the cytoplasm to the site of reaction. 11
  12. 12. Analyzes for most bioreactors in each step involved i. Transfer through the bulk phase in the bubble is relatively fast ii. The gas-liquid interface itself contributes negligible resistance iii. The liquid film around the bubble is a major resistance to oxygen transfer iv. In a well mixed fermenter, concentration gradients in the bulk liquid are minimized and mass transfer resistance in this region is small, except for viscous liquid. v. The size of single cell <<< gas bubble, thus the liquid film around cell is thinner than that around the bubble. The mass transfer resistance is negligible, except the cells form large clumps. vi. Resistance at the cell-liquid interface is generally neglected vii. The mass transfer resistance is small, except the cells form large clumps or flocs. viii. Intracellular oxygen transfer resistance is negligible because of the small distance involved 12
  13. 13. Chapter IX Unit Operations in Fermentation(introduction to downstream processing) 13
  14. 14. Downstream processing, what and whyDownstream processing is any treatment of culture broth afterfermentation to concentrate and purify products. It follows a generalsequence of steps:1.Cell removal (filtration, centrifugation)2.Primary isolation to remove components with properties significantlydifferent from those of the products (adsorption, liquid extraction,precipitation). Large volume, relatively non selective3.Purification. Highly selective (chromatography, ultra filtration, fractionalprecipitation)4.Final isolation (crystallization, followed by centrifugation or filtrationand drying). Typical for high-quality products such as pharmaceuticals.Downstream processing mostly contributes 40-90 % of total cost. 14
  15. 15. FiltrationType of filtration unit:• Plate and frame filter. For small fermentation batches• Rotary-drum vacuum filter. Continuous filtration that is widely used inthe fermentation industry. A horizontal drum 0.5-3 m in diameter iscovered with filter cloth and rotated slowly at 0.1-2 rpm.The filtration theory and equation are not explained here since they areavailable in the course “Unit Operations of Chemical Engineering I”. 15
  16. 16. CentrifugationCentrifugation is used to separate materials of different density when aforce greater than gravity is desiredThe type of industrial centrifugation unit:• Tubular bowl centrifuge (Narrow tubular bowl centrifuge orultracentrifuge, decanter centrifuge, etc). Simple and widely applied in foodand pharmaceutical industry. Operates at 13000-16000 G, 10 5-106 G forultracentrifuge• Disc-stack bowl centrifuge. This type is common in bioprocess. Thedeveloped forces is 5000-15000 G with minimal density differencebetween solid and liquid is 0.01-0.03 kg/m3. The minimum particlediameter is 5 µm 16
  17. 17. Centrifugation (dry solid decanter centrifuge) 17
  18. 18. The centrifugation theoryThe terminal velocity during gravity settling of a small spherical particle indilute suspension is given by Stoke’s law: ρp − ρf 2 ug = Dp g 18µWhere ug is sedimentation velocity under gravity, ρp is particle density, ρfis liquid density, µ is liquid viscosity, Dp is diameter of the particle, and gis gravitational acceleration.In the centrifuge: ρp − ρ f 2 2 uc = D pω r 18µuc is particle velocity in the centrifuge, ω is angular velocity in rad/s, and ris radius of the centrifuge drum. 18
  19. 19. The centrifugation theoryThe ratio of velocity in the centrifuge to velocity under gravity is called thecentrifuge effect or G-number. ω 2r Z= gIndustrial Z factors: 300-16 000, small laboratory centrifuge may up to 500 000.The parameter for centrifuge performance is called Sigma factor Q Σ= 2u gQ is volumetric feed rate. The Sigma factor explain cross sectional area of agravity settler with the same sedimentation characteristics as the centrifuge. If twocentrifuge perform with equal effectiveness Q1 Q2 = Σ1 Σ 2 19
  20. 20. The centrifugation theory 2πω 2 ( N − 1) 3 3 Disc-stack bowl centrifuge Σ= 3 g tan θ ( r2 − r1 )N is number of disc, θ is half-cone angle of the disc.The r1 and r2 are inner and outer radius of the disc, respectively. πω 2b 2 2Tubular-bowl centrifuge Σ= 2g (3r2 − r1 )b is length of the bowl, r1 and r2 are inner and outer radius of the wall of thebowl. 20
  21. 21. Cell disruption Mechanical cell disruption methods •French press (pressure cell) and high-pressure homogenizers. In these devices, the cell suspension is drawn through a check valve into a pump cylinder. At this point, it is forced under pressure (up to 1500 bar) through a very narrow annulus or discharge valve, over which the pressure drops to atmospheric. Cell disruption is primary achieved by high liquid shear in the orifice and the sudden pressure drop upon discharge causes explosion of the cells. •Ultrasonic disruption. It is performed by ultrasonic vibrators that produce a high-frequency sound with a wave density of approximately 20 kHz/s. A transducer convert the waves into mechanical oscillations via a titanium probe immersed in the concentrated cell suspension. For small scale 21
  22. 22. Cell disruption                           22
  23. 23. The equation for Manton-Gaulin homogenizer  Rm  ln  = kNpα R −R  m  Rm: maximum amount protein available for release R: amount of protein release after N passes through the homogenizer k: temperature-dependent rate constant p: operating pressure drop α: resistance parameter of the cells, for S. cerevisiae is 2.9 23
  24. 24. Cell disruption Non mechanical cell disruption methods Autolysis, use microbe own enzyme for cell disruption Osmotic shock. Equilibrating the cells in 20% w/v buffered sucrose, then rapidly harvesting and resuspending in water at 4 oC. Addition of chemicals (EDTA, Triton X-100), enzymes (hydrolyses, β- glucanases), antibiotics (penicillin, cycloserine) 24
  25. 25. Chromatography Chromatographic techniques usually employed for high value products. These methods, normally involving columns of chromatographic media (stationary phase), are used for desalting, concentration and purification of protein preparations. Several important aspects are molecular weight, isoelectric point, hydrophobicity and biological affinity. The methods are: 1.Adsorption chromatography 2.Affinity chromatography 3.Gel filtration chromatography 4.High performance liquid chromatography 5.Hydrophobic chromatography 25 6.Metal chelate chromatography
  26. 26. Finishing steps (final isolation) Crystallization Product crystallization may be achieved by evaporation, low-temperature treatment or the addition of a chemical reactive with the solute. The product’s solubility can be reduced by adding solvents, salts, polymers, and polyelectrolytes, or by altering pH. Drying Drying involves the transfer of heat to the wet material and removal of the moisture as water vapor. Usually, this must be performed in such a way as to retain the biological activity of the product. The equipment could be rotary drum drier, vacuum tray drier, or freeze-drier. 26
  27. 27. Chapter XBioreactor 27
  28. 28. Bioreactor configurationsStirred tank bioreactorSimilar to CSTR; this requires a relatively high input of energy per unitvolume. Baffles are used to reduce vortexing. A wide variety of impeller sizesand shapes is available to produce different flow patterns inside the vessel; intall fermenters, installation of multiple impellers improves mixing.Typically, only 70-80 % of the volume of stirred reactors is filled with liquid;this allows adequate headspace for disengagement of droplets from exhaustgas and to accommodate any foam which may develop. Foam breaker may benecessary if foaming is a problem. It is preferred than chemical antifoambecause the chemicals reduce the rate of oxygen transfer.The aspect ratio (H/D) of stirred vessels vary over a wide range. Whenaeration is required, the aspect ratio is usually increased. This provides forlonger contact times between the rising bubbles and liquid and produces agreater hydrostatic pressure at the bottom of the vessel.Care is required with particular catalysts or cells which may be damaged ordestroyed by the impeller at high speeds. 28
  29. 29. Bioreactor configurations 29
  30. 30. Bioreactor configurationsBubble columnIn bubble-column reactors, aeration and mixing are achieved by gas sparging;this requires less energy than mechanical stirring. Bubble columns are appliedindustrially for production of bakers’ yeast, beer and vinegar, and fortreatment of wastewater.A height-to-diameter ration of 3:1 is common in bakers’ yeast production; forother applications, towers with H/D of 6:1 have been used. The advantagesare low capital cost, lack of moving parts, and satisfactory heat and masstransfer performance. Foaming can be problem.Homogeneous flow: all bubbles rise with the same upward velocity and thereis no back-mixing of the gas phase.Heterogeneous flow: At higher gas velocity. Bubbles and liquid tend to riseup in the center of the column while a corresponding down flow of liquidoccurs near the walls. 30
  31. 31. Bioreactor configurationsAirlift reactorAirlift reactors are often chosen for culture of plant and animal cells andimmobilized catalyst because shear level are low. Gas is sparged into onlypart of the vessel cross section called the riser. Gas hold-up and decreasedliquid fluid density cause liquid in the riser to move upwards. Gas disengagesat the top of the vessel leaving heavier bubble-free liquid to recirculatethrough the downcomer. Airlift reactors configurations are internal-loopvessels and external-loop vessels. In the internal-loop vessels, the riser anddowncomer are separated by an internal baffle or draft tube. Air may besparged into either the draft tube or the annulus. In the external-loop vessels,separated vertical tubes are connected by short horizontal section at the topand bottom. Because the riser and downcomer are further apart in external-loop vessels, gas disengagement is more effective than in internal-loopdevices. Fewer bubbles are carried into the downcomer, the density differencebetween fluids in the riser and downcomer is greater, and circulation of liquidin the vessel is faster. Accordingly, mixing is usually better in external-loopthan internal-loop reactors. 31
  32. 32. Bioreactor configurations 32
  33. 33. Stirred and air-driven reactors: comparison ofoperating characteristicFor low-viscosity fluids, adequate mixing and mass transfer can be achieved instirred tanks, bubble columns and airlift vessels. When a large fermenter (50-500 m3) is required for low-viscosity culture, a bubble column is an attractivechoice because it is simple and cheap to install and operate. Mechanical-agitated reactors are impractical at volumes greater than about 500 m3 as thepower required to achieve adequate mixing becomes extremely high.Stirred reactor is chosen for high-viscosity culture. Nevertheless, mass transferrates decline sharply in stirred vessels at viscosities > 50-100 cP.Mechanical-agitation generates much more heat than sparging of compressedgas. When the heat of reaction is high, such as in production of single cellsprotein from methanol, removal of frictional stirrer heat can be problem so thatair-driven reactors may be preferred.Stirred-tank and air-driven vessels account for the vast majority of bioreactorconfigurations used for aerobic culture. However, other reactor configurationsmay be used in particular processes 33
  34. 34. Other bioreactorsPacked bedUsed with immobilized or particulate biocatalysts, for example during theproduction of aspartate and fumarate, conversion of penicillin to 6-aminopenicillanic acid, and resolution of amino acid isomers. Damaged dueto particle attrition is minimal in packed beds compared with stirred reactors.Mass transfer between the liquid medium and solid catalyst is facilitated athigh liquid flow rate through the bed. To achieve this, packed are oftenoperated with liquid recycle. The catalyst is prevented from leaving thecolumns by screens at the liquid exit. Aeration is generally accomplished in aseparated vessel because if air is sparged directly into the bed, bubblecoalescence produces gas pockets and flow channeling or misdistribution.Packed beds are unsuitable for processes which produce large quantities ofcarbon dioxide or other gases which can become trapped in the packing. 34
  35. 35. Other bioreactorsFluidized bedTo overcome the disadvantages of packed bed, fluidized bed may be preferred.Because particles are in constant motion, channeling and clogging of the bedare avoided and air can be introduced directly into the column. Fluidized bedreactors are used in waste water treatment with sand or similar materialsupporting mixed microbial populations, and with flocculating organisms inbrewing and production of vinegar.Trickle bedIs another variation of the packed bed. Liquid is sprayed onto top of thepacking and trickles down through the bed in small rivulets. Air may beintroduced at the base; because the liquid phase is not continuous throughoutthe column, air and other gases move with relative ease around the packing.Trickle-bed reactors are used widely for aerobic wastewater treatment. 35
  36. 36. Other bioreactors 36