Presenter: Suprvisor: Selection, Scale up and Operation of Bioreactors

Transcription

1 In the Name of God Presenter: Maryam Shahmansouri Suprvisor: Dr.Reza Gheshlaghi Selection, Scale up and Operation of Bioreactors (Chapter 10 Shuler) 1

2 Outline types of Bioreactors problems in large reactors Scale-up Scale-down 3 Classification types of Bioreactors Operation modes: - batch: stirred tank. - continuous:chemostat, fluidized-bed. - modified types of the above modes:fed-batch, chemostat with recycle, multi-stage continuous reactors. Oxygen supply: - aerobic: airlift. - anaerobic Application of energy: -Mechanical (mixers) -Pneumatic -Hydraulic Control of cell growth - Chemostat - Turbidostat 4 2

3 Basic Reactor types Stirred-tank reactor Reactors with internal mechanical agitation. Bubble columns reactors that rely on gas sparging for agitation. Loop reactors reactors that mixing and liquid circulation are induced by the motion of an injected gas, by a mechanical pump, or by a combination of the two. (Airlift, propeller loop, jet loop reactor) Three-phase reactors are difficult to design 5 Mechanical (stirred tank reactors with different stirrers) deflector plate deflector Plate(baffels) supply air supply air marine impellers Axial flow For low viscosity media For cellular systems with high levels of shear sensitivity. Disc impellers Radial flow For high viscosity media Most commonly used bioreactor type(highly flexible, provide high heat & mass transfer) High energy consumption 6 3

4 Pneumatic Bubble column Advantages: Highly distributed gas bubbles Suitable for low viscosity Newtonian broths Provide a higher energy efficiency than STR Provide a low-shear environment. Absence of mechanical agitation reduces cost and eliminates one potential entry point for contaminants. supply air exhaust air Disadvantages: less vigorous mixing capabilities than STR Mixing may not be possible in highly viscous broths. Less flexible than STR. Work over a rather narrow range of gas flow rates (foaming,bubble coalescence,nature of the broth) 7 Pneumatic loop reactor (airlift) Advantages: low shear forces cultivation of animal ells Low energy consumption Can generally handle somewhat more viscous fluids than bubble column Coalescence is not so much of problem exhaust air inner catalyst tube Disadvantages: The interchange of material between fluid elements is small, so the transient time to circulate is important. supply air 8 4

5 Main problems in large reactors The abilities of the design to provide an adequate supply of oxygen. Remove metabolic heat efficiently. Foaming sterility 9 Aeration(oxygen supply) For industrial-scale fermenters, oxygen supply and heat removal are the key design limitations. Oxygen transfer from gas bubbles to cells is usually limited by oxygen transfer through the liquid film surrounding the gas bubbles. 10 5

6 Aeration The rate of oxygen transfer from the gas to liquid phase is given by: K L : Oxygen transfer coefficient(cm/h) a : gas-liquid interfacial area(cm 3 /Cm 2 ) K L a : volumetric oxygen transfer coefficient(h -1 ) C * :saturated DO concentration(mg/l) C L : actual DO concentration(mg/l) N O2 : rate of oxygen transfer(mg/l.h) OTR : oxygen transfer rate O 2 gas bubble cells transmission of oxygen 11 Aeration The rate of oxygen uptake is denoted as OUR(oxygen uptake rate) : specific rate of oxygen consumption (mg O2 /g dw cells.h) : yield coefficient on oxygen (g dw cells/go2) :cell concentration (g dw cells/l) When oxygen transfer is rate-limiting step, the rate of oxygen Consumption is equal to the rate of oxygen transfer. If the maintenance Requirement of O2 is negligible compared to growth, then 12 6

7 Aeration Demand side Supply side The value of OUR: In large-scale systems are 40 to 200 (mmol/l.h) In most systems in the range of 40 to 60 (mmol/l.h) 13 Aeration A wide range of equations has been suggested for the estimation of K l a. K:empirical constant :Power requirement in an aerated bioreactor :bioreactor volume :superficial gas exit speed N : rotational speed of agitator K:constant based on reactor geometry : power required in ungased fermenter : impeller diameter Q: aeration rate(volume of gas supplied per minute divided by the liquid volume in the reactor) 14 7

8 Aeration Main factors effect on K L a & C* presence of salts and surfactants can significantly alter bubble size and liquid film resistance around the gas bubble. Temperature Pressure Vessel geometry Operation Fluid properties Presence of biomass Note1: On the supply side, C L should be maintained at a value above the critical oxygen concentration but at a low enough value to provide good oxygen transfer. Note2: one complication, in all methods is the value of C * to use. C * is proportional to po 2.at the sparger point, po 2 will be significantly higher than at the exit. 15 Aeration Although K L a is difficult to predict, it is measurable parameter. Methods of Measurement of K l a in a Bioreactor Two basic methods for measuring K l a: Chemical methods(no cell present) Sodium sulfite oxidation method Absorption of CO2 Physical methods(with/without cells) Four approaches are commonly used :unsteady state, steady state, dynamic, sulfite test. 16 8

9 Aeration Methods of K l a Measurement: Steady-state method (best way) a) Whole reactor is used as a respirometer. b) uses a gaseous oxygen analyzer to measure the oxygen concentration both in the inlet and the outlet gas stream of the bioreactor c) uses a probe for measuring the dissolved oxygen concentration in the liquid An oxygen mass balance under steady-state conditions yields: 17 Aeration (Unsteady-state method) Unsteady-state method (without cell) a) Measured C*accurately. b) Oxygen is removed from the system by sparging with N2. c) Air is introduced and the change in DO is monitored until the solution is nearly saturated. integration 18 9

10 Aeration Sulfite method a) In the presence of Cu 2+ The sulfur in sulfite (SO 2-3 ) is oxidized to sulfate (SO 2-4 ) in a zero order reaction. b) This reaction is very rapid and consequently C L approaches zero. c) Rate of sulfate formation is monitored and is proportional to the rate of oxygen consumption (1/2 mol of O 2 is consumed to product 1 mol of SO 2-4. desadvantages: the sulfite method probably overestimates K l a. physicochemical properties are very different from those of fermentation broths. 19 Aeration Dynamic method a) this method shares similarities with the steady-state method in that it uses a fermenter with active cells. b) It is simpler in that is requires only a dissolved oxygen(do). c) It is requires that the air supply be shut off for a short period (eq<5 min)and then turned back on. The governing equation for DO levels is: There is no gas bubble when the gas is off The lowest value of C L must be above the critical oxygen concentration. Advantage K L a can be estimated under actual fermentation conditions. If q O2 is known, the value of OUR can be used to estimate X

11 DO 2 CONC. C L (mm O 2 /L) C L STEADY-STATE C L,CRIT AIR-OFF AIR-ON Aerator off Aerator on TIME (MIN) 3-5 The slope of the descending curve will give the OUR or. When air sparsing is resumed, the ascending curve can be used to calculate K L a. A plot of versus result in a line with a slope of K L a 21 Heat removal In aerobic fermentation,since oxygen is the final electron acceptor, the rate of metabolic heat evolution can roughly be correlated to the rate of oxygen uptake. The total amount of cooling surface (either jacket or coil) required can be calculated by: given the temperature of the cooling water the maximum flow rates allowable the desired temperature differential between the exiting coolant and the reactor and the overall heat transfer coefficient

12 Scale-up What is Scale-up? Study of problems associated with the transfer of experimental data from laboratory and pilot-plant equipment to large scale industrial equipment. No actual data or correlation exist for scale-up. Stages Bench Scale ( 2 20 L) Pilot Scale ( L) Plant Scale (500 20,000 L) 23 Scale-up scale-up the box below To make a box twice as big, just 3 multiply the dimensions by 2 and 6 3 you have the scaled dimensions. 6 The same type of bioreactors of different size may be: Geometrically similar Geometrically non-similar The scale-related volume and surface area are fundamental physical parameters that can not be maintained at a constant state. As a result of scaling up, whether geometrically similar or not, the scale related surface area per unit volume will be decrease

13 Scale-up Preservation of Geometrical Similarity: H L1 /D t1 = H L2 /D t2 =.. = H L3 /D t3.=2/1 or 3/1 Unlike a vessel s dimensions, manufacturing process parameters should not be scaled linearly. Linear scaling of process parameters would produce undesired results and can greatly affect cell growth. HL3 HL1 HL2 DT1 DT2 DT3 LAB SCALE PILOT SCALE COMMERCIAL SCALE Geometric Scale-up of Bioreactors 25 Scale-up Problems in scale-up Surface-to-volume ratio decreases dramatically during scale up Wall growth in bacterial and fungal fermentation. Physical conditions in a large fermenter can never exactly duplicate those in a smaller fermenter if geometric similarity is maintained

14 Scale-up Scale up rules can be used to establish which parameters will be varied and how? Rules o Constant power input(p 0 /V) implies constant OTR o Constant impeller rotation number(n) give constant mixing time. o Constant impeller tip speed(ndi) give constant shear. o Constant Reynolds number( ) implies geometrically similar flow pattern. 27 Scale-up Important depended variables used in scale-up. 1. Energy input P N 3 D i 5 2. Energy input/volume P/V N 3 D i 2 3. Pump rate of impeller Q N D 3 i 4. Pump rate of impeller/volume Q/V N 5. Impeller Tip Velocity V t = (2 R)(N) = ( D i )(N) 6. Reynolds Number N Re = ND i2 /. Scale-up criterion in general are a function of independent variables N, Di

15 Scale-up the choice of scale-up criterion depends On two considerations: a) Nature of the fermentation and morphology of the microorganism. b) During scale-up, what is the objective parameter of fermentation we wish to optimize (maximize). 29 Scale-up Example geometric similarity was applied. N i2 N i1 SCALE-UP 80 L 10,000 L Volumetric scale-up ratio = V 2 /V 1 = 10,000/80 = 125 Impeller diameter scale-up ratio = D i2 /D i1 =

16 Independence of scale up parameters Constant, Re Scale-up Traditional scale up is highly empirical and make sense only if there is no change in the controlling regime during scale up. In empirical scale up operating parameters for the large scale are often determined experimentally (i.e. trial and error). Example: if constant DO is desirable, then the setpoint value for DO is maintained at the large scale, and other parameters (agitation speed, aeration rate,...) are varied to ensure the setpoint is achieved

17 Scale-down What is Scale-down? The basic concept is to provide at a smaller scale an experimental system that duplicates exactly the same heterogeneity in environment that exists at the large scale. Advantages: In many case scale-up will require using existing production facilities, but scale-down dose not. At the smaller scale many parameters can be tested more quickly and inexpensively than at the production scale. A small-scale system can be used to evaluate proposed process changes for an existing operating process. 33 Sterility: means the absence of any detectable viable organism. Sterility is an absolute concept ;a system is never partially or almost sterile. Pure culture: means that only the desired organism is detectably present. Disinfection: means reduce the number of viable organisms, often specific type of organisms, to a low, but none zero value. Death: means the failure of the cell, spores, or virus to reproduce or germinate when placed in a favorable environment

18 Reasons for sterilization 1. Economic penalty for contamination is high. 2. Many fermentation must be absolutely devoid of foreign organisms. 3. Recombinant DNA fermentations-exit streams must be sterilized

19 37 Spores of bacillus stearothermophilus E.coli N = number of viable spores or cells at any time, N 0 = original number of viable spores or cells

20 39 Dependence of the specific death rate on temperature is given by Arrhnius equation: R:gas constant T:absolute temperature E 0d :activation energy for the death of the organism( kcal/g-mol) Spores of bacillus stearothermophilus E.coli E 0d =127( kcal/g-mol) Vitamins and growth factors in many media E 0d =70( kcal/g-mol) E 0d =2-20( kcal/g-mol) Most thermal sterilizations take place at 121 c.the values for k d in such situations are very high for vegetative cells(often> min -1 ). For spores the values of k d typically range from min

21 The main factors in any sterilization protocol are: Temperature and ph of environment Time of exposure Initial number of organisms that must be killed Nature of microbes in the population Presence of solvents, organic matter, or inhibitors 41 Problems of sterilization increase with scale up Example: Consider the probability of an unsuccessful sterilization in a1l and 1000L reactor, where each contains the same identical solution, if k d t=15 and n0( concentration of particles)= 10 4 spores/l then: 42 21

22 43 Use of sterilization chart: 1. Spesify1-P 0 (t) which is acceptable 2. Determine N 0 in the system 3. Read k d t from the chart 4. With knowing k d for spores or cells, obtained requred time,t. Example:1-P 0 (t)=0.001, N 0= 10 8, k d =1 min c From the chart K d t=26 t=26min 44 22

23 Steam sterilization can be accomplished batch wise, often in situ in the fermentation vessel, or in a continuous apparatus. 45 Batch sterilization Disadvantages: Thermal lags Incomplete mixing Time required to heat (121 C)and to cool it back(37 C) is often much longer than the time of exposure to the desired temperature. For most spores Kd falls very rapidly with temperature so heat-up and cool-down periods do littleto augment spore killing. Elevated temperature during Heat-up and cool-down can be very damaging to vitamins and proteins

24 Continuous sterilization Advantages: Particularly a high-temperature, short exposure time, can achieve complete sterilization. Both the heat-up and cool-down period are very rapid Easier to control and reduce downtime in the fermenters. Disadvantages: Dilution of medium with steam injection Foaming

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