Bài giảng Kỹ thuật phản ứng sinh học: Chương 5 Quá trình truyền khối khí lỏng, cung cấp cho người học những kiến thức như: Khái niệm cơ bản; Tính toán quá trình truyền khối khí lỏng. Mời các bạn cùng tham khảo!
http://buihongquan.com Chương Quá trình truyền khối khí lỏng 5.1 Khá i niệ m bả n 5.2 Tính toá n quá trình truyề n khó i khí lỏ ng 12/23/2018 Bioreaction engineering 198 http://buihongquan.com BIOREACTORS 12/23/2018 Bioreaction engineering 199 http://buihongquan.com Contents Introduction O2 uptake and Stoichiometry Surface aeration Methods of aeration Mechanically stirred bioreactors Bubble driven bioreactors Airlift bioreactors Packed bed and trickle flow bioreactors Fluidized bed bioreactors 12/23/2018 Bioreaction engineering 200 http://buihongquan.com Bioreactors- Introduction Importance of considering process engineering factors when culturing cells Biological factors include the characteristics of the cells, their maximum specific growth rate, yield coefficient, pH range and temperature range The productivity of a fermentation is determined by the mode of operation of the fermentation process; eg the advantages of fed-batch and continuous fermentations over batch fermentations 12/23/2018 Bioreaction engineering 201 http://buihongquan.com Bioreactors Introduction Bioreactors The oxygen demand of an industrial process is generally satisfied by aeration and agitation Productivity is limited by oxygen availability and therefore it is important to the factors that affect a fermenters efficiency in supplying O2 O2 requirement, quantification of O2 transfer and factors influencing the transfer of O2 into solution 12/23/2018 Bioreaction engineering 202 http://buihongquan.com Bioreactors- Introduction Mass transfer, in particular, oxygen transfer are important factor which determined how a reactor must be designed and operated Cost was also described as an important consideration The larger the reactor or the faster the stirrer speed, the greater the costs involved 12/23/2018 Bioreaction engineering 203 http://buihongquan.com MASS TRANSFER and PHASES The rate of oxygen transfer = driving force / resistance E.g resistance to mass transfer from medium to mo`s are complex and may arise from; 12/23/2018 Diffusion from bulk gas to gas/liquid interface Solution of gas in liquid interface Diffusion of dissolved gas to bulk of liquid Transport of dissolved gas to regions of cell Diffusion through stagnant region of liquid surrounding the cell Diffusion into cell Consumption by organism (depends on growth/respiration kinetics) Bioreaction engineering 204 http://buihongquan.com The following diagram serves to illustrate the different phases and material that are relevant in general transport processes associated with fermentation technology; M AS S TR AN S FE R D is p e rs e d g a s e s Solid and Dis s olv ed n u tr i e n ts Im m i s c i b l e liquid n u tr i e n ts P r o d u c ts i n w a te r Ce l l s Fl o c 12/23/2018 Bioreaction engineering 205 http://buihongquan.com Phases present in bioreaction / bioreactor Non aqueous phase Aqueous phase Solid phase (Reactants / products) Dissolved reactants / products Reaction Gas (O2, CO2, CH4 etc) Liquids (e.g oils) Solid (e.g particles of substrate) 12/23/2018 Cells Sugars Organelles Minerals Enzymes Enzymes = reactant supply and utilisation = product removal and formation Bioreaction engineering 206 http://buihongquan.com Mass Transfer • One of the most critical factors in the operation of a fermenter is the provision of adequate gas exchange •The majority of fermentation processes are aerobic • Oxygen is the most important gaseous substrate for microbial metabolism, and carbon dioxide is the most important gaseous metabolic product • For oxygen to be transferred from a air bubble to an individual microbe, several independent partial resistance’s must be overcome 12/23/2018 Bioreaction engineering 207 http://buihongquan.com Airlift bioreactor 12/23/2018 Bioreaction engineering 242 http://buihongquan.com Airlift bioreactor The region into which bubbles are sparged is called the air-riser The air-riser may be on the inside or the outside of the draft-tube The latter design is preferred for large scale fermenters as it provides better heat transfer efficiencies The rising bubbles in the air-riser cause the liquid to flow in a vertical direction To counteract these upward forces, liquid will flow in a downward direction in the downcomer This leads to liquid circulation and thus improved mixing efficiencies as compared to bubble columns The enhanced liquid circulation also causes bubbles to move in a uniform direction at a relatively uniform velocity This bubble flow pattern reduces bubble coalescence and thus results in higher kLa values as compared to bubble column reactors 12/23/2018 Bioreaction engineering 243 http://buihongquan.com Airlift bioreactors - Disengagement zone 12/23/2018 Bioreaction engineering 244 http://buihongquan.com Airlift bioreactors - Disengagement zone The roles of the disengagement zone are to 12/23/2018 add volume to the reactor, reduce foaming and minimise recirculation of bubbles through the down comer Bioreaction engineering 245 http://buihongquan.com Airlift bioreactors - Disengagement zone The sudden widening at the top of the reactor slows the bubble velocity and thus disengages the bubbles from the liquid flow Carbon-dioxide rich bubbles are thus prevented from entering the downcomer The reduced bubble velocity in the disengagement zone also leads to a reduction in the loss of medium due aerosol formation The increase in area will also helps to stretch bubbles in foams, causing the bubbles to burst The axial flow circulation caused by the draft tube also helps to reduce foaming 12/23/2018 Bioreaction engineering 246 http://buihongquan.com Packed bed and trickle flow bioreactors The topic of packed bed bioreactors was discussed in another lecture on immobilisation 12/23/2018 Bioreaction engineering 247 http://buihongquan.com Packed bed bioreactors The rate of mass transfer between the cells and the medium depends on the flow rate and on the thickness of the biomass film on or near the surface of the solid particles Packed bed reactors often suffer from problems caused by poor mass transfer rates and clogging Despite this they are used commercially with enzymatically catalysts and with slowly or non-growing cells They are also used in the anaerobic treatment of high strength wastewaters (eg food processing wastes) Large plastic blocks are used as solid supports for the cells These blocks have a large surface area for cell immobilization and when packed in the reactor are difficult to clog 12/23/2018 Bioreaction engineering 248 http://buihongquan.com 12/23/2018 Bioreaction engineering 249 http://buihongquan.com Trickle flow bioreactors Trickle bed reactors are a class of packed bed reactors in which the medium flows (or trickles) over the solid particles In these reactors, the particles are not immersed in the liquid The liquid medium trickles over the surface of the solids on which the cells are immobilized They are used widely in aerobic treatment of sewage 12/23/2018 Bioreaction engineering 250 http://buihongquan.com Trickle flow bioreactors Oxygen transfer is enhanced by ensuring that the cells are covered by only a very thin layer of liquid, thus reducing the distance over which the dissolved oxygen must diffuse to reach the cells 12/23/2018 Bioreaction engineering 251 http://buihongquan.com Trickle flow bioreactors Because stirring is not used, considerable capital costs are saved However, oxygen transfer rates per unit volume are low compared with spared stirred tank systems Trickle flow systems are used widely for the aerobic treatment of sewage They are used to polish effluent from the activated sludge or anaerobic digestion process and for the nitrification of ammonia 12/23/2018 Bioreaction engineering 252 http://buihongquan.com Fluidized bed reactors 12/23/2018 Bioreaction engineering 253 http://buihongquan.com Fluidized bed reactors Fluidised bed bioreactors are one method of maintaining high biomass concentrations and at the same time good mass transfer rates in continuous cultures Fluidised bed bioreactors are an example of reactors in which mixing is assisted by the action of a pump In a fluidised bed reactor, cells or enzymes are immobilised in and/or on the surface of light particles A pump located at the base of the tank causes the immobilised catalysts to move with the fluid The pump pushes the fluid and the particles in a vertical direction The upward force of the pump is balanced by the downward movement of the particles due to gravity This results in good circulation 12/23/2018 Bioreaction engineering 254 http://buihongquan.com Fluidised bed reactors For aerobic microbial systems, sparging is used to improve oxygen transfer rates A draft tube may be used to improve circulation and oxygen transfer Both aerobic and anaerobic fluidised bed bioreactors have been developed for use in waste treatment Fluidised beds can also be used with microcarrier beads used in attached animal cell culture Fluidised-bed microcarrier cultures can be operated both in batch and continuous mode In the former the fermentation fluid is recycled in a pump-around loop 12/23/2018 Bioreaction engineering 255 http://buihongquan.com Fluidized bed reactors 12/23/2018 Bioreaction engineering 256 ... http://buihongquan.com Airlift bioreactor Air-riser and down-comer An air-lift reactor is divided into three regions: - the air-riser - down-comer - disengagement zone 12/23/2018 Bioreaction... 3.92 H 6 .5 O 1.94 12/23/2018 Bioreaction engineering 212 http://buihongquan.com O2 Requirements 6.67CH2O + 2.1O2 = C 3.92 H 6 .5 O 1.94 + 2.75CO2 + 3.42H2O 7.14CH2 + 6.135O2 = C 3.92 H 6 .5 O 1.94... performed in testtubes containing between 5- 10 ml of media T-flasks used in the small scale culture of animal cells are another example of a standing culture T-flasks are normally incubated horizontally