Thus, small regions of suboptimal cells after they are acclimated to the environment. After they reach their conditions can have a drastic effect on the productivity of the maximum density, the cells begin to die. This is sometimes called cycling. Nutrient feed and aeration strategies affect not only inoculum tank are optimized for the production of more rate but also yield and product distribution in a bioreactor.
The conditions in the production fermenter may be Unlike chemical reactors, different reaction paths, or meta- different, depending on the product. There are sometimes bolic pathways, may be used by the organism depending fewer than three steps from shaker lask to production, and on conditions. After inoculation, they fer, mixing, heat transfer, reaction engineering, and process require some time to adjust to the new environment before optimization.
This is called the lag phase. After acclima- Mass transfer tizing, the bugs enter a period of rapid, exponential growth, Although not a major factor in anaerobic fermentation, called the growth phase. Without When competition for nutrients or oxygen becomes suficient transfer of oxygen from the air to the liquid, organ- limiting, growth stops and the organisms enter the station- isms may die, go into shock, or make the wrong product.
During this phase, they may continue to make Aerobic processes demand a certain oxygen uptake rate the desired product. If the end product is the organism OUR.
Under quasi steady-state conditions, this equals the itself, as in the production of yeast or single-cell proteins, oxygen transfer rate OTR. Although OUR varies through- the process is normally stopped after the growth phase out the process, engineers must design for the peak rate. Some newer After fermentation, the broth is sent to a harvest tank to processes occur at microaerobic conditions typically less await downstream processing.
Some modiied E. The basic mass transfer equation is: Bioreactor vs. There are no The term in parentheses is the driving force. For example, some yeast strains are more efi- form is adequate. In tall vessels, the log-mean driving force 22 www. Some processes and organisms are more sensitive than others. Unfortu- nately, there is presently no good way to model the actual concentration distri- bution in a fermenter.
Various research- Axial Turbine ers are attempting to use numerical Rushton Turbine methods to combine mixing, mass p Figure 3. Rushton turbines, which have six flat p Figure 4. Axial impellers improve mixing in transfer, and reaction kinetics to plot blades mounted vertically on a disk, were used in fermenters. They are often combined with radial dissolved oxygen DO distribution, but early bioreactors. High backpressures impede One might think that, in anaerobic bioreactors, mix- the release of CO2, which must be kept below reasonable ing would not be an issue, since reaction times are always partial pressures to maintain healthy organisms.
In practice, very long compared to blend times, and there seem to be fermenters usually have a backpressure of less than 1 bar. However, agitation in a The other term in Eq.
It is usually treated as a single variable because it yield, and maximum product concentration at speciic power is dificult to separate the interfacial area per unit volume a inputs an order of magnitude or more higher than needed from the ilm coeficient kL. It is a function primarily of the to mix nutrients or keep solids in suspension 4.
Unfortu- broth type, supericial gas velocity, and agitator power input, nately, the mechanism responsible for these effects is not and is often expressed as: clear. References 1—3 provide more detailed discussions of agi- Heat transfer tation, airlow, mass balances, and agitator impeller systems Heat-transfer requirements for bioreactors vary. Anaerobic fermenters and cell culture reactors typically have very low rates of heat production, whereas aerobic Mixing fermenters can have quite high heat-production rates.
The The irst bioreactors used multiple Rushton turbines rate usually correlates well to oxygen consumption. The heat impellers, or all-axial impellers, to improve mixing. Many fermenters remove heat p Figure 6. Helical coils are often installed p Figure 7. Vertical tube bundles, which double using tank jackets.
Most fermenters, For a single bank of vertical tube bafles and the same therefore, use internal surfaces. For icient is only about 0. Large power. Instead, if there is a problem, add area or change the temperature of the cooling medium. In such cases, the overall heat-transfer the process.
If the ves- sel is jacketed, a dimpled jacket or half-pipe jacket may be more effective than a plain jacket. Reaction engineering As long as conditions are suitable for the microorganisms inside the bioreactor, the bioreactions may be treated as any Vertical Tube Bundles other chemical reaction, although the kinetic expression may be more complicated than simple irst or second order, and less total power for a given OTR.
In addition, they are easier allowances must be made for variations in the biosystem. On the For a batch reactor, the process design procedure may be other hand, they may be more expensive, and the distribu- mostly empirical and speciied by the microbiologists. For a tion of dissolved oxygen and nutrients within the vessel is continuous-low bioreactor system, good knowledge of the less uniform.
Optimization consists of performing fairly A continuous-low reactor system requires more total detailed calculations for a series of aspect ratios to determine retention time than a batch reactor, and sound chemical the lowest present worth of capital and operating costs for engineering reactor-design principles are very important to designs that are technically feasible.
For anaerobic fermenta- the success of the system. Sometimes, the kinetics are too tions, the optimum aspect ratio is usually 1 from a mixing complex to calculate, and the continuous-low system must viewpoint, although vessel cost may dictate other geometries.
The pilot plant must have the Economics of feed strategy vs. For some processes, the production number of stages, volumetric mass-transfer rate, and feeding rate is inluenced by nutrient concentration. For instance, strategies as the proposed production system. Continuous stirred tank bioreactors 2. Bubble column bioreactors 3.
Airlift bioreactors 4. Fluidized bed bioreactors 5. Packed bed bioreactors 6. Batch Bioreactor Various aspects of batch reactors are being utilized in chemical and metal processing industries. The basic concept of batch reactor is to be considered that the molar or mass reactions are restricted in a variable temperatures, stirring and PH. However the inflow and out flow of the reaction products will remain constant in same close door open vessels, reaction containers or in small conical flasks.
High-pressure reactor. This reactor is generally used in polymer synthesis, where the process of polymerization requires high-pressure reactions. In high-pressure reactors, the pressure gauge is varied between 1. To design a high-pressure reactor, special types of stainless steels are used. Additionally for better yields and selectivity, gradual addition or removal in semi batch reactor assists in controlling temperature particularly when the net reaction is highly exothermic.
Thus, the use of a semi batch reactor intrinsically permits more stable and safer operation than a batch operation. Fed batch reactors are mostly used in critical reactions and rarely used in wastewater treatments.
It involves the removal of culture medium continuously and replacement of this with a fresh sterile medium in a bioreactor. Both addition and removal are done at the same Rate so that the working volume remains Constant. Practical Issues for Bioreactors A Heat Transfer Configurations: The primary heat transfer configurations in fermentation vessels are: i. External jackets ii. Internal coils iii. External surface heat exchanger B Agitation gas transfer Mixing should produce homogeneous conditions and promote a Nutrient transfer b Gas transfer c Heat transfer Heat transfer is necessary during both sterilization and for temperature maintenance during operation.
Transport of Nutrients The performance of the reactor is affected if the rate of the transport of the limiting nutrients is slower than the rate of utilization by the cells. Efficiency of the bioprocess could be increased by increasing the rate of transport of a limiting nutrient. Transport of Oxygen Compressed air entering a fermenter is usually stripped of moisture and any oil vapors that may originate from the compressor. To prevent the risk of contamination, gases introduced into the fermenter should be passed through a sterile filter.
Related Papers Characteristics and techniques of fermentation systems. Articulo Biotecnologia By Marilyn Llumiquinga. Download File.
0コメント