bioreactors instrumentation and control

8/10/2019 Bioreactors Instrumentation and Control

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BIOREACTORS INSTRUMENTATIONAND CONTROL


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A well-stirred tank, one of the mostcommonly employed bioreactors,approximates reasonably well to the

idealized state of perfect mixing. It ensuresa very short mixing time and a high gasliquid mass transfer on a small scale. Thisparticular culture conditions are mostlyachieved in so-called high performance

bioreactors.

Introduction


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This is very important in microbialphysiological studies under conditionsclosely approaching those found in

industrial reactors to be able to distinguishbetween the dynamics of cell reactionsand the dynamics of the reaction vessel. Insuch bioreactors cells are subjected to anunchanging environment when circulated

throughout the liquid medium.


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A standard stirred tank bioreactor is normallysupplied with a means of measuring temperature,agitator speed, pH, the incoming air-flow rate anddissolved oxygen concentration . This basic

instrumentation can be complemented with severalother sensors capable of determining the pressuredrop, dissolved CO2 concentration, biomass, off-gasanalysis, foam control, power and torque, and liquidflow feeding.


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Temperature Thermocouples are the most widely used

temperatures sensors since they are cheap,provide rapid response and may be usedto measure both high and lowtemperatures.

They are less accurate (0.5 C to 2.2 C)than other sensors, however, and must becalibrated periodically.

Common instruments for

process automation


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General view of the laboratory of the research groupBioprocesses Control Laboratory with instrumentation


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Gas Flowrate Differential Pressure Producers are

frequently found in industrial applicationsgiven their reliability and easy of use. Theiraccuracy ranges from 0.8 % to 5%.

The basic principle is that any obstruction inthe fluid stream generates a pressure lossthat is related to the flowrate of the stream.

The main limitation of such devices is thatthe cost in energy from the loss of pressurecost can be considerable.


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In-gas converter (air and oxygen), prototype developedby Bioprocesses Control Laboratory group ICT Prague

utilises controllers F-201C-FZ (Bronkhorst Hi-Tec,Netherlands)


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Another commonly found air velocitymeasurement is the turbine anemometer.

Here, a rotating device is placed in thepath of the fluid, where its rotational speedis proportional t the fluid velocity andprovides accurate flow measurement overwide ranges.


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Liquid Flowrate There are many ways of measuring liquid

flowrates ranging from those based onsimple hydraulic properties to those basedon radioactive effects.

Instruments are usually sensitive to noise,however, interference from other electricdevices must be controlled.


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Turbine meters are small electric turbinesthat generate an electromotive forcewhich is proportional to the mean fluid

velocity. This instruments are reliable, precise and

can be used with clean liquids. More suited for the dirty liquids sensors are

magnetic sensors , since they are non-invasive, but which are very expensive.


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The instrument generates a magnetic fieldthat is perpendicular to the liquid flow.

Less expensive Doppler sensors , can alsobe used to measure flow rates of dirtyliquids.

Here, a continuous ultrasonic wave (0.5 to10 MHz) emitted by the instrument isreflected by the bubbles and suspendedsolids in the liquid stream.


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Radioactive sensors are basedon the tracking of radioactive

trace species. These are very precise

instruments and applicable toany kind of fluid, but they arevery expensive and dealing withradioactive species is difficult.


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Off-Gas Analysis Analysis of exhaust gases provides

information on the physiological state and

respiration rate of the culture. On-line measurement of CO2 and O2 can

be performed by gas chromatography(GC) or special purpose gas analyzers.

The main advantages of GC are that manycompounds can be monitored with thesame instrument over a wider range ofvalues


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Gas Analyzers, on the other hand, aremore precise and provide a faster response(few seconds). Several such instrumentsexist. Paramagnetic analyzers, available forCO2 and O2, are very precise, do notrequire periodic calibration, present lowinterference to other gases and have longlifetimes, but they are expensive.

Infrared instruments , are available for CO2only, are precise and having a longlifetime, though they are expensive andneed occasional calibration.


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Electrochemical analyzers, onlyfor O2, are low cost instruments

that provide good precision,however a fuel cell must bechanged periodically (betweenmonths and 2 years)


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Infrared analyzer CO 2 Servomex 1440B(Servomex International Ltd., GreatBritain) for off-gas analysis


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Paramagnetic analyzer CO 2 Servomex 1100A (Servomex International Ltd.,Great Britain) for off-gas analysis


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Analyzer of medium composition HPLCchromatograph with detectors WatersUV/VIS 2487 and Waters RI 2410 with twopumps Waters 515 (Waters Corp.,

Massachusetts, USA)


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pH The pH sensitive electrode (usually made of

glass) contains a buffer solution at a constant

pH. The glass behaves as a membrane thatseparates the sample from the buffer solution;a potential proportional to the pH difference isgenerated.

If pH control is critical to bioreactor operation,measurement redundancy is recommended.Using three sensors would be wise becausewhen one gives a very different reading fromthe other two it means that it needs cleaning.


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Ion Sensitive Field-Effect Transistor - ISFET


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Dissolved Oxygen When controlling dissolved oxygen in a

bioreactor, it is often sufficient to keep the

dissolved oxygen above a certain level. pO2 (partial pressure of dissolved oxygen) One of the most specific aspects of the

fermentation monitoring ispO2 measurement and control.pO2 control is characteristic only forfermentation processes. There are differentpO2 control principles:


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pO2 is commonly adjusted in % from thefixed one. The adjusted pO2 value has alower and upper limit. The differencebetween both these limits is usually 10% -20%.

Important parameters in pO2 control arethe control limits of the mixer's rotationalspeed n: nmin and nmax. It means that,when controlling pO2, n will vary only withinthis range. These limits are determined inconnection with eliminating of differentundesirable phenomena:


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Foam The appearance of foam is a very undesirable

phenomenon, since, in the course of itsappearance, there is a risk to loose an essential partof the fermentation broth. During the foaming, it isnot possible to perform high-quality analyses andmeasurements. For elimination of foam, 2 methodsor their combinations are commonly used:

Additional metering of an antifoam, based on theinformation provided by the foam sensor. The givenimpulses are relatively low, with long pauses and alimited metering time. This additional control isnecessary to avoid the possible overdose, since, inthis case, the mass exchange parameters candecrease dramatically.


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Mechanical metering of foam. For thispurpose, an upper drive with a special disk-type or other type of the mechanical foam

breaking mixer is installed in thebioreactor's upper cover. If an intensivefoaming begins, then the mechanicalbreaking of foam will not help any more.

An optimal solution is the combination ofboth the parameters. The application ofVariant 1 is more widely used in laboratorybioreactors.


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bioreactors instrumentation and control

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