chapter 8 ft

8/4/2019 Chapter 8 Ft

http://slidepdf.com/reader/full/chapter-8-ft 1/40

CHAPTER 8

INSTRUMENTATION AND CONTROL



- The success of fermentation depends upon the existence of definedenvironmental conditions for biomass & product formation.

- Thus the temperature, pH, degree of agitation, oxygen concentration

in the medium & other factor may have to be kept constant during the

process.

- The provision of such conditions requires careful monitoring of the

fermentation so that any deviation from the specific optimum might be

corrected by a control system.



CONTROL SYSTEM

A control loop consists of 3 basic components:

1. A measuring element (senses a process property such as flow,

pressure, temperature, etc. and generates a corresponding

output signal)

2. A controller (measurement signal with a predetermined desired

value set point and produce an output signal to counteract any

differences between the two)

3. A final control element (received the control signal and adjusts

the process by changing a valve opening or pump speed and

causing the controlled process property to return to these

points)



Automatic control system can be classified into four main

types:

1. Two-position controllers (On/Off)

2. Proportional controllers

3. Integral controllers4. Derivative controller

Two-position controllers (On/Off)

Simplest automatic controller, has a final control unit(valve, switch, etc), which is either fully open (On) or fully

closed (Off). The response pattern to such a change will be

oscillatory.



Proportional controllers

The change in output of controller is proportional to the input signal

produced by the environment change (commonly referred to as error)which has been detected by a sensor.

Expressed in following equation:

M=M0 + Kc ∑

Where,

M = output signal

M0 = controller output signal when there is no error

Kc = controller gain or sensitivity∑ = the error signal

Hence the greater the error (environmental change) the larger is the

initial corrective action which will be applied.



∆I → Controller → ∆O

Change in input change in output

Then ∆I = Kc∆O

Kc may contain conversion units if there is an electrical inputand pressure output or vive versa.

If the input to the controller gains of 1, the output will be 1

unit

If the input to the controller gains of 2, the output will be 2

units



Integral controller

Output signal of an integral controller is determined by integral

of error input over time of operation.

M = Mo + 1/Ti ⌠∑ dt

Where,

Ti = integral time

It is important to remember that the controller output signal

changes relatively slowly at first as time is required for thecontroller action to integrate the error.



Derivative controller

Controller sense of rate of change of the error signal and

contributes a component of the output signal that is proportional

to a derivative of the error signal.

M = Mo + Td d∑ / dt

Where,

Td = time rate constant

It is important to remember that if the error is constant there is no

corrective action with derivative control. In practice, derivative

control is never used on its own.



Combination of methods of control

Proportional plus integral control

The output response to an error gives rise to a slightly higher initial

deviation in the output signal compared with one which would beobtained with the proportional control on its own. This is due to

contribution in the signal from integral control. However, the

oscillations are soon reduced and there is finally no offset. This

mode of control finds wide applications since the proportional

component is ideal in a process where there are moderatechanges, whereas the integral component will allow for large load

changes and eliminate the offset that would have occurred.



Proportional plus derivative control

The output response to an error will lead to reduced deviation,

faster stabilization and reduced offset compared with proportional

control alone. Because the derivative component has rapid

stabilizing influence, the controller can cope with rapid load

changes.

Proportional plus integral plus derivative control

Provide the best control possibilities. The advantages of each

system are retained. The maximum deviation and settling time aresimilar to that for a proportional plus derivative controller whilst the

integral action ensures that there is no offset. This is method of

control finds the widest application because of its ability to cope

with wide variations of pattern of changes which might be

encountered in different processes.



Temperature

The temperature in a vessel or pipe is the most importantparameter to monitor and control in any process. It may be

measured by mercury-in-glass thermometers, bimetallic

thermometer, pressure bulb thermometers, thermocouples,

metal-resistance thermometer or thermistors.

Mercury-in-glass thermometers

May be used directly in small bench fermenter, but its fragility

restricts its use. In larger fermenter it would be necessary to insertin into a thermometer pocket in the vessel, which introduces a

time lag in registering the vessel temperature. It can be used solely

for indication, not for automatic control or recording.



Bimetallic thermometer

It consists of a bimetallic helical coil surrounded by a protecting tube or well. Thecoil winds or unwinds with changes in temperature and causes movement of a

fixed pointer. A pen can be fitted to the pointer so that temperature changes can

be monitored on chart. They are less subject to breakage than glass

thermometers but cost slightly more and are less accurate and once limited to

local indication.

Pressure bulb thermometer

It is basically pressure gauge connected by small-bore tubing, which may be up to

60m in length, to the detecting bulb (12x125mm). The whole system is gastight

and filled with an appropriate gas or liquid under pressure (2800-8000kNm-2). The

movement of the free end of the receiving element can be used to operate a pen

on a chart recorder or an electrical or pneumatic control. Response times of 5

seconds have been claimed. A variety of systems are used in this thermometer for

ambient temperature compensation in the pressure gauge.



Thermocouples

Seebeck discovered that if a circuit consisting of wires of two

dissimilar metals had the junction of the wires maintained atdifferent temperatures, a current flowed through the circuit. The

current produced can be measured on a calibrated instrument or

recorder and is a measure of point temperature at a joint.

Therefore holding the temperature at the all junctions, except one,

within a given circuit it is possible to measure temperature as a

function of the hot-junction temperature with reference to the

cold-junction temperature.

They have not been used much for temperature measurement infermenter because they are normally operated at ambient

temperatures and unfortunately tend to be susceptible to cold-

junction problems within 500 of this range.



Electrical resistance thermometer

It changes with temperature variation. The bulb of the

instrument contains the resistance element, a mica framework

(for every accurate measurement) or a ceramic framework

(robust but for less accurate measurement) around which the

sensing element is wound. A platinum wire of 100 Ω resistances

is normally used. Leads emerging form the bulb are connected

to the measuring element. The reading is normally obtained by

the use of a Wheatstone bridge circuit and is a measure of the

average temperature of the sensing element. It has greateraccuracy (±0.25) than some of the other measuring devices and

is more sensitive to small temperature changes.



Thermistors

It is semiconductor made from specific mixtures of pure

oxides of iron, nickel and other metals. Their main

characteristic is a large change in resistance with a small

temperature change. The change in resistance is a function

of absolute temperature. The temperature reading is

obtained with a Wheatstone bridge or a simple or more 3

complex circuit depending on the application. Thermistor is

relatively cheap and has proved to be very stable, give

reproducible readings and can be sited remotely from theread-out point. Their main disadvantage is the marked non-

linear temperature versus resistance curve.



Temperature control

In many small system there is a heating element, 300-400W

capacity is adequate for a 10dm3 fermenter, and cooling water

supply which are on or off depending on the need for heating or

cooling. The heating element should be as small as possible to

reduce the size of the “heat sink” and resulting overshoot when

heating is no longer required. In some cases it may be better to

run the cooling water continuously at a steady rate and only have

the heating element connected to control unit.



Flow measurement and control

Both gases and liquids are important in process management.

Gases- One of the simplest methods for measuring gas flow to a

fermenter is by means of a variety area meter. The most commonly

used example being a rotameter, which consist of a vertically

mounted glass tube with an increasing bore and enclosing a free-moving float which can be a ball or a hollow thimble. The position

of the float in the graduated glass tube is indicative of flow rate.

Difference sizes can for a wide range of flow rates.

- The accuracy depends on having the gas at a constant pressure,but error of up to ±10% of full scale deflection is quoted. The errors

are greatest at low flow rates. Ideally, rotameters, should not be

sterilized and are therefore normally placed between a gas inlet

and sterile filter.



There is no provision for on-line data logging with the simplest

rotameter. Metal tubes can be used in situations where glass is not

satisfactory. In these cases the float position is determined by

magnetic or electrical techniques, but this provision has not beennormally utilized for fermentation work. Rotameters can also be

used to measure liquid flow rates, provided abrasive particles or

fibrous matter is not present.



-The use of oxygen and carbon dioxide gas analyzers for effluent

gas analysis requires the provision of very accurate gas-flow

measurement if the analyzers are to be used efficiently.

-For this reason thermal mass flowmeters have been utilized for

the range 0 to 500dm3 min-1. These instruments have a ± 1% full-scale accuracy and work on the principle of measuring a

temperature difference across a heating device laced in the path of

the gas flow.

- Temperature probes such as thermistor are placed upstream and

downstream of the heat source, which may be inside or outside the

piping.



-The mass flowrate of the gas, Q can be calculated from the

specific heat equation:

H= QCp (T2 –T1)

Where,

H heat transferred,Q mass flow rate of the gas

Cp specific heat of the gas

T1 temperature of gas before heat is transferred to it.

T2 temperature of gas after heat is transferred to it.

- Control of gas flow is usually by needle valves. Often this

method of control is not sufficient, and it is necessary to

incorporate a self-acting flow control valves. At a small scale,

such valves as the ‘flowstat’ are available.



Liquids

On lab scale flow rates, may be measured manually using a sterile

burette connected to the feed pipe and timing the exit of a

measured volume. A more expensive is to use an electrical flowtransducer which can cope with particular matter in suspension

and measure range of flow rates from very low to high with an

accurate of ±1%.

Another indirect method of measuring flow rates aseptically is to

use a metering pump which pumps liquid continuously as a

predetermined and accurate rate. Some of example metering

pumps are commercially available including motorized syringes,

peristaltic pumps, piston pumps and diaphragm pumps.Motorized syringes are only used when very small quantities of

liquid have to be added slowly to a vessel. In a peristaltic pump,

liquid is moved for wards gradually by squeezing tubing held in a

semicircular housing.



- Piston pump contains an accurately machined ceramic or

stainless-steel piston moving in a cylinder normally fitted

with double ball inlet and outlet valves.

-The piston is driven by a constant-speed motor. Flow rates

can be varied within a defined range by changing the stroke

rate, the length of the piston stroke and by using a differentpiston size. Sizes are available form cm-3h-1 to thousand of

dm-3h-1 and all can be operated at relatively high working

pressure.

-Piston pumps are more expensive than comparable sizedperistaltic pumps but do not suffer from tube failure.

Unfortunately, it can not be used to pump fibrous or

particular suspension



Pressure measurement

One of the standard pressure measuring sensors is theBourdon tube pressure gauge which is used as a direct

indicating gauge. The partial coil has an elliptical cross-

section (A –A) which tend to become circular with increasing

pressure, and because of the difference between the

internal and external radii, gradually straightens out. The

process pressure is connected to the fixed socket end of the

tube, while the sealed tip of the other end connected by a

geared sector and pinion movement which actuates an

indicator pointer to show linear rotational response. When avessel or pipe is to be operated under aseptic conditions, a

diaphragm gauge can be used. Changes in pressure cause

movements of the diaphragm capsule which are monitored

by a mechanically levered pointer.



Pressure control

In normal operation a positive head pressure of 1.2 atmospheres(161kNm-2) absolute is maintained in a fermenter to assist in the

maintenance of aseptic conditions. This pressure wills obviously

be raised during a steam-sterilization cycle. The correct pressure

in different components should be maintained by regulatoryvalves controlled by associated pressure gauges.

Safety valves

Safety valves should be incorporated at various suitable places in

all vessels and pipe layouts which are likely to be operated under

pressure. The valves should be set to release the pressure as

soon as it increases markedly above a specified working

pressure.



Agitator shaft power

- A variety of sensor can be used to measure the power consumption of a

fermenter. On large scale, a watt meter attached to the agitator motor will

give a fairly good indication of power uptake. This measuring technique

becomes less accurate as there is a decrease in scale to pilot scale and

finally to lab fermenter, the main contributing factor being friction in

stuffing box.

- Torsion dynamometer can be used in small-scale applications. Since the

dynamometer has to be placed on the shaft outside the fermenter the

measurement will once again include the friction in the bearings in the

stuffing box. For this reason strain gauge mounted on the shaft within thefermenter are the most accurate method of measurement and over come

friction problems



Rate of stirring

In all fermenter it is important to monitor the rate of rotation (rpm) of

the stirrer shaft. The tachometer used for this purpose may employ

electromagnetic induction, voltage generation, light sensing or

magnetic force as detection mechanisms. Obviously the final choice of

tachometer will be determined by the type of signal which is required

for recording and / or process control for regulating the motor speedand other ancillary equipment.

Foam sensing and control



Foam sensing and control

The formation of foam is a difficult in many types of microbial

fermentation which can create serious problems if not controlled. It is

common practice to add antifoam to a fermenter when the cultures startforming above certain predetermined level. The method used for foam

sensing and antifoam additions will depend on process and economic

considerations. The properties of antifoams have been discussed

elsewhere as has their influence on dissolved oxygen concentration.

A probe is inserted through the top plate of the fermenter. Normally, the

probe is a stainless- steel rod, which is insulated except at the tip, and set

as a defined level above the broth surface. When the foams raise and

touches the probe tip, a current is passed through the circuit of the probe,

with the foam acting as an electrolyte and the vessel acting as an earth.

The current actuates a pump or valve and antifoam is released into the

fermenter for a few seconds.



- Process timers are routinely included in the circuit to ensure thatthe antifoam has time to mix into the medium and break down

the foam before the probe is programmed after a present time

interval to sense the foam level again and possibly actuate the

pump or valve. Alternatively antifoam may be added slowly at a

predetermined rate by small pump so that foaming never occursand there is therefore no need for a sensing system.



Measurement and control of dissolve oxygen

- In small fermenter (1dm3), the commonest are galvanic and have

a lead anode, silver cathode and employ potassium hydroxide,

chloride, bicarbonate or acetate as an electrolyte.

- The sensing tip of the electrode is a telfon, polyethylene or

polystyrene membrane which allows passage of the gas phase so

that equilibrium is established between the gas phases inside and

outside the electrode.



- Because of the relatively slow movement of oxygen across the

membrane, this type of electrode has slow respone of the order of 60 seconds to achieve a 90% reading of true value.

-These electrodes are suitable for monitoring very slow changes in

oxygen concentration and are normally chosen because of thecompact size and relatively low cost.

- Unfortunately, this type of electrode is very sensitive to

temperature fluctuations, and should be compensated for

temperature using a thermistor circuit. The electrodes also have a

limited life because of corrosion of the anode.



-Poloragraphic electrodes, which are bulkier than galvanic

electrodes, are more commonly used in pilot and productionfermenter, needing instrument ports of 19 or 25mm diameter. They

have silver anodes which are negatively polarized with respect to

reference cathodes of platinum or gold, using aqueous potassium

chloride as the electrolyte.

- Response times of 0.05 to 15 seconds to achieve a 35% reading

have been reported. The electrodes which can be very precise may

be both pressure and temperature compensated. Although a

poloragraphice electrode may initially cost 600% more than agalvanic equivalent, the maintenance costs are considerably lower

as only the membrane should b need replacing.



-Dissolved oxygen concentration may also be determined by a

tubing method. The probe consists of a coil or permeable telfon or

polypylene tubing within the fermenter through which is passed astream of helium or nitrogen.

-The oxygen which diffuses form the fermentation medium through

the tubing wall into the inert gas stream is then determined using a

paramagnetic gas analyzer.

- Times of 2-10 minutes are required before making reading. Thetubing will withstand repeated sterilization and has been used

continuously for up to 1000hours as pilot scale.



By observing the concentration of CO2 and O2 in the entry and

exits gases in the fermenter and knowing the gas flow rate it is

possible to determined the oxygen uptake of the system, the

carbon dioxide evolution rate and the respiration rate of

microbial culture.

The O2 can be determined by a

i) Paramagnetic gas analyzer

ii) Deflection analyzeriii) Thermal analyzer.



pH measurement and control



p easu e e t a d co t o

pH measurement is now routinely carried out using a combined glass

reference electrode that will withstand repeated sterilization at

temperature of 121oC and pressures of 138kN-m2. The electrode maybe silver/ silver chloride with potassium chloride as an electrolyte.

Occasionally calomel / mercury electrodes are used. The electrode is

connected via leads to pH meter / controller. Normally, pH electrodes

are autoclaveable.

Control unit may be simple On/Off or more complex. In the case of

the On/Off controller, the controller is set to a predetermined pH

value. When a single actuates a relay, a pinch valve is opened or

pump started, and acid or alkali is pumped into fermenter for shorttime which is governed by a process timer (0-5 seconds). The

addition cycle is followed by a mixing cycle which is governed by

another process timer (0-60seconds) during which time no further

acid or alkali can be added.



At the end of the mixing cycle another pH reading will indicate

whether or not there has been adequate correction of the

pH drift. In the small volumes the likelihood of overshoot is

minimal.

Carbon dioxide electrode

The measurement of dissolved CO2 is possible with electrode,

since a pH or voltage change can be detected as the gas

goes into solution.



ONLINE ANALYSIS OF OTHER CHEMICAL FACTORS

i Ion-specific sensors

ii Enzyme electrodes

iii Microbial electrodes

iv Mass spectrometers

v Fluorimeters



COMPUTER APPLICATION IN FERMENTATION TECHNOLOGY

Three distinct areas of computer function were recognized by

Nyiri (1972b)

1. Logging of process data

Data logging is performed by the data acquisition system which

has both hardware & software components. There is an interfacebetween the sensor and the computer. The software should

include the computer program for sequential scanning of the

sensor signals and the procedure of data storage.

2. Data analysis (Reduction of logged data)

3. Process control

chapter 8 ft

Documents