D ESCRIBE THE BIOREACTOR PARAMETERS AND HOW TO CONTROL THE TEMPERATURE

Introduction The standard stirred-tank bioreactor is normally supplied with means of measuring temperature, pressure, pH, and dissolved oxygen concentration. Basic control facilities consist of maintaining temperature, pH, and dissolved oxygen content steady, together with a means of suppressing foaming of the bioreactor contents (Figure 6.2).

ParameterInstrumentDescription Temperature 1. Mercury-in-glass thermometer 2. Bimetallic thermometer 3. Thermocouple 4. Thermistors The temperature in the vessel or pipe is the most important parameter monitor and control in any process.

Pressure 1. Bourdon tube pressure gauge 2. Diaphragm gauge Normal operation – positive head pressure of 1.2 atm absolute is maintained in the fermenter to assist in maintenance of aseptic conditions. Pressure was raised during steam-sterilization cycle. Correct pressure in different components were maintained by regulatory valves controlled by associated pressure gauges. Safety valves were incorporated at various suitable places in valves and pipes layout and operated under pressure. The valves are set to release pressure as soon as it increases above specified working pressure.

ParameterDescriptionInstrument Agitation speed Agitation speed is measured in the rate of rotation (rpm). 1.Tachometer Dissolved oxygen In most aerobic fermentation, it is essential to ensure that the dissolved oxygen concentration does not below a specified minimal level. Dissolved-oxygen electrodes 1.Galvanic 2.Polarographic

F OAM S ENSING A ND C ONTROL

F OAM Foam is a substance that is formed by trapping many gas bubbles in a liquid or solid.

Foaming sensing and control unit

When the foam (electrolyte) rises and touched the probe tip, a current is passed through the circuit. The current actuates a pump or valve. Antifoam is slowly released into the fermenter for a few seconds. Process timers-to ensure the antifoam has time to mix into the medium and break down the foam.

A NTI - FOAMING A GENT An anti-foaming agent is a chemical additive that inhibits the formation of foam.

P H M EASUREMENT AND C ONTROL

Carried out using a combined glass electrode. 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 a pH meter/controller. A recording unit maybe wired to the meter to monitor the pH pattern throughout the process cycle.

F LOW OF THE P ROCESS A signal actuates a relay A pinch valve is opened or a pump started Acid or alkali is pumped into the fermenter for a short time

The addition cycle is followed by a mixing cycle which is governed by another process timer At the end of the mixing cycle, another pH reading indicate (adequate correction of the pH drift)

HOW TO CONTROL TEMPERATURE ? Water jackets or pipe coils. In many small systems there is a heating element, 300 to 400 W capacity is adequate for a 10-dm 3 fermenter, and a 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.

For small-scale use, a unit will pump recirculating thermostatically heated water through fermenters for up to 10dm3 capacity and give temperature control of ±0.1º. In large fermenters, a regulatory valve at the cooling –water inlet may be sufficient to control the temperature. There may be provision for circulation of refrigerated brine if excessive cooling is required. Steam inlets to the coil and jacket must be present if a fermenter is being used for batch sterilization of media.