Question

biotechnology 2. For a variable that is controlled automatically

a. Describe the role of the sensor

b. Describe the role of the controller

c.Describe how temperature can be controlled for situations where its actual value is above the setpoint and for situations where its actual value is below the setpoint?

d. Describe how dissolved oxygen can be controlled for situations where its actual value is above the setpoint and for situations where its actual value is below the setpoint?

Answer 1

Industrial biotechnology-

a. Sensor- A sensor is an analytical instrument which senses and detects a particular component in the given environment. For example, a highly sensitive pH probe installed in a bioreactor is also a sensor which senses and detects pH of the media. Other sensors may detect oxygen, carbon dioxide and specific metabolites as required.

b. Controller- A controller is an instrument which maintains (or tries to maintain) the particular parameter of the bioreactor at optimum level. For example, a pH controller maintains pH in optimum range by adding buffering agents or other reagents that counter act change in pH. Similarly, a temperature controller maintains optimum temperature. The controller (say, pH controller) is functionally coupled to its sensor (pH probe) and both devices worked in coordination to maintain the optimum parameter.

C. Temperature control- The bioreactors are externally fitted with ‘heat exchangers’. Since most fermentation reactions are exothermic, they are used to optimize elevation in temperature by cooling. Heat exchangers are like cooling pipes wrapped around the bioreactor. So when the temperature rises, it’s sensed by temperature probes. The temperature probe then signals rise in temperature to the ‘temperature controller. The temperature controller then switches on circulation of chilled water through cooling pipes. Thus, the temperature is maintained at optima. The same can be used to counteract drop in temperature (rarely observed in fermentation processes) by flowing hot water (or steam) through heat exchanger.

D. Oxygen control- The bioreactors are externally fitted with oxygen probes which sense oxygen level. Sensing low oxygen level, it sends signals to oxygen control system (controller). The oxygen controlled system then triggers increased aeration (through aerator) and stirring (through inbuilt stirrer). Increased aeration rebuilds oxygen level by ‘flowing sterilized air or oxygen’ through media. Increased rate of stirring helps even distribution of oxygen and simultaneous removal of CO₂ or other metabolic gases.

What if oxygen level rises above the set point? Oxygen level in a bioreactor will only increase if the fermentation process is oxygenic (i.e. evolves oxygen), for example a culture of algae in bioreactor. Consider a theoretical case where an elevated oxygen level is needed to be counteracted. The oxygen probe senses it and sends signal to ‘oxygen control system’. The control system then increases the ‘stirring rate’. The bioreactor is coupled to a diffusion membrane (a sort of ventilation system) above media level through oxygen diffuses out because it has higher partial pressure in the bioreactor than the diffusion membrane.