Ethanol Production by fermentation

Three important type process variable:

Controlled variables (CVs): The process variables are controlled. The desired value of controlled variable is referred to its set point. In this fermentation process the controlled variables are the temperature of the fermentation, the pressure and level controller.
Manipulated variables (MVs): The process variables that can be adjusted in order to keep the controlled variables at or near their set points. In this case, our manipulated variable is the flow rate of the inlet as it will be easier to vary the variable compare if we want to manipulate the inlet temperature. 
Disturbance variables (DVs): Process variables that affect the controlled variables but cannot be manipulated. Disturbance generally are related to changes in operating environment of the process. In this case our disturbance variable is the temperature that coming from the source of heat from the reactor as it hard to manipulate the amount of heat that may need to be supply for the fermentation process.

Type of control algorithm (P, PI, or PID controller) that would be used. Why?

PID controller is the best controller that can be used is this experiment as it is the most dominant control technique for process control. The reason PID controllers are so popular is that using PID gives the designer a larger options and those options mean that there are more possibilities for changing the dynamic system in way that help the designer.

Proportional controller is mostly used in first order processes with single energy storage to stabilize the unstable process. The main usage of the P controller is to decrease the steady state error of the system. Integral controllers give zero steady step input while the derivative control produces faster response. By using the PID controller, the proportional controller may not give steady state error performance needed in a system but the integral controller may give the steady state performance but slow down the system and by adding the derivative controller that may help cure both those problems. PID controller has the optimum control dynamic including zero steady state error, fast response (short rise time), no oscillation and higher stability.

One of the advantages using PID controllers is it can be used with higher order process including more than single energy stored. Besides, PID controllers give a faster response and less overshoot in the desired output.