Chemical Process Control
What is Process Control?
The manipulation of an object (actuation device) to maintain a parameter within an acceptable deviation from an ideally required condition. At it's core, process control is the transfer of variability from on variable to another.
There are two basic process control philosophies, feedback and feedforward control.
In feedback control, the controlled variable is measured and compared with a set-point. The deviation between the controlled variable and the set-point is the error signal. The error signal is then used to reduce the deviation of controlled variable from set-point.
Direct Acting Control
If the controlled variable increases as the manipulated variable increases, then direct acting control is used.
Reverse Acting Control
The conservation laws on mass, energy and momentum are fundamental bases for the development of models of chemical processes. The general form of the law for a variable , when applied to a control volume is
Rate of X IN to CV - Rate of X OUT of CV + Rate of GENERATION of X within CV - Rate of DISAPPEARANCE of X within CV = Rate of ACCUMULATION of X within CV
When applied to mass this becomes the Law of Conservation of Mass. Assuming no nuclear reactions take place, then the rate of generation or disappearance of mass is zero. Hence, we have
Rate of Mass IN - Rate of Mass OUT = Rate of ACCUMULATION of Mass
In symbols we may say
m(in) - m(out) = dM/dt where M stands for the total mass within the CV
Process Reaction Curve
Using Mathematical Models
- The mechanical device that cause the activation or movement of a final control element.
- Direct Synthesis
- Final Control Element
- A physical device whose activation or movement causes a change in a dynamic process. In process control, the most common final control elements are control valves.
- Frequency Domain
- Internal Model Control
- IMC-PID Tuning
- A method for PID tuning that selects tuning parameters to approximate an IMC-derived controller.
- Ladder Logic
- A semi-graphical programming language used to represent control algorithms. The language is expressed using symbols for logic devices. The arrangement of the device symbols and their connections has the appearance of a ladder.
- Laplace Transform
- An integral transformation from time domain to Laplace domain. Given a function of time , the Laplace transform is given by the following
- The use of to represent the Laplace transform of is a common convention; however, in dynamics and control it is common to use and to represent a time-domain function and its Laplace transform, respectively.
- PID Controller
- Programmable Logic Controller, a microprocessor-based electronic device for implementing control algorithms.
- Time Domain
- Ziegler-Nichols Tuning