Chemical Process Control

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What is Process Control?[edit]

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.

Feedback Control[edit]

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[edit]

If the controlled variable increases as the manipulated variable increases, then direct acting control is used.

Reverse Acting Control[edit]

Feedforward Control[edit]

Advanced Control[edit]

Mathematical Modeling[edit]

Conservation Laws[edit]

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[edit]

Statistical Models[edit]

Using Mathematical Models[edit]


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