Engineering reference for Indian students

Reaction & separation

Process Dynamics & Control

5 self-contained study topics — notes, diagrams, formulas, and worked examples for exams and GATE.

Topics

Dynamic Modelling of Processes
Dynamic modelling turns unsteady mole and energy balances into differential equations, then linearises them about the steady state and Laplace-transforms into transfer functions G(s) that relate output to input deviations.
First and Second Order Responses
A first-order system responds to a step with an exponential rise reaching 63.2% at one time constant; a second-order system adds a damping ratio ζ that decides whether the response is overdamped, critically damped or oscillatory.
$63.2% of the final value is reached at t = \tau$
Controller Tuning
PID tuning sets the three controller parameters so the loop responds quickly without excessive oscillation; Ziegler-Nichols rules derive them either from the ultimate gain and period (closed loop) or from process gain, delay and time constant (open loop).
Stability of Process Loops
A control loop is stable if all roots of its characteristic equation lie in the left half-plane; this is checked with the Routh-Hurwitz test, root locus, or the Bode/Nyquist gain and phase margins.
$Open-loop transfer G_{OL} = G_{c} G_{p} G_{m}$
Feedforward and Cascade Control
Feedforward control measures a disturbance and cancels its effect before it reaches the output, while cascade control nests a fast inner loop inside a slow outer loop so that disturbances are corrected before they disturb the primary variable.