Introduction of process control

CHAPTER 1
INTRODUCTION OF PROCESS CONTROL

Objective
 Process control
 Example of controlled process
 Feedback control system
 Feed forward control system
 Classification of variables in chemical process
 Components of control system
 Summary

Why process control
 Structure of chemical process plant is very complex
 Any chemical plant consist of various process units which are inter connected with one another in
systematic manner
 Main objective of any plant is to convert certain raw materials into desired product using available
sources of energy
 Other objective- safety, product specification, environmental regulations, operation constraints,
economics
 These all these parameters are control by arrangement of various equipment like measuring
devices, valves, controller

Examples of controlled processes
1. Controlling the temperature of a water stream by controlling the amount of
steam added to the shell of a heat exchanger.

2. Operating a jacketed reactor isothermally by controlling the coolant that
flows through the jacket of a jacketed reactor.

3. Controlling the height of fluid in a tank to ensure that it does not overflow.

How process control
 Consider the tank heater system shown in Figure
 Assumption
Fi, Ti = flow rate (ft3/min) and temperature(°F) of entering
liquid into the tank
F = flow rate of steam which heated liquid (lb/mm).
F, T = the flow rate and temperature of the stream leaving the
tank.
 The tank is considered to be well stirred, which implies that the
temperature of the effluent is equal to the temperature of the
liquid in the tank.
 Objectives
1. To keep the effluent temperature T at a desired value Ts
2. To keep the volume of the liquid in the tank at a desired value Vs

1. To maintain the temperature of effluent ‘T’ at
desired temperature ‘Ts’
 The operation of the heater is disturbed by external factors such
as changes in the feed flow rate and temperature (Fiand Ti).
 If nothing changed, then after attaining T = Ts and V = Vs, we could
leave the system alone without any supervision and control.
 Consequently, some form of control action is needed to alleviate
the impact of the changing disturbances and keep T and V at the
desired values.
 In Figure we see such a control action to keep T = Ts when Ti or Fi,
changes.
 A thermocouple measures the temperature T of the liquid in the
tank.
 Then T is compared with the desired value Ts, yielding a
deviation ε = Ts - T.
 The value of the deviation ε is sent to a control mechanism which
decides what must be done in order for the temperature T to
return back to the desired value T.

1. To maintain the temperature of effluent ‘T’ at
 If ε > 0, which implies that T < Ts, the controller
opens the steam valve so that more heat can be
supplied.
 On the contrary, the controller closes the steam
valve when ε < 0 or T> Ts.
 It is clear that when T = Ts (i.e., ε = 0), the
controller does nothing.
 This control system, which measures the variable
of direct importance (T in this case) after a
disturbance had its effect on it, is called the
feedback control system.
 The desired value Ts is called the set point and is
supplied externally by the person in charge of
production.

1.To maintain the temperature of effluent ‘T’ at
 Returning to the tank heater example, we
realize that we can use a different control
arrangement to maintain T= Ts when Ti,
changes.
 Measure the temperature of the inlet stream T,
and open or close the steam valve to provide
more or less steam.
 Such a control configuration is called
feedforward control.
 The feedforward control does not wait until
the effect of the disturbances has been felt by
the system, but acts appropriately before the
external disturbance affects the system,
anticipating what its effect will be.

2. To maintain the height of liquid ‘h’ in the tank
at desired level ‘hs’
 In Figure we see a control action to keep h = hs
when Ti or Fi, changes. So that tank will not
overflow or go dry
 A level measuring device measures the height h of
the liquid in the tank.
 Then h is compared with the desired value hs,
yielding a deviation ε = h - hs.
 The value of the deviation ε is sent to a control
mechanism which decides what must be done in
order for the height h to return back to the desired
value hs.
 It may open or close the valve that affects the
effluent flow rate F
 It is also feedback control systems act post facto
(after the fact), that is, after the effect of the
disturbances has been felt by the process.

2. To maintain the height of liquid ‘h’ in the tank
at desired level ‘hs’
 If ε > 0, which implies that h < hs, the
controller opens the steam valve so that more
heat can be supplied.
 On the contrary, the controller closes the
steam valve when ε < 0 or T>Ts.
 It is clear that when T = Ts (i.e., ε = 0), the
controller does nothing.
 This control system, which measures the
variable of direct importance (T in this case)
after a disturbance had its effect on it, is called
the feedback control system.
 The desired value Ts is called the set point
and is supplied externally by the person in
charge of production.

Classification of variables in chemical process
1. Input Variable – This variable shows the effect of the
surroundings on the process.
2. Output variable- Also known as the control variable
These are the variables shows the effect of process on
the surrounding.
3. Error: In process controls, error is defined as: Error
= set point - process variable.
4. Set point: The set point is where you would like a
controlled process variable to be.
 Input variable – Ti, Fi, Fst, Tst
 Output variable – F, T
 Set point - Ts

Classification of variables in chemical process
There are two types of inputs variables.
 Manipulated inputs: if the value can be
adjusted freely by human operator or control
mechanism.
 Disturbances: inputs that can not be
controlled by an operator or control system
 Manipulated – Fst
 Disturbance – Ti, Fi, Tst

Components of control system
 Process (stirred tank heater)
 Measuring element (thermocouple,
thermometer)
 Controller
 Final control element (control valve)

Summary
 Controller —A device that outputs a signal to the process based on the magnitude of the error
signal. A proportional controller outputs a signal proportional to the error.
 Closed loop —In closed loop, the measured value of the controlled variable is fed back to the
controller.
 Feedback control system —this control system uses the direct measurements of control variable
to adjust the values of the manipulated variables.
 Feed forward control system —this control system uses direct measurement of the disturbances
to adjust the values of the manipulated variables.

Summary
 Block diagram —Diagram that indicates the flow of information around the control system and the
function of each part of the system.
 Error —The difference between the values of the set point and the measured Variable
 Manipulated variable —Process variable that is adjusted to bring the controlled variable back to
the set point.
 Set point —The desired value of the controlled variable.
 Controlled variable —The process variable that we want to maintain at a particular value.

Introduction of process control

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