THE COPPERBELT UNIVERSITY
School of mines and minerals sciences
CE 560 / MT 580
Process control
Lecture 01
Introduction to process control
C. Botha (Mr.)
Contacts: Chemical Engineering Department
Email: clyde.botha@cbu.ac.zm
Alt. Email: bthclyde@gmail.com
January, 2023
Introduction to process control
o Process engineers design, construct and operate physical processes.
 Such physical processes together with associated equipment must be maintained at
some desired conditions during operation.
 That is, some form of control is required to achieve this.
 Thus, process control is naturally involved in engineering activities, especially
operation.
 For example, plant operators must ensure that a proper response is made to
compensate for disturbances that occur constantly.
o Furthermore, processes usually involve fluctuations in process variables such as
flowrates, temperatures, pressures etc.
Introduction to process control
 By observing such processes, one would come to the realization that industrial
processes are in general dynamic.
 That is, they are characterized by constant change of key variables.
 To be able to control a process, a good understanding of the dynamic behaviour
of a process is required.
 This can lead to better design of control mechanisms to deal with dynamic
(changing) processes.
 Understanding the dynamic behaviour leading to better design of control
systems is important in control engineering.
Introduction to process control
 This is because it has a potential to ensure the safe and profitable operation of a
plant and equipment therein.
o Process control is also involved in engineering experiments.
 That is, conducting experiments require control of the laboratory equipment.
 This is in order to adhere to certain conditions as prescribed by the experimental
procedure.
o Thus process control can be defined as:
o An engineering discipline that deals with ways and means for monitoring and
maintaining the output of a specific process within a desired range by adjusting
appropriate input variables.
Examples of control system: (1) Room temperature control
o Consider the room heating system shown in Fig. 1.0
HX
(Sensor)
Thermostat
Controller
Furnace
Fuel flow
(Final control element)
Fig.1.0 Room temperature control system
 The control objective is to maintain the room temperature near a desired
(comfortable) temperature by circulating hot water through the HX.
Examples of control system: Room temperature control cont’d…
o How can this control objective be achieved?
 The room temperature is measured by the thermostat ( sensor).
 The measured room temperature is compared to the desired range, say 18 – 22℃.
 When the measured temperature is < 18℃, the furnace and the pump are turned on
which increases the fuel flow and the heat supply to the room.
 For room temperatures > 22℃, the furnace and fuel flow pump are turned off.
 Finally, for room temperatures between 18 – 22℃, the furnace and pump remain
unaltered, i.e. off.
o Fig. 2.0 illustrates the room temperature history for this type of a control strategy.
Examples of control system: Room temperature control cont’d…
Fig.2.0 Room temperature response for on/off control system
Examples of control system: Room temperature control cont’d…
 Room temperature fluctuates between the minimum and the maximum
temperatures as fuel flow-rate is turned on and off.
 The room temperature exceeds the upper limit slightly as the furnace and the heat
exchanger cannot respond instantly.
o Control strategy as described here is an example of on/off control and is one of the
control strategies.
 Often used where precise control at the desired level is not a primary concern.
o Better control strategies capable of maintaining variables of interest much closer to the
desired value are discussed later.
Examples of control system: (2) Driving an automobile
o Consider a person driving an automobile.
 Control objective is typically to stay in a specific lane.
o How can this control objective be achieved?
 Driver must determine location of the vehicle in relation to the road using eyes
(sensor).
 Driver must determine change required to maintain vehicle in that lane using the brain
(controller).
 Driver must then change the position of the steering wheel (final control element) by
an amount determined so as to keep the vehicle in lane.
Examples of control system: (2) Driving an automobile
o Performing the above three tasks will maintain the automobile close to the desired
position on the road in the presence of disturbances like bumps, curves etc.
o What does a control system do?
o For a process plant; defined as a systematic and rational arrangement of processing
units for converting raw materials into desired products,
o a control system generally performs the following three functions:
1) Suppressing the influence of external disturbances on a process.
2) Ensure stability of a process and (3) Optimizing the performance of a process.
Suppressing the effects of external disturbances
o A common function of a control system is to suppress the effect of external disturbances.
 A disturbance represent the effect that the surrounding has on a process.
 Control of such disturbances are usually out of reach of the operator.
 Hence, a control mechanism to bring about the necessary changes is required.
 That is, to cancel the negative impact that such disturbances may have on the
process.
o To understand this function better, consider the stirred tank heater (STH) system, Fig. 3.0
Suppressing external disturbance effect
Fi (ft3/min), Ti (oF)
T
h
Q
F (ft3/min), T (oF)
Condensate
Fst (Ib/min)
Steam
Fig.3 Stirred tank heater system
Suppressing external disturbance effect cont’d…
o Control objective for the STH system are as follows:
1) Maintain the temperature T of the outlet stream at a desired value Ts.
2) Ensure constant volume Vs of the liquid in the stirred tank heater.
o External factors (disturbances) are changes in
1. Feed flow-rate Fi (ft3/min)
2. Inlet temperature Ti (oF)
3. Surrounding temperature
o These external factors can disturb the steady-state operation of the stirred-tank
heater.
Suppressing external disturbance effect cont’d…
o In the absence of any changes in the feed conditions, then after attaining desired
operating conditions i.e., T = Ts and V = Vs
 STH system can be left without any supervision and control.
 However, this is not possible since inlet conditions (temperature and flow-rate)
may be subjected to frequent changes.
 Similarly, the surrounding temperature and other conditions may also change
frequently.
 Thus, some form of a control mechanism is needed to suppress the impact of
these disturbances and so keep T and V at or near the desired values Ts and Vs .
Suppressing external disturbance effect cont’d…
o The corresponding control scheme required to perform this task is shown in Fig. 4.
Fig.4 Feedback temperature control strategy for the stirred-tank heater system
Suppressing external disturbance effect cont’d…
 Temperature of the liquid is measure by a thermocouple (measuring instrument).
 Measured temperature T is compared to the set point value Ts yielding an error
or deviation given by
  Ts - T ..........................................Eq.1.0
 Computed value of the error is sent to a controller which in turn determines what
action to perform to keep T at or near the desired temperature Ts.
 When 𝜺 > 𝟎, the measured temperature T is less than set point temperature
Ts.
 Controller opens steam valve so that more steam is supplied to increase system
temperature.
Suppressing external disturbance effect cont’d…
 When 𝜺 < 𝟎, measured temperature T is greater than set point temperature Ts.
 Controller closes the steam valve so that less steam is supplied to reduce the
system temperature.
 If ε = 𝟎, which means that measured temperature T = Ts then no control action is
taken by controller.
o What to think about!
 Similar control strategy to maintain the volume of liquid in the tank at the desired
value Vs or to maintain the level in the tank h at or near hs.
Ensuring stability of a process
o The behaviour of a process variable 𝑥 is depicted in the Fig. 5.0 (a) and (b).
B
x
x
A
C
Time
to
(a)
Time
to
(b)
Fig.5.0 (a) Response of a stable system, (b) response of an unstable system
o At an initial time 𝑡 = 𝑡0 , the steady-state value of the variable 𝑥 is disturbed by some
external factor (disturbance).
Ensuring stability of a process cont’d…
o Response shown in Fig. 5.0 (a), the value of the variable returns to its desired value and
stays there as time elapses.
o Such a process is said to be stable or self-regulating and needs no external control.
o By contrast, if the variable 𝑥 does not return to its initial value after being disturbed as
shown in Fig. 5.0 (b), the process is said to be unstable.
o A control mechanism is required in this case to force 𝑥 to return to its original value.
o Example of such a process is an exothermic reaction whose temperature may continue to
rise if no mechanism to remove heat from the reaction vessel is provided.