Chapter 1: Introduction

Question 1

why do we need process control?

Answer 1

control makes it possible to operate the process by rejecting disturbances and keeping the process at its designed operating point. control keeps the process at steady state such that it is safe, efficient and profitable

Question 2

what process constraints must be fulfilled

Answer 2

mass balance control and energy balance control

Question 3

what producttion requirements must be fulfilled

Answer 3

production rate control and production quality control

Question 4

if we’re measuring temperature then what are we trying to control?

Answer 4

temperature, what we’re trying to measure is what we’re trying to control

Question 5

what do dotted lines represent

Answer 5

signals

Question 6

what is the manipulated variable if we’re measuring the level

Answer 6

the manipulated variable if the flowrate of the outlet stream

Question 7

what is the manipulated variable if we’re measuring the temperature?

Answer 7

the manipulated variable is the cooling water flowrate

Question 8

what rules do we have for gas recycle loops

Answer 8

for gas recycle loops set at a maxiumum circulation rate to improve product yield and compressor operation

Question 9

what rules do we have for liquid recycle loops

Answer 9

liquid recycle should have flow rate control to avoid snowball effect (variable flows of mass and energy round the recycle)

Question 10

what rules do we have for material balance control structure?

Answer 10

control levels in the direction of flow ( use outflow as mv)
or
control levels in the reverse direction of flow ( use inflow as mv)

Question 11

what does the flow controller do

Answer 11

the flow controller acts directly on the valve or pump to adjust the flowrate to the independent set point to ensure we meet production target

Question 12

whats a throughput control

Answer 12

a throughput control measures flowrate through the process stream and is independently set. it is also known as production rate control

Question 13

what is a sensor

Answer 13

a sensor is a measuring instrument and transmitter
LT, TT OR LI, TI

Question 14

what are examples of an actuator?

Answer 14

valve, pump

Question 15

what are examples of controllers

Answer 15

level controller, temperature controller, flow controller, pressure controller

LC,TC,FC,PC

Question 16

how do we know what type of controller to use?

Answer 16

look at the set point and measurement

Question 17

what relationship is there between level control and production rate control

Answer 17

there is an interaction between production rate control and level control

Question 18

when the operator changes the set point for feed rate of A what happens to the feed rate of A

Answer 18

the flow of feed stream is at steady state until teh set point is changed.the flow rate then gradually increases but it needs time fort he system to respond to the system change and won’t immediately go to steady state. once it reaches the new set point value it eventually reaches steady state

Question 19

what happen to level when the operator changes the set point for feed rate A

Answer 19

it goes to a maximum and then returns to level set point due to error signal from level controller

Question 20

what happen to the product stream flow rate whenthe operator changes the set point for feed rate A

Answer 20

it will gradually increase until it reaches the new set point value. because there is no accumulation of mass Fin=Fout. so the new set point for the feed flow rate will also be the flowrate for the product stream.

Question 21

what is a P&ID

Answer 21

piping and instrumentation diagram showing physical layout

Question 22

what is a cause and effect diagram

Answer 22

shows cause and effect relationships, also called block diagram

Question 23

what do the lines represent in a cause and effect diagram

Answer 23

the lines (wires) are not pipes. each wire represents a mathematical variable (also called a signal). the wires do not carry fluids

Question 24

what does a temperature sesnor do in a cause an deffect diagram

Answer 24

the temperature sesnor measures the temperature of the reactor (physical quantity) and sends an electronic signal of the measurement of the reactor temperature

Question 25

what does the valve do in a cause and effect diagram

Answer 25

the cooling water valve takes the output of the controller ( an electronic signal) and changes the flow rate of cooling water ( physical quantity)

Question 26

what does the reactor do in cause and effect diagram in terms of its temperature dynamics

Answer 26

the reactor is our control system and takes two inputs: the flow rate of feed A (physical quantity) and the flow rate of cooling water (physical quantity) and its output is the temperature of the reactor (physical quantity)

Question 27

what does the temperature controller do in cause and effect diagrams

Answer 27

the temperature contoller has two inputs: temperature set point ( electronic signal) and measurement of reactor temperature ( electronic signal) and its output is the output of the controller (electronic signal)

Question 28

what does feedback control mean

Answer 28

Control in which a measured variable is compared to its desired value to produce
an actuating error signal which is acted upon in such a way as to reduce the magnitude of the error.

Question 29

what is a controller

Answer 29

A device which operates automatically to regulate a controlled variable.

Question 30

what is a disturbance

Answer 30

An undesired change that takes place in a process that tends to adversely affect the
value of a controlled variable

Question 31

what is the final controlling element( actuator)

Answer 31

The element that directly changes the value of the manipulated variable.

Question 32

what is a sensor/ transducer?

Answer 32

An element or device which receives information in the form of one quantity
and converts it to information in the form of the same or another quantity. (e.g. converts
temperature to an electrical signal).

Question 33

what is the set point

Answer 33

An input variable which sets the desired value of the controlled variable. It may be
manually set, automatically set or programmed. Set point is expressed in the same units as the
controlled variable.

Question 34

what is a error signal

Answer 34

The signal resulting from subtraction of a particular return signal from its
corresponding input signal.

Question 35

what is the feedback signal

Answer 35

The return signal which results from a measurement of the directly controlled
variable.

Question 36

what is an output signal

Answer 36

A signal delivered by a device, element or system. Hence controller output is the
signal delivered by the controller.

Question 37

what is a controlled variable

Answer 37

In a control loop, the variable the value of which is sensed to originate a feedback signal

Question 38

what is a measured variable (process variable)

Answer 38

A quantity, property or condition which is measured. Common measured variables are temperature, pressure, rate of flow, film thickness, motor speed.

Question 39

what is the manipulated variable

Answer 39

A quantity or condition which is varied as a function of the error signal so
as to change the value of the controlled variable. The manipulation is done by the actuator.

Question 40

what is a measurement/ set point

Answer 40

measurements or set points determine whether process variables such as temperature are at the desired value

Question 41

what are actuators

Answer 41

actuators, generally in the form of control valves, make adjustments to flow rates of cooling water and process fluids

Question 42

what is a controller

Answer 42

a controller is an electronic device running a computer program which determines and implements a sensible sequeunce of adjustments to the actuators that will bring the measurements back to their set point

Question 43

what does a LT do

Answer 43

LT measures the levl of the liquid in the reactor and transmits the result to the LC through an electronic connection.

Question 44

what does the LC

Answer 44

The computer programme running in LC compares the measured level
with the set point value (sp)

Question 45

what does the controller do if the level is too high

Answer 45

If the level is too high then the controller will send a signal to the control valve to open a
little further compared to its previous operating point to let out the excess liquid

Question 46

what does the controller do if the level is too low

Answer 46

If the level is too low, the controller will send a signal to the control valve to close a little
compared to its previous operating point to allow the reactor contents build back up to the
correct level;