Chapter 1: Introduction Flashcards
why do we need process control?
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
what process constraints must be fulfilled
mass balance control and energy balance control
what producttion requirements must be fulfilled
production rate control and production quality control
if we’re measuring temperature then what are we trying to control?
temperature, what we’re trying to measure is what we’re trying to control
what do dotted lines represent
signals
what is the manipulated variable if we’re measuring the level
the manipulated variable if the flowrate of the outlet stream
what is the manipulated variable if we’re measuring the temperature?
the manipulated variable is the cooling water flowrate
what rules do we have for gas recycle loops
for gas recycle loops set at a maxiumum circulation rate to improve product yield and compressor operation
what rules do we have for liquid recycle loops
liquid recycle should have flow rate control to avoid snowball effect (variable flows of mass and energy round the recycle)
what rules do we have for material balance control structure?
control levels in the direction of flow ( use outflow as mv)
or
control levels in the reverse direction of flow ( use inflow as mv)
what does the flow controller do
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
whats a throughput control
a throughput control measures flowrate through the process stream and is independently set. it is also known as production rate control
what is a sensor
a sensor is a measuring instrument and transmitter
LT, TT OR LI, TI
what are examples of an actuator?
valve, pump
what are examples of controllers
level controller, temperature controller, flow controller, pressure controller
LC,TC,FC,PC
how do we know what type of controller to use?
look at the set point and measurement
what relationship is there between level control and production rate control
there is an interaction between production rate control and level control
when the operator changes the set point for feed rate of A what happens to the feed rate of A
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
what happen to level when the operator changes the set point for feed rate A
it goes to a maximum and then returns to level set point due to error signal from level controller
what happen to the product stream flow rate whenthe operator changes the set point for feed rate A
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.
what is a P&ID
piping and instrumentation diagram showing physical layout
what is a cause and effect diagram
shows cause and effect relationships, also called block diagram
what do the lines represent in a cause and effect diagram
the lines (wires) are not pipes. each wire represents a mathematical variable (also called a signal). the wires do not carry fluids
what does a temperature sesnor do in a cause an deffect diagram
the temperature sesnor measures the temperature of the reactor (physical quantity) and sends an electronic signal of the measurement of the reactor temperature
what does the valve do in a cause and effect diagram
the cooling water valve takes the output of the controller ( an electronic signal) and changes the flow rate of cooling water ( physical quantity)
what does the reactor do in cause and effect diagram in terms of its temperature dynamics
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)
what does the temperature controller do in cause and effect diagrams
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)
what does feedback control mean
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.
what is a controller
A device which operates automatically to regulate a controlled variable.
what is a disturbance
An undesired change that takes place in a process that tends to adversely affect the
value of a controlled variable
what is the final controlling element( actuator)
The element that directly changes the value of the manipulated variable.
what is a sensor/ transducer?
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).
what is the set point
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.
what is a error signal
The signal resulting from subtraction of a particular return signal from its
corresponding input signal.
what is the feedback signal
The return signal which results from a measurement of the directly controlled
variable.
what is an output signal
A signal delivered by a device, element or system. Hence controller output is the
signal delivered by the controller.
what is a controlled variable
In a control loop, the variable the value of which is sensed to originate a feedback signal
what is a measured variable (process variable)
A quantity, property or condition which is measured. Common measured variables are temperature, pressure, rate of flow, film thickness, motor speed.
what is the manipulated variable
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.
what is a measurement/ set point
measurements or set points determine whether process variables such as temperature are at the desired value
what are actuators
actuators, generally in the form of control valves, make adjustments to flow rates of cooling water and process fluids
what is a controller
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
what does a LT do
LT measures the levl of the liquid in the reactor and transmits the result to the LC through an electronic connection.
what does the LC
The computer programme running in LC compares the measured level
with the set point value (sp)
what does the controller do if the level is too high
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
what does the controller do if the level is too low
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;
