SKTK 3564 SECTION 04
PROCESS CONTROL & INSTRUMENTATION
SEMESTER 2, SESSION 2020/2021
FULL REPORT CASE STUDY 4
PREPARED FOR
Dr Muhd Yusuf Shahul Hamid
PREPARED BY
GROUP 4 (DYNAMIC)
CINDY POW (A18KT0048)
HAZZAWANNI NURULAINNI BINTI ABDUL MU’TI (A18KT0080)
LIM JIN HONG (A18KT0116)
TAN SIEW MEAN (A18KT0294)
DATE OF SUBMISSION
27TH JUNE 2021
1.0 INTRODUCTION
1.1 Process Description
Generally, the process operation of Kerry’s Wet-Mix process involves two spray dryer
plants available at Kerry, named as Dryer 1 and Dryer 2, where those consist of both semibatch and continuous processes throughout the production but we only focused on Dryer 2.
There are four tanks dedicated for wet mixing process in Dryer 2, which are oil melting tank,
a liquiverter, and two units of dissolvers. Oil melting tank and liquiverter are pre-mixture
preparation tanks, whereas dissolvers are the final product mixing tank. Dryer 2 contains two
sets of dissolvers due to batch sequencing, where product mixing and product drying occur at
the same time, alternately between these two tanks. Therefore, we identify eight control loops
involved in the wet mixing process in Dryer 2. Furthermore, we use the systematic procedure
for plantwide control design which includes the top-down and bottom-up approach. For the
top-down approach, we have identified the control objectives for each loop and determine all
process variables namely controlled variable, measured variable, manipulated variable and
disturbance. All the process variables are determined according to the selection guidelines.
Then, we evaluate the control degree of freedom which is the difference between the number
of control variable and manipulated variable. Lastly, for the bottom-up approach, we have
decided and justified the suitable control configuration for each loop.
1.2 Problem Statement
For case study 4, we are required to propose a preliminary design for an automatic
control system in the Wet-Mix process. We need to use the selection guidelines for the proper
selection of controlled, manipulated and measured variables and all explanations should be
included. Next, we need to use systematic procedure for control design including top-down and
bottom-up approaches together with their justification. Lastly, advanced control systems such
as feedforward, cascade, ratio need to be evaluated if the design required such a control system.
1.3 Objectives
● To design the control system based on the proper selection of controlled, manipulated
and measured variables.
● To evaluate and decide on which control systems need to be used based on systematic
procedure for plantwide control design including top-down and bottom-up approach.
2.0 LITERATURE REVIEW
2.1 Selection Guidelines for Controlle d, Manipulated and Measured Variables
In general, it is important to have at least as many MVs and CVs as possible. But that
is not always possible, so there are occasions when different types of control systems need to
be used. It may not be possible to control all output variables for a number of reasons:
1. It may not be feasible or economical to measure all outputs, in particular chemical
compositions.
2. MVs may not be enough.
3. Potential control loops may be impractical due to slow dynamics, low MV sensitivity,
or interactions with other control loops.
2.1.1 Controlled Variables
Guideline 1
All variables that are not self-regulating must be controlled. Non-selfregulating is defined as an output variable that exhibits an unbounded
response after a sustained disturbance such as a step disturbance.
Guideline 2
Choose output variables that must be kept within equipment and operating
constraints such as temperature, pressure and composition. The constraints
are due to safety, environmental, and operational requirements.
Guideline 3
Select output variables that are a direct measure of product quality such as
composition and refractive index or that strongly affect it for example
temperature or pressure.
Guideline 4
Choose output variables that seriously interact with other controlled
variables.
Guideline 5
Choose output variables that have favorable dynamic and static
characteristics.
2.1.2 Manipulated Variables
Guideline 6
Select inputs that have large effects on controlled variables. For a
conventional feedback control system, each manipulated variable should
have a significant, rapid effect on only one controlled variable. The effect
of a manipulated variable on another controlled variable should be
negligible.
Guideline 7
Choose inputs that rapidly affect the controlled variables where any time
delay and time constant should be small, relative to the dominant process
time constant.
Guideline 8
The manipulated variables should affect the controlled variables directly,
rather than indirectly.
Guideline 9
Avoid recycling of disturbances due to disturbances that tend to be
propagated forward or recycled back to the process. It can be avoided by
manipulating a utility stream to absorb disturbances or an exit stream that
allows the disturbances to be passed downstream.
2.1.3 Measured Variables
Guideline 10
Reliable, accurate measurements are essential for good control.
Guideline 11
Select measurement points that have an adequate degree of sensitivity.
Guideline 12
Select measurement points that minimize time delays and time constants
by reducing dynamic lags and time delays improve the closed-loop
stability and response characteristics.
2.2 Advanced Control System
2.2.1 Feedforward Control
Feedforward control is an anticipatory control configuration where it acts before a disturbance
affects the system. This control is normally used together with a feedback control.
Advantage for feedforward control
1) Act before the effect of disturbance felt by the system.
2) Good for slow and system with significant dead time.
3) Does not introduce instability in closed loop response
Disadvantages for feedforward control
1) Disturbance variable must be measured on-line.
2) Require good knowledge of process models.
3) Sensitive to process parameter variation
4) Cannot cope with unmeasured disturbances.
5) May not achieve perfect control.
2.2.2 Cascade Control
The defining feature that can be used to identify a cascade control,
1) Output signal of the master controller serves as the set point for the slave controller.
2) The two feedback control loops are nested with a secondary control loop placed inside
the primary control loop.
Disturbances arising within the secondary loops are corrected by the secondary controller
before they can affect the value of the primary controlled output.
2.2.3 Ratio Control
A ratio control is a special type of feedforward control. Two variables are measured and held
in constant ratio to each other. One is manipulated variables and the other is disturbance. Ratio
= manipulated / disturbance. Both are physical variables.
There are two method for ratio control with the most commonly used method are as shown
below,
2.3 Procedure in designing Plantwide control system
I.
Specify the control system design objectives.
A. State the plant production, economic, and control objectives, including
composition and production rates of all products.
B. Identify process constraints that must be satisfied, including safety,
environmental, and quality restrictions.
II.
Perform a top-down analysis.
A. Identify the process variables, control degrees of freedom, control structure, and
options for decomposition.
1. Identify the potential controlled variables.
2. Determine how the CVs can be measured or inferred, and identify other
process variables to be measured
3. Select the potential manipulated variables.
4. Perform a preliminary control degrees of freedom analysis (compare the
number of potential manipulated and controlled variables).
5. Identify the source and nature of the significant disturbances that must
be mitigated.
6. Perform a structural analysis based on a steady-state model, select the
final controlled and manipulated variables, and evaluate the possibilit ies
for decomposition of the control problem.
B. Establish the overall control structure (in conceptual form).
1. Identify where the production rate of each product will be measured and
controlled.
2. Identify how quality will be measured for each product, and how quality
will be controlled.
3. Determine how each recycle loop throughput/composition will be
controlled.
4. Specify how major constraints will be satisfied.
5. Determine how major disturbances will be handled.
6. Analyze the energy management scheme, and indicate conceptually how
it will be controlled.
III.
Develop a bottom-up design.
A. Develop a strategy for regulatory control.
1. Specify how the control system will respond to unsafe or abnormal
operating conditions and deal with constraints.
2. Identify control loops to regulate production rates and inventories.
3. Identify control loops that will mitigate major disturbances
B. Examine the potential of applying advanced control strategies.
1. Evaluate the use of enhanced single-loop control strategies, including
feedforward, ratio, cascade, and selective control schemes.
2. Employ MIMO control for highly interactive processes.
C. Evaluate the economic benefits of real-time optimization.
IV.
Validate the proposed control structure.
A. Perform a final control degrees of freedom analysis. Check the allocation of the
NFC degrees of freedom.
B. Check control of individual process units.
C. Check the effect of constraints and disturbances on manipulated and controlled
variables.
D. Stimulate control system performance for a wide range of conditions.
3.0 PLANTWIDE CONTROL SYSTEM DESIGN
3.1 Design of Control Loops in P&ID
3.2 Process Limitations
1. The oil temperature in the oil melting tank must be controlled within 60°C to 70°C.
2. The temperature in the liquiverter and both dissolvers must be controlled within 60°C
to 65°C.
3. The pH value in the liquiverter must be maintained at around 7, depending on the
product requirement.
4. The feed ratio from the oil melting tank and liquiverter into the two dissolvers must be
controlled to prevent losses.
5. The pH value in the dissolvers must be controlled depending on the product
requirement.
3.3 Oil Melting Tank
3.3.1 Loop 1
a) Top-down analysis
i)
Control Objective: To control the oil temperature in the oil mixing tank at the
desired temperature (60°C - 70°C)
ii)
Process variable
1) Controlled :
(a) Primary: Oil temperature in the oil melting tank
(b) Secondary: Steam pressure entering the oil melting tank
Selection criteria:
According to Guideline 1, oil temperature and steam pressure
must be controlled as they are not self-regulating. According to
Guideline 2, the operating constraints of loop 1 is the
temperature range of 60°C - 70°C, hence it must be controlled to
prevent any safety and environmental issues. According to
Guideline 4, the steam pressure that seriously interacts with oil
temperature must be controlled and well-regulated, or else it will
be a significant disturbance to the control loop.
2) Measured :
(a) Primary: Oil temperature in the oil melting tank
(b) Secondary: Steam pressure entering the oil melting tank
Selection criteria:
According to Guideline 10, the oil temperature and steam
pressure are reliable as they can be measured accurately.
According to Guideline 11, the measurement of oil temperature
at any point of the oil melting tank has an adequate degree of
sensitivity as the oil is well-mixed.
3) Manipulated: Flow rate of steam entering the oil melting tank
Selection criteria:
According to Guideline 6, 7 and 8, instead of outlet oil
flow rate from the oil melting tank, the inlet steam flow
rate has a larger, faster and more direct effect on the
steam pressure and oil temperature. Thus the time delay
and time constant can be minimised by varying the steam
flow rate.
4) Disturbance: Inlet temperature of steam
iii)
Control Degree of Freedom (DOF)
n(Controlled) = 2
n(Manipulated) = 1
DOF = n(Controlled) - n(Manipulated) = 2 - 1 = 1
b) Bottom-up analysis
i)
Control system: Cascade control
Justification:
Since the control DOF ≠ 0, a cascade control configuration is needed
where two variables are controlled by only one manipulating variable.
In this control configuration, the oil temperature loop will be dominating
the control system and the steam pressure will be the secondary loop
with its setpoint from the master loop. Cascade control systems can limit
the effect of disturbance on the primary loop. The secondary loop is
closer to the disturbance, inlet steam temperature. Thus, it has a faster
response than primary loop as it feedforward with the disturbance. This
allows for a quicker correction of the process.
3.4 Liquiverter-1
3.4.1
Loop 2
a) Top-down analysis
i)
Control Objective: To control pH of mixture in liquiverter1
ii)
Process variable
1) Controlled: pH of mixture in liquiverter-1
Selection criteria:
According to guideline 4, variables that seriously interact with
other controlled loops must be selected as the controlled
variable. The pH of mixture in liquiverter-1 must be controlled
to avoid the protein to be in insoluble condition.
2) Measured: pH of mixture in liquiverter-1
(a) Primary: Oil temperature in the oil melting tank
(b) Secondary: Steam pressure entering the oil melting tank
Selection criteria:
According to Guideline 11, the measurement point selected
should have an adequate degree of sensitivity. Therefore,
measuring the pH in the mixture directly from liquiverter-1 is the
most suitable.
3) Manipulated: Flowrate of process water feed into liquiverter-1
Selection criteria:
According to guideline 7 and 8, choose inputs that
rapidly affect the controlled variables where any time
delay and time constant should be small, relative to the
dominant process time constant and the manipulated
variables should affect the controlled variables directly,
rather than indirectly. Hence, flowrate of process water
feed into liquiverter-1 is the most suitable to be the
manipulated variable in loop 2 because water has a
neutral pH of 7 which can increase the pH by increasing
the flow rate.
4) Disturbance: Flowrate of protein/minerals entering liquiverter
iii)
Control Degree of Freedom (DOF)
n(Controlled) =
n(Manipulated) = 1
DOF = n(Controlled) - n(Manipulated) = 1 - 1 = 0
b) Bottom-up analysis
i)
Control system: Feedback control
Justification:
For loop 2, the most appropriate control configuration is Feedback
Control. To control the pH of mixture in liquiverter-1 at desired level so
that we can prevent the insoluble protein, feedback control can be used
where the action is taken after disturbance has occurred. Thus, the
feedback control loop for pH is significant for ensuring the product
quality.
3.4.2 Loop 3
a) Top-down analysis
i)
Control Objective: To control the temperature of liquiverter-1 within 60°C to
65°C.
i)
Process variable
1) Controlled:
(a) Primary: Temperature in the liquiverter-1
(b) Secondary: Steam pressure entering the liquiverter-1
Selection criteria:
Base on Guideline 2, we must select output variables that must
be kept within the equipment and operating constraints as the
controlled variables. Hence, the temperature and pressure in
liquiverter-1 is needed to be controlled in order to maintain the
value at the desired range.
2) Measured:
(a) Primary: Temperature of liquiverter-1
(b) Secondary: Steam pressure entering the liquiverter-1
Selection criteria:
Base on Guideline 12, select measurement points that minimize
time delays and time constants by reducing dynamic lags and
time delays improve the closed-loop stability and response
characteristics. Thus, directly measuring the temperature in the
tank and pressure of steam in the inlet pipe will be the
measurement point with the shortest time delay.
3) Manipulated: Flowrate of steam feeds into liquiverter-1
Selection criteria:
Base on Guideline 7, we need to choose inputs that
rapidly affect the controlled variables where any time
delay and time constant should be small, relative to the
dominant process time constant. Hence, the flow rate of
the steam feed into liquiverter-1 is more suitable to be
chosen as the manipulated variable for loop 3. This is
because the amount of steam used can cause a drastic
change in temperature Liquiverter-1.
4) Disturbance: Temperature of process water and protein/minerals feed
into liquiverter-1
ii)
Control Degree of Freedom (DOF)
n(Controlled) = 2
n(Manipulated) = 1
DOF = n(Controlled) - n(Manipulated) = 2 - 1 = 1
b) Bottom-up analysis
i)
Control system: Cascade control
Justification:
Cascade Control is suggested as the most suitable control configuration
for loop 3. To maintain the quality of the mixing process, we need to
ensure the temperature in Liquiverter-1 to be at their desired level.
Therefore, Cascade control can be used as it can significantly increase
the dynamic reaction to disturbances. This is due to the fact that the
secondary measurement point can detect an unsettled condition faster
than the controlled variable. Thus, pressure control is selected as the
secondary loop for cascade control as it can remove the disturbance and
maintain the process operation.
3.5 Dissolver 1 and Dissolver 2
3.5.1 Loop 5 and Loop 8
a) Top-down analysis
i)
Control Objective: To control the pH in both dissolvers at the desired pH
ii)
Process variable
1) Controlled: Mixture pH in both dissolvers
Selection criteria:
● Based on Guideline 1, the pH of the mixture must be controlled
as they are not self-regulating.
● Based on Guideline 2, the controlled variables must be kept
within equipment constraints. The constraint for loop 5 and 8 is
the pH of the mixture. It must be controlled to prevent any safety
and environmental issues.
2) Measured :Mixture pH in both dissolvers
Selection criteria:
● Based on Guideline 10, the pH of the mixture is reliable as it can
be measured accurately.
● Based on Guideline 11, the measurement of the pH of the
mixture at any point of the dissolver has an adequate degree of
sensitivity as the mixture is well-mixed.
3) Manipulated: Flow rate of process water stream entering both dissolvers.
Selection criteria:
● Based on Guideline 6, the process water stream has a larger
effect on the pH.
● Based on Guideline 7, the process water stream has a rapid effect
on thepH. Therefore, the time delay and time constant can be
neglected.
● Based on Guideline 8, the process water stream affects the pH
directly.
4) Disturbance: Inlet temperature of process water stream
iii)
Control Degree of Freedom (DOF)
n(Controlled) = 1
n(Manipulated) = 1
DOF = n(Controlled) - n(Manipulated) = 1 - 1 = 0
b) Bottom-up analysis
i)
Control system: Feedback control
Justification:
The controlled variable is measured and the correction action taken after
disturbance has occurred. The control loop is a simple unit and it does
not require a perfect control. Therefore, feedback control is chosen to
save cost.
3.5.2 Loop 6 and Loop 7
a) Top-down analysis
i)
Control Objective: To control the mixture temperature in both dissolver at the
desired temperature (60°C - 65°C)
ii)
Process variable
1) Controlled :
(a) Primary: Mixture temperature in the dissolvers.
(b) Secondary: Steam pressure entering the dissolver
Selection criteria:
● Based on Guideline 1, mixture temperature and steam
pressure must be controlled as they are not selfregulating.
● Based on Guideline 2,the controlled variables must be
kept within equipment constraints. The constraint for
loop 6 and 7 is the temperature of the mixture. It must be
controlled to prevent any safety and environmental
issues.
2) Measured :
(a) Primary: Mixture temperature in the dissolver.
(b) Secondary: Steam pressure entering the dissolver.
Selection criteria:
● Based on Guideline 10, the temperature of the mixture is
reliable as it can be measured accurately.
● Based on Guideline 11, the measurement of the
temperature of the mixture at any point of the dissolver
has an adequate degree of sensitivity as the mixture is
well-mixed.
3) Manipulated: Flow rate of steam entering the dissolver
Selection criteria:
● Based on Guideline 6, the inlet steam has a larger effect
on the temperature of the mixture.
● Based on Guideline 7, the inlet steam has a rapid effect
on the temperature of the mixture. Therefore, the time
delay and time constant can be neglected.
● Based on Guideline 8, the inlet steam affects the
temperature of the mixture directly.
.
4) Disturbance: Inlet temperature of steam
iii)
Control Degree of Freedom (DOF)
n(Controlled) = 2
n(Manipulated) = 1
DOF = n(Controlled) - n(Manipulated) = 2 - 1 = 1
b) Bottom-up analysis
i)
Control system: Cascade control
Justification:
The control configuration is cascade because the control loop employs
primary loop and secondary loop. The secondary feedback controller
and secondary measurement point which identify the upset situation
sooner than controlled variables but disturbance is not necessarily
measured. The primary loop monitors the mixture temperature in the
dissolver and sends a signal output as the new set point for the secondary
loop while the secondary loop measures the steam pressure entering the
dissolver and sends the signal to adjust the valve. Cascade control gives
enhanced performance since the control valve will be changed as soon
as the change in temperature is recognised.
3.5.3 Loop 4
a) Top-down analysis
i)
Control Objective: To control the composition of feed into the dissolver tank
from the outlet of oil melting tank and liquiverter tank.
ii)
Process variable
1) Controlled: Composition of feed into dissolver tank from the outlet of
oil melting tank and liquiverter into the dissolver.
-
According to Guideline 1, the composition of feed into the
dissolver tank from the outlet oil melting tank and liquiverter
must be controlled as they are not self-regulating.
-
According to Guideline 2, the operating constraints of loop 4 is
to ensure the set ratio or compositions of feed into the dissolver
tank from the outlet of the oil melting tank and liquiverter tank.
Hence it must be controlled to ensure operational requirements
are fulfilled and prevent loss of product.
2) Measured :
(a) Primary: Outlet flowrate from Liquiverter-1
(b) Secondary: Outlet flowrate from Oil Melting Tank
Selection criteria:
-
According to Guideline 10, the outlet flowrate
from Oil Melting Tank and Liquiverter-1 are
reliable as they can be controlled and measured
accurately.
-
According to Guideline 11, the measurement of
flowrate at any point at the outlet of oil melting
tank and liquiverter has an adequate degree of
sensitivity as the flowrate throughout those
outlets reacts quickly to changes.
3) Manipulated: Flow rate of outlet from Liquiverter-1
- Selection criteria:
According to Guideline 6, 7 and 8, manipulating
flowrate of outlet from Liquiverter-1 has a large effect on
the composition of inlet into the dissolver. The
manipulated input can also rapidly react to any
disturbance changes from the outlet of Oil Melting Tank
that can affect the inlet composition into the dissolver.
Manipulating the input also directly affects the inlet
composition into the dissolver.
4) Disturbance: Flowrate of outlet from Oil Melting Tank.
ii)
Control Degree of Freedom (DOF)
n(Controlled) = 1
n(Manipulated) = 1
DOF = n(Controlled) - n(Manipulated) = 1 - 1 = 0
b) Bottom-up analysis
i)
Control system: Ratio Control
Justification:
Only by using ratio control, the control loops will be able to respond to
the abnormal operating condition that might arise from the disturbance
in flowrate in Oil Melting Tank. Ratio control can also ensure the ratio
and composition of feed inlet into the dissolver can be accurately
identified and adjusted quickly and easily according to the specific
product requirement by changing only the set point ratio.
4.0 Conclusion
In this case study, we have studied the selection guideline, advance control system and
procedure in designing plantwide control systems. The control loops required for the process
operation of Kerry’s Wet-Mix Process have been identified with the use of proper guideline
and control system design procedure. There are a total of eight control loops needed by the
process. This includes three feedback controls, four cascade controls and one ratio control.