Transcript Slide 1
Integration of Design & Control
CHEN 4470 – Process Design Practice
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lecture No. 16 – Integration of Design and Control II
March 7, 2013
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Plantwide Control Design
Luyben et al. (1999) suggest a method for the conceptual
design of plant-wide control systems, which consists of the
following steps:
Step 1: Establish the control objectives.
Step 2: Determine the control degrees of freedom.
Simply stated – the number of control valves – with
additions if necessary.
Step 3: Establish the energy management system.
Regulation of exothermic or endothermic reactors, and
placement of controllers to attenuate temperature
disturbances.
Step 4: Set the production rate.
Step 5: Control the product quality and handle safety,
environmental, and operational constraints.
Plantwide Control Design
Step 6: Fix a flow rate in every recycle loop and control
vapor and liquid inventories (vessel pressures and
levels).
Step 7: Check component balances. Establish control to
prevent the accumulation of individual chemical species
in the process.
Step 8: Control the individual process units. Use
remaining DOFs to improve local control, but only after
resolving more important plant-wide issues.
Step 9: Optimize economics and improve dynamic
controllability. Add nice-to-have options with any
remaining DOFs.
Example 2: Acyclic Process
Select V-7 for
On-demand
product flow
Select V-1 for
fixed feed
Steps 1 & 2: Establish the control objectives and DOFs:
Maintain a constant production rate
Achieve constant composition in the liquid effluent from flash drum
Keep the conversion of the plant at its highest permissible value.
Example 2: Acyclic Process
Step 3: Establish energy management system:
Need to control reactor temperature: Use V-2
Need to control reactor feed temperature: Use V-3
Example 2: Acyclic Process
Step 4: Set the production rate:
For on-demand product: Use V-7
Example 2: Acyclic Process
Step 5: Control product quality, and meet safety, environmental, and
operational constraints:
To regulate V-100 pressure: Use V-5
To regulate V-100 temperature: Use V-6
Example 2: Acyclic Process
Step 6: Fix recycle flow rates and vapor and liquid inventories :
Need to control vapor inventory in V-100: Use V-5 (already installed)
Need to control liquid inventory in V-100: Use V-4
Need to control liquid inventory in R-100: Use V-1
Example 2: Acyclic Process
Step 7: Check component balances
N/A: Neither A or B can build up
Step 8: Control the individual process units
N/A: All control valves in use
Step 9: Optimization
Install composition controller, cascaded with TC of reactor
Example 2: Acyclic Process
Select V-1 for
fixed feed
Differences: Only step 6 is different
The liquid levels in R-100 and V-100 are now controlled in the
direction of the process flow, where before they were controlled in
the reverse direction.
Example 2: Acyclic Process
Example 3: Cyclic Process
This control structure for fixed feed has an inherent problem.
Can you see what it is?
Example 3: Cyclic Process
F0
D
F0 + B
B
B
Combined molar feed to the CSTR:
F0 B
Molar material balance around the flash vessel: F0 B D B
Overall molar material balance:
F0 D
Example 3: Cyclic Process
Molar balance on CSTR:
1 dnA
VR dt
kxActotal 1 xA F0 B kxActotalVR
1 xA F0 B kxActotalVR
Substitute: ctotalVR nT
1 xA F0 B kxAnT
xA F0 knT F0
Rearranging: B
1 xA
Balance on A for perfect separation: F0 kxAnT
B
F02
knT F0
Example 3: Cyclic Process
e.g., suppose knT = 200:
B
F02
knT F0
“Snowball” effect
F0
50
75
100
125
150
B
16.7
45
100
208
450
A more general result uses the dimensionless, Damköhler
number: Da = knT/F0 giving:
B
F0
Da 1
“Snowball” effect for Da 1
Example 3: Cyclic Process
Steps 1 & 2: Establish the control objectives and DOFs:
Maintain the production rate at a specified level
Keep the conversion of the plant at its highest permissible value.
Example 3: Cyclic Process
Step 3: Establish energy management system:
Need to control reactor temperature: Use V-2
Example 3: Cyclic Process
Step 4: Set the production rate:
For on-demand product: Use V-1
Example 3: Cyclic Process
Step 5: Control product quality, and meet safety, environmental, and
operational constraints:
To regulate V-100 pressure: Use V-4
To regulate V-100 temperature: Use V-5
Example 3: Cyclic Process
Step 6: Fix recycle flow rates and vapor and liquid inventories :
Need
Need
Need
Need
to
to
to
to
control
control
control
control
recycle flow rate: Use V-6
vapor inventory in V-100: Use V-4 (already installed)
liquid inventory in V-100: Use V-3
liquid inventory in R-100: Cascade to FC on V-1
Example 3: Cyclic Process
Step 7, 8 and 9: Improvements
Install composition controller, cascaded with TC of reactor
Summary
Part I: Previous Lecture
Provided motivation for handling flowsheet
controllability and resiliency as an integral part of
the design process
Outlined qualitative approach for unit by unit
control structure selection
Part II – This Lecture
Outlined a qualitative approach for plantwide
control structure selection
Other Business
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Next Lecture – March 19
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Equipment sizing and pinch analysis
Q&A Session with Consultant – March 21
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Bob Kline will participate via videoconference
Questions can be sent to Bob and/or me ahead of time