Integration of Design & Control
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Transcript of Integration of Design & Control
Integration of Design & Control
CHEN 4470 – Process Design Practice
Dr. Mario Richard EdenDepartment 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
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.
Steps 1 & 2: Establish the control objectives and DOFs:
Select V-7 for On-demand product flow
Select V-1 for fixed feed
Example 2: Acyclic Process
Need to control reactor temperature: Use V-2 Need to control reactor feed temperature: Use V-3
Step 3: Establish energy management system:
Example 2: Acyclic Process
For on-demand product: Use V-7
Step 4: Set the production rate:
Example 2: Acyclic Process
To regulate V-100 pressure: Use V-5 To regulate V-100 temperature: Use V-6
Step 5: Control product quality, and meet safety, environmental, and operational constraints:
Example 2: Acyclic Process
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
Step 6: Fix recycle flow rates and vapor and liquid inventories :
Example 2: Acyclic Process
Install composition controller, cascaded with TC of reactor
Step 7: Check component balances
Step 8: Control the individual process units
Step 9: Optimization
N/A: Neither A or B can build up
N/A: All control valves in use
Example 2: Acyclic Process
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.
Differences: Only step 6 is different
Select V-1 for fixed feed
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
B
D
B
F0 + B
0
0
0
Combined molar f eed to the CSTR:
Molar material balance around the fl ash vessel:
Overall molar material balance:
F B
F B D B
F D
Example 3: Cyclic Process
AA A 0 A
11 Rtotal total
R
dnkx c x F B kx c V
V dtR Ttotalc V n
Molar balance on CSTR:
A 0 A A 0 A1 1 R Ttotalx F B kx c V x F B kx n
A 0 0
A
1
Tx F kn F
Bx
Rearranging:
20
0
T
FB
kn F
Substitute:
Balance on A for perfect separation:
Example 3: Cyclic Process
16.750
450150
208125
100100
4575
BF0
e.g., suppose knT = 200:2
0
0
T
FB
kn F
A more general result uses the dimensionless, Damköhler number: Da = knT/F0
giving:
0
1F
BDa
“Snowball” effect for Da 1
“Snowball” effect
Example 3: Cyclic Process
Maintain the production rate at a specified level Keep the conversion of the plant at its highest permissible
value.
Steps 1 & 2: Establish the control objectives and DOFs:
Example 3: Cyclic Process
Need to control reactor temperature: Use V-2
Step 3: Establish energy management system:
Example 3: Cyclic Process
For on-demand product: Use V-1
Step 4: Set the production rate:
Example 3: Cyclic Process
To regulate V-100 pressure: Use V-4 To regulate V-100 temperature: Use V-5
Step 5: Control product quality, and meet safety, environmental, and operational constraints:
Example 3: Cyclic Process
Need to control recycle flow rate: Use V-6 Need to control vapor inventory in V-100: Use V-4 (already
installed) Need to control liquid inventory in V-100: Use V-3 Need to control liquid inventory in R-100: Cascade to FC on
V-1
Step 6: Fix recycle flow rates and vapor and liquid inventories :
Example 3: Cyclic Process
Install composition controller, cascaded with TC of reactor
Step 7, 8 and 9: Improvements
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
• Next Lecture – March 19– Equipment sizing and pinch analysis
• Q&A Session with Consultant – March 21
– Bob Kline will participate via videoconference– Questions can be sent to Bob and/or me ahead
of time
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