Transcript 2006 Emerson Exchange Powerpoint Template

Constraint Control on a
Distillation Column
Pamela Buzzetta
Process Engineer, MECS, Inc.
Presenters
• Pamela Buzzetta
Process Engineer
• Robert Heider
Adjunct Professor
[File Name or Event]
Emerson Confidential
27-Jun-01, Slide 2
Introduction
Why distillation columns?
• They are the largest source in energy and exergy
losses for industrial operations
• Conserving energy can keep a company
competitive, especially with rising energy costs
How do we use constraint control?
• Use an implied valve position (IVP) PID controller
in cascade with the overhead condenser controller
• IVP keeps the process at its minimum pressure by
holding the pressure control (PC) valve at a fixed
point
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Introduction
Outline
• Distillation Column Description
• Objectives for Constraint Control
• Schematic and Simulation for Solution
• Results and Analysis of Simulation
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Distillation Column Description
Bypass
Valve
Distillation
Column
Heat
Exchanger
Condenser
Tank
• Pressurized column for low b.p. compounds
• Condenser sub cools the condensate
• Vapor flow through bypass condenses on surface of liquid
in condenser tank
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Distillation Column Description
• Condenser, reboiler duties, feed composition, and
column design, etc. determine distillation column
pressure
• Motive force is the pressure drop across the
condenser and the bypass control valve
• Need bypass valve to control overhead pressure
that compensates for changes in condenser duty
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Example of Control Difficulty
• Severe thunder storm produces rapid ambient
temperature drop
• Efficiency (heat transfer rate) of condenser
increases
• Pressure drops rapidly
• Decreases boiling point of products
• Experience column flooding
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Example of Control Difficulty
• Losses Incurred:
– Distillation columns are a major energy consumer in
industry and also a major contributor to energy and
exergy losses
– Downtime to re-equilibrate the column is also money
lost
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Objectives for Constraint Control
• Overall goal: control pressure in short term and
valve position in long term
• IVP acts to keep valve nearly closed at steady
state (10% open in our model)
• IVP slowly decreases/increases PC SP
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DeltaV Schematic
Bypass
Valve
Heat
Exchanger
Ambient
Temperature
Distillation
Column
Condenser
Tank
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Process Assumptions
• All vapors from valve discharge condense at the
tank
• Reboiler held constant
• Condensate cooled to 10 °F above ambient
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DeltaV Control Studio Diagram
PID Pressure Controller
Analog Input
Pressure Transmitter
Analog Vapor
Flow Output
Analog Output
Valve Position
IVP Loop slower than
PC Controller
PC Gain: 0.5
IVP Gain: 1.25
Reset: 4.4
Reset: 600
PID Controller
Implied Valve Position
Variable Ambient
Temperature Output
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27-Jun-01, Slide 12
Real-time Cycle Control of Ambient Temp
Control Operation Outline
• PC mode is in cascade mode to output and set the
bypass valve position
• IVP SP is 10%, output is cascaded to the PC SP
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MATLAB® Integration Routine
• Set Initial Conditions
• Loop Start
– Read Ambient Temperature from DeltaV
– Read Valve Position from DeltaV
• Calculate valve flow properties (valve size is 26)
– Calculate pressures and temperature of column and tank
• Compute change in pressure between column and tank
• Compute pressure and temperature in Column
– Pcolumn = Ptank + dP
– Tcolumn calculated from properties of material, given pressure
• Compute heat reflux for tank heat balance (Q)
• Compute temperature and pressure in tank
– Ttank = Q / (mass * heat of vaporization)
– Ptank calculated from properties of material, given temperature
– Write pressure transmitter reading to DeltaV
– Pause
•
Loop End
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OPC Utility
• An OPC provides connectivity between MATLAB ®
and DeltaV
• OPCs are the Microsoft OLEs for process control
• OPC used is a MATLAB® add-in provided by
IPCOS TECHNOLOGY
Bosscheweg 145a
5282 WV Boxtel
The Netherlands
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27-Jun-01, Slide 15
Results of Temperature Changes
Key for plots:
•
Yellow/orange: ambient
temp (ºF)
•
Purple: IVP SP
•
Green: valve position
(%)
•
Blue: PC PV (psia)
•
Red: PC SP (psia)
Good control: Smooth and slow
response to small Tamb step increases
(Ex: sun rise)
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Results of Temperature Changes
Responses to Tamb step from 85 to 70 °F
(Ex: Rainstorm)
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Results of Temperature Changes
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18 hour real-time response
(Ex: day and night cycle)
Energy Savings Estimation
•
•
•
•
•
For 200,000 lb/hr pure pentane:
At P=60 psia, Qstream=-2.0657·108 Btu/hr
At P=20 psia, Qstream=-2.1442·108 Btu/hr
ΔQ = 7.85·106 Btu/hr
970.6 Btu/lb steam, $9/1000 lb steam
(sources: http://www.steamonline.com/loss_chart.html, http://www.pipingnews.com/steam3.htm)
• Savings: Assume operates at lower pressures 50%
of the time at 24 hr/day and 300 operating days/yr =
~$260,000 saved per year
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Summary
• Using an IVP in constraint control works to save
energy and thus, money, for a prominent
component of many industrial processes
• An OPC can integrate MATLAB® with DeltaV
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27-Jun-01, Slide 20
Summary
Questions?
Feedback?
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27-Jun-01, Slide 21
Where To Get More Information
• Sources:
– Shinskey, Francis G., Energy Conservation Through
Control, New York: Academic Press, 1978
– Sloley, Andrew, “Is achieving design conditions
realistic?”, Chemical Processing, Sept. 2005,
http://www.chemicalprocessing.com/articles/2005/535.ht
ml
• Thank you to:
–
–
–
–
Professor Robert Heider
Jason Hall, Washington University SSM Student
Washington University Chemical Engineering Dept.
Emerson and Emerson Users Group
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Emerson Confidential
27-Jun-01, Slide 22