Constraint Control on a Distillation Column Pamela Buzzetta Process Engineer, MECS, Inc.

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 2 PresentersPresenters Pamela Buzzetta Process Engineer Robert Heider Adjunct Professor

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 3 IntroductionIntroduction 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 4 IntroductionIntroduction Outline Distillation Column Description Objectives for Constraint Control Schematic and Simulation for Solution Results and Analysis of Simulation

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 5 Distillation Column Description Pressurized column for low b.p. compounds Condenser sub cools the condensate Vapor flow through bypass condenses on surface of liquid in condenser tank Bypass Valve Heat Exchanger Distillation Column Condenser Tank

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 6 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 7 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 8 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 9 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 10 DeltaV Schematic Ambient Temperature Condenser Tank Distillation Column Bypass Valve Heat Exchanger

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 11 Process Assumptions All vapors from valve discharge condense at the tank Reboiler held constant Condensate cooled to 10 °F above ambient

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 12 DeltaV Control Studio Diagram Analog Input Pressure Transmitter Analog Output Valve Position Variable Ambient Temperature Output PID Controller Implied Valve Position PID Pressure Controller IVP Loop slower than PC Controller Analog Vapor Flow Output Real-time Cycle Control of Ambient Temp PC Gain: 0.5 Reset: 4.4 IVP Gain: 1.25Reset: 600

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 13 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 14 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 15 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 16 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)

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 17 Results of Temperature Changes Responses to T amb step from 85 to 70 °F (Ex: Rainstorm)

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 18 Results of Temperature Changes 18 hour real-time response (Ex: day and night cycle)

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 19 Energy Savings Estimation For 200,000 lb/hr pure pentane: At P=60 psia, Q stream = ·10 8 Btu/hr At P=20 psia, Q stream = ·10 8 Btu/hr ΔQ = 7.85·10 6 Btu/hr Btu/lb steam, $9/1000 lb steam (sources: Savings: Assume operates at lower pressures 50% of the time at 24 hr/day and 300 operating days/yr = ~$260,000 saved per year

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 20 SummarySummary 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

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 21 SummarySummary Questions? Feedback?

[File Name or Event] Emerson Confidential 27-Jun-01, Slide 22 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, ml Thank you to: –Professor Robert Heider –Jason Hall, Washington University SSM Student –Washington University Chemical Engineering Dept. –Emerson and Emerson Users Group