Commissioning highly interactive process an approach for tuning control loops
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Commissioning highly interactive process an approach for tuning control loops

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The size of the process equipment used in a pilot plant dictates a little buffering and interaction between process units. We examine a skid mounted high temperature CO2 recovery process with a high ...

The size of the process equipment used in a pilot plant dictates a little buffering and interaction between process units. We examine a skid mounted high temperature CO2 recovery process with a high degree of process interaction. An effective tuning approach provided high performance control. A dynamic process simulation optimized performance by exploring various control strategies.

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Commissioning highly interactive process an approach for tuning control loops Presentation Transcript

  • 1. Commissioning Highly Interactive Process An Approach For Tuning Control Loops
  • 2. Presenters
    • Eric Chen
    • Terry Blevins
    • Willy Wojsznis
  • 3. Introduction
    • The design of a process often dictates that there is little buffering of interaction between process units. In this workshop we will examine:
    • A skid mounted process for CO2 recovery that is characterized by a high degree of process interaction.
    • Details on the approach taken in tuning the control and the performance achieved.
    • A dynamic process simulation that was developed to explore changes in the control strategy design .
    • An improvement in control response to process disturbances can be achieve when loops are tuned to minimize the impact of process interaction.
  • 4. SRP CO2 Capture Pilot Plant
    • Gas Capacity, m 3 /min = 25
    • Solvent Capacity, liter/min = 130
    • Inlet CO2 Composition, mol% =1-20
    • Capabilities:
    • - Solvent Screening
    • - Packing Performance
    • - Effect of Absorber Inter-cooling
    • - Solvent Regeneration Variations
    • - Evaluate Process Dynamics
    • - Evaluate Heat Exchangers
    • - Model Validation
  • 5. Field Trial – 2-stage Flash Skid
    • For CO2 recovery, a 2-stage flash skid is used in place of the existing stripping column with kettle reboiler.
    • Capital cost many be greatly reduced using this approach for CO2 recovery.
  • 6. 2-Stage Flash Skid
  • 7. SRP CO2 Capture Pilot Plant
  • 8. 2-Stage Flash Skid Design
    • Skid size did not allow buffering between units.
    • Flash tank temperature, pressure and Level controls are highly interactive
    • Process feedback through heat exchanger
  • 9. 8 m PZ Provides High P at 150 °C
    • Flash tank temperature and pressure must be maintained at design conditions for correct operation.
    Energy Analysis
  • 10. Initial Tuning
    • The 2-stage flash skid is a new process that was recently installed at the UT Pickle site.
    • Initial tuning was based on experience with similar loops. There was little opportunity to make tuning adjustments during the initial process startup.
    • Once the process was on-line the interaction associated with pressure, temperature and level control in the flash tanks made it difficult to hold desired conditions and to transition between target operating points.
    • A focused effort was made to re-tuning the skid controls to achieve more stable operation.
  • 11. Tuning Approach
    • Loops were addressed individually in the order of process flow
    • DeltaV On-demand tuner was used to minimize disturbances to the process.
    • The inner steam flow loop was tuned before the temperature loop.
    • The pressure loop was placed on Manual while flash tank level was tuned.
  • 12. Tuning Rules
    • Temperature is self-regulating. Typical PI and PID Tuning was selected for temperature and steam flow.
    • Pressure and level are integrating Processes. Lambda Averaging Level tuning was selected to minimize the impact of control action on downstream process and interaction between pressure and temperature .
  • 13. Tuning Results
  • 14. Before Re-tuning Control
    • Oscillatory behavior was observed on changes in feed rate, feed temperature, and loading.
    • Operator intervention was often required to break loop interaction
    30% Change in Feed Rate
  • 15. After Re-tuning Control
    • More stable response was observed.
    • Large changes can be made in process condition without the need for operator intervention.
  • 16. Tuning Impact – Operator Feedback
    • “ The plant is operating much better now.”
    • “ We have been running into various plugging problems with the absorber this morning with flows to the stripper changing quite a bit.  The stripper skid operation  has held up very well. “ 
  • 17. On-going Work
    • To allow the benefits of alternate control techniques such as MPC to be explored off-line, a dynamic simulation of the 2-Stage Flash Skid has been developed.
    • The dynamic response is designed to match the on-line behavior.
    • The variable speed pump curve, pressure drops across the heaters and heat exchanger, and impact of CO2 loading and temperature on CO2 partial pressure is accounted for in the simulation.
  • 18. 2-Stage Skid Simulation Implementation
  • 19. Simulation Implementation (Cont)
    • Seven(7) stream parameters are passed between simulation blocks using IN and OUT array parameters.
    • Downstream pressure/flow passed to block using BKCAL parameters.
  • 20. Control Module for Testing
    • All the loops associated with the 2-Stage Flash skid were combined in one module to facilitate testing.
    • External references in the Simulation module read the AO block output and provide simulated values to the SIMULATE_IN parameter of the AI blocks used in control.
  • 21. Business Results Achieved
    • The operation of the 2-stage Flash skid is more stable as a results of re-tuning the control loops to minimize interaction.
    • The time required to transition between different operating targets has been reduced and allows the time available for testing to be fully utilized.
    • Use of process simulation is allowing alternate control approaches to be studied and evaluated off-line.
  • 22. Summary
    • DeltaV Tune was used to establish tuning for the 2-Stage Flash Skid.
    • Commissioning loops in the order of process flow and selecting tuning rules that minimize abrupt control action is recommended when working with a highly interactive process.
    • The skid comprehensive tests can done faster when the loops are tuned for a optimal performance with minimal interaction.
  • 23. Where To Get More Information
    • Information on DeltaV Tune may be found in DeltaV Books On-line under Advanced Control Products/ DeltaV Insight.
    • Information of process simulation using DeltaV Modules address in Chapter 15 of “Control Loop Foundation – Batch and Continuous Processes”, ISA publication available at: http://www.isa.org/Template.cfm?Section=Find_Books1&Template=/Ecommerce/ProductDisplay.cfm&ProductID=11267
    • Example modules for process simulation may be downloaded at: http://www.controlloopfoundation.com/downloads-for-deltav-users.aspx