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 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

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