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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
Commissioning Highly Interactive
Process
An Approach For Tuning Control Loops
PresentersPresenters
Eric Chen
Terry Blevins
Willy Wojsznis
IntroductionIntroduction
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.
SRP CO2 Capture Pilot PlantSRP CO2 Capture Pilot Plant
4
Gas Capacity, m3/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
Absorber
P-128
Stripper
CrossExchanger
Coal-firedFlue Gas12% CO2
Lean AmineRich Amine
30-50 psia
80-90% Removal
120 °C
Clean Flue gas
To Compressor CO2 and H2O35 wt% MEA
Solvent
Field Trial – 2-stage Flash SkidField 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.
2-Stage Flash Skid2-Stage Flash Skid
SRP CO2 Capture Pilot Plant SRP CO2 Capture Pilot Plant
Steam Heater
Solvent Cooler
Flash Tank 13.5 atm150 °C
Flash Tank8 atm
150 °C
CO2 Makeup
Gas Out
Absorber
IntercoolingAbsorber Feedtank
CO2 Accumulator
Liquid Accumulator
HP Cross Exchanger
HP Pump
Two-stage Flash
2-Stage Flash Skid Design2-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
8 m PZ Provides High P at 150 °C8 m PZ Provides High P at 150 °C
Flash tank temperature and pressure must be maintained at design conditions for correct operation.
9
Energy Analysis
Initial TuningInitial 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.
Tuning ApproachTuning 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.
Tuning RulesTuning 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.
Tuning ResultsTuning Results
Before Re-tuning ControlBefore 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
After Re-tuning ControlAfter Re-tuning Control
More stable response was observed.
Large changes can be made in process condition without the need for operator intervention.
Tuning Impact – Operator FeedbackTuning 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. “
On-going WorkOn-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.
2-Stage Skid Simulation Implementation2-Stage Skid Simulation Implementation
Simulation Implementation (Cont)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.
Control Module for TestingControl 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.
Business Results AchievedBusiness 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.
SummarySummary
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.
Where To Get More InformationWhere 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
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