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

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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.

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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