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

Constraint Control on a Constraint Control on a Distillation ColumnDistillation ColumnPamela Buzzetta

Process Engineer, MECS, Inc.

[File Name or Event]Emerson Confidential27-Jun-01, Slide 2

PresentersPresentersPresentersPresenters

• Pamela Buzzetta

Process Engineer

• Robert Heider

Adjunct Professor


IntroductionIntroductionIntroductionIntroduction

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


IntroductionIntroductionIntroductionIntroduction

Outline

• Distillation Column Description

• Objectives for Constraint Control

• Schematic and Simulation for Solution

• Results and Analysis of Simulation


Distillation Column DescriptionDistillation Column DescriptionDistillation Column DescriptionDistillation 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

BypassValve

HeatExchanger

DistillationColumn Condenser

Tank


Distillation Column DescriptionDistillation Column DescriptionDistillation Column DescriptionDistillation 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


Example of Control DifficultyExample of Control DifficultyExample of Control DifficultyExample 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


Example of Control DifficultyExample of Control DifficultyExample of Control DifficultyExample 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


Objectives for Constraint ControlObjectives for Constraint ControlObjectives for Constraint ControlObjectives 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


DeltaV SchematicDeltaV SchematicDeltaV SchematicDeltaV Schematic

AmbientTemperature

CondenserTank

DistillationColumn

BypassValve Heat

Exchanger


Process AssumptionsProcess AssumptionsProcess AssumptionsProcess Assumptions

• All vapors from valve discharge condense at the tank

• Reboiler held constant

• Condensate cooled to 10 °F above ambient


DeltaV Control Studio DiagramDeltaV Control Studio DiagramDeltaV Control Studio DiagramDeltaV Control Studio Diagram

Analog InputPressure Transmitter

Analog OutputValve Position

Variable Ambient Temperature Output

PID ControllerImplied Valve Position

PID Pressure Controller

IVP Loop slower thanPC Controller

Analog VaporFlow Output

Real-time Cycle Control of Ambient Temp

PC Gain: 0.5 Reset: 4.4IVP Gain: 1.25 Reset: 600


Control Operation OutlineControl Operation OutlineControl Operation OutlineControl 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


MATLABMATLAB®® Integration Routine Integration RoutineMATLABMATLAB®® Integration Routine 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


OPC UtilityOPC UtilityOPC UtilityOPC 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 145a5282 WV BoxtelThe Netherlands


Results of Temperature ChangesResults of Temperature ChangesResults of Temperature ChangesResults 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)



Responses to Tamb step from 85 to 70 °F

(Ex: Rainstorm)



18 hour real-time response (Ex: day and night cycle)


Energy Savings EstimationEnergy Savings EstimationEnergy Savings EstimationEnergy Savings Estimation

• For 200,000 lb/hr pure pentane:• At P=60 psia, Qstream=-2.0657·108 Btu/hr

• At P=20 psia, Qstream=-2.1442·108 Btu/hr• ΔQ = 7.85·106 Btu/hr• 970.6 Btu/lb steam, $9/1000 lb steam(sources: http://www.steamonline.com/loss_chart.html, http://www.pipingnews.com/steam3.htm)

• Savings: Assume operates at lower pressures 50% of the time at 24 hr/day and 300 operating days/yr =

~$260,000 saved per year


SummarySummarySummarySummary

• 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


SummarySummarySummarySummary

Questions?

Feedback?


Where To Get More InformationWhere To Get More InformationWhere To Get More InformationWhere 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, http://www.chemicalprocessing.com/articles/2005/535.html

• Thank you to:– Professor Robert Heider– Jason Hall, Washington University SSM Student– Washington University Chemical Engineering Dept.– Emerson and Emerson Users Group

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

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