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Forget Laplace Transforms

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Industrial process control involves a lot morethan just Laplace transforms and loop tuning

Combination of both theory and practice

Understanding of core engineering principlesis key (thermodynamics, mass transfer, etc)

Control design requires collaboration withothers to understand objectives and provideprocess design guidance

Importance of both big picture and details

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Maintain the process at the desired state orset of conditions keep it out of the ditch Safety

Ensure the process conditions minimize risk

Optimal operation Running at the appropriate operating conditions

improves quality, yield, plant capacity, energyconsumption, etc

Recover from upsets or disturbances

Its not just about optimization; its aboutsuccessful operationof the entire plant

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A primary objective of the process control system isto keep the process running at the desired operatingconditions

Presumably these conditions have been chosenappropriately from a safety standpoint (hint, hint, design

engineer

)

Cruise control

The basic process control system should be able to handlemany disturbances, but not all

Cruise control on your car can handle hills and curves, butif theres an accident ahead, youll have to stop the caryourself

Safety Instrumented Systems (interlocks)

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A good process control system will keep the processrunning stably, even when hit with disturbances or upsets

This results in better efficiency, higher capacity, etc.

Improvementsto this tempcontrol strategyresulted in a

steam savingsof $260K/yr,or $1.1M NPV

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Running at the optimal operating conditionscan maximize production rate and yield,improve energy consumption, and is crucialfor product quality

However, these objectives often compete Best product quality may be attained at the cost of

additional energy consumption

Advanced Control techniques can help withbalancing this tradeoff

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Advanced control applications provide anadditional layer of control, to meet a variety ofcontrol objectives

Feed-back composition control based on lab data

Feed-forward to other unit operations or plant areas Perform complicated online calculations and close the

loop to manipulated variables

Plant-wide supervisory control strategies can balancerates, maximize throughput, minimize conversion costsor energy consumption

Model Predictive Control (MPC) incorporates a processmodel to optimize operation when there are multipleinput, output, and disturbance variables

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Youre a chemical engineerfirst and foremost!

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If you truly understand the chemical principles atwork in the process, then controlling it is easy!

Or easier, at least

You have to understand the fundamental stuffthats going on in order to determine:

What the control objectives are in the first place, andwhich variables should be controlled

What your control knobs are and how they will affect

the process as a whole how it all fits together If you increase the steam flow to a distillation columns

reboiler, what will happen to the composition on tray 15?What about the distillate? What about the pressure profile?

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Another way to think about it: the goal is to movevariability to some place where you dont careabout it

If the temperature in a reactor cycles or varies, thats bad

We can control this temperature (keep it stable) byimplementing a control loop which manipulates steamflow to the reactor jacket Who cares if the steam flow moves around? The reactor

temperature is constant, and thats what we want.

Comes back to fundamental process understanding

Must understand where variability is acceptable, andwhere its not

Must understand how everything fits together

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Distillation Control

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Need to understand manipulated variables(control knobs) available to us

Chemical Engineering knowledge tells us

Increasing the reflux will help purify the distillate The hotter the base, the more material will boil

overhead the entire composition profile will shift

The dynamics of liquid effects vs. vapor effects are

very different The temperature on each tray is a function of the

trays composition and pressure

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In order to maintain the desired top andbottom compositions, it is important toprevent the composition profile from moving

The temperatureprofile of a column isindicative of the compositionprofile

By selecting the right temperature to control, wecan actually peg the entire temperature profile

The appropriate temperature control strategy (traylocation, manipulated variable, etc) is highlydependent on the individual column design

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Manage inventory Need to ensure there is always reflux available

Likewise, need sufficient holdup in the column base

Maintain desired product compositions What are acceptable impurity ranges?

Is one product stream more important?

Other objectives Pressure control, column loading, minimize steam

Respond to certain upsets

What process upsets is this column likely to see?

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First, obtain or develop a steady-state model Need to know target compositions, normal flows,

pressures, the columns temperature profile, etc.

This gives you a snapshot of the desired operation

A steady-state model also yields insight on the controlknobs

Next, pair controlled variables with manipulatedvariables

Based on Chemical Engineering knowledge

Utilizing information regarding key control objectivesand predicted disturbances

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Steam

FFC

LC

LC

TCray 8

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PC

FC

LC

PC

LC

VACUUM LINE

TO

HEADER

CONDENSATE

FC

LC

CONDENSATE

FEED

600 PSIG

STEAM

REFLUX

RATIO

TARGETLC

REFLUX DRUM

HOT

CONDENSER

FI

FY

PRODUCT

HC

PC

LC

TO REACTORS

FC

FC

FC

XC

SGI

FI

TI

IX

COMPOSITION

And more Plant-wide

supervisory

control Feed-forward to

other unit ops orplant areas

Model predictivecontrol (MPC)

And so on

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Beneficial to create a dynamicsimulation of the columnusing this control strategy

Allows for testing of the strategyunder various disturbancescenarios

Gives valuable informationregarding dynamic behavior ofthe column

Provides initial tuning data

Steam

FFC

LC

LC

TCray 8

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Tray 8 toSteamControl Strategy

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Tray 42 toRefluxControl Strategy

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Double-Ended Temperature Control Strategy

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Once the control strategy framework hasbeen laid out, thenyou get into the nuts andbolts of configuration

Algorithm type

Controller action

Tuning (gain, time constants, etc)

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Capital Project Involvement

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For each unit operation, work closely with designengineer and other project/operationsrepresentatives to

Understand design intent, including steady-state flows,desired recoveries, conversions, etc.

Gain insight on potential process disturbances

Define key control objectives

Provide guidance on the actual process design

Determine residence times required for stable operation Specify instrumentation placement

Other recommendations based on dynamic simulation andother analysis (is desired steady-state operation feasible?)

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Provide guidance on plant-wide control Decouple interactions as much as possible

Control valve placement, piping layouts

Inventory management

Instrumentation selection

Safety considerations, interlocks

Control Narrative Detailed document describing control objectives

and strategies for each unit operation, the plan formanaging inventory plant-wide, etc.

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Remember: always think about processcontrol from the perspective of ChemicalEngineering fundamentals

Understand your process, as well as yourcontrol objectives What needs to be controlled? Which variables effect

each other (and how)? Where does variability hurtyou most? Etc.

Remember theres a dynamic component

Think about control early in design phase

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Next Lecture March 22

Integration of design and control part I

SSLW 322-340

Seminar Tomorrow

March 21, 3:00 PM Dr. Andreas Linninger, University of Illinois Chicago

Biomedical Engineering Problem Solving with SystemsEngineering Methods

Reception in ChemE Conference Room at 2:30 PM

Seminar in McMillan Auditorium 3:00 PM

Please, please attend if at all possible!

Other Business