Industrial Process Control - Process Design Practice - Lecture Slides, Slides of Process Engineering

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Forget Laplace Transforms…

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

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Combination of both theory and practice

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

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

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Importance of both “big picture” and details

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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, designengineer  ) 

“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 there’s an accident ahead, you’ll 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

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This results in better efficiency, higher capacity, etc.

Improvementsto this tempcontrol strategyresulted in asteam savingsof $260K/yr,or $1.1M NPV

 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 theloop 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

“You’re a chemical engineer

first and foremost!”

 Another way to think about it: the goal is to movevariability to some place where you don’t careabout it^ ◦ If the temperature in a reactor cycles or varies, that’s 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 that’s what we want.  Comes back to fundamental process understanding^ ◦ Must understand where variability is acceptable, andwhere it’s not ◦ Must understand how everything fits together

Distillation Control

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

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The

temperature profile of a column is

indicative of the

composition profile

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

 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?

Steam FFC LC LC TC Tray 8

PC FC LC PC LC VACUUM LINE TOHEADER CONDENSATE FC LC CONDENSATE FEED 600 PSIGSTEAM REFLUXRATIOTARGET LC REFLUX DRUM HOTCONDENSER FY FI PRODUCT HC PC LC TO REACTORS FC FC FC XC SGITI FI IX COMPOSITION And more… •^ Plant-widesupervisorycontrol

-^ Feed-forward toother unit ops orplant areas -^ Model predictivecontrol (MPC) -^ And so on…

“Tray 8 – to – Steam” Control Strategy

“Tray 42 – to – Reflux” Control Strategy