Building an Effective Operator Interface For Complex Ethylene APC Applications Erin S. Percell (

Baton Rouge Chemical Plant)

Louis Michaud (

Sarnia Chemical Plant)

ExxonMobil Chemical Company

Objectives

 Describe the content of an effective “overview display” for monitoring a system of multivariable control applications over a plant.

 Describe some possibilities for determining the throughput limiting constraint in a multi-application environment. Note: The authors are employed at two ExxonMobil Chemical ethylene plants where Aspen’s DMCplus® is the multivariable control software (with DMCplus Composite for feed maximization), but the discussion should be general enough for other solutions.

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Introduction

 Many ethylene plants now use multivariable model-based applications for advanced process control and feed maximization.

 Lots of applications = lots of information = lots of interface screens for operators.

 High service factor and good performance depend on having an effective method for monitoring the state of the whole plant/system without too much “screen hopping.”  Operators need to see what is happening and why at a reasonably high level.

 Constraint data is often of particular interest, especially when running full.

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An Interface Example

April, 2006     Consider your dashboard.

Few things you really need to see most of the time. Examples:    Speed (frequently) Fuel (occasionally) Trouble indicators (only to catch attention when needed) Also want access to controls for frequent actions (steering, climate control, cruise control, radio, etc.).

Don’t want to flip screens, but you know the owner’s manual is around somewhere just in case...

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The Overview Display (1)

 What should be on a “dashboard” for the multivariable control system?

 The ideal overview is a single display that operators would use for normal monitoring, eliminating most of the need to hop between individual application displays.

 The perfect content will vary according to site needs, and your operators are your best source of information. But some features have been found to be useful at our sites:  Status of individual applications. Both On/Off status and participation in feed maximization application (e.g., DMCplus Composite).

 Feed rates. Both current and target rates. By individual furnace and by feed type, if applicable. April, 2006 2006 Spring Ethylene Producers' Conference, Orlando, FL 5

The Overview Display (2)

 Useful features of the overview (continued):    Indicator of the throughput-limiting constraint. Formerly our #1 FAQ during periods where feed maximization was a primary objective. Useful for both short-term and long-term monitoring. More on this later.

Production rate data. Serves to verify how well production objective is being met.

Current value for significant process indicators. Very plant-specific. Your operators know what these are, and they watch them even when they aren’t limiting production.

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The Overview Display (3)

 Useful features of the overview (continued):     Entry mechanism for major operator-entered limits. Especially important for limits that change frequently for good reasons. Makes these changes convenient, and provides prominent display of their current values, which may have been entered by a previous (untrustworthy?) shift.

Limit violation indicators. If the multivariable control runs out of degrees of freedom, it may not be able to honor all constraints. Overview should at least highlight this circumstance for more investigation in the individual application. Countdown timer. Show time until next execution.

Some links. Covered later.

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Constraint Determination

 Former #1 question from operators at BRCP was, “What is limiting feed?”  When the operators know the answer, they can consider whether they have taken every manual step available to alleviate the constraint (extra pump, control valve bypass, creeping limits, etc.)  Is also important because it’s a question the operators get asked.

 But how will they know, without perusing page after page of constraint displays? ExxonMobil sites have implemented at least two different approaches for identifying “the” throughput constraint.

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Constraint Counting

 At the steady-state solution, in general, the LP solver in DMCplus maintains equality between constrained CV’s and unconstrained (free) MV’s.  The constraint counting method counts constrained CV’s and free MV’s in each downstream subcontroller. One subcontroller will usually have an “extra” CV (one in excess of the number of free MV’s), which is assumed to be tying up the feed degree of freedom. Individual subcontroller displays show which constraints are “normal.”  Potential difficulties: non-square subcontrollers, QP ranks in DMCplus, MV’s with LP Cost=0, MV’s with minimum move criterion.

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Constraint Expert System

 In many cases, the throughput limit may not be a single CV but a particular combination of constraints that are active simultaneously.  Can write a set of AND/OR rules to look at the constraints in the DMCplus LP solution and aggregate them. Rules help define circumstances where a CV is not a plant limit until DMC runs out of all other handles to mitigate it.  Drawback: can only identify circumstances for which rules have been written, so largely ineffective for situations not previously observed or anticipated.  For either method, External Targets may act like constraints. April, 2006 2006 Spring Ethylene Producers' Conference, Orlando, FL 10

Constraint Historization

 We have found it useful to provide a display that shows the throughput constraints (as percentages of time) over a user entered period.  So an operator can report, for example, that during his shift the plant was furnace-limited some portion of the time and cold-ends limited some other portion. April, 2006 2006 Spring Ethylene Producers' Conference, Orlando, FL 11

Links

 The overview display should also allow easy navigation to places where the operator can find more detailed information:   Detailed displays for each individual application Constraint history page   Optimization application information Application documentation April, 2006 2006 Spring Ethylene Producers' Conference, Orlando, FL 12

Frequent Actions

 The overview should also allow the operator to take some common actions without navigating to a new page:    Set certain limits subject to frequent change.

Put applications/furnaces into or out of the feed maximizer.

Turn individual applications on and off? Depends on your site expectations for reviewing limits first.

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Summary

 To be a good “dashboard,” the overview display should      show key information about the plant status (feed rates, production rates); show key information about application status (on/off, in/out of feed maximization); display the throughput-limiting constraint for the plant; allow execution of common operator actions; provide links to sources of more detailed information. April, 2006 2006 Spring Ethylene Producers' Conference, Orlando, FL 14