Reduced Safety Flaring through Advanced Control

31st Industrial Energy Technology Conference – New Orleans, LA

David Hokanson – ExxonMobil ChemicalKeith Lehman – Empirical Process SolutionsS. Matsumoto – Tonen ChemicalN. Takai – Tonen ChemicalF. Takase – TonenGeneral

May 20, 2010

2

Outline

• Problem Statement

• Plant Fuel Gas System Overview

• Solution

• Results

• Summary

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Problem Statement

• Reduce / eliminate fuel gas releases to safety flare system

• Stabilize fuel gas molecular weight to olefins furnaces

• Major issueAverage molecular weight of Fuel Gas is much different than fuel gas

make-up from propane / butane vaporizers

When demand / supply changes, change in propane/butane make-up causes system to cycle, causing a release to the safety flare system

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Fuel Gas System Overview

HP Fuel Gas

Unit1

GTG

Off- Gas

SC

BB/ PropaneVaporizers

Unit2

Unit6Unit5Unit4Unit3Unit1

Crude Fuel Gas

HP BB Gas

Dry Fuel Gas

Customers

PIC

Unit7

Town Gas

Unit8

LP BB / C3 GasPIC

Sphere

Unit9

LP Fuel Gas

FLARE

PIC

FLARE

PIC

C kPaG

D kPaG

B kPaG

BB/ PropaneVaporizers

PIC

A kPaG

A kPaG

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More Details – Fuel Gas System

• Heavily integrated refinery / chemical plant fuel gas system Older refining and olefins site with many changes Two control centers with different control systems

• Make-up fuel gas controlled using two sets of fuel gas vaporizers Mostly using various C4s; sometimes using propane

• Multiple users fired heaters, furnaces, and even the local city gas system

• Multiple suppliers Fuel gas produced from olefins production and many different refining processes Wide variety of molecular weights from fuel gas suppliers

• Pressure control Make-up pressure control on both fuel gas vaporizers set at ~330 kPaG Purge on pressure control to safety flare system set at 375-385 kPaG (5 pressure

controllers!)

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Solution• Develop a thorough understanding of the process

Developed detailed flow diagram of entire refining / chemical plant fuel gas system Used plant historical data to understand / model fuel gas changes due to changes in make-up flows

• Develop model-based advanced controller for the overall process Used DMCplusTM from AspenTech Basic DMCplus Design

Keep pressure control to flare valves in-place Use DMCplus to manipulate make-up from fuel gas vaporizers and cat cracker fuel gas flow

• Execute Pre-Test Confirm DMCplus design; obtain initial dynamic models for manipulated variables

Manipulated variables = plant variables such as flow setpoints or valves moved by DMCplus Collect additional data on fuel gas suppliers and users that will be “feed forward variables” to the

controller Design detail: Use “calculated heating value” for all fuel gas input streams and users to

develop pressure models

• Execute Plant Test / Commission Controller Utilized AspenTech’s SmartStepTM automated tester with in-house developed methodology to test and

commission the controller at the same time Testing and commissioning lasted 7 days

• Final DMCplus controller size 6 manipulated variables 13 controlled variables 46 feed-forward variables

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Results –Pressure & Flare Valve

Main Flare Valve

Fuel Gas Pressure

SmartStep Test Commissioning

Loss of DCS Comm

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Results – FG Mol Weight

SmartStep Test Commissioning

Loss of DCS Comm

Tight Control of Olefins MW without impacting MW of other users

Olefins Furnace Mol Weight

BH Mol Weight

Fuel Gas Mol Weight

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Tech Details

• Valve Linearization4 out of 6 MVs are valves; all are linearized1 CV is a valve (also linearized)

• Pressure “augmentation”Pressure CV is “augmented” by main flare valveIf flare opens (on PID pressure control), “augmentation” is added base on

flare valve openingAugmentation factor determined by analyzing test and plant historical data

• Feed Forward ModelingUtilized both test and plant historical data to get modelsUsed best estimate of mol weight to determine heating value for each stream

(usually typical lab result)Used test / plant data, not flow sheet, to make final decision whether to keep

or drop feed forwardsGetting an accurate flow sheet of the fuel gas system proved to be one of the

toughest challenges of this project

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Raw valve position vs. predictionValve positionPrediction

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Linearized valve position vs. predictionValve positionPrediction

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Summary

• Successful Fuel Gas DMC CompletedFuel Gas pressure controlled tightly

No fuel gas to flare when DMC is on control

Mol weight and other constraints obeyed / tightly controlled

Credits estimated by reducing C4s to flare by 75%(vs excellent 2008 operation)

• Actual reduction to date has been bigger than this estimate Continued little/no loss of fuel gas to flare when DMC is on control

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