Reduced Safety Flaring through Advanced Control 31 st Industrial Energy Technology Conference – New Orleans, LA David Hokanson – ExxonMobil Chemical Keith Lehman – Empirical Process Solutions S. Matsumoto – Tonen Chemical N. Takai – Tonen Chemical F. Takase – TonenGeneral May 20, 2010

2 Outline Problem Statement Plant Fuel Gas System Overview Solution Results Summary

3 Problem Statement Reduce / eliminate fuel gas releases to safety flare system Stabilize fuel gas molecular weight to olefins furnaces Major issue  Average 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

4 Fuel Gas System Overview

5 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 kPaG (5 pressure controllers!)

6 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 DMCplus TM 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 SmartStep TM 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

7 Results –Pressure & Flare Valve Main Flare Valve Fuel Gas Pressure SmartStep TestCommissioning Loss of DCS Comm

8 Results – FG Mol Weight SmartStep TestCommissioning 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

9 Tech Details Valve Linearization  4 out of 6 MVs are valves; all are linearized  1 CV is a valve (also linearized) Pressure “augmentation”  Pressure CV is “augmented” by main flare valve  If flare opens (on PID pressure control), “augmentation” is added base on flare valve opening  Augmentation factor determined by analyzing test and plant historical data Feed Forward Modeling  Utilized both test and plant historical data to get models  Used 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 forwards  Getting an accurate flow sheet of the fuel gas system proved to be one of the toughest challenges of this project

10 Raw valve position vs. prediction Valve position Prediction

11 Linearized valve position vs. prediction Valve position Prediction

12 Summary Successful Fuel Gas DMC Completed  Fuel 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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