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DESIGN ENHANCEMENTS TO Il\fPROVE FLARE EFF1CIENCY

Kenneth A Dooley G. Michael McLeod Mark D. LorenzSenior Chemical Engineer Assistant Department Superintendent Principal Chemical EngineerEastman Chemical CompanyLongview, TexasABSTRACfTwo flare systems used at separate Wlits within alarger chemical complex were modified to improveoverall performance and efficiency. One system wasa standard enclosed ground flare; the other was aless-conventional horizontal ground flare system. Inboth cases, restaging resulted in significantreductions in energy usage and continuing orimproved compliance with regulatory requirementswhile maintaining the flexibility to manage gaseshaving variable tlowrates and heat contents.OVERVIEWThe continuing importance of maintaining a highlevel of energy efficiency, the more recent emphasisplaced on the generation of greenhouse gases, and theever-tightening regulatory requirements for emissionssources provided a chemical company withopportunities to assess the performance of two flaresystems. The flare systems, an enclosed ground flareat a plastics production facility and a horizontalgroWld flare at an olefin cracker, were modified todecrease the energy required for operation of eachunit while insming compliance with applicableenvironmental regulations. Other benefits wererealized as well.THE ENCLOSED GROUND FLARE

Original DesignThe enclosed ground flare system was designedfor use at a plast ics production facility, to burnroutine vent and purge gases. It was configured as atwo-stage system with each stage consisting ofmultiple burners. The original burners were of a finplate design. The flare system was designed tooperate at low pressure due to venting constraintsimposed by plant equipment. Vent gases wereenriched with natural gas to maintain a minimlDll

heat content of 600 BTU/SCF, required for stablecombustion and complete destruction of volatileorganic compounds (VOC's).Actual Operations

As the plant was brought on line, the actual heatcontent of the vent gases was found to besignificantly lower than that used in the original

design. As a result, natural gas make-up to the flarewas excessive and costly.ModificationsThe flare system was reconfigured toaccommodate the lower heat content via the additionof an alternate first stage. The new stage wascomprised of three new burners designed for lowerflowrates and for gases with lower heating valuesthan the original fin-plate burners.

An existing on-line analyzer was respanned sothe range of heating values measured by the analyzermatched the capabilities of the new burners. Theminimum measurable value was decreased to 200BTU/SCF.

Finally, the control logic was reconfigured totake advantage of the combustion capabilities of thealternate first stage.ResultsNatural gas usage by the flare system wasreduced by 47% between 1994 and 1995 amountingto savings of 264,000 MMBTU per year whilemaintaining smokeless operation and the requiredcombustion efficiency of the unitTHE HORIZONTAL GROUND FLARE

Original DesignThe flare system was designed for use at anolefin cracker and was used for both routine ventgases and those generated as a result of shutdowns,maintenance activities. and plant upsets. The flaresystem consisted of horizontal 12" and 42" openended pipes with water spray racks at the ends tosuppress smoking.

Actual OperationsA continuous and significant methane purgefrom the plant was routed through the flare lines toprevent air ingress. Due to the variability in flows tothe flare system, smokeless operation was difficult tomaintain.

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Proceedings from the Nineteenth Industrial Energy Technology Conference, Houston, TX, April 23-24, 1997

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ModificationsSeveral changes were made to the flare system.Routine vent gases in the 42" flare line were divertedto the 12" line via a new 12" crossover. A steam-assisted flare tip was installed on the 12" flare line tominimize air ingress and to ensure smokelesscombustion of routine vent gases. Rupture pindevices (specialized relief devices) were installed onthe 42" flare line downstream of the crossover thatwould automatically open when the flare pressureexceeded predetermined limits such as would occurduring emergency venting; the main 42" flare linewas used only as required by venting conditions.Finally, the water spray rack on the 42" flare line wasrebuilt and the control logic was redefined to ensuresmokeless operation of the flare during emergencyventing.Results

The required methane purge to the flare wasreduced by 75%, a reduction of 80,000 MMBTU per

year for the one installation. The flare operateswithout smoke during routine venting and flaring.Capital expenditures were only 20% of those requiredfor a replacement vertical flare system. Similarmodifications were completed on other existing flaresystems with similar reSUlts.CONCLUSIONSVenting requirements are unique to eachproduction facility and are likely to change over thelife of the facility. The operation of flare systemsassociated with the production facility should bereviewed periodically. Flare systems should bedesigned (or retrofitted) to match the ventingrequirements; proper staging is vital to ensureefficient operation and proper performance of theflare system. For the subject flare systems, criteriaused in their redesign include energy requirements,environmental concerns, capital constraints, andapplicability to other systems.

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Proceedings from the Nineteenth Industrial Energy Technology Conference, Houston, TX, April 23-24, 1997