Steven E. Harper
Chevron Chemical Co. LLC
Port Arthur, Tex.
Stone & Webster Engineering Corp.
Stone & Webster's ARS technology was implemented in Chevron's ARS and refinery-gas dephlegmator coldboxes during the revamp in 1997.Chevron Chemical Co. LLC's Port Arthur, Tex., ethylene unit (EU-1544) was recently expanded from 1.0 billion lb/year to 1.7 billion lb/year.
The unit has been operating at capacity since November 1997.
Chevron and Stone & Webster Engineering Corp. have been working as a team since 1991, as the EU-1544 expansion project has evolved from a concept to construction.
The following project objectives were met:
- The unit started up in September 1997, ahead of the originally targeted start-up date of November 1997. Continuous operation at capacity was achieved in November of 1997.
- The total, all-inclusive capital cost was more than 10% below the amount appropriated for the expansion project.
- Ethylene-production capacity was increased by over 70%.
- Unit specific energy consumption was reduced by over 25%.
Project historyIn the late 1980s, Chevron identified an opportunity to address an increase in internal ethylene demand. It began to scope various capacity increase increments for an expansion in Port Arthur. These studies were initially pursued internally within Chevron, and then later with a Stone & Webster competitor. The chronology of the studies leading to the final expansion design is listed in Table 1 [25,809 bytes].
The final target capacity of about 1.7 billion lb/year of ethylene was set in 1991, using Stone & Webster's proprietary Ultra Selective Cracking (USC) and Advanced Recovery System (ARS) technologies. Stone & Webster's ARS technology was selected because it achieves the largest capacity increment with a single train.
Basic engineering was completed in 1992 to allow submittal of an appropriations request (AR) within Chevron. As a result of uncertainties about the olefins market, and the future of the Port Arthur site, Chevron shelved the project.
In 1994, Chevron reached an agreement with Clark Refining & Marketing Inc. to sell the Port Arthur refinery and retain the chemical operations at that site. With its commitment to remain in Port Arthur, Chevron turned its attention to making the entire chemical operation more efficient.
Although Chevron had a substantial need for additional ethylene, the need was not large enough to accommodate the excess ethylene from a world scale capacity new plant. Chevron's strategy was to continue to be a net buyer of ethylene rather than a net seller of ethylene.
Hence, in 1995, Chevron decided to revive the ethylene-expansion project. Stone & Webster was responsible for the engineering and procurement; Brown & Root Inc. was in charge of the construction. Exercises to reduce costs through value engineering and vendor alliances were key factors.
Plant schemeThe process scheme is shown in Fig. 1 [47,465 bytes]. It is described in detail elsewhere. 1
The existing caustic/water wash tower, the de-ethanizer, the depropanizer, the debutanizer, and the propylene fractionator had no modifications.
Major modifications and additions were:
- Furnaces. Chevron installed four new Stone & Webster USC 12M furnaces. Each furnace had a capacity of 22,500 lb/hr of ethylene and incorporated the newly developed Omega P USX double-pipe quench exchanger.
- Quench-water tower. In the quench-water tower, Chevron added a new feed nozzle from the new furnaces and a new parallel overhead nozzle to the cracked-gas compressor.
- Cracked-gas compressor. The work included replacing the compressor wheels and diaphragms on two compressor cases, as well as the gearbox, turbine wheels, and turbine diaphragms. The compressor-suction drums were modified with mesh/vane mist eliminators.
- Propylene-refrigeration compressor. The original compressor was replaced with a new 80M compressor. The new compressor had the same footprint and dry-gas seals. Like the cracked-gas compressor, the turbine wheels and turbine diaphragms were replaced. Also, the suction drums of this compressor received new mesh/vane mist eliminators.
- Ethylene-refrigeration compressor. The existing compressor was replaced with a new 38M compressor on the same footprint with dry-gas seals. Again, the turbine wheels and diaphragms were replaced, and new mesh/vane mist eliminators were installed in the ethylene-refrigeration compressor's suction drums.
- Demethanizer chilling/fractionation system. In this system, Chevron installed three new Air Product (APCI) dephlegmator coldboxes. The primary demethanizer was retrayed with UOP MD trays in the top and sieve trays in the bottom.
- Secondary demethanizer. A secondary demethanizer was added to the system as well as a new two-stage turboexpander/compressor.
- Refinery-gas unit. A new caustic tower was installed in the refinery-gas unit. One single-stage compressor was revamped. Chevron also added new chillers and dryers and one APCI dephlegmator coldbox.
- Second acetylene converter. In this converter, Chevron added a new parallel, two-stage converter system for the secondary demethanizer-bottoms stream.
- Ethylene fractionator. The company also retrayed the ethylene fractionator with UOP ECMD trays and retubed the fractionation system's condenser and reboiler with UOP HiFlux tubes.
- Flare. Chevron resized its flare, replacing the existing 48-in. flare with a new 60-in. John Zink flare tip.
- Cooling tower. In the cooling tower, Chevron added two cells, four fans, two pumps, and installed parallel piping.
Overall plant performanceThe Chevron Port Arthur ethylene plant has operated continuously at capacity since November 1997.
The plant feedstock ranged from a nominal 80/20 ethane/propane (E/P) feed to 50/50 E/P feed. The expansion design was based on two design cases: 70/30 E/P and 40/60 E/P. The design ethylene production was 1.73 million lb/year for 70/30 E/P and 1.68 million lb/year for 40/60 E/P.
The actual annual production for 1998 was 1.73 million lb/year.
Chevron established an energy index (CEI) for tracking energy consumption in plants. The CEI for Port Arthur's ethylene unit has, as a base, the energy consumption in 1991 of 100. Based on design information, Chevron targeted a CEI for 1998 of 76.9 (23% less than the 1991 energy consumption). The actual CEI for 1998 was 73.4 (26% less than the 1991 energy consumption).
The industry, represented by the Chemical Manufacturers Association, Arlington, Va., recognized Chevron's successful completion of this project and the resulting improved energy consumption by awarding the Port Arthur Plant the 1997 Energy Efficiency Award in November 1998.
Furnace performanceThe 70% capacity increase was achieved by adding four Stone & Webster USC 12M furnaces with the newly developed Omega-P USX quench exchangers. A detailed description of the furnace design, configuration, and operation is beyond the scope of this article.
The furnaces were guaranteed to produce 22,500 lb/hr of ethylene from feedstocks ranging from propane at 88% propane conversion to ethane at 70% ethane conversion, with a minimum 35-day run length and with a minimum thermal efficiency of 92.5%.
The furnaces have operated well beyond these ranges. They have produced more ethylene than the design rates, at higher conversions, from a range of feedstocks, with a higher thermal efficiency, without any adverse impact on run lengths. Moreover, emissions have been well below environmental permit limits.
- Prickett, R.D., Bush, K.E., Cruey, G., "Retrofit Your Ethylene Unit: the Chevron Experience," Hydrocarbon Processing, March 1998.
Steven E. Harper is area supervisor of ethylene unit 1544 at Chevron Chemical Co. LLC, Port Arthur, Tex. He has been with Chevron for 21 years. Harper has been intimately involved in all phases of the ethylene expansion project since the late 1980s. He holds a BSChE from Lamar University, Beaumont, Tex.
Wadie Malaty is a process technical coordinator at Stone & Webster Engineering Corp. He has been with Stone & Webster for 11 years. He previously held process positions with Coastal Corp. and Foster Wheeler for 13 years. Malaty has been the lead process engineer on the ethylene expansion project since 1991. Malaty holds a BS in chemical engineering from Columbia University in New York City and a masters degree in chemical engineering from the University of Houston. He is a registered professional engineer in Texas and a member of AIChE.
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