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A Stokes Engineering Publication Volume XX, Number 2 Second Quarter 2005 Reprint Ammonia Plants: ISL Process Saves Fuel

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From FIND$, A Stokes Engineering Publication Stokes Engineering Company 66 Davis Hill Road Weston, CT 06883 USA Tel: 203-454-0704 e-mail: stokeseng@yahoo.com website: www.finds.net Ammonia Plants: ISL Process Saves Fuel                                      Ammonia Plants:                                      ISL Process Saves Fuel                                      By Glenn Combs and Tom Czuppon The makeup gas to ammonia synloops contains water and carbon oxides, which are synthesis- catalyst poisons.  Some plants remove carbon oxides and water with molecular sieves or by scrubbing the syngas with ammonia or other solvent. However, these “dehydrators” are usually upstream from, or between, the syngas compressor’s low pressure and high pressure bodies. In contrast, our process, the Integrated Synthesis Loop Ultra Process (ISL), removes water and carbon oxides in an optimized syn loop and refrigeration system. The process, which saves fuel but can also increase ammonia production, has evolved from similar technology that a U.S. ammonia producer has used in a large plant for eight years. A retrofit scheme would introduce the ISL process equipment downstream from the syngas compressor, and, therefore, solids or liquid entrainment or blocked valves will not pose a threat to the compressor.  Molecular sieve dryers are more costly to install and maintain than ISL units and can shut ammonia plants down when they malfunction.   Figure I is a simplified flow diagram of an ISL retrofit in a plant that condenses the ammonia product with a multi-stage refrigeration system.  Makeup and recycle gas flow from the syngas machine’s high pressure body through the first part of the ISL unit.  The process uses ammonia to contact and remove moisture and carbon oxides in a unique way that saves more energy than earlier process schemes. The synthesis gas then flows to the ammonia converter. After heat recovery and cooling, the converter effluent flows to the second part of the ISL unit, which returns unconverted synthesis gas to the syngas compressor. The ISL retrofit reduces ammonia plant fuel requirement by 0.7 to 1.0 mm Btus HHV per ton of ammonia (0.2 to 0.28 mm kilocalories per tonne), or more, depending on the base plant’s operating conditions.  Energy savings improve at higher ammonia converter conversions.  Many ammonia converters operate with 11.5 to 13.5 percent ammonia conversion, corresponding to about 13.3 to15.3 percent ammonia in the converter effluent.  This ammonia conversion is the basis for the 0.7 to 1.0 mm Btu per ton fuel savings. The retrofit pays off in about two years when gas costs $4.50 per mm Btu.  The payback improves when production gains are included.  The retrofit achieves similar fuel saving for plants in warm and cold climates. Some plant owners may wish to combine an ammonia converter upgrade with ISL to save energy and increase production.  Depending on front-end limitations, this combined retrofit can increase capacity at lower cost than not using the ISL process.  A retrofit that saves 0.7 mmBtu per ton will typically reduce system loads as follows: Percent Load Reduction Synthesis Gas Compressor        8.6 Refrigeration Compressor          9.3 Steam System                              4.8 Fuel System                                 5.2 The ISL retrofit can increase production up to the limitations of front-end equipment when certain sections of the plant are bottlenecked.  Examples of equipment that ISL would debottleneck include: the synthesis compressor, refrigeration compressor, steam turbines drivers for these compressors, the boiler feed water pump, surface condensers, cooling tower, utility boiler, furnace fans, furnace coils, and furnace burners. Installing ISL can be accomplished in a typical two-week turnaround, following about four months of pre-shutdown construction. In summary, the ISL process offers an alternative to existing technologies and provides the following comparative advantages:       Higher energy savings       Equal or better payout       Greater Net Present Value from combined energy savings and capital       No equipment retrofit upstream from, or in, the synthesis compressor train       Lowers expansion cost       Lower maintenance costs       Higher reliability       Reduced downtime and production losses       Rapid start-up time

Glenn Combs has BS and MS Degrees in chemical engineering from Oklahoma State University.  He worked in plant support and process engineering for Olin Corporation, International Minerals & Chemicals, IMC Fertilizer, and Koch Industries.  He formed Chem-Engineering Services in 1998.  The firm offers process engineering, exchanger and furnace design, plant evaluation and problem-solving to the chemical industry.

Tom Czuppon has an MS degree in chemical engineering from New York University and a BS degree from C.W. Post College of Long Island University.  He is currently consulting in ammonia and related areas for energy conservation and production retrofits.  Previously, he was with KBR for more than 35 years in various process engineering, research, and management positions.