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