Incoporating Ammonia Synthesis for an Offshore Gas-to-Liquid Process
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The world energy demand is increasing, and so is the demand for fertilizer tosustain an exponential population growth. Currently, with low oil prices, asso-ciated natural gas is flared off or re-injected into oil reservoirs for enhancedoil recovery (EOR). A gas-to-liquid process (GTL) for offshore applicationsaboard a foating production, storage, and offoading vessel (FPSO) incorpo-rating Fischer-Tropsch Synthesis (FTS) seeks to reform natural gas into morevaluable liquid products. As the composition of natural gas feeds varies greatlydepending on location and other factors, a surplus of hydrogen production ismaintained in order to have a steady production of the desired FTS products.An alternative use for this surplus hydrogen is in ammonia synthesis, which ina relatively simple process design using readily available streams in the GTL process, canproduce considerable amounts of ammonia. A complimentary design to an existing GTL process designed for offshore appli-cations which seeks to incorporate ammonia synthesis is proposed. Two possibledesigns are tentatively suggested and evaluated, each design utilising differentstreams in the GTL process with a high nitrogen content, in addition to thesurplus hydrogen stream in the GTL process. The process design features asynthesis loop, as well as removal of compounds containing oxygen, such asCO2 and water, as these compounds are poisonous to the ammonia synthesiscatalyst. The ammonia synthesis reactor is simulated as three separate beds,with a refrigeration loop to cool the stream exiting the reactor to sub-zero tem-peratures in order for ammonia to condense and be separated from the synthesisloop. Uncondensed ammonia and unreacted hydrogen and nitrogen gas is recy-cled and reintroduced to the reactor. Two separate process designs were simulated in Aspen HYSYS V8.6, each witha different source of nitrogen. The basis for the nitrogen sources for the ammo-nia synthesis process are the streams pertaining to the GTL process proposedby Hillestad et.al. Two different kinetic models were also evaluated. Heatintegration is performed in Aspen Energy Analyzer V8.6, and a heat exchangernetwork (HEN) is proposed. The best process design features the N2 -rich stream from the membrane inthe air enrichment unit in the GTL process, as this offers a simpler processdesign compared to utilising the tailgas from the GTL process. The system isoptimised using the Temkin-Pyzhev kinetic model for the ammonia synthesisreactor. The power-optimal operating pressure is found to be approximately235 bar, while the optimal operating temperature is found to be approximately415 ◦C depending on the reaction bed. After heat integration, the process requires no external heat source, and thesystem power demand is suffciently low to be met by the gas turbine in theGTL process. The process converts 1889 kgmol/h of unprocessed hydrogen/CO2stream from the hydrogen selective membrane in the GTL process, and 613 kg-mol/h of nitrogen/oxygen stream from the air enrichment unit to produce 1167kgmol/h, or 19.9 tons/h, of ammonia. The total investment cost of the proposeddesign is estimated to 87.6 million US$.