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Analyser placement in ammonia plants

12 November 2009

Effective selection and placement of online gas analysers can improve product quality and production efficiency at ammonia plants.

Ammonia is used in the production of a wide variety of products, including fertiliser, urea, nylon, pharmaceuticals, and refrigerants. Due to its many uses, ammonia is one of the most highly produced inorganic chemicals. Effective and efficient production of high-purity product depends on careful use of process gas analysers and gas chromatographs placed at strategic points in the plant.

Ammonia production is a complex process beginning with steam and natural gas (methane, CH4). During the process, the gases pass through the steam reformer, high- and low-temperature shift converters, amine scrubber, methanator, and ammonia synthesis converter.

To achieve the ultimate benefits of online gas analyser installations, Michael Gaura, product manager for Emerson's Rosemount Analytical division, recommends analyser placement at the following points in the ammonia production process:

Measuring point 1: Steam reformer-Natural gas and steam react over catalyst to form hydrogen (H2) and carbon monoxide (CO). A process gas analyser measures unconverted CH4, as well as CO, which are monitored to determine reformer efficiency in preparation for the shift converters.

Measuring points 2A and 2B: High- and low-temperature shift converters-Shift converters remove CO by reacting with steam to form H2 and CO2. A process gas analyser measures CO content to determine shift converter efficiency.

Measuring point 3: Amine scrubber-At this point, CO2 is absorbed in amine solution in the absorber and then removed. An online gas analyser or gas chromatograph measures CO2 and CO to determine scrubber efficiency and quantify those levels prior to entering the methanator.

Measuring point 4: Methanator-This stage removes all remaining traces of CO and CO2, which are measured by a process gas analyser. CO must be removed completely or it can poison the ammonia converter catalyst, and the CO2 must be removed or it will react with ammonia and cause blockage in the ammonia converter. H2 and CH4 are also measured to control the feed for ammonia synthesis.

Measuring point 5: Ammonia synthesis converter-At this point, H2 reacts with nitrogen over a catalyst to create the actual ammonia product. Process gas analysers are used to measure ammonia before and after the converter to determine converter efficiency.

Measuring point 6: Recycle stream-The ammonia synthesis converter produces by-products, in addition to the ammonia product. Recycle gas sent to the compressor consists of H2 and N2 that must be maintained at optimal ratios to ensure proper synthesis conversion in the ammonia synthesis converter. A Rosemount Analytical gas chromatograph performs this analysis with minimal cycle time and utility requirements.

Measuring point 7: Purge gas-The process creates inerts, which are typically maintained at an optimal level of 10-15% and purged from the stream. The gas chromatograph installed at this point can monitor the inert levels and measure the BTU content if the purge gas is to be used as a fuel gas for process heaters.

"We recommend either Rosemount Analytical gas chromatographs or X-Stream process gas analyzers in either a flameproof enclosure or a field housing with purge/pressurisation systems for ammonia production plants," says Gaura. "These analysers are tried and true in this application, ensuring more efficient ammonia production, longer catalyst life and a higher quality end product."

Rosemount Analytical X-Stream process gas analysers are available in general purpose, general purpose compact, field housing, and flameproof (Class I, Zone 1, Group IIB + H2 hazardous areas certified) housing packages for maximum application flexibility. The gas chromatographs from Rosemount Analytical are available with ATEX/CSA/IEC-Ex certifications, with or without purge requirements. Regardless of the packaging option, Rosemount Analytical gas analysers and gas chromatographs have very small footprints and offer analysis utilising infrared, ultraviolet, and non-dispersive infrared (NDIR) photometry, paramagnetic and electrochemical oxygen, flame ionisation, and thermal conductivity detector technologies.

"With minimal installation and maintenance requirements, plants receive accurate and reliable measurements immediately, while saving maintenance time and the cost associated with expensive shelters and utilities," Gaura adds.

-Edited by Peter Welander, process industries editor, peter.welander@reedbusiness.com
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