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The Uhde dual-pressure process

Chemical plant capacities have for a long time been taking on ever greater dimensions. The reason lies in the reduction of specific production costs through economies of scale. More than ever before, the plant construction sector is facing the challenge of exploiting this advantage while at the same time continuing to employ proven technologies and equipment.

thyssenkrupp Industrial Solutions (then Uhde) and Johnson Matthey Catalysts have risen to this challenge and developed a process based on existing technology which now enables ammonia plants to produce very large capacities. This new process (see flowsheet) delivers a capacity of 3,300 mtpd using tried and tested equipment. It also provides the basis for even larger plants (e.g. 4,000 – 5,000 mtpd).

Technology highlights

  • Proven technology with several plants in operation.
  • Proven magnetite-based catalysts in all stages of the new process.
  • Improved energy efficiency.
  • A high conversion rate in the high-pressure synthesis loop and reduced production requirements result in smaller piping sizes in the high-pressure loop. Standard piping can be used for capacities of 4,000 mtpd and more.
  • The syngas compressor of a 3,300 mtpd dual-pressure plant is the same size as that in current 2,200 mtpd ammonia plants; several reference compressors are in operation.
  • There are no major deviations from conventional process conditions.

A proven process for world-scale ammonia plants

The first plant based on this process is the SAFCO IV ammonia plant in Al-Jubail, Saudi Arabia, started up in 2006. With a capacity of 3,300 mtpd it is by far the largest ammonia plant worldwide. The key innovation in Uhde's new dual-pressure ammonia process is an additional medium-pressure once-through ammonia synthesis connected in series with the conventional high-pressure ammonia synthesis loop as follows:

1. The once-through ammonia synthesis involves the compression of the make-up gas in a two-stage inter-cooled compressor. This is the low-pressure (LP) casing of the syngas compressor. The pressure at the discharge of the compressor is about 110 bar. At this pressure the three-bed, inter-cooled, once-through converter produces approximately one third of the total ammonia output. The syngas-ammonia mixture leaving this converter is cooled and 85% of the ammonia produced is separated from the gas as liquid.

2. The remaining syngas is then compressed in the high-pressure casing of the syngas compressor to the operating pressure of the ammonia synthesis loop (up to 210 bar). Since the syngas has been cooled down the HP casing can operate at a much lower temperature than in the conventional ammonia process. The high synthesis loop pressure is achieved through a combination of the chilled second casing of the syngas compressor and a slightly elevated front-end pressure. In this conventional ammonia synthesis loop the remaining two thirds of the total ammonia is produced.

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