Turning Ammonia Plants Into Real All-Rounders

A Multi-Faceted Concept

• The Linde Ammonia Concept – known as LAC – offers customers greater flexibility over their feedstock envelope. The process can also be used to create co-products such as nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide and synthesis gas.
• The LAC process flow requires fewer catalytic steps and thus reduces the volume of catalyst material and amount of energy required.
• Linde Engineering also offers this solution as a modular concept, which is a particularly attractive option for smaller ammonia plants.

“The Linde Ammonia Concept offers greater flexibility over the choice of co-products. Plant operators can also use a broader range of feedstocks – extending from natural gas through methanol to naphtha and heavy oil.”

Most ammonia plant operators would welcome greater flexibility – over both their feedstock envelope and their choice of useful co-products. At the same time, they are obviously keen to optimize plant economics. “We developed the Linde Ammonia Concept – or LAC for short – specifically to meet these challenges,” explains Dr Klemens Wawrzinek from Business Development & Technology for Hydrogen and Syngas at Linde Engineering. “It allows plant operators to use a broad spectrum of feedstocks to produce ammonia – ranging from natural gas through hydrogen-rich residual gas to naphtha and heavy oil,” adds Wawrzinek.

The fact that Linde’s engineers have integrated a clean syngas generation stage into the multi-step process is a further benefit as it gives customers the option of various gas streams comprising valuable co-products such as hydrogen or carbon monoxide. The LAC process flow starts with the production of pure hydrogen (H₂). This then reacts with nitrogen (N₂) in the presence of a catalyst to produce ammonia (NH₃). Unlike conventional plants, LAC was designed by Linde to avoid “ballast” so it does not carry inert gases such as argon and nitrogen along the process flow. This eliminates the need for complex scrubbing and removal processes, also avoiding three catalytic steps in total.

Linde’s Isothermal Reactor in Action

The catalytic reaction takes place in the fixed bed reactor. Spiral-wound tube bundles are integrated into the catalyst material to dissipate the heat released during the exothermic CO conversion reaction, thereby producing valuable steam. The spiral-wound design makes the tubes particularly effective at cooling, keeping the catalyst at an optimum operating temperature. This increases catalyst performance and service life. It also results in fewer by-products and efficient recovery of reaction heat with lower reaction costs.

Using Pressure Swing Adsorption to Purify the Gas Stream

The hydrogen-rich gas stream produced in the isothermal reactor must be scrubbed to remove carbon monoxide and carbon dioxide, which would otherwise contaminate the ammonia synthesis catalyst. With LACTM.L1, this is achieved using another technology developed by Linde known as pressure swing adsorption. During this step, the gas stream is passed at high pressure through an adsorption material comprising, for example, a molecular sieve and active carbon. The material efficiently traps components such as CO, CO₂ and any remaining methane from the natural gas. The hydrogen, however, passes through and flows on to the ammonia synthesis reaction. “The adsorption material has to be regenerated at regular intervals. To do this, we reduce the pressure in optimized cycles, allowing desorption of the gas molecules that have been retained,” explains Wawrzinek. In contrast, conventional processes require an energy-intensive CO₂ scrubbing and another catalytic step (methanization) to reduce CO and CO₂ to the required levels. “Our concept eliminates the need for three catalytic steps in total, which means it consumes around fifty percent less catalyst material than conventional processes,” says the Linde expert.

Alternative Uses for Hydrogen

Another highlight of the Linde Ammonia Concept is the fact that plant operators can use the syngas stream for purposes other than ammonia production. This is possible because the stream is not contaminated with nitrogen. Operators can also divert the pure hydrogen and use it for other applications – something that cannot be done with a conventional process scheme. “These benefits were key arguments for our customers in Al Jubail in Saudi Arabia and Togliatti in Russia,” explains Wawrzinek. Another customer in Salalah, Oman was won over by the fact that Linde’s concept can be used to process feedstocks other than natural gas: “In this case, we are using a hydrogen-rich gas provided by the plant operator to generate the pure H₂ stream for ammonia synthesis,” explains the Linde specialist. The plant is currently under construction and is set to go on stream in 2020. The nitrogen required for LAC is provided by an air separation plant. The N₂ stream is fed into the process just before the ammonia synthesis reaction. The high energy efficiency and feedstock conversion performance of LAC provide impressive testimony to the intelligent design of this concept.