Hydrogen and Synthesis gas plant, Al Jubail, Saudi Arabia. Photoshoot October 2016, picture taken by Torsten Pro?, Jürgen Jeibmann Photographik

Turning ammonia plants into real all-rounders

The Linde Ammonia Concept (LAC?) gives customers greater flexibility over both their feedstock envelope and their choice of possible co-products. Thanks to modular design options, this process flow is also an attractive prospect for smaller ammonia plants.

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 insert 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. 

Ammonia plants based on the Linde Ammonia Concept

In order to create pure hydrogen, a feedstock such as natural gas is converted into a synthesis gas comprising a mixture of H2 and carbon monoxide (CO). “The hydrocarbons are converted in the first process step. We only need one steam reformer to do this in LACTM.L1, the process variant for light hydrocarbons. Fed with natural gas, the LACTM.L1 reformer is designed for maximum conversion efficiency. Conventional ammonia plants also require a secondary reformer, which introduces ambient air into the system,” says Wawrzinek. This air provides the nitrogen source for the ammonia synthesis reaction. The downside of this approach is that the N2 gas in the air is transported through the system right from the start, even though it is only required at a later stage. In contrast, the LAC process does not draw in atmospheric air at this point. Instead, the nitrogen stream is provided by an air separation plant and only fed into the system shortly before the NH3 synthesis reaction. This eliminates two further process steps. One of which is the downstream CO conversion. While conventional plants require two steps for this, Linde’s concept uses a specially developed isothermal reactor, which gets the job done in a single step. “The CO has to be converted to increase the amount of hydrogen in the gas stream. Thanks to our isothermal reactor, our process requires a much lower catalyst volume for this step,” elaborates the Linde expert.

Linde-built ammonia plant in Togliatti, Russia, based on the Linde Ammonia Concept (LAC?).
The ammonia plant in Togliatti, Russia, uses the Linde Ammonia Concept.

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, CO 2 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 CO 2 scrubbing and another catalytic step (methanization) to reduce CO and CO 2 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.
Linde-Isotherm-Reaktor: Grafik eines Reaktors basierend auf der Kon-struktion spiralgewickelter W?rmetauscher

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 H 2 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 N 2 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.
The Linde-built hydrocarbon  and ammonia  plant
The HyCO and NH3 plant in Al Jubail, Saudi Arabia, produces hydrogen and carbon monoxide as co-products, which is then used in other process streams.

Ammonia goes modular

Whereas operating costs and efficiency are the main drivers with larger NH 3 plants, investment costs tend to play a more important role in the case of smaller plants producing less than 300 tons of ammonia a day. Linde offers a modularized approach that is an attractive solution for smaller plants in this high-growth segment. “We use our standardized ECOGANTM air separation plants to generate the nitrogen, and our modularized HYDROPRIME ® steam reformers for the hydrogen. We then bundle H 2 production, pressure swing adsorption and the cooling elements in a standard modules,” explains Wawrzinek. Ammonia production is integrated in another module. “As LAC is an inert-free process, it requires fewer plant components overall – and we can deliver those components on a smaller footprint.” This facilitates transport and on-site handling because there are fewer blocks to assemble and weld. “Our modularization concept is a great solution for sites in areas with harsh climates or where there is a lack of experienced workers available locally. Modularization has already proven highly successful for other plant types and now we are bringing its benefits to the world of ammonia,” summarizes Wawrzinek.

More flexibility on feedstock envelope and choice of useful co-products?