Two large-scale natural gas processing plants are currently being built in Russia and Australia. Linde is providing the nitrogen removal units (NRUs) for both locations. While both NRUs rely on the tried-and-tested principle of cryogenic rectification to separate the nitrogen from the methane in the natural gas mix, they are following very different approaches. For the Russian project, Linde is using a single-column process, whereas a double-column process has been chosen for the Australian unit. What are the advantages of each process and to what applications are they best suited?
The Amur GPP project in Russia
The Gazprom energy group is building the country’s largest natural gas processing plant, scheduled for completion in 2021. Located in the far eastern corner of Russia, the Amur GPP (Gas Processing Plant) project will provide the infrastructure to harness the region’s gas reserves. Linde was awarded a contract to deliver several building blocks within the plant – including the nitrogen removal unit. Natural gas typically contains up to ten percent nitrogen, which diminishes the value of gas as a fuel. The greater the nitrogen (N2) ballast, the lower the calorific value of the gas and the related Wobbe index. Linde’s task, therefore, was to design an NRU unit that would reduce the nitrogen fraction to one or two percent. But Gazprom expected even more from the plant; namely an additional unit to separate and subsequently commercialise the tiny quantities of valuable helium present in the gas (well under one percent) – and at the same time make the separated liquefied nitrogen available for use in other processes.
The plant modules being built for Gazprom by Linde Engineering and scheduled for shipment in 2017 essentially perform these functions in two stages. In the first stage, the methane and nitrogen (together with the helium fraction) are separated in the NRU. The helium is then separated. This stage makes use of the different boiling points of the other natural gas components to separate them in a cryogenic column (single-column process) – with liquid methane on one side and a gaseous mixture of 95 percent nitrogen and five percent helium on the other.
Stage 2 utilises a coldbox to cryogenically separate the remaining components so efficiently that over 98 percent of the helium present in the natural gas can be used. The advantage of this kind of plant is that most of the nitrogen fraction is captured in pressurised or liquefied form. Unlike many other plants, therefore, it is not wasted and instead is used in the downstream helium liquefaction process.
Two columns for Down Under
In Western Australia, one of the country’s largest natural gas processing plants is currently under construction. Linde’s role was to develop a nitrogen removal unit to improve the calorific value of the low-quality gas by reducing the nitrogen fraction. In other plants, a low-grade mixture of nitrogen and methane often remains unused, but here the plan is to feed it into the local gas network.
What sets the Australian project apart from the Russian one is the fact that the minimal helium fractions present in the comparatively low-grade Australian natural gas are not economically viable. For that reason, the only option is to remove the intrinsically worthless nitrogen and release it into the atmosphere. To make the plant as energy-efficient as possible, Linde decided to use a double-column process. The reasoning behind this was that highly pressurised liquid nitrogen could be drawn from the lower of the two superimposed cryogenic columns to provide a refrigerant for the process. The nitrogen is fed to the upper column, where it expands and thereby cools from around minus 150 degrees Celsius to minus 180 degrees Celsius.
In natural gas plants where liquid nitrogen is required for specific downstream processes, single-column separation is often the best solution. Where the nitrogen only needs to be removed and released into the atmosphere, however, a double-column process can be more efficient.