The researchers from the Mendeleyev University of Chemical Technology of Russia and the Lobachevsky State University of Nizhny Novgorod have tested a new hybrid technology for natural gas purification: they combined two different processes (gas hydrate formation and membrane separation) in a single unit. As a result, unwanted impurities such as carbon dioxide and hydrogen sulfide, which reduce gas energy value and damage equipment over time, have been efficiently removed from the gas. At the same time, losses of hydrocarbon, the most valuable component, turned out to be significantly lower than when using traditional solutions.
This combination is necessary because each method is ineffective on its own. Membranes effectively remove impurities from the gas, but they also allow some methane to pass through, which results in losses of useful raw material. On the other hand, the gas hydrate method leaves hydrocarbons largely unaffected, preserves them in a gas stream but is unable to thoroughly remove the acid gases. Therefore, the scientists combined both processes: first, the gas passes through a hydrate crystallizer, where the components are carefully redistributed, and then through a membrane, which removes the remaining impurities with a minimal impact on the hydrocarbons.
To conduct experiments, the researchers prepared a mixture with the composition as close as possible to natural gas from such gas condensate fields as the Urengoy or Kovykta fields. The mixture included methane, ethane, propane, butane, as well as carbon dioxide, hydrogen sulfide, nitrogen, and even small amounts of inert gases. At the first stage, the scientists manufactured polysulfone membranes in the form of thin hollow fibers. Then they measured the membrane permeability for different gases and based on the data, built a detailed mathematical model of the entire unit in Aspen Custom Modeler. This model allowed them to predict how the system would operate under various conditions, including when changing the proportion of the gas passing through the membrane.
The key parameter turned out to be the stage-cut indicator – the proportion of gas passing through the membrane. The researchers separately studied the two streams exiting the hydrate reactor: the normal gas phase and the gas released during hydrate decomposition. They found out that for the gas phase, optimal performance was achieved at membrane gas permeability equal to 20-40%. In this case, hydrogen sulfide was almost completely removed, carbon dioxide content reduced by more than tenfold, and methane losses remained moderate. For the gas obtained from hydrates, the best results were achieved at higher values – 40-60% – allowing for extraction of useful components even from the streams previously considered secondary.
Having completed the calculations, the scientists verified the results with a laboratory unit. The experiments showed a good agreement with the model: methane concentration deviation was only 2-3%, and for heavier hydrocarbons, it was 0.5-1.5%. They observed more significant differences, up to 10-20% only when assessing the overall hydrocarbon recovery, due to the errors in the gas flow measurement. Moreover, the model accurately describes the key effects – a rapid removal of acidic impurities and changes in the hydrocarbon composition.
Thus, the scientists were able to develop a highly practical and scalable process. Unlike traditional amine purification requiring heating of large units to regenerate the absorbent, the hybrid process requires less energy and is more easily adjusted to different gas volumes. Furthermore, it enables the recovery of useful components from the streams previously often lost or burned.
