SEPARATING NOBLE GASES
Chemists describe “noble” as those substances that bind to other substances only with relative difficulty; this applies to noble metals like platinum and gold and also to noble gases like helium.
Most people, on the other hand, understand “noble” to mean valuable—and they wouldn’t be wrong: Platinum is known to be considerably more expensive than, for example, iron. And one cubic meter of gaseous helium costs between €8 and €30 depending on the purchase quantity and the purity, among other factors. By comparison, one cubic meter of natural gas is available to a private customer for only about 60 euro cents. One reason for the high price of helium is its high demand. The gas is used to cool magnetic resonance tomographs (MRTs) in hospitals and to fill balloons and airships. It is also used in glass fibre production, welding, and the electronics industry, and in leak detection for systems and equipment. But helium is rare. It has been extracted from the few natural gas sources worldwide where it is present in proportions between two and eight percent. The world market is currently divided between the producing countries: the USA, Algeria, Qatar, Russia, and Poland.
Membrane technology for Canada
Since August 2016 Canada has also been producing helium. A unique plant has come on stream in Mankota, which extracts 99.999 percent helium fully automatically from the 250,000 cubic meters of gas produced daily. The plant was built by the Engineering Division of the Linde Group. Its distinguishing feature is that Linde has combined an established gas-separation process with Evonik’s new SEPURAN® Noble membrane technology. This hybrid process allows particularly efficient enrichment of helium in the Canadian gas source, where the helium content is well below two percent. Linde was thus able to make a persuasive case to Weil Group Resources, the owner of the gas source, and to come out ahead in global competitive bidding.
SEPURAN® membranes consist of polyimide, a high-performance polymer whose resistance to high temperatures and aggressive chemicals has been proven over many decades. For example, the polyimide is also used in hot-gas filtration in cement and steel smelting plants. Spinning systems at Evonik’s site in Schörfling (Austria) produce the material in the form of fibres with a very special architecture: Their interiors are hollow. The material is highly porous all the way up to this cavity. The pores become progressively smaller toward the exterior. A relatively dense shell, less than 100 nanometers thick, forms the surface of the fibres.
Gases whose molecules are very small (kinetic diameter) can permeate this skin faster than those whose particles are larger. In this way the skin can distinguish between, for example, carbon dioxide and methane, or helium and methane. The remaining porous part of the fibre does not help in gas separation because the pores are too large for this purpose; it serves as a supporting element, lending the material mechanical stability. Evonik bundles tens of thousands of these hollow fibres into a membrane module in a stainless steel housing.
The SEPURAN® Green membrane modules currently in use for biogas upgrading in more than 100 facilities worldwide are not perfectly suited for extracting helium: Due to the low proportion of helium in natural gas, SEPURAN® Green membranes do not upgrade the noble gas to the desired extent. What is needed is a membrane that can select helium from the rest of the gas even more effectively than SEPURAN® Green. Evonik’s specialists have succeeded in producing such a membrane by modifying the spinning process for fibre production. Evonik markets this membrane under the name SEPURAN® Noble.
CUSTOMIZED HOLLOW FIBRES
If you now think that a higher separation efficiency than that offered by SEPURAN® Green would also work for methane and carbon dioxide, and could thus also improve biogas upgrading, you wouldn’t be wrong. But SEPURAN® Noble would not allow sufficient permeation of either gas: The higher selectivity of this SEPURAN® variant comes at the cost of lower productivity. In the case of helium this is not critical, however, because it can pass through the membrane much faster than, say, carbon dioxide. The skill of the SEPURAN® team thus lies in customizing the properties of the hollow fibre membranes for the application in question. In this way it fully exploits the already very good gas separation properties of polyimide.
In the hybrid reference plant in Mankota, SEPURAN® modules upgrade the crude gas to a helium content of about 50 percent. From the resulting gas mixture, almost pure helium is then obtained by pressure swing adsorption (PSA). In this well-established method, the pressurized gas mixture is passed through a solid bed. Helium remains almost totally unadsorbed on this solid while the other gas components are deposited on or in it. As soon as the adsorption capacity of the solid is exhausted, it is regenerated by reducing the pressure. Two solid beds are operated concurrently so that PSA continuously provides helium; while one of these is in the adsorption mode, the other is being regenerated.
The PSA process only functions well with a helium content of at least 25 percent, and its effectiveness increases with the helium content. This is why initial upgrading of the crude gas by SEPURAN® membranes is necessary. The two processes complement each other perfectly because the membrane process produces an unpressurized helium gas mixture that is subjected to pressure by PSA. The pure helium finally obtained is thus also under pressure, which reduces transportation costs.
Collecting and upgrading used helium
Helium is expensive, and large users may find it worthwhile to recover the used noble gas; they too can benefit from SEPURAN® Noble. These large users include producers of optical fibres transmitting internet data and phone calls. The helium is particularly effective for cooling the glass fibres as these are drawn from the hot melt. This increases production speed: A single plant can produce more than two kilometers of fibre per minute. But helium cooling is costly: Many glass fibre production facilities spend hundreds of thousands of euros on helium annually. Nextrom, a leading global plant engineering firm for the glass fibre industry, has now developed a solution for fibre producers that is based on SEPURAN® Noble. It offers a system in which the used helium is collected, cleaned, and re-used for cooling. As much as 90 percent of the helium can be recovered in this way.
A mere two years after market launch, SEPURAN® Noble has now become established in the glass fibre industry; the SEPURAN® team owes this success partly to the support of their colleagues from Silanes, who are familiar with this sector. Moreover, membrane technology can be very easily integrated into glass fibre production because the upgraded helium need not be liquid nor ultrapure. But this is not the case for other helium applications, such as magnetic resonance tomography, where it will take somewhat longer for SEPURAN® Noble technology to gain a foothold. Membrane experts are nonetheless convinced that the technology will also establish itself in applications other than optical fibres.