Separating with fibers

The aerospace, energy, chemical, and food industries—all of these sectors require nitrogen at different levels of purity. With its newly developed hollow-fiber membrane SEPURAN® N2, Evonik is offering a needs-oriented, efficient, and energy-saving solution for obtaining the desired gas.

Incombustible, intert, odorless, and tasteless—these properties make nitrogen a highly needed gas for a number of branches of industry. It is obtained from air, which has a nitrogen content of up to 78 percent. Air is also made of up to 21 percent oxygen—an extremely reactive gas that facilitates fires. Gas separation processes are designed to separate oxygen and nitrogen.

Cryogenic air separation, also known as the “Linde process,” is particularly good at doing this. It can extract nitrogen at a purity of over 99.99 percent. But this level of quality comes at a price: Cryogenic air separation consumes a lot of energy and is only profitable in large-scale production. For many applications, though, technical nitrogen with a purity of 95 to 98 percent is sufficient.

One of the main applications for the inert gas of such qualities is active fire protection. In server rooms, for example, a supply of N2 ensures that the oxygen content in the indoor air drops below 16 percent. At this level, flames either die immediately or never form in the first place. The chemical industry frequently uses nitrogen for explosion protection in production plants or tank farms. For these purposes, nitrogen can be obtained far more cost-effectively with membrane technology.

This was Evonik’s starting point for its new development. A technological leader in high-performance polymers, the specialty chemicals company benefited in this development work from its extensive know-how in the design and production of hollow-fiber membranes. Based on this experience, Evonik has developed custom-designed membrane systems for gas separation for a wide variety of applications over the past few years. SEPURAN® Green, for example, is used for the treatment of biogas because it separates carbon dioxide and methane gas with high efficiency. The process was honored with the IKU Innovation Award 2014 of the German Federal Ministry of the Environment for outstanding innovations in climate and environmental protection.

From polymer solutions to the finished module

Building on their success with biogas treatment, the engineers at Evonik have now developed SEPURAN® N2, a hollow-fiber membrane specially developed for efficient nitrogen extraction. It combines high capacity, low air consumption, and superior energy efficiency.

SEPURAN® N2 is made from polyimide, an extremely heat- and chemical-resistant high-performance plastic that Evonik also produces itself, and that works based on the solution-diffusion model, whereby gases selectively permeate. The process involves directing compressed air into the hollow-fiber membranes, which look like long, extremely thin macaroni. This causes a partial pressure difference between the internal and external sides of the hollow fibers. The different gas components strive to balance the partial pressure, but this is impeded by the membrane.

Because of their small size, it is easy for oxygen molecules to pass through the membrane. So on the interior area, the retentate side, nitrogen is enriched to the desired purity, while on the exterior side of the hollow fibers, the permeate side, an oxygen-rich airstream forms. In some cases, this stream can be a welcome co-product that is used for such purposes as increasing the throughput of combustion or oxidation processes, for example.

The purity of the N2 can even be regulated through the quantity of compressed air injected: The lower the quantity injected the higher the quality of the nitrogen. But in this case, the process requires an overall larger volume of air, because the amount of nitrogen flowing through the plastic membrane increases along with the amount of oxygen. Commonly, the process is mostly used at a pressure of up to ten bar, but can also be used at higher pressures. Depending on the required purity, the ratio of injected air to produced nitrogen is between 2:1 and 3:1.

Evonik produces the polyimide hollow fibers at its site in Lenzing (Austria) which has extensive experience and expertise in the production of fibers. In a spinning plant, the polymer solution flows around a central jet of liquid, the subsequent hollow space of the fibers, into a precipitation bath. The liquid jet is also responsible for the required asymmetry and porosity of the membrane. The dense gas-active layer on the surface of the fibers, where the actual gas separation occurs, is less than 100 nanometers thick. The remaining area of the fibers, which is porous, serves primarily as support.

By the end, the spinning process has produced hollow-fiber membranes less than half of a millimeter in diameter, which can ultimately be fitted into stainless steel modules. The roughly 1.3-meterlong SEPURAN® N2 membrane module contains tens of thousands of these tiny pipes, which have been bundled and embedded in their own specially developed resin. Depending on the application and the size of the plant, any number of these modules can be connected with each other.

High capacity, high efficiency

For manufacturers of nitrogen generators, which are often medium-sized engineering companies, the membrane process constitutes an inexpensive alternative for customized, needs-based nitrogen production. In this context, Evonik experts were able to combine two positive attributes: First, the SEPURAN® N2 technology requires fewer membrane modules than alternative membrane systems to generate the required amount of nitrogen. Second, the process uses less air, which means that the upstream compressor can be build smaller. This not only reduces the overall amount of energy consumed in nitrogen production but also lowers the investment cost, much of which goes to the compressor.

Costs for operation and maintenance are also low with SEPURAN® N2. Another advantage is that the SEPURAN® N2 membrane module can be easily connected to existing compressed-air systems, which enables on-site production of the inert gas at low investment cost. With its modular design, the process offers high flexibility, and the plant can be started and shut down at short intervals. This is why membrane systems are ideal for combining with existing cryogenic air compressors or pressure-swing adsorption systems, to cover peaks in demand or expand load limits.

Reduced investment and operating costs enable more cost-effective production of nitrogen—and not just when compared to other membrane systems. SEPURAN® N2 technology is a particularly Attractive alternative to pressure-swing adsorption systems or supply by truck or cylinder rack when nitrogen purities lower than 98 percent are sufficient.

At Evonik, the value-added chain for Membrane modules extends from production of the monomers and polymers, through manufacture of the hollow fibers, to fabrication of the complete modules. Adaptations can be made, therefore, at every key point—on the molecular design of the chemical building blocks, the modification of the individual fibers, and on their embedding in the stainless steel elements. The result can be unprecedented module performance.