Nitrogen from the finest fibres – onsite

Nitrogen is a useful gas in many respects for various industries. The advantages are well known: It does not burn, is inert, tasteless and odourless. Depending on the application, this industrial gas exists in various forms of purity and quality. 99.999% purity is possible – but is it always the best choice? In fact, many companies use high quality nitrogen in processes where high purities are not needed.

Pure nitrogen not always necessary

Nitrogen is often used to prevent fires or explosions: anyone who supplies the inert gas to a room or a container can ensure in this way that the oxygen content drops to such an extent that flames cannot find food. In the chemical industry, oxidation of oxygen-sensitive products is prevented in a similar way, and in the food industry, nitrogen extends the shelf life of bottled products.

And this is precisely where the crucial issue lies: not all of these processes require 99.999% nitrogen purity. Many companies primarily want to prevent explosions, or have other processes that require nitrogen: in nine out of ten of these processes, 95% purity might be enough. So using especially pure nitrogen would be a waste of money.

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%. 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, technical nitrogen with a purity of 95-98% is sufficient.

In terms of efficiency and energy, membranes are unbeatable. In this case, nitrogen can be obtained directly from the air that surrounds us all. But in the process, the gas must be separated from oxygen, which makes up about 21% of this air. In membrane systems, compressed air is passed through long, thin hollow fibre membranes. This creates a partial pressure difference between the outside and the inside of the hollow fibre. The smaller oxygen molecules slip through the membrane, while the nitrogen molecules remain inside. The effect is that the less air is added, the purer the nitrogen in the end.

”In view of the high energy input involved, it is necessary to check what degree of purity is actually required for the processes in question”

However, this is also where the limits of the process lie: to achieve a purity of 99% or more, correspondingly less air would have to be fed into the membrane – and correspondingly less nitrogen would be obtained in the process. In view of the high energy input involved, it is therefore necessary to check what degree of purity is actually required for the processes in question. Companies are well advised to take a very close look here: in the case of inerting, safety considerations usually dictate exactly how much nitrogen is required. And purity levels between 95-97% are often sufficient – perfect for membrane systems.

As one of the technology leaders in high-performance polymers, Evonik has developed customised systems in recent years to separate different gases for a wide range of applications. The membranes specially developed by Evonik impress with their high selectivity. The hollow polyimide fibres are responsible for this. The gas-active layer on the surface of the fibre is less than 100 nanometers thick. The finished hollow fibre membranes have a diameter of less than half a millimeter.

As a chemical company, Evonik not only has access to a sophisticated basic polymer, but can also adapt it precisely for different applications. Especially in combination with the required capacity, for example how much air needs to be pumped into the membrane to produce one cubic metre of gas.

Saves space, costs and efforts

In the end, several tens of thousands of the polyimide tubes are embedded in stainless steel modules about 1.30 metres long. A space-saving system, and not just because of its size: depending on the location, the stainless steel tubes can be installed horizontally, vertically or even at an angle. Their modular design also makes them flexible. Due to their high capacity, SEPURAN® N2 also requires fewer membrane modules than alternative membrane systems.

In addition, they are energy-efficient: since less air has to be pumped, the upstream compressor can also be smaller. This saves both energy and investment costs. Especially compared to cryogenic systems, where the nitrogen is delivered. Operating costs and maintenance requirements are also low with SEPURAN® N2 – and guarantee additional flexibility. After all, the nitrogen is produced on-demand. As long as no nitrogen has to be produced, there are no costs.

As a producer of the polyimide plastic and the membrane technology, Evonik covers the entire value chain of membrane modules. As a result, the company can customise details for specific applications as needed, and also connect to established plant engineers who can take care of on-site installation.

Due to its flexibility, the SEPURAN® N2 membrane technology for onsite nitrogen generation is interesting for a wide range of possible applications. For example, even for companies that already work with cryogenic air separators but want to cover demand peaks at certain times. Or for situations where a system that is as mobile as possible is required – because a specific plant needs to be purged with nitrogen onsite. The situation is similar in pipeline construction.

The membranes can also be installed and transported in vehicles. Overall, the possibilities are extremely diverse – although the system itself is so compact and flexible, this makes it an asset for far more than just small applications. And it can be easily and quickly connected to existing control systems. It also can be used from long distances with automated steering systems, as in the case of the marine industry, for example. On top of that, it is durable: the membrane does not need to be replaced and can be used for several years without any problems. 

In the meantime, interested customers also have the option of using an online calculator to see for themselves how many membranes of what size they might need. This can be set variably according to parameters: interested parties can therefore specify that they need 120 cubic metres of nitrogen, or a purity level of over 97% – or both at the same time, for example.

Principle of selective permeation for gas separation

Gas separation membranes work according to the principle of selective permeation through the membrane surface.The permeation rate of each gas depends on its solubility in the membrane material and on the diffusion rate of the gas.

Gases with high solubility and small molecules pass through the membrane very quickly. Less soluble gases with larger molecules take more time to permeate the membrane. In addition, different membrane materials separate differently. The driving force needed to separate gases is achieved by means of a partial pressure gradient.

The driving force for a gas to permeate through a membrane is the partial pressure difference; in other words, the partial gradient between the inside of the hollow fibre (retentate side) and the outside of the hollow fibre (permeate side). The greater the difference, the more gas permeates through the membrane. For example, if carbon dioxide and methane are being separated, as is the case with biogas upgrading, carbon dioxide permeates through the membrane very quickly while the methane tends to be held back.

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 fibres, 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 nitrogen 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 polymeric 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.