Frequently Asked Questions

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Welcome to our Frequently Asked Questions (FAQ) page! Here, we have compiled a list of common questions and answers to help you better understand our products and services. Whether you are a new customer or a returning one, we hope this page will provide you with the information you need.

Our product experts are always ready to assist you. If you cannot find the answer to your question here, please do not hesitate to contact us.

Operating Evonik SEPURAN N2 membranes

You can use Evonik’s SEPURAN® N2 calculator in order to identify the right SEPURAN® N2 membrane product for your individual project.

Based on your process parameters (feed temperature, feed pressure, required nitrogen quantity and purity) the calculator will estimate the number and type of membrane fitting for your project.

We offer a product portfolio with membranes of different sizes. You can combine several membranes of the same type and operate them in parallel to further adjust the produced nitrogen quantity and purity. You can choose between different product sizes adjusting also your N2 purity to be achieved, your inlet pressure and inlet temperature in order to create the most economical solution for your project.

Just have fun playing around with our SEPURAN® N2 calculator!

First of all, our SEPURAN® N2 membranes can be installed in any direction (e.g. horizontally or vertically) in your skid. Installation direction does not affect separation performance of the membrane.

Secondly, when you install the Evonik SEPURAN® N2 membranes in your skid, please ensure to connect the feed, retentate and permeate ports with your surrounding process (i.e., pipes or hoses) as shown in the picture below. To avoid damage to the cartridge, you must ensure that the pressures at the feed inlet and retentate outlet (NEA) are always higher than the pressure on the permeate side (OEA).

Evonik SEPURAN® N2 membranes are operated in the so-called counter-current flow. This means, that the retentate flow (flow inside the fibers, „bore side“) is flowing from the feed port to the retentate port. The permeate flow however is leaving the system at the permeate port near to the feed port. This arrangement creates a counter-current flow of feed air to the one side and permeate, oxygen enriched air to the other side of the membrane module, optimizing the separation performance of the membrane.

When commissioning/using Evonik SEPURAN® N2 Membranes in your generator for the first time, please ensure to comply with the specifications listed in the Evonik SEPURAN® N2 User Manual (Chapter IV OPERATION OF SEPURAN® N2 SYSTEMS).

First slowly open the feed valve (≤ 1.0 bar/s) for pressurization.

Then slowly open the retentate valve (≤ 1.0 bar/s) to slowly start the gas flow through the membrane.

This way of commissioning prevents that oil (e.g. coming from the compressor that is normally retained by your pretreatment set-up during steady state operation) is pushed into the membrane during start-up. If you increase the feed flow too quickly during commissioning, the pretreatment set-up – that is designed for a certain flow rate – may not be able to retain enough oil to comply with pretreatment specifications necessary to keep the membrane intact.

When you store the membrane module, the fibers inside will lay more closely together in the tube due to their inactivity. The fibers will need to spread out again in order to achieve optimal flow profile inside the membrane module and therefore full separation performance. The fibers will be spread just by operating them with pressurized air at normal operating conditions.

For first commissioning, the separation performance, under normal operating conditions, will reach target performance within 24 hours from the initial start-up. The target performance of a membrane module is specified in the individual Evonik SEPURAN® N2 certificate of analysis (COA) of a membrane module that we ship with each membrane to our customers. After a longer standby time, the membrane will reach full performance within 12 hours. From our experiences in field, the time it takes to reach full separation performance might often be much shorter than this.

The N2 purity in your project retentate stream depends mainly on the process parameters listed below.

  • Feed inlet pressure
  • Feed flow rate
  • Feed  inlet temperature

By adjusting those process parameters (within the specifications stated in the Evonik SEPURAN® N2 technical information or user manual for the specific Evonik SEPURAN® N2 product) you can set the capacity and purity of your nitrogen enriched air. The table below summarizes how N2 capacity, N2 purity and air factor will change qualitatively when you adjust the process parameters feed temperature, feed pressure and feed air flow rate. (See table below.)

A. The process parameters as feed flow, inlet temperature, inlet pressure or inlet air composition are directly influencing membrane separation performance. If you have one of these parameters not at the design level you chose for your N2 generator, the product quality will vary from your originally defined design level as well.

B. Inlet air pretreatment protects the membranes from getting clogged by particles, water or oil. Ensure sufficient pretreatment according to ISO 8573-1:2010 class 1 for particles as well as for oil. For avoiding condensing water inside the fibers, the temperature of inlet air should be at least 10 Kelvin higher than the corresponding pressure dew point. If impurities for any reason can bypass the pretreatment set-up, this might cause clogged fibers resulting in reduced membrane surface and consequently reduced separation performance.

C. If the membranes are operated for the first time, or if they were not operated for a longer time, the fibers within the bundle can lay very closely together. The fibers then need to spread out again in order to achieve full separation performance. The fibers will spread out by operating them with pressurized air for some hours. Monitoring the oxygen concentration in the retentate over time during the first hours of operation will give you an indication of this process.

D. Ensure that no pressure at the permeate side of the membranes can build up. As gas separation via membrane technology is a pressure driven process, an increased permeate pressure will reduce the driving force of transporting gas components through the membrane. A decreased driving force will result in less purity of the nitrogen enriched air.

E. Measurement equipment always comes with measurement uncertainties. There are different technologies for gas flow measurement or measurement of oxygen concentration available on the market. Each of the different measurement technologies comes with advantages and disadvantages.

Electrochemical oxygen sensors for example need to be exchanged on a regular basis due to continuous measurement drift over time whereas oxygen sensors based on paramagnetic effect show extremely long lifetime but are sensitive towards vibrations.

Ensure to comply with the individual installation specification of each measurement equipment. Some types of flow sensors for example require straight inlet and outlet sections of a certain length to minimize measurement error.

And last, but not least, double-check your flow measurement or your gas sensors could not be the root cause for incorrect measurement! It is not the first time that we have seen brand new, just built-in sensors that were very unfortunately already broken.

F. If you have the possibility of measuring the O2 concentration on the permeate side of the membranes, this can give you insights on the integrity of the membrane fibers. Normally the expected O2 concentration for typical N2 applications should be in the range from 26 to 35 % depending on process parameters. You can check the expected O2 concentration in the permeate for your application by using Evonik’s SEPURAN® N2 calculator or the Excel calculation tool that we share with our customers. Significantly lower O2 concentrations could indicate broken fibers as ambient air can enter the permeate side reducing the O2 concentration on the permeate side.

You can operate the membranes at different process parameters as long as they comply with the specifications stated in the technical information sheet available for each SEPURAN® product or the SEPURAN® user manual. Especially when you are changing the feed inlet pressure, please comply with the pressure ramp stated in the SEPURAN® user manual.
The desired nitrogen capacity should match with the membrane type (2“ short/long, 4“ short, 4“, 6“ and 6“ long) capable of producing it. You can use Evonik’s SEPURAN® N2 calculator to estimate the number and type of membrane suitable for your process.

N2 capacity and N2 purity in the retentate flow will directly be effected by changing process parameters such as feed temperature, pressure or temperature (see also above question “How do I set the N2 purity in the retentate stream?”)

Separation performance is guaranteed only for N2 purity in the retentate from 95 % to 99.5 %, see also for this the individual technical information sheet of the individual SEPURAN® N2 product. There you will find a performance table with the framework criteria of 25° degree inlet temperature and 7 barg inlet pressure.

Below some limitations are listed that you should consider when operating at different process parameters.

  •  Operating with significantly decreased feed flow

As there are step changes in membrane area from one product to another,  there will be as well step changes in produced nitrogen capacity of the different products. 

  •  Operating at different feed pressures

By operating at increased feed pressure, the driving force for permeation is increased and therefore more oxygen enriched air and less nitrogen enriched air will be produced
By operating at a very low feed pressure under 5 barg inlet pressure, the separation mechanism between N2 and O2 will decrease and lose its economic efficiency as gas membrane separation technology is a pressure driven technology.

  • Considering compressor size

By operating the membrane under varying parameters, always keep the maximum compressor size also into consideration that secures enough air available in order to produce a certain amount of N2.


Before the membranes will be integrated into an N2 generator, they may have been in storage for some time. Thus, the fibers may lay close together and they may need to be spread again at first to achieve full separation performance. The fibers will be spread just by operating them with pressurized air at normal operating conditions (pay attention to correct prefiltration and avoid at all water condensation!). 

For first commissioning, the separation performance of the membrane - under normal operating conditions - will reach target performance as specified in the certificate of analysis (COA) of the individual membrane within max. 24 hours from the initial start-up. 
After a longer standby time, the membrane will reach full performance within 12 hours. From our experience, the time it takes to reach full separation performance will often be much shorter than this. 

An important prerequisite for reaching design performance as fast as possible is that process parameters (i.e., temperature, flowrate, etc.) reach design values as fast as possible in strict coherence with the specifications stated in the SEPURAN® user manual for the individual SEPURAN® membrane product.

For SEPURAN® N2 Membranes, we guarantee N2 purities from 95 to 99.5 % N2. 
Using SEPURAN® N2 membranes, it is generally technically possible to reach 99.9 % purity. 

The limiting factor for N2 purity is the low concentration of oxygen in the bore side gas. The lower the oxygen concentration gets in the bore side gas, the less driving force (i.e. partial pressure difference between bore side gas and shell side gas) for further mass transport is available and thus, less oxygen will permeate through the fibers.

This is a natural limitation inherent in all membrane gas separation processes.
For concentration above 99.9% inert gas purity, other separation technologies can be more advantageous compared to membrane gas separation technologies.

The lower the remaining oxygen concentration, the more sensitive the membrane separation system gets towards minor changes in oxygen concentration. The consequence is that minor changes in oxygen concentration can induce large variations in required feed air. For remaining oxygen concentrations of 0.1% and below, the separation process is strongly dependent on accuracy of measurement equipment and process parameter control system. We strongly recommend to our customers to choose high quality measurement equipment as well as high quality process parameter control systems.