There are many filters installed at compressor outlets that remove oil, water or dust from compressed air or other gases. However, they have no effect on microorganism retention. To be used in processes requiring an absolute sterility level, these gases must be treated using dedicated filters.
Many industrial processes require sterile air (or other gases), including fermentation, inerting, filling, storage, and more.
Operating conditions for these gases vary depending on their use. Whether the filter is installed on a pressurised line or used as a vent on a tank, sterile gas filtration solutions must be selected and adapted to each specific case.
Filtration mechanisms in gases
Using depth filtration techniques to filter air or gases involves three main filtration mechanisms:
Direct interception
During direct interception, particles collide with the surface of the filtration media and become trapped.
- Inertial impaction
As particles travel through the tortuous channels of the filtration media, they lose energy and eventually “settle” within the filter matrix.
- Diffusion or Brownian motion
Results from the random movement of particles in the air. Particles have a high probability of colliding with each other or with the filtration media, losing energy and becoming retained within the filter matrix.
These three classical mechanisms combine in gas filtration. In this context, the particles known as MPPS (Most Penetrating Particle Size) range from 0.1 to 0.3 microns. In sterile gas applications, industrial and scientific studies have demonstrated that microorganism retention is achieved using absolute 0.01 µm filters. However, due to Brownian motion, the same filter is significantly more efficient for submicron particles in the gas phase than in the liquid phase.
Which filters should be used on a sterile gas production line?
Importance of prefiltration
In addition to particulate and bacterial aspects, it is essential that the gas is dry and oil-free. If not, the use of coalescing filters is indispensable, installed well upstream of the process. These must be sized according to the operating flow rate, as well as the known or expected water and/or oil content in the gas.
Once this pretreatment has been carried out, particulate prefiltration (solid–gas separation) with an appropriate efficiency level is also necessary to protect the final sterilising filter media. For example, with typical gas velocities in connection pipework often exceeding 20 m/s, any particle such as scale originating from ageing air and steam distribution systems can have catastrophic consequences on sterilisation membranes.
It is therefore preferable to incorporate a depth filter (FDA-grade polypropylene is suitable) to provide adequate protection for the final sterilisation filter. These filters offer excellent combinations of impurity retention capacity and high flow rates to ensure that system performance is not compromised.
Final sterile filtration
Depending on the industry and operating constraints — for example, in-line filter integrity testing, required flow rates, operating cycle duration, etc. — the appropriate media and geometry must be selected.
In all cases, and to ensure effective blocking of any residual liquid microdroplets in the gas, hydrophobic and oleophobic media must always be used.
Borosilicate media impregnated with PTFE
(Material compliant with FDA CFR 21)
Manufactured using microfibres, these media can handle at least twice the flow rate of PTFE membrane filters at the same pressure drop.
Due to their high flow capacity, they are ideal for venting applications on non-vacuum tanks and for low-pressure sterile coverage on aseptic filling machines, particularly in the food and dairy industries.
PTFE media
(Material compliant with FDA CFR 21)
Mechanically more robust, these media can withstand a greater number of sterilisation cycles and allow integrity testing directly in place or in the laboratory.
Filters using these media are the preferred solution for all critical processes (pharmaceutical, biotechnological, etc.).
A way to improve flow rates without modifying installations or optimise energy costs related to gas production
One factor that generates pressure drop (at constant flow rate) — or conversely, limits flow capacity — is the change in cross-sectional area.
By using filter cartridges with larger internal passage areas than standard designs, it becomes possible to:
- Increase the gas production flow rate without modifying existing installations and without increasing pressure drop,
- Or, at the same flow rate, use less filtration surface (e.g. a 20″ filter cartridge instead of a 30″) without compromising operating conditions, while optimising associated costs,
- Or finally — particularly in the case of compressed air — significantly reduce the energy demand of the equipment required for gas production (such as compressors), or free up capacity (e.g. for production line expansions).
To illustrate this point, the graph below shows initial pressure drop curves for borosilicate media impregnated with PTFE (with comparable results for PTFE membranes) in:
Version with increased central passage.
Conventional version,
The impact is clearly significant:
At a 100 mbar pressure drop, the service flow rate almost doubles,
And typically, at the same flow rate, pressure drop decreases by 50%.
Careful consideration required regarding the filtration housings used!
Aside from their size, the living nature of microorganisms means they are capable of multiplying very rapidly under favourable conditions.
The requirements for filter elements must go hand in hand with those relating to filtration housings.
To achieve the expected performance, it is essential to ensure that:
The design promotes smooth airflow or gas flow without turbulence towards the filter element, to minimise pressure drop both at the filtration housing and the filter element.
The materials used are of high quality. Unless otherwise specified, 316L stainless steel filtration housings are commonly employed,
Internal surface finish is perfectly controlled. Ra (roughness coefficient) must be certified below 0.4 µm (or 0.8 µm, depending on the application),
The design has no ledges, corners or any potential retention areas,
Contact a filtration expert
We are at your service
Our experts place their knowledge and expertise at your disposal for liquid filtration, air and gas filtration, decontamination, and separation technologies.
