In the biopharmaceutical industry, filters are primarily used to remove impurities from filtration media or achieve sterilization, thereby ensuring the safe and smooth operation of production processes. Depending on the filtration medium, filters are categorized into gas filters and liquid filters. In the previous session of Alioth’s “Data in the ‘Core’” series, we explored the working principles of liquid filtration. Today, we will delve deeper into the retention mechanisms of gas filters.
How Do Gas Filters Work?
The retention mechanisms of gas filters are more complex than those of liquid filters and can be divided into:
1. Size exclusion: Removal of large particles.
2. Other mechanisms: Removal of small particles.
Size exclusion is straightforward to understand, so we will focus on how small particles are removed. These mechanisms typically include diffusion interception, inertial impaction, gravitational sedimentation, and electrostatic attraction.
Diffusion Interception
Brownian motion causes random particle movement, increasing the likelihood of contact between particles and the filter membrane, thereby enhancing retention probability. Smaller particles exhibit more intense Brownian motion, resulting in higher retention efficiency.
Inertial Impaction
The internal structure of the filter membrane is irregular. As gas passes through, particle inertia causes collisions with the membrane, increasing retention chances. Larger particles experience stronger inertial effects, leading to higher efficiency. For sterilizing-grade gas filters, the small pore size ensures most large particles are retained via size exclusion, making inertial impaction less significant. However, for pre-filters with larger pores, inertial impaction plays a substantial role.
Gravitational Sedimentation
This mechanism functions similarly to inertial impaction and is more effective for larger particles.
Electrostatic Attraction
Depending on the properties of particles and the filter membrane, intermolecular forces may retain particles near or in contact with the membrane. Even under pressure pulses, these particles remain captured, as the binding force from intermolecular interactions is several times stronger than the drag force from airflow. Efficiency depends on two factors: gas humidity and flow velocity. Drier gases carry higher charges, while flow velocity must ensure sufficient contact time between particles and the membrane.
Key Concept: MPPS
Introducing MPPS (Most Penetrating Particle Size), which refers to the particle size most likely to bypass retention under the combined effects of the above mechanisms. Net efficiency can be derived as a function of MPPS and air velocity.
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