How Filters Work: Mechanisms of Filtration Part 2
June 28, 2022
In the previous blog, we explored the principles of direct interception and the interplay between pore sizes, particle dimensions and process parameters on the performance of a simple filter. While this removal mechanism dominates the way in which many filters function within the biotech industry, it is not the only one and there are significant, and very useful exceptions. One of those exceptions is inertial impaction.
Imagine a warm summer’s day. The sun is shining, there is a cool breeze. Insects are making the most of the warm air and sunshine, as are you. If you decide to take the car out for a spin it is not long before you begin to observe insect-based debris. The observant may take note that the faster the car travels, the greater the number of impacts on the screen, others may also have noticed that some insects, such as butterflies are less likely to impact the screen than other species, like beetles. If we think of the car as a small part of a filtration matrix and the insects as particulates in a process fluid (air in this instance), we are starting to build a picture of inertial impaction as a removal mechanism.
Particles have mass. Moving particles have inertia and the faster they move the more inertia they have. A force is required to change their direction. In the car windscreen example, this force comes from the air movements as the air flows over the car when it is moving. Heavy particles, such as beetles, require more force to change their path than lighter particles, and smaller particles receive less force from the surrounding air than large particles. Light, large particles, such as butterflies change direction more easily than small, heavy particles such as beetles. Under the right circumstance they can both impact the windscreen, leaving part or all of themselves attached to the windscreen until being cleaned off.
Imagine a small particle passing through a filter matrix. The particle is smaller than the smallest pore and is too small to be removed by direct interception. As it passes through the filtration media it follows a tortuous path with many tight turns and many changes of direction. If these particles are moving fast enough their inertia becomes too great to flow around these corners and they become permanently embedded in the walls of the filtration matrix. This implies that smaller particles are more easily removed by filtration membranes that have a longer more tortuous path. This is especially applicable with filtration of gases. Inertial impact also occurs in liquid applications, however, the force exerted by the liquid on the particle as it changes direction is far greater than in gas applications. We can see this in action when watching a river in spate. When there is a sudden flood, the majority of floating debris is carried around a bend in the river but some debris close to the bank of the river, embeds itself in the outside radius of the bend.
The filtration mechanism of direct interception, as described in the previous blog, remains constant in both gas and liquid applications. It is true to say, however, that any complex filter matrix is capable of removing significantly more smaller particles in a gas application, than the same filter matrix in a liquid application. As a rule of thumb, the size rating of a filter can be as much as 10 times smaller in gas applications than liquid. Much of this difference between gas and liquid filtration is due to inertial impaction.
In the next blog, we will look at another removal mechanism that is also highly active in gas applications – diffusional interception.
Mark Ayles, Senior Marketing Manager
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