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Adsorptive Filtration: Chemisorption & Physisorption


Baking powder has been used by generations to remove undesirable odors from refrigerators – and various powders are used routinely on smelly sneakers in hopes of accomplishing the same. If your water source contains a high amount of Sulfur or Phosphorus, then you more than likely appreciate the value of an activated carbon water filter. So you might ask, “What do all of these scenarios have in common?” Well, it turns out that in each of these purification processes, there’s an adsorption process in action that of course has its roots in material science.

Hi, my name is Ken Milam. I’m an application engineer here at ThermoPore. Welcome to Thermo.TV. In this series, we’ll be discussing Adsorptive or Chemisorptive Filtration, also referred to “Gas Phase Filtration” when the filtration is specific to contaminants that are in a gas phase.

Our discussion will involve three parts. First, we’ll discuss the filtration mechanism that enables the capture of these often times molecular sized particles. Then, well discuss ThermoPore’s PolyMesh™ Chemisorptive filtration media and how it has been designed to excel in these challenging applications. Lastly, we’ll round out our discussion by comparing PolyMesh Chemisorptive filtration media to several other similar, yet different, technologies that are also available on the market.

Okay, so let’s get started. I think it’s first appropriate to differentiate two types of Chemisorptive Filtration from one another: adsorptive from absorptive filtration. Adsorption, spelled with an “Ad”, is the addition or accumulation of molecules on the surface of a material. This process creates a film of the adsorbate (the molecules being accumulated) on the adsorbent's surface. Now this is the basis for our discussion today so let me say it again. Adsorption is the addition of adsorbate particles onto the surface of an adsorbent material. Adsorption is different from absorption (spelled with an “Ab”). Absorption involves the diffusion of a one component into another; absorption goes beyond the sorbents surface layer. For today, however, we’ll only discuss adsorption, which again, is limited to surface interactions.

As you might recall from our earlier filtration video titled “Most Penetrating Particle Size”, diffusion tends to be the primary filtration capture mechanism for very small particles. Diffusion is influenced by a particle's Brownian motion. Brownian motion, as you might recall, inhibits a small particle, like a molecule, from traveling in a straight line. Fortunately for us, diffusion makes very small particles more susceptible to capture because it increases the chances that the small particle will come into contact with our filter media, or our adsorbent.

So once these molecules come into contact with our adsorbent, what happens? Well, it turn out that there are two sub classes of adsorption: phyisorption and chemisorption. These terms might sound intimidating, but hang in there. Their definition is actually rather straight forward and our discussion will quickly come into focus once these subclasses are defined. Phyisorption relies on attractive forces between the adsorbent (in our case, a filter component or additive) and the adsorbate (those particles or molecules that are being captured or filtered). Once in contact with one another, the adsorbates remain in contact with the adsorbent (the filter media) due to van der Waal forces. Van der Waal forces are relatively weak attractive forces. Nonetheless, they do enable an adsorbent to retain particles, and actually build layers of particles onto its surface.

Vander waal forces are more common than you might expect. They are the forces that keep dirt particles stuck to the fibers of your clothes, and they’re also the forces that keep cigarette smoke particles stuck to your hair or skin. Fortunately, these attractive forces are reversible, and adsorbates can be removed from the adsorbent. Phyisorption is the mechanism responsible for the capture of solvents, dispersed oil particles, hydrocarbons, and silicones. Okay, so that’s phyisorption.

Chemisorption also relies on diffusion and Brownian motion to bring an adsorbate into contact with an adsorbent. However, once these two items are in contact with one another, there is an actual chemical reaction that takes place between the adsorbent media and the adsorbate. This reaction creates a strong chemical bond between the two which is typically irreversible.

Unlike phyisorption that allow layers of adsorbates to form on adsorbent, chemisorption reactions take place in a single layer of the adsorbent. In other words, there are a limited number of reaction sites that facilitate chemisorption, and those sites are typically proportional to an adsorbent’s surface area.

When a porous adsorbent, like activated carbon is used, the adsorbates rarely can penetrate into the depth of the carbon particle, which means that an adsorbent’s efficiency is usually highly influenced by its available surface area. Now this is important, so let me repeat this point. An adsorbent’s chemisorption efficacy is highly influenced by its available surface area. In other words, more surface area means more bonding sites.

So with these facts in mind, let me ask you, “What features would you look for in a chemisorptive filter media to maximize the media’s ability to remove an unwanted gas phase contaminant?” You might answer this way. “I want a material that first and foremost maximizes capture via diffusion. By maximizing a filter's diffusion property, I maximize the filter’s ability to capture small particles. Then, I want to maximize the adsorbent’s surface area. By maximizing the adsorbent’s surface area, I maximize 1) the area that can be used in a physisorptive capacity, and 2) the area where chemisorptive reactions can occur. By simultaneously increasing particle capture via diffusion and the adsorbent's surface area, I maximize the filter’s ability to remove unwanted contaminates via adsorptive filtration, or its two sub classes, phyisorption and chemisorption.”

Wow! That’s a great answer, and I agree. So let’s look at how ThermoPore’s PolyMesh media has been engineered to deliver the same properties. ThermoPore’s PolyMesh Adsorptive filtration media is comprised of two outer scrims that function as a wrapper for the material’s internal adsorptive ingredients. In this case, the outer scrim is composed of a polyester non-woven material. Understand also that we have design and manufacturing flexibility to use any number of materials as the outer scrim.

Inside the scrim, you’ll notice our adsorbent material, in this case, activated carbon. We mentioned before the importance of maintaining active surface area on the adsorbent, and this is really where the PolyMesh Adsorptive media shines brightest. Through the patent use of a fibrous binder matrix, the PolyMesh media maximizes available surface area, as can be seen in this SEM image.

As we discussed earlier, active area plays to key role in performance, and notice how the use of a fiber binder maximizes the useful area of the adsorbent (activated carbon in this case). Not only that, but the fiber binder enhances diffusion capture by creating a tortuous path that the adsorbates must navigate during their travel through the PolyMesh matrix. Need even more adsorptive capacity? No problem, with the PolyMesh media, adsorbent loadings up to 1000 grams per square meter can be realized.

When you combine the way that PolyMesh maximizes available surface area with the design flexibility of various outer scrims, adsorbents, adsorbent weights, and converting options, you get best in class performance.

So what other competitive materials are on the market? Let’s take a look at two alternatives: sludge coated media and extruded media. Sludge coated media is essentially a non-woven substrate that has been submerged in a mixture of resin and activated carbon powder. The submerged substrate is then padded and dried in a curing oven. Typically, this material will have little dusting or particle shedding and a low differential pressure drop. This is a good feature. However, as is evident by the picture, the actual carbon content is low; therefore, the media suffers from poor adsorption efficiency.

A second competitive technology is an extruded carbon technology. The extrudate is actually an activated carbon matrix sandwiched between a top and bottom non-woven outer scrim. The carbon granules are secured in place with an adhesive or a binder. It’s a pretty good technology, but, as can be seen in this image, the binder often times occludes adsorbent surface area, and you should know by now what impact that can have on performance.

Company's requiring the best in class adsorption filtration efficacy, select ThermoPore’s PolyMesh Chemisorptive filtration media time and time again. I hope now, you know a little more about why that’s the case.

ThermoPore maintains a diversified portfolio of porous materials that can help you achieve any number of application involving filtering, wicking, diffusing, and venting functions and I hope that this tutorial has provided you with some insight into the types of variables that we’ll be able to tweak to satisfy the needs of your next chemisorptive filtration development project.

Stay on the look out for additional videos by signing up for our RSS feed and as always, if you have any additional questions or if there are some topics that you’d like to see added to the Thermo.TV channel, give us a call or drop us a line. For now, I’m Ken Milam saying thanks for watching this installment of Themro.TV – we’ll see you next time.

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