Sorbents for Indoor Air Cleaning: A Primer

Published 12/21/2021
By Udi Meirav
enVerid Sorbent Filter

With both indoor air quality and carbon emissions reduction a growing priority for building owners, managers, and tenants alike, there is an increased focus on the ability of HVAC systems to deliver these two priorities. In particular, there is a great deal of interest in air cleaning within HVAC systems, which has drawn attention to a well-established but underutilized technology – sorbents – as key to cleaning indoor air and reducing ventilation energy use.

Daikin’s recent announcement of the world’s first packaged total-air-quality system that combines the benefits of enVerid’s Sorbent Ventilation TechnologyTM (SVT) with class-leading, configurable airside systems to deliver superior air quality and cut energy use and carbon emissions has accelerated interest in sorbents.

Against this backdrop, I thought it would be helpful to share the answers to commonly asked questions about sorbents and the advancements that have made enVerid’s sorbents so uniquely effective at addressing indoor air quality and HVAC energy efficiency in buildings.

Q: What exactly are sorbents and how do they work?

A “sorbent” (a.k.a. adsorbent) is a material designed to adsorb large amounts of a particular gas. For decades, sorbents have been used in a wide range of applications including protective gear, industrial processes, food packaging (to control humidity), and air cleaning in submarines and space shuttles and now buildings. Activated carbon, silica gel, and zeolite are well known types of sorbents, but there are many others.

The capacity of sorbents is mostly a result of their highly porous structure. The microscopic “nooks and crannies” add up to an enormous amount of surface area, which increases the capacity to adsorb – in other words, to capture – gas molecules that come into contact with the material. One kilogram of activated carbon can have a surface area equivalent to a few hundred football fields. That’s a lot.

When a gas mixture – like air – comes into contact with a solid, there is a general tendency for some of the gas molecules to stick to the surface of that solid. That’s called adsorption. Almost all gases tend to adsorb, to some extent, with any solid. But only so much – there is a maximum “amount” of molecules that can be accommodated on the surface, after which no further adsorption is possible. Sometimes this is called “saturation.”

The actual amount that can be adsorbed depends on the type of (gas) molecule in question, as well as the type of solid doing the adsorption. It also depends on the temperature of the solid and the concentration of the particular molecules in the air. This is important as you will see in a moment.

enVerid sorbents selectively capture contaminants while allowing the components of healthy air to pass throughQ: What are the differences between sorbents?

There are important differences between sorbents in terms of their capacity and selectivity, meaning their ability to preferentially capture certain types of molecules – the contaminants or toxins – without adsorbing benign molecules. Selectivity is determined mostly by the molecular structure of the sorbent, and to some degree by the size of its pores. Selectivity is extremely important because sorbents have limited capacity and if they “use up” their limited capacity on common, benign air molecules (oxygen, nitrogen, argon, water) they will quickly saturate and become useless.

But selectivity can be a double-edged sword. For example, activated carbon can be effective for capturing benzene but not for carbon dioxide or formaldehyde, which makes it inadequate as a complete solution for improving indoor air quality (IAQ).

enVerid Sorbent Filter

Q: Are Are enVerid’s sorbents different from other sorbents? How so?

To address the limitations of existing sorbents like carbon and zeolite, enVerid developed a completely new sorbent material for commercial building IAQ applications. enVerid’s Sorbent Ventilation Technology or SVTTM is engineered to provide a highly efficient solution for capturing carbon dioxide (CO2) and formaldehyde (HCHO), the pillars of IAQ management. No other commercially available sorbent technology has this capability. But SVT’s capabilities go well beyond these two gases.

While CO2 and formaldehyde are the most common and problematic indoor contaminants, SVT ensures that all contaminants identified by ASHRAE Standard 62.1 (see pg. 106 of the User’s Manual) and by the USGBC’s performance-based indoor air quality design and assessment pilot credit are fully addressed through a combination of sorbent air cleaning and minimal outside air ventilation, allowing buildings to meet their IAQ goals with less outside air, which leads to energy savings.

Q: Why is formaldehyde adsorption such a big deal?

Formaldehyde is a common and highly toxic byproduct of manufactured items, and results from “off-gassing” of paint, carpets, laminates, and furniture. It is notoriously difficult to capture – most sorbents do not do a good job in removing formaldehyde from the air. This means that because of formaldehyde, carbon filters, despite their ability to treat various odors, do not actually make indoor air safer and are not well suited to enable reduction of outside air ventilation according to ASHRAE guidelines.

Q: What happens when a sorbent is saturated?

When a sorbent is saturated it can no longer capture airborne contaminants. There are two options at that point: Either (A) replace and discard the spent sorbent, or – if possible – (B) regenerate it. Regeneration means causing the sorbent to release the captured molecules, after which its surface is clean and ready to start working again. Usually that requires heat, and quite a bit of it. For this reason, sorbents that are deployed outside of a specialized facility are usually a use-and-dispose option.

Is that a showstopper? That depends. As long as the operating life of a sorbent is long enough, a limited use, replaceable sorbent can still be a great solution. For selectively removing small volume contaminants like formaldehyde, saturation times of months and even years are feasible. On the other hand, for removing CO2 – whose quantities are much larger – saturation time is measured in hours, and regular regeneration is the only way to go.

enVerid sorbents automatically regenerate inside the HVAC system at a low temperature

Q: Is on-site sorbent regeneration practical?

Regeneration of activated carbon in a HVAC system is not practical. The temperature and energy requirements are simply too high. The same is true for the vast majority of sorbents. One of the remarkable innovations of enVerid sorbents is their ability to regenerate at a modestly elevated temperature, which has enabled extended use of sorbent filters for CO2 capture.

This regeneration is essential for CO2 scrubbing, but not all buildings have to contend with excess CO2. That depends, among other things, on occupant density. When CO2 is not a concern, regeneration is needed less frequently or not at all. enVerid’s sorbent filters can be used in typical commercial building applications for two years, with or without regeneration depending on whether CO2 needs to be managed, before needing to be replaced.

For more information on sorbents and Sorbent Ventilation Technology, visit our SVT web page.

Udi Meirav

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