Tag Archive | sorbent

Should you buy an air Cleaner?

The first rule to improve your indoor air quality is to ventilate in order to recycle the fresh air. But what if the outdoor air is really bad? Or the climate extreme ? If you live in cold Norway, hot and humid Singapore or foggy Beijing, you may be considering to buy an air cleaner (also called air purifier). In Asia, air cleaners are becoming a common home appliance that people buy, like they would buy vacuum cleaners. Many technical words are used to describe working processes of air cleaners: electrostatic precipitators, HEPA filters, ionizers, sorbents, plasma, UVPCO, etc.

This post is aimed at helping you to see clearer and understanding the basic principles of these different technologies, as well as their potential risks. Be aware that there is no proper certification for air cleaners. So consumers are left with no choice but to believe the claims of air cleaner manufacturers. Indoor air pollution is not visible, which make it easy for companies to claim efficiency, even when the device provides no significant improvement. Finally, there is no requirement to test side-effects, such as ozone emissions. So consumers need to check the technical parameters to ensure a safety.

Most air cleaners combine different technologies. There are three major technologies commonly used to remove particles:

  1. Mechanical filters
  2. Electrostatic precipitators
  3. Ionizers

and three main technologies targeting gas pollutants

  1. UVPCO
  2. Sorbent
  3. Plasma System

I will  describe technologies in 2 different posts: Removing gas pollutants; let’s talk Tech!  and Removing PM: let’s talk Tech!

A good start to get a basic understanding of air cleaning technologies is the EPA summary on Residential Air Cleaners.

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Removing gas pollutants; let’s talk Tech!

Three main technologies are used to remove gaseous pollutants. They have different operating principles, energy requirements, potential side effects and efficiencies. Below is a basic description of these technologies.

Photo catalytic oxidation (PCO)

How does it work?

PCO technology uses ultraviolet light (UV) on a catalytic surface (often titanium oxide TiO2). It triggers the formation of  highly reactive species (hydroxyl radicals, ions, ozone,etc. ), which enable VOCs to be decomposed into CO2 and water.

Are there potential side effects?

UVPCO generates ozone, which is harmful in itself (increasing the risk of respiratory diseases) and may also react with other organic compounds to form harmful by-products such as formaldehyde. Alcohols can also create poisoning of the catalyst and cause incomplete oxidation leading to production of aldehydes or unwanted species.

Sorbents

How does it work?

Solid sorbents are materials with large internal surface, enabling electrostatic interaction between the gas molecules and a surface. Some examples include activated charcoal, silica gel, activated alumina, lithium chloride, zeolites to porous clay minerals. Gaseous pollutants removed by physical sorbents include ozone, nitrogen dioxide or VOCs (except low molecular weight carbon compounds such as formaldehyde and ammonia).

For chemisorption gas filters, the working principle is similar but in addition, a bond-forming chemical reaction between the adsorbed gas pollutant molecule and the adsorbing surface occurs, involving electron transfer. One common chemisorbent is potassium permanganate, an active oxidating reagent. It  can convert formaldehyde into water and carbon dioxide. The efficiency of the sorbent, measured via the rate of adsorption, decreases with the amount of pollutants captured. Similarly to mechanical air filters, sorbents require energy to pull the air through.

Are there potential side effects?

Carbon filters have high biocompatibility and microorganisms may multiply on filters.

Plasma cluster ions 

How does it work?

One type of ion generator is called cluster generators or plasma generator (the term DBD or Dielectric Barrier Discharge may also be used). It uses AC discharges to produce clouds of positive and negative ions successively. Electrons are separated from oxygen molecules and the electrons are combined with the oxygen molecules to produce oxygen cations and anions. The oxygen anions react with water in the air and produce reactive species, which then agglomerate and form ion clusters. Ion clusters can damage the surface of airborne microbial and oxidize gaseous volatile organic compounds and PM.

Are there potential side effects?

Ion generators may generate ozone and undesired by-products.  Plasma systems tend to be very popular and displace UVPCO systems although they use a lot more energy.