NAFA Guide To Air Filtration Fifth Edition Chapter 11 Airborne Molecular Contaminants presented by the National Air Filtration Association

Agenda Contaminant Control Removal Options Media Types Definitions Adsorption Chemisorption Catalysis Media Types Definitions Filter Options Here’s what we are going to cover in this chapter

Control Options Source Control Ventilation Removal Control Remove the contaminant source Ventilation Increase outdoor air Removal Control Remove contaminants via physical or chemical means There are three main ways to control molecular contaminants and they are the same as particulate contaminants – source control, Ventilation or dilution, and Removal control.

Molecular Filtration TYPES Physical - Adsorption Activated carbons Chemical - Chemisorption Potassium permanganate impregnated media Chemically treated activated carbons Catalysis The two main types of removal control include medias that utilize physical adsorption or chemical reaction, or a combination of these two with a catalyst

Types Adsorption - The process by which one substance is attracted and held onto the surface of another. Surface phenomena. Capacity is independent of particle size Adsorption rate is inversely proportional to particle size. High temperatures, may cause desorption. Humidity has adverse effect The contaminant being removed is called the adsorbate Adsorption is…

Types Chemisorption - The result of chemical reactions on and in the surface of the adsorbent. Fairly specific and depends upon chemical nature of media and the contaminant Irreversible & essentially instantaneous Oxidation - changes harmful gases to harmless solids Higher temperatures will increase the reaction rate in chemisorption Humidity is favorable toward the reaction

Types Catalysis Changing the rate of a chemical reaction, (usually increasing) where the catalyst is not consumed in the process Carbon impregnated with various metals can aid in the reaction of a product difficult to adsorb.

Media Overview Activated Carbon Chemically Treated Carbon adsorption may be reactivated Chemically Treated Carbon chemisorption Specific to a compound or family of compounds not reversible Potassium Permanganate broad based oxidizer

Definitions RESIDENCE TIME The time it takes air to cross a distance equal to the thickness of the filter without accounting for the resistance of the media through which it travels.

REMOVAL EFFICIENCY CAPACITY Definitions The fraction of the contaminant that, once in contact with the media, is removed by either physical or chemical means. CAPACITY The amount of contaminant the media is capable of removing. This determines the life of the filter.

Filter Options Partial bypass Total Retention easy to retrofit lower cost low efficiency low media weights shorter life Disposable Total Retention packed bed systems higher cost high efficiency high media weight longer life may be recycled depending upon media

Sample Questions Two main methods for molecular contaminant removal are? Adsorption and Chemisorption Humidity can hinder? Adsorption Particle bypass cells allow for retrofit without? Large pressure increases in the system

Thank you. Other Questions?