Abstract
Air filtration, as is commonly understood, is often what we read in advertisements of air-conditioners or what we read in newspapers and journals about the poor quality of air around us, which is mainly filtration of particulate contaminants.
This article, however, will discuss various aspects of air filtration at a molecular level, i.e. filtration/removal of gaseous contaminants. Air filtration at a molecular level is often also referred to as Gas Phase Filtration.
Today, when technologies and equipment, especially in the mission critical facilities require a clean environment to function at their optimum, molecular phase filtration and its understanding is essential.
This article will also discuss the concept, need, technology trends and equipment for molecular phase filtration.
First, let us understand the basics :
Molecular Phase Filtration – Filtration of gaseous contamination having size at the molecular scale.
Particulate Air Filtration – Filtration process that removes solid particulates from the air.
US ASHRAE standard 52.2 classifies particulate filtration as
The above classification caters more to various domestic, commercial and industrial needs. However, for various clean rooms and super clean rooms, we need to install
While HEPA filters can control contamination up to 0.3 micron (µm), ULPA can even control up to 0.12 micron (µm).
Various filters described above do take care of unwanted particulate contamination in the air. However, it is a very big challenge to control contamination of any matter smaller than particulate size of 0.12 micron (µm) and physically arrest them.
In depth knowledge of adsorption by desiccants help us to control contamination in such critical applications. Adsorption by various desiccants of matters (gases) having molecular diameter fraction of 0.1 micron (µm) in their micro/meso pores (diameter between 0.2 to 1.0 nm (nano meter)) is the starting point of filtration of gaseous contaminants.
Why do we need molecular filtration at such levels (less than 0.12 microns)?
Many of the unwanted gases which contaminate and can cause serious damage resulting in huge losses are either
These gaseous contaminants are potentially very harmful to humans as well to equipment, especially in an environment of controlled areas housing sensitive equipment like servers in data centers, etc.
As shown in Table below, gases typically have molecular diameter in the range of 0.0002 micron to 0.001 micron (µm). Unit used for measurement is Angstrom (Å) (1 micron = 10,000 Angstrom)
S. No. |
Unwanted Gases |
Molecular Diameter |
MW
|
Nature of Gas |
Sources of gases in Urban Areas |
Sources of gases in Industrial Areas |
|
Symbol
|
in Angstrom (Å) |
Odorous |
Corrosive |
||||
1 | H2S
(Hydrogen Sulfide) |
2.6 |
34.08 | Yes |
Yes |
|
|
2 | SO2 (Sulfur Dioxide) |
2.8 |
64.07 | Yes |
Yes |
|
|
3 | SO3 (Sulfur Trioxide) |
2.8 |
80.07 | Yes |
Yes |
||
4 | HF (Hydrogen Fluoride) |
1.8 |
20.01 | No |
Yes |
|
|
5 | NO2 (Nitrogen Dioxide) |
2.3 | 46.01 | Yes | Yes |
|
|
6 | NH3
(Ammonia) |
2.6 | 17.03 | Yes | Yes |
|
|
7 | Cl2
(Chlorine) |
3.2 | 70.9 | Yes | Yes |
|
|
8 | O3 (Ozone) |
2.6 | 48 | No | Yes |
|
|
9 | HCl (Hydrogen Chloride) |
3.2 | 36.47 | Yes | Yes |
|
|
10 | CnHn/VOC (Hydrocarbons/ Volatile Organic Compounds) |
4-4.9 | Yes | Yes |
|
|
This article is going to give more insight into the adverse effects of corrosive gases and potential filtration options currently available in the market.
How to remove corrosive gases?
As shown in figure 3, the process of filtration through adsorption and neutralization through chemical reaction is commonly known as Chemisorption. The air filtration systems remove corrosive gases through the process of adsorption and neutralization.
Adsorption
Chemisorption
Adsorption with Chemical Neutralization/Oxidation
What are the major elements to remove corrosive gases?
Evolution of Gas Phase Filtration Technologies
The above figure traces how the various types of filters have evolved from carbon media to honeycomb chemical filters.
How to classify the reactive environments?
International Society of Automation (ISA) defined severity levels on account of unwanted gases in instrumentation and control rooms way back in 1985. Keeping in view implementation of ROHS (Restriction Of use of Hazardous Substances) under the directive from EU (European Union) as per 2002/95/EC replacing lead (being carcinogenic) by silver and electronic circuits getting further miniaturized has led to ISA revising 1985 standard in 2013 which is as per below table below:
Class |
Severity Level |
Angstroms (Å) per 30 days |
Comments |
|
Copper |
Silver |
|||
G1 | Mild | <300 Å | <200 Å | Corrosion is NOT a factor in electronic equipment reliability |
G2 | Moderate | 300 Å – 999 Å | Effect of corrosion is measurable and may be a factor in electronic equipment reliability | |
G3 | Harsh | 1,000 Å – 1,999 Å | High probability that corrosive attacks will occur. Should prompt further evaluation and result in environmental controls | |
GX | Severe | > 2000 Å | Only specially designed and packaged equipment is expected to survive |
The ISA standard also defines in terms of gaseous concentration levels as per below table below:
Concentration of Gases (in ppb) as per ISA – 71.04-2013 |
||||
Contaminants |
G-1 (Mild) | G2 (Moderate) | G3 (Harsh) |
GX (Severe) |
H2S | <3 | <10 | <50 | >50 |
SO2 | <10 | <100 | <300 | >300 |
Cl2 | <1 | <2 | <10 | >10 |
NOX | <50 | <125 | <1250 | >1250 |
HF | <1 | <2 | <10 | >10 |
NH3 | <500 | <10000 | <25000 | >25000 |
O3 | <2 | <25 | <100 | >100 |
How to measure severity levels in corrosive environments?
Typically, there are two types of measurement methods as below:
Corrosion classification coupons have two pure metal strips of silver and copper.
These coupons are placed in the room where environment severity has to be measured for a period of 30 days.
The thickness of the layer of corrosion which forms on metal strips determines the “severity level” as per ISA 71.04-2013 standard.
These instruments help to access severity levels on real time basis. The real time measurements in typically 24 hours are extrapolated for 30 days to know the severity levels as per ISA standard.
In addition to severity levels due to airborne gaseous contaminants these instruments do also measure room temperature, RH and optionally, the differential pressures, to give the complete corrosion parameters.
Real time atmospheric corrosion monitors can be further classified in two technologies:
* One is based on Quartz Crystal Microbalance (QCM), which measures the rate of increase of corroded metal sensors mass
* The other determines the rate of electrical resistance increase of corroded metal strips
What are the types of equipment available for removing corrosive gases?
Equipment for removal of unwanted corrosive gases are broadly classified as under:
The above two concept categories can involve both types of chemical filters:
Deep bed and thin bed
Internal view of the system
Working principle of Deep bed and thin bed
How to install the equipment?
Gas Phase filtration systems are typically installed in three ways
Re-circulation option system used for server/data center application
Pressurization option system used for cleaning fresh air
Re-circulation + pressurization system type
What are the precautions to be taken for ensuring proper filtration?
Conclusion
Server rooms, data centers and mobile/base switching centers are mushrooming in urban areas. Knowledge of filtration at molecular level (more commonly referred as Gas Phase Filtration System) is helping in keeping such facilities with minimum downtime.
With the increase in automation in process industries, need of protecting their control rooms against corrosion from unwanted gases using Gas Phase Filtration has become the need of the hour.
We hope this article helps in better understanding of basics of air filtration at molecular level and need for gas phase filtration. This article, however, only gives the over view of the dynamics involved.