Environmental Remediation Terms & FAQs
What are Volatile Organic
Compounds (VOC's)?
Volatile Organic Compounds (VOC's) are a vapor form of hydrocarbons
that contribute to air pollution. These hydrocarbon compounds
can enter the atmosphere, and when exposed to sunlight, chemically
react with elements in the air to produce what is commonly
referred to as smog.
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What is a Catalyst?
A catalyst is a substance that increases the rate of a chemical
reaction without being consumed in the process.
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What is Oxidation?
Oxidation is a process that causes compounds to break apart
and reform into new compounds.
| The formula is: Cn CzHy + (z+y/4)O2 |
 |
(z)CO2 + (y/2)H2O |
The most effective way to neutralize VOCs is through thermal
or catalytic oxidation. On a very basic level, this involves
converting the molecules in the VOCs into harmless compounds
(carbon dioxide and water vapor) which can then be discharged
into the atmosphere.
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What is LEL?
It is important to understand the meaning of the term Lower
Explosive Limit (LEL), sometimes also referred to as Lower
Flammability Limit (LFL).
Lower Explosive Limit: Gases or vapors which form flammable
mixtures with air or oxygen have a minimum concentration
of vapor in air or oxygen below which propagation of flame
does not occur on contact with a source of ignition (LEL).
There is also a maximum proportion of vapor or gas in air
above which propagation of flame does not occur (UFL). These
boundary line mixtures of vapor or gas with air, which if
ignited will just propagate flame, are known as the "lower
and upper flammable or explosive limits", and are usually
expressed in terms of percentage by volume of gas or vapor
in air.
The LEL is based upon location specific normal atmospheric
temperatures and pressures. The general effect of an increase
of temperature or pressure is to decrease the lower limit
and increase the upper limit.
Applicable codes require thermal solvent processing systems
to operate no higher than 25% LEL without an LEL monitor
and control. Insurance companies may require LEL systems
if an oxidation system is added to existing machinery.
For further information concerning the maintenance of safe
LEL levels, please refer to NFPA Bulletin 86A and FM Loss
Prevention Bulletin No. 14.15. These bulletins will delineate
how safety interlocks can be implemented in the operator's
system.
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What is Thermal Oxidation?
Thermal oxidation uses high temperatures to heat the contaminated
air, causing the molecules in the VOCs to break apart and
yield carbon dioxide (CO2) and water (H2O). The formula is:
| HC + O2 |
|
H2O + CO2 |
1400-1800°F |
Key advantages of thermal oxidation:
· No risk of catalyst poisons
· Can handle higher LEL fume streams
· High temperature refractory reduces manufacturing
costs
· Extended residence time for higher destruction efficiencies
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What is Catalytic
Oxidation?
Catalytic oxidation is a chemical oxidation process in which
hydrocarbons (HC) are combined with oxygen at specific temperatures
to yield carbon dioxide (CO2) and water (H2O). The formula
is:
| HC + O2 |
|
H2O + CO2 |
400-800°F |
As its name suggests, catalytic oxidation uses a catalyst,
a substance that accelerates the rate of a chemical reaction
without itself being consumed. The catalyst allows the oxidation
process to occur at a lower temperature than is required
for thermal oxidation.
Key advantages of catalytic oxidation are:
- Much lower operating temperatures
- Longer heat exchange life (less stress because of lower
operating temperatures)
- Lower operating costs
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What is Catalyst
Poisoning?
In the presence of contaminates, the catalyst active sites
can become blinded and as a result, the catalyst surface
area and destruction efficiency is reduced.
The following partial list of poisoning agents and inhibitors
has been found to have a detrimental effect on the activity
of the noble metal catalyst.
Coating Agents
- rust
- dirt
- inorganic oxide
|
Covers catalyst active site |
Non-phosphate detergent washing usually
effective for removal.
|
Glass Forming
Coating Agents
- organic silicates (esters)
- silicones
- phosphorus containing materials
|
Covers catalyst active
site |
Factory reactivation
or replacement usually required. Non-phosphate detergent
washing may be effective. |
Poisons - Heavy Metal Complexes
- Mercury
- Lead
- Zinc
- Tin
- Arsenic
- Antimony, etc.
|
Permanent catalyst deactivation |
Factory replacement required |
| Sulfides |
Permanent catalyst
deactivation |
Depending on exposure
and sulfide concentration, factory reactivation, non-phosphate
detergent washing or replacement is required. |
Halogens
- fluorine
- chlorine
- bromine
- iodine
- halogenated hydrocarbons
|
Covers active site-resulting in temporary
or permanent deactivation |
Activity usually returns if exposed
to low concentrations (<10 ppmv) and upon removal
of halogen source. Prolonged exposure with water
(or protons) can corrode, dissolve the catalyst substrate
and require repair or replacement.
Note: Does not apply to chlorinated or fluorinated
catalysts which have been specifically designed to
be tolerant of and/or destroy halogenated hydrocarbons. |
| Organic Droplets
and Aerosols |
Covers active site.
Possible cause of catalyst hot spot |
Such materials may
carburize on the catalyst forming a refractory material
or become a hot spot source causing substrate deterioration.
Factory reactivation or replacement is required. |
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Will a Total Petroleum Hydrocarbon
(TPH) Catalyst be Affected by Trace Amounts of Chlorinated
Compounds in the air stream and will it affect the destruction
efficiency of the TPH?
As long as the chlorinated compound total does not exceed
10 ppmv the catalyst will perform as designed and you should
not see a reduction in destruction efficiency of the TPH.
The majority of the chlorinated compounds will pass through
untreated and will exit the discharge stack.
What happens to a TPH catalyst in the presence of chlorinated
compounds above 10 ppmv is that the chlorinated compounds
start to occupy active catalyst sites and the chlorinated
compounds slowly diffuse breaking down into inorganic acids,
which attack the silica substrate of the catalyst. As this
happens the catalyst looses geometric surface area. As a
result the destruction efficiency will start to decline.
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