ENVIRONMENTAL
ENGINEERING
Chemical Engineering department
Hydrocarbons and volatile organic carbons (VOCs):
Organic gases are those that contain both hydrogen and carbon, but may also
contain other atoms; hydrocarbons
(HCs) are organic gases that contain only
hydrogen and carbon. Volatile organic compounds (VOCs) are no methane
hydrocarbons (NMHC) and oxygenated hydrocarbons (which are hydrocarbons plus
oxygenated functional groups),
Methane, CH4, is the most abundant hydrocarbon in the atmosphere, found in
exhaust gas from automobiles, biomass burning, and agriculture activities.
Anthropogenic sources: indoor sources (e.g., formaldehyde emission), fossil fuel
combustion, and evaporation of gasoline (e.g., petroleum refineries; during fueling
of cars), Natural sources: HCs produced from decomposition of organic matter;
emitted by certain types of plains (e.g., pine trees, creosote bushes) Effects: some
HCs are indoor pollutants some HCs and VOCs contribute to ozone-containing smog
Example: pine trees produce VOCs such terpenes responsible for the smell of pines,
unfortunately these pleasant VOCs contribute to ozone formation that harms trees.
Control and Treatment of VOC and Hydrocarbons
Control and treatment of VOC and organic hazardous air pollutant emissions
are generally accomplished by adsorption, incineration, condensation and gas
absorption. The methodology is usually chosen depending upon the temperature,
composition and volumetric flow rate of the emission stream, space constraints and
allowable installation and operational costs. A brief description of each method is
given below:
1. Adsorption:
This is one of the most commonly used methods, especially for controlling emissions
from small sources. It can be physical adsorption or chemisorptions. The later is
rarely used for the VOC emission control because, it involves a less-reversible
chemical bonding of the adsorb (pollutant) and the adsorbing solid (packing) and is
relatively expensive. Physical adsorption uses the Van der Waals force, giving the
advantage of reversibility and regeneration due to the weaker bonding of the gas and
adsorbent material. The adsorbed material can be either recovered or incinerated.
Regeneration is usually accomplished by heating or extraction/displacement.
Activated carbon is a commonly used adsorbent because of its high surface area and
material hardness. It has between 800 and 1200 m2/g of surface area. In general,
activated carbon and other adsorbents such as hollow aluminum spheres coated with
a catalyst can be employed in a fixed, moving or fluidized bed system.
Fluidized bed systems, though more expensive to build and operate, yield high
contacting with low pressure loss and regeneration can be accomplished within the
system. The fixed beds are less expensive and provide longer packing life, but
provide less contacting per unit length and require a larger pressure loss; because
they are regenerated individually.
Moving beds have properties between fixed and fluidized beds. The useful life of
activated carbon can be determined using break through curves.
Regeneration can be achieved by contact with a hot, inert gas, contact with a
low
pressure
gas
stream
and
pressure
reduction
over
the
bed.
Steam desorption is the most commonly used process for regeneration .
2. Incineration:
Incineration or combustion is another common VOC control technology. Complete
combustion or oxidation of pure hydrocarbons produces carbon dioxide and water.
Sulfur and nitrogen compounds produce acid gases and limited air supply results in
the formation of carbon monoxide.
Complex organic compounds may not oxidize completely in the residence time and
ash may form. Most VOC oxidation must be done at high temperature, unless
catalysts are involved.
Flares, thermal oxidizers and catalytic converters all use oxidation chemistry to treat
VOC emissions. Flares mostly treat moderate to high temperature concentrations.
All of the heat produced by the combustion process is lost when the flares are used.
Most thermal oxidizers treat emission streams with maximum VOC concentrations
of 25% of the LEL (lower explosive limit). Catalyst beds especially when used to
enhance the oxidation of VOCs (usually noble metals like platinum and palladium)
must be able to withstand high temperatures and must be designed so that a minimum
pressure drop is created when the gas passes through the bed. For example by using
catalytic converters, thermal oxidation of the by-products of the incomplete engine
combustion can be safely accomplished at temperatures much lower than would be
required without the aid of catalysis
3. Condensation:
Condensation and gas absorption are most commonly used for highly
concentrated VOC streams that are advantageous to recover and the relatively large
expense is warranted. It employs a drop in temperature and/ or increase in pressure
to cause the VOCs in the emission stream to condense. The cleaned air stream is
separated from the condensate containing target pollutants. In many cases, very large
temperature drops are required to achieve effective condensation, requiring
significant energy investment to accomplish cooling.
Condensation is used to recover gasoline and fuel vapors at gasoline loading
terminals and in gasoline dispensing facilities. It is also used in the adsorbent
regeneration process to separate solvents from the stream to separate solvents from
the stream used to regenerate the activated carbon
4. Gas Absorption:
Gas absorption involves the absorption of a gas into a liquid. Water can be
used for recovery of water-soluble compounds such as acetone and low molecular
weight alcohols, which can later be separated from water using distillation.
Additives are often used to increase the effective mass transfer rate of the pollutant
from the gas phase into the liquid phase, affecting the surface tension, reducing
interfacial resistance and increasing the apparent solubility. Gas absorption can be
expensive; however it is generally used only to recover VOCs that have a secondary
market value. Gas absorption techniques are used for the recovery of a variety of
chemicals in the coke manufacturing industry. They are often called scrubbers.
WATER POLLUTANTS
The large number of water pollutants is broadly classified under the categories:
1. Organic pollutants,
2. Inorganic pollutants,
3. Sediments,
4. Radioactive materials and
5. Thermal pollutants.