Gas Absorption Tower

Gas Absorption Towers are industrial equipments that are used to remove pollutants and contaminants from a stream of gas. The aim of this study is to discuss how absorption towers can be used to control industrial pollution. The Gas Absorption Tower consists of vertically standing columns of metal tubes containing packed beds that absorb the pollutants from the gas passing through it.

Running head: GAS ABSORPTION TOWER
Gas Absorption Tower
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Author Note

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Introduction:
Gas Absorption Towers are industrial equipments that are used to remove pollutants
and contaminants from a stream of gas. This is an important part of the industrial process that
allows factories to control the emission of pollutants (Pouladi et al., 2016). Industrial
processes have been associated with the emission of various types of air pollutants and
greenhouse gases and such as carbon dioxide, surfer dioxide, nitrogen oxides as well as
various particulate matters such as carbon particles (El-Halwagi, 2017). These pollutants are
hazardous to human health and have an adverse impact on the environment. According to
Ghanbarabadi and Gohari (2014), the pollutants are majorly responsible for poor air quality,
can cause respiratory diseases when inhaled. It is therefore necessary to filter the pollutants
from the factory and industrial emissions as much as possible to mitigate the adverse impact
on human health and environment (Alessandrini, 2018).
The aim of this study is to discuss how absorption towers can be used to control
industrial pollution.
Discussion:
The Gas Absorption Tower consists of vertically standing columns of metal tubes
containing packed beds that absorb the pollutants from the gas passing through it. The basic
process consists of the stream of gas (emitted from the factories or industries) passing
through the tower along with a cleaning liquid (also called scrubbing liquid) that absorbs
pollutants and contaminants from the gas stream (Zhou et al., 2017). Based on the structure of
the absorption tower and the flow of the gas stream and scrubbing liquid, the Gas Absorption
Tower can be classified into different types such as Co Current Absorption Tower, Counter
Current Absorption Tower and Spray Column Absorbers (Wang et al., 2017)

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Counter Current Gas Absorption:
In this type of gas absorbers, columns have gas inlets at the bottom through the stream
of gases (emitted from the factories) flows in and washing liquid is fed through another inlet
at the top of the column that radially washes the column over the entire pipe’s cross section
(Lavalle et al., 2017). The streams of liquid and gas flows in the form of counter currents. As
the gas passes out of the outlet at the top of the column, it passes through the washing liquid
which scrubs the gas of pollutants. The structure of the absorption tower ensures maximum
contact between the gas and the scrubbing liquid and the waste is pumped out of the tower
through a separate outlet (De et al., 2018). The packing bed can be comprised of different
types of materials such as ceramic, plastic, metal or packing material that are loosely fitted
inside the tower and help to increase the contact area between the scrubber and the gas for
maximum absorption. This structure requires very little maintenance and can also be used for
emissions containing corrosive substances and a high volume of gas can be filtered using this
system (Boyadjiev & Boyadjiev, 2017). The downward flow of the scrubbing liquid and the
upward flow of the stream of gas leads to a counter current and the contact between the liquid
and vapor that allows the transfer of mass from the vapor or gas to the scrubbing liquid
(Lavalle et al., 2017). The diagram below shows the layout of a gas absorbing tower:

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Figure 1: Gas Absorbing Tower (source: Boyadjiev & Boyadjiev, 2017)
Co Current Gas Absorption Tower:
This is very similar to the counter current gas absorption, however with the difference
of the stream of gas and the scrubbing liquid flowing in the same direction instead of
opposing them. Both the inlets of the gas stream and the scrubbing liquid is at the top of the
tower and the outlets for the gas and pollutant laden scrubbing liquid at the bottom (Biard et
al., 2017). The same direction of the flow of gas and liquid causes a co current and helps to
transfer mass from the gas to liquid. However, this strategy is less effective compared to
counter current mechanism as it limits the amount of mass transfer (Collins et al., 2017). The
diagram below shows the layout of a co current gas absorber. The red arrow indicates the
flow of the gas and the blue arrow indicates the flow of the scrubbing liquid.

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Figure 2: Co Current Gas Abrorbing Tower (source: Collins et al., 2017)
Spray Columns:
This is another variant of the gas absorption tower in which the cleaning or scrubbing
liquid is injected into the tower in the form of spray of tiny droplets of aerosol. The tiny
droplets allow maximizing the surface area of the liquid and therefore maximize the contact
area with the gas (Raghunath & Mondal, 2017). Due to this, the spray tower allows a high
transfer rate of pollutants between the gases to the cleaning liquid. The spray columns are
similar to counter current gas absorption tower as the stream of the gases flow counter to the
flow of the droplets of cleaning liquid (Tamhankar et al., 2015).

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Figure 3: Spray tower (source: Raghunath & Mondal, 2017)
Operations:
Exhaust gas emitted from the factories and industries are injected into the towers
through their designated inlets, depending on the type of gas absorption tower. The gases are
generally laden with multiple pollutants such as particulate matter and hazardous gases or
chemicals. These substances when comes in contact with the liquid, condenses in to tiny
droplets which then flows into the stream of flowing liquid. The liquid then flows out of the
tower containing the pollutants and contaminants that was in the stream of gas and therefore
cleans the gaseous exhaust from the factories and industries which can then be released into
the air through the chimneys (Wang et al., 2017). This strategy can be used in various
industrial processes such as smelting of metal, manufacturing of chemicals, manufacturing of
cement and petrochemical processing (Lavalle et al., 2017). These industries have high levels
of emissions of particulate matter, volatile organic substances, oxides of sulfur and nitrogen

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as well as carbon dioxide (El-Halwagi, 2017). Additionally, industries that are associated to
the management of waste and conversion of waste to energy by pyrolysis, gasification or
plasma arc can also cause emission of various toxic air pollutants (Van Caneghem et al.,
2016). The gaseous emissions from these industries can be channeled through the gas
absorption towers in order to remove the contaminants (Wang et al., 2017).
The rate of absorption of the contaminants or pollutant into the cleaning liquid
depends upon the solubility of the liquid, the contact area between the liquid and the gas as
well as the rate of the flow of gas and liquid through the tower. These variables determine the
mass transfer coefficient of the tower and thus the efficiency of the equipment. The mass
transfer coefficient is the rate of movement of solutes from one phase (gas) to another (liquid)
and thus is an index of the efficiency of the gas absorption tower (Cao et al., 2017).
The figure below shows the equation to calculate the overall mass transfer coefficient
(KG) of a gas absorption tower where Gs is the molar gas flow rate through the cross section
of the tube, pAg is the partial pressure of the gas, pA is the equilibrium pressure, a is the area of
the liquid gas interface (contact area or effective area), z is the height of packing material
(Lee et al., 2015).
Figure 4: Equation of mass transfer coefficient (source: (Lee et al., 2015))
Conclusion:

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Gas absorption towers are equipments that can be used by factories and industries to
remove pollutants and contaminants present in their gaseous emissions before the exhaust are
discharged into the air. This helps to mitigate the adverse impacts of the pollutants and
contaminants on environment and human health. The process involves passing the gaseous
emissions from the factories and a cleaning liquid either in a counter current or co current and
the liquid can flow in the form of a stream or aerosol spray. When the gas comes in contract
with the stream or droplets of the liquid, it absorbs the pollutants from the stream of gas. The
rate of transfer of pollutants from the gas stream to liquid depends on the overall mass
transfer coefficient (KG).

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