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.
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)
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:
Figure 1: Gas Absorbing Tower (source: Boyadjiev & Boyadjiev, 2017)
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.
Figure 2: Co Current Gas Abrorbing Tower (source: Collins et al., 2017)
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).
Figure 3: Spray tower (source: Raghunath & Mondal, 2017)
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 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:
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).
References:
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