Air Pollution Scrubber
Air pollution in urban areas has become an issue affecting human health to a degree unprecedented in human history. The World Health Organization (WHO) recently prioritized the reduction of air pollution, citing: An estimated 4.2 million premature deaths globally are linked to ambient air pollution, mainly from heart disease, stroke, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections in children. Worldwide ambient air pollution accounts for: 29% of all deaths and disease from lung cancer. (World Health Organization, 2018). Additionally, air pollution is considered by many scientists as contributing to warming of the earth’s climate. Different gases absorb and re-emit thermal radiation in the earth’s atmosphere at different rates, and increased levels of gases produced as a result of burning of carbon-based fuels are suspected to be a factor in increased planetary surface temperatures (Lienhard 2011).
Reduction in atmospheric pollution is an ongoing challenge, addressed with alternative sources of power and increased treatment of exhaust gases. This paper presents the results of research into the feasibility of deploying open-air pollution control devices to selectively remove NO2 above areas experiencing severe air quality events. Different pollution control methods were investigated to determine the most appropriate technology based on effectiveness, cost, mobility, and social acceptance. The qualitative design objective is removal of 90 percent of the compound of interest. Statistical analysis will be performed on historical air pollution data from Los Angeles California. The historical analysis data will allow for the determination of an averaged value of severe air quality.
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Key Words: Air Pollution, Nitrogen Dioxide, Hydrogen Peroxide, Wet Scrubbers
Problem Analysis: Literature Review
The process of removal of NO2 as considered for this project was addressed by two different methods: diffusion of the NO2 into a liquid using a wet scrubber design, or removal using electrostatic forces. Equation
The removal of atmospheric NO2 by absorption into a liquid can be modeled using a mass balance equation:
d(V*Csys)/dt = Qin*Cin – Qout*Cout +/- V*r
where d(V*Csys)/dt = the derivative of the volume of the system and the concentration of NO2 within the system with respect to time
V = volume
Csys = concentration of NO2 within the system
Qin = mass of polluted air entering system
Cin = concentration of NO2 in the polluted air entering the system
Qout = mass of polluted air leaving the system
Cout = concentration of NO2 in the air leaving the system
and r = reaction rate
The characteristics of the system will be the environment in which it operates (the atmospheric conditions above a typical urban area such as Los Angeles), and the method by which it removes NO2. Since atmospheric conditions can vary considerably, the final design was evaluated at applicable temperatures as experienced during historical levels of pollution events.
The coefficients for NOx absorption into a liquid are :
The important parameters relating to modeling NO2 removal using a wet scrubber design are the rate of flow of air through the scrubber, the liquid used, and the internal surface area of the wetted tower. Additional parameters are the temperature of the operating environment, humidity, and atmospheric pressure.
The use of an airship (blimp) as the means to house the required air pollution removal equipment is the novel concept upon which this study was based. The use of an airship allows for mobility of the pollution-reducing device, allowing use at multiple sites by a single product, increasing economy. A blimp was chosen due to the large cargo carrying capacity, a current design by Lockheed Martin can carry a cargo of 20 metric tons (Dillow, 2016), matching the requirement of the ability to carry the equipment needed for removal of atmospheric pollution while processing the air through which it is moving. Blimps are also very fuel efficient, according to Van Wagner Aerial Media A blimp uses less fuel in two weeks than it takes a 747 airplane just to taxi to the runway (Van Wagner Aerial, 2014). It is also anticipated that alternate fuel and/or electric motors would be used for propulsion.
A wet scrubber is a method of removing gas-phase pollutants by absorption into a liquid. To remove gaseous impurities, wet scrubbers are usually configured to focus specifically on one type of pollutant, a correctly designed scrubber system should achieve a removal efficiency of up to 95% (IQS Directory, 2018). There are different ways to achieve the absorption, two methods considered for this study were a chamber with a spray system of small droplets of liquid, and a packed bed design using a liquid-coated media that increases surface area through which the absorption can occur.
A spray system is a simple design in which the pollutant gas passes through a spray of liquid droplets that allow the gas to be absorbed into the liquid. The pollutant gas is absorbed from the mixture of atmospheric gases by a liquid in which the pollutant gas is soluble (Treybal, 1988). Spray towers do not suffer from restrictions to gas flow by accumulated residues commonly found in packed scrubbers. However, spray towers have the least effective mass transfer capability and thus, are generally limited to use for PM removal and with high-solubility gases (EPA, 1992).
Advantages of spray towers include (AWMA, 1992; EPA, 1996):
- Relatively low pressure drop
- Can handle flammable and explosive dusts with little risk
- Fiberglass-reinforced plastic (FRP) construction permits operation in highly corrosive atmospheres
- Relatively low capital cost
- Relatively free from plugging
- Relatively small space requirements
- Ability to collect PM as well as gases
Disadvantages of spray towers include (AWMA, 1992; EPA, 1996):
- May create water (or liquid) disposal problem;
- Waste product collected wet;
- Relatively low mass-transfer efficiencies;
- Relatively inefficient at removing fine PM;
- When FRP construction is used, it is sensitive to temperature; and
- Relatively high operating costs.