Part II. Effect of Surface Properties on Sensitivity of Gas Sensor Using Carbon Nanotube and Graphene
Chapter 6. Estimation of Residual Life of Gas Filtration System
6.1 Background
6.1.1 Air Filter for Small Particles
The air filter is a filtering medium that filters small fine particles (such as fine dust and mold) floating in the air. The air filters are classified into three categories: pre-filter, medium filter and high efficiency particulate air (HEPA) filter. The pre-filter filters about 50% of the contaminants, the medium filter filters 80% of fine dust, and the HEPA filter filters 95% of fine dust. In particular, the HEPA filter is the most efficient filter to remove 99.97% of low-concentration micron particles. The HEPA filters are very small fibers with diameters less than 1 micron, so the HEPA filters have a much better performance than other filters in removing small dust particles. The HEPA filter unit is designed to have a deep wrinkle (Figure 6.1). The flow rate through the filter fiber is slowed down so that the HEPA filter has a high dust collection efficiency of 99.99%, and it is designed to reduce the pressure drop due to the filter.
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Cellulose paper, fiberglass, ceramics and plastics are examples of filter materials. The cellulose paper is easy to fold but needs special care to prevent it from tearing in folds. In addition, there is a limitation on the operating temperature, and it is not suitable to use at a temperature of 100 °C or more. The cellulose paper can be burned by spark. This cellulose paper is the least expensive and can be used under temperature, fire, and humidity-insensitive conditions. Fiberglass papers are made of very fine glass fiber base of 1 to 4 μm in diameter. It is used as the most popular filter materials. It is very resistant to heat and can withstand temperatures up to 500 °C. Because the fiberglass paper is a refractory material, the pyrotechnic itself does not expand the flame even if it is ignited by dust accumulated in the filter due to spark. It has good resistance to most chemicals and corrosive atmospheres. The plastic fibers are made of polyethylene, nylon, polystyrene, etc. However, the polystyrene materials are mainly used for HEPA filter. The fiber diameter range is 0.5 to 1.5 μm. It has good resistance to chemical reaction, but resistance to heat is low. Because it is flammable material, temperature is limited to less than 80 °C. The ceramic is a filter that can be used for a long time even at high temperatures over 500 °C. The ceramic filters are mainly made of thin paper or long type filters using aluminosilicate. It has good chemical resistance and can withstand up to 1000 °C.
Figure 6.1 HEPA filter image. It is manufactured in deep wrinkle form for high dust collection efficiency. The dust is trapped in the HEPA filter by three mechanisms: diffusion, interception, and impaction.
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6.1.2 Activated Carbon Filter for Small Molecule
The air filter can filter fine dust and small particles, but it do not have filtration capability for smaller size gas molecules. To filter gas molecules, we mainly use activated carbon filters. The activated carbon is an adsorbent that develops fine pores through activation process using various carbonaceous raw materials and has a large internal surface area. The typical activated carbon, as shown in Figure 6.2, has an internal surface area of 500 to 1500 m2/g [245]. Micro-meso-macro are classified in the pore structure of activated carbon. A pore with a diameter of 2 nm or less is called micro-porous. When it is greater than 2 nm and less than 50 nm, it is called meso-pore. For pores larger than 50 nm, it is called macro-pore.
Activation methods include physical activation and chemical activation. Physical activation is a method of using hot gases. It is generally necessary to convert organic precursor to a primary carbon, a mixture of ash, tar, amorphous carbon and crystalline carbon. The physical activation activate/oxidize organic precursor using a combination of carbonization in the presence of an inert gas [246].Chemical activation is preferred over physical activation because of the lower temperatures and shorter times required to activate the material [247]. The preparation of the activated carbon from agricultural byproducts/wastes has been hugely adopted in many studies because the chemical activation method provides many advantages over the physical activation method. In addition, activated carbon obtained by chemical activation exhibits a larger surface area than the physical activity and a better developed intermediate porosity. Figure 6.3 shows the process of manufacturing activated carbon.
In case of the chemical activation, before carbonization, agricultural byproducts/wastes are impregnated with acids, for example strong bases such as H3PO4, KOH and salts such as NaOH,
Figure 6.2 Different pore structure of activated carbon [9].
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ZnCl2. This step is required to prevent tar formation [248]. In this way, a well-developed carbonized product with a single porosity can be obtained. Agricultural byproducts/wastes are then carbonized at lower temperatures (450-900 ºC). The carbonization/activation step proceeds simultaneously with chemical activation. The chemicals incorporated into the precursor particles react with the product resulting from the pyrolysis of the precursor, thereby reducing the evolution of volatile materials and inhibiting shrinkage. In this way, the conversion rate of the precursor to carbon is high and a large amount of porosity is formed.
The most important atom in activated carbon is oxygen, which binds to other heteroatoms at the edge of the carbon plane or induces bonding. The most common functional groups formed by the
Figure 6.3 Scheme of the physical and chemical process for manufacturing activated carbon [4].
Figure 6.4 Oxygen functional groups affecting the physical adsorption process [9].
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role of oxygen include carboxyl group, carbonyl group, phenol group and lactone group (Figure 6.4).
These functional groups changes the acidity and basicity of the carbon itself, which are related to changes in surface properties. Furthermore, functional groups play an important role in the adsorption of non-polar molecules in the physical adsorption process and serve to make adsorption sites on the surface.
When the activated carbon is used to fabricate the filter system, the lifetime of the activated carbon can be known indirectly by measuring the weight before and after the test. This method is very inconvenient and inefficient. In the next chapter, we will discuss directly measuring the lifetime of activated carbon by inserting a carbon nanotube-based sensor into the activated carbon layer.