High-temperature thermal desorption

Based on the operating temperature of the desorbent, thermal desorption processes can be categorized into two types. The first is high-temperature thermal desorption (HTTD), in which waste is heated to temperatures from 320 to 560°C (600 to 1000°F). This process is frequently used in combination with incineration, solidification/stabilization, or dechlorination of SVOCs, PAHs, PCBs, and pesticides. The second type is low-temperature thermal desorption (LTTD), in which waste is heated to temperatures from 90 to 320°C (200 to 600°F). Nonhalogenated VOCs, SVOCs, and fuels can be effectively treated by LTTD. 

Oil-contaminated soil can also be treated by passing through a rotary kiln where it is allowed to remain for sufficient duration to achieve a soil temperature sufficient (between 371 and 482°C) to volatilize the contaminants (HTTP). The combustion gas in the HTTD system remains either in direct contact with contaminated soil (direct-fired high-temperature thermal desorption) or is separated from the vaporized petroleum hydrocarbons or volatilized residue (indirect-fired HTTP). The treated soil is then cooled and passed through the rehydration unit before being screened and stockpiled. The HTTD-treated soil is black in color, completely sterile, and lacks soil structure [15]. It is also expensive compared to bioremediation. 

Methods I and 2 are commonly used for regeneration of adsorbents used for gaseous phase adsorption. Naturally, method 2 can be applied for liquid phase adsorption if the equilibrium relation allows in specific cases. Fig. 9.1 shows these schemes of desorption. Desorption using an inert stream free of adsorbent is essentially the same operation as adsorption, which can be analyzed by the same basic equation with different initial, and boundary conditions. The same is true of desorption at high temperature (thermal desorption) except that the equilibrium relation is very different. Also, in the actual operation of thermal desorption, nonisothermal treatment becomes important in most cases. The combination of desorption at low pressure and adsorption at high pressure is the principle of pressure swing operation (PSA), which is discussed in Chapter II. 

High-temperature thermal treatment will mineralize the explosives contaminants to form mainly C02 and H20. The fate of the explosives contaminants is not as clear in the lower temperature thermal desorption case. Low-temperature thermal treatment involves some decomposition, but volatilization is the main removal mechanism. All of the low-temperature systems provide for methods to capture the volatilized organics from the off-gas. Residuals from the off-gas treatment may be organic liquid and/or spent carbon sorbents. These residuals will require further treatment or disposal. The low-temperature techniques provide good protection of human health and the environment by either destroying the contaminants or concentrating them in a controlled manner for further processing. 

Each type of compound discussed before, by its nature, lends itself to either direct reuse or some degree of treatment prior to reuse. Direct reuse includes utilizing the soil as a raw material for asphalt or concrete. Treatment options range from simple and inexpensive aeration to costly comphcated high temperature thermal treatment. Treatment operations discussed here include aeration, land farming, bioremediation, low-temperature thermal desorption, high temperature thermal treatment, asphalt incorporation, and concrete incorporation. 

High temperature thermal treatment It is similar to low temperature thermal desorption, but is hot enough to thermally destroy volatile and hydrocarbon compounds. The cost of high temperature thermal treatment is, therefore, much more than low-temperature desorption, but the soil is typically completely depleted of volatile and heavy hydrocarbon compounds. PCBs are typically destroyed to 99.99% by this method. The use of the soil after high temperature thermal treatment depends on the original material s quality, as the soil can typically be used as a clean fill. In some cases, it is used as a feed stock for asphalt or concrete. 

The accuracy on the activation values is calculated by several measurements. The two activation energies obtained for AI2O3 show that the recombination mechanism may be different for the two temperature ranges around 1200 K. In the thermal study, this difference is also encountered but around 1400 K. At high temperature, the desorption of atoms om the surface is faster than adsorption, so the overall activation energy goes from positive to negative values and therefore the recombination coefficient decreases as temperature increases. 

The primary process initiating dust surface chemistry is the collision of a molecule from the ISM with the surface. The sticking probability is a measure of how often molecules will stick to the dust surface but this depends on the collision energy, the temperature of the grain surface and the nature of the chemical surface itself. The silicate surface is highly polar, at least for a grain of sand on Earth, and should attract polar molecules as well as atoms. The adsorption process can also be reversed, resulting in thermal desorption, both as the reverse of adsorption and by new molecules as the product of surface reactions. 

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