Nitrogen dioxide residence time

This waste treatment sequence can reduce feed values of 68 mg/L cyanide and 42 mg/L copper to concentrations of about 0.1 mg/L for both contaminants [26]. This result requires 3.4 kg chlorine/kg cyanide in the effluent, and a 15-30 min residence time for destruction of the cyanide only. Three to four times this residence time is required for neutralization of copper cyanide. Ideally, sufficient oxidant should be added to convert cyanate to carbon dioxide and nitrogen to avoid the possibility of cyanate reversion back to cyanide after discharge (e.g., Eq. 13.34). 

The principal components of the atmosphere are nitrogen 78.09%, oxygen 20.95%, argon 0.932%, and carbon dioxide 0.03% (vol%, dry atmospheric air). The water content varies from 0.1 to 2.8vol%. However, there are some other components which in spite of their low concentrations exert strong influence on atmospheric chemistry [4]. Table 1 shows the natural content (i.e. average stationary concentrations) of the principal trace components, their average lifespans and rates of supply and removal from the atmosphere. Two latter values are equal to each other and are calculated as the ratio of the stationary concentration of an atmospheric component to its residence time in the atmosphere. 

In the ocean, elements that form insoluble hydroxides have relatively short residence times (e.g., A1 and Fe have residence times in the ocean of 100 and 200 years, respectively). Cations, such as Na (aq) and K (aq), and anions, such as Cl (aq) and Br (aq), have longer residence times in the ocean ( 7 x 10 to 10 years). In the atmosphere, the very stable gas nitrogen has a residence time of a million years or so, while oxygen has a residence time of 5,000-10,000 years. Sulfur dioxide, water, and carbon dioxide, on the other hand, have residence times in the atmosphere of only a few days, 10 days, and 4 years, respectively. Of course, residence times may be determined by physical removal processes (e.g., scavenging by precipitation) as well as chemical. 

Most plastics bum cleanly, producing emissions of carbon dioxide, nitrogen oxides, and water vapor, but some produce unwanted by-products such as polyvinyl chloride (PVC). However, PVC and such other by-products can be safely burned at high temperatures of 980 to 1,650°C (1,800-3,000°F), using controlled oxygen input, sufficient cycle (residence) time, typically i to 2 minutes, and appropriate auxiliary equipment like scrubbers and solid salts. However, most U.S. incinerators operate below 87°C (1,600°F) and use only limited auxiliary equipment. For example, incinerated PVC generates undesirable chlorine (and bleached paper generates much more chlorine). Exhaust scrubber systems must be used to remove this chlorine. 

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