Residence time nitrogen cycling

The Hamburg process can be varied by some simple process parameters such as pyrolysis temperarnre, type of fluidizing gas (nitrogen, steam, cycled pyrolysis gas), residence time to produce different products from plastic waste (Table 17.1). 

Hydrogeochemical cycles couple atmosphere, land, and water. The atmosphere is an important conveyor belt for many pollutants. The atmosphere reacts most sensitively to anthropogenic disturbance because it represents a much smaller reservoir than land and water furthermore, the residence times of many constituents of the atmosphere are smaller than those in the other exchange reservoirs. Water and atmosphere are interdependent systems. Many pollutants, especially precursors of acids and photooxidants, originate directly or indirectly from the combustion of fossil fuels. Hydrocarbons, carbon monoxides, and nitrogen oxides released in thermal power plants and, above all, by automobile engines can produce, under the influence of sunlight, ozone and other photooxidants. 

Table 12-8. Global Fluxes and Atmospheric Residence Times of Nitrogen Associated with Biological Nitrogen Fixation, Inorganic Atmospheric Oxidation, Agricultural Fertilizer Production, Combustion Process, and the Sedimentary Cycle...

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. 

In the case of the periodic runs, the flow rates of the two reactants H2 and Ci,H6 were divided into two sections. A portion of the feed of each reactant was fed at a constant rate and the balance in a periodic mode. The nitrogen flow rate was maintained at 60% of the total volumetric rate throughout each set of cyclic runs. The cycling policy was such that the mean composition, the overall volumetric flow rate and residence time, temperature and pressure were identical to those at the corresponding optimum steady state conditions. 

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