Impurity polynuclear aromatics

Examples of the need for multimedia models are found in contemporary problem areas. Polynuclear aromatic hydrocarbons and metals are emitted into the atmosphere as trace impurities with the products of coal combustion. The organics have low vapor pressure and partially condense on emitted particulates in a stack plume. The particulates are transferred to the soil by dry deposition, rainout or washout. The metals manifest... 

Residual fuel oil is generally more complex than distillate fuels in composition and impurities. Limited data are available, but there are indications that the composition of No. 6 fuel oil includes (volume basis) aromatics (25%), paraffins (15%), naphthenes (45%), and nonhydrocarbon compounds (15%). Polynuclear aromatic hydrocarbons and their alkyl derivatives and metals are important hazardous and persistent components of No. 6 fuel oil. 

Seventeen priority pollutant compounds can be classified as polynuclear aromatics (PNA). These compounds consist of two or more benzene rings that share a pair of carbon atoms. They are all derived from coal tar, with naphthalene being the largest constituent. Naphthalene derivatives such as alpha-naphthylamine and alpha-naphthol are used in some pesticide processes therefore, naphthalene is by far the most prevalent PNA priority pollutant in the industry. Acenaphthene, anthracene, fluorene, fluoranthene, and phenathrene are found as raw material impurities. Acenaphthene is found in one pesticide process as a raw material. The remaining ten PNAs are not suspected to be present in pesticide processes. 

A small amount of ammonia is also added to the feed during the adsorption cycle since this has been found to reduce coking and improve product purity, presumably by suppressing the adsorption of polynuclear aromatics on the external surfaces of the zeolite crystals. To avoid possible lifting of the bed the flow is ramped at the start of the adsorption cycle. During the first few seconds of the desorption cycle, impurities are removed from the interstices of the bed and the surface of the adsorbent, and this fraction is recycled to the feed in order to maintain high purity of the linear paraffin product... 

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