of which was lead itself. These additives are classed as synthetic organic chemicals. Fuel uses of TEL are obviously dissipative.

The following zinc chemicals were produced in 1988: zinc oxide (directly from ore by the so-called French process), 0.0345 MMT; zinc sulfate, 0.013 MMT; and zinc chloride, 0.085 MMT. Zinc oxide is used mainly in tire manufacturing and as a pigment. Zinc chloride is mainly used as an electrolyte in dry cells. Minor quantities of zinc were used in pesticides and to manufacture catalysts. Except for these uses, zinc is not consumed in the production of synthetic organic chemicals. All final uses are dissipative.

Synthetic Organic Chemicals30

Most organic industrial chemicals are based on petrochemical (hydrocarbon) feedstocks. There are three basic categories: (1) paraffins (alkanes), which are saturated aliphatic (straight or branched-chain) hydrocarbons, the most important of which are methane, ethane, propane, isobutane, and n-butane; (2) olefins (alkenes), unsaturated aliphatics with one or more double bonds (e.g., ethylene, propylene, butylene, butadiene); (3) cyclics and aromatics (e.g., benzene, toluene, xylenes, cyclopentane, cyclohexane, and naphthalene). There is a fourth, miscellaneous, group of nonhydrocarbons, including oxygenated compounds of organic origin, cellulose, fatty acids, and related chemicals.

Some of the primary feedstocks of alkanes and aromatics, totaling 32.44 MMT in 1988, were derived from natural gas liquids (22.46 MMT), refinery off-gas (1.12 MMT), and naphtha (8.864 MMT) (International Energy Agency, 1991). Separate data are no longer collected for ethane, propane, and butane, probably due to the prevalence of mixed streams generated and converted within the petroleum refining sector.

For our purposes, it is convenient to exclude the C2-C4 alkanes (ethane, propane, isobutane, and n-butane) from consideration, because virtually all organic chemical products, except methanol, are derived from the corresponding olefins. The latter, in turn, are almost entirely used for chemical conversion. Primary products for chemical conversion, including methane, C2-C4 olefins, C5 and ''other" aliphatics (including methane), and aromatics, consumed in the United States amounted to approximately 63.6 MMT in 1988, including net imports of 7.64 MMT, according to the Bureau of Mines (1991). The remainder was derived from petroleum refineries (32.44 MMT, see previous section) and natural gas (23.5 MMT, estimated).

Not all of this material was converted into petrochemical products. For example, U.S. refineries produced 10.61 MMT of benzene, toluene, and xylenes (known as BTX), but Chemical and Engineering News (1997) lists only 4.08 MMT as "chemicals"; the implication is that 6.5 MMT were used by the refineries as gasoline additives. In addition, refineries produce significant quantities of hydrogen from the dehydrogenation process. We have no exact figures, but chem-

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