densible refinery gases amounting to 34.072 MMT and salable products weighing 679.068 MMT, or 672.749 MMT after subtracting the 6.319 MMT of gasoil that was internally recycled. Petrochemicals are also derived to some extent from "light ends," dissolved volatiles that result from petroleum refining.24 Crude oil contains relatively little of these materials. A typical U.S. refinery using Texas or Louisiana crude oil might yield 1.3 percent light ends (as refinery off-gas) by volume from the initial distillation (Gaines and Wolsky, 1981). However, subsequent refining processes such as catalytic cracking, catalytic reforming, and delayed coking also yield large quantities of light ends. Most of these by-product volatiles are used internally within the refinery complex. The C4 gases (butylene, isobutane, and n-butane) are mostly alkylated or blended directly into gasoline. The C3 gases (propane, propylene) are collected and liquified under pressure for use as domestic fuel (i.e., liquid propane gas). The mixed C1 and C2 gases (methane, ethane, and ethylene) are mainly used as fuel for steam generation to provide heat energy for the refinery itself. Large amounts of hydrogen-rich off-gases are produced in refineries, but these are mostly used for hydrotreatment of naphtha or for hydroforming within the refinery. Similarly, catalytic reformate, derived from naphtha, is the major source of aromatic feedstocks, known as BTX (benzenetoluene-xylene), although most of this material is blended into gasoline to increase the octane number.

Nevertheless, light ends also constitute a source of aliphatic petrochemical feedstocks. In brief, light alkane feedstocks such as ethane (C2H6), propane (C3H8), and butanes (C4H10)—along with some naphtha and heavy gas oil—are dehydrogenated in a pyrolysis furnace within the refinery complex to yield ethylene (C2H4), propylene (C3H6), butadiene (C4H6), butene, butylene (C4H8), and other C4 olefins. In 1988, crude petroleum-based feedstocks consisted of liquified petroleum gas (22.459 MMT), naphtha (8.864 MMT), and light ends (1.12 MMT). (See section on organic chemicals, below.)

In summary, apparent mass losses during refining in 1988 amounted to 55.58 MMT (728.33 - 672.75), or 7.6 percent of input mass. In other words, the efficiency of mass conversion was 92.4 percent. Presumably, virtually all of the missing mass consists of carbon dioxide, carbon monoxide, or hydrocarbons, including fugitive volatile organic compounds (VOCs). The EPA estimated airborne effluents from the sector at 2 MMT (United States Environmental Protection Agency, 1991); we assume this figure does not include carbon dioxide. This is consistent with materials-balance arguments. VOCs from petroleum refining include significant quantities of BTX and other aromatics, many of which are carcinogenic.25

Crude oil contains small quantities, on the order of 0.1 percent depending on its origin, of sulfur and mineral ash. For example, Venezuelan oil is particularly high in sulfur. The petroleum refining industry recovers sulfur from crude oil and produces sulfuric acid as a by-product (2.4 MMT H2SO4 or 0.786 MMT S). Most of this sulfuric acid is used within the refinery for bleaching. However, over half

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