Page 311 - Introduction to Petroleum Engineering

300 MIDSTREAM AND DOWNSTREAM OPERATIONS
blend. Octane is a saturated hydrocarbon with eight carbon atoms. By contrast,
octane number is a measure of the compression a fuel can withstand before it ignites.
A fuel with high octane number can be compressed more than a fuel with low octane
number before ignition. Fuels with high octane number are used in high‐performance
engines. Straight chain molecules (paraffins) have a relatively low octane number,
while hydrocarbon molecules with rings (aromatics) have a relatively high octane
number. The catalytic reformer uses a catalyst in the presence of hydrogen and high
temperature to reform paraffin molecules into aromatic molecules.

15.2.3 Purification
Purification is the last step in the refining process following separation and conversion.
The primary purpose of purification is to remove sulfur in a process called hydrotreat-
ing. Unfinished products are fed into a hydrotreater that contains a catalyst and
hydrogen in a high‐temperature, high‐pressure chamber. The catalyst increases the
chemical reaction rate for a reaction that removes sulfur from molecules in the input
stream. The primary product of the reaction is hydrogen sulfide, which is removed by
extraction in other units. The hydrogen sulfide is separated into elemental sulfur and
hydrogen using a desulfurization process. The recovered sulfur can be sold as a
refinery by‐product and the hydrogen can be used in a hydrocracker or hydrotreater.

15.2.4 Refinery Maintenance
Refinery operations can wear out equipment and consume chemical catalysts. The
catalysts need to be replaced and equipment needs to be repaired or replaced. New
equipment may have to be installed. Operators shut down the refinery twice a year to
allow time to maintain the refinery. Typically, maintenance periods are scheduled dur-
ing periods when demand for petroleum products is lowest. As a rule, summer driving
season and winter heating result in periods of high petroleum demand during the
summer and winter. That leaves spring and fall for refinery maintenance. Maintenance
periods can require shutting down a refinery for a few days to a few weeks.

15.3 THE DOWNSTREAM SECTOR: NATuRAl gAS
PROCESSINg PlANTS

The typical components of natural gas and natural gas products from natural gas
processing plants are presented in Table 15.2. Natural gas consists of hydrocarbon
and nonhydrocarbon gases. It also contains water vapor. Water and other impurities
must be removed from natural gas before it can be used. Natural gasoline is a mixture
of pentane plus smaller amounts of hydrocarbon molecules with more than five
carbon atoms. The hydrocarbon molecules in natural gasoline usually do not have
more than 10 carbon atoms.
Countries or operators with excess natural gas production may choose to ship gas
to distant markets as LNG. LNG consists mostly of methane plus very small amounts
of ethane, propane, and nitrogen. LNG is shipped in specialized tankers as a liquid at