Page 63 - Petroleum and Gas Field Processing

Sour crude oils are those containing a high percentage of hydrogen sulfide.
However, many of the organic sulfur compounds are not thermally stable,
thus producing hydrogen sulfide during crude processing.
High-sulfur crude oils are in less demand by refineries because of the
extra cost incurred for treating refinery products. Naphta feed to catalytic
reformers is hydrotreated to reduce sulfur compounds to very low levels
(1 ppm) to avoid catalyst poisoning.
The following sulfur compounds are typical:
1. Mercaptans (H–S–R): Hydrogen sulfide, H–S–H, may be
considered as the simple form of mercaptan; however, the
higher forms of the series are even more objectionable in smell.
For example, butyl mercaptan (H–S–C 4 H 9 ) is responsible for the
unusual odor of the shank.
2. Sulfides (R–S–R): When an alkyl group replaces the hydrogen
in the sulfur-containing molecule, the odor is generally less
obnoxious. Sulfides could be removed by the hydrotreating
technique, which involves the hydrogenation of the petroleum
streams as follows:
R S R þ 2H H ! 2R H þ H S H
R S R þ H H ! R R þ H S H
The hydrogen sulfide may be removed by heating and may be
separated by using amine solutions.
3. Polysulfides (R–S–S–R): These are more complicated sulfur
compounds and they may decompose, in some cases depositing
elemental sulfur. They may be removed from petroleum
fractions, similar to the sulfides, by hydrotreating.

Nitrogen Compounds. Nitrogen compounds in crude oils are usually
low in content (about 0.1–0.9%) and are usually more stable than
sulfur compounds. Nitrogen in petroleum is in the form of heterocyclic
compounds and may be classified as basic and nonbasic. Basic nitrogen
compounds are mainly composed of pyridine homologs and have the
tendency to exist in the high-boiling fractions and residues. The nonbasic
nitrogen compounds, which are usually of the pyrrole and indole, also
occur in high-boiling fractions and residues. Only a trace amount of
nitrogen is found in light streams.
During hydrotreatment (hydrodesulfurization) of petroleum streams,
hydrodenitrogeneation takes place as well, removing nitrogen as ammonia
gas, thus reducing the nitrogen content to the acceptable limits for
feedstocks to catalytic processes.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.