Sulfur is commonly found in crude
oil, as it is a common element in the
earth’s crust. As shown in Table 1,
elemental sulfur and the sulfur-containing
mercaptans are very reactive
followed by sulfides. Reactive sulfur is
mainly in the form of organic sulfur
compounds like R-SH, where the
sulfur is attached at the end of an
organic molecule. When the molecule
is more complex, for instance when
the sulfur is surrounded or contained
within the molecule then the sulfur
compounds are more stable and less
reactive, like in R-S•S-R. Just a few
years ago, disulfides (of which DBDS
belongs) were once thought to be very
stable but it has been found that the
disulfide linkage can be susceptible to
cleavage resulting in the production of
mercaptans. Thiophenes are the most
stable of all these sulfur compounds.
Research at Doble has shown that
even thiophenes will break down
given enough time and thermal stress.
Whether they form corrosive sulfur
compounds or not is unknown. It is
known that a large percentage of the
breakdown products will reform into
smaller thiophene compounds. Crude
selection and the refining process
are the two main factors that dictate
the presence of any of the five sulfur
groups in a finished transformer oil