Lubricant Additives

260 Lubricant Additives: Chemistry and Applications

Any sulfurization process involves either the possible formation or the actual use of hydrogen-
sulfi de (H 2 S), an extremely toxic and corrosive gas. The smell of H 2 S is generally that of rotten
eggs, which is offensive to the human nose. In earlier times, the H 2 S released during the sulfuriza-
tion reaction (see Section 9.3.2.2) was just vented through a chimney or, if at all, scarcely absorbed
in alkaline scrubbers. Today, this procedure is no longer tolerated almost worldwide, and smaller
factories that have not yet done so need to invest into expensive safety equipment. This is another
reason why the smaller lubricant oil companies that still do a little bit of sulfurization are consider-
ing stopping. The processes are either pure batch or semicontinuous processes. For industrial appli-
cations, the volumes and varieties would not completely justify continuous productions.

9.3.2.2 Black Sulfurization

This is the simplest and oldest of all production technologies for sulfur carriers. The fi rst patented
sulfur carrier was done this way. The manufacturing equipment needs to withstand pressure above
1–2 bars (it may even be pressureless). Raw materials may be olefi ns as well as natural or synthetic
esters with a certain degree of unsaturation. The other reactant introduced into the olefi n-contain-
ing reaction vessel is sulfur fl ower. The mixture is heated above the melting point of sulfur. An
uncatalyzed reaction starts to become exothermic above 150–160°C, with the evolution of substan-
tial amounts of H 2 S. Catalyzed reactions start just above the melting point of sulfur, in the range of
120–125°C. Typical catalysts are organic amines, metal oxides, and acids.
Mechanistic studies of this reaction have been reported [32] and are very complex. At tempera-
tures between 120 and 160°C, intermolecular reactions are preferred. At 160–190°C, intramolecu-
lar reactions may be detected.
The fi rst reaction step in this so-called black sulfurization process is the ring opening of the
sulfur fl ower (S 8 -ring structure) and the subsequent oxidative attack of the sulfur on the vinylic pro-
tons (Equation 9.1). This quite uncontrolled reaction ends in the release of H 2 S and the formation of
vinylic mercaptans, vinylic thioethers, vinylic alkyl- and dialkylpolysulfanes, vinylic thioketones,
and even sulfur-containing heterocycles (thiophenes, etc.).

R


HCCH CH 2 R Sxx→R


HC CH CH SR()RHS()
 1 H S 2 (9.1)

O

O

O

O

O

O

S

S

S

S

S

S

S

S

S

FIGURE 9.6 Sulfurized fatty oil/olefi n mixture (light color).