Page 441 - Elements of Chemical Reaction Engineering Ebook

412 Nonelementary Reaction Kinetics Chap. 7

W-TC (Tribology) One of the major reasons for engine oil degradation is the oxida-
tion of the motor oil. To retard the degradation process, most oils contain an
antioxidant [see Znd. Eng. Chem 26, 902 (1987)J. Without an inhibitor to oxi-
dation present, the suggested mechanism at low temperatures is
12 k' > 21.

Z.+RH --% R*+HZ

R.+Oz --& RO,
RO, + RH '" > ROOH + R.

2RO; '1 > inactive
where Zz is an initiator and RH is the hydrocarbon in the oil. When the tem-
perature is raised to 100°C, the following additional reaction occurs as a result
of €he decomposition of the unstable ROOH:
ROOH 'I3 > RO. + *OH

RO*+RH krr > ROH+R*

>
*OH+RH '@ HZO+R.
When an antioxidant is added to retard degradation at low temperatures, the
following additional termination steps occur:

RO,. +AH > ROOH +A*
A*+ RO, ku 4 inactive

OH OH

Derive a rate law for the degradation of the motor oil in the absence of an
antioxidant at
(a) Low temperatures.
(b) High temperatures.
Derive a rate law for the rate of degradation of the motor oil in the presence
of an antioxidant for
(e) Low temperatures,
(d) High temperatures. Here assume that the inactive products formed with
antioxidant do not decompose (probably a bad assumption).
(e) How would your answer to part (a) change if the radicals I. were produced
at a constant rate in the engine and then found their way into the oil?
(f) Sketch a reaction pathway diagram for both high and low temperatures,
with and without antioxidant.