Составители:
Fig. 1 shows, that such technologically important parameter
as the pour point, dramatically increased from –16.2 to +11.2°C af-
ter thermal pre-treatment of the studied oil at T
pt
=37.5°C. Such in-
crease of the pour point was quite unexpected, as in conventional
technological recommendations heating of crude oils is proposed as
a method of decreasing the pour points. Our data, presented in
Fig. 1, show that the expected decrease was observed only after
thermal pre-treatment at temperatures, higher than 39–40°C,
while the anomalous increase of T
pour
is registered when the pre-
treatment temperatures exceed the characteristic value of
28–30°C. It should be noted, that the thermally pre-treated crude
oil is a strongly nonequilibrium system, so its altered pour point
eventually relaxes to its initial value. Slow relaxation of the de-
creased pour points is a fairly well-known phenomenon in the prac-
tice of petroleum industry [14]. In our studies, we have registered
sufficiently low relaxation rates for the increased pour points in oil
samples, stored at room temperature (18–22°C). For example, in a
sample, pre-treated at T
m
=37.5°C, after four months of R.T. stor-
age, the altered pour point changed to T
pour
≈8°C (i.e. only by
about 3°C).
Taking into account our previous results [4–6], the anomalous
pour point increase in Fig. 1 may be due to the formation in the
specific temperature range (28–30°C<T
pt
<39–40°C) of structurally
ordered oil domains reinforced by asphaltene nanoparticles. At
lower temperatures these domains interconnect, forming networks
extending over the entire oil volume. The probable mechanism that
trigger the observed anomalous effects is a structural phase transi-
tion in the asphaltene/resin subsystem of the crude oil [6]. Our op-
tical absorption studies [15] have shown that asphaltenes may
form molecular solutions in crude oils only at concentrations
<10
-3
–10
-2
wt. %. At higher concentrations, the structural unit of
the asphaltene disperse phase is evidently a quasispherical “mo-
lecular nanocluster” (MNC), which has a diameter of 1.5–2 nm, is
composed of four stacked asphaltene monomers (with
MW=750–1000) held together primarily by the π-π and the dipole-
43
Страницы
- « первая
- ‹ предыдущая
- …
- 41
- 42
- 43
- 44
- 45
- …
- следующая ›
- последняя »
