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treated at T
pt
=45°C, all types of the above bonds are effectively de-
stroyed, as indicated by a virtually constant activation energy in
the studied range of the flow (measurement) temperature.
The observed effect of structural ordering in a heated crude oil
strongly resembles the phenomenon of cold crystallisation, exten-
sively studied in solid polymers [21]. Moreover, the specific tem-
peratures, registered in our studies, are close to the temperatures
of cold crystallisation in solidified fractions of petroleum bitumen,
measured in [22]. Whether such resemblance is due to a common
molecular mechanisms, is an open question. The existence of cold
crystallisation in crude oils would imply a possibility of uncontrol-
lable spatial phase separation due to small changes of ambient
temperatures in a variety of technological processes. In the next
section we present preliminary evidence of such phase separation.
3.3. Composition stratification in the heated
crude oil, studied by optical absorption
The measurements were conducted in cylindrical glass ves-
sels, 15 mm in diameter, filled to a height of ≈105 mm with the
crude oil. Each oil vessel was introduced in a vertical position into
a thermostat at 26, 36 and 48°C for 1.5 hour.
At the end of that period, two 0.5 ml oil samples were collected
≈5 mm below the oil/air interface and ≈5 mm above the bottom of
the vessel. Each sample was diluted with toluene to the oil concen-
tration of ≈0.1 vol. %. Optical absorptivities ε of oil solutions were
measured at λ=364 and 670 nm, at room temperature (19–21°C),
and were normalised to the value of ε for the “lower layer” sample
of the oil, heated to 26°C.
The results are presented in Fig. 4. Two top curves and two
bottom curves show the effects of temperature on normalised ab-
sorptivities of the upper and lower oil layers, respectively. At tem-
peratures of 26°C and 48°C the respective values of ε are close to
each other, indicating a minor vertical gradient of oil composition.
In the crude oil, heated to 36°C (i.e. to the temperature, providing
the anomalous pour point increase in Fig. 1), absorptivites of the
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