Page 155 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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3. CHARACTERIZATION OF PETROLEUM FRACTIONS 135
the temperature is high, heavy hydrocarbons may be added
to a fraction to increase its flash point. The flash point of the 3.6.2 Pour Point
blend should be determined from the flash point indexes of The pour point of a petroleum fraction is the lowest tempera-
the components as given below [74]: ture at which the oil will pour or flow when it is cooled without
stirring under standard cooling conditions. Pour point repre-
2414 sents the lowest temperature at which an oil can be stored
(3.116) log BI F =−6.1188 + and still capable of flowing under gravity. Pour point is one
10
T F − 42.6
of low temperature characteristics of heavy fractions. When
where log is the logarithm of base 10, BI F is the flash point temperature is less than pour point of a petroleum product it
cannot be stored or transferred through a pipeline. Test pro-
blending index, and T F is the flash point in kelvin. Once BI F
is determined for all components of a blend, the blend flash cedures for measuring pour points of petroleum fractions are
point index (BI B ) is determined from the following relation: given under ASTM D 97 (ISO 3016 or IP 15) and ASTM D 5985
methods. For commercial formulation of engine oils the pour

(3.117) BI B = x vi BI i point can be lowered to the limit of −25 and −40 C. This is
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achieved by using pour point depressant additives that inhibit
where x vi is the volume fraction and BI i is the flash point the growth of wax crystals in the oil [5]. Presence of wax and
blending index of component i. As it will be shown later, the heavy compounds increase the pour point of petroleum frac-
blending formula by Eq. (3.117) will be used for several other tions. Heavier and more viscous oils have higher pour points
properties. Once BI FB is calculated it should be used in Eq. and on this basis Riazi and Daubert [73] used a modified ver-
(3.116) to calculate the flash point of the blend, T FB . Another sion of generalized correlation developed in Chapter 2 (Eq.
relation for the blending index is given by Hu–Burns [75]: 2.39) to estimate the pour point of petroleum fractions from
viscosity, molecular weight, and specific gravity in the follow-
(3.118) BI F = T F 1/x ing form:

where T F is the flash point in kelvin and the best value of x T P = 130.47 SG 2.970566 × M (0.61235−0.47357SG)

is −0.06. However, they suggest that the exponent x be cus- (0.310331−0.32834SG)
tomized for each refinery to give the best results [61]. The (3.119) × ν 38(100)
following example shows application of these methods.
where T P is the pour point (ASTM D 97) in kelvin, M is the
molecular weight, and ν 38(100) is the kinematic viscosity at
Example 3.20—A kerosene product with boiling range of 37.8 C (100 F) in cSt. This equation was developed with data
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175–260 C from Mexican crude oil has the API gravity of 43.6 on pour points of more than 300 petroleum fractions with
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(Ref. [46], p. 304). (a) Estimate its flash point and compare molecular weights ranging from 140 to 800 and API gravities --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
with the experimental value of 59 C. (b) For safety reasons it from 13 to 50 with the AAD of 3.9 C [73]. This method is also
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is required to have a minimum flash point of 65 C to be able accepted by the API and it is included in the API-TDB since
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to store it in a hot summer. How much n-tetradecane should 1988 [2] as the standard method to estimate pour point of
be added to this kerosene for a safe storage? petroleum fractions. As suggested by Hu and Burns [75, 76],
Eqs. (3.117) and (3.118) used for blending index of flash point
can also be used for pour point blending index (T PB ) with
Solution—(a) To estimate flash point we use either Eq.
(3.114) or its simplified form Eq. (3.115), which require ASTM x = 0.08 :
10% temperature, T 10 . This temperature may be estimated (3.120) BI P = T P 1/0.08
from Eq. (3.17) with use of specific gravity, SG = 0.8081, and
ASTM 50% temperature, T 50 . Since complete ASTM curve is where T P is the pour point of fraction or blend in kelvin. The
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not available it is assumed that the mid boiling point is the AAD of 2.8 C is reported for use of Eqs. (3.117) and (3.120)
same as T 50 ; therefore, T 50 = 217.5 C and from Eq. (3.17) with to estimate pour points of 47 blends [76].
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coefficients in Table 3.4, T 10 = 449.9 K. Since T 50 is less than
260 C, Eq. (3.115) can be used for simplicity. The result is 3.6.3 Cloud Point
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T F = 60.4 C, which is in good agreement with the experimen-
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tal value of 59 C considering the fact that an estimated value The cloud point is the lowest temperature at which wax crys-
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of ASTM 10% temperature was used. tals begin to form by a gradual cooling under standard con-
(b) To increase the flash point from 59 to 65 C, n-C 14 with ditions. At this temperature the oil becomes cloudy and the
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flash point of 100 C (Table 2.2) is used. If the volume frac- first particles of wax crystals are observed. The standard pro-
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tion of n-C 14 needed is shown by x add , then using Eq. (3.117) cedure to measure the cloud point is described under ASTM
we have BI FB = (1 − x add ) × BI FK + x add × BI Fadd where BI FB , D 2500, IP 219, and ISO 3015 test methods. Cloud point
BI FK , and BI Fadd are the blending indexes for flash points is another cold characteristic of petroleum oils under low-
of final blend, kerosene sample, and the additive (n-C 14 ), temperature conditions and increases as molecular weight of
respectively. The blending indexes can be estimated from oil increases. Cloud points are measured for oils that con-
Eq. (3.116) as 111.9, 165.3, and 15.3, respectively, which re- tain paraffins in the form of wax and therefore for light frac-
sult in x add = 0.356. This means that 35.6% in volume of n-C 14 tions, such as naphtha or gasoline, no cloud point data are
is required to increase the flash point to 65 C. If the blending reported. Cloud points usually occur at 4–5 C(7to9 F) above
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indexes are calculated from Eq. (3.118), the amount of n-C 14 the pour point although the temperature differential could be
required is 30.1%. in the range of 0–10 C (0–18 F) as shown in Table 3.27. The
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