Page 336 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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AT029-Manual
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316 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
2
The R for this equation is 0.99991 and the equation repro-
duces data with an average error of 0.37%. Triple point tem- 7.4.1 Enthalpy
perature of CO 2 as given in Table 7.1 is 216.58 K. Substitu- Enthalpy (H) is defined by Eq. (6.1) and has the unit of en-
tion of this temperature in the above equation predicts triple ergy per unit mass or mole (i.e., J/g or J/mol). This property
point pressure of 5.238 bar with 1% error versus actual value represents the total energy associated with a fluid and can
of 5.189 bar as given in Table 7.1. be measured by a calorimeter. Enthalpy increases with tem-
perature for both vapor and liquids. According to Eq. (6.1),
enthalpy of liquids increases with pressure, but for vapors
Example 7.4—Vapor pressure of ice at −10 C is 1.95 mm Hg enthalpy decreases with increase in pressure because of de-
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[21]. Derive a relation for sublimation pressure of ice and crease in volume. Effect of P on liquid enthalpy is small and
∼
estimate the following: can be neglected for moderate pressure changes ( P = 10
bar). However, effect of P on enthalpy of vapors is greater
a. Sublimation pressures at –2 and – 4 C. Compare calculated and cannot be neglected. Effect of T and P on enthalpy of
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values with experimental data of 3.88 and 3.28 mm Hg [21]. gases is best shown in Fig. 6.12 for methane.
b. The heat of sublimation of ice. In engineering calculations what is needed is the difference
between enthalpies of a system at two different conditions of
T and P. This difference is usually shown by H = H 2 − H 1
Solution—(a) Data available on solid vapor pressure are where H 1 is the enthalpy at T 1 and P 1 and H 2 is the enthalpy
P sub = 1.95 mm Hg = 0.0026 bar at T 1 = –10 C = 263.15 K. at T 2 and P 2 . Reported values of absolute enthalpy have a
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1
From Table 7.1 for water T tp = 273.16 K and P tp = 6.117 × reference point at which enthalpy is zero. For example, in the
10 −3 bar. Substituting these values into Eq. (7.28) gives A = steam tables values of both H and S are given with respect to
17.397 and B = 6144.3741. Thus the relation for sublimation a reference state of saturated liquid water at its triple point of
pressure of ice is determined from Eq. (7.27) as follows: 0.01 C. At the reference point both enthalpy and entropy are
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set equal to zero. The choice of reference state is arbitrary
6144.3741 but usually saturated liquid at some reference temperature
ln P sub (bar) = 17.397 − (T in kelvin) for ice
T is chosen. For example, Lenoir and Hipkin in a project for
(7.31) the API measured and reported enthalpies of eight petroleum
fractions for both liquid and vapor phases [23].
At T 1 =−2 C = 271.15 K we get P sub = 0.005871 = 3.88 mm This database is one of the main sources of experimen-
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Hg. Similarly at −4 C the vapor pressure is calculated as tal data on enthalpy of petroleum fractions from naphtha to
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
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0.004375 bar or 3.28 mm Hg. Both values are identical to kerosene and gas oil. The dataset includes 729 for liquid, 331
the experimental values. (b) Since coefficient B is equiva- for vapor, and 277 data points for two-phase region with total
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lent to H sub /R thus we have H sub = RB. H sub = 8.314 × of 1337 data points in the temperature range of 75–600 F and
6144.3741/18 = 2.84 kJ/g, where 18 is molecular weight of pressure range of 20–1400 psia. The reference state is satu-
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water. rated liquid at 75 F (23.9 C) with corresponding saturation
pressure of about 20–40 psia. Some values of enthalpy from
this database are given in Table 7.5. For all three fractions the
reference state is saturated liquid at 75 F and 20 psia. One
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7.4 THERMAL PROPERTIES should be careful in reading absolute values of enthalpy, en-
tropy, or internal energy since reported values depend on the
In this section, methods of estimation of thermal properties choice of reference state. However, no matter what is choice
such as enthalpy, heat capacity, heat of vaporization, and heat- of reference state calculation of H is independent of refer-
ing values for petroleum fractions are presented. These prop- ence state.
erties are required in calculations related to energy balances Heavy petroleum fractions possess lower enthalpy (per unit
around various process units as well as design and operation mass) than do light fractions at the same conditions of T and
of heat transfer related equipment. The fundamental equa- P. For example, for fractions with K W = 10 and at 530 K,
tions for calculation of enthalpy and heat capacity were dis- when the API gravity increases from 0 to 30, liquid enthalpy
cussed in Chapter 6. In this section application of those meth- increases from 628 to 721 kJ/kg. Under the same condi-
ods and some empirical correlations developed for prediction tions, for the vapor phase enthalpy increases from 884 to 988
of such properties are presented. Heat capacity, heats of va- kJ/kg as shown by Kesler and Lee [24]. Based on data mea-
porization, and combustion can be evaluated from enthalpy sured by Lenoir and Hipkin [23], variation of enthalpy of two
data, but independent methods are presented for convenience petroleum fractions (jet fuel and gas oil) versus temperature
and better accuracy. and two different pressures is shown in Fig. 7.12. Gas oil is
TABLE 7.5—Enthalpies of some petroleum fractions from Lenoir–Hipkin dataset [23].
Petroleum 20 psi and 1400 psi and 600 F and 20 psi and
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fraction K w API 300 F, liquid 500 F, liquid (P, psi), vapor (T, F), vapor
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Jet fuel 11.48 44.4 117.6 245.4 401.9 (100) 311 (440)
Kerosene 11.80 43.5 120 250.9 404.1 (80) 358.6 (520)
Fuel oil 11.68 33.0 115.8 243.1 346.0 (25) 378.4 (600)
Reference enthalpy (H = 0): saturated liquid at 75 F and 20 psia for all samples. H values are in Btu/lb.
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