Page 393 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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9. APPLICATIONS: PHASE EQUILIBRIUM CALCULATIONS 373
For undefined petroleum fractions the following correla-
tion may also be used to estimate true critical temperature discusses the nature of heavy compounds that are present
in petroleum residua and heavy oils. Precipitation of these
and pressure from specific gravity and volume average boil- heavy compounds under certain conditions of temperature
ing point (VABP) of the fraction [12]: and pressure or composition follow general principles of SLE,
−3
T tc = 358.79 + 1.6667 − 1.2827(10 ) 2 which were discussed in Section 6.8.3. In this section, the
problems associated with such heavy compounds as well as
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
(9.13) = SG (VABP − 199.82) methods that can be used to predict the certain conditions at
which they precipitate will be discussed. Based on the princi-
log (P tc /P pc ) = 0.05 + 5.656 × log (T tc /T pc )
10 10 ple of phase equilibrium discussed in Section 6.8.1, a thermo-
where T tc , T pc , and VABP are in kelvin and P pc and P tc are in dynamic model is presented for accurate calculation of cloud
bars. It is important to note that both T pc and P pc must be point of crude oils under various conditions. Methods for cal-
calculated from the methods given in Section 2.5 for criti- culating the amount of solid precipitation from sophisticated
cal properties of undefined petroleum fractions. The average thermodynamic models as well as readily available parame-
error for calculation of T tc from the above method is about ters for a petroleum fluid are also discussed in this section.
0.7% (∼3.3 K) with maximum error of 2.6% (∼12 K). Relia-
bility of the above method for prediction of true critical pres- 9.3.1 Nature of Heavy Compounds, Mechanism
sure of undefined petroleum fractions is about 5% as reported of their Precipitation, and Prevention Methods
in the API-TDB [12]. The above equation for calculation of
P tc is slightly modified from the correlation suggested in the Petroleum fluids, especially heavy oils and residues, contain
API-TDB. This correlation is developed based on an empirical heavy hydrocarbons from paraffinic, naphthenic, and aro-
graph of Smith and Watson proposed in the 1930s. For this matic groups. Generally, there are three types of heavy hydro-
reason it should be used with special caution. The following carbons that may exist in a heavy petroleum fluid: (1) waxes,
method is recommended for calculation of true critical vol- (2) resins, and (3) asphaltenes. As discussed in Section 1.1.3,
ume in some petroleum-related references [3]: the main type of waxes in petroleum fluids are paraffinic
waxes. They are mainly n-paraffins with carbon number range
Z tc RT tc
V tc = of C 16 –C 36 and average molecular weight of about 350. Waxes
(9.14) P tc that exist in petroleum distillates usually have freezing points

Z tc = x i Z ci between 30 and 70 C. Another group of waxes called crys-
◦
i talline waxes are primarily isoparaffins and cycloparaffins
Method of calculation of true critical points (T tc , P tc , and V tc ) (with long-chain alkyl groups) with carbon number range of
of defined mixtures through an equation of state (i.e., SRK) 30–60 and molecular weight range of 500–800. The melting
requires rigorous vapor–liquid thermodynamic relationships points of commercial grade waxes are in the 70–90 C range.
◦
as presented in Procedure 4.B4.1 in Chapter 4 of the API- Solvent de-oiling of petroleum or heavy residue results in
TDB [12]. At the true critical point, a correct VLE calculation dark-colored waxes or a sticky, plastic to hard nature material
should show that x i = y i . Most cubic EOSs fail to perform [14]. Waxes present in a petroleum fluid may precipitate when
properly at the critical point and for this reason attempts have the conditions of temperature and pressure change. When the
been made and are still continuing to improve EOS phase temperature falls, heavy hydrocarbons in a crude or even a
behavior predictions at this point. gas condensate may precipitate as wax crystals. The temper-
ature at which a wax begins to precipitate is directly related
to the cloud point of the oil [15, 16]. Effects of pressure and
9.3 VAPOR–LIQUID–SOLID EQUILIBRIUM— composition on wax precipitation are discussed by Pan et al.
SOLID PRECIPITATION [17].
Wax formation is undesirable and for this reason, different
In this section, practical application of three-phase equilib- additives usually polymer-based materials are used to lower
rium in the petroleum industry is demonstrated. Upon re- pour points of crude oils. Wax inhibitor materials include
ducing the temperature, heavy hydrocarbons present in a polyalkyl acrylates and methacrylates, low-molecular-weight
petroleum fluid may precipitate as a solid phase and the polyethylene waxes, and ethyl-vinyl acetate (EVA) copoly-
liquid becomes in equilibrium with both the solid and the mers. The EVA copolymers are probably the most commonly
vapor phase. In such cases, the solid is at the bottom, liquid used wax inhibitors [14]. These inhibitors usually contain
is in the middle, and the vapor phase is on top of the liquid 20–40 wt% EVA. Molecular weight of such materials is usu-
phase. A general schematic of typical vapor–liquid–solid equi- ally greater than 10 000. The amount of EVA added to an oil is
librium (VLSE) during solid precipitation in a petroleum fluid important in its effect on lowering pour point. For example,
is shown in Fig. 9.1. Solid precipitation is a serious problem when 100 ppm of EVA is added to an oil it reduces pour point
in the petroleum industry and the basic question is: what is from 30 to 9 C, while if 200 ppm of same inhibitor is added
◦
the temperature at which precipitation starts and under cer- to another oil, it causes an increase in the pour point from
tain temperature, pressure, and composition how much solid 21 Cto25 C [14].
◦
◦
can be precipitated from a petroleum fluid? These two ques- Asphaltenes are multiring aromatics (see Fig. 1.2) that are
tions are answered in this section. Since solids are formed insoluble in low-molecular-weight n-paraffins (LMP) such as
at low temperatures, under these conditions the amount of C 3 , n-C 4 , n-C 5 , or even n-C 7 but soluble in benzene, carbon
vapor produced is low and the problem reduces to SLE such disulfide (CS 2 ), chloroform, or other chlorinated hydrocar-
as the case for asphaltene precipitation. Initially, this section bon solvents [15]. They exist in reservoir fluids and heavy

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