Multicomponent paraffin waxes and petroleum solid deposits: structural and thermodynamic state

Fuel Vol. 77, No. 12, pp. 1253-1260, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/98 $19.OOtO.o0

PII: SOOl6-2361(%)00032-S

ELSEVIER

sits: thermodynamic state M. Dirand*, V. Chevallier, E. Provost, M. Bouroukba and D. Petitjean Laboratoire de Thermodynamique des Sbparations, Ecole Nationale Supkieure des Industries Chimiques, lnstitut National PO/y-technique de Lorraine, 1, rue Grandville BP 457 F54001, Nancy Cedex, France (Received 10 December 1997)

The X-ray diffraction analyses, carried out on eight commercial and industrial waxes and a heavy crude oil, show the following remarkable results: (i) each multicomponent paraffin wax (from 20 to 33 n-alkanes), which has a continuous distribution of consecutive n-alkanes (19 < n < 53), forms a single orthorhombic solid solution; (ii) the molecule packing identity period along the long c-axis of this solid solution corresponds to a chain length of a hypothetical orthorhombic n-alkane whose carbon atom number is equal to the average carbon atom number of nalkanes contained in each multicomponent paraffin wax. This multicomponent phase, whose orthorhombic structure is analogous to one of the two intermediate solid solutions, p’,, or p”,, of binary and ternary molecular alloys of consecutive n-alkanes, is also observed in the deposit of the heavy crude oil with the presence of an amorphous solid. 0 1998 Elsevier Science Ltd. All rights reserved (Keywords: petroleum; wax; n-aikanes; multicomponent;

NOMENCLATURE triclinic phases orthorhombic phases ‘Cristal’

phases at ‘room tempera-

ture’ -pure alkanes and terminal solid solution: index 0 Yo (Cz,) PO (C,,l)

triclinic phase Pi of even-numbered czp (n = 2P) orthorhombic phase Pbcm of oddnumbered Czr,+l (n = 2p + 1)

-orthorhombic intermediate solid solutions: fi’, and p,: index n = 1 or 2 to identify isostructural phases of different stoichiometries on both sides of the middle intermediate solid solution in the same binary or ternary systems ‘Rotator or Plastic’ phases at ‘high temperature’ orthorhombic phase (Fmmm); this B phase presents a ‘Rotator’ state called P-RI wRII

rhombohedral

Rotator phase (R3m)

solid solution)

fuel consumers. The development of adequate thermodynamic models lP4 to represent the behaviour of petroleum cuts necessarily requires the characterization of the state of these solid deposits. According to Srivastava et ak5, the petroleum waxes are multicomponent mixtures of high molecular weight saturated hydrocarbons, predominantly paraffins, in the range c18-c65. For these reasons, our laboratory has chosen to study the following points:

(1) the thermodynamic properties and structural states of pure n-alkanes (hereafter denoted by C,, 17 < n < 27)6, binary7-25 and ternary26 mixtures of consecutive C, (19 < n < 29) differing by one or two carbon atoms. (2) the solubility of pure C, and their binary mixtures in organic solvents with molecules consisting of seven carbon atoms (heptane, toluene, methyl cyclohexane)27-“4. Now, this research is extended to multicomponent systems: the aim of this study is to determine the structural states of solids, composed of a series of heavy C, (19 < n < 53) with different distributions, as obtained in solid deposits from waxy crude oils observed in pipelines and paraffin waxes produced in refineries.

INTRODUCTION of the solid deposits in crude oils and middle distillate fuels poses a constantly recurring problem in the petroleum industry and in very cold regions for the diesel-

The formation

* Author to whom correspondence

should be addressed

SOME LITERATURE RESULTS Pure n-alkanes The structural configuration of C, was described in 1925 by Muller and Saville35; then Muller36 observed the existence of a solid-solid transition was observed by X-ray

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solid deposits:

Table 1 Miscibility of binary n-alkane mixtures (C,,:C,,,) versus number difference of carbon atoms in solid state at ‘room temperature’ according to Kravchenko’s predictions 47 An = n - II’

Total miscibility

Partial miscibility No miscibility

1 2 4

n > 16” II > 33 II > 67

1l>n>l 34>n> 13 68 > n > 27

,I < 8 II < 14 n < 28

“In this case, if the two consecutive CzP and CQ,+~do not have the same crystalline structure, they cannot form a continuous solid solution

diffraction when the temperature increases, with the appearance of ‘Rotator’ phases below the melting point of C, (n = 18-44). Since then, the crystalline structures of C, have been the subject of several publications in the literature: particularly, Turner37, Heyding et a1.38, Gerson et a1.3g, and Craig et a1.40 have presented exhaustive monographs; therm&l namic data of C, are also to be found i! in literature6’23’25’41-4 . Binary, ternary and quaternary

(1) at ‘room temperature’, the existence of limited terminal solid solutions with the pure C, structures and many intermediate solid solutions with orthorhombic cells as the odd-numbered C 2p+1: in all these binary systems, they are isostructural with two phases only, called /3’, and p”,, but they are not isostructural with the orthorhombic pure CZp+r structures nor with their terminal solid solutions. laws of solid phase sequences (2) the phenomenological when the composition varies. (3) for the two orthorhombic intermediate solid solutions, P’, and P”,, an analogous behaviour to the pure oddnumbered C,r+, (C25 or C25)18’36,73-75 when the temperature increases with the appearance of ‘Rotator’ states of the orthorhombic P-RI and rhombohedral (YRI1 phases, respectively, with the space group (Fmmm) and (R3m)76.

Nouar et a1.26 have shown that these orthorhombic intermediate solid solutions p’, and p”, are also present in ternary molecular alloys of consecutive C, (C*2:C&*4), and Clavell-Grunbaum et a1.77 observed orthorhombic (room temperature) and hexagonal (high temperature) solid solutions in several model waxes consisting of 2, 3 and 4 C, with chain length differences of four carbons between their components.

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in organic solvents

The experimental results of Provost3”.34 regarding the solubility of pure C, (23 < n < 28) in organic solvents with molecules consisting of seven carbon atoms confirm the research works of Chang et a1.78, Arenosa et al.‘“, Domanska et a1.80-82, Ksiazcza et al.‘“, Madsen and Boistelles4, and Ghogomu et a1.27. However, the joint structural and thermodynamic studies33,34 show that:

(1) in the rich region of heavy C,, the solid, which forms

(2)

molecular mixtures

The binary mixtures of C, (n > 12) have been studied by many authors7-25,47-71. From the difference factor of the molecule lengths, Kravchenko47 predicted (Table 1) the type of probable equilibria in the binary systems of C,, as did Hume-Rothery’* for metallic alloys. However, for mixtures which did not show a continuous solid solution according to Kravchenko’s predictions (An/n > 0.06)47, Smith48 highlighted the existence of orthorhombic intermediate solid solutions in the binary systems of C, (18 < n < 36): particularly in (C&& mixtures with composition ratios 11 1 and 2/l (the structure of the pure C24 crystalline phase is triclinic, and that of CZ6 is triclinic or monoclinic35-40). The structure of one of these orthorhombic intermediate phases was described by Luth and Nyburg55, and Gerson and Nyburg@, respectively in the (C2O:C22)and (C2.+:&,) systems. For the binary molecular alloys of consecutive even:even, odd:odd and even:odd-numbered C, (19 < n < 27), Dirand et al.2o,22 established:

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M. Dirand et al.

the first deposit, is the ‘Rotator’ phase of the heavy pure C,: for these mixtures (C, + solvent) which are poor in solvent, the solid-solid transition temperature, measured by differential scanning calorimetry and simple thermal analysis with decreasing or increasing temperature, is always equal to that of heavy pure C,. then, when the concentration of the solvent increases, the solid, which forms the deposit, corresponds to the triclinic yo(C+) or orthorhombic fio(C2,,+,) ‘room temperature’ crystalline phase of the pure even-numbered C2r or odd-numbered CZp+,, respectively.

Concerning the studies of the solubility of heavy C, binary mixtures in the same organic solvents, carried out in our laboratory, Ghogomu et a1.27, and Provost et al.33334 observed a remarkable phenomenon: the addition of one long C, to a short one leads to an increase of the solubility of the latter in the solvents. Ghogomu et a1.27 also determined that the solubility of each binary mixture (C22 + C24 or C25 + CZ4) in ethylbenzene was greater than the solubility of the corresponding hypothetical ideal mixtures. Ghogomu’s results, confirmed recently by Floter et a1.85 who have studied the solubility of binary mixtures (C22 + C24) in heptane, showed that solid binary mixtures, which form the deposit, had a rather complex behaviour and could not be treated as an ideal solid solution. The joint structural and thermodynamic study of an equimolar mixture of (C26:C28) in heptane, carried out by Provost et al.33,34, leads to the following new results: (1) for the mixtures (C26:C28 + solvent) which are poor in solvent, the solid, which forms the deposit, is the Rotator phase as for solutions with a single heavy pure C,. (2) when the concentration of heptane increases, the solid which crystallizes is a phase whose orthorhombic structure is different from the triclinic or monoclinic ;;y4sture of CZ6, and the monoclinic structure of CZ8 This phase, observed by X-ray diffraction in equilibrium with the liquid, is isostructural to the orthorhombic intermediate solid solution @‘r, observed in the consecutive even-numbered Cz+, b&u-y systems 7-11,13,15,16and also in the consecutive odd-numbered C2p+l binary molecular alloys 14S18,19:the chemical analysis shows that the concentration of the phase which forms the deposit is greater in Czs. This research is extended to multicomponent mixtures in order to characterize the structural state of the solid deposits in crude oils and paraffin waxes. EXPERIMENTAL Nine samples notation:

have

been

examined

(1) No. 1: a heavy crude oil consisting (2)

with

the following

mainly of a solid deposit in equilibrium with a small amount of liquid. Nos 2-5: four paraffin waxes, purchased from

Molticomponent

paraffin

Wax No 2 co~~posbion

waxes and petroleum

solid deposits:

M. Dirand et al.

Wax No 4

16.00 14.00 12.00 b0 10.00

3 Z

E

8.00

6.00

Wax No 5 composition

12.00

I

e

b

$

10.00 8.00

1

g 6.00

r

4.00 1 2.00

Figure 1

Distribution

(a:I

t

variations of n-alkanes in the commercial

waxes Nos 2-5

,

18 19 20 21

I

I

I

I

I

I

2 3 4 5 6 7

I

I

1

IIIIII

8 91(

12 13 14 15 16 17

)

II 1

III/

III

II

I

I

I

I

I

I

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

B(“)hK, Cu

)

B(“)hK, Cu

@I,

I

z -

I

1 2 3 4 5 6

I

I

I

18 19 20 21

B(“)hK, Cu

7 8 9 10 11 12 13 14 15 16 17

EI(“)LKaCu

1234567

e(‘=)hKa Cu

Figure 2 X-ray diffractograms of the orthorhombic intermediate solid solution PI, observed in the binary mixture (Ca6:50 mol % C2s) (a), the heavy crude oil (b), the powder of wax No. 2 (c), and a sample of wax No. 2 (d) which has been melted and low cooled on a water surface

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Multicomponent

Table 2

c20 c21 c22 c23 C24 c25 C26 C27 C28 C29 c30 c31 C32 c33 c34 c35 C36 c37 C38 c39 c40 c41 C42 c43 c44 c45 C46 c47 C48 c49 c50 c51 C52

paraffin

waxes and petroleum

n-Alkane mixtures of molar concentrations

solid deposits:

in the commercial

M. Dirand

et al.

waxes Nos 2-S

No. 2 molar%

No. 3 molar%

No. 4 molar%

No. 5 molar%

0.070 0.812 4.778 13.057 20.360 18.464 14.594 9.236 6.568 4.682 3.35 1 2.051 1.050 0.424 0.198 0.104 0.062 0.045 0.030 0.029 0.014 0.014 0.007

0.046 0.448 2.885 8.004 13.105 14.346 15.458 13.255 11.438 8.853 6.237 3.445 1.560 0.563 0.206 0.078 0.034 0.016 0.008 0.008 0.008

0.016 0.061 0.437 1.814 5.081 9.194 14.461 15.629 15.808 14.003 11.374 7.143 3.284 1.101 0.378 0.129 0.054 0.017 0.008 0.008

0.038 0.182 0.678 2.078 4.254 6.594 8.476 10.6 I5 11.756 11.722 10.376 8.807 6.702 5.072 3.572 2.594 1.831 1.419 1.039 0.639 0.489 0.318 0.246 0.161 0.113 0.068 0.058 0.033 0.024 0.016 0.015 0.008 0.007

-

-

-

PROLABO, respectively, called paraffin 52-54°C 5456°C 58-60°C 60-62°C in its commercial catalogue. (3) Nos 6-9: four paraffin waxes, produced in a refinery of a petroleum company and quasi-exclusively composed of n-alkanes as imposed by legal standards for public consumption.

reasons, the compositions of samples Nos 6-9 cannot be published, but the petroleum company communicated the medium carbon atom number fi of C, contained in each sample (Table 3).

The X-ray diffraction experiments, using a copper anticathode X-beam (XCu KcY), were carried out on powder samples with:

RESULTS

(1) a Guinier-De Wolff Nonius camera: the positions of lines, observed on the photographic patterns, were determined with an accuracy of 0.25 mm for distances ranging from 10.5 to 125 mm. The calibration was obtained with spectroscopic pure gold as standard: this X-ray diffraction method provides a good separation of reflections and the observation of (h k Z) diffraction lines at wide Bragg angles. (2) an X-ray diffractometer (CGR theta 60): this method allows the exploration of the region of small Bragg angles, particularly the diffraction lines (0 0 I>, which defined the molecule packing identity period along the long c-axis. The calibration was obtained with the sample holder which is made of pure copper. The preparation of powder samples has already been described9-26. However, in order to obtain preferential crystallographic orientations and thus to increase the intensity of the diffraction lines (0 0 1), samples were obtained by melting and low cooling of waxes on a water surface, e.g. Langmuir’s phases. The n-alkane compositions of sample Nos 2-5 were determined by gas chromatography and mass spectrometry analyses; they are shown in Table 2 and Figure 1. For secret

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The experiments carried out on sample No. 1 by X-ray diffraction [Figure 2(b)] show that the deposit in equilibrium with the liquid in the heavy crude oil consists of:

(1) an amorphous solid; (2) an orthorhombic crystalline phase which is particularly characterized by the diffraction lines (1 1 0) + (1 1 1) and (0 2 0), as observed for the orthorhombic intermediate solid solution p”r of a binary mixture (Figure 2(a)), and near the origin of the diffractogram by the small peaks (0 0 2) and (0 0 4) which show the existence of a molecule packing identity period along the long crystallographic c-axis. Since it is not possible to separate these two solids, the observations of the other samples (Nos 2-9) allow us to define the type of crystalline phase and the molecule packing identity period along the long c-axis, supposing that the crystalline phase is predominantly composed of C,. For all these multicomponent mixtures of C,, the X-ray analyses (Figures 2 and 3) lead to the following results: (1) the amount of amorphous solid detected by this experimental method is very small or non-existent. (2) a single family of diffraction lines (0 0 2) appears on the diffractograms (Figure 3): for each sample, these peaks define a single molecule packing identity c period and

Multicomponent

paraffin waxes and petroleum

solid deposits: M. Dirand et al.

Table 3 Comparison of equivalent carbon atom number ii,, calculated from the molecule packing identity c period with the variation equation of the c-axis of orthorhombic pure C, (Fig. 4), and the average number fi of carbon atoms of C, contained in each wax clnm

Wax no.

9.29 7.14 7.43 7.61 8.08 6.92 7.41 8.52 9.69

A, It 0.5

A

An = fiC - ii

35 26.5 27.1 28.4 30.2 25.7 27.6 32 36.6

crude oil 25.52 26.43 27.76 29.37 25.1 27.45 31.02 36.06

0.98 I .27 0.64 0.83 0.6 0.15 0.98 0.54

thus

the presence of a single crystalline phase: c = l*dW1 with Bragg’s relation 2*dMl sin0 = X. (3) these orthorhombic solid solutions are isostructural with one of two intermediate phases fl’, or p”,, observed in the binary or ternary mixtures of consecutive C, 7-25. No3

From the linear equation of variations of orthorhombic pure C, c-parameters in the function of the carbon atom number II,, determined by optimization from the literature data36-40 (13 < n < 61, Figure 4), it is possible to establish a relationship between the molecule packing identity c period and a corresponding equivalent carbon atom number ii, (Figure 4, Table 3), and then to compare it to the average number ii of carbon atoms, determined from the molar in concentrations of C, (Table 2) by gas chromatography each sample. The equivalence between ii, and ii is almost perfect with an excess value lower than one carbon atom for most waxes: an excess value of the c parameter is indeed observed between the molecule packing identity c period of intermediate solid solutions firn or p”, of consecutive C, binary mixtures, and that of a corresponding hypothetical orthorhombic pure C, with an equivalent average carbon atom number fi (Figure 5, Table 4). This excess value is very likely to be due to the conformational disorder of chain packing in the multicomponent crystalline solid solution et a1.77: the such as that described by Clavell-Grunbaum equivalent carbon atom number ii, of the phase defines the space between the molecule packing crystalline planes; the chains which have a carbon atom number higher than E, must bend to insert themselves between these crystalline planes, and the molecule packing layers must organize themselves to adjust the considerable difference of molecule chain lengths (e.g. the smaller molecules are associated with the higher ones, etc. Figure 6): the disorder in the interfacial planes of molecule packing probably generates a molecule configuration order in packing layers.

Lmw 002

No7

No 6

L

004

004

1

2

006

006

3

4

1

3

2

4

00

W)

L

CONCLUSION

002

No8

004

I

2

3

W”)

4

_)

I

2

3

4

W”)

Figure 3 X-ray diffractograms of all the commercial and industrial multicomponents waxes: observation of a single family of diffraction lines (0 0 I) which allows the determination of a single molecule packing identity c,,~ period and the presence of a single crystalline phase in each sample

The X-ray diffraction analyses show the following remarkable results: the multicomponent paraffin waxes, which have a continuous distribution of consecutive C!,, as studied here, form a single orthorhombic solid solution; the molecule packing identity period along the long crystallographic c-axis of this solid solution corresponds to an equivalent carbon atom number ii, equal to the medium carbon atom number ii of the C,, which compose each paraffin wax, with an excess value lower than 1 carbon atom for most waxes. This multicomponent orthorhombic phase is identical to one of the two orthorhombic intermediate solid solutions /3’n or p”, of binary and ternary mixtures of consecutive C, 7-25. However, as in these binary and ternary systems where

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M. Dirand et al.

solid deposits:

15 13

c=

0.2545n, + 0.3842 R2 = 0.9993

131517192123252729313335373941434547495153555759 carbon atom number II,

Figure 4 Plot and variation equation of crystallographic c-axis length versus chain carbon number n, for the orthorhombic unit cells of pure C, (12 -==I n < 61, 37-4o)

Table 4 Comparison between the crystallographic c-axis parameters c,r of orthorhombic intermediate solid solutions fl’ and p”, of IZalkane binary mixtures 5-z and the c parameter values obtained from the variation expression of crystallographic c-axis of pure alkanes for an equivalent medium carbon atom number fi(Fig. 4):Ac = ceXp- c, excess value Intermediatephases

Binary mixtures mol % A

cex#nm

clnm

Achm

p”,

C*,:4% c*3 Cz,:80% c*3 Cz2:80% Cz3 C,,:40% c*‘j Cz,:lO% c*4

5.806 6.186 6.213 6.27 6.3 6.288 6.712 6.83 5.981 6.112 6.42 6.484 6.656 6.665 7.03

5.7491 6.1359 6.1868 6.1868 6.2632 6.253 6.6449 6.6704 5.8814 6.0748 6.3268 6.3904 6.4922 6.5431 6.8994

0.0569 0.0501 0.0262 0.0832 0.0368 0.0350 0.067 1 0.1597 0.0996 0.0372 0.0932 0.0936 0.1638 0.1219 0.1306

21.08 22.6 22.8 22.8 23.1 23.06 24.6 24.7 21.6 22.36 23.35 23.6 24 24.2 25.6

C&3% c25 C&:80%Czs C&35% c&j C21:30%c23 t&:18% CX Cz2:67.5%Cg4 t&:30% czs C&50% czs

0’”

CglO% c*fj C&:80% c*6

7.5

/

I

Figure 6 Probable scheme of the molecule configuration in packing layers: the molecules which have a carbon atom number higher than ii,, bend to insert themselves and associate themselves with the smaller ones 19

20

21

22

23

24

25

26

27

medium carbon atom number n

Figure 5 Excess values of crystallographic c-axis lengths of orthorhombic intermediate solid solutions /3’, and p”, observed in the binary mixtures of consecutive C, in relation to the c-axis values of hypothetical orthorhombic C, with a corresponding equivalent medium carbon atom number W(Table 4)

domains of two or three phases appear with varying concentration7-25, it is possible that two, three or many crystalline phases coexist in other multicomponent mixtures with different compositions and distributions of C,, according to the thermodynamic Gibbs’s laws, and the

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rules of Palatnik and Landau, concerning the phase equilibria in multicomponent systemsg6. But it appears that the crystalline phase, which forms a part of the solid deposit of the crude oil, is also identical to one of the orthorhombic intermediate solid solutions p’, or p”, of binary mixtures of consecutive C, that crystallize in the organic solvents33’“. In all likelihood, this solid solution is predominantly composed of heavy C, with an analogous distribution to those observed in commercial and industrial paraffin waxes that have been studied here. The question which is now raised about the development of an adequate thermodynamic model to represent the behaviour of petroleum cuts’-4s5-91, concerns the presence

Multicomponent

of the amorphous solid which is probably composed of all the other heavy hydrocarbons that are not found in the crystalline solid solution: this amorphous solid must be taken into account in liquid-deposit equilibria to determine the total quantity of the solid deposit.

paraffin

27 28 29

ACKNOWLEDGEMENTS

30

We gratefully acknowledge the Institut Franqais du P&role for the financial support of this research, and we are indebted to Mr Vacher (Manager of the Department of Paraffins, Centre Europken de Recherche Total, Harfleur, France) for the determination of commercial wax (Nos 2-5) compositions by gas chromatography analyses.

31

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24 25 26

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32 33 34

38 39 40 41 42

43 44 45 46

47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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solid deposits:

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al.

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