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Thermodynamics and mechanis of stabilization and precipitation of petroleum colloids
ELSEVIER
Fuel Processing Technology
53 (1997) 69-79
Thermodynamics and mechanism of stabilization and precipitation of petroleum colloids Horst Laux *, Iradj Rahimian, Thorsten Butz German Petroleum Institute, Walther Nernst Str. 7, Clausthal-Zellefeld Received 5 August 1996; accepted
D-38678, Germany
13 June 1997
Abstract The influences of different factors on the stability of colloid disperse crude oil systems are investigated by determining the flocculation points. The results are discussed on the base of theory of regular solutions considering the influence of solvation on the kinetics of chemical processes in liquid phases. Analogy to the behavior of polymer solutions was found. Differences result from the micelle structure of colloid particles. The criterion of metastability using nonpolar precipitants is defined by the Flory-Huggins interaction parameter. Using polar precipitants, the polar interactions and the solvation of asphalt require to use the Hansen solubility parameter to interpret the results. The influences of conditions of precipitation and of surfactants are discussed also. 0 1997 Elsevier Science B.V. Keywords:
Crude oil stability; Flocculation
points; Regular
solution theory
1. Introduction
The colloidal state of petroleum residues can cause several problems during processing stages, e.g., it determines the rheological behavior of products, decides about formation of coke in conversion processes and is the cause for fouling during transport. Therefore, detailed knowledge about the forming mechanism of the colloid disperse phase and about the factors responsible for stabilizing resp. precipitating the disperse phase is very important.
* Corresponding
author.
037%3820/97,/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO378-3820(97)00037-4
70
H. Laux et al./ Fuel Processing Technology 53 (1997) 69-79
The reasons for the formation of a colloid disperse phase are great differences in interaction forces among the components of residues of petroleum processing. Asphaltenes-condensed hydroaromatic, heteroatoms containing compounds-are only partially soluble under these conditions and form crystallites by T-T-bonds among the condensed aromatic rings. Yen et al. [ 11 proposed a corresponding model of asphaltenes crystallites. Determining the paramagnetism, Unger and Andrejeva [2] concluded, that the crystalline part of native asphaltene does not exceed 4 to 5%. However, in asphaltenes of crack residues it may amount up to 20%. Colloid disperse primary particles in the size of about 3 nm are formed by solvating these crystallites. Model investigations [3] and rheological examinations [3,4] have shown that the ratio of the volume of the colloid particles to the volume of asphaltenes is between 2.5 and 3.5. With that, the volume of the solvation layer is greater than the average content of resins. Decreasing metastability of the colloid disperse system effects the aggregation of primary particles and, finally, the precipitation of colloid disperse phase. Neumann [5] defines petroleum colloids as resoluble, polydisperse, spherical micellar colloids. The valuation of the thermodynamic and kinetic factors determining the stability of colloid disperse crude oil systems is difficult. Both the dispersion medium and the colloid disperse phase of petroleum residues are complex mixtures. The distribution of components of these phases overlaps considerably. A direct determination of the phase fractions and their composition is not possible. Conclusions on fraction and composition of the colloid disperse phase can only be drawn by means of precipitation. The precipitation result depends on the chosen precipitant and the precipitation conditions. Determining flocculation points is a practical method to investigate influences of different factors on the stability of colloid disperse crude oil systems.
2. Experimental The flocculation points were determined by a titration method developed at the German Petroleum Institute. A precipitant is added at a constant rate to a solution of the petroleum product under intensive stirring and under given conditions of pressure and temperature. The titration is monitored by means of a light intensity meter. Fig. 1 shows the measured light intensity as a function of the added amount of precipitant. The following phenomena can be observed during time: dilution of the sample by adding precipitant-increase of the light intensity; aggregation and coagulation of colloidsdecrease of the light intensity; end of the coagulation-increase of the light intensity by further dilution. The maximum of the light intensity curve is defined as flocculation point. Because it was found, that the concentration of the solution, the titration rate, the stirring speed and the shelf time of prepared samples influence the result, the standard conditions were chosen as: concentration: 5 wt.%, temperature: 25X titration rate: 1
H. LRUX et al. / Fuel Processing
Intensity 0.4
I
Temperature \ I
71
Technology 53 llY97) 6%7Y
I
[“Cl
I
132
31 30,s 30 29,5 29 26s 26 0
0,l
0,2
0,3
0,4
0,5
0,6
0,7
08
0,9
1
Time [h] Fig. 1, Dependence
of light intensity during determination
of flocculation
points.
cm3 min-‘, stirring speed: 200 rpm and shelf time of prepared samples: 5 days. A 20-cm3 solution was used for the flocculation point determination. To evaluate all influencing factors and to derive criteria of precipitation, experiments were carried out varying the solvent-precipitant pairs and the composition of the available products. The used solvents and precipitants, characterized by Hildebrand [6] and Hansen solubility parameters [7], are compared in Table 1. Table 2 gives a short characterization of the investigated crude oils.
Table 1 Solubility
parameters
of the applied solvents [ 141
Compound
Effect
Hildebrand
i-Oktane n-Pentane n-Hexane n-Heptane Cyclohexane Toluene Decalin Tetrahydrof. Ethyl acetate I-Butanol 1-Propanol Ethanol Methanol
Precip. Precip. Precip. Precip. Solvent Solvent Solvent Solvent Precip. Precip. Precip. Precip. Precip.
14.1 14.5 14.9 15.1 16.8 18.2 18.0 18.6 18.6 23.3 24.3 26.0 29.6
6 (mJ m-j j” ’
Hansen (m.l m ’)” ’
6.3
%
4
4
14.3
0.0 0.0 0.0 0.0 0.0 I .4 0.0 5.1 5.3 5.7 6.8 8.8 12.3
0.0 0.0 0.0 0.0 0.2 2.0 0.0 8.0 1.2 15.8 17.4 19.4 22.3
14.3 14.5 14.9 15.3 16.8 18.2
14.5
14.9 15.3 16.8 18.0 18.0 16.8 15.8 16.0 16.0 15.1 15.1
18.0 19.5 18.1 23.1 24.5 26.5 29.6
H. L.aux et al. /Fuel
72 Table 2 Characteristics
of investigated
Crude oil
Processing Technology 53 (1997) 69-79
crude oils
Density (g cme3)
H:C ratio
Yield (wt.%) Resins
Arabian heavy BCF Black minnel California Laguna Maya
0.8801 0.9121 0.9396 0.9855 0.9890 0.9201
1.67 1.65 1.60 1.58 1.52 1.60
Asphaltenes
0.51 1.67 0.69 0.47 1.21 0.20
esa
msb
lSC
0.6 1.2 2.9 2.0 2.0 0.9
0.4 0.8 1.2 1.1 1.4 0.1
3.6 3.7 6.0 7.4 7.6 9.7
aEasily soluble. bMiddle soluble. ‘Low soluble.
In addition, flocculation points of solutions For that, the asphaltenes were fractionated different cyclohexane-i-octane mixtures. The was 1 wt.%. The effect of adding surfactants
of asphaltene fractions were determined. in easy, middle and low soluble using concentration of the asphaltene solutions was investigated also.
3. Results To value the flocculation points, the solubility parameters of crude oil products were mixture at the flocculation calculated by Eq. (1) [8], th ose for the solvent-precipitant points by Eq. (2) [9]. 3.2285 0.00464 + ~
( ZRa -2)
+A&,
%
(2) Table 3 shows the results for four crude oils. It is shown: (1) Calculated solubility parameters of asphaltenes lie in the range of experimental results of Zenke [lo] and Lian et al. [II] (18.6 to 22.0 resp. 17.6 to 21.7 (mJ m-3)0.5>. (2) Using the polar precipitant ethyl acetate, the solubility parameter at flocculation points is higher than by applying nonpolar precipitants. (3) Solubility parameters at the flocculation point using nonpolar Table 3 Calculated solubility parameters of solvent-precipitant oils, characterized by the average solubility parameters Crude oil
8, (mJ m-3)o.5
Arabian heavy Black minnel Laguna Maya
21.65 20.75 21.17 21.33
aEthyl acetate.
mixtures at the flocculation points of stabilized crude of asphaltenes (solvent: toluene)
S, (ml m-3)o.5
using as precipitant
i-C 8
n-C,
n-C,
n-C,
etaca
15.3 15.4 15.1 15.6
15.5 15.8 15.4 15.8
15.6 15.7 15.6 16.0
15.9 15.8 15.8 16.2
18.5 18.4 18.4 18.4
H. L.aux et al. /Fuel
13
Processing Technology 53 (1997) 69-79
Table 4 Hansen solubility parameters (ml mm3 )’ 5, calculated ethyl acetate as precipitant and toluene as solvent
for solvent composition
at flocculation
points, applying
Crude oil
V,,,, (cm’)
6,
6,
6,
6,
Arabian heavy Black minnel Laguna Maya
34.5 22.6 22.6 17.9
16.6 16.8 16.8 17.0
3.9 3.5 3.5 3.2
5.3 4.8 4.8 4.5
17.3 17.8 17.8 17.9
precipitants are below that of cyclohexane, the solvent with the lowest solubility parameter. (4) Independent of applied solvents, the solubility parameter at the flocculation point increases with the solubility parameter of the applied nonpolar precipitant. (5) A significant influence of the solvent cannot be observed. The interactions between asphaltenes and other petroleum components can, nevertheless, only partly be attributed by the Hildebrand solubility parameter. Polarities and formation of hydrogen bonds should be considered. Therefore, the Hansen parameters of solvent-precipitant mixtures at the flocculation point were calculated to study especially the influence of polarity of precipitants. In Table 4, the data are listed for four crude oils taking ethyl acetate as precipitant. Using the alcohols methanol to n-butanol as precipitant, the determination of flocculation points with n-propanol and n-butanol did not result in a maximum in the light intensity curve (see Fig. 2). An insufficient coagulation of unstable colloid particles is to be presumed. If, in this case, the points of inflection are used for the calculation. the values in Table 5 are obtained.
I,4 3 B z
I,2
; .*
I,0
f
0,a
E
0,6
100
Precipitant Fig. 2. Dependence of light intensity during determination crude oil in decalin using alcohols as precipitants.
I [ml] of flocculation
points of a 5% solution of Maya
14
H. Lmx et al. / Fuel Processing
Table 5 Hansen solubility parameters using alcohols as precipitants V,,,
Methanol Ethanol n-Propanol
13.47 15.86 37” 165b 27.5
n-Butanol
(mJ rnd3 )“.5, calculated for solvent-precipitant (crude oil: Maya, solvent: decalin)
(cm’)
Precipitant
Technology 53 (1997) 69-79
mixtures
at flocculation
8,
6,
6
6,
16.7 16.7 16.7 16.2 16.8
4.8 3.9 4.4 6.1 3.3
8.7 8.6 11.3 15.5 9.1
19.4 19.2 20.6 23.1 19.4
points,
“First angle point. bSecond angle point (at Fig. 3).
Using a solution of 1% of asphaltene fractions in toluene, the dependence presented in Fig. 3 is obtained. The precipitant is i-octane. The higher the solubility parameter of the asphaltene fraction, the higher is the solubility parameter of solvent-precipitant mixture at the flocculation point. The addition of surfactants shows different effects on the flocculation points. The flocculation points were higher as well as lower than those of the original solutions. The influences of temperature on the flocculation point between 0 and 50°C and the influences of pressure from 1 to 70 bars were investigated. The following tendencies could be observed. (1) No significant influence of temperature was found, when using nonpolar precipitants. (2) By application of ethyl acetate, the necessary amount of precipitant increases slightly with increasing temperature. (3) Increasing pressure leads to a slight decrease of the necessary amount of precipitant at 25°C.
16,4 ---...
Arabian Heavy BCF BlackMinnel California Laguna Maya
____-~_____-r-_-__f-____l--___--~ I I I V: . . . . ..._._.__~__.___....__...___..I_.___...._._____.___.I___ : 0, I I _‘ _____+_-_-_-~______c-_-_-__~-__--~____-~ 16,0 ---I 0 %i : m I . ..____....__L_.__...__._...___...L_......__....._____._.._. . . . .. ,5,6___-___~_____J_____-L_ ______L_____I_-___J______ . . .. . . . . . . . . . ..I 15,6__-__--+_-__--;-_-_
16,2
----
.
p
0 m A
. . . . . . . .._~....
(&
. ..__....__.
l5,4__-___-;------~_---_-L___-_ . . . . . . . . . . . . . . . . . . . . ..~................
_.._____..I____....._.__....._..~........-.. ;_-_--_~__-__-~--_______-__~
----
--
...I r.____..__:..____._._)__.__.._..:...______.)...........
1 : ;-_-__ _______;-___-_ . .__.....__~_._____ :.___.. easily soluble asphaltenes .: ..___. .; I. . . . . . . . .. ~~,~_____-~-____~__-_-_~_-___+_---_~_--_--~-_-__~______l______ . .j.RD . . . .. _.....__.::._.._..___: ._____.__~ . . . ..__.._. _____..... i ._ .._._.: . I I I I
15,2
__-_-__;-#-“+_-_
14,8 19
20
21
23
22
6as Fig. 3. Dependence of solubility parameters solutions of crude oil asphaltenes.
of solvent-precipitant
mixtures
at flocculation
point using 1%
H. Laux et al. / Fuel Processing Technology 53 (1997) 69-79
75
4. Discussion To interpret the results of flocculation are taken in consideration: (1) solubility equilibrium of asphaltenes dissolved (2) adsorption particles
asphaltene
+ asphaltene
equilibrium
and
crystallites
interface
asc + solvate cover + dispersion (3) aggregation
point determination,
(asc)
equilibrium
medium
the following
processes
(3) during
formation
of primary
(4)
and coagulation
primary particles f particle aggregate + flock
(5)
The solubility equilibrium of asphaltenes can be neglected because the solubility of asphaltenes in organic solvents is very small. The behavior of colloid particles at the conditions of flocculation point determination was found like the behavior of polymer solutions [ 12,131. For example, the influence of concentration on the flocculation points of petroleum residues in solution is analogous to the dependencies of polymer solutions [15]. Eq. (6) was originally derived by Elias and Gruber [14] for polymer solutions. log @r,,, = log
@ait
-log
cpol
(6)
Where @r,,, is the volume fraction of precipitant at the flocculation point; cPO, is the concentration of the polymer at the flocculation point; and QCril is the critical volume fraction of precipitant at a polymer concentration of cpo, = 1 g cm ‘. Eq. (6) expresses the concentration influence on kinetics of coagulation and formation of flocks, as it is to be expected according to Eq. (5). Taking into account the behavior of polymer solutions, the following relationships were used to interpret the flocculation point results obtained by applying nonpolar precipitants. According to Kuhn [15], the solubility of polymers is determined by a critical value of the Flory-Huggins interaction parameter Xcrit. This critical value is to be calculated according to Eq. (7) neglecting the entropy term. XCrlt= 0.5( 1 + m-0.S)2
(7)
Accordingly, the solubility of a polymer is determined by the ratio of the molar volumes of polymer and solvent m. The upper limit of the molar mass distribution is decisive for the solubility of the polymer. On the other hand, the Flory-Huggins interaction parameter is a function of the solubility parameters of solvent and solute [6]: x=
-g-q,
-
8,)’
(8)
H. Laux et al./ Fuel Processing Technology 53 (1997) 69-79
76
As a criterion
for metastability,
we can express:
X S Xcrit
(9)
In colloid disperse petroleum systems, the limiting value of metastability should be determined by the particle volume of petroleum colloids. For the calculation of the Flory-Huggins interaction parameters, the solubility parameter at the surface of the colloid particle a,,,, is to use. With attention to the gradient in the solubility parameter among the solvate layer A6, it is valid: s part = &IS- A 6 On the base of flocculation
(10) point results, it was found [16]:
Xcrit = 0.62 + 0.04
(11)
These value corresponds to the colloid particle volume with a diameter dpart = 3 to 4 nm. The influence of solvation could be investigated by comparing flocculation points of solutions of asphaltenes and original complete residues. A &values between 0.4 and 1.4 (mJ m-3)o.5 were obtained. The reason of different values of solubility parameters at the flocculation point when applying n-alkanes as precipitants is their different molar volumes. Due to the influence of structure, results with i-octane as precipitant are not to be integrated into these relationships. If applying polar precipitants, Eqs. (8)-(10) are not valid. One has to consider: the theory of regular solutions requires the application of Hansen solubility parameters; and an influence on the solvation of asphaltenes cannot be excluded. According to the values in Table 1, the transition from solvent to precipitant, if applying polar compounds, lies in the range of the solubility parameters of tetrahydrofuran (solvent) and ethyl acetate (precipitant). Differences in the Hansen parameters can only be observed in a limited degree. The data given in Table 6, like dielectric constant E, dipole moment ,z, molar refraction R, and surface tension y do not exhibit significant differences either. When applying Hansen parameters, the limits in solubility of a polymer are given in Eq. (11) [9,17,18]. 4( %S - %Sl>’ + ($,s
- s,,.l>’ + (%,S - %S,)2
5 5
(12)
I.e., the solubility of a substance is given by a spherical space in the coordinates S,, S,, and S, by doubling the scale of the &axis. The center of this space is determined by the solubility parameters of the solute.
Table 6 Physical properties
of tetrahydrofuran
(thf) and ethyl acetate (etac) 1131
Compound
M (g/md
fb (“0
E
p (D)
R, km3)
y (dyn/cm)
thf etac
12.1 88.1
64-66 71.15
7.32 6.02
1.63
19.9 22.1
26.9 23.2
1.78
H. Lax
et al. /Fuel
Using the Hansen parameters
Processing
Technology 83 f 19971 69-79
of asphaltenes
estimated
17
by Zenke [lo]:
S, = 18.2 + 0.2; S, = 5.9 f 0.3; 6, = 6.4 f 0.2 [mJm-‘]0’5 the following values in the left side of Eq. (12) are obtained for tetrahydrofurane and ethyl acetate: for tetrahydrofurane, 4.31; and for ethyl acetate, 4.90. This result confirms the limitation of the solubility of asphaltenes in tetrahydrofurane as shown by Zenke
1101. The values in Tables 4 and 5 are in the expected range. A dominance of the disperse part of the solubility parameter 6, is recognized concerning the precipitation of petroleum colloids. Regarding the total solubility parameter a,, it is obviously impossible to make a statement about the solubility behavior of petroleum colloids in polar solvents. From the different results by application of nonpolar and polar precipitants, it may be concluded [ 131 that a polar precipitant as ethyl acetate destroys the solvate cover. The precipitation of resins by ethyl acetate proves this conclusion. In the case of nonpolar precipitants, the solvate cover is more or less stable. The observed stabilizing as well as destabilizing effects of surfactants depend on the petroleum product itself. A competition with native surfactants in the solvate cover must be assumed. The generally stabilizing effects of added petroleum resins confirm this presumption. An adaptation of the surfactants to the different crude oil systems is necessary if a stabilization is intended. From the positive effect of petroleum resins and of VD-III extracts of lubricant production on metastability of colloid disperse petroleum products [ 19,201, it can be concluded that aromaticity, polarity and-to a limited degree-molar mass of surfactants should be ranged between those of asphaltenes and dispersion medium. In this way, the gradual solubility gradient between the dispersion medium and the disperse phase is supported. This is a necessary condition for the colloid stability of petroleum systems. Nevertheless, the addition of petroleum resins and VD-III extracts is limited. Exceeding this limit leads to negative effects [ 191. An interpretation of the influence of temperature and pressure on the precipitation is very difficult because there is an influence of pressure and temperature on Eqs. (3) and (4) as well as on the process of aggregation and coagulation (Eq. (5)). The influence of temperature on the solubility parameters of compounds does not explain this picture [21]. Thiyagarajan et al. [22] found that entropic effects must be considered also.
5. Summary The determination of flocculation points of crude oil products allows to study the influences of different factors on the stability of the colloid disperse phase in these products. It is found, that the colloid disperse phase shows a behavior, which is well known from polymer solutions. Consequently, the theory of regular solutions provides an essential basis for the interpretation of precipitation results.
78
H. L.aux et al. /Fuel
Processing Technology 53 (1997) 69-79
Both the polarity and the formation of hydrogen bonds of asphaltenes and the solvation of particles are the reason that the application of nonpolar and polar precipitants make different results. The interpretation on the base of regular solution theory needs to distinguish between both. In case of nonpolar precipitants, the Hildebrand solubility parameters have to be used for valuation. If applying polar precipitants, the results may be evaluated by the Hansen solubility parameter. Different precipitation mechanisms are valid. Both mechanisms are borderline cases of the behavior of real systems. The overlapping of thermodynamic equilibria and kinetics of coagulation and aggregation complicates the quantification of all the factors influencing the process of precipitation. Further investigations are needed to study the influence of temperature and especially of pressure. Appendix A. Nomenclature C
m nC
R
44 T V ZRa
s’ A6
Ahe E
CL @
Concentration Ratio of molar volumes of polymers and solvent Carbon number Absolute gas constant Molar refraction Temperature (K> Volume, molar volume Hydrogen deficit Surface tension Solubility parameter Solubility parameter gradient Heteroelement correction of solubility parameter Dielectric constant Dipole moment Volume fraction
Subscripts
ale as cl-it d etac fP h P pre Part PO1 s, sl t
Alcohol Asphaltene Critical Dispersion Ethyl acetate Flocculation point H-bonding forces Polar precipitant Particle Polymer Solvent, solute Total
H. Lmx et al. /Fuel
Processing
Technology 53 (1997169-79
19
References [I] [2] [3] [4] [5] [6] [7] [8] [9]
T.F. Yen, J.G. Erdman, S.S. Pollak, Anal. Chem. 33 (1961) 1587. F.G. Unger. L.N. Andrejeva, Erdol u. Kohle 47 (1994) 18. H. Laux, I. Rahimian, H.-J. Neumann, ErdSl Erdgas Kohle 109 (1993) 368. D.A. Storm, E.Y. Sheu, Fuel 72 (1993) 233. H.-J. Neumann, Erdiil u. Kohle 34 (1981) 336. J.H. Hildebrand, R.L. Scott, Regular Solutions, New York 1962. CM. Hansen, Ind. Eng. Chem. Prod. Res. Dev. 8 (1969) 2. H. Laux, ErdGl Erdgas Kohle 108 (1992) 227. A.F.M. Barton, CRC: Handbook of solubility parameters and other cohesion parameters, CRC Press, Boca Raton, FL, 1985. [IO] G. Zenke, Zum Loseverhalten von ‘Asphaltenen’: Anwendung von Loslichkeitsparameter-Konzepten auf Kolloidfraktionen schwerer Erdolprodukte, PhD Thesis, TU Clausthal, 1989. [I 1’~H. Lian, J.-R. Lin, T.F. Yen, Fuel 73 (1994) 423. [I21 H. Laux, 1. Rahimian, A. Hatke, Chem. Techn. 44 (1992) 263. [13] H. Laux, I. Rahimian, A. Hatke-Antoniadis, Chem. Tech. 45 (1993) 222. [14] H.-G. Elias. U. Gruber, Makromol. Chem. 78 (1964) 72. [I51 R. Kuhn, Makromol. Chem. 177 (1976) 1525. [ 161 H. Laux, I. Rahimian, ErdBl u. Kohle 47 (1994) 430. [ 171 D.W. van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1990. [I81 B.-C. Ho, W.-K. Chin, Y.-D. Lee, J. Appl. Polym. Sci. 42 (1991) 99. 1191 Z.I. Sjunjaev. R.Z. Sjunjaev, R.Z. Safieva, Neftjanye Dispersnye Sistemy (Russ.), Chimija. Moskau. 1990. [20] H. Laux. I. Rahimian, Physikalisch-Chemische Grundlagen der Verarbeitung von Erdiiltickstlnden und Schweren Erdiilen, Verlag Papierflieger, Clausthal-Zellerfeld. 1994. [21] H. Laux, H. Kopsch, I. Rahimian, A. Hase, ErdSl Erdgas Kohle I IO (1994) 375. 1221 P. Thiyagarajan, J.E. Hunt, R.E. Winans, K.B. Anderson. J.T. Miller, Energy Fuels 9 (1995) 829.
Fuel Processing Technology
53 (1997) 69-79
Thermodynamics and mechanism of stabilization and precipitation of petroleum colloids Horst Laux *, Iradj Rahimian, Thorsten Butz German Petroleum Institute, Walther Nernst Str. 7, Clausthal-Zellefeld Received 5 August 1996; accepted
D-38678, Germany
13 June 1997
Abstract The influences of different factors on the stability of colloid disperse crude oil systems are investigated by determining the flocculation points. The results are discussed on the base of theory of regular solutions considering the influence of solvation on the kinetics of chemical processes in liquid phases. Analogy to the behavior of polymer solutions was found. Differences result from the micelle structure of colloid particles. The criterion of metastability using nonpolar precipitants is defined by the Flory-Huggins interaction parameter. Using polar precipitants, the polar interactions and the solvation of asphalt require to use the Hansen solubility parameter to interpret the results. The influences of conditions of precipitation and of surfactants are discussed also. 0 1997 Elsevier Science B.V. Keywords:
Crude oil stability; Flocculation
points; Regular
solution theory
1. Introduction
The colloidal state of petroleum residues can cause several problems during processing stages, e.g., it determines the rheological behavior of products, decides about formation of coke in conversion processes and is the cause for fouling during transport. Therefore, detailed knowledge about the forming mechanism of the colloid disperse phase and about the factors responsible for stabilizing resp. precipitating the disperse phase is very important.
* Corresponding
author.
037%3820/97,/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO378-3820(97)00037-4
70
H. Laux et al./ Fuel Processing Technology 53 (1997) 69-79
The reasons for the formation of a colloid disperse phase are great differences in interaction forces among the components of residues of petroleum processing. Asphaltenes-condensed hydroaromatic, heteroatoms containing compounds-are only partially soluble under these conditions and form crystallites by T-T-bonds among the condensed aromatic rings. Yen et al. [ 11 proposed a corresponding model of asphaltenes crystallites. Determining the paramagnetism, Unger and Andrejeva [2] concluded, that the crystalline part of native asphaltene does not exceed 4 to 5%. However, in asphaltenes of crack residues it may amount up to 20%. Colloid disperse primary particles in the size of about 3 nm are formed by solvating these crystallites. Model investigations [3] and rheological examinations [3,4] have shown that the ratio of the volume of the colloid particles to the volume of asphaltenes is between 2.5 and 3.5. With that, the volume of the solvation layer is greater than the average content of resins. Decreasing metastability of the colloid disperse system effects the aggregation of primary particles and, finally, the precipitation of colloid disperse phase. Neumann [5] defines petroleum colloids as resoluble, polydisperse, spherical micellar colloids. The valuation of the thermodynamic and kinetic factors determining the stability of colloid disperse crude oil systems is difficult. Both the dispersion medium and the colloid disperse phase of petroleum residues are complex mixtures. The distribution of components of these phases overlaps considerably. A direct determination of the phase fractions and their composition is not possible. Conclusions on fraction and composition of the colloid disperse phase can only be drawn by means of precipitation. The precipitation result depends on the chosen precipitant and the precipitation conditions. Determining flocculation points is a practical method to investigate influences of different factors on the stability of colloid disperse crude oil systems.
2. Experimental The flocculation points were determined by a titration method developed at the German Petroleum Institute. A precipitant is added at a constant rate to a solution of the petroleum product under intensive stirring and under given conditions of pressure and temperature. The titration is monitored by means of a light intensity meter. Fig. 1 shows the measured light intensity as a function of the added amount of precipitant. The following phenomena can be observed during time: dilution of the sample by adding precipitant-increase of the light intensity; aggregation and coagulation of colloidsdecrease of the light intensity; end of the coagulation-increase of the light intensity by further dilution. The maximum of the light intensity curve is defined as flocculation point. Because it was found, that the concentration of the solution, the titration rate, the stirring speed and the shelf time of prepared samples influence the result, the standard conditions were chosen as: concentration: 5 wt.%, temperature: 25X titration rate: 1
H. LRUX et al. / Fuel Processing
Intensity 0.4
I
Temperature \ I
71
Technology 53 llY97) 6%7Y
I
[“Cl
I
132
31 30,s 30 29,5 29 26s 26 0
0,l
0,2
0,3
0,4
0,5
0,6
0,7
08
0,9
1
Time [h] Fig. 1, Dependence
of light intensity during determination
of flocculation
points.
cm3 min-‘, stirring speed: 200 rpm and shelf time of prepared samples: 5 days. A 20-cm3 solution was used for the flocculation point determination. To evaluate all influencing factors and to derive criteria of precipitation, experiments were carried out varying the solvent-precipitant pairs and the composition of the available products. The used solvents and precipitants, characterized by Hildebrand [6] and Hansen solubility parameters [7], are compared in Table 1. Table 2 gives a short characterization of the investigated crude oils.
Table 1 Solubility
parameters
of the applied solvents [ 141
Compound
Effect
Hildebrand
i-Oktane n-Pentane n-Hexane n-Heptane Cyclohexane Toluene Decalin Tetrahydrof. Ethyl acetate I-Butanol 1-Propanol Ethanol Methanol
Precip. Precip. Precip. Precip. Solvent Solvent Solvent Solvent Precip. Precip. Precip. Precip. Precip.
14.1 14.5 14.9 15.1 16.8 18.2 18.0 18.6 18.6 23.3 24.3 26.0 29.6
6 (mJ m-j j” ’
Hansen (m.l m ’)” ’
6.3
%
4
4
14.3
0.0 0.0 0.0 0.0 0.0 I .4 0.0 5.1 5.3 5.7 6.8 8.8 12.3
0.0 0.0 0.0 0.0 0.2 2.0 0.0 8.0 1.2 15.8 17.4 19.4 22.3
14.3 14.5 14.9 15.3 16.8 18.2
14.5
14.9 15.3 16.8 18.0 18.0 16.8 15.8 16.0 16.0 15.1 15.1
18.0 19.5 18.1 23.1 24.5 26.5 29.6
H. L.aux et al. /Fuel
72 Table 2 Characteristics
of investigated
Crude oil
Processing Technology 53 (1997) 69-79
crude oils
Density (g cme3)
H:C ratio
Yield (wt.%) Resins
Arabian heavy BCF Black minnel California Laguna Maya
0.8801 0.9121 0.9396 0.9855 0.9890 0.9201
1.67 1.65 1.60 1.58 1.52 1.60
Asphaltenes
0.51 1.67 0.69 0.47 1.21 0.20
esa
msb
lSC
0.6 1.2 2.9 2.0 2.0 0.9
0.4 0.8 1.2 1.1 1.4 0.1
3.6 3.7 6.0 7.4 7.6 9.7
aEasily soluble. bMiddle soluble. ‘Low soluble.
In addition, flocculation points of solutions For that, the asphaltenes were fractionated different cyclohexane-i-octane mixtures. The was 1 wt.%. The effect of adding surfactants
of asphaltene fractions were determined. in easy, middle and low soluble using concentration of the asphaltene solutions was investigated also.
3. Results To value the flocculation points, the solubility parameters of crude oil products were mixture at the flocculation calculated by Eq. (1) [8], th ose for the solvent-precipitant points by Eq. (2) [9]. 3.2285 0.00464 + ~
( ZRa -2)
+A&,
%
(2) Table 3 shows the results for four crude oils. It is shown: (1) Calculated solubility parameters of asphaltenes lie in the range of experimental results of Zenke [lo] and Lian et al. [II] (18.6 to 22.0 resp. 17.6 to 21.7 (mJ m-3)0.5>. (2) Using the polar precipitant ethyl acetate, the solubility parameter at flocculation points is higher than by applying nonpolar precipitants. (3) Solubility parameters at the flocculation point using nonpolar Table 3 Calculated solubility parameters of solvent-precipitant oils, characterized by the average solubility parameters Crude oil
8, (mJ m-3)o.5
Arabian heavy Black minnel Laguna Maya
21.65 20.75 21.17 21.33
aEthyl acetate.
mixtures at the flocculation points of stabilized crude of asphaltenes (solvent: toluene)
S, (ml m-3)o.5
using as precipitant
i-C 8
n-C,
n-C,
n-C,
etaca
15.3 15.4 15.1 15.6
15.5 15.8 15.4 15.8
15.6 15.7 15.6 16.0
15.9 15.8 15.8 16.2
18.5 18.4 18.4 18.4
H. L.aux et al. /Fuel
13
Processing Technology 53 (1997) 69-79
Table 4 Hansen solubility parameters (ml mm3 )’ 5, calculated ethyl acetate as precipitant and toluene as solvent
for solvent composition
at flocculation
points, applying
Crude oil
V,,,, (cm’)
6,
6,
6,
6,
Arabian heavy Black minnel Laguna Maya
34.5 22.6 22.6 17.9
16.6 16.8 16.8 17.0
3.9 3.5 3.5 3.2
5.3 4.8 4.8 4.5
17.3 17.8 17.8 17.9
precipitants are below that of cyclohexane, the solvent with the lowest solubility parameter. (4) Independent of applied solvents, the solubility parameter at the flocculation point increases with the solubility parameter of the applied nonpolar precipitant. (5) A significant influence of the solvent cannot be observed. The interactions between asphaltenes and other petroleum components can, nevertheless, only partly be attributed by the Hildebrand solubility parameter. Polarities and formation of hydrogen bonds should be considered. Therefore, the Hansen parameters of solvent-precipitant mixtures at the flocculation point were calculated to study especially the influence of polarity of precipitants. In Table 4, the data are listed for four crude oils taking ethyl acetate as precipitant. Using the alcohols methanol to n-butanol as precipitant, the determination of flocculation points with n-propanol and n-butanol did not result in a maximum in the light intensity curve (see Fig. 2). An insufficient coagulation of unstable colloid particles is to be presumed. If, in this case, the points of inflection are used for the calculation. the values in Table 5 are obtained.
I,4 3 B z
I,2
; .*
I,0
f
0,a
E
0,6
100
Precipitant Fig. 2. Dependence of light intensity during determination crude oil in decalin using alcohols as precipitants.
I [ml] of flocculation
points of a 5% solution of Maya
14
H. Lmx et al. / Fuel Processing
Table 5 Hansen solubility parameters using alcohols as precipitants V,,,
Methanol Ethanol n-Propanol
13.47 15.86 37” 165b 27.5
n-Butanol
(mJ rnd3 )“.5, calculated for solvent-precipitant (crude oil: Maya, solvent: decalin)
(cm’)
Precipitant
Technology 53 (1997) 69-79
mixtures
at flocculation
8,
6,
6
6,
16.7 16.7 16.7 16.2 16.8
4.8 3.9 4.4 6.1 3.3
8.7 8.6 11.3 15.5 9.1
19.4 19.2 20.6 23.1 19.4
points,
“First angle point. bSecond angle point (at Fig. 3).
Using a solution of 1% of asphaltene fractions in toluene, the dependence presented in Fig. 3 is obtained. The precipitant is i-octane. The higher the solubility parameter of the asphaltene fraction, the higher is the solubility parameter of solvent-precipitant mixture at the flocculation point. The addition of surfactants shows different effects on the flocculation points. The flocculation points were higher as well as lower than those of the original solutions. The influences of temperature on the flocculation point between 0 and 50°C and the influences of pressure from 1 to 70 bars were investigated. The following tendencies could be observed. (1) No significant influence of temperature was found, when using nonpolar precipitants. (2) By application of ethyl acetate, the necessary amount of precipitant increases slightly with increasing temperature. (3) Increasing pressure leads to a slight decrease of the necessary amount of precipitant at 25°C.
16,4 ---...
Arabian Heavy BCF BlackMinnel California Laguna Maya
____-~_____-r-_-__f-____l--___--~ I I I V: . . . . ..._._.__~__.___....__...___..I_.___...._._____.___.I___ : 0, I I _‘ _____+_-_-_-~______c-_-_-__~-__--~____-~ 16,0 ---I 0 %i : m I . ..____....__L_.__...__._...___...L_......__....._____._.._. . . . .. ,5,6___-___~_____J_____-L_ ______L_____I_-___J______ . . .. . . . . . . . . . ..I 15,6__-__--+_-__--;-_-_
16,2
----
.
p
0 m A
. . . . . . . .._~....
(&
. ..__....__.
l5,4__-___-;------~_---_-L___-_ . . . . . . . . . . . . . . . . . . . . ..~................
_.._____..I____....._.__....._..~........-.. ;_-_--_~__-__-~--_______-__~
----
--
...I r.____..__:..____._._)__.__.._..:...______.)...........
1 : ;-_-__ _______;-___-_ . .__.....__~_._____ :.___.. easily soluble asphaltenes .: ..___. .; I. . . . . . . . .. ~~,~_____-~-____~__-_-_~_-___+_---_~_--_--~-_-__~______l______ . .j.RD . . . .. _.....__.::._.._..___: ._____.__~ . . . ..__.._. _____..... i ._ .._._.: . I I I I
15,2
__-_-__;-#-“+_-_
14,8 19
20
21
23
22
6as Fig. 3. Dependence of solubility parameters solutions of crude oil asphaltenes.
of solvent-precipitant
mixtures
at flocculation
point using 1%
H. Laux et al. / Fuel Processing Technology 53 (1997) 69-79
75
4. Discussion To interpret the results of flocculation are taken in consideration: (1) solubility equilibrium of asphaltenes dissolved (2) adsorption particles
asphaltene
+ asphaltene
equilibrium
and
crystallites
interface
asc + solvate cover + dispersion (3) aggregation
point determination,
(asc)
equilibrium
medium
the following
processes
(3) during
formation
of primary
(4)
and coagulation
primary particles f particle aggregate + flock
(5)
The solubility equilibrium of asphaltenes can be neglected because the solubility of asphaltenes in organic solvents is very small. The behavior of colloid particles at the conditions of flocculation point determination was found like the behavior of polymer solutions [ 12,131. For example, the influence of concentration on the flocculation points of petroleum residues in solution is analogous to the dependencies of polymer solutions [15]. Eq. (6) was originally derived by Elias and Gruber [14] for polymer solutions. log @r,,, = log
@ait
-log
cpol
(6)
Where @r,,, is the volume fraction of precipitant at the flocculation point; cPO, is the concentration of the polymer at the flocculation point; and QCril is the critical volume fraction of precipitant at a polymer concentration of cpo, = 1 g cm ‘. Eq. (6) expresses the concentration influence on kinetics of coagulation and formation of flocks, as it is to be expected according to Eq. (5). Taking into account the behavior of polymer solutions, the following relationships were used to interpret the flocculation point results obtained by applying nonpolar precipitants. According to Kuhn [15], the solubility of polymers is determined by a critical value of the Flory-Huggins interaction parameter Xcrit. This critical value is to be calculated according to Eq. (7) neglecting the entropy term. XCrlt= 0.5( 1 + m-0.S)2
(7)
Accordingly, the solubility of a polymer is determined by the ratio of the molar volumes of polymer and solvent m. The upper limit of the molar mass distribution is decisive for the solubility of the polymer. On the other hand, the Flory-Huggins interaction parameter is a function of the solubility parameters of solvent and solute [6]: x=
-g-q,
-
8,)’
(8)
H. Laux et al./ Fuel Processing Technology 53 (1997) 69-79
76
As a criterion
for metastability,
we can express:
X S Xcrit
(9)
In colloid disperse petroleum systems, the limiting value of metastability should be determined by the particle volume of petroleum colloids. For the calculation of the Flory-Huggins interaction parameters, the solubility parameter at the surface of the colloid particle a,,,, is to use. With attention to the gradient in the solubility parameter among the solvate layer A6, it is valid: s part = &IS- A 6 On the base of flocculation
(10) point results, it was found [16]:
Xcrit = 0.62 + 0.04
(11)
These value corresponds to the colloid particle volume with a diameter dpart = 3 to 4 nm. The influence of solvation could be investigated by comparing flocculation points of solutions of asphaltenes and original complete residues. A &values between 0.4 and 1.4 (mJ m-3)o.5 were obtained. The reason of different values of solubility parameters at the flocculation point when applying n-alkanes as precipitants is their different molar volumes. Due to the influence of structure, results with i-octane as precipitant are not to be integrated into these relationships. If applying polar precipitants, Eqs. (8)-(10) are not valid. One has to consider: the theory of regular solutions requires the application of Hansen solubility parameters; and an influence on the solvation of asphaltenes cannot be excluded. According to the values in Table 1, the transition from solvent to precipitant, if applying polar compounds, lies in the range of the solubility parameters of tetrahydrofuran (solvent) and ethyl acetate (precipitant). Differences in the Hansen parameters can only be observed in a limited degree. The data given in Table 6, like dielectric constant E, dipole moment ,z, molar refraction R, and surface tension y do not exhibit significant differences either. When applying Hansen parameters, the limits in solubility of a polymer are given in Eq. (11) [9,17,18]. 4( %S - %Sl>’ + ($,s
- s,,.l>’ + (%,S - %S,)2
5 5
(12)
I.e., the solubility of a substance is given by a spherical space in the coordinates S,, S,, and S, by doubling the scale of the &axis. The center of this space is determined by the solubility parameters of the solute.
Table 6 Physical properties
of tetrahydrofuran
(thf) and ethyl acetate (etac) 1131
Compound
M (g/md
fb (“0
E
p (D)
R, km3)
y (dyn/cm)
thf etac
12.1 88.1
64-66 71.15
7.32 6.02
1.63
19.9 22.1
26.9 23.2
1.78
H. Lax
et al. /Fuel
Using the Hansen parameters
Processing
Technology 83 f 19971 69-79
of asphaltenes
estimated
17
by Zenke [lo]:
S, = 18.2 + 0.2; S, = 5.9 f 0.3; 6, = 6.4 f 0.2 [mJm-‘]0’5 the following values in the left side of Eq. (12) are obtained for tetrahydrofurane and ethyl acetate: for tetrahydrofurane, 4.31; and for ethyl acetate, 4.90. This result confirms the limitation of the solubility of asphaltenes in tetrahydrofurane as shown by Zenke
1101. The values in Tables 4 and 5 are in the expected range. A dominance of the disperse part of the solubility parameter 6, is recognized concerning the precipitation of petroleum colloids. Regarding the total solubility parameter a,, it is obviously impossible to make a statement about the solubility behavior of petroleum colloids in polar solvents. From the different results by application of nonpolar and polar precipitants, it may be concluded [ 131 that a polar precipitant as ethyl acetate destroys the solvate cover. The precipitation of resins by ethyl acetate proves this conclusion. In the case of nonpolar precipitants, the solvate cover is more or less stable. The observed stabilizing as well as destabilizing effects of surfactants depend on the petroleum product itself. A competition with native surfactants in the solvate cover must be assumed. The generally stabilizing effects of added petroleum resins confirm this presumption. An adaptation of the surfactants to the different crude oil systems is necessary if a stabilization is intended. From the positive effect of petroleum resins and of VD-III extracts of lubricant production on metastability of colloid disperse petroleum products [ 19,201, it can be concluded that aromaticity, polarity and-to a limited degree-molar mass of surfactants should be ranged between those of asphaltenes and dispersion medium. In this way, the gradual solubility gradient between the dispersion medium and the disperse phase is supported. This is a necessary condition for the colloid stability of petroleum systems. Nevertheless, the addition of petroleum resins and VD-III extracts is limited. Exceeding this limit leads to negative effects [ 191. An interpretation of the influence of temperature and pressure on the precipitation is very difficult because there is an influence of pressure and temperature on Eqs. (3) and (4) as well as on the process of aggregation and coagulation (Eq. (5)). The influence of temperature on the solubility parameters of compounds does not explain this picture [21]. Thiyagarajan et al. [22] found that entropic effects must be considered also.
5. Summary The determination of flocculation points of crude oil products allows to study the influences of different factors on the stability of the colloid disperse phase in these products. It is found, that the colloid disperse phase shows a behavior, which is well known from polymer solutions. Consequently, the theory of regular solutions provides an essential basis for the interpretation of precipitation results.
78
H. L.aux et al. /Fuel
Processing Technology 53 (1997) 69-79
Both the polarity and the formation of hydrogen bonds of asphaltenes and the solvation of particles are the reason that the application of nonpolar and polar precipitants make different results. The interpretation on the base of regular solution theory needs to distinguish between both. In case of nonpolar precipitants, the Hildebrand solubility parameters have to be used for valuation. If applying polar precipitants, the results may be evaluated by the Hansen solubility parameter. Different precipitation mechanisms are valid. Both mechanisms are borderline cases of the behavior of real systems. The overlapping of thermodynamic equilibria and kinetics of coagulation and aggregation complicates the quantification of all the factors influencing the process of precipitation. Further investigations are needed to study the influence of temperature and especially of pressure. Appendix A. Nomenclature C
m nC
R
44 T V ZRa
s’ A6
Ahe E
CL @
Concentration Ratio of molar volumes of polymers and solvent Carbon number Absolute gas constant Molar refraction Temperature (K> Volume, molar volume Hydrogen deficit Surface tension Solubility parameter Solubility parameter gradient Heteroelement correction of solubility parameter Dielectric constant Dipole moment Volume fraction
Subscripts
ale as cl-it d etac fP h P pre Part PO1 s, sl t
Alcohol Asphaltene Critical Dispersion Ethyl acetate Flocculation point H-bonding forces Polar precipitant Particle Polymer Solvent, solute Total
H. Lmx et al. /Fuel
Processing
Technology 53 (1997169-79
19
References [I] [2] [3] [4] [5] [6] [7] [8] [9]
T.F. Yen, J.G. Erdman, S.S. Pollak, Anal. Chem. 33 (1961) 1587. F.G. Unger. L.N. Andrejeva, Erdol u. Kohle 47 (1994) 18. H. Laux, I. Rahimian, H.-J. Neumann, ErdSl Erdgas Kohle 109 (1993) 368. D.A. Storm, E.Y. Sheu, Fuel 72 (1993) 233. H.-J. Neumann, Erdiil u. Kohle 34 (1981) 336. J.H. Hildebrand, R.L. Scott, Regular Solutions, New York 1962. CM. Hansen, Ind. Eng. Chem. Prod. Res. Dev. 8 (1969) 2. H. Laux, ErdGl Erdgas Kohle 108 (1992) 227. A.F.M. Barton, CRC: Handbook of solubility parameters and other cohesion parameters, CRC Press, Boca Raton, FL, 1985. [IO] G. Zenke, Zum Loseverhalten von ‘Asphaltenen’: Anwendung von Loslichkeitsparameter-Konzepten auf Kolloidfraktionen schwerer Erdolprodukte, PhD Thesis, TU Clausthal, 1989. [I 1’~H. Lian, J.-R. Lin, T.F. Yen, Fuel 73 (1994) 423. [I21 H. Laux, 1. Rahimian, A. Hatke, Chem. Techn. 44 (1992) 263. [13] H. Laux, I. Rahimian, A. Hatke-Antoniadis, Chem. Tech. 45 (1993) 222. [14] H.-G. Elias. U. Gruber, Makromol. Chem. 78 (1964) 72. [I51 R. Kuhn, Makromol. Chem. 177 (1976) 1525. [ 161 H. Laux, I. Rahimian, ErdBl u. Kohle 47 (1994) 430. [ 171 D.W. van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1990. [I81 B.-C. Ho, W.-K. Chin, Y.-D. Lee, J. Appl. Polym. Sci. 42 (1991) 99. 1191 Z.I. Sjunjaev. R.Z. Sjunjaev, R.Z. Safieva, Neftjanye Dispersnye Sistemy (Russ.), Chimija. Moskau. 1990. [20] H. Laux. I. Rahimian, Physikalisch-Chemische Grundlagen der Verarbeitung von Erdiiltickstlnden und Schweren Erdiilen, Verlag Papierflieger, Clausthal-Zellerfeld. 1994. [21] H. Laux, H. Kopsch, I. Rahimian, A. Hase, ErdSl Erdgas Kohle I IO (1994) 375. 1221 P. Thiyagarajan, J.E. Hunt, R.E. Winans, K.B. Anderson. J.T. Miller, Energy Fuels 9 (1995) 829.