Energy & Fuels 2002, 16, 911-914
911
Characterization of Polar Components in Hydro-treated Lubricating Base Oil by Dilatometer Dianyu Chen,† Zhigang Xue,† Zhixing Su,*,† Shaoming Zhang,‡ and Xingguo Fu‡ Department of Chemistry, Lanzhou University, Lanzhou, 730000, China, and Petrochemical Research Institute, Petro-China Lanzhou Petroleum Processing and Chemical Complex, Lanzhou, 730000, China Received November 19, 2001
The polar components in hydro-treated lubricating base oil are discussed in this paper. Cationexchange resins, anion-exchange resins, and silica gel are employed for the separation of polar components from oil, and the relevant characteristics method of element analysis, hydrocarbon constituent analysis, and infrared spectroscopy are used. The effects of these fractions on free radical reaction studied via styrene radical polymerization at the temperature of 60 °C indicated that, with increasing acidity, the fractions restrain the free radical reaction. Finally, regarded as a special additive, the polar component effects on free radical reaction are studied, and the related kinetic equation shows that the reaction rate is directly proportional to the concentration of polar components.
Introduction Polar components are important constituents in oil because, even in small amounts, they can cause serious problems in processing and in the stability of the product fuel.1-4 To clarify the important role in oil stability, several methods have been employed by many oil researching workers to separate the polar components from oil and the further characteristics also have been studied.5-8 Mckay9-12 has separated the polar components into acid and base fractions and other compound types which are studied in detail. Korcek and Landis13,14 also reported the effects of sulfur and nitrogen compounds on lube oil oxidation stability. But all of the methods used before have two drawbacks, one is the complex separation operation, the other is the studying field not involving the effect on free radical reaction. The latter is just the mechanism of oil oxidation.15 * Corresponding author. E-mail:
[email protected]. † Lanzhou University. ‡ Petro-China Lanzhou Petroleum Processing and Chemical Complex. (1) Denison, G. H. Ind. Eng. Chem. 1944, 36, 477. (2) Banavali, R.; Karki, B. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1990, 35, 245. (3) Shah, D. J. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42, 224. (4) Aahvaryu, A.; Pandey, D. C.; Singh, I. D. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42, 229. (5) Harry, V.; Drushel, A. L. Anal. Chem. 1967, 39, 1819. (6) Choi, H. W.; Dines, M. B. Fuel 1985, 64, 4-8. (7) Schmitter, J. M.; Ignatiadis, I.; Patrick, A.; Georges, G. Anal. Chem. 1983, 55, 1685. (8) Pei, P.; Hsu, S. M. J. Liq. Chromatogr. 1986, 9, 3311. (9) Mckay, J. F.; Latham, D. R. Anal. Chem. 1980, 52, 1618. (10) Mckay, J. F.; Weber, J. H.; Latham, D. R. Anal. Chem. 1976, 48, 891. (11) Mckay, J. F.; Amend, P. J.; Harnsberger, P. M.; Cogswell, T. E.; Latham, D. R. Fuel 1981, 60, 14. (12) Mckay, J. F.; Harnsberger, P. M.; Erickson, R. B.; Cogswell, T. E.; Latham, D. R. Fuel 1981, 60, 17. (13) Landis, M. E.; Murphy, W. R. Lubr. Eng. 1991, 47, 595. (14) Korcek, S.; Johnson, M. D. Mech. Eng. 1993, 80, 177.
In this work, the polar components are separated into seven fractions, and characterized by element analysis, infrared spectra, and hydrocarbon constituents. Then the dilatometer apparatus is used for the study of effects in styrene free radical polymerization, and the related kinetic equation is obtained. Experimental Procedure Apparatus and Reagents. Infrared spectra are recorded using a Perkin-Elmer Model 521 infrared spectrophotometer. Element analysis is studied on the apparatuses of Foss Heraeus (CHN-O-RAPID, Germany), REN-1000 (NITROGEN ANALYZER, Jiangsu, China), and OS-1 (SULPHUR ANALYZER, Jiangsu, China). A mass spectrometer (SHIMADZU, QP-5050A, Japan) was employed to provide hydrocarbon constituents of polar components. Finally, free radical polymerization is operated on a dilatometer. Xi’an Chemical and Engineering Plant provided cationexchange resin and anion-exchange resin. Silica was purchased from Qingdao Chemical and Engineering Plant (Qingdao, China). The other reagents used in this experiment are all analysis grade. Preparation of the Polar Components. Hydro-treated lubricating base oil, which is provided by Lanzhou Petroleum Processing and Complex, is passed over silica, and some petroleum ether is used to wash off the nonpolar hydrocarbons. The materials retained by silica are defined as the polar components. Finally, the polar components are removed from silica using methyl alcohol. Separation of the Polar Components. A diagram of the scheme for separation of the polar components is shown in Scheme 1. Briefly, the polar components are diluted in petroleum ether and passed through the column containing cation-exchange resin and anion-exchange resin separately. Washing reagents of benzene and then methyl alcohol are used to obtain two acid fractions (weak and strong acid) and two basic fractions (weak and strong base) and remove the materi(15) Ingold, K. U. Chem. Rev. 1961, 61, 563.
10.1021/ef010273v CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002
912
Energy & Fuels, Vol. 16, No. 4, 2002
Chen et al.
Scheme 1. Separation of the Polar Components
Table 1. Hydrocarbon Testing by MS Method testing
term
saturated hydrocarbon
aromatic hydrocarbon
colloids
testing
value(%)
1.7
72.5
25.8
als retained on the column. The materials in petroleum ether are then passed through silica column and washed using benzene and methyl alcohol to obtain two other neutral fractions. With the acidity increasing order, these fractions are named as from fraction 1 to fraction 6. Free Radical Reaction Affects Experiment. Before polymerization, the monomer of styrene is pretreated according to the following process: washed by 5% of sodium hydroxide and distilled water, then dried in the presence of calcium chloride, and distilled with vacuum of 0.08 and the temperature of 60 °C. An amount of 7 mg of the polar fraction obtained above and 20 mg of 2,2′-azobis(isobutyronitrile) (AIBN) are dissolved in 15 mL of styrene, and 14 mL of the complex solution is removed into a dilatometer, and polymerization begins at the temperature of 60 °C, and the solution line in the dilatometer is recorded at different times.
Results and Discussion Percent of the Polar Components in Hydrotreated Lubricating Base Oil. Hydro-treated lubricating base oil is a new petroleum product obtained by mineral oil hydrogenation. After treatment by hydrogen under the plant operating conditions, the new oil contains smaller amounts of polar components than mineral oil. In the hydro-treated lubricating base oil studied here, only 0.1% of polar components are observed. Characteristic of the Polar Fractions. The six polar fractions are separated according to the adsorption power on cation-exchange resin, anion-exchange resin, and silica gel. To obtain more information about it, mass spectrometer and element analyses are used, and the results are shown in Tables 1 and 2. It is obvious that the most major constituent is alkyl-substituted aromatic compounds in every polar fraction but fraction 6 (C, H contents and the molar ratio of C to H); and in each fraction, elements of nitrogen and sulfur are observed. But the contents of sulfur and nitrogen in fractions 1,2,5,6 are obviously larger than those in others.
Figure 1. IR spectra of each fraction. 1, fraction 1; 2, fraction 2; 3, fraction 3; 4, fraction 4; 5, fraction 5; 6, fraction 6; 7, polar components. Table 2. Element Analysis of Each Polar Fraction samples
C%
H%
C/H(molar ratio)
S%
N%
fraction 1 fraction 2 fraction 3 fraction 4 fraction 5 fraction 6 polar components
61.29 73.77 82.79 77.07 61.12 49.48 79.35
9.08 9.72 12.17 11.90 8.83 9.55 11.60
1:1.778 1:1.579 1:1.764 1:1.853 1:1.734 1:2.312 1:1.754
5.22 6.25 0.67 5.28 7.05 9.73 2.73
1.00 1.00 0.25 0.48 0.90 0.21 0.72
The IR spectra characterization is summarized in Figure 1. Besides the diagnostic hydrocarbon of 2964 cm-1, 2925 cm-1, 2867 cm-1, 1455 cm-1, 1376 cm-1, 1286 cm-1, and 1006 cm-1, the differences are shown as: in fractions 1 and 2, the characteristic wavenumbers are 3392 cm-1 and 3286 cm-1 (might be N-H) and the absorbance of CdO is 1714 cm-1; in fractions 3 and 4, both N-H (3392 cm-1) and sulfur compounds (1170 cm-1, 1085 cm-1, and 1035 cm-1) are seen, the difference between these three spectra is the intensity of CdO (1714 cm-1); in fractions 5 and 6, sulfur compound wavenumbers are observed as 1240 cm-1, 1166 cm-1, 1031 cm-1, and 580 cm-1, especially in fraction 6, hydrobond absorbance of 3500 cm-1-3000 cm-1 is observed, and the absorbance of CdO is moved to 1708 cm-1. Considering the acidity order, it is thought that the basicity of the polar components may be caused by nitrogen compounds, and the acidity may be caused by sulfur and oxygen compounds.
Dilatometer Characterize Polar Components in Oil
Energy & Fuels, Vol. 16, No. 4, 2002 913
The degree of monomer conversion would then be
∆V V0 ∆[M] ) d2 - d1 [M] d2
Figure 2. Effects of each fraction on styrene free radical polymerization.
According to the results reported before,16 almost every nitrogen compound type, whether basic or not, accelerates the oxidation of oil, but most of the sulfur compounds restrain it, and the aromatic hydrocarbons have more complexes effects on oxidation. For these reasons and industrial application, the fraction effects are evaluated here by dilatometry as a special additive. Dilatometry for Evaluation Effects of Each Fraction. Dilatometry is used in this paper to determine the effects of the polar components on free radical polymerization, and free radical polymerization is a chain reaction consisting of a consequence of three steps: initiation, propagation, and termination. As we all know, the rate of free radical polymerization has been provided, that is
Rp ) Kp[M]
( ) fKd[I] Kt
1/2
W1 W2 d1 d2 ∆Vtotal fract ) W1 d1
d2 - d1 d2
∆V [M] [M]d2 ∆V ∆t V0 ∆[M] ) ) ∆t d2 - d1 V0(d2 - d1) ∆t d2
(5)
∆[M] d[M] ) Rp lim ) ∆t ∆tf0 dt
(6)
and
Usually, in terms of volume as a function of length, the change in height of the monomer solution in the capillary of the dilatometer may be expressed as a volume change, thus the equation can easily be converted to
Rp )
(2)
which simplifies to
∆Vtotal fract )
where ∆[M] is the incremental change in monomer concentration [M], ∆V is the change in volume from the initial volume V0, (d2 - d1)/d2 is the fractional volume change, ∆V/V0 is the fractional volume change at any time ∆t, W1 ) W2 is the grams of polymer of density d2 (or monomer of density d1). Now if both sides of eq 4 are divided by incremental time, ∆t, and rearranged, then
(1)
where Rp is the overall rate of polymerization, Kd is the rate constant for the initiator dissociation, Kp is the rate constant for the propagation step, Kt is the rate constant for the termination step, [I] is the concentration of the initiator species, [M] is the overall concentration of the monomer added, and f is the molar fraction of initiator radicals formed which actually add to the monomer and initiate polymerization. When the dilatometer is placed into the constanttemperature bath, solution volume changes; this is due to two factors of monomer thermal expansion and contraction for polymerization. A period of 5-10 min later, the liquid level begins to decrease, and this is the basis for dilatometry determination. The total fraction change in volume is given as
(3)
(16) Muray, D. W.; MacDonald, J. M.; White, A. M.; Wright, P. G. Proc. 11th World Pet. Congr. 1983, 447-457.
(4)
[M]d2S
∆h V0(d2 - d1) ∆t
(7)
where ∆h is the height change at any time ∆t, and S is the area of the capillary in the dilatometer. As
[M]d2S V0(d2 - d1)
) constant
(8)
so eq 7 can be described as
Rp ) k
∆h ∆t
(9)
in which k is a constant, and is defined here as the overall polymerization rate constant. Equation 9 is the basis of dilatometry determination, for d1 ) 0.8712 g/mL, d2 ) 1.050 g/mL, V0 ) 14 mL, [M] ) 8.377 mol/L, S ) 2.04 × 10-2 cm2, so k ) 7.174 × 102 mol/(L min). In this experiment, seven polar fractions are determined by this means, and the results are summarized in Figure 2. After regression to each line, the ∆h/∆t is obtained, and the relevant total polymerization rate is obtained too, which is summarized in Table 3. It is obvious that all of the six fractions restrain the styrene free radical polymerization, and as the basicity weakens from fraction 1 to fraction 6, the restraint power follows the order: basic fractions > neutral fractions > acid fractions. But this is not enough information to describe the effects of polar components
914
Energy & Fuels, Vol. 16, No. 4, 2002
Chen et al.
Table 3. Results from Dilatometry for Each Fraction samples
∆h/∆t
Rp(mol/L-min)
only AIBN fraction 1 fraction 2 fraction 3 fraction 4 fraction 5 Fraction 6
-0.0557 -0.0548 -0.0547 -0.0519 -0.0518 -0.0515 -0.0494
39.96 39.31 39.24 37.23 37.16 36.96 35.44
on free radical reaction, so further kinetic study of the components is presented. Kinetic Study of the Polar Components. On the basis of the equations discussed above, we consider the polar components as a special additive the same as AIBN, and suppose that the polar components and AIBN function in the polymerization independently, so eq 1 may be
(
)
fKd[I] Kt
Rp ) Kp[M] 1
1/2
Rp′ ) Kp′[M]
+ Kp′[M]
( ) fKd[p] Kt
( ) fKd[p] Kt
Table 4. Results from Kinetic Testing for the Polar Components sample
[p](mg/mL)
1 2 3 4 5 6 7 8 9 10 11 12
0 0.140 0.165 0.194 0.228 0.269 0.316 0.372 0.435 0.517 0.605 0.712
log[p]
Rp
Rp′
log Rp′
-0.853 -0.782 -0.712 -0.641 -0.571 -0.500 -0.430 -0.361 -0.287 -0.218 -0.147
0.03386 0.03429 0.03508 0.03616 0.03537 0.03730 0.03587 0.03623 0.03931 0.03809 0.04175 0.03924
0 0.00043 0.00122 0.00230 0.00151 0.00344 0.00201 0.00237 0.00546 0.00423 0.00789 0.00538
-3.366 -2.914 -2.638 -2.821 -2.463 -2.697 -2.625 -2.263 -2.374 -2.103 -2.269
After regression, the regression equation of log Rp′ log[p] is obtained as
log Rp′ ) -2.0937 + log[p]
(13)
x
(10)
so the index number is obtained as 1, and the kinetic equation may be expressed as
x
(11)
where Kp′ is the rate constant caused by the polar components, [p] is the polar component concentration in polymerization, and x is the index number of the polar components to rate increasing. The first term is the overall rate caused by AIBN in this experiment should be constant because AIBN content is not variable, and the other term (or Rp′) is the rate caused by the polar components. If both sides of eq 11 are treated by logarithm, then eq 11 can be described as
log Rp′ ) log(Kp′[M]) + x log(fKd[p]) - x log Kt (12) for only [p] is variable, so log Rp′ is directly proportional to log[p], and the slope of log Rp′ - log[p], is just the index number we want to know. Experiment results are summarized in Table 4..
Rp′ )
fKdKp′ [M] [p] Kt
(14)
that is, Rp′ is directly proportional to [p] in the content range between 0.140 and 0.712 mg/mL. Conclusion The polar components we studied are composed of alkyl-substituted aromatic hydrocarbons, including small amounts of nitrogen and sulfur compounds. These fractions affect the free radical reaction in the following order: acid fraction > neutral fractions > base fractions. And the kinetic equation of the polar components on styrene radical polymerization is also obtained as
Rp′ ) EF010273V
fKdKp′ [M] [p] Kt
