- Email: [email protected]
An e.s.r. study of CO2 on bitumen
Short Communications
An e.s.r. study
of CO2 on bitumen
K. C. Khulbe, R. S. Mann and B. C. Y. Lu Department of Chemical Engineering, University of Ottawa, Ottawa, Canada KIN 6N5 (Received 74 September 1992: revised 12 November 1992)
The effect of CO, on the paramagnetic properties of Athabasca bitumen was studied by electron spin resonance at room temperature. When CO, was passed through bitumen for 30min at 50ml mini, the intensity of the e.s.r. line due to free radicals increased by - 25%. However, when the CO,-treated bitumen was kept in an atmosphere ofC0, for 24 h the intensity ofthe free radicals decreased to its original level.
(Keywords: e.s.r. study; CO2 on bitumen; electron spin resonance)
Carbon dioxide is highly soluble in crude oil under reservoir conditions, considerably swells it on dissolution and decreases its viscosity. CO, is therefore gaining popularity as an enhanced oil recovery (EOR) agent. Presently, considerable research activity is directed towards predicting how CO, interacts with reservoir oil. One approach is to use a generalized PI/T (pressure-volume-temperature) correlation, based on thermodynamic considerations. In addition to physical intermolecular forces, there are specific forces of attraction which lead to the formation of new molecular species. Such forces are called chemical forces. There are numerous types of specific chemical effects that are of importance in the thermodynamics of solutions, but very few studies have been carried out to investigate the chemical effects of CO, on bitumen. The present communication reports the results of an electron paramagnetic resonance (e.s.r.) study of the association of CO2 with the components of Cold Lake dewatered bitumen (water ~0.5 wt%) at room temperature and atmospheric pressure. The e.s.r. measurements were made at an X-band microwave frequency of -9.57 GHz using a Bruker ESR 200 DSRC spectrometer, at room temperature. The bitumen sample was placed in a 3 mm i.d., 180 mm long e.s.r. glass tube, capped at the top. The height of the bitumen sample in the tube was -75 mm. First, the e.s.r. of the sample was measured. Next, CO, was bubbled at 50 ml min-’ through the sample at room temperature through steel tubing, one end of which dipped into the bitumen touching the bottom of the e.s.r. tube. After CO, had been passed through the bitumen sample for the required time, the e.s.r. tube was removed and capped with a plastic cap. After 5 min, the e.s.r. measurement was repeated. The number of spins of organic free radicals in the bitumen sample was calculated by comparing the area of the free radical e.s.r. signal with the area
determined with a standard sample of coke(supplied by Bruker)containing lOi spins measured under similar conditions. Figure 1 shows the e.s.r. spectrum obtained for Cold Lake bitumen. All the lines except the free radical line labelled fin the figure were due to the V02+ ion’. Figure 2 shows the e.s.r. spectrum of free radicals and the main line of V02+ at a low scan range of 100 G. When CO, was passed at a rate of 50 ml min- ’ through the bitumen for 30min, the intensity of the free radicals increased by -25% of the original level. When the sample was kept for a further 24 h at atmospheric pressure in CO, at room temperature the intensity of the free radicals decreased to its original level. However, no change was observed in the intensity of V02+ e.s.r. signals. It appears that new free radicals were formed.
I I
-60
(‘ 1993 Butterworth-Heincmann
Fuel 1993
Volume
Figure 1
Ltd.
72 Number
f
I
G-+
H
0016&2361,~93.06/0898-02 898
CO, is a linear molecule and has no dipole moment, although its quadrupole moment is sufficiently strong to produce its thermodynamic properties, which differ from those of other non-polar molecules of similar size and molecular weight. The quadrupole moment’ of CO, is 4.0 x 10mz6 erg’:’ cm512. The solubility of CO, in bitumen at -25°C and 1 atm pressure is quoted3 as 0.18 wt%, but it depends on the nature of the bitumen. CO, also dramatically reduces the viscosity of the oil, particularly at low temperature. Bitumen is a dark-brown viscous material with a high content of impurities. It contains saturated and unsaturated hydrocarbons, polymeric aromatic molecules, free radicals etc. It also contains substantial portions of asphaltenes and resins, which are non-volatile fractions of
6
E.s.r. spectum
of bitumen
over scan range
1200 G
Short Communications
--lo
When CO,-treated bitumen was kept for 24 h in a CO, atmosphere, the concentration of free radicals decreased its original level. Either the new unstable radicals formed earlier by interaction of CO, decomposed on long exposure to or radical coupling reactions CO,, occurred. Dipole-dipole interactions could also play a part in decreasing the intensity of the e.s.r. signal of the newly formed radicals. When CO, was passed for 24 h over a bitumen sample there was no change in the intensity of the e.s.r. signal for free radicals. However, when CO, became dissolved in the bitumen, the reduction in viscosity increased diffusion, which in turn allowed radical coupling reactions to occur.
G-t
ACKNOWLEDGEMENT
L Figure
2
The authors are grateful to the Natural Science and Engineering Research Council of Canada for financial aid (A-l 125).
E.s.r. spectrum of bitumen over scan range 100 G
petroleum and contain the highest portion of heteroatoms (S, N and 0). The asphaltene fraction contains mostly polar compounds. The concentration of free radicalsinbitumenis 10”~lO1sspins gg ‘. The change in the concentration of free radicals due to CO, dissolved in the bitumen, as observed by e.s.r.,
indicates the presence of donor-acceptor complexes. It is possible that COz forms a complex molecule in the transition state with the polar hydrocarbons of the bitumen:
REFERENCES Khulbe, K. C., Chen, B. W., Manoogian, A. and Patmore, D. J. Fuel 1986, 65, 1594 Sheng, W. and Lu, B. C.-Y. Fluid Phaar Equilibria
CO,+R+
X. free radicals
1990, 54, 167 Mehrotra, A. K. and Svreck, W. Y. Can. J. Chm. Eng. 1988, 66, 656
Fuel 1993
Volume
72 Number
6
899
An e.s.r. study
of CO2 on bitumen
K. C. Khulbe, R. S. Mann and B. C. Y. Lu Department of Chemical Engineering, University of Ottawa, Ottawa, Canada KIN 6N5 (Received 74 September 1992: revised 12 November 1992)
The effect of CO, on the paramagnetic properties of Athabasca bitumen was studied by electron spin resonance at room temperature. When CO, was passed through bitumen for 30min at 50ml mini, the intensity of the e.s.r. line due to free radicals increased by - 25%. However, when the CO,-treated bitumen was kept in an atmosphere ofC0, for 24 h the intensity ofthe free radicals decreased to its original level.
(Keywords: e.s.r. study; CO2 on bitumen; electron spin resonance)
Carbon dioxide is highly soluble in crude oil under reservoir conditions, considerably swells it on dissolution and decreases its viscosity. CO, is therefore gaining popularity as an enhanced oil recovery (EOR) agent. Presently, considerable research activity is directed towards predicting how CO, interacts with reservoir oil. One approach is to use a generalized PI/T (pressure-volume-temperature) correlation, based on thermodynamic considerations. In addition to physical intermolecular forces, there are specific forces of attraction which lead to the formation of new molecular species. Such forces are called chemical forces. There are numerous types of specific chemical effects that are of importance in the thermodynamics of solutions, but very few studies have been carried out to investigate the chemical effects of CO, on bitumen. The present communication reports the results of an electron paramagnetic resonance (e.s.r.) study of the association of CO2 with the components of Cold Lake dewatered bitumen (water ~0.5 wt%) at room temperature and atmospheric pressure. The e.s.r. measurements were made at an X-band microwave frequency of -9.57 GHz using a Bruker ESR 200 DSRC spectrometer, at room temperature. The bitumen sample was placed in a 3 mm i.d., 180 mm long e.s.r. glass tube, capped at the top. The height of the bitumen sample in the tube was -75 mm. First, the e.s.r. of the sample was measured. Next, CO, was bubbled at 50 ml min-’ through the sample at room temperature through steel tubing, one end of which dipped into the bitumen touching the bottom of the e.s.r. tube. After CO, had been passed through the bitumen sample for the required time, the e.s.r. tube was removed and capped with a plastic cap. After 5 min, the e.s.r. measurement was repeated. The number of spins of organic free radicals in the bitumen sample was calculated by comparing the area of the free radical e.s.r. signal with the area
determined with a standard sample of coke(supplied by Bruker)containing lOi spins measured under similar conditions. Figure 1 shows the e.s.r. spectrum obtained for Cold Lake bitumen. All the lines except the free radical line labelled fin the figure were due to the V02+ ion’. Figure 2 shows the e.s.r. spectrum of free radicals and the main line of V02+ at a low scan range of 100 G. When CO, was passed at a rate of 50 ml min- ’ through the bitumen for 30min, the intensity of the free radicals increased by -25% of the original level. When the sample was kept for a further 24 h at atmospheric pressure in CO, at room temperature the intensity of the free radicals decreased to its original level. However, no change was observed in the intensity of V02+ e.s.r. signals. It appears that new free radicals were formed.
I I
-60
(‘ 1993 Butterworth-Heincmann
Fuel 1993
Volume
Figure 1
Ltd.
72 Number
f
I
G-+
H
0016&2361,~93.06/0898-02 898
CO, is a linear molecule and has no dipole moment, although its quadrupole moment is sufficiently strong to produce its thermodynamic properties, which differ from those of other non-polar molecules of similar size and molecular weight. The quadrupole moment’ of CO, is 4.0 x 10mz6 erg’:’ cm512. The solubility of CO, in bitumen at -25°C and 1 atm pressure is quoted3 as 0.18 wt%, but it depends on the nature of the bitumen. CO, also dramatically reduces the viscosity of the oil, particularly at low temperature. Bitumen is a dark-brown viscous material with a high content of impurities. It contains saturated and unsaturated hydrocarbons, polymeric aromatic molecules, free radicals etc. It also contains substantial portions of asphaltenes and resins, which are non-volatile fractions of
6
E.s.r. spectum
of bitumen
over scan range
1200 G
Short Communications
--lo
When CO,-treated bitumen was kept for 24 h in a CO, atmosphere, the concentration of free radicals decreased its original level. Either the new unstable radicals formed earlier by interaction of CO, decomposed on long exposure to or radical coupling reactions CO,, occurred. Dipole-dipole interactions could also play a part in decreasing the intensity of the e.s.r. signal of the newly formed radicals. When CO, was passed for 24 h over a bitumen sample there was no change in the intensity of the e.s.r. signal for free radicals. However, when CO, became dissolved in the bitumen, the reduction in viscosity increased diffusion, which in turn allowed radical coupling reactions to occur.
G-t
ACKNOWLEDGEMENT
L Figure
2
The authors are grateful to the Natural Science and Engineering Research Council of Canada for financial aid (A-l 125).
E.s.r. spectrum of bitumen over scan range 100 G
petroleum and contain the highest portion of heteroatoms (S, N and 0). The asphaltene fraction contains mostly polar compounds. The concentration of free radicalsinbitumenis 10”~lO1sspins gg ‘. The change in the concentration of free radicals due to CO, dissolved in the bitumen, as observed by e.s.r.,
indicates the presence of donor-acceptor complexes. It is possible that COz forms a complex molecule in the transition state with the polar hydrocarbons of the bitumen:
REFERENCES Khulbe, K. C., Chen, B. W., Manoogian, A. and Patmore, D. J. Fuel 1986, 65, 1594 Sheng, W. and Lu, B. C.-Y. Fluid Phaar Equilibria
CO,+R+
X. free radicals
1990, 54, 167 Mehrotra, A. K. and Svreck, W. Y. Can. J. Chm. Eng. 1988, 66, 656
Fuel 1993
Volume
72 Number
6
899