- Email: [email protected]
Phase behavior of reservoir fluids
Fluid Phase Equilibria, 93 (1994) 353-362
353
Elsevier Science B.V.
Phase behavior of reservoir fluids VI. cosolvent effects on bitumen fractionation co, ’
with supercritical
J.M. Yu a, S.H. Huang b-2and M. Radosz b,* a Imperial Oil Resources Limited, Research Centre, 3535 Research
Road, Calgary, Alta. T2L 2K8 (Canada) b Exxon Research and Engineering Company, Annandale, NJ 08801 (USA)
(Received April 23, 1993; accepted in final form June 7, 1993)
ABSTRACT Yu, J.M., Huang, S.H. and Radosz, M., 1994. Phase behavior of reservoir fluids VI: cosolvent effects on bitumen fractionation with supercritical CO,. Fluid Phase Equilibria, 93: 353-362.
Equilibrium phase compositions (TPXY) are measured in a flow cell apparatus for a bitumen + decane + CO, system. Statistical Associated Fluid Theory (SAFT) [Huang, S.H. and Radosz, M., 1990a. Fluid Phase Equilibria, 60: 81-98; 1990b. Ind Eng. Chem. Res., 29: 2284-22941 and Peng-Robinson equations of state are found to give similar results for the condensed phase compositions. However, SAFT is found superior in predicting the supercritical fluid phase compositions. Hence, the bitumen partitioning coefficients (K-factors) can be reliably predicted with SAFT but not with PR. Moreover, the SAFT characterization approach based on measurable molecular properties is more appropriate for heavy, associating bitumen components than is the PR characterization approach based on hypothetical critical properties. Finally, SAFT-predicted K-factors allow for quantifying cosolvent selectivity and capacity in fractionating bitumen with supercritical CO*. Keywords: experiments, cosolvent effects, supercritical CO,, bitumen. INTRODUCTION
Economic recovery of rich bitumen reserves, especially in Alberta, Canada, are of strategic importance for North America. In the current * Corresponding author. ’ A preliminary account of this work was presented during the Annual AIChE Meeting in Los Angeles in November 1991. ’ Current address: DB Robinson and Associates, 9419-20 Avenue, Edmonton, Alta. T6N lE5, Canada. SSDI 03783812(93)02387-3
354
J.M.
Yu et al. 1 Fluid Phase Equilibria 93 (1994) 353-362
steam-based recovery method, one is interested in bitumen distribution between various phases under reservoir conditions. This may be even more important for the solvent-based recovery and solvent de-asphalting upgrading methods. For example, recently proposed solvent injection (Butler and Mokys, 1990), towards enhanced recovery, and solvent de-asphalting (Chow et al., 1991), towards surface upgrading, will certainly require the ability to quantify the partitioning of bitumen components between the equilibrium phases. For example, it will be crucial to quantify the solvent and cosolvent capacity and selectivity for bitumen phase equilibria. The most effective approach to modeling reservoir phase equilibria is that based on equations of state. The goal is to predict distribution (partitioning) coefficients, referred to as K-factors, from an equation of state for all the components, including the heavy components. K-factors, defined as ratios of the vapor-to-liquid mole fractions, are usually predicted from cubic equations of state for light components of the conventional oil reservoirs. However, for bitumen reservoirs, K-factors are much more difficult to measure and predict because the bitumen components are less volatile. Until very recently, there were no reliable data and no models available for bitumen K-factors. The objective of this series of papers, therefore, is to develop quantitative, predictive models for bitumen K-factors. Our measure of success is not the goodness of fit alone, but the quality of predictions as well. Yu et al. (1989) and Huang and Radosz (1990a) present experimental data and equations of state for model binary systems composed of bitumen, or its fractions, and supercritical C02, and predict the mutual solubilities of bitumen and CO2 with PHC and Soave equations of state. In two subsequent papers, Huang and Radosz (1991 b, c) propose lumping and molecular weight distribution approaches to characterizing reservoir fluids in a way that is compatible with equations of state. Finally, Huang and Radosz (1991d) demonstrate that an equation of state based on SAFT offers an attractive model for simulating bitumen phase equilibria. SAFT pure and binary parameters are found to be well behaved and, hence, easy to generalize with respect to measurable bitumen properties, such as molecular weight and aromaticity. This is not surprising because SAFT has theoretical justification and its parameters have physical significance that is not lost in the process of correlating experimental data. Our goal here is to demonstrate that SAFT retains its predictive power upon extension to pseudo-ternary bitumen systems, e.g. supercritical fluid, cosolvent and bitumen, and that SAFT can be used to evaluate cosolvency effects on bitumen fractionation. First, we measure phase equilibria in a model ternary system of CO2 + n-decane + bitumen, We refer to n-decane, our model aliphatic cosolvent, as decane. Next, we predict, without fitting, the measured phase equilibrium data with SAFT. Finally, we predict phase
J.M.
Yu et al. 1Fluid Phase Equilibria 93 (1994) 353-362
355
equilibria for which there are no experimental data available to evaluate the effect of cosolvent aromaticity on bitumen fractionation. We select CO, as our model solvent because most of the steam-based recovery processes at Alberta, Canada generate CO, as a product of chemical reactions between steam and oil sands. To improve the solvent power of CO2 for extracting as well as de-asphalting bitumen in situ, decane and tetralin are selected as model cosolvents to represent a naphtha stream that can be obtained from a surface partial upgrading scheme (Chow et al., 1991a,b). EXPERIMENTAL
A flow cell apparatus described by Yu et al. (1989) is modified for this work. A windowed cell serves as a two-phase separator. All the components are thoroughly equilibrated in a static mixer upstream of the cell. In addition to two feed lines used in the previous work, one for CO2 (HPLC reciprocating pump) and one for bitumen (heated reciprocating syringe pump), we use a third independent line for feeding the decane cosolvent (HPLC syringe pump). The decane stream is injected into the bitumen stream upstream of the CO2 injection point. As before, samples from both equilibrium phases are continuously separated, at ambient pressure, into CO2 and condensable fractions. The condensable fraction of each phase is analyzed for decane. The top phase (CO,-rich) decane concentration is determined from gas chromatography (GC). The calibration standards for such a GC analysis are prepared by adding a known amount of decane to a known amount of bitumen extract. The bottom phase (CO,-lean) decane concentration is determined from material balance after evaporating decane from the bottom phase condensate in a vacuum oven. The boiling temperature of decane overlaps with a small fraction of the bitumen light ends. This may contribute to an analytical error in measuring the decane concentration in bitumen by about 5%. However, we calibrate and fine-tune this procedure against bitumen (stripped with CO*) and decane mixtures of known composition. BITUMEN
CHARACTERIZATION
Properties of the Cold Lake bitumen used in this work are described in detail by Yu et al. (1989). Only a brief overview is given here. The bitumen is represented by eleven pseudo-components. Properties of the pseudo-components (distillation lumps) are characterized in terms of SAFT parameters which, in turn, are used to calculate bitumen phase equilibria. The SAFT parameters are presented by Huang and Radosz ( 1991d). In addition to SAFT, we include data calculated from the Peng-Robinson equation of
356
J.M.
Yu et al. /Fluid
Phase Equilibria 93 (1994) 353-362
state (PR) because this is a popular equation of state for reservoir and surface applications and, hence it serves as a useful frame of reference. PR is representative of other cubic equations of state such as Soave. The PR characterization of the Cold Lake bitumen is given by Yu et al. (1989). Conventional cubic equations of state, such as PR, require three purecomponent parameters, the critical temperature, critical pressure and Pitzer’s acentric factor. These, in turn, are estimated from empirical correlations and often have no physical significance, especially for heavy components, which, in contrast to light components, thermally decompose well below their critical temperatures. The purpose of such characterization is to mimic a hypothetical vapor pressure curve terminating in a hypothetical critical point. By contrast, SAFT characterization utilizes well-defined and measurable properties of heavy hydrocarbons. Specifically, the SAFT pure-component parameters (three for non-associating molecules: segment energy, segment volume and number of segments per molecule) are correlated against molecular weight and average aromaticity. These, in turn, can be estimated from experimental data, such as gas chromatographic distillation (GCD), specific gravity, hydrogen to carbon ratio and spectroscopy. The important advantage of SAFT is that each pseudo-component is characterized not only in terms of its average size (molecular weight), but also in terms of molecular structure (aromaticity). This way we can capture bitumen fractionation selectivity not only with respect to volatility but also with respect to molecular type. This is an advantage because supercritical solvents and cosolvents will fractionate bitumen by molecular size and structure. RESULTS
AND DISCUSSION
Experimentally determined phase compositions at 200°C and 12 MPa are listed in Table 1 for the CO, + decane + bitumen system at two feed compositions differing primarily in decane concentration. Also listed in Table 1 are phase compositions calculated from SAFT and PR. As expected, we find that the liquid phase composition is represented well by both SAFT and PR. This is shown in Fig. 1 for the CO, solubility in bitumen and in Fig. 2 for the decane solubility in bitumen as a function of decane weight percent in feed on a CO,-free basis. As we can see, decane increases the solubility of COZ in bitumen (Fig. 1). More importantly, both SAFT and PR predict reasonably the experimental solubilities, where available, as shown in Figs. 1 and 2. Also, extrapolating to higher decane concentrations results in similar predictions from both SAFT and PR. Incidentally, the small difference between the SAFT and PR curves shown
J.M. Yu ea al. / Fluid Phase Equilibria 93 (1994) 3S?-362
357
TABLE i Measured and SAFT- and PR-predicted liquid and vapor phase compositions CO,-decane-bitumen system at 200°C and 12 MPa Liquid phase wt.%
of the
Vapor phase wt.%
Measured
SAFT
PR
Measured
SAFT
PR
Set 1 a
COz C,, Bitumen
6.76 4.90 88.34
6.38 5.96 87.65
7.19 5.07 87.74
96.10 2.81 1.09
96.55 2.65 0.80
95.44 2.94 1.62
Set 2 b
CO, Cl0 Bitumen
7.18 11.00 81.82
7.35 13.41 79.24
8.27 11.26 80.47
93.4 5.72 0.88
93.87 5.42 0.72
92.75 5.95 1.31
3.24; C,,/bitumen, 4.62; C,,/bitumen,
0.146. 0.407.
a Set 1: Feed ratios (weight/weight): b Set 2: Feed ratios (weight/weight):
0
0
10
a0
30
40
50
COJbitumen, COJbitumen,
60
70
&cane wt% in Feed CCO$ree
80
90
loo
Basis)
Fig. 1. Measured and SAFT- and PR-predicted 12 MPa.
CO* solubilities in bitumen at 200°C and
in Figs. 1 and 2 can be explained by different results in fitting the CO,-decane binary. It should be noted that no attempt was made to predict the possible phase split in bitumen at high decane concentrations. For the record, the SAFT and PR binary parameters are estimated as follows: CO,-decane from binary VLE data (Reamer and Sage, 1963), 0.11 for PR and 0.00 for SAFT; CO,-bitumen from empirical correlations fitted to the CO&n-bitumen solubilities (Huang and Radosz, 1991d). While the liquid (bitumen-rich) compositions are predicted reasonably well by both models, the vapor (CO&h) compositions are realistically
358
J.M. Yu et al. / FItGd Phase Eq~ilib~~ 93 (1994) 353-362
0
10
20 30 40 59 60 70 a0 Decane wt% in Feed (CO$ea basis)
90
lly)
Fig. 2. Measured and SAFT- and PR-predicted decane solubilities in bitumen at 200°C and 12 MPa.
Fig. 3. Measured and SAFT- and PR-predicted
decane and bitumen solubilities in CO,.
predicted by SAFT but not by PR. This is illustrated in Fig. 3 where decane and bitumen concentrations in CO2 in weight percent are plotted versus decane weight percent in feed (C02-free basis). The PR predictions of bitumen solubility in CO2 are out. This is typical of other empirical equations of state also. Therefore, we use only SAFT for further calculations in this work. Another argument justifying selecting SAFT is that its adjustable binary parameter is applied only to a relatively minor dispersion term while the PR binary parameter is applied to the whole equation of state. This makes SAFT a more reliable predictive tool. Figure 3 shows that, while the decane solubility increases with increasing decane concentration in the feed, which is expected, the total bitumen
J.M.
Yu et al. 1 Fluid Phase Equilibria 93 (1994) 353-362
50
l60
260
-360
460
359
560
Molecular Weight of Bitumen Fractions
Fig. 4. SAFT-predicted mixtures.
K-factors of bitumen fractions in supercritical CO2 and COJdecane
solubility actually decreases with increasing decane concentration in the feed. However, decane does increase bitumen K-factors, relative to a decane-free case, because bitumen mole fractions in the bitumen-rich phase decrease at an even faster rate upon increasing the decane concentration in the feed. This is shown in Fig. 4. The K-factor, defined as the mole fraction ratio, vapor to liquid, is plotted in Fig. 4 versus the molecular weight of bitumen components. In all the cases, as expected, the bitumen K-factors decrease drastically upon increasing the molecular weight. The three curves shown in Fig. 4 correspond to the total concentrations of the cosolvent, decane, in solvent COZ, namely, 0 wt.%, 4 wt.% and 25 wt.%. Figure 4 shows that the addition of decane increases K-factors and therefore enhances the extraction of bitumen lights with supercritical COZ. Having established that SAFT realistically predicts such decane cosolvent effects, we are interested in using SAFT for other cosolvent candidates. Another example of a Cl,, hydrocarbon is tetralin, which is a hybrid hydrocarbon that contains an aromatic ring and a naphthenic ring. The binary interaction parameters of the CO,-tetralin pair for SAFT, obtained from the VLE data (Chou et al., 1990), is 0.100. A 25 wt.% tetralin in CO2 case is compared with the 25 wt.% decane and pure CO2 cases in Table 2 and Fig. 5. This comparison suggests that decane is more effective in extracting bitumen lights, if one simply compares K-factors. This can be explained by a greater affinity of decane to bitumen aliphatic (relative to bulk) lights. However, a more careful comparison of selectivity with respect to both molecular weight and aromaticity reveals a somewhat different picture. Figure 6 shows bitumen distributions of size and aromaticity, molecular
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J.M.
Yu et al. / Fluid Phase Equilibria 93 (1994) 353-362
TABLE 2 SAFT-predicted K-factors of bitumen fractions decane and tetralin at 200°C and 12 MPa Bitumen fraction
Composition (wt.%)
1 2 3 4 5 6 7 8 9 10 11
0.24 0.70 1.94 3.40 4.80 6.87 5.68 6.97 16.50 14.80 38.10
Molecular weight (gmol)
WC
130.7 147.1 166.2 188.7 215.2 246.4 283.7 327.6 437.9 813.0 2580
1.825 1.803 1.779 1.754 1.728 1.699 1.669 1.637 1.573 1.551 1.394
in supercritical
CO, and its cosolvents,
K-factors x 1000 Solvent CO*
Cosolvent 25 wt.% = decane
Cosolvent 25 wt.% = tetralin
57 36.4 21.3 11.1 5.11 1.99 0.63 0.16 0.005 0 0
123 85.8 55.6 33.1 17.9 8.57 3.52 1.21 0.08 0 0
107 68.1 39.8 20.9 9.72 3.87 1.27 0.33 0.01 0 0
a Cosolvent wt.% is in solvent CO2
0.15 wt% of comlvent in overall CO2 at 200 ‘C and 1!2MPa
0.1
0.05
0 50
150 250 350 450 Molecular Weight of Bitumen Fractions
Fig. 5. Comparison supercritical CO,.
of decane and tetralin
cosolvent
550
effects on bitumen
K-factors
in
weight (upper x-axis) and hydrogen to carbon ratio (H/C, lower x-axis), respectively. Comparing decane and tetralin, one finds that the tetralin extract is richer in lighter and less aromatic (high H/C) components. This means that the decane cosolvent results in a higher capacity (higher K-factors) but lower selectivity (heavier, more aromatic extract). A more
J.M.
Yu et al. 1 Fluid Phase Equilibria 93 (1994) 353-362
361
MolecularWeight of Bitumen Fractions 130 145 166 190 215 245 265 330 440 al.5 2680 40
0 1.83 1.8 1.78 1.75 1.73 1.7 1.67 1.64 1.57 1.55 1.39 WC of Bitumen Fractions
Fig. 6. Decane
and tetralin
cosolvent
effects on CO2 selectivity.
quantitative characterization of the solvent selectivity can be in terms of the ratio of K-factors (the separation factor) which can be easily estimated from SAFT. CONCLUSIONS
The SAFT and PR equations of state are found comparable in predicting the condensed phase compositions in CO,-decane-bitumen systems. However, SAFT is found superior in predicting the supercritical fluid phase compositions. Hence, the bitumen partitioning coefficients (Kfactors) can be reliably predicted with SAFT but not with PR. Moreover, the SAFT characterization approach based on measurable molecular properties is more appropriate for heavy, associating bitumen components, than is the PR characterization approach based on hypothetical critical properties. The SAFT predicted K-factors allow for quantifying cosolvent selectivity and capacity in fractionating bitumen with supercritical CO*. For example, decane (a model aliphatic cosolvent) is found to have a higher capacity but lower selectivity than tetralin (a model aromatic cosolvent). There are many challenges which will have to be addressed in the future. For example, for propane, butane and pentane de-asphalting, we will have to reconsider the heavy-end characterization as well as a detailed quantification of bitumen component distribution in both vapor and liquid phases. Also, the effects of supercritical water and asphaltene aggregation, important in many reservoir systems, will have to be studied.
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J.M. Yu et al. / Fluid Phase Equilibria 93 (1994) 353-362
REFERENCES Butler, R.M. and Mokys, I.J., 1990. A new process (VAPEX) for recovering heavy oils using hot water and hydrocarbon vapor. Paper no. CIM SPE 90-133, Proceedings of the International Technical Meeting, IO- 13th June, Calgary, Alta., Canada. Chou, G.F., Forber, R.R. and Prausnitz, J.M., 1990. High-pressure vapor-liquid equilibrium for CO&decane, CO,/tetralin, and CO&r-decane/tetralin at 71.1 and 104.4”C. J. Chem. Eng. Data, 35: 26-29. Chow, K.C., Overfield, R.E. and Yu, J.M., 1991a. Heavy oil product enhancement by diluent deasphalting. Meeting of Canadian Society of Chemical Engineering, 6-8th October, Vancouver, B.C., Canada. Chow, K.C., Overfield, R.E. and Yu, J.M., 1991b. Creating a new future for heavy oil through on-site product enhancement. Meeting of Canadian Society of Chemical Engineering, 6-8th October, Vancouver, B.C., Canada. Huang, S.H. and Radosz, M., 1990a. Phase behavior of reservoir fluids II: Supercritical carbon dioxide and bitumen fractions. Fluid Phase Equilibria, 60: 8 l-98. Huang, S.H. and Radosz, M., 1990b. Equation of state for small, large, polydisperse and associating molecules. Ind. Eng. Chem. Res., 29: 2284-2294. Huang, S.H. and Radosz, M., 1991a. Equation of state for small, large, polydisperse and associating molecules: Extension to fluid mixtures. Ind. Eng. Chem. Res., 30: 1994-2005. Huang, S.H., and Radosz, M., 1991b. Phase behavior of reservoir fluids III: Molecular lumping and characterization. Fluid Phase Equilibria, 66: 1-21. Huang, S.H. and Radosz, M., 1991~. Phase behavior of reservoir fluids IV: Molecular weight distributions for thermodynamic modeling. Fluid Phase Equilibria, 66: 23-40. Huang, S.H. and Radosz, M., 1991d. Phase behavior of reservoi fluids V: SAFT model of CO, and bitumen systems. Fluid Phase Equilibria, 70: 33-54. Reamer, H.H. and Sage, B.H., 1963. J. Chem. Eng. Data, 8: 508. Yu, J.M., Huang, S.H. and Radosz, M., 1989. Phase behavior of reservoir fluids: Supercritical carbon dioxide and Cold Lake bitumen system. Fluid Phase Equilibria, 53: 429-438.
353
Elsevier Science B.V.
Phase behavior of reservoir fluids VI. cosolvent effects on bitumen fractionation co, ’
with supercritical
J.M. Yu a, S.H. Huang b-2and M. Radosz b,* a Imperial Oil Resources Limited, Research Centre, 3535 Research
Road, Calgary, Alta. T2L 2K8 (Canada) b Exxon Research and Engineering Company, Annandale, NJ 08801 (USA)
(Received April 23, 1993; accepted in final form June 7, 1993)
ABSTRACT Yu, J.M., Huang, S.H. and Radosz, M., 1994. Phase behavior of reservoir fluids VI: cosolvent effects on bitumen fractionation with supercritical CO,. Fluid Phase Equilibria, 93: 353-362.
Equilibrium phase compositions (TPXY) are measured in a flow cell apparatus for a bitumen + decane + CO, system. Statistical Associated Fluid Theory (SAFT) [Huang, S.H. and Radosz, M., 1990a. Fluid Phase Equilibria, 60: 81-98; 1990b. Ind Eng. Chem. Res., 29: 2284-22941 and Peng-Robinson equations of state are found to give similar results for the condensed phase compositions. However, SAFT is found superior in predicting the supercritical fluid phase compositions. Hence, the bitumen partitioning coefficients (K-factors) can be reliably predicted with SAFT but not with PR. Moreover, the SAFT characterization approach based on measurable molecular properties is more appropriate for heavy, associating bitumen components than is the PR characterization approach based on hypothetical critical properties. Finally, SAFT-predicted K-factors allow for quantifying cosolvent selectivity and capacity in fractionating bitumen with supercritical CO*. Keywords: experiments, cosolvent effects, supercritical CO,, bitumen. INTRODUCTION
Economic recovery of rich bitumen reserves, especially in Alberta, Canada, are of strategic importance for North America. In the current * Corresponding author. ’ A preliminary account of this work was presented during the Annual AIChE Meeting in Los Angeles in November 1991. ’ Current address: DB Robinson and Associates, 9419-20 Avenue, Edmonton, Alta. T6N lE5, Canada. SSDI 03783812(93)02387-3
354
J.M.
Yu et al. 1 Fluid Phase Equilibria 93 (1994) 353-362
steam-based recovery method, one is interested in bitumen distribution between various phases under reservoir conditions. This may be even more important for the solvent-based recovery and solvent de-asphalting upgrading methods. For example, recently proposed solvent injection (Butler and Mokys, 1990), towards enhanced recovery, and solvent de-asphalting (Chow et al., 1991), towards surface upgrading, will certainly require the ability to quantify the partitioning of bitumen components between the equilibrium phases. For example, it will be crucial to quantify the solvent and cosolvent capacity and selectivity for bitumen phase equilibria. The most effective approach to modeling reservoir phase equilibria is that based on equations of state. The goal is to predict distribution (partitioning) coefficients, referred to as K-factors, from an equation of state for all the components, including the heavy components. K-factors, defined as ratios of the vapor-to-liquid mole fractions, are usually predicted from cubic equations of state for light components of the conventional oil reservoirs. However, for bitumen reservoirs, K-factors are much more difficult to measure and predict because the bitumen components are less volatile. Until very recently, there were no reliable data and no models available for bitumen K-factors. The objective of this series of papers, therefore, is to develop quantitative, predictive models for bitumen K-factors. Our measure of success is not the goodness of fit alone, but the quality of predictions as well. Yu et al. (1989) and Huang and Radosz (1990a) present experimental data and equations of state for model binary systems composed of bitumen, or its fractions, and supercritical C02, and predict the mutual solubilities of bitumen and CO2 with PHC and Soave equations of state. In two subsequent papers, Huang and Radosz (1991 b, c) propose lumping and molecular weight distribution approaches to characterizing reservoir fluids in a way that is compatible with equations of state. Finally, Huang and Radosz (1991d) demonstrate that an equation of state based on SAFT offers an attractive model for simulating bitumen phase equilibria. SAFT pure and binary parameters are found to be well behaved and, hence, easy to generalize with respect to measurable bitumen properties, such as molecular weight and aromaticity. This is not surprising because SAFT has theoretical justification and its parameters have physical significance that is not lost in the process of correlating experimental data. Our goal here is to demonstrate that SAFT retains its predictive power upon extension to pseudo-ternary bitumen systems, e.g. supercritical fluid, cosolvent and bitumen, and that SAFT can be used to evaluate cosolvency effects on bitumen fractionation. First, we measure phase equilibria in a model ternary system of CO2 + n-decane + bitumen, We refer to n-decane, our model aliphatic cosolvent, as decane. Next, we predict, without fitting, the measured phase equilibrium data with SAFT. Finally, we predict phase
J.M.
Yu et al. 1Fluid Phase Equilibria 93 (1994) 353-362
355
equilibria for which there are no experimental data available to evaluate the effect of cosolvent aromaticity on bitumen fractionation. We select CO, as our model solvent because most of the steam-based recovery processes at Alberta, Canada generate CO, as a product of chemical reactions between steam and oil sands. To improve the solvent power of CO2 for extracting as well as de-asphalting bitumen in situ, decane and tetralin are selected as model cosolvents to represent a naphtha stream that can be obtained from a surface partial upgrading scheme (Chow et al., 1991a,b). EXPERIMENTAL
A flow cell apparatus described by Yu et al. (1989) is modified for this work. A windowed cell serves as a two-phase separator. All the components are thoroughly equilibrated in a static mixer upstream of the cell. In addition to two feed lines used in the previous work, one for CO2 (HPLC reciprocating pump) and one for bitumen (heated reciprocating syringe pump), we use a third independent line for feeding the decane cosolvent (HPLC syringe pump). The decane stream is injected into the bitumen stream upstream of the CO2 injection point. As before, samples from both equilibrium phases are continuously separated, at ambient pressure, into CO2 and condensable fractions. The condensable fraction of each phase is analyzed for decane. The top phase (CO,-rich) decane concentration is determined from gas chromatography (GC). The calibration standards for such a GC analysis are prepared by adding a known amount of decane to a known amount of bitumen extract. The bottom phase (CO,-lean) decane concentration is determined from material balance after evaporating decane from the bottom phase condensate in a vacuum oven. The boiling temperature of decane overlaps with a small fraction of the bitumen light ends. This may contribute to an analytical error in measuring the decane concentration in bitumen by about 5%. However, we calibrate and fine-tune this procedure against bitumen (stripped with CO*) and decane mixtures of known composition. BITUMEN
CHARACTERIZATION
Properties of the Cold Lake bitumen used in this work are described in detail by Yu et al. (1989). Only a brief overview is given here. The bitumen is represented by eleven pseudo-components. Properties of the pseudo-components (distillation lumps) are characterized in terms of SAFT parameters which, in turn, are used to calculate bitumen phase equilibria. The SAFT parameters are presented by Huang and Radosz ( 1991d). In addition to SAFT, we include data calculated from the Peng-Robinson equation of
356
J.M.
Yu et al. /Fluid
Phase Equilibria 93 (1994) 353-362
state (PR) because this is a popular equation of state for reservoir and surface applications and, hence it serves as a useful frame of reference. PR is representative of other cubic equations of state such as Soave. The PR characterization of the Cold Lake bitumen is given by Yu et al. (1989). Conventional cubic equations of state, such as PR, require three purecomponent parameters, the critical temperature, critical pressure and Pitzer’s acentric factor. These, in turn, are estimated from empirical correlations and often have no physical significance, especially for heavy components, which, in contrast to light components, thermally decompose well below their critical temperatures. The purpose of such characterization is to mimic a hypothetical vapor pressure curve terminating in a hypothetical critical point. By contrast, SAFT characterization utilizes well-defined and measurable properties of heavy hydrocarbons. Specifically, the SAFT pure-component parameters (three for non-associating molecules: segment energy, segment volume and number of segments per molecule) are correlated against molecular weight and average aromaticity. These, in turn, can be estimated from experimental data, such as gas chromatographic distillation (GCD), specific gravity, hydrogen to carbon ratio and spectroscopy. The important advantage of SAFT is that each pseudo-component is characterized not only in terms of its average size (molecular weight), but also in terms of molecular structure (aromaticity). This way we can capture bitumen fractionation selectivity not only with respect to volatility but also with respect to molecular type. This is an advantage because supercritical solvents and cosolvents will fractionate bitumen by molecular size and structure. RESULTS
AND DISCUSSION
Experimentally determined phase compositions at 200°C and 12 MPa are listed in Table 1 for the CO, + decane + bitumen system at two feed compositions differing primarily in decane concentration. Also listed in Table 1 are phase compositions calculated from SAFT and PR. As expected, we find that the liquid phase composition is represented well by both SAFT and PR. This is shown in Fig. 1 for the CO, solubility in bitumen and in Fig. 2 for the decane solubility in bitumen as a function of decane weight percent in feed on a CO,-free basis. As we can see, decane increases the solubility of COZ in bitumen (Fig. 1). More importantly, both SAFT and PR predict reasonably the experimental solubilities, where available, as shown in Figs. 1 and 2. Also, extrapolating to higher decane concentrations results in similar predictions from both SAFT and PR. Incidentally, the small difference between the SAFT and PR curves shown
J.M. Yu ea al. / Fluid Phase Equilibria 93 (1994) 3S?-362
357
TABLE i Measured and SAFT- and PR-predicted liquid and vapor phase compositions CO,-decane-bitumen system at 200°C and 12 MPa Liquid phase wt.%
of the
Vapor phase wt.%
Measured
SAFT
PR
Measured
SAFT
PR
Set 1 a
COz C,, Bitumen
6.76 4.90 88.34
6.38 5.96 87.65
7.19 5.07 87.74
96.10 2.81 1.09
96.55 2.65 0.80
95.44 2.94 1.62
Set 2 b
CO, Cl0 Bitumen
7.18 11.00 81.82
7.35 13.41 79.24
8.27 11.26 80.47
93.4 5.72 0.88
93.87 5.42 0.72
92.75 5.95 1.31
3.24; C,,/bitumen, 4.62; C,,/bitumen,
0.146. 0.407.
a Set 1: Feed ratios (weight/weight): b Set 2: Feed ratios (weight/weight):
0
0
10
a0
30
40
50
COJbitumen, COJbitumen,
60
70
&cane wt% in Feed CCO$ree
80
90
loo
Basis)
Fig. 1. Measured and SAFT- and PR-predicted 12 MPa.
CO* solubilities in bitumen at 200°C and
in Figs. 1 and 2 can be explained by different results in fitting the CO,-decane binary. It should be noted that no attempt was made to predict the possible phase split in bitumen at high decane concentrations. For the record, the SAFT and PR binary parameters are estimated as follows: CO,-decane from binary VLE data (Reamer and Sage, 1963), 0.11 for PR and 0.00 for SAFT; CO,-bitumen from empirical correlations fitted to the CO&n-bitumen solubilities (Huang and Radosz, 1991d). While the liquid (bitumen-rich) compositions are predicted reasonably well by both models, the vapor (CO&h) compositions are realistically
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0
10
20 30 40 59 60 70 a0 Decane wt% in Feed (CO$ea basis)
90
lly)
Fig. 2. Measured and SAFT- and PR-predicted decane solubilities in bitumen at 200°C and 12 MPa.
Fig. 3. Measured and SAFT- and PR-predicted
decane and bitumen solubilities in CO,.
predicted by SAFT but not by PR. This is illustrated in Fig. 3 where decane and bitumen concentrations in CO2 in weight percent are plotted versus decane weight percent in feed (C02-free basis). The PR predictions of bitumen solubility in CO2 are out. This is typical of other empirical equations of state also. Therefore, we use only SAFT for further calculations in this work. Another argument justifying selecting SAFT is that its adjustable binary parameter is applied only to a relatively minor dispersion term while the PR binary parameter is applied to the whole equation of state. This makes SAFT a more reliable predictive tool. Figure 3 shows that, while the decane solubility increases with increasing decane concentration in the feed, which is expected, the total bitumen
J.M.
Yu et al. 1 Fluid Phase Equilibria 93 (1994) 353-362
50
l60
260
-360
460
359
560
Molecular Weight of Bitumen Fractions
Fig. 4. SAFT-predicted mixtures.
K-factors of bitumen fractions in supercritical CO2 and COJdecane
solubility actually decreases with increasing decane concentration in the feed. However, decane does increase bitumen K-factors, relative to a decane-free case, because bitumen mole fractions in the bitumen-rich phase decrease at an even faster rate upon increasing the decane concentration in the feed. This is shown in Fig. 4. The K-factor, defined as the mole fraction ratio, vapor to liquid, is plotted in Fig. 4 versus the molecular weight of bitumen components. In all the cases, as expected, the bitumen K-factors decrease drastically upon increasing the molecular weight. The three curves shown in Fig. 4 correspond to the total concentrations of the cosolvent, decane, in solvent COZ, namely, 0 wt.%, 4 wt.% and 25 wt.%. Figure 4 shows that the addition of decane increases K-factors and therefore enhances the extraction of bitumen lights with supercritical COZ. Having established that SAFT realistically predicts such decane cosolvent effects, we are interested in using SAFT for other cosolvent candidates. Another example of a Cl,, hydrocarbon is tetralin, which is a hybrid hydrocarbon that contains an aromatic ring and a naphthenic ring. The binary interaction parameters of the CO,-tetralin pair for SAFT, obtained from the VLE data (Chou et al., 1990), is 0.100. A 25 wt.% tetralin in CO2 case is compared with the 25 wt.% decane and pure CO2 cases in Table 2 and Fig. 5. This comparison suggests that decane is more effective in extracting bitumen lights, if one simply compares K-factors. This can be explained by a greater affinity of decane to bitumen aliphatic (relative to bulk) lights. However, a more careful comparison of selectivity with respect to both molecular weight and aromaticity reveals a somewhat different picture. Figure 6 shows bitumen distributions of size and aromaticity, molecular
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TABLE 2 SAFT-predicted K-factors of bitumen fractions decane and tetralin at 200°C and 12 MPa Bitumen fraction
Composition (wt.%)
1 2 3 4 5 6 7 8 9 10 11
0.24 0.70 1.94 3.40 4.80 6.87 5.68 6.97 16.50 14.80 38.10
Molecular weight (gmol)
WC
130.7 147.1 166.2 188.7 215.2 246.4 283.7 327.6 437.9 813.0 2580
1.825 1.803 1.779 1.754 1.728 1.699 1.669 1.637 1.573 1.551 1.394
in supercritical
CO, and its cosolvents,
K-factors x 1000 Solvent CO*
Cosolvent 25 wt.% = decane
Cosolvent 25 wt.% = tetralin
57 36.4 21.3 11.1 5.11 1.99 0.63 0.16 0.005 0 0
123 85.8 55.6 33.1 17.9 8.57 3.52 1.21 0.08 0 0
107 68.1 39.8 20.9 9.72 3.87 1.27 0.33 0.01 0 0
a Cosolvent wt.% is in solvent CO2
0.15 wt% of comlvent in overall CO2 at 200 ‘C and 1!2MPa
0.1
0.05
0 50
150 250 350 450 Molecular Weight of Bitumen Fractions
Fig. 5. Comparison supercritical CO,.
of decane and tetralin
cosolvent
550
effects on bitumen
K-factors
in
weight (upper x-axis) and hydrogen to carbon ratio (H/C, lower x-axis), respectively. Comparing decane and tetralin, one finds that the tetralin extract is richer in lighter and less aromatic (high H/C) components. This means that the decane cosolvent results in a higher capacity (higher K-factors) but lower selectivity (heavier, more aromatic extract). A more
J.M.
Yu et al. 1 Fluid Phase Equilibria 93 (1994) 353-362
361
MolecularWeight of Bitumen Fractions 130 145 166 190 215 245 265 330 440 al.5 2680 40
0 1.83 1.8 1.78 1.75 1.73 1.7 1.67 1.64 1.57 1.55 1.39 WC of Bitumen Fractions
Fig. 6. Decane
and tetralin
cosolvent
effects on CO2 selectivity.
quantitative characterization of the solvent selectivity can be in terms of the ratio of K-factors (the separation factor) which can be easily estimated from SAFT. CONCLUSIONS
The SAFT and PR equations of state are found comparable in predicting the condensed phase compositions in CO,-decane-bitumen systems. However, SAFT is found superior in predicting the supercritical fluid phase compositions. Hence, the bitumen partitioning coefficients (Kfactors) can be reliably predicted with SAFT but not with PR. Moreover, the SAFT characterization approach based on measurable molecular properties is more appropriate for heavy, associating bitumen components, than is the PR characterization approach based on hypothetical critical properties. The SAFT predicted K-factors allow for quantifying cosolvent selectivity and capacity in fractionating bitumen with supercritical CO*. For example, decane (a model aliphatic cosolvent) is found to have a higher capacity but lower selectivity than tetralin (a model aromatic cosolvent). There are many challenges which will have to be addressed in the future. For example, for propane, butane and pentane de-asphalting, we will have to reconsider the heavy-end characterization as well as a detailed quantification of bitumen component distribution in both vapor and liquid phases. Also, the effects of supercritical water and asphaltene aggregation, important in many reservoir systems, will have to be studied.
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