Solubility of n-alkanes in supercritical CO2 at diverse temperature and pressure
Graphical abstract
Introduction
Enhanced oil recovery processes have become increasingly important to the petroleum industry. Among numerous EOR methods, CO2-EOR is known as a promising, green, and economical oil recovery technology. It could recover 6–18% original oil in place after secondary oil recovery by oil viscosity reduction, oil swelling effect, interfacial tension reduction, and light-hydrocarbons extraction, etc. Furthermore, CO2-EOR could lighten greenhouse effect by injecting CO2 into oil reservoirs and storing it underground [1], [2], [3], [4], [5], [6], [7], [8].
Due to its low polarity, low dielectric constant, and relative high density at supercritical state, CO2 is a good solvent for petroleum [9], [10]. After being stored underground, liquid CO2 can effectively extract light (e.g., C3–C8) hydrocarbons from original light crude oil, leaving a much larger amount of heavy hydrocarbons flowing through the reservoir [2], [11]. Studies by Meng Cao et al. show that, as the injection volume of CO2 increase, produced oil becomes lighter and lighter [1]. This is because hydrocarbons’ polarity increases with their length. As a result, solubility of hydrocarbons in CO2 decreases. Owning to the fact that lighter hydrocarbons are easier to dissolve into CO2, the heavier hydrocarbons are left behind when CO2 passes through the reservoir. Besides, experimental studies indicate that the oil recovery factor will decrease if the C5–C19 mole fractions in the crude oil dwindle [1].
CO2-EOR is preferred to be used in light and medium oil reservoirs because of miscibility restriction [2], [4], [12], [13]. Moreover, asphaltene precipitation, caused by extracting phenomenon, makes the next stage of oil recovery even more difficult [3], [13], [14], [15], [16]. It is helpful to solve the problem with the settlement of heavy hydrocarbons’ solubility problem, which would be a significant step towards improving the CO2-EOR efficiency. Therefore, solubility of varying hydrocarbons in scCO2 deserves careful investigation [17].
Field of high pressure CO2-alkane systems phase equilibrium at elevated temperature and pressure has received the greatest attention [18], [19], [20], [21], [22], [23], [24], [25], [26]. Karen Chandler et al. reported capacity factors for n-alkanes (C9H20–C36H74) in CO2. The experimental temperature and pressure range are 308.2–348.2 K and 100–240 bar, respectively. The estimated solubility for solid n-alkanes (C24H50–C36H74) in CO2 is calculated from the capacity factors [18]. Eun-Joo Choi et al. measured critical point data (dew points and bubble points) of CO2/hexane, CO2/heptane, CO2/octane, and CO2/nonane, mole fractions of CO2 ranges from 0.974 to 0.834, and experimental temperature and pressure are up to 395.7 K and 153.41 bar, respectively [19]. Other phase equilibriums of systems like hexane/CO2 [20], octane/CO2 [21], decane/CO2 [22], [23], dodecane/CO2 [24], and tetradecane/CO2 [25], etc. were studied as well. Bo Wang et al. measured the solubility of hexane and other compounds containing Cl, F, or aromatic nucleus in scCO2. They discussed the effect of polar substitute on solubility of the compound in scCO2. Also, the dependence of solutes’ solubility on binary system density was investigated [27]. Zihao Yang et al. researched on the solubility of CO2 in hexane, octane, decane, and cyclohexane, and he looked at the effect of alkane molecular structure on volume expansion of CO2/alkanes system, basing on the data of CO2 solubility in alkanes [5]. Besides the aforementioned researches on phase equilibrium and intersolubility between alkanes and CO2 from different workers, Ali Zolghadr et al. measured the minimum miscible pressure (MMP) of heptane and hexadecane by vanishing interfacial tension technique to determine the effect of temperature and pressure on interfacial tension, which also represents, to some extent, their solubility in CO2 [28]. However, to make the study more systematic, we delve into the solubility of hydrocarbons in scCO2. There are literature data for systems similar to ours from other works: solubility of octane in CO2 at 333 K from Ref. [25], n-decane in scCO2 at 344.15 K, and n-hexadecane in the same solvent, at 308.15 K, both in the range 8–13 MPa [26]. In this study, we compare our experimental results with the above systems, using the literature data.
Chrastil, Bartle, Sung and Shim and many other workers proposed different semi-empirical models to correlate solute's solubility in CO2 with density of CO2 [29], [30], [31], [32], [33], [34], [35], [36]. These models are simple to use because it is not necessary to employ physicochemical properties. Instead, semi-empirical models were employed and widely used to provide correlations of the experimental solubility in scCO2 with density of CO2. These models are density-based methods in which the effects of temperature and pressure are considered crucial. These models also indicate that there is a linear relationship between solvent density and solute's solubility. Chrastil first develops a model to correlate the solute solubility in scCO2 with density of CO2, and this model is now commonly used by other researchers [29]. The equation is as follows:where ρ (kg/m3) is the density of CO2, and T (K) the system temperature; k, m and n are characteristic constants; S (kg/m3) is the solubility of the solute, which can be calculated using the formula below:where M0 and M1 are the molar weight of CO2 and the solute, respectively, x is molar fraction of the solute in the binary system.
One objective of this study is to examine the solubility of n-alkanes (C6–C18), which help us to have a better understanding on how much hydrocarbon could dissolve into the scCO2. Furthermore, temperature and pressure effects on the solubility of n-alkanes as well as the correlations between CO2 density and n-alkanes solubility are primary objectives as well. And the solubility results correlate with Chrastil model.
Section snippets
Materials
Provenance and purity of all chemical used in this study are included in Table 1. Carbon dioxide with a purity greater than 99.5% was purchased from Dalian Gas Co. Ltd.; n-hexane (AR) was supplied by Tianjin Fuyu Fine Chemical Co. Ltd.; n-octane (AR) was imparted by Tianjin Bodi Chemical Company; n-decane (AR) and n-tetradecane (AR) were purchased from Tianjin Guangfu Fine Chemical Research Institute; n-dodecane (AR) was provided by Tianjin Kemiou Chemical Reagent Co. Ltd.; n-hexadecane (GC)
Accuracy and repeatability test
Fig. 2 shows the condition of n-hexane (5.21 g)/CO2 binary system under different pressures at 318 K. Picture a illustrates n-hexane in the cell. After gently injecting CO2 into the cell, we notice the liquid level raises (picture b), which is caused by some CO2 dissolved into the n-hexane phase. Picture b and c shows that it is a two-phase system at 5.445 MPa. Additionally, the system appearance changes from turbid (d, 7.712 MPa) to transparent (e, 8.630 MPa) by adding more CO2 into the cell.
A
Conclusions
The experimental study on the solubility of n-alkanes in CO2 at 318–343 K demonstrated that alkane's solubility has a direct relation with pressure and CO2 density, while it has inverse relation with chain length and temperature. The cloud point pressure of n-alkanes exhibits a positive liner trend with temperature; it also increases with the chain length of n-alkane, attributed to that molecule chain length growth increases instantaneous dipole of n-alkane in CO2, and strengthen Vander Waals
Acknowledgement
We thank the Daqing Oil Field Co. (DQYT-0508003-2011-JS-362) for supporting this project.
References (36)
- et al.
J. Supercrit. Fluids
(2002) - et al.
J. Chem. Thermodyn.
(2013) - et al.
Fluid Phase Equilib.
(2012) - et al.
Fuel
(2013) - et al.
Fluid Phase Equilib.
(2006) - et al.
Fluid Phase Equilib.
(2001) - et al.
Fluid Phase Equilib.
(2006) - et al.
Fluid Phase Equilib.
(1983) - et al.
Chem. Eng. Sci.
(2008) - et al.
Fluid Phase Equilib.
(2014)
Energy Fuels
Energy Fuels
Energy Fuels
Energy Fuels
J. Chem. Eng. Data
J. Chem. Eng. Data
Energy Fuels
J. Chem. Eng. Data
Cited by (50)
High-pressure fluid-phase equilibria: Experimental methods, developments and systems investigated (2013–2016)
2024, Fluid Phase EquilibriaEffective viscosification of supercritical carbon dioxide by oligomers of 1-decene
2022, iScienceCitation Excerpt :There is a consistent trend of increase in solubility with increase in temperature. This is opposite to the solubility trend for n-alkanes (Shi et al., 2015) but in agreement with that of aromatics like toluene as reported in the literature (Ng and Robinson, 1978; Kim et al., 1986). Figures 3 and 4 reveal that the effect of pressure is greater than the effect of temperature in increasing the polymer solubility.
Optimization of supercritical CO<inf>2</inf> extraction by response surface methodology, composition analysis and economic evaluation of bamboo green wax
2022, Journal of Cleaner ProductionCitation Excerpt :However, at 30 MPa, the yield of alkanes decreased due to the increase in temperature, which may be because the intermolecular force between CO2 and n-alkanes as well as between the two n-alkanes molecules decreased. And the former is falling faster than the latter (Shi et al., 2015). Long chains of fatty acids vary in length from C12 to C20.
Waste-water purification through a countercurrent system driven by supercritical carbon dioxide (SC-CO<inf>2</inf>). Part I: Experimental investigation and process evaluation
2020, Separation and Purification Technology