High Pressure Phase Behavior of the Homologous Series CO2 + 1

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

High Pressure Phase Behavior of the Homologous Series CO2 + 1‑Alcohols Cara E. Schwarz* Department of Process Engineering, Stellenbosch University, Banghoekweg, Stellenbosch 7600, South Africa ABSTRACT: The phase behavior of the supercritical CO2 + 1-alcohol homologous series for 1-octanol and higher alcohols is investigated. A literature survey revealed that sufficient data are available lower alcohols (1-decanol and lower) but data are limited for higher 1-alcohols. Data for CO2 + 1-dodecanol, CO2 + 1-tetradecanol, and CO2+1-hexadecanol were thus measured using a visual static synthetic method for T = 313− 353 K. Measured and literature data indicated that the phase transition pressures are very high (>28 MPa) for CO2 + 1decanol and higher alcohols in the mixture critical region. Additionally, a temperature inversion (increasing temperature leading to decreasing phase transition pressure) is present in the mixture critical region and is more pronounced in systems containing higher 1-alcohols. The temperature inversion is postulated to result from the formation of alcohol multimers due to hydrogen bonding between alcohol molecules. From the phase behavior data as well as published upper critical end point and solid−liquid−liquid−vapor equilibria data, Type III phase behavior according to the classification of Scott and van Konynenburg is assigned to the systems CO2 + 1-octanol to CO2 + 1tetradecanol. However, for higher alcohols additional investigations are required to assign a phase behavior type. This systematic study hopes to aid future thermodynamic modeling attempts to describe the complex phase behavior of CO2 + 1-alcohol systems.

1. INTRODUCTION Detergent range alcohols (decanol through to eicosanol) are used in the production of surfactants and are often produced by grafting a hydroxyl group onto an alkane. However, during the grafting process, the conversion of alkane to alcohol is not complete resulting in a product stream that contains alcohols as well as unreacted alkanes. Alternatively, these alcohols can be produced through hydration of an alkenes. Here residual alkane may also be present in the product stream, this time originating either as a byproduct of the reaction or from the alkene feedstock. Also, as the alkane/alkene feedstock has a distribution of chain lengths, so too do the alcohols. As a result, a mixture of alkanes and alcohol with between 10 and 20 carbon atoms need to be separated. The alkane + alcohol mixtures have crossover boiling and melting points1 making transitional separation processes such as distillation and crystallization difficult, if not impossible to achieve. In addition, processes using organic solvents result in a problematic solvent residue that is difficult to remove. Therefore, an alternative separation technique is required. Supercritical fluid fractionation with CO2 as supercritical solvent is such an alternative technique. Here the supercritical CO2 acts in a way similar to a liquid solvent and preferentially extracts the alkanes above the alcohols. Such a process runs at mild temperatures (typically 310−370 K) and the solvent is readily removed from the product stream. It has previously been shown that such a technique is technically possible on pilot plant scale.2−5 However, further studies into this separation technique require detailed phase equilibria data for © XXXX American Chemical Society

both alkanes and alcohols with CO2 in a suitable temperature range. The literature is abundant with studies on the phase behavior of CO2 + alkanes (see the reviews of Dohrn and coworkers6−9) but for the CO2 + 1-alcohol homologous series the literature is at times lacking. In particular, for 1-alcohols with 12 or more carbon atoms data is scarce. In addition, although some studies have considered the phase behavior of a particular 1-alcohol in CO2 to date an overview of the general phase behavior present in this homologous series and summary of the available data is lacking. The aim of this work is to provide a comprehensive view of the CO2 + 1-alcohol homologous series for 1-alcohols with eight or more carbon atoms. The aim of the current contribution will be achieved by 1) Considering the available literature data for CO2 + 1alcohols, evaluating the various data sets and highlighting gaps in the literature. 2) Conducting additional phase behavior measurements where required. 3) Analyzing each system in terms of the pressure− temperature−compositional and critical phase behavior. 4) Highlighting trends observed in the homologous series. Special Issue: Emerging Investigators Received: November 15, 2017 Accepted: March 7, 2018

A

DOI: 10.1021/acs.jced.7b01000 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Summary of Available Literature Data for Fluid−Liquid Phase Transitions for System CO2 (1) + 1-Alcohol (2) Including the Temperature (T), Pressure (P), and Alcohol Mass Fraction (w2) Range alcohol 1-octanol

1-nonanol

1-decanol

1-undecanol

1-dodecanol

1-tridecanol 1-tetradecanol 1-pentadecanol 1-hexadecanol

1-octadecanol

sourcea 50

Byun and Kwak Chang et al.18 Chiu et al.51 Chrisochoou et al.52 Fourie et al.21 Feng et al.53 Hou et al.54 Hwu et al.55 Ke et al.26,b Lam et al.27,b Lee and Chen56 Li et al.57 Scheidgen10 Weng and Lee11 Weng et al.58 Artal et al.19 Chang et al.18 Lam et al.27,b Pfohl et al.36 Secuianu et al.28,a Chang et al.18 Ghaziaskar et al.15 Ioniţă et al.16,a Lam et al.27,b Lee and Chen56 Patton and Luks59,b Scheidgen10 Pöhler14 Weng et al.58 Zamudio et al.17 Artal et al.19 Lam et al.27,b Pöhler14 Scheidgen10 Secuianu et al.20,a Hölscher23 Katzenski-Ohling and Schneider60 Kordikowski37 Kordikowski and Schneider43 Lam et al.27,b Scheidgen10 Secuianu et al.42,a Spee38 Spee and Schneider39 Artal et al.19 Lam et al.27,b Jan et al.22 Lam et al.27,b Artal et al.19 Breman et al.61 Hölscher23 Jan et al.22 Kramer and Thodos62 Uchida and Kamijo63,b Yau and Tsai64 Jan et al.22 Kramer and Thodos65 Uchida and Kamijo66,b Yau and Tsai64

T/K 313.2−393.2 308.2−328.2 313.2−348.2 313.2 K 308.2−348.2 328.2 K 313.2−323.2 328.3 K 300.0−333.0 250.3−309.1 348.2−453.2 303.2 K 287.1−389.1 313.2−348.2 403.2−453.2 323.2 K 308.2−328.2 258.8−308.0 303.2 K 285.6−353.2 308.1−328.2 323.0 K 289.5−343.2 270.5−307.2 348.2−453.2 271.0−279.6 305.2−393.6 313.2−393.2 348.2−453.2 308.5−348.6 323.2 K 279.0−306.7 313.2−393.2 311.5−392.9 384.2−333.2 333.2−393.2 393.2 K 298.2−353.0 353.2 K 287.4−306.1 316.6−392.6 287.6−353.2 293.2−393.2 293.2−393.2 323.2 K 294.8−305.8 373.2−573.2 301.6−305.4 323.2 K 372.6−515.6 393.2 K 373.2−573.2 318.0−338.0 312.6−322.3 318.0−328.0 373.0−573.0 328.0−338.0 320.6−330.9 328.0−338.0

K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K (308.0 K omitted) K K (318.0 K omitted) K K (302.0−318.0 K omitted)

B

P/MPa

w2/g·g−1

0.5−2.20 MPa 1.5−15.1 MPa 3.3−18.3 MPa 0.5−15.2 MPa 6.8−17.9 MPa 3.0−13.4 MPa 1.1−8.0 MPa 3.0−13.3 MPa 6.6−22.9 MPa 1.8−8.0 MPa 1.0−5.0 MPa 0.3−0..9 MPa 4.9−98.9 MPa 4.0−19.0 MPa 6.5−19.0 MPa 9.5−22.2 MPa 2.2−15.6 MPa 2.3−7.9 MPa 11.3−34.0 MPa 1.2−10.3 MPa 2.2−15.2 MPa 9.0−18.0 MPa 1.7−15.1 MPa 3.2−7.8 MPa 1.0−5.0 MPa 0.1−3.2 MPa 21.1−97.1 MPa 10.0−32.3 MPa 6.0−19.0 MPa 7.6−27.5 MPa 9.5−23.9 MPa 4.0−7.7 MPa 10.0−26.1 MPa 23.7−99.5 MPa 1.1−15.4 MPa 10.0−28.7 MPa 9.0−27.0 MPa 10.0−25.2 MPa 13.0−25.2 MPa 4.9−7.6 MPa 22.2−99.3 MPa 0.8−9.4 MPa 10.0−27.5 MPa 10.0−27.5 MPa 8.4−24.1 MPa 5.9−7.6 MPa 1.0−5.1 MPa 6.9−7.6 MPa 9.1−23.7 MPa 1.9−3.0 MPa 1.0 o 32.3 MPa 1.0−5.1 MPa 14.2−41.5 MPa 0.1−23.0 MPa 5.0−20.3 MPa 1.0−5.1 MPa 14.0−45.3 MPa 0.1−23.0 MPa 4.9−20.4 MPa

0.0969−0.91 0.000−0.961 0.0459−0.915 0.000592−0.984 0.0163−0.712 0.00177−0.935 0.705−0.981 0.000887−0.930 not given 0.0224−0.776 0.00169−0.992 0.973−0.993 0.00200−0.908 0.00112−0.920 0.0161−0.908 0.0212−0.313 0.000−0.944 not given 0.0633−0.636 0.000655−0.998 0.000−0.952 0.000385−0.00674 0.650−0.965 0.0233−0.761 0.000593−0.990 0.763−1.000 0.290 0.00400−0.817 0.000899−0.920 0.0180−0.697 0.0125−0.211 not given 0.00100−0.801 0.290 0.00117−0.989 0.00100−0.845 0.00200−0.805 0.00500−0.831 0.010−0.381 not given 0.0390−0.530 0.00506−0.982 0.0200−0.853 0.0200−0.853 0.000660−0.0972 not given 0.00775−0.990 not given 0.00179−0.0593 0.000−0.976 0.00400−0.870 0.0136−0.992 0.0104−0.137 not given 0.00187−0.0480 0.00184−0.9920 0.00636−0.126 not given 0.00160−0.0305



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Table 1. continued a Indicates sources that also include three phase, (VLLE or SLVE) or critical data. bIndicates sources that only contain three phase, (VLLE or SLVE) or critical data.

considered in this work are available. Because of limited overlap, the data cannot be directly compared, but the data trends are all in agreement with one another and generally in agreement with the CO2 + 1-octanol and CO2 + 1-decanol data. In general, the data has also only been measured up to 15 MPa with limited data in the mixture critical region. However, as the CO2 + 1-octanol and CO2 + 1-decanol (see below) systems are very well-defined, no additional measurements here are justified. • CO2 + 1-decanol: This system has also been very well studied in the temperature and compositional range investigated in this work. The various sources are generally in agreement with one another. However, the pressure− composition data of Pöhler14 and Ghaziaskar et al.15 are not in complete agreement with the work of Ioniţă et al.16 or that of Zamudio et al.17 However, the work of Ioniţă et al. and Zamudio et al. are in agreement with each other and other sources, such as Chang et al.18 Pö hler presented both pressure−composition (constant temperature) as well as temperature−composition (constant pressure) data. The pressure−composition (constant temperature) data were measured in a separate work by Scheidgen but published in the thesis of Pöhler. The temperature−composition (constant pressure) data were part of the Ph.D. work of Pöhler. These two sets of data are not in complete agreement with one another yet the temperature−composition data is in agreement with other published data. There is thus some doubt regarding the pressure−composition data in the Ph.D. thesis of Pöhler, measured by Scheidgen. Ghaziaskar et al. used a flow technique to measure their data. It may be that their data may not have been fully saturated with the alcohol at the measurement point as their noted alcohol compositions in the vapor phase are lower than that of other researchers. However, in totality considering the reasonably complete data sets available and the fact that the data of Chang et al., Ioniţă et al., and Zamudio et al. as well as the temperature−composition data of Pöhler et al. are in agreement, no additional measurements are required. • CO2 + 1-undecanol: As for the CO2 + 1-nonanol system, considerably fewer data sets are available for this system. However, the data covers the majority of the temperature and compositional range and is in general agreement. The only exception is that of Artal et al.19 The data of Artal et al. is not in agreement with that of Pöhler14 or with Secuianu et al.,20 yet the work of Pöhler and Secuianu et al. are in agreement with one another. Artal et al. also measured data for the systems CO2 + 1-nonanol, CO2 + 1-tridecanol, and CO2 + 1pentadecanol, however nowhere does their data overlap with that of others. If all the data of Artal et al. for their various systems are compared to that of other homologous systems at the same temperature, it appears as if their CO2 + 1-undecanol data is not in agreement with other data (e.g., CO2 + 1-octanol and CO2 + 1-decanol data generated from the correlations of Fourie et al.21 and Zamudio et al.,17 respectively, and data for CO2 + 1-tetradecanol measured in this work). However, their other systems (CO2 + 1-nonanol, CO2 + 1-tridecanol, and CO2 + 1-pentadecanol) do seem to be in agreement with other data sets. Secuianu et al. studied the CO2 + 1-undecanol system in detail, and although the pressure range in their study was

The study will consider only the liquid−liquid, liquid−vapor, and liquid−liquid−vapor phase behavior at temperatures from the critical point of CO2 (304.1 K1) to 373 K and will focus on compositions in and near the mixture critical region. This is typically the temperature range in which supercritical processes would operate in order to take advantage of the volumetric behavior of the supercritical solvent near the critical point. It is also in the mixture critical compositional range where complex phase behavior is present and where thermodynamic models are lacking. No phase behavior involving solids will be considered except data that indicates the limits of the fluid only region and very briefly the solid−liquid−liquid−vapor point (Q-point). The study will be limited to linear, primary alcohols with eight or more carbon atoms. Additional measurements will be limited to system for which 1-alcohols are commercially available and measurements at conditions where the maximum pressure is expected to be 28 MPa or less (due to equipment limitations). All analyses will be presented as mass fractions alcohol as presentation of data in mass fractions (w) rather than mole fractions (x) allows for the data to be spread across a wider compositional range.

2. OVERVIEW OF AVAILABLE DATA A summary of available data for the phase behavior of CO2 + 1alcohols is given in Table 1. For completeness, the temperature range of the data included is up to 473 K. As seen, there is a large amount of data for the systems CO2 + 1-octanol and CO2 + 1-decanol. However, as the number of carbon atoms increases, the amount of available data decreases. This decrease in data can in all likelihood be attributed to a combination of an increase in cost of the 1-alcohol and an increase phase transition pressure as the number of carbon atoms increases. It is also noted that there is significantly more data available for 1alcohols with an even number of carbon atoms compared to those with an uneven number of carbon atoms. It is postulated that this is due to the lower cost and increased availability of the even numbered 1-alcohols relative to that of the uneven numbered 1-alcohols. A summary of the available data and critical comparison of the data sets and consistency between sources is briefly presented. • CO2 + 1-octanol: A large amount of data exists for this system in the temperature and compositional range considered in this work. In general, most data sets are in agreement with one another and a regular trend is noted in the temperature and compositional dependence. At times, the data of Scheidgen10 and Weng and Lee11 depart slightly from other published data (up to 1.2 MPa or 0.03 mass fraction). Both these sources used an analytical setup, and it has been shown that unless extreme care is taken, including sufficiently long waiting times for equilibrium and visual observation of the cell contents to ensure correct sampling, errors in measurement may be possible.12,13 However, considering the system as a whole sufficient information is present and additional measurements would merely be a repetition of previous studies. • CO2 + 1-nonanol: Numerous studies have been considered for this system. Although less studied than the CO2 + 1-octanol system, data covering the majority of the temperature range C



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result as in this region the pressures are estimated to be above 28 MPa. • CO2 (1) + 1-dodecanol (2): w2 = 0.08−0.70 for T = 313− 353 K, where possible. • CO2 (1) + 1-tetradecanol (2): w2 = 0.03−0.70 for T = 313−353 K, where possible. • CO2 (1) + 1-hexadecanol (2): w2 = 0.03−0.80 for T = 323−353 K, where possible.

limited no additional measurements will be conducted in the current work as the system CO2 + 1-decanol has been well studied. Also, if the system CO2 + 1-dodecanol is well characterized or additional measurements are conducted therefore sufficient information can be obtained from the two adjacent systems for the purpose of this work. • CO2 + 1-dodecanol: Some data has been measured for this system and in general these data sets are all in agreement with one another. However, in the temperature range of interest in this work only limited data is available and the majority thereof is at pressures lower than 10 MPa. Additional measurements are thus justified and, where possible, will be conducted. • CO2 + 1-tridecanol: Artal et al.19 are the only authors that have considered this system. As mentioned above it is believed this data is true as it is consistent with other homologues at the same temperature. However, as additional data for CO2 + 1dodecanol are to be measured no additional measurements for CO2 + 1-tridecanol will be conducted. • CO2 + 1-tetradecanol: Very little data is available for this system with the only known source being that of Jan et al.22 who measured data at 373−573 K at pressures up to 5.07 MPa. Therefore, where possible additional measurements will be conducted. • CO2 + 1-pentadecanol: Similarly to the CO2 + 1-tridecanol system, only Artal et al.19 published data. Again, the same comments hold true, the data is believed to be of good quality and again no additional data will be measured as additional measurements for the system CO2 + 1-tetradecanol will be conducted. • CO2 + 1-hexadecanol: There are a few sets of data that consider this system but only one set of data (data of Hölscher23) considers the entire compositional range and was measured at 393.2 K. However, some low mass fraction data are available. Data will therefore be measured where possible (measurements may be limited due to high pressures required for total solubility) and compared to literature data in the low mass fraction range. • CO2 + 1-octadecanol: Limited data are available for this system when 1-octadecanol is a liquid and the majority of the data sets is at very low 1-octadecanol concentrations. However, even at these low 1-octadecanol concentrations high pressures are required. Although additional data would be desired, from the data already available in the literature it is suggested that very high pressures beyond the limits of the equipment available for this work are required. Therefore, no additional measurements will be conducted. For the systems CO2 + 1-hexadecanol and CO2 + 1octadecanol as well as for CO2 + 1-eicosanol additional data exists for what is believed to be solid−vapor equilibrium (at temperatures well below the normal melting point of the 1alcohol or below the documented solid−vapor−liquid equilibrium locus). These data sets have been omitted from the current study as the nature of solid−vapor and liquid−vapor equilibrium are totally different. From the literature overview presented here it is noted that data for CO2 + 1-dodecanol and higher 1-alcohols are scarce. Therefore, this work will contribute by measuring additional data (phase transition pressures (P)) for the below mentioned three systems in the stated compositional and temperature (T) ranges. These compositional and temperature ranges were selected to ensure that the measurements complement already existing data and are above the solid−vapor−liquid line. For all three systems, a gap in the mixture critical region will likely

3. ADDITIONAL PHASE BEHAVIOR MEASUREMENTS 3.1. Materials and Methods. Additional phase behavior measurements were conducted on two previously constructed variable volume view cells using a static synthetic method.21,24 The view cells have a maximum volume of 4524 and 8021 cm3, are very similar in operation, and can be used interchangeably. The larger volume view cell was usually used for lower alcohol mass fractions and the smaller volume view cell was used for higher alcohol concentrations. The cells have been previously verified to produce reliable and repeatable results.17,21,24 Additionally, comparison with literature data in this work reinforces the reliability and repeatability of the view cells. The phase behavior measurements were conducted by visually observing the phase transition pressure for a known composition at a set temperature. The alcohol and CO2 were gravimetrically loaded into the cell. Thereafter the cell was closed and heated to the desired temperature. The cell was then pressurized into the one phase region. To measure the phase transition pressure, the pressure was slowly decreased until the phase transition pressure was visually observed. The phase transition measurement was repeated to ensure its accurate determination. After the phase transition pressure was determined at the desired temperature, the cell was heated to the next temperature and the measurement procedure repeated. The alcohol and the CO2 were loaded into the cell with an uncertainty of 0.0002 and 0.002 g, respectively, based on the accuracy of the balances used. Additionally, the alcohol can be transferred with an uncertainty of 0.0002 g whereas the uncertainty in the transfer of the CO2 is 0.12 g. Therefore, the relative uncertainty in the mass fraction alcohol (u(w2)) is at most u(w2) = 0.01· w2. In both cells, the temperature was measured with a regularly calibrated 4-wire PT100 accurately calibrated to 0.1 K and throughout the measurements temperature fluctuations no greater than 0.1 K were observed. The absolute uncertainty in the temperature measurement u(T) is therefore no greater than u(T) = 0.2 K. In both cells, the pressure was measured with a regularly calibrated ONEHalf20 pressure transmitter. The pressure transmitter is accurate to 0.0025 times the full scale (35 MPa). The uncertainty in the pressure measurement is therefore 0.0875 MPa. The phase transition pressure (P) was observed to within accuracy of 0.075 MPa, in most cases to within 0.05 MPa. It is noted that the accuracy of the phase transition observation is not as accurate as previous measurements on the same setups.17,21,24 However, at times very steep pressure−composition and pressure−temperature gradients complicated the measurements and therefore reduced the accuracy with which the phase transition pressure could be observed. Similar difficulties were previously observed for the systems CO2 + high molecular mass acids.25 The absolute uncertainty in the phase transition pressure measurement u(P) is therefore no greater than u(P) = 0.16 MPa. D



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Figure 1b, which shows pressure−temperature plots for compositions displaying temperature inversions and those which do not (see wC10‑OH = 0.126 and wC10‑OH = 0.605 which display a temperature inversion versus wC10‑OH = 0.679 and wnC10 = 0.126 which do not). Temperature inversions have previously been noted for the CO2 + 1-octanol21 and CO2 + 1-decanol17 system. From the data, it can also be seen that the relationship between phase transmission pressure and temperature is not linear and the data displays a temperature inversion. From the measurements conducted in this work, it is noted that temperature inversions are present for the CO2 + 1-dodecanol, CO2 + 1-tetradecanol, and CO2 + 1-hexadecanol system. The presence of temperature inversions are thus common in CO2 + 1-alcohol system. The presence of temperature inversions in the CO2 + 1-alcohol homologous series and possible explanation thereof is discussed in detail in the phase behavior analysis section below. The data were not measured at constant temperature. However, in order to conduct a meaningful analysis data should be presented as isothermal data. Previous publications17,21,29 have used linear, and second and/or third order polynomials to describe the pressure−temperature relationship. The same approach was followed in this work. Should a linear expression fit the data with an R2 value of 0.98 and be able to reproduce the data within 0.3 MPa and an error of no greater than 0.02 times the value, the fit was regarded as suitable. Should a linear fit not be suitable, preferably a second order polynomial, else a third order polynomial was fitted, again with the same criteria. In all other cases, a third order polynomial was found to be suitable. The fitted expressions together with the applicable temperature ranges, have been included in Table 3, Table 4 and Table 5, and where data was omitted to ensure a suitable fit was attained it has been noted. These expressions are able to estimate the phase transition pressure at a given temperature provided the temperature is within the fitted range and can now be used for further data comparison to ensure all data are compared at the same temperature. 3.3. Comparison with Literature Data. Figure 2 provides a comparison of the measured data with literature data for the systems CO2 + 1-dodecanol and CO2 + 1-hexadecanol. The selected temperatures used were based on the temperature of literature data. For the system CO2 + 1-tetradecanol, no meaningful comparison can be made as the literature data is at temperatures higher than considered in this work (T > 373 K) and the pressures are very low (P < 5 MPa). The comparison shows good agreement between the literature and measured data considering the accuracy of the measurements and the magnitude of scatter generally observed in high pressure phase behavior measurements. The measurements again show that the equipment and experimental procedure is able to produce reliable results.

The materials used, their suppliers, and supplier purities are listed in Table 2. The purities of the materials were checked Table 2. Materials Used, Their Suppliers, CAS Numbers, and Stated Purities component

CAS number

supplier

product number

purities

carbon dioxide 1-dodecanol 1-tetradecanol 1-hexadecanol

124-38-9 112-53-8 112-72-1 36653-82-4

Airproducts Sigma-Aldrich Sigma-Aldrich Fluka

K243C 126799 87158 52240

0.99995 0.98 0.99 0.99

with GC and in all cases the verified purities were greater than those listed by the suppliers. All materials were used without further purification. 3.2. Experimental Measurements. The experimental measurements for the system CO2 + 1-dodecanol, CO2 + 1tetradecanol and CO2 + 1-hexadecanol are given in Table 3, Table 4, and Table 5, respectively. The phase transition pressures are very high with measurements often very close to the pressure limits of the experimental setup. Additionally, at times large overpressures were required to achieve total solubility. The high pressures, large over pressure requirement, and steep pressure−composition and pressure−temperature gradients made measurements difficult. Thus, at times reliable, repeatable measurements were not possible even though the estimated phase transition pressure was within the pressure limit of the equipment. Experimentally, it was observed that at low temperatures liquid−liquid equilibrium was present. This qualitative observation was noted for selected CO2 + 1-dodecanol and CO2 + 1-tetradecanol measurements in the temperature range of 313−318 K. This observation of liquid−liquid equilibria is in agreement with literature.10,26−28 In general, with possible exception at very low temperatures where liquid−liquid equilibrium was observed, the nature of the phase transitions were generally all of a critical or near critical transition. The bubble point measurements did not form fine bubbles but rather what appeared to be a fine suspension with varying degrees of critical opalescence. Similarly, the dew point measurements did not result in condensation of fine droplets but rather the formation of a fine mist, again the in the presence of critical opalescence. These observations may also shed light into the underlying molecular interactions present. In most cases for supercritical systems, an increase in temperature leads to an increase in phase transition pressure. However, for highly nonideal systems with significant molecular interactions the opposite may be true in the mixture critical region in the vicinity of the critical temperature. Such observations where an increase in temperature leads to a decrease in phase transition pressure is termed a temperature inversion. The concept of the temperature inversion is illustrated in Figure 1. Here the phase behavior of the CO2 + 1-decanol17 system is compared to that of the CO2 + ndecane17 system, where the former displays a temperature inversion while the latter does not. From Figure 1a, it can be seen that in the alcohol mass fraction region wC10‑OH = 0.12 to wC10OH = 0.61 an increase in temperature leads to a decreases the phase transition pressure at low temperatures. Conversely, outside this region and for the CO2 + n-decane system at constant composition the phase transition pressure increases with an increase in temperature. This can also clearly be seen in

4. PHASE BEHAVIOR ANALYSIS Analysis of the phase behavior of the CO2 + 1-alcohol homologous series will be conducted mainly by considering the pressure−composition, temperature−composition, and pressure−temperature behavior. Additionally, the critical behavior of the system will be evaluated and where possible the type of phase behavior, according to the classification system of van Konynenburg and Scott30 (later refined by Privat and Jaubert31) as well as that of Bolz et al.32 will be assigned. A detailed comparison of the CO2 + alcohol homologous series E


0.276

0.250

0.0667

0.0430

0.0294

0.0215

0.617c

0.585d

0.232d

0.160d

0.114e

0.0850e

T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa

308.0 8.6 307.9 14.0 317.8 21.2 318.5 27.0 338.4 27.3 333.4 26.7 327.8 24.5 313.2 24.5

318.0 10.0 318.0 12.5 328.4 18.3 327.6 22.2 343.3 26.3 338.3 25.6 333.0 23.5 318.1 22.5

328.0 11.4 328.0 13.0 338.2 18.0 332.8 21.6 348.3 25.9 343.4 25.0 338.3 23.1 328.3 20.2

338.0 12.6 338.3 14.0 348.4 18.4 340.9 20.9 353.6 25.6 348.1 24.7 343.2 22.9 333.3 20.3

348.0 13.7 348.2 15.2 358.0 19.0 346.7 20.8 358.2 25.6 353.3 24.6 348.2 22.9 338.2 20.6 358.1 24.7 353.1 23.1 348.2 21.1

353.0 21.2

358.0 14.7 358.3 16.3

phase transition pressure (P) at a set temperature (T)

358.2 23.4 358.0 22.0

−04

−01

1.89408 × 10−01

−1.82283 × 10−04

−4.33902 × 1000

6.16368 × 10−03 1.14697 × 10−01

−4.22621 × 1000

5.94907 × 10−03

−1.07134 × 10−04

−1.22795 × 1002

3.55322 × 10−01

−3.42632 × 10−04

−6.55066 × 1001

−4.08787 × 1001

−6.12266 × 1001

1.75904 × 10−01

−1.68262 × 10−04

−3.87407 × 1001

5.52179 × 10

C/MPa·K−1

1.13906 × 10−01

−6.46429 × 10

B/MPa·K−2

−1.11237 × 10−04

A/MPa·K−3

7561.590

4873.600

788.184

776.128

14162.640

7113.880

4390.630

−100.162

D/MPa

0.992

0.996

0.994

0.994

0.993

0.998

0.995

0.994

R2

313−358

328−358

333−358

338−358

318−353

318−358

308−358

308−358

range/K

pressure−temperature correlation: P = A·T3 + B·T2 + C·T + D (pressure in MPa, temperature in K)b

a Standard uncertainties: u(w2) = 0.01·w2, u(T) = 0.2 K and u(P) = 0.16 MPa. bExpressions correlate data with AAD of 0.080 MPa and %AAD of 0.41%. cBubble point type of phase transition. dPhase transition in the vicinity of the mixture critical point. eDew point type of phase transition.

0.335

0.384

x2/mol·mol−1

0.681c

0.725

c

w2/g·g−1

composition

Table 3. Fluid−Liquid Phase Transition Pressure (P) Measurements at Selected Temperatures (T) for Specified Weight Fractions (w2) and Mole Fractions (x2) for the System CO2 (1) + 1-Dodecanol (2)a

Journal of Chemical & Engineering Data Article

F


G

0.268

0.250

0.218

0.0166

0.0103

0.00821

0.00580

0.640c

0.619c

0.575c

0.0758d

0.0483d

0.0388d

0.0276d

T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa

318.4 17.73 317.9 26.71 318.3 26.39 323.5 27.70 332.9 26.83 314.3 28.42 313.5 16.46 313.3 14.35 313.2 11.79

323.6 16.70 323.1 22.70 323.3 23.14 333.0 23.18 338.1 25.08 323.5 23.76 323.2 17.73 323.4 15.39 323.2 13.58

328.4 16.25 333.4 20.49 333.0 19.96 338.2 22.12 343.2 24.23 333.2 22.85 333.0 18.34 331.9 16.57 333.0 14.88

333.5 15.94 338.1 20.04 338.1 19.11 343.4 21.86 348.5 23.63 342.8 23.19 343.1 19.33 343.0 18.05 342.9 16.20

338.2 16.31 343.3 20.03 343.5 19.56 347.6 21.72 353.7 23.49 353.5 24.10 353.4 20.34 353.1 19.47 353.1 17.67

343.4 16.60 348.3 20.17 348.1 19.80 353.5 21.46

phase transition pressure (P) at a set temperature (T) 353.3 17.60 353.3 20.36 353.3 20.08

−6.50706 × 1000

4.59634 × 10−01 9.24948 × 10−03

−4.41955 × 10−04

−3.24159 × 10−04

3.33974 × 10−01

−1.59360 × 1002

3.22064 × 10−01

−3.07498 × 10−04

13117.800 −12.839 −26.551 −33.315

9.38640 × 10−02 1.30104 × 10−01 1.44527 × 10−01

1167.927

18441.390

13077.680

17695.820

5383.186

D/MPa

−1.14592 × 1002

−1.12362 × 1002

−1.54323 × 1002

4.48987 × 10−01

−4.62968 × 10 01

−4.35294 × 10−04

−1.26788 × 10

1.32849 × 10

C/MPa·K−1

−01

B/MPa·K−2 −04

A/MPa·K−3

0.997

0.997

0.991

0.997

0.996

0.998

0.998

0.991

0.991

R2

313−353

313−353

313−353

313−353

333−353

323−353

318−353

318−353

318−353

range/K


Standard uncertainties: u(w2) = 0.01·w2, u(T) = 0.2 K and u(P) = 0.16 MPa. bExpressions correlate data with AAD of 0.093 MPa and %AAD of 0.44%. cBubble point type of phase transition. dDew point type of phase transition.

a

0.277

0.310

x2/mol·mol−1

0.652c

0.687

c

w2/g·g−1

composition

Table 4. Fluid−Liquid Phase Transition Pressure (P) Measurements at Selected Temperatures (T) for Specified Weight Fractions (w2) and Mole Fractions (x2) for the System CO2 (1) + 1-Tetradecanol (2)a



H

0.309

0.261

0.2212

0.0124

0.0101

0.00830

0.00640

0.686c,e

0.633e

0.581e

0.0576f

0.0475d,f

0.0392d,f

0.0304f

T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa T/K P/MPa

329.4 10.42 329.2 13.20 329.1 18.35 325.9 26.11 343.3 25.76 323.5 24.89 323.3 21.47 323.1 20.66 322.9 17.11

337.7 11.38 338.7 13.98 334.1 17.90 329.5 24.39 348.7 24.83 333.2 23.32 338.2 21.11 332.9 19.96 333.1 18.78

348.1 12.43 349.0 14.83 338.9 17.87 334.4 22.52 353.4 24.35 343.0 23.20 353.0 22.63 342.6 20.43 343.4 19.83 353.3 20.95 353.5 20.37

358.1 13.39 358.7 15.36 343.5 17.90 338.8 21.79 358.0 24.21 353.1 23.35 346.4 18.04 348.5 21.07

352.7 18.27 358.5 21.15

phase transition pressure (P) at a set temperature (T)

358.7 18.13

−3.05251 × 1000

4.44071 × 10−03

1.93032 × 1000

−5.21690 × 1000

7.28821 × 10−03

−2.69693 × 10−03

−9.85305 × 1001

2.78987 × 10−01

−2.63280 × 10−04

−5.87904 × 1001

−324.987

547.586

957.772

11619.020

6819.130

−11.176

7.42042 × 10−02

1.03032 × 10

−23.468

D/MPa −01

C/MPa·K−1

1.69283 × 10−01

B/MPa·K−2

−1.62367 × 10−04

A/MPa·K−3

1.000

0.961

1.000

0.999

0.099

0.992

0.998

R2

323−353

323−353

343−358

328−358

328−353

328−358

328−358

range/K


Standard uncertainties: u(w2) = 0.01·w2, u(T) = 0.2 K and u(P) = 0.16 MPa. bExpressions correlate data with AAD of 0.048 MPa and %AAD of 0.29%. cData point at highest temperature omitted from correlation as polynomial is unable to capture the curvature of the data point correctly. dNo pressure−temperature correlation as polynomials are unable to capture curvature of data. eBubble point type of phase transition. fDew point type of phase transition.

a

0.376

0.455

x2/mol·mol−1

0.746e

0.803

e

w2/g·g−1

composition

Table 5. Fluid−Liquid Phase Transition Pressure (P) Measurements at Selected Temperatures (T) for Specified Weight Fractions (w2) and Mole Fractions (x2) for the System CO2 (1) + 1-Hexadecanol (2)a




Article

Figure 1. Comparison of the phase behavior of the CO2 + 1-decanol and the CO2 + n-decane system to illustrate the concept of a temperature inversion (a) pressure−composition plot and (b) selected pressure−temperature relationships (key: for (a) ◊ = 313.2 K, ○ = 323.2 K, and △ = 333.2 K with CO2 + 1-decanol as nonfilled markers and CO2 + n-decane as filled markers; for (b) ◊ = (wC10‑OH = 0.126), △ = (wC10‑OH = 0.605), ○ = (wC10‑OH = 0.679), and ■ = (wnC10 = 0.126); all data from Zamudio et al.17).

Figure 2. Comparative pressure−composition plot of measured and literature data for the system (a) CO2 + 1-dodecanol and (b) CO2 + 1hexadecanol. (Key: ◊ = 313.2 K for (a) and 323.15 K for (b), ○ = 333.2 K for (a) and 328.2 K for (b), and △ = 353.2 K for both (a) and (b); nonfilled markers this work from correlations in Table 3 and Table 5, gray filled markers from Secuianu et al.42 for (a) and Yau and Tsai64 for (b), and black filled markers from Hölscher23 at 333.2 K and Kordikowski and Schneider43 at 353 K for (a) and Kramer and Thodos62 for (b)).

CO2, the concentration of alcohols dimers and especially multimers is low, therefore resulting in regular behavior. At high alcohol concentrations only a small quantity of CO2 molecules are dissolved in the alcohol. These small number of CO2 molecules can easily be accommodated in the multimer matrix and therefore regular behavior is observed. However, in the mixture critical region interesting transitional phenomena are observed. Here the alcohol’s concentration is sufficiently high so that the alcohols form mainly multimers. Thus, a relatively large amount of CO2 needs to penetrate and dissolve in the alcohol multimer matrix. While nonpolar, CO2 has a quadrupole moment thus adding to the difficulty to penetrate the generally nonpolar matrix. It is thus very difficult for the CO2 molecules to penetrate the alcohol multimer matrix and therefore high pressures are required in order to form a homogeneous mixture of 1-alcohols with CO2. As temperature increases, the hydrogen bonds holding the multimers together become weaker and more flexible, and the alcohol thus allows the CO2 molecules to penetrate its multimer matrix more easily. Therefore, at higher temperatures the pressure required

with other homologous series containing either CO2 or 1alcohols is beyond the scope of the current work. 4.1. Pressure−Composition Relationship. The pressure−composition relationships at 313.2, 333.2, and 353.2 K for CO2 + 1-octanol, CO2 + 1-dodecanol, and CO2 + 1hexadecanol are given in Figure 3. The plots show (or imply in the case of the higher 1-alcohols) a maxima in the region of 0.2−0.5 1-alcohol mass fraction with the 1-alcohol mass fraction where the maxima occurs increasing with increasing temperature. In all cases, a temperature inversion is noted with the temperature inversion becoming more prominent for higher 1-alcohols. Linear primary alcohol molecules are known to hydrogen bond with one another resulting in dimers and particularly multimers. These multimers are essentially a number of alcohol molecules bonded together with hydrogen bonds to form a pseudo molecule. This pseudo molecule has a significantly higher molecular mass and results in a highly structured liquid matrix. As the hydroxyl groups generally bond together, the resultant matrix has a highly nonpolar nature. At low alcohol concentrations, where a small amount of alcohol is dissolved in I



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Figure 3. Pressure−composition plots for the systems (a) CO2 + 1-octanol (data from Byun and Kwak,50 Chiu et al.,51 Fourie et al.,21 Hou et al.,54 Scheidgen,10 and Weng and Lee11) (b) CO2 + 1-dodecanol (data from this work and from Hölscher,23 Kordikowski and Schneider,43 and Secuianu et al.42) and (c) CO2 + 1-hexadecanol (this work) at T = 313.2, 333.2, and 353.2 K. (Key: ◊ = 313.2 K, △ = 333.2 K, ○ = 353.2 K; nonfilled markers this work from correlations in Table 3 and Table 5, gray-filled markers indicate literature data).

homologous series, the supercritical fluid is a nonpolar alkane which more readily dissolves in the relatively nonpolar alcohol multimer matrix. The quadrupole of CO2 may make penetration of CO2 into the multimer structure rather difficult compared to ethane and propane, hence presence of a temperature inversions for the CO2 + 1-alcohol systems. Comparing the CO2 + 1-alcohol homologous series with that of the CO2 + n-alkane homologous series where the n-alkane and the 1-alcohol have the same hydrocarbon backbone (i.e., 8−16 carbon atoms), it is noted that no region of temperature inversion occurs for CO2 + n-alkanes for hydrocarbon backbone lengths considered in this work (i.e., up to 16 carbon atoms).35 The n-alkanes do not have a hydroxyl group that can form dimers and/or multimers, and therefore do not form such large “pseudo molecules”. CO2 molecules can therefore more easily dissolve in the n-alkane matrix. For higher n-alkanes (e.g., n-eicosane and higher), regions of temperature inversion occur but the region of temperature inversion is smaller35 (the region of temperature inversion of the CO2 + neicosane and CO2 + 1-octanol systems are comparable). Here the n-alkane will probably behave in a similar fashion to that of a multimer 1-alcohol of similar molecular mass. This observation is noted here but further analysis and comparison of the homologous series is beyond the scope of the current work.

for complete miscibility with CO2 decreases as experimentally observed. It is postulated that for higher molecular mass 1-alcohols the pseudo multimer molecules are larger and most likely less polar, making penetration of the CO2 more difficult. This is in agreement with the observed phase behavior. At constant temperature, higher pressures are required for the CO2 molecules to penetrate the multimer matrix and allow the formation of a homogeneous phase, ultimately resulting in more prominent temperature inversion regions for higher alcohols. The above discussion proposes an explanation for the occurrence of the temperature inversion. The formation of multimers may also explain the observations at the phase transition (the formation of a suspension and fine mist vs the usual fine droplets and bubbles) but further investigation is required. Future studies incorporating techniques such as IR or Raman spectroscopy of the CO2 + 1-alcohol mixture are suggested. These techniques may shed light on the molecular interactions present and hopefully experimentally verify the proposed explanation. As pointed out by Lam et al.,27 the phase behavior for 1alcohols in ethane, propane, and CO2 differs significantly. The phenomena of temperature inversion is not observed for the homologous series ethane +1-alcohol33 nor for propane +1alcohols34 for alcohols up to 1-docosanol. In these two J



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Figure 4. Pressure−composition plots for CO2 + various 1-alcohols at (a) 313.2 K, (b) 333.2 K, and (c) 333.2 K. Data from this work and from Byun and Kwak,50 Chiu et al.,51 Fourie et al.,21 Hou et al.,54 Hölscher,23 Ioniţă et al.,16 Kordikowski and Schneider,43 Pöhler,14 Scheidgen,10 Secuianu et al.,42 Weng and Lee,11 and Zamudio et al.17 (Key: + = C8-OH, × = C10-OH, ◊ = C12-OH, △ = C14-OH and ○ = C16-OH).

Figure 5. Pressure−carbon number plots for the homologous series CO2 + 1-alcohols at (a) w2 = 0.600 and (b) w2 = 0.070. Data interpolated from data in this work and from Byun and Kwak,50 Chiu et al.,51 Fourie et al.,21 Hou et al.,54 Hölscher,23 Ioniţă et al.,16 Kordikowski and Schneider,43 Pöhler,14 Scheidgen,10 Secuianu et al.,42 Weng and Lee,11 and Zamudio et al.17 (Key: ◊ = 313.2 K; △ = 333.2 K, ○ = 353.2 K with dashed lines indicating trends in the data).

the phase transition pressure and the number of carbon atoms (CN) at constant alcohol mass fraction (walcohol = 0.600 and walcohol = 0.070) and constant temperature. The increase in phase transition pressure is seen but, in agreement with Figure 4, it is noted that the increase is not necessary linear with a sharper increase at lower alcohol molecular masses. This sharper increase is especially noted at lower temperatures

In addition to the more prominent temperature inversions noted for the higher 1-alcohols, significantly higher pressures are required for total solubility. Comparing the pressure− composition relationships of various 1-alcohols in CO2, Figure 4 shows that the increase in phase transition pressure with increase in 1-alcohol molecular mass is more prominent at lower temperatures. Figure 5 shows the relationship between K



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where the difference in the phase behavior due to the presence of temperature inversions is observed. Importantly, an increase phase transition pressure with alcohol molecular mass is observed clearly, indicating that fractionation of alcohols according to hydrocarbon backbone is possible. The complex molecular interactions in the CO2 + 1-alcohol systems also manifest itself in low temperature liquid−liquid equilibria and barotropy. Scheidgen,10 Pfohl et al.36, and Secuianu et al.20 observed liquid−liquid phase behavior for the system CO2 + 1-octanol, CO2 + 1-nonanol, and CO2 + 1undecanol, respectively, at low temperatures. Additionally, in this work it was noted that at times the nature of the coexisting phases at 313 K was liquid−liquid like rather than vapor−liquid like. However, neither the work of Schiedgen nor the current work were able to clearly quantify the exact region of liquid− liquid phase behavior. In addition to low temperature liquid− liquid phase behavior, low temperature barotropy has also been observed. Barotropy is where the vapor-like phase is more dense than the liquid-like phase and the point where the inversion occurs is called the barotropic inversion point. Lam et al.27 found that a barotropic inversion occurs for the systems CO2 − 1-hexanol to CO2 + 1-undecanol but at temperatures below that considered in this work. Both Scheidgen10 and Kordokowski37 in their Ph.D. studies also noted a barotropic inversion, again generally at temperatures below that studied in this work. However, Spee38 found that for CO2 + 1-dodecanol the barotropic inversion occurred between 313.2 and 323.2 K at 25 MPa. The measurements conducted in the current work were determined via a static analytic technique and were of a critical nature. Therefore, the current work cannot provide any further information regarding barotropic behavior. However, cognizance needs to be taken that liquid−liquid phase behavior and barotropy are possible for CO2 + 1-alcohol systems at low temperatures and the presence thereof needs to be investigated should a process involving 1-alcohols be considered at temperatures close to the critical temperature of CO2. 4.2. Temperature−Composition Relationship. Generally, phase behavior studies concentrate on pressure− composition behavior. However, some studies10,14,37−39 have specifically investigated the temperature−composition relationship at constant pressure. Additionally, from pressure− composition data at constant temperature, temperature− composition data at constant pressure can be inferred. Thus, sufficient information is available to obtain insight into the temperature−composition behavior of CO2 + 1-alcohol systems. Typical temperature−composition plots showing the effect of pressure and 1-alcohol molecular mass are given in Figure 6 and Figure 7, respectively. The region of immiscibility is the region at compositions between the two branches of the phase boundary. The effect of the temperature inversion is clearly seen. At a specific temperature, the vapor phase has a maximum alcohol composition and at another temperatures the liquid phase has a maximum CO2 composition. These two temperatures are often similar but not necessarily the same. At both lower and higher temperatures, the distance between the two phases increases indicating lower solubility and a temperature inversion. Should a temperature inversion not be present, the phase boundary would have been continuous with a temperature minimum and a two phase region above the phase boundary line. From Figure 6, it can be seen that as pressure increases the two branches of the phase boundary approach one another and

Figure 6. Temperature−composition plots for the systems CO2 + 1dodecanol at 15, 16, and 20 MPa. Data from Kordikowski43 and Spee and Schneider.39 (Key: ◊ = 15 MPa, △ = 16 MPa, ○ = 20 MPa.)

Figure 7. Temperature−composition plots for various CO2 + 1alcohol systems at 15 MPa. Data from Chang et al.,18 Kordikowski,43 Pöhler,14 Scheidgen,10 and Zamudio et al.17 (Key: + = C8-OH, × = C10-OH, and ◊ = C12-OH).

the temperature at which the minimum occurs also increases. At pressures higher than that illustrated here the two branches of the phase boundary will merge. However, due to the temperature inversion at even high pressures two separate branches of the phase boundary will form: one at high temperatures with a temperature minimum and a region of immiscibility at temperatures higher than the phase boundary and one at low temperatures with a temperature maximum and a region of immiscibility at temperatures lower than the phase boundary. From Figure 7, it is clear that for higher alcohols the mutual solubility of the phases is lower resulting in a larger region of immiscibility. 4.3. Pressure−Temperature Relationship. The presence of the temperature inversion also results in interesting pressure−temperature relationships. In general, systems containing a supercritical fluid and a high molecular mass compound result in either a linear temperature−pressure relationship or a relationship with a positive gradient that can be fitted with a second or third order polynomial.17,25,34,40 In fact, the pressure−temperature relationship for propane with 1alcohols34 as well as that of high molecular acids with CO225 are generally linear. However, for CO2 + 1-alcohol systems in the L



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selected CO2 + 1-alcohol systems.26,28,41,42 Some researchers have tried to define the limits of the three-phase region and have published upper critical end point (UCEP) data.26,28,41,42 As the molecular mass of the alcohols increase, the temperature and pressure of the UCEP decreases. Additionally, the lower limit of the three phase region, that is, where solidification occurs in the presence of two liquid phases and a vapor phase (Q-point), has also been determined. As the molecular mass of the alcohol increases the pressure and temperature of the Qpoint increases. Figure 9 show the movement of the UCEP and the Q-point in terms of temperature and pressure as a function of carbon number. Extrapolation of the data suggests that for 1pentadecanol and higher no three phase region would observed as, should a three-phase region be present, it will be hidden by solidification. The information presented can aid in classifying the type of phase behavior present. Van Konynburg and Scott30 classified phase behavior into six main systems and the classification was later updated by Privat and Jaubert.31 Previously Ke et al.,26 Kordikowski and Schneider,43 and Secuianu et al.28,42 classified the CO2 + 1-octanol, the CO2 + 1-nonanol, and CO2 + 1dodecanol systems as type III. The information presented and discussed in this work is in agreement with this classification. For type III classification two branches of the critical curve exist. The one part of the critical curve has an s-shape, starting at the critical point of the pure 1-alcohol. Part of this s-shape manifests itself as the temperature inversion. The second part of the critical curve is noted at lower temperatures and pressures, starts at the critical point of CO2, and terminates in an UCEP. Thus, a three-phase region is present at temperatures and pressures below the UCEP with the locus of the threephase region terminating at the UCEP. This work therefore concludes that the systems CO2 + 1-octanol through CO2 + 1tetradecanol display type III phase behavior. Considering the further classification of Privat and Jaubert, these CO2 + 1octanol through CO2 + 1-tetradecanol systems can further be classified as type III-a-a′ systems. The low-temperature threephase line has a positive slope, indicating a-type behavior for this region. Additionally, the s-shape of the higher-temperature critical curve indicates a′-type of behavior. For systems CO2 + 1-pentadecanol and higher, information available suggests that either the UCEP is hidden through solidification or no longer exists. Therefore, for these systems at present no outcome as to the type of phase behavior can be given, however, the phase

mixture critical region in the temperature range observed in this work, the gradient is negative at low temperature, increases and proceeds through a minimum and is then positive at higher temperatures. As seen here and in the work of Fourie et al.21 and Zamudio et al.,17 for CO2 + 1-alcohol systems third order or higher polynomials are usually required. In his Ph.D. thesis, Scheidgen10 specifically considered the pressure−temperature relationship in the mixture critical region over a wide temperature and pressure range. A selection of the data from Scheidgen is shown in Figure 8. It is noted that at

Figure 8. Constant composition pressure−temperature for the systems CO2 + 1-octanol (w = 0.310), CO2 + 1-decanol (w = 0.290), and CO2 + 1-dodecanol (w = 0.290). Data from Scheidgen.10 (Key: + = C8-OH, × = C10-OH and ◊ = C12-OH.)

low temperatures the pressure−temperature gradient is extremely steep. At very high pressures, the gradient is highly positive, then infinitive, and then highly negative. Then, as temperature increases the gradient increases resulting in a pressure minima and finally at high temperatures mild positive gradient is observed. The pressure−temperature behavior observed in this work at compositions where temperature inversions occur is in agreement with the measurements of Scheidgen and illustrates why at times it was difficult to fit a polynomial through the measured data. 4.4. Phase Behavior Classification. At temperatures close to the critical point, three-phase behavior has been noted for

Figure 9. Plot of the UCEP and Q-point (a) temperature and (b) pressure as a function of alcohol carbon number. Data from Ke et al.,26 Lam et al.,27 and Secuianu and co-workers28,42 (Key: △ = UCEP; □ = Q-point; dashed lines indicate trends extrapolated to 1-pentandecanol.) M



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Phase Equilibria and Viability Study. J. Supercrit. Fluids 2011, 57, 101− 111. (3) Schwarz, C. E.; Bonthuys, G. J. K.; van Schalkwyk, R. F.; Laubscher, D. L.; Burger, A. J.; Knoetze, J. H. Separation of Alkanes and Alcohols with Supercritical Fluids. Part II. Influence of Process Parameters and Size of Operating Range. J. Supercrit. Fluids 2011, 58, 352−359. (4) Zamudio, M.; Schwarz, C. E.; Knoetze, J. H. Methodology for Process Modelling of Supercritical Fluid Fractionation Processes Illustrated for the Separation of Alkane/Alcohol Isomer Mixtures Using CO2. J. Supercrit. Fluids 2015, 104, 272−280. (5) Zamudio, M. The Separation of Detergent Range Alkanes and Alcohol Isomers with Supercritical Carbon Dioxide. Ph.D. Thesis in Chemical Engineering; Stellenbosch University, Stellenbosch, South Africa, 2014. (6) Dohrn, R.; Brunner, G. High-Pressure Fluid-Phase Equilibria: Experimental Methods and Systems Investigated (1988−1993). Fluid Phase Equilib. 1995, 106, 213−282. (7) Christov, M.; Dohrn, R. High-Pressure Fluid Phase Equilibria: Experimental Methods and Systems Investigated (1994−1999). Fluid Phase Equilib. 2002, 202, 153−218. (8) Dohrn, R.; Peper, S.; Fonseca, J. M. S. High-Pressure Fluid-Phase Equilibria: Experimental Methods and Systems Investigated (2000− 2004). Fluid Phase Equilib. 2010, 288, 1−54. (9) Fonseca, J. M. S.; Dohrn, R.; Peper, S. High-Pressure Fluid-Phase Equilibria: Experimental Methods and Systems Investigated (2005− 2008). Fluid Phase Equilib. 2011, 300, 1−69. (10) Scheidgen, A. L. Fluidphasengleichgewichte Binärer Und Ternärer Kohlendioxidmischungen Mit Schwerflüchtigen Organischen Substanzen Bis 100 MPa. Doctoral Dissertation, Rohr-Universität Bochum, Bochum Germany, 1997. (11) Weng, W. L.; Lee, M. J. Phase Equilibrium Measurements for the Binary Mixtures of 1-Octanol plus CO2, C2H6 and C2H4. Fluid Phase Equilib. 1992, 73, 117−127. (12) Fourie, F. C. v. N.; Schwarz, C. E.; Knoetze, J. H. Analytic Setup for Multicomponent High-Pressure Phase Equilibria via Dual Online Gas Chromatography. Chem. Eng. Technol. 2015, 38, 1165−1172. (13) Fourie, F. C. v. N.; Schwarz, C. E.; Knoetze, J. H. Analytic HighPressure Phase Equilibria. Part II: Gas Chromatography and Sampling Method Development. Chem. Eng. Technol. 2016, 39, 1475−1482. (14) Pöhler, H. Fluidphasengleichgewichte Binärer Und Ternärer Kohlendioxidmischungen Mit Schwerflüchtigen Organischen Substanzen Bei Temperaturen von 303 K Bis 393 K Und Drücken von 10 MPa Bis 100 MPa. Doctoral Dissertation, Rohr-Universität Bochum, Bochum, Germany, 1994. (15) Ghaziaskar, H. S.; Daneshfar, A.; Rezayat, M. The Co-Solubility of 2-Ethylhexanoic Acid and Some Liquid Alcohols in Supercritical Carbon Dioxide. Fluid Phase Equilib. 2005, 238, 106−111. (16) Ioniţa,̆ S.; Feroiu, V.; Geană, D. Phase Equilibria of the Carbon Dioxide + 1-Decanol System at High Pressures. J. Chem. Eng. Data 2013, 58, 3069−3077. (17) Zamudio, M.; Schwarz, C. E.; Knoetze, J. H. Phase Equilibria of Branched Isomers of C10-Alcohols and C10-Alkanes in Supercritical Carbon Dioxide. J. Supercrit. Fluids 2011, 59, 14−26. (18) Chang, C. J.; Chiu, K.-L.; Day, C.-Y. A New Apparatus for the Determination of P−x−y Diagrams and Henry’s Constants in High Pressure Alcohols with Critical Carbon Dioxide. J. Supercrit. Fluids 1998, 12, 223−237. (19) Artal, M.; Pauchon, V.; Embid, J. M.; Jose, J. Solubilities of 1Nonanol, 1-Undecanol, 1-Tridecanol, and 1-Pentadecanol in Supercritical Carbon Dioxide at T = 323.15 K. J. Chem. Eng. Data 1998, 43, 983−985. (20) Secuianu, C.; Ioniţa,̆ S.; Feroiu, V.; Geană, D. High Pressures Phase Equilibria of (Carbon dioxide+1-Undecanol) System and Their Potential Role in Carbon Capture and Storage. J. Chem. Thermodyn. 2016, 93, 360−373. (21) Fourie, F. C. v. N.; Schwarz, C. E.; Knoetze, J. H. Phase Equilibria of Alcohols in Supercritical Fluids: Part I. The Effect of the

behavior type needs to take the highly prominent temperature inversion into account. More recently Bolz et al.32 suggested an alternative classification. According to their classification the observed phase behavior of the CO2 + 1-octanol to CO2 + 1-tetradecanol is 1PA ln Q. This observation is in agreement with Secuianu et al.42 and is equivalent to the observed Type III phase behavior. Again, insufficient information is available for systems CO2 + 1pentadecanol and CO2 with higher 1-alcohol to provide a definitive classification.

5. CONCLUSIONS This work has achieved its aim of investigating the phase behavior of the CO2 + 1-alcohol homologous series. Additional phase equilibria measurements were conducted to complement existing literature and thus allow for a comprehensive analyses of the homologous series. All data sets considered displayed a so-called temperature inversion and the literature indicates barotropy and liquid−liquid phase behavior is present at low temperatures. It is postulated that the observed phenomena are a result of the formation of alcohol dimers and multimers. However, experimental evidence confirming the postulation is lacking and IR or Raman spectroscopy investigations may shed light on this phenomena. Generally these phenomena, especially the temperature inversion, are more pronounced at higher molecular mass alcohols. The available information is sufficient to classify the CO2 + 1-alcohol homologous series as type III according to the classification of van Konynenburg and Scott,30 type III-a-a′ according to Privat and Jaubert31 and type 1PA ln Q according to the classification of Bolz et al.32 for homologues up to and including 1-tetradecanol. For higher homologues. further studies are required to allow definitive classification. The current work has not considered the thermodynamic modeling of this homologous series. Because of the complicated phase behavior resulting from complex molecular interactions, such a study is not an easy task and beyond the scope of the current work. Models need to have a good fundamental backbone to be able to describe the observed phenomena, and cubic equations of state are unable to capture the complexities of the alcohol multimers and their impact on phase behavior.5,44,45 Secuianu and co-workers,16,20,28,42 and well as the group of Jaubert and Privat46−49 among others, have recently been making strides toward thermodynamic comprehension of these type of systems. However, implementation of these more complex models is not straightforward. Further work is required in this regard, and it is hoped the current work is able to aid in the development of such models.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +27 21 8084487. E-mail: [email protected]. ORCID

Cara E. Schwarz: 0000-0001-5513-2105 Notes

The author declares no competing financial interest.

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REFERENCES

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High Pressure Phase Behavior of the Homologous Series CO2 + 1

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