Ind. Eng. Chem. Res. 1989,28, 1693-1696 yields a ratio of mass 9l/mass 92 of about 1.75 under E1 conditions, which (from 3, shows a condition number of about 30. Accurate data are therefore required for toluene, and if isotope effects in the fragmentation process are significant, these will have to be accurately modeled in the computation of a simulation matrix in order to generate accurate results. Conclusions CI processes involving mainly hydrogen addition are useful for the determination of deuterium content in hydrocarbons, particularly if the hydrogen addition occurs without significant isotope effects and fragmentation is minimal. CI processes resulting primarily in hydrogen abstraction cannot generally be solved directly, though the boundary method (Price and Iglesia, 1989b) may be useful in these instances. Further experimentation with CI-MS for the analysis of deuterium content is warranted because the lack of isotope effects associated with fragmentation processes may yield a new level of accuracy in computed deuterium distributions.
1693
Literature Cited Conte, S. D.; deBoor, C. Elementary Numerical Analysis, 3rd ed., McGraw-Hill: New York, 1980. Dibeler, V. H.; Mohler, F. L.; deHemptinne, M. J. Res. Natl. Bur. Stand. 1954,53(2), 107. Harrison, A. G. Chemical Ionization Mass Spectrometry; CRC: Boca Raton, FL, 1983. Lenz, D. H.; Conner, W. M. C. Computer Analysis if the Cracking Patterns of Deuterated Hydrocarbons. A m i . Chim. Acta 1985 173, 227-238. Price, G. L.; Iglesia, E. Matrix Method for Correction of Mass Spectra in Deuterium Exchange Applications. Ind. Eng. Chem. Res. 1989a,28, 839-844. Price, G. L.; Iglesia, E. Use of CI-MS for the Determination of Deuterium Content in Hydrocarbons. 1. The Boundary Method for Hydrogen Abstraction Spectra. Ind. Eng. Chem. Res. 1989b, 28,1089-1095. Schissler, D. 0.;Thompson, S. 0.;Turkevich, J. Behavior of Paraffin Hydrocarbons on Electron Impact: Synthesis and Mass Spectra of some Deuterated Paraffin Hydrocarbons. Discuss. Faraday SOC.1951,10, 46.
Received for review March 16, 1989 Accepted July 28, 1989
Critical Pressures and Temperatures of Isomeric Alkanols Daniel J. Rosenthalt and Amyn S . Teja* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
T h e critical pressures and temperatures of 18 isomeric alkanols containing up to 12 carbon atoms have been measured with a low residence time flow apparatus. The effects of thermal decomposition of the higher alkanols were minimized by the short times of heating in the experiments. Our data generally agree with the data reported in the literature when these were available, except in the case of a few substances for which the literature data are apparently erroneous. The critical properties of fluids are required in many equation of state and corresponding state correlations. In spite of many studies of these properties (see, for example, the compilations of Ambrose (1980) and DECHEMA (1987)), reliable experimental data are still lacking for many substances. This is particularly true for high molecular weight materials that decompose or associate before attaining their critical states. As part of a continuing effort in the area of critical point measurement in our laboratory, we have developed a novel sealed ampule technique for the measurement of the critical temperatures and densities of thermally stable and unstable substances and reported the results for alkanes (Smith et al., 1987) and isomeric alkanols (Smith et al., 1986; Anselme and Teja, 1988a,b). The sealed ampule technique, however, can only be used to obtain the critical temperature and density. Another method must be used to measure the critical pressure. In this paper, we describe a low residence time flow technique for the measurement of the critical temperature and pressure of thermally stable and unstable fluids. New results for the critical properties of 1-and 2-alkanols using the flow apparatus are presented, and comparisons between the two methods developed in our laboratory are shown. Experimental Section In the flow method, the fluid of interest is pumped rapidly through a heated visual cell at a temperature and 'Present address: Soltex Polymer Corp., P.O. Box 10o0,Deer Park, T X 77536. 0888-588518912628-1693$01.50/0
pressure that result in critical opalescence and/or meniscus disappearance or reappearance in the observation chamber. The residence time of the fluid at elevated temperatures is kept low in order to minimize thermal decomposition or association. A schematic diagram of the flow apparatus is shown in Figure 1. An Aldex high-pressure pump (Model llOA) was used to pump a degassed sample of the fluid under study through a heated view cell, where the pressure and temperature of the fluid were measured and any phase changes noted. The pump was capable of maintaining a constant flow rate of the fluid between 0 and 10 mL/min. The view cell was made of stainless steel and had two windows made of borosilicate glass to admit light into the chamber and for visual observation of any phase changes. The internal volume of the cell was 4.36 mL. Two type K thermocouples were inserted into the observation chamber at distances of approximately one-third of the length of the cell from each end. The cell was placed inside a thermostated air bath where its temperature could be maintained constant within fO.l K. The pressure in the cell, which was measured with a digital Heise gauge, could be adjusted with a micrometering valve located downstream from the cell. The fluid leaving the cell through the micrometering valve was discarded. At the beginning of the experiment, the apparatus was cleaned and evacuated with a vacuum pump. The vacuum pump was then disconnected and the flow pump switched on at the maximum flow rate setting. A t this stage, the air bath temperature was set at a value approximately 20 K above the estimated critical temperature of the fluid, 1989 American Chemical Society
1694 Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 A 0
-
1-DECANOL I-UNDECANOL 1-DODECANOL
710
Y, w [r
3
I-
u
I 'I WSd
-
-
J.
p-1
1
sa
700
I-
690
B I 680
V l d
0
chembcll
20
40
60
80
100
RESIDENCE TIME (SEC)
Figure 1. Schematic diagram of the apparatus. b
and the micrometering valve was closed completely in order to build up pressure in the system. When the system pressure approached the critical pressure of the fluid, a flat meniscus separating the vapor and ligand phases could be observed in the visual chamber-provided the fluid was at its critical temperature in the cell. If the temperature of the fluid was lower than the critical temperature, then the meniscus had pronounced curvature because of the difference in the density between the vapor and liquid phases. It should be added here that the temperature of the fluid in the cell was not necessarily equal to the preset temperature of the air bath. When the pressure exceeded the critical pressure, the micrometering valve was adjusted in order to obtain steady flow and a decrease of pressure in the cell. Pressure reduction at the critical temperature led to the appearance of an orange-red band, which was first observed in the fluid at pressures slightly above the critical. The pressure was recorded when the color was the most intense (maximum opalescence). The position of the red band in the observation chamber could be manipulated by changing the flow rate of the fluid. The flow rate was therefore varied until the red band moved to a position near one of the thermocouples. The temperature measured by that thermocouple at maximum opalescence was assumed to be the critical temperature of the fluid and was recorded. It should be added that the typical temperature gradient in the cell was of the order of 5 K, and the thickness of the band varied from a few millimeters for the stable alkanols to about 8 mm for the unstable alkanols. The accuracy of our measured critical temperatures is estimated to be *0.6 K for the stable substances and that of the pressure is estimated to be *0.02 MPa. The precision of the temperature and pressure measurements was 0.1 K and 0.01 MPa, respectively. In the case of unstable substances, even the low residence time in the heated zone is not enough to prevent decomposition or reaction. Figure 2 shows the critical temperature versus residence time behavior in this case. The data obtained must be extrapolated back to zero residence time to obtain the critical temperature of the pure fluid as shown on the diagram. The actual critical temperature is assumed to be the average of the temperature at zero residence time and the temperature of the first data point and the error is half the difference between the two temperatures. This extrapolation procedure assumes that the pure substance and its products of decomposition yield a linear critical locus, which is valid for small amounts of decomposition (Anselme and Teja, 1988a).
I
2 51
A 2 4 I
2
E 2 3
E
3
0 0 2 2
1-UNDECANOL 1-DODECANOL
2 0 0
20
40
60
80
100
RESIDENCE TIME (SEC)
Figure 2. Change in the (a) critical temperature and (b) critical pressure of the higher alkanols with residence time. Table I. Source and Purity of the Substances Studied substance suDDlier lot no. purity, % ._ 99.90 ethanol Fisher 1-DroDanol 99.5 Fluka 275816 1-but'anol Fisher 871182 99.9 1-pentanol Fluka 252122 99 1-hexanol Fluka 31179 99 1-heptanol Fluka 22543 99.5 1-octanol Fisher 872023 99 1-nonanol Fluka 24788 99 1-decanol 98 Wiley 1-undecanol Aldrich 07305BT 99 1-dodecanol Fluka 254801 99.5 2-propanol Fisher 872063 99 2- butanol Fisher 875027 99 2-pentanol Fluka 237198 99 2-hexanol Kodak A17C 99 2-heptanol Fluka 31051 99 2-octanol Kodak A14D 99 2-nonanol 99 Wiley Purified by reactive distillation.
Source and Purity of the Substances Studied The 18 alkanols studied in this work are listed in Table I. The purest commercially available materials were used in the experiments, and all chemicals, except ethanol, were used as received. Table I gives the purity, supplier, and lot number of the materials used. Pure ethanol was obtained via reactive distillation of Fisher HPLC grade ethanol with magnesium metal catalyzed with iodine. The purity of the resulting ethanol was determined to be
Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 1695
>99.9% by gas chromatography. 6 -
Results and Discussion The results of our measurements of the critical properties of the alkanols are presented in Table 11. Alkanols smaller than 1-decanol were found to be thermally stable in our experiments; i.e., their critical temperatures did not change with changing residence time. The critical opalescence band for many of the alkanols was not as well-defined as that observed in the case of the stable n-alkanes in an earlier study (Rosenthal and Teja, 1989). Instead, the band was broad and fuzzy, probably due to hydrogen bonding in the alkanols. Smith et al. (1986) also observed the effects of self-association in their
curve is very smooth and decreases regularly with carbon number. The literature values also fall along this curve with the exception of the values for 1-pentanol and l-octanol. It is therefore likely that the literature values of the critical pressure for these two alkanols are not as reliable as the values obtained in the present work. In our initial experiments with 1-octanol, we obtained a critical pressure of 3.18 MPa and a critical temperature of 650.9 K for this substance. The value for the critical temperature was in agreement with the literature value (650.6 K), but the critical pressure was significantly different from the literature value (3.04 MPa). The sample used in our experiments was the same as that used in the sealed ampule experiments by Anselme and Teja (1988b). A new sample was ordered and the experiment repeated.
f .: p
I AUT
Inow 8
I
P
4
8
-I
C
E
8
I n
3 -
8 2 -
3
5
7
9
.. 11
7
13
TIME (MN)
Figure 4. Critical temperature versus time trace for 1-octanol obtained in the sealed ampule experiments. The points represent meniscus disappearances (on the rising part of the curve) or disappearance (on the falling part of the curve).
The critical temperature and pressure of this sample were found to be 652.4 K and 2.78 MPa. After running a gas chromatographic analysis of both samples, it was apparent that the first sample had become contaminated and the latter sample was nearly 100% pure. The sealed ampule experiment was therefore repeated with the new sample, and the results are presented in Figure 4. The new sealed ampule experiments yielded a critical temperature of 652.9 f 0.4 K, which is in agreement with the value obtained in
1696 Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989
t-
au
t
g
540
520
1
+
LIT FLOW
I 4
500 1
3
5
7
9
11
13
CAWON N U M R
Figure 5. Critical temperature as a function of the carbon number for the 2-alkanols.
the flow apparatus and the value reported in the literature. 1-Octanol was also the first to show signs of decomposition in the sealed ampule experiments (Anselme and Teja, 1988b). On the other hand, significant decomposition was only observed with 1-decanol in the flow apparatus. Thus, extrapolation of the data was only necessary for 1-decanol and higher alkanols in the present study and is shown in Figure 2. A small cloud was also observed near the orange-red band at the critical point in the case of 1-decanol. This cloud is most likely due to retrograde condensation and makes the observation of the critical opalescence conditions imprecise. However, the disappearance of the meniscus could still be observed with a precision of fO.O1 MPa. 2-Alkanols. The secondary alkanols (C,-C,) did not decompose in the flow apparatus. Also, the critical temperatures of these alkanols obtained in this study generally agree with those obtained in our laboratory using the seaaled ampule method. The exceptions are the values obtained for 2-heptanol and 2-nonanol. The critical temperature of 2-heptanol and 2-nonanol measured in the flow apparatus are 2.9 K lower and 1.0 K higher, respectively, than those measured by using the sealed ampule method. Figure 5 shows the results for the 2-alkanol series. The 2-heptanol value obtained in the sealed ampule method appears to be too high when compared with the values for the rest of the series, whereas the value obtained in the present study appears to lie on the curve. Both values for 2-nonanol are consistent with the homologous series curve, although they disagree with each other by 1.0 K. The value from the sealed ampule method, however, is significantly affected by the extrapolation technique. The extrapolation to zero time, which includes all of the data points obtained in the sealed ampule experiments, leads to a critical temperature of 649.9 K, in excellent agreement with the value obtained in the present work (650.1). However, excluding the points that have a rate of change of temperature of 0.5 K / s (i.e., those showing appreciable thermal lag) gives a value of the critical temperature of 649.1 K, as reported by Anselme and Teja (1988b). It should be noted that, in the flow method, 2-nonanol remained stable and no extrapolation was necessary. The most serious disagreement between our critical temperatures and those found in the literature was in the
case of 2-octanol, where there was a difference of 8 K. The literature value in the Ambrose compilation was obtained from the work of Brown (1906). Ambrose notes that pre-1910 measurements of critical temperatures differ by as much as 15 K from modern values. Therefore, the disagreement in the case of 2-octanol between the literature and this work is not unexpected. Only two critical pressures have been reported for the 2-alkanols. The critical pressure of 2-propanol measured in this work agrees well with the literature value for that substance, but that of 2-butanol differs from the literature value by an amount slightly greater than the experimental error. The literature value appears to be consistent with the behavior of the homologous series in this case. The flow experiment was repeated but the same value was obtained in the second experiment. The error could therefore be caused by the small amount of impurity present in the starting material.
Conclusions New data for the critical temperatures and pressures of isomeric alkanols containing up to 12 carbon atoms have been obtained with a low residence time flow technique. The data generally are in agreement with literature values when these were available. However, in some cases, the new data reveal several inconsistencies in the literature values. The critical pressures and temperatures of the 1- and 2-alkanols were shown to be smooth functions of the carbon number of the alkanol. The shifting of the OH group from the end position was shown to result in a significant lowering of the critical temperature and a small decrease of the critical pressure. This is due to the decreasing tendency of the 2-alkanol to hydrogen bond because of the relative inaccessibilityof the OH group relative to that in the corresponding 1-alkanol. Registry No. Ethanol, 64-17-5; 1-propanol, 71-23-8 1-butanol, 71-36-3; 1-undecanol,112-42-5; 1-dodecanol, 112-53-8; 2-pentanol, 6032-29-7; 2-hexanol, 626-93-7; 2-heptanol, 543-49-7; 2-octanol, 123-96-6; 2-nonanol, 628-99-9.
Literature Cited Ambrose, D. Vapor-liquid critical properties. National Physical Laboratory Report Chem. 102, 1980. Anselme, M. J.; Teja, A. S. The prediction of critical properties of dilute multicomponent mixtures. Proc. Znt. Symp. Supercritical Fluids, Nice, Fr. 1988a, 1, 169-176. Anselme, M. J.; Teja, A. S. The critical temperatures and densities of isomeric alkanols with six to ten carbon atoms. Fluid Phase Equilib. 1988b,40, 127-34. Brown, J. C. The Critical Temperature and Value of mL/B of Some Carbon Compounds. J. Chem. SOC.1906,89,311-5. DECHEMA. Critical Properties; Chemistry Data Series; Dechema: Frankfurt, 1987, Vol. 5. Rosenthal, D. J.; Teja, A. S. The critical temperatures and pressures of the n-alkanes using a low residence time flow apparatus. AIChE J. 1989,in press. Smith, R. L.; Anselme, M. J.; Teja, A. S. The critical temperatures of isomeric pentanols and heptanols. Fluid Phase Equilib. 1986, 31, 161-170. Smith, R. L.;Kay, W. B.; Teja, A. S. Measurement of the critical temperatures of thermally unstable n-alkanes. AZChE J. 1987 33, 232-8.
Received for review February 21, 1989 Accepted July 25, 1989
