- Email: [email protected]
Measurement of the critical temperatures and critical pressures of some thermally stable or mildly unstable alkanols
M-2534 J. Chem. Thermodynamics 1991, 23, 67-76
M e a s u r e m e n t of the critical t e m p e r a t u r e s and critical pressures of some t h e r m a l l y stable or mildly unstable alkanols S. K. Q U A D R I , a KI C. K H I L A R , " A. P. K U D C H A D K E R , a and M A H E N D R A J. P A T N I b
aDepartment of Chemical Engineering or bMaterials Science Centre, Indian Institute of Technology, Powai, Bombay 400 076, India (Received 3 July 1990; in final form 28 August 1990) An apparatus for the measurement of critical temperatures and critical pressures of thermally stable or mildly unstable organic substances has been designed, fabricated, and tested. A special feature is an arrangement for heating the sample rapidly to temperatures near the critical so as to reduce decomposition. Temperatures were measured with a chromel-to-alumel thermocouple with an accuracy of _+.+0.1K. Pressures were measured with a transducer assembly with an accuracy of _+0.001 MPa. Measurements of T~ and Pc for 3 n-alkanes, and 4 straight-chain and 10 branched alkanols, are reported, and compared with the literature and predicted values using the methods of Ambrose and of Somayajulu.,
1. Introduction An extensive literature survey revealed that out of about 600 industrially important chemicals, values of the critical temperature T~ for about 70 per cent, critical pressure, Pc for about 50 per cent, and critical volume Vc for about 30 per cent, are available. Values for oxygen compounds, especially for higher alcohols, aldehydes, glycols, acids, ketones, ethers, and esters, are scanty. The critical constants (T~, Pc, V~) of a substance are important in many thermodynamic and transport-property predictions, and are needed for (vapour + liquid) processes designed using the equation-of-state or corresponding-states models. The techniques of measurement of critical constants have been reviewed by Kobe and Lynn, (1) Kudchadker etal., (2) Hicks and Young, (3) and Quadri and Kudchadker. (4) However, most of the methods reviewed are suitable primarily for stable substances. Smith et al. (5'6) have described a rapid-heating sealed-ampoule technique which is used for the measurement of Tc and Pc of thermally unstable substances. (v) This technique as used was not suitable for the measurement of Pc. We have designed and fabricated a modified Kay-type apparatus (s) for the measurement of T~ and Pc of either thermally stable or mildly unstable organic compounds. The novelty of the apparatus is the intermittent heating of the heating block and the 0021-9614/91/010067 + 10 $02.00/0
© 1991 Academic Press Limited
68
S.K. QUADRI E T AL.
sample by means of a heated furnace which can be lowered to and raised from the heating block when needed. The heating block and the sample can thus be heated to the meniscus-disappearance temperature and cooled to the reappearance temperature with ease. We have measured Tc and Pc for 3 n-alkanes and octan-l-ol for testing performance of the apparatus, and of 4 straight-chain and 10 branched alcohols to increase the available results for organic oxygen compounds. Based on the measurement technique, the accuracy of Tc and Pc measured have been estimated to be _+0.7 K and +0.03 MPa, respectively.
2. Apparatus The rapid heating furnace 5 (figure 1) consisted of 0.559 mm diameter Kanthal A-l wire wound on a fused alumina tube of i.d. 75 mm and thickness 5 mm. The wire was
I I~
He/Ne laser 440
FIGURE 1. Assembly diagram of apparatus for the determination of the critical temperature and critical pressure. 1, Experimental tube-holding block; 2, compressor block; 3, experimental tube; 4, thermowell; 5, furnace; 6, heating block (brass piece); 7, four-way valve; 8 to 10, needle valves; 13, 14, timer circuit; 15, 16, 12 V d.c. power supply.
T~AND Pc FOR ALKANOLS
69
-3
1
!
FIGURE 2. Sectional view of compressor block. 1, 2, Compressor block; 3, experimental tube; 4, thermowell;5, Viton O-ring; 6, glass flange;7, Teflonpacking; 8, end cap; 9, cylinder; I0, 11, seal flanges; 12, 13, Viton O-rings; 14, 15, socket screws; 16, 17, steel rings; 18, 19, end caps.
wound over a length of 300 mm. It was insulated by ceramic wool of thickness 90 mm and surrounded by aluminium sheet. The current was regulated by means of an autotransformer to maintain a constant power supply. The temperature of the furnace up to 1000 K could be attained within l0 min by passing a current of 6 A. The sample heating block 6 was a brass piece of 100 mm length and 35 mm diameter with three viewing slits; two diametrically opposite windows 75 mm long and 5 mm wide were provided to observe the critical point visually. A third window of the same size at right angles to the axis of the other two was provided so as to view the sample during the heating process. The sample heating block could easily achieve the temperature of interest, i.e. 700 K or so, within about 3 rain by sliding the furnace on the sample heating block. The experimental tube 3 (figure 1) used in this study was constructed from borosilicate glass tubing of 2.5 mm precision bore and an o.d. of 8 mm, one end of which was sealed with a tiny well 4 for a thermocouple. The open end of the tube was shaped in the form of a flange 6 (figure 2) and a glass-to-metal seal connected the tube to the compressor block 1, through a small Viton O-ring 5 in the manner shown in figure 2. Gas pressure applied at the mercury surface in block 2 was transmitted through the rfiercury to the sample in the tube. Figure 2 is a sectional view of the compressor block showing its construction and the method of holding the experimental tube. The compressor block was a mercury-in-steel U-tube, one leg of which was connected to the four-way valve, while the other leg was fitted with a specially
70
S.K. QUADRI ET AL.
designed Teflon packing 7 and Viton O-ring 5 for holding securely the experimental tube in an inverted position. Finally, the end cap 8 which compressed the Teflon packing and O-ring, made the seal. Block 2 was constructed of a middle cylinder 9 which contained sufficient mercury; detailed construction of block 2 is shown in figure 2. The pressure-transmission system consisted of a four-way valve 7 one end of which was connected to a regulated supply of dry nitrogen gas and one to the compressor block assembly (figure 1). Pressure was adjusted by using three needle valves 8 to 10 and two solenoid valves 11 and 12 connected in parallel. The solenoid valves were used only for ease of pressure adjustment of the sample because the pressure adjustment and lowering and raising of the furnace had to be simultaneously carried out quickly for making the observation of T¢ and p~. Valves 8, 9, and 11 were used to increase the pressure while valves 10 and 12 were used to release the pressure. Needle valves 8 and 10 were used for sudden increase or release of pressure. Solenoid valves were operated by using two separate timer circuits LM555 (13 and 14) and two 12 V d.c. power supplies (15 and 16). A variable period was provided in the timer circuit which facilitated fine and coarse adjustments of the pressure of nitrogen (0.001 MPa to 0.1 MPa). One end of the four-way valve was connected to the pressure transducer (0 to 13.8 MPa, Sensotec, U.S.A.), provided with an IEEE 488 interface for digital output of results. The temperature was measured by a mineral-coated chromel-to-alumel K-type thermocouple located in a tiny thermowell 4 (figures 1 and 2) sealed to the top of the experimental tube 3. The thermowell was filled with a small amount of silicone oil for better thermal contact. The thermocouples were calibrated against a platinum resistance thermometer certified by the U.S. National Bureau of Standards. In addition, the standard thermometer had been checked by comparison with the ice point. Thermocouple e.m.f, was sensed by a Hewlett-Packard digital multimeter (Model 3468A), _+1 gV sensitivity. The multimeter was connected to the temperature recording system series 500 (Keithley, U.S.A.). The method of heating the sample did not permit the sample to attain a steady temperature during the period of observation. Hence additional temperature measurements were made both inside the sample at the meniscus as well as in the thermocouple well to find the difference in the two temperatures as the meniscus temperature varied from 470 K to 700 K. A special high-pressure adapter was provided for inserting the thermocouple in the sample tube. These measurements were repeated with different organic liquids under the present study. A calibration curve was prepared between the meniscus temperature and the temperature difference as stated above and was used to correct all thermocouple-well temperatures to obtain the 'true' meniscus disappearance- and real~pearancetemperature of each sample. The critical point i.e. disappearance of meniscus was determined visually and also by passing a He/Ne laser beam through the sample during the heating process. The scattered beam was received by a silicon detector head (EG and G Gamma Scientific Model 460-1), which was connected to a laser power meter (Model 460-1). The laser power meter provided a 3.5 digit bipolar display and also output which was
71
T~ AND Pc FOR ALKANOLS
interfaced to the recorder (series 500) for continuous recording of the scattered beam. This laser system was installed for future use and did not contribute to the results reported here.
3. Experimental The purity, grade, and source of compounds used in this investigation are reported in table 1. The purities of the alkanes were checked by g.c. (Shimadzu, Japan) on an SE 30, 10 mass per cent on chromosorb G, SS 3 mm × 2 m column at a column temperature of 393 K with f.i.d. (0.005 dm 3. s-1 flow rate), and of the alkanols on PEG, 15 mass per cent on chromosorb G, at a column temperature of 423 K with t.c.d. Triply distilled mercury obtained from B.D.H., U.K., was used for the experimental tube calibration and as confining fluid. As a safety precaution, each tube was tested to a pressure of 4 MPa. The experimental tube was thoroughly cleaned with chromic acid, then with distilled water, and finally with acetone, and dried completely in an oven. The determination of the critical constants required a sample to be loaded at room temperature so that the critical density was achieved upon heating to the critical temperature. This required knowledge of the capillary volume and the critical density. Critical densities were estimated using the method of Ambrose39~ A 250 mm 3 hypodermic syringe with a 0.5 m long needle was used to introduce the sample into the capillary. Capillary volume was determined from the mass and density of freshly distilled mercury required to fill the capillary. The weighing was done using an electronic balance with a sensitivity of + 0.1 mg. Because of the method of heating the sample, it was not possible to measure V~ of the samples under investigation.
TABLE 1. Mole-fraction purities x of samples as determined by g.c., their sources, and grades Compound
102 .x
Grade
n-Pentane n-Hexane n-Heptane Pentan-2-ol 2-Methyl-butan-l-ol 2-Methyl-butan-2-ol 3-Methyl-butan-l-ol 3-Methyl-butan-2-ol Hexan-l-ol a Hexan-3-ol 2-Methyl-pentan-l-ol 2-Methyl-pentan-3-ol 3-Methyl-pentan-3-ol Heptan-l-ol Octan-l-ol" Octan-2-ol Nonan-l-ol Decan-l-ol
99.9 99.9 99.8 99.9 99.8 99.97 99.2 99.83 99.6 99.42 98.6 98.9 99.9 99.0 98.7 99.9 99.7 98.98
spectroscopic spectroscopic spectroscopic synthesis laboratory laboratory puris purum synthesis laboratory laboratory laboratory laboratory laboratory purum laboratory laboratory gold label
a Purified by fractional distillation.
Source Spectrochem, Bombay Spectrochem, Bombay Spectrochem, Bombay E. Merck, Switzerland Aldrich, U.S.A. Aldrich, U.S.A. Fluka, Switzerland Fluka, Switzerland E. Merck, Switzerland Aldrich, U.S.A. Aldrich, U.S.A. Aldrich, U.S.A. Aldrich, U.S.A. B.D.H., U.K. Fluka, Switzerland Aldrich, U.S.A. Aldrich, U.S.A. Aldrich, U.S.A.
72
S.K. QUADRI E T AL.
The sample should be free of dissolved gases so as to obtain the 'true' Tc and Pc. For this purpose the sample was degassed (or de-aerated) in the experimental tube by successive freeze-melt cycles under vacuum to remove non-condensable gases. Each substance was frozen with liquid nitrogen and evacuated. The successive freeze-melt cycles were continued until no gas evolved upon melting. Then mercury was slowly and carefully transferred on top of the frozen sample under vacuum. The trapped air bubble between mercury and the wall of the tube was removed by inserting a fine steel capillary. After degassing the sample and transfer of mercury in the experimental tube, the tube was fastened in one leg of a mercury-in-steel U-shaped compressor block as shown in figure 2. The other leg 2 of the block was connected to a source of highpressure dry nitrogen gas for pressurizing the system. In preparation for the determination of the critical constants, the pressure was adjusted using needle and solenoid valves to the predicted critical pressure and the heating block was placed around the experimental tube and positioned so that the sample in the tube was in line with the slits of the block. The furnace was heated to 800 K on top of the assembly, then brought down around the heating block and the tube. Within 3 min the heating block attained the desired temperature after which the furnace was raised to its original position. Simultaneously, the sample was heated. The disappearance and reappearance of the meniscus were visually observed in the middle of the tube clearly within the field of vision on both the heating and cooling modes at a unique temperature, the critical temperature. The pressure corresponding to this critical temperature was the critical pressure. For each filling of the experimental tube with the sample, a number of measurements ranging from 7 to 10 of T~ and Pc were made by observing the disappearance and reappearance of the meniscus. In most cases the transition was sharp, and reproducible results within +0.3 K for T~ and _+0.01 MPa for Pc were obtained. This indicated that the samples were stable and did not decompose with the repeated heating and cooling cycles. For mildly unstable substances such as branched alkanols the earliest temperature of disappearance and reappearance was the same but successive values changed with time. Therefore, the earliest disappearance and reappearance temperatures were averaged and used as the criteria for the critical point. For rapidly decomposing substances such as decan-l-ol, reappearance of the meniscus was much more difficult to detect, because of the formation of a milky band and dissociation. Therefore, the disappearance of the meniscus was used as the criterion for the critical point which was reproducible within _+0.5 K for Tc and _+0.02 MPa for p~. Decomposition of the sample resulted in general in increasing the Pc and decreasing the Tc values as a function of time. This was observed for hexan-3-ol, octan-l-ol, octan-2-ol (slight decomposition), 3-methyl-pentan-3-ol, nonan-l-ol, and decan-l-ol (rapid decomposition), and hence for one sample charge in the tube, only one experimental observation was made by one heating and cooling cycle. Repeated measurements were carried out with a fresh filling. Reproducible results were thus obtained.
73
T~ AND p~ FOR ALKANOLS
The determination of critical temperature and critical pressure of a few slowly decomposing and rapidly decomposing samples required some stability test. To carry out this test we collected the sample after measurements in a small sample bottle through a 250 mm 3 hypodermic syringe with a 0.5 m long needle. The sample was analysed by using a glass capillary column of i.d. 0.75 mm, length 60 m, phase SPB-1, film thickness 1 gm (Supelco) at column temperature 453 K with f.i.d. Results obtained before and after measurement by g.c. are reported in table 3. If the chromatogram of the sample before and after the measurement was the same within the g.c. reproducibility, then the substance was considered stable (figures 3 and 4).
4. Results Experiments were carried out with n-pentane, n-hexane, n-heptane, and octan-1-ol in order to check the performance of the apparatus. The pressure as determined by the pressure transducer was corrected for the difference in the levels of mercury (corrected to standard temperature of 273.15 K) in the experimental tube and the compressor block, as well as for the partial pressure of mercury calculated by the equation proposed by Kay: ~8) lg(p/MPa) = 3.76862- 3039.30-
K/T,
(1)
where T is the temperature and p is the partial pressure of mercury. Some experiments were carried out with mercury within the heating zone and also outside the heating zone to check whether it was the vapour pressure of mercury or the partial pressure of mercury which affects the critical pressure. It was confirmed that equation (1) was applicable and hence it wa~ used in correcting the critical-pressure values as shown by the transducer. As stated earlier it was observed that temperature measured within the sample tube at the meniscus were always lower than those measured at the thermocouple well by 0.3 K to 0.5 K for most of the alkanols except for the C8 alkanol where it was 0.6 K and for C 9 and Clo alkanols, it was 0.8 K. The measured thermocouple-well temperatures were accordingly corrected. These corrected Tc values as well as Pc values are reported in table 2 for the alkanes and alkanols and are compared with reliable literature values ~1°-14) and those predicted using the methods of Ambrose o, 15) and of Somayajulu. ~16) The estimated accuracy in measured T¢ and Pc values for each compound are also reported in table 2. The results of stability tests by g.c. before and after 1 measurement and 3 to 5 measurements are given in table 3 as well as the chromatograms in figures 3 and 4.
5. Discussion The accuracy of the T~ measurements using the present apparatus is about + 0.7 K arid of Pc about _0.03 MPa. This was probably due to the method of heating the sample by which the temperature of the sample did not remain steady for a reasonably long period. This "unsteady state" in the sample tube might be responsible for higher inaccuracies in the measurements as compared with other
s . K . Q U A D R I E T AL.
74
TABLE 2. Experimental values of the critical temperature and critical pressure of the alkanes and alkanols with their estimated accuracies
Tc/K
Compound This work
lit.
n-Pentaue n-Hexane n-Heptane Pentan-2-ol
469.7_+0.4 507.5 +__0.4 540.4_+0.4 560.0 __+0.5
2-Methylbutan- 1-ol 2-Methylbutan-2-ol 3-Methylbutan-l-ol 3-Methylbutan-2-ol Hexan-l-ol Hexan-3-ol 2-Methylpentan-l-ol 2-Methylpentan-3-ol 3-Methylpentan-3-ol Heptan-l-ol
575.4 + 0.5 543.7-t- 0.5 577.2+0.5 556.1+0.5 610.4_+0.6 582.2-t-0.6 604.4+0.5 576.0_+0.5 575.6_+0.6 632.0+0.6
Octan-l-ol
652.2___0.6
Octan-2-ol
630.2+0.6
Nonan-l-ol
671.3-t-0.8
Decan-l-ol
687.7__+0.8
469.69" 507.70" 540.30" 560.40 b 560.40 c -545.0 ~ 579.40 e -611.35 c 582.48 c ---632.30 b 632.10 c 652.501 650.56 ~ 638.00 629.70 c 671.0 ~ 670.52 ~ 687.0" 687.30 c
a
pc/MPa lit.
predicted Ref. 15 Ref. 16
This work
predicted Ref. 15 Ref. 16
469.2 507.0 539.9 558.7
469.7 507.0 539.9 561.7
3.36___0.01 3.364a 2.99__+0.01 3.010 a 2.73+0.01 2.756" 3.64 ___0.02 --
3.34 3.00 2.71 3.87
3.34 3.00 2.71 3.77
576.8 540.6 580.7 548.2 611.0 578.63 603.4 568.2 563.1 632.9
576.8 536.7 576.5 551.7 611.0 585.0 602.2 568.23 562.7 633.7
3.94 _ 0.02 3.71 + 0.02 3.93___0.02 3.87+0.02 3.42+0.02 3.36___0.02 3.45__+0.02 3.46 +0.02 3.52-1-0.02 3.16_+0.02
----------
4.04 4.05 3.91 3.87 3.47 3.54 3.60 3.65 3.72 3.12
3.91 3.78 3.91 3.78 3.52 3.37 3.54 3.38 3.42 3.18
653.5
655.5
2.85+0.02
2.8601
2.84
2.89
630.1
632.1
2.78+__0.02
--
2.81
2.80
673.7
671.1
2.51+0.03
--
2.59
2.62
694.0
687.0
2.31+_0.03
--
2.39
2.39
Reference 10. b Reference 5. c Reference 7. d Reference 11. e Reference 12. y Reference 13. g Reference 14.
methods.
However,
3-methyl-pentan-3-ol,
for substances nonan-l-ol,
we were able to obtain
rapidly
decomposing
and decan-l-ol,
reproducible
and
showed
t h a t all t h e c o m p o u n d s was reduced
near
r e l i a b l e v a l u e s o f Tc a n d
specified accuracy. The detailed analysis of chromatograms decomposition
at and
and
Pc w i t h i n
the
( t a b l e 3, f i g u r e s 3 a n d 4)
were stable at the first experimental
due to the method
To, s u c h a s
used in the present investigation,
rapidity
run. Thermal
of heating.
As the
TABLE 3. Mole-fraction purities x and comparison of experimental values of the critical temperature and critical pressure of the alkanols initially, after one, and after three-to-five measurements Compound
initial 102X
after 1 measurement 102X TdK po/MPa
after 3 to 5 measurements 102x Tc/K p¢~MPa
2-Methylbutan-2-ol 3-Methylbutan-2-ol Hexan-3-ol 3-Methylpentan-3-ol Octan-2-ol Nonan-l-ol Decan-l-ol
99.97 99.83 99.42 99.99 99.99 99.7 98.98
99.8 99.8 99.32 99.74 99.83 99.4 98.67
99.66 99.78 99.31 99.1 99.54 99.0 98.47
543.7 556.1 582.2 575.6 630.2 671.3 687.7
3.71 3.87 3.36 3.52 2.78 2.51 2.31
543.7 556.1 582.1 574.8 630.1 671.3 687.6
3.72 3.88 3.37 3.59 2.80 2.55 2.41
T~ AND Pc FOR ALKANOLS
75
(a), x = 0.997 (b), x = 0.994 (c), x = 0.990 FIGURE 3. Stability test on nonan-l-ol. Chromatograms (a), prior to measurement; (b), after one measurement; and (c), after three measurements.
mercury-to-sample interface in our apparatus was at the sample temperature, it was necessary to correct the Pc values for the partial pressure of mercury. The T~ and Pc values for 2-methyl-butan-l-ol, 3-methyl-butan-2-ol, 2-methylpentan-l-ol, 2-methyl-pentan-3-ol, and 3-methyl-pentan-3-ol, a n d Pc of other alkanols, were measured for the first time in the present investigation. In general, the critical temperature of the alkanols decreased but the critical pressure increased with time due to thermal decomposition. This p h e n o m e n o n was m o r e rapid as the chain length increased. Thus the change in values with time for d e c a n - l - o l was greater than that for n o n a n - l - o l (decan-l-ol is more rapidly ,decomposing than nonan-l-ol). Finally, our results agree with those found in the literature, except for the thermally unstable higher alkanols. As established in the stability test the samples were stable at the first experimental run; we believe that the experimental technique used in this work gives reliable results within the specified accuracy.
(a), x = 0.9898 (b), x = 0.9867 (c), x = 0.9847 FIGURE 4. Stability test on decan-l-ol. Chromatograms (a), prior to measurement; (b), after one measurement; and (c), after three measurements.
76
S.K. QUADRI E T AL.
This w o r k was c a r r i e d o u t u n d e r the N a t i o n a l D a t a P r o g r a m m e f u n d e d b y the D e p a r t m e n t of Science a n d T e c h n o l o g y , N e w Delhi, I n d i a .
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Kobe, K. A.; Lynn, R. E. Chem. Rev. 1953, 52, 117. Kudchadker, A. P.; Alani, G. H.; Zwolinski, B. J. Chem. Rev. 1968, 68, 659. Hicks, C. P.; Young, C. L. Chem. Rev. 1975, 75(2), 119. Quadri, S. K.; Kudchadker, A. P. Measurement of the Critical Constants of Organic Substances--State-of-the-Art Report. National Data Program: D.S.T. 1989. Smith, R. L., Jr.; Anselme, M.; Teja, A. S. Fluid Phase Equilibria 1986, 31, 161. Smith, R. L., Jr.; Teja, A. S.; Kay, W. B. AIChe Journal 1987, 33,232. Anselme, M.; Teja, A. S. Fluid Phase Equilibria 1988, 40, 127. Kay, W. B.; Sung, C. P. 3". Chem. Thermodynamics 1980, 12, 673. Ambrose, D. Correlation and Estimation o f Vapour-Liquid Critical Properties. 11. Critical Pressure and Critical Volume of Organic Compounds. NPL Report Chem 98, 1979. Ambrose, D.; Townsend, R. Vapour-Liquid Critical Properties. NPL Report Chem 107, 1980. Brown, J. C. J. Chem. Soc. 1906, 89, 311. Kr~glewski, A. Roczniki Chem. 1955, 29, 754. Ambrose, D.; Broderick, B. E.; Townsend, R. J. Appl. Chem. Biotechnol. 1974, 24, 359. Ambrose, D.; Ghiassee, N. B. J. Chem. Thermodynamics 1990, 22, 307. Ambrose, D. Correlation and Estimation o f Vapour-Liquid Critical Properties. 1. Critical Temperature of Organic Compounds. NPL Report Chem 92, 1978. Somayajulu, G. R. J. Chem. Eng. Data 1989, 34, 106.
M e a s u r e m e n t of the critical t e m p e r a t u r e s and critical pressures of some t h e r m a l l y stable or mildly unstable alkanols S. K. Q U A D R I , a KI C. K H I L A R , " A. P. K U D C H A D K E R , a and M A H E N D R A J. P A T N I b
aDepartment of Chemical Engineering or bMaterials Science Centre, Indian Institute of Technology, Powai, Bombay 400 076, India (Received 3 July 1990; in final form 28 August 1990) An apparatus for the measurement of critical temperatures and critical pressures of thermally stable or mildly unstable organic substances has been designed, fabricated, and tested. A special feature is an arrangement for heating the sample rapidly to temperatures near the critical so as to reduce decomposition. Temperatures were measured with a chromel-to-alumel thermocouple with an accuracy of _+.+0.1K. Pressures were measured with a transducer assembly with an accuracy of _+0.001 MPa. Measurements of T~ and Pc for 3 n-alkanes, and 4 straight-chain and 10 branched alkanols, are reported, and compared with the literature and predicted values using the methods of Ambrose and of Somayajulu.,
1. Introduction An extensive literature survey revealed that out of about 600 industrially important chemicals, values of the critical temperature T~ for about 70 per cent, critical pressure, Pc for about 50 per cent, and critical volume Vc for about 30 per cent, are available. Values for oxygen compounds, especially for higher alcohols, aldehydes, glycols, acids, ketones, ethers, and esters, are scanty. The critical constants (T~, Pc, V~) of a substance are important in many thermodynamic and transport-property predictions, and are needed for (vapour + liquid) processes designed using the equation-of-state or corresponding-states models. The techniques of measurement of critical constants have been reviewed by Kobe and Lynn, (1) Kudchadker etal., (2) Hicks and Young, (3) and Quadri and Kudchadker. (4) However, most of the methods reviewed are suitable primarily for stable substances. Smith et al. (5'6) have described a rapid-heating sealed-ampoule technique which is used for the measurement of Tc and Pc of thermally unstable substances. (v) This technique as used was not suitable for the measurement of Pc. We have designed and fabricated a modified Kay-type apparatus (s) for the measurement of T~ and Pc of either thermally stable or mildly unstable organic compounds. The novelty of the apparatus is the intermittent heating of the heating block and the 0021-9614/91/010067 + 10 $02.00/0
© 1991 Academic Press Limited
68
S.K. QUADRI E T AL.
sample by means of a heated furnace which can be lowered to and raised from the heating block when needed. The heating block and the sample can thus be heated to the meniscus-disappearance temperature and cooled to the reappearance temperature with ease. We have measured Tc and Pc for 3 n-alkanes and octan-l-ol for testing performance of the apparatus, and of 4 straight-chain and 10 branched alcohols to increase the available results for organic oxygen compounds. Based on the measurement technique, the accuracy of Tc and Pc measured have been estimated to be _+0.7 K and +0.03 MPa, respectively.
2. Apparatus The rapid heating furnace 5 (figure 1) consisted of 0.559 mm diameter Kanthal A-l wire wound on a fused alumina tube of i.d. 75 mm and thickness 5 mm. The wire was
I I~
He/Ne laser 440
FIGURE 1. Assembly diagram of apparatus for the determination of the critical temperature and critical pressure. 1, Experimental tube-holding block; 2, compressor block; 3, experimental tube; 4, thermowell; 5, furnace; 6, heating block (brass piece); 7, four-way valve; 8 to 10, needle valves; 13, 14, timer circuit; 15, 16, 12 V d.c. power supply.
T~AND Pc FOR ALKANOLS
69
-3
1
!
FIGURE 2. Sectional view of compressor block. 1, 2, Compressor block; 3, experimental tube; 4, thermowell;5, Viton O-ring; 6, glass flange;7, Teflonpacking; 8, end cap; 9, cylinder; I0, 11, seal flanges; 12, 13, Viton O-rings; 14, 15, socket screws; 16, 17, steel rings; 18, 19, end caps.
wound over a length of 300 mm. It was insulated by ceramic wool of thickness 90 mm and surrounded by aluminium sheet. The current was regulated by means of an autotransformer to maintain a constant power supply. The temperature of the furnace up to 1000 K could be attained within l0 min by passing a current of 6 A. The sample heating block 6 was a brass piece of 100 mm length and 35 mm diameter with three viewing slits; two diametrically opposite windows 75 mm long and 5 mm wide were provided to observe the critical point visually. A third window of the same size at right angles to the axis of the other two was provided so as to view the sample during the heating process. The sample heating block could easily achieve the temperature of interest, i.e. 700 K or so, within about 3 rain by sliding the furnace on the sample heating block. The experimental tube 3 (figure 1) used in this study was constructed from borosilicate glass tubing of 2.5 mm precision bore and an o.d. of 8 mm, one end of which was sealed with a tiny well 4 for a thermocouple. The open end of the tube was shaped in the form of a flange 6 (figure 2) and a glass-to-metal seal connected the tube to the compressor block 1, through a small Viton O-ring 5 in the manner shown in figure 2. Gas pressure applied at the mercury surface in block 2 was transmitted through the rfiercury to the sample in the tube. Figure 2 is a sectional view of the compressor block showing its construction and the method of holding the experimental tube. The compressor block was a mercury-in-steel U-tube, one leg of which was connected to the four-way valve, while the other leg was fitted with a specially
70
S.K. QUADRI ET AL.
designed Teflon packing 7 and Viton O-ring 5 for holding securely the experimental tube in an inverted position. Finally, the end cap 8 which compressed the Teflon packing and O-ring, made the seal. Block 2 was constructed of a middle cylinder 9 which contained sufficient mercury; detailed construction of block 2 is shown in figure 2. The pressure-transmission system consisted of a four-way valve 7 one end of which was connected to a regulated supply of dry nitrogen gas and one to the compressor block assembly (figure 1). Pressure was adjusted by using three needle valves 8 to 10 and two solenoid valves 11 and 12 connected in parallel. The solenoid valves were used only for ease of pressure adjustment of the sample because the pressure adjustment and lowering and raising of the furnace had to be simultaneously carried out quickly for making the observation of T¢ and p~. Valves 8, 9, and 11 were used to increase the pressure while valves 10 and 12 were used to release the pressure. Needle valves 8 and 10 were used for sudden increase or release of pressure. Solenoid valves were operated by using two separate timer circuits LM555 (13 and 14) and two 12 V d.c. power supplies (15 and 16). A variable period was provided in the timer circuit which facilitated fine and coarse adjustments of the pressure of nitrogen (0.001 MPa to 0.1 MPa). One end of the four-way valve was connected to the pressure transducer (0 to 13.8 MPa, Sensotec, U.S.A.), provided with an IEEE 488 interface for digital output of results. The temperature was measured by a mineral-coated chromel-to-alumel K-type thermocouple located in a tiny thermowell 4 (figures 1 and 2) sealed to the top of the experimental tube 3. The thermowell was filled with a small amount of silicone oil for better thermal contact. The thermocouples were calibrated against a platinum resistance thermometer certified by the U.S. National Bureau of Standards. In addition, the standard thermometer had been checked by comparison with the ice point. Thermocouple e.m.f, was sensed by a Hewlett-Packard digital multimeter (Model 3468A), _+1 gV sensitivity. The multimeter was connected to the temperature recording system series 500 (Keithley, U.S.A.). The method of heating the sample did not permit the sample to attain a steady temperature during the period of observation. Hence additional temperature measurements were made both inside the sample at the meniscus as well as in the thermocouple well to find the difference in the two temperatures as the meniscus temperature varied from 470 K to 700 K. A special high-pressure adapter was provided for inserting the thermocouple in the sample tube. These measurements were repeated with different organic liquids under the present study. A calibration curve was prepared between the meniscus temperature and the temperature difference as stated above and was used to correct all thermocouple-well temperatures to obtain the 'true' meniscus disappearance- and real~pearancetemperature of each sample. The critical point i.e. disappearance of meniscus was determined visually and also by passing a He/Ne laser beam through the sample during the heating process. The scattered beam was received by a silicon detector head (EG and G Gamma Scientific Model 460-1), which was connected to a laser power meter (Model 460-1). The laser power meter provided a 3.5 digit bipolar display and also output which was
71
T~ AND Pc FOR ALKANOLS
interfaced to the recorder (series 500) for continuous recording of the scattered beam. This laser system was installed for future use and did not contribute to the results reported here.
3. Experimental The purity, grade, and source of compounds used in this investigation are reported in table 1. The purities of the alkanes were checked by g.c. (Shimadzu, Japan) on an SE 30, 10 mass per cent on chromosorb G, SS 3 mm × 2 m column at a column temperature of 393 K with f.i.d. (0.005 dm 3. s-1 flow rate), and of the alkanols on PEG, 15 mass per cent on chromosorb G, at a column temperature of 423 K with t.c.d. Triply distilled mercury obtained from B.D.H., U.K., was used for the experimental tube calibration and as confining fluid. As a safety precaution, each tube was tested to a pressure of 4 MPa. The experimental tube was thoroughly cleaned with chromic acid, then with distilled water, and finally with acetone, and dried completely in an oven. The determination of the critical constants required a sample to be loaded at room temperature so that the critical density was achieved upon heating to the critical temperature. This required knowledge of the capillary volume and the critical density. Critical densities were estimated using the method of Ambrose39~ A 250 mm 3 hypodermic syringe with a 0.5 m long needle was used to introduce the sample into the capillary. Capillary volume was determined from the mass and density of freshly distilled mercury required to fill the capillary. The weighing was done using an electronic balance with a sensitivity of + 0.1 mg. Because of the method of heating the sample, it was not possible to measure V~ of the samples under investigation.
TABLE 1. Mole-fraction purities x of samples as determined by g.c., their sources, and grades Compound
102 .x
Grade
n-Pentane n-Hexane n-Heptane Pentan-2-ol 2-Methyl-butan-l-ol 2-Methyl-butan-2-ol 3-Methyl-butan-l-ol 3-Methyl-butan-2-ol Hexan-l-ol a Hexan-3-ol 2-Methyl-pentan-l-ol 2-Methyl-pentan-3-ol 3-Methyl-pentan-3-ol Heptan-l-ol Octan-l-ol" Octan-2-ol Nonan-l-ol Decan-l-ol
99.9 99.9 99.8 99.9 99.8 99.97 99.2 99.83 99.6 99.42 98.6 98.9 99.9 99.0 98.7 99.9 99.7 98.98
spectroscopic spectroscopic spectroscopic synthesis laboratory laboratory puris purum synthesis laboratory laboratory laboratory laboratory laboratory purum laboratory laboratory gold label
a Purified by fractional distillation.
Source Spectrochem, Bombay Spectrochem, Bombay Spectrochem, Bombay E. Merck, Switzerland Aldrich, U.S.A. Aldrich, U.S.A. Fluka, Switzerland Fluka, Switzerland E. Merck, Switzerland Aldrich, U.S.A. Aldrich, U.S.A. Aldrich, U.S.A. Aldrich, U.S.A. B.D.H., U.K. Fluka, Switzerland Aldrich, U.S.A. Aldrich, U.S.A. Aldrich, U.S.A.
72
S.K. QUADRI E T AL.
The sample should be free of dissolved gases so as to obtain the 'true' Tc and Pc. For this purpose the sample was degassed (or de-aerated) in the experimental tube by successive freeze-melt cycles under vacuum to remove non-condensable gases. Each substance was frozen with liquid nitrogen and evacuated. The successive freeze-melt cycles were continued until no gas evolved upon melting. Then mercury was slowly and carefully transferred on top of the frozen sample under vacuum. The trapped air bubble between mercury and the wall of the tube was removed by inserting a fine steel capillary. After degassing the sample and transfer of mercury in the experimental tube, the tube was fastened in one leg of a mercury-in-steel U-shaped compressor block as shown in figure 2. The other leg 2 of the block was connected to a source of highpressure dry nitrogen gas for pressurizing the system. In preparation for the determination of the critical constants, the pressure was adjusted using needle and solenoid valves to the predicted critical pressure and the heating block was placed around the experimental tube and positioned so that the sample in the tube was in line with the slits of the block. The furnace was heated to 800 K on top of the assembly, then brought down around the heating block and the tube. Within 3 min the heating block attained the desired temperature after which the furnace was raised to its original position. Simultaneously, the sample was heated. The disappearance and reappearance of the meniscus were visually observed in the middle of the tube clearly within the field of vision on both the heating and cooling modes at a unique temperature, the critical temperature. The pressure corresponding to this critical temperature was the critical pressure. For each filling of the experimental tube with the sample, a number of measurements ranging from 7 to 10 of T~ and Pc were made by observing the disappearance and reappearance of the meniscus. In most cases the transition was sharp, and reproducible results within +0.3 K for T~ and _+0.01 MPa for Pc were obtained. This indicated that the samples were stable and did not decompose with the repeated heating and cooling cycles. For mildly unstable substances such as branched alkanols the earliest temperature of disappearance and reappearance was the same but successive values changed with time. Therefore, the earliest disappearance and reappearance temperatures were averaged and used as the criteria for the critical point. For rapidly decomposing substances such as decan-l-ol, reappearance of the meniscus was much more difficult to detect, because of the formation of a milky band and dissociation. Therefore, the disappearance of the meniscus was used as the criterion for the critical point which was reproducible within _+0.5 K for Tc and _+0.02 MPa for p~. Decomposition of the sample resulted in general in increasing the Pc and decreasing the Tc values as a function of time. This was observed for hexan-3-ol, octan-l-ol, octan-2-ol (slight decomposition), 3-methyl-pentan-3-ol, nonan-l-ol, and decan-l-ol (rapid decomposition), and hence for one sample charge in the tube, only one experimental observation was made by one heating and cooling cycle. Repeated measurements were carried out with a fresh filling. Reproducible results were thus obtained.
73
T~ AND p~ FOR ALKANOLS
The determination of critical temperature and critical pressure of a few slowly decomposing and rapidly decomposing samples required some stability test. To carry out this test we collected the sample after measurements in a small sample bottle through a 250 mm 3 hypodermic syringe with a 0.5 m long needle. The sample was analysed by using a glass capillary column of i.d. 0.75 mm, length 60 m, phase SPB-1, film thickness 1 gm (Supelco) at column temperature 453 K with f.i.d. Results obtained before and after measurement by g.c. are reported in table 3. If the chromatogram of the sample before and after the measurement was the same within the g.c. reproducibility, then the substance was considered stable (figures 3 and 4).
4. Results Experiments were carried out with n-pentane, n-hexane, n-heptane, and octan-1-ol in order to check the performance of the apparatus. The pressure as determined by the pressure transducer was corrected for the difference in the levels of mercury (corrected to standard temperature of 273.15 K) in the experimental tube and the compressor block, as well as for the partial pressure of mercury calculated by the equation proposed by Kay: ~8) lg(p/MPa) = 3.76862- 3039.30-
K/T,
(1)
where T is the temperature and p is the partial pressure of mercury. Some experiments were carried out with mercury within the heating zone and also outside the heating zone to check whether it was the vapour pressure of mercury or the partial pressure of mercury which affects the critical pressure. It was confirmed that equation (1) was applicable and hence it wa~ used in correcting the critical-pressure values as shown by the transducer. As stated earlier it was observed that temperature measured within the sample tube at the meniscus were always lower than those measured at the thermocouple well by 0.3 K to 0.5 K for most of the alkanols except for the C8 alkanol where it was 0.6 K and for C 9 and Clo alkanols, it was 0.8 K. The measured thermocouple-well temperatures were accordingly corrected. These corrected Tc values as well as Pc values are reported in table 2 for the alkanes and alkanols and are compared with reliable literature values ~1°-14) and those predicted using the methods of Ambrose o, 15) and of Somayajulu. ~16) The estimated accuracy in measured T¢ and Pc values for each compound are also reported in table 2. The results of stability tests by g.c. before and after 1 measurement and 3 to 5 measurements are given in table 3 as well as the chromatograms in figures 3 and 4.
5. Discussion The accuracy of the T~ measurements using the present apparatus is about + 0.7 K arid of Pc about _0.03 MPa. This was probably due to the method of heating the sample by which the temperature of the sample did not remain steady for a reasonably long period. This "unsteady state" in the sample tube might be responsible for higher inaccuracies in the measurements as compared with other
s . K . Q U A D R I E T AL.
74
TABLE 2. Experimental values of the critical temperature and critical pressure of the alkanes and alkanols with their estimated accuracies
Tc/K
Compound This work
lit.
n-Pentaue n-Hexane n-Heptane Pentan-2-ol
469.7_+0.4 507.5 +__0.4 540.4_+0.4 560.0 __+0.5
2-Methylbutan- 1-ol 2-Methylbutan-2-ol 3-Methylbutan-l-ol 3-Methylbutan-2-ol Hexan-l-ol Hexan-3-ol 2-Methylpentan-l-ol 2-Methylpentan-3-ol 3-Methylpentan-3-ol Heptan-l-ol
575.4 + 0.5 543.7-t- 0.5 577.2+0.5 556.1+0.5 610.4_+0.6 582.2-t-0.6 604.4+0.5 576.0_+0.5 575.6_+0.6 632.0+0.6
Octan-l-ol
652.2___0.6
Octan-2-ol
630.2+0.6
Nonan-l-ol
671.3-t-0.8
Decan-l-ol
687.7__+0.8
469.69" 507.70" 540.30" 560.40 b 560.40 c -545.0 ~ 579.40 e -611.35 c 582.48 c ---632.30 b 632.10 c 652.501 650.56 ~ 638.00 629.70 c 671.0 ~ 670.52 ~ 687.0" 687.30 c
a
pc/MPa lit.
predicted Ref. 15 Ref. 16
This work
predicted Ref. 15 Ref. 16
469.2 507.0 539.9 558.7
469.7 507.0 539.9 561.7
3.36___0.01 3.364a 2.99__+0.01 3.010 a 2.73+0.01 2.756" 3.64 ___0.02 --
3.34 3.00 2.71 3.87
3.34 3.00 2.71 3.77
576.8 540.6 580.7 548.2 611.0 578.63 603.4 568.2 563.1 632.9
576.8 536.7 576.5 551.7 611.0 585.0 602.2 568.23 562.7 633.7
3.94 _ 0.02 3.71 + 0.02 3.93___0.02 3.87+0.02 3.42+0.02 3.36___0.02 3.45__+0.02 3.46 +0.02 3.52-1-0.02 3.16_+0.02
----------
4.04 4.05 3.91 3.87 3.47 3.54 3.60 3.65 3.72 3.12
3.91 3.78 3.91 3.78 3.52 3.37 3.54 3.38 3.42 3.18
653.5
655.5
2.85+0.02
2.8601
2.84
2.89
630.1
632.1
2.78+__0.02
--
2.81
2.80
673.7
671.1
2.51+0.03
--
2.59
2.62
694.0
687.0
2.31+_0.03
--
2.39
2.39
Reference 10. b Reference 5. c Reference 7. d Reference 11. e Reference 12. y Reference 13. g Reference 14.
methods.
However,
3-methyl-pentan-3-ol,
for substances nonan-l-ol,
we were able to obtain
rapidly
decomposing
and decan-l-ol,
reproducible
and
showed
t h a t all t h e c o m p o u n d s was reduced
near
r e l i a b l e v a l u e s o f Tc a n d
specified accuracy. The detailed analysis of chromatograms decomposition
at and
and
Pc w i t h i n
the
( t a b l e 3, f i g u r e s 3 a n d 4)
were stable at the first experimental
due to the method
To, s u c h a s
used in the present investigation,
rapidity
run. Thermal
of heating.
As the
TABLE 3. Mole-fraction purities x and comparison of experimental values of the critical temperature and critical pressure of the alkanols initially, after one, and after three-to-five measurements Compound
initial 102X
after 1 measurement 102X TdK po/MPa
after 3 to 5 measurements 102x Tc/K p¢~MPa
2-Methylbutan-2-ol 3-Methylbutan-2-ol Hexan-3-ol 3-Methylpentan-3-ol Octan-2-ol Nonan-l-ol Decan-l-ol
99.97 99.83 99.42 99.99 99.99 99.7 98.98
99.8 99.8 99.32 99.74 99.83 99.4 98.67
99.66 99.78 99.31 99.1 99.54 99.0 98.47
543.7 556.1 582.2 575.6 630.2 671.3 687.7
3.71 3.87 3.36 3.52 2.78 2.51 2.31
543.7 556.1 582.1 574.8 630.1 671.3 687.6
3.72 3.88 3.37 3.59 2.80 2.55 2.41
T~ AND Pc FOR ALKANOLS
75
(a), x = 0.997 (b), x = 0.994 (c), x = 0.990 FIGURE 3. Stability test on nonan-l-ol. Chromatograms (a), prior to measurement; (b), after one measurement; and (c), after three measurements.
mercury-to-sample interface in our apparatus was at the sample temperature, it was necessary to correct the Pc values for the partial pressure of mercury. The T~ and Pc values for 2-methyl-butan-l-ol, 3-methyl-butan-2-ol, 2-methylpentan-l-ol, 2-methyl-pentan-3-ol, and 3-methyl-pentan-3-ol, a n d Pc of other alkanols, were measured for the first time in the present investigation. In general, the critical temperature of the alkanols decreased but the critical pressure increased with time due to thermal decomposition. This p h e n o m e n o n was m o r e rapid as the chain length increased. Thus the change in values with time for d e c a n - l - o l was greater than that for n o n a n - l - o l (decan-l-ol is more rapidly ,decomposing than nonan-l-ol). Finally, our results agree with those found in the literature, except for the thermally unstable higher alkanols. As established in the stability test the samples were stable at the first experimental run; we believe that the experimental technique used in this work gives reliable results within the specified accuracy.
(a), x = 0.9898 (b), x = 0.9867 (c), x = 0.9847 FIGURE 4. Stability test on decan-l-ol. Chromatograms (a), prior to measurement; (b), after one measurement; and (c), after three measurements.
76
S.K. QUADRI E T AL.
This w o r k was c a r r i e d o u t u n d e r the N a t i o n a l D a t a P r o g r a m m e f u n d e d b y the D e p a r t m e n t of Science a n d T e c h n o l o g y , N e w Delhi, I n d i a .
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Kobe, K. A.; Lynn, R. E. Chem. Rev. 1953, 52, 117. Kudchadker, A. P.; Alani, G. H.; Zwolinski, B. J. Chem. Rev. 1968, 68, 659. Hicks, C. P.; Young, C. L. Chem. Rev. 1975, 75(2), 119. Quadri, S. K.; Kudchadker, A. P. Measurement of the Critical Constants of Organic Substances--State-of-the-Art Report. National Data Program: D.S.T. 1989. Smith, R. L., Jr.; Anselme, M.; Teja, A. S. Fluid Phase Equilibria 1986, 31, 161. Smith, R. L., Jr.; Teja, A. S.; Kay, W. B. AIChe Journal 1987, 33,232. Anselme, M.; Teja, A. S. Fluid Phase Equilibria 1988, 40, 127. Kay, W. B.; Sung, C. P. 3". Chem. Thermodynamics 1980, 12, 673. Ambrose, D. Correlation and Estimation o f Vapour-Liquid Critical Properties. 11. Critical Pressure and Critical Volume of Organic Compounds. NPL Report Chem 98, 1979. Ambrose, D.; Townsend, R. Vapour-Liquid Critical Properties. NPL Report Chem 107, 1980. Brown, J. C. J. Chem. Soc. 1906, 89, 311. Kr~glewski, A. Roczniki Chem. 1955, 29, 754. Ambrose, D.; Broderick, B. E.; Townsend, R. J. Appl. Chem. Biotechnol. 1974, 24, 359. Ambrose, D.; Ghiassee, N. B. J. Chem. Thermodynamics 1990, 22, 307. Ambrose, D. Correlation and Estimation o f Vapour-Liquid Critical Properties. 1. Critical Temperature of Organic Compounds. NPL Report Chem 92, 1978. Somayajulu, G. R. J. Chem. Eng. Data 1989, 34, 106.