I d . Eng. C b m . Fundam. 1981, 20, 315-317
315
Superheat-Limit Temperatures of Polar Liquids Jennie R. Patrlck-Yeboah and Robert C. Reld' Department of Chemlcal Engineering. Massachusetts Instttute of Technobgy, Cambr&jge,Massachusetts 02139
The superheat-limit temperatures (SLT) of seven highly polar liquids and four polar liquid binary mixtures were measured at 1 bar pressure in a bubble column. For all liquids except acetic acid, the reduced SLT values were between 0.91 and 0.93. Mixture SLT were close to a mole fraction average of values for the pure components. Estimates of the SLT values using equations of state and chsslcal thermodynamic stability theory gave values slightly higher than those measured experimentally.
It is well established that liquids may be heated above their expected boiling temperatures if surface nucleation sites are minimized (Skripov, 1974; Blander and Katz, 1975). The maximum degree of superheat, termed the superheat-limit temperature or SLT, can be estimated from classical thermodynamic stability theory or from kinetic theory models. In the laboratory, the SLT is normally measured by heating small liquid drops that are immersed in an immiscible, hotter liquid. In this case there are few nucleation sites at the drop boundary and superheating is relatively easy to achieve. Unless some event triggers nucleation, boiling does not occur until the drop temperature attains the superheat-limit value. The rapidity of the vaporization of a liquid at or near its SLT had led to the supposition that a number of laboratory and industrial explosions may involve superheated liquids (Reid, 1976,1978). Such events are normally associated with the rapid contacting of two immiscible liquids, one cold and volatile while the other is hot and relatively nonvolatile. At the boundary between the two liquids, nucleation is suppressed, and the colder liquid is superheated. If the SLT is attained a local explosive vaporization results, and this may act as a trigger to mix further the hot and cold liquids and yield high rates of heat transfer with boiling. The violence and potential hazard of such an event is strongly dependent upon attaining the SLT in the colder liquid. If the hot liquid temperature is less than the SLT of the colder, then it is impossible to heat the colder liquid to its SLT, and any vaporization which eventually occurs is far less violent. It is therefore important to be able to estimate the superheat-limit temperature of a liquid or liquid mixture in order to assess the potential hazard in any operation where there is a possibility of this colder liquid contacting a hotter one. While there are sufficient experimental data (with comparisons to theory) to provide confidence in estimating the SLT of most nonpolar liquids, almost no data exist for polar liquids. To measure and correlate the SLT values of several typical polar liquids was the principal objective of this work. Experimental Section SLT values were measured in a bubble column (Moore, 1959; Wakeshima and Takata, 1958) shown schematically in Figure 1. The column was 2.8 cm in diameter and 50 cm in length. It was f i e d with an appropriate host liquid and heated externally with spiral-wound Nichrome resistance wire. A close-fitting insulating Pyrex jacket (not shown) extended over the entire length. The column was capped with a charcoal bed to adsorb vapor of the heated host liquid. 0196-4313/81/1020-0315$01.25/0
The pitch of the Nichrome spiral was carefully adjusted to yield a smooth vertical temperature profile. It was noted that any sharp variations in temperature resulted in spurious nucleation of the test liquid droplet even at temperatures well below the expected SLT. After a stable temperature profile was established in the column (with the temperature at the top above the expected SLT of the test liquid and, at the bottom, below the normal boiling point of the test liquid), drops of test liquid were injected through a side port near the bottom. The host fluid was chosen to be immiscible with-and more dense than-the test liquid, so the test-liquid drops (99.9% pure); however, the dissolved gas content was unknown. Forest and Ward (1978) and Mori et al. (1976) among others have shown that the presence of any significant amount of dissolved gas could profoundly decrease the SLT of a liquid. Thus we carried out our experiments, in moat instances, with thoroughly degassed liquids. The procedure used was modified slightly from that used by Ronc and Ratcliffe (1976). The test liquid was filtered under vacuum into a flask. The vacuum was maintained at the vapor pressure of the liquid for several hours; some test liquid was vaporized in this step. The flask was immersed in liquid nitrogen and the contents were frozen. Thawing was carried out slowly and under vacuum. Dissolved gas would appear as small bubbles 0 1981 American Chemical Society
316 Ind. Eng. Chem. Fundam., Vol. 20, No. 4, 1981
Table I. Superheat-Limit Temperatures (SLT) for Pure Liquids at 1 Bar est. from thermodynamics
Charcoal Adsorber
exptl Tr =
Thermocouple
T, K
T/T,
468 470 493 473 497 489 526 421 488 504
0.91 0.91 0.92 0.93 0.91 0.91 0.88 0.90 0.90 0.90
modified Soave, Berthelot,
K
K
~~
Exploding Droplet increasing Temperature Gradi en t
Heating Medium (Host Fluid)
Nichrome
/ Heating Wire
t-Syringe
Teflon Stopcock
-&+4-
Rubber Septum
Figure 1. Bubble column. One Test
?-Pentane
n-HeDtane
Temperolure
Benzene
K
Figure 2. Superheat limit temperatures for nonpolar liquids.
during the warming step. The freeze-thaw cycle was repeated a t least three times-more if bubbles were still evident. The final degassed liquid was withdrawn from the flask with an air-tight syringe through a rubber septum. The procedure was slightly modified for liquid mixtures (see Patrick-Yeboah, 1979). Results Three nonpolar liquids (n-pentane, n-heptane, and benzene) and seven polar liquids (methanol, ethanol, 1propanol, 2-propanol, acetic acid, acetonitrile, and acrylonitrile) were studied. Binary mixtures of acetic acid with both methanol and ethanol as well as the acetonitrileacrylonitrile and ethanol-acetonitrile pairs were included. The pressure was 1 bar. While it would have been most gratifying to have had all superheat-limit temperature measurements identical, in actual fact a range was found. For each liquid studied, a large number of tests were carried out. Typical results in the form of histograms are shown for the three nonpolar liquids in Figure 2. The highest measured temperature was selected as the SLT. These are shown in Table I. For nonpolar liquids, no difference was noted between degassed liquids and undegassed. When comparing the nonpolar SLT values with those in the literature, for benzene, the 504 K result is slightly higher than the 499 K value of Sinitsyn and Skripov (1968). Much lower values were reported by Sinha and Jalaluddin (1961) and Kenrick et
methanol ethanol 1-propanol 2-propanol acetonitrile acrylonitrile acetic acid n-pentane n-heptane benzene
477 481 500 474 503 493 550 430 498 513
477-483 502 476 503 493 550 432 499 514
al. (19241, who used different experimental techniques. The measured SLT for n-pentane (422 K) is 1' higher that reported by Blander et al. (1971). Again, earlier investigators (Wakeshima and Takata, 1958; Skripov and Kukushkin, 1961; Skripov and Ermakov, 1964; Siniteyn and Skripov, 1968)report slightly lower values. (The range was from 417 to 419 K.) For n-heptane our value of 488 K for the SLT is in good agreement with the 487 K reported by Eberhart et al. (1975) and Skripov and Ermakov (1964). For the polar liquids, degassing usually led to higher superheat limit temperatures and, therefore, only the degassed-liquid values are shown in Table I. For 2-propanol, acetonitrile, acrylonitrile, and acetic acid, no prior data were located. The acetic acid SLT may, in fact, be low. A large number of experiments were conducted and nucleation very often occurred at temperatures below the 526 K maximum. We did try to measure the SLT for water, but spurious nucleation inevitably resulted at temperatures well below the expected SLT. Our highest temperature was 516 K-well below the value of 553 reported by Apfel (1972). For methanol, the SLT shown in Table I is 468 K. Eberhart et al. (1973) found a value of 459 K using a method which involved heating the alcohol in a capillary tube until it explosively boiled. Kenrick et al. (1924) reported an SLT of 453 K while employing a similar technique. For ethanol, we found an SLT of 470 K while Eberhart et al. (1973) and Kenrick et al. (1924) published values of 463 and 474 K, respectively. Skripov and Pavlov (1970) suggested a method by which the SLT could be inferred by monitoring the explosive boiling near a rapidly heated, s,ubmerged wire. Their values for methanol, ethanol, and 1-propanol are 466,472, and 496 K; these agree quite well with the values shown in Table I. An SLT histogram for the acetonitrile-acrylonitrile binary is shown in Figure 3. The mixture SLT is close to a mole-fraction average of the SLT values of the pure componenb. Similar results were found for the other polar mixtures studied. Estimation of SLT Values from Thermodynamics Using the stability criterion outlined by Beegle et al. (1974) and estimating liquid P-V-T properties with both the Redlich-Kwong-Soave (Soave, 1972) and modified Berthelot (Eberhart, 1976) equations of state, SLT values were estimated for all pure liquids. These calculated values are shown in Table I. In all cases,the thermodynamic SLT values exceeded those found experimentally. Assuming these equations of state do, in fact, provide an adequate P-V-T model for the superheated liquids, it is not unex-
Ind. Eng. Chem. Fundam., Vol. 20, No. 4, 1981 917
Table 11. Experimental and. Thermodynamic Superheat-Limit Temperatures for Liquid Mixtures of Acetonitrile-Acrylonitrile at 1 Bar Pressure SLT (K) at various kti‘s and with RKS and MB equations of state 0.0
mole fraction acetonitrile 0.00 0.13 0.49 0.82 1.00 a
exptl, K 489 489 491 492 491
RKS a 493 495 498 502 503
Redlich-Kwong-Soave equation of state.
IO0
70
0.05
MB b 493 495 498 501 503
L
401 30
IO
IL
i-
‘ 0 460
I
470
I
1
480 490 500 Temperature, K
1
RKS
MB
RKS
MB
492 492 498
490 489 496
489 485 494
486 419 490
Modified Berthelot equation of state,
r
t
0.1
I
510
Figure 3. SLT values for the acetonitrile-acrylonitrile binary.
pected to find the thermodynamic SLT higher than experimental. The former should represent the ultimate SLT limit whereas the latter reflect experimental uncertainties such as nucleation from small motes, stray radiation, etc. Also, one would not expect the equations of state tested to model accurately the P-V-T properties of highly polar liquids; at the present time, however, no better equations of state exist. When using the equations of state to predict mixture SLT values, a binary interaction parameter, ki.,must be specified. Since k , values were not available /or the binaries studied, we carried out a parametric analysis using values of 0, 0.05, and 0.1. We show the results for the acetonitrile-acrylonitrile binary in Table 11. Increasing kij lowered the predicted SLT. Also, for this binary a kij value of about 0 would give the same ratio of predicted to experimental values of SLT. It is interesting to note that for low kij, the predicted mixture SLT is almost a mole fraction average of the pure component predicted values;
this is not true at higher values of kij where a slight minimum SLT is found in the mid-composition range. Concluding Remarks . A bubble column study of the superheat-limit temperatures of polar fluids indicated that the 1 bar SLT values occurred at reduced temperatures of about 0.91-0.93, somewhat higher than for nonpolar liquids. Mixture SLT results were essentially mole fraction averages of the pure component values. Thermodynamic modelling with two equations of state yielded SLT values a few degrees higher than those found experimentally. Finally, degassing was found to have a strong effect on polar liquid SLT but not on nonpolar results. Acknowledgment The financial support of the National Science Foundation under Grant ENG 75-21977is gratefully acknowledged. J. R. Patrick-Yeboahwas also partially supported by the Ford National Fellowship Foundation. Literature Cited Apfei, R. E. Net. phys. Scl. 1972, 238, 63. Beegle, B.; Modell, M.; Reld, R. C. AIChEJ. 1974, 20, 1200. Blander. M.; Hengstenberg, D.; Katz, J. L. J. phys. Chem. 1971, 75, 3613. Blander, M.; Katz, J. L. A X E J. 1975, 27, 833. Eberhart, J. 0. J. colloid Interfece Scl. 1976, 56, 262. Eberhart, J. G.; Hathawayand, E. J.; Blander, M. J. Wbkf Interfew Scl. 1873, 44, 389. Eberhart, J. G.; Kremsner, W.; Blander, M. J . Co/bki Interface Sci. 1975, 50, 369. Forest, T. W.; Ward, C. A. J . Chem. phys. 1978, 69, 2221. Kenrlck, F. B.; Gllbert, C. S.; Wismer, K. L. J . phys. Chem. 1924, 28, 1297. Moore, G. R. AIChE J. 1959, 5 , 458. Morl, Y.; Hljlkata, K.; Nagatanl, T. Int. J. M a t Mass Transfer 1976, 79,
1153.
Patrick-Yeboah, J. R. Sc.D. Thesis, Massachusetts InstlMe of Technology, -. Cambridge, MA, 1979. ReM, R. C. Am. Sci. 1976, 84, 146. ReM, R. C. Chem. Eng. Educ. 1978. XII(2), 60; (3), 108; (4), 194. Ronc, M.; Ratcllffe, 0. R. Can. J. Chem. Eng. 1976, 54, 326. Sinitsyn, E. N.; Skripov, V. P. Russ. J. phys. Chem. 1966, 42(4), 440. Slnha, D. 6.; Jalaluddln, A. K Ind. J. phys. 1961, 35, 311. Skripov, V. P. “Metastable Llqulds”; Wlley: New York, 1974, pp 55-82. Skripov, V. P.; Ennakov. G. V. Russ. J. phys. Chem. 1964, 38, 208. Skripov, V. P.; Kukushkln, V. 1. Rws. J . phys. Chem. 1961, 35, 1393. Skripov, V. P.; Pavbv, P. A. Temp. 1970, 8, 782. Soave, G. Chem. Eng. Sci. 1972. 27, 1197. Wakeshima, H.; Takata, K. J. phys. Soc.Jpn. 1956, 13, 1398.
Received for review August 19,1980 Accepted June 15, 1981
