INDUSTRIAL A N D ENGINEERING CHEMISTRY
2128
Entropy values were determined from the enthalpy informal against H a t constant pressure.
Graphical integration under such curves lead to entropy values, as indicated by the expression
+ 7.653(-57" to 12.8'C.)
Maas and Boomer
log p =
Moor et al.
l o g p = __
Coles and Popper
log p =
tion by preparing plots of
Vol. 42, No. 10
- 1410 + 7.839 (-5' - 1355 + 7.659 (0'
to 40" C.) to 32' C.)
In these equations the vapor pressure, p , is in mm. ot mwcury and temperature is K. The entropy results are also shown in Tables I and 11.
LITERATURE CITED
VAPOR PRESSURE DATA
From the measured enthalpy-pressure data shown in Figure 3 it was possible to evaluate vapor pressures of ethylene oxide from 120' to 300' F.with m estimated accuracy of 2 pounds per square inch absolute except at the highest temperatures where the errors may be somewhat larger. These results are compared with those of Mans and Boomer (7), Moor et al. (a), and Coles and Popper (2) in Figure 5. The several sets of information agree well. The solid line represents a correlation (by method of least squares) of the data obtained in this investigation and has the following equation: log p =
-
T 4-7.72 (49' to 127" C.)
(2)
(1) Christensen. L. D., and Smith, J. M., IND. ENG.CHEM.,42, 2128 (1950). (2) Coles, K. F., and Popper, F., Ibid., 42, 1434 (1950). (3) Giaque, W. F., and Gordon, J., J. Am. Chem. Soc., 71, 2176 (1949). (4) Godnev, J., and Morozov, V., J. Phw. Chim. (U.S.S.R.), 22, 801 (1948). (5) G i h t h a r d , H., and Heilbronner, I., Helv. Chirn. Acta, 31, 2128 (1948). (6) Hess, L. G.. and Tilton. V. V., IND.ENQ.CHEM..42, 1251 (1950). 44, 1709 (1922). (7) Maas, O., and Boomer, E. H., J. Am. Chem. SOC., (8) Moor, V. G., Kangs, E. K., and Dobkin, J. E., Trans. Bzptl. Reeearch Lab., Khengas (Leninjrod),3, 320 (1937). (9) Moureu, M., and Dode, M., Bull. SOC.Chim.,(5) 4,637 (1937).
(10) Nelson. J. M.. and Holcomb, D. E., Ph.D. thesis of J. M. Nelson, Purdue University (1949). (11) Prengle, W. H., Greenhaus, L. R., and York, R., Chem. Eng.
Progress, 44, 863 (1948). (12)
The equat#ionspresented by the previous authors are:
Smith, J. M., Ibid., 44,521
(1948).
RECEIVED April 12, 1950.
Pressure-Enthalpy Diagram for Acetaldehyde J
L. D. CHRISTENSEN AND J. M. SMITH Purdue University, Lafayette, Ind. Enthalpy data were determined calorimetrically for acetaldehyde from 180" F. to 300' F. and 60 to 400 pounds per square inch absolute with an estimated accuracy of 2%. . - Entropy _ _ values were computed from the enthalpy measurements, and all the results are summarized in tables and on a pressure-enthalpy diagram. Both vapor and liquid regions were investigated.
I
N A COMPANION paper (6) enthalpy and entropy data were presented for ethylene oxide. In this paper similar information is given for a r ~ t a l dehyde. As for ethylene oxide, the thermodynamic data for acetaldehyde are meager. However, specific heats, enthalpies, entropies, and free energies have been computed in the ideal gas state by Pitrer (6) and by Smith (8). In addition, specific heats have been recently determined at atmospheric pressure and over a small temperature range by Coleman and DeVries (2). Coles and Popper (3) have compared existing vapor pressure data with new information of their own which extends up to a temperature of 32' C. In this investigation enthalpy and entropy values were determined from 180' to 300" F. at pressures from 60 to 400 pounds per square inch absolute. The measurements were limited to an upper temperature of
300' F. because of the tendency for acetaldehyde to react at high temperatures. EXPERIMENTAL METHOD AND ACCURACY OF RESULTS
The methods and equipment used were the same as those emin the study of ethylene oxide and are described in the -ployed previous paper (6): However, it was necessary to double-distill
INDUSTRIAL AND ENGINEERIN G CHEMISTRY
October 1950
2129
Figure 1 was prepared on a temperature-enthslpy diagram. The final results are given in Table I for saturated liquid and vapor stated and in Table I1 for the superheated vapor regions These data are also illustrated graphically on a. pressure-enthalpy diagram in Figure 2. The dotted curves on Figure 2 refer to extrapolated regions. Entropy values were determined from the enthalpy data by graphical integration in accordance with the equation
a
sn - 8,
--
I -
(constant pressure)
(1)
The results of these calculations are included in Tables I and I1 and shown as lines of constant entropy in Figure 2.
--
.ENTHALPY, BTU/LB
k2
Figure 2. Pressure-Enthalpy Diagram for Acetaldehyde
COMPARISON WITH EXISTING DATA
Vapor pressures from 60 to 400 pounds per square inch absolute were evaluated from the pressure-enthalpy data shown in Figure 1 with an estimated accuracy of 3 pounds per square inch absolute. The results are compared with the low tcinperature data of Coles and Popper (8)in Figure 3. From this figure it is apparent that a straight line on a plot of log p against 1/Tfits both sets of data approximately, but the line lies slightly below the experimental results a t intermediate pressures and above at high pressures. This type behavior is exhibited by other oxygenated compounds such ~9 water and methanol. On the other hand the straight-line relation fib the data for hydrocarbons well. The specific heat of acetaldchgde vapor determined by Cole-
TABLE I. T
Figure 3.
ture. 0
Log Vapor Pressure os. 1/T for Acetaldehyde
F.
160 180 200 220 240 260 280 300
~
PROPERTIES OF
SATURATED A~CETALDEHYDE
Enthaliw, Entrow, B.t.u./Lb./O F: pressure ~ ~ ~ B.t.u./Lb. ~ ~ Lb./Sq.' B-aporiSstd. Satd. Vapori- Satd. Inch Aba. liquid eation vapor liquid ration vapor 49 242 0 IO86 0.311 0.397 62 244 0.105 0.284 0.389 74 245 0.123 0.259 0.382 86 246 0.141 0.234 0.375 99 246 0.159 0.210 0,369 112 245 0.177 0.186 0.363 126 0.196 0.161 0.357 244 142 0.218 0.131 242 0.349
the acetaldehyde immediately before introduction to the system in order to be certain of the purity of the material used in the runs. In the distillation all the material not boiling between 20.7" and 20.8O C. was discarded. Also it was necessary to make check runs ~eriodicallyat a standard low temDerature todetermine when a fresh sample of material Rhould be introduced into the apparaVAPOR TABLE 11. PROPERTIES OF ACETALDEHYDE tus. The Drocedure followed was to the equipment-and add a new sample whenever the Te np.. Pressure Lb./Sq.' Tzm Satd. Satd. __ check runs indicated a deviation of more than Inch A b . Vapor 180 ZOO 220 240 269 khthalnv (H), B.t.ii./LI). 0.5% from the standard value. In view of these precautions and the accuracy of the calibration 60 148 242 258 265 273 281 289 100 175 243 249 261 270 279 288 tests with n-butane (6),it is believed that the 140 196 244 249 264 276 286 measured enthalpies are accurate within 2%. 180 217 245 250 268 281 220 234 246 254 273 259 The tendency of acetaldehyde to react chemi260 250 246 300 266 245 calIy was more pronounced than for ethylene 340 279 244 oxide, especially at the higher temperatures. 380 291 243 O
F?
400
297
60 100 140 IS0
146 175 196 217
300 340 380
:
243
300
298 297 296 293 287 277 264
308 308 807 305 302 296 287 274 258 248
Entropy (SI,B.t.u./Lb./O F.
RESULTS
The measured enthalpy differences were adjusted to a base state (H = 0, S = 0 ) of saturated liquid a t 78" F. (vapor pressure = 17.8 pounds per square inch absolute). These experimental results are shown on prewum-enthalpy coordinates in Figure 1. To obtain smoothed values of entblpy, a cross plot of
280
;;: 400
234
$5;
0.403 0.430 0.391 0.398 0.383 0.376
9.311
5:i:
0.442 0.418 0.390
, 0 .500
0.483 0.471 0.459 0.447 0.433 0.417 0.397 0.372 0.317
2130
INDUSTRIAL AND ENGINEERING CHEMISTRY
man and DeVries (8) a t atmospheric pressure increased from C, 0.346 calorie per gram per O C. a t 99.5' C. to C, = 0.370 a t 149.2O C, Although the enthalpy data measured in the present work cannot be used to obtain accurate valuesof the this quantity can be estimated from Table 11. At the lowest pressure, 60 pounds per square inch gage, the average value between 82.20 and 148.90 c. (1800 to 3000 F,) is 0.38 calorie per gram per a C. Since a significant increase in specific heat from 14.7 to 60 pounds per square inch absolute pressure would be expected for acetaldehyde, these two results are in reasonable agreement. ip
Vol. 42, No. 10
BIBLIOGRAPHY (1) Berthelot, Ann. C h h . Phus., (SI9, 178 (1876). (2) Colemanl c. F.v and DeVriesv T., J - Am. C h m . S0C.t 71, 2839 (1949). (3) Coles, K. F., and Popper, F., IND.ENO.CHEM., 42,1434 (1950). (4) Gilmour, R., J . Soc. Chem. I&., 41,293 (1922). (5) Mock, J. E., and Smith, J. M., IND.ENQ.CHEY.,42, 2125 (1950). (6) Pitzor, K. S., and Weltner, J . Am. C h m . soc., 71, 2842 (1949). (7) Roth, A., L a n d o l t J h m t e i n Tabellen, 6th ed., supp. I, Julius Springer, Berlin, 1936. (8) Smith, J. M., TTans. Am. Inst. chm. ~ n g r ~42,983 ., (1946). RECEIVED April 12, 1950.
Heating and Ignition of Small Wood Cylinders '
J WALLACE L. FONS
California Forest and Range Experiment Station, Forest .Service, United States Department of Agriculture T h e literature provides limited information on the time of ignition of wood under conditions of rapid heating such as occur in forest and structure fires. An investigation was made of ease of ignition as affected by such physical properties of wood as initial temperature, size, and moisture content and by temperature of ambient gas or rate of heating. Temperature-time history curves are shown for two radii of wood specimens. With the temperature-time data and an equation for unsteady-state heat conduction, the surface temperature of wood when flame appears is calculated. With the method used, the surface ignition temperature of wood under conditions of rapid heating was found to be nearer 650' than the 550' F. temperature often quoted in the literature.
E
XPERIMENTAL determination of the ignition temperature and ignition period of solids is usually done at a low rate of heating. This paper presents a method and the results of an investigation using wood cylinders under rapid heating, like the conditions that occur in forest and structure fires. Only one species of wood was used, but the results establish some fundamental relations believed to be applicable in principle to all types and kinds of fuels. This information is useful in several fields. In studies of the ways in which forest fires start and spread (4, 6, 7), there is frequent need for accurate information on ignition temperature of forest fuels under conditions of rapid heating. Fire prevention and control work require knowledge of how ease of ignition is affected by physical properties of the fuel, such as initial temperature, size, and moisture content and by temperature of the ambient gas or flame. In studies of the use of water and chemicals for fire suppression, knowledge of temperatures associated with the appearance of flame and with glowing fuels is needed to determine the cooling necessary to prevent rekindling, particularly for forest-type fuels. Brown's summary ( 8 ) of studies by many investigators reveals that there is no general agreement as to the ignition temperature of any solid fuel. The principal sources of disagreement are (1)the definition of ignition temperature, which may refer to the surface or mean temperature of the specimen or to the external temperature of a bar, an ambient gas, or an oven and (2) the criteria used to indicate the ignition poet, such as appearance of glow or flame or some critical point on the time-temperature curve.
Landt and Hausmann ( 9 ) give ignition periods for several sizes of specially processed materials and for a few species of woods, presumably of one size, with several values of moisture content. METHOD
The method employed in this study ww to measure the time required for self ignition and the internal temperatures during the ignition period of wood cylinders inserted in an electric furnace. Air was admitted to the furnace through an opening in the door, The initial temperature of each specimen was known, and the furnace was maintained at a constant, known temperature considerably higher than the ignition temperature of the wood. The time interval between insertion of the specimen and first appearance of flame was recorded. The length of this interval was defined for the purposes of this experiment as ignition time. The temperature a t some point within each specimen was recorded throughout the heating interval. A series of these records for a number of specimens toget.her with the measurement of ignition time provided the experimental data from which surface temperature of the specimens a t ignition were calculated. APPARATUS
The apparatus used to measure ignition time and temperature (Figure 1) consisted of an electric furnace with No. 20 gage Chromel-Alumel thermocou lee in porcelain insulatin tubes, a precision potentiometer, a piotoelectric cell and amplifer, a strip chart recorder with two magnetic marker pens, a photoelectric recording potentiometer e uipped with one magnetic marker pen, and a device for inserting &e wood specimens. On both the strip chart recorder and recording potentiometer one pen was operated by a solenoid connected to two parallel electric circuits in such a way that the insertion device opened the solenoid a t the inatant the specimen entered the furnace. The amplified impulse from the photoelectric cell closed it again a t the instant the specimen was i nited. The second pen on the strip chart was operated by a soyenoid that was in series with a timer driven by a synchronous motor. this pen checked the speed of the chart by recording marks on the strip chart at 0.5-second intervals, The speed waa approximately 18 inches per minute. The specimen insertion mechanism consisted of a holder with an adapter for dserent size specimens, mounted on a V-shaped runner. This slid along a horizontal runway, about 21 inches in length, to an opening in the furnace throu h which the specimen passed. The runner was drawn by a weigft and was released by a hand-operated trigger. When the specimen was half wa in the furnace, the runner opened the circuit switch and recordelthe insertion time. The movement of the runner was sufficiently rapid
