Pressure-Enthalpy Diagram for Ethvlene Oxide J
J. E. MOCK AND J. M. SMITH Purdue Uniwrsity, Lqfoyette, I d .
R
A
LOW PRESSURE FEED TANK
C
HIOH PRESSURE SURGE TANK OIAPHRAQM BY-PASS VALVE RE REQULATINQ VALVE
D
H HEATER K INLET THERMOCOUPLE L LINE TO PRESSURE QAUQE M CALORIMETER N CaLLECTlNQ CONDENSER 0 REFLUX CONDENSER P TWO-WAY VALVE R FREON RECEIVER I) OUTLET THERMOCOUPLE T !XOW CONTROL N E E W W E U FLOWRATOR v SAMPLE BOMB W,X,Y,Z NEEDLE VALVES
ELATIVELY complete thermodynamic data, such as pressure-enthalpy or temperature-entropy diagrams, are available for only a few organic compounds containing oxygen. Such information was recently developed for methanol (fg),and as a continuation of this project, pressure-enthalpy diagrams are presented for ethylene oxide in this paper and for acetaldehyde in (I). The published information on ethylene oxide which is of value in preparing a tabulation of thermodynamic properties is limited to vapor pressures up to 40' C. (.a), a single value of the heat of vaporieation a t atmospheric pressure (7, 9),and thermodynamic properties in the ideal gas state evaluated from spectroscopic data (36). This latter information is not of value at present for determining properties a t finite pressures because of the lack of volumetric data. Therefore, the development of a pressure-enthalpy diagram required additional experimental data, In this investigation enthalpies were measured experimentally at pressures from 50 to 540 pounds per square inch absolute and temperatures from 120' to 300' F. in the liquid and vapor regions. Entropy values were computed from the enthalpy data. EXPERIMENTAL METHOD3
Enthalpy data were obtained directly in a flow calorimeter originally developed by Nelson and
Figure 1. Calorimeter Assembly
*,-
TO RECLUX CONDENSERS
r n
XNE OXIOE W
T
Enthalpy data were determined calorimetrically for ethylene oxide from 120' to 300' F. and 50 to 540 pounds per square inch absolute with an estimated accuracy of 2%. Both vapor and liquid regions were included in the investigation. Entropy values were calculated from the experimental data and the final results are presented in tables and on a pressure-enthalpy diagram. Vapor pressures evaluated from the experimental data from 120' to 300' F. were in reasonably good agreement with values predicted by extrapolating previously published information applicable a t lower temperatures.
A
INNER CALORIMETER GAR CUTER CALCRIYLTER CAN VAPOR LNTRAINMCNT SEPARATOR D VAPOR T H E R M O M E T C R L FREON VAPOR L I N K I TWO-WAY VALVE 0 FREON CONDENSCR H TREON-II LlOUlD
I)
C
ETHYLENE OXIDE
OU~LET
2125
e
J
CONDENSINO
* a R E o N
I(
WCIOHINd
L
COOLINQ COIL QLASI WOOL INSULATION FREON REFLUX LINE
Figure 2.
Calorimeter
2126
INDUSTRIAL A N D ENGINEERING CHEMISTRY
VoL 42, No. 10
were measured to 0.10' F. with copper-constantan thermocouples calibrated by comparison with a platinum resistance thermometer calibrated a t the U. S. Bureau of Standards. The pressure determinations were made with a Heise gage which had a 12-inch face and a range of 0 to 1000 pounds per square inch absolute. This gage was frequently calibrated for hysteresis with a dead weight gage. The precision of the pressure measurements was 0.5 pound per square inch absolute. The mass flow of ethylene oxide through the calorimeter and the quantity of Freon vaporized in the inner jacket were determined by weighing with a precision of 0.1 %. Purity of Materials. EthyIene oxide obtained from Carbide and Carbon Chemicals Company and having a purity of 99.5 weight % ENTHALPY 'BTU/LB was used for the calorimeter measurements. Figure 3. Pressure-Enthalpy Diagram for Acetaldehyde Freon-1 1 certified by Matheson Chemical Company to have a pirity of 99.9% was used without further treatment. The normal butane Holcomb (IO). The ethylene oxide was cooled a t constant presemployed in the calibration tests was obtained from Phillips sure conditions in the calorimeter, and the heat transfer was Petroleum Company. The n-butane content of this material measured by the quantity of Freon-I1 evaporated in the inner was greater than 99%. calorimeter jacket. Under the operating conditions used, kineticand potential-energy changes were negligible so that the measured ACCURACY OF RESULTS heat transfer was equal to the enthalpy difference between inlet and exit conditions of the calorimeter. The state of the ethylene To determine the suitability of the apparatus for measuring enoxide entering the calorimeter could be adjusted to any presthalpy data, a series of test runs was made using n-butane. This sure and temperature, but in every case the exit temperature w,w material was chosen because its vapor pressure was approximately approximately 78" F. the same R S that of ethylene oxide and because accurate thermodynamic data were available. The published information for nApparatus. The e uipment as a whole is shown in Figure 1. The start of the ethyqene-oxide system is storage tank A , from butane has been summarized by Prengle, Greenhaus, and York which the liquid ethylene oxide passed to piston pump B. The (11). discharge line from the pump was connected to surge tank C , On the basis of twenty test runs, the standard deviation of the while the remainder passed through valve E and on to preheater F . Preheater F consisted of a constant-temperature bath filled experimental results from those of Prengle et al. was 0.72%. This with Fino1 and was used to adjust the temperature of the eth lene would imply that 99% of the time the error would fall within the oxide to the desired value before it entered calorimeter M . Prom range -2.28 to 2.04%. In view of these results and the fact that the calorimeter the fluid moved through rotameter U , which was used as a qualitative indication of the flow rate, and finally into no individual deviation was more than 1.50%, the accuracy with sample bomb V . which enthalpies could be measured with the apparatus is believed The details of the calorimeter are illustrated in Figure 2. Ethto be 2% or better. ylene oxide passed through coil J which was made from stainless steel tubing, '/&-inch inside diameter. Surrounding the coil were two concentric jackets, A and B, both filled with Freon-11. The outer jackets served as a shield to prevent heat transfer between the surroundings and the inner jacket. Since the surrounding temperature was maintained above that of the calorimeter, the temperature in the outer jacket was maintained constant by vaporization of Freon. The vaporized material was returned to the jacket from reflux condensers operating a t atmospheric pressure. The only connection between the jackets was near the bottom through ports N . These openings permitted a constant liquid level in the two chambers but did not allow vapor passage. Since the temperature. in the two jackets was essentially the same, the vaporization of Freon in the inner one was entirely due to heat transfer from the ethylene oxide in coil
J.
Measurements. Temperatures and pressures entering and leaving the calorimet,er were determined at K and S (Figure 1). Temperatures
ENTHALPY
Figure 4.
- BTU/LB
Pressure-Enthalpy Diagram for Ethylene Oxide
I N D U S T R I A L A N b E N G I N E E R I N G CHEMISTRY
October 1950 TABLE
I.
PROPEaTIES OF
pressure, c
T$y.,
Lb./Sq. Inch
120 140 160 180 200 220 240 280 280 300
55 75 105 145 192 245 305 375 455 555
Ab.
2127
SATWAT5D ETHYLENE OXIDEI
-
Enthalpy, B.t.u./Lb. Entropy, B.t.u./Lb./O F. Satusatusatu88tU rated VapOI'i- rated rated VaPOri- rated liquid zation vapor liquid cation vapor 0.054 0.070 0.087 0.104 0.121 0.138 0.154 0.171 0.188 0.205
208 204 198 191 182 172 161 148 134 118
30 40 51 62 73 84 96 108 120 132
0.359 0.340 0.320 0.299 0.277 0.254 0.231 0,207 0.181 0.154
0.413 0.410 0.407 0.403 0.398 0.392 0.385 0.378 0.369 0.359
Because of the tendency of ethylene oxide to react a t high temperatures, the system was frequently purged and the runs continued with fresh material. ks a further check on the purity of the material in the apparatus a t any time, check runs were made a t a standard low temperature a t definite time intervals. These intervals were shortened as the operating temperature waa incrertsed, In no caae did the check runs a t low temperatures show deviations more than 0.5%. It is believed that the stability of the ethylene oxide was due to the high purity of the material and to the fact that it was subject to high temperatures for a short section of the apparatus, and hence the exposure time was short. The tendency of ethylene oxide to react with explosive violence, especially under acidic or basic conditions, should be emphasiaed. The hazardous nature of the material has recently been discussed by Hess and Tilton (6).
Ib!T.- l/tV
Figure 5.
RESULTS
The measurernenta for each run permitted the evaluation of an enthalpy difference between the calorimeter inlet temperature and the outlet temperature (about 78' F.) a t the constant pressure of the run. These results were all adjusted to a base state (H = 0) of saturated liquid a t 78" F. (vapor pressure = 26.2 pounds per square inch absolute). Figure 3 shows the experimental results plotted directly on a pressure-enthalpy diagram. The dotted curves were obtained by extrapolation.
Vapor Pressure Curve
To prepare the final tabulation of enthalpies, a cross plot of Figure 3 was made with temperature and enthalpy as coordinates. Then values read from this chart were replotted to give the smoothed pressure-enthalpy diagram shown in Figure 4. The final enthalpy data for the saturated liquid and vapor states are presented in Table I and for the superheated region in Table 11.
OF ETHYLENE OXIDEVAPOR TABLE11. PROPERTIES
Pressure, Lb./Sq. Inch Abs.
Satd. Temp., F.
Satd. Vapor
..
no
46 80 100 140 180 200 240 280 300 340 380 400 440 480
2ib
160
id2 144 157 178 196 '205 219 232 238 251 26 1 266 276 284 289 297
.. 6.410 0.408 0.404 0.399 0.397 0.392 0.388 0.386 0.381 0.377 0.375 0.371 0.366 0.364 0.360
180
200
220
240
260
280
Enthalpy (H), B.t.u./Lb. 243 24 1
250 249
258 257 255 253
255 256 257 257 257 257 257 256 255 254 253 251
540 .~ 0
140
Ef
500
40
Temperature, F. 120
266 265 263 26 1 256
274 273 272 270 266 260
281 280 279 278 275 271
289 288 287 285 283 280 276 272 267 261
2a7
262
297 296 295 294 292 289 286 282 278 274 268
304 301 303 302 300 298 296 293 289 286 282 276 270 262
300
312
309 311 310 308 307 306 304 301 298 295 292 288 282 275 267
---
255
Entropy (S), B.t.u./Lb.p 0.428
0,440
0.451 0.420 0.410
240 280 300 a40 380 400 440 480 500 540 Enthalpy valuer, obtained by extrapolating data to zero pressure.
0.461 0.432 0.424 0,408
0.471 0.445 0.437 0.422 0.405
0.482 0.457 0.450 0.434 0.418 0.410 0.396
F. 0.493 0.469 0.462 0.446 0.431 0.424 0.410 0.396 0.389
0.614 0.494 0.487 0.474 0.460 0.453 0.442 0.430 0.425 0.412 0,398 0.391 0.377
0.524 0.606 0.499 0.487 0.474 0.468 0.468 0.447 0.441 0.429 0.417 0.410 0.897 0.W 0.379 0 . a67
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
(1)
Christensen. L. D., and Smith, J. M., IND. ENG.CHEM.,42,
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:
(2) (3)
Coles, K. F., and Popper, F., Ibid., 42, 1434 (1950). Giaque, W. F., and Gordon, J., J. Am. Chem. Soc.,
log p =
-
2128 (1950).
T 4-7.72 (49' to 127" C.)
(2)
The equat#ionspresented by the previous authors are:
71, 2176
(1949).
(4)
Godnev, J., and Morozov, V., J. Phw. Chim. (U.S.S.R.), 22,
(5)
Gihthard, H., and Heilbronner, I., Helv. Chirn. Acta, 31,
801 (1948). 2128
(1948).
(6) Hess, L. G.. and Tilton. V. V., IND.ENQ.CHEM..42, 1251 (1950). Maas, O., and Boomer, E. H., J. Am. Chem. SOC.,44, 1709 (1922). 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, (7) (8)
(11)
Purdue University (1949). Prengle, W. H., Greenhaus, L. R., and York, R., Chem. Eng.
(12)
Smith, J. M., Ibid., 44,521
Progress, 44, 863 (1948). (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 employed in the study of ethylene oxide and are described in the - previous paper (6): However, it was necessary to double-distill
