April, 1929
Ih'DCSTRIAL A V D ELVGIArEERI;VGCHEMISTRY
353
A Thermodynamic Consideration of the Synthetic Methanol Process' A Revision Kenneth K. Kelley STANFORD UNIVERSITY, CALIF
I
N AN earlier paper the author? has considered the syn-
thetic methanol process from the standpoint of thermodynamics and obtained a free energy equation which was offered as a f i s t approximation only. Since that time Christiansens has calculated the equilibrium a t various temperatures by means of the Kernst approximation formula and data for the reactions CH30H
+ CO = HCOOCH, and HCOOCHa + 2H2 = 2CH30H
and Smith4 and Francis5 have also derived free energy equations. I n the previous paper the free energy of formation of liquid methanol obtained by Parks6 from thermal data and the third law of thermodynamics was used. Since Parks's specific heat measurements were carried down to only 90" IC, some uncertainty was involved in the extrapolation to 0" K. Recently the author' has made specific heat measurements on methanol down to 16' K. and obtained from them a value of 30.3 * 0.2 E. U. per mol for the entropy of liquid methanol a t 298.1" K. This value is considerably lower than that obtained by Parks, the difference being due almost entirely to the use of the n formula of Lewis and Gibson8in extrapolating below 90" K. This formula should not be applied to organic compounds, as will be shown elsewhere. Corresponding to this entropy value one obtains the free energy of formation and heat of formation given in Tsble I by utilizing the com, ~ entropy values for bustion data of Richards and D a v i ~the graphitic carbon and oxygen previously adopted,' and the value 31.25 E. U. for the entropy of one mol of hydrogen gas a t 298.1" K. The value previously used for hydrogen was 29.6 E. G. However, Giauque and Johnston'O have suggested that the absolute entropy of hydrogen should be 4.39 E. U. larger than the latter value and Gibson and Heitler'l have shown t'hat the spin effect ( R In 4 E. C. in this case) may be neglected in calculating the entropies of reactions involving monatomic and diatomic substances. It is not known at; present whether or not the spin effect must be considered in cases involving more complex molecules but tentatively Giauque'? recommends the use of 29.6 4.39 - R l n 4 = 31.25E. U., for hydrogen, in therniodynamical calculations. Considerable care has been taken in the choice of all other data for this paper. The values for carbon monoxide given in Table I are the best available a t the present tinie.13
+
Received October 24, 1928. Kelley, IND.ENG.CHEM.,18, 78 (1926). 3 Christiansen, J . Chem. Soc., 1926, 413. Smith, IRD.ERG.CHEM.,19, 801 (1927). 5 Francis, I b i d . , 20, 283 (1928). 6 Parks, J . A m . Chem. Soc., 41, 338 (1925). Kelley, I b i d . , 61, 180 (1929). Lewis and Gibson, Zbid., 39, 2554 (1917). 9 Richards and Davis, I b i d . , 42, 1599 (1920). l o Giauque and Johnston, I b i d . , 60, 3221 (1928). Cibson and Heitler, Z.Physik, 49, 465 (1928). 1 2 Giauque, personal communication. I t The author is indebted to E. D. Eastman, of the University of California, for these values. 2
Table I-Thermal SUBSTANCE CHiOH (liquid) C O (gas)
D a t a in
C. Calories AHzss -60,260
1 5 O
AF02~8
-42,970 -32,260
-26,310
Proceeding as in the earlier paper and using the same symbols one obtains for the reaction CO ( g )
+ 2H2 (8) = CH30H (1) = -33,950
A F o Z g 8= -10,710 calories
AH298
calories
The free energy change involved in vaporizing 1 mol of methanol a t 298.1 O K. to give vapor a t 1 atmosphere pressure is 1070 calories calculated from the equation, AF
=
P PI
R
T 2~
where Pz = 760 mm. and PI = 125 mm., the vapor pressure of methanol a t 298.1 " K. The heat of vaporization of methanol a t its boiling point is 8430 calories per m01.I~ By means of the specific heats of the gas and liquid the heat of vaporization a t 298.1' K. is calculated t o be 8800 calories. Therefore, for the reaction CO (8)
+ 2H2 ( g ) = CHIOH ( g )
we have AFozg8= -9640 calories and AHZ98= -25,150 calories. To obtain the free energy changes involved at' other temperatures the thermodynamic relationships
'AF
- = -
6T
AH and A H -T2
f C,dT
=
are used. The specific heat equations for hydrogen and carbon monoxide given below are taken from the earlier paper and accurately represent the experimental data.15 The equation for gaseous methanol was calculated by Francis6 and fits the existing experimental values as well as is possible.
+ ++
H P :C, = 6.65 0.00070 T CO: C, = 6.84 0.00038 T CH30H: C, = 0.60 0.0335 T
From these heat capacity equations, ACp = and A H = f (-19.54
+ 0.03172 T +- 19.54 0.03172 T) = A H 0
+
- 19.54 T 0.01586 T 2
AHo being the constant of integration which is evaluated as -20,740 calories using the value of AH a t 298.1" K. .'. A H = -20,740 - 19.54 T 0.01586 T 2
+
AF
19.54 +7 - 0.01586
6? 20740 and - = T2
6T
+
+
.'. A F o -20,740 45.00 T loglo T - 0.01586 T2 I T where I is the constant of integration. Using the value of the free energy change at 298.1 K.-namely, -9640 calories-Z is found to be -69.4 and the resulting free energy equation becomes A F o = -20,740 45.00 T loglo T - 0.01586 T2 - 69.4 T This equation is somewhat different from the one previously 1 4 Mathews, J . Am. Chew&. Soc., 48, 562 (1926).
+
15 Partington and Schilling, "Specific Heat of Gases," p. 145, Ernest Benn, Ltd., London, 1924.
INDUSTRIAL A.VD ENGINEERING CHEMISTRY
354
calculated and is less uncertain, because the values a t 298.1 "
K. used in evaluating the constants of integration are now known with more accuracy. Table I1 contains values of M" and K , the equilibrium constant, a t various temperatures which may be compared with the former results.
where J is the thermodynamic pressure or fugacity of the substance designated by the subscript, and is calculated from the equation M" = -RT In K . Table I1 ToK 300 400 500 600 700 800 900
AF' Cal. 9,650 4,200 1,330 6,920 12,620 +18,090 23,560
-
+ + + +
K 895 196 262 303 124 116 191
x
104
X 10-3 X 10-6 X 10-6 x 10-7 X 10-8
The values given in this table substantiate the qualitative conclusions which were drawn previously. They are not, however, in agreement with the recent results of Lewis and Frolich,l6 Audibert and Raineau,17 Brown and Galloway,'* and Morgan, Taylor, and Hedley,lg all of whom worked a t Lewis and Frolich, IND.END.CHEM.,20, 285 (1928). Audibert and Raineau, Ann. Office Combustibles Liquids, Vol. IV (1927). Abstracted, IND. E N G . CHEM.,20, 1105 (1928). 18 Brown and Galloway, I b i d . , 20, 960 (1928). 19 Morgan, Taylor, and Hedley, J . SOL.Chem. I n d . , 47, 117T (1928). 16
17
Vol. 21, No. 4
high pressures where correction from pressure to fugacity could not be made with any certainty owing to inadequate knowledge concerning the equations of state of the gases, Then, too, these workers were interested primarily in practical yields and not in studying the equilibria. The results of these investigations have been summarized by Fieldner and Brown.*o Nore recently Smith and Branting*l have studied the methanol equilibrium a t 576.9" K. and 1 atmosphere total pressure and have obtained K = 5.57 X 10-4. This value is probably correct to 5 per cent and is the only value in which any confidence should be placed for purely theoretical purposes. The calculations in this paper are not in agreement with this value, but the situation has been much improved and the new experiment31 data on methanol and carbon monoxide bring the calculations considerably closer to Smith and Branting's experimental value than were the former results. Obviously, something is wrong somewhere. Part of the difference may be due to the value chosen for the entropy of hydrogen, part to lack of sufficient heat-capacity data on methanol vapor, etc. The author feels, however, that the entropy value of methanol liquid a t 298.1" K. cannot be in any great error. This would also be true of the free energy if the question of the entropy of hydrogen were satisfactorily settled. Certainly more experimental work is necessary in order to clear up these difficulties. 20
21
Fieldner and Brown, IND. E N G .CuEM., 20, 1110 (1928). Smith and Branting, J . Am. Chem. Soc., 61, 129 (1929).
Specifications for Cellulose for Use in the Manufacture of Smokeless Powder',' Fred Olsen PICATINNY ARSENAL, DOVER, N. J.
T
H E type of cellulose ordinarily employed in the manufacture of nitrocellulose for use in smokeless powders is a high grade of cotton linters. This is perhaps due to the fact that the feature which has dominated nitrocellulose for the past two generations has been high stability. To this end an elaborate system for processing nitrocellulose has been developed, in the belief that the best results can be achieved only through the use of a high grade of cellulose. During the World War the demand for cotton linters became so heavy that the cutting of lint made a t the gins became closer and closer. Normally about 80 pounds of linters are removed from a ton of seed which has previously yielded about a ton of spinnable cotton and about 25 pounds of firstcut linters. The cut of second-cut linters was increased to about 150 pounds per ton, with a correspondingly larger amount of hull included with the linters. A still further supply of cotton was obtained by grinding the hulls after they had been cracked open to permit the recovery of the meat from which cottonseed oil is made, the ground hulls being separated by air into bran and hull fiber. This hull fiber is, of course, very short and indeed quite dusty. The purification process to which the cotton linters or hull fibers were subjected during the war period was by no means so thorough as is now practiced. Consequently there may be some justification in attributing the reduced stability of 1 Presented before the Division of Cellulose Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 t o 14, 1928. * Published by permission of the Chief of Ordnance, United States Army.
certain lots of propellent powders to the poorer quality of the cellulose employed. Cellulose for use by the Ordnance Department of the United States Army was expected to meet the following requirements: It shall contain not more than (1) 7 per cent material soluble in 10.0 * 0.1 per cent potassium hydroxide or in 7.14 * 0.1 per cent sodium hydroxide solution; (2) traces of lime, chlorides, sulfates, or hypochlorites; (3) 7 per cent moisture; (4) 0.4 per cent etherextractive matter; (5) 0.8 per cent ash. Alkali-Soluble Limit The purpose of the alkali-soluble limitation was to restrict the amount of oxy- and hydrocellulose. However, as most of the wartime cotton was of poor color, it is doubtful if very much of the material contained an excess amount of oxycellulose due to overbleaching. The nitrates of oxy- and hydrocellulose are reported in the literature as being highly unstable, although little is actually known about their properties. As heavier cuts of linters were made the cooking in the digesters became correspondingly more severe in order to effect the suitable disintegration of the highly colored hulls. However, little trouble was experienced in securing material complying with this requirement. Post-war developments of nitrocellulose lacquers have resulted in the production of cotton linters having an alkalisoluble content of less than 3.5 per cent. Apparently, cellulose of this quality can be obtained readily, and Ordnance Department specifications are being revised to require not more than 3.5 per cent material soluble in 7.14 per cent sodium
