F ig u re 1. V a p or P ressu re o f P u re C o m p o n e n ts
Figure 2. Acetylene-Ethane Binary System after , Kuenen (14)
674
Figure 3. Diagram o f Apparatus
M ETH ODS AND APPARATUS
The apparatus was composed of purification systems, equilib
rium cell, and gas analysis apparatus. The equilibrium cell had a volume of 92 cc. and had double glass windows for observing the condition of the sample within the cell. Figure 3 shows the cell. A, enclosed in a constant-temperaturc air bath, K , cooled by brine coils S and heated by heater X . M ercury was used as the confining fluid. The temperature of the coll was measured by a double-junction copper-constantan thermocouple embedded in the wall of the cell and a mercury-in-glass thermometer. A thermoregulator gave temperature control to =*= 0.1 ° F. Pressure was measured with a calibrated Bourdon-tube pressure gage with 10-pound scale divisions and with estimations to the closest pound.
The apparatus was supported on axis PR and was rocked to agitate the fluid phases and thus bring them into equilibrium. It was necessary to adjust the composition so that two phases could be obtained, since the two-phase region for the two com ponents is relatively narrow at 40 0 and 60 0 F. After being rocked for 15 minutes, the cell was placed in a vertical position ready for sampling the equilibrium phases through valve C. Mercury in cylinder M , driven by compressed nitrogen, was allowed to enter the equilibrium cell through valve D at the same rate that gas phase was allowed to leave the cell. The gas phase was used to purge the sampling line to receivers Q, and a portion of this I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 675
is a smooth curve connecting the two critical points, whereas the critical loci for the other two binary systems exhibit minimum points with respect to temperature. These minimum temperature points are associated with binary systems which form constant- boiling mixtures.
One of the earliest systems showing that hydrocarbons may form constant-boiling mixtures is the ethane-acetylene system, investigated by Kuenen (14). Figure 2 shows the critical locus and the border curve for the 68 mole % ethane mixture as pre
sented b y Kuenen. The two-phase region is not bounded by the vapor pressure curves of the two constituents for mixtures of this type, and two phases m ay exist for ethane-acetylene mixtures at pressures above the vapor pressure of the most volatile constitu
ent. The volatility of azeotropie mixtures, therefore, m ay not be predicted from the position of the vapor pressure curves o f the pure constituents. Kuenen’s data are of little use for quantitative vaporization calculations because of impurities which apparently entered the system. The dotted curve of Figure 2 is the revised position of the critical locus and minimum temperature curves as obtained from this research.
Acetylene is an endothennic compound and has been the source of several explosions when in the pure state under pres
sure. Acetylene is transported in cylinders containing a porous solid and acetone to decrease the possibility of explosion (7). If acetylene is suddenly decomposed in a closed vessel, the heat of decomposition causes the pressure to rise ten or eleven fold, ac
cording to the size o f the container (1, 22). Apparently the con
ditions for decomposition of pure acetylene are unpredictable, as judged by miscellaneous reports in the literature (2, 3 ,1 0 ,1 2 , 24, 26). It is evident, however, that up to pressures o f 25 pounds per square inch absolute, acetylene in the pure state is relatively safe.
A t pressures of 400 pounds per square inch upward, pure acety
lene is not safe unless it is in a porous medium or is diluted with some other material such as another hydrocarbon; even then ex
treme caution should be exercised. The presence of gas oil in acetylene mixtures at pressures up to 20 atmospheres has been shown to reduce the tendency for acetylene to decompose (23,25).
Acetylene is known to form metallic acetylides which are very unstable, especially when dry (10). These acetylides serve as a convenient basis for the analytical determination of acetylene. Notable among them are copper, silver, and mer
cury acetylides.
Figure 4. Ethane-Ethylcne-Acetylene l’ hase Equilibria
A . 761 11». per Hq. in. aha., 60° F.
I t . 511 11». per aq. in . aba., 10° F.
C. 561 lb . per aq. in ., 40° F.
676 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 7
T E M P E R A T U R Ę - 'P .
Figuro 5. Pressure-Teinperature Diagram for 0.16 Acetylene—0.51
Ethylene
Ordinary gas absorption methods were used for determining the concentrations of acetylene and ethylene in phase samples.
Approximately 100 cc. o f the gas at atmospheric pressure were measured in a buret, using mercury as the confining fluid with a covering layer o f 12.5% sodium su lfate-2.6% sulfuric acid solu
tion. The gas was passed through a pipet containing potassium iodomercurate solution which absorbed the acetylene. It was found that some ethylene was, also absorbed and calibration was necessary, especially for fresh solutions. Whenever three passes through the potassium iodomercurate solution (lasting one minute each) aid not heavier constituents by partial condensation at low temperature and cylinder pressure. The concentrated propane and heavier fraction was discarded. N o attempt was made to remove the ethylene present since its presence was not objectionable.
T h e acetylene was obtained from an ordinary Prest-O-Lite welding cylinder. Analysis showed a purity of 97.5% acetylene with 2.4 4% nitrogen, 0.05% oxygen, 0.01% carbon monoxide.
Acetylene was also purified by liquefaction with a dry ice-acetone bath around a container which had been packed with asbestos.
A portion of the liquefied material was exhausted so as to remove noncondensed nitrogen, carbon monoxide, and oxygen, and leave relatively pure acetylene. The noncondensable gases vented were approximately ten times the volume of the nitrogen, oxygen, and carbon monoxide present in the sample, and should have left quite pure acetylene. Each time the equilibrium cell was charged with a hydrocarbon mixture, a new batch of acetylene was pre
pared.
In all cases ethane was placed in the equilibrium cell before acetylene was admitted, to ensure that pure acetylene would not be present under high pressure in a container which did not con
tain a porous solid or diluent. In order to admit acetylene to the equilibrium cell, the liquefaction cell was separated from the Prest-O-Lite cylinder and the cooling medium removed. The acetylene pressure could, therefore, be controlled by the tempera
ture rise of the liquefaction cell, and acetylene could be added to the equilibrium vessel when the pressure was sufficiently high.
0i u a
I/)CD
M O L F R A C T IO N A C E T Y L E N E
Figure 6. Summarized Binary and Ternary Diagram at 40° F.
July, 1944 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 677 values determined at the several compositions. A plot was made of mole fraction acetylene in the vapor and mole fraction acety
lene in the liquid; with this plot the slope of the tie lines connect
ing bubble and dew points could be determined. Using these and analysis, small differences occur between smooth values and the original experimental data. The temperature control of
=*=0.1° F. had little effect on the results obtained. The pressure control and observation probably were within =*=3 pounds per square inch of the true value, and in some cases this tolerance per
mitted relatively large changes in the phase composition. The analytical method gave results differing by not more than 0.2% , based upon the entire sample, but with increasing percentages for the concentration o f the individual constituents.
In addition to determinations of the composition of vapor and liquid phases, pressure, volume, and temperature measurements were made on specific mixtures of known composition. The mix
ture was maintained in the equilibrium cell, and the border curves for bubble and dew points were determined by visual observation as well as by the percentages o f vapor and liquid between the two extremities. Figure 6 shows such a diagram for the 54 mole % ethylene-46 mole % acetylene mixture. Similar diagrams were
MOL FR AC TIO N A C E T Y LE N E
Figure 7. Summarized Binary and Ternary Diagram al 60° F.
made for five other mixtures as described in Table III. The re
sults of these phase diagrams were primarily to estimate the criti
cal locus for the 60° F. investigation. In addition, they serve as an independent check upon the vapor-liquid compositions given by the other triangular diagrams.
The relation between the binary and ternary systems is shown by Figure 6 for the 40° F. data; the binary phase diagrams at constant temperature are plotted opposite the side of the ternary diagram which represents that binary system. The pressure in
tersection with the bubble and dew point lines on a binary dia
gram when projected to the foot of that diagram gives the border intercepts of the bubble and dew point curves for the ternary dia
gram. Thus the 464-pound per square inch absolute line for the ethane-acetylene binary diagram intersects the bubble and dew point lines at two places, and forms two two-phase regions at the edge of the triangular diagram. The constant-boiling mixture of ethane and acetylene moves across the triangular diagram to the azeotropic mixture of ethylene and acetylene. Further, the bubble and dew point lines become tangent at each pressure as they cross the azeotropic locus, and the tie lines at these points have a length of zero, i.e., the equilibrium constants of the three components of ternary mixtures along the azeotropic locus are all unity.
The constant-boiling mixtures represented along this locus mean that, if liquid of any composition along this line were vapor
ized, the vapor would have the same composition as the liquid and thus give the impression o f a pure compound.
678 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 7
Figure 9. Qualitative Phase Behavior Diagram
Figure 7 gives a similar diagram for the 60° F. results. It dif
fers from Figure 6 in that the temperature is above the critical temperature for pure ethylene. As a result, mixtures of vapor and liquid cannot be formed at conditions closer to ethylene than the dotted curve shown; this curve is the locus of compositions which have critical temperatures at 60° F. and critical pressures varying from that of the ethane-ethylene system to that of the ethylene-acetylene system at 60°.
Figure 8 indicates the effect of pressure and composition upon the two-phase region at 40° F. and is, in effect, a series of tri
angular diagrams superimposed above one another and spaced according to the pressure scale. The three faces of the figure are the constant-temperature binary diagrams; the solid, represented by the bubble and dew point surfaces, shows the continuous change in phase behavior. The thickness between the bubble and dew point surfaces becomes zero along the azeotropic locus.
With the information made available in this investigation, it is possible to draw qualitatively the type of behavior which would be obtained at temperatures between those of the present investigation. Several of these qualitative diagrams are pre
sented in Figure 9. A is drawn at 47.5° F., which corresponds to the minimum temperature of the critical locus curve o f the ethylene-acetylene system and is the lowest temperature at which any mixture of ethylene and acetylene may exist in a criti
cal state. Below 47.5° F. there is no critical locus within the ternary diagram. As the temperature rises above 47.5° F., a critical locus begins to appear which, at 48.5°, would be o f the type shown in B. This temperature is below the critical of ethylene and above the minimum critical temperature o f the ethylene-acetylene system. A t 49.3° F., the critical temperature of ethylene, the diagram would be the type shown as C. The critical locus loop has expanded and encloses a greater portion of the ternary diagram. A t 55° F. the diagram would be the type shown as D . This temperature is above the critical of ethylene, and therefore a critical point exists in the ethane-ethylene system as well as in the ethylene-acetylene system.
Figure 8. l’ res- s u re-Compos i tion Diagram at 40° F.
L I T E R A T U R E C 1 T E D
(1) Azetylen TFiss. Ind., 33, 73 (1929).
(2) Berthelot and Vieill, Compt. rend., 121, 424 (1895); 124, 1000 (1897).
(3) Burrell and Oberfell, Bur. Mines, Tech. Paper 112 (1915).
(4) Cardoso and Baume, Compt. rend., 151, 141 (1910).
July, 1944 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 679
680 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 7 casein and rennet casein, and showed that over a wide range
sein than could be accounted
for by assuming reactions with a-amino groups, side-chain amino groups o f the diamino acids, and amide groups. It was suggested that the binding of aldehyde by hydroxyamino acids to form aeetals should be considered a possibility, inasmuch as the intera
tom ic distances of the aeetals involved were compatible with ideas o f protein structure already obtained from x-ray studies.
Recent studies on aldehyde binding by amino acids by Dunn and co-workers (8) and from this laboratory (5) have done much to clarify the general problem. W e showed that the amide group of asparagine does not react with formaldehyde and that, in gen
eral, the a-amino group reacts with one mole of aldehyde to give a fairly stable methylol derivative; the latter, in turn, reacts with a second mole o f aldehyde to form an unstable compound which gives up aldehyde readily and which is probably an acetal of the type R N H C H 2O CH 2OH. This structure is indicated by its properties and the fact that only in the presence of an alkyl group (such as in iV-methylleucine) is the second hydrogen of the amino group reactive with aldehyde. No reaction occurs with aldehyde when an acyl group is attached to nitrogen of the amino group (3). The length and structure o f the side chain, even conclude that arginine binds one mole of aldehyde on the Sakaguchi reaction (H ) for the guanidino group is negative afte aldehyde and arginine have reacted for some time.
With respect to lysine, the e-amino group apparently reacts with one mole of aldehyde before the a-amino group, and the latter reacts eventually with the usual two moles of aldehyde.
Histidine reacts with one mole of aldehyde at the a-amino group and with a second mole at higher aldehyde concentrations.
The great reactivity of the latter reaction leads one to suspect that the second aldehyde m ay react with the imidazole ring rather than at the a-amino as is usual.
Proline reacts with only one mole of formaldehyde. The five-membered ring structure of proline and oxyproline makes it difficult to fit them into peptide chains according to modern ideas o f protein structure. The least difficulty is encountered by as
suming proline to occur at the end of a peptide chain with the imino group in peptide linkage. This rules out a reaction with aldehyde.
In the first paper (4) we overlooked the possibility of aldehyde binding with the tyrosine side chain. However, Koebner {IS) prepared 2,6-dimethylol-4-methylphenol from p-crcsol and form
Tlie combining ratios between formaldehyde and deamin
ized casein are established over a concentration range up to 6.85% formaldehyde. Th e general law, relating bound being hound at any aldehyde concentration by deaminized casein as by acid casein. The aldehyde hound by acid casein and deaminized casein agrees closely with that ex
pected from the content o f certain individual amino acids in the respective proteins.