September, 1930
INDUSTRIAL AND ENGINEERING CHEMISTRY
more than 10 per cent; for the saving is not only in the soap formed by the neutralization of the fatty acids but also in the soap otherwise required to keep them in suspension. Spparently the amount of degumming was negligible, since the ‘(take-up,” amounting to 6.22 per cent, which is the difference between the bone-dry weights of the silk before and after soaking and which therefore takes into account the degumming factor, was only slightly below total fatty matter of 6.68 per cent extracted from the silk. The neutral fat seems to interfere with the free adsorption by the silk, since the amount of soap and fatty acids adsorbed was less than when soaked in a straight soap solution. Other data given in Table I1 are the absorption of liquor and the deposit of soaking ingredients on the silk, in each case calculated on the dry raw silk and the original liquor. Uneven Soaking
The instability of the emulsion and the ultimate concentration of neutral fat in a comparatively narrow layer would lead
985
one to suspect that the deposit of fat on the silk may not be uniform throughout. This was actually found to be the case. Two skeins of silk soaked in the same batch, one on the bottom of the tub and the other near the surface, were analyzed for total fatty matter. The bottom skein contained 9.68 per cent, compared with 14.90 per cent for the top skein. On the other hand, except for the silk close to the surface, the absorption of fatty matter in commercial soakings were found to be fairly uniform and close to the amount calculated from the liquor retained by the silk. Rapid penetration of the liquor and entanglement of the oil among the fibers may account for the comparatively uniform results obtained in practice. Nevertheless, the need of an emulsion of greater stability, at the same time retaining the lubricating and penetrating properties and pH value of the present formulas, is obvious. Literature Cited (1) Seem, “Raw Silk Properties, etc.,” p. 115 (1927).
The Problem of Establishing the Identity and Purity of a Hydrocarbon Obtained from Petroleum’.’*3 Edward W. Washburn BUREAUOF
STANDARDS, WASHINGTON,
D . C.
HE various handbooks The evidence in support of the identity and purity Evidence of Purity and monographs dealof most of the hydrocarbons reported as having been ing with the chemistry isolated from petroleum is inadequate. Suitable tests Before attempting to idenand technology of petroleum for purity and evidence for identity are described and tify a fraction as someparticuusually contain r a t h e r discussed and a procedure is suggested. It is recomlar hydrocarbon, it is neceslengthy lists of hydrocarbons mended that at least the five properties-refractive sary-xcept perhaps in the which, the author states, have index, density, boiling point, freezing point, and case of the lower hydrocarbeen isolated from petroleum. halogenation behavior-be reported on all petroleum bons, where the number of i?len, however, one examines fractions for which identification is claimed. possibilities is so small and the properties are so well known the evidence for the identification and purity of these supposed hydrocarbons, it becomes and so widely different that both purity and identity can be espainfully apparent that in the majority of cases this evidence is tablished by comparison of properties alone-to prove that the either utterly worthless or, when properly interpreted, proves fraction is substantially a single substance, not a mixture. that the material is not the hydrocarbon claimed. It may in- Since in general it cannot be assumed that, even if pure, the deed be and probably is true that all the reported hydrocarbons, fraction is a known hydrocarbon for which reliable physical as well as a great number of others, actually do occur in and data are on record, it is clear that the criteria for purity should, can be obtained from some one or more petroleums, but this as far as possible, be independent of the availability or nonprobability is one thing and actual proof of the isolation, by availability of corresponding data for the pure hydrocarbons. fractionation, of a given hydrocarbon from a given petroleum The most generally used criteria for purity are those which is something quite different. The purpose of this paper is depend upon (a) vaporizing or condensing behavior and (b) to discuss the nature of the evidence which must, be obtained crystallizing or melting behavior. before identity and purity can be considered to be established (a) VAPORIZING OR CONDENSING REHAVIOR AS A TESTFOR ’PuRITY-This test for purity is usually applied by determining with reasonable certainty. the distillation range (boiling-point range) under Constant 1 Received May 28, 1930. Presented before the Division of Petroleum Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, barometric pressure Or by determining the initial and final Ga., May 28, 1930. vapor pressures during an isothermal vaporization or conPublication approved by the Director of the Bureau of Standards densation, Neither of these procedures is very sensitive. of the U. S. Department of Commerce. To obtain a greater sensitivity the test may be conducted as The author disclaims all originality for the discussion herewith fOllOWS: presented. The principles involved and the methods and technic suggested
T
have been used and described many times in the literature. They have not, however, always been observed and utilized by workers in the field of petroleum chemistry, and it is hoped that this discussion may be helpful to investigators in this field. The principles and methods here described are those which are being employed in an investigation on “The Separation, Identification, and Determination of the Chemical Constituents of Commercial Petroleum Fractions,” listed as Project 6 of American Petroleum Institute Research. Financial assistance in this work has been received from a research fund ,of the American Petroleum Institute donated by John D. Rockefeller. This fund is being administered by the institute with the cooperation of the Central Petroleum Committee of the National Research Council.
Using an efficient fractionating apparatus carry out a complete vaporization fractionation of the whole sample under conditions which avoid all danger of cracking. Set aside the first fraction obtained as soon as it amounts to a few cubic centimeters; or, if the size of the total sample warrants it, discard this small initial distillate and collect and preserve the next small fraction which comes over. Continue the fractionation until only a few cubic centimeters of residue remain and set this aside also; or, if preferred, reject this small residue and collect and set aside the last small fraction of distillate. For these two small fractions (initial and final) determine
INDUSTRIAL AND ENGINEERING CHEMISTRY
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accurately either or both: (1) the difference in boiling point under the same external pressure ( 1 , 4, 7); ( 2 ) the difference in vapor pressure a t the same temperature (2, 5). Provided constancy is assured, it is not necessary to know the pressure in (1) or the temperature in (2). The higher the temperature the more sensitive is the test, maximum sensitivity being reached in the neighborhood of the critical temperature. All dissolved gases, especially air and moisture, must be removed in procedure (2).
As evidence of purity it is necessary, but not sufficient, that the difference shall be zero. If it is not zero, the sample is not a pure substance. Under certain conditions the degree of punty can be calculated, if only one impurity is present (6). Obviously this test will fail to distinguish pure substances from constant-boiling mixtures, whether the constant-boiling mixture is of the maximum or minimum type, or of the idealsolution type for components having the same boiling point. ( h ) FREEZINQ BEHAVIORAS A TESTFOR PuRrm-This test is customarily applied by determining the time-temperature cooling curve through the freezing range. The sensitivity of the test may be increased, if it can be carried out as follows: Cool the whole sample and separate (a) the first small crop of crystals which appears. Continue the cooling, accompanied by stirring, until most of the sample is frozen. Separate ( b ) the liquid fraction-e. g., by centrifuging or suction-as completely as possible from the crystals. Determine the timetemperature cooling curves of fractions (a) and ( b ) . Important elements of the technic are (1) a sufficiently high temperature gradient between sample and cooling bath; ( 2 ) a sufficiently low rate of cooling; (3) effective stirring.
As evidence of purity it is necessary, but not sufficient, that each cooling curve shall exhibit a region parallel to the time axis and that these regions shall coincide as to temperature. If desired, this coincidence may be established by the simultaneous cooling of equal masses of both (a) and (b) in the same bath with a differential thermocouple connecting them. As an index of purity this test will fail in the case of a constant-freezing mixture, whether this mixture be of the eutectic type or of the mixed-crystal type for components having the same freezing point. It will also fail for certain other types of rare occurrence. If the material contains only small amounts of impurities, the degree of purity can be calculated from the cooling curve of the whole sample, if certain con$tions can be assumed to be fulfilled (8). Instead of carrying out the test as described above, it is possible to conduct it in the reverse manner. I n this procedure the well-stirred mass is completely frozen and cooled several degrees below the freezing point. It is then transferred to a suitable calorimeter and the total heat content, or any quantity proportional thereto, determined as a function of the temperature. If the material is a pure substance, the heat content-temperature curve will exhibit a region parallel to the heat-content axis and meeting sharply the heat-capacity curves for the solid and liquid forms; and equal masses of the two fractions (a and b) will give identical curves. This method of conducting the test is a very sensitive one. I n principle, however, it gives the same kind of information as the cooling method and fails under the same conditions. If, when cooled, the liquid becomes very viscous before reaching its freezing point, it may solidify to the vitreous state. When this occurs the technic of the freezing test becomes very difficult and the test fails or loses its sensitivity. This difficulty can sometimes be avoided by dissolving an appropriately chosen pure substance, B, in the hydrocarbon and determining the cooling curve through either of the following points which are non-variant for a two-component system under constant pressure: (a) the eutectic; (b) the system, crystalline hydrocarbon liquid saturated with gase-
+
Vol. 22, No. 9
ous B at atmospheric pressure. The substance B should be one which can be readily removed later and whose presence results in the requisite incresse in fluidity. The eutectic procedure (a) may actually be more sensitive than the straight freezing test on the hydrocarbon alone, even when this test offers no experimental difficulties. Procedure (b) will, however, usually be less sensitive. Obviously for the successful application of procedure (b) the substance B in the gaseous state at atmospheric pressure must be sufficiently soluble, but not too soluble, in the cold hydrocarbon. I n other words, the non-variant system postulated in (6) must be realizable. Whenever a freezing test is carried out in contact with the atmosphere, it is, as a matter of fact, PI-0cedure (b) which is ordinarily realized, dissolved air playng the role of the added substance, B. From the foregoing discussion it is evident that some additional information and at all events a valuable check on the testing procedures described above can be obtained by making the freezing test also on the two end fractions obtained by the vaporization fractionation and vice versa. Discussion
Neither of the above criteria alone is proof of purity. Nor, taken together, are they entirely conclusive. However, a material which behaves like a pure substance by both of these tests can be only a pure substance or a mixture which is both constant boiling and constant freezing. Note-A eutectic mixture of the isomers A-tetrahydrotoluene (b. p. 1 0 3 O C.) and 4-methylcyclohexane (b. p. 102.2' C.) might be close t o such a mixture. The number of possible cases of this kind among the hydrocarbons is disconcertingly large.
If the material meets both of the tests and at the same time has a boiling point, a freezing point, a density, a refractive index, and a bromine or iodine number identical with those recorded for a known hydrocarbon, both purity and identity may be considered as substantially, but not absolutely, established. I n the absence of reliable comparison data on a pure hydrocarbon, however, there will always be a possibility that the material may be the peculiar mixture mentioned above. To eliminate this possibility the material may be subjected to other types of fractionation procedure-for example, distillation at very low pressures, crystallization, and/or distillation, precipitation, or extraction after admixture with some readily removable substance, such as alcohol or carbon dioxide. Or, the material may be transformed into chemical derivatives, which in turn may be fractionated. The fractions obtained are then tested for identity by the methods described above. Additional evidence may also be obtained by determining the empirical formula of the hydrocarbon. Such evidence by itself is of little value, since it fails to distinguish between a pure substance and a mixture of isomers or of substances of close molecular weight and hydrogen content. It is valuable confirmatory evidence, however, especially if the material has a zero bromine number. A petrographic examination of the crystalline material will also yield valuable evidence and in many cases will exclude the possibility of a eutectic mixture. By these methods it should in the majority of cases be possible to prove purity and establish identity. About the only case in which failure would result would be a mixture of two isomeric hydrocarbons having substantially identical vaporpressure curves and freezing points and forming a complete series of mixed crystals with each other. To separate and identify two such substances may be, practically speaking, impossible by physical methods. In this connection it should be remembered that, in addition to the purely physical methods of identification discussed
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 193(1
above, the organic chemist can in some cases more or less quantitatively convert the hydrocarbon into one of its derivatives, which in turn can be identified as described above. It is the writer’s impression, however, that the adequacy of this type of evidence must be separately considered in each case, no general statement being possible. Examples
1-A constant-boiling fraction obtained from gasoline is claimed by the investtigator to be normal hexane “to a high degree pure.” The only evidences presented are the boiling point, density, and refractive index, which are as follows, the corresponding data for synthetic n-hexane being added for comparison : n-Hexane from gasoline Synthetic n-hexane
B. P. ( ” C.)
4;
68.9-69.0 69.0
0.6622 0.6606
n$ ‘ 1.3851 1.375
It is clear from these data that the fraction is probably a constant-boiling mixture. It cannot be even approximately pure n-hexane and have the properties found. Neither can it consist solely of a mixture of the hexanes, if its refractive index is that given. 2-A petroleum fraction obtained from a chemical supply company and catalogued as “hexane (from petroleum) b. p. 65-70”” was found to have the following properties: B.P. ( “ C . ) “Hexane from petroleum” Synthetic n-hexane
65-70 69 0
F P. ( “ C) -86to -95 -94 3
2; 1 3855 1 373
This material is neither %hexane nor a mixture of the hexanes. The freezing range given was obtained by visual observation. It could not be detected on the time-temperature cooling curve itself. It is misleading to call such a material “hexane from petroleum.’’ It may, probably does, contain some of the hexanes, although there is no proof of this. 3-A petroleum fraction resulting from a rather extended fractionation by both distillation ,and crystallization was found to have the properties shown below. The corresponding data for the only hydrocarbon (for which properties were available) which appeared to have similar properties are shown for comparison. B.P.(’C) Petroleum fraction Tetramethylethylene
71 6-73 73
d’:
0 715 0 712
?L?
1 406 1 413
FP(’C)
-112 Sotonrecord
On the basis of comparison of physical properties we might conclude that this petroleum fraction is impure tetramethylethylene. The evidence for such a conclusion is much better than the evidence usually given in identification of hydrocarbons obtained from petroleum, for not only do we have what is ordinarily considered to be good agreement for three different properties, but we have the added information that the material exhibits quite a respectable freezing behavior. Fortunately in this instance we have a very easy method of testing the above conclusion According to the literature, tetramethylethylene readily adds bromine and iodine. When this test was applied the petroleum fraction was found to give zero bromine and iodine numbers. We are forced to conclude, therefore, that not only is the fraction not an impure tetramethylethylene, but it contains no appreciable amount of this hydrocarbon.
987
degree of probability the purity and identity of a petroleum fraction. FORFRACTIONS WFUCHARE LIOUIDAT ROOMTEMPERATURES-(~) Determine the boiling point, density, refractive index, and bromine or iodine number. (2) If these properties do not agree with available reliable data on a pure hydrocarbon, further purification must be resorted to or consideration must be given to the possibility that the fraction is a hydrocarbon for which reliable data for comparison are not available. (3) If the above properties can be identified with those of a known pure hydrocarbon, it is still necessary to obtain additional evidence. This additional evidence in some cases is not easy to secure, but it may not be omitted. The freezing behavior together with the above data may usually be taken as conclusive. That is, if the freezing behavior is that of a pure substance and the freezing point checks the other properties for the same hydrocarbon, the identification may be considered as established. If the freezing behavior is that of a pure substance but the freezing point of the presumed hydrocarbon is not available for comparison, there is still a high degree of probability for the assumed identification, in the case of hydrocarbons of not too high molecular weight. (4) If purity has been indicated by both vaporizing and crystallizing behavior but identification is in doubt on account of lack of available data for comparison, the empirical formula of the fraction should be determined as accurately as possible. If this formula corresponds with that of a single hydrocarbon, identity to this extent may be considered as tentatively established.‘ If now identity as to isomer cannot be established from available data and if such identification is desired, the problem is one for an organic chemist. FORFRACTIONS WHICHARE CRYSTALLINE AT ROOM TEMPERATURES-These are usually hydrocarbons of high molecular weight, and reliable boiling-point data for comparison purposes may in many cases be unavailable. (1) Apply the freezing test for purity, using preferably the most sensitive procedure. If melting is accompanied by decomposition, use the eutectic procedure. (2) If at all feasible, apply also the vaporization test, using if necessary the initial small distillate (6) and the whole of the remainder of the sample. (3) Examine the crystalline material with the petrographic microscope. (4) Determine the empirical formula and the bromine or iodine number. If purity is indicated by all these tests, identification will depend upon the availability of reliable data on pure hydrocarbons, and in some instances it may not be feasible to attempt to exclude the possibility that the material may be a mixture of isomers. In the case of all petroleum fractions for which purity and/ or identification is claimed the record should, as far as possible, show the palues of the following properties, determined for at least the phase form (crystal or liquid) which exists at room temperature : (1) Refractive index or indices (2) Density (say a t 20’ C.) (3) Boiling point or vapor pressure (4) Freezing point or appropriate eutectic (5) Bromine or iodine number, corrected, if necessary for substitubon of halogen for hydrogen
Suggested Procedure
For practical purposes and with due consideration to the present state of our knowledge of the composition of petroleum and the large amount of work which must be done, the following procedure is suggested for demonstrating to a reasonable
These properties are suggested, not because they are neoessarily the most distinctive characteristics, but because they are the ones most likely to be utilizable either now or in the 4
See, however, note under “Discussion.”
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near future, for comparison purposes, because the equipment required for their determination is generally available in chemical laboratories. It may well be that other properties, such as viscosity, surface tension, thermal conductivity, heat capacity, latent heats, dielectric characteristics, absorption spectra, and x-ray diffract,ionpatterns (S), are more distinctive and sensitive than some of these listed, and the time may come when the chemist will be in a position to employ them more generally than at present.
Vol. 22, No. 9
Literature Cited (1) Cottrell, J . A m . Chem. Soc., 41,721 (1919). (2) Shepherd, Bur. Standards J . Research, 2 , 1169 (1929). (3) Stewart, Chem. Rev., 6, 500 (1929). (4) Swietoslawski, Bull. acad. pol. sci. l e K , [AI,434 (1929). ( 5 ) Washburn, Z . physik. Chem., 130, 692 (1927). (6) Washburn, Bur. Slandards J . Research, 2 , 476 (1929); gives method of distilling without decomposition.
(7) Washburn and Read, J . A m . Chem. Soc., 41,799 (1919). (8) White, J . Phys. Chem., 24, 393 (1920).
Trends in Heat Transfer’ D. F. Othmer EASTMAN KODAKCOMPANY, ROCHESTER, N. Y.
I
N T H E innumerable places where heat is generated, applied, or dissipated in various ways in almost every
type of industry it must be passed from some relatively hot body to one a t a lower temperature. This process is called “heat transfer” and its aspects are among the most familiar of physical phenomena. It is not the purpose of this article to consider the physical laws governing the molecular, electronic, or wave motions involved in the theory, nor yet to consider the mechanics, often empirical, involved in the practical utilization of such abstruse discussions. Leaving such scientific aspects, however, there have been numerous examples of interesting applications of the principles of heat-transfer technology in recent years in that wide field involving the effects of heat, from the circulation of a warming fluid in the body or building to the cooling coils intricately blown of glass in some high-powered thermionic valves for radio transmitters. Boilers and Power Generation
The commercial utilization of heat received from the sun and stored in coal and oil has required the development of the modern boiler. Inefficient as the production of power from such combustion is, it converts a tremendously larger amount of the heat received than does that other solar-heat engine: the evaporation of surface water, condensation as rain and utilization of water power developed as the streams run down hill to the sea. There are recent experimental successes with a more direct type of solar-heat energy utilization from the thermal standpoint-i. e., the reflection by huge parabolic mirrors of the heat of the sun’s rays (7 B. t. u. per minute per square foot) on water tubes at the mirror’s focus. These boilers transform solar heat into power without waiting for the passage of geological eons as with coal, nor earthly seasons or lunar months as with hydroelectric or tidal energy transformation. Another of the heat engines in which the actual thermal units are as free as the air, but not quite so immediately valuable, is the proposed Claude power plant utilizing the difference in the temperature of surface and deep sea water in tropical seas. A continuous stream of warm surface water is drawn into a closed chamber maintained under high vacuum. Part of the water flashes into steam at this pressure, and the very low pressure steam is drawn through the blading of special turbines and thence through condensers cooled by water drawn up from the bottom of the sea.. The effective head on these water pumps supplying condensing water is very low, even though the water is lifted for miles, because the hydrostatic pressure nearly balances the water inside the suction pipe. The power required for water 1
Received May 21, 1930.
and vacuum pumps is subtracted to give the net amount available. Much interest is evidenced in this system, not alone because of the previous achievements of the proposer, but also because of the limitless power made available to a system which can use it between such narrow limits of high and low temperatures. Turning from equatorial islands to Arctic steppes, another Frenchman, Barjot, desires to take the cold of Canadian nights and make it turn the wheels of industry or fire the arc of an electric furnace. Insulated from the rigor of Arctic winter by the surface ice pack, vast quantities of fresh water in northern rivers never fall below 0” to 4” C. The average atmospheric temperature during months of the cold winters may be approximately -20” C., and various types of heat engines may be devised to operate between these two temperatures. A very ingenious one proposed by Doctor Barjot uses a material which is gaseous a t ordinary temperatures and boils a t approximately 0” C. Water pumped from the river bottom is intimately mixed with a substance such as liquid butane, the water gives up its latent heat of fusion to freeze and the butane absorbs it as heat of evaporation to boil at a pressure just below atmospheric. Again a special turbine wheel is turned and the vapors pass to a vacuum condenser space filled with blocks of cryohydrate, the ice-salt solid formed by freezing saturated brine. Through the use of a working substance mutually insoluble with water or brine, and therefore readily separated by gravity decantation, heat is transferred from ice water to the low-boiling liquid in the boiler by direct contact, and from vapors to blocks of cryohydrate in the condenser. This minimizes the loss of available temperature range caused by the use of heat interchanging surface. If the difference of temperature between subsurface water and cryohydrate in this paradoxical use of ice water to supply rather than remove heat were equal to the difference of temperature available to a Claude system, thermodynamics shows that the Arctic zone process would supply more power in a perfect engine than that of the torrid zone. Even the terrestrial heat stored under the thick crust of the earth has been utilized in this age of power. I n various cold countries in which hot springs are located it has been the practice to circulate such hot water to heat adjacent homes. I n Italy the mixture of vapor and spray formed by the flashing of superheated geyser water is separated and the steam used for power and heating. So-called “volcanic boilers,” in which water is circulated and heated in pipes suitably placed where the earth’s internal heat may be tapped and used near the surface, have been successfully experimented with. None of these newer methods are worrying producers of power from coal or waterfalls because of the small amount of
