e
Mechanism of the Steam-Carbon Reaction G. S. SCOTT Central Experiment Station, U. S. Bureau of Mines, Pittsburgh, Penna.
Studies of published data on the kinetics of steam decomposition by incandescent carbonaceous matter reveal one clear-cut case of a second-order reaction in the temperature range 700-1000° C. (for lignite char), The remaining data show local indications of a second-order reaction but, in general, show divergences when studied from the standpoints of initial gas composition and reaction velocities, respectively. These divergences may have been caused by insufficient compensation for factors which might tend to distort the results. Further research is necessary to clarify the situation.
N CONNECTION with research on the rate of oxidation of anthracite, the writer reviewed literature on the mechanism of the steam-carbon reaction. The experimental procedure of the different investigators whose data are considered in this paper was, essentially, to pass steam a t different rates through a tube containing some form of carbon at elevated temperatures, to measure the amount of steam decomposed, and to analyze the gaseous products. The salient features of the different apparatus and experimental procedures are tabulated and discussed later in this paper. The first data examined were those of Brewer and Reyerson (1). Times of contact (not given in their paper) were calculated, temperature, pressure, and volume changes during the reaction being taken into account and 42 per cent of voids arbitrarily assumed. The assumption of any reasonable figure for voids, whether or not it corresponds to the actual figure, can have no effect on the relations discussed here, provided the voids remain constant. Volume changes during the reaction were calculated by integration over the experimentally determined variation of total volume with percentage of steam decomposed. Against these times of contact were plotted the dry gas concentrations of carbon dioxide (Figure 1),hydrogen (Figure 21, and carbon monoxide (Figure 31, in the oxygen- and nitrogen-free gases. The carbon dioxide and hydrogen both increase with decrease in contact time; the former (Figure 1) appears to converge at 331/* per cent and the latter (Figure 2) at 662/a per cent. The equations for the decomposition of steam by hot carbon are given as
Figures 1 and 2 indicate that the initial products at temperatures below about 1000" C. are carbon dioxide and hydrogen only, according to reaction 2. For higher temperatures the experimental data are insufficient to permit extrapolation to zero time. If this be true, the carbon monoxide must approach zero at zero contact time, and reaction 1 does not take place initially. When plotted on logarithmic paper as in Figure 3, all curves for temperatures below 1100" C . appear to approach a 45" slope with decreasing time of contact. This means that, when the contact time reaches zero, the carbon monoxide will reach zero. If there is one primary reaction and it is in accordance with reaction 2, the rate of disappearance of water is given by the equation,
--d[Hzol at
+
+
+ He
+ 2IL
(l)
(2)
IC [Ha0]2
which when integrated (assuming time to start from zero) becomes 1 h i = - - -1 [Ha01 [HzOlc where [H20]i = initial partial pressure of steam [HaO] = partial pressure of steam at end of time t b = reaction velocity constant
I
C H 2 0 =: CO C 2Ha0 = COz
PII
33:
y
3 0, io
:: 2 g 5 8
E 3 1
0
4
a
12 16 20 CONTACT TIME, SECONDS
24
28
32
FIGURE 1. CARBON DIOXIDE IN DRYGASES(OXYGENAND NITROGEN-FREE), FROM BREWER AND REYERSON
If the steam is 100 per cent pure when it enters the reaction zone and the total pressure is 1 atmosphere, then [HnO]i is 1, and the above equation becomes kt = 1
- [Ha01 [&01
If the steam decomposes according to reaction 2 and is a second-order reaction in accordance with the above equation, then a plot of (1 - [HzO])/[HzO] against t should yield a straight line. 1279
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INDUSTRIAL AND ENGINEERING CHEMISTRY
would pass through the origin ([H,O] = 1, t = 0); but if all points are considered, these curves can hardly be called straight lines. That the effects of diffusion are not great seems to be indicated by Figure 5. The reaction rate increases with temperature a t a faster rate than does diffusion, so that if diffusion were a factor to be considered, the curves of Figure 5 should tend to straighten out a t the higher temperatures, whereas the opposite is the case.
66
w
Vol. 33, No. 10
60
3
9 & c
" s yi
s'
24
I
50
I
I
I
I
I
a
i; CONTACT TIME SECONDS
IN DRYGASES (OXYGEN-AND NITROFIGURE2. HYDROGEN GEN-FREE), FROM BREWER AND REYERSON
Such a plot is shown in Figure 4. Two sets of ordinates are given. The points were plotted against whichever set gave the longest line by moving the decimal points to right or left as required. Straight lines were drawn to represent the data, bearing in mind that all the lines must pass through the origin. The points for the longest contact times are omitted because of their obvious approach to equilibrium. Had this reaction been of the first order, its rate would supposedly have followed the equation, --= d'Hzol at
k [HzO]
which becomes when integrated
or when [HzO]s = 1, -kt
= In[HzO]
Upon plotting [HtO] against t on semilogarithmic paper, straight lines with negative slopes should have resulted. Figure 5 shows such a plot. Here and there it is possible to draw straight lines through two points which, if continued,
CONTACT TIME, SECONDS
FIGURE 3. CARBON MONOXIDE IN DRYGASES(OXYGEK-AND NITROGEN-FREE), FROM BREWER AND REYERSON
0
2
4 'CONTACT TIME, SECONDS
FIGVRE 4. TESTPLOT FOR SECOND-ORDER
I
R
I
RESC-
mos, FROM BREWER AND REYERSON
Since steam decomposition is determined by direct weighing, while the dry-gas composition is determined by chemical volumetric analysis (with, perhaps, weighing of the carbon dioxide), the two determinations are independent of each other, and plots of reaction rates do not necessarily have to indicate the same thing as plots of dry-gas compositions. I n the above instance they constitute independent checks on each other. That is, the curves of Figures 1, 2, and 3 for temperatures below about 1000° C.point toward a secondorder reaction; Figures 4 and 5 and some calculations for zero and third-order reactions independently show that the data are in better agreement with the assumption of a secondorder reaction than with one of any other order. However, it should be emphasized that while the second-order nature of the over-all reaction is consistent with the possibility that the primary reaction is between carbon and steam to form carbon dioxide and hydrogen, it does not constitute definite proof that such is the case. Although complete gas analyses were made by these investigators, the fuel samples were not analyzed after each test so that weight balances are not available. Calculations based on changes in gas composition with increase in time of contact can tell us little, if anything; such calculations must necessarily (in the absence of weight balances) assume that the hydrogen and the oxygen in the carbon dioxide and carbon
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1941
1281
1
5 10
5
0
N
.1
1
, .05
5
oo7 1.200~C
1 0.0006
.o 1
CONTACT TIME,
0.0007
0.0008
0,0009
0.0010
RECIPROCAL OF ABSOLUTE TEMPERATURE T1 OK
SECONDS
FIGURE 6. VARIATION OF REACTION V~LOCITY WITH TEMPERATURE, FROM BREWER AND REYERSON
FIQURE5. TEST PLOTFOR FIRBT-ORDER REACTION, FROM] BREWER AND REYERSON
monoxide both came from the steam decomposed, and therefore bear fixed relations to each other. When any two of the three constituents (carbon dioxide, carbon monoxide, and hydrogen) are fixed, the third is determined thereby, regardless of which of the water-gas reactions is the primary one. Figure 6,prepared from the velocity constants corresponding to the straight lines of Figure 4, shows the variation of reaction rate with temperature. A calculation of the activation energy from these data yields 26,000 calories per mole. This value is much smaller than those for homogeneous gas reactions at these temperatures but is of the order of magnitude to be expected for a surface reaction involving activated adsorption. Figures 1 to 6 were prepared from data on a lignite char having the following analysis (as received) : Ultimate analysis, yo Sulfur Carbon Hydrogen Ox gen nitrogen (by difference) A& Heating value, oal./gram Proximate analysis, % volatile matter
1.15 75.50 3.13 6.51 13.71 6761 17.57
+
indicating the initial formation of both oxides of carbon. Test plots for first- and second-order reactions (Figures 8 and 9) are inconclusive. The curves of Figure 8 might be interpreted as indicating that diffusion played a part in these tests. However, if diffusion at 1200' C. were an important factor in the case of 5-mm. coke, it should also be an important factor with 3 4 mesh lignite char whereas it apparently was not (Figure 5). Taylor and Neville (14) in 1921 considered the primary stage to be C 4- H20 = CO H1 and that a secondary stage, C 2H20 = C02 2H2, was favored by excess steam, suitable catalysts, and low temperatures. In 1924 Pexton and Cobb ( l a ) used nitrogen to dilute their steam. Figure 10 was drawn from such of their data as
+
333
+
I
I
1
I
+
1
I
I
I
I
I
J
The composition of the ash of the lignite char was given in a later paper (9) as Loss at 800' C. Si01 Ti02 AliOa FeaOa
MnO
0.77% 20.55 0.66 9.88 5.37 0.19
CaO MgO
NaaO Ea0 80s
Pa06
25.40% 9.75 2.89 0.44 22.82 0.043
c1
BaOs BaO
CUO
ZnO
0.123% 0.66 0.165 0.025 0.05 99.79
Data of the earlier important investigations were examined by the same procedure as was used for Brewer and Reyerson data (1). The first important work was carried out thirty years ago by Clement, Adams, and Haskins (3)with a sample of coke of unstated origin and ash composition. A plot of their dry-gas concentrations of carbon dioxide against contact time is shown in Figure 7. These curves do not indicate an approach to 33l/8 per cent at Eero time. On the contrary, the curves for temperatures of goo', lOOO", and 1100" C.appear to approach a considerably lower value,
CONTACT TIME, SECONDS
FIQURB7. CARBON DIOXIDE IN DRYGASES,FROM CLEMENT, ADAMS,AND HASHINS
I N D U S T R I A L A N D rE N G I N E E R I N G C H E M I S T R Y
1282
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Vol. 33, No. 10
1,300'C
.01
,011
1
0
I
I
II
I
2
3
4
5
I
1
7
6
I
I
8
9
CONTACT TIME SECONDS
FIGURE8. TESTPLOTFOR FIRST-ORDER REACTIOS,FROM CLEMENT, ADAMS,ARTD HASKINS
showed a relatively constant nitrogen flow in the wet gases; the dry gas-carbon dioxide concentrations are plotted against contact time. The 1000° C. curve for by-product coke strongly resembles the curves of Figure 1. The data for the other tefnperatures are inadequate to provide any evidence for or against convergence a t 331/3 per cent carbon dioxide a t zero time. Velocity plots (not shown here) indicate a secondorder reaction a t 1100" C. when using the same type of data for llOOo C. as shown in Figure 10. The other plots do not resemble either a first- or second-order reaction, but this may be due to the relatively small number of points and the variable nitrogen concentrations. Pexton and Cobb used cokes prepared by different processes from the same coal, the analysis being Loss at 105' C.
6.5% 33.9 5.8
Volatile matter
Anh
In an earlier paper (11) Pexton and Cobb gave the u1timat.e analysis of this coal as Moisture Carbon Hydrogen Nitrogen Sulfur
4.6%
Oxygen (by difference)
TESTPLOTFOR SECOND-ORDER REACTION, FROM CLEMENT, ADAhfS, AND HASKINS
and the composition of the ash as Si02 AlzOa Fez08 CaO
36.47, 19.1 32.4 4.0 1.4 6.7
+ Ti02
SOa Alkalies (by difference)
I n later experiments (1926) Marson and Cobb (10) prepared a series of coke samples containing synthetic ashes of various composition, using for the purpose a nearly ash-free coal. Chemical analyses of the ashes are given in their original paper. The quantities of ash and its principal constituent are as follows:
Silica coke Pure coke Lime coke Iron oxide coke Calcium carbonate coke
Volatile Matter, % 2.3 3.0 3.4 2.9 4.1
Ash, % 7.2(5% SiOz) 1.3 8 . 5 ( 5 % CaO) 7.9(5% FerOs) 9 . 7 ( 5 % CaO)
Figure 11is a plot of the dry-gas carbon dioxide concentrations against time, prepared from their data. Velocity plots (not shown here) resemble neither a first- nor a secondorder reaction, although six of the seven points for the "pure"
73.1 5.7 1.44 2.5 5.2 7.5
Ash
FIGURE 9.
33: w
3
B
m >
5
10
Y
B y.
g 5 0 z
8
r: 1 0
1
2
0
3
i
CONTACT TIME SECONDS
FIGURE10.
CARBON
DIOXIDE I N DRYGASES, FROM AND
COBB
2
3
CONTACT TIME SECONDS
PEXTOX
FIGURE 11.
C.4RBON
FROM
DIOXIDEI N DRYGASES MARSONAND COBB
(loooo c,),
October, 1941
1283
INDUSTRIAL AND ENGINEERING CHEMISTRY
c
TABLE I. EXPERIMENTAL FEATURES OP THE APPARATUS OB VARIOUS Pexton and Cobb
Marson and Cobb
Thiele and Haslam
Porcelain
Silica
Morgan reaction tube
Quartz
A . Sillimanite B. Quartz
1.5
1.9
2.2
2.2
2.22
A. 2.2 B. 2 . 5
?
26.4 om."
10 grams
10 grams
20 grams
A . 19 om." B . 14 cm.O
5 mm.
5-8 mm.
Reaction tube Material Porcelain Inside diam., em. Sample Quantity Screen size
INVESTIGATORS
Haslam, Hitchcock, Rudow
Clement, Adams, Haskins
SO% 20%
I/+/g I/s-~/IQ
...
in. f in.
Brewer and Reyerson
3-4 Tyler mesh
Thermocouple Material Position
Pt to Pt-Rh
Position of reaction tube
Vertical (steam down)
Inclined (steam up)
Horizontal
Preliminary preparation
?
?
Evacuation and NP Evacuation and Na 5-min. purge to replacement replacement waste
Check on euperheat
None
None
?
Precaution against activation
1
Coke changed after each temperature
Kept coke wt. loss None less than 60%
Gas analysis
Babb Orsat and COz, CO, HP, and COP by soda lime, COP by soda lime; COz by soda lime; U. 8 . Steel Corp. apparatus and CO and Hs on CO, Hz, and CHI others by WilCH4 on Hempel Hg for COZ,HI, by modified Bone liams or Burrell detn. of COa, 0 2 Bone and apparatus CO, CH4, 01, NP and Wheeler Wheeler appaHI, C O , CH4, N* apparatus ratus
Gaa aampling
Collected over Hg
a,
In charge
C h r om el-Alumel ? ? Outside reaction Sheathed in charge Sheathed in charge tube
Grab samples?
Horizontal
1
Through soda lime Through soda lime to gas holder to gas holder
Chromel-Alumel Outside reaction tube
C h r o m el-Alumel In sheath beside reaction tube
Vertical (steam up)
Horizontal Degassing
None
Thermometer in superheater
Fresh or once-used sample
Fresh samples for each test
Collected over water
Sampled glycerol
over
Length of the fuel column in the reaction tube.
coke definitely fit a second-order relation. It will be noted that the carbon dioxide plot (Figure 11) does not agree with this. In 1923 Haslam, Hitchcock, and Rudow (7) studied the effect of pressure variation on the water-gas reaction. Although their data are not amenable to treatment in the manner shown above, the interesting way in which they plot their results indicates definitely that both oxides of carbon are formed simultaneously from coke or electrode carbon and independently of the temperature, at least within the range of their experimental temperatures (650 to 1200" C.). In 1925 Haslam, Entwhistle, and Gladding (6) considered that the following four reactions were taking place in the reduction zone of a water-gas generator:
8C $-f Ei% 1 Ei "o,'+% COz 2co CO
+ Ha0 = COa + Hz
Probably all of these reactions occur, but the data and discussion given by Haslam, Entwhistle, and Gladding do not indicate definitely which is the primary reaction. Haslam, Ward, and Boyd (8) found no radical variation between five different cokes produced from coal from the same mine in their ability to reduce carbon dioxide or to decompose steam. In 1927 Thiele and Haslam (16)found that the weight of
steam decomposed per unit time by active charcoal at 875" C. was almost independent of the steam pressure, provided the same weight of steam per unit time was introduced a t all pressures. This means that the reaction was first order with respect to steam, for obviously at constant contact times the steam decomposed per unit time would have been directly proportional to the pressure. For the reaction of steam with retort carbon these authors found that the weight of steam decomposed per unit time a t temperatures from 1010" to 1125' C. was inversely proportional to the steam pressure when the same weight of steam per unit time was introduced a t all pressures. This effect is probably the combination of a zeroorder surface reaction with a diffusion effect such that in a critical range of steam velocity the reaction products are removed at a rate proportional to this velocity. Pexton and Cobb's data (1%)also make i t possible to calculate the effect of pressure on the reaction of by-product coke with steam a t 1000" C. Such calculations show that under the conditions of their experiments the reaction was between first and second order with respect to steam. I n considering the data as a whole the following facts may be noted: Brewer and Reyerson's data (1) clearly indicated a second-order reaction in the temperature range 7001000" C., both from dry gas analyses and from velocity plots. Pexton and Cobb's data (1%)for 1000' C. indicated a second-order reaction from dry-gas analyses and a velocity plot.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
In two other papers velocity plots for 1000" C. (10) and 1100' C. (3), respectively, indicated a second-order reaction, while the dry-gas analyses did not. The contradictions could conceivably have been brought about by one or both of two things: First, an experimental technique that did not adequately provide for all factors tending to befog the results, and secondly, a mechanism so dependent on its environment that it changes radically upon the slightest alteration of the environment. The question has been raised as to whether the sample itself did not exert great influence on the mechanism of the reaction. We have a t least a partial answer to this in the statement of Thiele and Haslam (16) that their previous results (7) were entirely a t variance with their present (1927) results, and for reasons not known to them at that time. They stated further that the divergence could not have been due to differences in samples, for in some instances they had used the same samples in both investigations. Although this does not prove that the mechanism is independent of the nature of the sample, it does prove that wide divergences may occur which cannot be attributed to differences in samples. Coal and coke are not simple substances, and investigation of their properties is beset with difficulties. As certain data, well known at present, were not readily available before the 1930's, it seems logical to review the apparatus and methods of the different investigators to see if the divergences shown above correlate with any particular type of apparatus or method. Table I shows what the writer considers to be the important features of the apparatus and methods of the various experimenters. The rate of a heterogeneous reaction depends, among other things, on the temperature, the contact surface area, and the partial pressure of the reacting gas in the gas phase. As previously pointed out, reactionvelocity plots and dry-gas analyses should constitute an independent check on each other. It is advantageous, therefore, to consider them separately.
Factors Affecting Rates As is well known, temperature has an enormous effect on the rate. Failure to make accurate temperature measurements of the reaction zone might readily result in rate data indicating almost any type of mechanism. Check readings between thermocouples placed outside a reaction tube and inside the coal charge with no steam flowing do not indicate that the same agreement would hold when steam is passing through the charge. For those cases of steam decomposition by carbon where the order of the reaction is greater than zero, i t is apparent that although the percentage of steam decomposed may decrease with increase in rate of steam supplied, the actual weight of steam decomposed per unit of time is greater. Therefore, because of the endothermicity of the reaction, the higher steam flow rates would create a cooling effect which would increase with increase in rate of steam supply and set up steep temperature gradients. A thermocouple located outside the reaction tube would not detect more than a minor ripple of this cooling, and hence a series of tests at a supposedly constant temperature with different rates of steam supply would in reality be a series varying both in temperature and time of contact simultaneously. A variable and undetermined amount of superheat with possible condensation between boiler and reaction zone would augment any such difference. A detailed discussion of the subject of heat transfer between flowing gases and a bed of broken solids, together with experimental data, was made in 1932 by Furnas (6). Variable contact surface areas may arise from two causes. First, two succeeding samples of the same weight or volume
Vol. 33, No. 10
may contain different surface areas. Large particles and small samples accentuate this. Secondly, the surface area may change during the test. Starting each test with a fresh sample is a partial compensation which becomes total only when the weight loss of the solid is the same for each test; it would be natural not to meet the latter condition experimentally because of the time that would be required to complete some tests. Increase in surface area of coals due to differential oxidation may be considerable (4). It takes place in steam most efficiently a t about 925" C. The effect of catalytic materials such as might be present in the ash of the carbon reactant must be quite large, as is apparent from the data of Marson and Cobb (10). Undoubtedly some of the discrepancies noted in this paper may be due to differencesin composition and amount of ash.
Factors Affecting Composition of Dry Gas Nearly all carbonaceous solids absorb oxygen and most of them contain it as an integral part of their composition, When heated, much of this oxygen is removed as water and oxides of carbon. When a sample has been heated to, say, 900" C., most of this oxygen is in the form of carbon monoxide (producer gas reaction). Unless some means of effectively removing this carbon monoxide is employed, it will be found in the gas sample and create the impression that carbon monoxide was formed initially, regardless of whether i t actually was so formed or not. The fact that the oxygen and hydrogen in the dry gases a g pear in about the same ratio as in water does not necessarily indicate that the oxygen came entirely from the decomposed steam. Ray (13)showed that during the interaction of steam and carbonaceous material, the latter loses a considerable proportion of its hydrogen. Collection of gases over water may result in erratic figures for carbon dioxide, since it is much more soluble than the other gases involved (according to any table of gas solubilities, such as in Lange's handbook, 9). High carbon dioxide concentrations will decrease on account of solubility; and if succeeded by samples lower in carbon dioxide, the latter will increase because a portion of the previously dissolved carbon dioxide comes out of solution to balance the ratio of carbon dioxide dissolved to the partial pressure of the carbon dioxide in contact with the solution. Oxygen has generally been found to be present in small amounts in the gas samples and, unless determined separately, will be recorded as carbon monoxide if either acid or alkaline cuprous chloride is used.
Summary Most of the data pertaining to the mechanism of the watergas reaction are reviewed so as to ascertain the nature of the primary reaction and the order of the over-all reaction. When steam is decomposed by lignite char a t temperatures less than 1000" C., the primary reaction is probably C 2Hz0 = COz 2Hz; this is shown by the fact that the drygas composition of the products approaches 33I/a per cent carbon dioxide and 6 6 2 / ~per cent hydrogen with decreasing contact time. The over-all reaction in the temperature range 700-1200" C. appears t o be close to second order and has an activation energy of 26,000 calories per mole. When the carbon reactant is coke, the available data, while in partial agreement with the lignite char results, are generally inconclusive. The importance of precise control of experimental conditions and of the presence of adventitious catalytic materials in the ash of the carbon reactant is emphasized. The most logical first step for further research appears to be a critical study of the laboratory methods.
+
+
October, 1941
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Acknowledgment The writer is deeply indebted to H. H. Storch, of the Bureau of Mines, whose advice and criticism were sought and generouuly given.
Literature Cited (1) Brewer, R. E., and Reyerson, L. H., IND. ENG.CHEM.,26, 734 (1934). (2) Ibid., 27, 1047 (1935).
(3) Clement, J. K., Adams, L. H., and Haskins, C. N., U. S. Bur. Mines, Bull. 7 (1911). (4) Fieldner, A. C., Hall, R. E., and Gallaway, A. E., U. S. Bur. Mines, Tech. Paper 479 (1930). (5) Furnas, C. C., U. 5. Bur. Mines, Bull. 361 (1932). (6) Haslam, R. T., Entwkistle, F. E., and Gladding, N. E., IND. ENG.CHEM.,17, 686 (1925).
1285
(7) Haslam, R. T., Hitchcock, F. L., and Rudow, E. W., Ibid., IS, 115 (1923). (8) Haslam. R.T.,Ward, J. T., and Boyd, J. H., Am. Gas Assoc. Proc., 1926,1083. (9) Lange, N. A., Handbook of Chemistry, pp. 1036-8 (1939). (10) Marson, C. B., and Cobb, J. W., Gas J.,175,882 (1926). (11) Pexton, S., and Cobb, J. W., Ibid., 163, 100 (1923). (12) Ibid., 167,161 (1924). (13) Ray, A. B., Chem. & Met. Eng., 28, 977 (1923). (14) Taylor, H. S., and Neville, H. A., J. Am. Chem. Soc., 43, 2066 (1921). (15) Thiele, E. W., and Haslam, R. T., IND.ENG. CHEW.,19, 882 (1927).
PRDS~NTDD under the title, “A Review of the Experimental Data Concerninn the Mechanism of the Water Gas Reaotion”. before the Division of Gas and Fuel Chemistry at the lOlst Meeting of the American Chemical Society, 8t. Louis. Mo. Published by permission of the Direotor, U. 8. Bureau of Mines.
Colloidal Stability of Asphalts Spot Tests with Partial Solvents that high-molecular asphalREDICTION of the servHANS F. WINTERKORN tenes surrounded by resinous ice behavior of asphaltic AND GEORGE W. ECKERT’ dispersing agents are distribmaterials by means of University of Missouri and Missouri State uted in a medium of oily and simple laboratory tests is imHighway Department, Columbia, Mo. resinous materials. According portant to both the asphalt to this picture and to well producer and the road builder. established colloidal principles The conventional physical concerning the stability of specification tests, which were The Oliensis spot test appears to be essendispersions, the colloidal stavaluable as quality indicators tially a method for the colloidal stability of an asphalt should debility as long as only a few well-deof a bitumen. The great interest of users of pend on the effectiveness of fined asphalts were on the asphalts in this test indicates the practical the adsorbed resins as a transimarket, are failing in this importance of the colloidal stability and of tion zone from the physical role in view of the increasand chemical properties of ingly varied products which methods for its determination. Data are the asphaltenes to the propnow are sold under the collecpresented on the appearance of spots oberties of the surrounding liquid tive name of “asphaltic bitutained with a number of different solvents medium. The differences in men”. and solvent mixtures as well as results obtype and quantity of the It hrts been found that tained with the standard Oliensis proseveral phases obtaining in failure of bitumen to function various asphalts are reflected properly in a pavement is usucedure. It is hoped that these data help in in the behavior of the asally associated with excessive the understanding of the Oliensis test, and phalts toward partial solvents. hardening. AB long as this also that some of them may provide a basis Consequently, it is probable hardening is caused only by loss for the extension of this test into fields not that the colloidal behavior of volatiles, i t can easily be yet covered. of systems consisting of parremedied. On the other hand, tial solvents and bitumen may this hardening may be due to a not only permit specific conchange in the colloidal structure clusions concerning the colloidal stability of the systems of the bitumen. Therefore, methods which indicate or measunder consideration, but may also indicate the general colure the stability of this structure are likely to become inloidal stability of the bitumen, provided due regard is given creasingly important in the future testing of bitumen. Acto the chemical and physical character of both bitumen and cordingly, any method now available for this purpose desolvent. The latter provision indicates the necessity of workserves theoretical and experimental examination of its possiing with more than one solvent or one solvent ratio if we debilities and limitations. sire to obtain a general indication of the colloidal stability. From a colloidal viewpoint, asphalts may represent sols or any of the many possible gel-like atructures, depending upon I n the work described here, the spot test was employed as the prevailing conditions of temperature and pressure, their an indicator of the colloidal Stability. I n the standard prochemical and mechanical composition, and their previous cedure, as worked out by Oliensis ( I ) , 10.2 cc. of naphtha are history. However, the same general picture of bituminous added to 2 cc. of asphalt. After the asphalt is dissolved under structure is common to all these possible states-namely, controlled conditions, a spot is made by applying a drop to a No. 50 Whatman filter paper. Spots which have a dark nu1 Present address, The Texas Company, Beaoon, N. Y.
P
