September, 1923
,
I N D U S T R I A L A N D ENGINEERING CHEiWISTRY
947
Measurement of t h e Relative Absorption Efficiencies of Gas-Absorbent Oi Is'" By Robert E. Wilson and Harold S. Davis MASSACHUSETTS INSTITUTE
OF
TECHNOLOGY AND ARTHUR D. LITTLE,INC.,CAMIBRLDGE, MASS.
I
T IS well recognized The relatiue absorption eficiencies for benzene of eight gasthe methods described in absorbent oils were measured by two vapor tension methods developed detail in the Previous artithat there is a need The results, together independently by the writers. The results show that the sewn for Some reliable methpetroleum distillates, which uaried considerably in boiling range ~ i t other h significant data Od of testing the absorbby T * G. ing capacity of gas-absorand method of production, were strikingly similar in the volume of benzene which could be absorbed by a given volume of the oil under Delbridge, of the Atlantic bent Oils, for the recoT'erY Refining Company, for the of "light oil'' from gas, and specified conditions, The differences in molecular weight of the oils should have produced uariations in absorption eficiencies if first fxven oils, are given in for some information as to Table 1. Plots ofthe mobRaoult's law held, but, strangely enough, these differences were the desirability of including ular weight data are given such 8. test in specifications. just about counterbalanced by he deviations from Raoult's law. The coal-tar distillate was found to be a somewhat better absorbent, in Figs. 1 to 3, inclusive. The writers in CoOPeration with Subcommittee 11 of but certain other properties constitute serious objections to its use The molecular weight Committee D-2 of the in most commercial installations. determinations were all made without any knowiAmerican Society for TestIn the light of the results, it does not appear necessary to preed@ of any other Properties ing &laterials have applied scribe any absorption or uapor pressure test for gas-absorbent oils of the oils, but when the their resPectil'e methods of of the ordinarg range of compositions. data were compared with measuring vapor pressures The molecular weights, specific gravities. and boiling points of the specifications, the cont o this Problem. Work by the uarious oils are giuen. sistency of the results was the two methods was carried on quite independently, and quite gratifying. Thus it the results were compared upon its completion. However, appears that: the fundamental data involved were so similar that it seemed ( a ) The higher boiling cuts had molecular weights about desirable to combine the results of the two sets of experi- forty points higher than the corresponding lower boiling cuts. ( b ) For a given boiling range there is very little difference ments into a single paper. between the molecular weights of the cracked and uncracked 011,s
USED ASD MOLECULAR WEIGHTDATA
At the request of the committee, seven samples of gasabsorbent oil3 were prepared by the iltlantic Refining Company to meet t'he following specifications: 1--4 straight-run, midcontinent distillate, over point about 450' F., 80 per cent distilling below 680" F. 2-Representing approximately one-half Sample 1 of low boiling point. 3-Representing approximately one-half Sample 1 of high boiling point. 4-Oil of approximately the same distillation ranges as Sample 2 but acid-treated. 5-Oil of approximately the same distillation ranges as Sample 3 but acid-treated. 6-Highly cracked, midcontinent distillate of about the same distillation ranges as Sample 2. 7-Highly cracked, midcontinent distillate of about the same distillation ranges as Sample 3.
I n addition to these seven, the writers desired to test a coal-t,ar oil* similar to those frequently used in England for benzene recovery. Since absorption efficiency might be expected to vary inversely with the molecular weight of the oil5 (if Raoult's lam held), it seemed desirable to make careful determinations of this constant. Most of this work was done by F. J . Guerin at the Massachusetts Institute of Technology, by 1 Published as a joint contribution from the Research Laboratory of Applied Chemistry of Massachusetts Institute of Technology (Contribution No. 67) and from the laboratories of Arthur D. Little, Inc. 2 Presented before t h e Division of Petroleum Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8 , 1922. 3 These oils were obtained through t h e courtesy of C. A . Lunn, chairman of Subcommittee I1 on Gas-Absorbent Oils of the American Society for Testing Materials. 4 Ohtained through the courtesy of The Barrett Company. THIS JOURNAL, 10, 718 (1918).
products. (c) Acid treating reduces the average molecular weight of the oils from eight to ten points. Since the loss on treatment is reported by Dr. Delbridge as 1 to 2 per cent, it would appear that the average molecular weight of the material removed in acid treatment was over 500; possibly much of it was really colloidal. TABLE I-PROPERTIES
O F GAS-ABSORBENT OILS TESTED Savbolt Initial Viscosity Roiling 50 Per cent at Point Point Dfy Mol. 70'F. O F . O F . Point Wt. 46 449 546 760 228 40 666 426 504 214 56 776 497 588 253 504 204 41 668 432 ~24 5.. 588 _ 57 774 502 47 567 21s 680 464 54 514 590 250 708 620 455 180 167 796 ~
No. 1 2 3 4 5 6 7 8
Gravity A . P . I . Sp. Gr. 0.841 36.8 0,832 38.6 0.851 34.8 0,832 38.6 0.850 35.0 0.853 34.4 0.858 33.4
1.10
~
~
~
~~
~~~~
~
METHODS FOR MEASURING ABSORBINGCAPACITY The two methods employed were similar in that they measured the vapor tensions of dilute solutions of benzene of definite concentration in the various gas-absorbent oils, the better absorbers giving the lower vapor tensions. In using the Davis method, described in detail in the subsequent a r t i ~ l e ,solutions ~ containing four parts by volume of benzene in 100 parts by volume of the absorbent oils were made up by measurement of definite quantities of the liquids from standardized, graduated pipets (2 cc. and 50 cc.). To measure the vapor tension of benzene from each of the resulting solutions, 2.3 cc. (1 per cent of the volume of the flask of the apparatus) were measured into one of the small glass containers. The upper stem of the container was then drawn out to a fine capillary, which was cut off to about 2 em., but not sealed. Freshly distilled, C. P. benzene was, used. The greater part came over within less than 0.5" C. 8
7
'
Wilson a n d Wylde, THISJOURNAL, 16, 801 (1923). Davis and Davis, to appear in October issue of THISJOURNAL.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
948
Vol. 15,:No. 9
.Feu
260
ti
B
$
Bd5 '5
1
Res&!/?, M ?z J p . s m L c m ,
0 0
FIG.1
FIG. 2
To illustrate the methods of calculation and correction employed, full details are given below for the first test on absorbent oil No. 1.
The Wilson-Wylde method and calculations were described in detail in the preceding article.6 Most of the oils were tested with only one concentration of benzene-approximately 5 per cent by weight (the precise concentrations differed slightly and were of course taken into account in plotting the results). Measurements were made a t a series of temperatures between 20" and 120" C. The temperature coefficients were in all cases small and similar to those for gasabsorbent oils Nos. 2 and 3, for which the complete curves are given in Fig. 5 . Since the 60" C. values are considered the most reliable, they were used for comparative purposes.
Time Min. 0 3
Pressure
PROCEDURE P u t apparatus in bath, closed stopcocks
Mm.of Oila
o n
-13.4 -13.8 -13.9 -13.9 -13.9 Broke container inside flask 61.3 0 84.0 10 93.1 15 97.2 20 101.0 25 102.5 30 103.9 35 103.9 40 Total pressure developed (uncorrected) = 117.8 Temperature of bath = 25' C. Barometer pressure = 76.7 cm. mercury Gas-absorbent oil No. 1 was used as the manometer liquid in all tests.
IO 20 25 30 5
a
This result must be corrected for two factors. ( a ) The changes in the volumes of the two flasks of the apparatus due to the movement of the manometer liquid. This was calculated from the following data:
1
DISCUSSION OF RESULTS
Unfortunately, since the problem was originally attacked independently by the two laboratories, the concentrations used in the two cases were appreciably different, making direct comparison impossible. Furthermore, the Davis measurements were all a t 25" C., while the Wilson-Wylde results were most reliable at 60" C. It therefore seemed best to compare the results graphically in order to study the deviations from Raoult's law as shown by the two sets of observations. The observed vapor tensions were therefore
Volume of I-cm. length of manometer tube = 0.052cc. = 234 cc. Average volume of the two flasks = 767 mm. mercury Barometer pressure
The change in volume of the air in each flask for a change 0.052 in the levels of the manometer liquid of 1 cm. = 2 cc., corre0.052 X 767 234 = 0.085 mm. of sponding to a change in pressure of mercury, so that the total correction due to the changes in pressure in both flasks is 0.17 mm. of mercury. Now, 1 cm. of the oil used for manometer liquid (sp. gr. = 0.833) = 0.0614 cm. of mercury. Consequently, the observed 0.17 X 100 = 27.5 per cent to reading must be increased by o,614 correct for this factor. ( b ) The change in concentration of the solution due to the partial evaporation of benzene into the flask: Let v = volume of flask in cubic centimeters w = grams of benzene in the solution in the container P = benzene pressure developed in millimeters of mercury Then a t 25" C. the percentage of benzene which evaporates 0.00042 P v into the flask is
I n the case of the test above this correction is 13.9 per cent,
so that the total correction factor by which the readings
should be multiplied is 1.275 x 1.139 = 1.45. The corrected vapor tension of a 4 per cent solution of benzene in this absorbent oil is therefore 170.8 mm. The detailed results on all the oils are discussed in a subsequent section.
calculated as percentages of the saturation tensions at the same temperature and plotted against the molecular fraction of benzene in the solution, as shown in Fig. 4. The lines are drawn straight through the origin and the observed points, as d6manded by Henry's law, which should hold fairly accurately in this region. By comparing the intersection of the two sets of lines with the 12 per cent saturation lines a t which they end, it is possible to compare closely the results obtained by the two methods. The consistency of the results is on the whole quite satisfactory, considering the large corrections which must be
September, 1923
I N D UXTRIAL A N D ENGINEERING CHEMIi3TRY
949
I
J7
MOL ECULRR
PRESSURE OF BENIL-NC IN SOLUTIOH
FIG.4
applied to the first method and the manipulative difficulties of the second. I n both sets of results, oils Kos. 2, 4, and 6 (the three low boiling ones) are grouped together with deviations slightly above Raoult's law, while the three high boiling cuts, Nos. 3, 5, and 7, are similarly grouped with negative deviations, oil No. 1 occupying an intermediate position. Oil No. 8, the coal-tar distillate, gave much higher results in both sets of experiments. Even the relative positions of the oils in the two groups are fairly consistent, but the variations between the members of the groups are so small that this must be considered as more or less accidental (note that the figure is drawn on a very large scale). As B whole, the Davis results are 2 to 3 per cent lower than those of Wilson and Wylde, which is not surprising under the circumstances. The ratio of liquid volume to air space is about four hundred times greater in the latter than in the former case, so the tendency is to get high results by the latter method and low ones by the former. The average should be fairly near the truth. The discrepancy would be a little greater if the less reliable 25" C. values of Wilson and Wylde were used. At any rate, there can be no serious doubt as to the reliability of both methods within a very few per cent, and the comparative results between different oils by the same method are probably good within 1 per cent if proper precautions are taken. Since the method of plotting in Pig. 4 placed the seven oils in two groups as far as deviation from Raoult's law is concerned, it might be expected that this division signifies real differences in absorption efficiencies. Surprisingly enough, however, the magnitude and direction of these deviations are very nearly such as to counterbalance the differences which would be caused by the variations in the molecular weight if Raoult's law held.
MOLECULAR PCR C E N T
B E N Z E ~IN ESOLUTION
FIG. 5
In other words, the oils which would be expected to give better absorption because of lower molecular weights deviate from the predictions of Raoult's law just about enough to nullify this apparent advantage. As a result the percentage by volume of benzene absorbed by the different oils under given conditions is substantially the same for all, as indicated by the following tabulation of results: RELATIVE AMOUNTS O F BENZENE ABSORBED FROM AIR 12 PER CENT SATURATED WITH BENZENE VAPORBY DIFFERENTGAS-ABSORBENT OILS
MOLARPER CENT ABSORBED Wilson PER CENT Molecular Sp. Gr. and BY BY SAMPLZ Weight -60"F. Wylde Davis Average Weight Volume 1 228 w0.841 11.42 11.93 11.68 4.33 4.15 2 214 0.832 11.22 11.62 11.42 4.48 4.26 4 204 0.832 11.09 11.29 11.19 4.60 4.38 6 218 0.853 1 1 . 0 0 11.46 11.23 4.32 4.18 A v e r a g e 2 , 4 , 6 212 0.839 11.10 11.46 11.28 4.47 4.27 0.851 3 253 1 2 . 5 8 13.06 12.82 4.33 4.22 5 245 0.850 12.00 12.40 12.20 4 . 2 3 4 . 1 3 7 250 0.858 12.30 13.37 12.83 4.39 4.32 A v e r a g e 3 , 5 , 7 249 0.853 12.29 12.94 12.61 4.32 4.22 Averageof all 230 0,845 11.66 12.16 11.91 4.38 4.26 1.10 9.32 9.32 9.32 8 167 4.57 5.64
While there are certain variations in the last column for the first seven oils, they are barely beyond the experimental error, are not consistent with any of the properties of the oils, and are quite without significance commercially-a difference of 1" to 2' C. in temperature making more difference in the absorption capacity in any one than any of the variations observed between the oils themselves. Since there is no consistent difference in the percentages absorbed by volume, the slight advantage of the lighter oils shown in the weight per cent column is due to their lower specific gravities. Gas-absorbent oil No. 8 stands out from the petroleum products in all the tests. Its deviation from Raoult's law is very large and positive, as might be expected from the chemical dissimilarity between an oxygen compound and a
INDUSTRIAL A N D ENGINEERING CHEMISTRY
950
hydrocarbon. Kevertheless, this apparent disadvantage is more than overcome by the very low molecular weight of the coal-tar oil, and the percentage of benzene absorbed by weight is slightly higher than that taken up by the petroleum oils under similar conditions. This oil is so dense, however, that the volume percentage held is considerably greater. Its excessively high viscosity (especially when cold), greater cost, and difficulty of handling in steam distillation would almost certainly rule it out under present conditions in this country, although the English claims of greater absorption efficiency appear to be justified. At the same time, the possibility of using coal-tar products under special conditions should not be overlooked.
Vol. 15, No. 9
conclusions regarding the problem of testing gas-absorbent oils: I-Cost considerations and certain necessary physical properties-especially viscosity a t low temperatures and initial boiling point-practically limit the choice of gas-absorbent oils for benzene in this country to a comparatively narrow range of mineral-oil fractions. All the samples tested appear to have absorptive capacities so nearly identical that the addition of a rather complicated absorption test t o the other physical tests required does not appear t o be justified. 2-While the method used for molecular weight determination is quite satisfactory for the purpose, the result is not directly significant as a measure of the absorptive capacity of the oil, and hence should not be made the basis for any requirement. &-For research purposes, involving the study of a wider range of absorbent oils for benzene or absorbents for other volatile solvents, either of the two vapor tension methods referred to, with the specified precautions, will give satisfactory results. For the comparison of absorptive capacities a t a single moderate temperature, the Davis method is undoubtedly simpler, but if information is desired a t a number of different temperatures, and especially a t temperatures above 40' C., the Wilson-Wylde method is more reliable and requires less time.
Motion Picture Films The International Committee of Young Men's Christian Associations, New York, N. Y., has secured a number of new films since the publication of the list in the March issue of THIS JOURNAL. Among these are the following:
MOLLCULHR PER CLNT ELNZCNL IN FIG.6
5OtuTION
*
In order to study the deviations from Raoult's law over the whole range of concentrations, more observations were made by Wilson and Wylde on the three representative absorbent oils. The results on Kos. 2 and 3 are shown in Fig. 5 and those on No. 8 in Fig. 6. The curves indicate that the deviations from Raoult's law are consistent over the whole range of concentrations. They also bring out the effect of temperature on the relative vapor tensions of the different solutions, higher temperatures giving slightly lower relative vapor tensions. A comparison of the benzene absorption capacities on a weight basis, of gas-absorbent oils Nos. 2, 3, and 8, is shown in Fig. 18 of the previous article. Throughout the whole range, the similarity between all the hydrocarbon oils is indeed striking, considering the wide variations in molecular weight, etc.
RECOMMENDATIONS REGARDIXG A N ABSORPTION TESTFOR GAS-ABSORBENT OILS After a careful comparative study of these two methods and the results obtained, the writers agree to the following
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