CORRESPONDENCE
CORRELATIONS FOR THE COEFFICIENTS OF GASEOUS DIFFUSION
S I R : \Ve: the authors of the article ‘:New Generalized Equation for Gas Diffusion Coefficient” IJ, Chem. E r g . Data 7 , 41 (1962)]. are pleased. indeed: with the care with which Dr. Scott has revie\ved our paper. Although this article was published in Januaiy 1962. most of the work was done in 1958. At that time. experimental dara on binary gas diffusion coefficients \\-ere scattered and not too plentiful. Lye made reference to the available data lvhich had just then been collected and published in the contemporary book b? lleid and Sherbvood ( J l and in the article by Lt’ilke and Lee (0)of a little more than two years earlier. These authors may have considered as experimental those systems noted in Dr. Scott‘s table. Lt-ilke and Lee also considered those systems in their correlation ,,recommended” by Emmert and Pigford ( 2 ) as follo\vs : ”\Vhen experimentally determined diffusivities are not available. several prediction methods based on the kinetic theor)- are available to provide estimates. \Vhen accurate estimnfr., are desired, the \Vilke and Lee modification. . . .is recommended.” (Emphasis is theirs.) Hence, a number of reputable \corkers have included those systems in their studies. h-o one commented adversely- as to studies on these data until the article by Scott and Cox (5) which appeared after the development of our correlation. Unfortunately, the Scott and Cox article was published in a foreign journal and did not attract our attention after \ve made the correlation, and before i t \vas published. \.e apologize sincerely to Dr. Scott for missing his articleand yet the ease of missing articles in the press of another countr)- is exemplified in his o\vn letter received by the editor in November 1963. In the type\rritten text of this letter, he made an insertion in ink (presumably just before the letter was mailed) Ivhich only then noted a n Othmer and Chen article (.?) ichich had breii published a full year before-and only seen when Dr. Srutt was mailing his letter to the editor. W e apolugizr a5ain to Dr. Scott that we were not so fortunate as to chancr upon his paper---say when ours was in galleys! It is inter 20--1>26-7, 31-2. 49, 53. 59. Of these 71 in our table: only 15: including t\vo from (a) for misplacing the reference, are listed in his table: Nos. 18, 19, 30, 40, 41, 43, 47: 51, 58, 61>63, 66: 68, 69: and 70.
I t is quite impossible to compute Dr. Scott’s value of 53. Dr. Scott’s “14” probably means items (3) ivhich have been answered in a previous paragraph. His “3” may refer to item (a) which is a typographical mistake. T h e literature reference is the source where the data can be found and this may seldom be the original. However, the reader can trace back to the original if he wants. This method of referencing is universal, in view of the necessity of saving space; and D r . Scott’s own letter. also the Scott and Cox article? does not follow the Euclidian logic of going back to axioms. T h e listing of Arnold and Reid and Shenvood as references in items ( c ) and (d) shows the reader of our paper that those values were considered and justified by those well knoLvn scientists. Also. \Vilke and Lee utilized much of these data to come u p Lvith a correlation recommended by Emmert and Pigford; and their evaluation. in turn. \vas given credence by Perry‘s Handbook. I n the table of Dr. Scott’s letter. each of the VOL. 3
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“quoted by” entries of the Othmer and Chen article which he says were “described erronerously,” is in excellent company. We had two feiver “errors” than the leader, Reid and SherLvood, but three more than Wilke a n d Lee. Quite correctly, D r . Scott characterizes all sin as wickedness in his fulsome admonition, “This repetition of erroneous data, and data of lois accuracy of uncertain origin, emphasizes the wellk nown dangers of using third- or fourth-hand information in correlation work.” But we would like to submit that data or correlations going through the hands of three or four competent scientists in their attempts to correlate o r determine mechanisms or mathematical models d o not necessarily lose thereby as w~ouldthird- or fourth-hand evidence examined in a court of law. As long as all the correlations are based on, or used with, the same experimental data, the respective value of the correlations may be compared. It’hat difference if some are a third- or fourth-hand attempt? O r would Dr. Scott consider only the first correlation made with any given data? We quite agree that only accurate data should be used-if available; and if there are ever sufficient, then no correlation for prediction will be necessary-one merely uses experimental results ! Also. kve agree that no data should be of “uncertain origin,” and we believe there is no question of the origin of the experimental data which we quoted. T h e geneology can always be traced. IVe confess that we didn’t point out a bar sinister here and there-some data reported by others were definitely calculated instead of measured ! Above: in item (e) of the 71 examples in our table, only 15
are listed by Scott as being calculated rather than measured, and he calls ”foul” on these. However, for our purpose a comparison of the existing correlations still can be made by eliminating these 15 systems. T h e results of such a comparison for the average absolute deviation d o not change relative positions greatly: Arnold, 15.71%; Gilliland, 15.20cc; HBS (estimated force constants), 9.57%; HBS (known force constants), 5.75%; Slattery, 9.2070; and our proposed correlation, 5.26%. As noted above. it is worthwhile to study the same data a third or fourth time; this may now be fifth h a n d ! But the result is about the same! literature Cited
(1) Boyd, C. A , , Stein, N., Steingrimsson, V., Rumpel, W. F., J . Chem. Phys. 19, 548 (1951). (2) Emmert, R. E., Pigford, R. L., “Gas Absorption and Solvent Extraction,” in Perry’s “Chemical Engineers Handbook,” 4th ed., McGraw-Hill, New York, 1963. (3) Othmer, D. F., Chen, H. T.: Ind. Eng. Chem. Process Design Dcuelop. 1, 249 (1962). (4) Reid, R. C., Sherwood, T. K., “ T h e Properties of Gases and Liquids,” McGraw-Hill, New York, 1958. (5) Scott, D. S., Cox, K. E.? Can. J . Chem. Eng. 38, 201 (1960). (6) Wilke, C. E., Lee, C. Y., Ind. Eng. Chem. 47, 1253 (1955). .Ving Hring Chen Donald F. Othmer Distinguished Professor Polytechnic Institute of Brooklyn 333 J a y St. Brooklyn, S.Y .
CORRESPONDENCE
CORRELATIONS FOR THE COEFFICIENTS OF GASEOUS DIFFUSION
SIR: M r . Scott calls attention to a situation which certainly needs correction. I suspect that many authors are in the habit of quoting published physical constants without checking on the original work and that this is done in fields other than molecular diffusion. Critical reviews going back to the experimental studies are much needed in this and other areas. Mr.
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FUNDAMENTALS
Scott would serve the profession well if he \could provide such a review, on which he has obviously made a good start. Thomnr K . Shemood
Departmelltof Chemicai Massachusetts Institute of Technology Cambridgz, M a s s .
