SIR JAMES DEWAR'S COCONUT CHARCOAL H. B. H A S , Purdue University, Lafayette, Indiana
C. T. KNIPP, University of Illinois, Urbane, Illinois
AND
A. R. PADGETT* Purdue University, Lafayette, Indiana
A study of the method of preparation and adsorptive pro#erties of the charcoal reported by Dewar in 1908 shows that it was comparable to the best modern commercial activated charcoals and was far superior to charcoals used for defense against military toxic gases i n 19151918. The cause of this high adsorptinrity i s explained as being due to a carbon dioxide-steam activation produced a limited access of gaseous combustion products and atmospheric oxygen.
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HE versatile genius of Sir James Dewar has for many years excited the admiration of chemists and physicists. He is known as the inventor of vacuum flasks and of the propellant explosive "cordite"
and as the man who first liquefied and froze hydrogen. In connection with his investigations of the gas-ad'Present address: Baytown. Texas.
sorbing powers of charcoal cooled to low temperatures and the application of this phenomenon to the production of high vacua he published in 1908l an article containing the following sentences. "All charcoals possess the property of gas absorption a t low temperatures. Light charcoal, such as is used in the manufacture of gun-powder, or the variety got from animal substances like blood, both act, hut experiments a t the Royal Institution show that charcoal prepared by careful carbonization from coconut is one of the most convenient varieties to use. While using the same process of carbonization, the absorptive effect is enhanced by making i t take place slowly and with
gradually increasing temperature. With the samples a year ago, it was possible to get an absorption of about
' DEWAR,J., Chem. N m s , 96, 6 (Jan. 3, 1908)
150 cc. of air per g. of charcoal a t -185'C., but with care the amount can be raised to as much as 350 to 400 cc. per g. "The amount absorbed a t atmospheric pressure and a t the temperature of liquid air can he quickly determined. A gram of charcoal, previously heated to a red heat is placed in a glass bulb connected by an India rubber &be and stopcock to a graduated vessel containing air kept over strong sulphuric acid or a high-boiling oil, so that on cooling the charcoal and opening the stopcock the absorption is measured." The adequacy of Dewar's admittedly abbreviated description of his method of producing coconut charcoal has been the subject of some differenceof o p i n i ~ n .For ~ this reason additional details have been added here concerning the method of producing coconut charcoal a t Cambridge a t a time approximately contemporaneous with the article containing the sentences just quoted. These are available from the personal recollections of one of the present authors (C. T. Knipp). This coconut charcoal was prepared by the writer of these paragraphs to assist him in producing the highest vacuum possible necessary in a research on "Rays of positive electricity from a Wehnelt (hot lime) ~ a t h o d e . " ~A brief account of its preparation and how used in the above research may be of interest. Coconut shell broken into convenient sized bits was placed in an open porous crucible of about one pint capacity. A cover was improvised out of heavy sheet asbestos previously burned to remove the volatile material. I t usually fit quite snugly. For the ignition and distillation of the shell an oven was crudely built up of fire brick into the open front of which was directed a blast operated by a foot bellows. After combustion was completed (judged by the absence of visible vapors and smoke) the glowing coals were quenched by dumping them on a sheet of thick asbestos (previously burned) and quickly covered by burned sheets of thin asbestos snugly pressed down. The charcoal thus produced reduced to granules about the size of a grain of wheat by placing it between sheets of tough paper and applying a hammer. Considerable powder resulted. No attempt was made to screen this out. For immediate use the charcoal was kept in glass bottles with ground (not luhricated) stoppers; however, for future use it was placed in vacuum bulbs and thoroughly out-gased by sub-
merging same in boiling water before sealing off from the pumping system. No attempt was made at the Cavendish Laboratory to study quantitatively the conditions in the preparation of the charcoal that would result in the best product. My interest a t the time in coconut charcoal and liquid air was a means to an end-was in the production
was
% National Carbon Company us. The Western Shade Cloth Company, Equity Case No. 13,520, Northern District of Illinois. a KNIPP, C. T., Phil. Mag., 22, 926-33 (1911).
and maintenance of a high vacuum, and this was successfully accomplished. Two of the writers (H. B. Hass and A. R. Padgett) had occasion in 1934 to correlate the air absorption test of Dewar with the "service life" and "retentivity" tests which were developed for the evaluation of charcoal for use in gas masks. The service life test consisted essentially in passing a definite mixture of dry air and chloropicrin vapor a t a certain rate through a specified column of dry charcoal granules of an arbitrary range of sizes until a detectable amount of chloropicrin es-
capes the adsorbent. Except for the detail of drying the air, this test is described by Fieldner.' The retentivity test consisted in first saturating the charcoal with chloropicrin vapor and then exposing it to a temperature of 100°C. a t an absolute pressure of 2 mm. of mercury. It was weighed a t half hour intervals and the curve plotted showing the per cent. of chloropicrin retained (based on the original weight of dry charcoal as one hundred per cent.) as ordinate and time 'as abscissa. When the losses became practically constant the straight portion of the curve was extrapolated to zero time and the retentivity read off the vertical axis.
Now let us return to the coconut shells in the unglazed crucibles. As the temperature is raised destructive distillation occurs, and the vapors and gases evolved expel the air from the crucible. Dewar's "gradual carbonization" yields a charcoal of a desirably high density compared to the product of a rapid carbonization. As the temperature approaches bright redness the destructive distillation becomes more and more complete and finally
METHOD OF PREPARING CHARCOAL SAMPLES
Unglazed crucibles of the fire-assay'type were filled with coconut shells, covered with lids of unglazed clay, and gradually, over a period of several hours, brought to a bright red temperature. If the charcoal was cooled and tested shortly after noticeable fumes ceased to be evolved, it absorbed about 150 cc. of air per gram a t - 185OC., indicating that this was substantially Dewar's procedure when he obtained charcoal of that activity a year prior to the publication cited. If the charcoal is maintained in this temperature range its activity gradually increases. This is equally true whether the heating is done continuously or intermittently. By prolonging this treatment, charcoal of activity equal to or greater than that of Dewar's later samples can easily be obtained. The correlation between service life and Dewar's air absorption test is shown in Figure 1. The relation between retentivity and Dewar's air absorption test is evident from Figure 2. Figure 3 is a typical retentivity curve. I t appears from these data that Dewar's samples must have had a service life of approximately seven hundred minutes and a retentivity oi about thirty-eight per cent. I t has been stated that a service life of about one thousand minutes and a retentivity of about forty per cent. is the maximum exhibited by adsorptive carb0n.j When i t is remembered that ordinary crude coconut charcoal shows only 0-5 minutes and zero retentivity when these same tests are applied, i t is evident that Dewar had a highly activated product. The question naturally arises as to how such carbon results from the simple procedure of heating coconut shells in an unglazed crucible. This can best be nnderstood in the light of one of the well-known commercial processes for making activated coconut char~oal.~ The shells are first subjected to destructive distillation and the crude charcoal thus obtained is partially oxidized by steam and/or carbon dioxide a t temperatures in the range 800-100O0C. The partial oxidation imparts activity, perhaps to some extent by removing tarry impurities, but chiefly by an etching action which increases the extent and "the unbalanced molecular attraction" of the internal surface.
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'FIELDNER, A. C., G . G . OBERFELL, M. C. TEADUE, AND J. N. LAWRENCE, I. Ind. Eng. Chem.. 11, 519-24 (1919). CHANEY, N . K., U.S. Pat. 1,497,543,
air diffuses inwardly and reacts with the pyroligneous vapors, yielding superheatedsteam and carbon dioxide. The gases produced by the burner which is usedfor heating the crucible contain steam and carbon dioxide as combustion products. Steam reacts with the charcoal yielding carbon monoxide and carbon dioxide; the latter reacts with the charcoal producing carbon monoxide. These combustible gases react with any inwardly diffusing oxygen and prevent it from reaching the charcoal. If the crucible cover is slowly removed a t this point the formation of a blue flame gives evidence of the presence of carbon monoxide in the atmosphere which surrounds the charcoal when the cover is in place. The charcoal is thus being activated by partial oxidation with steam and carbon dioxide. The length of time required for the development of optimum adsorptive capacity depends upon many variables such as the porosity of the crucible and cover, the closeness of fit of the cover, the size of the crucible, and how much charcoal is in it, but with care and patience Dewar's values can always be obtained. When this has occurred the charcoal is invariably found to be highly active toward chloropicrin.
ACKNOWLEDGMENT
The authors wish to thank Dr. 0. L. Barnebey for calling the attention of one of us to the Dewar article
and to Dr.F. J. Allen for the establishment and measurement of the temperature of -185'C. for the air absorption tests.
