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same, pure silica, no single gel is consistently the best adsorbent for all gases. T a b l e IV-Comparison of Adsorption in Various G e l s (Air stream saturated a t 30’ C.) “Gel from “Gel from Silica Gel iron” nickel” Per cent weight Per cent weight Per cent weight SUBSTANCE ADSORBED of gel of gel of gel Carbon disulfide 42.5 81.2 18.6 Carbon tetrachloride 46.6 91.5 69.2 Chloroform 50.7 88.3 57.2 Ether 24.6 66.2 25.8 Benzene 32.5 50.0 96.8
Effect of Silica Gels on Petroleum
Silica Gel was successfully used to remove objectionable sulfur compounds from petroleum fractions. The writers tried filtration of Lima crude oil containing 0.96 per cent sulfur through “gel from iron,” and found it decidedly more eficient in sulfur removal than the ordinary type of silica gel prepared according to Patrick’s patents. Both gels were powdered so that half passed through screens between 40 and 100 mesh and half between 100 and 200 mesh. Ten grams of powder were well shaken with 40 grams of oil and the mixture was thrown on a filter bed of 10 grams of powdered gel in a tube 1.6 cm. in diameter. Filtration was hastened by air pressure equal on both tubes so as t.0 secure 10-gram filtrates for analysis. T a b l e V-Removal
of S u l f u r C o m p o u n d s f r o m L i m a Crude Oil b y Different Types of Silica G e l
CIULFUR CONTENT OF OIL-
ADSORBENT GEL Silica Gel “Gel from iron”’
Before filtering Per cent 0.96 0.96
After filtering Per cent 0.67 0.33
A little more sulfur could have been removed by longer filtration, but the results indicate clearly that a silica gel with larger capillaries can be more efficient in sulfur removal than the usual type containing only those capillaries left by evaporation of water. Analysis of duplicate samples checked within 0.02 per cent. The first fractions filtered through “gel from iron” were as colorless as water, the next fractions gradually taking on a clear yellow, then a deeper color, until finally the coarser capillaries were clogged with colloidal material. The usual type of silica gel removed no color whatever from Lima crude oil. Adsorbed material may be burned out and the gels used again. Nitrogen Adsorption at Low Temperatures
R. B. Moore, of the Bureau of Mines, compared “gel from iron” with Silica Gel as to capacity to separate nitrogen from helium a t liquid air temperature. The time period for Silica Gel was 26.0 minutes, whereas for “gel from iron” it was 35.5 minutes. By “time period” he meant the point a t which the break in the adsorption of nitrogen took place. Chloropicrin Test
Galloway and Hall, of the Pittsburgh Station, Bureau of Mines, reported a chloropicrin test on “gel from iron.” The tube test showed 21.03 minutes with an activity of 20.03 per cent. This is evidently not equal to the “40-minute carbon” prepared by the Bureau of Mines. Toluene Retentivity
Katz and Hall, of the Bureau of Mines, compared the writers’ silica gel with the bureau’s best carbon in toluene adsorption and in retentivity. By their saturation test they mean the amount of toluene adsorbed per gram of activated adsorbent when exposed to saturated toluene vapor at 25’ C . By retentivity they mean the amount of adsorbed gas held apparently in a form not readily removed. The carbon took
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up only 42.1 per cent of toluene and retained 21.5 per cent, whereas the silica gel prepared by the writers adsorbed 56.25 per cent and came out again cleanly, as it should, with only 3.9 per cent retained. Discussion
The loss of water molecules in the drying process gives capillarity to the usual silica gel. I n the writers’ gels are found all of these capillaries and, in addition, coarser ones formed by removal of ferric oxide or other metallic oxide. It is well known that the best gas-adsorbent charcoals contain very fine capillaries whereas the best decolorizing charcoals contain coarser capillaries. I n this connection it is noteworthy that Lima crude oil is perfectly decolorized by “gel from nickel” and by “gel from iron,” while the better known Silica Gel fails to remove any color. Our acid-treated gel is chalk-white and less dense than the clear, glassy Silica Gel. Fortunately for the economy of the process, the ferric chloride used is all recovered by the treatment of the red gel with hydrochloric acid. Nickel chloride and other metallic salts may be recovered in the same way and used againin precipitation of the gel. The effort to secure porosity by mixing with any gel some soluble material which may later be washed out after drying the gel did not yield very satisfactory results. I n an application for patent the writers pointed out that superior porosity is obtained by securing an intimate mixture of a hydrated metallic oxide with hydrated silica in the act of mutual precipitation; then drying to a rigid solid and later removing the metallic oxide by treatment with a suitable acid. Patrick states that when ferric chloride, for example, precipitates silicic acid it is the acid of hydrolysis of the ferric chloride that really precipitates silicic acid from the sodium silicate. That this accounts for only part of the action is evident. When enough acid to equal that liberated by complete hydrolysis of the ferric chloride used was added to the water glass no precipitation occurred. The trivalent positive ferric ions must have a strong precipitating effect on the colloidal silicic acid always found in a solution of sodium silicate. The porosity of these gels is greatly affected by the rate of drying-the slower the better. The composition of the water glass is also a factor, as is the amount of excess (beyond neutrality) of ferric chloride used. The intermediate dry red gel used in preparation of white “gel from iron” showed a ratio of Fez03 to Si02 of 1:3. The uses of these various forms of silica gel must include solvent recovery, drying of air for the blast furnace and for the vacuum ice process, recovery of sulfur dioxide and oxides of nitrogen, removing offensive odors from air, removing sulfur compounds from petroleum fractions, decolorizing certain liquids, recovery of gasoline from natural gas, and other uses to be developed. Nok-Since writing this paper the authors prepared a more efficient “gel from iron” by slower drying of a thicker layer. The final product adsorbed 67 per cent of its own weight of benzene from an air stream saturated at 30’ C.
Decrease in Production of Sulfur in 1924 The production of sulfur in the United States in 1924 dropped to 1,220,600 long tons from 2,036,097 long tons in 1923, a decrease of 40 per cent, as shown by figures compiled in the Geological Survey. The shipments also decreased, but only to the extent of 5 per cent. The shipments in 1924 were thus the second largest on record and for the first time since 1920 were greater than the production. The estimated value of the shipments in 1924 was $25,000,000, compared with $26,000,000 in 1923, at approximately the same rate per ton. About 300,000 tons (10 per cent) of the stocks in hand at the end of 1923 were shipped in 1924.
