Ih’DUSTRIAL A N D EhTGINEERING CHEMISTRY
August, 1928
points between 1 :1 and 1 : 4. Special apparatus and treatment are necessary to dissolve any but the most alkaline silicates. These are not so useful as the more siliceous ones, so for convenience silicates are sold mostly in solution. They are, however, shipped in as concentrated a form as is consistent with easy handling. Thus a liquid silicate in which the ratio is NanO:2 Si02 will flow somewhat like molasses when it contains 54 per cent solids. At this concentration a solution of Naz0:3.25 Si02 ratio will be a glassy solid. It will pour freely a t a concentration of 38 per cent. In order to save freight on shipping large quantities of water, new factories sprang up closer to the consuming markets. In addition to those a t Anderson and Chester, plants have been established a t Buffalo, N. Y.; .Kansas City, Kans. ; Rahway, N. J. ; and St. Louis, Mo. Recently the silicate plant of the Central Commercial Company a t Utica, Ill., was acquired. In 1917 the Philadelphia Quartz Company of California was formed, establishing a plant a t Berkeley, Calif.
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Many other uses for silicates have been found. Most of these depend on the colloidal properties. The Philadelphia Quartz Company maintains engineering and chemical staffs, who not only work on manufacturing problems but who are available for consultation and, when desirable, for cooperative work to help the consumers secure the full advantages of silicate. The general headquarters of the Philadelphia Quartz Company are at 121 South Third St., Philadelphia, Pa. A Chicago office was recently opened a t 205 West Wacker Drive. Among the executives are: President, William T. Elkinton, vice president, Alfred C. Elkinton, both grandsons of the founder, Joseph Elkinton; vice president, Thomas W. Elkinton; secretary and chemical director, James G. Vail; treasurer, F. Algernon Evans; general representative, George W. Goudy; sales director, J. Passmore Elkinton; manager of Purchasing and Traffic Departments, William Martin; chief engineer and production manager, Lloyd B. Edgerton.
NOTES AND CORRESPONDENCE Number of Plates in Fractionating Columns Editor of Industrial and Engineering Chemistry: In the article on the “Design of Fractionating Columns” by Keyes, Soukup, and Nichols, in the May, 1928, issue of INDUSTRIAL AND ENGINEERING CHEMISTRY, it is shown that if one chooses a reflux ratio of 5 mols of reflux per mol of product, there results a more or less constant proportionality between ( a )the number of plates computed from an operating line having a slope O / V = 0.833 and (b) the number of plates figured from the diagonal which has the slope O / V = 1, and that this assumption leads to a slight simplification of the McCabe and Thielel method of determining rectifying column heights. It should be definitely pointed out that if a very small reflux ratio is chosen the above proportionality fails, and the simplified method will give results that are too low. Reflux ratios that are considerably less than 5 are of practical significance. As the reflux ratio is increased, it is true that the height of the column does decrease, but per unit of product both the volume of vapor and the necessary cross section increase. The following table shows the effect of changing the reflux ratio upon the relative dimensions of the column. It is based on the continuous separation of an equimolal mixture of benzene and toluene, to give a distillate containing 99.9 mol per cent benzene and bottoms containing 0.1 mol per cent benzene. REFLUX RATIO
o/v
0,582 0.636 0.667 0.767 0.833 1.000
REVLUX RATIO
VAPOR
OIP 1.39 1.75 2.00 3.29 5.00
2.39 2.75 3.00 4.29 6.00
Infinite
V/P
Infinite
NO. OF RELATIVE PERFECT VOLUME PLATES O F COLUMN N Vxn/P
41 31 28 24 21
15
98 84 84 101 126
Infinite
By interpolation from the table, one finds that the minimum volume of the column, 83 relative units of volume, corresponds to a reflux ratio O / P = 1.8. For this case the minimum reflux ratio, corresponding to an infinite number of plates, is 1.34. Even with heat costing nothing, one would always here use a reflux ratio greater than 1.34 but less than 1.8. Considering the cost of heat by making an economic balance, the optimum reflux ratio would be even less for this case. IND. ENG.CHEM., 19, 907 (1927).
The modified method suggested by Keyes, Soukup, and Nichols, then, should be applied very cautiously if the optimum reflux is much less than 5. W. I,. MCCABE DEPARTMENT OF CHEMICAL EXGINEERING UNIVERSITY OF MICHIGAN ANN ARBOR,MICH. June 24. 1928
.......... Editor of Industrial and Engineering Chemistry: In reply to a criticism of our recent article, I quite agree with Doctor McCabe when he states that our system of determining the number of plates will give a lower figure than if his very low reflux ratios are used. The difference is not great from a practical standpoint. The desirability of using very low reflux ratios and designing the column accordingly is debatable and should not be argued here. It may be‘of interest to point out that our method of determining the number of plates in a fractionating column has been used ten years and we have yet to find any serious defect in it. D. B. KEYES UNIVERSITY OF ILLINOIS URBANA, ILL. June 25, 1928
Corrected Nitrogen Figures Editor of Industrial and Engineering Chemistry: In the article entitled “Present Status of Coal By-product Nitrogen,” by Mildred S. Sherman, IXD. ENG.CHEM.,20, 80 (1928), exports of nitrogen salts are neglected. In the last paragraph on page 80 it is stated that the excess of consumption of nitrogen over domestic production is 178,080 tons. From government statistics I note that in 1926, 40,000 tons of nitrogen were exported, mainly in the form of ammonium sulfate. Consequently, the excess of consumption of nitrogen over domestic production in 1926 was nearer 138,080 tons than the quantity calculated by Mrs. Sherman. CHAPLINTYLER LAZOIE,INC. WILIIIINGTON, DEL. May 23, 1928
