Dec., 1921
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
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of flow through a filter cake varies directly as the first power of the pressure and inversely as the first power of the thickness must be accepted as true.” That the rate does not vary directly with the pressure is shown by experimental results obtained under careful laboratory conditions and published in THISJOURNAL in 1912. 3-If we assume the filtration law used to have the same coefficients in both constant pressure and constant rate runs and if the values of n in the different runs gave close checks, then Mr. Sperry’s method of computing n and ours should agree. However, both of the above assumptions are incorrect. Mr. Sperry’s coefficient of n = 0.7 applies to the constant pressure runs, whereas our coefficient n = 2.0 applies to the constant rate runs. The text referred to n = 2 for constant rate runs and the curvesin Graph 2 are labeled “constant rate.” Mr. Sperry’s method could have been used on the constant
dV rate runs (i.e., for constant thickness of sludge, v, plot log d% against n log P, the slope of the resulting line giving the value of n). When the rates were 158 and 194 cu. ft. per hr., respectively, the points lie too close for accuracy, but even so, this method of calculation indicates n a t least equal to 2. In our
n
- €or each constant rate run was obtained article the value of m by plotting the log of pressure vs. the log of volume. The three n n runsgave? = 1.7; ; = 2 . 1 and - = 2 . 3 . Fromtheconstant m m pressure runs Ve was proportional to 8 and, therefore, m = 1, and since in all sludges studied m has had values of 0 . 8 to 1 0
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poses probably holds for incompressible sludges such as sand or kieselguhr, but it will not hold for gelatinous precipitates. 6-In regard to the criticism that our plant tests were made on commercial apparatus of too crude a nature For determining factors of a fundamental forrimla, I wish to call attention to the fact that we were not trying to determine the formula, as that had been done by Lewis’and Almy years before. We were simply applying the formula to plant-scale equipment for the purpose of testing the application of theory to practice. While we admit that Lewis’ equation is not a t present complete for constant rate of flow, its simplicity and also its complete applicability to constant pressure conditions made it satisfactory as a starting point for investigations in constant rate of flow filtrations. That the mathematical treatment of filtration is still undeveloped is apparent when one tries to determine for a given sludge the best filtration method, i. e., the proper thickness of cake and whether the cycle should be a t constant pressure or constant rate of flow, together with the proper pressure or rate. SCIKOOL OF CHEMICAL ENGINEERING PRACTICE M A S S A C H U S E T T S I N S T I T U T E O F TECHNOLOGY
F.P. BAKER
CAMBRIDGE, MASS. September 6, 1921
The Variability of Crude Rubber
Editor of the Journal of Industrial and Engineering Chemistry: In a recent paper [THISJOURNAL, 13 (1921), 5191 Mr. J. B. Tuttle describes results of experiments which led him to the we took m = 1 ; therefore, n was equal to 2. If m did equal conclusion that a simple mixing of rubber and sulfur is not suitm able for the purpose of testing the variability in rate of cure of 0.8 instead of 1.O the value of n would be approximately 1 . 6 plantation rubber, because (1) the variability shown by such a instead of 2. What interested us and what we were trying to simple mixture is much greater than that met with in the factory show was that for constant rate runs n was greater than unity. where it is usual to add varying quantities of organic and in4-‘With respect to the fourth criticism we believe it better organic accelerators, and ( 2 ) because one particular synthetic to plot all data and draw a representative line than to compare product is found actually to retard vulcanization except in point with point. Graph 2 in our article shows that the points the presence of zinc oxide, when it acts as a powerful accelthemselves vary and we took the general trend or slope of these erator. The author therefore concludes that all tests on crude points as our representative line. rubber should be performed on mixtures to which have been 5-Although not pointed out by Mr. Sperry, the equation added from 2 to 5 per cent zinc oxide. which he proposes as fundamental is designed to be applicable As my name is mentioned with others as favoring the simple only to runs made under constant pressure, and hence is not mixing of rubber and sulfur for testing purposes, I would take general. This is true because it is derived by means of an in- the opportunity of pointing out that I have used for some time tegration in which the pressure P is placed outside of the sign a mixing containing zinc oxide but of late have confined myself of integration, and the expression integrated, holding P constant. to a simple rubber-sulfur mix for routine testing purposes as As shown by the tests described in our article, constant rate of I did not find that the presence of zinc oxide had any appreciable flow filtration is fundamentally different from constant pressure effect on the results. Mr. Tuttle will admit that it is only a and the equation derived for one is not directly applicable to few powerful accelerators such as the CS, condensation product the other. of dimethylamine and homologs, termed by Bruni “ultraaccelFurther, the real use of a law of filtration should be to afford erators” which are much influenced by the presence of basic a means of calculating sizes of filter equipment, which involves oxides, in particular zinc oxide, and he has not shown that accelan area term. At no place in Mr. Sperry’s equation does such erators of this type are present in crude rubber. Unless such are an area term appear, nor is it readily apparent how the equation present the addition of zinc oxide is superiluous. could be translated into terms of area of filtering medium. I am in many ways in agreement with Mr. Tuttle’s general It is acknowledged that the equation of Lewis and Almy used arguments, in that I consider the basis of the mixture employed in calculating the results of the tests described was derived for for testing should be broadened so as to include ingredients constant pressure operation and is not strictly applicable in commonly used in the rubber factory. It is however impossible its present form to constant rate of flow conditions. As a result, to make up and test a large number of different mixtures for important progress is being made in research work to alter the each sample of plantation rubber under examination. Under equation so that it will hold for both constant pressure and con- these circumstances it is wisest to employ the simplest mixture stant rate of flow conditions. However, the equation as it is possible, namely, rubber and sulfur only. Moreover, provided holds rigidly for constant pressure runs a t different pressures, a sufficiently large proportion of sulfur is used, this mixture has whereas the equation proposed by Mr. Sperry does not, as can the advantage that it accentuates variations inherent in the be shown by plotting the data of Lewis and Almy, and deter- raw rubber. It is consequently more sensitive than mixings mining the constants in Mr. Sperry’s equation at various pres- containing organic and inorganic accelerators. On the other sures. These are found not to be constant a t all fordifferent hand, the almost exclusive use of this simple mixture by chemists pressures, when the runs are made with the same sludge under in the East has given rise to a false impression as to the variaotherwise identical conditions. The equation Mr. Sperry pro- bility of plantation rubber, as it has been assumed that variations
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similar to those found with this mixture would also be experienced with the technical mixtures in the factory. If Mr. Tuttle will refer to my publications he will note that I have carried out many experiments with litharge and other ingredients and have emphasized the importance of a wider basis for raw rubber testing.
Vol. 13, No. E2
sarily are not comparable). I feel, and from what he has written I believe that Dr. Stevens agrees with me, that the amount of this variability is very much less than has been popularly supposed to exist. We have been sticking to an antiquated and wholly insufficient test because of the seeming impossibility HENRYP. STEVENS of getting a new and better one adopted. Perhaps I did not go far enough in my suggestion regarding the improvement in the 15 B O R O U G H HIGH ST. LONDON BRIDGS,s. E. 1 testing formula; it might have been better to have included LONDON, ENGLAND the suggestion that to the rubber, sulfur, and zinc oxide, we add August 4, 1921 sufficient organic accelerator to cure properly the standard high grades of plantation rubber. There can be no logical objection Editor of the Journal of Industrial and Engineering Chemistry: to organic accelerators; we know that some crude rubbers contain them, and they materially aid in the vulcanization, and to Dr. Stevens states that he has tried zinc oxide and did not find it to have any appreciable effect on the results. It will add such an accelerator would merely tend to a greater uniformity. be quite apparent that zinc oxide will eliminate any retarding Just which one should be used is a matter which could be decided effect caused by small amounts of acid and hence cannot fail by the leading plantation chemists. The consumers are in the to have some effect in reducing that part of the variability unfortunate position that by reason of their ignorance as to the which is due to impurities in the rubber, and not to variations manner of preparation, they are unable to make these tests effectively-they must be made at the place where the rubber is in the rubber itself. He is quite mistaken in assuming that it is only a few accel- prepared. It will be a great step forward if the planters and conerators, such as the dimethyldithiocarbamates, which are in- sumers could get together on this subject and find out the true fluenced by the presence of basic oxides; some of the weaker amount of variability, its causes, and the meansfor its elimination. JOHN B. TUTTLE accelerators, such as thiocarbanilide, are absolutely ineffective 68 BANKS?. in the absence of basic fillers. In this country, it is an almost NEWYORK, N. 1‘. universal custom to have some basic oxides present in any comAugust 20, 1921 pound containing organic accelerators, although there are some with which this is not necessary. The Nitrogen Supply of the World There can be no question regarding the impossibility of using a large number of formulas for testing rubber; one should suffice, Editor of the Journal of Industrial and Engineering Chemistry: and the simpler the formula, the better. However, we must The nitrogen demand and supply has of late been the subject not lose sight of the fact that we are testing the quality of the among us of much thought and discussion. It is indeed one of . crude rubber, and not whether or not there are minute amounts the most important and far-reaching problems with which the of organic accelerators which have been retained during the chemist has to deal. For this reason, it has seemed to me worth process of coagulation, etc. Hence, we should use the simplest while to prepare for the benefit of the readers of THISJOURNAL formula which will develop the facts regarding the crude rubber a brief digest of a recently published German book’ dealing with only. If the simple formuIa of rubber and sulfur would do this, this subject. there could be no objection to its use, but the facts are against The title of the book is “The Nitrogen Supply of the World;” it. Dr. Stevens’ experiments with zinc oxide and with litharge the author, Walter Eucken. It discusses the subject solely show that he, like many others, has felt the urgent need for some from the point of view of economics, mentioning only incidentally better way of testing crude rubber than by the simple formula the chemical, biological, and technical aspects of the subject. of rubber and sulfur. He admits that the almost exclusive The periods before, during, and after the world war are treated in use of this formula by the chemists in the East has given rise separate sections. to a false impression as to the variability of plantation rubber. In the section devoted to the pre-war period an account is Well, if the use of this formula by the plantation laboratories given of the development, both of the demand for and supply produces false impressions, what else can we expect in the labora- of.nitrogen throughout the world. There is also a careful detories of the consumers? Is it not logical to suppose that they scription of the world’s nitrogen market as it then existed, inwill have as large variations in their crude rubber tests as have cluding its organization, its method of fixing prices, and the been shown by the chemists of the Far East? And when they syndicates and cartels which dominated it. do obtain these differences, will they not feel a certain prejudice The section devoted to the war period is divided into two against plantation rubber which may be, and in my opinion is, parts, one dealing with the situation in Germany, the other absolutely unwarranted? with that in allied and neutral countries. The first part is Dr. Stevens has surely compared the rate of vulcanization especially interesting to us. The author states that the total by means of the rubber-sulfur mixture, of an exceptionally clean, German nitrogen supply on hand in April 1914 plus that captured thoroughly washed rubber. with the so-called “slab rubber.” later in Antwerp, Ostende, and northern France amounted to The former, containing practically no impurities, will hardly 50,000 tons. In 1913 to 1914 the coke ovens and gas works vulcanize a t all; the latter, with all sorts of fermentation prod- in Germany produced 110,000 tons, but with the initial slump ucts, vulcanizes rapidly. A vulcanization test of this type in the iron industry at the beginning of the war this dropped to would condemn the former; in other words, we’are placing a 65,000 tons of nitrogen per year. There was a small arc instalpremium on fermented rubber; on delay in coagulation: on lation (Pauling) at Muldenstein, near Bitterfeld, in the brown poorly washed rubber; on general carelessness and poor manage- coal region designed to operate op off-peak power in peace time, ment. In the face of the excellent results obtained in the in- and another similar plant was started a t Zschornewitz in the dustry with the use of plantation rubber, we dare not accept same region in 1915. There was also an arc plant a t Rhina the logical conclusion. Obviously there is an error in our argu- in Baden operated by electricity supplied by Swiss water power. ment and equally obviously, the error lies in the means by In 1917 the plant a t Muldenstein had to shut down onaccount which we measure quality. 1 “Die Stickstoffversorgung der Welt, Eine volkswirtschaftliche Practically every one interested in the subject believes that Untersuchung,” by Walter Eucken. Deutsche Verlags-Anstalt, Stuttgart, there is some variability in plantation rubber (confining our 1921. 1 8 5 p p . 2 3 . 5 X 1 5 . 5 cm. Price M. 28. Bound in half linen, argument to standard high grades only, the poorer grades neces- M. 35. ..e...........
