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regardless of whether you start with products containing water or not, and this explains why, in some of our own experiments, we have demonstrated that the highest dielectric properties in these phenolic resi’ns have been obtained in processes which started from watery solutions. Dr. Redman says: “This cannot possibly be true, as the formaldehyde and ammonia would leave 60 per cent of water present in the reaction, which is troublesome a t any time.” May I remind him of the plain fact that it does not matter a t all whether 60 per cent of water-which worries him so much-is present a t the beginning of the reaction, as long as it is not present in the end-product; and, this is easy to take care of. His reasoning is about the same as if he would say that rubber cannot be made dry because it has been washed with water, or that shellac cannot be freed from water, after it has been dissolved, bleached and precipitated in aqueous solutions. He might extend this reasoning and say that saltpeter cannot be obtained anhydrous because it has been made from a watery solution. &‘or is it difficult to explain our practical results. For instance, the phenolic resin molding mixtures which have found the most extensive use on the market, on account of their great mechanical strength, as well as their dielectric properties, are all made with wood-fiber as a filler. This is covered by my U. S. Patents Nos. 942,852 and 949,671. It is a known fact that wood-fiber, in itself, even after drying, contains water, which can be set free by heating a t somewhat higher temperatures. The same is true of cotton and paper, and yet, we know that all these products are excellent insulators. It all depends upon the condition in which the water is present But there are other considerations. Unduly large amounts of ammonia in presence of wood-fiber act in some way to lessen its dielectric properties Ammonia seems to fasten itself on the wood-fiber, perhaps decomposes some of the substances contained in the fiber, and thereby attracts some of the water which heretofore was tied and harmless. Furthermore, the presence or absence of f r e e water is objectionable only for electrical purposes. But the presence of alcohol, glycerin, free cresol, or free phenol, is just as harmful for these purposes, because they all tend to lower the dielectric properties. In view of this fact, it is rather illogical that Dr. Redman, after being so emphatic about “dry” products, should be so lenient as to recommend glycerin, or alcohol, and as to use an excess of phenol or cresol, as indicated by the proportions specified in his patents-“Straining at a gnat and swallowing a camel ” As to his statement that “The use of free formaldehyde and a small amount of ammonia makes a resin with a blinding, stifling odor of formaldehyde, which cannot be used commercially,’’ this is hard to reconcile with the plain fact that here and abroad, millions of pounds of articles made from such products have been successfully manufactured and sold, and thelr use is all the time increasing. If n r . Redman will give himself the trouble of purchasing, for instance, in any store, a sample of a Bakelite cigar-holder, he can easily verify what everybody knows, but what he seems not to desire to know, t h a t the product is absolutely tasteless and odorless. I should like to submit the following questions to Dr. Redman. ( I ) Does he know of the existence of any industry here or abroad, based on infusible, insoluble products, before I had started publishing my results on this subject, and before I had shown what can be accomplished with this material?” And now let us come to “the test of the pudding ( 2 ) Has he ever manufactured with his “dry” process-which he claims is SO superior-molding mixtures containing wood-fiber as filler and which show any higher dielectric properties than, or as good dielectric properties, as those which we are able to obtain by the use of our “wet” processes? If so, can he give me
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the name and address of anybody or any concern in New York City who uses this molding mixture? I am willing to submit my samples for a comparative test before impartial, competent witnesses. YOLKERS, N. Y.,May 19, 1916 L. H. BAEKELAND
ON THE EFFICIENCY OF AIR-DRYERS Editor of the Journal of Industrial and Engineering Chemistry: In the 1909 and 1910 issues of the Journal of Metallurgical and Chemical Engineering appeared a series of articles on the efficiency of air dryers. This discussion apparently ended in a misunderstanding, and inasmuch as the fundamental factors involved were not developed during that discussion, it seems highly important that the matter should be taken up again. The concept of efficiency always involves comparison with a perfect method for accomplishing the result desired. That this is true will be more readily granted if one will call to mind the fact that to designate the efficiency of an apparatus or process as greater than 100 per cent always arouses suspicion and leaves in the mind the impression that an error, either in the work or calculation, is unquestionably involved. It is undoubtedly true that efficiency can be defined in other ways than by involving this concept of perfection, but the fact quoted certainly indicates the close connection between this concept and our ordinary use of the word efficiency. With this point in mind the following definition of efficiency is offered. The efficiency of a machine can be considered from either one or the other of two points of view: ( I ) the input can be considered as constant, in which case the efficiency is the output of that machine divided by its theoretical output: or, ( 2 ) the output of the machine may be considered as constant, the efficiency being the theoretical input divided by the actual input. That these two definitions are entirely equivalent will be evident without further explanation. With regard to the use of the word “theoretical” as applied to output and input in the above definitions, the theoretical performance of an apparatus must be considered as the performance of a perfect type of machine or process under conditions which eliminate all losses due to friction, radiation or other irreversible processes. I t will always be impossible actually to realize such a theoretical apparatus or process because such losses as have been mentioned will always be present, and furthermore, such an apparatus will in general operate under substantially equilibrium conditions so that the driving forces will be infinitesimal, the rate of the reactions correspondingly small, and the capacity in consequence negligible. On the other hand, these idealized conditions must serve as the basis of our concept of efficiency. When we come to test the efficiency of a steam boiler, of an electric motor, or of other such types of apparatus no disagreement in the application of our definition will be met. The reason for this is that everyone can agree as to the amount of both input and output of such a machine and as to exactly what a perfect machine should accomplish. For example, an electric generator should take the mechanical energy delivered to it in the form of work and convert it quantitatively into electrical energy. No electric generator does this, but knowing as we do the mechanical equivalent of electrical energy we can calculate the electrical work which should be obtained from a certain amount of mechanical energy, and thereby can immediately express the efficiency of any given generator. In the same way the perfect steam boiler should take the heat energy of the fuel and deliver it quantitatively as energy in the steam produced, and while no steam boiler does this, the actual amounts transferred are readily determinable and the heat efficiency of such a boiler is a thing on which all engineers readily agree. On the other hand, certain types of apparatus are met concerning the efficiency of which agreement is not so easy. For
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example, a steam engine takes energy in the form of heat in steam and converts a part of it into work. The fraction of this energy converted into work is small. This fraction is generally called the thermal efficiency of a steam engine and rarely exceeds 2 0 per cent. Every engineer realizes, however, that even the perfect steam engine could not possibly convert all of the energy of the steam consumed into work even under ideal conditions of operation, but when engineers attempt to calculate the fraction of the energy content of the steam which the perfect steam engine should be able t o convert into work, misunderstandings develop as to conditions of operation, allowable assumptions, and similar points, which make it impossible for an agreement to be realized. As a result of this condition of affairs engineers have very wisely given up all attempt to express the efficiency of a steam engine in any terms comparable to those indicated by the above definition, agreeing to express efficiency by what is called performance, the pounds of steam consumed reduced to definite though hypothetical conditions-per horsepower-hour of mechanical energy produced. With regard to the efficiency of hot air dryers, the question immediately presents itself : “Theoretically what is the minimum heat necessary for the removal of water from an inert material such, for example, as sand, rock, etc.?” The consideration of two methods for the evaporation of water from such material will enable us to answer this question. First, let us assume that the material is dried, not in an air dryer, but by introducing the material into an ordinary vacuum dryer. The steam coming from this dryer is to be utilized, not by sending it to a condenser, but as the source of heat in the heating surface of another vacuum dryer of the same type. The steam from this dryer is to be utilized for evaporating more water in another similar apparatus and so on. I n an ideal apparatus only a differential temperature difference between the steam condensed in any dryer and the material being dried in that same apparatus will be required. In other words, with any finite temperature difference between the heat supply a t one end of the series of dryers and the temperature corresponding to the vacuum maintained a t the other end, an indefinitely large number of effects may be interposed. If now the heat contained in the dried material from each one of these effects be utilized to preheat the material entering that particular effect in a suitably designed counter-current preheater, it is evident the heat supplied to the first dryer can be re-utilized an indefinitely large number of times. I n other words, the heat consumption necessary t o evaporate a pound of water under such conditions can, theoretically a t least, be reduced to any desired quantity, however small. Since this theoretical heat consumption is negligibly small, but since the actual heat consumption in any real apparatus cannot possibly be reduced indefinitely, the efficiency of any actual dryer of this type as compared with the theoretical performance of a perfect machine is zero, or a t least indefinitely small. It may be objected that this argument applies only to the use of a dryer of the vacuum type and not of an air dryer. This, however, is not the case. The second way to accomplish air drying with a minimum expenditure of energy is to spread the material out under a shed and let the wind blow over it. The material can be dried perfectly in this way, at least provided the material is really inert toward water; and while heat has been consumed in the process, this heat is obtained from the inexhaustible supply available in our surroundings and need not be furnished from any exterior source. While the actual energy consumption of such an operation is finite. it can be carried on if desired to an unlimited extent, and the fuel consumption required would, of course, be zero. This outline will convince the reader that the theoretically minimum heat consumption for the drying of an inert materiala t least the heat consumption which it is necessary to furnish in the form of fuel-is a quantity both infinitesimal and in-
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definite. It will be agreed that some definite, tangible method of reporting the performance of such dryers should be adopted and made uniform, so that the comparison of the performance of different dryers of this type will be rendered easy and intelligible. This is exactly the line which the mechanical engineering profession has followed in adopting the steam consumption per horse-power-hour as the unit for the expression of the performance of the steam engine. The writer suggests the adoption by the chemical engineering profession of the actual heat consumption, expressed in B. t. u. per pound of water evaporated, as the unit for stating the performance of all drying and evaporating systems. If the dryer uses steam as its source of heat the dryer should be charged with the heat content of the steam supplied less the heat content of the condensed steam which is available for return to the boiler. I n the case of a dryer fired directly with fuel, the dryer should be charged with the total heat of combustion of the fuel without credits, unless a t some point in the apparatus or process heat is recovered for utilization in other processes than that of drying alone, this other apparatus or process being considered as essentially a part of the drying unit. It is evident that this point regarding the theoretically minimum heat consumption for the evaporation of water from inert materials applies also to the evaporation of water itself. I n other words if water is merely to be redistilled it is theoretically possible to use an indefinite number of multiple-effect evaporators for this distillation, and the theoretical heat Consumption necessary for the evaporation of a pound of water can be reduced to any desired extent. Therefore, the performance of any evaporative system should as before be expressed as the heat consumption per pound of water evaporated. Personally, the writer feels that this method of stating performance should apply to the evaporation of solutions as well as to evaporation of water, whether or not mixed with inert materials. On the other hand, the energy consumption in the evaporation of solutions cannot be reduced indefinitely, and furthermore is capable of exact formulation, so that the efficiency of any given evaporative system can be expressed as a per cent, if so desired. The reason for this lies in the fact that the heat consumed in evaporating a solvent from a solution is not again available a t the temperature of the boiling solution, because the vapor evolved from that solution is superheated to the extent of the boiling point raising of the solution, and will not give out its latent heat of vaporization until it is cooled to the extent of that superheat. There is, therefore, necessarily a finite drop in temperature between adjacent effects of a multipleeffect evaporating system, and this drop in temperature will be very large for strong solutions. This drop could be made the basis of a calculation of the theoretically minimum heat consumption of an evaporator, but this calculation can be carried out somewhat more simply as follows: Assume a solution boiling under definite pressure: this pressure, p , is less than the pressure, Po,of the pure solvent a t the same temperature. Assume a certain amount of solvent removed from this solution by evaporation. The heat content of the vapor can be recovered without loss in temperature by first compressing i t a t constant temperature until its pressure is equal to Po,that of the pure solvent. The vapor thus obtained can now be introduced into the heating coils of the effect from which the vapor itself came and there condensed a t a temperature differentially higher than that of the solution on the other side of the heating surface. The heat of condensation will thus be available for the evaporation of more water from the original solution. The work necessary for the isothermal compression of the vapor from the pressure p to the pressure Po is the necessary energy consumption for the separation of water from the solution in question. If we assume that the vapor obeys the gas laws this work is equal to the expression R T log p o / p . Xf
T I I E J O L - R S A L O F I-VDZ’STRZAL AAVD E.!VGILVEERING C H E M I S T R Y this assumption that the vapor follows the gas laws be not allovrrable, the work can be calculated from the characteristic equation of the vapor. The heat available for evaporation will be not only the heat of condensation of the saturated steam, but also the heat produced by isothermal compression of the vapor between the pressure limits indicated. This heat supply will in general be nearly though not exactly equal to the heat of vaporization of the solution in question. The difference must be supplied or disposed of as the case may be. It is evident from the above that the thing required for the separation of solvent from a solution is not primarily heat, but energy in the form of work. It is possible to furnish this energy through the mechanism of a drop in temperature, so that heat falls from a higher temperature to a lower one. This furnishes the work through a mechanism which is essentially equivalent t o a Carnot heat engine. This is the explanation of the necessity of a drop in temperature between succeeding effects of a multiple-effect evaporating system concentrating solutions, even though that system be ideal in all respects. The possibility of operating a commercial evaporating unit upon the principles outlined above was first suggested t o the writer by P. H. Sadtler, of the Swenson Evaporating Company, in 1910, and we understand that such a unit is now in actual industrial operation on an experimental scale in this country. While it is theoretically possible thus to calculate the minimum consumption of both heat and energy for the removal of solvent from any solution, the determination of the necessary data and agreement as t o the necessary conditions of operation would be so difficult that the method is inadvisable, a t present a t least, for reporting commercial tests. The writer therefore recommends that the heat consumption of all types of apparatus for the evaporation of volatile substances be reported as B. t. u. per pound of substance evaporated and that this method of reporting experimental results be made mandatory by the American Chemical Society upon its members. W. K. LEWIS RESEARCH L A B O R A T O R Y OF APPLIEDSCIENCZ MASSACHUSETTS INSTITUTE OF BOSTON,
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May 2, 1916
IMMEDIATE AND CONTINUED LIME REQUIREMENT vs. ACTIVE AND LATENT SOIL ACIDITY Editor of the Journal of Industrial and Engineering Chemistry: In THISJOURNAL, 8 (1916j, 341, there appeared an article cntitled “A New Apparatus for the Determination of Soil Carbonates and New Methods for the Determination of Soil Acidity,” by Prof. E. Truog, of the University of Wisconsin. This article contained reference to an article entitled “A Method for the Determination of the Immediate Lime Requirements of Soils,” by the undersigned, which appeared in THISJOURNAL, 7 (191j)>864. From comment in the same paragraph containing reference to the work of the undersigned one might easily infer that no previous lime requirement method had taken in consideration this difference between immediate and continued lime requirement or “soil acidity.” To quote Prof. Truog, “Akllof the methods just described have been of value in that the results indicate in a comparative way the degree of acidity. None of them, however, indicate the absolute amount of acidity as has sometimes been assumed. Lately, Hutchinson and MacLennan and also MacIntire have described methods in which a solution of CaCOd in carbonated water is used.” Again t o quote from the preface t o the description of the method devised by the undersigned and cited above-“The studies further led t o the conclusion that there is a considerable difference between a soil’s immediate ability to decompose CaC03 and its propensity t o continue the decomposition when soil and an excess of CaC03 continue in moist contact. That this observation is to be found in practice is show3 hy the analysis of the lime-treated plats of the Pennsylvania Station. Thirty-five per cent of the lime accumulated on these plats after 3 2 years of treatment is to be found as silicates. In offering the method, the differentiation is, therefore, made between immediate and continuous lime requirements of soils.”
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It is thus plain that this difference was recognized and emphasized by italicized type in the method of the writer for lime requirement, which was quoted by Prof. Truog. However, Prof. Truog’s article did not state that this prior differentiation in lime absorption by soils had been made. Therefore, one might readily conclude that no mention of the distinction had appeared in print, prior to the appearance of the findings and conclusions of Prof. Truog. Instead of the terms immediate and continued lime requirements as advanced by the writer, Prof. Truog used the terms ective and latent acidity. This distinction of terms is in harmony with the attributed differences as to the causes of the lime absorption; the writer attributing the decomposition primarily to the absorption of calcium by acid-silicates, while Prof. Truog states in his conclusions: IV-“Data are given indicating as follows: Soil acidity is due to true acids and not selective ion adsorption by colloids.” After careful and repeated readings of the article by Prof. Truog, the undersigned has failed t o find these data, above mentioned In Bulletin I 0 7 of the Tennessee Station, 1914., the writer, together with Messrs. Willis and Hardy, offered data t o demonstrate the continued evolution of COZ from excessive treatment of CaC03 upon soils (where questions sf biological influences and organic matter were eliminated) lor a period of over two years, while the longest period of observation given in the article by Prof. Truog was 1 7 hrs. This phenomenon also continued in lesser degree after igniting a soil a t white heat for a period of ~6 hrs., even where the soils heated contained a large excess of CaC03. Such an acid as would resist this treatment and still remain, even as “latent acidity,” would indeed be an unusual “true acid.” The same might be said of acid silicates and colloids in general. Apparently, however, Si02 and Ti02 are possessed of such unusual properties that upon moistening after ignition they will continue to evolve CO, from CaC03, but more particularly from MgCOa. After describing his procedure and giving his method for quantitatively distinguishing between the two forms of lime requirement or soil acidity, Prof. Truog stated in the concluding paragraph of his article: “The enormous supply of latent acid substances in many soils of the humid region as indicated by these methods is of the greatest importance in preventing excessive losses of bases by leaching. It also offers a further explanation why MacIntire, Hardy and Willis were able to secure large decompositions of MgC03, when this material was left in contact with soil t h a t supposedly had been completely neutralized.” One migh-c infer from this statement that Prof. Truog was doubtful whether the full absorption coefficient of the particular soils had really been fully met by the materials applied, i. e., that the “active acidity” had been neutralized while the “latent acidity” had not. If such an explanation alone were capable of accounting for the heavy decomposition of precipitated M g C 0 3 there would needs be attributed to the “latent acidity” a wonderful power of selective absorption of bases; for although the MgC03 treatment equivalent t o 16 tons of ground limestone per acre 6 in. had entirely decomposed a t the end of 8 weeks, there remained a t the end of 9 months approximately 2 2 , 0 0 0 Ibs., of CaC03 where this equivalent treatment was given as ground limestone. Furthermore, a t the end of 8 weeks approximately zo,ooo Ibs. of CaC03 remained where equivalent treatment of precipitated CaC03 had been applied. Many data reported in Bzdletin 107 of the Tennessee Station and many additional subsequent findings have seemed to justify the conclusion that the extensive decomposition of MgC03 must be attributed t o absorption by acid silicates and reaction with hydrated S O 2and TiO?. W ,H. MACINTIRE ITHACI,N E W Y O R K April 13, 1916
