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Vol. 20, No. 4
Thermodynamics in the Service of Technology' Merle Randall UNIVERSITY OF CALIFORNIA,
BERKELEY, CALIF.
so produced in our home, the electrical energy responsible for the heat. But if we were to construct a reversible heat engine with the inside of the building serving as the hot reservoir and the outdoor air as the cold reservoir, and if by means of a motor the electrical energy were used to operate this Application to Engines engine, the heat could be taken from without and given up inside the building. The amount of heating thus produced Thermodynamics had its origin with the discovery that would, in the limiting case of ideal efficiency, be more than heat and work could be transformed, the one into the other, sixteen times the electrical energy expended, if the outdoor and for many years it was'the primary function of this science temperature were 0" C. and the indoor temperature 18" C. to increase efficiency in the design and use of engines for the Thermodynamics tells us that this process is possible, but production of work. This function of thermodynamics has the engineer and economist tell us that it is not yet profitable. groan steadily in importance. Other fluids besides mercury and steam, such as sulfur, If a given amount of heat is taken from a boiler along with a diphenyl, etc., have been suggested which would increase vapor and allowed to do work in a perfect engine, the engine the efficiency of the power cycle. Sulfur is impractical bewill finally reject a certain amount of heat along with the cause of the difficulty of finding suitable materials for the vapor to the condenser. The theoretical conversion factor boiler and turbine, and organic compounds crack under the of such a perfect engine is the quotient of the difference successive or prolonged heating. It has yet proved impracbetween the temperature of the boiler and condenser divided tical to utilize the high-temperature gases and vapors proby the absolute temperature of the boiler. This simple duced in the process of refining of oils. Here the stili is the law has been responsible for the development from the simple boiler of a power plant, the opportunity of doing work or non-condensing erigine of Watt to the multicylinder condens- producing power is rejected in the name of expediency, and ing engine of Corliss and others, to the single-stage turbine, correctly so if the application of the idea will not yield the to the multi-stage high-pressure condensing turbine using necessary profit. high initial superheat in the steam. At the same time, beThe heat of combustion of coal or of hydrogen does not cause the designer knew the theoretical conversion factor measure the maximum amount of energy which may be of his engine, he forced himself to bring the mechanical effi- applied to useful purposes. This is measured rather by the ciency of his plant to a maximum and to make the cycle decrease of the free-energy content (shades of Garabed) of of his engine approach the conditions of the theoretical. the system. This amount may be larger or smaller than The gas engine and the Diesel engine are further develop- the decrease in the heat content of the system, depending ments of the attempt to increase the initial temperature upon whether heat is absorbed from or rejected to the surof entering gases and, therefore, the conversion fartor of the roundings when the process is carried out reversibly and heat engine. Perhaps the efforts of the chemist to find a isothermally. I n the case of the combustion of graphite practicable alloy which will withstand the severe conditions to form carbon dioxide, the decrease in heat content is 94,250 of high temperature, pressure, and erosion will at last make calories per mol, while the free-energy decrease is the same the gas turbine practical. (94,260 calories) within the limits of the experimental error. Meanwhile the chemist has insisted on thinking and talking Many attempts have been made to oxidize carbon reversibly, about the thermodynamic properties of substances other and thus obtain the available free energy as work, but these than steam. The engineer has turned to mercury in order attempts have so far proved futile. to increase the conversion factor of his machines, and the Applications to Physics and Chemistry mercury boiler is already in successful semicommercial operation. The installation of the first experimental mercury The function of thermodynamics in the design and use of boiler and turbine cost many thousands of dollars. The faith of the executives in the soundness of the thermodynamic engines is now overshado wed by the numerous important principles involved was so great that money was forthcoming. applications to physics and especially to chemistry. The On the other hand, millions of dollars have been wasted in methods of thermodynamics have made quantitative prefutile construction of secret devices which were unknowingly cision take the place of the old vague ideas of chemical affinity. designed to defeat the second law of thermodynamics. And Thus chemistry has made the greatest advance toward the still other millions have been saved because luckily some one status of an exact science since the early chemists, Lavoisier, with a knowledge of the fundamentals of thermodynamics had Richter, and Dalton, laid the foundations of stoichiometry. the opportunity to tell the investor that the inventor's idea Today thermodynamics is the unerring guide to the investigator of stellar evolution, radioactivity, radiation, atomic was impractical. Many of us look upon the use of the ordinary electric structure, and the humble ,industrial process. The successful chemical engineer is he who can guess right radiator as 100 per cent efficient for the purpose of heating our homes. We are badly mistaken, however. True, all more often than his competitor. Correct guesses are profitthe electrical energy is converted into heat energy. But able, incorrect guesses are charged off in the red, and if the reversibility is the fundamental concept of a perfect thermo- profits sufficiently overbalance the losses the chemical engidynamic process, and we cannot re-obtain from the heat neer is acclaimed a valuable man. Formerly, intuition played a great part in the guesses of the expert. Today intuition 1 An address delivered October 7 , 1927, before the Southern California has been largely replaced by a sound grasp of the fundaSection and December 9, 1927, before the California Section of the American mental principles and data of thermodynamics. Chemical Society. HEMISTS prefer to speak of our present era as a chemical age. But chemistry cannot be fully employed in meeting the various and complex needs of society without the constant use of the methods of thermodynamics.
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April, 1928
It is well to realize the limitations as well as the power of thermodynamics. To quote from the eminent French chemist, LeChatelier :* These investigations of a rather theoretical sort are capable of much more immediate practical application than one would be inclined to believe. Indeed the phenomena of chemical equilibrium play a capital role in all operations of industrial chemistry.* * * Unfortunately, there has been such an abuse of the applications of thermodynamics that it is in discredit among experimenters.
This was written in 1888, and thermodynamics is still discredited by many who ought to know better. Modern physical chemistry still has its propagandists who a t times have shown more zeal than scientific caution. T o quote from Lewis and Randall:3 We have seen “cyclical processes” limping about eccentric and not quite completed cycles, we have seen the exact laws of thermodynamics uncritically joined to assumptions comprising half truths or no truth a t all, and worst of all we have seen illbegotten equations supported by bad data. However, the fact that errors are constantly made in numerical calculations does not diminish our confidence in the principles of arithmetic; nor can any reasonable person question either the possibility of an exact application of thermodynamics to practical chemistry, or the great value to be gained thereby. Let us quote once more the words of LeChatelier: It is known that in the blast furnace the reduction of iron oxide is produced by carbon monoxide, according to the reaction Fe203 3CO = 2Fe 3C02 but the gas leaving the chimney contains a considerable proportion of carbon monoxide, which thus carries away an important quantity of unutilized heat. Because this incomplete reaction was thought to be due to an insufficiently prolonged contact between carbon monoxide and the iron ore, the dimensions of the furnaces have been increased. In England they have been made as high as thirty meters. But the proportion of carbon monoxide escaping has not diminished, thus demonstrating, by an experiment costing several hundred thousand francs, that the reduction of iron oxide by carbon monoxide is a limited reaction. -4cquaintance with the laws of chemical equilibrium would have permitted the same conclusion to be reached more rapidly and far more economically.
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Willard Gibbs a n d the Phase Rule
It is significant that such a remarkable scientific accomplishment as the great monograph, “The Equilibrium of Heterogeneous Substances,’’ by Josiah Willard G i b b ~should ,~ have remained so long hidden. At the recent jubilee celebration held in his honor by the Chemical Society of Holland, fifty years since the publication of this paper, many eminent scientists ranked his contribution rn one of the mightiest works of genius the human mind has ever produced. Gibbs was not a propagandist; but his character, his industry, his wonderful ability for taking pains, and his commendable lack of self-interest in research are now beginning to be appreciated. Meanwhile industry has profited immeasurably. His work was practically unknown for fifteen years after its publication. Ostwald, hearing rumors of the work, and being interested in chemical equilibrium, republished in German (with the author’s permission) the work of the gifted American physicist in 1892. It was not until 1906, three years after the author’s death, that Longmans in England reprinted the few hitherto inaccessible papers of Gibbs. Thus, it was that American students, after overlooking the great scientist a t home, went to Germany to learn of the thermodynamics of Gibbs. The name of Gibbs is usually associated with the phase rule. The number of phases plus the number of degrees of freedom in a system is equal to the number of components * A n n . mines, [SI 13, 157 (1888). 3 “Thermodynamics and t h e Free Energy of Chemical Substances,” p. 2, McGraw-Hill Book Co , New York, 1923 Trans. Conn. Acad S L S, 3, 228 (1878).
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plus two, for systems subject to changes of pressure, temperature, and composition. This rule has been of vast industrial importance. The classical investigations of van’t Hoff upon the salts of the Stassfurt deposits were founded upon the work of Gibbs and laid the foundation of the successful exploitation of this wonderful source of wealth. Millions of dollars have been spent to isolate the constituents of Searles Lake, Calif. Some spurned and doubted the value of so simple a rule as that of Gibbs. Without investigating the equilibrium relations of the various salts making up these complex brines, they built evaporators, ponds, railroads, leased fine offices, printed pretty stock certificates, and devised industrial processes. Experiments were made, based mostly on hopes that a chance result would somehow be duplicated. After many failures, which are a blot upon the record of American chemists, scientists with a fundamental knowledge of thermodynamics took hold, studied the equilibrium existing in the various systems a t various temperatures, and the rate of attainment of equilibrium. Then, with a knowledge of the fundamental facts, processes were devised which, with the aid of sound engineering, have, in some cases, produced a profitable business. Some concerns, until recently, have continued to print pretty certificates. One could add indefinitely to the record of the successful application of the phase rule. Of course, the industrial process may precede the working out of the equilibrium conditions. Sometimes a process is invented by a workman who has never heard of highbrow thermodynamics. But some are unconsciously guided by thermodynamic principles, which are, after all, just an easy way to express our everyday experience. T h e r m o d y n a m i c s i n Metallurgy
About twenty years ago the zinc industry was much interested in the Lungwitz zinc-smel.ing process. For many years it has been recognized that the process of zinc reduction is wasteful of metal and inefficient of fuel. Lungwitz proposed to smelt the zinc ore in a blast furnace and, in order to prevent the escape of zinc vapor with the waste gases, to increase the pressure so that the zinc would condense as a liquid and be recovered by tapping in the same manner as iron is tapped from a blast furnace. He claimed to have actually produced spelter on a small scale in this wag. In 1906 a large test blast furnace was built a t Warren, N. H., to resemble a small iron blast furnace but to operate a t 60 pounds pressure per square inch. This furnace was provided with elaborate arrangements for charging, tapping slag and spelter, and relieving the blast gases a t 60 pounds pressure. The cost of construction and operation over a period of several years was more than $100,000. The experiment was abandoned, although later one of the staff built a slightly larger furnace a t Philadelphia. A condenser near the bosh was added to this furnace, and a few thousand pounds of spelter were produced. The experiments were finally abandonedin 1916 after the expenditure of much time, money, and effort. Little or nothing was learned about the zincsmelting process, although adequate funds had been available. Recently Ralston and Maier, of the Pacific Experiment Station of the United States Bureau of Nines a t Berkeley, Calif., determined the free energy of zinc oxide, and of liquid and gaseous zinc. These experiments cost a trifling sum. At a recent meeting of the California Section of the SOCIETY, Mr. Maier told of the calculations relating to the zinc-reduction process that they were then able to make. If pure oxygen instead of air, and 95 pounds per square inch pressure rather than 60, had been used, the Warren furnace would have produced spelter. That is, the process under these conditions is a t least possible. Success of the process would,
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of course, depend upon many other factors. With ordinary air the pressure required would be between 95 and 5000 pounds per square inch, probably nearer the latter figure. Surely a small appropriation for the study of the thermodynamic properties of the zinc compounds would have paid a handsome profit. The record of failures and successes in the metallurgical field could be much extended. Much time and money have been spent in the effort to produce cast iron by means of gaseous or oil fuel. Most of the experimenters have ignored the thermodynamic requirements of the process, and have failed. Others have studied the conditions for obtaining sponge iron, which can be melted to steel in an electric furnace. Sponge iron has been successfully produced by reduction with gaseous fuels. Although the units are small, the results indicate favorable competition with the older established process. Chemical Equilibrium
The application of thermodynamics in the field of chemical equilibrium is new, Few new processes have been definitely thought out in this way. A friend asked me for my preliminary value for the free energy of hydrochloric acid gas. He then combined this value with other published values and predicted that chlorine and steam should react with carbon to produce hydrochloric acid and carbon dioxide. The possibility of side reactions was also investigated. He tried the process in the laboratory, but before recommending the expenditure of the money to build a small-scale unit, he insisted that I check his calculations. The plant was designed, the operating conditions predicted, and the plant and process have continued to operate satisfactorily. The great advantage of this particular process is that the byproduct chlorine from an electrolytic plant can be burned to hydrochloric acid without using the otherwise valuable hydrogen, from which ammonia is made. The direct union of hydrogen and chlorine requires more elaborate apparatus. Also, precautions must be taken against destructive explosions, which are absent in the carbon-steam process. Heat Balances
Thermodynamics might justly claim the profit which has come through the widespread use of heat balances. The blast-furnace operator, the boiler-room foreman, the refinery executive, in fact any operator who uses heat, looks upon the heat balance as a necessary part of the accounting system. It is just as important to him as the cash book is to the merchant, for heat units cost money and must be conserved. The heat balance is merely an application of the first law of thermodynamics. Free Energies and Catalysts
If we find that the free-energy increase in a reaction (taking into account the concentrations or partial pressures of the various substances) is negative, then the reaction will pro. ceed spontaneously. That is, it will proceed if the speed of the reaction is measurable. But thermodynamics cannot predict the speed, and at present actual trial in the laboratory is the only means of determining the rate of a reaction. Much effort is being spent in the search for new catalysts. The theory of homogeneous catalysis requiress that the free-energy increase in each reaction of a series of reactions, the resultant of which is the desired main reaction, must be negative. Of course, we speak of the actual rather than the standard free-energy increase a t unit concentration. Some 5 Bray and Livingston, J . A m . Chem. SOC.,46, 1251 (1923); Bray, Ibid., 43, 1262 (1921).
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progress has also been made in the extension of this idea to heterogeneous catalysis. Sometimes the concentration of the supposed intermediate substance may be small, or the supposed intermediate solid may not be a recognized chemical i n d i v i d u a l 4 e., manganese dioxide which has lost a fraction of a gram atom of oxygen-so that the result of the calculations may be of more qualitative than quantitative value. An extensive table of free energies would, then, enable us in a few hours to try on paper possibilities which would require months in the laboratory. But the speed of our possible reactions making up the series of reactions may be no greater than that of the uncatalyzed reaction. Our tables of free energy are woefully inadequate, but a t that a little light is better than no light at all. Tables of Thermodynamic Properties
The widespread use of thermodynamics by the designer of engines is probably due to the existence of adequate tables from which the theoretical properties of his fluid can be obtained, and to the simplicity of the indicator card as a record of the actual work produced in the cycle. The chemist has been greatly handicapped by the lack of fundamental data in the form of easily understood compact tables of thermodynamic properties of substances. After a dozen years, during which Professor Lewis and I devoted the major portion of our time to the collection of such data, we were finally able in 1923 to publish a table which listed the values for the free energy of formation of about one hundred and forty of the more important chemical substances. Since that time with the aid of several graduate students, I have calculated the values for about sixty additional substances. These values will be published shortly. In order to establish such a table of the thermodynamic properties of substances, we must select certain standard conditions for the various elements. For the elements which are gaseous a t 25' C. the hypothetical gas a t unit fugacity is chosen. For those which are liquids or solids a t 25" C. the element under a pressure of one atmosphere is chosen. It is not always easy to choose the standard condition of a solid, for the form chosen must be such that various samples are reproducible and their properties independent of their previous history. It is also difficult to choose values for the various physical constants, for in order to combine data it is absolutely necessary to use the same constant in all the calculations. If a change is made in one of these constants, the values throughout the table may possibly be varied. It is also necessary to be always consistent in the choice of the heat capacity which is to be assigned to each substance. This leads to many apparent anomalies. For example, we may choose to take the heat capacity of hydrogen gas equal to 6.5 plus 0.0009 times the absolute temperature, calories per mol per degree, and having made this choice, we must retain this value with an arithmetical precision which may be far beyond the experimental error of the heat capacity for the substance with which the hydrogen reacts. Correlation of Experimental Data
One of the greatest advantages of the systematic methods which we have evolved is the ability to gather together the results of experiments of many different kinds. Thus, in determining the free energy of gaseous water, the following are some of the experimental data which were used: vapor pressure of water, equilibrium in the Deacon process, dissociation of water vapor; dissociation of silver oxide and of mercuric oxide; solubility product of silver oxide and of silver chloride; dissociation constant of water; single potential of silver ion and of chloride ion; the equilibrium between silver chloride, silver oxide, chloride ion, and hy-
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droxide ion; the potential of the chlorine-silver-silver chloride electrode; the potential of the mercury-mercuric oxide electrode; the vapor pressure of hydrochloric acid over aqueous acid; the potential of the hydrogen-silver chloride and hydrogen-calomel electrodes; the freezing-point lowering of dilute solutions of hydrochloric acid; the dissociation of hydrochloric acid vapor; the equilibria involving magnesium chloride, magnesium oxide, oxygen, water vapor, and hydrochloric acid vapor; the equilibrium in the water-gas reaction; the speciiic heat and heats of transition of the various forms of hydrogen, oxygen, and water a t very low temperatures, and many other data. The proper combination of the data may show that large errors may be expected to exist in a particular set of data. How important it is from a technical standpoint to be able to fit a new experiment into a sort of jig-saw puzzle, where the picture becomes more and more apparent as more and more different sorts of experiments weld themselves into a consistent picture. I n the example just cited there is a possible uncertainty of about 300 calories in the free energy of water vapor, or in some of the other equilibria. It will require many days of careful checking, calculation, and even experiment to reduce the uncertainty to a few calories. It would be a great step forward to reduce the experimental uncertainty of the free energy of several of our more important reactions to a few calories. The power of thermodynamics to bring into a single relation data determined under avarietyof conditions is also illustrated by some unpublished calculations involving the vapor pressure of mercury. Such measurements are more numerous than those of any other substance with the possible exception of water. A list of the early investigators is also a list of well-known pioneers of physical chemistry-Dalton, Avogadro, Helmholtz, McLeod. The pressure measurements by various direct and indirect methods extend from -38" C., where the pressure is about 2 billionths of an atmosphere, to 1435" C., where the pressure is about 2000 atmospheres. Yet the calculated thermodynamic constant Z in the freeenergy equation for the vaporization of liquid mercury is practically constant over the entire range when activities are used, and the ideal heat of vaporization calculated agrees with measured value. This is the most extreme test of the equations involving a single directly measured reaction which has been made. Relation of Theoretical to Practical Data
Physical chemists have been criticized because the laws which they have announced were not applicable to actual operations, but only valid for ideal conditions. Many of these laws have as their basis a mechanical picture of a process involving particular species of substances. The great advantage of the thermodynamic method is that it does not inquire into mechanism of a process, but takes the system as it exists in the initial condition and predicts from the system in its final condition the various changes in properties, witbout asking how this was brought about. It is true that if we make certain simple arbitrary choices of units of composition-that is, a system consists of so many mols of A and so many mols of B-we may find that the system may obey certain very simple laws. Thus we say that a solution of sodium chloride is made up of z mols of sodium ion and 5 mols of chloride ion per 1000 grams of water. We do not inquire into the molecular complexity of the water, but as the composition can always be expressed as multiples of H20all in rapid equilibrium, we ordinarily say that the water exists as H,O. By assuming the mechanism of ions and taking into account the electrical work of dilution, we may arrive a t very simple laws, much less complicated than if we had chosen
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always to speak of the activity of the undissociated substance. But the thermodynamics on such a choice of composition would have given the same answer. We have now chosen to utilize fundamental equations which are true for the ideal systems, and then determine experimentally such approximations as are necessary to express the properties of individual substances. Thus the activity coefficient id a number by which we multiply the pressure of a gas, or the mol fraction of a solvent, or the molality of an ion constituent to determine the activity which must by definition fit into the true thermodynamic formulas. We have made much progress in determining these activity coefficients for a wide variety of substances, in gaseous solutions, for pure aqueous solutions, or pure non-aqueous solutions, and in mixtures. Where the determination of the activity coefficient in the particular mixture desired has not been determined, we have been able to fix upon empirical laws and relations which enable us to guess with a considerable degree of confidence the approximate value of the needed constant. Thus we are no longer limited to dilute solutions, or perfect gases, but by means of not more than five very simple formulas and tables of properties of the ideal substances, we make an approximate calculation of any equilibrium. And if more accuracy is needed, we may have recourse to the tables of activity coefficients which will enable us to make calculations with an accuracy as great as the accuracy of the data or approximations used. These formulas are simple algebraic formulas; the substitutions can be made by one unskilled in theoretical thermodynamics, and thus the results of scientific research are made available to the busy operator. Compilation of Thermodynamic Data
I have attempted to show that if thermodynamics is to be of the greatest use to the chemical industry, the data of thermodynamics must be put into compact form. This involves the preparation of tables of the free energy of formation of the known chemical substances a t some standard temperature, say 25" C. I n order that these data may be useful a t other temperatures, we need to select algebraic equations for the heat capacity of the several substances, and tabulate the values of the heat of formation. In addition, we need tables of the activity coefficients of actual substances under a variety of conditions, and all these must be mutually consistent. Revisions of the whole body of the tables should be made and published from time to time. but if consistency of the tables is to be maintained it is obvious that such revisions cannot be made piecemeal. There was chaos in the field of stoichiometry until the International Committee on Atomic Weights took hold of the situation, made a careful study of the existing measurements, and made recommendations for definite periods of time. Revisions in the table of atomic weights are made in such a way as to maintain the consistency in mutual relations of the atomic weights of the elements. In preparing the tables of thermodynamic properties, many calculations must be made in standardized form. These calculations are retained on file and become of value when adjustments must be made in the fundamental constants, or in the values of heat capacities, or other free energies which are combined in the final free energy of formation or when new experimental values appear. For instance, I have on file, for each substance, plots which give the values for the freezing point or equivalent function for all published measurements. When a new determination of a colligative property of a substance appears, I am able to determine very rapidly whether the new measurements warrant a change
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in the activity coefficient of that substance. So with the determination of solubilities in mixtures, or equilbria in special reactions. So far this work has been done without regular assistance. Only a portion of the literature has been searched and many experiments have been overlooked. New data appear every day. It will require the services of several full-time calculators and a clerk or so to catch up, and continue to fit in the various data as they are published. We should look forward to the establishment of a permanent headquarters for thermodynamic data, where the various results of experiments could be critically reviewed and the necessary standardized calculations made and retained.
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The annual cost would be perhaps $10,000 per year. Aside from the publication of consistent tables of thermodynamic data, such a committee would be able to suggest to individual investigators the kinds of experiments which would be of greatest value from both theoretical and practical standpoints. The demand for new data is very great. Nearly every week we receive letters requesting data on the free energy of some additional substance. It is hoped that this brief account will give some little insight into the service that thermodynamics has rendered technology in the past. However, the application of our science is still in its infancy, and we may confidently expect still greater returns in the future.
Factory Operation of Automatic Electrometric pH Control of Cane Juice Defecation' R. T. Balch and H. S. Paine CARBOHYDRATE DIVISION,BUREAUOF CHEMISTRY AND SOILS, WASHINOTON, D. C.
Automatic control of the liming of cane juice in factor in the defecation of the manufacture of raw sugar, operating on electrocane juice. M o r e o v e r , an sugar p r o d u c t i o n t h e metric principles, was tested in a Porto Rican central electrometric control appears chemical-te c hnological over a period of four months. The principles of the to be the most feasible bephase of the factory process control were fully established, and the results indicate cause electrical energy aphas been strongly stressed a very promising means of controlling this important plied in proportion to the hyafter a number of years of process. drogen-ion concentration of relative quiescence and has The equipment consisted of (1) a recording pothe juice may be made to c e n t e r e d around studies of tentiometer fitted with a controller time switch which, operate a liming apparatus juice clarification and means through relays, caused the motor of the liming device through suitable mechanical of controlling it more accuand electrical devices. The to operate in the required direction at definite interratelya2 The importance of vals; (2) tungsten and calomel electrodes fitted in a inability to transform juice proper control of clarification reactions proportionately into continuous-flow juice chamber; (3) a temperature needs no comment, and it is compensator which automatically corrected for the electrical or mechanical enbelieved t h a t any way in ergy, rather than lack of inchange in pH due to temperature variations; and which t h i s feature of the t e r e s t o r study, has been (4) a tilting weir box for the liming device. sugar-manufacturing process m a i n l y responsible for the Operators who do not desire full automatic control may be improved will always slow progress made in this of the liming process could benefit by adopting means be of economic importance to phase of factory operation. of recording the pH of the juice, which is made possible the industry; especially is this It is believed that the pracby similar equipment. This could be adapted to true during periods of keen ticability of automatic recordbatch as well as to continuous methods of liming. competition and low margin ing and control of the pH will of profit. I n 1926 the authors made a preliminary study3 of methods be fully demonstrated in spite o? the fact that unforeseen for controlling cane juice clarification and proposed schemes difficulties pertaining solely to the handling of the juice before for recording and controlling automatically the reaction of the liming prevented the authors from obtaining as fully satisfacjuice after liming. As certain mechanical features of the equip- tory a control as was desired. The principles of the control ment were lacking, however, it was impossible at that time have been established, however, and with a little experience to control the addition of lime to the juice. Confidence in it should be possible to apply them in future installations the adaptability of recently improved equipment to successful with entirely successful results. The investigation described here was conducted at a Porto automatic control and the potential value of this control to the industry were reasons for continuing the investigation, Rican sugar central4 employing the Petree-Dorr system of juice clarification. Since this was a double-defecation system, the results of which are reported in this paper. As in the previous investigation, a hydrogen-ion concen- it was possible to apply the equipment to a control of the tration method employing electrometric principles formed secondary as well as the primary juice clarification. It is the basis of the control, since the pH value is a determining unnecessary to discuss in detail the various steps taken in the development of the automatic control, but there is given 1 Presented before the Division of Sugar Chemistry at the 74th Meeting a full description of the equipment that was finally adopted of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927. and precautions regarding its use and care that were found 2 The number of references pertaining to this subject is large and includes publications by such agencies as the Carbohydrate Division, U. S. to be necessary for satisfactory results, in so far as the operaBureau of Chemistry and Soils; Experiment Station of Hawaiian Sugar tion of the equipment itself was concerned.
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N RECENT studies of raw
Planters' Association; British Empire Sugar Research Association; Association of Technical Advisers of Java Sugar Industry; and numerous individual investigators. 3 Balch and Paine, Planter Sugar Mfr., 75, 347 (1925).
4 The authors wish to express their appreciation of the many courtesies and excellent cooperation received from the otfcials and operating staff of Sucesion J. Serralles, Central Mercedita, Ponce, P. R.
