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make-up, would be at least 25 per cent of the sales price of phthalic anhydride as given in Table I. The naphthalene used by the old German process had to be pure, otherwise the yield of phthalic anhydride based on sulfuric acid would be correspondingly reduced. The vaporphase catalytic process uses a lower grade, cheaper naphthalene. The corrosion and erosion of the German reactors was very high and thus involved a much higher depreciation cost. The plant investment, not including that for the sulfuric acid recovery, was much higher and occupied much more space. The older process required equipment, such as centrifuges, for separating and washing water-soluble acids from the phthalic acid, which now is superfluous. The loss of mercury catalyst was an important cost factor. The operating labor was much greater and the yields were lower. It is an historical fact that the old synthesis of phthalic anhydride was one of the most important factors in the perfection of the contact sulfuric acid process, but if we were limited to its use today, phthalic anhydride could not be the source of most of its important derivatives now on the market. The simplicity of oxidation equipment for producing phthalic anhydride by the vapor-phase process is shown in Figure 1. This shows a single oxidation unit capable of producing 600,000 pounds of phthalic anhydride per year which alone, or if necessary in multiples, will make enough product for many consumers of phthalic anhydride. The potential supplies of naphthalene in the United States, if all coal tar should be processed to recover naphthalene, appear to be ample for any conceivable demand for phthalic anhydride. For example, from past experience, coal tar production may be expected to average about 600,000,000 gallons per year and if all this should be processed for naph-
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thalene and converted to phthalic anhydride, about 280,000,000 pounds of the latter could be produced. Of course, not all the tar will be distilled, but the domestic supplies appear adequate and it seems certain that in normal times a t least we can depend upon imports to make up any deficiency. The economical separation of naphthalene from coal tar distillates also depends to a large extent upon the simultaneous recovery of phenol and cresylic acids for which there appears to be a comparable market. As a last resort, naphthalene may also be recovered from oil-cracking operations.
Literature Cited (1) Badische Anilin- und Sodafabrik, Brit. Patent 18,221 (1896). (2) Canon, F. A. (to American Cyanamid Co. and Heyden Chemical Corp.), U. S. Patent 2,010,217 (Aug. 6, 1935). (3) Conover, Courtney (to Monsanto Chemical Works), Ibid., 1,645,180 (Oct. 11, 1927). (4) Downs, C. R. (to Barrett Co.), Ibid., 1,604,739 (Oct. 26, 1926); 1,789,809 (Jan. 20, 1931); 1,873,876 (Aug. 23, 1932). ( 5 ) Downs, C. R., J . SOC.Chem. I n d . , 45, 188 (1926). (6) Gibbs, H. D. (to du Pont Co.), U. S. Patent 1,599,228 (Sept. 7, 1926). (7) Gibbs, H. D., and Conover, Courtney, U. S. Patent 1.285.117 (Nov. 19,1918). (8) Graebe, C., Be?., 29, 2802 (1896); Brunck, H., J. Sac. Chem. I n d . , 20, 239 (1901); Levinstein. Herbert. Ibid.. 20. 332. 802 (1901). (9) Kirst, W. E., Eagle, W. M., and Castner, J. B., Trans. Am. I n s t . Chem. Engrs., 36, 371 (June 25, 1940). (10) Thomas, B. E. (to iMonsanto Chemical Co.), U. S. Patent 1,936,610 (Nov. 28, 1933). (11) Walter, Johann, German Patent 168,291 (1906). (12) Walter, Johann, J . prakt. Chem., 51, 107 (1895). (13) Wohl, Alfred (to I. G. Farbenindustrie A. G.), U. S. Patent 1,971,888 (Aug. 28, 1934). PRESENTED as part of the Symposium on Unit Processes before t h e Division of Industrial and Engineering Chemistry a t the 100th Meeting of t h e American Chemical Society, Detroit, Mich.
Economics of Catalytic Oxidation
in the Vapor Phase
COURTNEYCONOVER hfonsanto Chemical Company,
st. Louis, Mo.
Certain features of catalytic vapor-phase oxidation processes which are important in the economics of these processes are reviewed. Prominence is given to factors, generally economically unfavorable, which in the evaluation of new processes may at times receive insufficient attention.
N P H E manufacture of chemicals, air, because of its cheapness, is the oxidizing agent to be preferred in any case where it can be successfully used. Its field of application is rather severely limited, however, by its properties as a reagent. Except a t high temperatures or in the presence of active catalysts, it is inert toward oxidizable substances of many classes; and when reaction is induced by elevation of temperature or by the use of catalysts, the most frequent result is, in the case of a carbon compound, complete combustion sustained by the heat of reaction and, in the case of an inorganic substance, an unfavorable equilibrium or reaction of an undesired kind. Only in exceptional cases can the extent of oxidation be controlled to give a desired product in good yields.
I
In spite of its limitations air has been found useful as a reagent in two classes of reactions. In one class, substances which are by nature easily oxidized are exposed in the liquid state to air a t temperatures generally below 100' C. The heat of reaction is easily dissipated a t the low temperature level in the presence of liquids. No unusual apparatus is required; hence oxidations of this kind do not differ in any marked manner from other unit processes carried out with liquid reactants at relatively low temperatures. In another group of reactions, which are of the greatest importance in the chemical industries, air is used as a reagent a t temperatures above 400" C. and in the presence of catalysts. Reactions in this group are carried out in the vapor phase either from necessity or as a matter of convenience. The use
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INDUSTRIAL AND ENGINEERING CHEMISTRY
of catalysts and of high temperatures, together with operation in the gas phase, makes the processes based on these reactions somewhat exceptional in chemical nianufacturing practice. Troublesome problems in chemical engineering are involved in their application on a large scale, and these problems have been perhaps one of the factors which have prevented the widespread use of this type of process. Although scores of these processes have been demonstrated experimentally, those which appear to be well established and are operated on a large scale are few in number. They are in fact used in the preparation of only six chemicals-sulfuric acid, nitric acid, formaldehyde, acetone, phthalic anhydride, and maleic anhydride. Although in important respects individual catalytic vaporphase oxidation processes differ widely from one another, they have as a group a number of characteristics in common, especially with regard to the economics of their operation. A review of these economic aspects may be helpful in the evaluation of new processes, even though little specific information concerning commercially important processes can be included.
THE feature probably most favorable to low costs in catalytic vapor-phase oxidation processes is the use of air as the oxidizing agent. While air is not free to the manufacturer of chemicals, i t is nevertheless extremely cheap in comparison with commercial chemicals used as oxidizing agents. Its cost varies widely, depending on the degree of compression required, the size and type of compressors used, and the cost of power; but even under unfavorable conditions the oxygen contained in air will scarcely cost one tenth as much as the available oxygen contained in the cheapest chemical oxidizing agent, oleum. I n spite of the cheapness of the oxygen contained in air the economy of its use is evidently insufficient in many cases to counterbalance the unfavorable factors which are present rather frequently in proposed air-oxidation processes. For example, the process for making benzaldehyde and benzoic acid by the catalytic oxidation of toluene with air-a process which has been known in a n imperfectly developed state for many years-has not been able to compete with the process depending upon the chlorination of toluene followed by hydrolysis of the chlorination products, even though the chlorine used in the latter process costs perhaps twenty times as much as the equivalent amount of oxygen in the form of air. I n this case the failure of the air-oxidation process may be simply a result of incomplete development or i t may be a result of conditions intrinsic in the nature of the reaction which are reflected in Ijoor yields or in exorbitant cost of plant for a given output. As a n example of a more successful catalytic air-oxidation process, the modern process for making phthalic anhydride may be cited, This process since 1918 has rapidly displaced the older one in which oleum was the oxidizing agent, undoubtedly because of a double advantage-a greatly reduced cost of oxidizing agent and an improvement in yield. Another feature of catalytic vapor-phase oxidation processes favorable to low costs, a t least in the oxidation step proper, is the comparative ease with which automatic or semiautomatic controls are applied. This is a consequence of the fact that the processes under discussion are operated as nearly as possible continuously and with conditions held constant. In some processes complete automatic control offers no technical difficulties; and although such control may be considered inadvisable, it can be applied to any extent thought safe. I n the more modern contact sulfuric acid plants, operating labor has been cut down to what is considered the irreducible minimum-one man for each shift. All operations are carried on with this trifling amount
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of labor, regardless of the size of the unit; the result is very low expense for operating labor and supervision in the larger plants and, so far as the oxidation step proper is concerned, reduction of these expenses to negligible proportions. Although the large quantities of heat generated in catalytic oxidation reactions bring up many troublesome problems in the design of equipment, they may constitute an asset of some value in a process operated on an extensive scale. I n the contact sulfuric acid process, steam generated in the plant often yields a substantial credit which offsets an appreciable part of the operating costs. I n the phthalic anhydride process the credit for steam is of less importance but is still by no means negligible. T H E expense of maintaining catalysts in the processes under discussion is highly variable; i t depends on several obvious factors but especially on the frequency of required replacements. Processes have been described in which it was necessary to replace the catalyst almost continuously. It would seem that in such processes the maintenance of the catalyst would constitute a major expense. On the other hand, in certain established processes, such as the contact sulfuric acid process or the phthalic anhydride process, while the first cost of the catalyst adds an appreciable fraction to the total for the plant, the expense for catalyst per pound of product is still very low because of the high production capacity and long productive life of the catalyst. The best types of vanadium oxide catalysts, such as those used in the processes just mentioned, appear to suffer no deterioration whatever from service under favorable conditions. I n the history of the commercial use of such catalysts, replacements have been necessary only when the catalyst had been damaged by accident (as by fusion or by contamination with extraneous substances) or when it became necessary to replace the containing vessel (reactor) because of obsolescence or structural failure. Some of these catalysts even survive replacement of the reactor; they can be discharged from one reactor and charged into another without suffering any damage. A characteristic feature of catalytic vapor-phase oxidation processes (one that is unfavorable to low costs and hence should not be overlooked when the utilization of a new process is being considered) is the exceptionally high cost of developing these processes from the time of discovery of a promising catalyst to a stage of development at which the process is ready for commercial use. Even if this development is wholly successful, the cost may be so high as to lay a considerable burden of expense on the product when the process is put into commercial operation; and the possibility must be recognized that a profitable process may never be worked out. This high cost of development is a consequence of the large number of variable conditions which may affect the results of a catalytic gas-phase reaction and the marked effects which may result from slight variations in many of these conditions. Usually an exceedingly prolonged and tedious course of experimentation must be pursued, a seemingly end-. less succession of variables being surveyed in short steps, before the main features of the process are known. Then a t this stage so much may remain unknown that there is a standing temptation to try to improve the process through limitless experimentation. The attractive possibility is always present that the behavior of the main catalytic metal will be affected favorably by the addition of one or more other metals; every metal in the periodic system that would be stable under the conditions of the catalysis seems to merit testing as an auxiliary catalytic metal, and the testing of metals for this function, using different combinations in varied proportions, can be carried on endlessly. Work on carriers may be extremely tedious, comprising a search for an inert material hav-
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ing ideal physical properties, tests to determine the most favorable size and shape of carrier granules, and with tests of different methods of incorporating the active catalytic materials in the granules or of coating them on the granules. I n planning apparatus for carrying out the process, so many possibly advantageous forms suggest themselves that invention and experimentation in design are constantly invited. Exceptional judgment is required to determine the point a t which a search of this kind should be stopped. An obvious stopping point may not present itself until exorbitant expenses have been incurred. I n the oxidation of organic materials by catalytic gasphase methods, especially in the reactions which have not been extensively studied, yields are commonly poor, and processes which are attractive in other ways have failed in commercial practice for this reason. Even in some processes that are commercially profitable, yields are poor in comparison with those theoretically possible; this is apparently unavoidable. While excellent yields can be demonstrated experimentally in other processes, those obtained in actual manufacturing practice are lower, partly because yields are designedly sacrificed in favor of other factors having more weight in profitable operation I n the manufacture of phthalic anhydride, for instance, a t least one catalyst is known that would increase the usual plant yield 20 per cent or more; but this catalyst would still be uneconomical because it has a low productive capacity-less than 15per cent of that of the more common catalysts. I n the manufacture of phthalic anhydride, yields also suffer to some extent from the practice of adjusting operating conditions in the oxidation step so as to reduce expenses in the later steps of the process.
A SOURCE of trouble and expense in some of the processes under discussion is the dilute gaseous mixture processed. Thirty pounds or more of air may be used for the preparation of one pound of product. The cost of recovering phthalic anhydride from the dilute mixture used in its preparation, while making a rough separation from congeneric substances, .constitutes a large fraction of all costs of this product other than that of raw materials. I n some proposed processes the recovery of unreacted raw materials from dilute gas mixtures is contemplated, a practice which might in many .cases involve a burden of expense so great as to endanger the commercial success of the process in which it was attempted. A feature of many processes in the group under discussion which is highly unfavorable economically is the high initial .cost and maintenance cost of the apparatus required for a given output of product-that is, in contrast to costs in processes of other types which are operated on comparable scales of production. One reason for these relatively high "costs of apparatus is that problems of economy in the construction of plants have been considered secondary to the purely technical problems involved. An extraordinary amount of study and experimentation has been devoted to the solution of the baffling chemical engineering problems involved in the design of plants suitable for carrying out catalytic gas-phase processes a t high temperature. (One of the problems which may be mentioned as an example is the maintenance of a constant temperature above 400" C. and often above 450" C., while withdrawing enormous quantities of heat generated by the reaction.) The main problems have been solved fairly satisfactorily in the sense that largescale operation under well-controlled conditions has been made easy; but low cost in the building of plants has not been a major objective and has not been realized. Costly materials have been freely used (the use of mercury in the mercuryboiler type of reactor is an example) as have also expensive
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methods of construction. Most pieces of apparatus are not factory-built, but are specially designed and made in the shops of chemical manufacturers, a practice that cannot be expected to result in low costs. Not only are expensive materials and methods of construction used in many cases but in some processes, both actual and proposed, the plant required for a given output is disproportionately large in comparison with plants used for many other unit processes. One reason is that the reactants, besides being processed in the gaseous state at high temperatures, are diluted initially with excess air. This is particularly true of the processes for making phthalic anhydride and maleic anhydride in which the quantity of air used is several times that theoretically required. As a consequence of processing the very dilute gas mixtures, even though the reaction is completed in a fraction of a second, apparatus of relatively great volume is necessary; and this is true not only of the apparatus for the oxidation step but of accessory a p paratus also, such as preheaters, heat exchangers, condensers, and absorbers. The large excesses of air used in the phthalic anhydride and maleic anhydride processes appear to be indispensable as aids in controlling temperatures in the reacting gases, the quantity of heat generated in either process being so great that all practicable means must be utilized for keeping temperatures within the desired narrow ranges. I n the phthalic anhydride process the oxidation of naphthalene to phthalic anhydride, without further reaction, liberates about 450 kg.cal. per mole. If i t were possible to bring about the reaction quantitatively with only the theoretical volume of air and without dissipation of heat, the rise in temperature would be around 1800"C. Excess air, by reducing the concentration of hydrocarbon vapor in the gas mixture in contact with the catalyst and by serving as an internal heat absorbing medium, provides a valuable accessory means of controlling the rate and extent of temperature rise. I n addition to high initial costs, high maintenance costs prevail in many plants; corrosion is difficult to avoid in parts of the plant making acidic products, apparatus continually in service a t high temperatures may warp or crack, instruments in large numbers may entail a n appreciable expense for upkeep. It may be pointed out that even in processes as highly developed as the contact sulfuric acid and the phthalic anhydride processes, costs for repairs of apparatus and for amortization of plant make up a large fraction of the expenses other than those for raw materials, a fraction which ranges from 40 to 70 per cent in modern plants. I n view of the facts pointed out, the apparatus needed for proposed new catalytic gas-phase processes should be given extended study, with first cost and maintenance costs in view, and great care should be taken to avoid underestimating these items. SUMMARIZING the main economic features of the group of processes under discussion, i t may be said that they have in their favor the use of an extremely cheap reactant (air), that they are adapted to automatic and semiautomatic control, and that expense for heat in all forms is low or negative; opposed to the favorable characteristics are the high cost of development and of attempted improvement of processes, the low yields commonly obtained in the organic field, the high cost in some processes of recovering the product or unreacted raw material from the dilute gaseous mixture processed, and in particiilar the high first cost and maintenance cost of apparatus. PRESENTED as p a r t of the Symposium on Unit Processes before t h e Division of Industrial and Engineering Chemistry a t t h e 100th Meeting of t h e American Chemical Society, Detroit, Mich.
