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ployed heretofore, but complete purification, which means the removal of the last traces of impurities, has not been commercially feasible on account of its great cost. It has been considered better practice to operate the ammonia process a t higher temperatures, say 550” C., where the effect of impurities is not so great. At these higher temperatures the differences between catalysts are not so apparent and the advantages of highly reactive catalysts, which are capable of operating at much lower temperatures, are not fully realized. Recently, however, a method of gas purification has been developed by means of which it is possible to obtain a gas of high purity a t practically no additional cost.3 It is believed that this process will make possible the commercial operation of very reactive catalysts at their highest efficiencies.
PREPARATION OF CATALYST Since the properties of an ammonia catalyst of the iron type may be modified by the addition, intentional or otherwise, of small quantities of foreign materials, it is evident that the methods of its preparation must permit very close chemical control if the most active product is desired. Briefly stated, the preparation of an iron catalyst involves two steps: :For a brief statement of this purification process see Chem. Met.
En#., 80,948 (1924).
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(1) the preparation of an iron oxide admixed with suitable promoters, and (2) its reduction by hydrogen. I n this second step the nitrogen-hydrogen mixture prepared for the ammonia synthesis is usually employed. The reduction of the oxide involves no particular difficulty; the real problem lies in the preparation of the oxide mixture. It will not be possible to discuss the various forms of iron oxides which might be employed and their methods of preparation. It is sufficient a t this time to state that an artificial magnetite has been found to give the most satisfactory results. The problem of preparing an oxide is, therefore, one of fusing iron oxide and the promoter under such conditions that the chemical composition of the oxide mixture may be rigidly controlled. On account of the great chemical reactivity of fused iron oxide it cannot be melted in any crucible so far developed. Many refractory materials have been investigated for this purpose, but all have been found to contaminate the molten oxide. The method finally adopted consists in fusing the oxide in a protecting bed of the same material. The electrical conductivity of the oxide is sufficiently great so that the mateiial may be melted-between water-cooled iron electrodes. Promoters are readily added to the iron oxide in this method of fusion, and all those substances which are known to react unfavorably can be carefully excluded.
Petroleum Cracking Processes and Future Production Problems‘ By T. A. Boyd GENERAL MOTORS RESEARCH CORP., DAYTON, OHIO
HE cracking of heavy oils to gasoline hydrocarbons is considered to be the most outstanding development in the petroleum industry during the past decade. First the automobile made the cracking of heavy oils necessary, and then the gasoline produced by cracking made the phenomenal expansion of motor transportation possible. Were it not for the gasoline produced in the cracking plant, the “gasless” Sundays of 1918, during the last months of the war, would have been only the beginning of a curtailment of automotive transportation that later might have reached as much as one-fourth of the total volume of motor traffic. While, of course, the pyrogenic decomposition of petroleum hydrocarbons has been known and practiced for two generations, particularly in the procedure called “cracking distillation” which is used in refining many crude oils, it was not until about 1913 that cracking a t pressures above atmospheric began to be actively pursued. I n that year American refineries produced around 1,100,000,000 gallons of gasoline or naphtha, which was about 12.5 per cent of the crude oil run to refinery stills. Ten years later, in 1923, the volume of gasoline produced amounted to 7,500,000,000 gallons, and the percentage of the crude oil which entered the refinery that came out as gasoline had risen to over 30. Much of this increase has been made possible by cracking, of course. The present capacity of the commercial cracking plants in the United States is estimated to be sufficient to make 2,250,000,000 gallons of gasoline per year-twice the volume of all the gasoline produced in 1913.2
T
Received August 13, 1924. Presented under the title “Chemistry in the Cracking Industry” before t h e Division of Petroleum Chemistry a t the 68th Meeting of the American Chemical Society, Ithaca, N. Y., September 8 t o 13, 1924. T h e L a m 9 (published by Standard Oil Company of New Jersey), 6, h‘o. 6, 7 (1924).
The largest increases in cracking capacity have been made during the past five years. It has been estimated that in 1918 ten per cent of the gasoline produced was made by cracking heavier oils.a Ten per cent of the 1918 gasoline production would be 350,000,000 gallons, which is only about 15 per cent of the present estimated capacity of American cracking plants as given above. The progress made since 1918 has not all been along the line of increasing the number and capacity of plants. The greater part of the cracked gasoline produced in 1918 was probably made by the original Burton process. As a result of the research that has been conducted with especial vigor since the war, processes that were in use then have been improved, and others which incorporate many modifications of method have been developed. “CRACKOR BE CRACKED” And so cracking has now come to be an important part of refinery economics. This fact was one of the things brought out by a recent nation-wide survey of refinery operations.4 The results of the investigation showed $hat of the 563 petroleum refineries in the United States 164 were equipped to use one or more types of cracking processes; and that, while 201, or 35 per cent of all the refineries, were shut down entirely on May 1, 1924, only 10 per cent of those fitted with cracking plants were not operating on that &ate. To keep the figures on the right side of the ledger under present conditions it is imperative that a high yield of gasoline be obtained from every barrel of petroleum that goes into the refinery, a thing which can be done only by cracking the heavier oils. The total rated refining capacity of the plan+ ( N . Y.), as, 230 (1920). J.,28, No. 25, 58 (1924).
a Padgett, Chem. Age 4
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that were equipped for cracking but were shut down on May 1 represcnted less than 2 per cent of the capacity of all the American refineries that do cracking. As is shown in the table, there are about forty cracking processe:r now in use commercially in American refineries. Many of these are much the same in principle and in operative details, but some of them differ widely in construction of apparatus and in methods of operation. One of the important developments of the past few years in the field of cracking has been the adaptation of the design of pressure stills in such a way as to keep the source of heat away from the main body of the oil that is being cracked, and so t o reduce materially the hazard that necessarily attends the subjection of an inflammable liquid to a high temperature. This result has been accomplished in a number of ways. Thus, many of the large pressure stills are now equipped with external tubes of relatively small diameter, as in tho Burton-Clark apparatus, for example, in which the circulating oil is subjected to the highest temperature. Again in some processes, such as the Dubbs and the Cross, which are also equipped with tubes, the volume of oil in the entire cracking apparatus at any one time is comparatively small. Heating the moving oil in tubes does more than increase the safety factor. It also helps to minimize the formation of carbon in the heating zone, which, of course, is where it does the most harm. Another step has been to maintain a higher rate of oil flow through the tubes by using some means of forced circulation, as is done, for example, in the Cross, the Dubbs, the Isom, and the tube-and-tank processes. The increased velocity in the tubes tends to reduce the amount of carbon deposited still further. A third step has been to increase the operating temperature by raising the pressure above that used in earlier processes. This is an especial advantage when cracking the more volatile oils which do not decompose rapidly a t temperatures corresponding to the usual pressure-still pressures. Commercial examples of this are the Holmes-Manley and the tubeand-tank processes, the latter of which operates a t pressures around 360 pounds per square inch. Still another development is the cracking of oils a t pressures high enough to prevent distillation in the high-temperature apparatus. Distillation is then carried out subsequently in a still a t lower pressure, the heat in the cracked oil being utilized for the purpose. One of the advantages of cracking without simultaneous distillation is that it enables the fuel consumption to be kept lower than in the case of those processes in which heat is lost to a condenser by distillation and refluxing.j The Cross process is a commercial example of this met hod of procedure.
PASTPROGRESS MOREPHYSICAL THAN CHENICAL The progress that has so far been made in the petroleum refining industry has been largely along physical and engineering lines. The chemist of inquiring mind and the ability to get accurate information on the fundamental structure and chemistry of petroleum oils has not yet gotten into this field as much as he should. Of course there have been and are some notable exceptions to this, as the work of such men as lllendelejeff, Silliman, Mabery, Brooks, Johns, Curme, and many others testifies. Of necessity the petroleum refiner has been so deeply engrossed in meeting the demand for gasoline somehow that he has not had adequate opportunity for studying the fundamental chemical problems of his business, even if it be assumed that he is qualified to do so. As Brooks has pointed out, thc: petroleum industry “owes its present strength and 5
Kansas City Testing Laboratory, Bull. 17, 320 (1924).
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magnitude to skilful business management plus good engineering, and probably no great industry is less indebted to chemistry.”6 Bacon also has referred to the same thing by saying, “there is no industry of anything like the magnitude of the petroleum industry which has such a dearth of fundamental chemical knowledge on which to base its operation^."^
PETROLEUM REFINIKGRAPIDLYBECOMING A CHEMICAL INDUSTRY Although petroleum refining in the past has been largely physical in character, the essential operations being fractional distillation, condensing, freezing, filtering, and so on, for the separation of crude oil into various fractions, it is rapidly becoming a chemical industry. That is to say, it is becoming an industry which produces from petroleum substances that do not exist in crude oil as it runs into the refinery. That part of petroleum refining which has changed from a physical to a chemical industry is centered around cracking, or splitting some of the naturally occurring petroleum molecules into other molecules which are more valuable-which are better suited for meeting the needs of society. As one result of the active entrance of the chemist and the chemical engineer into this industry, many materials can now be made from the by-products of the cracking still, particularly from the unsaturated constituents of pressurestill gases. For some time isopropyl alcohol has been manufactured commercially from propylene in the gases from cracking stills, the first announcement of the development having been made by Ellis* in 1920. Production capacity for this compound is now being considerably enlarged. Many other materials have been made from the by-products of the cracking still, a t least in an experimental way. A partial list of such compounds would include ethylene and propylene; secondary butyl, amyl, hexyl, heptyl, and octyl alcohols; ethyl ether; ethylene chlorohydrin; ethylene glycol; ethylene dichloride and dibromide; diethyl sulfate, and the esters of other acids; ethylene oxide; oxalic acid; formaldehyde; formic acid; acetone; propylene glycol; propylene chlorohydrin; isopropyl acetate and chloride; di-isopropyl anilines and toluidines; not to mention many other materials, including even such a compound as “mustard gas.” Marked progress has been made during the past few years in improving upon old procedures and in working out new methods for the chemical treatment of cracked naphthas to make them meet commercial requirements more satisfactorily. The sulfuric acid and caustic soda method of treatment has been so varied as to refine many types of cracked naphthas, including those quite high in sulfur.9 Considerable effort has been expended in developing methods for treating distillates in the vapor phase. Thus, in one process a water solution of chemicals is brought into direct contact with vapor from the stills, so as to serve the double purpose of a condensing and a treating medium.”J I n another the vapor is passed through a bed of fuller’s earth, which is said to polymerize and remove some of its undesirable constituents.l1 A recent development in the application of chemistry to the refining of cracked distillates, which already has wide commercial application, is the use of hypochlorite solutions, either alone or in conjunction with the usual acid and alkali THISJOURNAL, 16, 185 (1924). Ibid., 15, 588 (1923). 8 Chem. Met. Eng., 23, 1230 (1920); U. S. Patents 1,365,044;1,365,046, 1,366,048(1921). Egloff and Morrell, Chem. Met. Eng., 29, 59 (1923); Morrell, I b t d . , 30, 786 (1924): 10 Fleming, U.S. Patent 1,325,668 (1919). 11 Gray, U. S. Patent 1,340,889(1920). 6
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treatment.12 Some of the advantages claimed for this interesting chemical method of treating cracked gasolines are that it is cheaper than the old processes, the refining loss is lower, more oil can be treated in a given equipment, the refined gasoline is more stable in storage, and it is entirely free from those materials, present in some gasolines, which corrode or darken copper.
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7-What is the character of the polymers formed by treating cracked oils with sulfuric acid? Can the acid-refining loss be turned into profit? It should not be forgotten that compounds of the alcohol type make ideal motor fuels from the standpoint of freedom from detonation. 8-What is it that causes gradual darkening and gumming of some refined cracked gasolines? Is it the oxidation of phenolic compounds, as has been suggested by Brooks and Parker,ld or the polymerization of aldehydes formed by oxidation of olefins, as supposed by Smith and Cooke?16 9-What causes the formation of acidic materials in unsaturated gasoline when in storage? Is it due partly to the oxidation of aldehydes to organic acids,l6 or entirely to the hydrolysis of unremoved alkyl sulfuric esters?16 10-What are the possibilities of making cyclic hydrocarbons economically by cracking? On account of their freedom from detonation, hydrocarbons of the ring structure make superior motor fuels which can be used in more efficient engines.
Needless to say, too little is known about the chemistry of cracking. It is true that some theoretical study of the mechanism of the action of heat on petroleum hydrocarbons has been made, notably that of Rittman and others.13 For the most part, however, men who have studied the cracking of heavy oils have done so in an empirical way, and they have These, and other problems that cannot be enumerated necessarily been working largely in the dark, or a t least in twilight. They have not even been able to get much de- here, indicate that there are yet many things for the organic pendable information on the composition of the oils which and the physical chemist, and the chemical engineer &s well, they have been trying to make into more valuable products. to do in this field of such great economic importance, the pyroAnd so, as has often been pointed out before, there is still genic decomposition of petroleum oils. a need for chemists of the highest type to direct their efforts OB COMMERCIAL CRACKINGPLANTS IN THE UNITEDSTATES along this line. The Petroleum Division of the AMERICAN CLASSIFICATION AS OF MAY 1, 1924O CHEMICAL SOCIETYis trying to foster research on petroleum Total rated capacity of refineries in products and to disseminate information on the fundamental which plants are chemistry of petroleum. Classification installed -NUMBER OF PLANTS-Barrels per dayb That this field is far from being an easy one in which to NAMEOF PROCESS of process Operating Not operating work is evidenced by the small amount of progress that has Beacon 1 20,000 1 1 3,000 Blaigdell been made so far, in spite of the fact that much endeavor Burk-Hintz 1 1,000 27 710,500 has been concentrated in it; but it is one which has great Burton Central 1 600 scientific interest and very large practical importance to Chem. Oil Ref. 1 2 500 9:500 A,2-C 2 society, as well as one which holds out the possibility of large Coast 2,000 Col. De Camp 1 A,2-C 1 35,000 rewards to the successful investigator. The oil industry is Cosden-Coast 173,250 Cross 19 A,4 fast awakening to the necessity of obtaining more funda- Crump-Steele 1 1,000 15,500 3 mental information on the materials upon which its business Doherty 292,000 5 c Dubbs 29 112,400 7 and profits are built. As in many other industries, the p e Ellis or Tube-and.-Tank 1,000 1 Emerson troleum refiners who are operating at a profit in these times Experimental 2 93,100 6 1 3,000 B-D of stress are those who have been, and are now, acting on the Ferrox 1 110,300 15 Fleming $2 1,500 1 Forward advice of the chemist and the chemical engineer. 12,000 1 1 Gillons We have space to mention briefly only a few of the chemical Greenstreet 2,500 1 100,000 Gulf 1 problems which, if solved, would help the cracking industry. Hanson 1 3,000 86,000 8 Holmes-Manley Many of these have been enumerated before. l d 52,500 4 1-Just what types of hydrocarbons are present in the complex mixture that we call crude oil? Much has been written about the chemical composition of petroleums, but the dependable data are very fragmentary. Information on the physical properties of petroleum hydrocarbons is also incomplete. For example, the cracking industry needs accurate data on the specific heats and other thermal properties of petroleum oils at temperatures within the cracking range, from 300' to 900" C. 2-What is the action of heat on the pure petroleum hydrocarbons of high molecular weight? What are the conditions that affect or influence the character of the products, and how can the yield of desired products be kept a t a maximum? The future development of oil shale as a source of motor fuel would be greatly assisted by a knowledge of how to produce a highly cracked oil of low unsaturated content. 3-Have the possibilities Of catalysis in breaking up large hydrocarbon molecules been exhausted? Does electricity exert anything in the nature of electron effects when it is used for cracking, or is its action due altogether to heat? 4-How can we make fuel oil in the cracking process that is free from colloidal or suspended carbon? 5-What are the chemical and physical properties of pure unsaturated hydrocarbons? The work of past investigators constitutes an excellent beginning along this line, but more adequate information would illuminate the field of cracking much better and settle a number of questions. 6-How can the percentage by volume of the unsaturated constituents in cracked gasolines be measured? Absorption in cold sulfuric acid and other methods now in use do not give a real measure of the volume of unsaturated constituents.
Isom 12,900 3 Jenkins 2,000 1 Landis 1,000 1 Leamson 67,500 4 Lewis $2 1,000 1 Miller 1 1,500 Muehl 4 2 12,500 3 National A2 1,500 1 Ohio 7,000 1 Prudential 500 1 Richey 2,000 1 Ryan 7,500 2 Slagter 6,360 6 B Snodgrass 36,000 3 Specia 1 77,000 B 4 Trumble 4,000 1 B Wellman Key to classification of process column: A-Li uid phase Atmospheric pressure ( 2 ) Pressure up t o 250 pounds. (3) Pressure 250 t o 500 pounds. (4) Pressure 500 t o 750 pounds (or high enough t o prevent distillation in cracking apparatus) B-Vapor phase! or combined liquid and vapor phase C-Steam used in process D-Catalytic material employed in process a This table is based in part on data given in Oil Gus J . , 23,, No. 25 58 (1924). Because it is very difficult t o get dependable informatlon a b o u i the petroleum refining industry, t h e d a t a in this table should not be taken as. entiiely quantitative. The table serves rather t o give a good qualitative picture of the situation. b The figures in this column do not give the capacity of the cracking plants, but the total rated,refining capacity of t h e refineries in ,which t h e cracking processes are used. No reliable statistics on the capaclties of ali the various types of cracking plants are ,available. c Two of these in course of constructlon. d I n course of construction.
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14
12 18
Dunstan and Brooks, T H IJOURNAL, ~ 14, 1112 (1922). Rittman, I b z d . , 7, 945 (1915); Bur. Mines. Bull. 114.
THISJOURNAL, 16, 587 (1924). Mines, Report of Investigations 2394. Brooks and Humphrey, J . Am. Chem. Soc., 40, 822 (1918).
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