US2904497A - Process for the high temperature reaction of hydrocarbons - Google Patents

Description

United States Patent PROCESS FOR THE HIGH TEMPERATURE REACTION OF HYDROCARBONS James H. Shapleigh, Wilmington, Del., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Application May 31, 1952 Serial No. 291,065

3 Claims. (Cl. 208-47) This invention relates to high temperature reactions and more particularly to an improved process for the high temperature reaction of hydrocarbons in thermal or catalytic systems.

In the decomposition, cracking, or reforming of hydrocarbons, it has been realized for some time that advantages are obtained by the operation of metal tube furnaces. However, when the operating temperature is increased above about 900 F., certain disadvantages arise as a result of the increased metal tube temperatures. These disadvantages, as heretofore recognized in the art, have to a large extent outweighed the advantages and have served to retard improvement and, accordingly, the use of metal tube furnaces for high temperature reactions.

Some of the disadvantages recognized in the art were associated with lack of knowledge of scaling properties, creep strength, and resistance of metals to sulfur compounds; especially, in systems operated at high temperature for long periods of time. Consequently, it was an objective in the art to carry out hydrocarbon decomposition at low temperatures. For example, in the production of hydrogen under conditions suitable for concurrent high conversion of CO to CO large quantities of steam were used to obtain better low temperature, theoretical equilibrium conditions. Also, for example, in the cracking of hydrocarbons to produce olefins and products of increased value, metal tube furnaces have not been readily accepted for operation at high temperatures and such furnaces as have been used have been limited to light hydrocarbons as raw material. Disadvantages in the nature of carbon and tar formation have retarded acceptance. Moreover, difliculties have existed in attaining proper control of both desirable and undesirable catalytic reactions, some of these caused by the catalytic elfect arising from the surface instability of the metal tubes.

Materials of construction for high temperature systems have been one of the factors retarding the progress toward such systems. The current problems encountered with materials of construction in systems for the cracking of hydrocarbons at high temperature are mixed problems such as strength of the material, its resistance to scaling, its ability to transfer heat, and the like. One of the principal problems lies in the actual physical and chemical changes which take place in a particular metal during heating, particularly where the metal is held at high temperature for long periods of time. It is known that stainless steels employed in high temperature equipment absorb carbon from organic reactants under certain circumstances. Furthermore, it is known that under reducing conditions and particularly where substantial quantities of hydrogen are present, such steels decarburize. Consequently, where a reaction furnace is being designed for long, substantially uninterrupted operation in high temperature hydrocarbon cracking processes, it is difficult if not impossible to predict the actual chemical analysis of the metal at any particular time. In Welding procedures'where the metal is at red heat for a period in the neighborhood of thirty minutes, it is. possible to employ stabilizing agents which will prevent chromium impoverishment adjacent the weld and thus be reasonably certain that the weld and adjacent metal will resist intergranular attack. Such predictions are not possible with cracking tubes which, in their lifetime, will be subjected, at high temperatures to a variety of physical and chemical conditions.

In relatively. recent years metallurgists have identified a metallic phase which forms in austenitic steels at temperatures between about 1000 F. and about 1700 F. This phase is known as sigma phase and it hasbeen definitely established that its formation is accelerated in the presence of stabilizers such as columbium and titanium which have been added to prevent intergranular corrosion. It has further been asserted that sigma formation brings about increased brittleness in the metal and some researchers have recorded tests which show that sigma is responsible for hot shortness in welding operations which results in cracking. Consequently, the art has generally considered the substantial formation of sigma phase to be detrimental and to be avoided where structural strength and resistance to cracking are important considerations.

A further problem involved in high temperaturehydrocarbon cracking is that of the catalytic eifect of materials employed in a given system. Moreover, the initial catalytic effect is of minor importance in comparison with the overall catalytic effect of a material during its life. For example, a material which is not substantially stable as to catalytic effect throughout. its life introduces another variable into a given process. Unless the process is continually adjusted to compensate for this variable, the maximum yield is not attained. Some processes are not sufficiently flexible to afiord such close control and at best such compensating manipulations are time consuming and expensive. Processes, in general, are subject to close control to compensate for variation in the composition of reactants, impurities in the reactants, variation in the catalyst activity, and the like. The substantial elimination of any one factor necessitatingprocess manipulation for compensating purposes is a definite contribution to the art.

, Thus, this invention has for its principal object an improved process for the high temperature reaction of hydrocarbons which process may be operated for long periods of time without appreciable deterioration of the equipment and without appreciable variation in the process due to the catalytic effect of materials of construction. A further object is to provide an improved process for the high temperature reaction of hydrocarbons. in

which thermal or noncatalytic conditions are employed as.

well as catalytic. An additional object is a process for high temperature hydrocarbon cracking which can be carried on continuously for greater periods than is now customary due to increased resistancev to tube failure. Other objects of the invention will appear hereinafter with reference to the specification and appended claims.

In accordance with this invention, these objects are accomplished by a process for the high temperature reaction of hydrocarbons in which the hydrocarbons in the vapor phase undergo reaction in the presence of a star bilizecl stainless steelv having an austenitic structure. In general, in the process of the invention, hydrocarbon and steam are passed through a heated alloy tube which may be wholly or partially externally or internally heated and 3 as columbium, molybdenum, and titanium, and mixtures thereof.

More specifically, the invention is a process for the high temperature reaction of hydrocarbons which comprises admixing the hydrocarbon with steam, passing the hydrocarbon-steam mixture through at least one thin-walled, austenitic alloy cracking tube, applying heat to the tube, and maintaining a section of the tube with a metal temperature gradient between a low of about 900 F. and a high of between about 1500 F. and about 2400 F., said alloy tube comprising essentially carbon, chromium, nickel, iron, and at least one material from the group consisting of columbium, molybdenum, and titanium, said ingredients being present in amounts from about 0.07 to about 025% for the carbon, from about 20 to about 30% for the chromium, from about 12 to about 25% for the nickel, from about 4 to about 16 times the carbon content for the columbium, from about 2 to about 6 times the carbon content for the molybdenum, from about 3 to about 7 times the carbon content for the titanium, and the remainder substantially iron; said alloy tube containing a substantial amount of sigma phase when main tained at temperatures between about 1000 F. and about 1700 F. The temperature gradient in the tube may be increasing or decreasing depending on the particular reaction conditions desired. Furthermore, the temperature gradient may extend throughout substantially the length of the cracking tube. In the zone of principal reaction, metal temperatures of from about 1500" F. to about 2400 F. are maintained. The exact metal temperatures at particular points are regulated in relation to the other conditions of treatment to give the desired temperature and temperature gradient within the reaction zone. Catalysts may be employed as desired, depending on the nature of the cracking reaction.

In a preferred embodiment of the invention the hydrocarbon is introduced, in accordance with the process disclosed in my U.S. Patent 2,525,276, into the alloy cracking tube or tubes along an inner path which is normally a concentrically disposed injection tube. superheated steam is introduced into the tube and led along an outer path concentric with the inner path. The steam in the outer path is heated indirectly by the externally heated cracking tube and is normally at a temperature of from 1000 F. to 1600" F. at injection level. Mixing of the two streams and flash heating of the hydrocarbon to substantially cracking temperature is obtained at the junction of the inner and outer paths or injection level. Tube wall temperatures at this level are generally in the range of about 1500 F. to about 1900 F. The resulting mixture is immediately passed through the zone of principal reaction and cracking is obtained without appreciable carbon deposition. In this embodiment the temperature of the hydrocarbon in the inner path is kept below that at which carbon deposition will occur. The dilution effected by the secondary steam flowing from the outer path brings about a desirable lowering of partial pressure of the hydrocarbon where that hydrocarbon feed is an oil and substantially completes vaporization at injection level. The highly superheated steam and indirect heating fi'om the cracking tube then effects substantially instantaneous flash heating through the danger zone for carbonization, without any appreciable carbon deposition.

The hydrocarbon feed in the process of the invention may be gaseous or normally liquid. The liquid stock is finely diffused prior to entry into the cracking tube, preferably by atomization by means of steam. Depending on the nature of the liquid hydrocarbon,-it may be desirable that partial vaporization also be effected prior to entry into the cracking tube. 1

It will be appreciated that the alloy employed in the invention may contain additional ingredients usually found in alloys of this class, such as manganese, silicon, sulfur and phosphorus. However, the alloy normally 4 will not have more than about 2% manganese, 1.5% silicon, 0.05% sulfur and phosphorus, present with the essential ingredients heretofore set forth.

Examples of the operation of the process of this invention are given in the following in which the steel numbers designated are American Iron and Steel Institute type numbers.

Example 1 In the production of olefins, a domestic fuel oil and steam in a weight proportion of about 1 to 1 were passed through a cracking tube 8 inches in diameter and 30 feet long. The cracking tube was disposed, substantially throughout its length, in a furnace aifording external heat to the tube. The tube was heated to a temperature of about 1300 F. in the region just below the furnace arch to a maximum of about 1800 F. in the lower portion of the tube. The temperature of the exit gases was about 7 1250 F. The tube was composed of type 310 stainless steel containing 0.08% carbon and stabilized with 1.12% columbium. No detrimental carbon was formed and an ethylene-propylene weight ratio of 0.67 to 1 was obtained.

With reference to the above example, the attainment of an ethylene-propylene ratio of 0.67 to 1 represents a marked improvement in the art and the results were consistent over a prolonged period of operation. Ordinarily, an ethylene-propylene ratio of about 3 to l is considered reasonably good process performance even where high propylene yield is desired. The ability of the process of this invention to efliciently operate at high temperatures makes it possible to obtain low ethylene-propylene ratios, although the process is readily adaptable to obtain high ethylene-propylene ratios where high ethylene yield is desired.

Example 2 In the production of hydrogen, a crude oil containing 2.5% sulfur and steam in a weight proportion of about 1 to 5 were passed through a plurality of cracking tubes each of which was 8 inches in diameter and 30 feet long. The cracking tubes contained a nickel catalyst and were disposed, substantially throughout their length, in a furnace aifording external heat to the tubes. The tubes were maintained at an increasing temperature gradient from about 1300 F. in the region immediately below the arch to a maximum of about 2000 F. in the region adjacent the opposite end of the tube. The temperature of the reactants in the zone of principal reaction was between about 1500 F. to about 1900 F. The tubes were composed of type 310 stainless steel containing 0.08% carbon and stabilized with 0.93% columbium. A cracking efliciency of 96.7% was obtained and the system was substantially free of sulfur poisoning and detrimental carbon formation.

Example 3 In the production of hydrogen, natural gas and steam in a volume proportion of 1 to 1.8 were passed through a plurality of cracking tubes containing nickel catalyst. The tubes and furnace were similar to that described for Example 2. The tubes were maintained at an increasing temperature gradient of from about 1000 F. in the region immediately below the arch to a maximum of about 1800 F. in the region adjacent the opposite end of the tube. A cracking efficiency of 98 to 99% was obtained in an experimental furnace operated for a test period of over two years with no appreciable change in efficiency.

In the furnace described in the above examples, the hydrocarbons and steam were passed downwardly and the combustion gases were passed upwardly. Thus, the high temperature combustion gases were in contact with a very hot tube. A plurality of burners was placed at various levels to control the temperature of the various 7 zones as desired.

sistance to tube failure during extended operation under the high temperature conditions illustrated in the ex-v amples. Upon examination of such tubes after long periods of substantially continuous exposure to such elevated temperatures, it was surprisingly discovered that the portion of the tubes below the arch which was maintained at temperatures below about 1700 F. contained very substantial amounts of sigma phase. In some instances, the sigma concentration was found to be as high as 62%. Still despite the stress, elongation, distortion, and shock conditions which occur during the course of extended high temperature cracking, no cracks or tube ruptures occurred in these regions and the crack-free life of the entire tube was considerably greater than with the 310 metal previously employed.

With reference to the above examples, it will be seen that the performances obtained under prolonged periods of operation were remarkably consistent. Although it is not alleged that the alloy used in the present invention is a predominant factor in obtaining such remarkable consistency in performance, there is no doubt but what it is an important contributingfactor. in control of catalytic elfect. This can readily be determined by microscopic examination and comparison of stainless steel tubes with and without stabilizing material present in the alloy. When the stabilizing material is present in the alloy, microscopic examination at 150x reveals a uniform distribution of the ingredients throughout the alloy. This condition exists even after prolonged operation at elevated temperature. In contradistinction thereto, when the stabilizing material is not present, microscopic examination reveals a uniform distribution of the ingredients throughout the alloy initially; but after pro,- longed operation at elevated temperature, microscopic examination reveals a definite precipitation of carbides at the grain boundaries of the alloy.

This carbide precipitation causes irnpoverishmentof chromium in the alloy. For example, in comparing a 25 20-tube (25% chromium-20% nickel) after service and containing stabilizing material with a 25720 tube after service and without stabilizing material, in the latter tube the chromium impoverishment produced adjacent to the grain boundaries provides a condition of chromium content below 12% over an appreciable area as compared to 25% in other areas. The precipitated carbides may run as high as 90% chromium at the grain boundaries. Therefore, there is a marked nonuniformity of chromium content and a surface of nonuniform metal analysis. Catalytically expressed, the exposed surface of the tube contains from 90% chromium to less than 12% chromium. It will be appreciated that the transitory characteristics of unstabilized stainless steel alloys introduce an undesirable variable in the process of cracking hydrocarbons which variable has been substantially eliminated in accordance with this invention.

In addition to the undesirable transitory characteristics of unstabilized stainless steels from the catalytic standpoint, carbide precipitation at the grain boundaries reduces the strength of the steels and renders them unsuitable for high temperature processes.

In service, nonstabilized 25-20 stainless steel tubes, produced by different methods consistently gave heavy carbide precipitation. This resulted in weakened crystal boundaries with eventual opening up of large cracks. The effect was one of shortened tube life or decreased capacity in order to sustain tube life.

In the catalytic steam-hydrocarbon process, carried out in a tubular furnace, the capacity of throughput of a system is dependent upon the pressure which is allowable or practical within a tube. Pressure may represent resistance to flow or it may represent in addition the resistance of subsequent equipment. In either case, the ability to Withstand increased pressure through years of operation is a much needed and distinctive improvement in the art.

' undesirable.

Furthermore, in accordance with the present invention there is provided a uniform composition of metal surface. This is desirable in catalytic processes where high. degree control of unwanted side reactions is an advantage. A uniform composition of metal surface is also desirable in thermal processes such as the production of olefins and valuable by-products since the elimination of unwanted side reactions improves the yield and quality of product obtained.

One distinguishing characteristic between steels conducive and nonconducive to undesired reaction of hydrocarbons is magnetic susceptibility. It was found that nonstabilized type 310 steel after long service at temperatures above 800 F. and notably above red heat showed substantially developed magnetic qualities. In contradistinction thereto stabilized type 310 steel containing colum-- bium in the same service for similar long periods showed only the slightest trace of magnetic qualities. Samples suspended by thread were easily turned out of a vertical line by a magnet in the case of service exposed nonstabilized type 310 steel, whereas service exposed stabilized type 310 steel containing columbium could not be attracted out of line but could be made to turn around its axis of suspension with difiiculty, there being no noticeable diiference in this case between new and service exposed material. It is believed that this characteristic is a contributing factor to improved catalytic result in the case of hydrogen production and to improved thermal cracking result in the case of olefin production through absence or impedance of side reactions.

The advantage of the present invention are multifold. The uniformity of surface composition provided in ac cordance with the present invention is a distinct improvement, particularly in processes where side reactions are In the production of olefins from crude oils, Where during high temperature cracking, conditions accelerative or conducive to carbon formation are un- Wanted, the process of this invention has been found-advantageous since no appreciable decomposition to carbon takes place. It has further been found that control of the cracking reaction has been improved whereby ethylenepropylene weight production ratio can be easily varied from 3 to 0.75 and even lower with less facility. It is quite normal to obtain a high ratio of ethylene to propylene, but the low ratio of ethylene topropylene as hereinbefore set forth is difiicult to obtain. This same case of control enables other desired and valued products to be obtained in maximum quantity. Furthermore, the resistance to cracking of the thin walled tubes employed is increased despite the formation of sigma phase which has formerly been thought to accelerate rather than retard the formation of cracks. In general, the process of the present invention provides improvement in operability, maintenance and longevity of a thermal or catalytic process and provides for an improvement in the quality of the products derived from such processes.

The present invention has wide applicability in proc esses such as: thermal or catalytic production of hydrogen; production of hydrogen by partial combustion of hydrocarbons; purification of hydrogen gases containing NO; production of acetylene, ethylene, propylene, and butylene; production of gas of specific B.t.u. content; production of other liquid hydrocarbons; and in general, process use at controlled temperature ranges from about 900 F. to highest practical temperatures of use of the metal, at pressures below or above atmospheric with variable quantities of steam, CO or other oxygen-containing material from zero to several times the weight ratio of hydrocarbon used, and including a large excess of steam. The operation of such processes including ratio of reactants, catalysts, quality of reactants, etc., may be carried out in accordance with normal practice with the present invention serving as a valuable adjunct when incorporated with such processes. In addition, such processes may utilize normally gaseous hydrocarbons or normally 7 liquid hydrocarbons or mixtures thereof while obtaining the benefits afiorded by this invention.

The process of the invention has been illustrated in the examples with type 310 metal, modified by addition of columbium. However, the other austenitic alloys, such as type 309 and type 314, modified by addition of the prescribed amounts of columbium, titanium, or molybdenum, are also operable and may be employed as desired.

The present invention is particularly applicable and advantageous in the processes disclosed in my US Patent Re. 21,521, and my copending applications: Serial No. 576,481, filed February 6, 1945, now US. 2,524,840; Serial No. 678,163, filed June 20, 1946; and Serial No. 692,345, filed August 22, 1946, now US. 2,525,276. Although the above processes disclose at least one externally heated elongated tube as a preferred means in conjunction with the process operation, it is evident that the tube or tubes may be of other configuration such as a coil, or other means constituting a suitable reaction zone, with out departing from the spirit and scope of the invention.

This application is a continuation-in-part of my copending application Serial No. 779,420, filed October 11, 1947, now abandoned, which is a continuation-in-part of my copending application Serial No. 692,345, filed August 22, 1946, and now issued as US. 2,525,276.

What I claim and desire to protect by Letters Patent is:

1. A hydrocarbon cracking process which comprises passing a hydrocarbon together with steam through an austenitic alloy cracking tube, applying heat to said tube so as to maintain the temperature of same within the limits of about 9002400 F. including a temperature gradient across at least an upstream section thereof within the limits of about 1000 to about 1700 F., said alloy tube comprising carbon, chromium, nickel, iron and columbium and the said ingredients being present in amounts of from about 0.07 to about 0.25 percent carbon, from about 20 to about 30 percent chromium, from about 12 to about 25 percent nickel, from about 4 to about 16 times the said carbon content of columbium and the re mainder substantially iron, said tube alloy containing sigma formed in the presence of said columbium and exhibiting thereby improved resistance to tube failure ordinarily caused by the presence of sigma formed within said range of about 1000 to about 1700 F.

2. A process of claim 1 wherein said hydrocarbon is finely difi'used and passed along an inner path within said tube, said steam is passed through said tube concurrently with flow of said hydrocarbon and along an outer path of said tube concentric with said inner path and in contact with the inner surface of said cracking tube, and wherein said hydrocarbon and said steam are caused to flow into direct contact within said heating tube in a downstream section thereof.

3. Apparatus for cracking hydrocarbons comprising a furnace, at least one tube within said furnace and means for heating said tube at a temperature within the range of 900 and 2400 F., said tube being formed from an austenitic-type stainless steel alloy comprising carbon, chromium, nickel, iron and columbium, the said ingredients being present in amounts, on a weight basis, from about 0.07 to about 0.25 percent carbon, from about 20 to about 30 percent chromium, from about 12 to about 25 percent nickel, and the remainder substantially iron; said stainless steel containing sigma formed in the presence of said columbium, whereby said tube exhibits improved resistance to failure Within the said 1000-1700" temperature range ordinarily caused by the presence of sigma.

References Cited in the file of this patent UNITED STATES PATENTS 1,842,221 Wade Jan. 19, 1932 1,927,829 Harnsberger et al Sept. 26, 1933 2,081,927 Hassler et a1 June 1, 1937 2,101,835 Alcorn Dec. 14, 1937 2,218,495 Balcar Oct. 15, 1940 2,525,276 Shapleigh Oct. 10, 1950 OTHER REFERENCES Morton: Nickel-Bearing Alloys Used in Petroleum Refining, paper presented at 8th mid-year meeting, American Petroleum Institute, Wichita, Kansas, May 25, 1938 (10 pages).

Metals Handbook, 1939 edition, pages 535, 536 (2 pages), pub. by American Society for Metals, Cleveland, Ohio.

Kinzel et al.: The Alloys of Iron and Chromium, vol. II, pages 413-429, 435, 436 (19 pages), pub. by McGraw- Hill Book Co., New York (1940).

Archer: Refiner and Natural Gasoline Manufacturer, vol. 20 (July 1941), pages 66-72, (8 pages).

Transactions of American Society for Metals, vol. 39, pages 404, 437 and 438 (3 pages), 1947.

Metals Handbook, 1948 edition, pages 5 63, 564 (2 pages), pub. by American Society for Metals, Cleveland, Ohio.

Claims (1)

1. AHYDROCARBON CRACKING PROCESS WHICH COMPRISES PASSING A HYDROCARBON TOGETHER WITH STEAM THROUGH AN AUSTENTIC ALLOY CRACKING TUBE, APPLYING HEAT TO SAID TUBE SO AS TO MAINTAIN THE TEMPERATURE OF SAME WITHIN THE LIMITS IOF ABOUT 900-2400* F. INCLUDING A TEMPERATURE GRADIENT ACROSS AT LEAST AN UPSTREAM SECTION THEREOF WITHIN THE LIMITS OF ABOUT 10000* TO ABOUT 1700* F., SAID ALLOY TUBE COMPRISING CARBON, CHROMIUM, NICKEL,IRON AND COLUMBIUM AND THE SAID INGREDIENTS BEING PRSENT IN AMOUNTS OF FROM ABOUT 0.07 TO ABOUT 0.25 PERCENT CARBON, FROM ABOUT 20 TO ABOUT 30 PERCENT CHROMIUM FROM ABOUT 12 TO ABOUT 25 PERCENT NICKEL, FROM ABOUT 4 TO ABOUT 16 TIMES THE SAID CARBON CONTENT OF COLUMBIUM AND REMAINDER SUBSTANTIALLY IRON, SAID TUBE ALLOY CONTAINIGN SIGMA FORMED IN THE PRESENCE OF SAID COLLUMBIUM AND EXHIBITING THEREBY IMPROVED RESISTANCE TO TUBE FAULURE WITHIN NARILY CAUSED BY THE PRESENCE OF SIGMA FORMED WITHIN SAID RANGE OF ABOUT 1000 TO ABOUT 1700* F.