I N D U S T R I A L A N D E N G I N E E R I N G C H E M I ST R Y
Februarl-, 1934
inherent necessity of maintaining in the liquid in the still enough crackable oil to act as a solvent for the tar.
HALLPROCESS Hall was one of the pioneers in vapor-phase cracking. The process (Figure 3) operated prior to 1917 at substantially atmospheric pressure and a temperature of about 1000" F.
n
FIGURE1.
ORIGINALBURTONSTILI,
Charging stock was fed through the heating coil made of 1inch tubes formed into hairpin shaped elements suspended in the furnace. At about 1000° F. the vapors passed into a vertical chamber. Leaving the chamber, the vapors were partially cooled and the heaviest portion separated in a drum as liquid tar. The remaining vapors were further cooled and passed into a second drum, where the condensate was separated and collected. The vapors leaving this drum were a t substantially atmospheric pressure and consisted of the lighter cracked vapors and fixed gas. To effect the maximum condensation, the vapors were compressed to 100 pounds and subsequently cooled under this pressure to produce the gasoline yield of the process. The tar and intermediate condensate collecting in the drums were further cooled and removed, the former as a final product of the plant and the latter serving as charging stock for subsequent operations. The yields were about 30 per cent gasoline, 20 per cent fixed gas, and 45 per cent recycle stock. No coke was formed in normal operation with cracking stock suitable for the process. Kerosene was the most successful stock,
DUBBSPROCESS The Dubbs process, as shown in Figure 4 prior to 1919, operated a t about 900" F. and 150 pounds pressure. Fresh feed was introduced either to the dephlegmator or directly to the furnace tubes. In either case it mixed with the condensate from the bottom of the dephlegmator. The oil passing through the reaction zone was composed of one part of fresh feed and four to six parts of recycle stock. Thermosiphon circulation through the furnace coil was aided by the gravity head of the elevated dephlegmator. From the furnace the mixture of vapor and liquid a t about 900" F. entered the relatively large expansion drum where the vapors separating from the liquid were passed on to the dephlegmator. Here they were cooled either by the fresh feed or the product reflux, or both. The heavier portion was condensed and returned to the furnace for further treatment. The vapors leaving the dephlegmator, consisting of light cracked fractions and fixed gas, passed to the condenser coil and thence
191
to the receiving tank where the liquid and gas were separated. Pressure was regulated by the release of gas. The distillate usually contained about 60 per cent gasoline. The liquid remaining in the expansion chamber was continuously withdrawn as tar or allowed to remain in the chamber until by polymerization it formed coke. Gasoline yields of 40 to 60 per cent varied with the charging stock and with the method of operation. The process v a s characterized by a crack per pass of 6 to 10 per cent a t intermediate temperature and pressure. Because of the low crack per pass, residual stocks could be handled without serious coke formation in the heating zone. However, the liquid tar produced by the process was found to have undesirable amounts of polymerized products in the form of coke or asphalt held in suspension which settled out and interfered wjth its normal use as fuel oil. Like the Burton process and for the same reason, although the pressure was higher, kerosene could not be as successfully cracked as heavier stocks. The operating conditions of the processes thus far described and others such as Fleming, Rittman, Coast, Greenstreet, Trumble, etc., which were being developed simultaneously, were limited by the materials and methods of construction then available. I n a great many cases the inventor was forced to adopt operating conditions which were less favorable than those which he wished to attain. Attempts to pump oil under high temperature and pressure had not been very successful. Control instruments of all classes (temperature, pressure, liquid level, rate of flow, etc.) had not been adapted to cracking plant use. Valves and fittings for this service had not been standardized, and the situation was
w.. ..-. - - - - -... FIGURE 2. BURTON-CLARK TUBULAR STILL
sufficiently acute to cause one refinery t o manufacture the valves and fittings for its cracking plant installations. The vessels used were of riveted construction and, because of being alternately cooled and heated, leaked a great deal. The use of corrosion-resistant and high-temperature strength alloys was unknown. CROSS PROCESS Walter Cross and Roy Cross proposed to crack oil a t 850" to 900" F. and under 600 pounds pressure even though handicapped by these construction conditions. To meet these requirements new methods of construction and fabrication were devised. The first process, arranged as shown in Figure 5, mas constructed in Rosedale prior t o 1916. Fresh feed a t the rate of 500 barrels per day was pumped through the heating coil and into the reaction chamber held a t a pressure of 600 pounds. The coil outlet temperature approximated 850" F. The converted oil after passing through the chamber
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was discharged through a pressure regulating valve and immediately cooled under a pressure of 40 pounds. The fixed gas and liquid, separated under this pressure, were then discharged. The liquid was termed “synthetic crude oil” and contained all the liquid products of the reaction-gasoline, recycle oil, and tar. The coke made during the reaction was deposited in the reaction chamber and the accumulation there determined the duration of operations, which on gas oil was about 4 days.
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20 per cent based on gasoline output. Little effort was made to utilize the excess heat in performing other functions such as topping, stabilizing, rerunning, etc. Length of run naturally varied with the process, but 10 days was unusually long Bubble columns to produce end-point gasoline were coming into use, but their design was almost completely empirical. Lack of qualitative knowledge of heat transfer in tubular furnaces gave rise to furnaces which heated unevenly, resulting in coked and split tubes. These conditions focused attention upon the design of a plant which would : (1 Handle distillate and residuum stocks with equal facility. (2{ Decrease fixed charges by greater capacity. (3) Lower fuel consumption by more effective use of heat exchangers and hot oil pumps. (4) Utilize the excess heat for other purposes. ( 5 ) Fractionate the distillate into an end-point product. (6) Increase the length of run and improve the quality of the
NONACE
FIGURE3. HALL’SPROCESS, 1916
tar by preventing coke deposition. (7) Increase the yield by recovering all the gasoline fractions in the gas and reducing the tar to a minimum. (8) Offer greater ease and safety in operation.
From gas oil and kerosene the yields were from 30 to 40 per cent gasoline, 50 to 55 per centcycle stock, 5 to 10 per cent Great advances along each one of these lines have been made. tar, and 5 per cent gas. The high crack per pass employed GYROPROCESS necessitated a charging stock of distillate character. The pressure being sufficiently high to maintain a solvent liquid in the reaction zone, a good quality of tar was produced. On the other hand, the high crack per pass prohibited the economical use of residual oils as charging stock.
OTHER PROCESSES There were other processes being developed at that time, among which were the Holmes-Manley and the Tube and Tank. I n the Holmes-Manley process, fresh feed and recycle stock were passed through a furnace coil and discharged into four upright stills which were mildly fired. These stills acted as reaction chambers and depositories for coke. Vapors from these stills were fractionated in a tower from which cracked distillate was taken overhead, the recycle being returned with a hot oil pump to the furnace coil. The Tube and Tank process resembled the Cross process quite closely, the distinguishing difference being operation at a lower pressure and the vertical position of the soaking drum. Both of these processes operated more successfully on gas oil than on any other charging stock.
LIMITATIONS OF EARLY PROCESES Until 1924 the capacity of cracking units remained small, the fresh feed rate ranging from 200 to 1000 barrels per day with a maximum gasoline production of 350 barrels. No process had been developed which handled a wide variety of stocks with equal facility. Processes which could crack residuum satisfactorily could not successfully handle light distillate. Of distillate plants the reverse was true. I n general, gas oil, which had to be predistilled from the crude, was the universal charging stock. Cracked distillate (sometimes called “pressure distillate”) contained only 60 to 75 per cent of gasoline. Mechanical limitations imposed such mild temperatures and pressures that some crackable oil was eliminated with the final tar to decrease the refractoriness of the oil to be recycled. The gasoline contained in the fixed gas was either not recovered or only partially recovered by an extraneous absorption plant a t considerable expense. Lacking efficient heat exchangers and hot oil pumps, fuel consumption was high-from 15 to
The Pure oil Company in developing the C;JTO process has followed along Hall’s general principle, Figure 6. The Gyro process has overcome the inherent inability of a vapor-phase process to handle residuum stocks by making provisions to predistill heavy charging stock. This is accomplished by passing a heavy stock combined with recycle oil from the cracking process through a pipe still and into a vaporizer. The vapors leaving this vessel, carefully freed from all entrained liquid, are passed through the cracking furnace, where
D€W.if GMATD..
FIGURE4. DUBBSPROCESS,1918 their temperature is raised rapidly to l l O O o F. To eliminate the injurious effects of polymerization and overcracking, the temperature of these vapors is lowered almost instantaneously to about 700’ F. by directly contacting them with cooled recycle oil. The process operates under atmospheric pressure, The gasoline has a high Cooperative Fuels Research Committee octane value, the gas production is high, and the tar production low, I n some installations excess heat has been utilized to generate steam and to rerun the distillate after its chemical treatment. Gas production has been reduced in some cases by polymerizing the cracking plant gases to motor fuel in a separate furnace. Careful design has made long runs possible.
Februarv. 1931
I S D U S T R 1.4 L A N D E N G I N E E R I N G
RECENT IMPROVEMENTS Recognition of the fact that a low-boiling range, cleanly distilled stock could be cracked successfully in the vapor phase led to the development of the de Florez process (5) which has heen widely used. Fresh feed, which may vary from kerosene
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the trouble previously experienced with coke in the tar. By Providing for increased Operating Pressures, the Plant could satisfactorily Process kerosene and other lighter Oils. Further design changes are to be found in the recently completed by the Shell Company a t East Canada. The feed stock is crude oil which, after being topped with waste heat of a predetermined percentage of light gasoline, is separated into two fractions, one light and one heavy. The heavy fraction is cracked at about 920" F. and a pressure of 250 pounds. The light stream, which with the recycle contains all the natural naphtha that it is necessary to reform, is cracked a t approximately 980" F. and a pressure of 250 pounds. End-point gasoline vapors leaving the installation are passed through vapor-phase clay-treating towers. The heat for stabilization of the gasoline is furnished by the
FIGURE 5 . CROSSPROCESS, 19J5
to pitches heavier than water, is introduced into an evaporator, having been heated in exchangers. I n the evaporator the fresh feed, being either distilled and/or cracked, produces a stock which is a suitable furnace charge. Absence of any cn appreciable coke deposition in the tubes allows runs as long as 60 days. Usually the furnace outlet temperature is above 1000" F. a t a pressure of 200 pounds. Excess heat is utilized in an FIGURE6. GYROPROCESS integral stabilizing and absorbing unit and in some installations is used to top crude. One particularly interesting unit charges a previously cracked fuel oil having an A. P. I. gravity cracking unit. The fixed gases are conducted to a separate of 3 to 5. A coke drum has been interposed between the absorbing system to recover their gasoline content. The furnace and the evaporator. The residue from the evaporator length of run is from 20 to 25 days. The Cross process was improved by the addition of fracis pumped into the coke drum, where, under the influence of a long soaking time, high temperature, and pressure of ap- tionating towers, whereby the hot synthetic crude was sepaproximately 100 pounds, it is reduced to coke. Plants of rated by contained heat into tar, cycle stock, and end-point this type possessing two such drums used alternately have gasoline. Subsequently a hot oil pump suitable for the produced over 1,000,000 pounds of coke during a run. With extremely high pressure was added, by which the recycle stock a crude topping tower as an adjunct, the process allows crude was reprocessed after blending hot with the fresh feed. The to be refined directly to gas, gasoline, and coke. draw-off from the reaction chamber was moved from the side The capacity of the early Dubbs units was limited to 500 to the bottom, and this change so decreased coke formation as barrels per day of feed by the gravity oil circulation system to extend the length of runs from 8 days to 20-25 days. employed. The installation of a hot oil pump to effect this The crack per pass employed was still too high to utilize circulation removed this limitation and simultaneously re- residuum oils as charging stocks. To overcome this limitaduced the fuel consumption by allowing all the fresh feed to tion, primary cracking was employed. This process consists of be fed into the dephlegmator. Another improvement was wbjecting the heavy oil to an initial crack per pass of 8 to 10 the i n s t a l l a t i o n of a per cent. Coincidental flash tower. The oil with the formation of was not allowed to rethis amount of gasoline main in the p r e s s u r e there is formed a large VAPOR-PHUf fUtNACL reaction chamber a percentage of oil of a sufficient length of tjime lower boiling point and for coke t o form but molecular weight than was discharged as fast the feed stock. This a s i t c o l l e c t e d i n t o gggL f r a c t i o n can satisfacanother drum, where torily be cracked at a it was distilled under h i g h c r a c k per pass. reduced p r e ss u r e , by I t is characteristic of its contained heat, to this primary cracking produce a satisfactory o p e r a t i o n t h a t the STRA/Cin%QU%r a m o u n t of residuum final tar. The distillate from this operat'ion was produced from a topped fractionated to remove crude having a given a small percentage of viscosity a n d pour g a s o l i n e , and the repoint is less than that cycle stock returned to amount which would the process' This imFIGURE 7 . FLOWD l A C R a M OF 20,000-BARREL TOPPING,C R . 4 C K I S G , be produced by normal provement. o v e r c a m e REFORMING, AND ST.4BILIZING UNIT distillation.
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I n the Cross plants of this type the residuum oil is cracked in the primary cracking furnace and discharged into an evaporator tower. The gas oil distillate and gasoline are passed overhead into a bubble tower, and the gas oil fraction following condensation is pumped with a high-pressure hot oil pump as fresh feed to an improved Cross unit. Units of this type are built to handle 15,000 barrels per day of fuel oil. Various other modifications of the plant were made to adapt it to charging stocks ranging from heavy fuel oil to the lightest kerosenes. Further advances in the Cross process have made possible the recent crude topping combination cracking unit installed by the Standard Oil Company of Indiana a t their Whiting Refinery (Figure 7). This unit is designed to handle 20,000 barrels per day of Midcontinent crude. By utilizing waste heat, it removes from the crude that part of the natural gasoline cut which has a satisfactory octane value. Subsequently the natural gasoline of a low octane value is removed and is reformed into a gasoline of a higher octane value under a pressure of 750 pounds and a t 1000" F. The topped crude is divided into two fractions, one composed of residuum oil and the other of light gas oil. The residuum oil and heavy recycle stock are cracked in a primary cracker a t a temperature of 890" F. and a pressure of 250 pounds. The light gas oil, together with the light recycle stock, is cracked a t 200 pounds and 940' F. in a furnace and a reaction chamber. This light stock has been so prepared that it exists completely in the vapor state in this portion of the plant, and in order more effectively to utilize the reaction chamber the flow through it is from the bottom to the top. Cracked gasoline is condensed and separated from the dry gas a t 200 pounds. Gasoline is stabilized a t 250 pounds, utilizing cracking plant heat for this purpose. The length of run is from 20 to 25 days. The improvements in the materials of construction and methods of building are exemplified in this unit. Electrically welded vessels, built to withstand almost any pressure and/or temperature required, can be constructed. Hot oil pumps have been developed to such a point that they are no longer a limitation. Enlarged-capacity centrifugal pumps, pumping as much as 40,000 barrels per day a t a temperature of 700" F. and against a discharge pressure of 650 pounds, have been manufactured. The danger attendant to corrosion is reduced by the utilization of such noncorrosive metals as 4 to 6 per cent chrome-molybdenum. Instrument manufacturers produce reliable temperature, pressure, and rate-of-flow controllers. So great has been the advance in instrument manufacture that in a modern cracking unit all essential temperatures and/or pressures are controlled automatically. To
CHEMISTRY
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summarize, mechanical improvement has been so great that the ultimate capacity of a cracking unit is determined by the size of vessels which can be shipped. Illustrative of the advance is the fact that a t the present time there is being erected a unit of 32,000 barrels per day crude capacity. Gasoline production will be in excess of 18,000barrels per day. These figures are startling when one realizes that the total production of gasoline in this country is just under 1,000,000 barrels per day and the cracked gasoline production slightly more than 400,000 barrels per day. It is obvious that the greatest progress has been the development of combination topping and cracking units which have been described. The widest possible flexibility of charging stock is provided for. Unnecessary pumping and gaging costs are eliminated. The crude oil now being entirely refined in one installation, the total fuel consumption is materially decreased. The fixed charges, which are probably the largest single cost item in the refining of oil, are curtailed, and the production of a uniiormly high quality of gasoline is assured. I n those refineries where lube oil is not manufactured, a combination unit becomes the major part of the complete refinery. The Shell Company's plant a t East Montreal, Canada, and the refinery now being built for the Pan American Refining Corporation a t Texas City, Texas, are examples. It is a coincidence that the recent Whiting installation of the Standard Oil Company (Indiana) is built on the site of the first Burton still battery. As has been mentioned, the early Burton stills charged 200 barrels each cycle, which lasted 72 hours and made a yield of gasoline of 30 to 35 per cent. The maximum pressure employed was 75 pounds and the maximum temperature 800" F. This is to be compared with temperatures in excess of 1000" F. associated with pressures of 750 pounds and a cracked and reformed gasoline production of 11,000 barrels per day, with length of runs approximating 25 days. The Burton stills cracked a prepared gas oil while the present unit cracks all but the lightest fraction of the crude. While never equaling the gasoline quality, it would have required 470 Burton stills charging gas oil to have equaled the present unit's cracked gasoline production. LITERATURE CITED (1) (2) (3) (4) (5) (6)
Atwood, Luther, U. 5. Patent 28,246 (1860). Benton, Ibid., 342,564 (1886). Burton, William, Ibid., 1,049,667 (1913). Dewar and Redwood, Ibid., 426,173 (1890). Florez, Louis de, Ibid., 1,437,045 (1922). Young, James, British Patent 3345 (1865).
RECEIYZD
September 7 , 1933.
MILL AND Box FACTORY OF THE
FRUIT GROWERS'
SUPPLYCOMPANY, SWANVILLE, CALIF.
Photo b y W . I . Hutchinson .Courlesr J
L'. 5'.
Fore31 Serrice
