US1983664A - Process of cracking hydrocarbons - Google Patents

Description

Dec. l1, 1934. E. c. HERTHEL 1,983,664

PROCESS OF CRACKING HYDROCARBONS Filed Aug. 18, 1926 ATTORNEYS ff Patented Dec. 11, 1934 UNITED STATES PATENT OFFICE Eugene C. Herthel, Chicago, Ill., assignor to Sinclair Refining Company, New York, N. Y., a

corporation of Maine Application August 18, 1926, Serial No. 130,049

1 Claim.

My invention relates to improvements in the cracking of higher boiling hydrocarbon oils to form lower boiling hydrocarbon oils therefrom by distillation at a cracking temperature under pressure. This application is in part a continuation of my prior application filed July 12, 1922, Serial No. 574,380, and my present invention relates particularly to improvements in the treatment of relatively heavy hydrocarbon oils, such as heavy gas oil, or Wax distillates, for the production of gasoline or pressure distillate or m0- tor spirit according to the general process there described.

In carrying out the process of my said prior application, a liquid body of the hydrocarbon oil to be cracked is subjected to distillation at a cracking temperature and under superatmospheric pressure, vapors being taken off under superatmospheric pressure, and fresh oil is supplied to and tar-laden oil Withdrawn from the liquid body as the operation proceeds, the supply of fresh oil to and withdrawal of tar-laden oil from the liquid body being regulated to prevent saturation of the oil in the liquid body with pitch or pitchy constituents and so that such pitch and pitchy constituents areheld in solution in the oil body and in the tar-laden oil as Withdrawn. The formation of carbonaceous deposits upon the heating surfaces of the still by coking thereon of pitch or pitchy constituents precipitated from the oil is thus prevented; several important advantages being thereby secured, among which may be mentioned prolongation of the period of operation and the avoidance or retardation of the formation of carbonaceous deposits upon the heating surfaces of the still acting as an insulating layer upon these surfaces. In carrying out my present invention, likewise, the operation is conducted so that the body of oil undergoing distillation does not become saturated With pitch or pitchy constituents during the operation or at least not until just prior to the time when it is desired to terminate the operation.

My present invention relates particularly to the treatment of hydrocarbon oils of Which not more than about 25 to 50% boils off at 600 F., and my present invention is of special value and application in connection with the treatment of such oils of which not more than about 30% boils off at 600 F. In the foregoing reference to percentages boiling off at 600 F., I refer to fractional distillation under atmospheric pressure, The following distillation figures were taken from ran oil typical of those to the treatment of which my present invention relates. This oil had a gravity of 30.5 B. and an initial boiling point of 226 F., 8.2% came off up to 500 F., 28% up to 600 F. and 72% up to '700 F.

Petroleum oils of this character tend to crack very readily, particularly when they contain wax. There is, in fact, a marked tendency to overcracking, the decomposition proceeding with the direct formation of coke or coky material instead of With theformation ofv pitches soluble in the oil. For example, at temperatures normally reached in distillation carried out under a pres- F. F. F. F. F.

'sure of 95 pounds per square inch (above atmospheric pressure), decomposition tends to proceed directly to the formation of gasoline, gas? and coke with an excessive production of the two latter materials. Such stocks, however, will remain in the liquid phase at relatively high temperatures.

According to my present invention, such relatively heavy oils are subject-ed to distillation at a cracking temperature under comparatively loW pressures such that the decomposition proceeds with the formation of pitches rather than of coke or coky material, and fresh oil is supplied to and f tar-laden oil Withdrawn from the charge of oil undergoing pressure distillation during the operation, the rate of supply of fresh oil and the rate of Withdrawal of tar-laden oil being correlated with the pressure maintained to prevent saturation of the charge of oil With pitch. With lower pressures, the proportion of the total oil supplied to the operation withdrawn as tar-laden oil may be less than with higher pressures. In general,

in carrying out my present invention, the supply of fresh oil and the Withdrawal of tar-laden cil is regulated so that the amount of oil taken off as distillate docs not substantially exceed about 40%, or better 35%, of the total oil including the original charge of oil and the fresh oil supplied during the operation. The percentage of the oil taken olf as distillate may with advantage be maintained as low as 30% or less of the total oil, particularly with stocks of higher boiling range, for example stocks not more than about 30% of which boils off at 600 F. under atmospheric pressure. As an example of the correlation of the rates at which fresh oil is supplied, tar-laden oil withdrawn, and distillate taken off, with a pressure of not more than about 60 pounds per square inch, say about 55 pounds per square inch, the percentage taken off as distillate may be as high as 40%, with a pressure of not more than about 95 pounds per square inch, say pounds per square inch, the percentage taken olf as distillate may be as high 35%, and with a pressure of 125 pounds to 130 pounds per square inch or more the percentage taken oif as distillate may be restricted not to exceed 30% or less. Likewise, in carrying out my invention, where the vapors from the pressure still are subjected to a refluxing operation, the refluxing action may be diminished to assist in malntaining lower temperatures and less intensive cracking conditions in the charge of oil undergoing pressure distillation. Where the vapors from the ,pressure still are subjected to a reuxing operation by direct contact and heat exchange with fresh oil, for example, the rate at which fresh oil is supplied to the reiuxing operation is with advantage restricted and such additional fresh oil as is necessary to maintain the still charge and to replace tar-laden oil withdrawn supplied directly to the charge of oil in the pressure still. It is particularly advantageous to reduce the refluxing action to which the vapors from the pressure still are subjected when higher pressures are used.

One form of apparatus adapted for carrying out the present invention is diagrammatically illustrated in the accompanying drawing, and the invention will be further described in connection therewith. The pressure still illustrated is of the general construction described and illustrated in Letters Patent No. 1,285,200 issued to the Sinclair Rening Company, November 19, 1918, on an applicationof Edward W. Isorn, but it will be appreciated that my invention is useful in connection with other types of pressure cracking stills.

Referring to the drawing, the pressure still proper comprises a bulk Supply tank 1 adapted to contain a body of oil and located away from a battery of heating tubes 2 with circulating connections 3, 4 and 5 including a. circulating pump 6 for circulating oil from the bulk supply tank upwardly through the heating tubes and back to the bulk supply tank. A connection is provided at 'l for withdrawing tar-laden oil from the body of oil in the bulk supply tank 1, and a connection is shown at 8 for supplying fresh oil directly to the stream of oil circulating through the pump 6 to the heating tubes. A reflux tower 9 is arranged above the bulk supply tank and is connected thereto by a vapor line 10 through which vapors escaping from the bulk supply tank enter the lower end of the reflux tower. The vapors escaping uncondensed from the reiiux tower pass to a suitable condenser (not shown) through connection 11 where they are condensed to form the pressure distillate. rlhe pressure within the system may be maintained by valves (not shown) located in the vapor line between the refluxqtower and the condenser or by valves arranged beyond the condenser. A connection 12 is provided for introducing fresh oil directly into the upper end of the reux tower. Any oil employed as a supplemental reuxing agent may be introduced into the reflux tow-er through this connection in admixture with fresh oil, or a separate connection may be provided for introducing such oil, ad-

vantageously at a point in the reflux tower above the point at which the fresh oil is introduced. Reflux and admixed fresh oil collecting in the lower end of the reflux tower are supplied to the circulating stream of oil entering the circulating pump 6 through connection 13.

As an example of the carrying out of my invention in apparatus of the type illustrated, the pressure still may be charged with about 8,000 gallons of a heavy oil of the character specified, say an oil of which not more than about 30% boils off at 600 F., and brought to cracking conditions of temperature Vand pressure. The introduction of fresh oil both into the upper end of the tower and into the circulating pump is begun and vapors are taken off and condensed to form the pressure distillate. Operating under a pressure of about 50 or 55 pounds per square inch, distillate may be taken ofi, for example, at a rate of about '700 gallons per hour, fresh oil supplied through the circulating pump at a rate of about 200 gallons per hour, and fresh oil supplied to the reflux tower at a rate of about 1400 gallons per hour. Before the concentration of pitch in the still charge reaches the saturation point, the withdrawal of tar-laden oil is begun. The still may, for example, be operated for about 3 hours before the withdrawal of tar-laden oil is started. initially, the tar-laden oil may be withdrawn at a rate upwards from about 1000 gallons per hour or somewhat more until the still charge is reduced approximately to the amount of the charge before the introduction of fresh oil is begun, and the rate of withdrawal of tar laden oil then somewhat reduced but maintained sufficient to prevent the concentration of pitch exceeding the saturation point. The distillation is advantageously terminated as the percentage of the oil taken off as distillate approaches 35 to 37% of the total oil supplied to the operation, including the original charge and the fresh oil supplied during the operation. The distillation may for example be terminated under the above conditions approximately 20 to 28 hours after the taking off of vapors from the pressure distillation has begun.

As a further example of the carrying out of my invention in apparatus of the type illustrated, the pressure still may be charged with about 8000 gallons of a similar stock and brought to cracking conditions under a pressure of, for example, to 100 pounds per square inch. With higher pressures, and consequent higher temperatures, the saturation concentration of the pitches formed is somewhat reduced andthe percentage taken off as distillate is therefore reduced as compared, for example, to the previous example in which a lower pressure is used. Distillate may be taken off, for example, at a rate of about 700 gallons per hour, fresh oil supplied to the reflux tower at a rate of from something less than about 700 gallons up to about 1000 gallons or somewhat more per hour, and additional fresh oil supplied through the circulating pump to maintain the total rate at which fresh oil is supplied in the neighborhood of from 2000 to 2500 gallons per hour, tar-laden cil being withdrawn at a rate approximating the difference between the total rate at which fresh oil is supplied and the rate at which distillate is taken off. Distillation is advantageously terminated before the percentage of the oil taken off as distillate substantially exceeds 35% or better 28 to 30%, of the total oil supplied to the operation, including the original charge and the fresh oil supplied during the operation.

The distillation may be continued under the above conditions for from approximately 100 to 400 hours after the taking 01T of distillate from the pressure distillation operation has begun, or somewhat longer.

After carrying out the major portion of the distillation in accordance with my invention, for example as described in either of the two foregoing examples, withdrawal of tar-laden oil may be stopped or diminished and the operation of the pressure still continued for a very brief interval to obtain an additional amount of distillate. This, however, entails danger of deposition of carbonaceous materialwithin the still, although only at the end of the operation and just before it is terminated.

In carrying out my invention, accordingly, heavy oils are cracked under conditions such that the decomposition proceeds with the formation of pitches, which are soluble in the oil undergoing distillation, rather than with the formation of coke or coky materials relatively insoluble in the oil, and the distillation is carried out to prevent the concentration of these pitches as formed from exceeding the saturation point, by supplying fresh oil and withdrawing tar-laden oil in amounts regulated to maintain the pitch concentration below the saturation point, so that the heavier products of the cracking operation are kept in solution in the oil thus preventing, or minimizing, the formation of carbonaceous deposits within the pressure still, a diiiculty particularly acute with very heavy stocks. In carrying out my invention, it is advantageous to maintain a very low temperature differential between the oil and the hot products of combustion, or other heating medium, employed for supplying heat to the cracking operation. 'I'his may be accomplished in several ways; for example, by taking off distillate at a reduced hourly rate with reduction in the amount of heat supplied per hour, or by mixing excess air with the heating gases, or by recirculating a part of the heating gases which have passed over the heating surfaces of the pressure still again over these heating surfaces in admixture with fresh heating gases.

I claim:

A process of cracking higher boiling hydrocarbons to form lower boiling hydrocarbons, which comprises heating a liquid body of hydrocarbon oil of which not more than about 50% boils off at 600 F. at atmospheric pressure to a cracking temperature under a pressure of not more than about 60 pounds per square inch by circulating oil from said liquid body through heating tubes and back to said body, and taking off vapors from said liquid body under pressure while continuing the heating operation, supplying fresh hydrocarbon oil of the same character to and withdrawing tar laden oil from the said liquid body of oil as the operation progresses, subjecting the vapors taken off from said liquid body to a refluxing operation and returning reflux condensate formed thereby to said liquid body, and regulating the supply of fresh oil and the withdrawal of tar laden oil so that throughout the major portion of the operation the amount of oil taken off as final distillate does not substantially exceed about 40% of the total oil supplied to the operation including the original liquid body of oil and the fresh oil supplied during the operation.

EUGENE C. HERTI-IEL.

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