US2472669A - Preventing coke formation in preheater tubes - Google Patents

Description

Patented June 7, 1949 PREVENTING COKE FDRMAZIJION? IN PREHEATERTUBES Eugene 'V. Mathy, Bartlesville, Okla 'a'ssignor-to Phillips PetrOIeum 'Compa-n-y, a corporation of Delaware ,No Drawing. ApplicationNovember 2 .1945, SeriaILNO. 626,432

2 Claims.

This invention relates to the heating'of hydrocarbons. In one of its more specific aspects it relates to a method of heating oils to catalytic cracking temperatures with substantially no coke formation in the tubes of the heating coil.

In the cracking of high boiling hydrocarbon oils for the production of lower boiling hydrocarbons normally included in the gasoline boiling range two general .methods, thermal andcatalytic, are known, and both are widely used. .In the thermal method the oil to be cracked is ordinarily passed through a tube heater under sufficient pressure to maintain the material in the liquid state then expanded into a" vessel in which the vapors formed are separated from'the liquid. In the catalytic process the oil tobe cracked .is passed through heater tubes in which the oil becomes heated and finally vaporized before leaving the tubes. As the oil becomesheated to progressively higher temperatures, higher boiling hydrocarbons vaporize until a point is reached at which all oil has vaporized and no liquidremains. At this point of final vaporization, or drypoint, coking ordinarily occurs so that after some time such a furnace must be taken off process for tube cleaning. Further, some coking occurs even prior to the dry point, the general tendency being,however, the closer to the point of complete vaporization the greater is the amount of coke deposition. Low operating pressures and short residence time result in high vapor velocity in heater tubes. Liquid remaining in this'high velocity vapor stream gives a scrubbing action on the tube walls. This action continues until a point of complete vaporization is reached at which point there is no liquid to assist'in keeping the walls washed free of carbon or coke.

An object of my invention is to provide a method for preventing or minimizing coke formation in heater tubes in which hydrocarbon oil is heated prior to catalytic conversion.

Another object of my invention is to provide amethod for minimizing coke formationin catalytic conversion heater tubes, the process 'of which requires substantially complete vaporization of the charge stock.

Still another object of my'invention is coprovide a method for the removal of carbon inadvertently formed on the walls of heater tubes used for complete vaporization of hydrocarbon Still other objects and advantages of my inven-- tion Will be apparent to those skilled in the art from a careful study of the following disclosure.

I have found that carbon or coke formation can '2 be prevented or materially decreased in heater tubes 'in which complete vaporization of the charge oil-is desired by having present at all times and at allpoints in-theheater-tubes, some liquid. To maintain some liquid in the heater tubes and yet-topermit complete vaporization of all liquid to be cracked catalyticallygI add to the charge oil a "small percentage of a substantially noncoking, higher boiling oil. Iprefer to add this extraneous oilto-the extent of from about 1 to 10 per cent by volume. This oil, which is preferably a fraction of acrude oil, is of such a boilingrange that it-remains-inthe liquid state during passage through the tube heater. In this manner "the small amount pf'difficultly vaporizable material from the original charge oil is maintained in solution or suspension in place of being vaporized or cracked to coke at the hereinbefore mentioned dry point. ,In addition to preventing occurrence of a dry point with its attendant coke deposit, this high boiling oil and residual unvaporized'heavy endsof the charge stock pass through the remaining tubes at a high velocity andexert a scrubbing action on the walls of the heater tubes which action, I have found, contributes materially in scrubbing orv removing coke already formed.

.In athermal cracking process, such a liquidas I use would have to be discarded, that is, not recycled to the heater-aftereachpass sinceit would contain'those portions of the original charge .011 which have the greatest coke-eforming tendencies.

According to my catalytic process, this liquid may :beseparated "from the vapor stream by a separator just prior 9130 thercatalyst chamber. As .an-alternativethe heater effluent containing this unvaporizedoilmaybe passed directly into the catalyst chamber. Although .this methodof operation increases the amount-of carbon deposition on'the ca'talyst,:it hasbeen; found that it does not cause excessive pressure drop at any point in the catalyst bed.

The same results could be obtained by operating the heating coil at azpressure sufficiently high ingly limits the scouring action on the tube walls.

Since these disadvantages overshadow any possible advantages, I prefer to solve the problem in a difierent manner and one in which such inherent disadvantages do not exist.

In one catalytic preheater in which a wide cut gas oil having an ASTM boiling range of 400 F. to 725 F. was heated to a temperature of 1000 F. at a coil outlet pressure of 90 pounds per square inch, I added to the gas oil prior to heating 5 per cent by volume of a noncoking portion of the original crude oil. This added oil was not vaporized at the conditions of heater operation. The oil kept the furnace tubes clean and without removal from the vapor stream prior to the catalyst chamber increased the carbon deposition on the catalyst by only approximately 1 per cent by weight of the charged gas oil.

This crude oil fraction had an average boiling point above 1050 F. and was from a naphthenic base crude 011.

While some catalytic cracking stocks tend to form more carbon in the preheater tubes than others, I have found that from 1 to per cent by volume of a high boiling, noncoking oil is ordinarily sufiicient to maintain heating and vaporizing tubes relatively free from carbon.

My invention, as herein disclosed, may be used with gas oil wherein the preheater transfer temperatures are as high as from 850 F. to 1150 F. some pressure is ordinarily maintained in preheating coils and I prefer coil outlet pressures from, for example, 50 to 200 pounds per square inch. While the Conradson carbon value of such a heavy, high boiling oil is more or less unimportant, it should preferably not be much over 4 per cent by weight. Such an oil employed as herein disclosed eliminates the dry point in the heating coil and, furthermore, at high gas velocities in the coil the oil exerts a scrubbing action which exerts a marked influence in maintaining the tube walls free of carbon.

One indirect, yet important, advantage resulting from the use of my invention is that clean tube walls promote more eflicient heat exchange and lengthen the life of the heater tubes.

In a preferred operation the oil-vapor efiluent from the heating coils is passed to a tar or heavy oil separator in which the liquid is separated from the vapors, the latter passing then to a catalytic conversion step. This tar or heavy oil separator bottoms may be sent to fuel oil or similar product. It preferably is not recycled since in addition to the carbon residue from the added heavy oil it contains the carbon residue from the gas oil and any tarry material which may have been formed in the heating coils.

From the tar separator the overhead vapors are conducted to the reaction chamber in which they are contacted with a suitable cracking catalyst. The effluent from the cracking chamber is then fractionated and worked up into fractions and stocks as desired.

As an alternative subsequent step, the tar separator may be entirely eliminated. In such a case the liquid present in the preheater coil efliuent is deposited on the catalyst where it is cracked. This method of operation gives a slightly greater carbon deposit on the catalyst but it has been found that this carbon deposit is ordinarily well distributed throughout the catalyst and, therefore, excessive pressure drop in a localized zone in the bed is not encountered.

It will be obvious to those skilled in the art 4 that many alterations and changes may be made in the process herein described such as in temperatures and pressures of operation Of the preheating coil and even in the specifications and properties of the high boiling oil added to the gas oil to be vaporized. The percentage of the oil added may even be some value not specifically stated herein and yet remain within the intended spirit and scope of my invention.

Having disclosed my invention, I claim:

1. A process for heating and vaporizing in the absence of a catalyst and to a vapor phase catalytic cracking temperature a gas oil charge stock consisting essentially of a wide cut gas oil having a boiling range of 400 to 750 F. in a tube heater with substantially no deposition of coke on the heater tube walls comprising adding to said gas oil to be heated and vaporized a liquid non-coking oil in an amount of from 1 to 10 per cent by volume of the charge oil, said non-coking oil having an average boiling point above 1050 F. and above any temperature in excess of 1050 F. to which the subsequently formed gas oil vapors are heated, passing this mixture of oils into the heating coil tube, heating said mixture therein to a temperature at which the gas oil is substantially completely vaporized without cracking and the noncoking oil remains as liquid, and removing the gas oil vapors and the liquid non-coking oil from the heating and vaporizing coil.

2. A process for continuously heating and vaporizing in the absence of a catalyst and to a vapor phase catalytic cracking temperature a gas oil charge stock consisting essentially of a wide cut gas oil havin a boiling range of 400 to 750 F. in a tube heater with substantially no deposition of coke on the heater tube walls comprising continuously adding to said gas oil to be heated and vaporized a liquid non-coking oil in an amount of from 1 to 10 per cent by volume of the charge oil, said non-coking oil having a Conradson carbon value not over 4 per cent by weight having an average boiling point above 1050 F. and above any temperature in excess of 1050 F. to which the subsequently formed gas oil vapors are heated, continuously passing this mixture of oils into a heating coil tube,,continuously heating said mixture therein to a temperature at which the gas oil is substantially completely vaporized without cracking and the non-coking oil remains as a liquid at all points of the tube downstream of the point of complete vaporization of the gas oil, and continuously removing the gas oil vapors and the liquid non-coking oil from the heating and vaporizing c'oil.

EUGENE V. MATHY.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,324,766 Fleming Dec. 9, 1919 1,408,698 Hanna Mar. '7, 1922 1,428,339 Manley Sept. 5. 1922 1,573,532 Alexander Feb. 16, 1926 1,752,004 Jaqua Mar. 25, 1930 1,861,399 Leamon May 31, 1932 2,286,447 Thomas June 16, 1942 2,374,338 Dunham Apr. 24, 1945