US2487324A - Conversion of hydrocarbons - Google Patents

Description

Nov. 8, 1949 5. J. FoRsETH CONVERSION OF HYDROCARBONS v Filed Aug. 12, 1946 35' :5o 3 BUBBLE POINT 15 CURVE U m .T m

HIGH PRESSURE 26 PRE EATING 23\ v A NONCATALYTIC f 33 CRACKING 3| 25% 32 l ,4 SEYPARATINYG LOW PRESSURE 1 MEANS PREHEATING GASOUNE, HEAVY PRODUCT on.

INVENTOR.

GLEN N J. FORSETH TTo NEYs l atentecl Nov. 8, 1949 i CONVERSION OF HYDROCARBON S Glenn J. Forseth, Spring Valley, Wiswassignor to Phillips Petroleum Company. a corporation .of

Delaware Amflimim u 6,-seria1 No.- 6893873 12 Claims.

This invention relates toflthe conversion .of-

tinuous operation is "the formation of solid carbonaceous deposits, generally termed coke, within the cracking equipment. Even .whena relatively low pressure and low temperature are used, together with a catalyst "for such reactions, the

catalystis often fouled by coke-deposits, andin any event, coke formation often takes. place within the coils of the preheating equipment. Such coke formation oftenmakes it necessary for the heating furnace to be shut down for removal of the cokeby a'mechanicalmeans or by oxidation. 'Sucha situation "ultimately reduces the totalprodu'ction of the "cracking unit. v

Iihave now discovered thatby proper control of the pressure in'correlation withthe temperature, coke formation can be considerably decreased. Such control andcorrelation applies in the heating coils, preceding both catalytic and non-catalytic crackingsections, or zones, of a cracking plant. I have determined that themajor portion ofthe coke-deposit occurs at that point where the oil or hydrocarbon material being treated is'present both in liquidphase and vapor phase. Since the 'oil is generally charged to'rthe process as a'liquid, and is desirable-to operate the craokingstep'at a'relatively low superatmospheric pressure, there generally isa-distinct-transition within the heating and/or cracking zones fromliquid'to vapor. (.Joke formation is substantially greaterthan it would be if such a direct transition were avoided. .I have foundthat operation I with a minimum formation of coke-results if the oil to be cracked is charged at an initial pressure :sufiiciently high that during the heating a separatephase does-not appear, and if the pressure is then reduced'tothe-desired cracking pressure at an elevated temperature sufficiently' high that two phases do not appearduring this reductionin pressure.

An object of this invention'is toproducehyd-rocarbons inthe gasoline range.

higher-boiling hydrocarbons .to form. lower=boilinglhydrocarbons with-a minimum production .of solidxcarbonaceous.=materia1.

A! further :obj ect of .this invention is to -.eifecta lowvpressure crackingcotiheavy hydrocarbons to form. lower-boiling .hydrocarbonsin the gasoline range .without excessive; production of coke.

-.F.urther .objectsand advantages of my invention will .becomeiapparent, to .one skilled. in the art, from. the accompanyingidisclosure. andudiscussion.

.My invention will bemore fully described in connection with the accompanying drawings which. iformaI part of this specification. .In. these drawings, .Figure .l ..-is a temperaturegpressure diagram diagrammaticallyshowing a typicalborder curve. for a typical gas oil, and illustrates two .alternativeheatin-g curves whereby such .an voil mayibe heatedtoa desired cracking temperature. .Figurez isva diagrammatic flow sheet illustrating .two arrangements of .apparatus for practicing two specific modifications of my invention.

Referring now toFigure 1, curve .lll is a.v border line curve-for agas oiLsuch as is well known to those skilled in the :art. It is-desiredtointro- .duce thisgas. oil into a. catalytic cracking chamber ata pressure of about 100 pounds per .square inch gage-and atemperature of about 1050 This generally accomplished by pumping the-oil .to the unit under zapressure of about 250 pounds per square inchgage, and heating the charge in a tube coil in a furnace. As the oil passes through the tube coil itreceives.heat,..thereby having its temperature raised, and thepressure on the .oil ialls ofii as .a result of resistance encountered in the passage of the stream of oilthrough the tube coil. The pressure. and temperature on the .oil at variouspoints along the length of the tube coil arerepresented by line .I I. which runs from the point-.124 to the point ,TzPa, which corresponds to the temperature and pressure of the oil at an inlet to the cracking. zone. At the point H where line H crossescurve Ill, the oil becomes a mixture of gas and liquid and remains such a mixture, with increasing amounts .of .gas and decreasing amounts .ofliquimuntil the curve! is crossed a second time at the point I3. It is'in this region that coke formation takes place to the greatest extent, especially when the amount of liquid oil becomesquitelow.

In accordance with *my invention, the oilis pumped'at a sufficientlyhigh inlet pressure that the line representing the heating of this oil completely-avoids'touc'h-ing curve ill. Thus, Iinitially pump the oil at a higher pressure Pr, and heat-1t 3 under this higher pressure until it is at a temperature of about 1000 F., this being a pressure sufiiciently high that curve ID has been avoided. At this point, T1P2 on Figure 1, I release the pressure and continue the heating so that the oil again corresponds in temperature and pressure to its former point T2P3. It will be seen that, in this second procedure, at no time do two phases appear. I have found, when following this latter procedure, that the oil can be brought to the point T2P3 with markedly less deposit of coke in the tube coils than when the usual heating procedure is followed.

Referring now to Figure 2, my invention may be practiced as follows: A suitable hydrocarbon charge stock, such as a naphtha to be reformed, or a gas oil to be cracked, or a mixture of two such stocks for simultaneous treatment, is introduced through line and pumped by pump 2| at a pressure sufficiently high that it will not cross the border curve It] for the specific hydrocarbon material being treated. At this high pressure, the oil is passed through economizer coil 22, highpressure preheating coil 23, a point of pressure reduction 24 where the pressure is reduced at least about 151) pounds per square inch and to below about zuu pounds per square inch absolute, and a second or low-pressure preheating coil to a cracking zone. The cracking operation itself may then take place in the absence of a catahlst, in the non-catalytic cracking zone 26, or, alternatively, in a catalytic cracking zone 21. While both or tnese have been shown on a single drawing, it is to be understood that, in general practice, one may be used without the other being incorporated. In operation, after a suitable reaction period, eiIluents of the cracking zone are cooled below a reaction temperature, as by introduction of a cool heavy oil through line 3i). The cooled oil is then introduced into separating means 3| for recovery or various products and byproducts. A desired hydrocarbon fraction boiling within the gasoline range is recovered as a product of the process through line 32. Undesired light hydrocarbons may be discharged through line 33. A heavy fraction, corresponding to or heavier than the hydrocarbon being charged, may be separated and recycled through line 34. A suitable oil may be separated through line and passed to line 30 for cooling efiluents of the cracking zone. Any desired portion of this latter stream may be discharged through line 36.

It will be appreciated that Figure 2 is completely diagrammatic and many specific items of equipment which are familiar to those skilled in the art, have not been included. Separating means 3| comprises separators, coolers, condensers, heat exchanger, accumulators, fractional distillation columns, and the like, and associated equipment, such as is well known to those skilled in the art. If desired, steam or some other diluent, may be added to the hydrocarbon material. Such an addition will, of course, lower the partial pressure of the hydrocarbon material; in such a case the total pressure should be such that the partial pressure of the hydrocarbon material will be within the limits elsewhere discussed herein. In most cases it will be desirable to add such a diluent, at an appropriate temperature, to the low-pressure preheating zone.

In order to successfully apply my process, it is, of course, necessary first, to determine the general course of the border curve In of Figure 1 for the specific charge stock to be treated. Although it is desired that the major portion, preferably at least about 70 percent, of the total amount of cracking per pass take place in the cracking zone, it is appreciated that a small amount of cracking will take place during the latter part of the preheating steps. In usual practice, the amount of cracking which will take place in each of the high-pressure preheating step, represented by line M of Figure 1, and of the low-pressure preheating step, represented by line [5 of Figure 1, will be between about 12 and 15% of the total amount of cracking per pass. It is preferred that this total amount of cracking per pass will be between about 33 and about 65 per cent of the total material charged through pump 2l of Figure 2. In practical operation the initial high-pressure preheating will take about 3 to about seconds. The low-pressure preheating will take about 0.5 to about 2.0 seconds and, in the presence of a catalyst such as granular bauxite, the time of residence in catalytic cracking zone 21 will be about 15 to about 100 seconds.

Since the temperature of the material undergoing reaction iscontinuously changing, as the stream of hydrocarbon material passes through the apparatus, it is more accurate and convenient to refer to the equivalent time at 950 F. for each of the three zones. The equivalent time at 950 F. is calculated from the equation of Neppe (Journal of the Institute of Petroleum, vol. 28, No. 218, pages 27-35, 1942) which relates the temperature interval for which the cracking is doubled to the cracking temperature.

where Ft=the temperature interval. t=cracking temperature, F.

From this the equation for the rate of cracking is developed.

(t950) (2) 450 and the equivalent time at 950 F. equals the con tact (residence) time times K from the above equation. When H so considered, the residence time of the stream in each of the two preheating zones will be about 8 to about 15 equivalent seconds at 950 F. and in the cracking zone will be about 17 to about 100 equivalent seconds at 950 F. When cracking a gas oil to produce a gasoline stock the initial inlet pressure, at the discharge of pump 2|, will generally be at least about 300 pounds per square inch and generally need not begreater than about 500 pounds per square inch. As previously stated, the pressure at the point of pressure reduction 24 should be reduced to below about 200 to 215 pounds per square inch absolute and should also be reduced at least about 150 pounds per square inch. The temperature of the hydrocarbon material at this point of pressure reduction will generally be between 900 and about 1050 F. and the tempera- 5 ture at the inlet to the cracking zone will generally be between about 950 and about 1000 F. The average temperature of the hydrocarbon material during the cracking operation will be between about 850 and about 1050 F., depending somewhat upon how much heat is introduced to the cracking zone.

Inasmuch as some small amount of cracking will take place during the preheating steps, with the production of lower-boiling hydrocarbons and perhaps also some hydrogen, the composition of ;Log K= 15.0515 L the hydrocarbon material will change somewhat, as the actual position and shape of the border curve 10, for the mixture just before, and shortly after, the stream passes point of pressure reduction 24, will be different than the border curve for the cold oil passing through pump 2|. This factor should, of course, be taken into consideration when designing the heating furnace and setting up the operation conditions, so that the lines I4 and I5, representing the passage of the material through the preheating coils, will safely miss the border curve [0 of the mixture in the coils as it actually exists. A satisfactorily close approximation can be readily calculated, or determined from synthetic mixtures, by one skilled in the art.

As will be obvious, the desired pressure drop at the point 24 can be obtained by any satisfactory means. design and manipulate a satisfactory pressure reduction valve. A more satisfactory procedure is to use small tubing up to this point, such as tubing having an internal diameter between about 2 and about 4 inches, and to use appreciably larger tubing from this point on, such as tubing having at least about twice the cross sectional area of the first tubing. In some instances there may be used sections of tubing having smaller differences in cross sectional area, or having-the same cross sectional area, by placing a suitable constriction or orifice at the point 24.

In place of a tubular non-catalytic cracking zone, such as is illustrated diagrammatically by coil 26, an empty reaction chamber, such as is well known to the art, may, of course, be utilized. While my invention finds successful application in using a single large bed of granular bauxite as a catalyst in catalytic cracking zone 21, other well known catalytic cracking catalysts and cracking procedures may, of course, be incorporated as a part of the total process.

My invention will now be further illustrated by the following specific example. A gas oil of the following characteristics is heated and charged to a catalytic cracking chamber, with a temperature of about 1003 F. and a pressure of 100 p. s. i. g. at the'inlet to the cracking chamber.

ASTM distillation, F.:

Bureau of Mines index 22.9 Pseudocritical temperature, F. 920 Pseudocritical pressure, p. s. i. g 187 This oil is charged, at a rate of about 4.8 barrels per ton of catalyst per hour, to the first section of the heating coil at about 300 p. s. i. g. and is brought up to a temperature of about 1000 F. with a pressure drop of about 90 p. s. i. g. The heated oil is then expanded to a pressure of about 100 p. s. i. g., and the temperature is raised to 1025 F. at the outlet of the second section of the heating coil. Since the critical point of the oil is 920 F. and 187 p. s. i. g., the heating is accomplished without entering the mixed phase region at any point. Near the first part of the low-pressure heating section steam at about 1000 F. is added to the stream of hydrocarbons,

It will, of course, be difficult to 6 in an amoiint of-about 64 pounds per barrel of the charged oil. The heated hydrocarbon-steam mixture is then passed through a mass of granular, hard Arkansas bauxite, wherein conversion to hydrocarbons in the gasoline range takes place. The average temperature in the catalyst chamber is about 940 F., the average pressure about pounds absolute, and the average time of residence is about 24.6 equivalent seconds at 950 F. With a preheating time of about 11 equivalent seconds at 950 F., about 16 per cent of the charge undergoes noncatalytic cracking during the preheating and about 50 per cent of the charge is cracked in the presence of the bauxite catalyst.

The eflluent of the cracking zone has the following composition.

Component:

C3 and lighter weight per cent 10.8 C4. volume per cent.., 7.6 C5 to- 400 F. do 35.8 Recycle Stock do 50.0 Carbon weight per cent 1.6

The fraction in the gasoline range (05 to 400 F.) has an octane number (ASTM, clear) of 76.5 and a Reid vapor pressure of 5.0 pounds.

It is to be appreciated that my invention is not to be restricted unnecessarily by any of the foregoing specific discussion. Various modifications of my invention can be practiced by one skilled in the art without departing from the scope or from the spirit of the disclosure, or without departing from the scope of the claims.

I claim:

1. An improved process for converting a heavier hydrocarbon material, capable of existing in a single phase under subsequent reaction conditions at a pressure below 200 pounds per square inch absolute and at a temperature in the range of 950 F. to 1050 F., to hydrocarbons in the motor fuel boiling range in which no coking occurs due to a liquid hydrocarbon phase and a gaseous hydrocarbon phase existing together, which comprises rapidly heating a liquid stream of such a hydrocarbon material in a first heating zone under a pressure of at least about 200 pounds per square inch absolute and sufiicient to avoid the formation of two phases of said hydrocarbon material during said heating to a temperature between about 900 F. and about 1050 F., the time of said heating in said first heating zone being between about 8 and 15 equivalent seconds at 950 F., subsequently reducing the pressure at least about 150 pounds per square inch and below 200 pounds per square inch absolute by passing said heated stream directly into a second enlarged heating zone having a greater cross sectional area while continuing said heating in said second heating zone so as to maintain a temperature in the range of about 950 F. to about 1050 F. and such that two phases of said hydrocarbon material do not appear, with a time of said heating in said second heating zone being between about 8 and 15 equivalent seconds at 950 F., subsequently passing said heated stream to a catalytic cracking zone containing a fixed bed of solid, granular bauxite as a cracking catalyst, the total extent of cracking per pass being about 33 to about 65 per cent by volume of the total hydrocarbon material charged, the residence time of said heated stream in said catalytic cracking zone being between about 17 and equivalent seconds at 950 F., andrecovering from efiluents of

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