US2633449A - Conversion of heavy hydrocarbons - Google Patents

Description

March 31, 1953 L. K. CHENEY 2,633,449

CONVERSION oF HEAVY HYDRocARBoNs Filed Dec. 28, 1949 /4 TTRNEYS Patented Mar. 31, 1953 CNVERSION'OF'HEAVY HYDROCARBONS Leroy K. Gheney,'.Lansing,1ll.assignorto :Sin-

Aclair Rening Company,l\lew YorkN. Ylra cor porationof Maine fappneatm December 2s, isiaiserial Noxissazv 1 claim. l

'.'My' invention relates :to improvements "inthe fconversion -of heavy H.hydrocarbonl oils. 'More `particularly, myfinvention resides in anl improved Ymethod for operating Vthermal cracking units charging-resi'dual stocksso as'to increase conversion Whileeffecting' heat economies.

The disproportionate.demand for gasoline and light fuels as compared to heavy fuels and resi dual products puts petroleum renners under heavy pressure to produce the maximum obtainable yieldszof Vs ,peciiication quality gasoline and distillatefuelsfrom crude "oil-'stocks--Whi-le lmini- :mizing productionoi theless valuable heavyjfuels and residual stocks. Tremendousguantities of Y'the latter, however, are inherently jpro'duced'in refining crude eiland in cracking 'heavy ils Ito light oils. Crude oil ordinarily contains substantial quantities of heavy ends which areasphaltic orcokefformirrg in nature. .Cracking .oil .to .produce.' lighter andmoredesirable molecular aggre gations also inherentlyproduces molecularcombinations.ofheavy cokeflike or cokefforming materials under .the .pyrolytic conditions involved.

As la consequence,.reners Vnd the .problem ,of

mere disposal ofheavy residual materialsin view .of their quantityan extreme 4problem intimes of 4peak demands for gasoline and" light jfuels.

iIn application Serial No. 1136,069, iled December128, 1949, of .HarryLouis `Pelzera method is `describedfor reducing heavy Yoils AYtopitch which as Yresiduum represents a .substantial reduction compared to .the Aquantities of tar or. heavy .oils

produced Lin conventional .cracking operations.

Becauseofits high .melting 'point and easy pul- 'verizability the .pitch product represents 'a more Vdesirable product withoutrequiringlthe costlycre- `moval'and'handling of "petroleum coke. 4Attthe same time, improved yields 'of 'gas oil are `ojotairied. 'I have *now found that this pitching process may .be ...adapted with particular advantage to combination with ordinary thermal vcracking systems charging-'residualstocks lwhich utilize a tar separator between fthe heating or cracking .'-zonezand thefractionating zone and which Acharge clean cycle oilcor mixed feedto the heating zone. I have found 'that the combination results in 'significant heat :savings and -in "higher conversion by improvingithe quality and quantity of cycle vstock available within "the cracking system.

According 'tomy invention, "tarsseparated 'from ithe gas voil evaporator or ntar separator of a thermal unitv charging 'residual stock is 'charged to a tower operated under pitching conditions to produce maximum overhead of heavy gas oil and about a 300 F. ring and ball M. P. or higher 2 pitch. Thetowenis :operatedatfa'flash' temperatture 'zo'fsahou't '7901to 380 vladventaugeou'sly '.'abQut;800 to :850 F., ,utilizing :steam tozreduce :the :hydrocarbon partial pressure and cool :the

5 tower :boot below about 650;1F. Anintermediate black oilheatenmay:beemployed,y ifthe tarseparator temperature is Vlow. wIn Aany event, the sensible lfieatof vthe evaporator bottoms iis utilized and a high yield of gas oil:isxstrimaedl out with- Aout tower coking. vLight :overhead isfseparated tovgas, -water .andgasoline -A- light., gas-oil: cutris advantageously withdrawn for use 1 as a vcutter stock or to net gasoil lmake yinorder-.tofreduce Ithe recirculation of highly. cyclic' and refractory i5 .oil Awithin the :cracking system. The bulk-oi the gas :oil produced, however, `is 4returned fto the -lcrackingheaterssas miXedfeed with the `ordinary cycle stockfrom the crackingunit fractionator.

`Irl-thermal cracking, recycle Yof .gas oil' is limited *l0 by decline in conversionivhena point ofdiminishingl'return isreachedfin :the concentration of refractory cyclic .stock recirculating within the unit. "Some of the .lighter rmore .refractory .gas oil, therefore, .is .usually withdrawn. On the z5 ,other lhand, ,useful convertible `stockisf lostto tar pbe .recovered for `,recycle while `a proportionate Y. Quantity `of unsatisfactory `refractory stock may ''be withdrawn so that the quality of mixed v,feed u to theicrakingheaterand hence the conversion 3" 'is significantly improved.

iMy invention will be' better understood by reference tothe Aaccormoanying drawing which rep- ;resents `conventionally and VVscheinatically "a 'sim- "plie'diiowplanofimy process. inthe drawing 40 `crude oilor-a reduced crude 4oil'stocl: is charged vto-primary tower I0 through'A line I I. Gasoline and lighter are taken overhead by line I'2 to receiver I3 `from-which `gas isremove'd by line i4, gasolineis removed by line -I5v, 'and reflux is re- `turned 'tothe `tower 'top'throughline I5. vThe kerosene orr light Voil cut maybetaken asafsi'de 'streamthrough" line H. The bottomsV is charged by'iline I8 Vthroughheater I-S .and linei'20 to *sec- `vondarytowerZ I A diesel for light gas voil-cut is ctaken 'overhead from tower 21I `through line'22 'whilefa heaviergas oil cut may be'taken through line `2-3.

Reduced 'crude f is delivered from the l bottoms of tower 2i through line 24 together with any extraneous feed, charged through line 25, through a heater and to gas oil evaporator 26 of the cracking system. Overhead from evaporator 26 is passed to bubble tower 21 through line 28. Clean cycle stock is charged to cracking heaters 29 from the bottom of bubble tower 27 through line 30 and is returned to gas oil evaporator 25 by means of line 3l. Gasoline and gas made in the cracking process are taken overhead from bubble tower 27 by line 32 to receiving drum 33. Gas is removed through line 34, reflux is returned to the tower top through line and product is taken through line 36.

According to my invention a bottom stream is taken from gas oil evaporator 26 and is passed through heater 37 in which it is raised to a temperature approximately 850 to 950 F. The evaporator bottoms is held in the heater for a period of time suiiicient under these temperature conditions to limit the percent crack (in terms of 400 F. end point and lighter) to 10% or less.

The hot evaporator bottoms are then flashed into pitching tower 3S through line 39. Pitching tower 38 is advantageously divided into a lower flash section and an upper fractionating section. Steam is introduced to the bottom of tower 39 through line 40. Sucient steam is introduced to the tower to cool the tower boot below coking conditions and to reduce the hydrocarbon partial pressure in the tower.

In general, about 10 to 25% by weight of steam 'i on the feed is required, with a hash temperature of 790 to 880 F. 'Ihe tower boot temperature should be maintained below approximately 650 F. to prevent coking, which, however, depends somewhat on the size of the unit. The lower portion of I the tower is advantageously baiiled so as to .improve heat transfer between the steam and the charge in the tower. With a large unit, obviously the residence time of the oil` on the tower wall will be less so that the tendency to coke is reduced. The tower is operated at approximately atmospheric pressure and up to about 10 p. s. i. g. The gas oil is flashed from the feed and produced either as side stream cuts taken through lines 4| and 42 or is taken overhead through line 43. As shown, heavy gas oil produced is taken as a fuel oil blending stock while the lighter gas oil through line 4| is returned to the cracking system in admixture with the clean oil recycled from bubble tower 21 through line 30 to cracking heaters 29. Also, as shown, bubble tower 27 is utilized as a fractionator for light products produced in the pitching tower with the accompanying heat economies. The light products passing overhead through line 53 are collected in receiver d4 from which water is removed as bottoms through line 45. Gaseous materials from receiver 44 are compressed by compressors 46 and are delivered to tower 2l' through line 41. Light gas oil collected from receiver 4,4 is delivered to tower 27 through line 48 for re-cracking in the thermal cracking system.

The following example of an operation embodying my invention in comparison with a similar run in a thermal cracking unit charging a residual stock and utilizing a vacuum evaporator to produce heavy cycle oil will further illustrate the principles underlying my invention. The gas oil evaporator tower bottoms are ashed to the pitching tower at a flash temperature of 820 F. Steam at 230 F. in the amount of 34,400

pounds per hour is introduced to the bottom of the pitching tower. The tower bottom temperature is maintained at 570 F. and the tower top temperature is maintained at 650 F. The crude oil charged to the combination crude running and thermal cracking unit is advantageously heated by indirect exchange in the top of the pitching tower. A water condenser may also be utilized in the tower top to act as a partial condenser. The tower is operated at 2 p. s. i. g. Fifty-seven thousand sci/hr. of gas are removed from the overhead receiver which is held at 100 F. and atmospheric pressure, for delivery to the cracking unit bubble tower. Twenty-four barrels per hour of light gas oil similarly are delivered from the overhead receiver to the bubble tower. The iight gas oil side stream from the pitching tower produces barrels per hour to net yield. Three hundred fty-ve barrels per hour of heavy gas oil are removed through a 250 F. condenser and charged to the cracking still heaters. A portion of the side stream may be returned to the pitching tower as reflux. One hundred thirty barrels per hour of pitch are produced as product. A comparison of the yields operating with pitching as against conventional vacuum evaporation of tar operated at maximum tower top tempera ture which will avoid coking follows:

in a hydrocarbon thermal cracking process charging a straight run residual stock and employing a tar separator between the cracking zone and the fractionating zone, the improvement which comprises heating tar bottoms from the separator at about 850 to 950 F. under cracking conditions for a period of time sufficient to limit cracking to lessl than 10% by weight, based on the 400 F. end point and lighter hydrocarbons, charging the so-treated tar bottoms to a pitching tower at about 790 to 880 F. and at approximately atmospheric pressure with about 10 to 25% by weight of steam on the feed, maintaining the boot temperature of the pitching tower at below approximately 650 F., separating water and light products overhead, removing heavy pitch as bottoms, recovering gas oil as distillate and returning gas oil to the cracking system as cycle oil stock.

LEROY K. CHENEY.

REFERENCES crrD The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,673,854 Watson June 19, 1928 1,953,013 Gray Mar. 27, 1934 2,023,619 Shefstead Dec. 10, 1935 2,096,589 Koch Oct. 19, 1937 2,101,088 Nofsinger et al Dec. 7, 1937 2,172,276 Dunham Sept. 5, 1939 2,045,458 Gary et al June 23, 1946

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