US2546349A - Apparatus for distillation of gasoline containing hydrocarbon fractions - Google Patents

Description

March 27, 195i P. scovlLLE APPARATUS FOR DI 2,546,349 sTILLATIoN oF GAsoLINE CONTAINING HYDROCARBON FRACTIONS 2 Sheets-Sheet l Original Filed Jan. l5

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March 27, 1951 P. scovlLLE APPARATUS FOR DIS 2,546,349 TILLATION 0F GAsoLINE CONTAINING HYDRocARBoN FRAcTIoNs Original Filed Jan. l5,

TORNEY QQ Inni Patented Mar. 27, 1951 l UNITED STATES PATENT OFFICE APPARATUS FOR DISTILLATION OF GASO- INE CON TAINING HYDROCARBON FRAC- IONS 1 Claim. (Cl. ZOB- 153) l This invention relates to the manufacture of gasoline, such as aviation gasoline andmotor fuel, from normally gaseous and liquid hydrocarbons. More particularly, the invention relates 2 Still another object of the invention is to provide an improved method of stabilizing raw cracked naphtha and natural gasoline, and alkylating overhead 10W-boiling or normally gastothe stabilization and fractionation of refinery 5 ecus isoparaifms with olens to produce aviation hydrocarbon fractions, such as cracked naphtha gasoline, wherein a greater recovery of low-boiland natural gasoline, and the catalytic convering olefins and isoparahins is effected from the sionof eluent W-boiling or normally gaseous raw cracked naphtha and the natural gasoline hydrocarbons to gasoline hydrocarbons of high to` thereby' materially increase the yield of aviaanti-knockvalue. 10 tion gasoline, While low-boiling normal parafin ThSS a-d-VSOH 0f my @pending aDPH-Ca'GOIl v components are retained in the stabilized. naph- Seriall No. 472,439, iiled January 15,I 1943, new tha or gasolinefor volatility, Patent N`0\2,.l42,440 dated TU-Ile 1,1948- A further object of the inventionI is to provide One of the. DriHCiDalObJ'eC'GS-Of theinventml iS apparatus for carrying out the 'method of this t0 DISOVde all improved prOCeSS 0f stablzingjand 15 invention, which apparatus is simple in construcfractionating, petroleum products OI' flaCOnS, tion, is readily available andv easily controlled, Such as Cracked naphtha, and 0f Converting IOW- and' economical in operation andl maintenance. bllg or normally gaseous eiiluents of such sta- Other objects and advantages of the invention bilization and fractionation to gasoline hydro- Wm. be apparent from the following description CaTbOIISOf high anti-RUOCK Value, t0 thereby 'll- 20 when taken inconjunction with the accompanycrease' the yields of aviation gasoline and' motor ing drawing andV appended claim. fuel produced froml available refinery fractions, In the drawing Whichrmusl-,rates preferred em, elimina-te or minimize corrosion', and secur'eother bodments 0f.1,he invention; advantages. Fig. 1 is a diagrammatic illustration of appa- Another object of the invention is to provide a ral-,us for carrying out the method of this invennovel methodof stabilizing and fractonating non as applicato the stabilization of raw cracked hydrocarbon mixtures to obtain a better separanaphtha and the polymerization of el'uent gases tion of desired fractions, while at the same time therefrom; and providing a' higher Stabilization pressure and a Fig..2 is a diagrammatic illustration of apparagreateroverhead condensateV recovery with availtus for carrying out the method of this invention able) cooling Water or otherv medium for the same as applied' to a modification involving the stabilireboiler temperature. zation andi fractionation of raw cracked naphtha Still another object of the invention is to` proand natural gasoline, and the alkylation of eiliuvide an improved method of stabilizingand frac'- ent low-boiling or normally gaseous isoparaflins tionating hydrocarbon mixtures containing both and olens therefrom. parains and olens of the same molecular In conventional rerlnery practice', it is cus- Weight, whereby a portion of a normal paraffin tomary to stabilize' raw cracked naphtha as obmay be retained in the stabilized bottoms for tained" from the cracking of petroleum hydrovolatility While effecting a greater recovery of the carbons" to remove' normalv butane and lighter, oleflns of the same molecular Weight in the over- 40 the eluent gases then being treated and conhead verted in part to gasoline hydrocarbons of high Another object of the invention isA tok provide anti-knock value by variousprocesses including an improved method of stabilizing raw cracked polymerization and alkylation. The resulting naphtha and polymerizing overhead low-boiling conversion products are stabilized and frac'- or normally gaseous unsaturated hydrocarbons to tion'ated: to recover an aviation gasoline, and a polymer gasoline, wherein a portion of a lowportion of the" normal butane may be blended boiling normal parafn is retained in the stabi'- back with the stabilized cracked naphtha for lized cracked naphtha for volatility, while at the volatility. As applied to polymerization, it is same time a greater proportion of the oleiins of customary' to stabilize the raw cracked naphtha the same molecular weight as the low-boiling so to remove overhead C4 and lighter; The overnormal parafn are removed in the overhead to thereby :increasethe yield of polymer gasoline;

head gases may then be fractionated to separate a' selected C4' fraction, and the resulting C4' frac'- tion is then polymerized to polymer gasoline, stabilized and hydrogenated to provide a saturated isoparanic aviation gasoline. On the other hand, the overhead gases may be condensed, and the condensate comprising mainly Csi-C4 hydrocarbons polymerized and stabilized to form a high octane motor gasoline. In an effort to secure maximum yields of this gasoline, the raw cracked naphtha may be deeply stabilized to remove substantially all of the normal butane and lighter, so that practically all of the C4 olens are obtained in the overhead. In this operation, difliculties may be encountered in condensing the desired amount of overhead gases with the use of available cooling water without refrigeration due to the lower stabilizer pressure required for a given available reboiler temperature; and the increased yield of polymerizable oler-ins in the stabilizer overhead may be counter-balanced by the oleiins remaining uncondensed and passing to the renery gas lines, rather than being passed in the condensate to the polymerization unit. On the other hand, if the stabilizer pressure is maintained constant, this deep stabilization of the cracked naphtha requires a higher reboiler temperature, which may be above that normally obtainable with the available renery high pressure steam supply. Also, the deep stabilization lowers the front end volatility of the stabilized cracked naphtha so that it will not meet specifications for motor uel, and necessitates the blending back oi normal butane with the stabilized cracked naphtha to meet front end volatility speciiications.

Consequently, in many reneries, a compromise is effected on the basis or" the available high pressure steam supply and the available cooling water temperature between the deep stabilization of the raw cracked naphtha described above and an operation in which a substantial proportion of the C4 hydrocarbons are left in the stabilized cracked naphtha for volatility. While this latter operation retains normaldoutane inV the stabilized cracked naphtha, it is also accompanied by a Substantial loss of the C4 olens, particularly butylene-2, in the stabilizer bottoms. This results in a loss of desirable olens in the polymer charge, with a resultant decrease in yield of aviation or motor gasoline from that theoretically possible from the available refinery stocks.

In order to operate at a suciently high stabilizer pressure to condense the major portion of the desirable constituents of the overhead from the raw cracked naphtha stabilizer, it has been previously proposed toinject water into the reboiler of the stabilizer. However, this method is necessarily limited to an increase in stabilizer pressure of approximately fifty pounds per square inch for a given reboiler temperature and, in'addition, may result in increased corrosion.

In accordance with the present invention, an improved method of operation is provided for stabilizing the raw cracked naphtha directly to desired volatility and R. V. P., while at the same time the portion of C4 retained in the stabilizer bottoms consists largely of normal butane, and a materially increased proportion of C4 oleiins for the particular relatively low reboiler temperature used is secured in the overhead. Moreover, ths method at the same time enables the stabilizer operating pressure to be increased substantially more than the above-noted fty pounds 4 for a given reboiler temperature without the use of water injection, whereby an increased recovery of the valuable unsaturated hydrocarbons is obtained in the overhead condensate by the use ofv available cooling water, or the same recovery can be obtained with a reduced overhead condenser surface area. This is accomplished by injecting into the stabilizer an essentially saturated hydrocarbon fraction, such as a butane ,fraction which is heavier than the overhead from the stabilizer but is lighter than the average of the heavier stabilized fraction removed as bottoms. This injected hydrocarbon fraction may be obtained from an external source, but preferably is obtained as a product of the stabilization and fractionation of the resulting polymerization products from which unsaturated hydrocarbons or olefins have been essentially removed. The injected hydrocarbon fraction may be introduced at any point within the tower below the point of entry of the normal reflux condensate, but preferably is added below the point of entry of raw cracked naphtha charge. Very good results have been secured by introducing this saturated C4 recycle stream into the stabilizer reboiler.

The invention is more particularly illustrated in Fig. 1, which shows the raw cracked naphtha as obtained from a refinery cracking operation being charged by pump lil through line H into an intermediate point of stabilizer l2. The latter may be a conventional fractionating tower of the multiple bubble plate type, and is equipped with a line I3 for withdrawing liquid from a pool within the base of the tower and passing the same through a reboiler I4 heated by high pressure steam supplied to heating coil l5, the heated hydrocarbon being returned to` the base of the tower by line i6. A bottoms of stabilized cracked naphtha is withdrawn from the base of the tower by line Il. Overhead gases composed of C4 and lighter pass by line i8 to water-cooled condenser I9 and thence by rundown line 2!) to accumulator 2i, Uncondensed gas, mainly C2 and lighter, is bled oiT from accumulator 2| by overhead line 22 to suitable gas-collecting means not shown. Liquid condensate, consisting largely of Cs-C4 is withdrawn from the accumulator through line 23 and forced by pump 24 through line 25. The latter communicates with branched lines so that a portion of the condensate may be returned by line 2l to the upper portion of tower i2 to serve as reiiux therein, while another portion passes by line 28 to a polymerization unit indicated generally at 29.

Any conventional catalytic or thermal polymerization unit for converting the olenic con-- stituents of the charge to polymer may be ernployed. Since the polymerization unit per se forms no part of the present invention, further illustration thereof is deemed unnecessary. By way of example, the unit 29 may be a conventional phosphoric acid unit provided with a heater in which the charge is raised to a temperature of about S-500 F. and then passed through a catalytic tower or converter containing kieselguhr on which is absorbed the phosphoric acid oatalyst. The operation thus far described is a socalled non-selective polymerization in which both C3 and C4 olefins are polymerized in the presence of each other. l-Iowever, it is to be understood that the overhead condensate from the stabilizer l2 could be further fractionated to separate mainly a C4 fraction, whereby cross-polymerization of normal and isobutylenes occurs in the unit 29;. Infany event, it is; to be understood'. that the `(Iroleiins of the charge are largely consumed in thev polymerization step to. form higher boiling normally liquid polymers.

The resulting polymerization products are passed by line 30 to a depropanizer 3|- where Cs and lighter are removed overhead by line. 32 and the depropanized polymer gasolineY is withdrawn as bottoms by line 33.. The overhead gas; passes to condenser 34 and thence byrundown line 35.v to an accumulator 36 from which condensate is withdrawn by liney 3l and returned by pump 38 through line 39 to an upper portion of tower 3 I- to serve as reflux therein. It is also to be understood that the depropanizer 3l may be equipped with a suitable heating coilor reboiler in the conventional manner.

The depropanized polymer gasoline passes by line 33 into a debutanizer 40 also equipped with a conventional heating coil or reboiler at thebase of the. tower (not shown), and from which a saturated C4 stream consisting essentially of normal butane and isobutane is removed as overhead by line: 4I and a stabilized polymer gasoline is withdrawn as bottoms by line 42 for passage to motor gasoline storage or to a hydrogenator (not shown) in the case. of selective polymerization in the production of aviation gasoline as is Well understood. The saturated C4 gases pass by line 4I to condenser 43 and rundown line 44 to an accumulator 45. The saturated C4 condensate is Withdrawn from accumulator 45 by line 46 and forced by pump 41 in part through line 48 to the top of tower 4I! to serve as reilux therein, and in part by line 49 to. the reboiler I4 of cracked naphtha stabilizer I 2.

Thel amount of saturated C4 which is recycled through line 49 to stabilizer I2 can be varied within substantial limits. For example, a relatively small amount of recycle on the basis of the raw cracked naphthal charged will give improved operation, and this can beA varied up to the recycle` of substantially all of the saturated C4 condensate not required as reflux in debutanizer 40. Thus, a recycle rate of as little' as one part by volume of saturated C4 to a hundred parts by volume ofraw cracked naphtha charged and up to equal parts by volume or more may be employed. In the latter case, it is obvious that additional saturated C4 from an external source would be required in initiating operation and until there had been substantial buildup of the, saturated C4 in the system. Generally, about one part of saturated C4 to twenty parts by volume of raw cracked naphtha charged up lto one part of recyclev to ten parts of naphtha charged are preferred to maintain the proper balance in the system'. It will be understood that this Will vary for different reboiler temperatures and dilerent temperatures of available cooling Water and can readily be determined in actual operation by those skilled in this art for the particular conditionsencountered to provide the most enicient operating conditions. In operation, the amount of recycle may be controlled by any conventional rate of flow controller in accordancer with the rate of feed of the raw cracked naphtha. to stabilizer I2. This is illustrated diagrammatically by ratio flow controller 50 between line I I and line 49 with air connection 5I regulating a valve 52 in recycle line 49. It is to be understood that the hydrocarbon recycle can also be combined with direct. water injection into the stabilizer tower, if.desiredto ob.- tain the cumulative effecty on increase. in'tOWer.

pressurefor agiven reboiler temperature. of both '6 the. hydrocarbon injection and they water in- J'ection.

The following results were obtained in comparative runsv in commercial operation on a combined raw cracked naphtha stabilization operation and a polymerization of the stabilizer overhead condensate by a conventional phosphoric acid polymerization process. In one run, the raw cracked naphtha stabilizer was operated in conventional manner without hydrocarbon recycle and injection into the reboiler of the stabilizer. In another run, a saturated C4 fraction obtained in the stabilization of the polymerization products as in Fig. l above was recycled at the rate of 14.6 b. p. h. into the reboiler of the raw cracked naphtha stabilizer to which the raw cracked naphtha was fed at the same rate as in the previous run, namely, at about 208 b. p. h. The following were the conditions and results of the run:

Without Wth 14'6' b. p. h. -b-ut am butanc mjiton injected in o reboiler reboel.

Rates in barrels per hour:

Charge to raw cracked naphtha. 208 208 Butane injection into reboiler 0 14.6 Charge to polymerization unit 45. 55 60.98 Reflux to stabilizer tower 124. 45 131.02 Reflux ratio 2.321 2. 0:1 Stabilizer bottoms, R. V. in 1bs 10. l l0. 5 Reflux material, Dist. I. B..P. F.. -44 -35 Reflux material, Dist. 98%,011 at. p +30 +34 Pressure, lbs. per sq. in. gauge:-

Stabilizer Tower Top. 215 212 Redux accumulator drum 211 208 High pressure steam for'reboiler- 174 172 Temperature, F.:

Stabilizer tower top 136 146 Rau/'naphtha to tower 208- 208 Reboiler vapor 345 342 Reux accumulator drum.-. 88 88 Water supply to condenser 8l 82 Water discharged from condenser 90 94 Charge plusbutane recycle combined Liquid,

Vol. B. p. h Per Cent Liquid,

Vol. B. p. h. Per Cent Analysis Chg. to

Stabilizer:

Methane 0.42 0. 87 0. 39 0.87 Ethylene. 0. 73 l. 52 0. 68 1. 52 Ethauc 3. 79 7. 88 3.54 7. 88 Propylene 3. 77 7. 84 3. 52 7. S4 Propane 6. 26 13. 02 5. 85 13.02 Isobut'ane; 2. 30 4'. 78 3. 04 6. 76 Isobutylenc+ butylene-l'. 3. 53 7. 34 63 8. 09 Normal butane 6. 5l 13. 54 10. 06 22. 25 Buty1ene-2 3. 71 7. 72 4. 57 l0. 18 Pentaues+ (i8. 98 143. 49` 64. 78 144. 19 Analysis, butano recycle:

Isobutane 13. 58 l. 08

5. 15 0. 75 59. 04 S. 7l 16. S4 2. 4G Pentanes+ .7e l o. 7o Analysis feed to 2. 26 1.03 1.50 0. 91 4. 48 2. 04 2. 24 1.36 9. 54 4. 34 13. 90 8. 48 Propylene ll. 30 5. ,15 13. 32 8. 12 Propane 33. 34 15.19 28. 92 17. 63 lsobutaue; 6. 35 2. 89 (i. 78 4. 13 lsobutylene+ butylene-l 8. 71 3. 97 il. 66 5. 89 Normal butano 15. 91 7. 25 15.79 fl. 63 Butylene2 6. 7l 3. 05 (l. 49 3. 96 Pentmesnl. 40 O. 64 1. 40 0. 85 Analysis stabilized bottoms:

I s o b u t a n e -I- normal butano- 4. 33 6. 73 6. 96 11. 04 Butylenos 3; 72 5. 78 2. 68' 4. 25 Pentanes+ 91. 95 142.85 90. 3G' 143. 34

Without Vltg l' .hmm butn 1n] ect1on im. ected mt-o into rcboilei reboel.

Total C in stabilizer charge, b. p. h 33. 38 47. 28 Total saturated butane in stabilizer charge b. p. h 18. 32 29.01 Total unsaturated butylenes in stabilizer charge b. p. h 15.06 18. 27 Saturated butanes in feed to polymerization unit b. p. h 10.14 13.76 Unsaturated butylenes in feed to polymerization unit b. p. l1 7.02 9. S5 Saturated butanes to stabilizer bottoms b h 6. 73 11.04 to stabilizer bottoms b. p. h 5. 78 4. 25 Percentage of available saturated butanes Fecd to polymerization unit 55. 35 47. 43 Stabilizer bottoms 36. 74 38. 06 Percentage of available unsaturated butylencs to- Feed to polymerization unit 46. 61 53.91 Stabilizer bottoms 38. 38 23. 26 Overhead gas from stabilizer accumulator drum, b. p. h 7.14 2. 99

From the above, it will be noted that with substantially the same high pressure steam supply to the stabilizer reboiler and substantially the same reboiler vapor temperature, and with approximately the same cooling water temperature for the stabilizer overhead condenser, very material advantages were secured with the butane recycle and injection into the reboiler of the stabilizer. Thus, the percentage of available butylenes left in the stabilizer bottoms dropped from approximately 38% to 213%, while the percentage of available butanes left in the bottoms was increased from approximately 37% to 38%, and the Reid vapor pressure of the stabilized bottoms was maintained or even slightly increased from 10.1 to 10.5 pounds. Moreover, the percentage of available butylenes in the feed to the polymerization unit was increased from approximately 46.5% to 54% while the percentage of available saturated butanes decreased from approximately 55.5% to 47.5%. At the same time, there was a substantially increased condensation of the stabilizer overhead and consequently an increase in the charge to the polymerization unit, as is shown by the drop in the overhead gas from the accumulator drum from '7.14 to 2.99 barrels per hour.

It is thus seen that, by the method of .the present invention, the raw cracked naphtha was stabilized to desired volatility and Reid vapor pressure while at the same time a substantially improved separation between saturated butanes and unsaturated butylenes was secured so that a greater proportion of the saturated butanes remained in the stabilized cracked naphtha while a greater proportion of the unsaturated butylenes was removed overhead, condensed and thus recovered for the polymerization feed. These beneficial results were secured solely by recycling the available butane fraction from the polymer debutanizer to the stabilizer reboiler, and without vention has been described in connection with 8 the stabilization of raw cracked naphtha to obtain a separation between saturated butanes and unsaturated butylenes, it is to be understood that the invention is also applicable to effecting a desired separation between pentanes and amylenes, hexanes and hexylenes, etc.

Referring to Fig. 2, the invention is shown as applied to the stabilization of raw cracked naphtha and natural gasoline and the alkylation of isobutane with C4 and C5 oleiins to thereby materially increase the yields 0f aviation gasoline from available charge stocks. As shown, raw cracked naphtha from a conventional cracking unit is introduced by line 60 to stabilizer 6I equipped with reboiler 62, and operated to take overhead by line 63 a C5 and lighter fraction containing a substantial proportion of the C5 olens. A stabilized cracked naphtha containing a substantial proportion of normal pentane for volatility is withdrawn as bottoms by line 64. The overhead fraction is passed through condenser 65 to accumulator 66. Condensate is forced by pump S1 in part through line 68 to serve as reflux in tower El, and in part by line B9 to a depropanizer 10. In the latter, the charge is fractionated to remove overhead through line 1| C3 and lighter, and obtain a bottoms cut consisting essentially of C4 and C5 which is passed by pump 12 through line 13 to a conventional alkylation unit 14.

While any conventional alkylation process can be employed, the invention is described for purposes of illustration in connection with a sulfuric acid alkylation step. In this process, an additional supply of isobutane introduced by the recycle line 'i5 to be hereinafter further described, or from any other suitable source, is mixed with the olefinic Cil-C5 charge from line 13 in the presence of strong sulfuric acid of about 88-100% concentration at temperatures of about 35-'70o F. and under sufficient pressure to maintain the hydrocarbon in liquid phase. Any conventional type of unit, such as the well known pump and time tank reactor, the jet reactor, or the mixer of the turbo type can be used. The isobutane is maintained in substantial molar excess of the olefins;

and preferably the operation is of the well known emulsion recycle type in which contact ratios of from 50:1 up to 200:1 or higher are employed. Under these conditions, the isobutane is alkylated by the C4 and C5 oleflns to produce good yields of gasoline hydrocarbons or alkylate of high anti-knock value. It is to be understood that other well known alkylation catalysts, such as hydrofiuoric acid, aluminum chloride-hydrocarbon complex, BFS-water complex, and the like can be used; although strong sulfuric acid is preferred for this operation. As the alkylation unit per se forms no part of the present claimed invention, further description thereof is unnecessary. It is understood that in conventional practice, a stream of the reaction products is withdrawn to a suitable settler where the catalyst is separated from the hydrocarbons, and the latter are then neutralized before being passed to the stabilizing and fractionating equipment. These various steps and the equipment therefor are well known, and it is to be understood that the diagrammatic illustration of the alkylation unit represented by the numeral 14 includes the alkylation reactor, settler and neutralizer.

The neutralized hydrocarbon reaction products from the alkylation unit are passed by line 11 to a debutanizer 78 where unreacted C4 hydrocarbons consisting essentially of excess isobutane and normal butane are removed overhead by line 19 and a debutanized alkyl'ate is withdrawn as bottoms by line 80. As the alkylation reaction consumes the olenns in the charge, the overhead C4 stream is further 'fractionated in a butane fractionator 89 where Aan isobutane out is removed overhead and recycled by line 15 to the alkylaton unitV While a normal butane stream is removed as bottoms by line 8| Vand passed in whole or part by pump 82 through line 83 to the stabilized cracked naphtha in line 64 to increase the yield and volatility of the motor gasoline.

The debutanized alkylate withdrawn by line 84 is `passed by pump 85 through a line 8S to a depentanizer 81 where C5 is removed overhead by line 88 and depentanized alkylate is removed as bottoms by line 89. It is to be understood that the above-described order of towers 'can be changed. For example, the raw alkylate can rst be depentanized, and the resulting C5 and lighter offgases fractionated in la series of towers to obtain the various fractions described, including an isobutane cut, a normal butane cut, an isopentane out and a normal p'entane cut. The C5 overhead fraction consisting essentially of isopentane and normal pentane is mixed with a stream of debutanized natural gasoline introduced by line 99, and the mixture passed vinto a de-isopentanizer 9|. The latter is operated to remove isopentane overhead by line 92, and the de-isopentanized natural gasoline containing the normal pentane from the alkylate is removed as bottoms by line 93. The latter is passed by pump 94 to a depentanizer 95 where normal pentane is removed roverhead by line 96 and the depentanized natural gasoline discharged as bottoms by line 91. It is to be understood that the fractionating towers 10, 18, 89, 81 and 9| are equipped with the usual reboilers or other heating means for the base of the towers, and with suitable condensers and accumulators for the overhead and pumps for supplying condensate as reflux to the tops of the towers as well as forcing the remaining portion of the condensate to the next tower or unit in series, which elements are not shown for simplicity in illustration.

The overhead normal pentane fraction passes by line 96 to condenser 99 and thence by rundown line |99 to accumulator lill from which condensate is withdrawn through Yline |92 by pump |93 and forced in part through reflux line |04 to the top of tower 95, and in part by recycle line |05 to the reboiler 92 of the cracked naphtha stabilizer `[il In this manner, a saturated C5 stream consisting largely of normal pentane is provided for injection into stabilizer 6| to thereby obtain a more complete separation of C5 clef-ins in the overhead, while retaining a desired .proportion of normal pentane in the stabilized cracked naphtha bottoms. While the above described introduction of debutanized natural gasoline into the fractionating system is advantageous for the production of more isopentane, it is to be understood that this is not essential in accomplishing the broader objects of the present invention. For example, the C5 cut from the stabilization of the alkylate may be directly fractionated into isopentane and normal pentane fractions, without addition of any natural gasoline cut. l

The depentanized alkylate removed from tower 81 by line 89 is forced by pump 01 through line |08 to the alkylate fractionator |09 where a desired aviation gasoline fraction boiling up to about S50-375 F. is removed overhead by line Il!) and alkylate bottoms are discharged by line and forced by pump H2 through line l3 to storage or for further treatment. Preferably, the alkylate bottoms are passed by line ||3 to suitable blending tanks (not shown) where they are mixed with the stabilized cracked naphtha from line 64 to increase the yield of motor gasoline. The overhead aviation fraction is condensed in condenser l l 5 and passed through rundown line ||6 to accumulator ||1 from which the aviation gasoline is discharged by line |8 to storage or for further treatment. The overhead isopentane fraction from line 92 is passed to suitable condenser and accumulator equipment (not shown) and the condensate blended with the aviation gasoline from line I8 to thereby increase the yield and adjust the volatility of the aviation gasoline. It is to be understood that the depentanized natural gasoline from line 91 may, if desired, be passed to a further fractionating tower (not shown) serving as a, de-isohexanizer from which an overhead isohexane out may be obtained for blending with the aviation gasoline from line l|8 to further increase the yield thereof. The resulting natural gasoline bottoms from this further fractionating operation may then be passed to reforming or other treatment, or may be blended directly with the stabilized cracked naphtha from line 94 for motor gasoline.

It is to be understood that the debutanized natural gasoline introduced byline has been previously stabilized to separate C4s and lighter, and the offgases from this stabilization fractionated to `separate an isobutane-rich fraction which is introduced by line |29 into the isobutane feed for the alkylation unit 14 to thereby serve as an external source of Ysupply of additional isobutane. A normal butane also recovered from the natural gasoline in this latter operation may be added to the normal butane stream passed by line 83 to the stabilized cracked naphtha in line B4. These elements are not illustrated in the drawing for the sake of simplicity in illustration.

While the invention has been described above in combination with a suitable polymerization or alkylation unit, from which the desired recycle stream is obtained for injection into the cracked naphtha stabilizer, it is to be understood that the invention is not limited to these particular catalytic conversion units. Thus, it is obvious that the stabilization method described herein can be applied in conjunction with other catalytic or thermal conversion units, such as isomerization, cyclization and aromatization, and various combinations of these with polymerization and alkylation. Likewise, the invention is not limited to the stabilization of cracked naphtha and natural or straightrun gasoline fractions, since the particular stabilization and fractionation method is more broadly applicable to the distillation of olefin and paraffin containing hydrocarbon mixtures generally.

In addition, the stabilizing and fractionating method of the present invention can be used to eifect more complete separation between an overhead isoparafiin and lighter out from a bottoms cut containing olefins of the same molecular Weight as the overhead isoparamn. In this case, the recycle stream or hydrocarbon stream injected into the stabilizer will be one which is enriched in the said olens to be retained in the bottoms and which is relatively lean in the isoparaffin to be removed overhead.

The invention can also be employed in a stabilizing operation where the overhead condensate capacity is insufficient for normal operation, to thereby enable the stabilizer to operate at a higher pressure for the same reboiler temperature, with resultant increased condensation of the overhead at the higher pressure with the same condenser cooling surface. Likewise, the invention can lbe applied to stabilizing operations where difficulty is encountered due to the temperature of available cooling water for the overhead condensate being too high, since the higher stabilizer pressures resulting from the hydrocarbon injection permit increased overhead condensation with the higher temperature cooling water.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be made as are indicated in the appended claim.

I claim:

Apparatus of the character described, comprising in combination, a fractionating tower of the multi-plate type, a liquid feed line opening into an intermediate portion of said tower, a vapor offtake from the upper portion of said tower, a condenser and accumulator connected to said vapor offtake, a liquid discharge line from the lower portion of said accumulator, a pump connected to said liquid discharge line and in turn connected to a return line opening into the upper portion of said fractionating tower above said liquid feed line to supply reux to the tower, a rst liquid discharge line from the base of said fractionating tower, a second liquid line discharging from a lower portion of said tower above said first liquid discharge line, a vessel equipped with an indirect heating coil connected to said second liquid line to receive liquid discharged thereby and to heat said liquid to a reboiling temperature, a return line from an upper portion of said vessel and opening into a lower portion of said tower above said first liquid discharge line to return heated liquid and vapor from said vessel to said tower, a liquid feed line for extraneous liquid opening into the lower portion of said vessel, a flow controller for said extraneous liquid feed line, and operative connections from said ow controller to said liquid feed line opening into said tower and to said extraneous liquid feed line opening into said vessel to thereby regulate the rate of feed of said extraneous liquid to said vessel in accordance with the rate of feed through said liquid feed line to the tower.

LOREN P. SCOVILLE.

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

UNITED STATES PATENTS Number Name Date 2,006,186 Stines June 25, 1935 2,072,093 Blakey Mar. 2, 1937 2,168,316 Brandt Aug. 8, 1939 2,172,560 Kemp Sept. 12, 1939 2,221,425 Ruthruff lNov. 12, 1940 2,286,504 Parker June 16, 1942 2,307,024 Carney Jan. 5, 1943 2,348,931 Schulze May 16, 1944 FOREIGN PATENTS Number Country Date 492,567 Great Britain Sept. 22, 1938

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