US2367284A - Processing pressure distillate - Google Patents

Description

Jan. 16, 1945. w. KAPLAN I PROCESSING PRESSUE DISTILLATE F-iled May 7, 1942 Patenied Jari. 16, 1945 UNITED STATES ,M PATENTl OFFICE e 2,367,284 I PROCESSmG PRESSURE DIS'lIl'lLA'lEk William Kaplan, Texas City, Tex., assignor to Cities Service Oil Company, New York, N. Y., a

vcorporation of Pennsylvania l Application May 7, 1942, serial No. 442,039

6 Claims.

This invention relates to a process for treating distillates containing low boiling constituents for the elimination ,of such constituents, and more particularly to a process for stabilizing pressure distillates.

In the conversion of petroleum oils and hydrocarbons in general, for the manufacture of motor fuels. the desired motor fuel constituents are 4usually recoveredvat relatively high pressure by con densing them in contact with lower boiling con stituents such as methane, ethal'le. propane and propylene which tend to remain absorbed to a considerable extent and render thegmotor fuel unstable. In view of; this more or less general condition, it is the usual practice to subject the pressure distillate Obtained in almostevery con version operation to a fractional distillation procedure generallyil referred to as stabilization. This `procedure involves the rectification of the pressure distillate under conditions adapted to eliminate all of the undesired normally-gaseous Constituents while retaining suflicient of the C4 hydrocarbons to give the motor fuel product the desired vapor pressure and distillation characteristics. Y y

Conventional methods of stabilization require the use. of high pressures because of the V.presence of relatively large proportions of low boiling con# ysttuents which otherwise could not be condensed to produce relluxwith ordinary cooling water temperatures. Thev high pressure necessarily employed increases the temperaturek required at the bottom of the stabilizer and decreases the relative volatility between the C3 and C4 hydro-v carbons, thus making it more difcult' than otherwise to. separate these constituents.- These conditions furthermore require a high reflux ratio in order to eiect a given separation,

The primary object of the' present invention is to provide an improvement in the handlingl of hydrocarbon mixtures to facilitate the elimination, of undesirable low boiling constituents.

A further object ofY the invention is to provide an improved process for the treatment of pressure distillates prior tck their stabilization, to

eliminate substantial proportion of the low boiling constituents which tend to make Athe stabilization difficult. v

A still further object ofthe invention is to provide an improved process by which the capacity of present stabilizing equipment maybe greatly increased.

Accordingly, the improved process of' the present; invention comprises the steps of passing an unstable distillate such as a pressure distillate,

initially at a relatively high pressure, through-a series of Vaporizing zones maintained at progressivelylower pressure, in each of Vwhich portions of the distillate are vaporized, passing the individualy vapor fractions into separate iractionators or absorbers in which the vapors are brought into contact with an absorption oil to prevent loss of desired hydrocarbons, discharging unabsorbed gaseous constituents from the absorbers, collecting the unvaporized constituents from the vaporizing zones and the rich ab'- sorption medium from the absorbers, and subjecting the resulting mixture to stabilization inl accordance with the usual procedure.

The improved process of the present invention preferably includes other features and objects which will be apparent to those skilled in the art from the following more detailed description of tion in connection with the treatment of pressure distillate obtained in an oil conversion operation.

Referring to the drawing, the charging stock which may be taken directly from the receiving drum of a conversion unit, is conducted at a pressure usually in excess of 200 lbs. per square inch, which may be the pressure employed in the receiving drum of the conversion unit, through a'valved charging line 2, into the iirst of a series of flash distilling chambers 4, 6, 8 and Ill. These chambers may be empty or provided with trays, and may be mounted separately or in the form of ak column, as shown. If the conversion unit receiving drum pressure is at 250 lbs., a pressure of approximately 180 lbs. per square inch is maintained in the flash distilling chamber 4 and the reduction of pressure by the valve in line 2 causes the vaporization of an equilibrium mixture of light hydrocarbons. The unvaporized distillate in the chamber 4 is conducted therefrom through a float valve controlled line I2 into the chamber Ki5' in which a pressure of approximately 125 lbs.

trolled line I4, into the iiash chamber 8 where a pressure of approximately 75 lbs. per square inch is maintained and additional distillation effected. Finally the distillate remaining unvaporized in chamber 8 is conducted through a float valve controlled line I6 into the flash distilling chamber i where light hydrocarbons are also vaporized because of the pressure reduction. The unpressure is reduced, particularly if a substantial portion of the total lean absorption oil used is introduced into the upper portion of the low pressure absorbers, as hereinafter described.

The vapor fractions produced in the chambers 4, 6, 8 and i8 which reach the top of these chambers, are conducted respectively through vapor lines 22, 24, 26 and 28, into the lower portions of a series of absorbers 30, 32, 35 and 36, in which the individual Vapor fractions are brought in contact withan absorption medium, such for example as a heavy gasoline, under conditions adapted to absorb all C and heavier hydrocarbons and as much of the C4 as desired. The

absorbers 30, 32, 34 and 36 are preferably provided with the usual bubble cap contact trays, and the absorbers may be mounted separately or in a tower, as shown. The pressures maintained in the absorbers approximately correspond to those maintained in the corresponding ash distilling chambers. The rich absorption oil reaching the bottom of the absorbers 30, 32 and 34 is preferably introduced and ashed into the mid-portion of the next lower absorber, as for example through oat valve controlled lines 38, 4D and 42 respectively, the rich absorption oil collected 'in the bottom of the absorber 36 being conducted through a float valve controlled line 44 into the surge tank 28. A small amount of absorption medium is introduced at the top of each absorber and the unabsorbed gas reaching the upper portion of each of the absorbers is discharged through a line 46 provided with a control valve set to maintain the desired pressure in the respective flash chamber and absorber.

The distillate mixture collected in the surge tank 26 and which may be substantially free of light hydrocarbons such as C1, C2 and C3, but which may contain some Cs, is forced through a line 48 by means of a pump therein, heated by means of a heat exchanger in the line, and then conducted into the mid-portion of a stabilizer tower 50, in which the distillate mixture is subjected to rectification for the elimination of constituents which would render the pressure distillate unstable. The excess of C4 and lighter hydrocarbons are conducted overhead from the stabilizer 53 through a vapor line 52, subjected to condensing conditions in a condenser therein, and the resulting condensate and gas mixture discharged into a receiving drum 54. sure maintained in the stabilizer Eril may vary from 150 lbs. to 200 lbs. per square inch, but may be considerably lower than that normally used for the stabilization of pressure distillate because The presof the relatively low content of C3 and lower hydrocarbons in the distillate mixture handled in the stabilizer 50. In any case, the desired pressure is maintained in the stabilizer 58 by the discharge of gas from the receiver 54 through a pressure valve controlled line 56. The distillate collected in the receiver 54, which may be largely C3 and C4 hydrocarbons, is discharged through a float valve controlled line 58, butI a regulated portion of this is preferably used as reflux in the stabilizer 5l) and introduced thereinto through a valve controlled line 68.

The gasoline reaching the bottom of the stabilizer 55 is conducted through a line 62 into a reboiler 64. Excess C4 and lighter components are stripped out of the gasoline in the lower trays of stabilizer 58 by means of reboiler 64 which is used for maintaining the desired bottoms temperature in the stabilizer. The heating medium (not shown) for reboiler 64 may be steam or hot oil.

When it is desired to utilize the heavy ends of the gasoline as absorption media, the stabilized gasoline from reboiler 64 is passed through a line 66 into fractio-nator 68 where a light gasoline fraction is distilled overhead and the desired heavy gasoline absorption medium is removed from the bottom. This fractionator is operated at 5 to 20 pounds gauge pressure, using steam for stripping. The heavy gasoline from the bottom of fractionator 68 may be passed through a line 'I0 to the heat exchanger in the line 48 to heat the distillate mixture therein, and then through a line 12, which includes a cooler and a pump for distributing the heavy gasoline to the absorbers, the excess heavy gasoline being passed through a float valve controlled line 'i4 to storage or to be mixed with the light gasoline. The absorption medium in the line 'l2 is supplied to the absorbers 30, 32, 34 and 36 respectively, from valve controlled lines 16, 18, 80 and 82, each of which discharges into the upper portion ofthe respective absorbers. The absorption medium may be advantageously cooled in the cooler in line 72 prior to being introduced into the absorbers.

Instead of using the heavy gasoline from the fractionator 68 as absorption media in the absorbers, other suitable absorption oil may be utiilzed, and Where such other oil includes constituents of higher boiling point than gasoline, the entire gasoline production of the process may be distilled overhead in the fractionator 68, while the absorption medium is recovered in the lower portion of the fractionating tower and recycled to the absorbers. Absorption media from an outside source may be introduced into the line l2 through a valved line 84. The high boiling absorption oil may be recovered in a different manner by providing a still in the line 48 to separate the pressure distillate as a vapor which would then be stabilized in the tower 50, as described, and the entire stabilized product withdrawn to storage from the line 66.

While, in describing the invention in connec- 4 tion with the foregoing example, certain speciic pressures have been mentioned for the vaporizing chambers and the corresponding absorbers, it is to be understood that the process may be carried out at greatly diierent pressures. For example, the distillate recovered in a recycling operation (including any absorption oil) may have an initial pressure of from 700 to 2000 lbs. per square inch. The pressure reduction in the series of fin theavaporizing chambers xmay fvaryv toasornel extent 'but -will usually rnot ibe "higher than apuproximatelyl100 "to il1-50-1.in the-'chamberllfand 4fconsiderab'lyflower lin vthe chamber 10. "-:The'temaccurately-controlled byrireg'ulatingthe temperaitureof *the` absorption medium f use'd''therem.

-The-jquantity=of absorptionmediumiintroduced finto the idiierent 'absorbers may vary 'considern stably; the egreatest-amountpreferablybeingiintroducediinto fthe" first absorber'. *It 'rwill be-`-note`dk y"that -this absorption medium, '"vtogether" withiits" absorbed vconstituents, `--isilafshed into thef-lo'we'r pressures-absorber32eandibroughti in contact-with :'vaporsf'from the lirici-24. This yprocedure;carried Lout inAl th'ef'series YAof absorbers; promotes theis'elecvtive l`elimination/of ylight gases'suchfas ethane and methane. `TheiCs and Ci hydrocarbons, forexample, *conductedinto the absorber 32 from the I linev 24, will tend to replace C1 and C2 hydrocarbons` which may ber present in the absorption medium introduced through the line 38 or present in the absorption oil in the upper part of the absorber 32. With certaintypes of hydro-- carbon mixtures, it may be found that only relatively small proportions of absorption oil will need to be introduced through the lines 18, 48l) and82.` .f

The advantages resulting .from removing a large proportion of the C1, C2 and C3 from the pressure distillate prior to its stabilization, is apparent from the fact .that -a lower reux rate will be requiredin the stabilizer. The proportion of constituents taken overhead will obviously be reduced, and it is therefore possible to considerably increase the capacity of given equipment by the -use of the improved process of the present invention. Because of the smaller proportion of.'

the light constituents referred to,A the stabilizer can be operated at considerably lower pressures csubstantialsproportions of low boiling-undosirablel Hconstituents, which comprises subjectingl the disttillate'to 'flash distillation in ka series of zones maintained 'at progressively Alower pressures, sep- 'larating a 'vapor'fractionffrom the-'distillate-in *each'f-zone Vand passing f such fractions intoseparate absorption "zonesin "contact with lan absorp- `tion medium adapted to'recover desirable `convvstituen'tsf'fromY the vapors;` said absorption zones 1being separate from said 4"flash-zones and `being maintained'at progressively I-lower Apressures inv *the* direction lfof 'theprogressively lower pressures of 'saidf'iias'h zones, passing r-ich-'Yabsorption'umeyfdium vfrom each absorptionrzoneexcept the last into i the-absorption cone of' next-lower pressure,

collecting the rich absorption media from the absorption zones together with the unvaporized portion yof the distillate from said ash distilling zones, and rectifying the resulting mixture at a high pressure to produce a distillate substantially free of undesired low boiling constituents.

3. In a process for stabilizing distillates produced at 'relatively high pressures and which conandtemperatures than normal, and a lower vapor load will be impose'dupon the reboiler andlower "50 section of the stabilizer in order to supply the n heat for carrying vout the fractionation. The use of a lower bottom temperature decreases the liquid expansion and reduces the volume of liquid flowing down through the trays. This effect may be appreciable when the operating bottom temperature is reduced from above 310 to below- 290 F. which is the temperature range in which the expansion of the butanes is greatest.l

From the foregoing description of the invention, it will be apparent to those skilledr in the art that various modifications may-be made in the procedure, without departing from the spirit of the invention.

Having described the invention in its preferred form, what is claimed as new is; y

1.'In the stabilization of unstable distillates, the improvement which comprises subjecting the distillate to successive vaporization steps in a series of zones of progressively decreasing pressure to produce a vapor fraction in each zone, passing the vapor fraction from each zone to al separate absorption zone of approximately the pressure of the distilling zone in which the fraction was produced, introducing a lean absorption tain undesirable relatively low boiling point constituents and at the same time avoiding substantial loss of desirable constituents 'from such distillates, the improvement which comprises passing theunstable distillate at. a relatively high lpressure into and through a series of ash distilling zones of progressively lower pressure in each of which a'vapor fraction is produced containing desirable and undesirable constituents,

passing each of the said fractions into a separate absorber of a series of absorbers in contact with an absorption medium to recover the desirable constituents `from each vapor fraction, the absorbers being separate from the distilling zones, introducing a lean absorption oil into the' upper portion of each absorber, and passing rich absorption medium from the absorberreceiving vapor from the distilling zone of highest pressure through the other absorbers in the series in con'- tact with the vapors therein.

4. The process of refiningk pressure distillates produced at relatively high pressures and which contain substantial proportions of low boiling constituents tending to make such distillates unstable, which comprises passing the pressure distillate into the first of aseries of ash distilling Zones maintained at progressively lower pressures, passing unvaporized distillate from each zone into the next succeeding lower pressure zone, producing a vapor fraction from the distillate in each zone and conducting such fractions into separate `absorption zones in contact with an absorption medium therein, the absorption vzones being separate from the distilling zones, maintaining the absorption zone for each vapor fraction at approximately the pressure maintained in, I

the distilling zone in which the Vapor fraction wasproduced, passing rich absorption medium from each absorption zone except the last into the absorption zone of immediately lower pressure and contacting it with the vapor fraction introduced thereinto, thereafter contacting each vapor fraction With lean absorption oil, and carrying out the absorption operations in-said absorption zones so as to recover desirable constituents from said vapor fractions.

5. In a process for stabilizing distillates produced at relatively high pressures and which contain undesirable relatively low boiling point constituents and at the same time avoiding substantial loss of desirable constituents from such distillates, the improvement which comprises passing the unstable distillate at a relatively high pressure into and through a series of flash distilling zones of progressively lower pressure in each of which a vapor fraction is produced containing desirable and undesirable constituents, passing each of the said fractions into a separate absorber of a series of absorbers in contact with an absorption medium to recover the desirable constituents from' each vapor fraction, the absorbers being separate from the distilling zones, passing rich absorption medium from the absorber receiving vapor from the distilling zone of highest pressure through the other absorbers inthe series in contact with the vapors therein, mixing the rich absorption ,medium from the last absorber of the series with the unvaporized portion of the distillate from the last ash-distilling Zone, and subjecting the mixture to rectification in a tower to produce a high boiling bottoms fraction, and utilizing said high boilingl bottoms fraction as absorption media in said absorbers.

6. In the stabilization of unstable distillates, the improvement which comprises subjecting the distillate to successive vaporization steps in a series of Zones of progressively decreasing pressure to produce a vapor fraction in each zone, passing the vapor fraction from each Zone to a separate absorption Zone of approximately the pressure of the distilling Zone in which the fraction was produced, producing a rich absorption oil containing the recovered desirable constituents in each absorption zone, passing the rich absorption oil from each absorption zone except the last into the next lower pressure absorption zone, and passing the vapor fraction introduced thereinto first in contact with said rich absorp* tion oil and then in Contact with a lean absorpe tion oil, and withdrawing rich absorption oil containing the desirable constituents recovered in the absorption zones from the absorption zone of lowest pressure.

WILLIAM KAPLAN.

Images