US2733286A

US2733286A - Purification of a benzene fraction by mild liquid

Info

Publication number: US2733286A
Authority: US
Publication date: 1956-01-31
Anticipated expiration: 1973-01-31

Description

Jan. 31, 1956 D. s. MAlsEL ETAL I 2,733,286

PURIFICATION OF A BENZENE FRACTION BY MILD LIQUID PHASE CLAY TREATMENT AND DISTILLATION To REMOVE RESULTANT PDLYMERS Filed March 19, 1952 @ave-Labors QENSS W M Gbborn United States Patent PURIFICATION OF VA BENZENE FRACTION BY IVIILD LIQUID PHASE CLAY TREATMENT AND DISTILLATION T REMOVE RESULTANT POLY- MERS Daniel S. Maisel, Union, and -Brook I. Smith, Elizabeth,

N. J., assignors to Esso Research and Engineering Company, a corporation of Delaware Application March 19, 1952, Serial No. 277,485

4 Claims. (Cl. 260-674) This invention relates to a `process for preconditioning a cracked petroleum naphtha fraction containing mainly Cri-C7 unsaturated hydrocarbons with benzene for purification of the benzene by extractive distillation and recovery of separated unsaturated hydrocarbon products.

More particularly, the present invention is concerned with steps of separating the proper benzene cut from the described naphtha fraction for subsequent treatment in liquid phase with clay that polymerizes some of the diolelin and highly reactive olefin components of the cut, and removal of the polymers thus formed prior to extractive distillation of the remaining benzene cut. These steps of securing the proper cut for carrying ont the clay treatment and of removing the resulting polymers are interdependent and highly important in preconditioning the feed stock for eiiicient purification of the benzene, simplification of the clay treatment, and best recovery of the non-aromatic components.

With increasing need for benzene, research has been directed to methods of separating benzene from highly cracked petroleum products. Some processes have dealt with recovery of benzene from either virgin naphthas or naphthas containing relatively small amounts of benzene and unsaturated hydrocarbons. Such processes more simply indicated that the unsaturated hydrocarbons, and especially the dienes or diolefins, should be completely polymerized for removal from the fraction containing the aromatic hydrocarbon that is to be extractively distilled to mainly separate paraflins. They contemplated using rigorous chemical treatments for removing substantially all of the dienes, e. g., by treatment with concentrated sulfuric acid, or the like, without regard to recovery of the dienes as valuable monomer hydrocarbon products and valuable polymerized resin products. They also ignored effects on the treating agents and on the components treated, including the aromatic components, since they were concerned with eliminating relatively small proportions of dioleiins completely to prevent any dioleins from being present in the extractive distillation zone wherein it was believed the diolefns caused undesired sludge formation with resultant loss of valuable extraction solvent.

Now, it is found that an important source of benzene is the highly aromatic and highly unsaturated hydrocarbon naphtha obtained by vapor phase thermal cracking of heavy virgin naphtha, kerosene, gas oil, and the like, covering petroleum fractions boiling from 200 F. through 900 F. and higher, at elevated temperatures of 1050 to 1400 F. under low pressures of l5 to 45 p. s. i. g. in a few seconds. In these high temperature-low pressure cracking processes, the vaporized cracking stock is preferably mixed with 50 to 90 volume percent of steam to avoid excessive decomposition,

The typical high-temperature cracked Cs-Cr naphtha fractions boiling in the range of 63 100 C. (145 212 F.) contain principally Ce-C'I dienes, cyclodi- 2,733,286 Patented Jan. 31, 1956 enes, and `alkenes with the benzene but very little paraflins or naphthenes.

Although th'efpri'or art indicated that an aromatic hydrocarbon fraction had to be entirely freed of diolens to be suitable for extractive distillation to avoid sludgeforming reaction of the extracting solvent with the dienes, it can be seen that such a drastic treatment would be wasteful of the dienesv and too expensive for a benzene-containing fraction which is rich in unsaturated hydrocarbons; therefore, there arose a need for determining how this ltype of fraction should be most practically pre'conditioned Vfor extractive distillation with any of the various known extraction solvents, such as phenol, cresylic acids, furfural, or triethylene-glycol.

The following factors have now been discovered to be important in processing the highly unsaturated type naphtha for the purposes of the present invention:

l. Preliminary fractionation of the steam-cracked naphtha to control its lowest, or initial, boiling point, as well as its highest, or end, boiling point, to keep a maximum of benzene in the resulting intermediate cut that is next treated in the liquid phase with clay but to eliminate from this cut the kinds or" dienes which do not form the proper polymers in limited amount on treatment with a clay catalyst.

2. Simplilication of the clay treatment of the benzene cut in liquid phase so that the treated benzene-rich cut can carry away the polymers formed to prevent fouling of the clay. This treatment is relatively mild and is easily controlled.

3. Fractional distillation rerunning of the clay-treated benzene cut carrying the polymers to separate the benzene cut without re'contamination by breakdown of the polymers so that the thus obtained benzene cut contains only such olefins and dioleins as do not adversely effect subsequent extractive distillation for recovery of the purified benzene.

It will be appreciated from the further description that the lirst fractional distillation step to obtain the benzene cut has an important effect on the eiiiciencies of the subsequent steps by eliminating dienes that even under mild conditions polymerize into either highly viscous or semisolid polymers which tend to foul the clay and tend to form unstable polymers which would decompose in the rerunning of the clay-treated benzene cut if extracted from the clay. The semi-solid polymers are diflicult to extract from the clay and necessitate frequent regeneration of the clay by burning olf the adhering polymers.

By using the method of the present invention the polymers readily recovered from the treated benzene solution have valuable'properties for use as core oil, ink base, or plastic extenders.

The preferred method of operation will first be outlined with reference to the flow plan in the drawing to explain the sequence of steps. The control requirements of these steps will then be explained in detail.

In the drawing, a debutanized cracked nap'ntha fraction boiling in the range from 50 to 216 C. (122- 420 F.) or higher is fed by line Il into a first fractionating column 2. This fractionating column 2 is equipped with a bottom drawoi iine 3, a reboiler 4, an overhead drawofr" line 5, condenser o', a redux line 7, and a light end drawoif line 8. Column 2 is operated so that the light ends distilled overhead through line 5 have an end point in the range of l55l65 F. so that the components of the side stream cut 'withdrawn by line 9 boil in the range of 165 to 185 F. The residual portion of the naphthapfraction, or heavy ends boiling above F., are withdrawn through line 3,

except for the portion recirculated through the reboiler 4 to the bottom of column 2 for supplying heat.

The intermediate side stream cut boiling in the range of 165-185 F. is passed through line 9 to one end of the clay treating unit 10. It is to be noted that the initial fractionation makes it possible for only a small fraction of the total naphtha to be clay treated. In addition, part of the diolefin content has been eliminated.

The liquid cut is passed preferably in upow manner through clay in the vessel for avoiding channeling, and to minimize packing and excessive pressure drop. However, a downow treatment of the liquid cut may be used alternatively. In the clay-treating unit the liquid benzene cut is made to contact the clay at temperatures of 250-400 F. at a rate of 1/z to 10 volumes of the liquid per volume of the clay per hour, while under a pressure of 100 to 200 p. s. i. g. to maintain substantially all the hydrocarbons in the liquid phase. Various types of clay may be used in vessel 10 for the polymerizing treatment, e. g., fullers earth, activated bentonite, and commercially available clays known by the names Attapulgus, Floridin, and Supertiltrol, or similar catalysts of mild polymerizing action.

Fortunately, it was found that the polymerizing activity of the dienes decreased with increasing molecular weight or increased boiling point in the liquid benzene cut so that only part of the dienes and particularly the lower boiling dienes were essentially polymerized without using severe clay treatment. With a thus limited amount of polymerization, the benzene and unreacted unsaturated hydrocarbons were found able to carry away the polymer from the clay as the polymer was formed, and were able to prevent formation of zones of high temperatures produced from the exothermic heat of reaction, thus permitting the clay bed to be used for a long period of about 40 days and longer in plant operations. The concentration of the benzene in the cut, being in the amount of about 35 to 50 volume percent is also an important factor in extracting the polymers from the clay, and thus keeping the clay from becoming fouled by deposited polymers.

The resulting clay-treated benzene cut carries the polymers from the clay treating vessel through line 11 into an intermediate part of rerun column 12. In column 12, the benzene cut boiling in the range of 165 to 185 F. is distilled overhead for withdrawal by line 13, leaving the polymers as residue which is withdrawn from the bottom of column 12 by line 14. A reboiler 15 at the bottom of the rerun column 12 may be used to supply heat for the distillation. In the reboiling bottoms part of the column 12 it is desirable to prevent the temperature from rising to above 450 F. in order to minimize decomposition of any polymer present that is most unstable, because decomposition of the polymer would cause the resulting diene monomers that are reactive to pass overhead with the benzene distillate which is being preconditioned for the next step of extractive distillation. n addition the period of residence at this high temperature must be made as small as practical, e. g. less than l() minutes. A portion of the benzene overhead may be condensed in cooling condenser 16 to supply reflux which is returned by line 17 to the top of column 1.2. A remaining portion of the benzene overhead concentrate is passed via line 18 into an intermediate part of the extractive distillation column 19, wherein the conventional extractive distillation operation is carried out for distilling all but a small amount of unsaturated hydrocarbons overhead to leave the benzene with a small amount of unsaturated hydrocarbons in the solvent extract. The solvent for the extraction is introduced at an upper part of the column 19 by line 20 to flow countercurrently to the vapors in column 19. The distilled vapor, or unsaturated hydrocarbon raffinate, is taken overhead through line 21, the final extract is withdrawn as bottoms through line 24 to a recovery unit. The extractive distillation need only be briefly discussed since it involves conventional operations known in the art. In the extractive distillation, employing phenol as the solvent, about 2 volumes of phenol are introduced through line 20 per volume of hydrocarbon feed entering from line 18. A portion of the overhead is condensed in cooling condenser 22 to be refluxed by line 23 to the top of the column 19 in a reflex ratio of, for example, 4.5 volumes of ratlnate per volume of hydrocarbon feed using a column having about 40 or more plates. Over 90 weight percent of the benzene is then extracted by the phenol, and is then recovered in being distilled from the phenol, and this benzene is only slightly contaminated with unsaturated hydrocarbons which can be removed by sulfuric acid treatment that gives a very small loss in hydrocarbons which are removed in the spent sulfuric acid. The extracting solvent is kept satisfactorily low in sludge.

One of the controlling factors, as already mentioned, depends on the composition of the initial naphtha feed which can be characterized as being principally unsaturated, in containing from 25 to 35 volume percent benzene, 10 to 25 volume percent dienes, and 50 to 70 volume percent alkenes boiling within the range of 50 to 216 C. (122 to 420 F.). It is advantageous to have a high benzene content in the initial feed and to retain as much of this benzene as possible in the cut that is sent through the clay treatment to the rerun fractionation prior to the extractive distillation, not only for the purpose of recovering a maximum amount of purified benzene at the end, but also for preventing fouling of the clay, as already explained. However, in the initial fractional distillation for obtaining the proper benzene cut to be clay treated, it was found that in order to eliminate such dienes as tend to give difficulty when polymerized, on account of their reactivities and tendencies to form detrimental polymers, some small loss of the benzene may have to be incurred in keeping the benzene cut substantially free of diolefin components which have boiling points below F. Analytical investigations have shown that by controlling the initial boiling point of the side stream benzene cut in this manner the kind of dienes which can be rejected overhead are the methyl cyclopentadienes and conjugated hexadienes which are troublesome if present in the benzene cut subjected to the clay treatment. Of course, the upper end boilingpoint of the benzene cut is necessarily limited to avoid having high boiling components extracted with the benzene by the solvent in the extractive distillation. For this reason, a suitable upper end boiling point for the benzene cut is about 180 to 185 F. These temperatures are substantially the true boiling point temperatures.

The intermediate benzene cut subjected to the mild polymerization treatment by contact in liquid phase with clay typically should contain 35 to 50 vol. percent benzene, l0 to l5 Vol. percent dienes, 40 to 45 vol. percent alkenes, and only a small amount of parains or naphthenes.

By having precluded the relatively low boiling Cs dienes from being present in the benzene cut which is sent to the clay treatment the clay treatment can be satisfactorily carried out under moderate conditions both as regard to temperature as well as to pressure necessary to cause the reaction to occur entirely in the liquid phase which leaves a substantial amount of the higher molecular weight dienes uppolymerized. The amount of polymers formed is readily extracted by the olefin and benzene rich liquid passed through the clay. The claytreated benzene eut efiluent from the clay treatment can thus be made to contain 35 to 50 volume percent benzene, 40 to 45 volume percent alkenes, 2 to 8 volume percent dienes, 5 to 15 volume percent polymers and only small amounts (less than 5%) or traces of paraffinic hydrocarbons. The polymerization makes an overall change in volume amounting to less than 5 vol. percent.

Test data demonstrating the utility and advantages of the polymerization treatment of appropriate benzene cuts are given in the following example:

Example Comparative runs were made on a benzene cut: (1) containing substantial amounts of low-boiling Ce dioleins (boiling below 165 F), and (2) substantially freed of the low-boiling C6 dioleiins. Relative data is tabulated The conditions of the clay treatments were comparative (.5 vol. of feed/vol. of clay/hr. at about 320 F. and 100 p. s. i. g. in clay bed). The viscosities of the polymers (14 vol. per cent) diluted with the same kind of naphtba was measured with a Saybolt Universal viscosimeter.

These test results substantiated that the inclusion of the lower boiling diolens greatly increased the viscositgI of the polymer formed.

Plant operations carried out on the various stocks showed that the run length could be more than doubled by using the feed stocks that form the lower viscosity polymers without severe fouling.

What is claimed is:

1. ln a process for separating substantially pure benzene from a cracked petroleum hydrocarbon naphtha containing mainly unsaturated hydrocarbons with benzene, the steps of preparing a benzene concentrate from said naphtha for extractive distillation which comprise fractionally distilling from said naphtha an intermediate benzene cut boiling in the range from 165 F. to 185 F. to obtain a cut which contains from 35 to 50 volume per cent benzene cut which is substantially freed of dienes having boiling points below 165 F., contacting said benzene cut in liquid phase with a clay under conditions at 250 F.

benzene but substantially freed of dienes having boiling points below 165 F., contacting said intermediate benzene cut in liquid phase at 250 F. to 400 F. with a clay,r polymerization catalyst to polymerize a portion of the diene components in said cut, owing the benzene cut containing unreacted alkenes, unreacted dienes, and resulting polymers from the clay into a second fractional distillation zone, and fractionally distilling the benzene with unreacted alkenes and dienes away from the polymers in said second fractional distillation zone wherein the residual polymers are heated to a temperature no higher than 450 F. and for a period of less than 10 minutes.

3. In a process for separating benzene from a hydrocarbon mixture containing principally unsaturated hydrocarbons with benzene, the steps of preparing a benzenerich cut boiling in the range from 165 F. to 185 F. and containing to 50 volume per cent of benzene but substantially free of dienes having boiling points below 165 F., contacting said benzene-rich cut in liquid phase with a mild polymerization clay catalyst at 250 F. to 400 F. to polymerize a portion of the diene components in 'said cut, extracting the polymers that are formed by the remaining unsaturated benzenerich cut from the clay catalyst, passing the thus treated benzene-rich cut containing the extracted polymers into a fractional distillation zone, and fractionally distilling the benzene with unreacted unsaturated hydrocarbons away from the polymers, and recovering said polymers as a bottoms fraction from the fractional distillation zone.

4. In a process for separating benzene from a hydrocarbon mixture containing principally unsaturated hydrocarbons with benzene, the steps of preparing a benzenerich cut boiling in the range of 165 F. to 185 F., said cut containing 35 to 50 volume per cent benzene, 10 to l5 volume per cent dienes, and 40 to 45 volume per cent alkenes, contacting said benzene-rich cut in liquid phase with a mild polymerization catalyst at 250 F. to 400 F. to polymerize a portion of the diene complement in said cut,

40 extracting the polymers that are formed by the remaining to 400 F. for polymerizing a portion of diene cornponents in the cut, ilowing the clay-contacted benzene cnt containing unreacted unsaturated hydrocarbons with polymers thus formed into a second fractional distillation zone, fractionally distilling the benzene with monomeric C6 to C1 unsaturated hydrocarbon components away from the polymers under conditions to prevent decomposition of the polymers, and recovering as a distillate from said second fractional distillation zone the benzene concentrate for the extractive distillation.

2. In a process for separating substantially pure benzene from a cracked petroleum hydrocarbon naphtha containing mainly unsaturated hydrocarbons with benzene and relatively small amounts of paratnic hydrocarbons, the steps of preparing a benzene concentrate from said naphtha for extractive distillation which comprises fractionally distilling from said naphtha an intermediate benzene cut boiling in the range from 165 F. to 185 F. and containing 35 to 50 volume per cent of unsaturated benzene-rich cut from the clay catalyst to thereby obtain a hydrocarbon mixture containing 35 to 50 volume per cent benzene, 40 to 45 volume per cent alkenes and 2 to 8 volume per cent of dienes with 5 to 15 volume per cent of polymers, passing the thus treated benzene-rich cut containing the extract polymers into a fractional distillation zone, and fractionally distilling the benzene with unpolymerized alkenes and dienes away from the polymers.

References Cited in the iile of this patent V UNITED STATES PATENTS 2,232,761 Balthis Feb. 25, 1941 2,346,401 Shanley Apr. 11, 1944 2,366,570 Souders et al Jan. 2, 1945 2,400,355 Jones et al May 14, 1946 OTHER REFERENCES Olens and Conjugated Dienes, etc. (Tropsch et al.), Indust. and Engr. Chem., vol. 30, No. 2, pages 169-127 relied on.

Claims

1. IN A PROCESS FOR SEPARATING SUBSTANTIALLY PURE BENZENE FROM A CRACKED PETROLEUM HYDROCARBON NAPHTHA CONTAINING MAINLY UNSATURATED HYDROCARBONS WITH BENZENE, THE STEPS OF PREPARING A BENZENE CONCENTRATE FROM SAID NAPHTHA FOR EXTRACTIVE DISTILLATION WHICH COMPRISE FRACTIONALLY DISTILLING FROM SAID NAPHTHA AN INTERMEDIATE BENZENE CUT BOILING IN THE RANGE FROM 165* F. TO 185* F. TO OBTAIN A CUT WHICH CONTAINS FROM 35 TO 50 VOLUME PER CENT BENZENE CUT WHICH IS SUBSTANTIALLY FREED OF DIENES HAVING BOILING POINTS BELOW 165* F., CINTACTING SAID BENZENE CUT IN LIQUID PHASE WITH A CLAY UNDER CONDITIONS AT 250* F. TO 400* F. FOR POLYMERIZING A PORTION OF DIENE COMPONENTS IN THE CUT, FLOWING THE CLAY-CONTACTED BENZENE CUT CONTAINING UNREACTED UNSATURATED HYDROCARBONS WITH POLYMERS THUS FORMED INTO A SECOND FRACTIONAL DISTILLATION ZONE, FRACTIONALLY DISTILLING THE BENZENE WITH MONOMERIC C6 TO C7 UNSATURATED HYDROCARBON COMPONENTS AWAY FROM THE POLYMERS UNDER CONDITIONS TO PREVENT DECOMPOSITION OF THE POLYMERS, AND RECOVEING AS A DISTILATE FROM SAID SECOND FRACTIONAL DISTILLATION ZONE THE BENZENE CONCENTRATE FOR THE EXTRACTIVE DISTILLATION.