US2433286A - Extractive distillation of hydrocarbon mixtures using hydrocarbon solvents - Google Patents

Description

Patented Dec. 23, 1947 i EXTRACTIVE -DISTILLA'ION 0F HYDRO- CARBON MIXTUBES USING l-IYDBOCAR- BON SOLVENTS Art C. McKinnis, Los Angeles, Calif.. assigner to Union Oil Company o! California. Los Angeles, Calif., a corporation of California Application September 22, 1943, Serial No. 503,448

7 Claims. (Cl. 2oz-39.5)

1 This invention relates to the preparation of pure hydrocarbons from hydrocarbon fractions or, more particularly, from narrow boiling hydrocarbon fractions. Morespeccally it relates to a method of treatment of a complex hydrocarbon mixture to cause the separation of relatively unsaturated or oleflnic hydrocarbons or of aromatic hydrocarbons from relatively saturated or paraffinic hydrocarbons or naphthenic hydrocarbons.

An object of this invention is to provide a means of separating individual hydrocarbons from hydrocarbon mixtures having boiling points so close as to prevent separation by means of ordinary distillation processes. The method is less complex and involves fewer steps than chemical methods oi separation and yields a purer product than that obtained by processes involving solvent extraction. r

Another object of the invention is to separate relatively olefinic hydrocarbons from relatively non-olefinic hydrocarbons having approximately y the same boiling point range and particularly to separate the dioleiins from monoolens and paraffine in the case of relatively narrow boiling fractions containing these hydrocarbons, said liractions being those such as may be obtained by the fractional distillation of the products of petroleum cracking or dehydrogenation processes.

It is also an object of my invention to separate mixtures of two or more hydrocarbons which form minimum boiling point azeotropes thus preventing separation by ordinary fractional distillation processes.

The difficulty in separating hydrocarbons having similar boiling points is well recognized and many methods for effecting such separations have been suggested including chemical processes, highly efficient fractional distillation proc-A esses, solvent extraction processes, and combinations of solvent extraction and distillation processes which, in many instances, may be described as extractve distillation processes and also azeotropic distillation processes. Some of these prooesses, particularly azeotropic distillation processes and extractive distillation processes have resulted in the separation of relatively pure hydrocarbons with high yields although the types of compounds used as azeotrope formers in the azeotropic distillation processes and as solvents in the extractive distillation processes are often costly, undesirably chemically reactive and diilcult to separate from the hydrocarbons after the treatment. The present invention relates to an improvement in an extractive distillation process.

The drawing shows a flow sheet ot the process. According to my invention the separation of a specic hydrocarbon from a mixture of hydrocarbons or a hydrocarbon fraction whose components have substantially the same boiling points as said specic hydrocarbon is accomplished by extractiva distillation using as a solvent a hydrocarbon or relatively narrow boiling hydrocarbon mixture having a boiling point or boiling point range higher than the boiling point or boiling point range of the mixture of hydrocarbons being treated. The addition of the higher boiling hy drocarbon or hydrocarbon mixture to the hydrocarbon fraction to be treated has the eiIect of changing the relative vapor pressures of the hydrocarbon components of the hydrocarbon fraction, the extent of the change in vapor pressure of each component of said hydrocarbon fraction being dependent upon the relative saturation or unsaturation of the particular component as defined hereinbelow. Thus the effect of adding the solvent is to change the relative boiling points of the hydrocarbons present in the hydrocarbon fraction thereby allowing separation of the hydrocarbons by controlled fractional distillation. The extractive distillation may be effected at any desired pressure, the particular pressure employed in any given case being dependent in part upon the boiling range of the complex hydrocar- 80 bon fraction being treated. Thus when extractively distilling complex hydrocarbon fractions comprising C4 hydrocarbons it is usually desirable to operate under pressures in the order of about 80 to 100 pounds per square inch gage and 35 in some instances even much higher pressures are employed such as for example about 300 pounds per square inch, although when treating complex hydrocarbon fractions comprising Cs hydrocarbons it is usually preferred to operate at ordinary 4o atmospheric pressures.

Solvents which may be employed in the extractive distillation process may be classified into first, those which have the effect of depressing the vapor pressure of the relatively saturated 45 hydrocarbons to a greater extent than the relatively unsaturated hydrocarbons and, second, those solvents which act in the opposite manner and depress the vapor pressure of the relatively unsaturated hydrocarbons to a greater extent 50 than the saturated hydrocarbons. For the purises of this description, by the term relatively saturated hydrocarbons is meant those hydrocarbons having relatively high hydrogen to carbon ratios, and specically those having the gen- 55 eral formula CnHzHz or the parailins and CnHn or the monoolens and the naphthenes. Moreover, by the term reiatlvebr unsaturated hydrocarbons is meant those hydrocarbons having relatively low hydrogen to carbon ratios and speciflcally those having the general formula CnHzn-z or dioleilns, and CnHzn-e or aromatics. Solvents of the first type include the saturated straight and branched chain paran hydrocarbons having three or more carbon atoms per molecule, as for example, propane, butane, isobutane, normal and isomeric pentanes, hexanes, heptanes, etc., up to those hydrocarbons containing about 30 carbon atoms per molecule, or mixtures of two or more of these parafin hydrocarbons, also relatively narrow boiling hydrocarbon fractions comprising paraffin hydrocarbons. Solvents of the second type include olefin and diolefin hydrocarbons having three or more carbon atoms per molecule, such as propene, propadiene, the butenes and butadienes, pentenes and pentadienes, etc.. and also aromatic hydrocarbons, such as benzene, toluene, xylene and higher homologs of benzene, naphthalene, and the various alkyl substituted naphthalenes, such as methyl naphthalene, ethyl naphthalene, etc.

The selection of a solvent in any case will dei pend upon the hydrocarbon fraction being treated and upon the particular hydrocarbon or hydrocarbon component which is to be separated from said hydrocarbon fraction. In general, the solvent will be selected upon the basis of its boiling point or boiling point range. The boiling point of the hydrocarbon or the initial boiling point of the hydrocarbon fraction used as solvent should be at least 25 F. and preferably 50 F. or more above the maximum boiling point of the hydrocarbon fraction being treated. Solvents of the first type disclosed above will be used in those cases in which it is desired to vaporize and distill overhead the relatively unsaturated hydrocarbons leaving the relatively saturated hydrocarbons as a residue and solvents of the second type will be selected when it is desired to vaporize the relatively saturated hydrocarbons and leave the relatively unsaturated hydrocarbons as a residue.

The advantages which may be gained by using hydrocarbon solvents as above disclosed are manifold. These solvents are in general chemically stable, low in cost, particularly because narrow boiling range mixtures of hydrocarbons or hydrocarbon fractions may be successfully employed, they are readily separated from the hydrocarbon being treated, and, due to their relatively low viscosities, the plate efficiency in fractionating columns operating on mixtures of hydrocarbons containing the hydrocarbon solvents. is particularly high.

The amount of solvent to be employed in any case will be dependent upon the particular hydrocarbon fraction being treated, upon the hydrocarbon or hydrocarbon fraction used as the extractive solvent and upon the efficiency of the fractionating equipment in which the extractive distillation is carried out. Generally, as the ratio of solvent to hydrocarbon fraction being treated is increased the efficiency of the extractive distillation increases although at the same time the effective capacity of the fractionating equipment decreases. Thus in any given case there is an ideal ratio of solvent to hydrocarbon fraction, smaller ratios producing less efcient separation and larger ratios being less economical. While I may use any ratio of solvent to hydrocarbon fraction which is great enough to produce the desired separation I prefer to employ. weight ratios of about 2:1 to about 25:1, respectively.

In separating relatively olenic hydrocarbons, such as dioleflns from brelatively non-oleflnlc hydrocarbons, a narrow boiling point range mixture of these hydrocarbons to which is added an appropriate amount of a solvent which, in this case may be a paraffin hydrocarbon, is extractively distilled. 'I'he relatively olefinic hydrocarbons are vaporized and distilled thereby leaving the relatively non-olef'lnic hydrocarbons together with the solvent as a distillation residue. More specifically in separating, for example diolefins from mcnooleflns and parafiins, an appropriate amount of a suitable paraffin hydrocarbon is added to the mixture of dioleflns, oleflns and parafilns and upon extractive fractional distillation the diolefins vaporize at temperatures lower than those required to distill the monoolens and parafns and are obtained as an overhead distillate, the mixture of monoolens, parafns and solvent being obtained as a residue substantially completely separated from the diolef'ln hydrocarbons. This distillation residue may then be distilled to separate the monoolens and paraflins from the higher boiling paraffin used as a solvent in the above extractive distillation.

It is possible to segregate hydrocarbons or hydrocarbon components by adding a solvent, as defined hereinabove, to a mixture of hydrocarbons and distilling the resulting mixture batchwise, however, in this method of operation the solvent remains as a liquid and comes into contact with the unvaporized hydrocarbons only, and not with the vaporized hydrocarbons. The desirable effect of the solvent is greatly increased by effecting the distillation in such a manner that the liquid solvent contacts and scrubs the vapors of the hydrocarbon mixture being treated. This efect is realized to some extent in the case of a flash-type distillation in which the mixture of solvent and hydrocarbon is passed through a heated zone where it is substantially completely vaporized and the vapors are lead into a fractionating column which is maintained at a temperature such that the solvent and some of the higher boiling hydrocarbons of the vaporized mixture are condensed and ow downward through the column and the lower boiling hydrocarbons remain as vapor and ascend the column. The temperature ln the column may be controlled by regulating the temperature of the feed to the column or by using a reboiler at the bottom of the column or both. In this type of distillation the solvent, due to its appreciably higher boiling point, condenses first and the thus formed liquid solvent contacts the vapors of the hydrocarbon mixture being treated.

The preferred method of extractive distilla-I tion, which is more efficient than either of the above described methods comprises passing the hydrocarbon mixture, as a liquid or as a vapor, into a fractionating column at a point below about the middle of the column and passing the liquid solvent into the same column at a point above the point of entry of the hydrocarbon mixture and preferably at a point near the top of the column. Heat is supplied to the column by means of heating the incoming hydrocarbon stream and/or by means of a reboiler at the base of the column, the temperature being maintained at -such a level that at least one hydrocarbon is vaporized and distills overhead from the column. The solvent flows downward through the column contacting and scrubbing the ascendassenso ing hydrocarbon vapors vcarrying with it those hydrocarbons present in the hydrocarbon mixture which have the lowest vapor pressure in the presence of the solvent. This mixture of solvent and hydrocarbons'is withdrawn from the bottom of the column and transferred to a second column where it is distilled at a somewhat higher Distillati'on is `eiiected by vaporizing'the mixture temperature to vaporize the 'hydrocarbons there by leaving the solvent as'a distillation residue. This solvent may be recycled to the extractive distillation column.

In some cases lt is preferable to employ a two stage extractive distillation process using one type of solvent, as disclosed hereinabove, in the :first stage and the other type of solvent in the second stage. lation process is of particular value in those instances in which the hydrocarbon fraction being treated has a relatively wide boiling point range or where large number of hydrocarbons or hydrocarbon components are present in the hydrocarbon mixture. Thus the hydrocarbon fraction may be distilled in the presence of a solvent of the rst type, the distillation temperature being maintained at such a point that the hydrocarbon which is to be separated from the cornplex hydrocarbon mixture just distills. For the purpose of this description the hydrocarbon or hydrocarbon component which is to be separated will be referred to as component A, At the distillation temperature employed, some of the other hydrocarbons originally present in the complex mixture, i. e., those which vaporize at the temperature employed in this extractive distillation process, such hydrocarbons being referred to as This two stage extractive distilcomponent B, will distill together with component A. The residue will consist of component C and solvent, where component C comprises those hydrocarbons which do not vaporize at the temperature employed in this distillation. This residue may be separately distilled at a somewhat higherl temperature to vaporize component C leaving solvent as the distillation residue. 'I'he overhead distillate from the first extractive distillation stage, comprising components A and B, is then extractively distilled using a solvent of the second type and maintaining a distillation temperature such that component B vaporizes and distills leaving component A together with solvent as a residue. This vresidue is separately distilled at a somewhat higher temperature to vaporize component A leaving solvent as the distillationresidue.

Treatment of the overhead distillate from the first extractive distillation stage comprising components A and B for the separation of component A may in some instances be effected by careful fractional distillation since in the absence of solvent the relative vapor pressures of components A and B are changed in a manner favorable to such separation, that is, the spread between the boiling points of component A and component B is wider in the absence of solvent.

The process of my invention is shown diagrammatically in the drawing.

The following specific examples serve to illustrate further the invention, but they are not to be taken as in any way limiting the invention.

Example I To 100 parts by weight of a mixture comprising 80 parts by Weight of butadiene and 20 parts by weight of n-butane is added 1500 parts by l oi solvent and hydrocarbons at a temperature of about 195 F. and passing the vapor into o fractionatim column at a point near the middle o! the column. The fractionating column is maintained at such a temperature that the va'- por outlet temperature is about 125 F. Butadiene distills and is obtained as an overhead product from the fractionating column and a mixture of n-butane and n-pentane is obtained as a. bottoms fraction, This bottoms is transferred to a second column maintained at the saine pressure and at a somewhat higher temperature (stillhead temperature of about 130 F.) where the n-butane is vaporized .leaving n-pentane as a residue.

` Example II A mixture of C4 hydrocarbons comprising about 50 parts by weight of butadiene, 20 parts by weight of n-butane, 15 parts by weight of butenel and 15 parts by weight of isobutene is heated and passed into an extractive distillation column at a point near the bottom of the column, and npentane is passed into the same column at a point near the top of the column at the rate of 10 parts by weight of the n-pentane tol part by weight of the C4 hydrocarbon mixture. The co1- umn is maintained at a pressure of pounds per square inch absolute and a top or vapor outlet temperature of about F. Under these conditions the overhead distillate is a mixture comprising butadiene, butene-l and isobutene, while n-butane together with n-pentane is obtained as a distillation residue, substantially completely separated from butadiene, butene-l, and isobutene.

The overhead distillate from this extractive distillation is passed to a second extractive distillation column maintained at the same pressure where it enters at a point near the bottom and benzene, which is used as the extractive solvent, is pumped into the column at a point near the top of the column at a rate' of 5 parts of benzene to l part of the C4 hydrocarbon mixture. The distillation temperature (vapor temperature) is maintained at about 122 F. and the butene-l and isobutene vaporize and distill overhead leaving butadiene and benzene as a distillation residue. The mixture of butadiene and benzene is fractlonally distilled in a third fractionating column to vaporize the butadiene and leave benzene as a residue.

The foregoing description of my invention is not to be taken as in any way limiting but merely illustrative of my invention for many variations may be made by those skilled in the art without departing from the spirit or scope of thefollowing claims.

I claim:

1. A method for the treatment of a mixture of normally gaseous hydrocarbons comprising paraflin, monoolen and dioleiin hydrocarbons to separate said mixture of hydrocarbons into its components which comprises extractively distilllng said mixture in the presence of a solvent comprising a paraflin hydrocarbon boiling at least 25 F. above the boiling point of said hydrocarbon mixture to reduce the vapor pressure of the paraflin contained in said hydrocarbon mixture to a greater extent than thevapor pressure of the monooleiin and diolefin is reduced, thereby permitting the vaporization of said monoolefln and diolefln thereby leaving said paraflin together with said solvent as a distillation residue suband diolefln, separately distilling said residue to vaporize said paraflin thereby leaving said solvent stantially completely separated from monoolefln residue, and separately distilling said last named residue to vaporize said diolefln thereby leaving said second solvent in the residue.

' 2. A method for the treatment of a mixture of C4. hydrocarbons comprising butadiene, butane and butenes to separate butadiene therefrom W-hich comprises extractively distilling said mixture of C4 hydrocarbons in the presence of a suilicient amount of a. solvent comprising a parafiin hydrocarbon boiling at least 25 F. above the boiling point of said mixture of C4 hydrocarbons to reduce the vapor pressure of said butane to a greater extent than the vapor pressure of butadiene and butenes is reduced, thereby permitting the vaporization of said butadiene and said bu- `tenes thereby leaving said butane together with said solvent as a distillation residue substantially completely separated from butadiene and butenes; separately distilling said residue to vaporize butane thereby leaving said solvent in the residue; and separately extractively distilling said vaporized butadiene and butenes in the presence of a sufiicient quantity of a second solvent selected from the class consisting of unsaturated and aromatie hydrocarbons and having a boiling point at least 25 F. above the boiling point of said butadiene and butenes Ito reduce the vapor pressure of the butadiene to a greater extent than the vapor pressure of the butenes is reduced, to vaporize said butene thereby leaving butadiene together with said second solvent in the residue and separately distilling said last named residue to vaporize butadiene thereby leaving said second solvent in the residue.

3. A method as in claim 1 wherein said solvent comprising a paraffin hydrocarbon is normal pentane.

4. A method as in claim 1 wherein said solvent comprising a paraiin hydrocarbon is normal hexane.

5. A method as in claim 1 wherein said second solvent is benzene.

6. A method as in claim 1 wherein said solvent comprising a paraiiln hydrocarbon is normal pentane and said second solvent is benzene.

7. A method for the treatment of a mixture of C4 hydrocarbons comprising. butadiene, normal butane and butenes to separate butadiene therefrom which comprises extractively distilling said mixture of C4 hydrocarbons in the presence of a suiilcient amount of normal pentane to reduce the vapor pressure of said normal butane to a greater extent than the vapor pressure of butadiene and butenes is reduced thereby permitting the vaporization of said butadiene and said butenes thereby leaving said normal butane together with said normal pentane as a, distillation residuo substantially completely separated from butadiene and butenes, separately distilling said residue to vaporize norma1 butane thereby leaving normal pentane in the residue and separately extractively distilling said vaporized butadiene and butenes in the presence of a sufcient quantity of benzene to reduce the vapor pressure of tlv, butadiene to a greater extent than the vapor pressure of the butenes to vaporize said butene:

thereby leaving butadiene and benzene in the residue and separately distilling said residue to vaporize butadiene thereby leaving benzene in the residue.

ART C. MCKINNIS.

REFERENCES CITED The following references are of record in the OTHER REFERENCES Ser. No. 289,710, Natta (A. P. C.), pub. May 18, 1943, t

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