US2325379A - Distillation process - Google Patents

Description

Fully 2'?, 1Q43.

E'. L. DURRUM DISTILLATION PROCESS l Filed May 26, 1942 2 Sheets-Sheet l Figli Digium@ Commn 5s C Y Sowzni Ezcovzrg Commn Ju|y 27, |943. E L DURRUM 2,325,272

DISTILLATION PROCESS Filed May 26, 1942 2 Sheets-Sheet 2 v table or animal products and the like.

Patented July 27, 1943 UNITED STATES PATENT OFFICE DISTILLATTON PROCESS Emmett L. Durrum, Palo Alto, Calif., assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application May 26, 1942, Serial No. 444,604

6 Claims.

AThe present invention relates to a process for separating a mixture of components by distillation. More particularly, it relates to a process for the separation of multi-compo-nent mixtures by distillation in the presence of a relatively high boiling selective solvent.

The present invention is applicable to the separation of relatively wide boiling range multicomponent mixtures containing components of over-lapping boiling range, some of which components exhibit markedly lower vapor pressures in the presence of a relatively high boiling solvent than do other components of the mixture. Mixtures of this type are particularly apt to occur in natural products or as derivatives thereof, such as petroleum coal tar, products of the destructive distillation of wood and other vege- Sometimes synthetic mixtures of this type also arise in manufacturing processes. Examples of such mixtures are the catalytic oxidation products of petroleum hydrocarbons such as contain acetic and propionic acids along with unoxidized hydrocarbons boiling in the range of acetic and propionic acids; products 0f hydrocarbon isomerization, reforming, catalytic cracking, hydrogenation, dehydrogenation, cyclization, halogenation, etc., which give rise to mixtures of compounds of neighboring boiling range but difierent degrees of saturation or polarity. Because these compounds have different solubilities in liquid polar solvents their vapor pressures and boiling temperatures are depressed to different extents in the presence of such liquid solvents, the compound of greatest solubility normally showing the greatest depression.

More specifically, such a mixture is illustrated by a straight run gasoline fraction having an A. S, T.M. boiling range between about 90 and 120 C. and containing benzene, toluene and saturated hydrocarbons boiling within this range. Such a mixture is considered for present purposes to be a 4-component system with benzene and toluene as the components that have relatively lower vapor pressures in the presence of a relatively I high boiling polar solvent, than do the other two components comprising the saturated hydrocarbons boiling near the boiling temperature of benzene and the saturated hydrocarbons boiling near the boiling temperature of toluene.

As is well known, it is impossible to separate such a mixture into its components by ordinary fractional distillation owing to the formation of azeotropes between the saturated hydrocarbons and the aromatcs, ordinary distillation being defined as distillation inthe absence of azeotrope forming agents or selective solvent for one of the components to be separated, or both. Processes have been proposed for the production of a pure component from such a mixture, for example benzene, but they have involved either very careful prefractionation to exclude high boiling components, followed by distillation in the presence of a relatively high boiling selective solvent; or else they have required a liquid phase solvent extraction treatment followed by distillation in the presence of a solvent.

It is an object of the present invention to provide an improved process for separating multicomponent mixtures of the type described. It is an object to provide a process capable of separating difcultly separable multi-component mixtures of relatively Wide boiling range into individual components or groups of components. Further objects Willbe evident from the following description;

The present invention is carried out by causing the vapor pressure relationships of the mixture to be distilled to be altered in such a sequence and manner that the formation of normally occurring azeotropes is suppressed and individual componentsare thus readily separated by distillation as hereinafter described.

The problem is to separate a multi-component mixture, say one containing components A, C, X and Z. In this mixture A and C, on the one hand, and X and Z, on the other, have overlapping boiling ranges, A and X normally having relatively higher vapor pressure than- C and Z. However, X and Z have relatively lower vapor pressures in the presence of a relatively high boiling solvent than A and C, respectively.

The invention comprises subjecting this mixture to a continuous distillation in the presence of a relatively high boiling selective solvent under conditions to take A overhead and to remove a bottom product comprising solvent containing dissolved X, C and Z. The bottom product is separately distilled to take C and X overhead and to leave a second bottom product comprising solvent containing Z. The solvent is separated from Z by further distillation and the former is returned to the first distillation step. The overhead product comprising C and X is then distilled in the absence of any solvent, whereby they are separated from each other by virtue of their different vapor pressures.

The present invention may better be understood from a consideration of the figures. Figure I is a diagrammatic representation of the process indicating the principle applied therein; while Figures II and III are schematic flow diagrams of different embodiments thereof.

With reference to Figure I, be it assumed that a relatively wide boiling range distillate containing components A, X, B, Y, C, and Z shall be separated into its components; Components X, Y and Z are or such nature that their vapor pres sures are lower in the presence of a relatively high boiling liquid solvent than are the components A, B and C, respectively. Further, in the absence of solvent components A, B, C, and X, Y, Z have overlapping boiling ranges, i. e. the vapor pressures of A and X are closer together but are higher than those of B and Y, which likewise are close to each other and are higher than those of C and Z, the vapor pressures of the latter also being close to each other. An example of sucha mixture is a straight run gasoline fraction boiling from about 60 to 150C. Such a fraction contains aromatic hydrocarbons such as benzene corresponding to X, toluene corresponding to Y, and the xylenes and ethyl benzene corresponding .to Z, and in addition saturated hydrocarbons normaliy haring vapor pressures in the neighborhood of each of said components X, Y, Z, or else tending to form azeotropes with them. The satn urated hydrocarbons may be divided into groups designated as component A. which comprises hydrocarbons having vapor pressures in the neighborhood oi the vapor pressure of benzene, component B, comprising hydrocarbons having vapor pressures in the neighborhood of the vapor pressure of toluene; and component C, comprising hydrocarbons having vapor pressures in the neighborhood of the vapor pressures of xylenes and ethyl benzene. If such a mixture is distilled in the absence of solvent, an overhead mixture is rst produced containing benzene (X) and saturated hydrocarbons (A) followed by toluene (Y) and saturated hydrocarbons (B), and nally the xylenes and ethyl benzene (Z) and saturated hydrocarbons (C). In other words, itis impossible to produce thevarious components in a pure state.

In Figure I the six components of the mixture are graphically arranged to show their mutual relationship with regard to separability.

The initial material as existing in the absence of a selective solvent is represented by state 1, wherein the horizontal lines divide the various components into pairs having normally similar vapor pressures while the vertical lines divide the components as to type, that is, by chemical constitution. Different types have different solubilities and vapor pressures in the presence of a relatively high boiling selective solvent, and thus in the presence of such a solvent the vapor pressure relationships of the components are represented by state 2, wherein components X, Y and Z all have their vapor pressures lowered with respect to the vapor pressures of A, B and C. Now, if the mixture is distilled While in this state component A may be taken overhead, and B, C, X, Y, and Z may be withdrawn with solvent as a first bottom product. This first bottom product is further distilled to take overhead B, C, X and Y,

while Z is withdrawn with solvent from which it may be separated in a separate distillation step.

'I'he mixture of B, C, X and Y, now free from or at least containing a substantially reduced amount of solvent, is represented by state 3. It is seen that A normally associated with X, and

Z normally associated with C, have been removed.-

Consequently C and X can now be separated by ordinary distillation in the absence of solvent. Thus, when the mixturerepresented by state 3 is distilled, X can be taken overhead alone while the bottom product comprising B, Y and C can be distilled in a separate step to take B and Y overhead and leave C as a bottom product. If the top product containing B and Y is now distilled in the presence of solvent, the vapor pressure relationships are represented by state 4 and upon distillation in the presence of solvent, B can be taken overhead while solvent and Y can be taken off as a bottom product from which Y can be separated.

Figure H is a ilow diagram representing a continuous distillation for separating pure components A, X, B, Y, C and Z from a feed mixture containing them, in accordance with the scheme of Figure I. The feed mixture comprising components A, X, B, Y, C and Z is continuously fed via line i to extractiva distillation column 2 equipped with reboiler 3. Herein the feed is distilled in the presence of a relatively high boiling solvent which is admitted to column 2 via line S and ows down the column countercurrently to ascending vapors. Vapors of A pass overhead via line ti and are condensed in condenser t and collect in accumulator l, whence a portion of the condensed product A is returned to column il as reiiux via line iii, while the remainder passes to storage not shown via line 9.

From the bottom of column 2 are withdrawn through line it components X, B, Y, C and Z, and solvent, which pass to extractive distillation column il equipped with reboiler l2. Vapors comprising X, B, Y and C are taken overhead via line i3 to condenser it and condensed vapors collect in accumulator l5, whence a portion of condensed vapors may be returned to column il as reflux via line le while the remaining portion passes to column 2l to be treated as described subsequently.

From the bottom of column il is withdrawn solvent containing dissolved Z through line il and passes to solvent recovery column i8 equipped with reboiler i9. Herein component Z is stripped from solvent, which latter is withdrawn from column i3 via line l and recycled to column 2 to contact further quantities of feed. A portion of the recycled lean solvent may be returned to column il via valved line 20 to aid in the separation of X, B, Y and C from solvent and Z therein, if desired. Vapors of Z pass overhead from column i8 through line 2l to condenser 22 and accumulator 23, whence a portion of component Z is returned to column i3 as reiiux via line 2d, while the remainder passes to storage not shown via line 25.

The top product from column il containing X, B, Y and C passes via line 26 to fractionating column 2l equipped with reboiler 28. Herein X is separated from B, Y and C, X passing overhead via line 28 to condenser 29. Condensed vapors collect in accumulator 30, whence a portion is returned to column 21 as reflux via line 3l, while the remainder passes to storage not shown via line 32. The latter product comprising B, Y and C is withdrawn via line 33 and passes to fractionating column 34 equipped with reboiler 35. Herein the B and Y are taken overhead, while C is withdrawn as bottom product via line 36 and passes to storage not shown. Vapors of B and Y pass overhead via line 3l to condenser 38. Condensed vapors collect in accumulator 39 whence a portion'is returned to co1- umn 34 as reflux vla line 40, while the remainder passes via line 4| to extractive distillation column 42 equipped with reboiler 43. Herein B and Y are distilled in the presence of solvent admitted to column 42 via line 44. Vapors of B pass overhead via line 45 to condenser 46. Condensed B collects in accumulator 41, whence a portion is returned to column 42 as reux vial line 48, while the remainder passes to storage not shown via line 49.

From the bottom of column 42 solvent and Y are withdrawn and pass vla line 50 to solvent recovery column 5| equipped with reboiler 52. Lean solvent is withdrawn from the bottom of column 5| and recycled to column 42 via line 44. Vapors of Y pass overhead via line 53 to condenser 54. Condensed Y collects in accumulator 55 whence a portion is returned to column 5| as redux via line 56 while the remainder is withdrawn via line 51 `and passes to storage not shown. Make-up quantities of solvent may be added to the cycle via valved lines 58 and 59.

Figure III is a now diagram representing another embodiment of the present invention which is particularly suitable for the separation of a four-component system of the nature described, because in this case it is possible to effect the separation of the components in a pure condition with but four distillation columns. A feed comprising A, X, C and Z is continuously fed via line to extractive distillation column |02 equipped with reboiler |03. Herein the mixture'is distilled in the presence of a relatively high boiling solvent which is admitted'to column |02 via line |04, which flows down the column countercurrently to ascending vapors as the distillation proceeds. Vapors comprising A are taken overhead via line and pass to condenser |06. Condensed vapors collect in accumulator |01 whence a portion may be returned to column |02 via line |08 as reflux, while the remainder is withdrawn through line |09 and passes to storage not shown. From the bottom of column |02 is withdrawn the solvent with components X, C and Z, which pass via line ||0 to extractive distillation column equipped with reboiler H2. This column is operated in such a manner as to take overhead components X and C, while withdrawing a bottom product through line I3 comprising solvent and Z, which latter product passes via line H3 to solvent recovery column H4 equipped with reboiier H5. Lean solvent is withdrawn from column H4 which is recycled to column |02 via line |04. If desired, a portion of lean solvent may be returned to column via valved line H6 to aid in the separation of X and C from solvent and Z. Overhead product from column H4 comprises vapors of Z, which pass through line H1 to condenser H8. Condensed vapors collect in accumulator H9, whence a portion is returned to column H4 as reflux via line |20, while the remainder passes to storage not shown via line |2|.

The overhead vapors from column cornprising X and C pass via line |22 to condenser |23. |24 whence a portion is returned to column Hl as reflux whil-e the remainder passes via line |26 to fractionating column |21, equipped with reboiler |23, which is operated in such a manner as to take overhead component X via line |29 to condenser |30. Condensed vapors of X collect in accumulator I3 whence a portion is returned to column |21 as reflux via line |32, while the remainder is withdrawn and passes to storage not shown via line |33. Component C is withdrawn Condensed vapors collect in accumulator from the bottom of column |28 via line |34. Make-up quantities of solvent may be added to the system via valved line |35.

In the ow diagrams pumps, heat exchangers, valves, now control equipment, and other auxiliaries, the proper placement of which is evident to one skilled in the art, have been omitted.

From the foregoing it is evident that the present invention is applicable to the separation of any multi-component mixture of overlapping boiling range provided a solvent exists in which the vapor pressure of some of the components is reduced to a greater degree than that of the other components.

In general, solvents reduce the vapor pressure of the more soluble components to a greater degree than the less soluble components, and further, in general, the solvent will tend to depress more the vapor pressure of the components more closely chemically (structurally) related thereto.

In separating hydrocarbons of different degrees of saturation various polar solvents may be employed, e. g. phenol, cresylic acids, alkyl phenol mixtures, aniline, alkyl anilines, diphenyl amine, ditolyl amines, carbitols (diethylene glycol monoethers) such as methyl, ethyl, propyl, carbitols; chlorinated dialkyl ethers such as beta-betadichlorethyl ether, nitrobenzene, nitrotoluene, nitroxylenes, naphthols, alkyl naphthols, benzoplienone, phenyl tolyl ketone, diphenylene ketone; alkyl phthalates, such as dimethyl phthalate; alkyl salicylates such as methyl salicylate; benzyl alcohol, benz chlorides, i. e. benzyl, benzal, benzo chlorides, benzonitrile, diphenyl oxide, ditolyl oxide, hydroxy pyridine, nitropyridine, chlorinated pyridines, quinoline, isoquinoline, chlorinated quinoline, hydroxy quinolines, 5nitro quinoline, tetra-hydrofurfuryl alcohol, furfural alcohol, furfural; the monoglycerol ethers such as l-methoxy glycerol, 2-methoxy glycerol, lethoxy glycerol, 2ethoxy glycerol, lpropoxy glycerol, 2-propoxy glycerol, 1-isopropoxy glycerol, 2-isopropoxy glycerol; the glycerol di-ethers such as 1,2`dimethoxy glycerol, 1,3-dimethoxy glycerol, 1,2-diethoxy glycerol, 1,3-diethoxy glycerol, 1,2-dipropoxy glycerol, 1,3-dipropoxy glycerol, 1,2-di-isopropoxy glycerol, and 1,3di-isopropoxy glycerol; the mixed diglycerol ether esters such as l-ethoxy,2methoxy glycerol, 1- methoxy,3propoxy glycerol, l-ethoxy,2isopro poxy glycerol; acetone or acetone-water mixture, dioxane, acetonitrile, etc.

`Suitable solvents should have high enough boiling temperatures that theyrmay be separated from the dissolved components Without the necessity of employing an excessively large number of plates or high reflux ratios. employ solvents which are relatively stable under the distillation conditions in the presence of the substance to be separated.

I claim as my invention: f

1. A process for separating a mixture comprising a rst pair of components A and X and a second pair of components C and Z, the components of said first pair normally having vapor pressures higher than the vapor pressures of the components of said second pair, said components X and Z having relatively lower vapor pressures in the presence of a substantial concentration of a relatively high boiling selective solvent than said components A and C respectively, comprising the steps of fractionally distilling said mixture in the presence of a substantial concentration of said solvent under conditions to form. a first overhead product rich in said component A It is. preferable to and leave a iirst bottom product comprising said solvent and said components C, X and Z, further distilling said lrst bottom product under conditions to form a second overhead product rich in said components C and X and leave a second bottom product comprising said solvent and Z,

` further distilling at least a portion of said second bottom product under conditions to separate said solvent and Z, recycling at least a portion of the resulting separated solvent to said first mentioned distillation step, and further distilling at least a portion oi' said second overhead product with substantially reduced concentration of solvent to separate said components C and X.

2. The process of claim 1 wherein said mixture is a hydrocarbon mixture comprising benzene and toluene and non-aromatic hydrocarbons, said benzene corresponding to component X, said toluene corresponding to component Z, the hydrocarbons boiling near the boiling temperature of benzene comprising component A, the hydrocarbons boiling near the boiling temperature of toiuene comprising component C, and said solvent comprising a relatively higher boiling polar solvent.

3. The process of claim l wherein said mixture is a hydrocarbon mixture comprising toluene and the Xylenes and ethyl benzene, and nonaromatic hydrocarbons, said toluene corresponding to component X, said xylenes and ethyl benzene corresponding to component Z, the hydrocarbons boiling near the boiling temperature of toluene comprising component A, the hydrocarbons boiling near the boiling temperature of the xylenes and ethyl benzene comprising component C, and said solvent comprising a relatively higher boiling polar solvent.

4. A process for separating a mixture comprising a irst pair of components A and X and a second pair of components C and Z, the components within each of said pairs having such similar vapor pressures as to make their separation diihcult by ordinary fractional distillation but the components of said rst pair normally having vapor pressures higher than the vapor pressures of the components of said second pair, said components X and Z having relatively lower vapor pressures in the presence of a substantial concentration of a relatively high boiling selective solvent than said components A and C respectively, comprising the steps of fractionally distilling said mixture in the presence of a .substantial concentration of said solvent under conditions to form a rst overhead product rich in said component A and leave a i'lrst bottom product comprising said solvent and said components C, X and Z, and further distilling said rstbottom product under conditions to form a second overhead product rich in said components C and X and leave a second bottom product comprising said solvent and Z, further distillirig at least a portion of said second bottom product under conditions to separate said solvent and Z, recycling at least a portion of the resulting separated solvent to said rst mentioned distillation step, and further distilling at least a portion of said second overhead product substantially in the absence of solvent to separate said components C and X.

5. A process for separating a mixture comprising a first pair of components A and X and a second pair of components C and Z, the components of said iirst pair normally having vapor pressures higher than the vapor pressures of the components of said second pair, said components AX and Z having relatively lower vapor pressures in the Apresence of a substantial concentration of a relatively high boiling selective solvent than said components A and C respectively, comprising the steps of fractionally distilling said mixture in the presence of a substantial concentration of said solvent under conditions to take overhead substantially pure component A and leave a first bottom product comprising said solvent; and said components C, X and Z, further distilling said iirst bottom product under conditions to form a second overhead product comprising components C and X and to leave the second bottom product consisting of said solvent and component- Z substantially free from said components A, C and X, further distilling at least a portion of said second bottom product under conditions to separate said solvent and component Z, recycling at least a portion of the resulting separated solvent to said rst distillation step, and further distilling at least a portion of said second overhead product substantially in the absence of solvent to separate components C and X.

6. A process for separating a mixture comprising 'a first pair of components A and X, a second pair of components B and Y, and a third pair of components C and Z, the components of said rst pair normally having vapor pressures higher than the vapor pressures of the components of said second pair which latter normally have vapor pressures higher than the components of said third pair, said components X, Y and Z having relatively lower vapor pressures in the presence of a substantial concentration of a relatively high boiling selective solvent than said components A, B and C respectively, comprising the steps of fractionally distilling said mixture in the presence of a substantial concentration of said solvent under conditions to form a iirst overhead product rich in said component A and leave a first bottom product comprising said solvent and said components B, C', X, Y and Z, further distilling said rst bottom product under conditions to form a second overhead product rich in said components B, C, X and Y and leave a second bottom product comprising said solvent and Z, further distilling at least a portion of said second bottom product to separate component Z from said solvent and recycling a1; least a portion of the resulting separated solvent to said rst distillation step, further distilling said second overhead product in the presence of a substantially reduced concentration of solvent under conditions to produce third top and bottom products rich in said components X and C respectively, and produce an intermediate fraction Icomprising said components B and Y, and further distilling said intermediate product in the presence of a substantial concentration of a relatively high boiling selective solvent to separate said components B and Y.

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