US2375478A - Distillation process - Google Patents

Description

May 8, 1945. G. R. LAKE DISTILLATION PROCESS Filed NOV; 4, 1941 Patented May 8, 19.45

'21,375.4'18 DISTILLATION PROCESS Gieorre4 B. Lake, Long' Beach, Calif., assignor to Union Oil Company of California, LosAng'eles, Calif., a corporation of California Application November 4, 1941,` Serial No. 417,781

This invention relates toa process of vazeotropic distillation to prepare pure hydrocarbons from complex petroleum fractions which are diflicult to separate by ordinary fractional distillation due to the small differences in boiling points of the hydrocarbons contained in the petroleum fraction. The invention is particularly directed to an improved process for separating the hydrocarbons and the azeotrope former that are contained in the azeotropic distillate produced by the azeotropic distillation The process of separating one hydrocarbon component from another'hydrocarbon component of substantially the same boiling point contained in a complex hydrocarbon fraction by azeotropic distillation is well known. This process consists in distilling the hydrocarbon fraction in the presence of an extraneous substance which has a preferential afnty for one of the components contained in the complex hydrocarbon fraction, thus causing a change in the vapor pressure equilibrium that formerly existed in the fraction in such manner that the partial vapor pressure o r fugacity of at least one component in the fraction is changed sufliciently to permit itsseparation by controlled fractional distillation. vIn such processes, the distillation effects the separation of the relatively more parafiinic hydrocarbons toseparate the azeotropevformer substantially-completely fromthe azeotropic distillate .by'washing with water since inl many eases-the' azeotrope gether with the extraneous substance, leaving as undistilled bottoms the relatively less parafilnic hydrocarbons which may or may not contain a portion of the extraneous substance. In the present description of my invention the aforesaid type of fractional distillation will be referred to as azeotropic distillation, the extraneous. substance or substances which are added to the complex hydrocarbon fraction to effect the aforementioned change will be referred to as azeotrope formers and the overhead from the. azeotropic distillation will be referred to as the azeotropic distillate.

One of the main difiiculties in the azeotropic distillation process is in the separation or recovery of the azeotrope former from the hydrocarbons contained in theazeotropic distillate. One of the methods proposed for this purpose resides in washing the azetropic distillate with water which is adapted to dissolve the azeotrope former from the azeotropic distillate and thus the azeotrope former maybe separated from the hydrocarbons by settling vand stratiiication. The solution of,` azeotropeformer and water may be distilled to separate the azetropeformer from the water. y

However, difculty has been experienced to former vhas a preferential solubility inthe hydro'- carbonsfas compared'with the solubility /in-the water. The resultis thatthe hydrocarbons must be washed with lan,excessively"large amount of Water in order to remove thelasttraoes ofthe azeotrope former sov th'a.t1,the hydrocarbons may be utilized andthe azeotrope former recovered without sustaining a substantial loss of this more valuable materiaLjTo illustrate, it v4has been foundv that certainwazeotrope* formers such, as methanol or lmethyl ethyl ketone, particularly containing about 10% by volume of'water, is a very efll-cient azeotrope former to eiect the sepa' ration of non-aromatic hydrocarbonsfrom a hy.-` drocarbon fraction containing toluene. Yet the use of these azetrope formersoifer the serious difiiculty of recovering'the azetrope former fromv the azeotropic distillate. While the separation of the azeotropel vformer may be accomplished by washing with water, this has required about five or six volumes of water for each volume of azeotropic distillate. Even by washing the-azeotropic distillate with this large amount of water, A

the hydrocarbons `thus separated still contained about one percent of azeotrope former.v Furthermore, in order torecover the azeotrope former from the very dilutev Wash solution, it is'nece'ssary J to heat an excessively large volume of dilute' azeotrope former While distillation to separate the azeotrope former from the hydrocarbons. is perhaps the most economical manner and perhaps the most frequently employed method to separate a mixv ture of components of differentboiling points, the distillation of the azeotropic distillate under substantially the same conditions employed in the primary azeotropic distillation of the original complex hydrocarbon charge merely results in distilling overhead a mixture of the hydrocarbons and azeotrope former,insubstantially'the same proportions as theyl distilled from the original complex lhydrocarbon charge.v r4Hence, a separation of the.azeotrope-formerfrom the azeotropic distillate cannot be made .undersuch conditions.

It is thus van object- -of my. invention to effect a substantially complete'r'separation ofthe azeo-v trope former from 'the hydrocarbons contained in the azeotropic distillate and it is a particular ob`y ject to effect the aforesaid separation iny an.

veflicient and economical manner without loss of azeotrope former.

It 1s a further bict dif- 'my invention meneer,

tion by distilling the azeotropic distillate under such conditions as to separate a portion oi.' one of the components contained in the azeotropic diistillateas a, substantially pure product and then washing or" extracting the 'remaining-faz'r-ro# 1 tropic distillate to separate the remaining portion of the component to leave the other component contained in the azeotropic distillate asv a substantially pure product.

I have discovered that if the azeotropic' d istillate is subjected to fractional distillation 4at ,au

pressure lower than the pressure employed Vfor the azeotropic distillation, the component contained in the azeotropic distillate l`having the lowest boiling point at the reduced pressure, i; e. either the hydrocarbons or the azeotrope former, will be distilled as al fraction containing a prtion of the higher boiling component, leaving as a distillation bottoms the remaining portion of the higher boiling component as a fraction substantially free from the lower boiling component. Thus, if the azeotropic distillate is composed of non-aromatic hydrocarbons having an average boiling point of, for example 220 lF. and an azeotrope former having a boilingl point of, for example 180 F., and if this azeotropic distillate was distilled from a mixture of aromatic and non-aromatic hydrocarbons at a distillation temperature of, for example 160 F. and atmospheric pressure, the separation of the azetotrope former from the non-aromatic hydrocarbons may be accomplished by distillation at a lower temperature than 160 F. which will necessitate a corresponding reduction in pressure below atmospheric in order to eiiect a distillation. Since in this case, the azeotrope former boils at a lower temperature than the non-aromatic hydrocarbons, this component will be separated' as an overhead product and the non-aromatic hydrocarbons will remain as a distillation bottoms. However, av portion of the hydrocarbons may be taken overhead with the azeotrope former. The non-aromatic hydra.' carbons will be substantially free from the azeotrope former sol that these may be withdrawn from the system without resulting in loss of azeotrope former.

The extent of the temperature reduction and corresponding pressure reduction in order to effect the desired separation of azeotrope former from non-aromatic hydrocarbons will, of course, vary with the composition and characteristics of the particular azeotropic distillate and will fur-r ther `depend upon the economical and practical considerations with respect to the amount of non-aromatic Vhydrocarbons which may be tolerated with the recovered azeotrope former. In other words, if the subsequent treatment to be described of the overhead resulting from the aforesaid low pressure distillation may be satisfactorily and economically carried out to recover the azeotrope former therefrom in the presence of a relatively large amount of the non-aromatic hydrocarbons, the extent of temperature and pressure reduction in the aforesaid low pressure distillation to produce the mixtureof azeotrope formercontaining'such amount of non-aromatic hydrocarbons from the azeotropic distillate need not be as great as when it is necessary that the overhead contains-4a relatively small amount of the non-aromatic hydrocarbons in order to recoverthe azeotrope kformer therefrom. Since a portion of the hydrocarbons have been separated as a fraction free from azeotrope former, the

washing of the remaining azeotropic distillate, i. e. the overhead vfrom the low pressure distilla- ,tion stage will result in a greater recovery of azeotrope former than is possible when the same amount oi azeotrope former must be washed in the presence of. a largenamount, of hydrocarl bons, assumingthat the same-percentage, fi. e. 1% of azeotrope former must be lost to the hydrocarbons when washing with the same proportion of water. In general, I have found that reducing' the'temperature for the distillation approximately F. below the temperature of the pri- 1,5.

maryaz'eotropic distillation with a corresponding reductionvin pressure will produce a satisfactory overhead azeotrope former which may be satisfactory and economically treated by the process hereinafter described.

The separation of the lower boiling constituent, i. e. azeotrope former, from 'the higher boiling constituent, i. e. non-aromatic hydrocarbons present inthe overheadmixture resulting from the low pressure distillation ofthe azeotropic distillate, is accomplished byiwashing the mixture with water in the usual manner in order to dissolve the water soluble azeotrope former from the hydrocarbons. Water soluble azeotrope former which may be separated from hydrocarbons in this manner includes methyl ethyl ketone. In some cases, particularly when an azeotrope former is used which is water insoluble and hence cannot be separated by water washing, the separation of the remaining azeotrope former may be accomplished by extraction with a solvent which is adapted to selectively dissolve the azeotrope former and substantially none of the hydrocarbons at the temperature of extraction. The extraction of the azeotrope former by means of the selective solvent may also be employed in the case of the water soluble azeotrope formers.

Selective solvents adapted to separate the azeotrope former `from the non-aromatic hydrocarbons include particularly suitable selective solvents namely the polyhydric alcohols, the ethanv olamines, diethylene trlamine and nitromethane.

As a modication of my invention as described above, I may subject the overhead from the low pressure distillation of the azeotropic distillate to fractional distillation in a second distillation stage at a higher pressure than maintained in the low pressure stage which will result in distilling all of the non-aromatic hydrocarbons together with a portion of the azeotrope former, leaving substantially pure azeotrope former as a distillation bottoms which may -be returned to the azeotropic distillation process. The small amount of overhead resulting from this second fractional distillation consisting of a mixture of the azeotrope former and non-aromatic hydrocarbons may then be Washed or extracted as above described to separate the azeotrope former from the hydrocarbons. Thus, by operating the azeotrope former recovery system in the two distillation stages as in the above manner, a hydrocarbon fraction is produced which is relatively free from azeotrope formertin the first distillation stage and an azeotrope former fraction is produced which is relatively free from hydrocarbons in the second distillation stage. Also, in the second distillationv vas'rrsfi'ra ried out at a higher pressure and higher temperature than the first stage distillation. These temperature and pressure conditions may be lower than the temperature-pressure conditions of the primary azeotropic distillation but higher than the `first stage distillation or they may be the same or higher than the primary azetropic distillation.

While I have described the distillation of the azeotropic distillate as being carried out at a low pressure and temperature, I may also effect separation of the azeotrope former from the hydrocarbons by carrying out the distillation at a highva portion of the azeotrope former, leavingl the remaining portion of the azeotrope former free from hydrocarbons as a distillation bottoms. The overhead mixture may then be washed or extracted, as described above, to separate the remaining azeotrope former contained in the overhead from the hydrocarbons. This procedure will result in the employing of a smaller amount of water or solvent than when treating the entire azeotropic distillate or by using the same quantity of water or solvent will result in greater reduction in loss of azeotrope former to the hydrocarbons.

By carrying out the distillation part of the azeotrope former recovery system in two stages as above described', it is only necessary to Wash or extract a relatively small amount of the 4mixture of azeotrope former and non-aromatic hydrocarbons. This is advantageous in that it will require the use of considerably less water or solvent and will result in considerably less loss of azeotrope former to the hydrocarbons than in the case where the entire azeotropic distillate is washed with water.

Other objects, features and advantages of my invention will be apparent to those skilled in the art from the following description of the invention which represents a diagrammatic arrange-` ment of apparatus for carrying out my invention. In the drawing, the hydrocarbon feed to be resolved into its component parts is taken from tank I0 via line I I and is pumped by pump I2 through line I4 controlled by valve I5 into line I6, Azeotrope former is taken from tank I1 via line I8 controlled by valve I9 and is pumped by pump 20 through lines 2I and 22 and valve 23 into line I6 Where it is mixed with the hydrocarbon feed from tank Ill. The mixture of hydrocarbon feed and azeotrope former is passed into fractionating column 24 where the mixture is subjected to fractionation, heat being supplied by 4closed steam coil 25. If desired, the azeotrope former may be introduced directly into the fractionating column at any other point as near the top of the column in which case it will act in part as reux for thev fractionation. In the fractionation column, the4 distillation is controlled so as to distill overhead an azeotrope consisting of the relatively non-aromatic hydrocarbons together with substantially all of the azeotrope former. If desired, the' azeotropic distillation may be carried out either at atmospheric or superatmospheric pressure or under a vacuum. The above overhead mixture is removed from the fractionating column via line 26, controlled by valve 21, condensed in condenser 28 and passed via line 29 into collecting tank 30.

The condensate consisting of theazeotrope former and non-aromatic hydrocarbons is withdrawn from the bottom of the collecting tank 38 lby pump 3l and part thereof may be passed via line 32 controlled by valve 33 to fractionatlng column 24 to serve as reflux for the fractionation. The re'- I covery system as will be described hereinafter.

The bottoms in the fractionating lcolumn 24 consisting of the relatively more aromatic hydrocarbons are withdrawn via line 36 controlled by valve 31 and are pumped by pump 38 through line 39. If the hydrocarbon feed to the azeotropic distillation has been carefully fractionated to produce a, fraction free from aromatic hydrocarbons other than the desired aromatic hydrocarbon, for example, toluene and if the azeotropic distillation has been carried out under such conditions as to remove all of the non-aromatic hydrocarbons and azeotrope former, the bottoms from the fractionating column may be passed directly via lines 36, 39, 40 controlled by valve 4I, 42, 43 controlled by valve 44 and 45 into storage tank 46. However, in

vthe event the charging stock has not been caretain all or substantial amounts of such aromatic hydrocarbons. In the event, it is desired to recover the aromatic hydrocarbon mixture per se, it is passed directly to tank 46 as described above. However, if it is desired to separate the toluene from the remaining aromatic hydrocarbons, the bottoms are passed via lines 36, 39, 40, 42 into line 41 controlled by valve 48 from which it passes through heater 49 and line 50 into fractionating column 5I where the mixture is fractionated to remove the toluene as an overhead product aided by heat from the heater 52. The vaporized toluene is removed from the top of the fractionating column 5I via line 53, condensed in condenser 54 and'passed via line 55 into collecting tank 56. I he condensate may be Withdrawn from the collecting tank by pump 51 and passed into line 58. If desired, part of the condensate may be cycled via line 59 controlled by valve 60 to the fractionating tank 46, The bottoms from the fractionating col-v umn, consisting of xylene or a'mixture of xylene and higher boiling aromatic hydrocarbons, is i withdrawn via line 63 controlled by valve 64 and pumped lby pump 65 and line 66 into storage tank 61.

In the event thebottoms fraction from the fractionating column 24 contains a portion of the azeotrope former, this may be removed by passing the bottom fraction via lines 36, 39 and 68 controlled by valve 69 through heater 10 and line 1I into fractionatin'g column 12 provided with a heater 13 and reflux cooling coil 14 where the azeotrope former may be fractioned and removed via line 15, condensed in condenser 16 and passed via line 11 into collecting tank 18 from which it may be returned to the fractionating column 24 by 4 l carbons obtained m tank s1 may be treated with clay which may be accomplished at a temperature `column 85 provided with heater 86.

be at sub-atmospheric.

`azeotropic column.

of about 230 F. employing 1 to 5 pounds of clay per barrel of the hydrocarbon fraction. If desired, the clay treatment may precede the frac- 1 tionation in fractlonating column 5| in which case the fractionation in 5| may serve either to rerun the clay treated stock and/or to fractionate the lhigh boiling aromatic hydrocarbons from the toluene. In place of clay treatment, the aromatic f `fraction may be cooled and then treated with i 1 to 10 pounds of sulfuric acid per barrel of the hydrocarbons followed by neutralization with clay or caustic alkali.` The acid treatment serves to remove small traces of undesirable unsaturated hydrocarbons which may be detrimental in color `stability and nitration of the toluene.

In order to recover the azeotrope former vfrom the azeotropic distillate, the distillate is passed via line 34 and valve 35 into fractionating As stated above; the distillation in column 85 is carried `out at a lower temperature and a corresponding lower pressure than the azeotropic distillation in column 24. This may be controlled by `means of a vacuum pump or compressorl 81 prodistillation is carried out at atmospheric pressure, the pressure maintained in column 85 vmay If the pressure of the distillation is superatmospheric, either a lower super-atmospheric pressure 'or a 4vacuum may be maintained in the column 85 and if the azeotropic distillation is carried out under vacuum, a higher vacuum must be maintained. in column 85. In the fractionating .column 85, the azeotrope former is distilled and removed via line 88 and compressor or vacuum pump 81. This fraction contains a relatively hydrocarbons. `head contains an unobjectionable amount of hydrocarbons, it may be passed via line 89 controlled by valve 90 into line 22 and thus recycled to the azeotropic distillation. However, the relatively non-aromatic hydrocarbons may be removed from the overhead product by closing `valve 90 and passing the overhead fraction via line 9| controlled by valve 92 through. heater 93 and line 94 into fractionating column 95 provided with heater 96 and reflux control 91 where the distillation is carried at a higher pressure than in column 85 which may be either the same, higher or lower than the pressure of the azeotropic distillation in column 24. The pressure 'in column 95 is controlled by the compressor or vacuum pump 81 and the regulation of valves in the inlet and outlet lines to the fractionatin'g The overhead from fractionating column 95'consisting of a mixture of azeotropic former and relatively non-aromatic hydrocarbons is withdrawn via line 98 and if desired may be recycled to the fractionating column 24 -via line 99 and valve |00 and lines 22 and I6'. Preferably, this overhead is passed via line and valve |02 and treatedv as hereinafter described.

The bulk of the azeotrope former is withdrawn as bottoms from the fractionating column 95 via line |03 controlled by valve |04 and is pumped by pump |05 through linel |06 into azeotrope former storage tank |1. The relatively nonaromatic hydrocarbons are withdrawn as bottoms from the fractionating column 85 via line |01 controlled by valve |08 and are pumped by pump |09 through line ||0 into storage tank small amount of the relatively non-aromatic In such cases where the over- The overhead from column 95 passed into line |0| and consisting of a, mixture of the azeotrope former and the relatively non-aromatic hydrocarbons is'p'assed via line ||2 into the bottom of washer ||5 lprovided with packing material, such as broken tile H6, for effecting intimate countercurrent-contact with water which is introduce'd-into the washer from tank ||1 via line |8 controlled by valve I I9 and pumped by pump |20 through line |2| into top of the washer:

The Contact of the distillate of azeotrope former of the non-aromatic hydrocarbons and a lower phase consisting of azeotrope former and water. The upper phase is withdrawn via linef |22 and valve |23 andv is pumped by pump |24 through lines |25 and ||0 into storage tank The washing operation is preferably carried out at an elevated temperature of approximately 300 F. under superatmospheric pressure in those cases where the azeotrope former is diicultly soluble v in water. While I have described the use of water to extract the azeotrope former from the azeotropic distillate in washer I5, it will be observed that the separation of the azeotrope former may be accomplished byextraction with a solvent whichis selective for the azeotrope former, such as for example, tetra ethylene glycol.

The lower phase is withdrawn via" line |26 controlled by valve |21 and is pumped by pump |28 through line |29 and heater |30 into fractionating column |3| provided with heater |32 where the azeotrope former is-removed as an overhead 'Vapor via line |33 controlled by valve |34, condensed in condenser |35 and returned via lines |36 and |06 to storage tank |1. The undistilled water or solvent substantially free from azeotrope former is removed via line |31 controlled by valve |38 and is pumped by pump |39 through line |40 to storage tank l| |1. It will be understood that when the water or s'olvent has a lower boiling point than the azeotrope former, the distillation in column |3| will distill l the water or solvent as an overhead fraction leaving the azeotrope former as a bottoms fraction in which case the overhead is condensed and passed to storage tank I1 and the bottoms will be -passed to storage tank ||1.

Instead of passing the overhead from column consisting of the mixture of azeotrope former and hydrocarbons to the higher pressure column in'order to separate a mixture of the azeotrope former and hydrocarbons from a bottoms fraction and pressure maintained in column 24 for the azeotropic distillation. In such case, the overhead product from column 85 will consist of substantially all of the hydrocarbons together with a portion of the azeotrope former while the bottoms in the column will consist of the remainder of the azeotrope former substantially free from `temperature than in column 85 to produce an overhead mixture of azeotrope former and'hydrocarbons, leavingl the remaining hydrocarbons as a distillation bottoms in the column. The overl head mixture lfrom column 95 may then be passed to the washer H as above described. The bottoms from 'column 85 consisting of substantially pure aze'otropeformer may then berecycled to the column 24 or passed to storage. The bottoms from column 95 consisting of substantially'pure hydrocarbons may be passed to storage.

Preferably, when vthe column 85 is operated Iunder a lower pressure and temperature than the azeotropic distillation in column 24, the overhead consisting of all of the azeotrope former with Aa portion of the hydrocarbons is passed to the higher pressure distillation column 95 in order to remove Yas much of the azeotrope former as is possibleas a -fraction free from hydrocarbons, thereby reducing the amount of. azeotrope former ketone wasthen countercurrently. contacted with yabout 500 liters of water at a temperature of to be recovered by Washing and thus reducing 'V the amount of water or solvent required to eil'ect the separation of the azeotrope former from the hydrocarbons. at a higher pressure than column 24, a substantial portion of the azeotrope former will have been removed as a bottoms fraction in column 85 and hence the overhead therefrom may be passed directly to the washer I I5. However, it is within the/ scope of my invention to refractionate this overhead at a reduced pressure in column 95 and to wash or extract the overhead from this column.v 1

The following is a specic example of, my invention as applied tospecichydrocarbon fractions employing a speclc azeotrope former.

In .one experiment, approximately 100 liters of a carefully fractionated hydrocarbon fraction having a boiling range of about 200 to 240 F. consisting of approximately 20% by volume of toluene and the remainder, parailin and naphthene hydrocarbons, was fractionally distilled in the presence of about 160 liters of a mixture of 144 liters of methyl ethyl ketone and 16 liters of water at an overhead temperature of 170 F. and at atmospheric pressure. The distilled overhead was condensed at a temprature of 70 F. and allowed to settle in a settling chamber which caused the condensate to stratify into two layers which were separated from each other. The upper layer consisted of 80 liters of non-aromatic'hydrocarbons anc/L 142 liters 'of methyl ethyl ketone containing a` trace of water while the lower layer consisted of about 16 liters of water and.2 liters of methyl ethyl ketone. The distillation bottoms consisted of about 20 liters of toluene of about 100% purity.

The upper layer consisting of the 142 liters of methyl ethyl ketone and 80 liters of hydrocarbonsv was then distilledl at a temperature of about 250 When the column 85 is operated F. under a pressure of about three atmospheres which resulted in producingan overhead consisting of about 80 liters of hydrocarbons and 80 liters of methyl ethyl ketone. The bottoms from this distillation consisted of abouty 62 liters of methyl ethyl keton'esubstantially free from hydrocarbons which was passed to storage.

The overhead from this last named .distillation under pressure Vand consisting of 'about- 80 liters each of hydrocarbons and methyl ethyl about 300" F. under a pressure of about two atmospheres which resultedin producing as separate fractionsabout 580 liters of a dilute methyl ethyl ketone and about liters ofhydrocarbons which were substantially free from methyl ethyl ketone.' The dilute methyl ethylv ketone was mixed with the 16 liters of water and 2 liters of methyl ethyl ketone previously separated and thisfmixture was then distilled at a temperature of about 164 F. which resultedI in distilling overhead an-azeotrope of methylethyl ketone and water consisting of about 82 liters of methyl ethyl ketone and 9 liters of water which was employed for Vthe azeotropic distillation of' further amounts of the mixture of aromatic and non-aromatic hydrocarbons. l

The. foregoing description of my invention is not to be taken as limiting my invention but only as illustrative thereof since many variations mayv be made by thoserskilled inthe art without departing from the scope of the following claim.

I claim: A process for the treatment of a complex normally liquid hydrocarbon fraction having avboilfraction together with said azeotrope former,

thereby leaving at least one of the remaining hydrocarbons contained in said complex hydrocarbon fraction in the residue, distilling said azeotropic distillate at a dilerent temperature and pressure than the temperature and pressure of distillation of said complex hydrocarbon fraction -to produce a fraction relatively free from one `of the components contained in said azeotropic distillate and a fraction containing a mixture of hydrocarbons and azeotrope former, extracting said last named mixture with an agent adapted to disf solve one of said components and substantially none of the other components contained in said mixture and separating a solution of said agent and dissolved component from the undissolved component.

`C'irlEIORGrE R. LAKE.

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