US2236723A - Process for the separation of azeotropic mixtures - Google Patents

Description

Apr w41, l

PROCESS FOR THE SERA D. L. YABRoFF E-rm. 2.236.723

RATION OF 'AZEOTR'IC MIXTUES Filed March 4, 1939 Mercapmm Mercpmnv l tion; they must be inert lutions containing condensate water, even Patented pr. 1, 1941 v UNITED STATES PATENT OFFICE A PROCEYSS FOR 0F AZEO- l TROPIC.

David Louis Yabroft, Berkeley,

Albany, Calif., assignors to Company, San Francisco,

of Delaware and Ellis R. White,- Shell Development Calif., a corporation Application March 4, 1939, Serial No. 259,760 '8 Claims. (Cl. 260-609) This invention deals with the separation of three component azeotropic mixtures consisting essentially of mercaptans, alkyl phenols and Water. Such mixtures are obtained when steam stripping aqueous solutions of caustic alkali containing substantial amounts of both mercaptides and alkyl phenolates.

It is known to extract mercaptans from sour distillates containing same by treating such distillates Wi-th strong aqueous solutions of a strong base, preferably an alkali metal hydroxide, in which is dissolved a substantial amount of a solubility promoter or solutizer for the mercaptans. The spent solutized caustic alkali solutions resulting from such extractions containing mercap- 1 tides are then regenerated, preferably by simple l steam stripping, whereby the mercaptans are liberated and taken overhead together with the stripping steam.`

solutizers 4to be suitable in the extraction must have the following properties: 'Ihey must beV at least partially soluble in the aqueous base soluto the base even under conditions of steam stripping; they mustl boil higher than water, be insoluble in hydrocarbon distlllates in the presence of 15% or less Water and be good solvents for the mercaptans. Among the several compounds which are known to be suitable as solutizers, alkyl phenolates are particularly useful, especially in admixture With 3' other solutizers such as alkali metal salts of hydrocarbon carboxylic acids, e. g., fatty acids, having -from 1 to 8 carbon atoms, diamines or alkanolamines, the alkylene radicals of which ,contain 2 or 3 carbon atoms; glycols, amino glycols or diamino alcohols o1' 3 to 5 carbon atoms; dihydroxy, diamine, or aminol hydroxy diethyl ethers, thioethers ormino others, etc. Reasons Afor the exceptional suitability of alkyl phenolates.

are explained in detail in our copending applica- 4 tion Serial No. 255,684, led February 10, 1939.

However, alkyl phenolates have the disadvantage of being partially decomposed during regeneration by steam stripping of spent alkaline sothem, taken overhead, together with the stripping steam and the mercaptans. We have found that l the mixtures of steam, mercaptans and alkyl phenols thus produced lform azeotropes which rcordinary reuxing with 50 sist being separated by though alkyl phenols in the mixture arenormally much higher boiling than water or the mercaptans. the boiling temperature of phenol (mono hydroxy benzene), the lowest boiling hydroxy aromatic compound. being 5 182 C., and alkyl phenols except .ortho-cresci boiling above 200 C.; while practically all mercaptans removed from hydrocarbon oils by solu- .tized aqueous caustic alkali solutions in substantial amounts boil below about 180 or 200 C.

The alkyl phenols coming overhead in this -manner are effectively lost, being discarded together with the mercaptans, unless special steps are taken -to recover them. ,Since losses of alkyl phenols from 'this source materially raise the cost of the mercaptan extraction with solutized alkaline solutions, it is important that these alkyl phenols be recovered.v One method in which the problem was solved has been described in our copending application Serial No, 235.572 led 0ctober 18, 1938. According to this method, a'

small amount of a caustic alkali, i. e., from .5 to 5 mol equivalent of the alkyl phenols contained in the overhead stream, may be added to the latter to produce an aqueous layer containing alkyl phenolatesand an oil layer consisting predominantly of mercaptans.

Now we have discovered a simple manner, not requiring the addition of an alkaline solution to the overhead mixture, in which to separate the azeotropes consisting essentially ot steam, alkyl phenols and mercaptans. Our method comprises the steps of cooling zthe azeotropic mixture to condense the steam and to form hot water, simultaneously .to condense the alkyl phenolsland' a portion ot the mercaptans. The condensed mixture is then allowed to stand, whereupon three iiuid layers are formed, two liquid layers and a vapor-layer. The lower liquid layer consists esfree alkyl phenols being 45 sentially of water. The upper liquid layer being an oily layer consists essentially of mercaptans and alkyl phenols; and the vaporlayer normally consists essentially of mercaptans boiling below .the boiling point of water and also contains other non-condensible substanoes which, for instance, may 1 have been introduced together with the steam. 'I'he lthree layers are 'withdrawn -separately, and the middle oily layer is redistilled,

preferably in the absence of added live steam.

Fractions/tion may be so regulated to produce an overhead product substantially free from alkyl phenols'havin` an end boiling point below the boiling temperature of the lowest 'boiling alkyl phenols present in the mixture substantially all of the mercaptans boiling below the alkyl phenols, as well as small amounts of water which may have been dissolved inthe mixture. The residual bottom fraction consisting predominantly of alkyl phenols may contain a small amount of relatively high boiling'mer'capl' and containing 2 4 tans. If desired, this bottom fraction may be further puriiied by oxidizing the mercaptans to disulfldes, for example, by air blowing at temperatures between about 130 to 150 C., and thereafter redistilling the oxidized product, preferably under vacuum, to take the alkyl phenols overhead ond distillation residue.

Alkyl phenols so produced' and purified are substantially free from mercaptans, and are excellent gum inhibitors for gasolines. having a greater inhibiting power than the average (alkyl phenols recoverable from petroleum oils,

In the attached drawing is shown an illustrative flow diagram of one form of our process which may be used to carry out our invention.

An alkaline aqueous solution containing substantial amounts of alkyl phenolates and mercaptides such as may result from the sweetening of sour gasolines by extracting same with a solutized aqueous caustic alkali solution, is introduced preferably while hot, e. g., at a temperature near its boiling point, through line I into the upper part of steam stripper 2. The stripper may be of conventional design, and may contain baiiles, bubble plates or a packing, etc. The hot solution ows downwardly through the stripper,

in countercurrent to steam introduced through line 3. Steam from an outside source not shown may be used, or it may be produced by boiling the stripped solution emerging from the stripper through line` l, in reboiler 5.

In the course of its upward path through the stripper and the counteriiowing aqueous solution, the steam llberates a considerable portion of the mercaptans and in the solution in the form of their salts. The resulting vaporous mixture of steam, mercaptans and alkyl phenols leaves The condensing operation is advantageously controlled so that the condensate emerges from the condenser while hot, preferably as close to the boiling temperature of the water as is practical. The hot condensate is accumulated in tank 8 wherein 3 layers are formed: A lower liquid layer consisting essentially of water; an upper oily liquid layer consisting essentially of alkyl -phenols and mercaptans boiling above the condensation temperature; and a vapor layer comprising mercaptans boiling below the condensation ternperature.

Maintenance of the relatively high temperature in ta'nk 8 is desirable to prevent dissolution of lighter mercaptans, particularly methyl mercaptan, in water, these lighter mercaptans being somewhat soluble in water. VWater containing dissolved light mercaptans useless for most purposes. On the other hand, if the water is substantially free from mercaptans it may, for example, be returned, preferably while hot, by means of pump S through line I0, into line I forthe purpose of diluting the incoming alkaline solution in orderto facilitate its steam stripping in accordance withV the principles disclosed in our copending application Serial No. 174,512, led November 15, 1937. If

desired, stripping may be conducted under considerable superatmospheric presure.

The vaporous mercaptans are withdrawn from tank 8 through line I I and may be utilized in any manner desired.

The oily liquid middle layer in tank 8 is taken through line I2 and heater I3 to be fractionally while leaving the disuldes in the secalkyl phenols contained Y the stripper through overhead line B and is cooled in condenser 1 to effect condensation of thesteam to water.

is obnoxious and is distilled in fractionator I4. 1n this distillation, it is usually possible to produce mercaptan fractions substantially free from alkyl phenols boiling up to 200 C. and even higher, by reason of the fact that the alkyl phenols which are taken overhead by the steam in the steam stripping of the strongly alkaline alkyl -phenolate solutions are those which are relatively weakly acidic containing one or several alkyl radicals and having boiling temperatures above about 200 C.

The distillation is carried out in the absence of added steam or water to avoid as much as possible recurrence of the three component azeotropic mixtures, and is so controlled to produce an overhead fraction, removed through line I5, which fraction consists essentially of mercaptans and a small amount of water and which is substantially free from alkyl phenols. The distillation residue comprises the alkyl phenols and high boiling mercaptans tillation range of the alkyl phenols, mercaptans, however, normally being present in small amounts only. This residue fraction emerges from fractionator I4 through line IB and may go to storage through line I1 or through line Il to tank I9. Y

In tank I9, the alkyl phenols may be air blown with air from line 20 to oxidize the residual mercaptans to disulfldes, preferably at a temperature between about to 150 C. This temperature may be controlled in any desired manner by means not shown. Waste air is vented through vent line 2l.

The air blown alkyl phenols. while still hot, are then released through release valve 22 in line 23 into vacuum fractionating tower 2, where alkyl phenols are vaporized and are condensed in condenser 25 situated in line 2B. Heat necessary for the distillation may be provided by heating coil 21. Disultldes are withdrawn through bottom line 28.

Condensed alkyl phenols substantially free from mercaptans are accumulated in receiver tank 29, line il connecting this tank with source of vacuum not shown.

While in the foregoing we have described a simplified now diagram of our process, it is understood that other modifications, are possible. In particular is the installation of additional pumps, heaters, coolers, condensers, heat exchangers, by-passes, tanks, etc., wherever necessary, considered within the skill of the average designer for such equipment.

We claim as our invention:

1. In the process of separating an azeotropic vapor mixture of steam, mercaptans and alkyl phenols, said mercaptans boiling above and below the boilingpoint'of watenthe steps comprising cooling said mixture to condense said steam and to produce hot water, simultaneously to condense said alkyl phenols and a portion of said mercaptans. thereby producing three uid layers, a lower liquid layer consisting essentially of hot water free from dissolved light mercaptans normally soluble in Water, an upper liquid oily layer comprising mercaptans and alkyl phenols, and a vaporous layer comprising uncondensed mercaptans, separately withdrawing the three layers, and fractionally distilling the oily layer substantially in theabsence of added steam or water to produce. an overhead mercaptan fraction having an end boiling point below the boiling temperature of the alkyl the mixture and a residual alkyl phenol fraction.

2. The process of claim l in which the merboiling within the disphenols present in captan fraction and fractionally distilling has an' end honing point below 200 C. A f

3. A process for removing mercaptans from .aqueous alkaline solutions containing mercaptides and alkyl phenolates comprising the steps of stripping said aqueous solution with live steam, thereby .producing a vaporous a'zeotropic mixture of steam, mercaptans and alkyl phenols, separating said vaporous mixture from the aqueous solution, `,cooling the separated mixture to condense said steam and to produce hot water,

simultaneously to condense said alkyl phenols and a portion of the mercaptans, thereby producing three iiuid layers, a lower liquid layer consisting essentially of hot water free from dissolved light mercaptans normally soluble in water, an upper liquid oily layer comprising mercaptans and alkyl phenols, and a vaporous layer comprising uncondensed mercaptans, separately withdrawing the three layers and fractionally distilling the oily layer in the absence ofl added steam to produce an overhead mercaptan fraction having 'an end boiling point below the boilsteam to produce a vapor consisting. predominantly of uncondensed mercaptans and two liquid layers, a lower layer consisting essentially of hot water free from dissolved light mercaptans normally soluble in water and an upper oily layer comprising mercaptans and alkyl phenols, separately withdrawing the two liquid layers, fractionally distilling the oily layer in the absence of added steam to produc-e an overhead mercaptan fraction having an end boiling point being temperature of the alkyl phenols present in said mixture and a' residual alkyl phenol fraction.

4. 'A process for removing mercaptans from aqueous alkaline solutions containing mercaptides and alkyl phenolates comprising the steps of stripping said aqueous solution with live steam, thereby lproducing a vaporous azeotropic mixture of steam, mercaptans and alkyl phenols, separating said vaporous mixture from thejaqueous solution, cooling the separated mixture to condense said steam and to produce hot water,

simultaneously to condense said alkyl phenols and a portion of the mercaptans, thereby producing three uid layers, a lower liquid layer consisting essentially of hot water free from dissolvedl light mercaptans normally soluble in water, an upper liquid oily layer comprising mercaptans and alkyl phenols, and a vaporous layer comprising uncondensed mercaptans, separately withdrawing the three layers; returning the condensed water to said aqueous solution, the oily layer in the absence of addedl steam to produce an overhead mercaptan fraction having an end .boiling point below the boiling temperature of the alkyl phenol Imation.

water to produce an overhead mercaptan frac-v tion having an end boiling point below the boil-v low the boiling temperature ofthe alkyl vphenols present in the mixtureand a residual alkyl phenol fraction.

6.- In thel process of separating an azeotropic vapor mixture of steam, mercaptans and alkyl phenols, said mercaptans boiling above and below water, the steps comprising cooling said mixture to condense said steam and to produce hot water of suiciently high temperature to prevent dissolution of lighter mercaptans, simultaneously to condense said alkyl phenols and a portion of said mercaptans, thereby producing three uid layers, a lower liquid layer consisting essentially of hot water free from dissolved light mercaptans normally soluble n water, an upper liquid oily layer comprising mercaptans and alkyl phenols, and a vaporous layer comprising uncondensed mercaptans, separately withdrawing the three layers, and fractionallydistilling the oily layer substantially in the absence of added steam or ingtemperature of the alkyl phenols present in the mixture and a residual alkyl phenol fraction.

ADAVID LOUIS YABROFF.

ELLIS R.. WHITE.

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