US2270491A - Process for regenerating caustic alkali solutions - Google Patents

Description

Jam. 20, 194-2- D. L. YABROFF ETAL 2,270,491

' PROCESS OF REGENERATING CAUSTIC ALKALI soLUTIoNs Filed Feb. 15, 1959 Solufion N N v N 4" 5" I 2 F I 3 T'feafeti Minute Fresh Solvent Z] Hewl' F fl-"g Span! solvent Extractor L. Phen ls Fig. 2.

lnven'fi'ors: David Louis Yubr'oh Ell is l2. Whi'l'e Patented Jan. 20, 1942 PROCESS FOR REGENERATING CAUSTIC ALKALI SOLUTIONS David Louis Yabrofl, Berkeley, and Ellis R. White, Albany, Calif., assignors to Shell Development Company, San Francisco, Calilt, a corporation of Delaware Application February 13, 1939, Serial No. 256,180

6 Claims.

This invention relates to the removal of weakly acidic organic compounds from aqueous caustic alkali solutions containing same and substantial amounts of solubility promoters for said compounds, and more particularly relates to a process oils or may be produced by cracking, e. g. mercaptans, phenols, thiophenols, alkyl phenols, etc. from aqueous caustic alkali solutions, particularly those caustic alkali solutions which have been used for the extraction of the acidic compounds from hydrocarbon oils and which contain substantial amounts of solubility promoters for said acidic compounds.

As described in our U. S. Patents Nos. 2,152,720; 2,152,166; 2,149,379; 2,160,632; 2,164,851; 2,152,- 722, we have found that certain substances, when added to aqueous caustic alkali treating solutions in substantial amounts, greatly enhance the solubility of organic acidic substances therein.

It has been shown that solubility promoters to be suitable must combine the following properties: high specific solubility promoting power, 1. e., ability to raise the solvent power of the aqueous caustic alkali for the free organic acids by a relatively large amount for a unit weight ad- 2 examples being propylene glycol, butylene glycols, alkyl glycerines in which the alkyl radical has from 2 to 4 carbon atoms, mono alkyl glycerine ethers in which the alkyl radical has 1 or 2 carbon atoms, di-- and tri-ethylene glycol, beta amino beta hydroxy diethyl ether, diand tri-ethylene glycol mono alkyl ethers in which the alkyl radical has'from 1 to 3 carbon atoms; diamino alcohols of 3 to 5 carbon atoms, etc. Very effective are also the alkali metal salts of alkyl phenols and of certain carboxylic acids, such as fatty acids having 2 to 6 carbon atoms, notably potassium isobutyrate, or of amino or hydroxy fatty acids having 3 to 7 carbon atoms, especially potassium alpha hydroxy n-butyrate; or of phenylacetic acids, etc.

The amounts of solubility promoters normally employed in solution of the aqueous caustic alkali vary between the approximate limits of to 85%, and preferably to 75% in the case of neutral and basic compounds. In the case of salts, the aqueous caustic alkali is preferably sub stantially saturated therewith, or at least nearly so.

Various caustic alkalis may be used. While alkali metal hydroxides are preferred, ammonia, quaternary ammonia bases, alkali metal carbonates, etc., may also be suitable.

The efliciency of the extraction of the organic acids from their solutions in organic water-insoluble soluble solvents by means of aqueous caustic alkalis has been expressed by the factor K. in

which concentration of salts of the organic acid in aqueous phase concentration of free organic acids in organic liquid phase dition of the promoter to the aqueous causticalkali; high solubility of the promoter in the aqueous caustic alkali; and very low solubility of the promoter in the hydrocarbon or other organic water-insoluble liquid containing the organic acids. Moreover, the promoter must be chemically and physically inert to the action of caustic alkalis even at the elevated temperatures of steam stripping. r l Among the many substances which combine the above properties in varying degrees, the following were found most suitable: mono-, diand tri-amino or hydroxy alkyl amines in which the alkyl groups contain 2 or 3 carbon atoms; diamino propanol; polyhydric alcohols or derivatives thereof having at least 1 carbon atom in excess of polar radicals, a-ratio of carbon atoms to hydroxyl radicals of at least 1 and a ratio of carbon atoms to polar radicals not in excessor 55 by first diluting the solutions with water and It may, therefore, be seen that when it is desired to do the opposite, i. e., to remove weak organicacids from their caustic alkali aqueous solutions by scrubbing with an organic solvent for the acids, the most efficient removal may occur when the caustic alkali solution is diluted, i. e., when the value of K is low.

Our invention thus comprises removing weak organic acids, particularly those having dissociation constants below about 10- from aqueous solutions of caustic alkali containing said acids and substantial amounts of solubility promoter,

thereafter scrubbing the resulting diluted solutions with a solvent for organic acids.

Solvents to be suitable should be substantially immiscible with the aqueous caustic alkali solutions, and should be chemically inert to the action of the caustic alkali under the conditions of the extraction.

Suitable solvents are, for example, hydrocarbon distillates such as gasoline, kerosene; substantially pure liquid hydrocarbons as pentane, hexane, benzene, toluene, xylene; chlorinated hydrocarbons as ethylene dichloride, chlorpropane; higher alcohols as octyl alcohols; ethers as diisopropyl ether, dibutyl ethers, etc. While many polar solvents such as diisopropyl ether, etc., may be better solvents for the organic acids than hydrocarbons and may even dissolve some salts of the organic acids, we often prefer to use the latter because of certain advantages which are disclosed later.

The above-mentioned effect of dilution on the K value is more fully explained in connection with Fig. 1 of the drawing. In Fig. 1' a co- )rdinate system is shown in which K is plotted m the ordinate against the normalities of caustic 0d8 solutions of various degrees of dilution, when :xtracting normal butyl mercaptan therewith- :rom a gasoline. Three cases may be distinmished as indicated by curves l, 2 and 3, re- :pectively. v

Curve I shows the change of K for the normal iutyl mercaptan with concentration of the cansic soda solution in the absence of a solubility )romoter. It begins at about zero value for K, ises approximately as a straight line over a hort distance and then drops'ofi with increasng normality of the caustic soda solution; This lrop in the value of K in spite of increasing alkainity of the aqueous solution beyond a certain voint is due to the salting out of the free meraptan by the dissolved caustic soda in the aqueus solution, the free mercaptan being in equilibium with the dissolved mercaptide.

Curve 2 represents the change of K with hanging concentration or dilution of a caustic oda solution containing two parts by weight of odium isobutyrate foreach part of caustic soda. )bviously, the ratio of caustic soda to isobutyrate emains the same over the entire range of conentrations up to the point of saturation of one f the dissolved components. This curve begins ke curve i at about zero value for K, but instead f dropping back, turns strongly upward. In ther words, the presence of the isobutyrate comletely overcomes the salting out effect, and with icreasing alkalinity of the solution and a proortionate increase in the isobutyrate concentraon, K increases at a greater rate than is proortional to the increase of the normality of the rustic soda solution.

Curve 3 represents the boundary case in which 1e caustic soda solution contains the minimum mount of solubility promoter necessary to overime the salting out effect and is therefore es- :ntially a straight line.

Now when attempting to improve the removal r scrubbing of weak organic. acids from their lution in caustic alkali by the expedient of dition, another factor besides the value of K ust be taken into consideration, namely, the

diluting it with an equal volume of water, no increased removal efficiency of the organic acids will result unless the K value of these acids is reduced to less than half of its former value.

Referring again to curve I, it will be seen that in the low normality range below about 1N, dilution has the effect of increasing the volume and lowering the K value in approximate inverse proportion. In the range of higher normality, however, the change in K is less than inversely proportional to the increase in volume,.the latter resulting, within a certain range, in an increase in K. For example, K for a 3N caustic soda solution in the absence of a solubility promoter is 7.2, as indicated by point A1. If now this caustic solution is diluted with an equal volume of water, the volume of the solution is doubled, but the K is reduced only from 7.2 to 6.5, the latter point being indicated by B1. This means that the scrubbing efliciency on the basis of equal volumes of caustic soda solution is improved by the factor but on the basis of actual volumes the net efficiency is'lowered by the factor 0.55. If we now consider points A: and Be on curve 2, it will be seen that K is lowered from 25 to 9.2 upon diluting the caustic soda solution with an equal volume of water. This will result in a net improvement of the scrubbing efficiency based on actual volumes of The above comparison clearly shows that dilution for the'purpose of removing weakly acidic organic substances from alkaline solutions of creased volume of the aqueous solution. In

their salts by scrubbing with an organic solvent for the acids is beneficial for all solutions which contain amounts of solubility promoters at least suflicient to compensate for the salting out of the free acids from their aqueous solutions by the caustic alkali.

A further increase in removal efliciency may be obtained by raising the temperature at which the acidic substances are extracted from the caustic aqueous alkali solution. The temperature of the removal, therefore, is preferably as high as practical, and. is limited only by the boiling temperature of the solutions and/or solvents used in this treatment. Superatmospheric pressure may be employed so that the extraction may be carried out at a temperature higher than the normal boiling temperatures of any of the solutions. Preferably the temperature is over C.

To give a specific example of the way in which the present invention can be utilized, reference is had to our copending patent application Serial No. 174,512, filed November 15, 1937. In that application it was shown that in extracting mercaptans from hydrocarbon distillates with caustic alkali solutions containing solubility promoters for mercaptans, the spent caustic alkali solution may be regenerated by steam stripping most effectively if the solution is diluted with water before being steamed. However, in that process it is frequently desirable to extract hydrocarbon solutions, which contain alkyl phenols as well as mercaptans. Since alkyl phenols are set free only to a small degree by steaming, it has been found that alkyl phenols remain in the caustic alkali treating solution, and build up upon recirculation to an undesirable degree, an exthrough lines 22 and 6 to the stripping column S cess of alkyl phenols causing the treating solution to become excessively viscous. However, by combining the two inventions, it has been found that hydrocarbons containing alkyl phenols can be treated with caustic alkali solutions, which can be regenerated with steam followed by a solvent extraction process, so that alkyl phenols do not unduly build up in the treating solutions.

The above example is illustrated in Fig. 2, reppresenting a flow diagram of a regenerative extraction process for weak organic acids from bydrocarbon distillates, which diagram includes our invention of scrubbing the spent caustic alkali solution with a solvent-for the acids, preceded by dilution with water.

A hydrocarbon distillate, which may contain both mercaptans and alkyl phenols, is introduced near the bottom of extractor E1 through line I, E1 being of conventional design adapted to the contacting of two substantially immiscible liquids and preferably containing packing or other means of effecting intimate contact between the two liquids. A caustic alkali solution containing suflicient solubility promoter at least to compensate for the salting out of mercaptans and alkyl phenols by the dissolved caustic alkali is introduced near the top of extractor E1 through line 2. Treated hydrocarbon distillate being at least partially freed of mercaptans and alkyl phenols,

is removed from the top of E1 through line 3. The foul caustic alkali solution, which now contains salts of mercaptans and alkyl phenols, is removed from E1 through line 4. Water of dilution may be added through line 5, and the resulting diluted solution is introduced into a column S, preferably under superatmospheric pressure. Steam is admitted into-the column S through line 6, either from an outside source not shown or from within the process as will be explained later, and the mercaptans, along with the steam and a small amount of the alkyl phenols are taken overhead through line I, are condensed by condenser 8 and introduced into separator 9. In this separator, two layers are formed, an upper oily layer containing the mercaptans and those alkyl phenols which were taken overhead, and a lower aqueous layer. The upper layer is withdrawn through line Ill and the lower layer may be reintroduced as water of dilution into the foul caustic alkali solution through line 5, or ,may be discarded. The caustic alkali solution, which is now substantially free of mercaptans but which may be diluted by the condensed steam used for steaming ofl the mercaptans is removed from F through line I I. If it is undiluted or insufficiently diluted, water of dilution is added through line I: and the diluted solution is introduced into scrubber-extractor E2 near its top, E2 being similar in design to E1. A solvent for alkyl phenols, which may or may not be the same distillate which was treated in extractor E1, is introduced through line l3 near the bottom of the scrubberextractor E2 to scrub out as much as possible the alkyl phenols and other remaining weak acids from the caustic soda solution. Spent solvent which contains a substantial portion of the alkyl phenols is removed from the top of the extractor E: through line H. The scrubbed dilute caustic alkali solution, now having substantially reduced contents of both the mercaptans and alkyl phenols, is withdrawn through line l5, and is reconcentrated in boiler- IE to its original strength, to be recirculated through line 2 into extractor E1. Steam produced in the boiler may bewithdrawn through line 2| or may be returned to be used in the stripping of the mercaptans. Since the regenerated caustic alkali solution is hot as it leaves boiler I6, it can advantageously be placed in heat exchanging relationship with the distillate entering extractor E2 by means of heat exchanger I'I, since, as pointed out above, increasing the temperature in the scrubbing step lowers the value of K which governs the efficiency of this step. Alkyl phenols being valuable'as gum inhibitors for the distillate, the distillate originally containing the 'alkyl phenols may be used advantageously for scrubbing the dilute caustic alkali solution. However, if desired, a different solvent may be used, provision beingmade for its introduction through line l8 and for the withdrawal of the distillate treated in E1 through line l9. As an alternative of the above process, if it is not desired to include a steam stripping operation, the spent caustic alkali solution from extractor E1 may be by-passed through lines 20 and Il directly to the top of the scrubber-extractor E2.

While in the foregoing we have described a preferred form of our process, it shall be understood that many modiflcations are within the scope of our invention. In particular, it shall be understood that the addition of pumps, coolers, heat exchangers, heaters, by-passes, etc., not shown in the flow diagram, is within the skill of the average designer for such equipment.

The following example further illustrates our invention:

A sample of an aqueous 6N KOH solution containing 135 grams per liter potassium isobutyrate and 135 grams per liter alkyl phenolate was scrubbed with 10 volumes of a gasoline. The amount of alkyl phenols dissolved in the gasoline after the scrubbing treatment was estimated by measuring the oxygen bomb induction period of the resulting gasoline, which period was 4.0 hours.

Another sample of the same aqueous KOH solution was then diluted with an equal amount of water, and the resulting dilute solution was scrubbed with 5 volumes of the same gasoline,

stants below 10-, are extracted from hydrocarbon oils containing them with an aqueous solution of caustic alkali containing dissolved therein alkali metal saltsof fatty acids having 2 tot carbon atoms in an amount 'sufiicient at least to compensate for salting out of said weak acids from the aqueous caustic alkali solution, to produce a treated oil and a fat aqueous solution, the latter containing absorbed said weakly acidic organic substances, and wherein said fat solution is regenerated by scrubbing to produce a regenerated aqueous solution containing substantially all of said solubility promoter but having a reduced content of said organic acids, the improvement comprising diluting said fat solution with water and thereafter scrubbing the diluted solution with an organic solvent for said weakly acidic substances, which solvent is insoluble in and inert to said caustic alkali solutions under conditions of scrubbing, thereby removing from the fat solution weakly acidic substances having dissociation constants below 10" without removing fatty acids.

ing point of either the solvent or the caustic 2. The process of clairn 1 in Which the scrubbing is carried out at a temperature above60 F'.

' but below the boiling temperatures of the solvent and the caustic alkalisolution.

3. The process of claim 1 in which the extraction is carried out under superatmospheric pressure at temperatures higher than the normal boilalkali solution.

4. The process of claim 1 in which the solvent

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