US2228295A - Regeneration of spent aqueous solutions of caustic alkali containing mercaptides - Google Patents

Description

Jan. 14, 1941. D. YABROFF 2,228,295

REGENERATION OF SPENT AQUEOUS SOLUTIONS OF CAUSTIC ALKALI CONTAINING MERCAPTIDES Filed Nov. 25, 1959 fiaso/lhe Lager 5k/mmed Off Wafer fiolufion /nven1'or.- David L. Viz? By his Affarneg Patented Jan. 14, 1941 PATENT OFFICE REGENERATION F SPENT AQUEOUS SO- LUTIONS 0F CAUSTIC ALKALI CONTAIN- ING MERCAPTIDES David Louis Yabroll', Berkeley, Calif., assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application November 25, 1939, Serial No. 306,148

6 Claims. (Cl. 260-609) This invention relates to the regeneration of spent aqueous solutions of caustic alkali containing absorbed mercaptides and solutizers for mercaptans, and more particularly is concerned with a simple pretreating step preceding the regeneration proper.

It is known that sour hydrocarbon distillates and, more particularly, gasoline distillates can be sweetened by extracting mercaptans contained 10 therein with aqueous solutions of alkali metal hydroxides containing solutizers. In this process, the 'role of the solutizer is, to increase the solvent power of the aqueous solution for free mercaptans. solutizers, therefore, are substances which are readily soluble in aqueous solutions of caustic alkali, are substantially insoluble in the hydrocarbon oil to be extracted, are good solvents for free mercaptans, are inert to the caustic alkali even at the elevated temperatures of steam strip- 20. ping, and preferably have boiling'temperatures substantially above that of water. To be effective, the solutizer must be present in an amount sumcient to materially raise the solvent power of the aqueous solution for mercaptans, e. g., normally inamounts in excess of about 15%.

Among the many substances having solutizing properties, the following are the most useful: Amino and hydroxy alkyl amines in which the alkyl radicals have 2 to 3 carbon atoms; glycols, amino glycols and diamino alcohols of 3 to 5 carbon atoms; diamino, dihydroxy or amino hydroxy dialkyl ethers or thioethers in which the alkyl radicals have 2 to 3 carbon atoms; alkali metal, and particularly potassium salts of fatty acids having from 3 to 5 carbon atoms, or of hydroxy or amino fatty acids having from 4 to 7 carbon atoms, or of phenyl acetic or hydroxy or amino phenyl acetic acids, or of alkyl phenols; or mixtures of the above. V

More detailed information about the various solutizers and the amounts in which they are effective has been disclosed in U. S. Patents 2,152,720, 2,152,723, 2,152,166, 2,149,379, 2,149,380, 2,156,577, 2,152,722, 2,164,851 and Serial Nos. 255,100 (filed February '7, 1939) and 271,962 (filed May 5, 1939).

It has been found that even though the solutizers are substantially insoluble in hydrocarbon oils, their presence in caustic alkali solutions in amounts sufiicient materially to raise the solvent power of these solutions for mercaptans causes dissolution of appreciable amounts of the hydrocarbon oils in the aqueous phase when extracting the oils with the 'solutized solution. Thereafter, 55 when regenerating spent extracting solution con- .volume hydrocarbons. Upon the addition of wataining mercaptides and the dissolved oil, as by steam stripping, hydrocarbon oil and mercaptans are expelled together. Upon condensation of the resulting overhead vapors, a condensate is obtained which segregates into two layers, a water layer and an oily mercaptan layer, the latter being highly contaminated with hydrocarbon oil. Frequently it is desired to produce relatively pure mercaptans and the content of hydrocarbon oil in the mercaptan layer is, therefore, of distinct disadvantage. The mercaptans and hydrocarbons cannot be separated by distillation, since they usually have similar boiling ranges. Analyses of mercaptan layers have shown that they sometimes contain 80% and more hydrocarbons.

Now I have discovered that when diluting with Water a spent aqueous solution containing an effective amount of solutizer, absorbedmercaptans and hydrocarbons, a portion of the oil is released and separates out, without a simultaneous release of a proportionate amount of the absorbed mercaptans. The separated oil may be removed as by settling, centrifuging or the like. While it is usually sour, its content of mercaptans is P not normally much greater than that of the hydrocarbon oil from which it had been extracted. Hence, if desired, it may be returned to the sour distillate receiving treatment, or it may be used for any other desired purpose. Q0

The diluted aqueous caustic alkali solution freed from a large portion of the absorbed hydrocarbon oil may then be regenerated by steam stripping in the usual way and preferably by the method described in the Yabrofi-White Patent 2,152,724. Mercaptans thus recovered, though normally still containing a small amount of neutral oil, are of a much higherdegree of purity than prior to the separating step described. Whereas prior to the separating step, contents of 80% and more neutral oil are the rule, after the separating step this content is frequently below 15% and occasionally even below 5%.

The percent of dissolved oil separated depends primarily on the degree of dilution. The following example shows the effect which dilution with various amounts of water may have on the solubility of hydrocarbons in the solutized aqueous caustic alkali solution.

A sour gasoline was extracted with an aqueous alkaline solution containing 6 gm. mol KOH and 3 gm. mol potassium isobutyrate per liter. The spent solution was analyzed for its hydrocarbon oil content, and was found to contain .45% by ter, the content of hydrocarbon decreased as My process is further illustrated in the attached drawing which represents a simplified flow diagram of a preferred form of my process.

A sour gasoline distillate is introducedfrom a source notshown into extractor I through line 2, and is sweetened by extraction with an aqueous solution of a caustic alkali containing sumcient solutizer to enable the desired sweetening to take place. Sweet distillate emerges through line 3 and fat aqueous solution containing ab-.

sorbed 'mercaptans and hydrocarbons passes through bottom line 4 to a surge or settling tank 5. Water recovered from the stripping operation in. a manner presently to be described is added from line 6. to the fat solution in line 4. Immediately upon dilution, the fat solution releases a port on of its dissolved hydrocarbons, which in tank& is allowed to rise to the top and is withdrawn through line I. If desired, the separated oil may be returned through line 8 to the sour distillate entering extractor. I

The resulting lean solution, which still contains substantially all of the absorbed mercaptans passes from tank 5 through heat exchanger 9 in line H, and the resulting hot solution enters steam stripper i2 at a point immediately below tray i3. Tray l3 divides the stripper I 2 into an upper smaller reflux section and a lower large stripping section. The hot lean solution flows down through the stripper section in countercurrent to stripping steam which enters the stripper near its bottom through line H. The stripped lean-solution then passes from the stripper l2 through line I 5 to reboiler l6, where the water of dilution, which had been added to the fat solution in line B, is vaporized, and steam so produced is used for stripping, returning to the steam stripper through line H.

Regenerated and reconcentrated caustic alkali solution returns to the top of extractor I through line ll to sweeten further amounts oi sour distillate.

The steam in stripper l2 rises through plate I3 into the upper reflux section in which a portion of the small amount of residual dissolved hydrocarbons, which have remained in the lean solution after dilution and which are'expelled in the stripper, may be condensed and returned to tank 5 together with the water of dilution, through lines l8 and 6 and heat exchanger 9.

The small amount of mercaptans dissolved in the reflux added through line 22 to stripper 12 are steamed out in the reflux section (above plate i3) and the water of dilution flowing through line [8 is thus substantially free of mercaptan. This reflux also serves as a spray catcher.

The remaining vapors emerge from the reflux section of stripper l2 through line l9 and are condensed in condenser 20. The resulting condensate flows into receiving tank 2|, where two liquid layers are formed, an upper relatively pure mercaptan layer which is withdrawn through line 23, and a lower water layer, containing small amounts of mercaptans, which layer is tially free from mercaptans.

returned'through line 22 to the stripping sections where it serves as reflux. In the course of the refluxing. the small amounts of mercaptans contained in the. reflux water are revaporized so'that the water withdrawn from plate I3 and used for dilution purposes in tank 5 is substan- The amount or. water available for dilution in tank 5, in a completely closed system as the one described above, is controlled by the amount oi! steam required to effect the regeneration. The more steam is used for stripping, the more water is available for diluting. Experience has shown that the amount oi steam required varies between wide limits. depending not only on the nature of the hydrocarbon distillate to be sweet- -ened, but also on the number or theoretical stages in both the extraction and stripping equipment available. Thus, the amount of water available for dilution may normally vary between about 5% to 200% by volumeor the spent aqueous caustic alkali solution.

Even in a system which is not completely closed, 1. e., in which water for dilution may be introduced into line 6 and tank I from an outside source through line 24, in which further excessive water of condensation in. tank 2| may be disposed of through line 25. and in which steam for stripping may be introduced from an outside source through line 26, the amount of water of dilution may not exceed that which is removed by evaporation in the reboiler l6, otherwise the circulating solution would become more and more diluted and incapable of sweetening the sour distillate.

' I claim as my invention:

1. In the process of regenerating by steam stripping a spent aqueous solution of caustic alkali which contains an amount of solutizer suflioient materially to raise the solvent power of the aqueous solution for mercaptans, which further contains mercaptans and. dissolved hydrocarbon oil, and which has been used to extract mercaptans from sour hydrocarbon 'distillates, the improvement comprising diluting said spent solution with water, whereby at least a portion of said dissolved oil is precipitated to form a separate phase, separating the precipitated oil from the diluted solution and thereafter subjecting the latter to steam stripping to expel mercaptans.

2. In the process of regenerating by steam stripping a spent aqueous solution of caustic alkali which contains an amount of solutizer suflicient materially to raise the solvent power of the aqueous solution for mercaptans, which further contains mercaptans and dissolved hydrocarbon oil, and which has been used to extract mercaptans from sour hydrocarbon distillates, the improvement comprising diluting said spent solution with an amount by weight of water not greater than the amount by weight of steam required in the steam stripping, whereby at least a portion of said dissolved oil is precipitated to Y form a separate phase, separating the precipitated oil from the diluted solution, and -thereafter stripping the latter with said amount of steam.

3. The process of claim 2 in which the amount,

of water of dilution is between 5% and 200% of the spent aqueous solution.

further contains mercaptans and dissolved hydrocarbon oil, and which has been used to extract mercaptans from sour hydrocarbon distillates, the improvement comprising diluting said spent solution with an amount by weight or water substantially equal to the amount by weight of steam required in the steam stripping, whereby at least a portion of said dissolved oil is precipitated to form a separate phase, separating the precipitated oil from the diluted solution, and thereafter stripping the latter with said amount or steam.

5. In the process of regenerating by steam stripping a spent aqueous solution of caustic alkali which contains an amount of solutizer sufficient materially to raise the solvent power of the aqueous solution for mercaptans, which further contains mercaptans and dissolved hydro. carbon oil, and which has been used to extract mercaptans from sour hydrocarbon distillates, the improvement comprising diluting said spent solution with water, whereby at least a portion of said dissolved oil is precipitated to form a separate phase, separating the precipitated oil from the diluted solution, stripping the latter with steam to remove mercaptans, thereby producing a foul steam containing mercaptans, condensing the foul steam to form a condensate which separates into two liquid layers, an upper 30 mercaptan layer and a lower aqueous layer,

separating the layers and using at least a portion of the condensed water for diluting said spent solution.

6. In the process of regenerating by steam stripping a spent aqueous solution of caustic alkali which contains an amount of solutizer sumcient materially to raise the solventpower of the aqueous solution for mercaptans, which further contains mercaptans and dissolved hydrocarbon oil, and which has been used to extract mercaptans from sour hydrocarbon distillates, the improvement comprising diluting said spent solution with water, whereby at least a portion or said dissolved oil is precipitated to form a separate phase, separating the precipitated oil from the diluted solution, stripping the latter with steam to remove mercaptans, thereby producing a foul steam containing mercaptans, refluxing said foul steam with water, condensing the refluxed foul steam to form a condensate which separates into two layers, an upper mercaptan layer and a lower water layer containing small amounts of mercaptans, separating the layers, using at least a portion of said water layer for said refluxing of said 'foul steam, whereby mercaptans dissolved in said portion are vaporized, and further using the resulting portion substantially free from mercaptans for diluting said spent solution.

DAVID LOUIS YABROFF.

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