US2560178A - Regenerating mercaptan solvent - Google Patents

Description

J. H. KRAUSE El' AL REGENRATING MERCAPTAN SOLVENT July 1o, 1951 2 Shee'tshShet l Filed June 25, 1949 July l0, 1951 J. H. KR-AusE ETAL REGENERATING MERCAPTAN SOLVENT 2 Sheetshsheet 2 Filed Jude 25, 1949 no v|20 TEMPERATURE -OF EXTRACTION, E

2.5.3. SSD? s .cz zmz VOLUMES METHANOL/l00 VOLUMES AQUEOUS CAUSTIC MMT,

mKe A Hm W Jr B Patented July 10, 1951 REGENERATING MERCAPTAN SOLVENT .lack H. Krause and Theodore B. Tom, Hammond,

Ind., assignors to Standard Oil Company,

Chi-

cago, Ill., a corporation of Indiana Application June 25, 1949, Serial No. 101,476

3 Claims.

This invention relates to the separation of mercaptans by extraction from hydrocarbon distillates such as gasoline, kerosene and heater oil obtained from the distillation of crude petroleum and cracking still distillates. More particularly the invention relates to the regeneration of caustic alkali solutions employed in the extraction process. The invention is illustrated by a drawing which shows in Figure 1 a diagram of an ap` paratus suitable for carrying out the process. Figure 2 shows the effect of temperature on extraction efciency. Figure 3 is a graph showing the behavior of a KOH-methanol solution on removal of methanol by distillation.

This application is a continuation-in-part of `our application Serial No. 45,538 filed August 21,

1948, now abandoned.

In the extraction of mercaptans with solutions of alkali metal hydroxides, it has long been found that the solubility of mercaptans in the caustic solution is substantially increased by employing methanol, ethanol, isopropyl and normal propyl alcohols in the solution. Methanol in particular has numerous advantages partly because of its lower oil solubility and partly because of its lower boiling point making it easier to recover. Solutions of various compositions have been employed including substantially anhydrous sodium hydroxide in methanol on the one hand -and relatively dilute solutions of sodium hydroxide in Water containing varying amounts of methanol on the other hand. After contacting the sour hydrocarbon oil, the solution has generally been regenerated by distilling off the vmethanol and the mercaptans employing steam rto hydrolyze the mercaptides dissolved in the caustic solution. After separating the methanol from the mercaptans in the distillate, it has been recombined with the regenerated caustic solution for further use in the extraction of sour oil.

In the extraction of heavier stocks such as kerosene and heater oil, regeneration of the caustic solution has been found to be more difficult than in the case of lighter stocks because of vthe diiiculty of distilling the high molecular -weight mercaptans from the caustic solution.

They usual method of removing the mercaptans from the caustic by steaming at elevated temperature requires much larger amounts of steam in the case of heavier mercaptans, thereby sub stantially increasing the cost of the operation. High regeneration temperatures cannot be used because the mercaptans break down in the presence of caustic, forming alkali metal suldes which are troublesome to handle and which represent a loss of alkali metal hydroxide from the process. Furthermore, higher temperatures increase corrosion problems. Unless the caustic 'is substantially free of mercaptans in the regeneration operation, contaminating fmercaptans 2 will be returned to the hydrocarbon oil being treated when the caustic solution is recycled. Thus a high mercaptan re-entry value limits the extent to which mercaptans can be extracted Yfrom the oil and results in a product which is unacceptable for marketing, it being generally desirable to reduce lthe mercaptan content of the oil to a value below 10 mercaptan number, preferably about 1 to 5 mercaptan number. The mercaptan number is the milligrams of mercaptan sulfur per ml. of oil and is usually determined by titration with a standardized copper salt solution. An oil having a mercaptan number of less than 1 is generallyconsidered to be sweet It has been found that the extraction of higher boiling stocks such as kerosene and heater oil containing higher molecular weight mercaptans is considerably improved by incorporating in the extraction solution in addition to methanol the alkali metal salt of an acidic organic solvent, preferably a heavy alkyl, phenol, cresol or xylenol. Napthenic acids can also be employed. Itis important from the standpoint of operating procedure that the alkali metal salt of the acid reagent be a liquid or be easily liquefied by heat. When present in the solution to the extent of about 2 to 20 per cen't by Weight, the alkyl phenols, hereinafter referred to as phenols, form phenolates with the caustic alkali and substantially increase the solubility of mercaptans in the alcoholic caustic solution.

Certain petroleum oil fractions contain phenols in small amounts, e. g. 0.1 per cent, which are relmoved from the oil by the methanol-caustic solution along with the mercaptans and these have been found to exert a beneficial influence on the mercaptan separation, particularly when treating the higher boiling hydrocarbon distillates. Phenols obtained from gasoline have a lower boiling range than those obtained from heavier oils such as heater oil. rlhe following distillation analysis gives the boiling range of two typical examples of such phenols:

Dismz'aaon analysis- ASTM Phenols vtion is to provide a method for regenerating caustic methanol solutions containing high molecular weight mercaptans diicult to eliminate from the solution by vaporizaton. Another object of our invention is to eliminate the use of steam in the regeneration of mercaptans containing caustic solutions. Still another object of our invention is to eiect the recovery of causticmethanol solutions at moderately low temperatures without the formation of undesirable color bodies which contaminate 'the oil undergoing treatment. tion is to simplify the recovery of mercaptancontaminated caustic solutions and thereby eifect substantial economies in the operation.

According to our invention, the foregoing objects are attained by maintaining a ratioof caustic alkali and water in the solution within a critical range corresponding to a concentration of 50i to 65 per cent and by maintaining close control of methanol concentration. Because of the tendency of sodium hydroxide to crystallize in solutions of this concentration, we have found that caustic soda is unsatisfactory for use in our process and we accordingly -employ a solution of potassium hydroxide. The amount of methanol employed in the solution is usually about 15 to 60 volumes per 100 volumes of aqueous KOH solution, a typical solution containing about 60 per cent KOH in water and 30 volumes of methanol per 100 volumes of aqueous KOH solution.

We have discovered that when operating within this range of caustic concentration, most of the methanol can be distilled from the solution at moderately low temperatures substantially free of mercaptans, apparently as a result of the combined eiect of the high KOH concentration, the low temperature of distillation, and the absence of steam ordinarily employed for the purpose of hydrolyzing the alkali mercaptides. For this purpose, it is desirable to employ temperatures in the range of about 150 to 200o F. Higher temperatures may be used, e. g., 200 to 400 F., but usually cause development of color bodies in the recovered caustic solution, making it brown. When using reduced pressure of about 50 mm. mercury, it is preferred to operate at a temperature in theV range of about 13G-180 F.

Altho the boiling point of methanol is 148 F., it is not practicable to distil off all the methanol at this temperature because` of the effect of the KOH in depressing the vapor pressure of the methanol. Accordingly, we prefer to employ reduced pressure in the methanol distillation step, a pressure of the order of to 1.00l mm. mercury, absolute, being satisfactory. In general, it is desirable to distil oit part of the methanol at atmospheric pressure and employ reduced pressure in a second distillation stage. For the practical operation of our process, it is necessary to reduce the methanol concentration below about 18 volumes per 100 volumes of aqueous caustic solution and best results are obtained by reducing the methanol concentration below about volumes per 100 volumes of aqueous caustic solution. When operating at 50 mm. pressure at a temperature of 172 E. the methanol concentration of a 59 per cent KOH solution was reduced` from volumes to less than 5 volumes per 100 volumes of 59 per cent aqueous KOH solution.

We have discovered that, when using highly concentrated solutions of KOH, the removal of at least one-third ofthe methanol from the KOH-methanol solution causes theV dissolved mercaptans to separate fromthe solution as a A still further object of our invenseparate phase, usually in the form of a liquid or an emulsion which floats on the surface of the strong aqueous KOH solution. When ernploying aqueous KOH solutions in the concentration range of about 50 to 65 per cent, the solubility of the mercaptans in the solution after removal of methanol is so low that the KOH solution is suitable for recycling in the process without further treatment and no stripping of Inercaptans or heating to high temperature is required.

This is well illustrated by the effect of removing methanol from a solution of 55 per cent KOH in water containing 25 volumes of methanol per 100 volumes of aqueous solution. This solution wasl saturated with mercaptans by contacting it with a sour heater oil from West Texas crude having a mercaptan number of 69. The mercaptan number of the extract was 239. After distilling off l5 volumes of methanol, the residue solution was allowed to separate into a mercaptide layer and a caustic layer, and the caustic layer was found to have a mercaptan number of 4.3.

Referring to Figure l, sour hydrocarbon oil, for example heater oil of 40 A. P. I. gravity having a 10 per cent point-ASTM-of 360 F'. and per cent point of 527 F., is introduced to the process by line i0 and contacted with an alkaline solution in mixer Il and settler l2 for the removal of H23 and CO2. The alkaline solution, for example dilute caustic soda, can be recycled to the mixer by line I3. The washed heater oil free of H25 passes by line I4 to mixer l5 which is the initial stage of contacting with KOH-methanol solution introduced by line I6. The mixture from l5 is settled in l'l from which caustic-methanol solution can be recycled by line I8 to mixer l5 to increase the efficiency of contact with the hydrocarbon stock. The volume of solution introduced by line ly isl generally about 5 toY 20 per cent of the volume of the oil treated, l0 per cen-t being a satisfactory amount, altho-we may use proportions of caustic solution to oil as lowv ask l to 50 by volume, depending on the character of the oil treated and the amount of mer-captans` contained therein.

From settler l'l the hydrocarbon oil passes by line I9 to mixer 20 and settler 2| where it is contacted with KOH-methanol introduced by line 22. Again the ratio of solution tooil in mixer 20 cany be increased by recycling the aqueous phase from` settler 21, using linesA 23 and 2li for this purpose. A suitable solution composition for charging to mixer 20 by line 22 is about. 25 volumes of methanol per 100. volumes of aqueous 55` per cent KOH. The temperature of contact is usually aboutSO to FI, preferably 90 F. Higher temperatures up to about F.' may be employed but result in somewhat lower mercaptansolubility in they caustic solution, while temperatures below 80 F. may produce. solidcation of the KOH solution, particularly at. the. higher concentrations. The effect. of! temperature. on extraction emciency is illustrated by thei curve in Figure 2,' showing removal of mercaptans from heater oil by two successive treats with'. KOH- methanol solution (100 volumes. 59%A KOH, 25 volumes` methanol):, which were equivalent to four countercurrent stages;l

Altho the drawing` shows only' two treats of caustic-methanol, any numberof stages maybe employed, e. g. three to. five stages, depending upon the economics ofthe process'whereininstallation costs, operation and. maintenancev are balanced against extractionv efficiency. Instead Vstage 21 the water may be recycled-thru mixer 26 by line 29. The nished oilvsubstantially free -of'methanol and mercaptans is Withdrawn from the process by line 30.

Regeneration of the spent caustic solution is effected by conducting the solution by line- 3l' to heat exchanger 32 and thence thru heater Sito ilash tower 34. In heater v33 the vcaustic solution is preferably heated to a temperature of about 130 F. to 300 F. The pressure on the heated caustic solution which may be in the range of about 10 to 100 p. s. i. is suddenly reduced by valve 35 and vaporation of the methanol results. Methanol vapors pass upwardly thru tower 34 and methanol may be refluxed by cooling coil 33. Methanol vapors are conducted by line 37 thru exchanger 32 and thence to cooler 33 where the methanol is condensed for recycling in the process. Tower 34 may be operated at'a reduced pressure maintained by vacuum pump'39 connected by vacuum line i5 to methanol receiver L'the methanol being withdrawn by line pump x 42 and line 43 to the regenerated caustic solution line 44. If tower 34 is operated at atmospheric pressure,'however, receiver 4I may be by-passed by a suitable by-pass line das indicated.

The heating coil 46 in the base of tower 34 may be employed to assist in elimination of methanol from the caustic-solutionif the amount eliminated by flashing is not sufficient for the purpose. From the base of the tower the caustic solution is withdrawn by line 41 leading to separator i3 equipped with suitable baffles for separation of the insoluble mercaptides which form as a layer on the surface of the regenerated KOH. Inasmuch as substantially no water is removed with the methanol in tower 34, the concentration of KOH in separator 48 is substantially the same as that employed in the original extraction solution, for example 55 per cent. This solution now containing very little mercaptans, as indicated by a mercaptan number of only 5,v is recycled to the extraction process by pump 49 and line 44 in combination with the recovered methanol.

The separated potassium mercaptides are withdrawn from separator i8 by line 50 and may be discarded from the system, if desired, or ernployed for various purposes. However, it is usually desirable to treat `the mercaptide fraction further for the recovery of KOI-I and small amounts of methanol contained therein. This is readily accomplished by adding water by line while forcing the mercaptides by pump 52 into mixer 53. Suicient water is introduced to produce a KOH concentration of about to 40 per cent resulting in hydrolysis of the mercaptides and separation of the oily mercaptan layer in separator 54, which is withdrawn by line 55.

The aqueous KOH solution is withdrawn by line 56 leading to still 51 which may be an externally red kettle operated either continuously or batchwise, preferably the latter, considering 6 that-'the amount-...of KOH to be recovered is but asmallfraction ofthe total amount of KOH employed in the extraction process. In still 51 the water containing a small amount of methanol .andmercaptans is removed by line 58 leading to condenserx and receiver 60 where mercaptans .are separated and withdrawn by line 6 I, the water -beingwrecycled by pump 62 and line 93 back to "mixer 53. Methanol accumulating in the re- ?c'ycle. water may be recovered if desired, by a conventional methanol ractionator not shown.

The-.KOHisolution in still 5l is concentrated Yto about:.the,.same concentration as thatv originally employed in the extraction solution, e. g.

.50 .to 65 per cent. However, a higher concentration may be desirable'to oiset the accidental in- Auxof. water into the system, in which case the KOH solution in 5l may be concentrated to '75 or' 80 per cent. The hot solution can be transferred by line S4 and pump 55 to caustic recycle line 44, or it may be conducted by line 5t to the spent caustic stream charged to evaporator 34.

Phenols contained in the oil undergoing extraction accumulate in separator 43 in the form of potassium phenolates. These are finally eliminated from the system with the mercaptans by line 55. The presence of phenols has a Afavorable-i effect on the separation ofthe mercaptides because of their solvent eiect but they have been found to be unnecessary for the operation of the- V.process under the conditions described. TheV stocks treated, particularly if they are cracked' stocks, usually contain a small amount of phenols, cresols, xylenols, etc., which aid in the separation of mercaptides.

In atypical operation of our process of regenerating KOH-methanol solutions, 100 volumes of 59 per cent aqueous KOH and 25 volumes of methanol were charged to the evaporator at a temperature of 248 F. to 294 F. The solution charged had a mercaptan number of 239. Methanol vapor was removed from the evaporator at a temperature of 154 to 160 F., '70 per cent of the methanol being taken oi. The mercaptan number of the recovered methanol was 112. The aqueous caustic solution removed from the bottom of the evaporator was allowed to separate from the mercaptides and had a mercaptan number of only 4.8 with a trace of KzS, indicating very slight decomposition of mercaptans in the operation. The separated mercaptide layer amounting to 5 per cent of the charged solution had a mercaptan number of 4530. No methanol was present in the mercaptides but the KOH equivalent was 0.6 pound per barrel of oil extracted.

In another example, the same spent causticmethanol solution having a mercaptan number of 239 was charged to the evaporator maintained at 50 mm. Hg pressure absolute. The methanol distillate amounting to 8l per cent of the total methanol had a mercaptan number of only 9.

*y The temperature in the evaporator was about on mercaptan number of a KOH solution containing 59 parts KOH and 41 parts waterJ .by weight, and 25 volumes methanol per 100 volumes of aqueous KOH, .distillation at ordinary pressure being employed. When the .amount of methanol was reduced to about 20 Volumes, phase separation was observed which `increased rapidly until the amount of methanol remaining was about 15 volumes, at which point :nearly all mercaptans had separated as potassium mereoaptides. Mercaptan number is expressed as milligrams of mercaptan sulfur per 10,0 milliliters of KOH solution inthe .aqueous phase.

Having thus described our invention what we claim is:

1. The method of removing mercaptans from a petroleum distillate which is higher boiling than gasoline and is Vwithin the kerosene to heateroil boiling range, which method comprises extracting said distillate at .a temperature in the range of about 80 to about 100 F. with a solution of potassium hydroxide in Water and methanol wherein the weight ratio of potassium hy.- droxide to water is in the range of 50:50 vto 65:35 lto give an aqueous potassium hydroxide solution which contains at least 50 but not more than .65 per cent potassium hydroxide by weight and wherein the methanol to aqueous potassium hy.- droxide volumetric ratio is greater than 15:100 but less than 601100, separating aqueous potassium hydroxide methanol extract from extracted distillate and regenerating said extract by distilling at a temperature in the range fof 130 to 300 F. to remove a part but not al-l -of the amethanol, vwithout removing any substantial amount of water or mercaptans and without `Aaddition f Water in liquid or vapor state, to reduce the methanol content Abelow "15 volumes -but-notvbelow volumes per 100 volumes of aqueous potassium hydroxide. Separating the boi residual liquid .from the distillation ster)` inte e @Oneenirted aqueous potassium hydrOXde phase and a P0- tassium mercaptide phase, withdrawing said aqueous potassium hydroxide phase, adding ,C911- densed methanol thereto and returning the fesulting aqueous potassium hydroxide methanol solution for further extraction of hydrocarbon distillate.

`2. The method Q f `claim 1 which includes the Steps of separately withdreimflis .the Potassium merceoude phase. admixine it with euieeni water to produce a potassium hydroxide concentration lof about 10 to 40 Weight vper cent, then `.separating Ya resulting .oilyrnercaptan layer from dilute yaemeous potassium hydroxide .Solution and concentrating said dilute aqueous potassium `h ydroxide solution for reuse.

`3. The method of claim 1 wherein the amount of methanol remaining in the aqueous potassium hydroxide extract after the distillation step in the range o f 5 -to 10 volumes per i100 volumes of aqueous potassium hydroxide solution. A

JACK H. KRAUSE. THEODORE B. TOM.

REFERENCES CITED The following references are of record in vthe iile of this patent:

UNTED STATES PATENTS `Number Name Date 2,152,721 Yabroi V K V V Apr. 4, 1939 2,181,036 Wilson i Nov. 21", 1939 2,181,037 Wilson V Nov. 21, 1939 2,269.,.167 Mccuuougi; V v Jan. l13, 1942 2,317,053 Henderson Apr. 20, 1943

Claims (1)

1. THE METHOD OF REMOVING MERCAPTANS FROM A PETROLEUM DISTILLATE WHICH IS HIGHER BOILING THAN GASOLINE AND IS WITHIN THE KEROSENE TO HEATER OIL BOILING RANGE, WHICH METHOD COMPRISES EXTRACTING SAID DISTILLATE AT A TEMPERATURE IN THE RANGE OF ABOUT 80* TO ABOUT 100* F. WITH A SOLUTION OF POTASSIUM HYDROXIDE IN WATER AND METHANOL WHEREIN THE WEIGTH RATIO OF POTASSIUM HYDROXIDE TO WATER IS IN THE RANGE OF 50:50 TO 65;35 TO GIVE AN AQUEOUS POTASSIUM HYDROXIDE SOLUTION WHICH CONTAINS AT LEAST 50 BUT NOT MORE THAN 65 PER CENT POTASSIUM HYDROXIDE BY WEIGHT AND WHEREIN THE METHANOL TO AQUEOUS POTASSIUM HYDROXIDE VOLUMETRIC RATIO IS GREATER THAN 15:100 BUT LESS THAN 60:100, SEPARATING AQUEOUS POTASSIUM HYDROXIDE METHANOL EXTRACT FROM EXTRACTED DISTILLATE AND REGENERATING SAID EXTRACT BY DISTILLING AT A TEMPERATURE IN THE RANGE OF 130* TO 300* F. TO REMOVE A PART BUT NOT ALL OF THE METHANOL, WITHOUT REMOVING ANY SUBSTANTIAL AMOUNT OF WATER OR MERCAPTANS AND WITHOUT ADDITION OF WATER IN LIQUID OR VAPOR STATE, TO REDUCE THE METHANOL CONTENT BELOW 15 VOLUMES BUT NOT BELOW 5 VOLUMES PER 100 VOLUMES OF AQUEOUS POTASSIUM HYDROXIDE, SEPARATING THE HOT RESIDUAL LIQUID FROM THE DISTILLATION STEP INTO A CONCENTRATED AQUEOUS POTASSIUM HYDROXIDE PHASE AND A POTASSIUM MERCAPTIDE PHASE, WITHDRAWING SAID AQUEOUS POTASSIUM HYDROXIDE PHASE, ADDING CONDENSED METHANOL THERETO AND RETURING THE RESULTING AQUEOUS POTASSIUM HYDROXIDE METHANOL SOLUTION FOR FURTHER EXTRACTION OF HYDROCARBON DISTILLATE.

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