US2556414A - Separating mercaptans - Google Patents

Description

June 12, 1951 J. A. BROOKS ETAL 2,556,414

SEPARATING MERCAPTANS Filed Dec. 11, 1948 2 Sheets-Sheet l .12/(07 oysnog J0 '0 uqdoma /y cow ovm

- INVENTORS.

John A. Brooks Hire: se M 47 Nmhan Fragen Theodore 8. Tom ack BY M uoyn oy ,zuady 370,1 ga /5X07 ap oua fix J0 sawn/OA i atented June 12, 1951 UNITED STATES PATENT OFFICE siirAitArmG MERoAr'rANs JohnA. Brooks, Chicago, 111., and Jack H. Krause,

Theodore B. Tom; and Nathan Fragen, Hammend, Ind., assignors to Standard Oil Coinpany, Chicago, 111., a corporation of Indiana Application December 11, 1948, serial No. 64,802

mercaptans with a mixture of strong caustic a1 kali and methanol.

One object of the invention is to provide a method for more efficiently and more eoonomi cally recovering the caustic-methanol solution employed in the extraction of mercaptansfrom sour petroleum distillates. Another object of the invention is to process thespent caustic solution containing mercaptans in such a manner that the mercaptan re-entry value of the regenerated caustic solution and caustic-methanol mixture is lower than has heretofore been economically possible. As a result of the lower mercaptan reentry value of the recycled caustic-methanol solu tion, a more complete removal of mercaptans from the petroleum distillate is effected. Still another object of the invention is to recover caus tic-'methanol solutions which have been contaminated with mercaptans without subjecting the mercaptans to high temperature resulting in decomposition to hydrogen sulfide.

It has heretofore been the practice in extracting mercaptans from oils with caustic methanol solutions to regenerate the caustic solution by stripping the mercaptans therefrom. When treating stocks containing low-boiling mercaptans, it has been relatively easy to remove the mercaptans from the caustic by the action of steam, particularly if the steam is superheated. With the higher-boiling mercaptans from kerosene and heater oil, however, removal from the caustic by distillation is not accomplished soread ily and large amounts of steam are required, thusconsiderably increasing the cost of the re covery operation. Also, we have found that the use of temperatures of 250300 F. and higher is required, which results in decomposition of mercaptans in the presence of strong caustic with formation of H28 which neutralizes an :equivalent amount of KOH, destroying its usefulness in the process. The removal of the mercaptans from the caustic by distillation alsorequires that themethanol be distilled off either with the mer captansi or in a preliminarydistillation operation.

V 2 Themethanol recovered in this way is generally contaminated by mercaptans unless special measures are adopted for purifying it.

We have now discovered that both the methanol and the caustic may be recovered almost free of mercaptans by thesimple expedient of dilut ing the spent caustic methanol solution with Wat'er, providing that a highly concentrated causticsolut-ion is used for extraction andthe ainount of dilution (in regeneration s controuea within a specified water-concentration range. It is also desirable that a small amount, e. g. 0.2 to 5-per cent of phenols, cresols or xylenols bepre'se'iit in the solution to facilitate mercaptan' se aration. The term xylenol's will be used herein to designate higher boiling phenols which boil at about the boiling point of the cresols and the xylenols or somewhat higher. Such xylenols as occur naturally in petroleum distillates, especially the cracking stilldistillates, are wellsuited for our process. A typical xylenol fraction extracted from heater oil had a boiling range of 'abOut375 to 550 F.

The solutions employed should containabout 15 to '75 volumes of methanol for each 100 vol um'es'of concentrated aqueous caustic employed, corresponding to about 530% by weight. 'Usu' ally about 25 to 50 volumes of methanolare used per 100 volumes of aqueouscaustic. After dilu-'-' tion, the spent solution is allowedto' stratify in a settling chamber, preferably at a somewhat elevated temperature of the order of to 140 F. The mercaptans and xylenols separate from the solution predominantly in the form of their alkali metal salts, mercaptides and xylenolates. The caustic solution containing the methanol is then reconcent'rated to remove the added Water and is ready for recycling to the process. It is preferred to useKO'H because of its greater isolu= bility and freedom from crystallization at-high concentrations, altho NaOI-I can .be used with careful control of concentration and temperature. Concentrations of about50 to 65 per cent, basedon the KOI-I-water components, are'desirable.

The invention is illustrated by drawings in which Figure 1 is .a graph showing the effect of water addition to a spent caustic solution with respect to'the amount of xylenolates and mercaptides discharged from the solution and the mercap tan numberofthe diluted caustic-methanol 80111- tion. The mercaptan number, sometimes called the copper number, is the milligrams of mercapt'an sulfur per ml. of oil, generally determined by titration with a standardized copper salt solution.

Figure 2 is a diagrammatic representation of a plant for carrying out the process.

In one series of experiments, a, West Texas heater oil having a mercaptan number of about 60 was extracted with 55 per cent KOH solution containing twenty-five volumes of methanol and five volumes of potassium cresylates per 190 volumes of KOl-I solution. The ratio of causticmethanol to heater oil employed in the extraction was 1:5 by Volume. To the spent caustic solution containing mercaptans extracted from the heater oil there was added varying amounts of water and, after thorough agitation, the solution was allowed to settle for two hours. The aqueous layer was drawn off the bottom and analyzed for mercaptan content. The volume of the upper Xylenolate-meroaptide layer was de termined. The results are shown in the chart in Figure l. The mercaptan number of the spent caustic solution before adding water was 288.5. It will be observed that this dropped to a value of about 20 after adding from about 30 to 100 volumes of Water per 100 volumes of the spent caustic-methanol solution. Addition of more water beyond about 140 volumes caused an increase in the mercaptan number of the caustic layer, showing that a maximum amount of mercaptans is separated at a concentration corresponding to about 30 to 140 volumes of added water per 100 volumes of spent KOI-I solution. Expressed on the basis of percentage of water, this corresponds to about 55 to '72 per cent, based on the KOl-I and water components of the solution alone.

In our preferred operation, the amount of water added should be at least about twice the weight of the methanol in the spent solution and less than the weight of KOH present.

Referring to Figure 2, the process is carried out in the following manner: The sour hydrocarbon distillate, for example, heater oil containing mercaptans, is supplied to the system by line leading to mixer i l where it is contacted with a caustic solution, e. g. NaOI-I, primarily to remove I-IzS as well as other strongly acid constituents. A caustic solution of about 5 to per cent is adequate for this purpose. From the mixer ii the stock passes to settler l2 from which the caustic solution is recycled by line l3 and pump M. From time to time spent caustic can be removed from the system by drawoff line 13a and fresh caustic supplied by line l5.

From prewash settler E2 the distillate is conducted by line E6 to mixer ll where it is contacted with caustic-methanol solution for removal of mercaptans. A satisfactory solution may contain about 55 per cent KOH based on the water plus KOI-I present, and in addition thereto about 25 volumes of methanol per 100 volumes of aqueous KOH solution. From mixer I? the distillate passes to settler l8 where the causticmethanol solution is allowed to separate from the distillate. Line l9 then conducts the distillate to mixer where it is contacted with fresh causticmethanol solution introduced by line 2|. The distillate then passes to settler 22 where the caustic-methanol solution is again separated by gravity and withdrawn by line 23 leading to mixer l1, thus effecting a countercurrent extraction of mercaptans by the caustic-methanol solution. Altho only two countercurrent stages are shown, more countercurrent stages may be employed, e. g. 3 to 5 stages, and if desired a countercurrent tower contactor may be employed in lieu of the stage extraction system.

From settler 22 the extracted distillate, which may be substantially free of mercaptans or have a low mercaptan number of about 2 to 5, passes by line 25 to mixer 25 where it is contacted with water for the removal of small amounts of en-- trained caustic not separated in settler 22 and for the removal of a small amount of dissolved meth anol. Water for the purpose may be added to the system by line 28 and it is preferred that water for this purpose be free of CO2 which has the undesirable effect of partially neutralizing the caustic. From mixer 25 the distillate flows to settler 2i whence the finished sweetened distillate is withdrawn by line 28. Water settled in 2'! is recycled by line it back to mixer 25.

Caustic-methanol solution contaminated with mercaptans, having a mercaptan number of about 200 to 400, is withdrawn by line 30 and conducted to mixer 31 where it is intimately contacted with a carefully regulated amount of water introduced by line 32. The amount of water must be sufiicient to reduce the solubility of mercaptides and xylenolates to a low value and cause them to separate from the caustic-methanol solution, but an excessive amount of water must be avoided to prevent re-solution of the mercaptides and xylenolates. The addition of about 20 to 180. volumes of water per 100 volumes of the spent. caustic-methanol solution previously described is. satisfactory, from 30 to volumes being a preferred amount.

From mixer 3! the diluted caustic-methanol solution'passes to settler 33 where the xylenols, and mercaptans in the form of their alkali metal compounds, potassium xylenolates and potas sium mercaptides, separate as an upper layer which is a liquid and can be withdrawn from settler 33 by line 3G. The caustic-methanol solu tion, substantially free of mercaptans, is withdrawn by line 35 and charged to still 36 where it is reconcentrated for re-use in the system. Heat for still 36 can be furnished by reboiler 37. Vapors of methanol and water are conducted by line 38 to methanol fractionator 39 where the methanol is separated from the water, methanol vapors being withdrawn overhead by line to leading to condenser 4i and receiver 42. Water is withdrawn from the base of 39 by line 43 and recycled to mixer 3! for further use in the system. Because all the water can be reused in the system, it is not necessary to completely separate methanol from water in fractionator 39.

Alternatively, removal of water from the caustic solution can be effected at a lower temperature by first distilling 015 methanol from the caustic solution, employing a moderate vacuum, if desired, to reduce to temperature required, then, in a second distillation stage, removing water under relatively high vacuum, e. g. 10 to 50 mm. mercury pressure, absolute. By this method, discoloration of the caustic solution from heating is still further reduced and the methanol fraction recovered in the first distillation stage is sufficiently free of water to be recycled to the extraction step without further fractionation.

A part of the water stream from line 43a may be periodically conducted, e. g., via line 25 by connections not shown to the final water-washing stage 21 of the extraction system. When the amount of methanol and entrained KOH collected in 21 is sufficient to require replacing the. water therein, it can be withdrawn periodically by 5 pump 45 and lines '47:; and 32 leading to mixer 31.

Reconcentrated KOH solution from still 36 is withdrawn by lines 46 and 4'! to mixing chamber 48 wherein it is mixed with methanol drawn from receiver 42 by line 19. The caustic-methanol solution in chamber 48 is mixed to the desired composition and recycled to the extraction system by lines 50- and 2 l as hereinabove described.

The separation of mercaptides from the caustic solution in separator 33 is considerably facilitated by the presence of cresols, xylenols and higher molecular weight hydroxy aromatic compounds such as those normally occurring in petrol'eum distillates, particularly in higher boiling distillates such as heater oils. These acidic phenolic substances are herein referred to as xylenols and their alkali metal salts as xylenolates. The amount naturally present inheater'oilsgabout 0.05 per cent, is usually sufiicient to bring about a rapid separation of the alkali metal mercaptides from the caustic solution after Water dilution in 33. Additional xylenols may be added to the system by line ii, if desired, or by adding them directly to the stock treated. Furthermore, xylenols may be recycled in the system as will be hereinafter described.

The upper oily layer of mercaptides and xylenolates withdrawn by line 34 from separator 33 contains both combined and entrained caustic alkali which may be discarded by line 34a but which it is generally desirable to recover and recycle to thesystem. This may be accomplished in various ways, the following method being illustrated in the drawing: The mercaptide-xylenolate stream is heated by steam heater 52 or by steam injected thru line 53. Water may also be added, if'desired, to increase the dilution. After thoro mixing, the mixture is allowed to settle in settler 54, generally maintained at an elevated temperature and under pressure to increase the extent of hydrolysis. Oil or other solvent may be added to the mixture in s'e'p'aiatorb l if desired, to assist separation of the hydrolyzed mercaptans from the caustic alkali solution. Mercaptans are withdrawn by line- 55 continuously or'periodically. The aqueous caustic solution containing some mercaptans and xylenols is conducted by line 56 to stripper 5'! where most" of the remaining merc'aptans are stripped out by steam introduced by line 58. Additional heat may be supplied to the base of stripper 57 by reboiler 59. The mercaptans together with steam and a small amount of residual methanol vapors are conducted by vapor line 60 to condenser 6! and receiver 62 from which the mercaptans are withdrawn by line 63. The dilute aqueous methanol-caustic solution collecting in B2 is conducted by line 64 to the methanol fractionator 39. To prevent appreciable amounts of xylenols from being carried overhead with the vapors from 57, a small stream of caustic alkali solution may be introduced by line 55 into the top of stripper 51. For this purpose, caustic solution from settler 33 may be bypassed thru line 68 or it may be Withdrawn by line 61 from the base of methanol stripper 36.

The caustic solution collecting in the bottom of stripper 5'! together with alkali metal xylenolates is withdrawn by line 68 and recycled to the system by line 59. The amount of mercaptans carried back into the system in this way is relatively small, depending on the efficiency of the stripping operation conducted in 57. The mercaptan number of the recovered xylenolates is usually under 1000 and since the amount of this recycled stream is very small, this is not sufficient 6. toadv'ers'ely affect the extent of mercaptan re movai-fro'mthe oil treated in extractors -l8= and F22. KOH stream line 69 may be only 1 to 10 per cent of the volume of the main caustic-methahol solution in line 41.

When operating with stocks which are relatively high in their content of xylenols, it is sometimes desirable to eliminate partly or entirely the xylenolates from the system by line It. When treating lighter hydrocarbon stocks such as sour keroseneand heavy naptha, it is preferred to a'dd'to the system the xylenols or other mercaptan solvent extracted from a higher boiling stock such as heater oil, in order to increase the extent of the mercaptan separation in 33. Likewise, other mercaptan solvents may be employed for this purpose such as Cellosolve, tributylamine', octyl and'nonyl amines, lauryl alcohol, and other high-boiling alcohols and ethers which are normally liquids. These solvents may be recovered in a manner similar to that described and recycled in the system.

As an example of an operation employing our process, 3000 ml. of spent caustic-methanol so lution prepared from 55 per cent aqueous KOH and 25 volumes of methanol per cc. KOH solution'were diluted with 900 ml. of water. The spent KOH-methanol solution had a mercaptan number of 513 which corresponded to 15.4:grams of sulfur. The total weight of KOH in solution was 1865 grams. After dilution, there separated from the solution 480 ml. of mercaptide layer containing 13.7 grams of mercaptan sulfur determined by analysis. The remaining caustic solution measuring 3420 ml. had a mercaptan-number of 50 corresponding to only 1.71 grams of mercaptan sulfur. This solution was subjected to stripping with steam, methanol and'water being removed as a, distillate for further separation. The remaining regenerated KOH solution of 55 percent KOH concentration wassuitable for recycling after the addition of the proper amount of methanol.

The oily mercaptide layer from the dilution step, measuring 480 ml., was further diluted and heated with 960 ml. of water. On standing, it separated. into two layers. An upper mercaptan layer measuring ml. had a mercaptan number of 6650 corresponding to 12.64 grams of sulfur. The amount of KOH remaining in this mercaptan layer was only 0.22 gram by analysis. The remaining solution measuring 1250 ml. contained substantially all the methanol and KOH originally present in the mercaptide layer and was suitable for recycling to the dilution step for treatment in other ways for recovery of methanol and KOH.

Another method for recovering methanol and caustic from the alkali mercaptides and xylenolates separated in settler 33 can be employed with considerable simplification. According to this method, sufficient water and/or steam is added by line 53 to substantially entirely decompose the alkali mercaptides and xylenolates in settler 54. Hydrolysis of these materials is facilitated by operating at elevated temperature, for example 300 to 400 F. under pressure. Hydrolysis is also greater when operating with the higher boiling stocks which yield mercaptans and xylenols of higher molecular weight.

When operating by this simplified method, the aqueous solution of caustic alkali and methanol is withdrawn from the bottom of separator 54 by dotted line H and conducted directly to mixer Tl-ius the volume of the recycled Xylenol- 3| where it is employed for di1u'tingI the spent caustic stream from the extraction system. When operating according to this method, it is not necessary to provide stripper 51 and accessories including lines 64, 65 and 69.

Our process enables substantially all the caustic and methanol to be recovered substantially without mercaptan contamination by the simple expedient of diluting with water and settling or centrifuging. The simplicity of the operation will be quite apparent. After dilution and separation of the mercaptans, it is only necessary to eliminate the excess water from the caustic solution before it can be recycled in the system.

Having thus described our invention what we claim is:

l. The process of removing mercaptans from a concentrated caustic-methanol solution consisting essentially of an alkali metal hydroxide solution in aqueous methanol which process comprises diluting the solution with an amount of water sufficient to effect separation of the solution into an aqueous caustic-methanol phase and an oily mercaptide phase and separating said mercaptide phase from said aqueous causticmethanol phase.

2. The process of claim 1 wherein said causticmethanol solution contains about to 50 volumes of methanol per 100 volumes of aqueous caustic.

3. The process of claim 1 wherein the separation of mercaptans from the diluted causticmethanol solution is facilitated by the presence of a mercaptan solvent insoluble in the aqueous caustic-methanol phase resulting from dilution with water.

4. The process of claim 3 wherein said mercaptan solvent consists essentially of xylenols.

5. The process of removing mercaptans from a sour petroleum distillate oil higher boiling than gasoline which comprises extracting said inercaptans with a solution of KOH containing about 50 to 65 parts by weight of KOl-I and 50 to 35 parts of water respectively and, in addition, about to 75 volumes of methanol per 100 volumes of aqueous KOH, separating the extract containin mercaptans from said distillate, diluting the extract with sufficient water to effect separation into two phases, an aqueous KOH-methanol phase and an oily mercaptide phase, sepa-j rating said phases, removing water from said aqueous KOH-methanol phase to restore it to the concentration first described hereinabove and recycling it to the extraction step for removal of mercaptans from further quantities of petroleum distillate.

6. The process of claim 5 wherein said oily mercaptide phase is hydrolyzed with water to recover therefrom an aqueous solution of KOH and methanol.

7. The process of claim 6 wherein said aqueous solution of KOH and methanol is recycled to said dilution step instead of the water used therein.

8. The process of claim 6 wherein said aqueous solution of KOl-I and methanol is distilled to remove water therefrom and the recovered KOI-I and methanol is recycled to said distillate extraction step.

9. The process of removing mercaptans from a solution of KOH, water and methanol in which the weight of KOl-l is at least equal to the weight of water present and the methanol constitutes about 5 to 30 per cent of the weight of the solution which comprises adding to the solution an amount of water equal to at least about twice the weight of methanol and less than the weight of KOH, thereby effecting separation of an insoluble potassium mercaptide layer and separating said potassium mercaptide layer from the aqueous KOI-I-methanol solution.

10. The process of claim 9 wherein xylenols are added to the mercaptan-contaminated KOH- methanol solution to assist the separation of said potassium mercaptides.

JOHN A. BROOKS. JACK H. KRAUSE. THEODORE B. TOM. NATHAN FRAGEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,228,295 Yabroff Jan. 14, 1941 2,309,654 Leum Feb. 2, 1943

Claims (1)

1. THE PROCESS OF REMOVING MERCAPTANS FROM A CONCENTRATED CAUSTIC-METHANOL SOLUTION CONSISTING ESSENTIALLY OF AN AKALI METAL HYDROXIDE SOLUTION IN AQUEOUS METHANOL WHICH PROCESS COMPRISES DILUTING THE SOLUTION WITH AN AMOUNT OF WATER SUFFICIENT TO EFFECT SEPARATION OF THE SOLUTION INTO AN AQUEOUS CAUSTIC-METHANOL PHASE AND AN OILY MERCAPTIDE PHASE AND SEPARATING SAID MERCAPTIDE PHASE FROM SAID AQUEOUS CAUSTICMETHANOL PHASE.

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