US2152166A - Process of separating mercaptans contained in a hydrocarbon distillate - Google Patents

Description

March 28, 1939. D. L. YVABROFF 2;15 2,166

PROCESS OF SEPARATING MERCAPTANS CONTAINED IN A HYDROCARBON DISTILLATE Filed Sept. 28, 1936 WASHER SEPA/FA 7'01? WASTE W475? 57521114 BO/LER COOLER FRESH CAUST/C AND 50LUT/Z'E7? By I773 Affarney; I.

' stances from their solution in hydrocarbon type Patented Mar. 28', 1939 UNITED STATES} raocnss or scraiwrmc nmncmn n'sconramcn. n a HYDROOARBON ms- TILLATE David Louis Yabroll, Oakland, Calif., assignor to Shell Development Company, San Francisco, Calif., a corporation 01 Delaware Application September 2a, 1936, Serial No. 102.39.".

6 Claims. (01. 196-32) This invention relates to the removal of weakly acid-reacting organic substances from solutions in substantially water insoluble organic liquids,

and in particular deals with the removal of mercaptans from petroleum distillates.

It is frequently necessary to eliminate small quantities of organic acidic components such as mercaptans, phenols, naphthenic acids, fatty acids, etc. from their solutions in substantially water insoluble neutral or basic organic liquids. The organic liquids, herein used, are normally liquid organic substances which are neutral'or slightly basic, such as the liquid hydrocarbons derived from petroleum, benzene, toluene, xylene,

substituted normally liquid hydrocarbons which are substantially insoluble in water, for instance,

chlorinated hydrocarbons, of which chlor ethane, ethylene dichloride, trichlorethylene, carbon tetrachloride, chlor propane, chlorbutylene,

chlorbenzene, brom benzene, are examples; or-

nitro hydrocarbons, for example, nitroethane, nitrobenzene; or other nitrogen containing hydrocarbons such as amyl or higher amines, aniline, pyridine, petroleum bases, etc.

In the Yabrofi and Givens application, Serial No. 30,374, filed May 18, 1936, it was shown that the eflicacy of the removal of acidic organic subliquids by means of alkaline-reacting solutions which are substantialy immiscible with said liquids, depends largely on the solvent power of the solution for the organic acids and on the alkalinity of the former. As a means for producing aqueous alkaline solutions of good solvent power for organic acids the use of quaternary ammonium bases was suggested.

I have discovered that a variety of polar organic substances which are miscible with water in all proportions have the ability of increasing the solvent power of. aqueous alkali toward organic acids to a high degree. Substances possessing these characteristics are called solubility promoters, and the expression solubility enhancing as herein used relates to the increase insolvent power, i. e. the difference in solvent power of the aqueous caustic alkali for the organic acids before and after the addition of the solubility promoter.

The term caustic alkali refers to all strongly alkaline bases, 1. e. the alkali metal hydroxides, alkaline earth hydroxides, quaternary ammonium more effective.

bases, alkali carbonates and bicarbonates, etc. although the hydroxidesof the alkali metals are preferred. Especially useful are the sodium and potassium hydroxides, as being the strongest of the easily available bases. While sodium hydroxide is more generally used because of its lower cost, potassium hydroxide is usually considered A group of solubility promoters which is of particular interest consists of aliphatic polar compounds which contain at least two non-carboxyllc hydroxyl radicals, at leasto'ne carbon chain with not less than two oxygen atoms attached to separate carbon atoms, and which, in addition, may contain certain other polar radicals. The ratio of carbon atoms to hydroxyl radicals in these polar compounds is not less than 1 /2, the ratio of carbon atoms to total polar radicals is not greaterthan 2V andthemumber of carbon atoms is greater than the number of polar radicals by at least 1.

Suitable polar radicals other than hydroxyl radicals are those which are substantially resistant to hydrolysis in the presence of aqueous caustic alkali at elevated temperatures of the order of C. and consist of the group of carbonyl, ether,'carboxylate, sulfone, nitro, sulionate and primary, secondary, and tertiary amino radicals.

In determining all-around suitability of solu- 'bility promoters for the deacidifying of hydroto increase the solvent power of the aqueous caustic alkali is only one of the factors to be considered. Of about equal importance is the relative solubility of the solubility promoters in water and hydrocarbon liquids, which determines the partition of the solubility promoter between the aqueous and oil phases when they are brought into intimate contact. Loss of solubility promoter in the oil phase depends on this partition.

Different compounds of good solubility enhancing properties have widely different relative solubilities in substantially water insoluble organic liquids. Thus of a series of compounds having about equal solubility enhancing p operties for organic acids, some may be soluble in substantially water insoluble organic liquids to such an extent even in the presence of substantial amounts of water to make their use prohibitive,

100 parts of a gasoline were shaken with parts of a 2.5-normal aqueous caustic alkali prepared by dissolving the caustic alkali in aqueous solubility promoters of varying contents of water, and the amounts of solubility promoters dissolved in the gasoline were determined. 1

Table 'I Volume 1 t l l i l t ercen so u 1 Solubility promots promote;

' 1 left in gasoline Eth 1 alcohol so 2. a o 25 2. 9 Ethylene glycol ethyl ether IsobutyI e neEl col so 1030 Do 25 034 2-3 butylone glycoL. so 034 The solubility enhancing powers of the solubility promoters in the above table being of similar magnitude it will be seen that polyhydroxy compounds having the proper carbon to hydroxyl ratio are vastly superior to other solubility promoters such as monohydric alcohols or glycol ethers.

The rule governing suitability of solubility promoters is that they must not only be effective as solubility promoters in the lowest concentrations possible, i. e. in the presence of a substantial amount of water, but also be substantially insoluble in substantially water insoluble organic liquids in the presence of that amount of water. Only if the solubility promoters are capable of greatly increasing the solvent power for organic acids at concentrations at which they are substantially insoluble in substantially water insoluble organicliquids, can they be said to be suitable.

This combination of properties rules out a large number of polar compounds such as simple monohydric alcohols particularly those of more than 2 carbon atoms, ethylene glycol, glycerine,

ethers, glycol ethers, etc. While some of the unsuitable compounds such as ethyl alcohol, propyl alcohol, ethylene glycol ethyl ether, etc. have good solubility enhancing properties but are too soluble in the hydrocarbon type liquids, other compounds such as ethylene glycol and glycerine possess negative solubility enhancing powers, i. e. they actually lower the solubility of mercaptans and other weak organic acids in aqueous caustic alkali under the conditions of my treatment.

For example, the influence of solubility promoters on the partition coeilicient K for normal amyl mercaptan between 2.5-normal aqueous caustic soda and straight run gasoline, K being the concentration of the mercapta'n in the caustic soda solution divided by its concentration in the gasoline, is demonstrated by the following table:

Table II Solubility promoter (contains 2.5 n NaOH) K Q OaN A third property to be considered is regenerability. Unless the solubility promoters, or preferably the aqueous caustic-alkali containing the solubility promoters, can be regenerated in a simple manner, the application of solubility promoters has little practical value, if any.

The solubility promoter concentration in the aqueous caustic alkali to be eflective may vary from about 15 to 85% depending on the caustic soda concentration and is often kept between about 25 and 75%, this being the most economical range for most solubility promoters. The solutizing efiect normally increases with increasing concentration of both the solubility promoter and the caustic alkali. Within. limits, an increase in the caustic alkali concentration has a similar effect as an increase in the solubility promoter concentration. Thus, whereas for intance a 2- or 3-normal caustic alkali solution'in a 25% aqueous solubility promoter such as propylene glycol is relatively ineffective, a substantially saturated aqueous caustic alkali having a normality of about 10 or 12 in the same aqueous solubility promoter is highly effective.

To minimize losses of solubility promoters it is thus often advantageous to use relatively strong caustic alkali solutions containing a lesser amount of solubility promoter. For instance a solution containing 2.5 normal caustic alkali in 75% isobutylene glycols has approximately the same average extracting power as a. i-normal caustic alkali solution in 50% isobutylene glycol.

The water content of the aqueous caustic alkali containing solubility promoter may vary from about 5 to 70% and preferably from 15 to 50%. In general, the higher the carbon to hydroxyl ratio the greater is the amount of water and the lower the solubility promoter concentration required for optimum' economy and efiiciency.

Examples of suitable compounds of the aforedescribed group of polyhydroxy polar compounds are: propylene glycol, l-2- or 2-3-butylene glycol, isobutylene glycol,. l-3- or l-4-dihydroxy butane, alkyl glycerines, di-ethyleneglycol triethyleneglycol, glycerine mono-ethyl ether, l-2- 3-amino di-hydroxy butane, etc. 1

It appears that polyhydroxy compounds of the herein described group which possess hydroxyl radicals attached to vicinal carbon atoms are especially useful. The relation of solubility enhancing power to solubility in substantially water insoluble organic oils becoming less favorable the farther apart the polar radicals, and in particular hydroxyl radicals, are from each other.

Depending upon the type of organic acids which are primarily to be extracted from hydrocarbon type solutions, I may choose different solubility promoters. For instance, I have found that in general, solubility promoters having a relatively high ratio of carbon atoms to polar radicals possess a preferentially selective solua mai f 7 bility enhancing .power for tl ifilh filier f of a clas's'oi' organic acids, such as, tor instance;- the classof normal mercaptans, whereas solubility promoters having lower carbon ratios possess a more even solubility enhancing power for all members. .Se1ective solubility-enhancing". as

herein used refers to the relative improvement oi the. solvent powers .of aqueous alkaline solutions toward various organic acids upon addition of solubility'promoters. For example, it a so1u-1 bility promoter increases the solubility in aqueous caustic alkali of a '7-carbon -mercaptan toa.

greaterexten t than that'pf a 4-carbon mercantan, the solubility promoter is selective for, the heaviermercaptan-in spite of the fact thatusuallythe sol'ub'ilitybfjthe lower .mercaptan in the caustic alkali containing solubility promoter remains greater than that or the heavier'mercaptan, because oi the great difference-between the solubilities of the two mercaptans in aqueous caustic alkali free from solubility'promoter. The effect of the selective solubility promoter 'is,

thus not only to raise the general-level of solubilities, but also to minimize differences between solubilities of homologous members of a group of organic acids. The application of selective solu'-.

bility promoters is of great importance, for in stance, in the sweetening of sour West Texas gasolines which, due to their content of relatively large quantities of higher mercaptans, .are very diflicult to desulfurize andsweeten.

In cases where a substantially water insoluble organic liquid. contains both higher and lower" organic acids I may advantageously'use 'mi r' tures of solubility promoters. having difierent selectivities. For instance, I have found butylene' glycols to be highly selective for higher merca'ptans whereas ethanol amine described in my co pending application, Serial No. 102,892, filed September 28', 1936, is'not. Mixtures of the two have given excellent results in the sweetening of certain sour West .Texas gasolines. If desired, known solubility pr'omoters other than thosedescribed. herein or in my co-pendlng application Serial No. 102,892, filed September 28,-1 936,'such as methyl and ethyl alcohols, quaternary ammo nium bases, etc. may be. combined with one orseveral members of my group.

at or near ordinary room or atmospheric temper atures, although higher or lower temperatures may be employed. .As a general rule the emcacy-l of the extraction of acid compounds decreases with increasing temperature. On the other hand,

the improvement due to lowering thetreatingtemperature to subatmospheric temperatures is usually insufficient to warrant artificial refrigeration.

ity promoter solution, a small amount of soluf spent caustic alkali may be subjected to a treatment to recover at least the solubility promoter. For more eflicient extraction, however, we prefer to flow the caustic alkali solution and hydrocarbon type liquids in countercurrent to each other through a series of continuous treaters.

In spite of a substantial amount of water con-' tained in the caustic alkali containing solubilbility promoter is dissolved in and carried away by the hydrocarbon type liquid. To recover this small amount, or at least a major portion thereof,

eus

latinjg stream of water; The same water may be amount of' solubility in r'omoter in the treated iiquid'lsverysmalland since,- moreover, the solubility promoters are preierentlally soluble in lly'. wash ,treatedliquidwltlii a cir'cu water, asmall amount: of water will successively wash=a large amount or treated liquid. Usually the ratio ofwater necessfiry. to wash a treated substantially waterinsoluble' organic liquid is well below 111000. .Thesol itlzer contentin the wash water maybe 'allowedto 'accumulate'toa concentration of about 10 to 50%, after which the enriched'wash water may be added to the, caustic alkali.

- Methods recovery of the solubility promoterin the spent caustic alkali solutionor oi the caustic alkaliitself vary with the typeof acids Also solvent extraction with a solvent for the solubility promoterwhich -solvent is only partially miscible with the'aqueous caustic. alkali, may be applicable.

- If, however, the acids contained in the spent caustic alkali consist substantiallyzonly of mercaptans', the entire solution "can be regenerated by steaming, if desired under; superatmospheric pressures and preferably'under reflux of water or partof the distillate; In the steaming oper- 'ation mercaptan'sare carried off by the steam.

1 The" ease of steam regeneration, i. e. the amount of steam per pound or caustic alkali required to reach ,a' certaindesired low mercaptide content in the caustic alkali solution, depends upon the I concentration of both the solubility promoter and the caustic alkali lowerjc'oncentrations of either or both 'facilitatingfltheregeneration. 01 two caustic alkali containing solubility promoter so- My treating process is preferablycarried out" lu'tions having aboutjequai solvent power for niercaptans; one having a' higher caustic alkali and lower solubility promoter concentration than the other, the first one usually requires less steam forexpelling the mercaptans. Foriexample, two solutions containing 5-normal caustic alkali in 50% .isobutylene glyc'oL'and a 2.5-normal caustic alkali; in 75% isobutylene glycol, respectively, each containing .5. mols per liter normal butyl mercaptide, were steamed-under: identical conditions with 1.0 lb.,of steam per lb. of solution.

1 The mercaptide content of the 5-normal caustic alkali was reduced to .12- mols perliter, whereas that of the 2.5-norma1 caustic alkali was reduced only to .16 mols per liter. a

It appears that for all-around suitability a 3- to 6-no'rmal aqueous caustic alkali containing about 50% of a butylene glycol is one of the most satisfactory mixtures.

That the ease and completeness of the regeneration is of great importance may readily be seen by considering that, when extracting mercaptans from .an oil phase with a regenerated caustic alkali solution containing mercaptides, the mercaptan content in the oil phase cannot be reduced below the equilibrium concentration of the mercaptans in the oil phase with the mercaptides in absorbed. If; 'the'spent caustic alkali contains the regenerated caustic alkali. The higher th mercaptide content is. the higher is obviously the equilibrium concentration. content in the regenerated caustic alkali exceeds acertain maximum, sweetening of the oil phase may become impossible.

The steam used for expelling the mercaptans is preferably fractionated in an efficient bubble tower to separate vaporized solubility promoters therefrom. Since the preferred solubility promoters do not form azeotropic mixtures with steam, this fractionation can be achieved fairly completely. The steam containing mercaptans and normally mere traces of solubility promoter may then be condensed; mercaptans are allowed to segregate and are separated. If desired, the water of condensation may be returned to the steam boilers for the regeneration of steam and reused in the steaming. Solubility promoters,

that may have escaped with the steam, may be I recovered in this manner.

since the presence of acids stronger than mercaptans in the spent caustic alkali precludes steam-regeneration, I usually pretreat solutions ofmercaptans and stronger organic acids in hydrocarbon type liquids in a manner to separate the stronger acids only and then treat the pretreated solution according to my invention. In this manner a regenerable spent caustic alkali containing the solubility promoter and mercaptides only is obtained. Suitable pretreatment may consist of simple water wash, caustic alkali treatment, fractional distillation, etc.

The attached drawing represents a flow diagram of one form of my process. Hydrocarbon type liquid containing m'ercaptans, propelled by pump I in line 2, passes through a continuous conventional countercurrent treating system 3, preferably comprising several stages. An aqueous caustic alkali solution containing solutizer of the type hereinbefore described, which is pumped from tank 4 by pump 5 enters the treating system 3 through line 6 to flow in countercurrent to the hydrocarbon type liquid. Spent caustic alkali leaves treater 3 through line I.

Treated hydrocarbon type liquid substantially free from mercaptans but containing a small amount of solubility promoter is transferred through transfer line 8 to washer 9 to be washed continuously with water from tank Ill. The water is circulated by pump H in line [2 through the washer 9 in countercurrent to the treated hydrocarbon type liquid and then through line l3 back into tank Hi. The water is preferably circulated at such a rate thatthe ratio of water to oil in the washer 9 is of the order of about 1:2 to 1:10.

As the water circulates, it gradually picks up solubility promoter from the passing treated liquid, and when the solutizer in the water has reached a concentration of about 10 to 50% it is diverted through line I4 to join the spent caustic alkali in line 1. Fresh water from line I5 is introduced into the water tank l0, whereupon the circulation continues. If desired, instead of discarding the circulating water in batch, a small portion may be withdrawn and be added to the spent caustic alkali continuously.

Treated hydrocarbon liquid substantially free from solubility promoter is withdrawn from washer 9 through top line 6 to storage or further treatment. i

The spent caustic alkali in line 1 together with spent water from the water wash goes to storage tank ll, whence it is pumped by pump l8 in line If the mercaptide withdrawn,

ting column 2| containing bubble trays 22. The caustic alkali enters column 2| at an intermediate point. Steam generated in steam boiler 23 is blownthrough line 24 and column 2|, in countercurrent to the caustic alkali flowing over the bubble trays in the lower portion of the column. During this steaming, mercaptans are carried'oif by the steam together with some solubility promoter. In the upper portion of column 2| a fractionation is effected, the major portion of the solutizer being condensed and eventually together with regenerated caustic alkali, through bottom line 25 to reboiler 26.

Reboiler 26 is heated by the closed steam coil 21 operated with steam from boiiler 23. Excess water in the caustic alkali is driven off through vapor line 2.8, and regenerated caustic alkali of the proper concentration containing substantially all of its original solubility promoter and having substantially the composition of the original caustic alkali is withdrawn through line 29, cooler 30, into storage tank 4. Make-up caustic alkali and solubility promoter to compensate for the infinitely small losses may be introduced into tank l0 through line 3|.

The upper part of column 2| comprises an eflieient fractionating tower such as a bubble tower. The vapors leaving the column through vapor line 32 consist largely of steam and mercaptans, which form azeotropic mixtures, and a relatively small amount of solubility promoter. The vapors are condensed in condenser 33 and the resulting condensate is separated in separator 34. Water insoluble mercaptansrise to the top and are withdrawn through line 35. A portion of the water which usually contains some solubility promoter is returned by pump 36 in line 31 as reflux to the top of column 2|, while the remainder may be discarded through line 38, or preferably is pumped by pump 39 through line 40 to the steam boiler 23. Solubility promoter, together with the steam generated therein, returns through line 24 to the caustic alkali in column 2|. Make-up water for the steam boiler 23 is introduced through line 4|.

The advantages of my preferred solubility promoters over other polar compounds capable of acting as solubility promoters when applied to the described processes are demonstrated by the illustrative data below. West Texas straight run gasoline having a. mercaptan sulfur content of 0.116% was extracted in a l-stage treater with 25% by volume of a 50% aqueous solubility promoter containing 2.5 normal aqueous sodium hydroxide. The treated gasoline was then washed with 25% by volume of water for the recovery of the dissolved solubility promoters. The solubility promoters used in these tests were isobutylene glycol, 2-3-butylene glycol, and ethyl alcohol. The mercaptan sulfur content of the gasoline was reduced to about 04% with all three solubility promoters, while the amounts of solubility promoter left in the gasoline were 3X 10- 3 X 10- and 5x10 respectively, gallons solutizer per barrel of gasoline.

I claim as my invention:

1. In the process of separating organic acidreacting substances contained in a water-insoluble neutral or basic organic liquid, the steps comprising treating said liquid with an aqueous solution of a strongly alkaline base in an amount sufllcient to absorb a major portion of said acidreacting substances, said solution containing a substantial amount of a butylene glycol and from 15 to 50% water, under conditions to form two and the other consisting essentially of the treated organic liquid, and separating the layers.

2. The process of claim 1 in which the glycol is isobutylene glycol.

3. The process of claim 1 in which the glycol is 2-3-butylene glycol.

4. In the process of separating organic acidreacting substances contained in a water-insoluble neutral or basic organic liquid, the steps comprising treating said liquid with an aqueous solution of a strongly alkaline base in an amount sumcient to absorb a major portion of said acidreacting substances, said solution containing from 25 to 75% of a butylene glycol and from 15 to I 50% water, under conditions to form two liquid prising treating said distillate with'an aqueous solution or a strongly alkaline base in an amount suilicient to absorb a major portion of said mercaptans, said solution containing a substantial amount of a butylene glycol and from 15 to 50% water, under conditions to form two liquid layers,

one comprising the aqueous alkaline solution con- I tainlng mercaptides and the other consisting essentially of the treated hydrocarbon distillate, and v separating the layers. v

6. In the process of separating mercaptans contained in a hydrocarbon distillate, the steps comprising treating said distillate with an aqueous solution of an alkali metal hydroxide in an amount. suiiicient to absorb a major portion of said mercaptans, said solution containing. from 25 to 75% of a butylene glycol and from 15 to 50% water, under conditions to form two liquid layers, one comprising the aqueous alkali metal hydroxide solution containing mercaptides, and

the other consisting essentially of the treated bydrocarbon distillate, and separating the layers.

; DAVID LOUIS YABROFF.

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