US2769765A - Sweetening process - Google Patents

Description

Now-r. 6, 1956 R. L.. BETTS ET AL SWEETENING PROCESS 2 Sheets-Sheet 1 Filed Nv. 19, 1952 wmvOOd. VT. 042

R. l.. BETTs E1' Ar. 2,769,765

swEETENINGPRocEss 2 Sheets-Sheet. 2

L-Eztls Nov. 6, 1956 Filed Nov. 19, 1952 WlzO OSU/Q United States Patent O SWEETENING PROCESS Raymond L. Betts, Westieid, Horace F. Corning, Elizabeth, and Harold A. Ricards, Westfield, N. J., and Ilaines W. Ryder, North Reading, Mass., assignors to Esso Research and Engineering Company, a corporation of Delaware Application November 19, 1952, Serial No. 321,486

2 Claims. (Cl. 196-32) The present invention is concerned with van improved sweetening process. The invent-ion is more particularly concerned with the removal of mercaptan compounds, particularly those mercaptan compounds which have been heretofore relatively diicult to remove from petroleum fractions, especially from those petroleum fractions boiling in the motor fuel, kerosene, diesel oil and heating oil boiling ranges. In accordance with the present process `a feed hydrocarbon oil characterized by containing mercaptan compounds, is contacted with an aqueous alkali metal hydroxide solution containing solubility enhancing agents which are characterized by also containing iaromatic mercaptides.

A more specific 'adaptation of the present invention comprises a particular larrangement and sequence of stages wherein a catalytically cracked naphtha containing aromatic niercaptans is contacted with 1an alkali metal hydroxide solution and wherein the spent solution from this operation is utilized to contact aV virgin petroleum `fraction preferably one which boils in the motor fuel boil- 'ing range.

A particularly preferred adaptation of the present invention is to contact the catalytically cracked naphtba with a relatively weak caustic solution followed by concentrating the spent caustic solution to a caustic equivalent strength in the range from about 40% NaOH to 60% NaOH and to utilize this solution for the contacting of virgin oils.

it is well known in the 'art to treat petroleum oils by nl various procedures in order to remove objectionable compounds as for example sulfur compounds therefrom. For example, it is known to treat petroleum fractions boiling in the motor fuel boiling range and in the general range below about 700 F. with various chemical reagents such as sodium or potassium hydroxide solutions. Also, it has long been known that weakly `acidic materials, such as mercapt'ans, are reactive to various degrees with basic materials. One method for the removal of mercaptans from hydrocarbon streams has been to treat the streams with a reagent which is insoluble in the hydrocarbon stream but which contains'an alkali type of material which reacts with mercaptans. Such treating reagents have ncrmally been aqueous solutions. The reaction product is a basic salt of the mercaptan. Normally such salts exhibit some solubility in the aqueous phase; this solubility decreasing as the molecular weight or branchiness of the hy- 'drocarbon structure of the mercaptan increases.

The art has generally recognized in the treating 'of hydrocarbon streams with aqueous solutions such as' strong or weak caustic in a continuous type process, that the equilibrium is generally unfavorable for mercaptan extraction. Therefore, caustic solutions when they are only 'spent to' a small extent in mercaptide salts will not give any further reduction in mercaptan content despite the fact that these treating solutions have `a large residual-free caustic content. The art has aiso recognized that the efficiency 'of the spent caustic can be partially or fully restored by regeneration of. these solutions.' The regeneration normally consists of removal of a part or 'all of the Y t 2,769,765 Patented Nov. ti, i956 One method is to employ steam regeneration for the revivication of caustic solution spent in mercaptan removal from hydrocarbon streams. The steam regeneration is normally carried out in a packed tower or a tower provided with bubble plates or pierced type tray plate design. The steam hydrolizes the mercaptide salts to the corresponding mercaptans which are stripped out by the steam vapors and taken overhead. The regenerator also serves as a means of adjusting the gravity of the caustic solution which has been shown to be critical. The regenerated caustic is withd-rawn from the bottom of the regeneration tower and reused in the extraction stage of the system wherein the caustic contacts the hydrocarbon stream through suitable mixing devices for further extraction of mercaptans by the same caustic.

It has been found that certain constituents will function as solubility enhancing agents. These constituents tend to improve the partition coeiiicient of the acid mercaptan in the aqueous phase and to increase the solubility of the mercaptide salts in the aqueous phase, particularly when the mercaptan is of a relatively high molecular weight. As mentioned, mercaptans are difcult to extract from hydrocarbon solution by straight `aqueous alkali soiutions. This is particularly true of the higher molecular weight mercaptans. In general, this is occasioned by the fact that the two phases are not miscible and that good contact must be obtained to fallow the alkali metal ion to react with the mercaptan hydrocarbon. Furthermore, once the reaction has occurred, it is essential that if'the extraction of the mercaptan is to be accomplished the reaction product must be soluble in the aqueous layer. As pointed out heretofore, the higher the molecular weight of the mercaptan the lower the solubility of the corresponding mercaptide salt in the aqueous phase. The solubility enhancing agents tend to increase this solubility and thus increase the effectiveness of the alkali treating solution.

The exact chemical nature of many of these solubility enhancing agents is not entirely known. However, it is felt that they comprise the lower molecular weight aliphatic acids and the various substituted phenolic compounds. It has been found that phenolic type materials having alkyl side chains containing less than about 2 carbon atoms are not effective by themselves as solubility promoters and that organic carboxylic acids having less than about 3 carbon atoms per molecule `are likewise not very effective. However, as the length of the side chain of the phenols increases or the length of the aliphatic acid hydrocarbon chain increases, their effectiveness as solubility promoters increases greatly. However, the solubility of the acids or phenols themselves in the caustic solution decreases with increasing hydrocarbon ichain length and thus limits the molecular weight of acid or phenolic type material which can Vbe employed. It is believed that the optimum chain length should be from about 3-7 carbon atoms for the acids. Y

Particularly desirable aliphatic acids are for example, i'sobutyric acid, propionic acid and valerie acid. Other satisfactory :acids are normal butyric acid, caproic acid, alpha ethyl butyric and alpha methyl butyric acid. These acids may be substituted with an OH group or with a halogen. Various alcohols are also satisfactory, as for example, methyl alcohol and ethyl alcohol. Ethylene glycol, as well as, propylene glycol and other poly hydroxy eliicient of the acid mercaptan in the aqueous phase and to increase the solubility of the mercaptide salts in the aqueous phase, particularly when the mcrcaptan is of a relatively high molecular weight.

A specific discovery of the present invention iis that certain classes of these acid oils occur in certain streams while other classes of these acid oils are naturally present in different streams. For example, it has been discovered that catalytically cracked naphthas and other cracked oils boiling below about 700 F. are predominantly rich in aromatic mercaptans and phenols while generally virgin naphthas are substantially free of solubility enhancing agents. It is realized of course that certain specific virgin naphthas, such as those derived from Colombian and Coastal crudes, may contain some naphthenic acids. Thus, a particular adaptation of the present invention is to use a sequence and arrangement of stages for effectively removing mercaptans from naphtha Astreams secured from catalytically cracking operations and from virgin distillation processes.

The process of the present invention may be more 'fully understood by reference to Figure 1 illustrating a specific embodiment of the same.

Referring specifically to the figure, a cracked hydrocarbon oil, preferably one boiling in the range from about 100 F. to 430 F. and which has been secured from a catalytically cracking operation, is introduced into treating zone 1 by means of line 2. The product oil contains a relatively large concentration of aromatic mercaptans and substituted alkyl phenols.

In zone 1 the oil is promptly washed, i. e., before coming into contact with an oxygen-containing gas, with an aqueous alkali metal hydroxide solution which is introduced into the system by means ofpline 3. The fresh sodium hydroxide solution has a concentration in the range from about 3-30% NaOH by weight, preferably a concentration in the range from about 20-25% by weight. The treated oil substantially completely free of mercaptans is removed from zone 1 by means of line 4 and further refined or handled as desired. The spent caustic containing the mercaptans and phenols is removed from zone 1 by means of line S and recirculated by means of line 6 until about 10-90% spent of free caustic. The solution is passed to storage drum 40 from which it is withdrawn by means of line 41 and passed to regeneration zone 7 wherein it is reconcentrated to a concentration above 30%, preferably to a concentration in the range from Lt-60% by weight. The concentrated caustic solution is withdrawn from zone 7 by means of line 8 and used to treat a sour virgin naphtha which is introduced into the system by means of line 11. This virgin naphtha may be prewashed in zone 9 with recirculating caustic which may be withdrawn from the system by `means of line 12. The virgin naphtha is introduced into extraction zone 13 by means of line 14 while the concentrated caustic equivalent solution 1s introduced into zone 13 by means of line 8.

A treated virgin stream free of objectionable mercapv .cooling zone 18 and passed to a stripping zone 19 wherein the mercaptans are stripped out by means of steam introduced by means of line 20. The mercaptans are removed overhead from zone 19 by means of line 21, while the stripped condensate is removed by means of line 22 and recycled to regeneration zone 7. Steam is introduced into zone 7 by means of line 23. A typical analysis of the solution utilized in zone 13 for the treatment of .virgin naphthas is free NaOH 20%, alkali metal soaps of aromatic mercaptans and phenols 35%, water 45% by weight.

for the removal of the mercaptans.

By operating as described, a disadvantage heretofore secured in the art is overcome. For example, with solubility enhancing agents such as aliphatic acids, naphthenic acids, phenols and the like, the composition of effective reagents is limited by the formation of a second phase (often solid) in the reagent. However, by including in the acid oils or solutizing agents a critical amount of aromatic mercaptides, the two-phase formation is eliminated and only a single phase is secured irrespective of the concentration of the acid oils. This phenomenon is illustrated in Figure 2.

Referring specifically to Figure 2, a three-phase diagram is illustrated. The area defined by lines AB, AC and BC, is the area wherein the sodium hydroxide equivalent, that is, free NaOH and sodium soaps is in thc range from 30%-50% by weight.

The area defined by lines EB, EFC and BC is the area wherein the reagent forms two phases when utilizing a sodium hydroxide solution containing acid oils free of aromatic mercaptides.

Thus, it is evident that as the concentration of NaOH equivalent is increased above about 30%, two phases will be secured unless the concentration of the mercaptanfree acid oils 'is increased to about 30% to 45%.

Line GH illustrates the equilibrium when the acid oils contain 4% aromatic mercaptans. When 4% aromatic mercaptans are present a single phase exists above the line GH while two phases are secured below the line GH. Thus, the concentration of the caustic equivalent can be increased to about 34% without securing two phases irrespective of the concentration of the acid oils.

ALine JK illustrates the situation when the acid oils conlthe sodium hydroxide equivalent of the treating solution can be increased to about 44% without securing the formation of two phases irrespective of the concentration of the acid oils.

The line LM illustrates the situation when the acid oils contain 40% aromatic mercaptans. Under these conditions the sodium hydroxide equivalent concentration may be increased to 48% under which a single phase is always secured irrespective of the concentration of the acid oils.

The invention is broadly directed toward operations wherein mercaptan-containing oils are contacted with strong caustic equivalent solutions containing acid oils The caustic solutions are those having caustic equivalent strengths above about 30%, preferably in the range from about 40% by weight to 60% by weight. It is essential that the acid oils also contain substantial quantities of aromatic mercaptans as for example from 10-40% to 50% by weight. Under these conditions the formation of two phases of the treating solution is avoided irrespective of the concentration of the acid oils.

The invention is more particularly directed toward an arrangement of sequence of stages wherein a catalytically cracked oil boiling below about 700 F. preferably boiling in the range from about 100 F. to 430 F. is contacted with a 20-25% by weight caustic solution. The

. spent caustic solution is then concentrated to a concentration in the range from about 40% to 60%. This strong caustic solution contains acid oils which are characterized by having a concentration of mercaptans in the range from about 10% to 50% by weight. This strong caustic solution is then employed to treat a mercaptan containing-virgin naphtha under the conditions as described.

Temperatures employed may vary appreciably, but it is preferred that the treating temperatures be above atmospheric as for example in the range from F, to

v 210 F., preferably in the range from 100 F. to 150 F.

" n The ratio of free caustic to caustic soapsvmay vary appreciably asA forexample from 25% caustic to 75% Soapsto 39.0% Gausticto 10% caustic soaps.

What is claimed is:

1. An improved process for the removal of mercaptan compounds from catalytic cracked oils and from virgin oils which comprises in combination washing in a washing zone said catalytic oil with an alkali metal hydroxide solution containing no more than about 30% by weight of alkali metal hydroxide, withdrawing spent hydroxide solution comprising mercaptides from said washing zone, passing said solution to a concentration zone, concentrating said solution to a strength corresponding to`a hydroxide equivalent from about 40 to about 60%, thereafter contacting in a treating zone said virgin oil with said concentrated mercaptide-containing solution and maintaining a single phase operation in said last-named zone.

2. The process of claim l wherein the temperature in said last-named treating zone is in the range from about 100 to 150 F.

References Cited in the le of this patent UNITED STATES PATENTS 2,335,347 McNamara Nov. 30, 1943 2,460,277 Hart et al Jan. 25, 1949 2,525,152 Strickland et al Oct. 10, 1950 2,525,153 McClennan et al. Oct. 10, 1950 2,574,122 Ryder Nov. 6, 1951

Claims (1)

1. AN IMPROVED PROCESS FOR THE REMOVAL OF MERCAPTAN COMPOUNDS FROM CATALYTIC CRACKED OILS AND FROM VIRGIN OILS WHICH COMPRISES IN COMBINATION WASHING IN A WASHING ZONE SAID CATALYST OIL WITH AN ALKALI METAL HYDROXIDE SOLUTION CONTAINING NO MORE THAN ABOUT 30% BY WEIGHT OF ALKALI METAL HYDROXIDE, WITHDRAWING SPENT HYDROXIDE SOLUTION COMPRISING MERCAPTIDES FROM SAID WASHING ZONE, PASSING SAID SOLUTION TO A CONCENTRATION ZONE, CONCENTRATING SAID SOLUTION TO A STRENGTH CORRESPONDING TO A HYDROXIDE EQUIVALENT FROM ABOUT 40 TO ABOUT 60%, THEREAFTER CONTACTING IN A TREATING ZONE SAID VIRGIN OIL WITH SAID CONCENTRATED MERCAPTIDE-CONTAINING SOLUTION AND MAINTAINING A SINGLE PHASE OPERATION IN SAID LAST-NAMED ZONE

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