US2356980A - Toluene sweetening - Google Patents

Description

Patented Aug.. 29, 1944 UNITED Gysbert F. de Ridder, Houston, Tex., assignor to Shell Development Company, San Francisco, Calif., a corporation` of Delaware Application september 2'1, 1943, serial No. 503,285

(cl. 19e-29 11 Claims.

This invention is concerned with the sweetening of relatively low boilingaromatic hydrocarbons recovered from petroleum distillates with the aid of selective solvents, particularly from naphthas.

For many years the recovery of relatively pure low boiling aromatic hydrocarbons, specically benzene, toluene, xylenes, ethyl benzene, etc., from petroleum oil has been considered impracticable on a commercial scale, and only in recent years methods have been developed which have made such recovery feasible. Most of these methods employ selective solvents one way or another. Depending upon circumstances, liquid-liquid solvent extraction may be employed, or vapor-liquid extraction (better known as extractive distillation), or azeotropical distillation.

Many diiferent types of solvents maybe employed in the separation. For example, liquid SO'.- is suitable for liquid-liquid extraction, as well as most of the selective solvents which have in the past vbeen employed for extracting kerosene or lubricating oils, as for example phenol, cresols, nitrobenzene, nitrotoluene, benzaldehyde, aniline, furfural, beta beta dichlor diethyl ether, crotonaldehyde, various sulfolanes as sulfolane, methyl sulfolane, dimethyl sulfolane, methyl sulfolanyl ether, etc., and many others. Solventsv suitable for extractive distillation have been described in Dunn, U. S. 2,288,126. In azeotropical distillation, one may employ methyl alcohol, ethyl alcohol, acetone, methyl ethyl ketone, acetonitrile, propionitrile, nitromethane, nitroethane. SO2,

ammonia, methylamine, ethylamine, propylamine,

etc.

The petroleum naphthas which yield aromatic hydrocarbons may be straight run or cracked or re-formed (both catalytically and non-catalytically), iso-formed, dehydrogenated, isomerized and dehydrogenated, etc., distillates. Depending on the procedure employed, naphtha prior to the separation of the aromatics may or may not contain mercaptans. The present invention is concerned with the treatment of aromatic hydrocarbons recovered by extraction from naphthas which do not contain mercaptans, more particularly aromatic hydrocarbons having between 6 and 10 carbon atoms per molecule and boiling below about 400 F., such as benzene, toluene, xylenes, ethylbenzen'e, hemimellitene, pseudocumene, mesitylene, etc. l

In the separation of aromatic hydrocarbons from some naphthas with the aid of a selective covered by these methods, while otherwise of high purity, contain in many instances relatively large amounts of mercaptans. There are many methods known for refining hydrocarbon oils for the purpose of removing mercaptans, outstanding among them being the old sulfuric acid treatment and re-distillation. Sulfuric acid apparently may act in a dual capacity of an oxident and selective solvent, dissolving some mercaptans and converting others to relatively high boiling disulfides, which may then be separated by fractional distillation, provided the naphtha has a relatively narrow boiling range. Its drawback, however, is that it may fail to sweeten completelywhere mercaptan concentration is relatively high. When attempting to sweeten sour hydrocarbons such as benzene, toluene, etc., it was found that, as a rule, ordinary sulfuric acid treatment and re-distillation, Whether carried out in batch or in continuous treating equipment, would not result in complete sweetening. In all these cases it was possible, however, to sweeten successfully when blowing during acid treatment with a large amount of air, the only difficulty being in this case that the amount of air required was so large that serious vaporization losses of the aromatic hydrocarbon solvent, mercaptans accumulate in the aromatic extract fraction.

occurred. In an eiort to overcome losses by vaporization, mechanical agitation alone was tried in an attempt to Whip air into the mixture. This was also found to be inadequate in that not enough air could be whipped into the hydrocarbon to completely oxidize the undesirable ingredients and to produce a suiliciently sweet product unless excessively long agitation was employed, which made the process impractical.

The purpose of this invention is to economically, effectively and substantially completely sweeten sour aromatic light hydrocarbon distillates recovered by solvent extraction of naphthas. Another purpose is to reduce the harmful mercaptarl content in such aromatic hydrocarbons. A specic purpose is to sweeten sour 1 toluene recovered by extraction of petroleum distillates to produce nitration grade toluene. Other purposes will appear in the following description.

Now it has been discovered that sweetening of sour, relatively low boiling aromatic hydrocarbons can be achieved quite easily and without substantial loss of the valuable hydrocarbon by sulfuric acid treatment and re-distillation, if the sulfuric acid is thoroughly distributed throughout the hydrocarbon liquid by mechanical means and simultaneously a small controlled amount of airis sus-,- pended in the liquid together with the acid. This amount of air is substantially less than the stoichiometrical amount required to convert all of the mercaptans in the oil to disuldes, 4and preferably is between about t4; and 1/2 of said stoichiometrical amount. If less than this amount of air is employed the hydrocarbons will not be suiiiciently sweetened, and if a greater amount of air is employed unnecessary hydrocarbon losses will occur. lThe air together with the sulfuric acid serves to oxidize the last trace of mercaptans not extracted by the sulfuric acid. In addition to reducing losses by vaporization this procedure has the advantage of always producing a sweet product, and in cases where sweetening with acid alone was possible, of requiring only about half as much acid as is necessary to sweeten in the absence of air. The intensity of mixing the abovementioned limited amount of air, with the acid, and hydrocarbons must be at least such that during the entire contact time, which usually lasts from about 1/3 up tov 10 hours, the air and acid remain suspended in the hydrocarbon liquid. y In continuous treating equipment, contact times below about 8 hours are usually sufficient, while in batch treatment, owing to the greater difllculty of thoroughly distributing the air through a large volume of the hydrocarbon liquid, longer contact times are necessary as a rule. Y

Generally the steps in carrying out the process of this invention comprise: mechanically agitating the hydrocarbon with acid and air; separating the air from the acid and hydrocarbon; and, separating the acid from the hydrocarbon. In order to remove products of oxidation, sulfonation, etc., it is generally necessary to include the additional steps of:v neutralizing the hydrocarbon; and, re-distilling the neutralized product.

The accompanying drawing shoes a preferred flow diagram of the process of this invention.

Referring to the drawing, the sour aromatic hydrocarbons are introduced through valved line I into the mixer 2, wherein they are contacted with acid introduced through valved line 3. The resulting mixture is then passed through line 4, into a turbo-mixer or agitator 5, provided with an efcient stirrer 6, such as a lightning mixer, driven by a suitable means such as a. motor 1. A controlled amount of air is introduced into the agitator through valved line 8, joining line 4. Steam may also be introduced together with this air through valved line 9 to help to keep the temperature of the mixture above about 75 F., and preferably above 85 F. and below about 160- F., as well as toprevent clogging of the openings in line 4. Optimum treating temperature range is between about 110 F. and 120 F. Clogging may be caused by the formation of vand sulfuric acid react to form toluene-sulfonic acid which readily crystallizes out of solution. At higher temperatures, however, they remain in solution.

In the agitator the air, acid, and sour aromatic hydrocarbons are intensively mixed s0 that a suspension is produced of sufficient stability to prevent air separation in the agitator and to cause retention in the suspension of a major portion of the air while the latter remains in the agitator. The residence time in the agitator is between about 1A; to 3 hours. As a result substantially all the mercaptans in the hydrocarbons will either be extracted or oxidized.

This suspension of hydrocarbon, acid, and air passes from the top of the agitator 5, through line I0, into air separator II, provided-with an air vent line I2, containing automatic valve I I, operated by levers I5 connected. to a liquid level float I4 in the chamber of the separator II.

This automatically controlled valve I3 permits uniform and constant separation of the air bubbles from the foam produced in the agitator i and minimizes the loss of hydrocarbon vapors. It has been found desirable to separate the air from the suspension produced in the agitator 5 prior to the separation of the acid in that after the air is removed the acid may be separated by mere settling.

The air-free mixture of acid and hydrocarbon liquid is withdrawn from the bottom of separator II, through line I6, into the acid separator I1, which preferably comprises a settler and coalescer, not shown. In the settler the acid and hydrocarbon mixture is allowed to form two liquid phases, namely an upper hydrocarbon phase and a lower acid-sludge phase, the latter being Withdrawn vthrough valved line I8. phase is then passed through the above-mentioned coalescer such as a chamber iilled with pebbles or the like to remove amounts of acid which did not settle into the acid-sludge phase. After this the hydrocarbon phase usually sti11 contains a trace of acid which must be removed by neutralization. For this purpose it is passed through line I9, into mixer 20, wherein it is mixed with an aqueous alkaline solution such as aqueous sodium hydroxide, sodium carbonate, etc., introduced through valved line 2l in an amount sufficient to neutralize residual acid. The resulting mixture is then passed through line 22 into settler 23.

The neutralized oil is passed through line 25 to the still 28, provided with heating means 21, for redistillation of the sweetened hydrocarbon The redistilled and final pure and sweetened hydrocarbon product is withdrawn from the top of still 26 through line 28, and the remaining small percent of high boiling impurities is withdrawn through the bottom valved line 29 and usually discarded.

This redistillation of the aromatic hydrocarbons is preferably carried out at a temperature not much above the boiling temperature of the aromatic product, in that excess heat tends to decompose the disuldes formed in the process, thereby souring it again. To avoid this it is also desirable that copper be omitted in the bottom of the still 26, in that copper acts as a catalyst causing decomposition of these disuliides. This latter catalytic eect, howeverI may be overcome by addition of small amounts of ammonia and air during the distillation. An ordinary steel still has been found satisfactory in the redistillation of sweetened aromatic extracts. The amount of loss of aromatic hydrocarbon by 'redistillation generally ranges between about 1% and 21/% of the amount redistilled.

The sour aromatic hydrocarbons comprising the feed to this process must be free from the solvent used in extracting them from their respective hydrocarbon distillates. For example, in the extractive distillation of a toluene concentrate in accordance with Dunn et al. U. S. 2,288,126, in the presence of phenolic solvent. the resulting toluene should preferably be caustic alkali treated to remove all traces of the phenol prior to the sweetening treatment of this invention.

Generally, the amount of mercaptan sulfur present in such aromatics obtained by extraction The upper as described is not more than about .5% by weight and usually is about .3% by weight or less depending upon the type of crude from which it Was derived and its prior treatment, although in some cases it may exceed 1%.

The acid employed in treating the aromatic hydrocarbon is preferably relatively concentrated sulfuric acid having a concentration between about 87% and 100%, preferably about 93% to 98%. This acid may be fresh or reconcentrated and refortied spent acid which has primarily been employed in the same or other treating processes. The quantity of acid required is dependent primarily upon the composition of the sour aromatic feed and the content of mercaptans and olens, as well as on the concentration of the acid employed. Treating temperatures are another factor, optimum temperatures being bev tween 110 F. and 120 F. Thus for example in treating toluene, the acid requirement at 110 F. is only about half that at 85 F. The quantity of acid required may also depend to a minor extent upon the mechanical design of the acid injection apparatus or mixer 2. The quantity of acid usually ranges from about 2 to 50 pounds per barrel of sour aromatic hydrocarbon, depending upon the concentration of the acid, less acid being required the higher its concentration. Roughly, in the range of 87% to 100% concentration, 1% difference in the concentratloncorresponds to about 2 to 3 pounds per barrel difference in the amount required. Thus, when using 98% to 100% sulfuric acid, amounts required normally vary from about 2 to 15 pounds per barrel; when using 93% to "98% acid the amounts vary between about 5 and 25 pounds per barrel; and when using 87% to 89% acid the requirement is usually on the order of 30 to 50 pounds per barrel. More acid may be employed if color tests, such as the acid heat test, of the resulting sweetened product do not meet specifications.

The following specific example clearly illustrates the processI of this invention:

Example I A toluene concentrate derived by distillation of a West Texas petroleum naphtha was extractively distilled with phenol to produce a sour, 1 toluene containing .3% by weight of mercaptan sulfur. Theoretically, one barrel of this sour toluene would require 13.5 cubic feet of air to oxidize all of the mercaptan present in the toluene. pounds of 98%l sulfuric acid and 2 to 3 cubic feet of air per barrel of toluene in a mixer, agitator and air separator exactly as shown in the drawing. The residence time of the mixture of toluene, acid and air in the agitator was about 35 minutes, and the treating temperature was about 85 F.

After removal 4of the air in the air separator, the acid was removed by settling and coalescing. The resulting toluenewas then neutralized with 2.25 to 2.75 pounds per barrel of 80 B. aqueous sodium khydroxide solution andv redistilled in a still having a reboiler temperature of not over 300 F., whereby a doctor sweet toluene was produced meeting nitration grade specifications.

The loss of toluene caused by the sweetening treatment amounted to between 1% and 1.5%.

I claim as my invention: v

l. In a process for sweetening sour aromatic hydrocarbons of 6 to 10 carbon atoms per molecule containing mercaptans, the steps comprising contacting them at a temperature between 75 and 160 F. with between about 2 and 50 pounds per barrel of sulfuric'acid of 87100|% concentration and a controlled amount of air, mechanical- 'ly agitating said mixture to produce a suspension of said acid and air in said hydrocarbons, maintaining said suspension for a period of about 1/3 to 10 hours and then separating said air and said acid from said suspension, said amount of air being substantially less than the stoichiometrical quantity necessary to oxidize all the mercaptans contained in said hydrocarbons-to disuldcs.

2. The process of claim 1 wherein said aromatic hydrocarbon is toluene. Y

3. The process of claim l wherein said amount of air is between 1/8 and 1/2 of said stoichiometrical quantity.

4. The process of claim 1 wherein said sulfuric acid is between about 93 and 98% concentrated. 5. In a continuous process for sweetening sour aromatic hydrocarbons of 6 to 10 carbon atoms per molecule containing mercaptans, the -steps comprising contacting them at a temperature between 75 and 160 F. with between about 2 and 50 pounds per barrel of sulfuric acid of 87-100% concentration and a controlled amount of air, mechanically agitating said mixture to producea suspension of said acid and air in said hydrocarbons, maintaining said suspension for a period of about 1A; to 3 hours and then separating said air and said acid from said suspension, said amount of air being substantially less than the stoichiometricalh quantity necessary to oxidize all the mercaptans contained in said hydrocarbons to disulfides.

6. In a batch process for sweetening sour aromatic hydrocarbons of 6 to 10 carbon atoms per molecule containing mercaptans, the steps comprising contacting them at a temperature between v75 F. and 160 F. with between about 2 and 50 This sour toluene was admixed with 20.

pounds per barrel of sulfuric acid of 87% to 100% concentration and a controlledamount of air, mechanically agitating said mixture to produce a suspension of said acid and air in said hydrocarbons, maintaining said suspension for a period of about 3 to 10 hours and then separating said air and said acid from said suspension, said amount of air being substantially less than the stoichiometrical quantity necessary to oxldize all the mercaptans contained in said hydrocarbons to disuldes.

7. In a process for sweetening sour aromatic hydrocarbons of 6 to'10 carbon atoms and containing mercaptans, the steps comprising contacting them at a temperature between 75 F. and F. with between about 2 and 50 pounds per barrel of relatively concentrated sulfuric acid and a controlled amount of air, mechanically agitating the mixture to produce a suspension of said acid and air in said hydrocarbons, maintaining said suspension for a period of about 1A; to 10 hours, separating the air from said suspension, and then separating the resulting spent acidk 160 F. with between. about 2 and 50 pounds per barrel of relatively concentrated sulfuric acid and a controlled amount of air, mechanically agitating the mixture to produce a suspension of acid and air in said hydrocarbons, maintaining said suspension for a period of about /a to 10 hours, separating the air from said suspension, and then separating the resulting spent acid from said hydrocarbons, and distilling said hydrocarbons to produce a pure sweetened product, said controlled amount of air being between V3 and 1/2 the stoichiometric quantity necessary to oxidize all the mercaptans contained in said hydrocarbons to disuldes.

9. 'I'he process of claim 6 wherein said aromatic hydrocarbons boil below about 400 F.

10. In a process for sweetening sour toluene containing mercaptans the steps comprising contacting said toluene at a temperature between 75 and 160 F. with between about 2 and 50 pounds per barrel of concentrated sulfuric acid, a controlled amount of air, and a sumcient amount o! steam to maintain the temperature of the mixture above about 85 F., mechanically agitstinz said mixture to produce a suspension of said acid, air, steam and hydrocarbon, maintaining said suspension for a period of '/3 to 10 hours. separating the air irom the suspension, and then separating the resulting spent acid and resulting condensed steam from the hydrocarbons by settling, said controlled amount o! air being between 5/8 and 1/2 the stoichiometric quantity necessary to oxidize al1 the mercaptans contained in said hydrocarbons to disulfldes.

11. The process of claim 8 wherein said temperature is between about 110 and 120 F.

GYSBERT F. nn RIDDER.

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