US2431770A

US2431770A - Sweetening process

Info

Publication number: US2431770A
Application number: US516368A
Authority: US
Inventors: Donald E Payne; Voorhees Vanderveer
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1943-12-31
Filing date: 1943-12-31
Publication date: 1947-12-02
Anticipated expiration: 1964-12-02

Description

Dwz, 1947. D. E; PAYNE Em.-

SWEETENING PR'ocE'ss Filed Dec. 31, 1945 Patented Dec. 2, 1947 UNITED STATES PATENT OFFICE sWEETeeeEs-S Donald E. Payne, Chicago, ildvadetvee Vonrl hees, Homewood, Ill., assignors vto Standard Oil Company, Chicago, Ill., a corporation of Indiana Appiieation Deeember 31, 1942, serialV o. 516,368

(o1. ieee-s2) 5 Claims. 1

This invention relates to a process of sweetening hydrocarbon oils and particularly petroleum distillates such as gasoline and kerosene. The process is especially applicable to the swcetening of cracked products such as gasoline and kerosene `from thermal or catalytic cracking processes. One of the objects of the invention is to provide a process of removing mercaptans from gasoline and other light petroleum products by extraction with alkaline solutions.` Another object of the invention is to improve the alkaline extraction of mercaptans by employing a new method of regenerating the alkaline solutions which are recycled in the process. Still another object of the invention is to provide a method for producing sweet gasoline which will pass the doctor test without the use of expensive reagents or retreating operations. In brief, oui` invention, W'l-iich will be described more specically hereinafter, involves the use of a second stage of regeneration for at least a part of the caustic solution used in sweeten-ing, after the solution has been regenerated incompletely in the usual manner, and it involves more particularly the extraction' of the partially regenerated caustic with a stream of sweetened distillate from the process itself.

The process of sweetening gasoline involves primarily removal from the gasoline of mercaptans which are dissolved therein in a small amount. It has heretofore been demonstrated that the mercaptans of low molecular weight such as ethyl and propyl mercaptans are relatively easily removed by extraction with alkaline solutions such as caustic soda or caustic potash. The higher molecular weight mercaptans such as amyl and hex-yl mercaptans, being more soluble in the oil, are less readily removed by caustic solutions. To facilitate the removal of mercaptans it has heretofore been the practice to add to the caustic solution an organic reagent or solvent commonly called a solutizer which is soluble in the caustic, thereby increasing the solubility of the mercaptans in the caustic solution and facilitating their extraction from the gasoline. From 5 to l% of solutizer is usually eiective, and the amount of solution required to sweeten gasoline is" usually 2 to 20%, for example 4 to 10% by vollime based on gasoline treated.

As the extraction proceeds, for example in a continuous process where the gasoline is passed thru a body of the caustic solution or where the gasoline is continuously extracted in a tower thru which caustic solution is recirculated from a supeA ply, the mercaptan content of the caustic soon irlioielss t' poit Where micaptans are Ilo longer completely removed from the gasoline, at leastno't suiicient to provide a sweet product which will. pass the doctor test. This test has long been used the' `petrolouin industry and consists in shaking a sample of oil with a al-` kaline solution of sodium plunfibte in the presence oi a trace of sulfur. IiD the oil contains an ob eotionable lan'iciiint of m'ercaptan, Va colored precipitate of lead meroaptide Will `be observed.

In ord-ei" to obtain more complete removal of mercaptans with alkaline solutions, either with cr the addition of organic mercaptan solvent-s, it is necessary to regenerate the alkaline solution, by' which is' meant the removal of merc-ptiisi therefrom; Ina-y be accomplished several ways,- -one of the inost common methods being to' Heat the solution to an elevated temperdiz-ure, for example to the boiling point or near boiling point, and vaporiz'e oi the mercaptans by a current of air, steam or an inert gas, such as -ue gasa* This regen-eration technique is based oii the fact that' the alkaliriiercaptides, forexample sodium or potassiur'n mercaptides, contaidi th alkaline Solution dissociate at higher temperatures as the Vapor pressure of the meroaptan eduilibriui therewith increases. By removing`- the 'iercaptan vapor, the equilibrium is displaced and the' caustic solution may be sube stanti-ally freed of mercaptans. This solution is then .cooled and returned to the gasoline extractor for" further us.- The mercaptan vapors liberated from` such'- a process may be condensed aridthe rnecaptans recovered, or the vapors may lie-burned in a furnace.

Another method of regeneration which has certain advantages from the standpoint of saving oI llieat :comprises contacting .the mercaptancontaining alkaline solution with air or oxygen or other oxidizing agent at normal temperatures or ternperature'sA only slightly elevated, for example to' 130 F. The mercaptans are oxidized by the air in the presence of caustic to organic disul'ii'd'es which are no longer soluble in the caustic solutionandwhich may be separated therefrom, after'wliich the alkaline solution is returned to the extractor for further use. The oxidation of mercaptans to disuldes is not a very rapid reactiomparticularly in the case of the lower molecular weight mercaptallns,` andtherefore it is desirable to employ a catalyst to speed up this reaction. A number of catalysts have been proposed and used heretofore, both organic and inorgzmic.` ,In the latter class are the salts of lead, copper, cobalt. nickel, cadmium, zinc and vanadium, while the organic oxidation catalysts are generally phenolic in character including cresols, amino phenols, and particularly the polyhydroxy aromatic compounds, such as hydroquinone, catechol, pyrogallol, dihydroxynaphthalene and the various tannins. These catalysts are generally effective in amounts of 0.1% up to 2 or 3%.

When using the organic regeneration catalysts, diculty is sometimes encountered with resinication or overoxidation of the catalyst as a result of long contact with air or oxygen in the regenerator. This may occur, for example, when using tannin as the catalyst in which case it is desirable to avoid regenerating the caustic solution with air to a point where complete regeneration is obtained. As a result, a small amount of mercaptan may be returned to the gasoline extractor and incomplete sweetening of the gasoline follows, owing to the impossibility of completely removing mercaptans from gasoline with a caustic solution which contains mercaptans. Even when not confronted with the problem of regeneration catalyst destruction,rthere is always some diiculty in producing sweet gasoline by regeneration and recycling alkaline solutions owing to the diilculty of regenerating these solutions to a point Where the mercaptan concentration will be suiciently low to provide the necessary equilibrium for sweet gasoline. In order to pass the doctor test a gasoline must contain less than about .001% of mercaptan sulfur.

In our process we have provided a method of obtaining sweet gasoline by substantially completely removing the mercaptans from at least a part of the caustic solution after the solution has been regenerated in the usual manner and before recycling back to the gasoline extractor. One modication of our process is illustrated by the accompanying drawing which shows diagrammatically an apparatus for carrying out the process.

Referring to the drawing, a hydrocarbon feed stock, for example gasoline which may be cracked or straight-run gasoline, or an absorption naptha is charged to the process by line I leading to pretreating extractor I I wherein the gasoline is contacted with an alkaline solution introduced at I2. The principal purpose of the pretreatment in Il is to remove from the gasoline any hydrogen sulfide it may contain and other acidicmaterials such as naphthenic acids and phenols. On account of their high oil solubility the phenols are not completely removed by alkaline washing but their concentration is reduced to a point where they do not interfere with subsequent operation of the process. For this purpose We may use caustic soda, caustic potash, sodium carbonate, or other convenient alkaline` material. When washing cracked gasoline, a sodium hydroxide solution of to 30%, for example 20%, is satisfactory. The caustic for the purpose may be stored in supply tank I3 and recycled thru lines I2 and I4. Additional caustic may be supplied to the system by line I5 and spent caustic may be withdrawn by line I5. The spent caustic solution, which may contain acids and phenols dissolved therein in an amount of about to 40%, preferably 25 to 30%, may be acidied subsequentlyto recover phenols which are of value as insecticides and wood preserving agents.

The washed gasoline from pretreater II is conducted by line II to the main mercaptan extractor I8. Tower I8 may be provided with suitable baile plates or packed with broken tile, Raschig rings, Berl saddles, or other suitable contacting material. As the gasoline flows upward thru the tower I8 it encounters streams of caustic solution introduced at I9, 20 and 2| before it passes out at the top of the tower thru line 22 sufciently depleted of mercaptans to meet the desired specifications. The amount of caustic solution can be about 5 to 15%, for example. For the purpose of our process, caustic solutions containing from about 10 to 30% are preferred, but other concentrations may be used, e. g. 5 to 40%, an example of a suitable concentration being 20%. In general caustic potash is somewhat more effective for mercaptan removal than caustic soda. The caustic solution is withdrawn from the base of the extractor by line 23 and conducted thru heater 24 to regenerator 25. Tower 25 may also be a baffled or packed tower, or a mechanically agitated vessel, in which the caustic solution is contacted with a current of air introduced at 26 and vented at 2. At the base of the tower the solution is drawn into settling section 28 where disuldes are allowed to separate and are withdrawn by line 29. Gas oil or other low volatile solvent may be introduced into separator 28 to assist in removal of disulfides. Catalyst for regeneration which may consist of an aqueous or alkaline solution of catechol or tannin is introduced by line 30 in the desired amount. After having once been brought up to the desired concentration it is not necessary to add additional catalyst except at infrequent intervals to make up for mechanical losses due to leakage and entrainment and slight losses from chemical destruction of the catalyst due to side reactions.

The partially regenerated caustic solution, which may be regenerated, for example, to the extent of to 98%, is Withdrawn by line 3l and recycled thru cooler 32 and line I9 to the extractor. It is preferred to operate the extractor at ordinary temperature, for example 30 to 50 in winter and up t0 70 or 90 in summer, while the caustic regenerator 25 is preferably operated at a temperature of around to 130 F., although a temperature of F. may be used. We may operate our regenerator at the same temperature as the mercaptan extractor, however. Inasmuch as the partially regenerated caustic in line I9 contains mercaptans, it will not produce a sweet gasoline at that point in the extractor but serves to remove the bulk of the mercaptans in the gasoline and leave therein only a small residual amount. In order to reduce the mercaptan concentration still further, We withdraw a portion of the partially regenerated caustic solution by line 33 and conduct it to caustic purifier 34 where it is extracted by a stream of sweet gasoline withdrawn from line 22 by line 35 leading thru heat exchanger 36 and heater 36a. The amount of gasoline withdrawn to purier 34 may be about 10 to 50% of the gasoline charged to extractor I8, preferably about 20 to 30%. In purifier 34 the small amount of mercaptans remaining in the caustic are transferred counter-currently to the gasoline which is then conducted by line 3'I back thru exchanger 36 and cooler 36D to an intermediate point in the extractor I8. The purified caustic solution now substantially free of mercaptans is conducted by line 38 thru cooler 39 and line 20 back to the extractor, introducing it at a point substantially above the point of introduction of the partially regenerated caustic solution previously introduced at I9. The upper section of the extractor therefore forms a second extraction zone and if desired, separate towers may be used for the purpose.

In case separate towers are used, itis preferred to conduct the alkalineso'lution from the bottom of the second extraction zone to the top of the first extraction zone but it may also be conducted directly bac-k to the regenerator 25.

In operating our process, we prefer to maintain the temperature of the caustic purifier 34 above the temperature of the extractor I8 in order to obtain a favorable equilibrium for transfer of mercaptans from gasoline to caustic in I8 and caustic to gasoline in 34. For this purpose, a temperature differential of about 25 lto 150 F. is usually suiiicient, preferably about 100 F. The caustic purifier 34 may be operated, if desired, at temperatures in the range of 150 to 225 F. in order to obtain more co-mplete removal of mercaptans from the caustic solution. For this purpose suitable heating coils may be supplied in 34 and the caustic solution in line 33 may be diverted thru heater te. When the temperature is above the boiling point of the distillate, pressure will be needed.

The proportion of the caustic solution in line 3l which is diverted thru line 33 for the purpose of completing the sweetening at the top of extractor I does not need to be rigidly defined as there is considerable flexibility in the operation of our process. Thus when a fairly complete regeneration of caustic solution is obtained in 25, we may divert only 5 or 10% of the caustic solution thru purifier 313. Under other conditions, we may divert 35 to 50% of the caustic solution thru the purifier and we may even operate with all the caustic solution being conducted thru the purier 34, depending on the economics of the operation, a factor affected by many variables. In general we prefer to divert about 10 to 25% of the caustic thru the purier.

On acc-punt of unavoidable contamination it is sometimes 'desirable to withdraw caustic solution from the system and replace it with fresh caustic, and this may be done by line il and discharge valve d2. If desired, the caustic removed from the system in this way may be employed in the pretreating system by conducting it thru valve 43. Fresh caustic supplied thru line 2l may be of the same strength as that used in the system or it may have a higher strength if it is desired to build up the concentration of the caustic in the system.

By introducing the fresh causticl at a point above the point of introduction of regenerated caustic thru line 2G we obtain additional extraction of mercaptans by means of caustic solution which is entirely free of mercaptans, thus providing a still more favorable equilibrium for mercaptan removal than that obtained at the lower levels in the extractor I8 by solutions introduced at I9 and 20. If desired, a portion of the caustic solution regenerated in 25 may be completely stripped of mercaptans by steaming, in apparatus not shown, and this solution may be continuously supplied to the extractor by line 2| in order to maintain the desired differential in mercaptan concentration required in the process. When operating in this way, extractor I8 and caustic purifier 34 may be operated at the same temperature, eliminating the need for heat exchangers, heaters and coolers.

As indicated above, We may employ various organic reagents or solvents in the alkaline solution to increase the solubility of mercaptans therein. Examples of such solvents are the phenols and alkyl phenols, cresols, ethyl and propyl phenols, xylenols, organic acids, naphthenic acids, fatty acids, preferably of 3 to 6 carbon atoms, furoic' acid, alcohols, preferably those 4of low volatility such a glycol and glycerine, ethanol and propanol amine, and the alkyl substituted benzoic acids. The solvents are usually effective in concentrations of about 5 to 50%, depending on solubility and on the character of the solvent. Phenols contained naturally in gasoline, particularly cracked gasoline, may be permitted to collect in the caustic alkali solution to an effective amount, e. g. 10 to 25%, and the amount may be controlled by adding phenols recovered from gasoline or by regulating the amount of phenols removed in the prewash stage. The extracting solution can be prepared for example by starting with an approximately 30% caustic soda solution and operating on an alkyl phenol containing charging stock such as a cracked gasoline until about 2 mols of the caustic have been converted to sodium phenolate or cresylates, giving a washing solution containing about 20-25% of phenolates and 20% free caustic. The addition of about 1% of catechol, hydrcquinone, or tannin to this solution is suicient to enable it to be regenerated and recycled almost indefinitely, make-up being necessary only for mechanical losses.

Having thus described our invention what we claim is:

l. The process of sweetening mercaptan-containing petroleum distillates which comprises contacting said distillates countercurrently in a first extraction Zone with an aqueous caustic alkali solution containing a solutizer for mercaptans, separating said caustic alkali solution and dissolved mercaptans from partially sweetened distillate and contacting it with air in a first regeneration stage in the presence of a phenolic mercaptan oxidation catalyst, thereby partially regenerating the solution by oxidation, controlling the oxidation to convert a major proportion but not all of said mercaptans to organic disulfides insoluble in said caustic solution while avoiding destruction of said phenolic oxidation catalyst by excessive oxidation, recycling a portion of said partially regenerated caustic solution to said first extraction zone and contacting it with petroleum distillate to remove mercaptans from additional amounts of distillate supplied thereto, countercurrently contacting another portion of said partially regenerated caustic solution in a second regeneration stage with a stream of sweet distillate produced in the process, the volume of which is substantially less than the volume of mercaptancontaining distillate undergoing treatment, thereby transferring residual mercaptans from said partially regenerated caustic solution to said distillate stream and producing a completely regenerated caustic solution, separating said distillate and caustic solution in said second regeneration stage, recycling the separated distillate stream to said first extraction zone and combining it therein with the mercaptan-containing distillate first mentioned, countercurrently contacting in a second extraction zone said completely regenerated caustic solution with said partially sweetened distillate from said first extraction zone, thereby producing a sweet distillate product, separating said product from caustic alkali solution in said second extraction zone and transferring the lastmentioned solution to said rst extraction zone wherein it is contacted with said mercaptan-containing distillate rst mentioned.

2. The process of claim 1 wherein the second regeneration Stags is operated at a higher tem- 7 perature than the temperature of said distillate extraction zones.

3. The process of claim 1 wherein hydrogen sulfide, phenols and organic acids are separated from the distillate charged to said first extraction zone by initially contacting it with an alkaline solution.

4. The process of sweetening a petroleum distillate which comprises contacting a continuous stream of said distillate with an aqueous caustic alkali solution in two successive extraction zones, partially sweetening said distillate in the rst of said extraction zones wherein mercaptans are dissolved in said alkali solution, separating mercaptan-containing alkali solution from distillate in said first extraction Zone and subjecting it to oxidation by contacting with oxygen in a rst regeneration zone at a temperature of about 120 to 150 F., regulating the extent of oxidation to convert a major part of the dissolved mercaptans in said solution to disulfides while retaining a minor part of the mercaptans unoxidized, separating disuldes in said first regeneration zone and discarding them from the system, recycling a part of the partially regenerated alkali solution from said first regeneration zone to said rst extraction Zone for extraction therein of additional amounts of sour distillate, transferring another part of the partially regenerated alkali solution from said first regeneration Zone to a second regeneration zone wherein substantially all remaining mercaptans are removed by countercurrent contacting in the absence of oxygen with a substantially mercaptan-free liquid hydrocarbon stream, thereby transferring mercaptans from said solution to said hydrocarbon stream, the resulting hydrocarbon stream containing mercaptans being then charged to said first extraction Zone for further removal of mercaptans therein, conducting regenerated alkali solution from said second regeneration Zone to the second of said distillate extraction zones, and contacting it therein with partially sweetened distillate from said first extraction zone, thereby completely sweetening said distillate, separating alkali solution from distillate in said second extraction zone and combining it with the alkali solution in said rst extraction zone for the removal of mercaptans from said additional amounts of sour distillate therein.

5. The process of sweetening a petroleum distillate which comprises contacting a continuous stream of said distillate with an aqueous caustic alkali solution in two successive extraction Zones, partially sweetening said distillate in the iirst of said extraction zones wherein mercaptans are dissolved in said alkali solution, separating mercaptan-containing alkali solution from distillate in said first extraction zone and subjecting it to oxidation by contacting with oxygen in a rst regeneration zone at a temperature of about 120 to 150 F., regulating the extent of oxidation to convert a major part of the dissolved mercaptans in said solution to disulfides while retaining a minor part of the mercaptans unoxidized, separating disulfides in said rst regeneration Zone and discarding them from the system, recycling a, part of the partially regenerated alkali solution from said rst regeneration zone to said first extraction zone for extraction therein of additional amounts of sour distillate, transferring another part of the partially regenerated alkali solution from said rst regeneration zone to a second regeneration zone wherein substantially all remaining mercaptans are removed by extraction in the absence of oxygen with a hydrocarbon substantially free of mercaptans, conducting regenerated alkali solution from said second regeneration Zone to the second of said distillate extraction zones, and

contacting it therein with partially sweetened distillate from said rst extraction Zone, thereby completely sweetening said distillate, separating alkali solution from distillate in said second extraction zone, combining it with the alkali solution in said rst extraction zone for the removal of mercaptans from said additional amounts of sour distillate therein, and employing a portion of the completely sweetened distillate from said second extraction zone as the mercaptan-free hydrocarbon for extracting the alkali solution in said second regeneration zone, thereby transferring mercaptans from said solution to said sweetened distillate and charging the resulting distillate containing mercaptans to said first extraction zone.

DONALD E. PAYNE. VANDERVEER VOORHEES.

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

UNITED STATES PATENTS Number Name Date 2,336,109 Lowry et al. Dec. 7, 1943 2,324,927 Heilman July 20, 1943 2,317,770y Holloway et al Apr. 27, 1943 2,285,898 Caselli et al June 9, 1942 2,258,279 Caselli et al Oct. 7, 1941 2,337,467 Hewlett Dec. 21, 1943 1,998,863 Chaney et al Apr. 2'3, 1935 2,080,564 Craig May 18, 1937 2,001,715 Fischer May 21, 1935i 2,369,771 Bond Feb. 20, 1945 2,015,038 Pevere Sept. 17, 1935