US2739101A - Sweetening of thermally cracked naphthas with alkali phenolate oxygen and sulfur - Google Patents

Description

March 20, 1956 M. GORDON ET AL SWEETENING OF THERMALLY CRACKED NAPHTHAS WITH ALKALI PHENOLATE OXYGEN AND SULFUR Filed June 26, 1955 IN V EN TORS 1 Moses Gordan George E. Thompson ATTOI? Y United States Patent SWEETENING 0F THERMALLY CRACKED NAPH- THAS WITH ALKALK PHENULATE OXYGEN AND SULFUR Moses Gordon and George E. Thompson, Chicago, 111., asslgnors to Standard Gil Company, Chicago, Ill., a corporation of Indiana Application June 26, 1953, Seriai No. 364,234

11 Claims. (Cl. 196-2?) 7 This invention relates to the sweetening of sour thermally cracked naphthas. More particularly the invention relates to the sweetening of thermally cracked naphthas having ASTM end points of not more than about 425 F. and mercaptan numbers of more than about 1.2.

Virtually all naphthas derived from the distillation of crude petroleum or from cracking of petroleum fractions contain objectionable amounts of mercaptans. mercaptain-containing naphthas are commercially spoken of as being sour. A naphtha which has been treated to remove substantially all these mercaptans is known as sweet or doctor sweet. In the petroleum industry a sweet oil is one which gives a negative doctor test. A more quantitative description of a sweet naphtha is one which has a so-called mercaptan number or copper number of substantially less than 1, e. g., about 0.5. Mercaptan number and copper number are defined as the number of milligrams of mercaptan-sulfur per 100 milliliters of naphtha. Many processes of sweetening naphthas are known. U. S. 2,015,038 teaches a procedure wherein the sour naphtha is contacted with an aqueous caustic solution, phenolic compounds, and a free-oxygencontaining gas. (Satisfactory phenolic compounds for use in this process may be obtained by caustic extraction of naphthas.) However, very surprisingly, it has been found that thermally cracked napthas having a mercaptan Such number higher than about 1.2 are not sweetened by the process of U. S. 2,015,038.

Another procedure for sweetening sour naphthas is to contact the sour naphtha with aqueous caustic and freesulfur. While this method is relatively effective on virtually all stocks, it has been found to have a very serious disability, namely, the sweet naphtha almost always contains corrosive sulfur. It is very difficult to control the sulfur addition to just the amount needed for sweetening. When using this procedure the excess free-sulfur appears as corrosive sulfur in the product naphtha. Furthermore, since the sulfur must do all the oxidizing of-the agent.

mercaptaus to disulfides, a large amount of sulfur is re- 7 quired and the product naphtha has a poor vtetraethyl lead susceptibility. e

It is an object of this-invention to sweeten sour therwill'become apparent in the course of the detailed de;

scription thereof. v

It has been discovered that thermally cracked naphthas having mercaptan numbers of more than about 1.2can be sweetened by the process wherein the naphtha is contacted with an aqueous alkaline treating agent, preferably containing at least about 5 weight percent of free-caustic, which contains at least a catalytic amount and preferably at least about 2 volume percent of alkylphenols, freeoxygen, and at least an amount of free-sulfur effective to sweeten said feed but not enough to produce a corrosive naphtha product. The contacting is carried out at a temperature of at least about F., preferablyat between about F., and F., for a time sufiicient to give a product which is doctor sweet, i. e., a mercaptan number substantially less than 1, e. g., 0.5-0.7.

The feed to this process is naphtha derived from thermal cracking. By thermal cracking is meant petroleum refinery operations such as vis-breaking, coking, gas oil cracking, heavy naphtha reforming, etc. cracked naphtha feed has an ASTM end point of not more than about 425 F. and preferably has a boiling range between about 120 F. and 400 F. The naphtha feed may be prewashed with a weak caustic solution to remove HzS, but it is not essential that the feed be HzS-free. A thermally cracked naphtha which has been subjected to a conventional caustic-cresylate extraction process to remove a portion of the mercaptans is also a satisfactory feed for this process.

The feed to this process should have a mercaptan number of more than about 1.2; although lower mercaptan content naphthas may be charged to the process. Naphthas with a mercaptan number in excess of about 20 may be sweetened by this process, but preferably the mercaptan numbers of these naphthas should be reduced by other methods to about 20 or less.

The aqueous treating agent used in this process is present in an amount at least sufiicient to form a separate phase. sweetening can be obtained with amounts of agent just great enough to impart a haze to the naphtha. For most operations the amount of the aqueous treating agent will be about 10 to about 200 volume percent based on the feed naphtha. It is preferred to use about 25% to about 50% by volume of aqueous treating agent in this process.

The caustic component of the aqueous treating agent may be sodium hydroxide, potassium hydroxide, or mixtures thereof. The caustic is present in the treating agent in the form of free-alkali and in chemical combination with the alkylphenols. Furthermore, some of the caustic may be combined with acidic contaminants in the treating Free caustic must be present, preferably, at least about 5 weight percent. Higher concentrations of caustic may be used and in some cases a saturated solution may be desirable, e. g., about 50% of NaOH. It is preferred that the total caustic content, i. e., free and combined caustic, of the aqueous treating agent be between about 15 and 25 weight percent.

The alkylphenols used in this invention may be any .alkylhydroxybenzene, such as cresol, xylenol, etc. The

7 rived from thermal cracking and catalytic cracking processes. These alkylphenols may be obtained by contacting the petroleum hydrocarbon with concentrated aqueous caustic solution. The cresols derived from cracked naphtha have an ASTM boiling range from about 370 F. to about 440 F. Alkylphenols occur in petroleum hydrocarbons boiling in the heavier-than-gasoline range, i. e., hydrocarbons boiling from about 350 F. to about 600 F. These alkylphenols are usually called heavy xylenols and may be obtained by contacting cracked gas oils and many virgin gas oils with concentrated aqueous caustic solution such as a 40% solution. These heavy xylenols have an ASTM boiling range between about 375 F. and 560 F. or higher.

The alkylphenols present in the aqueous treating agent act as solutizers for the mercaptans. The alkylphenols The thermally also aid in preventing the presence of corrosive sulfur in V the naphtha, and it is possible that they do this by reacting with the excess free-sulfur. It is necessary to have at least a catalytic amount of alkylphenols present in the ll. of light ends, i. e., butanes and pentanes, to avoid substantial losses of these by evaporation during the processing. It is desirable to use higher temperatures as the sweetening reaction is accelerated by higher temperatures.

aqueous treating agent, and preferably at least about 2 5 Therefore, for thermal naphtha feeds boiling between volume percent. It is advantageous to have more alkylabout 120 F. and 400 F, temperatures of about 120 F. phenols present and in some cases a saturated solution to about 160 F. are preferred. For naphthas which are may be desirable. However, these alkylphenols increase more refractory, i. e., harder to sweeten, temperatures the viscosity of the agent and may produce phase separaabove 160 F. may be used. tion difiiculties. It may be desirable to limit the alkyl- The mixture of free-sulfur, agent, naphtha and freephenols to about 30 volume percent when using sodium oxygen should be maintained intermingled for a time sufhydroxide, preferably about 20; somewhat higher amounts ficient to complete the sweetening reaction, i. e., to promay be used when using potassium hydroxide. It is duce a naphtha having a mercaptan number substantially preferred to use between about 5 and 20 volume percent less than 1. The required contacting time is dependent alkylphenols in the aqueous treating agent. on the mercaptan number and the type of naphtha, but The free-oxygen may be introduced into the process normally this time will be between about 1 minute and either in the form of substantially pure oxygen, i. e., 60 minutes. For most feeds, a contact time of from cylinder oxygen, or in the form of a free-oxygen-conabout 2 to about minutes will be sufficient. taining gas, e. g., air. Very slight amounts of free-oxygen After the completion of the sweetening reaction the are needed to oxidize the mercaptans in the naphtha to 20 mixture of treating agent and naphtha is settled for a time disulfides, but at least an amount sufiicient to oxidize the sufiicient to separate the product naphtha from the lower easily oxidizable mercaptaus is desirable since the sulfur aqueous phase. The aqueous phase is usually suitable is particularly required for the oxidation of mercaptans for recycle to the initial contacting step. However, in which are difiicult to oxidize. The amount of free-oxygen the course of time there is a buildup of thiosulfates and required will increase with increasing mercaptan number other by-product salts which interfere with the sweetenof the sour naphtha. Usually 5 s. c. f. of air will be sufing process and reduce the free-caustic concentration. ficient for the production of a sweet product. It is pre- The cresols also build up in the treating agent, when the ferred to add between about 0.2 and 2 standard cubic naphtha feed has not been prewashed with caustic to refeet of free-oxygen per barrel of sour naphtha. When move HzS or extracted with caustic-cresylate solution air is used, from about 1 to about 10 s. c. f. of air are for mercaptan number reduction. It is therefore desirable added per barrel of sour naphtha. to periodically discard the treating solution and use fresh It has been found that thermally cracked naphthas havsolution; or to continuously withdraw a portion of the ing mercaptan numbers greater than about 1.2 cannot be treating agent and replace it with fresh solution. sweetened by contacting the naphtha with an aqueous The results obtainable by the process of the invention caustic-cresol solution in the presence of air even at are illustrated by a number of laboratory experiments. elevated temperatures and for prolonged contacting time. Naphthas for the laboratory experiments were mercaptan- It has been discovered that a sweet product can be obextracted using conventional caustic-cresylate extraction tained by operating in the presence of free-sulfur. The to various mercaptan numbers from 1.0 to 4.1. Sweetenamount of free-sulfur must be at least an effective amount ing of these naphthas was carried out using both the but less than the am unt whi h giv s a C rr pro 40 canstic-cresylate-air-sulfur method of sweetening and the p The use of free-Sulfur afificts favorably the caustic-cresylate-air method for comparison. The temcontacting temperature and contacting time. The amount perature at which all runs were conducted was 140 F. of free-sulfur requu'edincreases with increasing mercaptan The strength of the caustic solution used in the experi number of the naphtha i of more than about ments-varied from about 19 to 22 weight per cent based has?safest.22:1.a;zlisfsae sjzaszz on the In effect on the TEL Susceptibility of product naphtha, of naphtha and 100ml. of caustwcresylate solution were and a corrosive naphtha product may result. Amounts used In an runs i i added f the rate, i i we of free-sulfur as low as 0.5 lb./1000 bbls./mercaptan (standard condltlmfs) per mmute' Dlsnllfmogs P number or lower may be sufficient with some naphthas. f the naphthas used the l f expenments Preferably, between about 0.9 to about 1.6 pounds of dlcated that they all 1901164 Wlihln the range 0f sulfur per 1000 barrels of naphtha feed per mercapt to 400 F. Additional details of these runs and the results number of said feed is used to obtain a doctor sweet thereof are showninTable I.

Table I Runs 1 2 a 4 5 a 7 FeedMercaptanNo 12-.-... 4.1.

Time, Minutes s s 20.

Vol. Percent Cresols, based 20 20 10 20.--" 20.

on Agent. Sulfur, mg. None None- 2.4 None None- None. 8.2. Product: Sweet... Sour- Sweet... B0rder1ine-. Sour.- Sour- Sweet.

1 Sulfur added in Runs 3 and 7 is equivalent to 1.8 lbs/1,000 bbls. naphtha/mercaptan number.

product with thermally cracked naphthas having a boiling range between about 120 F. and 400 F. When the particular type of mercaptans present in the naphtha are very difficult to oxidize, the amount of sulfur required to obtain a sweet naphtha product will correspond more closely to the 1.6 figure than to the 0.9 figure; this is the case normally with naphthas having ASTM end points appreciably above 400 F.

Normally, the practical extremes of temperature in which sweetening will be carried out are from about 80 F. to 210 F. Temperatures below about 100 F. are preferred for use on naphthas having substantial quantities A large 'scale embodiment of thisprocess is illustrated in the accompanying drawing which forms a part of this specification. The, embodiment illustrated is schematic in nature; all pumps, valves, and many other pieces of equipment have been omitted, as anyone skilled in the art can readily add these items to the embodiment shown in the figure.

Sour thermally cracked naphtha feed which has previously been extracted by conventional caustic-cresylate mercaptan extraction fi'oma mercaptan number of 24 to a mercaptan number of about 7.5, and which has a boiling range of between about 120 F. to 400 F. is charged from source 11 to line 12 at the rate of about 600 barrels per hour. All references rnade to barrels in the description of the large scale embodiment refer to 42-gallon barrels. From line 12 the naphtha flows to heat exchanger 13. Heated naphtha from exchanger 13 is passed by way of line 14 to line 15. i

Free-sulfur is introduced into the system by -means of naphtha, which is preferably feed naphtha from source 11. This naphtha which contains about 0.4 to about 0.5 weight percent sulfur and at a temperature of about 120 F., is fed at a rate of about 0.6 barrel per hour from source 16 by way of line 17 to line 15. Free-sulfur may be metered directly into line 15 or dissolved in naphtha other than a portion of the feed. Normally adding the free-sulfur in solution in a portion of the naphtha feed is the most desirable method.

Air from source 18 is fed at a rate of about 2400 standard cubic feet per hour by way of line 19 to line 15.

Recycle aqueous caustic-cresylate solution, which may contain makeup fresh caustic-cresylate solution is fed from line 20 to line 15 at a rate of about 150 barrels per hour. This aqueous solution has a free-caustic concentration of about 8 weight percent and a cresol concentration of about 15 volume percent.

The naphtha, naphtha-containing free-sulfur, air, and :aqueous caustic-cresylate solution mix together in line 15. Additional mixing of the components is provided by mixer 21. Mixer 21 may be any conventional mixer, e. g., a knothole mixer.

Efiiuent from mixer 21 is fed by way of line 22 to contactor 23 at a point near the bottom of contactor 23. Contactor 23 is a cylindrical vessel of about 400 barrels capacity. Its length is about 30 feet and its diameter about 10 feet, and it is installed in a vertical position. Contacting is carried out at about 120 F. for about minutes.

An aqueous phase is withdrawn by line 24 from the bottom of contactor 23, either continuously or intermittently as desired. Naphtha containing some of the treating agent both dissolved and suspended, in addition to the air, is taken overhead from contactor 23 by way of line 25, and enters the second contactor 26 at a point near the bottom thereof.

Contactor 26 is a vessel of equal size and shape as contactor 23 and it is installed in a vertical position similar to contactor 23. Most of the aqueous phase carried into contactor 26 by the naphtha settles out in the bottom of contactor 26 and is drawn oh" by line 27, either continuously or intermittently as desired.

The naphtha efiluent from contactor 26 is fed by line 28 to settler 29. Settler 29 is a 400-barrel settler of the same size and shape as the contactors 23 and 26. To obtain a haze-free product directly from the plant, it is necessary to use more than one settler of the size of settler 29, and it may be necessary to use as many as 3 or 4 such settlers in series. Conventional coalescers such as coalescers packed with fibreglass or other suitably coalescing material may be used in place of settlers to remove haze, or salt drums may be used in place of settlers to remove haze from the sweet naphtha product. However, if the plant eil'luent naphtha is to be pumped to storage, it can be settled free of haze in the storage tanks and one settler is sufficient. It is also possible that'no settlers" will be required and all the settling can be accomplished in storage.

The naphtha product, withdrawn by way of line 30, which is sweet and noncorrosive, should be cooled to about ambient temperatures either by indirect heat exchange or by water washing. If direct cooling by water washing is used, a settler should be provided after the water wash step to allow the water to settle free of the naphtha.

Substantially all the aqueous phase which has not settied out in contactors 23 and 26, settles out in settler 29. The aqueous layer which settles out as a bottom layer in settler 29 is withdrawn by line 31 either continuously or intermittently as desired. a

The aqueous phase is withdrawn from the contactors and settler by lines 24, 27 and 31 as previously indicated. Lines 24 and 31 feed into line 27 to combine all the aqueous phases for recycle to mixer 21. Downstream from the junction of lines 24, 27 and 31, a portion of the combined aqueous phase may be withdrawn by way of line 32 either intermittently or continuously for discard as waste. Fresh caustic-cresylate solution may be added from source 33 by line 34either intermittently or continuously to fortify the recycle aqueous phase.

The air associated with the naphtha may be released at any time after sweetening has been attained and normally it will be released to a vent line, but the air may be released in storage if proper safety precautions are taken. 4

Thus having described the invention, what is claimed is:

l. A process for sweetening a thermally cracked naph- V tha feed having a mercaptan number of at least 1.2,

which process comprises contacting said feed, in the presence of free-oxygen, with an aqueous alkaline treating agent containing at least a catalytic amount of alkylphenols, wherein said agent is present in an amount at least sufiicient to exceed the solubility of the agent in the feed, and with an amount of free-sulfur at least efiective to substantially sweeten said feed but insuflicient to render the product naphtha corrosive, at a temperature at least about F. for a time at least sufiicient to produce a substantially sweet naphtha and separating a substantially sweet, non-corrosive product naphtha from an aqueous phase.

2. The process of claim 1 wherein said agent is present in an amount at least sufiicient to form a haze in said feed.

3. The process of claim 1 wherein said feed naphtha has an ASTM end point of not more than about 425 F. and a mercaptan number of not more than about 20.

4. The process of claim 1 wherein said alkylphenols are present in said agent in an amount of at least 2 volume percent.

5. The process of claim 1 wherein said free-sulfur is present in an amount of not more than 1.8 pounds per 1000 barrels of said feed per mercaptan number of said feed.

6. The process of claim 1 wherein said temperature is between about F. and about F.

7'. A process for sweetening a thermally cracked naphtha feed having a mercaptan number between at least 1.2 and 20, which process comprises contacting said feed, in the presence of about 0.2 to about 2 standard cubic feet of free-oxygen per barrel of said feed, with an aqueous treating agent, which contains at least about 5 weight percent free-caustic and at least 2 volume percent alkylphenols, in amount of between about 10 and 200 volume percent based on said feed, and with free-sulfur in at least an amount effective to sweeten said feed but not more than 1.8 pounds per 1000 barrels of said feed per mercaptan number of said feed, at a temperature between about 100 F. and F. for a time sulficient to reduce the mercaptan number of said feed to substantially less than 1, and separating a sweet, noncorrosive product naphtha from an aqueous phase.

8. The process of claim 7 wherein said feed has an ASTM end point of not more than about 425 F.

9. The process of claim 7 wherein said alkylphenols are derived from the thermally cracked naphtha feed.

10. A process for sweetening a thermally cracked naphtha feed having a mercaptan number between at least 1.2 and 20, and an ASTM end point of not more than about 425 R, which process comprises contacting said feed, in the presence of about 1 to about 10 standard cubic feet of air per barrel of said feed, with between about 25 and 50 volume percent based on said feed of an aqueous treating agent which contains between about 15 and about 25 weight percent of free and combined caustic and between about 5 and about 20 volume percent of cresols derived irorn cracked petroleum naphthas, and with frec-sulfur in an amount between about 0.9 and 1.6 pounds per 1000 barrels of said feed per mercaptan number of said feed, at a temperature between about 120 References Cited in the file of this patent UNITED STATES PATENTS 1,767,356 Fischer June 24, 1930 1,789,335 Fischer et a1. Jan. 20, 1931 2,143,405 Campbell et a1 Jan. 10, 1939 2,638,439 Browder et a1 May 12, 1953 2,645,602 Torn et a1. July 14, 1953

Claims (1)

1. A PROCESS FOR SWEETENING A THERMALLY CRACKED NAPHTHA FEED HAVING A MERCAPTAN NUMBER OF AT LEAST 1.2, WHICH PROCESS COMPRISES CONTACTING SAID FEED, IN THE PRESENCE OF FREE-OXYGEN, WITH AN AQUEOUS ALKALINE TREATING AGENT CONTAINING AT LEAST A CATALYTIC AMOUNT OF ALKYLPHENOLS, WHEREIN SAID AGENT IS PRESENT IN AN AMOUNT AT LEAST SUFFICIENT TO EXCEED THE SOLUBILITY OF THE AGENT IN THE FEED, AND WITH AN AMOUNT OF FREE-SULFUR AT LEAST EFFECTIVE TO SUBSTANTIALLY SWEETEN SAID FEED BUT INSUFFICIENT TO RENDER THE PRODUCT NAPHTHA CORROSIVE, AT A TEMPERATURE AT LEAST ABOUT 80* F. FOR A TIME AT LEAST SUFFICIENT TO PRODUCE A SUBSTANTIALLY SWEET NAPHTHA AND SEPARATING A SUBSTANTIALLY SWEET, NON-CORROSIVE PRODUCT NAPHTHA FROM AN AQUEOUS PHASE.

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