US2754251A - Two stage thermal naphtha sweetening process - Google Patents

Description

July 10, 1955 M. GORDON ErAL Two STAGE THERMAL NAPHTHA swEETENTNG Prwcsss Filed NOV. 50, 1954 ATTORNEY nited tates TWO STAGE THERMAL NAPHTHA SWEETENING PRCESS Application November 30, 1954, Serial No. 471,978 9 Claims. (Cl. 196-29) This invention relates to a combination process for the sweetening of sour thermal naphthas.

It is known that petroleum naphthas which have been derived from thermal cracking or thermal coking operations are very dithcult to sweeten by a process involving contacting the naphtha with aqueous caustic solution and free-oxygen in the presence of a polyhydroxybenzene catalyst. Not only does the sweetening of naphtha involve a very serious degradation in color, but also requires prohibitive amounts of the catalyst. This same thermal naphtha can be sweetened without appreciable degradation in color by a process involving contacting with aqueous caustic-cresylate solution, free-oxygen and freesulfur; unfortunately when contacting a naphtha having a mercaptan number above about 5, the response of the sweet naphtha to TEL is reduced to a prohibitive extent. When a sour naphtha which may have a mercaptan number ranging as high as 50 is sweetened by the aqueous caustic cresylate-sulfur process, the octane depreciation may be as high as 2 umts depending upon the mercaptan number of the sour naphtha charged.

An object of the invention is a process for sweetemng thermal naphthas having a mercaptan number above about 5 without prohibitive use of polyhydroxybenzene catalyst and without uneconomic octane degradation. A particular object is the sweetening of a thermal naphtha having a mercaptan number between about 30 and 50 without uneconomic octane degradation and without prohibitive usage of polyhydroxybenzene catalyst. Other objects will become apparent in the course of the detailed description.

The combination process of the invention involves contacting, in a rst mercaptan conversion zone, a thermal naphtha having a mercaptan number markedly in excess of 5, for example 15-20, with aqueous causticsolution containing a catalytically effective amount of a polyhydroxybenzene catalyst in the presence of freeoxygen. The mercaptan content of the sour naphtha is reduced in this rst zone to a level of about 5, for example 4-6. The naphtha etliuent from the rst zone is made doctor-sweet in a second zone by contact with aqueous caustic-cresylate solution in the presence of freeoxygen in at least the stoichiometric amount to convert the mercaptan content to disultdes and free-sulfur in an amount less than that theoretically required to convert the mercaptans to disulides. Preferably the second zone is maintained at a temperature between about 120 and 160 F. The naphtha eluent from the second zone is sweet to the doctor test and is non-corrosive as determined by the copper strip test, i. e., contains essentially no dissolved free-sulfur.

The sour naphtha charged to the combination process of this invention is commonly known as thermal naphtha or thermally cracked naphtha. This naphtha has been derived by distillation from the thermal cracking of gas oils or thermal reforming of heavy naphtha or from the thermal coking of petroleum materials. The naphtha attent l Patented July 10, 1956 may boill over the range of about F. to 425 F. or may be a narrower boiling range fraction, for example a heavy naphtha boiling between about 250 F. and 425 F. This thermal naphtha contains sulcient mercaptans not only to be sour to the doctor test, but to make sweetening by either thel Doctor Process or by aqueousl causticcresylate-sulfur undesirable because of the octane loss. This maximum mercaptan content is about 5 mg. per 100 ml.; the mercaptan content is hereinafter described as mercaptan number which corresponds to mg. of sulfur per 100- ml. of naphtha.

The sourv naphtha charged to the initial mercaptan reduction zonemay have a mercaptan number as high as 50 or more and usually will be in excess of 30. It is preferred that the mercaptan number of the naphtha charged to the rst zone wherein mercaptans. are convertedl to disuldes should be on the order of 20 to 30.

The sour naphtha feed to the process normally contains hydrogen sulfide and cresols, i. e., alkylphenols. This naphtha may be charged to the first mercaptan conversion zone, but in order to avoid degradation of caustic and catalyst, it is preferred that the H28 be removed prior to the entry to the irst'mercaptan conversion zone. This removal of H2S may be either by a conventional aqueous caustic prewash or treatment by other conventional processes.

The process is described in detail in conjunction with the annexed figure which forms a part of this specication.

In the illustrative embodiment, the feed consists of a raw thermally cracked heavy naphtha having a mercaptan number of about 45 which contains H28 and phenolic compounds. The raw naphtha from source. 11 is introduced by way of`line 12v to a a point near the bottom of a prewash vessel'1'3. Prewashvessel 13 is a vertical cylindrical vessel provided with means for intimately contacting two immiscible liquids..

Aqueous caustic solution from` source 16 is passed by way of line 17 into an upper portion of prewash vessel 13. The aqueous caustic solution comprises essentially water and alkali metal hydroxide such asy sodium hydroxide or potassium hydroxide. Although solutions containing as little as 5 weight percent of caustic may be utilized, it is preferable tol operate with solutions containing a sucient concentration of causticy to extract an appreciable amount of mercaptans and phenolic compounds from the raw naphtha. In general, between about 10 and 35 weight percent of caustic, i. e., alkali metal hydroxide, should be present in the aqueous solution. However, higher concentrations may be used. It is preferred to operate with. an. aqueous caustic solution containing between about 20 and 30 weight percent of NaOH. In this embodiment, the aqueous caustic solution contains 25 weight. percent of NaOH.

The amount of aqueous caustic solution used in prewash vessel 13 will be dependent upon the type of naphtha and the degree of contacting attained in the vessel. In general, the volume. ratio of aqueous solution to raw naphtha feed is between about 1:5 and 1:1 or in other terms, between about 20 and 100 Volume percent of aqueous caustic solution; are used, based on raw naphtha feed; preferably between about 25 and 50 volume percent, i. e., a volume ratio of solution to naphtha of between about l:4 to 1:2. In this embodiment, the volume ratio of aqueous caustic solution to raw naphtha is 1:3.

The temperature of prewashing is normally slightly above atmospheric temperatures.

An aqueousy solution comprising NaOH, sodium mercaptides and sodium cresylates is withdrawn from the bottom of vessel 13 and is passed by way of line 18 into valved line 19, for recycle to line 17. Periodically, solution is withdrawn and disposed of by way of valved line 2t?. HzS-ree naphtha is removed from the upper portion of vessel 13 and is passed by way of line'21 into mixer 23.

The euent from the prewash tower 13 has a mercaptan number of about 25 and has been reduced substantially in alkylphenol content. Mixer 23 may be any form or" device for intimately agitating a hydrocarbon and an aqueous medium. Make-up aqueous caustic solution from source 26 is passed by way of valved line 27 and line 28 into mixer Z3. Aqueous caustic solution is circulated throughout this rst mercaptan conversion zone and normally will contain, in addition to the water and caustic, an appreciable amount of petroleum cresols extracted from the naphtha feed. A suicient amount of aqueous caustic solution is introduced in mixer 23 to form a distinct aqueous phase. Itis not necessary that a large excess of aqueous caustic solution be present and just enough to form a definite haze in the sour naphtha is sucient. More than this amount is normally used and in general, the amount of aqueous caustic solution will be between about and 100% based on'naphtha in line 2l.

The catalyst for the conversion of mercaptans to disuldes is a polyhydroxybenzene which may be an alkylsubstituted polyhydroxybenzene as well as the unsubstituted polyhydroxybenzene. Better colors are obtained when the polyhydroxybenzene contains a carboxylic group. Hydroquinone and 1,2,3-trihydroxybenzene are the preferred unsubstituted materials and gallic acid is the preferred substituted material. In this embodiment, Vhydroquinone from source 31 is passed by way of line`32 into line 28. The sour naphtha and aqueous caustichydroquinone solutions are thoroughly intermingled in mixer 23 and passed into a vertical vessel 34.

Air or free-oxygen from Vsource 36 is passed by way of line 37 into vessel 34. lf desired, the free-oxygen may be introduced into line 23. Vessel 34 is maintained at a temperature between about 80 and 200 F., preferably between about 80 and 120 F. The contacting time in vessel 34 is suicient to reduce the mercaptan number of the naphtha present therein from 25 to 5. The eluent naphtha, mercaptan number 5, is withdrawn by way of line 39.

A bottoms layer of aqueous caustic-hydroquinone solution is withdrawn by way of line 41 and is recycled by way of valve line 42 and line 23 to mixer 23. Periodically aqueous caustic solution is withdrawn from the system by way of valved line 43.

At least suicient free-oxygen is introduced to convert the mercaptans to the corresponding disultides. More than this amount is usually desirable, although extremely large excesses are notrbeneicial. ln general, the free-oxygen usage in vessel 34 is between about 100% to 250% of the stoichiometric requirement for the conversion of mercaptan to disulfide.

Polyhydroxybenzene theoretically acts as a catalyst and therefore only that amount needed to obtain the desired mercaptan conversion need be present in vessel 34. However, as the mercaptan content of the naphtha is reduced,

side reactions occur which result in loss of hydroquinoneY apparently to highly colored oil-soluble materials. When the eluent naphtha has a mercaptan number of about 5, for example 4 6, it has been found that the usage of polyhydroxybenzene such as hydroquinone or gallic acid is between about 0.1 and 0.2 pound per 1000 barrels (42 gal.) of naphtha charged to vessel 34 per each mercaptan number reduction of mercaptan content of naphtha charged to vessel 34. For example, a mercaptanreduction of units requires between about 2 andV 4'pounds of hydroquinone per 1000 barrels of naphtha charged to vessel 34. Variation in polyhydroxybenzene usage is dependent upon the type of naphtha feed, Vthe phenol content, free-oxygen usage and other variables not fully understood.

The eluent naphtha from vessel 34 is passed by way of line 39 to mixer 46. A side stream is withdrawn from line 39 by way of valved line 47 and is passed into sulfur pot 48 and is returned vby way of valved line 49 to line 39. Sulfur pot 48 is iilled with roll sulfur and the amount of sulfur introduced is determined by the size of the side streamrwithdrawn by line 47. In general, insuicient free-sulfur is introduced into the naphtha to theoretically convert the mercaptans to disulfides. rthe amount of free-sulfur used is between about l and 1.8 pounds per 1000 barrels (42 gal.) per mercaptan number of the naphtha charged t0 the second mercaptan conversion Zone. More usually the free-sulfiul used is between about 1.4 and 1.6 pounds per 1000 barrels per mercaptan number.

Aqueous caustic-cresylate solution from source 51 is passed by way of valved line 52 and line 53 into mixer 46. rl"his aqueous caustic-cresylate solution contains alkylphenols which appear to have a catalytic etlect on the mercaptan oxidation. These alkylphenols are alkylrnonohydroxybenzenes such as cresol and xylenol. The preferred alkylphenols are those naturally occurring in petroleum hydrocarbons such as thermally cracked naphthas. Those alkylphenols derivable from thermal naphtha by extraction with aqueous caustic solution are commonly known as petroleum cresols. it is necessary to have at least 2 volume percent of these cresols in the form of alkali-metal cresylates present in the aqueous solution. More than this amount is advantageous. However, amounts approaching the saturation content very greatly increase the viscosity of the aqueous solution and introduce phase separation problems. lt is preferred to use between about 5 and 20 volume percent of these alkylphenols in the aqueous solution charged to the second mercaptan conversion zone.

Although the alkali-metal hydroxide content of the aqueous caustic-cresylate solution can be between about 5 weight percent and the saturation amount, it is preferred to have a total caustic content, i. e., free and uncorribined alkali-metal hydroxide, of between about 15 and 25 weight percent.

The amount of aqueous caustic cresylate solution present is enough to form a distinct separate phase preferably more than this amount, usually between about 10 and 100 volume percent based on naphtha charged.

in mixer 46 the aqueous caustic-cresylate solution and the naphtha are thoroughly intermineled Vand passed by way of line 56 into vessel 57. ln vessel 57, the sweetening reaction takes place assisted not only bv the free-sulfur, but also by air from source 58 which is introduced by Vway of line 59. Although the amount of air may be less than that theoretically needed to convert the mercaptaus to disuldes because of the presence of free-sulfur, amount of the free-sulfur is reduced and the quality of the sweet naphtha is improved when the amount of reeoxygen introduced into vesselV 57 is at least about the theoretical requirement. More than this amount, preferably' not more than about 250% of the theoretical may beused.

The contents of vessel E? are maintained at a temperature between about and 180" E. and preferably' between about "V and 160 F.

Aqueous caustic-cresylate solution is withdrawn from the bottom of vessel 57 by way of line 61 and isV recycled by way of valved line 62 and line 53; Periodically solution is withdrawn by way of valved line 61 and malte-up aqueous caustic-cresylate solution is introduced intoY the system.

A naphtha which is sweet to the doctor test is withdrawn from the upper portion'of vessel S7V by way of line 6o Vand is freed of occluded aqueous solution by any one ol' washing.v

the conventional methods, for example'water Herein the occluded aqueous solution is removed by means ofV salt drum 67 which is filled with Vcrushed rock salt. Brine is removed from the drum by way of valved line 68. A sweet non-corrosive product naphtha is passed by way of line 69 to storage not shown.

The results obtainable by the combined process of this invention are illustrated by the following sweetening eX- periments.

T est 1 In a large scale operation, a sour naphtha derived by distillation from the thermal cracking of a gas oil which had been prewashed to a mercaptan number of 28 and which boiled over the range of 250 F. to 420 F. was sweetened by contacting it with aqueous caustic-cresylate solution at a temperature of 140 F, in the presence of free-sulfur and free-oxygen. By the use of about 4.0 pounds of sulfur per 1000 barrels (42 gal.) of naphtha for each mercaptan number of the naphtha, a sweet product naphtha was obtained. This sweet naphtha had a color of 5 on the Saybolt scale representing a degradation of units on the Saybolt scale. Differential octane numbers were obtained on sweet naphthas containing 2 ccs. of TEL and an octane loss of 1.0 unit had been suffered as a result of sweetening this naphtha by this method as compared with conventional methods wherein no octane loss was obtained.

T est 2 A prewashed thermal heavy naphtha similar to that of Test 1 but having a mercaptan number of only 26 was sweetened by contact with aqueous caustic solution in the presence of hydroquinone catalyst and free-oxygen. At a throughput of 300 barrels (42 gal.) per hour, it was possible to obtain an eluent which was essentially sweet, i. e., a mercaptan number of 1.3. In order to attain this degree of mercaptan reduction, it was necessary to use 0.4 pound of hydroquinone per 1000 barrels of naphtha for each mercaptan number unit reduction. However, the eluent naphtha had a color of 2.5 N. P. A. No octane loss was detectable by differential measurements.

Test 3 A prewashed thermal heavy naphtha having a mercaptan number of 28 was contacted with aqueous caustic, free-oxygen and hydroquinone until the eiiuent had a mercaptan number of 5.0. This reduction in mercaptan number was obtained at a throughput of 367 barrels (42 gal.) per hour. The eluent naphtha had a color of 2 N. P. A. At this degree of mercaptan reduction, the hydroquinone usage was only 0.1 pound per 1000 barrels for each mercaptan number reduction.

T est 3A The etlluent naphtha from Test 3 was contacted with aqueous caustic-cresylate, free-oxygen and free-sulfur at a temperature of 140 F. until the naphtha was sweet to the doctor test. The free-sulfur usage was 1.5 pounds per 1000 barrels per mercaptan number reduction. The sweet naphtha had a color of 2 N. P. A. The octane loss as measured by diiferential octane number method was only 0.2 unit.

The above tests show that by the combination process of this invention it is possible to sweeten a sour thermal naphtha without signicant loss in octane number and with economic amounts of polyhydroxybenzene catalyst, thus attaining the objects of the invention.

Thus having described the invention, what is claimed 1s:

1. A sweetenng process which comprises, in a first zone, contacting a sour naphtha feed which has been derived from a thermal cracking operation and which has a mercaptan number markedly in excess of 5, with suticient aqueous caustic solution to form a distinct aqueous phase, in the presence of a catalytically elective amount of polyhydroxybenzene mercaptan oxidation catalyst and in the presence of free-oxygen under conditions such that the mercaptan number of the naphtha is reduced to about 5, separating a sour naphtha having a mercaptan number of about 5 from aqueous caustic solution, in a second Zone, contacting the sour naphtha of about 5 mercaptan number, with aqueous caustic-cresylate solution containing at least 2 volume percent of alkylphenols, at a temperature between about F. and about 180 F., in the presence of free-oxygen in an amount between about and 250% of the theoretical and free-sulfur in an amount between about 1 and 1.8 pounds per mercaptan number of naphtha feed in the second zone per 1000 barrels (42 gal.) and separating a sweet non-corrosive naphtha from aqueous caustic-cresylate solution.

2. The process of claim 1 wherein said sour naphtha feed to the irst Zone has been prewashed to remove hydrogen sulfide.

3. The process of claim 1 wherein said polyhydroxybenzene is hydroquinone.

4. The process of claim benzene is gallic acid.

5. The process of claim 1 wherein said polyhydroxybenzene is present in an amount between about 0.1 and 0.2 pound per mercaptan number per 1000 barrels (42 gal.) of sour naphtha feed to said rst zone.

6. The process of claim 1 wherein the temperature in said second zone is between about and 160 F.

7. The process of claim 1 wherein the free-sulfur usage is between about 1.4 and 1.6 pounds per mercaptan number per 1000 barrels of sour naphtha to said second zone.

8. The process of claim 1 wherein said aqueous caustic-cresylate solution contains between about 5 and 20 volume percent of petroleum cresols.

9. A sweetening process which comprises (l) washing a sour thermal naphtha having a mercaptan number between about 30 and 50 with aqueous caustic solution to remove essentially all of the hydrogen sulfide present in said naphtha and reducing the mercaptan number of said naphtha to a point between about 20 and 30, (2) separating an HzS-free sour naphtha from aqueous caustic solution, (3) contacting said sour naphtha from step 2 with aqueous caustic solution containing between about 0.1 and 0.2 pound of hydroquinone per mercaptan number of sour naphtha charged per 1000 barrels of sour naphtha charged, in the presence of sufficient free-oxygen to reduce the mercaptan number of said sour naphtha charged to about 5, (4) separating a sour naphtha of about 5 mercaptan number from aqueous caustic-hydroquinone solution, (5) contacting said sour naphtha of about 5 mercaptan number with aqueous caustic-cresylate solution containing between about 15 and 25 weight percent of total caustic and between about 5 and 20 volume percent of petroleum cresols, at a temperature between about 120 and F., in the presence of between about 100% and 250% of the theoretical requirement of free-oxygen and free-sulfur in an amount between about 1.4 and 1.6 pounds per mercaptan number of the naphtha charge per 1000 barrels of the naphtha charged and (6) separating a sweet non-corrosive naphtha from aqueous causti-cresylate solution.

3 wherein said polyhydroxy- Bond Sept. 9, 1947 Backensto Mar. 6, 1951

Claims (1)

1. A SWEETENING PROCESS WHICH COMPRISES, IN A FIRST ZONE, CONTACTING A SOUR NAPHTHA FEED WHICH HAS BEEN DERIVED FROM A THERMAL CRACKING OPERATION AND WHICH HAS A MERCAPTAN NUMBER MARKEDLY IN EXCESS OF 5, WITH SUFFICIENT AQUEOUS CAUSTIC SOLUTION TO FORM A DISTINCT AQUEOUS PHASE, IN THE PRESENCE OF A CATALYTICALLY EFFECTIVE AMOUNT OF POLYHYDROXYBENZENE MERCAPTAN OXIDATION CATALYST AND IN THE PRESENCE OF FREE-OXYGEN UNDER CONDITIONS SUCH THAT THE MERCAPTAN NUMBER OF THE NAPHTHA IS REDUCED TO ABOUT 5, SEPARATING A SOUR NAPTHA HAVING A MERCAPTAN NUMBER OF ABOUT 5 FROM AQUEOUS CAUSTIC SOLUTION, IN A SECOND ZONE, CONTACTING THE SOUR NAPHTHA OF ABOUT 5 MERCAPTAN NUMBER, WITH AQUEOUS CAUSTIC-CRESYLATE SOLUTION CONTAINING AT LEAST 2 VOLUME PERCENT OF ALKYLPHONOLS, AT A TEMPERATURE BETWEEN ABOUT 80* F. AND ABOUT 180* F., IN THE PRESENCE OF FREE-OXYGEN IN AN AMOUNT BETWEEN ABOUT 100% AND 250% OF THE THEORETICAL AND FREE-SULFUR IN AN AMOUNT BETWEEN ABOUT 1 AND 1.8 POUNDS PER MERCPTAN NUMBER OF NAPHTHA FEED IN THE SECOND ZONE PER 1000 BARRELS (42 GAL.) AND SEPARATING A SWEET NON-CORROSIVE NAPHTHA FROM AQUEOUS CAUSTIC-CRESYLATE SOLUTION.

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