US2916442A - Air sweetening process for naphthas - Google Patents

Description

. 8, 1959 E. J. NlEHAus, JR.. ETAL 2,916,442

AIR SWEETENING PRocEss FOR NAPHTHAS Filed Nov. 14, 1956 v z. OZ-Dn-OI ow www 1v m www r 3 5.5m 9 mmf wm l A a ll. T QQ .3 EXE E .Emw 1v motmz. n n. tmm oN l' Al Al A! Q l.' |v E T N @Mmmm mm oz .596mm $355 op? (wm mzmwommx m T 35.2 om ..mnw mzmmommx of the added inhibitor to convert the mercaptans to United States Patent O 2,916,442 AlR SWEETENING PROCESS FOR NAPHTHAS Edward J. Niehaus, Jr., and William H. Gay, Baltimore,

Md., assignors to Esso Research and Engineering Company, a corporation of Delaware Application November 14, 1956, Serial No. 622,110 9 Claims. (Cl. 208-204) This invention concerns an improved process for sweetening petroleum distillates. The invention is particularly directed to the sweetening of naphtha derived from catalytic cracking operations. The sweetening is effected by a novel process in which use is made of an agent that will dissolve in caustic, will absorb oxygen when blown with air and will give up this oxygen when it comes into contact with a cracked naphtha, the oxygen then reacting with the mercaptans in the naphtha to form disuldes.

It is well known in the art of petroleum refining to sweeten sour distillates such as naphtha, motor fuels and the like, by contacting them with various reagents such as caustic soda. Such treatment is not entirely satisfactory because not all of the mercaptans that are present in the petroleum distillates will dissolve in the caustic. It is also known that further removal of the mercaptans can be effected by what is known as air sweetening wherein the mercaptans are oxidized to form disuliides which remain in the sweetened distillate.

It is one object of the present invention to provide an improved process for the air sweetening of petroleum distillates. It is an ladditional object of the invention to provide a process wherein the sweeteningproceeds at a faster rate and `at a lower temperature than with previously known air sweetening processes.

In accordance with the present invention, a cracked naphtha or similar distillate such as gasoline is rst treated with caustic soda to remove hydrogen sulfide and caustic-soluble mercaptans, particularly thiophenols. A small amount of a compound selected from the class of amines, amides and phenols is then added to the treated naphtha. Since most compounds of this class are well known as oxidation inhibitors it is convenient to referto them as inhibitors in the subsequent discussion. The naphtha containing the added inhibitor is next contacted with caustic that has previously been employed for treating raw kerosene distillate. The naphtha, after separation from kerosene caustic, is placed in a holding tank or storage vessel for several hours, where the sweetening occurs by action of oxygen on the stored naphtha. The sweetened' naphtha is then ready for blending into gasoline.

The oxygen is preferably introduced into thev system by saturating the kerosene caustic with air or oxygen prior to contacting the kerosene caustic with the naphtha. Alternative procedures comprise introducing air or oxygen into the naphtha just prior to contacting the same with the kerosene caustic or else introducing the air or oxygen intothe holding tank.

The exact function of the kerosene caustic is not completely understood but it is believed that certain substances extracted from the kerosene by the caustic used for treating the same act as carriers of oxygen which is given up to the naphtha on contact. This oxygen reacts slowly with the mercaptans in the naphtha in the presence Patented Dec. 8, 1959 suldes, thus causing the naphtha to become sweet during its storage in the holding tank.

The nature of this invention and the manner of its operation will be more readily understood when reference is made to the accompanying drawing in which the single figure diagrammatically illustrates the process steps followed in practicing this invention.

Referring now to the drawing in detail, a raw kerosene fraction entering through line 12 is treated in a conventional agitating vessel 11 with a caustic soda solution entering through line 13. Agitation may be effected by blowing with air entering through line 14 and leaving through exit 17. Preferably, the caustic soda is of from l0 to 20 B. strength, and from l to 5 volume percent of caustic based on the kerosene is employed. The mixture is allowed to settle and the treated kerosene is drawn off through line 15 and sent to storage or to further treatment. The caustic from the kerosene treatment is removed through line 16 and sent to an accumulator 23. f Alternatively, the kerosene may be treated with caustic by sending the kerosene and caustic through lines 12a and 13a, respectively, into a line mixer 18 and from there through line 19 to a drum settler 20 where separation of caustic from kerosene may be effected, the kerosene being sent through line 21 to storage or further treatment and the separated caustic being sent through line 22 to the accumulator.

Any entrained kerosene that enters the accumulator will separate out and can be sent back to the settler 20 by means of line 24. The kerosene spent caustic, which may also be referred to as promoter solution, is drawn from the bottom of the accumulator, as required, through line 25, and added to the naphtha sweetening system via recycle line 26 which carries circulating promoter solution to a saturator tower 28 wherein the solution is brought into intimate contact with oxygen or an oxygencontaining gas such Ias air which enters the saturator tower through line 29. The saturator tower may be a packed tower, a bubble cap tower, or a similar tower designed for intimate contact between gases and liquids.

The sour catalytic naphtha that is to be sweetened is given a pre-sweetening caustic wash by contacting it with caustic soda in a line mixer 33, the caustic entering through line 35 and the sour naphtha entering through line 34. The concentration of caustic in'this step is not critical and could range from 5 to 45 B.; preferably a caustic soda of from l0 to 20 B. is employed. The mixture of naphtha and caustic is conducted through line 37 to =a drum settler 38 where separation of caustic and naphtha takes place. Caustic may be recycled to the mixing zone through line 36 and spent caustic may be removed through line 39. The treated naphtha leaves the settling drum through line 40 and is conducted to another line mixer 43, after a small amount of an inhibitor of the type referred to above has been added by means of line 41. The inhibited naphtha contacts promotor solution from the saturator tower which is fed into the mixer through line 32. Preferably the promoter solution is used in the proportion of from about 5 to 20 percent of the naphtha. The mixture of the naphtha and promoter solution is conducted through line 44 into a drum settler 45 where separation of naphtha from promoter solution occurs, the separated naphtha being then conducted by means of line 47 into the holding tank 4S Where the naphtha is stored Afor several hours to complete the sweetening action. The sweetened naphtha is then removed through line 49.

Promoter solution separated from the naphtha in settling drum 45 is sent back to the recycle surge 27 through line 46. Any entrained naphtha that accumulates in the recycle surge drum may be pumped to the naphtha holding tank by means of line 63.

It is preferred to saturate the promoter solution with oxygen or air in a saturator tower, as previously described. Alternative methods may be used, however.

'For example, thesaturatortower-may beeliminated'and the promoter solution sent directly'to the mixer 43 by ymeans of lines 5() and 32, -thus bypassing the saturator tower. Air or oxygen is then introduced into the 'naphtha stream following inhibitor injection, as for example 'bymeans of line S1. An alternative method for introducing air is to add the airor oxygen to the holding tank, as lfor example by means of line 52. Also, any combination of the three methods of air Vinjectioncou'ld be used. However, from the standpoints of safety and effectiveness of sweetening, it ispreferred'to employ the sat'urator tower.

Although this invention is directed primarily to the "sweetening of catalytic naphthas, it is possible to sweeten a mixture of virgin naphtha and catalytic naphtha. In the event vthat virgin naphtha is to be mixed with the racked naphtha, the sour virgin naphtha is given a presweetening caustic wash in the same manner as in the case of a cracked naphtha. Thus, sour virgin naphtha from line 54 is mixed with caustic soda from line 55 in 'a line mixer 53 and the mixture is sent throughline 57 to a settling drum 58 wherein separation between caustic and naphtha takes place, the pretreated virgin naphtha then being conducted through line 60 to be admixed with 'the pretreated cracked naphtha in line 40 at or nearthe point of introduction of the through line 32. As in the promoter solution entering case of the pre-sweetening yof the cracked naphtha, the caustic separated from the -reaction, are selected from the class consisting of amines,

amides and phenols. Compounds of this class which have been found effective in promoting oxidative sweetening include acetamide, n-butyramide, N-butyl benzamide, phenol, beta naphthol, p-amino phenol, the phenylene diamines, crude petroleum phenols, and triethanolamine. The preferred compounds for use in this invention are -the phenylene diamines such as para-phenylene diamine and N,Ndisec butyl-p-phenylenediamine. In practicing this invention the selected inhibitor is added to the dis- `tillate being treated in the amount of from about 0.05

to one pound per thousand gallons of distillate, and.pref erably in the range of from about 0.1 to 0.5 ,poundper thousand gallons.

Although when reference is made to caustic in this disclosure, caustic soda is usually meant, it is tobe understood that the use of alkali metal hydroxides other than sodium hydroxide, such as potassium hydroxide, is not precluded. Sodium hydroxide is usually employed because of its greater availability and generally llower cost. It is to be understood that in practicing this invention lthe step of subjecting the naphtha or gasolineto a pre- 'sweetening caustic wash will be necessary whenever hy- -drogen sulde and/or other acidic materials are present that would interfere with or impair the eliiciency of the subsequent air sweetening steps.

In those instances where it is definitely known that materials'such'as hydro gen suliide which are deleterious to the air sweetening Ymechanism are absent, the pre-sweetening caustic -wash may be dispensed with. This step necessary, however.

will in general be The following examples serve to illustrate the advantages of this invention:

EXAMPLE 1 A promoter solution was prepared by treating a kerosene from a sweet Louisiana mixed crude with one volume percent of 15 B. caustic soda at 110 F. by air blowing in a plant agitator. Two separate samples of a light catalytic cracked naphtha having an A.P.l. gravity of 67.2 and a boiling range of 108 to 340 F. were given a pre-sweetening wash with 10 volume `percent of 15 B. caustic soda. One of the pre-treated samples was given a second fresh caustic wash with 10 volumepercent of 15 B. caustic. The other pretreated sample was given a ywash with 10 volume percent ofthe promoter solution prepared as described. In each case a small quantity of N,Ndisec-butyl-p-phenylene diamine was added to each of the pretreated naphtha samples before the second caustic treatment. The quantity of inhibitor added was equivalent to 0.2 pound per 1000 gallons of naphtha.

After the second treatment, with fresh caustic in the one case land the promoter solution in the second case, the naphtha samples were separated from the caustic and placed in separate holding tanks where the naphtha samples were held vfor a number of hours at temperatures of from 75 to 77 F. In the case of the naphtha given two vfresh caustic washes, an air space amounting to 50 volume percent of the holding tank was maintained, whereas in thev case of the naphtha treated with the promoter solution, air was excluded from the holding tank and the vapor space was blanketed with nitrogen. In each `case the naphtha was agitated for one minute prior to the beginning of the storage period. Samples of the naphtha were taken from each of the storage vessels at periodic intervals to determine the amount of sweetening that had occurred. Test results are presented l Milligrams of mercaptan sulfur per milliliter-s of naphtha.

2 10 volume percent of 15 Baume caustic used in each instance.

-It will be noted that in the case in which the promoter solution was employed, i.e. the spent caustic from kerosene treatment, the sweetening was completed in vless than 18 hours, whereas in the case where fresh caustic was employed in the second caustic wash, the naphtha was still not sweet after hours, even though the naphtha in the latter case had the advantage of agitation in a tank having a 50 percent air space.

EXAMPLE 2 In a second series of tests, the elect of varyingthe method by which air was introduced was determined. Three separate catalytic naphtha samples were employed, identified as naphthas A, B and C. Naphtha A had an A.P.I. .gravity of 64.4 and a boiling range of 96 'to 356 F., naphtha B had an A.P.I. gravity of 67.2 and a boiling range of 108 to 340 F, and naphtha C had an A.P.I. gravity of 63.8 and a boiling range of 100 to 344 F. As in Example 1, each of the naphtha samples was given a pre-sweetening caustic wash with 10 volume percent 'of fresh 15 B. caustic. The samel oxidation inhibitor as used in Example 1 was then added to each of the samples in a concentration equivalent to 0.2 pound per 1000 gallons of naphtha. Portions of each of the naphthas were then treated with the same promoter solution as used in Example 1. In some cases the promoter solution was aerated before contacting the naphtha and in other cases it was not.

Also, in some cases an air space was provided in the holding tank in which the naphtha was placed after the treatment with the promoter solution and in other cases air was excluded. As in Example l the progress of the sweetening was followed by the removal of portions of thenaphtha at periodic intervals, and determinationV of the mercaptan numbers of the samples thus collected. The results obtained are presented in Table II. The table also contains footnotes indicating the aeration procedure used in each instance.

range of from 96 to 334 lF. was employed. sweetening tests were conducted with this naphtha alone as well as with mixtures of the virgin naphtha and a sourcatalytic naphtha having an A.P.I. gravity of 63.8 andy a boiling range of from 100 to 344 1F. Tests were also conducted with the sour catalytic naphtha alone. In each instance the naphtha was given a pretreat with l0 volume percent of 15 B. caustic, after which the phenylene diamine inhibitor employed in the previous examples was added to each naphtha sample, in the same concentration as in those examples, and the samples were then treated with 10 volume percent of the promoter solution used in Example 1. As in the previous examples, the treated naphthas were placed in storage vessels and the progress of the sweetening was followed over a period of time. The results obtained are presented in Table III.

Table III COMPARISON OF SWEETENING RATES OF VIRGIN NAPHTHA AND CATALYTIC NAPHTHA Table 1l Mercaptan Number 1 Naritha Naphtha B Naphthan C Procedure Used 1 2 3 3 4 Raw Naphtha 7. 96 11.1 11.1 20.1 20.1 After First Caustic Wash 2.- 2. 64 4. 6 4. 6 7. 8 7. 6 After Kerosene Spent Caustic Wash 2 2. 16 2. 95 2. 5 3. 9 4. 7 After 18 Hours Storage 0.36 0.49 0.12 After 24 Hours Storage 3. 30 Alter 48 Hours' Storaf'r` 0. 50 Hours to Pass Doctor Test.- 24 18 18 18 24 after admitting air Procedure 1Spent kerosene caustic not ecrated; 50 percent air sapce in holding tank.

Procedure 2-Spent kerosene caustic aerated; 50 percent air space in holding tank.

Procedure -Spent kerosene caustic aerated; air excluded from holding tank and naphtha blanketed with nitrogen.

Procedure 4-Speut kerosene caustic not aerated; air excluded trom holding tank during first 24 hou: period and then admitted.

1Mlligrams of mercnptan sulfur per 100 milliliters of naphtha.

l 10 volume percent of 15 Baume caustic used in each instance.

It will be noted from the first column of data in Table II that sweetening of naphtha A was successfully accomplished without previously aerating the promoter solution, the air in the tank providing the necessary oxygen for the sweetening. Comparison of the data of the second and third columns in the table indicates that the oxygen picked up by the promoter solution during aeration was sufcient to effect the sweetening, since in the case where procedure 3 was followed, wherein air was excluded from the holding tank, sweetening was essentially as rapid as when following procedure 2 wherein a 50 percent air space was provided in the holding tank. Comparison of the data in the fourth and iifth columns of the table indicates that air must be introduced somewhere in the system.

It will be noted, also that when procedure 4 was followed, Very little sweetening occurred during the iirst 24 hours when the naphtha was protected from any air contact and the promoter solution had not been aerated. After air was admitted to the tank at the end of the first 24 hours storage, sweetening proceeded rapidly during the next 24 hours, even though no additional agitation was applied between the first and second 24 hour storage periods.

Other sweetening tests with naphthas of the general nature of Naphtha A indicated that the sweetening reaction proceeds essentialy as rapidly at 40 to 50 F. as at 75 to 77 F. when employing the procedures of this invention.

EXAMPLE 3 An additional set of tests was conducted to study sweetening of virgin naphthas. For these tests a sour virgin naphtha having an A.P.I. gravity of 68.1 and a boiling Mercaptan Number Percent Virgin Na htha. 5

0 Percent Catalytic aphtha Raw Naphtha After First Caustic Wash... After Kerosene Caustic Wash.

After Hours Storage Hours to Pass Doctor Tes It will be noted from the data obtained that the blend containing 25 percent of the virgin naphtha sweetened as rapidly as the 100 percent catalytic naphtha and that a blend of 5-0 percent virgin naphtha and 50 percent catalytic naphtha was sweetened within a period of 18 to 24 hours. It is thus practical to admix up to 50 percent of a sour virgin naphtha with the cracked naphtha and obtain the benefits of this invention. A blend containing 75 percent virgin naphtha required from 48 to 72 hours for sweetening, whereas the sample of 100 percent virgin naphtha showed virtually no sweetening even after 120 hours.

It is to be understood that this invention is not to be limited by the specic examples given nor by any theory regarding its operation. The scope of the invention is defined by the following claims.

What is claimed is:

l. In a process for the sweetening of a sour petroleum distillate containing mercaptans, involving the oxidation of the mercaptans to disuliides, the improvement which comprises adding to the said distillate 0.05 to l pound of an inhibitor compound per thousand gallons of distillate, said compound being selected from the class consisting of amine, amide and phenol oxidation inhibitors, thereafter contacting the distillate in the absence of substantial amounts of added free sulfur Awith a caustic solution previously employed for treating a raw kerosene fraction, separating said distillate from said caustic solution, and storing the thus treated distillate for a period of at least several hours to eiiect sweetening, said process including the step of introducing oxygen into said distillate subsequent to the addition of said compound.

2. Improved process are defined by claim 1 wherein said introduction of oxygen is effected by aerating the caustic from kerosene treating, prior to contact with the said distillate.

3. A process for sweetening a sour cracked naphtha which comprises the steps of treating the naphtha in a irst zone with a caustic solution to remove caustic-soluble materials including hydrogen suliide and thiophenols, adding to the caustic-treated naphtha 0.05 to 1 pound of an inhibitor compound per thousand gallons of naphtha, said compound being selected from the class consisting of amine, amide and phenol oxidation inhibitors, thereafter :treatngthe naphtha in the absence of substantial amounts Aof 'added free Vsulfur in a secondzonewitha caustic solu- 'tion previously employed for treating a :raw :kerosene fraction, separating said naphtha from said caustic Vsoluytion-and storingthe Vthus treated naphtha Vfor .aiperiod ,of 'at least severalhours to eectsweeteningsaid processin- `cluding the step of introducing oxygeninto said naphtha Vsubsequent-to #the addition of the said compound.

A4. Process as defined by claim 3 wherein said -introduction of loxygen is effected by aerating the caustic from kerosene'treating, 'priorto contact with the said naphtha.

'5. Process as dened by claim 3 wherein the said added compound comprises a phenylene diamine.

6. Process as dened by claim 3 wherein'the compound comprises N,Ndisec butyl `p-phenylene diamine.

7. Process as defined by claim 3 wherein said cracked .B naphthaisgadmxed withup'to 5.0zpercent of a sourvirgin :naphtha :8. Processias dened .by claim `wherein -said introduc- 'tion of 'oxygen is effected .during fthe step :of rstoring 'the -said:naphtha.

`9. .Process .as defined by claim 3 wwherein said .caustic .employed for treating said kerosene :fraction is of from 10 to 20 B. strength and is employed'in thesproportion '.of from:5 1o 20 .volume percent of thenaphtha.

References -Cited inthe le of this patent UNITED STATES PATENTS 2,552,399 Browder 'May'8, 1951 '2,701,784 Cauley Feb. 8, 1955 y2,771,403 -Kalinowski et a'l NOv.'2U, 1956 l27812.96 Brown `Feb. '12, 1957

Claims (1)

1. IN A PROCESS FOR THE SWEETENING OF A SOUR PETROLEUM DISTILLATE CONTAINING MERCAPTANS, INVOLVING THE OXIDATION OF THE MERCAPTANS TO DISULFIDES, THE IMPROVEMENT WHICH COMPRISES ADDING TO THE SAID DISTILLATE 0.05 TO 1 POUND OF AN INHIBITOR COMPOUND PER THOUSAND GALLONS OF DISTILLATE, SAID COMPOUND BEING SELECTED FROM THE CLASS CONSISTING OF AMINE, AMIDE AND PHENOL OXIDATION INHIBITOR, THEREAFTER CONTACTING THE DISTILLATE IN THE ABSENCE OF SUBSTANTIAL AMOUNTS OF ADDED FREE SULFUR WITH A CAUSTIC SOLUTION PREVIOUSLY EMPLOYED FOR TREATING A RAW KEROSENE FRACTION. SEPARATING SAID DISTILLATE FROM SAID CAUSTIC SOLUTION, AND STORING THE THUS TREATED DISTILLATE FOR A PERIOD OF AT LEAST SEVERAL HOURS TO EFFECT SWEETENING, SAID PROCESS INCLUDING THE STEP OF INTRODUCING OXYGEN INTO SAID DISTILLATE SUBSEQUENT TO THE ADDITION OF SAID COMPOUND.

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