US2083917A - Process of sweetening hydrocarbon distillates - Google Patents

Description

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2,083,917 rnoonss or SWEETENING mnocannon ms'mrn'ms John M. McGee, Casper, Wyo., assignor to Standard Oil Company,

poration oi Indiana Chicago, 111., a cor- No Drawing. Application April 8. 1935, Serial No. 15,283

14 Claims. (Cl- 196-29) This invention relates to processes of sweetening petroleum distillates' such as gasoline, cracked vgasoline, naphthas and kerosene and it comprises processes wherein such a hydrocarbon distillate is sweetened with sulphur or oxygen and magnesium oxide.

Gasoline, kerosene, naphthas, and similar petroleum ,distiilates contain sulphur compounds such as mercaptans and in the customary re-' fining processes these distillates are treated,with

sodium plumbite and sulphur to convert the mercaptans to unobjectionable disulphide. ,This process is known in the art as sweetening and the treated distillate is referred to as being sweet 5 to thedoctor test. i a

As stated, most of the present day processes in commercial use employ sodium plumbite and sulphur. A small amount of sulphur is added to the gasoline or other distillate and then the distillate is treated with a solution of sodium plumbite.

Within recent years other methods have been suggested for accomplishing the sweetening of these distillates. In some instances cuprous chloride has been used. More recently others have suggested the use of magnesium hydroxide, preferably in the form of brucite. In the socalled brucite process, much emphasis has been laid upon the necessity for using thoroughly hydrated magnesium hydroxide and this process is supposed to lessen the cost of sweetening petroleum distillates because brucite is considerably less expensive than the litharge and caustic soda which constitutes the usual doctor solu- 5 tion and can be readily regenerated.

I have now investigated the use of substantially anhydrous magnesium oxide as contrasted with the magnesium hydroxide of the prior art and have discovered that very much better resuits can be obtained if magnesium oxide be used in place of magnesium hydroxide. I have, for instance, discovered that very much less sulphur need be added to the gasolinea or other petroleum distillate when magnesium oxide is used. Thus, for example, whereas excellent sweetening can be obtained with magnesium oxide and but three to four milligrams of sulphur added to one hundred cubic centimeters of gasoline, with magnesium hydroxide at least ten milligrams of sulphur must be added to the same gasoline to obtain satisfactory purification. Moreover, the sweetening action with magnesium oxide is much more rapid than that with magnesium hydroxide. Comparative data on the same gasoline shows that with magnesium oxide,

and a gasoline containing ee to tour minigrams of sulphur per one hundred cubic centimeters, over 13,300 gallons of gasoline can be passed through a 'ton'oi magnesium oxide per hour. when using magnesium hydroxide only 19 gallons of gasoline, containing ten milligrams of sulphur per cc. can be passed through a ton of hydroxide per hour to get equivalent sweetening action.

From the foregoing experimental data it will be apparent that not only is the quantity oi. sulphur greatly reduced but also the rate of sweetening is increased many times over that possible with magnesium hydroxide.

While I have no definite explanation for this unusual behavior of magnesium oxide as contrasted with the hydroxide I am inclined to attribute it in part to the fact that magnesium oxide must be a far more reactive catalyst for the conversion of mercaptans to disulphides. Aside from the fact that the rate of sweetening is very much faster and the quantity of added sulphurvery much less, I have discovered that the process of the present invention possesses additional advantages of importance. Because the quantity of added sulphur can be greatly reduced I find that the danger of corrosion due to excess sulphur is greatly reduced. Moreover, I have discovered that the used magnesium oxide can be readily reactivated for reuse. And of course it is a well known fact that magnesium oxide is less expensive than litharge.

I shall now describe practical embodiments of my invention.

I first provide a tower packed with granular magnesium oxide through which the petroleum distillate is to be passed. advantageously the distillate, for example gasoline, flows from the bottom of the tower to the top to avoid any entrapment of gasoline vapors or gases. This simple arrangement constitutes the apparatus required.

The anhydrous magnesium oxide is advantageously prepared from the mineral brucite (magnesium hydroxide) by first treating it with caustic soda solution, of about 10% strength, to decompose any carbonates it may contain. The brucite is then calcined to remove both free and combined water, in other words, to convert magnesium hydroxide to substantially anhydrous magnesium oxide. Although the calcining operation can be conducted at a temperature of 600 F. or lower, I find that very much better results are obtained if the calcining temperatureis about "100 to 1200 F. Even higher temperatures up lac-1600 F. maybe used. V

The .tower referred, to above can, as stated,

be packed with substantially anhydrous granular' oughly dehydrating at temperatures of 600 F. 15 or higher.

Regardless of how the magnesium oxide is supported in the tower I do, as stated, provide a relati'veiy large volume of the oxide in such physical condition that a petroleum distillateto be sweet- 20. ened can be passed therethrough. I prefer to have as much oi-the catalyst as possible in the oxide tom; the hydroxide obviously lessens activity and efliciency oi the catalyst.

Next I prepare the distillate for treatment and 25 in this step. I incorporate small amounts of sulphur therein. In the distillate, which can be gasoline, either straight-run or cracked, kerosene, naphtha, and similar distillates usually subjected 'to the sweetening operation, I dissolve 30 about three to tour milligrams of sulphur per 100 cc. of distillate. As noted above, this amount of sulphur is very small in comparison to the quantity of sulphur hitherto used with magnesium hydroxide. I find that the best method 01' intro- 85 ducing sulphur into the distillate is by by-passing a portion of the distillate flowing to the magnesium oxide tower through a drum containing lump sulphur and then returning the lay-passed stream 01' distillate to the major quantity prior to 40 introduction thereof into the tower.

This stream of distillate then flows into contact with the magnesium oxide -Within the tower mercaptans in the distillate are decomposed and the distillate leaving the tower is sweet to the 45 doctor test.

In my process I find that I need not use elevated temperatures during the sweetening process and ordinarily operate at about 50 F. to 90 F. I find that high temperatures may be used it de- 50 sired, and when I employ heating it is desirable to prevent evaporation of the distillate by conducting the operation under pressures high enough to avoid evaporation.

Advantageously the rate of flow oi distillate 55 thru the magnesium oxide should be about 500 to 1,000 gallons per hour per ton of oxide in the tower. Generally the useful life of the oxide is about 3,000 to. 12,000 gallons per ton, depending on the rate of flow of the distillate through the magnesium oxide. Virgin gasoline gives even longer catalyst life. For some reason which I have been unable to explain I find that the eiiective life 01' the oxide is much greater if the rate of flow oi distillate is slow. Thus, with a flow of about.120 gallons per hour per ton of oxide I find that the effective life may be as much as 12,000 gallons of distillate. But because it is such a simple matter to reactivate the exhausted magnesium oxide for reuse, I find that on a plant 7 scale it is best to increase the rate of flow to 500 .or 1,000 gallons an hour per ton of oxide in the tower.

In the ordinary commercial use of my invention I provide a plurality of treating towers so ar- 75 ranged that distillate to be sweetened can be dias described above. tion, using the towers referred to, I pass the aosasiv rected into any of the towers, and thus obtain continuous operation. For example, I advantageous] provide three treating towers. Distillate first sent into tower 1 until the magnesiumoxide therein approaches exhaustion (as 5 indicated by the doctor test on eiiiuent distillate).

,. Then the distillate flow through towers 2 and 3 in series until the conte ts of tower 1 is wholly exhausted. Next, as tower 2 approaches exhaustion, the distillate passes through towers 2 10 and 3 in series until tower 2 is exhausted. Meanwhile the oxide in tower 1 is being reactivated or revivified and when tower 2 is exhausted distillate flows through towers 3 and 1 and so forth.

Various ways can be used to revivii'y the spent magnesium oxide. Since magnesium sulfide is one ot the reaction products of the sweetening I advantageously boil the spent oxide in a dilute (10 per cent) solution of caustic soda to decompose any sulfide present. This converts the spent oxide to magnesium hydroxide and this I dehydrate at temperatures oi'r 600 F. to 1,200 F.

In a commercial modificacaustic soda solution down through the spent oxide in a tower until the eflluent caustic soda liquor shows that most of the magnesium sulfide has been decomposed and I then dehydrate the magnesium hydroxide by passing hot flue gases upwardly through the tower. I may alternatively use hot water or steam instead of caustic to decompose undesired magnesium compounds and wash the catalyst.

. While in the foregoing I have specifically referred to the addition of about 3 to 4 milligrams of sulphur to 100 cc. of distillate prior to contact with the magnesium oxide I do not of course limit myself thereto. It is well known in the art that an excess of sulphur is to be avoided since this renders the treated gasoline corrosive. But one of the distinct advantages in the present process is that much less sulphur is required to get equal sweetening as compared with the use of litharge and caustic soda, or magnesium hydroxide. In addition, the amount of sulphur can be varied over quite wide limits, which is not true of prior processes. For example, I find that the amount of sulphur can vary from 10 to 40 grams in 100 gallons of distillate without the distillate showing any test for free sulphur (copper strip). Yet this wide variation gives uniformly good sweetening and in all instances the treated distillate. such as gasoline, is sweet to the doctor test. The advantages of this are readily apparent.

Great care need not be taken to insure that just enough sulphur is present for sweetening but yet not enough to cause corrosion.

Occasionally I find that some gasolines tend to develop color in my process. This is not ordinarily objectionable, especially when the marketed gasoline is given a tinted color by the addition of a dye. But if it is desired to obtain a waterwhite gasoline I find that I can combine my sweetening process with the usual vapor phase clay treating process. Thus I first pass the gasoline, in the vapor state, through a body of fullers earth, then fractionate and condense the vapors rand finally add sulphur to the gasoline and pass it through a bed of magnesium oxide as described above. I can also subject the sweetened gasoline to 'a final distillation step but Iflnd it best to distill at relatively low temperatures, below 300" F. and under reduced pressure ii necessary, in order to prevent the decomposition of sulphur compounds in the gasoline with the formation or mercaptans. Ordinarily, however, it is unnecessary. to redistill the gasoline after the sweetening process.

Instead of using sulfur in my magnesium oxide of air for four hours at a temperature of 700 F. Y 15 to 1200 F. I' have found that-optimum results are obtained when the magnesium oxide has been heated four hours at 800.F... When heated at a temperature lower than 600 F., magnesium hydroxide gave very poor results and when magne- 20 siufn oxide was calcined at 1400 F. to 1600 F. its

efliciency fell off to about one-half of that which had been held at 800 F. for four hours.

' It is not necessary to remove the spent MgO from the reaction chamber to revivify it. When 25 theMgO bed is spent it should first be drained of gasoline'and then thoroughly steamed to remove all oily matter. If desired, the bed may be flooded with water and heated to the boiling point of the latter by means of low pressure open 30 or closed coilsteam. The water, if used, should be drained off. Then by means of the heating coils in the bed of reagent, it should be heated first'to 250 F. to dry the Mg(OH)2 and then must be heated to about 800 F. by means of super heated steam oi flue gas to re-form MgO. The latter is activated by means of a stream of hot air. After heating and aerating for about four hours at 800 F. the heat is turned off and the bed cooled to atmospheric temperature by means 4 of a stream of cold air dried by passing through a bed of CaClz.

After the magnesium oxide has been revivified and cooled the gasoline or other sour distillate is pumped through the bed with air. I have found 45 that the air may be introduced into the base of the MgO tower and allowed to move upward in contact with sour oil while it is being treated. The amount of air required will vary with the sourness of the gasoline being treated; for exam- 50 ple I may use from .1 to 1.0 cubic feet of air per gallon of oil treated. The requisite amount of oxygen may be dissolved, by the aid of pressure if necessary, in the distillate by contact before the oil enters the MgO chamber.

55 A simple method of dissolving the oxygen in theoil is to by-pass a portion of the stream and subject it to air under pressure above atmospheric. The air charged oil is then returned to the main stream and the whole introduced into 60 the MgO chamber.

As an example of results obtained in carrying out the process a steel tower ten feet high and fourteen inches in diameter was charged with granulated magnesium oxide supported on a 65 Monel metal screen and dried by a suitable steam coil. A final heating to 850 was performed by a current of flue gas.

Cracked gasoline, having a mercaptan content equivalent to 0.2 to 0.25% mercaptan sulphur was 70 treated. In order to sweeten this gasolineby the conventional doctor method there was required about 0.28 pound of litharge and 0.35 pound of caustic soda with 0.07 pound of free sulphurper 100 gallons. The tower was charged with 279 sweetening process I may employ another oxidiz-' about 1200 F. Gasoline treated with air was passed through the tower at the average rate of 30 gallons per hour at a temperature of 60 F. A total of about 385gallons of gasoline was sweetened. The resulting gasoline was sweet as deter- 5 mined by the standard doctor test and 'did not show any corrosion when tested by the standard copper strip corrosion test.

r 750 F. respectively, yields of approximately 600 and 900 gallons were obtained. It is important tocontrol the rate of contacting the gasoline with magnesium oxide in order to allow adequate time for .the reaction to take place. The rate of treating in the foregoing runs varied between 0.01 and 0.5 gallon per pound of MgO per hour and lay in thevicinity of 0.1 gallon per pound MgO per hour.

The products from both runs gave negative corrosion tests, the product from the last run having a color of 20 Saybolt whereas the original stock had a color of between 19 and 21. The oxidation stability of the gasoline produced by this process is low, the copper dish determinations on the foregoing runs being on the average 2 of 300, 237 and 228 milligrams per 100 cc. respectively. The addition of oxidation stabilizers such as para benzylaminophenol, alpha naphthol, catechol, substituted catechols, pyrogallol, wood tar creosote, cresylic acid, etc. in amounts of .001 to .05% is suflicient to produce the desired oxidation and aging stability. Antioxidant may be added before or after sweetening but it is preferred to add it afterward. The gasoline may be likewise stabilized before sweetening or afterward, although it is preferred to operate the process on previously stabilized gasoline.

In the appended claims we mean the term petroleum distillates to embrace all those distillates customarily subjected to sweetening as a step in the refining thereof. These distillates, as is well known, are mostly in the gasoline boiling range and include kerosene, gasoline, naphthas, cracked gasoline and like relatively volatile petroleum distillates.

I claim:

1. The process of sweetening petroleum distillates comprising contacting said gasoline in the liquid phase with a catalyst consisting preponderantly of calcined magnesium oxide in the presence of an oxidizing agent from the group consisting of air, oxygen, and sulfur.

2. The process of sweetening petroleum distillates containing mercaptans which comprises dissolving oxygen therein by subjecting said distillates to air under pressure and subsequently contasting said oxygen-containing distillates with a sweetening agent consisting preponderantly of calcined magnesium oxide at a rate of about 0.01 to 0.5 gallon per pound of magnesium oxide per hour.

3. The process of claim 2 wherein the said magnesium oxide has been regenerated after exhaustion by treatment with steam and subsequent iliggiydration at a temperature between 600 and 4. The process of altering, mercaptans in petroleum distillates which comprises adding elemental sulphur to said distillate and passing the resulting sulphur solution through a mass consisting of substantially anhydrous calcined magnesium oxide.

5. The process as in claim 4 wherein the distillate is a gasoline.

6. The process of sweetening a petroleum distillate which comprises adding elemental sulphur thereto and passing theresulting solution through amass consisting of magnesium oxide calcined ata temperature vof 400 to 1000" F. 7.-The process as in claim 6 wherein the distillate is a gasoline.

8. The process or sweetening va petroleum dis- I tillate which comprises adding elemental sulphur thereto and passing the resulting solution through a mass consisting preponderantly of anhydrous calcined magnesium oxide supported by anvinert carrier. --9. The process as in claim 8 wherein the distillate is a gasoline.

'10. The process-oi sweetening gasoline which comprises adding to gasoline a quantity of elemental sulphur in quantity of the order of 10 to V 40 grams to 100- gallons of gasoline and passing the gasoline through a granular mass consisting preponderantly of substantially anhydrous calcined magnesium oxide.

11. The process as in claim 10 wherein the magnesium oxide has been calcined at a tem perature of 400 to 1000 F. I

12. The process of sweetening gasoline comprising subiecting the gasoline to the action of oxygen-containing gas in the presence of a catalyst consisting of magnesium oxide.

13. The process of sweetening petroleum distillates which comprises subjecting such a distillate containing a small quantity of free sulfur to the action of a sweetening agent consisting preponderantly of calcined magnesium oxide.

- 14. The process as in claim 13, wherein the distillate is in the liquid phase.

I JOHN M. McGEE.