US2574122A - Sweetening process - Google Patents

Description

Filed March 11, 1949 Il l:0 Qmml Lfdme JT-Ryder' Inventor bh). M/Gbboneg Patented ov. 62, 195i SWEETENING PROCESS James W. Ryder, Watchung, N. Jg, assignor to Standard Oil Development Company,y a corporation of Delaware Application March 11, 1949, sria1No.so,s59

, Y1 f f The present invention is concerned with an improved sweetening process. The invention is more particularly concerned with the removal of mercaptan compounds,especially those mercaptan compounds which have heretofore been relatively dcult to remove from hydrocarbon fractions, which fractions boil in the motorA fuel, kerosene, diesel oil and heating oil boiling ranges. In accordance with the present invention a feed hydrocarbon fraction which is characterized. by containing mercaptan compounds is contacted with a relatively strong caustic solution which contains an alkyl phenolate and an aromatic mercaptide. By treating a mercaptan containing feed oil with a treating solution of this character, unexpected and desirable vresults are secured with respect to the removal of the mercaptan compounds from the feed oil.

It is well known in the art to treat petroleum oils by various procedures in order to remove objectionable compounds, as for example sulfur compounds therefrom. For example, it is known to treat petroleum fractions boiling in the motor fuel boiling range and in the general range below about '700 F. with various chemical reagents such as, sodium or potassium hydroxide solutions. Also, it has long been known that weakly acidic materials, such as mercaptans, are reactive to various degrees with basic materials. One method for the removal of mercaptan compounds from hydrocarbon streams containing the same has been'to treat these streamswith a reagent which is insoluble in the hydrocarbon stream but which contains alkali typeof material which reacts with the mercaptan constituents. Such treating agents have normally been aqueous so-l lutions of an alkali material. The reaction product between the treating solution and the mercaptan is -a basic salt of the mercaptan. Normally, such salts exhibit some solubility in the aqueous phase. However, this solubility decreases as the molecular Weight or branchiness of the hydrocarbon structure of the mercaptan increases. Therefore, caustic solutions when they are only spent to a small extent in mercaptide salts will not give any further reduction in mercaptan content despite the fact that these treating solutions have a large residual free caustic content. The art has alsorecognized that the efficiency of the spent caustic .can be partially or fully restored by regeneration of these solutions. The regeneration normally consists of removal of a part or all of the mercaptide salts by means of hydrolysis and vaporization or by oxidation.

One method is to employ steam 4regeneration 6 Claims. (Cl. 196-32) 2 for the revivication of caustic solution spent in mercaptan removal from hydrocarbon streams. The steam regeneration is normally carried out in a packed tower or a tower provided with bubble plates or'pierced type tray plate design. The steam hydrolizes the mercaptidesalts to the corresponding mercaptans which are stripped out by the steam vapors and taken overhead. The regenerator also serves as a` means of adjusting the gravity of the caustic solution, which has been shown to be critical. The regenerated caustic is withdrawn from the bottom of the regeneration tower and reused in the extraction stage of the system wherein the .caustic contacts the hydro-.- carbon stream through suitable mixing devices for further extraction of mercaptans by thesame caustic.

It has-also been found that certain constituents of the oil itself Will function as solubility enhancing agents. These constituents tend to 'une prove the partitionv coefi'icient of the acid mercaptan in the aqueous` phase and to increase the solubility of the mercaptide salts in the aqueous phase, particularly when the mercaptan is of a relatively high molecular weight. As mentioned. mercaptans are diicult to extract from hydrocarbon solutions by straight aqueous alkali solutions. This is particularly true of the higher molecular weight mercaptans. In general, this is occasioned bythe fact that the two phases are not miscibleand that good contact must be obtained to allow Vthe alkali metal ion to react with the mercaptan hydrocarbon. Furthermore, yonce the rea-ction has occurred, it is essential that if the extraction of the mercaptan is to be Yac-4 complished the reaction product must be soluble in the aqueous layer. As pointed outrhere-r tofore,` the higher the molecular weight of the mercaptan thelower the solubility of the corresponding mercaptide salt in the aqueous phase. The solubility enhancing agents tend to increase this solubility and thus increase the effectiveness of the alkali treating solution.

The exact chemical nature of these solubility enhancing agents is not entirely known. How-Y ever, it is felt that they comprise the lower molecularweight aliphatic acids and the various -alkyl substituted phenolic compounds. It 4has been found that phenolic type materials having alkyl side chains containing less than about 3 carbon atoms are not effective by themselves as solubility promoters and that organic carboxylic acids having less than about 3 carbon atoms per. molecule are likewise not very effective. However, as the lengthof the side Vchain of the phenols `irl-,-

` amica creases or the length of the aliphatic acid hydrocarbon chain increases, their eiectiveness as solubility promoters increases greatly. However, the solubility of the acids or phenols themselves in the caustic solution decreases with increasing hydrocarbon chain length and thus limits the molecular weight of acid or phenolic type material which can be employed. It is believed V that the optimum chain length should be from about 3 carbon to 7 carbon atoms for the acids.

As pointed out heretofore, alkyl substituted phenols are very desirable as solvent enhancing agents, but that as the length or number of side chains of the phenols increase, their eiectiveness is impaired due to the Afact that these phenols themselves lose solubility in the alkali. Furthermore, with respect to any particular-alkyl phenol or class of alkyl phenols, their solubility in the alkali solution decreases as the .strength of the alkali solution increases. However, it has beengiiscpyeredthat providing an aromatic mer- .Gipltn is utilized in :conjunction with the alkyl phenol and caustic solution, the solubility of the alkyl phenol in the alkali is material increased resulting in substantially increasing the effec- Ativeness of .the alkyl phen'ols as solvent enhancing agents.

vThus it has now been discovered that providnga `iced oil is treated with an alkali solution containing an alkyl phenolate and an aromatic mercaptide, unexpected and desirable results are secured with respect to the removal of mercap- .tan .constituents from the feed oil. The present invention may be readily understood by reference to the drawing illustrating one embodiment of the same.

Referring specifically to the drawing,. a feed oil Awhich for .the .purpose of illustration is .assumed to be a fraction boiling in the motor fuel boiling range and derived by thermally cracking a gas oilis introducedr into zone .I by means of feed line I. In zone I5 the oil is handled 4in a manner to free the oil of hydrogen sulphide. This may be accomplished by a washing oper-ation with a suitable solvent, as for .example caustic soda, or `the removal may be accomplished by means of a distillation process. If a distillation process be utilized the hydrogen sulphide is usually removed from the propane in a stabilization-and debutanization operation. Zone :I5 may comprise any suitable number and arrangement offstages. If a washing `solvent is utilized, it is introduced -into zone 4I5 by means of line 5 and withdrawn from -zone-Iiby--means of line 6. A suitable washing solvent for example comprises aweak caustic solution. The Voil free of hydrogen sulphide is withdrawn from zone I5 by means of` line 2 and introduced into caustic treating zone I6. In zone I6 the -oil is contacted with a caustic solution which for the purpose of illustration is assumed to be ay sodium Vhydroxide solution. Fresh caustic `is, introduced into zone I6 by means of lines I0 and 1. Zone I6 may comprise any suitable arrangement of treating and mixing zones. rThe spent caustic solution is withdrawn from zone I6 by `means of line 8 and passed to regeneration zone l1 wherein the solution is handled in a manner to regenerate the same. This is usually accomplished by introduclngsteam into zone I1 by means of line 2l and removing overhead by means of line I I the mercaptans. The regenerated .caustic is withdrawn from the bottom of zone I1 by means of line 9 and `recycled to zone I6. Spent caustic may be withdrawn from the system by means of line 22 while fresh caustic may be added by means of line Ill. Since thermally cracked petroleum oils contain alkyl phenols, it may not be necessary to add these particular constituents from an external source. However, these relatively low molecular weight phenols are not particularly effective and it is therefore desirable to add to the oil, boiling iin I themotor'iilel boiling range, higher boiling phenolic constituents However, since thermally cracked petroleum hydrocarbons do not contain, to any substantial extent, aromatic mercaptans, it is also necessary in accordance with the present invention to add these mercap- Vtans 4to the caustic preferably by means of line .23. Thus, the treating solution comprises a caustic solution :containing an alkyl phenolate and aromatic mercaptides.

The treated oil withdrawn from zone I6 by means vof line 3 is passed into zone I8 wherein it is Water washed in order to further rene the same. Phe water `is introduced into zone I8 by means of line I2 and-withdrawn by means of line |53. The rened oil liswithdrawn from the system .by means of 'line .4 andfurther handled as desired.

The invention 'is broadly concerned with the removal .of mercaptan compounds from petrole- .um hydrocarbons-containing the .same utilizing an alkyl phenolate ,in .conunction with an aromatic mercaptide. By 'utilizing .a solution .of this character, the solubility of the alkyl phenol in the caustic solution, vas f or example, in the sodium hydroxide ,solution is. increased. It is thus, possible to utilize amuch ,stronger caustic solution. :with a resulting improvedefciency in the removalof the mercaptan compounds, particularly :the higher `mercaptan compounds Vfrom the oil stream.

The invention is particularly adapted for the treatmentof petroleum..oils `boiling below about '7.00J F. :especially those oils boiling in the motor fuel boiling range which fractions boil below about 420 F. The :processlhowever can be utilized inthe treatment of diesel fuels, kerosenes, gas .oils Vand the like. `A particularly desirable hydrocarbonl fraction for use in .conjunction with the .present inventionfis a fraction which has been secured by a thermal cracking operation. In these operations petroleum fractions, Ias for example, gas oils, reduced-etudes and the like, are crackedl at .elevatedftemperatures and pressures. Pressures areinf-the range above about A250 lbs. per sq. in. and as high as -1000 lbs. per sq. in. Cracking temperatures are in the range above about 8009 F. land-'are usually in the range of about 875 F. to 9509 F.

These thermally cracked fractions, particularly those Vf-ractionswhiclfiboil in the range from about 4009 F-.to 7009 F. contain naturally occurring alkylated phenols` and thus by adding to the caustic'solution -utilized an aromatic imercaptan, the desirabletreatingv solution is secured. Furthermore, the inventionmay be readily adapted to the :treatmentuof fractions .secured=from catalytic cracking operations. These fractions .ordinarily contain aromatic mercaptans which are .present as rthiophenols, v.thiocresols .or thioxylenols, .as yfor example, .thiop'henoL methyl fthiophenol, dimethyl thiophenol, ethylthiophenol, diethyl thiophencl, propylV .thiophenoL` .-and.l .dipropyl thiophenol. However, these aromatic llnercaptans,are.subject to an .unusualphenomenon inv @that .when oxygen is present oxidation .ofoleflns and .dioleilns in the gasoline-is .catalyzed byftheipresenceof these aro.- matic mercaptans. Thefnlens `ancljdioleilns :are

oxidized to peroxides, which inturn `destroythe aromatic mercaptans. Thus, unless special precaution is taken to exclude oxygen and thus prevent the destruction of the aromaticmercaptans, it will also be necessary to add these aromatic mercaptans to the treating solution containing the phenols if the present invention is to be carried out with respect to these particular feed oils.

The alkyl phenolate is selected from a class of alkyl phenolates which may be represented by the following formula:

in which X represents a metal selected from the group consisting of sodium, potassium, lithium, rubidium and cesium; and in which R represents hydrogen and at least one alkyl group. The alkyl group may contain from 2 to 7 carbon atoms in the group, however, it is prefered that the group comprise from about 3 to 4 carbon atoms. The number of alkyl groups on the ring likewise may vary in the range from about 1 to 4, however, 'it is preferred that at least one ortho position be left open. Satisfactory alkyl phenols for use in conjunction with the present invention are for example ortho, meta and para normal propyl phenol; ortho, meta and para isopropyl phenol; ortho, meta and para butyl phenol; ortho, meta and para secondary butyl phenol; vpara tertiary butyl phenol, para amyl phenol, para tertiary amyl phenol.V Specific satisfactory compounds are for example, p-tert. butyl-m cresol, p-tert. butyl phenol, p-tert. amyl phenol, m-isopropyl-o cresol (Carvacrol) and p-tert. butyl vm-ethyl phenol. A particularly desirable source of these alkylated phenols is from gas oils and heating oils which boil in the range from about 400 F. to 700 F. and which have been derived froma thermal cracking operation. these alkylated phenols is from heating oil fractions derived from a naphthenic type crude, as for example a quiriquire crude.

One method of segregating a mixture of desirable alkylated phenols is to treat a petroleum fraction boiling in the range from about 400 to 700 F. which has been secured from a thermal cracking operation with a B. sodium hydroxide solution. After extracting the cracked gas oil, the caustic solution is segregated and then neutralized with a strongfacid. The acid oils are thus sprung from the solution. These acid oils comprising alkylated phenols are partcularly desirable for use in accordance with the present invention.

The aromatic mercaptides employed in the present invention are selected from the class of compounds represented by the following structural formula:

in which X represents a metal selected from the group consisting of sodium, potassium, lithium, rubidium and cesium; and in which R represents A particularly desirable source ofY hydrogen or an alkyl group. The alkyl group may contain from 1 to '7 carbon atoms in the group. However, it is preferred that the number of carbon atoms in the alkyl group be not greater than 4. The number of alkyl groups on the ring may vary in the range from 0 to 4; however, it is preferred that at least one ortho position be left open. Satisfactory aromatic mercaptans are for example, thiophenol and thiocresol. Other satisfactory mercaptans are for example dimethyl. thiophenol, ethyl thiophenols and propyl thioplienol. Specific satisfactory compounds are for example, thiophenol, p-thio cresol, o-thio cresol, p-ethyl thiophenol and 1 thio-2,4 methyl benzene. The caustic solution used in the present invention should contain from about 10 to 30% free NaOH.v A desirable caustic solution contains from about 20 to 25% free NaOH or equivalent. The amount of caustic solution used based upon the feed oil being contacted is preferably from 5 to 50% by volume of caustic solution per volume of oil.

The concentration of the alkyl phenolates pres-V ent in the caustic solution should preferably be in the range from about 5 to 50% by weight based upon the Weight of NaOH in said caustic solution. A prefered concentration is in the range from about 10 to 25% by Weight. The concentration of the Varomatic mercaptide present is preferably in the range from about 5 to 20% by weight based upon the weight of the NaOH in said caustic solution. A preferred concentration of aromatic mercaptides is in the range from about 10 to 15% by weight based upon the caustic solution. n

The present invention may -be further illus-Y trated by the following examples illustrating embodiments of the same.

Example 1 A thermally cracked naphtha boiling in the motor fuel boiling range F.420 F.) and derived from a naphthenic type crude had a copper number of 31. This naphtha was treated with 15 volume per cent of a caustic solution. The caustic solution comprised 18 volume per cent of alkyl phenol, 6 volume per cent of p-thiocresol and 260 grams per liter of free NaOH.

The phenols were derived by treating a heating oil fraction boiling in the range from about 400 F. to 700 F. with a sodium hydroxide solution having a gravity of about 10 B. About l0 to 15 Volumepercent of the sodium hydroxide solution was employed, per volume of oil contacted. The heating oil fraction was derived by a thermal cracking operation from a naphthenic type crude. Thev phenols were freed by acidifying the spent caustic solution. The resulting mixture of phenols comprised p-tert. butyl-m cresol; p-tert. butyl phenol; p-tert. amyl phenol; m-iso propyl-o cresol and p-tert. butyl m-ethyl phenol. The treated naphtha had a copper number less than 0.5 and passed the doctor test1.

In another test wherein conditions were the same as described above except that the treating solution did not contain thio cresol, the treated 1 The doctor test is ya means cf making sure that hydrocarbon fractions are free of mercaptans whlch render the fraction soun A sample of the hydrocarbon 1s shaken with a sodium plumbite solution and a trace of free sulfur. If after shaking, the hydrocarbon, plumbite solution, and sulfur have not changed color, mercaptans are absent, the stock is sweet and the doctor test is Pass. If discoloration occurs mercaptans are present and the test does not pass (DNP.).

oi'lhad a copper number. of loanddid-not pass the .doctor test. I

Example 2 In one operation a treating solution contained 280 grams of free NaOH per liter. Alkylphenols derived from a petroleum hydrocbarbon'fraction, as described in Example 1, were added to this solution. Approximately volume percent of alkyl phenols were dissolved. The solution was used'to contact a cracked naphtha having a copper number of 35. The treated fraction had a copper num' ber of 10 and did not pass the doctor test.

In a similar operation to that described above the concentration of the sodium hydroxide was again 280 grams per liter. Similar phenols were added except that 5.4Y volume percent of p-thio' cresol was also added, In this operation the caustic solution dissolved 21.4 volume percent of phenols rather than 10% as described above. When this solution was used to treat a cracked naphtha having a copper number of 35 the treated naphtha had a copper number less than 0.5 and passed the doctor test.

From the above it is apparent that the use of aromatic mercaptans increased the solubility of phenolates in the caustic solution and resulted in an improved eiciency in the removal of undesirable mercaptans from petroleum fractions containing the same.V

Having described the-invention it is claimed:

1. Process for the removal of mercaptan com'- pounds from a petroleum oil which boils in the range from about 80 F; to about 420 F., which comprises treating said petroleum oil with a caustic 4solution containing therein an alkyl phenolate which boils above about 420 F. and an aromatic mercaptide, said alkyl phenolate having been derived from the acidication of the caustic extract oi a thermally cracked Stock boiling in the range of about 420 to 700 F., and said aromatic mercaptide having been derived from the caustic extract of a catalytcally cracked hydrocarbon fraction.

2. Process as dened by claim 1 wherein said alkyl phenolatey is selected from theclass of' alkyl phenolates which may be'represented bythe following formula: v

in which X represents a metal selected from the" group consisting of` sodium, potassium, lithium',

2The copper number is the milligrams of mercaptan sulfur present in 100 ml. of sample under test which will react with a, standard copper ammonium sulphate solution;

rubidium'and cesium, and in which R represents hydrogen and at least one alkyl group, andwhere in the aromatic mercaptide is selected from the class of' compounds represented by the following formula:

in which X represents a metal selected from the group consisting of sodiuml potassium, lithium, rubidium and cesium, and in which R represents hydrogen or an alkyl group.

3. Process as defined by claim 2 wherein said alkyl group of the phenolate contains from 3 to 4 carbon atoms in the chain.

4. Process as dened by'claim 3 wherein the mercaptide. contain at least one alkyl group, which group contains from 1 to 4 carbon atoms in the alkyl chain.

5. Process as defined by claim 1 wherein said caustic solution comprises a sodium hydroxide solution, the concentration of which comprises from about 10% to 30% free sodium hydroxide and wherein the concentration of the alkyl phenolate is in the range from about 5% to 50% by weight based ,upon the weight of the sodium hydroxide and wherein the concentration of the aromatic mercaptide is in the range from about 5% to20% by weight based upon the weight of the sodium hydroxide present in said caustic so- REFERENCES CITED Thefollowing references are of record in the fll'e of this patent:

UNTTED STATES PATENTS Number A Name Date 2,228,028 Brower Jan. 7, 1941 2,317,770 Holloway et al. Apr. 27, 1943 2,364,582; Davis et al. Dec. 5, 1944 .2,394,662 Camp et al. Feb. 12, 1946 2,457,975 Bolt Jan. 4, 1949 Hart et al. Jan. 27, 1949

Claims (1)

1. PROCESS FOR THE REMOVAL OF MERCAPTAN COMPOUNDS FROM A PETROLEUM OIL WHICH BOILS IN THE RANGE FROM ABOUT 80* F. TO ABOUT 420* F., WHICH COMPRISES TREATING SAID PETROLEUM OIL WITH A CAUSTIC SOLUTION CONTAINING THEREIN AN ALKYL PHENOLATE WHICH BOILS ABOVE ABOUT 420* F. AND AN AROMATIC MERCAPTIDE, SAID ALKYL PHENOLATE HAVING BEEN DERIVED FROM THE ACIDIFICATION OF THE CAUSTIC EXTRACT OF A THERMALLY CRACKED STOCK BOILING IN THE RANGE OF ABOUT 420* TO 700* F., AND SAID AROMATIC MERCAPTIDE HAVING BEEN DERIVED FROM THE CAUSTIC EXTRACT OF A CATALYTICALLY CRACKED HYDROCARBON FRACTION.

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