US2570277A

US2570277A - Sweetening process

Info

Publication number: US2570277A
Application number: US78042A
Authority: US
Inventors: Jr Harold A Ricards; James W Ryder
Original assignee: Standard Oil Development Co
Current assignee: Standard Oil Development Co
Priority date: 1949-02-24
Filing date: 1949-02-24
Publication date: 1951-10-09
Anticipated expiration: 1968-10-09

Description

Oct. 1951 H. A. RICARDS, JR, ET AL 2,570,277

SWEETENING PROCESS Filed Feb. 24, 1949 SODIUM 122x325: 9\ W F E E.l j STAGE 2 2o l SPENT CAUSTIC I SODIUM L HYDROXIDEN 15 v ,1?

.. Y (1(0- STAGE 5 30 T CoO- (Inventors 7W abhor-neg improved sweetening process. more particularly concerned with the removal of ing oil boiling ranges.

Patented Oct. 9, 1951 *tiNlTED STAT-ES PATENT OFFICE SWEETENING PROCESS .Harold A.-Ricards, .Jr., Cranfordand James W. RydenvwatchungpN. J., assignors to Standard Oil Development Company, a corporation of Delaware ApplicationFebruary 24, 1949, SerialNo. 78,042

9Claims. (Cl. 19632) Q The present invention is concerned with-an The invention is mercaptan compounds, particularly those mercaptan compounds which have heretofore been relatively diflicult to remove :from hydrocarbon fractions especially from thosefractions boiling in themotor fuel, kerosene, diesel oil andheat- In accordance with the present invention a feed hydrocarbon oil-characterized by containing mercaptan compounds, various phenolic compounds andother acidic compounds, such as lower" molecular weight-aliphatic acids, is contacted in a series oftreating zones wherein a particular arrangement-and sequence of the stages-are employed. In accordance with the broadcast concept of-thepresent process a feedoil containing mercaptan-compounds is contacted in a primary zone with a 'relatively concentrated I caustic solution under conditions to remove from the feedoil objectionable compounds as for example phenolic --compounds. The oil is then contacted in a secondary zone with -a less concentrated caustic solution than that employed in the prima-ry-zone and with solubility enhancing agents under "conditions to remove from-theoilthe objectionable mercaptan compounds.

Thisapplication contains material claimed incopending"application Serial No. 76,802;'fi18d "February 16, 1949 for the same inventors.

Itiswell known inthe-art totreatpetroleum oils -byvarious procedures in order to remove objectionable compounds, as for'example sulfur compounds therefrom. For example, it is known. to treat petroleum fractions boilingin the motor fuel boiling range and in the-general range below about '7 F. with various chemicalreagentssuch as sodium or potassium hydroxide solutions.

Also it has long beenknown-'that 'weaklyacidic materials, suchas mercaptans, are reactive to various degrees with basic materials. One

method for the removal of mercaptans from hydrocarbon streams has been to treat the streams -withareagent which is-insolublein the hydro- -carbon stream but which contains alkali type-of material which reacts With-mercaptans Such treating reagentshave normally been aqueous solutions. The reaction product is abasic-salt of the mercaptan. Normally such salts exhibit some solubility in the aqueous phase, this solu- -bility decreasing as the molecularweight or branchiness of the hydrocarbonstructure of the mercaptan-increases. The art hasgenera-llyrecognized inthe treating of hydrocarbon streams:

tan extraction.

25531011 or by oxidation.

One method is to employ steam regeneration for the revivification of caustic solution spent in mercaptan removal from hydrocarbon streams.

The steam regeneration is normallycarried out in. a packedtower or a tower provided withbubble plates or pierced type tray plate=design. ,.The

steam hydrolizes the mercaptide. salts to the corresponding mercaptans which are strippedout By the steam vapors and taken overhead. The regenerator also serves as ameans 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 hydrocarbon stream through suitable mixing. devices for further extraction of mercaptans byJthe same caustic.

It has. also been found that certain constituents .will function as solubility enhancing .agents. These constituents tend to improve the partition coeflicient of the acid mercaptanin. theaqueous phase and to increase thesolubility ofthemercaptide salts in theaqueous phase, particularly when the mercaptan is of a relatively high molecular .weight. As mentioned, .-mercapt ans are difificult to extract from hydrocarbonsolutionby straight aqueous alkali solutions. Fhis is particularly true of the higher molecular'weight mercaptans. .In general, this is occasioned bythe fact that the two phases are notmiscibleland that good contact-must be obtained to allow the alkali metal ion to reactwvitnthe mercaptan hydrocarbon. Furthermore, .once .the reaction has occurred,-it isessential thatif the extraction of the mercaptan is tobe accomplished the re- .action product. must be soluble in the .aqueous layer. Astpointed out heretofore the higher "the molecular Weight-of the mercaptanthe lowenthe solubility of the corresponding mercaptidesalt 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 many of these solubility enhancing agents is not entirely known. However, it is felt that they comprise the lower molecular weight aliphatic acids and the various substituted phenolic compounds. It has 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 like-wise not very effective. However, as the length of the side chain of the phenols increases or the length of the aliphatic acid hydrocarbon chain increases, their effectiveness 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 limitsthe molecular weight of acid or phenolic type material which can be employed. It is believed that the optimum chain length should be from about 3 carbon to '7 carbon atoms for the acids.

Particularly desirable aliphatic acids are for example, isobutyric acid, propionic acid and valeric acid. Other satisfactory acids are normal butyric acid, caproic acid, alpha ethyl butyric and alpha methyl butyric acid. These acids may be substituted with an OH group or with a halogen. Various alcohols are also satisfactory, as for example, methyl alcohol and ethyl alcohol. Ethylene glycol, as well as, propylene glycol and other poly hydroxy alcohols may also be used. Naphthenic acids, as for example dichlorocatechol naphthenic acid is also satisfactory.

A particularly desirable class of solubility enhancing agents to be used in conjunction with the present invention is a naturally occurring agent which is present in petroleum hydrocarbons. This is segregated from petroleum hydrocarbons by treating the same with a relatively weak caustic solution.

The fresh sodium hydroxide employed for the segregation of naturally occurring solubility enhancing agents should have a Be. gravity below about 10 and preferably have a gravity in the range from about 1 to 5". Thus, the concentration of the sodium hydroxide in the .fresh caustic solution should be below about 7% and is preferably in the range from about .7 to 3 /2%. When the concentration of the caustic solution is specifled in the present invention, it is meant for example, the grams of sodium hydroxide present in a hundred grams of mixture comprising sodium hydroxide and water. By utilizing a solution of this character it has been found that the most effective solubility enhancing agents can be readily removed from the oil. Furthermore, it has been found that when utilizing a caustic solution having these critical concentrations the various phenolic compounds and other constituents which apparently greatly adversely affect the efiiciency of the solubility enhancing agents are not picked up by the caustic solution but remain in the oil stream being treated.

Thus, a very sharp selective separation is made between the desired solubility enhancing agents which are picked up in the caustic solution and the other acidic constituents which adversely affect the efficiency of the solubility enhancing agents. While a once through operation may be employed'with respect to the caustic stream, it is preferred to employ at least a partial recircula tion of the caustic stream withdrawn from the initial zone. The spent caustic stream withdrawn from the contacting zone and not recirculated is controlled so as to be spent in the range from about 30% to 100%. A preferred method of operation is to control the various factors so that the spent caustic stream withdrawn and not recirculated is spent to a degree in the range from about 30% to 50%. It is to be understood that the term spent when used in the present invention means that percentage of sodium ion which has reacted with organic carboxylic acids, as for example with aliphatic acids, to produce sodium salts of these acids. In the operation, upon initial contact a portion of the sodium might react with the various phenols to form sodium phenolates. However, upon recirculation the phenolates are reconverted to the phenols and are displaced from the aqueous solution with the resulting formation of sodium salts of aliphatic acids. Thus, the amount of sodium not spent may be available either in the form of the hydroxide or as a sodium phenolate.

The concentration of the caustic employed in the present invention with respect to the segregation of naturally occurring solubility enhancing agents is a concentration at which effective results are secured. It is obvious that stronger caustic solutions could be initially used, as for example 10 B. or higher and somewhat similar results obtained by recirculating the spent caustic until from about to 100% is spent, that is, until 70% to 100% of the sodium ion is reacted to form sodium salts of aliphatic acids. When employing caustic solutions having greater concentrations than about 7 B., it is preferred that the caustic solution withdrawn from the initial stage be spent in excess of about 90%. As pointed out heretofore, the broad concept of the present invention is to treat a feed oil in an initial stage of a two-stage process with a relatively strong caustic solution andthen to treat the oil in a secondary stage with a less concentrated caustic solution in the presence of solubility enhancing agents.

The process of the present invention may be readily understood by referring to the drawing illustrating an embodiment of the same. Refer ring specifically to the drawing a feed oil which for the purpose of illustration is assumed to be a petroleum fraction boiling in the range from about to 420 F. is introduced into a hydrogen sulphide treating zone I by means of feed line 2. In zone i the feed oil is processed in a manner adapted to remove substantially completely the hydrogen sulphide from the feed oil. The treating reagent, if one be employed, is introduced into zone I by means of line 3 and withdrawn by means of line 4. It is to be understood that if the feed oil is free of hydrogen sulphide, this preliminary treatment in zone I may be dispensed with. A very desirable method of removing hydrogen sulphide is by a distillation operation. This is usually accomplished in a stabilization and debutanization operation wherein the hydrogen sulphide is removed overhead with the propane and lighter constituents.

The treated oil is removed from stage i by means of line 8 and is introduced into an initial caustic treating stage 20 wherein the same is contacted with a relatively concentrated sodium hydroxide solution. The sodium hydroxide solution is introduced into stage or zone 20 by, means of line 9 and is withdrawn by means of line ll.

50. and withdrawn by meansof line IS. The finished oil having a relatively low copper number is arrears The treated oil is removed from stage 20 by means of line I2 and isintrOduced into a secondary caustic treating stage '30, wherein the same'is contacted with a sodium hydroxide solution having a'c'onc'entration less than the solu- 1 tion employed in the initial stagean'd containing solubility enhancing agents. The sodium hydroxide solution is introduced into stage 39 by means of line I5 and the spent solution withdrawn by meansof line 22.

line I6 and is preferably water Washed in zone Water is introduced by means of line I1 withd'rawn'from zone '50 by means of line l9 and further refined or handled as desired.

The spent caustic solutionre'moved'fro'm initial caustic treating stage may be withdrawn are reconverted tomercaptans by hydrolysis and are removed overhead by means of line 24. Steam is introduced into zone 60 by means of line 23. The regeneration operation conducted in zone 60 also is controlled to adjust the gravity of the regenerated solution. The regenerated caustic may be removed from the system by means of line but is preferably recycled in a manner as hereinafter described to secondary zone by means of lines 26 and I5.

The invention is broadly directed toward a two stageprocess wherein a feed oil is contacted with a relatively concentrated caustic solution in an initial stage and with a lessconcentrated caustic solution in a second stage. The treatment in the initial stage serves to remove various constituents as for example various low molecular weight phenolic compounds and related constituents which would otherwise serve to greatly adversely aifect the efiiciency of the solubility enhancing agents used in conjunction with the caustic treating solution employed in the second stage.

The invention may be utilized for the removal of mercaptan compounds from any type of feed oil. It is particularly adapted however for the removal of mercaptan compounds from petroleum hydrocarbon fractions boiling below about 700 F. as for example for the removal of mercaptan compounds from hydrocarbon fractions which boil in the motor fuel, kerosene, diesel oil and gas oil boiling ranges.

While the present invention may be used in the treatment of any type of feed oil, as for example for the treatment of a mixed cracked and virgin fraction, it is apparent that the present invention is particularly adapted for the processing of cracked fractions derived from aromatic type crudes containing naphthenic acids. It is desirable in accordance with the present invention that hydrogen sulphide be substantially completely removed from the feed oil in a preliminary treating zone.

The strength of the fresh caustic added in the initial caustic treating stage is preferably in the range from about 24% to 30% NaOH. Here although a once through operation may be utilized, it is preferred to employ at least partial recycling under conditions so that the total caustic stream utilized in the treatment of the oil in the secondary stage does not have a concentration less The treated oil is 'withdrawn'from tertiary stage 30 by means of than about 12% t'o 15% NaOI-I. If thelsperit eaustic stream has approximately these critical m inimum concentrations a portion of 1 the stream is with'drawn from the system while a :portion of the stream may be recycledand mixed with fresh caustic solution having a concentration as specified above. 'By operating in this manner the phenolic and other acidic constituents whichadversely affect the efiici'ency of the solubility enhancing agents in the secondary stage :for the removal of mercaptans are effectively removed from the oil. The amount of caustic solution employed by volume based upon the oil may vary appreciably. However, it is preferred that in the initial stage the amount of causticsolution employed based upon the oilbe sufiicient'to'obtaingood mixing and remove desiredacidic constituents, which by volume'is in the range from about 5%to 50%.

The concentration .of the caustic solution employed in the second stage is less thanthe concentration of the caustic used to contact the oil in the initial stage. As previously mentioned the concentration of the caustic solution employed in the initial stage is preferably inthe range-from about 12% to 15%. The concentration of the caustic employed in the secondary stage for any particular operation is less than these specified concentrations for the initial stage, and is in the general range from about 9% to 12%. It is preferred that the concentration of the caustic employed in the secondary stage be fromabout 1% to 3% less than the caustic :employed in "the initial stage. I

These desired concentrations of the caustic for utilization inthe secondary stage may be secured by controlling the amount'of fresh caustic added and the extent to which the spent caustic is regenerated. i

The amount of caustic utilized in the secondary stage is from about 5 to 50% by volume based upon the volume of oil being treated. The concentration of the solubility enhancing agents present in the caustic solution introduced into the tertiary stage is preferably in the range so that from about 25% to 75% of the total sodium or other alkali metal is present as a sodium salt.

By operating in the manner described and controlling the concentration of the caustic stream, phenolic and other acidic constituents which were not removed from the oil in the initial stage will not be removed from the oil in the secondary stage. Thus, these objectionable phenolic compounds and other acidic constituents will not build up or accumulate in the circulating caustic stream of the secondary stage with a resulting impairment of efficiency with respect to the removal of mercaptan compounds from th oil stream. The spent caustic stream may be recirculated to a revivification or regeneration unit as hereinbefore described.

It is to be understood that the present invention may be employed in the treatment of various petroleum fractions other than motor fuels although it is particularly adapted for the production of high quality motor fuels. Heating oils, kerosenes, and the like may be treated in a manner as described. Although the invention has been particularly described with respect to the use of a sodium hydroxide solution, potassium hydroxide, and other alkali metal hydroxide solutions may also be used.

Having described the invention it is claimed:

1. Process for the removal of mercaptan compounds from hydrocarbon mixtures containing the same which comprises contacting the hydrocarbon fraction in an initial stage with a caustic solution, removing the caustic solution therefrom followed by contacting the hydrocarbon mixture in a secondary stage with a caustic solution of at least about 1 to 3 per cent lower concentration than that employed in the initial stage and using therewith agents which increase the solubility of mercaptans and mercaptides in said caustic solution. I

2. The process defined by claim 1 in which at least a portion of the caustic solution employed in the said initial stage is recycled to the said initial stage and in which at least a portion of the said caustic solution employed in the said secondary stage is recycled to the said secondary stage after at least partial removal of mercaptans contained in the solution.

3. Process as defined by claim 1 wherein the hydrocarbon mixture comprises a petroleum hydrocarbon fraction boiling below about 700 F.

and wherein the concentration of the caustic solution employed in the initial sta is in the range from about 12% to 15% 'andwherein the concentration of the caustic solution employed in the secondary stage is in the range from about 10% to 12%.

4. Process as defined by claim 1 wherein said agents used in conjunction with the caustic in the secondary stage comprise aliphatic acids containing from about 3 to 7 carbon atoms in the molecule.

5. Process as defined by claim 1 wherein said solubility enhancing agent comprises a naphthenic acid.

.6. Process as defined by claim 1 wherein said solubility enhancing agent comprises an alcohol. 7. Process as defined by claim 1 wherein said solubility enhancing agent comprises alkali metal salts of aliphatic acid which have been segregate from a petroleum hydrocarbon.

8. An improved process for the removal of mercaptan compounds from a f ed oil which is characterized by Containing hydrogen sulphide and mercaptan compounds which comprises processing said feed oil in a treating zone to substantially completely remove the hydrogen sulphide from said feed oil, then contacting said hydrogen sulphide-free feed oil with a relatively strong caustic solution in an initial caustic treating stage followed by contacting said oil in a secondary caustic treating stage with a caustic solution which is at least about 1 to 3 per cent less concentrated than that employed insaid initial stage and using in conjunction therewith solubility enhancing agents. j

9. Process as defined by claim 8 wherein the hydrocarbon mixture comprises a petroleum hydrocarbon fraction boiling below about 700 F. and wherein the concentration of the caustic solution employed in the initial stage is in the range from about 12% to 15% and wherein the concentration of the caustic solution employed in the secondary stage is in the range from about 10% to 12%.

HAROLD A. RICARDS, JR. JAMES W. RYDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,935,725 Perl Nov. 21, 1933 2,317,053 Henderson Apr. 20, 1943 2,431,770 Payne et al. Dec. 2, 1947 2,462,810 Ferguson Feb. 22, 1949

Claims

1. PROCESS FOR THE REMOVAL OF MERCAPTAN COMPOUNDS FROM HYDROCARBON MIXTURES CONTAINING THE SAME WHICH COMPRISES CONTACTING THE HYDROCARBON FRACTION IN AN INITIAL STAGE WITH A CAUSTIC SOLUTION, REMOVING THE CAUSTIC SOLUTION THEREFROM FOLLOWED BY CONTACTING THE HYDROCARBON MIXTURE IN A SECONDARY STAGE WITH A CAUSTIC SOLUTION OF AT LEAST 1 TO 3 PER CENT LOWER CONCENTRATION THAN THAT EMPLOYED IN THE INITIAL STAGE AND USING THEREWITH AGENTS WHICH INCREASE THE SOLUBILITY OF MERCAPTANS AND MERCAPTIDES IN SAID CAUSTIC SOLUTION.