US2152721A - Process for the removal of mercaptans from hydrocarbon distillates - Google Patents

Description

' April 4f,4 1939. D. L.. YABROFF I 2,152,721

PROCESS FOR THE REMOVAL 01:'V NERCAPT'NS FROM HYDROGRBQN DISTILLATES,

Filed May 26, 1937 METHYL ,aco/10.4 /N .5N-MOH Patented Apr. '4, *1939- I PATENT OFFICE PROCESS FOR` THE REMOVAL F IVIEBUAP- 'rANs FROM LATES v HYDRQOARBQN DISTIL- DavidLouis Yabrofl, Berkeley, Calif., assignor to Shell Development Company, San Francisco, Calif., a. corporation oi' Delaware Application May 26, 1937, Serial No. 144,867

1 Claim.

This invention deals with the removal of mercaptans from their solutions in hydrocarbon type liquids by treating same 'with an aqueous valcoholic solution of an alkali metal hydroxide.

vPetroleum .distillate's andy in particular gasolines,`,corrtaininganundesirable amount of mercaptans *havey a 'fsour" reaction'. and unpleasant odor.'A ,It is known,` that mercaptans canl be oxidized readily todisulildes which 'are sweet" but have thedis'advantage of reducing the knock-rating and lead susceptibilityl of gasoline toward tetra* ethyl lead.' lFor this reason, and because the sulfurcontent of A.gasolines should be reduced to a minimum, it'v is usually more desirable to remove atleast a major portion of the `mercaptans by extraction, and ii a lsmall remainder thereof 4is left in the solution of the hydrocarbon oil, to subject the same to a known` sweetening process as, for instance, by oxidation.

It is the purpcse of this invention to describe a process for extracting mercaptans from hydro.

carbon type liquids', particularly from gasoline distillates. It is another purpose., to carry out this process under conditions of minimum loss of treating material; and itisanother purpose to produce a spent extracting reagent which can-be recovered at avminimum cost.'

My process consistsA essentially of extracting mercaptans from hydrocarbon type liquids containingl same, but which are substantially free from acids other than mercaptans, Vwith an aqueous methyl or ethyl alcoholic alkali metal hydroxide solution containing from 25 to 75% by volume of the alcohol, under conditions to form two layers, a treated layer of hydrocarbon type liquid, and an alkali sludge layercontaining mercaptides, and separating the layers. In'order to remove mercaptans successfully it isnecessary that the extractionV bec'arried out in the absence,l

the extraction i's then regenerated by oxidationto convert mercaptides to disuliides.y Disuldes are separated from the oxidized sludge and the latter is used to extract further quantities of hydrocarbon type liquids containing mercaptans.

For this reason, the extraction should (Cl. IBS-32) In order that the alkali sludge be regenerable for reuse, it is essential that the hydrocarbon type liquidbe free from acids other than mercaptans, as no other acids can be removed from alkali hydroxide by simple oxidation to neutral compounds. Water or dilute alkali wash is suitable for the removal of suchacids from the hydrocarbon type liquid.

I am aware that it was known to treat hydrov`carbon oils with sulfuric acid and thereafter with aqueous alcoholic caustic containing about 50% alcohol, for the'purpose of removing. acid sludge dissolved in' the oil. In this treatment mercap' tans are largely dissolved in the sulfuric acid or are oxidized to disuldes so that in the subsequent treatment with alcoholic alkali hydroxide' few, if any, mercaptans are removed. Moreover, the acid sludge in the oil produces a spent alkali sludge which is `substantially non-regenerable because ofthe presence of sulfuric acid esters, sulfonic acids and the like, and the free alkalinity of the alkali sludge is generally reduced'to such a point, that its power to remove mercaptans is substantially lessened, if not completely destroyed.

I am also aware that ethyl an'd methyl alcohols have been used for the breaking of petroleumv o emulsions, as crude oil emulsions oremulsions of acid and alkali treated distillates. However, wherever such emulsions occur, the agent responsible for the emulsion contains an acid which is materially stronger than mercaptans, so that even if an aqueous alcoholic caustic of the required concentration would result, it would contain salts of acids which preclude a substantially complete regeneration for the production of a recovered aqueous alcoholic alkali hydroxide suitable for the continued extraction of mercaptans,

I am `further aware that petroleum distillates containing mercaptans have been treated with alcoholic caustic containing less than about 15% v to about 50%, above this point further increases containing a substantial amount of water, l. e., more than about 15%, because itwas believed that on account of the low acidity of the mercaptans Water caused hydrolysis of mercaptides to free mercaptans, which are preferentially sol-l uble in the hydrocarbon type liquid. However, I have found that this view is erroneous. Generally speaking, the eillciency of extracting mercaptans from hydrocarbon type liquids increases with both the concentrationsof the alcohol and of the alkali hydroxide. Thus from the point of view of extraction eiiiciency alone, it may appear desirable to use substantially anhydrous alcohol, rather than 1 aqueous alcohol, containing alkali hydroxide. However, lI have found that, Whereas the percent mercaptans extracted` increases,`

rapidly with increasing alcohol concentrations up in the extraction of mercaptans are small, and are insignificantv above about '7.5% alcohol. concentration. On the other hand, the loss of hydrocarbon type liquidin the alcoholic alkali hydroxide is very small up to'about 50% alcohol concentration and then increases rapidly.

The effect of alcohol concentration on the extraction eiciency and loss of hydrocarbons intov the extract phase is demonstrated in Fig. 1` and in Fig. 2, curve 2, respectively.

In Fig. 1 the percent mercaptans removedfrom a solution thereof in iso-octane, when treating same with 10% by volume oi' dilute methyl alcoyholic'5N sodium hydroxide in a single stage at 20 C. is plotted against the concentration of the V alcohol in the-sodium hydroxide solution. Four n-butyi,

pure mercaptans, namely, n-propyl, n-amyl and n-heptyl are represented by curves I to 4, respectively. As is clearly shown, a break occurs ineach ofthese curves at about 50% alcohol concentration, the curves being very steep at concentrations belowthat point but rapidly flattening out above it. At 75% alcohol concentration'from 95 to 99% of the mercaptans are extracted and further concentrating the alcohol has at best the effect of removing the remaining 1 to 5%.

Against this relatively small improvement in mercaptan removal, which under many circumstances is quite insignificant, stands the loss of the hydrocarbons inthe alcohol phase. In Fig. 2, curve 2,A the concentration of methyl alcohol in 5N sodium hydroxide is Vplotted against volume percent of a typical-gasoline absorbed by the alcoholic sodium hydroxide. when treating 75 parts of the gasoline with 5 parts of the hydroxide at.

20C. Up to about 50%Aconcentration of methyl alcohol, little, if any, gasolinefis absorbed, whereas above this point the absorption increases rapidly.

A further' unfavorable effect of increasing concentration of the alcohol inthe alkali metal hydroxide solution is its effect on the color of the hydrocarbon type liquid' to be treated. Alcoholic alkali metal hydroxide, besides extracting mercaptans, apparently is capable of eiecting condensation and the like reactions, resulting in the formation of colored products, many of which are soluble in the hydrocarbontype liquids to a considerable extent. The extent of these condensation reactions is increased with increasing concentration of the alcohol. Thus, I have found that upon agitating a raw Califorinia cracked gasoline for several minutes with 10% by volume of sodi-um hydroxide solutions of varying methyl alcoholconcentratlons, then separating the sodium hydroxide from the gasoline, washing the latter with water and diluting it with 3 parts by volume of a gasoline having a 30+ color on the Saybolt colorimeten the following colors were obtained It has been stated hereinbefore that one -of the important features of this process consists of regenerating the alkali sludge, and it'has been pointed out that the absence oi lacids other than mercaptans is-'esse'ntial for that purpose. I have nowfound that in order to'enable regeneration of the alkali sludge a further limitation mustbe imposed' thereon; i. e., Ivhave found that the economical regeneration may become dirlic'ult, if not impossible, if the concentration of the alcohol in the alkali sludge lis toohigh.

The usual method of regenerating `"alkali sludges used in the extraction of mercaptans convsists of steaming or oxidizing vsame with air.

Alkali hydroxides containing alcohols, however," cannot readilybe steamed because of the volatility ofthealcoholsand their readyv solubility in water. Moreover, mercaptans which are removedoverhead in the steaming, togethervvith `the alcohol, are considerably soluble even in relatively dilute aqueous a'lcohols, so that separation of the two would be extremely' dimcult. It therefore appears that oxidation is the onlyfpractical u means for regenerating. the alcoholic alkali sludge. l

Upon oxidation, mercaptides are converted to disuldes, which canbe separated ,fromthe aqueous alcohblic alkali vmetal hydroxide. Here again I-fourd that the completeness of separating the disuldes depends `largely fon the concentration of the alcohol, disulfldesbeing considerably soluble .in alcohols of4 high concentrations. trends in the' solubilities of disuliides in aqueous Fig,` 2, which shows that-up to concentrations of about 50% alcohol the solubllities of disulfldes 'alkali metal hydroxides ciV varying contents .of

alcohol are very similar to those for gasoline 'and' can be represented by`curves such a's curveZ in soy arequit'e low but that above this point they rapidly increase. When the concentration of alcohol is materially higher` than about'75%, frequently no separation at, all of disulfides takes place, all of the disuldes formed being soluble. While dissolved disuldes can be removed by scrubbing the oxidized alkali sludge with a hydrocarbon solvent or the like, such scrubbing is undesirable, because it not only adds several extra steps to the treatment, but also results in the loss of some alcohol since alcoholscontaining 25% or less water are considerably soluble in hydrocarbon solvents. On the other hand, vif the oxidized regenerated alkali sludge containing substantial amounts of disulfldes is used to extract further quantities of mercaptans from hydrocarbon type liquids, the disulfides contained therein are reintroduced into the hydrocarbon type liquid, with the effecty that the purpose of removing mercaptans rather than converting them to disulldes may be completely negatived.

The curves` in Fig. 2 graphically illustrate they a'n original mercaptanl sulfur content of .162%, with N methyl alcoholic sodium hydroxide which has been regenerated by oxidation, disulfides having beenseparated by skimming. The volume ratio of gasoline to alcoholic spdium hydroxide in thealcoholic hydroxide in terms of the right solution was l5 to lvthroughoutl the tests.

'Curve I shows the removal of mercaptans', as

read on the-left handscale. Asfwillbe noted. that curve closelyresembles thefcurves of Fig. 1. Curve v2 is the solubility curveifor the disuldes hand scale. Fromthis curve. curve 3 has been calcu1ated,.showing the amountof disulildesulfur which is returned with every' part of regenerated alkali sludge ingpercent of the gasoline to which-itl is returned las read on the left hand I and`.3- and lshows the -sum of mercaptan and disulfide sulfur in the gasoline after treatment. It is seen that for thisparticular gasoline the optimumconcentration of alcohol which results in the lowest sulfur content is between 50 and 60%. This optimum may Vary somewhat for different gasolines. Y

If, instead of merely skimming the oxidized solution to remove disulfides, scrubbing is resorted to, curve 3 is moved tothe right. Since, however', 'in the scrubbing the loss of alcohol must be balanced against the scrubbing efficiency in removing the disuliides, both of which are unfavorably inuencedby increasing alcohol concentration,` I have found that. the optimum alcohol concentration usually remains below r[5% and f always below 85%.

The oxidation for the purpose of regeneration must be carried out under conditions toconvert mercaptans to disulfides, whilesubstantially suppressing the .formatlon of sulfonic acids and the oxidation of the alcohol. I have found that the `desired oxidation can be effected satisfactorily byA agitating the alcoholic alkali hyf droxide with a moderate excess of air at about normal room temperature, preferably between about. 20 to 50 C. and inthe presence of acatalyst known to promote the oxidation of mercaptans or mercaptides to disulfldes', examples of such catalystv being PbO, PbS, NiO, NiS, C00, CoS, CuO, CuS, etc. The rate -of the oxidation increases with increasing oxygen pressure and temperature.. Ifair iszused `the air pressure maintained on the system in advantageously b'etween about 15 to 30 poundsabsolute.

In general the amount of oxygen absorbed is substantially independent of temperature and pressure within the above`- limitsv but .increases with both theconcentrations of the alcohol and of the alkali metal. hydroxide.

At low concentrations of alcohol andl alkali metal hydroxide the amount of oxygen is about 25 to 30% in excess' of that required to convert all vof the mercaptides to disulfdes. Since the oxidized solution normally contains n o aldehydes Thus when 'oxidizing'unden the above described conditions a series of sodium hydroxides of varying normalities, containing 50% alcohol and mercaptides, the following amounts of oxygen in percent of the theoretical amount required to convert the mercaptides to disulildes, were absorbed:

een o eore ca Norrnality of NaOH amount, form di sulildes.

2.5:. 5 130 7 155 7.3 (saturated) .e

Within the suitable yrange of alkali` metal hy-` droxide concentration of 2 mols per liter less than saturation theinfiuence of the concentration of the alcoholup -to about 75% is small. While there is no noticeable difference. in vthe oxygen consumption with variations in the alcohol concentrations below about 50%, there are usually some .variations between 50 and 75% alcohol concentrations. In general, from a purely practical standpoint it is therefore desirable to keep the alcohol concentration below about 75%, particu-l 'Ihe influence of the concentration of the alcohol on the oxygen absorption is well illustrated by the following data:

Oxygen bl'ptiou ir percen o more ca Concentration ofmethylalcoholln vaOH amount to mm M suldes From the above it is seen that when considering the various aspects which affect the efiiclency of mercaptan removal Afrom hydrocarbon type liquids with alcoholic alkali metal Ahydroxides, it

is advantageous to employ solutions containing substantial amounts of water. 'Ihe alcohol concentration shouldbe between the limits of about 25 to 75% and preferably between about 50 to 60%; and thealkali metal hydroxide concentranot take into account the amount of alkali metal4 hydroxide added. For instance, a 5N-NaOH solution containing 60% alcohol means a mixture of 40% water and60% alcohol in which 5 Moreover, its susceptihols, are too soluble in hydrocarbon type liquids to be of practical value and moreover, are readily salted out from their aqueous solutions by alkali metal hydroxldes. 4 Although the hydrocarbon type liquids hereinbefore mentioned vare either gasolineV distillates stricted to them. AAny organic liquid which is Ysubstantially immiscible with water, is neutral or slightly basic, resistant to the action of strong" l alkali hydroxide at ordinary temperatures and substantially resistant to oxidation under' the-',- conditions hereinbefore described, may be treated i by this process to separate mercaptans therefrom. The termv hydrocarbon type liquids as herein used is therefore meant to include, asidefrom mineral oil distillates, organic liquids such asicoal tar distillates, benzenetoluene, xylene, pentanes, hexanes, chlorinated hydrocarbo'nsof which carbon tetrachloride, chlor ethane, ethylene di-chloride," tri chlor ethylene,y tetra chlor ethane, chlorpro#A @ser mercaptans from said/oliwizt'han by a-treatedhydr@carboniiigiuiuiiini. l y,lution containing mercaptides .are forni conditions to" maires-mercatino@to disulfide' y 'least a portionfof` which formi-a;separate`v aye The term, however, dones notinclude'y air oxidation only', it shall be understood thatother mild oxidation methods may be used instead. For instance, -electrolytic oxidation under .carefully controlledconditions sov as to prevent'formation of sulfonic acids,}as described in-'the-Ya'bro` and Givens application, Serial Number`g96,708, flied circumstances.

erating said spent solution" by yainbliriu oil A withl the 1, resulting' regenerated `solution ha ing essentiallyv the same lcomosition'ortne orig-"l inal aqueoussolution, the-improvement comprisz' '1 ing employi'rigvlix'i saidextractidn av solution consistng essentially- 01;'50 `to "60%o'f an y alcohol .selected from*- the ,group consisting' oimethyl and ethyl alcoholss atleast 25%water-and'` an alkali-''v metal hydroxide, l 1 y DAVID lLouis 'YABROFE "soy

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