US2744054A - Sweetening process using oxygen, alkali, and a peroxide - Google Patents

Description

SWEETENING PROCESS USING OXYGEN, ALKALI, AND A PEROXIDE Willem Johan Pieters, Amsterdam, Netherlands, assignor to Shell Development Company, Emeryville, Califl, a corporation of Delaware No Drawing. Application March 22, 1954,

Serial No. 417,917

Claims priority, application Netherlands April 2, 1 953.

Claims. (Cl. 196-29) This invention relates to the" conversion of mercaptans or mercaptides into disulfides by free oxygen in a twophase system, one phase of which is formed by a hydrocarbon'oil, particularly a light distillate such as gasoline and kerosine, and the other by an aqueous alkaline solution, particularly an aqueous alkali metal hydroxide solution.

It has been proposed to oxidize the mercaptans while they are still dissolved in the hydrocarbon oil by the use of certain oil-soluble oxy compounds, such as hydrocarbon peroxides and hydroperoxides, free oxygen itself being substantially without direct effect on the mercaptans while present in the oil. However, not only is the oxidation of the mercaptans while present in the oil phase *comparati'vely slow, so that such processes are time consuming, but the use of such relatively expensive substances to efiect the total' oxidation is economically disadvantageous.

On the other hand, it has been proposed previously to oxidize mercaptans present in hydrocarbon oils to disulfides by contacting the oil in the presence of free oxygen or its equivalent with an aqueous alkali metal hydroxide. The mercaptans are extracted from the oil by the aqueous solution, and in this solution, in which they are present as mercaptides, they are oxidized to disulfides by the oxygen or equivalent oxidizing agent, and the disulfides then pass into the hydrocarbon oil. The oxygen required for the oxidation is only very slightly soluble in the aqueous solution, but it is fairly soluble in the oil. During the oxidation process the oxygen is therefore supplied from the hydrocarbon oil. Since the transfer of oxygen from the hydrocarbon oil to the aqueous solution is relatively slow, attempts have been made to minimize this factor by elfecting an intensive contact between the two phases, by increasing the pressure and/or the temperature of the system, and by other means. However, none of the previous means has proved to be entirely satisfactory.

It has now been found that the quantity of oxygen required to oxidize the mercaptans in a two-phase system, as already referred to, can be considerably decreased by the simultaneous use of a comparatively small quantity of a peroxide. Even when a quantity of peroxide is used which as such supplies only a small fraction of the excess of oxygen otherwise required, the quantity of oxygen can be reduced to the quantity which is theoretically necessary, without retarding thereby the conversion of the mercaptans (mercaptides) into disulfides. The simultaneous use of a small quantity of peroxide renders it therefore possible to decrease considerably the required quantity of oxygen and accordingly the pressure to be applied, so that the previous drawbacks accompanying the use of larger quantities of oxygen and of higher pressures are avoided. If a small quantity of peroxide is simultaneously used While the oxygen pressure is maintained, the oxidation of the mercaptans (mercaptides) to disulfides is appreciably accelerated, so that in a given apparatus a considerably larger quantity I erably at a somewhat lower temperature.

oil can be treated per unit of time.

For the purpose of simplicity the term mercaptans will be used in a generic sense to denote boththe neutral substance as well as the mercaptide ions as the substance exists in aqueous alkaline solutions.

The desired activation of the effect of oxygen is generally accomplished by using a quantity of peroxide amount to l0% 40%, particularly l5%'25%, of the stoichiometrical quantity with respect to the mercaptans 3 to be converted, calculated for conversion to disulfides;

Organic peroxides can in most cases be dissolved di rectly in the hydrocarbon oil to be treated in the quantities mentioned. Hydrogen peroxide can' only be dissolved in hydrocarbon oils with difiiculty and may conveniently be mixed with the hydrocarbon oil in the form of an alcoholic solution or it may be injected in the hydrocarbon oil in the form of a concentrated solu-- oxygen or air may, for instance, first be passed through I an ozonizer and then dissolved in the hydrocarbon oil.

For the present purpose peroxides of various types are- In addition to the hydrogen peroxide and ozone already mentioned examples of suitable peroxides are the suitable.

following: dimethyl peroxide, diethyl peroxide, propane peroxide prepared by incomplete combustion of propane, benzoyl peroxide, tetralin peroxide, tertiary butyl hydroperoxide, cumene hydroperoxide. It is also possible to form peroxides in the hydrocarbon oil to be treated by treating the hydrocarbon oil at a suitable temperature, for instance, C.-l25 C. and, more particularly C.- C., with oxygen, whereupon the hydrocarbon oil is brought into contact with an aqueous alkali metal hydroxide solution in the presence of oxygen, pref- The initially formed peroxides then activate the oxygen which is used for converting mercaptans into disulfides. It is preferable to treat only a fraction of the hydrocarbon oil to be freed from mercaptans at a temperature of from 75 C. to C. and, more particularly from 80 C. to 100 C., with oxygen to form peroxides and then combine this fraction with the rest of the hydrocarbon oil after which the whole is treated, preferably at a somewhat lower temperature, in the presence of an aqueous alkali metal hydroxide solution, with oxygen to convert th mercaptans (mercaptides) into disulfides.

The present process for converting mercaptans into disulfides is generally put into practice at temperatures of from 0 C. to 70 C., although, if desired, temperatures outside this range may be used. The preferred temperature range is from 10 C. to 45 C.

The oxygen required for the process may be supplied to the two-phase system to be treated, either as such or in the form of a mixture of oxygen with another gas which is inert under the operation conditions. As such a mixture air is particularly suitable.

The oxygen may be either dissolved in the hydrocarbon oil in advance or injected into the hydrocarbon oil while the latter is in contact with the aqueous alkali metal hydroxide solution. The quantity of oxygen is at least so large that, together with the oxygen which may be supplied by the peroxide, it is theoretically sufiicient for converting the total quantity of the mercaptans present in the hydrocarbon oil into disulfides. Generally the required quantity of oxygen (exclusive of the oxygen.

which may be supplied by the peroxide) does not exceed 3 100%-130% of the stoichiometrical quantity with respect to the mercaptans to be converted.

When applying the process for removing mercaptans from gasoline or kerosine with a mercaptan sulfur content not exceeding 0.040.05% by weight and the gasoline or kerosine is at equilibrium with the atmosphere, the quantity of oxygen present in the gasoline or kerosine will generally suffice to bring about the desired oxidation. However, the process for removing mercaptans from gasoline and kerosine is frequently employed shortly after the gasoline or kerosine has been produced from the crude oil and has been subjected to other pretreatments, if any, so that it is not saturated with air. It is then often necessary for air or another oxygen-containing gas to be dissolved in the hydrocarbon oil before or during con tact between the latter and the aqueous alkali metal hydroxide solution.

In general the process is carried out under atmospheric pressure. If the process is applied for removing mercaptans from hydrocarbon oils with a relatively high mercaptan content, for instance a mercaptan sulfur content of 0.06% by weight or higher, and air is used as oxygencontaining gas, it may indeed be advisable to operate under somewhat increased pressure so as to dissolve a sufiicient quantity of oxygen in the hydrocarbon oil, but the pressure increase may then be considerably less than in the case Where no peroxides are used for activating the effect of oxygen.

In order to promote the transfer of the oxygen from the hydrocarbon oil to the aqueous alkali metal hydroxide solution in which the oxidation proper takes place, care should be taken to bring about an intensive contact between the two phases.

Both aqueous sodium and potassium hydroxide solutions of widely differing concentrations are suitable as alkali metal hydroxide solution. It is preferred to make the concentration 2-normal or higher, since higher caustic alkali concentrations promote extraction of the mercaptans and accelerate oxidation to disulfides in the aqueous alkali metal hydroxide solution.

In order to increase the extracting efiect of the aqueous alkali metal hydroxide solution for mercaptans still further, suitable solutizers may be dissolved therein. Examples of suitable solutizers in aqueous alkali metal hydroxide solutions are: aminoand hydroxyalkyl amines, in which the alkyl groups contain 2-3 carbon atoms, glycols, amino glycols and diamino alcohols with 3-5 carbon atoms, diamino-, dihydroxyor amino hydroxy dialkyl ethers or -thio others in which the alkyl groups have 2-3 carbon atoms, alkali salts, in particular potassium salts, of fatty acids with 3-5 carbon atoms (e. g. isobutyric acid) or of hydroxyor amino fatty acids with 47 carbon atoms, or of phenylacetic acid or hydroxyor amino phenyl acetic acids, or of alkyl phenols, or mixtures of two or more of the compounds mentioned. Especially suitable are the aqueous solutions of an alkali metal hydroxide and a phenolate (which may or may not be substituted by alkyl groups with a total of not more than 3 carbon atoms and does not contain any other substituents) and which contain at most 54% by volume of water and at least 2 mol/liter of free alkali metal hydroxide.

The ratio of the quantity of hydrocarbon oil to the quantity of the aqueous alkali metal hydroxide solution may vary between wide limits. The ratio of the volume of the aqueous alkali metal hydroxide solution to the volume of the hydrocarbon oil generally lies between 0.5 and 5.

If the process is applied for removing mercaptans from a hydrocarbon oil by bringing the latter into contact with an aqueous alkali metal hydroxide solution in the presence of oxygen and a peroxide, the desired result may, in general, be obtained by treating the hydrocarbon oil with a quantity of the aqueous alkali metal hydroxide solution which is considerably smaller than the quantity of hydrocarbon oil. Generally a quantity of the aqueous alkali metal hydroxide solution from 5% to 50% by volume and more particularly from 10% to 20% by volume, calculated on the hydrocarbon oil, is very suitable.

If the process is used for regenerating an alkali metal hydroxide solution containing mercaptides by treating this solution in the presence of a. hydrocarbon oil, preferably a light hydrocarbon oil, with oxygen and a peroxide, it is preferred to take the ratio of the volume of the aqueous alkali metal hydroxide solution to the volume of the hydrocarbon oil between 0.2 and 5, more particularly between 0.5 and 2.

The process may be carried out either continuously or batchwise.

After the hydrocarbon oil and the aqueous alkali metal hydroxide solution have been in contact with each other for a sufficiently long time, they are separated from each other.

When the process is carried out batchwise, the entire quantity of hydrocarbon oil and aqueous alkali metal hydroxide solution which have been in contact with each other, are allowed to, stand until the phases have separated from each other, which occurs after a short time.

\Vhen carrying out the process continuously, the hydrocarbon oil may be supplied to the aqueous alkali metal hydroxide solution in such a manner that the oil is in contact with the aqueous solution for a sufficiently long time, and the continuously discharged hydrocarbon oil passed into a separate settling vessel, in which the entrained aqueous solution separates out and is recycled to the process.

The process may be applied for removing mercaptans from hydrocarbon oils, particularly light hydrocarbon oils (i. e. hydrocarbon oils with a boiling point or end boiling point of not more than 350 C.), especially gasoline and kerosine of different origin including gasoline or kerosine obtained by straight distillation from crude oils and gasoline and kerosinc obtained from heavy base materials by cracking. The so-called reformed gasolines may also be freed from mercaptans according to the present process. However, when applying the process to hydrocarbon oils containing unsaturated components, particularly cracked gasoline and reformed gasoline, it is necessary to add to the oil an anti-oxidant, e. g. an aryl amine or an alkyl phenol the alkyl groups of which contain a total of 4 carbon atoms or more to inhibit the formation of gum from the unsaturated components of the oil. In general a quantity from 0.0001% to 0.01 by weight of such an anti-oxidant will sutlice.

With the comparatively small amounts of peroxides used for promoting the conversion of mercaptans into disulfides, the hydrocarbon oil no longer contains peroxides at the end of the treatment. This is apparently due to the fact that the peroxide is completely decomposed during contact of the hydrocarbon oil with the aqueous alkali metal hydroxide solution. use of a peroxide does not impair the stability to gum formation of the hydrocarbon oil.

It may further be desirable to remove from the hydrocarbon oils any acids present such as hydrogen sulfide, which are stronger than the mercaptans, by means of a diluted aqueous alkali metal hydroxide solution before oxidizing the, mercaptans in the manner indicated. A pretreatment with diluted caustic alkali solution has the further advantage of aromatic mercaptans, which on the one hand possess a considerably stronger acidic character than aliphatic mercaptans, and on the other are more difficult to oxidize than aliphatic mercaptans, being removed, at any rate in a considerable quantity, from the hydrocarbon oil to be treated according to the manner indicated. Since aromatic mercaptans especially occur in light hydrocarbon oils obtained by cracking heavy hydrocarbons, particularly in products produced by catalytic cracking, a pre-treatmentwith diluted caustic alkali solution is especially suitable for such products. It is preferable to: carry out this pro-treatment before the cracked products come Further the into contact with oxygen or an oxygen-containing gas so as to prevent the formation of gum.

When applying the process for removing mercaptans from hydrocarbon oils, the disulfides formed during oxidation pass again into the hydrocarbon oil and for this reason the process is primarily suitable for treating hydrocarbon oils with a low mercaptan content, i. e. lower than 0.05% by weight and preferably lower than 0.02% by weight, calculated as mercaptan sulfur. In this case the quantity of disulfides returned into hydrocarbon oil is also small. As a rule a low content of organic disulfides does not appreciably affect the lead susceptibility of gasoline.

-When a hydrocarbon oil with a considerable mercaptan sulfur content, for example, 0.05% by Weight or more, is to be freed from mercaptans, the greater proportion of the mercaptans, if desired, together with other sulfur compounds, may be first removed by one of the hitherto usual methods and then the remainder of the original quantity of mercaptans oxidized according to the process of the invention. The pro-treatment for removing the greater proportion of the mercaptans may, for instance, be effected by extracting (without oxidation) the hydrocarbon oil with an aqueous alkali metal hydroxide solution containing a solutizer.

The present process provides a very simple method by which hydrocarbon oils, particularly gasoline or kerosine, can be freed from mercaptans in a short period, which in many cases varies between 2 and 20 minutes. If the hydrocarbon oil contains mercaptans which are diflicult to oxidize, it may be necessary to keep the oil and the aqueous alkali metal hydroxide solution in contact with each other in the manner described for a somewhat longer period. With a sufiiciently intensive contact between the hydrocarbon oil to be treated and the aqueous alkali metal hydroxide solution, it is, however, also possible in the latter case to free hydrocarbon oil from mercaptans to such an extent that the oil gives a negative doctor test within one hour. The invention will be illustrated by the following examples.

EXAMPLE I A number of tests was carried out whereby a gasoline as a percentage by weight. In the third column of Table I the quantity of the peroxide absorbed by the gasoline is expressed as a percentage of the stoichiometrical quantity with respect to the mercaptan content of the initial gasoline. For purposes of comparison two tests were made without the addition of peroxide.

In the fourth column of Table I the quantity of oxygen is shown which was injected into the gasoline in each of the tests before the gasoline was contacted with the aqueous solution of potassium hydroxide and cresolate. The quantity of oxygen is expressed by the percentage of the stoichiometrical quantity with respect to the mercaptan content of the initial gasoline.

The fifth column of Table I shows the total content of oxygen available, i. e. the sum of the quantity of atmospheric oxygen and the oxygen which can be supplied by the peroxide. This total quantity of oxygen is also shown as a percentage of thestoichiometrical quantity with respect to the mercaptan content of the gasoline.

. Each of the tests was carried out continuously and every 4 hours both the mercaptan content of the initial gasoline and that of the gasoline discharged from the turbo mixer were determined. The sixth column of Table I shows the feed rate of the gasoline expressed in liters per hours. The seventh column of Table I relates to the average contact time, expressed in seconds, of the gasoline with the aqueous solution of potassium hydroxide and cresolate.

Columns 8 and 9 of Table I show the mercaptan content of the initial gasoline and that of the gasoline discharging from the turbo mixer. The figures stated are the average figures of six observations made every 4 hours. The figures indicate the percentage by weight of mercaptan sulfur.

Finally, the figures in the last column of the table are a measure of the rate at which mercaptans are oxidized to disulfides. The figures relate to the factor A from the equation C ln -A. 15

wherein Co represents the mercaptan sulfur content of the initial gasoline and Cr, the mercaptan sulfur content of the gasoline after treating it for t seconds.

Table l Atmos- Total Mercaptan sul- Peroxide fi ggg pheric oxoxygen (105- for content as dosa e egent I ygen dosage, Feed rate, Contact percent W.X10 Axle; m Test No. age, percent percent of liter/h. time, seesec w 6 metrical of stoiehlostolehioends quantity metrical metrical Feed Final quantity quantity prod.

with a boiling range of from 40 C. to 100 C. produced by straight distillation from a Kuwait crude oil was treated in a three-stage turbo mixer with an aqueous alkali metal'hydroxide solution in the presence of oxygen in order to convert the mercaptans present in the gasoline into disulfides. As aqueous alkali metal hydroxide solution, use was made of a solution having the following composition: 34% by weight of water, 33 by weight of potassium hydroxide, 33% by weight of cresol (this percentage is actually calculated as cresol, although the cresol is in effect combined witha part of the potassium hydroxide to form potassium cresolate) In a number of the tests a 1% solution of cumene hydroperoxide in a sample of the same gasoline which was to be treated, was injected into the latter gasoline, in'a quantity which is shown for-each of the tests under consideration in the second column of Table I, by indicating the peroxide content absorbed by the gasoline, expressed It will be seen from the results of tests 1 and 2 that without the simultaneous use of a peroxide the theoretical quantity of oxygen gives a factor A of not more than 4.5 against a factor A of 10.0 when the double quantity of oxygen is used, This corresponds to an average increase in A 10 (delta A 10 of 0055x10 for each 1% excess of free oxygen over the stoichiometrical quantity required.

Comparison of the results of tests2 and 3 shows that when 8% excess of oxygen is provided as peroxide the average increase in A 10 per 1% excess is now 0.12 10 Then comparing the results of tests 3 and 5 shows that for the next 12% excess of oxygen provided as peroxide the average increase in AX 10 per 1% excess is 0.3 X 10 However, the next increase, from 20% up to 35% as peroxide (tests 5 and 6) shows an average increase'per 1% increase, of only 006x103. In general, the advantage in further increase in rate does not go beyond 40% of excess of peroxide, the most significant increases occurring by the use of up to about 25% of the stoichiometrical quantity of peroxide.

manner as Tables I and II. For purposes of comparison the result of test No. of Table I is again shown in Table HI.

Table III Peroxide $g Total oxy- 3 3E252;- Peroxide oxygen gen dosage gen dosage Feed Contact percent w.X1O '1 t No ature eroxlde dosage percent of percent of Damien? of rate time m es percent stoichiostoichm sto1ch1omgr/h seconas sccr \v. metrical metrical metrical Final quantity quantity quantity Feed prod.

Cumenehydroperoxide- 3 20 100 120 75 400 119 3 9. l Tert. butylhydroperoxide. 5 38 118 156 75 400 142 2 1g. 6 Benzoylperoxide S 19 87 106 75 400 190 8 A. 9 Hydrogen peroxide 0.7 19 92 111 400 121 2 10. n

Further significance of the test results is shown by a comparison of the results of tests 3 and 4. While maintaining the same percentage of peroxide (8% of stoichiometrical quantity), a 10% excess of free oxygen (110 vs. 100), resulted in an average increase in A 10 per 1% excess of free oxygen, of 0.07 X lO Finally, the result of test 7 demonstrates that a considerable value for factor A is obtained by using a quantity of curnene hydroperoxide amounting to 15% of the stoichiometrical quantity. In test 7 the total quantity of oxygen was 102% of the stoichiometrical quantity. Comparison of the result of test 7 with those of tests 2 and 3 according to which the total quantity of oxygen amounted to 100% and 108% respectively of the stoichiometrical quantity, shows that with the same total quantity of oxygen available the combination of atmospheric oxygen with a comparatively small quantity of peroxide results in a considerably higher oxidation rate than that which is obtained when using atmospheric oxygen solely, i. c. without peroxide.

EXAMPLE II The tests of this example were carried out in a manner similar to that of Example I except that a gasoline was used with a boiling range of from about 80 C. to 200 C. and which was obtained by thermal cracking. Just as in Example I for the removal of mercaptans a solution was used which consisted of 34% by weight of water, 33% by weight of potassium hydroxide and 33% by weight of cresol. The results of the tests of this example are shown in Table 11 arranged in exactly the same way as Table I.

As will be seen from the results shown in Table III the various peroxides tested are all suitable for the present purpose.

I claim as my invention:

1. A process for converting mercaptans into disulfides by means of oxygen in a two-phase system, one phase of which is formed by a hydrocarbon oil, and the other by an aqueous alkali metal hydroxide solution, which comprises carrying out the conversion in the presence of a peroxide in a quantity of 10% to 40% of the stoichiometrical quantity with respect to the mercaptans to be converted and of free oxygen in a quantity of no more than 130% of the stoichiometrical quantity with respect to said mercaptans.

2. A process according to claim 1, wherein the quantity of the peroxide is from 15% to 25% of the stoichiometrical quantity with respect to the mercaptans to be converted.

3. A process according to claim oxide is cumene hydroperoxide.

4. A process according to claim oxide is tertiary butyl hydroperoxide.

5. A process according to claim oxide is hydrogen peroxide.

6. A process according to claim oxide is benzoyl peroxide.

7. A process according to claim 1, wherein the quantity of oxygen used is 100-430% of the stoichiometrical quantity with respect to the mercaptans to be converted.

8. A process according to claim 1, wherein the process is applied for converting into disulfides mercaptans pres- I, wherein the per- 1, wherein the per- 1, wherein the per- 1, wherein the per- Table II Atmos- Total Mercaptan sul- Peroxme Eigggg phcrlc oxoxygen dosfur content as dosa e ercent a ygen dosage, Feed rate, Contact percent W.X10 AXIO, in Test N o. g pswicmm age, percent percent of liter/h. time, secsec w X 103 metrical of stoichiostoichio onds Imam metrical metrical Feed Final q y quantity quantity prod.

0.0 0.0 200 200 40 654 482 11 5. 8 0.0 0. 0 100 100 40 654 285 80 l. S 32 35 200 235 75 400 378 19 7. 5 12 I 9 89 98 40 654 372 30 3. 9

The results of the tests of Table II show that also a gasoline obtained by thermal cracking is advantageously treated by combining the oxygen with a relatively small quantity of a peroxide.

EXAMPLE III ent in sour gasoline containing no more than 0.05% by Weight of mercaptan sulfur.

9. A process according to claim 1, wherein the process is applied for converting into disulfides mercaptans present in sour gasoline and the aqueous alkali metal hydroxide solution is 1020% by volume of the volume of the gasoline.

10. A method of sweetening hydrocarbon oils contaminated with mercaptans which comprises oxidizing said mercaptans by means of no more than of the stoichiometrical amount of free oxygen in the presence of an aqueous alkali metal hydroxide solution and of a catalytic amount of peroxide.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Borgstrom Jan. 5, 1932 Bosing Mar. 13, 1934 5 Wilson Nov. 11, 1939 10 Wilson Nov. 11, 1939 Viles Mar. 11, 1947 Henderson et a1 Sept. 9, 1947 Browder et a1. May 12, 1953

Claims (1)

10. A METHOD OF SWEETENING HYDROCARBON OILS CONTAMINATED WITH MERCAPTANS WHICH COMPRISES OXIDIZING SAID MERCAPTANS BY MEANS OF NO MORE THAN 130% OF THE STOICHIOMETRICAL AMOUNT OF FREE OXYGEN IN THE PRESENCE OF AN AQUEOUS ALKALI METAL HYDROXIDE SOLUTION AND OF A CATALYTIC AMOUNT OF PEROXIDE.