US2146353A - Process for desulphurization of hydrocarbons - Google Patents

Description

T. `W. ROSEBAUGH PROCESS FOR DESULPHURIZATION OF HYDROCARBONS` Filed April 13, 1936 Feb. 7, 1939.

Patented Feb. 7A, 1939 PROCESS FR. DESULPHURIZATION F l HYDROCARBON@ Theodore W. Rsebaughakland, Calif., assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application April 13, 1936, Serial No. '14,005

6 Claims.

This vprocess relates to '-the desulphurlzation of hydrocarbons particularly those of 2 to 6 carbon atoms. It is the purpose of this invention to enable the removal of sulphur and/or sulphur 5 compounds to an inilnitesimally low content.

In the manufacture of solvents of the type of alcohols, ketones, ethers, etc., from the corresponding hydrocarbons which may be produced by distillation and/or cracking of mineral oils,

l0 shale oils, coal tar oils, oils synthesized by the interaction of carbon oxides and hydrogen, etc., it is 'of importance that the sulphur content of the hydrocarbons be reduced to a point of the order of .0005% or lower. A higher sulphur content tends to impart to the solvents an .unpleasant odor and corrosiveness. Malodorous solvents are unacceptable to many industries such as the perfume or food industries, and corrosiveness in the slightest degree often causes severe diiiiculties in the rayon and similar industries, by attacking the nozzles used for making threads. Moreover, it was found that traces of sulphur compounds have an adverse eiect on the storage stability of ketones causing them to go off distillation specication in a relatively short time.

The problem arising in the attempt to reduce the sulphur content to the indicated low level is quite different from those heretofore encountered. Whereas with the usual means for desulphurization such astreating with alkali hydroxide or other alkaline reacting substances, metal oxides and hydroxides, sulphuric acid, etc., reductions to sulphur contents of the order of .1% to .01% can be achieved under normal conditions,

it was found that new combinations of means are necessary to reduce the sulphur content still further.

I have discovered that hydrocarbons containing such low concentration of objectionable sulphur compounds can be desulphurized to the degree required for the production of odorless and non-corrosive solvents by first treating them,

with a substantially anhydrous alcoholic solution of an alkali hydroxide, preferably potassium hydroxide, although sodium hydroxide may also be used, and thereafter withv an aqueous solution of any alkali hydroxide, for instance,` sodium hydroxide. By substantially anhydrous alcoholic alkali hydroxide solution, as herein used, is meant one which contains less than about 10%'water in the solution. The'treatment with alcoholic alkali hydroxide may be, and usually is, preceded by a treatment with other desulphurizing agents, such as alkaline reagents of the type of tri- `potassium phosphate, tri-ethanolamine, sodium (ci. 19a-s2) hydroxide, to remove acidic sulphur compounds, or sulphuric acid to remove sulphur compounds of the type of thiophene and thioethers. Such pretreatment, although often not essential in my treatment, greatly reduces its cost by saving 5 alcoholic alkali hydroxide, which is expensive in comparison with other treating reagents. The process is particularly suited for refining hydrocarbon distillates boiling below about C. or which are substantially free of thiophenes. 10

Mineral oil hydrocarbons of more than 6 carbon atoms and aromatic coal tar hydrocarbons of the type of benzene, occasionally do not respond to my desulphurization treatment, possibly because of their relatively high content of thio- 1li phene and homologues. However, by eliminating sulphur compounds of this type by suitable treatment such as sulphuric acid treatment, this diiilculty can be overcome and the hydrocarbons then respond to the treatment as herein de- 20 scribed.. i

The process oi my invention is preferably carried out under such conditions as to maintain the hydrocarbons in the liquid state, although the contact maybe effected in the case of Volatile 25 hydrocarbons, by bubbling the hydrocarbon vapors through the liquid caustic solutions. The liquid phase treatment is the more effective of the two. I have found that ordinary temperatures between 10 and 30 C. are very suitable. 30 However. higher or lower temperatures may be used.

According to my invention, the alcoholic solution of alkali hydroxide may be prepared by dissolving substantially anhydrous alkali hydroxide 35 in any suitable non-acidic substantially anhydrous alcohol, particularly an aliphatic alcohol, which may be primary, secondary or tertiary, and may be substituted or not. In general, I prefer to use for this purpose one, or a mixture 40 of several alcohols in which the ratio of carbon atoms to hydroxyl group is not more than 3, and of which the following are examples: methyl, ethyl, propyl alcohols, ethylene glycol, alkyl glycols of less than '7 carbon atoms, glycerine, 45 alkyl giycerines of less than 10 carbon atoms. However, other stable alcohols or non-acidic hydroxy organic substances, capable of dissolving alkali hydroxides or forming alcoholates, may be employed, such as certain hydramines, hydroxy 50 ethers. and higher alcohols. Alcohols in which the ratio of carbon atoms to hydroxy radicles is greater than 3 are poor solvents for alkali hy droxides, but may be used in the form of alcoholates, which alcoholates may, for instance, be 55 prepared by heating a mixture oi' an alcohol having a boiling point higher than water with the alkali hydroxide until all the water liberated in the formation oi' the alcoholate has been expelled.

The prepared solution containing at least 2% and preferably above 5% alkali hydroxide is used in my process as such, without additional chemicals, like known oxidizing or sweetening agents, such. as alkali plumbite, the essential and active components being the alkali hydroxide and alcohol used in the indicated concentrations.

A typical flow diagram of a plant designed to dcsulphurize hydrocarbons as herein described substantially in the liquid phase, is shown in the attached drawing. According to this flow diagram the hydrocarbons, such as cracked distillate, boiling up to about 80 C. and containing about 1.6% hydrogen sulphide and other sulphur compounds, enter from a source now shown at ordinary temperature and under suil'icient pressure to keep them in the liquid state, through line I into mixer 2, where they are thoroughly mixed with .an aqueous solution of an alkaline reacting substance such as tri-potassium phosphate or the like, which is introduced through line 3. The resulting mixture is transferred through line 4 to settler 5, in which spent treating solution settles out and is withdrawn through bottom drain 6, and partly desulphurized hydrocarbons are transferred through line 1 to the vertical treater 8 in which the treatment with the regenerative treating solution may be repeated to achieve further desulphurization. Spent treating solution is withdrawn from treater 3 through drain 9 and after joining the spent solution from drain 6 in line III, proceeds through heating coil II in furnace I2 through transfer line I3 to regenerator I4, in which absorbed acidic sulphur compounds, mainly hydrogen sulphide, are removed from the treating solution with the aid of steam from stem line I5. Sulphur compounds and steam are ejected through line I8, while regenerated treating solution is recirculated by pump I1 in line I8, through lines 3 and I9 to the mixer 2 and treater 8, respectively.

The hydrocarbons, the acidic sulphur content of which is now considerably reduced, for instance to .04% HzS, leave treater 8 through overhead line 20 and are conducted through an aqueous solution of sodium hydroxide contained in vessel 2|, to remove last traces of acid sulphur compounds therefrom. The aqueous caustic solution in vessel 2I is frequently renewed either in batch or continuously, fresh caustic entering through line 33, and used caustic being withdrawn through line 34 to preclude incomplete absorption or return of acidic compounds to the hydrocarbons.

The hydrocarbons now substantially free from acidic compounds but still containing, say, about .03% non-acidic sulphur compounds, such as carbon oxysulphide, carbon disulphide and alkyl sulphides are transferred through line 22 to mixer 23 in which they are mixed with a solution of alcoholic potassium hydroxide, which is circulated by pump 24 in line 25. Normally I use an excess of the alcoholic solution so that the resulting mixture which passes through line 26 to settler 21, separates into three layers: a bottom sludge layer containing crystalline substances, found to comprise potassium carbonate; a middle layer of alcoholic potassium hydroxide, contaminated with varying amounts of potassium sulphide and other sulphur compounds; and a top layer of hydrocarbons substantially free from organic sulphur compounds.

The sludge is withdrawn intermittently through drain line 28. Potassium hydroxide solution is circulated by means of pump 24 in line 25, through the mixer 23 to be contacted with further quantities of hydrocarbons from line 22, back into settler 21 until' spent and incapable of eiecting further reduction of sulphur in the hydrocarbons passing through the mixer 23. The spent solution may then be drained through drain line 28 and fresh alcoholic potassium hydroxide is charged to the system through line 28. The renewal of alcoholic hydroxide may be carried out continuously by withdrawing spent solution through line 35 and continuously introducing an equal amount of fresh hydroxide into the circulating system through line 29.'

The hydrocarbons constituting the top layer in settler 21 are transferred through line 30 to treater 3l, in which they are contacted with an aqueous solution of fresh caustic, frequently renewed, which enters through line 31 and'goes out through drain 38. The so-treated hydrocarbons finally proceed through line 32 to the part of the plant not shown, in which they are converted to solvents or other derivatives.

Instead of admixing an excess of alcoholic hydroxide to the hydrocarbon in mixer 23, I may dissolve in the hydrocarbons an amount of alcoholic caustic which is insuilicient to form er cause separation of two liquid phases in separator 21. In this case, I conduct the resulting mixture or solution through lines 38 and 30 directly to the soda treater 3i, in which the reaction products between alcoholic caustic and sulphur compounds are washed out from the hydrocarbon liquid.

Contrary to expectations, the nal treating step with aqueous caustic alkali has been found to be an essential part of the desulphurization process. The alcoholic potassium hydroxide, as soon as it has been used for a short period and long before it is spent and incapable of eiecting removal of non-acidic sulphur compounds, begins to liberate varying quantities of hydrogen sulphide, at least part of which re-enter the hydrocarbons rendering them slightly acidic. This necessitates the final alkali wash.

'I'he sulphur compounds responsible for this anomalous behavior seem to be carbonyl sulphide and carbon disulphide, and the reaction leading to the formation of hydrogen sulphide under the influence of alcoholic potassium hydroxide although not definitely established is believed to proceed as follows:

While the alcoholic potassium hydroxide solution is fresh it is to be expected that free HzS appearing in the above reactions is immediately converted to KzS according to the equation:

In the presence of an alcohol such as methyl alcohol. however, an equilibrium reaction appears to take place as follows:

According to the last equation, free hydrogen sulphide makes.. its appearance in the alcoholic caustic as soon as potassium sulphide is being formed. The concentration of hydrogen sulphide obviously increases with increasing concentration of KzS, with the effect that within a short period after beginning of thetreatment, hydrogen sulaudace suiiicient quantities to cause solvents subsequent- 1y produced from these hydrocarbons to be malodorous. The subsequent aqueous caustic treat ment removes the secondary hydrogen sulphide, and thereby eliminates this defect.

The degree of sulphur reduction which can be achieved by my treatment varies somewhat with the nature an-d concentration of sulphur compounds contained in the hydrocarbons. Normally it is possible to reduce the sulphur content to ,0005% or lower.

In a typical example in which a butane-butylene cut produced in the cracking of mineral oils was treated in a plant as shown in the attached drawing the following average figures on sulphur content were found:

Sample taken Treatment H1B content Totglfnpthm None 1. 6% K|PO4 1 KgPOl 04 Aqueous NaOH. Bare trace 0.03 Alcoholic K OH U01- Aqueous NaOH. Less than .0005

I claim as my invention:

1. In the process of desulphurizing a mixture of hydrocarbons and sulphur compounds of the type of carbonyl sulphide, the improvement consisting of contacting the mixture with alkali alcoholate under conditions to form hydrogen sulphide thereby producing a treated hydrocarbon mixture free from carbonyl sulphide type sulphur compounds, but containing hydrogen sulphide and further treating said treated mixture with aqueous alkali hydroxide to remove hydrogen sulphide.

2. In the process of desulphurizing a mixture of hydrocarbons and sulphur compounds of the type of carbonyl sulphide, the improvement consisting of contacting the mixture with a substantially anhydrous alcoholic potassium hydroxide solution, under conditions to form free hydrogen sulphide thereby producing a treated hydrocarbon mixture free from carbonyl sulphide type sulphur compounds, but containing hydrogen sulphide and further treating said treated mixture with aqueous alkali hydroxide to remove hydrogen sulphide.

3. The process of claim 2, in which the alcoholic solution contains at least 2% potassium hydroxide.

4. In the process of desulphurizing a mixture of hydrocarbons and sulphur compounds of the type of carbonyl sulphide, the improvement consisting of contacting the mixture .in the liquid state with an amount of substantially anhydrous alcoholic alkali hydroxide solution whereby free hydrogen sulphide is produced, said amount being suiiicient to form two layers, an alkaline alcoholic layer and a hydrocarbon layer containing hydrogen sulphide, separating the two layers and treating the hydrocarbon layer with aqueous alkali hydroxideto remove hydrogen sulphide.

5. The process of desulphuriaing a hydrocarbon mixture free from sulphur compounds of the type of thiophene and containing sulphur compounds of the type of carbonyl sulphide, to reduce the sulphur content to .0005% or less, consisting of contacting said hydrocarbons while in the liquid state with an amount of a substantially anhydrous alcoholic solution containing not less than 5% potassium hydroxide whereby free hydrogen sulphide is produced, saidamount being suillcient to form two layers. an alkaline alcoholic layer and a hydrocarbon layer containing hydrogen sulphide, separating the alcoholic layer from the hydrocarbon layer and treating the latter with aqueous alkali hydroxide to remove hydrogen sulphide.

6. In the continuous process of desulphurizing a mixture of hydrocarbons and sulphur compounds of the type of carbonyl sulphide, the steps consisting of circulating a stream of substantially `anhydrous alcoholic alkali hydroxide solution, continuously admixing therewith a quantity of said hydrocarbon mixture sufllcient to produce three layers, a hydrocarbon layer, an alcoholic alkali hydroxide layer, and a sludge, separating said layers from each other, continuing the circulation o1' the alcoholic layer, maintaining the volume o! the circulating alkali hydroxide substantially constant by adding thereto fresh alcoholic alkali hydroxide, and treating the separated hydrocarbon layer with aqueous alkali hydroxide.

THEODORE W. ROBEBAUGH.

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