US3827974A - Process for the purification of light paraffinic petroleum distillates - Google Patents

Abstract

A CONTINUOUS PROCESS FOR THE PURIFICATION OF LIGHT PARAFFINIC FRACTION BOILING IN THE RANGE OF 25* TO 100*C. AND CONTAINING AS MAJOR PORTION STRAIGHT CHAIN PARAFFINS, BRANCHED PARAFFINS AND CYCLOPARAFFINS AND AS A MINOR PORTION AN IMPURITY SELECTED FROM THE GROUP CONSISTING OF AROMATIC HYDROCARBONS, SULPHUR-CONTAINING COMPOUNDS, NITROGEN-CONTAINING COMPOUNDS, AND OTHER NON--HYDROCARBON IMPURITIES, AND MIXTURES THEREOF, THE SULPHUR CONTENT BEING NO GREATER 500 P.P.M., SAID PROCESS COMPRISING: HYDROGENATION SAID LIGHT PARAFFINIC FRACTION AT A TEMPERATURE IN THE RANGE OF 150 TO 375*C., UNDER A PARTIAL PRESSURE OF HYDROGEN OF 10 TO 100 KG./CM.2, AT A S VELOCITY OF 0.5 TO 20 V./V./H. AND A HYDROGEN TO HYDROCARBON RATIO OF 100 TO 3,000 NM.3/M.3, IN THE PRESENCE OF A HYDROGENATING CATALYST CONTAINING BETWEEN 0.1 AND 2% OF PLATINUM ON A SILICA-ALUMINA CARRIER, SILICA BEING IN MAJOR QUANTITY, AND AT LEAST 30% OF THE PLATINUM BEING DISTRIBUTED ON THE CATALYST SURFACE WHICH IS ACCESSIBLE TO THE REACTANTS, FRACTIONATING THE PRODUCTS OF THE HYDROGENATION TO SEPARATE THE HYDROGENATED SULPHUR- AND NITROGEN-CONTAINING COMPOUNDS AND TO RECOVER A PARAFFINIC FRACTION SUBSTANTIALLY CONTAINING ONLY PARAFFINIC HYDROCARBONS WITH AN INCREASED CONTENT OF CYCLOPARAFFINS.

Description

99,431 i ept. 019 23 02, 23/04 v ABSTRACT; OF THEDISCLOSURE A continuous process for the' purification of light paraflinic fraction boiling in the range of 25 to 100 C. and containing as major portion straight chain paraffins, branched parafiins and cycloparaffins and as a minor portion-an impurity selected from the group consisting of aromatic hydrocarbons, sulphur-containing compounds, nitrogen-containing compounds,-and other non-hydrocarbon impurities, and mixtures-thereof, the sulphur content being no greater than 500 p.p.m.,;said process compr n hydrogenating said light parafiinic fraction at a temperature' in" the range fof 1 50 to 375 C., under a partial fpress'ureofhydrogen of to 100 kg./cm. at a space i velocity' of 0.5t0 2O v'./v.'/'h. and a hydrogen to hydrocarbon ratio of100to3l000 Nmfi/mfi, in the presence or a' hydrogenating catalyst containing between 0.1 and i 2% of platinum ona silica-alumina carrier, silica being "iii'major' quantity, and at least 30% of the platinum being distributed on the catalyst surface which is accessible to' the-reactants, fractionating the products of the hydrogenation to sepa'rate'the hydrogenated sulphurand nitrogen-containjing compounds and to recover 'aparaflinic fraction substantially containingonly paraflinic hydrocarbons with '5 an increased content of cycloparaflins.

, This invention relates to a process for the purification be practically free from aromatic hydrocarbons and other impurities, namely s'ulphur containing compounds, In par- Iticular, the use" of unleaded gasoline results in an increasing demand for branched paraffinic hydrocarbons, some of these compounds having a. particularly high octane number. Therefore, it is desirable to isomerizethe straight chain parafiinsagf these light parafiinic fractions or, at

1 least, to increase the content of branched parafiins in these fractions. However, with many isomerization processes,

the content of aromatic hydrocarbons and 'sulphur-con- J taining compounds in the feeds isomeri zed is of great importance. Many isomeri zation' catalyst'sare very sensitive to these impurities, must, be, removed as; completely a s .pos'sih1e... '1' =5l .ovrn methods for refininglight parafiinic hydrocarbons generally require special techniques, such as a two-step pretreatment comprising the States Patent 0 3,827,974 Patented Aug. 6, 1974 hydrodesulphurization of the feed and then the hydrogenation of any unsaturated hydrocarbons. Such processing is expensive.

An object of the present invention is to provide a process for the treatment of light parafiinic fractions in order to remove simultaneously the aromatic hydrocarbons and the sulphur-containing compounds. Another object of this invention is to provide a process whereby this treatment is performed with reduced investments and operating costs.

According to the present invention, there is provided a continuous process for the purification of a light parafiinic fraction boiling in the range of 25 to C. and containing straight-chain parafiins, branched paraffins and cycloparafiins with a lower content of aromatic hydrocarbons, sulphur-containing compounds, nitrogen-containing compounds and other impurities, the sulphur content being preferably not greater than 500 ppm, said process comprising subjecting said parafiinic fraction to hydrogenation at a temperature in the range of to 375 C., under a partial pressure of hydrogen of 10 to 100 kg./cm. at a space velocity of 0.5 to 20 v./v./h. and a hydrogen to hydrocarbon ratio of 100 to 3.000 Nmfi/mfi, in the presence of a hydrogenating catalyst containing between 0.1 and 2% by weight of platinum on a silicaalumina carriers, silica being in major quantity, and at least 30% of the platinum being distributed on the catalyst surface which is accessible to the reactants,

fractionating the products of said hydrogenation to separate the hydrogenated sulphurand nitrogen-containing compounds and to recover a parafiinic fraction substantially containing only parafiinic hydrocarbons with an increased content of cycloparafiins.

Several methods are known for the hydrogenation of petroleum fractions containing aromatic hydrocarbons, particularly for the hydrogenation of kerosene and fuels, in order to improve their combustion value. But these methods most frequently relate to the treatment of fractions boiling in the range of 60 to 350 C., to reduce their content of aromatic hydrocarbons. However, the hydrogenation yield is generally not higher than 98%, this yield being nevertheless more than enough to increase the combustion value of these fractions.

By contrast, the process of the present invention is applied to lighter fractions and it must perform to practically completely remove the aromatic hydrocarbons and also the sulphur-containing compounds. Therefore the present process differs from the prior processes in that a specific feed is treated in the presence of a specific catalyst in order to obtain a product which must have specified properties.

The feed to the present process mainly comprises parafiinichydrocarbons containing from 5 to 7 carbon atoms inclusive, more generally 5 to 6 carbon atoms, the amount of straight-chain paraffins being at least 30% and more often at least 50%.by weight. The feed contains also b ranched paraflins and cycloparatfins. The amount of aromatic hydrocarbons, more particularly benzene and toluene, may be present within wide limits, but generally is present in an amount within the range of 0.1 and 10% by weight. v

.The amount of sulphur-containing compounds present in the feed to the process of the presentin-vention genv ,erally is between about 50 and 500 ppm. If ,the amount -.is higher, the parafilnic fraction is advantageously desulphurized prior to use in the present process. This desulphurization treatment need not be directed in such a manner'as to secure a complete removal of the sulphurcontaining compoundsas required with prior processes.

A caustic wash will likely suffice. Another means of such desulphurization comprises subjecting said petroleum cut to a sweetening treatment to convert mercaptans contained in said out into heavier disulphides so that a subsequent distillation gives a parafiinic fraction with a low sulphur content. The object of these desulphurizing treatments is only to reduce the sulphur content of the paraflinic fraction to a level lower than about 500 ppm. The prior processes generally require a complete removal of the sulphur-containing compounds before hydrogenationof the parafiinic fraction and therefore, are more expensive.

In accordance with the process of the present invention, the light paraflinie fraction is hydrogenated under the above specified conditions, preferably, however, hydrogenation is carried out at a temperature within the range of 175 to 325 C., under a partial pressure of hydrogen of between about to 75 kg/cm. and with a space velocity from 0.5 to 20. The optimum conditions depend not only On the composition of the parafiinic fraction and on its content of aromatic hydrocarbons, cycloparaffinic hydrocarbons and sulphur-containing compounds, but they depend also on each other. Therefore, the temperature to be applied for a substantially complete removal of the aromatic hydrocarbons varies as a function of the sulphur content of the feed, the other conditions remaining equal. Also, the temperature may be lowered when the pressure is higher. Determination of such optimum value within the above defined ranges may readily be made by those skilled in the art.

In order to economically perform, a substantially complete hydrogenation of the aromatic hydrocarbons and of the sulphur-containing compounds, the hydrogenating catalyst will contain from 0.1 to 2% by weight of platinum on a silica-alumina carrier, where silica is in major quantity and where at least 30% of the platinum is distributed on the catalyst surface which is accessible to the reactants. Indeed, it has been found that with catalysts, whose carrier does not contain .a greater amount of silica than alumina or whose platinum is poorly distributed onthe surface, the degree of aromatic hydrogenation is lower under the conditions which prevail than with the catalyst utilized in the present invention.

Many processes are known for the manufacture of catalysts containing platinum on a carrier. However, a convenient process for preparing the present catalyst comprises impregnating the silica-alumina carrier with a solution of a platinum copound, usually chloroplatinic acid and then evaporating the solution and thereafter drying and calcinating the catalyst at about 400 C. Im-

wpregnation of the carrier may be performed with solutions of water soluble platinum salts, but with the use of solutions having a pH from about 1 to 7. An extended drying and a moderate calcination of the catalyst are particularly suitable. Under these conditions, at least 30% of the platinum is distributed on the total surface of the catalyst which is accessible to the reactants. Determination of this percentage of platinum on the surface is carried out by using described methods, e.g. the method of D. E. Mears and R. C. Hansford, in Journal of Catalysis, vol. 9, pp. 125-127 (1967).

In order to illustrate the effect of the catalyst composition, comparative hydrogenation tests were carried out on a light paraflinic fraction having the following composition:

Percent (by weight) n-Pentane 61.0 Iso-pentane 13.8 Cy'clopentane 4.7 2.2-dimethylbutane 0.6 2.3-dimethylbutane 0.4 Z-methylpentane I I 3.5 3-rnethylpentane 3.3 n-Hexane 8.2 Methylcyclopentane 3.0

Percent (by weight) Cyclohexane V 0.7 Benzene Sulphur, 200 ppm.

The operating conditions for hydrogenation were the following:

The catalyst used in each test consisted of platinum on a silica-alumina carrier. However, the ratios of silica, alumina and platinum were ditferent in each test. The catalyst compositions and the results of these tests are summarized in Table I.

TABLE I Weight percent sum in Pt 0! On Residual Residual s Test number carrier catalyst surface benzene (p.p.rn.)

16. 2 0. 73 v 52 72 2 68. 7 0. 74 61. 0. 3 1 as. o o. as 42 o. 4 1 83. 9 0. 76 15 50 8 68. 7 0. 08 54 62 v 13 Results presented in the above Table I clearly demonstrate that the catalyst must fulfill all of the above specified conditions relating to platinum content, the platinum distribution on the surface of the catalyst and composition of the carrier, in order to obtain a substantially complete aromatic hydrogenation of the light parafiinic fraction. Tests 2 and 3 show that the hydrogenation yield is higher than 99.5%. 'By contrast, a catalyst carrier which has a lower content in silica (test 1) does not perform an adequate hydrogenation, although the content and the distribution of the platinum are substantially the same as those of the catalysts of tests 2 and 3. The same appears with a catalyst having a poor distribution of the platinum (test 4) or with a catalyst having a low content in platinum (test 5).

Other hydrogenation tests were carried out in order to show the effect of temperature on the removal of aromatic hydrocarbons. In these tests, the catalyst contained 0.72% by weight of platinum on a silica-alumina carrier with 03.8% of silica. The amount of platinum distributed on the accessible surface of this catalyst was 43%.

The composition of the used light paraflinic'frac'tion hydrogenated was as follows:

' Percent by weight 92 The sulphur content was varied during some of the tests by adding dimethyldisulfide. i

The operating conditions for hydrogenationwere as follows:

partial pressure of hydrogen: 50 lrg./crn.-' space velocity: 2 v./v./h. hydrogenzfeed ratio: 500 Nmfi/mfi,

The sulphur content of the feed'and the temperature employed in each test and the results are summarized in Table II.

TABLE II Residual Temperabenzene S content of feed ture C.) (p.p.m.)

50 .m 250 45 p p 270 7 290 5. 5 310 9 100 .m 250 200 p p 270 35 290 18 310 12 330 30 TABLE III Residual Partial pressure of Temperabenzene hydrogen (kg/cm!) ture C.) (p.p.m.)

During the hydrogenation of the aromatic hydrocarbons contained in the light par-aflin-ic fraction in accordance with the present invention, the sulphur-containing compounds are converted into H -S which is easily removed by distillation. Isomerization of some paraffinic compounds occurs also during this hydrogenating treatment. These advantages of this invention are shown by the results presented in Table IV of a test for the hydrogenation of a light parafiinic fraction under the following conditions:

temperature: 330 C.

partial pressure of hydrogen: 50 kg./cm. space velocity: 2 v./v./h. hydrogen/hydrocarbons ratio: 500 Nm. /m.

The hydrogenated products were distilled to remove H S formed during the hydrogenation.

TABLE IV Hydrogen- Compounds Feed ated product n-Pentane, percent by wt 95. 2 86. 1 Iso-pentane 1 9. 1 Dimethylbutanes 5 5 Methylcyclopentane Traces 3. 2 Cyclohexane 3. 3 1. 1 Benzene, p.p.m 9 31 Sulphur, p.p.m 100 1 1 Cr 9,000 p.p.m.

The above mentioned tests show that the aromatic hydrocarbons of a light paraffinic fraction may befhydrogenated with a practically quantitative yield; Thebest results are obtained when the sulphur content of the feed is not higher than p.p.m. In such instance, the hydrogenation preferably is carried out at a temperature between 275 and 330 C. and under a partial pressure of hydrogen varying between about 50 and 70 kg./ cm. and with the hydrogenation yield higher than 99.5%. The sulphur-containing compounds are simultaneously converted into His, which is easily distilled off, so that the end product is practically sulphur-free.

The results of Table IV show also that during this hydrogenation of the light paraffinic hydrocarbon, an isomerization occurs with a yield which is about 12% under the conditions employed in the example. This yield may be increased by operating at a higher temperature. However, the temperature is kept within the above stated limits in order to secure a practically complete hydrogenation of the aromatic hydrocarbons.

For carryin out the subsequent isomerization of the paraffinic fraction, it is essential that the hydrogenated fraction does not contain more than 100 p.p.m. of residual aromatics and not more than 30 p.p.m. of sulphur, in order to prevent rapid deactivation of the isomerization catalyst. These conditions are easily fulfilled by applying the process of this invention.

From the foregoing description, one skilled in the .art can easily select the optimum operating conditions which are the most convenient for a specific feed for the purpose of reducing the aromatic content below about 10 p.p.m. and the sulphur content below about 5 p.p.m.

What is claimed is:

1. A continuous process for the purification of light paraffinic fraction boiling in the range of 25 to 100 C. and containing as a major portion straight chain parafiins, branched parafiins and cycloparaflins and as a minor portion an impurity selected from the group consisting of aromatic hydrocarbons, sulphur-containing compounds, nitrogen-containing compounds, and other non-hydrocarbon impurities, and mixtures thereof, the aromatic hydrocarbon content being between 0.1 and 10% by weight the sulphur content being no greater than 500 p.p.m. said process comprising:

hydrogenating said light paraflinic fraction at a temperature in the range of to 375 C., under a partial pressure of hydrogen of 10 to 100 kg./cm. at a space velocity of 0.5 to 20 v./v./h. and a hydrogen to hydrocarbon ratio of 100 to 3.000 Nm. /m. ,f in the presence of a hydrogenating catalyst consisting essentially of between 0.1 and 2% of platinum on a silica-alumina carrier, silica being in major quantity, and at least 30% of the platinum being distributed on the catalyst surface which is accessible to the reactants, fractionating the products of the hydrogenation to separate the hydrogenated sulfurand nitrogen-conta" ining compounds and to recover a paraflinic fraction substantially containing only parafiinic hydrocarbons with an increased content of cycloparatllns, less than 100 p.p.m. aromatic hydrocarbons and less than 30 p.p.m. sulphur.

2. The process of claim 1 wherein the hydrogenation reaction is carried out at a temperature of between and 325 C., under a partial pressure of hydrogen ofbetween 15 and 75 kg./cm. and with a space velocitYof 0.5 to 20 v./v./h.

3. The process of Claim 1 wherein said light paratfinic fraction comprises one containing paraffinic hydrocarbons of 5 to 7 carbon atoms.

4. The process of Claim 1 wherein said light paraffinic fraction contains no greater than 10% aromatic hydrocarbons by weight.

7 5. The process of Claim 1 wherein said light paraifinic fraction eontains at least 30% straight chain paraffin hyclrdcarbons.

References Cited UNITED STATES PATENTS 2,911,357 11/1959 Myers et a1. 208138 Re.--26,883 5/1970 Marechal et al. 208143 3,203,891 8/1965 Holden 2082l6 8 OTHER REFERENCES Mears et al., The Stoichiometry for Hydrogen Titration of Oxygen on Supported Platinum, Journal of Catalysis, 9, 1967, p. 125.

DELBERT E. GANTZ, Primary Examiner S. L. BERGER, Assistant Examiner U.S. Cl. X.R.