GB2053959A - Purification of aromatic hydrocarbon cuts - Google Patents

Abstract

Process for selectively hydrogenating the olefinic and acetylenic hydrocarbons present in an aromatic hydrocarbon cut in a proportion of less than 5% by weight, by contacting said cut, at least partly in the liquid phase, with a supported palladium catalyst whose palladium content is from 0.05 to 5% by weight, previously subjected to a treatment with a sulfur, selenium or tellurium compound selected from hydrides, oxides, salts and organic derivatives, at a temperature from 0 to 450 DEG C, in the presence of hydrogen.

Description

SPECIFICATION Purification of aromatic hydrocarbon cuts This invention is concerned with removing, by a convenient treatment, undesirable products which may be contained in a hydrocarbon cut of high aromatic hydrocarbon content and, more particularly, removing the unsaturated constituents contained therein, i.e. mono-oleofinic, polyolefinic and acetylenic hydrocarbons, these unsaturated hydrocarbons generally amounting to less than 5% of said hydrocarbon cut.The hydrocarbon cut of high aromatics content (benzene, toluene, xylenes, etc...) is generally a product from various thermal or catalytic processes commonly used for producing them and, for example, is produced by catalytic reforming or results for an aromatization reaction, catalytic or not (for example reactions for producing aromatic hydrocarbons from saturated or unsaturated gasolines and, particularly, the so-called "Aromizing" reactions); this cut may also be obtained from steamcracking, cracking, coal distillation etc...

More precisely, the cuts of very high aromatics content (generally containing at least 70% by weight of aromatic hydrocarbons) obtained, for example, during the abovementioned reactions, i.e. after severe treatments, always contain, as a result of the high temperatures to which they have been subjected during these treatments, more or less substantial amounts of olefinic, diolefinic and also acetylenic products. It is known that these unsaturated products are unstable and tend, by polymerizing to produce gums and resins which colour and give odors to these cuts of high aromatic hydrocarbons content, thereby reducing the value of these cuts which can no longer be used for petrochemical purposes but only as fuels.

Moreover the aromatic hydrocarbons are known as being very desirable raw materials since they are essential to the progress of industrial organic chemistry.

The basic aromatics, for example benzene, toluene, orthoxylene and paraxylene, issued particularly from catalytic reforming gasolines or from steam-cracking gasolines or still from aromatization reactions (for example the so-called aromizing reaction) or produced by coal distillation, etc. . ., are destined to the manufacture of such products as plastic materials, detergents, rubbers, synthetic fibers etc. .. The aromatic hydrocarbons are used in the manufacture of these products after chemical conversions which often are very complex, such as oxidations, alkylations, hydrogenation, dehydrogenation, etc...

Thus, benzene is used to a very large extent for manufacturing ethylbenzene, itself destined to be converted to styrene which is the monomer used for the production of polystyrene and SBR rubber. The other utilizations of benzene, substantially to the same extent, are the synthesis of cumene and the manufacture of cyclohexane. Cumene is used for manufacturing phenol and plastic materials derived therefrom; cyclohexane leads, through the intermediary of adipidic acid and its derivatives, to the manufacture of 6 and 66 nylons. Benzene is also used for manufacturing alkyl benzenes, a large portion of which is used for manufacturing detergents.

As far as toluene is concerned, it is used in conventional synthesis and particularly for manufacturing toluene-diisocyanate (a raw material for the manufacture of polyurethanes) as well as for manufacturing phenol by oxidation of toluene.

Xylenes, in admixture, may be used as solvent, as well as toluene. They are also used, in part for the manufacture of pure xylenes and in part as fuel in admixture with other hydrocarbons for improving the octane number. From the three isomers of xylene (ortho, meta and para), the most important is paraxylene which is used for manufacturing polyester fibers. Orthoxylene is almost entirely converted to phthalic anhydride, which is used as raw material for the manufacture of polyester resins. The third isomer, metaxylene, is used for manufacturing certain resins and additives.

These aromatic hydrocarbons, in order to be used conveniently, must conform with specifications warranting their purity but, above all, they must not contain certain undesirable products which contaminate them although these products are present in very low proportions, which generally are too low for being efficiently removed by the various existing physical methods of separation.

The older known process for removing undesirable products from cuts of very high aromatics content, consists of washing hydrocarbons with sulfuric acid.

This operating manner has the disadvantage of resulting in substantial losses of the treated material: as a matter of fact, sulfuric acid not only produces the polymerization of the unsaturated materials, but it also results in a sulfonation of the aromatic hydrocarbons, and, consequently, in a loss of very valuable products.

It thus became quickly necessary to replace this "rough" acid treatment by other more selective processes and particularly by selective hydrogenation processes; but these hydrogenation processes which convert the olefinic and diolefinic hydrocarbons to paraffins having the same number of carbon atoms as the unsaturated hydrocarbons, are not sufficiently selective since they also result in a slight hydrogenation of the aromatic hydrocarbons. This hydrogenation is not substantial and concerns only a small percent of the aromatic hydrocarbons but it is however sufficient to reduce the economic efficiency of the operation.In addition, the known processes of selective hydrogenation are not satisfactory for solving the problem of the acetylenic hydrocarbons present in the aromatic cuts: as a matter of fact, the acetylenic hydrocarbons produce a quick deactivation of the catalysts used up to now, as a result of the formation and the deposit of polymers as well as a result of a progressive dissolution of the active metal of the catalyst which is the greater as the content of acetylenic hydrocarbons is the higher.

Thus, the French Patent Specification No.1,502,462 shows that, even in the case of a charge having a relatively low content of acetylenic hydrocarbons, a palladium catalyst deposited on calcined alumina has already lost a large fraction of its activity and of its selectivity after only 7 days of use.

The present invention proposes the use of catalysts that have an initial activity substantially equal to that obtained with known catalysts of the palladium type, but that remain stable during time and lead to an improved selectivity as compared to that obtained in the prior art. It is observed that the content of palladium metal present in the catalyst, does not substantially vary during time.The use of the catalyst according to the invention provides for a minimum and negligible hydrogenation of the aromatic hydrocarbons of the treated cut (said cut containing at least 70% or at least 75% or more of aromatic hydrocarbons) and, nevertheless, provides for the substantially complete conversion of the unsaturated monoethylenic, polyethylenic and acetylinic hydrocarbons to saturated products, the proportion of said unsaturated hydrocarbons in the cut being less than 5% by weight and preferably less than 1%, this percentage being generally higher than 0.01%, e.g. 0.05% The present invention thus provides a method of selectively hydrogenating an aromatic hydrocarbon cut at least 70% by weight of which is composed of aromatic hydrocarbons and less than 5% by weight of which is composed of monoolefinic, polyolefinic and acetylenic hydrocarbons, comprising passing the said cut, at least partly in the liquid phase, and hydrogen into contact with a supported palladium catalyst at a temperature in the range 100 to 3500 C, the catalyst containing from 0.05 to 5% by weight of palladium with respect to the carrier and having been previously treated with at least one compound of at least one of sulfur, selenium and tellurium at a temperature in the range 0 to 4500C.

The catalyst used according to this invention is prepared by incorporating a palladium compound in a carrier, then activating by heating at 200-11 000C, and preferably at 650-11 000C, and then treating the resulting product with at least one sulfur, selenium and/or tellurium compound.

In the present process there is preferably used a supported palladium catalyst whose crystallites have an average size of at least 50 Angströms. This size can be measured for example, by electronic microscopy. A catalyst complying with this definition may be prepared, for example, by incorporating a palladium compound in a carrier in a proportion from 0.05 to 5% by weight of palladium with respect to the carrier and more particularly 0.1 to 5%, this incorporation being followed with a heat activation at a temperature from 200 to 11 000C, for example from 650 to 11 000C, preferably from 750 to 9500 C, said heating being performed in a neutral atmosphere, for example in nitrogen, or in a reducing atmosphere, for example in hydrogen, or in an oxidizing atmosphere, for example, in a gas containing free oxygen. An oxidizing atmosphere is however preferred, since it provides for a particularly rapid formation of the palladium crystallites. Any operating pressure can be used, for example the normal pressure.

Any method of incorporating the palladium compound may be used, for example admixture in a dry state or in the presence of water or impregnation by means of a solution of a palladium compound.

The palladium compound may be any one of the palladium compounds known and/or proposed for a similar use, for example palladium nitrate, palladium chloride or palladium acetylacetonate. In some cases, other metals having a catalytic effect may be added.

As carrier, there can be used any conventional carrier, for example, alumina or silica.

The carrier is preferably an alumina having an initial specific surface (before incorporation of palladium and before activation) generally from 1 to 250 m2/g, preferably from 25 to 1 50 m2/g, with a pore volume for example from 0.4 to 0.8 cc/g, at least 75% of the pores having an average diameter from 10 to 50 nanometers. Another type of carrier is silica of specific surface from 10 to 250 m2/g (before incorporation of palladium). Generally it is preferred to use a very weakly acid catalyst.This weak acidity is determined by the known text of ammonia adsorption as described for example in "Journal of Catalysis, 2, 211-222 (1963)": the method consists of heating the catalyst to 6000C under vacuum (i.e. under a pressure lower than about 0.01 mm of mercury) up to a complete gas removal (in particular for removing water and undesirable impurities); the catalyst is then placed in a calorimeter at 3200C and ammonia is introduced in such an amount that the final pressure of the system at the equilibrium be 300 mm of mercury (40 kilo Pascal) and the amount of evolved heat is measured.

The preferred carriers to be used according to the present invention (particularly alumina) thus have a neutralization heat by ammonia adsorption lower than 10 calories per gram at 3200C under a pressure of 40 kilo Pascal (300 mm of mercury). It is noticeable that the neutralization heat of the carrier used in the catalyst is substantially identical to that of the catalyst itself and that, also, the specific surface and the pore volume,of the final catalyst are substantially identical to the values indicated above for the carrier itself.

The catalyst, before its use in the treatment of the cut of high aromatic hydrocarbon content, is subjected to the action of at least one compound of at least one element from the group VIA, selected from sulfur, tellurium and selenium. This treatment is performed after activation of the catalyst, conducted as mentioned above, after incorporation of palladium to the carrier. It is known that the activation of a catalyst for hydrocarbon conversion is generally followed with a reduction of the catalyst by means of hydrogen. In the process of the present invention, the treatment by a sulfur, tellurium or selenium compound may be performed either during the reduction of the catalyst or before said reduction or still after said reduction.Finally the catalyst may be subjected to no reduction at all, but only to the mere treatment with at least one sulfur, selenium or tellurium compound. This treatment with selenium sulfur or tellurium may also be performed, after activation of the catalyst, by first proceeding to a reduction of the catalyst and then continuing the reduction simultaneously with the treatment with the sulfur, selenium or tellurium compound, finally by continuing the reduction of the catalyst in the absence of sulfur, selenium or tellurium compound. It may also be comtemplated, after the treatment with at least one compound of the element selected from sulfur, tellurium or selenium, to proceed to a scavenging of the catalyst by means of hydrogen under the conditions commonly used for the reduction of the catalyst.

The treatment with at least one compound of at least one element selected from sulfur, tellurium and selenium is performed at a temperature from 0 to 4500 C, preferably from 0 to 3000C and is continued until the final catalyst contains preferably from 0.01 to 2 atoms of sulfur, selenium or tellurium per palladium atom.

The reduction of the catalyst, performed either simultaneously or not with the treatment with the sulfur, selenium or tellurium compound, is generally also conducted at a temperature from 0 to 4500C, and preferably from 0 to 300 C.

Preferably, according to the process of the invention, the catalyst is subjected to reduction, at least a part of the reduction being performed simultaneously with the treatment with the sulfur, selenium or tellurium compound, at a temperature from 0 to 4500 C, preferably from 0 to 3000 C. When the reduction has started before the treatment with the sulfur, selenium or tellurium compound and/or when the reduction is continued after the treatment with the sulfur, selenium or tellurium compound, said reduction which follows and/or precedes the treatment with the sulfur, selenium or tellurium compound, is performed at the same temperature as the temperature of said treatment.It is also possible to begin with the reduction of the catalyst and then to introduce into the hydrogen stream a sulfur, selenium or tellurium compound at a temperature which may be lower than the selected reduction temperature, the temperature being then progressively increased up to that of the reduction.

The sulfur, selenium and tellurium compounds which can be used are particularly selected among the hydrides, oxides, salts comprising a volatile anion and the organic derivatives. Examples thereof are mercaptans, sulfides, selenium and tellurium oxides and, preferably, hydrogen sulfide H2S, hydrogen selenide H2Se and hydrogen telluride H2Te.

According to a preferred embodiment, the treatment with at least one compound of at least one element selected from sulfur, selenium and tellurium, is performed simultaneously with the reduction of the catalyst; said compound is preferably in a gaseous state or has a sufficient vapor pressure for being in the gaseous state under the operating conditions selected for the treatment. The concentration of the compound in the hydrogen stream where this compound is present, is from 0.01 to 5% by mole and preferably from 0.1 to 4% by mole.

The preferred conditions for obtaining, in the presence of the catalyst treated according to the invention, the selective hydrogenation of charges containing aromatic hydrocarbons and more particularly charges from an "Aromizing" unit, are as follows: total pressure 1 to 50 bars and preferably 3 to 30 bars -Space velocity : from 1 to 20 and preferably 1 to 10 (volume of liquid charge/volume of catalyst/hour) molar ratio H2/hydrocarbons : from 0.05 to 5 and preferably from 0.01 to 1.

It is convenient, and this is possible in view of the nature of the charge, to select the temperature between 1000C and 3500C; in order, on the one hand, to remove in a single step, simultaneously with the acetylenic hydrocarbons, the polyolefinic and monoolefinic hydrocarbons and, on the other hand, to avoid as much as possible the hydrogenation of the aromatic hydrocarbons, which hydrogenation would result in a slight decrease of the octane number of the treated cut. The temperature of the treatment is then from 100 to 3500C, but preferably from 140 to 2000C and more particularly from 1 60 to 1 900C.

EXAMPLE The charge to be treated is issued from an "Aromizing" unit. It has the following composition by weight:

Benzene 1.40% Toluene 14.60% Aromatics 79.50% C, Aromatics 35.10% C9 Aromatics 23% C10 Aromatics 5.40% Olefins 0,40% (mono and polyolefins) Napthenes 1.00% Acetylenics 0.10% Paraffins 19.% Octane number (research) 115 -Other characteristics Bromine Number : 2200 for the total product (expressed in mg/g) -cut 65-900C 8200 105-1150C 4300 134-1470C 350 147-1800C 110 Acid wash color (which characterizes the presence of acetylenic hydrocarbons) -cut 65-900C 6 105-1150C 8 134-1470C 8 147-1800C 13 A palladium catalyst is prepared by impregnating with a nitric solution of palladium nitrate, an alumina carrier consisting of balls of a 2 mm diameter, having a specific surface of 100 m2/g and a total pore volume of 0.58 cc/g, so as to obtain in the final catalyst a palladium content of 0.2% by weight.

After the end of the impregnation, the catalyst is dried at 1 200C for 2 hours and then roasted at 7000C for 2 hours in a dry air stream (average size of the crystallites greater than 60 A).

1 st test A portion of this catalyst is charged in a reactor, where it is reduced with hydrogen containing 3% by mole of hydrogen sulfide, at a temperature of 2500C for 2 hours. The catalyst thus contains 0.09 sulfur atom per palladium atom.

The charge to be treated is passed over the catalyst at a space velocity of 3 volumes of liquid charge per volume of catalyst and per hour, the temperature of the reaction zone being 1 800C and the pressure 25 bars, the molar ratio of hydrogen to hydrocarbons in the charge being equal to 0.1 at the inlet of the reactor.

The products issued from the reaction zone are analyzed. The results obtained are reported in Table I, simultaneously with the results from the second test specified below.

2nd test Another portion of the catalyst is charged in a reactor where it is reduced with pure hydrogen containing no sulfur, selenium or tellurium derivatives, at 2500C for 2 hours. The above-defined charge to be treated is passed over this catalyst under the same operating conditions as for the first test. The results are reported in the following Table I.

TABLE I

- COMPOSITION by weight in % Charge 1st Test 1 2nd Test Total aromatics 79.50 79.35 77.80 Benzene 1.40 1.35 1.15 Tol uene 14.60 14.55 13.70 C8 Aromatics 35.10 35.05 34.80 Cg A romati cs 23.00 23.00 22.80 C19 Aromatics 5.40 5.40 5.35 Olefins 0.40 < 0.01 < 0.01 Naphthenes 1.00 1.25 2.80 Acetylenics 0.10 - 0.00 0.10 Paraffins 19.00 19.40 19.40 Consumed aromatic hydrocarbons - 0.2% 2.1% Octane number (Research) 115 115 112 Bromine index - Total product 2200 75 80 65- 90 C cut 8200 9 8 105 - 115"C cut 4300 14 15 134-1470Ccut 350 20 20 147-180'Ccut 110 25 30 Acid wash color 65 - 900C cut 6 0 1 105 - 115 C cut 8 0 1 134-l470Ccut 8 0.5 2 t47-l8O0Ccut 13 1 4 The results reported in Table I make obvious the advantage of proceeding according to the present invention: The use of a presulfided catalyst according to the invention reduces the maximum extent the loss of aromatics by hydrogenation and provides here for the substantially total removal of acetylenic hydrocarbons.

3rd and 4th test The first test is repeated while replacing, during the reduction of the catalyst, hydrogen sulfide either with hydrogen selenide in the third test, or with hydrogen telluride in the fourth test. Each of the resulting catalysts respectively contains 0.09 atom of selenium or tellurium per palladium atom. There is so obtained, for the treatment of the charge, substantially the same results as those obtained in the firs . step, including particularly the substantially complete removal of the acetylenic hydrocarbons.

Claims (11)

1. A method of selectively hydrogenating an aromatic hydrocarbon cut at least 70% by weight of which is composed of aromatic hydrocarbons and less than 5% by weight of which is composed of monoolefinic, polyolefinic and acetylenic hydrocarbons, comprising passing the said cut, at least partly in the liquid phase, and hydrogen into contact with a supported palladium catalyst at a temperature in the range 100 to 3500C, the catalyst containing from 0.05 to 5% by weight of palladium with respect to the carrier and having been previously treated with at least one compound of at least one of sulfur, selenium and teilurium at a temperature in the range 0 to 4500C.

2. A method according to Claim 1, in which the total amount of the monoolefinic, polyolefinic and acetylenic hydrocarbons is less than 1% by weight of the hydrocarbon cut.

3. A method according to Claim 1 or 2, in which, after treatment of the catalyst with the sulfur, selenium and/or tellurium compound(s), the catalyst contains from 0.01 to 2 atoms of the said element(s) per palladium atom.

4. A method according to any one of the preceding claims, in which the treatment by at least one compound of sulfur, selenium and/or tellurium is performed at a temperature in the range 0 to 3500C and the said compound is a hydride, an oxide, a salt having a volatile anion or an organic derivative.

5. A method according to Claim 4, in which the treatment is conducted in the presence of hydrogen so as to simultaneously reduce the catalyst initially prepared by incorporation of palladium onto a carrier, the sulfur, selenium or tellurium compound(s) being a volatile compound having a sufficient vapor pressure to be in a gaseous state under the selected operating conditions, the molar concentration of the said compound in the hydrogen stream being from 0.01 to 5%.

6. A method according to Claim 5, in which the said molar concentration is from 0.1 to 4%.

7. A method according to Claim 6, in which the carrier of the palladium catalyst is alumina having a specific surface between 1 and 250 m2/g, a pore volume from 0.4 to 0.8 cc/g, 75% of its porosity corresponding to pores-of an average pore diameter from 10 to 50 nanometers, a neutralization heat by ammonia adsorption lower than 10 calories per gram at 3200C under 40 kilo Pascal, and an average crystallite size of at least 50 Angströms.

8. A method according to Claim 7, in which the sulfur, selenium or tellurium compound is hydrogen sulfide-H2S, hydrogen selenide H2Se or hydrogen telluride H2Te.

9. A method according to any one of the preceding claims, in which the liquid phase and hydrogen are passed in contact with the catalyst at a temperature from 140 to 2000 C.

10. A method according to claim 9, in which the temperature is from 1 60 to 1 900C and the catalyst contains from 0.1 to 5% weight of palladium with respect to the carrier.

11. A method according to Claim 1, substantially as hereinbefore described in the Example.