NO144759B - AKARICID AND FUNGICID AGENTS CONTAINING ORGANIC FLUORIMIDES / AMIDES. - Google Patents

Description

Process for thermal or catalytic dealkylation of alkylaromatic hydrocarbons.

The present invention relates to an I

method to thermal or catalytic

dealkylation of alkylaromatic hydrocarbons in the presence of hydrogen and a sulphur- ■

connection.

In recent years, the need for benzene has

and the naphthalene of high purity increased considerably. For example, benzene with a high purity for technical purposes is an important starting material for the production of alkyl aromatic sulphonates which are useful as cleaning agents and as surfactants. Benzene can also be used in the production of

phenol and styrene and as an intermediate

in the production of synthetic fibres. Similarly, there is a great need for

•high-quality naphthalene for manufacturing

of phthalic anhydride.

It is known to prepare benzene, naphthalene and higher polynuclear unsubstituted ones

aromatic hydrocarbons by dealkylation

of the corresponding alkylated compounds.

Usually the alkylaromatic carbon is led

or containing a hydrocarbon fraction

alkylaromatics through a reaction zone

at high temperature and high pressure 1 presence

of hydrogen, as well as, if desired, a dealkylation catalyst, and the dealkylated aromatic hydrocarbon is recovered from the outlet quantity from the reaction zone. Temperature tours in the area from approx. 540° C to approx. 820°C

and pressure in the range from approx. 20 to approx. 68

atmospheres are usually appropriate for the reaction. The reaction is also called "hydrodealkylation", because the hydro-

gen replaces the alkyl group that is removed from the aromatic ring and at the same time saturates it

•removed alkyl radical.

The starting materials used

■for the reaction, often contains organics

•nitrogen and/or sulfur compounds in a sufficient amount to damage the catalyst if one is used (whereby

the effectiveness of the catalyst deteriorates to such an extent that it becomes necessary to regenerate or replace the catalyst) or in any case to contaminate the product.

Consequently, these nitrogen and sulfur impurities can be removed before the hydrodealkylation step of the present process by means of known methods of

the area. When such pre-treatment has been carried out, the starting materials will be in a relatively pure state when they are led to the hydro-dealkylation zone, i.e. they will be mainly free of sulfur or nitrogen compounds.

However, it has now been found that if the system's sulfur content is reduced to such an extent, this will lead to certain undesirable results. First of all, a demethylation of the aromatic will occur

ring. If demethylation takes place,

the reaction becomes highly exothermic, and the methane formed will in turn decompose to form free carbon. If this

decomposition of methane during the formation of

free carbon is allowed to take place, this will lead to free carbon being deposited on the reactor walls and on other equipment parts as well as in

the spaces surrounding the catalyst particles or other contact material, which will necessitate frequent stopping of operation for decoking of the system.

It is therefore an object of the present invention to provide a method for the hydrodealkylation of alkyl-substituted aromatic compounds in the presence of hydrogen and a regulated amount of a sulfur compound, whereby the reaction will proceed more efficiently and with less carbon formation.

According to the invention, a method for the dealkylation of an alkylaromatic hydrocarbon is consequently obtained, whereby a hydrocarbon fraction containing said alkylaromatic hydrocarbon in the presence of hydrogen is subjected to dealkylation at a temperature in the range from approx. 540° C to approx. 820° C and a pressure in the range from approx. 20 to approx. 68 atmospheres cg the dealkylated hydrocarbon product is separated from the reaction mixture. The characteristic main feature of the method is that the fraction containing dealkylaromatic hydrocarbon is subjected to the dealkylation reaction in the presence of a sulfur compound in an amount corresponding to from 20 to 200 parts by weight of sulfur per parts by weight of the alkylaromatic hydrocarbon.

The starting material to be dealkylated is treated in the presence of hydrogen and a regulated amount of sulfur compound either thermally or catalytically to obtain the desired aromatic product. Starting materials used in the method according to the invention can have different origins. For example, naphthalene can be produced from certain materials such as ethylene copolymers, light cycle oil extracts and coke oven by-products, while benzene can be produced from toluene, the isomeric xylenes and ethylbenzene, etc. which may be present in raw coal tar. After the coal tar is refined and the hydrocarbons present are separated from each other by fractional distillation, the above-mentioned xylenes and toluene which are then recovered can be dealkylated according to the method according to the invention. Other starting materials include the products obtained by reforming hydrocarbon mixtures, namely the products that boil in a temperature range higher than the hydrocarbons that boil in the boiling range of petrol. Furthermore, the aromatic and alkyl-substituted aromatic compounds obtained by the separation of a starting material containing a mixture of aromatic and paraffinic hydrocarbons which have been extracted using a liquid solvent are considered possible starting materials.

Very suitable starting materials include concentrates of one or more of the compounds toluene, m-xylene, o-xylene, p-xylene, ethylbenzene, o-diethylbenzene, p-m-diethylbenzene, p-diethylbenzene, methyl-naphthalene, 1,2-dimethylnaphthalene and 1 ,4-dimethylnaphthene.

The prescribed concentration of sulfur compounds in the reactant mixture can be ensured by adding sulfur in an amount that is determined based on any sulfur content in the hydrocarbon supply. The sulfur can be supplied in the form of a suitable sulfur compound, preferably one which will dissociate easily under the reaction conditions. The sulfur can thus be added in the form of a mercaptan, such as methyl mercaptan, ethyl mercaptan, propyl mercaptan, n-butyl mercaptan, t-butyl mercaptan or the like or as a thioether such as dimethyl sulphide, diethyl sulphide or dipropyl sulphide. The hydrocarbon fraction which constitutes the starting material in the hydrodealkylation process is suitably supplied first to a suitable heater together with a hydrogen-containing stream. The reaction temperature is preferably kept in the range between approx. 650 and approx.

760° C. The pressure is kept in the range from approx. 20 to approx. 68 atmospheres, preferably in the range from approx. 34 to approx. 41 atmospheres. The outlet from the bottom of the reactor is first led to a high-pressure separator which is maintained at the reaction pressure. A hydrogen-rich gas phase with a hydrogen content higher than approx. 50% is withdrawn at the top of the separator, compressed and recycled to the reactor together with a fresh supply of alkyl aromatics. Additional amounts of hydrogen are added as needed. The liquid hydrocarbon phase which is taken out from the high-pressure separator is usually led to a low-pressure separator which is maintained, for example, at atmospheric pressure, and in which light hydrocarbons such as ethane, methane and the like are separated and preferably collected and used as fuel.

The liquid hydrocarbon phase is taken out from the low-pressure separator, and a part of it is fed together with the outlet quantity from the reactor that goes to the high-pressure separator, with the intention of absorbing a part of the light hydrocarbons in the outlet quantity from the reactor and thus enabling the recovery of the hydrogen in a higher state of purity. Another part of the liquid hydrocarbon from the low-pressure separator is fed to the bottom of the dealkylation reactor where it is mixed with the outlet quantity from the reactor just before the latter is withdrawn, which has a restraining effect on the outlet quantity, lowers its temperature and thereby reduces any metallurgical problems in the outlet system. Because the demethylation of those; alkylaromatic hydrocarbons such as toluene, xylenes or methylnaphthalene are strongly exothermic, the temperature must necessarily be regulated so that it is kept within the desired range in order to remove it

.large amount of heat of reaction which has

, its to act destructively on the desired product by hydrocracking the benzene, or the naphthalene.

The remaining part of the liquid hydrocarbon that is taken out from the low-pressure separator is usually passed through a clay processor to remove any contamination, so that the final product can meet the requirements for acid wash color and bromine index. The treated product is then subjected to fractional distillation to separate any light hydrocarbons and unreacted alkyl aromatic hydrocarbons from the desired benzene or naphthalene product. The unreacted alkylaromatic hydrocarbons are of course recycled to the reactor together with additional fresh feed. . Various modifications of the process described above can be used. For example, three separate separators can be used (instead of 2) for separating the hydrogen-rich gaseous phase, light . hydrocarbons and liquid hydrocarbon fractions. If this modification is used, the intermediate separator is preferably kept at a pressure in the range from .- approx. 3.5 to approx. 10 atmospheres. The sulfur compound that is added for the reaction preferably adds to the stream of fresh hydrocarbon feed that goes to the heater, and either before or after this stream is fed together with the unreacted alkyl aromatic material that is recycled from the product fractionation column and with the hydrogeh stream. Alternatively, the sulfur compound can be supplied directly to the reactor. Tertiary butyl mercaptan is a preferred sulfur compound for supplying the required sulfur.

The sulfur compound that is added in such a quantity that the total sulfur content is from approx. 20 to 200 parts by weight of sulfur per million parts by weight fresh feed, prevents excessive demethylation of the aromatic ring and formation of carbon from methane and will dissociate to form hydrogen sulphide and corresponding hydrocarbons. This hydrogen sulphide is dissolved in the outlet quantity from the reactor, whereby it is removed from the dealkylation reaction zone and led to the high-pressure separator. Part of it will be entrained with the hydrogen-rich gaseous fraction and will be recycled to the dealkylation zone, which means that a smaller amount of sulfur compound needs to be added directly to the reactor to maintain the total sulfur content in the above area. Any remaining hydrogen sulphide dissolved in the liquid hydrogen fraction leading to the low-pressure separator is carried off over the top together with ethane, methane and other light hydrocarbons which go to the gas absorber and are removed together with the aforementioned light hydrocarbons.

As previously mentioned, the hydrodealkylation of the alkylaromatic hydrocarbons can be carried out either as a thermal or as a catalytic process in the presence of hydrogen and a regulated amount of sulfur compound. If it is desired to use a catalyst, the dealkylation reaction zone may contain a catalyst such as a noble metal of group VIII in the periodic table, such as platinum, palladium, rhodium, osmium or iridium impregnated on a suitable refractory oxide. Cesium, vanadium, chromium

or tungsten or combinations of these with each other or with a precious metal of type

VIII in the periodic table impregnated on a refractory oxide such as aluminum oxide, silicon oxide, zirconium oxide, silicon oxide-aluminum oxide, aluminum oxide-boron oxide or silicon oxide-zirconium oxide-aluminum oxide can also be used. A particularly effective catalyst for the dealkylation of alkylaromatic hydrocarbons that can be used in the method according to the invention is a chromium oxide aluminum oxide mixture, in which an aluminum oxide with a relatively large specific surface area such as gamma aluminum oxide, eta aluminum oxide or theta aluminum oxide is used which is impregnated with chromic acid, dried and then oxidized at a temperature of approx. 700° C. The finished catalyst will contain from approx. 10 to approx. 20% chromium oxide calculated on the aluminum oxide. In the following, some embodiments of the method according to the invention are illustrated as examples.

Example 1:

A toluene starting material with a peroxide number of 0.8 and which contained approx. 2.1 , ppm sulfur was fed to a stainless steel reactor containing a hydrodealkylation catalyst consisting of chromium oxide deposited on aluminum oxide. Sulfur in the form of t.butyl mercaptan (TBM) was added to the reactor in an amount of 50 parts by weight of sulfur per million parts by weight of the fresh supply. The dealkylation of the toluene was carried out at a pressure of 34 atmospheres, a liquid space velocity per hour of 1.0', a quantity ratio between hydrogen and hydrocarbon supply of 10', a reactor temperature in the range from 593-7(04° C) and a supply rate for the toluene of 1 gram mol/hour.

The operating conditions, flow rates,. compositions and material balances in the above-mentioned dealkylation process are listed in Table I below:

Example 2.

Another batch of the same toluene starting material used in Example 1 was subjected to hydrodealkylation in the presence of a corresponding catalyst, but without the addition of any sulfur compound. The other reaction conditions were identical to those used in example 1. The results of this experiment are listed in Table II:

As will be seen from a comparison between the two tables above, the addition of a sulfur compound in an amount within the above stated amount range results in a greater final yield of benzene.

Claims (4)

s, • l ■ 1. Process for the thermal or ^catalytic dealkylation of alkylaromatic ^hydrocarbons, whereby a hydrocarbon fraction containing said alkylaromatic hydrocarbons, in the presence of hydrogen, is dealkylated at a temperature in the range from 540 to 820° C, preferably in the range from 650 to 760° C, and a pressure in the range from 20 to 68 atmospheres, preferably in the range from 34 to 41 atmospheres, and the dealkylated hydrocarbon product is separated from the reaction mixture, characterized in that the fraction containing jxie alkylaromatic hydrocarbons is subjected to the dealkylation reaction in the presence of a sulfur compound in an amount corresponding to from 20 to 200 parts by weight of sulfur per million parts by weight of the alkylaromatic hydrocarbons. 2. Method according to claim 1, characterized in that it is used a sulfur compound that readily dissociates under the reaction conditions of the dealkylation. 3. Method according to claim 1 or 2, characterized in that the sulfur is added to the reaction in the form of a thioether or a mercaptan, preferably in the form of tert.butymercaptan. 4. Method according to any one of claims 1-3, characterized in that a hydrocarbon fraction which is rich in alkylaromatic hydrocarbons and which has previously been subjected to a purification treatment whereby the sulfur content is reduced to less than 20 parts per million, is subjected to the dealkylation reaction in the presence of a sulfur compound added in sufficient quantity to raise the fraction's sulfur content to within the range of 20 to 200 parts by weight per million.