Improved dehydrocyclodimerization process.

This invention relates to a process for the conversion of hydrocarbons, more particularly to a catalytic process called dehydrocyclodimerization, in which two or more molecules of a light, aliphatic hydrocarbon, such as e.g. propane or butane, are combined to form an aromatic hydrocarbon as a product. Non-aromatic hydrocarbons are also formed, especially when significant amounts of olefins are present in the feedstock.

There are a large number of publications describing the conversion of light, aliphatic hydrocarbons to aromatic hydrocarbons. For example, U.S. Pat. No. 2,992,283 discloses the conversion of propylene to a variety of higher molecular weight hydrocarbons using crystalline aluminosilicate as a catalyst. U.S. Patent No. 4,347,394 discloses the transfer of C 1-4 hydrocarbons to aromatics using a non-acidic zeolite bearing a platinum compound. In U.S. Pat.

329 532 describes the conversion of olefins or mixtures of olefins and paraffins to aromatic hydrocarbons. The catalyst comprises a crystalline silicate having a specified composition, crystallite size range and X-ray diffraction pattern. U.S. Pat. No. 4,444,988 discloses a process scheme for recovering the products from a similar process using a C What is emphasized in this patent specification is the use of heat exchange to improve the economic conditions in the condensation of hydrocarbons from the effluent stream from the reaction zone.

U.S. Pat. No. 4,180,689 discloses the conversion of C1-C8 aliphatic hydrocarbons to aromatic hydrocarbons by a process using a catalyst comprising gallium supported on an aluminosilicate. U.S. Pat. No. 3,761,389 discloses an improved process for the conversion of hydrocarbons in the range of hydrocarbons to hydrocarbons having a boiling point of 204.4 DEG C. to aromatics over a ZSM-5 type catalyst. The improvement lies in the use of two reaction steps in series, the first of which is carried out under stricter operating conditions. U.S. Pat. No. 4,528,412 also discloses catalysts, reaction zone operation methods and product recovery methods for dehydrocyclodimerization processes. For a discussion of the dehydrocyclodimerization process, see also page 191 in Volume 18, No. 2 of the 1979 edition of Industrial Engineering Chemistry - Process Design and Development.

The inventor of the present invention has now devised a distinctive process flow scheme which increases the yield of the valuable alkyl aromatic hydrocarbons in a dehydrocyclodimerization process. It has also been found that the invention leads to the unexpected result that a greater conversion of the supplied light hydrocarbons per review. This is achieved by introducing benzene and / or toluene into the reaction zone in admixture with the light hydrocarbons supplied. 1 according to a preferred embodiment of the invention, recycling of benzene and / or toluene which is recovered from the effluent stream from the reaction zone is carried out.

The invention thus provides a process for converting hydrocarbons in which a vapor phase feed stream containing an aliphatic hydrocarbon is introduced into a catalytic reaction zone containing a solid catalyst and converted to aromatic hydrocarbons comprising benzene, toluene and xylenes, which are recovered from a effluent stream. . The new process is characterized in that benzene and / or toluene are introduced into the reaction zone in an amount of from 1.0 to 10.0 mol% of the total amount of hydrocarbons introduced into it, in order to increase the formation of xylenes.

The drawing shows a simplified process flow diagram for a preferred embodiment of the method according to the invention and shows the preparation of C₀aromatics from propane, with o recycling of benzene.,

Processes for the conversion of light aliphatic hydrocarbons to aromatic or non-aromatic C⁺-hydro166406 carbons have been the subject of considerable development work, as will be seen from the publications discussed above. The main advantage of the process is for the transfer of the cheap and large amounts of C1 and / or C1 hydrocarbons to the more valuable aromatic hydrocarbons and hydrogen, or for the transfer of the hydrocarbons in the feedstock to higher molecular weight aliphatic products. This may be desirable simply to increase the value of the hydrocarbons. It may also be desirable to correct for an excessive amount of the C 1-4 and hydrocarbons or to satisfy a need for the aromatic hydrocarbons. The aromatic hydrocarbons are very useful for the preparation of a wide variety of petrochemical products, of which benzene is one of the most widely used. The aromatic hydrocarbons obtained as products are also useful as blending constituents in high octane motor fuels.

The feedstocks for a dehydrocyclodimerization process are light, aliphatic hydrocarbons having from 2 to 4 carbon atoms per liter. molecule. The feed stream may comprise a single compound or a mixture of two or more of these compounds. The preferred feedstocks are propane, propylene, butanes and butylenes, with saturated compounds being highly preferred. The feed stream to the process may also contain a certain amount of 9 ° C 2 hydrocarbons. It is preferred that the concentration of C The preferred products of the process are aromatic C * hydrocarbons. However, dehydrocyclodimerization processes are not 100% selective and a certain amount of non-aromatic Cg is formed.⁺-hydrocarbons also from saturated feedstocks. When treating a feedstock consisting of propane and / or butanes, the predominant amount of Cg⁺The hydrocarbons obtained as product consist of benzene, toluene and the various xylene isomers.

A small amount of C The presence y of olefins in the feed stream results in an increased formation of long chain C 1-4 hydrocarbons as products when the preferred catalyst system is used. Significant olefin concentrations in the feedstock can significantly reduce the formation of aromatics. The invention is directed to increasing the amount of the valuable C The design of the reaction zone and the composition of the catalyst used in the reaction zone are not basic elements of the invention or characteristics which limit the invention. Nevertheless, it is believed that, in order to provide a background to the invention, it will be useful to describe the preferred reactor system for use in the reforming and dehydrocyclodimerization variants of the invention. This system comprises a multi-stage reactor with radial flow and movable bed, such as described

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catalyst regeneration systems and various aspects of the operation of movable catalyst layers and the equipment used for this purpose. This reactor system has found extensive commercial application for reforming naphtha fractions. The use of this system for dehydrogenation of light paraffins has also been described.

In the preferred movable bed reactor system, a spherical catalyst with a diameter of between 0.4 and 3.2 mm is used. The catalyst preferably comprises a support material and a metallic component which is deposited on the support material such as by impregnation or coagulation. In the above publications it is pointed out that the trend at present is towards the use of a zeolite support material, and the catalyst which in the art is referred to as zeolite of the ZSM-5 type, is often stated as a preferred material. When properly formulated, this zeolite material itself appears to have a significant effect on the dehydrocyclodimerization reaction. For more information on zeolite-containing dehydrocyclodimerization catalysts, see European Patent Application no. 832011422.9. Nevertheless, it is preferred to use a metallic component in the catalyst system to increase the activity and / or selectivity of the catalyst. The preferred metallic component is gallium, as disclosed in U.S. Pat. Hydrogen-producing reactions are usually favored by lower pressures, and overpressures lower than 483 kPa at the outlet of the reaction zone are strongly preferred.

The drawing illustrates the preferred reactor system for carrying out the invention. One skilled in the art will appreciate that the flow chart has been simplified by omitting a number of equipment units, such as heat exchangers, process control systems, pumps, condenser and cooler systems for the fractionation columns, etc., which are not necessary to understand the process. It will also be easily seen that the flow chart shown in the drawing can be modified in many respects, without departing from the spirit of the basic invention. For example, the number of heat exchangers shown in the drawing is kept to a minimum to facilitate the overview.

One skilled in the art will appreciate that the choice of heat exchange methods used to achieve necessary heating and cooling at various points in the process may vary widely. In a process that is as complex as this, there are many possibilities for indirect heat exchange between different process streams. Depending on the location of the plant for the present process and the circumstances in general, it may be desirable to carry out heat exchange with steam, hot oil or process streams from process units not shown in the drawing.

Referring now to the drawing, a feed stream comprising propane is introduced into the process through line 1 and mixed with a recycled stream of aromatic hydrocarbon fed through line 2. This mixture of the supplied propane and the recycled aromatic hydrocarbons flows through line 3 and is mixed. with recycled hydrogen-rich gas from pipeline 4. The mixture of hydrogen, added propane and recycled aromatic hydrocarbons flows into reaction zone 6 through pipeline 5. In the reaction zone, these materials are contacted with a solid dehydrodimerization catalyst under appropriate reaction conditions to effect conversion of at least a substantial portion of the added propane to aromatic. hydrocarbons. Also during this reaction a significant amount of hydrogen is formed and, as by-products of the reaction, a much smaller amount of methane and ethane.

Therefore, a reaction zone effluent is formed which is taken out through pipeline 7, and which consists of a mixture of residual amounts of the propane supplied, the light by-products methane and ethane, the aromatic hydrocarbons desired as product, recycled aromatic hydrocarbons and hydrogen. The effluent from the reaction zone is passed through a heat recovery heat exchanger which in typical cases will be arranged into the reaction zone itself, and it is then passed through a cooling zone, e.g. through a heat exchanger 8. The resulting cooling must be sufficient to cause partial condensation of the effluent from the reaction zone, so that at least 85 mol% of C⁺the hydrocarbons condense.

The resulting mixture phase effluent stream is introduced into vapor-liquid separation zone 9. A light gas stream consisting mainly of propane, hydrogen and light hydrocarbons is taken out of the separation zone 9 through pipeline 10. Part of the gas stream in pipeline 10 is taken out of the process through pipeline 11 as a net gas flow at a flow rate that offsets the formation of hydrogen and / or light hydrocarbons in the process. The remaining part of the gas stream flows through pipeline 12 and is introduced into pipeline 4 to be returned to the reaction zone as a recycled gas stream. A gas purification zone may, if desired, be provided in either pipeline 10 or pipeline 12 to remove hydrocarbons from the gas stream and increase the hydrogen concentration in the net gas stream or in the recycled gas stream.

The liquid phase condensate collected in the vapor-liquid separation vessel 9 is taken out through line 13 and introduced into a first fractionation column 14 which is operated as a strip.

pecolumn, to remove unwanted light hydrocarbons and dissolved hydrogen from the supplied condensate stream. The composition of the material taken out over the top of the column 14 will depend on the desired composition of the top stream from column 18 immediately on the downstream side. The stripping column 14 is preferably operated as a pentane distillation column 15, so that substantially all of the amount of C to produce benzene of high purity suitable for use as a petrochemical feedstock. However, if it is only desired to use the prepared aromatic hydrocarbons as a mixture component for motor fuel, the C the stripping column will be operated as a propane distillation column or as a butane distillation column. In any case, the separated light hydrocarbons in the top stream from column will be taken out through pipeline 15, and this stream can, if desired, be divided into a recycled part which is passed through pipeline 16, and a light product stream which is taken out of the process through pipeline. 17. It is also possible to recycle the entire light fraction from pipeline 15 through pipeline 16.

The net bottom stream from column 14 will preferably contain all of them ⁺hydrocarbons which were present in the condensate 55 which was fed to column 14. This stream is passed through pipeline 19 to a column which is referred to herein as a benzene column. The function of the benzene column is to collect practically all the inflowing benzene in a net overhead stream which is taken out through pipeline 27. At least a part of this benzene is taken out through pipeline 30 as a first product stream from the process. The remaining small amount of benzene is passed through line 31 and into line 2 for recycling to the reaction zone. The net bottom stream from column 18 is taken out through pipeline 20 and contains practically all of the C⁺hydrocarbons fed to column 18. This bottom₁₀ current is passed into a toluene column 21. The function of the toluene column is to collect virtually all incoming toluene in a net overhead stream taken out through pipeline 26. Preferably, all the toluene flowing through line 26 is taken out of the process through line 28 as a product. ₁₅ current. Alternatively, a portion of the toluene may be recycled through an optional pipeline 29 to be introduced from there into the reaction zone through pipeline 2.

The net bottom stream from column 21 will contain virtually all of the Cs_Q⁺hydrocarbons introduced into toluene ₂₀ columns. This net bottom current is fed via pipeline 22 into a C_o⁺product column 23. This column has the function o to separate from the inflowing hydrocarbons a Cg-rich net overhead stream which is taken out through pipeline 25, and which contains aromatic Cg hydrocarbons, such as ortho-, meta25 and paraxylene. The remaining aromatic C8 hydrocarbons which have been formed in the reaction zone are taken out of the process as a net bottom stream through pipeline 24. This method of carrying out the fractionation will normally be used when it is desired to recover a relatively pure stream of the C8 aromatics.

_}0 This will be the case for most petrochemical applications. However, the final product stream from the process could be the bottom stream from the toluene column 21, in which case it would be a mixture of aromatic C⁺-hydrocarbons.

It is believed that one skilled in the art of designing petroleum refining processes and petrochemical processes will be able to determine favorable operating conditions, equipment designs and operating procedures for the present process using standard process design methods after now knowing the general process scheme. It is known in the art that it is undesirable to introduce compounds which may tend to freeze or otherwise form solids into the low temperature part of the process. For this reason, a drying zone can be used. The function of this drying zone will be to prevent water from flowing into the low temperature equipment used to obtain an outgoing hydrogen gas stream of high purity. The drying zone is required primarily to remove the small amount of water that may be dissolved in a feed stream to the process and / or water that may be present on regenerated catalyst introduced into the process or that has been discharged from stripping steam used to seal catalyst passages, etc. .

The vapor-liquid separation zones used in the process preferably comprise a suitably sized vertically oriented container provided with a mist removal pad or other device for removing entrained liquid arranged at the upper end. The fractionation zones used in the process preferably comprise a single multi-stage fractionation column which has holes of the perforated plate type and which is of a relatively conventional design. For example, a suitably designed column with 40 bottoms can function as a stripping column 14. If desired, however, multi-column fractionation zones of one or the other design can also be used to recover specific product streams. The fractionation zone can e.g. is designed so that an initial division of the effluent stream is made, or so that a heavy hydrocarbon is separated in one of the first columns. Suitable fractionation zones can easily be designed by a person skilled in the art. The operating conditions required in the fractionation zones depend on the compounds to be separated and on the desired separation. As used herein, the term rich is intended to indicate a concentration of the compound or class of compounds in question which is higher than 65 mol%.

The process of hydrocarbon conversion according to the invention can thus be carried out by passing a feed stream comprising an aliphatic C₃or β-hydrocarbon and a first process stream comprising benzene, into a dehydrocyclodimerization reaction zone maintained at dehydrocyclodimerization conditions and containing a solid catalyst comprising a zeolite, and forming a reaction zone effluent stream comprising benzene, toluene and xylenes; separates the reaction zone effluent stream in a series of steps comprising partial condensation and fractional distillation to form a product stream comprising xylenes and a second process stream comprising benzene; and recycling at least a portion of the second process stream to the reaction zone as the above-mentioned first process stream.

In the method according to the invention, e.g. benzene to the reaction zone, in which a light, aliphatic hydrocarbon is converted to benzene and other aromatic hydrocarbons. Preferably, the benzene is recycled from the reaction zone effluent, as shown in the drawing. However, there are conceivable cases where it would be economically advantageous to supply benzene from another source to the reaction zone rather than separating benzene from the reaction zone effluent. As also shown in the drawing, it may also be desirable to recycle toluene to the reaction zone in admixture with the added hydrocarbons. It is ensured that the amount of benzene fed to the reaction zone together with the feed stream will give a concentration of aromatic hydrocarbon in the feed stream which is in the range of 1.0 to 10 mol%. Preferably, the concentration of the aromatic hydrocarbon in the feed stream is from 2.5 to 8 mol%.

The method according to the invention entails two separate advantages. First, experiments have shown that the process according to the invention achieves a surprising increase in the conversion of a light, aliphatic feedstock into aromatics per liter. review. Second, the process of the invention can be used to change the product distribution of alkyl aromatic hydrocarbons by increasing the amount of alkylbenzenes formed relative to the amount of benzene. These C_o-alkylbenzenes are normally a preferred product in a dehydrocyclodimerization process. For example, they have higher octane numbers than benzene. They can also constitute a more desirable component of a motor fuel with a fuel boiling range in cases where they are desired to reduce the petrol content of the petrol to a minimum. In a petrochemical complex, the value of C8 aromatics such as xylenes is usually significantly greater than the value of benzene. The process according to the invention will therefore entail significant economic advantages when the product is to be used as motor fuel or as a raw material in the petrochemical industry.

The following results of an experiment carried out on a semi-technical scale illustrate the advantages obtained by using the method according to the invention. A high purity propane stream was passed through a dehydrocyclodimerization reaction zone which was operated under the above preferred conditions and which contained a catalyst comprising gallium on a support comprising a zeolite. Conversion of 60% by weight of the propane to aromatic hydrocarbons was achieved. In a comparative example, where the present process according to the invention was used, 6 mol% of benzene was added to the feed stream to the plant, while the other conditions remained unchanged. The result was an unexpected increase in the average propane conversion to 66% by weight, without any loss in the total aromatic selectivity, calculated on a weight basis. It was also found that the total number of moles of aromatics formed by the process was smaller, but that more alkylbenzenes were formed when benzene was added simultaneously.