EP0009236B1 - Hydrocarbon cracking process - Google Patents

Abstract

Hydrocarbons are subjected to hydrogenation, pressure reduction and separation into liquid and gaseous fractions. The gaseous fractions are purified and desulfurized. Hydrogen-rich components of the gaseous fraction are returned to the hydrogenation stage. Hydrocarbon-rich components of the gaseous fraction and components of the liquid fraction are cracked and fractionated. Residue is partially oxidized with oxygen and steam. Gas produced by the partial oxidation is desulfurized and separated, and hydrogen is returned to the hydrogenation stage. A polymer free fraction of the residue is returned to the feed stock and to the hydrogenation stage, a heavy residue component of the initial liquid fraction is partially oxidized with the residue.

Description

The invention relates to a process for splitting hydrocarbons, in which the hydrocarbons are first hydrogenated and then thermally split.

For the production of olefins, light hydrocarbons such as ethane or propane or hydrocarbon mixtures with a boiling point below 200 ° C., for example naphtha, are particularly suitable as inserts for thermal cracking. They lead to a high yield and result in few undesirable by-products.

However, since there is a great need for olefins which can lead to a shortage or price increase of these cheap inserts, attempts have been made for some time to develop processes which also allow the inexpensive utilization of a higher-boiling feed material.

The use of higher-boiling inserts basically has the problem that the olefin yield decreases and liquid fission products occur, the proportion of which increases sharply with the boiling range of the insert. The liquid cleavage products are generally separated into a fraction boiling below 200 ° C. and a fraction boiling above 200 ° C. The lower-boiling fraction is a high-octane fuel and contains valuable components such as benzene, toluene and xylene. The fraction boiling above 200 ° C, however, forms an undesirable product that contains highly condensed aromatics, polymeric compounds and sulfur compounds. The fraction of this fraction, hereinafter referred to as residue, in the cleavage of naphtha is in the range from about 1 to 5% by weight of the total products and increases when using gas oil in the order of 30% by weight and for even heavier uses such as vacuum gas oil or Crude oil or crude oil residues to even higher values. The sulfur contained in the feed material accumulates in the residue in such quantities that the combustion of this fuel without the addition of low-sulfur fuels leads to an unacceptably heavily polluted exhaust gas. Mixing with low-sulfur fuels is associated with further problems, however, because the residue is only miscible with crude oil distillates and can therefore only be partially blended with them. Another undesirable property of this fraction is that it can only be stored and transported to a limited extent.

The production of olefins by cleavage of hydrocarbon mixtures with a boiling range above 200 ° C, such as gas oil or vacuum gas oil, is not economically justifiable if no precautions are taken to reduce the amounts of the residue or if this fraction is not otherwise used commercially can be.

To solve this problem, it is already known from DE-A-2 164 951 to catalytically hydrogenate the feed material in the presence of hydrogen before the thermal cleavage. The hydrogenating pretreatment leads to a reduction in the content of polyaromatic compounds which are essentially responsible for the formation of the residue. In addition, the feed material is desulfurized. When this known method is used, the amount of the liquid fission products boiling above 200 ° C. is reduced compared to methods without prior hydrogenation, but considerable amounts of residue still occur due to the higher-boiling feedstocks used.

In addition, it has already been proposed (DE-A-2 805 721) to work up the cleavage products boiling above 200 ° C. by separating the polymeric constituents of this fraction, which make up about 20% by weight. While the polymer-free fraction is a usable heating oil, the polymer residues, on the other hand, form an economically low-valued product.

Furthermore, from FR-A-1 318 919 a process for the hydroconversion of hydrocarbons is known, the overall course of which also involves thermal fission. Regarding the thermal cracking residue, without mentioning any details, it is only mentioned that it can be used as fuel or for the production of hydrogen. Furthermore, from US-A-3380910 a method is known, according to which a heavy crude oil distillate is prepared by hydrocracking and a light distillate by hydrodesulfurization. From this process, which does not provide for thermal cleavage, it is known to work up a heavy residue liquid, separated by distillation from a hydrocracking product, by partial oxidation to hydrogen. However, this process does not provide any indications of the favorable design of a process for the production of olefins.

The invention has for its object to design a method of the type mentioned in such a way that no products are obtained whose boiling range is above that of gasoline.

This object is achieved in that at least part of the residue of the thermal cleavage is converted into a gas mixture by partial oxidation.

In the course of the process according to the invention, a gas mixture consisting essentially of carbon oxides and hydrogen is generated from the residue fraction, which - optionally after cleaning and / or Disassembly into its individual components - can be used for a number of different processes, for example as a reducing gas, synthesis gas or heating gas. The low-valued residue of the thermal fission is thus used to produce a gas mixture that can be used in a variety of ways and is economically interesting.

The partial oxidation can be carried out with air, with oxygen or with other oxygen-enriched gases or gas mixtures. In addition, it is advantageous to add water vapor as an additional gasification agent.

In a favorable further development of the method according to the invention, after its separation from the residue fraction, only the polymeric constituents of this fraction are converted into the gas mixture. This measure increases the economy of the overall process, since only a small amount of about 20% of the residue fraction gets into the partial oxidation, which results in the use of smaller components and the provision of smaller amounts of oxygen-containing oxidizing agent and possibly water vapor. The polymer-free portion of the residue fraction can either be used directly as heating oil or reused as a feed for the hydrogenation.

If the residue fraction freed from the polymeric compounds is reintroduced into the hydrogenation stage, particularly high yields can be achieved with regard to the desired process products, because the purified residue fraction in the hydrogenation and subsequent thermal cleavage gives products similar to the fresh feed.

Hydrogen is required for the reactions taking place in the hydrogenation stage. For this purpose, the hydrogen generated during thermal cracking can be used directly after it has been separated from the other cracked products. In this way, however, only about 10 to 30% of the hydrogen requirement can generally be met. To further cover the hydrogen requirement from direct process products, a hydrogen-rich fraction is therefore separated from the gas mixture formed in the partial oxidation in a further embodiment of the invention and fed to the hydrogenation. With such a procedure, the hydrogen requirement to be covered by an external supply is particularly low. In addition, it is advantageous that part of the gas mixture can be recycled in the process itself, so that no precautions are required for the export of gas to separate plants. The residual gas resulting from the separation of the gas mixture can be used, for example, as heating gas.

Since the hydrogenation is generally carried out with an excess of hydrogen, the hydrogenation product is a liquid fraction of hydrocarbons and a gaseous fraction which consists essentially of hydrogen and also contains light hydrocarbons and gaseous impurities such as hydrogen sulfide. While the light hydrocarbons from the gaseous fraction represent a favorable application for the thermal cracking, the excess hydrogen is returned to the hydrogenation stage after it has been separated off on a recycle base. For this procedure, a gas separation is required, in which the hydrogen and the impurities are separated from the light hydrocarbons. Since the gas mixture obtained in the partial oxidation also has to be subjected to decomposition in order to separate the hydrogen for the hydrogenation, it is advantageous in a further development of the process according to the invention to digest this gas mixture together with the gaseous fraction obtained after the hydrogenation in order to reduce the costs for investment and operation of a process:; lower the plant accordingly.

The method according to the invention is explained in more detail below on the basis of two exemplary embodiments, which are shown schematically in the figures. Both figures show a procedure in which a heavy hydrocarbon mixture is first hydrogenated and then thermally split. The heavy residues resulting from the process are converted into a hydrogen-rich gas by means of partial oxidation, the hydrogen being fed to the hydrogenation stage after it has been cleaned.

The feed material, for example a vacuum distillate, is fed to a hydrogenation stage 2 via line 1. The hydrogenation can be carried out using conventional sulfur-resistant catalysts with elements of VI-VIII. Subgroup of the periodic table or mixtures thereof in elemental, oxidic or sulfidic form on a support made of silica, silica / alumina or on a zeolite basis. Favorable hydrogenation conditions are present when a pressure between 10 and 300 bar, preferably between 15 and 150 bar, at temperatures between 100 and 500 ° C, preferably between 200 and 400 ° C, and at an hourly space velocity between 0.2 and 10 LI / h is worked.

The hydrogen required for the hydrogenation is fed to the hydrogenation stage 2 via line 3. The hydrogenation product passes via line 4 to the expansion valve 29, in which it is expanded to the pressure of the thermal cleavage, preferably to a pressure between 1 and 4 bar. The hydrogenation product then flows into a separator 5, where it is broken down into a gaseous fraction consisting essentially of hydrogen and into a liquid hydrogenation product. The liquid fraction reaches a fractionation device 6, in which a heavy residue is separated from the hydrogenation product and drawn off via line 7 is withdrawn via line 8 while a lighter fraction boiling in the gasoline range is withdrawn.

This fraction reaches the thermal cracking stage 9 and is split there into an olefin-rich gas mixture. The cleavage is advantageously carried out in a tube furnace at temperatures between 700 and 1000 ° C., a residence time between 0.01 and 1 sec and a steam dilution of 0.2 to 4.0 kg of water vapor per kg of hydrocarbons. The hot cracked gas is then cooled and fed to a decomposition unit 10. Here the individual fission products are isolated and drawn off separately from one another, which is indicated by the lines 11, 12, 13. The pyrolysis residue which boils over 200 ° C. during the disassembly is drawn off via line 14 and fed into a device 15.

In this device 15, the residues from lines 7 and 14 are converted into a hydrogen-rich gas mixture by means of partial oxidation. Water vapor is supplied via line 16 and air or oxygen via line 17 as the gasifying agent.

The raw gas formed in the partial oxidation is withdrawn via line 18. It consists essentially of hydrogen and carbon monoxide if oxygen is supplied via line 17, or of hydrogen, carbon monoxide and nitrogen if air is used as the gasifying agent via line 17. The raw gas also contains impurities, especially hydrogen sulfide. The gas is therefore subjected to desulfurization 19, the separated hydrogen sulfide being removed via line 30.

The desulfurized gas is then fed via line 20 to a separation unit 21, in which the hydrogen is separated off. The decomposition unit 21 can be, for example, a pressure swing adsorption system working with molecular sieves. The separated hydrogen is withdrawn via line 3 and returned to the hydrogenation stage 2. To cover the hydrogen requirement for the hydrogenation, further hydrogen is supplied via line 22, which may at least partially come from the decomposition stage 10. The residual gas consisting essentially of carbon monoxide or, in the case of partial oxidation with air, of carbon monoxide and nitrogen is withdrawn via line 23.

The gaseous fraction obtained in the separator 5 consists essentially of excess hydrogen from the hydrogenation 2 and, in addition, also contains light hydrocarbons which have formed during the hydrogenation, and also impurities, in particular hydrogen sulfide. This fraction is fed via line 24 into a purification stage 25, in which the light hydrocarbons are separated off and fed to the thermal cracking 9 via line 26. In addition, hydrogen sulfide is separated off in this purification stage and drawn off via line 27. The cleaned gas is then introduced via line 28 into the cleaning stage 21, where it is subjected to a further cleaning together with the gas mixture supplied via line 20.

The method shown in FIG. 2 differs from that of FIG. 1 in three points.

The first difference is that the liquid hydrogenation product obtained in the separator 5 is not disassembled, but is led completely into the thermal cleavage 8 via line 31.

The second difference from the method in FIG. 1 is that the residue fraction which accumulates in the decomposition unit 10 and boils above 200 ° C. is not completely fed to the partial oxidation 15. Instead, this fraction drawn off via line 32 is fed into a treatment unit 33, in which the polymeric constituents of the fraction are separated off, for example by solvent extraction. The polymer-free fraction is withdrawn via line 34 and returned to the hydrogenation 2 together with fresh feed. The polymeric constituents of the heavy fraction are drawn off via line 35 and fed to the partial oxidation 15.

The third difference from the method in FIG. 1 consists in the joint processing of the gas obtained in the partial oxidation and the gaseous fraction from the separator 5 in a cleaning unit 36.

It is not necessary that the three differences in the procedure described be carried out together. Rather, it is also possible to realize each of these differences individually or in any combination. The preferred method of operation is based on the requirements in each specific case, which may depend not only on the choice of feedstock used, but also on the desired process products and external operational requirements.

Claims (5)

1. A process for cracking hydrocarbons, in which the hydrocarbons are first hydrogenized and subsequently thermally cracked, characterised in that at least a part of the residue of the thermal cracking is converted into a gas mixture by a partial oxidation.

2. A process as claimed in Claim 1, characterised in that the partial oxidation is carried out in the presence of steam.

3. A process as claimed in Claim 1 or Claim 2, characterised in that a separation of polymeric constituents of the residue from the thermal cracking is carried out, that the polymeric constituents are converted into the gas mixture, and that the remaining constituents of the residue form the thermal cracking are recycled to the hydrogenation step.

4. A process as claimed in one of Claims 1 to 3, characterised in that a fraction which is rich in hydrogen is separated from the gas misture and fed to the hydrogenation step.

5. A process as claimed in Claim 4, characterised in that the hydrogenation product is separated into a liquid and a gaseous fraction, that a hydrogen-rich fraction and a hydrocarbon- rich fraction are separated from the gaseous fraction, that the hydrogen-rich fraction is fed to the hydrogenation step and the hydrocarbon- rich fraction is fed to the thermal cracking step, and that the separation of the gas mixture and of the gaseous fraction of the hydrogenation product are effected together.

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