EP0009807A1 - Heavy hydrocarbons cracking process - Google Patents

Abstract

To obtain olefins by the thermal cracking of hydrocarbons, e.g., vacuum gas oil, by hydrogenation and subsequent steam cracking, an intermediate fractionation of the hydrogenate is provided so that the light fraction enriched in branched isomers can be used as fuel and the heavy fraction only is subjected to the steam cracking.

Description

The invention relates to a process for splitting heavy hydrocarbon mixtures by hydrogenation and subsequent thermal splitting.

Light feedstocks are required for the splitting of hydrocarbons for the production of olefins, i.e. Hydrocarbons with a boiling point below 200 ° C, such as naphtha, are particularly suitable. They lead to high splitting yields and result in few undesirable by-products.

The great need for such inexpensive slit inserts can lead to a shortage or increase in the price of these materials. For this reason, attempts have been made for some time to develop processes which also permit the inexpensive utilization of a higher-boiling feedstock.

The utilization of higher-boiling inserts generally leads to lower yields of valuable fission products, while at the same time an increasingly difficult-to-use hydrocarbon fraction boiling above 200 ° C is obtained. In addition, other difficulties arise from the fact that higher-boiling inserts lead to increased coke and tar formation in the cracking plant. These products are located on the walls of the pipe elements, such as pipes and heat exchanger, deposit, cause a deterioration in heat transfer and also lead to narrowing of the cross-section. It is therefore necessary to remove these deposits more frequently than when using light hydrocarbons.

To solve this problem, DE-OS 21 64 951 describes a process in which the insert is catalytically hydrogenated before it is thermally cleaved. This pretreatment reduces the content of aromatic compounds in the feed material, which essentially lead to the undesired cleavage products. In addition, desulphurization of the feed takes place.

The invention has for its object to design a method of the type mentioned so that it can be operated under particularly favorable economic conditions.

This object is achieved in that the hydrogenation is carried out under conditions in which only part of the hydrocarbon mixture is converted to low-boiling products, whereupon the hydrogenation product is broken down into a light and a heavy fraction and only the heavy fraction is fed to the thermal cracking.

In the hydrogenation of a heavy hydrocarbon mixture, not only are the heavy constituents, in particular the polyaromatic compounds, hydrogenated or hydrogenated, but also isomerization of n-alkanes and n-alkyl chains takes place. In the hydrogenation, on the one hand, a product suitable for thermal cleavage is first produced, but on the other hand, in the isomerization, which increases with increasing hydrocracking sharpness during the hydrogenation, products are formed at the same time which only lead to slight increases in the cleavage yields and because of their high hydrogen consumption limit the economics of the process.

It was therefore investigated whether there was a difference when the hydrogenation product was broken down into fractions Boiling ranges in the individual fractions allow a distribution of the components to be achieved in which one fraction has an improved use for thermal cracking compared to the undigested hydrogenation product and the other fraction has favorable properties for another use.

It was found that the degree of isomerization of the higher-boiling components of the hydrogenation product is unexpectedly low compared to that of the lower-boiling components.

After the more isomerized low-boiling components have been separated off, the remaining heavy boiling cut during thermal cleavage leads to surprisingly high olefin yields which are equivalent to those of naphtha.

The process according to the invention has the advantage, among other things, that the thermal cleavage can be carried out under particularly favorable conditions. While in known processes of DE-OS 21 64 951 the entire hydrogenation product, which comprises a relatively wide boiling range, reaches the thermal cleavage, a much narrower boiling cut is used for this in the process according to the invention, as a result of which the cleavage conditions can be better optimized.

In addition to the high yield of valuable products in the thermal splitting of the heavy boiling section, the proportion of pyrolysis heating oil which is difficult to utilize and boils above 200 ° C is surprisingly low. In all experiments with hydrogenated vacuum gas oil cuts, it was below 20% by weight of the fission products and thus below the range of a conventional fission of atmospheric gas oil. By contrast, thermal decomposition of an unhydrogenated vacuum gas oil produces up to 40% by weight of pyrolysis heating oil.

The cause of the high cleavage yields is seen in the chemical structure of the hydrogenated vacuum gas oil boiling cut, which consists essentially of low isomerized paraffins and naphthenes, both of which produce high olefin yields to lead. In order to keep the proportion of these components in the high-boiling fraction of the hydrogenation product as large as possible, it is advantageous to carry out the hydrogenation under mild conditions. This is because only a small part of the high-boiling naphthenes and paraffins is isomerized and thus largely left unchanged, while mainly naphthenes are produced from the polyaromatic compounds, which account for up to 45% by weight in vacuum gas oil, for example.

The thermal cleavage of the fraction, which is heavier compared to conventional splitting inserts, does not pose any particular technical problems. However, it is necessary to increase steam dilution compared to conventional systems.

The separated low-boiling components of the hydrogenation product, which are not fed to the thermal cracking, consist of gasoline fractions which are suitable as low-sulfur gasoline or turbine fuels or can be blended with other refined products suitable as fuel. In addition, if these components are not already left in the heavy fraction, middle distillates are obtained which meet the requirements for heating oil of the specification EL and diesel fuel. These fractions are particularly valuable due to their low sulfur content. They can also be blended with other sulfur-rich products to make them economically viable.

Of course, the quality of the hydrogenation product depends essentially on the reaction conditions of the hydrogenation. It is favorable to control the hydrogenation in such a way that the undesired polyaromatic compounds of the feed-carbon hydrogen mixture are largely hydrogenated, whereas the content of monoaromatic compounds is hardly changed. Since most of the monoaromatics get into the low-boiling fraction when the hydrogenation product is broken down, its motor properties are improved. In addition, such. Process management does not unnecessarily use hydrogen for the monoaromatic hydrogenation.

A favorable hydrogenation product results, for example, from hydrogenation under mild conditions, i.e. at temperatures between 350 and 400 ° C, a pressure between 80 and 150 bar and at space velocities of more than 1 1/1 catalyst material and hour when using conventional hydrogenation or hydrocracking catalysts. When using gas oil as the feed hydrocarbon mixture, even space velocities of more than 2 1/1 catalyst material and hour can be maintained.

A hydrogenation carried out under such conditions leads to a low hydrogen consumption which, for example in a vacuum gas oil hydrogenation when 40% by weight of the hydrocarbons are converted to low-boiling components, is below 150 Nm ³ per ton of hydrocarbon mixture used. Favorable hydrogenation conditions are generally present when the hydrogen consumption is between 100 and 250 Nm3 per ton of hydrocarbon.

With such a hydrogenation, part of the paraffins and naphthenes contained in the hydrocarbon mixture are always reacted, i.e. isomerized or hydrolyzed. The majority of these products get into the low-boiling fraction during the decomposition of the hydrogenation product and improve their motor properties due to the isomerization.

It has proven to be advantageous to include in the fraction with the lower-boiling components either those below 200 ° C. or the components boiling below 340 ° C. If the cut is carried out at 200 ° C, a gasoline fraction results, which in turn can be broken down into a light gasoline and a heavy gasoline fraction. The cut below 340 ° C also contains kerosene and heating oil of the specification EL or diesel fuel.

The method according to the invention is explained in more detail below using a few examples.

In all cases, a vacuum gas oil was used with a Boiling range between 340 and 580 ° C, whose density (at 15 ° C) was 0.913 g / ml. It is composed of 85.78% by weight of carbon, 12.14% by weight of hydrogen, 1.94% by weight of sulfur and 0.14% by weight of nitrogen. 47.8% by weight of the hydrocarbons were present as paraffins and naphthenes, 19.2% by weight as monoaromatics and 33.0% by weight as polyaromatics. The proportion of asphaltenes was below 0.05% by weight.

Example 1:

First, for comparison purposes, a sample of the vacuum gas oil was thermally cracked without hydrogenation. With a steam dilution of 1 kg water vapor per kg vacuum gas oil and with a dwell time of 0.2 seconds. it was implemented in a can. The outlet temperature was 830 ° C. This thermal cleavage forms a cleavage product which contains 9.3% by weight of methane, 18.5% by weight of ethylene and 10.3% by weight of propylene. 33.5% by weight of the vacuum oil used was obtained as pyrolysis heating oil boiling above 200 ° C.

Example 2:

A vacuum gas oil with the same properties as in Example 1 was hydrogenated at 380 ° C. under a pressure of 100 bar and at a space velocity of 1.2 liters per liter of catalyst material and hour. For the hydrogenation, a catalyst was used which contained nickel and molybdenum on an acidic support as the hydrogenation-active components.

In a hydrogenation, 145 Nm ^{3 of} hydrogen per ton of vacuum gas oil used were converted.

A hydrogenation gave a product which contained 59.7% by weight of components boiling above 340 ° C. In addition, 40.3% by weight of products boiling below 340 ° C. were obtained, which were separated off by distillation and then further broken down. This (based on the total hydrogenation product), 2.0% by weight H ₂ S, 0.2% by weight NH ₃ , 0.3% by weight gaseous hydrocarbons with 1-4 carbon atoms and 37.8% by weight liquid Hydrocarbons. The liquid products were in a light petroleum fraction (C ₅ hydrocarbons up to 82 ° C), the 0.7 wt.% of the hydrogenation product, into a heavy gasoline fraction (82-180 ° C), which made up 6.4% by weight and into a boiling between 180 and 340 ° C fraction of kerosene and desulfurized heating oil EL (30.7% by weight). The essential properties of these three fractions are given in Table 1.

The fraction boiling above 340 ° C, the properties of which are given in Table 2, column (1), was used as an insert for the thermal cleavage. The cleavage conditions were the same as in Example 1. The result was a cleavage product which contained 9.2% by weight of methane, 26.9% by weight of ethylene and 14.4% by weight of propylene as valuable components. The proportion of the residue fraction boiling above 200 ° C. was 18.2% by weight.

Example 3:

The same feed as in Example 1 was hydrogenated under conditions that were somewhat sharper than those in Example 2. This resulted in a fraction boiling above 3400C, the properties of which are given in column (2) of Table 2.

This fraction was thermally cleaved under the same conditions as in the previous examples. The cleavage product contained 9.1% by weight of methane in valuable components, 32.0% by weight of ethylene and 17.0% by weight of propylene. The residue fraction boiling above 200 ° C made up only 7.4% by weight of the cracked gas input.

Claims (5)

1. A process for splitting heavy hydrocarbon mixtures by hydrogenation and subsequent thermal cracking, characterized in that the hydrogenation is carried out under conditions in which only part of the hydrocarbon mixture is converted to low-boiling products, whereupon the hydrogenation product into a light and a heavy fraction disassembled and only the heavy fraction of the thermal cleavage is fed. 2. The method according to claim 1, characterized in that the hydrogenation is carried out under conditions in which between 100 and 250 Nm3 of hydrogen are reacted per ton of hydrocarbon. 3. The method according to claim 1 or 2, characterized in that the hydrogenation product is broken down into a fraction sometimes 340 ° C and in a fraction with components boiling above 340 ° C. 4. The method according to claim 1 or 2, characterized in that the hydrogenation product is broken down into a fraction with less than 200 ° C and a fraction with components boiling over 200 ° C. 5. The method according to any one of claims 1 to 4, characterized in that the light fraction is fed to a further treatment.