CS231688B1 - A method for pretreating a hydrocarbon feedstock for pyrolysis by solvent extraction of aromatic hydrocarbons - Google Patents

Abstract

The invention relates to a method of pretreatment of hydrocarbon feedstock consisting in the extraction of aromatic hydrocarbons with a liquid solvent, in which the oil fraction boiling in the range of 250 to 550 °C is extracted with a mixture of solvents with a composition of 50 to 99.9 wt. % methanol, 0.05 to 50 wt. % isobutanol and 0.05 to 20 wt. % water at a temperature of 20 to 55 °C, atmospheric pressure and a weight ratio of solvent to feedstock of 0.5 to 6, and the feedstock is washed with water. The feedstock thus treated can be directly pyrolyzed without the need for further treatment by hydrotreating or mixing with the original feedstock containing a higher concentration of sulfur.

Description

The invention relates to a method of pretreatment of hydrocarbon raw material for pyrolysis consisting in the extraction of aromatics from the hydrocarbon fraction intended for pyrolysis.

It is known that by reducing the content of aromatic hydrocarbons, especially polynuclear ones, the yields of desired products obtained by pyrolysis of hydrocarbon raw materials can be increased. Pyrolysis here means the process of thermal decomposition of hydrocarbons in the presence of steam, with olefins being obtained as the main product.

Reducing the content of aromatics also has an effect on reducing the rate of pyrolysis carbon formation. The content of aromatic hydrocarbons in the oil fraction can be reduced by hydrogenation or extraction. Both methods have already been verified for the purpose of preparing the raw material for pyrolysis. In operation, hydrogenated atmospheric gas oil with the end of the distillation range below 370 °C is pyrolyzed, which provides results comparable to the pyrolysis of gasoline. The extraction of aromatics from this fraction with dimethylformamide and water was verified in the laboratory. When the content of aromatics in the raw material was reduced to 5 to 10% by weight, the yield of ethylene increased by 1 to 1.5% by weight.

As a result of the increase in the world price of oil, more and more attention is being paid to its more efficient use. One option is to pyrolyze pretreated vacuum gas oil with a distillation range of 300 to 550 °C. Pretreatment consists mainly of removing aromatic hydrocarbons. A number of processes have been developed that allow for the selective hydrogenation of mainly polynuclear aromatics, while at the same time there is minimal undesirable isomerization of hydrocarbons, and the reaction product is separated by distillation and the fraction boiling above 340 °C, which constitutes about 60% by weight of the original raw material, is pyrolyzed. Hydrogen consumption for

- 2 231 688 hydrogenation of aromatics is however high and amounts to about 225 m^/m^ of feedstock, therefore it is not always possible to cover the hydrogen consumption only by its production in the olefin unit. Depending on the amount of vacuum gas oil processed, it is necessary to supplement the required amount of hydrogen by further production, e.g. by partial oxidation or steam reforming of methane from the olefin unit, or to take hydrogen from other sources. During hydrogenation, which usually requires a higher hydrogen pressure than is commonly available in the olefin unit and therefore requires compression from about 3.5 to 25 MPa, undesirable deep desulfurization occurs. It is therefore necessary to mix the dehydrogenated feedstock with the sulfur fraction, so that the sulfur content is above 500 ppm by mass. Therefore, trying to replace the hydrogenation of aromatics by their extraction seems to be another option for using this fraction for pyrolysis. For the extraction of aromatics from these high-boiling fractions, it is generally not possible to use a solvent suitable for reformate, atmospheric gas oil, e.g. dimethylformamide has a comparable density to this fraction and therefore only very poor separation of the raffinate, i.e. depleted, and extract phases occurs. Furfural and phenol are most often used for the extraction of vacuum gas oils. The aim of these processes is to improve the quality of the raw material - the raffinate, e.g. viscosity index. The residual aromatics can be mixed into oil fractions, hydrogenated or used for the production of carbon black or needle coke. The disadvantage of the above-mentioned extraction processes, e.g. in comparison with dimethylformamide, is the need to work with a higher temperature in the extractor, about 90 °C, and solvent-hydrocarbon separation. When using extraction to prepare the feedstock for pyrolysis, it is also essential that the solvent can be easily separated from the raffinate phase, e.g., by washing with water, and that any traces of solvent do not have an undesirable effect on the pyrolysis.

All the above disadvantages are eliminated by a pretreatment method, the essence of which lies in the fact that the hydrocarbon raw material for pyrolysis processing is treated by solvent extraction of aromatic hydrocarbons, in which the oil fraction boiling in the range of 250 to 550 °C is extracted with a mixture of solvents with a composition of 50 to 99.9 wt. % methanol, 0.05 to 50 wt. % isobutanol and 0.05 to 20 wt. % water, at a temperature of 20 to 55 °C, atmospheric pressure and a mass fraction of 1000-15000 m3.

- 3,231,888 solvent to feed ratio 0.5 to 6 and the raffinate is washed with water.

The solvent mixture preferably has a composition of 90 to 99% by weight methanol, 0.05 to 0.95% by weight isobutanol, 0.05 to 0.95% water. After separation of the solvent from the raffinate phase, the thus treated raw material can be directly pyrolyzed without the need for further treatment by hydrotreating or mixing with the original raw material containing a higher concentration of sulfur.

The experiments were carried out in a laboratory continuous column with 35 theoretical plates, at temperatures of 20 to 55 °C, at a solvent to feed ratio of 0.5 to 6 and at atmospheric pressure. The feedstock was gas oils boiling in the range of 250 to 550 °C. The solvent was a mixture of the following composition: methanol 50 to 100 wt%, preferably 90 to 99 wt%, isobutanol 0 to 50 wt%, preferably 1 to 10%, water 0 to 20 wt%, preferably 0 to i%. The extraction products, i.e. the raffinate phase after washing, were pyrolyzed on a quarter-scale continuous unit allowing the injection of 0 to 0.5 kg of feedstock per hour. Water was added according to the selected ratios. After preheating the feedstock, they were fed into a pyrolysis reactor, where it was possible to continuously set and regulate the temperature. The reaction products were analyzed after cooling and separation. Chromatographic analysis was performed on the gaseous samples. The liquid samples were weighed and separated by distillation into fractions boiling at and above 205 °C, i.e. into pyrolysis gasoline and pyrolysis oil.

The effect of extraction of aromatic hydrocarbons from vacuum gas oil on its pyrolysis can be illustrated by an example. For comparison, the results obtained during the pyrolysis of hydrogenated vacuum gas oil are presented. The results were obtained under the conditions given in Table 1 and are summarized in Table 2.

Table 1

231,688

Extraction Ratio of Solvent/ Raw materials (kg/kg) Temperature °C Pyrolysis Composition (% by weight) Temperature °C Pressure Ratio MPa steam/ hydrocarbons (kg/kg) 4 : 1 50 methanol 97 .4 795 0.2 1:1 i-butanol 2.5 water 0.1 Table 2 - Raw material After extraction After hydrogenation and stabilization distillation °C curve 320 - 520 325 - 520 325 - 520 density at 20 °C 911 887 854 kg/ ^m3 sulfur content % wt. 1.8 0.7 0.030 aromatic content % wt. 43.4 10.2 9.5

Yields % wt.

_C4 gas 58.6 74.8 76.1 ethylene 20.8 23.2 25.3 propylene 15.1 17.6 16.1 pyrogasoline 8.3 13.7 13.4 pyrolysis oil 33,i 11.5 10.5

The results show that aromatic extraction positively affects the yields of the desired products and significantly reduces the formation of pyrolysis oil. The reduction in the formation of pyrolysis oils can also be inferred from the probable reduction in the formation of pyrolysis carbon, which is one of the main reasons why it is unlikely that direct pyrolysis of

231 688 untreated vacuum gas oil. Traces of solvent, OH groups, which may remain in the treated raw material, according to the literature, will probably initiate the desired reactions rather than have an undesirable effect on the operation of the pyrolysis furnace. The yields obtained from hydrogenated vacuum gas oil are even more favorable, especially a higher yield of ethylene and lower formation of pyrolysis oil. However, if we take into account that during the extraction of aromatics, hydrotreating can probably be omitted, i.e. reducing the sulfur content below the permissible value while maintaining the requirement for its minimum content, and if we compare the costs involved, hydrogenation may not necessarily be more advantageous.

The minimum and maximum permissible sulfur content in the raw material depends on the type of pyrolysis furnace, the method of processing the reaction products and the requirements for product purity. Therefore, it is not possible to establish a generally valid criterion for the sulfur content in liquid raw materials. However, it is usually recommended to work in the range of 0.05 to 1 wt. % sulfur.

It is obvious that the proposed solvent can also be used in the extraction of low-boiling oil fractions, but according to literature data, it seems more advantageous to use e.g. dimethylformamide for these fractions. It is also obvious that the extraction of aromatics from the oil fraction by the method according to the invention does not always have to be done due to the need to prepare the raw material for pyrolysis.

Claims (2)

SUBJECT OF THE INVENTION 231888 Process for the pre-treatment of a hydrocarbon feedstock for pyrolysis treatment by solvent extraction of aromatic hydrocarbons, characterized in that the fraction boiling in the range of 250 to 550 ° C is extracted with a solvent mixture of 50 to 99.9% by weight of methanol, 0.05 to 50 wt. % isobutanol and 0.05 to 20 wt. water at 20-55 ° C, atmospheric pressure and solvent to feed weight ratio of 0.5-6, and the raffinate is washed with water. 2. A process according to claim 1, wherein the solvent mixture has a composition of 90 to 99% by weight. methanol, 0.05 to 9.95% by weight, isobutanol, 0.05 to 0.95% water.