Patent Application
20080041764
This application claims the priority of German Patent Document No. 10 2006 038 891.7, filed Aug. 18, 2006, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for treating a fraction consisting predominantly of hydrocarbons having at least seven carbon atoms (C7+ fraction) as produced in a plant for the catalytic splitting of hydrocarbon-containing feedstock and an apparatus for carrying out the method.
Primarily heavy crude oil components such as are produced, for example, in crude oil distillation are processed in a plant for the catalytic splitting of hydrocarbon-containing feedstock.
In accordance with the prior art, the heavy crude oil components are taken as feedstock for catalytic splitting. In catalytic splitting in the presence of a catalyst, the heavy crude oil components are converted primarily into shorter-chain paraffins, olefins and aromatics. A fraction consisting predominantly of hydrocarbons having at least six carbon atoms (C6+ fraction) is separated from the reaction products from the catalytic splitting. This C6+ fraction contains aromatics, primarily benzene, as an economically utilizable product, which find a use as starting material for the synthesis of numerous plastic materials and to increase the knock resistance of gasoline.
In order to obtain the economically utilizable products from the C6+ fraction, primarily benzene, and to maximize the yield as much as possible, the following method is applied from the prior art. The C6+ fraction is separated into a fraction consisting predominantly of hydrocarbons having six carbon atoms (C6+ fraction) and a fraction consisting predominantly of hydrocarbons having at least seven carbon atoms (C7+ fraction). The economically utilizable product benzene can be separated directly from the C6+ fraction. The linear hydrocarbons are separated from the C7+ fraction by means of fluid-fluid extraction and processed further as a raffinate, for example, the raffinate can be returned to the feedstock for catalytic splitting. The C7+ fraction freed from the linear hydrocarbons now contains primarily aromatics having seven to eight carbon atoms and is separated into a fraction consisting predominantly of hydrocarbons having seven carbon atoms (mainly toluene) and into a fraction consisting predominantly of hydrocarbons having at least eight carbon atoms (primarily xylene). The fraction consisting predominantly of hydrocarbons having eight carbon atoms is taken as feedstock to a process for extracting paraxylene. The fraction consisting predominantly of hydrocarbons having seven carbon atoms is taken as feedstock to a process for hydro-dealkylation.
A process of this kind for hydro-dealkylation is described, for example, in WO2005071045. The hydrocarbons are contacted with hydrogen in the presence of a catalyst at a temperature of 400° C. to 650° C. and a pressure between 20 bar and 40 bar, where the hydrogen is present in a molar excess of three to six times the hydrocarbons. Under these conditions the alkyl groups are split off from the alkylated aromatics (such as toluene) so that benzene and the particular alkanes form (methane for example).
The consumption of hydrogen in the hydro-dealkylation of the hydrocarbons has a negative effect on the economics of this process from the prior art for extracting benzene.
In accordance with the invention, with respect to the method, the C7+ fraction is subjected to steam dealkylation where mainly the two utilizable products benzene and hydrogen are produced along with reaction products such as carbon monoxide and carbon dioxide.
The basic idea of the invention is to carry out dealkylation of the alkylated aromatics while generating benzene with the aid of steam dealkylation. Steam dealkylation requires only inexpensive steam as the starting material and produces the valuable by-product hydrogen in addition to the desired quality product benzene.
The C7+ fraction employed in the steam dealkylation contains primarily:
a) aromatic hydrocarbons having seven to ten carbon atoms,
b) cyclic paraffins (cycloalkanes) having six to ten carbon atoms,
c) iso- and n-paraffins having six to ten carbon atoms,
d) alkenes having seven to ten carbon atoms, or
any mixture of the aforementioned, where the precise composition is dependent on the composition of the specific heavier naphtha which is taken as feedstock for catalytic reforming. The inventive method is suitable for each of the compounds of the C7+ fraction described.
The hydrocarbons from the C7+ fraction react advantageously with steam in the gas phase with the introduction of heat to a solid catalyst. The gaseous C7+ fraction is dealkylated by the presence of gaseous water (steam) on a catalyst under the constant introduction of heat, whereby the desired products benzene and hydrogen are produced in addition to carbon monoxide, carbon dioxide and additional by-products.
Preferably the heat required for the dealkylation reaction is generated by the combustion of a starting material with air. It proves to be particularly advantageous to use gaseous by-products from the steam dealkylation, specifically carbon monoxide and methane, as starting material for combustion with air. A part of the gaseous reaction products from the steam dealkylation, in particular carbon monoxide and methane, is combustible and can therefore serve as starting material for combustion to generate the necessary reaction heat. This saves heating gas and this otherwise unused part of the reaction products is employed usefully.
Following compression, the gaseous reaction products are expediently separated by way of pressure swing adsorption into gaseous hydrogen and gaseous reaction by-products, specifically carbon monoxide, carbon dioxide and methane. The valuable by-product hydrogen is also present in gaseous form and can be employed much more usefully than for combustion. By way of pressure swing adsorption preceded by compression, the hydrogen can easily be separated from the combustible gaseous reaction by-products which can serve as starting material in the combustion.
The flue gases generated in combustion are advantageously cooled via a heat exchanger while heating the starting materials for the steam dealkylation. By using the heat from the flue gases to preheat the starting materials (C7+ fraction and steam) for the steam deakylation, the heat to be supplied which is needed to maintain the temperatures required for the dealkylation reaction is reduced. This achieves an economical use of energy resources.
The C7+ fraction and the steam are advantageously conducted past the solid catalyst, preferably in pipes from top to bottom, where the catalyst is in the interior of the pipes. Heat is expediently supplied to the pipes from the outside. The heat required for the dealkylation reaction is preferably transferred to the pipe by electromagnetic radiation, thermal radiation and/or convection. The actual dealkylation reaction takes place in the interior of the pipe where the catalyst is located. The two components of the reaction (C7+ fraction and steam) are taken from top to bottom through the pipes filled with the catalyst. The heat needed for the dealkylation reaction is generated outside the pipes and transferred by the mechanisms named to the pipe from which the heat reaches the inside of the pipes, the site of the reaction, by means of thermal conduction and convection.
A solid catalyst of a porous carrier material is preferably used, specifically γ-Al2O3, MgAl spinel and/or Cr2O3, and an active component on the surface of the carrier material, in particular Rh with 0.1-1.0% loading by weight and/or Pd with 0.2-2.0% loading by weight.
The steam dealkylation is advantageously performed at a temperature of 400° C. to 600° C., preferably 450° C. to 550° C., particularly preferably 480° C. to 520° C. and at a pressure of 1 to 15 bar, preferably 1.2 to 10 bar, particularly preferably 1.5 to 8 bar.
Steam dealkylation is expediently performed at a molar quotient of steam to hydrocarbons which lies in the range of 1 to 20, preferably from 2 to 15, when it enters the reactor. In another embodiment of the invention, steam dealkylation is performed at a molar quotient of steam to hydrocarbons which lies in the range from 3 to 12, preferably from 5 to 10, when it enters the reactor. Steam dealkylation is generally performed with a molar excess of water, where the exact ratio in the different embodiments of the invention depends on the precise composition of the C7+ fraction.
It proves advantageous to subject the C7+ fraction to a process to convert dienes and styrenes before steam dealkylation, where specifically hydrating methods are employed involving the consumption of hydrogen. It is similarly advantageous to subject the C7+ fraction to a process to convert and remove components containing sulfur, nitrogen and/or oxygen before steam dealkylation, where specifically methods are also employed involving the consumption of hydrogen. By employing the hydrating processes, the diolefins present in the C7+ fraction are converted into their corresponding olefins, just as components containing sulfur, nitrogen and oxygen can be converted and removed. Deactivation of the catalyst is reduced and the life of the catalyst is clearly increased.
The reaction products from the steam dealkylation are preferably cooled and separated in a 3-phase separation into gaseous reaction products, hydrocarbons and water. The reaction products coming from the steam dealkyation contain not only the desired quality products benzene and hydrogen but also reaction products such as carbon monoxide and carbon dioxide and reaction by-products. To obtain the desired quality products, the reaction products must be separated. This is done by way of a 3-phase separation of the cooled reaction products into the gaseous reaction products, specifically hydrogen, carbon monoxide, carbon dioxide and methane, into the hydrocarbons, specifically benzene, and into water.
The hydrogen generated in the steam dealkylation of the C7+ fraction is expediently fed completely or partially into the starting material for the hydrogen-consuming processes. The hydrogen generated in steam dealkylation can be used entirely or partially for the hydrogen-consuming processes described in the previous section so that the need for hydrogen to be supplied externally is minimized.
In one embodiment of the invention, the hydrogen produced in the steam dealkylation of the C7+ fraction is taken as starting material to any process consuming hydrogen, preferably to a process in the oil refinery for converting and removing sulfur-containing components or a process for splitting hydrocarbon-containing starting material by means of hydrogen.
For a good yield of the desired reaction product benzene from steam-dealkylation, the reduction of the sulfur content in the C7+ fraction prior to steam dealkylation to below 10 ppm, preferably below 3 ppm, particularly preferably below 1 ppm proves advantageous.
Preferably the benzene is separated from the hydrocarbons of the reaction products through rectification. Following rectification, the benzene advantageously undergoes adsorptive fine cleaning to dry and remove the trace components, where the benzene is directed across an adsorbent on which the trace components, as opposed to benzene, are adsorbed. By applying the inventive method, the benzene can be extracted from the reaction products by simple rectification and processed further or marketed. Expensive extraction or extractive rectification as when applying a process in accordance with the prior art is not necessary, thus reducing investment and process costs.
Components in the C7+ fraction boiling close to benzene or forming azeotropes are advantageously converted by steam dealkylation. All heavier boiling reaction products than benzene from rectification, consisting predominantly of non-converted feedstocks from the steam dealkylation, are expediently returned to steam dealkylation as starting material by way of optional hydration. In another embodiment of the invention, all heavier boiling reaction products than benzene from rectification, consisting predominantly of non-converted feedstocks from steam dealkylation are returned for hydration of the C7+ fraction, C6+ fraction or for hydration of a fraction consisting predominantly of hydrocarbons with at least five carbon atoms prior to steam dealkylation. By returning the non-converted feedstocks for hydration or for steam dealkylation, circulation is achieved without losing valuable feedstocks.
In another embodiment of the invention, a fraction consisting predominantly of hydrocarbons having at least eight carbon atoms (C8+ fraction) is separated through distillation from the C7+ fraction, where the separated C8+ fraction is taken as feedstock to a process for extracting paraxylene or for processing as gasoline.
Concerning the apparatus, the object is achieved by the apparatus comprising an oven 100 with a furnace 110 and pipes 120 located in the furnace. The actual steam dealkylation takes place in the pipes which in turn are located in the furnace of the oven where the heat required for the dealkylation reaction can be generated.
The pipes are advantageously installed vertically in the furnace and have heat expansion compensating elements 130 at the lower and/or upper end. The heat expansion compensating elements at the lower and/or upper end of the vertical pipes prevent mechanical stress from temperature differences which can lead to increased wear of the pipes.
Each pipe expediently has a supply for the C7+ fraction and the steam, 122, 124, respectively, and an outlet 126 for the reaction products.
It proves equally advantageous that each pipe is filled on the inside with a catalyst 128, where the catalyst consists of a porous carrier material, in particular γ-Al2O3, MgAl spinel and/or Cr2O3 and an active component on the surface of the carrier material, in particular Rh with 0.1-1.0% loading by weight and/or Pd with 0.2.-2.0% loading by weight.
Preferably the oven has at least one burner 102 on the wall, the ceiling and/or the floor. The pipes are expediently suitable for an internal pressure of 1 to 15 bar, preferably 1.2 to 10 bar, particularly preferably 1.5 to 8 bar, and for use in an oven with flame temperatures of up to 1400° C.
The present invention is successful specifically in creating an economical alternative to the prior art for treating a C7+ fraction in a plant for the catalytic splitting of hydrocarbon-containing feedstock. Through the application of the inventive method and the inventive apparatus, the valuable by-product hydrogen is generated in addition to the usable product benzene.
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 038 891.7 | Aug 2006 | DE | national |
