DE102011118482A1 - Increasing cetane number of naphtha comprises hydrogenation of used naphtha containing unsaturated and/or aryl compounds in presence of catalyst containing iridium or rhodium and/or oxidic carrier, and/or ring opening of cyclic compound - Google Patents

Abstract

Increasing the cetane number of naphtha comprises carrying out at least one catalytic hydrogenation step for the hydrogenation of used naphtha containing unsaturated and/or aryl compounds in the presence of a catalyst and/or subjecting ring opening of the cyclic compound, where the catalyst comprises at least one iridium and rhodium and/or an oxidic carrier and the amount of iridium and/or rhodium is at least 0.1 wt.%.

Description

The invention relates to a method for increasing the cetane number of naphtha and the use of a catalyst for the same purpose.

Internal combustion engines have at least one combustion chamber enclosed by a cylinder and a movable piston therein. The thermal energy generated by combustion of an air-fuel mixture introduced into the combustion chamber is converted into mechanical energy, which ultimately drives a crankshaft. Depending on the type of combustion process, a distinction is drawn between self-igniting (diesel) engines (so-called CI engines = compression ignition engines) and spark ignited engines (so-called SI engines = spark ignition engines).

Gasoline engines in which the air-fuel mixture is known to be actively spark-ignited by an in-cylinder spark plug are operated with gasoline fuel which is a mixture of hydrocarbons having 5 to 12 carbon atoms. These contain mainly alkanes (linear and branched saturated hydrocarbons, paraffins) and, in addition, still varying proportions of alkenes (linear and branched unsaturated hydrocarbons, olefins), cycloalkanes and alkenes, and aromatics. Gasoline usually has a boiling temperature in the range of 90 and 180 ° C. The quality feature of gasoline is the octane number (OZ), which is a measure of the knock resistance, the octane number is greater, the lower the tendency of the gasoline for auto-ignition.

In contrast to gasoline engines diesel fuel is used in the case of diesel engines. This is a mixture of several hundred different hydrocarbons and consists mainly of paraffins with admixtures of olefins, naphthenes (cyclic hydrocarbons) and aromatic hydrocarbons (aromatics). Mainly they are hydrocarbons with 10 to 18 carbon atoms. Typical diesel fuels have a density in the range of 0.83 and 0.88 g / cm ³ , a boiling temperature of 170 to 360 ° C and a flash point in the range of 70 to 100 ° C. The decisive feature for the usability of a diesel fuel is its ignitability, expressed by the cetane number (CZ). The higher the ignitability of the fuel, that is, the shorter the time (ignition delay) between fuel supply into the combustion chamber and its ignition, the greater the cetane number.

In addition, new combustion processes are currently being developed which are intended to combine the advantages of the diesel and Otto engine combustion processes, as well as the diesel process based on auto-ignition of the fuel. In particular, the lower pollutant emissions of homogeneous Otto combustion should be linked to the better efficiency of diesel combustion. As a suitable fuel for such a combustion process, naphtha (also referred to as naphtha) has proven with a cetane number between 35 to 47, with a cetane number of 44 to 47 is particularly advantageous. The use of naphtha leads to an increase in efficiency compared to the use of conventional diesel fuel. In addition, this process has the potential to reduce soot and NO _x emissions, making a significant contribution to meeting future emission limits. In addition, against the backdrop of a future increase in diesel demand in Europe and the current fuel mix from European refineries, a new fuel specification with a significantly lower boiling range could become realistic.

The raw material for conventional liquid fuels is petroleum (crude oil), a mixture of various hydrocarbons. The composition of crude oil varies and depends heavily on the origin. An atmospheric distillation (s. 1 ) is the first process step of a refinery to separate the oil into different boiling fractions. The composition of petroleum according to the recoverable petroleum fractions in an atmospheric distillation is highly dependent on the oil production area. The fraction in the boiling range of 38 to 180 ° C is called naphtha or naphtha. Naphtha has the same boiling properties as gasoline and can therefore be considered as an intermediate product of conventional gasoline production. Naphtha contains hydrocarbon compounds of carbon number 5 to 10 and consists mainly of n- and iso-alkanes (58-70%), cycloalkanes (20-39%) and BTX aromatics (9-14% benzene, toluene and xylene). The middle distillate is the fraction in the boiling range from 180 to 396 ° C (gas oil). This conventionally produces diesel fuel and light fuel oil. Naphtha is therefore the untreated petroleum distillate from the refining of crude oil and not a chemically uniform substance. One distinguishes between lighter and heavier naphtha according to the average molecular mass. The substance is currently used mainly for the production of gasoline.

After atmospheric distillation, the second step in the refinery process is desulfurization to remove the sulfur compounds present in the petroleum distillate. sulfur compounds The most common method of removing the sulfur compounds is hydrodesulphurisation (HDS), in which the petroleum distillate, together with hydrogen H _2, passes through a solid, hydrogenated H ₂ catalyst, not only deactivating catalysts used in refining downstream processing steps of refining. finely divided catalyst is passed, which has a catalytically active metal for hydrogenation, which is fixed on an alumina support. The catalytic material used, for example, nickel-molybdenum and chromium-molybdenum catalysts. The hydrogenation reaction taking place on the catalyst leads to H ₂ S and the hydrogenated hydrocarbon radical. In the reactor outlet is the partially hydrogenated and sulfur-reduced product, unused hydrogen and small amounts of cracked by light hydrocarbons (C ₁ -C ₄ ), and hydrogen sulfide. Typical operating conditions for heavy fuel desulphurisation are a temperature in the range of 260-380 ° C, a pressure of 14 bar and an H ₂ consumption of 2 to 3 kg t ^-1 with a residence time in the reactor of 1 to 5 h ^-1 , The hydrogen sulfide is then washed out with a suitable solvent.

The desulfurized desulfurized distillate obtained by conventional methods has a cetane number of about 35, so that further processing steps must be followed to produce an optimum fuel for a new combustion process.

DE 10 2009 045 564 A1 suggests, after the hydrodesulfurization step described above, to add another hydrogenation step using a second fixed bed catalyst containing a palladium-loaded zeolite catalyst. In the second stage, a further hydrogenation and a ring opening of cyclic constituents of the naphtha, whereby the cetane number is raised.

EP 1 284 281 B1 describes a process for processing a Fischer-Tropsch product (FT product) comprising hydrocarbon compounds having a carbon number of at least 5 to naphtha with a cetane number above 30. For this purpose, the FT primary product is separated into two fractions and those above 270 ° C boiling fraction catalytically cracked. Preferred catalysts include nickel, cobalt, molybdenum, platinum or palladium on a suitable oxidic support material. The process is only suitable for FT naphtha, but it is not a widely available product.

The present invention has for its object to propose a method for increasing the cetane number of naphtha. In particular, a hydrogenation process is to be provided which effects a high yield of as saturated and unbranched hydrocarbons as possible. The naphtha product obtained by the process should ideally provide a cetane number of at least 40 and the process should be as cost effective as possible.

These objects are achieved in full or at least in part by a cetane number elevation method of naphtha in which the naphtha is subjected to at least one catalytic hydrogenation step for the hydrogenation of unsaturated and / or aromatic compounds contained in the naphtha used and for ring opening of cyclic compounds. According to the invention, a catalyst used in at least one of the at least one catalytic hydrogenation stage comprises at least one of the elements iridium and rhodium and an oxidic support, wherein a mass fraction of iridium and / or rhodium at least 0.1% (mass%) based on the total mass of iridium and / or rhodium and carrier. The catalyst according to the invention can contain either only iridium (Ir) or only rhodium (Rh) or a mixture of both.

It has been found that the iridium and / or rhodium-containing catalyst causes hydrogenation of aromatic and / or unsaturated compounds and simultaneously ring opening of cyclic compounds of naphtha with a good yield and high selectivity. As a result, the product of the process has a comparatively high proportion of linear or branched saturated hydrocarbons (n- and iso-paraffins) and a small proportion of cyclic hydrocarbons (naphthenes). The naphtha product of the process according to the invention has in particular a cetane number of at least 40. In addition, in the hydrogenation of the aromatic and / or unsaturated compounds, the removal of heteroatoms such as sulfur takes place at the same time so that the sulfur content of the product is at most 10 ppm.

Starting material of the process according to the invention is the low-boiling naphtha fraction (or naphtha fraction) of the atmospheric distillation of crude oil or the product thereof after an upstream desulfurization. In other words, in a one-step process, the naphtha fraction of atmospheric distillation may be added to the low-sulfur and high-cetane product Be transferred to the invention or already subjected to a conventional desulfurization product by the inventive method.

The mass fraction of the catalytic metal of iridium and / or rhodium is advantageously in the range of 0.1 to 5% and in particular in the range of 0.2 to 3%. The lower limit is determined by a desired minimum yield at moderate residence times and the upper limit by the endeavor to expose the highest possible proportion of the atoms of the metal at the surface. Preferably, the mass fraction of iridium and / or rhodium is about 0.5 to 1% in each case based on the total mass of these metals and the carrier.

The carrier, whose function is the representation of a large specific surface area at high porosity, the fixation of the catalytic metal and the prevention of its sintering, according to a preferred embodiment comprises a refractory oxide. In particular, aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ), titanium oxide (TiO ₂ ), zirconium oxide (ZrO ₂ ), vanadium oxide (V ₂ O ₃ ), zeolites or mixtures of the abovementioned are suitable.

The hydrogenation step of the present invention is preferably carried out at a temperature in the range of 150 to 350 ° C, especially in the range of 200 to 300 ° C, preferably at about 250 ° C. In this case, a pressure in the range of 10 to 150 bar is preferred. in particular in the range of 10 to 100 bar, preferably adjusted in the range of 20 to 70 bar. These conditions are considered to be relatively mild as compared to conventional hydrocracking ratios used, for example, to digest fractions obtained by vacuum distillation of the residue of atmospheric distillation. Conventional hydrocracking generally has temperatures of 350 to 500 ° C and pressures of 70 to 200 bar. However, these comparatively high pressures require the use of special steels, since otherwise hydrogen can diffuse through the reactor walls. However, it has been found in the present case that, when the iridium- and / or rhodium-containing catalyst is used, naphtha can be hydrogenated and reacted at lower pressures, namely those which are customary for the conventional desulfurization of naphtha, and can be reacted by ring opening.

The hydrogen required to carry out the process, depending on the crude oil, is between 5 to 21 kg per 1000 kg (1 ton) of naphtha used.

In a preferred embodiment of the invention, in addition to the hydrogenation stage according to the invention using the iridium and / or rhodium-containing catalyst, a further, in particular upstream hydrogenation stage for desulfurization (hydrodesulfurization), wherein conventional methods and catalysts can be used. For this purpose, the already conventionally used catalysts, in particular nickel-molybdenum or chromium-molybdenum-based, or modern desulfurization based on alumina. In this embodiment, therefore, the naphtha fraction of the atmospheric distillation of crude oil is first subjected to a conventional hydrodesulfurization stage and the already strongly sulfur-depleted product is subsequently fed to the catalyst according to the invention for further desulfurization, hydrogenation and ring opening. This two-stage process results in a naphtha product with further lowered sulfur content and further increased cetane number, which can be used to operate internal combustion engines, particularly CI engines.

In the case of the two-catalyst process described above, both catalysts, namely the conventional catalyst for hydrodesulfurization and iridium- and / or rhodium-containing catalyst according to the invention for further desulfurization, hydrogenation and ring opening, on two fixed beds connected in series can be used in separate reactors or - preferably - can be arranged in the same reactor. Advantageously, both catalysts can also be present on one and the same fixed bed in the form of a mixed catalyst. In this case, the two hydrogenation stages are carried out virtually simultaneously. As a result, the overall process can be substantially simplified and production costs and production costs for the plant for the production of fuel with the desired quality for the new combustion process can be reduced.

With the aid of the process according to the invention, the naphtha obtained by atmospheric distillation can be converted into a fuel having a cetane number of from 40 to 47, in particular from 42 to 47 and preferably from 44 to 47. The cetane number increases with increasing hydrogen content in the hydrocarbon compounds (s. 2 ). The cetane number describes the ignitability of diesel fuels and comparable substances, eg. Petroleum, gas oil components and marine diesel oil. The cetane number indicates that a fuel behaves in the same way as a mixture of n-hexadecane (cetane) and its ignitability 1-Methylnaphthalene with a volume fraction of n-hexadecane, which corresponds to the cetane number given. A mixture of 30% n-hexadecane and 70% 1-methylnaphthalene has z. As the cetane number 30. Instead of 1-methylnaphthalene, the synthetically accessible 2,2,4,4,6,8,8-heptamethylnonane is used with a cetane number of 15 as an ignitable fuel. The more unbranched hydrocarbon molecules are contained in the fuel as a percentage, the easier it ignites. The determination of the cetane number can according to DIN 51773 respectively.

The naphtha fuel obtained by the process typically contains hydrocarbon compounds of carbon number 4 to 11 and has a sulfur content of at most 10 ppm. The aromatic content is usually below 15% by volume, in particular below 2% by volume. Start and end of boiling are at 60 and 220 ° C, in particular 80 and 180 ° C; a boiling point is between 100 to 150 ° C, in particular at 110 ° C. The fuel produced according to the invention also typically has an average molecular weight between 100 and 120 g / mol; it usually has at 15 ° C a density in the range of 690 to 730 kg / m ³ , in particular 700 to 720 kg / m ³ .

Another aspect of the present invention relates to the use of the catalyst according to the invention as described above for hydrodesulfurization and cetane number elevation of naphtha.

The invention will be explained in more detail with reference to the figures and an embodiment. Show it:

1 Process stages of refining petroleum;

2 Correlation between chain length and cetane number for different hydrocarbon substance classes;

3 Selectivity of the catalyst according to the invention by the example of the reaction of cyclohexane to n-hexane and

4 Sales and selectivity of the catalyst of the invention using the example of hydrogenation and ring opening of toluene.

The 1 shows schematically an overview of the refining of petroleum to its various end products, especially naphtha. The percentages are to be understood by way of example and vary in detail depending on the Erdölfordergebiet.

Petroleum, for example Brent crude oil, is first subjected to distillation under atmospheric pressure, where the crude oil enters a low-boiling fraction boiling in the range 38 to 180 ° C, namely naphtha (or naphtha), a middle fraction (gas oil) boiling in the range 180 to 390 ° C and the non-boiling residue is separated. Conventionally, the naphtha as well as the middle distillate are subjected to a desulfurization step (hydrodesulfurization HDS). The desulfurized product of the middle fraction is further processed into diesel fuel and heating oil. On the other hand, the sulfur-reduced naphtha is subjected to a process step for raising the octane to produce gasoline for driving gasoline engines, or used directly as naphtha.

The following equations illustrate the hydrogenation reactions in a typical HDS plant, where R is a hydrocarbon radical. It can be seen that not only are sulfur-containing groups such as mercapto groups removed by the hydrogenation, but also oxygen-containing groups such as carboxyl groups, halogens such as chlorides or nitrogen groups such as nitrides and the like: (a) RSH + H ₂ → RH + H ₂ S RCl + H ₂ → RH + HCl 2RN + 4H ₂ → 2RH + 2NH ₃ ROOH + 2H ₂ → RH + 2H ₂ O

The invention now relates to a hydrogenation stage of naphtha which replaces the conventional desulfurization stage (circled area in US Pat 1 ) or this is downstream (see optional field indicated by dashed frame in FIG 1 ).

(c) Ringöffnung

The catalytic hydrogenation stage according to the invention comprises contacting the naphtha fraction of atmospheric distillation or the product of conventional desulfurization with a catalyst comprising at least one of the elements iridium and rhodium and an oxidic support, wherein a mass fraction of iridium and / or rhodium is at least 0 , 1% based on the total mass of iridium and / or rhodium and carrier. In this process stage, a hydrogenation of aromatics and olefins contained in the naphtha and a ring opening of naphthenes takes place. These reactions are illustrated below by way of example using toluene or its hydrogenation product methylcyclohexane: (b) aromatics hydrogenation

(c) Ring opening

2 shows the dependence of the cetane number CZ on the different substance groups of hydrocarbons and their chain lengths. It can be seen that the cetane number CZ increases with increasing chain length, increasing degree of saturation and as little branching as possible. In other words, long-chain n-paraffins have the highest cetane numbers.

The catalyst used in the context of the present invention is distinguished by a high selectivity and a good conversion rate under relatively mild reaction conditions and represents a less expensive alternative to platinum or palladium-containing catalysts.

The catalyst of the present invention was tested on various hydrocarbons contained in naphtha as components to be hydrogenated.

1st example: cyclohexane

Pure cyclohexane (C ₅ H ₁₂ ) was suspended on a fixed bed containing a catalyst of iridium or rhodium-loaded alumina (see below) at various temperatures of about 250, 270 and 285 ° C and a pressure of 50 bar in the presence of Hydrogen H ₂ hydrogenated to achieve ring opening. The supplied material amount of hydrogen was n _{H 2} / n _cyclohexane = 15. In the case of the rhodium catalyst, the residence time τ '(Rh) = 30 g · s / cm ³ and, τ, in the case of the iridium catalyst' (Ir) = 15 g · s / cm ³ . Due to the different residence times, the different high noble metal loadings of the catalysts were compensated, ie the same residence times based on the noble metal set. Catalyst 1: 0.5 mass% rhodium (Rh) on alumina (Al ₂ O ₃ ) support Catalyst 2: 1.0 mass% iridium (Ir) on alumina (Al ₂ O ₃ ) support

The selectivity of both catalysts with respect to the desired reaction to n-hexane (C ₅ H ₁₄ ) as a function of cyclohexane conversion is in 3 shown. It shows for both catalysts, a high selectivity of the reaction to n-hexane, which is at 250 ° C at about 0.9 to 0.95 and expected to decrease with higher temperatures.

For the experiment with Ir / Al ₂ O ₃ at 285 ° C is in 3 also listed the composition of the product mixture. At this temperature, the conversion of cyclohexane (C ₆ H ₁₂ ) is 75% and the selectivity to n-hexane at 74.2%. In addition, the product contains the other n-paraffins with 1 to 5 carbons to a total of 25.5% and the undesirable iso-paraffins 2 or 3-methylpentene only to a total of 0.4%. 2nd example: benzene

Pure benzene (C ₆ H ₆ ) was placed on a fixed bed containing a rhodium (Rh) or iridium on alumina (Al ₂ O ₃ ) catalyst at a temperature of 285 ° C and a pressure of 50 bar in the presence of hydrogen H ₂ hydrogenated to achieve double bond saturation and ring opening. The fed amount of hydrogen was n _H2 / n _benzene = 15 and the residence time τ '= 30 g · s / cm ³ . Catalyst 1: 0.5 to 1 mass% rhodium (Rh) on alumina (Al ₂ O ₃ ) support Catalyst 2: 0.5 to 1 mass% iridium (Ir) on alumina (Al ₂ O ₃ ) support

A yield of n-hexane of 56% and an selectivity of n-hexane of 74% was achieved.

3rd example: toluene

Pure toluene (C ₇ H ₆ ) was added to a fixed bed containing a catalyst of 1.0% by weight iridium or rhodium loaded alumina (Al ₂ O ₃ ) at a temperature of 268 ° C and a pressure of 50 bar hydrogenated in the presence of hydrogen H ₂ to achieve saturation of the double bonds and ring opening. The amount of hydrogen introduced was n _H2 / n _toluene = 21 and the residence time τ '= 1.2 g · s / cm ³ . Catalyst 1: 0.5 to 1 mass% rhodium (Rh) on alumina (Al ₂ O ₃ ) support Catalyst 2: 0.5 to 1 mass% iridium (Ir) on alumina (Al ₂ O ₃ ) support

There was a selectivity for the formation of 2-methylcyclohexane of 38.6% at a conversion of 14.6% of achieved.

The selectivity and the conversion of the rhodium catalyst with respect to the reaction to the saturated intermediate methylcyclohexane (C ₇ H ₁₄ ) and the "open" products as a function of the temperature is in 4 shown.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102009045564 A1 [0009]
• EP 1284281 B1 [0010]

Cited non-patent literature

• DIN 51773 [0021]

Claims (12)

Process for increasing the cetane number of naphtha, in which the naphtha is subjected to at least one catalytic hydrogenation stage for the hydrogenation of unsaturated and / or aromatic compounds contained in the naphtha used and for ring opening of cyclic compounds, characterized in that a catalyst used in the catalytic hydrogenation stage at least one the elements iridium and rhodium and an oxidic carrier, wherein a mass fraction of iridium and / or rhodium is at least 0.1% based on the total mass of iridium and / or rhodium and carrier. The method of claim 1, wherein the mass fraction of iridium and / or rhodium in the range of 0.1 to 5%, in particular in the range of 0.2 to 3%, preferably in the range of 0.5 to 1%. Process according to claim 1 or 2, wherein the support comprises a refractory oxide, in particular aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ), titanium oxide (TiO ₂ ), zirconium oxide (ZrO ₂ ), vanadium oxide (V ₂ O ₃ ), a zeolite or a mixture of these. Method according to one of the preceding claims, wherein the hydrogenation stage at a temperature in the range of 150 to 350 ° C, in particular in the range of 200 to 300 ° C, preferably at 250 ° C, is performed. Method according to one of the preceding claims, wherein the hydrogenation stage at a pressure in the range of 10 to 150 bar, in particular in the range of 10 to 100 bar, preferably at 20 to 70 bar, is performed. A process according to any one of the preceding claims, wherein an amount of hydrogen supplied to the hydrogenation stage is in the range of from 5 to 21 kg H ₂ per 1000 kg naphtha. Method according to one of the preceding claims, wherein except for the hydrogenation stage using the iridium and / or rhodium-containing catalyst, a further hydrogenation step for desulfurization. A process according to claim 7, wherein the further hydrogenation step is preceded by desulfurization of the hydrogenation step using the iridium and / or rhodium-containing catalyst or both hydrogenation steps are carried out simultaneously. A process according to claim 7, wherein the further desulfurization hydrogenation step and the hydrogenation step using the iridium and / or rhodium-containing catalyst are carried out simultaneously on a uniform fixed bed. A process according to any one of the preceding claims, wherein the naphtha obtained by the process has a cetane number of at least 40. A process according to any one of the preceding claims, wherein the naphtha obtained by the process has a sulfur content of less than 10 ppm. Use of a catalyst comprising at least one of the elements iridium and rhodium and an oxidic support, wherein a mass fraction of iridium and / or rhodium is at least 0.1% based on the total mass of iridium and / or rhodium and support, for the cetane number increase of naphtha ,

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