EP1532087A1

EP1532087A1 - Isothermal method for dehydrogenating alkanes

Info

Publication number: EP1532087A1
Application number: EP03792326A
Authority: EP
Inventors: Götz-Peter SCHINDLER; Klaus Harth
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2002-08-16
Filing date: 2003-08-14
Publication date: 2005-05-25
Also published as: DE10237514A1; MXPA05001617A; EA008365B1; JP2005539034A; AU2003255444B2; CN1675146A; US20060004241A1; NO20050616L; KR100996220B1; TWI319394B; CN1274646C; BR0313434A; TW200418784A; CA2495290A1; EA200500366A1; WO2004018391A1; AU2003255444A1; KR20050056972A; MY140150A; JP4159545B2

Abstract

The invention relates to an isothermal method for dehydrogenating alkanes to form corresponding alkenes on a catalyst bed containing a dehydrogenating catalyst. Said method is characterised in that the catalyst bed contains a catalytically inactive, inert diluting material. Preferably, said catalytically inactive, inert diluting material is selected from the group consisting of the oxides of the main groups II, III and IV, and the subgroups III, IV and V, the mixtures thereof and nitrides and carbides of elements of the main groups III and IV, and preferably has a BET surface of < 10 m2/g. The presence of the catalytically inactive diluting material in the catalyst bed enables the volume/time yield relating to the alkenes formed to be limited to preferably 7,0 kg/(kgbed x h).

Description

Isothermal process for the dehydrogenation of alkanes

The invention relates to an isothermal process for the dehydrogenation of alkanes to alkenes, in particular an isothermal process for the dehydrogenation of propane to propene.

The dehydrogenation of propane to propene is strongly endothermic with a reaction enthalpy ΔH of 135 kJ / mol. Propane and propene only have a relatively low heat capacity of 160 J / (mol x K) or 135 J / (mol x K) at 600 ° C. This leads to the formation of large temperature gradients in the propane dehydrogenation within the dehydrogenation reactor, as a result of which the reaction is severely limited in terms of heat transfer.

Adiabatic processes such as the UOP-Oleflex process avoid heat transport limitation of the dehydrogenation reaction, that is to say limitation of heat transport from the reactor walls into the interior of the reactor, by providing the required heat of reaction in the form of the heat stored in the superheated inlet gas. Typically up to 4 reactors are connected in series. The inlet gas is overheated up to 300 K in front of each reactor. By using several reactors, excessive differences in the temperatures of the reaction gas mixture between the reactor inlet and the reactor outlet can be avoided. Overheating the inlet gas mixture on the one hand forms coke precursors which cause coking of the catalyst, and on the other hand the selectivity of the propane dehydrogenation is reduced by cracking processes (formation of methane and ethene).

The severe overheating of the inlet gases is avoided in the Linde and Krupp / Uhde isothermal processes (STAR process) by using directly fired reactor tubes. The educt gas mixture is only heated to the reaction temperature and the energy required for the endothermic reaction is fed to the system over the entire length of the reactor via the reactor wall, an isothermal temperature profile being sought in both the axial and radial directions. In order to avoid the formation of coke precursors during the preheating of the inlet gas mixture, this can be done Entry gas mixture are also fed to the reactor at a temperature lower than the temperature required for the reaction, and not only the amount of heat required for the endothermic reaction but also the additional amount of heat required to heat the reaction gas mixture to the reaction temperature are introduced into the reaction gas via the reactor wall ,

In fact, when the isothermal propane dehydrogenation is carried out on an industrial scale, a temperature profile which deviates more or less from the ideal temperature profile is obtained. In particular in the entrance area of the catalyst bed, that is, where the system is still far from thermodynamic equilibrium and large gradual conversions are achieved, strong temperature gradients occur both in the axial and in the radial direction. The lowest temperatures are set where the highest sales per unit volume are achieved.

The object of the invention is to provide an improved isothermal process for the dehydrogenation of propane to propene. The object of the invention is, in particular, to provide such a method in which the heat transport limitation in the catalyst bed is reduced and the formation of strong temperature gradients in the catalyst bed is avoided.

The object is achieved by an isothermal process for the dehydrogenation of alkanes to the corresponding alkenes on a catalyst bed containing a dehydrogenation-active catalyst, which is characterized in that the catalyst bed contains inert, catalytically inactive diluent material.

In the following, an isothermal process - in contrast to an adiabatic process - is understood to mean a process in which heat is supplied to the reacting gas mixture from the outside by heating the reactor from the outside.

The catalyst bed is preferably diluted with catalytically inactive inert material at those points where large axial and / or radial temperature gradients would occur without such dilution. This is particularly the case at the points of the catalyst bed where high gradual conversions are achieved, in particular in the entrance area of the dehydrogenation reactor. For example, the oxides of LT., HI are suitable as catalytically inactive inert materials. and IV. main group, the HI., IV. and V. subgroup as well as mixtures of two or more of these oxides, and nitrides and carbides of elements of HI. and IV. main group. Examples are magnesium oxide, aluminum oxide, silicon dioxide, steatite, titanium dioxide, zirconium dioxide, niobium oxide, thorium oxide, aluminum nitride, silicon carbide, magnesium silicates, aluminum silicates, clay, kaolin and pumice. The catalytically inactive inert diluent materials preferably have a low BET surface area. This is generally <10 m / g, preferably <5 m / g and particularly preferably <1 m ² / g. A low BET surface area can be obtained by annealing said oxides or ceramic materials at high temperatures of, for example,> 1000 ° C.

The catalytically inactive, inert dilution material preferably has a thermal conductivity coefficient at 293 K of> 0.04 W / (m x K), preferably> 0.4 W / (m x K) and particularly preferably> 2 W / (m x K). The radial thermal conductivity of the catalyst bed diluted with catalytically inactive inert material is preferably> 2 W / (m x K), particularly preferably> 6 W / (m x K), in particular> 10 W / (m x K).

The catalytically inactive, inert diluent material can be used in the form of grit or in the form of moldings. The geometry and dimensions of the catalytically inactive diluent are preferably chosen so that the diluent and the dehydrogenation-active catalyst mix well. This is generally the case if the catalyst particles and the particles of catalytically inactive diluent have approximately the same particle diameter.

The geometry of the particles of catalytically inactive diluent material can be chosen so that the resulting pressure loss over the entire length of the bed is smaller than the pressure loss that would occur over an undiluted bed containing the same amount of the dehydrogenation-active catalyst. For example, rings or hollow strands made of catalytically inactive dilution material can be used for this purpose. These also result in an even better uniform distribution of the temperature (isothermal), since they force the gas flowing through them in a direction that deviates from the main axial direction of the reactor tubes. The resulting improved convective mixing increases the heat transport in the reaction gas mixture. The pressure drop decreases and the radial thermal conductivity increases with increasing size of the rings or hollow strands. However, the use of molded articles which are too large is then poorly mixed with the (Smaller) catalyst particles less preferred. Small catalyst particles are preferred over large catalyst particles because of the otherwise encountered mass transfer limitation.

Examples of suitable mold geometries are tablets or strands with a diameter of on average 2 to 8 mm and a height of on average 2 to 16 mm. The height is preferably 0.5 to 4 times the diameter, particularly preferably 1 to 2 times.

Rings or hollow strands with an outside diameter of on average 6 to 20 mm and a height of on average 6 to 20 mm are also suitable. The height is preferably 0.5 to 4 times the diameter, particularly preferably approximately 1 to 2 times the diameter. The wall thickness is usually 0.1 to 0.25 times the diameter. As stated, the rings and hollow strands additionally have the advantage of better convective mixing of the reaction gas mixture and in particular the lower pressure drop. The pressure loss of the dilute bed can, in spite of the increased volume and thus the increased reactor length, even be lower than that of an undiluted bed.

A spherical shaped body geometry is also suitable. Shaped body balls preferably have an average diameter of 1 to 5 mm.

In particular, shaped catalyst bodies and shaped inert material bodies have similar or even the same geometry and dimensions.

The void fraction of the catalyst bed diluted with the catalytically inactive dilution material is preferably at least 30%, preferably 30 to 70%, particularly preferably 40 to 70%.

The hydrogenation-active catalyst and catalytically inactive inert diluent are generally in the ratio catalyst: inert material from 0.01 1: 1 1 to 10 1: 1 1, preferably from 0.1 1: 1 1 to 2 1: 1 1, in each case based on the bulk volume of catalyst or inert material.

A suitable reactor form for carrying out the alkane dehydrogenation according to the invention is the fixed bed tube or tube bundle reactor. With these is the Catalyst (dehydrogenation catalyst and, when working with oxygen as a co-feed, possibly a special oxidation catalyst) as a fixed bed in a reaction tube or in a bundle of reaction tubes. The reaction tubes are usually heated indirectly in that a gas, for example a hydrocarbon such as methane, is burned in the space surrounding the reaction tubes. It is favorable to use this indirect form of heating only for the first approx. 20 to 30% of the length of the fixed bed and to heat the remaining bed length to the required reaction temperature by means of the radiant heat released as part of the indirect heating. Usual reaction tube inner diameters are about 10 to 15 cm. A typical dehydrogenation tube bundle reactor comprises approximately 300 to 1000 reaction tubes. The temperature in the interior of the reaction tube is usually in the range from 300 to 700 ° C., preferably in the range from 400 to 700 ° C. The working pressure is usually between 0.5 and 12 bar, the pressure at the reactor outlet is often between 1 and 2 bar when using a low water vapor dilution (according to the BASF Linde process), but also between 3 and 8 bar when using a high water vapor dilution ( corresponding to the so-called "steam active reforming process" (STAR process) from Phillips Petroleum Co., see US 4,902,849, US 4,996,387 and US 5,389,342). Typical catalyst analyzer loads (GHSV) with propane are 500 to 2000 h ^"1 , based on the reaction to be implemented alkane.

The dilution of the catalyst bed with catalytically inactive material leads to an increase in the volume of the diluted catalyst bed compared to an undiluted catalyst bed. The larger reactor volume required as a result is preferably provided by an extension of the individual reactor tubes. An increase in the tube diameter of the reactor tubes is less preferred since this reduces the surface: volume ratio of the reactor, which counteracts good heat transport. An increase in the number of reactor tubes with a constant length of the individual tubes is also less preferred, since complex welding and connections, which cause high costs, are additionally required. Extending the reactor tubes with a constant tube diameter only entails increased material costs and is therefore preferred. If necessary, the described measures for increasing the reactor volume can be combined in order to achieve an optimum in technical and economic terms. The heat transfer coefficient of the reactor tubes is preferably> 4 W / m ² K, particularly preferably> 10 W / m ² K, in particular> 20 W / m ² K. Examples of suitable materials which have such a heat transfer coefficient are steel or stainless steel.

The dehydrogenation-active catalyst is diluted, for example, in the sections of the reactor with catalytically inactive inert material in which, without dilution, the space / time yield, based on the alkene formed, is> 7.0 kg / (kgs _C bed xh). The space / time yield can be limited to the stated value as the upper limit by the dilution. This upper limit is preferably 4.0 kg / (kgs _C hüttung xh), more preferably from 2.5 kg / (kgs _C hüttun _g x h), and especially 1.5 kg / (kgs ₀ hüttung xh). The resultant lower gradual conversions prevent the formation of strong radial and / or axial thermal gradients. The catalyst can be diluted in the sections of the reactor in which the conversion would be> 0.3 kg / (kg fill h) without dilution, and is preferably diluted in the sections in which the conversion without dilution> 0.5 kg / (kgs _C rg _tt ung xh), more preferably> 1.0 would be kg / _(g kgschüttun h) and especially> 1.5 kg / (kg _Sc hüttun _g xh).

The dehydrogenation-active catalyst can also be applied as a shell on a shaped body made of catalytically inactive dilution material. Preferred shaped articles are rings or hollow strands which bring about a low pressure loss in the catalyst bed.

In one embodiment of the process according to the invention, the catalyst bed is diluted in the sections of the reactor with catalytically inactive inert material in which, when the catalyst is regenerated by burning off coke deposited thereon in an oxygen-containing gas in an undiluted catalyst bed of dehydrogenation-active catalyst, an internal temperature of> Would set 650 ° C, preferably> 700 ° C and particularly preferably> 750 ° C.

Some of the heat required for the dehydrogenation can be generated in the catalyst bed itself by burning hydrogen, hydrocarbons and coke with mixed oxygen. The combustion takes place catalytically. The dehydrogenation catalyst used generally also catalyzes the combustion of the hydrocarbons and of hydrogen with oxygen, so that in principle no special oxidation catalyst different from this is required. In a The embodiment is carried out in the presence of one or more oxidation catalysts which selectively catalyze the combustion of hydrogen to oxygen in the presence of hydrocarbons. The combustion of the hydrocarbons with oxygen to CO and CO ₂ takes place only to a minor extent, which has a clearly positive effect on the selectivities achieved for the formation of the alkenes. The dehydrogenation catalyst and the oxidation catalyst are preferably present in different reaction zones.

The catalyst, which selectively catalyzes the oxidation of hydrogen in the presence of hydrocarbons, is preferably arranged at the points where there are higher oxygen partial pressures than at other points in the reactor, in particular in the vicinity of the feed point for the oxygen-containing gas. Oxygen-containing gas and / or hydrogen can be fed in at one or more points in the reactor.

A preferred catalyst that selectively catalyzes the combustion of hydrogen contains oxides or phosphates selected from the group consisting of the oxides or phosphates of germanium, tin, lead, arsenic, antimony or bismuth. Another preferred catalyst that catalyzes the combustion of hydrogen contains a noble metal from VDT. or I. subgroup.

The dehydrogenation catalysts used generally have a support and an active composition. The carrier consists of a heat-resistant oxide or mixed oxide. The dehydrogenation catalysts preferably contain a metal oxide, which is selected from the group consisting of zirconium dioxide, zinc oxide, aluminum oxide, silicon dioxide, titanium dioxide, magnesium oxide, lanthanum oxide, cerium oxide and mixtures thereof, as a carrier. Preferred carriers are zirconium dioxide and / or silicon dioxide; mixtures of zirconium dioxide and silicon dioxide are particularly preferred.

The active mass of the dehydrogenation catalysts generally contain one or more elements of the VHI. Secondary grapple, preferably platinum and / or palladium, particularly preferably platinum. In addition, the dehydrogenation catalysts can have one or more elements of I. and / or π. Main group, preferably potassium and / or cesium. Furthermore, the dehydrogenation catalysts can contain one or more elements of the HL secondary group including the lanthanides and actinides, preferably lanthanum and / or cerium. Finally, the dehydrogenation catalysts can be one or more Elements of HI. and / or TV. Have main group, preferably one or more elements from the group consisting of boron, gallium, silicon, germanium, tin and lead, particularly preferably tin.

In a preferred embodiment, the dehydrogenation catalyst contains at least one element of the VIH. Sub-group, at least one element of the I. and / or H. main group, at least one element of the HI. and / or IV. main group and at least one element of HI. Subgroup including the lanthanides and actinides.

The alkane dehydrogenation is usually carried out in the presence of water vapor. The added steam serves as a heat carrier and supports the gasification of organic deposits on the catalysts, which counteracts the coking of the catalysts and increases the service life of the catalyst. The organic deposit rods are converted into carbon monoxide and carbon dioxide.

The dehydrogenation catalyst can be regenerated in a manner known per se. Steam can be added to the reaction gas mixture or an oxygen-containing gas can be passed over the catalyst bed at elevated temperature from time to time and the separated carbon can be burned off.

Suitable alkanes which can be used in the process according to the invention have 2 to 14 C atoms, preferably 2 to 6 C atoms. Examples are ethane, propane, n-butane, isobutane, pentane and hexane. Ethane, propane and butanes are preferred. Propane and butane are particularly preferred, and propane is particularly preferred.

The alkane used in the alkane dehydrogenation need not be chemically pure. For example, the propane used can contain up to 50% by volume of further gases such as ethane, methane, ethylene, butanes, butenes, propyne, acetylene, H ₂ S, SO and pentanes. The butane used can be a mixture of n-butane and isobutane and can, for example, up to 50 vol .-% methane, ethane, ethene, propane, propene, propine, acetylene, C ₅ - and C ₆ hydrocarbons and H ₂ S and SO ₂ included. The raw propane raw butane used generally contains at least 60% by volume, preferably at least 70% by volume, particularly preferably at least 80% by volume, in particular at least 90% by volume and very particularly preferably at least 95% by volume of propane or Butane. In the alkane dehydrogenation mixture, a gas mixture is obtained which contains secondary constituents in addition to alkene and unreacted alkane. Common secondary components are hydrogen, water, nitrogen, CO, CO ₂ , and cracking products of the alkane used. The composition of the gas mixture leaving the dehydrogenation stage can vary widely. For example, if the dehydrogenation is carried out with the addition of oxygen and additional hydrogen, the product gas mixture will have a comparatively high content of water and carbon oxides. In modes of operation without feeding in oxygen, the product gas mixture of the dehydrogenation will have a comparatively high content of hydrogen. For example, in the case of the dehydrogenation of propane, the product gas mixture leaving the dehydrogenation reactor contains at least the components propane, propene and molecular hydrogen. In addition, however, it will generally also contain N ₂ , H ₂ O, methane, ethane, ethylene, CO and CO. Usually it will be under a pressure of 0.3 to 10 bar and often have a temperature of 400 to 700 ° C, in favorable cases 450 to 600 ° C.

The invention is illustrated by the examples below.

example 1

catalyst preparation

5000 g of a split ZrO ₂ / SiO ₂ mixed oxide from Norton (sieve fraction 1.6-2 mm) were mixed with a solution of 59.96 g SnCl ₂ -2H ₂ O and 39.43 g H ₂ PtCl ₆ -6H ₂ O soaked in 2000 ml ethanol according to the solvent absorption. The composition was

Rotated at room temperature for 2 hours, then dried at 100 ° C. for 15 hours and

Calcined at 560 ° C for 3 hours.

The catalyst was then impregnated with a solution of 38.55 g of CsNO ₃ , 67.97 g of KNO ₃ and 491.65 g of La (NO ₃ ), which were made up to 2000 ml with total solution, in accordance with the water absorption. The catalyst was rotated at room temperature for 2 hours, then dried at 100 ° C. for 15 hours and calcined at 560 ° C. for 3 hours.

The catalyst had a BET surface area of 84 m ² / g. Example 2

Dehydration from propane to propene

125 ml corresponding to 140.57 g of the catalyst prepared according to Example 1 were intimately mixed with 1375 ml steatite balls (diameter 1.5 to 2.5 mm) and installed in a tubular reactor with an inner diameter of 40 mm and a length of 180 cm. The 114.5 cm long catalyst layer was placed so that the catalyst was in the isothermal area of the electrically heated reactor tube. The remaining volume of the reactor tube was filled with steatite balls (diameter 4 to 5 mm). The reactor was heated to 500 ° C. (reactor wall temperature) with a nitrogen stream of 250 Nl / h and a reactor outlet pressure of 1.5 bar.

The catalyst was successively for 30 minutes at 500 ° C first with dilute hydrogen (50 Nl / h H ₂ + 200 Nl / N ₂ ), then with undiluted hydrogen (250 Nl / h H ₂ ), then with flushing nitrogen (1000 Nl / h N ₂ ), then with lean air (50 Nl / h air + 200 Nl / h N ₂ ), then with undiluted air (250 Nl / h air), then with flushing nitrogen (1000 Nl / h N ₂ ), then with dilute hydrogen (50 Nl / h H ₂ + 200 Nl / N ₂ ) and then charged with undiluted hydrogen (250 Nl / h H ₂ ).

Subsequently, the catalyst at 612 ° C. (reactor wall temperature) was charged with 250 Nl / h of propane (99.5%) and with 250 g / h of water vapor. The reactor outlet pressure is 1.5 bar. The reaction products were recorded by gas chromatography. After a reaction time of two hours, 47% of the propane used was converted with a selectivity to propene of 97%. After a reaction time of 10 hours, the conversion was 42% and the selectivity 97%.

Comparative example

125 ml corresponding to 140.57 g of the catalyst prepared according to Example 1 were installed undiluted in a tubular reactor with an internal diameter of 40 mm and a length of 180 cm.

The 9.5 cm long catalyst layer was placed so that the catalyst in the Isothermal area of the electrically heated reactor tube was. The remaining volume of the reactor tube was filled with steatite balls (diameter 4-5 mm). The reactor was heated to 500 ° C. (reactor wall temperature) at a nitrogen stream of 250 Nl / h and a reactor outlet pressure of 1.5 bar.

The catalyst was activated with hydrogen and air as described in Example 2.

Subsequently, the catalyst at 612 ° C. (reactor wall temperature) was charged with 250 Nl / h of propane (99.5%) and with 250 g / h of water vapor. The reactor outlet pressure was 1.5 bar. The reaction products were recorded by gas chromatography. After a reaction time of two hours, 25% of the propane used was converted to propene with a selectivity of 96%. After a reaction time of 10 hours, the conversion was 24% and the selectivity was 97%.

Claims

claims

1. Isothermal process for the dehydrogenation of alkanes to the corresponding alkenes on a catalyst bed containing a dehydrogenation-active catalyst, characterized in that the catalyst bed contains catalytically inactive, inert diluent material.

2. The method according spoke 1, characterized in that the catalytically inactive, inert dilution material is selected from the Grappe consisting of the oxides of H., HI. and IV. main group, the HL, IV. and V. subgroup and their mixtures, and of nitrides and carbides of elements of the HL and IV. main group.

3. The method according spoke 1 or 2, characterized in that the catalytically inactive, inert dilution material is selected from the group consisting of magnesium oxide, aluminum oxide, silicon dioxide, steatite, titanium dioxide, zirconium dioxide, niobium oxide, thorium oxide, aluminum nitride, silicon carbide, magnesium silicate, aluminum silicate, Clay, kaolin, pumice and their mixtures.

4. The method according to any one of claims 1 to 3, characterized in that the catalytically inactive, inert diluent has a BET surface area of <10 m ² / g.

5. The method according to any one of claims 1 to 4, characterized in that the catalytically inactive, inert dilution material has a coefficient of thermal conductivity of> 0.04 W / (m x K).

6. The method according to any one of claims 1 to 5, characterized in that the space / time yield, based on the alkene formed, limited to 7.0 kg / (kg _Sc bed h) by the presence of the catalytically inactive diluent material in the catalyst bed becomes.

7. The method according to any one of claims 1 to 6, characterized in that the catalytically inactive, inert diluent material in the form of moldings, selected from the group consisting of tablets or strands with a Diameters of 2 to 8 mm on average, a height of 2 to 16 mm on average, the height being 0.5 to 4 times the diameter, rings or hollow strands with an outer diameter and a height of 6 to 20 on average mm, the height being 0.5 to 4 times the diameter and the wall thickness being 0.1 to 0.25 times the diameter, and spheres with an average diameter of 1 to 5 mm are included.

Method according to one of claims 1 to 7, characterized in that the leeward portion of the bed is at least 30%.

9. The method according to any one of claims 1 to 8, characterized in that the dehydrogenation-active catalyst one or more elements of the VHT. Sub-group, one or more elements of the I. and / or H. main group, one or more elements of the HI. Subsidiary group including the lanthanides and actinides and one or more elements of HI. and / or IV. Main group contains on an oxidic support.

10. The method according to any one of claims 1 to 9, characterized in that it is carried out in a tube or tube bundle reactor.

11. The method according to any one of claims 1 to 10, characterized in that propane is dehydrated.