DEHYDROGENATION OF ALKANES TO ALKENES

Abstract

Process for dehydrogenation of alkanesor alkylbenzenes by using metal sulfide catalyst under the presence of small amounts of hydrogen sulfide.

Claims

1. Process for the dehydrogenation of alkanes, alkenes or alkylbenzenes to the corresponding unsaturated chemical products and hydrogen (H.sub.2) comprising contacting the alkane, alkene or alkylbenzene with a metallic sulfide (MeS) catalyst in which the dehydrogenation is conducted in one or more dehydrogenation reactors without using steam (H.sub.2O) as carrier gas for the alkanes, alkenes or alkylbenzenes, and in the presence of hydrogen sulfide (H.sub.2S) without formation of H.sub.2S as a reaction product.

2. Process according to claim 1 in which the molar ratio of hydrogen sulfide to alkanes, alkenes or alkylbenzene is between 0.01 and 0.2.

3. Process according to claim 1 in which the molar ratio of hydrogen sulfide (H.sub.2S) to hydrogen (H.sub.2) is between 0.01 and 0.2.

4. Process according to claim 3 in which the molar ratio of hydrogen sulfide (H.sub.2S) to hydrogen (H.sub.2) is 0.05-0.06.

5. Process according to claim 1, in which the metal (Me) of the metallic sulfide (MeS) is selected from Fe, Co, Ni, Mn, Cu, Mo, W and combinations thereof.

6. Process according to claim 1, in which the process is conducted at temperatures in the range 500-700° C.

7. Process according to claim 1, in which the alkane is selected from ethane, propane, butane, pentane and combinations thereof.

8. Process according to claim 1, wherein the alkene is selected from ethylene, propylene, butene, pentene and combinations thereof.

9. Process according to claim 1, in which the alkylbenzene is ethylbenzene and the corresponding unsaturated chemical product is styrene.

10. Process according to claim 1, in which unreacted alkanes, alkenes, alkylbenzenes and by-products are recycled to the one or more dehydrogenation reactors.

11. Process according to claim 10 in which the unreacted alkanes, alkylbenzenes and by-products include methane, ethane, ethylbenzenes, benzene, toluene, and combinations thereof.

12. Process according to claim 1, in which benzene, toluene, or combinations of both, are used as carrier gas.

13. Process according to claim 1, in which methane, ethane, or combinations of both, are used as carrier gas.

14. Process according to claim 1, in which the dehydrogenation is conducted in adiabatic reactors with a reheating step and selective oxidation of hydrogen produced in the process in between the reactors.

15. Process according to claim 1, in which off-gas containing H.sub.2 and CH.sub.4 produced in the process is used as carrier gas and selective oxidation of hydrogen is conducted upstream the first dehydrogenation reactor.

Description

[0048] The accompanying FIGS. 1-4 serve to illustrate the present invention.

[0049] FIG. 1 corresponds to Example 3 and shows a process according to the prior art for conversion of ethylbenzene to styrene, where steam is used as carrier gas.

[0050] FIG. 2 corresponds to Example 4 and shows a process according to the present invention where ethylbenzene is converted to styrene, in which the carrier gas is a mixture of benzene/toluene and H.sub.2S is also used in the process.

[0051] FIG. 3 corresponds to Example 5 and shows a process according to the prior art for conversion of ethylbenzene to styrene, where steam is used as carrier gas, and where selective oxidation of hydrogen produced in the first reactor is conducted.

[0052] FIG. 4 corresponds to Example 6 and shows a process according to the present invention where ethylbenzene is converted to styrene, in which the carrier gas is a mixture of benzene/toluene, H.sub.2S is also used in the process, and selective oxidation of hydrogen produced in the first reactor is conducted.

[0053] FIG. 5 corresponds to Example 7 and shows a process according to the present invention where ethylbenzene is converted to styrene, in which the carrier gas is a mixture of benzene/toluene and off-gas recycle containing H.sub.2 and CH.sub.4, H.sub.2S is also used in the process, and selective oxidation before the first and second reactor is conducted.

EXAMPLE 1

Invention

Dehydrogenation of Ethylbenzene

[0054] The catalyst used consisted of CoMo oxides supported on a MgAl.sub.2O.sub.4 carrier the catalyst was sulfidized before use, i.e. presulfidized. The amount loaded in the reactor was 10.Math.g catalyst mixed with 10.Math.g inert material also consisting of MgAl.sub.2O.sub.4 (spinel) carrier.

TABLE-US-00001 TABLE 1 Ethyl Styrene Styrene Temp. benzene Styrene H.sub.2 H.sub.2S conv. select. [° C.] [vol %] [vol %] [vol %] [vol %] [%] [%] 550 4.39 1.81 4.64 0.31 28.8 90.9 550 7.42 2.19 5.01 0.31 22.5 94.8 550 2.97 0.88 3.71 0.31 22.8 94.2 575 2.61 1.10 3.92 0.31 29.1 95.2 575 2.47 1.00 3.82 0.31 28.4 95.2 575 2.54 0.94 3.77 0.31 26. 95.30 575 4.26 1.31 4.10 0.31 23.3 95.2 600 3.57 2.04 4.82 0.31 35.3 92.9 600 3.48 1.85 4.63 0.31 33.7 92.8 600 3.65 1.92 4.70 0.31 33.51 93.0 600 5.77 2.96 5.74 0.31 36.0 97.0 600 3.55 1.58 4.36 0.31 30.3 95.6 600 3.41 1.43 4.21 0.31 29.1 95.4

[0055] The loading zone in the reactor was 635 mm to 244 mm from the top flange. The total flow has been around 48 Nl/h throughout the testing period for ethylbenzene dehydrogenation. The pressure was 3 barg, which is the minimum pressure the experiment could be conducted using the present setup. The nonselective products were benzene, toluene, methane, ethane and ethylene. Nitrogen was used as carrier gas. Hydrogen sulfide was added from a hydrogen sulfide/hydrogen mixture thereby adding hydrogen. The surplus of hydrogen may give rise to increased cracking besides limiting the conversion which is relatively close to the calculated equilibrium value.

[0056] Only small amounts of H.sub.2S as measured by the molar (vol.) ratio H.sub.2S/H.sub.2 are used and in the absence of steam. As shown in Table 1, high styrene conversions (close to equilibrium) and high styrene selectivity are obtained with no carbon formation on the catalyst. Particular high conversion and selectivity are obtained at higher temperatures, which is expected since the reaction is endothermic, yet surprisingly high conversion and selectivity where the H.sub.2S/H.sub.2 value is particularly in the range 0.05-0.06, specifically here 0.054 (antepenultimate row of Table 1). The molar ratio of hydrogen sulfide (H.sub.2S) to hydrogen (H.sub.2) is the initial molar ratio and not after the reaction has been performed.

EXAMPLE 2

Invention

Dehydrogenation of Propane

[0057] The dehydrogenation of propane was conducted on the same catalyst as the dehydrogenation of ethylbenzene in Example 1. The conditions with respect to pressure and nitrogen as diluent as well as addition of a hydrogen hydrogen sulfide mixture for keeping the catalyst sulfided were the same. The by-products found were methane, ethane and ethylene. No mercaptanes were found. As shown in Table 2 conversions close to equilibrium and high selectivity are obtainable. No carbon formation on the catalyst was detected.

TABLE-US-00002 TABLE 2 Propane Propene Temp. Propene Propane H.sub.2 H.sub.2S conversion selectivity [° C.] [vol %] [vol %] [vol %] [vol %] [%] [%] 550 1.41 4.14 2.20 0.24 25.2 95.3 575 1.43 4.19 2.20 0.24 25.0 92.1 550 0.56 5.60 2.20 0.24 9.00 94.1 550 0.46 5.74 2.20 0.24 7.38 93.6

EXAMPLE 3

Prior Art

Classic Styrene Process

[0058] Styrene is produced in very large quantities worldwide and used in a variety of products. The dominant process today is based on catalytic dehydrogenation of ethylbenzene using an iron based catalyst.

[0059] FIG. 1 shows a typical process layout. The figures in this patent application are provided without a detailed heat exchange network, e.g. the streams to be heated and cooled are divided into convenient segments where heating and cooling is carried out to the dew or bubble points in order to provide input to a pinch analysis. The pinch analysis provides answers on what will be the minimum needed hot and cold utility provided a predetermined minimum temperature approach in the heat exchanger network. A minimum approach of 20° C. has been used in all cases (Examples 3-7).

[0060] In FIG. 1, ethylbenzene 1 is mixed with steam 2 to form mixed feed stream 3 and preheated to the inlet temperature of 645° C. at the inlet of the first reactor 20. The temperature drops adiabatically to 542° C. across the first reactor due to the reaction. The effluent stream 4 from reactor 20 is reheated to 645° C. before entering the second reactor 30, where effluent 5 leaves at 600° C. It is cooled down to 20° C. before the separator 40, where water is decanted off and the crude styrene/benzene/toluene mixture 6 together with unconverted ethylbenzene is separated out. The remaining traces of aromatics 7 are recovered by means of zeolites or the like in unit 50 and added to the crude styrene stream 6. The off-gas 8 consisting of mainly hydrogen, methane, ethylene and carbon dioxide is compressed to slightly above ambient pressure and used as fuel 9.

[0061] The crude styrene stream is sent to the distillation section 60, where styrene product 10 is obtained. A first column (not shown) separates benzene/toluene from ethylbenzene/styrene and a second column (not shown) separates ethylbenzene from styrene. Final purification of the styrene and benzene/toluene separation has not been included in the process comparisons as they are assumed identical in all cases.

[0062] The conversion of ethylbenzene to styrene has been fixed to 70% by varying the steam content resulting in a steam to carbon ratio of around 1 or a steam to oil weight ratio of around 1.4 as it is usually expressed in the styrene industry.

[0063] The loss of ethylbenzene due to side reactions where ethylbenzene is respectively converted to benzene, ethylene and to toluene, methane is calculated by transforming 2% of the styrene into ethylene and benzene and 3% into toluene and methane.

[0064] All the calculations have been made with a production of 100 MT per hour of styrene leaving the ethylbenzene/styrene splitter column of the second distillation column.

[0065] The result of the pinch analysis of the process is shown in Table 3. The hot utility is 307 MW (Relative hot utility=100).

EXAMPLE 4

Invention

New Styrene Process

[0066] In this example, the other two functions of steam, namely dilution of reaction mixture and heat carrier, are performed by recycling a benzene/toluene mixture, i.e. a benzene/toluene mixture is used as carrier gas. This stream can be obtained by recycle from the product separation columns, i.e. the first distillation column.

[0067] The process is illustrated by FIG. 2.

[0068] Recycled benzene/toluene stream 2 is added instead of steam to fresh and recycled ethylbenzene 1 together with enough H.sub.2S in stream 11 to keep the catalyst sulfide throughout the reactor train according to the equilibrium

9Co+8H.sub.2S=Co.sub.9S.sub.8+8H.sub.2

[0069] For which the equilibrium constant can be estimated from K.sub.p=0.004907*exp(98105/T).

[0070] The feed to the first reactor comprises therefore ethylbenzene, a small amount of H.sub.2S and benzene/toluene as carrier gas. Hydrogen sulfide is recuperated as stream 12 after the benzene/toluene recovery 50.

[0071] The result of the pinch analysis in Table 3 shows that the hot utility requirement is only 278 MW or about 9% lower than the process of Example 4 (prior art).

EXAMPLE 5

Prior Art

Improved Classic Styrene Process

[0072] In an improved version of the classic process (Example 3, FIG. 1) the reheat between the two reactors is provided by selective oxidation in unit 70 using air 13 over a noble metal catalyst of part of the hydrogen produced in the first reactor. FIG. 3 illustrates this concept. The result of the pinch analysis (Table 3) show that this process configuration reduces the need for hot utility to 263 MW which is a substantial improvement compared to the classic process of Example 3 (14% better energy efficiency, from 100 to 86 in terms of relative hot utility), especially because the medium used for reheat between the two reactors in Example 3 is steam superheated to very high temperature, as high as 890° C., which however also requires expensive high alloy steels for construction.

EXAMPLE 6

Invention

[0073] New Styrene Process with Selective Hydrogen Oxidation

[0074] In this embodiment the selective hydrogen oxidation 70 using air 13 in between dehydrogenation reactors is combined with the recycle of benzene/toluene as carrier gas 2 and the addition of small amounts of H.sub.2S in stream 11 according to the invention. This is illustrated in FIG. 4. The result of the pinch analysis in Table 3 shows a surprisingly higher effect on energy efficiency giving rise to about 23% percentage points improvement (from 278 MW to 214 MW of hot utility). The expected result would have been a 14% improvement, as for the prior art processes as described in Example 5.

EXAMPLE 7

Invention

[0075] New Styrene Process with Selective Hydrogen Oxidation Before Both Reactors

[0076] Even better energy efficiency is achieved by introducing also an off-gas recycle back to the first reactor. The principle is illustrated in FIG. 5. The off-gas recycle 14 consisting mainly of H.sub.2, CH.sub.4 and N.sub.2 is also serving as diluent and heat carrier but a small amount of benzene/toluene 2 is also used. Selective oxidation unit 70 is also added upstream the first reactor 20. The result from the pinch analysis in Table 3 shows that the need for hot utility is decreased all the way down to 186 MW and 29% below that of the improved prior art process of Example 5 (263 MW of hot utility). The need for compression energy increases from 1 to 8.5 MW, yet the benefits in energy efficiency are still surprisingly high.

[0077] The results of the calculations are summarized in Table 3:

TABLE-US-00003 TABLE 3 Relative Hot Process Hot Utility, MW Cold Utility, MW Utility Ex. 3 - Prior art 307 145 100 Ex. 4 - Invention 278 238 91 Ex. 5 - Prior art 263 167 86 Ex. 6 - Invention 214 197 70 Ex. 7 - Invention 186 203 61

[0078] The benefits of the inventive process using a sulfur passivated dehydrogenation catalyst are i.a.: [0079] A) The need for hot utility, which will have to be provided by burning additional fuel in a boiler, is significantly lower when avoiding the use of steam as carrier gas but using a benzene/toluene mixture instead. [0080] B) Where off-gas recycle and selective hydrogen oxidation is introduced (Ex. 7) the need for hot utility is reduced even more significantly and it is almost 40% (Ex. 3) or 29% below that of the prior art processes, Ex. 3 and Ex. 5 respectively.