A PROCESS FOR THE DEHYDROGENATION OF ALKANES TO ALKENES AND IRON-BASED CATALYSTS FOR USE IN THE PROCESS

Abstract

In a process for the catalytic dehydrogenation of lower alkanes to the corresponding alkenes, a regenerable catalyst comprising iron carbide supported on a carrier is used. A small amount (below 100 ppm) of a sulfur compound, such as H.sub.2S, is added during the process.

Claims

1. A process for the catalytic dehydrogenation of lower alkanes to the corresponding alkenes according to the reaction
C.sub.nH.sub.2n+2<->C.sub.nH.sub.2n+H.sub.2 in which n is an integer from 2 to 5, wherein the catalyst comprises a catalytically active iron compound supported on a carrier, and wherein a sulfur compound is added during the process.

2. The process according to claim 1, wherein the catalytically active iron compound is iron carbide.

3. The process according to claim 1, wherein the sulfur compound is hydrogen sulfide, which is added in an amount from above 0 to below 100 ppm.

4. The process according to claim 3, wherein the hydrogen sulfide is added in an amount from above 0 to below 50 ppm.

5. A catalyst for use in the catalytic dehydrogenation of lower alkanes to the corresponding alkenes according to claim 1, which is a regenerable catalyst that comprises iron carbide supported on a carrier, said iron carbide being formed during the catalytic dehydrogenation process.

6. The catalyst according to claim 5, wherein the steps for regeneration comprise oxidation in dilute air, conversion of the carbide into the corresponding oxide and conversion back to the sulfide by reduction in dilute hydrogen containing hydrogen sulfide in an amount below 100 ppm, and conversion of the sulfide into the catalytically active carbide by reaction with a carbon-containing gas.

7. The catalyst according to claim 6, wherein the carbon-containing gas is the reaction mixture for dehydrogenation.

Description

EXAMPLE 1

Activity Test

[0040] The test was done using a quartz reactor placed inside a stainless steel reactor by use of the catalyst prepared in Example 2. The catalyst was heated to the process temperature using nitrogen with 2% hydrogen and 0.02% H.sub.2S. Thus, at the start of the experiment, the iron sulfate had been converted into iron sulfide.

[0041] The influence of the propane/hydrogen ratio was studied. Experiment runs exceeding 24 hours each were done using 20 Nl/h of propane, 0.5 Nl/h of a gas containing 1% H.sub.2S and 99% H.sub.2 plus 5, 10 and 20 Nl/h hydrogen. The sulfur level thus corresponds to 100-200 ppm. The temperature was 620° C. and the pressure was 2 barg. The amount of propene formed declined proportionally with time. Besides, carbon was formed on the catalyst.

[0042] The amount of carbon formed was determined by treating the catalyst with dilute air and measuring the carbon dioxide formed. A subsequent reduction and sulfidation completely restored the activity. In the last sample, the carbon was determined using a LECO instrumental analysis. The selectivity was assessed by relating the amount of carbon (CO.sub.2) formed to the amount of C.sub.3H.sub.6 formed on a molar basis. The results are given in Table 1 below.

TABLE-US-00001 TABLE 1 Selectivity results Initial activity, Deactiv. rate, % Selectivity, Nl C.sub.3H.sub.8/H.sub.2 % C.sub.3H.sub.8 C.sub.3H.sub.6/hour CO.sub.2/Nl C.sub.3H.sub.6 4 13.5 0.21 0.0125 2 10.5 0.08 0.008 1 6.2 0.0013 0.002

EXAMPLE 2

Preparation of 5% Fe Catalyst

[0043] 12.44 g of FeSO.sub.4.7H.sub.2O is dissolved in water, and the volume of the solution is adjusted to 40 ml. The solution is used to impregnate 47.5 g of a support (with a pore volume of 0.75 ml/g). The sample is rolled for 1 hour, dried overnight at 100° C. and then calcined at 600° C. for 2 hours (4 hours heating ramp). Subsequently, the sample is washed in 100 ml of a 2% K.sub.2CO.sub.3 solution for 1 hour (rolling board). Afterwards, the sample is washed three times with 250 ml water (1 hour each, rolling board). Finally, the sample is filtered and dried overnight at 100° C.

[0044] Data obtained by testing the prepared catalyst are given in Table 2 below.

TABLE-US-00002 TABLE 2 inlet exit temp flow flow % % % % % % P ° C. Nl/h Nl/h C.sub.3H.sub.8 C.sub.3H.sub.6 H.sub.2 CH.sub.4 C.sub.2H.sub.6 C.sub.2H.sub.4 atm 630 45.8 49.6 44 9.5 40 2.6 2.1 0.9 3.2 620 30.5 32.9 44 9.4 40 2.5 2.1 0.7 3.2 610 30.5 32.6 48 8.1 40 1.9 1.7 0.4 3.2 600 30.5 32.4 52 7.0 40 1.1 1.0 0.3 3.2 620 61 65.0 51 7.0 41 1.7 1.1 0.6 3.5

EXAMPLE 3

Preparation of 5% Fe Catalyst

[0045] First, 133.3 g of carrier is impregnated with a solution of 26.24 g of Mg(NO.sub.3).sub.2.6H.sub.2O in 100 ml water, rolled for 1 hour, dried overnight at 100° C. and then calcined at 350° C. for 2 hours.

[0046] Then, 6.22 g of FeSO.sub.4.7H.sub.2O is dissolved in water, and the volume of the solution is adjusted to 19 ml. This solution is used to impregnate 23.2 g of the support previously prepared (with a pore volume of 0.75 ml/g). The sample is rolled for 1 hour and dried overnight at 100° C.

EXAMPLE 4

Temperature Effect

[0047] The carbon formation was measured on the catalyst prepared in Example 3 at three different temperatures using 20 or 10 Nl of Propane 10 or 5 Nl of hydrogen and 0.25 Nl of 1% H.sub.2S in hydrogen. This corresponds to a sulfur level of 70-150 ppm. The low flow was applied at 580 and 600° C.

[0048] The results are summarized in Table 3 below.

[0049] Using thermodynamic considerations, the phase boundary between iron sulfide and iron carbide can be calculated using the reaction:

9 FeS+C.sub.3H.sub.8+5 H.sub.2<->3 Fe.sub.3C+9 H.sub.2S

[0050] The results are given in the last column of Table 3.

TABLE-US-00003 TABLE 3 Temperature effect Initial ppm H.sub.2S at Temp. activity, Deactivation rate, Selectivity, Nl phase ° C. % C.sub.3H.sub.6 % C.sub.3H.sub.6/hour CO.sub.2/Nl C.sub.3H.sub.6 boundary 580 5.8 0.009 0.0017 850 600 7.0 0.017 0.003 1140 620 8.5 0.061 0.006 1500

EXAMPLE 5

Preparation and Testing of 2 wt % Fe Catalyst

[0051] 24.3 g of a carrier (with a pore volume of 0.75 ml/g) is placed in a beaker, and 25 ml 1-pentanol is added. The carrier is soaked in the alcohol for 10 minutes.

[0052] 2.49 g of FeSO.sub.4.7H.sub.2O is dissolved in water, and the volume of the solution is adjusted to 20 ml. This solution is used to impregnate the pre-wet support. The sample is rolled for 1 hour, air dried for 3 hours and then dried overnight at 100° C. 10 g of the sample was placed in a quartz reactor and heated to 620° C. in a stream of H.sub.2, N.sub.2 and H.sub.2S. At this condition, the state of iron is expected to be sulfidic.

[0053] Testing then took place, using 10 Nl of propane along with 5 Nl of hydrogen and 0.25 Nl of a mixture of H.sub.2 and 1% of H.sub.2S. After 60 hours of testing, the catalyst was regenerated using a mixture of 1% O.sub.2 in N.sub.2. The sulfur content is insufficient for keeping iron in the sulfide state; instead it is expected that it is carbidic as observed in previous tests.

[0054] After regeneration and resulfidation, the catalyst is tested again, this time in a mixture of 20 Nl propane and 10.25 Nl hydrogen without addition of sulfur. After 30 hours of testing, it was regenerated again and tested for 20 hours before being regenerated and resulfided.

[0055] The catalyst was tested for 15 hours in the gas containing 20 Nl propane and 10.25 Nl hydrogen. During this treatment, it deactivated from 7.7% propene to 5.8% propene. At the same time, the formation of CH.sub.4 increased from 1.6% to 2.4%.

[0056] During the regeneration, 1.5 Nl of CO.sub.2 was produced. This corresponds to a carbon content of 8% on the catalyst.

[0057] After regeneration and resulfidation, the catalyst was tested in a gas containing 20 Nl propane, 10 Nl hydrogen and 0.25 Nl of a mixture of 1% H.sub.2S in H.sub.2. During the run, there was hardly any change in the propene content, which was 7.4%. The formation of CH.sub.4 remained at 1.5%. During the regeneration, around 0.06 Nl CO.sub.2 was produced. This amount corresponds to a carbon content of 0.3%.

[0058] Thus it has been demonstrated that a sulfur content as low as 80 ppm in the gas is sufficient to drastically reduce the carbon formation and thereby the deactivation of the catalyst. Also the reduced formation of methane will result in a better selectivity.

EXAMPLE 6

Behavior of an Iron Catalyst Under Various Conditions

[0059] The experiments are typically run at a propane/hydrogen ratio of 2 with a gas containing 200 ppm H.sub.2S and an SV of 2000. The catalyst was made by impregnation of a spherical alumina carrier with iron sulfate. It is observed that the propene content in the exit gas rises to around 11% and then falls slowly due to carbon formation which leads to clogging of the pore system.

[0060] After about 20 hours, the catalyst is regenerated with dilute air, i.e. 1-2% O.sub.2 in N.sub.2, and the content of CO.sub.2 is measured. That is the black top in the FIGURE.

[0061] No matter to which iron compound the catalyst may have ended up during the dehydrogenation process, be it carbide or sulfide, then it has been converted to oxygen during the regeneration. Alternatively, by regenerating at lower temperatures, the sulfide can be converted to sulfate, but at .sup.˜620° C. iron oxide is formed. This iron oxide must be activated by a reduction. The reduction after .sup.˜20 hours takes place in a gas mixture consisting of 16% H.sub.2 and 0.16% H.sub.2S, the rest being N.sub.2. The reduction itself only takes .sup.˜1 hour. Then, shifting to the reaction mixture, the reaction is run for .sup.˜7 hours each at SV 4000, 2000 and 1000, respectively.

[0062] During the subsequent regeneration, 0.54 Nl CO.sub.2 is formed after 45 hours. The experiment is repeated at 600° C. In this case, only 0.16 Nl CO.sub.2 has been formed after 70 hours. When the catalyst is subsequently regenerated and tested under standard conditions (SV 2000), a very high initial activity of 12%, declining to 10.3% in 85 hours, is seen. The catalyst is regenerated, and 0.19 Nl CO.sub.2 is formed. After reduction with a gas without H.sub.2S, the catalyst is tested under standard conditions for 92-100 hours. This time, a much lower initial activity which increases, is observed. Furthermore, formation of methane is seen and, in the subsequent regeneration, much more CO.sub.2 (2.5 Nl in 100-105 hours) is observed. The catalyst is not reduced this time, but directly started under standard conditions after regeneration.

[0063] Again, a low initial activity and a large degree of methane formation can be seen. The amount of CO.sub.2 formed is 4.4 Nl corresponding to 2.4 g carbon on the catalyst, i.e. slightly above 20 wt %.

[0064] Thus it has been demonstrated that the presence of even very small amounts of sulfur content, typically down to .sup.˜50 ppm, leads to a catalyst with a high initial activity and a very limited tendency to carbon formation.