NO310807B1 - Catalyst pretreatment - Google Patents

Description

Field of the invention.

The present invention relates to a method for a pretreatment activation of a Pt/Sn-based catalyst and a catalyst activated by such pretreatment.

Background for the invention

During industrial operation, catalysts are permanently deactivated due to poisoning and sintering and must be replaced at given time intervals, generally after 1 to 5 years of operation, depending on feed streams and reaction conditions. Generally, the catalysts must be treated according to a special pretreatment procedure before the catalyst can operate at optimum performance (see in particular WO 92/06784 where conditions for optimum reduction of a cobalt catalyst are described). Especially when catalysts have to be replaced at short intervals, it is of great importance to reduce the pretreatment time. The lifetime of a typical Pt/Sn-based dehydrogenation catalyst is up to 3 years. The catalyst used in the STAR process (Pt/Sn on zinc aluminate) has an expected lifetime of 1 to 2 years in commercial operation, while the catalyst used in the Oleflex process (Pt/Sn/Cs on Al203) has an expected lifetime of 1 to 3 year [Catalytica (1993)]. Significant losses in alkene production can be anticipated if a long activation period is required after each catalyst replacement. Such catalysts or variations thereof can also be used for several chemical processes which include mainly or partially dehydrogenation or hydrogenation steps. A long activation period is also a disadvantage associated with unexpected stops and experimental development in the laboratory.

The catalysts mentioned in the present invention have previously been described in patent applications number 932173 and 981126.

Summary of the invention.

There is a need for methods for more quickly obtaining Pt/Sn-based dehydrogenation catalysts with the maximum catalytic activity than has been possible with the previously known technology, which may require many hundreds of hours in current before the approximately maximum activity is achieved. Such a method would improve the overall yields of dehydrogenation products that are obtained during the lifetime of a dehydrogenation catalyst.

The disadvantages of the prior art are overcome by a method for a pretreatment activation of a Pt/Sn-based dehydrogenation catalyst, where a Pt/Sn/Mg(AI)O catalyst is subjected to several (repeated) cycles of 1. ) reduction with H2 by elevated temperatures. 2. ) alkane dehydrogenation at elevated temperatures; 3. ) oxidation/regeneration with an O2-containing gas at elevated temperatures;

Preferably, the alkane dehydrogenation is carried out at temperatures in the range of 500 to 700 °C.

Likewise, the oxidation/regeneration is preferably carried out at temperatures in the range of 500 to 700 °C.

Likewise, the reduction with H2 is preferably carried out in the range 500 to 700 °C.

In particular, the chemical process dehydrogenation is carried out on a lower alkane.

More preferably, propane is dehydrogenated.

In a preferred process, the oxidation is carried out with a gaseous O2/N2 mixture in which the O2 content is gradually increased.

In a preferred process, the final phase of the oxidation is carried out in air.

With regard to the reduction step, this is preferably carried out in H2 in a temperature range of 500 to 700 °C.

With regard to the dehydrogenation time, this is less than half of the dehydrogenation time under normal dehydrogenation process conditions.

A typical dehydrogenation time is 2 hours.

Good results are obtained when the first part of the oxidation is carried out for approx. 3 hours, while the final phase of the oxidation is carried out for approx. 1 hour.

The reduction in H2 is especially carried out in approx. 2 hours.

The best results are obtained when the number of cycles is 6 to 12, but fewer cycles also have a significant effect.

It is expected that an optimal pretreatment activation procedure will largely depend on the exact preparation and method of preparation of the catalyst. This means that there will be wide variations in the preferred number of cycles, gas mixtures and required times for each stage.

A further object of the present invention is a catalyst activated by the pretreatment indicated above.

Detailed description of invention

As already mentioned above, the object of the present invention is to define a pretreatment procedure which reduces the time required to reach an optimum conversion level in an alkane dehydrogenation unit using a Pt/Sn/Mg(AI)0 catalyst described in NO patent 179131. In this earlier invention, we describe a pretreatment procedure which leads to an optimum dehydrogenation activity of a Pt/Sn/Mg (Al)O catalyst obtained by impregnation of Pt and Sn on a precalcined Mg(AI)0 support material. The pretreatment procedure used in this earlier invention consisted of in situ reduction, followed by oxidation and a further period of reduction (ROR). In the present invention, it was surprisingly observed that when using a slightly modified metal impregnation procedure, the ROR pretreatment procedure did not lead to an optimal dehydrogenation activity. During testing of the dehydrogenation catalyst, it was observed that the conversion level after the first regeneration was higher than the initial conversion level after ROR. After the next test cycle, the conversion level was even higher. A stable catalyst performance was not achieved until after 300 hours on stream, when the catalyst had been subjected to 9 cycles consisting of a dehydrogenation period followed by regeneration and reduction. A catalyst pretreatment procedure that dramatically reduced the time required to achieve optimum conversion levels in dehydrogenation of propane was developed.

The following examples will serve to illustrate the advantages of using the pretreatment procedure described here for the activation of the Pt/Sn-Mg(Al)0 dehydrogenation catalyst.

Generally

The catalyst was produced according to a slight modification of the method described in patent NO 179131 using a water-based acidic solution during the deposition of platinum and tin on hydrotalcite or precalcined hydrotalcite. The catalysts used in the examples have a Mg/Al ratio of 4.8. The content of platinum and tin is respectively approx. 0.25 and 0.5% by weight. The catalyst was calcined at 560°C after the impregnation. The catalyst was compressed into tablets, crushed and sieved to a pellet size of 0.64-1.0 mm before testing.

All pretreatment procedures were performed in situ in a laboratory-scale fixed-bed titanium reactor with subsequent testing for propane dehydrogenation. The internal diameter of the reactor was 9 mm. A titanium tube with an external diameter of 3 mm was located in the center of the reactor. The reactor temperature was controlled with a thermocouple placed in the 3 mm tube inside the reactor. The catalyst pellets (about 3

g) was placed on a titanium sinter in the reactor. The total pressure in the reactor was 1.1 bar and the reactor temperature was 600°C. However, due to the reaction being endothermic

nature a temperature gradient observed in the reactor. The reactor temperature was regulated

to 600°C at a distance of 1/3 of the total catalyst bed length from the top of the catalyst bed, while the temperature at the bottom of the catalyst bed, measured at a distance of approximately 1/3 of the catalyst bed length from the bottom of the catalyst bed, was in the range of 590 to 615°C. The GHSV was 1000 hours"<1> based on propane and the reaction gas contained 4.5% hydrogen, 32% propane and the rest steam on a mole basis.

The dehydrogenation times usually lasted about 20 hours and were followed by a regeneration of the catalyst. The conversion levels were calculated from on-line GC analysis. The analysis was taken at approximately 1-hour intervals. Conversion levels presented were obtained after 5 hours in the dehydrogenation period.

The regeneration of the catalyst was carried out by burning the coke formed using air diluted with nitrogen. The content of oxygen was first reduced to about 2% and was gradually increased to a final level of 21% in which clean air was used. The regeneration period was followed by catalyst reduction using hydrogen. Both the regeneration and the reduction of the catalyst were carried out at 600°C.

Example 1 (ROR)

The catalyst was subjected to a pretreatment procedure consisting of the following steps:

1. Heating from ambient temperature to 600°C in nitrogen, 100 ml/min.

2. Reduction in H2. 20 ml/min, 600°C, 2 hours.

3. Oxidation in a stream containing 40 ml/min N2 and 10 ml/min air. 600°C. 1 hour.

4. Oxidation in air. 50 ml/min, 600°C, 1 hour.

5. Reduced in H2. 600°C. 2 hours. 20 ml/min.

The results from the subsequent propane dehydrogenation tests are shown in Figure 1 and Table 1.

Example 2: (ROR - (PDH(600°C)-OR)<*>X)

The catalyst was subjected to a pretreatment procedure consisting of the following steps: 1. The catalyst is first pretreated according to the ROR procedure described in Example 1. 2. PDH (Propane dehydrogenation) at 600°C in a stream consisting of C3H8 (70 ml/min) . H2 (10 ml/min) and steam (140 ml/min) for 2 hours. 3. Oxidation/regeneration using an 02/N2 mixture with a gradually increasing 02 content. (1, 5 and 10%). Each step lasted 1 hour. Different O2 contents were obtained by diluting air with N2. Total flow: 84 ml/min., T=600°C.

4. Oxidation in air. 80 ml/min., 600°C, 1 hour.

5. Reduction in H2. 20 ml/min., 600°C, 2 hours.

Steps 2 to 5 were repeated 9 times, at which time no further increase in the conversion level of propane in the PDH period was observed. The results from the subsequent propane dehydrogenation test are shown in Figure 1 and Table 1.

Example 3 (R-OR<*>X)

The catalyst was subjected to a pretreatment procedure consisting of the following steps:

1. Heating from ambient temperature to 600°C in nitrogen, 100 ml/min.

2. Reduction in H2. 20 ml/min., 600°C, 2 hours.

3. Oxidation/regeneration using an O2/N2 mixture with a gradually increasing O2 content (1, 5 and 10%). 1 hour in each step. Different O2 contents were obtained by diluting air with N2. Total flow: 84 ml/min. T=600°C.

4. Oxidation in air. 80 ml/min., 600°C, 1 hour.

5. Reduction in H2. 20 ml/min., 600°C, 2 hours.

Steps 3 to 5 are repeated 12 times. The results from the following PDH conversion tests are shown in Figure 1 and Table 1.

Example 4: (ROR - (PDH-OR)<*>X)

The catalyst was subjected to a pretreatment procedure consisting of the following steps: 1. The catalyst is first pretreated according to the ROR procedure described in Example 1. 2. PDH (Propane dehydrogenation) at 650°C in a stream consisting of C3H8 (70 ml/min) . H2 (10 ml/min) and steam (140 ml/min) for 2 hours. 3. Oxidation/regeneration in an 02/N2 mixture with a gradually increasing 02 content (1.5 and 10%). 1 hour in each step. Different O2 contents were obtained by diluting air with N2. Total flow: 84 ml/min., T=600°C.

4. Oxidation in air. 80 ml/min., 600°C, 1 hour.

5. Reduction in H2. 20 ml/min., 600°C, 2 hours.

Steps 2 to 5 were repeated 6 times, so that no increase in the conversion level of propane in the PDH period was observed anymore. The results from the subsequent propane dehydrogenation tests are shown in Figure 1 and Table 1.

Example 5: R-(PDH-OR)<*>X

The catalyst was subjected to a pretreatment procedure consisting of the following steps: 1. Heating from ambient temperature to 600°C in nitrogen, 100 ml/min.

2. Reduction in H2. 20 ml/min., 600°C, 2 hours.

3. PDH (Propane dehydrogenation) at 630°C in a stream consisting of C3H8 (70 ml/min). H2 (10 ml/min) and steam (140 ml/min) for 2 hours. 4. Oxidation/regeneration using an 02/N2 mixture with a gradually increasing 02 content. (1, 5 and 10%) 1 hour in each step. Different O2 contents were obtained by diluting air with N2. Total flow: 84 ml/min. T=600°C.

5. Oxidation in air. 80 ml/min., 600°C, 1 hour.

6. Reduction in H2. 20 ml/min, 600°C, 0.5 hours.

Steps 3 to 6 were repeated 5 times so that it was found that no further increase in conversion level during the PDH period (Step 3) is observed. The results from the subsequent propane dehydrogenation test are shown in Figure 1 and Table 1.

Example 6: R-(PDH-OR)<*>X

The catalyst was subjected to a pretreatment procedure consisting of the following steps:

1. Heating from ambient temperature to 600°C in nitrogen, 100 ml/min.

2. Reduction in H2. 20 ml/min., 600°C, 2 hours.

3. PDH (Propane dehydrogenation) at 630°C in a stream consisting of C3Hs (70 ml/min). H2 (10 ml/min) and steam (140 ml/min) for 2 hours. 4. Oxidation/regeneration using an 02/N2 mixture with a gradually increasing 02 content. (1, 5 and 10%) 1 hour in each step. Different 02 contents were obtained by diluting air with N2. Total flow: 84 ml/min. T=600°C.

5. Oxidation in air. 80 ml/min., 600°C, 1 hour.

6. Reduction in H2. 20 ml/min, 600°C, 2 hours.

Steps 3 to 6 are repeated 6 times, whereby no further increase in conversion level was observed during the PDH period (Step 3). The results from the subsequent propane dehydrogenation test are shown in Figure 1 and Table 1.

Normalized conversion data from Ex. 1 to Ex. 6 is shown in Figure 1.

Conversion level data for Example 1 to Example 6 are all given as normalized values, where the stabilized conversion level achieved on the fully pretreated catalyst is given the value 100%. A typical example of the achieved conversion level of propane to propene is given in Table 2. Similar conversion levels were also achieved in the other examples. However, the conversion level shows little variation due to small changes in the reaction conditions, such as the temperature profile in the reactor and different partial pressures in the gases entering the reactor.

The results show that when applying the ROR pretreatment procedure described in NO patent 179131 to the present catalyst, the activity of the catalyst increases after each conversion cycle, consisting of a dehydrogenation period followed by regeneration/oxidation and reduction. The catalyst does not work with an optimal performance until after 300 hours in power. By using a pretreatment procedure consisting of repeated cycles of reduction and oxidation and a final reduction (Example 3), the time to reach the optimum performance is reduced to about 50-150 hours. However, several dehydrogenation-regeneration cycles must be performed after the pretreatment procedure in sequence, which provides a high and stable conversion level.

A pretreatment procedure comprising the above ROR procedure followed by many cycles consisting of a short dehydrogenation period (2 hours, at 600°) followed by regeneration (oxidation) and reduction in hydrogen (Example 2) reduced the pretreatment time to approximately 80 hours. When the short dehydrogenation period during the pretreatment was carried out at a higher temperature, i.e. 630-650°C, an even shorter pretreatment time was achieved (Example 4). The initial ROR pretreatment could also be excluded by using this type of catalyst pretreatment. (Examples 5 and 6). A pretreatment time of 40 hours was achieved using a pretreatment consisting of an initial reduction followed by 5 cycles consisting of dehydrogenation, regeneration and reduction. A detailed description of this procedure is given in Example 5. After this, the pretreatment catalyst operates with a conversion level of 90% of the optimum conversion level. During two conventional 20 hour dehydrogenation periods at 600°C, the catalyst performance was at an optimum level.

Failed preprocessing

Example 7: OR

The catalyst was subjected to a pretreatment procedure consisting of the following steps:

1. Heating from ambient temperature to 600°C in nitrogen, 100 ml/min.

2. Oxidation in air. 50 ml/min., 600°C, 10 hours.

3. Reduction in H2. 20 ml/min, 600°C, 2 hours.

The conversion after 5 hours of propane dehydrogenation (same test conditions as described in examples 1 to 6, 600°C) was 11% and the selectivity was 94%.

Example 8: O-Vapor-R

The catalyst was subjected to a pretreatment procedure consisting of the following steps: 4. Oxidation/regeneration of the catalyst from Example 7 using an O 2 /N 2 mixture with a gradually increasing O 2 content. (1, 5 and 10%) 1 hour in each step. Different O2 contents were obtained by diluting air with N2.

Total flow: 84 ml/min. T=600°C.

5. Steaming. 140 ml/min., 600°C, 10 hours.

6. Reduction in H2. 20 ml/min, 600°C, 2 hours.

The conversion after 5 hours of propane dehydrogenation (same test conditions as described in examples 1 to 6, 600°C) was 26% and the selectivity was 95%.

Example 9: O-Vapor-R

The catalyst was subjected to a pretreatment procedure consisting of the following steps: 7. Oxidation/regeneration of the catalyst from example 8 using an 02/N2 mixture with a gradually increasing 02 content. (1, 5 and 10%) 1 hour in each step. Different O2 contents were obtained by diluting air with N2.

Total flow: 84 ml/min. T=600°C.

8. Steaming. 140 ml/min., 700°C, 10 hours.

9. Reduction in H2. 20 ml/min, 600°C, 2 hours.

The conversion after 5 hours of propane dehydrogenation (same test conditions as described in examples 1 to 6, 600°C) was 7.5% and the selectivity was 93.5%.

Conclusion

The activation period of the Pt/Sn/Mg(Al)0 dehydrogenation catalyst is reduced from 300 hours to about 50 hours using the pretreatment procedure developed according to the present invention. The pretreatment procedure consists of an initial reduction of the catalyst using hydrogen, followed by many cycles comprising a short dehydrogenation period.

Literature cited

Catalytica Oxidative Dehydrogeation and Alternative Dehydrogenation Processes Study No,4192 OD; Catalytica Studies Division: Mountain View, CA, 1993

Akporiaye, D., Rønnekleiv, M., Hasselgård, P., NO 179131, 1996, transferred to Statoil.

Claims (15)

1. Method for pretreatment activation of a Pt/Sn-based catalyst, characterized in that a Pt/Sn/Mg (Al)O catalyst is subjected to several (repeated) cycles of 1. ) reduction with H2 at elevated temperatures. 2. ) alkane dehydrogenation at elevated temperatures; 3. ) oxidation/regeneration with an O2-containing gas at elevated temperatures; 2. Method according to claim 1, characterized in that the alkane dehydrogenation is carried out at temperatures in the range 500 to 700 °C. 3. Method according to claim 1, characterized in that the oxidation/regeneration is carried out at temperatures in the range 500 to 700 °C 4. Method according to claim 1, characterized in that the reduction with H2 is carried out in the range 500 to 700 °C. 5. Method according to claims 1 and 2, characterized in that a lower alkane is dehydrogenated. 6. Method according to claims 1, 3 and 5, characterized in that propane is dehydrogenated. 7. Method according to claims 1 and 3, characterized in that the oxidation is carried out with a gaseous 02/N2 mixture gradually increasing in 02 content. 8. Method according to claims 1 and 7, characterized in that the last part of the oxidation is carried out in air. 9. The method according to claim 1, characterized in that the reduction in H2 is carried out in a temperature range of 500 to 700 °C. 10. Method according to claims 1, 2, 5 and 6, characterized in that the dehydrogenation is carried out for approx. 2 hours. 11. Method according to claims 1, 3, 7 and 8, characterized in that the first part of the oxidation is carried out for approx. 3 hours, while the last part of the oxidation is carried out for approx. 1 hour. 12. Method according to claim 1, characterized in that the reduction in H2 is carried out in approx. 2 hours. 13. Method according to claims 1 to 12, characterized in that the number of cycles is more than one. 14. Method according to claims 1 to 12, characterized in that the number of cycles is 6 to 12. 15. Catalyst, characterized in that it is activated during the pretreatment according to claims 1 to 13.