Reactivating propane dehydrogenation catalyst

Abstract

Increase propane dehydrogenation activity of a partially deactivated dehydrogenation catalyst by heating the partially deactivated catalyst to a temperature of at least 660° C., conditioning the heated catalyst in an oxygen-containing atmosphere and, optionally, stripping molecular oxygen from the conditioned catalyst.

Claims

1. An improved process for dehydrogenating an alkane, the process comprising placing an alkane in operative contact with a heated alkane dehydrogenation catalyst in a reactor, the catalyst comprising a Group VIII noble metal and a Group IIIA metal, removing from the reactor a partially deactivated catalyst, rejuvenating the partially deactivated catalyst in a regenerator, and transporting the rejuvenated catalyst from the regenerator to the reactor, wherein the improvement comprises a combination of treatment within the regenerator and transport of the rejuvenated catalyst from the regenerator to the reactor, the treatment within the regenerator comprising sequential steps: a. heating the partially deactivated catalyst to a temperature of at least 660 degrees Celsius using heat generated by combusting both coke contained on the partially deactivated catalyst and a fuel source other than said coke, said heating yielding a heated, further deactivated catalyst which has an alkane dehydrogenation activity that is less than that of the partially deactivated catalyst wherein combusting the fuel source other than said coke occurs concurrent with combusting said coke contained on the partially deactivated catalyst; b. subjecting the heated, further deactivated catalyst to a conditioning step which comprises maintaining the heated, further deactivated dehydrogenation catalyst at a temperature of at least 660 degrees Celsius while exposing the heated, further deactivated dehydrogenation catalyst to a flow of an oxygen-containing gas for a period of time greater than two minutes to yield an oxygen-containing reactivated dehydrogenation catalyst that has an activity for dehydrogenating alkane that is greater than that of either the partially deactivated catalyst or the further deactivated catalyst; and, optionally, c. maintaining the oxygen-containing reactivated catalyst at a temperature of at least 660 degrees Celsius while exposing the reactivated catalyst to a flow of stripping gas that is substantially free of molecular oxygen and combustible fuel for a period of time to remove from the oxygen-containing reactivated dehydrogenation catalyst at least a portion of molecular oxygen trapped within or between catalyst particles and physisorbed oxygen that is desorbable at said temperature during that period of time and yield a rejuvenated dehydrogenation catalyst; with transport from the regenerator to the reactor being effected by a combination of gravity and motive force imparted by an inert transport gas.

2. The process of claim 1, wherein the oxygen-containing gas has an oxygen content within a range of from 5 mole percent to 100 mole percent, each mole percent being based upon total moles of oxygen-containing gas.

3. The process of claim 1, wherein the period of time in step b is within a range of from at least three minutes to no more than 14 minutes.

4. The process of claim 1, wherein the temperature for step b lies within a range of from 660 degrees Celsius to 850 degrees Celsius.

5. The process of claim 1, wherein the partially deactivated catalyst comprises a Group VIII noble metal, and a Group IIIA metal, wherein the Group VIII noble metal is platinum and the Group IIIA metal is gallium.

6. The process of claim 1, wherein one or more of steps a), b) and c) are physically separated from one another.

7. The process of claim 6, wherein steps b) and c) are physically separated from one another and an oxygen-containing atmosphere with an oxygen content within a range of from 5 mole percent to 100 mole percent, each mole percent being based upon total moles of oxygen-containing atmosphere, is used to effect catalyst transfer between steps b) and c).

8. The process of claim 1, wherein the partially deactivated catalyst comprises a promoter metal.

Description

EX 1

(1) Evaluate catalyst performance using 100 mol % air as a regenerating gas and a regeneration step time of 15 minutes (min) Effect heating for the reaction step and the regeneration step using a clam-shell furnace. As noted above, use flowing He between the regeneration step and the subsequent reaction step. Table 1 below shows propane conversion percentage and propylene (C.sub.3H.sub.6) selectivity percentage after a number of cycles as specified in Table 1.

(2) TABLE-US-00001 TABLE 1 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 41.8 99.2 2 42.1 99.2 5 41.7 99.2 8 41.7 99.2 10 42.8 99.2

(3) The results in Table 1 show that heating in the absence of combustion gases together with a conditioning step using an oxygen-containing gas, in this case air, yields relatively stable catalyst performance in terms of C.sub.3H.sub.8 conversion percentage.

EX 2

(4) Replicate Ex 1, but determine C.sub.3H.sub.8 conversion % and C.sub.3H.sub.6 selectivity % 15 seconds after time zero. Summarize results in Table 2 below.

(5) TABLE-US-00002 TABLE 2 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 35.4 99.1 2 37.3 99.4 5 38.0 99.4 8 37.5 99.4 10 37.1 99.4

(6) The results in Table 2 show that catalyst deactivation occurs to some extent in a relatively short time (15 seconds). Once again, heating in the absence of combustion gases leads to relatively stable catalyst performance.

CEX A

(7) Replicate Ex 1, but change the time under PDH reaction conditions from 15 minutes to 30 seconds, substitute a gaseous mixture of 25 mol % water, 5 mol % oxygen and 70 mol % inert gas (19 mol % N.sub.2 and 51 mol % He), each mol % being based upon total moles of gas in the mixture for air, and reduce the regeneration step time to 8 minutes. The gaseous mixture simulates composition of a gas composition within a PDH reactor after combustion of a fuel source and at least a portion of coke contained on the catalyst. Summarize results in Table 3 below.

(8) TABLE-US-00003 TABLE 3 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 40.6 99.2 2 37.7 99.2 5 35.6 99.2 8 34.4 99.1 10 33.5 99.1

(9) The results in Table 3 show that a process that includes only step a) leads to progressively lower catalyst performance results in terms of C.sub.3H.sub.8 conversion % and C.sub.3H.sub.6 selectivity % as the number of cycles increases.

CEX B

(10) Replicate CEx A, but add a regenerating step c) that is a flow of pure He at a flow rate of 120 sccm for 10 minutes subsequent to the 8 minute regeneration time with the gaseous mixture. Summarize results in Table 4 below.

(11) TABLE-US-00004 TABLE 4 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 41.7 99.4 2 38.0 99.2 5 35.5 99.2 8 33.7 99.1 10 33.0 99.1

(12) The results in Table 4 demonstrate that addition of a step c) to step a) provides no improvement in catalyst performance relative to step a) alone.

EX 3

(13) Replicate CEx B, but introduce a conditioning step b) by adding, subsequent to step a), a two minute flow of 100% air at a rate of 150 sccm. Follow step b) with a flow of He (120 sccm) for 20 minute. Summarize results in Table 5 below.

(14) TABLE-US-00005 TABLE 5 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 41.5 99.3 2 37.4 99.2 5 35.7 99.2 8 34.9 99.2 10 34.8 99.2

(15) The results in Table 5 show that a conditioning step of as little as two minutes leads to some improvement in catalyst performance relative to CEx C and CEx D as the number of cycles reaches 5 cycles.

EX 4

(16) Replicate Ex 3, but change the step b) time to five minutes. Summarize results in Table 6 below.

(17) TABLE-US-00006 TABLE 6 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 41.8 99.4 2 38.1 99.3 5 37.9 99.3 8 37.6 99.2 10 37.6 99.3

EX 5

(18) Replicate Ex 4, but change the step b) time to ten minutes. Summarize results in Table 7 below.

(19) TABLE-US-00007 TABLE 7 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 41.1 99.4 2 39.5 99.3 5 39.3 99.3 8 39.5 99.3 10 39.4 99.3

(20) The results in Tables 6 and 7 show that as the length of conditioning step b) increases, catalyst performance levels out to a relatively steady state, with a longer step b) time of ten minutes providing a relatively steady state performance more rapidly than a step b) time of five minutes and at a higher level of performance once it does reach a relatively steady state.

CEX C and D

(21) Replicate Ex 1, but add a hydrogen gas treatment step using 99.99 percent pure H.sub.2 flowing at a rate of 60 sccm for three minutes for CEx C and 15 minutes for CEx D after treatment with flowing He and just prior to the subsequent reaction step. Summarize results in Table 8 below. Table 8 also includes results from Ex 1.

(22) TABLE-US-00008 TABLE 8 Ex/ Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity CEx Of Cycles (%) (%) 1 1 41.8 99.2 1 2 42.1 99.2 C 1 39.0 99.2 D 1 30.9 98.9 D 2 30.9 98.8

(23) The data in Table 8 simulate use of a fuel gas as a motive fluid in CEx C and CEx D and show that it adversely affects catalyst performance in terms of C.sub.3H.sub.8 conversion relative to using He as a motive fluid.

EX 6 and CEX E

(24) Replicate Ex 1 with several changes. First, change the temperature for catalyst evaluation to 625° C. Second, change the feed stream to 90 mol % C.sub.3H.sub.8 and 10 mol % N.sub.2, each mol % being based upon combined moles of C.sub.3H.sub.8 and N.sub.2, and use a reaction time of 60 seconds and a C.sub.3H.sub.8 WHSV of 10 hr.sup.−1. Third, following the reaction time of 60 seconds, heat the catalyst (step a) for three minutes at a temperature of 750° C. with a gaseous mixture of 16 mol % water, 4 mol % oxygen, 8 mol % CO.sub.2 and 72 mol % He, each mol % being based upon combined moles of water, oxygen, CO.sub.2 and He. Fourth, condition the catalyst (step b.) using air as a regeneration gas for 15 minutes at 750° C. Fifth, cool the reactor from 750° C. to 625° C. and strip oxygen from the catalyst (step c) using flowing He as described above. Sixth, effect 142 reaction/regeneration/cooling cycles before collecting data for Ex 6 as shown in Table 9 below for cycle 143.

(25) For CEx E, change the procedure used for the first 143 cycles by adding an intermediate treatment step between steps b and c wherein the catalyst is subjected to two minutes of flowing (60 sccm) methane (CH.sub.4) before collecting data at time zero of cycle 144 as shown in Table 9 below.

(26) TABLE-US-00009 TABLE 9 Ex/ C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity CEx (%) (%) 6 46.1 96.4 E 29.3 94.1

(27) The data in Table 9 demonstrate that exposure to flowing CH.sub.4 compromises catalyst activity and leads to further catalyst deactivation as shown by the drop in both C.sub.3H.sub.8 conversion and C.sub.3H.sub.6 selectivity.

CEX F

(28) Replicate Ex 5, but change the temperature for conditioning from 700° C. to 600° C. and summarize results in Table 10 below.

(29) TABLE-US-00010 TABLE 10 Number C.sub.3H.sub.8 Conversion C.sub.3H.sub.6 Selectivity Of Cycles (%) (%) 1 41.5 99.0 2 38.7 99.0 5 37.4 99.1 8 36.5 99.1 10 35.8 99.1

(30) The data in Table 10 above demonstrate that, as between a conditioning temperature of 700° C. (Ex 5 and associated Table 7) and a conditioning temperature of 600° C. (CEx F and associated Table 10), the latter temperature leads to a less satisfactory result in terms of a more pronounced reduction in C.sub.3H.sub.8 conversion.

CEX G

(31) Replicate Ex 1 with changes. First, change the feed stream to 95 mol % C.sub.3H.sub.8 and 5 mol % N.sub.2, each mol % being based upon combined moles of C.sub.3H.sub.8 and N.sub.2, and use a reaction time of 60 seconds and a feed stream weight hourly space velocity of 8 hr .sup.−1. Second, use a mixture of 25 mol % water (H.sub.2O) and 75 mol % air as a regenerating gas and change the regeneration step time from 15 min to 10 min. Third, during stripping over a period of 30 minutes under flowing He (flow rate of 120 sccm), determine relative mol % of H.sub.2O in stripping step effluent by measuring using mass spectroscopy with H.sub.2O content at a time specified in Table 11 below and dividing it by H.sub.2O content at time zero of the stripping step.

(32) TABLE-US-00011 TABLE 11 Time (sec) Relative Mol % H.sub.2O 0 1.0 15 0.09 59 0.01 240 0.003 1800 0.004

(33) The data in Table 11 suggest that system water content rapidly drops off during stripping and reaches a baseline after 240 seconds that is consistent within experimental error and is believed to represent a “dry” system. When combined with the data from CEx B, this suggests that simple removal of water is not sufficient to promote recovery of catalyst activity. CEx B provides for an additional six minutes (min) of stripping (beyond the 4 min (240 seconds (sec)) shown in Table 11, but catalyst activity, in terms of C.sub.3H.sub.8 conversion continues to drop as the number of cycles increases.

CEX H

(34) Using the conditions of Ex 1 with a C.sub.3H.sub.8 weight hourly space velocity (WHSV) of 20 hr.sup.−1, a reaction temperature of 625° C., a catalyst platinum (Pt) concentration of 200 parts by weight per million parts by weight (ppm) of catalyst and a reaction time of 15 min, effect C.sub.3H.sub.8 dehydrogenation to establish a coke loading on the catalyst. Condition the catalyst as in Ex 1 (100 mol % air at 700° C. for 15 min) and use mass spectrometry to monitor coke combustion via detection of CO.sub.2. As shown in Table 12 below, the CO.sub.2 concentration (mol %) rapidly peaks (at 15 seconds) then drops off dramatically such that it below the detection limit after 2 minutes.

(35) TABLE-US-00012 TABLE 12 Time (sec) Mol % CO.sub.2 (%) 15 0.34 29 0.11 57 0.01 142 0.00

(36) The data in Table 12, when combined with the data for Ex 3 and, especially, Ex 4, suggest that simple coke removal is not sufficient to fully restore catalyst activity.

EX 7

(37) Replicate Ex 6 with several changes. First, change the temperature for catalyst evaluation to 620° C. and C.sub.3H.sub.8 WHSV to 8 hr.sup.−1. Second, vary oxygen concentration in the oxygen-containing atmosphere used for step b) as shown in Table 13 below. Third, vary time of treatment with 100% air at those O.sub.2 concentrations also as shown in Table 13 below. Fourth, collect C.sub.3H.sub.8 conversion data in step a) 30 seconds after starting feed stream flow. Fifth, use a modification of CEx E wherein regeneration is effected in two sequential sub-steps, with sub-step i) exposing the catalyst to a 150 sccm flow of simulated combustion byproducts (4 mol % O.sub.2, 8 mol % CO.sub.2, 16 mol % H.sub.2O and 72 mol % N.sub.2, all mol % being based upon total moles of simulated combustion byproducts) for a period of three minutes followed by sub-step ii) exposing the catalyst to a 150 sccm flow of oxygen-enriched air (see Table 13). Data shown in Table 13 show conversions for the 25.sup.th reaction-regeneration cycle (through steps a) through c) of each variation shown in Table 13.

(38) TABLE-US-00013 TABLE 13 C.sub.3H.sub.8 conversion at specified oxygen concentrations in step b) air Time (min) 21 mol % O.sub.2 45 mol % O.sub.2 75 mol % O.sub.2 4 45.2 46.7 48.7 6 46.7 48.2 50.7 8 48.6 51.1 52.0 10 49.3 50.7 Not measured

(39) The data presented in Table 13 demonstrate that increased oxygen concentration in air used for step b) has a beneficial effect upon C.sub.3H.sub.8 conversion under the conditions set forth in this Example 7. The data also show that oxygen is involved in the rejuvenating process of recovering catalyst activity

(40) Based upon a review of the data presented in Ex 1-7 and CEx A-H, it appears that catalyst activity regeneration or restoration requires a period of exposure to an oxygen-containing atmosphere after the heating or combustion step a). It also appears that oxygen participates in catalyst activity restoration. It further appears that simple H.sub.2O removal and carbon dioxide (CO.sub.2) removal are not sufficient to effect catalyst activity restoration.