US 2500482 A
Description (OCR text may contain errors)
March 14, 1950 c. BARTl-:R 2,500,482
CATALYTI C DEHYDROGENATI ON Filed July so, 1947 75 N 5 4o 7o B U 30 Ag s @5 D 2O 5..... .2 @o A E ro Y i 55 o @o :2o lao 24o soo soo 42o Process Period, Mnui-zs 8 50 Z5 O I 2 5 4 5 (o Process Period, Hours 5a+um+or F iq. II
Produd Rzqznemor Flu@ G05 -r- L -vfRmdor- :C l5 '4 .-4 l -2 u .2Q IO IH/ HHH 'w f l Feed I \f '2A' 5 In 7 f n l g k Patented 14, 195o CATALYTIC DEHYDBOGENATION Cyril Barter, Berkeley, Calif., assigner to Shell Development Company acorporation of Delaware San Francisco, Calif.,
Application .my so, um, sensi No. 164,841
4 claims. (ci. 26o-ess) This invention relates to catalytic dehydrogenation with metal oxide dehydrogenation catalysts which are periodically regenerated by burning carbonaceous deposits therefrom. I The term catalytic dehydrogenation as used throughout thepresent description and the appended claims. is used to denote a process or Ireaction in which one or more atoms of hydrogen is split and removed from a dehydrogenatable dehydrogen-containing"compound by the catalytic action of a catalyzer. In dehydrogenation the splitting of the C-H bond generally requires relatively severe conditions which closely approach those causing cracking or the breaking of C-C bonds. a consequence, the dehydrogenation of dehydrogenatable organic compounds is generally accompanied by a minor amount of such degradation or cracking reactions which lead to the formation of some tarry carbonaceous material. This tarry carbonaceous material deposits on the surface of .the catalyst and soon renders it ineffective.
It is known that the formation and depositionA of such tarry material can be appreciably repressed by the application of a suilicient partial pressure of hydrogen in the dehydrogenation zone. Thus, in some cases it is recommended to recycle a large amount of hydrogen (product gas) through the reaction zone, preferably under considerable pressure. Since, however, hydrogen is a product of reaction in dehydrogenation and since, furthermore, dehydrogenation is a reaction which results in an increase in volume, the use process to distinguish it from non-regenerative or continuous processes where regeneration, if carried out at all, is carried out only at very infrequent intervals. The regenerative manner of operation requires a plant provided with adequate facilities for Vperiodic regeneration of the catalyst.
Onthe other hand this method of operation has the ,advantages that it usually requires lowerl temperatures and pressures, and a considerable portion of-'the heat liberated in the regeneration can be stored in the catalyst and utilizedin the subsequent dehydrogenation period. For this latter reason it isoften desirable to use quite short: process periods. Short process periods down -toabout 10 minutes may be used with xed beds of catalysts. However, for operation with short process periods a system in which the catalyst is continuously recycled through the dehydrogenation zone and the regeneration zone is more advantageous.
In'spite of the advantages of the regenerative method of operation at low pressure lover operaof added hydrogen under pressure has the material disadvantage of considerably curtailing the rate of dehydrogenation and extent of dehydrogenation possible. Consequently, it is preferable in many dehydrogenation processes to forego the use of added hydrogen.
When added hydrogen` is not used undery sutilcient pressure, it is necessary to remove the carbonaceous deposits from the catalysts at frequent intervals. This process is known as regeneration and is carried out by carefully burning the carbonaceous deposits from the catalysts with a suitable regeneration gas, such for example, as air diluted with flue gas. The period during which dehydrogenation is carried out between the successive regeneration is called the process period. This process period may vary anywhere from about 10minutes up to several hours, depending upon the particular catalyst used, the particular conditions, and the particular material being 'dehydrogenated A process that is carried out in this manner with frequent regeneration of the catalyst may be called a regenerative had very little commercial success.
tion .'athigher temperatures and pressures in the presence of addedl hydrogen, this method has This is due largely to the fact that the ec'iency of the process d rops oi vas the process period is shortened; thus, as the process period is shortened the dehydrogenation declines and the amount of the feed treated that is converted to tarry deposits is increased.
Since in the regenerative method of operation the catalyst is' subjected to frequent regeneration, this type l0fk Operation is limited to the use of metal oxide dehydrogenation catalysts. Examples of a few applicable metal oxide dehydrogenation catalysts are the oxides of zinc, aluminum, titanium, thorium, chromium, molybdenum, tungsten, iron, nickel and cobalt and mixtures and Icombinations of these. While the use of dehydrogenating suliides, halides, etc., is often suggested these can not be regenerated in the simple manner described and are not suitable for practicaloperation with this type of process.
The various applicable metal oxide dehydrogenation catalysts may be prepared by any of the conventional methods. For example, in the preparation of multi-compound catalysts the powdered ingredients may be simply mixed and then formed into pellets by piling or by extruding the moistened mixture. vIn many cases the oxide catalysts maybe prepared by the decomposition of a compoundof the metal or metals, as the nitrate, acetate, ammonium salts, or the like, by heating, reduction, oxidation, or the like. Also and the like.
the metal oxide may in some cases be prepared by burning the powdered metal or the metal carbonyl. Many excellent catalysts of this type are prepared by precipitating the hydrous oxides. These various metal oxides or mixtures or compounds thereof, may be used per sev or in comv binaton with an inert diluent or other less active substance such, for example, as magnesia, kieselguhr, silica gel, zirconia, clay, silicon, silicon carbide, pumice, asbestos, bauxite or any one of the various forms of alumina. Particularly effective catalysts are prepared by impregnating a suitable support o r carrier substance, such as one of the porous forms of alumina, with the dehydrogenating metal oxide or oxides.
The catalyst may be prepared in a powdered form or in the form of lumps or granules, or in the form of cast or shaped particles of any desired size and shape, such as cylinders, balls, saddles In general after combining the desired ingredients-f the catalyst and forming into particles of the desired size and shape for considerable time before it is replaced. Various applicable metal oxides, their preparation, and their use in dehydrogenation are described in United States Patent No. 2,184,235, to which reference may be had.
It has now been found that the dehydrogenation of various dehydrogenatable organic hydrogen-containing materials with metal oxide dehydrogenation catalysts in a regenerative process may be unexpectedly and materially improved if the catalyst is saturated with carbon monoxide prior to contacting it with the material to be dehydrogenated, and the material to be dehydrogenated is contacted with the catalyst so saturated with carbon monoxide. Thus, when operating such a process in accordance with the present invention the catalyst, after each regeneration, is treated with carbon monoxide in the absence of any appreciable amount of hydrogen for a period until it is substantially saturated with carbon monoxide; then the material to be dehydrogenated is contacted with the catalyst so saturated with the carbon monoxide. This result in a considerably improved dehydrogenation activity which is particularly advantageous for short-cycle operation.
The treatment with carbon monoxide is carried out at an elevated temperature between about 350 C. and about 590 C. It is preferably carried out at a temperature Ibetween that prevailing at the end of regeneration, for example 550 C., and
i that prevailing during the dehydrogenation.
This not only allows the treatment to be effected without change or adjustment of the temperature of the catalyst, but is also advantageous since when the treatment is carried out at approximately the dehydrogenation temperature `the maximum amount of carbon monoxide that can be retained under the dehydrogenation conditions is absorbed. The treatment with carbon monoxide may be carried out lat any pressure from somewhat below atmospheric pressure up to several hundred atmospheres; however, the treatment is preferably carried out at atmospheric pressure or a pressure slightly above atmospheric pressure, for example 1.5 to 3 atmospheres.
The surface of the catalyst upon completion of the regeneration treatment is saturated with nitrogen, oxygen and carbon dioxide; however, these materials arerelatively weakly adsorbed and are easily displaced by any one of a number of more strongly adsorbed materials. Thus, if the catalyst at this stage is contacted with a material which is more strongly adsorbed, this latter material displaces the less strongly adsorbed material. When this displacement takes place to its maximum extent under theconditions prevailing, the surface of the catalyst is substantially covered with the more strongly adsorbed material and the catalyst is then said to be saturated with the latter material. If the catalyst is then treat- `ed with an even more strongly adsorbed material, this covers the catalyst by displacement, and the catalyst is then saturated with this latter material. It will be understood that the adsorption referred to is not limited to simple Van der Waals adsorption. At the temperatures under consideration activated adsorption and chemi-sorption may take place. According to the present invention the catalyst is saturated with carbon monoxide. This gas is only weakly adsorbed.
As the carbon monoxide is adsorbed on the surface of the catalyst a small temperature rise will be noted. The time required for substantially complete saturation is only a matter of a few minutes. Thirty minutes is ample.
The carbon monoxide may contain a minor amount of carbon dioxide or nitrogen or any other `material which is less strongly adsorbed than carbon monoxide; however, such materials as hydrogen and water vapor which are more strongly adsorbed than carbon monoxide should not be present in any substantial amount. Even relatively small amounts of water vapor are particularly detrimental. The carbon monoxide should, therefore, be substantially dry, i. e. contains not more than about 3% water vapor and preferably less than 2% water vapor.
During the treatment with carbon monoxide some reduction of the catalyst may take place. However, since treatment of the catalyst with hydrogen does not give the improvement in question, it appears that the improvement is not due to any reduction which may take place.
As pointed out above the catalyst is not only saturated with carbon monoxide after the regeneration and prior to contacting it with the material to be dehydrogenated, but the material to be dehydrogenated is contacted with the catalyst saturated with carbon monoxide. In other words, if the catalyst is treated with hydrogen or any other gas which is more strongly adsorbed than carbon monoxide, it can not be saturated with carbon monoxide, and, on the other hand, if the catalyst is treated with hydrogen or any other gas more strongly adsorbed than carbon monoxide after treating it with carbon monoxide and before contacting it with the material to be dehydrogenated, the beneficial effect of the carbon monoxide treatment is lost.
As will be explained further, the beneficial results obtained by having the catalyst saturated with carbon monoxide at the start of the process period is not permanent, but gradually diminishes, becoming substantially lost in about 5 to 6 hours. This is due to the displacement of the adsorbed carbon monoxide from the catalyst by anodine vthe hydrogen liberated in the course of the dehydrogenation. Since the partial pressure of product hydrogen under dehydrogenation conditions is relatively low, it requires about 5 to 6 hours for. the hydrogen to destroy the effect of the carbon monoxide. If a higher partial pressure of hydrogen were present. as in operation with added hydrogen under pressure. the beneflcial effect of the treatment with carbon monoxide would be lost in a much shorter time. It is forvthis reason that the use of any appreciable pressure of added hydrogen in the dehydrogenation is not recommended when operating in accordance with the process of the present invention. The partial pressure of added hydrogen, if this is used during the dehydrogenation, should be less than one atmosphere.
Certain aspects of the invention will be illustrated in the following examples:
Example I.Isobutane was `dehydrogenated under the following conditions:
Temperature 550 C. Pressure 1 atm.
Space velocity 16 vol. per min. Process period 6% hours The catalyst (Cr-183) was prepared by impregnating M; inch pellets of gamma alumina with chromium oxide (7.9% Cr) and magnesium oxide (1.4% Mg). Prior to use the catalyst was reduced with hydrogen for about one hour at about 550 C.
After each regeneration of the catalyst and before contacting it with the isobutane in the subsequent process period, the catalyst was saturated with carbon monoxide by passing a stream of carbon monoxide through the catalyst bed under the following conditions:
Temperature 550 C. Pressure 1 atm. Timel-- 30 min.
1This time was chosen to insure thorough saturation. As indicated by the tem rature rise, the absorption of carbon monoxide was su stantially complete in a much shorter time.
For comparison, the catalyst was treated with hydrogen instead of carbon monoxide in the same manner. v
'I'he results obtained during a typical process period are shown graphically in the attached drawing, Figure I. The curves A and B show the instantaneous conversions to oleilns in per cent at various times throughout the entire process period for the carbon monoxide treated catalyst and hydrogen treated catalyst, respectively. The substantial improvement obtained by the method of the invention is evident.
The two types of curves could be alternately obtained in successive process periods by alternately treating with carbon monoxide and hydrogen. This is shown in the following Table I Table I Tim Conversion To Isobutane, Catalyst Saturated Withe, Minutes H C E C0 3rd cycle 4th cycle 5th cycle othcycl'e 5 20. 8 30. 9 14. 9 22. 4 31. 8 17. 7 30; 0 30 24. 2 31. 5 18. 0 29. 2 60 24. 7 30. D Z). 8 28. 5 90 25. 6 21. 5 27. 0 120 26. 2 B. 8 17. 6 27. 1 180 24. 8 27. 4 Z3. 0 25. 5 240 25. 5 21. 7 21. 0 24. 2 23. 2 21. 4 22. 0 360 21. 6 22. 8 22. 4 21. 6
Example II.-lsobu tane was dehydrogenated under the following conditions:
Temperature 550 C. Pressure 1 atm.
Space velocity 16 vol. per min. Process period l0 min.
Temperature 550 C. Pressure 1 atm. Time 30 min.
For comparison, the catalyst was treated with hydrogen instead 'of carbon monoxide in the same manner.
In this case also the improvement was very marked and very similar to that shown in Example I.
It was thoughtv that the improvement might possibly be due to the presence of the carbon monoxide liberated from the catalyst during the To check this possibility, comparable runs were made in which small amounts of carbon monoxide were added with `the isobutane.
This added carbon-monoxide, however, did not exert any beneficial effect.
Example IIL-Methylcyclohexane was dehydrogenated under the following conditions:
Temperature 490 C. Pressure 1 atm. Liquid hourly space velocity 0.36 Process period 6hours Hydrogen/feed, mol ratio 1:1
The catalyst (250W-HES) was prepared by incorporating chromium oxide (10.6% Cr) on the surface of pelleted gamma alumina. Prior to use, the catalyst was heated at about 600 C. for about two hours, and finally reduced with hydrogen for about one hour at about 490 C. The regenerations wer carried out at about 600 C.
After regeneration, and before contacting the catalyst with the methylcyclohexane in the next process period, the catalyst was saturated with hydrogen by passing a stream of hydrogen through the catalyst at about 490 C. for about thirty minutes. The per cent toluene in the product and the per cent conversion during a typical process period are shown in the following Table II:
Table II Per Cent Per Cent Timo, Hours Tglgeilocn Conversion Following the process period, the conversions of which are given in the above table, the catalyst was regenerated in the 'usual manner and then saturated with carbon monoxide by passing a stream of carbon monoxide through the catalyst bed at about 490 C. for about thirty minutes. The results-obtained in the'following process period 'are shown in the following Table DI:
Table III Per ce!" Per Cent Time' HOU Tgllectm Conversion 0.1 s2. 1 79. s 1 2 15.1 73. t 2.3 73. 3 71. 0 3 4 70. 4 67. 7 :i 4.5 e2. 3 60 0 5 6 60. 6 58. 4
The results shown in Tables II and III are shown graphically in the attached drawing Figure II as curves A and Bv respectively.
Merely'for the sake of comparison, the re` sults obtained duringa typical process period when operating under the following conditions:
It will be seen that .the conversions are very much lower vin spite of the lower liquid hourly space velocity. lThe rate of decline of the activity of the catalysthis materially decreased, n-fact at this low temperature level the deposition of carbonaceous deposits is substantially eliminated. In order to obtain comparable conversions the temperature would have'to be materially vin.- creased'anl,l the depositionv of carbonaceous deposits Vwould then again become appreciable. This method, of using a substantial pressure of hydrogen, is therefore not suitable for short cycle operation, asfor example, when using the fiuidized catalyst technique. A
Example IV.-Isobutane was dehydrogenated' under the following conditions:
using diil'erent catalysts having. diilerent structures as well as compositions. The regenerations were carried' out in the usual manner with air.
improvement over no treatment at all between the -regeneration and the subsequent process pe- Temperature 500 C.
Pressure l atm. Space velocity 16 vol. per min. Process period 6 hours' riod, and consequently,`treatment with hydrogen has been used as a comparison.) The comparative results are shown below in Table V:
Table V Conversion, percent Time, catalyst 1 Pre- Catalyst 2 Precatalyst s Pre- M treatment treatment l treatment Ha CO Hg C0 Ha CO 5 l0. 0 25. 5 37. 5 18. 8 30. 9 l5 14. 2 24. 0 28.8 38. 1 m. 3 3l. 8 30 16.3 23. 8 32. 0 37. 4 22. 8 3l. 5 60 18. 0 32. 5 34. 8 24. 6 30. 0 18. 6 3l. 0 32. 2 1m 19.4 22.8 29.6 30.2 24.9 28.8 19.8 26.5 27. l 23.8 27.4 240 19. 7 Z). 8 23. 5 24. 2 300' 19.6' 20.2 23.3 23.2 360 18.8 17.2 17.4
After discovering the described unexpected effect of saturating the catalyst with carbon monoxide prior to contacting the feed to be dehydrolgenated with it, tests were made in which the catalyst was pretreated'with other agents. including nitrogen, water vapor and carbon dioxide. None of the agents or combinations thereof were comparable in effect to carbon monoxide. In combination with carbon monoxide the other agents gave different effects which could be correlated with their tendency to be adsorbed by the catalyst. Thus, for example, a. treatment with nitrogen either before or after, or before and after, treating the catalyst with carbon monoxide had very little effect. On the'other hand treatment of the catalyst with-hydrogen either before or after treating the catalyst with carbon monoxide destroyed the described and illustrated effect of the carbon monoxide.
The' beneficial effect of the described treatment of the catalyst with carbon monoxide is greatest at the start of the process period and gradually diminishes as the process period is continued. If the process period were made sufliciently long, the beneficial effect would be sub* stantially reduced. The process is of most practical advantage when the process period is less than about 10 hours. It becomes more advantageous asthe process period is` shortened. It is generally impractical to operate with process periods shorter than aminimum of about one hour with a fixed vbed of catalyst, although shorter process periods have ybeen used in exceptional cases. l' This is due primarily to the fact that shorter process periods'require very rapid temperature changes, rand to the fact that as the has not found any commercial application for dehydrogenation, because when using the usual type of catalyst under the usual conditions no advantage is gained in decreasing the eiective process period. In fact it is seen that under the usual conditions,l it is generally distinctly disadvantageous vto decrease the process period to below two or three hours. The iluidized catalyst tech- 9 nique has, however, certain other advantages and considerable attention has been given to the possibility oi' its proiitable application in this type of process. In applying the uidized catalyst technique to this type of process, however, it has been necessary to increase the residence time oi' the catalyst in the reaction zone between successive regenerations by recycling the catalyst through the reaction zone while withdrawing only a small slip stream for regeneration. This is simply a means of increasing the process period (note U. S, Patent Nos. 2,325,516 and 2,303,083).
While this expedient allows the iluidized catalyst technique to be applied without the adverse effect of shortening the process period, it has the disadvantage that it precludes one of the chief advantages of this technique. Thus, when the process period is lengthened by this expedient, the amount of catalyst cycled through the regeneration zone is small and consequently the heat supplied to the reaction zone with the regenerated catalyst is likewise small. It has been suggested to overcome this dimculty by mixing a large amount of inert heat carrier material with the catalyst. This, however,'greatly increases the cost of circulation and has other disadvantages which so far have precluded its commercial application.
The process Lof the present invention allows the tluidized catalyst technique to be applied to catalytic dehydrogenation of this type without these disadvantages inherent in the previously suggested processes. On the other hand, the short process periods in the order of one to thirty minutes which can be most easily maintained using this technique can be employed to great advantage when operating according to the processes of the present invention. Thus, the conversion may be considerably increased and at the Sametime all of the considerable heat required for the reaction can be supplied by the hot regenerated catalyst cycled to the reaction zone without reducing the catalyst with inert heat carrier material. A suitable application of the process of the invention using the luidized catalyst technique is illustrated diagrammatically in Figure III of the drawing. Referring'to Figure III, the feed to be dehydrogenated, for example, butane or propane, is introduced via line lI into the bottom of reactor 2. Reactor I is lled u p to about the level indicated by A with a nely divided dehydrogenation catalyst in the pseudo liquid state. The catalyst maybe any one of the various applicable metal oxide dehydrogenation catalysts, for example, a chromium oxidealumina-magnesia catalyst or molybdenum oxide impregnated on a spinel base. The catalyst l is ground to a suilciently ilne state that it mayv be fluidized. A catalyst ground to pass a 100 mesh sieve is suitable. Another particularly suitable catalyst is one prepared yby impregnating the dehydrogenating metal oxide or oxides on the surface of microspheres of porous alumina having diameters in the order of 100 microns. The regenerated catalyst, after being saturated with carbon monoxide, is introduced into the reactor at the top via 'line 3. The catalyst passes downward countercurrentto the ascending reactant vapors which are withdrawn through the centrifugal separator 4 and line 5. Bailles 6 in the reactor tend to diminish end-over-end mixing of the catalyst and reactant in the reaction zone, thus providing a more nearly perfect countercurrent operation. The temperature in the reaction zone is, for example,f550 C. and the presl0 sure is substantially atmospheric. The partially spent and partially cooled catalyst lleaves the reactor via line 1 and valve l. This catalyst is picked up by a stream of gas entering via line 0 and is carried via line I 0 to the top of the regenerator II. This gas not only serves to transport the catalyst from the reactor to the regenerator, but also serves to ilush or strip the catalyst of volatile combustible material. The line I0 is, therefore, not only a transfer line but also serves as a catalyst stripper. The ilue gas usually contains a small amount of non-reacted oxygen and this also helps in freeing-the catalyst of volatile material. The partially spent catalyst passes downward through the regenerator countercurrent to the regeneration gas, such as air or oxygen, introduced near the bottom via line I2. The spent regeneration gas (flue gas) leaves the regenerator through centrifugal separator I3 and line I4. Part of the ilue gas is withdrawn via line I5 and forced by blower I6 to line 9. Another portion is passed via line I1 to pick up the hot regenerated Acatalyst (withdrawnl via line I8 and valve I9) and carry it via 20 to the saturalarge, consequently there is little or no tendency to carry catalyst out of the saturator via line 25 with the exit gas. The rate of addition of carbon monoxide should, however, be sufficient to maintain the catalyst within the saturator in a free flowing condition. Excess carbon monoxide issuing via line 25 may, if desired, be reused. However, the gas issuing via line 25 will be considerably contaminated and for this reason it is pre- `ferred to restrict the ilow of carbon monoxide tov such an extent that little carbon monoxide passes out via line 25 and to forego recycling.
Other arrangements of apparatus and flow may be used with the iluidized catalyst technique.
Also the invention is applicable and advantageous when using lthe so-called moving-bed technique in which the catalyst is used as a solid moving bed rather than in the iluidized state. While the invention is particularly advantageous in the systems where the catalyst is cycled through a reaction zone anda separate regeneration zone, it is to be understood that it can alsobe advantageously employed using the conventional xed bed technique.
I claim as my invention:
1. In the dehydrogenation of a saturated hydrocarbon with a chromium oxide catalyst in the absence of added hydrogen wherein the catalyst is periodically regenerated by burning carbonaceous deposits therefrom, the improvement which comprises saturating the catalyst with substantially dry carbon monoxide after said regeneration and prior to contacting it with the material to be dehydrogenated, said saturation of the catalyst being effected without preheating the catalyst after regeneration to a temperature higher than the conversion temperature by treating the unpreheated catalyst with substantially dry carbon monoxide at substantially the temperature 11 at which the catalyst is used in the dehydrogenation, and then contacting the catalyst saturated with carbon monoxide with the material to be dehydrogenated.
2. In the dehydrogenation of a saturated hydrocarbon with a chromia-alumina catalyst in the absence of added hydrogen wherein the catalyst is periodically regenerated by burning carbonaceous deposits therefrom, the improvement which comprises saturating the catalyst with substantially dry carbon monoxide after said regeneration and prior to contacting it with the material to be dehydrogenated. said saturation of the catalyst being effected without preheating the catalyst after regeneration to a temperature higher than the conversion temperature by treating the unpreheated catalyst with substantially dry carbon monoxide at substantially the temperature at which the catalyst is used in the dehydrogenation, and then contacting the catalyst saturated with carbon monoxide with the material to be dehydrogenated.
3. In the dehydrogenation of a saturated hydrocarbon with a chromium oxide catalyst in the` absence of added hydrogen wherein the catalyst is periodically regenerated by burning carbonaceous deposits therefrom, the improvement which comprises saturating the catalyst with 'substantially dry carbon monoxide after each regeneration and prior to contacting it with the'material to be dehydrogenated.- said saturation of the catalyst being effected without preheating the catalyst after regeneration to a temperature higher than the conversion temperature by treatycatalyst in the absence of added hydrogen wherein the catalyst is continuously recycled through a regeneration zone wherein carbonaceous deposits are burned therefrom and through a dehydrogenation zone wherein it is contacted with the material to be dehydrogenated, the improvement which comprises treating the catalyst with substantially dry carbon monoxide during transfer from said regeneration zone to said dehydrogenation zone, said treatment being eiIected Without preheating the catalyst after regeneration to a temperature higher than the conversion temperature and at substantially the temperature at which the catalyst is introduced into the dehydrogenation zone.
REFERENCES lCITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 2,274,988 Matuszak Mar. 3, 1942 2,366,531 Ipatie et al Jan. 2, 1945 2,406,112 Schulze Aug. 20. 1946