US2730556A - Method for effecting endothermic dehydrogenation reactions - Google Patents

Description

Jan. 10, 1956 a. E. LIEDHOLM METHOD FOR EFFECTING ENDOTHERMIC DEHYDROGENATION REACTIONS Filed March 13, 1955 Fri DRYI N6 H15 REMOVAL a E 2 m g a EU w m PM Rm WW m m m D HEATING FURNACE PRODUCT GAS INVENTOK GEOK LI DHOLM BY Z HIS A RN Y 1 United States Patent METHOD FUR EFFECTENG ENDOTHERMIQ DEHYDRUGENATION REACTIGNS 7 George E. Liedholrn, Berkeley, Calif., assiignor to Shell Development Company, Emeryville, Calif a corporation of Delaware "Application March 13, 1953, Serial No. 342,139 11 Claims. Cl. 260-668 a This invention relates to an improved method for carrying out'endothermic dehydrogenation of various dehydrogenatable materials in the presence of added hydrogen. It relates more particularly to a particular method of contacting the reactants, hydrogen, and catalyst in commercial' practice.

Hydrogenation and dehydrogenation are the reversed directions of a single reaction. The concentrations of a dehydrogenated compound and its undehydrogenated parent in equilibrium in any case is dependent upon the pre- 7 vailing conditions and may be calculated from known thermodynamic properties of the compounds in question.

Dehydrogenation is favored at higher temperatures and hydrogenation at lower temperatures. The reaction rate is, of course, favored by higher temperatures.

' It is well known that various dehydrogenatable compounds may be dehydrogenated by bringing them into contact under suitable conditions with any one of a number of known hydrogenation-dehydrogenation catalysts. In order to obtain a suitably fast reaction rate and at-the same time. to allow a favorable equilibrium, it isdesired to employ a temperature near the maximum possible temperature. The maximum possible tempera- I hire is determined in any caseby the onset of excessive -s'ide reactions such in particular as cracking. When operating near this maximum applicable temperature the catalyst becomes fouled with carbonaceous deposits and horny short periods of operation can be applied. In some 1' cases the catalyst can be regenerated by burning off the ca'rbonaceous deposits but the necessity of such regenerationmakes such regenerative processes relatively costly and inefficient.

lowering the maximum possibleconversion by shifting the flrnentioned equilibrium in the-unfavorable direction. This disadvantagectnbe compensated for by some further increase in the reaction temperature, but the improvement obtained by increasing the partial pressure of hydrogen and'the temperature is limited :by the onset of destructive hydrogenation. Destructive dehydrogenation is to be avoided becauseit leads to the formation of large amounts of gas other than hydrogen which usually ac- 'cumulates in the recycled product gas thereby decreasing the protective eifect of the gas, Also, the destructive hydrogenation reaction is quite exothermic and, consequently, the initiation of destructive hydrogenation results in a temperature increase which further fosters destructive H hydrogenation and cracking. As a result of these factors, any appreciableamount of destrictive hydrogenation, if

allowed to take place, is prone to result in deactivation of the'catalyst'and increase in cracking until destruction 'of the feedis'the predominant reaction.

r 2,730,556 Patented Jan. 10, 1956 Dehydrogenation is endothermic; therefore, when appreciable extents of conversion are desired an appreciable amount of heat must be supplied to the reaction zone. In commercial application this presents a serious problem. Transfer of the desired heat to the catalyst bed through the reactor walls is costly, difiicult, and not very satisfactory. It is more practical to supply the heat with the feed to be dehydrogenated and/ or the recycled hydrogen gas. This requires, of course, that these materials be supplied at a temperature above the desired reaction temperature. However, the maximum temperature at which the reactant stream may be heated is strictly and seriously limited firstly, by the tendency for the feed to crack thermally during the preheating step, and secondly, by the necessity of avoiding localized temperatures sufficiently high to initiate the mentioned exothermic destructive hydrogenation in the presence of the catalyst. Also, even when preheating the feed stream to'the maximum permissible temperature, there is a very substantial temperature drop in the direction of the path of travel of the reactant through the catalyst bed. Thus, in a typical case, the temperature at the forepart of the catalyst bed is dangerously close to that causing thermal cracking and/ or destructive hydrogenation but the temperature near the exit end of the catalyst bed has dropped so far that the conversion is slow and is limited by a less favorable equilibrium. This undesired condition can be improved by eifecting the conversion in a series of small steps while reheating the reactant mixture between the steps, but this requires a series of reactors with interstage heaters and at best gives a saw toothed temperature profile.

An object of my invention is to provide a method of operation and apparatus therefore which allows catalytic vapor phase dehydrogenation to be carried out more effectively in a more selective and practical manner. Generally speaking, this object is attained by the method of operation about to be described in which all, or substantia'lly all of the heat of the endothermic dehydrogenation reaction is supplied with a portion of recycled and highly heated product gas consisting essentially of hydrogen. Sensible heat is thereupon transferred from the hydrogen to a solid finely divided dehydrogenation catalyst which acts as a heat carrier and is transported in the system by a separate portion of hydrogen gas. In this system the material to be dehydrogenated is passed in vapor form diluted with hydrogen, up through a fluidized downward moving bed of the preheated catalyst. The catalyst, cooled by the reaction continues to pass downward countercurrent to preheated hydrogen. By this arrangement, reactant vapors are removed from the recirculated catalyst before the same is fully heated thereby avoiding the severe coking that would otherwise occur at the high temperatures applied. A further and important characteristic of the process is that the catalyst is saturated with hydrogen before it contacts the materials to be dehydrogenated. This helps to prevent localized contact of the catalyst with the material to be dehydrogenated in the absence of sufiicient hydrogen to protect the catalyst.

The process will be more fully explained in connection with the following description of an example, namely the dehydrogenation of a naphthenic straight-run gasoline to produce aromatics. In this description reference will be had to the flow diagram given in the accompanying drawing. Referring to drawing, a straight-run gasoiine faction boiling between about 113 and 228 F. is introduced by line 1 at the operating pressure, preferably after having been partially preheated by heat exchange with one of the product streams (not illustrated). The operating pressure may be from about p. s. i. g. up to about 1000 p. s. i. g. In the example in question, the operating pressure is 200 p. s. i. g. The feed is preheated in preheating furnace 2 up to approximately the desired reaction temperature or as near thereto as possible without causing decomposition in the furnace coils. In the case of the particular feed in question the preheat temperatures are between about 920 F. and 975 F. A small amount of recycled gas consisting mainly of hydrogen is preferably added by line 3 and the mixture is passed to the reactor 4. The amount of hydrogen introduced by line 3 may be quite low, for instance, 0.1 to 1 mole/mole of feed. This amount is not sufiicient in itself to protect the catalyst under the temperature conditions prevailing and merely supplements the main ficw of hydrogen. It is used to prevent localized high concentrations of hydrocarbon in the catalyst bed near the point of feed injection.

The reactor is essentially an elongated vertically disposed cylindrical vessel provided with means for transporting catalyst from the bottom to the top. The vessel may be of uniform or non-uniform diameter throughout its length, e. g., the upper part may be of larger diameter than the bottom part. In the case illustrated, transportation of the catalyst is effected by withdrawing catalyst through line 5 and valve 6 and transporting it to the top of the reactor by line 7 by means of a stream of hot hydrogen gas introduced by line 8. Line 7 can it" desired be placed within the reactor shell. The hydrogen (recycled gas) used for this purpose is again only a minor amount of the total hydrogen used, c. g., l-Z moles/mole of hydrocarbon feed, and is heated to a high temperature, e. g., l1501450 F. Line 7 discharges in the upper section of the reactor, preferably in the socalled disengaging space above the fluidized bed of catalyst. A cylindrical bafile 9 which is open at both ends causes the incoming mixture of catalyst and gas to swirl in the annular space between the cylindrical baffle and the vessel wall thereby dropping out most of the catalyst which falls to the fluidized bed It A cyclone-type separator 11 is provided in the disengaging space of the reactor to effect a more thorough separation of catalyst particles from the vapors leaving the reactor. The loading of the cyclone separator 11 (which may have one or more stages) is materially decreased by the relatively open grid 12 which is in the disengaging space above the level of the fluidized bed of catalyst.

The amount of catalyst transported to the top of the reactor described may vary from about 2 to about parts by weight per part of reactant feed. The catalyst thus transported gradually flows downwardly through the vessel to the bottom and is then recycled. The reactor is preferably provided with grid plates 13-16. These grid plates have a fairly large open area to allow the catalyst to sift downwardly countercurrent to the uprising vapors. in the absence of these grid plates the composition of the reacting vapors and the temperature throughout the catalyst bed would be essentially uniform. This is the result of undesirable back mixing and is a characteristic property of unbafl'led fluidized beds. T e grid plates substantially decrease this mixing; consequently, the dehydrogenated product is not retained in the system for an inordinate length of time, unreacted material is not passed out of the reactor without suificient contact, and there is a generalty increasing temperature gradient from the level of introduction of the feed up to the top. More or less uniform conditions prevail, however, in the individual spaces between the plates. it will be understood that while four such grid plates or trays are used in this example, either a greater number or a lesser number may be used. The grid plate 15, situated just below the feed inlet, is relatively important for the reasons which will be later pointed out. The grid plates illustrated consist of concave plates provided with suitable holes or slots. They are placed in depending position, i. e., with the convex side downward. This is to counteract the normal tendency in fluidized beds for the uprising vapor to pass mainly up through the center of the supporting plates. In the arrangement illustrated the pressure required to pass the gas up through the plates and their superimposed beds is somewhat greater near the center than near the periphery. Other arrangements and construction of the plates are possible.

The main stream of the hydrogen gas is preheated to a temperature much above that applicable in the reaction zone, e. g., ll50l450 F., in heating furnace 17, and is passed via line 18 into the reactor near the bottom. This hydrogen transfers almost all of its sensible heat to the descending catalyst and at the same time removes reactant vapors from the catalyst and is itself cooled. This is most important since, if the catalyst is not substantially free of reactant vapors, it becomes severely coked upon raising its temperatures to the levels in question. Also this hydrogen is the main source of the considerable amount of hydrogen required in the reaction zone when operating at the relatively high temperatures used. The amount of hydrogen supplied by line 18 may vary somewhat depending upon the particular feed stock and the temperature and pressure conditions but is usually between about 1 and 10 moles per mole of reactant feed. The total amount of hydrogen heated in heating furnace 17, on the other hand, may be between 2 and 12 moles per mole of reactant feed.

The effluent mixture of product vapors and hydrogen leaving the reactor at the top by line 19 is at essentially the highest reaction temperature, e. g., 9l0990 F. This hot mixture carries in suspension a small amount of catalyst dust which escapes separation in the cyclone separator 11. This catalyst may be recovered in various ways to avoid fouling of the product recovery equipment. In the case illustrated a small part of the product is condensed upon passing through a partial condenser 21. The small amount of condensate containing the small amount of catalyst is collected in the separating tank or knock-out drum 22. The product vapors and hydrogen are then passed through a condenser 23 to a high pressure separator 24. The liquid product is passed through the low pressure separator 25 and is then withdrawn by line 26 for such further handling as may be desired. The low pressure vent gases consist largely of hydrogen released from solution in the liquid product upon decreasing the pressure.

The uncondensed material in the high pressure separator consists essentially of hydrogen but contains small amounts of hydrocarbon vapors, diluent gases, and traces of hydrogen sulfide. This gas is passed by line 27 to a conventional unit 28 for removing the hydrogen sulfide, and it may be passed to unit 29 for the removal of moisture and preferably also some of the hydrocarbon constituents. The excess, clean, dry gas is removed from the system by line 30 and the major portion is heated in heater 17 up to a temperature between about 1150" and 1450' F. to supply the heat of the dehydrogenation reaction. As previously pointed out, the main part of this hydrogen stream is passed into the bottom of the reactor where it transfers its heat to the catalyst and then passes upward to dilute the reactant vapors and protect the catalyst in the upper section of the reactor. A minor amount of the heated, recycled gas is passed by line 8 to transfer the heated catalyst to the top of the reactor, and a minor amount of the recycled gas may be advantageously mixed with the feed.

It will be noted that, in the case just described, the temperature of the catalyst transported to the top of the reactor is above the temperature of the feed introduced near the middle of the reactor. Also, the temperature of the catalyst at the bottom of the reactor approaches the high temperature of the preheated recycled gas. There is, therefore, a very desirable temperature gradient set up in the reactor in which the highest temperature is at the bottom. This temperature is above that applicable in the presence of the reactant feed. The temperature at the top may be somewhat lower but is substantially the highest gen mixture. This not only allows a fast reaction rate .to

ry h n rsi n e rer complet b a presents the most'faverable equilibrium from the standpoint of the dehydrogenation reaction. The temperature near the middle of the reactor justahove the points where the feed isinjected is the lowest. Herea particularly favorable equilibrium is not necessary since at the start of the reaction the reaction rate rather than the equilibrium is limiting. As the dehydrogenation takes place during the upward passage of the reactant the reacting vapors become more and more refractory and are .thus capable of withstanding'high'er temperatures without cracking and de- 7 activation of'the catalyst. While the described method can be -advantage ously applied at lower or moderate temperature levels, it has the important advantage of allowing the dehydrogenation to be carried out at higher average temperatures than applicable for the feed in question when using the hitherto employedmethods of operation.

The methodof the invention is applicable with the various'kn own hydrogenation-dehydrogenation catalysts. It

is particularly advantageous with the super-active hydro genation-dehydrogena'tion catalysts such as those containing metallic nickel, platinum, or palladium since with these catalysts theprotective effect of the hydrogen is especially important and,.on the other band, due to extreme rapidity of the dehydrogenation reaction, the 'maintenance'of a temperature gradient such as described is practically impossible by conventional methods. In the case of the relatively' rugged but less active catalysts such as iron oxide, molybdenum sulfide, nickel sulfide, tungsten sulfide,and

chromium oxide, the same problem exists but it is not so 3 serious.

The process of the invention is applicable and can be substituted for the conventional; processes for endothermic dehydrogenation 'of various dehydrogenatable organic compounds which can be vaporized. When using the mentioned shper-active metal catalysts, the reactant material is preferably free of compounds containing oxygen, nitrogen,sul fur, and halogen but they may contain boron,

phosphorous, or silicon. The process is particularly suitable for the dehydrogenation of naphthenic hydrocarbons,

. either individually or in kariousadmixtures, to their corresponding aromatic hydrocarbons. For this operation one of the super-active catalysts is preferred and the naphthenic hydrocarbon or .naphthenichydrocarbon fraction is-"-preferabl-y substantially free of sulfur.

/ The use of the relatively open grid plate or tray in the disengaging space above the fluidized bed in the reactor to decrease the loading on the cyclone separator is an improvement discovered by a coworker and constitutes no part of my invention. I

' I claim as my invention:

1. In a catalytic endothermic vapor phase dehydrogenation process the improved method of contacting the catalyst with the reactant to be dehydrogenated which comprises passing a powdered dehydrogenation catalyst in the form of a-plurality of semi-isolated fluidized beds downward through an elongated reaction vessel, introducing'preheated reactant vapors into the reaction vessel near the mid-height thereof, withdrawing reacted vapors in admixture with recycle gas consisting mainly of hydrogen from the top of said reaction vessel, separating from said withdrawn mixture a recycle gas consisting largely of hydrogen, heating said recycle gas to a temperature higher than the average reaction temperature, introducing the larger part of the heated recycle gas into the said reaction vessel near the bottom thereof below the point of intro duction of said feed whereby the catalyst in the lower zones of said vessel is preheated and stripped of reactant vapors and the thereby cooled recycled gas passes upward and mixes with the feed in the upper zones of the said vessel, and utilizing a separate minor portion of heated recycle gas to separately transport the heated catalyst from near the bottom of said reaction vessel to near the top of that the catalyst is a powder containing a metal selected fromthe group consisting of nickel, platinum, and palladiufn as the active catalyst promoter.

3. Process according to claim 1 further characterized in that the catalyst is largely present in a series of semiisolated but inter-communicating superimposed zones, all of which are traversed by recycled products gas seriatim, whereas only the upper of which are traversed by the said reactant vapors.

4. Process according to claim 1 further characterized in that a minor portion of said recycled gas is introduced into the reaction zone in admixture with the feed vapors.

5. Process according to claim 1 further characterized in that the temperature of the reactant vapors introduced a into the reaction zone is essentially the maximum temperature at which the reactant may be heated without thermal cracking and the temperature of the catalyst above and below the point of injection of the feed exceeds this temperature.

6. Process according to claim 1 further characterized in that theheat required for the endothermic dehydrogenation is supplied as sensible heat in recycled product gas and said sensible heat is largely transferred to the catalyst prior to commingling the main portion of said recycled gas with the reactant vapors.

7. A process for effecting dehydrogenation of a hydrocarbon with a finely divided dehydrogenation catalyst in the presence of a recycled hydrogen which com prises in combination the steps of separating from the product stream of such process a product gas consisting essentially of hydrogen, dividing said product gas into four separate streams, withdrawing one of said streams as product gas, utilizing the second of said streams as a carrier gas to transport powdered dehydrogenation catalyst through a confined passage from the bottom to the top of an elongated vertically disposed contact Zone, mixing the third portion with hydrocarbon to be dehydrogenated and injecting the mixture near the middle of said elongated contact zone, and passing the fourth portion up through the entire length of said elongated contact zone thereby stripping occluded hydrocarbon from the catalyst in the lower portion of said elongated contact zone and to serve a diluent for the hydrocarbon introduced as aforesaid near the middle of said elongated contact zone, and withdrawing product vapors consisting of reacted hydrocarbons and the combined four streams from the top of said elongated contact zone as the aforesaid product stream.

8. The method of effecting an endothermic dehydrogenation of a hydrocarbon with a finely divided dehydrogenation catalyst in the presence of hydrogen which comprises in combination the steps of providing a vertically disposed elongated contact zone substantially filled with downwardly flowing fluidized dehydrogenation catalyst, dividing said elongated contact zone into a plurality of semi-isolated smaller zones by means of barriers restricting free intermixing of finely divided catalyst between said zones while allowing downward flow of said catalyst serially through said zones, maintaining the temperature in the upper end of said contact zone at an elevated temperature above a given average dehydrogenation temperature by continuously introducing into said zone preheated dehydrogenation catalyst suspended in preheated product gas consisting essentially of hydrogen, maintaining the temperature at an intermediate point in said elongated contact zone at an elevated temperature below said given average dehydrogenation tempera ture by introducing into said zone at said point vapors of the hydrocarbon to be dehydrogenated preheated to a temperature which is below said given average dehydrogenation temperature and hydrogen separately cooled to a temperature which is below' said given averagedehydrogenation temperature, maintaining the temperature at the bottom of said elongated contact zone at a temperature above said given average dehydrogenation temperature by the introduction of recycled product gas consisting essentially of hydrogen and preheated to a temperature above said given average dehydrogenation temperature, passing said latter gas in the absence of hydrocarbon vapors other than those stripped from the descending catalyst up through the lower part of said elongated contact zone to the said point of introduction of said hydrocarbon vapors, and passing said gas in admixture Withsaid hydrocarbon vapors through the upper portion of said elongated contact zone countercurrent to the descending stream of preheated catalyst therein.

9. Process for the endothermic dehydrogenation of a hydrocarbon according to claim 8, further characterized in that the product gas consisting essentially of hydrogen is split into four streams which are introduced into said elongated contact zone as follows: a minor part is mixed with the hydrocarbon prior to introduction into said zone, a minor part is used to transport preheated and stripped catalyst from the bottom of said zone to the top thereof, a minor part is removed from the system as product gas, and the major part is introduced into said zone near the bottom thereof.

10. The method of effecting an endothermic dehydrogenation of a hydrocarbon with a powdered dehydrogenation catalyst which comprises in combination the steps of preheating hydrogen to a temperature above a desired average dehydrogenation temperature, passing said preheated hydrogen up through a bed of dehydrogenation catalyst cooled to below the said average dehydrogenation temperature, thereby to strip said catalyst of occluded hydrocarbons and to transfer heat to said catalyst, preheating hydrocarbon vapors to be dehydrogenated to a temperature below the said average dehydrogenation temperature, commingling the preheated hydrocarbon with partially cooled hydrogen passed up through said bed as aforesaid and passing the resulting mixture upward countercurrent to preheated dehydrogenation catalyst thereby to increase the temperature of the mixture up to above the said average dehydrogenation temperature and to dehydrogenate said hydrocarbon, suspending dehydrogenation catalyst preheated to a temperature above the said average dehydrogenation temperature by the aforesaid passage therethrough of preheated hydrogen in a separate portion of hydrogen and combining the resulting suspension with the mixture of hydrogen and hydrocarbon vapors passed upwardly countea-current to preheated dehydrogenation catalyst as aforesaid, separating the vapors from the suspended catalyst in the last resulting mixture, withdrawing the hydrocarbon vapors together with the hydrogen used for stripping and the separate portion of hydrogen as a product vapor stream, and passing the separated catalyst at a temperature above the said average dehydrogenation temperature countercurrent to the first aforesaid mixture to heat the same and effect dehydrogenation of the hy drocarbon therein under conditions of an increasing temperature gradient.

11. In a process for efiecting dehydrogenation of a hydrocarbon with a finely divided dehydrogenation catalyst in the presence of recycled hydrogen, the improved method of operating such process which involves in combination the steps of introducing a stream of pre heated dehydrogenation catalyst in hydrogen at the top of a descending stream of fluidized dehydrogenation catalyst, passing preheated vapor of the hydrocarbon to be dehydrogenated countercurrently to said descending stream of the powdered dehydrogenation catalyst preheated to a temperature above the temperature of said preheated hydrocarbon vapors so that there is an in creasing temperature gradient in the hydrocarbon vapors along the path of contact with said catalyst and a decreasing temperature gradient in the stream of powdered catalyst as it descends in contact with said hydrocarbon vapors, stripping the dehydrogenation catalyst of occluded hydrocarbons after said countercurrent contact with said hydrocarbon vapors by passing up through said catalyst in a bed a stream of hydrogen preheated to a temperature above the temperature of the said preheated hydrocarbon vapors whereby said hydrogen is partially cooled by said catalyst, passing the partially cooled hydrogen containing stripped hydrocarbons from said bed with said preheated hydrocarbon vapors countercurrently to said descending stream of powdered dehydrogenation catalyst thereby to dilute the said hydrocarbon vapors and be reheated, and passing the stripped catalyst in a confined stream in hydrogen up to the top of said descending stream.

References Cited in the file of this patent UNITED STATES PATENTS 2,602,771 Munday et al. July 8, 1952 2,643,214 Hartwig June 23, 1953 2,656,304 MacPherson et a1 Oct. 20, 1953

Claims (1)

1. IN A CATALYTIC ENDOTHERMIC VAPOR PHASE DEHYDROGENERATION PROCESS THE IMPROVED METHOD OF CONTACTING THE CATALYST WITH THE REACTANT TO BE DEHYDROGENATED WHICH COMPRISES PASSING A POWDERED DEHYDROGENATION CATALYST IN THE FORM OF A PLURALITY OF SEMI-ISOLATED FLUIDIZED BEDS DOWNWARD THROUGH AN ELONGATED REACTION VESSEL, INTORUCING PREHEATED REACTANT VAPORS INTO THE REACTION VESSEL NEAR THE MID-HEIGHT THEREOF, WITHDRAWING REACTED VAPORS IN ADMIXTURE WITH RECYCLE GAS CONSISTING MAINLY OF HYDROGEN FROM THE TOP OF SAID REACTION RESSEL, SEPARATING FROM SAID WITHDRAWN MIXTURE A RECYCLE GAS CONSISTING LARGELY OF HYDROGEN, HEATING SAID RECYCLE GAS TO A TEMPEATURE HIGHER THAN THE AVERAGE REACTION TEMPERATURE, INTRODUCING THE LARGER PART OF THE HEATED RECYCLE GAS INTO THE SAID REACTION VESSEL NEAR THE BOTTOM THEREOF BELOW THE POINT OF INTRODUCTION OF SAID FEED WHEREBY THE CATALYST IN THE LOWER

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