US2854402A - Method of heat transfer in catalyst reforming - Google Patents

Description

Sept. 30, 1958 w. A. REX 2,854,402

METHOD OF HEAT TRANSFER IN CATALYST REFORMING Filed Dec. 10, 1952 Producf gas Regenerafor Reacfor 32 II\ v25 f /2 I -Sfri er J g All J I J Recgcle gas a 2 Feed gas Lfilbew Q/Qex -5nvenbor Unite States Patent 0,

HEAT TRANSFER IN CATALYST REFORMING Application December 10, 1952, Serial No. 325,094

6 (Ilaims. (Cl. 208-135) METHOD OF The present invention relates to improvements in the catalytic conversion of hydrocarbon fractions, and particularly to the conversion of hydrocarbon fractions within the naphtha boiling range and having a low knock rating to produce high octane number motor fuels. More specifically, the invention relates to an improved process for upgrading naphtha hydrocarbons by catalytic hydroforming or aromatization processes, employing a fluidized solids reactor system.

Hydroforming and aromatization are chemically related processes which are well known and widely used for treating hydrocarbon fractions boiling in the motor fuel range. Their purpose, broadly speaking, is to convert straight chain and naphthenic hydrocarbons into branched chain and aromatic products of roughly the same molecular weight and volatility. The conversion products obtained in this way have greatly enhanced value as motor fuels, because of their improved anti-knock quality and, in some cases, because of improved volatility.

The hydroforming process, for example, may be used to convert a naphthenic heavy naphtha of poor octane quality and low volatility into a motor fuel of premium octane quality and improved volatility, containing a very substantial proportion of the total product as material in the aviation gasoline boiling range. The hydroforming operation is a catalytic process which includes a variety of dehydrogenation, cyclization, hydrogen transfer, isomerization, dealkylation and aromatization reactions.

' There is a net production of hydrogen and a significant increase in aromatics content, with the particular reactions which take place depending on the character of the feed stock and the operating conditions employed. The net production of hydrogen and aromatics means that the overall reaction is endothermic in character, requiring some means for continuously supplying the necessary heat of reaction.

The term aromatization refers broadly to any process carried out in the presence or absence of hydrogen which results in the formation of aromatics from non-aromatic hydrocarbons. The process is particularly valuable when the feed stock can be one consisting predominantly of paraffinic hydrocarbons. The aromatization of segregated naphthenes or olefinic compounds is also well known. The most desirable process, however, is one which does not require careful and expensive feed preparation. Since the naphthenes, and particularly the cyclohexane derivatives, are easily converted into aromatics, processes which permit the conversion of parafiins or mixed parafiin-naphthene stocks to aromatics are particularly important.

Regardless of the feed stock, both hydroforming and aromatization consist essentially of dehydrogenation type reactions, and the processes are characteristically endothermic. The catalysts suitable for these operations are well known. They consist ordinarily of compositions such as molybdenum oxide or chromium oxide supported on a base or spacing agent such as alumina gel, an activated or promoted alumina, or a zinc aluminate spinel.

Other oxides and sulfides of metals of groups 4, 5, 6 and 8 of the periodic system may be used as such, or after previous sulfiding or reduction to the free metal or to a lower oxidation state, in such a manner as may be known in the art for the individual catalyst. In the aromatization of a parafiinic feed stock, which requires high severity conditions, pressures for the catalytic conversion may be from atmospheric to at most about lbs/sq. in. Hydroforming conditions employing catalyst of the type described above may be carried out in the pressure range from about 50 to 1000 lbs/sq. inch, with a continually replenished hydrogen atmosphere which is ordinarily supplied by recycling the hydrogen-containing portion of the product stream at the rate of about 500 to 5000 cubic feet per barrel of oil fed. Either the low pressure aromatiza tion or the higher pressure hydroforming reaction may be carried out at temperatures of 750 to 1150 F.

In the ordinary case where saturated hydrocarbons such as parafiins and naphthenes are being treated by catalytic hydroforming, which will be understood hereinafter to include catalytic aromatization, the process is quite strongly endothermic. This means that large amounts of heat must be supplied to maintain the catalytic conversion zone at the desired temperature level. Theproblem of constructing apparatus to satisfy this requirement is seriously complicated by the fact that the catalysts employed have very poor heat transfer properties. Thus, heat applied to one portion of a fixed solid mass of discrete cata-' lyst particles flows only slowly to other portions of the mass. This means that in the ordinary fixed bed reactor, the heat of reaction is taken largely from the flowing stream of reactant and product vapors. The inlet temperature of such a hydroforming reactor at, say, 900 F. will, therefore, fall ofi rapidly to a temperature which may be or even 250 F. lower at the exit point from the catalyst bed. The catalyst, of course, is nothing like as active at these lower temperatures.

Unfortunately, in the attempt to correct this defect and improve the conversion in the catalyst bed by raising the average bed temperature, an increase in inlet temperature to asubstantially higher level such as 1000 F. causes very poor selectivity in the catalytic reaction at the initial reaction portion, where the feed is so hot that some thermal decomposition takes place.

The application of the fluidized solids technique to the hydroforming process offers several real advantages in handling this problem of heat supply. One immediate efiect of the high degree of turbulence and mixing of the solid particles within the bed is that the temperature within the bed is equalized, so that the normal gradient or temperature drop observed in the fixed bed reactor is erased. Another important effect is that heat supplied anywhere to the catalyst bed is distributed promptly and uniformly throughout. Such heat may be supplied either by reactant vapors, or with a recycle stream of hydrogencontaining gas which is employed in the hydroforming process, or in any other suitable manner.

A particular advantage in fluid bed type of hydroforming reaction appears in the possibility of supplying heat to the reaction by recirculating to the reaction zone a hot inert solid heat carrier. Various proposals to utilize this type of operation have beenmade. In general, it

is desired to circulate the solid heat carrier and the catalyst separately, since this permits the supply of controlled amounts of heat by a separate control of the feed rate and temperature of the solid heat carrier. Such an operation involves, as a necessary step, the ready separation of the heat carrier from the catalyst, so that it can be separately circulated at a controlled rate as desired.

In previous proposals of this nature, the use of relatively large non-catalytic particles has been proposed. Systems of this type have been called shot circulation,"

referring to the. large inert particles which can be separated from the more finely divided catalyst by some suitable simple means as by elutriation. The inert solid is then reheated and suppliedagainrto the catalyst bed, where it settles through the fluidized-'rnaterialandtrane fers heat to the catalyst with which it comes in contact, It also has been proposed in many cases to make the, solid heat carrier out of some material offmuchhighen'density. than the catalyst, such as fi'ne metaljshot,tofacilitajtetlie separation between'the catalyst nd h ?fll v One serious disadvanage of such proposals isthe fact that it is quite diflicult to obtain aheat carrier, which is, truly inert under hydroformingconditions, andparticu s.

larly so when the inert solid; is. to be supp ied. at. a; m er t re ubs antia ly. i her. a v that ofl "the: lytic reaction itself The metals, andindeed a large variety of high density solids'have been found to induce a hydrocarbon cracking reaction, giving a product of very poor quality, which is; more, closely. related: to .that obtained by simple thermal cracking. At the very least, thissubtracts from the total amount of feed:stock;javailabletor the desired catalytic conversion, andresults in;

a seriousdegradationof overall product quality.-

The use, of large-sized heat carrier-particles also suflf'ers from certaindisadvantages even apart from their com:

position. Theproblem of catalyst attrition-depends materially upon the composition; of the catalyst. But in anyv case it. is,,aggravated,- by the presence .of relatively large particles, particularly when their amount is large relative to the total amount of catalyst present. This attrition problem bearsbothnpon thecatalyst and upon. the,heat.,car rier particles, since. thelarge-sized particles. themselveswear away more rapidly than small-sized particlcs of the, same inherent hardness. Other defects appear.in the fluidization characteristics. of 'the mass of catalyst and heat carrier when large amounts of largesized particlesuareipresent and particularly when independent variationsnin. the flow rates ofthese particles may beimposedonthesystem. There are also. certain difficulties in the transfenof the tlarger-particles indilute phase suspension, either mixed with the catalyst or in a separate. .stream bythemselves.

According. to the ipresent-invention the advantages of" a separate circuit of heatcarrying solid are realized in a fluid catalyst. operation, of-thegeneral type of catalytic hydroforming, by using a solid heat carrier which is closely-sized and definitely finer than the average size of the catalystparticles; This type of-heat transfer operation has a number of advantages. Itm inimizes the problem of catalyst andinertparticle attrition, ,which isaggravated by the use oflarge-sized heat-carrying solids. Thereareno difficulties with fluidization, since the mixture of catalystand inertparticles lies entirely within the range characterized by good fluidity under hydroforming reaction conditions More, importantly, however, the system offers significant advantages inthe distribution of heat through the reactor.

Experience with; the hydroforming reaction hasshOWn that there is a real advantage' in carryingout the opera:v tion under conditions where an iny'erse temperature gradient can be imposed- This is finverse by way of contrast with the normal experience infixed bed operation as described above, .where the temperature falls 'off rapidly from the inlet point to the outlet point from. the catalyst bed. Improved results are, obtained WhfiILthiS temperature is kept uniform throughout the bed. A still further improvement appears when the temperature, in.

the outlet point can be held higher than that of the inlet point so that the gradient through the reaction zone.is the inverseor that ordinarily-observed. The reasons for this phenomenon are not exactly known. It appears, however, that a low temperature in the initial stages of the reaction is sufiicient to convert some components of the feed stock to more stable constituents, such 'as aromatics; these labile constituents may otherwise be degraded by hydrocracking, isomerization or other side reactions if they are exposed initially to a higher temperature sufiicient to cause the conversion of more refractory constituents of the feed blend. With the inverse temperature gradient type of operation, however, the mixture initially treated at a low inlet temperature can then be subjected to somewhat more severe conditions at later stages of the reaction with a minimum loss due to these side reactions. The simple turbulent fluid bed causes rapid and uniform heat distribution throughout the dense phase of the catalyst. Therefore, some special means must be provided if it is desired to impose an inverse temperature gradient on the reaction zone in fluid solids operation.

The use of the inert solid heat carrier has important advantages in this respect. Thus, when large-sized heat carrier particles are employed they may be introduced at the top of the reaction zone, employing baflles or stages within the fluid bed to cut down overall turbulence and backv mixing, and the upper portions of the bed will be heated to a higher temperature thereby. The heat carryingsolid will continue to fall downward through the fluid bed after exchanging heat at the top of the reaction zone. Even assuming no back mixing of the catalyst particles, the heat carrier will thus leave the top of the reaction zone at the temperature of that portion ofthe bed, and carry heat into the lower portion of the bed from that temperature level. Accordingly, the heat carrier used in this way will heat both the top and bottom portion of the reaction zone.

Anadded advantage of the process of the present invention appears in this connection. The fine-sized heatcarrying solid is one which can beselectively entrained overhead from the average catalyst particle in the fluid bed: that is to say, the total product gas stream and entrained solids on being passed through a simplegassolids separating means can continue to carry these fine solid particles after all except the finest of the catalyst particles have been separated out and returned directly to the fluid bed. The gas solids separatin means empioyedmay be a single-stage cyclone or even a tall settling zone-above the bed level in the reaction vessel.

According to this procedure, therefore, the heat carryingtsolid does not necessarily continue downward into the bottom portion of a staged reactor when it is introduced at the-top, for inverse temperature gradient operation.

The extent to which heat introduced at the top of the rcactor iscarried into the bottom section can thus be controlled by controlling the degree of back mixing throughout the catalyst bed as a whole. As a result, heat introduced at the top can be employed to maintain a larger temperature differential between the top of the reaction zone and the bottom section than when the heat-carrying particles are larger and heavier than the catalyst.

The large-sized particles give a heat transfer effect which..is equivalent to undesired-back mixing when an inverse temperature gradicntis desired. This effect may be quite large, since large amounts of inert solid of the order of l to 10 times the catalyst circulation rate are ordinarily required to keep the operation in heat balance.

Whentusing the fine inert solidrof the present invention however, the heat-carrying material can be caused to go overhead with the gas stream, so that the eflect of back mixing or its heat equivalent can be minimized.

Thehigh temperature differential between the top and bottom of the reaction zone which can be reaiizcd by this technique has great advantages. In the first place, it makes it possible to maintain a desired high exit temperature from the reaction zone, to cause the conversion of more refractory feed constituents, without excessive inlet temperatures. This can be accomplished with a feed rate of inert solid as high as may be'desircd. By proper adjustment of .the rates of inert solid hot catalyst and reactants fed to the system, the temperature difier- 3 en'tial of the inverse temperature gradient thus realized can be controlled independently within wide limits. As a result the inlet temperature of both the hydrocarbon feed stock and the recycle gas stream can be held down at levels where thermal cracking is minimized. In many cases, also, an important saving can be realized in the total amount of recycle gas fed to the system, since the burden on this gas stream to carry heat into the re action zone can be materially decreased.

The extent of the inverse temperature gradient which is desired depends materially upon the feed stock and upon the severity of the hydroforming operation being carried out. Ordinarily, inverse temperature gradients within the range of about 25 to 70 F. should be maintained in the reactor, with the highest such temperature gradients being desired when the feed stock contains a high percentage of paraflinic constituents.

A particularly desirable finely divided solid for use as the heat carrying inert in the process of the present invention many be prepared by using a heat-treated fines fraction of the carrier base employed to produce the catalyst. The catalysts employed for the hydroforming process are uniformly of the supported metal oxide or metal type, where the catalytic component is carried on an adsorptive base or spacing agent having a high surface area. Various activated gels have been prepared for this use by the partial dehydration of a suitable hydrous oxide, with the most common ones being those containing active alumina as a major constituent. The activation process consists usually of a careful drying to remove excess water associated with the hydrous oxide, followed by further heating and dehydration to remove a portion of what may be considered as water of constitution in the hydrous oxide gel. This stage of the activation process results in the formation of an adsorptive material of high surface area. If it is continued, however, at a progressively higher temperature level, the surface area of the metal oxide is found to decrease slowly at first and then rapidly after a certain degree of dehydration is reached at high temperatures. The partly dehydrated material in the activated form can adsorb and desorb water and many other gases or vapors, reversibly. Its high adsorptive capacity for hydrocarbons and other reactant gases is responsible in large measures for the usefulness of the material as a catalyst support. The more highly heated material, however, no longer shows any significant capacity for reversible adsorption and desorption. With the loss of effective surface area it shows progressively less and less effectiveness as a catalytic support.

Accordingly, if an alumina base is to be used to support the catalyst in the present invention, this alumina will have been activated in the usual manner by careful partial dehydration at a moderately elevated temperature which may be, for example, in the range of about 900 to 1500 F. A portion of the same alumina or aluminacontaining support material may be rendered substantially inert by heating it to a much higher temperature. This desurfacing or deactivating treatment might be carried out in various ways such as by heating, for example, to a temperature in excess of 2000 F. in the presence of water vapor. The catalytic portion of the carrier gel will, of course, be impregnated or compounded in some other suitable manner with a catalytic substance, while the non-catalytic inert portion will contain no such added material.

A particular advantage of the use of inert material having a composition basically similar to that of the catalyst carrier is that any residual activity not removed by the high temperature treatment will be fundamentally similar to that of the catalyst carrier itself, so that side reactions which might be catalyzed by material of different composition will be still further minimized.

The hydrous oxide to be treated in this manner can obviously be prepared in different ways. With an alumina-containing carrier, for example, the alumina may be prepared by any one of a variety of methods including the neutralization of an aluminum salt solution or an aluminate, the interaction of two such solutions, or the oxide, which in turn may be precipitated at various stages of the preparation or may be dried directly from a hydrogel state. a

The method of gel drying is also capable of numerous variations. Where careful sizing is desired, some advantage may be realized by the spray-drying of Washed gel particles, so as to produce finely divided spherical panticlesin a desired narrow size range. For a catalyst to be used by the fluidized solids technique as in the present invention, spray drying may be thus controlled to produce spherical particles essentially all of which are in the range of about 20 to micron size. The dried gel particles may then be sized into two fractions by screening or by air elutriation. One portion containing the particles of a size greater than, say, 50 microns in diameter can then be used as the base for the hydroforming catalyst, while the finer particles are treated separately to produce the heat transfer medium.

The fines fraction to be used as the heat transfer medium in the process as further described below will be mixed with the catalyst and separated, repeatedly, in each stage of the operating cycle. The separation according to size is'never entirely complete in passing through a simple separating device such as a cyclone or settling chamber, and the cut size chosen between the fines fraction and the catalyst particles in the initial preparation will depend to some extent upon the particular separating equipment to be used in the operating cycle. Whatever the cut point chosen, there will be some overlapping of inert fines into the catalyst fraction and the finest of the larger catalyst particles into the inert fines fraction. If the effective cut point in the plant operating equipment is, say, at 40 microns, this means that at normal gas velocities through the separator the bulk of the residual solid entrained in the exit gas stream will be finer than 40 microns in size, while most of the solid particles larger than 40 microns in size will be removed and returned directly back into the catalyst bed. The total percentage of the catalyst removed overhead and recycled with the inert fraction might then be minimized by making the cut point in the initial fines preparation at a size smaller than 40 microns, say at 35 microns. In this case relatively less of the inert fines will remain in the catalyst bed, where they would act as a diluent resulting in an increased bed volume requirement. On the other hand, this means that at a given total entrainment, relatively more of the catalyst is removed overhead and recirculated repeatedly throughthe regenerator along with the inert fines fraction, and this may not be desired. Conversely, therefore, if such an excess recirculation of catalyst fines along with inert fines is to be avoided, the initial cut point between fines and catalyst particles might be made at, say, 45

microns in the same example. In this case, less of the catalyst would be carried over with the inert fines. through the cyclone separator, with correspondingly more of the inert particles being left in the catalyst bed where they would circulate with the catalyst and appear as a diluent in the reaction space.

Catalyst fines selectively entrained out of the reactor and the regenerator together with the fine inert heatcarrying particles will have no harmful effect on heat carrying capacity, since both come out of the regenerator and the reactor at the same temperature. A less desirable result, however, appears from the fact that the catalysts used are materials of variable valence which are oxidized in the regeneration zone and reduced in the reactor (or in a separate pretreating zone). Extra catalyst recirculation means an extra load on reoxidation. This accelerates catalyst deactivation and is expensive at best, particularly when the extra catalyst circulation is in a form 7 w ch 995 no u t e acti it of me ire h es n a ed ata ys Thi ype o eci ula on should therefore be minimized.

Still. another alternative is t carry out a two-stag separation of untreated fines from the coarser catalyst particles in the first place. Thus, for example, 20 to 100 micron size spray-dried particles might initially be sieved through a 400-mesh screen, which passes 37 micron-sized particles, to give a fines fraction essentially 20 to 37 microns in diameter. The relatively coarser fraction obtained at the same time might then be further screened, or elutriated, to reject an intermediate fraction which might be that passing a 3251-1115511 screen, .at about 44 microns in diameter, or might be somewhat smaller or larger in size range as desired. A similar separation might be efiected by sieving the initial 20-l00 micron material through a 325.-mesh screen to recover the catalyst base and a fines fraction. Suspending these fines in a gas stream and passing this mixture through a separator of efficiency comparable to that in the refinery plant, the desired fines fraction would pass on through while removing an intermediate fraction from the entraining gas.

The apparatus illustrated shows a two-vessel system for fluid hydroforming, comprising a reactor 10 and a regenerator 30 together with the necessary separators and lines connecting the different vessels. In this apparatus, the catalyst within the hydroforming reactor 10 is maintained as a fluid bed 11 which may be separated by a grid plate or baflle 12 into an upper zone 13 and a lower zone 14. The reactant gases are introduced through the hydrocarbon vapor feed line ;16 and recycle gas feed line 17 .as upfiow streams entering the bottom of reactor 10. Product vapors pass overhead through line 18 to a gas-solids separating device 20, and these vapors freed of entrained solid particles are conveyed from separator 20 through the product line 22 to suitable product recovery equipment, not shown.

Spent catalyst from the .hydroforming reaction is withdrawn in any suitable manner, such as through catalyst withdrawal line 24 to a stripping zone shown as an external stripper 25. In this zone the spent solid passes countercurrent to a suitable stripping gas such as steam or a recycled gas stream introduced through line 26, for removing adsorbed and occluded .hydrocarbon from the spent catalyst. The stripped catalyst from line 27 at the bottom of stripper is withdrawn at a rate controlled by valve 28 and picked up as a dilute suspension in a stream of aerating gas such as air or diluted air and carried therewith through line 29 into the regeneration vessel 30.

In the transfer line 29 and regenerator 30, the carbonaceous deposits formed in the hydroforming reaction which have caused the deactivation of the catalyst are at least partly removed by a controlled combustion reaction. Additional amounts of air or an extraneous fuel, if desired, may be introduced into regenerator 30 through a suitable inlet line or lines 32 to effect the desired regeneration. At the vsame time, the regeneration stage of the process may be controlled by this means to be sure of producing enough heat to supply the necessary heat of reaction for the hydroforming process.

Regenerated catalyst is withdrawn from vessel 30 by way of a bottom draw-01f line 34, conveying a dense stream of fluidized solid directly into the bottom portion 14 of the fluid bed of catalyst within reactor 10. The finer particles entrained overhead from vessel 30 with the spent regeneration gases are carried by line 36 to separator 38 which separates the solid from regeneration fumes vented by way of line 39 to the stack.

It is an essential feature of the present invention that the finer solid particles entrained overhead through line 18, and separated out in separator 20, can be directed primarily through the return line 21 into the regeneration vessel 30, where they pick up heat from the regeneration P 'Q S Th se sam fines, heated i the e ator, may then be selectively entrained overhead from the total fl dized b dy o solid within fl i e .31 in re en rat r 30. The solid particles-thus entrained a parate ou in separator 33;, and returned by way of 'lin 0 into-wue 13 at the .top of the :fiuid bed 11 in the hydroforming reactor. Thisestablishesa separate circuit forwfine particles which essentiallyserve to carry heat from regenerator 30 to reactor .10 and back .again to pick up more heat, by .way of theseparators '38Yand 20, without passing this part of the solid rthrough the rest of the catalyst circulating system,

In addition .to this arrangemenh which constitutes the main path "for the fiow of these fines through the system, branch line 41 may :be supplied to return a portion of the catalyst separated in vessel 20 directly back to bed 13, and branch line 42 similarly to return a portion of the catalyst =fr-om separator 38 directly back to bed 31. The use of thesehranch lines permits an independent control at the rate at which fine solids entrained overhead from each bedare returned to the same bed rather than to the other .bed, so as to control the rate of fincs circulation to and from each bed for purposes of heat exchange.

According to another modification of the invention, the inert fines particles and catalyst particles may be entrained together-out of the two fluid beds in the system and conveyed overhead "by this means from each bed to the other. In this method of operation, as in the other, the rate of entrainment of the fine particles will be substantially greater than that of the coarser catalyst particles, providing a relatively high recirculation rate of fi s. thr u h t s em! t he sa ti e the total a o n fsq eatra amen n he he d h m thf d i e by at f a tm a v l c y s that ys particles are carried from one vessel to the other with the entrained fines. Since a significant separation of finer particles from the coarser takes place in the free space or outage above the bed 'level within the reactor 10 and regenerator ,30, the relative amounts of catalyst and fines recirculated from one vessel to the other according to this modification .of the invention may be controlled by controlling the bed level in each of these vessels. A higher bed level giving less outage, will increase the amount of the larger catalyst particles leaving with the vent gases for a given antount of total solid entrainment. This bed level can .be varied by varying the total inventory of the catalyst particles plus inert fines in the system. By operating in this manner, the amount of solid transferred from pne vessel to the other, by way of the catalyst withdrawal lines 24 and 34, is materially decreased. It may be possible in this manner to entirely eliminate these dense phase withdrawal lines, resulting in a significant simplification of the plant equipment and important savings in construction cost.

A study of relationships of gas velocity andrelative entrainment rates shows clearly certain important advantages of using a fine-sized heat carrying solid according to the present'invention instead of using relatively coarse or shot-sized particles.

Example I .Considering a typical hydroforming operation with an alumina-based catalyst at about 900 F. and 200 pounds operating pressure, the amount of heat to be transferred from the-regeneration zone to the reaction zone in one example corresponds to a heat-carrier to oil weight ratio of 3/1 superimposed upon a catalyst-to-oil weight ratio of l/ 1. Using a fine-sized heat carrier the circulation of this amount of inert fines requires the entrainment of about 0.8 lb./cu. ft. of inert solids overhead in the product vapor stream. Using an inert fines of 20 to 40 microns size having an average particle diameter of about 30 microns, this corresponds to a superficial vapor velocity (at conditions) of about 0.9 foot per second. At this velocity, catalyst particles of 55-75 microns in diameter or microns average size are entrained only at the rate of about 0.08 lb./cu. ft. and 75-85 micron particles at about 0.04 lb./cu. ft. at the same outage. The total solids entrained out of the bed from a mixture of these fines and coarser particles would thus correspond to about 85-90% fines without any other solids separation then the settling zone above the fluid bed in the reaction zone. These figures are for a 4-foot outage. Reducing the outage would make it possible to lower the gas velocity and get still better size selectivity at a given total entrainment rate, if such a change is desired.

Example II.The effect of particle size on attrition is a function of particle weight; Particles of 400 micron size may be taken as typical of a coarse or shot-sized material which can be readily separated from the catalyst by virtue of its larger size. The ratio of particle weights is over 2,000 to 1 as compared to fines of the same heat carrying material having an average particle diameter of 30 microns. This, of course, "is heavily in favor of avoiding the very large particles, not only because of the attrition of the particles themselves but also because of their grinding action on the catalyst and the relative rates of equipment erosion.

The effect of attrition is also to be considered using the inert fines of the present invention, since there is some tendency for the relatively coarser catalyst particles to wear down into the size range of the fines. The attrition of either the inert or catalytic materials knocks otf extremely fine particles which are blown out of the system as dust in the vent gases. With a catalyst of 40-90 micron size containing 1% of 80+ material, a catalyst replacement rate of 1% per day is found to correspond at equilibrium to a catalyst inventory having about of material below 40 microns in size. Increasing the proportion of coarser particles in the fresh catalyst feed to 10% of 80+ corresponds to about 4% of catalyst less than 40 microns in size.

While this amount of overlap between the catalyst and the inert fines can be considered negligible, even this can be cut down by removing an intermediate sized fraction of material during the initial preparation to give a dumbbell size distribution with substantially nothing in the middle size range, as previously described. A lower overall rate of attrition, corresponding to a lower catalyst replacement rate, will also give an equilibrium size distribution with less of the catalyst in the fines size range below 40 microns.

In addition to the advantage of inert fines over the shotsized inert particles from the viewpoint of attrition, the use of the fines material gives better mixing and more uniform temperature distribution Within the fiuid bed. Equally important, it avoids troublesome difficulties of separating according to particle size in solids transfer lines. These difficulties and the mere problem of imparting the necessary motion to the solid are both aggravated by the very large increase in particle weight for a given increase in the ratio of particle diameters, when the inert particles are significantly larger than the catalyst instead of being smaller.

These advantages of the present invention are not limited to the use of a divided bed or the inverse temperature gradient type of operation. However, when such an operation is desired, the ability to remove the heat carrying solid overhead contributes the added advantage of giving a larger temperature gradient for the same total heat supply.

Example III.In a particular inverse temperature gradient operation of the type shown in the drawing, zones 13 and 14 are maintained as fluid beds of equal size. The hydrocarbon feed and recycle gas temperatures are both 1000 F., taking advantage of the use of a solid heat carrier to avoid the necessity of heating either of these streams to higher temperatures. Two cases are now to be considered, where the heat carrier is 20-40 micron material which is finer than the catalyst and removed overhead from the upper bed and where it is coarser 300-500 micron material being removed from the bottom. In both cases the regenerated catalyst and heat carrier are entering the upper bed, at the same temperature, with 1/1 catalyst to oil, 3/1 heat carrier to oil, and a severity of 0.3 pound of oil per hour per pound of catalyst in the reactor. Under these conditions, the following bed temperatures appear at equilibrium for solid feed temperatures of 1000" F., or 1050 F with the two different types of heat carrier:

The fine sized carrier gives a larger temperature difference and a significantly. lower bottom zone tempera ture'for exactly the same total heat input. This results in greatly improved selectivity by avoiding over-activity in the initial stage of the two-stage upfiow reaction, where the inverse temperature gradient is desired. At the same time, an adequate amount of heat is supplied for the second stage of the reaction.

It will be understood that while the fluid bed 11 in reactor 10 has been shown as separated by a single bafile to prevent the overall mixing of material from the upper zone 13 into the lower zone 14, a plurality of such baffies may be supplied for the same purpose.

.Where more than one bafile is used in this way it may also be advantageous to use downcomers so as to permit a controlled amount of back flow of solid from an upper level within bed 13 to a lower level within the same maintained can be seen from the following data, which Efiecr of recirculation between stages Recirculation, Equivalent C/O Ratio 0 4 Bed Temperatures, F

Any recirculation between the top and bottom zones brings the temperature of these beds closer together, lessening the advantage of the inverse temperature gradient. However, as the number of stages is increased with heat supplied only to the top and bottom zones in a staged reactor the temperatures in each end of the reactor approach the inlet temperature of the material fed to the end zones, and temperatures throughout the reactor go through a minimum. This is undesirable from a process standpoint. Nevertheless, it may be desirable to use a multistage vessel from the standpoint of better fluidization' and contacting efliciency', even-though additional:

staging'is directionally undesirable for inverse-temperature gradient operation; The use'ofa two-zone' reactor providing baflies' containing" down'comers, to promote catalyst circulation between stages within each: zone, overcomes this difliculty. Thus, it is possible to obtain satisfactory inverse temperature gradients without sacri-- ficing the-improved contacting obtained with bafiies.

It will also be noted that the present system: allows catalyst to be circulated in concurrent flow with feed through the reactor. This is frequently desirable from a process standpoint.

Ordinarily it may not be'convenient to operateunderconditionswhere there is absolutely no recirculation between beds. Even under conditions of limited recirculation, however, the same trends appear as an advantage for the use of the fine-sized heat carrier, as'long. as an inverse temperature gradient is being maintained. In any case, of course, it is desirable to design a given plant for maximum plant throughput. This fixesthe vapor velocity, and the solids entrainment rate for a given outage. Outage in turn is controlledto maintain the necessary solids entrainment rate for recirculation of the heat carrier. It may ordinarily be preferred to design with a little more than the necessary amount of entrainment for this purpose, and return excess entrained solid by return lines 41.and 42 to effect the desired control of heat carrier circulation rates. Catalyst-to-oil. ratio may be independently controlled, as in the usual plant operation, by thte proper use of the catalyst withdrawal lines 24 and 34.

It is also possible to control the circulation of fine solids b y'installing storage vessels, not shown, between separators 20 and 38-and their respective return lines 21 and 40. The amount of fine inert solids carried overhead in the regenerator and reactor vessels can then be controlled by changing the inventory of fine inert solids in these vessels. When the fluidized beds in the reactor and regenerator become deficient in fines content, less total entrainment of solids will occur and more actual entrainment of the coarser catalyst particles. Thus, it is possible to change the amount of fines circulated in the dilute phase -by controlling the holdup in these vessels. This method of control is particularly advantageous when the catalyst and fine inert solids are circulated together, and makes it possible to adjust the ratio of catalyst/fines when operating in this manner. This inventory control technique may be superimposed upon the bed level control method mentioned above.

While the invention has been described particularly useful as applying to the catalytic conversion of hydrocarbons it is also useful for many other processes. example, in controlling a highly exothermic reaction such as the combustion of coke, oil shale, or other carbonaceous solids, the fine solids circulation system described herein can be advantageously used as a'method of heat removal. This principle has already been described in the catalytic reforming regenerator, where the fine'inert solids are used to control regenerator temperature and remove heat from this vessel. In general, where the invention is used for controlling an exothermic reaction, the fine solids carried overhead may be cooled in another fluid vessel or by other exchange methods and returned to the reaction zone as previously described. The same principles may, of course, be applied independently to supply heat to an endothermic reaction when this is desired.

What is claimed is:

1. The method of maintaining heat-balance between the regeneration stage and the reaction stage of an endothermic catalytic process for the conversion and treatment 'of naphtha hydrocarbons which comprises maintaining within a reaction zone a fluid bed of finely divided catalyst particles consisting of material having a particle size essentially greater than 40 microns in diameter, add- For 1 ingt'o'the catalyst in-said fluid bed a finely divided inert solid rnaterial' consisting of particles essentially smaller :than 40-microns in diameter having a substantially smaller free surface area and substantially no separate catalytic effect-as compared with said catalytic particles, continuously withdrawing from said fluid bed catalyst particles contaminatedby a carbonaceous deposit formed during said' catalytic reaction process, simultaneously withdrawing frornisaid fluid bed inert fine particles entrained overhead in the gas stream leaving. said zone, separating said fine particles. from said gas stream, conveying said separated inert fines. and saidwithdrawn spent catalyst to a second fl'uid'bed 'rnaintained within a common regeneration zone, introducing. into said regeneration zone a fluidizing stream of regeneration gas and removing therein at least a substantial portion of said carbonaceous deposits from said spent catalyst by an exothermic reaction between said catalyst particles and said regenerating gas, withdrawing hot regenerated catalyst particles from the fluid bed within the said regeneration zone and conveying them to a lower portion of the fluid bed in said reaction zone, withdrawing hot inert fine particles from said regeneration zone entrained in the spent regeneration gas passing overhead, separatingsaid hot fine particles from said spent regeneration gas, carrying a major portion of the heat released by said exothermic regeneration in the form of sensible heat of said inert fines and returning said hot fines to an upper portion of said reaction zone, the amount of inert fines being substantially greater than the amount of regenerated catalyst similarly recirculated, whereby the temperatureinvthe upper portion'of said reaction zone is maintained at'a level of about 25 -70 F. higher than the temperature inthe lower portion of said zone.

2. The'method according to claim 1 in which said inert fine particles are-selectively entrained overhead in the gas streams leaving the reaction zone and the regeneration zone, separate streams of dense fluidized spent catalyst and regenerated catalyst are withdrawn directly from the fluid bed in. saidreaction zone and said regeneration zone respectively, and said inert fines and said catalyst particles are' conveyed as separate streams from each of said zones to the other.

3. The-method according to claim 2 in which the mass rate at which the inert fines are circulated between said reaction zone and said regeneration zone is substantially in excess of that at which the catalyst particles are circulated between said zones.

4. The method according to claim 2 in which the rate at which said inert fines are recirculated from each of said zones to the other is independently controlled, by separating the entrained fine particles from each zone, conveying at least a major portion of the separated inert fines from each of said zones into the fluid bed in the other zone, simultaneously returning a variable portion of said fines'directly to the bed from which they have been entrained, and controlling the proportion of the total entrained fines which are returned directly to each bed without recirculation through the other bed.

5. The method according to claim 1 in which the stream of gas leaving the fluid bed in both the reaction zone and the regeneration zone carries with it a substantial proportion of catalyst particles entrained overhead from the fluid bed in each of said zones, and said inert fines and entrained catalyst particles are conveyed together from each of said zones to the other.

6. In a process for the catalytic reforming of a hydrocarbon feed stock in the naphtha boiling range wherein the vaporized feed stock is brought in contact with a catalyst supported on an alumina-containing base to pro- .duce a product stream enriched in aromatic hydrocarbons and other high-octane-quality components while at the same time forming a contaminating coke deposit on the catalyst, the improvement which comprises using a catalyst supported on an alumina-containing carrier consisting essentially of particles free of material finer than 13 about 40 microns in diameter, adding to said catalyst a body of finely divided inert particles consisting essentially of alumina-containing material having a size smaller than about 40 microns in diameter, said inert particles having substantially the same chemical composition as the alumina base for the catalyst but with a much smaller free surface area and moisture content, introducing said fine-sized inert material and said coarser catalytic particles into the body of a fluid bed of catalyst maintained within a reforming zone under conversion conditions of temperature, pressure, naphtha feed and catalyst feed rates, withdrawing said inert particles and coked spent catalyst from said fluid bed and conveying them to a common fluid bed maintained within a separate regeneration zone, selectively entraining said fine inert particles overhead from the fluid beds in said reaction zone and in said regeneration zone, separating said entrained fine particles from the entraining gas leaving each of said zones and returning said separated inert particles to an upper portion of the other of said zones, thus establishing a separate circuit whereby heat is extracted from said regeneration zone as sensible heat of said fine inert material and conveyed thereby to said reaction zone, the amount of inert fines being substantially greater than the amount of regenerated catalyst similarly recirculated, whereby the temperature in the upper portion of said reaction zone is maintained at a level of about 25 -70 F. higher than the temperature in the lower portion of said zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,446,247 Scheineman Aug. 3, 1948 2,455,915 Borcherding Dec. 4, 1948 2,464,616 Schwarzenbek et al. Mar. 15, 1949 2,642,381 Dickinson June 1 6, 1953 2,721,167 Nicholson Oct. 18, 1955

Claims (1)

1. 6. IN A PROCESS FOR THE CATALYTIC REFORMING OF A HYDROCARBON FEED STOCK IN THE NAPHTHA BOILING RANGE WHEREIN THE VAPORIZED FEED STOCK IS BROUGHT IN CONTACT WITH A CATALYST SUPPORTED ON AN ALUMINA-CONTAINING BASE TO PRODUCE A PRODUCT STREAM ENRICHED IN AROMATIC HYDROCARBONS AND OTHER HIGH-OCTANE-QUALITY COMPONENTS WHILE AT THE SAME TIME FORMING A CONTAMINATING COKE DEPOSIT ON THE CATALYST, THE IMPROVEMENT WHICH COMPRISES USING A CATALYST SUPPORTED ON AN ALUMINA-CONTAINING CARRIER CONSISTING ESSENTIALLY OF PARTICLES FREE OF MATERIAL FINER THAN ABOUT 40 MICRONS IN DIAMETER, ADDING TO SAID CATALYST A BODY OF FINELY DIVIDED INERT PARTICLES CONSISTING ESSENTIALLY OF ALUMINA-CONTAINING MATERIAL HAVING A SIZE SMALLER THAN ABOUT 40 MICRONS IN DIAMETER, SAID INERT PARTICLES HAVING SUBSTANTIALLYTHE SAME CHEMICAL COMPOSITION AS THE ALUMINA BASE FOR THE CATALYST BUT WITH A MUCH SMALLER FREE SURFACE AREA AND MOISTURE CONTENT, INTRODUCING SAID FINE-SIZED INERT MATERIAL AND SAID COARSER CATALYTIC PARTICLES INTO THE BODY OF A FLUID BED OF CATALYST MAINTAINED WITHIN A REFORMING ZONE UNDER CONVERSION CONDITIONS OF TEMPERATURE, PRESSURE, NAPHTHA FEED AND CATALYST FEED RATES, WITHDRAWING SAID INERT PARTICLES AND COKED SPENT CATALYST FROM SAID FLUID BED AND CONVEYING THEM TO A COMMON FLUID BED MAINTAINED WITHIN A SEPARATE REGENERATION ZONE, SELECTIVELY ENTRAINING SAID FINE INERT PARTICLES OVERHEAD FROM THE FLUID BEDS IN SAID REACTION XONE AND IN SAID REGENERATION ZONE, SEPARATING SAID ENTRAINED FINE PARTICLES FROM THE ENTRAINING GAS LEAVING EACH OF SAID ZONES AND RETURNING SAID SEPARATED INERT PARTICLES TO AN UPPER PORTION OF THE OTHER OF SAID ZONES, THUS ESTABLISHING A SEPARATE CIRCUIT WHEREBY HEAT IS EXTRACTED FROM SAID REGENERATION ZONE AS SENSIBLE HEAT OF SAID FINE INERT MATERIAL AND CONVEYED THEREBY TO SAID REACTION ZONE, THE AMOUNT OF INERT FINES BEING SUBSTANTIALLY GREATER THEN THE AMOUNT OF REGENERATED CATALYST SIMILARLY RECIRULATED, WHEREBY THE TEMPERATURE IN THE UPPER PORTION OF SAID REACTION ZONE IS MAINTAINED AT A LEVEL OF ABOUT 25*-70*F. HIGHER THAN THE TEMPERATURE IN THE LOWER PORTION OF SAID ZONE.

Images