US3042729A - Cyclic endothermic reaction processes - Google Patents

Description

July 3, 1962 B. F. MULASKEY CYCLIC ENDOTHERMIC REACTION PROCESSES Filed Sept. 30, 1959 E L C Y C L A E m E M E R G E R N U E P m,

mmDtxmumpzukl REACT FIG. 1

EXCESS COKE 'lll REGENERATE L-PURGE TIME FIG.2

INSUFFICIANT COKE INVENTOR BERNARD F. MULASKEV m umikmmmzwz E United States Patent ass 42,729 CYQLIC ENDGTHERMES REACTIGN PROCESSES Bernard F. Mulaskey, El Sobrante, (Ialifi, assignor to Cali ornia Research Corporation, San Francisco, Caiih, a corporation oi Delaware Filed Sept. 39, 1959, Ser. No. 843,580 6 Claims. (Cl. 268-680) This invention relates to improvements in catalytic conversion processes which are carried out in a cyclic fashion. More particularly, the invention is concerned with endothermic reactions wherein a substantial portion of the heat absorbed by the reaction is supplied by the combustion of carbonaceous deposits formed during the reaction. The invention is especially applicable to dehydrogenation processes wherein a heat retentive body of particulate catalyst solids is alternately contacted with a reactant stream to which it delivers heat to sustain the endothermic reaction, and with an oxygen-containing gas which serves to burn off combustible deposits formed during the reaction period, thereby raising the temperature of the heat retentive body to the initial condition.

In many catalytic reactions which are sustained by the absorption of heat, such as hydrocarbon cracking, deh drogenation, and the like, coke-like deposits are laid down on the catalyst as a by-product of the reaction. It has been found that many such reactions may be carried out in a substantially adiabatic fashion by utilizing the heat of combustion of the carbonaceous deposit released during regeneration of the catalyst to supply the necessary endothermic heat of the reaction. For example, the Houdry butane dehydrogenation process, as described in the December, 1953, issue of The Houdry Pioneer, is a process wherein a bed of catalyst, which may or may not contain inert heat retentive material, at an elevated temperature is contacted with the feed undergoing conversion. The temperature of the catalytic mass gradually decreases as it gives up heat absorbed by the reaction, and at the same time a carbonaceous deposit or coke is laid down on the catalyst. The bed is then contacted with a heated stream of oxygen-containing gas, referred to as an air blow, which serves to burn ofl the carbonaceous deposit and thereby to restore the bed to the initial elevated temperature, whereupon the cycle is repeated. It would be extremely fortuitous if the heat of combustion of the quantity of carbonaceous material formed exactly equalled the heat of reaction of the material converted during the corresponding on-stream period of the cycle. In actual practice the amount of coke formed will be either too great or too little to make the process truly adiabatic in the sense that no heat need be removed to or supplied from an external source to maintain a given conversion level.

Where the coke deposits are excessive, which in the case of butadiene production from butane may result from too high a concentration of butenes in the feed, or

I increase the coke concentration.

because the inlet temperature is too high, or the catalyst v too active, the bed temperatures on regeneration may tend to be excessive, leading to a gradually increasing temperature level and even a temperature runaway. Such a situation may be compensated for by extending the regeneration or air blow time, decreasing the inlet temperature, shortening the s n-stream time, or by adding an inert heat retentive material to the catalyst bed.

My invention is applicable to the converse situation, where the quantity of coke deposits is insufiicient and too poorly dis ributed. Such a situation may arise in the case of butane dehydrogenation, for example, where the feed consists essentially of normal butane, or the inlet temperature is too low, or the catalyst is relatively inactive with respect to coke formation. Insufiicient coke deposition may result in a gradually declining temperature level and in poor temperature distribution in that the lower ice portions of the bed with respect to the direction of flow will tend to be colder than the upper portions. It is known that such a situation may be partially compensated for by extending the regeneration or air blow time with heated gases, or by increasing the temperature of the preheated feed and regeneration air, or by eliminating the inert heat retentive material from the catalyst bed.

Extending the air blow time at a given rate of gas flow so as to raise the bed temperature to the desired level by use of the sensible heat of the air or other blow gas is undesirable in that over a period of several cycles the net on-stream time is correspondingly reduced, and plant capacity is thereby limited. Furthermore, the process is no longer adiabatic in that a substantial portion of the heat of reaction is supplied in an ineflicient and indirect manner, namely by heating the blow gas. Where the situation of insufiicient coke lay down is compensated for by increasing the feed preheat temperature, this also amounts to supplying a substantial portion of the reaction heat by indirect heat exchange with the feed. Moreover, there are definite limits to the maximum preheat temperature set by the properties of the materials of construction. Also, at elevated temperatures above the desired optimum the tendency toward thermal cracking of portions of the reactant-s to light gases is increased. Since the coke deposits form only on the catalyst portion of the bed, the elimination of inert heat retentive material will tend to However, the total amount of carbonaceous deposits may still be inadequate.

It is an object of this invention to provide a means of compensating for inadequate coke deposition by inducing an increased tendency toward the formation of carbonaceous or polymer-like deposits in a controlled manner.

Another object of this invention is to improve the vertical temperature profile of the catalyst bed by increasing the tendency toward formation of carbonaceous deposits in the cooler portions of the bed, such that on regeneration the bed temperature is substantially more uniform throughout.

Another object is to decrease the tendency toward cracking of the feed stock to light normally gaseous products, while operating at the optimum temperature level with respect to rate of conversion to the desired products.

Additional objects will appear from the description and examples which follow.

I have discovered that the tendency toward formation of carbonaceous deposits or polymers in a process of the type described above may be increased and the deposits more advantageously distributed by adding to the reactant or feed stock basic nitrogen compounds or hydrocarbon streams containing basic nitrogen compounds. Moreover, I have discovered that the addition of basic nitro gen compounds or streams containing basic nitrogen compounds to the reactant stream results in a decreased tendency toward cracking of the feed or intermediate products to light normally gaseous products at any given conversion level. I

In the practice of my invention many nitrogencontaining hydrocarbon streams available in a conventional refinery will be found to be useful. The only criteria would seem to be that the stock used should be one which is capable of being vaporized under the conditions of the process and that it contain suflicient basic nitrogen in the form of organic compounds such that inordinately large quantities need not be employed. It is especially advantageous to use hydrocarbon streams containing rela: tively large concentrations of nitrogen impurities since these stocks are generally considered unsuitable for many applications, such as reforming, without extensive purification. A particularly preferred additive is a distillate obtained from the coking of gilsonite, a solid hydrocarbon material of relatively high nitrogen content.

3 Where it is desired to minimize the quantity of diluents added, or if nitrogen-containing hydrocarbon streams are not readily available, the basic nitrogen compounds themselves may be used. For example, pyridine, substituted pyridines, or nitrogen compounds of the general type where R may be H or a hydrocarbon group, would be suitable additives. Since such compounds are relatively expensive in the pure form, the use of hydrocarbon streams containing naturally occurring nitrogen bases is preferred.

While the exact mechanism by which increased coking is effected by the basic nitrogen compounds is not known, and there is no intention to limit the invention by any particular theory, it is believed that the basic nitrogen compounds are chemisorbed at acidic sites on the catalyst surface. Apparently this chemisorption occurs preferentially in the cooler portions of the catalyst bed, which portions were less productive of coke than the hotter portions. On regeneration the combustion of the nitrogen compounds raises the temperature of the formerly cooler portions such that these areas are more active in subsequent cycles both for dehydrogenation and coking. Thus, only a very small concentration of basic nitrogen in the feed, less than 0.1 percent by'weight in nearly all cases and usually only 0.005 to 0.02 percent, is sufficient to cause an increase in coke production of up to two percent of the hydrocarbon feed. When the addition of basic nitrogen is discontinued, the beneficial eifect is observed for many cycles before the temperatures gradually decline to the original pattern.

The invention will be better understood by reference to the following examples and appended graphs. For convenience, the invention will be explained with reference to a butane dehydrogenation process; although it is understood that the procedure is applicable to other processes of like nature. r

-In the cyclic adiabatic butane dehydrogenation process a normal butane feed together with a recycle stream, which may contain butenes, is preheated to a predetermined temperature in the neighborhood of 10004200 F. and passed at sub-atmospheric pressure'in the neighborhood of' 1-10 p.s.i.a. over a preheated catalyst bed of relatively large cross-sectional area with respect to the'bed depth. Suitable catalysts for the reaction are group VI metal oxides on an alumina support, which may contain other activating compounds or promoters. A particularly preferred catalyst comprises chromium oxide on activated alumina. Various heat retentive particulate solids of high heat capacity, such as Alundum, may be mixed in with the catalyst. In passing downward over the catalyst bed, the butaneis converted to butene and butadiene with the release of hydrogen and absorption of heat. Light normally gaseous hydrocarbons are also formed bycracking side-reactions,"and a deposit referred to as coke, but which is believed to be a polymer-like material, is deposited on the catalyst. The average temperature of the bed decreases due to the endothermic character of the reaction while at the same time the rate of conversion of the butane decreases due to the lower temperature and catalyst fouling. After a short period of time, such as 5-20 minutes, the rate of conversion has become uneconomic, and the flow of feed is interrupted.

'A heated oxygen-containing gas is then passed over the catalyst bed to burn off the carbonaceous deposits, thereby raising the bed temperature to the initial level, desirably. This may be followed by a period of blowing with heated air or other gas to displace lateral and vertical temperature inequalities. After a short purging or evacuation step to remove residual oxygen, the cycle is repeated. A purge step may also intervene between the reaction .and regeneration steps. By providing a multiplicity ofreactors the operation is made continuous.

Referring now to FIGURE 1, there is shown schematically a typical temperature cycle under the ideal conditions where the heat of combustion of the coke deposited matches thev endothermic heat of reaction. FIGURE 2 shows the situation arising when excess coke is deposited and no corrective action is taken. It can be seen that the temperature tends to increase, and ultimately it increases beyond allowable limits. FIGURE .3 shows the situation arising when'there is insufiicient coke deposition. In this case there will also tend to be temperature inequalities throughout the bed, with the lower downstream portions tending to be cooler than the upper portions, due to the fact that ordinarily the coke tends to deposit Where the temperatures are higher. If no corrective action is taken, the temperatures tend to decline over a period of cycles and stabilize at a lower level, where the rate of conversion is unsatisfactory.

Example I This example is illustrative of a prior art process where coking is inadequate.

Normal butane preheated to about 1100 F. was passed over a chromium oxide on alumina catalyst, at a pressure of about .18 centimeters of mercury absolute and a space velocity of about 0.9 LHSV, for 7 /2 minutes. The feed was then interrupted, and air at 1100 F. was passed over the catalyst for 7 /2 minutes. Valve changes, evacuation and an intervening purge with inert gas (nitrogen) consumed an additional 2 /2 minutes. The cycle was then repeated. -As shown by the data presented in Table I, the lower portion of the catalyst bed was cooler than the upper portion. 'The'coke deposition, particularly in the lower portion, was insuflicient to sustain a desirable conversion level at this inlet temperature.

Example II This example illustrates the effect of adding a basic nitrogen-containing hydrocarbon stream.

A light distillate fraction derived from a prefractionator bottoms stream in a gilsonite coking process was employed. The light fraction had a gravity of 38.0 API, a boiling range of 385-47l F., and basic nitrogen content of 0.63 percent by weight. One volume of the light fraction was diluted with three volumes of 250 Thinner, a light solvent containing principally parafiins and naphthenes and having a mid-boiling point of 212 F. The resulting solution was injected with the n-butane feed to give a mixed feed containing about 4.4 weight percent of the additive solution. The mixed feed, preheated to 1100 F. as before, was passed over the chromium oxidealumina catalyst at the same conditions of pressure, space velocity, and cycle timing as in Example I. As shown in Table I, not only was coke deposition increased, but also the conversion of butane to butenes and butadiene was raised. Moreover, the catalyst bed temperatures were more uniform.

Example III This example illustrates the efiect of adding a higher concentration of basic nitrogen.

A heavier distillate fraction,. a light gas oil, was derived from a gilsonite coking process. The heavy fraction had a gravity of 30.9 API, a boiling range of 382- 613 F., and a basic nitrogen content of 0.96 percent by weight. One volume of the heavy fraction was diluted with three volumes of 250 Thinner, and the resulting solution was injected with n-butane to give a mixed feed containing about 4.4 weight percent of the additive solution. The mixed feed was then processed at the same conditions as in the preceding examples. As shown in Table I, the conversion to coke and desired products was further increased. Due to the increased coke laydown in the lower portion of the catalyst bed, the bottom of the bed Was at the same temperature as the top portion, after regeneration.

Example I V chromium oxide-alumina catalyst utilized in the above examples, although the results will not necessarily be equivalent. Thus, when a chromium oxide-alumina catalyst promoted with potassium oxide was employed in the butane dehydrogenation reaction, the increase in coke layis applicable to processes usin catalysts other than the This example fUI'thBI demonstrates that the eifects Obd n indu ed by basic nitrogen was not as proserved are attributable to the presence of basic nitrogen. Rimmed, but the general fl t was h ame The Solvent, Thinner, Was itself added to the From the foregoing it is apparent that the process of .n-butane feed to provide a concentration of 4.4 weight my invention id a means h b 11 d i i Percent Solvent in the miXed feed, and the experiment of carbonaceous deposits may be increased in a process Was repeated as AS Shown in Table Ll otherwise deficient in this respect. In addition, undesira- Slllts Were 3 slightly better than When P fl-blltane ble cracking reactions are minimized, and by improvwas the feed (Example I). ing the vertical temperature profile of the catalyst bed Example V increased conversion is obtained. ll/Ioreover, it has been 15 found that the incremental increase in carbon deposition This example Illustrates the use Of a basic nltrogen is directly related to the quantity of basic nitrogen incompound P troduced with the feed. Consequently, the increased cok- Qllinoline Was added to the nbutane feed to give a ing may be controlled to any desired degree by adding concentration of 0.13 weight percent quinoline, and the more or less f the nitrogemcomaining diluent as the experiment was repeated as before. As indicated in situation dictates Table I, the increased conversion to butenes and buta- Al h h p ifi examples herein have dealt h diene Was comparable to that Obtained using the heavy a preferred embodiment of this invention, namely, the fraction Of gilsonite coker distillate at about the same butane dehydrogenation reaction the procedure is ob. basic nitrogen coneentfafioll p D- Although viously applicable to endothermic reaction processes in the total increase 111 coke production Was apparently not general which are carried out in a cyclic manner using as great (as measured by C0 analyses f th fll n a fixed bed of catalyst and wherein the combustion of r g n it is elidellt from the temPel'a'iure carbonaceous deposits laid down on the catalyst is relied pattern that coke laydown in the lower portion of the bed on to some extent to supply the heat of reaction during was increased a considerable amount. the on-stream period.

TABLE I Example Number I II III IV V F d nn-Butane n-Butane n-Butane n-Butane Butane I- light heavy I- 250 Quinfraction fraction Thinner oline Percent Basic Nitrogen 0 0- 007 0.011 ml 0. 014 Inlet Temperature, F 1,100 1,100 1,100 1,100 1,100 Temperature after Regeneration:

Top of Catalyst Bed, F 1, 025 1, 035 1, 100 1, 070 1, 090

Bottom of Catalyst Bed, F 910 1, 050 1, 100 965 1,050 Percent Butane Converted:

To Butenes Butadiene 18.3 26.5 32.0 18. 8 33. 2

To Gases, C1-C3 0.9 2. 5 5. 5 1.2 5.9

To Hydrogen 1 2 2.2 3.2 1.4 2.4

To Coke 1 5 .4 4.2 1.6 1.9

In another series of examples, comprising n-butane, I claim: n-butane plus the light nitrogen-containing fraction in 1. In a cyclic process for carryn'ng out an endothermic 250 Thinner, and n-butane plus the heavy nitrogen-conhydrocarbon dehydrogenation reaction, comprising altaining fraction in 250 Thinner, the inlet temperatures ternate on-stream and ofi-stream periods wherein heat for were adjusted to obtain the same total butane conversion the endothermic reaction is supplied by combustion. durin each case. The data obtained appear in Table II. ing the elf-stream regeneration period of carbonaceous TABLE H material deposited in a body of dehydrogenation cata-' l t-containin ticulat in te 1 do n th tre ,'s I gpar e aria ri eons am reaction period, and which process is charactenzed by Feed n'butane insufiicient deposition of carbonaceous material to supply Fraction Fraction the heat necessary to sustain a desired high conversion level in the dehydrogenation reaction at a predetermined 0. g o p iced inlet temperature, the improvement of adding organic percent Butane Converted: asic nitrogen compounds to the hydrocarbon feed In an o Butenes+Butadiene amount sufiicient to cause increased deposition of carto Gases, C1-C3 5.8 5. 5 5.0 1

to Hydmgem 23 oonaceous material during the reaction period such that to Coke during the regeneration period increased heat i absorbed by said body of catalyst-containing particulate material,

It is apparent that the addition of the basic nitrogenwhereby said increased-heat is supplied to sustain a higher containing streams permitted operation at lower inlet temconversion in the dehydrogenation reaction. peratures with the same or better yield of desired prod- 2. In a cyclic process for carrying out an endothermic ucts as compared to the run without added nitrogen. In hydrocarbon dehydrogenation reaction, comprising alteraddition, the loss of butane due to cracking to light nate on-stream and ofi-stream periods wherein heat for gases, C to C was less when the basic nitrogen was the endothermic reaction is supplied by combustion duradded. The increased hydrogen production reflects a ing the oil-stream regeneration period of carbonaceous somewhat higher ratio of butadiene to butenes in the material deposited in afixed bed of dehydrogenation cataproduct. lyst-containing particulate material during the on-stream I have also found that the invention disclosed herein reaction period, and which process is characterized by a tendency for the lower portions of said fixed bed with heat of combustion of coke deposited in a bed containing chromium oxide-alumina catalyst to sup-V respect to the direction of flow to be cooler than the upper portions of said bed at the end of the off-stream regeneration period, 7 basic nitrogen compounds to the hydrocarbon feed in an amount suflicient to'cause increased deposition of carbonaceous material'in the cooler portions of said bed during the reaction period-such that at the end of the regeneration period the temperature is substantially uniform throughout said bed. V

3. In a butane dehydrogenation process utilizing the ply a substantial portion of the heat absorbed in the dehydrogenation reaction, by burning said coke, thereby raising the temperature of said bed, and then contacting said bed with a butane-containing feed whereby the temperature of said bed decreases and coke is deposited therein, in which process the coke deposition is insufiicient to sustain a desired high percent conversion of butane and portions of said bed tend to be cooler than other portions; the improvement of adding organic basic nitrogen the improvement of adding organic heat retentive 3 compounds to the butane feed in an'amount sufticient to cause increased coke deposition in the cooler portions of said bed during the dehydrogenation reaction such that after burning said coke the bed temperature is more uniform throughout, whereby a higher percent conversion of butane is sustained.

4. The improvement in the process according to claim 3 wherein said organic basic nitrogen compounds are contained in a hydrocarbon stream containing naturally occurring basic nitrogen compounds.

5. The improvement in'the process according to claim 4 wherein said hydrocarbon stream comprises a hydrocarbon distillate derived from gilsonite.

6. The improvement-in the process according to claim 3 wherein said organic basic nitrogen compounds consist essentially of quinoline.

References Cited in the file of this patent UNITED STATES PATENTS Singleton Nov. 20, 1945 -2,474,014 Seebold June 21, 1959

Claims (1)

1. IN A CYCLIC PROCESS FOR CARRYING OUT AN ENDOTHERMIC HYDROCARBON DEHYDROGENATION REACTION, COMPRISING ALTERNATE ON-STREAM AND OFF-STREAM PERIODS WHEREIN HEAT FOR THE ENDOTHERMIC REACTION IS SUPPLIED BY COMBUSTION DURING THE OFF-STREAM REGENARATION PERIOD OF CARBONACEOUS MATERIAL DEPOSITED IN A BODY OF DEHYDROGENATION CATALYST-CONTAINING PARTICULATE MATERIAL DURING THE ON-STREAM REACTION PERIOD, AND WHICH PROCESS IS CHARACTERIZED BY INSUFFICIENT DEPOSITION OF CARBOMACEOUS MATERIAL TO SUPPLY THE HEAT NECESSARY TO SUSTAIN A DESIRED HIGH CONVERSION LEVEL IN THE DEHYDROGENEATION REACTION AT A PREDETERMINED FEED INLET TEMPERATURE, THE IMPRROVEMENT OF ADDING ORGANIC

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