US2377579A - Process for dehydrogenation of hydrocarbons - Google Patents

Description

ETHANE 8. LIGHTER VAPOR RECOVERY w. A. scHuLzE. 2,377,579

PROCESS FOR DEHYDROGENATION OF HYDROCARBONS Filed Aug. 25, 1940 2 Sheets-Sheet 1 BOLVNOI LDVHA BUTENE 2 BUTANE cATALYsT A ACASES REcYcLE INVENToR w.A. scHuLzE i produce olefins and diolens.

Patented June y5, 1945 (PROCESS FOR DEHYDROGENATION HYDROCABBONS Delaware Application August 243, 19410, Serial No. 353,962 4 (c1. 26o-eso) 5 Claims.

This invention relates to'improvements in the catalytic dehydrogenation of .hydrocarbons to It relates more particularly to an improved process of producing butadiene by catalytic dehydrogenation of normal butane. v,

In the production of the valuable dioleiin butadiene from normal butane, two catalytic dehydrogenation reactions are involved; namely, the

conversion of butane to monolenic butenes, and the conversion of butenes to the diolelnic butadiene. These two reactions may becombined in a single stage operation in which butane is charged and butadiene extracted while a large volume of butane-butene recycle is handled. It is a disadvantage of this type of operation that the optimum conditions of temperature and pressure are quite different for the two reactions. Thus in a process which depends yon concurrence of the two reactions a compromise on operating conditions is necessary whereby neither reaction is most efliciently carried out.

When a two stage catalytic dehydrogenation I process is employed, butenes are produced in the first stage under 'conditions of temperature, pressure and reactant'l concentrations most suitable for their formation with a minimum of decomposition. suitable means and subjectedvto a further dehydrogenation step with conditions carefully selectedv to allow the ruse of higher conversion temperatures along with higher concentrations of unsaturated hydrocarbons and the relatively unstable products. The dehydrogenation of butenes is accordingly carried out atlow partial pressures of reactants and hence of diolefinic products .to avoid excessive decomposition.

One expedient for obtaining the necessary low partial pressures of butenes during dehydrogena tion is the use of a suitable inert diluent 'gas whereby the partial pressure of butenes may be varied with the total pressure of the dehydrogenation system. In this connection it has been found thatV the lower-boiling hydrocarbon gases are satisfactory diluents provided that said gases are relatively easily separated from. the C4 `re- The butenes are then separated by actants and products, and that the diluents are' suiciently inert so that dehydrogenation of the diluent does notproduce hydrogen in quantities large enough to affect the equilibrium in the dehydrogenation of the Ct hydrocarbons. Hydrohydrogenation of propane during the conversion of propane-butene mixtures at low partialpressures of said butenes hasproved disadvantageous.

I have now discovered a method of operation which retains the physical advantages of a" Ca hydrocarbon diluent while substantially elimiycarbon fractions comprising propane have been found satisfactory in applications involving the dehydrogenation of butane, but the minor de-lI nating the possible formation of hydrogen by dehydrogenation of propane. I have found that by my mode of operation, my preferred diluent not only does not hinder but promotes dehydrogenation of butenes. In addition, my process provides use for a by-product of the dehydrolgenation step which furnishes said butenes for my process. These and other objects and advantages of my invention, such as simplification of the butane dehydrogenation step, will be evident from the disclosure following.

I have found that by operating an initial cata- Y lytic dehydrogenation step at low pressures to convert butane to butenes in the absence of any diluent a considerable quantity of C3 hydrocarbons occurs at conditions which aiord a satisfactory yield of butenes per pass. This indicates that a number of reactions involving butane proceed according to the following equations:

The reactions illustrated by the rst two equations are of greatest importance since they acy count for the formation of normal butenes (both isomers), and propylene. The rst ythree reactions l are so-called-primary-reagtions whilethe fourth isa secondary reaction dpendent upon primary j vreaction products. Thus, propylene is present to a considerable degree in the eiliuents vwhereas the secondary product, propane, is not given `much chance to form and the amount is relatively small. 1`

In treating an eiiluent gas i containing all the reaction products'y from the dehydrogenation of butane to segregate a suitable charge for a second catalytic dehydrogenation step, a gas of the following composition must be processed with an This initial separation may be accomplished by compressing and cooling the vapors to condense the Cs and C4 hydrocarbons with a possible secondary recovery operation to strip Ca hydrocarbons out of the uncondensed gases if desired. The required pressure will of course depend on the degree of cooling applied, and usually presi 2,377,579` UNITED sTATEsPATr-:Nr OFFICE sures of 250 to 350 pounds per square inch gage are sufiicient. Some C2 hydrocarbons will remain in the liquid condensate, but the amount retained is not harmful and may be reduced during subsequent fractionation of the condensate.

By fractionation of the condensate I obtain a satisfactory charge to the second dehydrogenation step of my process. This fractionation may be accomplished in one or in a sequence of columns. In the former case a highly eiicient fractionating column is used to separate C3 hydrocarbons and butene-l overhead while the higherboiling C4 hydrocarbons are the kettle product which I recycle with fresh butane feed to the catalyst in my first step. The overhead product containing propylene, propane and butene-l will have a composition approximating the following.

Analysis A shows that under one set of conditions about 85 per cent of the Ca fraction is propylene, while the volume per cent of Ca hydrocarbons is 1'7 per cent of the total.

I have also noted that by choosing even. more severe conditions for the dehydrogenation of butane that a larger'percentage of C3 hydrocarbons may be produced with satisfactory ratio of propylene to propane. duce in my first dehydrogenation step a propylene-butene-l fraction containing as high as about 25 per cent of C: material of which the major portion is propylene as shown by analysis B above. By proper regulation of conditions, propylene-butene charge stocks containing -30 volume per cent of propylene may be obtained, and such charge stocks may be utilized in the process.

In this case, I may pro- The process may be more readily understood by reference to the accompanying drawings which is a diagram of one form of apparatus in which the invention may be practised.

In the figure, butane enters the system through line 32, and is pumped by pump I at about 15 pounds per square inch gage pressure into heater 2, through line 34. It is heated in coil 3, and then passes through line 35 to catalyst cases 4. The cases are filled with catalyst, and means for maintaining the temperature within a narrow range are provided. 'I'he effluent vapors leave the cases through line 36, and enter heat exchange system 5, where they are cooled to the desired level. This system may consist of one or a plurality of heat exchangers, condenser coils and the like and may contain a separator for heavy liquid polymer if desired.

Leaving the cooling section the eluent vapors enter the multistage compressing section 6, through line 31, this comprises usually at least two stages of compression, comprising compressors 1, coolers (not shown), and accumulators 8. The liquid accumulator products pass to the fractionating column 9 through line l38. The 'uncondensed gas, comprising hydrogen, methane, ethane, and ethylene and minor amounts of heavier hydrocarbons and other gases, leaves the compression system and passes to the vapor recovery plant I4, through line 45. IThis plant may be of the oil absorber-stripper type and serves to remove nearly all Ca and C4 hydrocarbons from the ethane and lighter gases which are removed from the system through line 41. The recovered C3 and C4 hydrocarbons are returned to the fractionating column through line 46. Column 9 is a highly eflcient column, operated at such temperature and pressure that the overhead fraction consists of Ca hydrocarbons and the lowest boiling butene, namely, butene-l. Ordinarily/ pressures of about to 250 pounds gage are used.

'Ihe propylene and the butene-l formed by the rst stage dehydrogenation are takenoverhead through line 46, are cooled in condenser I0, and enter reflux accumulator. I I. A portion of the liquid condensate is returned through lines 4| and 42, and pump I2 to the column as reflux. The excess accumulator liquid comprising propylene and butene-l is removed through line 43 to storage tank I5. The gas from the reflux accumulator is passed to the vapor recovery plant I4, through line 44, and the heavier hydrocarbons are recovered from it and returned to the column as above described. The bottoms fraction separated in the fractionator comprises butane-2 and unconverted butane and this fraction is recycled through lines 39 and 33 and pump I3 into the feed to the first dehydrogenation in line 34 along withfresh butane.

Liquid from the tank I5 is fed to the second dehydrogenation stage by pump I6, through lines 48 and 66. The volume of Cs hydrocarbons is maintained in the feed system at the desired level by the addition of a regulated amount of recycle material through line 6I. The feed enters furnace I1, where it is heated in coil I8 and passed to catalyst cases I9 through line 50. The eiuent vapors leaving the cases through line 5I, are cooled in system 26, by any convenient means, as indicated in the first dehydrogenation step. The cooled gas enters the compression system 2I through line 52. The compression system is a multiple stage system composed of compressors 22 and accumulators 23. 'I'he nal accumulator liquid passes to depropanizing column 24 through line 53. The accumulator gases pass to vapor recovery plant through line 54. Overhead vapors from the column, comprising C3 hydrocarbons leave in line 55, are cooled in cooler 25, and flow into accumulator 26. Part of the liquid from 26 is returned to the column as reflux through lines 56 and 51 and pump 21. Uncondensed gases from accumulator 26 may pass to the vapor recovery system through line 5B. The excess reflux liquid is recycled through lines 59 and 6I by means of pump 3|. Any excess is drawn off through line 66. The hydrocarbons re` moved from the bottom of the tower pass through line 62 to a butadiene 'separation step 28. This may consist of any of the well known processes for extracting butadiene from butane-butylene mixtures. Butadiene concentrate is removed through line 63 to storage 23. The'unconverted butenes are removed through line 64 to pump 30, which recycles the stream through line 65.

Alternately, the Ca-C4 condensate may pass from accumulator 23 through lines 53 and 61 to the butadiene separator 23 without being fractionated to separate the C3 hydrocarbons. In this case the recycle stream through line 64 comprises both Ca and C4 hydrocarbons. Propane removal in this operation is accomplished by higher temperature or lower pressure on accumulator 23.

In the rst stage of my process pressures of 15 to 50 pounds per' square inch gage are em'- ployed for the dehydrogenation of butane. Temperatures and pressures are selected within a range suitable for the catalyst used, and temperatures within the range of 1000 to 1200 F. are ordinarily employed. At these conditions `flow rates of the order of 1 to 10 liquid volumes of-butane per hour per volume of catalyst are usually maintained, although at the higher temperatures still higher ilow rates may be used. Particular conditions of flow rates, temperature and pressure are usually chosen to conform to the characteristics of the speciiic catalyst employed.

.The catalysts which are useful in the first stage of my process are those having suitable activity in promoting the dehydrogenation of parain hydrocarbons at the preferred operating temperatures. These may include the natural and/or synthetic metal oxide catalysts either alone or mixed with each other or promoted by oxides of metals of groups IV to VIII of the periodic table.

In the operation of the second dehydrogenation step the propylene-butene-l stock is` charged to a suitable catalyst together with a C3 hydrocarbon diluent to maintain the partial pressure of C4 material within a preferred range of, 0.2 to one atmosphere. Dehydrogenating conditions "suitable to produce a good yield of butadiene are maintained. The effluents from this treatment are compressed and cooled to effect a separation of material lower-boiling than propylene, and the Ca-C4 condensate is depropanized by fractionation. The C3 fraction obtained by this last fractionation is available for recycling as the required Ca hydrocarbon ,diluent. The C4 hydrocarbons are treated for the extraction of butadiene, after which the unconverted C4 material is recycled to the catalyst. After steady-state conditions are obtained, the charge stock `to the catalyst is composed of fresh propylene-butene-l feed, recycle C4 hydrocarbons, and enough of the C3 hydrocarbon recycle to produce the desired partial pressure ratio. This mode of operation requires that a Volurne of C3 hydrocarbons approximately equal to the propylene and propane formed in the two stages of dehydrogenation be removed from the system at some point along the recycle line in orderto maintain a constant volume of the diluent. The C: hydrocarbon fraction recycled is largely saturated and represents the equilibrium propanepropylene ratio obtained during the second catalytic treatment.

An important alternative feature of the second stage of my process is that the effluents of the second catalytic treatment subsequent to the deethanizing step may be cooled to suitable subat mospheric temperatures and processed for the extraction of butadiene. In this case the depropanizing step is eliminated, and the recycle Cs diluent returns to the catalyst along with the unconverted C4 hydrocarbons following the butadiene extraction step. The necessary withdrawals of Ca hydrocarbons described above may be accomplished by allowing the requisite volume of propane to escape with the ethane and lighter material inl the de-ethanzing step.

The propylene which is added with the fresh feed thus represents an excess of propylene over the equilibrium value, and is a rough measure of the extent to which hydrogenation of propylene will occur during the dehydrogenation of butenes. This displacement of the propane-propylene equilibrlum suppresses the dehydrogenation of the propane and thus largely restricts the dehydrogenation reaction to the C4 hydrocarbons. Further, I have found that tothe extent that propylene is present in excess of `equilibrium values an increased dehydrogenation of butenes to butadiene occurs. This effect is undoubtedly due to a reduction of the hydrogen concentration in the treated vapors caused by hydrogenation of propylene. In other words, the propylene acts asa hydrogen acceptor.

Ethane and lighter gases must be satisfactorily removed from the eiuents of the first stage in order to facilitatethe segregation of the propylenebutene-l stock by fractionation. Compression of the vapors to fairly high pressures isone method of accomplishing this de-ethanizing step, but other variations of this method are possible. For

example, refrigeration of the vapors prior to a low-temperature flashing operation at lower pressures to remove ethane and lighter material is possible. Fractionation of the condensate to separate propylene and butene-l may be accomplished in a single column, or if desired the C3 hydrocarbons may `be separated in `one column and butene-l in another column and the fractions combined ahead of the dehydrogeination step.

The overhead fraction will contain minor amounts of dissolved light hydrocarbons, but these may be retained as diluents. The other C4 hydrocarbons likely to be included are isobutane and isobutene, but' in the process described the amounts of these compounds formed are minor.

The second stage of dehydrogenation is operated at partial pressures of butenes in the range of .0.2 to one atmosphere Withtotal pressures usually between zero and 50 pounds per square inch gage. Low total pressures are desirable in order to operate with maximum volume concentrations of C4 material.

Higher temperatures are required in the second stage than in the first stage to obtainsatisiactory yields of butadiene from butenes. Thus, temperatures in the range of 1100 to 1300 F. are ordinarily employed. Flow rates in the second stage are usually 4maintained between 1 and l0 liquid volumes of` charge per hour per volume of catalyst. The particular combination of ow rate, temperature and pressure for a specic operation will depend on the catalyst used and the degree Ordinarily some of the C3 hydrocarbons are lost with the light gases separated during the compression steps. This amount is very small, however, when using a secondary recovery system and ordinarily would be a considerably smaller quantity than that formed by decomposition of C4 hydrocarbons in the two reaction stages. Insofar as a loss occurred it would serve to reduce the quantity of excess propane to be removed from the propane recycle stream following the second stage.

Butadlene separation may be carried out by any conventional method such as chemical extraction by suitable solutions, such as cuprous salt solutions, solvent extraction by sulfur dioxide and/or other solventsor other satisfactory physical extraction processes.

'I'he butene stream which is recycled to the second catalytic treating section comprises the equilibrium mixture of the normal butenes, containing both butene-1 and butene-2. This mixture, however, is also readily dehydrogenated to butadiene under essentiallythe same conditions as butene-1 and is quite suitable for inclusion in the butene-1 charge. When operating with a non-hydrogenatable diluent, a small proportion of butane is also formed. However, with the propylene-rich diluent used in our process the quantity of butenes so hydrogenated is suppressed. Any butane formed is readily included in the recycle stream without trouble.

The following example will further illustrate the ymanner in which my process may be carried out.

Example Normal butane was charged to the apparatus diagrammed in the drawings at a fiow `rate of one liquid volume of butane per hour per volume of catalyst. It entered the catalyst chamber at 1125* F. and 30 pounds per square inch gauge pressure. The catalyst used was calcined bauxite, and the cases were so constructed that the temperature drop through the cases was minimized.

Conversion of the butane was about 20 per cent and the effluents contained about 15 weight per cent butenes; about per cent by weight of ethane, hydrogen and other light gases; and about two per cent by Weight C3 hydrocarbons, almost exclusively propylene.

The effluent vapors were substantially deethanized and passed in liquid form to a fractionating column so operated as to take the butene-1, comprising about one-third of the butenes, and the C3 hydrocarbons overhead. When the system has attained a steady state relative to recycle and fresh feed, conversion of n-butane per pass is about 20 'per cent and approximately two-thirds by weight of the conversion products are butene-1. This high conversion to butene-1 results from the equilibrium concentration of the butylenes, the greater part of the butene-2 being isomerizecl to butene-1 by keeping a relatively high concentration of butene-2 infthe recycle stream. As the run progressed, the-temperature was gradually increased to maintain conversion at the desired level. At the end of 24 hours the .temperature had reached 11'75u F., and the run was discontinued for the purpose of regenerating the catalyst.

The propylene-butene-l product from the first dehydrogenation step containing about per cent of propylene was charged to the second stage, combined with suiiicient added C3 diluent to reduce the concentration of butene-l to volume per cent. The gases entered the catalyst cases at l180 F. and emerged at 1175 F. A pressure of five pounds per square inch gage was maintained. The catalyst used was calcined bauxite impregnated with 5 per cent by weight of barium hydroxide.

Conversion of butene to butadiene was approxima'tely 25 per cent. TheA remaining butene was recycled as was the major portion of the Ca material while the light gases, (ethane, etc.) equivalent to about l5 per cent of the butene were removed. In this process the C3 diluent approached the equilibrium mixture of propaneproylene in which propane predominated.

Approximately five per cent of the butene-1 was I converted to propylene, so that in all, about two` and one half per cent of the original butane charged to the first step was removed as excess C3 diluent from the second stage to maintain constant volume.

During the run the temperature was gradually v increased to 1220 F. to maintain conversion at a constant level. At the end of six hours the run was discontinued for regeneration of the catalyst.

I claim:

l. In the two step dehydrogenation of paraflln hydrocarbons of at least four carbon atoms to straight chain diolefins in which the conversion of a paraffin to the corresponding olefin is the principal reaction of the first dehydrogenation step and the conversion of said olefin tothe corresponding dioleln is the principal reaction of the second dehydrogenation step, the improvement comprising removing hydrogen from the eilluent of the first dehydrogenation step, passing the said corresponding oleiin from the first dehydrogenation step to the second dehydrogenation step in adi mixture with at least an equal volumeI of hydrocarbon diluent comprising an olefin of lower molecular weight as hydrogen acceptor and the paraffin corresponding to thev olefin of lower molecular weight, the olefin of lower molecular weight being present in the diluent in excess of the concentration required for equilibrium with the said corresponding parain in the second dehydrogenation step and acting as hydrogen acceptor to promote the principal reaction of the second dehydrogenation step.

2. In the two step dehydrogenation of butane to butadiene in which the conversion of butane to butenes is the principal reaction of the first dehydrogenation step and conversion of butenes to butadiene is the principal reaction of the second dehydrogenation step, the improvement comprising removing hydrogen from the ellluent of the first dehydrogenation step, and passing the butene from the first dehydrogenation step to the second dehydrogenation step in admixture with at least an equal volumey of hydrocarbon diluent comprising propane and propylene, the propylene being present in excess of the equilibrium value of the propylene-propane reaction under the conditions prevailing in the second dehydrogenation step whereby at least a portion of the propylene is converted to propane acting as hydrogen acceptor to promote the conversion of butylene to butadiene.

3. In the two step dehydrogenation of butane to butadiene in which the conversion of butane to butenes is the principal reaction of the first dehydrogenation step and conversion of butenes to butadiene is the principal reaction of the second dehydrogenation step, the improvement comprising removing hydrogen from the eiliuent of the first dehydrogenation step, and passing the butene from the first dehydrogenation step to the second dehydrogenation step in admixture with at least an equal volume of Ca hydrocarbon diluent comprising propane and propylene, separating Ca hydrocarbons from the effluent of the second dehydrogenation step and recycling a part of said C3 hydrocarbons as said diluent, and introducing with said butenes and said recycled hydrocarbons to the second dehydrogenation step propylene in addition to that contained in, said recycle such that the concentration of the propylene in the hydrocarbon diluent is in excess of the concentration required for equilibrium with propane under the operating conditions of the second dehydrogenation step whereby at least a portion of the propylene acts as a hydrogen acceptor to promote the conversion of butylene to butadiene.

4. The process of manufacturing conjugated straight chain diolefins from the corresponding parafns which comprises catalytically dehydrogenating the paramn to the corresponding olefinA in a rst dehydrogenation step, removing hydrogen from the effluent of the first dehydrogenation step, forming a mixture containing said corre-A sponding oleiin contained in the eiiiuent and at least an equal volume of diluent comprising an olen of lower molecular weight and the paraiin corresponding to the olen of lower molecular weight, and catalytically dehydrogenating said corresponding olen to the corresponding conjugated diolen by subjecting said mixture to contact with a dehydrogenation catalyst in a second dehydrogenation step, said mixture containing said olen of lower molecular weight in amount in substantial excess of the concentration required for equilibrium with the parafn corresponding to the olefin f lower molecular weight in the second dehydrogenation step and in greater concentration than the concentration of said lowerv olefin in the eiiiuent of the first dehydrogenation step whereby at least a portion of said olefin of lower molecular weight acts as a hydrogen acceptor to promote dehydrogenation of said rst mentioned oleiin.

5. Ina process for the manufacture of conjugated straight chain diolefins from the corresponding oleilns, the improvement which comv prises preparing a mixture containing said corresponding olen and at least an equal volume of hydrocarbon diluent comprising an oleiin of lower molecular weight and the paraiin corresponding to the olen of lower molecular weight, and subjecting said mixture to catalytic dehydrogenation in a dehydrogenation step to convert said corresponding olefin to the dioleiiln as the principal reaction of said dehydrogenation, said olefin of lower molecular weight being present in the mixture in amount in substantial excess of the concentration required for equilibrium with the paraiiin corresponding to the olefin of lower molecular weight under the conditions prevailing in the dehydrogenation step whereby at least a Y portion of said olefin of lower molecular weight acts as a hydrogen acceptor to promotethe principal reaction of the dehydrogenation.

WALTER A. SCHULZE.

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