US2381693A - Process for the dehydrogenation of hydrocarbons - Google Patents

Description

Patented Aug. 7, 1945 PROCESS FOR THE DEHYDROGENATION OF HYDBOCARBONS Walter A. Schulze and John C. Hillyer, Bartlesvilie, kla., assignors to Phillips Petroleum Company, a corporation of Delaware Application September 7, 1940, Serial No. 355,710

8 Claims.

This invention relates to the production of the valuable-dloleflnic hydrocarbons from the corresponding paraflinic and/or oleilnic hydrocarbons. It relates more particularly to the process of dehydrogenating normal pentane in successive stages to produce successively pentenes and pentadiene.

In a, more specific sense this invention is concerned with a new and improved process in which the mono-olefin pentene-l, Obtained as one of the products from the catalytic dehydrogenation of normal pentane or of the C fraction of thermally cracked hydrocarbon gases is separated by fractional distillation and is subsequently dehydrogenated in a second catalytic step to produce a good yield of pentadiene.

This is a continuation-impart of our co-pendlng application Serial No. 352,786 filed August 15, 1940, in which the preparation of butadiene is specifically disclosed.

It has already been proposed to produce diolefins by the catalytic treatment of the total oletln mixture resulting from the dehydrogenation of normal pentane. However, it has been the practice of those attempting to carry out such a process to first dehydrogenate pentane to produce mixed pentenes and to recycle the treated vapors until the desired concentration of pentenes was reached or the desired percentage of the pentane charge had been converted. Because of the limitations imposed by thermodynamic equilibrium, it has not been possible to secure satisfactorily pure mixed pentenes by this proccdure without resorting to such severe conditions and excessive recycling that considerable quantities of the pentane and pentenes were decomposed, and the resulting yields of pentenes were commercially not feasible.

Also attempts have been made to produce mixed pentenes by dehydrogenating normal pentane and separating the resulting pentenes after each passage over the catalyst for further dehydrogenation. Such attempts have involved various complex solvent extraction and chemical separation methods which have proved expensive and generally unsatisfactory.

We have now discovered a novel process for the production of Pentadiene which eliminates the unsatisfactory practices of the art to date. Our invention not only provides for the economical production of pentenes from normal pentane by catalytic dehydrogenation but also includes the segregation from said pentenes of pentene-l as the most suitable charge to a second dehydrogenation treatment to produce pentadiene. By the practice of our invention we have further discovered thatv the dehydrogenation of normal pentane over suitable catalysts may be operated to yield larger amounts of pentene-l when pentene-2 is recycled to the catalyst along with unconverted pentane. Thus, it is an object of this invention to convert normal pentane to pentadiene in two stages of dehydrogenation using a stream of substantially pure pentene-l separated from the products of the first dehydrogenation step as a, charge to the second dehydrogenation step.

In a mixture of C5 hydrocarbons such as is produced by dehydrogenating normal pentane and comprising pentene-l, pentene-2 and normal pentane, the boiling points at 760 mm. are 862 F. for pentene-l, 97.5 F. for pentene-2, and 97.1 F. for n-pentane. Up to the present, most attention hasbeen centered on separating unconverted n-pentane from the pentene mixture, and even the very efllcient commercial fractionating equipment available is not satisfactory for separating n-pentane from pentene-2 in such a mixture. For this reason the above-mentioned solvent extraction and chemical separation processes for the segregation of pentenes have been attempted in spite of obvious disadvantages. However, we have discovered that a highly satisfactory fractional distillation may be conducted to separate pentene-l from the C5 hydrocarbon mixture produced by the dehydrogenation of n-pentane. By our process the C5 hydrocarbons remaining after the separation of pentene-l and comprising pentene-2 and unconverted n-pentane are returned to the. first dehydrogenation step for further conversion to pentene-l.

We have further discovered that the efliciency of our process based on the segregation of pentene-l is not impaired by the continuous recycling of pentene-2, but that the concentration of pentene-2 in the effluents from the first dehydrogenation step does not at any time exceed the equilibrium value at any given temperature of dehydrogenation. This has been found to be due to the isomerizaticn of pentene-2 during the catalytic treatment to produce substantially the ratio of the isomeric pentenes produced by dehydrogenation of pure n-pentane regardless of the amount of pentene-2 in the charge. This isomerization accompanying dehydrogenation enhances the value of our process by promoting the ultimate conversion of substantially all the C5 hydrocarbons in our raw feed stock, and by assuring a feed of substantially constant pentene-l content for our fractional distillation step.

In its broader aspects, our invention comprises conducting the conversion of n-pentane to pentadiene by a series of steps listed below in the order employed: (1) catalytic dehydrogenation of n-pentane; (2) separation of the Cs hydrocarbon mixture so produced by means of fractional distillation into an overhead fraction comprising substantially pure pentene-l and a bottoms fraction comprising pentene-2 and n-pentane; (3) continuously recycling the bottoms fraction to the initial dehydrogenating unit, together with fresh n-pentane, and producing thereby additional pentene-1, both by dehydrogenation of n-pentane and by isomerization of pentene-2; (4) subjecting the overhead traction comprising pentene-l to a second catalytic treatment under suitable conditions to eifect a considerable dehydrogenation to pentadiene; (5) separating the pentadiene so produced by any convenient means; and (6) recycling the unconverted pentenes remaining after separation of the pentadiene to either or the dehydrogenation steps as may be convenient.

If we use the Cs fraction from refinery cracked gases instead of n-pentane in our process, the steps of our invention are not materially altered. In such a fraction comprising n-pentane and pentenes, the pentene concentration usually is not great enough to justify preliminary fractionation for the segregation of pentene-l, and the entire stock is then dehydrogenated by the first step of our process to produce additional pentenes prior to the separation of the pentene-1 fraction. Obviously, it such a Cs fraction is rich enough in pentenes to approximate or exceed the pentene content resulting from the initial dehydrogenation step we may first separate pentene-l by fractional distillation, and then return the pentene-2 and n-pentane as charge to the initial dehydrogenation operation.

In order that the invention may be more clearly understood, reference will be made to the drawing, which is a flow diagram according to which the steps of the invention may be carried out.

In the drawing, the raw n-pentane or suitable Cs hydrocarbon feed comprising n-pentane and pentenes enters by line 2| into heater I where the feed stream is raised to the desired temperature. The hot vapors then pass by line 22 into catalyst cases 2. These cases contain a catalyst capable of eflecting the desired degree of dehydrogenation of n-pentane to yield pentenes. From 2, the treated vapors pass with some cooling (not shown) through line 23 into polymer separator I where small amounts of heavy material are removed by line 24. From I the vapors pass through line 26 with required compression and/or cooling (not shown) into Iractionating column 4. In 4 a fractionation is effected to remove hydrogen and C4 and lighter hydrocarbons overhead while the C5 hydrocarbons constitute the bottoms fraction. The overhead fraction leaving by line 26 may be sent to further processing units through valve 21, or a portion may be returned by line 28 into the raw pentane stream ahead of the heater, providing the quantity of hydrogen gas thus returned is not allowed to pyramid in a fashion unfavorable to the dehydrogenation reaction. The Cs fraction leaves column 4 by line 29 and is passed to iractionating column 5 wherein a fractional distillation is carried out to take pentene-1 overhead, while pentene-2 and n-pentane are removed from the kettle by line 3| and recycled to the raw feed stream ahead of the heater. The pentene-l traction passes through line GI and is collected in storage 8. The auxiliary equipment for columns 4 and 5, including heat exchangers, condensers, reflux accumulators and the like is familiar to the art, and thus is not shown in this flow diagram.

From storage I, the pentene-l concentrate passes by line 32 into a heater 1, where the stream is heated to the temperature required for the second dehydrogenation. The heated vapors pass I by line It to catalyst cases I containing a suitable dehydrogenation catalyst. The treated vapors exit through line 24 with some cooling, and into polymer separator I, wherein small amounts of heavy material are removed through line 35. From 8, the stream passes through line 20 into fractionating column ll after suitable compression and cooling (not shown). In column ll,-

hydrogen and hydrocarbons including butane and lighter are removed overhead through line 21 while Cs hydrocarbons constitute the kettle product. The overhead product Irom I! may be passed to further processing units through valve II, or optionally a portion or a component thereof may be sent through line II to the feed stream ahead of heater I to serve as a diluent. In the latter operation, the quantity of hydrogen gas recycled is regulated so as not to influence the reaction uniavorably. The Cs fraction from column ll passes through line 40 to the pentadiene extractor where pentadiene is removed by suitable reagents. The unconverted mono-olefin leaves the extractor through line 4| and is recycled to the second dehydrogenation step into line 32 ahead of the heater I. The pentadiene in combination with the extracting medium is taken through line 42 to a suitable desorbing or recovery unit (not shown).

In the operation of the first dehydrogenation step, the hydrocarbon vapors may be subjected to two or more successive treatments with dehydrogenation catalyst in a series of catalyst chambers, or the vapors or any fraction thereof may be recycled with the fresh teed vapors through the catalyst chamber. This may be accomplished, if desired, by splitting the stream of hot treated vapors leaving the catalyst tower with one part passing through a compresosr or its equivalent wherein the pressure is raised enough to force the recycled vapors into the stream of heated vapors prior to passage into the catalyst tower. Some additional heat, also, may be supplied to the recycled vapors, if desired.

Other possible arrangements of the conventional equipment used in the practice of our invention will be apparent to those skilled in the art, and thus are held within the scope of our invention. Also, the conditions of temperature, pressure, flow rate and the like used in operating this equipment will depend largely on the selection of the catalyst to be used and on the desired degree 01 conversion, since each catalyst has a specific range of conditions within which it operates with maximum efliciency.

Many catalysts have been found for the dehydrogenation of hydrocarbons, and some of these may be used more or less successfully. Among the types suggested are metals, metallic oxides, particularly diflicultly reducible oxides, but including oxides of metals in groups 11 to VIII inclusive of the periodic table, actiyated or lustrous carbon, clays, some silicates, and many others. The great variety of oxide catalysts makes them of the most importance.

In the practice of our invention the charging a,ss1,ses q 3 stock to the initial dehydrogenation operation is normally heated to temperatures in the range 850 to 1150 F. and passed over the catalyst at such velocities that contact time is quite short,

of the order of 0.5 to 10 seconds. Pressures only slightly above atmospheric, from about to 50 pounds are normally used, although higher pressures, up to 200 or 300 pounds gage may be used, if desired. Conditions of operation are selected with reference to economic and technical factors in any given installation.

In the second dehydrogenation step of our process, the charge stock is heated suiliciently to maintain temperatures between about 1050 and 1250 F. in the catalyst cases. The catalysts used may be those which give a suitable degree of conversion of pentene-l to pentadiene and do not induce excessive polymerization or cracking reactions. Further, it is usually desirable to maintain low partial pressure of pentene-l in the charge to the second dehydrogenation step, for example, by addition of an inert diluent in order to suppress deleterious side reactions involving pentene-l.

Ordinarily two or more catalyst cases would be provided. Those cases not on stream will generally be under preparation for subsequent use, either by replacement of spent catalyst or by regeneration. Regeneration is contemplated whenever the activity has declined to any predetermined level, The regeneration may be car ried out by means such as controlled treatment with an oxygen-containing gas.

The following example will serve to further illustrate one method of practicing our invention.

Example Normal pentane was charged to the system diagrammed in the drawing, operated at a pressure of 30 pounds gage. The heated vapors passed through the catalyst cases which were maintained at a temperature of 1050 F. at the inlet and 1020 F. at the outlet. The catalyst used was bauxite-chromium oxide. A flow rate of 1.5 liquid volumes of pentane "per hour per volume of catalyst was used and the total unsaturated Ct hydrocarbons in the dehydrog'enated vapors after the recycle volume had been built up averaged about 30 per cent or the charge and pentene-l averaged about per cent.

The eiiluent vapors were cooled and condensed and fixed gases were separated. The condensate was fractionated in two columns to remove butane and lighter overhead in the first while pentene-l I was taken overhead in the second. The bottoms fraction 01' pentane and pentene-2 was recycled to the first catalytic step while the pentane-1 rich fraction was charged to the second dehydrogenation step.

The pentene-l was diluted with 3 parts by volume of a substantially inert Ca-Ca hydrocarbon mixture and heated to 1080 F, for passage over the second catalyst. This catalyst was calcined bauxite and a flow rate of 1 liquid volume of feed per hour per volume of catalyst was maintained with an inlet temperature of 1080" F. and an outlet temperature of 1050 F. The pressure was 5 pounds gage, Conversion of the pentane-1 amounted to about 50 per cent, and 40 per cent of the pentene-l charge was recovered as 1,3-pentadiene.

The etlluents from the second step were cooled and condensed and fractionated after the separation of light gases to separate a C2-C3 fraction for recycle as diluent, and a. C4-C5 fraction from which diolefins were extracted. The diolefin extraction was accomplished by a cuprous salt reagent, after which the diolefins were recovered and the non-diolefinic material was recycled to the second catalyst. The C4 material in this recycle was included as part of the diluent gas when steady state conditions had been obtained by the recycling operations.

The diolefinic material recovered included some butadiene, and this was easily separated from the pentadiene by fractionation. v

The foregoing specification and example have disclosed and illustrated the invention, but since it is of generally wide application and the number of examples off results obtainable by its use might be multiplied greatly, the scope of the invention is limited only by the following claims.

We claim:

1. A process for preparing pentadiene from n-pentane which comprises contacting said n-pentane with a dehydrogenation catalyst in a first dehydrogenation step to convert at least a portion of the pentane to olefins comprising pentene-l and pentene-2, separating pentene-2 and unconverted pentane from the eiil'uent of the first dehydrogenation step, recycling pentene-2 and pentane to the first dehydrogenation step for isomerization of the pentane-2 to pentene-l and conversion of pentane to olefins, separatin pentene-1 from the eiliuent of the first dehydrogenation step, and contacting said pen-, tene-I with a dehydrogenationcatalyst in "a second dehydrogenation step for, conversion of pentane-1 to pentadiene.

2. A process for preparing pentadiene from npentane which comprises contacting said npentane with a dehydrogenation catalyst ,in a first dehydrogenation step to convert at least a portion of the pentane to olefins comprising pentene-l and pentene-2, separating pentene-2 and unconverted pentane from the eflluent of the first dehydrogenation step, recycling pentene-2 and pentane to the first dehydrogenation step for isomerization of the pentane-2 to pentene-l and conversion of pentane to olefins, separating pentene-l from the eiiluent of the first dehydrogenationstep, contacting said pentene-l with a dehydrogenation catalyst in a second dehydrogenation step for conversion of pentene-l to pentadiene, separating the pentadiene from the efliuent of the second dehydrogenation step, and recycling unconverted pentenes to the-second dehydrogenation step.

3. A process for preparing pentadiene from Ii-pentan which comprises contacting said n,- pentane with a dehydrogenation catalyst in a first dehydrogenation step to convert at least a portion of the pentan'e to olefins comprising pentene-l and pentene-2, separating pentene-g and unconverted pentane from the efliuent of the first dehydrogenation step, recycling penteneand pentane to thefirst dehydrogenation step for isomerization of the pentene-2 to pentane-l4 and conversion of pentane to olefins, separating pentene-l from the eiiiuent of the first dehydrogenation step, admixing a. relatively inert,

diluent with the pentene-l to obtain low partial pressur of pentene-l in the resulting mixture, and passing the mixture into contact with a dehydrogenation catalyst in a second dehydrogenation step for conversion of pentane-l to penta diene. g

4. A process for preparing pentadiene from n-pentane which comprises contacting said npentane with a dehydrogenation catalyst in a first dehydrogenation step to convert at least a portion of the pentane to olefins comprising pentene-l and pentene-2, separating pentene-2 and unconverted pentane from the efliuent of the first dehydrogenation step, recycling pentene-2 and Dentane to the first dehydrogenation step for isomerization of the pentene-2 to pentene-l and conversion of pentane to olefins, separating pentene-l from the efliuent of the first dehydrogenation step, admixing with the pentene-l at least an equal volume of relatively inert diluent comprising hydrocarbons of two to four carbon atoms to obtain low partial pressure of pentene-l in the resulting mixture, passing the mixture into contact with a dehydrogenation catalyst in a second dehydrogenation step to convert at least a portion of the pentene-l to pentadiene, separating the pentadiene from the effluent of the second dehydrogenation step, recycling unconverted pentenes to the second dehydrogenation step in admixture with the pentene-l from the first dehydrogenation step, and recycling hydrocarbons of two to four carbon atoms to the second dehydrogenation step as said diluent.

5. A process for preparing pentadiene from n-pentane which comprises contacting a hydrocarbon mixture containing n-pentane with a dehydrogenation catalyst at temperatures within the range of about 850 F. to about 1150 F. in a first dehydrogenation step to convert at least a portion of the n-pentane to oleflns comprising pentene-l and pentene-2, separating pentene-Z and unconverted pentan from the eifiuent of the first dehydrogenation step, recycling pentene-2 and pentane to the first dehydrogenation step in admixture with fresh n-pentane for isomerization of the pentene-2 to pentene-l and conversion of the pentane to olefins, separating pentene-l from the efiluent of the first dehydrogenation step, contacting the pentene-l with a dehydrogenation catalyst at temperatures within the range of about 1050 F. to about 1250 F. in a second dehydrogentation step to convert pentene-l to pentadiene, separating the pentadiene from the eifiuent of'the second dehydrogenation step, and recycling unconverted pentenes to the second dehydrogenation step in admixture with pentene-l from the first dehydrogenation step.

6. A process as defined in claim 1 wherein the catalyst in the first dehydrogenation step is bauxite-chromium oxide, and the catalyst in the second dehydrogenation step is bauxite.

7. A process as defined in claim 3 wherein the catalyst in the first dehydrogenation step consists of bauxite and chromium oxide, and the catalyst in the second dehydrogenation step consists of bauxite.

8. A process as defined in claim 5 wherein the catalyst in the first dehydrogenation step consists of bauxite and chromium oxide, and the catalyst in the second dehydrogenation step consists of bauxite.

WALTER A. SCHULZE. JOHN C. HIILYER.

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