US3088986A - Dehydrogenation method - Google Patents

Description

ed States 3,0883% DEHYDR6GENATIN METHOD Donald H. Stevenson, Milrnont Park, Pa., assignor to Arr Products and Chemicals, Inc., a corporation of Delaware No Drawing. Filed June 17, 1959, Ser. No. 820,849 1 Claim. (Cl. 260680) This invention relates to the dehydrogenation of a mixture consisting essentially of C and C hydrocarbons. The term consisting essentially of as used herein is intended to exclude the presence of reactants significantly affecting the products, but to permit the presence of the minor amounts of impurities ordinarily present in the commercially employed compositions of an analogous nature. For example, a recycle stream from a hydrocarbon dehydrogenation may contain the minor amounts of lower hydrocarbons and/or higher hydrocarbons because the separation procedures are intended to remove such by-products cheaply and are not intended to reduce the impurity levels to those of ultra pure materials.

Heretofore, various proposals have been made for the preparation of butadiene by the cracking of normal pentane and by the catalytic dehydrogenation of normal butane. It has been proposed that a catalyst be employed for dehydrogenating hydrocarbons of four carbon atoms until partially deactivated, and that the partially deactivated catalyst be employed for dehydrogenatin-g hydrocarbons of five carbon atoms. Such blocked out operation has permitted the separation of the desired products from relatively simple mixtures. Similarly, the use of the combination of entirely separate dehydrogenation units and entirely separate recovery systems for the C hydrocarbons and the C hydrocarbons has been favored for engineering reasons. It has long been recognized that the catalytic dehydrogenation of a mixture of C and C hydrocarbons would produce an efiluent containing many components. Previously, it has been known that as the number of components present in the efiiuent from the catalyst zone is increased, the difficulties in recovering good yields of high purity products from the effluent from a catalyst chamber are increased much more than proportionately to the number of components. If one starts with a mixture of C and C hydrocarbons, and if one desires to obtain as one product a high purity unsaturated C hydrocarbon and as another product a high purity unsaturated C hydrocarbon, then the separation costs are much lower if the fractions are separately dehydrogenated after the relatively simple feedstock mixture is separated into a 0., fraction and a C fraction than if a high purity unsaturated C hydrocarbon fraction and a high purity unsaturated C hydrocarbon fraction are isolated from the extremely complex efiluent from a zone for the catalytic dehydrogenation of a mixture of C and C hydrocarbons. Accordingly, those familiar with the economics of such production of high purity unsaturated hydrocarbon having fewer than 6 carbon atoms by dehydrogenation (and with minimized cracking of hydrocarbons of the same number of carbon atoms) had reason to discredit the several suggestions indicating that it might be possible to subject a mixture of C and C hydrocarbons to a dehydrogenation zone. The prior art literature pertinent to the catalytic dehydrogenation of mixtures of C and C hydrocarbons taught those skilled in the art that the concept of dehydrogenating a mixture of C and C hydrocarbons had been described, but that such a procedure was quite unattractive because the potential advantages from such mixed dehydrogenation were insuflicient to compensate for the difficulties of recovering pure products from the complex mixture.

If the amount and composition of a recycle stream for an established continuous conversion of a gas stream are stabilized and known and if the calculations concern merely the fresh feed and the withdrawn products, then the recycle stream can be ignored'in such calculations. In research runs, however, the recycle stream (or the portion simulating recycle) is desirably deemed not only a part of the feedstock but also a part of the crude product stream, and the calculations are desirably based upon such classifications. To the extent that the total iso C feedstock is not recovered as unreacted iso C hydrocarbons, it is deemed converted to a mixture of desired products and less desired by-products. If the feedstock contains more isopentenes than the product, then the thus consumed isopentenes are a part of the iso C hydrocarbons which disappear and/or are converted. The extent of iso C conversion is thus equivalent to the disappearance of total iso C feed. The amount of isoprene per pass represents a particular percentage of the converted iso C feed for that run, and such percentage is designated as the ultimate isoprene selectivity if isopentenes are consumed, and as isoprene selectivity if there is a net production of isopentenes. If there is a net production of isopentenes, then the amount thereof can be added to the amount of isoprene and such sum constitutes the total desired iso C unsaturates per pass. The amount of total desired iso C unsaturates per pass represents a particular percentage of the converted iso C feed, and such percentage is designated as total selectivity. Such requirements for selectivities can be expressed by formulae, such as:

Ultimate isoprene selectivity Isoprene per pass Disappearance of isopentenes and isopentane Total selectivity Isoprene plus isopentenes per pass Disappearance of total iso C feed In accordance with the present invention, isoprene is prepared from a gas mixture containing isopentane as the predominant C hydrocarbon but the mixture may contain recycle C hydrocarbons comprising isopentenes. Said gas mixture is also designated as an isopentane stream containing not more than 49% isopentenes. The isoprene is prepared with advantageous selectivity, and simultaneously butadiene is prepared from a gas mixture containing normal butane as the most abundant C hydrocarbon but the mixture may contain normal butenes. Said gas mixture is also designated as a normal butane stream containing not more than 49% normal butenes. For example, the recycle of C hydrocarbons may contain butenes. Such dehydrogenation of the C hydrocarbons produces advantageously small amounts of by-products. A hydrocarbon gaseous mixture (preferably including recycled C C hydrocarbons) consisting essentially of from 10% to 60% by weight (8.2% to 54.7% by volume) iso C hydrocarbons (predominantly isopentane) and from 40% to by weight (45.3% to 91.8% by volume) normal C hydrocarbons (normal butane being the most abundant C hydrocarbon) is directed through a catalytic dehydrogenation zone ata space rate of about 1 to 6 liquid volumes per catalyst volume per hour at a temperature within the range from 1050 F. and 1200 F. at an absolute pressure less than 15 inches of mercury at a severity converting at least 10% of the iso C hydrocarbons in a dehydrogenation-regeneration cycle maintaining adiabatic dehydrogenation conditions. The gaseous efliuent from the catalytic zone contains butenes (1- butene; Z-cisbutene; and Z-transbutene), butadiene, isopentenes (Z-methyl l-butene; 3-methyl l-butene; and 2- methyl Z-butene), and isoprene (Z-methyl 1,3-butadiene). The selectivity for the formation of isoprene in the catalytic zone is superior to that attainable at equivalent conditions in the absence of the C hydrocarbons. The selectivity for the formation of butenes and butadiene is substantially as good as that attainable at equivalent conditions in the absence of the C hydrocarbons. A stream of unsaturated C hydrocarbons and a stream of isoprene are separated as products of the process.

The technical subject matter pertinent to the present invention is further clarified by reference to a plurality of examples.

EXAMPLE I An adiabatic dehydrogenation apparatus for laboratory preparation of butadiene by the Houdry method can be described as comprising a flowmeter measuring the amount of hydrocarbon fed to a heater, a catalyst chamber containing heat retention material and Houdry catalyst (approximately 20% chromia on a sorptive alumina carrier), a vacuum system maintaining an absolute pressure of about 177 mm. of mercury in the catalyst chamber, a quenching system to cool quickly the etlluent from the catalyst zone, and a recovery system for isolating all of the products of the reaction. During the periodic regeneration, the combustion of the carbonaceous deposit on the catalyst provides heat to the catalyst bed, but the dehydrogenation portion of the cycle cools the catalyst bed. In maintaining heat balance between the dehydrogenation and regeneration portions of the cycle, the temperatures of the inlet, middle and outlet portions of the bed are recorded, and average bed temperatures are determined and utilized in evaluating runs. The dehydrogenation results may be influenced by factors such as the extent of conversion, pressure, space rate, temperature, and the surface area of the catalyst. The iso C products are best described as a percentage by weight of the corresponding iso C feed, without regard to whether the iso C hydrocarbons are pure or mixed with other gases. The products from the dehydrogenation of C hydrocarbons are also best described as a percentage of the C hydrocarbon feed. Although the same proportion of butane is converted to butadiene in the presence or absence of iso C hydrocarbons, the actual amount of butane subjected to dehydrogenation is less when iso C hydrocarbons are admixed, thus producing a total quantity of butadiene smaller than in the absence of iso C hydrocarbons.

A mixture consisting of 25% isopentane and 75% normal butane was passed through the adiabatic dehydrogenation unit at 177 mm. Hg absolute pressure to simulate the conditions which might be employed in oncethrough operation for manufacture of butene as the principal product plus butadiene, isoprene and isopentenes as supplemental products. The results were compared with a run for a similar dehydrogenation of 100% normal butane. The results are indicated in Table I.

Wt. Percent Conv. of n0 H Net Butene, wt. Percent of nC4H1n. Butadiene, Wt. Percent of nC4Hl0- Total Selectivity, Percent Bed Inlet, F Bed Avg, F Bed Outlet, F Percent Oonv. of i051 Net Isopentene, wt. Percent of iCfiHla Isoprene, wt. Percent of iC Hn Total i0 Select., Percent--- Attention is called to the fact that in the improved method the extent of iso C hydrocarbon conversion was 36% and to the fact that the total iso C selectivity was 75 This advantageously high total selectivity for dehydrogenation of isopentane to form isoprene and and isopentenes makes the mixed C -C hydrocarbon dehydrogenation process attractive. Moreover, 16% of the isopentane was dehydrogenated to isopentenes and 11% of the isopentane charge was converted to isoprene by the improved method. The conversion of isopentane to isoprene is achieved more satisfactorily in a zone in which a significant amount of normal butane is also being dehydrogenated than in the absence of such normal butane. Surprisingly, however, the presence of the isopentane undergoing dehydrogenation in the same zone does not significantly impair the effectiveness of the process for converting normal butane to butene and/or butadiene. The percentage of butene obtained, the proportion of butadiene obtained and the extent of conversion achieved are all substantially the same whether the isopentane dehydrogenation is or is not conducted simultaneously in the same dehydrogenation zone.

EXAMPLE II In many commercial dehydrogenation units, a portion of the product gas is recycled through the dehydrogenation unit. A synthetic mixture comprising normal butane and recycle butenes, simulating the type mixture sent to an industrial dehydrogenation unit, is passed through the previously described adiabatic apparatus for the laboratory study of dehydrogenation reactions. In the control ru the feedstock consists of normal butane as fresh feed and recycle C hydrocarbons. A similar mixture, combined with recycle 0;, hydrocarbons, plus fresh isopentane is passed through the catalytic zone in the improved method, thereby obtaining the data shown in Table II.

Table II Data Control Improved Method F tool! 100% 0 Es... 25% 0 's,

- 75% O 4's. Catalyst Surf. Area 27 m /g. Liquid Hourly Space Velocity 1 3 1.6 Butadiene, wt. percent of 04's 12. Recycle Butenes, Wt. percent of O is 28. Butadiene Select. percent 55. Bed Inlet, F 1,095. Bed Avgz, F 1,065. Bed Outlet, F 1,140. Wt. percent Conv. of i0 rs 18. 9. Net Isopentene, Wt. percent of i0 5S -3. 2. Isoprene, wt. percent of i0 sS 15. 8. Ultimate Isoprene Select. percent 71. 5.

The conditions employed permitted the same percentage yield of butadiene to be produced in both methods. As the conditions employed in the run for the improved method, 15.8% of the iC hydrocarbons charged to the catalytic zone were recovered as isoprene. By reason of the 71.5% selectivity for isoprene at 18.9% iso C conversion, the method is superior to some competitive procedures available for converting isopentane to isoprene.

EXAMPLE III A laboratory unit for the dehydrogenation of hydrocarbons having fewer than 6 carbon atoms per molecule is adapted to permit the recycling of the portion of the product which is not withdrawn by the separation procedures. The apparatus is operated continuously with variations in the proportions of the feedstock introduced to the system. Operating data permit the calculation of estimated costs of production of isoprene in a commercial plant. The necessity for recovering C products involves both higher equipment costs and higher operating costs than involved in a process from which only C, hydrocarbons are recovered, and all such marginal costs must be carried by the isolated C products. When a mixture consisting of by weight C hydrocarbons and 5% by weight C hydrocarbons is subjected to the standard dehydrogenation conditions, the quantities of isoprene produced are sufficiently small that the cost of recovery of the isoprene is excessive. In treating a mixture of 65% by weight C hydrocarbons and 35% by weight C hydrocarbons, the selectivity for isoprene production from iso C hydrocarbons so nearly approaches the selectivity achieved in the absence of C hydrocarbons that the savings attributable to the marginally superior selectivity are insuflicient to compensate for the equipment costs and operating costs for mixed dehydrogenation. It is established that the dehydrogenation of the mixture of normal C and iso C hydrocarbons by the method of the present invention must be conducted using a mixture within the range from to 60% by weight iso C hydrocarbons and from 90% to 40% by weight normal C hydrocarbons.

EXAMPLE IV A gas mixture was prepared consisting of:

Percent by weight Ethane 0.1 Propane 0.1 Isobutane 1.9 N-butane 59.3 Isopentane 31.8 Isopentenes 5.9 N-pentane 0.8 N-pentenes 0.1 Total C 38.6

The gas mixture was passed through a catalytic dehydrogenation zone at conditions of 1.2 LHSV, 177 mm. Hg absolute pressure, over a bed containing 40% alumina granules and 60% chromia-alumina catalyst having a surface area of 25 m.*/ g. to convert 36% of the butane at a total selectivity of 65%. Simultaneously, 24% of the iso C hydrocarbons were converted at 90 total selectivity. The isoprene yield per pass was The temperature was raised to convert 48% of the butane at a total selectivity of 62%, and at the same time, 29% of the iso C hydrocarbons were converted at a total selectivity of 76%. The isoprene yield per pass was 18%.

Not only is the dehydrogenation of the iso C hydrocarbons more selective in the presence of normal C hydrocarbons undergoing dehydrogenation but also it is feasible to convert a higher percentage of the iso C hydrocarbons without encountering difiiculties in maintaining a heat-balanced operation. Higher conversions per pass permit lower recycle ratios, thereby permitting smaller units (involving less capital investment) for a given capacity to produce isoprene. The combination of higher conversion and higher selectivity for isoprene production while still utilizing the Houdry method of frequently regenerating the catalyst makes attractive the conversion of butadiene plants to plants producing both butadiene and isoprene.

EXAMPLE V A mixture of 40% by weight butane and 60% isopentane is passed through a bed containing 40% heatretentive inert alumina granules and 60% chrornia-alumina dehydrogenation catalyst at a liquid hydrocarbon space velocity of 1.3 volumes per volume of catalyst per hour at 1075 F. at 127 mm. Hg absolute pressure to obtain 45% conversion of the isopentane at a total selectivity of 73%. The isoprene yield per pass is 12% and the piperylene yield is only 2.5%, providing an advantageous ratio of almost 5 to 1.

EXAMPLE VI Several mixtures of normal C and iso C hydrocarbons were prepared and stored in tanks, the mixtures simulating various combinations of once-through and recycle processing. Each mixture consisted of about 75% normal C hydrocarbons and about 25% iso C hydrocarbons.

Sample A represented a mixture containing both recycle normal C hydrocarbons and recycle iso C hydrocarbons. If only the normal C hydrocarbons were to be recycled, the composition might be like B and if only the iso C hydrocarbons were recycled, the composition might be like C. Sample D represented once-through feedstock. Each of the mixtures (A, B and C) simulating a mixture containing a recycle stream contained more saturated than olcfinic hydrocarbons. Commercial operation has indicated that in dehydrogenation units having a recycle stream, normal butane is the most abundant nC hydrocarbon component in the total feed if normal butane is the most abundant nC hydrocarbon in the fresh feed, and such relationships would be expected to prevail in the dehydrogenation of mixtures of C and C hydrocarbons.

A dehydrogenation unit containing 40% inerts, 60% catalyst (chromia-alurnina aged to a surface area of 27 m. g.) particles was employed adiabatically on a oncethrough basis. The dehydrogenation was conducted at 177 mm. Hg absolute pressure on a 9 minute dehydrogenation, 9 minute regeneration, 3 minute purge cycle. Data relating to several runs are shown in Table III. The abbreviation NC is employed to indicate that the information was not available for some reason such as for example, the value was not calculated or the value was not measured.

Table III Feedstock A B C D L' uid Hourly Space Velocity 1.6 1. 7 1. 2 1.2 Inlet. F 1,072 1,124 1,108 1,099 Average, F. 1,052 1,080 1,058 1,055 Outlet, F a 1,100 1,154 1,088 1, 080 04113, wt. Percent of C4Feed. 6. 4 4. 0 19. 5 20. 9 CAHG, wt. Percent of OilFeed... 10. 5 12. 6 6.8 6.6 Percent C4 Conversion 13. 2 28.2 35.9 38. 6 Total C; Selectivity, Percen NO N O 74 71 Ultimate C4Hs Selectivity Per 54 52 NO N 0 10 E10, wt. Percent 0110 Feed -0. 5 13. 9 0.5 16. 4 Isoprene, wt. Percent of 0 Feed 14. 9 10. 9 17.6 10. 7 Percent 05 Conversion 15. 3 40. 9 22.6 36.0 Total 0 Selectivity, Percent N G 01 75 Ultimate Isoprene Selectivity, Percent 94 NC 79 NC A comparison of such data with comparable runs which were not C -C mixtures established that the presence of the C hydrocarbons had no significant effect upon the dehydrogenation of the C hydrocarbons but that the presence of the C hydrocarbons had an important influence upon the C dehydrogenation. The isoprene to piperylene unit ratios when the dehydrogenation is conducted in the presence of normal C hydrocarbons are as high as 11, in contrast with unit ratios such as 2 or 3 in the absence of such normal C hydrocarbons. The presence of normal C hydrocarbons permits operation of the isopentane dehydrogenation step at higher temperatures While still restricting coke production to commercially satisfactorily low levels.

By a series of tests, it is established that the limits for the method are the inclusion of at least 10% but not more than 60% by weight iso C hydrocarbons in the mixture of normal C and iso C hydrocarbons, the maintenance of a pressure less than 15 inches of mercury, the use of a dehydrogenation-regeneration cycle maintaining heat balance in the approximately adiabatic dehydrogenation, the maintenance of dehydrogenation severity sufficient to convert at least 10% of the C hydrocarbons, the maintenance of an average bed temperature of at least 1050 F. and less than 1200 F. and a space velocity of at least 1 but less than 6 volumes of liquid hydrocarbon per volume of catalyst per bed hour. Because the dehydrogenation of C -C hydrocarbon mixtures in accordance with the present invention permits adiabatic operation at higher conversion levels and with better isoprene selectivity than is possible in the dehydrogenation of isopentane, there is more than ample economic justification for the relatively expensive separation system necessary for recovering isoprene and butadiene from the effluent from the catalytic dehydrogenation zone.

-'It should be noted that the method of the present invention involves the treatment of a hydrocarbon gas stream by passage through an adiabatic bed of granules of dehydrogenation catalyst at a space rate of about 1 to 6 liquid volumes per catalyst volume per hour at a temperature within the range between 1050 F. and 1200 F. at an absolute pressure lower than about 15 inches of mercury to produce an efilnent comprising hydrogen, C -C hydrocarbons, a product stream of unsaturated C hydrocarbons, a product stream of isoprene, a C hydrocarbon recycle stream, and a C hydrocarbon recycle stream. A particularly important feature of the present invention is the attainment of better selectivity in the conversion of C hydrocarbons to isoprene than would be obtained at equivalent conditions in the absence of the C hydrocanbons. This advantageous result of improved selectivity is achieved by controlling the composition of the gas stream directed to the catalyst bed so that the weight concentration of the C hydrocarbons is within the range between 40 and 90% and the weight concentration of iso C hydrocarbons is within the range between and 60%, normal butane being the most abundant component of the C hydrocarbons, and isopentane being the most abundant component of the C hydrocarbons in said gas stream.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claim.

The invention claimed is:

The method of preparing isoprene and butadiene simultaneously which includes the steps of:

directing C hydrocarbons from a source of supply as a stream of fresh C hydrocarbons toward a mixing zone;

directing C hydrocarbons from a source of supply as a stream of fresh C hydrocarbons toward a mixing zone;

directiong C hydrocarbons from a hereinafter designated separation zone as a stream of recycled C hydrocarbons toward a mixing zone;

directing C hydrocarbons for a hereinafter designated separation zone as a stream of recycled C hydrocarbons toward a mixing zone; preparing a hydrocarbon gas stream in the mixing zone consisting essentially of a mixture of said:

(1) recycled C hydrocarbons, (2) recycled C hydrocarbons, (3) fresh C hydrocarbons, and (4) fresh C hydrocarbons, said gas stream consisting of from 40% to 90% by weight normal 0.; hydrocarbons and from 10% to iso C hydrocarbons, normal butane being the most abundant component of the C hydrocarbons and isopentane being the most abundant component of the C hydrocarbons in said gas stream; directing said hydrocarbon gas stream through chromia on alumina catalyst at a space rate of about 1 to 6 liquid hydrocarbon volumes per volume of catalyst per hour at an absolute pressure less than 15 inches of mercury at a temperature Within the range between 1050 F. and 1200 F.; directing the efiiuent from the catalyst bed to a separation zone in which the efliuent is separated into a plurality of streams, including a stream of recycled C hydrocarbons, a stream of recycled C hydrocarbons, and product streams; separating in said separation zone as a product of the method a stream of unsaturated C hydrocarbons, the quantity of butadiene constituting a conversion of the normal butane to butadiene substantially as good as that attainable at equivalent conditions in the absence of the C hydrocarbons; and separating as a product of the method a stream of isoprene, the quantity of isoprene constituting a conversion of the isopentane to isoprene better than attainable at equivalent conditions in the absence of the C hydrocarbons.

References Cited in the file of this patent UNITED STATES PATENTS 2,401,973 Seyfried et a1. June 11, 1946 2,440,492 Seyfried et a1. Apr. 27, 1948 2,458,082 Kilpatrick Jan. 4, 1949 2,900,429 Heinemann et a1 Aug. 18, 1959