Abstract:
An integrated method that comprises a hydrocarbon thermal cracking operation to form at least one olefin product, coupled with dimerization and metathesis operations, the dimerization operation forming additional feed material for the metathesis operation, and the metathesis operation forming additional amounts of olefin product.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to the thermal cracking and disproportionation (metathesis) of hydrocarbons in an integrated method. More particularly, it relates to the thermal cracking of ethane to form, in part, ethylene, and the disproportionation of 2-butene in the presence of ethylene to form propylene.  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In an olefin production plant, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to severe hydrocarbon thermal cracking at elevated temperatures (1,450 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace (steam cracker or cracker).  
         [0003]     The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C 1  to C 35 , inclusive). This product contains aliphatics, alicyclics, aromatics, saturates, and unsaturates, and molecular hydrogen (hydrogen).  
         [0004]     This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate and individual product streams such as hydrogen, ethylene, propylene, fuel oil, and pyrolysis gasoline. After the separation of these individual streams, the remaining cracked product contains essentially C 4  hydrocarbons and heavier. This remainder is fed to a debutanizer wherein a crude C 4  stream is separated as overhead while a C 5  and heavier stream is removed as a bottoms product.  
         [0005]     Such a C 4  stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, and diolefins such as butadiene (both cis and trans isomers).  
         [0006]     Separately from the cracking process aforesaid, crude C 4  streams have heretofore been subjected to selective hydrogenation of diolefins to convert them to the corresponding monoolefins with simultaneous isomerization of alpha olefins to internal olefins followed by etherification of the isoolefins, and finally metathesis of internal olefins with ethylene to produce propylene, U.S. Pat. No. 5,898,091.  
         [0007]     Also separately from the cracking process aforesaid, ethylene has been dimerized followed by a metathesis operation to form polymer grade propylene.  
         [0008]     It is advantageous for a number of reasons which will be discussed hereinafter in detail, to have a single, integrated process which employs cracking, dimerization, and metathesis therein, particularly when directed to the formation of ethylene and propylene products.  
       SUMMARY OF THE INVENTION  
       [0009]     In accordance with this invention a single, integrated process is provided which cracks a hydrocarbon such as an ethane containing feed to form at least one product olefin, metathesizes internal olefins to form additional product olefin, and internally generates additional feed for the metathesis operation. This method has the flexibility to produce an ethylene product, or a propylene product, or both, all from ethane. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a flow diagram of a conventional hydrocarbon cracking plant that produces polymer grade propylene.  
         [0011]      FIG. 2  is a flow diagram of a conventional ethylene dimerization plant that employs a metathesis unit to produce polymer grade propylene.  
         [0012]      FIG. 3  is a flow diagram that demonstrates one embodiment within this invention that produces chemical grade propylene. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  shows a typical cracking plant wherein a hydrocarbonaceous feed  1  is introduced into a thermal cracking furnace  2 . It should be noted that there are many other cracking plant processing configurations than that shown in  FIG. 1 . This invention is applicable to all such other configurations,  FIG. 1  being just a single example of an applicable configuration. In furnace  2 , a plurality of differing hydrocarbon compounds are formed as aforesaid to produce a cracked product  3 . Hot, gaseous effluent  3  invariably includes, among many other compounds, ethylene, propylene, and butenes, both alpha (1-butene) and internal (2-butenes). Product  3  is subjected to separate oil and water quenches and other fractionation, collectively unit  4 , to liquefy heavier compounds (C 5  and heavier) therein to form liquid streams such as fuel oil stream  5  and automotive gasoline grade (pyrolysis gasoline) stream  6 , which streams are removed from the overall cracking process for other use elsewhere. The gaseous product  7  of fractionation operation  4  is transferred to a compression zone  8  wherein it undergoes several stages of compression. The compressed product  9  then undergoes a process  10 , such as caustic washing, to remove acid gases therefrom, including hydrogen sulfide and carbon dioxide  11 . The product  12  of zone  10  passes to a separation zone  13  which is a combination of cryogenic cooling and fractional distillation, and from which is separated a high purity molecular hydrogen stream  14  and a separate methane stream  15 , both of which are removed as products of the overall cracking plant.  
         [0014]     The remainder of effluent  3  is transferred by way of line  16  to fractional distillation zone  17  wherein ethane and ethylene are separated and recovered in line  18 , with the remainder of stream  16  being recovered as bottoms  24 .  
         [0015]     Stream  18  is subjected to selective hydrogenation step  19  to convert acetylenics and diolefins to their corresponding monoolefins, after which it is passed by way of line  20  to another fractional distillation zone  21  wherein ethylene is separated from ethane, and each are recovered as separate products  22  and  23 , respectively, of the cracking plant.  
         [0016]     Stream  24  is also subjected to additional fractional distillation in zone  25  to separate propane and propylene therefrom as an overhead stream  27 , and leave a C 4  containing stream  26  as a bottoms product.  
         [0017]     Stream  27  is subjected to selective acetylenic and diolefin hydrogenation to convert same to their corresponding monoolefins in zone  28 , and the resulting hydrotreated stream  29  passed to a fractional distillation zone  30  wherein propane is separated from propylene to form separate streams  31  and  32 , both of which are removed as products of the overall cracking process.  
         [0018]     Propylene product  32  is a very pure polymer grade material. As such, product  32  has a propylene content that is substantially greater than that of chemical grade propylene. Chemical grade propylene has numerous uses of value. In fact, most processes involving propylene and outside the polymer industry, e.g., the production of propionitrile, propylene oxide, and the like, requires only chemical grade propylene. The use of polymer grade propylene in such processes is neither practical nor necessary. The separation of propane from propylene in tower  30  is quite difficult due to the close proximity of their respective boiling points. Accordingly, distillation column  30  is very large in size, and is expensive as to both its construction and operating costs. If a process produced chemical grade propylene it would have enhanced flexibility because the chemical grade material could, if desired, be transformed into polymer grade material, or it could be used, without more, in the numerous commercial processes that call for chemical grade propylene. This invention provides that flexibility.  
         [0019]     Stream  26  is subjected to a butene recovery fractional distillation process in zone  33  wherein a C 4  containing stream  34  is separated as a product of the overall cracking process. The C 5  and heavier materials in stream  26  are separated as stream  35  for various uses such as addition to the automotive gasoline pool.  
         [0020]     Thus, the cracking plant of  FIG. 1  produces, among other materials, ethane, ethylene, polymer grade propylene, and propane, with essentially no flexibility for doing otherwise.  
         [0021]      FIG. 2  shows a particularly useful commercial process known as “Product Flexibility” as employed in its dimer mode. In this Figure, ethylene feed  40  and catalyst  41  are fed into ethylene dimerization reactor  42  which is maintained under conditions that favor the dimerization of ethylene to butenes, 2-butenes being favored over 1-butene. The butene containing product  43  of reactor  42  is passed to butene recovery zone  44  wherein an automotive grade gasoline stream  45  is separated therefrom, and a C 4  rich stream  46  is produced. Butene stream  46  is subjected to a drying step  47  to prepare it for use as feed  48  to metathesis zone  49 . Additional ethylene feed  57  can be employed if necessary to ensure an excess of ethylene is present. The product  50  of reactor  49  is passed to fractional distillation zone  51  wherein ethane and ethylene are separated therefrom and returned as feed to reactor  49  by way of line  52 . The C 3  and heavier materials are passed by way of line  53  to a fractional distillation zone  54 . In zone  54 , polymer grade propylene  55  is separated out as a product of the overall dimerization/metathesis process, the remaining C 4  and heavier materials being returned by way of line  56  as feed to butene recovery zone  44 .  
         [0022]      FIG. 3  employs units of both  FIGS. 1 and 2 . For sake of clarity, the reference numbers used in  FIGS. 1 and 2  are carried over to  FIG. 3  for those units that are present in  FIGS. 1 and 2 , and are carried over into FIG.  3 . Accordingly, elements  1  through  16 , inclusive, in  FIG. 3  are identical to the elements similarly marked in  FIG. 1 , and will not, for sake of brevity, be described in greater detail at this point because the process is well known, and further detail is not necessary to inform one skilled in the art. At line  16 , this invention starts to take over.  
         [0023]     Fractionation zone  17  is the same unit as set forth in  FIG. 1  but has a different feed thereto because of the addition of the stream in line  50  which will be discussed in more detail hereinafter. In this invention, zone  17  also separates an overhead stream  18  that contains essentially ethane and ethylene, leaving the remainder of stream  16  as a bottoms product stream  24 .  
         [0024]     Stream  24  is subjected in zone  63  to selective hydrogenation of its acetylenic and diolefinic components to monoolefins as aforesaid. The hydrogenated product  64  is then passed to fractional distillation zone  25  wherein a chemical grade propylene product  65  is recovered as a product of the overall integrated process of  FIG. 3 . The remainder of stream  64  is recovered from zone  25  and passed to butene recovery zone  44 , see  FIG. 2 .  
         [0025]     Stream  18  is passed to a selective hydrogenation zone  19 , followed by fractional distillation in zone  21 , just as explained hereinabove for  FIG. 1 . At this point this invention really takes over. An ethane product stream  23  can, if desired, but is not required, be recovered as in  FIG. 1 , but, in any event, ethylene stream  22  is treated much differently in this invention. If desired, of course, a relatively pure ethylene product stream  22  can be removed from the overall process, but, in accordance with this invention, some, even a substantial amount, if not all, of stream  22  can be passed into line  59 . In addition to, or in lieu of, stream  59  containing all or part of the contents of stream  22 , stream  59  can contain, for example, in whole or in part 1) a side draw of an impure ethylene stream from unit  21  (e.g., an impure ethylene stream taken from the tower above the feed but below the product stream  22 ), and/or 2) ethylene fractionation feed stream  20 .  
         [0026]     Stream  59  is split between lines  60  and  61 . The relative amounts that go into steams  60  and  61  can vary widely depending on how the process is desired to be operated at any given time, it only being required that some of stream  59  goes into each of streams  60  and  61 . However, at least about 67 wt. %, but less than all, of stream  59  can go into stream  60  and about 33 wt. %, but less than all, can go into stream  61 , both wt. % based on the total weight of stream  59 .  
         [0027]     Stream  60  passes to ethylene dimerization zone  42 , while stream  61  is passed to metathesis reactor  49 , compare with  FIG. 2 . The operation of zones  42  and  49  are the same as in  FIG. 2 , zone  42  producing a stream  43  that is rich in butenes, and zone  49  producing a propylene containing stream  50 . As in  FIG. 2 , additional ethylene feed can be supplied by way of line  57 , if desired.  
         [0028]     Propylene rich stream  50  from reactor  49  is added to stream  16 , and after processing in units  17  and  63 , the propylene newly formed in zone  49  finds its way to zone  25 , and, therefore, to propylene product stream  65 .  
         [0029]     Stream  43  passes to butene recovery unit  44 , from which is separated an automotive grade gasoline stream  45 . The butenes rich product  46  is subjected to drying in unit  47  to prepare it as feed for disproportionation, and then passed by way of line  48  as feed to metathesis reactor  49 .  
         [0030]     A comparison of  FIGS. 1-3  shows that large and expensive fractionation tower  30  and butene recovery unit  33  of  FIG. 1  have been eliminated by this invention without eliminating the function thereof. This same comparison shows that fractionation towers  51  and  54  of  FIG. 2  have similarly been eliminated without loss of their function. Although this comparison will also show that selective hydrogenation zone  28  of  FIG. 1  is not present in  FIG. 3 , this function has not been eliminated because a new selective hydrogenation zone  63  ( FIG. 3 ) is employed in this invention.  
         [0031]     Thus, it can be seen that a major advantage of this invention is the elimination of the difficult and costly operation of separating propane from propylene (tower  30 ). This results in a substantial savings in both construction and operating costs. But this is not the only advantage. A significant advantage for this invention is the gain in flexibility of operation in a number of ways. There is greater product flexibility because this invention produces a chemical grade propylene product, the grade that most processes require, without losing the ability to upgrade to the more pure polymer grade of propylene later, if desired. This invention also provides the flexibility to significantly vary the relative production volumes of ethylene and propylene from an ethane cracking plant. This invention also provides the flexibility to produce a propylene product from a plant that cracks a feedstock that contains essentially only ethane. Finally, flexibility is improved in that the metathesis reactor has two sources of ethylene feed, i.e., from the cracking operation itself and from any residual ethylene from dimerization unit  42 .  
         [0032]     The disproportionation reaction employed in reactor  49  is well known. It is a double displacement mechanism that starts with two different compounds. The reaction involves the displacement of groups from each compound to produce two new compounds. There is displacement cleavage at a double bond on each different compound, and the new compounds have double bonds where the old double bonds were cleaved. Thus, the metathesis of one mole of 2-butene and one mole of ethylene yields two moles of propylene. These reaction conditions can vary widely, but generally will include a temperature of from about 300 to about 800 F., a pressure of from about 200 to about 600 psig, and a weight hourly space velocity of from about 1 to about 100 reciprocal hours (based on butene and tungsten trioxide catalyst). Suitable catalysts that favor the disproportionation reaction include at least one of halides, oxides and/or carbonyls of at least one of molybdenum, tungsten, rhenium, and/or magnesium carried on an acidic support such as alumina, silica, alumina/silica, zeolites, and the like. This process is in commercial use, and further detail is not necessary in order to inform the art.  
         [0033]     The ethylene dimerization reaction is a homogeneous liquid phase reaction that is also well known, and in commercial use. Its reaction conditions will also vary widely, but will generally include a temperature of from about 80 to about 150 F., a pressure of from about 100 to about 300 psig, and a residence time of from about 15 to about 60 minutes. Suitable catalysts that favor the homogeneous liquid phase dimerization reaction include at least one from the aluminum alkyl halide family, such as ethyl aluminum dichloride, and a nickel salt-phosphine complex. This process is well known, see U.S. Pat. Nos. 3,485,881; 3,627,700; and 3,726,939.  
       EXAMPLE  
       [0034]     A feed consisting essentially of ethane with less than 10 weight percent (wt. %) of impurities such as propane is cracked at a temperature of from about 1,500 to about 1,600 F. at a pressure of from about 15 to about 25 psig. The cracked product is cooled and then subjected to oil quenching followed by water quenching to a temperature of about 100 F. at about 10 psig, after which it is subjected to compression to a pressure of about 520 psig. The compressed stream is cooled to about 60 F., dried, and then chilled and partially condensed in stages to a temperature of at least about minus 240 F. to separate from the compressed stream a high purity hydrogen stream. Methane is next separated from the remaining hydrocarbons via distillation as an overhead product from a demethanation tower.  
         [0035]     This cracked product ( 16 ,  FIG. 3 ) from the bottoms of the demethanizer is passed to distillation tower  17  which operates at a bottom temperature of about 170 F. at a pressure of about 350 psig to form an overhead stream that consists essentially of ethane and ethylene, line  18 , and a bottoms stream  24  that contains C 3  and heavier hydrocarbons.  
         [0036]     Stream  24  is subjected to selective hydrogenation  63  at a temperature of about 100 F., a pressure of about 300 psig, and a weight hourly space velocity of about 10 reciprocal hours, using a catalyst composed of palladium on an aluminum support. Thereafter the hydrogenated stream is distilled at a bottom temperature of about 200 F. and 110 psig to separate out an overhead product  65  that consists essentially of chemical grade propylene.  
         [0037]     Stream  18  is selectively hydrogenated using similar conditions and catalyst used on stream  24  followed by distillation of the hydrogenated stream in a tower with a bottom temperature of about 20 F. and pressure of about 280 psig to remove a stream consisting essentially of ethane therefrom, and leaving a separate stream  22  consisting essentially of ethylene.  
         [0038]     About 50 wt. % of ethylene stream  22  is removed as product of the overall process. The remainder of stream  22  is split, about 67 wt. % to stream  60  and about 33 wt. % to stream  61 . All wt. % are based on the total weight of the stream.  
         [0039]     Stream  60  is passed to an ethylene dimerization reactor operating at a temperature of about 100 F., a pressure of about 150 psig, and a residence time of about 30 minutes, using a mixture of ethyl aluminum dichloride and a nickel salt-phosphine complex to catalyze the reaction. After quenching the reaction and removing residual catalyst, the dimerized product  43  is subjected to fractional distillation at a bottom temperature of about 230 F. and 70 psig to remove C 5  and heavier hydrocarbons as an automotive gasoline product and produce a butene rich stream  46 . Stream  46  is dried at ambient temperature and about 60 psig pressure using a molecular sieve adsorbent, and then returned as feedstock to metathesis reactor  49 .  
         [0040]     Streams  48  and  61  are introduced as feedstock into metathesis reactor  49  which is operated at a temperature of about 600 F., a pressure of about 400 psig, and a weight hourly space velocity of about 15 reciprocal hours (based on butenes and the tungsten oxide catalyst), using a catalyst consisting essentially of a mixture of tungsten oxide on a silica support and magnesium oxide. In reactor  49 , 2-butene is disproportionated in the presence of an excess of ethylene to form a product  50  that is rich in propylene. This product is combined with cracked stream  16  and the resulting combination stream passed to fractionation zone  17  as feedstock therefor.