Patent Publication Number: US-2003225306-A1

Title: Process and apparatus for preparing olefins

Description:
[0001] The invention relates to a process for preparing olefins from a hydrocarbon-containing feed, where the feed is fed to a treatment plant, in particular a refinery or olefin plant, in which various hydrocarbon-containing fractions are produced, at least part of the fractions comprising relatively long-chain olefins, in particular olefins having at least four carbon atoms, is fed to an olefin conversion stage in which at least part of the relatively long-chain olefins is converted into shorter-chain olefins and the reaction products are fed together with any unreacted hydrocarbons to a conversion product fractionation stage in which a fraction comprising hydrocarbons having not more than three carbon atoms is produced, and this fraction is separated off and recirculated to the treatment plant, and also an apparatus for carrying out the process.  
       [0002] The market for chemical raw materials has shown an increasing demand for propylene in recent years. In the past, propylene has been produced mainly as by-product in olefin plants, in particular ethylene plants. Since the future demand for propylene will no longer be able to be covered by this route alone, processes for increasing the propylene yield in olefin plants have already been developed. These processes are based essentially on the principle of converting relatively long-chain olefins, e.g. those having from four to six carbon atoms, obtained in olefin plants into shorter-chain olefins having, for example, two and three carbon atoms, in particular into propylene (three carbon atoms). Here, use is usually made of a catalyst which effects the appropriate conversion of the olefins. Such processes are known under the names “Propylur process” (cf. Hydrocarbon Engineering, May 1999, pages 66 to 68) and “Meta-4 process” (cf. Petrochemicals PTQ Spring 1997, pages 109 to 115) and can be summarized under the collective term olefin conversion processes. The economics of such processes depend greatly on the achieved yield of the desired olefins.  
       [0003] It is an object of the present invention to configure a process of the type mentioned at the outset and an apparatus for carrying out the process in such a way that an increase in the yield of the desired olefins having up to three carbon atoms is made possible.  
       [0004] Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.  
       [0005] According to the invention, a process aspect is achieved by at least part of the fractions comprising hydrocarbons having more than three carbon atoms which are obtained in the conversion product fractionation stage being fed to a paraffin/olefin separation stage in which olefins and paraffins are separated from one another, with at least part of the paraffins being fed to the treatment plant or taken off and passed to another use and at least part of the olefins being recirculated to the olefin conversion stage.  
       [0006] The treatment plant for the preparation of olefins is usually an olefin plant comprising a cracking furnace for cracking the hydrocarbons and a downstream fractionation stage for separating the cracking products into individual fractions. However, it is also possible to utilize particular product streams from a refinery for the preparation of olefins, for example so as to provide additional utilization opportunities for the refinery.  
       [0007] The idea on which the invention is based is to separate the hydrocarbon mixture obtained in the olefin conversion stage into paraffins and olefins and to increase the yield of desired olefins by recirculating at least part of the olefins to the olefin conversion stage. The paraffins which have been separated off form a suitable feed for the cracking furnace of the olefin plant, which is why they are preferably fed to the cracking furnace. Alternatively or in addition, the paraffins can also be taken off and passed to another use. Overall, this improves both the economics of the total olefin plant or refinery and the yield of desired olefins.  
       [0008] In a particularly preferred embodiment of the invention, at least one fraction comprising hydrocarbons having four and/or five carbon atoms is produced in the conversion product fractionation stage and is fed to the paraffin/olefin separation stage. In addition, at least one second fraction comprising hydrocarbons having at least five carbon atoms is produced and is taken off. The conversion product fractionation stage thus produces a total of three fractions, namely a fraction which comprises hydrocarbons having not more than three carbon atoms and is recirculated to the treatment plant, e.g. to the fractionation stage of the olefin plant, a fraction comprising hydrocarbons having four and/or five carbon atoms and a fraction comprising hydrocarbons having at least five carbon atoms. In the downstream paraffin/olefin separation stage, a paraffin stream is separated off from the fraction comprising hydrocarbons having four and/or five carbon atoms and this is fed to the treatment plant, in particular the cracking furnace of the olefin plant, and contributes to improved utilization of the capacity of the treatment plant, in particular the cracking furnace, by suitable feedstocks. On the other hand, an olefin stream is separated off in the paraffin/olefin separation stage and this is recirculated to the olefin conversion stage, thus increasing the yield of olefins.  
       [0009] A further development of the concept of the invention provides for only the fraction comprising hydrocarbons having not more than three carbon atoms being separated off in the conversion product fractionation stage and being recirculated to the treatment plant, in particular to the fractionation stage of the olefin plant, with the remainder being fed in its entirety to the paraffin/olefin separation stage. The stream fed to the paraffin/olefin separation stage is then separated into at least three fractions, with a first fraction comprising paraffins having four and/or five carbon atoms being fed to the treatment plant, in particular the cracking furnace of the olefin plant, a second fraction comprising olefins having four and/or five carbon atoms being recirculated to the olefin conversion stage, and a third fraction comprising hydrocarbons having at least five carbon atoms being taken off. In this variant of the invention, the conversion product fractionation stage can be simpler, since only one fraction comprising hydrocarbons having not more than three carbon atoms has to be separated off. In this case, the paraffin/olefin separation stage is designed so that separation into the three product streams mentioned is made possible.  
       [0010] The paraffin/olefin separation stage advantageously comprises an extractive distillation. This extractive distillation is preferably configured so that the three product streams mentioned can be produced.  
       [0011] The olefin conversion stage preferably comprises a catalytic reaction, e.g. catalytic cracking, of the relatively long-chain hydrocarbons to form shorter-chain hydrocarbons. Such catalytic conversion processes are known per se from the prior art.  
       [0012] The invention further provides an apparatus for preparing olefins from a hydrocarbon-containing feed, which comprises a treatment plant, in particular a refinery or olefin plant, which has a fractionation apparatus for producing various hydrocarbon-containing fractions, with the fractionation apparatus being connected to an olefin conversion apparatus for converting relatively long-chain olefins into shorter-chain olefins, downstream of which there is a conversion product fractionation apparatus in which the hydrocarbons reacted in the olefin conversion apparatus and any unreacted hydrocarbons are fractionated and which is connected via an olefin recirculation line to the fractionation apparatus.  
       [0013] An apparatus aspect of the invention is achieved by the conversion product fractionation apparatus being followed by a paraffin/olefin separation apparatus for separating olefins and paraffins which is connected via a paraffin line to the treatment plant and via an olefin recirculation line to the olefin conversion apparatus.  
       [0014] The treatment plant for preparing the olefins can be a refinery or olefin plant. In the case of an olefin plant, it is provided with a cracking furnace for cracking the hydrocarbons which is followed by a fractionation apparatus for separating the cracking products into fractions. The paraffin/olefin separation apparatus is in this case connected via the paraffin line to the cracking furnace of the olefin plant.  
       [0015] The paraffin/olefin separation apparatus is advantageously equipped with distillation columns designed for extractive distillation. At least two distillation columns are provided, with one serving to separate paraffins and olefins and the second distillation column being configured as a regeneration column for recovering the extractant.  
       [0016] According to a further development of the concept of the invention, at least three distillation columns are provided, as a result of which it is additionally possible to separate off hydrocarbons having at least five carbon atoms. In this case, the upstream conversion product fractionation apparatus can be simpler, since only the short-chain hydrocarbons having fewer than three carbon atoms have to be separated off.  
       [0017] The olefin conversion apparatus preferably has a catalytic reactor, in particular a fixed-bed reactor. As catalysts, it is possible to use the types of catalyst known from the prior art, in particular a zeolite catalyst.  
       [0018] The invention makes it possible, in particular, to achieve a significant increase in the propylene yield in olefin plants or refineries for a justifiable engineering outlay. The additional process step of paraffin/olefin separation substantially increases the yield of target products in olefin plants. At the same time, circulated streams are minimized so that the specific capital costs for propylene production are reduced. This compensates for the additional capital investment for the additional process step of paraffin/olefin separation. Overall, the economics of the olefin plant or refinery are improved.  
       [0019] The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German Application No. 102 17 866.6, filed April 22, 2002, is hereby incorporated by reference.  
       [0020] The invention is illustrated below with the aid of a comparison of the prior art with the illustrative examples of the invention shown schematically in the figures. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0021] Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:  
     [0022]FIG. 1 shows a flow diagram of an olefin plant having an integrated olefin conversion stage according to the prior art;  
     [0023]FIG. 2 shows an olefin plant having an integrated olefin conversion stage and a downstream conversion product fractionation stage according to the prior art;  
     [0024]FIG. 3 shows an olefin plant having an integrated olefin conversion stage with conversion product fractionation stage and a downstream paraffin/olefin separation stage;  
     [0025]FIG. 4 shows a flow diagram of an olefin plant having an integrated olefin conversion stage with conversion product fractionation apparatus and a downstream paraffin/olefin separation stage with integrated fractionation; and  
     [0026] FIGS.  5 - 10  show various embodiments of the paraffin/olefin separation stage. 
    
    
     [0027] The olefin plant  1  according to the prior art shown in FIG. 1 is operated using naphtha which is fed via line  2  to a cracking furnace  3 . It is also possible for the plant to be provided with a plurality of cracking furnaces which convert the naphtha into a cracking gas which is passed to a water scrub  4  and, by means of a compressor  5 , to a fractionation stage  6 . In the fractionation stage  6 , olefins, pyrolysis gasoline, pyrolysis oil and light gases are taken off via lines  7  and  8 . Hydrocarbons having four or five carbon atoms obtained in the fractionation stage  6  are fed via line  9  to a diene/acetylene removal unit  10 . The remaining paraffins and olefins having four and/or five carbon atoms are passed via line  11  to an olefin conversion stage  12 . The effulent streams in lines  11  and  13  both contain paraffins and olefins. The olefin conversion stage  12  is usually configured as a catalytic fixed-bed reactor and converts the relatively long-chain olefins (C 4 /C 5 -olefins) into shorter-chain olefins (C 2 -C 4 -olefins). Butanes discharged via line  13  are supplied directly as feed to the cracking furnace  3  where they are cracked to form olefins and by-products. The olefins having from two to four carbon atoms produced in the olefin conversion stage  12  and gasoline and light gases obtained are conveyed via line  14  to the water scrub  4  of the olefin plant  1 . A substream can also be conveyed directly via line  15  to upstream of the pump  5 . This arrangement represents the simplest variant disclosed in the prior art for increasing the propylene yield in olefin plants.  
     [0028]FIG. 2 shows a variant of the prior art, in which a conversion product fractionation stage  17  is provided downstream of the olefin conversion stage  12 . In this arrangement, the same plant-components a-re denoted by the same reference numerals as in FIG. 1. In contrast to the plant shown in FIG. 1, the hydrocarbons produced in the olefin conversion stage  12  are separated into various fractions, with the hydrocarbons having not more than three carbon atoms being recirculated via line  18  directly to the fractionation stage  6  of the olefin plant  1 , the relatively long-chain hydrocarbons having at least five carbon atoms and gasoline product being discharged via line  19  and the hydrocarbons having four and/or five carbon atoms being taken off from the conversion product fractionation stage  17  via line  20  and partly fed via line  21  directly to the cracking furnace  3  of the olefin plant  1  and partly recirculated via line  22  to the olefin conversion stage  12 . This arrangement corresponds to the prior art as is described, for example, in “Hydrocarbon Engineering May 1999, page 66 to page 68”.  
     [0029]FIG. 3 shows an embodiment of the invention which is based on the plant according to the prior art shown in FIG. 2. The same plant components are denoted by the same reference numerals. This variant of the invention differs from the prior art in that a paraffin/olefin separation stage  23  is provided downstream of the conversion product fractionation stage  17 . This additional process step serves to separate the paraffins from the olefins. While the paraffins (butane, pentane) are conveyed as feed to the cracking furnace  3  of the olefin plant  1  via line  24 , the olefins (butene, pentene) are recirculated via an olefin recirculation line  25  to the olefin conversion stage  12 . This enables a significant increase in the propylene yield of the olefin plant to be achieved.  
     [0030]FIG. 4 shows another variant of the invention, in which the paraffin/olefin separation stage  23  is designed so as to serve simultaneously as fractionation stage. This makes it possible to omit the removal of hydrocarbons having more than five carbon atoms in the conversion product fractionation stage  17  as shown in FIG. 3. These relatively long-chain hydrocarbons are thus not discharged via line  19  as shown in FIG. 3 but instead from the propylene/olefin separation stage  23  via line  26 . The other process steps correspond to the variant shown in FIG. 3.  
     [0031] FIGS.  5  to  10  show variants of the paraffin/olefin separation stage in detail. The paraffin/olefin separation stage is in each case configured as an extractive distillation. The variants shown in FIGS. 5 and 6, which each have two distillation columns, correspond to the arrangement shown in FIG. 3 in which only a separation into a fraction comprising butane and pentane and a fraction comprising butene and pentene takes place in the paraffin/olefin separation stage. In the illustrated embodiments shown in FIGS.  7  to  10 , the propane/olefin separation stage has three distillation columns in each case, which makes it possible for hydrocarbons having more than five carbon atoms to be additionally separated off as shown in FIG. 4.  
     [0032] The paraffin/olefin separation stage shown in FIG. 5 is supplied via lines  25  and  1  with a mixture of butane, pentane, butene and pentene which is taken off from the conversion product fractionation apparatus. This mixture is introduced into a first distillation column  1000 . The distillation column  1000  is operated using an extractant which is introduced via lines  12 ,  13  and  2  into the upper part of the distillation column  1000 . The extractant can comprise the following substances: NMP (N-methylpyrrolidone), DMF (dimethylformamide), NFM (N-formylmorpholine), acetone, acetonitrile, furfural, DMAC (dimethylacetamide), sulpholane, diethylene glycol (glycol mixtures) or dimethyl sulphoxide. Solvent mixtures can also be used, and water or methanol can be employed as additives. The paraffins butane and/or pentane are taken off from the top of the distillation column  1000  via line  3  and are finally recirculated via line  6  to the cracking furnace of the olefin plant. Part of these paraffins is recirculated via line  5  to the distillation column  1000  for backscrubbing. Water can be separated off via line  7 . The olefins butene and pentene obtained at the bottom of the distillation column  1000  are conveyed via line  8  and a heat exchanger  104  and line  9  to a second distillation column  2000  which serves as regeneration column. For backscrubbing, water or an external runback stream is fed via line  11  to the top of the distillation column  2000 . The olefins butene and pentene are finally taken off from the top of the distillation column  2000  via line  10  and recirculated via a compressor  5000  and line  27  to the olefin conversion stage. The distillation column  1000  is, in the present example, operated at a pressure of 9 bar which is sufficient for further processing of the paraffins in the cracking furnace. If the paraffins are intended only for liquefaction or for introduction into a fuel gas supply, a pressure in the distillation column  1000  of from 4 to 6 bar is sufficient. The pressure at which the distillation column  2000  is operated is as low as possible in order to avoid decomposition of the extractant. For this reason, the pressure in the present example is from 1 to 4 bar, in particular less than 2 bar. If an auxiliary (e.g. higher hydrocarbons) is introduced, the pressure should be high enough for condensation to be achieved or for the top product to be able to be taken off in gaseous form. In this case, the pressure is from 2 to 6 bar (condensation: 4-5 bar, gaseous top product: &gt;3 bar). The temperatures in the distillation columns  1000  and  2000  are matched to the respective pressures and the separation task to be achieved. In the regeneration column  2000 , a boiling temperature which is sufficiently below the decomposition temperatures of the solvents is set at the bottom.  
     [0033] In FIGS.  6  to  10  below, the same plant components as in FIG. 5 are denoted by the same reference numerals. The variant depicted in FIG. 6 differs from that shown in FIG. 5 by an additional measure for removing acetylene which comprises a third distillation column  3000  which is connected via lines  14  and  17  to the distillation column  2000 . Acetylene is taken off from the top of the distillation column  2000  via line  15 . Backscrubbing with water or an external runback stream takes place via line  16 .  
     [0034] In the embodiment depicted in FIG. 7, an additional fractionation is integrated into the paraffin/olefin separation stage. The distillation columns  1000  and  2000  correspond to the embodiment shown in FIG. 5, with the same plant components being denoted by the same reference numerals. However, the plant is provided with an additional distillation column  4000  which serves to separate off the relatively long-chain hydrocarbons (hydrocarbons having more than five carbon atoms) and gasoline product. Paraffins (via lines  3 ,  4  and  6  to the cracking furnace), olefins (via lines  10 ,  29  and  30  to the olefin conversion stage) and relatively long-chain hydrocarbons having more than five carbon atoms (via lines  19 ,  20  and  22 ) are taken off as top products from the respective distillation columns. Part of the top products is in each case recirculated to the distillation columns for backscrubbing (via lines  5 ,  11  and  21 ). In addition, water is separated off from the top product by means of water separators  202 ,  204  and  203 .  
     [0035] The embodiment depicted in FIG. 8 differs from that shown in FIG. 7 only in that part of the top product from the distillation column  2000  is branched off from the line  10  and recirculated via the line shown as a dotted line to the distillation column  1000 . This enables the purity of the top product to be improved at relatively high pressures in the distillation column  2000 .  
     [0036] In the variant shown in FIG. 9, preliminary separation of the hydrocarbon mixture supplied via line  25  takes place in a small column  205 .  
     [0037] Finally, FIG. 10 shows a variant in which the top product from the distillation column  2000  is taken off directly in gaseous form via line  10 . This is possible when the pressure in the distillation column  2000  is high enough. In this case, recirculation of part of the top product for backscrubbing can be dispensed with and instead an external runback stream can be supplied via line  11 .  
     [0038] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.  
     [0039] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.