Abstract:
An improvement in a process for the production of ethylbenzene from a dilute ethylene stream wherein an ethylene-containing stream derived from a cracking process is directed to an ethylene fractionator for separation of ethylene and ethane. The improvement includes providing the dilute ethylene stream by liquefying and separating out a portion of the ethylene-containing stream prior to directing the remainder of the ethylene-containing stream to the ethylene fractionator and/or by drawing off a side stream from the ethylene fractionator; and, directing the dilute ethylene stream as a feed to an alkylator for alkylation with benzene to produce ethylbenzene.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to processes related to the production of ethylbenzene from dilute sources of ethylene. 
     2. Description of the Related Art 
     Ethylene is a commonly used unsaturated organic compound which may be used in various processes for the synthesis of more complex compounds. For example, ethylene can be reacted with aromatics to produce alkylaromatics, such as ethylbenzene, and with butenes to produce propylene. Ethylbenzene is commonly used to produce styrene, which may be polymerized to produce polystyrene. Propylene is commonly used for the manufacture of polypropylene. 
     The process for producing high purity ethylene is well known, and involves pyrolysis of a hydrocarbon feed and subsequent separation of ethylene and reaction by-products by distillation. The process generally comprises the following: A feedstock comprising ethane, propane, butane, naphtha, gas oils or hydrocracked vacuum gas oils is fed to an ethylene plant, where it is thermally cracked in the presence of steam in a bank of pyrolysis furnaces. An olefin-bearing effluent gas is formed and is quenched progressively by generating steam and through direct contact with oil and/or water. The effluent is compressed in a multi-stage centrifugal compressor, acid gases are removed by amine treating and/or a caustic wash, and then the gases are dried over a molecular sieve. Methane offgas is recovered under cryogenic conditions in a demethanizer. Ethylene and ethane are then recovered together in a deethanizer. Acetylene is normally catalytically removed and then an ethylene product recovery takes place under low temperature conditions in a final fractionation column. Just prior to final fractionation the ethylene stream will include significant amounts of ethane (15% to 35%) and relatively small amounts of hydrogen, methane and propylene. Final fractionation results in a high purity (polymer-grade) ethylene (at least about 99.95 mol %) and recycle ethane, which may be used to produce more ethylene. Variations of this scheme also exist including schemes where the deethanization, depropanization and/or acetylene removal steps may precede the demethanization step. In all cases a final fractionation of ethylene-ethane is employed to produce high purity ethylene product. 
     The final fractionation of the ethylene mixture is relatively energy intensive and it would be preferable to reduce the amount of ethylene/ethane processed in this manner, or to eliminate this step altogether. However, many processes, including those to produce propylene and ethylbenzene, typically are carried out with a feed of high purity ethylene. Ethylene streams diverted from the ethylene plant, after acetylene removal but before final fractionation, typically contain only about 65 mol % ethylene when ethane crackers are the source of the ethylene, and about 85 mol % ethylene when naphtha crackers are the source of the ethylene; the primary difference between the two processes being the feedstock used and a somewhat simpler recovery section for the ethane cracker (i.e., the ethane cracker has fewer distillation columns since heavy byproduct formation is reduced). 
     A number of processes for producing alkylaromatics, such as ethylbenzene, are also known, and may employ fixed-bed or catalytic distillation type processes. The fixed-bed process generally comprises the following: Benzene is sent to an alkylator containing a fixed bed of alkylation catalyst and reacted with ethylene to yield a mixture of alkylated benzenes and excess benzene. The mixture is fractionated to recover ethylbenzene, recycle benzene, and higher ethylated benzenes. The recycle benzene is sent back to the alkylator to react with additional ethylene and to a transalkylator, where the higher ethylated benzenes are transalkylated with the benzene to form additional ethylbenzene. 
     While polymer-grade ethylene is preferable for these processes, ethylbenzene can also be produced from relatively dilute ethylene feeds. In this event, catalytic distillation reactors are preferred because ethylene feeds as dilute as about 15 mol % can be utilized to produce ethylbenzene. If the fixed-bed process is used with dilute ethylene feeds, ethylene with a purity as low as about 60 mol % can be used, provided the remaining 40 mol % of the feed contains minimal hydrogen and methane content. Dilute ethylene from an ethane cracker may have relatively low amounts of methane and hydrogen, but this may not always be the case since, for example, dilute ethylene from an ethylene plant with a front-end deethanizer may contain larger quantities of hydrogen and methane. Alternatively, dilute ethylene from a fluid catalytic cracker (FCC) may contain very large quantities of hydrogen or methane if they are not separated at a FCC vapor recovery unit by compression and distillation of FCC off-gas. Typically, the ethane and lighter gases from the FCC do not undergo further separation—rather they are sent to a fuel gas system in the refinery. In any event, fixed-bed processes will incur an energy penalty when the ethylene feed purity is below about 83 mol %. 
     The energy penalty includes additional energy which must be used in the ethylbenzene plant when ethylene sources used are very dilute. For example, in the ethylbenzene plant described above, additional energy may be needed to recover aromatics from vent gases. Such additional processing may involve refrigerated vent condensers and/or an absorption/stripping system with reboilers and condensers. 
     The advantages of the invention include significant energy, and consequently cost, savings by eliminating or reducing the final fractionation of ethylene in the ethylene plant. 
     SUMMARY 
     An improved process is provided herein for the production of ethylbenzene from a dilute ethylene stream wherein an ethylene-containing stream derived from a cracking process is directed to an ethylene fractionator for separation of ethylene and ethane. The improvement comprises (a) providing the dilute ethylene stream by (i) liquefying and separating out a portion of the ethylene-containing stream prior to directing the remainder of the ethylene-containing stream to the ethylene fractionator and/or by (ii) drawing off a side stream from the ethylene fractionator; and, (b) directing said dilute ethylene stream as a feed to an alkylator for alkylation with benzene to produce ethylbenzene. 
     The process advantageously saves costs by reducing the amount of energy required for ethylene fractionation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments are described below with reference to the drawings wherein: 
         FIG. 1  is a schematic diagram illustrating an ethylbenzene process of the invention wherein a dilute ethylene stream is provided by condensing a portion of an ethylene-containing stream to form a dilute ethylene liquid stream which is sent to an alkylator; 
         FIG. 2  is a schematic diagram illustrating an ethylbenzene process of the invention wherein an ethylene-containing stream is partially condensed to form a dilute ethylene liquid which is sent to an alkylator; 
         FIG. 3  is a schematic diagram illustrating an ethylbenzene process of the invention wherein a dilute ethylene stream is taken as a side draw from the stripping section of an ethylene fractionator; 
         FIG. 4  is a schematic diagram illustrating an ethylbenzene process of the invention wherein a dilute ethylene stream is taken as a side draw from the rectification section of an ethylene fractionator; and 
         FIG. 5  is a schematic illustration of a process for producing ethylbenzene from ethylene and benzene. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention relates to processes for producing dilute ethylene, and using the dilute ethylene to produce ethylbenzene. Dilute ethylene streams from cracking processes typically contain from about 60 mol % to about 85% ethylene with the remainder being mostly ethane with minor amounts of methane and/or hydrogen. 
     While individual processes for producing ethylene, and ethylbenzene are known, the present invention combines the processes in a manner which is designed to improve overall efficiency and, consequently, reduce the total costs associated with the production of ethylbenzene. 
     Generally, preferred processes of the invention comprise diverting a dilute ethylene stream from an ethylene plant at a point subsequent to acetylene removal. The ethylene plant includes the cracking of a hydrocarbon feed such as ethane, propane, butane, naphtha, gas oil, hydrocracked vacuum gas oil and combinations thereof. 
     In one embodiment the method of the present invention provides dilute ethylene for the production of ethylbenzene wherein a dilute ethylene stream containing ethylene and ethane is withdrawn from the ethylene plant at a point downstream of acetylene removal and upstream of final ethylene product fractionation. The ethylene content of the dilute ethylene feed is at least about 60 mol %. By withdrawing dilute ethylene from the effluent of the actetylene remover and before the final fractionation step, significant capital and energy saving can be achieved in the ethylene plant because the ethylene/ethane mixture sent to the ethylbenzene plant does not need to be fractionated, thereby saving the fractionation energy. Dilute ethylene can be supplied by subjecting the entire effluent of the acetylene remover to a cooling process in which part of the effluent is condensed to a liquid, or fully condensing only a portion of the effluent. Partial condensing produces a slightly more dilute feed to the ethylbenzene plant and enriches the remaining stream sent as feed to the ethylene fractionator. Fully condensing only a portion of the effluent has the advantage of producing the richest ethylbenzene feed (about 83% for a typical naphtha cracker). Savings are achieved by reducing the ethylene/ethane processed in the ethylene fractionator. In the case of partial condensation, some savings in reflux are also achieved due to feed enrichment. These two cases are detailed below. 
     In another embodiment the dilute ethylene stream is withdrawn from the ethylene plant as a side draw from the ethylene fractionation column. By withdrawing dilute ethylene as a side-draw from the final fractionation step, significant capital and energy savings can be achieved in the ethylene plant because the ethylene/ethane mixture sent to the ethylbenzene plant does not need to be fully fractionated, thereby saving the fractionation energy. Side-draw can be taken as a liquid or vapor from either the stripping or rectification section. If taken from the rectification section, dilute ethylene of at least 83% purity can be readily produced, thereby eliminating any energy penalty in the ethylbenzene plant. These cases are summarized below. 
     The dilute ethylene streams are then sent to an alkylator for alkylation with benzene to produce ethyl benzene. Various processes for the production of ethylbenzene from the alkylation of ethylene and benzene are known. Suitable processes for the alkylation of dilute ethylene streams are set forth in commonly assigned copending patent application Ser. Nos. 10/372,449 filed Feb. 25, 2003 and Ser. No. 10/376,683 filed Feb. 28, 2003, both of which are incorporated by reference herein in their entirety. 
     Referring now to  FIG. 1 , a process  10  for the production of ethylbenzene from a dilute ethylene stream is shown. More particularly, the deethanizer  11  of a conventional ethylene plant provides a C 2  vapor stream  12  containing ethylene, ethane and acetylene, which is then sent to an acetylene removal unit  13  for the removal of acetylene preferably by partial hydrogenation in a catalytic reactor to form more ethylene. Optionally, unit  13  can alternatively be an acetylene recovery unit which separates out acetylene rather than converting it. The resulting effluent stream  14 , containing at least about 60 mol % ethylene is then divided into a dilute ethylene stream  14   a  and a remainder  14   c  of the ethylene-containing steam which is directed to the ethylene fractionator  17  as a feed. The ethylene fractionator  17  separates the ethylene-containing stream  14   c  into a high purity ethylene fraction  18  (at least about 99.95 mol % pure ethylene) and an ethane fraction  19 . The ethane is preferably recycled to the cracking units. The dilute ethylene stream  14   a  containing at least about 60 mol % ethylene is sent to a condenser  15  wherein stream  14   a  is totally condensed to liquid stream  14   b , which is then sent to a conventional ethylbenzene process  16 . The amount of dilute ethylene stream  14   a  drawn off from the acetylene reactor effluent  14  depends upon the ethylene requirements for the ethylbenzene process. In this case the ethylene content of the dilute ethylene stream  14   b  is the same as the ethylene content of the ethylene fractionator feed  14   c . The high purity ethylene stream  18  from the ethylene fractionator  17  can be used for polymerization processes, alkylation processes, or any process where high purity ethylene is required, as well as those in which dilute ethylene can be employed. 
     Referring to  FIG. 2  an alternative embodiment  20  of the method of the present invention is illustrated wherein a C 2  vapor stream  22  from deethanizer  21  is sent to an acetylene removal unit  23  for the removal of acetylene by partial hydrogenation or acetylene recovery. The entire resulting effluent stream  24  is sent to a cooler  25  wherein a part of the ethylene-containing stream is condensed to a liquid dilute ethylene stream  24   a  which is then sent to a conventional ethylbenzene process  26 . The remaining cooled but uncondensed portion  24   b  of the ethylene-containing effluent stream is sent as a vapor to ethylene fractionator  27  for separation into an overhead high purity ethylene stream  28  and bottom ethane stream  29 . In this case the uncondensed portion  24   b  of the ethylene-containing stream is richer in ethylene content than the dilute ethylene stream  24   a.    
     Referring now to  FIG. 3 , yet another embodiment  30  of the present invention is illustrated wherein a C 2  vapor stream  32  from a deethanizer  31  is sent to an acetylene removal unit  33  for the removal of acetylene by partial hydrogenation or acetylene recovery. The resulting effluent stream  34  is sent to ethylene fractionator  37  for separation into an overhead high purity ethylene stream  38  and an ethane bottoms  39 . A liquid or vapor stream  35  of dilute ethylene is drawn from the side of the stripping portion ethylene fractionator at point  37   a , and is then sent to the ethylbenzene process  36 . The point  37   a  at which the dilute ethylene is withdrawn from the ethylene fractionator may be the feed tray or one or two trays below the feed tray such that the ethylene content of the sidedraw is at least about 60 mol %. The actual location will vary depending on the ethylene fractionator feed composition. The amount of liquid or vapor drawn off depends upon the ethylene requirements of the ethylbenzene process. 
     Referring to  FIG. 4 , yet another embodiment  40  of the invention is shown wherein a C 2  vapor stream  42  from a deethanizer  41  is sent to an acetylene removal unit  43  for the removal of acetylene by partial hydrogenation or acetylene recovery. The resulting effluent stream  44  is then sent to ethylene fractionator  47  for separation into an overhead high purity ethylene stream  48  and an ethane bottoms  49 . A liquid or vapor stream  45  of dilute ethylene is drawn from the side of the ethylene fractionator rectification section at point  47   a  and is then sent to the ethylbenzene process  46 . The point  47   a  at which the dilute ethylene is withdrawn from the ethylene fractionator may be ten to fifteen trays above the feed tray such that the ethylene content of the sidedraw is at least about 82 mol %. The actual location will vary depending on the ethylene fractionator feed composition. The amount of liquid or vapor drawn off depends upon the ethylene content of the stream  45  and the ethylene requirements of the ethylbenzene process. 
     Referring now to  FIG. 5  a process for producing ethylbenzene by the alkylation of benzene with ethylene is illustrated in a schematic diagram wherein ethylbenzene process  100  receives a dilute ethylene feed E such as that provided from the process described above (e.g., streams  14   b ,  24   a ,  35 , and  45  in  FIGS. 1 to 4 ), which is fed into alkylator  110  along with benzene feed B. The overhead from the alkylator  110  is sent to fractionator  130  from which a benzene overhead  131  is recycled back to the alkylator  110  and transalkylator  120 . The bottoms  132  from fractionator  130  is sent to fractionator  140 . The overhead  141  from fractionator  140  is an ethylbenzene product P. The bottom stream  142  from fractionator  140  are sent to fractionator  150 . The overhead  151  from fractionator  150  contains polyethylbenzene which is recycled back to the transalkylator  120  for conversion to ethylbenzene. The bottom stream  152  includes a heavy end fraction. The product ethylbenzene P can be sent to further processing for conversion to styrene monomer. The heavy ends  152  can be used as fuel. 
     Various aspects of the invention are illustrated by the Examples given below. 
     EXAMPLE 1 
     In a system such as that illustrated in  FIG. 1 , a portion of the effluent  14  is drawn off as dilute ethylene stream  14   a  which is totally condensed to produce a liquid equivalent to the amount of ethylene required for ethylbenzene production and is sent to ethylbenzene plant  16  as a liquid stream  14   b . The remaining stream  14   c  is sent to the ethylene fractionator  14  as a vapor. This case reduces the feed to the ethylene fractionator  17 . Energy is saved in the ethylene fractionator  17  but partly lost in the form of lower recuperation from ethane recycle and in the form of higher reflux requirements in the deethanizer  11  to reduce the propylene content of the deethanizer overhead (dilute ethylene). Propylene content of the dilute ethylene must be sufficiently low to prevent the excessive formation of cumene in the ethylbenzene unit. Cumene, if formed, will be present as an impurity in the ethylbenzene product and is typically limited to 500 ppm or less. The purity of the ethylene feed  14   b  in this case is about 80 mol % to about 83 mol %. The operating cost savings for a 950,000 kta ethylene plant combined with a 550,00 kta ethylbenzene plant are estimated to be about $560,000 per annum at current energy price levels. 
     EXAMPLE 2 
     In a system such as that illustrated in  FIG. 2 , the effluent  24  is sent to condenser  25  where it is partially condensed. The liquid equivalent to the amount of ethylene required for ethylbenzene production is sent to the ethylbenzene plant  26  via line  24   a . The chilled but uncondensed portion of the effluent  24  is sent to the ethylene fractionator as a vapor via line  24   b . In this case the feed to the ethylene fractionator is both reduced as well as enriched. Energy is saved in the ethylene fractionator but is partly lost in the form of lower recuperation from ethane recycle and in the form of higher reflux requirements in the deethanizer  21  to reduce the propylene content of the deethanizer overhead (dilute ethylene). The purity of the ethylene feed  24   a  to the ethylbenzene plant  16  in this case is about 72 mol % to about 78 mol %. The purity level is expected to cause reduced steam production in the ethylbenzene plant. However, without considering the impact in the ethylbenzene plant the savings in operating costs for a 950,000 kta ethylene plant combined with a 550,000 kta ethylbenzene plant are estimated to be about $580,000 per annum at current energy price levels. 
     EXAMPLE 3 
     In a system such as that illustrated in  FIG. 3 , the entire effluent  34  from the acetylene converter  33  is sent to the ethylene fractionator  37  and the dilute ethylene feed to the ethylbenzene plant  36  is drawn off via stream  35  as a vapor from the stripping portion of the ethylene fractionator  37 . Energy is saved in the ethylene fractionator  37  but is partly lost in the form of lower recuperation from ethane recycle and in the form of a higher reflux requirements in the deethanizer  31  to reduce the propylene content of the deethanizer overhead (dilute ethylene). The purity of the dilute ethylene stream  35  is about 60 mol % to about 65 mol %. The purity level is expected to cause reduced steam production in the ethylbenzene plant. However, without considering the impact in the ethylbenzene plant the savings in operating cost for a 950,000 kta ethylene plant combined with a 550,000 kta ethylbenzene plant are estimated to be about $850,000 per annum at current energy price levels. 
     EXAMPLE 4 
     In system such as that illustrated in  FIG. 4 , the entire effluent  44  from the acetylene converter  43  is sent to the ethylene fractionator  47  and the dilute ethylene feed to the ethylbenzene plant  46  is drawn off via stream  45  as a liquid from the rectification portion of the ethylene fractionator  47 . Energy is saved in the ethylene fractionator  47  but is partly lost in the form of lower recuperation from ethane recycle. A higher reflux requirement in the deethanizer is not needed for this case since the side-draw is taken above the feed where propylene concentration is already sufficiently low. The purity of the dilute ethylene stream  45  is about 82 mol % to about 85 mol %. This case produces the highest purity of dilute ethylene feed and has no significant impact in the ethylbenzene plant. The savings in operating cost for a 950,000 kta ethylene plant combined with a 550,000 kta ethylbenzene plant are estimated to be about $780.000 per annum at current energy price levels. 
     While the above description contains many specifics, these specifics should not be construed as limitations on the scope of the invention, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision many other possible variations that are within the scope and spirit of the invention as defined by the claims appended hereto.