Abstract:
A process for preparing styrene via the catalytic dehydrogenation of ethylbenzene, comprising recirculation of reaction byproducts to the initial reaction stream as an oil based diluent, providing an effective means for reducing the steam to oil ratio required to operate the catalytic dehydrogenation reactor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application, pursuant to 35 U.S.C. §119(e), claims priority to U.S. Provisional Application Ser. No. 61/717,772, filed Oct. 24, 2012, which is herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    Embodiments disclosed herein relate generally to a process for the production of styrene monomers by the catalytic dehydrogenation of ethylbenzene. More specifically, embodiments disclosed herein relate to catalytic dehydrogenation of ethylbenzene at lower overall weight ratios of water to ethylbenzene (lower overall weight ratios of steam to oil) by recycling a portion of benzene and toluene present in the dehydrogenation effluent to the reactant feed stream as an oil-based diluent. 
       BACKGROUND 
       [0003]    The production of styrene by the catalytic dehydrogenation of ethylbenzene may be performed as illustrated by the process of  FIG. 1 . Feed stream  2 , which includes ethylbenzene and primary steam, is combined with superheated steam from  6  and fed into a dehydrogenation reactor  12 , which contains any appropriate solid-phase dehydrogenation catalyst. The effluent from reactor  12  is then cross-exchanged with superheated steam  8  and introduced to a second dehydrogenation reactor  14 . Following the dehydration reaction, the effluent from reactor  14  contains a mixture of styrene monomer, unreacted ethylbenzene, benzene, and toluene. The effluent is then passed through the a series of waste heat exchangers  4 , fed by vaporized feed stream  2  and steam  5 . Prior to offgas processing, effluent  15  is heat exchanged against cold water at  16  and  17 . 
         [0004]    The offgas component  36  is processed in offgas processing zone  26 . In zone  26 , the offgas  36  is compressed and passed through a flux oil scrubber  38  and a flux oil stripper  40  (stripped with steam  28 ). The non-codensables are recovered as offgas stream  30 , hydrocarbons are recycled from stripper  40  via overhead line  31 , and hydrocarbon condensates  32  are collected and returned for further processing along with dehydrogenation effluent  18 . 
         [0005]    The dehydrogenation effluent  18  is collected in a phase separator  19 , which isolates the crude styrene-containing product mixture  20  from the aqueous fraction  22 . The aqueous fraction  22  is distributed to a skimming tank for recovery of dissolved hydrocarbons and volatile organics, and the crude styrene product  20  is fractionated to obtain a purified styrene product, such as using multiple distillation columns (not shown). 
         [0006]    In commercial dehydrogenation processes, such as those described above with respect to  FIG. 1 , vaporized ethylbenzene is placed into contact with a fixed catalyst bed in the presence of steam, converting a portion of the ethylbenzene to styrene monomer and hydrogen gas. The dehydrogenation of ethylbenzene to form styrene is an endothermic equilibrium reaction, which limits the overall conversion of ethylbenzene because of the reversible nature of the process. As an added concern, the production of styrene monomer occurs simultaneously alongside various side reactions, such as the pyrolytic cracking of ethylbenzene to benzene and toluene, and the oligomerization of styrene monomers to form insoluble residues. While the latter can be suppressed with polymerization inhibitors, cracking must be limited by the reduction of reactor temperatures. 
         [0007]    However, due to the endothermic nature of the conversion of ethylbenzene to styrene, the temperature drops rapidly across the catalyst bed of the reactor, reducing or eliminating catalyst activity, leading to decreased production of styrene monomer. Within the field several approaches have been applied to overcome this limitation, including the use of multiple reactor stages with interstage heating. A typical application of interstage heating in multi-reactor setups is the use of indirect heating with steam to restore dehydrogenation effluent to reaction temperature prior to introduction to subsequent reactors. Thus, the inlet temperatures of the reactor are kept at range that is high enough to initiate catalytic conversion, but low enough to avoid excessive loss of ethylbenzene to decomposition reactions. 
         [0008]    The use of steam is widely known in the field as a method of introducing at least some of the heat needed to initiate conversion of ethylbenzene to styrene. Steam also acts as a diluent, reducing the partial pressure of the styrene and hydrogen within the reactor and shifting the reaction equilibrium towards the production of styrene. Steam within the reactor also functions as a means of extending the life of the catalyst by removing deposits from reaction surfaces. 
         [0009]    The mass steam to oil ratio, i.e., the ratio of steam to ethylbenzene (“oil”) contained in a feed stream on a weight basis (the S/O ratio), is an important factor in the dehydrogenation of ethylbenzene. In commercial applications, reducing the S/O ratio is desirable, because of the costs associated with energy consumption in the vaporization process. Furthermore, excessive use of steam dilutes the reaction mixture, reducing reactor capacity and negatively affecting the overall styrene output of the system. Thus, there have been ongoing efforts in the field to develop dehydrogenation catalysts with enhanced activity under reduced S/O ratios. 
         [0010]    Many commercially available catalysts operate at reduced S/O ratios, from about 1.3-1.7, with some catalysts capable of operating at a S/O ratio as low as about 1.0. However, while improvements in the catalysts have reduced the need for lower partial pressures and continuous decoking of the catalysts, because of the multiple roles steam plays in the reaction, further limiting steam feed (i.e., further lowering the S/O ratio) presents other challenges. 
       SUMMARY OF INVENTION 
       [0011]    In one aspect, embodiments disclosed herein relate to a process for the dehydrogenation of ethylbenzene. The process may include: (a) passing a mixture of ethylbenzene, steam, and an oil-based diluent comprising at least one of benzene and toluene through a catalytic dehydrogenation reactor to produce a dehydrogenation effluent comprising unreacted ethylbenzene, styrene monomer, benzene, toluene and water; (b)separating the dehydrogenation effluent to recover a water fraction and a hydrocarbon fraction comprising ethylbenzene, styrene monomer, benzene, and toluene; (c) fractionating the hydrocarbon fraction to recover a styrene fraction and one or more fractions comprising ethylbenzene, benzene, and toluene; and (d) returning at least a portion of benzene, toluene, or a combination thereof in the one or more fractions recovered in step (c) to step (a) as the oil-based diluent. 
         [0012]    In another aspect, embodiments disclosed herein relate to a process for the dehydrogenation of ethylbenzene. The process may include: (a) passing a mixture of ethylbenzene, steam, and an oil-based diluent comprising at least one of toluene and benzene through a catalytic dehydrogenation reactor to produce a dehydrogenation effluent comprising unreacted ethylbenzene, styrene monomer, benzene, toluene and water; (b) separating the dehydrogenation effluent to recover a water fraction and a hydrocarbon fraction comprising ethylbenzene, styrene monomer, benzene, and toluene; (c) separating the hydrocarbon fraction to recover a styrene fraction and a fraction comprising ethylbenzene, benzene, and toluene; and (d) recycling at least a portion of the fraction comprising ethylbenzene, benzene, and toluene to step (a) as an oil-based diluent. 
         [0013]    In yet another aspect, embodiments disclosed herein relate to a system for the dehydrogenation of ethylbenzene. The system may include (a) a conduit for feeding a mixture of ethylbenzene, an oil based diluent comprising at least one of benzene and toluene, and steam to a catalytic dehydrogenation reactor to produce a dehydrogenation effluent comprising unreacted ethylbenzene, styrene monomer, benzene, toluene and water; (b) a first separation system for separating the dehydrogenation effluent to recover a water fraction and a hydrocarbon fraction comprising ethylbenzene, styrene monomer, benzene, and toluene; (c) a second separation system for fractionating the hydrocarbon fraction to recover a styrene fraction and one or more fractions comprising ethylbenzene, benzene, and toluene; and (d) a conduit for feeding at least a portion of the one or more fractions comprising ethylbenzene, benzene, toluene, or a combination thereof to the dehydrogenation apparatus of step (a) as the oil-based diluent. 
         [0014]    In processes where the total amount of steam is reduced, temperature drops quickly as the reaction proceeds in the reactor, due to the loss of the heat previously provided by the steam fraction. At lower temperatures current catalysts exhibit reduced dehydrogenation efficiency, lowering styrene yields, leading to increased production times in order to generate the same amount of styrene produced at higher S/O ratios, thereby eliminating the cost benefit of reducing steam input. Catalyst performance is also compromised by the absence of diluent, shortening the catalyst lifetime due to poisoning by the buildup of hydrocarbon deposits. 
         [0015]    The processes and systems described herein advantageously add oil-based diluents into the reactant feed that provide the additional heat necessary to increase reactor temperatures and styrene yield in reduced steam conditions. By recycling a fraction of the benzene and toluene generated following the removal of styrene as disclosed herein, the dehydrogenation reactor can advantageously be operated at a steam/oil ratio of 1.0 or below. 
         [0016]    As a diluent, benzene and toluene are stable at higher reactor temperatures, are compatible with the ethylbenzene reactant, and reduce the further production of benzene and toluene byproducts in accordance with Le Chatelier&#39;s Principle. By introducing vaporized diluent into the reactant feed, the temperature remains high enough to preserve catalyst activity, promoting complete conversion to products, and overcoming the primary limitations of the reduction of the S/O ratio. Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a simplified process flow diagram of a prior art process for the dehydrogenation of ethylbenzene to form styrene. 
           [0018]      FIG. 2A and 2B  are simplified process flow diagrams of a process for the production of styrene monomer (SM) according to embodiments disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Referring to  FIG. 2A , a method for the production of styrene from the catalytic dehydrogenation of ethylbenzene is illustrated. Feed stream  202 , which includes ethylbenzene and primary steam, is combined with oil based diluent  150  and superheated steam  206  and fed into a dehydrogenation reactor  212 , which contains any appropriate solid-phase dehydrogenation catalyst. 
         [0020]    Reactor setup can vary from multiple beds contained in a single reactor, or single beds in multiple reactors, or a mixture of these arrangements. As illustrated, two reactors  212 ,  214  in series are used for the desired conversion. The effluent from reactor  212  is cross-exchanged with superheated steam  208  and introduced to a second dehydrogenation reactor  214  for continued reaction. Following the dehydration reaction, the effluent from reactor  214  contains a mixture of styrene monomer, unreacted ethylbenzene, benzene, and toluene. The effluent is then passed through a series of waste heat exchangers  204 , fed by vaporized feed stream  202  and steam  205 . Prior to offgas processing in zone  226 , cooled effluent  215  is heat exchanged against cold water in exchangers  216 ,  217  to further reduce the temperature of the effluent and condense the hydrocarbons. 
         [0021]    In offgas processing zone  226 , the offgas fraction  236  (uncondensed reactor effluent) is compressed and processed through a flux oil scrubber  238  and a flux oil stripper  240 . The non-condensable lights are evacuated via stream  230 , recycle hydrocarbons (C6&#39;s-C8&#39;s or higher, for example) are recovered in the stripper overheads and combined with steam  228  for combination with effluent  215  and further processing. The recirculating flux oil may be recovered as a bottoms from stripper  240  and recycled as the scrubbing fluid in flux oil scrubber  238  (accumulated heavies may be purged, although not shown). Condensates  232  are also collected and returned for further processing along with the dehydrogenation effluent  218 . 
         [0022]    The dehydrogenation effluent  218  is collected in a phase separator  219 , which isolates the crude styrene-containing product mixture  220  from the aqueous fraction  222 . Aqueous fraction  222  is distributed to a skimming tank for the recovery of dissolved hydrocarbons and volatile organics, and the crude styrene hydrocarbon mixture  220  is processed to obtain a purified styrene as illustrated in  FIG. 2B . 
         [0023]    Referring now to  FIG. 2B , following removal of offgas and aqueous condensates, the crude styrene-containing product  220  is mixed with a polymerization inhibitor  102  and fed into a first distillation column  104  for separation of the dehydration effluent into a styrene-rich bottoms  106  and an overheads fraction  108 , including the unreacted ethylbenzene, benzene and toluene. In some embodiments, overheads  108  may be used to partially vaporize reactant feed stream  110  via cross exchange with overheads fraction  108  in exchanger  152 . 
         [0024]    Overheads fraction  108  is then condensed (or further condensed) via heat exchanger  109 , a portion of which is returned to column  104  as reflux. The remaining overhead condensate is recovered via flow line  114 . 
         [0025]    A portion of the ethylbenzene, benzene, and toluene in stream  114  is used as oil based diluent  150  and combined with the ethylbenzene/water reactant feed stream  110 / 202  (as shown in  FIG. 2A ). Ethylbenzene in stream  150  is a recycled reactant, while the toluene and benzene are an oil-based diluent. Oil-based diluent  150  may be combined with the ethylbenzene reactant stream prior to vaporization, and/or may be vaporized separately prior to admixture with reactant stream  202 . 
         [0026]    The remainder of stream  114  is then fed to distillation column  112  for further separation. In column  112 , ethylbenzene is recovered as a bottoms fraction  124  and the lower boiling benzene and toluene reaction by-products are recovered as an overheads fraction  120 . The ethylbenzene-containing bottoms fraction  124  may be recycled as additional reactant for the dehydrogenation reaction. 
         [0027]    Overheads fraction  120  is then condensed, a portion of which is fed via stream  122  to distillation column  126 . In column  126 , the benzene and toluene are separated, the benzene being recovered as overheads fraction  127  and the toluene product being recovered as bottoms fraction  130 . Overheads fraction  127  may then be condensed, a portion being returned to column  126  as reflux, and the remaining being recovered as benzene product stream  128 . 
         [0028]    Styrene-rich fraction  106  is passed from column  104  to a styrene purification column  132  for separation of the styrene product from oligomers and other heavies. The substantially pure styrene monomer product is recovered as an overheads, condensed and returned as reflux to the column or isolated as styrene product stream  134 . The bottoms fraction  138 , including styrene, oligomerized styrene, and tars, are vaporized and returned to the column as reboil, or transferred to thin film evaporator  140 . Within thin film evaporator  141 , steam  142  vaporizes the low boiling volatiles, such as styrene monomer, which are recovered and returned to column  132 , while oligomers and tar exit as heavies stream  144 . 
         [0029]    As described above, oil-based diluent may be provided to the dehydrogenation reactors via flow stream  150 . Oil-based diluent may alternatively or additionally be obtained from other streams along the distillation train. For example, it may be advantageous to recycle a benzene and/or toluene-containing vapor draw from one or more of the distillation columns in order to obviate the need for vaporization prior to admixture with the ethylbenzene feed stream. For example, in  FIG. 2B , a vapor draw may be taken from streams  108 ,  120 ,  127 , or a combination thereof and recycled to reactant stream  202  as an oil-based diluent. 
         [0030]    In yet other embodiments, benzene and/or toluene-containing fractions may be diverted from a liquid stream and vaporized prior to use as a return to the ethylbenzene feed stream. For Example, in  FIG. 2B , a fraction of the liquid streams  114 ,  124 ,  122 ,  130 ,  128 ,  134 , or a combination thereof may be recycled as an oil-based diluent. 
         [0031]    When a portion of a liquid draw is returned as the oil-based diluent, such benzene and/or toluene-containing fractions may be vaporized or partially vaporized by recovering heat from a process stream associated with the distillation column(s) prior to admixture with the ethylbenzene feed stream as an oil-based diluent. Suitable heat sources would include excess steam from heat exchangers  107 ,  125 ,  131 ,  139 , steam condensate stream  146 , cross exchange with product streams  106 ,  124 ,  130 ,  144 , or heat exchange with appropriate overheads streams. 
         [0032]    In some embodiments, the amount of oil-based diluent in the feed may be in the range from about 5 wt. % to about 50 wt. %, based on the total feed to the reactors (including steam, ethylbenzene, and the oil-based diluent, which may include toluene, benzene, or a combination thereof). In other embodiments, the amount of oil-based diluent in the feed may be in the range from about 10 wt. % to about 40 wt. %, based on the total feed to the reactors; from about 15 wt. % to about 35 wt. % in other embodiments; and from about 20 wt. % to about 30 wt. % in yet other embodiments. 
         [0033]    Using oil-based diluents according to embodiments disclosed herein, the catalytic dehydrogenation reactors may be operated at a steam to oil (EB) ratio (S/O ratio) of less than 1.0; in other embodiments, the S/O ratio may be in the range from about 0.30 to about 0.75; in other embodiments, the S/O ratio may be in the range from about 0.45 to about 0.55; and in yet other embodiments the S/O ratio may be in the range from about 0.48 or 0.49 to about 0.51 or about 0.52, such as about 0.5. 
         [0034]    The operating temperature of the dehydrogenation reactor should be in a range from about 500° C. to about 1000° C., preferably in a range from about 550° C. to about 750° C., and more preferably in a range from about 600° C. to about 650° C. In some embodiments, the dehydrogenation reactor pressure may vary from about 40 to about 80 kPa. It is important that sufficient pressure be maintained at the reactor inlet to overcome the pressure drop through the catalyst bed(s) contained in the reactor vessel or in separate vessels if each such bed is contained in a separate reactor. Suitable catalysts include palladium oxide, platinum metal , supported palladium, molybdenum-bismuth oxide, ferrous oxide-potassium oxide, other metal oxides and/or sulfides, including those of calcium, lithium, strontium, magnesium, beryllium, zirconium, tungsten, molybdenum, titanium, hafnium, vanadium, aluminum, chromium, copper, and mixtures of two or more including chromia-alumina, alumina-titania, alumina-vanadia, etc. Dehydrogenation may be conducted at atmospheric pressure, although in some cases, subatmospheric or superatmospheric pressure may be desirable. 
         [0035]    As a diluent, benzene and toluene are stable at higher reactor temperatures, and are compatible with the ethylbenzene reactant. By introducing vaporized diluent into the reactant feed, the temperature remains high enough to preserve catalyst activity, promoting complete conversion to products and overcoming the primary limitations of the reduction of the S/O ratio, the oil-based diluent acting as an additional heat source. 
         [0036]    Recycled benzene and toluene in the diluent stream may also increase the efficiency of the catalytic conversion. In accordance with Le Chatelier&#39;s principle the increased concentration of benzene and toluene may shift the reaction equilibrium to favor the production of styrene monomer, resulting in an overall decrease in the production of byproducts. 
         [0037]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.