Abstract:
The fouling in a styrene production process involving the dehydrogenation of ethylbenzene is reduced by removing polymerizable components of the gaseous dehydrogenation effluent prior to the condensation of the effluent in the main condenser system. This involves the scrubbing of the gaseous effluent with organic condensate from the main condenser system to remove styrene, divinylbenzene and other polymer precursors which may be present and sometimes some of the ethylbenzene. The scrubber may include a reboiler and stripping section and function as a full fractionator thereby reducing the need for downstream distillation.

Description:
FIELD OF THE INVENTION 
     This invention relates to a process for the production of styrene by the dehydrogenation of ethylbenzene in the presence of steam and more particularly to a method of reducing the fouling of certain process components due to polymer formation. 
     BACKGROUND OF THE INVENTION 
     In styrene manufacturing processes, many plants experience troublesome fouling and even plugging problems in certain equipment and particularly in the main condenser system, off-gas compressor and downstream cooler and lean oil scrubber/stripper system. This polymer formation is mainly due to the presence of uninhibited styrene and in many cases is aggravated by the presence of small concentrations of divinylbenzene and other polymer precursors produced along with styrene in the dehydrogenation reactor as a side reaction and also as a product of the dehydrogenation of diethylbenzene which may be present in the feed. 
     SUMMARY OF THE INVENTION 
     The invention relates to the reduction or elimination of the fouling in a styrene production process by removing polymerizable materials from the process stream upstream of the process components subject to fouling. More particularly, the invention relates to the scrubbing/prefractionation of the dehydrogenation effluent to remove divinylbenzene and styrene prior to the condensing of the effluent in the main condenser system with the scrubber/prefractionation being refluxed by organic condensate from the main condenser system. Alternately, the scrubber may include a reboiler and a stripping section and function as a full fractionator thereby reducing or eliminating the need for downstream distillation equipment and systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a process flow diagram of a prior art styrene production process involving the dehydrogenation of ethylbenzene. 
     FIG. 2 is a styrene production process flow diagram incorporating the scrubbing/prefractionating of the present invention. 
     FIGS. 3,  4  and  5  are process flow diagrams similar to FIG. 2 but illustrating modified embodiments of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present commercial process for the production of styrene comprises the dehydrogenation of ethylbenzene using a conventional catalyst for this purpose such as iron oxide and using conventional, known operating conditions. Typically, the dehydrogenation is carried out at 600° C. or higher using low pressure and dilution steam. FIG. 1 of the drawings generally illustrates such a prior art process flow diagram. A steam superheater  10  produces a major portion of the diluent steam  12  for the process at a steam temperature above the dehydrogenation temperature. The ethylbenzene feed  14  is vaporized in the vaporizer/reboiler  16  and passed to the ethylbenzene separator drum  18  where vapor and liquid are separated and the liquid  20  recycled to the vaporizer/reboiler  16 . The ethylbenzene vapor  22  is further heated in the waste heat exchanger  24  and fed to a conventional catalytic dehydrogenation reactor  26  along with the superheated diluent steam  12 . The effluent gas  28  from the dehydrogenation reactor  26  contains primarily styrene, hydrogen, unreacted ethylbenzene, divinylbenzene and small amounts of benzene, toluene, methane, ethane, carbon monoxide, carbon dioxide, various polymeric materials and tars as well as an aqueous component. The effluent gas  28  is partially cooled in the waste heat exchanger  28  against the incoming ethylbenzene and sometimes against other streams and then fed to the main condenser  30 . The styrene, unreacted ethylbenzene, divinylbenzene, polymeric materials, tars and the aqueous component are condensed while the hydrogen, methane, ethane and carbon monoxide and dioxide and most of the benzene and toluene remain in the gaseous phase. From the main condenser  30 , the now partially condensed effluent is fed to the phase separator  32 . The gaseous phase  34  is separated and treated by means including compression  36  followed by recovery of the benzene and toluene. 
     Also separated in the phase separator  32  is the aqueous phase  38 , which will normally be treated in a condensate stripper (not shown). The organic dehydrogenation mixture  40  from the separator  32  comprises primarily the crude styrene and the unreacted ethylbenzene which are fed to the distillation column  42  which is often referred to as an ethylbenzene/styrene monomer splitter. This distillation may be in a single column or a plurality of columns in series. The key separation is between the ethylbenzene and lighter materials  44  and the styrene and heaver materials  46 . The column is operated at reduced pressure to lower the distillation temperature and thereby reduce styrene polymerization. The ethylbenzene  48  is separated from the lighter materials  50  in the ethylbenzene recovery distillation column  52  and the ethylbenzene  48  is recycled. The styrene monomer product  54  is separated from the heavier materials  56 , primarily tar, in the styrene monomer recovery distillation column  58 . 
     The problem encountered with these prior art systems such as shown in FIG.  1  and described above is that polymers can form primarily in the main condenser  30  and all downstream equipment including the off-gas compressor  36 . This polymer formation is due to the presence of uninhibited styrene and aggravated by the likely presence of small concentrations of divinylbenzene and other polymer precursors produced along with styrene in the dehydrogenation reactor as a side reaction and/or as a product of the dehydrogenation of diethylbenzene which may be present in the feed. 
     One embodiment of the present invention is shown in FIG.  2 . In this embodiment, a scrubber  60  is added and the effluent gases  28  from the dehydrogenation reactor  26  are fed to the lower end of this scrubber. The scrubber may be any type of liquid/gas contactor such as a packed bed column. Fed into the top of the scrubber  60  is reflux  62  which comprises a portion of the organic dehydrogenation mixture  40  from the phase separator  32 . Scrubbing with this organic dehydrogenation mixture scrubs a significant amount of the styrene and divinylbenzene from the dehydrogenation effluent gases  18  and also forms a condensate aqueous phase. The aqueous phase is removed at  64  and either sent to separator  32  or combined with the aqueous phase  38  from the phase separator  32  for subsequent treatment as shown. Depending on the operating conditions chosen, the amount of aqueous phase formed in the scrubber will vary and in some cases may be totally avoided. In other cases, intermediate side draws from the scrubber can be used to withdraw the aqueous phase. The organic phase  66  from the scrubber  60  is fed to the distillation column  42  for separation of the ethylbenzene and styrene monomer. The overhead  44  from the distillation column  42  is relatively pure ethylbenzene which is combined with the ethylbenzene  48  from the ethylbenzene distillation column  52  for direct recycle. The styrene monomer stream  46  from the distillation column  42  is processed as in FIG. 1 in the distillation column  58 . The remaining overhead gases  68  from the scrubber  60  are fed to the main condenser just as in FIG.  1 . 
     In the invention shown in FIG. 2, most of the divinylbenzene and most of the styrene monomer are removed from the gaseous dehydrogenation effluent before feeding this gaseous effluent to the main condenser  30 . Removing most of these two materials eliminates the fouling and plugging problems in the main condenser  30  and the off-gas compressor  36  as well as the other downstream equipment and piping. An added advantage is that there is a partial separation in the scrubber  60  between the ethylbenzene and the styrene monomer. This permits a reduction in the size and duty of the ethylbenzene/styrene monomer splitter or distillation column  42  and in the size and duty of the ethylbenzene recovery distillation column  52 . A further advantage is that there is a drastic reduction in the styrene monomer concentration in the organic liquid in contact with the aqueous condensate in the separator  32  from over 60% to less than 2% which will tend to alleviate polymer fouling in the downstream condensate stripper system. 
     Another advantage is the absence of lights such as benzene, toluene and dissolved gases such as CO 2  in the feed to the ethylbenzene/styrene splitter distillation column  42 . This permits a higher temperature and increased driving force for the azeotropic boiler/condenser of the distillation column  42  to either reduce the surface requirement or reduce the operating pressure. Reducing the pressure would be another factor in reducing polymer formation in the column. Also, corrosion problems in the overhead system are reduced due to the absence of carbon dioxide. 
     Computer simulations of refluxed scrubber  60  indicate that about one quarter to one half of the ethylbenzene content of the dehydrogenation effluent  28  is prefractionated from styrene monomer in the scrubber  60  and the styrene content of the overhead  68  is reduced to less than 2 wt. % without any external heat source for the scrubber. As a result, the reboiler duty for the ethylbenzene/styrene monomer splitter  42  and the ethylbenzene recovery column are reduced significantly. 
     Another embodiment of the invention is shown in FIG. 3 in which a stripping section with a reboiler  70  is added to the bottom of the refluxed scrubber  60 . This serves to further fractionate ethylbenzene and lighter from styrene monomer and heavier to further reduce the reboiler duty of the ethylbenzene/styrene monomer splitter  42 . FIG. 4 illustrates a variation of this embodiment in which the size and duty of the reboiler  70  and the stripping section of the scrubber  60  are increased to the extent that there is a complete separation of ethylbenzene and styrene monomer thereby completely eliminating the need for a ethylbenzene/styrene monomer splitter. In this case, the bottoms  72  from the scrubber  60  are fed directly to the styrene monomer recovery distillation column  58  while the aqueous side stream  74  is combined with the aqueous phase  38  from the phase separation  32 . 
     A further embodiment of the invention is illustrated in FIG. 5 in which the primary separation in the scrubber  60  is between the toluene and lighter as overhead, now designated  76 , and the ethylbenzene and heavier as bottoms, now designated  78 . The bottoms  78  now contain essentially all of the ethylbenzene which is separated as the overhead stream, now designated  80 , from the ethylbenzene/styrene monomer splitter  42 . This completely eliminates the need for an ethylbenzene recovery distillation column  52  as included in the previous embodiments.