Abstract:
A process for making ethylbenzene and/or styrene by reacting toluene with methane in one or more microreactors is disclosed. In one embodiment a method of revamping an existing styrene production facility by adding one or more microreactors capable of reacting toluene with methane to produce a product stream comprising ethylbenzene and/or styrene is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/047,930, filed Mar. 13, 2008, now U.S. Pat. No. 8,071,836. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a process for the production of ethylbenzene and styrene. 
     2. Description of the Related Art 
     Styrene is an important monomer used in the manufacture of many of todays plastics. Styrene is commonly produced by making ethylbenzene, which is then dehydrogenated to produce styrene. Ethylbenzene is typically formed by one or more aromatic conversion processes involving the alkylation of benzene. 
     Aromatic conversion processes, which are typically carried out utilizing a molecular sieve type catalyst, are well known in the chemical processing industry. Such aromatic conversion processes include the alkylation of aromatic compounds such as benzene with ethylene to produce alkyl aromatics such as ethylbenzene. Typically an alkylation reactor, which can produce a mixture of monoalkyl and polyalkyl benzenes, will be coupled with a transalkylation reactor for the conversion of polyalkyl benzenes to monoalkyl benzenes. The transalkylation process is operated under conditions to cause disproportionation of the polyalkylated aromatic fraction, which can produce a product having an enhanced ethylbenzene content and a reduced polyalkylated content. When both alkylation and transalkylation processes are used, two separate reactors, each with its own catalyst, can be employed. The alkylation and transalkylation conversion processes can be carried out in the liquid phase, in the vapor phase or under conditions in which both liquid and vapor phases are present. 
     In the formation of ethylbenzene from alkylation reactions of ethylene and benzene, other impurities and undesirable side products may be formed in addition to the desired ethylbenzene. These undesirable products can include such compounds as xylene, cumene, n-propylbenzene and butylbenzene, as well as polyethylbenzenes, and high boiling point alkyl aromatic components, sometimes referred to as “heavies,” having a boiling point at or above 185° C. As can be expected, reduction of these impurities and side products is important. This is especially true in the case of xylene, particularly the meta and para xylenes, which have boiling points that are close to that of ethylbenzene and can make product separation and purification difficult. 
     Ethylene is obtained predominantly from the thermal cracking of hydrocarbons, such as ethane, propane, butane or naphtha. Ethylene can also be produced and recovered from various refinery processes. Ethylene from these sources can include a variety of undesired products, including diolefins and acetylene, which can act to reduce the effectiveness of alkylation catalysts and can be costly to separate from the ethylene. Separation methods can include, for example, extractive distillation and selective hydrogenation of the acetylene back to ethylene. Thermal cracking and separation technologies for the production of relatively pure ethylene can account for a significant portion of the total ethylbenzene production costs. 
     Benzene is obtained predominantly from the hydrodealkylation of toluene which involves heating a mixture of toluene with excess hydrogen to elevated temperatures (500° C. to 600° C.) in the presence of a catalyst. Under these conditions, toluene can undergo dealkylation according to the chemical equation: C 6 H 5 CH 3 +H 2 →C 6 H 6 +CH 4  This reaction requires energy input and as can be seen from the above equation, produces methane as a byproduct, which is typically separated and used as fuel within the process. 
     In view of the above, it would be desirable to have a process of producing ethylbenzene, and styrene, which does not rely on thermal crackers and expensive separation technologies as a source of ethylene. It would also be desirable if the process was not dependent upon ethylene from refinery streams containing impurities which can lower the effectiveness and can contaminate the alkylation catalyst. It would further be desirable to avoid the process of converting toluene to benzene with its inherent expense and loss of a carbon atom to methane. 
     SUMMARY 
     One embodiment of the present invention is a process for making ethylbenzene which involves reacting toluene and methane in one or more microreactors to form a first product stream comprising ethylbenzene and/or styrene. The first product stream may also have one or more of benzene, toluene, methane, or styrene present. The process may comprise at least one separation process for at least partial separation of the components of the first product stream. 
     Methane may be separated from the first product stream which may be recycled back to the microreactors or may be utilized as fuel within the process. Toluene may also be separated from the first product stream and recycled to the microreactors. At least a portion of the components of the first product stream can be further processed in a styrene production process. The reactors can include a reaction zone and can be capable of dissipating heat to maintain the reaction zone within a desired temperature range for reacting toluene and methane to form ethylbenzene and/or styrene. 
     A further embodiment of the invention is a method of revamping an existing styrene production facility by adding one or more microreactors capable of reacting toluene with methane to produce a new product stream containing ethylbenzene. The new product stream containing ethylbenzene may then be sent to the existing styrene production facility for further processing to form styrene. The existing styrene production facility can include separation apparatus to remove at least a portion of any benzene from the new product stream, an alkylation reactor to form ethylbenzene by reacting the benzene with ethylene, and a dehydrogenation reactor to form styrene by dehydrogenating ethylbenzene. 
     Yet another embodiment of the present invention is a process for making ethylbenzene which includes reacting toluene and methane in one or more microreactors to form a first product stream comprising one or more of ethylbenzene, styrene, benzene, toluene and methane. The first product stream is sent to a separation zone where at least a portion of any methane and toluene are removed for recycle to the one or more microreactors. At least a portion of the benzene is removed from the first product stream and at least a portion of the benzene removed is reacted with ethylene in an alkylation reactor to form ethylbenzene. The ethylbenzene is dehydrogenated in one or more dehydrogenation reactors to form styrene. 
     The one or more microreactors may have one or more reaction zones and be capable of dissipating heat to maintain one or more of the reaction zones within a desired temperature range to promote reacting toluene and methane to form ethylbenzene and/or styrene. The one or more microreactors can comprise a plurality of microstructured panels creating a reaction zone comprising a plurality of microchannels. A portion of the microstructured panels can create reaction zones comprising a plurality of reaction zone microchannels and a portion of the microstructured panels can create a plurality of cooling microchannels for the flow of a cooling medium capable of dissipating heat to maintain the reaction zones within a desired temperature range for reacting toluene and methane to form ethylbenzene. The plurality of microstructured panels can be arranged in an alternating manner so the reaction zone microchannels and the cooling microchannels are capable of dissipating heat to maintain the reaction zones within a desired temperature range for reacting toluene and methane to form ethylbenzene. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating a process for making ethylbenzene and styrene. 
         FIG. 2  is a schematic block diagram illustrating a process for making ethylbenzene and styrene according to an embodiment of the present invention. 
         FIG. 3  is an illustrated example of a microstructured panel. 
         FIG. 4  is an illustrated example of two microstructured panels, each having microchannels, one for reactants and the other for a cooling medium. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , there is illustrated a schematic block diagram of a typical alkylation/transalkylation process carried out in accordance with the prior art. A feed stream of toluene is supplied via line  10  to reactive zone  100  which produces product streams of methane via line  12  and benzene via line  14 . The benzene via line  14  along with ethylene via line  16  are supplied to an alkylation reactive zone  120  which produces ethylbenzene and other products which are sent via line  18  to a separation zone  140 . The separation zone  140  can remove benzene via line  20  and send it to a transalkylation reaction zone  160 . The benzene can also be partially recycled via line  22  to the alkylation reactive zone  120 . The separation zone  140  can also remove polyethylbenzenes via line  26  which are sent to the transalkylation reaction zone  160  to produce a product with increased ethylbenzene content that can be sent via line  30  to the separation zone  140 . Other byproducts can be removed from the separation zone  140  as shown by line  32 , this can include methane and other hydrocarbons that can be recycled within the process, used as fuel gas, flared or otherwise disposed of Ethylbenzene can be removed from the separation zone  140  via line  34  and sent to a dehydrogenation zone  180  to produce styrene product that can be removed via line  36 . 
     The front end of the process  300 , designated by the dashed line, includes the initial toluene to benzene reactive zone  110  and the alkylation reactive zone  120 . It can be seen that the input streams to the front end  300  include toluene via line  10 , ethylene via line  16  and optionally oxygen via line  15 . There can also be input streams of benzene from alternate sources other than from a toluene reaction, although they are not shown in this embodiment. The output streams include the methane via line  12  which is produced during the conversion of toluene to benzene in reactive zone  110  and the product stream containing ethylbenzene via line  18  that is sent to the back end of the process  400 . The back end  400  includes the separation zone  140 , the transalkylation reaction zone  160  and the dehydrogenation zone  180 . 
     Turning now to  FIG. 2 , there is illustrated a schematic block diagram of one embodiment of the present invention. Feed streams of toluene supplied via line  210  and methane supplied via line  216  are supplied to one or more microreactors  200  which produces ethylbenzene along with other products, which can include styrene. In some embodiments an input stream of oxygen  215  may be supplied to the microreactors  200 . The output from the microreactor  200  includes a product containing ethylbenzene which is supplied via line  218  to a separation zone  240 . The separation zone  240  can separate benzene that may be present via line  220  which can be sent to an alkylation reaction zone  260 . The alkylation reaction zone  260  can include a transalkylation zone. The separation zone  240  can also remove heavy molecules that may be present via line  226 . The alkylation reaction zone  260  can produce a product with increased ethylbenzene content that can be sent via line  230  to the separation zone  240 . Other byproducts can be removed from the separation zone  240  as shown by line  232 , this can include methane and other hydrocarbons that can be recycled within the process, used as fuel gas, flared or otherwise disposed of. Ethylbenzene can be removed from the separation zone  240  via line  234  and sent to a dehydrogenation zone  280  to produce styrene product that can be removed via line  236 . Any styrene that is produced from the reactive zone  200  can be separated in the separation zone  240  and sent to the dehydrogenation zone  280  via line  234  along with the ethylbenzene product stream, or can be separated as its own product stream, (not shown), bypassing the dehydrogenation zone  280  and added to the styrene product in line  236 . 
     The front end of the process  500  includes the one or more microreactors  200  which can be in series or parallel arrangements. The input streams to the front end  500  are toluene via line  210  and methane via line  216  and optionally oxygen via line  215 . The output stream is the product containing ethylbenzene via line  218  that is sent to the back end of the process  600 . The back end  600  includes the separation zone  240 , the alkylation reaction zone  260  and the dehydrogenation zone  280 . 
     A comparison of the front end  300  of the prior art shown in  FIG. 1  against the front end  500  of the embodiment of the invention shown in  FIG. 2  can illustrate some of the features of the present invention. The front end  500  of the embodiment of the invention shown in  FIG. 2  has a single microreactor zone  200  rather than the two reactive zones contained within the front end  300  shown in  FIG. 1 , the reactive zone  100  and the alkylation reactive zone  120 . The reduction of one reactive zone can have a potential cost savings and can simplify the operational considerations of the process. 
     Both front ends have an input stream of toluene, shown as lines  10  and  210 . The prior art of  FIG. 1  has an input stream of ethylene  16  and a byproduct stream of methane  12 . The embodiment of the invention shown in  FIG. 2  has an input stream of methane  216 . The feed stream of ethylene  16  is replaced by the feed stream of methane  216 , which is typically a lower value commodity, and should result in a cost savings. Rather than generating methane as a byproduct  12  which would have to be separated, handled and disposed of, the present invention utilizes methane as a feedstock  216  to the microreactor  200 . 
     A comparison of the back end  400  of the prior art shown in  FIG. 1  with the back end  600  of the embodiment of the invention shown in  FIG. 2  can further illustrate the features of the present invention. It can be seen that the back end  400  of the prior art shown in  FIG. 1  is essentially the same as the back end  600  of the embodiment of the invention shown in  FIG. 2 . They each contain a separation zone, an alkylation reaction zone and a dehydrogenation zone and are interconnected in the same or essentially the same manner. This aspect of the present invention can enable the front end of a facility to be modified in a manner consistent with the invention, while the back end remains essentially unchanged. A revamp of an existing ethylbenzene or styrene production facility can be accomplished by installing a new front end or modifying an existing front end in a manner consistent with the invention and delivering the product of the altered front end to the existing back end of the facility to complete the process in essentially the same manner as before. The ability to revamp an existing facility and convert from a toluene/ethylene feedstock to a toluene/methane feedstock by the modification of the front end of the facility while retaining the existing back end can have significant economic advantages. 
     The microreactor  200  of the present invention can comprise one or more single or multi-stage microreactors. In one embodiment the microreactor  200  can have a plurality of microreactors connected in series (series-connected microreactors). Additionally and in the alternative, the microreactors may be arranged in a parallel fashion. The microreactor  200  can be operated at temperature and pressure conditions to enable the reaction of toluene and methane to form ethylbenzene, and at a feed rate to provide a space velocity enhancing ethylbenzene production while retarding the production of xylene or other undesirable products. The reactants, toluene and methane, can be added to the plurality of series-connected microreactors in a manner to enhance ethylbenzene production while retarding the production of undesirable products. For example toluene and/or methane can be added to any of the plurality of series-connected microreactors as needed to enhance ethylbenzene production. 
     The microreactor  200  can be operated in the vapor phase. One embodiment can be operated in the vapor phase within a pressure range of 4 psia to 1000 psia. Another embodiment can be operated in the vapor phase within a pressure range of atmospheric to 500 psia. 
     The feed streams of methane and toluene can be supplied to the microreactor  200  in ratios of from 2 to 50 moles methane to toluene. In one embodiment the ratios can range from 5 to 30 moles methane to toluene. 
     In one embodiment of the invention oxygen is added to the microreactor  200  in amounts that can facilitate the conversion of toluene and methane to ethylbenzene and styrene. The oxygen content can range from 1% to 50% by volume relative to the methane content. In one embodiment the oxygen content can range from 2% to 30% by volume relative to the methane content. 
     In one embodiment the microreactor  200  of the present invention can comprise multiple microreactors and oxygen can be added to the plurality of series-connected microreactors in a manner to enhance ethylbenzene and/or styrene production while retarding the production of undesirable products. Oxygen can be added incrementally to each of the plurality of series-connected microreactors as needed to enhance ethylbenzene and/or styrene production, to limit the exotherm from each of the microreactors, to maintain the oxygen content within a certain range throughout the plurality of microreactors or to customize the oxygen content throughout the plurality of microreactors. In one embodiment there is the ability to have an increased or reduced oxygen content as the reaction progresses and the ethylbenzene and/or styrene fraction increases while the toluene and methane fractions decrease. There can be multiple series-connected microreactors which are arranged in a parallel manner. 
     The oxygen can react with a portion of the methane and result in a highly exothermic reaction. The heat generated by the exothermic reaction can be regulated to some extent by the use of microreactors which can have a large surface area to reactant contact area ratio. The small contact area for the reactants can result in a short residence time for the reaction, which in some embodiments can be as short as less than a second. The shortened residence time and large surface area to reactant contact area ratio can facilitate heat dissipation from the microreactor. These factors, along with the ability for incremental oxygen addition to the plurality of series-connected microreactors, can be used to control the reaction temperatures within a range to facilitate the production of ethylbenzene and/or styrene and reduce the production of undesired components. Microreactors with integrated cooling can also be used, thus a short residence time reactor with an integrated heat exchanger can be used. 
     When a plurality of series-connected microreactors are utilized, counter-flow micro heat exchangers can be used to dissipate heat and provide temperature control for the reaction. In one embodiment a plurality of series-connected microreactors are utilized and one or more counter-flow micro heat exchangers are located between two or more of the microreactors used to dissipate heat and provide temperature control for the individual microreactors and the overall reaction. One temperature range to facilitate the production of ethylbenzene and/or styrene is from 550° C. to 1000° C. Another temperature range to facilitate the production of ethylbenzene and/or styrene is from 600° C. to 800° C. The heat generated by the exothermic reaction can be removed and recovered to be utilized within the process. 
     In one embodiment the microreactor zone  200  of the present invention can comprise one or more single or multi-stage microreactors which can contain one or more single or multi-stage catalyst sites. The catalyst that can be used in the microreactor  200  can include any catalyst that can couple toluene and methane to make ethylbenzene and/or styrene and are not limited to any particular type. It is believed that the oxidation reaction of toluene and methane can be accelerated by base catalysis. In one non-limiting example the catalyst can comprise one or more metal oxides. In one non-limiting example the catalyst can contain a metal oxide which is supported on an appropriate substrate. It is believed that with a metal oxide catalyst the oxygen/oxide sites can function as the active reaction centers which can remove hydrogen atoms from the methane to form methyl radicals and from the toluene to form benzyl radicals. The C8 hydrocarbons can be formed as a result of cross-coupling between the resulting methyl and benzyl radicals. The catalysts may contain different combinations of alkali, alkaline earth, rare earth, and/or transition metal oxides. In another non-limiting example the catalyst can comprise a modified basic zeolite. In yet another non-limiting example the catalyst can be a base zeolite, such as an X, Y, mordenite, ZSM, silicalite or AIPO4-5 that can be modified with molybdenum, sodium or other basic ions. The zeolite catalyst may or may not contain one of more metal oxides. 
     A catalyst can be introduced into one or more parts of the process. In one embodiment the microchannels of the microreactor can have a catalyst deposited or impregnated on or within them. The catalyst can also be affixed to an article, such as a rod, that can be contained or inserted into the microreactor in a manner which can contact the catalyst with the reactant streams. Alternatively, a process for wash coating a carrier with catalyst can be used where the carrier is capable of contacting the reactants within one or more of the microreactors. The catalyst can be contacted with the reactants at one or more points of the plurality of series-connected microreactors. The catalyst can alternately be contacted with the reactants at one or more points between the plurality of series-connected microreactors, such as for example at a location between two microreactors in conjunction with a counter-flow micro heat exchanger. 
     Referring now to  FIG. 3 , the microreactor can comprise a number of microstructured panels  700  that can have recesses or channels of small depth that serve as flow channels or microchannels  710 . These types of microreactors can be similar to typical plate-and-frame type heat exchangers known in the art, but of much smaller size. In one embodiment the microreactor panels  700  can range from about 30 to 50 mm in length and from about 30 to 50 mm in height. The microreactor panels  700  can be constructed by micro-machining or etching a panel made of metals, silicon, glass or ceramic materials, which can be referred to as a substrate material. Microchannels  710  can be etched or otherwise formed in a pattern on the surface  712  of the substrate panel material. In one embodiment the number of microchannels formed on the surface of the panel can range from 10 to 5000. The microchannels  710  can be in fluid communication with openings through the panels which can serve as inlet  714  and outlet  716  passages between the microreactors and/or microchannels so that the reactants can enter and exit the microchannels. The panel  700  may also have pass-though holes  718 ,  720  that can allow a fluid or gas to pass through the panel  700  without being in contact with the panel inlet  714 , outlet  716  or the microchannels  710 . The pass-though holes  718 ,  720  in one embodiment have a diameter of from 0.5 mm to 2.0 mm. The width and depth of the microchannels  710  in one embodiment can range from 100 μm to 300 μm while the total depth of the panel  700  can range from 400 μm to 600 μm. In another embodiment the width and depth of the microchannels can range from 100 μm to 500 μm while the total depth of the panel  700  can range from 700 μm to 1 mm. In yet another embodiment the width and depth of the microchannel can range from 300 μm to 600 μm while the total depth of the panel  700  can range from 800 μm to 1.5 mm or more. The width and depth of the microchannels do not have to be consistent or have the same dimensions of the other microchannels. While in some embodiments the width may be of a larger dimension than the depth, in other embodiments the depth may be of a larger dimension than the width. If plugging is a concern, having the depth and width of the microchannels be of similar dimension to create a more uniform cross sectional flow area of the microchannel may be desired. 
     In yet another embodiment the microreactor panels  700  can range from about 300 mm to 900 mm in length and from about 300 mm to 900 mm in height. The width of the microchannel  710  of these larger panels can be as large as 5 mm while the depth of the microchannel  710  would still be limited to a dimension less than that of the substrate panel material. The size of the panels and dimensions of the microchannels can vary greatly while still being within the scope of the present invention. 
     Referring now to  FIG. 4 , in one embodiment a reactant inlet stream  740  supplies an inlet stream  742  to the microchannels  710  of panel  700 . The reactants can flow through the microchannels  710  and exit panel  700  in outlet stream  744  to combine in the product stream  750 . The reactant inlet stream  740  can pass through the opening  814  of panel  800  without being in contact with the fluids flowing through the microchannels  810  of panel  800 . The product stream  750  can likewise pass through the opening  816  of panel  800  without being in contact with the fluids flowing through the microchannels  810  of panel  800 . A cooling medium stream  840  supplies an inlet stream  842  to the microchannels  810  of panel  800 . The cooling medium can flow through the microchannels  810  and exit panel  800  in outlet stream  844  to combine in the cooling medium exit stream  850 . The cooling medium inlet stream  840  can pass through the opening  718  of panel  700  without being in contact with the reactants flowing through the microchannels  710  of panel  700 . The cooling medium outlet stream  850  can pass through the opening  720  of panel  700  without being in contact with the reactants flowing through the microchannels  710  of panel  700 . The microreactor would comprise a plurality of panels that are pressed together in a manner to enable the reactants and cooling medium stream to be contained within their respective flow paths and not in communication with each other. A gasket material, a solder material, or a brazing material can be used to provide a seal between the panels. 
     Multiple microreactors can be utilized in a facility. In one embodiment the number of panels can range from 2 to 100. In an alternate embodiment the number of panels can range from 100 to 3000. In a commercial scale petrochemical plant the number of panels that can be used can reach hundreds or thousands, with up to a million channels or more per reactor. 
     As can be seen in  FIG. 4 , in one embodiment the microreactor can comprise alternating panels, to provide reactant flow through the microchannels of every other panel, while a different fluid, such as a cooling medium, can be flowing through the alternate panels. The different fluid, such as a cooling medium, can be flowing through the alternate panels in a counter-flow or co-current flow in relation to the reactant flow. Dissipation of the exotherm is through the panel material that make up the microchannel walls containing the reactants and into the cooling medium that is flowing through the microchannels created by the adjacent panels. This enables a rapid heat dissipation and the ability to control the reaction temperature within the microchannel in a manner that conventional reactors can not achieve. 
     The substrate material used for panel construction can act as a catalyst, or the microchannels may be coated with a catalyst layer, for example by using a wash coating or thin-film deposition of a catalyst material within or adjacent to the microchannels. A catalyst material can also be placed within a recess of the panel material that is in fluid contact with the reactants flowing through the microchannels, such as just before the reactants enter the microchannels. 
     Other types of microreactors can be used within the scope of the present invention. The description of multiple panel microreactors is not meant to be a limiting example of the microreactor. Another microreactor that can be used is a Falling Film microreactor which utilizes a multitude of thin falling films flowing through a multi-channel reactor. 
     Microreactors can be provided by sources such as Atotech located in Berlin, Germany; Veloeys located in Plain City, Ohio, USA; Microinnova located in Graz, Austria; and Ehrfeld Mikrotechnik BTS GmbH located in Wendelsheim, Germany. Further, other types or brands of microreactors can be used in conjunction with the present invention. 
     The foregoing description of certain embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or limit the invention to the precise form disclosed, and other and further embodiments of the invention may be devised without departing from the basic scope thereof. It is intended that the scope of the invention be defined by the accompanying claims and their equivalents.