Patent Publication Number: US-7713386-B2

Title: Apparatus for producing ethylbenzene or cumene

Description:
FIELD OF THE INVENTION 
     The invention is directed to an apparatus for producing ethylbenzene or cumene using a dividing wall distillation column. 
     BACKGROUND OF THE INVENTION 
     Ethylbenzene is a valuable product that is used mainly for the manufacture of styrene monomer. Most ethylbenzene is produced by alkylation of benzene with ethylene. A byproduct also produced is polyethylbenzene. Therefore, ethylbenzene production processes contain two reaction sections, alkylation and transalkylation. The polyethylbenzenes produced from minor side reactions are recycled back to the transalkylation section and reacted with benzenes to produce more ethylbenzene. The alkylator and transalkylator effluents undergo separation operations to separate recycle benzene, ethylbenzene product, recycle polyethylbenzene and by-product streams using distillations. Traditionally three distillation columns are used. The first is typically a benzene column, used to recover excess benzene from the reactor effluents. The benzene column overhead, which is largely benzene, is typically recycled to the alkylator and transalkylator. The second distillation column is typically an ethylbenzene column used to recover the ethylbenzene product from the benzene column net bottoms. The ethylbenzene product is recovered as overhead, typically the net overhead, from the ethylbenzene column. The ethylbenzene product may be routed directly as feedstock to a styrene processes unit, or may be sent to storage. The third distillation column is usually a polyethylbenzene column used to recover recycle polyethylbenzene from the ethylbenzene column bottoms stream. Polyethylbenzene is recovered in the overhead of the polyethylbenzene column and is typically recycled to the transalkylator. The high boiling bottoms, flux oil, is usually cooled and sent to storage. Optionally, a fourth column, a light ends column, may be used to remove a small amount of light ends, light non-aromatics, and water from the recycle benzene stream. 
     Cumene, or isopropylbenzene, is a valuable product that is used mainly for the manufacture of phenol and acetone. Cumene has been produced by a catalytic process using a solid phosphoric acid that is made by impregnating kieselguhr with phosphoric acid. Now, zeolitic catalysts are used to produce higher quality cumene at a lower investment cost. 
     In a typical commercial process for the production of cumene, liquid benzene and liquid propylene are charged into an alkylation zone containing one or more reactors containing an alkylation catalyst. In order to minimize the production of polyalkylated products of benzene it has been the practice to maintain a molar excess of benzene throughout the reaction zone ranging from about 4:1 to about 16:1, and more preferably about 8:1 of benzene to propylene. Two competing reactions with the desired production of isopropylbenzene have created problems in some commercial processes. One of these has been the formation of polyalkylated benzenes such as di- and triisopropylbenzene rather than the desired monoalkylated product. This competing reaction may be partially controlled by employing large molar excesses of benzene. However, a transalkylation reactor is employed to react polyalkylated benzenes with benzene to form additional cumene. The other competing reaction causing losses in the yield of cumene based on propylene reactant charged is the formation of oligomers of propylene such as propylene dimer and trimer which occur to a limited extent even with the large molar excesses of benzene present. Propylene trimers and some of the propylene tetramers boil with cumene. Since the presence of these olefins interfere with the oxidation reaction used to make phenol from cumene, this oligomerization side reaction must be minimized to make a high purity product. 
     The alkylator and transalkylator effluents undergo separation operations to separate benzene, cumene product, polyisopropylbenzene, and by-product streams using distillation columns. Traditionally three distillation columns are used. The first is typically a benzene column, used to recover excess benzene from the reactor effluents. The benzene column overhead, which is largely benzene, is typically recycled to the alkylator and transalkylator. The second distillation column is typically a cumene column used to separate the cumene product from the benzene column bottoms. The cumene product is recovered as overhead from the cumene column. The cumene product may be used in applications such as phenol or acetone processes, or may be sent to storage. The third distillation column is usually a polyisopropylbenzene column used to recover polyisopropylbenzene from the bottoms of the cumene. Polyisopropylbenzene is recovered as overhead from the polyisopropylbenzene column and is typically recycled to the transalkylator. The high boiling bottoms, the heavy ends, is usually cooled and sent to storage. 
     The present invention provides an improvement over current process flow schemes by replacing the benzene column and the ethylbenzene or cumene column with a single divided wall column. The resulting advantages include a savings in the high pressure steam typically used in the reboilers, a reduction in condenser duty, a capital costs savings due to a reduction in equipment count and heat exchanger surface area, and a higher ethylbenzene recovery. Additional advantages include a reduction in plot space required, lower flare equipment, and less hydrocarbon inventory which improves the inherent safety of the process unit. 
     The dividing wall or Petyluk configuration for fractionation columns was initially introduced some 50 years ago by Petyluk et al. A recent commercialization of a fractionation column employing this technique prompted more recent investigations as described in the article appearing at page s14 of a  Supplement to The Chemical Engineer,  27 Aug. 1992. 
     The use of dividing wall columns in the separation of hydrocarbons is also described in the patent literature. For instance, U.S. Pat. No. 2,471,134 issued to R. O. Wright describes the use of a dividing wall column in the separation of light hydrocarbons ranging from methane to butane. U.S. Pat. No. 4,230,533 issued to V. A. Giroux describes a control system for a dividing wall column and illustrates the use of the claimed invention in the separation of aromatics comprising benzene, toluene and orthoxylene. 
     Using a dividing wall column in the present invention provides significant advantages over ethylbenzene or cumene production processes that do not employ a dividing wall fractionation column, as is shown below. 
     SUMMARY OF THE INVENTION 
     An ethylbenzene or cumene generation apparatus requiring a dividing wall fractionation zone has been developed. The process apparatus has an alkylation zone, where a feed stream comprising at least ethylene or propylene and a first benzene recycle stream comprising at least benzene is contacted with an alkylation catalyst under alkylation conditions to convert at least a portion of the ethylene and benzene into ethylbenzene or the propylene and benzene into cumene and form an alkylation zone effluent comprising benzene and ethylbenzene or cumene. The apparatus also requires a transalkylation zone, where either (1) a polyethylbenzene recycle stream comprising at least polyethylbenzene and a second benzene recycle stream comprising at least benzene are contacted with a transalkylation catalyst under transalkylation conditions to convert at least a portion of the polyethylbenzene and benzene into ethylbenzene and form a transalkylation zone effluent comprising benzene and ethylbenzene or (2) polyisopropylbenzene recycle stream comprising at least polyisopropylbenzene and a second benzene recycle stream comprising at least benzene are contacted with a transalkylation catalyst under transalkylation conditions to convert at least a portion of the polyisopropylbenzene and benzene into cumene and form a transalkylation zone effluent comprising benzene and cumene. 
     The alkylation zone and the transalkylation zone are in fluid communication with a dividing wall fractionation column which is operated at fractionation conditions. The dividing wall fractionation column is divided into at least a first and a second parallel fractionation zone by a dividing wall, with the first and the second fractionation zones each having an upper and a lower end located within the fractionation column, with the first and second fractionation zones being in open communication at their upper ends with an undivided upper section of the fractionation column and in open communication at their lower ends with an undivided lower section of the fractionation column. Two reactor effluent conduits provide the fluid communication with the column at one or more intermediate points of the first fractionation zone. 
     A product conduit is in fluid communication with an intermediate point of the second fractionation zone of the dividing wall fractionation column and is used to remove a stream comprising ethylbenzene or cumene. A first end conduit is in fluid communication with the first end of the dividing wall fractionation column. The second benzene recycle stream is removed from an intermediate point of the second fractionation zone of the dividing wall fractionation column located between the product conduit and the first end of the dividing wall fractionation column. A second end conduit is attached to the second end of the dividing wall fractionation column and is used to remove a polyethylbenzene or polyisopropylbenzene stream. The second end conduit is in fluid communication with a polyalkylbenzene fractionation column to separate either the polyethylbenzene recycle stream from a flux oil stream or the polyisopropylbenzene recycle stream from a heavy aromatics stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The FIGURE is a schematic illustration of one embodiment of the present invention. The FIGURE does not show a number of details for the process arrangement such as pumps, compressors, valves, stabilizers and recycle lines which are well known to those skilled in the art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the FIGURE, the process of the invention requires two reactors, an alkylation reactor  2 , or alkylator, and a transalkylation reactor  4 , or transalkylator. Ethylene or propylene feedstock  6  and an excess of benzene  8  are introduced to alkylator  2 . A typical ethylene feedstock may contain polymer grade ethylene (99.9 vol-% min.) acetylene (10 ppm vol-% max.), acetylene (1 ppm vol.-% max.), dienes (1 ppm vol.-% max), propylene (25 ppm vol-% max.), and C3 and heavier components (100 ppm vol.-% max). A typical benzene feedstock may contain benzene (99.9 wt.-% min.) and toluene (0.05 wt.-% min). Alkylation reactors may be operated in the vapor phase, liquid-phase or mixed-phase. It is preferred to operate the alkylation reactor in the liquid phase. At the lower temperatures of the liquid phase operation, xylene impurities are not produced and an ethylbenzene or cumene product of superior quality is produced. In one embodiment, the temperature of the alkylation reactor is selected from the range of 100° C. to 310° C. (212 to 590° F.) and the pressure is selected from the range of 800 to 5100 kPa (116 to 740 psig). In a more specific embodiment the temperature is in the range of 150 to 280° C. (303 to 536° F.) or 120 to 280° C. (248 to 536° C.) and the pressure is in the range of from about 1000 to 3900 kPa (145 to 570 psia). Alkylation reactor  2  contains an effective amount of alkylation catalyst. Suitable alkylation catalysts include solid acid catalysts and preferably a solid oxide zeolite. Examples are zeolite beta, zeolite X, zeolite Y, mordenite, faujasite, zeolite omega, MCM-22, MCM-36, MCM-49, MCM-56 and UZM-8. Alkylation reactors, operating conditions and catalysts are known in the art and not further discussed here. 
     In alkylation reactor  2 , the benzene is alkylated with the ethylene or propylene to form ethylbenzene or cumene, respectively. Some polyalkylbenzenes, which are mainly di- and tri-substituted ethylbenzenes or propylbenzenes, are also formed. Benzene is fed to the alkylator in excess so that virtually all the ethylene or propylene is reacted. Therefore, when the feed is ethylene, the alkylation reactor effluent  10  contains primarily benzene, ethylbenzene and polyethylbenzenes. When the feed is propylene, some heavy aromatics (C 16  to C 24 ) may be formed, and alkylation reactor effluent  10  contains primarily benzene, cumene and polyisopropylbenzenes. 
     Transalkylation reactor  4  is used to transalkylate the polyalkylbenzene produced in the alkylation reactor and recycled in line  43  with benzene recycled in line  20  to form additional ethylbenzene. Suitable conditions and catalysts may be the same as described for the alkylation reactor. In one embodiment, the temperature is selected from the range of 170° C. to 270° C. (228 to 518° F.) and the pressure is selected from the range of 800 to 5100 kPa (116 to 740 psia). When the feed is ethylene, the transalkylation effluent  12  from transalkylation reactor  4  contains primarily benzene, ethylbenzene and polyethylbenzene. Transalkylation effluent  12 , for example, may contain from 30 to 70 wt.-% benzene, 10 to 50 wt.-% ethylbenzene, 5 to 30 wt.-% polyethylbenzene and from 0 to 5 wt-% flux oil. When the feed is propylene, the transalkylation effluent  12  from transalkylation reactor  4  contains primarily benzene, cumene, and polyisopropylbenzene. Suitable transalkylation catalysts include solid acid catalysts and preferably a solid oxide zeolite. Examples are zeolite X, zeolite Y, mordenite, faujasite, zeolite omega, MCM-22, MCM-36, MCM-49, MCM-56 and UZM-8. Transalkylation reactors, operating conditions, and catalysts are known in the art and not further discussed here. 
     Both alkylation effluent  10  and transalkylation effluent  12  are introduced to dividing wall distillation column  14 . Within the dividing wall distillation column are two parallel fractionation zones. A first fractionation zone occupies a large portion of the left-hand side of the mid-section of the distillation column. Note that the terms “left-hand” and “right-hand” are used herein as relative to the drawings. In actual practice the placement of the zones as to the left side or the right side of the column is not critical. This first fractionation zone is separated from a parallel second fractionation zone occupying the other half of the column cross section by a substantially fluid tight vertical wall  16 . The vertical wall is not necessarily centered in the column and the two fractionation zones may differ in cross sectional area or shape. The vertical wall divides a large vertical portion of the column into two parallel fractionation zones. The two zones are isolated from each other for the height of this wall, but communicate at both the top and bottom ends of the column. There is no direct vapor or liquid flow between the two fractionation zones through the dividing wall, but the upper end of the fractionation zone is open to the internal volume of the distillation column containing an undivided fractionation zone preferably having additional trays. Liquid may pass under the dividing wall at the bottom of the two fractionation sections although vapor flow is preferably restricted or controlled. Thus, vapor and liquid can freely move around the wall between the two portions of the column. 
     During operation, the components of both effluents are separated in the first fractionation zone with the more volatile compounds moving upward out of the left-hand first fractionation zone and emerging into the undivided upper portion of the distillation column. As with the first fractionation zone, the upper end of the right-hand second zone is in open communication with the upper section of the distillation column which may optionally contain additional fractionation trays extending across the entire column cross section. 
     The components of the effluents will separate according to boiling point or relative volatilities, which is the main factor in determining their behavior in the distillation column. The component having a relatively low boiling point is the benzene from each of the effluents. The mid-range boiling component is the desired product, ethylbenzene or cumene. The components having relatively high boiling points are the polyalkylbenzenes and any flux oil or heavy aromatics. 
     The transalkylation effluent  12  and the alkylation effluent  10  are introduced into a first vertical fractionation zone occupying a large portion of the left-hand side of the midsection of the distillation column. The effluents may be combined before being introduced, but advantages may be realized by introducing the effluents at different heights along the dividing wall distillation column. For example, the alkylation reactor effluent may contain a higher concentration of polyalkylbenzene and therefore it may be advantageous to introduce the alkylation reactor effluent at a relatively lower height along the column as compared to the transalkylation reactor effluent. In one embodiment, the alkylation zone effluent is introduced into the dividing wall fractionation column at an intermediate height which is in between the height at which the transalkylation zone effluent is introduced and the first end of the of the column where the overhead is withdrawn. 
     The benzene present in the effluents is driven upward in the first fractionation zone and enter the top section of the column where it is removed in overhead  18  and side draw  20 . In one embodiment, the dividing wall column is operated so that the overhead is at a temperature ranging from about 88° C. to about 104° C. (190 to 220° F.) and a pressure ranging from about 103 to 241 kPa (15 to 35 psia). In a more specific embodiment, the dividing wall column is operated so that the overhead is at a temperature of about 98° C. (208° F.) and a pressure of about 172 kPa (25 psia). Benzene in overhead  18  is removed from the top of the dividing wall column and passed through an overhead condenser  38  to form liquid delivered to receiver  22 . Recovered water stream  24  may be removed from receiver  22  for further processing. Receiver  22  may also have vent stream  21 . A liquid phase stream of benzene  26  is removed from the receiver and divided into a first portion  28  which is returned to the top of the dividing wall fractionation column as reflux and a second portion  8  which is recycled to the alkylation reactor  2 . Benzene may also be removed from dividing wall column in stream  20  which is a side draw. Benzene stream  20  may be recycled to transalkylation reactor  4 . Two benzene stream may be withdrawn from the benzene column because the catalysts typically used in the alkylation reactor can tolerate some water present during the alkylation reaction, but the catalysts typically used in the transalkylation reactor are less tolerant of water. Therefore, the overhead contains benzene which may be saturated with water and is appropriate for the alkylation reactor, while the side draw contains dry or semi-dry benzene which is appropriate for the transalkylation reactor. As used herein the term “rich” is intended to indicate a concentration of the indicated compound or class of compounds greater than 50 and preferably greater than 75 mol percent. 
     The bottom of the dividing wall fractionation column  14  also comprises an undivided fractionation zone. This zone can receive liquid draining from both the first and second fractionation zones. This liquid is subjected to distillation which drives the ethylbenzene or cumene upwards as vapor while concentrating the less volatile polyethylbenzene and flux oil into a liquid bottoms stream  34 . This separation is effected through the use of a reboiler  36  providing vapor to the bottom undivided fractionation zone. At least a portion of bottoms stream  34  is passed to polyalkylbenzene separation column  40 . The product ethylbenzene or cumene stream  32  is withdrawn from the dividing wall fractionation column in a side draw from the right-hand side fractionation zone. The product may be collected, or passed to another process system  50  which may be a process system such as a styrene, phenol or acetone generation process. 
     In a more specific embodiment of the invention, the undivided bottom section of the dividing wall fractionation column is depicted as separated from the two parallel fractionation zones by a gas flow control or gas trap out tray located just below the bottom of the wall. A slight gap at this point allows horizontal liquid flow between the parallel fractionation zones. This tray may have liquid sealed perforations allowing the normal downward flow of liquid, but its structure is such that the upward flow of vapor is at least greatly restricted or controlled. The tray may totally block the upward vapor flow. The use of this tray may provide a means to positively control the partition of the upward gas flow between the two fractionation zones, which is a prime means of controlling performance of the two zones. The vapor from the column bottom is, therefore, preferably routed from the column via a line and split between two lines which feed the vapor to the bottom of the two parallel fractionation zones separately. The gas flow may be controlled by one or more flow control valves or by adjusting the relative liquid levels in the bottom of the two zones. This is described in some detail in U.S. Pat. No. 4,230,533, hereby incorporated by reference. 
     A net bottoms stream  34  containing primarily polyalkylbenzene is passed to polyalkylbenzene column  40  for the separation of polyalkylbenzene into polyalkylbenzene column overhead  42 , and flux oil or heavy aromatics into polyalkylbenzene column net bottoms  44 . In one embodiment, where the feed to the overall process is ethylene, the column is operated so that the overhead temperature is from about 121° C. to about 138° C. (250 to 280° F.) and the pressure is from about 21 to about 23 kPa (3.0 to 3.3 psia). In a more specific embodiment the column is operated so that the overhead is at about 130° C. (266° F.) and 22 kPa (3.2 psia). Polyethylbenzene column overhead  42  containing primarily polyethylbenzene or polyisopropylbenzene is passed through receiver  46  and a portion may be returned to the top portion of polyalkylbenzene column  40  as reflux. The net overhead stream  43  is recycled to transalkylation reactor  4 . 
     In another embodiment of the invention, there may be a need to generate low pressure steam, usually for use elsewhere at the plant site. In this embodiment, cooling water in optional line  19  may be heat exchanged with dividing wall overhead stream  18 , second benzene recycle stream  20  using optional heat exchanger  21 , and product stream  32  using optional heat exchanger  23  to form low pressure steam in optional line  29 . If less steam is required, fewer heat exchange operations with fewer streams are employed. For example, water in line  19  may be exchanged only with overhead  18  to generate the steam; only with second benzene recycle stream  20  using optional heat exchanger  21  to generate low pressure steam; or only with product stream  32  using optional heat exchanger  23  to form low pressure steam. Any combination of heat exchange operations may be employed depending upon the application and the requirements for low pressure steam. 
     The theoretical modeling results of an ethylbenzene production process shown in Table 1 indicate that the separation performed in the present invention would compare favorably to that achieved using a conventional scheme employing a benzene column followed by an ethylbenzene column. The data is based solely upon engineering design calculations and indicates that two conventional fractionation columns (the benzene column and the ethylbenzene column) could be replaced with the dividing wall column of the present invention to provide significant benefits and costs savings. For example, the dividing wall column of the present invention requires a total reboiling duty of 20.4 MW (69.7 MMBTU/hr) versus 36.5 MW (124.7 MMBTU/hr) or 28 MW (96 MMBTU/hr) for the two conventional columns, depending upon the pressures of the conventional benzene column. The dividing wall column requires a total condenser duty of −41.7 MW (−141.6 MMBTU/hr) versus −46.6 MW (−159 MMBTU/hr) for the two conventional columns. At the same time, the dividing wall column requires a total number of stages of 56, versus 72 or 62 stages for the conventional columns, depending upon the pressure of the benzene column. Finally, the data shows there is a small increase in ethylbenzene recovery which translates into a smaller loss of ethylbenzene product. Therefore, the present invention, through the use of the dividing wall column reduces the capital costs as to the number of trays and equipment count as well as the utility costs as compared to a conventional fractionation column. 
     In a conventional process the benzene column may be operated at an overhead pressure of 672 kPa (97.5 psia) so that the condenser can be used to produce an LP stream that can be used elsewhere at the plant site, and the first simulation in the example was modeled at this higher overhead pressure in the benzene column. However, the dividing wall column was modeled in the example at an overhead pressure of 172 kPa (25 psia) so that the column could be reboiled with high pressure steam. The lower pressure gives the dividing wall column of the present invention two additional advantages over the conventional two column sequence with the benzene column being operated at a higher pressure. First, the feed streams will flash to a higher vapor fraction which will in turn, reduce the amount of vapor that must be generated in the column. Second, the relative volatilities in the column will increase, leading to an easier separation. For a complete comparison a second simulation was modeled with the benzene column of the conventional two column scheme operating at an overhead pressure of 172 kPa (25 psia). In Table 1, “Case 1” signifies the conventional benzene column followed by an ethylbenzene column where the benzene column is operated at an overhead pressure of 672 kPa (97.5 psia), and “Case 2” signifies the conventional benzene column followed by an ethylbenzene column where the benzene column is operated at an overhead pressure of 172 kPa (25 psia). As the data demonstrates, regardless of the operating pressure of the benzene column, the divided wall column of the present invention provides advantages and benefits. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Benzene Column 
                 EB Column 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Case 1 
                 Case 2 
                 Case 1 
                 Case 2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Overhead PkPa (psia) 
                 672 
                 (97.5) 
                 172 
                 (25) 
                 175 
                 (25.4) 
                 175 
                 (25.4) 
               
               
                 Overhead Toc (deg. F.) 
                 156 
                 (314) 
                 98 
                 (208) 
                 158 
                 (316) 
                 158 
                 (316) 
               
               
                 Cond QMW (MMBTU/hr) 
                 −35.1 
                 (−119.8) 
                 −36.0 
                 (−123) 
                 −11.4 
                 (−39) 
                 −10.5 
                 (−36) 
               
               
                 Bottoms Toc (deg. F.) 
                 236 
                 (457) 
                 174 
                 (346) 
                 226 
                 (439) 
                 226 
                 (439) 
               
               
                 Reb QMW (MMBTU/hr) 
                 25.5 
                 (87.2) 
                 14.6 
                 (50) 
                 11.0 
                 (37.5) 
                 13.4 
                 (46) 
               
            
           
           
               
               
               
               
               
            
               
                 Total # Stages 
                 38 
                 30 
                 34 
                 32 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Column Diameter M (ft) 
                 4.7 
                 (15.5) 
                 5.2 
                 (17) 
                 3.4 
                 (11) 
                 3.4 
                 (11) 
               
            
           
           
               
               
               
               
            
               
                   
                 (Stg 1-15) 
                 (Stg 1-15) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 5.0 
                 (16.5) 
                 4.0 
                 (13) 
               
            
           
           
               
               
               
               
            
               
                   
                 (Stg 16-38) 
                 (Stg 16-30) 
                   
               
            
           
           
               
            
               
                 EB in Benzene to TA Reactor (wt. 
               
               
                 %) 
               
               
                 Benzene in EB Product (ppmw) 
               
               
                 DEB in EB Product (ppmw) 
               
               
                 EB in bottoms (wt. %) 
               
               
                 EB Recovery 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 DWC Savings 
               
            
           
           
               
               
               
            
               
                   
                 Compared 
                 Compared 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Total 
                   
                 to 
                 to 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Case 1 
                 Case 2 
                 DWC 
                 Case 1 
                 Case 2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Overhead PkPa (psia) 
                 — 
                 — 
                 172 
                 (25) 
               
               
                 Overhead Toc (deg. F.) 
                 — 
                 — 
                 98 
                 (208) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Cond QMW 
                 46.5 
                 (−158.8) 
                 −46.5 
                 (−159) 
                 −41.5 
                 (−141.6) 
                 10.8% 
                 10.9% 
               
            
           
           
               
               
               
               
               
            
               
                 Bottoms Toc (deg. F.) 
                 — 
                 — 
                 223 
                 (433) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Reb QMW 
                 36.5 
                 (124.7) 
                 28.1 
                 (96) 
                 20.4 
                 (69.7) 
                 44.1% 
                 27.4% 
               
            
           
           
               
               
               
               
               
            
               
                 Total # Stages 
                 72 
                 62 
                 56 
                   
               
            
           
           
               
               
               
               
               
            
               
                 Column Diameter M (ft) 
                 — 
                 — 
                 5.8 
                 (19) 
               
            
           
           
               
               
               
            
               
                   
                 (Stgs 1-44) 
                   
               
            
           
           
               
               
               
            
               
                   
                 4.6 
                 (15) 
               
            
           
           
               
               
               
            
               
                   
                 (Stg 45-56) 
                   
               
            
           
           
               
               
               
               
               
            
               
                 EB in Benzene to TA Reactor 
                 2.5 
                 2.5 
                 2.5 
                   
               
               
                 (wt. %) 
               
               
                 Benzene in EB Product (ppmw) 
                 126 
                 126 
                 86 
               
               
                 DEB in EB Product (ppmw) 
                 50 
                 50 
                 50 
               
               
                 EB in bottoms (wt. %) 
                 2 
                 2 
                 1.6 
               
               
                 EB Recovery 
                 96.6 
                 96.5 
                 96.8 
               
               
                   
               
            
           
         
       
     
     Similarly, the theoretical modeling results of a cumene production process shown in Table 2 indicate that the separation performed in the present invention would compare favorably to that achieved using a conventional scheme employing a benzene column followed by a cumene column. The data is based solely upon engineering design calculations and indicates that two conventional fractionation columns (the benzene column and the cumene column) could be replaced with a dividing wall column to provide significant benefits and costs savings. Energy costs as well as the total number of stages are reduced with the dividing wall column as compared to the conventional two columns in series. 
     When designing distillation units, often the number of stages in a unit must be balanced with the energy requirements of the unit. In the theoretical modeling described herein, two different dividing wall column (DWC) designs were modeled. One (case 1) has a greater number of stages and a lower energy requirement, while the other (case 2) has a lower number of stages but a higher energy requirement. The design data for each of the systems modeled are provided in Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 2 Column Design 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Benzene 
                 Cumene 
                 DWC Design 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Column 
                 Column 
                 Case 1 
                 Case 2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 overhead pressure kPa, 
                 55(8)  
                 7(1) 
                 7(1) 
                 7(1) 
               
               
                 (psia) 
               
               
                 overhead temperature 
                  38(100) 
                 155(311) 
                  38(100) 
                  38(100) 
               
               
                 ° C., (° F.) 
               
               
                 Condensor work MW, 
                 4.57(15.6) 
                 4.89(16.7) 
                 4.50(25.6) 
                 7.76(26.5) 
               
               
                 (MMBTU/hr) 
               
               
                 Bottoms temperature 
                 199(391) 
                 233(452) 
                 235(455) 
                 234(454) 
               
               
                 ° C., (° F.) 
               
               
                 Reboiler work MW, 
                 4.66(15.9) 
                 4.42(15.1) 
                 7.41(25.3) 
                 7.70(26.3) 
               
               
                 (MMBTU/hr) 
               
               
                 Total # Stages 
                 36 
                 42 
                 67 
                 62 
               
               
                 Column Diameter 
                 183(6)  
                 213(7) 
                 259(8.5)  
                 274(9)  
               
               
                 cm(ft.)183(6) 
               
               
                   
               
            
           
         
       
     
     Table 3 below shows the results of the modeling. As clearly shown by the data, each of the dividing wall column designs provided both an energy savings and a reduction in the number of stages as compared to the conventional two column design. For example, the dividing wall column case 1 requires a total condenser duty of 25.6 MMBTU/hr, and the dividing wall column case 2 requires a total condenser duty of 26.5 MMBTU/hr versus 32.3 MMBTU/hr for the two conventional columns. Similarly, the dividing wall column case 1 requires a total reboiler duty of 25.3 MMBTU/hr, and the dividing wall column case 2 requires a total reboiler duty of 26.3 MMBTU/hr versus 31 MMBTU/hr for the two conventional columns. As to the number of stages, the dividing wall column case 1 requires 67 stages, and the dividing wall column case 2 requires 62 stages versus 78 total stages for the two conventional columns. Finally, the data shows there is a small increase in cumene purity and a decrease in the amount of benzene lost to vent. Therefore, the use of the dividing wall column reduces the capital costs as to the number of trays as well as the utility costs as compared to a set of conventional fractionation columns. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 2 Column 
                   
                   
               
               
                   
                 Design 
                 DWC Case 1 
                 DWC Case 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Total Cond QMW(MMBTU/hr) 
                 9.45(32.3) 
                 7.50(25.6) 
                 7.76(26.5) 
               
               
                 Total Reb QMW(MMBTU/hr) 
                 9.08(31)   
                 7.41(25.3) 
                 7.70(26.3) 
               
               
                 Total # of Stages 
                 78 
                 67 
                 62 
               
               
                 % Savings in Cond Q 
                   
                 20.74% 
                 17.96% 
               
               
                 % Savings in Reb Q 
                   
                 18.39% 
                 15.16% 
               
               
                 % Savings in # Stages 
                   
                 14.10% 
                 20.51% 
               
               
                 Cumene Purity (wt %) 
                 99.944 
                 99.955 
                 99.955 
               
               
                 Benz in Cumene (wt ppm) 
                 4 
                 4 
                 4 
               
               
                 Cymenes in Cumene (wt ppm) 
                 16 
                 16 
                 15 
               
               
                 Benzene Lost to Vent* (wt %) 
                 1.37 
                 0.18 
                 0.18 
               
               
                   
               
               
                 *based on benzene in fresh feed