Abstract:
This invention is an improved distillation sequence for the separation and purification of ethylene from a cracked gas. A hydrocarbon feed enters a C2 distributor column. The top of the C2 distributor column is thermally coupled to an ethylene distributor column, and the bottoms liquid of a C2 distributor column feeds a deethanizer column. The C2 distributor column utilizes a conventional reboiler. The top of the ethylene distributor is thermally coupled with a demethanizer column, and the bottoms liquid of the ethylene distributor feeds a C2 splitter column. The ethylene distributor column utilizes a conventional reboiler. The deethanizer and C2 splitter columns are also thermally coupled and operated at a substantially lower pressure than the C2 distributor column, the ethylene distributor column, and the demethanizer column. Alternatively, a hydrocarbon feed enters a deethanizer column. The top of the deethanizer is thermally coupled to an ethylene distributor column, and the ethylene distributor column utilizes a conventional reboiler. The top of the ethylene distributor column is thermally coupled with a demethanizer column, and the bottoms liquid of the ethylene distributor column feeds a C2 splitter column. The C2 splitter column operates at a pressure substantially lower than the ethylene distributor column, the demethanizer column, and the deethanizer column.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under United States Department of Energy Cooperative Agreement No. DE-FC07-01ID14090. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     The use of distillation to purify products from olefins plants is well known in the art. Conventional distillation schemes typically have utilized “sharp-split” distillation, wherein each distillation column is used to make a sharp separation between adjacent components of a homologous series. In a sharp-split distillation sequence, each component leaves the distillation column in a single product stream, either as overheads or bottoms. An inherent inefficiency in sharp-split distillation can be observed by considering the number of phase changes necessary to produce a recoverable hydrocarbon component. For example, a hydrocarbon gas feed typically containing C1+ hydrocarbons, such as ethylene, is first condensed in a demethanizer, then revaporized in a deethanizer, and is finally condensed again as a liquid product from a C2 splitter. A total of three complete phase changes must be accomplished for all the ethylene. The same number of phase changes applies to ethane and propylene. 
     The number of phase changes needed to produce a hydrocarbon component such as ethylene can be reduced by utilizing a refinement upon conventional, sharp-split distillation. Such a refinement is known as distributed distillation. In a distributed distillation scheme, one or more of the feed components is “distributed” between the top and bottom of one or more distillation columns. Such schemes require less energy to operate than conventional sharp-split schemes. In addition, they provide additional degrees of freedom for energy optimization—namely the distribution ratio of the distributing components in each column. Finally, concepts of thermal coupling of columns can also be applied to olefins plant separations, further reducing energy requirements. Thermally coupled columns are those where at least some of the reboiling or condensing duty for one column is provided by a vapor or liquid sidedraw from another column. By doing so, the thermodynamically undesired “remixing” phenomenon can be minimized. 
     A discussion of distributed distillation that incorporates the features of thermal coupling is found in Manley (U.S. Pat. No. 5,675,054.) Manley recites fully thermally coupled embodiments for ethylene separation, including an embodiment that recites a front-end depropanizer ethylene recovery and purification process that utilizes full thermal coupling of the C2s distributor and ethylene distributor. The thermal coupling of the columns is integral to the claimed energy efficiency of this prior art process. It is important to note that all of the columns recited in Manley&#39;s embodiments operate at substantially the same pressure, with any differences in pressure due to typical hydraulic pressure drops through the columns, exchangers, and piping. Substantial differences in pressure between the columns would require vapor compression or liquid pumping between columns. 
     Manley recites that such a fully-coupled distributed distillation system has lower energy requirements than systems that are not thermally coupled. Conventional wisdom suggests that such an arrangement, being fully thermally coupled, would be more energy efficient than a scheme that has no couples or is only partially thermally coupled. 
     Surprisingly, we have found out that such a fully distributed distillation sequence is not as energy efficient as this invention. Two of the thermal couples taught by Manley, specifically the thermal couple between the C2 distributor and deethanizer columns and the thermal couple between the ethylene distributor and the deethanizer or C2 splitter, actually increase the energy requirement for the process when implemented in a conventional cracker with conventional refrigeration equipment. The distillation system of this invention, therefore, does not include these couples and represents an unexpected improvement in energy savings as compared to Manley. 
     In addition, it has been found that removing these two thermal couples allows the deethanizer/C2 splitter to be operated at a lower, more optimal pressure than the rest of the distillation sequence. The full thermal coupling recited by Manley, on the other hand, requires that all columns be operated at roughly the same pressure, or utilize energy intensive vapor recompression between columns. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, the hydrocarbon feed comprising hydrogen, methane, ethane, ethylene, propane, propylene, and optionally heavier components, is introduced into a C2 distributor to produce a first overhead and a first bottom stream. The first overhead stream is introduced into an ethylene distributor, and the first bottom stream is introduced into a deethanizer. The C2 distributor and the ethylene distributor are thermally coupled, but the C2 distributor and the deethanizer are not thermally coupled. The C2 distributor utilizes a conventional reboiler exchanger and is refluxed with a liquid side draw from the ethylene distributor. The hydrocarbon feed to the ethylene distributor is distributed as a second top stream and a second bottom stream. The second top stream is introduced into a demethanizer to produce a fourth top stream and fourth bottom stream, and the second bottom stream is introduced into a C2 splitter. The ethylene distributor and the demethanizer are thermally coupled, but the ethylene distributor and C2 splitter are not thermally coupled. The fourth top stream is sent for hydrogen recovery, and the fourth bottom stream is recovered as ethylene. The ethylene distributor utilizes a conventional reboiler exchanger and is refluxed with a liquid side draw from the demethanizer. The hydrocarbon feed to the deethanizer is distributed as a third top stream and a third bottom stream. The third top stream is introduced into the C2 splitter, and the third bottom stream can be introduced into a C3 splitter for recovery propane and propylene components. The deethanizer and the C2 splitter are thermally coupled. The hydrocarbon feeds to the C2 splitter are distributed as a fifth top stream as recoverable ethylene, and a fifth bottom stream for ethane recycling. 
     In another aspect of the invention, a hydrocarbon feed comprising hydrogen, methane, ethane, ethylene, propane, propylene, and optionally heavier components, is introduced into a deethanizer to produce a first top stream and a first bottom stream. The first top stream is introduced into an ethylene distributor to produce a second top stream and a second bottom stream. The deethanizer is refluxed with a liquid draw from the ethylene distributor. The second top stream is introduced into a demethanizer to produce a third top stream and a third bottom stream. The third top stream is sent for hydrogen recovery and the third bottom stream is recovered as ethylene. The ethylene distributor is refluxed with a liquid draw from the demethanizer. In addition, the ethylene distributor is reboiled with a reboiler exchanger. The second bottom stream is introduced into the C2 splitter to produce a fourth top stream and a fourth bottom stream. The fourth top stream is recovered as ethylene and the fourth bottom stream is sent for ethane recycling. 
     The process shall be described for the purposes of illustration only in connection with certain embodiments. However, it is recognized that various changes, additions, improvements and modifications to the illustrated embodiments may be made by those persons skilled in the art, all falling within the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  depicts a front-end depropanizer ethylene recovery and purification design containing both a C2s distributor and an ethylene distributor. 
         FIG. 2  depicts a front-end depropanizer ethylene recovery and purification design containing an ethylene distributor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , the feed stream to the process begins as an overhead stream from a depropanizer tower (not shown) and enters the process via stream  100 . Stream  100  comprises a mixture of hydrogen, methane, ethane, ethylene, propane, and propylene. Stream  100  has optionally been passed through a hydrogenation reactor in order to remove essentially all of the acetylene as well as portions of the methylacetylene and propadiene. The depropanizer overhead is optionally cooled in exchanger E 101  before entering column C 101  as stream  101 . 
     Column C 101  is a distillation apparatus that serves as a C2s distributor. It can be either a trayed or packed column. The overheads of the column exit in stream  102  and contain essentially all of the hydrogen and methane present in the column feed, as well as a fraction of the ethane and ethylene. Column C 101  is controlled such that little or no propane or propylene is contained in stream  102 . The bottoms of C 101  exit in stream  112  and contain essentially all of the propane and propylene present in the column feed, as well as the remainder of the ethylene and ethane. Column C 101  is controlled such that there is little or no methane in the column bottoms. 
     Stream  102  is fed to column C 102 , which acts as an ethylene distributor. Columns C 101  and C 102  are thermally coupled in that a liquid side draw from C 102 , depicted as stream  103 , provides reflux to C 101 . Stream  112  is fed to column C 104 , which acts as a deethanizer tower. The C2s distributor, depicted as column C 101 , and the deethanizer, depicted as column C 104 , are not thermally coupled. Column C 101  is reboiled in the conventional manner with reboiler exchanger E 102 . It has been surprisingly found that removing the thermal couple between columns C 101  and C 104  actually improves the energy efficiency of the process. Also important to this invention is that the pressure of stream  112  is decreased before it is fed to column C 104 . The figure shows this pressure reduction being accomplished through a pressure letdown valve, V 102 , though other methods are available and known to those skilled in the art. 
     The overheads of C 102  are removed as stream  104  and the bottoms are removed as stream  108 . The overheads of C 102  contain hydrogen, methane and ethylene and are fed to a demethanizer column C 103 . Columns C 102  and C 103  are thermally coupled in that a liquid side draw from C 103 , depicted as stream  105 , provides reflux to C 102 . Column C 103  can employ one or more side condensers, depicted in  FIG. 1  as E 104 . 
     The bottoms of C 102  contain ethylene and ethane and are fed to an ethylene/ethane (C2) splitter column, C 105 . Column C 102  is not thermally coupled with either C 104  or C 105 . Instead, column C 102  is reboiled in the conventional manner with reboiler exchanger E 103 . It has been surprisingly found that removing the thermal couple between columns C 102  and C 104  or C 102  and C 105  actually improves the energy efficiency of the process. Also important to this invention is that the pressure of stream  108  is decreased before it is fed to column C 105 . The figure shows this pressure reduction being accomplished through a pressure letdown valve, V 101 , though other methods are available and known to those skilled in the art. 
     The overheads of C 104  contain mixtures of ethylene and ethane and exit in stream  114 . This stream is fed to column C 105 . Columns C 104  and C 105  are thermally coupled in that a liquid side draw from C 105 , depicted as stream  115 , provides reflux to C 104 . Columns C 104  and C 105  are operated at a pressure that is significantly lower than that of columns C 101 , C 102 , and/or C 103 . The bottoms of Column C 104 , depicted as stream  116 , contain essentially all of the propylene and propane and are sent to a C3 splitter (not shown). 
     The overheads of C 105  contain product quality ethylene and are removed as stream  110 . Column C 105  is refluxed in the conventional manner with a condensing exchanger E 107 . Column C 105  is reboiled with exchanger E 108  and the bottoms contain ethane which can be recycled to the cracking furnaces. There are many ways in which column C 105  can be designed.  FIG. 1  shows a simple design where C 105  is reboiled in a conventional manner using a reboiler exchanger E 108 . 
     The overheads of C 103  contain hydrogen, methane and small amounts of ethylene. They are cooled and at least partially condensed to provide reflux for C 103 .  FIG. 1  shows this being accomplished with a standard partial condenser exchanger E 105  and separation drum. Other methods of supplying reflux can be employed (e.g. dephlegmators) and are well known to those skilled in the art. Overhead vapors from the partial condenser exit in stream  106  and are sent to a cryogenic section to recover refrigeration value and optionally a hydrogen product. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Stream Flows and Properties for FIG. 1 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 101 
                 102 
                 104 
                 106 
                 107 
                 108 
                 110 
                 111 
                 112 
                 114 
                 116 
               
               
                   
               
             
          
           
               
                 Temperature (Deg F.) 
                 −39.0 
                 −49.0 
                 −70.0 
                 −205.0 
                 −10.0 
                 2.5 
                 −33.6 
                 9.6 
                 24.3 
                 −13.6 
                 110.9 
               
               
                 Pressure (psia) 
                 340 
                 338 
                 323 
                 322 
                 323 
                 333 
                 220 
                 240 
                 345 
                 235 
                 240 
               
               
                 Molar flows (lb mol/hr) 
               
               
                 CO 
                 32.6 
                 34.1 
                 34.4 
                 32.6 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 HYDROGEN 
                 8253.1 
                 8378.6 
                 8378.3 
                 8253.1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 METHANE 
                 4714.5 
                 5356.1 
                 5554.3 
                 4712.6 
                 0.8 
                 0.7 
                 1.1 
                 0.0 
                 0.4 
                 0.4 
                 0.0 
               
               
                 ETHYLENE 
                 9572.9 
                 10944.9 
                 10074.5 
                 13.0 
                 3953.7 
                 3616.7 
                 5593.3 
                 12.9 
                 1989.5 
                 3829.7 
                 0.0 
               
               
                 ETHANE 
                 2562.5 
                 3008.6 
                 1.5 
                 0.0 
                 1.0 
                 1702.7 
                 1.4 
                 2559.3 
                 858.8 
                 2517.3 
                 0.8 
               
               
                 ACETYLENE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 PROPYLENE 
                 660.2 
                 0.7 
                 0.0 
                 0.0 
                 0.0 
                 1.7 
                 0.0 
                 1.5 
                 658.5 
                 0.2 
                 658.7 
               
               
                 PROPANE 
                 140.5 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 140.5 
                 0.0 
                 140.5 
               
               
                 PROPADIENE 
                 7.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 7.0 
                 0.0 
                 7.0 
               
               
                 METHYLACETYLENE 
                 3.5 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 3.5 
                 0.0 
                 3.5 
               
               
                 ISOBUTANE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 ISOBUTENE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 1,3-BUTADIENE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 BUTENE1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 n-BUTANE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T-BUTENE2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 C-BUTENE2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
               
             
          
         
       
     
     Contrary to the prior art, columns C 103  and C 105  are not thermally coupled. The bottoms of C 103  are reboiled in the conventional manner with reboiling exchanger E 106 . The bottoms stream  107  contains product-quality ethylene. The embodiment shown in  FIG. 1  has significant energy benefits over the prior art design. The features attributed to such an energy savings are the lack of thermal coupling of the bottoms section of the C2s distributor and the deethanizer, the lack of thermal coupling of the bottoms section of the ethylene distributor and the C2 splitter, and the operation of the deethanizer and C2 splitter at a substantially lower pressure than the other columns. Table 1 represents the compositions and properties of selected streams from  FIG. 2 . 
     As depicted in Table 1, the deethanizer and C2 splitter operate at a pressure substantially lower than the pressure of the other columns. 
     Replacing the thermal coupling between the C2 distributor and deethanizer with a separate reboiler on the C2 distributor is beneficial from an energy standpoint and results in a 382.1 horsepower (HP) savings in total energy. This energy savings is brought about because part of the deethanizer reboiler duty (requiring relatively high temperature heat) is shifted to a lower temperature level on the C2 distributor reboiler, where it becomes a useful heat sink for condensing 50 F propylene refrigerant. The changes in energy consumption can be found in Table 2. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Changes in energy consumption by replacing thermal coupling 
               
               
                 between the C2 distributor and the deethanizer 
               
             
          
           
               
                   
                 Without thermal coupling 
                 With thermal coupling 
                   
               
               
                   
                 (this invention) 
                 (Manley &#39;054) 
               
             
          
           
               
                   
                   
                 Duty 
                   
                 Duty 
                 Temp 
                   
                 Horsepower 
               
               
                 Column 
                   
                 (MMBTU/hr) 
                 Temp (F.) 
                 (MMBTU/hr) 
                 (F.) 
                 Utility 
                 Savings 
               
               
                   
               
             
          
           
               
                 C2 distributor 
                 Q reb   
                 7.19 
                 24.1 
                 0 
                 24.1 
                 50° F. 
                 448 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                 Deethanizer 
                 Q con   
                 8.42 
                 3.4 
                 7.73 
                 3.4 
                 25° F. 
                 −65.9 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                   
                 Q reb   
                 20.36 
                 134.9 
                 26.86 
                 134.9 
                 150 psi 
               
               
                   
                   
                   
                   
                   
                   
                 stream 
               
               
                   
                   
                   
                   
                   
                   
                 Net 
                 382.1 
               
               
                   
               
             
          
         
       
     
     In addition, replacing the thermal coupling between the ethylene distributor and C2 splitter with a separate reboiler on the ethylene distributor is beneficial from an energy standpoint in that removing the thermal couple costs very little energy, but allows other process changes that provide significant energy savings. The changes in energy consumption brought about by removing the thermal couple between the ethylene distributor and the C2 splitter can be seen in Table 3. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Changes in energy consumption by replacing thermal coupling 
               
               
                 between the ethylene distributor and the C2 splitter 
               
             
          
           
               
                   
                 Without 
                 With thermal 
                   
               
               
                   
                 thermal coupling 
                 coupling 
               
               
                   
                 (this invention) 
                 (Manley &#39;054) 
               
             
          
           
               
                   
                   
                 Duty 
                 Temp 
                 Duty 
                 Temp 
                   
                 Horsepower 
               
               
                 Column 
                   
                 (MMBTU/hr) 
                 (F.) 
                 (MMBTU/hr) 
                 (F.) 
                 Utility 
                 Savings 
               
               
                   
               
             
          
           
               
                 Ethylene distributor 
                 Q reb   
                 10.26 
                 0.8 
                 0 
                 0.8 
                 0° F. 
                 978.8 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                 C2 splitter 
                 Q con   
                 96.69 
                 −12.7 
                 93.89 
                 −12.7 
                 −45° F. 
                 −583.2 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                   
                 Q reb   
                 20.36 
                 134.9 
                 26.86 
                 134.9 
                 50° F. 
                 −464.8 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                   
                   
                   
                   
                   
                   
                 Net 
                 −68 
               
               
                   
               
             
          
         
       
     
     As seen here in Table 3, removing this thermal coupling causes little, if any, energy penalty. Removing this couple, however, does allow the deethanizer and C2 splitter to be operated at a lower, more efficient pressure, which results in a significant energy savings. When the aforementioned thermal couples are removed from the design, it is possible to operate the deethanizer and C2 splitter at a pressure lower than the rest of the columns. Operating these columns at a lower pressure is not possible with the fully coupled prior art, since lowering the C2 splitter pressure would require all other columns to be operated at lower pressure also, and any energy savings from a lower pressure C2 splitter would be offset by energy penalties elsewhere in the system. Operating the C2 splitter and deethanizer at a lower pressure than the other columns results in a significant energy savings because it reduces the condensor and reboiler duties and allows column reboiling and feed vaporizing to occur at lower temperatures, thus providing greater recuperating ability. These energy saving can be seen in Table 4 below. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Energy consumption with low pressure deethanizer and C2 splitter 
               
             
          
           
               
                   
                   
                 Low pressure 
                   
               
               
                   
                   
                 DeEth &amp; C2 
               
               
                   
                 Split feed &amp; thermal coupling 
                 splitter (this 
               
               
                   
                 (Manley &#39;054) 
                 invention) 
               
             
          
           
               
                   
                   
                 Duty 
                   
                 Duty 
                 Temp 
                   
                 Horsepower 
               
               
                 Column 
                   
                 (MMBTU/hr) 
                 Temp (F.) 
                 (MMBTU/hr) 
                 (F.) 
                 Utility 
                 savings 
               
               
                   
               
             
          
           
               
                 C2 splitter 
                 Q con   
                 96.69 
                 −12.7 
                 92.33 
                 −33.6 
                 −45° F. 
                 908.2 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                   
                 Q reb   
                 76.88 
                 31.1 
                 70.27 
                 9.6 
                 25° F. Propylene 
                 1921 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                 Ethane recycle 
                   
                 9.06 
                 −44 
                 10.84 
                 −44 
                 −45° F. 
                 370.8 
               
               
                   
                   
                   
                   
                   
                   
                 Propylene 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
               
                 Deethanizer 
                 Q con   
                 0 
                 3.4 
                 0 
                 −15.7 
                   
                 0.0 
               
               
                   
                 Q reb   
                 14.45 
                 134.9 
                 12.27 
                 110.7 
                 150 psi steam 
               
               
                   
                 Q feed   
                 6.7 
                 40 
                 6.6 
                 18.3 
                 25° F. Propylene 
                 210.9 
               
               
                   
                   
                   
                   
                   
                   
                 refrigerant 
               
             
          
           
               
                   
                 Net 
                 3411.0 
               
               
                   
                   
               
             
          
         
       
     
     Table 5 compares the propylene and ethylene system refrigeration horsepower required for the two designs for equivalent total ethylene production. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Ethylene and Propylene Refrigeration 
               
               
                 Compressor Energy Requirements 
               
             
          
           
               
                   
                 Manley ‘054 
                 Embodiment of FIG. 2 
               
               
                   
                   
               
             
          
           
               
                   
                 Total Refrigeration 
                 45,778 
                 42,053 
               
               
                   
                 Compressor Energy (HP) 
               
               
                   
                   
               
             
          
         
       
     
     The invention, as embodied in  FIG. 1 , saves a significant amount of energy over the prior art design. Those skilled in the art will also recognize that because the invention of  FIG. 1  contains fewer thermal couplings between columns, it will be easier to operate and control than the prior art design. 
     With reference to  FIG. 2 , the feed stream to the process begins as an overhead stream from a depropanizer tower (not shown) and enters the process via stream  200 . Stream  200  comprises a mixture of hydrogen, methane, ethane, ethylene, propane, and propylene. Stream  200  has optionally been passed through a hydrogenation reactor in order to remove essentially all of the acetylene, as well as portions of the methylacetylene and propadiene. The depropanizer overhead is optionally cooled in exchanger E 201  before entering column C 201 , as stream  201 . 
     Column C 201  is a distillation device that serves as a deethanizer column. It can be either trayed or packed. The overhead of the column exits as stream  202 , which contains essentially all of the hydrogen, methane, ethane, and ethylene. The bottoms of C 201  exit as stream  204  and contain all of the propane and propylene that enter column C 201 . This bottoms stream can be directed to further downstream purification columns if desired. 
     Stream  202  enters column C 202 , which acts as an ethylene distributor. Columns C 201  and C 202  are thermally coupled such that a liquid side draw from C 202 , depicted as stream  203 , provides reflux liquid to C 201 . The overheads of C 202  exit as stream  205  and contain essentially all of the hydrogen and methane that enter the column, as well as a portion of the ethylene. The ratio of ethylene to ethane in stream  205  is such that product-quality ethylene can be made without further separation of ethylene and ethane. 
     The bottoms of column C 202  exit in stream  207  and contain the remainder of the ethylene and essentially all of the ethane that enters C 202 . The pressure of stream  207  is reduced by a pressure letdown valve, V 201 , though other methods are available and known to those skilled in the art. Stream  207  is fed to column C 204 , which acts as an ethylene/ethane separation column. Columns  202  and  204  are not thermally coupled. Column  202  is reboiled using a conventional reboiler exchanger E 203 . Optionally, the feed to column C 204  can be split and partially vaporized in exchanger E 208 , as shown in  FIG. 2 . 
     The overheads of C 204  exit in stream  212  and contain product-quality ethylene. The bottoms of C 204  exit in stream  213  and contain ethane and possibly a small amount of ethylene. The overheads of column C 202 , depicted as stream  205 , enter column C 203 , which acts as a demethanizer. Columns C 202  and C 203  are thermally coupled such that a liquid sidedraw from C 203 , depicted as stream  206 , provides reflux liquid to C 202 . Column C 203  can employ one or more side condensers, depicted in  FIG. 2  as E 204 . 
     The overheads of C 203  contain hydrogen, methane and small amounts of ethylene. They are cooled and at least partially condensed to provide reflux for C 203 .  FIG. 2  shows this being done with a standard partial condenser exchanger E 205  and a separation drum. Other methods of supplying reflux (e.g. dephlegmators) can be employed and are well known to those skilled in the art. Overhead vapors from the partial condenser exit in stream  208  and are sent to a cryogenic section to recover refrigeration value and optionally a hydrogen product. Columns C 203  and C 204  are not thermally coupled. The bottoms of C 203  are reboiled in the conventional manner with reboiling exchanger E 206 . The bottoms stream of C 203 , depicted as stream  209 , contains product-quality ethylene. 
       FIG. 2  retains the key features of  FIG. 1 , including the lack of thermal coupling of the bottoms section of the ethylene distributor and the operation of the C2 splitter at a substantially lower pressure than the other columns. Table 6 represents the compositions and properties of selected streams from  FIG. 2 . 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Stream Flows and Properties for FIG. 2 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 201 
                 202 
                 204 
                 205 
                 207 
                 208 
                 209 
                 212 
                 213 
               
               
                   
               
             
          
           
               
                 Temperature (Deg F.) 
                 5.0 
                 −11.0 
                 176.3 
                 −70.0 
                 35.7 
                 −194.6 
                 18.1 
                 −33.6 
                 9.6 
               
               
                 Pressure (psia) 
                 512.0 
                 510.0 
                 515.0 
                 500.0 
                 513.0 
                 480.0 
                 485.0 
                 220.0 
                 240 
               
               
                 Molar flows (lb mol/hr) 
               
               
                 CO 
                 32.6 
                 36.1 
                 0.0 
                 32.9 
                 0.0 
                 32.6 
                 0.0 
                 0.0 
                 0.0 
               
               
                 HYDROGEN 
                 8253.6 
                 8651.7 
                 0.0 
                 8268.8 
                 0.0 
                 8253.6 
                 0.0 
                 0.0 
                 0.0 
               
               
                 METHANE 
                 4714.5 
                 5958.5 
                 0.0 
                 4815.4 
                 0.9 
                 4712.6 
                 1.0 
                 0.9 
                 0.0 
               
               
                 ETHYLENE 
                 9573.0 
                 18478.3 
                 0.0 
                 5637.5 
                 4460.0 
                 13.0 
                 5100.0 
                 4447.1 
                 12.9 
               
               
                 ETHANE 
                 2562.4 
                 6610.0 
                 0.7 
                 1.5 
                 2560.4 
                 0.0 
                 1.3 
                 1.1 
                 2559.3 
               
               
                 ACETYLENE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 PROPYLENE 
                 658.9 
                 2.4 
                 655.9 
                 0.0 
                 3.0 
                 0.0 
                 0.0 
                 0.0 
                 3.0 
               
               
                 PROPANE 
                 140.2 
                 0.0 
                 140.2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 PROPDIENE 
                 6.9 
                 0.0 
                 6.9 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 METHYLACETYLENE 
                 3.6 
                 0.0 
                 3.6 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 ISOBUTANE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 ISOBUTENE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 13BUTADIENE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 BUTENE1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 n-BUTANE 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T-BUTENE2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 C-BUTENE2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
               
             
          
         
       
     
     The invention embodied in  FIG. 2  has significant energy benefits over the prior art design. The lack of a thermal couple between the ethylene distributor, C 202 , and the C2 Splitter, C 204 , that is characteristic of this invention allows a number of process options that reduce the energy requirements of the system. Table 7 illustrates some of the major energy benefits of the design shown in  FIG. 2 . 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Energy Savings From Embodiment of FIG. 2 
               
             
          
           
               
                   
                   
                 Embodiment 
                   
               
               
                   
                   
                 of FIG. 2 
               
               
                   
                 Manley &#39;054 
                 (this invention) 
               
             
          
           
               
                   
                   
                 Duty 
                 Temp 
                 Duty 
                 Temp 
                   
                 Horsepower 
               
               
                 Column 
                   
                 (MMBTU/hr) 
                 (F.) 
                 (MMBTU/hr) 
                 (F.) 
                 Utility 
                 Savings 
               
               
                   
               
             
          
           
               
                 Ethylene 
                 Q reb   
                 0 
                 0.8 
                 19.3 
                 35 
                 50° F. Propylene 
                 1009.4 
               
               
                 distributor 
                   
                   
                   
                   
                   
                 Refrigerant 
               
               
                 C2 splitter 
                 Q con   
                 93.9 
                 −13 
                 81.9 
                 −35 
                 −45° F. Propylene 
                 2379.6 
               
               
                   
                   
                   
                   
                   
                   
                 Refrigerant 
               
               
                   
                 Q reb   
                 26.9 
                 135 
                 59.8 
                 10 
                 25° F. Propylene 
                 5106.9 
               
               
                   
                   
                   
                   
                   
                   
                 Refrigerant 
               
               
                   
                   
                   
                   
                   
                   
                 Net 
                 8495.9 
               
               
                   
               
             
          
         
       
     
     Table 7 shows the duties and temperatures for the ethylene distributor and the C2 splitter for the Manley reference and the embodiment of  FIG. 2 . One benefit is the additional recuperation of 50° F. propylene refrigeration in the ethylene distributor reboiler. Another significant energy savings is brought about by shifting the C2 Splitter reboiler duty from 150 psi steam to recuperation of 25° F. propylene refrigeration. Finally, the C2 Splitter condenser duty decreases, producing an additional savings in −45° F. propylene refrigeration. 
     Note that in Tables 2, 3, 4, and 7, Q con  refers to the heat duty of the condensor, and Q reb  refers to the heat duty of the reboiler. 
     It should be noted that these savings are partially offset by energy penalties elsewhere in the system. For example, the reflux requirement of C 201  is significantly higher than that of C 101 , and the duty is required at a significantly lower temperature. This offsets a portion of the savings outlined in Table 6, but a rigorous energy analysis of the overall system indicates that there is a net energy benefit for the process of  FIG. 2  compared with the prior art. Table 8 compares the total propylene and ethylene system refrigeration horsepower required for both the prior art design and the invention design in  FIG. 2  for equivalent total ethylene production. It is clear that there is an overall energy savings for the process of  FIG. 2  over the prior art design. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Ethylene and Propylene Refrigeration 
               
               
                 Compressor Energy Requirements 
               
             
          
           
               
                   
                 Manley ‘054) 
                 Embodiment of FIG. 2 
               
               
                   
                   
               
             
          
           
               
                 Total Refrigeration 
                 45,778 
                 42,185 
               
               
                 Compressor Energy (HP) 
               
               
                   
               
             
          
         
       
     
     All major separation, heating, and cooling steps have been shown in the description of the preferred embodiments. Some details of the process design that are well known to those skilled in the art, such as vapor-liquid separation drums, process control valves, pumps, and the like, have been omitted in order to demonstrate more clearly the important concepts of the invention.