Abstract:
A process for recovering hydrogen from a mixed hydrocarbon stream wherein the mixed hydrocarbon stream is subjected to a separation technique to produce a substantially hydrogen enriched stream, which is then recovered as hydrogen product. A process for providing refrigeration duty to the process is also disclosed, wherein a substantially methane enriched stream arising from the separation technique is expanded to provide cooling duty for the process.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001]     This invention was made with government support under United States Department of Energy Cooperative Agreement No. DE-FC07-01ID14090. 
     
    
     CROSS REFERENCE TO RELATED APPLICATIONS  
       [0002]     None.  
       BACKGROUND OF THE INVENTION  
       [0003]     For many years, distributed distillation has been suggested as a basis for the design of refinery systems, ethylene recovery systems, and other commercial chemical, petroleum, and petrochemical separations systems. Distributed distillation is best understood by contrasting it with sharp split distillation. In sharp split distillation, a separation is made between light and heavy components that are adjacent to each other on the volatility curve of the mixture being separated. That is, there are little or no compounds in the mixture that have a volatility that is intermediate to those of the light and heavy components.  
         [0004]     For example, a typical sharp split deethanizer column in an ethylene recovery system performs a sharp split between ethane and propylene. The overheads of the column contain essentially no propylene and the bottoms contain essentially no ethane. The overheads therefore contain all components lighter than the light component (e.g. ethylene, methane, etc.), and the bottoms contain all components heavier than the heavy component (e.g. propane, C4+, etc).  
         [0005]     In a distributed distillation operation, a sharp split is not made between components that are adjacent on the volatility curve. A distributed distillation analog to the deethanizer is a “C2 distributor”. A C2 distributor column produces a sharp split between methane and C3 components while distributing ethane and ethylene between the column overhead and bottoms. In a C2 distributor column, the light component is methane and the heavy component is propylene. These components are not adjacent to each other on the volatility curve; ethane and ethylene have a volatility that is intermediate between methane and propylene. In this case, then, ethane and ethylene distribute between the column overheads and bottoms. The overheads contain some ethane and ethylene, as well as methane and lighter components, but essentially no propylene. The bottoms also contain some ethane and ethylene, as well as propylene and heavier components, but essentially no methane. Of course, further purification of the components is done in downstream columns.  
         [0006]     A benefit of a distributed distillation system is that it requires less total energy to produce the final purified components than an analogous sharp split distillation sequence. A way of understanding the energy savings provided by distributed distillation is that it accomplishes the separation of components with fewer overall phase changes. Phase changes (condensation or vaporization) require energy, and reducing the number of phase changes also reduces the energy consumption of the system.  
         [0007]     U.S. Pat. No. 5,675,054 issued to Manley, and Manley and Hahesy (Hydrocarbon Processing, April 1999 p. 117) describe distributed distillation in ethylene recovery and purification. Manley describes the use of an ethylene distributor column in which both the bottoms and overheads stream contain ethylene, and in which a product-quality separation of ethane and ethylene in the overhead product is achieved.  
         [0008]     In Manley &#39;054, the ethylene distributor is thermally coupled to the downstream demethanizer column. The vapors from the ethylene distributor flow directly into the demethanizer for further separation, while a liquid side draw from the demethanizer is used to reflux the ethylene distributor column. The overheads of the demethanizer column contain primarily hydrogen and methane, and can be further treated to recover methane and hydrogen if desired. The  Hydrocarbon Processing  article states that after removing sufficient ethylene from the demethanizer overhead stream, it can be sent to a standard adiabatic hydrogen purification section where Joule-Thompson expansion is utilized to further chill and separate a purified hydrogen product from the demethanizer overhead stream.  
         [0009]     U.S. Pat. No. 5,035,732 issued to McHue is directed to a process for recovering C2 hydrocarbons from a mixed stream that utilizes dephlegmation of the feed and introduction of the dephlegmator liquid product into a demethanization system, characterized by the presence of two demethanization steps, one at moderately low cryogenic temperatures and one at very low cryogenic temperatures.  
         [0010]     U.S. Pat. No. 4,629,484 issued to Kister is directed to cooling a gaseous hydrocarbon feed mixture in a plurality of cooling stages and introducing a bottoms portion from at least one cooling stage to a hydrogen stripper, in which hydrogen is removed from the bottoms portion before the bottoms portion is introduced to a demethanizer fractionating column, and where relatively pure hydrogen and methane streams are produced.  
         [0011]     U.S. Pat. No. 4,900,347 issued to McHue is directed to a process for recovering ethylene from cracked hydrocarbon feed gas which utilizes a plurality of dephlegmator units, the liquid products of which are fed into serially connected demethanizer fractionators.  
         [0012]     Surprisingly, we have found that making a rough separation of methane and hydrogen downstream of the ethylene distributor and upstream of the hydrogen recovery and purification section of the plant significantly increases the hydrogen recovery of the process with only a small increase in energy levels. In contrast to standard distributed distillation systems, a hydrogen depleted gas is expanded and used for refrigeration, so less hydrogen is degraded from chemical to fuel value. This overcomes one of the disadvantages of a typical distributed distillation system, namely, low hydrogen recovery. We have further found that the methane rich gas from the aforesaid rough separation can be expanded and chilled to provide a cooling duty to the overall process.  
       SUMMARY OF THE INVENTION  
       [0013]     This invention describes a process for recovering hydrogen. The process comprises the steps of introducing a mixed hydrocarbon feed stream comprising a mixture of hydrogen, methane, and C2 components into an ethylene distributor column to obtain a first stream. This first stream is subjected to a separation technique to produce a second stream substantially hydrogen enriched and substantially methane depleted, and a third stream substantially hydrogen depleted and substantially methane enriched. A purified hydrogen product is recovered from the second, substantially hydrogen enriched and substantially methane depleted stream.  
         [0014]     This invention also describes a process for recovering hydrogen and recovering refrigeration duty. The process comprises the steps of introducing a mixed hydrocarbon feed stream comprising a mixture of hydrogen, methane, and C2 components into an ethylene distributor to obtain a first stream. The first stream is subjected to a separation technique to produce a second stream substantially hydrogen enriched and substantially methane depleted, and a third stream substantially hydrogen depleted and substantially methane enriched. This third stream is introduced into a second separation column to recover a fourth stream substantially methane enriched. This fourth stream is directed to an expansion device to produce a lower-temperature, lower pressure fifth stream. This fifth stream is reheated to provide cooling duty to the overall process. A purified hydrogen product is recovered from said second stream.  
         [0015]     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 scope and spirit of the invention. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1  depicts an olefins plant demethanization section that utilizes an ethylene distributor and demethanizer tower to depict the hydrogen recovery of this invention.  
         [0017]      FIG. 2  depicts an embodiment which utilizes dephelgmator technology to provide the partial separation of hydrogen and methane between the ethylene distributor overhead and the demethanizer inlet.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     With reference to  FIG. 1 , the feed to the process enters as stream  101 . Stream  101  comprises a mixture of hydrogen, methane, ethane, and ethylene and optionally heavier hydrocarbons. Stream  101  enters C 101 , which serves as an ethylene distributor. A liquid side stream, stream  102 , can optionally be withdrawn from C 101  at or near the feed point to provide reflux to an upstream column or columns. The overhead stream arising from C 101 , stream  103 , comprises essentially all of the hydrogen and methane that enters the column as well as a fraction of the ethylene. Stream  103  enters a partial condenser, E 101 , which provides at least a portion of the reflux to column C 101 . One or more side condensers can optionally be utilized on C 101 , such as E 102  as shown. Column C 101  is controlled such that the ethane/ethylene ratio in stream  103  is sufficiently low that a product quality ethylene can be produced from this stream by removal of the hydrogen and methane. Typically, the ethane content of stream  103  is controlled by adjusting the reflux liquid provided to C 101  by exchangers E 101  and/or E 102 .  
         [0019]     The bottoms product of C 101 , stream  104 , comprises ethane and heavier hydrocarbons leaving column C 101 , as well as the remaining fraction of the ethylene. Stream  104  can be further purified in downstream columns.  FIG. 1  shows column C 101  operated with a conventional reboiler exchanger, E 103 . Column C 101  is typically operated so that there is essentially little or no methane in stream  104 . The methane content of stream  104  can be controlled by adjusting the amount of stripping vapor provided to column C 101  by exchanger E 103 .  
         [0020]     In  FIG. 1 , the uncondensed vapor from E 101 , shown as stream  105 , is sent to a hydrogen/methane separation step, designated S 101  in  FIG. 1 . In this step, at least a partial separation is made between the hydrogen and the heavier components (primarily methane and ethylene) present in the net overhead vapor from C 101 . This separation step can include the use of partial condensation and separation, rectification, membrane separation, dephlegmation, and adsorption, individually or in a combination of these or other operations known to those in the art to be beneficial for separation of hydrogen from a mixed hydrocarbon stream.  
         [0021]     The separation step S 101  produces two streams. The first stream, stream  106 , is enriched in hydrogen relative to stream  105 . The second stream, stream  107 , is depleted in hydrogen relative to stream  105 . A portion of stream  106  can be optionally fed to an expander system as shown by dotted line  108 . The remainder of the hydrogen-enriched stream, stream  109 , would advantageously be sold as product hydrogen or sent to a further hydrogen purification step if needed. Beneficially, the flow in stream  108  would be minimized in order to maximize the fraction of the hydrogen-enriched stream  106 , which is sent to the hydrogen product recovery via stream  109 . In some cases, however, the refrigeration requirement for the overall process and the process energy balance will dictate that some of stream  106  be directed to the expander system, X 101 , via stream  108 . The material from stream  108  would provide additional chilled expander outlet gas which would in turn to provide the required refrigeration duty as described below.  
         [0022]     Stream  107  is fed to column C 102 , which serves as a demethanizer. If C 102  operates at a pressure significantly lower than that of S 101 , a valve or other expansion device may be employed in line  107 . The overheads of C 102  are partially condensed in E 104  to provide reflux to the tower C 102 .  FIG. 1  shows this to be accomplished by a standard partial condenser arrangement, though other means, such as dephlegmation, could also be used. One or more side condensers can be utilized on C 102 , shown as exchanger E 105 , to provide additional reflux liquid to the tower. The uncondensed vapor from E 104 , stream  110 , is the net overhead product from the demethanizer and comprises methane, optionally hydrogen, and very little ethylene. Stream  110  is fed to an expander system X 101  for recovery of refrigeration.  FIG. 1  shows a single stage of work expansion being accomplished on the net demethanizer overhead to provide a chilled lower-pressure stream, stream  111 . This chilled lower-pressure stream  111  is reheated against other process or external refrigeration streams to provide cooling duty to the rest of the process.  FIG. 1  shows this being done in a single exchanger E 106 , though multiple exchangers can also be used. A Joule-Thompson expansion could also be employed in place of the work expansion device that is shown, as is well known to those skilled in the art. The final reheated expanded stream, stream  112 , comprises primarily methane and some hydrogen and can be used as plant fuel if desired. The bottoms product of C 102 , stream  113 , comprises product-quality ethylene.  
         [0023]      FIG. 1  shows column C 102  being operated with a conventional reboiler exchanger E 107 . Column C 102  is typically operated so that there is essentially little or no methane in stream  113 . The methane content of stream  113  can be controlled by adjusting the amount of stripping vapor provided to column C 102  from exchanger E 107 . A benefit presented by this invention is that of a partial separation of hydrogen and methane that takes place between the ethylene distributor and demethanizer. This results in production of a stream, stream  110 , that is relatively depleted in hydrogen, which can be preferentially fed to the expander system to provide refrigeration for the rest of the process. As a result, the amount of the hydrogen-enriched stream, stream  108 , that is directed to the expanders is reduced or eliminated, and hydrogen loss to fuel is therefore reduced. An additional benefit of this invention is the lack of thermal coupling between the ethylene distributor and the demethanizer columns. The elimination of thermal coupling between these two columns allows the columns to operate more independently, each with its own condenser for providing reflux. This improves the controllability of the system compared to the prior art by providing more direct control of the column overhead compositions. Another benefit from this arrangement is that the demethanizer overheads are depleted in hydrogen relative to the prior art and thus the temperature of the top of the demethanizer is higher than in the prior art. This means that relatively cheaper, higher-temperature refrigeration can be used for the demethanizer condenser compared with the prior art.  
         [0024]     The process shown in  FIG. 1  has significant benefits over the prior art in terms of hydrogen recovery. This invention recovers significantly more hydrogen than the prior art, and those skilled in the art will recognize that the reason for the improved hydrogen recovery is the partial separation of hydrogen and methane that occurs between the ethylene distributor and the demethanizer towers, which allows preferential feeding of a relatively hydrogen-depleted stream to the expansion system for recovery of refrigeration.  
       EXAMPLE  
       [0025]      FIG. 2  depicts an embodiment which utilizes dephelgmator technology to provide the partial separation of hydrogen and methane between the ethylene distributor overhead and the demethanizer inlet.  
         [0026]     A mixed hydrocarbon feed, stream  201 , enters column C 201 . In  FIG. 2 , stream  201  is the chilled overhead vapor from a deethanizer column (not shown) and comprises a mixture of hydrogen, methane, ethylene, and ethane and optionally heavier hydrocarbons. Column  201  contains  105  theoretical trays, and the feed, stream  201 , enters on theoretical tray  75  (as measured from the top). The overhead vapor of column C 201  is partially condensed in exchanger E 201 . The liquid and vapor from exchanger E 201  are separated in drum D 201  and the liquid is returned to C 201  as reflux. Stripping vapor is provided to the bottom of C 201  by reboiler E 202 .  
         [0027]     Column C 201  serves as an ethylene distributor, making a sharp split between ethane and methane, and allowing ethylene to distribute between the overhead and bottom streams. The overhead vapor from D 201 , stream  202 , comprises a mixture of hydrogen, methane and ethylene, but little, if any, ethane. The bottoms liquid, stream  203 , comprises a mixture of ethane and ethylene, but little, if any, hydrogen or methane. Stream  203  can be further purified in downstream columns. A liquid sidestream, stream  204 , is taken from C 201  at a point just above the feed. Stream  204  is used as reflux liquid to the upstream deethanizer tower (not shown).  
         [0028]     Stream  202  is further chilled to about −145° F. in exchanger E 203  against an external refrigerant circuit, shown as stream Ref, and reheating hydrocarbon vapors as described below. The partially condensed stream is sent to separator drum D 202 . The liquid from D 202 , stream  205 , is split into two streams. One portion, stream  206 , is reheated to about −45° F. in E 203  and then sent to the demethanizer column C 202 . The remaining portion, stream  207 , is sent directly to the demethanizer column C 202 .  
         [0029]     The vapor from D 202 , stream  208 , is sent to dephlegmator C 203 . The dephlegmator C 203  is chilled with a variety of reheat streams as described below. Within the dephlegmator C 203 , both heat and mass transfer operations are carried out. The dephlegmator C 203  has been simulated as having ten theoretical separation stages, with equal heat removal at each stage. The overhead vapor from C 203 , stream  210 , is enriched in hydrogen and depleted in methane and ethylene. Stream  210  is sent to further purification and recovery of a salable hydrogen product as described below. The bottoms liquid from C 203 , stream  211 , is combined with stream  207  to become stream  212  and sent to the demethanizer column C 202 .  
         [0030]     The demethanizer column C 202  contains 45 theoretical stages. The upper feed, stream  212 , enters at theoretical stage  9  (as measured from the top), and the lower feed, reheated stream  206 , enters at theoretical stage  15 . The overhead vapor of column C 202  is partially condensed in exchanger E 204 . The liquid and vapor from E 204  are separated in drum D 203  and the liquid is returned to C 202  as reflux. Stripping vapor is provided to the bottom of C 202  by reboiler E 205 . The overhead vapor from D 203 , stream  213 , is enriched in methane and contains little, if any, ethylene. Stream  213  is sent to an expansion device to provide cooling for the dephlegmator C 203  and other process cooling as described below. The bottoms liquid from C 202 , stream  214 , contains product-purity ethylene.  
         [0031]     The hydrogen-enriched stream emerging from the overhead of the dephlegmator C 203 , stream  210 , is split. In this example the process cooling requirements are greater than can be supplied by expansion of only the demethanizer overhead vapor which is described below. Therefore, a minor fraction of stream  210 , designated stream  215 , is reheated in the dephlegmator C 203  and then sent to the first stage expander X 201  to provide additional chilled expander outlet vapor and therefore additional process cooling capacity. The majority of stream  210 , designated stream  216 , is sent to hydrogen recovery. A typical 2-stage adiabatic hydrogen recovery section is shown in  FIG. 2 . It consists of two heat exchangers, E 206  and E 207 , and two drums, D 204  and D 205 . The operation of the hydrogen recovery section is well known to those skilled in the art and will not be described in detail here. The operation results in a high-pressure, high-purity hydrogen stream, stream  217 , and a lower-pressure methane-rich fuel gas stream, stream  218 . These streams are reheated first through the dephlegmator C 203  and then through E 203  as shown. These streams will typically also be further reheated elsewhere in the process to recover additional cooling capacity. The reheated hydrogen product stream is designated stream  219  and the reheated lower-pressure fuel stream is designated stream  220 .  
         [0032]     The overhead vapor from the demethanizer column, stream  213 , is directed, along with a minor fraction of the hydrogen-enriched stream, stream  215 , to the first expansion stage X 201 . The expander reduces the pressure of the stream to an intermediate pressure, thereby cooling the stream. The cold intermediate-pressure stream, stream  221 , is reheated through the dephlegmator C 203 , and then sent to the second stage expander, X 202 . This second expander reduces the pressure of reheated stream  221  to a lower pressure, further cooling it. The cold lower-pressure stream, stream  222 , is also reheated through the dephlegmator C 203 , and then through exchanger E 203 . This stream will typically be further reheated elsewhere in the process to recover additional cooling capacity and then used as fuel. The reheated second lower-pressure fuel stream is designated as stream  223 . Those skilled in the art will recognize that the expanders X 201  and X 202  could be part of expander/compressor (compander) sets.  
         [0033]     Table 1 compares the hydrogen recovery for a prior art process and the process of this invention for a plant producing 1000 kilotonnes of ethylene per year. It also compares the total compressor energy requirement for the two processes (measured as the sum of the cracked gas compressor and the ethylene and propylene refrigeration compressor horsepower requirements). The process of this invention recovers approximately 2,200 lb/hr more hydrogen product than the prior art process, with a relatively modest 275 HP increase in energy requirement. The value of the additional hydrogen product more than offsets the slightly higher energy use of the process of this invention.  
                                           TABLE 1                           Hydrogen Production, Hydrogen Recovery, and Energy Requirement                Prior Art Design               (Manley U.S. Pat. No. 5,675,054)   This Invention                        Hydrogen Product   9939   12186       flow (lb/hr)       Hydrogen Recovery   60.2%   73.8%       to Product (%)       Total Compressor   99736   100011       Horsepower (HP)                  
 
         [0034]     Conditions and compositions of streams shown in  FIG. 2  are given in Table 2 and exchanger duties are given in Table 3. The data in Table 2 shows that the arrangement in  FIG. 2  produces a partial separation of the hydrogen and methane that are present in the ethylene distributor overhead.  
                                                                                                                               TABLE 2                       Flows and Conditions for Streams in  FIG. 2                                      Stream No.                201   202   203   204   205   206   208   210   211   213               Temperature (Deg F.)   −12.7   −73.4   36.0   −11.4   −145.0   −145.0   −145.0   −206.2   −149.9   −142.1       Pressure (psig)   515   500   515   511   498   498   498   495   498   480       Molar flows (lb mol/hr)       CO   34.4   32.6   0.0   1.8   3.7   2.2   29.0   28.5   0.5   4.2       HYDROGEN   8458.2   8254.6   0.0   203.6   147.3   88.4   8107.3   8087.3   20.0   167.3       METHANE   5349.3   4713.9   0.9   634.5   1643.5   986.1   3070.5   2829.1   241.4   1883.8       ETHYLENE   16353.0   5178.1   4423.3   6751.6   4565.9   2739.5   612.2   11.0   601.2   2.1       ETHANE   5956.9   1.6   2533.8   3421.6   1.5   0.9   0.1   0.0   0.1   0.0       ACETYLENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       PROPYLENE   2.9   0.0   1.4   1.6   0.0   0.0   0.0   0.0   0.0   0.0       PROPANE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       PROPDIENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       METHYLACETYLENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0                        Stream No.                214   215   216   217   218   219   220   221   222   223               Temperature (Deg F.)   18.1   −206.2   −206.2   −219.7   −219.7   −161.3   −161.3   −212.0   −213.3   −161.3       Pressure (psig)   485   495   495   490   42   487   41   135   42   41       Molar flows (lb mol/hr)       CO   0.0   4.4   24.1   16.0   8.1   16.0   8.1   8.5   8.5   8.5       HYDROGEN   0.0   1236.4   6850.9   6093.0   757.9   6093.0   757.9   1403.7   1403.7   1403.7       METHANE   1.0   432.5   $$   255.2   2141.4   255.2   2141.4   2316.3   2316.3   2316.3       ETHYLENE   5165.0   1.7   9.3   0.0   9.3   0.0   9.3   3.7   3.7   3.7       ETHANE   1.6   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       ACETYLENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       PROPYLENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       PROPANE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       PROPDIENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0       METHYLACETYLENE   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0                  
 
         [0035]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 Heat Exchanger Duties for  FIG. 2   
               
             
          
           
               
                 Exchanger 
                 Service 
                 Duty (mBTU/hr) 
               
               
                   
               
             
          
           
               
                 E201 
                 Ethylene Distributor Condenser 
                 −82.71 
               
               
                 E202 
                 Ethylene Distributor Reboiler 
                 19.61 
               
               
                 E203 
                 Cracked Gas Chiller 
                 −36.03 
               
               
                 E204 
                 Demethanizer Condenser 
                 −5.69 
               
               
                 E205 
                 Demethanizer Reboiler 
                 17.67 
               
               
                 E206 
                 First Stage Hydrogen Recovery 
                 −5.82 
               
               
                 E207 
                 Second Stage Hydrogen Recovery 
                 −4.02 
               
               
                 C203 
                 Dephlegmator 
                 −8.68 
               
               
                   
               
             
          
         
       
     
         [0036]     Table 4 compares the hydrogen and methane contents of stream  202  (net ethylene distributor overhead), stream  219  (the final hydrogen product stream) and the combination of fuel streams (streams  220  and  223 ). This table demonstrates that through the application of this invention, about 74% of the hydrogen present in stream  202  is recovered as salable hydrogen product, while only about 26% of the hydrogen present in stream  202  is lost into the fuel streams. The drawings contain depictions of certain embodiments of this invention. All major separation, heating, and cooling steps have been shown.  
                                                                                     TABLE 4                           Hydrogen and Methane Recovery                Stream and Stream Number                    Ethylene   H2   Fuel   Fuel   Total Fuel               Distr. Ovhd   Product   Stream 1   Stream 2   Stream               202   219   220   223   (220 + 223)                    Hydrogen Flow   lbmol/hr   8254.6   6093.0   757.9   1403.7   2161.6       Methane Flow   lbmol/hr   4713.9   255.2   2141.4   2316.3   4457.7       Hydrogen Recovery   %       73.8%           26.2%       Methane Recovery   %       5.4%           94.6%