Abstract:
The invention provides a process and system for regenerating a solvent used to remove carbon dioxide from feed gases, such as natural gas and synthesis gas. The invention employs one or more hydraulic turbochargers to transfer energy from a higher energy solvent stream to a lower energy solvent stream. This provides for a significant reduction in operating expenses.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention is related to the removal of acid gases from a feed gas. More particularly the invention relates to acid gas removal from high carbon dioxide and hydrogen sulfide containing feed gases. A process is provided for reduced energy requirements in the regeneration of the solvent used to remove the acid gases. 
         [0002]    Absorption systems are commonly used for the removal of CO 2  from natural gas or synthesis gas. A physical solvent such as a dimethylether of polyethylene glycol (DMPEG) can be used to wash out carbon dioxide and other acid gases such as hydrogen sulfide and carbonyl sulfide. DMPEG solvents are used in systems licensed by UOP LLC under the trademark Selexol™. Cryogenic methanol systems are also known to those skilled in the art for this use including the Rectisol™ process currently licensed by Lurgi AG. Other physical solvents that may be used include a mixture of N-formyl and N-acetyl morpholine, N-methyl-2-pyrrolidone and sulfolane. 
         [0003]    After absorption of carbon dioxide and/or hydrogen sulfide and/or carbonyl sulfide by a physical solvent, the solution is regenerated to remove absorbed gases. The regenerated physical solvent can then be recycled for further absorption. Absorption and regeneration are usually carried out in different columns or drums containing packing or bubble cap tray for efficient operation. Regeneration is generally achieved in two stages. First, the absorbent solution&#39;s pressure is reduced so that absorbed carbon dioxide is vaporized from the solution in one or more flash vessels, sometimes terminating with a vacuum flash drum. Next, if thermal regeneration is required, the flashed absorbent is stripped with steam in a stripping regenerating column to remove residual absorbed carbon dioxide. Low carbon dioxide levels are needed in order to achieve the required carbon dioxide specifications for treated gas. 
         [0004]    The prior art processes have significant power requirements. The solvent processes employ pressures that range from about 2758 to 7584 kPa (400 to 11 psia) and solvent flow rates that range from 3000 to 20000 gpm. Some of the energy is recoverable from the solvent during pressure let down via turbines. However, conventional turbines have been found to be unreliable and cost prohibitive. The release of relatively large amounts of vapor combined with the high solvent flow rates often means that two or more conventional turbines are required to adequately recover the pump energy. However, the capital costs associated with multiple turbines usually makes this option economically unattractive. 
         [0005]    It has now been found that a hydraulic turbocharger can be used to recover energy at a relatively low cost compared to turbines 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention involves the use of a solvent stream as the motive fluid at the elevated pressures typical of the process eliminating the high costs previously encountered in regenerating the solvent stream. The motive fluid that is used can be the bottom stream from a carbon dioxide absorber unit or the liquid from the recycle flash drum depending upon the configuration of the system. The pressure of the recycle flash drum can be varied as needed and in certain circumstances the drum can be removed if there is a need for additional energy. The operating pressure of the carbon dioxide absorber ranges from about 2758 to 6550 kPa (400 to 950 psia) so that sufficient energy would be available to be transferred from the fluid exiting the absorber to increase the pressure of another solvent stream. This invention is applicable to any process in which flash or thermal regeneration of a solvent is used in conjunction with a high pressure absorber. 
         [0007]    The present invention comprises a process for treating a carbon dioxide containing gas comprised of sending a feed gas containing carbon dioxide through a carbon dioxide absorber unit and contacting it with a lean solvent to produce a loaded solvent containing a majority of said carbon dioxide from said feed gas and a treated gas, regenerating the loaded solvent in a carbon dioxide removal system, and recovering energy from at least one solvent stream to boost pressure of at least one other solvent stream through use of one or more hydraulic turbochargers. There are a number of different configurations that utilize the hydraulic turbocharger that are within the scope of the invention. The hydraulic turbocharger may be used to recover energy from a semi-lean solvent stream that exits the carbon dioxide absorber unit and transfers the energy to pump a lean solvent stream. Alternatively, a hydraulic turbocharger may be used to recover energy from a semi-lean solvent stream that exits the carbon dioxide absorber unit and transfers this energy to pump a semi-lean solvent stream from the carbon dioxide removal system. Also, there may be another hydraulic turbocharger to recover energy from a hydrogen sulfide absorber unit with transfer of this energy to pump a lean solvent stream. Another hydraulic turbocharger can be used instead to recover energy from a hydrogen sulfide concentrator solvent stream to use in pumping a lean solvent stream. The invention also involves a system for removal of carbon dioxide from a carbon dioxide containing solvent comprising a carbon dioxide absorber, a recycle drums a vacuum flash drum, an eductor and a carbon dioxide venting apparatus. The invention also comprises a system that is designed to run the processes described above. 
         [0008]    The invention is generally applicable to physical solvents for which flash or thermal regeneration is used on the solvent stream to produce a solvent stream that contains almost no acid gas. Among the physical solvents that can be used are a dimethylether of polyethylene glycol (DMPEG), methanol, a mixture of N-formyl and N-acetyl morpholine, N-methyl-2-pyrrolidone and sulfolane. Dimethylether of polyethylene glycol is a preferred solvent for use in the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a prior art process that does not use any turbines for power recovery. 
           [0010]      FIG. 2  shows a flow scheme in which a hydraulic turbocharger is used to recover energy from a semi-lean solvent stream to use in pumping a lean solvent stream. 
           [0011]      FIG. 3  shows a flow scheme in which a hydraulic turbocharger is used to recover energy from a semi-lean solvent stream to use in pumping a semi-lean solvent stream. 
           [0012]      FIG. 4  shows a flow scheme in which a hydraulic turbocharger is used to recover energy from a semi-lean solvent stream to use in pumping a semi-lean solvent stream and to recover energy from the bottoms of a hydrogen sulfide absorber to use in pumping lean solvent. 
           [0013]      FIG. 5  shows a flow scheme in which a hydraulic turbocharger is used to recover energy from a carbon dioxide absorber bottoms to use in pumping a semi-lean solvent stream and to recover energy from the bottoms of a hydrogen sulfide absorber to use in pumping lean solvent. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention includes at least one hydraulic turbocharger to recover energy from a solvent stream and transfer the energy to another solvent stream in a solvent process to remove impurities from gasification synthetic gas or natural gas. The main impurities that are being removed are hydrogen sulfide, carbon dioxide and carbonyl sulfide. The low cost of the hydraulic turbocharger results from the turbine side and pump side being coupled within a single component. There is no need for external equipment to transfer energy from the turbine to the pump, and there is no need for external shaft seals to prevent the process from leaking to the atmosphere. A single stage turbine is beneficial on the turbine side of the turbocharger because single stage turbines are more capable of handling large vaporization rates that are typically found in gas processes than the multiple stage turbines that are often required for conventional power recovery. 
         [0015]    In the present invention, a hydraulic turbocharger is used to boost the pressure as necessary in the process solvent streams. In some processes there are often both lean and semi-lean solvent streams whose pressure must be increased from near ambient to over 7500 kPa or to the operating pressure of the absorber. The carbon dioxide absorber bottoms stream which is the highest pressure stream in the carbon dioxide absorption section can be used to boost the pressure of either the lean or semi-lean streams. In one embodiment of the invention, as the pressure of the carbon dioxide absorber bottoms stream is let down in the hydraulic turbocharger, the energy from this stream is used to boost the lean solution pressure. In another embodiment of the invention, the energy from letting down the pressure of the carbon dioxide absorber bottoms stream can be used to boost the pressure of the semi-lean solution. Additional embodiments of the invention employ hydraulic turbochargers at different locations within the process where it is advantageous to recover energy from one solvent stream and use it to increase the pressure of another solvent stream. 
         [0016]    A prior art system is shown in  FIG. 1  in which a feed gas  2  and a recycle gas  4  from a hydrogen sulfide concentrator are combined in line  6  to enter a hydrogen sulfide absorber  8 . Also entering hydrogen sulfide absorber  8  is a loaded solvent  62 . A rich solvent stream  10  leaves the bottom of the hydrogen sulfide absorber  8  and a gas stream containing carbon dioxide leaves the top of the hydrogen sulfide absorber in line  12 . The gas stream in line  12  enters carbon dioxide absorber  14  where it is contacted with a semi-lean solvent that enters through line  42  and a lean solvent stream  64  that is chilled by chiller  66  and enters carbon dioxide absorber  14  through line  68 . The gas stream is treated in the carbon dioxide absorber and the treated gas exits in line  70 . The loaded solvent containing carbon dioxide exits the carbon dioxide absorber in line  16 . A portion of this loaded solvent is sent through line  54  to loaded solvent pump  56  to line  58 , loaded solvent chiller  60  and line  62  back to the previously described hydrogen sulfide absorber  8 . The other portion of the loaded solvent passes in line  18  to recycle flash drum  20  where some of the carbon dioxide passes through line  44  to recycle compressor  46  through line  48  to recycle cooler  50  to line  52  and then enters the carbon dioxide absorber. The solvent stream exits the recycle flash drum through line  22  and passes to carbon dioxide vent drum  24  from which more of the carbon dioxide is vented through line  26 . The solvent stream then passes through line  28  to carbon dioxide vacuum drum  30  from which most of the remaining carbon dioxide is vented as shown in line  32 . The resulting solvent is now semi-lean and passes through line  34  to semi-lean solvent pump  36 , then to line  38 , semi-lean solvent chiller  40  and then to line  42  to enter the carbon dioxide absorber. 
         [0017]    In  FIG. 2 , a gas treating process is shown that uses a hydraulic turbocharger to recover energy from a semi-lean solvent stream for use in pumping lean solvent. A feed gas  102  enters a carbon dioxide absorber  114  where it is contacted with a semi-lean solvent that enters through line  142  and a lean solvent stream  167  that is chilled by chiller  166  and enters carbon dioxide absorber  114  through line  168 . The gas stream is treated in the carbon dioxide absorber and the treated gas exits in line  170 . The loaded solvent containing carbon dioxide exits the carbon dioxide absorber in line  116 . A portion of this loaded solvent is sent through line  162  to another absorber or to be regenerated. The other portion of the loaded solvent passes in line  117  to hydraulic turbocharger  165 . Also entering the hydraulic turbocharger is a lean solvent stream in line  164 . The lean solvent stream exits in line  167  and a carbon dioxide-containing solvent stream exits the hydraulic turbocharger in line  118  and then enters recycle flash drum  120  where some of the carbon dioxide passes through line  144  to recycle compressor  146  through line  148  to recycle cooler  150  to line  152  and then enters the carbon dioxide absorber. The solvent stream exits the recycle flash drum through line  122  and passes to carbon dioxide vent drum  124  from which more of the carbon dioxide is vented through line  126 . The solvent stream then passes through line  128  to carbon dioxide vacuum drum  130  from which most of the remaining carbon dioxide is vented as shown in line  132 . The resulting solvent is now semi-lean and passes through line  134  to semi-lean solvent pump  136 , then to line  138 , semi-lean solvent chiller  140  and then to line  142  to enter the carbon dioxide absorber. 
         [0018]    In  FIG. 3 , a gas treating process is shown that uses a hydraulic turbocharger to recover energy from a semi-lean solvent stream for use in pumping semi-lean solvent. A feed gas  202  enters a carbon dioxide absorber  214  where it is contacted with a semi-lean solvent that enters through line  267  and a lean solvent stream  264  that is chilled by chiller  266  and enters carbon dioxide absorber  214  through line  268 . The gas stream is treated in the carbon dioxide absorber and the treated gas exits in line  270 . The loaded solvent containing carbon dioxide exits the carbon dioxide absorber in line  216 . A portion of this loaded solvent is sent through line  262  to another absorber or to be regenerated. The other portion of the loaded solvent passes in line  217  to hydraulic turbocharger  265 . Also entering the hydraulic turbocharger is a semi-lean solvent stream in line  242 . A solvent stream exits in line  267  to return to the carbon dioxide absorber and a carbon dioxide-containing solvent stream exits the hydraulic turbocharger in line  218  and then enters recycle flash drum  220  where some of the carbon dioxide passes through line  244  to recycle compressor  246  through line  248  to recycle cooler  250  to line  252  and then enters the carbon dioxide absorber. The solvent stream exits the recycle flash drum through line  222  and passes to carbon dioxide vent drum  224  from which more of the carbon dioxide is vented through line  226 . The solvent stream then passes through line  228  to carbon dioxide vacuum drum  230  from which most of the remaining carbon dioxide is vented as shown in line  232 . The resulting solvent is now semi-lean and passes through line  234  to semi-lean solvent pump  236 , then to line  238 , semi-lean solvent chiller  240  and then to line  242  to enter the hydraulic turbocharger. 
         [0019]    In  FIG. 4  is shown a gas treating process that uses a first hydraulic turbocharger to recover energy from a semi-lean solvent to use in pumping semi-lean solvent and a second hydraulic turbocharger to recover energy from a hydrogen sulfide absorber to use in pumping lean solvent. A feed gas  302  and a recycle gas  304  from a hydrogen sulfide concentrator are combined in line  306  to enter a hydrogen sulfide absorber  308 . Also entering hydrogen sulfide absorber  308  is a loaded solvent  362 . A rich solvent stream  310  leaves the bottom of the hydrogen sulfide absorber  308  and a gas stream containing carbon dioxide leaves the top of the hydrogen sulfide absorber in line  312 . The gas stream in line  312  enters carbon dioxide absorber  314  where it is contacted with a semi-lean solvent that enters through line  367  and a lean solvent stream  364  that is chilled by chiller  366  and enters carbon dioxide absorber  14  through line  368 . The gas stream is treated in the carbon dioxide absorber and the treated gas exits in line  370 . The loaded solvent containing carbon dioxide exits the carbon dioxide absorber in line  316 . A portion of this loaded solvent is sent through line  354  to loaded solvent pump  356  to line  358 , loaded solvent chiller  360  and line  362  back to the previously described hydrogen sulfide absorber  308 . The other portion of the loaded solvent passes in line  317  to a second hydraulic turbocharger  365 . A solvent stream passes through line  323  to recycle flash drum  320  where some of the carbon dioxide passes through line  344  to recycle compressor  346  through line  348  to recycle cooler  350  to line  352  and then enters the carbon dioxide absorber. The solvent stream exits the recycle flash drum through line  322  and passes to carbon dioxide vent drum  324  from which more of the carbon dioxide is vented through line  326 . The solvent stream then passes through line  328  to carbon dioxide vacuum drum  330  from which most of the remaining carbon dioxide is vented as shown in line  332 . The resulting solvent is now semi-lean and passes through line  334  to semi-lean solvent pump  336 , then to line  338 , semi-lean solvent chiller  340  and then to line  342  to enter hydraulic turbocharger  365 , where it is increased in pressures sufficient to enter carbon dioxide absorber  314  through line  367 . In addition, the bottom stream  310  from hydrogen sulfide absorber  308  passes to a second hydraulic turbocharger  319 . A lean solvent stream  315  also enters this second hydraulic turbocharger and shown exiting the second hydraulic turbocharger is a rich solvent stream  313  and a lean solvent stream  364 . 
         [0020]    In  FIG. 5  is shown a gas treating process that uses a first hydraulic turbocharger to recover energy from a bottom stream leaving a carbon dioxide absorber to use in pumping semi-lean solvent and a second hydraulic turbocharger to recover energy from a hydrogen sulfide concentrator to use in pumping lean solvent. A feed gas  402  and a recycle gas  404  from a hydrogen sulfide concentrator are combined in line  406  to enter a hydrogen sulfide absorber  408 . Also entering hydrogen sulfide absorber  408  is a loaded solvent  462 . A rich solvent stream  413  leaves the bottom of the hydrogen sulfide absorber  408  and a gas stream containing carbon dioxide leaves the top of the hydrogen sulfide absorber in line  412 . The gas stream in line  412  enters carbon dioxide absorber  414  where it is contacted with a semi-lean solvent that enters through line  467  and a lean solvent stream  464  that is chilled by chiller  466  and enters carbon dioxide absorber  414  through line  468 . The gas stream is treated in the carbon dioxide absorber and the treated gas exits in line  470 . The loaded solvent containing carbon dioxide exits the carbon dioxide absorber in line  416 . A portion of this loaded solvent is sent through line  454  to loaded solvent pump  456  to line  458 , loaded solvent chiller  460  and line  462  back to the previously described hydrogen sulfide absorber  408 . The other portion of the loaded solvent passes in line  417  to a hydraulic turbocharger  465 . A solvent stream passes through line  423  to recycle flash drum  420  where some of the carbon dioxide passes through line  444  to recycle compressor  446  through line  448  to recycle cooler  450  to line  452  and then enters the carbon dioxide absorber. The solvent stream exits the recycle flash drum  420  through line  422  and passes to carbon dioxide vent drum  424  from which more of the carbon dioxide is vented through line  426 . The solvent stream then passes through line  428  to carbon dioxide vacuum drum  430  from which most of the remaining carbon dioxide is vented as shown in line  432 . The resulting solvent is now semi-lean and passes through line  434  to semi-lean solvent pump  436 , then to line  438 , semi-lean solvent chiller  440  and then to line  442  to enter hydraulic turbocharger  465  where its pressure is increased sufficient to enter carbon dioxide absorber  414  through line  467 . In addition, the bottoms stream  425  from hydrogen sulfide concentrator passes to a second hydraulic turbocharger  419 . A lean solvent stream  415  also enters this second hydraulic turbocharger and shown exiting the second hydraulic turbocharger is a rich solvent stream  421  and a lean solvent stream  464 . 
         [0021]    Other embodiments may be employed that employ the basic principles of the present invention.