Abstract:
The present invention relates to an integrated membrane/adsorbent process and system for removal of carbon dioxide from natural gas on a ship that houses natural gas purification equipment. Additional membrane units or adsorbent beds are used to reduce the amount of product gas that is lost in gas streams that are used to regenerate the adsorbent beds. These systems produce a product stream that meets the specifications of less than 50 parts per million carbon dioxide in natural gas for liquefaction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Provisional Application No. 61/358,433 filed Jun. 25, 2010, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a process and system for removing carbon dioxide from natural gas in a floating environment, such as on a ship. More specifically, the invention relates to an integrated membrane/adsorbent system for removal of carbon dioxide from natural gas on a ship that houses natural gas purification equipment. 
         [0003]    In an LNG (Liquefied Natural Gas) plant, carbon dioxide content in the feed gas stream must be reduced to less than 50 ppmv before liquefaction to avoid formation of dry ice within the system. Commercially this can be achieved by using a solvent absorption process such as contacting the natural gas with an amine solvent to remove the carbon dioxide, which is then followed with the natural gas being sent through a molecular sieve dehydration unit to remove water down to below 1 ppmv. 
         [0004]    Depending on the amount of carbon dioxide and the volume in the inlet gas stream, membrane processes have also been used to remove the bulk of the carbon dioxide in front of a downstream amine unit. One of the benefits of this membrane-amine hybrid system is the reduction of the size of amine column that is needed and as well as a reduction in its energy consumption. Adsorption systems have also been used for front-end feed purification for LNG plants. TSA (Temperature Swing Adsorption) processes employing molecular sieves such as 4A or 13X zeolites can remove both carbon dioxide and water from natural gas streams. A growing application for a TSA process is for peak shaving of pipeline gas, where a portion of the pipeline gas is converted and stored as an LNG when demand is low. In the TSA process, the adsorbed carbon dioxide and water in the molecular sieve column are regenerated using a hot purge gas, typically from the feed or the product gas stream. The hot regeneration gas is cooled to knock out most of the water and is then returned to the pipeline. The carbon dioxide removed from the adsorbent, which is not condensable at the cooler temperature, is also returned to the pipeline. 
         [0005]    There has been a renewed interest in floating liquefied natural gas (FLNG) systems as a way to develop stranded gas fields, isolated and remote from land. These fields generally are too small for permanent platform installation. An FLNG system will use a ship or barge to house necessary recovery, gas treatment, liquefaction and offloading equipment. Compared to a land based LNG plant, an FLNG system will have a greater need for a modular design to minimize the equipment footprint and weight. An additional challenge for FLNG systems is the effect of sea motion on the performance of processing equipment, especially for systems containing liquid. The removal of carbon dioxide by use of an amine system can be impacted by a loss of efficiency from rocking and tilting of the column internal components. While both membrane and TSA systems have been used commercially in offshore platform installation, nearly no operating experiences for amine systems have been reported for offshore platform applications. 
         [0006]    In general, membrane processes that use carbon dioxide-selective polymers such as cellulose acetate can not generate a residue or product stream that meets the specification levels of less than 50 ppmv CO 2 , as the process is limited by the driving force or the CO 2  partial pressure across the membrane. Molecular sieve TSA processes typically can not handle a feed stream with more than 3% CO 2 , since the size of the adsorbent beds that is required become too large and the necessary regeneration gas flow then becomes prohibitively large. Furthermore, for an FLNG application, there is no existing solution to treat or recycle the effluent regeneration gas, which contains the CO 2  removed from the feed stream. 
         [0007]    There exists a need to develop an improved process or integrated processes that can remove carbon dioxide and moisture to meet FLNG requirements. The desired processes should be compact and robust, and not susceptible to producing natural gas that is below specification due to winds and waves. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a process of treating a natural gas stream comprising sending a natural gas stream to a ship, barge or platform that is carrying equipment for purification of natural gas. The natural gas stream is sent to a membrane unit on the ship, barge or platform to remove carbon dioxide and other impurities from the natural gas stream and to produce a partially purified natural gas stream. Then, the partially purified natural gas stream to a temperature swing adsorption unit to remove carbon dioxide and produce a purified natural gas stream, and sending a regeneration gas stream to the temperature swing adsorption unit to desorb carbon dioxide from adsorbents within the temperature swing adsorption unit. The regeneration gas is then preferably subjected to additional treatment by an additional membrane unit or an additional adsorbent bed to remove the carbon dioxide and to recover natural gas from the regeneration stream to be included in the product stream. The regeneration gas may be returned to the same temperature swing adsorption unit or to a second temperature adsorption unit for additional treatment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows an integrated membrane/adsorbent bed system for purifying natural gas. 
           [0010]      FIG. 2  shows an integrated membrane/adsorbent bed system for purifying natural gas with a membrane unit to purify the regeneration stream. 
           [0011]      FIG. 3  shows an integrated membrane/adsorbent bed system for purifying natural gas with a membrane unit and an adsorbent bed to purify the regeneration stream. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    In this invention, membrane and adsorption processes are combined to remove CO 2  from a natural gas stream to below 50 ppm. The inlet gas stream is first processed by a membrane unit to lower the CO 2  level to below about 3%. The product gas or the residue gas from the membrane is sent to a molecular sieve TSA unit to further reduce the CO 2  to below 50 ppmv. In most embodiments of the invention, regeneration gas from the TSA unit, which contains the non-condensable CO 2  is recycled back to the inlet of the membrane or further processed by a combination of membrane and TSA units. The invention is particularly useful for offshore application such as FLNG and operates without the use of a solvent absorption system such as an amine solvent. 
         [0013]    The prior art practice for the front-end purification of an LNG or FLNG plant to remove CO 2  and water. The membrane section may not be needed, especially if the CO 2  content in the feed is low. The TSA regeneration gas generally can be recycled to the feed of the TSA because water is removed from a knock out after cooling the regeneration gas. 
         [0014]      FIG. 1  shows one embodiment of the present invention, where a membrane unit first treats the feed gas to lower the CO 2  level to below about 3%, and preferably below about 2%. The resulting partially purified natural gas feed stream is then cooled to about 45° C., preferably to about 35° C., more preferably to about 24° C. and even more preferably cooled to below about 5° C. This is followed by sending the gas to a molecular sieve TSA unit to further reduce the CO 2  level to below 50 ppmv. The regeneration gas from the TSA unit, which contains the non-condensable CO 2  is recycled back to the inlet of the membrane. The membrane can be of a single stage or multi-stage for increasing hydrocarbon recovery. More specifically, in  FIG. 1 , a feed  2  is shown entering a membrane unit  4 , with carbon dioxide being removed in a permeate stream  8  and the treated natural gas going to a TSA unit  12  in line  10 . The natural gas is further treated with carbon dioxide levels being reduced below 50 ppm by TSA and the fully treated natural gas is now sent to a liquefier in line  14 . A small portion of the fully treated natural gas is shown sent back to the TSA unit as a regeneration stream  16  to remove the adsorbed carbon dioxide from the adsorbent and returned to feed  2  so that the majority of this carbon dioxide may be removed by the membrane unit. 
         [0015]      FIG. 2  is another embodiment of the present invention. Referring back to the first embodiment in  FIG. 1 , the regeneration effluent stream  16  from the TSA unit may contain, on average, 2 to 5% of CO 2 , which is mixed with the inlet feed stream before the membrane unit. The feed stream can contain higher than 5% CO 2 , more typically higher than 10% CO 2  and sometimes much higher amounts of carbon dioxide. Mixing of these two streams with disparate CO 2  concentrations may result in an inefficiency of separation. Therefore, in the second embodiment of this invention, a separate membrane unit is used to treat the TSA regeneration gas. This second membrane unit removes a certain amount of CO 2  from the regeneration gas and generates a residue gas that has the same CO 2  composition as the residue gas from the first membrane unit. The residue gases from both membrane units are sent to the TSA. More specifically, in  FIG. 2  is seen a feed  2  is shown entering a membrane unit  4 , with carbon dioxide being removed in a permeate stream  8  and the treated natural gas going to a TSA unit  12  in line  10 . The resulting partially purified natural gas feed stream is then preferably cooled to about 24° C. and more preferably cooled to below about 5° C. The natural gas is further treated with carbon dioxide levels being reduced below 50 ppm by TSA and the fully treated natural gas is now sent to a liquefier in line  14 . A small portion of the fully treated natural gas is shown sent back to the TSA unit as a regeneration stream  16  to remove the adsorbed carbon dioxide from the adsorbent and then sent to a second membrane unit  24  to remove carbon dioxide in line  28  and then return the treated regeneration stream in line  26  to line  10 . 
         [0016]    Without mixing of the two gas streams with disparate CO 2  concentrations, the combined size of the two membrane units is expected to be smaller than one single membrane unit as in  FIG. 1  of the first embodiment. However, the TSA size can still remain quite large if the feed stream to the TSA contains high concentration of CO 2 , e.g. greater than 1%. This can be improved by the third embodiment of this invention as shown in  FIG. 3 , where the residue gas from the second membrane unit is kept at a low CO 2  composition, e.g. about 0.5%, before it is sent to a second TSA unit. As the feed to this second TSA unit contains only about 0.5% of CO 2 , the TSA size and its associated regeneration gas can be reduced significantly. This regeneration gas is recycled back to the inlet of the second membrane unit. The size of the first TSA unit can also be reduced as its regeneration gas is no longer recycled to its feed as in the embodiment of  FIG. 2 . More specifically in  FIG. 3  is shown a feed  2  entering a membrane unit  4 , with carbon dioxide being removed in a permeate stream  8  and the treated natural gas going to a TSA unit  12  in line  10 . The resulting partially purified natural gas feed stream is then preferably cooled to about 24° C. and more preferably cooled to below about 5° C. The natural gas is further treated with carbon dioxide levels being reduced below 50 ppm by TSA and the fully treated natural gas is now sent to a liquefier in line  14 . A small portion of the fully treated natural gas is shown sent back to the TSA unit as a regeneration stream  16  to remove the adsorbed carbon dioxide from the adsorbent and then sent to a second membrane unit  24  to remove carbon dioxide in line  28 , go to a second TSA unit  32  to remove carbon dioxide through line  30  and then return the treated regeneration stream in line  34  to line  14 . A regeneration stream  36 , which is a portion of the treated regeneration stream  34 , is shown passing through second TSA unit  32  to remove carbon dioxide from the adsorbent with the TSA unit  32  and then to return to line  16  before it enters second membrane unit  24 . 
         [0017]    In summary, the features of the current invention are that the system is not susceptible to vibration or rocking from sea motion due to absence of liquid solvent amine unit. In addition to CO 2 , water can be removed by the membrane unit and the molecular sieve TSA unit. Both the membrane and the TSA units are integrated by further processing the regeneration gas from the TSA unit using a second membrane unit or a combination of a second membrane and a second TSA unit. 
         [0018]    Membrane materials that can be used for CO 2 /CH 4  separation include cellulose acetate, polyimide, perfluoro polymer, etc. Adsorbents that can be used for the CO 2  removal in the TSA process include zeolite A, X or Y with different levels of Si/Al ratios and with various cationic forms such as Na, Ca, Li, K, Ba, Sr, etc. The current invention is not limited to the materials used for the membrane or the adsorption process. 
         [0019]    The following examples demonstrate various applications of the current invention. 
       Example 1 
       [0020]    A natural gas stream with a CO 2  composition of 20%, a flow rate of 5,663,000 m 3 /day (200 MMSCFD) at 5171 kPa (750 psia) and 24° C. (75° F.) is to be converted to LNG. Based on the first embodiment of the current invention in  FIG. 2 , the gas first enters into a membrane unit to remove the bulk of CO 2  and the membrane residue gas is then sent to a molecular sieve TSA unit to remove CO 2  down to 50 ppm level. The regeneration off-gas from the TSA unit is recycled back to the feed of the membrane unit. The permeate gas from the membrane is burned as a fuel. The required membrane and TSA sizes and the flow rates are summarized in Table 1 for four different cases of 0.5, 1, 2 and 3% of CO 2  compositions in the membrane residue stream or the feed to the TSA unit. The results are all referenced to the case with 0.5% CO 2 , where the calculated flow rates and equipment sizes are scaled to 100 for this case. Also included in the table are the recoveries of CH 4  from the integrated process and the relative total equipment weight and footprint. As can be seen, increasing the CO 2  concentration for the feed to the TSA increases the size or the sorbent volume and the regeneration gas flow of the TSA unit. However, the size of the membrane unit decreases. Overall, decreasing the CO 2  concentration of the TSA feed reduces the total equipment weight, but the minimum footprint appears at about 1% CO 2 . The methane recovery increases with the increasing CO 2  concentration for the membrane residue gas or the feed to the TSA unit. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Simulation Results from Example 1 
               
             
          
           
               
                   
                 Feed to TSA, CO 2  % 
                 0.5% 
                 1% 
                 2% 
                 3% 
               
               
                   
                   
               
             
          
           
               
                   
                 relative flow to TSA 
                 100 
                 141 
                 222 
                 374 
               
               
                   
                 relative reg flow of TSA 
                 100 
                 214 
                 556 
                 1305 
               
               
                   
                 number of TSA beds 
                 3 
                 4 
                 12 
                 20 
               
               
                   
                 relative sorbent volume 
                 100 
                 243 
                 631 
                 1433 
               
               
                   
                 relative membrane size 
                 100 
                 75 
                 62 
                 63 
               
               
                   
                 relative flow to membrane 
                 100 
                 110 
                 141 
                 207 
               
               
                   
                 C 1  recovery 
                 100 
                 123 
                 137 
                 137 
               
               
                   
                 relative total equipment wt. 
                 100 
                 123 
                 246 
                 588 
               
               
                   
                 relative total footprint 
                 100 
                 94 
                 147 
                 244 
               
               
                   
                   
               
             
          
         
       
     
       Example 2 
       [0021]    The example is based on the first embodiment of the current invention and the same conditions as in Example 1 except that the feed temperature of the gas entering the TSA unit is cooled down to 1.7° C. (35° F.). As the treated gas from the TSA unit will be sent to a downstream liquefaction plant, cooling the gas to 1.7° C. (35° F.) is not expected to incur energy penalty. The results are summarized in Table 2, expressed in terms of relative values to the case of 0.5% CO 2  in Table 1 of Example 1. Lowering feed temperature to the TSA unit not only reduces the TSA size, but also reduces its regeneration flow. Consequently, the membrane sizes are also reduced. Total equipment weight and footprint are all lower than those of Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Simulation Results from Example 2 
               
             
          
           
               
                   
                 Feed to TSA, CO 2  % 
                 0.5% 
                 1% 
                 2% 
                 3% 
               
               
                   
                   
               
             
          
           
               
                   
                 relative flow to TSA 
                 98 
                 135 
                 199 
                 294 
               
               
                   
                 relative reg flow of TSA 
                 83 
                 175 
                 427 
                 876 
               
               
                   
                 number of TSA beds 
                 3 
                 3 
                 8 
                 16 
               
               
                   
                 relative sorbent volume 
                 77 
                 161 
                 485 
                 999 
               
               
                   
                 relative membrane size 
                 99 
                 73 
                 58 
                 55 
               
               
                   
                 relative flow to membrane 
                 98 
                 107 
                 129 
                 169 
               
               
                   
                 C 1  recovery 
                 101 
                 126 
                 141 
                 145 
               
               
                   
                 relative total equipment wt. 
                 94 
                 99 
                 191 
                 351 
               
               
                   
                 relative total footprint 
                 98 
                 85 
                 116 
                 185 
               
               
                   
                   
               
             
          
         
       
     
       Example 3 
       [0022]    The example is also based on the first embodiment of the current invention and the same conditions as in Example 1 except that a two-stage membrane is used instead of a single stage membrane. The results are listed in Table 3, again expressed in terms of relative values to the case of 0.5% CO 2  in Table 1 of Example 1. In addition to the increasing sizes of the membrane units, the sizes of the TSA units are also increased due to the higher CH 4  recoveries and higher membrane residue gas flows into the TSA unit. As expected, the overall equipment weight or footprint is more than that of Example 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Simulation Results from Example 3 
               
             
          
           
               
                   
                 Feed to TSA, CO 2  % 
                 0.5% 
                 1% 
                 2% 
                 3% 
               
               
                   
                   
               
             
          
           
               
                   
                 relative flow to TSA 
                 197 
                 228 
                 321 
                 533 
               
               
                   
                 relative reg flow of TSA 
                 197 
                 342 
                 804 
                 1854 
               
               
                   
                 number of TSA beds 
                 4 
                 8 
                 12 
                 20 
               
               
                   
                 relative sorbent volume 
                 218 
                 372 
                 900 
                 1997 
               
               
                   
                 relative membrane size 
                 201 
                 128 
                 97 
                 98 
               
               
                   
                 relative flow to membrane 
                 195 
                 177 
                 203 
                 296 
               
               
                   
                 C 1  recovery 
                 197 
                 199 
                 198 
                 195 
               
               
                   
                 relative total equipment wt. 
                 214 
                 227 
                 346 
                 639 
               
               
                   
                 relative total footprint 
                 222 
                 185 
                 213 
                 337 
               
               
                   
                   
               
             
          
         
       
     
       Example 4 
       [0023]    The feed gas conditions are the same as Example 1, but one additional membrane unit is added to process the TSA regeneration effluent gas, following the second embodiment of the current invention in  FIG. 2 . This second membrane generates a residue gas with the same CO 2  concentration as the residue gas from the first membrane unit. The two membrane residue gases are combined, cooled to 1.7° C. (35° F.) and sent to the TSA unit. The results in terms of relative numbers are shown in Table 4. Compared with Table 2 of Example 2, the overall membrane sizes are deceased because the regeneration off-gas from the TSA unit, with average CO 2  compositions ranging from 2.7 to 4.9%, is sent to the second membrane unit, which avoids the inefficiency of mixing with a high CO 2  composition of 20% in the feed. The improved separation efficiency of the membrane unit also increases the C 1  recovery, in comparison with Example 2. However, the increased membrane residue gas flow also increases the feed to the TSA unit, and increases its size. Consequently, only a small reduction in overall equipment weight or footprint is achieved in this example. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Simulation Results from Example 4 
               
             
          
           
               
                 Feed to TSA, CO 2  % 
                 0.5% 
                 1% 
                 2% 
                 3% 
               
               
                   
               
             
          
           
               
                 relative flow to TSA 
                 101 
                 139 
                 208 
                 314 
               
               
                 relative reg flow of TSA 
                 92 
                 180 
                 447 
                 939 
               
               
                 number of TSA beds 
                 3 
                 3 
                 8 
                 16 
               
               
                 relative sorbent volume 
                 80 
                 166 
                 508 
                 1067 
               
               
                 relative total membrane size 
                 94 
                 69 
                 51 
                 46 
               
               
                 relative membrane size (M2) 
                 5 
                 7 
                 9 
                 11 
               
               
                 feed to Mem. (M2), CO 2  % 
                 2.7 
                 3.8 
                 4.6 
                 4.9 
               
               
                 C 1  recovery 
                 105 
                 129 
                 148 
                 156 
               
               
                 relative total equipment wt. 
                 92 
                 98 
                 192 
                 359 
               
               
                 relative total footprint 
                 95 
                 83 
                 113 
                 183 
               
               
                   
               
             
          
         
       
     
       Example 5 
       [0024]    The example is also based on the second embodiment of the current invention, but with a two-stage membrane for the first membrane as in the Example 3 and a single stage membrane for the second membrane. The relative results are summarized in Table 5. In comparison with the results in Table 3, the sizes of both the membrane and the TSA units are reduced and the overall equipment weight or footprint is also decreased. Because the permeate from the second membrane is not recovered by a second stage membrane, the C 1  recovery is lower than in Example 3. Consequently, the TSA sizes are reduced due to the decreased feed flows to the TSA units. 
         [0025]    A variation of this example is to use a two-stage membrane for the second membrane and a single-stage membrane for the first membrane. The choice depends on the quality and quantity of the permeate gas, which typically can be used as a fuel source. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Simulation Results from Example 5 
               
             
          
           
               
                 Feed to TSA, CO 2  % 
                 0.5% 
                 1% 
                 2% 
                 3% 
               
               
                   
               
             
          
           
               
                 relative flow to TSA 
                 188 
                 216 
                 303 
                 503 
               
               
                 relative reg flow of TSA 
                 188 
                 326 
                 759 
                 1753 
               
               
                 number of TSA beds 
                 4 
                 8 
                 12 
                 20 
               
               
                 relative sorbent volume 
                 209 
                 372 
                 851 
                 1921 
               
               
                 relative total membrane size 
                 180 
                 109 
                 74 
                 68 
               
               
                 relative membrane size (M2) 
                 9 
                 10 
                 13 
                 16 
               
               
                 feed to Mem. (M2), CO 2  % 
                 2.4 
                 3.2 
                 3.9 
                 4.2 
               
               
                 C 1  recovery 
                 188 
                 189 
                 187 
                 184 
               
               
                 relative total equipment wt. 
                 185 
                 201 
                 309 
                 595 
               
               
                 relative total footprint 
                 180 
                 152 
                 178 
                 290 
               
               
                   
               
             
          
         
       
     
       Example 6 
       [0026]    This example is based on the third embodiment of the current invention in  FIG. 3 . The example is the same as Example 4, but one additional TSA unit is used to process the residue gas from the second membrane unit, which is kept at a CO 2  composition of 0.5%. The relative results are presented in Table 6, where the case for the 0.5% CO 2  to the first TSA unit is omitted because the results are the same as in Table 4 of Example 4. As can be seen in Table 6, the sizes of the membrane units are increased because more CO 2  is removed to reach a residue gas composition of 0.5% for the second membrane. On the other hand, the overall TSA sizes are reduced, mainly due to the reduced CO 2  feed composition to the second TSA unit. This effect is more pronounced for the two cases with 2 and 3% CO 2  to the first TSA unit, which results in a significant reduction of overall equipment weight or footprint. The increased size of the second membrane unit also contributes to the reduction of C 1  recovery, compared to Table 4. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Simulation Results from Example 6 
               
             
          
           
               
                   
                 Feed to TSA, CO 2  % 
                 1% 
                 2% 
                 3% 
               
               
                   
                   
               
             
          
           
               
                   
                 relative flow to TSA1 
                 109 
                 128 
                 137 
               
               
                   
                 relative reg flow of TSA1 
                 136 
                 261 
                 390 
               
               
                   
                 relative flow to TSA2 
                 24 
                 44 
                 61 
               
               
                   
                 relative reg flow of TSA2 
                 26 
                 47 
                 51 
               
               
                   
                 number of beds, TSA1 
                 3 
                 4 
                 6 
               
               
                   
                 number of beds, TSA2 
                 2 
                 2 
                 3 
               
               
                   
                 relative total sorbent volume 
                 152 
                 350 
                 414 
               
               
                   
                 relative total membrane size 
                 73 
                 66 
                 70 
               
               
                   
                 relative membrane size (M2) 
                 11 
                 24 
                 37 
               
               
                   
                 feed to Mem. (M2), CO 2  % 
                 3.5 
                 4.2 
                 4.7 
               
               
                   
                 C 1  recovery 
                 125 
                 134 
                 131 
               
               
                   
                 relative total equipment wt. 
                 105 
                 152 
                 187 
               
               
                   
                 relative total footprint 
                 93 
                 104 
                 131