Abstract:
Low-energy, low-capital hydrogen production is disclosed. A reforming exchanger  14  is placed in parallel with an autothermal reformer (ATR)  10  to which are supplied a preheated steam-hydrocarbon mixture. An air-steam mixture is supplied to the burner/mixer of the ATR  10  to obtain a syngas effluent at 650°-1050° C. The effluent from the ATR is used to heat the reforming exchanger, and combined reformer effluent is shift converted and separated into a mixed gas stream and a hydrogen-rich product stream. High capital cost equipment such as steam-methane reformer and air separation plant are not required.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of our earlier provisional application U.S. Ser. No. 60/320,015, filed Mar. 18, 2003. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates to the production of a synthesis gas (syngas) using an autothermal reactor (ATR) and a reforming exchanger. 
     Reforming of hydrocarbons is a standard process applying a plurality of generally endothermic reactions for the production of hydrogen-containing synthesis gas used for manufacturing ammonia or methanol, for example. A conventional autothermal reforming reactor (ATR) is a form of steam reformer including a catalytic gas generator bed with a specially designed burner/mixer to which preheated hydrocarbon gas, air or oxygen, and steam are supplied. Partial combustion of the hydrocarbon in the burner supplies heat necessary for the reforming reactions that occur in the catalyst bed below the burner to form a mixture of mostly steam, hydrogen, carbon monoxide (CO), carbon dioxide (CO2), and the like. Effluent from the steam reformer is then usually further converted in shift converters wherein CO and steam react to form additional hydrogen and CO2, especially for ammonia or other syntheses where hydrogen is a main desired syngas constituent. 
     Advantages of ATR are low capital cost and easy operation compared to a conventional catalytic steam reformer, for example. Disadvantages of commercial ATR processes are the capital costs, operating difficulties, and plot area requirements associated with the air separation unit (ASU), especially where operating personnel and plot area are limited or other factors make an ASU undesirable. Where the synthesis gas is used for ammonia production, low temperature distillation has been used to remove excess nitrogen and other impurities to obtain a 99.9% purity level. 
     The present invention addresses a need for producing hydrogen from an ATR without using an ASU and/or low temperature distillation, by operating the ATR with excess air, supplying the ATR process effluent to a reforming exchanger to provide heat for additional syngas production, and partially purifying the product hydrogen stream without the need for low temperature processing for nitrogen rejection. Reforming exchangers used with autothermal reformers are known, for example, from U.S. Pat. Nos. 5,011,625 and 5,122,299 to LeBlanc and U.S. Pat. No. 5,362,454 to Cizmer et al., all of which are hereby incorporated herein by reference in their entirety. These reforming exchangers are available commercially under the trade designation KRES or Kellogg, Brown and Root (KBR) Reforming Exchanger System. 
     SUMMARY OF INVENTION 
     The present invention uses a reforming exchanger in parallel with an autothermal reactor (ATR) in a new hydrogen plant with reduced capital costs, reduced energy requirements, greater ease of operation, and reduced NOx and CO2 emissions, or in an existing hydrogen plant where the hydrogen capacity can be increased by as much as 40-60 percent with reduced export of steam from the hydrogen plant. The resulting process has very low energy consumption. 
     The present invention provides in one embodiment a process for producing hydrogen. The process includes: (a) catalytically reforming a first hydrocarbon portion with steam and air in an autothermal reactor to produce a first syngas effluent at a temperature from 650° to 1050° C., desirably 650° to 1000° C.; (b) supplying the first syngas effluent to a reforming exchanger; (c) passing a second hydrocarbon portion with steam through a catalyst zone in the reforming exchanger to form a second syngas effluent; (d) discharging the second syngas effluent from the catalyst zone adjacent the inlet to form a syngas admixture with the first syngas effluent; (e) passing the admixture across the catalyst zone in indirect heat exchange therewith to cool the admixture and heat the catalyst zone; (f) collecting the cooled admixture from an outlet of the reforming exchanger; (g) shift converting the admixture to obtain a carbon dioxide-rich gas stream lean in carbon monoxide; and (h) separating the carbon-dioxide-rich gas stream to form a hydrogen-lean, mixed gas stream comprising nitrogen and carbon dioxide and a hydrogen-rich product stream. 
     If desired, the reforming, shift conversion and mixed gas separation can be at a process pressure from 10 to 200 bars, e.g. above 30 bars. The nitrogen and carbon dioxide removal can consist of membrane separation or pressure swing adsorption, or a like unit operation that can simultaneously remove a mixture of gases from the hydrogen at the process pressure and desirably does not require separate sequential steps for carbon dioxide and nitrogen removal. The process desirably includes compressing air to the catalytic reforming with a gas turbine drive and recovering heat from exhaust from the gas turbine. The catalyst zone can include catalyst tubes, and the process can further include: supplying the first syngas effluent to a shell-side of the reformer; supplying the second hydrocarbon portion with steam through the catalyst tubes; and discharging the second syngas effluent from the catalyst tubes adjacent the shell-side inlet to form the syngas admixture. The autothermal reformer can be operated with excess air. The hydrogen-rich gas stream from the shift conversion can have a molar ratio of hydrogen to nitrogen less than 3. The nitrogen and carbon dioxide removal is desirably free of cryogenic distillation, and the process is desirably free of air separation. The proportion of the first hydrocarbon portion relative to a total of the first and second hydrocarbon portions is desirably from 55 to 85 percent. The proportion of the first hydrocarbon portion relative to a total of the first and second hydrocarbon portions is more desireably 60 to 80 percent. The hydrogen product stream can have a purity of at least 70% up to 99.5%, desirably at least 90%, more desirably at least 95%, even more desirably at least 97%, and especially at least 98.5%, by volume. The process can include supplying the hydrogen product stream to a fuel cell for the generation of an electrical current, or to a hydrotreater, e.g. to up-grade a crude oil, or to other refinery processes. 
     In another embodiment, the invention provides an apparatus for preparing syngas. The apparatus includes: (a) autothermal reactor means for catalytically reforming a first hydrocarbon portion with steam and air to produce a first syngas effluent at a temperature from 650° to 1050° C.; (b) means for supplying the first syngas effluent to an inlet of a reforming exchanger; (c) means for passing a second hydrocarbon portion with steam through a catalyst zone in the reforming exchanger to form a second syngas effluent; (d) means for discharging the second syngas effluent from the catalyst zone adjacent the inlet to form a syngas admixture with the first syngas effluent; (e) means for passing the admixture across the catalyst zone in indirect heat exchange therewith to cool the admixture and heat the catalyst zone; (f) means for collecting the cooled admixture from an outlet from the reforming exchanger; (g) means for shift converting the admixture to obtain a carbon dioxide-rich gas stream lean in carbon monoxide; and (h) means for separating the carbon-dioxide-rich gas stream to form a hydrogen-lean, mixed gas stream comprising nitrogen and carbon dioxide and a hydrogen-rich product stream. The separation means of the apparatus can include a pressure swing adsorption unit or a membrane separator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The FIGURE is a simplified schematic process flow diagram of the ATR-reforming exchanger process according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     One embodiment of a process according to the present invention has the general configuration shown in FIGURE. Desulfurized natural gas or other hydrocarbon supplied from line  2  is mixed with process steam from line  4  and the mixture is preheated in a feed preheat exchanger  6 . The steam to carbon ratio of the mixture is desirably from 2.0 to 4.0, e.g. about 3. A first portion of the preheated steam-hydrocarbon mixture is fed via line  8  to the burner in autothermal reformer (ATR)  10 , and a second portion is supplied via line  12  to the tube-side inlet of reforming exchanger  14 . If desired, additional steam can be added via line  36  to line  8 . 
     Air is supplied via line  16  and mixed with steam from line  18 , and the steam-air mixture is preheated in preheater  38 , e.g. to a temperature from 200° C. to 650° C., and sent to the burner via line  20 , taking due care to maintain the flame temperature in the burner below 1500° C. The air is desirably excess air, by which is meant that the resulting molar ratio of hydrogen to nitrogen (following shift conversion) in the syngas is less than about 3 (the typical stoichiometric ratio for ammonia syngas make-up). Using air instead of oxygen or oxygen-enriched air can be economically beneficial where the nitrogen content and/or hydrogen purity of the syngas is not critical, for example, in fuel cells, in the hydrotreatment of crude oil or heavy fractions thereof, or in applications where the nitrogen is inert and the presence thereof does not significantly affect the economics of the method for the use of the syngas. Air can be used as a substitute for pure oxygen when economic or space consideration restrict the use of a conventional air separation unit (ASU), such as when an ATR/reforming exchanger is used for producing hydrogen for use on a floating production storage and offtake (FPSO) facility. If desired, the air can be supplied by a compressor that driven by a gas turbine, and heat recovered from the gas turbine exhaust, for example, to preheat process feed streams, generate process steam, or the like. 
     The molar ratio of steam to molecular oxygen in the air-steam mixture is desirably from about 0.8 to about 1.8, more desirably about 1 to about 1.6, and the molar ratio of oxygen to carbon in the hydrocarbon feed to the ATR can be from about 0.5 to about 0.8, desirably from about 0.6 to 0.7. The split of the hydrocarbon feed to the ATR  10  (line  8 ) relative to the total hydrocarbon feed to the ATR  10  and the reforming exchanger  14  (line  2 ), is desirably from 55 to 85 percent, more desirably from 60 to 80 percent, and particularly 65 to 75 percent to the ATR. The operating conditions and flow rates are generally optimized for maximum hydrogen production. 
     The syngas effluent in line  22  from the ATR  10  can be supplied to the shell-side inlet of the reforming exchanger  14 . The reformed gas from the outlet ends of the catalyst tubes  24  mixes with the ATR effluent and the mixture passes across the outside of the catalyst tubes  24  to the shell-side outlet where it is collected in line  26 . The combined syngas in line  26  is cooled in the cross exchanger  6  and waste heat boiler  28  to produce steam for export, and supplied to downstream processing that can include a shift section  30  (which can include high temperature, medium temperature and/or low temperature shift converters), heat recovery  32 , mixed gas separation  34  such as CO2 removal (pressure swing adsorption (PSA) or membrane separation, for example), and the like, all unit operations of which are well known to those skilled in the art. The separation  34  is desirably free of low temperature or cryogenic separation processes used to remove excess nitrogen in ammonia syngas production, which require a separate upstream removal system for carbon dioxide that can solidify at the low temperature needed for nitrogen removal. 
     The heat requirement for the reforming exchanger  14  is met by the quantity and temperature of the ATR effluent. Generally, the more feed to the reforming exchanger, the more heat required to be supplied from the ATR effluent. The temperature of the ATR effluent in line  22  should be from 650° to 1000° C. or 1050° C., and can desirably be as hot as the materials of construction of the reforming exchanger  18  will allow. If the temperature is too low, insufficient reforming will occur in the reforming exchanger  14 , whereas if the temperature is too high the metallurgical considerations become problematic. Care should also be taken to ensure that operating conditions are selected to minimize metal dusting. Operating pressure is desirably from 10 to 200 bars or more, especially at least 25 or 30 bars, and can be conveniently selected to supply the hydrogen product stream at the desired pressure, thereby avoiding the need for a hydrogen compressor. 
     The present invention is illustrated by way of an example. A reforming exchanger installed with an ATR as in the FIGURE where air is used in place of oxygen for 50 MMSCFD hydrogen production has a total absorbed duty in the fired process heater of 38.94 Gcal/hr, and has the associated parameters shown in Table 1 below: 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 ATR-Reforming Exchanger Process with Excess Air 
               
             
          
           
               
                   
                 Catalyst 
                 ATR 
                 ATR 
                 Shell-side 
                 Air-steam 
               
               
                   
                 tube inlet, 
                 feed, 
                 effluent, 
                 outlet, 
                 to ATR, 
               
               
                 Stream ID: 
                 line 12 
                 line 8 
                 line 22 
                 line 26 
                 line 20 
               
               
                   
               
             
          
           
               
                 Dry Mole Fraction 
               
             
          
           
               
                 H2 
                 0.0200 
                 0.0200 
                 0.3578 
                 0.4492 
                   
               
               
                 N2 
                 0.0190 
                 0.0190 
                 0.4628 
                 0.3561 
                 0.7804 
               
               
                 CH4 
                 0.9118 
                 0.9118 
                 0.0013 
                 0.0036 
               
               
                 AR 
                 0.0000 
                 0.0000 
                 0.0055 
                 0.0042 
                 0.9400 
               
               
                 CO 
                 0.0000 
                 0.0000 
                 0.0835 
                 0.1026 
               
               
                 CO2 
                 0.0000 
                 0.0000 
                 0.0891 
                 0.0843 
                 0.0300 
               
               
                 O2 
                 0.0000 
                 0.0000 
                 0.0000 
                 0.0000 
                 0.2099 
               
               
                 C2H6 
                 0.0490 
                 0.0490 
                 0.0000 
                 0.0000 
               
               
                 C3H8 
                 0.0002 
                 0.0002 
                 0.0000 
                 0.0000 
               
               
                 Total Flow 
                 312.6 
                 713.9 
                 4154.2 
                 5414.7 
                 2446.2 
               
               
                 KMOL/HR (dry) 
               
               
                 H2O 
                 947.7 
                 2164.0 
                 2827.0 
                 3380.6 
                 728.9 
               
               
                 KMOL/HR 
                   
               
               
                 Total Flow 
                 1260.3 
                 2878.0 
                 6981.2 
                 8795.3 
                 3175.1 
               
               
                 KMOL/HR 
               
               
                 Total Flow 
                 22288 
                 50896 
                 134887 
                 156700 
                 83990 
               
               
                 KG/HR 
               
               
                 Pressure 
                 25.9 
                 25.9 
                 22.4 
                 22.1 
                 24.0 
               
               
                 (kg/cm 2  abs) 
               
               
                 Temperature 
                 601 
                 601 
                 1011 
                 747 
                 621 
               
               
                 (° C.) 
               
               
                   
               
             
          
         
       
     
     In addition, the data in Table 1 are for an example that represents low capital cost, low energy consumption, easy operation, and reduced NOx and CO2 (56 percent less than a comparable steam reforming hydrogen plant of the same capacity) and CO2 emissions. This process is an attractive option for construction of new hydrogen production facilities where excess nitrogen is desired or can be tolerated, or can be economically removed from the sythesis gas. 
     As another example, a reforming exchanger is installed with an ATR as shown in the FIGURE wherein air is used as the oxygen source, for a 50 MMSCFD hydrogen production. Typical pressures and temperatures are indicated in the FIGURE for this example, and other associated parameters are shown in Table 2 below: 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 ATR-Reforming Exchanger Process with Excess Air Oxidant 
               
             
          
           
               
                   
                 Catalyst 
                   
                 ATR 
                 Shell-side 
                 Air-steam 
               
               
                   
                 tube inlet 
                 ATR feed 
                 effluent, 
                 outlet, line 
                 to ATR, 
               
               
                 Stream ID: 
                 12 
                 line 8 
                 line 22 
                 26 
                 line 20 
               
               
                   
               
             
          
           
               
                 Dry Mole Fraction 
               
             
          
           
               
                 H2 
                 0.0200 
                 0.0200 
                 0.4115 
                 0.4792 
                   
               
               
                 N2 
                 0.0023 
                 0.0023 
                 0.4020 
                 0.3089 
                 0.7804 
               
               
                 CH4 
                 0.9612 
                 0.9612 
                 0.0026 
                 0.0227 
               
               
                 AR 
                 0.0000 
                 0.0000 
                 0.0048 
                 0.0037 
                 0.0094 
               
               
                 CO 
                 0.0000 
                 0.0000 
                 0.0803 
                 0.0875 
               
               
                 CO2 
                 0.0150 
                 0.0150 
                 0.0987 
                 0.0980 
                 0.0003 
               
               
                 O2 
                 0.0000 
                 0.0000 
                 0.0000 
                 0.0000 
                 0.2099 
               
               
                 C2H6 
                 0.0013 
                 0.0013 
                 0.0000 
                 0.0000 
               
               
                 C3H8 
                 0.0002 
                 0.0002 
                 0.0000 
                 0.0000 
               
               
                 Total Flow 
                 371.5 
                 754.3 
                 4069.7 
                 5299.5 
                 2094.1 
               
               
                 KMOL/HR (dry) 
               
               
                 H2O 
                 1074.8 
                 2182.2 
                 2610.9 
                 3325.1 
                 656.2 
               
               
                 KMOL/HR 
                   
               
               
                 Total Flow 
                 1446.3 
                 2936.5 
                 6680.5 
                 8624.6 
                 2750.3 
               
               
                 KMOL/HR 
               
               
                 Total Flow 
                 25395 
                 51557 
                 124039 
                 149434 
                 72482 
               
               
                 KG/HR 
               
               
                 Pressure 
                 25.5 
                 23.6 
                 22.8 
                 22.5 
                 23.6 
               
               
                 (kg/cm 2  abs) 
               
               
                 Temperature 
                 601 
                 601 
                 884 
                 659 
                 621 
               
               
                 (° C.) 
               
               
                   
               
             
          
         
       
     
     The data in Table 2 are also for an example that represents low capital cost, low energy consumption, easy operation, and reduced NOx and CO2 emissions. The effluent recovered from the reforming exchanger includes 47.9% H2, 30.9% N2, 8.8% CO, and 9.9% CO2. The reforming exchanger effluent undergoes shift conversion, as shown in the FIGURE, resulting in an effluent having a composition of 51.9% H2, 28.6% N2, 0.5% CO, and 16.6% CO2. Purification by PSA results in a purified product having a composition of 98.0% H2, 0.80% N2, and 1.0% CH4. 
     The foregoing description of the invention is illustrative and explanatory of the present invention. Various changes in the materials, apparatus, and process employed will occur to those skilled in the art. It is intended that all such variations within the scope and spirit of the appended claims be embraced thereby.