Abstract:
Low-energy hydrogen production is disclosed. A reforming exchanger is placed in parallel with a partial oxidation reactor in a new hydrogen plant with improved efficiency and reduced steam export, or in an existing hydrogen plant where the hydrogen capacity can be increased by as much as 20–30 percent with reduced export of steam from the hydrogen plant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of our earlier provisional application U.S. Ser. No. 60/320,011, filed Mar. 16, 2003. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates to the production of a synthesis gas (syngas) using a partial oxidation (POX) reactor and a reforming exchanger. 
     Reforming of hydrocarbons is a standard process for the production of hydrogen-containing synthesis gas used for ammonia or methanol, for example. Conventional POX reactors are unpacked, free-flow, non-catalytic gas generators to which preheated hydrocarbon gas and oxygen are supplied, optionally with a temperature moderator. The partial oxidation reactor effluent is then quenched or cooled, typically to 200–300° C., optionally cleaned to remove soot, and usually further converted in high and low temperature shift converters wherein CO and steam react to form additional hydrogen and CO2. Syngas with high hydrogen content is especially desirable for ammonia or other synthesis processes where hydrogen is the main reactant from the syngas. The steam to hydrocarbon weight ratio in the POX reactor feed is generally from 0.1 to 5, the atomic ratio of oxygen to carbon in the hydrocarbon is in the range from 0.6 to 1.6, and reaction times vary from 1 to 10 seconds. 
     POX reactors are described, for example, in U.S. Pat. Nos. 2,896,927; 3,920,717; 3,929,429; and 4,081,253, which are hereby incorporated herein by reference in their entirety. 
     POX reactors produce a syngas effluent at a very high temperature prior to quenching, e.g. from 1100° to 1650° C. This means that much of the hydrocarbon feed must, in effect, be used as a rather expensive fuel to preheat feeds and generate high- or medium-pressure steam. However, the steam production is usually far in excess of plant requirements and must therefore be exported, and frequently there is little or no market for the steam. 
     There is a need in the art for a way to improve efficiency of hydrogen plants that use POX reactors and reduce or eliminate the steam export. It is also frequently desired to maximize or increase hydrogen production from an existing hydrogen plant; however, the POX reactor is frequently a capacity-limiting operation. POX reactors cannot easily be expanded to increase production. 
     The present invention addresses these needs by supplying the partially cooled POX reactor process effluent to the shell side of a reforming exchanger to provide heat for additional syngas production. 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 Reforming Exchanger System. 
     SUMMARY OF INVENTION 
     The present invention uses a reforming exchanger in parallel with a partial oxidation (POX) reactor in a new hydrogen plant with improved efficiency and reduced steam export, or in an existing hydrogen plant. In one embodiment, the hydrogen capacity can be increased by as much as 20 to 30 percent with reduced export of steam from the hydrogen plant. The resulting process has very low energy consumption. 
     The present invention provides a process for preparing syngas. The method includes: (a) partially oxidizing a first hydrocarbon portion with oxygen in a partial oxidation reactor to produce a first reactor effluent; (b) cooling the first reactor effluent to a temperature from 650° and 1000° C.; (c) supplying the first reactor effluent to a reforming exchanger; (d) passing a second hydrocarbon portion with steam through a catalyst zone in the reforming exchanger to form a second reactor effluent; (e) discharging the second reactor effluent from the catalyst zone to form an admixture with the first reactor effluent; (f) passing the admixture across the catalyst zone in indirect heat exchange therewith to cool the admixture and heat the catalyst zone; and (g) collecting the cooled admixture from the reforming exchanger. 
     The cooling can include introducing water into the first reactor effluent as a quench fluid, indirect heat exchange, or a combination of water quenching and indirect heat exchange. The indirect heat exchange can be used to preheat the second hydrocarbon portion in a cross exchanger. The catalyst zone can include catalyst tubes. The method can also include supplying the second hydrocarbon portion to a tube side of the reforming exchanger and passing it through the catalyst tubes, and supplying the cooled first reactor effluent to a shell side inlet of the reforming exchanger. The shell side inlet can be adjacent an outlet end of the catalyst tubes. The method can further include supplying the first and second hydrocarbon portions in a weight ratio of from 40:60 to 95:5. More desirable, the first and second hydrocarbon portions can be supplied in a weight ratio of from 40:60 to 60:40 (for more efficient hydrogen production), or from 80:20 to 95:5 (if more CO is desired). 
     The present invention also provides a syngas production apparatus. The apparatus includes: (a) partial oxidation reactor means for partially oxidizing a first hydrocarbon portion with oxygen to produce a first reactor effluent; (b) means for cooling the first reactor effluent to a temperature from 650° to 1000° C.; (c) means for supplying the first reactor effluent to a reforming exchanger; (d) means for passing a second hydrocarbon portion with steam through a catalyst zone in the reforming exchanger to form a second reactor effluent; (e) means for discharging the second reactor effluent from the catalyst zone to form an admixture with the first reactor effluent; (f) means for passing the admixture across the catalyst zone in indirect heat exchange therewith to cool the admixture and heat the catalyst zone; and (g) means for collecting the cooled admixture from the reforming exchanger. 
     The present method further provides a method for retrofitting a syngas process comprising a partial oxidation reaction step for converting a first hydrocarbon stream to a first reactor effluent, a heat recovery step for cooling the first reactor effluent and producing steam with the recovered heat, and a downstream processing step for receiving the cooled reactor effluent and producing a product syngas of enhanced hydrogen content. The retrofit includes: (a) a step for partially cooling the first reactor effluent to a temperature from 650° to 1000° C.; (b) a step for diverting the partially cooled first reactor effluent to a reforming exchanger; (c) a step for passing a second hydrocarbon portion with steam through a catalyst zone in the reforming exchanger to form a second reactor effluent; (d) a step for discharging the second reactor effluent from the catalyst zone to form an admixture with the first reactor effluent; (e) a step for passing the admixture across the catalyst zone in indirect heat exchange therewith to cool the admixture and heat the catalyst zone; and (f) a step for supplying the admixture from the reforming exchanger to the heat recovery step. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified schematic process flow diagram of a conventional prior art POX process that can be retrofitted according to one embodiment of the present invention. 
         FIG. 2  is a simplified schematic process flow diagram of a syngas process with a POX reactor and a reforming exchanger integrated according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The retrofit candidate plants for the present invention have the general configuration shown in  FIG. 1 . 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 (not shown). The preheated steam-hydrocarbon mixture is fed via line  6  to a POX reactor  8  (or a plurality of POX reactors) with oxygen  10  and the effluent is collected in line  12 , quenched with water injected via line  14 , and then supplied to downstream processing  15  that can include a shift section (high temperature, medium temperature and/or low temperature shift converters), heat recovery, CO2 removal (pressure swing absorption or PSA, for example), and the like. A hydrogen-rich syngas stream  17  is produced. 
     The plant of  FIG. 1  is retrofitted, or a new plant is built, in accordance with one embodiment of the present invention as shown in  FIG. 2 . The POX reactor(s)  8  and lines  2 ,  4 ,  6 ,  10  are conventional as described in reference to  FIG. 1 . The process effluent in line  12  from the POX reactor(s)  8  is quenched with process water via line  14  to 700°–1100° C., desirably 750°–1000° C., and the mixture supplied via line  16  to the shell-side inlet of the reforming exchanger  18 . A heat exchanger  15  can be used in addition to, or in lieu of, quench line  14 . The heat exchanger  15  can be used to preheat feed stream  19 . 
     A preheated mixture in line  19  of steam and hydrocarbon, which can be the same or different as the hydrocarbon in line  2 , is supplied to a tube-side inlet of the reforming exchanger  18 . The mixture passes through the catalyst tubes  20  to form additional hydrogen-containing gas. The reformed gas from outlet openings of the catalyst tubes  20  mixes with the POX reformer effluent and the mixture passes across the outside of the catalyst tubes  20  to the shell-side outlet where it is collected in line  22  in a conventional manner. The combined syngas in line  22  is then supplied to conventional downstream processing  24  as in  FIG. 2 , which can include a shift converter, a heat exchange unit for the recovery of heat, and further purification, producing purified molecular hydrogen. In the retrofit application, the downstream processing units can be modified or expanded as necessary to handle the additional syngas supplied via line  22  that results from the addition of the reforming exchanger  18 . 
     The heat requirement for the reforming exchanger  18  is met by the quantity and temperature of the POX reactor effluent. Generally, the more feed in line  19  to the reforming exchanger  18 , the more heat required from the POX reactor effluent  16  to sustain the generally endothermic reforming reaction in the catalyst tubes  20 . The temperature of the reformer catalyst tube effluent gas is desirably as hot as the materials of construction of the reforming exchanger  18  will allow, e.g. from 750° to 1000° C. in the standard KRES unit. If the temperature is too low, insufficient reforming can occur in the reforming exchanger  18 , whereas if the temperature is too high the metallurgical considerations might become problematic. Care should also be taken to ensure that the temperature is selected to minimize metal dusting. 
     The proportion of hydrocarbon feed to the POX reactor(s)  8  can range from 40 to 95 percent of the total, whereas the proportion to the reforming exchanger  18  can be from 5 to 60 percent of the total hydrocarbon feed. The feed split between the POX reactor(s)  8  and the reforming exchanger  18  is desirably such that the POX reactor(s)  8  must produce a suitable volume of hot effluent to provide the heat requirements of the reforming exchanger  18 . A feed split to the POX reactor(s)  8  of from 40 to 60 percent of the total is beneficial for improved energy efficiency and maximizing the hydrogen production rate, whereas feeding from 80 to 95 percent of the total hydrocarbon feed to the POX reactor(s)  8  is beneficial for making more CO in the syngas. 
     The present invention is illustrated by way of an example. Preliminary process design parameters for an integrated POX-reforming exchanger unit installed as in  FIG. 2  were developed based on the retrofit of the typical POX process of  FIG. 1  with the stream composition and flow rate for line  16  indicated in Table 1 below. Compositions, properties and flow rates for selected streams in the process modified in accordance with the configuration of  FIG. 2  are also shown in Table 1. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 POX Reactor-Reforming Exchanger Configuration 
               
             
          
           
               
                   
                 Stream ID: 
               
             
          
           
               
                   
                 POX 
                 Catalyst 
                 Catalyst 
                 Shell-Side 
               
               
                   
                 Effluent 
                 Tube 20 
                 Tube 20 
                 Outlet Line 
               
               
                   
                 Line 16 
                 Inlet 
                 Exit 
                 22 
               
             
          
           
               
                 Component 
                 Stream Composition, dry mole percent 
               
               
                   
               
             
          
           
               
                 H 2   
                 62.35 
                 1.80 
                 73.79 
                 64.21 
               
               
                 N 2   
                 0.66 
                 1.80 
                 0.47 
                 0.63 
               
               
                 CH 4   
                 0.66 
                 94.40 
                 3.04 
                 1.05 
               
               
                 Ar 
                 0.11 
                 0.00 
                 0.00 
                 0.09 
               
               
                 CO 
                 33.26 
                 0.10 
                 16.52 
                 30.54 
               
               
                 CO 2   
                 2.96 
                 0.20 
                 6.17 
                 3.49 
               
               
                 C 2 H 6   
                 0.00 
                 1.20 
                 0.00 
                 0.00 
               
               
                 C 3 H 8   
                 0.00 
                 0.30 
                 0.00 
                 0.00 
               
               
                 i-C 4   
                 0.00 
                 0.10 
                 0.00 
                 0.00 
               
               
                 i-C 5   
                 0.00 
                 0.10 
                 0.00 
                 0.00 
               
               
                 Total Flow, kmol/hr 
                 636.2 
                 32.1 
                 123.5 
                 759.7 
               
               
                 H 2 O, kmol/hr 
                 153.2 
                 85.8 
                 50.3 
                 203.5 
               
               
                 Total Flow, kmol/hr 
                 789.4 
                 117.9 
                 173.8 
                 963.1 
               
               
                 Total Flow, kg/hr 
                 10,528 
                 2,073 
                 2,073 
                 12,601 
               
               
                 Pressure (bar (a)) 
                 32.4 
                 35.5 
                 32.4 
                 32.1 
               
               
                 Temperature (° C.) 
                 999.7 
                 308.8 
                 938.1 
                 702.3 
               
               
                   
               
             
          
         
       
     
     In the base case with a POX reactor only, the syngas produced from the reforming section of the plant will have the composition and flow rate of the POX reactor effluent in line  16 . Using the reforming exchanger in parallel with the POX reactor according to this embodiment of the invention, the effluent in line  16  is mixed with the gas exiting the catalyst tubes  20  to obtain a syngas having the composition in line  22 . This example shows that an integrated POX-reforming exchanger process can be used to recover waste heat in the reforming exchanger and increase hydrogen production by 20 to 25 percent. Using process heat for the additional hydrogen generation in this manner yields a corresponding reduction in steam export. 
     The invention is described above with reference to non-limiting examples provided for illustrative purposes only. Various modifications and changes will become apparent to the skilled artisan in view thereof. It is intended that all such changes and modifications within the scope and spirit of the appended claims be embraced thereby.