Patent Publication Number: US-2022219975-A1

Title: Steam reforming with carbon capture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT International Application No. PCT/US2021/071206, filed Aug. 17, 2021, titled “STEAM REFORMING WITH CARBON CAPTURE,” which claims the benefit of U.S. Provisional Application Ser. No. 63/066,467, filed Aug. 17, 2020, titled “STEAM REFORMING WITH CARBON CAPTURE,” both of which are incorporated by reference herein in their entirety. 
    
    
     FIELD 
     The present disclosure relates to systems and methods for the production of hydrogen by steam reforming. 
     SUMMARY 
     In a first aspect, a method of producing hydrogen comprises reforming a mixture of a steam and a feedstock containing carbon and hydrogen in a steam reforming unit to produce syngas comprising hydrogen, carbon dioxide, and at least one of methane and carbon monoxide; separating the syngas in at least one separation unit into a carbon dioxide rich stream, a hydrogen rich stream, and a tail gas stream containing remaining contents of the syngas; and converting at least a portion of the tail gas stream to hydrogen and carbon dioxide. 
     In some embodiments, the at least a portion of the tail gas is recycled to the steam reforming unit wherein the at least a portion of the tail gas is converted to hydrogen and carbon dioxide. In some embodiments, nitrogen is separated from the at least a portion of the tail gas before the at least a portion of the tail gas is recycled to the steam reforming unit. 
     In some embodiments, water is separated from the syngas in a water separator to produce an outlet stream of water and a stream containing remaining contents of the syngas entering the water separator. 
     In some embodiments, at least a portion of the hydrogen rich stream is used as a combustion fuel for the steam reforming unit. In some embodiments, the at least a portion of the hydrogen rich stream is turbo-expanded to perform work before being used as the combustion fuel. In some embodiments, a temperature of the at least a portion of the hydrogen rich stream is increased by at least 50° C. before being turbo-expanded. 
     In some embodiments, the carbon dioxide rich stream is separated from the syngas in a carbon dioxide separation unit, wherein the hydrogen rich stream is separated from remaining syngas in a hydrogen separation unit having the hydrogen rich outlet stream and the tail gas outlet stream, and wherein at least a portion of the tail gas stream from the hydrogen separation unit is compressed and fed into the hydrogen separation unit to separate additional hydrogen from the tail gas. 
     In a second aspect, a method of producing hydrogen comprises reforming a mixture of a steam and a feedstock containing carbon and hydrogen in a steam reforming unit to produce syngas comprising hydrogen, carbon dioxide, and at least one of methane and carbon monoxide; separating the syngas in at least one separation unit into at least a hydrogen rich stream and a tail gas stream comprising at least hydrogen and one or more of carbon dioxide, methane, and carbon monoxide; and recycling at least a portion of the tail gas stream into the steam reforming unit. 
     In some embodiments, recycling the at least a portion of the tail gas stream comprises mixing the at least a portion of the tail gas stream with the mixture of the steam and the feedstock in a line connected to the steam reforming unit. 
     In some embodiments, flue gases generated by the method do not contain carbon dioxide. 
     In some embodiments, a portion of the hydrogen rich stream is used as a combustion fuel for the steam reforming unit. In some embodiments, the portion of the hydrogen rich stream used as a combustion fuel comprises at least 40% of hydrogen entering the at least one separation unit. 
     In some embodiments, the method further comprises recompressing and recycling a portion of the tail gas stream into the at least one separation unit. 
     In some embodiments, the tail gas stream comprises at least one of carbon monoxide and methane. In some embodiments, the recycling causes carbon in the at least one of carbon monoxide and methane in the tail gas to be converted to carbon dioxide in the steam reforming unit. In some embodiments, the at least one separation unit comprises at least a carbon dioxide scrubber, and wherein carbon dioxide removed from the syngas at the carbon dioxide scrubber is sequestered. In some embodiments, any carbon compounds not removed from the syngas at the carbon dioxide scrubber are included in the at least a portion of the tail gas recycled into the steam reforming unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an example hydrogen production unit in accordance with the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     Steam reforming processes include reacting a hydrocarbon with steam and/or carbon dioxide over a catalyst to produce syngas, a mixture of hydrogen and oxides of carbon. The hydrogen content of the syngas is often increased by cooling the syngas and reacting it over a catalyst to convert some of its carbon monoxide and remaining water vapor to additional hydrogen and carbon dioxide. A final treatment is to separate the hydrogen product from the remainder of the syngas comprised of steam, CH 4 , CO, and CO 2 . The steam is condensed and separated by cooling the syngas and passing the resulting two-phase mixture through a gas-liquid separator. Hydrogen is then separated in a pressure swing adsorption (PSA) unit containing a molecular sieve, resulting in a high pressure outlet stream containing approximately 90% of the inlet hydrogen at greater than 99% purity and a low pressure outlet stream of “tail gas” comprising the remaining H 2 , and CH 4 , CO, and CO 2  constituents of the inlet syngas to the PSA unit. 
     The fuel value of the low-pressure tail gas is conveniently used to fulfill a large portion of low-pressure burner fuel requirements for heating the reformer. The process provides extremely high purity hydrogen used as a reactant in other processes. Conventional steam reforming processes are not effective, however, for lowering undesirable CO 2  emissions to the atmosphere of hydrocarbons used as fuels for heating or power generation compared to the emissions of burning the inlet feedstock. 
     CO 2  may be scrubbed from a gas mixture such as flue gas from combustion furnaces and electric power plants by which the CO 2  portion of a gas preferentially dissolves in a solvent or forms chemical or physical bonds with a liquid. Such methods include the cycling of the gas to be scrubbed between a low and/or high temperature or high- and low-pressure baths of amines, ammonia, hydroxides, or the like. Other methods of isolating CO 2  from other gas species include distillation and adsorption. 
     Although conventional CO 2  separation methods exist, they are not suitable for removing the CH 4  and CO components of the tail gas which is combusted to heat the reformer. Thus, conventional CO 2  separation methods still result in substantial CO 2  stack emissions. Removal of dilute CO 2  from a stack at atmospheric pressure is less effective and more expensive than its removal from a high concentration of CO 2  in a gas at high pressure. 
     It would be desirable to produce hydrogen by the relatively inexpensive steam reforming process while also capturing a high portion and preferably all the carbon contained in the feedstock in order to lower and preferably eliminate CO 2  emissions to the environment. In systems described herein, the carbon components of the feedstock are not exhausted to the atmosphere, but rather can be removed at high pressure as CO 2 . This technology can prevent carbon emissions from a steam reformation facility. Other objectives of the present technology will be observed by one reasonably skilled in the art. 
     Conventional steam methane reforming (SMR) wisdom is to make as much hydrogen product for export as possible from a plant of a given size, to process the process gases no more than one time, and to burn all of the carbon entering the plant to provide heat for the reforming process. In contrast, the present technology substantially reduces or eliminates carbon emissions by departing from this conventional practice in some or all respects. Whereas about 89% of the hydrogen produced in the reformer is recovered as product hydrogen for export in conventional practices of SMR and PSA hydrogen production (with the other 11% being inseparable from the fuel used for heating), the present technology utilizes approximately 43% of the hydrogen produced in the reformer to be intentionally combusted for heating purposes. Accordingly, approximately 57% of the hydrogen at high purity is recovered as product hydrogen for export in embodiments of the present technology. This in turn entails the novel and counterintuitive consumption of some of the purified PSA hydrogen as fuel and the recycling of carbon (in the form of PSA tail gas) from the syngas to the reformer feedstock rather than combusting carbon in the SMR furnace or scrubbing carbon from flue and/or exhaust gasses. This sacrificial combustion of fully refined hydrogen product is non-intuitive to one skilled in the art. However, by the discretionary use of hydrogen as fuel, by recycling PSA tail gas to the reformer mixed feed, and by forgoing the combustion of carbon to heat the reforming process, the present technology differs substantially from the strategy and methods of conventional SMR processes and reduces or eliminates carbon emissions. 
     Mixed feed of steam and a hydrocarbon or other feedstock containing hydrogen and carbon is reformed over a catalyst to form a syngas. In some embodiments the syngas undergoes the following three separations. First, steam is separated from the syngas, preferably in a separate process by condensation of the steam followed by phase separation of condensed water from the syngas, producing a water outlet stream and a stream of the remaining syngas. Second, carbon dioxide is separated from the syngas, preferably in a separate process and preferably by preferential dissolution of carbon dioxide in a solvent such as an amine, producing a carbon dioxide rich outlet stream and a stream of the remaining syngas. Third, hydrogen is separated from the syngas, preferably in a separate process and preferably by PSA, producing a high-pressure outlet stream of enriched hydrogen and a low-pressure outlet stream (e.g., a tail gas stream) containing the remaining components of the syngas. The separations of water, carbon dioxide, and hydrogen may be performed in any sequence, combination, or subcombination, and by any alternative processes. In some embodiments, water is separated first, and hydrogen is separated last, such that the tail gas from the PSA unit contains substantial volume percentages of methane, carbon monoxide, and hydrogen, and nominal or trace volume percentages of steam and carbon dioxide. For example, in some embodiments the tail gas may be less than 1% steam and less than 5% CO 2  by volume. 
     The carbon dioxide outlet stream exits the process and may be sequestered or used for some purpose such as a chemical reactant or addition or to enhance the recovery of subterranean oil or natural gas. 
     The hydrogen rich stream from the PSA unit is separated into a first hydrogen stream used as a combustion fuel, such as for heating the reforming furnace, and a second hydrogen stream that is exported or used as a product for a purpose outside the steam reforming unit. 
     The first hydrogen stream may be turbo-expanded prior to entering the reforming furnace as a fuel. The first hydrogen stream may also be raised in temperature (e.g., by at least 50° C. and in some embodiments to at least 800° C.) prior to turbo-expansion at a temperature which may be less than 300° C. The turbo-expander may also perform useful work such as to drive a compressor or turn an electric generator, etc. The hydrogen may be expanded in multiple stages of alternating reheating and further expansion. 
     In some embodiments, the tail gas containing CH 4 , CO, and H 2  is compressed and used as a feedstock in a steam reforming unit, such as the reforming unit described above. 
     In some embodiments, the tail gas stream is compressed and conveyed to a PSA unit, such the PSA unit described above, wherein hydrogen is separated from the other contents of the tail gas. 
     In some embodiments, the tail gas is compressed and divided into a first tail gas stream used as feed to a reformer and a second tail gas stream fed to a PSA unit for separation of hydrogen from the other contents of the second tail gas stream. The first tail gas stream can be used to purge the CH 4  and CO content of the syngas and tail gas. The second tail gas stream can be used to increase the recovery of hydrogen from the tail gas. 
     In some embodiments, nitrogen undesirably introduced with the feedstock is separated and purged from the second tail gas stream. For example, the second tail gas stream may be compressed and then subjected to membrane separation wherein nitrogen is retained by the membrane and H 2 , CH 4 , and CO permeate the membrane at a pressure suitable for entry into the reformer as feedstock. 
     Referring now to  FIG. 1 , an example hydrogen production unit implementing carbon capture aspects of the present technology will be described. As shown in  FIG. 1 , line  1  conveys a hydrocarbon feedstock to a heater  2 . Within the heater  2 , the feedstock can be preheated to a temperature suitable for desulfurization. The preheated feedstock is conveyed by line  3  from the heater  2  to a desulfurization unit  4  wherein the feedstock is desulfurized. Line  5  conveys the desulfurized feedstock from desulfurization unit  4  to line  6  carrying steam, wherein the feedstock mixes with the steam in line  6 . Line  7  conveys boiler feed water to a boiler  8  wherein the boiler feed water is raised to steam. Line  9  conveys the steam from the boiler  8  to line  6  wherein the steam mixes with the feedstock from line  5  to form a mixed feed. Line  6  conveys the mixed feed to a reforming reactor tube  10  wherein the mixed feed is heated and converted over a catalyst to a syngas containing H 2 , CH 4 , CO, CO 2 , and steam. Reforming reactor tube  10  resides at least partially within, and is heated by, a furnace  11 . Line  12  conveys the syngas from the reforming reactor tube  10  to a water gas shift (WGS) unit  13  wherein some of the CO and steam in the syngas react to form additional H 2  and CO 2 . The syngas may be cooled from its peak temperature within the reforming reactor tube  10  to a lower temperature in line  12  and in WGS unit  13  via heat exchange against mixed feed within a bayonet reforming reactor tube  10  as shown, or via other forms of heat exchangers or coolers. Line  14  conveys the shifted syngas from the WGS unit  13  to a water knockout unit  15  wherein the syngas is cooled, some of the steam condenses to water from the syngas, and the condensed water is separated from the remaining syngas and exits via line  16 . Line  17  conveys the syngas from the water knockout unit  15  to a carbon dioxide scrubber  18  wherein carbon dioxide is dissolved in an amine solvent at a first temperature and the solvent is isolated from the syngas and heated to a second temperature at which carbon dioxide comes out of solution in gaseous state. The separated CO 2  exits via line  19 . The separated carbon dioxide may be further compressed for a specific use or for sequestration. The syngas in the carbon dioxide scrubber  18  may be at a pressure of greater than 10 bar. The carbon dioxide may be expelled at a pressure greater than 10 bar and preferably 30-60 bar. 
     Line  20  conveys syngas from the scrubber to a pressure swing adsorption (PSA) unit  21 . Within the PSA unit  21 , about 90% of the hydrogen (e.g., 89%) is separated from the remaining constituents of the syngas. Line  22  conveys high purity, high pressure hydrogen from the PSA unit  21  to line  23 , and can be separated into 2 streams, for example, into lines  23  and  24 . In some embodiments, line  24  conveys a portion of the hydrogen from line  22  through the heater  2  wherein the hydrogen is heated (e.g., to at least 300° C. and in some embodiments to at least 800° C.) and then conveyed to a turbo-expander  25  wherein the hydrogen expands and performs work. In some embodiments, the line  24  can carry about 40% of the hydrogen in line  22  form the PSA unit  21 . In some embodiments, the line  24  can advantageously carry about 43% of the hydrogen in line  22 . One of skill in the art, guided by this disclosure, will understand that the amount of the hydrogen product from line  22  can be varied as required to meet operational requirements. The turbo-expander  25  may provide power to turn an electric generator  40 , for example. Line  26  conveys at least a portion of the expanded hydrogen from the turbo-expander  25  to line  27  and a regenerative burner  28  of the heater  2 . Line  26  also conveys at least a portion of the expanded hydrogen from the turbo-expander  25  to line  29  and a regenerative burner  30  of the boiler  8 . Line  26  further conveys at least a portion of the expanded hydrogen from the turbo-expander  25  to line  31  and a regenerative burner  32  of the furnace  11 . Each of the said regenerative burners can be fed air by a line  33  and can exhaust cooled flue gas via line or stack  34 . 
     Line  35  conveys low pressure tail gas from the PSA unit to a compressor  36  wherein the low pressure tail gas is compressed. As discussed above, the tail gas may include the remaining H 2 , CH 4 , CO, and CO 2  constituents of the inlet syngas. Line  38  conveys the compressed tail gas from the compressor  36  to line  6 , wherein the compressed tail gas mixes with the other mixed feed to be reformed in the reforming reactor tube  10 . Thus, as described herein, the carbon-rich tail gas can be recycled as a portion of the mixed feed to be reformed rather than being emitted, thus reducing or eliminating carbon emissions (e.g., CH 4 , CO, and/or CO 2  emissions). Recycling carbon-rich tail gas can require a larger reforming reactor tube  10  than in conventional systems. 
     In some embodiments, line  41  may convey a portion of the compressed tail gas in line  38  through a membrane unit  42  and back to line  38  and to line  6  wherein the tail gas mixes with mixed feed and is conveyed into reformer tube  10 . The membrane unit  42  can separate and purge some nitrogen introduced in the feedstock in line  1  from the tail gas. The purged nitrogen exits via line  43 . Line  37  may convey some of the compressed tail gas from the compressor to line  20 , which conveys it into the PSA unit  21  for additional hydrogen separation from the syngas. 
     Other advantages and other embodiments of the current invention will be obvious to those skilled in the art. Their omission here is not intended to exclude them from the claims advanced herein. 
     Although the present invention has been described in terms of certain preferred embodiments, various features of separate embodiments can be combined to form additional embodiments not expressly described. Moreover, other embodiments apparent to those of ordinary skill in the art after reading this disclosure are also within the scope of this disclosure. Furthermore, not all the features, aspects and advantages are necessarily required to practice the present technology. Thus, while the above detailed description has shown, described, and pointed out novel features of the present technology as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the apparatus or process illustrated may be made by those of ordinary skill in the technology without departing from the spirit or scope of the present disclosure. The present technology may be embodied in other specific forms not explicitly described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner.