Abstract:
The present invention is a method of reducing the carbon dioxide balance from a reformer furnace flue gas to the high pressure syngas exit water gas shift reaction unit. Introducing a heated gas mixture into at least one pre-reforming chamber. The heating being provided by indirect heat exchange with one or more of an SMR furnace flue gas or an SMR furnace syngas introducing the gas mixture into a standard H2 PSA unit, wherein the gas mixture is separated into a hydrogen enriched stream and a PSA tail gas stream; introducing the PSA tail gas stream into a CPU system, wherein the PSA tail gas stream is separated into a carbon dioxide enriched stream, a hydrogen rich stream, and a residual stream, and introducing the residual stream as fuel into the reformer furnace along with natural gas.

Description:
FIELD OF THE INVENTION  
       [0001]    This invention relates to a method of reducing the total carbon dioxide production and shift the balance of carbon dioxide from a reformer furnace flue gas to the high pressure syngas exit water gas shift reaction unit. 
       BACKGROUND  
       [0002]    A process for making hydrogen with low to no CO2 production is disclosed in the present invention. It incorporates the concepts described in co-pending US patent application 2010-0037521, herein incorporated by reference, describes a process for making hydrogen by adjusting the conditions in the steam methane reformer (SMR) to produce more hydrogen and CO by converting more methane and subsequently converting more of the CO to Hydrogen in a lower temperature medium temperature shift or the combination of high temperature and low temperature shift reactors. This CO2 in the syngas is then removed by contacting with an amine wash and the hydrogen is purified in a pressure swing adsorption (PSA) unit—with the residue (tail gas) of the PSA being sent to the SMR furnace to provide the necessary fuel for the furnace. Supplemental fuel is provided typically by natural gas to provide the additional fuel needed to control the temperature of the SMR furnace. This process removes about 67% of the CO2 produced in the Hydrogen plant compared to a conventional steam methane reformer equipped with an amine contactor in which about 57% of the CO2 can be removed. The remaining CO2 is produced from remaining CO and Methane in the PSA tail gas and the supplemental natural gas fuel are combusted in the SMR furnace to CO2 and contribute the remaining CO2 which is not recovered and emitted in the Furnace flue gas. Co-pending US patent application 2010-0037521 further teaches that the CO2 recovery can be further increased to about 90% by increasing the SMR feed by 33% and reducing the hydrogen recovery in the PSA such that enough more hydrogen is passed to the tail gas and subsequently to the SMR furnace and no supplemental natural gas is supplied to the SMR Furnace. 
         [0003]    Co-pending, as-yet unpublished patent application Ser. No. 12/970,041, herein incorporated by reference, teaches that the extent of pre-reforming can be increased by utilizing higher amounts of waste heat for pre-reforming. The reaction products from a first stage of pre-reforming is heated to a higher temperature by exchanging heat with flue gas or process gas and sent to a second adiabatic catalytic reactor in which the endothermic reforming reactions drop the temperature. The process can be repeated through up to 4 or 5 pre-reformers in series and subsequently increasing the amount of pre-reforming from about 8-10% per a single bed pre-reformer to up to 20-25%. With higher degree of pre-reforming, the firing duty of the main reformer is reduced. 
         [0004]    Referring to Co-pending US patent application 2010-0037521, the inventors teach that CO2 emissions from an SMR can be reduced by reducing the amount of CO2 produced by burning hydrocarbons in the SMR furnace. Co-pending, as-yet unpublished patent application Ser. No. 12/970,041 teaches that by increasing the extent of pre-reforming utilizing waste heat as the heat source, that the firing duty of the main reformer is reduced. For example by using three stages of pre-reforming instead of one stage of pre-reforming, the CO2 emissions from a conventional SMR can be reduced by 5-6%. By utilizing the increased pre-reforming concepts disclosed in Ser. No. 12/970,041 in addition to the increased CO2 capture taught in invention 2010-0037521, the CO2 removed can be increased from about 67% to about 90% without lowering the PSA H2 recovery as taught in 2010-0037521. Another benefit of the invention is that by using waste heat from the SMR furnace to do additional pre-reforming, steam production is reduced and when combined with CO2 removal by an amine contactor, there is no net export of steam from the SMR. 
         [0005]    CO2 recovery utilizing the present invention can be further increased to 100%. This is achieved by taking the flue gas from SMR furnace through a dryer to remove water and compressing it. Typical specification for Nitrogen used for Enhanced Oil Recovery is &gt;95% nitrogen. The resulting flue gas from the present invention will contain &gt;95% Nitrogen+Argon, &lt;3.1% CO2 and less than 1.9% Oxygen and would be an excellent gas to be used for enhanced oil recovery. By utilizing the flue gas for Enhanced Oil Recovery, no flue gas is emitted from the SMR and therefore no CO2 or NOx emissions. 
         [0006]    A preferred gas for enhanced oil recovery would contain very low oxygen content. To produce a flue gas with low oxygen content, the flue gas from the SMR is combined with purified hydrogen from the PSA and contacted over a bed of catalyst to promote combustion of H2 with O2 to form water. The resulting flue gas stream is dried to remove excess water and compressed and used for enhanced oil recovery. The composition of the flue gas stream would be &gt;97% N2+Argon, &lt;3% CO2 and &lt;0.1% O2, &lt;0.1% H2. 
         [0007]    The production of hydrogen by the steam reforming of hydrocarbons is well known. In the basic process, a hydrocarbon, or a mixture of hydrocarbons, is initially treated to remove, or convert and then remove, trace contaminants, such as sulfur and olefins, which would adversely affect the reformer and the down stream water gas shift unit catalyst. Natural gas containing predominantly methane is a preferred starting material since it has a higher proportion of hydrogen than other hydrocarbons. However, light hydrocarbons or refinery off gases containing hydrocarbons, or refinery streams such as LPG, naphtha hydrocarbons or others readily available light feeds might be utilized as well. 
         [0008]    The pretreated hydrocarbon feed stream is typically at a pressure of about 200 to 400 psig, and combined with high pressure steam, which is at a higher than the feed stream pressure, before entering the reformer furnace. The amount of steam added is much in excess of the stoichiometric amount. The reformer itself conventionally contains tubes packed with catalyst through which the steam/hydrocarbon mixture passes. An elevated temperature, e.g. about 1580° F., or 860° C., is maintained to drive the endothermic reaction. 
         [0009]    Prereforming of hydrocarbons upstream of the SMR or ATR is a well known process. It converts heavier hydrocarbons (ethane and heavier) to methane. It may also convert some of the methane to hydrogen, CO, and CO2, depending upon the chemical equilibrium under the given conditions. 
         [0010]    Prereformer utilizes waste heat in the flue gas or process stream, which otherwise may be utilized in raising steam. Utilization of high level heat (at about 1600° F. to about 900° F.) is thermodynamically more efficient when used for prereforming than for raising steam with boiling temperature of about 400° F. to 600° F. Disposal of excess steam is a problem in many plants. 
         [0011]    Typically the feed (hydrocarbon and steam mixture) to the prereformer is preheated in the range of 850° F. to 1000° F. before contacting with a catalytic bed in an adiabatic reactor. The reactants come to a chemical equilibrium. The extent of conversion of methane to H2/CO/CO2 is a function of the reaction temperature, higher temperature favoring the conversion. 
         [0012]    The inlet temperature of the feed to prereformer is limited by its potential to crack hydrocarbons and deposit carbon on the catalyst and the preheat coils. Heavier the feedstock, lower is the potential cracking temperature. For example, the feed temperature for typical light natural gas is limited to about 1000° F., while feed temperature for naphtha feed is limited to 850° F. The amount of waste-heat utilization for prereforming depends on the preheat temperature of feed mixture. There is a need for a process that can utilize larger amounts of waste heat for prereforming. 
         [0013]    The effluent from the reformer furnace is principally hydrogen, carbon monoxide, carbon dioxide, water vapor, and methane in proportion close to equilibrium amounts at the furnace temperature and pressure. The effluent is conventionally introduced into a one- or two-stage water gas shift reactor to form additional hydrogen and carbon dioxide. The shift reactor converts the carbon monoxide to carbon dioxide by reaction with water vapor, which generates additional Hydrogen. This reaction is endothermic. The combination of steam reformer and water gas shift converter is well known to those of ordinary skill in the art. 
         [0014]    If CO2 capture from the high pressure syngas stream exiting the water gas shift unit is desired, the shift converter effluent, which comprises hydrogen, carbon dioxide and water with minor quantities of methane and carbon monoxide is introduced into a conventional absorption unit for carbon dioxide removal. Such a unit operates on the well-known amine wash or other solvent processes wherein carbon dioxide is removed from the effluent by dissolution in an absorbent solution, i.e. an amine solution or potassium carbonate solution, respectively. Conventionally, such units can remove up to 99 percent or higher of the carbon dioxide in the shift converter effluent. 
         [0015]    The effluent from the carbon dioxide absorption unit is introduced into a pressure swing adsorption (PSA) unit. PSA is a well-known process for separating essentially pure hydrogen from the mixture of gases as a result of the difference in the degree of adsorption among them on a particulate adsorbent retained in a stationary bed. 
         [0016]    Conventionally, the remainder of the PSA unit feed components, after recovery of pure hydrogen product, which comprises carbon monoxide, the hydrocarbon, i.e. methane, hydrogen and carbon dioxide, is returned to the steam reformer furnace and combusted to obtain energy for use therein 
         [0017]    To practice CO2 emissions capture from such hydrogen plants, one must consider total emissions resulting from the plant, which includes CO2 recovery from reformer furnace flue gas as well. 
       SUMMARY  
       [0018]    The present invention is a method of reducing the carbon dioxide balance from a reformer furnace flue gas to the high pressure syngas exit water gas shift reaction unit, comprising; providing a first gas mixture; heating said first stream mixture to a first temperature, then introducing said heated first gas mixture into at least one pre-reforming chamber, thereby producing a pre-reformed mixture; said heating being provided by indirect heat exchange with one or more of an SMR furnace flue gas or an SMR furnace syngas further heating said pre-reformed mixture in a primary reformer, thereby generating a second gas mixture comprising hydrogen, carbon monoxide, carbon dioxide, and a flue gas, wherein said primary reformer comprises tubes filled with catalyst; introducing said second gas mixture into at least one isothermal shift reactor, or a combination of high followed by a low temperature shift reactor, or a medium temperature shift reactor, thereby generating a third gas mixture; introducing said third gas mixture into a standard H2 PSA unit, wherein said third gas mixture is separated into a hydrogen enriched stream and a PSA tail gas stream; introducing said PSA tail gas stream into a CPU system, wherein said PSA tail gas stream is separated into a carbon dioxide enriched stream, a hydrogen rich stream, and a residual stream, and introducing said residual stream as fuel into the reformer furnace along with natural gas, a portion of the feed hydrocarbon stream, a portion of the hydrogen enriched stream, or any other external make-up fuel for the reformer furnace, wherein step b) is repeated twice, for a total of three pre-reforming steps, the temperature at the second pre-reformer is higher than at the first pre-reformer, and the temperature at the third pre-reformer is higher than at the second, where the pre-reforming chamber comprises at least three beds of catalyst, wherein an outlet gas from each pre-reformer is heated up in a coil in exchange with the SMR furnace flue gas or process syngas before going to a next pre-reformer reactor, and an outlet gas from the third pre-reformer or before entering the main reformer tubes. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, and in which: 
           [0020]      FIG. 1  illustrates the use of a CPU in accordance with one embodiment of the present invention. 
           [0021]      FIGS. 2A-2F  illustrate various permutations in accordance with various embodiments of the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
         [0023]    It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0024]    Turning to  FIG. 1 , which illustrates one embodiment of the present invention, a first stream comprising hydrocarbons  101 , steam  102 , and possibly PSA offgas stream  144  is heated to a first temperature in first heat exchanger  103 , by indirect heat exchange with hot gas stream  117 , thereby producing first pre-reformer inlet stream  104 . First pre-reformer inlet stream  104  is then introduced into first pre-reforming chamber  105 , thereby producing first pre-reformed stream  106 . 
         [0025]    First pre-reformed stream  106  is heated to a second temperature in second heat exchanger  107 , by indirect heat exchange with hot gas stream  116 , thereby producing second pre-reformer inlet stream  108 . Second pre-reformer inlet stream  108  is then introduced into second pre-reforming chamber  109 , thereby producing second pre-reformed stream  110 . 
         [0026]    Second pre-reformed stream  110  is heated to a third temperature in third heat exchanger  111 , by indirect heat exchange with hot gas stream  115 , thereby producing third pre-reformer inlet stream  112 . Third pre-reformer inlet stream  112  is then introduced into third pre-reforming chamber  113 , thereby producing third pre-reformed stream  114 . Third pre-reformed stream  114  may then be heated once again in a fourth heat exchanger (not shown) prior to usage downstream. Note in one embodiment, hot gas stream  117 , hot gas stream  116 , and hot gas stream  115  may come from different sources (not shown). 
         [0027]    The second temperature may be greater than said first temperature. The third temperature may be greater than said second temperature. The indirect heat exchange may be with a flue gas from an SMR furnace. The indirect heat exchanger may be with one or more process streams. The indirect heat exchange may be with SMR furnace syngas. 
         [0028]    The amount of steam mixed with hydrocarbons depends on the catalyst, and the type of hydrocarbon feedstock. The skilled artisan will be able to select the proper amount of steam for any application without undue experimentation. 
         [0029]    Each pre-reforming chamber may be a stand alone reactor. At least two pre-reforming chambers may be contained in a single vessel. All three pre-reforming chambers may be contained in a single vessel. The three pre-reforming beds may be stacked in one vessel with internal heads. The first pre-reforming chamber may have a first space velocity, the second pre-reforming chamber may have a second space velocity, and the third pre-reforming chamber may have a third space velocity, where the first space velocity is lower than said second space velocity or said third space velocity. 
         [0030]    The pre-reformer chambers may consist of a bank of tubes filled with catalyst which are heated in contact with SMR furnace flue gas or syngas. 
         [0031]    Third pre-reformed stream  114  is introduced to a novel primary reformer  119 , wherein a syngas stream  130  comprising at least carbon dioxide and hydrogen is produced. Novel primary reformer  119  may be configured and operated as defined in co-pending US patent application 2010-0037521, herein incorporated by reference. Either at least a portion  152  of the reformer furnace flue gas stream  134  or a portion  151  of the syngas stream  130  may be directed to the pre-reformer, as hot gas stream  115 . 
         [0032]    A portion of the syngas stream  130  may be sent to a waste heat recovery unit  120  to produce steam  121 . The exit of waste heat recovery, stream  131  is then introduced to a high temperature shift reactor followed by a low temperature shift reactor, or alternatively either an isothermal or a medium temperature shift reactor (symbolically represented by  122 ). This produces a carbon dioxide richer stream  132 . Carbon dioxide richer stream  132  is further cooled in waste heat recovery unit  123  to generate steam  124 , and a cooler syngas stream  133 . 
         [0033]    The cooler syngas stream  133  is introduced into the PSA unit  127  wherein relatively pure hydrogen  128  is recovered, and residual stream  129  may be compressed in compressor  141  to produce compressed stream  150 , and introduced into a CO2 separation unit  147  (such as a CPU, i.e. cryogenic purification unit). CO2 separation unit  147  may be a CPU or a combination of CPU and membrane units. In the CO2 separation unit  147 , stream  150  is separated into a CO2 stream  148  and a hydrogen rich stream  142  which may be recycled to PSA  127  and a residual stream  149 . A portion  144  of residual stream  149  may be recycled upstream of reformer  119 . At least a portion  143  of residual stream may be used as fuel in steam reformer  119 . 
         [0034]    A portion of reformer furnace flue gas stream  134  may be sent to waste heat recovery unit  135 , to produce steam  136  or preheat other process streams (not shown). The total carbon dioxide recovered by the amine wash may represent greater than 80% of the overall carbon dioxide generated by the SMR, preferably 90%. The total carbon dioxide recovered by the amine wash may represent greater than 85% of the overall carbon dioxide generated by the SMR, preferably 95%. 
         [0035]    In one embodiment of the present invention, the catalyst in the first pre-reformer consists of conventional pre-reforming catalyst, and the catalyst in following pre-reformers of typical main catalyst bed reforming catalyst. 
         [0036]    A portion of the heat for the reforming reaction may be provided by exchange with exit gas through the helical shaped tubes. The temperature of the exit gas from the top of the helical tubes may be between 1200 and 1300 degrees F. 
         [0037]    As illustrated in  FIGS. 2A-2F , the various pre-reformers may be provided heat by either a portion  152  of the reformer furnace flue gas stream  134 , or a portion  151  of the syngas stream  130 , in any appropriate combination, but portion  151  and portion  152  will typically be at different pressure and of different composition, so physically blending these two portions will ordinarily not occur. 
         [0038]    The flue gas from the SMR furnace may be utilized for industrial purposes resulting in 100% recovery of the CO2 and no emission of nitrogen oxides from the SMR. The SMR furnace flue gas may be compressed and used for “Enhanced Oil Recovery (EOR).” The SMR Furnace Flue gas may be dried to remove water by passing through a bed of adsorbent. The 
         [0039]    Nitrogen +Argon composition of the flue gas downstream of the drier may be greater than or equal to 95%. The SMR flue gas may be contacted with Hydrogen from the PSA and passed over a bed of catalyst to promote combustion of H2 with O2. The oxygen content of the flue gas downstream of the combustion zone may be less than 0.1 mol %. The SMR Furnace Flue gas may be dried to remove water by passing through a bed of adsorbent. The Nitrogen+Argon composition of the flue gas downstream of the drier may be greater than or equal to 97%, preferentially 99%. 
         [0040]    Illustrative embodiments have been described above. While the method in the present application is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings, and have been herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the method in the present application to the particular forms disclosed, but on the contrary, the method in the present application is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the method in the present application, as defined by the appended claims. 
         [0041]    It will, of course, be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but, would nevertheless, be a routine undertaking for those of ordinary skill in the art, having the benefit of this disclosure.