Abstract:
A method of heating process streams fed to a boiler incorporating an oxygen transport membrane device that includes an oxygen-containing stream and a boiler feed water stream. The membrane device separates oxygen to support combustion of a fuel and generate heat to raise the steam. Heat is recovered and process streams are heated by separately heating portions of the oxygen-containing stream and the boiler feed water stream with a retentate stream produced from the oxygen separation and a flue gas stream generated from the combustion. The flow rate of the portion of the oxygen-containing stream heated by the retentate stream is greater than that heated by the flue gas stream to help minimize heat transfer area and thus, fabrication costs. Also, water is condensed from the flue gas stream during the heat exchange involved in the heat recovery to increase thermodynamic efficiency.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of heating process streams fed to a boiler in which the process streams include a boiler feed water stream and an oxygen-containing stream and the boiler utilizes an oxygen transport membrane device to separate oxygen from the oxygen-containing stream to support combustion of the fuel stream to in turn generate heat to produce steam by heating the boiler feed water stream with a retentate stream and a flue gas stream. More particularly, the present invention relates to such method in which heat is recovered and the process streams are heated by separately heating portions of the oxygen-containing stream and the boiler feed water stream with the retentate stream and the flue gas stream. 
       BACKGROUND OF THE INVENTION 
       [0002]    A variety of boilers and like devices have been proposed in the prior art that make use of oxygen transport membranes to separate oxygen from a heated oxygen-containing feed to support combustion of a fuel. The heat produced by such combustion can be indirectly transferred to boiler feed water flowing within steam tubes to raise steam. A central advantage of such boilers and devices is that carbon dioxide produced by the combustion can be sequestered for environmental purposes and for use in other processes. 
         [0003]    Oxygen transport membranes are known devices that incorporate a ceramic material that is capable of oxygen ion transport at elevated temperatures. When an oxygen-containing gas is exposed to one side of the membrane, conventionally known as a cathode side, the oxygen is ionized and oxygen ions are transported through the membrane to the opposite side known as the anode side. The oxygen ions react with a fuel species that consumes the oxygen ions. This consumption of oxygen ions creates a partial pressure difference of oxygen between the cathode side and the anode side of the membrane that provides a driving force for the oxygen ion transport. The partial pressure difference can also be created by compressing a feed stream containing the oxygen and/or reducing the pressure on the anode side. 
         [0004]    Electrons are made available for oxygen ionization at the cathode side by electrons being lost from the oxygen ions at the anode side. Certain ceramic materials, formed from perovskites, exhibit both oxygen ion and electron conductivity and thus are known as mixed conductors. In such materials, the electrons flow through the material from the anode to the cathode side. Other ceramic materials are ionic conductors and are capable of only ionic transport. Such materials are thus used in combination with an electrically conductive phase for the electron transport or with an external circuit for electrical circuit. A typical example of such an ionic conductor is yttrium stabilized zirconia. 
         [0005]    As indicated above, the heat generated by the combustion of the fuel introduced to the anode side of the membrane can be used to generate steam. Membranes utilized in such boilers can be driven under a positive oxygen partial pressure that is produced by combusting a fuel at the anode side of the membrane. For example, in U.S. Pat. No. 6,394,043 a boiler is disclosed in which steam tubes and ceramic membrane elements are interspersed. Fuel is introduced into the device that reacts with oxygen ions that have been transported through the membrane to generate heat to raise steam in boiler feed water flowing within the steam tubes. This type of boiler has been optimized in a paper entitled, “Cost and Feasibility Study on the Praxair Advanced Boiler for the CO 2  Capture Project Refinery Scenario”, Switzer et al., Elsevier (2005). In this paper a boiler is illustrated having rows of oxygen transport membrane tubes located within a housing and alternating with steam tubes to superheat saturated steam by combustion of a fuel supported by oxygen separation. The resulting heated and oxygen depleted retentate is used to heat heated boiler feed water and thereby to generate the saturated steam. Such heating takes place within the housing upstream of the oxygen transport membrane tubes. Part of the flue gas is recirculated, mixed with the fuel and also introduced into the housing. 
         [0006]    As can be appreciated, it is desirable to recover heat energy from both the heated flue gas stream and the retentate stream for use in preheating the air feed to the oxygen transport membrane device and for heating boiler feed water. In Switzer, the incoming air is heated against flue gas after having passed through a heat exchanger being used to preheat the boiler feed water. Preheated air is passed through a heat exchanger in which the air is further heated by the retentate stream after having been used to generate the saturated steam. Thereafter, the retentate stream flows into another heat exchanger to further heat the boiler feed water. 
         [0007]    The boiler, described above and like systems, operates at high temperatures and therefore require that the air be preheated to a temperature of generally about 900° C. Such high temperature operation requires expensive, high temperature heat exchangers that are necessary to recover heat and thereby capture a sufficient thermal efficiency to make the use of such boilers and systems practical. As will be discussed, the present invention provides an inherently efficient process for recovering heat energy and thereby heating the oxygen-containing stream, the boiler feed water stream and the fuel stream that optimizes the use of the heat exchangers to decrease the costs involved in fabricating such boilers. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a method of heating process streams fed to boiler utilizing an oxygen transport membrane unit. In accordance with the method, the process streams that are fed to the boiler include a heated boiler feed water stream and a heated oxygen-containing stream. 
         [0009]    The heated boiler feed water stream is heated within the boiler through indirect heat exchange with a retentate stream and a flue gas stream to generate steam. The flue gas stream is produced by combustion of a fuel supported by oxygen separated from the heated oxygen-containing stream by the oxygen transport membrane device. The separation of the oxygen thereby also produces the retentate stream with a higher mass flow rate than the flue gas stream. 
         [0010]    Heat is indirectly transferred from the retentate stream to a first subsidiary oxygen-containing stream and thereafter, to a first subsidiary boiler feed water stream. The heat exchange produces a heated first subsidiary oxygen-containing stream and a first heated boiler feed water stream. The flue gas stream indirectly exchanges further heat to a second subsidiary oxygen-containing stream and thereafter, to a second subsidiary boiler feed water stream, thereby to produce a second heated oxygen-containing stream and a second heated boiler feed water stream. 
         [0011]    The heated first subsidiary oxygen-containing stream and the heated second subsidiary oxygen-containing stream are combined to form the heated oxygen-containing stream and the heated first boiler feed water stream and the heated second boiler feed water stream are combined to form the heated boiler feed water stream. 
         [0012]    The heat exchange area required for the indirect heat exchange between the retentate stream and the flue gas stream to the oxygen-containing stream and the boiler feed water stream is minimized by providing the first oxygen-containing stream with a greater mass flow rate than that of the second oxygen-containing stream. This minimization of the required heat transfer area for the heat recovery allows fabrication costs to be reduced. 
         [0013]    In an embodiment of the present invention, the water condenses during the indirect heat exchange of the flue gas stream and the second boiler feed water stream. This increases overall thermal efficiency of the heat recovery process. 
         [0014]    The indirect heat exchange between the retentate stream and the first subsidiary oxygen-containing stream and the flue gas stream and the second subsidiary oxygen-containing stream are each conducted within two heat exchangers operating at higher and lower temperatures and upstream of the indirect heat exchange with the first boiler feed water stream and the second boiler feed water stream. The use of higher and lower operational temperatures for such heat exchangers allows fabrication costs to be further reduced by the reduction of the requirement for the use of expensive high temperature materials. 
         [0015]    In an alternative embodiment, the indirect heat exchange between the retentate stream and the first subsidiary oxygen-containing stream and the flue gas stream and the second subsidiary oxygen-containing stream are each conducted within two heat exchangers operating at higher and lower temperatures. The indirect heat exchange of the retentate stream and the first boiler feed water stream and the indirect heat exchange of the flue gas stream and the second boiler feed water stream occurring between the two heat exchangers. In such embodiment, water may condense during the indirect heat exchange of the flue gas stream with the second subsidiary oxygen-containing stream and within the other of the two heat exchangers. This embodiment is applicable for situations in which the boiler feed water is available at high temperature. Again, such embodiment minimizes the use of expensive high temperature materials required for heat exchange at high temperatures. 
         [0016]    The heated oxygen-containing stream can be further heated by introducing the oxygen-containing stream into a duct burner and combusting a fuel within the duct burner. In any embodiment, the oxygen-containing stream can be air. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    While the specification concludes with claims distinctly pointing out the subject matter that applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
           [0018]      FIG. 1  is a schematic, sectional view of a boiler used in connection with the present invention; 
           [0019]      FIG. 2  is a schematic, fragmentary process flow diagram of a heat recovery system of the present invention; 
           [0020]      FIG. 3  is a graphical representation of a required total product of the heat transfer coefficient and heat transfer area against the part of the air that exchanges heat solely with the flue gas stream; 
           [0021]      FIG. 4  is a schematic, fragmentary process flow diagram of an alternative embodiment of a heat recovery system in accordance with the present invention; 
           [0022]      FIG. 5  is a graphical representation of a product of the overall heat transfer coefficient and the heat transfer area versus the fraction of the air that exchanges heat solely with the flue gas; and 
           [0023]      FIG. 6  is a schematic, fragmentary process flow diagram of an alternative embodiment of a heat recovery system in accordance with the present invention. 
       
    
    
       [0024]    The same reference numbers having been used in the Figures for elements having the same description to avoid needless repetition in the description of such elements. 
       DETAILED DESCRIPTION 
       [0025]    With reference to  FIG. 1 , a boiler  1  is illustrated that is to be used in connection with a method in accordance with the present invention. It is understood, however, that boiler  1  is discussed herein for exemplary purposes and is not intended to limit the application of the present invention as the present invention has application to similar devices in which, as will be discussed, water is heated to steam by heat generated through combustion supported by oxygen ion transport. 
         [0026]    Boiler  1  is provided with a housing  10  that contains an oxygen transport membrane device formed by tubular oxygen transport membrane tubes  12 . In boiler  1 , oxygen transport membrane tubes  12  are formed from a dual phase conductor, that is, a mixture of ionic and electronically conducting phases. However, it is understood that the present invention would have equal applicability to a boiler incorporating oxygen transport materials formed by mixed conductors or ionic conductors used in a manner described above and also, possibly for combined cycles in which the oxygen ion transport occurred within a fuel cell type of device incorporating an ionic conducting membrane. 
         [0027]    A heated oxygen-containing stream  14  is introduced into the interior of oxygen transport membrane tubes  12  through inlets  16 . At the same time, a heated fuel stream  17  is introduced into housing  10  to combust at the outer surface of oxygen transport membrane tubes  12  by combination with oxygen ions permeating through oxygen transport membrane tubes  12 . The consumption of oxygen ions establishes a partial pressure differential to drive oxygen ion transport through oxygen transport membrane tubes  12  and electronic transport to ionize the oxygen contained within heated oxygen-containing stream  14  to accomplish the oxygen separation. As a result of this separation operation, a flue gas stream  18  is created that is discharged from the housing  10  of boiler  1  and a retentate stream  20  that by way of conduit  22  is introduced into a heat recovery steam generator  24  and discharged from outlets  25  thereof. 
         [0028]    A heated boiler feed water stream  26  is introduced into steam tubes  28  of heat recovery steam generator  24  and is indirectly heated by retentate contained in retentate stream  20  to form a saturated steam stream that by way of conduit  30  is collected in steam drum  32 . The saturated steam is thereafter introduced into a heat recovery steam generator  33  having steam tubes  34  intermingled with oxygen transport membrane tubes  12  to superheat the steam through indirect heat exchange with the flue gas that is evolved from the combustion occurring at the outer surfaces of oxygen transport membrane tubes  12  and that is discharged as flue gas stream  18 . The superheating thereby forms a product steam stream  36  that is discharged from boiler  1  for use in downstream processes. 
         [0029]    As illustrated, fuel stream  17  can be combined with a recirculated subsidiary flue gas stream  37  with the use of a recirculation blower  38 . A steam stream  39  can then be combined to adjust the steam to carbon ratio in the fuel to be combusted to control carbon formation on oxygen transport membrane tubes  12 . The combined stream is then preheated in a preheater  40  and passed through heat recovery steam generator  24  and to the oxygen transport membranes  12  as indicated by the arrowheads “A”. 
         [0030]    With reference to  FIG. 2 , heat from retentate stream  20  and flue gas stream  18  is recovered by a heat recovery network  41  that is designed to carry out a method in accordance with the present invention for heating oxygen-containing stream  42  and a boiler feed water stream  44  to form heated oxygen-containing stream  14  and heated boiler feed water stream  26  that constitute the process streams being fed to boiler  1 . 
         [0031]    Oxygen-containing stream  42 , for example, air, is introduced into a heat recovery flow network  41  and eventually boiler  1  by way of a blower  46 . No compression is required given that the combustion of the fuel drives the transport. It is to be noted, however, that the present invention has applicability to a system in which oxygen ion transport is driven by a positive total pressure and as such, the oxygen-containing stream  42  could be compressed for such purposes. A first subsidiary oxygen-containing stream  48  derived from oxygen-containing stream  42  is introduced into a heat exchanger  50  to effect indirect heat exchange with retentate stream  20  and thereby to produce a heated first subsidiary oxygen-containing stream  52 . At the same time a second subsidiary oxygen-containing stream  54  derived from oxygen-containing stream  42  is introduced to a heat exchanger  56  to effect indirect heat exchange with flue gas stream  18  and thereby form a heated second subsidiary oxygen-containing stream  58 . First heated subsidiary oxygen-containing stream  52  and second heated subsidiary oxygen-containing stream  58  are then combined to form heated oxygen-containing stream  14 . Optionally, a fuel stream  60  can be also combined with heated oxygen-containing stream  14  within a duct burner  61  by means of a blower  62  for partial combustion and further heating of the heated oxygen-containing stream  14 . 
         [0032]    Retentate stream  20  after passage through heat exchanger  50  and flue gas stream  18  after passage through heat exchanger  56  are then introduced into heat exchangers  64  and  66  that are located downstream of heat exchangers  50  and  56  used in heating oxygen-containing stream  42 . Boiler feed water stream  44  is pumped by a pump  68  and thereby pressurized to a desired operational pressure of product steam stream  36 . A first subsidiary boiler feed water stream  70  made up of boiler feed water stream  44  is heated by retentate stream  20  within heat exchanger  64  to produce first heated subsidiary boiler feed water stream  72 . A second subsidiary boiler feed water stream  74 , also made up of boiler feed water stream  44 , is heated by flue gas stream  18  within heat exchanger  66  to form second heated subsidiary boiler feed water stream  76 . First heated subsidiary boiler feed water stream  72  is then combined with second heated subsidiary boiler feed water stream  76  to form heated boiler feed water stream  26 . 
         [0033]    It is to be noted that all of the heat exchangers  50 ,  56 ,  64  and  66  can be of shell and tube design. In order to increase the thermal efficiency of the heat exchange process, water contained in flue gas stream  18  can be condensed within heat exchanger  66  as the dew point for such water is at a high temperature and the heat of condensation is therefore significant and can be recovered within second subsidiary boiler feed water stream  74 . However, since flue gas stream  18  also contains carbon dioxide, the resulting acid can be corrosive and require special materials in the fabrication of heat exchanger  66  that can increase the fabrication costs. 
         [0034]    As described above, a process of the present invention is conducted with the aim of reducing the costs involved in fabricating the heat exchangers, described above, in the heat recovery network  41 . As indicated above, the mass flow rate of retentate stream  20  is greater than that of flue gas stream  18  by virtue of the fact that air contains about 80 percent nitrogen. By diverting the flow within oxygen-containing stream  42  into first subsidiary oxygen-containing stream  48  that is subjected to indirect heat exchange with the retentate stream  20  having the higher mass flow rate than flue gas stream  18 , temperature differences at the inlet and outlet of heat exchanger  50  between the streams can be minimized to also increase the amount of heat able to be transferred. As a result, a product of the heat transfer coefficient and area is reduced in heat exchanger  50 . Since there is a closer flow rate match in heat exchanger  56  and less heat will be transferred the size of heat exchanger  56  can be optimized. The resulting closer correspondence of outlet temperatures of the retentate stream  20  and the flue gas stream  18  upon their discharge from heat exchangers  50  and  56  allows for a closer approach in temperatures at the downstream heat exchangers  64  and  66  used in heating boiler feed water stream  44  to also result in an area savings for the total required heat exchange. 
         [0035]    A calculated example in tabular form is set forth below for the operation of the heat recovery network  41  illustrated in  FIG. 2 . 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 
               
             
             
               
                   
                   
               
               
                   
                   
                   
                   
                 Mass 
                   
               
               
                   
                 Vapor 
                   
                 Pressure 
                 Flow 
                 Composition 
               
             
          
           
               
                 Stream # 
                 Fraction 
                 Temperature C. 
                 [psia] 
                 [lb/hr] 
                 Methane 
                 Ethane 
                 Nitrogen 
                 Oxygen 
                 CO2 
                 H2O 
               
               
                   
               
             
          
           
               
                 42 
                 1.00 
                 25 
                 14.7 
                 121500 
                 0 
                 0 
                 0.79 
                 0.21 
                 0 
                 0 
               
               
                 42 after 
                 1.00 
                 55 
                 19 
                 121500 
                 0 
                 0 
                 0.79 
                 0.21 
                 0 
                 0 
               
               
                 blower 46 
               
               
                 14 
                 1.00 
                 550 
                 18 
                 121500 
                 0 
                 0 
                 0.79 
                 0.21 
                 0 
                 0 
               
               
                 60 
                 1.00 
                 25 
                 14.7 
                 1028 
                 0.95 
                 0.03 
                 0.02 
                 0 
                 0.01 
                 0 
               
               
                 60 after 
                 1.00 
                 44 
                 18 
                 1028 
                 0.95 
                 0.03 
                 0.02 
                 0 
                 0.01 
                 0 
               
               
                 blower 62 
               
               
                 14 after 
                 1.00 
                 882 
                 18 
                 122500 
                 0 
                 0 
                 0.78 
                 0.18 
                 0.01 
                 0.03 
               
               
                 firing within 
               
               
                 duct burner 
               
               
                 61 
               
               
                 20 
                 1.00 
                 600 
                 16.5 
                 105600 
                 0 
                 0 
                 0.89 
                 0.06 
                 0.02 
                 0.03 
               
               
                 20 after 
                 1.00 
                 150 
                 15.5 
                 105600 
                 0 
                 0 
                 0.89 
                 0.06 
                 0.02 
                 0.03 
               
               
                 passage 
               
               
                 through heat 
               
               
                 exchanger 50 
               
               
                 20 after 
                 1.00 
                 70 
                 14.8 
                 105600 
                 0 
                 0 
                 0.89 
                 0.06 
                 0.02 
                 0.03 
               
               
                 passage 
               
               
                 through heat 
               
               
                 exchanger 64 
               
               
                 44 
                 0.00 
                 25 
                 14.7 
                 100000 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                 44 after 
                 0.00 
                 25 
                 140.5 
                 100000 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                 having been 
               
               
                 pressurized 
               
               
                 by pump 44 
               
               
                 26 
                 0.00 
                 97 
                 140 
                 100000 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                 18 
                 1.00 
                 600 
                 16.5 
                 21330 
                 0 
                 0 
                 0 
                 0.01 
                 0.33 
                 0.65 
               
               
                 18 after 
                 1.00 
                 257 
                 15.5 
                 21330 
                 0 
                 0 
                 0 
                 0.01 
                 0.33 
                 0.65 
               
               
                 passage 
               
               
                 through heat 
               
               
                 exchanger 56 
               
               
                 18 after 
                 0.50 
                 70 
                 14.8 
                 21330 
                 0 
                 0 
                 0 
                 0.01 
                 0.33 
                 0.65 
               
               
                 passage 
               
               
                 through heat 
               
               
                 exchanger 66 
               
               
                   
               
             
          
         
       
     
         [0036]      FIG. 3 , set forth a further calculation based on the data developed in the above table in a graphical form. As is apparent from the graph, a minimum UA is obtained where the flow rate of second subsidiary oxygen-containing stream  54  is roughly 18 percent of the flow rate of oxygen-containing stream  42  and therefore the remainder of the flow is concentrated in first subsidiary oxygen-containing stream  48 . For a constant heat transfer coefficient, this also represents the minimum heat transfer area required to conduct the process of the above example and therefore, the minimum costs to fabricate the heat exchangers. 
         [0037]    With reference to  FIG. 4 , costs can further be reduced by splitting the heat exchange duty of heat exchangers  50  and  56  into two heat exchangers  50   a  and  50   b  and  56   a  and  56   b . Heat exchangers  50   a  and  56   a  operate at higher temperatures than heat exchangers  50   b  and  56   b . As such, the use of expensive, high temperature materials can be concentrated within the higher temperature heat exchangers  50   a  and  56   a  to also reduce fabrication costs. 
         [0038]    With reference to  FIG. 5 , again using the data of the above table, the calculated split of oxygen-containing stream  42  involves second subsidiary oxygen-containing stream  54  being roughly 26 percent of the total flow within oxygen-containing stream  42  and with the remainder of the flow within first subsidiary oxygen-containing stream  48 . 
         [0039]    With reference to  FIG. 6 , another economizing method can be taken when boiler feed water stream  44  is available at high temperature. In such embodiment, the heat exchange duty for oxygen-containing stream  40  can be split between two sets of heat exchangers  50   a ′;  50   b ′ and  56   a ′;  56   b ′, each set operating at higher and lower temperatures. In such embodiment, heat exchangers  50   b ′ and  56   b ′ are located downstream of heat exchangers  64  and  66  with condensation of water within flue gas stream  18  occurring in heat exchanger  56   b ′. The use of expensive, high temperature materials are therefore confined to heat exchangers  50   a ′ and  56   a ′ to also produce a cost savings. 
         [0040]    While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and scope of the present invention as set fort in the appended claims.