Abstract:
A fixed bed membrane reactor is disclosed. The reactor has a housing including an inlet for receiving reactants and an outlet for discharging retentate streams of reaction products. The inlet and outlet are in fluid communication with a reaction zone in which the reactants may pass downstream from the inlet to the outlet with the reactants reacting to produce reaction products including water. The reactor further includes a membrane assembly disposed in fluid communication with the reaction zone. The membrane assembly includes at least one porous support with a water permselective membrane affixed thereto. The membrane allows water produced in the reaction zone to be selectively removed from the reaction zone as a permeate stream while allowing retentate reaction products to remain in the reaction zone and be discharged as a retentate stream. Ideally, the membrane assembly locates most of the membrane proximate the downstream portion of the reaction zone where accumulated produced water may be selectively removed from the reaction product as opposed to the upstream portion where relatively little water accumulation has occurred. A method for using the reactor to perform in situ water dehydration of reactions, such as Fischer-Tropsch reaction, is also disclosed.

Description:
This application claims priority to Provisional Application Ser. No. 61/018,378, filed Dec. 31, 2007 as allowed under 35 USC 119(e). This application claims priority to and benefits from the foregoing, the disclosure of which incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to methods and apparatus wherein water is produced as part of a chemical reaction and the water is removed in-situ from the reaction products, and more particularly, to methods and apparatus which use in-situ dehydration during the synthesis of F-T products using membranes to remove produced water. 
     BACKGROUND OF THE INVENTION 
     Water vapor is a primary by-product in a Fischer-Tropsch (FT) reaction and its presence is generally detrimental to the overall efficiency of the FT reaction. In a FT reaction, a synthetic gas mixture of carbon monoxide (CO) and hydrogen gas (H 2 ), referred to hereinafter as “syngas”, is converted in the presence of a FT catalyst into hydrocarbon products, water vapor and other byproducts. The syngas may be generated from a number of carbon containing sources such as natural gas, coal (fossil), or bio-mass (renewable). It is often desirable to convert these carbon sources into a liquid hydrocarbon form from their original gas or solid states. There are two major types of catalysts used to catalyze this reaction: iron (Fe)-based catalysts and cobalt (Co)-based catalysts. The FT reaction is a relatively high temperature catalytic reaction. Accordingly, the water produced is generally in the form of water vapor. 
     Due to the adverse effects of water on this reaction, conventional FT reactors have a relative low rate of per-pass CO conversion. Conventional FT reactors separate water from other reaction products and un-reacted CO and H 2  gas after they exit the reactor&#39;s outlet. The un-reacted CO is often recycled back to a FT reactor inlet so that it may again potentially be converted into a hydrocarbon. 
     Efforts with respect to in-situ dehydration in F-T conversion of syngas to hydrocarbon products and water has described in several references. A first example is Espinoza et al., U.S. Pat. No. 6,403,660, which describes the use of slurry and fluidize beds to produce F-T hydrocarbon products. In the case of a slurry bed, a membrane apparatus is disposed within the liquid slurry and is used to remove water from the slurry. In another embodiment, a fluidized bed is used with a membrane apparatus again being disposed in a bed of catalyst. This membrane removes water from the bed during the production of F-T products and accompanying water. However, slurry and fluidized beds have shortcomings relative to using fixed bed reactors. 
     Rohde et al. proposed a fixed bed reactor with silica membrane or a Ceramic Supported Polymer (CSP) membrane with iron catalyst. For example, see M. P. Rohde, et al., “Membrane Application in Fischer-Tropsch Synthesis Reactor—Overview of Concept,” Catalysis Today 106 (2005) 143-148; and D. Unruh, et al., “In-situ Removal of H 2 O During Fischer-Tropsch Synthesis—A Modeling Study,” and DGMK-Conference, “Chances For Innovative Processes at The Interface Between Refining and Pertochemistry,” Berlin, 2002, Germany. However, these references fail to address heat management in terms of using commercial viable methods. Also, the use of membranes is not optimized to perform water separation where most produced water has been accumulated. 
     There is a need for improved designs for reactors in which water is removed in-situ during reactions in which the presence of produced water is detrimental and wherein heat management issues and water removal are also addressed as well as efficient distribution and use of membrane materials. 
     SUMMARY OF THE INVENTION 
     A membrane reactor is disclosed. The reactor includes a housing including an inlet for receiving reactants and an outlet for discharging retentate streams of reaction products. The inlet and outlet are in fluid communication with a reaction zone in which the reactants may pass downstream from the inlet to the outlet with the reactants reacting to produce reaction products including water. The reactor further includes a membrane assembly disposed in fluid communication with the reaction zone. The membrane assembly includes at least one porous support with a water permselective membrane affixed thereto. The membrane allows at least some of the water produced in the reaction zone to be selectively removed from the reaction zone as a permeate stream while allowing retentate reaction products to remain in the reaction zone and be discharged as a retentate stream. In one embodiment, the membrane assembly locates most of the membrane proximate the downstream portion of the reaction zone where accumulated produced water may be selectively removed from the reaction product as opposed to the upstream portion where relatively little water has been produced and accumulation has occurred. A method for using the reactor to perform in situ water dehydration of reactions, such as a Fischer-Tropsch reaction, is also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended accumulated claims and accompanying drawings where: 
         FIG. 1  is a schematic drawing of an FT packed bed membrane reactor (PBMR), including an in-situ water removal membrane, which was used in a computer model to estimate the effects of in situ dehydration during FT reactions; 
         FIG. 2  is a graph showing the computational effect of utilizing and not utilizing a membrane to remove water on CO conversion in a PBMR and in a comparable Plug Flow Reactor (PFR), in which no in situ water removal occurs; 
         FIG. 3  is a graph showing the computational effect of utilizing a membrane on H 2 O partial pressure using a PBMR and a PFR; 
         FIGS. 4(   a ) and ( b ) show graphs depicting the computational effect of utilizing a membrane on CO 2  yield and hydrocarbon yield in PBMR and PFR; 
         FIGS. 5(   a )-( d ) show the computational effect of sweep ratio on CO conversion, water partial pressure, hydrocarbon yield and on the amount of extra catalyst weight required in a PFR to achieve the same hydrocarbon yield as in a PBMR; 
         FIGS. 6(   a )-( d ) show the computational effect of permeate side pressure on CO conversion, water partial pressure, hydrocarbon yield and the amount of extra catalyst weight required in a PFR to achieve the same hydrocarbon yield as in a PBMR; 
         FIGS. 7(   a ) and ( b ) show the computational effect of membrane separation properties on hydrocarbon yield and CO conversion for a non-ideal membrane using an inert sweep gas; 
         FIGS. 8(   a ) and ( b ) show the computational effect of membrane separation properties on hydrocarbon yield and CO conversion for a non-ideal membrane with hydrogen (H 2 ) as the sweep gas; 
         FIG. 9  is a schematic drawing of a second embodiment of a packed bed membrane reactor (PBMR) which utilizes a water coolant to maintain the reactor at a desired operating temperature, however, the reactor does not utilize a sweep gas to remove water vapor; 
         FIG. 10  is a schematic drawing of a third embodiment of a packed bed membrane reactor (PBMR) which does utilize a water coolant and a sweep gas; 
         FIG. 11  is a schematic drawing of a fourth embodiment of a generally cylindrical packed bed membrane reactor (FBMR) which utilizes water coolants and axially spaced apart membranes to remove water at select axial locations along the reactor; and 
         FIGS. 12(   a )-( c ) show (a) that water vapor accumulates when forming in a FT reaction with the majority of the accumulation near the downstream end portion, (b) a membrane assembly wherein the radius of membrane material increases from the upstream to the downstream end to provide greater water vapor permeability proximate the downstream end as compared to proximate upstream end, and (c) a membrane assembly containing spaced apart membrane disks wherein the disks are spaced closer together on the downstream portion of the membrane assembly as opposed to the upstream portion to provide an increasing amount of membrane material available for water removal from the upstream end to the downstream end. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a schematic drawing of a first embodiment of a packed bed membrane reactor (PBMR)  20  in which chemical reactions, which produce water as a by-product, can be dehydrated in situ. This embodiment was computer modeled to study the effects of varying process variables during in-situ dehydration of conversion reactions. By way of example and not limitation, examples of these types of reactions are Fischer-Tropsch (FT) reactions in which syngas (CO and H 2 ) will react over an iron-based or cobalt-based catalyst to produce hydrocarbons and water. Examples of potential reactions include: 
     Paraffin Formation
 
 n CO+(2 n+ 1)H 2 →C n H 2n+2   +n H 2 O ( n ≧1);  (1)
 
Olefin Formation
 
 n CO+2 n H 2 →C n H 2n   +n H 2 O ( n ≧2); and  (2)
 
     Water Gas Shift Reaction
 
CO+H 2 O→CO 2 +H 2 .  (3)
 
Reactor:
 
     Reactor  20  is a fixed bed or packed bed membrane reactor (PBMR). Reactor  20  includes an outer shell  22  and a coaxially aligned inner membrane tube  24  which cooperate with one another to form an annular reaction zone  26  there between. Upstream and downstream perforated end caps  28 ,  30  also assist in defining reaction zone  26  and capturing the packed FT catalyst  32  within reaction zone  26 . Inner membrane tube  24  includes a porous support member  34  upon which a water permselective membrane  36  is affixed. The membrane material may be affixed either on the radial inside or outside of support member  34 . Water permselective membrane  36  is ideally chosen to permit water vapor to radially pass there through while inhibiting the passage of other reactants and products contained with reaction zone  26 . For example, the membrane may be made of an appropriate zeolite or other permselective membrane known in the art. 
     Reaction Conditions: 
     Typically, the reaction conditions include using a suitable FT catalyst such as an iron-based or cobalt-based catalyst or a mixture of both. The pressure in reaction zone  26  is ideally maintained at an elevated pressure of 5-40 bar. The temperature in reaction zone  26  is maintained in the range of 170-400° C. More preferably, the temperature is kept at about 180-220° C. for cobalt-based catalysts and about 250-280° C. for iron-based catalysts. A clam shell heater (not shown) may surround reactor  20  during operation to maintain reaction zone  26  at a desired generally isothermal operating temperature. The pressure within inner membrane tube  24  is maintained at a much lower pressure than that in reaction zone  26  where the FT conversions take place. A sweep gas can be used optionally to further reduce the partial pressure of water on the permeate side of membrane  36  and hence increase the driving force for the water separation. The syngas feed H 2 /CO molar ratio may be on the range of 1-3 and more preferably is about 1:2. 
     Operation: 
     In operation, a syngas feed is introduced to an upstream end cap  28  and into reaction zone  26 . Under suitable reaction conditions, as described hereinafter and in Table 1 below, reactions identified in equations (1), (2), and (3) and others occur. Reaction products include hydrocarbon products of varying carbon chain lengths, CO 2  and water and a variety of other compounds. Under these conditions, the water is in the form of water vapor. Accordingly, water vapor preferentially passes through the permselective membrane  36  as a permeate while the other reaction products and un-reacted feed preferentially remain in the annular reaction zone  26  and are eventually discharged as a part of a retentate stream through the perforated downstream end cap  30 . Ideally, un-reacted H 2  an CO gases will be separated from the discharged retentate stream and recycled and reintroduced (not shown) into the upstream portion of reaction zone  26  and/or to a syngas reformer (not shown) using processes known in the art. 
     As an alternate design to that shown in  FIG. 1 , the catalyst may be packed inside the inner membrane tube  26  rather than in the annular reaction zone  24 . In this case, a syngas feed can enter inside the inner membrane tube  26  and be converted therein to FT products and water. A sweep gas may be introduced in the annular reaction zone  24 . The retentate stream will exit from within the inner membrane tube  26  while the permeate stream will exit from the annular reaction zone  24 . 
     Computer Model and Modeling Results 
     As described below, a computer simulation study shows that using a zeolite membrane in a Packed Bed Membrane Reactor (PBMR) utilizing a cobalt catalyst enhances the overall FT process performance. In order to be able to optimize the membrane usage in this reaction, it is important to understand the effect of water removal on reaction rates and hydrocarbon yields. Ideally, membrane properties are matched with reaction rates to optimize the reactor design. 
     Computer models were generated corresponding to the reactor embodiment shown in  FIG. 1 . Table 1 describes base case conditions used in the computer simulation. It assumed that the reactor  20  of  FIG. 1  will be placed into a clam shell heater to maintain reaction zone  26  generally in an isothermal state. As the following examples show, in-situ water vapor removal has a definitive role in enhancing the F-T conversion process. 
     
       
         
               
             
               
               
             
               
               
               
               
             
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Conditions used for simulation 
               
               
                   
               
             
             
               
                 Base Case Conditions 
               
             
          
           
               
                 Reactor Dimensions 
                 Operating Conditions 
               
               
                   
               
             
          
           
               
                 Reactor diameter 
                 1 
                 Operating temperature (° C.) = 
                 240 
               
               
                 (in.) = 
                   
                 Feed inlet pressure (bar) = 
                 21 
               
               
                 Membrane diameter 
                 1 
                 Permeate pressure (bar) = 
                 7 
               
               
                 (cm) = 
                   
                 Feed H2/CO molar ratio =  
                 2 
               
               
                 Reactor length 
                 10 
                 Feed flow rate (mol/sec) = 
                 4.70E−03 
               
               
                 (in.) = 
                   
                 Sweep ratio = 
                 1 
               
               
                 Catalyst weight 
                 81 
                 W/F co  (gr · hr/mol) = 
                 7 
               
               
                 (gr) = 
                   
                 Superficial velocity (m/sec) = 
                 25 
               
               
                 Cobalt catalyst 
               
               
                   
               
             
          
           
               
                 Ideal Membrane §  Properties 
               
               
                   
               
             
          
           
               
                   
                 Perm H2O (mol/(sec · cm 2  · bar)) = 
                 1.38e−5 
               
               
                   
                 Perm H2 (mol/(sec · cm 2  · bar)) = 
                 0 
               
               
                   
                 Perm CO (mol/(sec · cm 2  · bar)) = 
                 0 
               
               
                   
                 Perm CO2 (mol/(sec · cm 2  · bar)) = 
                 0 
               
               
                   
                 Perm CH4 (mol/(sec · cm 2  · bar)) = 
                 0 
               
               
                   
                 Perm C8 (mol/(sec · cm 2  · bar)) = 
                 0 
               
               
                   
                 Perm others (mol/(sec · cm 2  · bar)) = 
                 0 
               
               
                   
               
               
                   § Ideal membrane: will pass only water 
               
             
          
         
       
     
     Example 1 
     Effect of Membrane on CO Conversion 
     Integrating a membrane in a FT reactor will enhance CO conversion as compared with a comparable plug flow reactor (PFR). A comparable PFR is defined as a reactor having an equivalent size and configuration as the PBMR shown in  FIG. 1 , except that the inner tube is not porous, has no membrane, and has no provisions for a sweep gas. CO conversion results from running the simulation are shown in  FIG. 2 . CO conversion in the PBMR is better than PFR the farther downstream CO conversion occurs in a respective reactor, as indicated in  FIG. 2 . 
     Example 2 
     Effect of Membrane on H 2 O Partial Pressure 
     A water permselective membrane will remove water vapor from the reaction medium and hence lowers water partial pressure in the reactor. This reduces catalyst deactivation and hence will increase life of a catalyst.  FIG. 3  shows the calculated effect of how using a membrane in the PBMR will decrease water partial pressure relative to the use of a comparable PFR. 
     Example 3 
     Effect of In-Situ Water Vapor Removal on FT Reaction 
       FIGS. 4(   a ) and  4 ( b ) show the calculated effect of in-situ water removal on FT reactions. By removing water, less CO 2  will form through water-gas shift reactions. Furthermore, partial pressure of the other components will increase and the rate of hydrocarbon formation will increase. The overall result is that less CO 2  will form and hydrocarbon yield will increase. Hydrocarbon Yield (HC Yield) in the graphs refers to the amount of produced (all) hydrocarbons (minus C 1  and C 2 ) per mole of CO feed to the reactor. 
     Example 4 
     Effect of Sweep Ratio 
       FIGS. 5(   a )-( d ) illustrate the calculated effect of sweep ratio on CO conversion, water partial pressure, hydrocarbon yield and the amount of extra catalyst weight required in a PFR to achieve the same hydrocarbon yield as in a PBMR. Sweep gas ratio is defined as the mole of sweep gas per total mol of feed gas. 
     Example 5 
     Effect of Permeate Side Pressure 
       FIGS. 6(   a )-( d ) show the calculated effect of permeate side pressure on CO conversion, water partial pressure, hydrocarbon yield and amount of extra catalyst weight required in a PFR to achieve the same hydrocarbon yield as in a comparable PBMR. 
     Example 6 
     Effect of a Membrane Separation Properties on Hydrocarbon Yield and CO Conversion for Non-Ideal Membrane with Inert Sweep Gas 
       FIGS. 7(   a )-( b ) show calculated results of membrane separation properties variation on hydrocarbon yield and CO conversion in FT reaction using a non-ideal membrane when an inert sweep gas has been used. That is, when components other than water in the reaction gas are permitted to selectively pass through the membrane. In these figures, water permeance was kept constant and separation properties of membrane have been changed. This study suggests targets for required membrane properties when a PBMR is compared with a comparable PFR, which does not utilize in situ dehydration. 
     Example 7 
     Effect of Membrane Separation Properties on Hydrocarbon Yield and CO Conversion for Non-Ideal Membrane with Hydrogen Sweep Gas 
       FIGS. 8(   a )-( b ) illustrate the calculated results of this study as described in example 7 above, however, using hydrogen (which is a reactant) as the sweep gas rather than the inert gas. 
       FIG. 9  shows a schematic of a second embodiment of a fixed bed membrane reactor (FBMR)  120  which does not employ a sweep gas to remove a permeate stream. Reactor  120  is a multi-double-tubular type reactor with catalyst  121  placed in a reaction zone  122  between outer and inner annular shell was  124  and  125  and an inner tubular membrane assembly  126 . A removable catalyst support grid  127  is provided which can be opened to allow catalyst particles drain out for catalyst replacement. 
     Membrane assembly  126  includes multiple tubes  128  which are made of a porous material such as stainless steel or alumina. Tube wall  128  works to support a membrane or membrane film to withstand the pressure difference between reaction zone  122  and a vapor zone  133 . An end cap  132  seals one end of tube  128  and forms water vapor zone  133 . A water permselective material or membrane  135 , such as a zeolite membrane, is affixed to either the inner or outer radial surface of tube  128  to allow water vapor to readily pass there through into vapor zone  133  from reaction zone  122  while inhibiting the passage of other reactants and products. The top of membrane apparatus  126  is sealed with tube sheet  134  (a tube sheet is a circular plate with multiple holes drilled with specific pattern to pass the membrane tubes.) to an upper end cap  140  which has a water vapor outlet  142 . The downstream end of reactor  120  has an end cap  144  with a products outlet  146 . An outer shell  150  provides a water bath chamber  151 , surrounding reaction zone  122 . Water inlet  152  and steam outlet  154  are in fluid communication with water chamber  151 . Controlling the water flow and the pressure and boiling temperature of water in water bath chamber  151  allows the temperature in reaction zone  122  to be controlled. Reactor  120  also has a reactant inlet  156  for receiving a syngas feed into reaction zone  122 . 
     In operation, reactants are introduced into reactor  120  by way of reactant inlet  156  into reaction zone  122 . Reactants (H 2 , CO, CO 2 , H 2 O) come in from the top of the tubular reactor and flow downward into the catalyst bed. In order to aid with the heat management, a small portion of liquid hydrocarbons may be added with the reactants to provide latent-heat of vaporization. FT conversions take place in reaction zone  122  with water vapor also being produced. A portion of the water vapor permeates from reaction zone  122  through permselective membrane  135  and into vapor zone  133  and exits reactor  120  by way of water vapor outlet  142 . Water vapor zone  133  is operated at low pressure or even at vacuum conditions to improve the permeability of water vapor. Reaction conditions are selected to maintain only gas phase in the catalyst bed and ensure high permeability of vapor through the membrane. 
     Meanwhile, the FT products, un-reacted CO and H 2  gas pass downstream through the catalyst  121  in reaction zone  122  and exit through reactor outlet  146 . Again, the in-situ dehydration of water in the reactor during the FT conversion provides enhancements in the FT conversion as demonstrated in the examples previously discussed. Water entering inlet  152  passes through cooling chamber  151 , receives heat from reaction zone  122 , becomes steam and exits out of reactor  120  by way of steam outlet  154 . 
     A third embodiment of a FT reactor  220  is shown in  FIG. 10 . In this embodiment, the FT reactor has the capability of providing a sweep gas to enhance the in-situ water vapor removal from FT reactor  220 . Similar to FT reactor  120 , reactor  220  has catalyst  221  packed into a reaction zone  222  formed between tubes  224  and  230 . Partially mounted in reaction zone  220  is a membrane assembly  226  which has multiple tubes with porous wall  230  and an end plate  234  which seals the tube, thereby defining an annular vapor zone  233 . Membrane materials are affixed to support wall  230 , such as a zeolite membrane  235 , to permit water vapor to readily pass from reaction zone  222 , through membrane  235  and into water vapor zone  233 . 
     A mini-tubular sweep gas assembly  236  is provided for introducing a sweep gas into vapor zone  233 . Sweep gas assembly  236  has multiple tubes  237  which are inserted into water vapor zone  233  and serve to deliver sweep gas to the lower end of water vapor zone  233 . Sweep gas assembly  236  is in fluid communication with an end cap  240  which has a sweep gas inlet  242 . 
     Located between tubes  224  is a water jacket  243  having a cooling water inlet  244  and a steam outlet  246 . Reactor  220  has a reactant inlet  250  which introduces reactants, i.e. syngas, into reaction zone  222  and end cap  252  which receives FT products and un-reacted feed from reaction zone  222 . Products outlet  254  allows FT products to exit reactor  220 . Ideally, these products are then separated with un-reacted CO and H 2  gas again being recycled (not shown) back to reactant inlet  250 . 
     In operation, a syngas feed is introduced into reactor  220  by way of reactant inlet  250  and into reaction zone  222 . FT conversions take place in reaction zone  222  with FT products being produced and water vapor. The FT products and un-reacted feed stream is then allowed to exit FT reactor  220  by way of products outlet  254 . 
     A significant portion of the water vapor produced passes through membrane  235  and into water vapor zone  233 . The pressure in water vapor zone  233  is maintained at a relatively low pressure compared to reaction zone  222 , in part, due to a sweep gas being provided to water vapor zone  233 . Sweep gas is introduced into sweep gas inlet  242 ; passes inside the sweep gas tubes  237  to the lower end of water vapor zone  233 ; and then flows counter current to the syngas feed along membrane  235  to assist in the removal of water vapor. The sweep gas may be an inert gas or may be a gas such as reactant H 2  gas or other desired gases or gas mixtures. The water vapor is then swept out reactor  222  by way of water vapor outlet  256 . 
     Water is introduced into cooling water inlet  244  and surrounds reaction zone  222  to maintain the temperature in reactor  220  at a predetermined temperature. Heat supplied from reaction zone  222  transforms the water into steam which exits reactor  220  by way of steam outlet  246 . 
     A fourth conceptual embodiment of a FT reactor  320  is shown schematically in  FIG. 11 . Reactor  320  includes a cylindrical outer shell  322  and an upstream inlet  324  and a downstream outlet  326 . FT catalyst  325  is packed within outer shell  322 . Axially spaced along reactor  320  are cooling coils  330  which receive cooling water, allowing the water to receive heat from the FT reactions and outlet steam from reactor  320 . 
     Also, spaced intermittently along reactor  320  are membrane assemblies  332 , which might be in the form of coils or radially-extending stakes. Membrane assemblies  332  include porous support members  334  which support permselective membrane materials forming membranes  336 . Membrane assemblies  332  allow water vapor formed in a FT reactions to pass through membranes  336  and out of FT reactor  320 . Preferably, the majority of membranes  336  are located closer to the downstream end  326  of FT reactor  320  than the upstream end  324 . At the upstream end of FT reactor  320 , relatively little water vapor has been formed as the syngas has just entered reactor  320 . At the downstream end of FT reactor  320 , all of the water vapor that will be formed has been formed. Accordingly, it is beneficial to place more of the membranes  336 , i.e., membrane materials, and hence the ability to remove water vapor, toward the downstream outlet  326  rather than the upstream inlet  324 . 
     There are numerous ways in which the goal of providing more water vapor removal capability in the downstream portion as opposed to the upstream portion of PBMR can be accomplished.  FIG. 12(   a ) suggests that most of the water accumulation occurs in the downstream portion of the reactor. One way to get continuous vapor removal while increasing membrane capacity is to use membrane assembly  400  having a porous generally frusto-conical shaped support member  402  supporting a membrane  404 , as shown in  FIG. 12(   b ). Membrane support  402  and membrane  404  will replace the membrane tube  24  of  FIG. 1 . The radius of this membrane  404  increases from its upstream end to it downstream end, i.e. r 0 &lt;r 1 . Corresponding to the fourth embodiment described above,  FIG. 12(   c ) shows a simplified membrane assembly  500  wherein the spacing between porous support disks  502  affixed with membrane material decreases from an upstream end to a downstream end. A central conduit may serve to carry sweep gas and water vapor away from a reaction zone in a FT or other reactor. Alternatively, the diameter or width of membrane  504  on each of the disks  502  may be increased from the upstream end to the downstream end of membrane assembly  300 . 
     While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention.