Abstract:
An outlet system in a reforming catalytic reaction apparatus for cracking hydrocarbons for the production of hydrogen (H 2 ) includes differently dimensioned inlet and outlet reaction tubes attached to and in flow communication with one another; and an external layer of thermal insulation material surrounding a part of the outlet reaction tube

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to an apparatus for the production of hydrogen by steam reforming. In particular, the invention relates to a furnace outlet system for transferring a stream of reactants including hydrogen from a heating furnace towards a manifold conduit of the apparatus for the production of hydrogen.  
         BACKGROUND OF THE INVENTION  
         [0002]    Purified hydrogen is an important gas source for many energy conversion process devices and can be produced by a steam reforming process implementing chemical reaction which results in producing hydrogen and certain byproducts or impurities later removed.  
           [0003]    Under steam reforming, steam and a hydrocarbon react in the presence of a catalyst. Steam reforming requires elevated temperature, e g, up 1500° F., and produces primarily hydrogen and carbon dioxide. Some trace quantities of unreacted reactants, and byproducts such as carbon monoxide also result from steam reforming.  
           [0004]    Typically, the steam reforming process includes distributing a mixture of hydrocarbons and steam over many parallel passes of the catalyst-filled reaction tubes which are subjected to elevated temperatures sufficient to perform the reforming reaction in a primary reformer section. At the furnace outlet system, parallel part streams are passing through downstream portions of the reaction tubes and are further collected in a single stream guided towards a waste heat boiler in which hydrocarbons and hydrogen are quenched for further separation.  
           [0005]    Steam reforming catalytic apparatuses are classified as either side-fired or top-fired implement the steam reforming process. A side-firing construction results in long and narrow boxes and often in several furnace boxes that have a common convection bank. Top-fired furnaces, however, can be built for very large capacities just by adding more tube rows in a common box.  
           [0006]    Regardless of the type of the steam reforming apparatus, hydrogen purification attempts to always maximize production of hydrogen from the reforming process. To increase the amount of hydrogen obtained, attempts have been made to decrease thermal exposure of the reaction tubes and, thus, to minimize their potential failure.  
           [0007]    A structure illustrated in FIGS. 1 and 2 is representative of the steam reforming catalytic reaction apparatus, as disclosed in U.S. Pat. Nos. 3,467,503 and 3,600,141 In particular, the steam reforming catalytic apparatus  10  includes a reaction tube  12  extending through and below the floor of the furnace (not shown) and shaped to conduct a stream of gas mixed with steam through a removable catalyst support grid  16  above where decomposition of hydrocarbons takes place in the presence of a nickel-containing catalyst. A mixture of CO, CO 2 , H 2 , some other minor reactants, and steam thus leave the furnace and is further conducted through an outlet system into a refractory-lined collector manifold  38  (FIG. 2).  
           [0008]    There is no reformer design that can avoid exposing the reformed tubes to very high temperatures approximating 1600° F. as the mixture leaves the furnace. The greater the temperature differential between the inside and outside temperatures, the greater the chance for the tube&#39;s failure.  
           [0009]    To establish a relatively smooth and gradual temperature transition and to minimize the axial and radial expansion of the reaction tube, its inner surface is lined with a thermal insulating layer. Thus, the inlet reaction tube  12  and an outlet reaction tube  18 , which is welded to the inlet reaction tube  12  and to a wall  20  of the collector manifold  38 , as indicated by a reference numeral  30 , are provided with first  26 , second  28 , third  32  and forth  34  thermal insulating internal layers. These thermal insulating layers are aligned with one another along a longitudinal direction of the reaction tube between a cone  24  and a lower portion of the outlet reaction tube  18  and are composed of various thermal insulations. Such a structure allows a temperature to decrease gradually from approximately 1600° F. inside the reaction tube to approximately 300° F. corresponding to the outside temperature of the wall.  
           [0010]    Still another temperature differential that affects structural integrity of the reaction tube and its expansion in both axial and radial directions is observed between the wall  20  of the collector manifold  38  and the inner space of the collector manifold  38 , which is in flow communication with a plurality of the reaction tubes. More particular, the outlet reaction tube  18  is provided with a gas conducting tube  22  positioned centrally and guiding the stream of the reactants and being in flow communication with a plug system  36 , which extends into the collector manifold  38 . Accordingly, while the wall  20  of the collector manifold  38  is about 300° F., the temperature inside the manifold reaches 1600° F. To minimize the chance of the conduit&#39;s failure, its inner wall has a single thermal insulating layer  40 .  
           [0011]    Overall, the steam reforming catalytic reaction apparatus  10  has a complex structure, which is difficult to assemble and maintain. Numerous internal thermal insulating layers are difficult to install and the reaction tube&#39;s shape, which is cylindrical and has a uniform diameter along its entire length, is not instrumental in reducing the gas temperature. Furthermore, a single thermal insulating layer provided on the inner side of the collector manifold is not always sufficient to prevent the overheating of this inner wall.  
           [0012]    It is, therefore, desirable to provide a steam reforming catalytic reaction apparatus having a simple, cost-efficient structure, which can be easily assembled and maintained A furnace outlet system facilitating relatively rapid cooling of reactants flowing from the furnace of the steam reforming catalytic reaction apparatus is also desirable.  
         SUMMARY OF THE INVENTION  
         [0013]    A reforming catalytic reaction apparatus for cracking hydrocarbons for the production of hydrogen (H 2 ) constructed in accordance with this invention attains these objectives.  
           [0014]    In particular, an outlet system of the inventive reaction apparatus has a multiplicity of reaction tubes, each of which includes differently dimensioned inlet and outlet portions attached to and in flow communication with one another.  
           [0015]    It is known that radiant heat flux, not reaction kinetics, is the controlling factor in determining the effectiveness of the reaction tubes. The efficient reaction tube heat transfer surface area for the specified average heat flux is associated with the high velocities of reactants guided along the reaction tubes, an optimal volume of catalyst required to affect the endothermic steam reforming reaction and with a relatively uniform, acceptable temperature of the walls of the reaction tubes. A structure of the inventive reaction tube including differently dimensioned portions provides such an effective heat transfer surface area.  
           [0016]    In accordance with another aspect of the invention, an outlet portion of a reaction tube is an assembly of an intermediate tube, which extends from the furnace of the reaction apparatus furnace and is provided with at least one external layer of thermal insulating material. The outlet portion further has a bottom tube attached to a collector manifold, which receives multiple streams of reactants exiting a plurality of reaction tubes.  
           [0017]    In practice, application of thermal insulating material to the outer surface of the intermediate tube requires a relatively short installation time. Furthermore, by eliminating an inner insulating layer associated with the known prior art, the flow path, along which the reactants flow inside this tube, is straight forward.  
           [0018]    According to still another aspect of the invention, the collector manifold is thermally insulated by utilizing multiple internal layers of thermal insulating material 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The above and other objects, features and advantages will become more readily apparent from the detailed description of the preferred embodiment illustrated by the following drawings, in which:  
         [0020]    [0020]FIG. 1 is an axial sectional view of an outlet system of a reforming catalytic reaction apparatus for cracking hydrocarbons for the production of hydrogen in accordance with the known prior art;  
         [0021]    [0021]FIG. 2 is a sectional view of a collector manifold of the reforming catalytic reaction apparatus illustrated in FIG. 1;  
         [0022]    [0022]FIG. 3 is an axial sectional view of the outlet system of the reforming catalytic reaction apparatus for cracking hydrocarbons for the production of hydrogen in accordance with the present invention;  
         [0023]    [0023]FIG. 4 is an axial sectional view of the collector manifold of the inventive apparatus shown in FIG. 3; and  
         [0024]    [0024]FIG. 5 is a cross-sectional view of the inlet portion of the reaction tube of the inventive apparatus shown in FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Referring to FIGS.  3 - 5 , a reforming catalytic reaction apparatus  50  for cracking hydrocarbons for the production of hydrogen receives a mixture of a reformable hydrocarbon and steam fed in a direction indicated by arrows  51  as the raw materials (feedstock) into a plurality of reaction tubes Each of the reaction tubes is a combination of an elongated fired portion  52  including a fire tube  78 , and an unfired outlet portion  76  having an intermediate tube  56  and a bottom tube  57 , which fire  78 , intermediate  56  and bottom  57  tubes are spaced apart along a longitudinal direction of the apparatus  50  To provide the reaction tube with an efficient flow velocity, the intermediate tube  56  has an outer diameter which is smaller than a uniform outer diameter of the fire  78  and bottom  57  tubes.  
         [0026]    The mixture of hydrocarbon feedstock and steam flows through the catalyst bed supported in the fire box by the reaction tube  78  on a ledge, and as the mixture passes through the catalyst bed, it receives heat from a heating furnace  70 . As a result of the endothermic steam reforming reaction, the hydrocarbon feedstock is converted into hydrogen (H 2 ), carbon dioxide (CO 2 ) and carbon monoxide (CO), which collectively form a stream of reactants flowing downwards through the outlet portion  76  of the reaction tube at a high temperature of about 1600° F.  
         [0027]    An upper cone  54  and a lower cone  60  serve as connecting elements between the fire, intermediate and bottom tubes and are field-welded, as denoted by  62 , to opposing ends of these tubes. Due to the geometry of the catalyst tube  78 , intermediate  56  and bottom  57  tubes, the upper cone  54  has a peripheral surface converging downwards from the catalyst tube, whereas the lower cone  60  has an inverted structure with a peripheral wall diverging downwards from a lower end of the intermediate tube  56 .  
         [0028]    The catalyst tube  78  disposed within the heating furnace and terminating approximately at the level of the furnace floor  70  does not have an inner layer of insulating material. However, as shown in FIG. 5, the catalyst tube  78  is thermal insulated along its outer periphery by a multi-layer insulating structure including layers  86 ,  90 ,  88  and  94  composed of high temperature cloth seal which is made up of silica inner layer, ceramic fiber and chopped fiber, as well as a firebrick layer  92  The penetration of the inlet portion  52  of the reaction tube through the furnace floor  70  is sealed completely by resilient elements such as flexible bellows  68  allowing the compensating axial and radial thermal expansion of the reactor tube due thermal loads applied to this tube.  
         [0029]    As shown in FIG. 3, to minimize thermal effects of heat produced by the stream of reactants that flows along the outlet portion  76  of the reactor tube, the intermediate tube  56  has an external layer  64  of heat insulating material The heat insulating material can be selected from ceramic fiber blanket, chopped fiber, firebrick of their combinations and can include a few sub-layers concentrically attached to one another. The external layers  64  extends preferably between the upper  54  and lower  60  cones and is surrounded by a jacket  66  made from stainless steel. The jacket  66  covers a part of the intermediate tube  56  stretching between the bellows  68  the lower cone  60 .  
         [0030]    Covering the intermediate tube  56  by the external layer  64  offers a simple and reliable structure reducing a temperature from about 1600° F. inside the reactor tube to about 300° F. on the outside of the external layer  64 . Accessible from outside, the external layer can be easily modified by adding additional sub-layers of insulating material. Furthermore, if a reaction tube fails, isolation of tube can be easily provided in a very short down time without cooling down the heating surface by removing the external layer  64 , the jacket  66  and either replacing the failed tube with a new one or providing a cap on the bottom tube  57 .  
         [0031]    The outlet portion  76  has a mixture conducting tube  58  positioned centrally in the bottom tube  57  and projecting into a collector manifold  69  which supports a multiplicity of reaction tubes having a construction identical to the one disclosed above and guides multiple streams of reactants to a waste heat boiler (not shown). The inner surface of the collector manifold  69  is insulated by multiple concentric layers of thermal insulating material including an outer layer  84  and an inner layer  82 . The outer layer  84  extends from the collector manifold upwards into a space formed between the mixture conducting tube  58  and the bottom tube  57  and includes insulating quality castable material. The inner layer which has heat-insulating properties inferior to the outer layer  84  is made up of high temperature erosion resistant castable material.  
         [0032]    Overall, the reforming catalytic reaction apparatus  50  featuring a combination of the inventive variously dimensioned reaction tube, external layers of thermal insulating material covering the intermediate tube of the outlet portion of the reaction tube and the concentric thermal insulating layers mounted in the collector manifold has a simple structure which is easy to assemble and maintain. The invention is not limited to the disclosed preferred embodiments subject to numerous modifications without, however, departing from the scope of the invention as recited in the following claims.