Abstract:
An apparatus ( 1 ) for producing synthesis gas is described, the apparatus comprising a substantially cylindrical shell ( 2 ) closed by opposite bottoms ( 3,   4 ), at least one inlet opening ( 8 ) for feeding a gaseous flow comprising oxygen, at least one inlet opening ( 7 ) for a gaseous flow comprising hydrocarbons and at least one outlet opening for a flow of synthesis gas and at least one burner ( 9 ) in fluid communication with a reaction chamber ( 15 ) for partially oxidising and/or reforming said hydrocarbons obtaining said flow of synthesis gas, and being characterised in that it comprises a pipe ( 12 ) of a ceramic material extended inside said shell ( 2 ), said pipe ( 12 ) of ceramic material internally defining said reaction chamber ( 15 ).

Description:
FIELD OF APPLICATION 
       [0001]    In its most general aspect, the present invention concerns an apparatus for producing gaseous mixtures comprising hydrogen, carbon monoxide and optionally nitrogen such as synthesis gases for methanol and ammonia. 
         [0002]    In particular, the present invention concerns an apparatus of the aforementioned type comprising a substantially cylindrical shell closed at opposite ends, at least one inlet opening for feeding a gaseous flow comprising oxygen, at least one inlet opening for feeding a gaseous flow comprising hydrocarbons, at least one outlet opening for a flow of synthesis gas, at least one burner in fluid communication with a reaction chamber for carrying out reforming reactions of the hydrocarbons and/or for partially oxidising said hydrocarbons obtaining said flow of synthesis gas, said reaction chamber being in fluid communication with said at least one outlet opening and optionally containing a suitable catalytic bed to promote said reforming reactions. 
         [0003]    In the remainder of the description and in the subsequent claims, the term “gaseous flow comprising oxygen” generically indicates a comburent gas comprising oxygen and optionally steam and/or nitrogen, whereas the term “gaseous flow comprising hydrocarbons” indicates a combustible or process gas comprising hydrocarbons, and optionally also carbon dioxide, hydrogen, carbon monoxide and steam. 
       PRIOR ART 
       [0004]    As it is well known, apparatuses of the aforementioned type are widely used to carry out secondary reforming or autothermal reforming of hydrocarbons (ATR) in the presence of catalyst in the aforementioned reaction chamber or partial oxidisation of hydrocarbons (POX) in the absence of catalyst, obtaining synthesis gas for the production of various chemical compounds, in particular synthesis gas for the production of ammonia and methanol. 
         [0005]    Within the use for secondary reforming, the combustible gas generally comes from a primary reforming section where hydrocarbons, for example natural gas, fuel oil, LPG (liquid petroleum gas) or refinery gas and mixtures thereof, have been made to react with steam. Such a combustible gas generally contains light hydrocarbons (for example C 1 -C 4 ) as well as hydrogen, carbon monoxide, carbon dioxide and steam. In the secondary reforming apparatus, the hydrocarbons are subjected to partial oxidation in a first zone of the reaction chamber beneath the burner, also known as combustion zone, with consumption of the oxygen contained in the comburent gas so obtaining a gaseous mixture. Such oxidation or combustion is highly exothermal and produces the heat necessary to promote the reforming reactions of the gaseous mixture that, instead, are endothermal, in a second zone of the reaction chamber beneath the combustion zone in which a suitable catalytic bed is installed, so as to obtain a gaseous synthesis flow comprising carbon monoxide and hydrogen. 
         [0006]    Within the use for autothermal reforming, the partial oxidation of the hydrocarbons and the reforming with steam are combined in a single apparatus (so called autothermal reformer) without previously subjecting the hydrocarbons to primary reforming. Therefore, in this case, the combustible gas directly contains the hydrocarbons of the predetermined supply source (for example natural gas) to which carbon dioxide coming for example from a recycled gaseous flow can optionally be added and the reforming is carried out on a suitable catalytic bed. 
         [0007]    In apparatuses intended for the partial oxidation of hydrocarbons (POX), so called partial oxidators, the feeding of the comburent and combustible gaseous flows is similar to that of an apparatus for autothermal reforming. However, in this case, the partial oxidation of hydrocarbons and optionally reforming reactions with steam are carried out together in a reaction chamber without catalyst. 
         [0008]    It is also known that that aforementioned apparatuses operate, at least in the combustion zone, at rather high temperatures, generally 1000-1600° C. and this makes it necessary to protect the metallic structure of the shell. For this purpose, the inside of the shell is lined with some layers of a refractory materials, generally made of preformed bricks or casts of refractive cements, which however has various drawbacks. 
         [0009]    First of all, it should be noted that such refractory materials must satisfy different characteristics of resistance to high temperatures, of heat conductibility and of heat expansion coefficient, according to whether they constitute a more inner layer (therefore more subject to high temperatures) or a more outer layer (less subject to high temperatures) of the lining. This implies that the lining must necessarily consist of multiple layers of different materials. 
         [0010]    In order to obtain a suitable resistance to high temperatures and a low heat dispersion one is also forced to make refractory linings of substantial radial thickness (300-500 mm), generally consisting of alternating layers of different materials. 
         [0011]    Moreover, the installation of such refractory linings is very laborious requiring very long time periods and highly specialised personnel since it is necessary to arrange the bricks or cement casts in such a way to avoid the formation of channels or slits allowing the diffusion of heat and gas towards the shell. 
         [0012]    As well as being difficult to make and install, conventional refractory linings impose substantial restrictions upon the operation of the aforementioned apparatuses. At first, after installation, a long time is needed for curing, drying and sintering of the refractory lining. In normal operation, during the heating and cooling steps of the apparatus, the presence of the refractory lining forces the adoption of suitable temperature gradients per unit time (dT/dt) that allow for the differential expansion of the various refractive layers (due to the temperature differences in the thickness of the refractory lining and to the different materials used) without the occurrence of thermal stresses greater than those acceptable for the materials. In fact, the exceeding of the maximum thermal gradient can often lead to ruinous damages to the refractory lining itself. 
         [0013]    Indeed, it should be noted that, for example, during the heating which proceeds from the inside towards the outside of the apparatus, the thermal stresses are generated by the fact that the thermal expansion of the hotter inner layers is prevented by the outer ones. 
         [0014]    For these reasons, the heating and cooling steps of the aforementioned apparatuses can require a number of days. 
         [0015]    The technical problem at the basis of the present invention is that of providing an apparatus for producing synthesis gas that overcomes the aforementioned drawbacks and in particular an apparatus for producing synthesis gas that is simple to make and allows high production capacities to be obtained with low investment, operating and maintenance costs. 
       SUMMARY OF THE INVENTION 
       [0016]    Such a technical problem is solved by an apparatus for producing synthesis gas comprising a substantially cylindrical shell closed at opposite ends, at least one inlet opening for feeding a gaseous flow comprising oxygen and a gaseous flow comprising hydrocarbons, at least one outlet opening for a flow of synthesis gas, at least one burner in fluid communication with a reaction chamber for partially oxidising and/or reforming said hydrocarbons obtaining said flow of synthesis gas, the apparatus being characterised in that it comprises a pipe of ceramic material extended inside said shell, said pipe of ceramic material internally defining said reaction chamber. 
         [0017]    Preferably, said pipe of ceramic material is substantially coaxial with said shelf. 
         [0018]    In the apparatus for producing synthesis gas according to the invention, an (annular) interspace is defined between the ceramic pipe and the shell, the interspace being delimited by the ceramic material outer surface and the shell inner surface. 
         [0019]    According to an aspect of the present invention, said interspace is substantially filled with an insulating material, preferably with at least a layer of said insulating material. 
         [0020]    The apparatus according to the invention can be used independently for secondary reforming, for autothermal reforming and for partial oxidation of hydrocarbons and it is particularly suitable for producing synthesis gas to be used for the production of methanol, ammonia or other chemicals. 
         [0021]    In the case of use for secondary or autothermal reforming, the pipe of ceramic material contains on the inside a catalytic bed so as to separate the reaction chamber into two zones, notably a first reaction zone, in direct contact with the burner, for carrying out partial oxidation, and a second reaction zone in which there is said catalytic bed for carrying out reforming reactions. 
         [0022]    In the apparatus according to the invention, the pipe of ceramic material has refractive properties that make it suitable for resisting the high operating temperatures, even over 1600° C., which are reached above all in the first reaction zone as a result of the exothermicity of the oxidation reactions of the hydrocarbons. Moreover, the pipe of ceramic material has a much lower thickness than that of conventional refractive coatings, preferably between 5 and 50 mm. According to a particularly preferred embodiment, the thickness of the pipe of ceramic material is between 20 and 50 mm. 
         [0023]    Preferably, the ceramic material for the pipe is chosen from the group comprising refractory oxides, refractory nitrides, refractory carbides and combinations thereof. 
         [0024]    Moreover, it should be noted that the ceramic material of the pipe intrinsically has a high capability to resist much greater thermal shocks than those that can be withstood by conventional refractory materials thanks to its low thickness and high thermal conductivity allowing the temperature gradients in the ceramic pipe to be considerably reduced, thus minimising the thermal stresses. 
         [0025]    The insulating material may be any material of any type such as for example a fibrous material, a non-fibrous material or a mixture thereof. In addition, the insulating material can be of any suitable shape such as for example loose fibres, tape, felt, woven or unwoven fabric etc. 
         [0026]    Preferably, the insulating material (fibrous or non-fibrous) is chosen from the group comprising refractory oxides, refractory nitrides, refractory carbides and combinations thereof. 
         [0027]    Preferably, the insulating material (fibrous or non-fibrous) includes one or more metallic oxides chosen among alumina, silicon oxides such as silica, rare earth (lanthanides) oxides and mixtures thereof. 
         [0028]    Most preferably, the insulating material is in the form of ceramic fibres. 
         [0029]    The ceramic fibres may be continuous or can have a predetermined length (for example cut fibres). In addition, the ceramic fibres may contain crystalline and vitreous phases and are obtained from different materials like for example metallic oxides, metallic nitrides, metallic carbides and combinations thereof. 
         [0030]    For example, the ceramic fibres can mainly or completely comprise fibres formed from metallic oxides like for example alumina, alumina-silica, alumina-boron oxide-silica, silica, zirconium oxide, silica-zirconium oxide, titanium oxide, titanium oxide-silica, rare earth oxides and combinations thereof. 
         [0031]    Preferably, in the present invention, the filling comprises ceramic fibres formed from allumina and silica. Fillings of this type that are particularly preferred are the commercial products known as kaowool. 
         [0032]    In the present invention, the insulating material has suitable high insulating and refractive properties as well as low mass due to its porosity. It is also characterized by a low mechanical resistance, i.e. it yield under load. 
         [0033]    In this way, it is advantageously possible to suitably protect the metallic structure of the shell from the high operating temperatures, to avoid thermal stresses in the ceramic pipe and to withstand high thermal loads, all in a simple manner and with low installation costs thanks to the fact that it is no longer necessary to fix any refractory material directly onto the shell, but rather it is necessary to simply install a pipe of ceramic material with the respective insulating material filling inside the shell. 
         [0034]    In particular, the insulating material filling, since it is yielding, it does not contrast the thermal expansions of the ceramic pipe thus neutralizing the thermal stresses that are generated during the operation of the apparatus according to the invention. 
         [0035]    This arrangement advantageously avoids the formation of cracks or breaks in the pipe of ceramic material with a consequent increase in its useful life. Moreover, the increased resistance to thermal shocks allows the times necessary to heat and cool the apparatus according to the invention to be substantially reduced. 
         [0036]    In the case the pipe of ceramic material and the insulating material filling have to be replaced, the removal is made just as simple as installation, to the great advantage of a reduction in costs and maintenance times. 
         [0037]    In the apparatus according to the invention, the pipe of ceramic material is obtained through per se conventional procedures, in particular through procedures that foresee the extrusion and the sintering of the ceramic material or plasma deposition procedures in which the formation of tubular structures is carried out by spraying powders of a powdered ceramic material into a plasma burner kept at a temperature of 5000-10000° C., so as to melt said powders and thus make them stick together on a rotating body. 
         [0038]    Preferably, the ceramic material used to produce the aforementioned pipe comprises sintered aluminium oxides and/or zirconium oxides and/or carbides, such as silicon carbides and/or rare earth (lanthanides) oxides. 
         [0039]    In order to reduce the passage of gas, from the reaction chamber towards the insulating filling, through the intrinsic pores of the ceramic material, the inner surface of the ceramic pipe can be subjected to a vitrification process to eliminate surface porosity. 
         [0040]    Preferably, the pipe of ceramic material is formed from modules of pipe pieces fitted together. 
         [0041]    Further characteristics and advantages of the present invention shall, moreover, become clear from the following description of some preferred exemplary embodiments, given for representative and not limiting purposes with reference to the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0042]      FIG. 1  shows a schematic view in longitudinal section of an apparatus for producing synthesis gas according to a first embodiment of the invention, 
           [0043]      FIG. 2  shows a schematic view from above and in partial section of the apparatus of  FIG. 1 , 
           [0044]      FIG. 3  shows a schematic view in longitudinal section of an apparatus for producing synthesis gas according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    With reference to  FIGS. 1 and 2 , an apparatus according to the invention for producing synthesis gas is globally indicated with  1 . 
         [0046]    The apparatus  1  can be used particularly for partial oxidation and/or reforming of hydrocarbons in the absence of catalyst and it is particularly suitable for producing synthesis gas for methanol, ammonia or other chemicals. 
         [0047]    The apparatus  1  generally operates at temperatures of between 800-1700° C. and pressures of between 5-200 bar. 
         [0048]    The apparatus  1  comprises a substantially cylindrical shell  2  with vertical axis A-A closed at the opposite ends by respective lower bottom  3  and upper bottom  4 . The upper bottom  4  is provided with a nozzle  4   a  having an opening for the insertion, in a suitable housing  6 , of a per se conventional burner, globally indicated with  9 . The body of the burner  9  projects from said nozzle  4   a  towards the outside of the shell  2  and ends at the top with a nozzle  7  constituting an opening for the entry of a gaseous flow comprising hydrocarbons and a nozzle  8  constituting an opening for the entry of a gaseous flow comprising oxygen. 
         [0049]    A collector  10  for collecting synthesis gas is instead envisaged close to the lower bottom  3 , said collector  10  extends in a nozzle  3   a  of the lower bottom and ends with a nozzle (not shown) constituting the outlet opening of the synthesis gases from the shell  2 . 
         [0050]    In accordance with the present invention, the apparatus  1  also comprises a pipe  12  of ceramic material with a smaller diameter than that of the shell  2  and coaxial with it so as to define an annular interspace  13 . More specifically, the pipe  12  of ceramic material has opposite vertical walls  12   a  (parallel to the axis A-A of the shell  2 ) as well as, at the opposite ends, a upper bottom  12   b  and a lower bottom  12   c  of suitable shape, like for example flat, as shown in  FIG. 1 , hemispherical or semi-elliptical. The bottoms  12   b  and  12   c,  top and bottom respectively, are each equipped with an opening so as to place the pipe  12  of ceramic material in fluid communication at the bottom with the collector  10  for collecting the synthesis gases and at the top with the burner  9 . 
         [0051]    On the inside, the pipe  12  of ceramic material defines a reaction chamber  15  for carrying out reforming reaction and/or partial oxidation reaction of hydrocarbons, said reaction chamber  15  being delimited at the top by the upper bottom  12   b  of the pipe  12  and by the burner  9 , and at the bottom by the lower bottom  12   c  of the pipe  12 . 
         [0052]    Of course, the pipe  12  of ceramic material may have any shape, i.e. it can have a circular or polygonal cross section, for example hexagonal. 
         [0053]    In the apparatus  1 , the annular interspace  13  is substantially filled by a layer  14  of ceramic fibres. In this way it is advantageously possible to adequately protect the metallic structure of the shell from high operating temperatures of the apparatus  1 . 
         [0054]    Moreover, it should be noted that the layer  14  of ceramic fibres is also provided close to the lower bottom and upper bottom  3  and  4  and in the corresponding nozzles  3   a  and  4   a  so as to advantageously protect the burner  9  and the outlet collector  10  of the synthesis gases from the high operating temperatures. As far as the operation of the apparatus  1  is concerned, a gaseous flow comprising hydrocarbons and a gaseous flow comprising oxygen entering the respective nozzles  7  and  8  is fed to the burner  9  and conveyed from it to the reaction chamber  15  of the pipe  12  of ceramic material. 
         [0055]    In the reaction chamber  15  the partial oxidation reaction of the hydrocarbons is carried out with consumption of oxygen reaching high temperatures (1000-1700° C.), due to the high exothermic content of the oxidation reactions, obtaining a “hot” gaseous mixture, i.e. with high thermal content. 
         [0056]    Such a gaseous mixture can thus undergo, again in the reaction chamber  15 , highly endothermal reforming reactions of the hydrocarbons, promoted by the reaction heat deriving from the partial oxidation. Therefore, a synthesis gas comprising carbon monoxide, hydrogen and optionally nitrogen is obtained which through the collector  10  comes out from the apparatus  1 . 
         [0057]      FIG. 3  illustrates a further embodiment of the apparatus according to the invention for producing synthesis gas. In such a figure, structural elements that are common or functionally equivalent to those of the apparatus  1  represented in  FIGS. 1 and 2  have been given the same reference numerals. 
         [0058]    The apparatus illustrated in  FIG. 3 , globally indicated with  30 , differs from the apparatus  1  in that it has a catalytic bed  31  in a bottom portion of the pipe  12  of ceramic material suitably supported by a support element  34 . In this way, the reaction chamber  15  defined by the pipe  12  of ceramic material is divided into two zones namely a first zone  32  or top zone in contact with the burner  9  and a second zone  33  or bottom zone filled with catalyst of the catalytic bed  31 . 
         [0059]    In particular, the top zone  32  is delimited at the top by the upper bottom  12   b  of the tube  12  and by the burner  9 , and at the bottom by the maximum level reached by the catalyst, indicated by the line  31   a  in  FIG. 3 . 
         [0060]    The apparatus  30  can be used particularly for obtaining synthesis gas through autothermal or secondary reforming in which the partial oxidation of the hydrocarbons is substantially carried out in the first reaction zone  32  and the reforming in the second reaction zone  33  with the aid of the catalyst of the catalytic bed  31 . 
         [0061]    The main advantage of the apparatus for producing synthesis gas according to the invention lies in the simplicity of installation and maintenance of the refractive material, consisting of the pipe of ceramic material and optionally of the outer coating, all at reduced costs compared to known apparatuses. 
         [0062]    Another advantage of the apparatus according to the invention lies in the high capability of ceramic material pipe to resist to resist high thermal stresses and shocks due to its low thickness and which high thermal conductivity (compared to the conventional refractory materials of the prior art). 
         [0063]    A further advantage of the apparatus according to the invention lies in the capability of reducing operating costs since, thanks to the fact that capability of the filling of refractive insulting material (in particular ceramic fibres) can efficiently absorb thermal expansion of the ceramic pipe, the start up and shutdown times are greatly reduced compared to the prior art. 
         [0064]    A further advantage of the apparatus according to the invention lies in its greater production capacity thanks to the fact that it is possible to reduce the radial thickness of the filling of refractive insulating material to a few tens of millimetres (for example 100 mm) to have adequate protection against heat of the shell, which consequently allows to increase the overall volume available for carrying out the oxidation and reforming reactions to be increased, at the same external size of the apparatus. 
         [0065]    A still further advantage of the reforming apparatus according to the invention lies in the fact that the pipe of ceramic material and the filling of refractive insulating material have relatively low costs which allows the investment costs to be reduced. 
         [0066]    Of course, a man skilled in the art can bring numerous modifications and variants to the apparatus according to the invention, all of which are in any case covered by the scope of protection of the following claims. For example, for some specific applications, the opening or inlet mouth for the gaseous flow comprising hydrocarbons could be envisaged on the shell instead of on the burner so as to enter the hydrocarbons directly into the reaction chamber, for example through a duct outside the burner and coaxial with it or a certain number of burners may be installed at convenient positions on the shell and/or the upper bottom of the apparatus.