Abstract:
The invention provides a process which facilitates high incremental capacity expansion of existing syngas-based plants by providing an adiabatic prereformer and an independent heater which is configured to (1) receive a prereformed and preheated mixed feed stream from the prereformer, (2) heat the prereformed and preheated mixed feed stream to a reheat temperature of as high as around 700° C. without local overheating of heat exchange surfaces and without the risk of carbon formation (on, e.g., fired heater heating coils), and which (3) feeds the prereformed and preheated mixed feed stream at the reheat temperature to the inlet of reformer radiant section catalyst tubes, wherein the prereformer and independent heater are not contained within the reformer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     In hydrocarbon reforming, a hydrocarbon feedstock and steam is reacted catalytically in a reformer furnace to form a synthesis gas comprised of hydrogen, carbon monoxide, and carbon dioxide. The reforming furnace is a critical component of hydrogen production facilities and plants which use synthesis gas to produce methanol and ammonia, and can account for almost half of the operating costs and energy expenditures of such installations.  
         [0002]     A reforming furnace typically contains a fired radiant section, a transition section, and a convection section. Tubes filled with a reforming catalyst (e.g., nickel on an alumina support) are disposed in the radiant section. A hydrocarbon feedstock and steam is fed through and reformed in the tubes. Combustion of a conventional fuel in the radiant section produces a hot flue gas which heats the tubes and provides the thermal energy necessary for the endothermic reforming reaction. The transition section receives hot flue gas from the radiant section and passes it to the convection section. One or more heating coils disposed in the convection section may be used for different preheat purposes, including preheating the hydrocarbon feedstock and steam before that feed stream is reformed in the radiant section catalyst tubes. Flue gas from the transition section heats the convection section coils.  
         [0003]     The overall efficiency of a reforming furnace is determined by the absorbed heat duty of the radiant section catalyst tubes. In general, greater catalyst tube heat duties require increased temperatures and firing rates in the radiant section. Operating the radiant section in this way requires increased maintenance and shortens the radiant section&#39;s useful life. Increasing the heat duty provided by the radiant section can also lead to coke formation in the radiant section catalyst tubes.  
         [0004]     Maximizing the temperature of the hydrocarbon feedstock and steam at the inlet of the radiant section catalyst tubes (the “preheat temperature”) improves reformer efficiency. At higher preheat temperatures, the reforming reaction is initiated closer to the inlet of the catalyst tubes, which improves the efficiency of the reforming reaction.  
         [0005]      FIG. 1  illustrates a conventional reforming process in which a mixed feed  1  comprising a hydrocarbon feedstock and steam is preheated in the convection section  2  of the reformer  10 . Preheated mixed feed  3  then flows through catalyst-filled tubes  4  located in radiant section  5 . Synthesis gas product stream  6  is collected at the other end of the tubes and is supplied to a customer after additional purification. A conventional reforming process such as that illustrated in  FIG. 1  can only achieve a preheat temperature of around 500° C. to around 600° C. due to the risk of carbon formation from the heavy hydrocarbons present in the feedstock.  
         [0006]     A reduction in radiant section heat load can be achieved by reforming at least the hydrocarbon feedstock heavy hydrocarbons in a prereformer prior to feeding the mixed feed through the radiant section catalyst tubes. This approach is illustrated in  FIG. 2  and in U.S. Pat. No. 5,264,202. Referring to  FIG. 2 , a mixed feed  1  of a hydrocarbon feedstock and steam is preheated in convection section  2  of reformer  10  and is then fed to prereformer  7 . Prereformer effluent stream  3  may be heated to a reheat temperature of around  680 ° C in convection section  2  prior to being fed to the inlet of radiant section catalyst tubes  4 .  
         [0007]      FIG. 3  illustrates a variant of the process of  FIG. 2  in which a prereformer is positioned within the reformer convection section. In the process illustrated in  FIG. 3 , a mixed feed comprising a hydrocarbon feedstock and steam is fed to prereformer  1  which is positioned in and heated to a reheat temperature by convection section  2  of reformer  10 . The prereformed mixed feed is then fed at the reheat temperature to radiant section catalyst tubes  4 . Prereforming within a convection/transition section prereformer is also described in U.S. Pat. No. 6,818,028. In general, prereforming proves useful for reforming a natural gas feed as a means to reduce steam generation or primary reformer duty and in instances where the conversion from one feedstock to another is required.  
         [0008]     The preheat and prereforming process designs described above prove unsuitable for expanding the production capacity of an existing reformer above around 25% of existing capacity because they are constrained by the energy available in the flue gas, radiant section firing duty, convection section space limitations, and overall plant heat balance.  
         [0009]     Exposure to reformer section radiant heat and variation in radiant section flue gas temperature make it difficult to regulate the reheat temperature of the effluent stream from a prereformer located in a reformer convection section as well.  
         [0010]     Installation of a prereformer in the transition section of an existing reformer is very expensive; the reformer must be taken off-line, thereby disrupting the output of all associated facilities. Limited space in the convection section often precludes installation of an adequately-sized prereformer. Further, the convection section coil design may interfere with the positioning of the prereformer in the convection section.  
         [0011]     These problems can be compounded by the fact that heat in the flue gas coming from the reformer radiant section may be inadequate to heat both the convection section coils and the prereformer. In such cases, installing a prereformer in the convection section could disrupt the energy balance of the reformer and all associated plants. Major changes to all of the convection coils downstream of the prereformer might also be required.  
         [0012]     Accordingly, the need exists for a cost-effective process which expands the production capacity of an existing reformer through preheat and prereforming without disrupting the output or energy balance of the either the reformer or any associated plant.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The invention provides a process for generating a hydrogen-containing product gas in a reformer comprising a radiant section, a plurality of reforming catalyst-filled tubes disposed in the radiant section, a transition section, and a convection section, the process comprising: 
    (a) firing the reformer radiant section to generate a flue gas which passes from the radiant section, through the transition section, and to the convection section and which heats the radiant section catalyst-filled tubes and the convection section;     (b) feeding a mixed feed stream comprising a hydrocarbon feedstock and steam through the convection section to yield a preheated mixed feed stream;     (c) feeding the preheated mixed feed stream to a prereformer which adiabatically prereforms the preheated mixed feed stream and generates a prereformed and preheated mixed feed stream;     (d) feeding the prereformed and preheated mixed feed stream to an independent heater which heats the prereformed and preheated mixed feed stream to a reheat temperature; and     (e) feeding the preheated and prereformed mixed feed stream at the reheat temperature through the radiant section catalyst-filled tubes, thereby generating a hydrogen-containing product gas,     wherein (1) the prereformer is configured to receive the preheated mixed feed stream from the reformer convection section and to feed the prereformed and preheated mixed feed stream to the independent heater; (2) the independent heater is configured to receive the prereformed and preheated mixed feed stream from the prereformer and to feed the preheated and prereformed mixed feed stream at the reheat temperature to the inlet of the radiant section catalyst tubes; and (3) neither the prereformer nor the independent heater are located within the reformer.    
 
         [0020]     In another embodiment, the invention provides a process for increasing the hydrogen production capacity of a reformer comprised of a radiant section, a plurality of reforming catalyst-filled tubes disposed in the radiant section, a transition section, and a convection section comprising one or more coils, the process comprising: 
    (a) providing a prereformer which is configured to receive a preheated mixed feed stream comprising a hydrocarbon feedstock and steam from the reformer convection section and which adiabatically prereforms the preheated mixed feed stream and generates a prereformed and preheated mixed feed stream; and     (b) providing an independent heater which: (i) is configured to receive the prereformed and preheated mixed feed stream from the prereformer, (ii) heats the prereformed and preheated mixed feed stream to a reheat temperature without causing local overheating of heat exchange surfaces (e.g., independent heater reheat coils) or carbon formation on surfaces contacted by the preheated mixed feed stream (e.g., inside a heater reheat coil), and which (iii) feeds the prereformed and preheated mixed feed stream at the reheat temperature to the inlet of the plurality of reforming catalyst-filled tubes disposed in the reformer radiant section, wherein     (1) the reformer radiant section is fired to generate a flue gas which passes from the radiant section, through the transition section, and to the convection section and which heats the radiant section catalyst-filled tubes and the one or more convection section coils;     (2) a mixed feed stream comprising a hydrocarbon feedstock and steam is fed through and heated in the convection section to yield the preheated mixed feed stream;     (3) feeding the preheated and prereformed mixed feed stream through the radiant section catalyst-filled tubes generates a hydrogen-containing product gas; and     (4) the prereformer and independent heater are not contained within the reformer.    
 
         [0027]     In a preferred embodiment of the invention, the prereformed and preheated mixed feed stream is heated from about 400° C. to a reheat temperature of between about 650° C. to about 700° C. in the independent heater.  
         [0028]     In another preferred embodiment of the invention, the independent heater is a fired heater.  
         [0029]     In a particularly preferred embodiment, the reformer is a steam-methane reformer (SMR), the hydrocarbon feedstock is natural gas, the independent heater is a fired heater, the prereformer effluent reheat temperature is between about 650° C. to about 700° C., and the hydrogen-containing product gas is synthesis gas which is recovered and purified. Existing prereformers or independent heaters which are located outside of an operating reformer can be reconfigured in accordance with the instant invention to increase the hydrogen-containing product gas production capacity of the reformer and overall hydrogen production of an associated plant.  
         [0030]     Processes of the invention facilitate the expansion of the hydrogen-containing product gas production capacity of an existing reformer and/or an associated plant (conceivably by around 25% to around 60%) without disrupting the output or energy balance of either the reformer or any associated plant. Implementation of processes of the invention is not constrained by reformer convection section size or coil configuration. Substantially uniform reheat temperatures are achieved by processes of the invention because the prereformer effluent reheat coil is not exposed to a non-uniform heating environment, e.g., reformer radiant section flames. Positioning the prereformer and independent heater outside of the reformer achieves uniform temperature control of the reformer feed; does not interfere with reformer heat duty; and eliminates carbon formation on heat exchange surfaces such as fired heater heating coils while reheating prereformed feed to the reformer to temperatures of as high as around 700° C.  
         [0031]     The independent heater used in the invention may be adjusted to vary reheat temperature, e.g., in response to variations in process conditions and product demand. A lower reheat temperature of the prereformer effluent stream may be maintained if the reformer&#39;s rate of production of hydrogen-containing product gas is reduced and a higher reheat temperature may be maintained if a greater rate of production of hydrogen-containing product gas is required. The independent heater may be relatively small and may have a substantially smaller heat duty compared to the reformer. Therefore, the rate of production of hydrogen-containing product gas can be varied in a matter of hours.  
         [0032]     Another advantage offered by processes of the instant invention is that when the independent heater is taken off-line, e.g., for purposes of servicing, the prereformer effluent may be fed directly to the reformer radiant section catalyst tubes, thereby avoiding a complete shut-down of reformer operation.  
         [0033]     Further, because of all of the aforementioned advantages, processes of the instant invention prove particularly useful in optimizing the incremental expansion of hydrogen production networks.  
         [0034]     These and other aspects of the invention are described further in the following detailed description of the invention. 
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0035]      FIG. 1  illustrates a conventional reforming process, as described above.  
         [0036]      FIG. 2  illustrates a conventional reforming process that employs adiabatic prereforming, as described above.  
         [0037]      FIG. 3  illustrates a conventional reforming process that employs convection section prereforming, as described above.  
         [0038]      FIG. 4  illustrates one embodiment of a process of the instant invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]     The following definitions apply unless indicated otherwise.  
         [0040]     A “hydrocarbon feedstock” includes any hydrocarbon-containing stream which can be reacted chemically to produce a hydrogen-containing product gas. Hydrocarbon feedstocks include but are not limited to light hydrocarbons such as natural gas, naphtha, and refinery offgases, and high molecular weight liquids or solid carbonaceous materials. “Reforming” and “reacting hydrocarbon feedstock chemically to produce a hydrogen-containing product gas” include but are not limited to: steam reforming of light hydrocarbons (primarily natural gas, naphtha, and refinery offgases); dry reforming; the partial oxidation of carbon-containing feedstocks (ranging from natural gas to high molecular weight liquid or solid carbonaceous materials); non-oxidative catalytic decomposition; and autothermal reforming of light hydrocarbon feed (which combines features of both partial oxidation and steam reforming in a single reactor).  
         [0041]     Steam reforming is a preferred reforming process. In steam reforming, a hydrocarbon and steam mixture reacts in the presence of a catalyst to form hydrogen, carbon monoxide and carbon dioxide. Since the reforming reaction is strongly endothermic, heat must be supplied to the reactant mixture, e.g., by heating the tubes in a furnace or reformer. The amount of reforming achieved depends on the temperature of the gas leaving the catalyst.  
         [0042]     “Hydrogen-containing product gas” is produced by reforming a hydrocarbon feedstock, as defined above. Hydrogen-containing product gas (e.g., synthesis gas) produced by processes of the invention can be separated to yield a substantially pure hydrogen product by separation apparatus and processes that are well-known to those of ordinary skill in the art.  
         [0043]     “Reformers” used in the processes of the invention include but are not limited to conventional steam methane reformers and modular steam reformers, including Modular Partition Reformers (MPR&#39;s) such as those described in commonly-owned U.S. Pat. No. 6,793,700 (&#39;700 Patent) and U.S. patent application Ser. No. 10/746,577, the complete disclosures of which are hereby incorporated by reference.  
         [0044]     One embodiment of a MPR that is disclosed in the &#39;700 Patent and that can be used in processes of the invention includes: a combustion chamber, a convection chamber in fluid communication with the combustion chamber, at least one burner disposed in the combustion chamber, and a reaction chamber. The combustion chamber has a first end and a second end opposite the first end. The convection chamber has a first end and a second opposite the first end, the first end of the convection chamber being adjacent the second end of the combustion chamber. The at least one burner is disposed in the combustion chamber and is adapted to combust a fuel, thereby generating a flow of a flue gas from the combustion chamber to the convection chamber, the flue gas having a sensible heat. The reaction chamber has a first part and a second part in fluid communication with the first part. A substantial portion of the first part is disposed in the combustion chamber and a substantial portion of the second part is disposed in the convection chamber. The second part is a tube-in-tube having an annular portion between an inner tubular portion and an outer tubular portion surrounding the inner tubular portion. The apparatus also includes a means for flowing a first mixed-feed through the first part of the reaction chamber, and a means for flowing a second mixed-feed through the annular portion of the second part of the reaction chamber counter-currently with the flow of the flue gas in the convection chamber.  
         [0045]     MPR&#39;s disclosed in the &#39;700 Patent can combine combustion and convection chambers in one compact unit that can be built in the shop and serve as a modular unit, so that several units can be added with relatively simple connections in the field to achieve or to expand synthesis gas production capacity.  
         [0046]     One embodiment of a MPR disclosed in U.S. patent application Ser. No. 10/746,577 which can be used in processes of the invention includes a vessel having at least one partition wall disposed in the vessel. The at least one partition wall divides the vessel into a plurality of chambers, including at least one combustion chamber and at least one convection chamber. Each of the chambers has a first end and a second end opposite the first end. At least one burner is disposed in the combustion chamber. The burner is adapted to combust a fuel, thereby generating a flue gas having sensible heat. The apparatus also includes communication means between the combustion chamber and the convection chamber whereby at least a portion of the flue gas flows from the combustion chamber to the convection chamber at a first location adjacent the first end of the convection chamber. The apparatus also includes transfer means whereby at least a portion of the flue gas flows to a second location in the convection chamber adjacent the second end of the convection chamber. The apparatus also includes multiple reaction chambers, including a first reaction chamber and a second reaction chamber. A substantial portion of the first reaction chamber is disposed in the combustion chamber, and a substantial portion of the second reaction chamber is disposed in the convection chamber.  
         [0047]     MPR&#39;s disclosed in U.S. patent application Ser. No. 10/746,577 are in the form of a compact unit that may be built in the shop and may be used as a modular unit in a configuration where several units set side-by-side are connected with simple connections at a field site to achieve or to expand synthesis gas production capacity.  
         [0048]     “Prereformers” used in processes of the invention are adiabatic prereformers which can comprise an insulated vessel filled with a prereforming catalyst. Other types of useful adiabatic prereformers are well-known to those of ordinary skill in the art and can be also be used in the invention.  
         [0049]     Reforming and prereforming catalysts used in processes of the invention can be the same or different and include but are not limited to metallic catalysts such as structured or unstructured metallic catalysts, e.g., structured or unstructured nickel reforming catalysts Conventional steam-methane reforming and prereforming catalysts such as nickel-alumina, nickel-magnesium alumina and the noble metal catalysts can also be used in steam-methane reforming embodiments of the invention.  
         [0050]     In certain embodiments, reformer catalyst tubes have an inside diameter of at least about 125 mm, the pressure drop through the reformer catalyst tubes is less than about 0.1 MPa, the catalyst in the catalyst-filled tubes preferably has a substantially uniform size distribution, and the catalyst in the reformer catalyst-filled tubes preferably has a nickel content from about 15 to about 20 weight percent and is optionally promoted with potassium. Other useful reforming and prereforming catalysts are well-known to those of ordinary skill in the art and can be used in the invention.  
         [0051]     An “associated plant” means any facility that receives or uses hydrogen-containing product gas produced by a reformer.  
         [0052]     A “mixed feed stream comprising a hydrocarbon feedstock and steam” typically is comprised of mixture of natural gas with steam, mixture of vaporized naphtha with steam, mixture of refinery off gases with steam, or combination of those hydrocarbons with steam  
         [0053]     A “prereformed and preheated mixed feed stream” typically is comprised of hydrogen, carbon monoxide, carbon dioxide, water, as well as methane and nitrogen.  
         [0054]     “Conventional fuels” include “hydrocarbon feedstock” as defined herein, as well as other hydrogen or hydrocarbon containing fuels.  
         [0055]     “Independent heaters” used in processes of the invention can include any means of heating the prereformer effluent, including but not limited to convective, electrical, and other heating means. The independent heater preferably employs heating elements, e.g., pipes or coils, which reheat the prereformer effluent at a substantially constant reheat temperature. Achieving a uniform reheat temperature minimizes carbon deposition in the prereformer effluent stream.  
         [0056]     Well-known infrastructure (e.g., pipes, valves, compressors, etc.) can be used to: (1) configure the prereformer to receive the preheated mixed feed stream from the reformer convection section and to feed the prereformed and preheated mixed feed stream to the independent heater; (2) configure the independent heater to receive the prereformed and preheated mixed feed stream from the prereformer and to feed the preheated and prereformed mixed feed stream at the reheat temperature to the inlet of the radiant section catalyst tubes; and (3) to recover and optionally purify the hydrogen-containing product gas from the radiant section catalyst tubes.  
         [0057]     “Control means” can be associated, e.g., with the reformer, prereformer, and independent heater used in processes of the invention. The control means can perform a variety of functions, including regulation and optimization of the amount of hydrocarbon feedstock and steam in the mixed feed stream, the mixed feed preheat temperature, and the reheat temperature.  
         [0058]     Control means can include computer systems comprising central processing units (CPU&#39;s) for processing data related to network parameters (e.g., hydrocarbon feedstock flow rates and hydrogen-containing product gas production and consumption rates), associated memory media including floppy disks or compact discs (CD&#39;s) which may store program instructions for CPU&#39;s, one or more display devices such as monitors, one or more alphanumeric input devices such as a keyboard, and one or more directional input devices such as a mouse. Computer systems used in control means can include a computational system memory such as DRAM, SRAM, EDO DRAM, SDRAM, DDR SDRAM, or Rambus RAM, or a non-volatile memory such as a magnetic media (e.g., a hard drive) or optical storage. The memory medium preferably stores a software program or programs for event-triggered transaction processing. The software program(s) may be implemented in any of various ways, including procedure-based techniques, component-based techniques, and/or object-oriented techniques, among others.  
         [0059]     Control means can include instrumentation for: (1) monitoring analog input data relating to process parameters (e.g., hydrocarbon feedstock flow rates and hydrogen-containing product gas production rates); (2) converting such analog input data to CPU input digital signals for CPU processing and generation of CPU digital output signals; and (3) converting CPU digital output signals to analog signals that vary process parameters such as hydrocarbon feedstock flow rates and preheat and reheat temperatures in accordance with CPU digital output signals. Thus, control means can provide real-time, feedback control of process operation.  
         [0060]     In a preferred embodiment, the independent heater is a fired heater having a combustion section which generates a flue gas which is passed to a fired heater convection section. One or more heating coils may be positioned in the fired heater convection section in manner which avoids direct exposure of the coils to the radiation heat from the fired heater combustion section. The fired heater flue gas temperature may be controlled by varying the number and/or duty of fired heater combustion section firing elements (e.g., burners) and by using, e.g., 180% to about 280% of a stoichiometric amount of air. Such a firing system provides substantial temperature control of the fired heater flue gas temperature in proximity to the fired heater convection section coils. The temperature of the flue gas in proximity to the fired heater convection section heating coils may be held slightly above that required for prereformer effluent stream reheat.  
         [0061]     In still another embodiment, the combustion air used to fire the reformer radiant section is preheated in the independent heater.  
         [0062]     Generating a flue gas in the fired heater combustion section through combustion of a conventional fuel with excess air eliminates high temperature gradients between the fired heater flue gas and reheated prereformer effluent and minimizes the differential between the prereformer effluent reheat temperature and fired heater convection section coil metal peak temperatures. This minimizes the risk of carbon formation on the fired heater convection section coils and enables a high reheat temperature for the prereformer effluent stream.  
         [0063]     In another embodiment, the fired heater convection section contains rows of convection coils configured in a split design, with the first several rows of the coils arranged to channel fired heater flue gas co-currently with the prereformer effluent stream to reduce the coil&#39;s metal peak temperatures.  
         [0064]     In preferred embodiments, the prereformed and preheated mixed feed stream is heated from about 370° C. to a reheat temperature of about 700° C. in the independent heater, and is most preferably heated from about 400° C. -500° C. to a reheat temperature of about 680° C.  
         [0065]      FIG. 4  illustrates one embodiment of a process of the instant invention. Referring to  FIG. 4 , mixed feed stream  1  comprising a hydrocarbon feedstock and steam is preheated in convection section  2  of reformer  10 . An adiabatically prereformed and preheated mixed feed stream is then generated by feeding the preheated mixed feed stream  1  through prereformer  7 .  
         [0066]     The prereformed and preheated mixed feed stream is then heated to a reheat temperature by independent heater  8 . As indicated by dashed line  1   a,  preheated mixed feed stream  1  may also be preheated prior to prereforming using independent heater  8 . Prereformed and preheated mixed feed stream  3  is then fed at the reheat temperature to the inlet of a plurality of reforming catalyst-filled tubes  4  disposed in radiant section  5  of reformer  10 , thereby generating a hydrogen-rich synthesis gas  6  which is recovered and optionally purified through the use of processes and apparatus that are well-known to those of ordinary skill in the art.  
         [0067]     The invention is illustrated further in the following non-limiting example.  
       EXAMPLE 1  
       [0068]     An optimized steam reforming process of the instant invention was modeled based on the use of an independent fired heater which heats an adiabatically prereformed and preheated mixed feed described in Table 1 below to a reheat temperature of around 650° C.  
         [0069]     The fired heater is designed with a separate combustion chamber which operates on natural gas fuel with an excess air of 130% above stoichiometry (corresponding to 11.2% (wet) oxygen in the fired heater flue gas). A reheat coil is located in the fired heater convective section and is not exposed directly to the fired heater combustion chamber radiant heat. The temperature of the flue gas at the coil inlet is 990° C. The reheat coil has nine rows of 2″ tubes; the first two rows of tubes are configured in a co-current arrangement with the flue gas flow to reduce peak temperatures, and the remaining seven rows are configured in a counter-current arrangement with the flue gas. The inner gas film temperature at the first and hottest tube row of the coil is 690° C., which ensures than no carbon formation can occur on the internal heat exchange surface. The corresponding maximum (peak) metal temperature is 733° C. and the bulk gas temperature at the first row exit is 505° C. The first five rows of the coil are designed based on the use of bare tubes; finned tubes are used for the last four rows of the coil to extend surface area at the part of the coil with lower flue gas temperatures.  
                                                                 TABLE 1                           PREREFORMER EFFLUENT       STREAM       (Inlet to the Fired Heater       reheat coil)                COMPOSITION                    COMPONENTS   kmol/hr   % mol.                            H2   21.42   7.9%           C1   59.11   21.8%           CO   0.11   0.0%           CO2   6.79   2.5%           N2   0.25   0.1%           H2O   183.17   67.6%           TOTAL   270.84   100.0%           Pressure,   24.6           Bara           Temperature ° C.   453                      
 
         [0070]     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.