Abstract:
A hydrocarbon dehydrogenation process in which a hydrocarbon feed, normally a straight run naphtha, comprising acyclic and cyclic paraffins is dehydrogenated at elevated temperature of at least 540° C. with process heat provided at least in part by a solar or nuclear thermal energy source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates and claims priority to U.S. Provisional Patent Application No. 61/268,774, filed on Jun. 16, 2009. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to hydrocarbon dehydrogenation processes and more particularly to catalytic naphtha reforming processes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hydrocarbon dehydrogenation processes are important commercially in the petroleum refining and petrochemicals industries for the production of unsaturated hydrocarbons such as olefins and aromatics from saturated or more fully saturated precursors. The most important commercial process utilizing hydrocarbon dehydrogenation is the catalytic reforming process in which straight-run naphthas are converted to more highly aromatic products over a catalyst usually containing platinum as an active component. The products may be either high octane gasolines or petrochemical feedstocks. 
         [0004]    The catalytic reforming process is generally regarded as comprising four individual reactions:
       1. Dehydrogenation of naphthenes to produce aromatics and hydrogen. The dehydrogenation reactions are strongly endothermic (˜70 kJ/mol) and have the highest reaction rates of the reforming reactions; this necessitates the use of interstage heaters between successive reactors to maintain the reactant hydrocarbons at the required high temperatures for the reactions to proceed at acceptable rates.   2. Isomerization of paraffins, mainly isomerization of n-paraffins to branched chain products and isomerization of alkyl cyclopentanes to cyclohexanes which are subsequently converted to benzenes. This reaction is rapid and is mildly exothermic but is favored by the use of high temperatures to increase reaction rate.   3. Dehydrocyclization of paraffins. This reaction involves difficult molecular rearrangements of paraffins to naphthenes. Higher molecular weight paraffins are relatively easier to rearrange since ring formation tendency is higher. Dehydrocyclization reactions are endothermic (˜44 kJ/mol) and favored by low pressure, high temperature and require acid and metal catalyst functions.   4. Hydrocracking and dealkylation. Paraffin hydrocracking is a high probability reaction due to the difficulty of naphthene isomerization and paraffin cyclization reactions. This reaction is kinetically the slowest and is mildly exothermic but is favored kinetically by the use of higher temperatures along with high pressures. Dealkylation involves either reducing the chain length on the aromatics or completely removing the side chain. Dealkylation is favored by high temperature and high pressure.       
 
         [0009]    Overall, the reforming process is favored by the use of high temperatures by a balance of the thermodynamic equilibria and the kinetics of the respective endothermic and exothermic reactions participating in the process. While metallurgical design considerations may at some point become a limiting factor in operating at higher temperatures other factors also enter into consideration including the economics and environmental factors of fossil fuel use for process heating, the activity and selectivity of the catalyst system at higher temperatures and the extent to which competing reactions will become progressively favored. Process economics would obviously be favored by reductions in the use of fossil fuels for satisfying process heat requirements which would also result in a reduction in the generation of carbon dioxide emissions. Substitution of fossil fuel process heat by other heat sources such as nuclear and solar energy is therefore an economically, environmentally and politically attractive option since in a carbon-constrained economy, the potential to dramatically reduce flue gas and extend fossil resource capacity may depend on nontraditional uses of alternative energy. 
         [0010]    A major consideration in reforming is the balance which must be achieved between the hydrogenation/dehydrogenation function of the catalyst and its cracking function at any selected temperature. Generally, with present platinum-based reforming catalysts, the platinum provides the catalytic sites for the hydrogenation/dehydrogenation activity while an acidic function required for the initial cracking step of the hydrocracking and for the paraffin dehydrocyclization is provided by partly by the alumina of the carrier and some may be provided by the feed but since these quantities are usually insufficient, the support material or separate carrier is usually halogenated with the degree of halogenation optimized to provide the desired degree of activity to promote isomerization. Typically, platinum-containing alumina-based reforming catalysts are usually manufactured having a predetermined amount of halide, particularly chloride, on catalyst, sometimes up to about 3 wt. %, depending on the active metals content of the catalyst. As the catalyst ages, chloride loss becomes appreciable and, inter alia, contributes to loss of catalyst activity and chloridation during the catalyst cycle becomes necessary to maintain activity. 
         [0011]    At higher temperatures catalyst activity and selectivity may become unfavorable as paraffin cracking becomes more significant even though aromatization of naphthenes will be promoted thermodynamically. A balance between the desired aromatization reactions and the less desired cracking reactions can therefore be achieved in principle by operation at higher temperatures with a catalyst in which cracking activity is inhibited. 
         [0012]    Current catalytic reforming such as UOP&#39;s Platforming™ process operate with a platinum-containing catalyst at temperatures in the range of 525 to 540° C. and hydrogen pressures of 345 to 3450 kPa n the case of fixed bed processes and rather lower pressures in the moving bed CCR Platforming™ process. There is a need to reduce, eliminate or use lower cost dehydrogenation catalysts in catalytic reforming process technology. This would be possible if the dehydrogenation process can be run more efficiently, e.g., with greater production of aromatics, at higher temperature while considering the factors discussed above relative to the overall scheme of the process. 
       SUMMARY OF THE INVENTION 
       [0013]    We now propose that hydrocarbon dehydrogenation processes such as reforming are to be provided with process heat from nuclear and solar thermal energy sources. One of the unique attributes of nuclear and solar thermal technologies is their ability to provide high temperature (800 to 1500° C.) high pressure steam. Heat at these temperatures can be utilized effectively for the highly endothermic high temperature dehydrogenation of acyclic and cyclic paraffins to aromatics. The process occurs may be operated in the absence of catalyst or with reduced amounts of lower cost dehydrogenation catalyst or with a catalyst additive for inhibiting excessive cracking and dealkylation reactions at higher temperature. 
         [0014]    Among the advantages of using nuclear and/or solar thermal heat for hydrocarbon dehydrogenation processes are the following:
       1 use of lower cost dehydrogenation catalysts arising from conducting the dehydrogenation at high process temperatures;   2 conservation of fossil fuel energy required to provide process heat;   3 elimination of carbon dioxide evolution associated with burning of a hydrocarbon resource to generate process heat;   4 driving aromatization vs. isomerization reactions at higher temperature.       
 
         [0019]    The present invention therefore provides a hydrocarbon dehydrogenation process in which a hydrocarbon feed, normally a straight run naphtha, comprising acyclic and cyclic paraffins is dehydrogenated at elevated temperature of at least 540° C. with process heat provided at least in part by a solar or nuclear thermal energy source. The process is preferably operated with a co-catalyst or additive which is effective to inhibit cracking reactions including dealkylation reactions at the selected operating temperatures. The use of cheaper catalysts which are less active at conventional reforming temperatures also becomes possible at the higher temperatures enabled by the use of nuclear or solar heat sources. 
     
    
     DETAILED DESCRIPTION 
       [0020]    Although the present invention may be applied to dehydrogenation processes other than reforming, for example, the high temperature dehydrogenation of ethane to ethylene in the steam cracking of ethane and the conversion of ethylbenzene to form styrene, its main application will be to the naphtha reforming process as carried out conventionally for the production of high octane gasoline and aromatic petrochemical feedstocks. The general catalytic reforming process configuration will remain unchanged with an incoming low sulfur (&lt;10 ppmw sulfur, preferably &lt;2 ppmw) naphtha feed being heated to reaction temperature after which the feed is passed over the reforming catalyst in successive reactors with interstage heating between successive reactors. The process may be operated in a in fixed bed units either in a semi-regenerative mode with the catalyst being regenerated and reactivated at extended intervals, in a cyclic mode with the catalyst being regenerated in the reactors at shorter intervals, feed and regeneration gases being switched between reactors in a cyclic manner. Continuous catalytic reforming is also possible with a moving bed of catalyst passing through successive reactor vessels, either in a stacked or side-by-side configuration and then to a regenerator in which coke is burned off and the catalyst re-activated by halogenation to the extent necessary to maintain activity before being returned to the reactor section. The proposed high temperature operation may also be applied to a hybrid fixed/moving bed unit such as shown in U.S. Pat. No. 4,498,873; U.S. Pat No. 5,190,638; U.S. Pat. No. 5,1909,639; U.S. Pat. No. 5,196,110; U.S. Pat. No. 5,211,838; U.S. Pat. No. 5,221,463; U.S. Pat. No. 5,354,451; U.S. Pat. No. 5,368,720; U.S. Pat. No. 5,417,843 as well as in the technical, for example, in the NPRA Papers No. AM-96-50 and AM-03-93. Units converted to moving bed reactor configuration from older, fixed bed units as described in US 2004/0129605 A1, US 2005/0274648 A1 and WO2006/102326 are also amenable in principle to modification for operation with solar and nuclear heat sources provided that metallurgical constraints are observed. 
         [0021]    The heat from the solar or nuclear thermal energy sources will be supplied to the process in the form of feed pre-heat and by interstage heating. When solar energy sources provide the heat, the feed and/or the reaction stream may be passed directly through a solar furnace, e.g. at the focus of the furnace, to provide the heat directly or by heat exchangers passing a heat transfer medium from the solar source to the exchanger. With nuclear energy sources where circulation directly through the nuclear reactor is not possible, the feedstream and reaction stream will be passed through heat exchangers fed from the nuclear reactor. The heat exchanger will normally be fed with heat transfer medium in a secondary loop heated in a heat exchanger with the nuclear reactor primary coolant in its own loop passing through the nuclear reactor core but if the primary coolant does not become radioactive in the reactor core, e.g. with helium in a gas-cooled reactor, the heat exchangers for the reformer heat stream and interstage reaction streams may be fed with the primary coolant. 
       Solar Thermal Energy Sources 
       [0022]    Solar thermal energy is provided by the conversion of light to heat energy. This is typically achieved by focusing solar radiation onto a point source using mirrors, and the point source increases in temperature thus generating heat. For commercial applications, multiple mirrors are required to be installed to increase light capture. Once the solar radiation is focused on a point, the heat is transferred to fluid heat transfer medium. Three types of solar thermal device designs have been explored: solar tower, solar trough, and solar reactors. 
         [0023]    Solar thermal installations with a tower design use mirrors to focus incoming solar radiation on to a point that is often located on a central tower. Typically, the mirrors in a heliostat system are motorized to follow the sun over the course of the day. At this focal point, a liquid heat transfer medium is heated to the required temperature. Solar trough power plants use curved, trough-shaped mirrors to focus light on to a heat transfer fluid that flows through a tube above them. These trough reflectors tilt throughout the day to track the sun for optimal heating. The heat transfer fluid is heated in the troughs and then flows to a heat exchanger, which is used to produce superheated steam. A modified version of the parabolic trough design, the Fresnel reflector design, is uses a series of flat mirrors with a number of heat transfer receivers. Solar reactors, or Concentrated Solar Power (CSP), are useful for applications such as the present that take advantage of the high-temperature capabilities of tower technology which uses reactors similar to closed volumetric receivers except that a rhodium or another catalyst is dispersed on the surface of the ceramic mesh, directly absorbing the solar energy to produce syngas, hydrogen, and carbon monoxide as disclosed by Moller, S. et al., in 2002: Solar production of syngas for electricity generation: SOLASYS Project Test-Phase, 11 th  SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, Zurich. In its application to the present invention a solar reactor is used for directly heating the heat transfer fluid to high temperatures. 
         [0024]    The solar energy source may be augmented with natural gas or nuclear heat at times the solar thermal reactor output is diminished due to lack of availability of solar radiation. 
       Nuclear Thermal Energy Sources 
       [0025]    The high temperatures required for the present invention can also be provided by certain nuclear thermal energy sources. While conventional light water reactors are not adequate to supply these high temperatures, high temperature gas-cooled reactors and others have appropriate characteristics. One example is the Toshiba 4S (super safe, small, and simple) nuclear power system is based on a low-pressure, liquid-sodium design which is therefore capable of supplying the required high temperatures. It can be transported in modules and installed in a building measuring 22×16×11 metres and therefore commends itself for appropriate adaptation to refinery usage. High-temperature gas-cooled reactors (HTGRs) which typically use helium as a coolant are another next-generation reactor design that have the potential for driving endothermic chemical reactions, e.g., the regeneration reactions in the sulfur sorption cycle. One factor making HTGRs advantageous for the present application is that in principle the HTGCR can operate at temperatures well above 800° C., a range of refining operations including cracking, reforming and solid contact sulfur sorption as described above. The Siemens PBMR (the pebble bed modular reactor, or PBMR) is an example of a HTGCR which would be particularly useful for these purposes. The pebble bed modular reactor (PBMR) potentially meets US safety standards and includes a required airtight steel-lined reinforced-concrete containment structure. Operation of the PBMR is based on a single helium coolant loop, which exits the reactor core at 900° C. and 70 bar and therefore can be used to heat a heat transfer medium to comparable temperatures for use in refining processes. The PBMR is described in Weil, J., 2001:  Pebble - Bed Design Returns, IEEE Spectrum,  38 (11), 37-40. 
         [0000]    Heat Transfer from Source to Process Unit 
         [0026]    As noted above, the heat from solar sources may be applied directly to the feed and reaction streams by passing them through heating coils in the solar furnace. I other cases, the heat from the solar or nuclear high temperature heat source will be applied by the use of a heat transfer medium and heat exchange device transferring the heat from the solar or nuclear power source to the reforming process unit. The heat transfer medium will be routed from the solar or nuclear source to a heat exchanger providing pre-heat for the process, direct heat to the process environment e.g. by a heating jacket on the reactor used for carrying out the process or by heat transfer coils or tubes inside the reactor. Heat from solar and nuclear heat sources at temperatures potentially in excess of 1500° C. and heat of this quality can be used very effectively to provide process heat to the reforming reactions, even when transferring heat to the process streams in a heat exchanger. Heat transfer at the high temperatures contemplated, typically above 800° C. and ideally higher, e.g. 900, 1000° C., even as high as 1500° C., can be effected using transfer media such as liquids, gases, molten salts or molten metals although molten salts and molten metals will often be preferred for their ability to operate at the very high temperatures required for high energy densities without phase changes; in addition, corrosion problems can be minimized by appropriate choice of medium relative to the metallurgy of the relevant units. Molten salt mixtures such as mixtures of nitrate salts, more specifically, a mixture of 60% sodium nitrate and 40% potassium nitrate are suitable but other types and mixtures of molten salts may be used as a heat transfer and a thermal storage medium. Liquid metals such as sodium as well as alloys such as sodium-potassium alloy, bismuth alloys such as Woods metal, (m.p. 70° C.) and alloys of bismuth with metals such as lead, tin, cadmium and indium; the melting point of gallium (30° C.) and its alloys would, but for the aggressiveness of this metal towards almost all other metals, generally preclude it from consideration. Mercury is excluded for environmental reasons. Hot helium from a HTGCR can be used in a single loop heat exchange circuit from the nuclear reactor to the hydrocarbon process unit since helium is incapable of becoming radioactive and HTGCR reactor design is inherently safe: in the event of a loss of coolant, the temperature in the core will increase until Doppler broadening leads to a breakdown in the fission chain reaction. Outlet temperature and pressure for the helium coolant of the HTGCR are 850° C. and 70 bar, respectively, making it suitable for the present purposes. If required for safety or other reasons, the primary heat exchange fluid can be used to heat a secondary heat exchange fluid in a secondary circuit with this secondary fluid passing to the hydrocarbon process unit. 
       Application to Reforming 
       [0027]    By conducting the reforming process at temperatures above 540° C. and desirably, higher, the potential exists for reducing the amount of catalyst or enabling use of lower cost catalysts and so providing significant cost savings associated with catalyst use. Thus, by expanding the operating temperature envelope significant process economic benefits are to be expected. In addition, the use of higher temperatures may favor the production of the more highly valued aromatics, potentially in greater yields as the conditions for reactions producing aromatics and their precursors become more favorable. 
         [0028]    Reaction temperatures which are higher than the current norm for the process are enabled by the use of process heat from nuclear and solar sources: temperatures potentially as high as 1300° C. could be achieved but for practical reasons resort will normally not be made to such values. Temperatures of at least 600, 650, 700, 750 or 800° C. are readily achievable and metallurgies are capable of handling such values. The optimal range of temperatures would be from 650-800° C., in most cases 700-800° C. Pressures will depend on the type of operation, fixed bed, continuous or hybrid. Fixed bed reforming processes generally require relatively high pressures of at least 15 bar for adequate catalyst life between successive regenerations whereas continuous reforming is operated at the lower pressures allowed by the shorter time between successive regenerations, typically about 3 to 10 bar. 
         [0029]    Operation at the higher temperatures enabled by the use of solar and/or nuclear thermal energy sources permits catalysts which do not exhibit adequate activity at lower temperature ranges to be used successfully. In particular, base metal catalysts come into consideration, for example, chromium, molybdenum and chromium/molybdenum catalysts as well as those containing other transition metals of groups 5-10 of the long form Periodic Table, for example, iron, vanadium, cobalt, nickel. The base metals may be used in combination with platinum and the promoter metals normally associated with platinum in reforming catalysts including rhenium, iridium, rhodium, tin but the amount of the noble metal may be reduced relative to that conventionally used in the platinum-based catalysts for a more economic catalyst, for example, below about 0.6 wt. percent, e.g. below 0.3 or 0.25 wt. pct. Pt. If used alone, the base metals would typically be at levels of 0.5 to 20, more usually 2 to 10 wt. pct base metal on the total catalyst. Carrier materials will be alumina, silica or silica-alumina with preference given to the alumina-containing materials as these anchor the metal component more effectively. 
         [0030]    The use of the higher temperatures will favor the strongly endothermic dehydrogenation reaction which is the most desired reaction in the reforming process. It will also favor the less desired cracking reactions even though some measure of cracking (acidic) functionality must be retained in order to promote the isomerization reactions. Cracking is a first order reaction dependent on time at a given temperature and since most of the reforming reactions are favored kinetically at higher temperatures, their use will enable shorter reaction times to be utilized while attaining similar equilibrium concentrations. In addition, the relative reaction rates will play a role: hydrocracking as the slowest reaction will tend to be kinetically disfavored by the shorter reaction times which can be sued at the higher temperatures and therefore can be expected to result in a net increase in the more difficult napthene isomerization and paraffin cyclization reactions, both of which are desirable. Space velocities can be increased commensurately with the shorter reaction durations, so permitting greater capacities to be achieved within given equipment size or smaller vessel size for the same capacity. 
         [0031]    Another potential advantage offered by the use of the higher temperatures is that the acid function of the catalyst may be decreased while retaining the dehydrogenation activity. In this way, the undesired cracking reactions will be less favored relative to dehydrogenation. Control of catalyst acidity may be affected by reduction or elimination of the halide content of the catalyst, the use of less acidic carriers, for example, by using the less acidic forms of alumina such as boehmite (gamma alumina oxide hydroxide) and by less halogenation during the actual processing. Another possibility is to add a co-catalyst or catalyst additive such as calcium carbonate or another alkaline solid to the carrier formulation. 
         [0032]    The process might be operated non-catalytically at the contemplated higher reaction temperatures although some loss in reaction selectivity patterns may be encountered. In this case, the process could be carried out in an extended tubular reactor fitted with heating coils, preferably finned coils, fed by the nuclear or solar heat source distributed along the path of reactant flow to maintain reactant temperature. 
         [0033]    Naphtha feeds will be conventional in type but may be liberated from the ultra-low sulfur feed specification when the base metal catalysts are used as these are less susceptible to sulfur poisoning than the platinum-based catalysts. The use of the nuclear and solar heat sources may therefore permit a reduction in hydrotreating capacity to be made with consequent further economies of operation. A wider range of naphthas may also fall for consideration with this possibility.