Abstract:
Process for the oxidation of a hydrocarbon substrate to a corresponding oxidised hydrocarbon compound by contacting the hydrocarbon substrate with an oxygen containing atmosphere in presence of a catalyst being active in the oxidation of the hydrocarbon substrate, which process comprises the steps of: passing a process gas containing the hydrocarbon substrate through a reaction zone with the catalyst; introducing the oxygen containing atmosphere divided into a plurality of substreams successively into the reaction zone; within the reaction zone reacting the process gas continuously with the oxygen containing atmosphere; and withdrawing from the reaction zone a product gas containing oxidised hydrocarbon compound and carbon monoxide.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/152,081, filed Sep. 2, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to selective oxidation of a hydrocarbon substrate. More particularly, the invention is directed to oxidation of a hydrocarbon substrate to a corresponding oxidised hydrocarbon compound by reacting the substrate with oxygen in presence of an active:catalyst arranged in an improved reactor system for carrying out the selective oxidation process. 
     SUMMARY OF THE INVENTION 
     It is a general object, of this invention to provide an improved process and reactor for catalytic oxidation of a hydrocarbon substrate to an oxidised hydrocarbon compound through controlled admission of oxygen reactant and at high conversion rate and selectivity for the desired oxidised hydrocarbon compound. 
     In accordance with the above object, this invention is a process for the oxidation of a hydrocarbon substrate to a corresponding oxidised hydrocarbon compound by contacting the hydrocarbon substrate with an oxygen containing atmosphere in presence of a catalyst active in the oxidation of the hydrocarbon substrate, which process comprises the steps of: 
     passing a process gas containing the hydrocarbon substrate through a reaction zone with the catalyst; 
     introducing the oxygen-containing atmosphere via a plurality of substreams successively into the reaction zone; 
     within the reaction zone, reacting the process gas continuously with the oxygen containing atmosphere; and 
     withdrawing from the reaction zone a product gas containing oxidised hydrocarbon compound and carbon monoxide. 
     The process is particularly useful in the selective oxidation of paraffinic and olefinic hydrocarbons and organic hydroxy compounds for the preparation of unsaturated hydrocarbon compounds and oxygenates. 
     As used herein, the term “oxidised hydrocarbon compound” means any compound resulting from selective oxidation of a hydrocarbon substrate, including alkenes, hydroxy, oxo, and carboxyl compounds and acid anhydrides. 
     A reactor system useful for carrying out the process according to the invention comprises at least one reactor tube provided with a plurality of perforations along a wall of the tube and a reaction zone with an active catalyst arranged on the tube side and/or shell side of the reactor tube; and 
     a bed of particulate material surrounding the at least one reactor tube, the bed of particulate material being adapted to be fluidised by an oxygen containing atmosphere and to transport heat from the reactor tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the appended drawings: 
     FIG. 1 shows a multi-tube reactor immersed in a fluidized bed of sand. 
     FIG. 2 schematically shows an arrangement of the tubes. 
     FIG. 3 shows a different arrangement of the inert material. 
     FIG. 4 illustrates a three layer asymmetric structure. 
     FIG. 5 is a graphic comparison of the thermal evolution of N 2  permeability through a VPO/αAl 2 O 3  membrane after a reducing treatment and after an oxidizing treatment. 
     FIG. 6 illustrates the n-C 4 H 10  or O 2  permeability at 500° C. after 30 minutes. 
     FIG. 7 graphically illustrates the difference in gas permeability through oxidized and reduced membranes. 
     FIGS. 8 and 9 graphically illustrate the increased MA yield when the CO 2  percentage in the feed is increased. 
     FIG. 10 graphically illustrates oxygen conversion with good MA selectively with varying butane percentages. 
     FIG. 11 graphically illustrates the increase in MA selectively with increasing CO 2  content in the feed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When carrying out the invention in practice, oxygen or oxygen containing atmosphere will be added in a controlled manner by successive permeation through a plurality of perforations along the walls of a reactor tube. 
     Tubes with a sufficient number of perforations for the successive addition of oxygen reactant by controlled permeation into the reaction zone may be prepared from metallic or porous ceramic material, either in the form of perforated tubes or in the form of membranes with a porous structure. 
     Membranes useful in the invention are generally prepared by deposition, coating or impregnation of membrane material on a surface of a commercially available porous tubular membrane support, such as macroporous aluminium oxide tubes. 
     The membrane may further be catalytically active when coated or impregnated with catalytically active material. 
     Suitable catalytically active material for oxidation of the hydrocarbon substrate is known in the art and conventionally employed in the industry. 
     Those catalytic materials include the known vanadium phosphor oxide compounds (VPO), vanadium on magnesium oxide support (VMgo), vanadium on titanium oxide support, molybdates, silver catalysts or supported silver catalysts, including mixtures of the above compounds. 
     A multi-tube reactor immersed in a fluidized bed of sand according to a specific embodiment of the invention is shown in FIG.  1 . 
     The reactor comprises a number of perforated reaction tubes with a fixed bed catalyst of conventional vanadium phosphate (VPO) oxidation catalyst arranged on the tube side. Cooling tubes arranged within a fluidized bed shell surround each reaction tube. 
     The arrangement of the tubes is schematically shown in FIG. 2. A tubular membrane forms the reactor tubes. The shortest centre to centre distance between adjacent tubes is in the shown arrangement 1.35 times the diameter of the tube, and the calculated free section for fluidization is about 57% of the total area. A filter on the upper side of the reactor avoids intrusion of the solids from the fluidized bed. 
     As mentioned above and described in more detail below, the reactor. tube may be in the form of a perforated tube or in the form of a porous membrane to provide for the desired successive and gradual addition of oxygen to the process gases. The membrane may be formed from inert material (IMR) or catalytic active material (CMR). Useful material for the reactor shell includes carbon steel. The design temperature and pressure are typically about 700° C. and 150 psi. 
     The fluidized bed particles may be inert material or catalytic material. The main function of the fluid-bed is as a heat transfer medium. 
     Use of metallic membranes or perforated metal tubes facilitates necessary welding for the desired final length of a reactor tube and connection between adjacent tubes within the reactor shell. 
     A typical unit for testing a membrane tube is shown in FIG. 3 with a VPO catalyst located inside the membrane tube and a stainless steel shell, where the membrane is fixed. Hydrocarbon feed, e.g. butane is introduced at the tube side and oxygen at the shell side. Inert gas is introduced with the hydrocarbon feed to the tube side or partly with the oxygen reactant. Graphite gaskets are used to seal the space between the membrane and the shell. 
     In a different embodiment of the IMR test unit as shown in FIG. 3, the VPO catalyst is arranged in the space between the membrane and the stainless steel shell, feeding the butane to the shell side and the oxygen to the tube side and permeating the oxygen outwards. This embodiment is referenced as an outward flow reactor (OFR). 
     The above membrane reactor according to the invention is particularly useful for the selective oxidation of a hydrocarbon substrate. The IMR oxygen can be maintained at a sufficient level to avoid decreases in selectivity, but at the same time maintaining the oxygen partial pressure at a level outside of the explosion limits. Since oxygen is fed gradually into the membrane reactor, higher conversion than in the conventional reactor may be obtained. In addition, since no depletion of oxygen at any location in the membrane reactor occurs, changes in the catalyst, which may occur at very low oxygen concentration, can be avoided, and a reasonable level of selectivity can be achieved. 
     EXAMPLES 
     Example 1 
     Preparation of an Inert Membrane According to the Invention 
     Membranes were deposited in and/or on a 15 cm or 25 cm commercial SCT tubular alumina macroporous supports (in/out diameter=7/10 mm) with a sufficient length of enamel at each end. The basic supports have a three layers asymmetric structure with mean pore sizes of 10 μm (external layer), 0.8 μm (intermediate layer) and 0.2 μm (internal layer) (FIG.  4 ). 
     Either top-layers or composite membranes were prepared by depositing the membrane material on top of the support or inside its pores (loaded supports), respectively. Thus, the prepared inert membrane has the advantages of lower influence of defects, a higher thermo-mechanical resistance, a higher reproducibility, and a higher barrier effect due to greater thickness and higher diffusion resistance. 
     To obtain sufficient membrane stability at reaction conditions (i.e. at 400-500° C.), the membranes were thermally treated at 600° C. or higher, preferably at 800° C. For preparing silica mesoporous membranes, commercial support material was immersed (multiple impregnation) in a commercial silica sol (Ludox A40-Dupont). The impregnated support was dried at 100° C. and calcined at 800° C. The resulting membrane was impregnated with a LiNO 3  solution, dried and heated at 600° C. Finally, both ends of the loaded support were enamelled and heated at 800° C. in order to vitrify the ceramic enamel. Thus, the prepared membrane was typically mesoporous with a large pore size distribution (the smaller pore diameters were 4 nm according to porosimetry). The amorphous silica-based materials of this membrane were mainly inside the pores of the macroporous support materials. The membrane gas permeability is a linear function of the trans-membrane pressure and is typically in the range of (15+15 Pm) 10 −3  mole/(sq.Pa.m 2 ), i.e. lower than the permeability of the above-mentioned -alumina-based membranes. The permeation selectivity at 400-500° C. was lower than the Knudsen selectivity. The silica-loaded membrane was stable after a number of experiments (200 h in the range 400-500° C., with 20 heating-cooling cycles) at industrial operating conditions. 
     Example 2 
     IMR Preparation 
     MFI zeolite/αAl 2 O 3  composite membranes were prepared at 175-190° C. by hydrothermal synthesis of zeolite in/on α-alumina supports in contact with a sol of SiO 2 /TPAOH/H 2 O. The template including the pores was eliminated at 600° C. and membranes were treated at 450° C. in N 2  overnight before gas permeation tests were conducted. 
     The quality of the synthesised MFI membranes was first tested by studying the N 2  permeability at room temperature as a function of trans-membrane pressure (ΔP). A stable or even slightly decreasing N 2  permeability was observed when 
     increasing ΔP from 0.5 and 3 bars. The membranes were then considered free of macro-defects (&gt;20 nm) because no viscous flow was observed. Permeability was typically within the range from 10 −8  to 10 −6  mol/sq.Pa.m 2 . 
     The textural characteristics (N 2  adsorption-desorption) of this type of membrane were not modified by calcination at 600° C. for 3 h in wet air (95% humidity). Furthermore, single-gas (N 2 ) permeability behaviour of the membranes was unchanged after more than 20 heating-cooling cycles between room temperature and 500° C. with about 10 hours per cycle in the range of 450-500° C. 
     Example 3 
     CMR 
     Sol-gel derived VPO membrane materials from the polymeric synthesis route. 
     The synthesis of VPO catalysts was also investigated from alkoxide precursors in organic media in order to increase the specific surface area and V 4+ /V 5+ -ratio of the VPO membrane materials. A heteroatomic liquid VPO precursor was prepared by reacting di-n-propylphosphate with freshly prepared alkoxide VO(OR) 3  in 1-methoxy 2-propanol (ROH). The precipitate was refluxed in ROH, yielding a blue solution, which was used as VPO precursor. The dried VPO precursor was amorphous (XRD) although TEM observations showed presence of VOHPO 4 , 0.5H 2 O small crystallites. Different heat 
     treatments have been used for this polymeric precursor, including thermal treatment in air (T=500, 650° C.), in N 2  (T=500° C.), and in a reaction mixture (T=400° C.). XRD, LRS,  31 P NMR, elemental analysis, and XRD demonstrated that the VPO catalyst obtained at 650° C. in air consisted of VOPO 4  micro-domains (β, γ, δ phases) dispersed in a (VO) 2 P 2 O 7  pyrophase. The detected presence of the di-phosphate phase VO (PO 3 ) 2 was consistent with the high molar ratio P/V=1.2 in the catalyst. In all cases, the n-C 4  conversion was very low due to the low specific surface area of the catalysts (S BET &lt;5 m 2 /g). As in the aqueous sol-gel method, the insertion of 1 wt % cobalt (Co (acac) 2 )in the starting sol and use of surfactant additives or modifications of the thermal treatments did not improve the specific surface area of the final catalyst. 
     The above-mentioned blue solution was used for deposition on or impregnation in the alumina tubular support. The membranes were then dried and calcined at 650° C. in air. As described in Example 1, the procedure was repeated typically 5 times until the support reached a stable weight. 
     The above-prepared composite membranes exhibit an advantageous gas permeability property. FIG. 5 compares the thermal evolution of N 2  permeability through the VPO/αAl 2 O 3  composite membrane after a reducing treatment (n-C 4 H 10  at 500° C.) and after an oxidising treatment (O 2  at 500° C). The ratio of 
     N 2  permeability values through the oxidised and reduced membrane is 3.6 at 20° C. and 18 at 500° C. The observed reduction in the N 2  permeability through the oxidised membranes was reversible and reproducible, probably due to a modification of the membrane porous texture. Nine cycles with n-C 4 /O 2  at 500° C. were performed on a VPO/αAl 2 O 3  membrane. The n-C 4 H 10  or O 2  single gas permeability (FIG. 6) was measured at 500° C. after 30 min. The next step was to switch off the gas and purge with N 2  for 5 min. The reduction-oxidation process was found to be reversible at these conditions. This type of membrane is reproducible, thermally and chemically stable. The single gas permeability ratio, n-C 4 H 10 /O 2 , was about 40 at 500° C. 
     Advantageously, the gas permeability through the membrane is higher when in reduced form and lower in oxidised from, resulting in a self-regulating permeability of O 2  through the membrane and controlled oxygen concentration in the IMR (FIG.  7 ). Depending on the operating conditions such as reactants&#39; partial pressure and internal and trans-membrane pressures, O 2  flow through the membrane is stable after several minutes. If the n-C 4 H 10  concentration profile decreased along the reactor (consumption), a non-uniform O 2  permeability is obtained by this redox-sensitive membrane, decreasing O 2  flow from the inlet to the outlet. 
     Example 4 
     Metallic Membrane or Flow Distributors 
     A metallic membrane may be made by use of sinter metal tubes available from e.g. Krebsöge or other producers. Alternatively, the same effect can be obtained by perforating a metal tube. Preferably, the holes have a diameter between 1-50 μm. Perforations may conveniently be obtained by laser drilling. 
     Example 5 
     Several inert gases were tested: nitrogen, methane and carbon dioxide. Whereas no changes in reactor performance were observed when methane or nitrogen was employed instead of helium, strong changes were observed with CO 2 . FIG. 8 shows an increased MA yield when the CO 2  percentage in the feed is increased. As obvious from FIG. 9, the effect was stronger when the concentration of butane is increased. 
     Example 6 
     In a reactor according to a further embodiment of the invention, the VPO catalyst is arranged in the space between the membrane and the stainless steel shell. N-butane is introduced into the shell side and the oxygen into the tube side of the reactor tube. Oxygen permeates thereby outwardly from tube side to shell side and the reactor is referenced as Outward Flow Reactor (OFR). As obvious from FIG.  10 , it is possible to reach a high oxygen conversion and still maintain a good MA selectivity. MA yield obtained by the OFR membrane reactor was about 30% with 5% butane. With a proper introduction of oxygen, MA yield close to 30% can be obtained even with 10% butane in the feed. 
     Example 7 
     The effect of feeding a mixture containing CO 2  to a fixed bed of VPO catalyst was tested with rich butane (20%) and lean (2.5%) butane concentration. The results show a clear increase in MA selectivity when the CO 2  content in the feed was increased (FIG.  11 ). The impact of CO 2  was higher with the butane-rich mixture than with the butane-lean: an increase in selectivity of 20% in the first case and only 5% in the second case was observed. Only small changes in butane conversion were obtained with CO 2 . 
     Example 8 
     A process according to a specific embodiment of the invention is based on the above described membrane technology. n-Butane and oxygen react exothermically to maleic anhydride according to the following reaction scheme: 
     
       
         C 4 H 10 +3.5O 2 →C 4 H 2 O 3 +4H 2 O  (1) 
       
     
     Additionally, side reactions take place according to the following reactions: 
      C 4 H 10 +6.5O 2 →4CO 2 +5H 2 O 
     
       
         C 4 H 10 +4.5O 2 →4CO+5H 2 O 
       
     
     The above synthesis may be carried out in an IMR reactor according to a specific embodiment of the invention. 
     The dimensions of the membrane tube were: length 2 m; internal diameter=1″; thickness=0.78″, the fixed-bed reactor volume is approximately 1015 cm 3  with about 1830 g of VPO catalyst. 
     The reactor system included an inert membrane reactor with the tube dimensions given above. The reactor was operated by addition of one of the reactants (oxygen) and by heat removal and temperature control of the reactor. One of the reactants is used as a fluidisation agent for particles of, for example, sand. The fluidised bed is arranged on the shell side of the membrane tubes to remove heat from the walls of the membrane tubes. The pressure on the shell side was higher than on the tube side; thus, a flow of the reactant(s) from the fluidised bed to the reaction zone inside the tubes allows control of concentration of the reactant(s). Thereby, the temperature of the fluidised bed was maintained below the temperature of the reaction zone and the flow from the fluidised bed to the reaction: zone, so that the temperature could be adjusted in the reaction zone. 
     Use of CO 2  as carrier gas increased selectivity of the reaction to maleic anhydride. 
     Below are listed further process conditions and reactor dimensions employed in this example. 
     The ratio between the shell and tube area was 2.7. Oxygen content at the high-pressure side inlet to the reactor was controlled at 86% implying a recycle to makeup ratio of 2.6. 
     Oxygen was introduced into the membrane reactor at a velocity of 5 cm/s to fluidise the sand particles. 
     The fluidised bed and oxygen back mixing result in a substantial constant temperature of the oxygen stream throughout the entire reactor. 
     Diffusion through the membrane tubes was adjusted in such a manner that the reactor effluent at the low pressure side had an oxygen content of 5%. 
     Content of n-butane at the reactor inlet was maintained at 5% by adjusting the n-butane feed to recycle ratio. 
     The n-butane feed was controlled to achieve a production of 50.000 MTPY maleic anhydride. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Feedstocks 
                   
                   
               
               
                   
                 n-Butane 
                   
               
               
                   
                 Pressure, kg/cm 2  g 
                   
                 2.9 
               
               
                   
                 Temperature, ° C. 
                   
                 25 
               
               
                   
                 Composition 
                   
                 100% 
               
               
                   
                 Oxygen 
               
               
                   
                 Pressure, kg/cm 2  g 
                   
                 5 
               
               
                   
                 Temperature, ° C. 
                   
                 25 
               
               
                   
                 Composition (mole %): 
                 O 2   
                 95 
               
               
                   
                   
                 N 2   
                 1.9 
               
               
                   
                   
                 Ar 
                 3.1 
               
               
                   
                 Product 
               
               
                   
                 Maleic Anhydride 
               
               
                   
                 Pressure, kg/cm 2  g 
                   
                 0.7 
               
               
                   
                 Temperature, ° C. 
                   
                 50 
               
               
                   
                 Composition (mole %): 
                 C 4 H 2 O 3   
                 13.76 
               
               
                   
                   
                 H 2 O 
                 86.24 
               
               
                   
                   
               
             
          
         
       
     
     Example 9 
     A process according to a specific embodiment of the invention is based on the above described membrane technology. Propane and oxygen react exothermically to propylene and water according to the following reaction scheme: 
     
       
         C 3 H 8 +0.5O 2 →C 3 H 6 +H 2 O 
       
     
     Several side reactions occur according to the following reaction scheme: 
     
       
         C 3 H 8 +5O 2 →3CO 2 +4H 2 O 
       
     
     
       
         C 3 H 8 +3.5O 2 →3CO+4H 2 O 
       
     
     The above synthesis may be carried out in the above-described IMR reactor. 
     The dimensions of the membrane tube are: length=2 m; internal diameter=1″; thickness=0.78″, the fix-bed reactor volume is approximately 1015 cm 3  with about 1800 g of vanadium magnesium oxide catalyst (VMgO). 
     The reactor system comprises an inert membrane reactor with tube dimensions as given above. The reactor was operated with addition of one of the reactants (oxygen) and at heat removal and temperature control of the reactor. One of the reactants was used as a fluidization agent for particles of, for example, sand. The fluidized sand bed was arranged at the shell side of the membrane tubes to remove the heat from walls of the membrane tubes. The pressure on the shell side was higher than the pressure on the tube side. Flow of the reactant(s) from the fluidized bed to the reaction zone inside the tubes allowed control of concentration of the reactant(s). The temperature of the fluidized bed was below the temperature of the reaction zone. The flow from the fluidised bed to the reaction zone allows controlled temperature in the reaction zone. 
     Further process conditions and reactor dimensions employed in the above process were: 
     The ratio between the shell and tube area was 2.7. Oxygen content at the high-pressure side inlet to the reactor was 34%. Oxygen was introduced into the membrane reactor at a velocity of 0.2 m/s to fluidize the sand partiles. 
     The fluidized bed and oxygen back mixing gave a substantially constant temperature of the oxygen stream throughout the entire reactor. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Feedstocks 
                   
               
               
                   
                 Propane 
               
               
                   
                 Pressure, kg/cm 2  g 
                 2 
               
               
                   
                 Temperature, ° C. 
                 25 
               
               
                   
                 Composition 
                 100% 
               
               
                   
                 Oxygen 
               
               
                   
                 Pressure, kg/cm 2  g 
                 3 
               
               
                   
                 Temperature, ° C. 
                 25 
               
             
          
           
               
                   
                 Composition (mole %): 
                 O 2   
                 100 
               
             
          
           
               
                   
                 Product 
                   
               
               
                   
                 Propylene 
               
               
                   
                 Pressure, kg/cm 2  g 
                 0.75 
               
               
                   
                 Temperature, ° C. 
                 60 
               
               
                   
                 Composition (mole %): 
                 Propylene 100%