Thermochemical Processes Episode 6

140 Thermochemical Processes: Principles and Models Before leaving metallic catalysts, it is interesting to note that it was at first thought that the formation of iron carbide, Fe3C played an important intermediate role in the Fischer–Tropsch process. Although this has not been proved to occur, nevertheless some metal carbides, such as WC, Mo2C and VC are finding useful application in the production of organic species. One aspect of these compounds is that the tendency to form surface oxycarbide phases, which also act as catalysts, makes some new organic syntheses possible in an oxygen and sulphur-containing atmosphere. Catalysis by metal oxides Metal oxides present structures of a wide range from the alkaline earth oxides with the simple rocksalt structure to the cage-like structures of the zeolites, and the Ruddlesden–Popper phases in which layers of rocksalt structure are interspersed with perovskite unit cells, as in the ceramic superconductors. The oxygen anions, which dominate the surface structures of oxides are co-ordinated with metal ions of varying ionic charge and cation size, and thus the overall spacing between the anions varies with the cationic radius, providing a significant variable in the fit of adsorbed molecules on the oxide surface. Many cations can exist in more than one valency in the same oxide, leading to semiconduction, or even metallic conduction, depending on the particular cation, and the oxygen potential of the gas phase. It is clear that oxides are very versatile catalysts, and a wide range of studies have been made to compare the efficiencies of a number of oxides for the catalysis of a particular reaction. One feature of oxides is that, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than the bulk of the cations, referred to as ‘posi-tive holes’ because their effect of oxygen partial pressure on the electrical conductivity is the opposite of that on free electron conductivity. The inter-stitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. ZnC interstitial ions in the zinc oxide ZnO structure. An important species which occurs on the surface of oxygen-deficient compounds is the singly charged oxygen ion. This results from the filling of an oxygen vacancy by an adsorbed oxygen atom according to the equation .1/2/O2 C V.O2 / C O12 D 2O where V.O2 ) is an oxygen vacancy, and O12 is an oxygen ion on a neigh-bouring lattice site. The presence of the singly-charged oxygen ion confers positive hole conduction on the oxide. When the cation has a number of Heterogeneous gas–solid surface reactions 141 valencies the adsorption of oxygen more usually leads to the formation of higher valency cations. There is some distortion at the surface, the oxygen ions being displaced towards the underlying cations when compared with anions in the bulk of the material, in part because the co-ordination of a surface ion is less than in the bulk, especially on ledges and kinks. It has been estimated that in a crystal of MgO, the Madelung constant, which measures the binding of an ion to its environment, decreases from the bulk value of 1.748, to 1.567 on a ledge site and 0.873 on a kink corner ion. Since the second ionization potential of oxygen is endothermic, it is quite probable that the singly-charged oxygen ion could occur on the low Madelung constant sites, such as these corner sites, as a predominant species. Because of the existence of surface point defects, such as vacancies in the anion structure and O ions, oxides can function as receptors of water molecules by reactions involving the formation of surface hydroxyl groups. These were invoked above in the catalytic interactions at the nickel–alumina interface referred to in the context of methane reforming. They can be quite stable at room temperature, leading to the formation of a hygroscopic layer on the surface, e.g. of BaO, but are largely evaporated at high temperature. It should be noted that this hydroxylation of the surface is possible in the transition state of a surface reaction, and that other oxidizing gaseous reagents, such as nitrous oxide, can undergo analogous reactions. N2O C V.O2 / D N2 C O12 An effect which is frequently encountered in oxide catalysts is that of promo-ters on the activity. An example of this is the small addition of lithium oxide, Li2O which promotes, or increases, the catalytic activity of the alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of the major point defect which is the Ni3C ion. Since the valency of the cation in the alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacancy formation. Schematic equations for the two processes are Li2O C V.Ni2C/ ! 2Li1C C O2 C Ni3C; on NiO and Li2O ! 2Li1C C O2 C V.O2 /; on BaO 142 Thermochemical Processes: Principles and Models Coupling reactions of methane The reaction shown above for the steam reforming of methane led to the forma-tion of a mixture of CO and H2, the so-called synthesis gas. The mixture was given this name since it can be used for the preparation of a large number of organic species with the use of an appropriate catalyst. The simplest example of this is the coupling reaction in which methane is converted to ethane. The process occurs by the dissociative adsorption of methane on the catalyst, followed by the coupling of two methyl radicals to form ethane, which is then desorbed into the gas phase. A closer analysis of the equilibrium products of the 1:1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Exper-imental results for the coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as the temperature and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. The reaction of CH4 with hydrogen, at the other end of the oxidation scale, produces mainly acetylene, C2H2, ethylene C2H4 and ethane, C2H6. These reactions are favoured by operating at high temperatures. In fact the production of acetylene is most efficient if the gas mixture is passed through an arc struck between carbon electrodes, which probably produces a reaction temperature in excess of 2500K. It would seem that the coupling of methane can be carried out at oxygen potentials in between these two extremes using a catalyst which is to some extent reducible at moderately high oxygen potentials. A further constraint on the selection of the oxide is that the volatility of the oxide must be low at the operating temperature, about 1000–1200K. Manganese forms a series of oxides MnO2, Mn2O3, Mn3O4 and MnO spanning an oxygen dissociation pressure between 1atmos for the MnO2/Mn2O3 equilibrium, about 10 3 atmos for Mn2O3/Mn3O4, to less than 10 9 for the Mn3O4/MnO equilibrium. The oxide Mn2O3 can therefore undergo reduction on adsorption of methane with subsequent regeneration by oxygen in the gas phase. Methyl radicals produced by the adsorption process undergo coupling to form ethane on the surface, which is then desorbed into the gas phase. Alternatively, it has been proposed that methyl radical combination can take place primarily in the gas phase. The lithium oxide-promoted barium oxide also functions as a catalyst for the methane coupling reaction, but the mechanism is not clearly understood at the present time. The only comment that might be offered here is that the presence of O ions on the surface of this material might enhance the formation of methyl radicals through the formation of hydroxyl groups thus CH4 C O D CH3 C OH0 Heterogeneous gas–solid surface reactions 143 followed by the desorption reaction 2OH0 D H2O(g) C O12 A comparative study of oxides which were closely related, but had different electrical properties, showed that both n- and p-type semiconduction promoted the oxidation reaction, forming CO as the major carbon-containing product. In a gas mixture which was 30% methane, 5% oxygen, and 65% helium, reacted at 1168K the coupling reactions were best achieved with the electrolyte La0.9Sr0.1YO1.5 and the p-type semiconductor La0.8Sr0.2MnO3 x and the n-type semiconductor LaFe0.8Nb0.2O3 x produced CO as the major oxidation product (Alcock et al., 1993). The two semiconductors are non-stoichiometric, and the subscript 3 x varies in value with the oxygen pressure and tempera-ture. Again, it is quite probable that the surface reactions involve the formation of methyl radicals and O ions. Reactors for catalytic processes The industrial production of compounds by catalytic reactions is carried out mainly in one of two types of reactor. In the fixed, or packed bed reactor, particles of the catalyst are held in close contact in a cylindrical container. The gases flow through the unoccupied volume of this packed bed, and the temperature of reaction is achieved by a combination of control of the container temperature, pre-heating the inlet gas, and by the generation or absorption of heat on the catalyst as a result of the gaseous reaction. The transfer of heat to and from the gas phase and the rate of reaction are therefore important in fixing the dimensions of the catalyst particles which at a small diameter will restrict gas flow, and at a large size will present too little surface, and hence catalyst, to the reactants. The overall diameter of the containing vessel will determine the throughput of gas to the reaction, once the optimum particle diameter has been decided. The pressure drop, P, across a packed bed of length L consisting of particles of average diameter dp, for a gas of density g, and viscosity g, flowing at a velocity ug, is given approximately by the empirical Ergun equation P D K1gug C K2u2 K1 D 150.1 ε/2 ; K2 D 1.753dp ε/ Here ε is the porosity of the bed, which is equal to the difference between the bed volume and the volume of particles, divided by the bed volume. It can 144 Thermochemical Processes: Principles and Models be assumed that the gas phase is in turbulent, and hence well mixed, motion throughout the reaction volume. It follows that the position of thermodynamic equilibrium will change along the reactor for those reactions in which a change of the number of gaseous molecules occurs, and therefore that the degree of completion and heat produc-tion or absorption of the reaction will also vary. This is why the external control of the independent container temperature and the particle size of the catalyst are important factors in reactor design. In the fluidized bed the catalyst is suspended as separate particles in the gaseous reactants, which have been suitably pre-heated. The advantages of this form of reactor include excellent heat transfer to and from the catalyst particles, maximum contact between the catalyst and the gas, and the elimination of the possibility of particle–particle sintering during the production run. There is also very little pressure drop across the reactor, and so there is negligible effect on the position of equilibrium. The principal disadvantages include the necessity of particle size control of the catalyst to minimize sweeping of the light particles from the reactor, and the settling of the oversize particles into the reactor entry port. The gas transit time is also not as easily controlled as in the fixed bed because of the need to suspend the catalyst particles. The problem of fine particle entrainment can be decreased by reducing the gas velocity to a level where the mass of particles has the appearance of a boiling liquid, which decreases the overall rate of reaction. Alternatively at high gas input rates, the entrained particles can be separated from the effluent gases in a precipitator and recycled with the fresh particle input. The gas velocity at which fluidization occurs is given by d2 g.s g/ mf 1650g for large particles, when the Reynolds number, NRe, of the gas is large (>1000), and dpg.s g/ mf 24.5g for small particles, with a small gas Reynolds number (<20). This dimen-sionless number is defined in the case of a particle suspended in a gas by the equation dpugg Re g and for a particle of diameter 1mm, and density 3gcm 3, suspended in air (viscosity 0.04cp, and density3 ð 10 4 gcm 3) which is flowing at ug cms 1, ... - tailieumienphi.vn