Boreskov Institute of Catalysis From the SelectedWorks of Andrey N Zagoruiko October, 26 Anaerobic catalytic oxidation of hydrocarbons in moving heat waves. Case simulation: propane oxidative dehydrogenation in a packed adiabatic V-Ti oxide catalyst bed Andrey N Zagoruiko Available at: https://works.bepress.com/andrey_zagoruiko/2/
Anaerobic catalytic oxidation of hydrocarbons in moving heat waves. Case simulation: propane oxidative dehydrogenation in a packed adiabatic V-Ti oxide catalyst bed Andrey N. Zagoruiko Boreskov Institute of Catalysis, Novosibirsk, Russia
Anaerobic oxidation concept Separate feeding of reagents Use of lattice (chemisorbed) oxygen instead of molecular one J.E.Bailey, F.J.M.Horn (1968), A.Renken (1972), Yu.Sh.Matros (1982) theoretical conclusions on possibility of selectivity control in cyclic periodic operation mode V.A.Doroshenko et al. (1986), S.A.Veniaminov (1993), P.L.Silveston (1998), D.Creaser et al. (1999), R.Grabowski, S.Pietrzyk et. al. (22) and many others experimental confirmation of advantages of oxidative dehydrogenation of hydrocarbons in anaerobic mode E.H.Stitt (1998), S.D.Jackson et al. (21) practical application efforts
Anaerobic oxidation process development basis Complicated process detailed mathematical modelling is required Account of reaction heat effects in adiabatic fixed catalyst beds A.N.Zagoruiko (25) simulation of model system of exothermic reactions (applying Eley-Rideal mechanism) in adiabatic fixed catalyst bed: increased selectivity and aim product yield; significantly lower maximum temperature; possibility of reaction performance in reversed heat wave, moving countercurrently to gas flow direction (with further decrease of temperature and increase of selectivity) Great variety is hard for analysis case studies are required
Case study: propane oxydative dehydrogenation at V/Ti oxide catalyst Mechanism and kinetic data: R.Grabowski, S.Pietrzyk et. al. (22) Eley-Rideal type mechanism: C 3 H 8 + [V 2 ] C 3 H 6 + H 2 O + [V 2 O 4 ] C 3 H 6 + 9 [V 2 ] 3 CO 2 + 3H 2 O + 9 [V 2 O 4 ] C 3 H 8 + 1 [V 2 ] 3 CO 2 + 4H 2 O + 1 [V 2 O 4 ] [V 2 O 4 ] + ½ O 2 [V 2 ] w w w w 1 2 3 4 = = = = k k k k 1 2 3 4 c C 3H 8 c c c C 3H 6 C 3 H 8 O 2 θ θ θ 2 2 (1 θ ) θ- surface fraction of oxidized sites (V 2 ) Reason of choice transient kinetic available in literature, commercially important application
Model formulation Mathematical model: one-dimensional adiabatic fixed catalyst bed one-temperature model without heat/mass transfer limitations (quasi-homogeneous model) account for change of reaction mixture volume account for change of reaction mixture heat capacity solution of energy balance equation in enthalpy terms (both for gas and solid phases) without direct application of reactions heat effects and adiabatic heat rise approximation of reagents enthalpies (both gas and solid phase) by linear functions of temperature ( uci ) = ν ij w l θ amax = ν t T ( 1 ε ) γ + amax t j l = u = u ; c = l = t = T ( l) j ij w j u = u H вх вх ; θ = T нач ( u θ = t T i c = T вх i вх i ( l); θ ( l) = θ c h ( T )) i l нач i ( l);
Calculation of heat effects C 3 H 8 + ½ O 2 C 3 H 6 + H 2 O + 27.9 kcal/mol С 3 H 8 + 5 O 2 3 CO 2 + 4 H 2 O + 488.3 kcal/mol С 3 H 6 + 4.5 O 2 3 CO 2 + 3 H 2 O + 46.4 kcal/mol ============================================= C 3 H 8 + [V 2 ] C 3 H 6 + H 2 O + [V 2 O 4 ] 1.1 kcal/mol C 3 H 6 + 9 [V 2 ] 3 CO 2 + 3H 2 O + 9 [V 2 O 4 ] + 198.3 kcal/mol C 3 H 8 + 1 [V 2 ] 3 CO 2 + 4H 2 O + 1 [V 2 O 4 ] + 199.4 kcal/mol [V 2 O 4 ] + ½ O 2 [V 2 ] + 29. kcal/mol Heat effect of stages depend upon the bonding energy of oxygen at catalyst surface Process temperature regime depends upon the catalyst surface thermodynamics
Moving heat wave of anaerobic oxidation Co-current heat front propagation (counter-current propagation is impossible due to endothermic character target reaction) Propane inlet concentration 1% 3 Oxygen inlet concentration % 25 Inlet temperature 3 C 2 Initial catalyst temperature 2 C 15 Initial state of the catalyst completely 1 oxidized Catalyst bed length 1 m 5 Gas flow superficial inlet velocity 1 m/sec Catalyst chemisorption capacity 2.8 st.m 3 O 2 per 1 m 3 l/l bed Difference between inlet and maximum temperature is insignificant T, С,2,4,6,8 1 Increased selectivity due to temperature decrease towards bed outlet 125 25 375 5
Counter-current (reverse-flow) feeding of propane and air Two-phase operation cycle Phase 1. Propane anaerobic oxidative dehydrogenation (catalyst reduction phase) inlet feed 1% propane USED AIR inlet temperature ambient Phase 2. Catalyst reoxidation by air inlet feed air inlet temperature ambient PROPANE Initial catalyst state fully oxidized, AIR preheated to 3 C Phases alternation until achievement of established cyclic operation (accurately repeating of all process parameters from cycle to cycle) PROPANE+ PROPYLENE
Established autothermal cyclic operation Temperature in the catalyst bed Catalyst reduction phase Catalyst reoxidation phase 5 4 7 14 21 28 35 5 4 35 28 21 14 7 Temperature, C 3 2 1 Propane Temperature, C 3 2 1 AIR,2,4,6,8 1,2,4,6,8 1 l/l l/l
Established autothermal cyclic operation Catalyst oxidation degree profiles Catalyst reduction phase Catalyst reoxidation phase 1 1 35,8,8 [O],6,4 7 14 21 [O],6,4 28 21 14 7,2 28,2,2,4,6,8 1 l/l 35,2,4,6,8 1 l/l
Autothermal cyclic operation Cycle duration influence 7 Maximum temperature, C 6 5 4 3 2 1 3 325 35 375 4 425 45 Half-cycle duration, sec Required duration of reduction/reoxidation phases may be generally different duration conjugation is necessary (application of different flow rates)
Selectivity/yield issues Selectivity in cyclic anaerobic mode is higher than in steady-state, though the propane conversion may be lower Reason - self-sufficient structure of the moving reaction zone in the bed, efficient residence time may be too low Possible solutions for anaerobic process: conversion depth improvement (increase of catalyst oxygen capacity) extension of reaction zone (increase of axial heat conductivity) The propylene yield maybe competitive with steady-state, but propylene output per unit catalyst volume will be higher (due to undiluted propane use) Propylene yield 5% 4% 3% 2% 1% % 5 1 15 2 25 Surface area, sq.m/g
Simulation fundamentals anaerobic oxidative dehydrogenation of propane may be performed in cyclic autothermal mode with high selectivity; maximum operation temperature (both at anaerobic oxidation and reoxidation phases) is much lower than in steady-state process due to low inlet gas temperatures and efficient use of solid catalyst heat capacity; temperature regime of the process depends upon the thermodynamic properties of the catalyst (oxygen bonding energy); propane conversion and propylene yield may be controlled by variation of oxygen storage capacity (catalyst surface area) and catalyst bed axial heat conductivity; Generally more parameters to be independently optimized, more degrees of freedom for process development and optimization
Advantages of anaerobic cyclic process compared to conventional steady-state process Simple and cheap reactor design (fixed bed reactors, minimized heat-exchange environment) High oxidation selectivity and low operation temperature Use of undiluted propane as feed, increase of unit production capacity of the catalyst Increased process safety due to absence of direct contact between hydrocarbons and molecular oxygen Use of air instead pure oxygen without separation complications, arising from nitrogen presence in product stream Efficient coke incineration in each reoxidation cycle no coke accumulation, probable increase of catalyst lifetime
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