Ni/MgO Catalysts on Structured Metal Supports for the Air Conversion of Low Alkanes into Synthesis Gas

Abstract

Heat-resistant, heat-conducting, and selective catalysts based on nickel highly porous foam-cellular material (HPFCM) and a mesh support are developed for the air conversion (partial oxidation) of low alkanes into additives that initiate combustion for fuel supplied to an engine as synthesis gas. The catalysts are developed in several stages: preparing a support based on nickel HPFCM (amount of Ni, 99.95%; PPI = 40) or an FeCrAl mesh; creating the support surface; forming structured blocks; the heat treatment of samples; applying an active component via the repeated co-impregnation of magnesium and nickel acetates; and stepwise heat treatment. NiO-MgO/(HPFCM or FeCrAl) catalysts tested in the air conversion reactions of propane, propane-butane, and natural gas, and in tri-reforming are prepared using this technique. The catalysts exhibit conversion of 90–96% over 80–100 h in all experiments at hourly space velocities of 32 000–71 000 h⁻¹ and coefficients of air excess of 0.31–0.43 with no formation of coke. A two-phase two-temperature mathematical model of the air conversion of liquefied petroleum gases (LPGs) that is in good agreement with experimental data on the temperatures of the catalyst and flow, and on the composition of the gas mixture at the output, is developed for numerical analysis of the results. Results from calculations for a generator of the air conversion of LPGs at a thermal power of 100 kW are presented as an example.

Appendices

APPENDIX A

С_g—mass heat capacity of a gas mixture, J/(kg K)

C—molar density of a mixture at a flow temperature, mol/m³

d_eq—equivalent diameter of transport channels in a catalytic block, m

${{D}_{i}}$—binary diffusion coefficeint of a gas component in nitrogen, m²/s

$G$—mixture mass velocity , kg/(m² s)

$k_{{{\text{OX}}}}^{{}}$—propane oxidation reaction constant, mol(${{{\text{C}}}_{{\text{3}}}}{{{\text{H}}}_{{\text{8}}}}$)/(m³ s atm)

$k_{{{\text{RSH}}}}^{{}}$—backwards CO steam reforming reaction constant, mol(СО₂)/(m³ s atm)

$k_{{{\text{SR}}}}^{{}}$, $k_{{{\text{SR}}}}^{{\text{0}}}$—reaction constant and pre-exponential factor, respectively, for the steam reforming of propane, mol(${{{\text{C}}}_{{\text{3}}}}{{{\text{H}}}_{{\text{8}}}}$)/(m³ s atm)

$K_{{{\text{MET}}}}^{{}}$, $K_{{{\text{RSH}}}}^{{}}$—thermodynamic equilibrium constants for the CO methanation and backward CO steam reforming reactions, respectively, atm⁻²

L—length of a catalytic block, m

m_i—molar mass of a gas mixture component, kg/mol

${{N}_{i}}$—molar flow of the ith mixture component, from the flow to the catalytically active walls of block transport channels, mol/(m² s)

${{Q}_{{{\text{OX}}}}}{\text{,}}\;{{Q}_{{{\text{SR}}}}}{\text{,}}\;{{Q}_{{{\text{MET}}}}}{\text{,}}\;{\kern 1pt} {{Q}_{{{\text{RSH}}}}}$—specific heats of reaction stages, J/mol

${{P}_{i}}$—partial pressure of the ith mixture component in a stream, atm

$P_{i}^{{\text{s}}}$—partial pressure of the ith mixture component on the catalytically active walls of a block’s transport channels, atm

S_V—specific surface area for the walls of transport channels in a catalytic block, m⁻¹

T_S—catalyst temperature, °C or K

T_g—gas temperature, °C or K

T1—temperature of the gas mixture at the inlet to the layer catalyst, °CT2—catalyst temperature in the inlet section catalyst bed, °CT3—catalyst bed outlet temperature, °C

${{{\omega }}_{{{\text{OX}}}}}$, ${{{\omega }}_{{{\text{SR}}}}}$, ${{{\omega }}_{{{\text{MET}}}}}$, ${{{\omega }}_{{{\text{RSH}}}}}$—reaction stage rates per unit block volume, mol(${{{\text{C}}}_{{\text{3}}}}{{{\text{H}}}_{{\text{8}}}}$, СО or СО₂)/(m³ s)

Х(С₃Н₈)—propane conversion, %

$x_{i}^{{}}$—dimensionless mass fraction of a gas mixture component in a flow

$y_{i}^{{}}$—dimensionless molar fraction of a gas mixture component in a flow

$y_{i}^{{\text{S}}}$—dimensionless molar fraction of a mixture component near the catalytically active walls of block transport channels

z—coordinate along the block length, m

$\alpha = \frac{{{{{\text{O}}}_{{\text{2}}}}}}{{{\text{5}}{{{\text{C}}}_{{\text{3}}}}{{{\text{H}}}_{{\text{8}}}}}}$—dimensionless coefficient of air excess

${{\beta }_{i}}$—gas–solid mass transfer coefficient, m/s

${{{\sigma }}_{{{\text{rad}}}}}$—Boltzmann radiation constant, W/(m² K⁴)

$\gamma$—gas–solid heat transfer coefficient, W/(m² K)

${{{\lambda }}_{{\text{g}}}}$—gas mixture heat conductivity, W/(m K)

${{{\lambda }}_{{\text{S}}}}$—foam nickel heat conductivity, W/(m K)

${{{\lambda }}_{{{\text{Ni}}}}}$—nickel heat conductivity, W/(m K)

${{{\mu }}_{{\text{g}}}}$—gas mixture dynamic viscosity, kg/(s m)

${{{\rho }}_{{\text{g}}}}$—gas mixture density, kg/m³

${{\varepsilon }_{0}}{\kern 1pt}$—foam nickel porosity coefficient

${{{\varepsilon }}_{{{\text{rad}}}}}{\kern 1pt}$—degree of balckness for the catalytic block ends

Nu—Nusselt number

Pr—Prandtl number

Re—Reynolds number

Sh—Sherwood number

Sс—Schmidt number

APPENDIX B

in—Input to catalytic block

$i = \left\{ {{{{\text{O}}}_{{\text{2}}}}{\text{,}}\,\,{\text{C}}{{{\text{H}}}_{{\text{4}}}}{\text{,}}\,\,{\text{C}}{{{\text{O}}}_{{\text{2}}}}{\text{,}}\,\,{{{\text{H}}}_{{\text{2}}}}{\text{O}}{\text{,}}\,\,{\text{CO}}{\text{,}}\,\,{{{\text{H}}}_{{\text{2}}}}} \right\}$—Gas component numbers

g—Gas flow

OX—propane oxidation stage

SR—propane oxidation stage

МЕТ—methanation reaction stage

RSH—backward CO steam reforming stage

S—catalytic block channel wall