Natural Gas Reforming

Reaction of natural gas reforming is catalytic reaction between natural gas and steam by uses nickel catalyst that is supported by alumina (NI/Al₂O₄). Overall equilibrium reaction of natural gas reforming is endothermic. In industrial world, reaction of natural gas reformation with steam is main process to produce synthesis gas which consists of CO and H₂. If natural gas is represented by CH₄, so main reactions of natural gas reforming can be written in equations as follow:

Equation 1:

CH₄ + H₂O ↔ CO + 3H₂          ΔH_{298 K} = +206.2 kJ/mol
Equation 2:

CO + H₂O ↔ CO₂ + H₂           ΔH_{298 K} = -41.1 kJ/mol 
Equation 3:

CH₄ + 2H₂O ↔ CO + 4H₂       ΔH_{298 K} = +165 kJ/mol 

At high temperature and low pressure, conversion of CH₄ based on Equation 1 and Equation 3 thermodynamically will increase. Equilibrium reaction of equation 2 is known as water-gas shift reaction which has exothermic properties and does not depend on operating pressure.

The new study of reforming reaction is CO₂ reforming as shown in Equation 4. The reaction is endothermic and conversion of CH₄ thermodynamically will increase at high temperature and low pressure.

Equation 4:
CH₄ + CO ↔ 2CO + 2H₂                ΔH_{298 K} = +247.4 kJ/mol 

This last reaction is utilized to get synthesis gas with low ratio of H₂/CO by replace some or all steam with CO₂ in feed gas process. High content of CO₂ can increase carbon forming.

Equilibrium composition in reaction of natural gas reforming, free Gibbs energy thermodynamically can be estimated as shown in Equation 5. For ideal gas, fugacity coefficient (Фi) can be assumed as one, so Equation 5 can be written as Equation 6. Equilibrium composition is reached at minimum free Gibbs energy is mathematically shown in Equation 7.

Where:

a_i : activity of component i

G_i^o : free Gibbs energy in forming component i

n_t : total mol

n_io : initial mol of component i

n_i : mol of component i

y_i : fraction of component i

ɛ : extent of reaction

Фi : fugacity coefficient of component i

Finally, equation of equilibrium reaction can be written as following equations:

K = exp (-ΔG / RT)       

K = Π C_i^vi

K = P^Δv Π (y_i)^vi

Where:

C_i = equilibrium concentration of component i

K = constant equilibrium reaction

P = total pressure

Δv = difference coefficient of product and reactant

Constant equilibrium reaction (Ki) can be calculated from empirical equation as function of temperature in following equations:

Where: T in Kelvin unit.