Calculations of effectiveness factors and the criteria of mass transfer effect for high-temperature methanation (HTM) catalyst
International Journal of Low-Carbon Technologies
Natural gas is an extremely important bridge fuel to a low-carbon energy economy for improving local air quality. Coal to synthetic natural gas (SNG) is an effective way to convert the high-carbon energy (coal) into the low-carbon energy with rich hydrogen (natural gas). For the modern coal to SNG industry, the high-temperature methanation (HTM) catalyst plays an important role, and the advanced evaluation process should necessitate the elimination of mass transfer effect. Some simple but effective model catalysts, such as slab and sphere, can be very helpful in defining the reaction conditions, and thus facilitating the evaluation process for real HTM catalysts. In this work, slab and sphere model catalysts were adopted to derive mass transfer and reaction-coupled equations, the numerical methods were used to solve the coupled equations for the concentration profiles in catalysts, and the effectiveness factors were accordingly calculated. By taking advantage of the Thiele module w and the Weisz - Prater module F, the criteria for the elimination of mass transfer effect in the HTM catalyst evaluation process were successfully defined. This work also complements the Weisz - Prater criterion by incorporating 'negative reaction orders'.
*Corresponding author; zhaolijun@nicenergy; com
1 INTRODUCTION
Natural gas, composed mainly of methane, is known as a quality
clean-and-low-carbon energy. However, natural gas is very limited
in many parts of the world [1]. With rapid urbanization and
increasing living standard, the demand for natural gas is huge, and
the short supply will get even worse in future without
unconventional input.
Coal reserves are abundant, and therefore coal to synthetic natural
gas (SNG) has been seriously considered as one of the solutions for
natural gas shortage. The H/C ratio in coal can be raised four to six
times higher by converting into natural gas. The total thermal
efficiency for coal to SNG can be as high as 62–65%, by comparison,
coal to oil is 40–50% and coal to electricity is as low as 36–38% [2].
The high efficiency of coal to SNG process reduces relatively the CO2
emission, and the CO2 gets highly concentrated in the process for
treatment or utilization. In addition, pollutants such as sulfur can be
reclaimed as useful byproducts in the coal to SNG process.
The high-temperature methanation (HTM) catalyst is crucial
for the modern coal to SNG industry, and is of much superior
hydrothermal stability at high temperatures to the conventional
methanation catalysts [3, 4]. Usually, there are two evaluation
processes applied. By comparison, the process for ‘activity
change’ measurement has been regarded most favorable for the
evaluation of the HTM catalyst [4]. To apply the process, the
measurement conditions must be carefully defined. Only under
the conditions of the elimination of mass transfer effect, can
intrinsic reactions occur, and the measured reaction rates can be
used to quantify the activities of catalyst.
Mass transfer in porous catalysts has been of great interest for
a very long time, and the first papers appeared in the late 1930s
[5]. Due to the ubiquitous nature of mass transfer, the
concentrations of reactants are always higher on the surfaces than in the
interior of catalyst particles. The effectiveness factor h has been
defined as ‘the ratio of the real reaction rate of the catalyst
particle to the imaginary reaction rate when the whole particle is
assumed to bathe in the surface reactant concentration’ [6, 7].
Numerous efforts have been directed to the approximate solution
of model catalyst systems for the effectiveness factor h changing
with complex reaction kinetics. In contrast, the Weisz – Prater
criterion, which was introduced in 1954 [8], has been widely applied
in reaction kinetics measurements since it dictates the conditions
for the elimination of mass transfer effect.
The Weisz – Prater criterion has been derived by analyzing a
model catalyst with ‘zero and positive reaction orders’ and by
using the general assumption for mass transfer elimination if
jh j 0:05 [8]. However, the methanation reaction has been
known to proceed via a negative order of 20.5 with respect to CO
in high concentrations [9, 10], for which the Weisz– Prater
criterion could not give definite answers for the mass transfer effect.
Therefore, detailed investigation, including positive to negative
reaction orders, should be made of the effectiveness factor h in
relation with the Thiele module w and the Weisz – Prater module F, so
that the effect of mass transfer can be evaluated and eliminated in
the advanced evaluation process of ‘activity change’ measurement
for the HTM catalyst.
2 CATALYST MODELS AND CALCULATION
METHODS
Catalyst models, mass transfer and reaction-coupled equations
and the solving methods and the equation for effectiveness factor
h calculations will be discussed successively in this section.
2.1 Slab and sphere (...truncated)