(174aa) Cooling and Dilution Effects in a Thermally Integrated Microreactor for Sabatier Reaction

Cooling
and dilution effects in a thermally integrated microreactor for Sabatier
reaction

CO₂
methanation, i.e., the catalytic reduction of CO₂ to methane, has
recently gained wide attention in the context of PtG (Power to Gas) where
renewable energy is stored as methane, a fuel. CO₂ methanation, also
called Sabatier reaction is a reversible and exothermic reaction. It is
accompanied by several side reactions such as reverse water gas shift reaction
which produces CO as a side product. The reaction is kinetically limited at
lower temperatures and thermodynamically limited at higher temperatures.
Previous investigation from our group on reactor modelling indicated the possibility
of autothermal operability with feed at ambient conditions in a thermally
integrated microreactor. However the maximum temperatures attained in the
system during some cases of operation are high which kindled the interest in
exploring alternative reactor configurations and to analyse the cooling and
dilution effects in the microreactor.

In this work, we analyse
the effects of external cooling and feed dilution on the performance of a
thermally integrated microreactor. Two reactor geometries shown in Figure 1 are
studied using ANSYS Fluent with methanation kinetics incorporated using user
defined function. The first geometry is that of an air-cooled microreactor,
where air is fed in counter-current direction to the reactant mixture (80% H₂
and 20% CO₂). The second geometry is where an equivalent reactant
mix is fed in both channels in a counter-current “heat recirculating” microreactor
geometry. Preliminary simulations reveal that a favourable temperature profile
that helps cross the kinetic barrier near the inlet and thermodynamic barrier
near the outlet forms in both the reactors. This temperature profile in turn
helps improve CO₂ conversion and CH₄ selectivity. In the
air-cooled reactor, very high selectivity to CH₄ is observed even
for cases where the inlet is highly preheated due to the effective cooling by
the air coolant. Feed dilution also resulted in improved selectivity due to
reduced maximum temperatures. The counter-current reactor exhibited comparatively
better stability from quenching as well as higher selectivity than air-cooled
reactor.

Effect of design and
operating parameters such as wall thermal conductivity and inlet velocities
will be presented for better understanding and design of the reactor.

Figure 1: Model of (a) air-cooled and (b) counter-current
reactors.