(265a) Towards Tailored Monolithic Sponges for Highly Exothermic Catalytic Processes in Chemical Energy Storage

Towards Tailored Monolithic
Sponges for Highly Exothermic Catalytic Processes in Chemical Energy Storage

Lars Kiewidt (kiewidt@uni-bremen.de) and Jorg Th?ming (thoeming@uni-bremen.de)

Center for Environmental
Research and Sustainable Technology (UFT), University of Bremen, Leobener
Straµe, 28359 Bremen, Germany

Monolithic
ceramic and metallic sponges, also known as open-cell foams, have been
generating substantial interest as catalyst support for fixed-bed reactors in
current and recent years. In contrast to conventional packed beds composed of
pellets or extrudates, they combine low pressure drops and large volumetric
surface areas with superior heat and mass transport properties, and are thus
well-suited as catalyst support for highly endo- or exothermic processes. Three
prominent examples of exothermic processes, which are currently
discussed as promising options for effective storage of excess renewable
energy, are the Sabatier process (Power to Gas, PtG), the Fischer-Tropsch
process (Power to Liquid, PtL), and the synthesis of methanol (PtL) from green
syngas. The performance of these processes, however, is vitally dependent on
the effective removal of heat from the reaction zone to avoid hot spots that
induce thermodynamic limitations on conversion and yield, promote undesirable
side reactions, and decrease catalyst lifetime by thermal sintering. In
addition, thermal runaway has to be prevented for safe operation. Consequently,
catalyst supports with enhanced and adjustable heat transport properties are
required to enable efficient small-scale units for decentralized Chemical
Energy Storage as proposed in our former study [1].

Figure 1: Sensitivity of
the volumetric surface area, the effective thermal conductivity, and the pressure
drop of monolithic sponges on variations of porosity and pore count.

In
this study we demonstrate how the structure of monolithic sponges, represented
by their pore count and open porosity, and further their spatial
distribution can be utilized to tune the heat transport properties locally,
and thus intensify heat transport where necessary to avoid hot spots. In order
to show the potential of tuning the volumetric surface area, the effective
thermal conductivity, and the pressure drop by adjusting the sponge structure,
we conducted a sensitivity analysis at typical reaction conditions (300 °C, 10
bar, 4:1 H₂/CO₂-mixture) using recently published
correlations [2–4]. Figure 1 shows that the volumetric surface area and
the pressure drop of monolithic sponges can effectively be tuned by adjusting
the pore size, whereas the effective thermal conductivity can be altered by
changing the porosity. Consequently, the heat transport properties of
monolithic sponges and the release of heat can be tuned locally by acutely
adjusting the structure of the sponge.

Figure 2: Qualitative
temperature profiles for graded (A), uniform (B), and tailored (C) monolithic
sponge catalyst supports in the yield-temperature plane.

Therefore,
we propose tailored catalyst supports based on monolithic sponges with a
graded structure to balance high heat transport, low pressure drops, and
high space-time yields (see Fig. 2). As a first proof of principle we utilized
a 2-d fixed-bed reactor model to simulate the methanation of CO₂ over
a 20 wt.-% Ni/Al₂O₃ catalyst. Figure 3 shows the
temperature distributions in a 2.5 m long and 25 mm in diameter single-tube
reactor with external cooling and a 4:1 H₂/CO₂-mixture at
325 °C and 5 bar(a) as feed, for a uniform sponge (Fig. 1a; 40 ppi, 80 %
porosity), and a graded sponge with linear radial porosity profile (Fig. 1b; 40
ppi, center: 75 % porosity; wall: 90 % porosity). The axial temperature
profiles are qualitatively depicted in Fig. 2 (profiles A and B). The hot spot
close to the inlet can effectively be reduced from 625 °C to 360 °C using the
graded sponge. Because of the lower temperature, the CH₄-yield drops
simultaneously from 80 % to 30 % for the same reactor length. Therefore, we use
the developed model to find the optimal porosity profile that leads to the
optimal temperature profile (Fig. 2, profile C) [1] and provides
high yields, tolerable hot spots, and low pressure drops in order to intensify
the catalytic process. Further, we give an outlook on the experimental
realization of graded sponges and their application in CO₂-methanation.

Figure 3: Temperature
distribution in a sinlge-tube methanation reactor for (a) a uniform sponge (80
% porosity, 40 ppi), and (b) a graded sponge (40 ppi) with linear porosity
profile (axis: 75 % porosity; wall 90 % porosity) at 325 °C inlet temperature
and 5 bar(a) (isobaric). The tube is 25 mm in diameter and 2.5 m long.

References