(712e) Structured Multiphase Reactors for Electrocatalytic CO2 Conversion

There is a broad consensus that the chemical industry will have to make a transition to renewable feedstocks. An important route will be to use electrochemical processes, driven by electricity from sustainable sources, to produce hydrogen from water, hydrocarbons from CO₂ and water, and ammonia from nitrogen and water. This will require novel reactor designs, with special attention to the structured design of electrodes. Although there is progress in electrochemical flow reactors, scalable macro- and micro-reaction environments need to be developed involving e.g. gas-liquid electrolyte flow [1].

We examine the use of millichannels in designing multiphase reactors for electrocatalytic applications. We give a general analysis based on dimensionless numbers, followed by an illustration of this approach to catalytic packed-bed reactors. Finally, we discuss how such reactors can play an important role of increasing the number of electrocatalytic processes in the chemical industry.

Although the electrocatalysts required for CO₂ reduction receive substantial research interest, the research efforts devoted to the design of electrocatalytic reactors remains behind. Traditionally electrochemical reactors are designed as semi-2D geometries (one dimension much smaller than the other two) because of the use of plate electrodes. However, when gas bubbles are involved (e.g., due to hydrogen formation), the use of small channels might be attractive. Here we study the influence of channel size (hydraulic diameter d_h and length L), possibly filled with particles of diameter d_p, at porosity e. We consider flow at superficial velocity U of two types of fluids through these reactors, one gas-like and one liquid-like, for which we will for the moment use the physical properties of air and water at 20 ⁰C temperature and 1 atm pressure.

The differential pressure drop necessary to reach significant convective improvement of mixing and heat and mass transfer (typically Re>100) becomes prohibitive for small channels. For example, for an empty structured channel of hydraulic diameter d_h, the pressure drop associated with wall friction can be described by the Darcy-Weisbach equation [2]. Fig. 1 shows Dp/L as a function of d_hfor liquid-like (black solid line) and gas-like (black dashed line) fluids at Re = 100, assuming f = 64/Re. At this fixed Re, U decreases like 1/d_h, leading to 1/d_h³ scaling of the required pressure drop: for d_h= 0.1 mm, the pressure drop to reach Re=100 is already substantial, approximately 10 bar/m, while for d_h= 1 mm the necessary pressure drop is a more manageable: 0.01 bar/m.

For a channel filled with catalytic particles of diameter d_p, the differential pressure drop can be described by the Ergun equation [3]. Under these conditions, for d_p= 0.1 mm, the pressure drop necessary to reach Re=100 is enormous, approximately 1000 bar/m, while for d_p = 1 mm, the necessary pressure drop is much more reasonable, approximately 1 bar/m.

Pressure drop and mass and heat transfer are not the only important consideration. We also need to consider that smaller reactors are also more prone to contaminants and agglomerates getting stuck in corners of the channel or pore space. Moreover, it is important to tune the residence time in the channel to optimize reaction yield or selectivity. The average residence time is τ = εL/U. At fixed Re, this leads to τ/L = ρd_h/(μRe) or τ/L = ρεd_p/(μRe), for the two respective systems. In the regime where inertial flow enhancement becomes relevant (Re>100), reaching sufficiently long residence times requires sufficiently wide channels or sufficiently large particles.

Millichannels have clear advantages over traditional, non-structured reactors, in electrocatalytic processes. Analysis based on dimensionless numbers shows that the use of millichannels has important advantages over microchannels. The walls can act as electrode, while the channel configuration prevents problems with gas bubbles encountered in other types of electrocatalytic reactors.

References

[1] Walsh, F. C. & Ponce De León, C. 2018, 280, 121-148.

[2] Romeo, E., Royo, C. & Monzón, A. 2002, Chemical Engineering Journal, 86, 369-374.

[3] Ergun, S. 1952, Chem. Eng. Progress., 48, 89-94.