Thermally induced evolution of dissolved gas in water flowing through a carbon felt sample

Abstract

Carbon felts are widely used as flow through porous electrodes in great variety of electrochemical devices due to their superior electrochemical properties and low price. In plant-scale electrochemical reactors stacked with large carbon felt electrodes, uniformly and efficiently feeding liquid electrolytes to the entire area of individual electrode is frequently challenged by the complicated gas bubble evolution phenomena induced by dissolved gas exsolutions or/and gassing side reactions. To explore the hydraulic characteristics of this locally generated two-phase flow in fibrous porous media, thermally induced evolutions of dissolved air in water flowing through a carbon felt sample are experimentally studied. When 20°C air-saturated water is started to be pumped through the felt and simultaneously heated up to 30°C at the outlet, evolved gas bubbles are observed to emerge several minutes later and accumulate to considerable amount after a few hours. The correspondingly increasing average gas saturations in the felt are measured by a weighing method, maximum values of which fall in the range from 6% to 14% at different liquid velocities. The most surprising finding is that although the volume evolution rate of gas bubbles is only 0.36% of the liquid flow rate, the resulting liquid-phase relative permeability drops significantly and continuously for a couple of hours and down to 0.42 at the lowest liquid velocity.