Monitoring phase transition of aqueous biomass model substrates by high‐pressure and high‐temperature microfluidics

Abstract

Aqueous‐Phase Reforming (APR) is a promising hydrogen production method, where biomass is catalytically reformed under high pressure and high temperature reaction conditions. To eventually study APR, in this paper, we report a high‐pressure and high‐temperature microfluidic platform that can withstand temperatures up to 200°C and pressures up to 30 bar. As a first step, we studied the phase transition of four typical APR biomass model solutions, consisting of 10 wt% of ethylene glycol, glycerol, xylose or xylitol in MilliQ water. After calibration of the set‐up using pure MilliQ water, a small increase in boiling point was observed for the ethylene glycol, xylitol and xylose solutions compared to pure water. Phase transition occurred through either explosive or nucleate boiling mechanisms, which was monitored in real‐time in our microfluidic device. In case of nucleate boiling, the nucleation site could be controlled by exploiting the pressure drop along the microfluidic channel. Depending on the void fraction, various multiphase flow patterns were observed simultaneously. Altogether, this study will not only help to distinguish between bubbles resulting from a phase transition and/or APR product formation, but is also important from a heat and mass transport perspective.

Towards controlled bubble nucleation in microreactors for enhanced mass transport

“…), which is reminiscent of our previous work. 59 The additional 15°C above the nominal boiling temperature is indicative of superheating, which is also consistent with the observed boiling mechanism. In contrast, in devices with micropits, the first vapor bubbles already formed at 85°C (Fig.…”

“…DRIE-etching of silicon gives rise to defects on the silicon walls, or so-called scallops, which can also be hydrophobic due to the presence of fluorocarbon residues from the dry-etching. 59 Examination of both the micropits and the silicon microchannel wall using highresolution SEM demonstrated that the micropits were very similar in size as the etching defects, around 2 μm (see ESI-7 †), so that both structures could equally act as bubble nucleation sites by lowering the thermodynamic barrier for nucleation.…”

Aqueous‐Phase Reforming in a Microreactor: The Role of Surface Bubbles

“…At low protrusion angles (below 45°) positive slip lengths were expected, and negative slip at higher angles . The low temperature value (525 K) corresponded to a typical APR reaction temperature , whereas the upper temperature limit (725 K) refers to the maximum temperature, at which our microreactor can operate .…”

“…). This 2D geometry was derived from a previously described silicon/glass microfluidic device . Briefly, a 250‐µm deep fluidic channel was created in a silicon substrate and covered by a glass top layer.…”