An evaluation of a microchannel reactor for the production of hydrogen from formic acid

Abstract

This dissertation evaluates the performance of a microchannel reactor for the decomposition of vaporised formic acid as a promising technology for the production of hydrogen for proton exchange membrane fuel cell applications. Accordingly, a combined experimental and modelling approach was used to evaluate the microchannel reactor coated with a gold supported on alumina (1.15 wt. % Au/Al2O3) catalyst. For the experimental evaluation, two phase of experiments were carried out where pure formic acid (99.99 %) and dilute formic acid (50 vol. %) were taken as the feed to the reactor respectively. The first phase of the experimental evaluation involved measuring key performance parameters such as, formic acid conversion, formic acid residual concentration, selectivity to hydrogen and hydrogen yield at different temperatures of 250 – 350°C and formic acid (99.99 %) vapour flowrates of 12 – 48ml/min. Overall, the reactor performed well in decomposing pure formic acid (99.99 %), achieving conversions (98 to 99 %) close to equilibrium at 350 oC and all studied vapour formic acid flowrates of 12 – 48 ml/min. At all studied temperatures however, both dehydrogenation (HCOOH → H2 +CO2) and dehydration (HCOOH → H2O+CO) reactions occurred and the dehydrogenation reaction was found to be dominant. The dehydration reaction was mostly favoured at high temperatures and carbon monoxide concentrations ranged between 4 – 15 % while the corresponding selectivity towards H2 production ranged between 0.7 and 0.88. Effort was made to improve the H2 yields in the second phase of the experiments through decomposing a mixture of formic acid and water (50/50 vol. %) thereby promoting the occurrence of the forward water gas shift reaction. Under these conditions, carbon monoxide concentrations decreased to a range of 2 – 7 % while selectivity towards hydrogen production increased to a range of 0.84 – 0.94. Overall, for both pure FA (99.99 %) and dilute FA (50 vol.%), the best microchannel reactor performance was achieved at a reactor operating temperature of 350 oC and FA vapour flowrate of 48 ml/min (17.1 Nml.gcat-1.h-1). At these conditions, H2 production rate (11.8 NL.gcat-1.h-1) was maximised with pure FA (99.99 %) while selectivity (0.81) and H2 yield (80) were maximised with dilute FA (50 vol.%). Overall, the reactor was found stable at a continuous period of 144 hours after running for approximately 1 200 hours. A computational fluid dynamic model was developed for concentrated formic acid (99.99 %) experiments aimed at describing reaction-coupled transport phenomena relating to velocity, mass and temperature profiles within the microchannel reactor. Kinetic rate expressions that best described the experimental results were successfully estimated using a model-based parameter optimisation and refinement on Comsol Multiphysics™ 4.3b. Validation of the model against the experimental results showed that the developed model was an acceptable fit to the experimental conversions and hydrogen yields especially at temperatures higher than 250 oC. Overall, this dissertation highlights the first steps in the development and use of microchannel reactors in promoting formic acid as a future hydrogen storage medium for portable and distributed fuel cell applications.