Biohydrogen purification using methyldiethanolamine and caustic in a two stage absorption system

Abstract

As the research on biohydrogen production is growing, there is a need to explore the most suitable way to purify the biogas produced from the fermentation process so that pure biohydrogen can be obtained. The common biohydrogen gas composition produced from biological process is mainly hydrogen (H2) and carbon dioxide (CO2). The presence of CO2 in biohydrogen will affect its application if biohydrogen is used in fuel cell application. In this work, solvent absorption was selected due to its simplicity and low cost. The CO2 removal was performed by using two stage solvent absorption at which the activated methyldiethanolamine (MDEA) was used in the first stage and caustic (NaOH) in the second stage. The MDEA was activated by adding piperazine (PZ) into the MDEA solution. It was added to improve the reaction between CO2 and MDEA. The operating pressure and temperature were 1 bar and 33°C respectively. The simulated untreated biohydrogen gas contained 50 mol% of H2 and 50 mol% of CO2 . At first, 40 wt% MDEA was activated with different concentration of PZ and evaluated. It was found that 6 wt% of PZ was required to fully activate the 40 wt% MDEA. Next, the CO2 removal was improved by implementing an improvised gas distributor at the first stage. Lastly, the CO2 removal was further improved by utilizing a wire mesh packing at the first stage as to improve the contact area between the gas and the chemical solvent. At the same time caustic stage containing 20 wt% NaOH was added as the second stage. By doing so, the highest CO2 removal achieved in this study at the first stage was 86% and 99.59% at the second stage, producing treated biohydrogen gas with purity of >99 mol%. Hence, this system has great potential to be used as a proper CO2 removal system for biohydrogen gas produced by fermentation process.