Understanding Hydrogen in Bayer Process Emissions. 1. Hydrogen Production during the Degradation of Hydroxycarboxylic Acids in Sodium Hydroxide Solutions

• Published in: Industrial & engineering chemistry research 2011-11, Vol.50 (22), p.12324-12333
• Main Authors: Costine, Allan, Loh, Joanne S.C, Power, Greg, Schibeci, Mark, McDonald, Robbie G
• Format: Article
• Language: English
• Subjects: Aliphatic compounds; Applied Chemistry; Applied sciences; Bayer process; Carboxylates; Chemical engineering; Degradation; Digestion; Exact sciences and technology; Hydrogen production; Organic compounds; Wet oxidation
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/ie201387x

The formation of potentially explosive gas mixtures during Bayer process digestion and the wet oxidation of Bayer process liquors underscores the need for an improved understanding of the degradation reactions of organic compounds that produce flammable gases. This study is the first of a series investigating the production of hydrogen from different classes of organic compounds in sodium hydroxide solutions. The alkaline degradation of a range of aliphatic and aromatic carboxylates and hydroxycarboxylates was investigated under anaerobic conditions in an autoclave. It was found that aliphatic C4 carboxylates possessing a single β-hydroxy substituent are particularly reactive under these conditions, generating hydrogen gas and a range of low molecular weight (LMW) carboxylates. The effect of temperature (175–275 °C) and NaOH concentration (0–6 M) on the degradation of 3-hydroxybutanoate and hydroxybutanedioate (malate) was investigated in detail for reaction times up to 120 min. Under conditions that promote the total degradation of the organic compound, the β-hydroxycarboxylates have similar hydrogen production capacities, each generating about 1 mol of hydrogen gas per mole of organic compound consumed. The results provide direct evidence for an ionic degradation mechanism involving base-catalyzed oxidation by water, consistent with the stoichiometry of the formation of hydrogen gas and the other main reaction products (LMW carboxylates). These findings have important implications for the production of hydrogen in Bayer process digestion and the safe application of wet oxidation technologies for the treatment of organic compounds in alkaline liquors, such as those found in the Bayer and Kraft processes. The results also shed new light on the mechanism of hydrogen production during the base-catalyzed gasification of biomass at low temperature.