Dilute strong acid pretreatment is the most studied and developed of lignocellulosic ethanol pretreatment methods. In this pretreatment there is always a tradeoff of pH of the reaction mixture and temperature of the pretreatment. Lowering the pH allows lower temperatures to be employed at the expense of added acid. This added acid has a small OPEX impact, but more significantly makes subsequent operations (saccharification, fermentation, water treatment) more difficult and expensive due to salts formed during the required pH adjustment following pretreatment. Operating at a higher pH requires higher temperatures which consumes fragile sugars and generates toxic materials which also impacts downstream operations. To overcome this paradigm, we have developed a pretreatment system employing a mineral acid strong, reversible acid utilizing alphahydroxysulfonic acids. The formation of alphahydroxysulfonic acids takes place spontaneously and exothermically when an organic carbonyl is mixed with sulfur dioxide and water. These acids are as strong as mineral acids yet have the capability to be “reversed” to their starting components and if these components are volatile they are easily removed from the reaction mixture. The pretreatment system shown focuses on the uses of acetaldehyde as the starting organic carbonyl, which reacts with SO2 to make alpha-hydroxyethanesulfonic acid (HESA). Pretreatment reactions carried out at temperatures below 130 °C and a pH of &lt;1 have shown effective results, providing a substrate that is easily hydrolyzed enzymatically with little inhibitor or toxin formation and excellent sugar retention. Recycle of the acid through the flashing of the components (acetaldehyde and SO2) and subsequent reforming the acid in an overhead stream has been demonstrated with high recovery of HESA components (~99% per pass). The outlet pH from the RAPT system has a pH set by the organic acids (primarily acetic acid) formed from the hydrolysis of biomass esters in the feedstock.

RAPT (reversible acid pretreatment) for cellulosic ethanol production

249th National Meeting (2015)

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