Minimizing Precipitated Lignin Formation and Maximizing Monosugar Concentration by Formic Acid Reinforced Hydrolysis of Hardwood Chips

Prehydrolysis Kraft involves high-temperature steam treatment of wood chips followed by Kraft pulping. In industrial practice, the acidic prehydrolysate is mixed with alkaline spent Kraft black liquor because separate handling of the hydrolysate causes plugging in downstream equipment due to lignin precipitation. However, dilution of black liquor by the hydrolysate significantly increases evaporation cost, and the prehydrolysate sugars are lost as potential feedstock for fuels and chemicals. Thus, there is a strong incentive for reducing or eliminating the formation of the troublesome lignin precipitates in the prehydrolysate. Earlier we reported that addition of 10 g/L formic acid (FA) during hot water (160 °C) treatment of hardwood chips significantly minimized the lignin precipitates in prehydrolysate. Also, FA reinforced prehydrolysates contained more hemicellulose sugars, with more in monomeric form. In the present paper, we show that further increasing the FA concentration and/or addition of a small amount of sulfuric acid leads to almost complete hydrolysis of the oligomers and, more importantly, also further decreases the lignin precipitate concentration. The effect of FA, hydrolysis time, L/W ratio, and H2SO4 concentration on dissolved sugar and lignin precipitate concentrations will be described. The molecular weight and polydispersity of the lignin precipitates decreases with increasing FA concentration. A new mechanism is proposed to explain the effect of FA and other operating conditions on the insoluble lignin concentration. Recovery of FA charged at 10 g/L at L/W ratio of 3.5 L/kg can be achieved by reactive distillation using methanol as reactant. At 95% FA recovery, the net charge of fresh FA to the process is 0.9 g/100 g wood because FA is partially carried forward into the Kraft process with the drained extracted chips. However this chemical requirement is compensated by the production of acetic acid (AA) at 2.3 g/100 g wood, furfural at 1.4 g/100 g wood, and hemicellulose sugars at 10 g/100 g wood, 90% of which are present as monosugars when prehydrolysis is performed at 15 g/L of FA or 10 g/L FA plus 0.25 g/L H2SO4.