Effect of reactor configuration on the subcritical water hydrolysis of recycled paper mill sludge

Recycled paper mill sludge (RPMS) was hydrothermally treated by subcritical water (SBW) in two modes of operation, namely semi-continuous and batch, at temperatures in the range 150–250 °C, and 100 bar. The performance of both systems was analyzed in terms of degree of hydrolysis, product yield and formation of degradation products. The highest degree of RMPS hydrolysis was achieved using the batch reactor configuration, while in the semi-continuous system higher yields of water soluble compounds (WSC) and monosaccharides were achieved. By using the semi-continuous system at 100 bar, 250 °C and a water flow rate of 5 mL/min, 40% of the organic carbon content of RPMS was recovered in the form of soluble compounds. Additionally 20% of total carbohydrates and 75% of total lignin of RPMS were recovered as soluble material. Detailed mass balances performed for both operational modes revealed that under batch mode, the naturally occurring compounds found in RPMS underwent significant transformation, generating derivatives referred to as degradation products. An analysis of the severity factor was performed for both operational modes. It was concluded that significant values of WSC were only obtained for severity factors higher than four.