FRACTIONATION OF RESIDUAL LIGNOCELLULOSICS BY DILUTE-ACID PREHYDROLYSIS AND ALKALINE EXTRACTION - APPLICATION TO ALMOND SHELLS

A two-stage process based on a hydrolytic pretreatment and an alkali e xtraction was applied to the fractionation of almond shells into cellu lose, pentosan and lignin. Autohydrolysis and dilute-acid hydrolysis w ere studied as pretreatment methods in two different reactor configura tions: an isothermal plug-flow reactor and a stirred non-isothermal, b atch autoclave. Yield and composition of the pulps resulting from the hydrolytic pretreatment were equivalent for both reactors when compare d through the reaction severity concept in this approach, pretreatment severity is quantified by R(OH), a semi-empirical parameter that comp rises the effect of temperature, time and acid concentration used duri ng the pretreatment into a single reaction ordinate. Results showed th at a pretreatment severity of log(10)R(OH)=4.3 was the optimal to maxi mize pentosan recovery, with 66% of the potential yield. At this sever ity, lignin obtained was 64% of the potential yield and 9% of the init ial cellulose was degraded. Viscosity average degree of polymerization of the alpha-cellulose remaining in the pulp was reduced to 500 at th is severity. Lignin recovery increased with severity to a value of 82% at log(10)R(OH)=4.7, allowing a better separation of the cellulose an d the lignin, but also producing a higher depolymerization of the alph a-cellulose. Polydisper sity of the alpha-cellulose also decreased wit h the severity increase, from 6.3 at log(10)R(OH)=4.0 to a value of 3. 4 at log(10)R(OH)=4-7.