Organosolv

Organosolv refers to a class of pulping and biomass pretreatment processes that utilize organic solvents to dissolve lignin and, to a lesser extent, hemicelluloses from lignocellulosic feedstocks. The process produces a cellulose-rich solid pulp and a lignin-containing liquor. Common solvents include ethanol, methanol, acetone, and ethyl acetate, often used with water as a co-solvent. An acid or base catalyst, such as dilute sulfuric acid or p-toluenesulfonic acid, is typically employed to catalyze lignin solubilization and hemicellulose hydrolysis at elevated temperatures (generally 150–210 C) under pressurized or near-atmospheric conditions.

After treatment, the solid cellulose-rich pulp is separated and can be washed and dried for pulping or further enzymatic hydrolysis for bioethanol production. The lignin is recovered from the solvent stream by precipitation, phase separation, or solvent withdrawal, and can be processed into chemicals and materials.

Advantages include the absence of sulfur compounds associated with Kraft pulp, easier lignin valorization due to relatively low condensation and high purity lignin, higher carbohydrate preservation, and potential integration with chemical or thermochemical valorization. Organosolv pulps generally display high brightness and digestibility.

Limitations involve high solvent recovery costs, energy requirements, and the need for corrosion-resistant equipment; solvent flammability and volatility pose safety concerns; and the economic viability depends on solvent recycling efficiency, plant scale, and product prices. Lignin quality varies with solvent and process conditions, influencing downstream use.

Organosolv has been studied since the early 20th century and has been implemented in pilot plants and demonstration facilities. It is considered attractive for woody biomass pretreatment and biorefinery concepts, where lignin and sugars are valorized separately.