Organosolv Pretreatment

The organosolv process is a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose. The process was first developed as an environmentally benign alternative to kraft pulping. Its main advantages include the production of high-quality lignin for added values and easy recovery and recycling of solvents used in the process, thereby alleviating environmental stress on the water stream.

In this type of pretreatment of lignocellulose, an organic solvent (etha­nol, butanol, or methanol) is added to the pretreatment reaction to dissolve and remove the lignin fraction. In the pretreatment reactor, the internal lignin and hemicellulose bonds (refer to Figure 4.1) are broken and both fractions are solubilized, whereas the cellulose remains as a solid. After leaving the reactor, the organic fraction is removed by evaporation (distil­lation) in the liquid phase. The lignin then precipitates and can be removed by filtration or centrifugation. Thus, this process cleanly separates the feed­stock into a solid cellulose residue, a solid lignin that has undergone a few condensation reactions, and a liquid stream containing xylon, as shown in Figure 4.9.

The organosolv process is usually carried out at an elevated temperature of 140-230°C under pressure. High temperature is somewhat dictated by the desired bond cleavage reactions involving the liberation of lignin, and the high pressure is needed to keep the solvent process operation in the liquid phase. Ethanol has been regarded as a preferred solvent for organosolv due to its low price, availability, and easy solvent recovery. Butanol, also, has shown promise because of its superior capability of high lignin yield and immisci — bility with water which make solvent recovery simple without energy-inten­sive distillation. Although butanol’s effectiveness is quite appealing, its cost is considered to be somewhat prohibitive. As explained, a principal concern in these processes is the complete recovery of the solvent, which affects the overall process economics; as such, process engineering and optimization become important factors in process economics.

Results have shown that there are some reactions occurring during the organosolv process that strongly affect the enzymatic rate [53]. These reac­tions could be due to the physical or chemical changes in lignin or cellulose.



Organosolv pretreatment process. (Modified from Wright, 1988. Ethanol from biomass by enzymatic hydrolysis, Chem. Eng. Prog, 84: 62-74.)

In general, organosolv processes have higher xylose yields than the other processes because of the influence of organic solvent on hydrolysis kinetics. In a recent study, Pan et al. [54] applied the ethanol organosolv pretreatment to lodgepole pine killed by mountain beetle and achieved 97% conversion to glucose. They recovered 79% of the lignin using the conditions of 170°C, 1.1 wt% H2SO4, and 65 vol% ethanol for 60 minutes.