In organic acid fractionation process, the obtained pulp after cooking is generally washed with fresh organic solvent to avoid the lignin precipitation on the pulp, and then washed with water. After the solid is filtrated, the black liquors rich in lignins can be diluted with water to obtain crude lignin. It was observed that a water to liquor ratio of 6 in the lignin-precipitation step resulted in lignin recovery of 88.0 and 77.2% (based on the Klason lignin content in the original material) for formic acid and acetic acid fractionation processes, respectively [99]. In another study, most of the lignin was centrifuged with a water to liquor ratio of 7, and the solid phase (lignin) was washed repeatedly with water to remove the maximum possible amount of carbohydrates to recover a purified lignin [90].

The main operate cost in organic acid pulping, especially for formic acid pulping, is the solvent-recovery section. The black liquor, together with washing liquor, is evaporated to recover the solvent. The organic acid staying in the evaporation residue is recovered by spray drying. Then the organic acid is con­centrated for further use in cycle. Distillation, azeotropic distillation, extractive

distillation, extraction, membrane and adsorption are potential methods for con­densation of organic acid [111]. By using pinch technology for heat integration of the Milox process, the needs for external heating and cooling were reduced by about 40 and 50%, respectively.

A simplified process for simulation of material balance of acetosolv fraction­ation has been proposed [112, 113] (Fig. 11.3). In this process, wood is mixed with the cooking reagents and recycle streams and reacts in the reactor. After cooking, the resulting suspension is subjected to separation by filtration, and the resulting pulp is washed with water in a percolation unit. In the solvent — and HCl-recovery section, liquors and a recycle stream from the secondary recovery section are processed by flash evaporation, stripping and distillation to recover HCl, formic acid and water. In the by-products recovery section, the concentrated liquors are mixed with water to precipitate hydrophobic materials, and the precipitations are recovered by centrifugal filtration. The liquors from the lignin-precipitation unit are sent to heteroazeotropic distillation unit to give a head stream that is rectified to obtain furfural and a bottom stream. In the secondary solvent-recovery section, the above liquors are evaporated to separate the non-volatile solutes as a con­centrated solution, whereas the vapor is recycled. According to the computer simulation [112], the proposed process allowed recovery of 97.6% of acetic acid and 91.7% of HCl, whereas furfural with a high purity of 99.5% was obtained as a valuable by-product.

Simulation of solvent recovery in peroxyacid pulping has been carried out by shortcut simulations and pinch technology [2]. It was found that simple distillation seems to be the preferable solution for recovering the solvents. The presence of a low percentage of acetic acid in formic acid/water system was not favorable to the distillation. However, this can be avoided by using a high percentage of acetic acid in the pulping process since pulping with formic acid/acetic acid/water was also feasible. By using pinch technology, up to 40% energy savings were achieved during the separation of formic acid/acetic acid/water mixture.