A process that employs ethanol fractionation as a pretreatment approach to separate cellulose, hemicelluloses, lignin and extractives from woody biomass has been proposed by Lignol Innovation (Fig. 11.1) [42]. The obtained cellulose fraction is claimed to be highly susceptible to enzymatic hydrolysis, and the generated glucose of a high yield is readily converted into ethanol, or possibly used as sugar platform chemicals via saccharification and fermentation. In addi­tion, the liquor rich in lignin, furfural, xylose, acetic acid and lipophylic extrac­tives, can be separated by well-established unit operations. The ethanol is recovered and recycled back in the whole process. The recycled process water is of high quality, low BOD5 and suitable for the overall system process closure. The proposed steps for product separation are shown as follows: (1) After the cooking, the cooking liquor is turned into a black liquor, which is further subjected to

Cellulose

I

Saccharification

and

Fermentation

precipitation to recover lignin by diluting the black liquor with enormous process steam and filtering, washing and drying the precipitated lignin. (2) The ethanol in the black liquor is recovered and recycled by flashing the black liquor and com­pensating the vapors. With respect to the filtrate and washing liquor, they are distilled to achieve a higher concentration. (3) Acetic acid, furfural, xylose and extractives are separated from the distillation column. (4) Oligosaccharides are converted into sugars for fermentation to produce more ethanol using mild acid hydrolysis. Based on the economic evaluation, it has been claimed that this process can be operated in a plant as a small scale as 100 mt per day.

Ethanol fractionation process in combination with ultra-filtration has been designed by Garcia et al. [43]. The main unit operations are cooking, flash oper­ation, washing stage and ultra-filtration. The cooking operation is conducted in a pressurized reactor. Flash operation is used to recover stream mixtures of ethanol and water. In the washing stage, the obtained fibers are washed with mixtures of ethanol and water under the same concentration of the cooking liquor. The lignin dissolved in the black liquor is separated into homogeneous fractions by using ultra-filtration and then is subjected to precipitation with water. To achieve fully solvent recycle, liquor faction after lignin precipitation is sent to distillation unit to recover the ethanol/water mixtures, whereas the residue composed of water and co-products is treated by heating in a flash unit to recover a clean water stream for lignin precipitation. By using the simulation software Aspen Plus, the energetic and economical efficiencies of the ethanol fractionation are evaluated considering several units, including reaction, solid fraction washing, products recovery and liquid fraction processing. Mass and energy balances are evaluated in terms of yield, solvents/reactants recovery and energy consumption. In addition, pinch technology has been applied to improve the heat exchange network of the ethanol fractionation process reducing the associated utilities requirements, making the process more competitive as compared to the soda process.

In a recently proposed ethanol extraction process, a pre-hydrolysis step is applied to remove the hemicelluloses of wood chips [44]. The open diagram in Fig. 11.2 shows the steps required for recovering the hemicelluloses from the pre­hydrolysis liquor (PHL) in a separate stream. In the pre-hydrolysis step, hemicelluloses are extracted accompanying removal of a part of lignin.

Subsequently, the dissolved lignin is precipitated by decreasing the pH of the PHL to 2 using sulfuric acid. Then the precipitated lignin is subjected to a filter washer. The recovery of hemicelluloses is conducted by the addition of ethanol into the acidified PHL and further separated from the ethanol/water solution in a filter washer. The pre-hydrolyzed feedstock, with increased porosity, is subjected to the ethanol fractionation for removing the remaining lignin, similar to the ethanol pulping process. The dissolved lignin in the ethanol extraction step is recovered by acidification and then separated in a filter washer. After the ethanol extraction, cellulose is remained as a solid residue associated with a small amount of lignin, which can be further removed in an elemental chlorine free (ECF) based bleaching process.

Separation of lignin from spent liquor is generally based on the lignin insolu­bility in acid water. The recovery of lignin in an acid process consists of the following stages: precipitation of the lignin fraction with higher molecular weight; separation of the precipitate by decantation, thickening, centrifugation or filtration; washing with water to reduce impurities; further thickening to remove the water retained in the washing stage; drying of lignin. However, lignin dissolved in alkaline ethanol liquor is difficult to precipitate because the process decreasing the spent liquor pH to around 2 requires a large amount of acid to neutralize.

Generally, there are two typical methods to recover lignin from acidic ethanol — soluble liquor. One is dilution of spent liquors in water directly [20, 45], which is characterized by low speeds and sometimes difficult to filtrate or centrifuge due to the generation of a rather stable colloidal suspension. The other way consists of recovery of the alcohol from spent liquor in recovery tower under reduced pres­sure, then precipitation of lignin in water [46]. This procedure is usually ineffective and difficult to control, because lignin tends to precipitate as a sticky tar in the internal surfaces of the recovery tower, reducing the recovery of alcohol. The modified method is the evaporation of 60-65% alcohol in a flash tank, cooling the spent liquor to a temperature above 70°C (to avoid the precipitation) and diluting by injection of the liquor into water through a Venturi tube [47].

In a recent study, two feasible laboratory-scale ways are proposed to recover lignin by precipitation [48]. The laboratory-scale representation of a system involving the reduction of ethanol concentration in the spent liquors by evapora­tion in a flash tank to 30% (v/v), dilution ratio of 1:1, at 40°C and centrifugation, appeared as the best alternative for lignin recovery (45% of precipitate with a purity of 94%, yielding 42% pure lignin). Another feasible procedure involved lignin precipitation and recovery from the spent liquors by dilution with water under a dilution ratio of 1:2. This method yielded 41% pure lignin, yet from a precipitate of 48% with 87% purity (much more contaminated, mainly with car­bohydrates). The temperature of the treatments affects the recovery process. In both cases, the most suitable dilution conditions are at room temperature or 40°C.

In addition, ultra-filtration membrane allows to recover lignin with specific molecular weight, but the cost is relatively high [49, 50]. For instance, ultra­filtration has been used to fractionate the lignin dissolved in ethanol-soluble liquor. The ultra-filtration module is a pilot unit equipped with a stainless steel tank with water jacket for temperature control, a recirculation pump and a set of tubular ceramic membranes of different cut-offs in the interval 5-15 kDa [51]. Four different cutoff fractions are obtained: less than 5 kDa fraction; 5-10 kDa fraction; 10-15 kDa fraction and more than 15 kDa fraction. After the ultra-filtration, the obtained lignin has a relatively homogeneous molecular weight distribution.