Lignin removal

The normal limit for lignin removal in the digester is set both by pulp strength requirements, usually indicated by pulp viscosity, and by yield requirements. As noted earlier, direct control of hydroxide and hydrosulftde concentrations can maximize lignin removal while maintaining pulp strength. 

Alkaline delignification in the form of the Kraft or Sulfate process is now the most widely used method of lignin removal. It uses a mixture of sodium hydroxide and sodium sulfide—the latter being produced in the recovery process by the reduction of sodium sulfate... 

Figure 3.5 The effect of lignin removal upon reactivity of acetic anhydride with MDF fibre (a) (from data of Hague and ffill, 2000) and wood flour (b) (from data of ( etin, 1999).

Jain, R.K. Gupta, A. Dixit, A.K. Mathur, R.M. Kulkami, A.G. Enhanced biomethanation efficiency of black liquor througb lignin removal process in small agro based paper mills. Bioenergy... 

Ganjidoust, H. Tatsumi, K. Yamagishi, T. GhoUan, R.N. Effect of synthetic and natural coagulant on lignin removal from pulp and paper wastewater. Water Sci. Technol. 1997, 35 (2-5), 291-296. 

When autohydrolysed aspen was treated with cellulase enzyme, poor results were obtained. Based on a ton of dry wood, only about 300 lb of fermentable sugars were obtained, and the fermentation was very inefficient with low yields of ethanol. These results compare poorly with those shown in Figure 5 when autohy-drolysed-caustic extracted aspen was treated with cellulase enzyme, and then fermented. It would appear from these results that the caustic extraction step for lignin removal would be necessary if the enzymatic hydrolysis process were to be adopted and advisable when acid hydrolysis is used. 

Dispersal of lignin as a colloid is a close relative of true solution as a means of lignin removal. Dispersing agents include alkalis. Effective dispersion may involve molecular weight reduction as a necessary condition. 

Yang, B., and Wyman, C. E., 2000, Effect of xylan and lignin removal by batch and flowthrough pretreatment on the enzymatic digestibility of com stover cellulose, Biotechnol. Bioeng. 86 88-95. 

Fig. 7. Change in lignin removal with fluid velocity for hot water only pretreatment of corn stover.

Fig. 1. Effect of oxygen delignification conditions on lignin removal and carbohydrate loss.

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