Monosaccharides from sulfite spent liquors

In principle, monosaccharides and their conversion products including furfural can be isolated from sulfite spent liquors. Because of the complicated separation technique needed and since alternative raw material sources, such as wood and agricultural wastes are available, these processes have so far been of very limited practical interest. Because of their carbohydrate content, sulfite spent liquors find use either directly or after some fractionation as a feed component for cattle. 

Sulfite and sulfate ions were precipitated from the spent birch liquor (Fig. 1) with barium hydroxide. Monosaccharides and other low molar mass non-electrolytes and weak electrolytes were separated quantitatively from the lignosulfonates by means of ion exclusion chromatography (5). 

Table 3.5s4 shows the retention of sodium base spent sulfite liquor by a series of UF membranes. The multistage UF was carried out in succession from the largest to the smallest pore size-i.e., the permeate from the first stage became the feed for the second stage. Although 90% of the sugar passed through membranes between 100,000 and 20,000 MWCO, the retention increases for 10,000 and 500 MWCO membranes up to 36%. It has been estimated that about 76 of the sugars are bound in complexes with molecular weights above 10,000 and an additional 10% in complexes above 500 daltons. In other words, A of all the monosaccharides present in spent sulfite liquor are bound. On the other hand, 100,000 MWCO membranes retained over 50% of the lignosulfonates. It appears (from Table 3.5)M that about 70% of the lignosulfonate compounds have a MW over 50,000 only 7.2% had a MW less than 10,000.

In order to study birch lignosulfonates, spent sulfite liquor, from which monosaccharides had been removed by ion exclusion chromatography, was fractionated on the basis of molecular size by preparative GPC (Fig. 1). 

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