Pentose and Furfural in the Sulfite Process

The least problematic stream is the flash condensate obtained when depressurizing the digesters to 1 ATM. Stemming from a vapor, this stream is, of course, free of pentose as the latter is nonvolatile, and for the same reason it is free of calcium sulfate. Consequently, this small stream containing some furfural is readily submitted to distillation to recover the furfural. 

A somewhat different furfural stream is obtained when the total waste water of a sulfite plant is submitted to thickening by a multieffect evaporator. In this case, the vapor 

One of the reasons why the pentose in calcium sulfite waste liquors is not being utilized for making furfural lies in the fact that the liquor from the digesters is actually super- 

At 30 C, the solubility of calcium dithionate in water exceeds that of calcium sulfate by a factor of 105. According to the principle of Le Chatelier, under a high sulfur dioxide pressure the equilibrium of the given reaction lies on the right-hand side, while at a low pressure of sulfur dioxide essentially only calcium sulfate exists. This implies that when the digesters are depressurized, the high solubility of calcium sulfate is terminated, thus leading to calcium sulfate precipitation. 

Hodgeman et al.. Handbook of Chemistry and Physics, 42 Edition, The Chemical Rubber Publishing Company, Cleveland, 1960. 

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