Additional Loss Reactions in Sulfite Liquor

All available publications on the kinetics of furfural formation are based on xylose in water. Thus, it is hardly surprising that these kinetics are found to be far from correct when they are applied to the pentose contained in sulfite liquor, the obvious reason being that this liquor contains substances known to react with furfural and with intermediates of the pentose-to-furfural conversion [19], with lignosulfonate being the main culprit, so that the quantity of furfural produced per unit mass of pentose is very much smaller than what kinetics in water predict. In other words, the kinetics of furfural formation in water must he supplemented by further loss terms. So far, none of the respective rate constants have been determined. Only an overall yield for special circumstances can he given in a later chapter. 

With the sealed ampoule process used for their kinetic studies, Root, Saeman, Harris, and Neill [20] achieved fiirfiiral yields well in excess of 70 % at temperatures above 220 °C, whereas industrial furfural processes, operating at lower temperatures and featuring a continuous removal of the furfural by steam stripping, have typical yields below 60 %. By contrast, in analytical chemistry, at a proven yield of 100 % [21], the formation of furfural from xylose or pentosan is routinely used for the quantitative determination of these substances. It is of great importance to elaborate the reasons for this yield paradox . 

The situation is quite different when a small furfural concentration is generated in the second case, and when the heating is effected by condensing steam of 100 °C (atmospheric analogue of an industrial furfural process) as this results in point B lying in the liquid field where furfural can react with itself and with the first intermediate of the xylose-to-furfu-ral conversion. 

The crucial point is that loss reactions can be completely avoided in a boiling liquid but can not be avoided in a non-boiling liquid, and it is the inherent disadvantage of the 

In simple terms, it can be said that the yield tragedy of industrial furfural reactors is due to the fact that the steam used for heating and stripping is simply not hot enough to bring the reaction medium to boiling. 

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