1. How the Severity Factor in Biomass Hydrolysis Came About

The search for a useful index that could provide a simple descriptor of pretreatment operations motivated us, in the 1970s, to explore the concept of severity in chemical reactions. The literature showed that industrially there had been numerous efforts to define such indices. For example, pre-hydrolysis in kraft pulping is an operation which includes steam and aqueous pretreatment methods similar to those that we were pursuing in our labs at the time. This had been modeled by Brasch and Free via the P-factor, as well as the H factor = ∫ (k(T)/k(373)dt, between times 0 and t, with the temperature in °K, with k being the rate constant which traditionally has been considered to follow the Arrhenius temperature dependency. Delignification rates are proportional to the H factor, hence its usefulness in controlling the final pulp consistency. Outside of wood processing, the high temperature cracking of petroleum fractions resulted in similar indexes which had been developed and implemented in plant operations. With increasing analytical insight into petroleum chemistry and processing, the cracking indices have gradually been replaced by a chemical kinetic-based “lumping” approach used for each of the petroleum fractions in a given crude oil.