Process Description

The first application of enzymes for hydrolysis of wood in an ethanol process was obvious — simply replace the acid hydrolysis step with an enzyme hydrolysis step. This configuration, now often referred to as separate hydrolysis and fermentation (SHF) is shown in Figure 1¹. Pretreatment of the biomass is required to make the cellulose more accessible to the enzymes. Many pretreatment options have been considered, including both thermal and chemical steps.

Illustration of the enzyme process with separate hydrolysis and fermentation, as described in the text.

Figure 1: The enzyme process configured as separate hydrolysis and fermentation

The most important process improvement made for the enzymatic hydrolysis of biomass was the introduction of simultaneous saccharification and fermentation (SSF), as patented by Gulf Oil Company and the University of Arkansas^2,3. This new process scheme reduced the number of reactors involved by eliminating the separate hydrolysis reactor and, more importantly, avoiding the problem of product inhibition associated with enzymes. In the presence of glucose, ß-glucosidase stops hydrolyzing cellobiose. The build up of cellobiose, in turn, shuts down cellulose degradation. In the SSF process scheme, cellulase enzyme and fermenting microbes are combined. As sugars are produced by the enzymes, the fermentative organisms convert them to ethanol. The SSF process has, more recently, been improved to include the cofermentation of multiple sugar substrates. This new variant of SSF, known as SSCF for Simultaneous Saccharification and CoFermentation, is shown schematically in Figure 2.

Illustration of the enzyme process with simultaneous saccharification and cofermentation, as described in the text.

Figure 2: The enzyme process configured for simultaneous saccharification and cofermentation (SSCF)

References

¹Wilke, C.R.; Yang, R.D.; von Stockar, U. "Preliminary Cost Analyses for Enzymatic Hydrolysis of Newsprint." Biotechnology and Bioengineering, No. 6, 1976, pp 155-175.

²Gauss, et al, U.S. Patent No. 3,990,944, November 9, 1976.

³Huff, et al, U.S. Patent 3,990,945, November 9, 1976.