Techno-economic analysis of a softwood ethanol process featuring sugars concentration and continuous fermentation with cell recycle

Many second generation biofuel processes are constrained by low sugars and high inhibitor concentrations in the hydrolysate.  Low sugars concentrations negatively impact economics by increasing distillation costs and requiring larger process equipment, while inhibitors reduce fermentation rates, requiring larger fermentor sizes.  Sugars concentration operations and advanced fermentation techniques can help mitigate these problems; unfortunately, inhibitors can be concentrated along with the sugars.  This analysis compares the use of thermal evaporation and nanofiltration (NF) to concentrate glucose in a ponderosa pine hydrolysate from 30 g/l to 100, 150 and 200 g/l before fermentation by S. cerevisiae.  To ferment this high gravity, inhibitor containing feed, batch fermentation is compared with continuous fermentation with microfiltration (MF) cell recycle.

Detailed capital and operating cost analyses were used to calculate the equivalent annual operating cost (EAOC) , which quantifies how these improvements impact overall process economics. Evaporation offers the benefit of complete furfural and partial acetic acid and HMF removal while nanofiltration has the potential to remove 65-70% of all inhibitors.  Evaporation is more capital and energy intensive than distillation and does not reduce EAOC, though batch fermentor size is reduced by 60% compared to an unconcentrated feed.  Nanofiltration requires replacement of expensive membranes; however, it has the potential to lower EAOC if high membrane performance is maintained.  Continuous fermentation can improve fermentor productivity by over 10 times, reducing the EAOC.  Continuous fermentation coupled with nanofiltration was the most economical system studied, but EAOC strongly depends on membrane (MF and NF) cost and performance.