Thin stillage

Yeast separation and concentration. Extraction of ethanol from biomass requires several separation steps, traditionally by centrifuging, sedimentation and cake filtration. First, after the fermentation, the yeast is removed from the fermentation broth and may be recycled. Additionally, after ethanol is stripped from the fermentation broth by steam followed by the removal of solid fractions, the remaining material called the thin stillage can be clarified for reuse upstream as the process water. 

Alumina and other ceramic membranes of various microfiluaiion pore sizes have been used for the separation of yeast (saccharomyces cerevisiae) from the broth and the clarification of thin stillage [Cheryan, 1994]. A typical flux of 110 L/hr-m can be obtained with a crossflow velocity of 4 m/s and a transmembrane pressure of 1.7 bars. The crossflow velocity is found to markedly affect the membrane flux. Concenuation factors (ratios of final to initial concentrations) of 6 to 10 for both the broth and the stillage can be achieved. Backflushing with a frequency of every 5 minutes and a duration of 5 seconds helps maintain the flux, particularly in the initial operating period. The permeate flux for both types of separation reaches steady state after 30 to 90 minutes. 

Pressure-driven membrane processes have also been applied for recovery of valuable residual or side-products present in the spent fermentation broth, which is called whole stillage. It contains fiber, oil, protein, diverse unfermented components of the grain, and yeast cells (Abels et ak, 2013). The whole stillage can be separated into two streams—namely, thick stillage and thin stillage—via centrifugation or vacumn belt filtration. Membrane separations can be used for the recovery of valuable solids from thin stillage (Arora et ak, 2009). 

Arora et al. (2009) extensively studied the posttreatment of thin stillage via ultralil-tration (Figure 11.6) to recover valuable nutrients such as ethanol, lactic acid, acetic acid, and saccharides. Several cellulosic ultrafiltration and microfiltration membranes were tested, and the ash content of the feed solutions could be reduced by at least 50% by applying the tested membranes. Leberknight, Wielenga, Lee-Jewett, and Menkhaus studied the recovery of proteins in a corn-to-ethanol process. In order to recover... 

Dereli, R. K., Urban, D. R., Heffeman, B., Jordan, J. A., Ewing, J., Rosenberger, G. T., et al. (2012). Performance evaluation of a pilot-scale anaerobic membrane bioreactor (AnMBR) treating ethanol thin stillage. Environmental Technology, 33, 1511—1516. 

Wu, J.-M., Liu, R.-H., 2012. Thin stillage supplementation greatly enhances bacterial cellulose production by Gluconacetobacter xylinus. Carbohydrate Polymers 90 (1), 116-121. 

Naidu, K., Singh, V., Johnston, D.B., Rausch, K.D., and Tumbleson, M.E. 2007. Effects of ground com particle size on ethanol yield and thin stillage soluble solids. Cereal Chem. 84 6-9. 

Thin stillage A liquid portion of stillage that is separated by screening and/or centrifugation. It contains suspended fine particles and dissolved material. 

Yen, H.-W., Yang, Y.-C., Yu, Y.-H., 2012. Using crude glycerol and thin stillage for the production of microbial lipids through the cultivation of Rhodotorula glutinis. Journal of Bioscience and Bioengineering 114, 453—456. 

---