Dryhouse operational issues

Reprinted from The Alcohol Textbook 5th Edition 2009 with permission from Lallemand Ethanol Technology and Nottingham University Press

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Major unit operations comprising the dryhouse incorporate numerous control points to enable plant personnel to optimise operations. Aside from the designated control mechanisms, numerous 'external' parameters can have a major impact on dryhouse operations. These parameters primarily relate to two general categories of process fluid physical properties: particle size distribution and fluid viscosity. These properties are, to a degree, controllable by plant design and/or operator decisions.

Solids distribution studies reveal that fine-grind milling results in high concentrations of fine particulate matter in process streams. This serves to reduce decanter centrifuge separation efficiency, increases non-Newtonian viscosity of thin stillage and stillage concentrates and adversely impacts DDGS dryer operations by disrupting wet cake, syrup and DDGS mixing. Dryhouse operations are further compromised by high fermentation and stillage acidity concentrations. The presence of elevated populations of acid-producing microbes or the use of excessive quantities of mineral acids increases process-fluid acidity and degrades fibre structure, producing fibre fines that reduce the separation efficiency of decanter centrifuge solids. Because fibre destruction is a function of acid concentration, temperature and time, technology providers must be concerned with beer-still operation temperatures as well as with whole-stillage storage conditions.

In addition to non-Newtonian viscosity generated by the presence of suspended solids, numerous front-end processes can serve as sources of Newtonian viscosity. Sources of non-Newtonian viscosity include

• unconverted and retrograded starch resulting from inefficient cooking and liquefaction operations,
• unfermented dextrin and glucose concentrations resulting from poor temperature control and microbial activity,
• elevated lactic acid concentrations resulting from microbial contaminant activity and
• elevated glycerol concentrations resulting from environmental stress on yeast.

As the concentration of undesirable solids in the stillage increases during dryhouse operations, so does fluid viscosity. These additional solids not only detract from ethanol yield but also increase dryhouse duty and associated energy consumption. Therefore, optimisation of dryhouse operations is greatly dependent on nondryhouse processes.

Conclusion

The dryhouses found in today's dry-grind fuel ethanol facilities are not significantly different from those found in early whisky distilleries. Decanter centrifuges, thin-stillage evaporators and thermal dryers continue to be widely used to convert whole stillage to DDGS. Recent growth in grain-based fuel ethanol production has focused more attention on the industry. Opportunities are being explored to improve production economics, focus research activities and develop traditionally undervalued stillage streams.

These efforts include the application of dry-grind corn fractionation technologies that recover higher value protein and lipid-rich concentrates in advance of the ethanol process. This reduces mass flow through the ethanol process as well as solids loading on dryhouse operations. In addition, groups continue to research stillage-refining technologies that promise to recover and concentrate higher-value constituents of whole stillage such as glycerol, organic acids, amino acids, peptides, proteins and lipids.

The application of these new technologies, individually or in various combinations, will change the technical definition of DDGS as well as the unit operations and processes used in their production. As the industry continues to grow, the dryhouses of the past will not be easy to find in the ethanol production facilities of the future.

References

• Anonymous (2008). College of Food, Agricultural and Natural Resource Sciences, University of Minnesota, St. Paul, MN.
• Belyea RL, Rausch KD and Tumbleson ME (2004) Composition of corn and distillers dried grains with solubles from dry grind ethanol processing. Bioresource Technology 94 293-298.
• Croll Reynolds (2008) Process Vacuum and Power Systems Thermocompressor Theory. http://www.croll.com/_website/pr/thermhome.asp (accessed April 2008).
• Green DW and Perry RH (2007) Perry's Engineering Handbook 8th ed. McGraw-Hill, New York, NY.
• Minton PE (1986) Handbook of Evaporation Technology. Noyes Publications, USA.
• Rausch KD and Belyea RL (2005) Coproducts from bioprocessing of corn. ASAE Annual International Mtg., Tampa, Fl. July 17-20. Paper No. 057041.
• Shurman J (2008) Distillers Grains By-products in Livestock and Poultry Feeds. University of Minnesota, Department of Animal Science. http://www.ddgs.umn.edu/profiles.htm (accessed 10 April 2008).
• Woon-Feung Leung Wallace (1998) Industrial Centrifugation Technology 1st ed. McGraw-Hill, New York, NY.

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