The stillage from the distillation process contains nutrients, minerals and unfermented sugars, and in the case of cereal feedstocks, a high proportion of protein, fats and fibre. These substances are concentrated and are valuable as animal feed, providing an additional output product from the distillation process.
Processing options depends on the degree to which the value of the co-products can be increased. Wet spent grains have a limited shelf life and must be transported to animal feedlots within proximity of the distillery. DDGS (distiller's dried grains with solubles) can be handled, stored and transported to satisfy international demand from beef, dairy, pig and poultry feed markets.
Decanting. Decanting centrifuges mechanically dewater the spent grains carried in the stillage. Thin stillage, containing both dissolved and suspended solids, is produced as the centrate. Some of the thin stillage is recycled as backset to the fermentation process, where it reduces the pH of the mash and lowers the risk of infection, while providing nitrogen and nutrients that assist in the fermentation process. The remaining thin stillage, normally with a concentration ranging between 5 and 10% TS(total solids), is then concentrated in an evaporator to produce the so-called condensed distiller's solubles ingredient of DDGS.
Drying. The decanting centrifuges simultaneously produce a wet cake of spent grains, or WDG (wet distiller's grains), which is either sold as cattle feed or processed further in a dryer to produce the more valuable DDGS. Product quality has improved in recent years as producers recognize the benefit of advanced drying systems that limit protein denaturing and improve animal digestion rates. This is achieved by improved management of drying temperatures and residence times, and by limiting the fibre content (bran) in the product to increase its suitability for monogastric (non-ruminant) digestion.
Evaporation. Falling film evaporators are used to pre-concentrate the thin stillage. Depending on the energy situation, these can be heated using multiple effects, dryer exhaust vapour or by using an MVR (mechanical vapour recompression) system.
The integration of the energy streams between the dryer exhaust vapour and evaporator heating requirements is a feature of ZAVKOM Industries DDGS process plants. The dryers are designed to operate at optimal efficiencies in a closed loop, gas recycle mode. This limits the amount of dry air and raises the dew point temperature of the exhaust vapour. The energy in the exhaust can therefore be utilized as the heating source for the evaporation plant, recovering condensate as makeup water and reducing the vapour plume, while evaporating the thin stillage. Non-condensable and volatile organic gases are combusted in the process.
Condensate Recovery. Process condensates are recovered and recycled as process makeup water. Since the condensate streams contain unwanted organic substances and are slightly acidic, these are treated using combinations of biological and/or membrane separation plants. Up to 80% of the condensate streams are recovered in this manner.