In bio-ethanol production, a large portion of a plant’s energy demand is consumed at the rotary dryers where moisture content of wet distiller grains with solubles (WDGS) is reduced to values of around 10%. Using fluidized bed Multi-Effect Evaporators (MEE), instead of conventional forced recirculation evaporators, prior to these dryers allows one to significantly reduce the moisture content of the feed. Hence the energy demand of the dryers and the plant can be drastically reduced.
Stillage is a large by-product of the distillation step in bio-ethanol production. Stillage is the heaviest product of the distillation process. It is a solution rich in carbon sources like glycerol, glucose and maltose containing up to 15% solids. From the bottom of the distiller, stillage is sent to centrifuges which separate the coarse grains from the solubles. The solubles, called thin stillage, that come out of the centrifuge are sent through a MEE where water is removed resulting in a 35% solids mixture called syrup.
Condensate removed through evaporation is treated by bio-methanation so it can be reused in the process. The coarse grains from the centrifuge are mixed with the syrup from the evaporators to form WDGS, which have a moisture content of over 60%. The WDGS are further dried in rotary drum dryers to reduce the moisture content to approximately 10%. The product is called dried distillers grains with solubles (DDGS) and it is sold as a feedstock for cattle. The energy used in drying is mainly from natural gas used to fire the dryer and is approximately 42% of all thermal energy consumed in the facility.
To a large extend, energy consumption of bio-ethanol plants could be reduced if the WDGS moisture content at the entrance of the dryer could be reduced below 60%. One way to achieve this is by further concentrating the syrup in the MEE. This is not common practice because the MME would suffer from severe fouling when concentrating above 35% solids.
If the MEE makes use of fluidized bed heat exchangers in the high concentration stages, syrup can be further concentrated without fouling problems in the MEE. The limits are still to be tested. However, Klaren International is confident that 50% concentration can be achieved. Assuming 50% concentration of the syrup at the exit of the MEE and a moisture content of the WDGS of 45% would be achieved. This would mean a load reduction of the dryer of 43%. The MEE energy consumption would increase but the savings in the dryer would be far over this increase.
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