Features:

The hunt for cleaner, more successful drying

25th September, 2007

When producing ethanol from grain a byproduct is left behind, stillage. This can be further processed to produce animal feed additives, but in order for it to be used for this application it must be dried. The first step is spinning the stillage in a centrifuge to increase its solids content to 30-35%. The liquid and dissolved solids spun out during centrifuging form a thin stillage that is reduced by an evaporator system to a syrup. Next, the centrifuge solids and the syrup are recombined and mixed with enough recycled final dry product to achieve a blend with 65% to 75% solids. This blend is fed to a drying system, which yields a final dry product called distiller’s dried grain with solubles (DDGS), which ethanol plants can sell for use as a high protein animal feed additive.

The drying process is one of the most energy intensive in an ethanol plant. Having an energy optimised drying system that is integrated into the overall production process can reduce this energy consumption significantly.

Conventional drying systems

Most of the conventional drying systems are direct-heated rotary drum dryers. Hot, dry flue gas from a natural-gas-fired furnace is in direct contact with the product to be dried in the rotary drum. High temperatures cause the particles to burn and increase the gasification of organic compounds.

The exhaust gas from the dryer has to pass further process steps to remove entrained particles and air pollutants such as VOCs. This end-of-pipe solution such as a scrubber, filter or regenerative thermal oxidiser, adds another layer of maintenance and fuel cost. To reduce the exhaust gas flow and the heat consumption a part of the hot exhaust gas can be recycled back to the furnace.

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