Ethanol producers are under constant pressure to improve performance as the price of feedstocks and energy continues to increase. Traditional corn-based ethanol plants use fermentation to produce ethanol and carbon dioxide (CO₂) from ground corn, which results in the production of unfermented residuals (the parts of the kernel that are not converted to ethanol and CO₂).

Unfermented residuals usually are separated into two streams, consisting of wet cake (distillers grain) and syrup (soluble), by means of a centrifuge. The syrup is concentrated in multi-effect evaporators to approximately 35 percent solids and combined with the wet cake. The wet cake is dried to produce dried distillers grain (DDG), or dried in combination with the syrup to produce dried distillers grain with solubles (DDGS). These products are used for high-protein animal feeds and provide a critical income stream for the plants. However, drying of the wet cake and syrup is the largest energy consumer in a typical corn-based ethanol plant.

Rotary drum drying has been the primary drying technology used in the dry corn ethanol industry. The method has proven reliable and robust enough to handle process upset conditions. Modified flash dryers (ring dryers) and steam-tube rotary dryers have been used to a lesser extent. This article will focus on the use of rotary drum dryers to provide an economical drying solution and how including a counterflow rotary cooler, regardless of dryer style, can both reduce energy consumption and improve product quality.

Rotary Dryer Basics

Enlarge this picture In a typical 55 MGPY ethanol plant, the DDGS drying and cooling system might be laid out similar to this schematic.

The workhorses for many industrial applications, direct-fired rotary dryers and rotary coolers have been around for more than 100 years. When designed properly, these heavy-duty units typically can run continuously for more than 30 years while requiring minimal maintenance.

A typical rotary dryer/cooler system for DDGS consists of the following major components:

• Mixer.
  
• Dryer with air system.
  
• Dry recycle system.
  
• Cooler with air system.

The mixer is located prior to the dryer and used to condition the wet cake and syrup -- both of which usually contain about 65 percent moisture -- with dried, recycled material. The resulting mixture has about 30 percent moisture and is less prone to sticking and buildup in the dryer.

The mixed materials feed directly into the rotary dryer. Retention time and temperature in the dryer are controlled to provide a dried product at the desired moisture content (usually 10 percent to 12 percent). A substantial portion of the dried DDGS is recycled back to the mixer to condition the incoming wet cake and syrup. The remainder is sent to the cooler.

Cooling of the dried DDGS is required to improve product-handling characteristics and reduce the potential for mold in the stored, finished product. Co-current-flow coolers are used at most dry mill ethanol facilities. In most dry corn ethanol plants, these systems are not capable of providing the necessary cooling. A few later-generation plants have installed fluid bed (crossflow) coolers. Also, a counterflow-style cooler is now on the market.

The rotary dryer is direct fired, which means the hot gas comes in direct contact with the material, and is co-current, meaning the direction of flow of the hot gases and material are the same. Wet DDGS enters the inlet of the dryer where spiral feed-flights advance the DDGS into the lifting zone. In the lifting zone, specially designed lifters (or flights) lift and cascade the DDGS in the hot gas stream.

The air heater of the rotary dryer typically consists of a refractory-lined combustion chamber, burner, combustion air fan and dilution air fan. The combustion chamber completely encloses the flame so that it does not come in direct contact with the DDGS. The fuel source can be natural gas, fuel oil or biomass. The burner system is controlled by the discharge gas temperature. The dilution airstream is made up of recycled dryer exhaust gases and fresh air.

The entire dryer exhaust gas stream and entrained solids are filtered in a cyclone with a portion of the stream recycling back to the combustion chamber and the remaining portion is directed to a regenerative thermal oxidizer to handle the VOCs. The use of high recycle rates for the exhaust gases is critical for improved dryer efficiency. By raising the wet-bulb discharge temperature of the exhaust gas, the mass of air necessary to exhaust the evaporated water is reduced. The dryer efficiency (BTUs required per pound of water evaporated) is directly related to wet-bulb temperature of the exhaust gas.