Distilleries generate a lot of spent grain during the regular distillation process. At the end of the day, all those spent grains need to be disposed of in a safe and environmentally friendly way.
This particular distillery disposes of its grain slurry waste sustainably and economically by separating the grain from water using a centrifugal sifter. The grain is donated to a local farmer as livestock feed, while the water is treated before being safely piped into the public wastewater system.
Wet Stillage for Livestock Feed is Difficult to Transport
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The waste produced from the distillation process, called stillage or spent grains, is about 40 percent grain solids and 60 percent water by weight, says Greg Lowe, distiller. The distillery processes 3,000 lb (1363 kg) of grain weekly, which produces roughly 7,500 lb (3,409 kg) of stillage.
A local farmer previously picked up eight 300 gal (1136 liter) totes per week of the watery grain mixture, but at 940 lb (423 kg), the totes were difficult to transport, and required covers to prevent spillage. “This wet approach would not scale for higher production,” Lowe explains, so the grain needed to be separated from the water. To complicate matters, any water piped to the municipal wastewater treatment system needed to be free of solids content and have a pH in an acceptable range.Multiple Ways to Separate Grain from Water Evaluated
The stillage produced by some distilleries contains grain that remains intact, and can therefore be separated from water using a settling and rinsing process called “lautering.” However, the fine grain particles resulting from the grinding process employed by 8 Feathers precludes lautering as a means of separation.
Also ruled out was flocculation, which causes particles to clump using chemical additions that would render the dewatered grain unsafe as animal feed.
Mechanical separation emerged as the logical choice. Options included filter presses, screw presses, circular vibratory screeners and centrifugal screeners, Lowe said. “Initially, all mechanical options appeared as viable choices.”
“The filter press offered excellent performance but the cost of the press, air diaphragm pumps to fill the press, and a compressor to run the pumps put the total cost at more than double that of a screw press or centrifugal separator,” Lowe says. “Further, without adding some form of chemical solids coagulator into the flow, the filter cloths would require extensive cleaning after each use. Chemical use would also contaminate the recovered grains sent for feed and raise the cost of continuous operation to an unacceptable level.”
“We also considered screw presses, but smaller units that fit our budget and footprint were unavailable.”
“This narrowed our choices to mechanical sifter-type separators. Kason Corporation manufactures both circular vibratory and centrifugal screeners, so we provided the company with a 10 gal (38 l) sample of stillage for testing on both machine types in its laboratory.”
The circular vibratory unit proved unable to handle the 50 gal/min (189 l/min) infeed rate required to handle the stillage pump capacity, but the centrifugal screener handled the flow rate with capacity to spare.
This distillery purchased an “MO-SS” CENTRI-SIFTER model with a 140-mesh (105 micron) heavy duty reinforced stainless steel screen to provide effective separation and minimize the risk of screen blinding.
Installing the Sifter
Lowe says, “The results were as good as or better than anticipated. The sifter processed a 1500 gal (5678 l) test sample with 40 percent solids, fed at 40 gal/min (151 l/min) without any degradation of performance. The separated grain was at an acceptable level of dryness for transport. After the test run, the unit was opened to inspect the screen for evidence of blinding and none was found.”
Separating grain from water centrifugally
Installed 8.5 ft (2.6 m) above the plant floor, directly over the vat that receives the water, the unit measures 21 in. (533 mm) wide, 32.8 in. (832 mm) tall and 74.8 in. (1899 mm) long, including its 3 hp (2.2 kW) motor.
The water/grain mixture is fed by a lobe pump into the inlet at the upstream end of the centrifugal sifter, where a screw feeds the mixture into the open end of a horizontal screen cylinder. Rotating helical paddles propel the water/grain mixture against the screen, accelerating the passage of water through the screen and against the wall of the chamber which funnels the water by gravity through a 10 in. (254 mm) outlet into a 2000 gal (7570 l) polyethylene vat below. It is further treated with soda ash to adjust pH before finally flowing into the public wastewater system.
Grain solids unable to pass through screen apertures are propelled in a helical pattern through the screen cylinder until being discharged through its open end, into a chute that funnels the dewatered solids into a plastic tote on a pallet below. When a tote is full with about 2,500 lb (1,136 kg) of the damp grain, a pallet truck replaces it with an empty tote. The full totes are stored in a holding area to be picked-up by a farmer.
The screener separates 22 to 33 gal/min (83 to 125 l/min) of 40 percent solids (by weight) Lowe says. “For our solids content, the optimal rate is 20 to 25 gal/min (76 to 95 l/min).”
Cleaning and Maintenance
The centrifugal sifter’s end housing can be opened, allowing tool-free removal of the screen cylinder and paddle assembly from the cantilevered shaft for cleaning. However, operators simply backwash the sifter with water as it runs, cleaning the pipes and the screens at the same time. “It takes two or three minutes and has worked well, eliminating the need to climb a ladder and physically access the machine,” notes Lowe. He says no maintenance has been required, or reliability issues encountered. “We have not opened the door on that thing in six months,” he says.