American Crystal Sugar is a beet-sugar-processing agricultural cooperative operating five facilities in the Red River Valley along the border between Minnesota and North Dakota, as well as a sixth facility in Sidney, Montana.
Formed in 1973, the company plants across 500,000 acres, which yields 12 million tons of sugar beets annually. Its three-stage evaporative crystallization system is responsible for 13% of total U.S. production, making it the largest in the nation. “The U.S. sugar market is essentially closed,” explained Kevin Smith, American Crystal Sugar’s control system engineer in Crookston, Minnesota, who spoke at Emerson Exchange. “Only so much foreign import is allowed each year.”
Because of the closed market, American Crystal Sugar looks for processing efficiencies where it can. An automation project was implemented to regulate the concentration of sugar (Brix) in its feedstock for the third stage of its crystallization system, which contains the most impurities and least sugar of all the feedstocks.
American Crystal Sugar packages around 250 consumer products, including byproducts such as beet pulp and molasses. “The soil of the Red River Valley lends itself to agriculture,” said Smith. “In addition to fertile soil and good rainfall, it’s 17 °F in January. The low temperatures allow us to store those beets outside until springtime. Most manufacturers can only run factories from August to February. We can extend well into May.”
When American Crystal Sugar is running, it’s 24/7 continuous processing. “We do that up to 275 days/year,” explained Smith. “When not processing, we completely shut down and do maintenance and capital upgrades. We have 23 people per shift on four rotating shifts. We’re facing some operational challenges—increasing cost of labor and labor shortages in general.”
Another challenge comes in the form of crop variability. “We rely on natural irrigation,” explained Smith. Late planting because of cold temperatures combined with unique moisture conditions to yield a crop with high sugar content this year.
American Crystal Sugar’s processing begins well before the crop reaches the multi-stage evaporators. The sugar beets are stored outside and then brought on trucks that tip them into the factory, where they’re blasted with water and then sliced into cassettes using rotary cutters. “Then we pull out the sugar and beet pulp, and we remove smaller impurities,” said Smith. “We go through vacuum evaporation. We have three stages. The first one produces the white sugar we sell. The others remove more impurities.”
In the first stage, the sugar liquid is brought to its evaporation point. “We spin off the liquid from the crystal and then that liquid gets sent to a second stage,” said Smith. The second stage goes through more centrifuges to produce intermediate sugar. All of the sugar in the second and third stages get recycled back into the first stage.
“In the second stage, after we boil it up and separate the crystals, it’s a very high-density liquid,” explained Smith. “It’s hard to pump that up because it’s too thick. We’d have issues with pumping it. We added water to make it easier to pump, but that’s inefficient, and we would see inconsistencies in the Brix.”
The existing system consisted of a Rosemount 8705 magnetic flowmeter and an analog control valve. “We were relying on lab measurement one time an hour, which wasn’t frequent enough,” said Smith. Operators manually adjusted the flow of water added. “We’d run inefficiently for the sake of being able to run,” admitted Smith.
To refine the process with more accurate measurement, American Crystal Sugar investigated several alternatives, including a Coriolis meter and a fork densitometer, which were dismissed due to the viscous fluid and relatively high cost of instrumentation.
An overflow pipe offered a place to grab the fluid between the second and third stages to measure Brix. A Rosemount 3051AL level transmitter for pressure measurement was installed. “When we think density, we think of a Coriolis meter,” said Matt Sanders, account manager, Rosemount Measurement, Emerson.
Pressure transmitters have been used for hydrostatic tank gauging (HTG). In a vented tank, two pressure transmitters are mounted toward the bottom of the tank, and one is subtracted from the other for the differential pressure, which gives the density.
At American Crystal Sugar, the pressure transmitter was mounted on the overflow pipe, so only one was needed. The 3051AL was mounted to the 3-inch vertical pipe 114 inches below the overflow.
“Pressure is measured from the 3051 in inches of water and can be divided by the known value of 114 inches of water column in the pipe to determine density and specific gravity,” explained Smith. “Brix is then calculated from the specific gravity. It’s all just derived equations.”
The water addition control is automated, replacing the manual process, and is controlled to 82 degrees Brix, a pretty high density. “The Brix controller determines cascade setpoint for dilution water addition,” explained Smith. Because this occurred between the second and third stages, it was a low priority for operators.
The solution yielded an increase of about a 10th degree Brix compared to the previous three years realizing annual savings of $10,000 per year for 40% sugar recovery from molasses. The installation cost of $12,000, which was relatively low because the sample line and pressure transmitter were already available, meant a full payback in the second year.