Southern Minnesota Beet Sugar Co-op Reduces Waste and Saves Over $500,000
Southern Minnesota Beet Sugar Cooperative (SMBSC) in Renville, Minnesota, produces several varieties of sugar from sugarbeets grown regionally. Juice is extracted from the beets and a lime solution is used to purify the juice. The juice is then evaporated and crystallized to form sugar. The cooperative processed approximately 10,000 tons of beets per day.
Through this intern project, the facility reduced lime use and waste by 20-25%, or 22,000 tons a year and saved $500,000 to $1,000,000 a year in lime purchases and lime waste.
Incentives for Change
Each year SMBSC generated approximately 98,700 tons of lime waste which it stockpiled in a holding pond. The lime waste had potential to contaminate storm water discharges. With changes in environmental rules, the lime waste became regulated as a process waste stream and needed a greater level of management under the U.S. Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) requirements.
To meet the requirements, SMBSC covered the storage ponds with sod. Lime waste that is now generated is taken by a farmer for use as an amendment to adjust soil pH.
SMBSC decided to decrease the overall volume of lime waste it needed to manage. A MnTAP intern worked with the co-op in summer 1997 on this task. At the time of the project, the co-op was processing 61 percent more beets than the system’s purification and filtration stage was designed to handle.
Because limestone becomes waste at a rate almost equal to the volume used, the key to reducing waste was to use less lime in the process. The intern investigated the two operations related to lime: 1) the lime kilns, and 2) the purification and filtration process.
The co-op uses limestone to create the lime (calcium oxide) in its “milk of lime” solution. SMBSC had two kilns in which limestone was heated with coal and converted to lime and carbon dioxide. The intern evaluated how to improve the kiln’s conversion efficiency by maximizing the distribution of coal throughout the limestone in the kiln using mechanical means.
After further investigation, SMBSC made adjustments to the air-to-fuel ratio to improve the limestone to lime conversion. Also, an improved control system was added to achieve optimum operation.
Purification and Filtration Stage
In the first stage of purification, the beet juice passes through three reaction tanks of the liming system. Milk of lime is added to each tank in the series to chemically react with the different soluble non-sugars in the raw beet juice. The juice then moves to the two carbonation tanks where it is recarbonated to form a calcium carbonate precipitate, allowing the impurities to be filtered out.
Adjusted Alkalinity Targets
Employees measured the alkalinity of the three liming tanks to determine how much lime to add. SMBSC was using the alkalinity levels set when its system was designed in 1988. But to process the higher volume of beets, more water was being used to extract sugar out of the beets. This higher volume of water diluted the tanks’ alkalinity, indicating the need for more lime.
The company changed equipment to enable it to process beets using less water. A more concentrated juice improved reaction rate, increased settling rates and lowered the filtration rate. The intern ran calculations to determine what new alkalinity targets/lime addition would provide the optimum lime to sugar ratio. Changing the alkalinity targets reduced lime use and waste by approximately four tons per day, saving $100,000 each year.
Increase Size of Liming and Carbonation Tanks
The co-op had gradually outgrown the capacity of its original liming and carbonation tanks used for purifying the beet juice. In 1997, SMBSC was processing an average of 9,200 tons of beets per day. The original liming and carbonation tanks were constructed for 8,820 and 6,500 tons per day, respectively.
To process the increased volume of beets, the co-op shortened the residence time in the tanks of the purification and filtration step. The beets had less time to react with the lime. To compensate, a greater volume of lime was added to ensure reaction and precipitation of the non-sugars.
The intern investigated ways to improve the reaction and precipitation of non-sugars. The intern suggested replacing the two liming tanks and two of the three carbonation tanks with greater capacity tanks-ones that could handle 14,500 tons of beets per day-to manage the higher volume of processing. However, improvements in pre-liming reduced the hydraulic load-increasing juice concentration. This allowed retention time in the tanks to increase, reducing the need for excess lime.
The co-op will install the larger tanks in the carbonation system after 2000, during its planned processing capacity expansion. These new tanks will reduce the use of lime by 56 to 60 tons of lime waste per day. SMBSC is saving $443,000 to $604,000 per year between reduced lime purchases and reduced costs for managing the lime waste. The tanks cost approximately $488,000, giving a payback period of under two years.
Optimizing alkalinity in the pre-limer and reducing the hydraulic load, as well as educating employees, reduced lime use and increased processing capacity without incurring capital costs. Increasing the size of the purification tanks allowed the co-op to further increase its processing capacity.
By evaluating its process, the co-op was better positioned for its expansion. As a result of the intern project at Southern Minnesota Beet Sugar Cooperative lime use and waste was reduced by 20 to 25 percent, or 22,000 tons a year. The co-op is saving over $500,000 dollars a year.