| intern summary |
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Southern Minnesota Beet Sugar Co-op
Reduces Waste and Saves Over $500,000
Optimizing Beet
Sugar Processing Reduces Lime Use by 20 to 25%
| Company |
Southern Minnesota
Beet Sugar Cooperative
Renville, Minnesota |
| Results |
Reduced lime use
and waste by 20 to 25 percent, or 22,000 tons a
year. Saved $500,000 to $1,000,000 a year in lime
purchases and lime waste. |
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Process Background
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.
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.
Kiln Operation
Lime kilns. 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.
Results. 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.
Results. 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.
Results. 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.
Overall Results
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.
More Information
MnTAP has a variety of technical assistance services available
to help Minnesota companies reduce and manage their industrial
waste. If you would like assistance or more information about
MnTAP's Intern Program, call 612.624.1300 or 800.247.0015
from greater Minnesota.
This project was conducted in summer 1997
by MnTAP intern Cathleen von Lehe, a chemical engineering
senior at the University of Minnesota.
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