05-05-2012, 10:39 AM
Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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Sugar production from sugar beets
This paper describes the plant stations in a beet sugar factory. It is intended to give you an overview of
the individual processes and interrelations in such a factory to allow for a better understanding of the
sugar production process and facilitate the selection of the appropriate process measurement and control
equipment. The nominal diameters of the measuring instruments and final control elements described
here have to be adapted accordingly to the actual capacity requirements of the respective sugar factory.
The examples detailed here are based on the assumption that the described factory has an average daily
throughput of 8000 t of sugar beets. Sugar beets are usually sowed in March or April and reach their maximum
weight in September or October. The sugar is a result of photosynthesis in the leaves of the plants.
The sugar content of a beet usually amounts to 16 to 18 % of its total weight. As sugar beets are perishable
agricultural products, sugar factories normally work in 4 shifts 24 hours a day to avoid considerable
sugar loss. The processing season ("campaign") ends before Christmas every year, depending on the
sugar beet harvest in the catchment area of the respective sugar factory.
In Germany, sugar beets are transported from the farm to the factory by truck or farm vehicle. The trucks/
vehicles first pass the weighing station, where the beets are weighed and tested automatically. The samples
are taken to determine the sugar content, degree of dirtiness, etc. Subsequently, the beets are
unloaded through stationary or mobile tipping units or by washing them off. The unloaded - or intermediately
piled - sugar beets are dumped into a wet hopper where water is used to transport them to the beet
washer. Here the beets undergo a separation from dirt and rocks sticking to them. The portion of dirt and
rocks varies between 10 % and 30 %, depending on the weather conditions. After washing, the beets are
transported on a belt conveyor into a hopper above the slicers. The slicers cut the beets into slices called
“cossettes” that are fed to the extraction tower and extracted with hot water of about 70 °C.
Juice production – Extraction
In the extraction station most of the sugar is washed out of the sliced beets. The produced raw juice
should be as pure, concentrated and processable as possible. The following section describes a juice
extractor with a cossette scalder using the reverse direction flow principle. After washing, the beets are
transported to the beet hopper above the slicers. This buffer provides for a constant supply of cossettes
to the juice extractor, even if the quantity of delivered beets varies. The beet hopper is hence an important
interface in the sugar process. The "beet hopper volume" is a decisive process variable for the beet conveyors
and washers as well as for all subsequent stations like the juice extractor station, juice purification
station, evaporator, and sugar house.
The most reliable and precise method for determining the "beet hopper volume" of a new plant is a measurement
using load cells. In an already existing plant the beet hopper volume can also be measured with
capacitive gauges. The cossette quantity is measured by a belt weigher in the cossette conveyor and
input to the slicer speed control unit (2) as the reference value. This control unit ensures that the specified
cossette quantity required for constant juice extraction is always met.
Juice purification
In the purification process (Figure 2-4) the non-sugars are to be removed from the raw juice to the greatest
possible extent. The result of this process considerably influences the total sugar yield and the sugar
quality. The means and methods used for juice purification must be as gentle as possible to avoid
unwanted destruction of the sugar.
Usually, burned lime is used as an auxiliary agent for purifying the raw juice. A by-product of lime production
is carbon dioxide (CO2). Adding carbon dioxide to the juice in the purification process removes
excess lime and, thus, improves the juice quality. Various level and temperature control loops are
required for continuous juice purification. They contribute to the required process control by ensuring the
appropriate response time, i.e. they provide for a constant work process even with different beet material
and a varying raw juice flowrate.