BLEACHING EARTHS
Filtration is an important step in the bleaching process of edible oils which depends on factors such as the oil being processed, bleaching temperature, the amount of impurities in the oil and the type of bleaching earths and filtration aids used Rick Veldkamp Filtration is an important step in the bleaching process of edible oils and is an intensive operation in terms of manual activity and constant surveillance by operators. The length of the filtration cycle is critical and is dependant on the oil being processed, the bleaching earth used, bleaching temperature, and the amount of soaps, gums and moisture in the oil. The frequency of filter changes affects the cost of bleaching, with a longer filter cycle lowering the cost. A pre-coating agent will extend filtration cycles and reduces stress on filter screens, minimising their chemical cleaning. Filter aids also allow less frequent changes of polishing filters and can enhance filter cake release.
Filter selection and size
Selecting and sizing filters is based on the bleaching process, the solids that have 20 OFI – JUNE 2022
Bleaching earths filtration June 2022.indd 2
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Optimising filtration to be moved, and capital and operational expenditures. Feedstocks before filtration will contain certain pollutants including 0.1% H20. The water is evaporated during vacuum bleaching and other contaminants like phosphatides will be removed with the bleaching clay. In physically refined oil, the maximum phosphatide level should not exceed 25ppm. Caustic or chemically refined oil should be filtered with a maximum of 0.1% H2O, 25ppm soap and 5ppm phosphatides. Contaminant levels have a significant effect on filterability and even more on the frequency of washing filter leaves. In all cases, it is important that when choosing a filter type and supplier, a dedicated design that uses only a specific clay, adsorbent, filter aid or filter that makes the use of other products impossible should be avoided.
screen or cloth. Below 0.3% solids, a precoat or body feed is required. The flow rate ranges from 0.02-2.5m3/m2/h. Polishing/police filters are safety filters which inform the processor that the main filtration is working correctly. They can have disposable elements such as bags or cartridges for applications with around 1-100ppm solids and work most economically at <25ppm solids. There are good reasons to avoid or minimise the use of disposable elements as replacing bags, cartridges or paper sheets requires the filter to be opened, which requires operator time to change them and can also result in health/quality issues. Bags and catridges will also be soaked in oil, resulting in oil loss. The answer are pulse type filters using filter cloths as the filter media, which can be retained for 12-24 months.
Filtration options
In the bleaching process of edible, vegetable, animal and fish oils, there has been a continuous quest for improved performance, bigger filters and higher yields. In the early stages of oil bleaching, such as Hercules filtration in 1939, the industry was already focused on lowering bleaching clay consumption or obtaining better yields from clays. Refining and removal of the substances that caused high clay consumption were the first improvements and major steps were also made with improved designs of centrifuges used in refining. The relation between the presence of factors – such as phosphatides, soap,
Filtration options can be divided into continuous filters (centrifuges and decanters, vacuum belt filters, vacuum drum filters, continuous settlers, hydro cyclones and belt presses) (see Figure 1, p22) and discontinuous filters (sand and media filters, filter presses, cartridge, bag and other disposable element filters, pressure leaf/tube type filters and single plate Nutsch filters) (see Figure 2, p22). Process filters (pressure leaf or tube filters) (see Figure 3, p22) can be used for applications ranging from 0.001-5% solids, up to a maximum of 15%. With 0.3% solids or higher, oil can be filtered directly on
Choice of bleaching systems
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13/06/2022 15:13:20
