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De Smet has supplied over 180 preparation plants. Purpose  The purpose of preparation is to modify the seed into a shape that is suitable for fast and effective extraction by mechanical pressing or by solvent extraction. It is a succession of several of the following processes: seed Dehulling, to increase the protein content of the de-oiled meal, Cracking, to reduce the particle size of the seeds, Cooking, to reduce the viscosity of the oil and adjust the moisture content, Flaking, to rupture the cell structure of the broken seeds, and sometimes Expanding, to enhance extraction parameters. [Top] Home page] Dehulling Dehulling (decorticating), or partial dehulling, is an additional and sometimes complex process. Its purpose is to increase the protein content and reduce the fibre content of the extracted meal. In this case, the decision to decorticate or not is dictated by the local market, which offers or not a premium for a higher percentage of proteins. Sunflower and soya are typical seeds for which  the decision to dehull or not is based on the market. In the case of cottonseed, partial delinting and dehulling at least, is required by the process. Rape or canola seed is almost never dehulled.Hulls are lighter than the kernel or meat. Dehulling is generally a succession of unit operations: impacting or cracking, screening, and gravity separation through multiple aspirators. Several steps are often applied, both to enhance the protein and fibre levels of the meal, and for reducing the amount of oil lost with the hulls. The hulls are generally sold as fibre complement, to feed cattle, or burnt as fuel. [Top] Home page] Cracking  Seeds are cracked to reduce the particle size to ensure uniform cooking. Cracking is achieved on corrugated roll stands. Cracking mills with one or two pairs of rolls are used. Number of rolls and size of corrugations depend on the average dimension of the seeds. Today's unit capacities reach up to 1000 tons per day. Hammermills are used for certain raw materials like palm kernel. [Top] Home page] Cooking - Conditioning  During cooking, the seeds are heated and conditioned to a desired moisture. The main objectives of the process are: 1. Give plasticity to the seeds or meats. 2. Decrease the viscosity of the oil. 3. Coagulate the proteins. 4. Sterilise the seeds. 5. Detoxify certain undesirable seed constituents. Multi-stage vertical stack cookers or large, single stage, rotary horizontal conditioners are used. In either case the heating medium is invariably steam at 6-10 bar gauge pressure. The seed is usually heated to 90-110°C for pressing or 65-72°C for direct extraction. [Top] Home page] Flaking  All seeds must be flaked. Oil extraction is aided by passing the seed between large diameter smooth rolls to produce flakes of, typically, 0.3 to 0.4mm thickness. The flakes are produced by friction between the rolls that turn at different speeds. The thickness of the flakes is controlled by adjusting the pressure of the cylinders. The pressure is adjusted hydraulically and automatically. Today's unit capacities reach up to 350 tons per day. Larger machines are being tested. [Top] Home page] Expanding Some oil millers think of expanding as part of the usual configuration to prepare seeds.  Expanders are however found along different lines and their use varies considerably from country to country. An expander consists of a simple screw rotating in a barrel. The addition of steam creates the desired expansion effect, The screw is sometimes equipped with a drainage cage to recover a portion of expelled oil when high-oil content seeds are processed. The basic advantage of expanding, used in combination with some other conventional preparatory equipment, is to render the conduct of the preparation less critical to good extraction, by enhancing the extractability of the seed. Steam and hexane consumption are lower at extraction level, but power and steam consumption are higher in the preparation. Expanders are often the simplest and most economical way to increase the capacity of deep bed extractors. Expanders are sometimes used in combination with expellers in pre-press solvent extraction operations.[Top] Home page]  Hulls & Meal grinding Hulls separated in the dehulling process and de-oiled meal from the Desolventiser-toaster-dryer-cooler are often ground to a granulometry demanded by the market. Hammermills are used for this purpose. In large plants, meal grinding is combined with sifting. In some countries, hulls and/or meal are pelletised. Pelletising increases the bulk density and reduces transportation costs.[Top] Home page] |