In the modern technology of vegetable oil production, pressing as a method of extracting oil from seeds often precedes the final degreasing of the material with an organic solvent – extraction. Purely pressed oil is used only in relatively small volumes.
At the beginning of squeezing the oil from the pulp, the material is compacted, the particles of the pulp come together and the surface layers of the oil come into contact. The thickness of the oil layers increases, and due to the continuous increase in the thickness of the layer, the oil is no longer held by the surface forces on the particles, especially since the total surface of the particles is constantly decreasing, and it begins to be released in a free state.
With further compaction of the pulp, its particles are deformed and oil begins to flow not only from the surface, but also from the internal capillaries of the deformed particles. A further increase in pressure on the materials leads to fusion of the particles. This is facilitated by an increase in the temperature of the material due to the conversion of part of the mechanical energy into thermal energy. As a result of the increase in pressure and temperature, the plasticity of the pulp increases significantly, the pulp ceases to perceive pressure, and oil extraction practically stops, although some amount of oil still remains inside the material.
The screw press consists of a collapsible stepped cylinder, the walls of which are made of steel plates placed in the frame of the cylinder so that there are narrow gaps between the plates for the exit of pressed oil, and a screw shaft composed of separate helical links separated by intermediate cylindrical or conical rings.
In the place of the connector, two sets of shaped knives are placed, which are necessary to prevent the material from turning with the screw shaft and to clean the rings and screws from the pulp that adheres.
The pulp enters through the feeder into the stepped cylinder, the so-called zeer (seeder), or zeer drum, is captured there by the turns of the screw shaft and moves to the exit from the press. The peculiarity of the screw press is the continuous reduction of the conveying capacity (productivity) of the screw shaft from the point of entry of pulp into the press and its exit from the press. This is achieved in several ways, primarily by reducing the pitch of the turns, the screw shaft and the clearance between the screw shaft body and the inner surface of the press drum before exiting the press.
Therefore, the squeezing of oil from the pulp in the screw press occurs as a result of its constant compaction due to the reduction of the productivity of the screw shaft in the direction of the exit from the press and the continuous reduction of the free volume inside the Zeer cylinder for the passage of the pulp through the press.
In order to increase the pressure on the pressed material, a regulating device (cone, diaphragm, rings) is installed at the exit of the press, which allows changing the width of the output annular opening and, accordingly, the thickness of the output cake.
In the presses designed to produce cakes of a given shape and size, the device at the exit from the press is a matrix – a die with holes of different sizes and shapes. This allows you to get a granular cake.
The value of the maximum pressure developed by the screw press depends on the physical and mechanical properties of the pulp created during wet-heat treatment, its humidity and temperature. Maximum extraction of oil in the press is possible only with a specific combination of temperature, humidity and the resulting plastic properties of the prepared pulp for this type of press.
The maximum pressure developed by screw presses is 25-30 MPa. The degree of pulp compression – the ratio of the volume of pulp entering the press to the volume of material leaving the press is in the range of 2.8-4.4, the duration of pressing in different types of presses is 78-225 seconds. The temperature of the pulp during pressing increases by 15-30 °C compared to the temperature of the pulp entering the press.
Deep removal presses work with much lower productivity (18-30 t/day also in terms of sunflower seeds), but the oiliness of the cake in them is 4-7%. This is due to the long stay of the material in the press (220-225 s) due to the slow rotation of the shaft (5-18 revolutions per minute) and the small width of the exit annular gap for the cake – the thickness of the shell coming out of the press is within 3-5 mm.
In the oil and fat industry, the M-68 forepress is used for preliminary squeezing of oil from the pulp of oilseeds. The seer drum of the press is four-stage, and consists of two hinged and integrally cast halves.
A screw shaft with nine turns passes inside the zeer drum. The shaft rotation frequency (18, 24 or 37 rpm) is regulated by a three-speed electric drive motor.
In addition to the MP-68 press, the KhSP-26 and ETP-20.1 screw presses, as well as the G-24 granulator presses, are used in the oil and fat industry.
The first two are intended for preliminary squeezing of oil from the pulp.
The screw press-granulator G-24 is designed for squeezing oil from the pulp of oilseeds with the simultaneous production of strictly calibrated cake pellets of a given shape. The main difference of the press-granulator G-24 from the already considered screw presses is that instead of a conical device that regulates the thickness of the cake at the exit from the press, a device for forming cake pellets is provided. This device contains removable matrix filters with holes of various shapes and sizes. To feed the pulp to the matrix, a three-way unloading loop is installed on the press shaft.
In recent years, RZ-MOA-10 screw presses have been used in the oil and fat industry, which should replace other types of presses. They are produced by machine-building enterprises as part of the RZ-MOA oil pressing unit.
The RZ-MOA unit consists of a seven-chamber roaster, a screw press with a feeder, a crane with an electric hoist, which is used for disassembling and assembling the press’s press cylinder. The unit is equipped with automatic pulp level adjustment in the upper vat of the roaster. The cake coming out of the press can have a shell or granule structure. The rotation frequency of the screw shaft of the press is 42-72 min-1, the productivity of the press is 300 t/day in terms of sunflower seeds, the oiliness of the cake is 14-16%. The unit’s semi-chamber roaster consists of vats with a diameter of 3000 mm, the total heating surface of the roaster is 80 m2.
