Process selection criteria
Selection of the right process route for Full Pressing is based on several criteria, related to both the economics (CAPEX and OPEX analysis, product YIELDS) and the specificity of the products (CAKE and OIL).
The key market reason to select dry Extrusion + full pressing is to maximize oil yield and produce a cake with a high PDI value. A Dry Extruder is used to perform a short duration but intense thermal treatment of the seeds prior to pressing. It is very commonly used for Soya processing, however the extrusion pressing process can be adapted for other seed types. The extruder consists in a barrel with central shaft and high shear screw. The material enters from one end into the barrel and is forced by the screw through a die at the other end. The pressure generated by the screw on the material creates shear and friction forces ending in tremendous release of heat that brings temperature up to 150-160°C in a matter of seconds. As the extrudate leaves the extruder, steam is flashed off from the product helping to reduce the moisture of the material fed to the press. That unique combination between high temperature and low residence time makes the extrusion the ideal trade-off to perform substantial reduction of anti-nutritional factors (when soybean for instance) while preserving protein digestibility.
Other selection criteria are:
Oil Yield: due to extrusion thermal treatment, material to be pressed is soft and liposome membrane are easily broken, in the press hence releasing a high amount of oil. This is the reason for the high oil yield with a typical value of 6% residual oil in cake for soybean. For applications in which a low PDI meal is required, Hot Double Stage Full Pressing could be considered.
Oil Value: due to the very short time of the heat treatment, most nutritional components of the oil are preserved from degrading, and oil value is relatively high compared to conventional hot-pressing oil. However, higher nutritional or commercial value can be achieved with Cold Single Stage Full Pressing.
Cake PDI: Thanks to a reduced heat load (product of temperature by time of exposure) compared to Hot Single Stage full pressing, the resulting PID after Extrusion + pressing is still very good. Higher PDI can only be achieved by Cold Single Stage Full Press.
OPEX Saving: The Extrusion pressing process produces specialist products with high commercial values, however the extruder has a high operating cost due to a high electrical power consumption and a high level of spare parts replacement. On the other hand, dry extrusion does not require steam, making it attractive for plant sites without boiler facility.
CAPEX Saving: A dry Extruder is not such an expensive technology, and in many cases, it will replace both the milling and the cooking, making it a more affordable process line compared to Hot pressing. Moreover, the cost of installation is also lower because of its simpler design (no need of steam) and the fact that a screw press can typically be operated at higher speed and throughput than in conventional pressing applications. If steam is available at good condition and the electrical power supply is expensive then Hot Single Stage Full pressing can be considered as an alternative.
The seed from storage may contain up to 2% foreign material. The purpose of the cleaning process is to remove as much of the foreign material as possible. An optional weighing process can be foreseen to get an accurate inventory of the material being sent through the seed preparation process.
Milling is aimed at reducing inlet raw material into smaller size particles. It is an optional process step to reduce the wear rate in the subsequent Extrusion process. Milling can be achieved in a hammer mill, or cracking rolls.
In some cases, the cracking rolls milling can be combined with dehulling system, generally aimed at increasing protein content on the press cake.
Extrusion is a continuous mechanical process creating high pressure and friction in the inlet material (often soybean), resulting in quick and sharp temperature increase, up to 150ºC (300F). At the discharge of the extruder pressure is released and moisture flashes off, this can reduce moisture in the feed down to 3%. Extrusion can be seen as replacement for flaking and cooking steps. Extrusion is aimed at turning hard and harsh cracks/beans into soft and easy to squeeze material in pressing step. The dry extrusion process is much more efficient and simpler than other hot full pressing processes because no steam is required to heat up the material.
Full Pressing is the heart of the process. Properly extruded material is fed continuously into the screw press where the cellular structure of the seed is severely distorted, tearing open the cell walls and releasing the oil. The high work already done by the extruder means press throughputs are typically higher than in other full pressing applications. Most of the oil will be extracted while producing a discharge cake with both integrity of shape and porosity, ideal for downstream cake cooling. Desmet Ballestra offers a full range of ROSEDOWNS screw presses for all kind of seeds, at all capacities.
Cake cooling is required to remove the heat from the cake coming out from the press. It is achieved through convective and evaporative cooling via fully counter-current air flow through the cake layer. After the Cake Cooler, the cooled cake has a temperature within the range of 20 to 40°C (68 - 104F), and a moisture contains below 3%.
Oil clarification is required as it is inevitable there will be a level of residual solids in the extracted Press oil. Clarification is a two-step process that separates solid matter from the press oil. The first separation step removes most coarse solids by screening or settling. Then, a second separation step removes the small fines and impurities from the oil by filtration using a hermetic leaf filter or, in larger plants, by a centrifugal decanter.