Selection of the right process route for Full Pressing is based on several criteria, related to both the economics (CAPEX and OPEX analysis, products YIELDS) and the specificity of the products (CAKE and OIL).
The key market reason to select Cold/Hot Double stage full pressing is to optimize the cost/benefit of the operation by achieving the best combination of quality and quantity through a two steps extraction process. The cold pressing step will give a Cold Pressed oil with low content of gums and high level of natural antioxidant while the hot-pressing step will allow for further oil expelling and consequently an oil yield increase. This will also give a higher cake protein content. This process is recommended to maximize the overall product value when a premium selling price can be expected for cold pressed oil, typically when products are for both human and animal consumption.
Other selection criteria are:
Oil value: In first pressing step, rotating speed of the shaft will be reduced, resulting in a relatively low friction avoiding heating too much the pressed material. In addition, cooling of the cake can be performed by a press cooling system. These operating conditions will keep temperature from rising but, on the other hand, will limit the oil extraction rate and press capacity. In the second pressing stage, most of the extractable oil will be recovered thanks to the stronger pressing effect at higher temperature and controlled moisture.
Cake Value: As a result of second pressing, residual oil in cake is typically lower than 8% and protein content relatively high. Protein is partially denatured during the cooking and the second stage pressing does affect the PDI quite significantly, (however no more so than for solvent extracted meals). The meal will have an increased RUP value and will be well suited to ruminant feeds. If higher PDI meal is desired with lower residual oil, then consider Extrusion + Full Press.
OPEX Saving: due to two steps process sequence, and cooking stage, cold/hot double stage full pressing has a relatively high consumption of steam and electricity.. For lower utilities consumption, see Hot Single Stage Full Pressing.
CAPEX Saving: due to two steps process sequence, and need of steam, the Cold/Hot Double Stage Full Pressing requires a relatively high investment. For lower CAPEX, see Hot Single Stage Full Pressing.
The seed from storage may contain up to 2% foreign material. The purpose of the cleaning process is to remove as much of 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. For smaller and softer feed materials Milling may not be required ahead of the cold pre-pressing stage. When Milling is required, it may be achieved in a hammer mill, or cracking rolls. In some cases, the milling can be combined with dehulling system, generally aimed at increasing protein content on the press cake.
The Cake after prepressing should be broken or milled to increase the heat transfer rate in the subsequent cooking process. Depending on the feed material this Milling may be sufficient using a cake breaker built into the Pres Press but on certain feeds it may be achieved in a hammer mill, or cracking rolls.
First Pressing can also be considered a pre-pressing. Its purpose will be to recover the easily available oil while maintain the highest possible oil quality. Properly prepared material is fed continuously into the cooled screw press where cellular structure of the seed is severely distorted tearing open many of the cell walls. Roughly 50 to 60% of the seed oil content will be extracted while producing a cake with an oil content about 20%, ideal for downstream cooking and second pressing. DesmetBallestra offers a full range of ROSEDOWNS screw presses with specific arrangement for cold pressing, for all kind of seeds, all capacities.
Cooking is a thermal process which heats the press feed material to approximately 110ºC (230ºF) by indirect steam contact. It is aimed at both reducing the moisture down to 3% range and to increase the fluidization and availability of oil. Both parameters will guarantee the highest oil reclaim from the squeezing of the material into the press. DesmetBallestra offers full range of horizontal and vertical cookers design.
Second Pressing also called full pressing. The pre-pressed cake with a typical residual oil in cake about 20% is fed continuously into the screw press where the cellular structure of the seed is further distorted, tearing open many of the remaining cell walls. Most of the oil in the pre-pressed cake will be extracted while producing a full-pressed cake with both integrity of shape and porosity, ideal for downstream cake cooling. DesmetBallestra offers a full range of ROSEDOWNS screw presses with specific arrangement for full pressing, for all kind of seeds, all the capacities.
Cake cooling is requested to removing excess of heat from the cake coming out of the press. Cooling is achieved through convective and evaporative heat transfer from cake to air passing through the cake layer on a fully counter-current flow. After the Cake Cooler, the cooled cake has a temperature within the range of 20 to 40°C (68 - 104ºF), and a moisture contains below 3%.
Oil clarification is required as it is inevitable that there will be a level of residual solids in the extracted Press oils. In cold/hot press plant there will need to be separate oil clarification systems for the cold press oil and the second press oil as they will be of different qualities and likely be sold to different markets.
The process for both the hot and cold press oils will be very similar. The 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 large plants, by a centrifugal decanter.