Introduction
If you're involved in the edible oil extraction business, you will generally have three criteria to measure your plant's productivity: high-quality oil which has minimal undesirable components, high oil extraction yields, and high-value meal which can be redirected to animal feed and other industries.
There are many ways to achieve these objectives, the most popular routes being mechanical extraction and solvent extraction. Both of these processes have their pros and cons. For instance, mechanical extraction techniques cannot recover as much oil as solvent extraction techniques can. On the other hand, the low capital cost of expeller presses makes mechanical extraction far more accessible than its chemical counterpart.
Regardless of which approach you take to extraction, you can maximise your edible oil plant's productivity by investing in a continuous extractor. Whether it is for mechanical pressing or solvent-based separation - in fact, even if you're looking to enhance the productivity of the alkali refining or deodorisation stages of oil refining - continuous extraction apparatus may be just what your plant needs to eliminate bottlenecks, speed up oilseed processing, and increase extraction yields.
Batch processes vs continuous processes
The evolution from batch processing to continuous processing has taken years.
In the 1800s, small batch-type oil extraction operations were the norm. In batch extraction, oil is extracted from one batch of seeds completely before the vessel is cleaned out completely and the process begins again for the next batch. In this approach, the same tank or vessel is used to extract oil and desolventise the meal.
Today, specialty oil extraction, high-value oil extraction, and oil recovery from spent bleaching clay are carried out in batch-type systems. These batch extractors usually comprise either a rotating drum or a vertical cylindrical kettle with a vapour-tight cover. Even though batch extractors are still in use today, they are nowhere close to as popular as continuous extractors, especially in large-scale industrial operations.
Large-scale continuous extractors began to be used in the early 1920s. In the continuous solvent extraction method used today, the extractors are generally of either immersion or percolation types. In both cases, continuous extraction involves oilseeds being continuously fed into the extraction apparatus, with the machine extracting oil from the seeds for as long as it is on.
Using a continuous extractor for mechanical extraction
Mechanical extraction is an age-old, fairly straightforward approach to extracting oil from oilseeds, fruit, or other oil-bearing plant material. It involves applying pressure to the oil-bearing material to expel the oil from it.
A century ago, hydraulic presses (which use pistons to apply pressure to the oilseeds) were common. Seeds packed in filter bags were loaded into the extraction equipment, with the exerted pressure removing oil from the seed and allowing it to flow through the filter bag.
Over time, the filter bags were replaced by cages. The hydraulic presses were replaced by screw presses. And the labour-intensiveness of older, manual processes was replaced with more automation.
Today, hydraulic pressing is still an option for anyone in the oil extraction business. It offers the advantage of easier maintenance and additional pressure on the oilseeds to extract that last bit of oil. But if you're committed to high processing capacities, high oil yields, and lesser downtime, screw presses a.k.a. continuous extractors in the mechanical extraction world, are the way to go. Let us tell you why.
Hydraulic press vs screw press.
Hydraulic presses use a piston to apply pressure to the oilseeds, while screw presses use a rotating screw. This screw is positioned within a barrel, and as the seeds are fed into the barrel, the rotating helical screw presses the oil from the seeds and forces it through a filter.
What's the difference between the two? Screw presses offer higher oil yields compared to hydraulic pressing. Screw presses can also process larger quantities of oilseeds, partly because, unlike hydraulic presses, they allow for continuous processing. So when we speak of continuous extractors in mechanical extraction, we are talking about screw presses.
Screw presses have some drawbacks - difficulty in maintenance and faster wear-and-tear because of the moving parts, high pressure, and high temperature. However, improvements in this continuous extraction apparatus have helped increase the life of the parts that wear out most quickly. Larger screw presses also mean that this continuous extraction apparatus can now have processing capacities as high as 100 tonnes per day for full pressing and 800 tonnes and more daily for pre-pressing.
Given these advances, there is no reason for large-scale operations to rely on manual feeding and cleaning after each batch of oil extraction. Continuous extraction using screw presses is the best way to maximise mechanical extraction efficiency.
Continuous extraction in solvent extraction
Even when you go the solvent extraction route, you can leverage all the benefits of continuous extraction.
Solvent extraction involves using a solvent, most commonly hexane, to preferentially dissolve the oil in the oilseed. Subsequently, the oil-solvent mixture is separated from the oilcake. Then both, the oil-solvent miscella and the solvent-soaked cake must be desolventised, yielding two safe products - the crude oil and the meal.
The efficiency of solvent extraction depends on multiple factors. How the oilseeds are prepared, temperature, equipment design, and of course, mode of operation i.e. batch or continuous processing. In general, the industrial standard is to have less than 1% residual oil in the meal after commercial solvent extraction.
The continuous solvent extraction method has been shown to yield better results compared to batch solvent extraction. As we've mentioned earlier, continuous extraction in solvent extraction can be carried out either using immersion-type continuous extractors or percolation-type continuous extractors.
In the immersion setup, small-sized, flaked oilseeds are submerged in a solvent like hexane. While this helps achieve maximum contact between the seed surface and the solvent, the difficulty of separating the solid and miscella makes this process suitable only for continuous extraction in solvent extraction operations of a small scale.
For efficient large-scale solvent extraction, percolation-type continuous extractors are the answer. In this approach, flaked seeds are positioned into a fixed bed. The solvent is sprayed over the bed and washes out the oil. Over time, the design of this type of continuous extractor has evolved greatly to make it more efficient. Today, continuous extraction apparatus for solvent extraction has a horizontal, rotary-type design.
For anyone setting up an edible oil plant today, investing in a range of continuous extractors across oilseed processing stages is investing in efficiency, productivity, and profitability. The ease and efficiency that they offer is unparalleled. If you're committed to building a plant that will stand the test of time, continuous extractors are the answer.
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