Expeller pressing (also called oil pressing) is a mechanical method for extracting oil from raw materials. The raw materials are squeezed under high pressure in a single step. When used for the extraction of food oils, typical raw materials are nuts,

seeds A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, includ ...

and

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular m ...

, which are supplied to the press in a continuous feed. As the raw material is pressed, friction causes it to heat. In the case of harder nuts (which require higher pressures) the material can exceed temperatures of .

Overview

An expeller press is a screw-type machine that mainly presses oil seeds through a caged barrel-like cavity. Some other materials used with an expeller press include meat by-products,

synthetic rubber A synthetic rubber is an artificial elastomer. They are polymers synthesized from petroleum byproducts. About 32-million metric tons of rubbers are produced annually in the United States, and of that amount two thirds are synthetic. Synthetic rubbe ...

and animal feeds. Raw materials enter one side of the press and waste products exit the other side. The machine uses friction and continuous pressure from the screw drive to move and compress the seed material. The oil seeps through small openings that do not allow seed fiber solids to pass. Afterward, the seeds are formed into a hardened press cake, which is removed from the machine. Pressure involved in expeller pressing creates heat in the range of . Raw materials are typically heated up to to make the pressing more efficient, otherwise the pressing itself will heat the oil to . Some companies claim that they use a cooling apparatus to reduce this temperature to protect certain properties of the oils being extracted.

Efficiency

Expeller processing cannot remove every last trace of liquid (usually oil) from the raw material. A significant amount remains trapped inside the cake remaining after pressing. In most small-scale rural situations this is of little importance, as the remaining cake after oil extraction finds uses in local dishes, in the manufacture of secondary products, or in animal feed. Some raw materials do not release oil by expelling, the most notable being

rice Rice is the seed of the grass species ''Oryza sativa'' (Asian rice) or less commonly '' Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and ''Porteresia'', both wild and domesticate ...

bran Bran, also known as miller's bran, is the hard outer layers of cereal grain. It consists of the combined aleurone and pericarp. Corn (maize) bran also includes the pedicel (tip cap). Along with germ, it is an integral part of whole grains, ...

. To remove oil from commodities that do not respond to expelling or to extract the final traces of oil after expelling, it is necessary to use

solvent extraction A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

Design

Continuous screw

The earliest expeller presses utilized a continuous screw design. The compression screws were much like the screws of a

screw conveyor A screw conveyor or auger conveyor is a mechanism that uses a rotating helical screw (simple machine), screw blade, called a "''flighting''", usually within a tube, to move liquid or granular materials. They are used in many bulk handling in ...

—that is, the

helicoid The helicoid, also known as helical surface, after the plane and the catenoid, is the third minimal surface to be known. Description It was described by Euler in 1774 and by Jean Baptiste Meusnier in 1776. Its name derives from its similari ...

flighting started at one end and ended at the other.

Interrupted screw

Valerius Anderson invented the interrupted screw design and patented it in the year 1900.Twin screw press with interrupted flights – US Patent 6550376 Description
/ref> Anderson observed that in the continuous flighting arrangement of a compression screw, there are tendencies for slippery materials either to co-rotate with the screw or to pass through with minimal dewatering. He wrote that "

brewer Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and fermenting the resulting sweet liquid with yeast. It may be done in a brewery by a commercial brewer, ...

s' slops,

slaughterhouse A slaughterhouse, also called abattoir (), is a facility where animals are slaughtered to provide food. Slaughterhouses supply meat, which then becomes the responsibility of a packaging facility. Slaughterhouses that produce meat that is no ...

refuse" and other "soft and mushy" materials dewater poorly in continuous screw presses. His invention consisted of putting interruptions in the flighting of a compression screw. It was much like having a hanger bearing in a screw conveyor: there is no flighting on the shaft at that point, so material tends to stop moving and pile up. It is only after solids accumulate in the gap that the downstream flighting catches material. When this happens, material is forced along its way. The result is better dewatering and thus a more consistent press cake.

Resistor teeth

After the 1900 patent, a major improvement was made with the addition of resistor teeth. Fitted into the gaps where there is no flighting, these teeth increase the agitation within the press, further diminishing co-rotation tendencies.

Expanded applications

As the years passed, applications of the interrupted screw design expanded beyond slippery and slimy materials. This happened because competing continuous screw presses worked best only under conditions of constant feed, at constant consistency. If either the consistency or the flow rate diminished, squeezing would diminish until it was inadequate for proper moisture removal. At the same time, if the consistency increased, the press could jam. To counteract these tendencies it was necessary to build a heavy press, frequently with a costly variable speed drive. In contrast, it was found that the interruptions in the flighting of the Anderson screw would provide cushion within the press. If consistency decreased, compression was still effective. A plug of sufficiently solid material had to build up at each interruption before solids could progress toward the discharge. This self-correcting performance prevents wet material from purging at the cake discharge. It is achieved without varying the speed of the screw. The economic advantages of these characteristics led to interrupted screw presses being used to dewater fibrous materials that are neither slippery nor slimy. Examples would be

alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, and silage, as ...

corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...

husk Husk (or hull) in botany is the outer shell or coating of a seed. In the United States, the term husk often refers to the leafy outer covering of an ear of maize (corn) as it grows on the plant. Literally, a husk or hull includes the protective ...

, and, more recently,

paper mill A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags, and other ingredients. Prior to the invention and adoption of the Fourdrinier machine and other types of paper machine that use an endless be ...

fibers.

References

{{reflist Oils High lipid content microalgae Liquid-solid separation