Extract oil by pressing is a traditional method of vegetable oil extraction. With the development of oil extraction technology, three types of oil extraction methods are created from manpower press, hydraulic press to screw oil press. The widely used oil machine today is the screw oil press, and the hydraulic oil press is still in use. Screw oil press is an advanced continuous oil extraction equipment, which is widely adopted at home and abroad in recent years. Compared with manpower press, hydraulic oil press (hydraulic machine for oil extraction) and other equipment, screw oil press has the following advantages: large processing capacity, continuous production, low labor intensity and high yield efficiency. The screw oil press is suitable for oil plants of various kinds, such as the rapeseed, soybean, cottonseed, rice bran, especially the oil plants that need to maintain its original natural flavors or some characteristics, such as the tea seed, sesame, cocoa beans, castor bean, etc.

Characteristics of vegetable oil extraction by pressing:
Simple process, less supporting equipment, well-adapted to oil variety, flexible production, good oil quality, light color and pure flavor. However, the amount of residual oil in the cake is higher after pressing with low oil efficiency, large power consumption and the parts wear easily.

First, the basic principle of vegetable oil extraction by pressing.

1.Pressing process

In the process of pressing, the particles of flakes are subjected to strong pressure that the liquid part of oil and the gel of the non-fat material would have different changes respectively, namely the oil is pressed from the gap of pressed material and material particles would become hard cake by elastic deformation.

The separation process of oil from the pressed material: at the beginning of pressing, the deformation of particles occurs and combines in the individual contact point. The gap between particles is reduced and the oil begins to be extracted. At the main stage of pressing, the particles are further deformed and combine. The gap gets smaller and smaller and the oil is pressed out in large quantity. At the end of pressing, the particles combine completely, the cross section of the inner space is suddenly narrowed, the oil line is closed, and the oil is rarely pressed out. After relieving the pressure, the oil cake expands by elastic deformation and forms fine holes in it. Sometimes, there are thick cracks, which would absorb the oil not discharged.

Cake formation process: in the process of pressing, under the effect of pressure, flake particles are pressed with the discharging of oil constantly. The plastic deformation of a certain particle would be produced by direct contact of particles and mutual pressure, and forms an organic whole especially in the broken place of oil film. Then, the pressed material is no longer a loose body and begins to form a complete plastic body, which is called the cake. The forming of the cake is the prerequisite for the setting up of oil discharging pressure during the process of the oil pressing and the necessary condition of oil discharging.

2. The basic principle of vegetable oil extraction by pressing

In the process of pressing, pressure, viscosity and cake forming are the three elements of vegetable oil extraction by pressing. The pressure and viscosity are the main drivers and possible conditions determining the oil discharging of pressed material, and the cake forming is the necessary condition.

(1) Oil discharging power. When the material is pressed, the gap between the material is compressed and the air is discharged that the density of flakes increases rapidly with the movement of extrusion, deformation and displacement. Thus, the outer surface of the material is closed, and the porous channel of inner surface shrinks rapidly. When the porous channel is small enough, the ordinary pressure fluid oil becomes the high-pressure oil. High pressure oil produces the flow of energy. In the flow, small oil droplets are gathered into large oil droplets, and even become the independent liquid phase in the gaps. When the pressure increases to a certain degree, high pressure oil opens the flow oil line and gets rid of the frictional resistance of protein molecules of pressed material and oil molecules, oil molecules and rushes out of the high pressure force field of pressed material and separate from plastic cakes.

The viscosity and power act as the function of temperature in the process of pressing. When pressing the material, mechanical energy is turned into heat energy, the material temperature rises, molecular motion intensifies, frictional resistance between the molecules reduces as well as the surface tension and oil viscosity, thus provides convenient for fast flowing and gathering of oil and separation from plastic cake.

(2) Oil discharging depth. When the oil is extracted, the amount of oil remaining in the pressed material may reflect the depth of oil discharging. The lower the residual quantity, the deeper the discharging depth. The depth of oil discharging is related to the pressure size, the increase of pressure and the influence of viscosity.

In the pressing process, a certain amount of pressing pressure must be provided to make flakes extruded and deform in order to increase the density, discharge the air, narrow the gap and internal and external surface area. Large pressure would lead to large material deformation.

In the process of pressing, reasonable increase of pressure can obtain good oil discharging depth. The pressure increasing change should be small and the pressurizing time should be short. In this way, the gap between the material becomes smaller gradually, and the oil is gathered and flows in a sufficient amount of time and extracted out of the material that the depth of the oil discharging can be improved. The principle of “press light and often” is applied to the pressurizing design of all oil presses.

(3) The necessary condition for the forming and oil extraction of cake is the forming of cake. If the plasticity of pressed material is low, the material would not deform or difficult to deform after compression that the cake can’t be formed and there is not enough oil discharge pressure. In addition, the outside surface of flakes cannot be closed, pore channels in the inner surface are not compressed and the density cannot be increased. Under this condition, the oil can not turn into continuous phase from the discontinuous phase, from small oil drop into large oil drop and normal pressure oil cannot be closed up into high pressure oil, also the flow of oil extraction power cannot be produced, let alone depth of oil discharging. Smooth forming of the cake is a necessary condition for oil discharging. Flakes are pressed into cakes that the pressure can be set up smoothly. Proper control of temperature and reduce of oil discharging resistance can improve the depth of oil discharging.

The forming of cake is related to the following factors: (1) appropriate moisture content, temperature and certain compressive deformation plasticity. When the compressive ability reduces to a reasonable value, the pressure effect can be given full play; (2) appropriate amount of slag and oil discharge; (3) The material should be enclosed in a container, forming the stress and the space field force of plasticity denatured.

Second, the factors that affect the effect of vegetable oil extraction.

There are many factors that determine the quality of oil extraction. It mainly includes the material structure and pressing condition.

1. The influence of the properties of the material structure on the oil extraction effect – the properties of pressed materials mainly depend on the composition of the material and the pretreatment effect.

The pretreatment effect:

The number of the damaged cells in the pressed material is the more the better, which is beneficial to oil extraction.

The particle size of pressed material should be appropriate. If the particle is too large, it is easy to crust and block the oil line, and not good for oil extraction. If the particle is too thin, it is also bad for the oil extraction, because the fine particles would be taken away when pressing, increasing the resistance of oil flow, and even blocking the oil line. At the same time, the fine particle would make the pressed material more plastic, bad for the increasing of pressure.

Unit weight of pressed material is the higher the better under the premise of not affecting the internal and external structure, which is beneficial to the improvement of the processing capacity of equipment.

The pressed material should have appropriate moisture and good mobility. It should also have necessary temperature to reduce the viscosity and surface tension in the pressed material so as to ensure the good mobility of oil in the process of pressing.

Enough plasticity of the pressed material particles. The plasticity of the pressed material must in a certain range. On the one hand, it should not below a certain limit to ensure that the particles have quite complete plastic deformation; on the other hand, the plasticity can’t be too high. Otherwise, the mobility of pressed material is large and not easy to set up the pressure. Thus, “extrusion” phenomenon would appear when pressing, increasing the unnecessary return of material. At the same time, high plasticity would lead to early forming and oil discharging, and the cake would be hard and harmful for oil extraction. The quality of oil is poor as well.

Enough plasticity of the pressed material particles. The plasticity of the pressed material must in a certain range. On the one hand, it should not below a certain limit to ensure that the particles have quite complete plastic deformation; on the other hand, the plasticity can’t be too high. Otherwise, the mobility of pressed material is large and not easy to set up the pressure. Thus, “extrusion” phenomenon would appear when pressing, increasing the unnecessary return of material. At the same time, high plasticity would lead to early forming and oil discharging, and the cake would be hard and harmful for oil extraction. The quality of oil is poor as well.

Properties of the pressed material:

The properties of the pressed material not only include the gel part, but also related to the existence form, quantity and degree of separability of the oil. Factors affecting the properties of pressed materials are moisture, temperature and protein denaturation.

Moisture content: as the moisture content increases, the plasticity would also increase. When the moisture reaches a certain point, the pressing condition is the best. Once a little more than that, it would produce sharp “extrusion” phenomenon known as “mutation”. On the other hand, if the moisture is slightly lower, it would also make the plasticity drop suddenly and the particles combining loosely, which is harmful for oil extraction.

Temperature: heating the pressed material can increase its plasticity; while cooling the material would reduce the plasticity. If the temperature drops significantly when pressing, the combination of the pressed particles is bad, and the cake is loose and not easily to be formed. However, the temperature should not be too high, otherwise certain substances would break down into gas or produce scorched flavor due to the high temperature. Therefore, insulation is one of the important conditions of the pressing process.

Protein denaturation: it is necessary for the extraction of oil by pressing. But excessive protein denaturation would reduce the plasticity of the pressed material, increases the “extrusion” pressure of the oil press, which is contrary to the effect of increasing the moisture and temperature. The extent of colloid structural damage of the material is measured by the protein denaturation. Due to the combined heating and high pressure in pressing, it would lead to continuous protein denaturation. But improper temperature and pressure would result in excessive protein denaturation, which is also not conducive to the oil extraction. The protein denaturation, therefore, can neither be too much to make the plasticity too low, nor too low to affect the oil efficiency and product quality, for example, the undenatured colloidal substance in the oil would influence the oil refining.

2. The influence of pressing condition on the vegetable oil extraction effect

The pressing condition, the process parameter (pressure, time, temperature, material thickness, oil drainage resistance, etc.), is the decisive factor that increases the oil extraction efficiency.

(1) The pressure inside the pressing chamber. The pressure on the material must be proper and the pressure changes must be consistent with oil discharge rate, namely “make the oil flows continuously”. The highest pressure section of the screw oil press is small. The maximum pressure is general distributed in the main pressing part. For the single pressing of oil seed with low oil content, the highest pressure point is usually at the beginning of the main pressing stage; while in pressing or prepressing of oil seed with high oil content, the highest pressure point is generally distributed in the posterior part of the main pressing stage. At the same time, the pressing method summarized in the long-term practice, “the first light after, press light and often” is effective.

(2) The pressing time. The pressing time is an important factor affecting the production capacity of oil press and the depth of oil discharge. It is commonly believed that long pressing time means high oil extraction rate. However, too long pressing time would lead to unnecessary heat loss, also affect the oil extraction rate and the capacity of equipment. A good control of proper pressing time must be a comprehensive consideration of the pressing method, pressure, material thickness, oil content, heat preservation condition and equipment structure, etc. On the premise of meeting the oil extracting rate, the pressing time should be reduced as much as possible.

(3) The influence of the temperature. The temperature change directly affects the plasticity of material and oil viscosity, further affects the efficiency of oil extraction. It is also related to the quality of extracted oil and the cake. If the temperature of the pressing chamber is too high, the cake color would deepen and even burn, and the rate of residual oil in the cake would increase, and the color of the oil would deepen as well. It is impossible to get the hard pressed cake and get the most oil extracted when pressing by cold and unheated oil press. Therefore, maintaining the appropriate pressing temperature is not negligible.

Suitable pressing temperature range usually refers to the pressing temperature of the material (100~135℃). Different pressing modes and materials have different temperature requirements. However, the parameter is only necessary to control the initial temperature of pressing, and it is difficult to control the temperature change within the range in the process of pressing.

Third, oil extracting equipment.

Currently, there are mainly two types of pressing equipment: the intermittent hydraulic oil press and continuous screw oil press. Wide varieties of oil plants require the pressing equipment to satisfy various requirements in the structure design as much as possible. At the same time, the oil extraction equipment should have large production capacity, high efficiency, convenient operation and maintenance, multi-usage, less power consumption, etc.

1. Hydraulic oil press

Hydraulic oil press is a kind of intermittent pressing device that makes use of the liquid to transmit the pressure to press the material in the cake ring. It has simple structure, convenient operation, low power consumption, oil cake of good quality and can process a variety of materials. Thus, the hydraulic oil press is suitable for small oil plants with various and small quantity materials in small regions for small amount oil processing. But its labor intensity is higher with strict craft condition. Thus, it has been replaced by continuous pressing equipment gradually.

2. Screw oil press

Screw oil press is an advanced continuous oil pressing machine widely adopted internationally. Its working principle is that the material is pushed forward continuously by the rotating screw axis in the pressing chamber. At the same time, due to the shorten spiral lead of material or increase of root diameter, the space volume of pressing chamber is continuously reducing to press the material and extract the oil content of the material. The oil flows out from the crack in the press cage. At the same time, the residues are pressed into cakes and discharged from the end of the pressing shaft.

Characteristics of screw oil press: continuous production, large capacity, low labor intensity, high efficiency, thin cake and easy crushing, comprehensive utilization, so its application is very extensive.