The oil extraction process is widely used in the field of oil production because it can improve production efficiency and oil yield.

The main principle is to mix the pretreated oil with a low boiling point solvent (industrial n-Hexane), extract the oil from the meal, and finally separate the solvent from the meal by evaporation. The mixed solution of oil and solvent is evaporated and stripped to separate the oil and solvent, and finally obtain the oil product, and the solvent is recycled.

The extraction process has been widely used in other oils, but it is less used in the palm oil industry. However, the oil extraction process has broad prospects for the extraction of residual oil from palm kernel meal (PKM) and palm fiber after pressing, and can reduce 8-10% residual oil to ≤1%.

Because the extraction process has high investment and high explosion-proof requirements, it is more suitable for large-scale production.

 The main process includes: material extraction, mixed oil desolventization, wet meal desolventization, solvent recovery and exhaust treatment

The main equipment includes: conveying and metering equipment, extractor, degassing machine (DTDC), suspension separator, collector, evaporator, stripper, condenser, vacuum pump, etc.

Introduction to extraction process:

1. Material extraction

The pre-treated material to be extracted is transported to the material storage box in the extraction workshop through a conveying scraper, and then the material is sent to the extractor for extraction through an auger. In addition to achieving continuous, stable and controllable feeding to the extractor, the auger also has a key role in sealing, that is, preventing the solvent gas in the extractor from "slipping" to the pretreatment workshop through the conveying equipment.

After entering the extractor, the material to be extracted comes into contact with the solvent. Depending on the type of extractor, there are three main contact methods: immersion, spraying and immersion spraying mixed.  The common movement direction of materials and solvents in the extractor is reverse movement, that is, the material moves from the feed end of the extractor to the discharge end of the extractor, and the oil content of the material becomes lower and lower, while the movement of the solvent is just the opposite, that is, it moves from the discharge end of the extractor to the feed end, and the oil content in the solvent becomes higher and higher. This leaching method is called "counter current leaching". The advantage of counter current leaching is that it can reduce the solvent consumption, obtain a larger solvent ratio, and at the same time increase the leaching rate and shorten the leaching time.

2. Mixed oil desolventization

Through reverse leaching, the mixed oil with a higher concentration is pumped into the mixed oil tank for temporary storage through the mixed oil pump. Before entering the tank, it is generally necessary to pass through a set of impurity removal equipment. The most common one is a combination of a self-cleaning filter and a suspension separator. The impurity removal equipment separates most of the meal contained in the mixed oil from the mixed oil. After the cleaner mixed oil enters the mixed oil tank, the meal sinks to the bottom of the mixed oil tank by natural sedimentation, and then the meal is discharged from the mixed oil tank by regular slag discharge.  The mixing oil tank is usually installed above the extractor so that the slag can be discharged directly into the extractor.

After the purified mixed oil is evaporated and separated in the first long tube evaporator (first evaporator), the concentration is increased to 70%-80%. After evaporation and separation in the second long tube evaporator (second evaporator), the concentration can be increased to more than 90%. Finally, the content of solvent in the mixed oil is reduced to less than 100ppm by the stripping action of the stripping tower, and qualified crude oil is obtained.

The most commonly used mixed oil evaporation equipment in the oil extraction plant is the rising film long tube evaporator, and the most commonly used mixed oil stripping equipment is the layer disc stripping tower. The steam in the shell side of the first and second evaporators quickly heats the mixed oil in the tube and makes it boil. The solvent in the mixed oil evaporates and expands in volume, pushing the mixed oil to the surrounding of the tube wall and entering the top separator as it rises. Due to the increase in the volume of the separator, a large amount of solvent begins to vaporize and is vacuum-pulled into the condensation system to condense into liquid.

 The mixed oil concentrated by the first and second evaporators is sprayed evenly into the stripping tower from the top of the stripping tower and forms an oil film on the layer disc under the action of gravity. Steam is sprayed from the bottom and the oil flows down from the top. The steam and oil are counter currently contacted and stripped. The solvent and other low-boiling substances contained in the oil are taken out with the stripping steam under vacuum conditions.

3. Wet meal desolventization

The wet meal desolventization equipment commonly used in leaching oil plants is the DTDC desolventizer. According to the functional division, it can be divided into pre-desolventization, desolventization, drying and cooling layers. Different functional areas will be designed with different numbers of layers according to the capacity and requirements.

The wet meal enters the DTDC desolventizer through the conveying scraper. In the pre-desolventization, the material is heated and part of the solvent is removed under the action of the interlayer steam. Under the stirring action, the material is pushed to the discharge port and falls to the next layer. The pre-desolventization is generally designed to be two to three layers, and the material layer height is not high. The size of the discharge port can be adjusted by a slider and fixed with bolts.

 After the material enters the desolventizing layer, direct steam will directly contact the material and strip the material. Direct steam is sprayed from the interlayer of the bottom layer of the desolventizing layer. The upper surface plate of the interlayer is distributed with direct steam holes of specific aperture and number (as shown above). Under the stripping action of direct steam, the solvent in the material is transferred to the steam and moves upward with the steam. The mixed gas mixed with solvent gas passes through the pores of specific number and aperture designed on each desolventizing layer in turn and strips the material. Except for the direct steam layer, the interlayers of the other desolventizing layers are passed through indirect steam to heat the material. The solvent contained in the wet meal after steam stripping is basically removed, and the anti-nutritional factors are also fully destroyed.

The meal discharged from the desolventizing layer has a high temperature and a high moisture content, so it needs to be dried and cooled.  The cooling medium used in the drying and cooling layer of the DTDC degassing machine is air provided by a centrifugal fan. In the drying layer, the air is heated to a certain temperature by a heater and then introduced into the interlayer of the drying layer. There are pores with specific apertures and numbers on the upper surface of the interlayer. The hot air is ejected through the pores to contact the material. The material is in a semi-fluid state under the action of airflow and stirring. The centrifugal fan inlet damper is adjusted according to the moisture content of the material to obtain the appropriate air volume. The air temperature can also be adjusted to dry the meal and remove the excess moisture contained in the meal. The air introduced into the cooling layer does not need to be temperature-regulated. It is directly sprayed into the material through the pores on the upper surface of the interlayer to contact the material and cool the material. According to the temperature of the material, the centrifugal fan inlet damper is adjusted to obtain the appropriate air volume so that the material temperature meets the requirements for safe storage.

4. Solvent recovery

Solvent recovery mainly involves two sources of solvents: DTDC degassing machine and evaporation system.

①Recovery of solvents generated by DTDC degassing machine

There is a large pipe on the top of the DTDC degassing machine, called the gas phase pipe, which is the channel for the mixed gas of steam and solvent in the degassing machine to be discharged. Because the mixed gas contains a certain amount of meal, a manhole is generally set at the lowest position of the gas phase pipe to facilitate maintenance and cleaning of the meal deposited inside. Because this manhole is opened only a few times a year, and the steam condensate contained in the mixed gas will also be deposited here. Therefore, every time the manhole is opened to clean the meal, the smell is quite sour and refreshing. I believe that those who have cleaned it have a very deep memory.

The temperature of the mixed gas is generally around 80℃, and the heat is huge. If it can be used, it will greatly save steam consumption. Therefore, it can be used to heat up the mixed oil.  However, in order to prevent the meal contained in the mixed gas from entering the first steam shell pass and adhering to the outer surface of the tubes to affect the heat exchange efficiency or reduce the shell space to increase the resistance of the mixed gas flow and affect the negative pressure of the DTDC steam degassing machine, which may cause blockage of the first steam shell pass in severe cases, it is generally necessary to capture the meal in the mixed gas. There are many ways to capture meal. The most common one I have encountered is the wet capture method, which is to use the hot water in the wastewater cooking tank to spray it evenly into the mixed gas through the nozzle. The meal is captured by the water in the wastewater tank, and the clean mixed gas enters the first steam shell pass to exchange heat with the mixed oil. Corresponding to the wet capture method is the dry capture method, which is to capture with solvent liquid at room temperature. Each method has its advantages and disadvantages, which will not be introduced in detail here. If there is still a part of the mixed gas that has not condensed into liquid after heat exchange, it will be passed into a condenser and cooled by cooling circulating water to become liquid.  The mixed liquid that becomes liquid after heat exchange or cooling enters the water separation tank and the solvent liquid obtained after water separation is recycled, but there will still be a small amount of solvent gas that has not been condensed. This part of the gas will enter the exhaust system for recovery. The exhaust fan can make the exhaust treatment system and the equipment connected to it, such as the condenser, in a slightly negative pressure state, which can provide power for the gas flow.

② Recovery of solvents produced by the evaporation system

The leached mixed oil contains a large amount of solvent. After one, two and stripping, the solvent content in the leached crude oil has been reduced to a very low level. The solvent content in qualified leached crude oil is generally less than 100ppm. The solvent vaporized by the evaporation system enters the condensation system, and after heat exchange with cooling water, it becomes condensate and flows into the water distribution tank for water distribution. The solvent after water distribution is recycled again.

The vacuum pump draws the gas in the condensation system and the evaporation system to make the inside of the system in a negative pressure state. After the solvent evaporates and becomes gas, it begins to move backward and enters the shell side of the condenser (the space between the outer wall and the tubes is called the shell side), and cooling water is introduced into the tubes in the condenser (referred to as the tube side). The solvent gas in the shell side exchanges heat with the condensed water in the tube side. The solvent gas that loses heat is condensed into liquid and enters the water distribution tank for water distribution and recycling. The cooling water in the tube side takes the heat obtained from the solvent gas to the cooling tower for cooling and then circulates for heat exchange.  Since there is no direct steam in the first and second steaming, and the temperature is not too high, the value of heat recovery is not high, but the amount of solvent gas is relatively large, so the solvent gas from the first and second steaming is combined and directly condensed in a condenser with a large heat exchange area. Although the amount of solvent gas coming out of the stripping tower is small, a large amount of direct steam is used during stripping. The steam temperature is high and contains a lot of heat, which has a very considerable recycling value. If it is directly cooled and condensed, it will cause a waste of heat. It can be used as needed to exchange heat with the material that needs to be heated (a sufficiently large temperature difference is required), and then enter the condenser for cooling and condensation. In addition to the above two main sources of solvents that need to be recovered, there is also a certain amount of solvent in the solvent free gas produced by the extractor and the non-condensable gas in the condenser that needs to be recovered.