Grease dewaxing is a process in which the high melting point wax and high melting point solid fat in the oil are separated out by cooling and crystallization, and then filtered or centrifuged to remove them.
One is to remove waxes containing high melting points including rice bran oil, sunflower oil, corn oil, safflower seed oil, wheat germ oil, etc. These waxes are essentially C 20 -C 28 high-grade fatty acids and C 22 -C 30 high-grade fats Alcohol constitutes wax fat.
The second is to remove all solid components that produce turbidity in the oil during storage. These solid components include wax components, oil polymers, saturated triglycerides and other components.
Strictly speaking, the former should be called dewaxing, and the latter should be called winterization. These are two different concepts, and the two should not be confused.
Wax mostly exists in the seed coat and germ of vegetable oil, and the content of wax varies with different vegetable oils and different processing methods. The wax content in rice bran oil is about 3%-9%, corn oil is 0.05%, sunflower oil is 0.01%-0.35%, and rapeseed oil is 0.0016%. Most of the wax in sunflower seeds is contained in the shell. The crude oil is prepared without dehulling, and the wax content is 0.02%-0.35%, while the wax content in crude oil prepared after dehulling is 0.011%-0.015% .
Generally, when the wax content of sunflower oil, safflower oil, corn oil, rapeseed oil, etc. is low, the oil is generally cooled to about 7-14°C, and then the precipitated crystals are filtered with a filter press. When filtering wax, there are often traces of viscous substances, oil-soluble gums, soapstock components, etc. in the grease that adhere to the wax crystals. These substances often cause clogging of the filter cloth and bring obvious difficulties to the filtration operation.
Whether the dewaxing is successful or not, how to filter to make the crystals easy to precipitate, and how to improve the filtration efficiency is related to the difficulty of all filtration operations. In order to reduce the precipitation of such impurities and non-crystalline substances, try to avoid difficult filtration operations. As the oil that needs to be dewaxed, it is usually not crude oil, and it is best to semi-refined oil after deacidification and decolorization. For dewaxing target oils with different impurities, moisture, fatty acid content, and wax content, different dewaxing processes can be selected to ensure the quality of the final product oil.
Ⅰ.The meaning and mechanism of dewaxing
1. Meaning
The oil contains trace amounts of wax fat, which will increase the cloud point of the oil, reduce the transparency and digestion and absorption rate of the oil, and make the smell and palatability worse, thereby reducing the edible quality, nutritional value and industrial use value of the oil. In addition, Wax is an important industrial raw material and can be used to make wax paper, waterproofing agent, glossing agent, etc. Removal and extraction of wax from oils can not only improve the quality of edible oils, nutritional value and the quality of oily foods, but also improve the industrial use value of oils and the purpose of comprehensive utilization of vegetable oil wax sources.
There are many dewaxing methods: conventional method, solvent method, surfactant method, combined degumming, deacidification method, etc., in addition to coagulant method, urea method, electrostatic method, etc. Although the auxiliary methods used in various methods are different, the basic principles all belong to the category of freezing and crystallization and then separation. That is, according to the melting point difference between wax and grease and the solubility (or decomposition) of wax in grease decreases with temperature, the crystalline wax (or wax and crystallization aid mixture) is precipitated by cooling, and then filtered or centrifuged. To achieve the purpose of oil-wax separation.
2. Mechanism
The presence of acyl groups in wax molecules makes the wax weakly polar, so wax is a lipophilic compound with weak hydrophilic groups. When the temperature is higher than 40℃, the polarity of wax is weak and it dissolves in grease. As the temperature decreases, the mobility of wax molecules in the oil decreases, and the polarity of the ester bond in the wax molecules increases, especially below 30℃ At this time, the wax crystallizes and forms a relatively stable colloidal system. When the low temperature continues, the wax crystals agglomerate into larger crystal grains, and the density increases to form a suspension. It can be seen that the interfacial tension between oil and wax varies with Changes with changes in temperature. There is an inverse relationship between the size of the interfacial tension and the viscosity, which is the theoretical basis why the dewaxing process must be carried out at a lower temperature.
Ⅱ.Factors affecting dewaxing
The main factors affecting dewaxing are temperature, cooling rate, crystallization time, stirring rate, auxiliary agents, conveying and separation methods, etc.
1. Temperature and cooling rate
Because the two hydroxyl carbon chains in the wax molecule are both long, supercooling occurs during the crystallization process, and the lipophilicity of the wax hydroxyl group makes it supersaturated when it reaches the freezing point. In order to ensure the dewaxing effect, the dewaxing temperature must be fixed. It must be controlled below the freezing point of the wax, but not too low, otherwise not only the viscosity of the grease will increase, which will cause difficulties in oil-wax separation, but also the solid grease with a higher melting point will also be precipitated. The solid grease will be separated from the oil together with the wax during separation. Separate, increase the dewaxing loss of grease. Conventional methods are mostly used in China, and the crystallization temperature is 20-30°C, while the solvent method is controlled at 20°C.
Crystallization is a physical process and changes slowly. The whole crystallization process can be divided into three steps: melting waxy grease, supercooling, supersaturation, crystal nucleation and crystal growth. Wax has a high melting point and can crystallize naturally at room temperature. Naturally crystallized crystals are small and have different sizes. Some are peptized in oil, making it difficult to separate the oil and wax. Therefore, the oil temperature must be adjusted before crystallization to melt the wax crystals, and then the crystallization process is artificially controlled. In order to create good separation conditions-the crystal grains are large and strong. The size of the crystal grain depends on two factors: the rate of nucleation (W) and the rate of crystal growth (Q). The dispersion of crystal grains is directly proportional to W/Q. During the crystallization process, W should be reduced and Q should be increased.
The cooling rate has a great relationship with W and Q. When the cooling rate is slow enough, the wax with a high melting point will crystallize first, and the heat of crystallization will be released at the same time. The temperature continues to drop, and the wax with a lower melting point also crystallizes. The wax molecules that are about to be precipitated collide with the wax crystals that have crystallized, and they grow up with the precipitated wax crystals as the core, so that the crystal grains are large and few. If the cooling rate is faster, the high melting point wax will precipitate, and it will have time to collide with the low melting point wax in the future, and the lower melting point wax will have been precipitated separately, making the crystal grains small and large, and entrained grease will inevitably be more.
In order to maintain a suitable cooling rate, the temperature difference between the coolant and the grease must not be too large, otherwise a large number of crystal nuclei will be formed on the cooling surface, which is not conducive to heat transfer and oil-wax separation. The cooling process should be carried out slowly, but it should not be too slow in terms of production. The appropriate cooling rate can be determined by cooling experiments.
2. Crystallization time
In order to obtain easy-to-separate wax crystals, cooling must be done slowly. And when the temperature gradually drops to the predetermined crystallization temperature, it is necessary to maintain the temperature for a certain period of time for crystallization (or aging, maturation). During the crystallization process, the crystal grains continue to grow, that is to say, it takes enough time from the formation of the crystal nucleus to the growth of the crystal into a large and strong crystal.
3. Stirring rate
Crystallization requires low temperature, but it is an exothermic process, so it must be cooled. Stirring can not only cool the grease uniformly, but also collide the wax molecules that are about to crystallize, and promote more opportunities for crystal grains to grow uniformly. Stirring can reduce the formation of crystal clusters. In addition to the growth of crystal nuclei during crystallization, several crystals may also gather into crystal clusters, which can entrain oil and increase the loss of grease during dewaxing. On the contrary, it can only rely on Brownian motion without stirring, and the crystallization rate is too slow, but the stirring cannot be too fast, otherwise the crystal grains will be broken (generally 10-13r/min).
4. Adjuvant
Different dewaxing methods require different auxiliary agents.
(1) Solvent
Grease and wax have different structures and have different affinity for solvents, especially at low temperatures. The presence of the solvent makes the wax easy to crystallize and helps the solid (wax crystal) and liquid (grease) phases to reach equilibrium faster, the resulting crystals are firm (less oil entrained), and the cooling rate can also be higher. At the same time, the solvent can reduce the viscosity of the system and improve the oil-wax separation effect.
(2) Surfactant
The addition of surfactants helps wax crystallization. The non-polar group in the surfactant molecule has a strong affinity with the hydroxyl group of the wax to form a copolymer. The surfactant has a strong polar group, so the polarity of the copolymer is much greater than that of the monomer wax, which makes oily -The surface tension of the wax interface is greatly increased, and the copolymer crystals are enlarged, the growth rate is fast, and it is easy to separate from the grease.
Phospholipids, monoglycerides, diglycerides, free fatty acids and soap produced in alkali refining in crude oil are all good surfactants, which can "pull" wax out of the oil under low temperature conditions. This is the main basis for rice bran oil to be dewaxed at the same time as low-temperature degumming and alkali refining. However, there is still a certain affinity between wax and oil. The surface active substances in the above-mentioned oils do not have enough "tension". Separate all the wax molecules in the oil, and add some powerful surfactants to achieve the ideal dewaxing effect. Commonly used are polyacrylamide, aliphatic alkyl sulfates and glycolipids.
(3) Flocculant
Flocculant is a kind of electrolyte crystal promoter. Add a proper amount of electrolyte solution to wax-oil sol to increase the ion concentration in the sol, create favorable conditions for the negatively charged wax crystal grains to attract oppositely charged ions, and reduce the colloid. The potential difference in the electric double layer structure reduces the repulsive force between the particles, and the stable system of the sol is destroyed, which causes the wax crystal grains to condense.
(4) Urea
Urea can selectively contain wax in the spiral tube formed by crystallization. The inclusion compound is easy to precipitate and separate from oil. Because of the different solubility of wax and urea in water, wax and urea are easy to separate.
(5) Electrostatic dewaxing
It is a method of polarizing wax molecules by applying an uneven electric field, and the negatively charged wax crystal particles are enriched and settled at the anode under the action of the electric field, so that the oil-wax is separated.
5. Conveying and separating methods
When various delivery pumps deliver fluids, the turbulence caused by them varies in strength. The stronger the turbulence, the greater the shear force received by the fluid. In order to prevent wax crystals from being broken by shearing force, when transporting grease containing wax crystals, a reciprocating plunger pump with weak turbulence and low shear force should be used, or compressed air, preferably vacuum filtration. When wax-oil separation, the filtration pressure should be moderate, because wax is compressible, too high pressure will cause the wax crystal filter cake to deform, block the filtration gap and affect the filtration rate, but too low pressure will cause the filtration rate to decrease. produce. The use of filter aids can increase the filtration rate.
6. Oil quality
The colloidal impurities in the oil will increase the viscosity of the oil, not only affect the formation of wax crystals, reduce the firmness of the wax crystals, and cause difficulties in oil-wax separation, but also reduce the quality of the wax (both oil and gum impurities are high) Therefore, the grease should be degummed before dewaxing. Wax has an adverse effect on alkali refining, decolorization and deodorization processes. It is more reasonable to dewax the crude oil after degumming, and then to alkali refining, decolorization and deodorization. However, domestic oil plants often perform dewaxing after deodorization. This is determined by the refining process adopted in our country. my country generally uses conventional dewaxing without adding filter aids, which can be combined with refined oil filtration. Investment in a set of filtration equipment.
