This paper mainly introduces the application of industrial refrigeration crystallization technology in the oil and fat industry, as well as the principle of frozen crystallization, the separation method using frozen crystallization technology in the separation and purification of oil industry, and its operation and application characteristics. The advantages and disadvantages of frozen crystallization technology and some feasible improvement methods are analyzed.
1 Overview
The process by which crystals form in solution is called crystallization. There are generally two methods for crystallization: one is an evaporation solvent method, which is suitable for substances whose temperature does not have a significant influence on solubility. The “salt salt” in coastal areas is the method used. The other is to cool the hot saturated solution method.
This method is applicable to substances with increased temperature and increased solubility. For example, in the salt lakes of northern China, the summer temperature is high, and there is no crystal on the lake surface; every winter, the temperature is lowered, and the materials such as sulphate (Na2CO3·10H2O) and thenardite (Na2SO4·10H2O) are precipitated from the salt lake. In order to obtain larger intact crystals in the laboratory, a method of slowly lowering the temperature and slowing down the crystallization rate is often used.
One cannot simultaneously see the macroscopic phenomenon in which a substance dissolves and crystallizes in a solution, but there are actually two reversible movements in which the particles of the constituent substance dissolve and crystallize in the solution. By changing the temperature or reducing the solvent, the crystallization rate of the solute particles can be made larger than the dissolution rate at a certain temperature, so that the solute crystallizes out from the solution.
Crystallization can be divided into two stages: nucleation (nucleation) and crystal growth. The driving force of both stages is the supersaturation of the solution (the concentration of solute in the crystallization solution exceeds the value of its saturated solubility). There are three forms of nucleation: primary homogeneous nucleation, primary heterogeneous nucleation, and secondary nucleation. Under high supersaturation, the process of spontaneously forming nucleation of the solution is called primary homogeneous nucleation; the process of generating nucleation under the induction of foreign matter (such as dust in the atmosphere) is called primary non-uniformity. Phase nucleation; and the nucleation process in a solution containing solute crystals is called secondary nucleation. Secondary nucleation also belongs to the heterogeneous nucleation process, which occurs under the induction of tiny grains generated between crystals or crystals colliding with other solids (walls, agitators, etc.).
The requirement for crystallization operation is to produce crystals that are pure and have a certain particle size distribution. The particle size and distribution of crystal products mainly depend on the rate of nucleation (the number of crystal nuclei produced per unit volume of solution per unit time), the crystal growth rate (the increase in the linear size of the crystal per unit time), and the crystal in the crystallizer. Average residence time in the middle. The supersaturation of the solution is related to the rate of nucleation and the rate of crystal growth, and thus has an important influence on the particle size and distribution of the crystalline product. In a solution with low supersaturation, the ratio of crystal growth rate to nucleation rate is large, so the crystal obtained is larger and the crystal form is more complete, but the crystallization rate is slow. In an industrial crystallizer, the supersaturation is usually controlled in the metastable zone, in which case the crystallizer has a high production capacity and a crystal product of a certain size can be obtained. Make the crystals intact.
2. The application of frozen crystallization technology in industry
The industrial freeze crystallization technique mainly relies on cooling (freezing) the solution to reach saturation to produce crystals. This method is used for substances whose solubility decreases with temperature, for example: ammonium nitrate, potassium nitrate, ammonium chloride, sodium phosphate, thenardite, etc. The solubility temperature coefficient of these substances varies greatly, and the solubility of these substances when the temperature drops When it falls, a supersaturated solution is formed, which is in a thermodynamically unstable state, and the solute crystallizes out from the solution. These chemicals are particularly suitable for separation by cryocrystallization [1]. When the uranium hydrometallurgical plant of the nuclear industry extracts uranium from ore with sulfuric acid, a uranium-containing stripping solution is obtained, from which uranium is precipitated to produce a Na2CO3+NaOH solution containing a large amount of Na2SO4, which must be removed in order to recover the lye. The Na2SO4, uranium plant is a freeze crystallization method. The dehydrated Glauber's salt is crystallized at about 0 ° C. After filtration and separation, the obtained lye is returned to the production. The process of recovering the lye reduces the production cost of the plant and recovers the useful by-product Glauber's salt.
3. Application of frozen crystallization technology in oil industry
Freeze crystallization technology is mainly used to separate and purify specific high-value components in oils and fats, such as oleic acid, linoleic acid and other unsaturated fatty acids. At present, there are more than ten methods for separating and purifying polyunsaturated fatty acids, including molecular vacuum distillation, supercritical fluid CO2 extraction, low temperature crystallization, urea encapsulation and lipase enrichment. There are two main methods used in the freeze crystallization technology: low temperature crystallization and urea encapsulation.
4. Conclusion
As a common separation technology, frozen crystallization technology has the advantages of simple process principle and convenient operation. It has also been commercialized in industries such as oils and fats, but in order to obtain a good separation effect, the crystallization process needs to take into account many aspects such as solvent properties, temperature, stirring frequency and crystallization time. On the other hand, if the purity of the product separated by the freeze crystallization technique is not high, it is necessary to combine with other separation methods such as molecular distillation, supercritical extraction, column chromatography, etc. to optimize the process to obtain the final high-purity product.
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