Crystallization is a technique used by chemists to purify solid compounds. It is one of the fundamental procedures that each chemist must master in order to master the laboratory. Crystallization is based on the principles of solubility: compounds (solutes) tend to be more soluble in hot liquids (solvents) than in cold liquids.
If a hot saturated solution is allowed to cool, the solute is no longer soluble in the solvent and forms pure compound crystals. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration.
Crystallization is a very old technology, and information related to salt and sugar crystallization dates back to the beginning of civilization. In the last 30 years, the technical means to measure nucleation in operating equipment have been developed.
This has given a new perspective on the design of crystallization equipment with the result that modern crystallization designs are much more flexible in terms of their apacity to control the size and size distribution of crystal products than previous equipment. Crystallization has long been considered an art rather than a science, although recent theoretical discoveries and new analytical techniques have produced a change in this historical position.
Jabir Bin Hayyan conducted his great experiments in science and chemistry in the eighth century during the Abbasid caliphate of Haroon al-Rashid Jabir introduced experimental research into science that quickly changed its character in modern chemistry. His fundamentally important contribution to chemistry included the perfection of scientific techniques such as crystallization, distillation, calcinations, sublimation and evaporation and the development of various instruments for this purpose.
The still is his great invention, which made the process of distillation easy and systematic. Jabir placed great emphasis on experimentation and precision in his work based on its properties, he has described three different types of substances.
There is a wide variety of equipment used to carry out the crystallization process, called crystallizers.
Such equipment can be classified into four general types:
All of these types of equipment have aspects in common:
The glass crystallizer is generally made of borosilicate which is a glass with high thermal resistance and very good chemical resistance to water, salt solutions, acids, bases and organic solvents.
Industrial crystallization processes are aimed at large-scale production of crystalline products by forming a suspension of growing particles in a solution. The quality of the product is defined by the type of crystalline phase produced, the crystal size distribution, the morphology of the crystal and the purity of the product. These aspects of product quality are determined by the crystallization sub-processes, of which the nucleation and growth of the crystals are generally of major importance.
The driving force of these subprocesses is usually established by evaporating the solvent to increase concentration or by cooling the solution to decrease solubility. Recent pharmaceutical research interest in continuous crystallization processes is reinforced by claims of improved product quality, efficient use of materials and energy resources, and reduced waste stream.
Improvements in the understanding of the fundamentals of crystallization have led to the successful resolution of problems encountered in industrial practice. A reduction in process variability and improvements in product quality (crystal shape, product handling, post-processing performance, etc.)
Crystallization is one of the most widely used technologies in the chemical industry, and the robustness of the process governs the productivity and economy of the process. In particular, the pharmaceutical and food sectors are using crystallization for optimized separation, purification and solid form selection. For example, crystallization is the most common method of forming pharmaceutical solids for the development of active pharmaceutical ingredients (APIs).
Optimization of particle properties, such as particle size and shape distribution, is paramount, as the physical form determines the quality and efficacy of the drug. Many pharmaceutical drugs have poor physiochemical profiles, such as low solubility in biological fluids. Significant research and development efforts have been made to develop a solid shape landscape covering all possible solid structures, including polymorphs, solvates, co-crystals, salts and the amorphous phase to enhance the development of the active pharmaceutical ingredient (API).
Snowflake formation, Honey crystallization in a jar, Stalactite and stalagmite formation, Deposition of gemstone crystals
Examples of artificial crystallization include: Cultivation of sugar crystals in a jar Production of synthetic gems
Jabir bin Hayyan was born in 721 A.D. in the Persian city of Tus. He achieved excellence in the fields of Alchemy, Astronomy, Physics, Pharmacy, Philosophy, Astrology and Geography. It has been discovered that it recognizes the early works of Plato, Socrates, Aristotle and Pythagoras, as well as the knowledge of the prominent Muslim jurist Imam Jafar as-Sadiq on alchemy, chemistry, philosophy and astronomy.
Jabir bin Hayyan prepared chemicals, discovered many acids and prepared, as well as, improved many chemical processes. He emphasized the importance of experiencing one’s theory, and that is why we see many inventions and discoveries made by him. In fact, he was the one who introduced experimental techniques into the field of chemistry.
He discovered such significant chemical procedures as crystallization, fusion, distillation, calcination, reduction, liquidation and sublimation. The dyeing of cloth and leather, as well as the preparation of steel are also associated with this great Islamic scientist.
He gave a detailed description of acetic acid, tartaric acid and citric acid. The discovery of hydrochloric acid, sulfuric acid and nitric acid are some of the major contributions made by Jabir bin Hayyan. He combined nitric acid with hydrochloric acid and invented another acid called today “Aqua Regia”. The latter is strong enough to dissolve gold. His division of the substance into three different classes functioned as the basis for the modern classification of metals and non-metals.
The crystallization process produces a physical change in the objects. It leads to the formation of crystalline structures. It is not possible to distinguish clearly between crystallization processes. But we can identify two categories of crystallization processes, namely crystallization by cooling and crystallization by evaporation.
Cooling Crystallization: Many substances, when dissolved in solvent, form crystals when cooled under favorable conditions in this process. However, this method is not very common and has many limitations. Generally, the substance is heated and then allowed to cool in this method. Evaporative crystallization is more common than cooling crystallization.
The evaporation of seawater leaves salt behind. The pure crystals of substance are deposited after the evaporation of the solutions. But this method requires a high concentration of the substance in the solution. Therefore, its mass ratio solute / solvent must be high.
Simple experiments to demonstrate crystallization:
Experiment 1
Experiment 2
Crystallization by cooling is attractive when the solubility of the product increases significantly with increasing temperature. In a cooling crystallization process, the feed is cooled in a heat exchanger, which can be located inside the crystallizer or in an external circuit.
The wall of the crystallizer can be used as an internal heat exchanger, but the heat exchanger can also be integrated into the crystallizer in the form of cooled tubes or plates. Crystallization can take place when the liquid is cooled down to a temperature below the equilibrium solubility. The lowest temperature in the system is on the surface of the heat exchanger.
Therefore, cooling must be done carefully to avoid nucleation on the cold surface of the heat exchanger, which will cause scaling. Generally, measures to prevent this unwanted phenomenon are to reduce the temperature difference between the refrigerant and the crystallization solution.
Increase the velocity of the liquid along the surface of the heat exchanger to level the temperature difference along the heat exchanger or use a scraper to keep the surface of the heat exchanger free of solids. Alternative cooling methods that do not require a heat exchanger are flash cooling involving (partial) evaporation of the solvent or direct cooling by inserting a cold gas or refrigerant.
Fractionated crystallization is a term used to describe a process in which repeated crystallization steps are used to increase product purity and/or increase process yield. Applications can be found in metal, metal refining, oil and gas, e.g. dewaxing of oil, food, e.g. fractionation of palm oil and freezing concentration and in chemical industries, e.g. degreasing with kerosene wax or ultra-purification of chemicals.
In a fractionated crystallization process, the crystals formed in the first stage are separated from the mother water with devices such as filters, centrifuges or washing columns and are rewound for use as feed in a second crystallization stage.
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