Chemical separation technology is an important branch of chemical engineering, whether it is petroleum refining, plastics chemical fiber, hydrometallurgy, isotope separation, or the refining of biological products, the preparation of nano-materials, desulfurization of flue gas and fertilizer pesticide production, and so on can not be separated from the chemical separation technology. Chemical production of raw materials and products in the vast majority of mixtures, the need to use the system of differences in the physical properties of the components or with the help of the separator to make the mixture to be separated and purified. It is often a key step to obtain qualified products, fully utilize resources and control environmental pollution.
Along with the rapid development of the chemical industry, separation technology has also gained high speed development. On the one hand, the research and application of traditional separation technology has been progressing continuously, the separation efficiency has been improved, the processing capacity has been increased, the engineering enlargement problem has been solved gradually, and new separation devices have been appearing continuously; on the other hand, in order to adapt to the technological progress and put forward new separation requirements, the development, research and application of membrane separation technology, supercritical extraction technology, adsorption technology and other existing separation technologies have become the frontiers of the separation engineering research. The topic.
The importance of chemical separation process
Chemical separation process is the operation of separating a mixture into two (or several) products of different compositions. A standard chemical production plant consists of a
Reactor and a number of separators for the purification of raw materials, intermediates and products. Firstly, the separation process supplies the chemical reaction with raw materials of the right quality, removes hazardous substances and improves yields; secondly, the reactants are separated and purified to obtain the right products and to recycle unreacted products; furthermore, it plays an invaluable role in the full utilization of resources and in the protection of the environment. In addition, the separation process in the full utilization of resources and protection of the environment to play an indispensable role, so the separation process in the chemical industry production occupies a very obvious position.
Classification and Characteristics of Separation Process
Separation processes commonly used in chemical production can be divided into two categories: mechanical separation and mass transfer separation. The separation object of mechanical separation process is a mixture composed of more than two phases. The purpose is simply to separate the phases, as long as a simple mechanical method can be separated from the two phases, and there is no material transfer phenomenon between the two phases; for example, filtration, sedimentation, centrifugal separation, cyclone separation and electrostatic precipitation and so on.
The mass transfer separation process for the separation of various homogeneous mixtures, which is characterized by the phenomenon of mass transfer occurs, according to the different physicochemical principles based on the mass transfer separation process commonly used in industry is divided into equilibrium separation process and the rate of separation process, that is, the process of separation of energy and matter.
1. Equilibrium separation process
The process is to make the homogeneous mixture system into a two-phase system with the help of a separation medium, and then the components of the mixture in the phase equilibrium of the two phases in different distribution based on the realization of the separation. Examples are: evaporation, distillation, absorption, adsorption, extraction, leaching, drying, crystallization, ion exchange, etc.
For example, in the process of traditional extraction, its energy is transferred to the extractant without rules, and then the extractant diffuses into the substrate material, and finally the substrate is dissolved or entrapped with a variety of components diffuse out. Microwave extraction is a new technology to improve the efficiency of microwave energy extraction, due to the existence of substances with different dielectric constants, the degree of absorption in the microwave energy will be different, so the heat generated and the heat transferred to the surrounding environment are also different.
In the microwave field, the size of the absorption capacity of the substrate material part of the region to be selectively heated, from which the extracted material through the substrate to separate, and then into the microwave absorption capacity is relatively weak, the dielectric constant is relatively small extractant.
Microwave extraction process:
Microwave extraction process is roughly as follows: raw material pretreatment (cleaning, crushing or slicing) → material mixing and solvent → microwave extraction → filtration → concentration → separation → extraction of components
Balanced separation process has experienced a long period of application practice, with the progress of science and technology and the rise of high-tech industries, increasingly perfect and constantly developing, evolved a variety of new separation technology with characteristics. In the traditional separation process, distillation is still listed as the first petroleum and chemical separation process, so strengthen the method in the continuous research and development.
2. Rate separation process
Rate separation process is in some kind of driving force (concentration difference, pressure difference, temperature difference, potential difference, etc.) under the action, sometimes in the selective permeability of the membrane with the use of the components of the diffusion rate of the difference between the components to achieve the separation of components. The raw materials and products handled by this type of process usually belong to the same phase, with only compositional differences.
The principle of membrane separation technology is a unit operation that utilizes the difference in permeation rates of each component in the fluid to the membrane to achieve separation of components. The membrane can be solid or liquid, the fluid being processed can be liquid or gas, and the driving force for the process can be a pressure difference, concentration difference, or potential difference.
Microfiltration, ultrafiltration, reverse osmosis, dialysis and electrodialysis are the more mature membrane separation technologies with large-scale industrial applications and markets. Among them, the common point of the first four is used to separate the liquid containing dissolved solute or suspension material, solvent or small molecule solute through the membrane, solute or macromolecule solute is retained by the membrane, the different membrane process of solute particles of different sizes of retention. Electrodialysis is the use of charged membrane, driven by the electric field force, from the aqueous solution or electrolyte enrichment.
Gas separation and osmotic evaporation are two membrane technologies that are being developed and applied. Gas separation is more mature, with industrial-scale applications such as separation of oxygen and nitrogen in air, separation of hydrogen from ammonia plant mixtures, and separation of carbon dioxide from methane in natural gas. Osmotic evaporation is a membrane separation process with phase change, which utilizes the difference in dissolution and diffusion properties of different components of the mixed liquid in the membrane to achieve separation. Because it can be used to remove trace water in organic matter, trace organic matter in water, as well as to realize the separation between organic matter, the application is promising.
Emulsion membrane is a branch of liquid membrane separation technology, which is a membrane separation operation with liquid membrane as the separation medium and concentration difference as the driving force. Liquid membrane separation involves three phases of liquid, the raw material phase containing the separated components, the product phase receiving the separated components, and the membrane phase between the above two phases. Liquid membrane separation is mainly used in hydrocarbon separation, wastewater treatment, and extraction and recovery of metal ions.
Mass transfer separation process of distillation, absorption, extraction and some other unit operations with a long history has been widely used, membrane separation and field separation and other new separation technologies in product separation, energy saving and environmental protection has shown their superiority.
Types of separation methods and principles of selection
1. Types of separation methods
There are many different types of material separation methods, that is because there are a variety of chemical production materials, and in the process of selecting the separation method, often in accordance with the separation of various components of the material is separated in accordance with the different chemical and physical properties to determine the choice; in accordance with the chemical and physical properties to distinguish between, there are the following five types of common separation methods: ① solid mixtures of separation methods, ② gas-solid-phase mixtures of separation methods, ③ liquid mixtures of separation methods, ③ liquid mixtures of separation methods, separation methods, separation methods, separation methods, separation methods, separation methods, separation methods, separation methods ③ liquid mixture separation method, ④ liquid-solid phase mixture separation method, ⑤ gas mixture separation method.
2. Principle of separation method selection
In the selection of separation methods, the degree of refinement of the product and the production value of the product to be considered, for a high degree of refinement and high production value of the product, do not need to consider the cost of separation, you can choose some of the high efficiency separation methods, for some relatively low production value and a large number of products, you need to consider the cost of separation, you can choose those separation steps less or relatively simple separation methods.
Try to avoid the presence of solid-containing logistics in the production process, should be as far as possible in advance to remove the solids in the logistics, due to their relatively large energy consumption in the transportation, and the liquid or gas-containing logistics is quite easy to form pipeline blockage.
In the separation of materials mixed with many different substances, the separation order should be considered as follows: in order to avoid the process being affected, should try to separate the substances that are easy to lead to extremely harmful and side reactions, and at the same time, the substances that need to be separated under high pressure should also be considered to be separated first; in addition, the first to be separated out of the easiest to separate the components, and left to the last to be separated is the most difficult to separate the components.
Selection of separation methods or the main principles of economic rationality and technical reliability to consider. For example, distillation and extraction are both methods of separating liquid mixtures, according to the degree of technological maturity, distillation lies above the extraction, if you can take the distillation of separated materials, should avoid using extraction, if the boiling point of the mixture of large deviations, the use of distillation can be simple to carry out the separation, there is no need to use distillation, so that the operating costs and selection of investment are relatively low.
The selection of separation method must be targeted, because it is a technical work, only to be separated from the chemical and physical properties of the material, as well as the separation requirements are clearly grasp the best choice.
Wide range of chemical applications, the needs of the environment are illustrated in the chemical separation process in the national economy and people's livelihood in the status and role, and demonstrates the broad prospects for the separation process, modern society can not be separated from the separation technology, the separation of technology development in the modern society.
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