The main task of evaporation of the evaporator
In the pharmaceutical, chemical, light industry and food industries, it is often necessary to concentrate a solution containing a solid solute to obtain a solid product or to prepare a solvent. The concentration method commonly used in the industry generally heats a dilute solution to boil some solvent. Vaporization and continuous removal to increase the concentration of solutes in the solution is called evaporation. The task of the evaporation operation is to separate the non-volatile solutes from the volatile solvents.
(top) is a simplified diagram of the flow of nitric acid in aqueous solution. The evaporator is mainly composed of a heating chamber and a separation chamber. The lower part is a heating chamber and the upper part is a separation chamber. The heating chamber is a heat exchanger composed of a plurality of heat exchange tubes. Steam is passed between the tubes as a heating heat source, and the inside of the tube is a solution space. The separation chamber is used for gas-liquid separation. The dilute nitric acid is preheated by the preheater according to the solution (called the liquid), and then added to the evaporator, heated and boiled in the heat exchange tube of the heating chamber, and the concentrated nitric acid is used as a solution (referred to as a completion liquid) from the evaporator. The bottom is discharged, and the water vapor generated by vaporization is separated by gas and liquid, sent to a condenser, and condensed and removed.
The evaporation operation is mainly heated by saturated steam, and the material to be evaporated is mostly an aqueous solution, so the steam generated during evaporation is also water vapor. For the sake of distinction, the former is called heating steam (if it is from a boiler, also called raw steam), the latter is called For secondary steam. In the operation, the secondary steam is generally removed by condensation, otherwise the steam and the boiling solution tend to balance, so that the evaporation process cannot be performed. The operation of directly condensing secondary steam without using its heat of condensation is called single-effect evaporation. If the secondary steam is introduced to the next evaporator as heating steam to utilize its heat of condensation, this series evaporation operation is called Multi-effect evaporation.
The evaporation operation can be carried out under normal pressure, pressure or reduced pressure. The atmospheric pressure evaporation can be carried out by using an open device to discharge the secondary steam into the atmosphere. When the vacuum is evaporated, the operating pressure on the solution side is lower than the atmospheric pressure. The vacuum pump is used to extract the non-condensable gas and maintain the vacuum of the system. The pressurized evaporation is carried out under pressure, so the boiling point of the solution rises. The temperature of the secondary steam generated. Also high, it is possible to use secondary steam as a heating agent for other equipment.
From the above, the essence of the evaporation process is the heat transfer process between the vapor condensation on one side of the heat transfer wall and the boiling of the solution on the other side. Therefore, the evaporator is also a heat exchanger. However, the evaporation process is different from the general one. The particularity of the heat transfer process.
1 The solution contains a non-volatile solute, so its vapor pressure is lower than that of the solvent at the same temperature (its pure water), in other words, at the same pressure, the boiling point of the solution is higher than the boiling point of pure water. Under the same conditions, the heat transfer temperature difference of the evaporation solution is smaller than the heat transfer temperature difference of the evaporated pure solvent, and the higher the solution concentration, the more significant the phenomenon. Therefore, an increase in the boiling point of the solution is an important issue that must be considered in the evaporation operation.
2 Under the industrial scale, the evaporation of solvent is often very large, it requires a large amount of heating steam, and a large amount of secondary steam is generated. How to use the latent heat of secondary steam is a key issue to be considered in the evaporation operation.
3 The specificity of the solution determines the special structure of the evaporator. For example, some solutions may foul or precipitate crystals during evaporation. In the structural design of the evaporator, try to prevent or reduce the formation of scale layer, and the heating surface should be made. Easy to clean. Some materials are heat sensitive, others have large viscosity or are highly corrosive, etc. It is necessary to design or select an evaporator of suitable structure according to these characteristics of the materials.
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