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Original Title: Evaporator Classification Design Selection Information All You Want Is Here 1 Type of evaporator With the continuous development of industrial evaporation technology, the structure and type of evaporation equipment are constantly improved and innovated, with a wide range of different structures. At present, there are more than 60 kinds of evaporation equipment in industry, of which more than 10 types are most commonly used. This section only introduces a few commonly used types. The common evaporator is mainly composed of a heating chamber and a separation chamber. There are many types of heating chambers. At first, jacketed or coiled heating devices are used, followed by horizontal and vertical short-tube heating chambers. Then the vertical long tube liquid film evaporator and the scraper film evaporator were invented. According to the flow of solution in the evaporator, the indirect heating evaporator commonly used in industry can be roughly divided into two types: circulation type and one-way type. 1. Circulation type evaporator The characteristic of this kind of evaporator is that the solution circulates in the evaporator. According to the principle of liquid circulation, it can be divided into two types: natural circulation and forced circulation. The former is the natural circulation caused by the density difference of the solution due to the different heating degree of the solution at different positions of the heating chamber, while the latter is the forced circulation of the solution by external power. At present, the commonly used circulating evaporators are as follows: (1) Central circulation tubular evaporator Expand the full text The structure of the central circulation tube evaporator is shown in the picture. Its heating chamber is composed of a vertical heating tube bundle (boiling tube bundle). In the center of the tube bundle, there is a tube with a larger diameter, called the central circulation tube, whose cross-sectional area is generally 40 ~ 100% of the total cross-sectional area of the heating tube bundle. The central circulation tube evaporator has the advantages of compact structure, convenient manufacture and reliable operation, so it is widely used in industry and is called “standard evaporator”. But in fact, due to the limitation of structure, its circulation speed is relatively low (generally below 0.5m/s); and because the solution circulates continuously in the heating tube, its concentration is always close to the concentration of the finished solution, so the boiling point of the solution is high and the effective temperature difference is reduced. In addition, the cleaning and maintenance of the equipment is not convenient enough. (2) Basket evaporator Since the boiling liquid is in contact with the evaporator shell at a lower temperature, there is less heat loss. The basket-type evaporator is suitable for evaporating the solution which is easy to scale or has crystal precipitation. Its disadvantage is that the structure is complex and the amount of equipment materials required for unit heat transfer surface is large. (3) Externally heated evaporator The characteristic of the external heating evaporator is that the heating chamber is separated from the separation chamber, which not only facilitates cleaning and replacement, but also reduces the total height of the evaporator. Because the heating tube is long (the ratio of the tube length to the tube diameter is 50-100) and the solution in the circulating tube is not heated, the circulating speed of the solution is high, which can reach 1.5m/s. (4) Levin evaporator The advantages of Levin evaporator are high circulation speed and good heat transfer effect. Because the solution does not boil in the heating tube, it can avoid the precipitation of crystals in the heating tube, so it is suitable for the treatment of solutions with crystal precipitation or easy scaling. Its disadvantage is that the equipment is huge and the required plant is high. In addition, since the static pressure of the liquid layer is large, the pressure of the heating steam is required to be high. (5) Forced circulation evaporator This kind of evaporator has the advantages of large heat transfer coefficient and good adaptability to materials with high viscosity or easy crystallization and scaling, but its power consumption is large. 2. One-pass type evaporator The characteristic of this kind of evaporator is that the solution flows along the wall of the heating tube in the form of a film and reaches the required concentration once through the heating chamber, while the residence time is only a few seconds or more than ten seconds. The main advantages of the single-pass evaporator are high heat transfer efficiency, fast evaporation speed and short residence time of the solution in the evaporator, so it is especially suitable for the evaporation of heat-sensitive materials. According to the flow direction of materials in the evaporator and the reasons for film formation, it can be divided into the following types: Rising film evaporator; Falling film evaporator; Rising-falling film evaporator; Scraper film evaporator. (1) Rising film evaporator The heating chamber of the rising film evaporator is composed of one or several vertical long tubes. The diameter of the heating tube is usually 25 ~ 50mm, and the ratio of the tube length to the tube diameter is 100 ~ 150.
The climbing film evaporator is suitable for the solution with large evaporation capacity (i.e. dilute solution), heat sensitivity and easy foaming, but not suitable for the solution with high viscosity, crystal precipitation or easy scaling. (2) Falling film evaporator The falling film evaporator can evaporate the solution with higher concentration, and is also suitable for the material with higher viscosity. However, it is not suitable for the solution which is easy to crystallize or scale. In addition, because the liquid film is not easy to distribute evenly in the tube, its heat transfer coefficient is smaller than that of the climbing film evaporator. (3) Rising-falling film evaporator The rising film and falling film evaporators are installed in a casing to form a rising-falling film evaporator. After being preheated,nutsche filter dryer, the raw material liquid is first raised by the rising film heating chamber, then lowered by the falling film heater, and finally separated from the secondary steam in the separation chamber to obtain the finished liquid. This kind of evaporator is mostly used in the situation where the viscosity of the solution changes greatly in the process of evaporation, the evaporation of water is not large and the height of the workshop is limited. (4) Scraper film evaporator In some cases, the solution may be evaporated to dryness to obtain a solid product directly from the bottom. The disadvantages of this kind of evaporator are complex structure, large power consumption, small heat transfer area, generally 3 ~ 4 m2, the maximum is not more than 20 m2, so its processing capacity is small. 3. Direct contact heat transfer evaporator In actual production, in addition to the above two types of circulating and single-pass evaporators with dividing wall heat transfer, direct contact heat transfer evaporators are sometimes used. Fuel (usually coal gas or heavy oil) is mixed with air and the resulting high-temperature flue gas is directly sprayed into the evaporated solution. The high-temperature flue gas is in direct contact with the solution, making the solution boil and vaporize rapidly. The evaporated water is directly discharged from the top of the evaporator together with the flue gas. Generally, the depth of the combustion chamber of this evaporator in the solution is 200 ~ 600mm, and the temperature of the high temperature flue gas in the combustion chamber can reach more than 1000 ℃. However,, due to the fast heat transfer rate when the gas and liquid are in direct contact, the temperature of the gas leaving the liquid level is only 2 ~ 4 ℃ higher than that of the solution. The nozzle of the combustion chamber is easy to be damaged due to its use at high temperature, so it should be made of high temperature and corrosion resistant materials, and its structure should be considered to be easy to replace. The submerged combustion evaporator is characterized by simple structure and high heat transfer efficiency. The evaporator is especially suitable for the evaporation of materials which are easy to crystallize, scale or corrode. At present, this kind of evaporator has been widely used in the treatment of waste acid and the evaporation of ammonium sulfate solution. However, it is not suitable for the treatment of materials that cannot be contaminated by flue gas, and its secondary steam is also difficult to use. Due to the limitation of space, the working principle of various evaporators is not introduced in detail in this article. If you want to know this content, you can search for evaporation crystallization in the chemical 707 app to see more details. There are so many evaporators, so how to design and calculate the evaporation process? Let’s learn together! 2 Evaporator design steps 1. The calculation of multi-effect evaporation generally adopts iterative calculation method. (1) According to the process requirements and the nature of the solution, determine the operating conditions of evaporation (such as heating steam pressure and condenser pressure), the type of evaporator (rising film evaporator, falling film evaporator, forced circulation evaporator, wiped film evaporator), process and number of effects. (2) According to the production experience data, preliminarily estimate the evaporation capacity of each effect and the composition of the finished liquid of each effect. (3) Based on experience, assume that the pressure drop of steam passing through each effect is equal, and estimate the boiling point of solution in each effect and the effective total temperature difference. And (4) calculating the evaporation capacity and the heat transfer capacity of each effect according to the enthalpy balance of the evaporator. And (5) calculating the heat transfer area of each effect according to the heat transfer rate equation. If the obtained effective heat transfer areas are not equal, the effective temperature difference shall be redistributed as described below, and steps (3) to (5) shall be repeated until the obtained effective heat transfer areas are equal (or meet the accuracy requirements given in advance). 2. Calculation method of evaporator The calculation method of multi-effect evaporation is introduced by taking the evaporation device with three-effect cocurrent feeding as an example. (1) Estimate the evaporation capacity of each effect and the composition of finished liquid In the process of evaporation, the total evaporation is the sum of the evaporation of each effect. The feed liquid in any one effect is composed of In general, the evaporation capacity of each effect can be estimated according to the average value of that sent by the General Political Department, namely For the multi-effect evaporation of cocurrent operation, due to the self-evaporation phenomenon, it can be estimated according to the following proportion.
For example, In the above categories: W — total evaporation capacity, kg/H; W 1 ，W 2 ，… , W n — evaporation capacity of each effect, kg/H; F — Flow of raw material solution, kg/H; x 0 , x 1 ,… , X n — composition and mass fraction of raw material solution and finished solution of each effect. (2) estimate that difference between the boiling point and the effective total temperature of each effect solution In order to obtain the boiling point temperature of each effect, it is necessary to assume the pressure. Generally, the heating steam pressure and the pressure in the condenser (or the final effect pressure) are given, and the pressure of other effects can be determined according to the assumption that the steam pressure drop between effects is equal. That is Where: -difference between heating steam pressure and secondary steam pressure of each effect, Pa; P 1 — pressure of the first effect heating steam, Pa; P ‘K — pressure of secondary steam in the last effect condenser, Pa. The total temperature difference of effective heat transfer in multi-effect evaporation can be calculated by the following formula: Where: -total effective temperature difference, which is the sum of effective temperature difference of each effect, ℃; T 1 — temperature of the first effect heating steam, ℃; T ‘K — the saturation temperature of the secondary steam at the operating pressure of the condenser, ℃; — Total temperature difference loss, which is the sum of temperature difference loss of each effect, ℃. In the formula — loss of temperature difference due to drop in vapor pressure of the solution, ℃; — loss of temperature difference due to the static pressure of the solution in the evaporator, ° C; — Temperature difference loss caused by pressure drop due to pipeline fluid resistance, ℃. The solutions of, and are respectively introduced as follows: a. The temperature difference loss caused by the decrease of the vapor pressure of the solution can be corrected by the correction factor method and the Duhring method. The rule is obtained. In the formula — temperature difference loss caused by solution vapor pressure drop under normal pressure, ℃; The boiling point, t a, of some solutions at atmospheric pressure can be obtained from the manual; — Correction factor, dimension one. Where: T ‘1 — Boiling point of water at operating pressure, i.e. saturation temperature of secondary steam, ℃; R ‘— vaporization heat of secondary steam under operating pressure, kJ/kg. Turing’s rule: The boiling point of a solution is linearly related to the boiling point of a standard liquid (usually water) at the same pressure. A set of straight lines, called Turing lines, can be obtained on a rectangular coordinate graph with the boiling point of water as the abscissa, the boiling point of the solution as the ordinate, and the composition of the solution as the parameter. According to the composition of the solution and the boiling point of water at the same pressure, the boiling point of the solution at the same pressure can be found out by using the Duhring diagram, and then the value can be obtained. An approximation of the boiling point of a liquid at various pressures can also be calculated from Turing’s rule. This method is based on the fact that the ratio of the difference between the two boiling points of a liquid at two different pressures to the difference between the two boiling points of water at the same pressure is a constant, namely Obtaining the value of K, the t ‘A of the boiling point at any other pressure can be obtained from the following equation: （1-11） Therefore, the value of the solution can be calculated without using the Duhring diagram. b. Loss of temperature difference due to the static pressure of the solution in the evaporator Some evaporators are in the operating room, and the solution in the evaporator needs to maintain a certain liquid level, so the pressure inside the solution in the evaporator is greater than the pressure at the liquid level, resulting in a higher boiling point inside the solution than at the liquid level. The difference between the two is the temperature difference loss caused by the static pressure of the solution. For the sake of simplicity, the boiling point inside the solution can be found by the average pressure of the liquid surface and the bottom layer. The average pressure is approximately estimated according to the static equation: （1-12） Where: P m — average pressure between the liquid level and the bottom in the evaporator, Pa; P ‘— pressure of secondary steam, i.e. pressure at liquid surface, Pa; ρ — average density of solution, kg/m3; L — height of liquid layer, m; G — acceleration of gravity, m/S2. （1-13） Where: TPM — the boiling point of water calculated according to the average pressure, ℃; TP — boiling point of water calculated according to the secondary vapor pressure, ℃.
Temperature difference loss caused by pressure drop due to pipeline flow resistance In multi-effect evaporation, when the secondary steam of each effect before the last effect flows to the heating chamber of the second effect, the pressure decreases due to pipeline resistance, and the saturation temperature of the steam also decreases accordingly. The resulting temperature difference loss is. According to the experience, the temperature difference loss caused by the pipeline resistance between each effect is taken as 1 ℃. According to the estimated secondary steam pressure of each effect And that Los of the temperature difference, the boiling point t of each effective solution can be estimate from the following equation. （1-14） 3. Preliminary Calculation of Heating Steam Consumption and Evaporation Water Volume of Each Effect The enthalpy balance formula of the first effect is The evaporation capacity Wi of the first effect can be obtained from the formula (1-15). If the concentration heat of the solution and the heat loss of the evaporator are taken into account in the enthalpy equation, the heat utilization coefficient η should also be taken into account. For the evaporation of general solution, η can be taken as 0.98-0.7 (where X is the composition change of the solution, expressed in mass fraction). Where: Di — heating steam quantity of the ith effect, kg/H, when no additional steam is extracted, R I — vaporization heat of heating steam of the first effect, kJ/kg; R ‘I — vaporization heat of secondary steam of the first effect, kJ/kg; C PO — Specific heat capacity of feed solution, kJ/ (kg · ℃); C PW — specific heat capacity of water, kJ/ (kg · ℃); T I, t i-1 — boiling point of the solution of the i-th effect and the (i-1) -th effect, ℃; ηi — heat utilization coefficient of the ith effect, dimension is one. For the consumption of heating steam (live steam), it can be obtained by listing the enthalpy balance formula of each effect and solving it with the formula (1-2). 4. Distribution of Heat Transfer Area and Effective Temperature Difference of Evaporator in Each Effect The heat transfer rate equation for any effect I （1-17） Where: Q I — heat transfer rate of the ith effect, W; K I — heat transfer coefficient of the ith effect, W; Si — heat transfer area of the ith effect, m2; T I — heat transfer temperature difference of the ith effect, ℃. The purpose of effective temperature distribution is to calculate the heat transfer area Si of evaporation. Now take the triple effect as an example, namely （1-18） In the formula （1-19） （1-20） In multi-effect evaporation, in order to facilitate manufacturing and installation, evaporators with equal heat transfer area of each effect are usually used. If the heat transfer areas obtained from Equation (1-18) are not equal, the areas shall be redistributed according to the principle of each effective area. Effective temperature difference. The method is as follows: Let t ‘denote the effective temperature difference when the effective surfaces are equal, then Compared with the formula (1-18), Add the three formulas in formula (1-22) to get In the formula, ∑ t is the sum of the effective temperature difference of each effect, which is called the total effective temperature difference, ℃. After the heat transfer area S is obtained from Equation (1-23), the effective temperature difference of each effect can be redistributed from Equation (1-22). Repeat the above steps until the obtained heat transfer area of each effect is equal, and the area is the required one. 3 Selection of evaporator As mentioned above, there are many types of evaporator structures. When selecting the type of evaporator or designing the evaporator, on the premise of meeting the requirements of production tasks and ensuring product quality, it is also necessary to take into account the simple structure, easy manufacture, convenient operation and maintenance, good heat transfer effect and so on. In addition, it is also necessary to have good adaptability to the process characteristics of the material to be evaporated,, including the viscosity, heat sensitivity, corrosiveness of the material and whether it crystallizes or scales.
The climbing film evaporator is suitable for evaporating low-boiling point alcohols or low-boiling point volatile organic compounds; The falling-film evaporator is suitable for the concentration of water-soluble substances and the evaporation of materials which are not easy to crystallize or scab; The horizontal falling film evaporator is suitable for the evaporation of volatile organic compounds with medium boiling points such as ethanol; The natural internal circulation evaporator is suitable for the concentration and evaporation of general materials; The forced external circulation evaporator is suitable for the evaporation and crystallization of high-concentration and easy-to-crystallize materials; The combination of falling film and forced external circulation evaporator is suitable for the concentration and crystallization of low-concentration and easy-to-crystallize materials; The combination of natural circulation and forced external circulation evaporator is suitable for the crystallization of the thickener of the medium and low concentration materials which are easy to crystallize; The combination of forced external circulation and multi-effect evaporation crystallization is suitable for secondary concentrated crystallization containing high-boiling inorganic or organic substances; Selection Criteria for Common Evaporation Equipment Return to Sohu to see more Responsible Editor:.