It is mainly composed of heating chamber and evaporation chamber.Heating chamber directionliquidProvide the heat required for evaporation to make the liquid boilvaporization；The evaporation chamber completely separates the gas and liquid phases.The vapor generated in the heating chamber contains a large amount of liquid foam. After reaching the evaporation chamber in a larger space, these liquids can be separated from the vapor by their own condensation or demister.The demister is usually located on the top of the evaporation chamber.
Evaporators are divided into circulation type and membrane type
The evaporator is divided into normal pressure, pressurization and decompression according to the operating pressure.According to the movement of the solution in the evaporator, it can be divided into: ① circulation type.Boiling solution passes through the heating surface several times in the heating chamber, such as central circulation tube type, hanging basket type, external heating type, Levin type and forced circulation type.
② One way type.The boiling solution passes through the heating surface at one time in the heating chamber without circulating, i.e., the concentrated solution is discharged, such as rising film type, falling film type, stirred film type and centrifugal film type.
③ Direct contact type.Direct contact heat transfer between heating medium and solution, such asSubmerged combustion evaporator。During the operation of evaporation device, a large amount of heating steam will be consumed. In order to save heating steam, multi effect evaporation device and steam recompression evaporator can be used.Evaporators are widely used in chemical and light industries.
Evaporator in medicine
Evaporator in medicine
Volatile inhalation anesthetics are liquid at room temperature.The evaporator can effectively evaporate volatile anesthetic liquid into gas, and can accurately adjust the concentration of anesthetic vapor output.The evaporation of anesthetic requires heat, and the temperature around the evaporator is the main factor determining the evaporation rate of volatile anesthetic.Temperature flow compensation evaporators are widely used in contemporary anesthesia machines, that is, when the temperature or fresh air flow changes, the evaporation speed of volatile inhalation anesthetics can be kept constant through the automatic compensation mechanism, so as to ensure that the output concentration of inhalation anesthetics leaving the evaporator is stable.Due to the different boiling points andSaturated vapor pressureDue to different physical characteristics, evaporators are drug specific, such as enflurane evaporators and isoflurane evaporators, which cannot be used universally.The evaporators of modern anesthesia machines are mostly placed outside the anesthesia breathing loop, with a separate oxygen flow connected to it. The vaporized inhaled anesthetic vapor is mixed with the main flow before being inhaled by patients.
characteristic
Announce
edit
In engineering, the evaporation process only separates part of the solvent from the solution, while the solute remains in the solution. Therefore, the evaporation operation is to makeVolatile solventSeparation process with nonvolatile solute.Since the vaporization rate of solvent depends on the heat transfer rate, the evaporation operation is a heat transfer process, and the evaporation equipment is a heat transfer equipment. However, compared with the general heat transfer process, the evaporation operation has the following characteristics:
Increase of boiling point of solution
Because the solution contains nonvolatile solute, the vapor pressure of the solution is lower than that of the pure solvent at the same temperature, that is, the boiling point of the solution is higher than that of the pure solvent at the same pressure. The higher the concentration of the solution, the more significant the effect.
Material and process characteristics
In the process of material concentration, solutes or impurities often deposit and crystallize on the heating surface to form a scale layer, which affects heat transfer;Some solutes are heat sensitive and easy to deteriorate if they stay at high temperature for a long time;Some materials have higher corrosivity or viscosity.
energy recovery
The evaporation process is a solvent evaporation process. Due to the great latent heat of solvent evaporation, the evaporation process is a large energy consumption unit operation.Therefore, energy conservation is an important issue that should be considered in evaporation operation.
application
Announce
edit
Evaporation is a unit operation in which the solution containing nonvolatile solute is heated to boiling state by heating, so that part of the solvent is vaporized and removed, thus increasing the concentration of solute in the solvent.Evaporation operation is applied in industrial production in the following occasions:
1. Concentrate the dilute solution to directly produce products or reprocess the concentrated solution (such as cooling crystallization) to produce solid products, such as the concentration of electrolytic caustic soda, the concentration of sugar aqueous solution and the concentration of various fruit juices;
2. Concentrate the solution and recover the solvent at the same time, such asOrganophosphorus pesticideConcentrate and remove benzene from benzene solution, evaporation of alcohol extract in the production of traditional Chinese medicine, etc;
3. In order to obtain pure solvents, such as seawater desalination.
In short, evaporation is widely used in chemical industry, food industry, pharmaceutical industry and other industries.[1]
Classification of evaporation operation
Announce
edit
1. According to evaporation mode:
Evaporator in medicine
Natural evaporation: that is, the solution evaporates at a temperature lower than the boiling point, such as salt drying with seawater. In this case, the solvent vaporizes only on the solution surface, so the solvent vaporization rate is low.Boiling evaporation: The solution is heated to boiling point to evaporate under boiling condition.Industrial evaporation operations are basically of this type.
2. According to heating mode:
Direct heat source heating refers to the evaporation process of mixing fuel and air to make the high-temperature flame and flue gas generated by its combustion directly spray into the evaporated solution through the nozzle to heat the solution and vaporize the solvent.
Indirect heat source heats the wall between vessels and transfers it to the evaporated solution.That isIntermediate wall heat exchangerThe heat transfer process carried out in.
3. According to operating pressure:
It can be divided into normal pressure, pressurization and decompression (vacuum) evaporation operations.Obviously, heat sensitive materials, such as antibiotic solution and fruit juice, should be carried out under reduced pressure.High viscosity materials should be heated by pressurized high temperature heat source (such as heat transfer oil, molten salt, etc.) for evaporation
4. Score by effectiveness:
It can be divided into single effect evaporation and multiple effect evaporation.If the secondary steam generated by evaporation is directly condensed and no longer used, it is called single effect evaporation.If the secondary steam is used as the next effect heating steam and multiple evaporators are connected in series, the evaporation process is called multi effect evaporation.
Characteristics of evaporator
At present, the commonly used interwall heat transfer evaporators can be roughly divided into circulation type and one-way type according to the situation of the solution staying in the evaporator.[2]
Circulating evaporator
In this type of evaporator, the solution circulates in the evaporator.Due to the different causes of circulation, it can be divided into natural circulation and forced circulation.
oneCentral circulating tubular evaporatorThis evaporator is also called standard evaporator.Its heating chamber is composed of vertical tube bundles, with a central circulation tube with a large diameter in the middle, and the other heating tubes with smaller diameters are called boiling tubes.Because the central circulation tube is large, the heat transfer surface occupied by the unit volume solution is smaller than that occupied by the unit solution in the boiling tube, that is, the solution in the central circulation tube and other heating tubes are heated differently, so the density of the vapor liquid mixture in the boiling tube is smaller than that of the solution in the central circulation tube, plus the upward suction of rising steam,It will cause the solution in the evaporator to form a circulating flow that drops from the central circulating pipe and rises from the boiling pipe.This cycle is mainly caused by the density difference of solution, so it is called natural cycle.This effect is conducive to improving the heat transfer effect in the evaporator.
Pulse counter
In order to make the solution have a good circulation, the sectional area of the central circulation pipe is generally 40~100% of the total sectional area of other heating pipes;The height of heating pipe is generally 1~2m;The diameter of heating pipe is 25~75mm.The evaporator is widely used because of its compact structure, convenient manufacture, good heat transfer and reliable operation.However, due to structural constraints, the cycle speed is not large.In addition, the solution circulates continuously in the heating chamber, so that 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 decreases.This is a common disadvantage of circulating evaporators.In addition, the cleaning and maintenance of the equipment are not convenient enough, so this evaporator is difficult to fully meet the production requirements.
2．Suspended basket evaporatorIn order to overcome the shortcomings of the circulating evaporator, such as easy crystallization, easy scaling and difficult cleaning, the structure of the standard evaporator has been improved more reasonably, which is the hanging basket evaporator.The heating chamber 4, like a basket, is suspended at the lower part of the evaporator shell, and the central circulation pipe is replaced by an annular hole between the outer wall of the heating chamber and the inner wall of the evaporator.The solution rises along the center of the heating tube, and then flows downward along the annular gap between the outer wall of the hanging basket type heating chamber and the inner wall of the evaporator to form a cycle.Since the annular area is about 100 to 150% of the total cross-sectional area of the heating tube, the solution circulation speed is larger than that of the standard evaporator, up to 1.5m/s.In addition, the heating chamber of the evaporator can be taken out from the top for maintenance or replacement, and the heat loss is small.Its main disadvantage is its complex structure and high metal consumption per unit heat transfer area.
3．Levin evaporatorThe aboveNatural circulation evaporatorIts circulation speed is not large enough, generally below 1.5m/s.In order to make the evaporator more suitable for evaporating solutions with high viscosity, easy crystallization or serious scaling, and increase the solution circulation speed to extend the operation cycle and reduce the cleaning times.
The structure features that a boiling chamber is added on the heating chamber.The solution in the heating chamber is not boiling in the heating tube due to the effect of the additional static pressure of the liquid column in the boiling chamber. It cannot start boiling until it rises to the boiling chamber and the pressure it receives decreases. Therefore, the boiling vaporization of the solution is moved from the heating chamber to the boiling chamber without heat transfer surface, thus avoiding the formation of crystals or dirt in the heating tube.In addition, the sectional area of the circulating pipe of this evaporator is about 2-3 times of the total sectional area of the heating pipe, and the solution circulation speed can reach more than 2.5 to 3 m/s, so the total heat transfer coefficient is also large.The main disadvantage of this evaporator is that the temperature difference loss caused by the static head effect of the liquid column is large (see 6.3.1 for the meaning). In order to maintain a certain effective temperature difference, the heating steam must have a high pressure.In addition, the equipment is huge, and the materials consumed are large, which requires large workshops.In addition to the above natural circulation evaporator, when evaporating materials with high viscosity, easy crystallization and scalingForced circulation evaporator。In this evaporator, the circulation of solution mainly depends on the external power, and the pump forces it to flow in a certain direction to generate circulation.The circulating speed can be controlled through the flow regulation of the pump, generally above 2.5m/s.The heat transfer coefficient of forced circulation evaporator is also larger than that of normal natural circulation.However, its obvious disadvantage is that the energy consumption is large, and the heating area per square meter needs about 0.4~0.8kW.[3]
once-through evaporator
The main feature of this kind of evaporator is that the solution passes through the heating chamber only once in the evaporator and is discharged as concentrated solution without circulating flow.When the solution passes through the heating chamber, it flows in the form of film on the pipe wall, so it is also known as the liquid film evaporator.According to the different flow directions of materials in the evaporator, one-way evaporators are divided into the following types.
1. literMembrane evaporatorThe heating chamber is composed of many vertical long tubes.The diameter of commonly used heating pipe is 25-50mm, and the ratio of pipe length to pipe diameter is about 100-150.After preheating, the feed liquid is introduced from the bottom of the evaporator, heated and boiled in the heating tube and vaporized rapidly. The generated steam rises at a high speed in the heating tube. Generally, the appropriate outlet steam speed is 20~50m/s under normal pressure, and the steam speed can reach 100~160m/s or more under reduced pressure.The solution is driven by the rising steam, rises along the pipe wall in a film shape and continues to evaporate. The vapor liquid mixture is separated in separator 2, and the liquid is discharged from the bottom of the separator, while the secondary steam is discharged from the top.It should be noted that if the amount of water evaporated from the feed liquid is not large, it is difficult to reach the vapor speed required above, that is, the rising film evaporator is not suitable for evaporation of concentrated solution;It is also not suitable for materials with high viscosity, easy crystallization or scaling.
2．Falling film evaporatorThe difference between a falling film evaporator and a rising film evaporator is that the feed liquid is added from the top of the evaporator, falls along the tube wall to form a film under the action of gravity, evaporates and thickens in the process, and gets concentrated liquid at the bottom.Because the film forming mechanism is different from that of the rising film evaporator, the falling film evaporator can evaporate materials with high concentration, high viscosity (for example, in the range of 0.05~0.45Ns/m2) and heat sensitivity.However, due to the uneven distribution of the liquid film in the tube and the smaller heat transfer coefficient than that of the rising film evaporator, it is still not suitable for materials that are easy to crystallize or scale.
As the solution flows in a film form in a one-way evaporatorConvective heat transfer coefficientIt is greatly improved, so that the solution can reach the required concentration through no circulation in the heating chamber at one time. Therefore, it is better thancirculation evaporator It has greater advantages.The advantages of solution non circulation include: (1) the residence time of solution in the evaporator is very short, so it is especially suitable for the evaporation of heat sensitive materials;(2) The concentration of the whole solution is not always close to the concentration of the finished solution like that of the circulating type, so the effective temperature difference of this evaporator is large.Its main disadvantage is that it is quite sensitive to the fluctuation of feed load, and it is not easy to form film when the design or operation is not suitable, at this time, the convective heat transfer coefficient will decline significantly.
3．scraped film evaporator The evaporator shell is equipped with a heating steam jacket, which is equipped with a rotatable blade, namely a scraper.There are two types of scraper: fixed type and rotor type. The clearance between the former and the inner wall of the shell is 0.5~1.5mm, and the clearance between the latter and the wall of the container varies with the number of revolutions of the rotor.The feed liquid is added from the upper part of the evaporator along the tangent direction (also added to the throw tray coaxial with the scraper).Due to gravity, centrifugal force and the effect of rotating scraper, the solution forms a downward spiral film on the inner wall of the device, and is evaporated and concentrated during this process to complete the liquid discharge at the bottom.This evaporator is a one-way evaporator that uses external power to form film. Its outstanding advantage is that it has strong adaptability to materials, and its residence time is short, usually several seconds or tens of seconds, so it can be adapted to high viscosity (such as tannin extract, honey, etc.) and materials that are easy to crystallize, scale, and heat sensitive.However, its structure is complex and its power consumption is large. The heat transfer surface needs about 1.5~3kW per square meter.In addition, its handling capacity is very small and its manufacturing and installation requirements are high.
Evaporator in direct contact with heat transfer
In actual production, evaporator with direct contact heat transfer is sometimes used.It is to mix fuel (usually gas and oil) with air and burn it in the combustion chamber immersed in solution. The high temperature flame and flue gas generated will be directly injected into the evaporated solution through the nozzle at the lower part of the combustion chamber.High temperature gas is in direct contact with the solution, and heat transfer is carried out at the same time to evaporate and vaporize the water. The generated water vapor and waste flue gas are discharged from the top of the evaporator together.The immersion depth of the combustion chamber in the solution is generally 0.2~0.6m, and the gas temperature out of the combustion chamber can reach more than 1000 ℃.Because of direct contact heat transfer, it has good heat transfer effect and high heat utilization rate.Because no fixed heat transfer wall is required, the structure is simple, and it is especially suitable for evaporation of easily crystallized, scaling and corrosive materials.At present, it has been widely used in waste acid treatment and evaporation of ammonium sulfate solution.However, if the evaporated liquid is not allowed to be polluted by flue gas, this type of evaporator is generally not applicable.Moreover, due to the existence of a large number of flue gas, the utilization of secondary steam is limited.In addition, the nozzle is easily damaged due to immersion in high-temperature liquid.It can be seen from the above introduction that there are many types of evaporator structures, each with its advantages, disadvantages and application occasions.When selecting the model, first of all, it is required to see whether it can adapt to the process characteristics of the evaporated materials, including the viscosity, heat sensitivity, corrosiveness of the materials and whether it is easy to crystallize or scale. Then, it is required to have simple structure, easy manufacturing, low metal consumption, convenient maintenance, good heat transfer effect, etc.
Energy consumption comparison
Announce
edit
Calculated with installed capacity of 19700Kg/h[4]
