Popularization of chemical equipment: types of evaporators
　　 Evaporation equipment classification: cyclic type (non-membrane type) and single-pass type (membrane type).
Circulating (non-membrane) evaporator
Features: The solution is continuously circulated in the evaporator to improve heat transfer and to alleviate solution fouling.
Classification (causes of cyclic motion): natural circulation and forced circulation
Natural circulation: due to the different degrees of heat of the solution in different positions of the heating chamber, a cyclical movement caused by the difference in density occurs;
Forced circulation: relies on external force to force the solution to circulate in one direction.
Circulating type (non-membrane evaporator)
Central circulation tube (or standard) evaporator
The heating chamber is composed of a vertical tube bundle having a tube having a larger diameter in the center of the tube bundle
Thick pipe - downcomer pipe or central circulation pipe, the cross-sectional area is 40-100% of the total cross-sectional area of ​​the heating pipe
Thin tube - boiling tube or heating tube, diameter Ф 25-75mm, length to diameter ratio 20-40
The reason for the circulation: the liquid volume per unit volume of the thin tube is large, and the heat is good, so that the vapor-liquid mixture in the thin tube is smaller than that in the thick tube, and the density difference causes the solution to circulate.
Advantages: good solution circulation, high heat transfer efficiency; compact structure, convenient manufacturing, reliable operation, wide application, known as "standard evaporator".
Disadvantages: The viscosity of the liquid is large and the boiling point is high; the heating chamber is not easy to clean.
Note: The central circulation tube evaporator is suitable for treating solutions with less fouling and less corrosive.
Suspended basket evaporator
This evaporator is an improvement of the central circulation tube evaporator. The heating steam enters the heating chamber from the central steam tube. The heating chamber is suspended in the device and can be taken out for cleaning and replacement. The circulation channel is composed of the heating chamber and the annular gap in the wall surface of the evaporator casing.
Advantages: The solution circulation speed is relatively high, about 1-1.5 m / s; the fouling in the heating tube is improved, and the heat transfer rate is improved.
Disadvantages: The equipment has a large amount of consumables, which occupies a large area and has a large amount of solution retention in the heating tube.
Note: This evaporator is suitable for evaporation of solutions with crystal precipitation.
External thermal evaporator
Structural features: The heating chamber is long, and its length to diameter ratio is 50-100. The heating chamber and the separation chamber are separated.
Movement of material: Since the solution in the circulation pipe is not heated by steam, its density is larger than that in the heating pipe, so the circulation of the solution falling along the circulation pipe and rising along the heating pipe is formed, and the circulation speed can reach 1.5 m/s.
Levin evaporator
Structural features: a 2.7-5m circular cylinder is used as a boiling chamber at the upper end of the heating chamber, so that the heating tube has to bear a large static pressure of the liquid column. The solution can only boil and vaporize when it rises into the boiling chamber, which can avoid crystal precipitation in the heating tube or Fouling; there is a longitudinal parallel partition above the boiling chamber to limit the growth of the bubble.
Advantages: circulation speed (2-3m / s), good heat transfer effect, no crystal blockage, suitable for processing solutions with crystal precipitation or scaling.
Disadvantages: The circulating pipe must maintain a certain height (7-8m), requiring high plant height and consumables at the same time; the liquid column static pressure of the evaporator is large, and the steam with higher pressure is required to maintain a certain temperature difference.
Forced circulation evaporator
Advantages: Circulation with external power (pump), suitable for handling solutions with high viscosity, easy to crystallize or easy to scale.
Disadvantages: The power consumption is usually 0.4-0.8kw / (m2 heat transfer surface), so that the heating surface of this evaporator is limited.
Membrane (single pass) evaporator
Comparison of membrane evaporators and non-membrane evaporators:
The main disadvantage of the non-membrane evaporator is that the amount of stagnant material in the heating chamber is large, so that the material stays at a high temperature for too long, and is not suitable for processing heat sensitive materials.
The membrane evaporator can reach the required concentration only once through the heating chamber, and the residence time is short. During operation, the solution has the best heat transfer effect along the heating tube.
Rising film evaporator
Structure: The heating chamber is composed of single or multiple vertical tubes with a length to diameter ratio of 100-150.
Principle: After the raw material liquid is preheated to the boiling point or near the boiling point, it is introduced into the tube from the bottom of the heating chamber, driven by the high-speed rising secondary vapor, flowing in the form of a membrane along the wall surface, and evaporating at the top of the heating chamber to achieve the desired The concentration of the completion liquid is discharged from the bottom of the separation chamber.
Note: The solution should be preheated to the boiling point or near the boiling point before being introduced into the evaporator; the speed of the secondary vapor in the heating tube should not be less than 10m/s, generally 20-50m/s, and can be as high as 100-160m/s under reduced pressure. . The evaporator is suitable for treating dilute solutions with large evaporation and heat-sensitive or foam-prone solutions; it is not suitable for treating solutions with high viscosity, crystallization or scaling.
Falling film evaporator
Structure: its heating chamber is similar to a rising film evaporator
Principle: The raw material liquid is added from the top of the heating chamber, and the liquid distributor at the end of the tube evenly flows into the heating tube. Under the action of the gravity of the solution, the solution flows down along the inner wall of the tube and evaporates.
In order to make the solution evenly distributed on the wall and prevent the secondary vapor from being directly pulled out from the top of the heating tube, a well-processed liquid distributor must be provided at the top of the heating tube. Falling film evaporators are suitable for the treatment of heat sensitive materials, but are not suitable for handling solutions that are easily crystallized, scaled or have a high viscosity.
Lit-falling film evaporator
Structure: The heating pipe is a vertical hollow pipe. The pipe has a jacket outside, and the inside is heated to vapor, and the pipe is equipped with a rotating stirring blade.
Principle: The raw material liquid enters the tube in the tangential direction. Due to the centrifugal force, gravity and the scraping action of the blade, a film with a falling rotation is formed on the tube wall, and is continuously evaporated, and the finished liquid is discharged from the bottom.
Disadvantages: the structure is complex, the power consumption is large, the heat transfer surface is small, and the processing capacity is not large. "
The evaporator is suitable for high-viscosity, easy to crystallize, easy to foul or heat-sensitive solution evaporation immersion combustion evaporator to mix the fuel gas and air into the solution, the temperature of the combustion gas can be as high as 1200-1800 ° C, due to gas The temperature difference between the liquids is large, and the gas has a strong bubbling effect on the solution, so that the water evaporates quickly, and the distilled secondary vapor is discharged from the top together with the flue gas.
Features: Simple structure, no need for fixed heat transfer surface, high heat utilization. Suitable for the treatment of evaporation that is easily fouled, easily crystallized or corrosive. However, it is not suitable for handling heat sensitive materials. Currently widely used in the waste acid treatment industry.
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