The Introduction to Evaporator（Two）

In the previous article, we already have a certain understanding of the evaporator. This article will continue the partition heat transfer evaporator.

 

Commonly used partition wall heat transfer evaporators can be roughly divided into two types: circulating type and single pass type according to the situation of the solution staying in the evaporator.

 

Circulating evaporator

 

In this type of evaporator, the solution circulates in the evaporator. Due to different causes of circulation, it can be divided into natural circulation and forced circulation.

 

Central circulation tube evaporator

 

This evaporator is also called a standard evaporator. Its heating chamber consists of vertical tube bundles, with a central circulation tube with a large diameter in the middle, and the remaining heating tubes with smaller diameters are called boiling tubes. This type of evaporator requires a certain vacuum clamp fitting to store solutions and gases. Due to the larger central circulation tube, the heat transfer surface occupied by the unit volume of 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 to different degrees. In this case, the density of the vapor-liquid mixture in the boiling tube is lower than the density of the solution in the central circulation tube. Coupled with the upward suction of rising steam, it will cause the solution in the evaporator to form a circulating flow that descends from the central circulation pipe and rises from the boiling pipe. This cycle is mainly caused by the density difference of the solution, so it is called natural cycle. This effect is conducive to the improvement of the heat transfer effect in the evaporator.

 

In order to have a good circulation of the solution, the cross-sectional area of the central circulation tube is generally 40 to 100% of the total cross-sectional area of other heating tubes; the height of the heating tube is generally 1 to 2m; and the diameter of the heating tube is between 25 to 75mm. In addition, its commonly used valve types are mainly 1.5 tri clamp valve or butterfly valve food grade. This type of evaporator is widely used due to its compact structure, convenient manufacturing, good heat transfer, and reliable operation. However, due to structural limitations, the cycle speed is not large. In addition, the solution is continuously circulated in the heating chamber, so that its concentration is always close to the concentration of the completion liquid. So the boiling point of the solution is high, and the effective temperature difference is reduced. This is a common drawback of circulating evaporators. In addition, the cleaning and maintenance of the equipment is not convenient enough, so it is difficult for this evaporator to fully meet the production requirements.

 

Suspended basket evaporator

 

In order to overcome the shortcomings of the evaporating liquid in the circulating evaporator, such as easy to crystallize, easy to scale and not easy to clean, the structure of the standard evaporator has been more rationally improved. Usually sanitary hose clamp or stainless steel tri clamp will be installed inside. The heating chamber is like a basket, suspended from the lower part of the evaporator shell, and the central circulation tube 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 down through the annular gap between the outer wall of the hanging basket type heating chamber and the inner wall of the evaporator to form a circulation. Since the annulus area is about 100 to 150% of the total cross-sectional area of the heating tube, the solution circulation speed is greater than that of the standard evaporator, up to 1.5m/s. In addition, the heating chamber of this evaporator can be taken out for repair or replacement, and the heat loss is also small. Its main disadvantage is that the structure is complicated, and the metal consumption per heat transfer area is relatively large.

 

Levin evaporator

 

The above-mentioned natural circulation evaporator, the 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 to crystallize or serious fouling, it needs to increase the solution circulation speed to extend the operation cycle and reduce the number of cleaning.

 

The structural feature is that a boiling chamber is added on the heating chamber. In addition, some distillation towers,even laboratory distillation column will be added, but the distillation column price is relatively high. The solution in the heating chamber is not boiled in the heating tube due to the additional static pressure of the liquid column in the boiling chamber until it rises into the boiling chamber. Only when the pressure is reduced can the boiling start, so the boiling vaporization of the solution moves 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 cross-sectional area of the circulation tube of this evaporator is about 2 to 3 times the total cross-sectional area of the heating tube, 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 type of evaporator is the large loss of temperature difference caused by the static head effect of the liquid column. In order to maintain a certain effective temperature difference, the heating steam needs to have a higher pressure. In addition, the equipment is huge and consumes a lot of materials, requiring a large factory building. In addition to the above natural circulation evaporator, a forced circulation evaporator is also used when evaporating materials with high viscosity, easy to crystallize and scale. In this evaporator, the circulation of the solution mainly depends on the external power, and it is forced to flow in a certain direction with a pump to produce circulation. The size of the circulation speed can be controlled by the flow adjustment of the pump, which is generally above 2.5m/s. The heat transfer coefficient of the forced circulation evaporator is also larger than that of general natural circulation. But its obvious shortcoming is that the energy consumption is large, and the heating area per square meter needs about 0.4 to 0.8kW.

 

Single pass evaporator

 

The main feature of this large type of evaporator is that the solution only passes through the heating chamber once in the evaporator, and becomes concentrated liquid without circulating flow. When the solution passes through the heating chamber, it flows in a film shape on the wall of the tube, so it is also commonly known as a liquid film evaporator. According to the different flow directions of materials in the evaporator, the single-pass evaporator is divided into the following types.

 

Rising film evaporator

 

Its heating chamber is composed of many vertical long tubes. The diameter of a commonly used heating tube is 25-50 mm, and the ratio of tube length to tube diameter is about 100-150. The feed liquid is introduced from the bottom of the evaporator after preheating, boils in the heating tube and quickly vaporizes, and the generated steam rises at high speed in the heating tube. Generally, the suitable outlet steam speed when operating under normal pressure is 20-50m/s, and the steam speed when operating under reduced pressure can reach 100 to 160m/s or more. The solution is driven by the rising steam, which rises in a film along the tube wall and continues to evaporate. The vapor-liquid mixture is separated in the separator 2, the completed liquid is discharged from the bottom of the separator, and the secondary steam is discharged at the top. It should be noted that if the amount of water evaporated from the feed liquid is not large, it is difficult to achieve the above-mentioned required steam speed, that is, the rising film evaporator is not suitable for the evaporation of a concentrated solution. It is also not suitable for materials with high viscosity and easy to crystallize or scale.

 

Falling film evaporator

 

Falling film evaporator is in charge of separating substances from the mixture. In cannabis-processing process, falling film evaporator is to get ethanol evaporated. In this way, we can get purer CBD, and ethanol evaporator can also be recycled. The difference between a falling film evaporator and a rising film evaporator is that the feed liquid is added from the top of the evaporator and falls down in a film shape along the tube wall under the action of gravity. And in this process, it is concentrated by evaporation, and a concentrated liquid is obtained at the bottom of it. Since the film forming mechanism is different from 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-sensitive materials. However, because the liquid film is not easily distributed in the tube, the heat transfer coefficient is smaller than that of the rising film evaporator, and it is still not suitable for materials that are easy to crystallize or scale. In addition, most of the evaporator for CBD extraction will use Dewaxing closed column extractor, which is very efficient and does not waste materials.

 

Because the solution flows like a film in a single-pass evaporator, the convective heat transfer coefficient is greatly improved, so that the solution can reach the required concentration in one pass in the heating chamber without recirculation, so it has a larger value than the circulating evaporator. advantage. The benefits of not circulating the solution are: (1) the residence time of the solution in the evaporator is very short, so it is particularly suitable for the evaporation of heat-sensitive materials; (2) the concentration of the entire solution is not always close to the completion liquid like the circulating type. The effective temperature difference of this evaporator is relatively large. The main disadvantage is that it is quite sensitive to fluctuations in the feed load, and it is not easy to form a film when the design or operation is not suitable. At this time, the convection heat transfer coefficient will decrease significantly.