Characteristics and disadvantages of double cone vacuum drying equipment


1 Overview

Double cone vacuum dryer (hereinafter referred to as double cone) is a vacuum indirect heating dynamic dryer with a long history, which is an energy-saving drying equipment. It features that the vacuum cone is driven 360 by a drive mechanism. Rotation promotes the mixing of materials in the cone and contact heating with the heating surface of the cone. Continuous mixing of materials will have a self-cleaning effect on the heating surface. The temperature difference is also very beneficial to the low-temperature drying of many heat-sensitive materials.

The double cone operates under vacuum without the intervention of other inert gases, which is very conducive to the evaporation and drying of materials containing various organic solvents, and the recovery of solvents with a high solvent recovery rate. The structural shape of the double cone is very conducive to the clean and quick discharge of the dry material, which is an operational advantage that cannot be compared with any other type of dryer.

The double cone is suitable for drying various loose materials without sticky walls. Because the exhaust gas filter is installed on the vacuum suction pipe inside the cone, the loss of product dust entrained by the exhaust gas is reduced, and the drying yield is high.

It can feed and unload in a closed manner at one time, avoiding operational pollution, and avoiding dust flying and polluting the environment during unloading. The double cone dynamic drying efficiency is much higher than that of various disc drying.

However, there are also some shortcomings in the double cone drying. First, the double cone is only suitable for intermittent drying.

Due to the use of friction mechanical vacuum seals, it is impossible to avoid the wear and foreign matter of the seal during dry friction. Once a vacuum leak occurs, the wear and foreign matter in the seal must be sucked into the double cone and mixed with the product being dried. If the product is a medicine, the medicine will be contaminated, making a good medicine become a bad medicine and harming people's medicine.

The double cone is a jacketed heating dryer. If the double cone is regarded as a sphere, when the double cone dryer is enlarged, its heating area F (m) will expand in direct proportion to the square of the diameter of the sphere D (mm), while The volume V(m) of the sphere expands proportionally to the cube of the diameter D(mm) of the sphere. Therefore, when the double cone expands from a small diameter to a large diameter, its heating area is far less than its volume expansion.

As a result, the ratio of the bicone heating area F(rfl) to the bicone volume V(m) becomes out of balance with the expansion of the diameter D (iTlm), resulting in the F/V value of the small-sized bicone being much larger than that of the large-sized bicone F/, value. For example, the F/V value of a 5 m double cone is about half smaller than that of a 0.3 m double cone, about 3.6 m/m, and when the volume of the double cone expands to 10 m, its F/V value decreases to 2.5 m/m.

As the name suggests, the double cone vacuum drying equipment is used for heating and drying, and its main function should be reflected in the heating and drying efficiency or the size of the heat exchange area.

Relatively speaking its volume is of secondary importance. But at present, almost all the product samples of double cone dryers use the volume to identify the specifications of the double cones. To a certain extent, the double cone manufacturers are misleading users intentionally or unintentionally. Because many users will not realize that there is a regular change in F/V when biconical magnification occurs.

It is misunderstood that both large and small volume bicones have similar drying efficiencies. Often buy a large-scale double-cone, and find that its drying efficiency is far lower than the drying efficiency produced by the small-sized double-cone test practice in the drying factory. Because the drying efficiency has doubled and cannot meet the desired output, it is necessary to purchase another large-scale double cone.

It not only increased the investment and increased the plant land, but also delayed the production plan.

The drying temperature of many heat-sensitive products is lower than 100°C, and many heat-sensitive products are still being dried<The hot water of 100~ is used as the heat source, so it needs to be equipped with auxiliary facilities such as hot water tank, hot water pump and hot water pipeline. Sensible heat heating of hot water must have a temperature difference between the inlet and outlet, which naturally reduces the temperature difference compared with steam latent heat heating, which is very unfavorable for low-temperature heating with high heat sensitivity.

The heat transfer coefficient of hot water heating is much lower than that of saturated steam heating, and the structure of jacketed hot water heating is also more complicated than that of steam heating, which not only increases equipment investment, but also is not conducive to management.
Due to the continuous mixing and rotation of the dry material in the double cone, some products with fine crystals and high water content, such as riboflavin, acetylspiramycin and other products, can easily form pellets of different sizes during the mixing process. Once the pellets are formed it is difficult to achieve complete drying.

In principle, double cones are not suitable for materials that have pellets in the drying process.
The double cone is a dynamic dryer that is constantly rotating with a heating boiler (heating jacket). Large-scale (big diameter) traditional double-cone cone liner and the wall thickness of the outer wall of the jacket are thick and heavy.

The inner tank of the cone is generally made of stainless steel, while the outer wall of the jacket is made of carbon steel. The diameter of the large double cone (2-10 m) is 1,600-2,600 mm, the wall thickness of the cone inner tank is 8-14 mm, and the wall thickness of the outer wall of the jacket is thicker. As we all know, the thermal resistance of stainless steel is 5 times greater than that of carbon steel. A thick stainless steel liner is not conducive to the heat transfer of the cone, which reduces the heating and drying efficiency of the double cone. Driving a heavy liner naturally increases the power of the drive motor. .

The outer jacket of the double cone is thicker and heavier, which increases the energy consumption of the driving motor. The double cone runs for thousands of hours a year, so how to correctly treat the long-term running of the double cone to save energy and reduce consumption is actively mobilizing and researching the new era of energy conservation all over the world. For the equipment structure and equipment materials that have been neglected in the past, It is worth our in-depth study and discussion.
The following discussion is based on double cones without GMP requirements.
2 Magnification of the bicone
With the development of fine chemical products, there is an increasing need for a double cone with improved efficiency and large size.

A traditional large-scale double-cone, its huge body shape is not commensurate with the heating surface it contains, the heating area of ​​a 5 m double-cone is only 18 m, and the F/V value is 3.6 m/m, while a The F/V value of the 10 m double cone has further dropped to 2.5 m/m, and it is not an exaggeration to describe the double cone as strong on the outside and dry in the middle.
The enlargement of the double cone vacuum dryer naturally refers to the enlargement of the heat exchange area. Fortunately, there is a huge space in the cone that can be used, and how to make good use of this space is the key.

Considering the cleaning of the equipment, the simpler the surface of the heat exchange element, the better. The thin hollow c,Di~ heating plate is selected, which meets the dual requirements of smooth surface, easy cleaning and compactness. Taking the 10 m double cone as an example, a thin inner heating plate can be installed in the huge cone space along the direction of material flow, leaving room for cleaning and necessary maintenance, such as keeping a distance of 110-160 mm between the inner heating plates, Generally, increasing the area of ​​the inner heating plate can be 1.5 to 3 times larger than the original cone heating area, so that the F and V values ​​of the double cone can be increased to 0.2 m.

A similar value of 9.0 m/m for the small bicone. In particular, the thickness of the inner heating plate is less than one-third of the thickness of the cone, and the heating area is doubled and the thermal resistance is greatly reduced, so that the drying efficiency of the enlarged double cone is more than 2 to 3 times that of the traditional double cone. The inner structure of the 10 m double cone after the inner heating plate is installed is shown in Figure 1.
Illustrated by Figure 1, 10 m.

The diameter of the cone is 2,600. On both sides of the maintenance channel with a width of 600 mm, there are 7 inner heating plates from # l to ≠}7, 14 in total. The total area of ​​the inner heating plate is F=56 m, and the thickness of the inner heating plate is 40 mm, the total volume of the inner heating plate V=2.2 m, the distance between the centers of the two inner heating plates d=200 mm, the actual gap between the two inner heating plates is 160 mm, and the heating area of ​​the cone jacket is known as F= 29.6 m , so the total heating area of ​​10 m double cones is F=85.6 m , the volume of the cone is known to be V=12.1 m , the actual volume of the cone after deducting the inner heating plate is V=9.9 m , by It is known that the F/V value of the new bicone reaches 8.6 m/m, which is very close to the 9.0 1TI/m of the small 0.2 in bicone. The F/V value of the 10 1TI double cone without the inner heating plate is 2.5 II1 /m, so the 10 1TI after adding the inner heating plate. The F/V value of the double cone is more than 3 times higher than that without the addition (as shown in Figure 2). If the F/V value is increased, the drying efficiency of one new double cone can be equal to the traditional drying efficiency of three sets. Double cone, the driving motor per hour can save about 60 kW/h of power consumption.

3 Comparison of saving stainless steel by adding inner heating plate with large double cone
Still take 10 m (actual operating volume) as an example. The overall dimensions of the 10 m double cone are 2600 X 3600 (total height). According to the above description, the area of ​​the double cone heating jacket is F=29.6 in, and the total area of ​​the inner heating plate is F=56 1TI. If the wall thickness of the double cone is 14 millimetres, and the wall thickness of the inner heating plate is 3 mm, then The stainless steel material of the double cone weighs 3-3 t, while the stainless steel material of the inner heating plate weighs 1.34 t.

It can be seen that the double cone needs 111.4 kg of stainless steel per square meter, while the amount of stainless steel required for the heating plate per square meter is only 24 kg, that is, the amount of stainless steel consumed per square meter of heating area of ​​the double cone is the same The heating area of ​​the inner heating plate is 4.6 times larger. Considering only the heating area ~ IE4 of the existing large-scale traditional double cone, and the consumption of a lot of stainless steel, the cone is hollow and empty, so it is possible to increase the internal heating plate, but it is also very necessary to increase the internal heating plate , to offset the negative effects of the cone wall thickness.
4 Structural strengthening of the inner heating plate
The conventional hollow and thin inner heating plate needs to be strengthened by plug welding. Because the wall thickness of the inner heating plate is only 2-3 mm, both sides of the hollow heating plate need to be drilled. It is difficult to process the groove on the thin plate strictly according to the welding specification, especially The heating plate required by the pharmaceutical GMP needs to be ground and polished after welding, so there are tiny weak links in many plug solder joints, which will suddenly cause the entire heating plate to break due to local cracks in future production operations. blasting.

The traditional thin plate plug welding reinforcement structure is easy to leave this kind of hidden danger.
We have learned a lesson deeply and adopted hinged non-destructive welding strengthening technology on the new double-cone inner heating plate reinforcement structure. There is no need to punch holes on the double-sided mother board of the inner heating plate, so as to avoid the disadvantages caused by the perforated plug welding operation. Eliminates the practical difficulty of processing the inner heating plate, which is necessary for biconical magnification.
5 Low temperature (100°C) heat source problem of double cone
When the double cone is used to dry heat-sensitive products, the traditional double cone adopts hot water heating method.

Hot water heating not only requires a whole set of cumbersome hot water tanks, hot water pumps and pipelines, but due to the sensible heat of hot water, there must be a temperature difference between the inlet and outlet, and the natural heating and drying effect at the hot water outlet is much lower than that at the hot water inlet. The temperature difference of hot water heating is very unfavorable for drying.
If the large-scale double cone is still heated by hot water, the heat load required will also increase significantly due to the substantial increase in the load of the drying equipment. Bring a lot of disadvantages to the efficiency of drying equipment. However, if saturated steam is used as the heat source, as long as the temperature or pressure in the double-cone heating jacket is strictly controlled to ensure that the temperature or pressure meets the physical parameters of saturated steam required by the heating temperature, it is ensured that the steam in the jacket can be condensed in time. Water pumped out.

The saturated water vapor of 0.2 MPa can be directly used as the heat source for the low-temperature heating of the double cone. Of course, the space inside the large-scale double-cone jacket is large, and there is a lot of inert gas in the jacket (especially in the inner heating plate) when the car is just started, which must be discharged, otherwise it will seriously affect the normal operation. , and then add saturated water vapor.
6. Research on thinning the wall thickness of large-scale double-cone equipment
Today's world is thinking about energy saving and emission reduction. From airplanes to trains and cars, they are all trying to reduce the thickness of the structure.

Reminiscent of a large boiler (heating jacket) on the back of the double cone, it seems that it is also necessary to conduct research on energy saving, emission reduction, saving stainless steel, and improving heating and drying efficiency. We must also make our contribution in the field of drying technology.
7 Conclusion
With the large-scale production of fine chemical industry, many large-scale double-cone vacuum dryers are needed. Because the large-scale double-cone structure has not kept up with the pace of the times, it has brought many problems to production and use. The ratio of the output value and energy consumption of our industrial production is much higher than that of developed countries such as Japan, ignoring the efficiency of many equipment, especially the misunderstanding that the price is not based on the cost performance of the equipment, but on the weight of the equipment. Double cone vacuum drying Equipment is probably typical in this regard.


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