1. Correctly select design parameters and reasonably configure drying equipment:
Sugar-containing materials do not necessarily refer to (pure) sugar materials. They also include sugar-containing plants such as angelica liquid in Chinese herbal medicine, sugar acid contained in papermaking night, vegetable (such as tomato juice) and fruit juice, starch, cellulose hydrolysate and other complex sugar compounds.
For different sugar contents, measuring the glass transition temperature, formulating drying operating conditions, and selecting design parameters are the most fundamental methods. First of all, the process flow must be smooth, non-sticky, non-agglomerated, and the product can guarantee quality. Secondly, the rationality of heat energy and power consumption is required to reduce blind amplification, reduce excess power consumption, and convenient operation.
For example, the outlet tail gas temperature of lactose drying should not be greater than 101°C. At the same time, the possibility of the crystallization of lactose should be considered as much as possible in the manufacturing process to make it tend to a stable structure. When designing the drying tower, the time of its constant speed section should be shortened as much as possible, so that the gas and liquid phases are mixed and contacted in the area with the highest possible speed to increase the heat transfer and mass transfer rate on the droplet surface. So that the particle surface appears dry in a short time. When designing the diameter of the drying tower, the spray distance of the liquid during atomization must be considered, and it is required that the liquid contacts the tower wall only after the surface is dried. This requires the selection of the atomizing device, including the viscosity, solid content, spray volume, atomizing wheel speed (or pressure and aperture), and the correct configuration of sufficient heat sources and fans.
2. Cooling of the drying tower wall
In the insulation area of the tower wall of the drying tower, remove the insulation cotton to allow it to enter the normal temperature air. This part of the air can be returned to the inlet of the air heater after being heated, so the heat is not wasted. The tower diameter required for designing this type of tower is appropriately enlarged, because the air layer near the tower wall has a lower temperature and does not belong to the normal drying area. Secondly, it is required to keep the cooling airflow flowing evenly around the outer wall of the drying tower, and no deviation and dead corners are allowed.
This method has been effectively used in the spray drying of sugar materials, and it has also been successful in other heat-sensitive materials such as polyoxin, which require the product to have biological activity (potency).
When using this method, close attention should be paid to the increase in the relative humidity of the air on the inner wall of the drying tower. The air contains a lot of moisture during normal operation of spray drying.
3. Fine powder return
The fine powder separated from the lower part of the cyclone separator returns to the atomization area to promote product agglomeration, particle size increase, and surface drying. When the fine powder is refluxed, it should be noted that it must be transported with dense phase airflow. If dilute phase transportation is used, even if the fine powder reaches the atomization area, the large amount of air relative to the small amount of dry powder makes it impossible for the droplets to collide with the dry powder and become ineffective.
The area for fine powder return can also be set on the inner wall of the drying tower. The air carrying fine powder enters the drying tower from the tangential direction of the circle, so that a thin air film is formed on the tower wall, and the airflow can blow away the dry powder adsorbed on the tower wall. The incoming airflow should also take into account that the relative humidity of the air cannot be too high. If it is a hot airflow after cooling the dry powder, there is no concern about condensation due to the decrease in relative humidity due to the increase in gas temperature. The gas using this method can be transported in dilute phase.
4. Low temperature and low humidity drying conditions
In order to avoid the glass transition temperature or heat-sensitive biologically active materials, it is required to use dehumidified and very low temperature air for spray drying, which is called BIRS process abroad. Its inlet air temperature is only 60℃. The whole process only changes the humidity but not the temperature. The drying rate of this process is relatively low, so the drying tower appears to be very high, so as to ensure the time it takes. There are similar processes in the production of tomato powder and disperse dyes.
5. Local introduction of cold air
At the end of the drying process, the sprayed droplets have been dried, but often the tail gas temperature is greater than the glass transition temperature of the material, causing the material to agglomerate and clog in the cyclone separator. The introduction of cold air and the tail gas to mix lowers the gas temperature and can reduce the viscosity of the powder surface, but it should be noted that the incoming air will increase the relative humidity of the mixed gas, causing the humidity of the powder surface to increase and reduce the Tg value. A more reliable way is to add a refrigerated dehumidifier before the cold air enters the system. It is especially necessary in the south of my country. This process can be seen in the spray drying of drugs or sugar-containing materials.
6. Tower wall cleaner
For powdery materials with low glass transition temperature and some synthetic resin glues, which are loosely attached to the inner wall of the drying tower after drying, a rotating vertical pipe can be used close to the tower wall, and compressed air can be passed into the pipe to blow away the attachments on the tower wall. This method is effective for drying Chinese herbal medicines with high sugar content. However, it is not recommended for drugs with biological activity, because it is inevitable that materials will accumulate on the rotating arm device, and once this part of the material enters, it will affect the product quality.