Reasons and solutions for spray drying and sticking

Stick type and cause

(1) The semi-wet material adheres to the wall: the ejected droplets come into contact with the tower wall before the surface is dried, and thus adhere to the hot wall. The material of the sticky wall is thicker and thicker, and when it reaches a certain thickness, it is free to fall off in a block form. This type of sticky wall often causes the surface of the bulk material to be charred, while the internal moisture content exceeds the standard, which affects normal production. The cause of the sticky wall of the semi-wet material is more complicated, mainly related to the structure of the spray drying tower, the structure of the atomizer, the installation, the operation and the movement state of the hot air in the tower.

(2) The hot-melt adherent wall particles of the low-melting material are melted and sticky at a drying temperature, resulting in adhesion to the hot wall. The main reason for this is that the softening point of the dried material is lower than the drying temperature, resulting in sticking.

(3) Dry powder surface adhesion: Since the sprayed dry powder is small in particle size and large in specific surface area, it moves in the limited space of the spray tower, and some particles always hit the wall and adhere to it. The degree of dry powder adhesion is related to the geometry of the tower wall, roughness, air flow rate, electrostatic force, and the like. The above classification is rough, and the actual operation may be based on a type of sticky wall, or several types of sticky walls may be more serious. In production, it should be targeted according to the specific situation.

Spray drying method

(1) Structure and material of the improved drying tower In the design of the tower body, if the tower diameter is smaller than the maximum diameter of the spray cone, a serious sticky wall will be generated on the plane of the maximum trajectory against the movement of the droplet. In order to prevent the material from sticking to the wall, it is possible to consciously increase the diameter of the tower wall so that the semi-dry material does not hit the wall and falls down. This method has the effect of relieving the sticky wall, but the tower diameter should not be too large, otherwise it will not only increase the material cost of the equipment and the equipment footprint, but also reduce the running speed of the hot air in the tower and affect the drying quality. The vertical conical spray drying tower is easy to stick to the wall at the cone. The vertical cylinder structure can overcome the above shortcomings.

(2) Reasonable selection of atomizers Nebulizers, also known as nozzles, are key components of spray drying equipment. The different structure directly affects the liquid atomization and dispersion effect, which in turn affects the particle size and performance of the particles. The paste material is extremely strong in adhesion and difficult to disperse, and it is easy to re-bond the dispersed materials into agglomerates, resulting in no drying and sticking to the wall. At the same time, the combined state of water and material in the paste material belongs to capillary water, permeate water, adsorbed water and structural water, so the transfer resistance of water in the material is large, if it cannot be managed to disperse the material into small particles to reduce Heat and mass transfer resistance and prolonged drying time are also important causes of sticking.

(3) Correct installation of the atomizer The standard spray pattern produced by the airflow nozzle and the pressure nozzle is a hollow cone that is symmetrical with the nozzle axis, and the droplets should be evenly distributed in the spray cone. When the gas passage of the air flow nozzle does not coincide with the axis of the liquid passage, or the pressure nozzle hole is not round, the generated mist cone is asymmetrical, and the deviation of the mist cone will cause local severe adhesion. If only one nozzle is installed in the spray tower, the axis of the nozzle is to be mounted on the centerline of the tower, ie the two coincide. If multiple nozzles need to be installed, the nozzles of each nozzle should not overlap. By adjusting the spray angle, the droplets should not be sprayed directly onto the opposite wall. The vibration of the nozzle is also a cause of the sticking of the wall. For the rotary atomizer, vibration is particularly prevented during operation.

(4) Selection of suitable operating conditions For the airflow spray drying tower, the operating process parameters mainly relate to drying temperature, feed rate and nozzle pressure. The temperature of the spray drying tower is divided into the inlet temperature and the outlet temperature. Increasing the temperature of the air inlet can increase the evaporation strength of the droplets, so that the surface of the droplets is solidified before contacting the tower wall, which can effectively reduce the stick wall loss and improve the product yield. In addition, the temperature has a great influence on the particle size. When the temperature is low, the time for the solution droplet to reach supersaturation is prolonged, the instantaneous nucleation rate is reduced, and the number of nucleation is reduced. Therefore, the particle size of the obtained particles increases, resulting in drying time. extend. When the temperature of the air inlet remains unchanged, increasing the temperature of the air outlet can reduce the temperature difference between the inlet and outlet, increase the average temperature of the hot air in the tower, accelerate the drying rate, and effectively prevent the sticking tower phenomenon.

In summary, the nozzle pressure also has a suitable range, which should be reasonably set according to the nature of the material and the characteristic parameters of the drying tower. For centrifugal spray dryers, the rotary disk speed should be chosen.