Spray drying is characterized by fast heat transfer, rapid water evaporation, and instant drying time. Besides, the product has good quality, crisp texture, and good solubility, which can improve the dissolution rate of some preparations and is suitable for thermosensitive drugs. In addition, spray drying can also be used to prepare microcapsules. The widely used method for dye drying.
A drying method for removing moisture by atomizing dyes. Suitable for drying various dyes. It can dry fine dye powder, ultrafine powder, dust-free powder, and hollow granules.
According to the method of dye atomization, it can be divided into three categories:
(1) Airflow atomization, using compressed air or water vapor to atomize the material liquid;
(2) Pressure atomization, using a high-pressure pump to press the material liquid out of the nozzle at high speed, forming a mist;
(3) Rotary atomization involves adding the material liquid to a high-speed rotating disc (7000~20000 r/min) in the atomizer to quickly eject the material liquid and atomize it. The third method has good results, short time, high labor productivity, and is favored by the dye industry. Its disadvantages are high equipment investment and high energy consumption.
The principle of spray drying is to disperse the materials to be dried into fine particles like fog through mechanical action, (increase the evaporation area of water, accelerate the drying process) contact with hot air, remove most of the water in an instant, and dry the solid materials in the materials into powder.
Advantages and disadvantages of spray drying Advantages of spray drying:
1. The drying process is very rapid;
2. It can be directly dried into powder;
3. Easy to change drying conditions and adjust product quality standards;
4. Due to instantaneous evaporation, the selection requirements for equipment materials are not strict;
5. The drying room has a certain negative pressure, ensuring hygiene conditions during production, avoiding dust flying in the workshop, and improving product purity;
6. High production efficiency with few operators.
Disadvantages of spray drying:
1. The equipment is relatively complex, covers a large area, and requires a large one-time investment;
2. The price of atomizers and powder recovery devices is relatively high;
3. Requires a large amount of air, increasing the electrical energy consumption of the blower and the capacity of the recovery device;
4. Low thermal efficiency and high heat consumption.
Requirements of spray drying on equipment:
1. The parts in contact with the product must be easy to clean and sterilize;
2. Measures should be taken to prevent coke powder from generating eddy currents and countercurrent in hot air;
3. Prevent air from carrying impurities into the product;
4. Equipped with temperature and pressure indicating and recording devices for easy inspection of production operation;
5. A dust recovery device with high recovery rate;
6. The powder should be quickly discharged and cooled to improve solubility and instant solubility;
7. The temperature inside the drying room and the exhaust temperature should not exceed 100 ℃ to ensure safety and quality;
8. When spray, the concentrated milk drops are in uniform contact with the hot air to improve the thermal efficiency;
9. Minimize wall sticking for viscous substances as much as possible.
The classification of spray drying equipment is classified according to the method of granulation:
1) Pressure spray drying method:
① Principle: Using a high-pressure pump, the material is condensed into 10-200 mist like particles through a atomizer (spray gun) at a pressure of 70-200 atmospheric pressure, which directly contacts hot air for heat exchange and completes drying in a short time.
② Pressure spray granulation device: M-type and S-type, with a guide groove that can make the liquid flow rotate. The axis of the M-type guide groove is perpendicular to the axis of the nozzle and does not intersect with it; The axis of the S-shaped guide ditch is at a certain angle to the horizontal. The purpose is to try to increase the turbulence of solution during spray.
2) Centrifugal spray drying method:
① Principle: A high-speed rotating disk in the horizontal direction is used to apply centrifugal force to the solution, causing it to be thrown out at high speed, forming a thin film, fine wire, or liquid droplet. Due to the friction, obstruction, and tearing of air, the tangential acceleration generated by the rotation of the disk and the radial acceleration generated by centrifugal force result in a combined velocity moving on the disk, with a trajectory of a spiral shape. After the liquid is thrown out from the disk above this spiral line, Dispersed into very small droplets, they move along the tangential direction of the disk at an average speed, and at the same time, the droplets fall under the gravitational force of the Earth’s center, due to the different sizes of particles sprayed. Therefore, their flying distances are also different, and the particles falling at different distances form a cylinder symmetrical about the axis of rotation.
② Requirements for obtaining more uniform droplets:
a. Reduce vibration during disk rotation
b. The amount of liquid entering the disk remains constant per unit time
c. The surface of the disc is flat and smooth. d. The circular velocity of the disc should not be too small, rmin=60m/s. If the emulsion (100-160m/s) is less than 60m/s, the spray droplets are uneven. The spray distance seems to be mainly composed of a group of droplets and a group of fine droplets sinking near the disc, and decreases with the increase of rotating speed.
③ Structure of centrifugal spray: requirements: the wetting perimeter is long, the solution can reach high rotation speed, the spray is uniform, the structure is firm, light, simple, no dead corner, easy to disassemble and wash, and the productivity is high. Classify according to the form of the drying chamber according to the direction of movement between the hot air and the dried particles in the drying chamber: parallel flow type, counter flow type, mixed flow type. Concurrent flow patterns are often used in milk. The parallel flow pattern can be dried using a higher inlet air temperature without affecting the quality of the product.
a. Horizontal parallel flow pattern
b. Vertical descent and flow pattern
c. Vertical descent mixed flow type.
d. Spray drying of vertically ascending and flowing food Some plant nutrients will be lost due to high temperature. For these plants, freeze drying is an effective method of concentration and preservation. However, some plants do need a certain temperature to remove toxicity. For these plants, spray drying technology is ideal. Take soybeans as an example. The concentration process requires a certain temperature to remove a substance called trypsin inhibitors (which will hinder digestion and protein decomposition).
Spray drying is often the last step in the manufacturing process, which is to change substances from liquid to powder through continuous spray, mixing and drying. Among many food storage technologies, spray drying has its unique advantages. Because the temperature used in this technology is not very high, it can effectively retain the taste, color, and nutrition of food while removing microbial contamination. Spray drying is usually used to remove water from raw materials. In addition, it has various other uses, such as changing the size, shape, or density of substances. It can assist in adding other ingredients during the production process, helping to produce products with the strictest quality standards. Operation of spray drying method Before spray drying, soybeans must be washed and peeled, and non protein ingredients such as fat must be reduced, so as to concentrate the most nutritious ingredients in soybeans. After such concentration, soybeans can be spray dried.
The actual process of spray drying can be divided into several steps:
Firstly, the liquid product is fed into the atomizer, where a high-speed rotating wheel atomizes the liquid, causing the product to become a solid liquid mixed particle state.
In the second step, the atomized particles are directed to a drying chamber with controllable temperature and airflow, and the hot air evaporates the liquid in the particles. In order for the final product to meet the standards, the contact time of particles with hot air must be appropriate to maintain a certain amount of moisture in the powder product; At the same time, the state in the drying room should also be well controlled, as the size of the drying room volume and airflow conditions can affect the nutrition of the product.
The third and final step in the entire program is to collect the powder in the airflow into a container through a separator, so that the final product can be packaged or mixed with other components.
Features: Fast drying speed. After centrifugal spray, the surface area of the feed liquid is greatly increased. In the high-temperature airflow, 95% – 98% of the water can be evaporated instantly, and the drying time is only a few seconds. The parallel flow spray drying can make the droplets flow in the same direction as the hot air. Although the temperature of the hot air is high, the hot air enters the drying room and immediately contacts with the spray droplets, resulting in a sharp drop in the indoor temperature, while the wet bulb temperature of the materials is basically unchanged, so it is also suitable for drying thermosensitive materials.
Post time: Apr-26-2023