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In China's pharmaceutical industry, spray technology is widely used, especially in the field of oral solid preparations, and is often used for the drying of highly heat-sensitive drugs and liquids that are easily decomposed during the concentration of feed liquids. The resulting product is fine and uniform with good flowability and fast dissolvability.
A spray dryer is a drying apparatus that directly dries a solution or suspension into solid particles. The dry product can be obtained directly from the feed liquid, eliminating the need for evaporation, crystallization, separation and crushing and other unit operations, continuous, automated production and stable operation. With this drying method, the gas-solid two-phase contact surface in a spray dryer has a large contact surface area and a short drying time. Generally, the drying time is 5 to 30 s, which is suitable for drying heat-sensitive materials. The product obtained by drying has good quality and can obtain particles of 30-50 μm. The product has good flowability and fast dissolvability. The disadvantage is that the dryer has a large volume and a low heat transfer coefficient, resulting in low thermal efficiency and high power consumption.
There are many classification methods for spray dryers, which can be divided into flow-type, counter-flow-type, and mixed-flow-type according to gas-liquid flow. The atomizer can be divided into upper spray-type and lower-spray-type. The structure of the device can be divided into centrifugal type, pressure type, and air flow type; according to whether the heating gas is circulated or not, it can be divided into open type, partial type, and closed type.
The basic process of spray drying (closed)
The material liquid is atomized by the atomizer and dispersed into small droplets to enter the drying chamber. At the same time, the heated gas is sent to the heater through the blower to raise the temperature, and then enters the drying chamber. The feed droplets and hot gases are mixed and contacted in the drying chamber. The liquid in the droplet evaporates instantaneously into gas, and the material dries into small particles. The mixed gas-solid two-phase induced draft fan is sucked into the cyclone separator to separate the two phases. The solid material settles to the bottom collector and the gas phase is led to the dust filter to trap the escaped material. After the filtered gas phase is condensed by the condenser, the solvent carried by the gas phase becomes a liquid, which can be collected and used. The gas as the carrier is dried and then recycled in the system.