The spray drying technology is a spray method, which makes the material dispersed in the hot gas flow in the fog drop state. The material is in full contact with the hot gas. The process of heat transfer and mass transfer is completed instantaneously, and the solvent (including water and organic solvent) is rapidly evaporated into gas to achieve the purpose of drying. The spray drying technology is widely used in the pharmaceutical industry of our country. In the field of oral solid preparation, it is often used in the drying of easily decomposed drugs in the process of high heat sensitive drugs and liquid concentration. The product has fine particle size, uniformity, fluidity and instant solubility.
  Characteristics and classification of spray dryer
  Spray dryer is a drying equipment for directly drying solution or suspension into solid particles. The drying product can be obtained directly from the liquid, and the unit operation such as evaporation, crystallization, separation and comminution can be saved, and it can be produced continuously and automatically, and the operation is stable. With this drying method, the surface area of gas-solid two phase contact surface is large, the drying time is short and the drying time is 5 ~ 30s. It is suitable for drying heat sensitive materials. The quality of dried products is good, and 30~50 micron m particles can be obtained. The product has good fluidity and instant solubility. The drawback is that the dryer has large volume and low heat transfer coefficient, resulting in low thermal efficiency and high power consumption.
 There are many kinds of classification methods of the spray dryer, which can be divided into flow type, countercurrent and mixed flow according to the flow direction of gas and liquid. According to the installation way of the atomizer, it can be divided into upper and lower jet. According to the structure of the atomizer, it can be divided into centrifugal type, pressure type and air flow type, and the heating gas can be divided into open and partial circulation according to whether the heating gas is circulating or not. Type and airtight.
  Basic flow of spray drying (airtight)
  The liquid is atomized through the atomizer and dispersed into the drying chamber as fine droplets. At the same time, the heated gas is fed into the heater through the blower to warm up, and then into the drying chamber. Material droplets and hot gases are mixed and contacted in the drying chamber. The liquid in the droplet evaporates instantaneously into gas, and the material is dried into fine particles. The mixed gas - solid two - phase air blower inhales the cyclone separator, two phases are separated, solid materials settle down to the end collector, and the gas phase is introduced to the dust filter to capture the escaping material. After condensation of the filter gas through the condenser, the solvent carried by the gas phase becomes liquid, which can be collected and used, and the gas as the carrier is circulate in the system after the gas is dried. The flow chart is as follows:
  Technical requirements for oral API
  There are various kinds of oral solid preparations, and the specific technological requirements are different. In the process of the design and manufacture of the spray dryer, it is best to make different designs for different varieties in order to achieve the best state of equipment and technology. In the selection of spray drying machines for solid pharmaceutical production, the following requirements are often raised:
  1. cleaning requirements (please change the following 7 subtitles into italics)
  The main process of API production needs to be carried out in a clean environment of 100,000 grade. Spray drying is an important process of quality control. Spray dryer contact material parts, to use stainless steel material, for acid corrosive materials, the best choice is 316L stainless steel. Drying chamber, cyclone separator, filter and other parts directly in contact with materials, should have smooth inner wall, no dead angle, easy to clean, easy to disassemble, easy to clean. The spray dryer often has a large system, so it can not be placed in the clean area as a whole, and often only the part of the collection of materials in the clean area, which requires the good sealing performance of the system, in order to prevent the external pollution from the system.
  2. avoid sticky wall
 So far, the phenomenon of wall sticking is still a prominent problem that hinders the normal operation of the spray dryer. The spray drying process of solid preparation (especially Chinese Medicine) is particularly obvious. Material sticking to the wall is not only unfavorable to the collection operation, but also with the extension of time, sensitive sticking materials will deteriorate and become unqualified materials. From the point of view of the process, the solution includes choosing the appropriate solvent, adding the auxiliary material and changing the process parameters, but the room for adjustment is not very large, so the fundamental solution must be sought from the angle of the equipment. Experts at home and abroad have conducted in-depth study on the problem of wall adhesion and caking in the drying process. It is agreed that the main cause of wall adhesion is wall temperature. Five possible ways to prevent wall sticking are listed below.
  * A wall drying tower was used to cool the walls and keep the wall temperature below 50 C.
  * The two air cooling tower walls are cut through the spiral wall of the tower wall.
  * A gas broom consisting of a row of nozzles is installed near the wall of the tower and slowly rotates along the wall of the tower.
  * The tower wall increases the air hammer, and the pneumatic material is used to force the sticky wall material to break away.
  * The accuracy of the machining is increased, and the inner wall polishing of the tower can also reduce the sticky wall.
  3. automatic control performance
 Spray drying is a continuous production process with many control points. For example, the control points of the closed loop system include feed concentration, feed speed, inlet speed, air inlet temperature, system pressure, pressure difference of each component, gas purity test and so on. It is very difficult to rely on traditional manual adjustment. A good automatic control system can make the whole spray drying system run smoothly, reduce the unqualified rate of products and reduce the potential safety hazards. At present, domestic products are relatively backward in self-control, and some advanced equipment abroad, such as the Danish spray dryer of the Danish company, have been very mature in the technology of self-control.
  4. drying effect
  Due to the need to further complete the preparation process, the physical properties of the spray drying products are required to improve the effect of the preparation and the absorption effect of the final drug. First of all, moisture content is the first index of dry products. The general requirements are controlled within a certain range, not too high or too low. Secondly, drug loading is generally controlled by volume, which requires stable bulk density and good liquidity. Oral granules often require quick solubility. In general, changing the dispersion of atomization or changing the operating conditions to control the drying rate can be used to produce products with different heap density, but the range of change is limited. One technique worth mentioning in improving physical properties is foam spray drying, in which the feed liquid is foamed before spray drying. The method was originally proposed to improve thermal efficiency and was later used to adjust the physical properties of products. It has been proved that the product prepared by foam spray drying has large particle size, porous, multi-depression, rough surface and good instant solubility. In addition, there are special requirements for oral raw material, such as particle shape. Some are spherical particles, and the size of the particles is uniform. Generally speaking, particles of different shapes can be obtained by changing the atomization mode. Before choosing the spray dryer, it is best to optimize the parameters of the equipment with the assistance of the equipment manufacturer to determine the best combination of parameters to ensure the drying effect of the spray.
  5．protective measures
  Due to the different nature of drugs, there are also higher requirements for the protection and design of equipment. When dissolved in organic solvents, the whole system must meet the requirements of explosion protection when it is dry. In the design of motor, instrument and control valve, explosion-proof type should be chosen. In addition, in order to prevent the mixture of solvent and air into explosive gas, the system can not be open, but the closed cycle system must be adopted, and the carrier should choose inert gas, such as nitrogen. In addition, the drying system of some easily oxidizable materials should also be sealed with nitrogen.
  6．Environmental protection
 The spray drying products are fine particles. In order to meet the requirements of environmental protection, the spray drying system only uses a cyclone separator to separate the product and purify the tail gas. Generally, the bag filter is used to purify the dust, so that the dust content in the tail gas is lower than 50mg/ (NM) 3 gas; or the wet scrubber can be used to reduce the dust content of the tail gas. To 15 ~ 35mg/ (nm) 3 gas. When nitrogen is used as carrier, the system should be kept tightly closed and the exhaust port should be outdoors.
  7．cost reduction
  The inlet temperature of hot air is 250-500 C in parallel operation, 200-300 C in countercurrent operation and 60-100 C in outlet after atomization and drying. The thermal efficiency of spray drying is low, and the thermal efficiency of the industrial scale spray dryer is generally 30% to 50%. The thermal efficiency of spray drying with waste heat recovery can reach 70% in foreign countries. However, this equipment has economic significance only when the production capacity is more than 100 kg (water) / h. But when the nitrogen carrier and the organic solvent are used, the closed circulation system can be used to use the nitrogen cycle and the solvent condensation to effectively reduce the production cost.
  The domestic spray dryer manufacturers are constantly improving the technical level and solving the technical problems of the spray drying in the field of oral material medicine. It is believed that in the near future, it will be able to produce more suitable, high-quality and inexpensive products, making the spray drying technology of our country reach the leading level of the world.