The process stages, advantages and disadvantages of spray drying were analyzed. The research progress of spray drying technology was reviewed, and the application prospect of spray drying technology was analyzed.
Finally, an application example of spray drying technology in the production of traditional Chinese medicine pharmaceuticals is given. One-step spray drying granulation of Chinese medicine liquid. The technology directly sprays the traditional Chinese medicine dilute liquid into dry granules.
The four steps of concentration, multi-effect concentration, granulation and drying of Chinese medicine processing Chinese medicine liquid are combined into one step, which greatly simplifies and shortens the process and time of Chinese medicine extract to semi-finished or finished product.
Increased production efficiency and product quality. It can provide reference and help for the promotion and application of spray drying technology and the improvement of Chinese medicine pharmaceutical level.
Spray drying is a drying process in which a raw material liquid is dispersed into a mist by an atomizer, and hot air (or other gas) is directly contacted with the mist to obtain a powdery product. The raw material liquid may be a solution, an emulsion or a suspension, or may be a melt or a paste. The dried product can be formed into powder, granules, hollow spheres or agglomerates as needed.
Spray drying technology has a history of more than 100 years. Since the earliest spray drying in 1865 for egg processing, this process of liquid atomization and drying has turned into a solid powder in a very short time, and considerable progress has been made. It allows many valuable but difficult to preserve materials to greatly extend the shelf life, making some materials easy to package, use and transport, while also simplifying the processing of some materials. Because spray drying has the characteristics of “instant dryingâ€, “good dry product quality†and “simple drying processâ€, it is obviously superior to other drying methods. By the 1930s and 1940s, this technology has been widely used in dairy products. Detergents, dehydrated foods, and the production of fertilizers, dyes, and cements. Currently, instant coffee, milk powder, and convenience food soups are products obtained by spray drying. The earliest use of spray drying in industrial scale production is the dairy industry, followed by the detergent and dye industries. Currently, it has been widely used in all industries mentioned above, especially in the ceramic and pharmaceutical industries. For the general.
For the Chinese medicine pharmaceutical industry, the application of spray drying technology has its unique function, which greatly simplifies and shortens the process and time of the Chinese medicine extract to the semi-finished or finished product, and improves the production efficiency and product quality. In this paper, the process stages and advantages and disadvantages of spray drying are analyzed. The research progress of spray drying technology is reviewed, and the application prospect of spray drying technology is analyzed. Finally, the application example of spray drying technology in traditional Chinese medicine pharmaceutical production is given. The liquid is spray dried in one step.
1 spray drying process stage and analysis of advantages and disadvantages
1.1 Process stage of spray drying
Spray drying can be divided into three basic process stages: one is atomization into droplets; the other is that the droplets are in contact with the drying medium, mixed and flowing, that is, drying; and the third is that the dried product is separated from the air.
1.1.1 The first stage of spray drying - atomization of liquid
The atomization of the liquid is the unique feature of spray drying in the contact and mixing of droplets and droplets with hot air. The purpose of atomization is to disperse the liquid into fine droplets, which have a large surface area. When it comes into contact with hot air, the moisture in the droplets is rapidly vaporized and dried into a powder or granular product. The size and uniformity of the droplets have a great influence on product quality and technical and economic indicators, especially for the drying of heat sensitive materials. If the size of the sprayed droplets is very uneven, there will be a phenomenon that the large particles have not reached the drying requirement, and the small particles have dried excessively and deteriorated. Therefore, the atomizer used for atomization of the liquid is a key component of spray drying. At present, the commonly used atomizers are airflow, pressure, rotary and acoustic energy atomizers.
1.1.2 The second stage of spray drying - contact between droplets and air
The contact, mixing and flow of the droplets and air is a simultaneous heat and mass transfer process, ie a drying process, which takes place in a drying tower. The contact mode, mixing and flow state of the droplets and air are determined by the structure of the hot air distributor, the installation position of the atomizer in the tower, and the exhaust gas discharge mode. In the drying tower, the droplet-air flow direction has a cocurrent, countercurrent, and mixed flow. The way in which the droplets are in contact with the air has a great influence on the temperature distribution in the drying tower, the trajectory of the droplets (or particles), the residence time of the particles in the column, and the nature of the product.
The drying process of the droplets also undergoes a constant speed and a slowdown phase. Study the movement and drying process of the droplets, mainly to determine the drying time and the main dimensions of the drying tower.
1.1.3 The third stage of spray drying - separation of dry products from air
Most of the spray-dried products are discharged at the bottom of the tower, and some fine powder is entrained in the exhaust gas. The exhaust gas must be collected before discharge to improve product yield and reduce production cost. The exhaust gas must comply with environmental protection emission standards to prevent environmental pollution.
1.2 Analysis of the advantages and disadvantages of spray drying
1.2.1 Advantages of spray drying
As long as the drying conditions are kept constant, the dry product characteristics remain constant; the spray drying operation is continuous, the system can be fully automatic control operation; the spray drying system is suitable for the drying of heat sensitive and non-heat sensitive materials, suitable for aqueous and organic solvent materials. Dry; raw material can be solution, mud, emulsion, paste or melt, even filter cake can be treated; spray drying operation has great flexibility, spray capacity can reach several kilograms per hour to 200 tons.
1.2.2 Disadvantages of spray drying
Spray drying investment costs are relatively high; spray drying is convective drying, and thermal efficiency is relatively low (unless using very high drying temperatures), typically 30% to 40%.
2 Research progress of spray drying technology
At the heart of the spray drying technology is fluidization technology, which has the outstanding advantage of instantaneous drying from fluid to solid. The equipment is generally composed of an atomizer (nozzle), a drying tower, an inlet and outlet gas, and a material collection and recovery system. The atomizer used to atomize the liquid is a key component of the spray drying device.
2.1 type of atomizer and atomization form
Generally, atomizers commonly used in production include airflow atomizers, pressure atomizers, and rotary atomizers. Different atomizers can produce different atomization forms. According to different atomization forms, spray drying can be divided into airflow atomization, pressure atomization and rotary atomization. The choice of atomization form depends on the nature of the feed liquid and the characteristics required of the final product. For the atomization mechanism of liquids, there are basically three types, namely, drip splitting, filamentous splitting, and membranous splitting. In the spray drying operation, the atomization mechanism is related to the atomization method, the operating conditions, the physical properties of the fluid, and the like. The atomization mechanism can guide us to design and operate a reasonable atomizer.
Airflow atomization uses compressed air (or water vapor) to eject from the nozzle at a high speed and mix with the liquid conveyed by the other channel. The friction is generated by the friction between the air (or water vapor) and the relative velocity of the liquid and liquid. The liquid is dispersed into droplets. According to the number of fluid passages of the nozzle and its layout, the airflow atomizer can be divided into two-fluid external mixing, two-fluid internal mixing, three-fluid internal mixing, three-fluid internal and external mixing, and four-fluid external mixing, four-fluid two. Inside and outside the mix and so on. The airflow atomizer has a simple structure and a wide range of processing objects, but consumes a large amount of energy.
Pressure atomization uses a pressure pump to eject the liquid from the high pressure in the nozzle hole, directly converting the pressure into kinetic energy.
The feed liquid is contacted with a drying medium and dispersed into droplets. The pressure atomizer has large production capacity and low energy consumption; less fine powder is generated, small particles can be produced, and solid matter recovery rate is high.
Rotary atomization uses a centrifugal force generated by a high-speed rotating disc or wheel to pry the liquid to contact the drying medium to form a droplet. The rotary atomizer is less affected by the influence of the feed (such as pressure); the control is simple.
Theoretical research on three atomization principles, mainly focusing on the key parameters and atomization performance of the atomizer, Huang Lixin, etc.
This is a summary report. This research will contribute to the improvement of the performance of the sprayer, and also to the selection of the atomizer according to the requirements of the spray liquid and its product requirements during the application.
Spray drying of traditional Chinese medicine extracts is basically carried out in the form of rotary atomization and airflow atomization, while the latter is more common in small test equipment. From the realization of atomization, pressure atomization requires high pressure pump and large atomization space, and airflow atomization energy consumption is high, which limits their application. Relatively speaking, the rotary atomizer technology is relatively low in requirements and is the easiest to implement.
2.2 Study on the mechanism of spray drying
There are many factors that affect the effect of spray drying. In addition to the atomizer, there are drying towers, inlet and outlet gas and material collection and recovery systems as well as the entire dryer system. Many scholars at home and abroad have studied the mathematical model of spray drying in order to give the distribution information of gas flow state and various thermodynamic parameters in the drying tower, which has very good design, optimization and drying effect of spray dryer. Significance. Wu Zhonghua et al. applied gas-particle two-phase flow theory and computational fluid dynamics (CFD), combined with the characteristics of spray drying, to establish a CFD model for simulating gas-particle two-phase turbulent flow in a spray drying tower, and to spray pulsating combustion in the laboratory. The drying process was numerically simulated. The results have detailed and intuitive characteristics; the distribution of the gas phase flow field and various thermodynamic parameters in the spray drying tower can provide reference for the design of the spray dryer and the optimization of the drying process. Dai Minghe et al. carried out thermodynamic modeling and simulation of spray drying process. Based on mass balance principle, heat balance principle and Newton's law, the one-dimensional two-way static thermodynamics mathematical model of countercurrent spray drying process was derived. It includes material temperature equation and hot air temperature. Equation, particle velocity equation, hot rheumatism content equation, material moisture content equation, after simulation with MA TLAB, the conclusion is that the increase of air volume is more technical and economic than the increase of air temperature.
2.3 Research on spray process optimization
In the experimental research of spray drying, Kang Zhiyong studied the effect of the nozzle aperture of the pressure spray drying tower on the powder. It is considered that the large pore size is more suitable for the distribution of the spray particles to concentrate on large particles. Wang Xiaolan studied the factors affecting the particle size distribution of spray-dried powder under the conditions of large-scale production in the factory, and analyzed the relationship between the viscosity of the ceramic billet, the water content, the spray pressure, the pore size of the sprayer and the particle size distribution of the powder, and the influence was obtained. The coefficients from large to small are sprayer pore size, pressure, viscosity, moisture content and the like. In the research on the spray drying process of pesticide water-dispersible granules, Yang Zhisheng analyzed the effects of dry air inlet temperature and feed amount on the suspension rate, particle density and particle shape of dry products.
In a wider range of applications, spray drying is not limited to the traditional drying mode. Liu Xiangdong et al. conducted a spray drying study of pulsating airflow. The spray drying test of the NaCl solution was carried out by using the high frequency pulsating gas flow generated by the pulsating combustion. The results showed that:
Spray drying at high temperature, high frequency oscillating airflow is 2.5 times higher than conventional spray drying.
2.4 Development trend of spray drying technology
Spray drying technology is widely used, and its advantages are obvious, but its theory still lags behind practice, and its performance is poorly guided by the practice of drying theory. The fields of drying kinetics, dry simulation of non-spherical particles, spray drying, etc. need to be further studied. Spray drying has low thermal efficiency. Therefore, the problem of energy saving and consumption reduction of spray drying is more prominent; sub-high temperature spray drying (inlet air temperature 60-150 ° C), normal temperature spray drying (inlet air temperature below 60 ° C), reducing energy consumption and multi-level Drying and so on will be the focus of future research. In addition, spray drying technology combined with specific application areas will also be used in spray cooling modeling, spray reaction, spray absorption, spray coating and spray granulation. The author believes that in the future, we should pay attention to strengthening the research and development in the following aspects.
(1) Combination drying. When spray drying itself does not accomplish the drying task, it is first necessary to think of combined drying. Such as spray drying plus fluidized bed (drying and cooling), spray drying and belt drying.
(2) Improvement of the atomizer. When it is difficult to atomize a certain material, the structure of the original atomizer can be improved to meet the atomization requirements of the new material. For example, various improvements have been made to rotary atomizers to atomize highly viscous materials and spray granulation.
(3) Research and development of electrostatic atomization technology. The technology is in the research and development stage, it can produce micron and sub-micron particles, manufacture functional particles, manufacture films and spray, etc., and predict its future development prospects.
(4) Develop and improve online measurement systems. Automate system operation to ensure product quality and yield.
(5) Development of a spray drying system for superheated steam. This is a closed loop system that saves energy and eliminates the need for nitrogen circulation.
(6) Solving the design problem of spray dryer by using computational fluid dynamics (CFD) method. In the future, some reliable experimental data (including flow patterns) can be used, and the CFD method can be used to calculate the size of the dryer and the distribution of hot air more accurately, instead of the current semi-theoretical and semi-empirical methods (the current method error is too large).
(7) Develop special spray dryers to meet the needs of special materials. Such as Chinese medicine, tomato powder, special foods, etc. with low softening point. Special dryers should reach this level - no experimentation, direct selection according to the nature of the material.
(8) Control environmental pollution. At the time of design, it must be considered that the system noise, dust, exhausted gas, wet dusting liquid, etc. do not pollute the environment.
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