Compared with other drying equipment, spray drying devices have some disadvantages such as lower drying volume intensity (relatively higher energy consumption; larger volume) and higher operation requirements. However, its advantages are as follows:
① Large single-unit processing capacity (about 90 m³ of liquid per hour);
② Short exposure to high temperature, thus suitable for heat-sensitive materials;
③ Good sphericity of the product and adjustable particle size within a certain range;
④ Simple process, no need for crushing and screening after drying.
Spray drying is often in a leading position when drying solutions and suspensions, and this technology is developing rapidly. With the rapid development of China's national economy, there have been over 20 enterprises capable of manufacturing spray drying devices in China in the past 20 years. However, most of them are small in scale, with insufficient production means and testing capabilities, and weak research and development capabilities. Only 3 to 4 factories have a relatively large scale, with an estimated annual output value of about 200 to 400 million yuan. China has fundamentally reversed the situation of relying solely on imports, and has even started to export a small amount, mainly to Southeast Asian countries and South Korea.
The specifications and capacity of the complete sets of spray drying equipment produced in China (measured by the spray liquid volume) range from 5 kg/h to 26,000 kg/h. Recently, a large-scale centrifugal spray dryer with a capacity of 45 to 90 tons per hour has been successfully developed (this device is used by the State Power Corporation for flue gas desulfurization in 125MW power generation units). It can now meet the needs of various industries and multiple varieties of dried products in China, covering approximately 400 types of materials. The quality and comprehensive performance of Chinese spray dryers have approached the level of the Danish NIRO company.
The main gap in the late 1990s was in appearance and the durability of the supporting equipment (such as fans), due to the significant price difference, it has obvious competitiveness.
I. Development Overview of Spray Dryers in China
(1) Centrifugal atomizer
A motor drives a speed-increasing machine to rotate the adjustable rotating impeller, and the liquid is atomized into droplet groups of tens of micrometers by centrifugal force. This is the working principle of the rotary spray dryer. The characteristics of this type of machine are: large flow rate, not easy to clog, stable operation performance, low energy consumption, large single-unit production capacity, and uniform product particle size. Therefore, it has become the mainstream model of spray dryers.
The rotational speed of the rotary spray dryers produced in China ranges from 10,000 r/min to 32,000 r/min, and the spray liquid volume can reach 90 m³/h. After 20 years of exploration, it has now become completely mature. The rotary spray drying units produced in China have taken into account the characteristics of Chinese materials from the design stage, and are slightly larger in size than the same specification products of the NIRO company, but operate more stably and have a longer service life. For example, the F160 unit of a certain foreign company uses traditional planetary gear transmission, which inevitably requires a fast-rotating planetary carrier. Due to insufficient installation accuracy of the planetary gears on the carrier, accidents often occur. The RW10TE and RW45T models produced in China use quasi-planetary gear transmission, with non-rotating planetary gears, eliminating the planetary carrier and greatly increasing the reliability of the mechanism.
Large-scale rotary spray dryers are mainly used in flue gas desulfurization in thermal power plants, waste liquid treatment in paper mills, waste incineration, and the drying of large-scale chemical products, such as sodium metasilicate and silica production.
(2) Airflow atomizer
The device that disperses liquid droplets by high-speed airflow is an airflow atomizer, usually using compressed air. Due to its simple processing, large processing capacity, and low clogging risk, it was widely used in China in the past. However, its energy consumption increases in a square relationship compared to the centrifugal atomizer, so it is generally not suitable for large-scale drying machines.
In the 1980s and 1990s, China conducted in-depth research on airflow atomizers. Compared with foreign products, the viscosity that can be atomized under the same airflow ratio has reached the international advanced level. It is reported that the viscosity of the United States is 100,000 MPa•s, that of Japan is 60,000 MPa•s, and that of China has also exceeded 100,000 MPa•s. Although the air flow atomizer has a relatively high atomization energy consumption, the viscosity of high-solid materials is often high. Since it can atomize high-viscosity materials, when the solid content is high, the comprehensive energy consumption of the material during spray drying is still reasonable (relatively low). However, when using a rotary atomizer, it cannot atomize well at high viscosity and is forced to add water, thereby increasing both thermal and power energy consumption.
The air flow atomizers made in China are of two types: two-fluid and three-fluid, with specifications ranging from 5 kg/h to 1500 kg/h (about 10 types of processing capacity), and the gas-liquid ratio can be below 0.5.
Typical materials applied include ammonium-based emulsions, disproportionated rosin soap, polyvinylpyrrolidone (PVP), and microcrystalline cellulose, etc.
⑶ Pressure atomizer
The device that generates static pressure in the liquid and atomizes it into droplets through a small hole when it is at high speed is called a pressure atomizer. Its atomization energy consumption is only half that of the rotary atomizer and is a type of atomizer. However, since it must pass through a small hole, it has higher requirements for the filtration of the material liquid to avoid frequent clogging of the holes during production. At the same time, the pressure pumps that match it are not many in large specifications (generally below 5 m³/h), thus limiting its use.
When the pressure atomizer is above 5 MPa, it is a high-pressure type and can produce very fine droplets. It is generally used in the food industry, such as the production of milk, soy milk, and soy protein isolate, etc. When it is below 5 MPa, it is a low-pressure type and can produce coarser droplets. To fully dry and save the height of the drying tower, a fountain type is usually adopted, that is, spraying from bottom to top. Typical materials are the drying and granulation of special ceramics such as alumina and ferrite. The product quality of these spray dryers can already fully compete with foreign products.
II. Progress of spray drying equipment in recent years
With the development of the national economy and the improvement of people's requirements for the quality of material life, many products not only require drying but also have higher requirements for the color, aroma, taste, re-dissolution performance (cold or quick dissolution), vitamin activity retention rate, etc. of the final product. For some expensive drugs, the total yield is also very strict. These requirements are exactly what the international market demands of us after China's entry into the WTO. The following summarizes the new types of equipment developed in the field of spray drying in China to meet the objective market requirements. The common feature of these devices is that they do not sell a single spray dryer as a commodity, but use the spray dryer as the core and combine other drying devices or auxiliary equipment according to the characteristics of the material to form a combined drying equipment system, so that the final product meets the user's requirements.
⑴ Three-stage combined drying device
A three-stage combined drying device consisting of spray drying, internal fixed fluidized bed, and vibrating fluidized bed. In the spray drying stage, a higher inlet air temperature is used to rapidly remove most of the surface water while forming particles. In the second stage, a lower temperature and longer time are used to remove the internal moisture of the powder. This not only saves the volume of the spray drying stage but also saves about 20% of energy. In the third stage of the vibrating fluidized bed, lecithin slurry can be sprayed to make the product agglomerate and achieve cold and quick solubility. Dry and cold air after dehumidification is also introduced to cool and package the product.
This three-stage combined drying device has been used in China for products such as milk, coconut milk, banana powder, powdered oil (palm oil raw material), and tomato juice, and has achieved success. The scale ranges from 5000 kg/h to 1000 kg/h.
⑵ Spray drying device for high-sugar materials
Many food and traditional Chinese medicine materials contain rich sugar components, such as lactose, maltose, sucrose, glucose, fructose, etc. For instance, Angelica sinensis contains a large amount of sucrose, and its "glass transition temperature" is only 61℃. When it absorbs water, the value is even lower. However, the conventional spray drying inlet air temperature is controlled at 70-100℃, which causes the already dried powder to stick to the drying tower, leading to production difficulties. Besides sugar, there are many other materials with low "glass transition temperature" that can be solved by the same method, such as sorbitol, VBt, urea-formaldehyde resin, polyvinyl acetal resin, etc. Several enterprises in China have already produced special equipment such as jacketed cooling, intermediate air sweeping, and bottom air dehumidification cooling.
⑶ Biochemical spray drying equipment
Most of the materials in this category are pharmaceuticals, which have high requirements for equipment cleanliness. During the production process, it is necessary to prevent the invasion of external bacteria. The products are expensive and have strict requirements for recovery rate. The common feature of these devices is that the hot air needs to be filtered before and after heating. The packaging room requires isolation, and the drying process should be carried out under positive pressure to meet the GMP (pharmaceutical grade) standards.
Currently, the equipment provided by China has been used for heparin sodium, penicillin, streptomycin, earthworm enzyme, VB2, and calcium pantothenate, etc. The dust content in the exhaust gas can be less than 20mg/m³, and the total recovery rate can exceed 99.98%.
⑷ Environmental protection spray drying equipment
During the "Seventh Five-Year Plan" period, China invested a huge amount of human and material resources in the research and development of flue gas desulfurization for thermal power plants. This research continued until the "Tenth Five-Year Plan" period. After continuous efforts and exploration of various methods, two relatively mature technologies have been developed so far. One is the semi-dry method of neutralizing SO2 with lime slurry; the other is the electron beam method of neutralizing with ammonia water to obtain ammonium sulfate and ammonium nitrate.
Both of these technologies require the dispersion of liquids into droplets to come into contact with SOx or NOx in the gas phase and undergo related reactions. Therefore, they are inseparable from spray drying engineering or equipment.
The characteristics of this type of equipment are large processing capacity. For example, the capacity of a RW10T high-speed centrifugal spray dryer is 10-20m³/h, which can only be matched with a 25MW small-scale generator set. Considering that the main generator sets in China are all above 125MW, the capacity of a single spray dryer should exceed 45m³/h. The development of such large-scale high-speed spray dryers is being organized by the State Power Corporation and implemented by Jiangsu Linzhou Drying Machinery Factory, with the design unit being the Institute of Forestry Chemical Industry, Chinese Academy of Forestry. This equipment has been put into industrial application. Currently, only the Danish NIRO company and China can produce this type of equipment on a large scale.
A similar device is the gas purification in waste incineration. Different harmful components in the gas are sprayed with corresponding liquid. China is still in the initial stage. In Japan and the United States, thousands of incinerators have been shut down recently. The reason is that the incineration of plastics produces dioxins, cyanides, and other toxic substances. After being adsorbed by activated carbon and buried in the soil, they will re-pollute the soil. Therefore, China should be cautious about the gas purification methods in waste incineration.
The black liquor discharged from paper mills has high COD and BOD values and cannot be treated by general sewage treatment methods. The discharge accounts for about 30% of the total discharge in China and is a major source of pollution. However, the components such as lignin in the black liquor can be converted into beneficial substances after processing. There are nearly 20 paper mills in China that use spray drying equipment to turn the concentrated black liquor into dry powder. The product, calcium lignosulfonate, produced by Guangzhou Paper Mill is a popular and profitable product, and its profit has accounted for more than 30% of the total profit of the factory for many consecutive years.
It is worth noting that in developed countries, the raw materials for papermaking are mostly wood, while in China, a large amount of grass-based raw materials (such as wheat straw, reeds, bagasse, etc.) are used. The black liquor from grass pulp is much more complex than that from wood pulp. Grass pulp has higher ash and sugar content than wood pulp, which brings many technical challenges (such as the adhesion problem caused by the low glass transition temperature of sugar). However, China has overcome these challenges one by one. In this sense, China has surpassed the international level in this field. ⑸ Oxygen-sensitive Material Spray Drying Equipment
Some substances are extremely sensitive to oxygen and undergo denaturation. For instance, when β-carotene is dried in the presence of oxygen, it loses 48% of its content. Some materials are prepared using organic solvents, and during drying, the evaporated moisture is the solvent. These materials must be isolated from oxygen both before and after drying. There are three processes available for different degrees of oxygen sensitivity requirements.
The system is filled with inert gases such as nitrogen or CO2. The gas is cooled after drying, and the organic solvent can be recovered and reused in the heating cycle. This type of equipment requires tight manufacturing of the fan and drying equipment to prevent air leakage for normal operation. Typical applicable materials include nanomaterials, Vc salts, and some special ceramics (super-hard materials). Medium and small-scale systems of this type have been mature in China and some have been exported.
Clean fuel combustion (such as natural gas, propane, etc.) is used as the heat source, which naturally eliminates oxygen after combustion. Some of the low-temperature gas containing moisture after drying and a small amount of fresh air are simultaneously mixed into the drying hot air flow to regulate and control the inlet air temperature for drying. The oxygen content in the hot air flow of this method is approximately 8%, which is suitable for some oxygen-sensitive substances. Its advantages include low operating costs, energy savings of about 30%, relatively simple equipment manufacturing and operation. The typical material is the spray drying of the dye intermediate H acid.
Using the above method, the tail gas after drying is cooled and dehumidified, and returned to the high-temperature gas flow, thereby controlling the oxygen content in the system below 2% without adding fresh air. No inert gas needs to be supplemented, and the solvent can be recovered. This method has been successfully used in the production of spirulina to ensure that the β-carotene is not oxidized. Other oxygen-sensitive substances such as proanthocyanidins and their glycoside derivatives can also be produced using this method. However, China is still in the initial stage.
III. Progress in Spray Drying Ancillary Equipment
⑴ Recovery System
The ultimate goal of spray drying is to recover qualified products. Even if the main equipment is excellent, users will not be satisfied if there is a low recovery rate and high energy consumption in the recovery system. Three conventional methods are commonly used in China's spray drying systems to recover fine powder:
① Cyclone separator - When the average particle size is greater than 2 μm, the recovery rate is greater than 98%, and the resistance can be lower than 1500 Pa. There are already standardized series products in China, with diameters up to 2.3 m. These indicators have reached the level of the late 1990s.
② Bag filter - Due to the needs of the metallurgy, chemical, and cement industries, there is a group of formal developers of bag filters in China. In recent years, significant achievements have been made in bag material and pulse cleaning methods, and they have been put into large-scale production and application. Some enterprises' products can ensure an efficiency of over 99.9% at a wind speed of 1.5 to 2 m/s, with an emission depth of more than 5 mg/m³. It can be said that the level of bag filters in China has caught up with the international level.
③ Tail gas scrubber - A device that uses water to clean the gas containing a small amount of dust coming out of the cyclone separator. The circulating scrubbing water gradually concentrates the dust. In paper mills, dilute black liquor can be used instead of water, and thermal energy can be fully utilized. In China, the efficiency can reach 99.98%, with the dust content in the discharged gas less than 20 mg/m³ and the resistance less than 1000 Pa.
⑵ Heating System
① Steam heater - Most domestic steam heaters use aluminum finned tubes and can fully utilize the superheated, high-pressure saturated, and low-pressure saturated sections after high-pressure water pressure relief flash evaporation in one heater. The temperature difference between the heated air and the saturated air is no more than 10°C, demonstrating a high heat transfer efficiency. This is called a three-stage steam heater and has been widely used in large-scale production.
② Fuel oil hot air furnace - This type of furnace uses light diesel combustion to indirectly heat air, with a total heat transfer coefficient of up to 40 kcal/h•㎡•℃. It is widely used in various spray dryers. Due to the insufficient performance of domestic burners, most of the time, the Italian brand LIAOYA, the German brand OUKO or the American brand MAXON are adopted to enhance the overall level of the spray drying machine. Another type of fuel-fired hot air furnace is to directly introduce the flue gas after combustion into the drying system. The common feature of these two types of furnaces is that they use ambient air to form a jacket, which can preheat the air and prevent heat dissipation, making full use of the heat. China's fuel-fired hot air furnaces are available in various specifications and models, and can be fully equipped with spray drying machines of all scales. The technology is mature and advanced.