Working principle and applications of centrifugal spray dryers and what materials they are suitable for

The centrifugal spray dryer is a continuous drying equipment that atomizes liquid materials through high-speed centrifugal atomization and brings them into contact with hot air, completing moisture evaporation in an extremely short time to obtain powdery or granular products. Its core advantages lie in fast drying speed, short material heating time, and wide adaptability to material forms, so it has relatively mature applications in food, pharmaceutical, chemical, ceramic and other industries. Starting from the working principle and combined with the characteristics of different materials, this paper sorts out which materials the centrifugal spray dryer is suitable for processing, as well as the key points that need attention in model selection and operation.

I. Working Principle

The process of centrifugal spray drying can be summarized as three continuous stages: “atomization—drying—separation.”

1. After filtration and heating, air is uniformly sent into the tower in a spiral manner by the hot air distributor at the top of the drying tower. The feed liquid is delivered by the feed pump to the high-speed centrifugal atomizer at the top of the tower. The atomizer disc rotates at a speed of 5,000—25,000 r/min. Under the action of centrifugal force, the feed liquid forms a film or fine filaments along the edge of the disc and is torn by air friction into tiny droplets. The lower limit of the peripheral speed of the atomizer disc is 60 m/s, and the recommended range is 100—160 m/s. Below this lower limit, differences in droplet flight distance will lead to uneven drying. The particle size of the atomized droplets is usually between 30—150 μm. After co-current contact with hot air, moisture evaporates by 95%—98% within 5—15 seconds. The dried powder is collected from the tower bottom and cyclone separator, and the exhaust gas is discharged after dust removal.

2. The drying tower adopts a co-current design, with hot air and droplets moving in the same direction. The wet-bulb temperature of the material surface is far lower than the hot air inlet temperature, which makes the loss of active ingredients in heat-sensitive materials relatively small during drying. The surface roughness of the working surface of the atomizer disc needs to be controlled within 0.8 μm, and the speed fluctuation is recommended not to exceed ±2%, so as to ensure that the liquid film spreads evenly on the disc surface without premature rupture.

II. Criteria for Determining Applicable Materials

To determine whether a material is suitable for centrifugal spray drying, the main consideration is the degree of matching in the following dimensions.

1. Viscosity and solid content:
Centrifugal atomization has relatively high tolerance for feed liquid viscosity and can process materials with viscosity not exceeding 5,000 mPa·s, including high-solid-content slurries and pumpable pastes. The solid content of the feed liquid is usually controlled between 10%—60%. Raw liquids with a moisture content of 40%—90% can be fed directly without the need for substantial pre-concentration. This characteristic gives it a clear advantage when processing materials that easily clog traditional nozzle atomizers, such as traditional Chinese medicine extracts and polymer solutions.

2. Heat sensitivity:
The co-current drying method keeps the material temperature throughout the entire drying process below the wet-bulb temperature of the air, and the heating time is only a few seconds, making it suitable for processing temperature-sensitive components such as enzyme preparations, probiotics, and plant extracts. For scenarios with higher activity retention requirements, low-temperature equipment can be selected, with the inlet air temperature controlled at 35—100°C, or inert gas can be used as the drying medium in a closed-loop system to further reduce the risk of oxidation.

3. Material form:
Suitable for solutions, emulsions, suspensions, and pumpable paste materials. Slurries containing fine solid particles are not prone to clogging the feed liquid channel under centrifugal atomization, and the finished product particles are relatively uniformly distributed.

4. Solvent type:
Materials with water as the medium can use a conventional open-loop system; slurries containing organic solvents such as ethanol, acetone, and hexane require a closed-loop system, using nitrogen as the drying circulation gas, while achieving solvent recovery and explosion-proof safety.

III. Applicable Materials in Various Industries

1. Food Industry

Dairy products are one of the typical application scenarios. Milk powder, whey protein, milk powder base materials, etc. can be processed by centrifugal spray drying to obtain powders with uniform particles and good solubility. In the production of fruit and vegetable powders such as citrus juice powder, tomato powder, and carrot powder, the drying time is usually within 1 second, and the retention rate of heat-sensitive nutrients such as vitamin C can reach over 85%. In the condiment field, salty flavors, bone broth powder, soy sauce powder, vinegar powder, and amino acid materials can all adopt this process. Among egg products, systematic process research has already been conducted on the centrifugal spray drying of egg yolk powder, involving the effects of inlet air temperature and centrifugal disc speed on the water reconstitution and instant solubility of egg yolk powder. In addition, maltodextrin, plant protein, instant tea powder, coffee powder, etc. are also within the applicable range.

2. Pharmaceuticals and Bioengineering

Traditional Chinese medicine extract is a representative application object of centrifugal spray drying in the pharmaceutical field. Traditional Chinese medicine extracts have high viscosity and high solid content, and traditional pressure nozzles are prone to clogging, while centrifugal atomizers have good adaptability to such materials. The national standard GB/T 32237-2015 specifically formulates technical specifications for spray dryers for traditional Chinese medicine extracts. For heat-sensitive bioactive materials such as enzyme preparations, probiotics, and antibiotic APIs, low-temperature centrifugal spray dryers can control the inlet air temperature within a relatively low range, and the retention rate of active ingredients can reach a relatively high level. For pharmaceutical intermediates processed in closed-loop systems, solvent residues after drying can be controlled below 500 ppm, meeting pharmacopoeia requirements.

3. Chemical Industry and New Materials

The chemical field covers pigments, dyes, catalysts, white carbon black, inorganic salts, detergents, and surfactants, etc. In terms of ceramic slurries, materials such as alumina, zirconium dioxide, aluminum nitride, kaolin, and bentonite can be processed by centrifugal spray drying to obtain spherical powders with good flowability, and the Span value of particle size distribution can be controlled within a relatively narrow range, facilitating subsequent forming and sintering. In terms of polymers, materials such as urea-formaldehyde resin and PVC can also adopt this process. Closed-loop systems have special value in the field of precision ceramics. Materials such as tungsten carbide, silicon nitride, and nano-ceramics need to be dried in an inert atmosphere to prevent oxidation. Closed-loop centrifugal spray dryers can produce spherical materials with good product consistency.

4. Other Fields

Materials such as feed additives, pesticide wettable powders, and dye intermediates are also applicable. In the environmental protection direction, zero liquid discharge treatment of desulfurization wastewater and drying and solidification of heavy-metal-containing wastewater have also begun to adopt centrifugal spray drying technology.

IV. Materials Not Applicable or Requiring Cautious Handling

The following situations require special evaluation during model selection. Materials with high fibrous content may wind around the atomizer disc, affecting atomization performance and equipment dynamic balance. Materials with viscosity exceeding 5,000 mPa·s are difficult to pump, and the load on the atomizer disc is too high, so dilution pretreatment or other drying methods may be required. Feed liquids containing large hard impurities will accelerate atomizer disc wear, and effective filtration is required before feeding. For scenarios requiring higher product bulk density (such as exceeding 0.6 g/cm³), the powder obtained by centrifugal spray drying is usually relatively loose, and it may be necessary to meet the requirements by adjusting tower height or adding subsequent compaction processes.

V. Reference Points for Model Selection and Operation

1. Equipment selection should be based on small-scale test data of the material. Evaporation capacity is the core parameter for determining equipment specifications. The evaporation capacity of small laboratory units is 5—10 kg/h, and industrial units can cover the range of 50—6,500 kg/h. For high-viscosity materials, priority should be given to centrifugal atomizers with variable-frequency speed control; for materials containing organic solvents, a closed-loop system with nitrogen protection can simultaneously meet safety and solvent recovery requirements; for extremely heat-sensitive materials, low-temperature equipment or vacuum-assisted drying solutions are worth considering.

2. Parameter coordination during operation is equally important. Feed rate, hot air temperature, and atomizer speed constrain each other, and adjusting any parameter requires evaluating its impact on the other parameters. The fluctuation of atomizer speed should be controlled within a small range; unstable speed will lead to an abnormal increase in droplet size and affect drying uniformity. For materials prone to wall sticking, material deposition can be reduced by installing a cold air jacket on the tower wall, adding an air sweep device, or adding a vibration device. The powder collection rate of traditional Chinese medicine extract materials can reach over 95% after being equipped with appropriate anti-wall-sticking measures.

3. The core value of centrifugal spray drying technology lies in compressing the drying process of liquid materials into a time scale measured in seconds, while establishing a controllable balance between efficiency and product quality through the coordination of atomization method and drying tower structure. Whether a material is suitable for this process requires comprehensive consideration of multiple factors such as viscosity, heat sensitivity, solid content, solvent type, and product particle size requirements, and judgment should be made based on experimental data rather than directly applying general parameters.