The spray drying process has a long history. The earliest description can be traced back to 1860, and the earliest design patent record was in 1872. The basic principle is to produce highly dispersible powder products by evaporating the solvent in the liquid feed. Specifically, the heated gas is mixed with the atomized (sprayed) liquid flow with a high surface mass ratio in the drying chamber, and the solvent is evaporated uniformly and quickly through direct contact. It is best if the droplet sizes are the same.
I. Application fields
Spray drying is widely used in the production of fluid powders. This dehydration method has been successfully applied in many fields:
- Pharmaceutical field.
- Bone and dental amalgam field.
- Beverage industry.
- Spice, dye and plant extraction field.
- Milk and egg products field.
- Plastic, polymer and resin field.
- Soap and detergent field.
- Textile and many other product fields.
Compared with other drying methods such as ovens, freeze drying and rotary evaporators, spray drying can more flexibly control the characteristics of powder particles, such as density, size, fluid characteristics and humidity, avoiding the production of particles with uneven sizes and inconsistent shapes and a large amount of materials that need further processing.
II. Design and control
For designers and users, the challenges are to increase production, improve the quality of powder products and reduce costs. This requires a deep understanding of the production process and efficient implementation of production control.
- Spray drying stages
- Feed preparation: It can be homogeneous, pumpable, impurity-free liquid, suspension or viscous material.
- Atomization (converting feed into droplets): This is the most critical step in the process. The degree of atomization determines the drying rate and thus the size of the dryer. Commonly used atomization technologies are:
- Pressure nozzle atomization: Make the liquid form a spray by rushing out of the nozzle under pressure. This method is more energy-efficient and the particle size obtained is the most uniform.
- Two-channel air flow nozzle atomization: Mix the feed with compressed air to achieve atomization. This method has the worst energy-saving effect but can be used to produce fine particles.
- Centrifugal atomization: Make the feed flow through the turntable to generate atomization. It is the most wear-resistant and can usually run for a long time.
- Drying: In the constant-rate stage, moisture evaporates rapidly from the particle surface. In the subsequent falling-rate stage, the drying rate depends on the speed at which moisture diffuses from the inside of the particle to the surface.
- Separating powder from moisture: Treatment methods such as recycling the drying medium are economical and pollution-free. Fine particles are generally removed by cyclones, bag filters, settlers or scrubbers.
- Cooling and packaging.
- Control system features
- Have precise loop control with set point curve programming.
- Have an easy-to-parameterize recipe management system.
- Be suitable for sequential control of complex control strategies.
- Be able to perform real-time safety data recording of the system for analysis and evidence.
- Have a local operator station with a clear graphical interface, monitoring function and parameter control access permission management.