Spray drying process removes the moisture from liquids to turn it into powdered form without considerably changing their nutritious value or core chemical properties. It increases the product shelf-life without the need for refrigeration, and thus, is a key process in food and pharmaceutical industry, along with other sectors where spray drying is also used. Quick processing, precise, consistent particle size, and solubility are the hallmarks of a perfect spray drying process.
Spray dryer requires consideration of factors related to two key aspects, the properties of the feed material and the efficiency of the dryer.
A) Factors to consider in feed properties
Technically, spray drying can convert any liquid product into powdered form. However, the chemical properties of the feed must be considered not only from particle consistency point of view, but also in terms of safety of the equipment. Here's how chemical properties of the feed can affect the dryer system and the drying process.
- pH factor: Feeds with a high pH factor can corrode the walls and damage the equipment.
- Flammability: Certain feeds can be flammable or prone to explosion. In such cases, adequate safety precautions need to be taken during the spray drying process.
- Hydroscopic properties: Products that tend to attract water, present additional challenges for drying
- Abrasive properties: If the feed is abrasive, it will damage the equipment at vital points as it runs through the spray dryer system. It could damage piping system, pumps, walls etc.
- Thermoplastic properties: Lower glass transition temperatures of thermoplastic materials such as certain resins tend to create additional challenges in drying.
The viscosity of the feed determines how smoothly it will flow through the spray dryer system. As the liquid moves, the motion of the particle relative to one another develops shear stress. Adjacent layers of the fluid tend to move at different velocities than one another.
Key decisions regarding batch processing and atomization techniques can be made only after considering whether the viscosity of the feed increases or decreases with the shear. This information also helps to determine the best way of cleaning the equipment after the operation is completed.
Spray drying is best suited to dry heat-sensitive products. However, if the liquid has very low heat sensitivity, it will be difficult to achieve a desired result by heating it. One the other hand, extremely high heat sensitivity also poses challenges in spray drying and can limit the dryer capacity.
One factor that deeply impact the product throughput is the amount of solid present in the feed. Learning the exact percentage of solid in the feed is key from dryer efficiency perspective. Firstly, it tells us whether the feed will be successfully evaporated or whether it would exceed the evaporation capacity of the dryer, which is not an ideal situation. Secondly, If the percentage of solid content is high, it translates into lower evaporation of the feed and a higher production rate.
In many cases, the feed material could be an emulsion, a suspension or a colloid. The efficiency in spray drying depends considerably on the structure of emulsions. Coalescence of emulsions is a serious problem in spray drying. In addition,
suspension materials will witness an uneven flow if there is excessive settling out. The final product will be affected and the dryer system will have jammed nozzles.
Specific gravity is a factor to consider when working with fluids other than water. Specific gravity is the ratio of a fluid's density in comparison with water. The specific gravity of water is always held as 1. Specific gravity helps to decide product
throughput in the spray drying operation.
B) Factors to consider in spray dryer
When nozzle atomizers are used, atomization of the feed is done under pressure. The chemical properties of the feed material and desired droplet size in turn influence the selection of the atomizer. The pressure applied during this process
influences the droplet size. The higher the pressure, the smaller the droplet size of the final product. You can find additional information about the working of different atomization systems in our blog here.
Inlet temperature is the temperature of the drying gas, measured just before it enters the drying chamber. The higher the inlet temperature, the higher the evaporation rate of the solvent.
However, simply increasing the inlet temperature does not necessarily yield the best final product.
On the contrary, high inlet temperature can affect the wet-bulb temperature of the surrounding air, and leads to thermal degradation of the final product. The calculations for level of inlet temperature to be maintained depend on the properties
of the feed material.
The volume of drying gas injected into the drying chamber per unit time is known as the gas flow rate. If the gas flow rate is high, it increases the efficiency of the cyclone within the drying chamber, but air-droplet interaction is minimized, and movement of particles increases. Thus, the gas flow rate should be maintained just enough to ensure evaporation of moisture and the separation procedure that follows.
Outlet temperature is the temperature of air holding the dried particles, measures prior to deposition of these particles in the collector. In theory, this is the maximum temperature to which the product can be heated. However, it does not stand true in all cases. IN case of counter-current dryers, the outlet air might have a lower temperature than the final product.
Residence time is the time feed material droplets spend inside the drying chamber. This factor directly impacts the quality of the final product. Residence time should be calculated correctly to dry the liquid feed yet retain its core chemical properties.
Ideally, the residence time is a few seconds, with fine particles not spending over 15 seconds in the chamber.
Conclusion From feed properties to movement of particles within the drying chamber, a number of factors must be considered for a successful spray frying process. Achieving precision in the process requires a combination of technology, expertise and experience. Shachi Engineering has over two decades of expertise in drying process for diverse industrial applications. We have proven experience and expertise in spray dryer designing, manufacturing till commissioning at customer sites that meets global standards. Apart from having pilot plants for development and testing purpose, Shachi Engineering provides advanced simulation and analysis of the drying process using CFD technology to make your process more efficient and cost effective.