Air Drying Particulate Solids
Key points
TL;DR Summary
Air drying of particulate solids is essential in food, pharmaceutical, chemical and mineral industries. The process uses heated air to remove moisture while maintaining product quality, balancing three key factors: energy use, throughput and quality. Different drying systems such as tray, belt, fluidised-bed, flash and ring dryers are suited to specific materials and requirements.
Key challenges include sticky or cohesive powders, inconsistent moisture control, energy consumption, and risks of caking or flow problems. Effective drying requires understanding drying curves, sorption isotherms, and bulk flow behaviour. Pilot trials, lab tests and expert support remain crucial to ensure reliable large-scale performance.
Drying wet particulate solids is part science, part craft. Striking the right balance between the nature of the product, equipment design, and operating conditions is essential, not just to maximise the yield in removing water, but to preserve product quality, and keep the whole process running smoothly in harmony with upstream and downstream operations.
Air drying, which uses heated air to dry particles, is a critical operation in many process industries, including food, pharmaceuticals, chemicals and minerals.
Although it is widely used, it still presents challenges, especially when handling powders that tend to be sticky, cohesive, or poor-flowing.
When it comes to handling, storage, and processing of bulk solids, including drying operations, the challenges tend to arise in three main scenarios which demands expert insight to address effectively:
From a decision-maker’s perspective, air drying operation is often a balance between three key factors: product quality, energy use, and processing time.
Quality refers to preserving the product’s key characteristics, such as texture, nutritional value, and stability, ensuring it meets required standards.
Energy efficiency measures how effectively the system removes moisture relative to energy consumed, impacting operational costs and sustainability; and throughput denotes the processing rate, balancing production targets with the need to maintain quality and energy efficiency.
To achieve this balance, engineers focus their efforts on how well the material, drying air, and equipment interact as each element plays a vital role in overall drying performance.
An air-drying system has three main functions: first, to heat the particles so that moisture inside them moves toward the surface; second, to provide the latent heat required to convert the water into vapour; and third, to carry away the evaporated water. In air drying systems, heated air is used for all three tasks.
The air heats the particles, evaporates the water, and has the capacity to hold and remove the resulting vapour.
However, if the air is insufficient to provide the required heat, alternative methods can be used, such as infrared radiation or conductive heating, where the product is brought into contact with a heated surface.
Volumetric heating can also be used, for example, by microwave heating, which involves exposing the wet material to a microwave field, or by ohmic heating, in which an electric current is passed directly through the wet material.
Figure 1 shows different types of the moisture content and ranked them according to the easiness of removing. Surface water is unbound and the easiest to evaporate; it can be removed effectively by an airstream, provided that sufficient heat is transferred to cause evaporation and that there is adequate airflow with a high moisture-holding capacity.
Heating the air not only supplies the heat required for evaporation but also increases its capacity to hold and remove the resulting vapour.
For example, warm air at 50 °C can carry up to six times more moisture than air at 20 °C, which makes drying air temperature control critical.
In contrast, capillary and interstitial moisture require more rigorous action to release the water to the surface and expose it to the drying air.
Various air-drying systems, such as tray, belt, flatbed, fluidised-bed, flash, and ring dryers, are suited to different materials and moisture levels. Tray dryers are ideal for small batches but slow, while belt dryers handle large volumes with higher energy use and space needs.
Fluidised-bed dryers provide efficient, even drying with adjustable airflow but require careful feed control. Flash and ring dryers offer very fast drying in compact spaces, perfect for fine or temperature sensitive powders, though they need precise feeding and can consume more energy.
Ultimately, the choice often depends on budget, available space, market requirements, and the specific needs of the material.
Nonetheless, obtaining technical insights is essential at every stage of management. Practical experience shows that any technical investigation is incomplete without addressing the following key questions:
To answer these questions, the right data must be collected and analysed by experts, either on-site or through specialised external support. Two key types of data are needed as shown in Figure 2, kinetic drying curves and sorption isotherms.
Sorption isotherms show how powders absorb and release moisture under different air humidities at specific temperatures, helping to predict storage behaviour and also estimate the heat required for drying.
Dynamic Vapour Sorption systems are used for tiny samples (a few milligrams), while climate chambers are suitable for larger samples of up to one kilogram.
Figure 2: Basic drying characteristics curves
Kinetic drying curves track how a sample’s weight changes over time during drying. These should be measured at several different temperatures to understand how drying rates change with various operating conditions.
It is important to generate kinetic curves using test rigs that reflect the real dryer design. For example, results from a tray dryer may be acceptable for general use, but a pilot-scale fluidised bed dryer will give better predictions for fluid bed drying systems.
There is well-established thermodynamic analysis procedure to determine the heat of desorption, the energy needed to evaporate one kilogram of water, using the sorption curves data. Comparing the theoretical value of desorption heat with respect to the actual heat used in practice reveals the system’s drying efficiency.
Fluidised bed dryers typically achieve an energy efficiency of around 20–65%, depending on the material and conditions. Flash dryers tend to be more efficient, sometimes reaching 75%. These figures are not fixed; they depend on how the system is operated, the product being dried, and the effectiveness of heat recovery measures.
While high energy consumption is often the main criticism of drying operations, other challenges must also be tackled, including inconsistent product quality, difficulties in maintaining target moisture levels, and environmental or safety risks. Beyond these, drying systems are still subject to common material flow and handling issues.
Most solutions to these challenges involve physical or mechanical interventions during drying, making it essential to understand the mechanical properties of powders, both as individual particles and in bulk, not only before and after drying but throughout the drying process itself.
Therefore, while drying curves provide valuable information, they do not indicate the likelihood of caking or changes in flow properties. Caking can cause significant problems, including quality risks and flowability issues that may halt operations and lead to costly downtime.
In other words, how a powder behaves in bulk is often overlooked, yet it is even more critical in drying because the material’s state changes significantly as it dries. Powder cohesiveness and flowability are closely linked to both moisture content and thermal treatment.
Figure 3 illustrates how caking can occur due to various reasons including moisture migration, mechanical loads, and thermal conditions.
Characterising a material’s flow and sticking behaviour helps define practical limits for airflows, bed depths, and feed rates. For example, knowing the maximum airflow that can pass through/over a layer of particulate material without excessive dust generation or loss of air–particle contact is essential.
Poor flowability can also limit feed systems that deliver wet product into dryers, especially for flash dryers, which rely on consistent feeding. Shear testing can help identify these limits.
Figure 3: Caking phenomena in powder handling
Thermal exposure is another crucial aspect in air-drying systems, where some powders are temperature-sensitive and may degrade or become sticky when exposed to higher temperatures.
This is why it is important to determine the sticky-point curve, which shows the temperatures and moisture levels at which a powder becomes tacky.
This information helps explain why certain materials cake during drying and how mechanical loads from equipment can make this worse.
Knowing the material’s drying and flow characteristics allows to predict potential risks and areas for improvement. However, lab tests alone may not be enough especially for large capacity drying systems.
Pilot-scale or full-scale trials are still needed to confirm that a system will work reliably. This is why it remains common practice for buyers to work closely with equipment suppliers to run preliminary trials, even when they already have lab test data and expert advice.
Independent expertise is invaluable for bridging the gap between what the buyer needs and what suppliers can offer.
In short, success depends on thoroughly understanding drying behaviour and bulk solids flowability before, during, and after the air-drying process, including storage and handling for the next stage. Expert laboratory analysis, pilot trials, and specialist advice are essential to achieve the right balance between product quality, energy efficiency, and consistent throughput.
The Wolfson Centre for Bulk Solids Handling Technology, part of the University of Greenwich since 1992, boasts over five decades of consultancy and research expertise. It stands as a globally unique centre offering independent advice on bulk solids handling and processing, including drying operations. Supporting industries worldwide, the Centre helps ensure powders and bulk materials to behave in the way they need them to!
What is air drying of particulate solids?
Air drying is the process of using heated air to remove moisture from powders, granules and bulk solids while preserving their key properties.
Why is air drying important in process industries?
It ensures product quality, reduces energy consumption, improves throughput and keeps operations aligned with upstream and downstream processes.
What are the main functions of an air-drying system?
Heating particles to release moisture, supplying the latent heat for evaporation and carrying away vapour with the airflow.
Which industries use air drying of powders and bulk solids?
It is widely used in food processing, pharmaceuticals, chemicals and minerals.
What are the main types of air dryers?
Tray dryers, belt dryers, fluidised-bed dryers, flash dryers and ring dryers each designed for specific capacities, materials and energy requirements.
What factors influence the choice of dryer?
Product characteristics, energy efficiency, space, throughput requirements, market standards and budget.
What are the main challenges in air drying?
High energy consumption, caking, poor flowability, variable moisture levels, thermal sensitivity and environmental or safety concerns.
How is drying efficiency measured?
By comparing theoretical heat of desorption with actual heat use, and by analysing drying curves and sorption isotherms.
What is a sticky-point curve and why is it important?
It identifies the temperature and moisture levels at which powders become tacky, helping prevent caking and processing issues.
Why are pilot trials and lab tests important?
They validate drying performance, predict flow behaviour, and bridge the gap between lab data, equipment design and real plant conditions.
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