Flash dryers are essentially a piece of industrial drying equipment. They are utilized to eliminate moisture from damp materials. It finds widespread use in an array of industries, including food processing, pharmaceuticals, minerals, and chemicals. The functioning of the flash dryer involves swiftly heating the wet material, leading to rapid evaporation of the moisture and yielding a dried product.
We understand not everyone will get their technicality. So, we’ll try our best to use the simplest language to introduce them to you. In this article, we will look at the following:
So, let’s start.
How Flash Dryers Work: The Engineering Behind Them
A flash dryer is a system in which a wet powder is introduced into a hot stream of air. Usually this is done by throwing the wet material into a very high-speed airflow in a venturi using a mechanical disperser. The powder has a large surface area surrounded by hot air. Since the powder is wet, the hot air can easily evaporate the water at the particle surface, simultaneously cooling the air down and carrying the water vapour away in the air stream.
You may have dried your hands in a public bathroom using a warm air dryer. When doing so, you will notice that if your hands are wet, they feel cool even though they are being held in a current of warm air. Not until the water is almost all evaporated do your hands start to feel warm. Your hands are protected from the heat by the evaporating water. This is the same process that happens at the feed point of the flash dryer.
The rapid evaporation which takes place in the venturi of the dryer is called flashing, hence the name “Flash Dryer”. In fact, most of the water is evaporated in this short section until the powder surface has no free water. The remaining moisture must diffuse through the inside of the particles, and this takes more time. So, the last part of the drying takes much longer than the initial flashing. The moisture content in the final product can vary from 0% to as high as 12%, influenced by the operating parameters or the amount of residual moisture present in the product. So, it’s an effective industrial solution to extract moisture from wet substances.
Flash dryers are quite simple and easy to construct and widely utilized across a range of industries, including:
- Food processing,
- and Minerals.
Operating Process of Flash Dryers
Flash drying is a continuous process that can be heated either directly or indirectly. The moist material is fed into the dryer and exposed to hot air at high velocity. This hot air quickly removes the moisture from the material, producing a dry output that can be easily transported and stored.
As a co-current dryer, the hottest air comes in direct contact with the wettest product. The inlet temperature in the dryer can vary, ranging from 340°F (170°C for heat sensitive products to more than 1,100°F (600°C) for products which will not be damaged by high temperatures. The system's low residence time and the flashing off moisture result in significant evaporative cooling, enabling the use of higher inlet temperatures without overheating the product than other types of drying process. Moreover, these higher inlet temperatures enhance the overall efficiency of the dryer.
This type of dryer can be constructed to handle very small capacities up to throughputs of hundreds of tons per hour. Evaporative capacity can be from a few kilos per hour to 40 tons per hour or more.
We at Dedert provide flash dryers that are uncomplicated yet adaptable. They can be engineered with inlet temperature levels as high as 1470°F (800°C). As such, these industrial giants can dry a broad spectrum of products, primarily: filter cakes, slurries, paste, sludge, friable filter cakes, powders, and granules.
- Native Starch
- Sodium Bicarbonate
- PET Flake
- Calcium Silicate
- Polystyrene (EPS)
- Copper Powder
- Corn Masa Flour
- Dicalcium Phosphate
- Coal Polyacrylonitrile (PAN)
Energy Management and Configuration of Flash Dryers
Flash dryers also vary in their configuration. Let’s take a look at a few of them.
1. OC (Open-Circuit) Configuration
An OC configuration flash dryer operates at the minimum exhaust temperature required for a specific application, resulting in a high-quality final product. In this setup, the product is constantly exposed to clean, pure air with low humidity to minimize the risk of thermal damage to the particles. The rapid evaporation at the feed point protects the product from thermal damage and it is only at the dryer outlet that the particles attain the highest temperature.
This is generally related to the relative humidity of the air in the dryer exhaust which must always be less than 100%. The dry product is in equilibrium with the air at the dryer outlet. Relative humidity of air changes with temperature and we test materials in our pilot plant to determine the air temperature and humidity which is required to give the correct product moisture content. These open circuit dryers generally product the highest quality products but have higher heat usage than the other systems which we will discuss below. Some heat can be recovered by using heat exchangers to recycle heat from the dryer exhaust to preheat the incoming fresh air at the dryer inlet.
However, for enhanced thermal efficiency and to address the need for inertization, the use of recycled exhaust gases can be employed. This partial gas recycling (PGR) setup is available for all our airstream drying systems and can also be retrofitted to existing drying operations belonging to customers.
2. PGR (Partial Gas Recycle) Configuration
In the Partial Gas Recycle (PGR) configuration, the majority of the dryer's exhaust stream is circulated back to the air heater as a preheated drying medium. This recycled hot air contains evaporated water, and this lowers the oxygen in the system. Most often this type of dryer is directly heated by burning gas which has the effect of consuming converting oxygen to CO2 and water during the combustion process. This increases the exhaust humidity even more and reduces the oxygen level to as low as 6%. Effectively this type of dryer is inertized at these low oxygen figures and will not support combustion. The high humidity of the exhaust requires that these dryers run at higher outlet temperatures to maintain the relative humidity necessary.
Typically, the recycle stream is about 70% of the dryer exhaust and the remaining 30% which contains the evaporated water and the combustion products from the burner is termed the bleed-off. A specialized type of flash dryer termed a Ring Dryer is most often used for this type of drying system to keep the exhaust temperature as low as possible. The bleed off stream has high humidity, so a Waste Heat Evaporator (WHE) or a condensing scrubber may be incorporated to recover energy for the concentration of a different stream or to heat other streams in the plant's operations.
Dedert's WHE is optimized for particle removal and stream saturation, maximizing the evaporator's performance. Additionally, it eliminates the need for a wet scrubber for the dryer bleed-off stream and reduces the volume of any exhaust which may need to be treated by a gas cleaning system.
3. Superheated Steam Dryers
In a superheated steam dryer, a gas/gas heat exchanger is used to heat the dryer and all the dryer exhaust except for the evaporated water is recycled.
This means that all air is purged from the dryer during start up and it runs with superheated water vapour circulating inside the dryer. With this type of dryer all the bleed off can be condensed so that much of the energy can be reused for other processes in the plant.
Benefits of Using Flash Dryers
Flash dryers offer several benefits.
1. Industrial Use
Flash dryers are widely utilized in industrial settings due to their ability to process materials quickly and effectively. They can handle high-volume production processes, making them suitable for industries with demanding production requirements.
2. Volumized Capacity and Rapid Drying
Industries need to process large quantities of materials efficiently and meet tight production schedules. As flash dryers achieve extremely fast drying times, often within seconds, their high-speed drying capability is advantageous for this.
3. Reducing Contamination
Flash dryers, by design, can help minimize the risk of contamination because of the simplicity of their design which makes them easy to clean. This aspect is particularly crucial in industries such as food and pharmaceuticals, where maintaining product purity and hygiene is essential.
Another crucial utilitarian aspect of flash dryers is that they are highly versatile and can effectively dry a wide range of materials. Whether it's powders, granules, slurries, or pastes, flash dryers can handle different material forms, making them suitable for companies that require drying diverse materials in a single production cycle.
5. Energy Efficiency & Compact Design
We can design flash dryers to have a high thermal efficiency, resulting in energy savings during the drying process. The efficient utilization of heat reduces energy consumption and lowers operational costs for businesses.
These dryers also typically have a compact footprint, allowing them to be easily integrated into existing production lines or facilities with limited space. This space-saving feature is advantageous for industries where optimizing floor space is crucial.
6. Improved Product Quality
The rapid drying process of flash dryers helps maintain the quality of heat sensitive materials by minimizing exposure to high temperatures. This ensures that the final product retains its desired characteristics.
When choosing a flash dryer for your application, there are several key factors to consider, including the type of material being processed, the required drying time, and the desired output quality. It's essential to pick a flash dryer that is specifically designed for your specific application and materials to guarantee optimal results.
To sum up, Dedert Flash Dryers are a flexible and effective means of extracting moisture from wet substances. Their quick drying ability makes them perfect for high-volume production processes, and their versatility allows them to handle a diverse range of materials. When choosing a flash dryer for your application, it's crucial to consider the specific needs of your process to attain the best outcomes.