The first industrial application of a fluid bed dryer was for a coal gasification project in 1922. The design has been vastly improved over the years, and a modern fluid bed can process many types of bulk solid materials at a wide range of moisture levels and operating temperatures. Drying rates can range from a few hundred pounds per hour to more than 100 tons per hour, with equipment size varying from a one-foot diameter circular pilot dryer to a rectangular production unit of 20 by 50 feet.
The heat and mass transfer between the drying air and the material being dried is highly efficient, giving the fluid bed dryer a short residence time. The dryer is also easy to operate, can run in batch or continuous mode, and has fewer mechanical components and a smaller footprint than many other dryers.
A fluid bed dryer operates according to the phenomenon of fluidization. Fluidization occurs when air or another gas of sufficient velocity is passed upward through a bed of bulk material, lifting the material and causing the particles to move in random order. In this type of dryer, where the fluidizing air is hot, this fluidization provides extremely rapid heat and mass transfer between the air and the material. The fluidized bed achieves a uniform temperature and moisture content and also has a low temperature compare with the air temperature. In fact, it is not uncommon to have a material temperature of no more than 200 degrees Fahrenheit in a fluid bed dryer operating with an air temperature of 1200 degrees Fahrenheit.
Almost any material, including chemicals, fertilizers, spent grains, clay, minerals and others, can be dried with a fluidized bed as long as the material has a suitable particle size distribution and other characteristics. Generally, only submicron powders or extremely large, dense solids are not candidates for this type of drying.
A typical fluidized bed dryer is part of a drying system consisting of a fluidizing fan, combustion chamber with a fuel or steam supply, the fluid-bed dryer itself, emissions control or dust collection equipment, an exhaust fan and controls. The dryer's main body includes an air distribution plenum, and a freeboard section (a free area at the dryer's top) with an exhaust air outlet ducted to the emissions control equipment, as well as a feed inlet and discharge outlet.
In operation, the fluidizing fan provides the motive force for the air that will fluidize the material. This fluidizing air flows from the fan through ductwork to the combustion chamber, where the air is heated. The hot air flows from the combustion chamber to the fluid-bed dryer's air distribution plenum, then flows upward through an air distribution plate (with a series of holes) or tuyeres (nozzle-like devices), evenly distributing the airflow upward into the dryer's drying chamber. A conveyor or rotary airlock valve at the feed inlet controls the wet feed to the drying chamber, where the material falls into a fluidized bed above the air distribution plenum. In the fluidized bed, the particles rapidly move about in random order. Each particle is fully exposed to the hot fluidizing air, which quickly transfers heat to the particle and promotes fast drying. As a result of the fluidization effect and the direct contact between the particles and hot air, the bed forms a mixture of almost homogeneous temperature and moisture.
In the freeboard section above the fluidized bed, the air velocity is lower, de-entraining the particles. The air in this section is also cooler and wetter. Dried material exits the dryer through the discharge outlet, typically controlled by a rotary airlock valve or knife-gate valve. This valve regulates the discharge flowrate to maintain the optimal fluidized bed height, in turn regulating the material residence time in the dryer. The system's exhaust fan draws dust-laden air from the freeboard section through the emissions control equipment, such as a cyclone or baghouse, so it can be safely exhausted. The controls regulate this exhaust airflow to produce a very slight vacuum in the dryer's freeboard section, which prevents dusting out of the dryer.
The fluidizing fan typically operates at higher pressures than an exhaust fan -- usually from 10-40 inches of water, depending on the application. The fan must be properly sized so it can provide adequate pressure to overcome the pressure drop from the fan through the combustion chamber, the dryer's air distribution plenum and the material itself.
The combustion chamber's heat source can be a standard fuel such as natural gas, propane, fuel oil or steam. Electric heat is typically not used because of the large electrical demand an industrial-scale fluid bed dryer would make. When a standard fuel is used, the chamber's combustion air can be supplied by the fluidizing air or air drawn by a dedicated combustion fan. For a high temperature application greater than 1000° Fahrenheit, the chamber is typically lined with refractory materials such as fire brick.
The fluid bed dryer can be circular or rectangular, or trough-type. A circular dryer has the smallest footprint, making it a good option for an application with limited floor space. A rectangular dryer requires more floor space, but provides tighter control of the discharge moisture content, making it suitable for an application where the discharge moisture content is critical. Unlike in a circular dryer, in which some material in the fluidized bed follows a circular path around the dryer periphery, expoosing some particles to more fluidizing air than others and producing areas of slightly hotter, dryer material, the rectangular dryer's incoming fluidized air is evenly distributed through the plenum running the length of the drying chamber. As material flows from the dryer's inlet end, through the chamber and over a dam at the chamber's discharge end, it follows a first-in/first-out flow pattern, exposing each particle to dry, hot fluidizing air all along its path toward the discharge. The particles at the dryer's discharge end are exposed to air that is as hot and dry as that at the inlet end, achieving dried material with a tightly controlled moisture content.
While fluidization can take several forms in a fluid bed dryer, including a bubbling bed, a slugging bed, and in an extreme case, pneumatic conveying, a turbulent bed is desired for most fluid bed drying applications.
For high temperature applications, the dryer's plenum can be lined with refractory materials. The dryer's freeboard section may be the same width or diameter as the drying chamber or may be larger, which will reduce the air velocity in this section and promote de-entrainment of particles before the air is exhausted to the emissions control equipment.
Heyl & Patterson fluid bed dryers are among the most efficient and cost-effective dryers on the market. We offer conventional designs available for powders and granular materials, as well as unique designs for materials which exhibit characteristics not normally conducive to fluid bed processing, such as sludges, filter cakes and agglomerates. Trough-type and circular models are available, and applications can be tested at Heyl & Patterson's pilot plant lab facility.
To learn more about fluid bed dryers from Heyl & Patterson, click here.
How a Fluid Bed Dryer Operates
The first industrial application of a fluid bed dryer was for a coal gasification project in 1922. The design has been vastly improved over the years, and a modern fluid bed can process many types of bulk solid materials at a wide range of moisture levels and operating temperatures. Drying rates can range from a few hundred pounds per hour to more than 100 tons per hour, with equipment size varying from a one-foot diameter circular pilot dryer to a rectangular production unit of 20 by 50 feet.
The heat and mass transfer between the drying air and the material being dried is highly efficient, giving the fluid bed dryer a short residence time. The dryer is also easy to operate, can run in batch or continuous mode, and has fewer mechanical components and a smaller footprint than many other dryers.
A fluid bed dryer operates according to the phenomenon of fluidization. Fluidization occurs when air or another gas of sufficient velocity is passed upward through a bed of bulk material, lifting the material and causing the particles to move in random order. In this type of dryer, where the fluidizing air is hot, this fluidization provides extremely rapid heat and mass transfer between the air and the material. The fluidized bed achieves a uniform temperature and moisture content and also has a low temperature compare with the air temperature. In fact, it is not uncommon to have a material temperature of no more than 200 degrees Fahrenheit in a fluid bed dryer operating with an air temperature of 1200 degrees Fahrenheit.
Almost any material, including chemicals, fertilizers, spent grains, clay, minerals and others, can be dried with a fluidized bed as long as the material has a suitable particle size distribution and other characteristics. Generally, only submicron powders or extremely large, dense solids are not candidates for this type of drying.
A typical fluidized bed dryer is part of a drying system consisting of a fluidizing fan, combustion chamber with a fuel or steam supply, the fluid-bed dryer itself, emissions control or dust collection equipment, an exhaust fan and controls. The dryer's main body includes an air distribution plenum, and a freeboard section (a free area at the dryer's top) with an exhaust air outlet ducted to the emissions control equipment, as well as a feed inlet and discharge outlet.
In operation, the fluidizing fan provides the motive force for the air that will fluidize the material. This fluidizing air flows from the fan through ductwork to the combustion chamber, where the air is heated. The hot air flows from the combustion chamber to the fluid-bed dryer's air distribution plenum, then flows upward through an air distribution plate (with a series of holes) or tuyeres (nozzle-like devices), evenly distributing the airflow upward into the dryer's drying chamber. A conveyor or rotary airlock valve at the feed inlet controls the wet feed to the drying chamber, where the material falls into a fluidized bed above the air distribution plenum. In the fluidized bed, the particles rapidly move about in random order. Each particle is fully exposed to the hot fluidizing air, which quickly transfers heat to the particle and promotes fast drying. As a result of the fluidization effect and the direct contact between the particles and hot air, the bed forms a mixture of almost homogeneous temperature and moisture.
In the freeboard section above the fluidized bed, the air velocity is lower, de-entraining the particles. The air in this section is also cooler and wetter. Dried material exits the dryer through the discharge outlet, typically controlled by a rotary airlock valve or knife-gate valve. This valve regulates the discharge flowrate to maintain the optimal fluidized bed height, in turn regulating the material residence time in the dryer. The system's exhaust fan draws dust-laden air from the freeboard section through the emissions control equipment, such as a cyclone or baghouse, so it can be safely exhausted. The controls regulate this exhaust airflow to produce a very slight vacuum in the dryer's freeboard section, which prevents dusting out of the dryer.
The fluidizing fan typically operates at higher pressures than an exhaust fan -- usually from 10-40 inches of water, depending on the application. The fan must be properly sized so it can provide adequate pressure to overcome the pressure drop from the fan through the combustion chamber, the dryer's air distribution plenum and the material itself.
The combustion chamber's heat source can be a standard fuel such as natural gas, propane, fuel oil or steam. Electric heat is typically not used because of the large electrical demand an industrial-scale fluid bed dryer would make. When a standard fuel is used, the chamber's combustion air can be supplied by the fluidizing air or air drawn by a dedicated combustion fan. For a high temperature application greater than 1000° Fahrenheit, the chamber is typically lined with refractory materials such as fire brick.
The fluid bed dryer can be circular or rectangular, or trough-type. A circular dryer has the smallest footprint, making it a good option for an application with limited floor space. A rectangular dryer requires more floor space, but provides tighter control of the discharge moisture content, making it suitable for an application where the discharge moisture content is critical. Unlike in a circular dryer, in which some material in the fluidized bed follows a circular path around the dryer periphery, expoosing some particles to more fluidizing air than others and producing areas of slightly hotter, dryer material, the rectangular dryer's incoming fluidized air is evenly distributed through the plenum running the length of the drying chamber. As material flows from the dryer's inlet end, through the chamber and over a dam at the chamber's discharge end, it follows a first-in/first-out flow pattern, exposing each particle to dry, hot fluidizing air all along its path toward the discharge. The particles at the dryer's discharge end are exposed to air that is as hot and dry as that at the inlet end, achieving dried material with a tightly controlled moisture content.
While fluidization can take several forms in a fluid bed dryer, including a bubbling bed, a slugging bed, and in an extreme case, pneumatic conveying, a turbulent bed is desired for most fluid bed drying applications.
For high temperature applications, the dryer's plenum can be lined with refractory materials. The dryer's freeboard section may be the same width or diameter as the drying chamber or may be larger, which will reduce the air velocity in this section and promote de-entrainment of particles before the air is exhausted to the emissions control equipment.
Heyl & Patterson fluid bed dryers are among the most efficient and cost-effective dryers on the market. We offer conventional designs available for powders and granular materials, as well as unique designs for materials which exhibit characteristics not normally conducive to fluid bed processing, such as sludges, filter cakes and agglomerates. Trough-type and circular models are available, and applications can be tested at Heyl & Patterson's pilot plant lab facility.
To learn more about fluid bed dryers from Heyl & Patterson, click here.
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