Drying Bulk Particulate Effectively
Some drying technologies are better suited for certain industrial applications than others. Learn where drying processes and fluid-bed, rotary drum, rolling-bed and belt dryer technologies intersect best.
When planning a new industrial process or updating an existing one, a number of different drying technologies can be used. The properties of the material being processed must be considered carefully for a qualified selection of the drying technology. Specific material properties include the product’s temperature sensitivity, size distribution, moisture content and any special characteristics. Depending on the material specifications, different types of dryer technology may be more or less suited for the application.
Proper specification of the material allows for proper consideration of the numerous drying technologies available — and the optimal selection of a type of industrial dryer. Incorrectly chosen dryers can lead to lower drying efficiency (leading to higher-than-desired moisture content), clumped product and, in some cases, burnt product. Additionally, some dryers have multiple process functions, including calcining, cooling, de-dusting and cleaning. Depending upon the material requirements, various supporting process roles often are best suited by a particular drying technology.
The fluid-bed dryer is suitable for many types of materials. In addition to drying, fluid beds also can be utilized to cool or stabilize a product temperature.
Fluid-bed — or fluidized-bed, as they also are known — dryers provide perpendicular gas flow to the solid flow of material in a shallow bed. Fluidized-bed dryers have demonstrated advantages when a material has a standard size. In an efficiently fluidized bed, the entire solid material is rendered into a quasi-fluid state by the air currents. At the end of the dryer, the same volume of solid runs over an overflow weir as wet solid material is added at the entry of the dryer. The solid material flows in a countercurrent to the upward current of air. The overflow weir is height adjustable and, by influencing of the height of the fluidized bed, enables adjustment of the solid-material residence time.
Due to the constant flowing of air around every solid particle, sensitive wet granulate from granulating plates or combined mixer-granulators can be dried gently in fluidized beds. Vibration of the fluid-bed dryer ensures even coarse particles that cannot be fluidized by the current of air are transported. Depending upon particle-size distribution and bulk density of the solid material, the use of fluidized-bed dryers is only recommended for particles less than 0.25” (6 mm) diameter. Otherwise, there is the possibility for coarse material to accumulate in the dryer because it cannot flow over the retaining weir at the end of the dryer. If a weir is not installed at the dryer discharge, the solid-material residence time cannot be varied by adjusting the bed height. Poorly fluidized solid material would no longer be sufficiently aerated.
Fluidized-bed dryers should be fed with the predominantly fine-grained solid materials for which they have been designed. The perpendicular flow of air can be adjusted to allow sufficient de-dusting of the product if required. If much coarser solids are fed to the dryer without adjusting the air, fluidization can collapse.
While fluidized bed dryers have demonstrated advantages when the material to be dried is a standard size, efficient dryer operation can become impossible when the inlet material has an inconsistent particle-size distribution. For instance, biofuel materials often have non-standard sizes. When drying bulky and fibrous products, fluidized-bed dryers can form carpet-like masses. Many organic materials such as tree cuttings can be bulky and fibrous, and they can stick to internal parts.
In some applications, the design of a fluidized-bed dryer can allow the drying air to become saturated with moisture easily. Energy consumption also can be a factor. Energy transfer can be improved by implementing integrated heat exchangers. Doing so can expedite reducing the overall length of the fluid-bed dryer by up to 70 percent. In addition to reducing the footprint of the fluidized-bed dryer, efficiencies also are realized due to thermal conservation and less air necessary to fluidize the particles. Up to 80 percent of the required drying heat can be supplied by the integrated heat exchangers, which reduces the required air.
Rotary Drum Dryers
Another common type of dryer for industrial applications is the rotary drum dryer. Drum dryers are comparatively large machines that run gas parallel with the solids while drying. This can be either counter-current or co-current, with the latter being the most popular. Modern drum dryers have blades or flights on the inside of the drum that lift moist solids as the drum rotates. The lifted product falls directly into the drying airflow, which averages between 1,112 to 1,652°F (600 to 900°C). For thermally insensitive materials such as silica sand, the flame can fire directly into the rotating drum. For other materials, however, firing chambers are used. Drum dryers can accommodate solid throughput of 5 to 150 tons per hour.
Although drum dryers have a comparatively simple operation, they are capable of emitting fine material with a low de-dusting effect. Additionally, some drum dryers can have two or even three drums for additional drying or cooling stages. Because accessibility for this type of dryer can be limited, it can present maintenance challenges. An advantage of drum dryers is that they are largely insensitive to fluctuations in the starting moisture-content level, feed rate, particle size or unwanted clumps and coarse particles (particle size distribution).
While suitable for sands, these dryers are particularly efficient at drying coarse and very coarse bulk solids. During product changeover, it is not necessary to adjust the air volume as it would be with a fluid-bed dryer. For these applications, the drum dryer is the ideal solution.
Similar to fluidized-bed drying, belt dryers have perpendicular gas and solid flows. A key advantage of this technology is the ability to control material residence time. Because the product is mechanically conveyed by the belt, the duration of thermal processing can be controlled exactly. Other drying technologies such as rotary drums and fluid-bed dryers mix the product during drying. While the mixing of the product results in more homogenous drying of product, free movement of individual particles of material is created. As a result, individual particles have a range of residence times.
Belt dryers solve the issue of potential carpet formation encountered by fluidized-bed dryers. However, they are best suited for low temperature drying and do not de-dust products. Further, belt dryers are limited in their ability to mix the product during drying. As noted, the product remains stationary relative to the other material during the process. The material fed into the belt dryer can become stratified as it is piles onto the belt. Without mixing, the layers of product have varying exposure to the drying air. Inner layers can retain moisture because the outer layers act as insulation. As a result, the quality of dried material may be inconsistent.
Rolling Bed Dryers
The rolling-bed dryer provides an alternative to the belt dryer. It includes a mixer to allow greater homogenized drying. Although the residence time cannot be exactly controlled, the dryer is able to provide a more homogenous drying of material.
This design is especially advantageous for biofuel-type organic materials (tree trimmings, wood chips, green waste, etc.) that have unique product shapes with a propensity to tangle and form clumps or carpets. Rolling-bed dryers provide gentle, low temperature drying that allows for energy efficiency while thoroughly drying organic materials. Rolling-bed dryers utilize a deep bed for the bulk material for optimum heat exchange and an adjustable air velocity. Rolling beds can operate with low air velocities or low pressure drops, which allows for low energy consumption. The rolling-bed dryer permits long retention time while still drying the content to an even moisture-content level. Rolling-bed dryers can also be operated utilizing waste heat (low heat or secondary heat).