Evaluating Dryers for New Services
This simple, reliable approach to rate dryers for continuous service combines simulation with the specific drying rate approach.
Drying is a complex operation that involves simultaneous mass and heat transfer to and from irregularly shaped particles (1). During this process, the paniculate matter undergoes primary transformation (e.g., loss of moisture upon exposure to heat) and secondary transformation (e.g., particle breakage, shrinkage, biochemical reaction and morphological change), the latter of which may alter the product’s bulk characteristics, quality and appearance, and cause problems in dryer performance.
For dryers with known mechanical features and operating parameters, “troubleshooting” means more than rinding and fixing a problem - it involves evaluating a dryer for “new process conditions.” The various troubleshooting guises include (2):
* How do we get this dryer, which has never worked properly, to perform as it should?
* Why does dryer performance deviate from the expected?
* What can be done to increase dryer capacity?
* How do we avoid problems during dryer scaleup?
* How do we evaluate a dryer for new services where operating parameters deviate from the design basis?
This article presents a straightforward, reliable procedure for rating continuous tray and plate dryers for new or unintended services. The continuous behavior of plate and tray dryers are predicted from tests that are carefully conducted in laboratory-scale, vendor-supplied batch simulators. Data are recorded on the drying behavior of solids inside the dryer, and are used in the specific drying rate (SDR) equation, which, in combination with other formulas, rates the dryer’s performance at the new feed conditions.
Continuous dryer configurations
Two types of continuous tray dryers are commercially available: the rotating-shelf tray dryer and the stationary-plate dryer with rotating conveyor arms (3). Both dryers are particularly useful when relatively long residence times are required to dry the product and when product containment is necessary. Both units are also relatively gentle in their tray-to-tray solids-conveying techniques. For moisture removal, tray dryers rely on convection heat transfer, while plate dryers use conduction heating (the plates are heated trays).
Krauss Maffei Corp. (Florence, KY; www.kraussmaffei.com) rolled out the plate dryer in the U.S. in late 1970, and is still the only company that manufactures a plate dryer. The tray dryer was introduced in the U.S. in 1930 by Wyssmont Corp. (Fort Lee, NJ; www.wyssmont.com), which marketed its product under the TurboTray brandname. This dryer’s widespread acceptance has made “TurboTray” nearly synonymous with continuous tray-drying technology. For this reason, the TurboTray dryer has also been selected for illustrative examples in this article. The general features of the Wyssmont’s TurboTray and Krauss Maffei’s plate dryer are compared in Table 1.
Krauss Maffei plate dryer. This indirectly heated dryer is vertically oriented and features horizontal plates, equipped with arms and plows that ease product transfer, mounted inside the housing. The plates are heated by hot water, steam or thermal oil to temperatures up to 320°C. Product enters through the top of the unit and is conveyed downward in a spiral fashion via a central-rotating shaft to which the plates are attached. Material movement is facilitated by the disc and doughnut arrangement of the plates (Figures 1 and 2). The even numbered plates have a larger diameter then the odd numbered plates. The entire operation is conducted under vacuum.
Wyssmont tray dryer. An external view of the Wyssmont TurboTray dryer is shown in Figure 3. This dryer consists of a stack of rotating annular shelves (or trays) at the center of which turbo-type fans revolve to circulate the air over the trays. Wet material enters through the roof, falling onto the top shelf as the shelf rotates beneath the feed opening (Figures 4 and 5). After the tray nearly completes one revolution, the material is swept by a stationary wiper through radial slots onto the shelf below, where it is spread into a pile of uniform thickness by a stationary leveler. This action is repeated on each shelf; material transfer occurs approximately once per revolution. Material is discharged from the last shelf through the bottom of the dryer.
Applications. It is beyond the scope of this article to discuss the types of materials that can and cannot be dried in plate and tray dryers. Perry’s Chemical Engineers’ Handbook (3) describes tray-dryer performance data for several materials, including antioxidants, water-soluble polymers, antibiotics and petroleum coke. In addition, Perry’s presents plate-dryer performance data for three applications: plastic additives, pigments and foodstuff. Both dryers require free-flowing or friable feed. If the feed is doughy or sticky, the material may adhere to the tray, plate, plows or level arm, thereby impeding solids transportation and causing poor dryer performance. Lumpy material could bridge the slots in a tray dryer or lodge between the plows of a plate dryer. Pre-treatment of the feed is sometimes necessary to facilitate controlled solids transport inside the dryer.