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Ind. Eng. Chem. Res. 2009, 48, 7004–7015
Effect of Distributor Design on the Bottom Zone Hydrodynamics in a Fluidized Bed Dryer Using 1-D X-ray Densitometry Imaging Michael Wormsbecker,† Todd Pugsley,*,† J. Ruud van Ommen,‡ John Nijenhuis,‡ and Rob Mudde§ Department of Chemical Engineering, UniVersity of Saskatchewan, 57 Campus DriVe, Saskatoon, SK S7N 5A9 Canada, DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, and Kramers Laboratorium Voor Fysische Technologie, Delft UniVersity of Technology, Prins Bernhardlaan 6, 2628 BW Delft, The Netherlands
The hydrodynamics near the base of a 15 cm i.d. fluidized-bed dryer having perforated, punched, and porous plate distributors have been studied using one-dimensional X-ray densitometry imaging. The time-averaged solids concentration profiles of dry pharmaceutical granule and wet granule undergoing drying were compared. The porous plate creates good gas distribution over the bed cross section with dry granule but results in defluidization at the distributor level when the granule is wet. The perforated and punched plates exhibit different solids concentration profiles with dry granule; however, the presence of moisture results in similar hydrodynamics between the plates during drying. Standard deviation analysis of the high-frequency solids concentration data was used to corroborate the results of the time-averaged solids concentration measurements. Introduction The crucial role of the gas distributor in promoting effective gas-solid contacting in fluidized beds is well-known. This is discussed in detail in the classic text of Kunii and Levenspiel1 who also present rules-of-thumb for distributor design. Effective gas-solids contacting in the distributor region of fluidized beds has a positive influence on the heat and mass transfer rates, which in turn improves the overall performance of the system. In the case of fluidized-bed drying, which is the subject of the present paper, bed performance is measured in terms of minimization of drying times while maintaining product quality (i.e., uniform moisture content, minimal attrition, meeting viability or activity targets). Recognition of the importance of the gas distributor to the drying process has led to a range of distributor types, which have been reviewed by Masters2 as well as in a recent publication by our group.3 These works also contain schematics of the various designs, namely, the perforated plate, porous plate, Dutch weave, and punched plate (referred to as the Gill plate by Masters2). Each of these plate designs has its advantages and disadvantages, with the main drawbacks related to the plugging of the small openings in the porous or perforated plates or insufficient momentum to fluidize large, wet particles when using the weave design. The potential for the attrition of highvalue granular product as drying proceeds is also an issue. The punched plate is a tuyere-type distributor design that introduces the drying air into the bed laterally through unidirectional caps on concentric circles. The punched plate is intended to create what many authors refer to as swirling flow in fluidized beds,4-6 which has been shown by our group (see below) to improve gas-solids contacting under specific operating conditions. The hooded design of the punched plate also minimizes weeping of solids back into the windbox, reduces plugging of the distributor orifices by wet solids, and reportedly facilitates cleaning of the bed for batchwise operation. * To whom correspondence should be addressed. Tel.: 1-306-9664761. Fax: 1-306-966-4777. E-mail:
[email protected]. † University of Saskatchewan. ‡ DelftChemTech, Delft University of Technology. § Kramers Laboratorium voor Fysische Technologie, Delft University of Technology.
Recent work by our group has studied the impact of distributor designs specific to the pharmaceutical industry. Of these studies, one has addressed the influence of distributor design on dry bed hydrodynamics in conical fluidized beds,3 while the other has focused on distributor effects during actual drying.7 Bed hydrodynamics and dryer performance were analyzed based on pressure fluctuation analysis, drying times, and temperature profiles. In our dry bed hydrodynamic study,3 pressure fluctuation analysis indicated that the punched plate distributor led to reduced bubble size and minimal segregation below inlet velocities of 2.0 m/s. The Dutch weave exhibited poor performance because of extensive segregation. Beyond 2.0 m/s the distributor plates demonstrated similar hydrodynamics between the designs. This was attributed to the velocity of the fluidizing gas being so far in excess of that required for minimum fluidization of the bed material that subtle differences between designs in terms of bubble coalescence and rise velocity were no longer discernible. Our drying study7 examined dryer performance based on hydrodynamics as well as batch drying times. Although this study found only small differences (
