Tray drying

Essentially large-scale, semicontin-uous tray drying. Suited to a wide variety of shapes and forms. Operation can be made continuous. Widely used Not apphcahle Suited for leather, wallboard, veneer. 

Design Methods for Vactium-Shelf Dryers Heat is transferred to the wet material by conduction through the shelf and bottom of the tray and by radiation from the shelf above. The critical moisture content will not be necessarily the same as for atmospheric tray drying [Ernst, Ridgway, and Tiller, Jnd. Eng. Chem., 30, 1122 (1938)]. 

At the wet milling step we encounter a situation similar to preblending that is, only two of the 30 study batches are prepared using the no. 5 drilled screen. The no. 7 is obviously the screen of choice. The purpose of this step is to produce particles of reasonably uniform size, which in turn will improve drying. From the records, we also know that the no. 5 screen was used only with batches that were tray dried. Elapsed drying time and residual moisture were compared for the two batches from the no. 5 screen process and the other 13 batches that were tray dried. No important differences were detected. Still, in fight of the limited use of the no. 5 screen, it would not be inappropriate to recommend this option be eliminated from the processing instructions. 

The moisture content of the 15 tray-dried batches following final mix remained essentially unchanged from the drying step. The batches from the fluid bed process gained moisture. This is probably attributable to handling very dry material in a relatively humid environment. Both groups are still below the target for this step of 1.5 %, however. 

Table 2 gives a comparison of the moisture contents following the drying and tumbling steps. The sizable increase in mean moisture content of the fluid bed-dried batches deserves further study. To determine whether or not all batches were uniformly affected, the mean moisture content was plotted in the order in which the batches were produced. Whereas the plot for the tray-dried batches is unremarkable, the fluid bed process chart (Fig. 4) depicts an unnatural pattern. Further investigation discloses that heating, ventilation, and air condition (HVAC) problems were experienced by the area in which a number of these batches were blended. 

K Constant in trays dry pressure mms2/m2 in-sVft2 V Linear velocity m/s ft/s... 

The sieve tray dry pressure drop is calculated next, applying the following equations ... 

Using these calculated ratios, AHAA and THDIA, a hole discharge coefficient factor CFCV is calculated from a curve-fitted equation in Fig. 18-14 in the Chemical Engineer s Handbook [14], CFCV is a factor in Eq. (3.112) for calculating the sieve tray dry pressure drop. 

The total pressure drop across a tray is the sum of the pressure drop across the disperser unit, hd (dry hole for sieve trays dry valve for valve trays), and the pressure drop through the aerated mass hh i.e.,... 

A few performance data of batch fluid dryers are in Table 9.14(a). This process is faster and much less labor-intensive than tray drying and has largely replaced tray drying in the pharmaceutical industry which deals with small production rates. Drying rates of 2-101b/(hr)(cuft) are reported in this table, with drying times of a fraction of an hour to several hours. In the continuous operations of Table 9.15, the residence times are at most a few minutes. 

Under normal tray drying conditions, NTU in the falling rate section is obtained fi om... 

Fig. 9 Tray drying of Softgels in controlled drying tunnels.

Fig. 2 Convective heat transfer applications in pharmaceutical dr5dng (A) tray-drying of a static solids bed and (B) fluid bed-drying of a spherical particle.

Historically, the most common method of drying of pharmaceutical powders has been tray-drying. With this method, wet powder or granulation is placed on paper-lined trays, usually solid or perforated metal, which are then placed directly onto racks in a drying chamber (oven) or onto movable racks, or trucks, that are wheeled into an oven. The heat and low relative vapor pressure of solvent provided by the flow of heated, dry air throughout the chamber provide a driving force for solvent transfer to and subsequent removal from the particle surfaces of the powder. This results in the gradual overall loss of solvent from the bulk powder. 

Tray-drying is also used as a method to remove water from soft elastic gelatin capsules,and can be model according to Eq. (39a) ... 

The term moisture, usually defined as wetness conferred by an unidentified liquid, is assumed here to be due to water. Thus, the scope of this article is the characterization of and consequences due to relatively small amounts of water associated with solids of pharmaceutical interest. Chemical stability, crystal structure, powder flow, compaction, lubricity, dissolution rate, and polymer film permeability are some properties of pharmaceutical interest that have been demonstrated to be influenced by the presence of moisture. Wet granulation, extrusion, spheronization, tray drying, freeze drying, spray drying, fluid-bed drying, tableting, and aqueous film coating are some unit operations that obviously depend on the amount and state of water present. 

As water diffuses through the bed of solid, it will carry with it any components of the formulation that are soluble in it. This will lead to a non-uniform distribution of these components in the solid. This is not usually a problem with fluidized bed drying, but with tray drying, significant differences in composition can occur between the upper and lower surfaces of the solid bed. This can give rise to non-uniform drug... 

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