Cross dryer

The wrinkle recovery angle provides a measure of the degree of chemical modification. This is calculated by blending a small sample and measuring the recovery to the flat configuration (180°). Whereas the untreated cotton recovers approximately 90°, the cross-linked cotton sample recovers 120—140°. If this is measured on dry fabric, it is termed conditional wrinkle recovery angle if on wet fabric, it is termed wet wrinkle recovery. At one point, wet wrinkle recovery was important, particularly in Europe. In the United States, the widespread use of clothes dryers has made conditional wrinkle recovery important. 

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

FIG. 12-34 Cross-flow of gas and solids in a cascade-type gravity dryer. (Link-Belt Co., Multi-Louvre piinciple.)... 

Thus, spray-dried xylan/ESlOO microparticles were produced at different polymer weight ratios dissolved in alkaline and neutral solutions, separately. More precisely, xylan and ESIOO were dissolved in 1 1 and 1 3 weight ratios in 0.6 N NaOH and phosphate buffer (pH 7.4). Then, the suspensions were spray-diied at the feed rate of 1.2 mL/min (inlet temperature of 120°C) using a Biichi Model 191 laboratory spray-dryer with a 0.7 mm nozzle, separately. Cross-linked xylan microcapsules were also coated by ESIOO after spraydrying at the same conditions. 

Dust explosions usually occur in two stages a primary explosion which disturbs deposited dust followed by the second, severe, explosion of the dust thrown into the atmosphere. Any finely divided combustible solid is a potential explosion hazard. Particular care must be taken in the design of dryers, conveyors, cyclones, and storage hoppers for polymers and other combustible products or intermediates. The extensive literature on the hazard and control of dust explosions should be consulted before designing powder handling systems Field (1982), Cross and Farrer (1982), Barton (2001), and Eckhoff (2003). 

The dryer diameter is then found from the allowable mass velocity of the air and the entering air flow and for a mass velocity of 0.95 kg/m2s, the cross sectional area of the dryer is ... 

All products require a volume and velocity of air to break the cohesive bonds between particles, wet, or dry, and to permit the batch to become fully fluidized. Laboratory trials will yield values for process air volume for the various stages of the process. Using this volume and the dimensions of the product container, a face velocity through the distributor plate can be estimated (permeability of the plate is not considered). It is reasonable to assume that approximately the same velocity will be needed in scale-up. In Table 4, estimates for process air volume are derived from the cross-sectional areas of the base of the product container for various sizes of fluid bed dryers. 

Through circulation dryers employ perforated or open screen bottom tray construction and have baffles that force the air through the bed. Superficial velocities of 150 ft/min are usual, with pressure drops of 1 in. or so of water. If it is not naturally granular, the material may be preformed by extrusion, pelleting, or briquetting so that it can be dried in this way. Drying rates are greater than in cross flow. Rates of 0.2-2 lb/(hr)(sqft tray area) and thermal efficiencies of 50% are realized. Table 9.7(d) has performance data. 

Say the solid occupies 8% of the cross section. With a solids density of 501b/cuft, the dryer volume,... 

One-dimensional mathematical models are widely applied to calculate drying processes, which can predict average values of different properties of the phases in cross-sections of the dryer [4-9]. 

As in spray dryers, a variety of devices have been used or suggested for producing droplets from the melt. Centrifugal devices, such as spinning discs and rotating perforated baskets impart an initial radial velocity to the droplets. Such devices require larger tower cross-sections and may lead to inefficient air/droplet contact due to non-uniform prill distribution across the tower [6]. These devices are best-suited to prill tubes of circular cross-section. Atomizing nozzles produce small droplets which are only suitable when fine prills are required. 

An aqueous gel (1 liter per min) is spray dried at 25°C in a counter-current spray dryer. The droplets produced by atomization are 50 pm in diameter and remain that size throughout drying. If the drying rate is limited by mass transfer in the boimdary layer, what height of the spray dryer of cross-sectional area 1 m is required if the drying air (100 liter per min) enters at 150°C and = 0% relative humidity. See problem 3 for the data. 

A gravity dryer consists of a stationary vertical, usually cylindrical housing with openings for the introduction of solids (at the top) and removal of solids (at the bottom). Gas flow is through the solids bed and may be cocurrent or countercurrent and, in some instances, cross-flow. By definition, the rate of gas flow upward must be less than that required for fluidization. 

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