Spray dryers particles-sizes

Spray characteristics of pressure nozzles depend on the pressure and nozzle-orifice size. Pressure affects not only the spray characteristics but also the capacity. If it is desired to reduce the amount of hquid sprayed by lowering the pressure, then the spray may become coarser. To correct this, a smaller orifice would be inserted, which might then require a higher pressure to produce the desired capacity, and a spray that would be finer than desired might result. Multiple nozzles tend to overcome this inflexible characteristic of pressure atomization, although several nozzles on a dryer complicate the chamber design and air-flow pattern and risk collision of particles, resulting in nonuniformity of spray and particle size. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Following carbonation, the product can be further purified by screening. This screening, also used to control the maximum size of the product, is followed by dewatering (qv). Rotary vacuum filters, pressure filters, or centrifuges are used in the mechanical removal of water. Final drying is accompHshed as with natural calcium carbonate in either a rotary, spray, or flash dryer. Products having mean particle sizes from submicrometers (- O-OS fiTo) to several micrometers are available. 

Spray Drying Detailed descriptions of spray dispersion dryers, together with apphcation, design, and cost information, are given in Sec. 17. Product quality is determined by a number of properties such as particle form, size, flavor, color, and heat stability. Particle size and size distribution, of course, are of greatest interest from the point of view of size enlargement. 

In the manufacturing of USY catalyst, the zeolite, clay, and binder are slurried together. If the binder is not active, an alumina component having catalytic properties may also be added. The well-mixed slurry solution is then fed to a spray dryer. The function of a spray dryer is to form microspheres by evaporating the slurry solution, through the use of atomizers, in the presence of hot air. The type of spray dr er and the drying conditions determine the size and distribution of catalyst particles. 

The catalyst manufacturers control PSD of the fresh catalyst, mainly through the spray-drying cycle. In the spray dryer, the catalyst slurry must be effectively atomized to achieve proper distribution. As illustrated in Figure 3-10, the PSD does not have a normal distribution shape. The average particle size (APS) is not actually the average of the catalyst particles, but rather the median value. 

Pneumatic dryers, also called flash dryers, are similar in their operating principle to spray dryers. The product to be dried is dispersed into an upward-flowing stream of hot gas by a suitable feeder. The equipment acts as a pneumatic conveyor and dryer. Contact times are short, and this limits the size of particle that can be dried. Pneumatic dryers are suitable for materials that are too fine to be dried in a fluidised bed dryer but which are heat sensitive and must be dried rapidly. The thermal efficiency of this type is generally low. 

The purpose of this study was to investigate the effect of particle size on encapsulated orange oil, a widely used flavor which is highly susceptible to oxidation, using a readily available laboratory scale spray dryer. A close examination of the surface morphology was also attempted. 

Detailed design of fluid spray dryers must, however, be accompanied by experimental work. It has been noted [22] that even quite similar materials may behave very differently in a spray granulator. Experimental work will indicate such information as the natural mechanism of particle growth (coalescence vs layering), attainable product sizes, the preferred temperature and moisture conditions for the bed material, suitable ranges of bed voidage... 

The catalyst slurry is fed to the top of the spray dryer where it is atomized in the form of small micron sized droplets. These droplets fall through gravity in a flow of hot air following a spiral downward movement. The particles with the desired size are collected at the bottom of the spray dryer and the fines are recovered with the aid of cyclones and recycled. This contained process explains why recoveries of 99% are achieved. The height of a commercial spray dryer is measured in meters and such a process is difficult to downsize, therefore explaining partly the lack of academic research in the field of zeolite catalysts spray drying (6). 

Once the conditions for making a processible catalyst slurry were established, microspheres were generated using the reference formula with and without the different alumina powders. Table II summarizes particle size distributions for typical commercial FCC and our experimental catalysts. Two commercial samples (Catalysts V and VI) had particle sizes much higher than we could produce with our spray dryer. However, the size distributions of the experimental catalysts approximate those of the other four commercial grades. 

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