Spray dryer bulk density

Spray. Direct type, continuous operation. Rotary atomizer, pressure nozzle, or two-fluid nozzle. Includes combined spray-fluid bed and spray-belt dryers Suited for large capacities. Product is usually powdery, spherical, and free-flowing. High temperatures can sometimes be used with heat-sensitive materials. Products generally have low bulk density. See comments under Liquids. Pressure-nozzle atomizers subject to erosion Requires special pumping equipment to feed the atomizer. See comments under Liquids. Not applicable unless feed is pumpable Not applicable Not applicable Not applicable Not applicable... 

Use countercurrent spray drying for products which are not heat-sensitive, but may require some degree of heat treatment to obtain a special characteristic, i.e., porosity or bulk density. In this case the final powder temperature may be higher than the dryer outlet temperature. 

All raw materials are procured either in liquid or powder form. While ensuring a certain quality, the selection is mostly based on price. The physical characteristics of the components is not important since they will be dispersed or dissolved and mixed with other liquids to form a solution or slurry. This slurry is pumped by low- or medium-pressure pumps to the spray dryer (Fig. 6.3-2). An air vessel (pressure accumulator) is used to even-out pressure peaks. The spray towers can work in con- or counter-current fashion [B.97]. Concurrent contact yields light powders (mostly made-up of hollow particles) with a bulk density of about 100-150 g/L and a moisture content of 3-10%. Also, the hollow beads tend to break-up and form dust. Counter-current drying (Fig. 6.3-2) produces powder with a bulk density of 300-500 g/L and a moisture content of 7-15% (commonly 10%). Most plants use this method because the higher bulk density is almost always desired. 

The choice of dryer is more difficult when the drying step is also used to shape the product. Typical designs here are the spray dryer and the sprayed fluidized bed. If necessary, drying can be followed by a classification step in which undesired particles, for example, fines, are reyded to the feed stream for the dryer. When the desired product properties, for example, freedom from dust, pourabUity, rate of dissolution, and bulk density, are not attainable in the drying step, additional steps such as compaction and granulation must be used. 

In a countercurrent flow system, the spray and hot drying air enter the drying chamber at the opposite ends of the dryer. It typically produces high bulk density powders. 

Since the choice of the atomizer is very crncial, it is important to note the key advantages and limitations of different atomizers (centrifngal, pressnre, and pneumatic atomizers). Other atomizers, e.g., ultrasonic atomizer, can also be nsed in spray dryers (Bittern and Kissel 1999) but they are expensive and have rather low capacity. Although different atomizers can be used to dry the same feedstock, the final product properties (bulk density, particle size, flowability, etc.) are quite different and hence a proper selection is necessary. 

Most food-processing companies use spray dryers to produce powdered products. Spray drying has the ability to handle heat-sensitive foods with maximum retention of their nutritive content. The flexibility of spray-dryer design enables powders to be produced in the various forms required by consumer and industry. This includes agglomerated and nonagglomerated powders having precise particles size distribution, residual moisture content, and bulk density. As examples, spray drying of milk, tomato juice, tea extracts, and coffee is discussed. 

A typical arrangement of an open-cycle spray dryer for pharmaceuticals is shown in Figure 33.10. Gas-liquid nozzles are commonly used to spray feeds that are solutions. However, in the production of certain pharmaceuticals, such as antibiotics, the powders obtained by spray drying low-concentration aqueous solutions have low bulk density. Higher bulk density can be obtained if the feed is partly precipitated. High bulk density antibiotics are produced... 

During the process, the aqueous silicate solution is introduced into the upper portion of the gas-fired spray dryer and passes through a spray nozzle or a disk atomizer (see Figure 22.5). The speed of the spray wheel may be about 11,000 rpm. The finely and evenly dispersed liquid comes into contact with upwardly directed hot air. Typical spray tower tanperatures are about 180°C [21] with inlet temperatures of about 260-300°C and outlet air temperatures of above lOO C. The resultant spray-dried droplets adopt the form of hollow microspheres. The silicate particles are collected at the spray dryer s bottom and are withdrawn by a screw conveyor. The amorphous sodium silicate may have a bulk density on the order of 250-500 g/L, an SiOjiNajO molar ratio of 2.04 1, and an ignition loss on the order of 19-20%. Its mean particle size can be on the order of 100-200 pm. The material may be subjected to further milling to modify the form and density of the powder [51,63]. 

Silicate agglomerates of relatively high bulk density and large particle size can be prepared by recycling the fine particles that are normally produced in spray drying operations back into the spray dryer. As these particles fall through the dryer, they encounter droplets of sodium silicate solution that have been atomized into the dryer. The particles may adhere to one another and agglomerate as they dry [67]. 

Based on an evaluation of the physicochemical properties of the active compound, several initial formulations (generally, four to six) are selected and screened in this step (Dobry et al. 2009). A small-scale spray dryer designed for maximizing yields from SDD batches of less than 100 mg is used. This dryer is not designed to replicate optimized bulk powder properties (e.g., particle size, density) of larger scale spray dryers, but rather is used to guide formulation decisions based on physicochemical properties and fast, efficient formulation-screening studies. 

Products produced using fluidized spray drying have a broader particle size distribution and lower bulk density than the particles produced by conventional spray dryers with a typical mean size particle size range of 150 00 pm. This process is not meant to replace conventional spray drying processes but instead is a feasible alternative for spray drying applications that require larger mean particle sizes. 

The quality of dry powder is the single most important factor that is considerably affected by the operating conditions of the process. Powder character and quality are usually determined by further processing or by consumer requirements. To meet the required bulk density of the dry powder, it is necessary to know how the particle size and size distribution are affected by various parameters. General information about the selection of spray dryer design to meet powder specifications can be found in Reffi. [11,23,24], and, for the particular case of food drying, in Ref. [17]. 

Liquid-form starting materials are commonly dried to produce a free-flowing, low bulk density powder with a size distribution using a spray dryer, or to a higher bulk density, flaky product using a drum dryer. Numerous variants of both dryer types are available, necessary... 

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