Spray Dryer Design

Spray drying involves four fundamental unit processes (see Fig. 7.2)  

The chamber shape is chosen to accommodate the type of atomizer used. Droplets are ejected in a narrow cone from a nozzle which therefore requires a tall tower to prevent particle contact with the walls before they are dry. 

Similarly, a chamber of relatively large diameter but low height is used to conform to the droplet pattern spun horizontally from a centrifugal atomizer. 

The water evaporative capacities of the range of standard industrial spray dryers offered by one manufacturer is given in Fig. 7.5 as a function of inlet drying air temperature. Units with capacities up to 40,000 Ib/h (18,100 kg/h) or more of water evaporation are available. Dryer size in Fig. 7.5 is related to the mass flow rate of drying air several drying chamber physical sizes are possible for each of these air rates, depending on inlet temperature, 

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Design Methods for Spray Dryers Design variables must be... 

This conceptual design and economic evaluation is based on theoretical considerations only and many of the assumptions used in this design need to be verified in an actual pilot-plant operation. These various design assumptions include (1) MgO/ash separation, (2) HC1 removal and purge, and (3) spray dryer design. 

The MgO system is based on a conservatively designed lime system (i.e., a MgO stoichiometry of 1.8 moles MgO/mole SO2 absorbed, a 20 F° approach to the flue gas saturation temperature in the spray dryer, etc.). All of the spray dryer design considerations will therefore have to be evaluated in a spray dryer MgO pilot plant. 

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. 

Masters, K., Impact of spray dryer design on powder properties, Drying 91, A.S. Mujumdar and I. Filkovd, eds., Elsevier Science Publishers, Amsterdam, the Netherlands, 1991, p. 56. 

An example is a skimmed drying system that consists of a spray and vibrofluidized bed dryer. This lowered the specific energy consumption to 4300 kJ/kg from 5550 kJ/kg water evaporated for the spray dryer alone [57]. The next substantial advance in spray dryer design was the compact spray dryer. In this system, a stationary fluid bed is positioned on the lower... 

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. 

Furuta, T., Hayashi, H., Ohashi, T, 1994. Some criteria of spray dryer design for food liquid. Drying Technol. 12 151—177. 

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]. 

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