Spray dryers residence time

Average drop diameters in a spray dryer range from 20 /rm when a disk atomizer is used to 180 /im with a coarse spray nozzle. Residence times vary from 3 to 6 s in cocurrent dryers to as much as 25 to 30 s in countercurrent dryers. 

Fig. 12. Material residence time profiles in A, a pneumatic conveyor, B, a spray, and C, a rotary or fluid-bed dryer.

In the jet spray dryer, cold feed is introduced(42) into preheated primary air which is blown through a nozzle at velocities up to 400 m/s. Very fine droplets are obtained with residence times of around 0.01 s, and an air temperature of 620 K. This equipment has been used for evaporating milk without adverse effect on flavour and, although operating costs are likely to be high, the system is well suited to the handling of heat-sensitive materials. 

The droplet has a relatively short residence time (on the order of seconds) in the spray-dryer, which minimizes the degradation of heat-sensitive components. Addi-... 

Figure 9.5. Residence time distribution in particle dryers, (a) Four types of dryers (McCormick, 1979). (b) Residence time distribution cf air in a detergent spray tower example shows that 27% (difference between the ordinates) has a residence time between 24 and 32 sec [Place et al., Trans. Inst. Chem. Eng. 37, 268 (1959)]. (c) Fluidized bed drying of two materials (Vanacek et al., Fluidized Bed Drying, 1966).

The mean residence time of the gas in a spray dryer is the ratio of vessel volume to the volumetric flow rate. These statements are made in the literature regarding residence times for spray drying ... 

The design of spray dryers is based on experience and pilot plant determinations of residence time, air conditions, and air flow rate. Example 9.10 utilizes such data for the sizing of a commercial scale spray dryer. 

Feed to a spray dryer contains 20% solids and is to be dried to 5% moisture at the rate of 500 lb/hr of product. Pilot plant data show that a residence time of 6 sec is needed with inlet air of 230T, H = 0.008 lb/lb, and exit at 100°F. Ambient air is at 70°F and is heated with steam. Enthalpy loss to the surroundings is 10% of the heat load on the steam heater. The vessel is to have a 60° cone. Air rate and vessel dimensions will be found. 

The droplet has a relatively short residence time (on the order of seconds) in the spray dryer, which minimizes the degradation of heat-sensitive components. In addition, the drug is exposed to a temperature much lower than that at the drying inlet owing to the cooling effect of the solvent evaporation. Control of droplet residence time and the lower temperature defines the amorphous versus crystalline nature of the material. 

FIGURE 15 Computer simulation indicates multiple particle formation paths exist in bench scale spray dryer particle formation rate and residence time dependent upon flow streamline. 

Spray Dryers A pumpable feed is atomized into droplets by a rotary or nozzle atomizer, as described under Entrainment Dryers. An integral fluid bed or belt may be added below the dryer to give longer residence time and some agglomeration. Semibatch and continuous operation is possible. 

A quick scoping estimate of the size of an industrial spray dryer can be made on this basis. The required evaporation rate or product rate can be multiplied by the relevant ratio from the table to give the mass flow rate of the drying gas. The next step would be to calculate the size of a spray drying chamber to allow the drying gas at outlet conditions approximately 25 s of residence time. A cylindrical chamber with diameter D and height H equal to D and a 60° conical bottom has a nominal volume of... 

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