Pulse combustion spray-dryers

The data, chart, and discussion points presented in this application section are based on several trials conducted at the PCS plant in Payson, AZ, ntilizing their pulsed combustion spray dryer technology. The trials were not specifically designed for this documentation, so some definitive protocols cannot be provided. However, we investigated how several parameters such as contact and exit temperatures (components of the dryer operating envelope), flow aid level, and percentage of feed solids affected capsule integrity as measured analytically and presented as % free core. 

Figure 23.9 shows three types of commercial pulse combustion spray dryers offered by the Pulsecom Dryer System, Japan. The pulse combustor used gaseous fuels such as coal gas and natural gas, and the drying capacity of the PC dryers varied from 25 to 4000 kg water/h. The energy consumption to evaporate unit water was about 800 kcal/kg water. The product temperature was 50°C-70°C due to short residence time, and the outlet temperature of the exhaust temperature was about 90°C-120°C. For the PC dryer with an evaporation of 4000 kg water/h, it has a dimension of 13 m length x 10 m width x 17 m height and an electric consumption of 75-110 kw - h. 

FIGURE 23.9 Types of commercial pulse combustion spray dryers offered by Pulsecom Dryer System, Japan, (a) The pulse combustion spray dryer with a cylindrical drying chamber, (b) The pulse combustion spray dryer with a cylindrical drying chamber and air recycle pipe, (c) The pulse combustion spray dryer with a big volume drying chamber. 

Following on from the aforementioned characteristics, an advantage of pulse combustion is that the strongly oscillating hot-gas jet from the combustion chamber, and also from the tailpipe, can promote the dispersion of liquids, pastes and clumps of particulate materials (Wu et al., 2012). Hence, in the case of liquids, pulse combustion spray-dryers can refrain from using conventional disk or nozzle atomizers, which translates into a simple design and lower capital and operating costs, as well as an enhanced product quality due to the creation of finer droplets with a narrow size distribution. 

Fig. 2.14 Streamlines pattern in a pulse combustion spray-dryer (pulsating flow) after (a) 19.5 s, (b) 19.502s, (c) 19.504s, and (d) 19.507s.

Tab. 2.3 Basic differences between classical and pulse combustion spray dryers (courtesy of Pulse Combustion Systems, Payson, AZ, USA www.pulsedry.com).

FIGURE 23.6 A vertical pulse combustion spray drying installation and its pulse combustor developed by Pulse Combustion Systems, the United States, (a) Schematic of the PC spray dryer (1, air 2, unidirectional air valve 3, eombustion chamber 4, fuel 5, pilot 6, tailpipe 7, atomizer 8, quench air 9, liquid 10, drying chamber), (b) Photo of pulse combustor used in the PC spray dryer. 

As noted above, pulse combustion spray-drying experiments are difficult to perform because of the high gas temperature in the chamber, the high noise level, and the short drying time. In order to avoid removing samples from the chamber, Zbicinski et al. (2002) applied a particle dynamic analysis (PDA) technique to determine the water evaporation level as a function of the distance to the atomizer in a horizontal valved PCD system. The laser-based PDA system counts the number of droplets in a cross-section of the dryer, which in turn makes it possible to estimate the flux of the discrete phase in the chamber. 

The only validated CFD model of a pulse combustion spray-drying process has been presented by Zbicinski et al (1999) and Smucerowicz (2000). The two-dimensional axi-symmetric CFD calculations of water evaporation and drying of salt solutions in the oscillating flow of a continuous phase were carried out for a valved pulse combustor. Extensive experiments (PDA, LDA, temperature, noise, pressure measurements) to determine the inlet conditions and also to validate the model were performed for a horizontal pulse combustion dryer (Smucerowicz, 2000). Sinusoidal axial velocity oscillations have been defined as follows ... 

Temperature profiles for the evaporation of salt solution (atomization ratio 18/15 kg h /kgh are presented in Fig. 2.17. A rapid depression of drying agent temperature near the atomizer, where an intensive evaporation of water takes place (typical also for classical spray drying), can also be observed. In pulse combustion spray drying the evaporation of water to the constrained volume of the spray envelope in the vicinity of the atomizer reduces the temperature of the drying agent in the dryer axis. However, this effect can be observed only near the atomizer where the radius of the spray envelope is small. Enirther extension of the spray envelope... 

Results from microcapsule spray drying runs on PCS pulsed combustion dryer will serve as the basis for this discussion. The rationale behind showing the different data sets is simply to illustrate the many variables to consider while drying microcapsule slurries with the pulsed combustion dryer technology. These data sets also show that different products tend to behave differently. The encapsulated material presented in this chapter is primarily fragrance based. 

Spray Spray/fluid bed combination Vacuum belt dryer Pulse combustion dryers... 

Comparison of Unison Pulse Combustion Dryer versus Conventional Spray Dryer for Selected Groups of Materials... 

Pulse combustion dryer Tunnel dryer Band dryer Impingement dryer Rotary dryer Huid bed dryer Hash dryer Spray dryer Drum dryer (pastes)... 

Pulse combustion dryer Conventional dryers Belt conveyor dryer Rash dryer Ruid bed dryer Rotary dryer Spray dryer... 

Pulse combustion drying combines high economic of direct fire heating system with high intensity of drying of disperse material. Typical power consumption is 3,000-3,500 kJ/kg H2O evaporated (4,500 to -11,500 for spray dryer and 3,200-6,500 for drum dryer) [53,54]. The main advantages of pulse combustion drying are as follows ... 

Fluid/spouted beds of inert particles Spray/fluid bed combination Vacuum belt dryer Pulse combustion dryers Spouted bed of inerts Fluid bed (with solid backmixing)... 

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