Pulse Combustion in Drying

Keller, J.O., German, R.S., and Ozer, R.W. 1992. Fundamentals of enhanced scalar transport in strongly oscillating and/or resonant flow fields as created by pulse combustion. In Drying 92 (A.S. Mujumdar, Ed.), Elsevier, New York, NY, pp. 161-180. 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

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. 

Benali, M. and Legros, R., Thermal processing of particulate solids in a gas fried pulse combustion system. Drying Technology, 2004, 22(1 2), 347-362. 

Swientek, R.J., Pulse combustion burner dries food in 0.01 sec.. Food Processing, 1989,50, 27-28. 

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... 

Wu and Liu (2002) analyzed the mechanism of the pulse combustion spray drying of salt solution in an oscillating flow field produced by a Helmholtz-type pulse combustor with an external flapper valve. The solution was atomized directly by the pulsating flow in the combustor tailpipe, which reduced the average droplet diameter by about 50% in relation to conventional nozzle atomization. An optical analyzer was applied to measure the droplet size distribution in the spray in order to determine the initial conditions for the CFD model of the process. The results of CFD simulations enabled the average residence time of droplets in the drying chamber (ca. 0.1 s) and the... 

Mujumdar, A. S., Wu, Z. H., 2004. Pulse combustion spray drying, in Topics in heat and mass transfer, (eds G. H. Chen, S. Devahastin, B. N. Thorat), IWSID-2004, Mumbai, India, pp. 79-91. 

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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. 

It should be pointed out that many new dryers cannot conveniently fit the classification suggested earlier. For example, a pulsed combustion dryer for pasty solids or waste sludge, the Remaflam process for drying textiles by controlled combustion of solvent (alcohol) on the wet fabric itself, and vibrated bed drying of pastes cannot be placed under any one single category of dryers. Such a coarse classification is stiU of interest in that it allows one to home in on a limited number of possible dryers, which can then be evaluated in depth. 

Use of superheated steam in direct dryers Increased use of indirect (conduction) heating Use of combined (or integrated) heat transfer modes Use of volumetric heating (microwave [MW]/radio-frequency [RF] fields) in specialized situations Use of two-stage (or multistage) dryers Use of intermittent heat transfer Use of novel combnstion technologies (e.g., pulse combustion for flash drying)... 

Although some processes can be carried on in a tailpipe of the pulse combustor, the typical pulse combustion system consists of a combustor and an applicator in which the pressure waves are amplified by acoustic resonance. The frequency of the pressure pulsations is generally close to the frequency of the fundamental acoustic mode of the combustion chamber and the applicator. If properly tuned, the pulsed combustor can excite large-amplitude pulsations in a process (e.g., drying, calcining, or incineration) carried out downstream of its tailpipe. 

Pulse combustion is a specific form of combustion-driven oscillation. Combustion oscillations can be an inherent problem or a potential benefit in enclosed combustion systems, such as gas turbine combustors, afterburners, furnaces, and rocket engines. Oscillations can produce beneficial increases in heat transfer rates and reduce pollutant formation. In other situations, these instabilities are undesirable because they may reduce the thermodynamic efficiency of a combustor or become a source of system failure if their amplitude is not kept within an acceptable range. Oscillations in the pulse combustion drying systems are desired and useful. Combustion with oscillations may be treated as some regular form of unstable combustion. 

An alternative mode of pulse combustion drying was suggested by Zinn et al. (1990), where pulse combustion is mainly applied to generate a large-amplitude pulsation in the drying chamber and, in this condition, provides small portion of heat for moisture evaporation. This kind of pulse combustion dryer consists of a combustor and a dying chamber where the hydro-dynamic action of pressure and velocity waves enhances drying rates. Under certain conditions, these pressure waves can be favorably amplified by an acoustic resonance. To excite... 

Another advantage of pulse combustion drying is the very short contact time of drying agent and drying material. Table 23.5 compares the residence times of the dried material in the pulse combustion dryer with selected convectional dryers. The typical residence time is 0.01-1 s for pulse combustion drying. For this reason, this technique can be applied even to extremely sensitive materials (Swientek, 1989). 

Pulse combustion dryers have been in the market for many years. The pilot test and technical reports of pulse combustion dryers indicate that the pulse combustion drying, as compared to classical (continuous) drying, enables one to (1) increase the drying rate by a factor of 1.2-3, depending on the configuration of PC dryer (2) reduce unit air consumption by up to 30% (3) eliminate distribution of characteristic process parameters (e.g., temperature, concentration, and moisture content) within the dryer, which improves the... 

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