Calculations of the pulse combustion drying system

Energy Conversion and Management 42 (2001) 1909±1918

www.elsevier.com/locate/enconman

Calculations of the pulse combustion drying system I. Zbici nski *, C. Strumillo, M. Kwapinska, I. Smucerowicz Faculty of Process and Environmental Engineering, Technical University of Lodz, ul. W olczanska 213/215, 90-924 Lodz, Poland

Abstract The paper describes modelling of spray drying system where valved and valveless pulse combustors were applied as a source of drying agent. Experimental analysis of pulse combustors operation was carried out in order to optimize their performance to achieve low emission of toxic substances, stable operation and high and smooth pressure oscillations. Optimized valved and valveless units were applied in the drying system. Extensive drying and evaporation tests were carried out to establish basic data required for modelling of the system. Laser techniques were applied to determine ¯ow ®eld in the drying chamber (LDA) as well as to analyse the structure of disperse phase (PDA). A modi®ed mathematical model of heat, mass and momentum transfer in spray drying, developed earlier, was used to describe the pulse combustion drying system. The model enables to obtain temperatures of continuous and disperse phases, percent of evaporation and positions of particular fractions in the dryer. Air velocity pro®les in the axial and radial directions in the drying chamber determined experimentally were substituted to the main programme calculating the spray drying process. A comparison of theoretical and experimental results shows good agreement also for complex hydrodynamics of pulsating ¯ow in the drying chamber. Experimental investigations and mathematical modelling proved that the pulse combustion spray drying system could be e€ectively applied in dewatering of solutions with low content of solids, which could be important for waste disposal. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Valveless, valved pulse combustor; Spray drying; Entrainment rate

1. Introduction Drying process is one of the most harmful to environment and energy demanding unit operations in chemical engineering. Therefore, for many years, studies concerning modi®cations of the

*

Corresponding author. Fax: +48-42-6364923. E-mail address: [email protected] (I. Zbici nski).

0196-8904/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 0 1 ) 0 0 0 5 0 - 4

1910

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

Nomenclature c CD d g h DH i k N q r S T U w Y

speci®c heat capacity, J/(kg K) drag coecient, ± diameter, m acceleration due to gravity, m/s2 distance in axial direction, m latent heat of vaporization, J/kg enthalpy, J/kg mass transfer coecient, m/s number of particles, 1/s heat ¯ux from dryer shell (heat losses), W/m2 distance in radial direction, m particle surface area, m2 temperature, °C velocity, m/s mass ¯ux, kg/s gas humidity, dry basis, kg/kg

Greek a heat transfer coecient, W/(m2 K) q density of gas, kg/m3 Subscripts av average c core e equilibrium evap evaporation g gas p particle, droplet H humid r radial x axial v vapour phase w water 0 bulk, initial

existing technologies, drying equipment and searching for new drying techniques have been carried out. One of the openings is the application of pulse combustors as a source of drying agent [1,2]. Pulse combustion spray drying technique uses the technology of combustion-driven oscillations to produce high temperature, high velocity pulsating jet which might atomize and dry a material.

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

1911

The gas stream, which leaves the combustor, is characterized by oscillations of velocities reaching 100 m/s and frequency of 50±250 Hz [3]. The enhanced mixing and transport processes cause highly ecient and compact combustion processes. The oscillatory mixing and high heat losses through the combustor walls produce oscillatory ¯ow conditions inside the combustion zone which minimizes NOx production. The aim of the research carried out in the Department of Heat and Mass Transfer, Technical University of è odz, was to construct the pulse combustion drying system and analyse its performance. Valved and valveless pulse combustors were designed to work as a source of a heat carrier in the drying system. The valveless pulse combustor was designed and constructed in cooperation with Washington State University, Pullman, USA and the Institute of Aeronautics, Warsaw, Poland. In the ®rst step of the project experiments were performed to obtain an optimum pulse combustion operation. To achieve high and sinusoidal pressure oscillations in the combustion chamber and low emission of toxic substances the pulse combustor geometry was subjected to several changes including: the volume of combustion chamber, the length of the tailpipe, its shape as well as the amount of delivered fuel and air. During the experiments, the amplitudes and frequencies of pressure oscillations inside the combustion chamber and at the outlet of the tailpipe, emission of NOx , CO, CO2 and noise level for various geometries and lengths of the tailpipe, for di€erent air/fuel ratios were measured and stored by the data acquisition system shown in Fig. 1. The system consists of a piezo-electrical pressure transducer produced by PELTRON, Poland and thermocouples to measure temperature inside the combustion chamber and at the outlet of the tailpipe (CZAKI, Poland). Fuel ¯ow was measured by a precision electronic ¯ow meter (COLE PALMER, USA), the content of ¯ue gases was also analysed (ELJACK, Poland). The probes were connected to ADVANTECH, USA data acquisition and controlling system installed on a PC. Optimized valved and valveless units were applied in the drying system.

Fig. 1. Data acquisition system: T1 ± temperature in the combustion chamber, T2 ± temperature at the tailpipe outlet, F1 ± air ¯ow meter, F2 ± propane ¯ow meter, P ± pressure transducer, C ± analysis of ¯ue gas composition.

1912

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

Fig. 2. Valveless pulse combustor drying installation.

2. Pulse combustion drying tests The experimental set-up of valveless pulse combustion drying system is presented in Fig. 2. The dryer consists of stainless steel section with diameter of 0.29 m and length of 1.2 m. Raw material is introduced to the drying chamber by a pneumatic nozzle. Dry product and water vapour are conveyed from the chamber into the cyclone where the dry particles are separated. The chamber was equipped with quartz windows to perform Laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA) measurements. Before drying tests, LDA measurements were carried out to determine ¯ow ®eld in the drying chamber. Flowlite System by DANTEC, Denmark was used in the trails. MgO was applied as seeding to increase data rate. The experimental results show a complex structure of the ¯ow®eld in the drying chamber, lack of axial symmetry of the ¯ow and oscillations of process parameters. Fig. 3 presents pro®les of axial velocity in the drying chamber. Each experimental point represents average values from several thousand of samples. It should be stressed that oscillations of the axial velocity exceeded a couple of hundred percent of average value which is relevant to the suggestions presented by Keller et al. [4].

Fig. 3. Axial velocities in the drying chamber.

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

1913

Extensive tests on drying and water evaporation were carried out for various feed rates and operation parameters of the pulse combustor. Each test included the analysis of temperature distribution in the dryer, evaporation degree and sprayed material structure. The latter measurement was performed using the PDA technique and Flowlite System by Dantec. For water evaporation this technique was used also to determine the evaporation degree as a function of the distance to the atomizer. The sprayed material and air temperature in the spray envelope was determined by the technique presented in Ref. [5] using shielded thermocouples. Data collected in this way can be a basis for modelling and scaling up of the pulse combustion drying system.

3. Modelling of the pulse combustion drying system An attempt of modelling of pulse combustion drying system was undertaken in the study. Calculations were made using a modi®ed mathematical model of momentum, heat and mass transfer during spray drying proposed by Zbici nski [6]. The model enables a calculation of temperatures and moisture content of continuous and disperse phase and description of the trajectory of motion of particular fractions taking into account spray expansion. In relation to the original model, the equations for particle movement were modi®ed taking into account horizontal position of the dryer (Fig. 2). Trajectory of the particles as a function of the distance to the atomizer can be calculated from the momentum equations that have the following form for the two dimensions and for the horizontal position of the dryer (no tangential movement of particles was assumed). Axial velocity: dUpx ˆ dh

Up …Upx Ugx †qg 1 3 CD 4 Upx qp dp

…1†

Due to horizontal position of the dryer two equations describing radial velocity for particles travelling up and down should be distinguished (Fig. 4). Radial velocity for particles travelling down:

Fig. 4. Balancing of the droplet-gas system.

1914

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

dUpr ˆ dh

1

! qg g qp

Up …Upr Ugr †qg 3 CD 4 qp d p

!

1 Upx

Radial velocity for particles travelling up: ! ! qg Up …Upr Ugr †qg dUpr 3 1 g CD ˆ 1 4 Upx dh qp qp dp

…2†

…3†

where Up is the relative particle velocity (Eq. (4)): Up ˆ ……Upx

Ugx †2 ‡ …Upr

Ugr †2 †1=2

…4†

A key problem in the calculation of such two-phase systems is to determine the amount of continuous phase which is in contact with the disperse phase. It was proposed that the amount of air was determined on the basis of the position of the largest droplet as a function of the distance from the atomizer (Fig. 4). Entrainment rate …dwg0 =dh† can be determined on the basis of the increment of spray envelope cross section:   dwg0 pr dr …5† qg Ugx ‡ Ugr ˆ 1‡Y dh dh where the increment of spray diameter and radial location of particles …dr=dh† is calculated from Eq. (6). dr Upr ˆ dh Upx

…6†

If the air velocity pro®le changes both as a function of spray radius and the distance to the atomizer, the air ¯ow in the spray core region wgc should also be modi®ed (Fig. 4). It was suggested to determine the change in air ¯ow in the spray core region on the basis of changes in the mean air ¯ow velocity in the region being analysed:   dwgc pr2 dUg …7† ˆ qg dh 1‡Y dh av where mean air velocity is de®ned by Eq. (8): 2 Ug ˆ …Ugx ‡ Ugr2 †1=2

…8†

The above system of equations enables to make heat and mass balances for the two-phase system taking into account the expansion of spray envelope and swirl ¯ow in the drying chamber. Consider mass balances for droplets of water-gas system, assuming that atomization is monodispersive or that only one fraction of the dispersed material is taken into account. Assume that in the stream cross section there is neither temperature nor air humidity gradient, and that the ¯ow of continuous and dispersed phase is co-current. The mass balance has the form of Eq. (9): dY Y0 Y dwg0 ˆ dh wg dh

Y dwgc wg dh

1 dww wg dh

where Y0 is the bulk humidity (outside the spray envelope).

…9†

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

1915

Heat balance of both phases leads to Eq. (10), which can be used to determine gas temperature in the stream cross-section taking into account entrainment e€ects.   dTg 1 dwg0 dwgc dY dTp dww dq …ig0 ig † ww cp ˆ ig wg …cv Tg ‡ DH † cp Tp wg c H dh dh dh dh dh dh dh …10† A complete description of the process requires the determination of humidity and material temperature in the process. Heat balance for a particle of sprayed material makes it possible to determine droplet temperature as a function of distance to the atomizer:
! w 1 6 a…Tg Tp † dw …DH ‡ cv Tg †=Sp dTp 1 dh N …11† ˆ dh Upx dp qp cp where the evaporation rate is determined by Eq. (12): dww 1 ˆ Nk Sp qg …Ye dh 1‡Y

Y†

1 Upx

…12†

where subscript ``e'' refers to the equilibrium between the gas and dispersed phase. The model described above was solved for polydispersive atomization. Initial atomization parameters (initial droplet size distribution, initial velocity of particular fractions and atomization angle) were found on the basis of PDA measurements. Constant air temperatures and humidities in the cross sections of the spray were assumed. In such a situation, to solve heat and mass balances for continuous phase (Eqs. (9) and (10)), the evaporation rate (Eq. (12)) should be calculated as a sum of evaporations from particular fractions ``i'' according to Eq. (13): n qg dww X 1 Ni ki Spi …13† ˆ …Ye Y † Upxi dh 1‡Y iˆ1 For the entire stream of atomized material, entrainment rate (Eq. (5)) was calculated on the basis of the motion of the largest droplet. Heat and mass balance and momentum equations for particles are solved separately for particular fractions. Results are used to obtain temperatures, rate of evaporation and positions of particular fractions in the dryer. Air velocity pro®les in the axial and radial directions in the drying chamber determined experimentally (Fig. 3) were substituted to the main programme calculating the spray drying process. In all calculations, heat transfer coecients were determined from the Ranz±Marshall equation and mass transfer coef®cients from the Chilton±Colburn analogy, where the psychrometric coecient and humidity potential coecient were taken as unity. Examples of calculated results are given in Figs. 5±8. Fig. 5 displays a calculated trajectory of particles. The graph shows trajectories of particles 50, 80 and 100 lm in diameter at gas ¯ow rate 0.4 m/s and inlet air temperature 600°C. After reaching the distance of about 35 cm, particles of diameter exceeding 80 lm will collide with the bottom of the dryer. A similar result was obtained during experimental investigations. Fig. 6 shows the impact of feed rate on evaporation capacity. Good agreement between theory and experiment (performed by the PDA technique) proved correctness of the model construction. Changes in air temperature as a function of the distance from the atomizer for various feed rates are illustrated in Fig. 7. An abrupt drop of air temperature and then its growth are connected with

1916

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

Fig. 5. Trajectories of particles at atomization angle 30°.

Fig. 6. Evaporation as a function of the distance from the atomizer.

Fig. 7. Air temperature as a function of the distance from the atomizer.

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

1917

Fig. 8. Air humidity as a function of the distance from the atomizer.

jet expansion and are typical of spray drying. Again experiments con®rm date obtained from the model. Fig. 8 shows a change of air humidity versus distance from the atomizer and the function of feed rate. One can observe a constant increase of air humidity during the drying process for feed rates: 9.2 and 13.8 kg/h. However, for the lowest feed rate 4.6 kg/h we can notice an increase and next about 0.2 m decrease of air humidity. In this moment, the material had already been dried but the spray envelope was still expanding, and fresh air ¯ew into the spray envelope which decreased average humidity. Experimental investigations proved that the proposed mathematical model could be successfully applied also for complex hydrodynamics of pulsating ¯ow in the drying chamber. 4. Summary Extensive experimental drying and evaporation tests were carried and in the pulse combustion drying system. A modi®ed, developed earlier mathematical model of heat, mass and momentum transport in spray dryer was used to describe this system. The comparison of experimental and model data proved that the proposed mathematical model could be successfully used also for complex hydrodynamics of pulsating ¯ow in drying chamber. Acknowledgements This work was supported by the US±Poland Maria Sklodowska±Curie Fund (grant no. MEN/ NIST-96-262). References [1] Zinn BT. Pulse combustion: Recent applications and research issues. 24th International Symposium on Combustion/The Combustion Institute. 1992. p. 1297. [2] Kudra T, Buchkowski AG, Kitchen JA. Pulse-combustion drying of white Pine, 4th IUFRO International Conference on Wood Drying, Rotorua, New Zealand 1994. p. 396.

1918

I. Zbicinski et al. / Energy Conversion and Management 42 (2001) 1909±1918

[3] Putnam AA, Belles FE, Kent®eld JAC. Pulse combustion. Prog Energ Comb Sci 1986;12:43. [4] Keller JO, Gemmen RS, Ozer RW. Fundamentals of enhanced scalar transport in strongly oscillating and/or resonant ¯ow ®elds as created by pulse combustion. In: Mujumdar AS, editor, DryingÕ92. Elsevier S.P. Part A 1992. p. 161. [5] Papadakis SE, King CJ. Air temperature and humidity pro®les in spray drying. Ind Eng Chem Res 1998; 27(11):2111±23. [6] Zbici nski I. Development and experimental veri®cation of momentum, heat and mass transfer model in spray drying. The Chem Eng J 1995;58:123±33.