APPLICATION OF PULSE COMBUSTION TECHNOLOGY IN SPRAY DRYING PROCESS

 

I.Zbicinski¹, I.Smucerowicz¹, C.Strumillo¹ and C.Crowe^2
1Technical University of Lodz, Faculty of Process and Environmental Engineering,
ul. Wolczanska 213/215, 93-005, Lodz, Poland
Phone +48 42 631 37 73, Fax +48 42 636 49 23, +48 42 636 56 63
²Washington State University, School of Mechanical and Materials Engineering,
Pullman, WA 99164-2920, USA

 

(Received: September 30, 1999 ; Accepted: April 6, 2000)

 

 

Abstract - The paper presents development of valved pulse combustor designed for application in drying process and drying tests performed in a specially built installation. Laser technique was applied to investigate the flow field and structure of dispersed phase during pulse combustion spray drying process. PDA technique was used to determine initial atomization parameters as well as particle size distribution, velocity of the particles, mass concentration of liquid phase in the cross section of spray stream, etc., in the drying chamber during drying tests. Water was used to estimate the level of evaporation and 5 and 10% solutions of sodium chloride to carry out drying tests.
The Computational Fluid Dynamics technique was used to perform theoretical predictions of time-dependent velocity, temperature distribution and particle trajectories in the drying chamber.
Satisfactory agreement between calculations and experimental results was found in certain regions of the drying chamber.
Keywords: Valved and valveless pulse combustors, Time dependent flow calculations, Drying process calculations

 

 

INTRODUCTION

The investigation on pulse combustion and its applications dates back to the 18th century. Heat excited acoustic oscillations were probably discovered by Higgins when, in 1777, he observed the so called hydrogen "singing" flames in tubes. Later, in 1859, Rijke discovered that strong acoustic oscillations appeared when a heated metallic grid was positioned in the lower half of a vertical tube opened at both ends (Putnam et al., 1986).

During pulse combustion intensive mixing and enhanced heat and mass transfer between the combustion products and fuel and between the flue gases and walls take place. The enhanced mixing and transport processes lead to highly efficient and compact combustion processes. The oscillatory mixing and high heat transfer through the combustor walls produce oscillatory flow conditions inside the combustion zone which minimizes NO_x production and the residence time of reagents compared to classical combustion systems (Zinn, 1992). Nowadays pulse combustors are found in many practical applications - mostly as air and water heaters, but also drying installations are built successfully where pulse combustors provide a high temperature and highly turbulent drying agent. The disadvantage of the pulse combustor is noisy operation. Noise can achieve the level of 130 dB.

There are three main sources of this noise:

(a) detonating character of combustion (this is an intrinsic feature of this combustor operation),

(b) vibrations of metal walls of the combustor,

(c) velocity difference between gases flowing from the combustor and ambient air.

During several decades of not always successful attempts of practical implementation, a number of methods to decrease efficiently the noise level have been developed (Hansen, 1994).

Apart from the obvious methods such as acoustic insulation or decreasing the combustion intensity, the most popular are the three following ones:

(a) coupling of two combustors so that they work in counter-phase,

(b) application of ejectors at the inlet and outlet,

(c) shielding the space between the ejectors and inlet and outlet.

Having this in mind and taking into account the general orientation towards environmental protection, one must not forget that in some cases noise is a useful phenomenon, e.g. in drying, when it intensify the process.

 

PULSE COMBUSTION DRYING APPLICATION

Industrial applications of pulse combustion in the drying process may be found mainly in the following techniques:

(a) spray drying,

(b) fluid bed drying,

(c) flash drying.

Hosokawa Bepex and Sonodyne Industries Inc., USA, have conducted the most extensive research on pulse combustion application in the spray drying process.

Hosokawa patented a new type of pulse combustors with cylindrical rotary valves. Bepex spray drying installation with pulse combustor is presented in Figure 1, (Ozer, 1993).

 

 

This device has been used to dry many chemical and pharmaceutical products, food, polymers and so on. Low production costs and high quality products have been obtained. During experiments an intensive heat transfer, low air consumption used on 1 kg of evaporated water (about 30 – 40 % less) and excellent atomization of pastes and sludges have been observed. However, further research has been terminated because of problems with feeding system and dried material deposition on walls near the air inlet.

Another spray drying installation was developed by Sonodyne Industries, Inc. (Sonodyne Industries, Inc., 1984). The mobile drying system with capacity of 2 ton/h of evaporated water is equipped in the valveless pulse combustor as a source of a drying agent. A device called SONO-DRI has been applied to dry over 100 various products such as sensitive thermal food products or industrial sewage. Drying tests confirmed advantages of the pulse combustion drying: high quality product, high energy efficiency, low air consumption on 1 kg of dry product, possibility of drying of a wide range of materials.

Nowadays pulse combustors are often used in the systems for drying industrial wastes. The example of such an installation, pulse combustion vibrofluidized bed dryer IMPULS (Foundation for Industrial Research, The Netherlands) has a capacity of 20,000 ton/year of evaporated water. The dryer was successfully used to dry acid wastes, biological deposits, spent brewery yeast, sawdust, deposits from tanneries, toxic wastes, urban deposits, deposits after plating, sludges, dangerous wastes and many more (Kudra and Mujumdar, 1995).

Novodyne Ltd, Canada, has conducted recently an intensive research to construct a pulse combustion drying system for wood industry (Buchkowski, 1999).

The flash dryer presented in Figure 2 is equipped with the valved pulse combustor is used for sawdust and wood waste drying. During drying of such big particles we cannot expect satisfying energy parameters of the drying process, but results obtained in this system show that

 

 

(a) the device operation is stable and safe,

(b) thermal efficiency is similar to the efficiency of flash dryers,

(c) electrical energy consumption is 40 – 50 % less than for conventional flash dryers,

(d) capital costs are 10 – 15 % less in comparison to classical flash dryers, because of a compact design of the system,

(e) emission of toxic substances is low.

Successful development of the Novodyne flash dryer arouses continuous interest in the application of pulse combustion technique to drying process.

 

PULSE COMBUSTION SPRAY DRYING UNIT

In the frame of this work the pulse combustion drying system was developed, optimised and modelled. The experimental set-up of valved pulse combustion drying spray system is presented in Figure 3. 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.

 

 

To provide sufficient safety level of the combustion and drying process the anti-explosion safety valves were installed. The methodology of experiments was the following; when the pulse combustor operation was stable and steady (temperatures inside the combustion chamber and at the outlet of the tailpipe were constant), the combustor was linked with the drying chamber.

Before drying tests, laser technique was applied to investigate the flow field produced by the pulse combustor. FLOWLITE laser system produced by DANTEC, Denmark was employed to carry out the measurements. An optical probe was installed on the traverse and measurements were made in 15 points along the diameter and in 5 points along the length of the drying chamber. 1500 validated samples were taken in each measuring point.

Figure 4 presents profiles of average axial velocity in the drying chamber. Analysis of the results confirms complex nature of the pulsating flow in the drying chamber. The flow in the chamber is not axi-symmetrical. This effect might be caused by non axi-symmetrical, rectangular outlet from the drying chamber.

 

 

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. (1992).

In this work 24 drying tests of the NaCl solutions and evaporation of water were performed for different operating process parameters. The kinetics of raw material drying was determined and the measurements of drying agent temperature in the spray envelope and material temperature were carried out.

An example of the results is presented in Figure 5. Data collected in this way can be a basis for modelling and scaling up of the pulse combustion drying system.

 

 

MODELLING OF EVAPORATION AND DRYING PROCESS IN TIME DEPENDENT PULSATING FLOW

There are a number of attempts of mathematical modelling of pulse combustion process encountered in the literature (Celik et al., 1993, Akulicz et al., 1998, Zbiciński et al., 1999). According to our knowledge there are no attempts to model pulse combustion drying process.

In the project an attempt was made to model the velocity, temperature and pressure fields of gaseous flow generated by a pulse combustor in a drying chamber.

Computational Fluid Dynamics (CFD) technique was used to calculate time dependent flow in the chamber. Software package Fluent 5.0.2 was used to perform all calculations.

Steady and unsteady cases were investigated to show the influence of velocity oscillations on the character of the flow. In both cases a standard k-e turbulence model, an implicit, segregated solver and the PISO algorithm for pressure-velocity coupling were used. The source of high temperature and velocity oscillations was the jet of flue gases from valved pulse combustor. Due to technical reasons the outlet part of the dryer was rectangular and a position of a pipe delivering flue gases to the cyclone which was situated lower than the dryer. This geometry caused three dimensional, not axi-symetrical flow of a drying agent in a drying chamber. Calculations with three-dimensional geometries were unsuccessful, because they required enormous storage for data files and the convergence rate obtained during these calculations was poor. For this reason all further calculations were performed for two-dimensional, axi-symmetric geometries.

In the paper results of calculations for time dependent flow will be presented only.

Modeled geometry of the system is presented in Figure 6. The preliminary grid consisted of 8650 nodes. This number was finally increased to 11220 nodes during a series of calculations to achieve non grid-dependent solutions. In all further calculations the dense grid was used.

 

 

All calculations were done on Hewlett-Packard workstations, a HP 9000 C100 with 128 Meg RAM, 300 Meg swap, running HP-UX 10.20.

Two-dimensional axi-symmetric, unsteady, turbulent flow was assumed. Due to operating frequency of the pulse combustor equal to 121 Hz, the sinusoidal axial velocity oscillations were defined as follows:

ux=5.76 + 12*(sin 754*t) [m/s]

Calculations were carried out for 10000 time steps at an increment of 0.0015 sec. After every 500 time steps the mean temperature along dryer radius at 5 distances from the inlet was checked and was found to decrease. Changes in the mean temperature in the period of time equal to 14.25 sec are presented in Table 1.

 

 

It was also noted that the difference in temperature between time step 9500 and 10000 was 1 K, so the solution was assumed to be stable.

After a total number of 13000 time steps equal to the time of 19.5 sec the calculations were stopped and a stable state was achieved. This final data file was later used for evaporation and drying calculations. Additional 20 time steps of 0.00065 sec were then performed to investigate the influence of the velocity oscillations on flow field in the drying chamber during a single cycle of combustor performance (equal to 8.33 msec). Finally 500 and 1000 time steps of 0.001 sec restarted from time 19.5 sec were calculated to have a reference with results obtained from two-phase flow calculations. Results obtained during these calculations are presented in Figures 7-10.

Figures 7-10 show the influence of velocity oscillations on the flow character. We can observe changes in the stream lines for 4 different times: 19.5; 19.502; 19.504 and 19.507 sec. The main flow is moving from the dryer axis to the walls. One big recirculation zone is formed near the dryer inlet and a second is formed temporarily in the conical section of the dryer as a result of reverse flow. Recirculation zones are responsible for negative values of axial velocity near the wall.

Generally, the stream lines obtained for time-dependent flow are similar to stream lines for the steady flow. However, the large recirculation zone oscillates between the wall and the axis which results in fast mixing and equalization of the temperature and velocity profiles in the drying chamber.

Figure 11 shows the effect of momentum exchange between continuous and dispersed phase. It was observed that particle injection caused an increase in the velocity of the flow. Gas velocity increased over a factor of two in the case where initial velocity of particles was the highest. In other cases the velocity of gas increased from 1 to 4 m/s in comparison with flow without particles. The influence of the particles presence is substantial only at a certain distance from the dryer inlet.

During injection of particles a drop of air temperature can be observed (Figure 12). Introduction of particles into a flow caused a rapid evaporation and cooling of the air inside the dryer. However, this effect is observed only in the vicinity of the atomizer (up to 0.1 m).

Strong oscillations of the flow field (Figures 8-11) cause rapid mixing and uniformization of the temperature along the radius of the chamber.

 

CONCLUSIONS

In this work an extensive research concerning pulse combustors, operating principles and applications in drying process, as well as the experimental investigations on pulse combustion drying process are presented. Computational Fluid Dynamics (CFD) calculations of steady and transient multiphase flow generated by pulse combustor in the drying chamber were performed and reported in the paper.

Both experiments and theoretical results show analogies in spray and pulse combustion spray drying process in terms of stream lines, velocity and temperature distributions of continuous and dispersed phase. However in the flow produced by a pulse combustor a rapid equalisation of property distributions in the drying chamber is observed.

Theoretical and experimental results show that pulse combustion drying seems to be an effective and competitive way of dehydration. Further research is necessary to perform a more detailed comparison of drying in steady and pulsating flow.

 

ACKNOWLEDGMENTS

The authors greatly acknowledge assistance of Mr Jan Kasznia during construction of the experimental set-up and carrying out the experiments.

 

REFERENCES

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