Asian Science Citation Index is committed to provide an authoritative, trusted and significant information by the coverage of the most important and influential journals to meet the needs of the global scientific community.  
ASCI Database
308-Lasani Town,
Sargodha Road,
Faisalabad, Pakistan
Fax: +92-41-8815544
Contact Via Web
Suggest a Journal
American Journal of Engineering and Applied Sciences
Year: 2009  |  Volume: 2  |  Issue: 1  |  Page No.: 61 - 69

High Temperature Convective Drying of a Packed Bed with Humid Air at Different Humidities

J. Sghaier, S. Messai, D. Lecomte and A. Belghith    

Abstract: Problem statement: Drying a packed bed of porous particle at high temperature with varying humidity of hot air is an attractive process. Many researches on experimental and simulation on a fixed bed drying at low and average temperature are proposed. Few studies showed drying at high temperature with humid air or using superheated steam. The latest is compared to dry air. Approach: In this study, we present an experimental and numerical study of humid air drying of a fixed bed of moist porous alumina particles. The air velocity, the air temperature and the vapor pressure were varied from 1.7-2.3 m sec-1, 120-160°C and 0.1-0.6 bar, respectively and the experiments were performed at atmospheric pressure. Then a mathematical model describing heat and mass transfer during drying is developed. This model is based on the averaging volume approach using two scale changes. Results: From the experimental works, the solid temperature and the bed moisture content have been presented at different drying conditions. The previous results show that an increase in humidity leads to an increase of the wet bulb temperature and a decrease in the drying time. At the same drying temperature, the variation in the gas velocity affects also the drying time. In addition, we note that the drying time increases if the bed depth increases. The predicted results deduced from the developed model were compared with the experiment. Conclusion: The experimental and predicted results obtained from this study describing drying of a packed bed illustrate clearly the effect of the air humidity on the drying kinetics.

Fig. 1. Initially designed for steam drying, the system allows using humid air as drying agent. For technical reasons-difficult control of air leakage in the circuit-superheated steam drying experiments could not be performed.

The circulation of the drying agent is realized by a fan. Gas flow rate and velocity in the chamber are controlled by adjusting the speed of the fan. A control system allows setting the gas temperature and vapor pressure. Water vapor is generated continuously in the steam generator and injected in the closed loop through a control valve. The pressure is maintained slightly above atmospheric pressure using a pressure control valve.

The drying cell as shown schematically in Fig. 1 is an opaque aluminum cylinder. The inside diameter and the length of the cell were approximately 0.18 and 0.32 m, respectively. The drying agent flows through the drying chamber and the experimental cell containing a fixed bed of alumina particles. At the inlet of the cell, a grid ensures a good distribution of the drying agent.

Fig. 1: Experimental set up. (1): Steam generator; (2): Pressure control valve; (3): Steam flow rate valve; (4): Convective cell; (5): Fan; (6): Hot gas heating system; Weight: v1 and v3 closed v2 opened; Normal operation: v1 and v3 opened v2 closed

Thermocouples are placed at different positions in the cell to measure the temperature of the alumina beads. Periodically, the cell is automatically moved down and supported by a stand connected to the weigh scales. The stand is heated at its base to avoid condensation. During the weighing period, the drying agent is by-passed (thus closing valves V1 and V3 and opening V2, (Fig. 1).

For humid air drying with low relative humidity, a standard humidifier was used.

Experimental procedure: Porous particles alumina beads were previously immersed in distilled water for 24h to insure full saturation within the sphere and preheat them at a temperature just below the saturation temperature. During the packing of the bed in the experimental cell, thermocouples were introduced in the bed at the particle surface in different axial positions.

Meanwhile, the drying equipment was heated to a desired experimental temperature and the steam generator was activated. Steady state conditions were reached. Then the experimental cell was inserted in the drying chamber (the drying agent was circulated through the by-pass during this operation). The data acquisition system was started immediately after closing the chamber door, with continuous measurement of the solids temperatures at different depths in the bed and discontinuous measurement of the mass loss of the bed, using the previously described experimental protocol. The experiment was stopped when the mass of the bed remained constant.

Mathematical model: The mathematical model describing the drying is developed in a previous study in the case of superheated steam[6]. This model is based on the averaging approach tacking into account the non validity of the thermal equilibrium and using the following assumptions:

The porous medium is homogeneous and isotropic
the bed porosity is uniform
The compression work and viscous dissipation are negligible
The dispersion and the tortuosity terms can be modeled as diffusive flux

The macroscopic equations governing heat and mass transfer in the packed bed are:

Energy conservation in the gas phase g:

(1)

Energy conservation equation in the solid phase s:

(2)

= The evaporation rate
A = The specific surface
lseff and λgeff = The effective thermal conductivity of solid and gas phases, respectively
hgs = The convective heat transfer coefficient (solid-gas) expressed as:

(3)

Where Nu is the Nusselt number

Initial Conditions: Initially temperatures and moisture content are constant:

(4)

(5)

Thermal boundary conditions:

The lateral surface of the bed is adiabatic
At the inlet of the cylinder, the temperature of the gas is assumed constant, the solid and gas temperatures are related by:

(6)

At the outlet of the cylinder, a heat transfer coefficient is introduced to describe the transfer:
Solid phase:

(7)

Gas phase:

(8)

Dying kinetics: The phenomenon of water evaporation and mass transfer in a solid particle and the surrounding gas phase is expressed by drying kinetic. The evaporated rate is given by:

(9)


Where,
A = The specific surface
Fm = The mass flux obtained by the numerical resolution of a single particle model[13] describing drying

Correlations for the later are given from a set of numerical tests carried out for various values of temperatures and drying fluid velocity. In a first step, correlations of maximum mass flux and moisture content are expressed as a function of the gas temperature for a given gas velocity. In a second step, the gas temperature is kept constant and we vary the gas velocity. With combination of the two functions, correlations of mass flux and moisture content are given function of the gas temperature and velocity in the form:

(10)

Where:

Fmax= The mass flux at the constant rate period

F1max,= The mass flux which corresponds to the point of intersection of two sub-periods of the falling-rate period is given by:

(11)

The equilibrium moisture content Xeq is a function of the gas temperature:

(12)

Xcr, the critical moisture content indicating the start of the falling-rate period is given by:

(13)

X1 is the moisture content which corresponds to the point of intersection of two sub-periods of the falling-rate period given by:

(14)

ai, bi, ci, di and ei are coefficients which depend of the drying agent and the medium used. The values of different constants of correlation are shown in Table 1.

Final correlations of mass flux are:

(15)

(16)


Table 1: Values of different constants for the drying kinetics of single particles

Table 2: Properties of the solid particles used in the simulations and numerical parameters
(17)

The resolution of the system of equation governing heat and mass transfer during drying packed bed is based on the finite volume method[14], with a regular mesh. A gauss siedel iterates method was used.

The properties of the alumina particle used in the simulation are shown in Table 2.

RESULTS AND DISCUSSION

In the series of experiments, the velocity and temperature of the drying air were varied from 1.7 to 2.3 m sec-1 and 120-160°C, respectively. On Fig. 2, we present the evolution of the bed moisture content and the solid temperature for two values of vapor pressure.

These experiments were carried out for the same velocity and drying temperature. A decrease of the temperatures in the bed is observed at the beginning of the drying process due to the heat transfer between the preheated alumina particles and the colder cell. Then the solid temperature reaches the wet bulb temperature.

The later depends on the drying conditions. For example, for a vapor pressure of 0.6 bar the wet bulb temperature is about 90°C whereas it falls to 58°C for 0.1 bar of vapor pressure. This phase lasts between 15 and 9 min depending on the drying conditions. Next the air temperature raises regularly, the falling rate period starts. When the solid temperature reaches the inlet temperature of the drying agent, the end of the drying process is reached. We note also a good consistency between the temperature curves and the observed mass For drying with low vapor pressure, the constant rate period is much shorter (less than 600 s) and hardly observable from the moisture curve, although the measured temperature in the bed seems rather steady.

Fig. 2: Drying kinetics during humid air drying ((a): Pv = 0.6 bar and (b): Pv = 0.1 bar): Tg = 140°C, Vg = 2 m sec-1, d = 4.3 mm, Z = H/2 and H = 8 cm

This is probably due to the more sensitive effect of external mass transfer. Figure 3 also shows that the drying agent velocity affects the time required to reach the drying temperature. When the drying agent velocity is increased for the same drying temperature, the convective heat transfer coefficient also increases and heat and mass transfers are enhanced.

Figure 4 shows that the variation in the temperature of the drying medium affects both, the bed moisture content and temperature profiles, for the two conditions of vapor pressure. The experimental curves show that at 120°C the drying time (defined as the time when moisture content reaches equilibrium) with humid air for Pv = 0.6 bar is shorter than for Pv = 0.1 bar. On the opposite, the drying time is shorter for high vapor pressure drying at 160°C. This suggests that an inversion temperature of the alumina particle fixed bed can be found.

Fig. 3: Effect of gas velocity on drying kinetics during humid air drying ((a): Pv = 0.6 bar and (b): Pv = 0.1 bar): Tg = 140°C, d = 4.3 mm, Z = H/2 and H = 8 cm

Fig. 4: Effect of gas temperature on drying kinetics during humid air drying ((a): Pv= 0.6 bar and (b): Pv= 0.1 bar): Vg = 2 m sec-1, d = 4.3 mm, Z = H/2 and H = 8 cm

Fig. 5: Evolution of drying time versus drying agent temperature: The inversion temperature: Vg = 2 m sec-1, d = 4.3 mm, Z = H/2 and H = 8 cm

Figure 5 shows the drying time versus drying agent temperature for experimental humid air drying (Pv = 0.6 bar and Pv = 0.1 bar) and clearly indicates an inversion temperature around 130°C.

Figure 6 presents the variations of the solid temperature and the bed moisture content over time for different bed depths during humid air drying. For the same axial position, the bed depth affects the drying time. For example, when bed depth decreases from 12 to 5 cm the drying time is reduced about 1000s. The bed with the thinnest depth reaches the gas temperature and the equilibrium moisture content more quickly and remains at this temperature for the shortest amount of time.

For the deepest bed length, three temperature measurements can be performed in the bed.

Fig. 6: Effect of bed depth on drying kinetics during humid air drying (Pv = 0.6 bar (a) and Pv = 0.1 bar (b)) Tg = 140°C, d = 4.3 mm, Z = H/2 and Vg = 2 m sec-1

Fig. 7: Product temperature at different bed depths during humid air drying (Pv= 0.6 bar (a) and Pv = 0.1 bar (b)) Tg = 140°C, d = 4.3 mm, Vg = 2 m sec-1 and H = 8 cm

At the entry of the cylinder (Z = 0), the product reaches the gas temperature more quickly than the centre (Z = H/2 = 4 cm) and the exit (Z = H = 8 cm) Figure 7 which was an expected result. We note the existence of a drying front that occurs at the start of the drying process. During drying, this front moves in the granular media[7].

Figure 8 shows that, the predicted curves are in agreement with the experiments carried out at the same drying conditions: Gas temperature (Tg = 140°C) velocity (Vg = 2 m sec-1), particularly in predicting the time to reach the drying agent temperature. However, the experimentally measured solid temperature rises earlier than the predicted value for (Pv = 0.1bar). This can be attributed to the position of the thermocouples. Moreover the assumption of a constant gas velocity is not completely exact In fact, the pressure air decreases due to friction between the gas and the walls as well as due to momentum losses caused by interaction of particles. The velocity between the particles and the gas is controlled by the interfacial drag and friction between the particles and the walls [15].

We can notice Fig. 9 that the predicted results are in good agreement with experimental results for the two case of pressure vapor. In particular at high humidity, the wet bulb temperatures and the time duration for the isenthalpic phase is appreciatively the same.

Fig. 8: Comparison of experimental and numerical curves during humid air drying ((a) Pv = 0.6 bar and (b) Pv = 0.1 bar): Tg = 140°C, Vg = 2 m sec-1, d = 4.3 mm, Z = H/2 and H = 8 cm

Fig. 9: Comparison of experimental and numerical curves during humid air drying ((a) Pv = 0.6 bar and (b) Pv = 0.1 bar): Tg = 160°C, Vg = 2 m sec-1, d = 4.3 mm, Z = H/2 and H = 8 cm

CONCLUSION

An experimental study of humid air drying of a packed bed of spherical alumina porous particles at atmospheric pressure at different vapour pressure was carried out. The influence of external and internal parameters was examined for two values of drying agent vapour pressure (Pv = 0.6 bar and Pv = 0.1 bar). The experimental solid temperature curve at different axial position indicates the existence of a drying front inside the bed. In fact the influence of vapour pressure on the drying kinetics is of great importance; therefore it will be interesting to study experimentally superheated steam drying.

A comprehensive heat and mass transfer model was developed for a packed bed drying process during humid air. This model is based on the averaging volume approach. Taking into account the non validity of the thermal equilibrium hypothesis, a two temperatures model is presented: particle temperature (solid phase) and the gas temperature. The mass transfer is introduced by drying kinetics deduced from the particle model. The correlations of mass flux and moisture content, deduced from the numerical resolution of the single particle model, were incorporated in the bed model. The predicted results compared to the experimental data, indicated good agreements despite the few discrepancies.

" target="_blank">View Fulltext    |   Related Articles   |   Back
   
 
 
 
  Related Articles

 
 
 
Copyright   |   Desclaimer   |    Privacy Policy   |   Browsers   |   Accessibility