научная статья по теме MASS TRANSFER AND FLUID FLOW VISUALIZATION IN PACKED AND FLUIDIZED BEDS BY THE ADSORPTION METHOD Химия

Текст научной статьи на тему «MASS TRANSFER AND FLUID FLOW VISUALIZATION IN PACKED AND FLUIDIZED BEDS BY THE ADSORPTION METHOD»

ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2009, том 83, № 9, с. 1726-1729

== PHYSICAL CHEMISTRY OF SEPARATION PROCESSES. ^^^^

CHROMATOGRAPHY

УДК 541.183

MASS TRANSFER AND FLUID FLOW VISUALIZATION IN PACKED AND FLUIDIZED BEDS BY THE ADSORPTION METHOD

© 2009 N. Boskovic-Vragolovic*, R. Garic-Grulovic**, Z. Grbavcic*, R. Pjanovic*

*Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia **Institute for Chemistry, Technology and Metallurgy, University of Belgrade, Belgrade, Serbia E-mail:garic@tmf.bg.ac.rs; nevenka@tmf.bg.ac.rs

Abstract — Mass transfer coefficient (jD) between fluid and column wall in liquid packed and fluidized beds of spherical inert particle has been studied experimentally using adsorption method. Experiments were conducted in column 40 mm in diameter for packed and fluidized beds. In all runs mass transfer rates were determined in presence of spherical glass particles 2.06 mm in diameter. This paper introduced adsorption method as very suitable method for studies of mass transfer and for fluid flow visualization. The adsorption method is based on the dynamic adsorption of an organic dye onto a surface covered with a thin layer of a porous adsorbent. Local and average mass transfer coefficients were determinated from the surface color intensity of the foils of silica gel. Correlation jD = f(Re) was derived using mass transfer coefficients data.

INTRODUCTION

Research of transport phenomena in liquid — particles systems, in past years, had more theoretical then practical importance [1—3].For industrial use, especially with fast development of bio and water cleaning processes, better knowing of these systems become more important. Industrial application of these systems requires determination of transfer characteristics, especially mass transfer. Mass transfer in fluidized beds has been widely investigated in terms of particle-fluid mass transfer by dissolution, by electrochemical and by ion-exchange methods [1, 2, 4—8]. Some of the results of the mass transfer in fluidized beds have been obtained as transfer between immersed surface and liquid [9, 10].

The adsorption method was introduced as a relatively simple mass transfer measurement technique, applicable for liquid flow investigations [11]. The method is based on the dynamic adsorption of an organic dye onto a surface covered with a thin layer of a porous adsorbent. The rates of the dye adsorption are supposed to be diffusion controlled. The quantity of the dye transferred during a fixed time period is a function of diffusion boundary layer conditions.

The rate of adsorption decreases from high values at the beginning to a constant value for each dye concentration. This constant value is the actual mass transfer rate through the completely formed boundary layer. For high concentration of the dye the concentration at the film's surface is cp ^ 0 and the adsorption process is controlled by mixed kinetics. Also, for high mass transfer rates mixed kinetics occur at lower concentration.

The adsorption method is based on the experimentally determined fact that under certain conditions mass transfer with adsorption can be treated as a sta-

tionary process governed by mass transfer only. Adsorption from a much diluted solution and far from equilibrium conditions is a very fast process. If the exposure time is short (less than 10 min) the mass transfer rate depends only on the diffusion through the boundary layer. The concentration of adsorbate just above the adsorbent's surface is c = 0.

Mass flux is:

Na = kC0 - C) = kc0 (1)

and for exposure time t:

Na = C,/1, (2)

where cp is the surface concentration of organic dye on adsorbent layer, c0 is the bulk concentration.

If the induction period can be short (in the case of thin boundary layers) a simplified expression to calculate mass transfer coefficient is obtained [12]:

k = cp/tc0. (3)

For this method, value of the surface concentration cp, is necessary for data quantification. This method is used since 1953, but determination of this parameter, cp which now can be done easily, using suitable software (Sigma Scan Pro), gives absorption method new actuality.

This paper presents an experimental study of the measurement of the local and average mass transfer coefficient in heterogeneous systems - packed and fluidized beds. This paper, also introduced adsorption method as very suitable method for determination of mass transfer and for fluid flow visualization.

MASS TRANSFER AND FLUID FLOW VISUALIZATION

1727

1

4

2

9

Fig. 1. Experimental system: (1) column 40 mm in diameter, (2) of silica gel, (6) tank, (7) pump, (8) valve (9) flowmetar.

Fig. 2. Chromatograms for flow in packed bed (a), in fluid-ized bed at minimum velocity (b) and in fluidized bed (c).

EXPERIMENTAL

Experiments were conducted in columns 40 mm in diameter (Fig. 1, 1), for packed and fluidized beds.

Very diluted solution of methylene blue (c0 = 2.5 x x 10-3 g/dm3) was used as a fluid in the presence of inert glass particles 2.06 mm in diameter (Fig. l, 4). The foils of silica gel (Fig. 1, 5) was used as adsorbent

liquid distributor, (3) water overflow, (4) glass particles, (5) foils

Fig. 3. Relationship between mass transfer coefficient and superficial liquid velocity (U), the particle diameter dp = = 2.06 mm.

("Merck", DC-Alufolien Kieselgel). Concentration profiles of methylene blue were measured in the flow of water through packed and fluidized beds. Colour intensity of the surface was determinated by "Sigma Scan Pro 5" software. The fluid flow was changed with valve (Fig. 1, 8) and measured by flowmetar (Fig. 1,9).

1728

BOSKOVlC-VRAGOLOVlC et al.

Fig. 4. Fluid flow visualization around spherical particle in packed bed.

RESULTS AND DISCUSSION

Average color intensity of silica gel surface for flow in packed bed (a), in fluidized bed at minimum velocity (b) and in fluidized bed (c) were shown on Fig. 2.

Chromatogram (a) gives clear visualization of fluid flow around particles in the packed bed. Color inten-

sity is proportional to local mass transfer rates. Chro-matograms (b) and (c) visualize fluidized beds for minimal velocity and for highly expanded fluidized bed. Uniform color intensity could be observed in both cases, with higher intensity for minimal fluidized bed velocity. This color uniformity indicates uniform mass transfer rates for fluidized beds. Lighter top parts of chromatograms (b) and (c) represents mass transfer in the single phase flow.

The color intensity of silica gel surface varies with fluid flows. Higher color intensity means higher mass transfer coefficient as can be seen on Fig. 3. With increasing liquid velocity in packed beds mass transfer coefficient increases while in fluidized beds mass transfer coefficient decrease. The highest mass transfer coefficient was at minimum fluidization velocity because of high concentration of moving particles.

Adsorption method is also very useful for determination of local mass transfer coefficient and fluid flow visualization as can be seen on Fig. 4. Values of local mass transfer coefficients at a different points are shown in table.

Experiments in packed and fluidized beds were conducted as liquid-to-wall mass transfer. Figure 5 present relationship between mass transfer factor and particle Reynolds number in packed and fluidized bed. Also this picture gives the comparison of our experimental data with tree correlation for mass transfer of liquid in packed and fluidized beds.

Dwivedy and Upadhyay [1]:

JDs = 0.765/ Rep82 + 0.365/ Re'

0.386

(4)

- Igfoe)

0

q dp = 2.06 mm (experimental data) □ dp = 1.94 mm (absorption method) [10] Д dp = 1.94 mm (dissolution method) [15]

-[1]

-—[7] ---[8]

Packed bed

л л

Fluidized bed O^

1 2 3

log Rep

Fig. 5. Mass transfer factor vs. particle Reynolds number

Jd 1.0

0.8

0.6

0.4

0.2

О

d dp = 2.06 mm (absorption method) О dp = 1.94 mm (dissolution method) [10, 15]

Packed bed : Fluidized bed

О

О

О О

□ □ п

□ □ J_I_L

8 Re x 10-3

Fig. 6. The relationship between mass transfer factor and Reynolds number.

1

2

0

4

6

MASS TRANSFER AND FLUID FLOW VISUALIZATION

1729

The local mass transfer coefficients (k) at different points on Fig. 4 (I is average color intensity)

Point I к x 106, m/s

1 11.36 5.57

2 9.11 3.85

3 14.01 7.82

4 26.53 20.2

5 29.93 24.2

6 25.143 18.7

7 16.43 10

8 6.26 2.02

9 3.74 0.825

10 21.77 15.2

11 26.32 19.9

12 28.57 22.5

13 26.122 19.7

14 24.08 17.6

15 20.95 14.4

16 19.1 12.6

where Rep is the Reynolds number for particle, (4) Rep = UdpPf/^; jD is the mass transfer factor jD = (k/U)Sc2/3, Rahman and Streat (1981) [13]:

jDe = 0.86Re/5, Rep = 2-25, (5)

Yutani et al. (1987) [14]:

D = 0.4Re/4, Rep = 0.5-1000. (6)

As can be seen, our experimental data are lower than data predicted by all three correlations. Our previous experimental data [10]. Show satisfactory agreement with Dwivedy and Upadhyay and Yutani et al. correlations.

Experimental research showed that adsorption method is very suitable for visualization of fluid flow and for determination of local and average mass transfer coefficients. Adsorption method could be easily employed using novel software solutions.

Figure 6 presents the mass transfer factor as a function of Reynolds number in packed and fluidized beds for different experimental techniques. It could be seen that there is no significant difference between mass transfer factors obtained by this two methods. [10, 15]. Often used dissolution method is very reliable, and agreement of data shows that the adsorption method gives good results also. Advantage of this method is possibility to obtain local mass transfer coefficients.

CONCLUSION

The wall-to-liquid mass transfer in packed and fluidized beds, with the adsorption method of methylene blue was investigated. This method is suitable for fluid flow visualization. The wall-to-liquid mass transfer factor in packed beds is higher then in fluidized beds for Reyn

Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.

Показать целиком