172 Chem. Eng. Technol. 13 (1990) 172- 184

Measurements with Multi-point Microprobes : Effect of Suspended Solids on the Hydrodynamics of Bubble Columns for Application in Chemical and Biotechnological Processes*

Christoph Wolff, Frank Ulrich Briegleb, Joerg Bader, Klaus Hektor, and Hans Hammer**

Many experimental studies reveal that suspended solids considerably alter the coalescence behaviour and hydrodynamic functions of two-phase flow. But no systematic efforts have yet been undertaken to separate the effects of different particle properties on local gas hold-up, bubble size and interfacial area gasiliquid. The aim of this paper is to present the local values of these parameters in three-phase fluidized beds of different solids, using miniaturized optical fibre and conductivity needle probes. It is shown that particle concentration, size and, in particular, density are decisive for the change in coalescence behaviour. Ranges of normal ( e , > eL) and inverse fluidization (e, < eL) must be distinguished and the flow regime also exerts a strong influence on the interactions between the dispersed phases, the transition point itself being a function of particle properties. For certain combinations of solid parameters, even increased interfacial areas gaslliquid can be observed. This effect is evaluated for different column diameters, between 0.1 and 0.3 m.

1 Introduction

A large number of industrial chemical processes are performed in slurry bubble columns or three-phase fluidized beds. It is customary to distinguish between reactions, in which the solid phase acts as educt, product or catalyst [I] . Three-phase bubble columns have furthermore found new areas of application in biotechnology and particularly in wastewater treatment, fermentation of single cell protein and antibiotics. The efficien- cy of these processes could be enhanced by techniques of cell immobilization, where enzymes or even complete cells are at- tached to different carrier materials [ 2 ] .

It is well documented that suspended solids considerably alter coalescence behaviour and hydrodynamic functions of a two- phase system. Different research groups reported that the inter- facial area per unit volume and, consequently, the mass transfer gasiliquid increased or decreased depending on the properties of the solid phase. The aim of this work is to determine the local experimental values of the relevant hydrodynamic parameters (gas hold-up, bubble size and rise velocity and interfacial area per unit volume) as functions of such properties as particle con- centration, density and size. Special importance was attached to the selection of technically relevant solid systems.

2 Hydrodynamics in Bubble Columns

Despite their simple construction, hydrodynamic modelling of bubble columns is very complex. Geometric boundary condi-

* Paper presented by Chr. Wolff at the “GVC Working Party on Multiphase Flow”, Schliersee, Fed. Rep. Germany, April 13 to 15, 1988.

** Dr.-Ing. Chr. Wolff, Dip1.-Ing. F.U. Briegleb, Dr. rer. nat. K. Hektor and Prof. Dr.-Ing. H. Hammer, Inst. of Fuel Chemistry and Phys.- Chem. Engineering, Aachen University of Technology, Worringerweg 1, D-5100 Aachen, and Dip1.-Ing. J. Bader, present address: Inst. of Technical Chemistry, University of Hannover, Callinstr. 1, D-3000 Hannover.

tions (column diameter, height and gas sparger), fluid proper- ties (density and viscosity of both phases and interfacial tension) as well as operating parameters (superficial liquid and gas velocities) form a highly interrelated network. The resulting ef- fect on the self-adjusting hydrodynamic functions can only be estimated within narrow operating limits. Hammer [3] pointed out that the assumption of integral mean values represents an oversimplification, because hydrodynamic functions in both gas and liquid phases show strong radially and axially overlapping profiles. If, in addition, solid particles are suspended, further interaction occurs according to their local distribution, due to:

- particle density, - particle volume concentration, - particle size, - particle shape, - particle wettability.

Fig. 1 represents the network of interacting hydrodynamic func- tions in three-phase flow, based on a diagram of Heijnen and Van’t Riet [4].

Detailed analysis reveals that the problems in bubble column flow modelling arise mainly as a consequence of insufficient description of gas phase coalescence in real fluids. Similarly, the location of the transition point from homogeneous to heterogeneous flow cannot be determined exactly and is subject to additional effects due to the solid particles.

In the past few years, considerable progress has been achieved with regard to measuring techniques for local multiphase flow parameters. Consequently, more sophisticated models of bub- ble column flow have been developed (see Fig. 2). Luebbert [5] adopted a comprehensive approach, incorporating the recent results of chaos research, and pointed out that all the above described mode structures occur and transform into one another, depending on the geometric boundary conditions of the system and its energy input through the gas sparger.

@ VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1990 0930-751619010306-0172 $3.50+ ,2510

Chem. Eng. Technol. 13 (1990) 172- 184 173

Type o f gas sparger hole

d iameter and pitch I Dispersion coefficient liquid phase

1 I 3T’d 1 1 rz DL

I I I I

Column geometry -_ I - -

Flow reglme Llquld

heterogeneous homogeneous/ __L

clrculatlo”

Superflclal gas

Bubble rlse

Heat transfer coefflclent h

Mass transfer Interfacial area per unit volume a

mass transfer Solld properties I.-- profile coefficient KLa

particle sire i

Liquid properties

coalescence properties

I I dlstrlbutlon

3 Experimental

Local properties of the following hydrodynamic functions were measured:

- hold-up of dispersed phase, - local liquid circulation velocity, - bubble size distribution and bubble rise velocities and ap-

- interfacial area per unit volume. propriate mean values of these distributions,

Recently, a number of highly efficient non-intrusive measuring techniques have been developed for the determination of local phase properties in two-phase flow. In three-phase flow, however, the available experimental equipment is still restricted to miniaturized needle probes especially for measuring bubble sizes. This study compares the performance of the two most widely applied probe systems, namely the conductivity and the optical fibre probes in their most advanced layouts, with respect to miniaturization and the amount of information that they can supply. The conductivity probe is the four-point microprobe, which was developed by Buchholz [6] (see Fig. 3 ) .

The optical fibre microprobe (Fig. 4) was designed at the In- stitute of Fuel Chemistry and Physico-Chemical Engineering of the Aachen University of Technology [7] . This probe provides

Fig. 1. Influence of material properties, apparatus geometry and operating conditions on hydrodynamic functions in three-phase fluidized beds.

a good mechanical stability and favourable signal-noise charac- teristics [8]. The signal processing unit, operating at 20 MHz, is capable of resolving bubble penetration lengths down to 0.1 pm [9 ] . The signal response behaviour was tested with high- speed recording by camera and computer which were time- synchronized. Thus, computed bubble diameters could be com- pared to the actual ones and good agreement was achieved with correct calibration. The signals, obtained with the four-point probe, were evaluated by means of an analytical model which allows the determination of the bubble velocity vector’s com- ponents as well as the angles of bubble rise. Starting from two- point probe signals, statistical models [lo] have to be applied for the calculation of bubble properties.

The experiments were conducted in two bench-scale bubble col- umns, 0.3 and 0.2m in diameter, and 11 and 5m in height, respectively. The set-up is depicted in Fig. 5. Compressed air is saturated before being sparged at the bottom of the main col- umn. It leaves the column at the top or is circulated when a highly volatile liquid phase or a mixture is processed. The large column can be operated as an air-lift loop reactor with an exter- nal loop. Because of the characteristics of the sparger (annular sparger with 316 holes, Imm each) the state of heterogeneous flow prevails at low superficial gas velocities. The smaller col- umn is equipped with a sieve plate (pore diameter 0.5mm) and its operating range includes both flow regimes and the transition zone.

174 Chem. Eng. Technol. 13 (1990) 172- 184

Bubble street model o f RIETEMA

Circulation cell model of JOSH1

Gulf stream model Energy balance model of FREEDMAN o f WHALLEY

stream lines of - liquid phase

stream lines of

gas phase area of gassing

---

I". Gas - a -

Shear flow model Macro-Vortex model o f FRANZ o f SACHOVA

f

Clrculatlon f low model of MlYAUCHl

Roller model o f Zehner

Fig. 2. Models of bubble column flow.

In the experiments, solid concentration was varied between 0 and 10 v01.-% and superficial gas velocity between 0 and 10 cm/s. All experiments were carried out with the system tap water/air at standard pressure and 20 "C. The particle systems used in the study covered the parameter range which is impor- tant for industrial application. We attempted to assess the in- fluence of the different solid parameters via their independent variation. In preliminary experiments it was established that all solids could be completely suspended already at low superficial gas velocities.

Table 1 lists the particle properties of the investigated solids. Polypropylene (PP) is lighter than the liquid phase, polystyrene has roughly the same density, whereas the ion exchanger amberlite (AMB) is heavier than the liquid phase.

Axial profiles of solid concentration were determined for the average volume concentration of 5%, using the method of Begovich and Watson [ 1 11. The gas hold-up was determined by averaging several one-point probe measurements over the col-

Table 1. Properties of investigated particles

Solid No. Shape Density Mean Particle [g/cm3] Size [mm]

Polypropylene PP 1 cylinder 0.897 4.43 Polystyrene PS 2 cylinder 1.029 3.18 Amberlite AMB 3 sphere 1.228 0.7

PP - Vestolen P5200 from Chem. Werke Huels; PS - Vestyron X-200 from Chem. Werke Huels; AMB - Amberlite IR 120 from Roehm & Haas Co.

umn cross-section. Fig. 6 depicts the oppositely directed con- centration profiles for normal and inverse fluidization. Note that, at higher gas velocities, the profiles for both solids become more uniform.

4 Conclusion and Results

Assessment of the quality of experimental results, obtained with the needle probe, is facilitated by a consistency check of the dif-

Chem. Eng. Technol. 13 (1990) 172- 184 175

\ 7

ferent hydrodynamic functions. It can be shown theoretically that, as a measure of bubble residence time, the gas hold-up must be inversely proportional to bubble size and rise velocity. Sauter mean bubble diameter, bubble rise velocity and inter- facial area per unit volume for particle systems listed in Table 1, are illustrated in Figs 7 - 9. Particle concentration ranged be- tween 0 and 10 vol.-%. All diagrams show that, without excep- tion, increase or decrease in bubble size and rise velocity coin- cides with an opposite tendency for the gas hold-up. The fact, that these results were obtained by independent techniques, con- firms that the relative trends have been read correctly by the needle probes. The hold-up measurements, recorded in these diagrams, were obtained by the widely used pressure head method. All results, discussed in this chapter, were obtained in the small column, applying the optical fibre probe, since parti- cle concentration exceeded the limit for the conductivity probe.

suppbrl handle

tainless steel tube

- 2 m m

Fig. 4. Two-point optical fibre microprobe.

Fig. 3. Four-point conductivity probe according to Buchholz [6] .

7 Probe tip; 2 Polyester resin; 3 Probe holder; 4 Stabilizing tube; 5 column wall; 6 PVC adapters; 7 Plug connecting board.

A~ = 231.9 pm; H3 = 212.1 pm; H4 = 282.8 pm; p3 = 117.5; p4 = 261.3.

= 181.1 pm; A4 = 181.4 pm; H2 = 303.0 pm;

4. I Effects of Particle Concentration and Density

The following conclusions can be drawn about the behaviour of hydrodynamic functions in suspensions of different particle density and concentration.

1 ) The gas hold-up (Figs 7a, 8a, 9a) shows for all systems a linear increase with superficial gas velocity in the homogeneous flow regime. For the light solids PP and PS, no dependence on

e

Compressed Air

Water Supply

)

Fig. 5. Bench-scale bubble column at the Institute of Fuel Chemistry and Physico-Chemical Engineering, Aachen University of Technology.

I I / % I

- 0 0.2 0.4 0.6 0.8 1 Axial height h/H 1-1 e

Fig. 6. Axial solid concentration profile in the systems waterlairl5 v01.-% AMB or PP.

solid concentration could be established. The transition from homogeneous to heterogeneous flow regime is marked by a relative maximum of the gas hold-up at 7 cm/s superficial gas velocity. This is displaced to smaller gas throughputs when solid concentration and density are increased. For the heavy amberlite, gas hold-up decreases with particle concentration even in homogeneous flow. The transition to heterogeneous flow occurs at lower gas throughputs for AMB than for PP and PS. In the heterogeneous flow regime, hold-up decreases with increasing superficial gas velocity and is inversely proportional to solid concentration. The decrease of gas hold-up in heterogeneous flow is caused by a shift in the bubble size spec- trum and the occurrence of large bubble agglomerates at the transition point.

2 ) The Sauter mean bubble diameter (Figs 7b, 8b, 9b) increases in the homogeneous flow regime for PP and PS, whereas the profile flattens out in the heterogeneous regime. The solid-free system features the lowest Sauter mean diameters, and the largest bubbles are observed in the suspension with the highest solid concentration. In agreement with the results of Calderbank [12], in a system with slight coalescence inhibition, such as tap water, the Sauter mean diameter varies only between 4 and 6 mm .

Chem. Eng. Technol. 13 (1990) 172- 184

At low superficial gas velocities, the Sauter mean diameters show some particularly interesting features, depending on solid density. For PP, i.e. in inverse fluidization, a steady shift of ob- tained Sauter mean diameters towards higher values can be observed at all superficial gas velocities. The heavy AMB, however, features extraordinarily large bubble sizes at low superficial gas velocities which exceed even those of heterogeneous flow for 10 v01.-% solid concentration.

The reason for this behaviour which, so far, has not been reported in literature lies in the development of axial solid con- centration profiles primarily at low superficial gas velocities. These results confirm the assumptions of Oeztuerk and Schumpe [ 141 and of Sauer [ 151 that local solid concentration at the gas sparger determines the hydrodynamic flow pattern. A layer of high solid concentration in this region offers an addi- tional resistance to bubble rise and increases the frequency of coalescence. At the same time, a large-scale liquid circulation, which is characteristic of heterogeneous flow, occurs at high solid concentrations around the gas sparger, at low gas throughputs. The circulation pattern is generated by radial im- parities in the gas hold-up, due to formation of large bubbles, which rise along the column axis.

Fig. 10 illustrates the behaviour of the mean bubble diameter and Fig. 9b that of the Sauter mean diameter. Hereby, it must be taken into account that the determination of the Sauter mean diameter faces a wider error range. Similar trends with respect to particle concentration can be observed for the mean diameter, although it decreases with gas velocity in the heterogeneous flow regime while the Sauter mean diameter increases. This fin- ding, which applies to all particle systems, is again based on the restructure of bubble size distribution in heterogeneous flow 181.

3) Figs 7a and 7b show gas hold-ups between 0 and 2 7 % , whereas the Sauter mean diameter ranges only between 4.5 and 6mm in the examined range of gas throughputs. Therefore, gas hold-up is the decisive factor determining the interfacial area per unit volume, which is diagrammed in Figs 7c, 8c and 9c. The observation by Sittig [ 161, that the interfacial area per unit volume a in three-phase flow exceeds that of the solid-free system, could only be confirmed for inverse fluidization, low particle concentration and homogeneous flow. In any case, in the heterogeneous regime, a decreases when solids are added. This is in agreement with the literature since the reported in- crease of the interfacial area per unit volume gadliquid in three- phase beds was measured in cocurrent, homogeneous flow. This also applies to the compact reactor, operated by Raebiger [17]'):

u g = 0.714 Jd,g .

4) Since, according to the Davies-Taylor equation, bubble size and bubble rise velocity are proportional they follow similar trends. The experimental confirmation is depicted in Figs 7d, 8d and 9d. Experimental values vary between 0.3 and 0.5 m/s and lie within the range of correlation, given in literature.

1) List of symbols at the end of the paper.

Chem. Eng. Technol. 13 (1990) 172- 184 177

0 2 4 6 8 10 Superficial gas v e l o c i t y U GL [ c m / s 1 -

C )

300

2 5 0

zoc

- E . c ; 150

W

5 - 0 > + ' E 100 L W

cl a

F

;; 50

._

L L W + c -

2 4 6

L W + W

5 5 4.5

4

0.5

0.45

; 0.4 \ E u - m

2

x 2 0.35 " 0 - >

W YI ._

0.3 - n n a m

I I . Polypropylene

Symbol N~ S o l i d c o n c . -

x I 1 I 0 vol -%

0 0 2 4 6 8 10

S u p e r f i c i a l gas v e l o c i t y u C I L I c m / s l - S u p e r f i c i a l g a s v e l o c i t y u G L f c m / s l - Fig. 7. Hydrodynamic functions at different superficial gas velocities and particle concentrations in the system wateriairlpolypropylene. a) Gas hold-up;

b) Sauter mean diameter; c) Interfacial area per unit volume; d) Bubble rise velocity.

178

a)

1

Chem. Eng. Technol. 13 (1990) 172- 184

0.5

0.45

: o 2 4 6 8 10 m

Superficial gas v e l o c i t y uCILIcrn/sl -

c>

30 0

2 50

200

- E . c I

0 150 W - 5 0

c 5 loo L aJ a

0 i

50 - 0 u 0 Y-

0 c c I

.-

2 4 6 B 0 2 4 6 0 10

superficial gas v e l o c i t y uCILfcrn/sl - Superficial gas v e l o c i t y ufiLfcm/sJ - Fig. 8. Hydrodynamic functions at different superficial gas velocities and particle concentrations in the system water/air/polystyrene. a) Gas hold-up; b) Sauter mean diameter; c) Interfacial area per unit volume; d) Bubble rise velocity.

Chem. Eng. Technol. 13 (1990) 172- 184 179

2 4 6 8

c e 4.5 E L

c Q)

a v)

4

0 0 2 4 6 8 10

S u p e r f i c i a l gas v e l o c i t y uGLlcrn/si -

0.5

0.45

..-. M 0.4 \ E " - m 3

h c .- 0.35

L

a 0.3 - 9 n a m

Superf ic ial gas velocity U GL l c m / s 1 -

0 I I I I z 4 6 8 10

S u p e r f i c i a l gas v e l o c i t y uGLIcrn/sl -- S u p e r f i c i a l gas v e l o c i t y ulSLIcrn/sl - Fig. 9. Hydrodynamic functions at different superficial gas velocities and particle concentrations in the system water/air/amberlite. a) Gas hold-up, b) Sauter mean diameter, c) Interfacial area per unit volume, d) Bubble rise velocity.

Chem. Eng. Technol. 13 (1990) 172- 184 180

L. 5

L

- 3.5 E E I

m 0

L al +

E B 4 al - n n 3 D

c 0

II 2.5

0

I i I I Arnberl i te I Symbol N o S o l i d c o n c .

1 4 0 4

Superficial gas v e l o c i t y uGLIcrn /s l - Fig. 10. Mean bubble diameter as a function of solid hold-up in the system waterlairiamberlite.

Figs 1 l a - d present the measured hydrodynamic functions for different particle concentrations. The results are presented as ratios of three-phase to the corresponding solid-free system values. An example of homogeneous and heterogeneous flow is included for each particle system and concentration.

For the gas hold-up (Fig. 1 la), the ratio of three-phase to two- phase values shows a maximum for homogeneous flow, which occurs at 3 - 4 % particle concentration for PP and at 1 - 2 % for the lighter PS. It does not occur for the heavy AMB. As a general rule, it can be stated that hold-ups of three-phase systems are always lower than those of the corresponding two- phase systems at high particle concentrations and heterogeneous flow, Furthermore, the ratio of these values decreases with par- ticle Concentration.

For the Sauter mean diameter (Fig. l l b ) , the corresponding ratio always exceeds unity and even increases with particle con- centration and density in homogeneous flow. In the heterogeneous regime, these trends are not as distinct.

The effects of the gas hold-up and Sauter mean diameter on in- terfacial area per unit volume become superimposed (Fig. 1 lc). This area remains smaller than that of the solid-free system for all investigated particle systems with the ratio of three to two- phase values even declining with particle concentration. This decrease can amount to 30 - 40% of the value in the solid-free system for the 10% particle suspension.

The same observation applies to the ratio of bubble rise velocities as to that of the Sauter mean diameters. While, in homogeneous flow, their values in the three-phase system ex-

ceed those of the two-phase system, in heterogeneous flow, their ratio oscillates around unity (Fig. 1 Id).

4.2 Effect of Particle Size

As the behaviour of hydrodynamic functions remains consistent for this part of the study, the discussion of the influence of parti- cle diameter can be limited to the evaluation of interfacial area per unit volume a. The solid concentration in experiments, described in this chapter, was 3 % .

For amberlite (Fig. 12b), no effect of particle size on a can be observed. On the other hand, interfacial area per unit volume increases with particle size for polypropylene, at 3 - 4 % parti- cle concentration and in the regime of inverse fluidization (Fig. 12a). Fine particles reduce a whereas, according to Table 1, the non-comminuted particles attain values which considerably ex- ceed those of the solid-free system. The results for the heavier AMB confirm that the distinct increase in a is typical of inverse fluidization. With increasing solid density, this trend with respect to particle size becomes weaker and disappears in the case of high solid concentration around the gas sparger. The related increase of bed expansion with particle size, often refer- red to in literature [16], was also measured in liquid fluidized particle beds, i.e. under similar operating conditions.

4.3 The Effect of Column Diameter

It was further investigated, how far the enhancement of inter- facial area per unit volume also depends on the column diameter. The comparison of the relative values of hydrodynamic functions in the three bench scale bubble col- umns, 0.1, 0.2 and 0.3m in diameter, is somewhat problematic since different gas spargers were used. However, the HID ratio of 12 was maintained. Nevertheless, it can be seen from Figs 13b and c that gas hold-up for inverse fluidization increases even more with increasing column diameter. In PP suspensions of 5 v01.-%, the reduction of bubble rise velocity amounts to 15 - 20% in the large reactor. At the same time, a correspon- ding increase of gas hold-up is observed. In this context, the results of Sittig [16] should be mentioned who reported a 20% increase in the volumetric mass transfer coefficient, compared to the solid-free system, in a l m diameter column, when 2 v01.- % polyoxymethylene granules were suspended. Sittig generally recommended suspending inert solids in order to in- tensify the mass transfer. If the trends, represented in Figs 13a-d could be extrapolated, this effect could be promising for large scale operations.

5 Summary

The following results can be stated:

1) At high gas throughputs and solid concentrations, the measured gas hold-ups in three-phase beds are lower than those of the solid-free system, whereas the Sauter mean diameter and bubble diameters show an opposite trend. However, in the

Chem. Eng. Technol. 13 (1990) 172- 184 181

0 2 L 6 8 10

Sol id concentrat ion ES [ v o l 7 % I

0 2 4 6 a 10

0 2 L 6 8 I0

S o l i d h o l d - u p c , l v o l . - % I

Superficial Symbol - ~- gas velocity

- 2 c m / s 0 2 PP a c m / s 0 3 PS 2 c m / s

- 2 PS 8 c m / s V 5 A M 0 2 c m / s T 6 A M B 8 c m / s

~ _ _ _ _ _ _ _ ~

~

~-

I I 1

2 4 6 8 10

S o l i d h o l d - u p c S I ~ o l . - % I - S o l i d h o l d - u p c S [ vol.-%l

Fig. 11. Ratios of hydrodynamic parameters in three-phase systems to those of the solid-free system at different superficial gas velocities and for different materials. a) Gas hold-up; b) Sauter mean diameter; c) Interfacial area per unit volume; d) Bubble rise velocity.

182

2 50

200

- 150 E

Chem. Eng. Technol. 13 (1990) 172- 184

Particle Symbol Nr diameter

x 1 without 0 2 0 8 - l O m m 0 3 063-08 mm 0 4 05 -063mm

5 0 3 - 0 5 m m

-- __.-- - _ _ _ -

- 1

Superf ic ial gas velocity U G ~ l c m / s l - S u p e r f i c i a l gas v e l o c i t y ufil[cm/sI - Fig. 12. Interfacial area per unit volume gadliquid as a function of particle size and superficial gas velocity for 3 v01.-% solid hold-up. a) System water/air/PP; b) System water/air/AMB.

homogeneous flow regime and at low solid concentrations (2 - 4 v01.-%), gas hold-up and interfacial area per unit volume exceed the corresponding values of the two-phase system. For inverse fluidization, this increase amounts up to 20% whereas, for high solid densities, - concentrations and gas throughputs, a decrease of 50% was observed.

4) Intensification of mass transfer by addition of inert solids in- creases in the case of investigated columns (D, = 0.1 - 0.3m) with the reactor diameter. If further extrapolation proves to be valid, the addition of inert granules can be recommended for large scale operation.

2 ) For dense particles and low superficial gas velocities, resulting in high local solid concentration near the gas sparger, large Sauter mean diameters are measured, which exceed those observed in heterogeneous flow. Comparison of the hydrodynamic functions in particle suspensions of different densities illustrates the effect of the axial particle concentration gradient on the resulting bubble size. While the gas hold-up and the interfacial area per unit volume decrease with solid density, the opposite applies to bubble size and rise velocity. This fin- ding confirms the observation of Oeztuerk and Schumpe [14], who noted the decisive effect of local solid concentration at the gas sparger on the flow structure. They recommended the use of local concentration instead of the mean system solid hold-up in dimensionless correlations.

3) Regimes of normal and inverse fluidization must be distinguished for the assessment of the effect of particle size. An increase of the interfacial area per unit volume was measured for the light polypropylene, while no influence of par- ticle size on hydrodynamic functions could be observed in high density solid suspensions.

Acknowledgements

The authors wish to express their gratitude to the National Ger- man Scholarship Foundation (Studienstiftung des Deutschen Volkes) for the dissertation grant to C.W.

Received: February 21, 1989 [CET 2081

interfacial area per unit volume gadliquid particle concentration mean bubble diameter particle diameter Sauter mean bubble diameter column diameter gravitational acceleration height coordinate column height bubble rise velocity

Chem. Eng. Technol. 13 (1990) 172- 184 183

S u p e r f i c i a l g a s v e l o c i t y uGL[crn/sl --

1.3

1.2

1.1

0.9 \ 01 W L

f 0.8

0 2 4 6 8 10

S u p e r f i c i a l gas v e l o c i t y ~ ~ ~ I c r n / s I -

0 2 4 6 8 10 0 2 4 6 8 10

S u p e r f i c i a l gas v e l o c i t y u G L I c m / s l - S u p e r f i c i a l g a s v e l o c i t y uGL[cm/s] - Fig. 13. Hydrodynamic functions as ratios of three-phase/two-phase values in the system water/air/PP for different column diameters. a) Sauter mean diameter; b) Gas hold-up; c) Interfacial area per unit volume; d) Bubble rise velocity.

184 Chem. Eng. Techno]. 13 (1990) 184- 196

' k L [m/sl superficial gas velocity [4] Heijnen, J.J., Van't Riet, K . , Chem. E n g J . (Lausanne) 28(1984) pp.

P [bar1 pressure [5] Luebbert, A., Chem-Ing.-Tech. 59 (1987) No. 6 , p. 513. EG [ - I gas hold-up (61 Buchholz, R., Steinemann, J . , Onken, U. , Chem.-Ing.-Tech. 54

VL [WmsI viscosity [7] Tuominen, O., Hektor, K . , Hammer, H., Chem.-Ing.-Tech. 59 0, ["I contact angle (1987) No.11, pp. 867-869 and 60 (1988) No. 5 , pp. 405-407. Qc [kg/m31 gas density [8] Wolff, Chr., Thesis, TH Aachen 1988; Fortschr. Ber. VDI Reihe 3 Q L [kg/m31 liquid density No. 176 (1989). Q S [kg/m31 solid density [9] Brueck, F.-J., Hektor, K . , Chem.-1ng.-Tech. 56 (1984) No. 12, pp.

00 [ - I index for single bubbles [lo] Werther, J . , Chem.-hg.-Tech. 45 (1973) No. 6, pp. 375-377.

UL [misl liquid circulation velocity B21- B42.

ES [ - I solid hold-up (1982) NO. 6, pp. 608-609.

UL "/ml surface tension 920 - 922.

[ l l ] Begovich, J.M., Watson, J.S., AIChE J . 24 (1978) No. 2 , pp.

[12] Wezorke, H. , Thesis, TU Dortmund 1986. 1131 Calderbank, P.H., Moo Young, M.B., Chem. Eng Sci. 16 (1961) p.

[ 141 Oeztuerk, S . S . , Schumpe, A . , Chem. Eng Sci. 42 (1987) No. 7, pp.

3.51 - 354.

References 39.

[ l ] Deckwer, W.-D., Reaktionstechnik in Blasensaeulen, Salk & 1781 - 1785. Sauerlaender, Frankfurt/M. 1985. [I51 Sauer, T. , Thesis, GH Paderborn 1986.

[2] Zlokarnik, M . , BTF Biotechnol. Forum 3 (1986) No. 4 , pp. [I61 Sittig, W., Verfahrenstechnik (Mainz) I 1 (1977) No. 12, pp.

[3] Hammer, H. , Habilirationschri', TU Berlin 1968. 211-218. 730 - 740.

[17] Raebiger, N . , Chem.-Zng.-Tech. 57 (1985) No. 3, pp. 248-249.

Determination of Flooding Gas Velocity and Liquid Hold-up at Flooding in Packed Columns for GaslLiquid Systems*

Jerzy Mafkowiak**

A new model of suspended bed of droplets for describing the vapour or gas velocity at the flooding point in packed columns for rectification and absorption under vacuum and normal pressure is presented. The model was verified by measurements on about 100 different types of randomly filled metallic, ceramic and plastic packings with diameters of 8 - 90 mm as well as on sheet metal and gauze packings, in a wide range of liquid and vapour loads. Approximately 650 literature measurements and own data were evaluated. The mean relative error in determining the gas velo- city at flooding point is less than + 5 % . On the basis of the double layer model, a new equation was derived for the hold-up at flooding point, which is needed for the calculation of the flooding gas velocity. An example of calculations for sample applications is also included.

1 Introduction

I . I Hydraulic Behaviour of Packed Columns

A reliable design of packed columns can only be ensured when hydraulic behaviour of the particular packing element is known throughout the whole load range. In order to attain small cross-

* Presented in part at the "GVC-Jahrestreffen der Verfahrens- Ingenieure", Strasbourg (France), September 17 to 19, 1986.

** Dr.-Ing. J. MaCkowiak, Soldnerstr. 16a, D-4630 Bochum 1; new ad- dress: Environ Engineering, BaBfeldshof 4-6, D-4220 Dinslaken.

sections, the apparatus should be loaded as much as possible; for this reason, the flood limit must be known. The hydraulic behaviour of a packed column with a two-phase countercurrent gas-liquid or vapour-liquid system can be explained on the basis of the relationship between the pressure drop A p of flowing vapour or gas per 1 m of the column height and on the velocity of the gas or vapour uv, or their capacity factor Fv = uvd&'). Fig. 1 b illustrates qualitatively the relationship between Ap/H and the vapour capacity factor F,. The parameter is the liquid

1) List of symbols at the end of the paper

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