clrcmical &z~inrcrhg Science. Vol. 47. No. 13/14. pp. 3557-3564. 1992. oam-2509i92 ss.oo+o.oo Printed in Great Britain. 01992klpunon PressLd

HYDRODYNAMICS AND MASS TRANSFER CHARACl-ERISTICS OF A LOOP-VENTURI REACTOR WITH A DOWNFLOW LIQUID JET EJECTOR

P.H.M.R. Cramers. A.A.C.M. Beenackers and L.L. van Dierendonck

Department of Chemical Engineering, University of Groningen 9747 AG Groningen, Netherlands

DSM Research, PO Box 18, 6160 MD Geleen. Netherlands

ABSTRACT The hydrodynamics and mass transfer characteristics of a loop-venturi reactor have been investigated using a downflow liquid jet ejector. The specific interfacial area of the ejector and the main holding vessel were determined separately. The cobalt catalyzed sulfite oxidation was used as a model system. T@e measured values of the interfacial area were in the range of 40.000 to 70.000 m*/m in the ejector and 500 to 2500 in the total system. From this it can be concluded that the Loop-venturi reactor is particularly suitable for very fast reactions, in which the liquid phase mass transfer is the reaction limiting step of the process. Further it was demonstrated that the flow regime in the ejector has a huge influence on the .mass transfer characteristics of the total system. From the results it is concluded that for a proper design of a loop-venturi reactor, the ejector and the main holding vessel should be considered as two reactors in series. which require individual modeling.

KEYWORDS

Ejector, venturi, jet, mass transfer, flow regime, modeling.

INTRODUCTION

In many chemical processes, the overall production rate is often limited by gas-liquid mass transfer. Recently gas-liquid contactors with ejector-type of gas distributors have been recommended for chemical processes if the interfacial mass transfer is the rate-controlling step of the process. Ejectors are devices which utilize the kinetic energy of a high velocity liquid jet in order to entrain and disperse the gas phase. Investigations on the hydrodynamics and the mass transfer of loop-venturi reactors (LVR1 with liquid driven ejectors have been reported by several authors {Zahradnik et al. (19821, Dutta et al. (19871 and van Dierendonck et al. (19881). Comparison of these literature data is difficult, since different ejector configurations and ranges of gas-liquid flow ratios were applied. Until now only one author studied the mass transfer rates of the ejector and the vessel separately [Dirix and van de Wiele. 19901. They demonstrated that the mass transfer of both the ejector and vessel are influenced by the .flow regime in the ejector. From this it can be concluded that there is a need to study the local hydrodynamics in ejectors, rather than lumping its properties with those of the reactor volume.

The present work presents an experimental study on the influence of the gas/liquid flow ratio on the hydrodynamics and the mass transfer characteristics of both the ejector and the main holding vessel. Besides that, the influence of the initial liquid volume in the vessel was studied on the overall reactor performance.

THEORETICAL CONSIDERATIONS: BASIC PRINCIPLES OF TURBULENT DISPERSION

The structural element through which the initial dispersion of the gas takes PlaCe iS

cts 47:13/14-z 3557

Cl0 Characteristics of loop-venturi reactor 3559

The mass transfer rates in the loop-venturi reactor were studied by using the cobalt catalyzed sulfite oxidation as a model system. Applying air at ambient pressure and sulfite concentrations higher than its critical value, the reaction orders of oxygen, sulfite and cobalt concentration are 2. 0 and 1 respectively (Linek et al., 1981). Since the reaction rate constant is very sensitive to traces of impurities, it is not allowed to adopt kinetic constants from the literature. Therefore the reaction rate constant (kr) was determined for each charge of sulfite solution used in the experiments_ The oxidation rate of the sulfite solution was measured with a standard flat interface stirred cell reactor.

Experimentally it was verified that all interfacial area measurements were carried out under the conditions of so-called fast reaction (Ha > 2). In this regime, the interfacial area can be calculated with Eqs. (3) and (4)

Further, the gas flow both through the ejector and in the main holding vessel iS assumed to be plug flow. A more detailed description of the experimental techniques and procedures used, is given by Cramers (1993).

EXPERIMENTAL RESULTS & DISCUSSION

m regimes in the eiector TWO different flow regimes have been observed in the ejector depending on the gas-liquid flow ratio, i.e. bubble flow and jet flow. If the gas phase dispersion occurs in the mixing tube of the ejector, bubble flow occurs. If on the other hand dispersion takes place in the diffuser or draft tube jet flow is observed. Bubble flow appears if Iow gas-liquid flow ratios are applied. This flow regime is characterized by the formation of very small bubbles in a continuous liquid phase (Fig. 3aI. Under these flow conditions the dispersed section of the holding vessel is relatively small. At higher gas flow rates there is a transition from bubble to jet flow. In the jet flow regime the gas and the liquid phase are ejected straightly through the mixing tube into the diffuser (Fig. 3b) or draft tube, depending on the gas-liquid flow ratio. Visual observations showed that in case the ejector entrains the maximum amount of gas, the mixing zone is located near the outlet of the ejector. In this flow regime the dispersed section in the column is large. The transition from bubble to jet flow is influenced by the liquid velocity through the nozzle as shown in figure 4. It is seen that the flow ratio at the transition point decreases by using higher liquid flow rates. This corresponds well with the data of Dirix and van de Wiele (1990).

DisDersion behavior in the main holding vessel Fig. 2 shows that the holding vessel can be divided into two and the clear liquid zone. In Fig. 5 the clear liquid heights vs. the gas-liquid flow ratios. Experimantally it has been liquid heig-ths in the column . It shows that the clear liquid the column decreases with increasing liquid flow velocities. increasing momentum of the faster two phase jet discharging

zones, i.e. the dispersed in the column are given verified that the clear height at the bottom of This is caused by the

from the ejector. For a constant liquid flow rate, the clear liquid height remains nearly constant up to a certain gas flow rate, where after the clear liquid level rapidly decreases. This sudden change is also caused by the change in flow regime in the ejector. These observations prove that the flow regime in the ejector has a significant effect on the hydrodynamics in the main holding vessel.

Overall averaeed snecif ic interfacial area and eas hold-un of total loop-venturi system reactor

Fig. 6 shows the the influence of the gas flow rate on the overall specific interfacial area and gas hold-up of the system as a whole as function of the liquid flow rate. It is seen that a increases with both the gas flow rate and the liquid

flow rate. At the lower gas gow rates the gas hold-ups and specific interfacial areas are almost proportional to the superficial gas velocity. For the higher liquid flow

3560 P. H. M. R. CRAM~RS et al. Cl0

rates this linear dependency vanishes rather abruptly, caused by the change in flow regime as mentioned in the previous sections.

Further it is seen from Fig. 6 that the initial diplength of the ejector has no systematic influence on either the overall gas hold-up and a ov Visual observations

indicated the the bubbles discharging from the ejector into the vessel did not coalescence (the sulfite solution is a coalescence inhibited system 1. The gas and liquid formed a stable dispersion; a swarm of spherical bubbles of a steady size. Therefore it can be concluded that the bubble swarm velocity remains nearly constant. The latter suggest that under the homogeneous bubble flow conditions the gas hold-up in the dispersed section is proportional to cis swarm; independently of the . dispersion height (as will be shown in the next sections).

Svecif ic interfacial a h m e iector The variation of the specific interfacial area in the ejector with the jet velocity and the gas-liquid flow ratio is shown in Fig. 7. In this, the total ejector volume is used as reactor volume. Since very small bubbles are formed in the ejector (in the range of 25-60 ~1, it is permissible to disregard the slip between both phases, so that the gas hold-up can be calculated from the gas and liquid throughputs.

Fig. 7 shows that extreme high specific interfacial areas are created in the ejector section. Further is seen that both the jet velocity and the change in flow regime have a significant effect on the specific interfacial area of the ejector. In the region of lower gas flow rates, an increase in gas flow rate results in a larger number of bubbles without appreciably Increasing the bubble diameter, thereby proportionally increasing the interfacial area. In the region of higher gas flow rates, however, due to the formation of larger bubbles and bubble coalescence, the increase of the specific contact area with gas flow rate decreases and eventually the specific contact area even starts to decrease with increasing Q,. The latter effect however is caused

by a decrease in reactor volume (change in flow regime). In fact in this regime the actual reactor volume becomes smaller than the ejector volume, causing the dramatic decrease of the total interfacial area in the ejector and thus is the specific area which is based on the total ejector volume.

Fig. 8 shows that Eq. (21 correlates the experimental data within 10 7. accuracy for C=19500. It is seen that the theoretical value of 0.4 indeed can be used as exponent for the energy dissipation rate, indicating that the flow in the ejector is locally isotropic. This is also in agreement with the results of Nagel (19761. who studied the two-phase pipe flow nozzle which is also an ejector-type of digtri$utog. Further. Nagel (19761 measured also mass transfer areas of the order of 10 m /m using the sulfite oxidation as a model system.

Soecific interfacial area in the main holding vessel The gas hold-up and specific interfacial area of the dispersed section of the holding vessel are shown in Fig. 9. It is seen that the power supplied by the two-phase jet discharging from the ejector has no effect on either Ed and adIs. This implies that

the mass transfer in this section is only influenced by the gas flow rate, confirming that this section behaves like a bubbIe column, were the ejector is used as a special gas distributor.

Further it follows that the dispersed section of the LVR has much higher interfaciai areas (a

dls 1 relative to conventional bubble columns in which gas distributors such aS

sparger rings and perforated plates are used. Therefore the benefits of the ejector are not restricted to a larger rate of mass transfer in the ejector section, but it also generates smaller bubbles in the holding vessel, particularly for non-coalescing systems.

The specific interfacial area of the column (including the clear liquid zone) depends on the clear liquid volume. As mentioned, the clear liquid level decreases using

Cl0 Characteristics of loop-venturi reactor 3561

higher liquid flow rates, resulting in higher acol values. The decrease of the clear

liquid level is attributed to the higher two-phase jet momentum with increasing liquid circulation rate, causing the bubbles to penetrate deeper into the column. For free submerged turbulent jets it is known that the penetration depth of the jet is proportional to ,,t&e discharging velocity (Davies. 19721. For liquid jets this length is equal to (P ,.a * .

Our data empirically correlate as

a co1 =(P 1 (51 Jet

which is shown in Fig. 10. The two-phase jet power is defined as

P = x/8*oL*(1-e let

More existing correlations for a and co contain the energy dissipation per unit time

and per unit reactor volume (kW/rn?. Our results however indicate that this approach is unsuccessful here, since the column characteristics are not influenced by the volume of the holding vessel. Changing the reactor volume by varying the vessel diameter was studied by Dutta et al. (19871. He noted that the mass transfer of a larger tank is more localized near the ejector than in a smatler tank, indicating that the mass transfer rates of the column will be influenced by the vessel geometry. Further, in the literature on looo-venturi reactors with down-f low ejectors, the penetration depth of the bubble -dispersion in the main holding vessel is never mentioned, though this is a very important design and scale-up parameter. Therefore, additional research on the bubble penetration depth and on ejector to vessel geometries is required to learn its influence on the mass transfer characteristics of the column section.

Effective eiector contribution to the overall interfacial area Fig. 11 gives an impression of the magnitude of the effective ejector contribution to the overall specific interfacia1 area created in the LVR. From this figure it is seen that in the bubble flow regime a large part of the overall gas-liquid mass transfer takes place in the ejector. Therefore, in this flow regime it should be considered as a separate reactor. i.e. the loop-venturi reactor should be modeled as two reactors in series.

CONCLUSIONS -.

In ejectors extreme high specific contact areas are created and in case of fast reactions the ejector and holding vessel should be considered as two reactors in series, which require separate modeling. Further it was shown that the flow regime in the ejector strongly influences the mass transfer characteristics of both the ejector and dispersed section of the holding vessel. The high mass transfer capacity of the Loop-venturi reactor makes this reactor particularly suitable for fast reactions, in which the liquid phase mass transfer is the reaction limiting step of the process. In case of downflow ejectors, the clear liquid height in the holding vessel is a very important design parameter.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the technical and financial support from DSM Research (Geleen. Netheriands) and Buss AC (Pratteln, Switzerland). This research would not have been possible without the contributions of graduating students G. Hartholt and C. van de Hek.

REFERENCES

Dirix C.A.M.C. and K. van der Wiele (19901, Mass tranfer in jet loop reactors, Chem.

3562 P. H. M. R. CRAMERS er ol. Cl0

Eng. Ski.. 45(8), pp. 2333-2340 Dierendonck van L-L.. G.W. Meindersma and and G. Leuteritz (19881, Scale up of Gas-Liquid reactions made simple with loop reactors, 6th European Conference on Mixing, 287-295 Dutta N-N. and K-V. Raghavan (19871, Mass Transfer and Hydrodynamic Characteristics of

Loop Reactors with Downflow Liquid Jet Ejector, Chem. Eng. J., 36, Pp. 111-121 Evans (19911, PhD thesis, A study of a Plunging Jet Bubble Column, University of Newcastle, N.S. W. Henzler H. J. (1981). Das Sogverhalten von Strahlsaugern fiir das Stoffsystem: flilssig-gasfbrmig. Vt-Verfahrenstechnik, lS(lO), 738-749 Hesketh R-P., A. W. Etchells and T.W.F. Russel (19871, Bubble Size in Horizontal Pipelines, AIChE Journal, a. 663-667 Levich V. G., Physicochemical Hydrodynamics (1962). Prentice Hail. New York Linek V. and V.Vacek (19811. Chemical Engineering use of Catalyzed Sulfite Oxidation Kinetics, Chem. Eng. Scl., 36, pp.1746 Rylek M. and J. Zahradnik (19871: Chisa, 9th International Congress of Chemical Engineering, Praha, August 31-September 4 Sande van de E. (1976). PhD thesis, Air entrainment by Plunging water Jets,, University of Delft, Netherlands Witte J.H. (19691, Mixing Shocks in two-phase flow, J. fluid Mech., 36(41, pp. 639-655 Zahradnik J.. F. Kastanek, J. Kratochvil and M. Rylek [19821. Hedrodynamic Characteristics of Gas-Liquid beds in Contactors with Ejector type Gas Distributors, Coil. Czech, Chem. COmrfL, 47. pp.1939-1949

NOMENCLATURE

Ii cat C d D E J kr

: [I V We E P

specific interfacial area interfacial area concentration concentration at interface diameter diffusion coefficient power-input per unit volume molar flux reaction rate constant power volumetric flow rate velocity volume Weber-number hold-up density

Q surface tension

m%.m -3

m kmol.mm3 kmol.mm3 m

2 -1 m .s W.mv3 kmol.m-2.s-1 m3. kmol-. s-l

w -1 m3.s

-1 mb m

kg. m3 N.m-

Subscriots:

A oxygen b,j two-phase jet

continuous phase : critical co1 column d dispersed phase D draft tube ej ejector G gas phase jet liquid jet L liquid phase max maximum N nozzle S sauter tot total

Cl0 Characteristics of loop-venturi reactor 3563

Fig. 1 Scheme of the hydrodynamics and pressure distribution across the ejector. (a) nozzle; (b) gas suction chamber; (cl mixing tube; (d) diffuser; (e) draft tube.

i

1

3

(a) (b) Fig. 3 Flow regimes in the ejector:

(1) liquid jet; (2) mixing zone; (3) bubbly flow.

Fig. 5 Clear liquid heights in holding Fig. 7 Specific interfacial area in the vessel. ejector.

Fig. 2 Schematic dia&am ot experlmental set-up.

2.o I 1.6 * Jet flow

Bubble flow

0.0 1. 0 2 4 6 8

QL (ml/h)

Fig. 4 Transition from bubble to jet flow regime in ejector.

100000

80000

~~~~ &

* Qe,=4

20000 - . oU=s

0.0 0.3 0.6 0.9 1.2 1.5

QGQ,

P. H. M. R. CRAMIW et al. Cl0

Fig. 6 - kxrk/s)

Effect of the liquid flow rate, gas flow rate and initial liquid volume on the overall specific interfacial area of the total system; (0: gashold-up: o: VL= 62.4 1; .VZ V = 70 1.; A: VL= 80 1.).

L

a- (m-7

Fig. 8 Predicted a-values vs. the experi- mental a-values.

1500 [

01 0.00 0.02 0.04 0.05 0.06

Fig. lb Specific interfacial are of the total column section (including the clear liquid section).

01 _ caw-4 I 0.0 0.0 0.5 1.0 1.5 2.0 2.5

- kmk.)

Fig. 9 Specific interfacial area and gas hold-up in the dispersed section of the column.

500 -

01 0 1 2 3

- bnLs) Fig. 11 Effective ejector contributio

to the overall interfacial area based on total system volume