Hydrodynamics and mass transfer characteristics of a loop-venturi reactor with a downflow liquid jet ejector

Hydrodynamics and mass transfer characteristics of a loop-venturi reactor with a downflow liquid jet ejector

clrcmical &z~inrcrhg Science. Vol. Printed in Great Britain. 47. No. 13/14. pp. 3557-3564. 1992. oam-2509i92 01992klpunon HYDRODYNAMICS AND MASS ...

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clrcmical &z~inrcrhg Science. Vol. Printed in Great Britain.

47. No.

13/14. pp. 3557-3564.

1992.

oam-2509i92 01992klpunon

HYDRODYNAMICS AND MASS TRANSFER CHARACl-ERISTICS REACTOR WITH A DOWNFLOW LIQUID JET P.H.M.R.

Cramers.

Department “DSM

A.A.C.M.

Beenackers

and

L.L.

of Chemical Engineering, University Netherlands 9747 AG Groningen,

Research,

PO Box

18, 6160 MD

Geleen.

ss.oo+o.oo PressLd

OF A LOOP-VENTURI EJECTOR

van of

Dierendonck” Groningen

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. [email protected] 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. transfer of loop-venturi reactors Investigations on the hydrodynamics and the mass (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 Until now only one author studied the mass of gas-liquid flow ratios were applied. 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 From this it can be concluded that there influenced by the .flow regime in the ejector. 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 the influence of the initial liquid ejector and the main holding vessel. Besides that, volume in the vessel was studied on the overall reactor performance. THEORETICAL CONSIDERATIONS: BASIC PRINCIPLES OF TURBULENT The

structural

cts 47:13/14-z

element

through

which

the

DISPERSION initial

3557

dispersion

of

the

gas

takes

PlaCe

iS

Characteristicsof loop-venturireactor

Cl0

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 0 and 1 respectively (Linek et al., 1981). are 2. it is not Since the reaction rate constant is very sensitive to traces of impurities, Therefore the reaction rate allowed to adopt kinetic constants from the literature. 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 In under the conditions of so-called fast reaction (Ha > 2). interfacial area can be calculated with Eqs. (3) and (4)

Further, the gas flow both through the ejector and assumed to be plug flow. A more detailed description and procedures used, is given by Cramers (1993). EXPERIMENTAL

RESULTS

in of

the the

were carried this regime,

out the

main holding vessel iS experimental techniques

& DISCUSSION

m

regimes in the eiector different flow regimes have been observed in the ejector depending on the If the gas phase dispersion gas-liquid flow ratio, i.e. bubble flow and jet flow. 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 This flow regime is characterized appears if Iow gas-liquid flow ratios are applied. by the formation of very small bubbles in a continuous liquid phase (Fig. 3aI. Under of the holding vessel is relatively small. these flow conditions the dispersed section from bubble to jet flow. In the jet At higher gas flow rates there is a transition straightly through the mixing flow regime the gas and the liquid phase are ejected 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. by the liquid velocity through The transition from bubble to jet flow is influenced transition the nozzle as shown in figure 4. It is seen that the flow ratio at the This corresponds well with the data point decreases by using higher liquid flow rates. of Dirix and van de Wiele (1990).

TWO

DisDersion behavior in the main holding vessel i.e. the dispersed Fig. 2 shows that the holding vessel can be divided into two zones, in the column are given and the clear liquid zone. In Fig. 5 the clear liquid heights that the clear the gas-liquid flow ratios. Experimantally it has been verified vs. liquid heig-ths in the column . It shows that the clear liquid height at the bottom of This is caused by the the column decreases with increasing liquid flow velocities. from the ejector. For a increasing momentum of the faster two phase jet discharging nearly constant up to a constant liquid flow rate, the clear liquid height remains certain gas flow rate, where after the clear liquid level rapidly decreases. This These sudden change is also caused by the change in flow regime in the ejector. observations prove that the flow regime in the ejector has a significant effect on the hydrodynamics in the main holding vessel. loop-venturi total area and eas hold-un of Overall averaeed snecif ic interfacial reactor system 6 shows the the influence of the gas flow rate on the overall specific Fig. of the liquid interfacial area and gas hold-up of the system as a whole as function increases with both the gas flow rate and the liquid flow rate. It is seen that a flow rate. At the lower are almost proportional

gas gow rates the to the superficial

gas gas

hold-ups velocity.

and specific interfacial areas For the higher liquid flow

Cl0

P. H. M. R. CRAM~RSet al.

3560

rates this linear dependency vanishes rather regime as mentioned in the previous sections. Further it is seen influence systematic

from Fig. on either

abruptly,

caused

by

the

change

in

flow

6 that the initial diplength the overall gas hold-up and

of the ejector has no Visual observations a ov’ indicated the the bubbles discharging from the ejector vessel did not the into 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 the dispersed in section is proportional to independently of the “ci’“s .swarm; 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, increase of the 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, is locally indicating that the flow in the ejector 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 using the m /m 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 that this section behaves like a bubbIe column, were the ejector gas distributor. Further it follows that 1 relative to areas (a dls

flow rate, confirming is used as a special

the dispersed section of the LVR has much higher interfaciai conventional bubble columns in which gas distributors such aS

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

of the column As mentioned,

(including the clear liquid zone) depends liquid level decreases using the clear

Characteristicsof loop-venturireactor

Cl0 higher

liquid

flow

rates,

resulting

in

higher

acol

3561

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 a

which

is shown P

More

co1

let

existing

=(P

correlate

as

1 Jet

in Fig.

(51 10. The two-phase

jet

power

is defined

as

= x/8*oL*(1-e 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 the down-f low ejectors, 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 vessel the bubble ejector to depth and on penetration 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 should be considered as two reactors in and holding vessel 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 transfer capacity of the and dispersed section of the holding vessel. The high mass 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 Research (Geleen. Netheriands) and Buss AC (Pratteln, without the contributions would not have been possible Hartholt and C. van de Hek.

financial support from DSM Switzerland). This research students G. of graduating

REFERENCES Dirix

C.A.M.C.

and

K.

van

der

Wiele

(19901,

Mass

tranfer

in

jet

loop

reactors,

Chem.

P. H. M. R. CRAMERS

3562

Cl0

er ol.

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. Stoffsystem: Das fiir das Sogverhalten von Strahlsaugern (1981). flilssig-gasfbrmig. Vt-Verfahrenstechnik, lS(lO), 738-749 Hesketh R-P., Russel (19871, Bubble Size in Horizontal A. W. Etchells and T.W.F. 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 Jets,, water Sande van de E. PhD thesis, Plunging Air entrainment by (1976). University of Delft, Netherlands Mech., 36(41, pp. 639-655 Witte J.H. (19691, Mixing Shocks in two-phase flow, J. fluid Hedrodynamic Zahradnik J.. F. Kastanek, J. Kratochvil and M. Rylek [19821. 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 Q

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 surface tension

-3 m%.m 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 b,j : co1 d D ej G jet L max N S tot

oxygen two-phase jet continuous phase critical column dispersed phase draft tube ejector gas phase liquid jet liquid phase maximum nozzle sauter total

Cl0

Fig.

3563

Characteristics of loop-venturi reactor

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.

Fig.

Schematic set-up.

2

dia&am

ot‘ experlmental

2.o I

i

1.6 *

Jet

flow

1

Bubble

3

0.0

1.

0

Fig. 3

2 QL

(b) (a) Flow regimes in the ejector: (1) liquid jet; (2) mixing zone; (3) bubbly flow.

Fig.

4

flow

4

6

8

(ml/h)

Transition from bubble flow regime in ejector.

100000 ~~~~

to

jet

&

80000

20000

0.0

0.3

*

Qe,=4

.

oU=s

0.9

1.2

area

in the

0.6 QG’Q,

Fig.

5

Clear liquid heights vessel.

in holding

Fig.

7

Specific ejector.

interfacial

1.5

P. H. M. R. CRAMIW

kxrk/s)

“-

Fig.

6

Cl0

et al.

Effect of the liquid flow rate, gas flow rate and initial liquid 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.).

volume

on

L

01

0.0

aFig.

8

1500

(m-7

Predicted a-values mental a-values.

vs.

the

0.5

Fig.

9

I

2.5

0.0

[

01 0.00

0.02

0.04

0.05

-

01

0.06

0

1

“lb

1.5 2.0

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

500

Fig.

‘caw-4

kmk.)

“-

experi-

_

1.0

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

Fig.

11

2

bnLs)

Effective ejector contributio to the overall interfacial area based on total system volume

3