Hydrodynamics and mass transfer in an impinging-stream loop reactor aerated from the top

Hydrodynamics and mass transfer in an impinging-stream loop reactor aerated from the top

Chemical Engineering and Processing, 33 (1994) 285-289 ELSEVIER Hydrodynamics and mass transfer in an impinging-stream reactor aerated from the top...

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Chemical Engineering and Processing, 33 (1994) 285-289

ELSEVIER

Hydrodynamics

and mass transfer in an impinging-stream reactor aerated from the top

loop

E. S. Gaddis Institut ftir Thermische

Dedicated

Verfahrenstechnik,

to ProjI Dr.-kg.

Technische

Universitiit

Dr. h.c.lINPL

ClausthaI, LeibnizstraJe

E.-U. Schhkder

15, 38678 Clausthal-Zellerfeld,

on the occasion

Germany

of his 65th birth&y

Abstract In this investigation, experimental results related to the hydrodynamics and mass-transfer performance of an impinging-stream loop reactor, in which the nozzles are placed at the top of the reactor, are presented and discussed. The experimental results cover the range over which the gas is sucked by means of the nozzles without the use of a compressor. Key words:

Hydrodynamics; Mass transfer; Impinging-stream loop reactor

1. Introduction It has been shown in earlier publications [ 1- lo] that the impinging-stream loop reactor is a very efficient device for carrying out mass-transfer operations between a liquid and a gaseous phase. This reactor type is thus very appropriate for liquid aeration as well as for chemical and biochemical reactions, particularly when such reactions are limited by the process of mass transfer. Two variants of this reactor have been presented in the above publications. In one of the variants, the two nozzles are placed at the top of the reactor; in the second variant they are placed at the bottom [3,7]. Mass-transfer measurements have shown that the variant with nozzles placed at the bottom of the reactor has, in general, a higher mass-transfer rate and thus requires a lower power compared with the variant with nozzles placed at the top. Nevertheless, the variant with nozzles placed at the top of the reactor has some advantages over the other variant. These are: (i) the required gas pressure is much lower; (ii) this variant can operate in a self-sucking manner and hence the installation of a compressor may thus be unnecessary; and (iii) easy access to the nozzles for maintenance purposes. In certain cases these advantages may be more important than the reduced power consumption associated with the variant with nozzles placed at the bottom. Moreover, under certain flow conditions, this variant 025%2701/94/$7.00 0 1994 SSDIO255-2701(94)01009-C

Elsevier Science S.A. All rights reserved

may have a mass-transfer performance which approaches that of the variant with nozzles placed at the bottom [3, 71. In this investigation, the hydrodynamics and the mass-transfer performance of an impingingstream loop reactor with nozzles placed at the top have been examined. The results reported in this paper were obtained under self-sucking conditions.

2. Experimental equipment and procedure

The reactor examined is shown in Fig. 1. It had an effective volume of between 11 .O and 26.7 1 depending on the level of the two-phase mixture inside the separation vessel at the top of the reactor. The main dimensions of the reactor (unless otherwise stated) were: internal diameter of the main reactor tube, 110 mm; internal diameter of the guide tubes, 50 mm. To measure the liquid side volumetric mass-transfer coefficient, the steady-state physical method with oxygen absorption was used. Water with a low oxygen concentration was fed to the reactor nozzles. Ambient air flowed through the aerating tubes (placed concentrically inside the nozzles) and water flowed through the annular areas confined between the internal surfaces of the nozzles and the external surfaces of the aerating tubes. The vacuum created in the water jets at the outlet of the nozzles caused air to be sucked inside the reactor.

E. S. Gaddis / Chemical Engineering and Processing 33 (1994) 285-289

286

caloric

Fig.

1. Impinging-stream

flow

from the bottom of the reactor by a pump, mixed with the feed and then recirculated inside the nozzles (Fig. 2). To measure the air flow rate in the present experiments, a caloric flow meter was inserted in the main air tube at the reactor inlet. This flow meter has the advantage that it does not disturb the velocity profile of the flowing medium and it does not cause any resistance to the air flow. A description of this device is given elsewhere [ 11, 121. The water leaving the reactor at high oxygen concentration was fed to another aerating device, where oxygen was desorbed to a greater extent by means of a nitrogen stream. The water flow leaving that aerating device with a low oxygen concentration, was then re-fed to the examined impinging-stream loop reactor.

meter

loop reactor

examined.

Shear forces acting on the air stream at the outlet of the aerating tubes resulted in the dispersion of the gaseous phase in the form of very small bubbles. After circulating inside the reactor, the air sucked by the nozzles left the reactor at the top. Separation of the liquid and the gaseous phases occurred inside the separation vessel placed at the top of the reactor. The level of the two-phase mixture was adjusted inside the separation vessel by means of a weir of variable height between 25 and 180 mm. The water outlet was placed at the bottom of the reactor; this mode of operation is similar to that used in laboratory and pilot plants based on the principle of the impinging-stream loop reactor and applied in the field of biological wastewater treatment. In such applications, the liquid feed is usually very small in order to create a high effective impinging velocity inside the impinging zone. Liquid is sucked -__-_~--_c-

gas inlet

pump

Fig. 2. Impinging-stream (aeration of the reactor

loop reactor with external liquid circulation takes place at the top).

3. Experimental results Since the main resistance to mass transfer lies in the water film at the interface between the liquid and the gaseous phase, the liquid-side mass-transfer coefficient and the overall mass-transfer coefficient are effectively equal. The liquid-side volumetric mass-transfer coefficient, &,a), was calculated from the following equation:

In Eq. (l), Vu is the effective reactor volume, pi_ is the water Bow rate and ci and c, are the oxygen concentrations in the water at the reactor inlet and the reactor outlet, respectively. Because of the high recirculation rate of the liquid and the gaseous phase, and because of the associated high turbulence level, ideal mixing may be assumed inside the reactor. The mean concentration difference AC can then be calculated from the equation Ac=c”-c

(2)

where c* is the mean oxygen saturation concentration at the air-water interface and c is the mean oxygen concentration in the water inside the reactor. For the calculation of c*, its value was measured at the top and bottom of the reactor while the reactor was aerated. The arithmetic mean was then calculated and corrected to take into consideration the oxygen depletion in the air due to oxygen absorption in the water. The oxygen concentration (c) in the water was also measured at the top and bottom of the reactor and the arithmetic mean calculated. The gas holdup was calculated from the measured displaced water volume. Figure 3 shows the dependence of the liquid-side volumetric mass-transfer coefficient @,a), on the power dissipation per unit effective reactor volume (P/ V,) for two different weir heights of 180 and 25 mm, respectively. For both weir heights, the k,a value in-

287

E. S. Gaddis / Chemical Engineering and Processing 33 (1994) 285-289

gas t

llouid

gas1

liquid



Fig. 3. Plots of @,a),

versus (P/V,)

for two different weir heights.

creases with increasing power dissipation (P), However, the k,a value is much higher with the weir of smaller height. The reason for the different mass-transfer performance of the reactor in the two examined cases becomes clear from Fig. 4, in which the gas holdup (co) is shown as a function of (P/V,> for both cases. It is seen from this figure that the gas holdup depends greatly on the weir height, with the weir with the smaller height leading to a much higher gas holdup. Since the interfacial area (A) between the liquid and the gaseous phase [and thus the interfacial area per unit effective reactor volume (u)] depends largely on the gas

h, = 25 mm

100

IO' iP/VRi/(

Fig. 4. Plots of Ed versus (P/V,)

A.

kWim3) for two different weir heights.

Fig. 5. Separation

vessel at the top of the reactor.

holdup, the weir with the smaller height leads to a much higher volumetric mass-transfer coefficient. Figure 5 shows in a schematic form the two-phase flow in both cases. At a large weir height [Fig. 5(a)], the nozzles are completely submerged in the liquid. The gas bubbles at the outlet of the upper part of the main reactor tube rise due to buoyancy forces and escape from the liquid-gas mixture. Separation of the phases is thus nearly complete in this case and gas circulation is reduced to a minimum. Through their outer surfaces the nozzles suck liquid which is almost free from gas bubbles, and place it back in circulation. The gas holdup associated with this flow pattern is very low. At small weir height [Fig. 5(b)] separation of the phases is also nearly complete as in the previous case. However, the nozzles are not submerged in the water. The two jets at the outlet of the nozzles now suck gas from the separation vessel through their outer surfaces and place it back in circulation through the guide tubes. The gas circulating through the reactor and the gas holdup are thus much higher than in the case of a large weir height. Figure 4 also shows that in the range examined the gas holdup with the smaller weir height is not very sensitive to power dissipation in the liquid but is linearly dependent on power dissipation with the larger weir height. In Fig. 6, the dependence of the air flow rate (v,) sucked by the nozzles on the power dissipation in the

E. S. Gaddis 1 Chemical Engineering and

288

Processing

33 (1994) 285-289

IO'

IO.”

100

IO' ( PIVRIl(

Fig. 6. Plots of r’G versus (P/V,)

IO-'-

lo2 h,/

Fig. 8. Dependence

of @,a),

mm

and vG on the weir height.

kWlm3)

for two different weir heights.

10-l

IO0 PI

kW

Fig. 7. Plots of vG versus P for two different weir heights

liquid per unit effective reactor volume (P/ V,) is shown for both examined weir heights. Since the effective reactor volume (V,) changes considerably with weir height, and since the gas flow rate (r’,) is not related to the reactor volume, To is plotted once more in Fig. 7 against the dissipated power (P). Figure 7 shows that the behaviour of the nozzles depends largely on the height of the two-phase mixture inside the separation vessel. The air sucked by the nozzles is not very sensitive to the power dissipated in the liquid when the level of the two-phase flow in the separation vessel is small (h, = 25 mm). On the other hand, vG depends to a larger extent on P when the level of the two-phase flow is increased (h, = 180). The higher rate of increase of pG with P in the latter case may be partly due to a reduction in the hydrostatic pressure in the liquid at the outlet of the nozzles as a result of the increase in the gas holdup in the separation vessel (Fig. 4), as well as to the increase in the upward

velocity of the two-phase flow at the outlet of the main reactor tube as power dissipation in the liquid is increased. Figure 7 also shows that the dependence of po on h, may differ greatly at different power dissipation levels. While vG may decrease with an increase in h, at low power dissipation in the liquid, it may remain virtually unchanged or even increase with an increase in h, at higher power dissipation. Figure 8 shows the dependence of &,a), and vG on the weir height (h,) in the separation vessel for a power dissipation in the liquid of 0.22 kW. The measurements illustrated in Fig. 8 were taken with a guide tube diameter of 30 mm. The above results show that the magnitude of the liquid-side volumetric mass-transfer coefficient obtained at a low level of two-phase mixture in the separation vessel when compared with that attained at high level of the mixture cannot be attributed to an increase in the flow rate of air sucked by the nozzles. At high power dissipation, the flow rate of air may not change or may even increase with an increase in the level of the twophase mixture; however, the k,a value still decreases with increasing h,. This suggests that the high k,a value attained at low level of the two-phase mixture in the separation vessel is primarily due to the high recirculation rate of the gaseous phase and the associated high gas holdup. The aerating tubes thus play a minor role in the mass-transfer process and may be removed without affecting the mass-transfer rate. In that case, to prevent depletion of oxygen in the separation vessel the cover of the vessel should be removed or the vessel should be ventilated with fresh air, allowing the exhausted air to leave the separation vessel through adequate holes.

4. Conclusions Irrespective of the fact that the variant of the impinging-stream loop reactor in which the nozzles are placed

E. S. Gaddis / Chemical

Engineering

at the top of the reactor has a lower mass-transfer performance compared with the other variant in which the nozzles are placed at the bottom, this variant still has some merits which may justify its use in certain circumstances. The differences between the mass-transfer performance of the two mentioned variants can be greatly reduced by adjusting the height of the twophase mixture inside the separation vessel at the top of the reactor to a low level by reducing the height of the weir inside that vessel.

and Processing

inac-

[l] E. S. Gaddis and A. Vogelpohl,

[Z]

[3]

[4]

[5]

Nomenclature A a C

Co

c*

AC &v W&

&G

interfacial area between the liquid and the gaseous phase = A / V,, interfacial area per unit effective reactor volume mean oxygen concentration in the water inside the reactor oxygen concentration in the water at the reactor inlet oxygen concentration in the water at the reactor outlet mean oxygen saturation concentration at the liquid-gas interface =C *_ c, mean oxygen concentration difference weir height liquid-side volumetric mass-transfer coefficient power dissipation inside the reactor volumetric gas flow rate volumetric liquid flow rate effective reactor volume gas holdup

289

References

Acknowledgement

Financial support from the ‘Arbeitsgemeinschaft dustrieller Forschungsvereinignngen’ is gratefully knowledged.

33 (1994) 2X5-289

[6]

[7]

[8]

[9]

[IO]

[ 111 [ 121

Der Prallstrahlreaktor: Ein Hochleistungsreaktor fiir Stoffaustauschlimitierte Reaktionen, Dechema Jahrestagung, Frankfurt/Main, Germany, May/June 1990, Programm und Vortrlge in Kurzfonn 216-217. E. S. Gaddis, D. Si-Salah and A. Vogelpohl, Hydrodynamics and mass transfer in a newly developed high performance impinging-stream loop reactor, 4th World Congr. Chem. Eng. STRATEGIES 2000, Karlsruhe, Germany, 16 21 June 1991, Preprints I, 3.6-13. E. S. Gaddis, D. Si-Salah and A. Vogelpohl, The impingingstream loop reactor: a newly developed reactor with a high mass transfer performance, Proc. 4th Int. Symp. Transport Phenomena in Heat and Mass Transfer, Sydney, Australia, 14 19 July 1991; Elsevier, Amsterdam, 1992, Vol. 2, pp. 1466-1476. E. S. Gaddis and A. Vogelpohl, Zur Hydrodynamik und rum Stoffaustausch in einem weiterentwickelten Hochleistungs-Prallstrahlreaktor, Jahrestrrflen der Verfahrensingenieurr, Kiiln, Germany, 25527 September 1991, Kurzfassung der Vortrage und Poster, 458-461. E. S. Gaddis and A. Vogelpohl, The impinging-stream loop reactor: a newly developed high performance reactor for mass transfer controlled chemical reactions, 2nd Japanese/German Symp. Bubble Columns, Kyoto, Japan, November 1991, Preprints 165-170. E. S. Gaddis and A. Vogelpohl, Der Prallstrahlreaktor: Ein neuartiger Schlaufenreaktor mit einer sehr hohen Stoffaustauschleistung, Chem-Zng.-Tech., 63 (1991) 1256-1257. E. S. Gaddis and A. Vogelpohl, Zur Hydrodynamik und rum Stoffaustausch in einem neuentwickelten Hochleistungs-Prallstrahlreaktor, Chem.-Zng.-Tech., 64 (1992) 185-187. E. S. Gaddis and A. Vogelpohl, The impinging-stream reactor: a high performance loop reactor for mass transfer controlled chemical reactions. 12th Int. Symp. Chem. Eng., Chemical Reaction Engineering Today, Turin, Italy, 28 June-l July 1992; Chem. Eng. Sci., 47 (1992) 2877-2882. E. S. Gaddis and A. Vogelpohl, Der Prallstrahlreaktor: Ein Schlaufenreaktor mit sehr hoher Stoffaustauschleistung, Mitteilungsbl. TU Clausthal, 75 (1993) (ISBN 0344-661 l), 46647. E. S. Gaddis, B. Genenger and A. Vogelpohl, Residence time distribution in an impinging-stream loop reactor, Prepr. 3rd German/Japanese Symp., Schwerte, Germany, 13-15 June, 1994, pp. 2377242. J. Hiiltje, Entwicklung eines GasdurchfluDmeBgerltes mit geringem Druckverlust. Studienarbeit, Institut fur Therm&he Verfahrenstechnik der Technischen Universitat Clausthal, 1986. B. Genenger, Messung und Korrelation des angesaugten Gasvolumenstroms und des optimalen Diisenabstands eines im Ejektorbetrieb arbeitenden Schlaufenreaktors, Diplomurbeit, Thermische Verfahrenstechnik der Technischen Universitat Clausthal, 1988.