Enhancement of gas-liquid mass transfer in a modified reversed flow jet loop reactor with three-phase system

S

eSes Pergamon

Chemical En#ineerin 0 Science, Vol. 50, No. 18, pp. 2997-3000, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0009-2509/95 $9.50 + 0.00

0009-2509(95)00149-2

Enhancement of gas-liquid mass transfer in a modified reversed flow jet loop reactor with three-phase system (Received 16 February 1994)

INTRODUCTION Different types of loop reactors are extensively being used in recent years in fermentation, biotechnology and wastewater treatment processes because of their advantages such as simple construction and operation, low investment and operational costs, definitely directed circulation flow, very fine gas dispersion, high mixing and mass transfer performance and relatively lower power requirements. In jet loop reactors (JLR), the liquid circulation and fine gas dispersion are achieved with hydrodynamic jet flow drive. The previous investigations (Blenke, 1979; Warnecke et al., 1988; Tebel and Zehner, 1989) on these reactors were carried out with a concentric draft tube and a two-fluid nozzle at the bottom of the reactor. This type of arrangement has been found to be disadvantageous when the reactor is used as a slurry reactor, due to the blockage of the nozzle, and in processes involving sparingly soluble gases due to the lower residence time of the gaseous phase (Kulkarni et al., 1983; Wachsmann et al., 1985). An improved design ofJLR (Wachsmann et al., 1985; Padmavathi and Remananda Rao, 1991; Velan and Ramanujam, 1991) with the two-fluid nozzle at the top of the reactor not only eliminated the blockage of the nozzle but also increased the residence time of the gas in the reactor as the gas bubbles were forced to move against buoyancy. However, in the above investigations the outlet of the liquid was provided at the bottom of the reactor where, only a partial recirculation of liquid takes place as the major part of the liquid leaves the reactor without being circulated into the annulus, resulting in a lower residence time of the liquid phase. With the liquid outlet at the top section of the reactor, the entire liquid entering the reactor should travel down the draft tube and leaves through the annulus at the top, thereby increasing the residence time of the liquid in the reactor which can enhance the gas holdup as well as the rate of mass transfer. A study of the fundamental hydrodynamic characteristics in gas-liquid-solid systems in a reversed flow JLR has been made by Padmavathi and Remananda Rao (1991) and of gas-liquid systems for both Newtonian and non-Newtonian fluids by Velan and Ramanujam (1991, 1992a), but only a little work has been reported on gas-liquid mass transfer (Wachsmann et al., 1985; Velan and Ramanujam, 1992b) with a two-phase system, and no work has been reported with a three-phase system so far. With the recent development of biotechnology and the extensive use of this type of reactors in fermentation and wastewater treatment, studies on a three-phase system are of significant importance. Particles for use in bioreactors have enzymes or microorganisms immobilized on them and serve as support particles. The densities of the particles normally are of the order of 1000-1300 kg/m 3 and diameter of 1-3 mm. The effect of such low-density particles on the hydrodynamics and mass trans-

fer characteristics has been reported in the literature on three-phase fluidized bed reactors (Sun et al., 1988; Miyahara et al., 1993) and on bubble column reactors (Koide et al., 1992). In the present investigation experiments were being performed to obtain the overall volumetric mass transfer coefficient, KLa in the modified gas-liquid-solid reversed flow jet loop reactor. Effect of solid particles on KLa has been studied and the results were compared with those in the other types of reactors reported in the literature. EXPERIMENTAL A schematic diagram of the experimental set-up is shown in Fig. 1. The apparatus was made of a perspex tube with 0.192 m i.d. and 1.4 m high. Five draft tubes of varying diameters and 1.0 m in length were used one at a time fixed coaxially inside the main reactor. The liquid was withdrawn continuously from the outlet provided at 1.2 m height from the bottom of the reactor and circulated back to the reactor by means of liquid circulation pump from a storage tank via Air inlet Liquid

Vent JL

__

/Liquid

~\

Iii

inlet

outlet

Probe I ~

fR

I

I---0T

I

Recorder

,,ds LI0u,d

2 ° "ed

drain

Pu Sf0raga tank

Fig. 1. Schematic diagram of the experimental set-up: R--reactor; DT--draft tube; TN--two-fluid nozzle; CB~conical bottom; DO--dissolved oxygen; RM--rotameter; SRG--solids retaining grid; V--control value; PG--pressure gauge.

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a calibrated flow meter and a two-fluid nozzle. The gas (compressed air), was metered through a rotameter calibrated for air at atmospheric pressure and ambient temperature, and fed through an aeration tube of 2.75 × 10-3 m i.d., fixed coaxially at the center of the two-fluid nozzle. A solidsretaining grid made of stainless steel wire mesh of 2 m m openings was fixed just below the outlet to prevent the solid particles from getting out of the reactor. The bottom of the reactor consisted of a cone placed at the center of a hemisphere as shown in Fig. 1. This design minimized the dead space and energy losses due to direction reversal. MATERIALS A N D M E T H O D S

The dimensions of the reactor and the ranges of the 'variables are shown in Table 1. Spherical polyurethane beads, cylindrical polystyrene particles and two different sizes of glass beads were used as the solid media. The physical properties of the solid particles are given in Table 2. Transient gassing-in (gassing-out) method was used for the determination of KLa. A batch of liquid of volume nearly equal to that of the reactor was deaerated to approximately zero oxygen concentration by rapid addition of 300 g/m 3 of sodium sulphite ( N a z S O a . 7 H z O ) together with 2 g/m 3 of cobaltous chloride (COC12 • 6 H 2 0 ) for a few minutes (Medic et al., 1989). Air flow to the reactor was started at a pre-set value. The time-change in dissolved oxygen concentration was monitored by a dissolved oxygen electrode (YSI 5739 with standard membrane) connected to a YSI model 58 dissolved oxygen meter. The assumptions of constant gasphase composition and well-mixed liquid phase were made

in the evaluation of KLa (Verlaan and Tramper, 1987). The influence of electrode dynamics on KLa was neglected as the time constant of the probe, zE is less than 10 s and the condition KLa ~ 1~rE was fulfilled (Nakanoh and Yoshida, 1983; Medic et al., 1989), KLa value for each run was obtained from the slope of the straight line in the plot ln[(C* - Co)/(C* - CL)] VS t (Benefield and Randall, 1980). Tap water was used in all the experiments. Six experimental runs were carried out with one liquid batch and the last run was always a repetition of the first run and the results were found to be reproducible within 10% experimental error. All the experimental runs were carried out at room temperature of 26 + 2°C and the KLa value at 20°C was calculated using the following general formula (Benefield and Randall, 1980).

(KLa)roc = (KLa)2oocl.O20(T-20(

RESULTS AND DISCUSSION The results of the study Of KLa in a modified reversed flow jet loop reactor for the three-phase system were presented on a comparative basis with those in bubble columns and other types of reactors. The variation of KLa as a function of gas flow rate for different solids loading is shown in Fig. 2. From the figure, it was observed that the KLa values increased with the increase in the gas flow rate according to the following equation:

KLa = ~U~s~

(2)

where ~ = 0.02; fl = 0.6 when KLa is in s - t and Use in m/s, which is similar to those reported in the literature by many

Table 1. Dimensions of the reactor and the ranges of variables Reactor Inner diameter Overall length Dispersion height

D (m) H (m) Ho (m)

Draft tube Inner diameter

DE (m)

0.192 1.4 1.2

0.064 0.074 0.084 0.094 0.112

(DE/D = 0.33) (De/D = 0.39) (DJD = 0.44) (DJD = 0.49) (DE/D = 0.58)

Length Bottom clearance

LE (m) Hn (m)

1.0 7.0 x 10 -2

Nozzle Inner diameter Immersion depth

Dz (m) HT (m)

1.2 x 10 -2 0.2

Aeration tube Inner diameter Outer diameter

Dol (m) DGo (m)

2.75 x 10 -a 3.0 × 10- 3

Qo (m3/h)

0.3-2.5 2.5-5.5 0.3-1.5 (0.0083~).042)

Gas flow rate Liquid flow rate Solids loading

(1)

QL (m3/h) Vs x 103 (m 3)

(es)

Table 2. Physical properties of the solid particles

Material

Average diameter (m) x 103

Average length (m) x 103

Density (kg/m 3)

Polystyrene cylinders Polyurethane beads Glass beads Glass beads

2.4 3.79 2.95 4.97

4.2 ----

1050 1100 2500 2900

2999

Shorter Communications other investigators for various types of reactors. However, the magnitude of the values of ~t and fl varied from reactor to reactor and was reported that they were dependent mostly on the reactor geometry and on the operating conditions. The presence of solids enhanced KLa by 10-20% at low concentrations as reported by Charinpanitkul et al. 0993) in three-phase reactors and by Smith and Skidmore (1990) in airlift loop reactors. An optimum value of solids loading 0.9 x 10-a m 3 (corresponding solids volume fraction, es = 0.025) was observed for maximum Kca as shown in Fig. 2. Kca decreased with the further increase in the solids concentration. Similar observation was reported in the literature by Smith and Skidmore (1990) for a concentric tube airlift reactor and the result was explained as a disruption in the liquid-film surrounding the air bubble which reduces the resistance to mass transfer in the presence of solids. At higher solids concentration the blocking effect of the particles dominates, resulting in the reduced interfacial area for mass transfer and hence, effect a reduction in the rate of mass transfer. An increase in Kza in three-phase reactors is mainly attributable to an increase in the value of liquid-film mass transfer coefficient, KL. The large bubbles formed due to bubble coalescence in the presence of solids, become more irregular in shape and oscillate unsteadily, leading to an increase in K L. In addition, large bubbles enhance circulation flow of liquid-phase because of high rise velocity and turbulent motion which also enhances the KLa. Figure 3 shows the comparison of the range of the experimental results on KLa in the present work with those of other investigations in various types of reactors. It is clearly observed that the entire range of KLa values obtained in the present study are comparatively higher than those in stirred tank reactors (Kojima et al., 1987), and in fluidized bed reactors (Sun et al., 1988), in bubble column reactors (Medic et al., 1989; Koide et al., 1992) and in airlift loop reactors (Smith and Skidmore, 1990; Lindert et al., 1992). Figure 4 shows the comparison of the range of the results in the present work with those in the other reversed flow jet loop reactors reported in the literature. It can be seen that the results in the present work are significantly higher when compared to those in similar reactors of Wachsmann et al. (1985) and Velan and Ramanujam (1992b). From Fig. 4, it is I

I

I

3.0--

[ Es ]

0.0 0.3 0.6 0.9 1.2

[0] [0.0086] [0.0168] [0.0253] [0.03371

'~

1.5=0.64

:

I

I

IlllIJ

L~ " 0.1

o_j

System : Air - Water / Air - Water - Solids

1

0.01

Present Work (RFJLR) Lindert ef al., 1992 {ELALR) Koide ef al., 1992 (BCR) Smith & Skidmore, 1990 (ALR) Medic et hi., 1989 (RLBER) Sun et ol., 1988 (FBR) Kojima et al., 1987 (STR)

2 3 4 5 6 7 I

I

t ,tl,,I

0.001

,

I

I

II,ltI

0.01 Superficial

gas

O.1 velocity, Us6(m.s -1)

Fig. 3. Comparison o f the range o f the experimental results

of KLa in the present work with those in other types of reactors.

I

I

I III

I

I

I

I

I

I II

i

System

10--

......

: Air-Water

Wachsmann et al., 1985

Velan and Ramanujam 1992o'Present

'~

work

3-

"~o

"]

~..J L....` ~

0.3 0.3

J I I IIll 0.6 1

Energy

[O.06211~

dissipation

. ---"

I 2 rate,

I 3

I

I I II 5 7

I0

( E / V ) L (kW. m-3)

Fig. 4. Comparison of the range of the experimental results on KLa in the present work with those in other downflow jet loop reactors.

.

-

1.C

I

I

1

J

0. 5

1.0

I. 5

2.0

Gas

I

0.6 -

~_j2£

0

, I I Ill

I

OEID

(E

×

I

1--

V s x 103 (m 3 )

l --

%

I

beads

5.0

4.0

I

I

System:Air-Water-Polyurethane

o [] A • •

10

flow rate, 0'13 (m31hr-1)

Fig. 2. Effect of gas flow rate on Kca at different solids loading.

also observed that the KLa values in the present study are significantly higher in the order of about 2-3 fold when compared to those of Velan and Ramanujam (1992b), who worked in a similar reactor with the liquid outlet at the bottom section of the reactor. This clearly indicates that the modification of the reactor enhances KLa values due to the increased residence time of the liquid which in turn influences an increase in the gas holdup and an increase in the gas-liquid inteffacial area available for mass transfer. CONCLUSIONS Based on the present investigation, the following conclusions can be made.

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(i) Overall volumetric mass transfer coefficient increases with an increase in the gas flow rate. (ii) The presence of solids at low concentrations increases the mass transfer rate. The optimum value of solids loading for maximum value of KLa was found to be 0.9 x 10 -3 m 3 corresponding to a solids volume fraction, es = 0.025. (iii) The KLa values in the modified reversed flow jet loop reactor are higher than those in the other types of reactors such as bubble columns, fluidized beds and airlift reactors. (iv) For the same energy input the KLa values in the modified reversed flow jet loop reactor with the liquid out let at the top section are comparatively higher than those in the other downflow jet loop reactors. (v) The modification of the reactor enhances the KLa values by 2-3 fold. K. YAGNA PRASAD T. K. RAMANUJAM t

Department of Chemical Engineering Indian Institute of Technology Madras 600 036, India A Azf C* Co CL D D~ Doi DGo Dz (E/V)L H Hn

Hr KL KLa LE Q t T Uj Uso V

NOTATION cross-sectional area of the column, m s flow area [ = n/4(D~ - D2o)], m 2 saturation (equilibrium) concentration of oxygen in liquid, mg/1 initial concentration of oxygen in liquid, mg/1 instantaneous concentration of oxygen in liquid, mg/l inner diameter of the column, m inner diameter of the draft tube, m inner diameter of the aeration tube, m outer diameter of the aeration tube, m diameter of the nozzle, m energy dissipation rate per unit reactor volume, [ = (pLAzfU3)/2 liD], kW/m 3 height of the reactor, m bottom clearance (height of the lower edge of the draft tube from the bottom of the reactor), m nozzle immersion depth (height of the upper edge of the draft tube from the tip of the nozzle), m liquid-film mass transfer coefficient, m/s overall volumetric mass transfer coefficient, s-1 length of the draft tube, m volumetric flow rate of the fluid, m3/h time, s temperature, °C linear liquid velocity based on Azs, m/s superficial gas velocity based on A, m/s volume of the fluid, m 3

Greek letters a constant in eq. (2), m-1 fl an exponent in eq. (2), dimensionless e fractional hold-up, dimensionless p mass density, kg/m a z~ first-order electrode time constant, s Subscripts D G L S

dispersed phase gas phase liquid phase solid phase

Abbreviations ALR airlift loop reactor BCR bubble column reactor ELALR external airlift loop reactor FBR fluidized bed reactor i.d. inner diameter

tCorresponding author.

JLR RFJLR RLBCR STR

jet loop reactor reversed flow jet loop reactor rectangular bubble column reactor stirred tank reactor REFERENCES

Benefield, C. R. and Randall, C. W., 1980, Aeration, in: Biological Process Desi#n for Waste Water Treatment, Chap. 5, pp. 281-293. Prentice-Hall, Englewood Cliffs, NJ. Blenke, H., 1979, Loop reactors, in Advances in Biochemical Engineering, 13 (Edited by T. K. Ghose, A. Fiechter and N. Blakebrough), pp. 121-214. Springer, Berlin. Charinpanitkul, T., Tsutsumi, A. and Yoshida, K., 1993, Gas-liquid mass transfer in a three-phase reactor. J. Chem. Engng Japan 26 (4), 440-442. Koide, K., Shibata, K., Ito, H., Kim, S. Y. and Ohtaguchi, K., 1992, Gas holdup and volumetric liquid-phase mass transfer coefficient in a gel particle suspended bubble column with draught tube. J. Chem. Engng Japan 25 (1), 11-16. Kojima, H., Uchida, Y., Ohsawa, T. and Iguchi, A., 1987, Volumetric liquid-phase mass transfer coefficient in gassparged three-phase stirred vessels. J. Chem. Engn# Japan, 20 (1), 104-106. Kulkarni, K., Shah, Y. T. and Schumpe, A., 1983, Hydrodynamics and mass transfer in downflow bubble column. Chem. Engng Commun. 24, 307-311. Lindert, M., Kochbeck, B., Pruss, J., Warnecke, H. J. and Hempel, D. C., 1992, Scale up of airlift loop bioreactors based on modelling the oxygen mass transfer. Chem. Engng Sci. 47 (11), 2281-2286. Medic, L., Cehovin, A., Coloini, T. and Pavko, A., 1989, Volumetric gas-liquid mass transfer coefficients in a rectangular bubble column with a rubber aeration pad. Chem. Enong J. 41, B51-B54. Miyahara, T., Lee, M.-S. and Takahashi, T., 1993. Mass transfer characteristics of a three-phase fluidized bed containing low density and/or small particles. Int. Chem. Engng 33 (4), 680~86. Nakanoh, M. and Yoshida, F., 1983, Transient characteristics of oxygen probes and determination of KLa. Biotechnol. Bioengng 25, 1653-1654. Padmavathi, G. and Remananda Rao, K., 1991, Hydrodynamics of reversed flow jet loop reactor as a gas-liquid-solid contactor. Chem. Engng Sci. 46 (12), 3293-3296. Smith, B. C. and Skidmore, D. R., 1990, Mass transfer phenomena in an airlift reactor: Effects of solids loading and temperature. Biotechnol. Bioengng 35, 483-491. Sun, Y., Nozawa, T. and Furusaki, S., 1988, Gas holdup and volumetric oxygen transfer coefficient in three-phase fluidized bed bioreactor. J. Chem. Engng Japan 21 (1) 15-20. Tebel, K. H. and Zehner, P., 1989, Fluid dynamic description of jet loop reactors in multiphase operations. Chem. Engng Technol. 12, 274-280. Velan, M. and Ramanujam, T. K., 1991, Hydrodynamics in downflow jet loop reactor. Can. J. Chem. Enono 69, 1257-1261. Velan, M. and Ramanujam, T. K., 1992a, Hydrodynamics and mixing in downflow jet loop bioreactor with a nonNewtonian fluid. Bioprocess Enon9 7, 193-197. Velan, M. and Ramanujam, T. K., 1992b, Gas-liquid mass transfer in a downflow jet loop reactor. Chem. Enon9 Sci. 47 (9-11), 2871-2876. Verlaan, P. and Tramper, J., 1987, Hydrodynamics, axial dispersion and gas-liquid mass transfer in an airlift loop bioreactor with three-phase flow, in Bioreactors and Biotransformations (Edited by G. W. Moody and P. B. Baker), pp. 363-373. Elsevier, London. Wachsmann, U., Rabiger, N. and Vogelpohl, A., 1985, Effect of geometry on hydrodynamics and mass transfer in the compact reactor. Ger. Chem. Enono 8, 411-418. Warnecke, H. J., Geisendorfer, M. and Hempel, D. C., 1988, Mass transfer behaviour of gas-liquid jet loop reactors. Chem. Engng Technol. 11, 306-311.