BIOTECHNOLOGY TECHNIQUES Volume 9 No.12 (Dec.1995) p.853-858 Received 2nd October.

INFLUENCE OF THE FLOW REGIME ON THE EFFICIENCY OF A GAS-LIQUID TWO-PHASE MEDIUM FTLTRATION

M. Mercier, C. Fonade and C. Lafforgue-Delorme”

Institut National des Sciences Appliquees, Centre de Bioingenierie G. Durand (UK4 CNRS 544, LA INRA), Complexe Scientifique de Rangueil, F3 1077 TOULOUSE cedex, France

* Author to whom correspondence should be addressed.

ABSTRACT An easy technique, consisting in injecting air into the liquid stream, is proposed to enhance the permeate frux in crossjlow @ration of a model jluid (i.e a bentonite suspension). The injected air promotes turbulence and increases the superjfcial crossjlow velocity that leads to a regular disturbance ofthe boundav layer. A systematic study of‘dif erent two-phase configurations points up that the slugjlow seems the most approprinte regime. The resulting permeate rate is increased up to 140’96, in comparison with the usualJiltration processes.

LNTRODLJCTION Crossflow filtration plays an important role in biochemical, biotechnological and food

industries for cell harvesting or recycling and enzymes recovery. The most serious limitation to

the wider utilization of this technique lies in the rapid flux decline combined with the membrane

fouling (VAN DEN BERG and SMOLDERS, 1988). Promoting turbulence at the membrane

surface can improve the performances of the filtration systems and consequently, several

different techniques have been developped, including pulsating flows (BEN AMAR et

al., 1987), turbulence promoters (FINNIGAN and HOWELL, 1990) and intermittent jets

(ARROYO and FONADE, 1993). However, their designs are quite complex.

The creation of a gas-liquid two-phase flow at the membrane surface by air sparging in the feed

stream increases both turbulence. within the system and superficial crossflow velocity of the

process fluid, leading to a regular disturbance of the concentration layer over the membrane.

Consequently, a high and stable permeate flux can be maintained.

This new type of unsteadiness has been recently investigated. LEE et al. (1993) used air slugs

entrapped in crossflow stream to prevent the flux declining during filtration of bacterial cell

suspensions. They were able to increase filtration flux up to 100% with an ultrafiltration

membrane (MWCO 300 kDa) and 30% with a 0.2 pm microfiltration membrane. Applying this

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technique to the ultrafiltration of several test solutions, CUI et al. (1994) have reported a

maximum enhancement of 60% for undyed 162 kDa dextran, 113% for 162 kDa dyed dextran

and 91% for BSA solutions. But these works do not pay particular attention to the

hydrodynamic conditions in the membrane.

The present study particularly investigates the influence of the kind of gas-liquid two-phase

flow on the efficiency of gas phase in improving permeate flux, using a tubular ultrafiltration

membrane and a model fluid (a bentonite suspension).

MATERIAL AND METHODS Membrane and solution The experiments were carried out using an ultrafiltration axisymmetric zirconia-coated alumina membrane : 15 mm internal diameter, 750 mm length (SCT France). This tubular membrane had a.& average porosity of 20 nm with an effective filtration surface of 0.0353 m*. The membrane was cleaned prior to each experiment using the backflush method with ultrapure water. Before each filtration operation, pure water permeability experiments were carried out to assess the cleanliness of the membrane. JO corresponded to the stable value of the permeate flux during pure water filtration. All the results will be expressed in relative flux terms : JR=J/J~. Bentonite has been widely used to characterize membranes and membrane processes. All experiments in this study were carried out using a bentonite suspension (1 g/l) in order to investigate the cake filtration behaviour.

Filtration system The set-up used for these experiments is shown schematically in figure 1.

Heat exchange1

Gas

Computer

Gas rotameter

Liquid flowmeter Pump unit

Y Gas tlowmeter

#in Fig. I : Experimental set-up

The system consisted of an airtight tank of 100 1, a filtration module, a visualization system and a centrigugal pump (FLYGT Clerinox, 2CEX 80/l 1) used for liquid recirculation. A heat exchanger allowed to maintain a constant temperature of 25°C.

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The visualization system was composed of a glass pipe with the same dimensions as the membrane unit and a glass box, filled with distilled water to avoid the distortion of the optical image. This system allowed to visualize the gas-liquid two-phase flow and to set the operative conditions before beginning the filtration experiment. A preliminary study using a high-speed photographic technique was carried out in order to determine the kind of air-water flow (bubble, dispersed bubbles, slug or chum) according to the gas and liquid flow rates. The compressed air (3.8 bars) was injected into the liquid stream through a porous plate. Two pressure gauges with pressure range O-6 bars were located upstream and downstream of the module. The flow rates and pressures were adjusted prior to each filtration experiment using in-line valves. The gas flow rate was estimated at the reservoir exit with a gas rotameter.

Proceahre and measurements A range of liquid flow rates up to 0.4 m3/h and gas flow rates between 0.25 and 0.5 m3/h were examined. The transmembrane pressure TMP determined according to TMP=(Pu + Pn)/2 (Pu : upstream pressure and P n : downstream pressure) was fixed to 1 bar. The recovered permeate was continually weighted using a PC interfaced electronic balance (SM?ORIUS) and recycled into the feed vessel in order to maintain a constant feed concentration.

RESULTS AND DISCUSSION Flow description

When gas-liquid mixtures flow upwards in a vertical tube, the two phases may distribute in

numerous flow patterns, each characterizing the radial and axial distributions of liquid and gas.

In our work, we could define five basic patterns for upflow (Fig. 2) in accordance with

previous classifications (TAITEL et aZ., 1980)

SMALL BUBBLE FLOW : The gas phase is uniformly distributed as discrete bubbles in a continuous liquid phase. In bubble flow, smaller bubbles will generate a more turbulent flow.

INTEFMEDIATE FLOW : Increasing the gas flow rate, the bubbles begin to deform and randomly collide and coalesce, forming larger individual bubbles with a spherical cap and a diameter smaller than the pipe one. In our classification, we have made a distinction between these two patterns for a better understanding of our filtration experiments. But small bubble and intermediate flows are usually regrouped as a single pattern called “bubble flow”.

SLUG FLOW : At higher gas flow rates, most of the gas is located in large builet-shaped bubbles which occupy most of the pipe cross-sectional area. They move regularly upwards and are sometimes designated as “Taylor bubbles”. The Taylor bubbles are axially separated by slugs of continuous liquid which bridge the pipe and contain small bubbles. The liquid confined between the Taylor bubble and the pipe wall flows downwards as a thin falling film.

CHURN FLOW : Chum flow is somewhat similar to slug flow. It is, however, much more chaotic, frothy and distorted, with a typical oscillatory motion of the liquid phase.

DISPERSED BUBBLE FLOW : At higher liquid flow rates (Qr, 2 1,3 m3 / h), turbulent forces act to break and disperse the gas phase into small bubbles. Consequently, the dispersed bubble flow pattern exists even for higher gas flowrates.

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Small bubble flow intermediate flow Slug flow Chum flow

Fig. 2 : Flow patterns in vertical flow Dispersed bubble flow

After the flow characteristics visualization, we plotted the existing data as a flow-pattern map in terms of system parameters (gas and liquid flow rates). This map was useful to define the fluid behaviour in the membrane unit (Fig. 3).

2.0

0.0

0.0 0.3 0.6 0.9 1.2 I .j

Gas flow rate (m3/h) Fig. 3 : Flow-pattern map

Air-water. vertical upward, D=Ij mm (a, q4 ,Q ,9) : Experimental points

Fig. 4 : Experimental visualization ofslug jlow ((II, -0.5 m?h and QG -0.49 m’.h)

Effect of a continuous gas injection on the membranepe$ormances

Figure 5 shows the results of the experiments with and without air sparging. As well as in usual

crossflow filtration, the permeate flux in the run without air injection decreased sharply during

the first few minutes due to the formation of a solid particle layer on the membrane surface

(Fig. 5). In the experiments with continuous air sparging, two main effects of the unsteady

operating conditions could be observed. First of all, the decline of the permeate flux was

slower : 35 min were needed to reach the half of the initial flux whereas under steady

conditions, 10 min were enough. Then, the final flux value was higher. The maximum

enhancement was achieved using the gas flow rate of 0.43 m”k and in this case, reached 140%

for the bentonite suspension. Obviously, the high and stable filtration flux is due to the

introduction of a gas phase in the feed stream.

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- Ihstcady (inlcrmediatc)

‘- IJnskady (slug flow)

0 40 80 120 160 0 40 x0 120 160

Filtration time (min) Filtration time (min)

Fig. 5 : Infuence qfga.s flow rate on benionite cross$‘ow~jiltration (Q,.=O. 5 rn’,:h)

Jntermediaie bubbEeMug. flow : C&=0.25 rn’,‘h Slug,flow : SC; --0.13 m3 ‘h

Fig. 6 : Evolution qf the relative hydraulic resistance under steady and unsteady condiiions (best condiEions oj$llration)

The combination (Qr.-0.5 m’/h, Qe=0.43 m”/h) corresponds here to an intermittent “slug flow”

(Fig. 2 and 4). Even if the inlet conditions are stationary, the flow, as an observer can see it, is

an unsteady phenomenon, dispersed flow appearing alternately with separated flow. These two

states follow in a random-like manner, inducing pressure and velocity fluctuations. In our case,

as Taylor bubbles move along the membrane, a drastic reduction of the channel section for the

feed stream results in a high shear rate between the membrane surface and the bubble interface.

The high shear rate improves the turbulence in the filtration unit and leads to a regular

disturbance of the particle layer. Moreover, the gas injection in the filtration module increases

the superficial crossflow velocity of the process fluid.

The influence of the gas phase on the membrane performances can also be observed through

the relative hydraulic resistance R defined as R=Jo/J-1 which represents the additional

resistance due to the particle layer formation. In our case, it can be divided by a factor nearby 3

(Fig. 6).

This unsteady technique is very efficient as, in the filtration of bentonite suspensions, the

limiting phenomenon is the external fouling. An intermittent gas-liquid flow can only remove

the particles on the membrane surface but not the matter plugging the membrane.

Steady and unsteady conditions with the same velocity in the membrane unit

In order to determine the main mechanism involved in the enhancement of the permeate f-lux

using a gas-liquid two-phase flow, a further experiment was carried out, consisting in

comparing the flux with and without air sparging, the gas and liquid flow rates being adjusted

to impose the same mean velocity in the membrane module (Fig. 7).

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’ Steady (Q,=O.85 m’/h)

The comparison of both curves shows that the

flux under unsteady conditions is mutiphed by

a factor 1.4. This means that the permeate rate

enhancement is not only due to the increase of

the fluid superficial velocity but also to the gas

itself, which generates instabilities in the feed

stream. 0.0 - ’ ’ ’ ’ I ’ ’ ’ I I ’ ’ ’ ’ I I ’ ’ ’ I ’ ’ I ’ 0 50 100 150 200 250

Filtration time (mm)

Fig. 7 : Flux decline under steady and unsteady conditions (mean veloci& of I. 3.5 mk)

CONCLUSION The preliminary studies presented in this paper showed that the use of air sparging in the membrane module to create a gas-liquid two-phase flow is a simple and successful technique to enhance effectively the ultrafiltration flux. Indeed, during the filtration of a bentonite suspension, the highest improvement of the permeate rate was 140%. In order to optimize the membrane performances, it seemed necessary to know, for each experiment, what kind of two-phase flow was generated in the membrane unit. So, our filtration assays were closely related to the flow characteristics. After these first experiments, the most appropriate flow seems to be the slug flow : the gas phase creates instabilities which lead to a regular disruption of the boundary layer. The combination unsteadiness-increased crossflow velocities at the membrane surface results in a significant enhancement of the filtration flux. Further experiments will be necessary, on the one hand, to test another gas flow rates and to find a compromise between the flux improvement and the economic cost, and on the other hand, to estimate the real possibilities of this process, for example, in high cell density fermentations.

REFERENCES ARROYO, G. and FONADE, C., (1993). Use of intermittent jets to enhance flux in crossflow filtration., .I Membrane. Sci. 80, 117-129 BEN AMAR, R., BOUZAZA, A., JAFFRIN, M.Y. and GLPTA, B.B., (1987). Augmentation du flux de permeat en UF et MF tangentielle par l’emploi de debit pulse., Rtcents Pro&s en G.&tie des Pro&d&s. 1,57-62 CUI, Z.F. and WRIGHT, KIT., (1994). Gas-liquid two-phase cross-flow ultrafiltration of BSA and dextran solutions., .I Membrane. Sci. 90, 183-189 FINNIGAN, S.M. and HOWELL, J.A., (1990). The effect of pulsatile flow on UF fluxes in a baffled tubular membrane system., Desal. 79, 18 l-202 LEE, C.K., CHANG, W.G. and JU, Y.H., (1993). Air slugs entrapped cross-flow filtration of bacterial suspensions., Biotech. Bioeng. 41,525-530 TAITEL, Y., BARNEA, D. and DUKLER, A.E., (1980). Modelling flow pattern transitions for steady upward gas-liquid flow in vertical tubes., AICHE J. 3,345-354 VAN DEN BERG, G.B. and SMOLDERS, CA., (1988). Flux decline in membrane processes., Filtr. Sep. March/April 88, 115-121

Aknowledgements : the authors thank K. Maccio for text language revision.

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