교차흐름 정밀여과에 있어서 맥동효과

노 성 희,  김 선 일

조선대학교 공과대학 화학공학과

Pulsation Effect in Crossflow Microfiltration

Sung-Hee Roh, Sun-Il Kim

Dept. of Chem. Eng., Chosun University, Kwangju 501-759, Korea

INTRODUCTION

CFMF is used to separate particles of the size range 0.01 to 10 □. CFMF is a pressure driven membrane process in which fluid to be filtered flows parallel  to the membrane surface which reduce the formation of filter cake allowing better permeate flux. Currently, CFMF is used for many applications, such as, water and wastewater treatment, medical and pharmaceutical treatment. food and beverage processing, biotechnical applications, etc[1-3]. There are certain problems encountered in  CFMF such as membrane compression, fouling, concentration polarization, gel layer formation, deposition, internal clogging etc, which reduce the flux and membrane life time and it was found that clogging cannot be avoided completely. Many declogging techniques are described in literature[4], some of which are : the periodical backflush with filtrate, the use of abrasive, pulsation of flow etc[5]. In this present study, the effect of operating parameters of pulsation of flux was studied using laboratory scale microfiltration experiments. Artificial suspension plastic clay was used in this investigation.

PRINCIPLE OF PULSATION CLEANING TECHNIQUE

Pulsation of flux in microfiltration can be achieved easily by operating three valves, one in the concentrate line, another in the permeate line and the third one in the recycle line. Figure 1 shows a membrane unit with valves for pulsating of flux.

Fig. 1. Arrangement of valves for pulsating of flux.

An automatic timer arrangement was provided to close and open these valves at required time interval. By closing valve (2), a shock wave is created through the membrane and this helps in the removal of particles deposited (including internal clogging). Closure of valve (1) modifies the pattern of deposited along the membrane surface. Opening of valve (3) at the same time (i. e., during the closure of valves (1) and (2) prevents this system from the impulse wave created by the sudden closure of valve (1). The deposited particles are disturbed and become resuspended in the suspension during the operation of closing the valves (1) and (2). Then both the valves (1) and (2) are open while the valve (3) is closed. So with crossflow, disturbed deposited particles and colloids which caused internal clogging are removed. In addition, a turbulence is created along the membrane surface, which reduce the boundary layer thickness, concentration polarization and gel layer formation. Thus the total resistance is further reduced. This operation is carried out at a certain time internal. The expected variation of permeate flux and of concentrate flow are shown in Figure 2.

Fig. 2. Variation of concentration and permeate flux during pulsating of flux process.

EXPERIMENTAL

Experiments were carried out to study the feasibility of using pulsating cleaning technique as membrane cleaning technique in CFMF. The membrane filter used was a tubular unit made of ceramic material. The effective filtration area of membrane used was 45.5 Cm2. Membrane housing unit made of stainless steel was used. Figure 3 shows the schematic diagram of experimental set up used. The test suspension of plastic clay used in the present study, was pumped from the base of the storage tank under pressure into the membrane filter. The effluent of the feed suspension was filtered out as permeate while most of the suspension was recycled back to the storage tank. The by-pass system was used to control the operating condition such as applied pressure and crossflow velocity. The cooling system was necessary for keeping temperature constant as 30±2℃. Three solenoid valves were used for pulsation control, the location and diameters of which are shown in Figure 3. During filtration, valves S1 , and S2 were opened while valves S3 was closed. But the first two valves were closed during closure period while the third was opened. The period of closure of valves S1 and S2 (which causes pulsation of flow in the unit) was programmed by a programmable controller (OMRON SYSMAC-S6). The details of solenoid operations are shown in Figure 4.

RESULTS AND DISCUSSION

The effect of pulsation frequency (Tpf) of 1 min., 2 min., and 5 min., was studied for three different break durations before pulsation (Tpb) of 1 sec., 2 sec,. and 5 sec.

The results obtained are given in Figure 5. In case of every 1 min., (Tpf), a maximum permeate flux was obtained for a (Tpb) of 1 sec. The difference in flux among the three different pulsation frequencies, though superior to that when no pulsating technique was used, was very little. In other words, and improvement of 80% in flux was achieved when 1 min., (Tpf) and 1 sec., (Tpb) was applied. When

Fig. 3. Experimental set-up of tubuar ceramic membrane with pulsating of flux as a declogging         technique.

A: Pulsation frequency(1, 2 and 5min)

B: Break duration before pulsation

(1, 2 and 5 sec)

Fig. 4. Mode of solenoid valves operation.

(Tpb) was increased to 2 sec., and 5 sec., no further improvement was observed. But, the net cumulative permeate volume was less when (Tpf) was long. In the case of 2 min., (Tpf) , maximum flux was obtained when (Tpb) was extended to 5 sec. This shows the requirement of longer break in filtration before pulsation at every 2 min., pulsation. But, improvement of flus in case of pulsation for every 2 min., was less than the pulsation for every 1 min. In the case of 5 min., (Tpf), no significant improvement in permeate flux was observed, which indicated the 5 min., duration was too long. These observation indicated that in pulsation of flux technique, the effect of pulsation frequency was important than the break duration in filtration before pulsation. In all the above experiments the permeate turbidity was observed to be same, ranging between 0.1 to 0.2 NTU, which shows about 99.6 to 99.9% of removal efficiency. In all the above experiments, the flux improvement by pulsating cleaning technique was noticed only after 8 minutes of filtration. The experimental results along with the percentage increase in permeate flux after 120 minutes of filtration, are tabulated in table 1. From this study, it was observed that maximum permeate flux was obtained when Tpf 1 min., and (Tpb) was used. In this case a significant percentage increase in permeate flux of about 30% was obtained.

Fig. 5. Effect of break duration on filtration before pulsation(Tpb) on pulsation               frequency(Tpb) in CFMF with pulsation technique using clay as suspension.

(Vs= 3m/s, □P= 100kPa, T=30±2℃, turbidity of clay suspension= 55NTU)

CONCLUSION

The following conclusions can be drawn the above study conducted and results obtained :

1. The higher permeate flux was obtained when the operation of pulsation was more frequent. In the ranges studied in this research, every 1 min., Tpf and 1 sec., Tpb was the optimum pulsation cleaning technique. An improvement of 80% in flux was obtained by this pulsation cleaning technique.

2. In every 2 min., pulsating flux, the longer time (5 sec.) of Tpb contributed to the improvement of permeate flux.

3. Pulsation cleaning technique was found to be ineffective when pulsation was applied once in 5 minutes (as it was long in duration). Membrane resistance, mainly due to deposition and internal clogging, would have developed significantly during this 5 minutes period.

REFERENCES

1. Aimar, P., Howell, J. A., Clifton, M. J. and Sanchez, V., J. Membr. Sci., 59, 81    (1991).

2. Knops, F. N. M., Futselaar, H. and Racz, I. G., J. Membr. Sci., 73, 153 (1991).

3. McGregor, W. C., J. Biotechnol., 10, 277 (1989).

4. Vigneswaran, S. and Ben Aim, R., ENSIC REVIEW, 31, AIT, Thailand (1990).

5. Mistry, V. V. and Kosikowski, F. V., J. Dairy Sci., 68, 2536 (1984).