Results and Discussion
4.1 Influence of Membrane Nature and Operation Parameters on Filtration Characteristics
4.1.3 Effect of transmembrane pressure
Transmembrane pressure has a significant effect on the permeate flux during membrane operation. The operating pressure for UF is usually about 0.1−0.7 MPa. In this study, the experiments were run at an emulsion concentration of 0.5 and 5 vol.%
and influent feed velocity of 0.9 m/s unless stated elsewhere. Experiments were performed at initial transmembrane pressures of 1, 2, 3, 4, 5 and 6 bar to evaluate the effect of transmembrane pressure. The effects of operating pressure for the selected membranes on permeate flux at two feed concentrations are shown in Figures 4.1.5, 4.1.6 and 4.1.7.
Taking Figures 4.1.5, 4.1.6 and 4.1.7 together, some common phenomena for the tested membranes can be observed:
(1) At a lower emulsion concentration three kinds of UF membrane have higher abilities to permeate water. At each experimental temperature the flux increased approximately with an increase in the operating pressure, especially in case of the PES membrane. It shows that the effect of concentration polarization or gel layer is not obvious.
(2) At a higher emulsion concentration the effect of the operating pressure on the flux becomes complicated. At a lower pressure, the permeate flux is directly proportional to the transmembrane pressure. When the operation pressure is over a critical value, the flux is not influenced by the operating pressure and reaches a plateau. This kind of variation tendency is the same on each membrane. It is estimated that the membrane fouling and concentration polarization become more serious with increasing the operating pressure at higher emulsion concentration.
(3) The critical pressure for different membrane is variable, which depends on the capillary pressure. When transmembrane pressure is over the capillary pressure, oil can easily pass and deposit in the membrane pores thus decreasing membrane pore
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Chapter 4.1. Influences of Membrane and Process
size and increasing membrane fouling [WILLIAMS and WAKEMAN 2000]. At higher operating pressure the effect of membrane fouling is more important than the effect of pressure. The critical pressures for FS 202-09 and FP 055A membranes are about 2 bar, whereas it is about 3 bar for TS 6V membrane.
(4) The fouling resistance of FVDF (FP 055A) membrane is lower than that of PES membrane (FS 202-09 and TS 6V-205) at lower feed concentration (see Figures 4.1.5 (a), 4.1.6 (a) and 4.1.7 (a)), in which the flux is almost not influenced by higher transmembrane pressure for PES membrane.
(5) The effect of pressure on the flux is controlled by the temperature for PES membranes (FS and TS 6V). At different temperatures the extent of pressure influence is different.
0 1 2 3 4 5 6
0 50 100 150 200 250 300
20oC 30oC 40oC 50oC 60oC Permeate flux, [l/m2h]
Pressure, [bar]
Figure 4.1.5 (a) Permeate flux as a function of transmembrane pressure for FP 055A at feed oil concentration of 0.5%
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Chapter 4.1. Influences of Membrane and Process
0 1 2 3 4 5 6
0 30 60 90 120 150 180
20oC 30oC 40oC 50oC 60oC Permeate flux, [l/m2h]
Pressure, [bar]
Figure 4.1.5 (b) Permeate flux as a function of transmembrane pressure for FP 055A at feed oil concentration of 5%
0 1 2 3 4 5 6
0 30 60 90 120 150 180
20oC 30oC 40oC 50oC 60oC
Permeate flux, [l/m2h]
Pressure, [bar]
Figure 4.1.6 (a) Permeate flux as a function of transmembrane pressure for FS 202-09 membrane at feed oil concentration of 0.5%
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Chapter 4.1. Influences of Membrane and Process
0 1 2 3 4 5 6
0 30 60 90 120 150
20oC 30oC 40oC 50oC 60oC Permeate flux, [l/m2h]
Pressure, [bar]
Figure 4.1.6 (b) Permeate flux as a function of transmembrane pressure for FS 202-09 membrane at feed oil concentration of 5%
0 1 2 3 4 5 6
0 30 60 90 120 150 180
20oC 30oC 40oC 50oC 60oC
Permeate flux, [l/m2h]
Pressure, [bar]
Figure 4.1.7 Permeate flux as a function of transmembrane pressure for TS 6V-205 membrane at feed oil concentration of 0.5%
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Chapter 4.1. Influences of Membrane and Process
0 1 2 3 4 5 6
0 30 60 90 120 150
20oC 30oC 40oC 50oC 60oC Permeate flux, [l/m2h]
Pressure, [bar]
Figure 4.1.7 Permeate flux as a function of transmembrane pressure for TS 6V-205 membrane at feed oil concentration of 5%
As well known, the transmembrane pressure is applied to overcome the membrane resistance and the pressure drop of the fluid on the membrane surface [WANG 1984].
The effect of transmembrane pressure depends on the membrane property and the behaviors of the boundary layer. It should be paid more attention to air bubble in permeate as treating an emulsion under a higher operating pressure. Because there are some surfactants in the feed emulsion, the air bubble can pass through the membrane and enter into permeate under a higher operating pressure. The air bubble may influence greatly the mass transfer coefficient and may lead to variation of permeate flux. The further study on the effect of air bubble is necessary for ultrafiltration of oily emulsion.
The experimental results of COD and oil concentration in permeate under their critical pressures at 40oC are presented in Table 4.1.5. It shows that the three investigated membranes have satisfactory results at low feed concentration (0.5 %): the permeate from both membranes of FS 202-09 and TS 6V-205 can achieve the level to discharge according to the National Standard of China (the oil concentration in permeate is <10 mg/l, COD <100 mg/l). At high feed concentration (5 %) their COD values increase, whereas their oil concentrations are lower.
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Chapter 4.1. Influences of Membrane and Process
When the pressure increased to 4 bar, the COD values and oil concentrations in the permeate were increased more rapidly, as shown in Table 4.1.6. Therefore these membranes are not suitable to treat the selected emulsion with a high feed concentration at high transmembrane pressure. This is because the membrane fouling (loss of permeability) can be negligible at lower operating pressure, the membrane fouling, however, becomes more important as the pressure is over a critical pressure.
The reason of this phenomenon seems to be that concentration polarization gives a higher probability for the oil drops to contact the membrane pores, and at some pores the operating pressure exceeds the capillary pressure so that the oil drops can be deformed and enter the membrane structure [LEE et al. 1984, SEIFERT and STEINER 1996].
a): Feed oil concentration 0.5%; b): Feed oil concentration 5%.
Table 4.1.6 Oil concentration and COD in permeate at 4 bar Membrane COD a)
a): Feed oil concentration 0.5%; b): Feed oil concentration 5%.