In spite of the advantages discussed in overview, gas transfer membranes have intrinsic problems that can be detrimental depending on application.
- Lumen condensation
Lumen condensation occurs when the sweep gas supplied to the hollow fiber lumen immersed in liquid/water transfers to shell. This phenomenon commonly occurs in membrane oxygenation as shown in Fig. 2 of overview. However, degassing and dehumidification applications are largely immune to this issue because either partial vapor pressure in the lumen is always lower than the vapor pressure of liquid (degassing) or vapor is lost through membrane (dehumidification). In humidification process, lumen condensation does not occur as long as all the sweep gas components are saturated in shell side liquid so that no sweep gas components transfer through the membrane.
Fig. 1 illustrates the mechanisms of lumen condensation in oxygenation process using pure oxygen. In this example, hollow fiber membrane is immersed in mixed liquor with microorganisms that treat COD using the oxygen supplied through membrane. The mechanisms of lumen condensation is as follow.
- As soon as oxygen gas enters the membrane lumen, oxygen starts to diffuse out to shell through membrane due to the partial pressure gradient. In the beginning, partial oxygen pressure in the lumen might be 1 bar while that in shell is ~0.02 bar assuming dissolved oxygen level is 1 mg/L.
- Water vapor, dissolved carbon dioxide, and nitrogen in shell side water diffuses into lumen since initial partial pressures of those gases in pure oxygen is zero.
- After gas proceeds a certain distance (or transitional length), H2O vapor in lumen reaches saturation point. No further water vapor intrusion occurs from that point.
- Likewise, CO2 and N2 also reaches equilibrium pressures with shell side liquid. No further intrusion occurs from that point on. Transitional lengths for H2O, CO2 and N2 are all different depending on permeance of each gas, initial partial pressure, etc.
- While the H2O, CO2 and N2 intrusions into lumen are ending, O2 is continuously transferred to liquid. This causes gas volume contraction, which effectively slows down the gas velocity in the lumen.
- Due to the loss of O2 to shell, partial pressures of H2O, CO2 and N2 increase. The excess gases can transfer back to shell due to the reverse concentrate gradient. Due to low boiling point, H2O would rather condense than back transferred to shell.
Due to the above mechanism, water condensation in lumen is unavoidable in membrane aeration. Once one fiber is plugged by condensate, inlet gas is distributed among other intact fibers, which eventually increases pressure loss and decrease apparent mass transfer rate. This is a fundamental limitation that needs to be overcome to make this process commercially viable in large scale.
Based on the literature data, the actual specific gas transfer rates per membrane area appear below or around 1/10 of the clean dry membrane’s in hydrogen based hollow fiber membrane biofilm reactor. There are various potential causes of the lackluster performance, but lumen plugging is likely one of the major causes. Detailed literature data analyses are not performed in this article.
Fig. 1. Conceptual diagram showing water condensation issue in hollow fiber lumen.
To combat lumen condensation in membrane oxygenation, one end of porous hollow fiber was hydrophilized and sealed as shown in Fig. 2. The other side was coated to make that area non-porous. While oxygen is supplied from the open end with pressure, the condensate is formed in the middle of the fiber. Since the oxygen in the downstream of the condensate drop continues to transfer to the shell, condensate drop can proceed to the hydrophilic area. Once the condensate reaches the hydrophilic area, it is pushed out by pressure.
- Pore wetting
It is self explanatory, but pore wetting occurs only when membranes have pores. Therefore, non-porous membranes do not have this issue whatever the application is.
As shown in Fig. 3a, when dry hydrophobic membranes are contacted with water, water does not intrude pores unless its pressure exceeds the bubble point. Bubble point can be calculated using the equation shown in direct integrity monitoring method. According to the equation bubble point is directly related with the cosine of the contact angle. Therefore, it is crucial to maintain the contact angle stable by keeping the membrane surface hydrophobic.
However, initially hydrophobic membrane surface can become partially hydrophilic over time. In practice, any raw water contains impurities more or less, which can deposit on membrane gradually. If any material deposits near the entrance of membrane pore, it compromises local contact angle that in turn decreases bubble point. Once bubble point drops below the water pressure, water intrudes into lumen and plug up the fiber. This gradual pore wetting problem appears unavoidable over the long run. Pore wetting will become more significant, if biological activity occurs in water phase, e.g. oxygenation of mixed liquor in biological wastewater treatment.
a) Clean membrane with intact meniscus
b) Contaminated membrane with fouling around pore entrance
Fig. 3. Conceptual diagram of pore wetting mechanism for porous hydrophobic membrane.
- Mass transfer barrier in shell
When gas is dissolved into liquid through membrane, e.g. membrane oxygenation, gas concentration gradient develops as shown in Fig. 4. The gas concentration is the highest in the immediate membrane surface and it decreases gradually until it reaches bulk concentration. Due to the high concentration on membrane surface, driving force for gas molecules to diffuse through membrane decreases. Though gas dissolution can be enhanced by agitating the liquid, which decreases the boundary layer thickness and increases the diffusion of gas molecules out of boundary layer. However, the energy required to agitate the liquid can partially or fully compromise the energy savings of the process.
Fig. 4. Mass transfer across membrane.
- Aging biofilm and its renewal
In most circumstance, biofilm formation on the wet side of membrane surface is unavoidable. If the membrane surface is intended to be clean especially in vacuum degassing, humidification / dehumidification, biofilm formation can be an issue because it hampers mass transfer through the membrane. On the contrary, biofilm formation is intended to enhance mass transfer, e.g. membrane oxygenation, membrane supported biofilm reactor, etc.
As shown in Fig. 5, biofilm consumes oxygen immediately on membrane surface, which accelerates oxygen transfer from the gas phase by making the concentration gradient steeper. However, as biofilm gets aged, dead cells in the biofilm eventually acts as mass transfer barrier that makes the concentration profile in the biofilm less steeper. Ideally, the dead and active cells are in equilibrium with much more active cells in biofilm. But, since there is no mechanism to remove dead cells completely from the biofilm due to the non-biodegradable portion of the cell mass, the activity of biofilm must decrease gradually. Eventually each segment of biofilm will slough off randomly when biofilm loses its ability to attach, which causes performance fluctuation.
Fig. 5. Mass transfer across membrane.
© Seong Hoon Yoon