Influence of membrane morphology on characteristics of porous hydrophobic PVDF hollow fiber contactors for CO2 stripping from water
Highlights
► Different membrane morphologies were obtained using dry–wet spinning technique. ► Phosphoric acid (PA) and PEG-400 were used as non-solvent additives in the PVDF dope. ► The plain PVDF membrane showed a finger-like structure with high permeability, overall porosity and low wetting resistance. ► The PVDF/PA membrane exhibited sponge-like structure with high surface porosity and wetting resistance. ► The PVDF/PA and PVDF/PEG-400 membranes presented relatively higher CO2 stripping flux than the plain membrane.
Introduction
Dissolved carbon dioxide (CO2) in water has been responsible for reduction of oxygen level in aquaculture systems [1] and increasing acidity, which results in corrosion nature of water. In fact, the higher ratio of CO2 to bicarbonate ion directly decreases pH of water, which causes corrosion of piping materials results in increasing maintenance costs and addition of contaminants to water. In order to remove CO2 from water, aeration process using columns has been a common method. However, attention should be paid to the technologies with high removal efficiency and minimum operational and environmental impacts. Recently, gas–liquid membrane contactors have attracted considerable attention for CO2 absorption and stripping due to the advantages over conventional contacting devices [2], [3]. CO2 stripping using membrane contactors can be an alternative to increase pH level of water and solve the associated problems. Indeed, increasing pH level of water without addition of chemicals using CO2 stripping is a general interest due to lower environmental and economical impacts. The removal of scaling species such as CaCO3, Mg(OH)2 and Ca3(PO4)2 present in brackish and wastewater is possible by increasing pH using CO2 stripping method [4].
Although membrane contactors have shown attentions for gas absorption application [[5], [6], [7], [8]], more studies and development are expected for gas stripping using gas–liquid membrane contactors in the near future. Koonaphapdeelert et al. [9] have studied CO2 stripping from monoethanolamine (MEA) solution at high temperature using ceramic hollow fiber membrane contactor. It was found that the membrane contactors could be operated very well even in the region of ordinary column showing flooding or loading. Mansourizadeh and Ismail [10] investigated the effect of main operating parameters on the efficiency of CO2 stripping from water through the polyvinylidene fluoride (PVDF) hollow fiber membrane contactor. The liquid phase temperature, the gas–liquid contact area and liquid phase residence time were the key parameters for enhancement of CO2 stripping efficiency.
In gas–liquid membrane contactor processes, the membrane used is not selective and the pores should be filled with the gas phase. The driving force of mass transfer is concentration difference between gas and liquid phases and there is no convective flow through the membrane pores. Hence, partial wetting of the membrane pores can significantly decrease separation performance [11]. On the other hand, in pressure driven processes such as microfiltration, ultrafiltration and nanofiltration, the membranes are wetted by separated solutions, where separation efficiency is controlled by selective layer on the surface of membrane [12]. Since the membrane adds extra resistance to the process compared to conventional contacting devices, the porous membrane required to possess essential properties in order to minimize resistance. The main properties of the membrane include high surface porosity (permeability), high wetting resistance and good mechanical and chemical stability. These requirements can be fulfilled by using hydrophobic polymers like polypropylene (PP), polytetrafluoroethylene (PTFE) and PVDF, which possess low values of surface energy [13]. Since PP and PTFE cannot be dissolved in common solvent at room temperature, the symmetric membranes are usually prepared by stretching and thermal methods, which are complicated processes to control the membrane structure. On the other hand, PVDF has been used for preparing asymmetric membranes via phase-inversion process due to its solubility in organic solvents. In fact, the fabrication parameters during phase-inversion can be controlled to develop the membrane structure for different applications. Simone et al. [14] prepared microporous PVDF hollow fiber membranes via a dry–wet spinning technique for vacuum membrane distillation (VMD). By considering fabricating parameters, the membranes showed excellent mechanical properties, high porosity (up to 80%) and an average pore size ranging from 0.12 to 0.27 μm. A long term stable VMD was reported for over 15 days. In another study, microporous membranes with sponge-like structure were prepared by changing pore forming additives in the polymer dope. The hollow fiber membranes were used to perform As(V) extraction experiments in a membrane contactor module, using Aliquat-336 as extractant. Around 70% arsenic removal efficiency was achieved after 6 h of the operation [15]. The structure of the PVDF hollow fiber membrane was improved for CO2 absorption through a gas–liquid membrane contactor [16]. Using 4 wt.% lithium chloride as a non-solvent additive in the polymer solution resulted in a sponge-like structure with small mean pore sizes and high wetting resistance. After a gradual CO2 flux reduction of 25% the membrane demonstrated a stable long term operation.
Since the membrane structure is a key factor for membrane contactor applications, it is required to improve the structure to enhance the membrane performance. Therefore, there is a need to investigate the effect of morphology on the characteristics and performance of hydrophobic asymmetric hollow fiber membranes. In the present study, an attempt was made to prepare different membrane morphologies by considering thermodynamic and kinetic aspects of phase-inversion process. The cloud point diagrams of PVDF/NMP/water were obtained to study the phase-inversion behavior. The effect of prepared morphology on the overall porosity, gas permeability, wetting resistance and outer surface contact angle was investigated. In addition, CO2 stripping from water was conducted through the gas–liquid membrane contactors to study the prepared membrane performance.
Section snippets
Preparation of PVDF hollow fiber membranes
Commercial PVDF polymer pellets (Kynar® 740) were supplied by Arkema Inc., Philadelphia, USA. 1-methyl-2-pyrrolidone (NMP, > 99.5%) (Merck) was used as polymer solvent. Polyethylene glycol 400 (PEG-400) and ortho-phosphoric acid (PA) were supplied by Merck and used as non-solvent additives in the polymer dope.
Ternary phase diagrams of polymer/solvent/water were obtained using cloud point measurement to evaluate the phase inversion behavior of the polymer dopes. In order to measure the cloud
Cloud point diagrams
Generally, phase-inversion behavior of the polymer solution is an important factor for preparing porous membranes. Indeed, faster solidification of the polymer dope results in the membranes with high surface porosity and ultra thin skin layer [21]. These two parameters result in the membranes with low mass transfer resistance, which is a favorable factor for membrane contactor application. The addition of non-solvent additives into the polymer dope is an option to enhance solidification of the
Conclusion
Porous PVDF hollow fiber membranes with different morphologies were prepared via a dry–wet spinning technique. The influence of membrane morphology on the wetting pressure, gas permeability, overall porosity and water contact angle was studied. The plain PVDF membrane with large finger-like morphology presented the higher N2 permeance, lower wetting pressure and larger mean pore size. By addition of phosphoric acid in the spinning dope a sponge-like morphology was obtained, which demonstrated a
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