Biocatalytic gas-liquid membrane contactors for CO2 hydration with immobilized carbonic anhydrase
Graphical abstract
Introduction
The gas-liquid membrane contactor is a promising candidate for the CO2 removal from flue gas, biogas and natural gas. Compared with the conventional packed towers and bubble columns, the gas-liquid membrane contactor has special advantages such as large contact area, independent control of gas and liquid flow rate, small footprint, modular design, easy to operate and scale-up/down [1]. The gas liquid contactor uses porous hydrophobic membranes as non-selective physical barriers to define the gas-liquid interfaces, thus it can be operated without significant pressurization, leading to lower energy consumption and easier installation [2].
There is extensive existing research in this area; however, most focus on the membrane modification (hydrophobic) and contactor design. For the gas-liquid contactor, even a slight membrane pore wetting would significantly increase the total mass transfer resistance [3]. As a result, hydrophobic membranes and membrane hydrophobic modifications are usually applied to prevent the membrane pore wetting. On the other hand, the contactor module design has been intensively investigated based on both experimental work and theoretical calculation, including longitudinal, crossflow and coiled hollow fiber modules [4], [5]. The proper selection of the membrane module can promote the mass transfer efficiency and eventually improve the contactor performance.
While these two aspects are relatively well studied, the selection of the optimal liquid absorbent is a still unsolved issue for membrane contactors. Considering the mass transfer within the membrane contactor is dominated by the liquid side, the liquid absorbent that can lead to instantaneous chemical absorption is preferable [6]. Conventional amine based liquids, including methyldiethanolamine (MDEA) monoethanolamine (MEA) and diethanolamine (DEA), have high CO2 absorption capacities and rapid reaction rates, thus their applications in the bubble columns have been well documented [7], [8]. However, these amine-based liquids have some problems: the low surface tension of these liquids would lead to a high tendency of membrane pore wetting; they could react with the polymer membranes and eventually damaging the membrane porous structure; and their volatility will complicate the membrane contactor design, construction and operation. In addition, the strong chemical bonding between liquid and CO2 requires extra parasitic energy to regenerate the amine solutions, leading to higher operational cost for CO2 capture [3]. On the other hand, ionic liquids have been studied as liquid absorbent. However, their compatibility with polymeric membrane still needs more investigation [9]. As a result, using more benign solvents for CO2 absorption are desirable, such as carbonate solution or even pure water.
However, these benign solvents usually have low CO2 hydration kinetics, and thus the contactor requires higher flow rate or larger interfacial contacting area. A promising solution is to use the biocatalyst, carbonic anhydrase (CA, E. C.4.2.1.1), to promote the hydration efficiency. The enzyme has very high turnover numbers up to 1×106 s−1, making it one of the most efficient enzyme to promote CO2 hydration [10], [11], [12]. In order to increase the stability and facilitate its reuse, immobilized CA would be preferable. In our previous work, we have successfully immobilized CA onto TiO2 nanoparticles, TiO2 functionalized membranes and carbon nanotube membranes [13], [14]. Satisfactory enzyme loading and apparent activity retention with the hydration of 4-nitrophenyl acetate (p-NPA) have been achieved. However, it has been demonstrated that the apparent activity does not directly reflect the real CO2 hydration efficiency for the immobilized enzymes. As a result, the construction of the biocatalytic reactor should be carried out to test the feasibility of using the immobilized CA for practical CO2 hydration.
So far, there are only a few bioreactors with immobilized CA for CO2 hydration, such as the biocatalytic membrane reactor [15], [16], Janus biocatalytic membrane reactor [14], integrated vacuum carbonate absorption process (IVCAP) reactor [10], [17], hydrogel reactor [18], [19] and bubbling reactor [20]. Zhang et al. [10], [17] developed a series of IVCAP reactors where the biocatalytic nanoparticles were suspended within carbonate solution, which was filled into a sealed glass vessel containing pure CO2. Subsequently, the gradual loss of the gas pressure indicated the hydration of CO2 into the liquid. However, it is difficult to allow continuous operation for such a reactor. In another line of research, Zhang et al. [18], [19] encapsulated CA within the hydrogel, which was then filled into the interstitial spaces between two intimately co-mingled hydrophobic polyvinylidene fluoride (PVDF) membranes. However, the high mass transfer resistance for the hydrogel limited the full utilization of the immobilized enzymes, and the regeneration of the denatured enzyme was difficult.
The combination of the biocatalytic process and a gas-liquid membrane contactor allows continuous operation and easier process control. An early attempt was to dissolve CA within a liquid membrane sandwiched by two hydrophobic membranes [21]. Yet the poor stability of the liquid membrane caused deterioration of the long-term operational performance of the biocatalytic system. To solve this problem, CA was dispersed into the ionic liquid, [bmim][Tf2N], to prepare a biocatalytic liquid membrane [15]. A potentially more efficient approach is to strategically immobilize CA on a hydrophobic membrane surface to minimize the mass transfer resistance. For example, Arazawa et al. applied the CA coated hollow fiber membrane as an artificial lung to accelerate CO2 removal from blood [16]. Recently, Yong et al. developed a series of biocatalytic membrane reactors: CA was immobilized onto polymer membrane surface via the direct layer-by-layer approach or with the silica nanoparticle intermediate supports [22], [23], [24]. Similarly, we recently developed a novel Janus-type hydrophilic/superhydrophobic membrane contactor to strategically immobilize CA near the gas-liquid interface so the mass transfer resistance could be minimized [14]. A significantly improved CO2 hydration efficiency was achieved, yet the low enzyme loading on the membrane and the complicated fabrication processes limit their wider application. In principle, biocatalytic nanoparticles can be suspended in the liquid phase of the membrane contactor, and it ensures easy reuse and regeneration of the immobilized enzymes. With this respect, the combined system could provide a facile route to promote the CO2 hydration efficiency of gas-liquid membrane contactors with benign solvents.
In this work, CA was covalently immobilized onto TiO2 nanoparticles via the sequential process reported in our earlier publication [13]. The gas-liquid membrane contactors were constructed with both virgin polypropylene (PP) and superhydrophobic functionalized polypropylene (PP) hollow fiber membranes. Comprehensive investigations were carried out to understand the effect of operational parameters on the biocatalytic membrane contactor performance, including enzyme concentration, gas flow rate, liquid flow rate, gas temperature, liquid temperature, pH and buffer concentration. Next, the reusability of the immobilized enzyme was demonstrated with 10 cycles of both CO2 and 4-nitrophenyl acetate (p-NPA) hydration tests. The mass transfer coefficients for the biocatalytic gas-liquid membrane contactor were calculated to provide the insights into the biocatalytic CO2 hydration process. Lastly, a preliminary reactor optimization was carried out with a superhydrophobic PVDF flat sheet membrane contactor, which had much lower membrane resistance.
Section snippets
Materials
The hydrophobic hollow fiber polypropylene (PP) membrane was purchased from Membrana (Accurel PPS6/2, 0.2 µm pore size, wall thickness 450 µm, inner diameter 1800 µm). The hydrophobic flat sheet PVDF membrane was purchased from Millipore with a pore size of 0.45 µm. Carbonic anhydrase (CA) from bovine erythrocytes (E. C.4.2.1.1,≥3,500 W-A units per mg protein), 4-nitrophenyl acetate (p-NPA), titanium isopropoxide (TTIP,≥97%), 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PDTS), glutaraldehyde and
Effect of superhydrophobic modification
Membrane pore wetting is a serious issue for the membrane contactor. It significantly increases the mass transfer resistance [3]. After the TiO2-based superhydrophobic modification, the water contact angle of the PP membrane increased from 105° to over 160°. We further investigated the penetration pressure of both virgin and modified membranes. The virgin membrane had the penetration pressure of 3.05 bar. After the modification, the LEP of the hollow fiber membrane increased to 3.75 bar. The
Conclusion
In summary, the incorporation of biocatalytic nanoparticles with the gas-liquid membrane contactor could effectively enhance the CO2 hydration efficiency, especially for the contactor with low membrane resistance operating at low liquid Reynolds number.. The feasibility of utilizing such a biocatalytic system for practical application was demonstrated by exposing the contactor to a wide pH range and high feed gas temperature. The biocatalytic nanoparticles also showed satisfactory reusability
Acknowledgements
This work is financially supported by the Australian Research Council funding scheme (DP1095930 and DP130104048) and the University of New South Wales Goldstar Award.
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