Hollow Fiber Membrane Modules for NOx Removal using a Mixture of NaClO3 and NaOH Solutions in the Shell Side as Absorbents

Nitrogen oxide (NOx) is one of the polluting gases harmful to humans and the environment. Nitrous oxide gas is mostly found in air, namely nitrogen monoxide (NO) and nitrogen dioxide (NO₂). Nitrogen oxide gas in the air, which mostly comes from exhaust gases, needs to be reduced to minimize the threats to humans and the environment and comply with applicable regulations regarding hazards. The absorption process with a membrane contactor is an alternative to reduce NOx concentrations in the air. This study evaluates the hollow fiber membrane modules' performance in the NOx absorption process using sodium chlorate (NaClO₃) and sodium hydroxide (NaOH) together as an absorbent solution. Based on the experimental results, the NOx reduction efficiency increased from 96.3 to 99.2% and from 99.4 to 99.7% with an increase in the concentration of NaClO₃ from 0.02 to 0.05 M and the number of fibers in the membrane module from 50 to 150. However, the absorption efficiency declined from 99.7 to 99.2% by increasing the feed gas flow rate from 100 to 200 mL/min. The highest value of NOx reduction efficiency, the overall mass transfer coefficient, the flux, and the NOx loading obtained in the study were 99.7%, 0.01743 cm s^-1, 9.510´10^-8 mmole cm^-2 s^-1, and 0.026 mole NOx/mole NaClO₃, respectively.

Absorption efficiency; Hollow fiber membrane module; NaClO3; NaOH; NOx

In the 21^st century, air pollution has become one of the global community's problems of concern. Pollutants cause air pollution from harmful gases, one of which is nitrogen oxide (NOx) such as NO and NO₂. Nitrogen oxide gas is generally formed from the combustion process with a high temperature above 300^oC (Tan et al., 2019). Fifty-five percent of NOx gas comes from motor vehicles, and 45% comes from the industries' combustions process. High NOx levels in the atmosphere are the leading cause of acid rain, smog formation, decreased water quality, and global warming (Skalska et al., 2010; Gao et al., 2018; Sun et al., 2019; Mohan et al., 2020). Moreover, exposure to NOx gas with a 50-100 ppm concentration can cause lung inflammation from a health perspective. If the NOx concentration reaches 500 ppm, the people who inhale will inevitably die within 2-10 days (Shaw and Chadwick, 1998).

        According to Government Regulation No. 45/1997, the quality standard for NOx in the air is 100µg/Nm³ or about 0.05 ppm (Ministry of Environment RI, 1997). Various technologies have been developed to reduce the NOx concentration in the air. These technological developments include dry methods,  such  as  Selective   Catalytic   Reduction (SCR)  and   Selective   Non-catalytic   Reduction  (Brandenberger et al., 2008) and wet methods, such as absorption using absorbents (Kartohardjono et al., 2019a; Fangyang et al., 2020). The SCR method uses NH₃ as a reducing agent over catalysts based on V₂O₅-WO₃/TiO₂ or Cu- and Fe-zeolite, which is very efficient to reduce NOx but requires high temperatures around 300 to 400°C (Grossale et al., 2008; Mehring et al., 2012; Wang et al., 2019).  The dry methods widely used are low-NOx burners and SCR, which have the disadvantages of low-efficiency and high investment costs, making the wet methods attractive to many researchers (Guo et al., 2018; Kartohardjono et al., 2019a). The wet methods through absorption in the conventional gas-liquid contactor still have disadvantages such as the relatively low contact surface area between 25-75 ft²/ft³, thereby reducing the mass transfer. One alternative technology for NOx gas absorption to increase the contact surface area is using a membrane module as a gas-liquid contactor (Cai et al., 2019).

Several previous studies have been conducted regarding NOx absorption through a membrane contactor using a mixture of solutions functioning as an oxidizer and absorbent. The effective oxidizing agents include NaClO₃, NaClO₂, KMnO₄, and H₂O₂ with the addition of NaOH or HNO₃ as an absorbent (Yan et al., 2018;  Kartohardjono et al., 2019a; Kartohardjono et al., 2020). Sodium chlorate and NaClO₂ showed good NOx absorption efficiency (> 90%) with the bubble column reactor media. A study by Shi et al. (2019) with NaClO₃/NaOH solvents conducted in the bubble column reactor media showed promising results with the highest NOx absorption efficiency achieved, namely 91.5%. This study aims to see the polysulfone-based hollow fiber membrane modules' ability as media for the NOx gas absorption process using a mixture of NaClO₃ and NaOH solutions as an absorber. The reaction mechanism of NOx absorption by NaClO₃ may occur as follows (Shi et al., 2019):

                                                  NaClO₃ + H⁺ ? Na⁺ + HClO₃                                                         (1)

                               13NO + 6HClO₃ + 5H₂O ? 6HCl + 3NO₂ + 10HNO₃                                        (2)

                                                         3NO₂ + H₂O ? 2HNO₃ + NO                                                       (3)

                                                2NO + H₂O + HClO₃ ? HCl + 2HNO₃                                                 (4)

                                  2NO + H₂O + NaClO₃ + H⁺ ? Na⁺ + HCl + 2HNO₃                                          (5)

                                       NaClO₃ + 2NO + H₂O ? 2HNO₃ + NaCl                                                     (6)

       The study has been conducted to reduce the NOx concentration from its mixture with N₂ in the hollow fiber membrane modules using an absorbent of a mixture of NaClO₃ and NaOH solutions. The experimental results confirmed that the gas stream's NOx concentration could be drastically reduced through the proposed process. The NOx's absorption efficiency increased with increasing NaClO₃ concentration in the absorbent solution and the amount of fibers in the membrane module. However, the NOx's absorption efficiency declined as the feed gas flow rate increased. The best results from experiments on the NOx absorption efficiency, the overall mass transfer coefficient, the flux, and the NOx loading were 99.7%, 0.01743 cm s^-1, 9.510´10^-8 mmole cm^-2 s^-1, and 0.026 mole NOx/mole NaClO₃, respectively.

    The authors wish to acknowledge the financial support for this study from the PDUPT Project via the Directorate of Research and Services Universitas Indonesia through Contract No. NKB-267/UN2.RST/HKP.05.00/2020.

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